Spatially localized photoelectric effect in ambipolar organic field-effect phototransistors

A numerical simulation has demonstrated that the conversion of incident radiation into the photocurrent in ambipolar phototransistors based on organic semiconductors at certain parameters of the materials and structure is not uniform over the entire channel length. Such a conversion occurs efficiently in a narrow region and the spatial position of this photosensitive region along the $x$ coordinate directed from the source to the drain is controlled by the gate voltage $V_{\mathrm{G}}$. Such a region exists because the electric field strength is high in it and strongly affects the efficiency of the separation of photogenerated charges in organic semiconductors. The dependences of the spatial position of the photosensitive region on the gate voltage $V_{\mathrm{G}}$ have been determined. The dependences of the ratio of the photocurrent to the dark current on the gate voltage $V_{\mathrm{G}}$ have been calculated for various spatial distribution profiles of the incident radiation intensity (step, rectangular, and Gaussian). It has been shown that these dependences after the transformation of the gate voltage scale $V_{\mathrm{G}}$ to the $x$ scale reproduce with a high accuracy the incident radiation profiles.