Strong light-matter coupling in organic photovoltaics

Abstract

Organic photovoltaic (OPV) solar cells are next-generation solar harvesting devices based on organic polymers and small molecules. Perylene diimides are a promising class of electron acceptor OPV molecule due to their excellent absorption properties, low cost, ease of synthesis and stability. These can be paired with the polymer electron donor PTB7-Th to create a suitable material system for solar energy harvesting. We show that perylene diimide monomers (PDIs) are plagued by strong intermolecular π-π interactions which inhibit device performance, but this can be partially overcome for twisted dimers (TPDIs). TPDI dimers are shown to experience reduced recombination losses due to their more favourable morphology. We also demonstrate dual-direction energy harvesting in both systems, since charge transfer states are formed from excitation of both the PTB7-Th donor and the (T)PDI acceptors. We provide a method to quantify the contribution of each material to the overall charge production, and show that TPDI blend films demonstrate more balanced dual-direction energy harvesting.

It is also possible to alter the properties of organic materials by strong light-matter coupling, where a photon in an optical cavity couples to the excitonic transition of a material. We fabricate and compare bare and cavity-based OPV devices and demonstrate strong coupling in cavity-based OPVs through both optical and electrical measurements. We demonstrate improved device functioning in cavity devices once the cavity structure is accounted for. We also fabricate cavity-based devices of different thicknesses in order to control the interaction between the exciton in the material and the photon in the cavity. Through these experiments, we demonstrate that the presence of a resonant cavity within an OPV device has the ability to alter properties which are normally intrinsic to a material without changing the material itself, having profound implications for the study of device physics in OPVs