Halogen-substituted fullerene derivatives for interface engineering of perovskite solar cells

Abstract

The interface between perovskites and charge transport layers is crucial for the power conversion efficiency and stability of perovskite solar cells (PSCs). We report for the first time the exploration of three novel fullerene derivatives, N-ethyl-2-arylvinyl-5-methyl fulleropyrrolidine (NAMF) with H, Cl or Br substitutions on the pyrrolidine side-chain (namely NAMF-H, NAMF-Cl and NAMF-Br), via a low cost and one-step reaction from C₆₀, as the interlayer to enhance the efficiency and eliminate hysteresis in planar heterojunction PSCs. The crystallinity of MAPbI₃ perovskite cast on these interlayer materials was found to be similar, but the grain size increased and the grain boundaries reduced. All three fullerene derivatives have well-aligned LUMO levels that facilitate electron transport from the perovskite to TiO₂, and reduce trap density and charge recombination in PSCs, leading to enhanced device efficiency and reduced hysteresis. Notably, the NAMF-Cl interlayer was found to chlorinate the TiO₂ electron transport layer, leading to an enhanced PCE of 19.2% in the reverse scan and 17.7% in the forward scan, which were greater than those of PCBM-based PSC devices. Our work adds a new direction to the design of new fullerene derivatives with favorable physical and electrical properties as interlayer materials to improve the performance of PSCs.