Cu2ZnSn(S,Se)4 solar cells with over 10% power conversion efficiency enabled by dual passivation strategy

Interfaces quality have always been considered the key factors in the fabrication of efficient Cu2ZnSn(S,Se)4 (CZTSSe) solar cells, and harmful defects at the interface can significantly reduce the performance of the device. Here, we propose a dual passivation strategy by adding Na to the surface of the absorber layer (CZTSSe) to passivate the CZTSSe/CdS interface and treating the buffer layer (CdS) using ultraviolet (UV) irradiation to passivate the CdS/ZnO interface. Due to the Na doping passivation, the absorber surface conductivity increases while the roughness decreases and detrimental defects that can lead to nonradiative recombination are reduced. The power conversion efficiency (PCE) of the cell is improved by about 1 percentage point by Na passivation. The contributions of open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF) to the improvement of PCE have been quantified as 50.64%, -5.58%, and 54.94%, indicating that Na doping can significantly boost Voc and FF. UV-ozone passivation reduces residual carbon organics in the CdS buffer layer, weakens carrier recombination, allows for more light absorption, and widens the depletion region, which favors efficient carrier collection. The estimated contribution of Voc, Jsc, and FF to the PCE of the device is 8.78%, 119.59%, and -28.37%, respectively, demonstrating that UV-ozone passivation can significantly improve the Jsc of the device. The two passivation strategies complement each other's advantages and achieve a synergistic effect. Our prepared champion device exhibits a PCE of 10.69% and remarkable long-term stability.