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 sensitively 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 voltages ($V_d$). 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. Furthermore, 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 violation 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.

Figure 1

(a) Schematic of a setup of GM imaging measurements, the furnace of the PVT, and a single-crystal FET, which is composed of a Cytop gate dielectric, which are used for the GM imaging measurements. (b) An optical micrograph of a pentacene signal-crystal FET with source (S) and drain (D) electrodes.

Figure 2

(a) Cross-section schematics of pentacene (left) and polymer (right) FETs with source (S), drain (D) and gate (G) electrodes, used for GM imaging measurements. (b)–(e) Microscope GM images of the pentacene single-crystal FET (b) and (c) and the PDVT-10 FET (d) and (e), obtained with a modulated gate bias at V_gmod= −80 V without drain bias (b) and (d) and with dc drain biases at V_ddc= −80 V (c) and (e). Profiles of GM signal distribution along the semiconducting channel layer are extracted from the respective GM images, and are presented in the lower panel of each microscope GM image.

Figure 3

(a) Bias-dependent GM signal profiles along the crystal channel layer (Cry.) of pentacene single-crystal FET with Cytop gate dielectric layer, measured at different dc drain biases (V_ddc= 0 to −80 V) under the same modulated gate bias (V_gmod= −80 V) and (b) under modulated drain and gate biases (V_dmod= V_gmod= −10 to −80 V). (c) Lateral electric field distributions derived from the spatial differentiation of surface potential (which is shown by a gray curve), measured at different dc drain biases (V_ddc= 0 to −8 V) and at constant dc gate bias (V_gdc= −8 V) by Kelvin probe force microscopy. (d) Variations of GM signal intensity are plotted as a function of modulated gate bias: with modulated drain bias (pink) and without drain bias (light blue) and the ratio between the cases with and without the drain bias (inset).

Figure 4

(a) Optical absorption spectrum and (b) GM spectra measured at different positions with the drain biases [at areas A, B, and C, as depicted in Fig. 2] and without the drain bias (at area D), for pentacene single-crystal FETs with Cytop gate dielectric layer. Comparisons of the GM spectra measured (c) with and (d) without drain biases, with the first and second derivatives of the absorption spectrum (red and blue dashed curves, respectively).

Figure 5

Schematics for energy (E) distribution of charge carriers near valence band (VB) states (a) in conventional trap-dominated organic FETs, and (b) in trap-eliminated organic FETs under drain bias. Top: Excitation of holes from the shallow trap states to the valence band states owing to the drain field. Bottom: Spatial extensions of carriers within organic crystals in the respective cases.