Impact of Window Layers on Selenium Distribution and Photovoltaic Performance in CdSe_xTe_1−x/CdTe Solar Cells

The incorporation of the Se element in CdTe solar cells is critical, while the low bandgap CdSexTe1−x, formed by the interdiffusion of CdTe and CdSe during device preparation, can promote the carrier lifetime. Different window layers formed by CdSe w/o MZO or CdS have different Se distributions. This paper systematically evaluates the influence of four types of window layers (CdSe, CdS/CdSe, MZO/CdSe and MZO/CdS/CdSe) on the performance of CdTe solar cells, and focuses on the correlation between the window layers and the Se distribution characteristic, carrier recombination mechanism, and device efficiency. The results show that CdSe and MZO/CdS/CdSe window layer devices achieve Eff of 15.21% and 14.40%, respectively. The CdSe and MZO/CdS/CdSe devices exhibit relatively high Ea of 1.41 eV and 1.39 eV from J-V-T measurements, coupled with high Rrec of 9458 Ω and 8293 Ω, respectively. This indicates suppressed recombination, suggesting that non-radiative recombination is reduced. In contrast, the CdS/CdSe and MZO/CdSe devices show lower performance. Their extrapolated Ea values are 1.25 eV and 1.31 eV, with correspondingly lower Rrec values of 2207 Ω and 3304 Ω. These results point to faster recombination rates and an increased proportion of non-radiative recombination, consistent with their suboptimal Eff. Detailed analysis of Se distribution reveals that S-Se interdiffusion in S-containing devices results in the x-value decrease (highest value of Se content in CdSexTe1−x), thereby suppressing the long-wavelength expansion. In devices of S-free window layers, the formed CdSexTe1−x alloy has a relatively higher x-value, allowing the long-wavelength response to extend beyond 850 nm. It can be concluded that CdSe is the optimal window layer for CdTe Solar Cells, which can both form a desirable Se distribution and a good junction with less interface recombination.