Dual Optimization of Back Electrode Interface and Bulk via the Synergistic Passivation Effect of Niobium Pentoxide Enables Efficient Kesterite Solar Cells

As compared to the predecessor Cu(In,Ga)Se 2 device, the current efficiency of kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells is still much lower mainly due to the known carriers recombination issue within interface and absorber bulk. In contrast to the majority of researches concerning recombination issues that focus on either single absorber bulk or interface passivation strategy, this study is pioneering in constructing synergistic passivation effects (SPE) to address the bulk and interface recombination issue simultaneously. By introducing a novel niobium pentoxide passivation layer into the back electrode interface (BEI), it is identified that SPE can be constructed due to Nb (& O) diffusion from Nb 2 O 5 layer to absorber bulk and BEI during high-temperature selenization. The chemical passivation effect is fulfilled via the intrinsic high resistance characteristic of Nb 2 O 5 layer, and also through the NbO x passivation aiming to absorber bulk benefited from Nb (& O) diffusion. Meanwhile, the occupations of Nb (& O) on the Mo (& Se) sites induce a conduction type inversion in MoSe 2 interfacial layer and create a preferable interface p + -Mo(Se,O) 2 :Nb/CZTSSe, achieving an interfacial field passivation effect. Ultimately, the promoted absorber quality and improved charge carrier transportation from SPE contribute to the boost of device performance beyond 10% efficiency.