Abstract
Surface-emitting organic light-emitting transistors (OLETs) could well be a core element in the next generation of active-matrix (AM) displays. We report some of the key characteristics of graphene-based vertical-type organic light-emitting transistors (Gr-VOLETs) composed of a single-layer graphene source and an emissive channel layer. It is shown that FeCl3 doping of the graphene source results in a significant improvement in the device performance of Gr-VOLETs. Using the FeCl3-doped graphene source, it is demonstrated that the full-surface electroluminescent emission of the Gr-VOLET can be effectively modulated by gate voltages with high luminance on/off ratios (~104). Current efficiencies are also observed to be much higher than those of control organic light-emitting diodes (OLEDs), even at high luminance levels exceeding 500 cd/m2. Moreover, we propose an operating mechanism to explain the improvements in the device performance i.e., the effective gate-bias-induced modulation of the hole tunnelling injection at the doped graphene source electrode. Despite its inherently simple structure, our study highlights the significant improvement in the device performance of OLETs offered by the FeCl3-doped graphene source electrode.
Highlights
Our first challenge relates to the structure and operating characteristics of our Gr-vertical OLETs (VOLETs), including the bottom indium tin oxide (ITO) gate, the Al2O3 gate dielectric layer, the single-layer graphene (SLG) source, the functional channel layers including the organic light-emitting layer (EML) of the Super Yellow (SY) conjugated copolymer, and the Al metal drain, in sequence (Fig. 1a, see Methods and Supplementary Fig. S1a for details of the structure and layer thicknesses)
During the operation of the Gr-VOLET, the electron injection occurs from the Al drain into the channel layer, and the hole injection from the SLG source can be modulated by adjusting the gate voltage VGS, as discussed below
To investigate its distinct characteristics further, we tested three types of SLG materials as source electrodes; (i) p-doped SLG with FeCl3, where FeCl3 doping is done spontaneously during the graphene transfer process, as shown previously[35]; (ii) pristine SLG, cleaned by an electrochemical process[35], as a comparative reference (SLG2 for Gr-VOLET2); and (iii) SLG coated with a conventional hole-injection layer (HIL) of (3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)[34] as a second comparative reference (SLG3 for Gr-VOLET3)
Summary
Organic light-emitting transistors (OLETs) have been developed by integrating the ability of organic light-emitting diodes (OLEDs)[1,2,3,4,5,6] to generate light with the electrical-switching functionality of organic field-effect transistors (FETs)[7,8] into a single device structure[9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]. Due to their compact architecture, they can be used to develop integrated organic opto-electronic devices such as highly efficient light sources, optical communication systems, and/or electrically driven organic lasers[11,12,13,14,15,16,17,18] For these principal reasons, the luminance from an OLET can be modulated without any additional driving devices, and displays using OLETs offer the additional advantage of reducing the number of high mobility-driving thin film transistors (TFTs) and simplifying the inherent complexity of the circuits of conventional active-matrix (AM) OLEDs14,17,20. To obtain highly increased source drain current flows even at low gate voltages, vertical OLETs (VOLETs) were constructed with short channel lengths by coupling a static induction transistor structure[21,22], constructing a metal insulator semiconductor structure[23,24], or using micro-networks with periodic vacancies in a vertical configuration[25,26] These devices have shown stripe-type or quasi-surface emission patterns. A reliable and high-resolution patterning method for electrodes and/or an insulating charge-restriction layer would be crucial to control the leakage currents and switching characteristics
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