On the Role of Charge Transfer Excitations in Non-Fullerene Acceptors for Organic Photovoltaics

TL;DR

This study combines computational and experimental methods to elucidate the role of charge transfer excitations in Y6 non-fullerene acceptors, revealing their impact on optical properties and guiding future material design for improved organic photovoltaics.

Contribution

It provides a detailed understanding of charge transfer excitations and their influence on Y6's optical and electronic properties, aiding the design of more efficient OPV materials.

Findings

Charge transfer excitations explain spectral broadening and red shift.

Polaronic states are polaron pairs, not free carriers.

FE-CT hybrid states dominate the excited state landscape.

Abstract

Through the development of new non-fullerene electron acceptor (NFA) materials, such as Y6 and its molecular derivatives, the power conversion efficiencies of organic photovoltaics (OPVs) have now exceeded 19%. However, despite this rapid progress, our fundamental understanding of the unique optical and electronic properties of these Y-series NFAs is lacking, and this currently limits progress in material design. In this work, we provide a detailed computational-experimental characterisation of the archetypal NFA, Y6. To explain the significant broadening and red shift of the absorption spectrum observed when moving from the solution phase to the solid state, we first rule out more typical causes, such as J-aggregation. Instead, by considering the role of charge transfer (CT) excitations and their mixing with Frenkel exciton (FE) states, we can computationally reproduce the experimental absorption spectra of Y6 with excellent accuracy. Using transient absorption spectroscopy, we provide evidence for this dense manifold of FE-CT hybrid electronic excitations in Y6 through the prominent sub-picosecond relaxation events following supra band gap excitation. Furthermore, through sub band gap excitation, we also find states with polaronic character in Y6 that are in a dynamic equilibrium with the FE-CT hybrid states. Magnetic resonance spectroscopies reveal that these polaronic states are polaron pairs, most likely located on neighbouring Y6 molecules, not free charge carriers, as has been previously suggested. Thus, this new understanding of how the solid-state packing motif directly controls the optical and electronic properties of Y-series NFAs opens the door to intelligently design NFA materials to further increase OPV performance.