Fabrication and investigation of extremely thin CdTe absorber layer solar cells

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2016
Hosseini, Arezoo
Extremely thin absorber layer (ETA) solar cells aim to combine the advantages of using very thin and cheaply produced absorber layer on nano structured substrates with stability of all-solid-state solar cells. This type of photovoltaic devices use a nano structured interpenetrating heterojunction of thin light-absorbing layer at the interface between an n- and p-type semiconductors. N-type nano structured TiO2 layer is deposited on a Transparent Conducting Oxide coated glass (TCO) substrate, following with the CdTe thin absorber layer deposition to cover the structured wide band gap semiconductor. Finally a void-filling ZnSnTe, optically transparent semiconductor, is deposited onto the absorber layer. This structure is followed by a metallic layer deposition to fulfill the ETA solar cell structure. These types of solar cells with nanostructured n-type material and thin CdTe absorber layer have ahigher absorption rate of light as compared to flat ones. The thickness of the absorber layer is an important parameter for both device operation and the ultimate cost of energy production of these structures. The aim of this thesis study is to investigate the fabrication and performance of different solar cell structures with extremely thin CdTe absorber layer. In this study, a variety of nano structured TiO2 layers were produced by different techniques and characterized as the n-type material for both two- and threecomponent ETA solar cells. The spin coated nano-porous, hydrothermally grown nanowire and electrochemically anodized nanotube TiO2 layers were used as the substrate for sputtered and electrodeposited CdTe layer. It was observed that the sputtered TiO2 layer should be present prior to the nanoporous TiO2 layer formation in order to prevent the shorting between FTO and CdTe layers. Also, ZnTe layer was investigated as the p-type layer for three-component ETA solar cells but due to the high resistance value of this material, ZnTe was substituted with low resistance ZnSnTe layer. Various novel two- and three-component ETA solar cells based on different nano structured TiO2withCdTe and ZnSnTe thin film layer deposited by sputtering and electrodeposition methods, were fabricated in the laboratory scale and characterized. The best efficiency and the device parameters were obtained for the two-component Glass/FTO/NWAsTiO2/Electrodeposited CdTe/Cu-Au structure. Both CdCl2surface treatment and post annealing of the CdTe surface, which were applied prior to the formation of the complete solar cell structure, are found to be essential for the efficiency improvement. The use of nano structured TiO2is also an important factor, which improves the light trapping in this structure. Also the use of electrodeposited CdTe absorber layer, which fills the pores of nano structured TiO2 substrates,improves the efficiency when comparedto the solar cell structure with the sputtered CdTe thin film layer.

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Citation Formats
A. Hosseini, “Fabrication and investigation of extremely thin CdTe absorber layer solar cells,” Ph.D. - Doctoral Program, Middle East Technical University, 2016.