Ultrafast Singlet Exciton Dissociation in Y-Series Acceptors

Y-series acceptors are critical components of the highest-performance organic photovoltaic devices. When prepared as nanoparticles (NPs), Y-series acceptors are also highly effective photocatalysts for hydrogen (H₂) evolution from water. Their performance is partially due to their unusual ability to dissociate excitons in the absence of an electron donor. The exciton dissociation is rapid and occurs within the pulse duration of most ultrafast spectrometers. Here, we use broadband compressed laser pulses to resolve exciton dissociation time constants in NPs of four Y-series acceptors: Y6, L8-BO, PC6, and PY-IT. We find that exciton dissociation is fastest in Y6 NPs, followed by PC6 NPs, PY-IT NPs, and L8-BO NPs, with time constants ranging from 28 ± 3 fs for Y6 NPs to 149 ± 16 fs for L8-BO NPs. The trend in the exciton dissociation rate correlates with the NP morphology, where fast exciton dissociation is associated with increased energetic disorder and mixed H/J aggregation. This study shows that exciton dissociation is a conserved property across a range of Y-series acceptors and occurs at sufficiently fast rates to outcompete most other photophysical processes.