In condensed matter, many-body electronic correlations play a strong role in determining the optical properties in semiconductors. These electronic correlations can give rise for example to biexcitons, a bound two-quantum quasiparticle. Optical signatures of biexcitons in inorganic semiconductors have been well characterized. However, in organic semiconductors, direct detection of Frenkel biexcitons is much more complex. Despite the extensive theoretical work that has pointed towards their existence in organic solids and the experimental approach which has inferred the presence of two-quantum states, there is a scant evidence of bound biexcitons. In this work, we report the two-quantum coherence correlation spectrum of a liquid-crystalline π-conjugated polymer, PBTTT, at 5 K, measured by means of two-dimensional photoluminescence excitation spectroscopy as shown in Figure 1. By implementing a Frenkel exciton model, we quantify the influence of the energy landscape on the stability of biexcitons. We found that excitons delocalized along polymer chains, form biexcitons via interchain exciton-exciton coupling and excitons delocalized between chains show to do so through intrachain excitonic coupling. The binding energy is related to inter-site hopping energy and to the exciton-exciton contact interaction. The results presented in this work, are of fundamental significance and of broad interest as it provides new opportunities for general insights into the many-body electronic structure in molecular aggregates, photosynthetic light-harvesting complexes, and DNA.

Two-quantum electronic correlations in semiconductor polymers: Frenkel biexcitons

ACS Spring Meeting 2021

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