Simultaneous carrier concentration regulation and grain boundary passivation enable over 14 % efficient Ag-alloyed kesterite solar cells

Silver (Ag) doping improves the efficiency of kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells, yet concurrently suppresses the hole carrier concentration. While lithium (Li) doping can mitigate this issue through Li Zn antisite defect formation to restore hole density, the inherent polycrystallinity of CZTSSe imposes a critical compromise: elevated carrier concentrations intensify grain boundary scattering and non-radiative recombination. This fundamental constraint highlights the necessity of strategic grain boundary engineering for efficiency enhancements. Herein, synergistic Na/Li co-doping was implemented to simultaneously attain high carrier concentration and modify grain boundary characteristics. This synergistic approach yields a remarkable 24 % relative efficiency enhancement (from 11.36 % to 14.08 %) attributed to simultaneous improvements in charge carrier mobility and minority carrier lifetime. In addition, the contribution of Na versus Li to the increase in photovoltaic performance is different in Ag alloying (Li > Na) and Ag-free (Li < Na) devices, implying that the preparation of highly efficient device cannot be based on previous understanding.