Effect of the chemical composition of co-sputtered Zn(O,S) buffer layers on Cu(In,Ga)Se2 solar cell performance

Abstract

High-efficiency Cu(In,Ga)Se2 (CIGSe) solar cells generally require a CdS buffer layer formed by chemical bath deposition (CBD). However, the incompatibility of this process with in-line vacuum-based production methods is a matter of concern. In this contribution, ZnO1−xSx thin films synthesized by RF co-sputtering of ZnO and ZnS targets in a confocal configuration are studied as a buffer layer in CIGSe solar cells. It was found that the sulfur content x can be easily tuned by controlling the power applied to the ZnO and ZnS targets. The band structure of the Zn(O,S) compound as a function of the sulfur content was measured by combining X-ray photoelectron spectroscopy and optical analysis techniques and found to follow the trends described in the literature. CIGSe solar cells using sputtered Zn(O,S) buffer layers with different sulfur contents were fabricated. The variation of the electrical parameters of the devices as a function the x ratio was attributed to the evolution of the conduction-band offset (CBO) at the CIGSe/ZnO1−xSx interface, as confirmed by means of SCAPS numerical simulations. However, even with an optimized band alignment between the two materials, relatively low Voc values were obtained, likely due to defect assisted interface recombination at the buffer/window layer interface.