Revealing aggregation of non-fullerene acceptors in intermixed phase by ultraviolet-visible absorption spectroscopy

Non-fullerene acceptor (NFA) aggregation is crucial in determining bulk-heterojunction (BHJ) organic photovoltaic (OPV) performance. However, it is still a big challenge to characterize the nanostructure of NFAs in the disordered donor-acceptor intermixed phase. Here, we demonstrate a method to characterize NFA aggregation and composition in the intermixed phase by measuring NFA concentration-dependent ultraviolet-visible (UV-vis) absorption spectroscopy of BHJ films. In various polymer:NFA films, an absorption shift as a function of increasing molecular concentration (ASIMC) phenomenon is observed, and different NFA aggregation behaviors can be distinguished. The ASIMC method was then applied to study the influence of processing conditions on the NFA concentration in the intermixed phase of devices to establish a correlation with device efficiency. The current work provides a feasible tool to study the nanostructure of NFAs in the complex polymer matrix and to understand the variations in the NFA concentration in the intermixed phase under non-equilibrium conditions. [Display omitted] •Concentration-dependent UV-vis spectra reveal NFA aggregation in blend film•NFA composition in the intermixed phase is determined even at a non-equilibrium state•Molecular interactions develop between NFA molecules in the intermixed phase The disordered intermixed phase that determines organic photovoltaic performance is difficult to probe. Fang et al. report that concentration-dependent UV-vis absorption spectra reveal NFA aggregation and composition in the intermixed phase, which is correlated with solar device performance.