Recent advances of organic emitters in deep-red light-emitting electrochemical cells.

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.

Light-Emitting Electrochemical Cells Based on Nanogap Electrodes.

In a light-emitting electrochemical cell (LEC), electrochemical doping caused by mobile ions facilitates bipolar charge injection and recombination emissions for a high electroluminescence (EL) intensity at low driving voltages. We present the development of a nanogap LEC (i.e., nano-LEC) comprising a light-emitting polymer (F8BT) and an ionic liquid deposited on a gold nanogap electrode. The device demonstrated a high EL intensity at a wavelength of 540 nm corresponding to the emission peak of F8BT and a threshold voltage of ∼2 V at 300 K. Upon application of a constant voltage, the device demonstrated a gradual increase in current intensity followed by light emission. Notably, the delayed components of the current and EL were strongly suppressed at low temperatures (<285 K). The results clearly indicate that the device functions as an LEC and that the nano-LEC is a promising approach to realizing molecular-scale current-induced light sources.

Binuclear Copper(I) Complexes for Near-Infrared Light-Emitting Electrochemical Cells.

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 (3 MLCT) 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.

Electrically Amplified Circularly Polarized Luminescence by a Chiral Anion Strategy.

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.

White Light-Emitting Electrochemical Cells Employing Phosphor-Sensitized Thermally Activated Delayed Fluorescence to Approach All-Phosphorescent Device Efficiencies.

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.

From Textile Coloring to Light-emitting Electrochemical Devices: Upcycling of the Isoviolanthrone Vat Dye.

Produced at ton scale, vat dyes are major environmental pollutants generated by the textile industry. However, they represent ideal and accessible candidates for chemical upcycling since they are usually composed of large π-conjugated scaffolds. Based on the valorization of "old" products, waste or even contaminant into high-added value goods, this concept can be easily transposed to the laboratories. As a contribution to the current environmental and ecological transition, we demonstrate herein the valorization/upcycling of wastewaters generated during the dyeing procedure. To do so, the reduced (leuco) form of vat violet 10, also known as isoviolanthrone, was functionalized to afford a readily soluble derivative that was subsequently and successfully used as active material in operating solution processed light-emitting electrochemical cells, that is, from textile dyeing to high-tech application.

Exploring the Electroluminescent Applications of Imidazole Derivatives.

Recently, the fields of organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs) have improved tremendously with regard to tunable emission, efficiency, brightness, and thermal stability. Imidazole derivatives are excellent deep blue-green light-emitting layers in the OLED or LEC devices. This Review summarizes the major breakthroughs of various electroluminescence (EL) layers with imidazole-containing organic or organometallic derivatives, the molecular design principles, and their light-emitting performances as effective EL materials. The highly tunable chemical structures and flexible molecular design strategies of imidazole-based compounds are advantages that provide great opportunities for researchers. They can provide a good basis for the design and development of new EL materials with narrower emission and higher efficiency. Moreover, imidazole compounds have demonstrated breakthrough performances in thermally activated delayed fluorescence (TADF) properties where triplet excitons are utilized to inhibit anti-intersystem quenching, showing great promise in breaking the theoretical external quantum efficiencies (EQE) limits in traditional fluorescent devices.

Long-Wavelength Light-Emitting Electrochemical Cells: Materials and Device Engineering.

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.

Ionic multiresonant thermally activated delayed fluorescence emitters for light emitting electrochemical cells.

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.

Stable and Bright Electroluminescent Devices utilizing Emissive 0D Perovskite Nanocrystals Incorporated in a 3D CsPbBr3 Matrix.

The 0D cesium lead halide perovskite Cs4 PbBr6 has drawn remarkable interest due to its highly efficient robust green emission compared to its 3D CsPbBr3 counterpart. However, seizing the advantages of the superior photoluminescence properties for practical light-emitting devices remains elusive. To date, Cs4 PbBr6 has been employed only as a higher-bandgap nonluminescent matrix to passivate or provide quantum/dielectric confinement to CsPbBr3 in light-emitting devices and to enhance its photo-/thermal/environmental stability. To resolve this disparity, a novel solvent engineering method to incorporate highly luminescent 0D Cs4 PbBr6 nanocrystals (perovskite nanocrystals (PNCs)) into a 3D CsPbBr3 film, forming the active emissive layer in single-layer perovskite light-emitting electrochemical cells (PeLECs) is designed. A dramatic increase of the maximum external quantum efficiency and luminance from 2.7% and 6050 cd m-2 for a 3D-only PeLEC to 8.3% and 11 200 cd m-2 for a 3D-0D PNC device with only 7% by weight of 0D PNCs is observed. The majority of this increase is driven by the efficient inherent emission of the 0D PNCs, while the concomitant morphology improvement also contributes to reduced leakage current, reduced hysteresis, and enhanced operational lifetime (half-life of 129 h), making this one of the best-performing LECs reported to date.

Achieving Record Efficiency and Luminance for TADF Light-Emitting Electrochemical Cells by Dopant Engineering.

Thermally activated delayed fluorescence (TADF) light-emitting electrochemical cells (TADF-LECs) are appealing due to their simple sandwich structure and potential applications in wearable displays and sensors. However, achieving high performance remains challenging. In this paper, we demonstrate that the use of TADF emitters with a low aggregated-caused quenching (ACQ) tendency is crucial to address this challenge. To verify it, two types of TADF-LECs are compared in parallel using different kinds of TADF emitters. The control device uses 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) as the dopant, which suffers from a serious ACQ issue and thus dramatically limits the doping concentrations of 4CzIPN in these TADF-LECs. At the best doping condition (0.5 wt %), insufficient host-to-dopant energy transfer (ET) does exist, thereby displaying very limited efficiency and luminance, i.e., 2.43% and 1483 cd m-2. By contrast, the TADF-LECs using 3,6-di(tert-butyl)-1,8-di(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)-9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenyl) carbazole (BPAPTC) can tolerate a much higher doping concentration because BPAPTC is a satisfactory TADF emitter featuring a low ACQ tendency. At the optimized doping condition of 18 wt %, the BPAPTC-based emissive layer possesses the best TADF property, including the longest τDF (2646 ns), the largest rDF (69%), and the highest kRISC of 7.50 × 105 s-1. Moreover, the corresponding TADF-LEC simultaneously displays the most efficient host-to-dopant ET. It thus achieves unprecedented performance, e.g., the highest external quantum efficiency (EQEmax.) of 7.6%, the highest luminance (Lmax.) of 3696 cd m-2, and an EQE of 7.01% at a practical high luminance of 1000 cd m-2.

Multivariate Analysis Identifying [Cu(N

White light-emitting electrochemical cells (LECs) comprising only [Cu(N^N)(P^P)]+ have not been reported yet, as all the attempts toward blue-emitting complexes failed. Multivariate analysis, based on prior-art [Cu(N^N)(P^P)]+ -based thin-film lighting (>90 papers) and refined with computational calculations, identifies the best blue-emitting [Cu(N^N)(P^P)]+ design for LECs, that is, N^N: 2-(4-(tert-butyl)phenyl)-6-(3,5-dimethyl-1H-pyrazol-1-yl)pyridine and P^P: 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene, to achieve predicted thin-film emission at 490 nm and device performance of 3.8 cd A-1 @170 cd m-2 . Validation comes from synthesis, X-ray structure, thin-film spectroscopic/microscopy/electrochemical characterization, and device optimization, realizing the first [Cu(N^N)(P^P)]+ -based blue-LEC with 3.6 cd A-1 @180 cd m-2 . This represents a record performance compared to the state-of-the-art tricoordinate Cu(I)-complexes blue-LECs (0.17 cd A-1 @20 cd m-2 ). Versatility is confirmed with the synthesis of the analogous complex with 2-(4-(tert-butyl)phenyl)-6-(3,5-dimethyl-1H-pyrazol-1-yl)pyrazine (N^N), showing a close prediction/experiment match: λ = 590/580 nm; efficiency = 0.55/0.60 cd A-1 @30 cd m-2 . Finally, experimental design is applied to fabricate the best white multicomponent host:guest LEC, reducing the number of trial-error attempts toward the first white all-[Cu(N^N)(P^P)]+ -LECs with 0.6 cd A-1 @30 cd m-2 . This corresponds to approximately ten-fold enhancement compared to previous LECs (<0.05 cd A-1 @<12 cd m-2 ). Hence, this work sets in the first multivariate approach to design emitters/active layers, accomplishing first-class [Cu(N^N)(P^P)]+ -based blue/white LECs that were previously elusive.

Perovskite Light-Emitting Electrochemical Cells Employing Electron Injection/Transport Layers of Ionic Transition Metal Complexes.

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.

25 Years of Light-Emitting Electrochemical Cells: A Flexible and Stretchable Perspective.

Light-emitting electrochemical cells (LECs) are simple electroluminescent devices comprising an emissive material containing mobile ions sandwiched between two electrodes. The operating mechanism of the LEC involves both ionic and electronic transport, distinguishing it from its more well-known cousin, the organic light-emitting diode (OLED). While OLEDs have become a leading player in commercial displays, LECs have flourished in academic research due to the simple device architecture and unique features of its operating mechanism, inviting exploration of new materials and fabrication strategies. These explorations have brought LECs to an exciting frontier in advanced optoelectronics: flexible and stretchable light-emitting devices. Flexible and stretchable LECs are discussed herein, presenting the LEC system as a robust and fault-tolerant development platform. The engineering of emissive composites is highlighted to control mechanical properties, and how the tolerance of LECs to electrode work function and roughness has enabled the incorporation of new electrode materials to achieve flexibility and stretchability. As part of this story, the solution processability of LECs has led to exciting demonstrations of flexible and printed LECs. An outlook is provided for LECs that builds on these strengths, potentially leading to flexible, stretchable, low-cost devices such as illuminated tags, smart packaging, flexible signage, and wearable illumination.

Surface Energy-Driven Preferential Grain Growth of Metal Halide Perovskites: Effects of Nanoimprint Lithography Beyond Direct Patterning.

Hybrid organic-inorganic lead halide perovskites have attracted much attention in the field of optoelectronic devices because of their desirable properties such as high crystallinity, smooth morphology, and well-oriented grains. Recently, it was shown that thermal nanoimprint lithography (NIL) is an effective method not only to directly pattern but also to improve the morphology, crystallinity, and crystallographic orientations of annealed perovskite films. However, the underlining mechanisms behind the positive effects of NIL on perovskite material properties have not been understood. In this work, we study the kinetics of perovskite grain growth with surface energy calculations by first-principles density functional theory (DFT) and reveal that the surface energy-driven preferential grain growth during NIL, which involves multiplex processes of restricted grain growth in the surface-normal direction, abnormal grain growth, crystallographic reorientation, and grain boundary migration, is the enabler of the material quality enhancement. Moreover, we develop an optimized NIL process and prove its effectiveness by employing it in a perovskite light-emitting electrochemical cell (PeLEC) architecture, in which we observe a fourfold enhancement of maximum current efficiency and twofold enhancement of luminance compared to a PeLEC without NIL, reaching a maximum current efficiency of 0.07598 cd/A at 3.5 V and luminance of 1084 cd/m2 at 4 V.

Electrogenerated Chemiluminescence and Electroluminescence of N-Doped Graphene Quantum Dots Fabricated from an Electrochemical Exfoliation Process in Nitrogen-Containing Electrolytes.

Artificial lighting sources are one of the most important technological developments for our modern lives; the search for cost-effective and efficient luminophores is therefore crucial to a sustainable future. Graphene quantum dots (GQDs) are carbon-based nanomaterials that exhibit exceptional optical and electronic properties, making them a prime candidate for a luminophore in a light-emitting device. Nitrogen-doped GQDs fabricated from a facile top-down electrochemical exfoliation process with a nitrogen-containing electrolyte in this report showed strong photoluminescent emission at 450 nm, and electrogenerated chemiluminescence at 660 nm in the presence of benzoyl peroxide as a coreactant. When introduced into solid-state light-emitting electrochemical cells, for the first time, the GQDs displayed a broad white emission centered at 610 nm, corresponding to Commision Internationale de l'eclairage (CIE) colour coordinates of (0.38, 0.36).

Revealing the Impact of Heat Generation Using Nanographene-Based Light-Emitting Electrochemical Cells.

Self-heating in light-emitting electrochemical cells (LECs) has been long overlooked, while it has a significant impact on (i) device chromaticity by changing the electroluminescent band shape, (ii) device efficiency because of thermal quenching and exciton dissociation reducing the external quantum efficiency (EQE), and (iii) device stability because of thermal degradation of excitons and eliminate doped species, phase separation, and collapse of the intrinsic emitting zone. Herein, we reveal, for the first time, a direct relationship between self-heating and the early changes in the device chromaticity as well as the magnitude of the error comparing theoretical/experimental EQEs-that is, an overestimation error of ca. 35% at usual pixel working temperatures of around 50 °C. This has been realized in LECs using a benchmark nanographene-that is, a substituted hexa-peri-hexabenzocoronene-as an emerging class of emitters with outstanding device performance compared to the prior art of small-molecule LECs-for example, luminances of 345 cd/m2 and EQEs of 0.35%. As such, this work is a fundamental contribution highlighting how self-heating is a critical limitation toward the optimization and wide use of LECs.

Combinational Approach To Realize Highly Efficient Light-Emitting Electrochemical Cells.

Light-emitting electrochemical cells (LECs) show high technical potential for display and lighting utilizations owing to the superior properties of solution processability, low operation voltage, and employing inert cathodes. For maximizing the device efficiency, three approaches including development of efficient emissive materials, optimizing the carrier balance, and maximizing the light extraction have been reported. However, most reported works focused on only one of the three optimization approaches. In this work, a combinational approach is demonstrated to optimize the device efficiency of LECs. A sophisticatedly designed yellow complex exhibiting a superior steric hindrance and a good carrier balance is proposed as the emissive material of light-emitting electrochemical cells and thus the external quantum efficiency (EQE) is up to 13.6%. With an improved carrier balance and reduced self-quenching by employing the host-guest strategy, the device EQE can be enhanced to 16.9%. Finally, a diffusive layer embedded between the glass substrate and the indium-tin-oxide layer is utilized to scatter the light trapped in the layered device structure, and consequently, a high EQE of 23.7% can be obtained. Such an EQE is impressive and consequently proves that the proposed combinational approach including adopting efficient emissive materials, optimizing the carrier balance, and maximizing the light extraction is effective in realizing highly efficient LECs.

Rational Design of Efficient Organometallic Ir(III) Complexes for High-Performance, Flexible, Monochromatic, and White Light-Emitting Electrochemical Cells.

Highly efficient light-emitting electrochemical cells (LECs) have attracted tremendous interest because of their simple structures and low-cost fabrication processing, showing great potential for full-color displays and solid-state lighting. In this work, we rationally designed and synthesized two red-emitting cationic Ir(III) complexes, [Ir(tBuPBI)2(biq)]PF6 (R1) and [Ir(tBuPBI)2(qibi)]PF6 (R2), in which a tert-butyl-functionalized 1,2-diphenyl-1H-benzo[d]imidazole (PBI) unit and conjugated 2,2'-biquinoline (biq) and 2-(1-phenyl-1H-benzo[d]imidazol-2-yl)quinolone (qibi) were employed as cyclometalated and ancillary ligands, respectively. The introduced tert-butyl group led to homogeneous and highly emissive thin films by increasing the solubility and suppressing the strong intermolecular interactions due to steric hindrance. Based on the abovementioned high-quality emissive layer, high-efficiency LECs were achieved. An efficient red-emitting LEC fabricated on a glass substrate achieved a current efficiency (ηC) of 7.18 cd/A and an external quantum efficiency (ηext) of 9.32%. By doping both complexes into a blue-green-emitting cationic Ir(III) complex, high-performance white LECs were also successfully fabricated with Commission International de L'Eclairage (CIE) coordinates of (0.39,0.39), a ηC of 17.43 cd/A, and a ηext of 8.92%. In addition, we also fabricated flexible red and white LECs with outstanding efficiencies and mechanical flexibilities. The ηC and ηext values of a flexible white LEC could be as high as 13.50 cd/A and 6.86%, respectively. The efficiency of the flexible device remained at approximately 95% of the initial value after 500 bends with a radius of curvature of 5 mm, demonstrating the great potential of these complexes for full-color displays and flexible optoelectronics.

Hybrid White-Light-Emitting Electrochemical Cells Based on a Blue Cationic Iridium(III) Complex and Red Quantum Dots.

Solid-state white light-emitting electrochemical cells (LECs) show promising advantages of simple solution fabrication processes, low operation voltage, and compatibility with air-stable cathode metals, which are required for lighting applications. To date, white LECs based on ionic transition metal complexes (iTMCs) have shown higher device efficiencies than white LECs employing other types of materials. However, lower emission efficiencies of red iTMCs limit further improvement in device performance. As an alternative, efficient red CdZnSeS/ZnS core/shell quantum dots were integrated with a blue iTMC to form a hybrid white LEC in this work. By achieving good carrier balance in an appropriate device architecture, a peak external quantum efficiency and power efficiency of 11.2 % and 15.1 lm W-1, respectively, were reached. Such device efficiency is indeed higher than those of the reported white LECs based on host-guest iTMCs. Time- and voltage-dependent electroluminescence (EL) characteristics of the hybrid white LECs were studied by means of the temporal evolution of the emission-zone position extracted by fitting the simulated and measured EL spectra. The working principle of the hybrid white LECs was clarified, and the high device efficiency makes potential new white-emitting devices suitable for solid-state lighting technology possible.