Fabrication of perovskite solar cells with PCE of 21.84% in open air by bottom-up defect passivation and stress releasement

The archetypical n-type semiconductor material, tin oxide (SnO2), is common and well-known electron transport layer (ETL) material in regular perovskite solar cells (PSCs) due to the superior electron mobility and suitable energy level alignment with perovskite. However, plentiful detrimental defects concentrated at perovskite/SnO2 ETLs interface and unfavorable residual stress within perovskite films, compromise and dampen the photovoltaic performance and shelf stability of PSCs. Herein, alkali metal salts (i.e., 2, 2′-biquinoline-4, 4′-dicarboxylic acid alkali metal salts (BCA-2H, BCA-2Na, and BCA-2K)) are employed as pre-buried modifiers in SnO2 films to achieve a holistic amelioration of ETLs, perovskite photoactive layer, and their interface heterojunction. The BCA modification facilitates crystal orientation and crystalline of SnO2 nanocrystals, instrumental in decreasing trap state densities within functional layers, releasing interfacial residual tensile strain, and upgrading perovskite film performance. In comparison to BCA-2H- and BCA-2Na-doped ETLs, the BCA-2K decoration corroborate better defect passivation and stress release effect originating from preferable interface enhancement and desirable surface chemical properties. Consequently, the PSCs based on SnO2-BCA-2K ETL fabricated under open-air conditions deliver the champion efficiency of 21.84% accompanied by negligible hysteresis. Moreover, the unpacked devices maintain 81.2% of its original value after 1200 h storage under an ambient condition of 55 ± 5% (relative humidity, RH) and 25 ± 5 ℃ with open-circuit conditions.