EPR Characterization of the Light-Induced Negative Polaron in a Functionalized Dithienylthiazolo[5,4-d]thiazole Acceptor for Organic Photovoltaics

Abstract

Functionalized 2,5-dithienylthiazolo[5,4-d]thiazole (DTTzTz) derivatives have attracted interest towards application as non-fullerene acceptors in solution-processed organic solar cells. Here, we present a combined high-field electron paramagnetic resonance and density functional theory study of the light-induced negative polaron on the novel acceptor 2,4-diCN-Ph-DTTzTz formed after charge transfer in bulk heterojunction blends with a donor polymer. Despite spectral overlap with the polymer cation, the g-anisotropy of the acceptor radical could be directly confirmed through detection of its unique N-14 hyperfine couplings using electron-electron double resonance (ELDOR)-detected nuclear magnetic resonance (EDNMR) for spectral filtering. The spectral assignment is further underpinned by quantum-chemical calculations, which also provide detailed information about the spin density and charge distribution of the polaron in the DTTzTz acceptor.