Effect of Visible Light on the Electrical Performance of difTES ADT Thin-Film Transistors

Photocurrent microscopy can be used to probe the underlying mechanisms of charge generation and charge transport in organic thin-film transistors. We report on the effects of monochromatic illumination on the electrical performance of bottom-gated bottom-contact solution-processed difTES ADT transistors. The wavelength, 532 nm, was selected since photons of that wavelength are readily absorbed. When incident light was focused at the center of the channel, the magnitude of the drain current was found to be higher than in the dark in both the linear and saturation regimes, signifying the dissociation of excitons into holes (and electrons) which migrated to the drain electrode due to the drain-source bias. In addition to the enhancement of the drain current under illumination, we also observe an increase in the hole mobility. Injected carriers from the electrodes along with photo-induced carriers that are generated midway in the channel fill localized in-gap (deep traps) states of the semiconductor, associated with impurities and defects in the channel. Mobile carriers that are near the mobility edge are able to traverse the active layer easier resulting in a larger mobility. We also noted a significant shift in both the threshold voltage and subthreshold swing under illumination.