Origin of the threshold voltage shift of organic thin-film transistors under light illumination

The apparent shift of threshold voltage of organic thin-film transistors under light illumination has been explained as a result of the superposition of a photo-generated current on the dark current overall biases. Our model has been confirmed by demonstrating that the apparent threshold voltages calculated under different illumination intensities matched perfectly with the experimental values, for two devices with different channel widths. Our model indicates that (1) there is a photo-current associated with the photo-excitation process in organic thin-film transistors and (2) the apparent threshold voltage under illumination is not the intrinsic threshold voltage of a device as measured in the dark; instead, it is monotonically shifted from the intrinsic value due to the increase in photo-current under normal laboratory conditions.