Air-Stable Radical Additives Modulate Spin-Dependent Recombination in High-Efficiency Organic Solar Cells

Triplet-associated recombination remains a potential loss channel in high-efficiency nonfullerene organic solar cells, particularly when separated charges re-encounter to form spin-correlated charge-transfer or polaron-pair states. Here, we introduce the air-stable organic radical TTM-1Cz as an additive to manipulate the triplet formation by the single electron-occupied doublet state characteristic of radicals. Electron paramagnetic resonance measurements confirm that the radical character of TTM-1Cz is retained after solution processing. Grazing-incidence wide-angle x-ray scattering measurements show that TTM-1Cz and its structurally related closed-shell analogue αHTTM-1Cz induce comparable, modest increases in crystalline coherence, indicating a structural contribution associated with the common molecular framework. In contrast, charge-separated-state-normalized transient absorption spectroscopy reveals a pronounced reduction in the late-time triplet-associated signal only in the TTM-1Cz-containing blend. Together with magneto-photocurrent measurements and local molecular-complex calculations, these observations are consistent with an additional, radical-specific perturbation of spin-dependent charge-transfer recombination. The optimized D18:L8-BO:TTM-1Cz device achieves a champion power conversion efficiency of 20.63%, with a certified efficiency of 20.45%. These results reveal a design principle for radical additives in organic solar cells, in which molecular-packing effects and spin-active modulation of recombination act as distinct but complementary contributions.