Bright and Efficient,Non‐Doped,Phosphorescent Organic Red‐Light‐Emitting Diodes

Ir^(III) metal complexes with formula [(nazo)₂Ir(Fppz)] ( 1 ), [(nazo)₂Ir(Bppz)] ( 2 ), and [(nazo)₂Ir(Fptz)] ( 3 ) [(nazo)H = 4‐phenyl quinazoline, (Fppz)H = 3‐trifluoromethyl‐5‐(2‐pyridyl) pyrazole, (Bppz)H = 3‐t‐butyl‐5‐(2‐pyridyl) pyrazole, and (Fptz)H = 3‐trifluoromethyl‐5‐(2‐pyridyl) triazole] were synthesized, among which the exact configuration of 1 was confirmed using single‐crystal X‐ray diffraction analysis. These complexes exhibited bright red phosphorescence with relatively short lifetimes of 0.4–1.05 μs in both solution and the solid‐state at room temperature. Non‐doped organic light‐emitting diodes (OLEDs) were fabricated using complexes 1 and 2 in the absence of a host matrix. Saturated red electroluminescence was observed at λ_max = 626 nm (host‐emitter complex 1 ) and 652 nm (host‐emitter complex 2 ), which corresponds to coordinates (0.66,0.34) and (0.69,0.31), respectively, on the 1931 Commission Internationale de l'Eclairage (CIE) chromaticity diagram. The non‐doped devices employing complex 1 showed electroluminance as high as 5780 cd m^–2, an external quantum efficiency of 5.5 % at 8 V, and a current density of 20 mA cm^–2. The short phosphorescence lifetime of 1 in the solid state, coupled with its modest π–π stacking interactions, appear to be the determining factors for its unusual success as a non‐doped host‐emitter.     

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.     

Enhancement of External Quantum Efficiency of Red Phosphorescent Organic Light‐Emitting Devices with Facially Encumbered and Bulky PtII Porphyrin Complexes

An enhancement in the external quantum efficiency (QE) of red phosphorescent organic light‐emitting devices (OLEDs) by using facially encumbered and bulky meso‐aryl substituted Pt^II porphyrin complexes is demonstrated. The maximum external QEs of phosphorescent OLEDs doped with the facially non‐encumbered Pt^II porphyrin complex 1 [5,15‐bis[4‐(4,4‐dimethyl‐2,6‐dioxacyclohexyl)phenyl]‐2,8,12,18‐tetrahexyl‐3,7,13,17‐tetramethylporphyrin platinum(II )], the facially encumbered Pt^II porphyrin complex 2 [5,15‐bis(2,6‐dimethoxyphenyl)‐2,8,12,18‐tetrahexyl‐3,7,13,17‐tetramethylporphyrinato platinum(II )], the Pt^II porphyrin complex 3 that bears bulkier 3,5‐di‐tert‐butylphenyl substituents [5,15‐bis(3,5‐di‐t‐butylphenyl)‐2,8,12,18‐tetrahexyl‐3,7,13,17‐tetramethylporphyrin platinum(II )], and the “doubly‐decamethylene‐strapped” Pt^II porphyrin complex 4 were 1, 4.2, 7.3, and 8.2 %, respectively. The trend of increasing QE values in the order of 1 < 2 < 3 < 4 may be related to facial encumbrance and steric bulkiness of meso‐aryl substituted Pt^II porphyrin complexes. Especially, in the case of the Pt^II porphyrin 4 , it is considered that the “double straps” play an important role in restricting rotational freedom of the meso‐aryl substituents. The triplet excited‐state lifetimes for Pt^II porphyrins 1 – 4 in OLEDs at an injection current density of 0.55 mA cm^–2 were 80, 103, 140, and 152 μs, respectively. We believe that the trend of increasing triplet lifetime in going from 1 to 4 is correlated with suppressing non‐radiative decay.     

Novel Heteroleptic CuI Complexes with Tunable Emission Color for Efficient Phosphorescent Light‐Emitting Diodes

A series of orange‐red to red phosphorescent heteroleptic Cu^I complexes (the first ligand: 2,2′‐biquinoline (bq), 4,4′‐diphenyl‐2,2′‐biquinoline (dpbq) or 3,3′‐methylen‐4,4′‐diphenyl‐2,2′‐biquinoline (mdpbq); the second ligand: triphenylphosphine or bis[2‐(diphenylphosphino)phenyl]ether (DPEphos)) have been synthesized and fully characterized. With highly rigid bulky biquinoline‐type ligands, complexes [Cu(mdpbq)(PPh₃)₂](BF₄) and [Cu(mdpbq)(DPEphos)](BF₄) emit efficiently in 20 wt % PMMA films with photoluminescence quantum yield of 0.56 and 0.43 and emission maximum of 606 nm and 617 nm, respectively. By doping these complexes in poly(vinyl carbazole) (PVK) or N‐(4‐(carbazol‐9‐yl)phenyl)‐3,6‐bis(carbazol‐9‐yl) carbazole (TCCz), phosphorescent organic light‐emitting diodes (OLEDs) were fabricated with various device structures. The complex [Cu(mdpbq)(DPEphos)](BF₄) exhibits the best device performance. With the device structure of ITO/PEDOT/TCCz:[Cu(mdpbq)(DPEphos)](BF₄) (15 wt %)/TPBI/LiF/Al (III), a current efficiency up to 6.4 cd A^–1 with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.61, 0.39) has been realized. To our best knowledge, this is the first report of efficient mononuclear Cu^I complexes with red emission.     

A Novel Fluorene‐Triphenylamine Hybrid That is a Highly Efficient Host Material for Blue‐, Green‐, and Red‐Light‐Emitting Electrophosphorescent Devices

TFTPA (tris[4‐(9‐phenylfluoren‐9‐yl)phenyl]amine), a novel host material that contains a triphenylamine core and three 9‐phenyl‐9‐fluorenyl peripheries, was effectively synthesized through a Friedel‐Crafts‐type substitution reaction. Owing to the presence of its sterically bulky 9‐phenyl‐9‐fluorenyl groups, TFTPA exhibits a high glass transition temperature (186 °C) and is morphologically and electrochemically stable. In addition, as demonstrated from atomic force microscopy measurements, the aggregation of the triplet iridium dopant is significantly diminished in the TFTPA host, resulting in a highly efficient full‐color phosphorescence. The performance of TFTPA ‐based devices is far superior to those of the corresponding mCP‐ or CBP‐based devices, particularly in blue‐ and red‐emitting electrophosphorescent device systems. The efficiency of the FIrpic‐based blue‐emitting device reached 12 % (26 cd A^–1) and 18 lm W^–1 at a practical brightness of 100 cd m^–2; the Ir(piq)₂acac‐based red‐emitting device exhibited an extremely low turn‐on voltage (2.6 V) and a threefold enhancement in device efficiency (9.0 lm W^–1) relative to those of reference devices based on the CBP host material.     

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.     

A Light‐Blue Phosphorescent Dendrimer for Efficient Solution‐Processed Light‐Emitting Diodes

We describe the preparation of a dendrimer that is solution‐processible and contains 2‐ethylhexyloxy surface groups, biphenyl‐based dendrons, and a fac‐tris[2‐(2,4‐difluorophenyl)pyridyl]iridium(III ) core. The homoleptic complex is highly luminescent and the color of emission is similar to the heteroleptic iridium(III ) complex, bis[2‐(2,4‐difluorophenyl)pyridyl]picolinate iridium(III ) (FIrpic). To avoid the change in emission color that would arise from attaching a conjugated dendron to the ligand, the conjugation between the dendron and the ligand is decoupled by separating them with an ethane linkage. Bilayer devices containing a light‐emitting layer comprised of a 30 wt.‐% blend of the dendrimer in 1,3‐bis(N‐carbazolyl)benzene (mCP) and a 1,3,5‐tris(2‐N‐phenylbenzimidazolyl)benzene electron‐transport layer have external quantum and power efficiencies, respectively, of 10.4 % and 11 lm W^–1 at 100 cd m^–2 and 6.4 V. These efficiencies are higher than those reported for more complex device structures prepared via evaporation that contain FIrpic blended with mCP as the emitting layer, showing the advantage of using a dendritic structure to control processing and intermolecular interactions. The external quantum efficiency of 10.4 % corresponds to the maximum achievable efficiency based on the photoluminescence quantum yield of the emissive film and the standard out‐coupling of light from the device.     

N∧C∧N‐Coordinated Platinum(II) Complexes as Phosphorescent Emitters in High‐Performance Organic Light‐Emitting Devices

A series of terdentate cyclometallated Pt^II complexes with remarkable luminescence properties are used as new phosphorescence‐emitting dopants in a blended host matrix as the emitting layer, resulting in very high electroluminescence efficiencies. Because of the high phosphorescence quantum yields of these Pt complexes and the efficient energy transfer from both singlet and triplet excited states of the host to the emitting guest, external electroluminescence quantum efficiencies as high as 4–16 % photons per carrier and luminous efficiencies of 15–40 cd A^–1 are achieved. Moreover, these high efficiency values were maintained over a four‐decade current intensity span with no significant roll‐off. Tuning of the electroluminescence spectra from the yellow to the green‐bluish region of the chromaticity diagram is obtained simply by changing the substituents at the central 5‐position of the cyclometallating ligand.     

Platinum Binuclear Complexes as Phosphorescent Dopants for Monochromatic and White Organic Light‐Emitting Diodes

Efficient blue‐, green‐, and red‐light‐emitting organic diodes are fabricated using binuclear platinum complexes as phosphorescent dopants. The series of complexes used here have pyrazolate bridging ligands and the general formula C^∧NPt(μ‐pz)₂PtC^∧N (where C^∧N = 2‐(4′,6′‐difluorophenyl)pyridinato‐N,C^2′, pz = pyrazole ( 1 ), 3‐methyl‐5‐tert‐butylpyrazole ( 2 ), and 3,5‐bis(tert‐butyl)pyrazole ( 3 )). The Pt–Pt distance in the complexes, which decreases in the order 1 > 2 > 3 , solely determines the electroluminescence color of the organic light‐emitting diodes (OLEDs). Blue OLEDs fabricated using 8 % 1 doped into a 3,5‐bis(N‐carbazolyl)benzene (mCP) host have a quantum efficiency of 4.3 % at 120 Cd m^–2, a brightness of 3900 Cd m^–2 at 12 V, and Commission Internationale de L'Eclairage (CIE) coordinates of (0.11, 0.24). Green and red OLEDs fabricated with 2 and 3 , respectively, also give high quantum efficiencies (～ 6.7 %), with CIE coordinates of (0.31, 0.63) and (0.59, 0.46), respectively. The current‐density–voltage characteristics of devices made using dopants 2 and 3 indicate that hole trapping is enhanced by short Pt–Pt distances (< 3.1 Å). Blue electrophosphorescence is achieved by taking advantage of the binuclear molecular geometry in order to suppress dopant intermolecular interactions. No evidence of low‐energy emission from aggregate states is observed in OLEDs made with 50 % 1 doped into mCP. OLEDs made using 100 % 1 as an emissive layer display red luminescence, which is believed to originate from distorted complexes with compressed Pt–Pt separations located in defect sites within the neat film. White OLEDs are fabricated using 1 and 3 in three different device architectures, either with one or two dopants in dual emissive layers or both dopants in a single emissive layer. All the white OLEDs have high quantum efficiency (～ 5 %) and brightness (～ 600 Cd m^–2 at 10 V).     

Highly Branched Phosphorescent Dendrimers for Efficient Solution‐Processed Organic Light‐Emitting Diodes

Intermolecular interactions play a crucial role in the performance of organic light‐emitting diodes (OLEDs). Here we report the photophysical and electroluminescence properties of a fac‐tris(2‐phenylpyridyl)iridium(III ) cored dendrimer in which highly branched biphenyl dendrons are used to control the intermolecular interactions. The presence of fluorene surface groups improves the solubility and enhances the efficiency of photoluminescence, especially in the solid state. The emission peak of the dendrimer is around 530 nm with a PL quantum yield of 76 % in solution and 25 % in a film. The photophysical properties of this dendrimer are compared with a similar dendrimer with the same structure but without the fluorene surface groups. Dendrimer LEDs (DLEDs) are prepared using each dendrimer as a phosphorescent emitter blended in a 4,4′‐bis(N‐carbazolyl)biphenyl host. Device performance is improved significantly by the incorporation of an electron‐transporting layer of 1,3,5‐tris(2‐N‐phenylbenzimidazolyl)benzene. A peak external quantum efficiency of 10 % (38 Cd A^–1) for the dendrimer without surface groups and 13 % (49.8 Cd A^–1) for the dendrimer with fluorene surface groups is achieved in the bilayer LEDs.     

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.     

Orange and Red Organic Light‐Emitting Devices Employing Neutral Ru(II) Emitters: Rational Design and Prospects for Color Tuning

A new series of charge‐neutral Ru(II ) pyridyl and isoquinoline pyrazolate complexes, [Ru(bppz)₂(PPh₂Me)₂] (bbpz: 3‐tert‐butyl‐5‐pyridyl pyrazolate) ( 1 ), [Ru(fppz)₂(PPh₂Me)₂] (fppz: 3‐trifluoromethyl‐5‐pyridyl pyrazolate) ( 2 ), [Ru(ibpz)₂(PPhMe₂)₂] (ibpz: 3‐tert‐butyl‐5‐(1‐isoquinolyl) pyrazolate) ( 3 ), [Ru(ibpz)₂(PPh₂Me)₂] ( 4 ), [Ru(ifpz)₂(PPh₂Me)₂] (ifpz: 3‐trifluoromethyl‐5‐(1‐isoquinolyl) pyrazolate) ( 5 ), [Ru(ibpz)₂(dpp?)] (dpp? represents cis‐1,2‐bis‐(diphenylphosphino)ethene) ( 6 ), and [Ru(ifpz)₂(dpp?)] ( 7 ), have been synthesized, and their structural, electrochemical, and photophysical properties have been characterized. A comprehensive time‐dependant density functional theory (TDDFT) approach has been used to assign the observed electronic transitions to specific frontier orbital configurations. A multilayer organic light‐emitting device (OLED) using 24 wt % of 5 as the dopant emitter in a 4,4′‐N,N′‐dicarbazolyl‐1,1′‐biphenyl (CBP) host with 4,4′‐bis[N‐(1‐naphthyl)‐N‐phenylamino]biphenyl (NPB) as the hole‐transport layer exhibits saturated red emission with an external quantum efficiency (EQE) of 5.10 %, luminous efficiency of 5.74 cd A^–1, and power efficiency of 2.62 lm W^–1. The incorporation of a thin layer of poly(styrene sulfonate)‐doped poly(3,4‐ethylenedioxythiophene) (PEDOT) between indium tin oxide (ITO) and NPB gave anoptimized device with an EQE of 7.03 %, luminous efficiency of 8.02 cd A^–1, and power efficiency of 2.74 lm W^–1 at 20 mA cm^–2. These values represent a breakthrough in the field of OLEDs using less expensive Ru(II ) metal complexes. The nonionic nature of the complexes as well as their high emission quantum efficiencies and short radiative lifetimes are believed to be the key factors enabling this unprecedented achievement. The prospects for color tuning based on Ru(II ) complexes are also discussed in light of some theoretical calculations.     

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.     

Color Tuning in Benzo[1,2,5]thiadiazole‐Based Small Molecules by Amino Conjugation/Deconjugation: Bright Red‐Light‐Emitting Diodes

Bipolar compounds (referred to in general as btza ) containing a benzo[1,2,5]thiadiazole core and peripheral diarylamines and/or 4‐tert‐butylphenyl moieties have been synthesized via palladium‐catalyzed cross‐coupling reactions of 4,7‐dibromobenzo[1,2,5]thiadiazole with appropriate stannyl compounds. These compounds are fluorescent and the emission color ranges from green to red. The fluorescence of the compounds originates from a charge‐transfer process and exhibits solvatochromism. These red‐light‐emitting materials are amorphous and devices of different configurations were fabricated: I) ITO/ btza /TPBI/Mg:Ag; II) ITO/ btza /Alq₃/Mg:Ag; III) ITO/ btza /Mg:Ag (where ITO = indium tin oxide, TPBI = 1,3,5‐tris(N‐phenylbezimidazol‐2‐yl)benzene, and Alq₃ = tris(8‐hydroxyquinoline)aluminum). The performance of some of the red‐light‐emitting devices appears to be very promising.     

High‐Efficiency and Color Stable Blue‐Light‐Emitting Polymers and Devices

Novel fluorene‐based blue‐light‐emitting copolymers with an ultraviolet‐blue‐light (UV‐blue‐light) emitting host and a blue‐light emitting component, 4‐N,N‐diphenylaminostilbene (DPS) have been designed and synthesized by using the palladium‐ catalyzed Suzuki coupling reaction. It was found that both copolymers poly [2,7‐(9,9‐dioctylfluorene)‐alt‐1,3‐(5‐carbazolphenylene)] (PFCz) DPS1 and PFCz‐DPS1‐OXD show pure blue‐light emission even with only 1 % DPS units because of the efficient energy transfer from the UV‐blue‐light emitting PFCz segments to the blue‐light‐emitting DPS units. Moreover, because of the efficient energy transfer/charge trapping in these copolymers, PFCz‐DPS1 and PFCz‐DPS1‐OXD show excellent device performance with a very stable pure blue‐light emission. By using a neutral surfactant poly[9,9‐bis(6'‐(diethanolamino)hexyl)‐fluorene] (PFN‐OH) as the electron injection layer, the device based on PFCz‐DPS1‐OXD5 with the configuration of ITO/PEDOT:PSS/PVK/polymer/PFN‐OH/Al showed a maximum quantum efficiency of 2.83 % and a maximum luminous efficiency of 2.50 cd A^–1. Its CIE 1931 chromaticity coordinates of (0.156, 0.080) match very well with the NTSC standard blue pixel coordinates of (0.14, 0.08). These results indicate that this kind of dopant/host copolymer could be a promising candidate for blue‐light‐emitting polymers with high efficiency, good color purity, and excellent color stability.     

Blue Single‐Layer Organic Light‐Emitting Diodes Using Fluorescent Materials: A Molecular Design View Point

Since the beginning of organic light‐emitting diodes (OLEDs), blue emission has attracted the most attention and many research groups worldwide have worked on the design of materials for stable and highly efficient blue OLEDs. However, almost all the high‐efficiency blue OLEDs using fluorescent materials are multilayer devices, which are constituted of a stack of organic layers to improve the injection, transport, and recombination of charges within the emissive layer. Although the technology has been mastered, it suffers from real complexity and high cost and is time‐consuming. Simplifying the multilayer structure with a single‐layer one, the simplest devices made only of electrodes and the emissive layer have appeared as an appealing strategy for this technology. However, removing the functional organic layers of an OLED stack leads to a dramatic decrease of the performance and achieving high‐efficiency blue single‐layer OLEDs requires intense research especially in terms of materials design. Herein, an exhaustive review of blue emitting fluorophores that have been incorporated in single‐layer OLEDs is reported, and the links between their electronic properties and the device performance are discussed. Thus, a structure/properties/device performance relationship map is drawn, which is of interest for the future design of organic materials.     

Organic Light‐Emitting Transistors Containing a Laterally Arranged Heterojunction

An approach to produce organic light‐emitting transistors (OLETs) containing a laterally arranged heterojunction structure, which minimizes exciton quenching at the metal electrodes, is described. This device configuration provides an organic light‐emitting diode (OLED) structure where the anode (source) electrode, hole‐transport material (field‐effect material), light‐emitting material, and cathode (drain) electrode are laterally arranged, thus offering a chance to control the electroluminescent intensity by changing the gate bias. Pentacene and tris(8‐quinolinolato)aluminum (Alq₃) are employed as the field‐effect and light‐emitting materials, respectively. The laterally arranged heterojunction structures are achieved by successively inclined deposition of the field‐effect and light‐emitting materials. After deposition of pentacene, a narrow gap of about 10–20 nm between the drain electrode and pentacene was obtained, thereby creating an opportunity to fabricate a laterally arranged heterojunction. In the OLETs, unsymmetrical source and drain electrodes, that is, Au and LiF/Al ones, are used to ensure efficient injection of holes and electrons. Visible‐light emission from OLETs is observed under ambient atmosphere. This result is ascribed to efficient carrier injection and transport, formation of a heterojunction, as well as good luminescence from the organic emissive layer. The device structure serves as an excellent model system for OLETs and demonstrates a general concept of adjusting the charge‐carrier injection and transport, as well as the electroluminescent properties, by forming laterally arranged heterojunctions.     

Correlation Between Triplet–Triplet Annihilation and Electroluminescence Efficiency in Doped Fluorescent Organic Light‐Emitting Devices

Triplet–triplet annihilation (TTA) is studied in a wide range of fluorescent host:guest emitter systems used in organic light‐emitting devices (OLEDs). Strong TTA is observed in host:guest systems in which the dopant has a limited charge‐trapping capability. On the other hand, systems in which the dopant can efficiently trap charges show insignificant TTA, an effect that is due, in part, to the efficient quenching of triplet excitons by the trapped charges. Fluorescent host:guest systems with the strongest TTA are found to give the highest OLED electroluminescence efficiency, a phenomenon attributed to the role of TTA in converting triplet excitons into additional singlet excitons, thus appreciably contributing to the light output of OLEDs. The results shed light on and give direct evidence for the phenomena behind the recently reported very high efficiencies attainable in fluorescent host:guest OLEDs with quantum efficiencies exceeding the classical 25% theoretical limit.     

Influences of Connecting Unit Architecture on the Performance of Tandem Organic Light‐Emitting Devices

The present work investigates the influence of the n‐type layer in the connecting unit on the performance of tandem organic light‐emitting devices (OLEDs). The n‐type layer is typically an organic electron‐transporting layer doped with reactive metals. By systematically varying the metal dopants and the electron‐transporting hosts, we have identified the important factors affecting the performance of the tandem OLEDs. Contrary to common belief, device characteristics were found to be insensitive to metal work functions, as supported by the ultraviolet photoemission spectroscopy results that the lowest unoccupied molecular orbitals of all metal‐doped n‐type layers studied here have similar energy levels. It suggests that the electron injection barriers from the connecting units are not sensitive to the metal dopant used. On the other hand, it was found that performance of the n‐type layers depends on their electrical conductivities which can be improved by using an electron‐transporting host with higher electron mobility. This effect is further modulated by the optical transparency of constituent organic layers. The efficiency of tandem OLEDs would decrease as the optical transmittance decreases.