EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Diffusion of excited-state energy is a key process in both photosynthesis and in organic optoelectronic devices. In organic heterojunction photovoltaic devices, the formed excitons must migrate to an interface, where charge transfer states are present, and then dissociate into free charge carriers. However, the diffusion length of excitons in organic materials is short compared to the optical path length, thus severely limiting the efficiency of planar heterojunction systems. In devices, a bulk heterojunction is therefore employed to reduce the distance excitons need to diffuse to reach an interface. We have explored an alternative method to increase the efficiency for excitons to reach the charge transfer state. The method is based on increasing the effective rate of exciton diffusion. Polaritons are quasiparticles formed when light and matter are strongly coupled together. Organic polaritons are formed when a molecular transition is on resonance with an optical cavity and the exchange of energy between the two is faster than the energy dissipates from the system. The formed polaritons inherit properties of both light and matter, effectively allowing the modification of the molecular potential energy landscape, and the delocalization of energy throughout the whole cavity. By creating these delocalized polaritons, a new channel to reach the interfacial charge transfer states is created. This exemplifies how a relaxation from a delocalized hybrid light-matter state to a molecular centered charge transfer state can be used to transport energy in photovoltaic devices.