Charge Generation in Neat Non-Fullerene Acceptor Domains

Non-fullerene acceptors (NFAs) are exciting molecules allowing high efficiency in organic photovoltaic (OPV) blends with conjugated polymers. Interestingly, charges can also be generated by neat NFA films without additional donor. To understand the origins of exciton dissociation in neat NFAs, we have looked at the impact of aggregation, external electric field end non-linear effects. We used solvatochromism in order to gain insight on charge redistribution after excitation in isolated NFAs. We found that unaggregated NFAs feature a more dipolar excited state, revealing intramolecular charge transfer (ICT) character. This ICT character, however, is not enough to generate separated charges. Aggregation is the key to exciton dissociation in neat NFAs, which we observe with TA of solutions and films of several different NFAs. To explore the impact of an electric field on exciton dissociation in neat NFA devices, we used bias-dependent external quantum efficiency (EQE) and transient absorption (TA) spectroscopy. Electromodulated differential absorption (EDA) measurements then allowed us to observe charge transport under bias. Excitation correlation spectroscopy and fluence-dependent TA finally revealed how non-linear effects can increase the charge yield. Lastly, we comment on whether the neat domain charge generation significantly affects the photophysics of blends or not.