Photoexcitations in Poly(2,5-Thienylene Vinylene)
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PHOTOEXCITATIONS IN POLY(2,5-THIENYLENE VINYLENE) I. D. W. SAMUEL*, A. J. BRASSETIT*, N. F. COLANERI*, R. A. LAWRENCE*, D. D. C. BRADLEY*, K. E. MEYER*, R. H. FRIEND*, H. MURATA**, S. TOKITO**, T. TSUTSUI** and S. SAITO** *Cavendish Laboratory, Madingley Road, Cambridge CB3 OBE, UK. "**Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka, 816 Japan ABSTRACT Poly(2,5-thienylene vinylene), PTV, can be conveniently prepared via a precursor route to give coherent films suitable for optical measurements. Photoexcitation above the band gap at 1.8 eV gives rise to two transient absorption peaks at 0.44 eV and 1.0 eV that are associated with bipolarons that live for times of order a few ms at 80 K. Photoluminescence, comparable in intensity to the Raman scattering, is found at energies above the band edge, and we assign this to a hot recombination process. Pump and probe measurements of the induced bleaching at 2 eV show a very fast rise and initial fall time, which are within the 100 fsec resolution of the measurement. INTRODUCTION Photoexcitation of conjugated polymers provides fundamental information about the electronic structure of these materials and of their excited states. The electronic excitations of the x electrons coupled to the a-bonded polymer chain take the form of self-localised polarons [1,21, or solitons for the special symmetry of trans- polyacetylene [3]. The relaxation of the structure has the effect of pulling energy levels away from the band edge into the semiconductor gap. Anisotropy in the electronic structure determines the dynamics for relaxation and recombination of photoexcitations, and it is appropriate to make a distinction between intra- and inter-chain excitations. We consider that intrachain photoexcitation across the x-x* gap creates predominantly intra-chain polaron excitons, which may then decay radiatively, and that separation of charge to form polarons or bipolarons requires interchain motion of charge [4]. The polaron-exciton can be studied by its radiative decay through measurement of photoluminescence, PL. Charged photoexcitations can be studied by measurement of photoinduced optical absorption, PA, due to transitions between the gap states and the band edges. Most measurements are sensitive only to the long-lived excitations, and the experimental evidence is that these are doubly-charged bipolarons, formed by coalescence of two like-charged polarons, as observed in poly(phenylene vinylene), PPV [4], in polythiophene [5] and its alkyl side chain derivatives [6,7]. Poly(arylene vinylene)s can be conveniently prepared via a solution-processible precursor polymer which is either a sulphonium polyelectrolyte or is derived from this. Polymers prepared by these routes can be obtained as coherent, dense films, and in a state of high purity. It is also possible to control bandgap over a range of energies, by selection of the arylene group, and these polymers provide therefore an excellent series for experimental study.
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