Band offset at the heterojunction interfaces of CdS/ZnSnP2, ZnS/ZnSnP2, and In2S3/ZnSnP2

Heterojunctions were formed between ZnSnP2 and buffer materials, CdS, ZnS, and In2S3, using chemical bath deposition. The band offset was investigated by X-ray photoelectron spectroscopy based on Kraut method. The conduction band offset, ΔEC, between ZnSnP2 and CdS was estimated to be −1.2 eV, which significantly limits the open circuit voltage, VOC. Conversely, ΔEC at the heterojunction between ZnSnP2 and ZnS was +0.3 eV, which is within the optimal offset range. In the case of In2S3, ΔEC was a relatively small value, −0.2 eV, and In2S3 is potentially useful as a buffer layer in ZnSnP2 solar cells. The J−V characteristics of heterojunction diodes with an Al/sulfides/ZnSnP2 bulk/Mo structure also suggested that ZnS and In2S3 are promising candidates for buffer layers in ZnSnP2 thin film solar cells, and the band alignment is a key factor for the higher efficiency of solar cells with heterojunctions.