Halogenation engineering of benzo[a]phenazine-based small-molecule acceptors enables nonhalogenated solvent processed organic solar cells with an efficiency 20.14%, revealing how halogenation modulates intermolecular interactions, energy loss, and morphology.

Abstract

Halogenation emerges as a key strategy to enhance the performance of organic solar cells (OSCs) by tuning molecular packing, energy levels, and charge dynamics. Here, we report three new benzo[a]phenazine-core small-molecule acceptors, namely NA5, NA6, and NA7, and systematically evaluate their photovoltaic properties in o-xylene-processed binary and ternary OSCs. Halogenation significantly strengthens intermolecular interactions, improves charge carrier mobility, and facilitates exciton dissociation, leading to a remarkable increase in binary device efficiencies from ∼2% (NA5) to over 17% (NA6, NA7). However, halogenation also increases charge-transfer state character, which can induce higher nonradiative recombination and energy loss. Despite this drawback, the enhanced driving force for charge separation and improved morphological order enabled by halogenation outweigh the negative effects on energy loss. Notably, incorporation of NA7 into the PM6:BTP-eC9 ternary system optimizes blend morphology, suppresses nonradiative recombination, and thus achieves a record power conversion efficiency of 20.14% (certified 19.93%)—the highest reported for OSCs processed with hydrocarbon solvents. These findings highlight the dual role of halogenation in modulating both beneficial and detrimental aspects of device energetics, providing new insights into molecular design strategies for high-performance, environmental-friendly OSCs.