On the optical tuning of the threshold voltage for DPPDTT-based organic field effect transistors

Understanding the threshold voltage tuning in photosensitive organic field effect transistors (OFETs) is crucial for many device applications like depletion-load unipolar inverters for logic circuits. Here, we report on the correlation between the illumination conditions and threshold voltage tuning in a bottom gate top contact OFET based on photoresponsive organic polymer Poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)], (DPPDTT). The shift in the threshold voltage as a function of illumination intensity and time shows an exponential decay where it finally saturates. The shift in threshold voltage is attributed to the trapped electrons at the dielectric/semiconductor interface that dissociates from the photogenerated excitons. An average positive shift of about 20 V was observed after successive illuminations for various times and intensities. The overall charge carrier mobility of the device remained constant before and after illumination, with average saturation mobility of 0.01cm2/V.sec.