THE INFLUENCE OF INTER-OLIGOMER SEPARATION DISTANCE ON OBSERVED HOPPING MOBILITIES IN TERTHIOPHENE-DOPED POLYCARBONATE

Carrier mobilities for alpha-terthienyl (alpha-3T) have been directly determined by dispersing the oligomer in an inert polycarbonate matrix . Over the temperature and concentration ranges studied, hole mobiliti es (mu(h)) are found to increase with applied electric field (F) accor ding to ln (mu(h)) is-proportional-to F1/2. The results may be success fully analysed using a recently developed theory of hopping within a G aussian distribution of localized states of energetic width sigma. A c omparison of sigma values deduced from the thermal variation of zero-f ield mobilities (sigma(T)), and those calculated from time-of-flight l ineshapes (sigma(LS)), indicates that sigma(LS) < sigma(T), and it is consequently necessary to invoke a self-localization energy E(p) almos t-equal-to 0.45 eV for the oxidized oligomer state. Concentration stud ies subsequently suggest that for dilute doping, where the average int er-oligomer separation distance rho exceeds 11 angstrom, polaron hoppi ng is nonadiabatic and controlled by wavefunction overlap for which a localization radius rho0 = 3.3 angstrom is deduced. In the concentrate d regime (rho < 11 angstrom), phase separation occurs and is accompani ed by a sharp transition to adiabatic behaviour for which [mu(h)(T = i nfinity)]/rho2 = 1.1 x 10(13) V-1 s-1. Detailed investigation of the a diabatic region is precluded however by the emergence of deep trapping states which reduce the effective polaron drift-length. It is argued that these states, as opposed to the short radical lifetime of the oli gomer, account for the inability to observe a field-enhanced current i n pure alpha-3T. The calculated field-effect mobility of 3 x 10(-9) cm 2 V-1 s-1 for pure alpha-3T is found to be in substantial agreement wi th the extrapolated value of 1 x 10(-8) cm2 V-1 s-1 inferred from publ ished field-effect mobilities in longer chain oligothiophenes.