Exploring the Quantum Character of Interfacial Excitations at the Donor-Acceptor Heterojunction – Towards Efficient Organic Solar Cells with Minimum Energy Offset

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Recent advances in organic solar cells (OSCs), enabled by the recent development of non-fullerene acceptors (NFAs) have been a game changer in the field. Importantly, NFA-based OSCs seem to require a smaller driving force for charge generation to work efficiently, with the benefit of a smaller open circuit voltage loss. This raises the question whether or not similar mechanisms as in fullerene-based solar cells dictate the efficiency of NFA-based devices and whether efficient OSCs with zero driving force may become feasible upon proper design of the NFA. This research will investigate the role of the quantum characteristics of the interfacial excited state (IFS), commonly called CT state, in the generation, separation and recombination of charges. To this end, we apply time resolved photoluminescence, femtosecond transient absorption spectroscopy, temperature and field dependent time delayed collection field and quasi-steady-state photoinduced absorption spectroscopy to polymer:NFA donor-acceptor systems where we fine-tune the HOMO of the polymer in steps of 60 meV. By moving from bilayers with abrupt interface to optimized bulk heterojunctions and finally dilute blends, we will investigate samples where the direct environment of the IFS is systematically varied. The unique combination of our transient and steady-state techniques, along with computational modelling, will provide detailed insight into the processes determining the rate of the formation, dissociation, reformation and decay of the IFS in these energetically and morphologically well-defined samples. By clarifying the role of the quantum characteristics in key photovoltaic processes, we aim at developing strategies to realize efficient organic solar cells with minimum energy offset.