Enhancing photovoltaic performance of inverted perovskite solar cells via imidazole and benzoimidazole doping of PC61BM electron transport layer

Although [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) has been widely used as electron transport layer (ETL) for inverted perovskite solar cells (PeSCs), it still remains to be improved. Herein, we report that amphoteric imidazole (IZ) and benzimidazole (BIZ) can act as effective and multifunctional dopants for PC61BM-based ETL. The resultant MAPbI3-xCl3-based PeSCs outperform the devices with pristine PC61BM ETL with power conversion efficiency (PCE) significantly improved from 14.38% to 15.62/16.47%. Detailed mechanism studies demonstrate that IZ and BIZ-doping cause remarkable reduction of ETL surface roughness and significant increase in electrical conductivity of ETL, both of which facilitate the electron transport from perovskite (PVK) and Ag cathode. More importantly, IZ and BIZ as dopants may passivate both Lewis-acid and Lewis-base type defects on the PVK surface due to their amphoteric nature. DFT computations further reveal that BIZ is a better Lewis-base and Lewis-acid than IZ due to its larger π-conjugation, which thus makes it a superior dopant over IZ. In addition, the PeSCs with PC61BM:BIZ based ETL display enhanced device stability by retarding the I− anion migration from PVK to Ag cathode.