Dithiafulvalene-Based Hole Transport Materials Enabling Efficient and Stable n‑i‑p Perovskite Solar Cells via a...

The stability of conventional hole transport materials (HTMs, e.g., Spiro‑OMeTAD) in n‑i‑p perovskite solar cells (PSCs) remains a critical challenge, particularly due to the unstable nitrogen‑centered radical cations ([Ar₃N]^·+) generated during their p‑type doping oxidation. Herein, three indenofluorene (IF)-based HTMs substituted with fluorene and 1,4-dithiafulvalene (DTF) units were designed and synthesized, denoted IF‑FF, IF‑TF and IF‑TT. The incorporation of the DTF unit endows IF‑TT with improved planarity, optimized energy‑level alignment, enhanced stacking orientation, and effective interfacial passivation. More importantly, through cooperative doping‑oxidation with Magic Blue and LiTFSI, IF‑TT generates highly delocalized, structurally stable sulfur‑centered radical cations ([DTF]^·+), which greatly accelerates the post-oxidation process. Notably, IF‑TT^·+ exhibits stronger interaction with LiTFSI than Spiro‑OMeTAD^·+, resulting in a higher concentration of more stable radicals upon doping. Consequently, IF‑TT displays superior doping stability and charge‑transport properties. As a result, IF‑TT‑based PSCs achieve a remarkable power conversion efficiency of 25.89%, outperforming Spiro‑OMeTAD‑based devices (25.29%). Furthermore, the enhanced radical stability and improved film characteristics endow IF‑TT‑based devices with exceptional operational stability. This study highlights that stabilizing the radical species generated during doping is a key strategy for synergistically improving both the efficiency and long‑term stability of PSCs.