Tailoring the CdS/CdSe/CdTe multilayer structure for optimization of photovoltaic device performance guided by mapping spectroscopic ellipsometry

Abstract

Thin film CdTe superstrate solar cells have been fabricated by sputtering starting from CdS/CdSe front layers deposited on transparent conductor coated glass. The performance of such devices is sensitive to the fabrication details including the temperature-time profile, which leads to CdSe/CdTe interdiffusion and formation of a CdTe1-xSex bandgap-graded absorber. Mapping spectroscopic ellipsometry (M-SE) has been applied to the CdS and CdSe thin films for process calibration, which involves determining the deposition rate in terms of effective thickness (volume/area) versus spatial position on the sample. The goal is to optimize the performance of the devices by correlating cell parameters with these two effective thicknesses. Intended variations in the thicknesses along with unintended spatial non-uniformities enable coarse and fine-scale optimization, respectively. Using these methods, the highest performance solar cells from the CdS/CdSe/CdTe structure are obtained with 13 nm CdS and 100 nm CdSe. An increase in the CdS thickness above 13 nm leads to a decrease in open-circuit voltage and fill-factor attributed to the formation of a CdSe1-zSz interdiffusion region with z approaching 0.5, where the alloy electronic properties are likely to suffer. Our results demonstrate that M-SE, exploited in conjunction with deposition non-uniformities, serve as a viable approach for process optimization of complex solar cell structures.