Tailored Band Alignment for Improved Carrier Transport in Composition-Controlled Sb₂(S,Se)₃

Sb₂(S,Se)₃ is a highly available energy material with a tunable bandgap by adjusting the S/Se ratio. Increasing the Se ratio can enhance the efficiency of Sb₂(S,Se)₃ solar cells, with a higher short-circuit current (J_SC). However, the accompanying decrease in the open-circuit voltage (V_OC) restricts further improvement. The defect passivation is important, since it can reduce carrier recombination, enhancing the V_OC. In this study, the relevance of the S/Se ratio, defect concentration, and V_OC was investigated. The samples with or without the deposition of Se-rich Sb₂(S,Se)₃ onto S-rich Sb₂(S,Se)₃ were used for defect characterization. Different surface compositions were confirmed by Raman spectroscopy. The complicated subdefect states of S-rich Sb₂(S,Se)₃ were shown through photoluminescence and conductive atomic force microscopy, and a decrease in the defect concentration was observed through surface photovoltage. The improvement of J_SC via bandgap grading and the simultaneous V_OC improvement by defect passivation resulted in efficient cell performance.