ESR Analysis of Charge-Transporting Materials in Non-Fullerene Organic Solar Cells

Charge-transporting materials significantly influence power conversion efficiency (PCE) and stability of non-fullerene organic solar cells (OSCs). Among them, self-doped materials have been used widely as electron-transporting layer (ETL) materials. Electron spin resonance (ESR) spectroscopy is one of the effective techniques for their characterization. However, detailed understanding of the origins of ESR signals of these ETL materials remains insufficient. In the present study, we thoroughly investigate the origin of the ESR signal of perylene diimide amino N-oxide (PDINO), which is one of the representative ETL materials for non-fullerene OSCs. It is revealed that the ESR signal of PDINO is composed of two Lorentzian components, each originating from PDINO anions with different orientation. On the other hand, ESR signal originating from PDINO cations was not observed because of large hyperfine coupling constants. Based on these findings, we quantitatively estimate the degree of self-doping in PDINO. Detailed investigation of radical states based on ESR spectroscopy provides valuable insights towards improving PCE and stability of non-fullerene OSCs.