Band-gap tuning of Cu2ZnSn(S,Se)4 solar cell absorbers via defined incorporation of sulphur based on a post-sulphurization process

We demonstrate that based on a post-sulphurization approach the mean sulphur concentration and bandgap of Cu2ZnSn(S,Se)4 (CZTSSe) solar cell absorber layers can be precisely controlled. However, X-ray diffraction and secondary ion mass spectrometry studies reveal an inhomogeneous sulphur incorporation at lower process temperatures resulting in a sulphur gradient in the absorber. Optimization of such gradients may enable further performance improvement of future devices. During the post-sulphurization process sulphur seems to accumulate at the back of the absorber first where there is already an enrichment of sulphur in the purely selenized absorbers. With higher temperatures more sulphur gets incorporated and the sulphur distribution gets evened out. Morphology studies show that the process of exchanging the chalcogenides leaves behind holes and results in a porous absorber layer. With higher process temperatures and therefore higher sulphur content the absorber gets more dense. The device performance of the post-sulphurized samples increases significantly with increasing process temperature and increasing sulphur content.