Enhancing the efficiency of CZTSSe solar cells via binary solvent induced microstructure regulation

A binary solvent system of dimethyl sulfoxide (DMSO) and isopropanol (IPA) has been developed for preparing high-crystalline kesterite Cu2ZnSn(S,Se)4 (CZTSSe) films. The absorbers made with the single DMSO solvent exhibit a granular microstructure, which can be altered by adding IPA to the solvent. Addition of IPA not only elevates wettability of the solution, but also enhances its volatility, thus enabling the regulation of the formation of cracks in the precursor thin film. Appropriate cracks allow Se vapor to penetrate the precursor film which promotes the grain growth during the selenization process. Hence, the incorporation of IPA into the solvent impacts the crystallinity of the absorber. Large grains can be obtained when the volume ratio of DMSO to IPA is 4 : 1. Both the open-circuit voltage (VOC) and fill factor (FF) are dramatically improved by around 24% and 47%, respectively, with the addition of 20% IPA. As a result, the efficiency of the CZTSSe solar cell has increased from 3.33% to 7.18%.