Optically accessible long-lived electronic biexcitons at room temperature in strongly coupled H- aggregates.

Photon absorption is the first process in light harvesting. Upon absorption, the photon redistributes electrons in the materials to create a Coulombically bound electron-hole pair called an exciton. The exciton subsequently separates into free charges to conclude light harvesting. When two excitons are in each other's proximity, they can interact and undergo a two-particle process called exciton-exciton annihilation. In this process, one electron-hole pair spontaneously recombines: its energy is lost and cannot be harnessed for applications. In this work, we demonstrate the creation of two long-lived excitons on the same chromophore site (biexcitons) at room temperature in a strongly coupled H-aggregated zinc phthalocyanine material. We show that exciton-exciton annihilation is suppressed in these H- aggregated chromophores at fluences many orders of magnitudes higher than solar light. When we chemically connect the same aggregated chromophores to allow exciton diffusion, we observe that exciton-exciton annihilation is switched on. Our findings demonstrate a chemical strategy, to toggle on and off the exciton-exciton annihilation process that limits the dynamic range of photovoltaic devices.