We investigated the effect of CdCl2 treatment of CdS films on the photovoltaic performance of polycrystalline CdTe/CdS solar cells. X-ray diffraction studies indicated that the diffusion of S into CdTe is qualitatively the same for CdTe/CdS films fabricated with both as-deposited and CdCl2-treated CdS. A major difference was observed in the extent of Te diffusion into CdS for the two types of CdS films. Full conversion of CdS into CdS1-yTey; was observed for films prepared with asdeposited CdS, while the formation of the ternary phase was below the detection limit for films prepared with CdCl2-treated CdS. Photoluminescence measurements confirmed this result. The difference in interdiffusion leads to differences in optical transmission of CdS films and spectral response of CdTe/CdS solar cells. An increase of 2.7 mA/cm2 in short-circuit current density was observed as a result of improved spectral response in the wavelength range of 500–600 nm for the CdCl2-treated CdS.
Vacuum-evaporated Cu-doped ZnTe films have been studied as the intermediate layer between CdTe and metal contacts in CdTe/CdS thin-film solar cells for the formation of low resistance back contacts. Different metals (Au, Ni, Co) have been investigated as the contact material to the ZnTe layer. The effects of Cu concentration, ZnTe:Cu layer thickness, and ZnTe post-deposition annealing temperature on the cell performances have been investigated. We found that different metal contacts on the ZnTe layer lead to different doping densities in the CdTe layer and different open-circuit photovoltages of the solar cells. The possible formation of a back contact diode at the CdTe/ZnTe interface was explored, based on capacitance-voltage analysis. The series resistance of the CdTe/CdS cells was reduced significantly by the introduction of the ZnTe layer. Fill factors greater than 0.76 and an energy conversion efficiency of 12.9% have been achieved using ZnTe back contacts on electrodeposited CdTe.
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