Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE
Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE
7/9 Conclusion – sodium incorporation Sodium incorporation into CIGS absorbers is beneficial for the performance of CIGS solar cells. The deposition of different layers thicknesses of sodium via post deposition treatment revealed an optimum of thickness a 20 nm for the low temperature growth process. It can be concluded, that the required sodium depends on the processing speed and the applied substrate temperature during the CIGS growth process. Based on the findings of an optimum Na-dosage, sodium was co-evaporated during the CIGS growth. With this technique, the low temperature growth process yielded efficiencies of approx. 12,5%, which are comparable or even higher than those gain with PDT. This result is important, as it opens the applicability of the sodium incorporation method in a in-line CIGS process. Investigations of the cell microstructure reveal a reduced grain size near the cell back contact, if sodium is present during the growth process (co-evaporation). It is assumed, that a Ga-rich structures forms within this absorber region. Further analyses, like SIMS and Raman spectroscopy are planed in cooperation with other project partners in order to determine composition gradients along the absorber thickness. 2) Modified CIGS absorber for buffer-free cells Introduction It is commonly accepted, that a buffer between the CIGS absorber and the transparent conducting front contact (TCO) is needed for the following reasons: � Improvement of electronic interface properties due to better band alignment between window front contact and absorber layer � Protection of CIGS absorber from sputter damage during window layer deposition � Doping of absorber to improve the homo-junction properties Thus, in most laboratories, the standard device structure of Cu(In,Ga)Se2 (CIGS)-based solar cells includes a very thin CdS buffer layer. In view of an industrial production process the non-vacuum chemical bath deposition process of this buffer involves technological problems. But also for ecological reasons efforts are undertaken to substitute the CdS buffer layer. Alternative buffer layers are e.g. indium-sulfide, zinc-sulfide and magnesium-oxide, which can be deposited via vacuum or non-vacuum techniques. As these buffer layers require additional process steps and equipment, a different approach could be, to modify the CIGS surface in such a way, that the buffer layer could be omitted. Even though this way is accompanied by significant cost reduction potential, it is desired to achieve efficiencies comparable to CIGS cells with other alternative buffer layers. Experimental At ETH Zurich state-of-the-art CIGS solar cells are grown on 1 mm thick soda-lime glass. A 1 µm thick Molybdenum back contact is deposited by DC sputtering. The ~2 µm thick CIGS absorber is grown by elemental co-evaporation in a high-vacuum chamber using a three-stage process. Our standard buffer layer is a 50 µm CdS layer deposited by chemical bath deposition. As front contact a bi-layer of 50 nm i-ZnO and 250 nm ZnO:Al is deposited by RF sputtering. The cells are finished by e-beam evaporated nickel and aluminium grids and mechanical scribing. Efficiencies generally yield 12% to 17%, depending on the substrate and processing temperature. In order to avoid the additional buffer layer deposition the finishing of the CIGS deposition process has been modified in this work. After a standard three-stage process the samples were cooled down to 200°C and a thin layer of i) InxSey, ii) InxGa1-xSey and iii) GaxSey was evaporated onto the CIGS. A reference sample was prepared without this additional step. Results Figure 2.1 shows IV parameters of solar cells, processed as described above. High open circuit voltage almost comparable to standard cells with CdS buffer layers can be reached with a 2:1 ratio of In:Ga for the CIGS finishing (sample 2 in figure 2.1). However, the short-circuit current strongly decreases with increasing gallium content and reaches almost zero for pure GaxSey. Due to very poor fill factors of around 50%, efficiencies of buffer free cells did not exceed 5% in these experiments. LARCIS, A. N. Tiwari, ETH Zurich 101/290
In a second experiment the amount of InxSey evaporated at the end of CIGS deposition was increased to 10 nm. With this thicker layer a significant improvement of solar cell performance is achieved, compared to the experiments with the 5nm thick layer. This is shown in figure 2.2, which contains I-V characteristics of cells with InxSey and In2S3 layer. In2S3 is a promising alternative buffer layer investigated by several groups, in order to replace the cadmium sulfide buffer layer used in standard devices. For these experiments In2S3 was deposited by ultrasonic spray pyrolysis (USP). Table 2.1 shows a comparison of the photovoltaic parameters of both solar cells. It is shown there that cells with InxSey layer achieve comparable properties to cells with other alternative buffer layer. Further experiments have to follow in order to optimize this simple deposition technique. Further improvements can be expected by varying the layer thickness, deposition temperature and gallium content. Fig. 2.1) Current-Voltage characteristics of buffer-free solar cells. Cell Parameters are shown as function of different In/Ga ratios. The sample notation is: 0) standard process, 1) 5 nm InxSey, 2) 5 nm (In2,Ga1)Sey, 3) 5 nm (In1,Ga2)Sey, 4) 5 nm GaxSey. Fig. 2.2) IV-curves of solar cells with a 10 nm thick InxSey finishing (solid line) compared to an alternative In2S3 buffer layer (dashed-dotted), deposited by ultrasonic spray pyrolysis (USP). Both cells included an antireflection coating. LARCIS, A. N. Tiwari, ETH Zurich 102/290 8/9
- Page 54 and 55: Eidgenössisches Departement für U
- Page 56: 3/3 on the front side. All the laye
- Page 59 and 60: Introduction / Project Goals Prior
- Page 62 and 63: Eidgenössisches Departement für U
- Page 64 and 65: 3/4 Effective light trapping scheme
- Page 66 and 67: Département fédéral de l’envir
- Page 68 and 69: 3/6 The Athlet consortium comprises
- Page 70 and 71: 5/6 Large area thin-film silicon ce
- Page 72 and 73: Eidgenössisches Departement für U
- Page 74 and 75: 3/4 Der verbesserte Lichteinfang is
- Page 76 and 77: Eidgenössisches Departement für U
- Page 78 and 79: 3/5 Results Microstructure characte
- Page 80: 5/5 Mechanical testing: The mechani
- Page 83 and 84: Introduction / Project goals The fo
- Page 85 and 86: Optimization of CdS chemical bath d
- Page 87 and 88: Fig. 5: J-V curve (left) and extern
- Page 90 and 91: Eidgenössisches Departement für U
- Page 92 and 93: 3/7 Work performed and results achi
- Page 94 and 95: 5/7 Fig. 3: Raman spectra recorded
- Page 96: 7/7 Measurements of solar cells per
- Page 99 and 100: Introduction / project objectives T
- Page 101 and 102: Optimum Na dosage Sodium layer with
- Page 103: application might become more impor
- Page 108 and 109: Eidgenössisches Departement für U
- Page 110 and 111: 3/9 After optimizing the In2S3 buff
- Page 112 and 113: 5/9 Indium sulfide layer characteri
- Page 114 and 115: 7/9 3) Semitransparent CIGS solar c
- Page 116: 9/9 Steps towards multi-junction so
- Page 119 and 120: Introduction / project objectives T
- Page 121 and 122: Figure 2: SEM picture of laser-abla
- Page 123 and 124: Introduction / project objectives T
- Page 125 and 126: Work progress and achievements duri
- Page 127 and 128: Furthermore, with a good reproducib
- Page 129 and 130: Main achievements - The association
- Page 131 and 132: Introduction Dye-sensitized solar c
- Page 134 and 135: Eidgenössisches Departement für U
- Page 136 and 137: 3/4 So far, experiments were perfor
- Page 138 and 139: Eidgenössisches Departement für U
- Page 140 and 141: 3/5 Tasks of the collaboration part
- Page 142: 5/5 Figure 4: Maximum achievable sh
- Page 145 and 146: Project Goals The goal is the estab
- Page 147 and 148: Evaluation 2008 and Outlook 2009 L'
- Page 149 and 150: Project Goals NAPOLYDE industrials
- Page 151 and 152: � Organic large solar panel and
- Page 153 and 154: Materials such as the conductive ca
In a second experiment the amount of InxSey evaporated at the end of CIGS deposition was increased<br />
to 10 nm. With this thicker layer a significant improvement of solar cell performance is achieved,<br />
compared to the experiments with the 5nm thick layer. This is shown in figure 2.2, which contains I-V<br />
characteristics of cells with InxSey and In2S3 layer. In2S3 is a promising alternative buffer layer<br />
investigated by several groups, in order to replace the cadmium sulfide buffer layer used in standard<br />
devices. For these experiments In2S3 was deposited by ultrasonic spray pyrolysis (USP).<br />
Table 2.1 shows a comparison of the photovoltaic parameters of both solar cells. It is shown there that<br />
cells with InxSey layer achieve comparable properties to cells with other alternative buffer layer.<br />
Further experiments have to follow in order to optimize this simple deposition technique. Further<br />
improvements can be expected by varying the layer thickness, deposition temperature and gallium<br />
content.<br />
Fig. 2.1) Current-Voltage characteristics of buffer-free solar cells. Cell Parameters are shown as<br />
function of different In/Ga ratios. The sample notation is:<br />
0) standard process, 1) 5 nm InxSey, 2) 5 nm (In2,Ga1)Sey, 3) 5 nm (In1,Ga2)Sey, 4) 5 nm GaxSey.<br />
Fig. 2.2) IV-curves of solar cells with a 10 nm thick InxSey finishing (solid line) compared to an<br />
alternative In2S3 buffer layer (dashed-dotted), deposited by ultrasonic spray pyrolysis (USP).<br />
Both cells included an antireflection coating.<br />
LARCIS, A. N. Tiwari, ETH Zurich<br />
102/290<br />
8/9
Change language