Simultaneously optimizing the heterojunction band alignment and passivating the absorber grain boundaries for kesterite solar cells with 14.26% efficiency

Abstract

As an emerging thin-film photovoltaic device, kesterite solar cells are affected by severe non-radiative recombination at the heterojunction and within the absorber bulk, which is a major cause of their low open-circuit voltage (V_OC). Herein, an effective strategy is proposed by spin-coating Na₂S solution onto the surface of the precursor film to reduce the recombination by simultaneously passivating the defective grain boundaries and tailoring the heterojunction band alignment. The obtained absorber shows improved crystallinity and passivated defects. A significantly reduced interface defect is observed. Besides, the increased S/Se ratio on the absorber surface elevates the conduction band minimum (CBM), making the electron transport barrier at the heterojunction smaller. As a result, the interface carrier recombination decreases, and the carrier lifetime increases. The devices treated with Na₂S show obvious improvement in V_OC and fill factor (FF), with a champion efficiency of 14.26%. This provides a new aspect for the application of Na doping method.