Optical probing of charge traps in organic field-effect transistors

Abstract

We report spatially resolved optical probing of charge traps in organic field-effect transistors using focussed laser illumination. By scanning a 635 nm laser across the transistor channel and simultaneously acquiring transfer characteristics, we observe persistent, localised shifts in transistor turn-on voltage correlated with illumination dose and position, with negligible impact on field-effect mobility. The effect is strongest 5-10 um from the source electrode and requires a drain-to-source scan direction with sub-10 um step size. Kelvin probe force microscopy confirms trapped negative charges along the scan path, consistent with exciton dissociation and electron trapping near the semiconductor-dielectric interface. The phenomenon is reproducible across multiple device geometries and organic semiconductors, including TMTES-pentacene, TIPS-pentacene, and diF-TES-ADT. These findings enable direct mapping of trap distributions and suggest new strategies for trap engineering, threshold voltage tuning, and development of organic optoelectronic memories.