Multifunctional zwitterion modified SnO2 nanoparticles for efficient and stable planar perovskite solar cells

SnO2 nanoparticles (NPs) have been widely employed as electron transport layers (ETLs) in planar n-i-p perovskite solar cells (PSCs). Perfect SnO2 ETL/perovskite interface plays a key role in high efficiency PSCs, however, the main issues caused by the self-aggregation of the SnO2 NPs and the interface recombination limit the further improvement of device performance. Here, l-carnitine hydrochloride (L-ch) zwitterion with multiple functional groups has been incorporated into SnO2 NP solution for high-efficiency PSCs. The results demonstrate that the unique coordination complexes assembled between L-ch zwitterion and SnO2 NPs can inhibit the aggregation of SnO2 NPs, benefiting for the formation of more compact film with reduced roughness. Simultaneously, the L-ch zwitterion can adjust the electronic properties of SnO2 and promote the carrier transport between perovskite and SnO2 ETL with well aligned energy level. Moreover, the -COO- and –N+ (CH3)3 groups can passivate uncoordinated Pb2+ and MA vacancies defects in the perovskite film, respectively. Therefore, by using SnO2/L-ch as the ETL, the efficiency of champion MAPbI3 PSCs can be increased from 18.77% to 20.56%, with reduced hysteresis and improved stability.