Improvement of red light response of Cu2Sn1−xGexS3 solar cells by optimization of CdS buffer layers

A cross-over anomaly has been observed in Cu2Sn1−xGexS3 (CTGS) solar cells, which degrades the solar cell performance under red light illumination. This is a critical drawback for bottom cells because it should work under red light illumination, filtered through top cells. We suppressed this cross-over anomaly by optimizing the deposition conditions of CdS buffer layers, which improved the performance of the cells under red light illumination. The mechanism of this phenomenon was discussed, and we concluded the origin of the anomalies was attributed to accepter-like defects in CdS buffer layers. Furthermore, we also inferred a model for the conduction band offset of the CTGS/CdS interface, which will aid the design of high efficiency solar cells.