Low-voltage-operating organic electrochemical light-emitting cells (LECs) and transistors (OECTs) can be realized in robust device architectures, thus enabling easy manufacturing of light sources using printing tools. In an LEC, the p-n junction, located within the organic semiconductor channel, constitutes the active light-emitting element. It is established and fixated through electrochemical p- and n-doping, which are governed by charge injection from the anode and cathode, respectively. In an OECT, the electrochemical doping level along the organic semiconducting channel is controlled via the gate electrode. Here we report the merger of these two devices: the light-emitting electrochemical transistor, in which the location of the emitting p-n junction and the current level between the anode and cathode are modulated via a gate electrode. Light emission occurs at 4 V, and the emission zone can be repeatedly moved back and forth within an interelectrode gap of 500 μm by application of a 4 V gate bias. In transistor operation, the estimated on/off ratio ranges from 10 to 100 with a gate threshold voltage of -2.3 V and transconductance value between 1.4 and 3 μS. This device structure opens for new experiments tunable light sources and LECs with added electronic functionality.

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Table of content:We report on a hybrid sheet-based manufacturing approach for organic electrochemical transistors comprising printing and a self-aligning laser ablation step. AbstractWe report on a hybrid manufacturing approach for organic electrochemical transistors (OECTs) on flexible substrates. The technology is based on conventional and digital printing (screen and inkjet printing), laser processing, and post-press technologies. A careful selection of the conductive, dielectric and semiconductor materials with respect to their optical properties enables a self-aligning pattern formation which results in a significant reduction of the usual registration problems during manufacturing. For the prototype OECTs, based on this technology, we obtained on/off ratios up to 600 and switching times of 100 milliseconds at gate voltages in the range of 1 V.3

Low-cost and low-voltage active matrix displays were fabricated by simply patterning gate electrode arrays on a polymer electrolyte (PE)-coated polymer light-emitting diode (PLED). Structurally, a PE capacitor seamlessly stacked on a PLED by sharing a common Al:LiF composite electrode (PEC|PLED). This monolithic integrated organic optoelectronic device was characterized and interpreted as the tunable work function (surface potential) because of the perturbation of accumulated ions on Al:LiF composite electrode by PEC charging and discharging. The modulation of electron injection by the PEC resulted in increases in the electroluminescent brightness, from <100 cd m to >8000 cd m, and the external quantum efficiency from <0.025% to 2.4%.

An understanding of the doping and ion distributions in light-emitting electrochemical cells (LECs) is required to approach a realistic conduction model which can precisely explain the electrochemical reactions, p-n junction formation, and ion dynamics in the active layer and to provide relevant information about LECs for systematic improvement of function and manufacture. Here, Fourier-transform infrared (FTIR) microscopy is used to monitor anion density profile and polymer structure in situ and for time-resolved mapping of electrochemical doping in an LEC under bias. The results are in very good agreement with the electrochemical doping model with respect to ion redistribution and formation of a dynamic p-n junction in the active layer. We also physically slow ions by decreasing the working temperature and study frozen-junction formation and immobilization of ions in a fixed-junction LEC device by FTIR imaging. The obtained results show irreversibility of the ion redistribution and polymer doping in a fixed-junction device. In addition, we demonstrate that infrared microscopy is a useful tool for in situ characterization of electroactive organic materials.