Biexcitons in mixed-stack charge-transfer solids, conjugated polymers, and molecular aggregates

Ultrafast spectroscopic studies of organic solids reveal features unexpected within simple noninteracting models for these systems. We consider: (a) organic mixed-stack charge-transfer solids, (b) conjugated polymers, and (c) aggregates of metal-halogen phthalocyanines, and show that in all cases the photophysics is dominated by excitons and bound multiexciton states. Theoretical modeling is simplest for charge-transfer solids, where stable multiexcitons are verified by femtosecond pump-probe spectroscopy as well as two-photon absorption. In conjugated polymers, pump-probe spectroscopy reveals features due to biexcitons as well as a low energy charge-transfer exciton. In both charge-transfer solids and conjugated polymers the biexcitons are bound by electron-electron Coulomb interactions. In contrast, the exciton-exciton binding in molecular aggregates can originate from various sources and the magnitude of the biexciton binding energy in these systems is considerably smaller. No evidence for a biexciton in molecular aggregates have found to date. We present the first experimental evidence for a biexciton in the H-aggregate of a metal-halogen-phthalocyanine. Biexcitons, well-established in conventional semiconductors, are therefore characteristic elementary excitations of several different classes of organic solids.