N-type Ag2S modified CZTSSe solar cell with lowest Voc,def

Abstract

One of the primary challenges impeding an improvement in the efficiency of kesterite (CZTSSe) solar cells is the significant open-circuit voltage deficit (V_oc,def), which is mainly due to high defect concentrations and energy level mismatches at the heterojunction interface. Here, we propose a novel low-temperature surface modification strategy by the in situ incorporation of n-type Ag₂S at the front interface of CZTSSe. We first found that the formation of narrow-bandgap Ag₂S induces secondary diffusion of microregion elements on the CZTSSe absorber surface. During annealing, Sn- and Zn-doped Ag₂S forms and serves three critical functions in CZTSSe devices: boosting of p–n conversion, front-interface bandgap grading, and defect passivation. These processes collectively reduce the carrier transport barrier and enhance charge extraction capability. Additionally, the outward diffusion of Ag⁺ to the absorber surface partially substitutes Cu⁺, reducing concentrations of Cu_Zn, Cu_Sn, and [2Cu_Zn + Sn_Zn] defects, thereby suppressing non-radiative recombination. Notably, the efficiency of an Ag₂S-modified CZTSSe device increases from 12.38% to 14.25%, achieving the highest V_oc to date at 0.584 V and the lowest V_oc,def of only 0.228 V. This novel strategy offers new insights for significantly promoting V_oc in p-type copper-based thin-film solar cells.