Open‐Shell Guest Acceptors Manipulate Charge‐Transfer Spin Dynamics in Organic Solar Cells

ABSTRACT Non‐radiative energy loss in high‐efficiency organic solar cells (OSCs) is closely associated with triplet‐mediated recombination involving triplet charge‐transfer ( 3 CT) states. Here, rather than suppressing this pathway solely through excited‐state energetic engineering, we explore a spin‐manipulation strategy using intrinsically open‐shell non‐fullerene acceptors (NFAs) as spin‐active guest acceptors in OSC blends. Two (thio)barbituric acid‐terminated open‐shell NFAs, MAZ‐1 and MAZ‐2, were incorporated into D18:N3 solar cells. Electron spin resonance (ESR), variable‐temperature ESR, and variable‐temperature 1 H NMR measurements confirm their open‐shell character and thermally accessible triplet states. Upon MAZ incorporation, the ternary devices exhibit weakened high‐field magneto‐photocurrent responses and accelerated decay of long‐lived CT states, consistent with modulated CT‐state spin evolution and suppressed triplet‐related recombination. Consequently, the non‐radiative energy loss is reduced by up to ∼20 meV, accompanied by enhanced charge transport, improved morphology, and a power conversion efficiency increase from 18.52% to 20.23%. This work establishes open‐shell guest acceptors as an intrinsic spin‐active platform for manipulating CT‐state spin dynamics and reducing non‐radiative recombination losses in OSCs.