Achieving high-efficiency Cu2ZnSn(S,Se)4 solar cells by Ag doping in Cu2ZnSn(S,Se)4 and substituting annealed In0.01Cd0.99S for CdS

To improve the performance of Cu₂ZnSn(S, Se)₄ (CZTSSe) solar cells, a strategy is proposed to replace CdS in traditional CZTSSe solar cells with In-doped CdS (In_xCd_1−xS) unannealed and annealed and to substitute Ag doped CZTSSe (CAZTSSe) for CZTSSe in this work. It is found that In doping in CdS can increase the electron density (n_e) of CdS and incident light passing through the buffer layer (I_(buffer)) when doping atomic ratio x = 0.01. When replacing the CdS buffer layer with In_0.01Cd_0.99S, the power conversion efficiency (PCE) of CAZTSSe solar cell increased from 10.21 % to 10.62 % without an anti-reflection layer. The improvement in the PCE is mainly attributed to the increase in photogenerated current density (J_L), which results from the In doping increase I_(buffer) and expands the width of the depletion region (W_d) by increasing n_e of CdS. When replacing the CdS with annealed In_0.01Cd_0.99S, the PCE of CAZTSSe solar cell increases further from 10.62 % to 12.12 % without an anti-reflection layer. The enhancement in PCE is mainly due to the decrease in reverse saturated current density (J₀) and series resistance (R_s). It is demonstrated that the decrease in J₀ stems from that the annealing promotes the migration of In of In_0.01Cd_0.99S towards the CAZTSSe surface, which passivates defects at the In_0.01Cd_0.99S/CAZTSSe interface, thereby decreasing interface recombination. While the decrease in R_s is attributed to that the diffusion of the In improves the crystallinity of CAZTSSe. It is also found that PCE increase from CZTSSe solar cells to CAZTSSe solar cells when the buffer layer is CdS is smaller than PCE increase when the buffer layer is annealed In_0.01Cd_0.99S.