Photoresponse and saturation behavior of organic thin film transistors

Abstract

Organic thin film transistors based on poly(3,3‴-didodecylquarter-thiophene) were characterized under illumination with a fixed wavelength but various intensities from dark to 1100 μW cm−2. Typically the illumination process should increase the drain current through the increase in the number of charge carriers in the channel in the form of polarons, as a result of generation and dissociation of excitons or electron-hole pairs. However, the rate of the current increase was found to decrease as the light intensity was increased, and eventually the level of drain current reached a maximum before declining. We suggest that the physics behind this oversaturation behavior is related to the increasing number of electron-hole recombination events associated with the increase in polaron density in the channel. When the polaron density goes above a threshold value at high light intensity, the number of polarons cannot increase further as they are already closely packed and the recombination overtakes generation, resulting in a decrease in the drain current from its peak value. We show that quantitative analysis agreed well with our model, and in our device the polaron diameter and mean free path are 19 and 2 nm, respectively.