Suppressing Deep‐Level Trap Toward Over 13% Efficient Solution‐Processed Kesterite Solar Cell

Abstract Cu 2 ZnSn (S,Se) 4 (CZTSSe), a promising absorption material for thin‐film solar cells, still falls short of reaching the balance limit efficiency due to the presence of various defects and high defect concentration in the thin film. During the high‐temperature selenization process of CZTSSe, the diffusion of various elements and chemical reactions significantly influence defect formation. In this study, a NaOH‐Se intermediate layer introduced at the back interface can optimize Cu 2 ZnSnS 4 (CZTS)precursor films and subsequently adjust the Se and alkali metal content to favor grain growth during selenization. Through this back interface engineering, issues such as non‐uniform grain arrangement on the surface, voids in bulk regions, and poor contact at the back interface of absorber layers are effectively addressed. This method not only optimizes morphology but also suppresses deep‐level defect formation, thereby promoting carrier transport at both interfaces and bulk regions of the absorber layer. Consequently, CZTSSe devices with a NaOH‐Se intermediate layer improved fill factor, open‐circuit voltage, and efficiency by 13.3%. This work initiates from precursor thin films via back interface engineering to fabricate high‐quality absorber layers while advancing the understanding regarding the role played by intermediate layers at the back interface of kesterite solar cells.