A comparative study of photoconductivity and carrier transport in oligomeric films

DC conductivity and transient photoconductivity (TPC) measurements on 1 µm films of pentacene and thienylene-vinylene oligomers deposited by thermal evaporation (TE) and by laser ablation are reported. Conduction properties are found to be relatively insensitive to both chemical composition and deposition method. DC conductivity prefactor magnitudes of order 0.1 S cm^-1, and the observation of field-dependent conduction, are indicative of hopping transport. In all formulations we observed a two-power-law photocurrent decay, consistent with transport initially by hopping down in energy in an exponential distribution of localised states followed by thermal excitation and multiple-trapping (MT). The temperature-dependence of photocurrent decay has been studied for pentacene and yields an exponential tail slope of 650 K (56 meV). A maximum mobility of 10^-5 cm² V^-1 s^-1 is estimated from the TPC data, 2–3 orders less than field-effect mobilities reported previously. This discrepancy may be due to differences between surface and interior conduction processes.