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ABSTRACT: Cu(In,Ga)Se2 and CdTe heterojunction solar cells grown on rigid (glass) or flexible foil substrates require p-type absorber layers of optimum optoelectronic properties and n-type wide-bandgap partner layers to form the p–n junction. Transparent conducting oxide and specific metal layers are used for front and back electrical contacts. Efficiencies of solar cells depend on various deposition methods as they control the optoelectronic properties of the layers and interfaces. Certain treatments, such as addition of Na in Cu(In,Ga)Se2 and CdCl2 treatment of CdTe have a direct influence on the electronic properties of the absorber layers and efficiency of solar cells. Processes for the development of superstrate and substrate solar cells are reviewed. Copyright © 2004 John Wiley & Sons, Ltd.
Progress in Photovoltaics Research and Applications 02/2004; 12(2‐3):93 - 111. · 5.79 Impact Factor
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ABSTRACT: Polycrystalline thin-film CdTe/CdS solar cells have been developed in a configuration in which a transparent conducting layer of indium tin oxide (ITO) has been used for the first time as a back electrical contact on p-CdTe. Solar cells of 7·9% efficiency were developed on SnOx:F-coated glass substrates with a low-temperature (<450°C) high-vacuum evaporation method. After the CdCl2 annealing treatment of the CdTe/CdS stack, a bromine methanol solution was used for etching the CdTe surface prior to the ITO deposition. The unique features of this solar cell with both front and back contacts being transparent and conducting are that the cell can be illuminated from either or both sides simultaneously like a ‘bi-facial’ cell, and it can be used in tandem solar cells. The solar cells with transparent conducting oxide back contact show long-term stable performance under accelerated test conditions. Copyright © 2004 John Wiley & Sons, Ltd.
Progress in Photovoltaics Research and Applications 12/2003; 12(1):33 - 38. · 5.79 Impact Factor
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17th European Photovoltaic Solar Energy Conference and Exhibition, Munich, Germany; 10/2002
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Proceedings of PV in Europe from PV Technology to Energy Solutions, Rome, Italy; 10/2002
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17th European Photovoltaic Solar Energy Conference and Exhibition, Munich, Germany; 10/2001
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17th European Photovoltaic Solar Energy Conference and Exhibition, Munich, Germany; 10/2001
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ABSTRACT: The measurement of quantum efficiency with bias voltage is a powerful tool to characterize CdTe/CdS solar cell. As the quantum efficiency changes drastically with bias it will be referred to as Apparent Quantum Efficiency AQE. The AQE gives insight to the spectral contents of the cell current and therefore resolves the spatial carrier collection in the cell at each working point. So it is possible to understand the influence of the junctions and changing resistances in the cell. The photoconductivity of CdS facilitates AQE well above unity, i.e. up to 100, at high forward bias. The spectral sensitivity of the CdS photoconductivity affects the cell current strongly. This can explain the dependence of fill factor and roll-over of the I-V characteristics on the spectral content of illumination. Further more back contact junction influence and defect related features, such as sub band gap generation, are evident in the AQE for high forward bias.
MRS Proceedings. 12/2000; 668.
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16th European Photovoltaic Solar Energy Conference and Exhibition, Glasgow, Scotland; 05/2000
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16th European Photovoltaic Solar Energy Conference and Exhibition, Glasgow, Scotland; 05/2000
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ABSTRACT: Recent developments in the technology of high vacuum evaporated CdTe solar cells are reviewed. High-efficiency solar cells of efficiencies up to 12.5% have been developed on soda-lime glass substrates with a low-temperature (<450 °C) process. This simple process is suitable for in-line production of large-area solar modules on glass as well as on flexible polymer films with a roll-to-roll deposition process. Flexible and lightweight CdTe solar cells with a record efficiency of 11.4% have been developed in a superstrate configuration, and 3.5% efficiency mini-modules have been realised in a preliminary development. Deposition of high-temperature stable ITO front contact layer on polyimide is important for high-efficiency cells, as the layer should withstand processing steps maintaining its high electrical conductivity and optical transparency. Another development is an application of a transparent conducting oxide (TCO) ITO as a back electrical contact on CdTe leading to first bifacial CdTe solar cells, which can be illuminated from either or both sides. Accelerated long-term stability tests show that light soaking improves the efficiency of CdTe solar cells with ITO back contacts and performance does not degrade.Stability of CdTe solar cells has been measured after irradiation with high-energy protons and electrons of different fluences. These solar cells exhibit superior radiation tolerance compared to conventional Si and GaAs solar cells for space applications. Because of extreme stability, and high specific power (kW/kg) of flexible solar cells, CdTe has a promising potential for space applications.
Solar Energy Materials and Solar Cells.
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ABSTRACT: The stability of CdTe/CdS solar cells depends on spatial changes of defects and impurities throughout the cell. Degradation effects are often associated with metal diffusion from the back contact of the cell, which is Cu in most cases. However, cells with stable back contact can also exhibit instability, as also all the other cell layers are potential sources of impurities causing instability. Cell degradation due to generation of defects from external influences like particle irradiation, e.g. in space, is another reason for instability. The development of stable back contacts as well as sources of instability in the cell performance are discussed with a special focus on the CdS layer and CdTe/CdS interface, which are very sensitive to the accumulation of impurities and defects. A non-destructive method to assess the stability issue is described. An analysis of performance stability with respect to defect generation caused by high-energy protons and electrons is presented. Additionally, effects of meta-stability and the capability to recover from degradation by defect relaxation are shown.
Thin Solid Films.
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ABSTRACT: Polycrystalline thin film solar cells of II-VI compound semiconductors are important because of their low cost, high efficiency and stable performance. Flexible and lightweight solar cells are interesting for a variety of terrestrial and space applications that require a very high specific power (ratio of output electrical power to the solar module weight). Moreover, light modules are advantageous in terms of transport and mounting. We have previously described the development of flexible CdTe/CdS solar cells on polyimide substrates with a novel method; efficiencies of 8.6 % were reported. Further improvements in the processing have now increased the solar cell efficiency to 11%. The CdTe solar cell is fabricated in a "superstrate" configuration where the light passes through the polyimide substrate. Absorption of the incident light and possible degradation of polyimide under UV and particle irradiation are the major limitations of this configuration. To overcome these a new approach is introduced here. The CdTe/CdS/TCO stacks are grown on a NaCl/glass substrate. After the solar cell has been processed, a polymer layer is spin coated on top and the flexible solar cells are detached from the glass by dissolving the NaCl buffer layer. Solar cells of 7.3% efficiency have been obtained.
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ABSTRACT: Conventional back contacts on CdTe/CdS solar cells are commonly made with Cu/Au or Cu/graphite. Often the contact limit the solar cells efficiency and the performance degrades because of Cu diffusion to the junction. In order to get stable and "non-rectifying" back contacts Sb has been applied. Pre-depo-sition etching treatments, post-deposition annealing, influence of Sb layer thickness and stability issues have been studied. Different etchants not only clean the surface but they also produce a conducting Te layer on the grain boundaries. Using a mixture of nitric and phosphoric acid and Sb/Au as a back con-tact, 12.5% efficiency cells are obtained. The stability of solar cells depends on the etching solution. Stability tests under continues 1 sun illumination suggest that under optimum condition stable cells with Sb/Au contact can be developed. A comparative analysis of the photovoltaic properties of solar cells with different back contacts will be presented.
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ABSTRACT: CdTe/CdS solar cells of 10% efficiency, developed with a vacuum deposition method were irradiated with high-energy protons of different fluences. The V oc and f.f. of irradiated cells increase or decrease depending on the fluence. The normal soda lime glass substrate darkens under the irradiation; therefore low Isc is measured. Measurements suggest that CdTe solar cells are highly stable under proton flux. Flexible and lightweight solar cells were developed in a superstrate configuration on polymer substrates. 8.6 % efficiency cells with Voc770 mV and Isc of 20.3 mA/cm 2 were achieved.
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ABSTRACT: To make CdTe/CdS solar cells highly efficient generally a Cu containing back contact is used which creates a quasi Ohmic, non-rectifying contact and additionally dopes the CdTe layer. These cells exhibit good efficiencies in the beginning, however they degrade with time due to Cu diffusion to the front contact which causes shunting. This is shown with secondary ion mass spectroscopy (SIMS) depth profiling. To get stable but still highly efficient CdTe/CdS cells different back contact materi-als and etching treatments were investigated. It was found that a chemical etching process prior to the deposition of back contact creates a back surface field (BSF) due to a p + -doped Te-rich surface of CdTe. To overcome the naturally existing Schottky barrier between p-CdTe and any metal a thin buffer layer was evaporated prior to the final metallization. Amongst many investigated back contact materials the most suitable are Sb or Sb 2 Te 3 as a buffer and Mo for the metallization. Cells with these combinations showed high stability under accelerated stability tests corresponding to 70 years stability in the field. IV-and CV-characteristics indicate that an increase in efficiency of these cells is still possible and is not limited by the back contact materials.
