Optimized Nb-doped SnO2 buffer layer for enhanced carrier extraction and Sb2S3 photovoltaic responses

Crystal vertical orientation, deep-level traps, interface charge non-radiative recombination, etc., for Sb2S3 solar cells are the main factors that constrain its optoelectronic performance. Suitable electron transport layers (ETLs) can improve Sb2S3 film deposition and enhance crystallization quality. Here, niobium-doped SnO2 (Nb-SnO2) as the ETLs was prepared to increase the carrier concentration and film conductivity. The optimized Nb-SnO2 film can promote efficient charge separation and transport. The high crystallinity of Nb-SnO2 can also improve fusions between nanocrystals and reduce grain boundaries. The incorporated Nb5+/Nb3+ ions may form the Nb2S5 (Nb2S3) with the S2− ions at the interfaces to induce Sb2S3 film deposition, increase grain sizes, enhance crystal orientation, and reduce deep-level traps. The Nb-doped SnO2 film together with the high-quality Sb2S3 layer and Nb-SnO2/Sb2S3 heterojunction can promote charge separation and extraction and alleviate carrier non-radiative recombination. The best device performance with a high photoelectric conversion efficiency (7.25%, a VOC of 0.703 V, a JSC of 16.87 mAcm−2, and an FF of 0.611) is achieved. The whole dynamic process of charge separation, extraction, and recombination is determined via detailed characterizations. Our simple doping strategy also provides useful guidelines for Sb2S3 solar cells to enhance SnO2 film conductivity, reduce deep-level traps in the Sb2S3 layer, optimize Nb-SnO2/Sb2S3 band alignment, etc.