Tracking triplet pair state in singlet fission using exciton-polariton: A search for light in the dark*

Exciton-polaritons, by virtue of mixed photon and exciton character, possess the potential to modify the properties of organic materials by manipulating their energetics under ambient conditions. Here we explore that potential using singlet fission (SF) in organic photovoltaics as a model process, where two individual excited triplets can be generated at the expense of one photoexcited singlet. This conversion is enabled by an intermediate triplet pair state, which remains enigmatic despite the recent wide-ranging efforts in developing new fission materials. The triplet pair states are generally non-emissive or "dark", hence traditional optical spectroscopic techniques have permitted determination of the triplet-pair energy in only a small subset of materials. Hence, we investigate the microcavities of TIPS-pentacene, a popular SF agent with uncharacterized triplet-pair energy. By analyzing the temperature-dependence and lifetime of lower polariton emission as a function of detuning, we identify contributions to the polariton population from two distinct intra-reservoir states. The lower of these tracks the reported behavior of triplet pairs in transient absorption, permitting direct assignment of the triplet-pair energy from the polariton emission. This finding opens a new avenue of using exciton-polaritons to map the energetic structure of functionally important but poorly characterized dark electronic states and turning them on in the process towards a promising direction.