Thiophene Terminated Fullerene Derivatives for Interfacial Modification toward High Efficiency MAPbI₃ Perovskite Solar Cells

Interface engineering has become a critical process to fabricate efficient and stable planar heterojunction perovskite solar cells (PSCs), as the degradation of perovskite, hysteresis, and photo and thermal instability can all be triggered by the defective interfaces of PSCs. Herein, two fullerene derivatives, NMPPF and NMTTF, have been designed as interlayers between the MAPbI3 and TiO2 electron transport layer. The thiophene terminals of NMTTF have been found to effectively passivate the uncoordinated Pb2+ of perovskite, reducing trap density and charge recombination of PSCs as well as facilitating charge transport at the perovskite and TiO2 interface. Compared to the reference MAPbI3 PSC with a maximum power conversion efficiency (PCEmax) of 18.7% and serious hysteresis (ΔPCE = 7.2%), PSCs incorporating NMTTF achieve a PCEmax of 19.5% with minor hysteresis (ΔPCE = 1.2%) and outperform the performance of NMPPF (PCEmax = 18.0%, ΔPCE = 2.3%) and PC61BM (PCEmax = 18.9%, ΔPCE = 1.4%) modified PSCs. Unencapsulated PSCs adopting these NMPPF and NMTTF interlayers also exhibit good storage stability, maintaining 80% of their initial PCEs after storing in ambient for 30 days. This work demonstrates that the thiophene moiety can be an efficient defect passivation unit to design new fullerene molecules for interfacial engineering of PSCs with high efficiency and stability.