Alleviating non-radiative losses in organic solar cells through side-chain regulation of low-bandgap non-fullerene acceptors

Non-radiative losses present a significant hurdle limiting the performance of organic solar cells (OSCs). Selenium substitution in non-fullerene acceptors (NFAs) is an effective approach for tuning intermolecular stacking and energy levels in state-of-the-art near-infrared absorbing OSCs. However, the inter-system crossing (ISC) induced by selenium may enhance the formation of triplet excitons in NFAs, aggravating non-radiative losses in OSCs. Herein, we have structurally engineered selenium-containing NFAs through introducing achiral N-alkyl substituents with varied branching sites at C2 or C3 position onto the pyrrole moiety of NFA core. We find that the C2-branched ones in our material system exhibit more intimate molecular packing, improved luminescence efficiency and reduced triplet exciton generation in both neat and blend films, reflected as improved external quantum efficiencies and open-circuit voltages in higher-performance devices. We utilize the best-performing NFA as a ternary component into the benchmark D18:L8-BO absorber to realize a high efficiency of 20.4% (certified as 19.88%). This work shows how precisely regulating the microstructure of NFAs by side-chain modification can reduce the non-radiative losses of OSCs. The inter-system crossing induced by selenium may undesirably enhance formation of triplet excitons in non-fullerene acceptors, leading to increased non-radiative losses. Here, the authors introduce achiral N-alkyl substituents, achieving maximum efficiency of 20.4% for ternary organic solar cells.