Interdiffusion of CdS and $Zn2SnO4$ layers and its application in CdS/CdTe polycrystalline thin-film solar cells

In this work, we found that the interdiffusion of the CdS and $Zn2SnO4$ (ZTO) layers can occur either at high temperature $(550–650 °C)$ in Ar or at lower temperature $(400–420 °C)$ in a $CdCl2$ atmosphere. By integrating a $Zn2SnO4$ film into a CdS/CdTe solar cell as a buffer layer, this interdiffusion feature can solve several critical issues and improve device performance and reproducibility of both $SnO2-based$ and $Cd2SnO4-based$ CdTe cells. Interdiffusion consumes the CdS film from both the ZTO and CdTe sides during the device fabrication process and improves quantum efficiency at short wavelengths. The ZTO film acts as a Zn source to alloy with the CdS film, which results in increases in the band gap of the window layer and in short-circuit current density $Jsc.$ Interdiffusion can also significantly improve device adhesion after $CdCl2$ treatment, thus providing much greater process latitude when optimizing the $CdCl2$ process step. The optimum $CdCl2-treated$ CdTe device has high quantum efficiency at long wavelength, because of its good junction properties and well-passivated CdTe film. We have fabricated a $Cd2SnO4/Zn2SnO4/CdS/CdTe$ cell demonstrating an NREL-confirmed total-area efficiency of 15.8% $(Voc=844.3 mV,$ $Jsc=25.00 mA/cm2,$ and fill $factor=74.82%).$ This high-performance cell is one of the best thin-film CdTe solar cells in the world.