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.     

High‐Performance Dibenzoheteraborin‐Based Thermally Activated Delayed Fluorescence Emitters: Molecular Architectonics for Concurrently Achieving Narrowband Emission and Efficient Triplet–Singlet Spin Conversion

Thermally activated delayed fluorescence (TADF) materials, which enable the full harvesting of singlet and triplet excited states for light emission, are expected as the third‐generation emitters for organic light‐emitting diodes (OLEDs), superseding the conventional fluorescence and phosphorescence materials. High photoluminescence quantum yield (Φ_PL), narrow‐band emission (or high color purity), and short delayed fluorescence lifetime are all strongly desired for practical applications. However, to date, no rational design strategy of TADF emitters is established to fulfill these requirements. Here, an epoch‐making design strategy is proposed for producing high‐performance TADF emitters that concurrently exhibiting high Φ_PL values close to 100%, narrow emission bandwidths, and short emission lifetimes of ≈1 µs, with a fast reverse intersystem crossing rate of over 10⁶ s^?1. A new family of TADF emitters based on dibenzoheteraborins is introduced, which enable both doped and non‐doped TADF‐OLEDs to achieve markedly high external electroluminescence quantum efficiencies, exceeding 20%, and negligible efficiency roll‐offs at a practical high luminance. Systematic photophysical and theoretical investigations and device evaluations for these dibenzoheteraborin‐based TADF emitters are reported here.     

Wavelength‐Emission Tuning of ZnO Nanowire‐Based Light‐Emitting Diodes by Cu Doping: Experimental and Computational Insights

The band‐gap engineering of doped ZnO nanowires is of the utmost importance for tunable light‐emitting‐diode (LED) applications. A combined experimental and density‐functional theory (DFT) study of ZnO doping by copper (Zn²⁺ substitution by Cu²⁺) is presented. ZnO:Cu nanowires are epitaxially grown on magnesium‐doped p‐GaN by electrochemical deposition. The heterojunction is integrated into a LED structure. Efficient charge injection and radiative recombination in the Cu‐doped ZnO nanowires are demonstrated. In the devices, the nanowires act as the light emitters. At room temperature, Cu‐doped ZnO LEDs exhibit low‐threshold emission voltage and electroluminescence emission shifted from the ultraviolet to violet–blue spectral region compared to pure ZnO LEDs. The emission wavelength can be tuned by changing the copper content in the ZnO nanoemitters. The shift is explained by DFT calculations with the appearance of copper d states in the ZnO band‐gap and subsequent gap reduction upon doping. The presented data demonstrate the possibility to tune the band‐gap of ZnO nanowire emitters by copper doping for nano‐LEDs.     

Emission Color Tuning in Ambipolar Organic Single‐Crystal Field‐Effect Transistors by Dye‐Doping

The effect of dye‐doping in ambipolar light‐emitting organic field‐effect transistors (LE‐OFETs) is investigated from the standpoint of the carrier mobilities and the electroluminescence (EL) characteristics under ambipolar operation. Dye‐doping of organic crystals permits not only tuning of the emission color but also significantly increases the efficiency of ambipolar LE‐OFETs. A rather high external EL quantum efficiency (～0.64%) of one order of magnitude higher than that of a pure p‐distyrylbenzene (P3V2) single crystal is obtained by tetracene doping. The doping of tetracene molecules into a host P3V2 crystal has almost no effect on the electron mobility and the dominant carrier recombination process in the tetracene‐doped P3V2 crystal involves direct carrier recombination on the tetracene molecules.     

High‐Quality White Organic Light‐Emitting Diodes Composed of Binary Emitters with Color Rendering Index Exceeding 80 by Utilizing Color Remedy Strategy

Implementing rigorous standards for high‐quality white organic light‐emitting diodes (WOLEDs) demands further investigation. Herein, a novel and feasible color remedy strategy (CRS) is proposed in WOLEDs composed of binary‐emitters, to arouse the green‐emission, thereby complementing the spectral deficiency in white‐emission. Thus, the color rendering indexes (CRIs) of binary‐emissive WOLEDs can be boosted from 63 to 80 threshold, and the Commission International de I'Eclairage‐(x, y) coordinates are precisely located inside the American National Standard Institute quadrangles, which can rival the WOLEDs integrating ternary or more emitters. Moreover, it is more feasible for CRS‐based binary‐emissive system to tune white‐emission from cool white‐emission (correlated color temperature (CCT) ≈ 5000 K) to eye‐friendly warm white‐emission (CCT ≈ 2000 K). Meanwhile, benefiting from the reduced energy loss and low driving voltage of CRS zone, all of the CRS‐based WOLEDs with diverse CCTs can exceed 20% external quantum efficiency, and the highest approach 25%, as well as the highest power efficiency beyond 60 lm W^?1, which is comparable with those reported employing light‐extracting techniques. In addition, it is evident that CRS‐based WOLEDs also exhibit outstanding color stability within the variation of luminance in several orders of magnitude (50–12 000 cd m^?2). Thus, this novel CRS provides an innovative pathway to fabricate high‐quality WOLEDs composed of binary emitters.     

Efficient Charge Carrier Injection and Balance Achieved by Low Electrochemical Doping in Solution‐Processed Polymer Light‐Emitting Diodes

Charge carrier injection and transport in polymer light‐emitting diodes (PLEDs) is strongly limited by the energy level offset at organic/(in)organic interfaces and the mismatch in electron and hole mobilities. Herein, these limitations are overcome via electrochemical doping of a light‐emitting polymer. Less than 1 wt% of doping agent is enough to effectively tune charge injection and balance and hence significantly improve PLED performance. For thick single‐layer (1.2 µm) PLEDs, dramatic reductions in current and luminance turn‐on voltages (V_J = 11.6 V from 20.0 V and V_L = 12.7 V from 19.8 V with/without doping) accompanied by reduced efficiency roll‐off are observed. For thinner (<100 nm) PLEDs, electrochemical doping removes a thickness dependence on V_J and V_L, enabling homogeneous electroluminescence emission in large‐area doped devices. Such efficient charge injection and balance properties achieved in doped PLEDs are attributed to a strong electrochemical interaction between the polymer and the doping agents, which is probed by in situ electric‐field‐dependent Raman spectroscopy combined with further electrical and energetic analysis. This approach to control charge injection and balance in solution‐processed PLEDs by low electrochemical doping provides a simple yet feasible strategy for developing high‐quality and efficient lighting applications that are fully compatible with printing technologies.     

Carbazole–Dibenzofuran Dyads as Metal‐Free Single‐Component White‐Color Photoemitters

White‐color light emitters from single organic molecule without heavy metals are valuable for practical applications in organic light‐emitting devices. In this study, carbazole (Cz)–dibenzofuran (DBF) donor–acceptor dyads are designed for white‐color light emitters. Originally, these molecules show photoluminescence (PL) in near ultraviolet region. However, upon successive ultraviolet (UV) irradiation, white‐color PL appears, comprising dual‐color phosphorescence from the amorphous and crystalline state of the dyad. A continuous UV irradiation makes the twisting angle between the Cz and DBF planes flatten through the triplet‐excited state, which proceeds crystallization. Thermal annealing and UV irradiation can switch the blue‐ and white‐color phosphorescences from the dyad. Furthermore, charge injection generates white‐color electroluminescence. The materials with PL color modulation ability by UV‐light irradiation and heating can be applicable as light‐ and thermo‐sensors.     

Cover Picture: White Electroluminescence from a Single‐Polymer System with Simultaneous Two‐Color Emission: Polyfluorene as the Blue Host and a 2,1,3‐Benzothiadiazole Derivative as the Orange Dopant (Adv. Funct. Mater. 7/2006)

New single‐polymer electroluminescent systems containing two individual emission species—polyfluorenes as a blue host and 2,1,3‐benzothiadiazole derivative units as an orange dopant on the main chain—have been designed and synthesized by Wang and co‐workers on p. 957. The resulting single polymers are found to have highly efficient white electroluminescence with simultaneous blue and orange emission from the corresponding emitting species. A single‐layer device has been fabricated that has performance characteristics roughly comparable to those of organic white‐light‐emitting diodes with multilayer device structures. New single‐polymer electroluminescent systems containing two individual emission species—polyfluorenes as a blue host and 2,1,3‐benzothiadiazole derivative units as an orange dopant on the main chain—have been designed and synthesized. The resulting single polymers are found to have highly efficient white electroluminescence with simultaneous blue (λ_max = 421 nm/445 nm) and orange emission (λ_max = 564 nm) from the corresponding emitting species. The influence of the photoluminescence (PL) efficiencies of both the blue and orange species on the electroluminescence (EL) efficiencies of white polymer light‐emitting diodes (PLEDs) based on the single‐polymer systems has been investigated. The introduction of the highly efficient 4,7‐bis(4‐(N‐phenyl‐N‐(4‐methylphenyl)amino)phenyl)‐2,1,3‐benzothiadiazole unit to the main chain of polyfluorene provides significant improvement in EL efficiency. For a single‐layer device fabricated in air (indium tin oxide/poly(3,4‐ethylenedioxythiophene): poly(styrene sulfonic acid/polymer/Ca/Al), pure‐white electroluminescence with Commission Internationale de l'Eclairage (CIE) coordinates of (0.35,0.32), maximum brightness of 12 300 cd m^–2, luminance efficiency of 7.30 cd A^–1, and power efficiency of 3.34 lm W^–1 can be obtained. This device is approximately two times more efficient than that utilizing a single polyfluorene containing 1,8‐naphthalimide moieties, and shows remarkable improvement over the corresponding blend systems in terms of efficiency and color stability. Thermal treatment of the single‐layer device before cathode deposition leads to the further improvement of the device performance, with CIE coordinates of (0.35,0.34), turn‐on voltage of 3.5 V, luminance efficiency of 8.99 cd A^–1, power efficiency of 5.75 lm W^–1, external quantum efficiency of 3.8 %, and maximum brightness of 12 680 cd m^–2. This performance is roughly comparable to that of white organic light‐emitting diodes (WOLEDs) with multilayer device structures and complicated fabrication processes.     

Rational Molecular Design for Deep‐Blue Thermally Activated Delayed Fluorescence Emitters

By simple modification of the functional groups on the donor unit, the thermally activated delayed fluorescence (TADF) properties of emitters can easily be manipulated. A series of deep blue to blue emissive TADF derivatives is developed, capable of deep‐blue emissions from 403 to 460 nm in toluene. Deep‐blue organic light‐emitting diodes (OLEDs) based on this series of TADF emitters are fabricated, resulting in an electroluminescence peak at 428 nm and a high external quantum efficiency of up to 10.3%. One deep‐blue OLED has achieved the commission internationale de l'eclairage (CIE) coordinates of (0.156, 0.063), which is among the best reported TADF performances for deep‐blue OLEDs with CIE_y < 0.07.     

Toward Highly Efficient Solid‐State White Light‐Emitting Electrochemical Cells: Blue‐Green to Red Emitting Cationic Iridium Complexes with Imidazole‐Type Ancillary Ligands

Using imidazole‐type ancillary ligands, a new class of cationic iridium complexes ( 1 – 6 ) is prepared, and photophysical and electrochemical studies and theoretical calculations are performed. Compared with the widely used bpy (2,2′‐bipyridine)‐type ancillary ligands, imidazole‐type ancillary ligands can be prepared and modified with ease, and are capable of blueshifting the emission spectra of cationic iridium complexes. By tuning the conjugation length of the ancillary ligands, blue‐green to red emitting cationic iridium complexes are obtained. Single‐layer light‐emitting electrochemical cells (LECs) based on cationic iridium complexes show blue‐green to red electroluminescence. High efficiencies of 8.4, 18.6, and 13.2 cd A^?1 are achieved for the blue‐green‐emitting, yellow‐emitting, and orange‐emitting devices, respectively. By doping the red‐emitting complex into the blue‐green LEC, white LECs are realized, which give warm‐white light with Commission Internationale de L'Eclairage (CIE) coordinates of (0.42, 0.44) and color‐rendering indexes (CRI) of up to 81. The peak external quantum efficiency, current efficiency, and power efficiency of the white LECs reach 5.2%, 11.2 cd A^?1, and 10 lm W^?1, respectively, which are the highest for white LECs reported so far, and indicate the great potential for the use of these cationic iridium complexes in white LECs.     

White Electroluminescence by Supramolecular Control of Energy Transfer in Blends of Organic‐Soluble Encapsulated Polyfluorenes

Here, it is demonstrated that energy transfer in a blend of semiconducting polymers can be strongly reduced by non‐covalent encapsulation of one constituent, ensured by threading of the conjugated strands into functionalized cyclodextrins. Such macrocycles control the minimum intermolecular distance of chromophores with similar alignment, at the nanoscale, and therefore the relevant energy transfer rates, thus enabling fabrication of white‐light‐emitting diodes (CIE coordinates: x = 0.282, y = 0.336). In particular, white electroluminescence in a binary blend of a blue‐emitting, organic‐soluble rotaxane based on a polyfluorene derivative and the green‐emitting poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole ( F8BT ) is achieved. Morphological and structural analyses by atomic force microscopy, fluorescence mapping, µ‐Raman, and fluorescence lifetime microscopy are used to complement optical and electroluminescence characterization, and to enable a deeper insight into the properties of the novel blend.     

Tetradentate Platinum Complexes for Efficient and Stable Excimer‐Based White OLEDs

A series of tetradentate platinum complexes that exhibit both efficient monomer and excimer emission are synthesized. Via small modifications to the cyclometalating ligands, both the monomer and excimer emission energy can be separately tuned. Devices employing all of the developed emitters demonstrate impressively high external quantum efficiencies (EQEs) within the range of 22% to 27% for concentrations between 2% and 16%. The halogen‐free design of the complexes also enables the fabrication of single, doped, white organic light‐emitting diodes (OLEDs) with long operational lifetimes. A balanced white device employing the complex Pt2O2, achieves a device operational lifetime to 80% of the initial luminance estimated at over 200 h at 1000 cd m^–2, while also achieving 12.5% peak EQE for a warm white light with a color rendering index of 80. Furthermore, a highly doped device exhibiting nearly exclusive excimer emission showed an impressive operational lifetime, which is estimated at more than 400 h for 1000 cd m^‐2.     

Hydrogen‐Bonding‐Assisted Intermolecular Charge Transfer: A New Strategy to Design Single‐Component White‐Light‐Emitting Materials

This study reveals the mechanism of the dual‐emission properties for asymmetrical diphenylsulfone and diphenylketone derivatives. A series of asymmetrical diphenylketone and diphenylsulfone derivatives with dual‐emission properties are designed and synthesized. By single crystal structure analyses, various photophysical studies, and 2D ¹H–¹H NOSEY NMR studies, the lower energy emission bands in the dual‐emission spectra are successfully assigned to hydrogen‐bonding‐assisted intermolecular charge transfer emission. The emission properties of these compounds can easily be tuned in both solid state and solution state by destroying or strengthening the intermolecular hydrogen bonding. In addition, thermally activated delayed fluorescence characteristics for the intermolecular charge transfer emissions are also observed. The control of the intermolecular and intramolecular charge transfers serves as the basis for the generation of the white‐light emission. For compound CPzPO, nearly pure white‐light emission with CIE coordinates of (0.31, 0.32) is easily achieved by precipitation from dichloromethane and hexane mixed solvent system. These results clearly give an insight into the dual‐emission properties and provide a rational strategy for the design and synthesis of single‐component white‐light‐emitting materials and mechanoresponsive light‐emitting materials.     

Ternary Emission of Fluorescence and Dual Phosphorescence at Room Temperature: A Single‐Molecule White Light Emitter Based on Pure Organic Aza‐Aromatic Material

Herein, a simple aza‐aromatic compound dibenzo[a,c]phenazine (DPPZ), which exhibits single‐molecule white light with a ternary emission, consisting of simultaneous fluorescence (S₁→S₀) and dual room‐temperature phosphorescence (RTP, T₂→S₀ and T₁→S₀) is reported. The Commission Internationale de l' Éclairage coordinates of DPPZ powder are (0.28, 0.33). To everyone's knowledge, this is the first case to achieve single‐molecule white emission with ternary emission of fluorescence and dual RTP. This finding provides a prototype strategy to realize low‐cost, stable pure organic single‐molecule white light emission with three standard primary colors through further precise modulation of excited states.     

A Multifunctional Bipolar Luminogen with Delayed Fluorescence for High‐Performance Monochromatic and Color‐Stable Warm‐White OLEDs

Increasing exciton utilization and reducing exciton annihilation are crucial to achieve high performance of organic light‐emitting diodes (OLEDs), which greatly depend on molecular engineering of emitters and hosts. A novel luminogen (SBF‐BP‐DMAC) is synthesized and characterized. Its crystal and electronic structures, thermal stability, electrochemical behavior, carrier transport, photoluminescence, and electroluminescence are investigated. SBF‐BP‐DMAC exhibits enhanced photoluminescence and promotes delayed fluorescence in solid state and bipolar carrier transport ability, and thus holds multifunctionality of emitter and host for OLEDs. Using SBF‐BP‐DMAC as an emitter, the nondoped OLEDs exhibit maximum electroluminescence (EL) efficiencies of 67.2 cd A^?1, 65.9 lm W^?1, and 20.1%, and the doped OLEDs provide maximum EL efficiencies of 79.1 cd A^?1, 70.7 lm W^?1, and 24.5%. A representative orange phosphor, Ir(tptpy)₂acac, is doped into SBF‐BP‐DMAC for OLED fabrication, giving rise to superior EL efficiencies of 88.0 cd A^?1, 108.0 lm W^?1, and 26.8% for orange phosphorescent OLEDs, and forward‐viewing EL efficiencies of 69.3 cd A^?1, 45.8 lm W^?1, and 21.0% for two‐color hybrid warm‐white OLEDs. All of these OLEDs can retain high EL efficiencies at high luminance, with very small efficiency roll‐offs. The outstanding EL performance demonstrates the great potentials of SBF‐BP‐DMAC in practical display and lighting devices.     

Highly Efficient Pure‐White‐Light‐Emitting Diodes from a Single Polymer: Polyfluorene with Naphthalimide Moieties

Light‐emitting diodes exhibiting efficient pure‐white‐light electroluminescence have been successfully developed by using a single polymer: polyfluorene derivatives with 1,8‐naphthalimide chromophores chemically doped onto the polyfluorene backbones. By adjusting the emission wavelength of the 1,8‐naphthalimide components and optimizing the relative content of 1,8‐naphthalimide derivatives in the resulting polymers, white‐light electroluminescence from a single polymer, as opposed to a polymer blend, has been obtained in a device with a configuration of indium tin oxide/poly(3,4‐ethylenedioxythiophene)(50 nm)/polymer(80 nm)/Ca(10 nm)/Al(100 nm). The device exhibits Commission Internationale de l'Eclairage coordinates of (0.32,0.36), a maximum brightness of 11 900 cd m^–2, a current efficiency of 3.8 cd A^–1, a power efficiency of 2.0 lm W^–1, an external quantum efficiency of 1.50 %, and quite stable color coordinates at different driving voltages, even at high luminances of over 5000 cd m^–2.     

Over 10% EQE Near‐Infrared Electroluminescence Based on a Thermally Activated Delayed Fluorescence Emitter

Significant effort has been made to develop novel material systems to improve the efficiency of near‐infrared organic light‐emitting diodes (NIR OLEDs). Of those, fluorescent chromophores are mostly studied because of their advantages in cost and tunability. However, it is still rare for fluorescent NIR emitters to present good color purities in the NIR range and to have high external quantum efficiency (EQE). Here, a wedge‐shaped D‐π‐A‐π‐D emitter APDC‐DTPA with thermally activated delayed fluorescence property and a small single‐triplet splitting (ΔE_st) of 0.14 eV is presented. The non‐doped NIR device exhibits excellent performance with a maximum EQE of 2.19% and a peak wavelength of 777 nm. Remarkably, when 10 wt% of APDC‐DTPA is doped in 1,3,5‐tris(1‐phenyl‐1H‐benzimidazol‐2‐yl)benzene host, an extremely high EQE of 10.19% with an emission peak of 693 nm is achieved. All these values represent the best result for NIR OLEDs based on a pure organic fluorescent emitter with similar device structure and color gamut.     

Emission Color Trajectory and White Electroluminescence Through Supramolecular Control of Energy Transfer and Exciplex Formation in Binary Blends of Conjugated Polyrotaxanes

White electroluminescence and fine‐tuning of the emission color from binary blends of a blue‐emitting polymer and a green/yellow‐emitting threaded molecular wire consisting of a conjugated polymer supramolecularly encapsulated by functionalized cyclodextrins are demonstrated. Encapsulation controls the minimum intermolecular distance on the nanoscale, resulting in suppressed energy‐transfer between the blend constituents and reduced formation of interchain charge‐transfer complexes. The use of a green‐emitting polyrotaxane significantly improves the electrical properties with respect to blends of a blue electroluminescent polyrotaxane and leads to a significant reduction in the turn‐on voltage required for achieving white electroluminescence (V_ON = 3 V), with only 20% by weight of the encapsulated material.     

Achieving High‐Performance Nondoped OLEDs with Extremely Small Efficiency Roll‐Off by Combining Aggregation‐Induced Emission and Thermally Activated Delayed Fluorescence

Luminescent materials with thermally activated delayed fluorescence (TADF) can harvest singlet and triplet excitons to afford high electroluminescence (EL) efficiencies for organic light‐emitting diodes (OLEDs). However, TADF emitters generally have to be dispersed into host matrices to suppress emission quenching and/or exciton annihilation, and most doped OLEDs of TADF emitters encounter a thorny problem of swift efficiency roll‐off as luminance increases. To address this issue, in this study, a new tailor‐made luminogen (dibenzothiophene‐benzoyl‐9,9‐dimethyl‐9,10‐dihydroacridine, DBT‐BZ‐DMAC) with an unsymmetrical structure is synthesized and investigated by crystallography, theoretical calculation, spectroscopies, etc. It shows aggregation‐induced emission, prominent TADF, and interesting mechanoluminescence property. Doped OLEDs of DBT‐BZ‐DMAC show high peak current and external quantum efficiencies of up to 51.7 cd A^?1 and 17.9%, respectively, but the efficiency roll‐off is large at high luminance. High‐performance nondoped OLED is also achieved with neat film of DBT‐BZ‐DMAC, providing excellent maxima EL efficiencies of 43.3 cd A^?1 and 14.2%, negligible current efficiency roll‐off of 0.46%, and external quantum efficiency roll‐off approaching null from peak values to those at 1000 cd m^?2. To the best of the authors' knowledge, this is one of the most efficient nondoped TADF OLEDs with small efficiency roll‐off reported so far.     

Doping‐Induced Charge Trapping in Organic Light‐Emitting Devices

By using pyran‐containing donor–acceptor dyes as doping molecules in organic light‐emitting devices (OLEDs), we scrutinize the effects of charge trapping and polarization induced by the guest molecules in the electro‐active host material. Laser dyes 4‐(dicyanomethylene)‐2‐methyl‐6‐[2‐(julolidin‐9‐yl)phenyl]ethenyl]‐4H‐pyran (DCM2) and the novel 4‐(dicyanomethylene)‐2‐methyl‐6‐{2‐[(4‐diphenylamino)phenyl]ethenyl}‐4H‐pyran (DCM‐TPA) are used as model compounds. The emission color of these polar dyes depends strongly on doping concentration, which we have attributed to polarization effects induced by the doping molecules themselves. Their frontier orbital energy levels are situated within the bandgap of the tris(8‐hydroxyquinoline)aluminum (Alq₃) host matrix and allow the investigation of either electron trapping or both electron and hole trapping. In the case of DCM‐TPA doping, we were able to show that electron trapping leads to a partial shift of the recombination zone out of the doped Alq₃ region. To impede charge‐recombination processes taking place in the undoped host matrix, a charge‐blocking layer efficiently confines the recombination zone inside the doped zone and gives rise to increased luminous efficiency. For a doping concentration of 1 wt.‐% we obtain a maximum luminous efficiency of 10.4 cd A^–1. At this doping concentration, the yellow emission spectrum shows excellent color saturation with CIE chromaticity coordinates x, y of 0.49 and 0.50, respectively. In the case of DCM2 the recombination zone is much less affected for the same doping concentrations, which is ascribed to the fact that both electrons and holes are being trapped. The experimental findings are corroborated with a numerical simulation of the doped multilayer devices.