A comprehensive study on the halogenation effect of non-fullerene acceptors for photovoltaic application

In this study, four non-fullerene acceptor (NFA) materials, namely ITC9-4F, ITC9-4Cl, ITC9-4Br, and ITC9-4I, were designed and synthesized by introducing 4F, 4Cl, 4Br, and 4I substituents within their end groups, respectively. A comprehensive investigation into the photovoltaic properties of these NFAs was conducted in organic solar cells (OSCs). Calculation results showed similar surface electrostatic potentials among the four materials, but significant differences in atomic radii during the halogen substitution process. Photoelectric property analysis indicated that halogen substituents influenced absorption spectra and molecular energy levels. Specifically, ITC9-4F exhibited a blue-shifted absorption spectrum and lower extinction coefficient compared to the other three NFAs. In OSC applications, ITC9-4F-based cells demonstrated distinct photovoltaic parameters, including a high open-circuit voltage (VOC) of 0.89 V but a lower power conversion efficiency (PCE) of 13.1%. On the other hand, OSCs based on ITC9-4Cl, ITC9-4Br, and ITC9-4I showed comparable PCEs of 14.6%, 14.1%, and 14.6%, respectively, with progressively increased VOC and decreased fill factor values. Trap-assisted recombination became more severe in the order of 4F-, 4Cl-, 4Br-, and 4I-based cells, while energy loss decreased gradually. These findings highlight the potential of halogenated acceptor-donor-acceptor-type NFAs in OSC applications and offer valuable insights for designing novel photovoltaic materials. Four small-molecule acceptors with the same conjugated backbone but different terminal halogen substituents (F, Cl, Br, I) were developed, the effects of these halogens on film morphology and optoelectronic performance were systematically studied.