Li, Ag Co‐Doping Enables Efficient Kesterite Solar Cell with a High Fill Factor of 74.30%

Abstract In addition to open‐circuit voltage (V_OC) loss, fill factor (FF) loss is considered another major factor restricting the further optimization of Cu₂ZnSn(S,Se)₄ (CZTSSe) device efficiency. In this work, a comprehensive investigation into the loss mechanisms of FF has been conducted, and implemented a Li&Ag co‐doping approach to enhance FF. The results indicate that the FF loss caused by insufficient carrier extraction is higher than that caused by non‐radiative recombination. The carrier extraction capability is significantly influenced by the band alignment of the CdS/CZTSSe interface and has little relationship with the carrier concentration of the absorber. Therefore, although Ag doping reduces the hole concentration and conductivity, it reduces the FF loss caused by carrier extraction due to the improvement of band alignment. Ag doping is also superior to Li in passivating harmful defects, which helps reduce FF losses caused by non‐radiative recombination. Correspondingly, Li performs better than Ag in increasing the hole carrier concentration and optimizing band alignment, greatly reducing FF losses caused by insufficient carrier transport. Finally, the Li and Ag co‐doping strategy enables a 14.91% efficient kesterite solar cell with the highest reported FF to date of 74.30% through collaborative optimization of carrier extraction and suppression of non‐radiative recombination.