Dual‐Functional ITO Interlayer for Effective Defect Passivation and Cationic Composition Engineering in Kesterite Solar Cells

ABSTRACT Excessive back interface interdiffusion and absorber defects are the primary limitations to the performance of Cu ₂ ZnSn(S,Se) ₄ (CZTSSe) solar cells. In this study, we address these limiting factors by introducing an ITO interlayer at the back interface. During the initial stage of selenization, the ITO functions as a diffusion barrier, effectively suppressing the migration of metal elements toward the Mo electrode and thereby mitigating elemental loss. In the middle and final stages of selenization, the ITO acts as a self‐sacrificing layer, supplying Sn and In elements to compensate for the absorber layer. The dual‐functional ITO interlayer facilitates the redistribution of elemental composition within the absorber layer. It reduces Sn‐related vacancy defects through supplemental Sn supply and alleviates Sn‐related substitutional defects via trace In doping. Through the optimization in this work, the defects in the absorber layer were effectively reduced. The reverse saturation current density decreased from 4.14 × 10 ⁻⁷ to 2.67 × 10 ⁻⁸ A/cm ² , indicating effective suppression of carrier recombination. Finally, the ITO‐10 sample achieved a photoelectric conversion efficiency of 13.11%. This work presents a new interlayer material that reveals the synergistic mechanism of bulk doping and defect regulation in transparent conductive oxide ITO for CZTSSe.