A comparative study of photoconductivity and carrier transport in oligomeric films

DC conductivity and transient photoconductivity (TPC) measurements on I mu m films of pentacene and thienylenevinylene 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 dep osition method. DC conductivity prefactor magnitudes of order 0.1 S cm(-1), and the observation of field-dependent conduction, are indicative of hoppi ng transport. Ln 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 dec ay has been studied for pentacene and yields an exponential tail slope of 6 50 K (56 meV). A maximum mobility of 10(-5) cm(2) V-1 s(-1) is estimated fr om the TPC data, 2-3 orders less than field-effect mobilities reported prev iously. This discrepancy may be due to differences between surface and inte rior conduction processes. (C) 2000 Elsevier Science B.V. All rights reserv ed.