Study of Thin Film Solar Cell based on Copper Oxide Substrate by ...
Study of Thin Film Solar Cell based on Copper Oxide Substrate by ...
Study of Thin Film Solar Cell based on Copper Oxide Substrate by ...
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GENERAL INFORMATIONHARTNELL COLLEGEADDRESS411 CENTRAL AVENUESALINAS, CA 93901GENERAL COLLEGE NUMBER 831-755-6700831-755-6711WEB ADDRESSwww.hartnell.eduATHLETICS WEB ADDRESSCOLLEGE MAPS WEB ADDRESSwww.hartnell.edu/athleticswww.hartnell.edu/about/hartnellATHLETIC DEPT. PHONE NUMBER 831-755-6830ATHLETIC DEPT. FAX NUMBER 831-755-6831GENERAL LOCATIONHartnell College is located 60 miles south <str<strong>on</strong>g>of</str<strong>on</strong>g>San Jose and 120 miles north <str<strong>on</strong>g>of</str<strong>on</strong>g> San LuisObispo in the produce capital <str<strong>on</strong>g>of</str<strong>on</strong>g> the world –Salinas. Nearest airports (San Jose 1 hr, S.F.and Oakland 2 hrs and M<strong>on</strong>terey 20 min.)
thin film structure created <strong>by</strong> reactiveDC sputtering at various temperatures 973.3.3 Photovoltaic effect in Cu/Cu 2 O/TiO 2 /Au(heterojuncti<strong>on</strong>) thin film structure created<strong>by</strong> reactive DC sputtering 993.3.4 Performance comparis<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> differentphotovoltaic cell build 103CONCLUSIONS 1064
1 Introducti<strong>on</strong>1.1 <str<strong>on</strong>g>Solar</str<strong>on</strong>g> energy c<strong>on</strong>versi<strong>on</strong>Availability <str<strong>on</strong>g>of</str<strong>on</strong>g> affordable energy has enabled spectacular growth <str<strong>on</strong>g>of</str<strong>on</strong>g>industrializati<strong>on</strong> and human development in all parts <str<strong>on</strong>g>of</str<strong>on</strong>g> the world. With growth(low accelerating In developing countries. demands <strong>on</strong> energy sources andinfrastructure are being stretched to new limits. Additi<strong>on</strong>al energy issues includethe push for renewable resources with reduced greenhouse gas emissi<strong>on</strong>s andenergy security affected <strong>by</strong> the uneven distributi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> energy resources around theglobe. Together, these issues present a field <str<strong>on</strong>g>of</str<strong>on</strong>g> opportunity for innovati<strong>on</strong>s toaddress energy challenges throughout the world and all al<strong>on</strong>g the energy flow.These energy challenges form the backdrop for this special expanded in [1]. Thereis introduced the global landscape <str<strong>on</strong>g>of</str<strong>on</strong>g> materials issues associated with energy. Itexamines the complex web <str<strong>on</strong>g>of</str<strong>on</strong>g> energy availability, producti<strong>on</strong>. Storage,transmissi<strong>on</strong>, distributi<strong>on</strong>, use, and efficiency. It focuses <strong>on</strong> the materialschallenges that lie at the core <str<strong>on</strong>g>of</str<strong>on</strong>g> these areas and discusses how revoluti<strong>on</strong>aryc<strong>on</strong>cepts can address them. At the Figure 1.1 presented the world total energyprimary energy supply [1].5
Figure 1.1 World total primary energy supply (2004) <strong>by</strong> source. Mtoe ismilli<strong>on</strong> t<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> oil equivalentPhotovoltaics (PV) are an important energy technology for many reas<strong>on</strong>s. As asolar energy technology, it has numerous envir<strong>on</strong>mental benefits. As a domesticsource <str<strong>on</strong>g>of</str<strong>on</strong>g> electricity, it c<strong>on</strong>tributes to the nati<strong>on</strong>'s energy security. As a relativelyyoung, high-tech industry, it helps to create jobs and strengthen the ec<strong>on</strong>omy. Asit costs increasingly less to produce and use, it becomes more affordable andavailable.Few power-generati<strong>on</strong> technologies have as little impact <strong>on</strong> the envir<strong>on</strong>mentas photovoltaics. As it quietly generates electricity from light, PV produces no airpolluti<strong>on</strong> or hazardous waste. It does not require liquid or gaseous fuels to betransported or combusted. In additi<strong>on</strong>, because its energy source - sunlight - isfree and abundant, PV systems can guarantee access to electric power.PV frees us from the cost and uncertainties surrounding energy supplies frompolitically volatile regi<strong>on</strong>s. In additi<strong>on</strong> to reducing our trade deficit, a robustdomestic PV industry creates new jobs and strengthens the world ec<strong>on</strong>omy.Photovoltaics is about to celebrate 50 years <str<strong>on</strong>g>of</str<strong>on</strong>g> its modern era.6
Figure 1.2 World photovoltaic module producti<strong>on</strong> (in megawatts), totalc<strong>on</strong>sumer, and commercial per country [1]). Most <str<strong>on</strong>g>of</str<strong>on</strong>g> this producti<strong>on</strong> isfrom crystalline or multicrystalline Si solar cells at present.During this time, the industry has grown from small satellite power supplies toutility-scale systems that are now produced and routinely installed in manycountries <str<strong>on</strong>g>of</str<strong>on</strong>g> the world (figure 1.2, 1.3). <str<strong>on</strong>g>Solar</str<strong>on</strong>g> cells capable <str<strong>on</strong>g>of</str<strong>on</strong>g> producing power inexcess <str<strong>on</strong>g>of</str<strong>on</strong>g> 500 MW were manufactured in 2002, providing electricity to a variety<str<strong>on</strong>g>of</str<strong>on</strong>g> applicati<strong>on</strong>s ranging from small c<strong>on</strong>sumer products, power systems for isolateddwellings and remote industrial equipment to building-integrated solar arrays andmegawatt-size power stati<strong>on</strong>s.7
Figure 1.3 Trend in worldwide PV applicati<strong>on</strong>s (From Reference [5] MaycockP, Renewable Energy World 3, 59–74 (2000).<str<strong>on</strong>g>Solar</str<strong>on</strong>g> electricity, also known as photovoltaics (PV), has shown since the 1970sthat the human race can get a substantial porti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> its electrical power withoutburning fossil fuels (coal, oil or natural gas) or creating nuclear fissi<strong>on</strong> reacti<strong>on</strong>s.Photovoltaics helps us avoid most <str<strong>on</strong>g>of</str<strong>on</strong>g> the threats associated with our presenttechniques <str<strong>on</strong>g>of</str<strong>on</strong>g> electricity producti<strong>on</strong> and also has many other benefits.Photovoltaics has shown that it can generate electricity for the human race for awide range <str<strong>on</strong>g>of</str<strong>on</strong>g> applicati<strong>on</strong>s, scales, climates, and geographic locati<strong>on</strong>s.Photovoltaics can bring electricity to a rural homemaker who lives 100 kilometersand 100 years away from the nearest electric grid c<strong>on</strong>necti<strong>on</strong> in her country, thusallowing her family to have clean, electric lights instead <str<strong>on</strong>g>of</str<strong>on</strong>g> kerosene lamps, tolisten to a radio, and to run a sewing machine for additi<strong>on</strong>al income. Or,photovoltaics can provide electricity to remote transmitter stati<strong>on</strong>s in themountains allowing better communicati<strong>on</strong> without building a road to deliver8
diesel fuel for its generator. It can help a major electric utility in Los Angeles,Tokyo, or Madrid to meet its peak load <strong>on</strong> hot summer afterno<strong>on</strong>s when airc<strong>on</strong>diti<strong>on</strong>ers are working full time. It allows homes and businesses a new level <str<strong>on</strong>g>of</str<strong>on</strong>g>guaranteed energy availability and security, and photovoltaics has been poweringsatellites orbiting the Earth or flying to Mars for over 30 years The examples <str<strong>on</strong>g>of</str<strong>on</strong>g>possible applicati<strong>on</strong> are presented at figure 1.4.Photovoltaics is an empowering technology that allows us to do totally newthings, as well as, do old things better. It allows us to look at whole new modes <str<strong>on</strong>g>of</str<strong>on</strong>g>supplying electricity to different markets around the world and out <str<strong>on</strong>g>of</str<strong>on</strong>g> the world(in outer space). It also allows us to do what we already do (generate electricity,which is distributed overthe transmissi<strong>on</strong> grid) but to do it in a sustainable,polluti<strong>on</strong>-free, equitable fashi<strong>on</strong>. Whyis photovoltaics equitable? Because nearlyevery <strong>on</strong>e has access to sunlight.Figure 1.4 <str<strong>on</strong>g>Solar</str<strong>on</strong>g> ro<str<strong>on</strong>g>of</str<strong>on</strong>g> and <str<strong>on</strong>g>Solar</str<strong>on</strong>g> Car Auburn University.9
Figure.1.5 Human development index (HDI) vs. per capita kW usage [3]Electricity is the most versatile form <str<strong>on</strong>g>of</str<strong>on</strong>g> energy we have. It is what allowscitizens <str<strong>on</strong>g>of</str<strong>on</strong>g> the developed countries to have nearly universal lighting <strong>on</strong> demand,refrigerati<strong>on</strong>, hygiene, interior climate c<strong>on</strong>trol in their homes, businesses andschools, and widespread access to various electr<strong>on</strong>ic and electromagnetic media.Access to and c<strong>on</strong>sumpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> electricity is closely correlated with quality <str<strong>on</strong>g>of</str<strong>on</strong>g> life.Figure 5 shows the Human Development Index (HDI) for over 60 countries,which includes over 90% <str<strong>on</strong>g>of</str<strong>on</strong>g> the Earth‘s populati<strong>on</strong>, versus the annual per capitaelectricity use [3]. The HDI is compiled <strong>by</strong> the UN and calculated <strong>on</strong> the basis <str<strong>on</strong>g>of</str<strong>on</strong>g>life expectancy, educati<strong>on</strong>al achievement, and per capita Gross Domestic Product.To improve the quality <str<strong>on</strong>g>of</str<strong>on</strong>g> life in many countries, as measured <strong>by</strong> their HDI, willrequire increasing their electricity c<strong>on</strong>sumpti<strong>on</strong> <strong>by</strong> factors <str<strong>on</strong>g>of</str<strong>on</strong>g> 10 or more, from afew hundred to a few thousand kilowatt-hrs (kWh) per year. How will we do it?Our choices are to c<strong>on</strong>tinue applying the answers <str<strong>on</strong>g>of</str<strong>on</strong>g> the last century such asburning more fossil fuels (and releasing megat<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 , SO 2 , and NO 2 ) orbuilding more nuclear plants (despite having no method <str<strong>on</strong>g>of</str<strong>on</strong>g> safely disposing <str<strong>on</strong>g>of</str<strong>on</strong>g> thehigh-level radioactive waste) or to apply the new millennium‘s answer <str<strong>on</strong>g>of</str<strong>on</strong>g>10
enewable, sustainable, n<strong>on</strong>polluting, widely available clean energy likephotovoltaics (figure 6) and wind. (Wind presently generates over a thousandtimes more electricity than photovoltaics but it is very site-specific, whereasphotovoltaics is generally applicable to most locati<strong>on</strong>s.) The largest solar-powerplant is located in Spain, Murcia (26 MW). Am<strong>on</strong>g top 50 solar-power plants, 31plants are in Spain. (cf. Germany: 14, U.S.A : 2, Korea: 1, Japan: 1, and Portugal:1) Korea‘s solar pant is the 7th in the world (see figure 1.7)Figure 1.6 <str<strong>on</strong>g>Solar</str<strong>on</strong>g> Arrays11
Figure 1.7 <str<strong>on</strong>g>Solar</str<strong>on</strong>g> <str<strong>on</strong>g>Cell</str<strong>on</strong>g> Plants1.1 <str<strong>on</strong>g>Solar</str<strong>on</strong>g> Energy C<strong>on</strong>versi<strong>on</strong>.On Figure 1.8. <str<strong>on</strong>g>Solar</str<strong>on</strong>g> phot<strong>on</strong>s c<strong>on</strong>vert naturally into three forms <str<strong>on</strong>g>of</str<strong>on</strong>g> energyelectricity, chemical fuel, and heat that link seamlessly with existing energychains.Figure 1.8 <str<strong>on</strong>g>Solar</str<strong>on</strong>g> phot<strong>on</strong>s c<strong>on</strong>versi<strong>on</strong>Despite the enormous energy flux supplied <strong>by</strong> the Sun, the three c<strong>on</strong>versi<strong>on</strong>routes supply <strong>on</strong>ly a tiny fracti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> our current and future energy needs. <str<strong>on</strong>g>Solar</str<strong>on</strong>g>electricity, at between 5 and 10 times the cost <str<strong>on</strong>g>of</str<strong>on</strong>g> electricity from fossil fuels,supplies just 0.015% <str<strong>on</strong>g>of</str<strong>on</strong>g> the world's electricity demand. <str<strong>on</strong>g>Solar</str<strong>on</strong>g> fuel, in the form <str<strong>on</strong>g>of</str<strong>on</strong>g>12
iomass, accounts for approximately 11 % <str<strong>on</strong>g>of</str<strong>on</strong>g> world fuel use, but the majority <str<strong>on</strong>g>of</str<strong>on</strong>g>that is harvested unjustifiably. <str<strong>on</strong>g>Solar</str<strong>on</strong>g> heat provides 0.3% <str<strong>on</strong>g>of</str<strong>on</strong>g> the energy used forheating space and water. It is anticipated that <strong>by</strong> the year 2030 the world demandfor electricity will double and the demands for fuel and heat will increase <strong>by</strong>60%, The utilizati<strong>on</strong> gap between solar energy's potential and our use <str<strong>on</strong>g>of</str<strong>on</strong>g> if can beovercome <strong>by</strong> raising the efficiency <str<strong>on</strong>g>of</str<strong>on</strong>g> the c<strong>on</strong>versi<strong>on</strong> processes, which are all wellbelow their theoretical limits[4].1.2 History <str<strong>on</strong>g>of</str<strong>on</strong>g> photovoltaic’sThe history <str<strong>on</strong>g>of</str<strong>on</strong>g> photovoltaics goes back to the nineteenth century, as shown inTable 1.1. The first functi<strong>on</strong>al, intenti<strong>on</strong>ally made PV device was <strong>by</strong> Fritts [5] in1883. He melted Se into a thin sheet <strong>on</strong> a metal substrate and pressed a Au-leaffilm as the top c<strong>on</strong>tact. It was nearly 30 cm 2 in area. He noted, ―the current, if notwanted immediately, can be either stored where produced, in storage batteries, . . .or transmitted a distance and there used.‖ This man foresaw today‘s PVtechnology and applicati<strong>on</strong>s over a hundred years ago. The modern era <str<strong>on</strong>g>of</str<strong>on</strong>g>photovoltaics started in 1954 when researchers at Bell Labs in the USA13
accidentally discovered that pn juncti<strong>on</strong> diodes generated a voltage when theroom lights were <strong>on</strong>. Within a year, they had produced a 6% efficient Si pnjuncti<strong>on</strong> solar cell [6]. In the same year, the group at Wright Patters<strong>on</strong> Air ForceBase in the US published results <str<strong>on</strong>g>of</str<strong>on</strong>g> a thin-film heterojuncti<strong>on</strong> solar cell <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong>Cu2S/CdS also having 6% efficiency [7]. A year later, a 6% GaAs pn juncti<strong>on</strong>solar cell was reported <strong>by</strong> RCA Lab in the US [8]. By 1960, several key papers <strong>by</strong>Prince [9], L<str<strong>on</strong>g>of</str<strong>on</strong>g>erski [10], Rappaport and Wysoski [11], Shockley (a Nobellaureate) and Queisser [12], developed the fundamentals <str<strong>on</strong>g>of</str<strong>on</strong>g> pn juncti<strong>on</strong> solar celloperati<strong>on</strong> including the theoretical relati<strong>on</strong> between band gap, incident spectrum,temperature, thermodynamics, and efficiency. <str<strong>on</strong>g>Thin</str<strong>on</strong>g> films <str<strong>on</strong>g>of</str<strong>on</strong>g> CdTe were alsoproducing cells with 6% efficiency [13]. By thistime, the US space program wasutilizing Si PV cells for powering satellites. Since space was still the primaryapplicati<strong>on</strong> for photovoltaics, studies <str<strong>on</strong>g>of</str<strong>on</strong>g> radiati<strong>on</strong> effects and more radiati<strong>on</strong>tolerantdevices were made using Li-doped Si [14]. In 1970, a group at the I<str<strong>on</strong>g>of</str<strong>on</strong>g>feInstitute led <strong>by</strong> Alferov (a Nobel laureate), in the USSR, developed a heter<str<strong>on</strong>g>of</str<strong>on</strong>g>aceGaAlAs/GaAs [15] solar cell which solved <strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g> the main problems that affectedGaAs devices and pointed the way to new device structures. GaAs cells were <str<strong>on</strong>g>of</str<strong>on</strong>g>interest due to their high efficiency and their resistance to the i<strong>on</strong>izing radiati<strong>on</strong> inouter space. The year 1973 was pivotal for photovoltaics, in both technical andn<strong>on</strong>technical areas. A significant improvement in performance occurring in 1973was the ―violet cell‖ having an improved short wavelength resp<strong>on</strong>se leading to a30% relative increase in efficiency over state-<str<strong>on</strong>g>of</str<strong>on</strong>g>-the-art Si cells [16]. GaAsheterostructure cells were also developed at IBM in the USA having 13%efficiency [17]. Also in 1973, a crucial n<strong>on</strong>technical event occurred called theCherry Hill C<strong>on</strong>ference, named after the town in New Jersey, USA, where agroup <str<strong>on</strong>g>of</str<strong>on</strong>g> PV researchers and heads <str<strong>on</strong>g>of</str<strong>on</strong>g> US government scientific organizati<strong>on</strong>s metto evaluate the scientific merit and potential <str<strong>on</strong>g>of</str<strong>on</strong>g> photovoltaics. The outcome wasthe decisi<strong>on</strong> that photovoltaics was worthy <str<strong>on</strong>g>of</str<strong>on</strong>g> government support, resulting in theformati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the US Energy Research and Development Agency, the world‘s firstgovernment group whose missi<strong>on</strong> included fostering research <strong>on</strong> renewableenergy, which ultimately became the US Dept. <str<strong>on</strong>g>of</str<strong>on</strong>g> Energy. Finally, in October1973, the first World Oil Embargo was instituted <strong>by</strong> the Persian Gulf oil14
producers. This sent shock waves through the industrialized world, and mostgovernments began programs to encourage renewable energy especially solarenergy. Some would say this ushered in the modern age <str<strong>on</strong>g>of</str<strong>on</strong>g> photovoltaics and gavea new sense <str<strong>on</strong>g>of</str<strong>on</strong>g> urgency to research and applicati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> photovoltaics in terrestrialapplicati<strong>on</strong>s. In the 1980s, the industry began to mature, as emphasis <strong>on</strong>manufacturing and costs grew. Manufacturing facilities for producing PV modulesfrom Si wafer pn juncti<strong>on</strong> solar cells were built in the USA, Japan, and Europe.New technologies began to move out <str<strong>on</strong>g>of</str<strong>on</strong>g> the government, university and industriallaboratories, and into precommercializati<strong>on</strong> or ―pilot‖ line producti<strong>on</strong>. Companiesattempted to scale up the thin-film PV technologies like a-Si and CuInSe 2 , whichhad achieved >10% efficiency for small area (1 cm 2 ) devices made with carefullyc<strong>on</strong>trolled laboratory scale equipment. Much to their disappointment, they foundthat this was far more complicated than merely scaling the size <str<strong>on</strong>g>of</str<strong>on</strong>g> the equipment.Most large US semic<strong>on</strong>ductor companies, gave up their R/D efforts (IBM,General Electric, Motorola) lacking large infusi<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> private or governmentsupport to c<strong>on</strong>tinue. One comm<strong>on</strong> result was the purchase <str<strong>on</strong>g>of</str<strong>on</strong>g> American companiesand their technologies <strong>by</strong> foreign companies. In 1990, the world‘s largest solarmanufacturer was Arco <str<strong>on</strong>g>Solar</str<strong>on</strong>g> (CA, USA), owned <strong>by</strong> oil company AtlanticRichfield, which had c-Si and thinfilm a-Si in producti<strong>on</strong> and thin-film CuInSe 2 inprecommercializati<strong>on</strong>. They were sold to the German firm Siemens and renamedSiemens <str<strong>on</strong>g>Solar</str<strong>on</strong>g> (in 2001, the Dutch company Shell <str<strong>on</strong>g>Solar</str<strong>on</strong>g> would buy Siemens,becoming another large internati<strong>on</strong>ally <str<strong>on</strong>g>based</str<strong>on</strong>g> company with multiple PVtechnologies in producti<strong>on</strong>). Also in 1990, Energy C<strong>on</strong>versi<strong>on</strong> Devices (MI, USA)formed a joint venture called United <str<strong>on</strong>g>Solar</str<strong>on</strong>g> Systems Corp. with the Japanesemanufacturer Can<strong>on</strong> to commercialize ECD‘s roll-to-roll triple-juncti<strong>on</strong> a-Sitechnology. In 1994, Mobil <str<strong>on</strong>g>Solar</str<strong>on</strong>g> Energy (MA, USA), which had developed aprocess for growing solar cells <strong>on</strong> Si ribb<strong>on</strong> (called the Edge defined film growthor EFG process) instead <str<strong>on</strong>g>of</str<strong>on</strong>g> more costly wafers, was sold to the German companyASE and renamed ASE Americas. The British solar company BP <str<strong>on</strong>g>Solar</str<strong>on</strong>g> acquiredpatents to electrodepositi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> thin-film CdTe solar cells in 1989, when it‘s parentcompany purchased the American oil giant Standard Oil <str<strong>on</strong>g>of</str<strong>on</strong>g> Ohio. At the sametime, it acquired the patents <str<strong>on</strong>g>of</str<strong>on</strong>g> the University <str<strong>on</strong>g>of</str<strong>on</strong>g> New South Wales (Australia) to15
fabricate the Laser-Grooved Buried-Grid (LGBG) cells, which became the mostefficient silic<strong>on</strong> cells in fabricati<strong>on</strong>. In 1996, it signed a license agreement withthe Polytechnic University <str<strong>on</strong>g>of</str<strong>on</strong>g> Madrid (Spain) to exploit the Euclides c<strong>on</strong>centrati<strong>on</strong>technology that used their LGBG cells as c<strong>on</strong>centrator cells. In 1999, BP <str<strong>on</strong>g>Solar</str<strong>on</strong>g>acquired <str<strong>on</strong>g>Solar</str<strong>on</strong>g>ex from Enr<strong>on</strong> (another huge fossil-fuel energy company) that hadcrystalline and amorphous Si solar cell technology. Thus, BP <str<strong>on</strong>g>Solar</str<strong>on</strong>g> establishedthemselves with manufacturing interests in all three technology opti<strong>on</strong>s (standardSi wafers, thin films and c<strong>on</strong>centrators).1 Meanwhile, the Japanese PV industrybegan to take <str<strong>on</strong>g>of</str<strong>on</strong>g>f. Producti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> c-Si modules and intensive research <strong>on</strong> thin-filmtechnology in Japan led to many innovative device designs, improved materialsprocessing, and growing dominance in the world PV market. Al<strong>on</strong>g with thematuring <str<strong>on</strong>g>of</str<strong>on</strong>g> the solar cell technology, the BOS needed to grow. Many productslike inverters, which c<strong>on</strong>vert the DC power into AC power, and sun trackers had<strong>on</strong>ly limited applicati<strong>on</strong> outside <str<strong>on</strong>g>of</str<strong>on</strong>g> a PV power system, so <strong>on</strong>ce again there was<strong>on</strong>ly limited technical and financial resources for development. In many systemevaluati<strong>on</strong>s, the inverter was repeatedly identified as the weak link in terms <str<strong>on</strong>g>of</str<strong>on</strong>g>reliability and AC power quality [24]. Their costs have not fallen nearly as fast asthose for the PV modules. While much effort and resources had been focused <strong>on</strong>the solar cell cost and performance, little attenti<strong>on</strong> had been paid to installati<strong>on</strong>and maintenance costs. It was quickly discovered that there was room for muchimprovement. An early development that helped many companies was to sell PVcells for c<strong>on</strong>sumer-sized, small-scale power applicati<strong>on</strong>s. The solar-poweredcalculator, pi<strong>on</strong>eered <strong>by</strong> Japanese electr<strong>on</strong>ics companies as a replacement forbattery-powered calculators in the early 1980s, is the best-known example. Thisled to the early use <str<strong>on</strong>g>of</str<strong>on</strong>g> thin-film a-Si PV cells for various applicati<strong>on</strong>s. Anotherexample was solar-powered outdoor lighting. These novel c<strong>on</strong>sumer applicati<strong>on</strong>s,where portability and c<strong>on</strong>venience were more valued than low price, allowed thePV companies to maintain some small income while c<strong>on</strong>tinuing to develop powermodules. Another applicati<strong>on</strong> was the rural electrificati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> remote villages in anattemptto help roughly <strong>on</strong>e-third <str<strong>on</strong>g>of</str<strong>on</strong>g> the world‘s citizens to gain access to a modestamount <str<strong>on</strong>g>of</str<strong>on</strong>g> modern communicati<strong>on</strong> and lighting. Most <str<strong>on</strong>g>of</str<strong>on</strong>g> these PV installati<strong>on</strong>swere very small, <strong>on</strong> the order <str<strong>on</strong>g>of</str<strong>on</strong>g> 10 to 40 W per household (100 times smaller16
than the ―needs‖ <str<strong>on</strong>g>of</str<strong>on</strong>g> a modern home in the developed world.) Most <str<strong>on</strong>g>of</str<strong>on</strong>g> theseinstallati<strong>on</strong>s were funded <strong>by</strong> some internati<strong>on</strong>al aid agency. Reviews and followupstudies <str<strong>on</strong>g>of</str<strong>on</strong>g> these programs have indicated very large failure rates, primarily dueto lack <str<strong>on</strong>g>of</str<strong>on</strong>g> technical infrastructure [25], training, cultural misunderstandings,design <str<strong>on</strong>g>of</str<strong>on</strong>g> the payment structure, and other n<strong>on</strong>technical reas<strong>on</strong>s [26].Rarely havethe PV modules failed. Even with subsidies from the internati<strong>on</strong>al agencies, thehigh initial cost <str<strong>on</strong>g>of</str<strong>on</strong>g> ownership ($100–1000) was still a major barrier in much <str<strong>on</strong>g>of</str<strong>on</strong>g> theworld where this represents a year‘s income for a family [27].On the opposite end <str<strong>on</strong>g>of</str<strong>on</strong>g> the size scale were the MW-size PV plants installed <strong>by</strong>utilities in developed countries in the 1980s to evaluate their potential in twoapplicati<strong>on</strong>s: as a peak-load-reducti<strong>on</strong> technology, where the photovoltaicsprovides additi<strong>on</strong>al power primarily to meet the peak demand during theafterno<strong>on</strong> [28]; or as distributed generators, to reduce transmissi<strong>on</strong> anddistributi<strong>on</strong> losses [29]. Several American utilities investigated these applicati<strong>on</strong>s,to assess the technical as well as financial benefits for photovoltaics in utilityscale applicati<strong>on</strong>s. Other novel c<strong>on</strong>figurati<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> grid-tied PV systems wereevaluated as so-called ―demand side management‖ opti<strong>on</strong>s where the <strong>on</strong>sitedistributed photovoltaics is used to reduce demand rather than increase supply[30].Although American utilities lost interest in PV in the late 90s due toderegulati<strong>on</strong>, gridc<strong>on</strong>nected applicati<strong>on</strong>s in Europe and Japan began to growrapidly, primarily owing to str<strong>on</strong>g government support. Both small- and largescalegrid c<strong>on</strong>nected PV installati<strong>on</strong>s are blossoming in these countries [31, 32].Yet another important development in the applicati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> PV in the late 1990s,was building integrated PV (BIPV [33]), where PV cells are incorporated into astandard building product, such as a window or a ro<str<strong>on</strong>g>of</str<strong>on</strong>g> shingle, or into anarchitectural feature like an exterior sun awning or semitransparent skylight. Inthis way, the cost <str<strong>on</strong>g>of</str<strong>on</strong>g> the PV is partially <str<strong>on</strong>g>of</str<strong>on</strong>g>fset <strong>by</strong> the cost <str<strong>on</strong>g>of</str<strong>on</strong>g> the building materials,which would have been required anyway, so the incremental cost <str<strong>on</strong>g>of</str<strong>on</strong>g> thephotovoltaics is much lower. The success <str<strong>on</strong>g>of</str<strong>on</strong>g> grid-c<strong>on</strong>nected residential or BIPVcommercial applicati<strong>on</strong>s has been possible because several countries led <strong>by</strong>Germany have established high rates to pay for the PV electricity produced <strong>by</strong>17
solar installati<strong>on</strong>s in private houses. In this scheme, the installati<strong>on</strong> owner receives$0.5/kWh for the electricity they feed into the public electric grid (as <str<strong>on</strong>g>of</str<strong>on</strong>g> 2001).But the owner buys the electricity c<strong>on</strong>sumed in their own house at the normal cost<str<strong>on</strong>g>of</str<strong>on</strong>g> ~ $0.1/kWh from the grid. Additi<strong>on</strong>ally, German banks provided generousloans for purchasing the installati<strong>on</strong>. Similar c<strong>on</strong>cepts are used in Spain, theNetherlands, and other countries in Europe. But, the success has been still biggerin Japan where homebuilders receive a rebate from the government for about 30%<str<strong>on</strong>g>of</str<strong>on</strong>g> the PV system cost. Then, their electric bill is determined <strong>by</strong> the utility usingthe ―net metering‖ where the customer pays <strong>on</strong>ly the net difference between whatthey used and what they generated. Rebates and net metering are available insome, but not all, states in the USA as <str<strong>on</strong>g>of</str<strong>on</strong>g> 2002. Interestingly, government support<str<strong>on</strong>g>of</str<strong>on</strong>g> photovoltaics in Japan has been decreasing while the market for PV homes hasc<strong>on</strong>tinued showing an impressive growth rate.Table 1.1 Notable events in the history <str<strong>on</strong>g>of</str<strong>on</strong>g> photovoltaics• 1839 Becquerel (FR) discovered photogalvanic effect in liquid electrolytes• 1873 Smith (UK) discovered photoc<strong>on</strong>ductivity <str<strong>on</strong>g>of</str<strong>on</strong>g> solid Se• 1877 Adams and Day (UK) discover photogenerati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> current in Se tubes;the first observati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> PV effect in solids• 1883 Fritts (US) makes first large area solar cell using Se film• 1954 First 6% efficient solar cells reported: Si (Bell Lab, USA) andCu2S/CdS (Air Force, USA)• 1955 H<str<strong>on</strong>g>of</str<strong>on</strong>g>fman Electr<strong>on</strong>ics (USA) <str<strong>on</strong>g>of</str<strong>on</strong>g>fers 2% efficient Si PV cells at $1500/W• 1958 NASA Vanguard satellite with Si backup solar array• 1959 H<str<strong>on</strong>g>of</str<strong>on</strong>g>fman Electr<strong>on</strong>ics (USA) <str<strong>on</strong>g>of</str<strong>on</strong>g>fers 10% efficient Si PV cells• 1963 Sharp Corp (JP) produces first commercial Si modules• 1966 NASA Orbiting Astr<strong>on</strong>omical Observatory launched with 1 kW array• 1970 First GaAs heterostructure solar cells <strong>by</strong> Alferov, Andreev et al. in theUSSR• 1972 First PV c<strong>on</strong>ference to include a sessi<strong>on</strong> <strong>on</strong> terrestrial applicati<strong>on</strong>s(IEEE)• 1973 A big year in photovoltaics: Worldwide oil crisis spurs many nati<strong>on</strong>s to18
c<strong>on</strong>sider renewable energy including photovoltaics; Cherry Hill C<strong>on</strong>ference inUSA (established photovoltaics‘ potential and legitimacy for government researchfunding); World‘s first solar powered residence (University <str<strong>on</strong>g>of</str<strong>on</strong>g> Delaware, USA)built with Cu 2 S (not c-Si!) solar modules• 1974 Project Sunshine initiated in Japan to foster growth <str<strong>on</strong>g>of</str<strong>on</strong>g> PV industry andapplicati<strong>on</strong>s; Tyco (USA) grows 2.5 cm wide Si ribb<strong>on</strong> for photovoltaics, firstalternative to Si wafers• 1975 First book dedicated to PV science and technology <strong>by</strong> Hovel (USA)• 1980 First thin-film solar cell >10% using Cu 2 S/CdS (USA)• 1981 350 kW C<strong>on</strong>centrator array installed in Saudi Arabia• 1982 First 1 MW utility scale PV power plant (CA, USA) with Arco Simodules <strong>on</strong> 2-axis trackers• 1984 6 MW array installed in Carrisa Plains CA, USA [18]• 1985 A big year for high-efficiency Si solar cells: Si solar cell >20% understandard sunlight (UNSW, Australia) [19] and >25% under 200X c<strong>on</strong>centrati<strong>on</strong>(Stanford Univ. USA) [20]• 1986 First commercial thin-film power module, the a-Si G4000 from Arco<str<strong>on</strong>g>Solar</str<strong>on</strong>g> (USA)• 1987 Fourteen solar powered cars complete the 3200 km World <str<strong>on</strong>g>Solar</str<strong>on</strong>g>Challenge race (Australia) with the winner averaging 70 kph• 1994 GaInP/GaAs 2-terminal c<strong>on</strong>centrator multijuncti<strong>on</strong> >30% (NREL,USA) [21]• 1995 ―1000 ro<str<strong>on</strong>g>of</str<strong>on</strong>g>s‖ German dem<strong>on</strong>strati<strong>on</strong> project to install photovoltaics <strong>on</strong>houses, which triggered the present favorable PV legislati<strong>on</strong> in Germany, Japanand other countries• 1996 Photoelectrochemical ―dye-sensitized‖ solid/liquid cell achieves 11%(EPFL, Switzerland) [22]• 1997 Worldwide PV producti<strong>on</strong> reaches 100 MW per year• 1998 Cu(InGa)Se 2 thin-film solar cell reaches 19% efficiency (NREL, US)[23] comparable with multicrystalline Si. First c<strong>on</strong>centrating array for spacelaunched <strong>on</strong> Deep Space 1 <strong>by</strong> US (5 kW using high efficiency GaInP/GaAs/Getriple juncti<strong>on</strong> cells)19
• 1999 Cumulative worldwide installed photovoltaics reaches 1000 MW• 2000 Olympics in Australia highlight wide range <str<strong>on</strong>g>of</str<strong>on</strong>g> PV applicati<strong>on</strong>s, and theawarding <str<strong>on</strong>g>of</str<strong>on</strong>g> the first Bachelor <str<strong>on</strong>g>of</str<strong>on</strong>g> Engineering degrees in Photovoltaics and <str<strong>on</strong>g>Solar</str<strong>on</strong>g>Engineering (UNSW, Australia)• 2002 Cumulative worldwide installed photovoltaics reaches 2000 MW. Ittook 25 years to reach the first 1000 MW and <strong>on</strong>ly 3 years to double it; producti<strong>on</strong><str<strong>on</strong>g>of</str<strong>on</strong>g> crystalline Si cells exceeds 100 MW per year at Sharp Corp. (Japan). BP <str<strong>on</strong>g>Solar</str<strong>on</strong>g>ceases R&D and producti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a-Si and CdTe thin-film modules in USA ending>20 years <str<strong>on</strong>g>of</str<strong>on</strong>g> effort1.3 Basic principles <str<strong>on</strong>g>of</str<strong>on</strong>g> photovoltaic.There are two general types <str<strong>on</strong>g>of</str<strong>on</strong>g> photovoltaic cells, the "wet" and the "dry"types. The "wet" type was discovered first, but the mechanism is still not wellunderstood. A review <str<strong>on</strong>g>of</str<strong>on</strong>g> this type <str<strong>on</strong>g>of</str<strong>on</strong>g> cell was given <strong>by</strong> Copeland, Black, andGarrett (1942). The cells are less stable than the solid state ("dry") type, perhapsinherently so, and have therefore received less attenti<strong>on</strong>.At the heart <str<strong>on</strong>g>of</str<strong>on</strong>g> any solar cell is the pn juncti<strong>on</strong>. Modeling and understanding isvery much simplified <strong>by</strong> using the pn juncti<strong>on</strong> c<strong>on</strong>cept. This pn juncti<strong>on</strong> resultsfrom the ―doping‖ that produces c<strong>on</strong>ducti<strong>on</strong>-band or valence-band selective20
c<strong>on</strong>tacts with <strong>on</strong>e becoming the n-side (lots <str<strong>on</strong>g>of</str<strong>on</strong>g> negative charge), the other the p-side (lots <str<strong>on</strong>g>of</str<strong>on</strong>g> positive charge).Silic<strong>on</strong> (Si), <strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g> the most abundant materials in the Earth‘s crust, is thesemic<strong>on</strong>ductor used in crystalline form (c-Si) for 90% <str<strong>on</strong>g>of</str<strong>on</strong>g> the PV applicati<strong>on</strong>stoday. Surprisingly, other semic<strong>on</strong>ductors are better suited to absorb the solarenergy spectrum. These other materials are in development or initialcommercializati<strong>on</strong> today. Some are called thin-film semic<strong>on</strong>ductors, <str<strong>on</strong>g>of</str<strong>on</strong>g> whichamorphous silic<strong>on</strong> (a-Si), copper indium gallium diselenide (Cu(InGa)Se 2 orCIGS), and cadmium telluride (CdTe). <str<strong>on</strong>g>Solar</str<strong>on</strong>g> cells may operate under c<strong>on</strong>centratedsunlight using lenses or mirrors as c<strong>on</strong>centrators allowing a small solar cell area tobe illuminated with the light from larger area. This saves the expensivesemic<strong>on</strong>ductor but adds complexity to the system, since it requires trackingmechanisms to keep the light focused <strong>on</strong> the solar cells when the sun moves in thesky. Silic<strong>on</strong> and III-V semic<strong>on</strong>ductors, made from compounds such as galliumarsenide (GaAs) and gallium indium phosphide (GaInP) are the materials used inc<strong>on</strong>centrator technology that is still in its dem<strong>on</strong>strati<strong>on</strong> stage. For practicalapplicati<strong>on</strong>s, a large number <str<strong>on</strong>g>of</str<strong>on</strong>g> solar cells are interc<strong>on</strong>nected and encapsulatedinto units called PV modules, which is the product usually sold to the customer.They produce DC current that is typically transformed into the more useful ACcurrent <strong>by</strong> an electr<strong>on</strong>ic device called an inverter. The inverter, the rechargeablebatteries (when storage is needed), the mechanical structure to mount and aim(when aiming is necessary) the modules, and any other elements necessary tobuild a PV system are called the balance <str<strong>on</strong>g>of</str<strong>on</strong>g> the system (BOS).As menti<strong>on</strong>ed before, solar cells are made <str<strong>on</strong>g>of</str<strong>on</strong>g> materials called semic<strong>on</strong>ductors,which have weakly b<strong>on</strong>ded electr<strong>on</strong>s occupying a band <str<strong>on</strong>g>of</str<strong>on</strong>g> energy called thevalence band. When energy exceeding a certain threshold, called the band gapenergy, is applied to a valence electr<strong>on</strong>, the b<strong>on</strong>ds are broken and the electr<strong>on</strong> issomewhat ―free‖ to move around in a new energy band called the c<strong>on</strong>ducti<strong>on</strong>band where it can ―c<strong>on</strong>duct‖ electricity through the material. Thus, the freeelectr<strong>on</strong>s in the c<strong>on</strong>ducti<strong>on</strong> band are separated from the valence band <strong>by</strong> the bandgap (measured in units <str<strong>on</strong>g>of</str<strong>on</strong>g> electr<strong>on</strong> volts or eV). This energy needed to free theelectr<strong>on</strong> can be supplied <strong>by</strong> phot<strong>on</strong>s, which are particles <str<strong>on</strong>g>of</str<strong>on</strong>g> light. Phot<strong>on</strong> is21
absorbed and energy is given to an electr<strong>on</strong> in the crystal lattice. Usually thiselectr<strong>on</strong> is in valence band, tightly bound in covalent b<strong>on</strong>ds. Energy given <strong>by</strong> thephot<strong>on</strong> ―excites‖ it into the c<strong>on</strong>ducti<strong>on</strong> band. Covalent b<strong>on</strong>d now has <strong>on</strong>e fewerelectr<strong>on</strong> (hole). B<strong>on</strong>ded electr<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> neighboring atoms can move into the ‗hole‘,leaving another hole behind – hole can propagate through lattice. Free electr<strong>on</strong>sflow through the material to produce electricity. Positive charges (holes) flow inopposite directi<strong>on</strong>. Different PV materials have different band gap energies.Phot<strong>on</strong>s with energy equal to the band gap energy are absorbed to create freeelectr<strong>on</strong>s. Phot<strong>on</strong>s with less energy than the band gap energy pass through thematerial. Phot<strong>on</strong>s absorpti<strong>on</strong> creates mobile electr<strong>on</strong>-hole pairs (see figure 1.9).Figure 1.9 Phot<strong>on</strong>s absorpti<strong>on</strong> in different materialsFigure 1.10 shows the idealized relati<strong>on</strong> between energy (vertical axis) and thespatial boundaries (horiz<strong>on</strong>tal axis). When the solar cell is exposed to sunlight,phot<strong>on</strong>s hit valence electr<strong>on</strong>s, breaking the b<strong>on</strong>ds and pumping them to thec<strong>on</strong>ducti<strong>on</strong> band. There, a specially made selective c<strong>on</strong>tact that collectsc<strong>on</strong>ducti<strong>on</strong>-band electr<strong>on</strong>s drives such electr<strong>on</strong>s to the external circuit. Theelectr<strong>on</strong>s lose their energy <strong>by</strong> doing work in the external circuit such as pumpingwater, spinning a fan, powering a sewing machine motor, a light bulb, or acomputer. They are restored to the solar cell <strong>by</strong> the return loop <str<strong>on</strong>g>of</str<strong>on</strong>g> the circuit via asec<strong>on</strong>d selective c<strong>on</strong>tact, which returns them to the valence band with the sameenergy that they started with. The movement <str<strong>on</strong>g>of</str<strong>on</strong>g> these electr<strong>on</strong>s in the external22
circuit and c<strong>on</strong>tacts is called the electric current.Figure 1.10 Schematic <str<strong>on</strong>g>of</str<strong>on</strong>g> a solar cell. Electr<strong>on</strong>s are pumped <strong>by</strong> phot<strong>on</strong>s fromthe valence band to the c<strong>on</strong>ducti<strong>on</strong> band. There they are extracted <strong>by</strong> ac<strong>on</strong>tact selective to the c<strong>on</strong>ducti<strong>on</strong> band (an n-doped semic<strong>on</strong>ductor) at ahigher (free) energy and delivered to the outside world via wires, where theydo some useful work, then are returned to the valence band at a lower (free)energy <strong>by</strong> a c<strong>on</strong>tact selective to the valence band (a p-type semic<strong>on</strong>ductor)Figure 1.11 The p-n juncti<strong>on</strong> in thermal equilibrium.23
The potential at which the electr<strong>on</strong>s are delivered to the external world isslightly less than the threshold energy that excited the electr<strong>on</strong>s; that is, the bandgap. Thus, in a material with a 1 eV band gap, electr<strong>on</strong>s excited <strong>by</strong> a 2 eV phot<strong>on</strong>or <strong>by</strong> a 3 eV phot<strong>on</strong> will both still have a potential <str<strong>on</strong>g>of</str<strong>on</strong>g> slightly less than 1 V (i.e.the electr<strong>on</strong>s are delivered with an energy <str<strong>on</strong>g>of</str<strong>on</strong>g> 1 eV). The electric power producedis the product <str<strong>on</strong>g>of</str<strong>on</strong>g> the current times the voltage; that is, power is the number <str<strong>on</strong>g>of</str<strong>on</strong>g> freeelectr<strong>on</strong>s times their potential.Sunlight is a spectrum <str<strong>on</strong>g>of</str<strong>on</strong>g> phot<strong>on</strong>s distributed over a range <str<strong>on</strong>g>of</str<strong>on</strong>g> energy. Phot<strong>on</strong>swhose energy is greater than the band gap energy (the threshold energy) canexcite electr<strong>on</strong>s from the valence to c<strong>on</strong>ducti<strong>on</strong> band where they can exit thedevice and generate electrical power. Phot<strong>on</strong>s with energy less than the energygap fail to excite free electr<strong>on</strong>s. Instead, that energy travels through the solar celland is absorbed at the rear as heat. <str<strong>on</strong>g>Solar</str<strong>on</strong>g> cells in direct sunlight can be somewhat(20–30ºC) warmer than the ambient air temperature. Thus, PV cells can produceelectricity without operating at high temperature and without mobile parts. Theseare the salient characteristics <str<strong>on</strong>g>of</str<strong>on</strong>g> photovoltaics that explain safe, simple, andreliable operati<strong>on</strong>.The p-n juncti<strong>on</strong> in thermal equilibrium w/ zero bias voltage applied.Electr<strong>on</strong>s and holes c<strong>on</strong>centrati<strong>on</strong> are reported respectively with blue and redlines. Gray regi<strong>on</strong>s are charge neutral. Light red z<strong>on</strong>e is positively charged; lightblue z<strong>on</strong>e is negatively charged. Electric field shown <strong>on</strong> the bottom, theelectrostatic force <strong>on</strong> electr<strong>on</strong>s and holes and the directi<strong>on</strong> in which the diffusi<strong>on</strong>tends to move electr<strong>on</strong>s and holes.At the Figure 1.11 is presented p-n juncti<strong>on</strong> in thermal equilibrium w/ zerobias voltage applied. Electr<strong>on</strong>s and holes c<strong>on</strong>centrati<strong>on</strong> are reported respectivelywith blue and red lines. Gray regi<strong>on</strong>s are charge neutral. Light red z<strong>on</strong>e ispositively charged; light blue z<strong>on</strong>e is negatively charged. Electric field shown <strong>on</strong>the bottom, the electrostatic force <strong>on</strong> electr<strong>on</strong>s and holes and the directi<strong>on</strong> inwhich the diffusi<strong>on</strong> tends to move electr<strong>on</strong>s and holes.There is much more to sunlight than the porti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the spectrum that can see.Sunlight incident <strong>on</strong> the earth‘s surface can deliver up to 1000 W <str<strong>on</strong>g>of</str<strong>on</strong>g> power to a 124
m 2 area. If the solar energy incident <strong>on</strong> 1 m 2 could be c<strong>on</strong>verted into electricitywith 100% efficiency, the resulting electrical power could operate ten 100 W lightbulbs, a toaster, or a computer with m<strong>on</strong>itor and printer. Unfortunately, thec<strong>on</strong>versi<strong>on</strong> efficiency from solar energy to electrical energy is nowhere near100%. Direct c<strong>on</strong>versi<strong>on</strong> efficiencies from 5% to 20% are typical.Photovoltaic cells c<strong>on</strong>vert sunlight directly to electricity. Their principle <str<strong>on</strong>g>of</str<strong>on</strong>g>operati<strong>on</strong> is the same as that for photodiode light detectors. As menti<strong>on</strong>ed beforethese devices are fabricated from semic<strong>on</strong>ductor materials, such as silic<strong>on</strong> (Si),gallium arsenide (GaAs), and copper sulfide (Cu 2 S) and other materials. In aphotovoltaic cell, incident sunlight phot<strong>on</strong>s remove electr<strong>on</strong>s in the semic<strong>on</strong>ductorfrom their b<strong>on</strong>ds so that they are free to move around inside the semic<strong>on</strong>ductormaterial. The empty electr<strong>on</strong> b<strong>on</strong>d states that are left behind act like positivecharges, are also mobile, and are called ―holes.‖Photovoltaic cells generally c<strong>on</strong>sist <str<strong>on</strong>g>of</str<strong>on</strong>g> at least two thin layers, <strong>on</strong>e atop theother, and each having different, intenti<strong>on</strong>ally added impurities called dopants(see Figure 1.12). The result is that <strong>on</strong>e layer is ―n-type,‖ having an excess <str<strong>on</strong>g>of</str<strong>on</strong>g>(negative) electr<strong>on</strong>s, while the other layer is ―p-type,‖ having an excess <str<strong>on</strong>g>of</str<strong>on</strong>g> positiveholes. This two-layer structure, called a p-n juncti<strong>on</strong>, results in a ―built-in‖ electricfield. When phot<strong>on</strong>s create free electr<strong>on</strong>s and holes near the p-n juncti<strong>on</strong>, thebuilt-in electric field makes them move in opposite directi<strong>on</strong>s; the electr<strong>on</strong>s moveto the n side and the holes move to the p side. Thus, an electromotive force (emf)is generated between the p and n layers, with the p side positive and the n sidenegative. If wires c<strong>on</strong>nect the p and n sides to a ―load,‖ for example a light bulb ora calculator, a voltage exists across that load and an electric current flows throughthe load. The power delivered to the load is simply:PVI25
Figure 1.12 Photovoltaic cell c<strong>on</strong>nected to a load (an incandescentlamp). Absorbed phot<strong>on</strong>s create free electr<strong>on</strong>s and free holes that movein the directi<strong>on</strong>s indicated because <str<strong>on</strong>g>of</str<strong>on</strong>g> the built-in electric field (E) at theThus, pn juncti<strong>on</strong>. a photovoltaic cell c<strong>on</strong>verts optical energy directly to electrical energy.Specific data about a photovoltaic cell is <str<strong>on</strong>g>of</str<strong>on</strong>g>ten presented in the form <str<strong>on</strong>g>of</str<strong>on</strong>g> acharacteristic curve, as shown in Figure 1.13. The curve gives the current supplied<strong>by</strong> the cell as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the voltage supplied <strong>by</strong> the cell, for a known value <str<strong>on</strong>g>of</str<strong>on</strong>g>photovoltaic irradiance. Two quantities identified <strong>on</strong> this current-voltage (or I-V)characteristic curve are frequently used when describing the performance <str<strong>on</strong>g>of</str<strong>on</strong>g> aphotovoltaic cell. These quantities are V oc and I sc .When no load is c<strong>on</strong>nected to a photovoltaic cell, the device is said to be in an―open circuit‖ c<strong>on</strong>diti<strong>on</strong>, meaning that no current can flow. A good voltmeter actslike an open circuit, and if it is c<strong>on</strong>nected to the cell terminals, the voltagemeasured is called the open circuit voltage, denoted V oc in Figure 1.14. When thephotovoltaic cell terminals are c<strong>on</strong>nected with a wire, the device is said to be in a―short circuit‖ c<strong>on</strong>diti<strong>on</strong>, meaning that the voltage between the terminals is zero.The current that flows through the wire is called the short circuit current, denotedI sc in Figure 1.14. A good ammeter acts nearly like a short circuit (that is, it has avery low resistance), so if the photovoltaic cell terminals are c<strong>on</strong>nected to anammeter, the measured current is nearly I sc . (See figure 1.14)26
Figure 1.13. Current vs. voltage characteristics for an “ideal” silic<strong>on</strong>photovoltaic cell. Notice that the short circuit current and the open circuitvoltage are identified <strong>on</strong> the axes. The actual quantitative data depends <strong>on</strong>the irradiance and the cell area.Figure 1.14 Energy Band <str<strong>on</strong>g>of</str<strong>on</strong>g> p-n Juncti<strong>on</strong> <str<strong>on</strong>g>Solar</str<strong>on</strong>g> <str<strong>on</strong>g>Cell</str<strong>on</strong>g>, Short circuit currentI sc open circuit voltageV os.27
The available solar radiati<strong>on</strong> is frequently described <strong>by</strong> giving an air massvalue, such as ―air mass zero‖ (AM0), ―air mass <strong>on</strong>e‖ (AM1), ―air mass 1.5‖(AM1.5), etc. AM0 refers to the solar radiati<strong>on</strong> available just outside the earth‘satmosphere. The corresp<strong>on</strong>ding total solar irradiance is 1353 W/m 2 , and this valueis called the ―solar c<strong>on</strong>stant.‖ AM1 refers to the solar radiati<strong>on</strong> available at theearth‘s surface when the sun is at the zenith (total solar irradiance ~925 W/m 2 ). Ingeneral, when x > 1, AMx is defined to be the solar radiati<strong>on</strong> available at theearth‘s surface when the sun is at angleθcos11xfrom the zenith. Rewriting this equati<strong>on</strong>, you can say that the air massvalue isxcos θ1If you sketch this situati<strong>on</strong> (as you will be asked to do later), you will see thatx is the ratio <str<strong>on</strong>g>of</str<strong>on</strong>g> the length <str<strong>on</strong>g>of</str<strong>on</strong>g> the path that sunlight takes through the atmosphere tothe vertical thickness straight up through the atmosphere from sea level. The value<str<strong>on</strong>g>of</str<strong>on</strong>g> x obviously depends <strong>on</strong> the latitude, the seas<strong>on</strong> (solar declinati<strong>on</strong>), and the hour<str<strong>on</strong>g>of</str<strong>on</strong>g> the day. The effective value <str<strong>on</strong>g>of</str<strong>on</strong>g> x also depends <strong>on</strong> the altitude, the weather, andthe air quality.28
Figure 1.15. <str<strong>on</strong>g>Solar</str<strong>on</strong>g> spectral irradiance curves for several air mass values andvery clear air. Each total irradiance value listed in the table in the center <str<strong>on</strong>g>of</str<strong>on</strong>g>the graph is equal to the total area under the corresp<strong>on</strong>ding curve.Sunlight c<strong>on</strong>sists <str<strong>on</strong>g>of</str<strong>on</strong>g> a broad spectrum <str<strong>on</strong>g>of</str<strong>on</strong>g> wavelengths, not all <str<strong>on</strong>g>of</str<strong>on</strong>g> them visible,with substantial power density from 300 nm (ultraviolet) to 2000 nm (infrared), asshown in Figure 1.15. Not all <str<strong>on</strong>g>of</str<strong>on</strong>g> the light that hits a photovoltaic cell is absorbedin a useful manner. Some is reflected and some <str<strong>on</strong>g>of</str<strong>on</strong>g> the phot<strong>on</strong>s do not havesufficient energy to remove electr<strong>on</strong>s from their b<strong>on</strong>ds. A significant fracti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g>the energy in sunlight is in the form <str<strong>on</strong>g>of</str<strong>on</strong>g> phot<strong>on</strong>s having energies less than 1 eV (orwavelengths greater than 1240 nm), as shown in Figure 1.18. However, in silic<strong>on</strong>,phot<strong>on</strong>s having energy less than 1.12 eV (or wavelengths greater than 1110 nm)cannot create free electr<strong>on</strong>s and holes. C<strong>on</strong>sequently, these phot<strong>on</strong>s are ―wasted.‖The highest energy phot<strong>on</strong>s in sunlight are also wasted or underutilized forreas<strong>on</strong>s that are bey<strong>on</strong>d the scope <str<strong>on</strong>g>of</str<strong>on</strong>g> this experiment. There are also other energyloss mechanisms in photovoltaic cells. For example, the quality <str<strong>on</strong>g>of</str<strong>on</strong>g> the p-n juncti<strong>on</strong>is an important factor. The net result is that the efficiency <str<strong>on</strong>g>of</str<strong>on</strong>g> most silic<strong>on</strong>29
photovoltaic cells is less than 20%, meaning that less than 20% <str<strong>on</strong>g>of</str<strong>on</strong>g> the incidentoptical power can be c<strong>on</strong>verted to electrical power.An important feature <str<strong>on</strong>g>of</str<strong>on</strong>g> any power source is that to extract the maximumpower from it, the load must be ―matched‖ to the source. In the case <str<strong>on</strong>g>of</str<strong>on</strong>g> aphotovoltaic cell that is used to power some device, such as a DC motor, a lightbulb, or a calculator, the ―effective resistance‖ <str<strong>on</strong>g>of</str<strong>on</strong>g> the load must be properlychosen to get the maximum power (and highest efficiency) from the cell.Main types <str<strong>on</strong>g>of</str<strong>on</strong>g> solid state cell structures are following.30
1.3.1 P- or n-type c<strong>on</strong>tact metal/semic<strong>on</strong>ductor Schottky [34-38]On figure Figure 1.16 are presented energy band structures <str<strong>on</strong>g>of</str<strong>on</strong>g> metalsemic<strong>on</strong>ductorphotovoltaic.p-type C<strong>on</strong>tact Metaln-type C<strong>on</strong>tact MetalFigure1.16 Metal n- p- semic<strong>on</strong>ductor c<strong>on</strong>tact Schottky solar cell1.3.2 Homojuncti<strong>on</strong> Device.-Single material altered so that <strong>on</strong>e side is p-type and the other side is n-type(see Figure 1.17):31
it.-p-n juncti<strong>on</strong> is located so that the maximum amount <str<strong>on</strong>g>of</str<strong>on</strong>g> light is absorbed nearFigure 1.17 P-n Juncti<strong>on</strong>s: Essential for <str<strong>on</strong>g>Solar</str<strong>on</strong>g> <str<strong>on</strong>g>Cell</str<strong>on</strong>g>32
1.3.3 Heterojuncti<strong>on</strong> solar cellFigure 1.18 Energy band diagram for the Cu 2 O/TiO 2heterostructure. [39]-Juncti<strong>on</strong> is formed <strong>by</strong> c<strong>on</strong>tacting two different semic<strong>on</strong>ductors.-Top layer - high bandgap selected for its transparency to light.-Bottom layer - low bandgap that readily absorbs light.1.3.4 P-i-n and n-i-p devices.-A three-layer sandwich is created,- C<strong>on</strong>tains a middle intrinsic layer between n-type layer and p-type layer.- Light generates free electr<strong>on</strong>s and holes in the intrinsic regi<strong>on</strong>.1.4 Advantages and disadvantages <str<strong>on</strong>g>of</str<strong>on</strong>g> photovoltaics.Photovoltaics is the technology that generates direct current (DC) electricalpower measured in Watts (W) or kiloWatts (kW) from semic<strong>on</strong>ductors when theyare illuminated <strong>by</strong> phot<strong>on</strong>s. As l<strong>on</strong>g as light is shining <strong>on</strong> the solar cell (the namefor the individual PV element), it generates electrical power. When the light stops,33
the electricity stops. <str<strong>on</strong>g>Solar</str<strong>on</strong>g> cells never need recharging like a battery. Some havebeen in c<strong>on</strong>tinuous outdoor operati<strong>on</strong> <strong>on</strong> Earth or in space for over 30 years.Table 1.2 lists some <str<strong>on</strong>g>of</str<strong>on</strong>g> the advantages and disadvantages <str<strong>on</strong>g>of</str<strong>on</strong>g> photovoltaics.Note, that they include both technical and n<strong>on</strong>technical issues.Often, the advantages and disadvantages (Table 1.2) <str<strong>on</strong>g>of</str<strong>on</strong>g> photovoltaics arealmost completely opposite <str<strong>on</strong>g>of</str<strong>on</strong>g> c<strong>on</strong>venti<strong>on</strong>al fossil-fuel power plants. For example,fossil-fuel plants have disadvantages <str<strong>on</strong>g>of</str<strong>on</strong>g>: a wide range <str<strong>on</strong>g>of</str<strong>on</strong>g> envir<strong>on</strong>mentallyhazardous emissi<strong>on</strong>s, parts which wear out, steadily increasing fuel costs, they arenot modular (deployable in small increments), and they suffer low public opini<strong>on</strong>(no <strong>on</strong>e wants a coal burning power plant in their neighborhood). Photovoltaicssuffers n<strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g> these problems. The two comm<strong>on</strong> traits are that both PV and fossilfueled power plants are very reliable but lack the advantage <str<strong>on</strong>g>of</str<strong>on</strong>g> storage.Notice that several <str<strong>on</strong>g>of</str<strong>on</strong>g> the disadvantages are n<strong>on</strong>technical but relate toec<strong>on</strong>omics and infrastructure. They are partially compensated for <strong>by</strong> a very highpublic acceptance and awareness <str<strong>on</strong>g>of</str<strong>on</strong>g> the envir<strong>on</strong>mental benefits. During the late1990s, the average growth rate <str<strong>on</strong>g>of</str<strong>on</strong>g> PV producti<strong>on</strong> was over 33% per annum.34
1.5 <str<strong>on</strong>g>Thin</str<strong>on</strong>g> film photovoltaic and materials1.5.1<str<strong>on</strong>g>Thin</str<strong>on</strong>g> filmThe definiti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> thin film solar cell technology given <strong>by</strong> Chopra et al. [40]provides a good starting point and also yields a criteri<strong>on</strong> to discriminate the term‗thin film‘ from ‗thick film‘. They define a thin film as a material ‗created abinitio <strong>by</strong> the random nucleati<strong>on</strong> and growth processes <str<strong>on</strong>g>of</str<strong>on</strong>g> individuallyc<strong>on</strong>densing/reacting atomic/i<strong>on</strong>ic/molecular species <strong>on</strong> a substrate. The structural,chemical, metallurgical and physical properties <str<strong>on</strong>g>of</str<strong>on</strong>g> such a material are str<strong>on</strong>glydependent <strong>on</strong> a large number <str<strong>on</strong>g>of</str<strong>on</strong>g> depositi<strong>on</strong> parameters and may also be thicknessdependent. <str<strong>on</strong>g>Thin</str<strong>on</strong>g> films may encompass a c<strong>on</strong>siderable thickness range, varyingfrom a few nanometers to tens <str<strong>on</strong>g>of</str<strong>on</strong>g> micrometers and thus are best defined in terms<str<strong>on</strong>g>of</str<strong>on</strong>g> the producti<strong>on</strong> processes rather than <strong>by</strong> thickness. One may obtain a thinmaterial (not a thin film) <strong>by</strong> a number <str<strong>on</strong>g>of</str<strong>on</strong>g> other methods (normally called thickfilmtechniques) such as <strong>by</strong> thinning bulk material, or <strong>by</strong> depositing clusters <str<strong>on</strong>g>of</str<strong>on</strong>g>microscopic species in such processes as screen-printing, electrophoresis, slurryspray, plasma gun, ablati<strong>on</strong>, etc.‘ The given definiti<strong>on</strong> still leaves room for abroad field <str<strong>on</strong>g>of</str<strong>on</strong>g> technologies to deposit the thin film (plasma, sputtering,evaporati<strong>on</strong>, depositi<strong>on</strong> from the liquid phase, etc.) and to tailor its electrical andmorphological properties (crystalline, amorphous and intermediary forms). Forthe inorganic n<strong>on</strong>-crystalline Si materials and technologies is presented in [40].1.5.2 <str<strong>on</strong>g>Thin</str<strong>on</strong>g> film photovoltaic materialsC<strong>on</strong>venti<strong>on</strong>ally, photovoltaic materials use inorganic semic<strong>on</strong>ductors. Thesemic<strong>on</strong>ductors <str<strong>on</strong>g>of</str<strong>on</strong>g> interest allow the formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> charge-carrier separatingjuncti<strong>on</strong>s. The juncti<strong>on</strong> can be either a homojuncti<strong>on</strong> (like in Si) or aheterojuncti<strong>on</strong> with other materials to collect the excess carriers when exposed tolight. In principle, a large number <str<strong>on</strong>g>of</str<strong>on</strong>g> semic<strong>on</strong>ductor materials are eligible, but<strong>on</strong>ly a few <str<strong>on</strong>g>of</str<strong>on</strong>g> them are <str<strong>on</strong>g>of</str<strong>on</strong>g> sufficient interest. Ideally, the absorber material <str<strong>on</strong>g>of</str<strong>on</strong>g> anefficient terrestrial solar cell should be a semic<strong>on</strong>ductor with a bandgap <str<strong>on</strong>g>of</str<strong>on</strong>g> 1–1.5eV with a high solar optical absorpti<strong>on</strong> (10 4 − 10 5 cm −1 ) in the wavelength regi<strong>on</strong><str<strong>on</strong>g>of</str<strong>on</strong>g> 350–1000 nm, a high quantum yield for the excited carriers, a l<strong>on</strong>g diffusi<strong>on</strong>length low recombinati<strong>on</strong> velocity. If all these c<strong>on</strong>straints are satisfied and the35
asic material is widely available, the material allows in principle themanufacturing <str<strong>on</strong>g>of</str<strong>on</strong>g> a thin-film solar cell device. From the point <str<strong>on</strong>g>of</str<strong>on</strong>g> view <str<strong>on</strong>g>of</str<strong>on</strong>g>processing, manufacturing and reproducibility, elemental semic<strong>on</strong>ductors providea simple and straightforward approach to manufacture thin-film solar cells. Thefirst material at hand is the material which dominates the PV market nowadays:silic<strong>on</strong>.Crystalline silic<strong>on</strong> is a semic<strong>on</strong>ductor material with a bandgap <str<strong>on</strong>g>of</str<strong>on</strong>g> 1.1 eV.Because <str<strong>on</strong>g>of</str<strong>on</strong>g> the indirect bandgap character <str<strong>on</strong>g>of</str<strong>on</strong>g> silic<strong>on</strong> for phot<strong>on</strong>s with energylower than 3.4 eV, it is clearly not an ideal material for thin-film solar cells.Nevertheless, there is a substantial R&D effort being put into developing thin-filmsolar cells <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> crystalline Si. The thin film <str<strong>on</strong>g>of</str<strong>on</strong>g> crystalline Si can be growneither <strong>by</strong> low-temperature depositi<strong>on</strong> techniques which yield microcrystalline Sior <strong>by</strong> high-temperature techniques. In the latter case the material properties <str<strong>on</strong>g>of</str<strong>on</strong>g> thegrown crystalline Si film are similar to the properties <str<strong>on</strong>g>of</str<strong>on</strong>g> bulk crystalline Si solarmaterial. Because <str<strong>on</strong>g>of</str<strong>on</strong>g> its relatively low absorpti<strong>on</strong> coefficient, crystalline Si layershave to be at least 30 μm thick to absorb sufficient light unless opticalenhancement techniques are used to improve the effective absorpti<strong>on</strong>. As a result<str<strong>on</strong>g>of</str<strong>on</strong>g> its high refractive index (4) crystalline Si allows efficient optical c<strong>on</strong>finementwith optical pathlength enhancements up to a factor 50. It will come as no surpriseto the reader that optical enhancement is therefore a substantial part <str<strong>on</strong>g>of</str<strong>on</strong>g> theresearch in the field <str<strong>on</strong>g>of</str<strong>on</strong>g> thin-film crystalline Si.Si can also be deposited in its amorphous form. Amorphous Si as such is amaterial <str<strong>on</strong>g>of</str<strong>on</strong>g> little use for photovoltaics because <str<strong>on</strong>g>of</str<strong>on</strong>g> the extremely high danglingb<strong>on</strong>d density and intragap state density in the material (>10 19 cm −3 ), resulting inimmediate recombinati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> photoexcited excess carriers and pins the Fermi level,excluding c<strong>on</strong>trollable doing. The properties <str<strong>on</strong>g>of</str<strong>on</strong>g> amorphous Si are drasticallyimproved <strong>by</strong> alloying it with H, which passivates most <str<strong>on</strong>g>of</str<strong>on</strong>g> the dangling b<strong>on</strong>ds andreduces the intrap state density to 10 16 cm −3 or less. The alloying with H takesplace in a natural way during the depositi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the film which is deposited <strong>by</strong>cracking a Si precursor (most <str<strong>on</strong>g>of</str<strong>on</strong>g>ten SiH 4 ) <strong>by</strong> means <str<strong>on</strong>g>of</str<strong>on</strong>g> a plasma and the materialformed is denoted as a-Si:H. In comparis<strong>on</strong> with crystalline Si, a-Si:H has anumber <str<strong>on</strong>g>of</str<strong>on</strong>g> properties which make it attractive as an absorber layer for thin-film36
solar cells. The bandgap <str<strong>on</strong>g>of</str<strong>on</strong>g> a-Si:H, is to some extent tailorable <strong>by</strong> the method andc<strong>on</strong>diti<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> depositi<strong>on</strong>. In additi<strong>on</strong>, the material is relatively easy to dope, whichallows the manufacturing <str<strong>on</strong>g>of</str<strong>on</strong>g> homojuncti<strong>on</strong> devices and, last but not least, it has ahigh optical absorpti<strong>on</strong> coefficient over the wavelength range <str<strong>on</strong>g>of</str<strong>on</strong>g> interest. Undersuitable depositi<strong>on</strong> c<strong>on</strong>diti<strong>on</strong>s and str<strong>on</strong>g hydrogen diluti<strong>on</strong>, nanocrystalline andmicrocrystalline materials are obtained. While the crystallite size and volumefracti<strong>on</strong> are very small, these crystallites catalyze the crystallizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> theremainder <str<strong>on</strong>g>of</str<strong>on</strong>g> the amorphous matrix up<strong>on</strong> annealing. Microcrystalline materialsdeposited <strong>by</strong> this method are found to have less defect density and are more stableagainst light degradati<strong>on</strong> compared with a-Si. Recently developed improvedefficiency materials c<strong>on</strong>sist <str<strong>on</strong>g>of</str<strong>on</strong>g> this heterogeneous mixture <str<strong>on</strong>g>of</str<strong>on</strong>g> the amorphous andan intermediate range order microcrystalline material. a-Si:H has emerged as anattractive material which, for some time, was challenging the supremacy <str<strong>on</strong>g>of</str<strong>on</strong>g>crystalline Si cells in the Eighties. Because <str<strong>on</strong>g>of</str<strong>on</strong>g> stability problems and the lowerefficiencies as compared with crystalline Si, the market share <str<strong>on</strong>g>of</str<strong>on</strong>g> a-Si:H <str<strong>on</strong>g>based</str<strong>on</strong>g>thin- film solar cells remained limited. Nevertheless, the present shortage <str<strong>on</strong>g>of</str<strong>on</strong>g>crystalline Si and the developments around multijuncti<strong>on</strong> structures andmicro/nanocrystalline Si extensi<strong>on</strong>s provide a new opportunity for this technologyto make it to the marketplace. Recently also carb<strong>on</strong> came into the picture as acandidate material for thin-film solar cells, with first results being reported forbor<strong>on</strong>-doped diam<strong>on</strong>d-like carb<strong>on</strong> [41] and fullerene films [42].III–V compound materials like GaAs, InP and their derived alloys andcompounds, which most <str<strong>on</strong>g>of</str<strong>on</strong>g>ten have a direct bandgap character, are ideal forphotovoltaic applicati<strong>on</strong>s, but are far too expensive for large-scale commercialapplicati<strong>on</strong>s, because <str<strong>on</strong>g>of</str<strong>on</strong>g> the high cost <str<strong>on</strong>g>of</str<strong>on</strong>g> the necessary precursors for thedepositi<strong>on</strong> and the depositi<strong>on</strong> systems itself. The depositi<strong>on</strong> systems for thesematerials are either <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> molecular beam epitaxy or metalorganic chemicalvapour depositi<strong>on</strong>.More appealing from the point <str<strong>on</strong>g>of</str<strong>on</strong>g> view <str<strong>on</strong>g>of</str<strong>on</strong>g> ease <str<strong>on</strong>g>of</str<strong>on</strong>g> processability and cost <str<strong>on</strong>g>of</str<strong>on</strong>g>material and depositi<strong>on</strong> are the ―II–VI compound materials‖ like CdTe orvariati<strong>on</strong>s <strong>on</strong> this like CuInSe 2 .37
The interest in these materials for thin-film solar cell manufacturing isessentially <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> two elements. Because <str<strong>on</strong>g>of</str<strong>on</strong>g> the chemical structure <str<strong>on</strong>g>of</str<strong>on</strong>g> thesematerials the internal (grain boundaries, interfaces) and external surfaces areintrinsically well passivated and characterized <strong>by</strong> a low recombinati<strong>on</strong> velocity forexcess carriers. The low recombinati<strong>on</strong> activity at the grain boundaries allowshigh solar cell efficiencies even when the material is polycrystalline with grainsizes in the order <str<strong>on</strong>g>of</str<strong>on</strong>g> <strong>on</strong>ly a few μm. This is to be c<strong>on</strong>trasted with crystalline Siwhere grain boundaries are normally characterized <strong>by</strong> a high recombinati<strong>on</strong>velocity. Moreover, the polycrystallinity allows a large number <str<strong>on</strong>g>of</str<strong>on</strong>g> substrates(glass, steel foil, . . . ) and is compatible with low-cost temperature depositi<strong>on</strong>techniques, as there is no need for epitaxial growth or high temperatures to obtainlarge grain sizes. A sec<strong>on</strong>d important property is the possibility to tailor thebandgap e.g. replacing Se <strong>by</strong> S in CuInSe 2 results in a material with a higherbandgap. This property allows <strong>on</strong>e to build in bandgap gradients aiding thecollecti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> excess carriers and, ultimately, could even be used to developmultijuncti<strong>on</strong> solar cells. With the increasing number <str<strong>on</strong>g>of</str<strong>on</strong>g> comp<strong>on</strong>ents in ternariesand even quaternaries, the number <str<strong>on</strong>g>of</str<strong>on</strong>g> possible material combinati<strong>on</strong>s increases.An interesting alternative to inorganic semic<strong>on</strong>ductor absorbers are organicsemic<strong>on</strong>ductors, which combine interesting optoelectr<strong>on</strong>ic properties with theexcellent mechanical and processing properties <str<strong>on</strong>g>of</str<strong>on</strong>g> polymeric/plastic materials. Inorganic semic<strong>on</strong>ductors, absorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> phot<strong>on</strong>s leads to the creati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> boundelectr<strong>on</strong>–hole pairs (excit<strong>on</strong>s) with a binding energy <str<strong>on</strong>g>of</str<strong>on</strong>g> 0.5 eV rather than freecharges. The excit<strong>on</strong>s carry energy, but no net charge, and have to diffuse todissociati<strong>on</strong> sites where their charges can be separated and transported to thec<strong>on</strong>tacts. In most organic semic<strong>on</strong>ductors, <strong>on</strong>ly a small porti<strong>on</strong> (30 %) <str<strong>on</strong>g>of</str<strong>on</strong>g> theincident light is absorbed because the majority <str<strong>on</strong>g>of</str<strong>on</strong>g> semic<strong>on</strong>ducting polymers havebandgaps higher than 2.0 eV.The typically low charge-carrier and excit<strong>on</strong> mobility require the activeabsorber layer thickness to be less than 100 nm. This thickness is sufficient toabsorb most <str<strong>on</strong>g>of</str<strong>on</strong>g> the incident solar phot<strong>on</strong>s if light trapping is used, although thelowrefractive index calls for adapted approaches.38
More importantly, organic semic<strong>on</strong>ductors can be processed from soluti<strong>on</strong>s ator near room temperature <strong>on</strong> flexible substrates using simple, cheap and lowenergydepositi<strong>on</strong> methods such as spinning or printing there<strong>by</strong> yielding cheaperdevices. Even though the efficiency <str<strong>on</strong>g>of</str<strong>on</strong>g> these devices is poor at present, they mayfind immediate applicati<strong>on</strong>s for disposable solar cell <str<strong>on</strong>g>based</str<strong>on</strong>g> small powerapplicati<strong>on</strong>s. Am<strong>on</strong>g the major issues to be addressed before reas<strong>on</strong>able marketpenetrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the organic devices takes place are the improvement <str<strong>on</strong>g>of</str<strong>on</strong>g> the stability<str<strong>on</strong>g>of</str<strong>on</strong>g> c<strong>on</strong>jugate polymers, and the matching <str<strong>on</strong>g>of</str<strong>on</strong>g> the bandgap <str<strong>on</strong>g>of</str<strong>on</strong>g> the organic materialswith the solar spectrum for higher c<strong>on</strong>versi<strong>on</strong> efficiency <strong>by</strong> usingblended/composite polymers and suitable dyes.1.5.3 Different thin film solar cell technologiesBased <strong>on</strong> the available semic<strong>on</strong>ductor absorber materials discussed above <strong>on</strong>ecan go systematically over the different related thin film solar cell technologies.1.5.3.1 Crystalline silic<strong>on</strong> thin film solar cellsThere are a large variety <str<strong>on</strong>g>of</str<strong>on</strong>g> crystalline Si thin film approaches. The <strong>on</strong>e that isclosest to the classical crystalline Si solar cell structure is the ‗epitaxial solar cellapproach‘. The basic idea behind this thin film approach, is the realizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> athin crystalline Si film <str<strong>on</strong>g>of</str<strong>on</strong>g> high electr<strong>on</strong>ic quality <strong>on</strong> a low cost Si carrier substrate<strong>by</strong> means <str<strong>on</strong>g>of</str<strong>on</strong>g> epitaxial growth as shown in Figure 1.19a. The depicted structurestr<strong>on</strong>gly resembles the structure <str<strong>on</strong>g>of</str<strong>on</strong>g> a classical, self supporting bulk crystalline Sisolar cell and, as a result, the basic solar cell process to produce the solar cell isvery similar to the practices used within the photovoltaics (PV) industrynowadays. Its structural similarity would result in a low acceptance threshold inthe solar cell industry, which is presently still dominated <strong>by</strong> crystalline Si. Aspecial case is the ‗lift-<str<strong>on</strong>g>of</str<strong>on</strong>g>f approach‘ where, <strong>by</strong> means <str<strong>on</strong>g>of</str<strong>on</strong>g> a crystalline Si template<str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> porous Si, a thin cell is realized which is lifted <str<strong>on</strong>g>of</str<strong>on</strong>g>f before or after the cellprocess.The epitaxial cell approach relies <strong>on</strong> the use <str<strong>on</strong>g>of</str<strong>on</strong>g> a Si substrate. There are alsoattempts to develop thin film crystalline Si solar cell structures <strong>on</strong> n<strong>on</strong>-Sisubstrates. Because <str<strong>on</strong>g>of</str<strong>on</strong>g> the high temperatures (>600 ◦C) used for the filmdepositi<strong>on</strong>, glass is not suitable as a substrate. Rather, low cost ceramic substrates39
and graphite are the substrates <str<strong>on</strong>g>of</str<strong>on</strong>g> choice. In case the substrate is n<strong>on</strong>c<strong>on</strong>ductive,novel solar cell structures are needed to c<strong>on</strong>tact the solar cell as shown in Figure1.19b. The Si layer, deposited <strong>on</strong> top <str<strong>on</strong>g>of</str<strong>on</strong>g> these substrates, will be micro orpolycrystalline with a grain size determined <strong>by</strong> the growth temperature andsupersaturati<strong>on</strong> c<strong>on</strong>diti<strong>on</strong>s during the silic<strong>on</strong> layer depositi<strong>on</strong>. It turns out to bedifficult to realize solar cells with proper energy c<strong>on</strong>versi<strong>on</strong> efficiencies inmaterial with a grain size <str<strong>on</strong>g>of</str<strong>on</strong>g> 1–10 μm, although substantial progress has beenmade recently in this field. On ceramic substrates which withstand hightemperatures, liquid phase recrystallizati<strong>on</strong> is <str<strong>on</strong>g>of</str<strong>on</strong>g>ten applied to increase the finalgrain size, whereas laser recrystallizati<strong>on</strong> and rapid thermal annealing is beingdeveloped for substrates which can <strong>on</strong>ly withstand process temperatures >650 ◦Cfor a limited time.Figure 1.19 a) Schematic <str<strong>on</strong>g>of</str<strong>on</strong>g> a epitaxial cell <strong>on</strong> a highly doped low cost Sisubstrate (ARC stands for anti-reflective coating; b) Novel solar cells with an<strong>on</strong>coductive substrate.1.5.3.2 Amorphous and microcrystalline silic<strong>on</strong> thin film solar cellsBecause a-Si:H can be doped efficiently p- and n-type, the cell structure is<str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> a homojuncti<strong>on</strong>. As a result <str<strong>on</strong>g>of</str<strong>on</strong>g> the short carrier lifetime and the lowcarrier mobility collecti<strong>on</strong> <strong>by</strong> pure diffusi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> excess carriers is not veryeffective. Therefore a-Si:H solar cells also include a drift z<strong>on</strong>e to improve thecarrier collecti<strong>on</strong>. As a result the structure <str<strong>on</strong>g>of</str<strong>on</strong>g> the solar cell device is a p-i-nstructurewhere an intrinsic a-Si:H layer is sandwiched between a thin n+ and p+typelayer. Because <str<strong>on</strong>g>of</str<strong>on</strong>g> the low doping in the intrinsic part, the electrical field willextend all over the intrinsic layer. As menti<strong>on</strong>ed before, the properties <str<strong>on</strong>g>of</str<strong>on</strong>g> thematerial and the juncti<strong>on</strong> device are severely affected <strong>by</strong> the light induced creati<strong>on</strong>40
<str<strong>on</strong>g>of</str<strong>on</strong>g> metastable defects, known as the Staebler–Wr<strong>on</strong>ski effect. Light induceddegradati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a-Si:H devices and, as a c<strong>on</strong>sequence, a reducti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the electricalfield in the intrinsic part <str<strong>on</strong>g>of</str<strong>on</strong>g> the cell is tackled <strong>by</strong> reducing the a-Si:H layerthickness so that the photogenerated carriers need to move <strong>on</strong>ly a short distancebefore they reach the electrode. However, thinning down also results in lowerlight absorpti<strong>on</strong> and thus optical c<strong>on</strong>finement techniques employing diffuselyreflecting fr<strong>on</strong>t and back c<strong>on</strong>tacts are required to increase the effective layerthickness in order to absorb the phot<strong>on</strong>s. Over a period <str<strong>on</strong>g>of</str<strong>on</strong>g> time, extensive researchand development work <strong>on</strong> depositi<strong>on</strong> techniques and device structure haveresulted in development <str<strong>on</strong>g>of</str<strong>on</strong>g> single juncti<strong>on</strong> and multijuncti<strong>on</strong> devices with highefficiency and moderately good stability. A typical a-Si:H <str<strong>on</strong>g>based</str<strong>on</strong>g> multijuncti<strong>on</strong>solar cell structure is shown in Figure 1.20 a.As the cost <str<strong>on</strong>g>of</str<strong>on</strong>g> the Ge precursors is high and the electr<strong>on</strong>ic quality <str<strong>on</strong>g>of</str<strong>on</strong>g> the a-SiGe:H layers is lower than for a-Si:H layers, a lot <str<strong>on</strong>g>of</str<strong>on</strong>g> effort is being d<strong>on</strong>e inreplacing the a-SiGe:H part <str<strong>on</strong>g>of</str<strong>on</strong>g> the cell <strong>by</strong> other soluti<strong>on</strong>s like microcrystalline Si.This leads to the ‗micromorph‘ cell c<strong>on</strong>cept combining an a-Si:H topcell with amicrocrystalline Si bottomcell as shown in Figure 1.20 b.41
Figure 1.20 a) Cross secti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> typical multijuncti<strong>on</strong> solar cell <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> a-Si:H. In order to improve the performance the lower a-Si:H subcells aresometimes replaced <strong>by</strong> a-SiGe:H alloys; b) In the sec<strong>on</strong>d approach (themicromorph c<strong>on</strong>cept) improved stability is obtained <strong>by</strong> replacing the a-Si:Hbottom cell <strong>by</strong> a microcrystalline Si solar cell (taken from [40]).1.5.3.3 <strong>Copper</strong> indium gallium selenide and cadmium telluride solar cellstructuresBoth superstrate and substrate device structures are currently being pursuedfor copper indium gallium selenide (CuIn(Ga)Se2, CIGS) device fabricati<strong>on</strong>. Thefilm growth and interdiffusi<strong>on</strong> and hence the device properties are dependent <strong>on</strong>the device structure. These CIGS solar cells, <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> the superstrate structure,are inferior to the substrate structure, because <str<strong>on</strong>g>of</str<strong>on</strong>g> the interdiffusi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> CdS duringhigh temperature CIGS film growth. A best device efficiency <str<strong>on</strong>g>of</str<strong>on</strong>g> 10.2% wasreported for the superstrate structure. On the other hand, a substrate c<strong>on</strong>figurati<strong>on</strong>like the <strong>on</strong>e shown in Figure 1.22 a with CdS buffer layer resulted in a 19.2%efficiency device. Cadmium telluride devices are fabricated preferably in thesuperstrate c<strong>on</strong>figurati<strong>on</strong> because the CdTe surface is exposed for c<strong>on</strong>tacting. In42
additi<strong>on</strong>, the benign feature <str<strong>on</strong>g>of</str<strong>on</strong>g> CdS diffusi<strong>on</strong> during the processing reduces thelattice mismatch between CdTe and CdS. Cadmium telluride solar cells useborosilicate glass for high temperature depositi<strong>on</strong> (600 ◦ C) and soda lime glass forlow temperature depositi<strong>on</strong> (60–500 ◦ C). Cadmium telluride has also beendeposited <strong>on</strong> thin metallic foils such as stainless steel, Mo, Ni and Cu.Molybdenum is best suited for CdTe depositi<strong>on</strong>, owing to better thermalmatching.Figure 1.22 a) Schematic view <str<strong>on</strong>g>of</str<strong>on</strong>g> the typical substrate structure <str<strong>on</strong>g>of</str<strong>on</strong>g> CIGS solarcells; b) Schematic view <str<strong>on</strong>g>of</str<strong>on</strong>g> the typical superstrate structure <str<strong>on</strong>g>of</str<strong>on</strong>g> CdTe solarcell[40].1.5.3.4 Basic structure <str<strong>on</strong>g>of</str<strong>on</strong>g> thin film organic solar cellsThe term ‗organic solar cell‘ has to be correctly defined. The term coversthose photovoltaic devices where the organic layer is an essential part <str<strong>on</strong>g>of</str<strong>on</strong>g> thephotovoltaic process. The basic steps in photovoltaic c<strong>on</strong>versi<strong>on</strong> are lightabsorbance, charge carrier generati<strong>on</strong>, charge carrier transport andextracti<strong>on</strong>/injecti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> charge carriers through externally accessible c<strong>on</strong>tacts.More specifically, the term ‗organic solar cell‘ is applicable whenever at least the43
two first steps are being realized <strong>by</strong> means <str<strong>on</strong>g>of</str<strong>on</strong>g> an organic layer. By this definiti<strong>on</strong>,full organic devices as well as hybrid devices are being covered.Organic solar cells have already been the subject <str<strong>on</strong>g>of</str<strong>on</strong>g> R&D efforts for a l<strong>on</strong>gtime because <str<strong>on</strong>g>of</str<strong>on</strong>g> the potentially very low cost <str<strong>on</strong>g>of</str<strong>on</strong>g> the active layer material.Originally, most <str<strong>on</strong>g>of</str<strong>on</strong>g> the attempts to realize organic solar cell devices were <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong>essentially the same c<strong>on</strong>cepts as thin film p-n or p-i-n devices. This resulted inenergy c<strong>on</strong>versi<strong>on</strong> efficiencies <str<strong>on</strong>g>of</str<strong>on</strong>g> about 1% with the main limitati<strong>on</strong> being theshort excit<strong>on</strong> diffusi<strong>on</strong> length and insufficient excit<strong>on</strong> dissociati<strong>on</strong>. Thebreakthrough for solar cells incorporating an organic part came with the advent <str<strong>on</strong>g>of</str<strong>on</strong>g>c<strong>on</strong>cepts that radically deviated from the planar hetero- or homojuncti<strong>on</strong> solarcells. The generic idea behind these c<strong>on</strong>cepts is the existence <str<strong>on</strong>g>of</str<strong>on</strong>g> a bulk distributedinterface to capture the excited carrier and to increase the excit<strong>on</strong> dissociati<strong>on</strong>rate. The ‗Graetzel cell‘ is a prominent example <str<strong>on</strong>g>of</str<strong>on</strong>g> this generic idea bel<strong>on</strong>ging tothe class <str<strong>on</strong>g>of</str<strong>on</strong>g> hybrid cells. Within the pores <str<strong>on</strong>g>of</str<strong>on</strong>g> a porous TiO 2 layer a m<strong>on</strong>olayer <str<strong>on</strong>g>of</str<strong>on</strong>g>an organic sensitizer is adsorbed <strong>on</strong> the pore walls as shown in Figure 1.23 a.After absorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a phot<strong>on</strong>, the excited electr<strong>on</strong> within the sensitizer molecule isimmediately transferred to the c<strong>on</strong>ducti<strong>on</strong> band <str<strong>on</strong>g>of</str<strong>on</strong>g> TiO 2 , after which the electr<strong>on</strong>diffuses through the porous network to the c<strong>on</strong>tact. The oxidized sensitizermolecule is reduced to the original state <strong>by</strong> supplying electr<strong>on</strong>s through a liquidelectrolyte within the pores. <str<strong>on</strong>g>Cell</str<strong>on</strong>g>s <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> this hybrid c<strong>on</strong>cept showc<strong>on</strong>firmedenergy c<strong>on</strong>versi<strong>on</strong> efficiencies up to 11%for small area cells, whereas upscaledmodules exhibit efficiencies between 5 and 7%. Standing issues <str<strong>on</strong>g>of</str<strong>on</strong>g> this type <str<strong>on</strong>g>of</str<strong>on</strong>g>hybrid solar cells are the replacement <str<strong>on</strong>g>of</str<strong>on</strong>g> the sensitizer <strong>by</strong> a material withincreased absorpti<strong>on</strong> in the red and infrared part <str<strong>on</strong>g>of</str<strong>on</strong>g> the spectrum, the replacement<str<strong>on</strong>g>of</str<strong>on</strong>g> the liquid electrolyte <strong>by</strong> a solid state hole c<strong>on</strong>ductor and the improvement <str<strong>on</strong>g>of</str<strong>on</strong>g>the cell stability.The full organic counterpart <str<strong>on</strong>g>of</str<strong>on</strong>g> the hybrid cell is the bulk d<strong>on</strong>or–acceptorheterojuncti<strong>on</strong> c<strong>on</strong>cept (see Figure 1.23 b), which is <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> blends <str<strong>on</strong>g>of</str<strong>on</strong>g> twoorganic compounds, <strong>on</strong>e with d<strong>on</strong>or character, the other with acceptor properties.The excit<strong>on</strong>s dissociate very efficiently at the interfaces between d<strong>on</strong>or andacceptor phases and flow through the percolated d<strong>on</strong>or and acceptor subnetworksto the c<strong>on</strong>tacts, which are carrier selective. Efficiencies up to 5% were reported44
for this type <str<strong>on</strong>g>of</str<strong>on</strong>g> cell <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> the P3HT/PCBM d<strong>on</strong>or–acceptor couple. Thisacceptor (PCBM) is <str<strong>on</strong>g>of</str<strong>on</strong>g>ten used, because excit<strong>on</strong> dissociati<strong>on</strong> turns out to beextremely efficient with transfer times in the 100 fs range. Alternatives in whichPCBM is replaced <strong>by</strong> a polymer with acceptor characteristics were also reported(e.g. CN-PPV/MEH-PPV pair). For this type <str<strong>on</strong>g>of</str<strong>on</strong>g> organic cell, standing issues arethe extensi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the active layer absorbance towards the red and infrared range,the use <str<strong>on</strong>g>of</str<strong>on</strong>g> materials with higher mobilities (in this c<strong>on</strong>text also tests with liquidcrystal materials should be menti<strong>on</strong>ed) and the critical issue <str<strong>on</strong>g>of</str<strong>on</strong>g> stability. In theframework <str<strong>on</strong>g>of</str<strong>on</strong>g> improving the red and infrared sensitivity <str<strong>on</strong>g>of</str<strong>on</strong>g> the d<strong>on</strong>ors, themajority <str<strong>on</strong>g>of</str<strong>on</strong>g> the activities are directed towards thiophenes, which could alsoimprove stability.For the sake <str<strong>on</strong>g>of</str<strong>on</strong>g> completeness it should be menti<strong>on</strong>ed that there is also an<strong>on</strong>organic counterpart <str<strong>on</strong>g>of</str<strong>on</strong>g> these three-dimensi<strong>on</strong>al juncti<strong>on</strong> devices. These are theETA (extremely thin absorber) structures, shown in Figure 1.23c.45
Figure 1.23 Structures <str<strong>on</strong>g>of</str<strong>on</strong>g> a) the Graetzel cell <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> nanocrystalline porousTiO 2 ; b) the bulk d<strong>on</strong>or–acceptor heterojuncti<strong>on</strong> c<strong>on</strong>cept; c) the structure <str<strong>on</strong>g>of</str<strong>on</strong>g>ETA cells.46
C<strong>on</strong>clusi<strong>on</strong>sPhotovoltaics c<strong>on</strong>stitute a new form <str<strong>on</strong>g>of</str<strong>on</strong>g> producing electric energy that isenvir<strong>on</strong>mentally clean and very modular. In stand-al<strong>on</strong>e installati<strong>on</strong>s, it must usestorage or another type <str<strong>on</strong>g>of</str<strong>on</strong>g> generator to provide electricity when the sun is notshining. In grid-c<strong>on</strong>nected installati<strong>on</strong>s storage is not necessary: in the absence <str<strong>on</strong>g>of</str<strong>on</strong>g>sunlight, electricity is provided from other (c<strong>on</strong>venti<strong>on</strong>al) sources.PV electricity is highly appreciated <strong>by</strong> the public. It is unique for manyapplicati<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> high social value such as providing electricity to people who lackit in remote areas.Often, internati<strong>on</strong>al d<strong>on</strong>or agencies are providing the funding, as many <str<strong>on</strong>g>of</str<strong>on</strong>g> theusers are very poor. Photovoltaics is very suitable as the power supply for remotecommunicati<strong>on</strong> equipment. Its use is increasing rapidly to produce electricity ingrid-c<strong>on</strong>nected houses and buildings in industrialized countries, despite a 5 to 10times higher cost than c<strong>on</strong>venti<strong>on</strong>al electricity. Often, publicly funded programsare required to enable photovoltaics to compete <strong>by</strong> partially <str<strong>on</strong>g>of</str<strong>on</strong>g>fsetting its highercosts.Largely, because <str<strong>on</strong>g>of</str<strong>on</strong>g> grid-c<strong>on</strong>nected PV applicati<strong>on</strong>s such as homes andbusinesses, the expansi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the PV market has been very rapid in the last years <str<strong>on</strong>g>of</str<strong>on</strong>g>the twentieth century and it is expected to c<strong>on</strong>tinue during the next few years <str<strong>on</strong>g>of</str<strong>on</strong>g>the twenty-first century. Then, the growth will probably c<strong>on</strong>tinue at a slower paceunless new technological advances are developed and commercialized. In thatcase, growth could accelerate. Photovoltaics is poised to become a large globalhigh-tech industry, manufacturing and selling modules in nearly every country.Governments and entrepreneurs should be aware <str<strong>on</strong>g>of</str<strong>on</strong>g> this. Public R&D support hasalways been generous. It must c<strong>on</strong>tinue to be so for those countries that want tomaintain leadership in this technology. Partial subsidizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> PV installati<strong>on</strong>s ispermitting an unprecedented development <str<strong>on</strong>g>of</str<strong>on</strong>g> the PV industry and will also helpthe industry <str<strong>on</strong>g>of</str<strong>on</strong>g> the countries involved in this endeavor to take the lead.Crystalline Si technology, both m<strong>on</strong>ocrystalline and multicrystalline is todayclearly dominant, with about 90% <str<strong>on</strong>g>of</str<strong>on</strong>g> the market. It will remain dominant at leastfor the next ten years. The trend is towards the multicrystalline opti<strong>on</strong>. Si is <strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g>47
the most abundant elements in the Earth‘s crust but the purified Si used in today‘ssolar cells is obtained primarily as <str<strong>on</strong>g>of</str<strong>on</strong>g>f-grade poly-Si and scrap wafers from themicroelectr<strong>on</strong>ic industry. So<strong>on</strong> it will not be enough for the growing PV industry.Thus, some c<strong>on</strong>cerns exist regarding a shortage <str<strong>on</strong>g>of</str<strong>on</strong>g> the purified silic<strong>on</strong> for the PVindustry. However, it is doubtful that Si technology will be able to reachcompetiti<strong>on</strong> with c<strong>on</strong>venti<strong>on</strong>al electricity. C<strong>on</strong>sequently, low-cost alternatives andhigh-efficiency novel c<strong>on</strong>cepts, many already in development, are needed.<str<strong>on</strong>g>Thin</str<strong>on</strong>g>-film technology is <strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g> the candidates to take over from Si technologyin the l<strong>on</strong>g-term. There are many technological opti<strong>on</strong>s regarding thin-filmmaterials and methods <str<strong>on</strong>g>of</str<strong>on</strong>g> depositi<strong>on</strong> but their primary claim to the thr<strong>on</strong>ecurrently occupied <strong>by</strong> Si is that they can be ultimately produced at much lowercost.C<strong>on</strong>centrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> sunlight is another candidate for mass penetrati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g>photovoltaics, although it will not be easily accepted for the grid-c<strong>on</strong>nectedhouses, <strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g> the most promising applicati<strong>on</strong>s today. C<strong>on</strong>centrators will probablyfind incipient niche markets in big stand-al<strong>on</strong>e applicati<strong>on</strong>s or as small, centralpowerplants during the present decade. Finally, new materials and device designs<str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> III-V semic<strong>on</strong>ductor alloys, such as GaInP allowing more efficient use<str<strong>on</strong>g>of</str<strong>on</strong>g> the solar spectrum are now being developed for space applicati<strong>on</strong>s. With theuse <str<strong>on</strong>g>of</str<strong>on</strong>g> c<strong>on</strong>centrators, they may be <str<strong>on</strong>g>of</str<strong>on</strong>g> interest for terrestrial applicati<strong>on</strong>s, with thepotential <str<strong>on</strong>g>of</str<strong>on</strong>g> reaching competitive costs with c<strong>on</strong>venti<strong>on</strong>al electricity. Otheropti<strong>on</strong>s, such as quantum dots and dye-sensitized solar cells, are still in the initialresearch phase. They must compete not <strong>on</strong>ly with the ubiquitous Si but also withthe other opti<strong>on</strong>s, menti<strong>on</strong>ed above for funding for further development.Thus, photovoltaics possesses a panoply <str<strong>on</strong>g>of</str<strong>on</strong>g> novel technologies that almostensures that alternatives will be available when the current Si wafer technologycannot reach prices low enough to compete with c<strong>on</strong>venti<strong>on</strong>al electricity. If thishappens, a str<strong>on</strong>g industry and infrastructure – first developed for the Sitechnology – will be able to seamlessly take this new PV technology and apply itworldwide. In any case, the present ―subsidies‖ to research or applicati<strong>on</strong> must bec<strong>on</strong>sidered as public investment in a policy with str<strong>on</strong>g public support and l<strong>on</strong>gtermhuman benefits.48
The widespread c<strong>on</strong>tributi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> PV electricity in electric grids will require anew type <str<strong>on</strong>g>of</str<strong>on</strong>g> grid management that can accept small generators as well as smallc<strong>on</strong>sumers. On the other hand, hybrid forms <str<strong>on</strong>g>of</str<strong>on</strong>g> electricity generati<strong>on</strong>, includingphotovoltaics, will play an important role in the future development <str<strong>on</strong>g>of</str<strong>on</strong>g> stand-al<strong>on</strong>eapplicati<strong>on</strong>s and minigrids. The general cost reducti<strong>on</strong> will make photovoltaicsavailable to more and more people in developing countries helping theirdevelopment with little degradati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> air quality associated with fossil-fuelgenerators.In summary, it is very likely that photovoltaics will become in the next halfcentury an important source <str<strong>on</strong>g>of</str<strong>on</strong>g> world electricity. Public support and globalenvir<strong>on</strong>mental c<strong>on</strong>cerns will keep photovoltaics viable, visible, and vigorous bothin new technical developments and user applicati<strong>on</strong>s. Nati<strong>on</strong>s which encouragephotovoltaics will be leaders in this shining new technology, leading the way to acleaner, more equitable twenty-first century, while those that ignore or suppressphotovoltaics will be left behind in the green, ec<strong>on</strong>omic energy revoluti<strong>on</strong>.49
2 Materials, experimental setup and methods.2.1 <strong>Copper</strong> (I) oxide (Cu 2 O):<strong>Copper</strong> oxide or cuprous oxide (Cu 2 O) or Cuprite:It is an oxide <str<strong>on</strong>g>of</str<strong>on</strong>g> copper. It is insoluble in water and organic solvents. <strong>Copper</strong>oxide is found as the mineral cuprite in some red-colored rocks. When it isexposed to oxygen, copper will naturally oxidize to copper (I) oxide, albeit at avery slow rate. With further heating, copper (I) oxide will form copper (II) oxide.Molar mass 143.09 g/molAppearance brownish-red solidDensity 6.0 g/cm 3Melting point 1235 °C, 1508 KBoiling point 1800 °C, 2073 KSolubility in water: Insoluble in water and soluble in acidThe cuprous oxide (Cu 2 O) electr<strong>on</strong>ic and transport properties presented in[43,44].Cu 2 O is usually p-type. As grown material has high resistivity (ρ > 10 6 Ωcm).Resistivity depends str<strong>on</strong>gly <strong>on</strong> annealing or photoexcitati<strong>on</strong> [43]. The resistivity(lowest bulk resistivity) are 42 Ω cm at T = 300 K for sputtered film [45], 35 Ωcm at T = 300 K undoped [46] and 9 Ω cm T = 300 K Cd-doped.Thermal activati<strong>on</strong> energy <str<strong>on</strong>g>of</str<strong>on</strong>g> the c<strong>on</strong>ductivity E a 0.20-0.38 eV at T = 298 Kfor oxygen saturated sample [47], 0.48-0.70 eV at T = 298 K sample annealed at1050°C [47] and 0.49-.0.70 eV at T = 298 K sample annealed at 1050°C and750°C [47], 0.01 eV at T < 200 K sputtered film [45], 0.18 eV at T > 200 Ksputtered film [45].Minority carrier lifetime τ is 10 –8 -10 –9 s at T = 300 K [48].Photoexcited carrier lifetime is 10 –8 s at T = 4 K for cyclotr<strong>on</strong> res<strong>on</strong>ance [49].Activati<strong>on</strong> energies <str<strong>on</strong>g>of</str<strong>on</strong>g> electr<strong>on</strong>ic c<strong>on</strong>ductivity in Cu 2 O have been ascribed t<strong>on</strong>umerous deep levels (reviews in [43,44]). Optical absorpti<strong>on</strong> [50] or emissi<strong>on</strong> inthe near infrared [51,52,53] shows the existence <str<strong>on</strong>g>of</str<strong>on</strong>g> three maxima <str<strong>on</strong>g>of</str<strong>on</strong>g> the density <str<strong>on</strong>g>of</str<strong>on</strong>g>states, the first near 1.7 eV, at 1.4 eV and 0.8 eV; sometimes a weaker peak at 1.5eV. The highest energy corresp<strong>on</strong>ds well to the deep acceptor level near 0.4 eV.50
Photoc<strong>on</strong>ductivity [54,55]. Photovoltaic effect [46]. Photoelectric effect [56],"Photomemory" (persistent photoc<strong>on</strong>ductivity) has been explained using twolevels at E c = – 1.1 eV (A trap) and E c =–1.0 eV (B trap) [57,58]. Temperaturedependence <str<strong>on</strong>g>of</str<strong>on</strong>g> the electrical c<strong>on</strong>ductivity and switching phenomena in Cu 2 Osingle crystal are reported from the ambient to 200 °C [47].Mobility is µ= 70 cm 2 V –1 s –1 at T = 298 K in oxygen saturated sample [47].The structure and crystallographic properties are following [59,60]:Name and formulaMineral name: Cuprite, synEmpirical formula: Cu 2 OCrystallographic parametersCrystal system: Cubica (Å): 4.2696b (Å): 4.2696c (Å): 4.2696Most important Peak (see Figure 2.1)No. h k l d [A] 2Theta[deg] I [%]2 1 1 1 2.46500 36.419 100.03 2 0 0 2.13500 42.298 37.051
Figure 2.1 <strong>Copper</strong> (I) oxide XRD Stick PatternCuprous oxide (Cu 2 O) is a well-known p-type semic<strong>on</strong>ductor with a directband gap <str<strong>on</strong>g>of</str<strong>on</strong>g> 2.0 eV. Cu 2 O thin films have high optical transmittance atwavelengths above 600 nm with a slightly yellowish appearance and a highoptical absorpti<strong>on</strong> coefficient at the wavelengths below. Cu 2 O in thin-film form isan attractive material for many device applicati<strong>on</strong>s because <str<strong>on</strong>g>of</str<strong>on</strong>g> its p-type semic<strong>on</strong>ductivity,high absorpti<strong>on</strong> coefficient, abundant availability, n<strong>on</strong>-toxic nature,and low producti<strong>on</strong> cost. Cu 2 O thin films find diverse applicati<strong>on</strong>s in oxygen andhumidity sensors, electro chromic devices [61], and photovoltaic devices such asthin-film hetero-juncti<strong>on</strong> solar cells [62–66]. Cu 2 O thin films can be prepared <strong>by</strong>various techniques such as activated reactive evaporati<strong>on</strong> [67], thermal oxidati<strong>on</strong>[68], vacuum evaporati<strong>on</strong> [69], chemical vapour depositi<strong>on</strong> [70], sol–gel process[71,72], electro-depositi<strong>on</strong> [73], and sputtering [74-79]. Am<strong>on</strong>g these depositi<strong>on</strong>techniques, direct-current (dc) reactive magnetr<strong>on</strong> sputtering is <strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g> the besttechniques for preparati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu 2 O thin films due to the advantages <str<strong>on</strong>g>of</str<strong>on</strong>g> highdepositi<strong>on</strong> rates, excellent uniformity over large area substrates, and goodc<strong>on</strong>trollability <strong>on</strong> the chemical compositi<strong>on</strong> and the physical properties <str<strong>on</strong>g>of</str<strong>on</strong>g> sputter-52
deposited thin films. The physical properties <str<strong>on</strong>g>of</str<strong>on</strong>g> the sputter deposited thin films arehighly influenced <strong>by</strong> the depositi<strong>on</strong> parameters such as oxygen partial pressure,oxygen flow rate, substrate temperature, substrate bias voltage, sputteringpressure, and sputtering power[80-83].2.2 <strong>Copper</strong> (II) oxide (CuO)<strong>Copper</strong> (II) oxide or cupric oxide (CuO) is the higher oxide <str<strong>on</strong>g>of</str<strong>on</strong>g> copper. As amineral, it is known as tenorite. It is a black solid with an i<strong>on</strong>ic structure, whichmelts above 1200 °C with some loss <str<strong>on</strong>g>of</str<strong>on</strong>g> oxygen. It can be formed <strong>by</strong> heatingcopper in air:2 Cu + O2 → 2 CuOEmpirical formula is CuO and chemical formula is CuO.Crystallographic parametersCrystal system:M<strong>on</strong>oclinica (Å): 4.6691b (Å): 3.4805c (Å): 5.1183Most important XRD Peak (Figure 2.2)No. h k l d [A] 2Theta[deg] I [%]2 -1 1 1 2.53415 35.392 100.03 1 1 1 2.34848 38.295 80.753
Figure 2.2 CuO XRD Stick Pattern<strong>Copper</strong> (II) oxide has applicati<strong>on</strong> as a p-and n-type semic<strong>on</strong>ductor, because ithas a narrow band gap <str<strong>on</strong>g>of</str<strong>on</strong>g> 1.2 eV. It is an abrasive used to polish opticalequipment. Cupric oxide can be used to produce dry cell batteries. It has also beenused in wet cell batteries as the cathode, with lithium as an anode, and dioxalanemixed with lithium perchlorate as the electrolyte. The film <str<strong>on</strong>g>of</str<strong>on</strong>g> cupric oxide can beobtained and deposited <strong>by</strong> reactive sputtering depositi<strong>on</strong>. Properties <str<strong>on</strong>g>of</str<strong>on</strong>g> the filmstr<strong>on</strong>gly depend <strong>on</strong> sputtering c<strong>on</strong>diti<strong>on</strong>.[69,80,81,85]54
2.3 Titanium <strong>Oxide</strong>Titanium dioxide occurs in nature as well-known minerals rutile, anatase andbrookite. Rutile and anatase have tetrag<strong>on</strong>al Crystal system and brookiteorthorhombic. The main properties is presented in Table 2.1Molecular formula TiO 2Molar mass 79.866 g/molAppearance White solid Density 4.23 g/cm 3Melting point 1843 °CBoiling point 2972 °CSolubility in waterRefractive index (n D )7.4x10 5 M2.488 (anatase)2.609 (rutile)2.583 (brooklite)Molecular formula TiO 2Molar mass 79.866 g/molAppearance White solid Density 4.23 g/cm 32.3.1 The main structure and crystallographic properties <str<strong>on</strong>g>of</str<strong>on</strong>g> Rutile [86]:Crystallographic parametersCrystal system:Tetrag<strong>on</strong>ala (Å): 4.6570b (Å): 4.6570c (Å): 3.0093Most important XRD Peak (see Figure 2.3)No. h k l d [A] 2Theta[deg] I [%]1 1 1 0 3.29300 27.056 100.055
Figure 2.3 Rutile XRD Stick Pattern2.3.2 Anatase [87]Name and formula:Empirical formula: O 2Ti 0.72Chemical formula: Ti 0.72O 2Crystallographic parametersCrystal system:Space group:Tetrag<strong>on</strong>alI41/amdSpace group number: 141a (Å): 3.7830b (Å): 3.7830c (Å): 9.497056
Most important XRD Peak (see Figure 2.4):No. h k l d [A] 2Theta[deg] I [%]1 1 0 1 3.51444 25.322 100.0Figure 2.4 Anatase XRD Stick Pattern2.3.3 Brookite [88,89]:Crystallographic parametersCrystal system:Orthorhombica (Å): 9.1660b (Å): 5.4360c (Å): 5.135057
Brookite XRD Peak list(see Figure 2.5):No. h k l d [A] 2Theta[deg] I [%]2 1 1 0 3.22000 27.681 100.0Figure 2.5 Brookite XRD Stick Pattern2.3.4 Titanium oxide thin film applicati<strong>on</strong>, doping and depositi<strong>on</strong> methodsTitanium oxide TiO 2 thin films are widely used in various fields such asoptical and protective coatings and optical fibbers because <str<strong>on</strong>g>of</str<strong>on</strong>g> their excellentchemical stability, mechanical hardness and optical transmittance with highrefractive index. Recently, TiO 2 thin films have become the most promisingmaterials in envir<strong>on</strong>mental cleaning such as photocatalytic purifier [90,91] andphotochemical solar cells [92,93].TiO 2 is a large band gap semic<strong>on</strong>ductor, which has gained a lot <str<strong>on</strong>g>of</str<strong>on</strong>g> importanceas catalyst for breakdown <str<strong>on</strong>g>of</str<strong>on</strong>g> organic substances under UV illuminati<strong>on</strong>. It is achemically and biologically inert material with good photostability [94,95]. Asmenti<strong>on</strong>ed before the three main crystal phases <str<strong>on</strong>g>of</str<strong>on</strong>g> TiO 2 , rutile is thethermodynamically stable phase, but anatase and brookite have a high kinetic58
stability. In thin film processing usually, the anatase phase is formed attemperatures below ca. 873 K [94]. The anatase (bandgap=3.2 eV) is the moreactive phase in photocatalysis compared to rutile (bandgap=3.0 eV) [94]. On theother hand, it has been argued that a mixture <str<strong>on</strong>g>of</str<strong>on</strong>g> anatase and rutile may show thehighest photocatalytic activity [96,97]. About the brookite phase very little isknown. Therefore, most studies discuss pure anatase or rutile thin films.Since the doping <str<strong>on</strong>g>of</str<strong>on</strong>g> nitrogen [100] into titanium dioxide was discovered toinduce photocatalytic activity under visible light irradiati<strong>on</strong> [98,99], the synthesisand characterizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> nitrogen-doped titanium dioxide (N-dopedTiO 2 ) haveattracted extensive interest with a visi<strong>on</strong> to efficiently utilize the solar energy.A number <str<strong>on</strong>g>of</str<strong>on</strong>g> techniques, such as CVD [101], sol–gel [102,103], plasmasource i<strong>on</strong> implantati<strong>on</strong> [104], pulsed laser depositi<strong>on</strong> [105], and magnetr<strong>on</strong>sputtering [106-108] have been used to deposit TiO 2 thin films <strong>on</strong>to several kinds<str<strong>on</strong>g>of</str<strong>on</strong>g> supporters. Magnetr<strong>on</strong> sputtering <str<strong>on</strong>g>of</str<strong>on</strong>g> TiO 2 films has aroused special interest,because it is an industrial process applicable to large-area depositi<strong>on</strong>, and highquality TiO 2 films can be achieved even at low substrate temperatures. Onlyanatase and rutile have been observed in the thin films. In additi<strong>on</strong>, amorphousTiO2 films are <str<strong>on</strong>g>of</str<strong>on</strong>g>ten observed if the substrate temperature is low duringdepositi<strong>on</strong> [109]. These three different phases as well as the grain size, crystallinestructure, morphology and orientati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the TiO 2 layers can be c<strong>on</strong>trolled <strong>by</strong> theprocess parameters such as sputtering gases, O 2 partial pressure, substratetemperature, annealing process and some special sputtering techniques [110-112].The properties <str<strong>on</strong>g>of</str<strong>on</strong>g> the titanium oxide film, prepared <strong>by</strong> reactive magnetr<strong>on</strong>sputtering, depend not <strong>on</strong>ly <strong>on</strong> the preparati<strong>on</strong> technique but also <strong>on</strong> the sputteringc<strong>on</strong>diti<strong>on</strong>s, such as discharge pressure [113], reactive gas partial pressure [114]and DC discharge current [115].59
2.4 <strong>Copper</strong> oxide and titanium oxide combinati<strong>on</strong> properties andapplicati<strong>on</strong>Cu 2 O is an attractive material for photovoltatic energy c<strong>on</strong>versi<strong>on</strong>. The bandgap is 2.0 eV, which is in the acceptable range for photovoltaic materials. Thetheoretical efficiency <str<strong>on</strong>g>of</str<strong>on</strong>g> a Cu 2 0 solar cell is about 20% under AM 1 solarilluminati<strong>on</strong>. Furthermore, Cu 2 0 is a candidate material for low-cost solar cells,since copper is very abundant, and since large-grain Cu 2 0 can be grown <strong>by</strong>oxidizing copper. Although Cu 2 0 rectifiers were studied from 1920 to 1940, verylittle effort has been devoted to actual solar-cell development. The best cellefficiencies obtained have been <strong>on</strong> the order <str<strong>on</strong>g>of</str<strong>on</strong>g> 1 %.[116] The primary cause forthis apparent "efficiency plateau" is that barrier heights <str<strong>on</strong>g>of</str<strong>on</strong>g> Schottky barrier solarcells are in the range 0.7-0.9 eV, regardless <str<strong>on</strong>g>of</str<strong>on</strong>g> the choice <str<strong>on</strong>g>of</str<strong>on</strong>g> metal. The mostprobable explanati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the barrier-height results is that most metals reduce Cu 2 0to form a copper-rich regi<strong>on</strong>, or that a large density <str<strong>on</strong>g>of</str<strong>on</strong>g> states exists at the Cu 2 0surface, which dictates the barrier-height magnitude.Surface analyses combined with barrier-height studies indicate that Cu 2 0Schottky barriers made with low-work- functi<strong>on</strong> metals are essentially Cu/Cu 2 0cells due to reducti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the Cu 2 0 surface and subsequent interdiffusi<strong>on</strong>phenomena. The copper-rich regi<strong>on</strong> essentially determines the barrier height. It isclear that to fabricate efficient Cu 2 0 solar cells; <strong>on</strong>e must either utilize a MISstructure or a heterojuncti<strong>on</strong> [116]. The interfacial layer in a MIS structure couldserve two purposes: it could inhibit reducti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the Cu 2 0 surface, and it wouldhopefully block out majority-carrier flow. The n-type partner in a heterojuncti<strong>on</strong>must not react with Cu 2 0. If such a material is determined, then very efficientsolar cells are possible.Heterojuncti<strong>on</strong> solar cell with 2% efficiency <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> a Cu 2 O substrate wasobtained in [117] made <strong>by</strong> depositi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> transparent c<strong>on</strong>ducting oxide (TCO)films <strong>on</strong> Cu 2 O substrates. TCO usually is <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> ZnO, In 2 O3, SnO 2 , and CdO.On the other hand for the reas<strong>on</strong>s menti<strong>on</strong>ed before [116] is a TiO 2 goodcandidate to n-type partner in heterojuncti<strong>on</strong> with Cu 2 O. Cu 2 O thin films60
deposited <strong>on</strong> a TiO 2 substrate typically form a p–n heterojuncti<strong>on</strong>. This systemhas been widely investigated for photocatalysis and solar cell applicati<strong>on</strong>s [118-120]. The band gap energies <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu 2 O and TiO 2 are 2.0 and 3.2 eV [121-123],respectively (see Figure 2.6). Both the c<strong>on</strong>ducti<strong>on</strong> band minimum and the valenceband maximum <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu 2 O lie above those <str<strong>on</strong>g>of</str<strong>on</strong>g> TiO 2 ; therefore, the electr<strong>on</strong>s excitedto the c<strong>on</strong>ducti<strong>on</strong> band <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu 2 O would transfer to TiO 2 , whereas the holesgenerated in the valence band in TiO 2 prefer opposite transfer to Cu 2 O. Chargecarriers separated in different semic<strong>on</strong>ductors effectively reduce the chance <str<strong>on</strong>g>of</str<strong>on</strong>g>electr<strong>on</strong>-hole pair recombinati<strong>on</strong> also prol<strong>on</strong>g their lifetime, thus increasing thequantum efficiencies. In additi<strong>on</strong>, working range <str<strong>on</strong>g>of</str<strong>on</strong>g> the wavelength is extended toa visible regi<strong>on</strong> due to absorpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> visible light <strong>by</strong> Cu 2 O, further enhancing theefficiency <str<strong>on</strong>g>of</str<strong>on</strong>g> the solar energy transiti<strong>on</strong>. Synergistic cooperati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> these effectsenables the Cu 2 O/TiO 2 system exhibit great potential for solar cell andphotocatalysis applicati<strong>on</strong>s [124-127]. Transport properties <str<strong>on</strong>g>of</str<strong>on</strong>g> the heterojuncti<strong>on</strong>sare, however, largely influenced <strong>by</strong> band disc<strong>on</strong>tinuities and interface states [128-129]. Band disc<strong>on</strong>tinuities at the interface are usually determined <strong>by</strong> the barrierheights for charge carriers to be overcome at the interface; therefore, theirmagnitudes dramatically influence the performance <str<strong>on</strong>g>of</str<strong>on</strong>g> the heterojuncti<strong>on</strong>s. Energy<str<strong>on</strong>g>of</str<strong>on</strong>g> the photogenerated electr<strong>on</strong>s will be also lost during the transfer process if theband disc<strong>on</strong>tinuities are large. In additi<strong>on</strong>, the interface states act as therecombinati<strong>on</strong> centers or carriers traps. The band structure at Cu2O/TiO2heterojuncti<strong>on</strong>s interface is str<strong>on</strong>gly depending <strong>on</strong> c<strong>on</strong>diti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> film depositi<strong>on</strong>, inparticular for the sputtering depositi<strong>on</strong> [130].61
Figure 2.6 Energetic diagrams <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu 2 O/TiO 2 [89]62
2.5 Methods and setups descripti<strong>on</strong>2.5.1 The reactive DC-Magnetr<strong>on</strong> Sputtering ProcessIn presented work thin film depositi<strong>on</strong> have been performed <strong>by</strong> dc-magnetr<strong>on</strong>sputter process. This process <str<strong>on</strong>g>of</str<strong>on</strong>g>fers the advantage <str<strong>on</strong>g>of</str<strong>on</strong>g> a homogeneous areadepositi<strong>on</strong>. For the required properties <str<strong>on</strong>g>of</str<strong>on</strong>g> the film depositi<strong>on</strong> parameters havebeen optimized. Their influence <strong>on</strong> the resulting layers is studied.2.5.1.1 The Principle <str<strong>on</strong>g>of</str<strong>on</strong>g> sputteringWithin the sputtering process, gas i<strong>on</strong>s out <str<strong>on</strong>g>of</str<strong>on</strong>g> plasma are accelerated towards atarget c<strong>on</strong>sisting <str<strong>on</strong>g>of</str<strong>on</strong>g> the material to be deposited. Material is detached (‘sputtered‘)from the target and afterwards deposited <strong>on</strong> a substrate in the vicinity. The processis realized in a closed recipient, which is pumped down to a vacuum base pressurebefore depositi<strong>on</strong> starts (see Figure 2.8). To enable the igniti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> plasma usuallyarg<strong>on</strong> is feed into the chamber up to a pressure between 10 -5 -10 -6 mbar. By naturalcosmic radiati<strong>on</strong>, there are always some i<strong>on</strong>ized Ar+-i<strong>on</strong>s available. In the dcsputteringa negative potential U up to some hundred Volts is applied to the target.As a result, the Ar-i<strong>on</strong>s are accelerated towards the target and set material free(see Figure 2.7), <strong>on</strong> the other hand they produce sec<strong>on</strong>dary electr<strong>on</strong>s. Theseelectr<strong>on</strong>s cause a further i<strong>on</strong>izati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the gas.Figure 2.7 Sputtering mechanism: a) incident i<strong>on</strong>s; b) i<strong>on</strong>s collide targetand erode it[131]63
The gas pressure p and the electrode distance d determine a break-throughvoltage U D —from which <strong>on</strong> a self sustaining glow discharge starts — followingthe equati<strong>on</strong> U D = A pd/(ln(pd) + B) with materials c<strong>on</strong>stants A and B.Graphically spoken the i<strong>on</strong>izati<strong>on</strong> probability rises with an increase in pressureand hence the number <str<strong>on</strong>g>of</str<strong>on</strong>g> i<strong>on</strong>s and the c<strong>on</strong>ductivity <str<strong>on</strong>g>of</str<strong>on</strong>g> the gas also increase. Thebreak through voltage drops. For a sufficient i<strong>on</strong>izati<strong>on</strong> rate a stable burningplasma results, where from a sufficient amount <str<strong>on</strong>g>of</str<strong>on</strong>g> i<strong>on</strong>s is available for sputtering<str<strong>on</strong>g>of</str<strong>on</strong>g> the material.Figure 2.8 The principle <str<strong>on</strong>g>of</str<strong>on</strong>g> the sputtering processTo increase the i<strong>on</strong>izati<strong>on</strong> rate <strong>by</strong> emitted sec<strong>on</strong>dary electr<strong>on</strong>s even further, aring magnet below the target is used in the magnetr<strong>on</strong> sputtering. The electr<strong>on</strong>s inits field are trapped in cycloids and circulate over the targets surface (see Figure2.9).64
Figure 2.9 Magnetr<strong>on</strong> sputtering system [131]By the l<strong>on</strong>ger dwell time in the gas, they cause a higher i<strong>on</strong>izati<strong>on</strong> probabilityand hence form a plasma igniti<strong>on</strong> at pressures, which can be up to <strong>on</strong>e hundredtimes smaller than for c<strong>on</strong>venti<strong>on</strong>al sputtering. On the <strong>on</strong>e hand, higher depositi<strong>on</strong>rates can be realized there<strong>by</strong>. On the other hand less collisi<strong>on</strong>s occur for thesputtered material <strong>on</strong> the way to the substrate because <str<strong>on</strong>g>of</str<strong>on</strong>g> the lower pressure andhence the kinetic energy at the impact <strong>on</strong> the substrate is higher (see also below).The electr<strong>on</strong> density and hence the number <str<strong>on</strong>g>of</str<strong>on</strong>g> generated i<strong>on</strong>s is highest, where theΒ-field is parallel to the substrate surface. The highest sputter yield happens <strong>on</strong>the target area right below this regi<strong>on</strong>. An erosi<strong>on</strong> z<strong>on</strong>e is formed which followsthe form <str<strong>on</strong>g>of</str<strong>on</strong>g> the magnetic field.The bombardment <str<strong>on</strong>g>of</str<strong>on</strong>g> a n<strong>on</strong>-c<strong>on</strong>ducting target with positive i<strong>on</strong>s would lead toa charging <str<strong>on</strong>g>of</str<strong>on</strong>g> the surface and subsequently to a shielding <str<strong>on</strong>g>of</str<strong>on</strong>g> the electrical field.The i<strong>on</strong> current would die <str<strong>on</strong>g>of</str<strong>on</strong>g>f. Therefore, the dc-sputtering is restricted toc<strong>on</strong>ducting materials like metals or doped semic<strong>on</strong>ductors. In our setup were usedAu, Cu and Ti target. The reactive sputtering in ambient <str<strong>on</strong>g>of</str<strong>on</strong>g> other gases likeoxygen or nitrogen are fed into the sputter chamber additi<strong>on</strong>ally to the arg<strong>on</strong>, toproduce oxidic or nitridic films.The reactive gas chemically reacts with deposited film <strong>on</strong> substratecreating desired material properties. However, reactive gas also reacts with targetand erodes it. This phenomen<strong>on</strong> is called as ―target pois<strong>on</strong>ing‖ and decriessputtering rate, so is appear difficulties to c<strong>on</strong>trol sputtering process. When65
eactive gas is introduced, <strong>on</strong>e part <str<strong>on</strong>g>of</str<strong>on</strong>g> the reactive gas is pumped other part reactwith sputtered material and create desired compositi<strong>on</strong>. For this reas<strong>on</strong>, reactivesputtering rate is low than pure metal sputtering.Descripti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> reactive sputtering process in arg<strong>on</strong> and oxygen gasesmixing (see Figure 2.10):q t : reactive gas flux c<strong>on</strong>summated <strong>on</strong> target surfaceq c : reactive gas flux c<strong>on</strong>summated <strong>on</strong> surfaces where target‘smaterial is sputtered;q p : pumped reactive gas flux and used in any process.Figure 2.10 Fluxes <str<strong>on</strong>g>of</str<strong>on</strong>g> gas during reactive sputtering2.5.1.2 The Sputter ParametersThe resulting film properties can be c<strong>on</strong>trolled <strong>by</strong> adjusting the followingsputter parameters:The sputter current I sp determines mainly the rate <str<strong>on</strong>g>of</str<strong>on</strong>g> the depositi<strong>on</strong> processand hence the time which remains for the arriving particles during the growthprocess for either surface diffusi<strong>on</strong> and agglomerati<strong>on</strong> <strong>on</strong> existing growth centersor nucleati<strong>on</strong> with other adatoms. The applied voltage determines the maximum66
energy, with which sputtered particles can escape from the target (reduced <strong>by</strong> thebinding energy). Energies <str<strong>on</strong>g>of</str<strong>on</strong>g> the sputtered particles show a broad distributi<strong>on</strong> witha maximum <str<strong>on</strong>g>of</str<strong>on</strong>g> the distributi<strong>on</strong> between 1 eV and 10 eV. The applied voltagedetermines also the sputter yield, which is the number <str<strong>on</strong>g>of</str<strong>on</strong>g> sputtered particles perincoming i<strong>on</strong>. The pressure p in the sputter chamber determines the mean freepath λ for the sputtered material, which is proporti<strong>on</strong>al to 1/p. Together with thetarget substrate distance (TS) the pressure c<strong>on</strong>trols, how many collisi<strong>on</strong>s occur forthe particles <strong>on</strong> their way from the target to the substrate. This can influence theporosity <str<strong>on</strong>g>of</str<strong>on</strong>g> the films. In additi<strong>on</strong>, the crystallinity, adhesi<strong>on</strong> and texture can beaffected. Via the gas mixture <strong>on</strong>e can c<strong>on</strong>trol the stoichiometry <str<strong>on</strong>g>of</str<strong>on</strong>g> films, which aresputtered from a metallic target. The oxygen flow q(O 2 ) is the parameter varied,whereas the desired total pressure is kept c<strong>on</strong>stant <strong>by</strong> regulati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the Ar-flowq(Ar).The substrate temperature can have a str<strong>on</strong>g impact <strong>on</strong> the growth behaviourwith respect to crystallinity or density <str<strong>on</strong>g>of</str<strong>on</strong>g> the samples. It can be adjusted betweenroom temperature and 700ºC. Nevertheless, even during sputtering withoutexternal heating the substrate temperature may rise c<strong>on</strong>siderably, especiallyduring l<strong>on</strong>g sputtering times.2.5.1.3 The Sputter SystemSsubstrate and target surface are parallel to each other and have a distance 13cm. Magnetr<strong>on</strong> with the target <str<strong>on</strong>g>of</str<strong>on</strong>g> 2 inch diameter were used(see Figure 2.11).67
Figure 2.11 Photo <str<strong>on</strong>g>of</str<strong>on</strong>g> magnetr<strong>on</strong> with Au target.It is b<strong>on</strong>ded to a water cooling system. Electrical potential was applied <strong>on</strong>target and vacuum chamber (see Figure 2.12) was grounded.Figure 2.12 Photo <str<strong>on</strong>g>of</str<strong>on</strong>g> vacuum chamber.68
The sample holder was heated be electrical furnace. Temperature wasc<strong>on</strong>trolled <strong>by</strong> thermo-couple. Vacuum chamber were pumped <strong>by</strong> rotary and turbopump. The pressure was c<strong>on</strong>trolled <strong>by</strong> full-range and capacitive vacuum detectors.The gases (Ar, O 2 and N 2 ) flows were c<strong>on</strong>trolled and operated <strong>by</strong> fluxmeter. Fullsetup photo is presented in Figure 2.13.Figure 2.13 Photo <str<strong>on</strong>g>of</str<strong>on</strong>g> sputtering system setup.2.5.2 <str<strong>on</strong>g>Thin</str<strong>on</strong>g> films characterizati<strong>on</strong>2.5.2.1 <str<strong>on</strong>g>Thin</str<strong>on</strong>g> film thickness measurementsIn order to verify the thickness <str<strong>on</strong>g>of</str<strong>on</strong>g> deposited films it has been usedpr<str<strong>on</strong>g>of</str<strong>on</strong>g>ilometer Veeco model Dektat 8 (see Figure 2.14). The instrument isc<strong>on</strong>stituted from a tip, which slides <strong>on</strong> a sample to measure pressing <strong>on</strong> it with ac<strong>on</strong>stant force. The roughness <str<strong>on</strong>g>of</str<strong>on</strong>g> the surface is reflected in a vertical movement <str<strong>on</strong>g>of</str<strong>on</strong>g>the tip and the cantilever in which it is fixed. The cantilever it is c<strong>on</strong>nected to ac<strong>on</strong>denser, and the vertical movement <str<strong>on</strong>g>of</str<strong>on</strong>g> the tip is translate in a variati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the69
ability to this c<strong>on</strong>denser, than the system it c<strong>on</strong>verts in a pr<str<strong>on</strong>g>of</str<strong>on</strong>g>ile visualized <strong>on</strong> thescreen <str<strong>on</strong>g>of</str<strong>on</strong>g> the PC (to which the pr<str<strong>on</strong>g>of</str<strong>on</strong>g>ilometer is c<strong>on</strong>nected).The sample to measure must be flat and sufficiently hard. Moreover he musthave a z<strong>on</strong>e not deposited in such way from being able to measure the difference<str<strong>on</strong>g>of</str<strong>on</strong>g> height between this and the z<strong>on</strong>e film covered. The scanning comes carried outfrom the deposited area to in order to prevent that an error in the selecti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> thelength <str<strong>on</strong>g>of</str<strong>on</strong>g> scanning.Figure 2.14 Photos <str<strong>on</strong>g>of</str<strong>on</strong>g> Dektat 8 Veeco pr<str<strong>on</strong>g>of</str<strong>on</strong>g>ilometerThe measure <strong>on</strong> the samples carried out executing 6 various scanning <str<strong>on</strong>g>of</str<strong>on</strong>g> 2millimetres length, <strong>on</strong> 3 various points <str<strong>on</strong>g>of</str<strong>on</strong>g> the film edge. For every scanning thethickness <str<strong>on</strong>g>of</str<strong>on</strong>g> the film was calculated medium value from measurements <str<strong>on</strong>g>of</str<strong>on</strong>g>deposited films. On the 6 values are averaged in order to obtain the mediumthickness, <str<strong>on</strong>g>of</str<strong>on</strong>g> the film.2.5.2.2 X-ray diffracti<strong>on</strong> microstructure analysis.One <str<strong>on</strong>g>of</str<strong>on</strong>g> the phenomena <str<strong>on</strong>g>of</str<strong>on</strong>g> interacti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> X-rays with crystalline matter is itsdiffracti<strong>on</strong>, produced <strong>by</strong> the reticular planes that form the atoms <str<strong>on</strong>g>of</str<strong>on</strong>g> the crystal. Acrystal diffracts an X-ray beam passing through it to produce beams at specific70
angles depending <strong>on</strong> the X-ray wavelength, the crystal orientati<strong>on</strong> and thestructure <str<strong>on</strong>g>of</str<strong>on</strong>g> the crystal.In the macroscopic versi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> X-ray diffracti<strong>on</strong>, a certain wavelength <str<strong>on</strong>g>of</str<strong>on</strong>g>radiati<strong>on</strong> will c<strong>on</strong>structively interfere when partially reflected between surfaces(i.e., the atomic planes) that produce a path difference equal to an integral number<str<strong>on</strong>g>of</str<strong>on</strong>g> wavelengths. This c<strong>on</strong>diti<strong>on</strong> is described <strong>by</strong> the Bragg law:2 d(hkl )sinnIn additi<strong>on</strong>, reticular parameter is defined as2 2 2a h k l dhklwhere n is an integer, λ is the wavelength <str<strong>on</strong>g>of</str<strong>on</strong>g> the radiati<strong>on</strong>, d is the spacingbetween surfaces and θ is the angle between the radiati<strong>on</strong> and the surfaces. Thisrelati<strong>on</strong> dem<strong>on</strong>strates that interference effects are observable <strong>on</strong>ly when radiati<strong>on</strong>interacts with physical dimensi<strong>on</strong>s that are approximately the same size as thewavelength <str<strong>on</strong>g>of</str<strong>on</strong>g> the radiati<strong>on</strong>. Since the distances between atoms or i<strong>on</strong>s are about10 -10 m (1Å), diffracti<strong>on</strong> methods require radiati<strong>on</strong> in the X-ray regi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> theelectromagnetic spectrum, or beams <str<strong>on</strong>g>of</str<strong>on</strong>g> electr<strong>on</strong>s or neutr<strong>on</strong>s with similarwavelength. So, through X-ray spectra <strong>on</strong>e can identify and analyse anycrystalline matter. The degree <str<strong>on</strong>g>of</str<strong>on</strong>g> crystallinity or order will c<strong>on</strong>diti<strong>on</strong>ate the quality<str<strong>on</strong>g>of</str<strong>on</strong>g> the obtained result. In this work Bragg Philps Xpert-Pro diffractometer is used(see Figure 2.15). As X-ray beam source Cu target X- ray tube used. To identifyand analyse deposited film the 2θ XRD spectra were measured [132].Diffractometer was set ―thin film c<strong>on</strong>diti<strong>on</strong>‖ measurement.71
Figure 2.15 Photo <str<strong>on</strong>g>of</str<strong>on</strong>g> Philips X’Pert Pro diffractometer used in experiment.Scherrer showed that the average dimensi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the crystals that compose acrystalline powder is related with the pr<str<strong>on</strong>g>of</str<strong>on</strong>g>ile <str<strong>on</strong>g>of</str<strong>on</strong>g> the peak <strong>by</strong> means <str<strong>on</strong>g>of</str<strong>on</strong>g> the equati<strong>on</strong>:DKcosWhere K is proporti<strong>on</strong>ality c<strong>on</strong>stant approximately similar to the unit, β theFWHM <str<strong>on</strong>g>of</str<strong>on</strong>g> the peak in radians (theoretically corrected from the instrumentalbroadening), D the size <str<strong>on</strong>g>of</str<strong>on</strong>g> the crystal in the directi<strong>on</strong> perpendicular to thereflecting planes and λ the wavelength <str<strong>on</strong>g>of</str<strong>on</strong>g> the X-rays used. This is the most usualand simple equati<strong>on</strong> that allows to evaluate grain size.2.5.2.3 Four-Point Probe method [133]In this work four-point method were used. The purpose <str<strong>on</strong>g>of</str<strong>on</strong>g> the 4-point probe isto measure the resistivity <str<strong>on</strong>g>of</str<strong>on</strong>g> any semic<strong>on</strong>ductor material. It can measure eitherbulk or thin film specimen, each <str<strong>on</strong>g>of</str<strong>on</strong>g> which c<strong>on</strong>sists <str<strong>on</strong>g>of</str<strong>on</strong>g> a different expressi<strong>on</strong>.72
The 4-point probe setup used c<strong>on</strong>sists <str<strong>on</strong>g>of</str<strong>on</strong>g> four equally spaced tungsten metaltips with finite radius. Each tip is supported <strong>by</strong> springs <strong>on</strong> the other end tominimize sample damage during probing. The four metal tips are part <str<strong>on</strong>g>of</str<strong>on</strong>g> an automechanicalstage, which travels up and down during measurements. A highimpedance current source is used to supply current through the outer two probes;a voltmeter measures the voltage across the inner two probes (see Figure 2.16) todetermine the sample resistivity. Typical probe spacing is ~ 1 mm.In a sheet resistance measurement, several resistances need to be c<strong>on</strong>sidered,as shown in Figure 1 (a). The probe has a probe resistance R p . It can bedetermined <strong>by</strong> shorting two probes and measuring their resistances. At theinterface between the probe tip and the semic<strong>on</strong>ductor, there is a probe c<strong>on</strong>tactresistance, R cp . When the current flows from the small tip into the semic<strong>on</strong>ductorand spreads out in the semic<strong>on</strong>ductor, there will be a spreading resistance, R sp .Finally, the semic<strong>on</strong>ductor itself has a sheet resistance R s .The equivalent circuit for the measurement <str<strong>on</strong>g>of</str<strong>on</strong>g> semic<strong>on</strong>ductor sheet resistance<strong>by</strong> using the four-point probe is shown in Figure. 2.16(c). Two probes carry thecurrent and the other two probes sense the voltage. Each probe has a proberesistance R p , a probe c<strong>on</strong>tact resistance R cp and a spreading resistance R spassociated with it. However, these parasitic resistances can be neglected for thetwo voltage probes because the voltage is measured with a high impedancevoltmeter, which draws very little current. Thus, the voltage drops across theseparasitic resistances are insignificantly small. The voltage reading from thevoltmeter is approximately equal to the voltage drop across the semic<strong>on</strong>ductorsheet resistance.73
Figure 2.16: Four-point probe measurement <str<strong>on</strong>g>of</str<strong>on</strong>g> semic<strong>on</strong>ductor sheetresistanceBy using the four-point probe method, the semic<strong>on</strong>ductor sheet resistance canbe calculated:R s =F(V/I),where V is the voltage reading from the voltmeter, I is the current carried <strong>by</strong>the two current-carrying probes, and F is a correcti<strong>on</strong> factor. For collinear or inlineprobes with equal probe spacing, the correcti<strong>on</strong> factor F can be written as aproduct <str<strong>on</strong>g>of</str<strong>on</strong>g> three separate correcti<strong>on</strong> factors:F=F 1 F 2 F 3F 1 corrects for finite sample thickness, F 2 corrects for finite lateral sampledimensi<strong>on</strong>s, and F 3 corrects for placement <str<strong>on</strong>g>of</str<strong>on</strong>g> the probes with finite distances fromthe sample edges. For very thin samples with the probes being far from the sampleedge, F 2 and F 3 are approximately equal to <strong>on</strong>e (1.0), and the expressi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> thesemic<strong>on</strong>ductor sheet resistance becomes:R s =πV/Iln2The four-point probe method can eliminate the effect introduced <strong>by</strong> the proberesistance, probe c<strong>on</strong>tact resistance and spreading resistance. Therefore, it hasmore accuracy than the two-point probe method.74
2.5.2.4 The hot point probe methodThe hot point probe method was used to determine whether the semic<strong>on</strong>ductoris N- or P- type. The potential difference when measured between the hot and thecold c<strong>on</strong>tacts (thermal Seeback effect) and the center-zero meter deflecti<strong>on</strong>identify the carrier type. The cold probe polarity therefore indicates thec<strong>on</strong>ductivity type (see Figure 2.17 for n – type.). For current directi<strong>on</strong>measurement standard digital multimeter were used.Figure 2.17 Hot-point probe measurements for n- type semic<strong>on</strong>ductor caseHot probes are effective in the resistivity range from 10 -3 to 10 4 Ohm∙cm.75
3. Experimental resultsIntroducti<strong>on</strong>In view <str<strong>on</strong>g>of</str<strong>on</strong>g> the severe future ecological impacts <str<strong>on</strong>g>of</str<strong>on</strong>g> energy producti<strong>on</strong> <strong>by</strong>combusti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> fossil fuels, solar energy is seriously c<strong>on</strong>sidered as an alternative.However, the development <str<strong>on</strong>g>of</str<strong>on</strong>g> new solar energy c<strong>on</strong>verters with improvedperformance and lower cost requires new approaches focused <strong>on</strong> the use <str<strong>on</strong>g>of</str<strong>on</strong>g> cheapand n<strong>on</strong>-toxic materials. Cuprous oxide (Cu 2 O) is an attractive semic<strong>on</strong>ductormaterial for fabricating low-cost solar cells and other optoelectr<strong>on</strong>ic devices[134]. Cuprous oxide attracts the most interest because <str<strong>on</strong>g>of</str<strong>on</strong>g> its high opticalabsorpti<strong>on</strong> coefficient in the visible range and its reas<strong>on</strong>ably good electricalproperties [135]. Cuprous oxide is a semic<strong>on</strong>ductor having a direct band gap <str<strong>on</strong>g>of</str<strong>on</strong>g> 2eV which has been studied previously for applicati<strong>on</strong> in solar energy c<strong>on</strong>vertingdevices [135]. <strong>Copper</strong> and copper oxide (metal-semic<strong>on</strong>ductor) is <strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g> the firstphotovoltaic cell invented [137].It has been shown in [74] that to fabricate efficient Cu 2 0 solar cells, <strong>on</strong>e musteither utilize a MIS structure or a heterojuncti<strong>on</strong>. TiO 2 has a band gap <str<strong>on</strong>g>of</str<strong>on</strong>g> р 3.2 eV,so interface with copper oxides must forms n-TiO 2 /p-Cu2O heterostructurejuncti<strong>on</strong> [138], so heterojuncti<strong>on</strong> photovoltaic cell can be created. The objective<str<strong>on</strong>g>of</str<strong>on</strong>g> the present study is to develop solar cell <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> Cu 2 O thin film substrateusing well know magnetr<strong>on</strong> sputtering technique.3.1 Cu2O thin film depositi<strong>on</strong> <strong>by</strong> reactive DC magnetr<strong>on</strong> sputtering.In presented work, the thin films <str<strong>on</strong>g>of</str<strong>on</strong>g> copper oxide (Cu x O) were deposited <strong>by</strong>direct current (DC) magnetr<strong>on</strong> reactive sputtering. Pure copper target were used.Oxygen plays role <str<strong>on</strong>g>of</str<strong>on</strong>g> reactive gas in order to obtain oxide films. The distancebetween target and substrate was 10 cm and after changed to 13 cm, becausesample-holder heater was installed. The main c<strong>on</strong>diti<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> the depositi<strong>on</strong> arepresented in Table 3.1.76
Table 3.1: Cu x O sputtering c<strong>on</strong>diti<strong>on</strong>.SputteringtypePowersupplymodeTargetReactivegasGas fluxc<strong>on</strong>trolmode<strong>Substrate</strong>DCMagnetr<strong>on</strong>C<strong>on</strong>stantcurrentCu O 2C<strong>on</strong>stanttotalpressureQuartz 10 x 10mm 2sputtering chamberSample heaterRoomtemperaturesample holderCu x OsampleIn order to improve sputtering process and obtain necessary crystallineproperties, electr<strong>on</strong>ic and adhesi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu x O sputtered films, it was performedstudy <str<strong>on</strong>g>of</str<strong>on</strong>g> influence <str<strong>on</strong>g>of</str<strong>on</strong>g> oxygen and arg<strong>on</strong> fluxes <strong>on</strong> menti<strong>on</strong>ed parameters. The besttemperature c<strong>on</strong>diti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> depositi<strong>on</strong> in range <str<strong>on</strong>g>of</str<strong>on</strong>g> 100-600° C was also determined.3.1.1 Influence <str<strong>on</strong>g>of</str<strong>on</strong>g> oxygen/arg<strong>on</strong> flux ratio <strong>on</strong> Cu x O sputtered films.The sputtering depositi<strong>on</strong>s at various flux ratios <str<strong>on</strong>g>of</str<strong>on</strong>g> arg<strong>on</strong> and oxygen in twoexperimental batches under room and 200°C temperature were d<strong>on</strong>e. It wasexperimentally shown that for good adhesi<strong>on</strong> total working pressure (oxygen andarg<strong>on</strong>) during sputtering process must be less than 8·10 -3 mbar. At higher total77
sputtering pressure, all deposited films are peeled. So the optimal workingpressure 5·10 -3 mbar was chosen.3.1.1.1 Sputtering at room temperature.The runs 1A-5A were d<strong>on</strong>e at room temperature at c<strong>on</strong>stant current <str<strong>on</strong>g>of</str<strong>on</strong>g>sputtering and at various fluxes <str<strong>on</strong>g>of</str<strong>on</strong>g> oxygen and arg<strong>on</strong> (see table 3.2).Table 3.2 Experimental data and sputtering c<strong>on</strong>diti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> DC magnetr<strong>on</strong>sputtering Cu x O films depositi<strong>on</strong>s (Runs from 1A to 5A) at room temperature.RunFluxArFluxO 2O 2 /ArFluxVITarget -SampledistanceLatticeParameterThicknessSputtering ratesccm sccm V A cm Å nm Å/s1A 7 3 0,43 465 0,3 10 4,2378 1340 7,442A 6,5 4 0,62 473 0,3 10 4,2538 1480 7,713A 5,2 5,2 1 484 0,3 10 4,2632 1357 7,544A 6 4,5 0,75 468 0,3 10 4,2620 1113 6,185A 6,2 4,2 0,67 454 0,3 10 4,2594 413 3,44On the Figure 3.1 is presented experimental data <str<strong>on</strong>g>of</str<strong>on</strong>g> lattice parameter as afuncti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> ratio oxygen arg<strong>on</strong> gases for the room temperature depositi<strong>on</strong>. Thecrystallizati<strong>on</strong> level <str<strong>on</strong>g>of</str<strong>on</strong>g> the sputtered films is increases with this ratio increasingand comes to saturati<strong>on</strong> after 0.7 value. Green line shows theoretic value <str<strong>on</strong>g>of</str<strong>on</strong>g>reticular parameter.78
eticular parameter4,2704,2654,2604,2554,2504,2454,2404,2354,2300,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1O 2/Ar ratioFigure 3.1 Reticular parameter versus O 2 /Ar fluxes ratio for roomtemperature DC magnetr<strong>on</strong> sputtering depositi<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu 2 O films (runs from1A to 5A).Figure 3.2 Room temperature depositi<strong>on</strong>s XRD 2Θ spectra (Runs from 1Ato 5A).The X-ray diffracti<strong>on</strong> patterns <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu 2 O films deposited at room temperatureare shown in Figure. 3.2. The ratio intensity <str<strong>on</strong>g>of</str<strong>on</strong>g> the diffracti<strong>on</strong> peaks79
Cu 2O /corresp<strong>on</strong>ding to (200) and (111) lattice orientati<strong>on</strong> as functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> oxygen/arg<strong>on</strong>fluxes ratio is increases rapidly after 0.8 value. Before this value (111) orientati<strong>on</strong>is prevalent for Cu 2 O sputtered films (see Figure 3.3).14 / Cu 2OT = 20 C700,3 0,6 0,9Flux ratio O 2/ArFigure 3.3 Cu 2 O diffracti<strong>on</strong> peaks ratio for (200)/(111) orientati<strong>on</strong> versusoxygen/arg<strong>on</strong> flux ratio for room temperature depositi<strong>on</strong>s.80
Cu / Cu 2O4Cu / Cu 2O Cu / Cu 2O T = 20°C200,3 0,6 0,9Flux ratio O 2/ArFigure 3.4 Cu and Cu 2 O diffracti<strong>on</strong> peaks ratio for Cu(111) /Cu 2 O (111)andCu 2 O (200) orientati<strong>on</strong> versus oxygen/arg<strong>on</strong> flux ratio for room temperaturedepositi<strong>on</strong>s.In the Figure 3.4 is shown that with increasing oxygen flux and decreasing <str<strong>on</strong>g>of</str<strong>on</strong>g>arg<strong>on</strong> the intensity <str<strong>on</strong>g>of</str<strong>on</strong>g> X-ray diffracti<strong>on</strong> peak Cu 2 O (111) and Cu 2 O (111) areincreasing, but decreasing Cu(111) is observed.3.1.1.2 Sputtering depositi<strong>on</strong>s Cu 2 O at 200°C (runs from 1B to 7B).It order to study <str<strong>on</strong>g>of</str<strong>on</strong>g> influence <str<strong>on</strong>g>of</str<strong>on</strong>g> O 2 and Ar gas fluxes at 200°C <strong>on</strong> theproperties <str<strong>on</strong>g>of</str<strong>on</strong>g> deposited Cu 2 O thin film and optimize sputtering process the series<str<strong>on</strong>g>of</str<strong>on</strong>g> experimental sputtering was performed (runs 1B-7B). The sputtering currentwas c<strong>on</strong>stant. In table 3.3 experimental data and parameters <str<strong>on</strong>g>of</str<strong>on</strong>g> the Cu 2 O filmsputtering are presented. In the Figure 3.5 is presented analysis <str<strong>on</strong>g>of</str<strong>on</strong>g> experimentalresults <str<strong>on</strong>g>of</str<strong>on</strong>g> dependence <str<strong>on</strong>g>of</str<strong>on</strong>g> the Cu 2 O sputtered thin film lattice parameter versusoxygen/arg<strong>on</strong> gases fluxes ratio. The green line is theoretical parameter. One cansee that lattice parameter is close to theoretical at oxygen/arg<strong>on</strong> gases fluxesrati<strong>on</strong> value higher than 1.5. In the Figure 3.6 are presented measured 2Θ XRDspectra <str<strong>on</strong>g>of</str<strong>on</strong>g> films deposited at 200°C.81
eticular parameterTable 3.3 Experimental data and c<strong>on</strong>diti<strong>on</strong> DC magnetr<strong>on</strong> sputteringdepositi<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu x O films (Runs from 1B to 7B).RunFluxArFluxO 2O 2 /ArFluxVITarget -SampledistanceLatticeParameterThicknessSputteringratesccm sccm V A cm Å nm Å/s1B 6 4,5 0,75 673 0,40 13 4,2540 5021 16,742B 5,5 5 0,91 678 0,40 13 4,2590 5317 17,723B 4,5 6 1,33 706 0,40 13 4,2649 6115 16,994B 3,5 7 2,00 713 0,40 13 4,2582 3702 15,435B 2,5 8 3,20 696 0,41 13 4,2654 3895 12,986B 4,1 6,2 1,51 653 0,40 13 4,2575 5096 16,997B 2,9 7,6 2,62 658 0,40 13 4,2576 3101 11,494,2704,2684,2664,2644,2624,2604,2584,2564,2544,2524,2500,5 1,0 1,5 2,0 2,5 3,0 3,5O 2/Ar flux ratioFigure 3.5 Reticular parameter as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> O 2 /Ar fluxes ratio for DCmagnetr<strong>on</strong> sputtering depositi<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu 2 O films (Runs from 18 to 26) at200°C.82
Cu 2O /Figure 3.6 200°C depositi<strong>on</strong>s XRD 2Θ spectra (Runs1B-7B).Cu 2O / T = 200°C0,300,240,181 2 3Flux ratio O 2/ArFigure 3.7 Cu 2 O diffracti<strong>on</strong> peaks intensity ratio for (200)/(111) orientati<strong>on</strong>versus oxygen/arg<strong>on</strong> flux ratio for 200°C depositi<strong>on</strong>s.83
Cu / Cu 2O3,63,0Cu / Cu 2O Cu / Cu 2O T = 200°C2,41,81,20,60,01 2Flux ratio O 2/ArFigure 3.8 Cu and Cu 2 O diffracti<strong>on</strong> peaks intensity ratio for Cu(111) /Cu 2 O(111) and Cu 2 O (200) orientati<strong>on</strong> versus oxygen/arg<strong>on</strong> flux ratio for 200°Cdepositi<strong>on</strong>s.In the films deposited <strong>by</strong> sputtering technique at 200°C at various ratiooxygen/arg<strong>on</strong> fluxes increase <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu 2 O (111) XRD peak intensity with increasing<str<strong>on</strong>g>of</str<strong>on</strong>g> fluxes ratio value is observed(see Figure 3.7). The diffracti<strong>on</strong> peaks intensity <str<strong>on</strong>g>of</str<strong>on</strong>g>Cu 2 O (200) is decreases with oxygen/arg<strong>on</strong> fluxes ratio. The peak <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu (111) isdecreased, but Cu 2 O (200) and (111) orientati<strong>on</strong> increased respect the Cu(111)peak with fluxes ratio increasing(see Figure 3.8).Summarizing data obtained for room and 200°C depositi<strong>on</strong>, X-ray diffracti<strong>on</strong>spectra <str<strong>on</strong>g>of</str<strong>on</strong>g> films at sputtered room and 200°C temperature were analyzed. Scherrerequati<strong>on</strong> used that allows to evaluate grain size. The results are presented inFigure 3.9.84
D, A300250Cu 2O , 2 36.52°20 ° C200 ° C2001501005000,5 1,0 1,5 2,0 2,5 3,0 3,5Ratio O 2/Ar fluxFigure 3.9 De<strong>by</strong>e-Scherrer calculati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> grain size for Cu 2 O (111)orientati<strong>on</strong> diffracti<strong>on</strong> peak versus oxygen/arg<strong>on</strong> flux ratio for room and200°C temperature sputtering depositi<strong>on</strong>s.85
3.1.2 Influence <str<strong>on</strong>g>of</str<strong>on</strong>g> temperature <strong>on</strong> cuprous oxide thin film propertiesInfluence substrate temperature <strong>on</strong> properties <str<strong>on</strong>g>of</str<strong>on</strong>g> deposited copper oxide filmwas studied. The reactive direct current magnetr<strong>on</strong> sputtering depositi<strong>on</strong>s wered<strong>on</strong>e at c<strong>on</strong>stant sputtering current 0.4 A, voltage 640 V, distance between targetand substrate 13 cm and c<strong>on</strong>stant oxygen/arg<strong>on</strong> fluxes ratio was 3.2. Thesputtering process has been performed in temperature range from 100 to 600°C. InTable 3.4 is presented measured characteristic <str<strong>on</strong>g>of</str<strong>on</strong>g> obtained copper oxide thin films.Table 3.4 Experimental data DC magnetr<strong>on</strong> sputtering deposited Cu x O filmsin temperature range <str<strong>on</strong>g>of</str<strong>on</strong>g> 100-600°C (Runs from 1C to 5C).Run Temperature, °C Lattice Parameter, Thickness, nm SputteringÅrate, Å/s1C 100 4,2808 2486 4,972C 200 4,2654 3895 12,983C 300 4,2654 3349 11,164C 400 4,2644 3475 7.305C 600 4,2626 3145 6,29Reticular parameter as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> reactive sputtering is presented in Figure3.10. The black line <strong>on</strong> the graph is theoretical value for cuprite Cu 2 O. One cansee that with temperature increasing experimental value is approximating totheoretical.86
eticular parameter4,504,45Cu 2O (1 1 1)2 36.52, cuprite4,404,354,304,254,204,154,100 200 400 600temperature, CFigure 3.10 Reticular parameter <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu 2 O as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> reactivesputtering temperature in range <str<strong>on</strong>g>of</str<strong>on</strong>g> 100°C- 600°C.In this work, it is experimentally proved that copper oxide semic<strong>on</strong>ductor filmsputtered at room temperature has n-type (electr<strong>on</strong>ic) c<strong>on</strong>ductivity. On the otherhand all thin film deposited at temperature higher 200°C has p-type (holes)c<strong>on</strong>ductivity. Measurements were performed <strong>by</strong> ―Hot point‖ method describedbefore.The specific resistance <str<strong>on</strong>g>of</str<strong>on</strong>g> sputtered copper oxide films sputtered attemperature lower 100°C is very low, close to 0. On the c<strong>on</strong>trary, samplesobtained at the temperature higher 200°C specific resistance have very high value<str<strong>on</strong>g>of</str<strong>on</strong>g> resistivity and after decreases with the temperature <str<strong>on</strong>g>of</str<strong>on</strong>g> sputtering (see Figure3.11).Specific resistance was measured <strong>by</strong> standard ―four – probe‖ method. In ―thinfilm‖ c<strong>on</strong>diti<strong>on</strong>, so sample area can be neglected.87
cm3000250020001500100050000 100 200 300 400 500 600T,°CFigure 3.11 Specific resistance Cu x O thin films versus sputtering temperature.Results:At the room temperature, we have obtained Cu 2 O phase, but copper or copperoxide CuO is also present. These films have electr<strong>on</strong>s (n-type) c<strong>on</strong>ducti<strong>on</strong>.<str<strong>on</strong>g>Film</str<strong>on</strong>g> depositi<strong>on</strong> <strong>on</strong> heated at 200°C substrate have higher crystal structure andP-type c<strong>on</strong>ductivity as have to be in Cu 2 O semic<strong>on</strong>ductor.Increasing temperature Cu 2 O and Cu 2 O peaks increase while Cupeak decrease significantly.The thin films <str<strong>on</strong>g>of</str<strong>on</strong>g> copper oxide deposited at room temperature have very lowresistivity.<str<strong>on</strong>g>Film</str<strong>on</strong>g>s prepared at high temperature depositi<strong>on</strong> have high resistivity.3.2 Reactive DC sputtering <str<strong>on</strong>g>of</str<strong>on</strong>g> TiOx thin filmTitanium oxide (TiO x ) thin film was deposited <strong>on</strong> quartz substrate <strong>by</strong> DCmagnetr<strong>on</strong> sputtering using Ti target in reactive oxygen gas ambient. The mainsputtering c<strong>on</strong>diti<strong>on</strong> is presented in Table 3.5:88
Table 3.5 Reactive sputtering c<strong>on</strong>diti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> Titanium oxide (TiO x ) thin filmSputtering:Regime:P-Supplymode:Target:ReactiveGas :Target -SampledistanceGasFluxC<strong>on</strong>trolMode:<strong>Substrate</strong>:Magnetr<strong>on</strong>DCC<strong>on</strong>stantCurrentandPulsemodeTi O 2 13 cmC<strong>on</strong>stanttotalpressure5.2 10 -3mbarQuartz10 x 10mm 2After film depositi<strong>on</strong>, thickness <str<strong>on</strong>g>of</str<strong>on</strong>g> the film has been measured in order todetermine depositi<strong>on</strong> rate. X-ray diffracti<strong>on</strong> spectra were measured to identify thinfilm properties. The main experimental results and sputtering parameters arepresented in Table 3.6. Where T 0 is temperature <str<strong>on</strong>g>of</str<strong>on</strong>g> sample-holder, measured <strong>by</strong>thermocouple at the beginning <str<strong>on</strong>g>of</str<strong>on</strong>g> sputtering process and T max - maximaltemperature measured during depositi<strong>on</strong> process, total working pressure <str<strong>on</strong>g>of</str<strong>on</strong>g> Ar andO 2 gases was c<strong>on</strong>stant 5.2 10 -3 mbar and measured <strong>by</strong> capacitance type vacuumdetector during depositi<strong>on</strong> process.RunTable 3.6 Experimental data and results <strong>on</strong> TiO x reactive sputtering:O 2 /ArFluxT 0,C°T max,C°Currentsupply modeVoltageVCurrentADepositi<strong>on</strong>ratenm/min<str<strong>on</strong>g>Film</str<strong>on</strong>g>Structuret1 1,33 32 70 DC 440 0,4 Amft2 1,33 27 120 DC 500 1 Amft3 0,11 33 91 DC 463 1 26,4 Tit4 0,40 33 78 DC 459 1 25,6 Tit5 0,91 33 86 DC 435 1 19,8 Tit7 3,35 30 215 DC 447 1 2,0 Rt8 2,67 35 155 DC 460 1 1,5 R+At9 2,50 200 245 DC 464 1 1,7 At10 2,53 400 408 DC 460 1 1,5 At11 2,50 300 339 DC 460 1 1,5 At12 2,50 30 156 Pulsed 800 1 0,7 Amft13 2,50 33 224 Pulsed 1000 1 # Amf89
XRD spectra (see Figure 3.12) analysis results that amorphous (Amf) structure(no peaks <strong>on</strong> XRD spectra) was obtained for the Runs 1t-2t for oxygen/arg<strong>on</strong>fluxes ratio 1.33 and Runs t7-8 in case <str<strong>on</strong>g>of</str<strong>on</strong>g> pulse mode <str<strong>on</strong>g>of</str<strong>on</strong>g> power source.Titanium Ti thin film was obtained for Runs t3-t5 when oxygen/arg<strong>on</strong> fluxesratio is below 1.TiO 2 rutile(R) phase(Run t7) is observed for oxygen/arg<strong>on</strong> fluxes ratio 3.35and with its decreasing to 2.5 value titanium oxide passed to mixed rutile andanatase (A) phase(Run t8).Pure anatase phase <str<strong>on</strong>g>of</str<strong>on</strong>g> titanium oxide was observed for Runs t9-t11 whensputtering depositi<strong>on</strong> was performed in temperature range from 200°C to 400°C.Figure 3.12 XRD spectra <str<strong>on</strong>g>of</str<strong>on</strong>g> thin film <str<strong>on</strong>g>of</str<strong>on</strong>g> titanium oxide, deposited <strong>by</strong>reactive magnetr<strong>on</strong> sputtering in temperature range 30-400°C. The mostsignificant peak for Anatase phase (101) 2Θ= 25.28° and for Rutile phase(110) 2Θ= 27.25°.90
eticular paramiterAnalyzing X-ray diffracti<strong>on</strong> spectra we obtain reticular parameter <str<strong>on</strong>g>of</str<strong>on</strong>g> titaniumoxide phases presented <strong>on</strong> Figure 3.13. The colour lines are representingtheoretical values. The value <str<strong>on</strong>g>of</str<strong>on</strong>g> reticular parameter shows very close experimentalvalue to theoretical <strong>on</strong>es.5,4AnataseRutile5,25,04,84,64,40 200 400temperature, CFigure 3.13 Reticular parameter for Anatase phase (101) 2Θ= 25.28 andRutile phase (110) 2Θ= 27.25° as functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> temperature. <str<strong>on</strong>g>Thin</str<strong>on</strong>g> film <str<strong>on</strong>g>of</str<strong>on</strong>g>titanium oxide, deposited <strong>by</strong> reactive magnetr<strong>on</strong> sputtering in temperaturerange 30-400°C. Colored lines are theoretic value.XRD spectra.ResultsIt has been obtained two crystalline phase <str<strong>on</strong>g>of</str<strong>on</strong>g> TiO 2 : Anatase and Rutile.The peak intensity <str<strong>on</strong>g>of</str<strong>on</strong>g> Anatase is increasing with the temperature <str<strong>on</strong>g>of</str<strong>on</strong>g> sputteringwhile Rutile‘s peak is decreasing.The sputtering c<strong>on</strong>diti<strong>on</strong> such partial pressure <str<strong>on</strong>g>of</str<strong>on</strong>g> gases and temperature <str<strong>on</strong>g>of</str<strong>on</strong>g>sputtering are str<strong>on</strong>gly influence <strong>on</strong> properties <str<strong>on</strong>g>of</str<strong>on</strong>g> obtained titanium oxide films.91
3.3 <strong>Copper</strong> oxide photovoltaic cell development.The ―wet‖ (copper/copper oxide/electrolyte), metal-semic<strong>on</strong>ductor(copper/copper oxide) and hetero-juncti<strong>on</strong> (copper oxide/titanium oxide) werec<strong>on</strong>structed and studied.3.3.1 Cu 2 O photovoltaic “wet” solar cell.The ―wet‖ copper oxide photovoltaic elements were produced. 3 copper discs(run 1) have diameter 12.5, and area <str<strong>on</strong>g>of</str<strong>on</strong>g> electrode <str<strong>on</strong>g>of</str<strong>on</strong>g> 117.2 cm 2 . Disk number 1 wasoxidized <strong>by</strong> ―methane torch‖ and slightly mechanically grinded in order to removeCuO residuals. Disk number 2 oxidized <strong>by</strong> ―methane torch‖ and not grinded (seeFigure 3.14). The black oxide coating was peeled under water. Disk 3 Notoxidized and used as electrode.Figure 3.14 Photo <str<strong>on</strong>g>of</str<strong>on</strong>g> disk 1 e disk 2 respectively after oxidati<strong>on</strong>:92
The electrodes were completely introduced in NaCl electrolyte soluti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> 4liters volume (see Figure 3.15). Distance between electrodes was 15 cm.Figure 3.15 Photo <str<strong>on</strong>g>of</str<strong>on</strong>g> disk 1 and electrode in NaCl electrolyte soluti<strong>on</strong>.The current and voltage were measured <strong>by</strong> Kiethley 195A multimeter (seetable 3.6). To illuminati<strong>on</strong> has been used usual bulb lamp (100W) in order toobtain photovoltaic effect. The distance between lamp and soluti<strong>on</strong> was about 2cm.CudiskU dark,m VU photo,m VI dark,AI photo,Adelta U,m Vdelta I,AelectrolyteNaCl, gr/l1 129 149 115 136 20 21 20 4202 49 96 40 92 47 52 20 24441 143 156 129 142 13 13 30 1692 90 106 80 102 16 22 30 3521 163 179 156 163 16 7 40 1122 112 146 97 130 34 33 40 11221 165 190 149 166 25 17 50 4252 113 153 97 133 40 36 50 14401 161 173 153 177 12 24 60 288P,nW93
2 112 144 94 130 32 36 60 11521 163 188 148 174 25 26 70 6502 105 154 93 138 49 45 70 2205Other kind <str<strong>on</strong>g>of</str<strong>on</strong>g> experiment has been d<strong>on</strong>e with the same Cu discs from Run 1(Run 2) after polishing. The mechanical and electrical setup was the same as atRun 1. Only 50 gr/l NaCl electrolyte c<strong>on</strong>centrati<strong>on</strong> was used. Disk 1(see Figure3.16) oxidized in electrical oven during 12 hours up to 850 °C, and after wasleaved for the night in order the temperature was going down slowly. Disk 2(Figure 3.17) oxidized in same c<strong>on</strong>diti<strong>on</strong> and together with Disk 1. After it wasannealed during 2 hours at 500°C and slowly cooled. Disk 3 was not oxidized andused as electrode.Figure 3.16 Photo <str<strong>on</strong>g>of</str<strong>on</strong>g> disk 1 after oxidati<strong>on</strong> in electric oven:94
Figure 3.17 Photo <str<strong>on</strong>g>of</str<strong>on</strong>g> disk 2 after oxidati<strong>on</strong> in electric oven and annealed at500°CTable 3.7 Run 2 experimental dataName <str<strong>on</strong>g>of</str<strong>on</strong>g>Cu disk1 (notannealed)Udark,m VUphoto,m VIdark,AI photo,AdeltaU,m VdeltaI,AelectrolyteNaCl, gr/lP,nW97 175 70 160 78 90 50 70202 (annealed) 148 160 124 144 12 20 50 24095
P, 10 -9 W80007000600050004000300020001000020 30 40 50 60 70electrolyte, gr/l1 RUN 1 grinded2 RUN 1, n<strong>on</strong> grinded1, RUN 2, not annealed2, RUN 2, annealedFigure 3.18 Electric power measured in short circuit c<strong>on</strong>necti<strong>on</strong> for bulb lampilluminati<strong>on</strong> for setup Run 1 and Run 2 versi<strong>on</strong>s versus electrolyte c<strong>on</strong>centrati<strong>on</strong>.From the experimental data presented in Table 3.7 and the figure 3.18 <strong>on</strong>e cancome to c<strong>on</strong>clusi<strong>on</strong> that ―wet‖ photovoltaic the performance cannot be improved<strong>by</strong> anealing and cleneaning <str<strong>on</strong>g>of</str<strong>on</strong>g> CuO layer. The proper chose <str<strong>on</strong>g>of</str<strong>on</strong>g> electrolitec<strong>on</strong>centrati<strong>on</strong> can significantly improve cell performace (see Figure 3.19, 3.20).96
I , micro AV, mV20018016014012010080604020disk 1, Udarkdisk 2 Udarkdisk 1, U photodisk 2, U photo00 20 40 60 80electrolyte, gr/lFigure 3.19 Dark and photo voltage versus electrolyte c<strong>on</strong>sentrati<strong>on</strong>.2001801601401201008060402000 20 40 60 80electrolyte, gr/ldisk 1, I darkdisk 2, I darkdisk 1, I photodisk 2, I phtoFigure 3.20 Dark and photo current versus electrolyte c<strong>on</strong>sentrati<strong>on</strong>.97
3.3.2 Cu/Cu 2 O/Cu (metal/semic<strong>on</strong>ductor) thin film structure created <strong>by</strong>reactive DC sputtering at various temperaturesThe main parameters <str<strong>on</strong>g>of</str<strong>on</strong>g> experimental setup used for creati<strong>on</strong> the metalsemic<strong>on</strong>ductorSchottky photovoltaic cell are presented in Table 3.8:Table 3.8 Sputtering c<strong>on</strong>diti<strong>on</strong> and parameters.Sputtering Regime PowerSupplymodeTargetReactiveGasTarget -SampledistanceGas FluxC<strong>on</strong>trolMode<strong>Substrate</strong>Magnetr<strong>on</strong> DC C<strong>on</strong>stantCurrentCu O 2 13 cm C<strong>on</strong>stantTotalPressureQuartz10 x 10mm 2Cu target were cleaned before every depositi<strong>on</strong> <strong>by</strong> pre-sputtering. Schematicpresentati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> prepared <strong>by</strong> magnetr<strong>on</strong> sputtering depositi<strong>on</strong>s presented in Table3.9. Cu/Cu 2 O/Cu structures is presented in Figure 3.21. <str<strong>on</strong>g>Thin</str<strong>on</strong>g> layer <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu weresputtered at quarz <strong>on</strong> the top <str<strong>on</strong>g>of</str<strong>on</strong>g> it were sputtered Cu 2 O thin film and after other Cuthin layer. For electrical characterizati<strong>on</strong> obtained metal/semic<strong>on</strong>ductors structurec<strong>on</strong>nected <strong>by</strong> tungsten needle for electrical characterizati<strong>on</strong> to standardmultimeter.Table 3.9 Sputtering operati<strong>on</strong> steps, sputtering parameters and depositedfilm characteristic.Step<str<strong>on</strong>g>Film</str<strong>on</strong>g>depositedTargetArFlux(sccm)O 2Flux(sccm)P10 -3mbarT(C°)V(V)I(A)Thickness(nm)Size(mm)1 Cu Cu 7 0 6 30 468 0.4 500 10x102Cu 2 OCu 2.5 8 7.6300 1400 2 557 0.4 2200600 33 Cu Cu 7 7.5 5.7 30 447 1 150 Ø 6Ø 91 Run 1D;2 Run 2D)3 Run 3D98
MultimeterLight from bulb lampCu, 150 nm,Ø 6 mmV, ACu 2O,2200 nmØ 9mmCu, 960nm10X10mmQuartz substrate10X10 mmCuCu 2 OCuFigure 3.21 Schematic view <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu/Cu 2 O/Cu structures.ResultsWe have obtained Cu/Cu 2 O/Cu. This structure does not show sensitivity tovisible light.The structure does not show diode behavior.The possible reas<strong>on</strong> is that wr<strong>on</strong>g combinati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> metal semic<strong>on</strong>ductor wasused.99
3.3.3 Photovoltaic effect in Cu/Cu 2 O/TiO 2 /Au (heterojuncti<strong>on</strong>) thin filmstructure created <strong>by</strong> reactive DC sputtering.Heterojuncti<strong>on</strong> photovoltaic thin film cell <str<strong>on</strong>g>based</str<strong>on</strong>g> <strong>on</strong> Cu 2 O/TiO 2 juncti<strong>on</strong> havebeen created (see Figure 3.22) The Cu and gold layers play role <str<strong>on</strong>g>of</str<strong>on</strong>g> ohmic c<strong>on</strong>tacts.The main producti<strong>on</strong> steps are shown in Table 3.11. TiO 2 film was sputtered inc<strong>on</strong>diti<strong>on</strong> to obtain anatase phase.Table 3.10: Main sputtering c<strong>on</strong>diti<strong>on</strong>s:SputteringRegimeP-SupplymodeTargetReactiveGasTarget -SampledistanceGas FluxC<strong>on</strong>trolMode<strong>Substrate</strong>Magnetr<strong>on</strong>DCC<strong>on</strong>stantCurrentCu, Ti,AuO 213 cmC<strong>on</strong>stantTotalPressureQuartz10 x 10mm 2Table 3.11 Experimental data and results <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu/Cu 2 O/TiO 2 /Au structureproducti<strong>on</strong>.Run Target ArFluxO 2FluxP w ,10 -3mbarT ,C°VoltageVCurrentAthicknessnmSizemm<str<strong>on</strong>g>Film</str<strong>on</strong>g>depositedD-1 Cu 7 0 6 30 468 0.4 960 10x10 CuD-2 Cu 2.5 8 7.55 600 557 0.4 2250 Ø 9 Cu 2 OD-3 Ti 3 7.5 5.66 300 447 1 8 Ø 9TiO 2 ,AnataseD-4 Au 7 0 6.54 29 684 0.4 219 Ø 3 Au100
MultimeterAu, 219 nm,Ø 3 mmTiO 2, 8nmØ 9mmV, ACu 2O,2250 nmØ 9mmCu, 960nm10X10mmCharge collecti<strong>on</strong> volumes, about 6 micr<strong>on</strong> width [139]Quartz substrate10X10 mmCuTiO 2 +Cu 2 OAuFigure 3.22 Schematic presentati<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> Cu/Cu 2 O/TiO 2 /Au structures.101
Figure 3.21 Photo <str<strong>on</strong>g>of</str<strong>on</strong>g> tested sample. Structure c<strong>on</strong>nected <strong>by</strong> tungsten needlefor electrical characterizati<strong>on</strong>The Cu/Cu 2 O/TiO 2 /Au heterojuncti<strong>on</strong> structures have been electricallycharacterized (see Figure 3.21). The phovoltaic characteristic and comparis<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g>―wet‖ photovoltaic cell versus ―sputtering‖ photovoltaic elements are presented inTable 3.12. From this table <strong>on</strong>e can see that heterojuncti<strong>on</strong> photovoltaic cell is <strong>on</strong>eorder higher light c<strong>on</strong>versi<strong>on</strong> performance respect to wet‖ photovoltaic.Other Runs 1H and Cu/Cu 2 O/TiO 2 /Au heterojuncti<strong>on</strong> photovoltaic cell wereprepared. As substrat have been used 2 inch in diameter Cu sheet <str<strong>on</strong>g>of</str<strong>on</strong>g> 0.08 mm102
thickness. The strutructures were created <strong>by</strong> the same technique used for thequartz substrate, menti<strong>on</strong>ed before. One structure is created <strong>by</strong> sputtering at600°C Run 1H (see Figure 3.13), but other at 300°C Run 2H (see Figure 3.14).In order to optimize fr<strong>on</strong>t Au c<strong>on</strong>tact, structure 1H was divided in two parts.On the <strong>on</strong>e part c<strong>on</strong>tact have shape <str<strong>on</strong>g>of</str<strong>on</strong>g> grid. Other part was totally covered <strong>by</strong>semitransparent 100 nm thick gold (see Figure 3.14).Figure 3.13 Run 1H. <str<strong>on</strong>g>Thin</str<strong>on</strong>g> film 2-inch heterojuncti<strong>on</strong> photovoltaic solar cellfabricated <strong>by</strong> sputtering technology at 600°C. As the substrate and back c<strong>on</strong>tactCu sheet 0.08 mm thickness is used.103
Figure 3.14 Run 2H. <str<strong>on</strong>g>Thin</str<strong>on</strong>g> film 2-inch heterojuncti<strong>on</strong> photovoltaic solar cellfabricated <strong>by</strong> sputtering technology at 300°C. As the substrate and back c<strong>on</strong>tactCu sheet 0.08 mm thickness is used.3.3.4 Performance comparis<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> different photovoltaic cell buildThe power <str<strong>on</strong>g>of</str<strong>on</strong>g> photovoltaic devices normalized to area measured under bulblamp light illuminati<strong>on</strong> is presented in Table 3.12. It was calculated <strong>by</strong> theformula:P = V os·I sc, /AWhere:V os -open circuit voltage (measurement d<strong>on</strong>e with good voltmeter satisfy thisc<strong>on</strong>diti<strong>on</strong>)104
I sc -short circuit current (measurement d<strong>on</strong>e with good amperemeter satisfiesthis c<strong>on</strong>diti<strong>on</strong>)A - active area:For ―sputtered‖ samples was taken in c<strong>on</strong>siderati<strong>on</strong> the ring around ohmic Auc<strong>on</strong>tact and for ―thermal‖ samples was taken in c<strong>on</strong>siderati<strong>on</strong> <strong>on</strong>ly <strong>on</strong>e side <str<strong>on</strong>g>of</str<strong>on</strong>g> Cuoxidized plate.Table 3.12 Performance comparis<strong>on</strong>s <str<strong>on</strong>g>of</str<strong>on</strong>g> different photovoltaic cells buildType Active Area ( cm 2 )Power, 10 -8(W/cm 2 )NoteCu/Cu 2 O/electrolyte(Thermal oxidati<strong>on</strong>)Cu/Cu 2 O/electrolyte(Thermal oxidati<strong>on</strong>)Cu/Cu 2 O/TiO 2 +Au(Sputtering)Cu+Cu 2 O+TiO 2 +Au(Sputtering)122,7 5,7 maximum value122,7 0,89 average value0,57∙10 -3 42,8 device 7.10,57∙10 -3 72,4 device 7.2Cu+Cu 2 O+TiO 2 +Au(Sputtering)8 95,7Run1H, 600°C,Grid shape c<strong>on</strong>tactCu+Cu 2 O+TiO 2 +Au(Sputtering)6 0,85Run1H, 600°C,plane shape c<strong>on</strong>tactCu+Cu 2 O+TiO 2 +Au(Sputtering)19.6 1,5Run2H, 300°C,Grid shape c<strong>on</strong>tact105
From Table 3.12 <strong>on</strong>e can see that performance <str<strong>on</strong>g>of</str<strong>on</strong>g> heterojuncti<strong>on</strong> thin filmphotovoltaic is in <strong>on</strong>e order higher than for ―wet‖ solar cell. The higherperformance <str<strong>on</strong>g>of</str<strong>on</strong>g> heterojuncti<strong>on</strong> photovoltaic was predicted in [74]. It isexperimentally shown that shape <str<strong>on</strong>g>of</str<strong>on</strong>g> the fr<strong>on</strong>t c<strong>on</strong>tact str<strong>on</strong>gly influence <strong>on</strong>performance <str<strong>on</strong>g>of</str<strong>on</strong>g> solar cell. Run 1H with semitransparent Au fr<strong>on</strong>t c<strong>on</strong>tact has verylow performance, <strong>on</strong> the other hand the same sample with a grid c<strong>on</strong>tact havehighest efficiency. The sample with Cu 2 O layer sputtered at 300°C has lowc<strong>on</strong>versi<strong>on</strong> efficiency.Short circuit current I sc measured in heterojuncti<strong>on</strong> photovoltaic has positivedirecti<strong>on</strong>. This fact is proving that p-n juncti<strong>on</strong> is obtained. From Cu 2 O part holescollecti<strong>on</strong> is observed, for TiO 2 side electr<strong>on</strong> are collected.106
CONCLUSIONSIf <strong>on</strong>e is interested to invest in this research see summarizing table presentedbelow. Photovoltaic Cu/Cu 2 O/TiO 2 /Au cells structures were created <strong>by</strong> DCmagnetr<strong>on</strong> reactive sputtering in O 2 reactive gas ambient. O 2 and Ar and havec<strong>on</strong>stant partial gas pressure and fluxes.DeviceFr<strong>on</strong>tc<strong>on</strong>tactCu 2 O,depositi<strong>on</strong>TargetActive Area(cm 2 )Power, 10 -8(W/cm 2 )7.1Plane 600°C Cu, Ti, Au0,57∙10 -3 42,87.2plane 600°C Cu, Ti, Au0,57∙10 -3 72,41.1HGrid 600°C Cu, Ti, Au8 95,71.2Hplane 600°C Cu, Ti, Au6 0,852HGrid 300°C Cu, Ti, Au19.6 1,5At the room temperature, we have obtained Cu 2 O phase, but copper or copperoxide CuO is also present. These films have electr<strong>on</strong>s (n-type) c<strong>on</strong>ducti<strong>on</strong>.<str<strong>on</strong>g>Film</str<strong>on</strong>g> depositi<strong>on</strong> <strong>on</strong> heated at 200°C substrate have higher crystal structure andP-type c<strong>on</strong>ductivity as have to be in Cu 2 O semic<strong>on</strong>ductor.Increasing temperature Cu 2 O and Cu 2 O peaks increase while Cupeak decrease significantly.The thin films <str<strong>on</strong>g>of</str<strong>on</strong>g> copper oxide deposited at room temperature have very lowresistivity.<str<strong>on</strong>g>Film</str<strong>on</strong>g>s prepared at high temperature depositi<strong>on</strong> have high resistivity.We have obtained two crystalline phase <str<strong>on</strong>g>of</str<strong>on</strong>g> TiO 2 : Anatase and Rutile.The peak intensity <str<strong>on</strong>g>of</str<strong>on</strong>g> Anatase is increasing with the temperature <str<strong>on</strong>g>of</str<strong>on</strong>g> sputteringwhile Rutile‘s peak is decreasing.107
The sputtering c<strong>on</strong>diti<strong>on</strong> such partial pressure <str<strong>on</strong>g>of</str<strong>on</strong>g> gases and temperature <str<strong>on</strong>g>of</str<strong>on</strong>g>sputtering are str<strong>on</strong>gly influence <strong>on</strong> properties <str<strong>on</strong>g>of</str<strong>on</strong>g> obtained titanium oxide films.It has been obtained Cu/Cu 2 O/Cu. This structure do not show sensetivity tovisible light. The structure does not show diode behavior. The possible reas<strong>on</strong> isthat wr<strong>on</strong>g combinati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> metal semic<strong>on</strong>ductor was used.It has been obtained Cu/Cu 2 O/TiO 2 /Au solid thin film photovoltaic elements<strong>by</strong> DC magnetr<strong>on</strong> reactive sputtering. This thin film structures are sensitive tolight. The specific power <str<strong>on</strong>g>of</str<strong>on</strong>g> these samples is in two orders higher than in―thermal‖ oxidized electrolyte solar elements. Photovoltaic performance <str<strong>on</strong>g>of</str<strong>on</strong>g> thesestructure is str<strong>on</strong>gly depends <strong>on</strong> sputtering temperature and fr<strong>on</strong>t electrode shape.Photovoltaics c<strong>on</strong>stitutes a new form <str<strong>on</strong>g>of</str<strong>on</strong>g> producing electric energy that isenvir<strong>on</strong>mentally clean and very modular. In stand-al<strong>on</strong>e installati<strong>on</strong>s, it must usestorage or another type <str<strong>on</strong>g>of</str<strong>on</strong>g> generator to provide electricity when the sun is notshining.Photovoltaics is very suitable as the power supply for remote communicati<strong>on</strong>equipment. Its use is increasing rapidly to produce electricity in grid-c<strong>on</strong>nectedhouses and buildings in industrialized countries, despite a 5 to 10 times highercost than c<strong>on</strong>venti<strong>on</strong>al electricity.Crystalline Si technology, both m<strong>on</strong>ocrystalline and multicrystalline is todayclearly dominant, with about 90% <str<strong>on</strong>g>of</str<strong>on</strong>g> the market.<str<strong>on</strong>g>Thin</str<strong>on</strong>g>-film technology is <strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g> the candidates to take over from Si technology.There are many technological opti<strong>on</strong>s regarding thin-film materials and methods<str<strong>on</strong>g>of</str<strong>on</strong>g> depositi<strong>on</strong> but their primary claim to the thr<strong>on</strong>e currently occupied <strong>by</strong> Si is thatthey can be ultimately produced at much lower cost.<strong>Copper</strong> oxide is a good candidate for low cost photovoltaic element. It is n<strong>on</strong>toxic and has high absobti<strong>on</strong> in visible spectra <str<strong>on</strong>g>of</str<strong>on</strong>g> light. In order to improve itperformance doping methods and ―partner‖ comp<strong>on</strong>ent for hetero- or homo –juncti<strong>on</strong> have to be studied.In summary, it is very likely that photovoltaics will become in the next halfcentury an important source <str<strong>on</strong>g>of</str<strong>on</strong>g> world electricity. Public support and globalenvir<strong>on</strong>mental c<strong>on</strong>cerns will keep photovoltaics viable, visible, and vigorous both108
in new technical developments and user applicati<strong>on</strong>s. Nati<strong>on</strong>s which encouragephotovoltaics will be leaders in this shining new technology, leading the way to acleaner, more equitable twenty-first century, while those that ignore or suppressphotovoltaics will be left behind in the green, ec<strong>on</strong>omic energy revoluti<strong>on</strong>.109
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