Rigid-flexible coupling: exquisite modulation of asymmetrical spiro-type hole-transporting materials toward efficient and stable perovskite solar cells

Hole-transporting materials (HTMs) are the cornerstone to ensure efficient charge extraction and transport in conventional perovskite solar cells (PSCs). Despite the development of numerous novel HTMs, their performance consistently lags behind that of the benchmark, Spiro-OMeTAD. However, the instability of Spiro-OMeTAD induced by heavy doping remains a critical limitation. Herein, based on a Spiro-type molecular skeleton, we design a series of HTMs (namely 3MPA, 2MPA, MPA, and 4MCz) by precisely regulating the peripheral flexible diphenylamine and rigid carbazole fragments, which enables synergistic modulation of multiple properties of the HTMs. Specifically, the introduction of more carbazole units leads to enhanced thermal stability, lowered HOMO energy level, more compact film morphology and strengthened passivation capacity, but inversely, decreased solubility, doping efficiency and conductivity of the HTMs. Consequently, 3MPA and 2MPA can enable high-performance PSCs at low doping concentrations with PCEs of 25.21% and 24.86%, respectively, and importantly, 2MPA based PSCs exhibit superior device stability under ISOS-D-1 and ISOS-D-2 conditions.