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.