Revitalizing Fulleropyrrolidine via Nonionic Sidechain Engineering: An Ethanol‐Processible Interlayer Enabling Efficient Organic Solar Cells

ABSTRACT Organic solar cells (OSCs) advancement hinges critically on cathode interlayer materials (CIMs) that meet multiple requirements: thickness‐insensitivity to facilitate device upscaling, mitigated hydrophilicity to enhance stability, and processability from green solvents to minimize environmental impact and reduce post‐processing costs. We design and synthesize bis‐sulfonimide‐functionalized fulleropyrrolidine derivatives grafted with nonionic sidechains. C60‐BSI‐PS, incorporating phosphonate ester sidechains, exhibits superior electronic properties compared to its carbonate‐ester counterpart and outperforms benchmark fulleropyrrolidine with ionic sidechains. These characteristics promote efficient charge‐transport and suppress recombination even under thick‐film conditions. The phosphonate ester groups also endow C60‐BSI‐PS with high solubility in ethanol, eliminating the need for toxic halogenated solvents or methanol. When processed from ethanol, the C60‐BSI‐PS interlayer delivers a power conversion efficiency (PCE) of 19.76% in PM6:D18:L8‐BO‐based OSCs, alongside excellent operational stability. It retains >94% of its optimal PCE even at an ultra‐thick coating of 81 nm. The broad applicability of C60‐BSI‐PS is further demonstrated across leading binary, ternary, and quaternary OSC systems, achieving PCEs 21.11% for small‐area devices (0.04 cm 2 ) and 19.69% for large‐area devices (0.6 cm 2 ) in D18:L8‐BO:BTP‐eC9‐based system. Thus, engineering fullerene‐based materials through judicious sidechain functionalization is a powerful strategy for creating high‐performance, sustainable interlayers, paving the way for next‐generation photovoltaic technologies.