Branched Fluorenylidene Derivatives with Low Ionization Potentials as Hole-Transporting Materials for Perovskite Solar Cells

A group of small-molecule hole-transporting materials (HTMs) that are based on fluorenylidene fragments were synthesized and tested in perovskite solar cells (PSCs). The investigated compounds were synthesized by a facile two-step synthesis, and their properties were measured using thermoanalytical, optoelectronic, and photovoltaic methods. The champion PSC device that was doped with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) reached a power conversion efficiency of 22.83%. The longevity of the PSC device with the best performing HTM, V1387, was evaluated in different conditions and compared to that of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD), showing improved stability. This work provides an alternative HTM strategy for fabricating efficient and stable PSCs.