Solvent Engineering-Enabled Surface Defect Passivation in Cu₂ZnSn(S,Se)₄ Solar Cells with Low Open-Circuit Voltage Losses and Improved Carrier Lifetime

The efficiency of earth-abundant kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells has been lagging behind the Shockley-Queisser limit primarily due to the presence of deep-level defects. These deep-level defects cause critical issues such as short carrier diffusion length, significant band tailing, and a large open-circuit voltage (V_oc) deficit, ultimately leading to low device efficiency. To address these issues, we propose a post-fabrication defect healing strategy by dip-coating the CZTSSe film in dimethylformamide (DMF) solvent. Immersing the absorber layer in DMF (a polar solvent), neutralizes Cu_Sn antisite defects through chemical bonding and facilitates the formation of a dense, smooth CZTSSe film with larger grain size. Deep-level transient spectroscopy revealed a remarkable increase in carrier diffusion length from 93 nm (control device) to 142 nm (champion device), confirming the beneficial effect of solvent-assisted post-treatment on mitigating Cu_Sn antisite defects. The reduction in defect densities led to a decrease in V_oc deficit by up to 289 mV, accompanied by an increased champion device efficiency of 11.4 %. This work highlights the huge potential of the DMF post-treatment strategy for defect healing in CZTSSe solar cells.