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The concept of a metal-free and all-organic electroluminescent device is appealing from both sustainability and cost perspectives. Herein, we report the design and fabrication of such a light-emitting electrochemical cell (LEC), comprising a blend of an emissive semiconducting polymer and an ionic liquid as the active material sandwiched between two poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) conducting-polymer electrodes. In the off-state, this all-organic LEC is highly transparent, and in the on-state, it delivers uniform and fast to turn-on bright surface emission. It is notable that all three device layers were fabricated by material- and cost-efficient spray-coating under ambient air. For the electrodes, we systematically investigated and developed a large number of PEDOT:PSS formulations. We call particular attention to one such p-type doped PEDOT:PSS formulation that was demonstrated to function as the negative cathode, as well as future attempts towards all-organic LECs to carefully consider the effects of electrochemical doping of the electrode in order to achieve optimum device performance.

The initial operation of a light-emitting electrochemical cell (LEC) constitutes the in-situ formation of a p-n junction doping structure in the active material by electrochemical doping. It has been firmly established that the spatial position of the emissive p-n junction in the interelectrode gap has a profound influence on the LEC performance because of exciton quenching and microcavity effects. Hence, practical strategies for a control of the position of the p-n junction in LEC devices are highly desired. Here, we introduce a "chemical pre-doping" approach for the rational shifting of the p-n junction for improved performance. Specifically, we demonstrate, by combined experiments and simulations, that the addition of a strong chemical reductant termed "reduced benzyl viologen" to a common active-material ink during LEC fabrication results in a filling of deep electron traps and an associated shifting of the emissive p-n junction from the center of the active material towards the positive anode. We finally demonstrate that this chemical pre-doping approach can improve the emission efficiency and stability of a common LEC device.

Organic semiconductors (OSCs) have been a significant focus of research for electronic devices over the years exclusively due to their superior mechanical and optical properties as opposed to conventional inorganic semiconductors (ISCs) such as silicon or germanium. These unique materials have smoothened the path for developing extremely light-weight and ultrathin electronic devices with built-in flexibility and transparency for a considerably low cost. However, the commercial application of these organic materials is limited by their inferior electrical conductivity and charge carrier mobility compared with inorganic counterparts. This review article presents an overview of the works published on how to control and adjust the electrical conductivity of OSCs, focusing on electrical doping. The main point of this review is related to the principles and fundamental mechanisms of charge generation efficiency (CGE) in OSCs, which are significantly different from those of ISCs. The reported CGE of OSCs, defined as the ratio of the generated charge carriers to the dopants, is found to be in the range of a few percent and is significantly small compared with the ISCs. The origin of this lies in the lower dissociation rate of the charge transfer complexes (CTCs) formed between the tightly bound ionized dopants and generated charges. The CTCs induce localized electron-hole pairs, and the dissociation of such CTCs into free charge carriers requires large activation energies to overcome the strong Coulombic forces exerted on the generated charges by the dopant ions. Therefore, high doping concentrations are required to generate a large number of free charge carriers in doped OSC films. In this review, the CGE of OSCs is discussed in detail from the point of view of CTC formation and charge separation efficiencies.