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

Abstract

The electrodeless flash-photolysis time-resolved microwave conductivity technique ($\mathrm{FP}\text{−}\mathrm{TRMC}$) has been used to study the photogeneration of charge carriers in spin-coated films of regioregular poly(3-hexylthiophene) $\left(\mathrm{P}3\mathrm{HT}\right)$, over the photon energy range from 1.9 to $5.2\mathrm{eV}$ for incident light intensities from ${10}^{13}$ to ${10}^{16}$ $\text{photons}∕{\mathrm{cm}}^2$ per $\left(3\mathrm{ns}\right)$ pulse. The initial, single-photon quantum yield of photoionization, $\varphi$, has been estimated from the low-intensity limit to the photoconductivity based on a charge carrier mobility of $0.014{\mathrm{cm}}^2∕\mathrm{Vs}$ (determined in separate pulse-radiolysis $\mathrm{TRMC}$ experiments on bulk $\mathrm{P}3\mathrm{HT}$). The value of $\varphi$ is constant at $\left(1.7\pm 0.4\right)\%$ within the range $1.9–3.0\mathrm{eV}$, which encompasses the first electronic absorption band of $\mathrm{P}3\mathrm{HT}$. Above $3.0\mathrm{eV}$, $\varphi$ increases, up to a value of $\left(7\pm 2\right)\%$ at $5.2\mathrm{eV}$. The activation energy of the photoconductivity was found to be approximately $50\mathrm{meV}$ 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 $2.3\times {10}^{-8}{\mathrm{cm}}^3∕\mathrm{s}$ and $1.1\times {10}^{-8}{\mathrm{cm}}^3∕\mathrm{s}$.