Synchronous amelioration of SnO2 surface aggregation and buried layer defects by sodium salts for high-efficiency and stable perovskite solar cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have received extensive attention due to their high efficiency, low cost and simple solution processability. In regular PSCs, tin oxide (SnO2), as the commonly used electron transport layer (ETL) material, suffers from undesirable aggregation and inferior charge transport during the low-temperature fabrication process, jeopardizing and dampening the photovoltaic performance and shelf stability of the devices. Herein, an alkali metal salt, 2,2'-biquinoline-4,4'dicarboxylate disodium salt trihydrate (BCA), was introduced into a SnO2 colloidal solution to overcome these obstacles. The BCA modifiers can optimize the agglomeration phenomenon of SnO2 colloids, thereby forming a SnO2 electron transport layer with favorable film coverage and outstanding electrical properties. The oxygen vacancy (-OH) defects at the SnO2 upper surface could be efficiently passivated through coordination bond interaction between the C=O groups of BCA molecules and -OH sites. Moreover, the C=N and sodium ions from BCA combined with the undercoordinated Pb2+ and halide vacancies distributed at the perovskite bottom side to modulate crystal growth and the resultant film morphology. Accordingly, the BCA molecules effectively passivated the buried layer defects and realized the benign energy level alignment of the perovskite heterojunction, which was conducive to the extraction of charge and the reduction of interfacial carrier recombination. As a result, the PSCs fabricated employing the SnO2-BCA layer under open-air conditions achieved a champion efficiency of 20.24% along with negligible hysteresis. The unencapsulated modified device retained 80.1% of its initial PCE after aging for over 1000 h at 55 +/- 5% relative humidity under ambient conditions. This work provides a simple and feasible pathway to synchronously minimize SnO2 colloid coagulation and enhance interfacial electron transport, hence contributing to further enhancement in the efficiency and stability of PSCs.