OPTICAL SPECTROSCOPY OF PHOTOINDUCED AND FIELD-INDUCED EXCITATIONS IN POLYACETYLENE PREPARED BY THE DURHAM PHOTOISOMER ROUTE

We present a study of the optical, electronic and electro-optical properties of polyacetylene prepared by the Durham 'photoisomer' route. This modification of the standard Durham precursor route gives enhanced thermal stability of the precursor, but the resultant polyacetylene is considerably more disordered than that produced from the standard precursor polymer. This is evident in the electronic properties in the increased pi to pi* bandgap, with the peak in the optical absorption at 2.6 to 2.7 eV. We show here that electronic and vibrational excitations of the polymer, although affected by the disorder, are not fundamentally changed in character and that, in particular, we still find evidence for electron states associated with soliton defects on the chain, which are of non-bonding character and lie in the centre of the gap. Photoinduced absorption measurements show the 'mid-gap' band to 'soliton' transition associated with charged photoexcitations at about 0.65 eV, significantly higher that that found for better ordered polyacetylene. We observe the doping-induced IR modes at high frequencies (pinned mode above 1000 cm-1). We see no evidence for the photoinduced absorption feature seen at 1.35 eV in Shirakawa polyacetylene, and at 1.5 eV in the unoriented standard Durham polyacetylene. The electronic and electro-optical properties of the field-induced charge accumulation layer formed in MIS device structures are also investigated. We calculate from these measurements charge-carrier concentrations in the range 6 x 10(14) to 7 x 10(17) cm-3, comparable with the standard Durham material. We have measured the 'mid-gap' optical absorption band associated with the charge present in the accumulation layer, and we find a broad absorption feature which peaks at 0.9 eV. We discuss the electronic properties of this form of polyacetylene and, in particular, the modelling of disorder on the electronic structure.