Chlorine-Substituent Regulation in Dopant-Free Small-Molecule Hole-Transport Materials Improves the Efficiency and Stability of Inverted Perovskite Solar Cells

Although doped hole-transport materials (HTMs) offer an efficiency benefit for perovskite solar cells (PSCs), they inevitably diminish the stability. Here, we describe the use of various chlorinated small molecules, specifically fluorenone-triphenylamine (FO-TPA)-x-Cl [x = para, meta, and ortho (p, m, and o)], with different chlorine-substituent positions, as dopant-free HTMs for PSCs. These chlorinated molecules feature a symmetrical donor–acceptor–donor structure and ideal intramolecular charge transfer properties, allowing for self-doping and the establishment of built-in potentials for improving charge extraction. Highly efficient hole-transfer interfaces are constructed between perovskites and these HTMs by strategically modifying the chlorine substitution. Thus, the chlorinated HTM-derived inverted PSCs exhibited superior efficiencies and air stabilities. Importantly, the dopant-free HTM FO-TPA-o-Cl not only attains a power conversion efficiency of 20.82% but also demonstrates exceptional stability, retaining 93.8% of its initial efficiency even after a 30-day aging test conducted under ambient air conditions in PSCs without encapsulation. These findings underscore the critical role of chlorine-substituent regulation in HTMs in ensuring the formation and maintenance of efficient and stable PSCs.