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

ABSTRACT

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 α HTTM‐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.