FAST TRANSIENT PHOTOCONDUCTIVITY STUDIES OF POLYACETYLENE AND POLYDIACETYLENE

We review the experimental results of fast transient photoconductivity studies of the two prototypical quasi-one-dimensional semiconducting polymers: trans-polyacetylene, trans - (CH)_X, and polydiacetylene(bis p-toluene sulfonate), PDA-TS. For both polymers, the measurements reveal two distinct photoconduction mechanisms: the initial transport is by carriers occuping extended band states; whereas at later times, the carriers are trapped on shallow defects, and transport is governed by trap-limited mobility. The quantum yield is independent of temperature and external field, demonstrating that the Onsager geminate recombination model, used extensively for analysis of the photoconductiivty in organic materials and molecular crystals, is not applicable to PDA-TS. Since the quantum yield is temperature independent, and since the de Broglie wavelength of the carriers is comparable to the width of the potential barrier which prevents the separation of bound geminate carriers, tunneling appears to play fundamental role in the geminate carrier dissociation process. Using carrier sweepout measurements, the mobility has been measured in PDA-TS in the nanosecond time regime; µ ≈ 5 cm²/Vs. The polarization dependence of the photoconductive response in both non-oriented and oriented trans -(CH)_X samples has been studied. As a consequence of the large anisotropy in the absorption coefficient, photoconduction in non-oriented samples is dominated by intrachain excitation and transport. In the case of oriented samples, it is possible to have interchain generation with relatively high quantum efficiency. Comparative studies of the properties of the photocurrent tail in polydiacetylene and in oriented and non-oriented trans -polyacetylene indicate that the magnitude of the tail provides a rough gauge for the quality of the material; in oriented polyacetylene, the enhanced photocurrent tail indicates the importance of shallow traps rather than deep sp³ defects.