Hamburg 2001 – wissenschaftliches Programm

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SYOF: Organische Festkörper

SYOF 4: Optical Properties

SYOF 4.4: Vortrag

Dienstag, 27. März 2001, 15:55–16:10, S6

Excitonic photoconductivity in polydiacetylene single crystals — •Gerhard Weiser and Sven Möller — Fachbereich Physik der Philipps-Universität Marburg, 35032 Marburg, Germany

Polydiacetylene single crystals show in striking contrast to other polymers a large gap of 0.5eV between the absorption and photocurrent threshold. This gap which is attributed to a large exciton binding energy disappears for 4-BCMU single crystals after a phase transition increases the exciton transition energy from 2.0 to 2.3eV. Electroabsorption spectra prove that the exciton binding energy has not changed. The excitonic photoconductivity is not activated since the ratio of generation efficiency for excitation above and below the bandgap is the same at 77K and 300K. The excitonic yield increases further as treatment above 135C introduces large disorder resulting in broad spectra similar to those of spin-cast films. We propose extrinsic carrier generation by Auger recombination of excitons which excites trapped carriers high into free states. Such non-equilibrium carriers in traps are obvious from large displacement currents observed after switching the applied voltage. The relative position of electron states of chain and side-group explains that a modest increase of the exciton transition energy has a significant effect on carrier generation. We suggest that similar processes may be active in other polymers.