Homology‐Guided Zwitterionic Interlayers for 21% Efficiency Non‐Fullerene Organic Solar Cells

The performance evolution of organic solar cells (OSCs) is increasingly constrained by a growing mismatch between state-of-the-art photoactive layers and conventional cathode interlayer materials (CIMs). Here, we report a homology-guided molecular design strategy to synchronize CIM with fused-ring electron acceptors (FREAs). Grafting zwitterionic sidechains onto the high-performance pentacyclic FREA, we created a novel CIM, SZ1. This design grants SZ1 a deeper lowest unoccupied molecular orbital energy level, higher electron mobility, and superior interfacial compatibility compared to the perylene diimides-based counterpart. SZ1 simultaneously lowers the cathode work function and elevates the active layer's work function, facilitating Ohmic contact and enhancing electron extraction. SZ1 also acts as a supplemental light-harvester, with hole/energy transfer at the SZ1/polymer interface contributing to photocurrent generation. These attributes make SZ1 a highly efficient and versatile CIM with an optimal thickness near 30 nm and exceptional thickness tolerance, retaining ∼89% of peak performance even at a thick interlayer of 90 nm. An impressive efficiency of 21.07% is achieved, ranking among the most efficient OSCs. The generality of this homology concept is demonstrated by its successful extension to non-fused ring electron acceptors. This work establishes a transformative design paradigm for multifunctional, thickness-insensitive interlayers, paving the way for commercially viable OSCs.