Blue Light-emitting Electrochemical Cells Research Articles

Cationic Ir(III) Complex with 5-Phenyl-1H-1,2,4-triazole and 2-(1-Pyrazolyl)pyridine Derivatives for Blue Light-emitting Electrochemical Cells

Cationic iridium complex for blue light-emitting electrochemical cells (LECs) was synthesized from 1-(2,6-dimethylphenyl)-3-methyl-5-phenyl-1H-1,2,4-triazole (dphtz) cyclometalating ligand and 2-(1-phenyl-1H-pyrazol-3-yl)pyridine (phpzpy) ancillary ligand. The photophysical and electrochemical properties were investigated by UV-vis absorption, photoluminescence spectroscopy and cyclic voltammetry, and compared to those of the previously reported [Ir(ppy)2(phpzpy)]+. The LECs emitted blue light peaking around 488 nm and max luminance of 77.35 cd/m2 under constant current density of 10 mA/cm2.

Intrinsically-ionic donor-acceptor molecules featuring thermally-activated delayed fluorescence for high-performance host-guest blue light-emitting electrochemical cells

High-performance blue light-emitting electrochemical cells (LECs) have remained a formidable challenge. Ionic donor–acceptor molecules, especially those featuring thermally-activated delayed fluorescence (TADF), are promising hosts for developing high-performance blue LECs, but such hosts have remained undeveloped. Here, we report two intrinsically-ionic donor–acceptor molecules, 1-(3-(4-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-6-phenylpyrimidin-2-yl)phenyl)-3-ethyl-1H-imidazol-3-ium hexafluorophosphate (H1) and 1-(3-(4-(2-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-6-phenylpyrimidin-2-yl)phenyl)-3-ethyl-1H-imidazol-3-ium hexafluorophosphate (H2), and their use as hosts for high-performance blue LECs. 3,6-di-tert-butylcabazole and imidazolium-substituted 2,4-diphenylpyrimidine are used as the donor and acceptor, respectively, which are located para (for H1) or ortho (for H2) to each other on the central phenyl linker. While H1 shows a low triplet energy (<2.6 eV), a large singlet–triplet energy gap (ΔEST) (>0.4 eV) and no TADF, H2 exhibits a high triplet energy (>2.8 eV), a small ΔEST (∼0.2 eV) and distinct TADF, due to its twisted donor–acceptor conformation. Both H1 and H2 show excellent electrochemical stability. Host-guest blue LECs using H2 as the host and a phosphorescent cationic Ir(III) complex as the guest afford sky-blue electroluminescence with high blue-color stability and peak brightness/peak external quantum efficiency/half-lifetime at 1078 cd m−2/8.6%/66 min under a constant-current driving. Upon adding LiPF6 into the active layer, a half-lifetime of ∼ 2 h is achieved at 918 cd m−2. These performances are the highest for host–guest blue LECs reported so far. The efficiencies and operational stability achieved at the high brightness are also the highest for blue LECs with inert-metal cathodes reported so far.

Blue light-emitting electrochemical cells incorporating aggregation-induced emission small molecules

The light-emitting electrochemical cells (LECs) have received extensive attention from researchers in recent years due to their simple single-layer structure, air-stable metal electrodes, and low manufacturing cost. So far, reports on the use of LEC active layer emitter materials have mainly focused on ionic transition metal complexes (iTMCs) and conjugated polymers (CPs). The application of the above materials in blue LECs is restricted due to high cost, complicated synthesis process, and poor stability, which results in the slow development of blue LECs. A novel blue LECs device based on aggregation-induced emission (AIE) small molecule (2,3-diphenylbenzo[b]thiophene S, S-sulfur dioxide, DP-BTO) with a peak luminance of 36.58 cd/m2, a CEmax of 1.14 cd/A, and an EQEmax of 0.46% was reported in this work. The blue light emission centered at ~460 nm was exhibited at a voltage above 6 V. This work successfully constructed blue light-emitting LECs devices based on AIE materials, opening up a new research avenue for the application of AIE materials in the field of thin-film device development.

Color‐Stable, Efficient, and Bright Blue Light‐Emitting Electrochemical Cell Using Ionic Exciplex Host

AbstractThe lack of high‐performance blue light‐emitting electrochemical cells (LECs) has remained a formidable challenge for fabricating white LECs for lighting applications. Here, a ionic exciplex host is used for color‐stable, efficient, and bright blue LECs by taking advantage of its facilitated carrier injection, bipolar charge‐transport, and efficient energy transfer to the guest dopant. A cationic donor molecule, 1‐(3‐(3,6‐di‐tert‐butyl‐9H‐carbazol‐9‐yl)phenyl)‐3‐methyl‐1H‐imidazol‐3‐ium hexafluorophosphate (tbuCAZ‐ImMePF6), and a cationic acceptor molecule, 1‐(3‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)phenyl)‐3‐ethyl‐1H‐imidazol‐3‐ium hexafluorophosphate (TRZ‐ImEtPF6), are developed to form the ionic exciplex host. The mixed film of tbuCAZ‐ImMePF6 and TRZ‐ImEtPF6 affords blue exciplex with fast reverse intersystem crossing and thermally activated delayed fluorescence. For the film doped with a blue‐emitting iridium complex, energy is efficiently transferred from the exciplex to the complex. Host‐guest LECs using the doped film as the active layer show stable blue emission color and high current efficiencies of up to 25.8 cd A−1. More importantly, they attain simultaneously high efficiency and high brightness (14.1/17.4/16.8 cd A−1 at 705/872/1680 cd m−2), which are the most efficient and bright host‐guest blue LECs reported so far. The primary host‐guest LEC also exhibits promising operational stability. The work reveals that the use of an ionic exciplex host is a promising avenue toward high‐performance blue LECs.

Utilization of novel phenanthrene–imidazole-based ionic small molecules for blue light-emitting electrochemical cells

Light-emitting electrochemical cells (LECs), which represent solid-state lighting (SSL) systems, are next generation displays to replace organic light-emitting diodes.

Aggregation induced emission small molecules for blue light-emitting electrochemical cells

Two blue aggregation induced emission (AIE) luminogens containing phenanthroimidazole and tetraphenylethylene were designed and synthesized. The optical properties, thermal stability, electronic properties, electrochemical properties and electroluminescence of both the derivatives were studied. The phenanthroimidazole derivatives show good thermal stability and AIE properties. The non-doped light-emitting electrochemical cell devices were fabricated using these luminogens as light-emitting materials. The LEC devices show blue color emission centered at 474 nm with good performances.

Enhancing the Overall Performances of Blue Light-Emitting Electrochemical Cells by Using an Electron-Injecting/Transporting Ionic Additive.

Light-emitting electrochemical cells (LECs) have emerged as a promising emissive thin-film technology for next-generation solid-state lighting. However, blue LECs show low performances, which has remained a bottleneck for the fabrication of white LECs for lighting applications. Here, we demonstrate a remarkable enhancement of overall device performance for blue LECs by using an electron-injecting/transporting ionic additive, that is, [Zn(bpy)3](PF6)2 (bpy is 2.2'-bipyridine, PF6- is hexafluorophosphate). It is revealed that adding [Zn(bpy)3](PF6)2 into the active layers of blue LECs accelerates the device response, simultaneously enhances the brightness and efficiency, reduces the efficiency roll-offs, significantly improves the blue color stability upon the continuous electrical operation, and enhances the device operational stability at optimized conditions. The remarkable enhancement of the overall device performance upon adding [Zn(bpy)3](PF6)2 results from facilitated electron injection/transport and thus more balanced electron-hole recombination and more centered recombination zone, as well as the reduction of phosphorescence concentration quenching in the LECs. The work demonstrates for the first time that the use of electron-injecting/transporting ionic additives such as [Zn(bpy)3](PF6)2 is a facile yet effective strategy to remarkably boost the overall performances of blue LECs.

Improving color saturation of blue light-emitting electrochemical cells by plasmonic filters

Abstract In consideration of the advantages of light-emitting electrochemical cells (LECs), it is desired to develop the saturated blue LECs for LEC display, which is hindered by the features of broad emission spectrum and emission peak not short enough. In this study, we demonstrated a novel method to improve blue saturation of the sky-blue LECs by engineering its emission spectrum through the plasmonic filters. These plasmonic filters composed of randomly distributed silver nanoparticles (Ag-NPs) can absorb the green and red emission tail of the sky-blue LECs due to localized surface plasmon resonance (LSPR). The LSPR wavelengths of Ag-NPs are tuned by manipulating the effective refractive index of materials around Ag-NPs through the accurate control of the TiO 2 thickness using atomic layer deposition technique. By integrating with the plasmonic filters, the CIE1931 coordinate of the blue LECs can approach to (0.14, 0.22), which is comparable to or even better than the reported bluest values of blue LECs. Combination with the green and red LECs, the color gamut increases from 34% (without filters) to 54% of National Television System Committee (NTSC) color gamut, corresponding to 1.6 times enhancement. In addition, the blue LECs integrated with plasmonic filters still have better efficiency than those of the reported bluest LECs.

Phenanthroimidazole derivatives for single component blue light-emitting electrochemical cells

ABSTRACTLight emitting electrochemical cells (LECs) are considered to be the future generation of lighting sources for full color displays and solid-state lightings. The recent report on LEC devices with small organic molecules presents a plausible way for this potential lighting technology. Novel blue emitting phenanthroimidazole derivatives were synthesized and characterized by thermal, photophysical, electrochemical and electroluminescence properties. Solution processed LEC devices were fabricated using both the emitters as a light-emitting layer. The fabricated single-component LEC devices exhibited blue-electroluminescence centered at 485 nm and 480 nm with CIEx,y of (0.18, 0.26) and (0.17, 0.16) for compound 1 and 2, respectively.

Blue Light-Emitting Electrochemical Cells Based on Angularly Structured Phenanthroimidazole Derivatives

Photophysical properties of four new ionic blue emitting phenanthroimidazole derivatives were investigated. Rarely utilized meta and ortho positions of C2 substituted benzene linker of phenanthroimidazole were functionalized herein to generate blue emitters for light-emitting electrochemical cell (LEC) applications. To this end, pyrene and anthracene units were coupled to phenanthroimidazole core through meta or ortho linkage. This molecular design strategy is anticipated to generate deeper blue emission than the corresponding para isomers due to lack of extended conjugation. Highly contrasting properties were observed between ortho- and meta-coupled fluorophores. Unexpectedly, solubility issues hinder the application of ortho-linked molecules in LEC devices. However, bright blue emissions were achieved in LEC devices when meta derivatives were mixed in the active layer of LEC with poly(ethylene oxide) and lithium triflate. Notably, the molecule having pyrene and phenanthroimidazole coupled through meta l...

Designing NHC-Copper(I) Dipyridylamine Complexes for Blue Light-Emitting Electrochemical Cells.

This study presents the influence of various substituents on the photophysical features of heteroleptic copper(I) complexes bearing both N-heterocyclic carbene (NHC) and dipyridylamine (dpa = dipyridylamine skeleton corresponding to ligand L1) ligands. The luminescent properties have been compared to our recently reported archetypal blue emitting [Cu(IPr)(dpa)][PF6] complex. The choice of the substituents on both ligands has been guided to explore the effect of the electron donor/acceptor and "push-pull" on the emission wavelengths and photoluminescence quantum yields. A selection of the best candidates in terms of their photophysical features were applied for developing the first blue light-emitting electrochemical cells (LECs) based on copper(I) complexes. The device analysis suggests that the main concern is the moderate redox stability of the complexes under high applied driving currents, leading to devices with moderate stabilities pointing to a proof-of-concept for further development. Nevertheless, under low applied driving currents the blue emission is stable, showing performance levels competitive to those reported for blue LECs based on iridium(III) complexes. Overall, this work provides valuable guidelines to tackle the design of enhanced NHC copper complexes for lighting applications in the near future.

Non-doped deep blue light-emitting electrochemical cells from charged organic small molecules

Pure deep-blue light-emitting LEC devices were fabricated utilizing charged organic small molecules. with CIE coordinates of (0.15, 0.09) and (0.16, 0.10) for compound 1 and compound 2, respectively.

Synthesis and photophysical characterization of an ionic fluorene derivative for blue light-emitting electrochemical cells

A highly fluorescent an ionic fluorene derivative 1 was synthesized and its photophysical, electrochemical and electroluminescence characteristics were investigated. Deep blue emissions were observed for compound 1 in solid as well as in dilute solutions. The synthesized compound shows high fluorescence quantum yield around 77% indicates that compound 1 can perform its role as efficient ionic emitter in LEC devices. Light-emitting electrochemical cell (LEC) devices were fabricated incorporating compound 1 without (device I) and with (device II) ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM·PF6). Devices I and II exhibited blue electroluminescence maximum centered at 455 and 454nm with CIE coordinates of (0.15, 0.21) and (0.16, 0.22), respectively. Maximum luminance and current efficiency of 1105cdm−2 and 0.14cdA−1 respectively, has achieved for device I while that of device II resulted in 1247cdm−2 and 0.14cdA−1 respectively.

Do the Intramolecular π Interactions Improve the Stability of Ionic, Pyridine-Carbene-Based Iridium(III) Complexes?

Throughout the last years one of the most intensive research topics in light-emitting electrochemical cells (LECs) focused on the design of blue-emitting, ionic iridium(III) complexes. To this end, the most recent strategy is the use of carbene-based ancillary ligands. Although blue LECs have been successfully fabricated, the stability has been noted as the main drawback. To overcome this problem, Zhang et al. have recently explored the use of π interactions to enhance the strength of pyridine-carbene-based complexes. The authors suggested that the use of intramolecular π–π stacking interactions by means of pendant phenyl rings to improve the stability of LECs is not as effective as in devices with diimine-based complexes. To interpret this phenomenon clearly, the features of a family of pyridine-carbene-based iridium(III) complexes, in which phenyl groups are sequentially attached to the carbene and the pyridine rings of the ancillary ligand, have been thoroughly studied by using a theoretical approach. ...

Fully transparent, organic light-emitting electrochemical cells

We report the fabrication and performance of fully transparent, organic blue light-emitting electrochemical cells (OLECs), in which both the anode and cathode are made of indium tin oxide. The active layer is a blend of polyfluorene with long and flexible alkyl side chains grafted on the 9,9 position and of a molten salt. Two identical spin-coated active layers are laminated together at high temperature to form the OLECs. The electroluminescence threshold is around 3.3 V and the light intensity exceeds 10 μW/cm2 at 5 V.