Coplanar indoline-functionalized fullerene with elevated LUMO level for tin halide perovskite photovoltaics

Raising the LUMO level of fullerene derivatives is essential for minimizing voltage losses in tin halide perovskite photovoltaics. Multi-adducts effectively raise the LUMO level but produce inseparable isomeric mixtures and weaken electron mobility. Mono-addition with electron-donating groups avoids these issues but typically provides only modest LUMO elevation. Herein, we show that simply replacing triphenylamine with indoline elevates the LUMO level by 90 meV, approaching the enhancement achieved by bis-PCBM. This single-substitution strategy simultaneously minimizes voltage loss and preserves electron mobility, circumventing the classic trade-off in fullerene electron-transport materials.