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

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 n-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 (J0) due to the stronger BSF effect in suppressing BEI recombination. As a result, an accompanying increase in open-circuit voltage (VOC) and short-circuit current density (JSC) 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.