Pyridine-Functionalized Fullerene Electron Transport Layer for Efficient Planar Perovskite Solar Cells

In regular perovskite solar cells (PSCs), the commonly used electron transport layer (ETL) is titanium oxide (TiO2). Nevertheless, the preparation of a high-quality TiO2 ETL demands an elevated-temperature sintering procedure, unfavorable for fabrication of PSCs on flexible substrates. Besides, TiO2-based devices often suffer from notorious photocurrent hysteresis and serious light soaking instability, limiting their potential commercialization. Herein, a novel pyridine-functionalized fullerene derivative [6,6]-(4-pyridinyl)-C-61-ethyl acid ethyl ester (PyCEE) was synthesized and applied as an ETL to replace TiO2 in n-i-p PSCs. PyCEE-based devices achieved a champion power conversion efficiency (PCE) of 18.27% with significantly suppressed hysteresis, superior to that of TiO2-based devices. PyCEE has suitable energy levels and high electron mobility, which facilitate electron extraction/transport. Besides, the pyridine moiety within PyCEE affords coordination interactions with the Pb2+ ion within CH3NH3PbI3, passivating the trap states of CH3NH3PbI3 and thus improving the device performance and suppressing hysteresis greatly. Moreover, PyCEE ETLs were applied in flexible PSCs, achieving a PCE of 15.25%. Our results demonstrated the applicability of PyCEE ETLs in flexible devices and provided new opportunity for the commercialization of PSCs.