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1 December 1997 Biexcitons in mixed-stack charge transfer solids, conjugated polymers, and molecular aggregates
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Abstract
Ultrafast spectroscopic studies of organic solids reveal features unexpected within simple noninteracting models for these systems. We consider: (1) organic mixed-stack charge- transfer solids, (2) conjugated polymers, and (3) 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 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.
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Sumitendra Mazumdar, Michael E. Chandross, Y. Shimoi, Aparna Chakrabarti, Neal R. Armstrong, and Nasser Peyghambarian "Biexcitons in mixed-stack charge transfer solids, conjugated polymers, and molecular aggregates", Proc. SPIE 3145, Optical Probes of Conjugated Polymers, (1 December 1997); https://doi.org/10.1117/12.295541
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