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

ABSTRACT 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.