Photocurrent Study of Oxygen-Mediated Doping States in Pentacene Thin-Film Transistors

We have measured and analyzed the gate-bias dependence of the photocurrent in pentacene organic field-effect transistors which have been doped using a UV–ozone treatment and compared these to the response of identical devices produced with no air or ozone exposure. The wavelength-dependent photocurrent spectrum shows intensified photocurrent peaks in oxygen-doped samples in the range of 350–480 nm, which corresponds to energy transitions (2.66, 2.76, 2.95, and 3.15 eV) larger than the pentacene HOMO–LUMO gap. These peaks are attributed to the formation of excitons and improved dissociation into electrons and holes, owing to the trap states formed at the interface between the UV-treated dielectric and the pentacene, which also account for positively shifted threshold voltage in the UV-treated sample. Our results are consistent with the trap-and-release transport model for pentacene. The gate-bias-dependent photocurrent spectrum shows that the photocurrent intensity is proportional to the mobility in the linear region, and this mobility relationship was confirmed via simultaneous transport measurement in the device.