A high effciency (11.06 %) CZTSSe solar cell achieved by combining Ag doping in absorber and BxCd1-xs/caztsse heterojunction annealing

Abstract

Many literatures have demonstrated that a smaller amount of Ag doping in Cu₂ZnSn(S, Se)₄ (CZTSSe) can elevate the open-circuit voltage (Voc) and fill factor (FF) of CZTSSe solar cells, but decrease short-circuit current density (Jsc) due to the increase in bandgap (E_g) of the CAZTSSe by Ag doping. The decreased Jsc limits the enhancement in PCE through Ag doping. In this paper, to compensate for the deficit in Jsc and further increase Voc and FF, a strategy is proposed to substitute CdS/CAZTSSe in CAZTSSe solar cells with annealed B-doped CdS/CAZTSSe. It is found that B doping can increase the electron density of CdS (n_e) and decrease the lattice mismatch between CdS and CAZTSSe. Annealing can decrease the hole density of CAZTSSe (n_p) and passivate the surface of CAZTSSe by diffusing B towards the surface of CAZTSSe. The increased n_e and reduced n_p widen the depletion region, leading to an increase in photogenerated carrier density (J_L), resulting in an increase in Jsc. The surface passivation and decreased lattice mismatch can reduce interfacial recombination, resulting in decrease in the reverse saturation current density (J₀), so further increases in Voc and FF. By optimizing the Ag doping content, B doping content, as well as the annealing temperature and time of the B_xCd_1-xS/CAZTSSe, the power conversion efficiency (PCE) increases from 8.96 % to 11.06 %. This study not only advances a deeper understanding of the mechanisms behind various parameters in CZTSSe solar cells but also proposes a method to boost the PCE of CZTSSe solar cells.