Giant Enhancement of Excitonic Electro-optic Response in Trap-Reduced Organic Transistors

Organic field-effect transistors (FETs) exhibit excellent switching characteristics when they involve trap-eliminated semiconductor interfaces with highly hydrophobic gate dielectric layers. In this study, we investigate the excitonic electro-optic response by delocalized carrier accumulation at the trap-eliminated interfaces of pentacene single-crystal FETs. We find that gate-modulation (GM) imaging, which sen-sitively visualizes the variation in optical microscope images between the gate-on and gate-off states, exclusively reveals the unique enhancement of electro-optic response under the application of drain volt-ages (V-d). The V-d-unbiased GM image exhibits a uniform spatial distribution, which is consistent with the accumulated carrier density in the channel. In contrast, the V-d-biased GM image presents a peculiar spatial distribution with fairly sharp increases around the edges of the source and drain electrodes. Further-more, the following intriguing features are observed: (1) The sharp increase in the GM signal distribution around the electrode edge is similar to the lateral electric field distribution as measured by Kelvin probe force microscopy, and (2) the GM spectra, extracted from the respective GM images measured at different wavelengths, present a second-derivative-like shape that implies the broadening of exciton absorption. Based on these observations, we investigate the origin of this unique effect in terms of the enhanced vio-lation of exciton coherence by delocalized carrier accumulations under drain bias. The gate-induced holes that are weakly bound to shallow traps should be detrapped by lateral electric fields, which eventually generate valence band holes and thus enhance the electro-optic response. These findings should elucidate the spatial coherence of the molecular excitons that are responsible for the various unique photoelectric characteristics of organic electronic devices.