Cooperatively controlling unpleasant defects in kesterite solar cells by introducing La3+ ions induced Na diffusion

Hindering the unpleasant bulk defects is considered an effective measure to improve the photoelectric conversion efficiency (PCE) of Cu2ZnSn(S, Se)4 (CZTSSe) thin film solar cells. In this work, rare earth La3+ ions were introduced into the Cu2ZnSn(S, Se)4 absorption layer and induced Na diffusion from the substrate, collaborating to optimize their unpleasant Sn-related and Cu-related defects. Na diffusion can reduce the CuZn defects and improve the crystallinity of CZTSSe films. The introduced La can locate at the grain boundaries of films, suppress the SnZn bulk defects, and enhance the surface potential distribution of film. Cooperation optimization helps increase the PCE of the device. With dual cation (Ag and La) doping, the PCE of the champion CZTSSe device is improved to 11.6%. Our conclusions supplement the understanding of the existence form and influence mechanism of La in kesterite materials and provide an insight into the performance optimization of kesterite devices.