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

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 cm2) and 19.69% for large-area devices (0.6 cm2) 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.