Thin oxide buffer layers for avoiding leaks in CIGS solar cells; a theoretical analysis

The purpose of this research is the performance improvement of the CIGS/CdS/ZnO solar cells, while the CdS buffer layer is too thin. Enhancement of photocurrent by decreasing the thickness of CdS buffer layer is expected from a reduced parasitic absorption at short wavelengths. However, the formation of pinholes due to the too-thin CdS buffer layer and a non-uniform coverage of the CIGS surface degrades the solar cell performance by reducing fill factor (FF) and open-circuit voltage (V_oc). This degradation is because the direct contact of ZnO and CIGS could exist in pinholes, and the formation of the cliff-like band alignment at CIGS/ZnO interface increases the recombination rate. In this work, to eliminate this destructive effect in the CIGS solar cells with a thin CdS layer, ZnO window layer has been replaced by a suitable thin oxide layer as an intermediate buffer layer. By the proposed oxides, the band alignment between the CIGS layer and the adjacent layer is optimized, while pinholes occurred. It is found that the proposed buffer-less CIGS solar cell leading to the efficiency improvement from 17.6% to 18.8%.