Signature of exciton annihilation in the photoconductance of regioregular poly(3-hexylthiophene)

Abstract

The transient photoconductance in drop-cast films of regioregular poly(3-hexylthiophene) has been studied over the photon energy range from $1.7\text{to}2.8\mathrm{eV}$ for incident light intensities from ${10}^{13}\text{to}{10}^{16}\text{photons}∕{\mathrm{cm}}^2$ per $4\mathrm{ns}$ pulse. Charge carriers were detected using an electrodeless, time-resolved microwave conductivity technique. The photon-energy dependence of the photoconductance for a $15\mu \mathrm{m}$ thick, optically dense film was found to be very different from that of a submicrometer thick film, which could not be explained by the differences in light absorption alone. A model based on exciton-exciton annihilation, however, with a rate coefficient of $(0.9\pm 0.8)\times {10}^{-8}{\mathrm{cm}}^3∕\mathrm{s}$, can reproduce the spectra as a function of film thickness and incident intensity. From kinetic fits of the conductance transients, bimolecular charge recombination can be excluded as the main origin of the nonlinear dependence of the photoconductance on the light intensity. The quantum yield of photoionization, $\varphi$, was found to be constant at $(2.5\pm 0.4)\%$ within the investigated photon energy range.