Geometrically Asymmetric Phosphonium Zwitterions Enable Efficient Printable Mesoscopic Perovskite Solar Cells.

Abstract

Carbon-based, fully printable hole-transport-layer-free mesoscopic perovskite solar cells (p-MPSCs) offer a low-cost, scalable photovoltaic technology, yet suffer from efficiency losses due to non-radiative recombination at electron-selective interface. Here, we design geometrically asymmetric zwitterions, featuring a bulky charge-dispersed triphenylphosphonium cation and a small charge-concentrated sulfonate anion linked by an alkyl chain, to weaken intermolecular and intramolecular charge attraction, thereby constructing a permanent dipole interlayer that mitigates such recombination. By extending the alkyl spacer and introducing methyl substituents on triphenylphosphonium, the optimized zwitterion, 4-(tri-p-tolylphosphonio)butane-1-sulfonate (4MePS), achieves an ultrahigh dipole moment of 14.92 Debye. 4MePS strongly interacts with both perovskite and TiO₂ to form surface dipoles, drastically lowering their work functions by 0.42 and 0.54 eV, respectively. Comprehensive characterizations confirm that 4MePS suppresses non-radiative recombination and accelerates charge extraction in p-MPSCs. Consequently, 4MePS-treated p-MPSCs deliver a champion power conversion efficiency (PCE) of 23.3% (vs. 21.8% for control) and a minimodule efficiency of 20.2% over 57.3 cm², among the highest reported for p-MPSCs. Encapsulated devices retain 90% of their initial PCE after 1200 hours of maximum power point tracking under 1‑sun illumination at 55 ± 5 °C. This work establishes a charge density-asymmetric molecular design strategy for engineering interfacial dipoles toward high-performance perovskite devices.