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1 December 1997 Biexcitons in mixed-stack charge transfer solids, conjugated polymers, and molecular aggregates
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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.
© (1997) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
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);

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