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This work describes the first use of blue‐emitting boron‐ and nitrogen‐doped carbon dots (BN‐CDs), rationalizing their photoluminescence behavior in solution and ion‐based thin‐films to prepare white light‐emitting electrochemical cells (LECs). In detail, a cost‐effective and scalable water‐based microwave‐assisted synthesis procedure is set for BN‐CDs featuring an amorphous carbon‐core doped with N and B. While they show a bright (photoluminescence quantum yield of 42%) and excitation‐independent blue‐emission (440 nm) in solution related to emitting n−π* surface states, they are not emissive in thin‐films due to aggregation‐induced quenching. Upon fine‐tuning the film composition (ion‐based host), an excitation dependent emission covering the whole visible range is noted caused by interaction of the ion electrolyte with the peripheral functionalization of the BN‐CDs. Moreover, the efficient energy transfer from the host to the BN‐CDs emitting species enabled good performing LECs with white emission (x/y CIE coordinates of 0.30/0.35, correlated color temperature of 6795 K, color rendering index of 87) and maximum luminance of 40 cd m−2, and stabilities of a few hours. This represents a significant improvement compared to the prior‐art monochromatic CD‐based LECs with similar brightness levels, but stabilities of <1 min. [ABSTRACT FROM AUTHOR]

Developing transparent and highly refractive environmentally friendly polymers has not been realized yet toward sustainable optoelectronics. This work describes poly(thiourea)s (PTUs) design following a new “polarizable group synergy” concept, combining highly polarizable hydrogen bonding groups and aromatic‐based spacers to form densely packed and high‐refractive‐index polymer networks. Specifically, PTUs containing m‐ and p‐phenylene spacers exhibit an easy synthesis, high thermostability (Tg = 159 °C), visible transparency (>92%T at 1 µm‐film), ultra‐high refractive index (nD = 1.81) based on the random H‐bonding arrays with a high packing constant (Kp = 0.738), and straightforward preparation of flexible films via solvent‐based techniques. Capitalizing on these assets, PTU‐films are integrated into benchmark graphene‐based lighting device architectures based on the light‐emitting electrochemical cells (LECs) concept. A joint optical modeling and experimental validation confirm the increase in external quantum efficiency expected by the enhanced light out‐coupling of PTU‐films. Finally, PTUs are efficiently depolymerized to low molecular weight compounds by simply adding diamines under heating, following the dynamic covalent bond exchange between thiourea moieties. Overall, this work highlights the PTU family as new promising materials with a unique polarizable H‐bond design to meet efficient and sustainable thin‐film lighting devices. [ABSTRACT FROM AUTHOR]

It is crucial to develop functional electronic materials that can be processed from green solvents to achieve environmentally sustainable and cost‐efficient printing fabrication of organic electronic devices. Here, the design and cost‐efficient synthesis of two hydrophilic and emissive conjugated polymers, TQ‐OEG and TQ2F‐OEG, are presented, which are rendered hydrophilic through the grafting of oligo(ethylene glycol) (OEG) solubilizing groups onto the thiophene‐quinoxaline conjugated backbone and thereby can be processed from a water:ethanol solvent mixture. The results show that the introduction of the OEG groups enables for a direct dissolution of salts by the neat polymer for the attainment of solid‐state ion mobility. These properties are utilized for the design and development of light‐emitting electrochemical cells (LECs), the active materials of which can be solution cast from a water:ethanol‐based ink. It is specifically shown that such an LEC device, comprising an optimized blend of the TQ2F‐OEG emitter and a Li salt as the active material positioned between two air‐stabile electrodes, delivers deep‐red emission (peak wavelength = 670 nm) with a radiance of 185 µW m−2 at a low drive voltage of 2.3 V. This study contributes relevant information as to how polymers and LEC devices can be designed and fabricated to combine functionality with sustainability. [ABSTRACT FROM AUTHOR]

Light‐emitting electrochemical cells (LECs) are kind of easily fabricated and low‐cost light‐emitting devices that can efficiently convert electric power to light energy. Compared with blue and green LECs, the performance of deep‐red LECs is limited by the high non‐radiative rate of emitters in long‐wavelength region. While various organic emitters with deep‐red emission have been developed to construct high‐performance LECs, including polymers, metal complexes, and organic luminous molecules (OLMs), but this is seldom summarized. Therefore, we overview the recent advances of organic emitters with emission at the deep‐red region for LECs, and specifically highlight the molecular design approach and electrochemiluminescence performance. We hope that this review can act as a reference for further research in designing high‐performance deep‐red LECs. [ABSTRACT FROM AUTHOR]

Enhancing low‐energy emitting Cu(I)‐ionic transition metal complexes (iTMCs) light‐emitting electrochemical cells (LECs) is of utmost importance towards Cu(I)‐iTMC‐based white‐emitting LECs. Here, the ancillary ligand design includes (i) extension of π‐systems and (ii) insertion of S‐bridge between heteroaromatics rings. This led to two novel heteroleptic Cu(I)‐iTMCs: 2‐(pyridin‐2‐yl‐l2‐azanyl)quinoline (CuN2) and 2‐(naphthalen‐2‐ylthio)quinoline (CuS2) as N^N and bis[(2‐diphenylphosphino)phenyl] ether as P^P, exhibiting improved photoluminescence quantum yields (ϕ) and thermally activated delayed fluorescence processes compared to their reference Cu(I)‐iTMCs: di(pyridin‐2‐yl)‐l2‐azane (CuN1) and di(pyridin‐2‐yl)sulfane (CuS1). Despite CuS2 stands out with the highest ϕ (38% vs 17 / 14 / 1% for CuN1 / CuN2 / CuS1), only CuN2‐LECs show the expected enhanced performance (0.35 cd A−1 at luminance of 117 cd m−2) compared to CuN1‐LECs (0.02 cd A−1 at6 cd m−2), while CuS2‐LECs feature low performances (0.04 cd A−1 at 10 cd m−2). This suggests that conventional chemical design rules are not effective towards enhancing device performance. Herein, nonconventional multivariate statistical analysis and electrochemical impedance spectroscopy studies allow to rationalize the mismatch between chemical design and device performance bringing to light a hidden design rule: polarizability of the ancillary ligand is key for an efficient Cu(I)‐iTMC‐LECs. All‐in‐all, this study provides fresh insights for the design of Cu‐iTMCs fueling research on sustainable ion‐based lighting sources. [ABSTRACT FROM AUTHOR]

Organic semiconductors that emit by the process of multi‐resonance thermally activated delayed fluorescence (MR‐TADF) can deliver narrowband and efficient electroluminescence while being processable from solvents and metal‐free. This renders them attractive for use as the emitter in sustainable light‐emitting electrochemical cells (LECs), but so far reports of narrowband and efficient MR‐TADF emission from LEC devices are absent. Here, this issue is addressed through careful and systematic material selection and device development. Specifically, the authors show that the detrimental aggregation tendency of an archetypal rigid and planar carbazole‐based MR‐TADF emitter can be inhibited by its dispersion into a compatible carbazole‐based blend host and an ionic‐liquid electrolyte, and it is further demonstrated that the tuning of this active material results in a desired balanced p‐ and n‐type electrochemical doping, a high solid‐state photoluminescence quantum yield of 91%, and singlet and triplet trapping on the MR‐TADF guest emitter. The introduction of this designed metal‐free active MR‐TADF material into a LEC, employing air‐stabile electrodes, results in bright blue electroluminescence of 500 cd m−2, which is delivered at a high external quantum efficiency of 3.8% and shows a narrow emission profile with a full‐width‐at‐half‐maximum of 31 nm. [ABSTRACT FROM AUTHOR]

Planar polymer light‐emitting electrochemical cells (LECs) with concentric circle electrodes are demonstrated. With a fixed outer electrode radius of 6.8 mm, the inner electrode radius is varied from 1 to 3 mm. All cells can be activated with a 40 V bias at 360 K. The extremely large electrode gap size allows for time‐lapsed imaging of the in situ electrochemical p‐ and n‐doping processes. When the advancing p‐ and n‐doping fronts meet, a light‐emitting junction in the shape of a jagged ring is formed near the center of the electrode gap. Reversing bias polarity pushes the light‐emitting junction outward to near the negative electrode. It is possible to achieve a perfectly centered emitting junction by fine tuning the inner electrode radius. The sensitivity of junction position to relative electrode radius indicates the p‐ and n‐doping reactions are strongly coupled. The concentric circular electrode configuration offers a cross sectional view of an LEC‐based light‐emitting fiber. In any fiber shaped light‐emitting device, the inner and outer electrodes always differ in size. This electrode asymmetry should be considered and exploited to achieve optimal cell performance. [ABSTRACT FROM AUTHOR]

Organic light‐emitting diodes (OLEDs) attract much research attention owing to their superior device characteristics in display and lighting applications. However, multi‐layered organic thin films deposited by thermal evaporation in a high‐vacuum chamber complicate the fabrication processes and increase the production costs. Alternatively, solution‐processable single‐layered light‐emitting electrochemical cells (LECs) possess easy fabrication and good device performance due to in situ electrochemical doping. To take advantages of both types of devices, the study proposes an efficient tandem white OLED/LEC hybrid device by stacking a red OLED on top of a blue LEC with a sophisticatedly designed charge‐generating layer (CGL). The proposed tandem device exhibits a high external quantum efficiency (EQE) of 21.53%, which is almost the sum of the EQEs of the two stacked devices. When fabricated on the diffusive substrate to recycle the trapped light in the substrate, the EQE can be further enhanced to reach 37.88%. Besides connecting the two stacked devices, the CGL improves the carrier balance of the blue LEC due to enhanced electron injection. This study demonstrates that the white tandem OLED/LEC hybrid device has a simpler device structure than all‐vacuum‐processed tandem OLEDs but it still delivers high device efficiency, revealing its potential in lighting applications. [ABSTRACT FROM AUTHOR]

Two binuclear heteroleptic CuI complexes, namely Cu−NIR1 and Cu−NIR2, bearing rigid chelating diphosphines and π‐conjugated 2,5‐di(pyridin‐2‐yl)thiazolo[5,4‐d]thiazole as the bis‐bidentate ligand are presented. The proposed dinuclearization strategy yields a large bathochromic shift of the emission when compared to the mononuclear counterparts (M1–M2) and enables shifting luminescence into the near‐infrared (NIR) region in both solution and solid state, showing emission maximum at ca. 750 and 712 nm, respectively. The radiative process is assigned to an excited state with triplet metal‐to‐ligand charge transfer (3MLCT) character as demonstrated by in‐depth photophysical and computational investigation. Noteworthy, X‐ray analysis of the binuclear complexes unravels two interligand π–π‐stacking interactions yielding a doubly locked structure that disfavours flattening of the tetrahedral coordination around the CuI centre in the excited state and maintain enhanced NIR luminescence. No such interaction is present in M1–M2. These findings prompt the successful use of Cu−NIR1 and Cu−NIR2 in NIR light‐emitting electrochemical cells (LECs), which display electroluminescence maximum up to 756 nm and peak external quantum efficiency (EQE) of 0.43 %. Their suitability for the fabrication of white‐emitting LECs is also demonstrated. To the best of our knowledge, these are the first examples of NIR electroluminescent devices based on earth‐abundant CuI emitters. [ABSTRACT FROM AUTHOR]

Light‐emitting electrochemical cells (LECs) have attracted considerable attention owing to their unique luminescence mechanisms and device structures, making them suitable for solution processing. However, these cells encounter problems related to color purity, which plays an important role in efficient and high‐resolution displays. Here, two types of solution‐processed LECs are reported that exhibit ultrapure blue emissions at 467 and 452 nm with narrow emission full‐widths at half‐maximum (FWHM) values of 21 and 30 nm, corresponding to the Commission Internationale de l'éclairage (CIE) coordinates of (0.12, 0.15) and (0.16, 0.10), respectively. These values are obtained using the multi‐resonance‐induced thermally activated delayed fluorescence (MR‐TADF) materials, namely, ν‐DABNA and DABNA‐4TBC. The suppression of structural relaxation and vibronic coupling in these MR‐TADF materials results in high‐color‐purity emissions with a small FWHM. Host‐guest‐type LECs employing these molecules as guest emitters in a spin‐coated single active layer exhibit both ultrapure blue electroluminescence (EL) and solution processability, strongly supporting their use in building a sustainable society in the future. [ABSTRACT FROM AUTHOR]

A rational implementation and optimization of thermally activated delayed fluorescent (TADF) dendrimer emitters in light‐emitting electrochemical cells (LECs) sets in the Dendri‐LEC family. They feature outstanding stabilities (90/1050 h for green/yellow devices) that are comparable to the best green/yellow Ir(III)‐complexes (450/500 h) and conjugated polymers (33/5500 h), while offering benefits of low‐cost synthesis and easy upscaling. In particular, a fundamental molecular design that capitalizes on exchanging peripheral substituents (tert‐butyl vs methoxy) to tune photophysical, electrochemical, morphological, and ion conductivity features in thin films is rationalized by temperature‐dependent steady‐state and time‐resolved emission spectroscopy, cyclic voltammetry, atomic force microscopy, and electrochemical impedance spectroscopy techniques. Herein, a TADF mechanism associated to a reduced photoluminescence quantum yield, but an enhanced electrochemical stability and ion conductivity enables to clarify the reduced device efficiency and brightness (4.0 lm W−1@110 cd m−2 vs 3.2 lm W−1@55 cd m−2) and increased stability (90 vs 1050 h) upon using methoxy groups. What is more, this substitution enables an excellent compatibility with biogenic electrolytes keeping device performances (1.9 lm W−1@35 cd m−2 and 1300 h), while graphene‐devices achieve on par figures to traditional indium–tin oxide‐devices. Overall, this work establishes the bright future of dendrimer emitters toward highly stable and truly sustainable lighting sources. [ABSTRACT FROM AUTHOR]

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The orientation of the emissive dipoles in thin‐film devices is important since it strongly affects the light outcoupling and thereby the device emission efficiency. The light‐emitting electrochemical cell (LEC) is particularly interesting in this context because its emissive dipoles are located in a high electric‐field p‐n junction, which is formed in situ by redistribution of bulky ions. This implies that the dipole orientation could be distinctly different in the driven LEC compared to the pristine device. This study develops the destructive‐interference microcavity method for the accurate in situ determination of the orientation of the emissive dipoles during LEC operation and apply it on a common LEC device comprising an amorphous conjugated polymer termed Super Yellow as the emitter. It is found that ≈95% of the emissive dipoles are oriented in the horizontal direction with respect to the thin‐film plane in both the pristine LEC and during steady‐state light emission. This finding is attractive since it enables for efficient outcoupling of the generated photons, and interesting because it shows that a horizontal orientation of the emissive dipoles can remain despite the existence of a strong perpendicular electric field and the nearby motion of bulky ions during LEC operation. [ABSTRACT FROM AUTHOR]

Phenanthroimidazole derivatives are currently frequently used chemical compounds in the active layers of various devices, such as organic light-emitting diodes (OLEDs) or light-emitting electrochemical cells (LECs). Their advantages include simple and cheap synthesis, good solubility in organic solvents, easy processing, high thermal stability, excellent electron transport ability, and very good optical and electroluminescent properties. The construction of the LEC is based on a very simple construction in which the active layer plays a significant role. Thus, the compounds used in it are extremely important because they play the role of more than just emitters. This review summarizes the current state of the art regarding phenanthroimidazole derivatives used in the active layer in LECs. The synthesis of the considered compounds is presented in detail in the first part of the review. Next, research on the physicochemical properties of phenanthroimidazole derivatives is presented. Finally, the latest reports on LEC test devices are presented. [ABSTRACT FROM AUTHOR]

The development of circularly polarized electroluminescence (CPEL) is currently hampered by the high difficulty and cost in the syntheses of suitable chiral materials and the notorious chirality diminishment issue in electrical devices. Herein, diastereomeric IrIII and RuII complexes with chiral (±)‐camphorsulfonate counteranions are readily synthesized and used as the active materials in circularly polarized light‐emitting electrochemical cells to generate promising CPELs. The addition of the chiral ionic liquid (±)‐1‐butyl‐3‐methylimidazole camphorsulfonate into the active layer significantly improves the device performance and the electroluminescence dissymmetry factors (≈10−3), in stark contrast to the very weak circularly polarized photoluminescence of the spin‐coated films of these diastereomeric complexes. Control experiments with enantiopure IrIII complexes suggest that the chiral anions play a dominant role in the electrically‐induced amplification of CPELs. [ABSTRACT FROM AUTHOR]

Solid‐state light‐emitting electrochemical cells (LECs) show promising advantages of simple device architecture, low operation voltage, and insensitivity to the electrode work functions such that they have high potential in low‐cost display and lighting applications. In this work, novel white LECs based on phosphor‐sensitized thermally activated delayed fluorescence (TADF) are proposed. The emissive layer of these white LECs is composed of a blue‐green phosphorescent host doped with a deep‐red TADF guest. Efficient singlet‐to‐triplet intersystem crossing (ISC) on the phosphorescent host and the subsequent Förster energy transfer from the host triplet excitons to guest singlet excitons can make use of both singlet and triplet excitons on the host. With the good spectral overlap between the host emission and the guest absorption, 0.075 wt.% guest doping is sufficient to cause substantial energy transfer efficiency (ca. 40 %). In addition, such a low guest concentration also reduces the self‐quenching effect and a high photoluminescence quantum yield of up to 84 % ensures high device efficiency. The phosphor‐sensitized TADF white LECs indeed show a high external quantum efficiency of 9.6 %, which is comparable with all‐phosphorescent white LECs. By employing diffusive substrates to extract the light trapped in the substrate, the device efficiency can be further improved by ca. 50 %. In the meantime, the intrinsic EL spectrum and device lifetime of the white LECs recover since the microcavity effect is destroyed. This work successfully demonstrates that the phosphor‐sensitized TADF white LECs are potential candidates for efficient white light‐emitting devices. [ABSTRACT FROM AUTHOR]

We designed and synthesized two new ionic thermally activated delayed fluorescent (TADF) emitters that are charged analogues of a known multiresonant TADF (MR-TADF) compound, DiKTa. The emission of the charged derivatives is red-shifted compared to the parent compound. For instance, DiKTa-OBuIm emits in the green (λPL = 499 nm, 1 wt % in mCP) while DiKTa-DPA-OBuIm emits in the red (λPL = 577 nm, 1 wt % in mCP). In 1 wt % mCP films, both emitters showed good photoluminescence quantum yields of 71% and 61%, and delayed lifetimes of 316.6 μs and 241.7 μs, respectively, for DiKTa-OBuIm and DiKTa-DPA-OBuIm, leading to reverse intersystem crossing rates of 2.85 × 103 s−1 and 3.04 × 103 s−1. Light-emitting electrochemical cells were prepared using both DiKTa-OBuIm and DiKTa-DPA-OBuIm as active emitters showing green (λmax = 534 nm) and red (λmax = 656 nm) emission, respectively.

Long‐wavelength light‐emitting electrochemical cells (LECs) are potential deep‐red and near infrared light sources with solution‐processable simple device architecture, low‐voltage operation, and compatibility with inert metal electrodes. Many scientific efforts have been made to material design and device engineering of the long‐wavelength LECs over the past two decades. The materials designed the for long‐wavelength LECs cover ionic transition metal complexes, small molecules, conjugated polymers, and perovskites. On the other hand, device engineering techniques, including spectral modification by adjusting microcavity effect, light outcoupling enhancement, energy down‐conversion from color conversion layers, and adjusting intermolecular interactions, are also helpful in improving the device performance of long‐wavelength LECs. In this review, recent advances in the long‐wavelength LECs are reviewed from the viewpoints of materials and device engineering. Finally, discussions on conclusion and outlook indicate possible directions for future developments of the long‐wavelength LECs. This review would like to pave the way for the researchers to design materials and device engineering techniques for the long‐wavelength LECs in the applications of displays, bio‐imaging, telecommunication, and night‐vision displays. [ABSTRACT FROM AUTHOR]

Emerging organic light‐emitting devices, such as light‐emitting electrochemical cells (LECs), offer a multitude of advantages but currently suffer from that most efficient phosphorescent emitters are based on expensive and rare metals. Herein, it is demonstrated that a rare metal‐free salt, bis(benzyltriphenylphosphonium)tetrabromidomanganate(II) ([Ph3PBn]2[MnBr4]), can function as the phosphorescent emitter in an LEC, and that a careful device design results in the fact that such a rare metal‐free phosphorescent LEC delivers broadband white emission with a high color rendering index (CRI) of 89. It is further shown that broadband emission is effectuated by an electric‐field‐driven structural transformation of the original green‐light emitter structure into a red‐emitting structure.

Single layer light‐emitting electrochemical cells (LECs) are amongst the simplest electroluminescent devices and operate with air‐stable electrodes. Transparent light‐emitting devices are of great interest as they can enable new applications in consumer electronics. In this work, a transparent ionic transition metal complex based LEC is fabricated by developing a transparent top contact based on tin (IV) oxide (SnO2) and indium‐tin oxide, processed by low‐temperature atomic layer deposition and pulsed laser deposition, respectively. The resulting devices present transparency in excess of 75% over the full visible spectrum (380–750 nm), with 82% transmission at the emission peak (563 nm). The devices emit from the front and the rear with high luminance (260 cd m−2) and long lifetime (176 h). These parameters place them among the highest performing single layer transparent electroluminescent devices. [ABSTRACT FROM AUTHOR]

Light‐emitting electrochemical cells (LECs) comprising metal‐free molecules that emit by the process of thermally activated delayed fluorescence (TADF) can be both sustainable and low cost. However, the blue emission performance of current TADF‐LECs is unfortunately poor, which effectively prohibits their utilization in important applications, such as illumination and full‐color displays. Here, this drawback is addressed through the development of a TADF‐LEC, which delivers blue light emission (peak wavelength = 475 nm) with a high external quantum efficiency of 5.0%, corresponding to a current efficacy of 9.6 cd A‐1. It is notable that this high efficiency is attained at bright luminance of 740 cd m‐2, and that the device turn‐on is very fast. It is demonstrated that this accomplishment is enabled by the blending of a carbazole‐based 9‐(4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)‐2‐methylphenyl)‐3,6‐dimethyl‐9H‐carbazole guest emitter with a compatible carbazole‐based tris(4‐carbazoyl‐9‐ylphenyl)amine:2,6‐bis(3‐(carbazol‐9‐yl)phenyl)pyridine blend‐host for the attainment of bipolar electrochemical doping, balanced electron/hole transport, and exciplex‐effectuated host‐to‐guest energy transfer. [ABSTRACT FROM AUTHOR]

Phenanthroimidazole derivatives are currently frequently used chemical compounds in the active layers of various devices, such as organic light-emitting diodes (OLEDs) or light-emitting electrochemical cells (LECs). Their advantages include simple and cheap synthesis, good solubility in organic solvents, easy processing, high thermal stability, excellent electron transport ability, and very good optical and electroluminescent properties. The construction of the LEC is based on a very simple construction in which the active layer plays a significant role. Thus, the compounds used in it are extremely important because they play the role of more than just emitters. This review summarizes the current state of the art regarding phenanthroimidazole derivatives used in the active layer in LECs. The synthesis of the considered compounds is presented in detail in the first part of the review. Next, research on the physicochemical properties of phenanthroimidazole derivatives is presented. Finally, the latest reports on LEC test devices are presented.

Light‐emitting electrochemical cells (LECs) are the simplest and cheapest solid‐state lighting technology for soft and/or single‐use purposes. However, a major concern is a transition toward eco‐friendly devices (emitters/electrolytes/electrodes) to meet green optoelectronic requirements without jeopardizing device performance. In this context, this study shows the first biogenic electrolyte applied to LECs, realizing self‐stable and highly performing devices with cellulose‐based electrolytes combined with archetypical emitters (conjugated polymers or CPs and ionic transition‐metal complexes or iTMCs). In contrast to reference devices with traditional electrolytes, self‐stability tests (ambient storage/thermal‐stress) show that devices with this bio‐electrolyte hold film roughness and photoluminescence quantum yields over time. In addition, charge injection is enhanced due to the high dielectric constant, leading to high efficacies of 15 cd A−1@3750 cd m−2 and 2.5 cd A−1@600 cd m−2 associated with stabilities of 3000/7.5 h and 153/0.7 J for CPs/iTMCs‐LECs, respectively. They represent four‐/twofold enhancement compared to reference devices. Hence, this novel biogenic electrolyte approach does not reduce device performance as in the prior‐art bio‐degradable polymer and DNA‐hybrid electrolytes, while the easiness of chemical modification provides plenty of room for future developments. All‐in‐all, this study reinforces the relevance of carbohydrate‐based electrolytes not only for energy‐related applications, but also for a new field in lighting. [ABSTRACT FROM AUTHOR]

Light-emitting electrochemical cells (LECs) hold potential for low-cost, large-area lighting applications due to their air-stable electrodes and various solution preparation methods. However, the high cost of noble metals poses a significant challenge to the development of LECs. To address this issue, this study designed a series of five copper(I) complexes within the framework of using 2-(2′-pyridyl)imidazole (PI) as a ligand to achieve efficient copper-based ionic transition metal complexes (iTMCs). In electroluminescence (EL) tests of LECs, complexes Cu-PI-P and Cu-PI-PFCN exhibited the highest efficiency of saturated red emission to date, with maximum external quantum efficiencies (EQE) of 0.63 and 0.82 %, respectively. Additionally, Cu-PI-PCN , Cu-PI-PFCN , and Cu-PI-PF are the first white-light emitting LECs known in the literature using a non-doped approach, with EQE of 0.34, 0.43, and 0.98 %, respectively. • Five copper(I) complexes using 2-(2'-pyridyl)imidazole to achieve efficient copper-based ionic transition metal complexes. • Complexes Cu-PI-P and Cu-PI-PFCN exhibited the highest efficiency of saturated red emission to date. • Cu-PI-PCN , Cu-PI-PFCN , and Cu-PI-PF are the first white LECs known in the literature using a non-doped approach. [ABSTRACT FROM AUTHOR]

Solid-state light-emitting electrochemical cells (LECs) offer promising benefits, including simple device architecture, low operating voltage, and insensitivity to electrode work functions. These advantages make them highly suitable for low-cost display and lighting applications, with significant potential for widespread adoption. However, great challenges remain in realizing stable and efficient white LECs. In this work, the blue LECs integrated with red or yellow/red quantum-dot (QD) color conversion layers (CCLs) are shown to achieve efficient and stable white emissions. When the blue LECs are combined with red QD CCLs, they achieve an external quantum efficiency (EQE) exceeding 20 % at a low current density of 0.003 mA cm−2. Alternatively, the blue LECs integrated with yellow/red QD CCLs deliver white emissions with a low CCT <2500 K and a high CRI up to 93 while the peak EQE at a low current density of 0.005 mA cm−2 still remains high (>16 %). The data effectively showcase that efficient and stable white emission can be achieved through partial energy down-conversion from the blue LECs to the QD CCLs, and the proposed white light sources are potential candidates for lighting applications. [Display omitted] • White light sources using blue LECs and red or yellow/red QD color conversion layers. • Peak external quantum efficiencies of the proposed devices are higher than 20 %. • The white EL spectra are stable in a wide range of operating current. [ABSTRACT FROM AUTHOR]

Solid‐state light‐emitting electrochemical cells (LECs) using sustainable and eco‐friendly materials and affording high brightness, efficiency, and stability are highly desired. Here, intrinsically ionic, thermally activated delayed fluorescence (TADF) materials 1–3 for efficient, bright, and stable LECs are reported. 1–3 feature carbazole‐type donors and cationic triazine‐type acceptors, which are located ortho to each other on the phenyl linkers. Through‐space charge‐transfer (CT) dominates the CT transitions in 1–3. In doped and neat films, 1–3 show blue and green TADF emission, respectively, with reverse intersystem crossing rates at around 7.0 × 105 s−1. 1–3 possess excellent electrochemical stability (except for the oxidation of 1) and film‐forming abilities. LECs using neat films of 1–3 as the single active layers afford green electroluminescence with peak brightness/peak external quantum efficiency (EQE) of up to 572 cd m−2/6.8% under 4.0 V and peak brightness/peak EQE/half‐lifetime of up to 860 cd m−2/5.4%/48 h under 50 A m−2. A longer half‐lifetime of 218 h has further been achieved at 162 cd m−2 under 10 A m−2. The work reveals the bright prospect for the development of efficient, bright, and stable LECs with intrinsically‐ionic TADF materials. [ABSTRACT FROM AUTHOR]

Light‐emitting electrochemical cells (LECs) have presented themselves as an alternative to light emitting diodes (LEDs) because of the simple device design which is accompanied by a lower driving power. LECs operate by rearranging ion and electron transfers that create a p‐n junction at a sufficient voltage, permitting LEC emissions. However, this rearrangement is not well understood. Therefore, the ion and electron transfer processes of the device during ion rearrangement, operation and at excessive overpotentials must be characterized for LEC devices. This paper reports on investigation of these LEC processes using electrochemical impedance spectroscopy (EIS). All processes were successfully characterized with simple equivalent circuits. To the best of our knowledge, an inductive low frequency loop was observed in an LEC for the first time. We propose that this inductivity was due to the p‐n junction resistance to low frequency potential changes. Consistent observations and inverse proportionality between overpotential and inductance provided additional evidence for our proposal. This p‐n junction at low frequency opposition combined with LEC operational stability data can provide a basis for judging the chemical resistance to deterioration and charge storage capacity of p‐n junctions in the future. Furthermore, the techniques and equivalent circuits presented here will help to identify failure mechanisms and increase LEC operational lifetimes. [ABSTRACT FROM AUTHOR]

The synthesis, photophysical/electrochemical characterization, and implementation in light‐emitting electrochemical cells (LECs) of three novel red‐emitting heteroleptic [Cu(N^N)(P^P)]PF6 complexes are reported. The complex design consists in combining i) the bridged di(pyrazin‐2‐yl)sulfane N^N ligand with expanded π‐conjugation and electro‐withdrawing 4‐(trifluoromethyl)pyrimidine moieties and ii) the [(diphenylphosphino)phenyl] ether (DPEphos) P^P ligand. The effect of the N^N ligand substitution on the photophysical, electrochemical, ion conductivity, and morphological features in their respective powders and thin films is thoughtfully rationalized. This is rounded by the trends noted in single‐layered red‐emitting LECs (λmax = 630–660 nm) featuring moderate performances with irradiances of around 60–90 µW cm−2 and total emitted energies in the range of 4–12 mJ. A further optimization using a multilayered architecture, in which hole injection/transport and exciton formation processes are decoupled, leads to a significant enhancement of the irradiance up to ≈150 µW cm−2 and total emitted energies of 91 mJ without affecting device chromaticity. Finally, the best red‐emitting complex in LECs is used to fabricate host:guest white devices, achieving luminances of 12 cd m−2 in concert with an excellent white color quality (x/y CIE color coordinates of 0.31/0.32; color rendering index of 90) stable over lifespan. [ABSTRACT FROM AUTHOR]

Soft and stretchable electronic devices are expected to offer technological advances in the field of robotics, human–machine interfacing, and healthcare. Employing biodegradable elastomers, hydrogels, and nontoxic conductors would add significant value to and minimize the ecological impact of such disposable and transient electronic applications. Here, the biodegradable and photo‐crosslinkable elastomer poly(glycerol sebacic) acrylate (PGSA) is characterized for its use in soft and stretchable electronics. Its mechanical properties are investigated in terms of their chemical composition and compared to commonly used gelatin hydrogels. Furthermore, these materials are combined with interconnects made of liquid Galinstan in order to create functional substrates with certified biodegradability under ISO standards. The combination of these materials produces elastic circuit boards that act as soft platforms for body‐mounted sensors or biodegradable stretchable light‐emitting devices. These soft platforms reveal linear elongations at a break of 130% to 350% and similar moduli to nondegradable elastomers and human tissue, without any decrease in conductivity. Advanced applications in biofriendly packaging, soft robotics, and healthcare will greatly benefit from these biodegradable devices. [ABSTRACT FROM AUTHOR]

Cyclometallated iridium complexes possess fascinating electrochemical and photophysical properties that make them excellent candidates for a variety of photonic and optoelectronic applications. In particular, light-emitting electrochemical cells (LEECs) based on iridium-containing ionic transition-metal complexes (Ir-iTMCs) are a promising alternative to conventional organic light-emitting diodes with several advantages, including a simpler device structure, solution processability, and reduced manufacturing costs. This review aims to provide a comprehensive and systematic overview of the current status of Ir-iTMC-based LEECs using the archetypal complex [Ir(ppy)2(bpy)]PF6 as a reference emitter. After a discussion of the device fundamentals and important photophysical and device parameters, key strategies for tuning the emission characteristics and device stability through LUMO and HOMO stabilization/destabilization are presented using numerous examples from the literature, with a particular focus on ligand modification with hydrophobic, electron-withdrawing, and electron-donating substituents, π-stacking interactions, and alternative ancillary and cyclometalated ligand skeletons. Comprehensive data tables summarizing the photophysical and LEEC properties of the various classes of iridium complexes reported to date are also provided. Finally, in an effort to highlight promising directions for future research, the current champion iridium complexes for fabricating state-of-the-art LEECs are identified, and the merits and limitations of existing approaches are discussed.

Recently, perovskites have attracted intense attention due to their high potential in optoelectronic applications. Employing perovskites as the emissive materials of light‐emitting electrochemical cells (LECs) shows the advantages of simple fabrication process, low‐voltage operation, and compatibility with inert electrodes, along with saturated electroluminescence (EL) emission. Unlike in previously reported perovskite LECs, in which salts are incorporated in the emissive layer, the ion‐transport layer was separated from the emissive layer in this work. The layer of ionic transition metal complex (iTMC) not only provides mobile ions but also serves as an electron‐injection/transport layer. Orthogonal solvents are used in spin coating to prevent the intermixing of stacked perovskite and iTMC layers. The blue iTMC with high ionization potential is effective in blocking holes from the emissive layer and thus ensures EL color saturation. In addition, the carrier balance of the perovskite/iTMC LECs can be optimized by adjusting the iTMC layer thickness. The optimized external quantum efficiency of the CsPbBr3/iTMC LEC reaches 6.8 %, which is among the highest reported values for perovskite LECs. This work successfully demonstrates that, compared with mixing all components in a single emissive layer, separating the layer of ion transport, electron injection and transport from the perovskite emissive layer is more effective in adjusting device carrier balance. As such, solution‐processable perovskite/iTMC LECs open up a new way to realize efficient perovskite LECs. [ABSTRACT FROM AUTHOR]

In the search for bio and eco‐friendly light sources, light‐emitting electrochemical cells (LECs) are promising candidates for the implementation of biomaterials in their device architecture thanks to their low fabrication complexity and wide range of potential technological applications. In this work, the use of the DNA derivative DNA‐cetyltrimethylammonium (DNA‐CTMA) is introduced as the ion‐solvating component of the solid polymer electrolyte (SPE) in the active layer of solution‐processed LECs. The focus is particularly on the investigation of its electrochemical and ionic conductivity properties demonstrating its suitability for device fabrication and correlation with thin film morphology. Furthermore, upon blending with the commercially available emissive polymer Super Yellow, the structure property relationship between the microstructure and the ionic conductivity is investigated and yields an optimized LEC performance. The large electrochemical stability window of DNA‐CTMA enables a stable device performance for a variety of emitters covering the complete visible spectral range, thus highlighting the universal character of this naturally sourced SPE. [ABSTRACT FROM AUTHOR]

The choice of an adequate electrolyte is a fundamental aspect in polymer light-emitting electrochemical cells (PLECs) as it provides the in situ electrochemical doping and influences the performance of these devices. In this study, a hyperbranched polymer (Hybrane DEO750 8500) blended with a Li salt is used as a novel electrolyte in state-of-the-art Super Yellow (a polyphenylenevinylene) based LECs. Due to the desirable properties of the hyperbranched polymer and the homogeneous and smooth films that it forms with the emitting polymer, PLEC with excellent electroluminescent properties are obtained using a pulsed current bias scheme. The devices are very stable, with lifetimes in excess of2000 h with initial luminance values above 450 cd m-2, a peak efficiency of 12.6 lm W-1, and sub-minute turn-on times. The stability of the devices is also studied by measuring the photolu-minescence (PL) of the semiconductor during electroluminescent operation. The findings suggest that it is possible to observe the quenching of the PL in vertically stacked devices due to the advancement of the doped fronts in the film and an immediate PL recovery when the bias is removed. [ABSTRACT FROM AUTHOR]

Semiconductor junctions are among the most important material interfaces in modern technology. The solid polymer light‐emitting electrochemical cell (PLEC) possesses a semiconducting polymer homojunction that is reminiscent of a conventional p–n junction but also exhibits distinct features that are profoundly intriguing. The PLEC junction is formed under bias when the propagating p‐ and n‐doping fronts make contact. The PLEC junction can be immobilized by cooling after the initial junction formation. Further, the resulting frozen junction can be relaxed, or partially de‐doped into a p–i–n junction by controlled heating/cooling cycles. It is on such a frozen polymer p–i–n junction that the authors observed one of the most puzzling phenomena in organic electroluminescence (EL). The frozen p–i–n junction displays the brightest reverse bias (RB) EL ever imaged in a polymer junction and a large photovoltage. Moreover, the RB EL exhibits hypersensitivity to temperature, increasing in intensity by 22.6% K−1 when the de‐doped cell is cooled from 250 to 200 K. A model based on the tunnel injection of charge carriers with fast transit time across the intrinsic region explains the RB EL's hypersensitivity to temperature and super‐linear dependence on cell current. [ABSTRACT FROM AUTHOR]

Blue electroluminescence is highly desired for emerging light‐emitting devices for display applications and optoelectronics in general. However, saturated, efficient, and stable blue emission has been challenging to achieve, particularly in mixed‐halide perovskites, where intrinsic ion motion and halide segregation compromises spectral purity. Here, CsPbBr3−xClx perovskites, polyelectrolytes, and a salt additive are leveraged to demonstrate pure blue emission from single‐layer light‐emitting electrochemical cells (LECs). The electrolytes transport the ions from salt additives, enhancing charge injection and stabilizing the inherent perovskite emissive lattice for highly pure and sustained blue emission. Substituting Cl into CsPbBr3 tunes the perovskite luminescence from green through blue. Sky blue and saturated blue devices produce International Commission on Illumination coordinates of (0.105, 0.129) and (0.136, 0.068), respectively, with the latter meeting the US National Television Committee standard for the blue primary. Likewise, maximum luminances of 2900 and 1000 cd m−2, external quantum efficiencies (EQEs) of 4.3% and 3.9%, and luminance half‐lives of 5.7 and 4.9 h are obtained for sky blue and saturated blue devices, respectively. Polymer and LiPF6 inclusion increases photoluminescence efficiency, suppresses halide segregation, induces thin‐film smoothness and uniformity, and reduces crystallite size. Overall, these devices show superior performance among blue perovskite light‐emitting diodes (PeLEDs) and general LECs. [ABSTRACT FROM AUTHOR]

The synthesis, characterization, and application in light‐emitting electrochemical cells (LECs) of two near‐infrared (NIR)‐emitting Pt‐porphyrins (Pt‐por) with none or 20 F atoms and their respective Pt‐por polyads containing from 1 to 4 BODIPY (BDP) units (PtBDP) connected via tetra‐fluorophenyl triazole spacer groups are reported. The functionalization of PtBDPs is tuned with respect to the number of BDP units and F atoms optimizing 1) the NIR emission through an efficient energy transfer from the BDP to the Pt‐por core, 2) the electronic structure to decouple charge transport via BDP and exciton formation at the Pt‐por, and 3) the ionic conductivity in thin films. A comprehensive rationale on how the molecular design rules the device performance is provided, achieving LECs with emission at 780 nm. This is realized using PtBDP‐2‐8, in which a host:guest strategy between BDP and Pt‐por in thin films is fully operative accompanied by an optimal ionic conductivity. This is supported by spectroelectrochemical findings and device analysis with both Pt‐por references and the PtBDP series. Overall, this work highlights that Pt‐porphyrins are efficient emitters in developing NIR LECs due to their rich functionalization, enabling to control charge transport, energy transfer, and ionic conductivities.

A rational host–guest concept design for the attainment of high efficiency at strong luminance from light‐emitting electrochemical cells (LECs) by suppression of exciton‐polaron quenching [Tang et al., Nature Communications2017, 8, 1190] has been reported. However, a practical drawback with the presented host–guest LEC devices was that the operational stability is insufficient for many applications. Here, a systematic study is performed, revealing that a major culprit for the limited operational stability is that the employed n‐type host, 1,3‐bis[2‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazo‐5‐yl]benzene (OXD‐7), has a strong propensity for crystallization and that this crystallization results in a detrimental phase separation of the constituents in the active material during device operation. The authors, therefore, identify an alternative class of concept‐functional n‐type hosts in the form of spirobifluorene‐phosphine‐oxide compounds, and report that the replacement of OXD‐7 with amorphous 2,7‐bis(diphenylphosphoryl)‐9,9′‐spirobifluorene results in a much improved operational lifetime of 700 h at >100 cd m−2 during constant‐bias driving at an essentially retained high current efficacy of 37.9 cd A−1 and a strong luminance of 2940 cd m−2. [ABSTRACT FROM AUTHOR]

In this article, organic light-emitting electrochemical cells (OLECs) have been fabricated on a substrate consisting of a standard woven polyester cotton textile. The textile substrate is presmoothed by first screen printing an ultraviolet (UV) curable polyurethane layer, termed the interface layer. Solution-processed spray coating is then utilized to deposit all the functional layers on the textile to achieve fully spray-coated flexible OLECs. OLEC devices were initially fabricated on ITO-coated glass substrates for process optimization. Then, the OLECs were fabricated on flexible textile using the optimized spray coating process. Finally, both OLECs devices on the glass and textile substrates were encapsulated by stencil printing epoxy before testing in an ambient environment. The OLECs on ITO-coated glass exhibit a turn-on voltage of 3 V, a brightness level of 200 cd/m2 and a lifetime of at least 60 min. The OLECs on textile exhibit a turn-on voltage of 4 V, a brightness level of 80 cd/m2 level and a lifetime of at least 30 min. [ABSTRACT FROM AUTHOR]

Crystalline and morphological defects in the perovskite film affect the operation of light-emitting devices. Thus, advanced and scalable fabrication techniques can improve device properties. In this work, we use slot-die coating at ambient conditions, followed by hot air drying, to produce CsPbBr 3 light-emitting electrochemical cells. We compare this method to spin-coating and analyze film morphology and optical properties. We reveal that annealing the film on a hot plate increases PLQY and Shockley-Read-Hole lifetime, but worsens film morphology. In contrast, hot air drying during deposition improves morphology but reduces photoluminescence. The slot-die coating shows better results for device fabrication. With InGa and Al top electrodes, we achieve luminance 8100 cd m−2 and 2900 cd m−2 at a 5 V bias, respectively. • Near 100% PLQY of CsPbBr 3 perovskite film fabricated at ambient condition. • Improving film morphology by hot air drying during the deposition process. • The first perovskite light-emitting electrochemical cell was fabricated by slot-die coating at ambient condition. [ABSTRACT FROM AUTHOR]

Light-emitting electrochemical cells (LECs) have enabled many novel solution-processable, low-cost, and flexible emission applications. Nevertheless, relatively inferior device efficiencies render LECs less competitive with other emission technologies. Light extraction would be a promising approach to enhance the device efficiencies of LECs. The diffuser film composed of a transparent photoresist layer doped with large and small TiO 2 nanoparticles (NPs) embedded between the indium-tin-oxide layer and the glass substrate has shown superior light extraction performance (>2.5X). However, the physical insight of optimization, e.g. , adjusting the concentration of small TiO 2 NPs in diffuser film, has not been explored yet. In this work, increasing the concentration of small TiO 2 NPs is found to increase the thickness, roughness, and refractive index of the diffuser film. The optical simulation indicates that the thickness of the diffuser film has little effect on the light extraction. However, a higher roughness but a lower refractive index is preferred for the diffuser film to achieve better light extraction. As such, it is rational to expect that a proper concentration of small TiO 2 NPs to reach a high enough roughness but still keep a relatively low refractive index is required for diffuser optimization. This study clarifies the physical mechanism responsible for diffuser optimization and thus shows a high potential for lighting applications of LECs. [Display omitted] • High light extraction performance (>2.5X) from LECs by using diffuser films. • A diffuser film is a transparent photoresist layer with large and small TiO 2 NPs. • Diffuser film is optimized by adjusting the concentration of small TiO 2 NPs. • Thickness, roughness, and refractive index of diffuser film are tuned simultaneously. • High roughness but low refractive index is preferred for the best light extraction. [ABSTRACT FROM AUTHOR]