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I N T R O D U C T I O N1Contents1.1 Hybrid interfaces for photovoltaics 11.2 Physical factors relevant for photoconversionat hybrid interfaces 81.3 Theoretical modeling of hybrid interfaces 121.4 Aims and outline of this Thesis 141.1 hybrid interfaces for photovoltaicsPhotovoltaics represents a promising and challenging fieldof inquiry in the area of renewable and sustainable energiesand the search of new and more efficient photovoltaicsmaterials is a constant stimulus for materials science.The photovoltaic market is currently dominated by thesilicon based materials, that provide high power conversionefficiencies (PCE) (up to 25% [8]) due to the excellentcharge transport properties and stability of high pure silicon[9]. The drawback of this trend consists in the highcosts and in the environmental impact needed to producehigh quality material.An alternative to the conventional silicon systems arethe organic solar cells [10, 11]. Organic materials have beentaken into account as possible candidates in replacement ofsilicon due to the discovery of organic molecules and polymershaving both conducting and semiconductor properties[9]. Polymer conductivity is due to conjugation, thatis the alternation of single and double bonds between thecarbon atoms [12]. Every bond contains a localised σ bondwhich forms a strong chemical bond and every doublebond contains a less strongly localised and weaker π bond.In these conditions two delocalized energy bands are formed,the bonding π and the antibonding π ∗ orbitals, also calledthe highest occupied molecular orbital (HOMO) and thelowest unoccupied molecular orbital (LUMO), respectively.HOMO and LUMO are separated by a bandgap (typically1
2 introduction1-3 eV) and the transition between these two levels can beexcited by light in the visible spectrum [13]. These propertiesmake conjugated organics very interesting for photovoltaicapplications.Furthermore organic semiconductors, having very highabsorption coefficients and quite good charge carrier mobility(0.1 cm 2 V −1 s −1 for the P3HT polymer [14]) allowthe use of very thin films but still absorbing a sufficientportion of the solar spectrum [9]. The reduction in materialused, the low cost manufacturing techniques and thepossibility to produce devices using solution phase methods,such as ink jet printing or various roll to roll techniques[15, 16], make organic materials very attractive tothe photovoltaic market [9] . Moreover their properties anddesigns can be finely tuned and optimized based on materialsversatility, solution-based processing, and mechanicalflexibility [17].Bilayer solar cells are composed by two layers of materials;the one with higher electron affinity and ionization potentialhas the role of electron acceptor, while the other materialis the electron donor and acts also as light absorber(see Figure 1.1). An important example of electron acceptormaterial is the buckminsterfullerene (C 60 ) [18], while thesemiconductor polymer most used as donor is the poly(3-hexylthiophene) (P3HT). The device is completed by twoelectrodes, a semi-transparent anode (e.g. the indium-tinoxide,ITO) and a metallic cathod having a low work functionvalue (e.g. aluminum, lithium) [19]. Special contact layershave been developed to obtain better performance, inparticular the PEDOT:PSS polymer [20] has shown good resultsused as anode due to its high transparency in the visiblerange, high mechanical flexibility, and excellent thermalstability [21].Unlike the silicon case, where the light absorption resultsin the formation of free electrons and holes, in organicsystems electrons are promoted from the HOMOto the LUMO, resulting in the formation of excitons composedby a hole and an electron strongly bound together(usually with a binding energy between 0.5 and 1 eV [13]);a large potential gradient is then necessary to drive thecharge carriers away from the dissociating interface [22],resulting in a lower efficiency of the system. For an ef-
Page 1 and 2: Università degli Studi di Cagliari
Page 4: A C K N O W L E D G M E N T SI woul
Page 7 and 8: example of a novel class of systems
Page 10 and 11: C O N T E N T S1 introduction 11.1
Page 12 and 13: L I S T O F F I G U R E SFigure 1.1
Page 14 and 15: List of FiguresxiiiFigure 2.15 Init
Page 16 and 17: List of FiguresxvFigure 4.6Figure 4
Page 18: Figure 5.7Figure 5.8Figure A.1Figur
Page 23 and 24: 4 introductionorder in such a compl
Page 25 and 26: 6 introductionprocesses that need t
Page 27 and 28: 8 introduction1.2 physical factors
Page 29 and 30: 10 introductionrecovered by the int
Page 31 and 32: 12 introductionA cheap and environm
Page 33 and 34: 14 introductiondensed phases [63, 7
Page 36 and 37: P 3 H T - P O LY ( 3 - H E X Y LT H
Page 38 and 39: 2.1 mechanism of assembling and mor
Page 40 and 41: 2.2 p3ht assembling and intermolecu
Page 42 and 43: 2.3 p3ht crystalline bulk phases 23
Page 44 and 45: 2.5 nanocrystalline p3ht 25Figure 2
Page 46 and 47: 2.5 nanocrystalline p3ht 27Table 2.
Page 48 and 49: 2.6 conclusions 29Figure 2.13.: Ini
Page 50 and 51: P O LY M E R / S E M I C O N D U C
Page 52 and 53: 3.3 adhesion of a single p3ht molec
Page 54 and 55: 3.4 p3ht/zno interface 35surface (t
Page 56 and 57: 3.5 p3ht/zno interface: low deposit
Page 58 and 59: 3.6 p3ht/zno interface: high deposi
Page 64 and 65: 3.7 effective model for the transpo
Page 66 and 67: 3.8 conclusions 47As for the low te
Page 68 and 69: T E R N A RY Z N O / Z N P C / P 3
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4.1 self assembling of znpcs on zno
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4.2 polymer interaction with znpcs
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4.3 electronic and optical properti
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4.3 electronic and optical properti
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4.4 conclusions 59ties. The absorpt
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I N T E R A C T I O N B E T W E E N
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5.2 solvent thf interaction with zn
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5.3 conclusions 69γ L/L ∼ 0.112
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C O N C L U S I O N SIn this thesis
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M O L E C U L A R D Y N A M I C SAa
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A.1 molecular dynamics 75a.1.2The t
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A.2 the force field 77Finally, τ t
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A.2 the force field 79Waals interac
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A.3 methods 81tional Theory (DFT) l
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B I B L I O G R A P H Y[1] http://w
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ibliography 85[17] J. D. Servaites,
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ibliography 87Zakeeruddin, and M. G
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ibliography 89Phys. Chem. Chem. Phy
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ibliography 91[65] D. J. Binks and
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ibliography 93cells,” Energy Envi
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ibliography 95[97] C. Ingrosso, A.
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ibliography 97298.15 k,” Journal
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ibliography 99charges. am1-bcc mode
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colophonThis document was typeset u
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