Fundamentals of Materials Selection for Light‐Emitting Electrochemical Cells

The in situ formation of a light‐emitting p–n or p–i–n junction in light‐emitting electrochemical cells (LECs) necessitates mixed ionic–electronic conductors in the active layer. This unique characteristic requires electronic, luminescent, and ionic ingredients that work synergistically in the LECs. The material requirements that lead to promising electroluminescent properties are discussed and the important components reported so far are surveyed. Particular attention is paid to the working mechanisms behind junction formation and stabilization to create efficient and stable electroluminescence in conjugated‐polymer‐based LECs. Keeping these fundamentals in mind explains how LEC devices have evolved from classic conjugated polymer blends into highly stable crosslinked, hybrid composite, and stretchable device architectures. To conclude, a future development strategy is proposed based on a dual approach: develop new materials specifically for LEC devices and explore novel ways to efficiently process and stabilize the p–i–n junction, which will drive improvements in both LEC external quantum efficiency and operating lifetime toward truly low‐cost solid‐state lighting applications.     

Fundamental Tradeoff between Emission Intensity and Efficiency in Light‐Emitting Electrochemical Cells

The characteristic doping process in polymer light‐emitting electrochemical cells (LECs) causes a tradeoff between luminescence intensity and efficiency. Experiments and numerical modeling on thin film polymer LECs show that, on the one hand, carrier injection and transport benefit from electrochemical doping, leading to increased electron‐hole recombination. On the other hand, the radiative recombination efficiency is reduced by exciton quenching by polarons involved in the doping. Consequently, the quasi‐steady‐state luminescent efficiency decreases with increasing ion concentration. The transient of the luminescent efficiency shows a characteristic roll‐off while the current continuously increases, attributed to ongoing electrochemical doping and the associated exciton quenching. Both effects can be modeled by exciton polaron‐quenching via diffusion‐assisted Förster resonance energy transfer. These results indicate that the tradeoff between efficiency and intensity is fundamental, suggesting that the application realm of future LECs should be sought in high‐brightness, low‐production cost devices, rather than in high‐efficiency devices.     

Small Molecules in Light‐Emitting Electrochemical Cells: Promising Light‐Emitting Materials

Light‐emitting electrochemical cells (LECs) are solid‐state lighting devices that convert electric current to light within electroluminescent organic semiconductors, and these devices have recently attracted significant attention. Introduced in 1995, LECs are considered a great breakthrough in the field of light‐emitting devices for their applications in scalable and adaptable fabrication processes aimed at producing cost‐efficient devices. Since then, LECs have evolved through the discovery of new suitable emitters, understanding the working mechanism of devices, and the development of various fabrication methods. LECs are best known for their simple architecture and easy, low‐cost fabrication techniques. The key feature of their fabrication is the use of air stable electrodes and a single active layer consisting of mobile ions that enable efficient charge injection and transport processes within LEC devices. More importantly, LEC devices can be operated at low voltages with high efficiencies, contributing to their widespread interest. This review provides a general overview of the development of LECs and discusses how small molecules can be utilized in LEC applications by overcoming the use of traditional lighting materials like polymers and ionic transition metal complexes. The achievements of each study concerning small molecule LECs are discussed.     

Efficient and Long Lived Green Light‐Emitting Electrochemical Cells

The combination of high efficiencies and long lifetime in a single light‐emitting electrochemical cell (LEC) device remain a major problem in LEC technology, preventing its application in commercial lighting devices. Three green light‐emitting cationic iridium‐based complexes of the general composition [Ir(C^{^}N)₂(N^{^}N)][PF₆] with 4‐Fppy (2‐(4‐fluorophenyl)pyridinato) as the cyclometalating C^{^}N ligand and 1,10‐phenanthroline ( 1 ), 4,7‐diphenyl‐1,10‐phenanthroline (bathophenanthroline, bphen, 2 ), and 2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline (bathocuprione, dmbphen, 3 ) as ancillary N^{^}N ligands are synthesized and characterized. Computational studies are carried out in order to compare the electronic structure of the three ionic transition metal complexes (iTMCs) and provide insights into their potential as LEC emitter materials. LECs are then fabricated with complexes 1 – 3 . Driven under a pulsed current, they display a high luminance and current and power efficiencies. As the LEC based on complex 2 displays the overall best device performance, including the longest lifetime of 474 h, it is selected for subsequent driving conditions optimization. An extraordinary power efficiency of 25 lm W^?1 and current efficiency of 30 cd A^?1 are achieved under optimized operation conditions with reduced current density, resulting in a long device lifetime of 720 h. Altogether, ligand design in iTMCs and optimization of the device driving conditions leads to a significant improvement in LEC performance.     

Salt Concentration Effects in Planar Light‐Emitting Electrochemical Cells

Incorporation of ions in the active layer of organic semiconductor devices may lead to attractive device properties like enhanced injection and improved carrier transport. In this paper, we investigate the effect of the salt concentration on the operation of light‐emitting electrochemical cells, using experiments and numerical calculations. The current density and light emission are shown to increase linearly with increasing ion concentration over a wide range of concentrations. The increasing current is accompanied by an ion redistribution, leading to a narrowing of the recombination zone. Hence, in absence of detrimental side reactions and doping‐related luminescence quenching, the ion concentration should be as high as possible.     

Dynamic Processes in Sandwich Polymer Light‐Emitting Electrochemical Cells

The operational mechanism of polymer light‐emitting electrochemical cells (LECs) in sandwich geometry is studied by admittance spectroscopy in combination with numerical modeling. At bias voltages below the bandgap of the semiconducting polymer, this allows the determination of the dielectric constant of the active layer, the conductivity of mobile ions, and the thickness of the electric double layers. At bias voltages above the bandgap, p–n junction formation gives rise to an increase in capacitance at intermediate frequencies (≈10 kHz). The time and voltage dependence of this junction are successfully studied and modeled. It is shown that impedance measurements cannot be used to determine the junction width. Instead, the capacitance at intermediate biases corresponds to a low‐conductivity region that can be significantly wider than the recombination zone. Finally, the long settling time of sandwich polymer LECs is shown to be due to a slow process of dissociation of salt molecules that continues after the light‐emitting p–n junction has formed. This implies that in order to significantly decrease the response‐time of LECs an electrolyte/salt combination with a minimal ion binding energy must be used.     

Molecularly Engineered Near‐Infrared Light‐Emitting Electrochemical Cells

The development of near‐infrared (NIR) luminescent materials has emerged as a promising research field with important applications in solid‐state lighting (SSL), night‐vision‐readable displays, and the telecommunication industry. Over the past two decades, remarkable advances in the development of light‐emitting electrochemical cells (LECs) have stunned the SSL community, which has in turn driven the quest for new classes of stable, more efficient NIR emissive molecules. In this review, an overview of the state of the art in the field of near‐infrared light‐emitting electrochemical cells (NIR‐LEC) is provided based on three families of emissive compounds developed over the past 25 years: i) transition metal complexes, ii) ionic polymers, and iii) host–guest materials. In this context, ionic and conductive emitters are particularly attractive since their emission can be tuned via molecular design, which involves varying the chemical nature and substitution pattern of their ancillary ligands. Herein, the challenges and current limitations of the latter approach are highlighted, particularly with respect to developing NIR‐LECs with high external quantum efficiencies. Finally, useful guidelines for the discovery of new, efficient emitters for tailored NIR‐LEC applications are presented, together with an outlook towards the design of new NIR‐SSL materials.     

Polymer Light‐Emitting Electrochemical Cells with Bipolar Electrode‐Dynamic Doping and Wireless Electroluminescence

This progress report reviews the background and recent developments of polymer light‐emitting electrochemical cells (PLECs). The PLECs of interest have a planar configuration and contain bipolar electrodes (BPEs). A BPE is an electrically floating conductor immersed in an electrochemical cell that contains redox species. When the electrochemical cell is polarized with an externally applied voltage bias, coupled redox reactions are induced wirelessly at the extremities of the BPEs due to the development of a sufficient interfacial potential difference. BPEs can have a dramatic effect on the doping pattern and the emission profiles of PLECs. In a bulk homojunction PLEC containing a large number of dispersed micro‐BPEs, the turn‐on response and light output are greatly enhanced when multiple light‐emitting p–n junctions form throughout the active layer. PLECs offer a solid‐state platform on which the bipolar electrochemistry phenomena can be investigated, and the understanding of the complex PLEC processes can be improved. This progress report highlights several new BPE types as well as their potential applications for device performance enhancement and for the visualized screening of functional materials.     

Advances and Challenges in White Light‐Emitting Electrochemical Cells

Since the birth of light‐emitting electrochemical cells (LECs) in 1995, white LECs (WLECs) still represent a milestone. To date, over 50 contributions have been reported, presenting record WLECs with brightness of up to 10 000 cd m^?2, efficiencies of >10 cd A^?1, and excellent color rendering index >90 in different contributions. This is achieved following three main strategies focused on modifying: i) the design of the emitters, that is, emissive aggregates, multiemissive mechanism, multifluorophoric emitters; ii) the active layer composition, that is, host–guest, multilayers, exciplex‐ and electroplex‐like emitting species systems; and iii) the device architecture, that is, tandem, photoactive filters, and microcavity/interfacial dipole effects. Herein, all of them are comprehensively discussed with respect to the above strategies in the frame of the type of emitters employed. Overall, this work highlights both the advances and challenges of the WLEC field.     

Self‐Heating in Light‐Emitting Electrochemical Cells

Electroluminescent devices become warm during operation, and their performance can, therefore, be severely limited at high drive current density. Herein, the effects of this self‐heating on the operation of a light‐emitting electrochemical cell (LEC) are systematically studied. A drive current density of 50 mA cm^?2 can result in a local device temperature for a free‐standing LEC that exceeds 50 °C within a short period of operation, which in turn induces premature device degradation as manifested in the rapidly decreasing luminance and increasing voltage. Furthermore, this undesired self‐heating for a free‐standing thin‐film LEC can be suppressed by the employment of a device architecture featuring high thermal conductance and a small emission‐area fill factor, since the corresponding improved heat conduction to the nonemissive regions facilitates more efficient heat transfer to the ambient surroundings. In addition, the reported differences in performance between small‐area and large‐area LECs as well as between flexible‐plastic and rigid‐glass LECs are rationalized, culminating in insights that can be useful for the rational design of LEC devices with suppressed self‐heating and high performance.     

Shedding Light on the Operation of Polymer Light‐Emitting Electrochemical Cells Using Impedance Spectroscopy

A combination of impedance spectroscopy, device characterization, and modeling is used to pinpoint key processes in the operation of polymer light‐emitting electrochemical cells (LECs). At low applied voltage, electric double layers with a thickness of ≈2–3 nm are shown to exist at the electrode interfaces. At voltages exceeding the bandgap potential of the conjugated polymer (V ≥ 2.5 V for superyellow), a light‐emitting p–n junction forms in situ, with a steady‐state structure that is found to depend strongly on the applied voltage. This is exemplified by that the effective p–n junction thickness (d_pn) for a device with an interelectrode gap of 90 nm decreases from ≈23 nm at 2.5 V to ≈6 nm at 3.9 V. The current increases with decreasing d_pn in a concerted manner, while the brightness reaches its peak at V = 3.4 V when d_pn ≈ 10 nm. The existence of an optimum d_pn for high brightness in LECs is attributed to an offset between an increase in the exciton formation rate with decreasing d_pn, due to an increasing current, and a simultaneous decrease in the exciton radiative decay rate, when an increasing fraction of excitons diffuses away from the p–n junction into the surrounding non‐radiative doping regions.     

Recent Progress on Light‐Emitting Electrochemical Cells with Nonpolymeric Materials

Light‐emitting electrochemical cells (LECs) have emerged as some of the simplest light‐emitting devices. Indeed, numerous LECs have been produced using fluorescent polymers; however, initial LEC structures require a mixture of polymers and electrolytes, thus strictly limiting their applicability. In contrast, recent advances in device technologies and material synthesis have opened a route for LECs using nonpolymeric materials. This progress report focuses on current developments in the device concepts, mechanisms, and characteristics of LECs that allow the utilization of nonpolymeric materials. First, the three primary device types, namely, electrochemically doped, ionic‐material, and electrostatically doped LECs, are categorized, and their distinct features are described. Second, electrochemically doped LECs based on small molecules and branched molecules are introduced. Then, an overview of the rapidly growing field of ionic‐material LECs, especially ionic transition metal complexes, ionic small molecules and perovskites, and their characteristics are provided. Following these results, recent achievements in solid‐state materials, such as inorganic single crystals, quantum dots, and 2D materials, as electrostatically doped LECs are highlighted. Finally, an overview and evaluation of these LECs reveal the key directions and remaining issues that must be overcome to further functionalize LECs, which provide a versatile approach for new lighting applications comprising emergent materials.     

Organic–Inorganic Perovskite Light‐Emitting Electrochemical Cells with a Large Capacitance

While perovskite light‐emitting diodes typically made with high work function anodes and low work function cathodes have recently gained intense interests. Perovskite light‐emitting devices with two high work function electrodes with interesting features are demonstrated here. Firstly, electroluminescence can be easily obtained from both forward and reverse biases. Secondly, the results of impedance spectroscopy indicate that the ionic conductivity in the iodide perovskite (CH₃NH₃PbI₃) is large with a value of ≈10^?8 S cm^?1. Thirdly, the shift of the emission spectrum in the mixed halide perovskite (CH₃NH₃PbI_3?xBr_x) light‐emitting devices indicates that I^? ions are mobile in the perovskites. Fourthly, this work shows that the accumulated ions at the interfaces result in a large capacitance (≈100 μF cm^?2). The above results conclusively prove that the organic–inorganic halide perovskites are solid electrolytes with mixed ionic and electronic conductivity and the light‐emitting device is a light‐emitting electrochemical cell. The work also suggests that the organic–inorganic halide perovskites are potential energy‐storage materials, which may be applicable in the field of solid‐state supercapacitors and batteries.     

Recent Advances in Optical Engineering of Light‐Emitting Electrochemical Cells

Since the first demonstration of light‐emitting electrochemical cells (LECs) in 1995, much effort has been made to develop this technology for display and lighting. A common LEC generally contains a single emissive layer blended with a salt, which provides mobile ions under a bias. Ions accumulated at electrodes facilitate electrochemical doping such that operation voltage is low even when employing high‐work‐function inert electrodes. The superior properties of simple device architecture, low‐voltage operation, and compatibility with inert metal electrode render LECs suitable for cost‐effective light‐emitting sources. In addition to enormous progress in developing novel emissive materials for LECs, optical engineering has been shown to improve device performance of LECs in an alternative way. Light outcoupling enhancement technologies recycle the trapped light and increase the light output from LECs. Techniques to estimate emission zone position provide a powerful tool to study carrier balance of LECs and to optimize device performance. Spectral tailoring of the output emission from LECs based on microcavity effect and localized surface plasmon resonance of metal nanoparticles improves the intrinsic emission properties of emissive materials by optical means. These reported optical techniques are overviewed in this review.     

The Dynamic Emission Zone in Sandwich Polymer Light‐Emitting Electrochemical Cells

In light‐emitting electrochemical cells (LECs), the position of the emission zone (EZ) is not predefined via a multilayer architecture design, but governed by a complex motion of electrical and ionic charges. As a result of the evolution of doped charge transport layers that enclose a dynamic intrinsic region until steady state is reached, the EZ is often dynamic during turn‐on. For thick sandwich polymer LECs, a continuous change of the emission color provides a direct visual indication of a moving EZ. Results from an optical and electrical analysis indicate that the intrinsic zone is narrow at early times, but starts to widen during operation, notably well before the electrical device optimum is reached. Results from numerical simulations demonstrate that the only precondition for this event to occur is that the mobilities of anions (μ_a) and cations (μ_c) are not equal, and the direction of the EZ shift dictates μ_c > μ_a. Quantitative ion profiles reveal that the displacement of ions stops when the intrinsic zone stabilizes, confirming the relation between ion movement and EZ shift. Finally, simulations indicate that the experimental current peak for constant‐voltage operation is intrinsic and the subsequent decay does not result from degradation, as commonly stated.     

Recent Progress in White Light‐Emitting Electrochemical Cells

Solid‐state white light‐emitting electrochemical cells (LECs) exhibit the following advantages: simple device structures, low operation voltage, and compatibility with inert metal electrodes. LECs have been studied extensively since the first demonstration of white LECs in 1997, due to their potential application in solid‐state lighting. This review provides an overview of recent developments in white LECs, specifically three major aspects thereof, namely, host–guest white LECs, nondoped white LECs, and device engineering of white LECs. Host–guest strategy is widely used in white LECs. Host materials are classified into ionic transition metal complexes, conjugated polymers, and small molecules. Nondoped white LECs are based on intra‐ or intermolecular interactions of emissive and multichromophore materials. New device engineering techniques, such as modifying carrier balance, color downconversion, optical filtering based on microcavity effect and localized surface plasmon resonance, light extraction enhancement, adjusting correlated color temperature of the output electroluminescence spectrum, tandem and/or hybrid devices combining LECs with organic light‐emitting diodes, and quantum‐dot light‐emitting diodes improve the device performance of white LECs by ways other than material‐oriented approaches. Considering the results of the reviewed studies, white LECs have a bright outlook.     

Scrutinizing Design Principles toward Efficient,Long‐Term Stable Green Light‐Emitting Electrochemical Cells

Enhancing the efficiency and lifetime of light emitting electrochemical cells (LEC) is the most important challenge on the way to energy efficient lighting devices of the future. To avail this, emissive Ir(III) complexes with fluoro‐substituted cyclometallated ligands and electron donating groups (methyl and tert ‐butyl)‐substituted diimine ancillary (N^{^}N) ligands and their associated LEC devices are studied. Four different complexes of general composition [Ir(4ppy)₂(N^{^}N)][PF₆] (4Fppy = 2‐(4‐fluorophenyl)pyridine) with the N^{^}N ligand being either 2,2′‐bipyridine ( 1 ), 4.4′‐dimethyl‐2,2′‐bipyridine ( 2 ), 5.5′‐dimethyl‐2,2′‐bipyridine ( 3 ), or 4.4′‐di‐tert ‐butyl‐2,2′‐bipyridine ( 4 ) are synthesized and characterized. All complexes emit in the green region of light with emission maxima of 529–547 nm and photoluminescence quantum yields in the range of 50.6%–59.9%. LECs for electroluminescence studies are fabricated based on these complexes. The LEC based on ( 1 ) driven under pulsed current mode demonstrated the best performance, reaching a maximum luminance of 1605 cd m^?2 resulting in 16 cd A^?1 and 8.6 lm W^?1 for current and power efficiency, respectively, and device lifetime of 668 h. Compared to this, LECs based on ( 3 ) and ( 4 ) perform lower, with luminance and lifetime of 1314 cd m^?2, 45.7 h and 1193 cd m^?2, 54.9 h, respectively. Interestingly, in contrast to common belief, the fluorine content of the Ir‐iTMCs does not adversely affect the LEC performance, but rather electron donating substituents on the N^{^}N ligands are found to dramatically reduce both performance and stability of the green LECs. In light of this, design concepts for green light emitting electrochemical devices have to be reconsidered.     

Fully Printed Light‐Emitting Electrochemical Cells Utilizing Biocompatible Materials

The use of biomaterials and bioinspired concepts in electronics will enable the fabrication of transient and disposable technologies within areas ranging from smart packaging and advertisement to healthcare applications. In this work, the use of a nonhalogenated biodegradable solid polymer electrolyte based on poly(ε‐caprolactone‐co‐trimethylene carbonate) and tetrabutylammonium bis‐oxalato borate in light‐emitting electrochemical cells (LECs) is presented. It is shown that the spin‐cast devices exhibit current efficiencies of ≈2 cd A^?1 with luminance over ≈12 000 cd m^?2, an order of magnitude higher than previous bio‐based LECs. By a combination of industrially relevant techniques (i.e., inkjet printing and blade coating), the fabrication of LEC devices on a cellulose‐based flexible biodegradable substrate showing lifetimes compatible with transient applications is demonstrated. The presented results have direct implications toward the industrial manufacturing of biomaterial‐based light‐emitting devices with potential use in future biodegradable/biocompatible electronics.