Passivating Sn_Zn Defect and Optimizing Energy Level Alignment via Organic Silicon Salt Incorporation toward Efficient Solution‐Processed CZTSSe Solar Cells

Abstract The Sn_Zn defect passivation and heterojunction interface energetics‐modification are crucial for further improvement of open‐circuit voltage (V_oc) and efficiency of Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells. Herein, a simple but effective cation substitution strategy is reported to promote the performance of CZTSSe devices by simultaneously modifying the absorber and heterojunction interface. This is achieved by introducing the organic silicon salt C₈H₂₀O₄Si into CZTSSe precursor solution as a source of silicon to partially substitute Sn with Si. Systematic studies reveal that there are two mechanisms synergistically contributing to the increase of efficiency: 1) This strategy can passivate the undesired Sn_Zn defect in CZTSSe absorber more efficiently, leading to at least one order of magnitude lower Sn_Zn defect density and beneficial carrier transport properties; 2) After Si incorporation, the conduction band minimum of the CZTSSe is upshifted, thus enlarging the bandgap, which ultimately optimizes the energy level structure at the CdS/CZTSSe interface and reduces interfacial energy loss by decreasing the carrier transport barrier. Consequently, the CZTSSe device with 7% Si substitution delivers a satisfactory efficiency of 13.02% owing to an increase in V_oc. It is hoped the Si substitution strategy reported here can provide inspiration for developing advanced CZTSSe devices with broad application prospects.