Ryosuke Maekawa

Cu2Sn1-xGexS3 (x = 0.17) Thin-Film Solar Cells with High Conversion Efficiency of 6.0%

We have fabricated Cu2Sn1-xGexS3 thin-film solar cells by cosputtering deposition of Cu and Sn followed by sulfurization in S and GeS2 vapors. The conversion efficiency was significantly improved to be as high as 6.0% compared with the values of Cu2SnS3 solar cells similarly fabricated. Scanning electron microscopy observation revealed that alloying with Ge accelerated the grain growth during the sulfurization process, contributing to the improvement in the conversion efficiency. The bandgap energy of Cu2Sn0.83Ge0.17S3 was about 1.0 eV, which is suitable for bottom cells used in double-junction solar cells.

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.