Frenkel biexcitons in polymeric semiconductors
Carlos Silva, Georgia Institute of Technology
Frenkel excitons are primary photoexcitations in organic semiconductors and are unequivocally responsible for their optical properties. Biexcitons are, moreover, critical intermediates because they are ubiquitous in many relevant photophysical processes that occur in these materials, including exciton bimolecular annihilation, which dictates the exciton population dynamics. Understanding the details of biexciton correlations is, thus, crucial to achieving a step-change in how we design and use this class of semiconductors. However, so far, due to their spectral ambiguity, there has been scant, direct evidence of bound biexcitons. Moreover, a quantum-mechanical basis for biexciton correlation/stability is missing. By employing nonlinear coherent spectroscopy, here, we identify bound biexcitons in a model polymeric semiconductor. We find, unexpectedly, that excitons with interchainvibronic dispersion reveal intrachain biexciton correlations and vice-versa. Using a Frenkel exciton model, we relate the biexciton binding energy to molecular parameters quantified by quantum chemistry, including the magnitude and sign of the exciton-exciton interaction the inter-site hopping energies. Therefore, our work will provide a window towards general insights into the many-body electronic structure in excitonic systems beyond polymeric semiconductors such as organic semiconductor crystals, molecular aggregates, photosynthetic light-harvesting complexes, and DNA.