Synthesis and characterization of blue-emitting Ir(III) complexes with multi-functional ancillary ligands for solution-processed phosphorescent organic light-emitting diodes

A series of Ir(III) complexes, (dfpmpy)₂Ir(pic), (dfpmpy)₂Ir(EO₂-pic), (dfpmpy)₂Ir(pic-N-O), and (dfpmpy)₂Ir(EO₂-pic-N-O), containing 2-(2,4-difluorophenyl)-4-methylpyridine (dfpmpy) based main ligand with varying ancillary ligands such as picolinic acid (pic), 4-(2-ethoxyethoxy)picolinic acid (EO₂-pic), picolinic acid N-oxide (pic-N-O), and 4-(2-ethoxyethoxy)picolinic acid N-oxide (EO₂-pic-N-O), respectively were successfully synthesized for highly efficient blue phosphorescent organic light-emitting diodes (PhOLEDs). The photophysical, electrochemical, and electroluminescent (EL) properties were systematically correlated. The solubilizing 2-ethoxyethanol (EO₂-) group was attached to the ancillary ligand through tandem reaction. All of the Ir(III) complexes show high thermal stability and good photoluminescence quantum yields (Ф_pl) in film state. Solution-processed PhOLEDs were fabricated using these Ir(III) complexes as dopants and achieved a maximum external quantum efficiency (EQE) of 10.9% and current efficiency of 21.15 cd/A for (dfpmpy)₂Ir(EO₂-pic). All the Ir(III) complexes emitted blue light with color purity at the Commission Internationale de L’Eclairage (CIE) coordinates of (0.15, 0.31).     

High‐Efficiency Deep‐Blue Light‐Emitting Diodes Based on Phenylquinoline/Carbazole‐Based Compounds

Highly efficient deep‐blue fluorescent materials based on phenylquinoline–carbazole derivatives (PhQ‐CVz, MeO‐PhQ‐CVz, and CN‐PhQ‐CVz) are synthesized for organic light‐emitting diodes (OLEDs). The materials form high‐quality amorphous thin films by thermal evaporation and the energy levels can be easily adjusted by the introduction of different electron‐donating and electron‐withdrawing groups on carbazoylphenylquinoline. Non‐doped deep‐blue OLEDs that use PhQ‐CVz as the emitter show bright emission (Commission Internationale de L'Éclairage (CIE) coordinates, x = 0.156, y = 0.093) with an external quantum efficiency of 2.45%. Furthermore, the material works as an excellent host material for 4,4′‐bis(9‐ethyl‐3‐carbazovinylene)‐1,1′‐biphenyl dopant to get high‐performance OLEDs with excellent deep‐blue CIE coordinates (x = 0.155, y = 0.157), high power efficiency (5.98 lm W^?1), and high external quantum efficiency (5.22%).     

Amorphous Diphenylaminofluorene‐Functionalized Iridium Complexes for High‐Efficiency Electrophosphorescent Light‐Emitting Diodes

Two new phosphorescent iridium(III ) cyclometalated complexes, [Ir(DPA‐Flpy)₃] ( 1 ) and [Ir(DPA‐Flpy)₂(acac)] ( 2 ) ((DPA‐Flpy)H = (9,9‐diethyl‐7‐pyridinylfluoren‐2‐yl)diphenylamine, Hacac = acetylacetone), have been synthesized and characterized. The incorporation of electron‐donating diphenylamino groups to the fluorene skeleton is found to increase the highest occupied molecular orbital (HOMO) levels and add hole‐transporting ability to the phosphorescent center. Both complexes are highly amorphous and morphologically stable solids and undergo glass transitions at 160 and 153 °C, respectively. These iridium phosphors emit bright yellow to orange light at room temperature with relatively short lifetimes (< 1 μs) in both solution and the solid state. Organic light‐emitting diodes (OLEDs) fabricated using 1 and 2 as phosphorescent dopant emitters constructed with a multilayer configuration show very high efficiencies. The homoleptic iridium complex 1 is shown to be a more efficient electrophosphor than the heteroleptic congener 2 . Efficient electrophosphorescence with a maximum external quantum efficiency close to 10 % ph/el (photons per electron), corresponding to a luminance efficiency of ～ 30 cd A^–1 and a power efficiency of ～ 21 lm W^–1, is obtained by using 5 wt.‐% 1 as the guest dopant.     

m‐Indolocarbazole Derivative as a Universal Host Material for RGB and White Phosphorescent OLEDs

The host materials designed for highly efficient white phosphorescent organic light‐emitting diodes (PhOLEDs) with power efficiency (PE) >50 lm W^‐1 and low efficiency roll‐off are very rare. In this work, three new indolocarbazole‐based materials (ICDP, 4ICPPy, and 4ICDPy) are presented composed of 6,7‐dimethylindolo[3,2‐a]carbazole and phenyl or 4‐pyridyl group for hosting blue, green, and red phosphors. Among this three host materials, 4ICDPy‐based devices reveal the best electroluminescent performance with maximum external quantum efficiencies (EQEs) of 22.1%, 27.0%, and 25.3% for blue (FIrpic), green (fac‐Ir(ppy)₃), and red ((piq)₂Ir(acac)) PhOLEDs. A two‐color and single‐emitting‐layer white organic light‐emitting diode hosted by 4ICDPy with FIrpic and Ir(pq)₃ as dopants achieves high EQE of 20.3% and PE of 50.9 lm W^?1 with good color stability; this performance is among the best for a single‐emitting‐layer white PhOLEDs. All 4ICDPy‐based devices show low efficiency roll‐off probably due to the excellent balanced carrier transport arisen from the bipolar character of 4ICDPy.     

Multifunctional Crosslinkable Iridium Complexes as Hole Transporting/Electron Blocking and Emitting Materials for Solution‐Processed Multilayer Organic Light‐Emitting Diodes

Here, a new series of crosslinkable heteroleptic iridium (III) complexes for use in solution processed phosphorescent organic light emitting diodes (OLEDs) is reported. These iridium compounds have the general formula of (PPZ‐VB)₂Ir(CˆN), where PPZ‐VB is phenylpyrazole (PPZ) vinyl benzyl (VB) ether; and the CˆN ligands represent a family of four different cyclometallating ligands including 1‐phenylpyrazolyl (PPZ) (1), 2‐(4,6‐difluorophenyl)pyridyl (DFPPY) (2), 2‐(p‐tolyl)pyridyl (TPY) (3), and 2‐phenylquinolyl (PQ) (4). With the incorporation of two crosslinkable VB ether groups, these compounds can be fully crosslinked after heating at 180 °C for 30 min. The crosslinked films exhibit excellent solvent resistance and film smoothness which enables fabrication of high‐performance multilayer OLEDs by sequential solution processing of multiple layers. Furthermore, the photophysical properties of these compounds can be easily controlled by simply changing the cyclometallating CˆN ligand in order to tune the triplet energy within the range of 3.0–2.2 eV. This diversity makes these materials not only suitable for use in hole transporting and electron blocking but also as emissive layers of several colors. Therefore, these compounds are applied as effective materials for all‐solution processed OLEDs with (PPZ‐VB)₂IrPPZ (1) acting as hole transporting and electron blocking layer and host material, as well as three other compounds, (PPZ‐VB)₂IrDFPPY ( 2 ), (PPZ‐VB)₂IrTPY(3), and (PPZ‐VB)₂IrPQ( 4 ), used as crosslinkable phosphorescent emitters.     

Influence of Cavity Thickness and Emitter Orientation on the Efficiency Roll‐Off of Phosphorescent Organic Light‐Emitting Diodes

This article describes the first systematic investigation of how the efficiency roll‐off in organic light‐emitting diodes (OLEDs) is influenced by the position and orientation of the emitter molecules within the OLED cavity. The efficiency roll‐off is investigated for two OLED stacks containing either the phosphorescent emitter Ir(MDQ)₂(acac) or Ir(ppy)₃ by varying the distance between emitter and metal cathode; a strong influence of emitter position and orientation on roll‐off is observed. The measurements are modeled by triplet‐triplet‐annihilation (TTA) theory yielding the critical current density and the TTA rate constant. It is found that Ir(MDQ)₂(acac) shows the lowest roll‐off when the emitter is located in the first optical maximum of the electromagnetic field, whereas the roll‐off of the Ir(ppy)₃ stack is lowest when the emitter is positioned closer to the metal cathode. Measurement and modeling of time‐resolved electroluminescence show that the different roll‐off behavior is due to the different orientation and the corresponding change of the decay rate of the emissive dipoles of Ir(MDQ)₂(acac) and Ir(ppy)₃. Finally, design principles are developed for optimal high‐brightness performance by modeling the roll‐off as a function of emitter‐cathode distance, emissive dipole orientation, and radiative efficiency.     

High Performance Polymer Electrophosphorescent Devices with tert‐Butyl Group Modified Iridium Complexes as Emitters

The synthesis and photophysical study of two novel tert‐butyl modified cyclometalated iridium(III) complexes, i.e., bis(4‐tert‐butyl‐2‐phenylbenzothiozolato‐N,C^2′) iridium(III)(acetylacetonate) [(tbt)₂Ir(acac)] and bis(4‐tert‐butyl‐1‐phenyl‐1H‐benzimidazolato‐N,C^2′) iridium(III)(acetylacetonate) [(tpbi)₂Ir(acac)], are reported, their molecular structures were characterized by ¹³C NMR, ¹H NMR, ESI‐MS, FT‐IR, and elementary analysis. Compared with their prototypes without tert‐butyl substituents [(bt)₂Ir(acac) and (pbi)₂Ir(acac)], (tbt)₂Ir(acac) and (tpbi)₂Ir(acac) both have shortened phosphorescent lifetimes[(tbt)₂Ir(acac) versus (bt)₂Ir(acac), 1.1 μs:1.8 μs; (pbi)₂Ir(acac) versus (tpbi)₂Ir(acac), 0.8 μs:1.82 μs]. Moreover, (tbt)₂Ir(acac) has much more improved phototoluminescence quantum efficiencies in CH₂Cl₂ solution, [(tbt)₂Ir(acac), 0.51; (bt)₂Ir(acac), 0.26]. Employing them as dopants, high performance double‐layer PLEDs were fabricated. The (tbt)₂Ir(acac)‐based and (tpbi)₂Ir(acac)‐based PLEDs have the maximum external quantum efficiencies of 8.71 % and 10.25 %, respectively, and high EL quantum efficiencies of 5.92 % and 7.21 % can be achieved under high driven current density of 100 mA cm^–2. PLEDs fabricated with both the two phosphors have much broadened EL spectra with FWHM of > 110 nm, which afford the feasibility to be used as dopants in white LEDs, and the best doping concentrations of the two complexes in fabrication of PLEDs were optimized.     

Deep‐Blue Phosphorescent Ir(III) Complexes with Light‐Harvesting Functional Moieties for Efficient Blue and White PhOLEDs in Solution‐Process

The photoluminescence (PL) efficiency of emitters is a key parameter to accomplish high electroluminescent performance in phosphorescent organic light‐emitting diodes (PhOLEDs). With the aim of enhancing the PL efficiency, this study designs deep‐blue emitting heteroleptic Ir(III) complexes (tBuCN‐FIrpic, tBuCN‐FIrpic‐OXD, and tBuCN‐FIrpic‐mCP) for solution‐processed PhOLEDs by covalently attaching the light‐harvesting functional moieties (mCP‐Me or OXD‐Me) to the control Ir(III) complex, tBuCN‐FIrpic. These Ir(III) complexes show similar deep‐blue emission peaks around 453, 480 nm (298 K) and 447, 477 nm (77 K) in chloroform. tBuCN‐FIrpic‐mCP demonstrates higher light‐harvesting efficiency (142%) than tBuCN‐FIrpic‐OXD (112%), relative to that of tBuCN‐FIrpic (100%), due to an efficient intramolecular energy transfer from the mCP group to the Ir(III) complex. Accordingly, the monochromatic PhOLEDs of tBuCN‐FIrpic‐mCP show higher external quantum efficiency (EQE) of 18.2% with one of the best blue coordinates (0.14, 0.18) in solution‐processing technology. Additionally, the two‐component (deep‐blue:yellow‐orange), single emitting layer, white PhOLED of tBuCN‐FIrpic‐mCP shows a maximum EQE of 20.6% and superior color quality (color rendering index (CRI) = 78, Commission Internationale de L'Eclairage (CIE) coordinates of (0.353, 0.352)) compared with the control device containing sky‐blue:yellow‐orange emitters (CRI = 60, CIE coordinates of (0.293, 0.395)) due to the good spectral coverage by the deep‐blue emitter.     

Simplified Bilayer White Phosphorescent Organic Light‐Emitting Diodes

We report on highly efficient blue, orange, and white phosphorescent organic light‐emitting diodes consisting only two organic layers. Hole transporting 4, 4,’ 4”‐tris (N‐carbazolyl)triphenylamine (TcTa) and electron transporting 2‐(diphenylphosphoryl) spirofluorene (SPPO1) are used as an emitting host for orange light‐emitting bis(3‐benzothiazol‐2‐yl‐9‐ethyl‐9H‐carbazolato) (acetoacetonate) iridium ((btc)2(acac)Ir) and blue light‐emitting iridium(III)bis(4,6‐difluorophenyl‐pyridinato‐N,C2’) picolinate (FIrpic) dopant, respectively. Combining these two orange and blue light‐emitting layers, we successfully demonstrate highly efficient white PHOLEDs while maintaining Commission internationale de l'éclairage coordinates of (, ). Accordingly, we achieve a maximum external quantum, current, and power efficiencies of 12.9%, 30.3 cd/A, and 30.0 lm/W without out‐coupling enhancement.     

White Organic Light‐Emitting Diodes with Evenly Separated Red,Green, and Blue Colors for Efficiency/Color‐Rendition Trade‐Off Optimization

A novel yellowish‐green triplet emitter, bis(5‐(trifluoromethyl)‐2‐p‐tolylpyridine) (acetylacetonate)iridium(III) (1), was conveniently synthesized and used in the fabrication of both monochromatic and white organic light‐emitting diodes (WOLEDs). At the optimal doping concentration, monochromatic devices based on 1 exhibit a high efficiency of 63 cd A^?1 (16.3% and 36.6 lm W^?1) at a luminance of 100 cd m^?2. By combining 1 with a phosphorescent sky‐blue emitter, bis(3,5‐difluoro‐2‐(2‐pyridyl)phenyl)‐(2‐carboxypyridyl)iridium(III) (FIrPic), and a red emitter, bis(2‐benzo[b]thiophen‐2‐yl‐pyridine)(acetylacetonate)iridium(III) (Ir(btp)₂(acac)), the resulting electrophosphorescent WOLEDs show three evenly separated main peaks and give a high efficiency of 34.2 cd A^?1 (13.2% and 18.5 lm W^?1) at a luminance of 100 cd m^?2. When 1 is mixed with a deep‐blue fluorescent emitter, 4,4′‐bis(9‐ethyl‐3‐carbazovinylene)‐1,1′‐biphenyl (BCzVBi), and Ir(btp)₂(acac), the resulting hybrid WOLEDs demonstrate a high color‐rendering index of 91.2 and CIE coordinates of (0.32, 0.34). The efficient and highly color‐pure WOLEDs based on 1 with evenly separated red, green, blue peaks and a high color‐rendering index outperform those of the state‐of‐the‐art emitter, fac‐tris(2‐phenylpyridine)iridium(III) (Ir(ppy)₃), and are ideal candidates for display and lighting applications.     

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.     

Synthesis,characterization of the phenylquinoline-based on iridium(III) complexes for solution processable phosphorescent organic light-emitting diodes

A new series of highly efficient Ir(III) complexes, (DPQ)₂Ir(pic-N-O), (F₄PPQ)₂Ir(pic-N-O), (FPQ)₂Ir(pic-N-O), and (CPQ)₂Ir(pic-N-O) were synthesized for phosphorescent organic light-emitting diodes (PhOLEDs), and their photophysical, electrochemical, and electroluminescent (EL) properties were investigated. The Ir(III) complexes, including picolinic acid N-oxide (pic-N-O) ancillary ligand, are comprised with the various main ligands such as 2,4-diphenylquinoline (DPQ), 4-phenyl-2-(2,3,4,5-tetrafluorophenyl)quinoline (F₄PPQ), 2-(9,9-diethyl-9H-fluoren-2-yl)-4-phenylquinoline (FPQ) and 9-ethyl-3-(4-phenylquinolin-2-yl)-9H-carbazole. Remarkably, high performance PhOLEDs using a solution-processable (DPQ)₂Ir(pic-N-O) doped CBP host emission layer were fabricated to give a high luminance efficiency (LE) of 26.9 cd/A, equivalent to an external quantum efficiency (EQE) of 14.2%.The calculated HOMO–LUMO energy gaps for (DPQ)₂Ir(pic-N-O), (F₄PPQ)₂Ir(pic-N-O), (FPQ)₂Ir(pic-N-O) and (CPQ)₂Ir(pic-N-O) were in good agreement with the experimental results.     

High efficiency, solution-processed, red phosphorescent organic light-emitting diodes from a polymer doped with iridium complexes

High efficiency, solution-processed, red emissive phosphorescent organic light-emitting diodes (PhOLEDs) have been developed. The PhOLEDs utilize bis[9-ethyl-3-(4-phenylquinolin-2-yl)-9H-carbazolato-N,C²′]iridium picolinate (Et-Cvz-PhQ)₂Ir(pic) and bis[9-(2-(2-methoxyethoxy)ethyl)-3-(4-phenylquinolin-2-yl)-9H-carbazolato-N,C²′]iridium picolinate (EO-Cvz-PhQ)₂Ir(pic) in a nonconjugated polymer host of PVK that contains the electron transport material of OXD-7 and the hole transport material TPD. The electroluminescence (EL) spectra of the PhOLEDs parallel those of (Et-Cvz-PhQ)₂Ir(pic) and (EO-Cvz-PhQ)₂Ir(pic) with maxima at 608 nm and a CIE (Commission International de l’Eclairage) coordinate of (0.62, 0.38). The red emitting PhOLEDs, based on ITO/PEDOT:PSS/PVK:OXD-7:TPD:Ir complex/cathode configuration, have a maximum external quantum efficiency of 10.6% and a luminance efficiency of 17.5 cd/A. The efficiency is significantly higher than those obtained using common solution-processed red emissive PhOLEDs.     

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.     

Efficient Organic Light‐Emitting Diodes based on Sublimable Charged Iridium Phosphorescent Emitters

The synthesis and characterization of two new phosphorescent cationic iridium(III) cyclometalated diimine complexes with formula [Ir( L )₂(N‐N)]⁺(PF₆^–) ( HL = (9,9‐diethyl‐7‐pyridinylfluoren‐2‐yl)diphenylamine); N‐N = 4,4′‐dimethyl‐2,2′‐bipyridine ( 1 ), 4,7‐dimethyl‐1,10‐phenanthroline ( 2 )) are reported. Both complexes are coordinated by cyclometalated ligands consisting of hole‐transporting diphenylamino (DPA)‐ and fluorene‐based 2‐phenylpyridine moieties. Structural information on these heteroleptic complexes has been obtained by using an X‐ray diffraction study of complex 2 . Complexes 1 and 2 are morphologically and thermally stable ionic solids and are good yellow phosphors at room temperature with relatively short lifetimes in both solution and solid phases. These robust iridium complexes can be thermally vacuum‐sublimed and used as phosphorescent dyes for the fabrication of high‐efficiency organic light‐emitting diodes (OLEDs). These devices doped with 5 wt % 1 can produce efficient electrophosphorescence with a maximum brightness of up to 15 610 cd m^–2 and a peak external quantum efficiency of ca. 7 % photons per electron that corresponds to a luminance efficiency of ca. 20 cd A^–1 and a power efficiency of ca. 19 lm W^–1. These results show that charged iridium(III) materials are useful alternative electrophosphors for use in evaporated devices in order to realize highly efficient doped OLEDs.     

Highly Efficient Red Phosphorescent OLEDs based on Non‐Conjugated Silicon‐Cored Spirobifluorene Derivative Doped with Ir‐Complexes

A novel host material containing silicon‐cored spirobifluorene derivative (SBP‐TS‐PSB), is designed, synthesized, and characterized for red phosphorescent organic light‐emitting diodes (OLEDs). The SBP‐TS‐PSB has excellent thermal and morphological stabilities and exhibits high electroluminescence (EL) efficiency as a host for the red phosphorescent OLEDs. The electrophosphorescence properties of the devices using SBP‐TS‐PSB as the host and red phosphorescent iridium (III) complexes as the emitter are investigated and these devices exhibit higher EL performances compared with the reference devices with 4,4′‐N,N′‐dicarbazole‐biphenyl (CBP) as a host material; for example, a (piq)₂Ir(acac)‐doped SBP‐TS‐PSB device shows maximum external quantum efficiency of η_ext = 14.6%, power efficiency of 10.3 lm W^?1 and Commission International de L'Eclairage color coordinates (0.68, 0.32) at J = 1.5 mA cm^?2, while the device with the CBP host shows maximum η_ext = 12.1%. These high performances can be mainly explained by efficient triplet energy transfer from the host to the guests and improved charge balance attributable to the bipolar characteristics of the spirobifluorene group.     

Highly Efficient Simple‐Structure Blue and All‐Phosphor Warm‐White Phosphorescent Organic Light‐Emitting Diodes Enabled by Wide‐Bandgap Tetraarylsilane‐Based Functional Materials

Two host materials of {4‐[diphenyl(4‐pyridin‐3‐ylphenyl)silyl]phenyl}diphenylamine (p‐PySiTPA) and {4‐[[4‐(diphenylphosphoryl)phenyl](diphenyl)silyl]phenyl}diphenylamine (p‐POSiTPA), and an electron‐transporting material of [(diphenylsilanediyl)bis(4,1‐phenylene)]bis(diphenylphosphine) dioxide (SiDPO) are developed by incorporating appropriate charge transporting units into the tetraarylsilane skeleton. The host materials feature both high triplet energies (ca. 2.93 eV) and ambipolar charge transporting nature; the electron‐transporting material comprising diphenylphosphine oxide units and tetraphenylsilane skeleton exhibits a high triplet energy (3.21 eV) and a deep highest occupied molecular orbital (HOMO) level (‐6.47 eV). Using these tetraarylsilane‐based functional materials results in a high‐efficiency blue phosphorescent device with a three‐organic‐layer structure of 1,1‐bis[4‐[N,N‐di(p‐tolyl)‐amino]phenyl]cyclohexane (TAPC)/p‐POSiTPA: iridium(III) bis(4′,6′‐difluorophenylpyridinato)tetrakis(1‐pyrazolyl)borate (FIr6)/SiDPO that exhibits a forward‐viewing maximum external quantum efficiency (EQE) up to 22.2%. This is the first report of three‐organic‐layer FIr6‐based blue PhOLEDs with the forward‐viewing EQE over 20%, and the device performance is among the highest for FIr6‐based blue PhOLEDs even compared with the four or more than four organic‐layer devices. Furthermore, with the introduction of bis(2‐(9,9‐diethyl‐9H‐fluoren‐2‐yl)‐1‐phenyl‐1H‐benzoimidazol‐N,C³)iridium acetylacetonate [(fbi)₂Ir(acac)] as an orange emitter, an all‐phosphor warm‐white PhOLED achieves a peak power efficiency of 47.2 lm W^?1, which is close to the highest values ever reported for two‐color white PhOLEDs.     

Modification of Exciton Lifetime by the Metal Cathode in Phosphorescent OLEDs,and Implications on Device Efficiency and Efficiency Roll‐off Behavior

Time resolved photoluminescence and electroluminescence measurements are used to study changes in the emission characteristics of materials typically used in phosphorescent organic light emitting devices (PhOLEDs). Studies on archetypical PhOLEDs with phosphorescent material, fac‐tris(2‐phenylpyridine) iridium (Ir(ppy)₃), show that the lifetime of triplet exciton is modified when in close proximity to a metal layer. Interactions with a metal layer ～30–100 nm away, as is typically the case in PhOLEDs, result in an increase in the spontaneous emission decay rate of triplet excitons, and causes the exciton lifetime to become shorter as the distance between the phosphorescent material and the metal becomes smaller. The phenomenon, possibly the result of the confined radiation field by the metal, affects device efficiency and efficiency roll‐off behavior. The results shed the light on phenomena affecting the efficiency behavior of PhOLEDs, and provide new insights for device design that can help enhance efficiency performance.     

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.     

Red Phosphorescent Iridium Complex Containing Carbazole‐Functionalized β‐Diketonate for Highly Efficient Nondoped Organic Light‐Emitting Diodes

A novel red phosphorescent iridium complex containing a carbazole‐functionalized β‐diketonate, Ir(DBQ)₂(CBDK) (bis(dibenzo[f,h]quinoxalinato‐N,C²) iridium (1‐(carbazol‐9‐yl)‐5,5‐dimethylhexane‐2,4‐diketonate)) is designed, synthesized, and characterized. The electrophosphorescence properties of a nondoped device using the title complex as an emitter with a device configuration of indium tin oxide (ITO)/N,N′‐diphenyl‐N,N′‐bis(1‐naphthyl)‐1,1′‐diphenyl‐4,4′‐diamine (NPB; 20 nm)/iridium complex (20 nm)/2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline (BCP; 5 nm)/tris(8‐hydroxyquinoline) (AlQ; 30 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) are examined. The results show that the nondoped device achieves a maximum lumen efficiency as high as 3.49 lm W^–1. To understand this excellent result observed, two reference complexes Ir(DBQ)₂(acac), where acac is the acetyl acetonate anion, and Ir(DBQ)₂(FBDK), [bis(dibenzo[f,h]quinoxalinato‐N,C²) iridium (1‐(9‐methyl‐fluoren‐9‐yl)‐6,6‐dimethylheptane‐3,5‐diketonate)], have also been synthesized, and as emitters they were examined under the same device configuration. The maximum lumen efficiency of the former compound is found to be 0.26 lm W^–1 while that for the latter is 0.37 lm W^–1, suggesting that the excellent performance of Ir(DBQ)₂(CBDK) can be attributed mainly to an improved hole‐transporting property that benefits the exciton transport. In addition, a bulky diketonate group separates the emitter centers from each other, which is also important for organic light‐emitting diodes.