Synergistic defect regulation via Rb and Cd double doping for efficiency enhancement in Cu2ZnSn(S,Se)4 solar cells

The efficiency of Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells has reached a record 15.8 %, but it still lags behind that of Cu(In,Ga)Se₂ (CIGS) solar cells. The efficiency limitation in CZTSSe solar cells, primarily manifested as a severe open-circuit voltage deficit (V_OC,def), stems from intrinsic Cu_Zn antisites, defect clusters, and band tailing states. While single-cation doping partially mitigates these issues, its efficacy in simultaneously suppressing these interconnected defects remains insufficient. To address this gap, we implement a novel Rb, Cd dual-cation doping strategy in CZTSSe (RCZTSSe:Cd) via sol–gel synthesis, hypothesizing that Rb⁺ suppresses Cu-Zn disorder (leveraging its larger ionic radius) while Cd²⁺ reduces Zn-related defects and band tailing, acting synergistically to attenuate non-radiative recombination. When the doping ratio of Cd is 7 %, structural and optoelectronic characterizations confirm improved crystallinity, reduced defect density, and suppressed bandgap fluctuations, leading to optimized devices with a significant 16.8 mV V_OC enhancement and a champion efficiency of 8.22 %. This work demonstrates dual-cation doping as a scalable approach to ameliorate band-tail states and boost CZTSSe performance.