Regulation of selenium composition by supercritical carbon dioxide for CZTSSe solar cells efficiency improvement

Se-rich selenization offers remarkable benefits for grain growth of Cu2ZnSn(S,Se)4 (CZTSSe) absorber layers, which is critical for solar cells fabrication. However, low-activity selenium often forms Se-clusters and trapped in CZTSSe absorber layers. These trapped Se-clusters increase carrier recombination in CZTSSe absorber layers, thus decreasing efficiency of CZTSSe solar cells. Herein, supercritical carbon dioxide (SCCO2) treatment was demonstrated to regulate selenium composition of CZTSSe absorber layers for solar cells efficiency improvement. Compared with untreated CZTSSe absorber layers, selenium composition of CZTSSe absorber layers were significantly reduced by SCCO2 treatment. The open circuit voltage (VOC) and fill factor (FF) of CZTSSe solar cells improved pronouncedly due to carrier recombination reduction and carrier transport enhancement. As a consequence, the power conversion efficiency (PCE) was increased from 9.26 % (ref device) to 11.43 % (SCCO2 device). This unique SCCO2 treatment may offer new ideas for other selenization-based optoelectronic devices performance improvement. In this work, supercritical carbon dioxide (SCCO2) treatment, a novel strategy was used to regulate the selenium composition of CZTSSe absorber layers for CZTSSe solar cells efficiency improvement. Benefiting from reducing of Se-clusters trapped in CZTSSe absorber layers, SCCO2 treatment gains the ability of improving carriers transport and collection in final CZTSSe solar cells. Finally, a champion device with 11.43 % PCEs was achieved, corresponding more than 23 % improvement compared with ref device. [Display omitted] •Low-activity selenium often forms Se-clusters and be trapped in absorber layers.•The trapped Se-clusters will increase carrier recombination in absorber layers.•Selenium composition in absorber layers can be regulated by supercritical carbon dioxide.•Extract of Se-clusters from absorber layers is beneficial for carrier collection.•The proposed strategy may be used for other Se-based photoelectric devices.