Cationic Ir(III) Complexes with 4-Fluoro-4'-pyrazolyl-(1,1'-biphenyl)-2-carbonitrile as the Cyclometalating Ligand: Synthesis, Characterizations, and Application to Ultrahigh-Efficiency Light-Emitting Electrochemical Cells.

Light-emitting electrochemical cells (LECs) promise low-cost, large-area luminescence applications with air-stabilized electrodes and a versatile fabrication that enables the use of solution processes. Nevertheless, the commercialization of LECs is still encountering many obstacles, such as low electroluminescence (EL) efficiencies of the ionic materials. In this paper, we propose five blue to yellow ionic Ir complexes possessing 4-fluoro-4'-pyrazolyl-(1,1'-biphenyl)-2-carbonitrile (ppfn) as a novel cyclometalating ligand and use them in LECs. In particular, the device within di[4-fluoro-4'-pyrazolyl-(1,1'-biphenyl)-2-carbonitrile]-4,4'-di-tert-butyl-2,2'-bipyridyl iridium(III) hexafluorophosphate (DTBP) shows a remarkable photoluminescence quantum yield (PLQY) of 70%, and by adjusting the emissive-layer thickness, the maximal external quantum efficiency (EQE) reaches 22.15% at 532 nm under the thickness of 0.51 µm, showing the state-of-the-art value for the reported blue-green LECs.

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.

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.

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.

Multifaceted Sulfone-Carbazole-Based D-A-D Materials: A Blue Fluorescent Emitter as a Host for Phosphorescent OLEDs and Triplet-Triplet Annihilation Up-Conversion Electroluminescence.

Electroluminescence (EL) from the singlet-excited (S1) state is the ideal choice for stable, high-performing deep-blue organic light-emitting diodes (OLEDs) owing to the advantages of an adequately short radiative lifetime, improved device durability, and low cost, which are the most important criteria for their commercialization. Herein, we present the design and synthesis of three donor-acceptor-donor (D-A-D)-configured deep-blue fluorescent materials (denoted as TC-1, TC-2, and TC-3) composed of a thioxanthone or diphenyl sulfonyl acceptor and phenyl carbazolyl donor. These systems exhibit strong deep-blue photoluminescence (422-432 nm) in solutions and redshifted emission (472-486 nm) in thin films. The solid-state photoluminescence quantum yield (PLQY) was estimated to be 78 and 94% for TC-2 and TC-3, respectively. TC-2 and TC-3 possess good molecular packing and large molecular cross-sectional areas, which not only improves the PLQY but enhances the triplet-triplet annihilation up-conversion (TTAUC) efficiency of fluorescent emitters. Furthermore, both compounds were applied as an acceptor for confirming their TTAUC property using bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) (Ir(MDQ)2acac) as the sensitizer. Non-doped OLEDs based on TC-2 and TC-3 exhibit blue EL in the 461-476 nm range. In particular, TC-3 exhibits a maximum external quantum efficiency (EQEmax) of 5.1%, and its EL maximum is 476 nm. In addition, the three emitters were employed as hosts in red OLEDs using bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) (Ir(piq)2acac) as the phosphorescent dopant. The red phosphorescent OLEDs based on TC-1, TC-2, and TC-3 achieve excellent EQEmax values of 21.6, 22.9, and 21.9%, respectively, and peak luminance efficiencies of 12.0, 14.0, and 12.3 cd A-1. These results highlight these fluorophores' versatility and promising prospects in practical OLED applications.

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.

TADF-based NIR-II semiconducting polymer dots for in vivo 3D bone imaging.

Intraoperative fluorescence imaging in the second near-infrared (NIR-II) region heralds a new era in image-guided surgery since the success in the first-in-human liver-tumor surgery guided by NIR-II fluorescence. Limited by the conventional small organic NIR dyes such as FDA-approved indocyanine green with suboptimal NIR-II fluorescence and non-targeting ability, the resulting shallow penetration depth and high false positive diagnostic values have been challenging. Described here is the design of NIR-II emissive semiconducting polymer dots (Pdots) incorporated with thermally activated delayed fluorescence (TADF) moieties to exhibit emission maxima of 1064-1100 nm and fluorescence quantum yields of 0.40-1.58% in aqueous solutions. To further understand how the TADF units affect the molecular packing and the resulting optical properties of Pdots, in-depth and thorough density-functional theory calculations were carried out to better understand the underlying mechanisms. We then applied these Pdots for in vivo 3D bone imaging in mice. This work provides a direction for future designs of NIR-II Pdots and holds promising applications for bone-related diseases.

Dual-Functional Enantiomeric Compounds as Hole-Transporting Materials and Interfacial Layers in Perovskite Solar Cells.

In this paper, we describe the application of the enantiomeric compounds YLC-1-YLC-4, each featuring a bulky spiro[fluorene-9,9'-phenanthren]-10'-one moiety, as both hole-transporting materials (HTMs) and interfacial layers in both n-i-p and p-i-n perovskite solar cells (PSCs). These HTMs contain an enantiomeric mixture and a variety of core units linked to triarylamine donors to extend the degree of π-conjugation. The n-i-p PSCs incorporating YLC-1(a) exhibited a power conversion efficiency (PCE) of 19.15% under AM 1.5G conditions (100 mW cm-2); this value was comparable with that obtained using spiro-OMeTAD as the HTM (18.25%). We obtained efficient and stable p-i-n PSCs having the dopant-free structure indium tin oxide (ITO)/NiOx/interfacial layer (YLC)/perovskite/PC61BM/BCP/Ag. The presence of the spiro-based compounds YLC-1 and YLC-2 efficiently passivated the interfacial and grain boundary defects of the perovskite and enhanced the sizes of its grains, more so than did YLC-3 and YLC-4. These spiro-based YLC derivatives packed densely and functioned as Lewis bases to coordinate Pb and Ni ions in the perovskite and NiOx layers, respectively. Together, the effects of smaller grain boundaries and defect passivation of the perovskite enhanced the optoelectronic properties of the PSCs. The photoinduced charge carrier extraction in the linearly increasing voltage (photo-CELIV) curves of NiOx/YLC-1(a) showed the faster carrier transport 3.3 × 10-3 cm2 V-1 s-1, which improved the carrier mobility, supporting the notion of defect passivation of the perovskite. The best-performing NiOx/YLC-1(a) device provided a short-circuit current density (JSC) of 22.88 mA cm-2, an open-circuit voltage (VOC) of 1.10 V, and a fill factor (FF) of 80.93%, corresponding to an overall PCE of 20.37%. In addition, the PCEs of the NiOx/YLC-1(a) and NiOx/YLC-4(b) PSC devices underwent decays of only 98.1 and 97.0% of their original values after 41 days under an Ar atmosphere. Thus, these YLC derivatives passivated the NiOx surface and optimized the film quality of perovskites, thereby leading to superior PCEs of their respective PSCs.

Imidazolyl-Phenylcarbazole-Based Host Materials and Their Use for Co-host Designs in Phosphorescent OLEDs.

In recent years, owing to the demand for high-efficiency phosphorescent organic light-emitting devices (PhOLEDs), many studies have been conducted on the development of bipolar host materials. A series of imidazolyl-phenylcarbazole-based host materials, i. e., im-CzP, im-CzPCz, im-CzPtBu, and im-OCzP, were synthesized to obtain high-efficiency green and red-emitting PhOLEDs. With im-OCzP as the host, satisfactory peak efficiencies of 22.2 (77.0 cd A-1 and 93.1 lm W-1 ) and 14.1 % (9.0 cd A-1 and 10.1 lm W-1 ) could be obtained, respectively. To further improve the performance of the devices, an electron transport material, bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PyMPM) was selected to construct a co-hosted system. The efficiency of im-OCzP combined with B3PyMPM forming co-hosts could also achieve high values of 23.0 (80.0 cd A-1 and 98.8 lm W-1 ) and 16.5 % (10.2 cd A-1 and 13.4 lm W-1 ) for green and red PhOLEDs, respectively. These results exhibited that the proposed bipolar hosts have great flexibility in adjusting the carrier balance of EML in OLEDs, demonstrating their ingenious design and high potential.

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

Invited for the cover of this issue are Chin-Wei Lu, Zu-Po Yang, Hai-Ching Su, and co-workers at National Yang Ming Chiao Tung University and Providence University. The image depicts electron transport for light-emitting electrochemical cells. Read the full text of the article at 10.1002/chem.202103739.

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.

Facile Fabrication of Highly Stable and Wavelength-Tunable Tin Based Perovskite Materials with Enhanced Quantum Yield via the Cation Transformation Reaction.

Metal halide perovskites have attracted great attention for their superior light energy conversion applications. Herein, we demonstrated a facile synthesis of zero-dimensional Sn2+ perovskite Cs4-xMxSnBr6(M = K+ and Rb+) material through the cation transformation reaction at room temperature. Cs4SnBr6 NCs was mixed with pure metal bromide salts (KBr and RbBr) via the mechanochemical process to successfully synthesize Cs4-xMxSnBr6 perovskite where transformation of Cs to mixed Cs/Rb and mixed Cs/K was achieved. By substituting different cations, the bright fluorescence of the Cs4-xMxSnBr6 was tuned from dim green to greenish-cyan while achieving the photoluminescence (PL) quantum yield of ∼39%. The crystal structure of Sn based perovskite with the substitution of K+ or Rb+ cations was determined by X-ray diffraction (XRD). Moreover, the Cs4-xMxSnBr6 demonstrated superior air stability and exhibited a better photocatalytic activity for CO2 reduction reaction (CO2RR) with high selectivity of CH4 gas with a higher yield rate compared to the pristine Cs4SnBr6 NCs.

Dicyano-Imidazole: A Facile Generation of Pure Blue TADF Materials for OLEDs.

A series of dicyano-imidazole-based molecules with thermally activated delayed fluorescence (TADF) properties were synthesized to obtain pure blue-emitting organic light-emitting diodes (OLEDs). The targeted molecules used dicyano-imidazole with a short-conjugated system as the electron acceptor to strong intermolecular π-π interactions, and provide a relatively shallow energy level of the lowest unoccupied molecular orbital (LUMO). The cyano group was selected to improve imidazole as an electron acceptor due to its prominent electron-transporting characteristics. Four different electron donors, that is, 9,9-dimethyl-9,10-dihydroacridine (DMAC), 10H-spiro(acridine-9,9'-fluoren) (SPAC), and 9,9-diphenyl-9,10-dihydroacridine (DPAC), were used to alternate the highest occupied molecular orbital (HOMO) energy level to tune the emission color further. The crowded molecular structure in space makes the electron donor and acceptor almost orthogonal, reducing the energy gap (ΔEST ) between the first excited singlet (S1 ) and the triplet (T1 ) states and introducing significant TADF property. The efficiencies of the blue-emissive devices with imM-SPAC and imM-DMAC obtained in this work are the highest among the reported imidazole-based TADF-OLEDs, which are 13.8 % and 13.4 %, respectively. Both of Commission Internationale de l'Eclairage (CIE) coordinates are close to the saturated blue region at (0.17, 0.18) and (0.16, 0.19), respectively. Combining these tailor-made TADF compounds with specific device architectures, electroluminescent (EL) emission from sky-blue to deep-blue could be achieved, proving their great potential in EL applications.

Spherical Hole-Transporting Interfacial Layer Passivated Defect for Inverted NiOx-Based Planar Perovskite Solar Cells with High Efficiency of over 20.

In this study, we achieved a facile and low-cost (18-22 USD/g) synthesis of spiro[fluorene-9,9-phenanthren-10-one]-based interfacial layer materials (MSs; designated MS-PC, MS-PA, MS-OC, and MS-OA). Carbazoles and dimethylacridine substituents with an extended π-conjugation achieved through ortho- or para-orientations were used as donors at the spiro[fluorene-9,9'-phenanthren-10'-one] moiety. Highly efficient and stable inverted perovskite solar cells (PSCs) with the device architecture of ITO/NiOx/MSs/perovskite/PC61BM/BCP/Ag can be achieved to improve the surface morphology of NiOx when MSs are adopted as the interfacial layer. During a morphological study, the ortho-orientated donor of MS-OC and MS-OA has spherical structures indicated that the films were smooth and that the films of perovskite deposited on them had large grain size and uniformity. The photoluminescence properties of the perovskite layers on the NiOx/MSs were showed better hole-transporting capabilities than the bare NiOx. The dual-functional interfacial layer has shown defect passivation effect, it not only improved the surface morphology of NiOx but also enlarged the perovskite layer grain size. The best PSC device performance of the NiOx/MS-OC was characterized by 22.34 mA cm-2 short-circuit current density (Jsc), 1.128 V open-circuit voltage (Voc), and 80.8% fill factor (FF), resulting in 20.34% power conversion efficiency (PCE). The NiOx/MS-OC PSCs showed good long-term device stability, even retained the original PCE of 93.16% after 370 days under argon (25 °C). Owing to the superior perovskite morphologies of the NiOx/MSs, the resulting devices outperformed the bare NiOx-based PSCs.

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.

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.

Facile Generation of Thermally Activated Delayed Fluorescence and Fabrication of Highly Efficient Non-Doped OLEDs Based on Triazine Derivatives.

A series of donor-acceptor-donor triazine-based molecules with thermally activated delayed fluorescence (TADF) properties were synthesized to obtain highly efficient blue-emitting OLEDs with non-doped emitting layers (EMLs). The targeted molecules use a triazine core as the electron acceptor, and a benzene ring as the conjugated linker with different electron donors to alternate the energy level of the HOMO to further tune the emission color. The introduction of long alkyl chains on the triazine core inhibits the unwanted intermolecular D-D/A-A-type π-π interactions, resulting in the intermolecular D-A charge transfer. The weak aggregation-caused quenching (ACQ) effect caused by the suppressed intermolecular D-D/A-A-type π-π interaction further enhances the emission. The crowded molecular structure allows the electron donor and acceptor to be nearly orthogonal, thereby reducing the energy gap between triplet and singlet excited states (ΔEST ). As a result, blue-emitting devices with TH-2DMAC and TH-2DPAC non-doped EMLs showed satisfactory efficiencies of 12.8 % and 15.8 %, respectively, which is one of the highest external quantum efficiency (EQEs) reported for blue TADF emitters (λpeak <475 nm), demonstrating that our tailored molecular designs are promising strategies to endow OLEDs with excellent electroluminescent performances.

Efficient and Saturated Red Light-Emitting Electrochemical Cells Based on Cationic Iridium(III) Complexes with EQE up to 9.4 .

Solid-state light-emitting electrochemical cells (LECs) have several advantages, such as low-voltage operation, compatibility with inert metal electrodes, large-area flexible substrates, and simple solution-processable device architectures. However, most of the studies on saturated red LECs show low or moderate device efficiencies (external quantum efficiency (EQE) <3.3 %). In this work, we demonstrate a series of five red-emitting cationic iridium complexes (RED1--RED5) with 2,2'-biquinoline ligands and test their electroluminescence (EL) characteristics in LECs. The Commission Internationale de l'Eclairage (CIE) 1931 coordinates for the LECs based on these complexes are all beyond the National Television System Committee (NTSC) red standard point (0.67, 0.33). The maximal EQE of the neat-film RED1-based LECs reaches 7.4 %. The reddest complex, RED3, is doped in the blue-emitting host complex, BG, to fabricate host-guest LECs. The maximal EQE of the host-guest LECs (1 wt % complex RED3) reaches 9.4 %, which is among the highest reported for the saturated red LECs.

Cationic IrIII Emitters with Near-Infrared Emission Beyond 800†nm and Their Use in Light-Emitting Electrochemical Cells.

Solid-state near-infrared (NIR) light-emitting devices have recently received considerable attention as NIR light sources that can penetrate deep into human tissue and are suitable for bioimaging and labeling. In addition, solid-state NIR light-emitting electrochemical cells (LECs) have shown several promising advantages over NIR organic light-emitting devices (OLEDs). However, among the reported NIR LECs based on ionic transition-metal complexes (iTMCs), there is currently no iridium-based LEC that displays NIR electroluminescence (EL) peaks near to or above 800 nm. In this report we demonstrate a simple method for adjusting the energy gap between the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) of iridium-based iTMCs to generate NIR emission. We describe a series of novel ionic iridium complexes with very small energy gaps, namely NIR1-NIR6, in which 2,3-diphenylbenzo[g]quinoxaline moieties mainly take charge of the HOMO energy levels and 2,2'-biquinoline, 2-(quinolin-2-yl)quinazoline, and 2,2'-bibenzo[d]thiazole moieties mainly control the LUMO energy levels. All the complexes exhibited NIR phosphorescence, with emission maxima up to 850 nm, and have been applied as components in LECs, showing a maximum external quantum efficiency (EQE) of 0.05 % in the EL devices. By using a host-guest emissive system, with the iridium complex RED as the host and the complex NIR3 or NIR6 as guest, the highest EQE of the LECs can be further enhanced to above 0.1 %.

Effects of tuning the applied voltage pulse periods on the electroluminescence spectra of host-guest white light-emitting electrochemical cells.

Solid-state white light-emitting electrochemical cells (LECs) are potential candidates in solid-state lighting due to their promising advantages of simple device structure, low-voltage operation and compatibility with inert cathode metals. Adjusting the correlated color temperature (CCT) of background illumination is highly desired for modern smart lighting systems. In this work, a novel technique to tune the CCT of electroluminescence (EL) from white LECs is proposed. Color tuning is based on adjusting the applied voltage pulse period on the host-guest white LECs and the working mechanism is illustrated. A shorter voltage pulse period is insufficient to completely charge the capacitive LEC device and thus the effective voltage applied on the device is lower. Since the host-guest energy level offsets favor carrier trapping, a lower effective applied voltage results in a more pronounced guest emission, rendering redder white EL with a lower CCT. On the other hand, a longer voltage pulse period facilitates more complete charging and the effective voltage applied on the white LEC is higher. A higher bias facilitates direct exciton formation on the host molecule and subsequent partial host-guest energy transfer generates bluer white EL with a higher CCT. By tuning the voltage pulse period from 0.2 to 20 ms, the CCT of EL resulting from white LECs ranges from 2482 to 5723 K. The CCT tuning range is sufficient for general lighting applications. In contrast to color tuning of white LECs under constant-voltage driving, in which >10× brightness enhancement is accompanied by higher-CCT white EL, the discharging half-period in pulse-voltage driving provides relaxation time to turn off the device and reduces the average brightness of the white LECs driven under a longer voltage pulse period. Therefore, similar brightness can be achieved for white EL with different CCTs. No additional optical filtering device is needed for this novel color tuning technique and it has potential for use in solid-state lighting.