Twisted or Planar? Side-Chain Tailoring of Fluorene-Based Hole-Transport Materials for Efficient and Stable Perovskite Solar Cells

The functionalization of small-molecule hole-transport materials (HTMs) heavily relies on the rational design of molecular geometry, which can optimize both intrinsic HTM properties and interfacial properties for realizing high-performance and stable lead halide perovskite solar cells (LHPSCs). Herein, two fluorene-based donor–π linker–donor HTMs are seen, FL01 and FL02, whose side chains are tailored with planar phenyl-carbazole groups and twisted triphenylamine groups, respectively. Benefiting from the high conformational flexibility of twisted side chains, the strong and oriented interaction via Pb.O bonding is well coordinated at the perovskite and FL02 interface, which favors the interfacial charge transfer as well as the protection of perovskite layer by effectively blocking or mitigating the diffusion of hygroscopic dopants toward the perovskite surface. Consequently, the performance of FL02 HTM-based n–i–p LHPSCs is significantly enhanced by achieving a power conversion efficiency of 17.8%, which is twice higher than that (8.6%) of FL01 HTM-based ones and comparable with the case (18.8%) of conventional spiro-OMeTAD HTM-based devices. More importantly, the FL02-based devices exhibit impressively high operation and storage stabilities with T80 and TS80 lifetimes of >98 h and ≈270 days, respectively, which are among the longest lifespans for the type of hygroscopically doped LHPSCs.