Competing Charge Separation Pathways Govern Charge Generation in Organic Photovoltaic Blends: Insights From Transient Electron Spin Resonance

Abstract

ABSTRACT Understanding charge generation in organic photovoltaics remains a central challenge despite recent efficiency gains. Here, competing charge separation pathways in donor:acceptor blends are identified by directly probing photoinduced spin states using transient electron spin resonance (ESR) spectroscopy, enabling distinction between charge‐transfer states and separated charges. The results reveal that, across fullerene and non‐fullerene acceptor blends, charge generation proceeds via two pathways: a fast pathway forming separated charges on timescales faster than spin mixing, and a slower pathway mediated by spin‐polarized interfacial charge‐transfer (CT) states. Modeling of the time‐dependent evolution of spin polarization across blends and temperatures quantifies the relative contributions of these charge separation pathways and resolves subsequent charge dynamics, governed by a balance of dissociation, re‐encounter, and spin‐selective recombination via CT states. Analysis of CT‐state ESR spectra provides a link between charge separation dynamics and morphology by identifying interfacial donor:acceptor geometries associated with the slower charge separation pathway. The results show that higher‐performing blends, particularly PM6:Y6, are characterized by dominance of the faster charge separation pathway and dynamic interconversion between CT states and separated charges. These findings emphasize the importance of balancing fast charge separation with efficient re‐dissociation of CT states for achieving high performance in organic photovoltaic materials.