Scrutinizing Design Principles toward Efficient,Long‐Term Stable Green Light‐Emitting Electrochemical Cells

Enhancing the efficiency and lifetime of light emitting electrochemical cells (LEC) is the most important challenge on the way to energy efficient lighting devices of the future. To avail this, emissive Ir(III) complexes with fluoro‐substituted cyclometallated ligands and electron donating groups (methyl and tert ‐butyl)‐substituted diimine ancillary (N^{^}N) ligands and their associated LEC devices are studied. Four different complexes of general composition [Ir(4ppy)₂(N^{^}N)][PF₆] (4Fppy = 2‐(4‐fluorophenyl)pyridine) with the N^{^}N ligand being either 2,2′‐bipyridine ( 1 ), 4.4′‐dimethyl‐2,2′‐bipyridine ( 2 ), 5.5′‐dimethyl‐2,2′‐bipyridine ( 3 ), or 4.4′‐di‐tert ‐butyl‐2,2′‐bipyridine ( 4 ) are synthesized and characterized. All complexes emit in the green region of light with emission maxima of 529–547 nm and photoluminescence quantum yields in the range of 50.6%–59.9%. LECs for electroluminescence studies are fabricated based on these complexes. The LEC based on ( 1 ) driven under pulsed current mode demonstrated the best performance, reaching a maximum luminance of 1605 cd m^?2 resulting in 16 cd A^?1 and 8.6 lm W^?1 for current and power efficiency, respectively, and device lifetime of 668 h. Compared to this, LECs based on ( 3 ) and ( 4 ) perform lower, with luminance and lifetime of 1314 cd m^?2, 45.7 h and 1193 cd m^?2, 54.9 h, respectively. Interestingly, in contrast to common belief, the fluorine content of the Ir‐iTMCs does not adversely affect the LEC performance, but rather electron donating substituents on the N^{^}N ligands are found to dramatically reduce both performance and stability of the green LECs. In light of this, design concepts for green light emitting electrochemical devices have to be reconsidered.     

Blue‐Emitting Cationic Iridium Complexes with 2‐(1H‐Pyrazol‐1‐yl)pyridine as the Ancillary Ligand for Efficient Light‐Emitting Electrochemical Cells

Two blue‐emitting cationic iridium complexes with 2‐(1H‐pyrazol‐1‐yl)pyridine (pzpy) as the ancillary ligands, namely, [Ir(ppy)₂(pzpy)]PF₆ and [Ir(dfppy)₂(pzpy)]PF₆ (ppy is 2‐phenylpyridine, dfppy is 2‐(2,4‐difluorophenyl) pyridine, and PF₆^? is hexafluorophosphate), have been prepared, and their photophysical and electrochemical properties have been investigated. In CH₃CN solutions, [Ir(ppy)₂(pzpy)]PF₆ emits blue‐green light (475 nm), which is blue‐shifted by more than 100 nm with respect to the typical cationic iridium complex [Ir(ppy)₂(dtb‐bpy)]PF₆ (dtb‐bpy is 4,4′‐di‐tert‐butyl‐2,2′‐bipyridine); [Ir(dfppy)₂(pzpy)]PF₆ with fluorine‐substituted cyclometalated ligands shows further blue‐shifted light emission (451 nm). Quantum chemical calculations reveal that the emissions are mainly from the ligand‐centered ³π–π* states of the cyclometalated ligands (ppy or dfppy). Light‐emitting electrochemical cells (LECs) based on [Ir(ppy)₂(pzpy)]PF₆ gave green‐blue electroluminescence (486 nm) and had a relatively high efficiency of 4.3 cd A^?1 when an ionic liquid 1‐butyl‐3‐methylimidazolium hexafluorophosphate was added into the light‐emitting layer. LECs based on [Ir(dfppy)₂(pzpy)]PF₆ gave blue electroluminescence (460 nm) with CIE (Commission Internationale de L'Eclairage) coordinates of (0.20, 0.28), which is the bluest light emission for iTMCs‐based LECs reported so far. Our work suggests that using diimine ancillary ligands involving electron‐donating nitrogen atoms (like pzpy) is an efficient strategy to turn the light emission of cationic iridium complexes to the blue region.     

Efficient and Long Lived Green Light‐Emitting Electrochemical Cells

The combination of high efficiencies and long lifetime in a single light‐emitting electrochemical cell (LEC) device remain a major problem in LEC technology, preventing its application in commercial lighting devices. Three green light‐emitting cationic iridium‐based complexes of the general composition [Ir(C^{^}N)₂(N^{^}N)][PF₆] with 4‐Fppy (2‐(4‐fluorophenyl)pyridinato) as the cyclometalating C^{^}N ligand and 1,10‐phenanthroline ( 1 ), 4,7‐diphenyl‐1,10‐phenanthroline (bathophenanthroline, bphen, 2 ), and 2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline (bathocuprione, dmbphen, 3 ) as ancillary N^{^}N ligands are synthesized and characterized. Computational studies are carried out in order to compare the electronic structure of the three ionic transition metal complexes (iTMCs) and provide insights into their potential as LEC emitter materials. LECs are then fabricated with complexes 1 – 3 . Driven under a pulsed current, they display a high luminance and current and power efficiencies. As the LEC based on complex 2 displays the overall best device performance, including the longest lifetime of 474 h, it is selected for subsequent driving conditions optimization. An extraordinary power efficiency of 25 lm W^?1 and current efficiency of 30 cd A^?1 are achieved under optimized operation conditions with reduced current density, resulting in a long device lifetime of 720 h. Altogether, ligand design in iTMCs and optimization of the device driving conditions leads to a significant improvement in LEC performance.     

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.     

Saturated and Multi‐Colored Electroluminescence from Quantum Dots Based Light Emitting Electrochemical Cells

Novel light emitting electrochemical cells (LECs) are fabricated using CdSe‐CdS (core‐shell) quantum dots (QDs) of tuned size and emission blended with polyvinylcarbazole (PVK) and the ionic liquid 1‐butyl‐3‐methylimidazolium hexafluorophosphate (BMIM‐PF₆). The performances of cells constructed using sequential device layers of indium tin oxide (ITO), poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS), the QD/PVK/IL active layer, and Al are evaluated. Only color saturated electroluminescence from the QDs is observed, without any other emissions from the polymer host or the electrolyte. Blue, green, and red QD‐LECs are prepared. The maximum brightness (≈1000 cd m^‐2) and current efficiency (1.9 cd A^‐1) are comparable to polymer LECs and multilayer QD‐LEDs. White‐light QD‐LECs with Commission Internationale d'Eclairage (CIE) coordinates (0.33, 0.33) are prepared by tuning the mass ratio of R:G:B QDs in the active layer and voltage applied. Transparent QD‐LECs fabricated using transparent silver nanowire (AgNW) composites as the cathode yield an average transmittance greater than 88% over the visible range. Flexible devices are demonstrated by replacing the glass substrates with polyethylene terephthalate (PET).     

Archetype Cationic Iridium Complexes and Their Use in Solid‐State Light‐Emitting Electrochemical Cells

The archetype ionic transition‐metal complexes (iTMCs) [Ir(ppy)₂(bpy)][PF₆] and [Ir(ppy)₂(phen)][PF₆], where Hppy = 2‐phenylpyridine, bpy = 2,2′‐bipyridine, and phen = 1,10‐phenanthroline, are used as the primary active components in light‐emitting electrochemical cells (LECs). Solution and solid‐state photophysical properties are reported for both complexes and are interpreted with the help of density functional theory calculations. LEC devices based on these archetype complexes exhibit long turn‐on times (70 and 160 h, respectively) and low external quantum efficiencies (～2%) when the complex is used as a pure film. The long turn‐on times are attributed to the low mobility of the counterions. The performance of the devices dramatically improves when small amounts of ionic liquids (ILs) are added to the Ir‐iTMC: the turn‐on time improves drastically (from hours to minutes) and the device current and power efficiency increase by almost one order of magnitude. However, the improvement of the turn‐on time is unfortunately accompanied by a decrease in the stability of the device from 700 h to a few hours. After a careful study of the Ir‐iTMC:IL molar ratios, an optimum between turn‐on time and stability is found at a ratio of 4:1. The performance of the optimized devices using these rather simple complexes is among the best reported to date. This holds great promise for devices that use specially‐designed iTMCs and demonstrates the prospect for LECs as low‐cost light sources.     

Rationalizing Fabrication and Design Toward Highly Efficient and Stable Blue Light‐Emitting Electrochemical Cells Based on NHC Copper(I) Complexes

Recently, the use of a new family of electroluminescent copper(I) complexes—i.e., the archetypal [Cu(IPr)(3‐Medpa)][PF₆] complex; IPr: 1,3‐bis‐(2,6‐di‐iso‐propylphenyl)imidazole‐2‐ylidene; 3‐Medpa: 2,2′‐bis‐(3‐methylpyridyl)amine—has led to blue light‐emitting electrochemical cells (LECs) featuring luminances of 20 cd m^?2, stabilities of 4 mJ, and efficiencies of 0.17 cd A^?1. Herein, this study rationalizes how to enhance these figures‐of‐merit optimizing both device fabrication and design. On one hand, a comprehensive spectroscopic and electrochemical study reveals the degradation of this novel emitter in common solvents used for LEC fabrication, as well as the impact on the photoluminescence features of thin‐films. On the other hand, spectro‐electrochemical and electrochemical impedance spectroscopy assays suggest that the device performance is strongly limited by the irreversible formation of oxidized species that mainly act as carrier trappers and luminance quenchers. Based on all of the aforementioned, device optimization was realized using ionic additives and a hole transporter either as a host–guest or as a multilayered architecture approach to decouple hole/electron injection. The latter significantly enhances the LEC performance, reaching luminances of 160 cd m^?2, stabilities of 32.7 mJ, and efficiencies of 1.2 cd A^?1. Overall, this work highlights the need of optimizing both device fabrication and design toward highly efficient and stable LECs based on cationic copper(I) complexes.     

Light‐Emitting Paper

A solution‐based fabrication of flexible and light‐weight light‐emitting devices on paper substrates is reported. Two different types of paper substrates are coated with a surface‐emitting light‐emitting electrochemical cell (LEC) device: a multilayer‐coated specialty paper with an intermediate surface roughness of 0.4 μm and a low‐end and low‐cost copy paper with a large surface roughness of 5 μm. The entire device fabrication is executed using a handheld airbrush, and it is notable that all of the constituent layers are deposited from solution under ambient air. The top‐emitting paper‐LECs are highly flexible, and display a uniform light emission with a luminance of 200 cd m^?2 at a current conversion efficacy of 1.4 cd A^?1.     

Efficient Light‐Emitting Electrochemical Cells (LECs) Based on Ionic Iridium(III) Complexes with 1,3,4‐Oxadiazole Ligands

Eight new iridium(III) complexes 1‐8 , with 1,3,4‐oxadiazole (OXD) derivatives as the cyclometalated C^N ligand and/or the ancillary N^N ligands are synthesized and their electrochemical, photophysical, and solid‐state light‐emitting electrochemical cell (LEC) properties are investigated. Complexes 1 , 2 , 7 and 8 are additionally characterized by single crystal X‐ray diffraction. LECs based on complexes 1‐8 are fabricated with a structure indium tin oxide (ITO)/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/cationic iridium complex:ionic liquid/Al. LECs of complexes 1 – 6 with OXD derivatives as the cyclometalated ligands and as the ancillary ligand show yellow luminescence (λ_max = 552–564 nm). LECs of complexes 7 and 8 with cyclometalated C^N phenylpyridine ligands and an ancillary N^N OXD ligand show red emission (λ_max 616–624 nm). Using complex 7 external quantum efficiency (EQE) values of >10% are obtained for devices (210 nm emission layer) at 3.5 V. For thinner devices (70 nm) high brightness is achieved: red emission for 7 (8528 cd m^?2 at 10 V) and yellow emission for 1 (3125 cd m^?2 at 14 V).     

Multifunctional Fluorene‐Based Oligomers with Novel Spiro‐Annulated Triarylamine: Efficient,Stable Deep‐Blue Electroluminescence,Good Hole Injection,and Transporting Materials with Very High Tg

A series of fluorene‐based oligomers with novel spiro‐annulated triarylamine structures, namely DFSTPA, TFSTPA, and TFSDTC, are synthesized by a Suzuki cross‐coupling reaction. The spiro‐configuration molecular structures lead to very high glass transition temperatures (197–253 °C) and weak intermolecular interactions, and consequently the structures retain good morphological stability and high fluorescence quantum efficiencies(0.69–0.98). This molecular design simultaneously solves the spectral stability problems and hole‐injection and transport issues for fluorene‐based blue‐light‐emitting materials. Simple double‐layer electroluminescence (EL) devices with a configuration of ITO/TFSTPA (device A) or TFSDTC (device B)/ TPBI/LiF/Al, where TFSTPA and TFSDTC serve as hole‐transporting blue‐light‐emitting materials, show a deep‐blue emission with a peak around 432 nm, and CIE coordinates of (0.17, 0.12) for TFSTPA and (0.16, 0.07) for TFSDTC, respectively, which are very close to the National Television System Committee (NTSC) standard for blue (0.15, 0.07). The maximum current efficiency/external quantum efficiencies are 1.63 cd A^?1/1.6% for device A and 1.91 cd A^?1/2.7% for device B, respectively. In addition, a device with the structure ITO/DFSTPA/Alq₃/LiF/Al, where DFSTPA acts as both the hole‐injection and ‐transporting material, is shown to achieve a good performance, with a maximum luminance of 14 047 cd m^?2, and a maximum current efficiency of 5.56 cd A^?1. These values are significantly higher than those of devices based on commonly usedN,N′‐di(1‐naphthyl)‐N,N′‐diphenyl‐[1,1′‐biphenyl]‐4,4′‐diamine (NPB) as the hole‐transporting layer (11 738 cd m^?2 and 3.97 cd A^?1) under identical device conditions.     

Organic Light‐Emitting Diodes with a White‐Emitting Molecule: Emission Mechanism and Device Characteristics

The unique and unprecedented electroluminescence behavior of the white‐emitting molecule 3‐(1‐(4‐(4‐(2‐(2‐hydroxyphenyl)‐4,5‐diphenyl‐1H‐imidazol‐1‐yl)phenoxy)phenyl)‐4,5‐diphenyl‐1H‐imidazol‐2‐yl)naphthalen‐2‐ol (W1), fluorescence emission from which is controlled by the excited‐state intramolecular proton transfer (ESIPT) is investigated. W1 is composed of covalently linked blue‐ and yellow‐color emitting ESIPT moieties between which energy transfer is entirely frustrated. It is demonstrated that different emission colors (blue, yellow, and white) can be generated from the identical emitter W1 in organic light‐emitting diode (OLED) devices. Charge trapping mechanism is proposed to explain such a unique color‐tuned emission from W1. Finally, the device structure to create a color‐stable, color reproducible, and simple‐structured white organic light‐emitting diode (WOLED) using W1 is investigated. The maximum luminance efficiency, power efficiency, and luminance of the WOLED were 3.10 cd A^?1, 2.20 lm W^?1, 1 092 cd m^?2, respectively. The WOLED shows white‐light emission with the Commission Internationale de l′Eclairage (CIE) chromaticity coordinates (0.343, 0.291) at a current level of 10 mA cm^?2. The emission color is high stability, with a change of the CIE chromaticity coordinates as small as (0.028, 0.028) when the current level is varied from 10 to 100 mA cm^?2.     

Origin of the Exclusive Ternary Electroluminescent Behavior of BN‐Doped Nanographenes in Efficient Single‐Component White Light‐Emitting Electrochemical Cells

White‐light‐emitting electrochemical cells (WLECs) still represent a significant milestone, since only a few examples with moderate performances have been reported. Particularly, multiemissive white emitters are highly desired, as a paradigm to circumvent phase separation and voltage‐dependent emission color issues that are encountered following host:guest and multilayered approaches. Herein, the origin of the exclusive white ternary electroluminescent behavior of BN‐doped nanographenes with a B₃N₃ doping pattern (hexa‐perihexabenzoborazinocoronene) is rationalized, leading to one of the most efficient (≈3 cd A^?1) and stable‐over‐days single‐component and single‐layered WLECs. To date, BN‐doped nanographenes have featured blue thermally activated delayed fluorescence (TADF). This doping pattern provides, however, white electroluminescence spanning the whole visible range (x/y CIE coordinates of 0.29–31/0.31–38 and average color rendering index (CRI) of 87) through a ternary emission involving fluorescence and thermally activated dual phosphorescence. This temperature‐dependent multiemissive mechanism is operative for both photo‐ and electroluminescence processes and holds over the device lifespan, regardless of the device architecture, active layer composition, and operating conditions. As such, this work represents a new stepping‐stone toward designing a new family of multiemissive white emitters based on BN‐doped nanographenes that realizes one of the best‐performing single‐component white‐emitting devices compared to the prior‐art.     

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.     

A Multifunctional Iridium‐Carbazolyl Orange Phosphor for High‐Performance Two‐Element WOLED Exploiting Exciton‐Managed Fluorescence/Phosphorescence

By attaching a bulky, inductively electron‐withdrawing trifluoromethyl (CF₃) group on the pyridyl ring of the rigid 2‐[3‐ (N‐phenylcarbazolyl)]pyridine cyclometalated ligand, we successfully synthesized a new heteroleptic orange‐emitting phosphorescent iridium(III) complex [Ir( L ₁ )₂(acac)] 1 ( HL ₁ = 5‐trifluoromethyl‐2‐[3‐(N‐phenylcarbazolyl)]pyridine, Hacac = acetylacetone) in good yield. The structural and electronic properties of 1 were examined by X‐ray crystallography and time‐dependent DFT calculations. The influence of CF₃ substituents on the optical, electrochemical and electroluminescence (EL) properties of 1 were studied. We note that incorporation of the carbazolyl unit facilitates the hole‐transporting ability of the complex, and more importantly, attachment of CF₃ group provides an access to a highly efficient electrophosphor for the fabrication of orange phosphorescent organic light‐emitting diodes (OLEDs) with outstanding device performance. These orange OLEDs can produce a maximum current efficiency of ～40 cd A^?1, corresponding to an external quantum efficiency of ～12% ph/el (photons per electron) and a power efficiency of ～24 lm W^?1. Remarkably, high‐performance simple two‐element white OLEDs (WOLEDs) with excellent color stability can be fabricated using an orange triplet‐harvesting emitter 1 in conjunction with a blue singlet‐harvesting emitter. By using such a new system where the host singlet is resonant with the blue fluorophore singlet state and the host triplet is resonant with the orange phosphor triplet level, this white light‐emitting structure can achieve peak EL efficiencies of 26.6 cd A^?1 and 13.5 lm W^?1 that are generally superior to other two‐element all‐fluorophore or all‐phosphor OLED counterparts in terms of both color stability and emission efficiency.     

Organic Light‐Emitting Diodes: Organic Light‐Emitting Diodes with a White‐Emitting Molecule: Emission Mechanism and Device Characteristics (Adv. Funct. Mater. 4/2011)

The unique and unprecedented electroluminescence behavior of the white‐emitting molecule 3‐(1‐(4‐(4‐(2‐(2‐hydroxyphenyl)‐4,5‐diphenyl‐1H‐imidazol‐1‐yl)phenoxy)phenyl)‐4,5‐diphenyl‐1H‐imidazol‐2‐yl)naphthalen‐2‐ol (W1), fluorescence emission from which is controlled by the excited‐state intramolecular proton transfer (ESIPT) is investigated. W1 is composed of covalently linked blue‐ and yellow‐color emitting ESIPT moieties between which energy transfer is entirely frustrated. It is demonstrated that different emission colors (blue, yellow, and white) can be generated from the identical emitter W1 in organic light‐emitting diode (OLED) devices. Charge trapping mechanism is proposed to explain such a unique color‐tuned emission from W1. Finally, the device structure to create a color‐stable, color reproducible, and simple‐structured white organic light‐emitting diode (WOLED) using W1 is investigated. The maximum luminance efficiency, power efficiency, and luminance of the WOLED were 3.10 cd A^?1, 2.20 lm W^?1, 1 092 cd m^?2, respectively. The WOLED shows white‐light emission with the Commission Internationale de l′Eclairage (CIE) chromaticity coordinates (0.343, 0.291) at a current level of 10 mA cm^?2. The emission color is high stability, with a change of the CIE chromaticity coordinates as small as (0.028, 0.028) when the current level is varied from 10 to 100 mA cm^?2.     

Molecular Engineering of Blue Fluorescent Molecules Based on Silicon End‐Capped Diphenylaminofluorene Derivatives for Efficient Organic Light‐Emitting Materials

Blue fluorescent materials based on silicone end‐capped 2‐diphenylaminofluorene derivatives are synthesized and characterized. These materials are doped into a 2‐methyl‐9,10‐di‐[2‐naphthyl]anthracene host as blue dopant materials in the emitting layer of organic light‐emitting diode devices bearing a structure of ITO/DNTPD (60 nm)/NPB (30 nm)/emitting layer (30 nm)/Alq₃ (20 nm)/LiF (1.0 nm)/Al (200 nm). All devices exhibit highly efficient blue electroluminescence with high external quantum efficiencies (3.47%–7.34% at 20 mA cm^?2). The best luminous efficiency of 11.2 cd A^?1 and highest quantum efficiency of 7.34% at 20 mA cm^?2 are obtained in a device with CIE coordinates (0.15, 0.25). A deep‐blue OLED with CIE coordinates (0.15, 0.14) exhibits a luminous efficiency of 3.70 cd A^?1 and quantum efficiency of 3.47% at 20 mA cm^?2.     

Blue‐Light‐Emitting Oligoquinolines: Synthesis,Properties, and High‐Efficiency Blue‐Light‐Emitting Diodes

The synthesis, photophysics, cyclic voltammetry, and highly efficient blue electroluminescence of a series of four new n‐type conjugated oligomers, 6,6′‐bis(2,4‐diphenylquinoline) (B1PPQ), 6,6′‐bis(2‐(4‐tert‐butylphenyl)‐4‐phenylquinoline) (BtBPQ), 6,6′‐bis(2‐p‐biphenyl)‐4‐phenylquinoline) (B2PPQ), and 6,6′‐bis((3,5‐diphenylbenzene)‐4‐phenylquinoline) (BDBPQ) is reported. The oligoquinolines have high glass‐transition temperatures (T_g ≥ 133 °C), reversible electrochemical reduction, and high electron affinities (2.68–2.81 eV). They emit blue photoluminescence with 0.73–0.94 quantum yields and 1.06–1.42 ns lifetimes in chloroform solutions. High‐performance organic light‐emitting diodes (OLEDs) with excellent blue chromaticity coordinates are achieved from all the oligoquinolines. OLEDs based on B2PPQ as the blue emitter give the best performance with a high brightness (19 740 cd m^–2 at 8.0 V), high efficiency (7.12 cd A^–1 and 6.56 % external quantum efficiency at 1175 cd m^–2), and excellent blue color purity as judged by the Commission Internationale de L'Eclairage (CIE) coordinates (x = 0.15,y = 0.16). These results represent the best efficiency of blue OLEDs from neat fluorescent organic emitters reported to date. These results demonstrate the potential of oligoquinolines as emitters and electron‐transport materials for developing high‐performance blue OLEDs.     

Highly Efficient Electroluminescence from Green‐Light‐Emitting Electrochemical Cells Based on CuI Complexes

The complexes [Cu(dnbp)(DPEphos)]⁺(X^–) (dnbp and DPEphos are 2,9‐di‐n‐butyl‐1,10‐phenanthroline and bis[2‐(diphenylphosphino)phenyl]ether, respectively, and X^– is BF₄^–, ClO₄^–, or PF₆^–) can form high‐quality films with photoluminescence quantum yields of up to 71 ± 7 %. Their electroluminescent properties are studied using the device structure indium tin oxide (ITO)/complex/metal cathode. The devices emit green light efficiently, with an emission maximum of 523 nm, and work in the mode of light‐emitting electrochemical cells. The response time of the devices greatly depends on the driving voltage, the counterions, and the thickness of the complex film. After pre‐biasing at 25 V for 40 s, the devices turn on instantly, with a turn‐on voltage of ca. 2.9 V. A current efficiency of 56 cd A^–1 and an external quantum efficiency of 16 % are realized with Al as the cathode. Using a low‐work‐function metal as the cathode can significantly enhance the brightness of the device almost without affecting the turn‐on voltage and current efficiency. With a Ca cathode, a brightness of 150 cd m^–2 at 6 V and 4100 cd m^–2 at 25 V is demonstrated. The electroluminescent performance of these types of complexes is among the best so far for transition metal complexes with counterions.     

Manipulating Charge‐Transfer Character with Electron‐Withdrawing Main‐Group Moieties for the Color Tuning of Iridium Electrophosphors

A new and synthetically versatile strategy has been developed for the phosphorescence color tuning of cyclometalated iridium phosphors by simple tailoring of the phenyl ring of ppy (Hppy = 2‐phenylpyridine) with various main‐group moieties in [Ir(ppy‐X)₂(acac)] (X = B(Mes)₂, SiPh₃, GePh₃, NPh₂, POPh₂, OPh, SPh, SO₂Ph). This can be achieved by shifting the charge‐transfer character from the pyridyl groups in some traditional iridium ppy‐type complexes to the electron‐withdrawing main‐group moieties and these assignments were supported by theoretical calculations. This new color tuning strategy in Ir^III‐based triplet emitters using electron‐withdrawing main‐group moieties provides access to Ir^III phosphors with improved electron injection/electron transporting features essential for highly efficient, color‐switchable organic light‐emitting diodes (OLEDs). The present work furnished OLED colors spanning from bluish‐green to red (505–609 nm) with high electroluminescence efficiencies which have great potential for application in multicolor displays. The maximum external quantum efficiency of 9.4%, luminance efficiency of 10.3 cd A⁻¹ and power efficiency of 5.0 lm W⁻¹ for the red OLED (X = B(Mes)₂), 11.1%, 35.0 cd A⁻¹, and 26.8 lm W⁻¹ for the bluish‐green device (X = OPh), 10.3%, 36.9 cd A⁻¹, and 28.6 lm W⁻¹ for the bright green device (X = NPh₂) as well as 10.7%, 35.1 cd A⁻¹, and 23.1 lm W⁻¹ for the yellow‐emitting device (X = SO₂Ph) can be obtained.     

The Design and Realization of Flexible,Long‐Lived Light‐Emitting Electrochemical Cells

Polymer light‐emitting electrochemical cells (LECs) offer an attractive opportunity for low‐cost production of functional devices in flexible and large‐area configurations, but the critical drawback in comparison to competing light‐emission technologies is a limited operational lifetime. Here, it is demonstrated that it is possible to improve the lifetime by straightforward and motivated means from a typical value of a few hours to more than one month of uninterrupted operation at significant brightness (>100 cd m^?2) and relatively high power conversion efficiency (2 lm W^?1 for orange‐red emission). Specifically, by optimizing the composition of the active material and by employing an appropriate operational protocol, a desired doping structure is designed and detrimental chemical and electrochemical side reactions are identified and minimized. Moreover, the first functional flexible LEC with a similar promising device performance is demonstrated.