TOF mobility measurements in pristine films of P3HT: control of hole injection and influence of film thickness

Time-of-flight (TOF) photocurrent measurements have been used to study charge transport in films of regioregular poly(3-hexylthiophene) (P3HT). Devices in which the P3HT film had been deposited directly onto an indium tin oxide (ITO) electrode produced high dark currents as a result of hole injection into P3HT from ITO. Photocurrent transients in such devices were disperse. It was found however, that these dark currents could be significantly reduced by inserting a dense TiO2 layer between the ITO and the polymer film. The resulting devices gave non-dispersive transients with hole and electron mobilities in the range of 1 - 2 10(-4) cm(2) V-1 s(-1) at room temperature. The mobility values were observed to be almost independent of film thickness over the range of 350 nm to 4.3 mu m. Temperature dependence studies showed a weak dependence on temperature with a low energetic disorder parameter according to analysis using the Gaussian Disorder Model (GDM) of 71 meV.