Electrodeless time-resolved microwave conductivity study of charge-carrier photogeneration in regioregular poly(3-hexylthiophene) thin films

The electrodeless flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to study the photogeneration of charge carriers in spin-coated films of regioregular poly(3-hexylthiophene) (P3HT), over the photon energy range from 1.9 to 5.2 eV for incident light intensities from 10(13) to 10(16) photons/cm(2) per (3 ns) pulse. The initial, single-photon quantum yield of photoionization, phi, has been estimated from the low-intensity limit to the photoconductivity based on a charge carrier mobility of 0.014 cm(2)/Vs (determined in separate pulse-radiolysis TRMC experiments on bulk P3HT). The value of phi is constant at (1.7+/-0.4)% within the range 1.9-3.0 eV, which encompasses the first electronic absorption band of P3HT. Above 3.0 eV, phi increases, up to a value of (7+/-2)% at 5.2 eV. The activation energy of the photoconductivity was found to be approximately 50 meV at all photon energies. The high-intensity, sublinear dependence of the photoconductivity can be described by the occurrence of either exciton-exciton annihilation or diffusional charge recombination with rate coefficients of 2.3x10(-8) cm(3)/s and 1.1x10(-8) cm(3)/s.