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

One of the primary challenges impeding the efficiency improvement of kesterite (CZTSSe) solar cells is the significant open-circuit voltage deficit (Voc,def), 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 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, the Sn- and Zn-doped Ag2S forms and serves three critical functions in CZTSSe devices: p-n conversion boosting, 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 CuZn, CuSn, and [2CuZn+SnZn] defects, thereby suppressing non-radiative recombination. Notably, the Ag2S-modified CZTSSe device efficiency increases from 12.38% to 14.25%, achieving the highest Voc to date at 0.584 V and the lowest Voc,def of only 0.228 V. This novel strategy offers new insights for significantly promote Voc in p-type copper-based thin-film solar cells.