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

Non-radiative energy loss in high-efficiency organic solar cells (OSCs) is closely associated with triplet-mediated recombination involving triplet charge-transfer (³CT) 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 ¹H 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.