Modulation of Field-Effect Passivation at the Back Electrode Interface Enabling Efficient Kesterite-Type Cu₂ZnSn(S,Se)₄ Thin-Film Solar Cells

Abstract

For further efficiency improvement in kesterite-type Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells, it is essential to address the carrier recombination issue at the back electrode interface (BEI) caused by the undesirable built-in potential orientation toward an absorber as an <i>n</i>-MoSe₂ interfacial layer formed. In this regard, back surface field (BSF) incorporation, i.e., field-effect passivation, shows promise for dealing with this issue due to its positive effect in decreasing recombination at the BEI. In this study, the BSF was realized with the p-type conduction transition in interfacial layer MoSe₂ by incorporating Nb into the back electrode. The BSF width can be tuned via modulating the carrier concentration of the absorber, which has been demonstrated by capacitance-voltage characterization. A beyond 7% efficiency BSF-applied CZTSSe solar cell is prepared, and the effects of a tunable BSF and the mechanism underpinning device performance improvement have been investigated in detail. The wider BSF distribution in the absorber induces a decrease in reverse saturation current density (<i>J</i>₀) due to the stronger BSF effect in suppressing BEI recombination. As a result, an accompanying increase in open-circuit voltage (<i>V</i>_OC) and short-circuit current density (<i>J</i>_SC) is achieved as compared to the BSF-free case. This study offers an alternative strategy to address the BEI recombination issue and also broadens the interface passivation research scope of potentially competitive kesterite solar cells.