Effect of dipole orientational order on charge transfer exciton states in a donor-acceptor single crystal

Heterojunctions between organic semiconductors are central to the operation of many optoelectronic devices. Here the charge transfer (CT) exciton proves to be a crucial component, fundamental to the operation of organic solar cells. However, the impact on CT excitons of dynamic disorder and permanent dipoles – both common features in organic materials - still lacks a rigorous understanding. We have grown perylene-tetrabromophthalic anhydride (TBPA) - a donor-acceptor cocrystal - to study the effect of disorder on the CT exciton. Perylene-TBPA exhibits dipolar disorder at room temperature which ceases below a phase transition at 250 K, thus providing an ideal system to study the effects of disorder without added mesoscale morphological influences. The CT exciton was studied in both the ordered and disordered phase using optical spectroscopy. We find that the absorption band consists of three separate CT exciton states in the disordered phase – a novel finding in donor-acceptor co-crystals. We use model calculations to attribute this splitting of the CT exciton to nearest-neighbour dipoles flipped from their lowest energy configuration. This work thus highlights the important role of dipole motion and dipole-dipole interactions in the energetics of CT excitons in organic materials.