All-inorganic and low-cost quantum-dot light-emitting diodes (QLEDs) are always desired considering the easy processing and outstanding physical and chemical stability of inorganic oxides. Herein, efficient all-inorganic QLEDs are demonstrated by using NiO and ZnO as the charge transport layers fabricated via ultrasonic spray processes. Excellent device performance is achieved thanks to the introduction of an Al2O3 interlayer between quantum dots (QDs) and an amorphous NiO layer. Transient photoluminescence and electricity measurements indicate that the Al2O3 layer can suppress the exciton quenching induced by the NiO layer and reduce the electron leakage from QDs to NiO. In consequence, relative to that of a device without an Al2O3 layer, the efficiency of an Al2O3-containing device is enhanced by a factor of 539%, increasing from 3.8 cd/A to 20.5 cd/A, and it exhibits color-saturated green emission (peak at 530 nm) and high luminescence (>20 000 cd/m2). These are the best performances for all-inorganic QLEDs reported to date. Meanwhile, it is demonstrated that ultrasonic spray is a feasible and cost-effective technology to construct efficient all-inorganic QLEDs. We anticipate that these results will spur the progress toward realization of high performance and mass production of all-inorganic QLEDs as a platform for QD-based full-color displays.Keywords: exciton quenching; inorganic metal oxides; insulating layer; passivation; quantum-dot light-emitting diodes (QLEDs);

Construction of donor–acceptor (D–A) molecules with a highly hybridized local and charge-transfer (HLCT) excited state has been shown to be an effective strategy to achieve the maximum electroluminescence (EL) efficiency through the synchronous harvest of high photoluminescence (PL) efficiency and exciton utilization. Herein, two novel D–A-structured bifunctional (emissive and hole-transporting) materials, PPI-2TPA and PPI-2NPA, have been designed and synthesized for application in deep-blue OLEDs. As revealed by theoretical calculations and comprehensive photophysical experiments, both of them exhibit significant HLCT excited-state characteristics and ambipolar properties. Using them as emitting layers (EML) in multilayer non-doped devices presents true deep-blue Commission Internationale de l’Eclairage (CIE) coordinates of ca. (0.15, 0.06), accompanied by record-setting performance with maximum external quantum efficiencies (EQEs) of 7.20% for PPI-2TPA and 6.33% for PPI-2NPA. Remarkably, the simple bilayer devices fabricated using them as non-dopant EML and hole-transporting layers (HTLs) still deliver EQEs as high as 4.69% and 4.10% with little changes in color purity (PPI-2TPA: CIE (0.150, 0.063) and PPI-2NPA: (0.152, 0.063)). To the best of our knowledge, this performance is the highest among the reported non-doped devices in this color gamut, irrespective of whether the two newly formed molecules functioned as EML or EML and HTL simultaneously.

•Two-dimensional-growth high quality TCTA thin film was fabricated by USC.•USC-TCTA film possesses more uniform surface topography.•The optical property, electrical property and formation mechanism of USC-TCTA film are studied.•The green OLEDs based on USC-TCTA film perform excellent performances.Two-dimensional-growth small molecular organic thin film with high quality is fabricated by ultrasonic spray coating technology (USC) from the toluene solution of 4,4′,4″-tris (carbazol-9-yl) triphenylamine (TCTA). In comparison to the vacuum thermal evaporation (VTE) TCTA film, the USC-TCTA film obtained from liquid-phase solution possesses more uniform surface topography. The differences between the USC-TCTA and the VTE-TCTA in optical property, electrical property and formation mechanism are also studied in detail. Besides, to evaluate the hole transport and electron blocking ability of USC-TCTA film, the organic light-emitting devices (OLEDs) employing USC-TCTA film as hole transport layer are fabricated successfully. Additionally, the green OLEDs based on USC-TCTA film perform as well as the ones with VTE-TCTA film in current density, luminance, efficiency and color stability, and show even better tolerance to high bias voltage.Download high-res image (563KB)Download full-size image

•ML- and SL-PHOLEDs with a bipolar host EBBPC are fabricated.•The maximum EQE are 14.9%, 15.6%, 16.0% and 15.4% for R, G, B, and white ML-PHOLEDs.•The maximum EQE are 10.1%, 14.6% and 9.8% for R, G, and B SL-PHOLEDs.•It could be attributed to the balanced bipolar transporting properties and appropriate energy level of EBBPC.Efficient multilayer (ML) and single layer (SL) phosphorescent organic light-emitting devices (PHOLEDs) with a bipolar host 9,9′-[4′-(2-ethyl-1H-benzimidazol-1-yl) [1,1′-biphenyl]-3,5-diyl] bis-H-Carbazole (EBBPC) are fabricated. The photophysical, electrochemical and carrier transporting properties of EBBPC host are investigated. The maximum external quantum efficiency (EQE) can reach 14.9%, 15.6%, 16.0% and 15.4% for red (R), green (G), blue (B), and white ML-PHOLEDs, respectively. And the maximum EQE can also reach 10.1%, 14.6% and 9.8% for R, G, and B SL-PHOLEDs, respectively. Besides, the SL-PHOLEDs show low efficiency roll-off due to the broader exciton formation zone in SL OLEDs. The excellent performance of ML- and SL-PHOLEDs could be attributed to the balanced bipolar transporting properties and appropriate energy level of the host.Download high-res image (306KB)Download full-size image

•Three Ir(III) dyes with different substituents on 1,2-diphenyl-1H-benzoimidazole ligands are designed.•They exhibit bright lights with almost identical emissions in neat films.•The relationship between the chemical structures and EL performances were investigated.•Non-doped device N3 shows high efficiencies of 18.6 cd A−1 and 16.2 lm W−1.•Device N3 simultaneously displays low efficiency roll-off at high luminance.To construct efficient emitters suitable for non-doped devices and deeply understand the relationship between structures and performances, we designed and synthesized two heteroleptic iridium(III) complexes based on 1,2-diphenyl-1H-benzoimidazole (PBI) ligands whose substituents are varied simply from methyl (complex 2) to tert-butyl groups (complex 3). The parent complex 1 with non-substituent on PBI ligand has also been presented for a better comparison. Their photophysical, electrochemical and electroluminescent (EL) performances are investigated systematically. Despite their structural modification, all complexes exhibit almost identical emission and excited-state characters, which are rationalized by the quantum-chemical calculations. However, the obvious differences on device performances are found. Non-doped device employing 3 as emitting layer displays the highest EL performance with maximum current efficiency (ηc, max) of 18.6 cd A−1 and power efficiency (ηp, max) of 16.2 lm W−1 accompanied by low efficiency roll-off values, which is much higher than those of complexes 1 and 2. The obtained results herein suggest that introduction of the simple substituent into PBI ligand is an effective and feasible approach to develop highly efficient non-doped phosphors.To construct efficient emitters suitable for non-doped devices and deeply understand the relationship between structures and performances, three heteroleptic Ir(III) dyes employing modified 1,2-diphenyl-1H-benzoimidazole ligands whose substituents are varied simply from non-substituent (1) to tert-butyl groups (3) are synthesized. Non-doped device using 3 as emitting layer displays the highest EL performance with ηc, max of 18.6 cd A−1 and ηp, max of 16.2 lm W−1 accompanied by low efficiency roll-off values.

•A bipolar host DBTPhPm with small singlet-triplet splitting was synthesized.•The PHOLEDs with DBTPhPm host exhibited high brightness, high efficiency and low efficiency roll-off.•DBTPhPm is an efficient host material for most common phosphorescent dopants.A bipolar host 4, 6-Bis[3-(dibenzothiophen-2-yl)phenyl] pyrimidine (DBTPhPm) with small singlet-triplet splitting has been synthesized and confirmed through a series of photophysical and electrochemical properties. Monochromatic phosphorescent organic light-emitting devices (PHOLEDs) based on different hosts [(4,4′-N,N'-dicarbazole) biphenyl, 2,7-bis (diphenylphosphoryl)-9-[4-(N,Ndiphenylamino) phenyl]-9-phenylfluorene, (3,3'-bicarbazole) phenyl and DBTPhPm] and dopants are fabricated. Compared to other hosts, the DBTPhPm-based PHOLEDs exhibited high brightness, high efficiency and low efficiency roll-off. The maximum power efficiency of the DBTPhPm-based red (R), green (G), blue (B), yellow (Y), and orange (O) PHOLEDs are 12.2, 47.2, 17.6, 42.6 and 15.1 lm/W, respectively. The current efficiency roll-off of the R, G, B, Y, and O PHOLEDs are 29.8%, 8.6%, 18.2%, 5.9%, and 22.4% from the maximum current efficiency to the high brightness of 5000 cd/m2. The detailed working mechanism of the DBTPhPm-based device is discussed.

Five neutral heteroleptic Ir(III) complexes 1–5 using the same cyclometalated ligand and different pyridine-1,2,4-triazolyl derivatives as ancillary ligands with fluorine substituents attached, were rationally designed and prepared. Their photophysical, electrochemical, and thermal properties were studied, and theoretical calculations were performed to understand the emission behaviors as well. Introducing fluorine atoms has little effect on the photophysical and thermal properties, but the performances of the resulting devices can be fine-tuned. Among them, a heavy doping level device employing a phosphor with five fluorine atoms delivers superior device efficiencies with ηc = 32.6 cd A–1 and ηp = 27.6 lm W–1, respectively, which is higher than those of other counterparts. Importantly, such a device exhibits almost negligible roll-off in luminance efficiency. Despite nondoped devices achieving good EL performance, more fluorine atoms lead to a relatively higher efficiency roll-off. The results suggest that rational incorporation of fluorine atoms into the ancillary ligands can significantly improve the performance of devices with features of high efficiency and small roll-off.

•Three Ir(III)-based dyes with N-heterocyclic phenyltriazole ligands are prepared.•They display blue emissions at 460–466 nm in CH2Cl2 at 298 K.•Their photoluminescence and electroluminescence properties are investigated.•They possess increased photoluminescence efficiencies with 14% and 18%.•They exhibit improved electroluminescence efficiencies with 4.1 and 4.6 lm W−1.Novel kinds of blue-emitting heteroleptic iridium(III) complexes (fpdmtz)2Ir(mpypz) (1), (fpmptz)2Ir(mpypz) (2) and (fpmptz)2Ir(pypz) (3) with N-heterocyclic phenyltriazole ligands were designed and synthesized under the guidance of theoretical calculations. Especially, the effect of substituent groups on their emission properties was systematically studied through introducing methyl and propyl moieties into the cyclometalated and ancillary ligands. The experimental results showed that 2 and 3 modified by propyl groups exhibited higher photoluminescence and electroluminescence efficiencies than 1 modified by methyl moiety in its cyclometalated ligands. Such greatly improved emission efficiencies were tentatively attributed to the suppressed intermolecular interactions in solid state caused by the introduction of propyl group, indicating that the photoluminescence and electroluminescence properties of iridium(III) complexes can be manipulated just by modifying simple small molecule groups into the cyclometalated ligands.Novel kinds of blue-emitting iridium(III) complexes adopting N-heterocyclic phenyltriazole ligands were designed and synthesized, which showed improved efficiencies through introducing propyl group into the cyclometalated ligands.

•Three pyridine-azole-based dyes modified by tetraphenylethene unit were prepared.•The dyes show intrinsic aggregated-induced emission features.•They dyes also exhibit the piezochromism with a high contrast ratio.•The emission color between blue and green can be reversibly and quickly switched.•Non-doped electroluminescent devices with ηc of 2.3 cd A−1 and ηp of 2.0 lm W−1 are achieved.In this work, three tetraphenylethene-functionalized pyridine-azole derivatives were successfully synthesized and characterized. Their photophysical properties in both solution and solid-state were investigated systematically. All luminogens are almost non-emissive in solution but highly emissive in the aggregated states, showing aggregated-induced emission. Importantly, their crystalline aggregates exhibit effective piezochromism with high contrast in both emission color and intensity. The emission color between blue and green can be reversibly and quickly switched by a grinding-heating process several times without any deterioration. The experimental data clearly demonstrate that the interconversion between crystalline and amorphous states is response for the present piezochromism. Non-doped electroluminescence devices using the dyes as light-emitting layers were fabricated. The devices display a peak current efficiency of 2.3 cd A−1 and power efficiency of 2.0 lm W−1, respectively. The obtained results will be useful in designing new efficient multifunctional materials and enriching piezochromic luminescent systems as well.Pyridine-azole based dyes functionalized with a tetraphenylethene moiety were synthesized and showed intrinsic AIE features and piezochromism. Efficient non-doped OLEDs based on the dyes were characterized.

•Four Ir(III)-based dyes with 1,2-diphenyl-1H-benzoimidazole ligands were prepared.•They displayed green emissions around 492 nm with high quantum yields in CH2Cl2.•Their electroluminescence properties were successfully investigated.•A non-doped device displayed high efficiencies of 19.8 cd A−1 and 20.4 lm W−1.•A device simultaneously displayed low efficiency roll-off at high luminance.Four novel iridium(III) complexes containing 1,2-diphenyl-1H-benzoimidazole as cyclometalated ligands were successfully synthesized and characterized. The complexes displayed strong emissions around 492 nm with high photoluminescence quantum yields of 70–92% in dichloromethane solution at 298 K. Doped OLEDs based on the complexes were prepared, which showed a peak current efficiency of 34.5 cd A−1, power efficiency of 40.1 lm W−1. Non-doped OLEDs using the complexes as emitters were then fabricated for further investigation of their electroluminescence properties. Encouragingly, one non-doped device possessed outstanding performance with a maximum current efficiency of 19.8 cd A−1 and power efficiency of 20.4 lm W−1 whilst simultaneously displaying low efficiency roll-off at high luminance.Novel kinds of iridium(III) complexes with 1,2-diphenyl-1H-benzoimidazole ligands were prepared to fabricate high-performance non-doped OLEDs, which showed favorable performance with a maximum ηc of 19.8 cd A−1 and ηp of 20.4 lm W−1 accompanied by low efficiency roll-off at high luminance.

•White inverted top-emitting organic light-emitting devices were demonstrated.•Bphen:Ag interlayer was used to enhance the electron injection.•Ag–Ge–Ag capping with light outcoupling layer was used as anode.•The device shows stable spectra with voltage and viewing angle.By adopting an 4,7-diphenyl-1,10-phenanthroline (Bphen):Ag interlayer with the gradient structure between the cathode and electron injection layer, the electron injection of white inverted top-emitting organic light-emitting devices (WITOLED) was enhanced. The structure of Ag–Ge–Ag anode capping with light outcoupling was used to ensure the spectral stability with the angles. The structure of the emission layer was carefully designed to guarantee the spectral stability with the voltages. The highest current efficiency of the WITOLED can reach 21.7 cd/A. The CIE 1931 coordinates varies only 0.022 in X and 0.015 in Y (0o–60o).

•Phosphor nanoparticles doped in TiO2 film improve polymer solar cell performance.•Short-circuit current and FF improved by phosphor nanoparticles dopant, giving 6.83% efficiency.•Phosphor nanoparticles improve light harvesting and charge transport property.Phosphor materials can be applied to enhance the energy conversion efficiency of a solar cell. Such materials convert low-energy transmitted photons to higher-energy photons that can be absorbed by the cell, substantially decreasing the spectral mismatch between the cell and the solar spectrum. In this paper, the efficiency of organic solar cells (OSCs) was improved by incorporating phosphor nanoparticles as dual functionality into TiO2 cathode buffer layer. The dependence of devices performance on doping concentration of NaYF4:Yb3+,Er3+ nanoparticles was investigated. A high power conversion efficiency of 6.83% was achieved, which mainly attributes to the increase of short-circuit density. The absorption spectrum indicates that light-harvesting of doped films is higher than undoped film, which originates from scattering effect and NIR spectrum sensitization of phosphor nanoparticles. The measurement of electron-only devices shows that electron transport property of doped devices was apparently improved. Impedance spectroscopy reveals that the diffusion coefficient and carrier mobility were greatly enhanced. This study demonstrates that phosphor nanoparticles doping is useful for fabricating high performance OSCs.The light-harvesting and electron carrier transport property are greatly improve by introduction of phosphor nanoparticles.

Mechanical resilience is important for future organic light-emitting devices (OLEDs) in lighting and displays. A key constraint is the electrode used in OLEDs. Here we demonstrate ultra-thin silver/germanium/silver (AGA)-based flexible organic light-emitting devices, with comparable efficiency to their ITO-based counterparts. The fabrication process of AGA electrodes is compatible with that of the OLEDs, and they show excellent optical, electrical and mechanical properties. They can be used as transparent cathodes and anodes for top-emitting, bottom-emitting and transparent flexible OLEDs.

Two novel iridium(III) complexes (pbi)2Ir(mtpy) (1) and (pbi)2Ir(pbim) (2) adopting 1,2-diphenyl-1H-benzoimidazole (Hpbi) as cyclometalated ligands were successfully synthesized and characterized. Strong emissions at 501 and 536 nm with high photoluminescence quantum yields of 48% and 91% in CH2Cl2 at 298 K were obtained for 1 and 2, respectively. The quantum chemical calculations and the photophysical properties indicated that the dominant 3MLCT (metal-to-ligand charge-transfer) state mixed with 3LLCT (ligand-to-ligand charge-transfer) and 3LC (ligand-centered 3π–π*) characters contributed to their phosphorescence emissions. Doped organic light-emitting diodes (OLEDs) based on 1 and 2 showed a peak current efficiency of 45.0 cd A−1 and power efficiency of 47.9 lm W−1 accompanied by very low efficiency roll-off values. In their non-doped OLEDs, high efficiencies of 24.4 cd A−1 and 26.3 lm W−1 were achieved as well. These appealing results reveal that complexes 1 and 2 open interesting perspectives for the development of high-performance OLEDs in the future.

•Effect of the greenish-yellow (GY) emission on the CRI of WOLEDs was demonstrated.•The simulation predicted that GY emission is beneficial to improve the CRI of three-component WOLED.•The corresponding hybrid WOLED with GY dopant was carefully designed and fabricated.Effect of the greenish-yellow emission on the color rendering index (CRI) of white organic light-emitting devices (WOLEDs) was demonstrated. The correlated color temperature and CRI of the three-component WOLEDs were numerically modeled. The simulation predicted that the greenish-yellow emission is beneficial to improve the CRI of three-component WOLED. The corresponding hybrid WOLED with greenish-yellow dopant was carefully designed and fabricated. In a tolerable deviation from the Planckian locus, such WOLED shows general CRI values above 85 and a power efficiency of 10.4 lm/W.

•Effective electron injection is achieved when Ag is deposited on Bphen.•Single OLED with Ag cathode shows comparable performance to that of device with Mg:Ag cathode.•Tandem WOLED using non-modified Ag film as cathode and interconnecting layer is demonstrated.•Tandem WOLED exhibits low driving voltage, high power efficiency and low efficiency roll-off.Tandem white organic light-emitting device (WOLED) using non-modified Ag film as cathode and interconnecting layer is demonstrated. Effective electron injection is achieved when Ag is deposited on 4,7-diphenyl-1,10-phenanthroline electron transporting layer without any modified layer. Single OLED with Ag cathode shows comparable performance to that of device with Mg:Ag cathode. Such tandem WOLED exhibits low driving voltage, high power efficiency (15.1 lm/W at 1000 cd/m2) and low efficiency roll-off. The working mechanisms of single and tandem devices were discussed in detail. These results could provide a simple method to fabricate high performance tandem white OLED.Graphical abstract

•Two Ir(III) complexes with carbene ligands are designed and synthesized.•Both complexes exhibit greenish-blue emission with high efficiency of ∼0.5.•The experimental results are rationalized by the theoretical calculations.•Greenish-blue OLEDs show good efficiencies of 11.78 cd A−1 and 11.43 lm W−1.•A white OLED using one of them as dopant is successfully fabricated.Two heteroleptic iridium(III) complexes using carbene as cyclometalated ligands and pyridine-triazole as ancillary ligand, namely (fpmi)2Ir(mtzpy) (1) and (fpmi)2Ir(phtzpy) (2) [fpmi = 1-(4-fluorophenyl)-3-methylimdazolin-2-ylidene-C,C2′, mtzpy = 2-(5-methyl-2H-1,2,4-triazol-3-yl)pyridine, phtzpy = 2-(5-phenyl-2H-1,2,4-triazol-3-yl)pyridine], were synthesized and their structural, photophysical and electrochemical properties investigated systematically. Both complexes exhibit bright greenish-blue phosphorescence (λmax ∼490 nm) with quantum yields of about 0.50. Comprehensive density functional theory (DFT) approach was then performed to gain insights into their photophysical and electrochemical characters. The fabrication of organic light-emitting diodes (OLEDs), employing complexes 1 and 2 as phosphorescent dopants, was successfully achieved. Among them, the device based on 1 exhibited considerable power efficiency (ηp) of 11.43 lm W−1 and current efficiency (ηc) of 11.78 cd A−1. With the merit of intrinsic characteristic of complex 1, a white OLED comprised of 1 and one orange phosphor (pbi)2Ir(biq) achieved a peak ηp of 9.95 lm W−1 and ηc of 10.81 cd A−1, together with Commission Internationale de l'Eclairage (CIE) coordinates of (0.34, 0.40). The results indicate that two iridium(III) complexes reported here are promising phosphorescent dyes for OLEDs.Two iridium(III) complexes containing carbene-based cyclometalated ligands and pyridine-triazole ancillary ligand exhibit intense greenish-blue phosphorescent emission. The greenish-blue and white OLEDs using the titled complexes as the dopant emitters show effective electroluminescence efficiencies.

A novel iridium(III) (pbi)2Ir(biq) with a strong orange emission at 573 nm was synthesized, which showed a high photoluminescence quantum yield (PLQY) of 45% in CH2Cl2 at 298 K. The quantum chemical calculations together with the photophysical properties indicated that the transition incorporation of 3MLCT (metal-to-ligand charge transfer), 3LLCT (ligand-to-ligand charge transfer) mixed with 3LC (ligand-centered 3π–π*) characters contributed to the emission of (pbi)2Ir(biq). Cyclic voltammetry (CV), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were then performed to investigate its electrochemical and thermal properties for fabrication of electroluminescent devices. Interestingly, complex (pbi)2Ir(biq) exhibited wide-bandwidth in electroluminescence (EL) of its orange electroluminescent devices, which made it effectively available to fabricate high-quality two-element white organic light-emitting diodes (WOLEDs). The resultant WOLED possessed a high current efficiency (ηc) of 22.1 cd A−1 and power efficiency (ηp) of 25.5 lm W−1 with a high color rendering index (CRI) of 80. Especially, the WOLED retained a favorable ηp of 10.9 lm W−1 and ηc of 16 cd A−1 at a high luminance of 1000 cd m−2. Moreover, it was worthwhile to note that this WOLED exhibited a qualified R9 of 13 and Duv of −0.0013, and these values together with the high CRI conformed to the required standard for lamp illumination of the ENERGY STAR.

•Ultra-high CRI WOLED was achieved by using a deep red phosphorescent dye.•Ultra-high general CRI and R9 of 96 and 91 and the average of all 14 CRIs is up to 95 at 5 V.•The luminance and power efficiency of the device can reach 2529 cd/m2 and 7.86 lm/W at 5 V.Ultra-high color rendering index (CRI) white organic light-emitting device (WOLED) was achieved by using a deep red phosphorescent dye Iridium(III)Bis[1-(3,5-dimethylphenyl)-7-methylisoquinoline] (acetylacetone). The WOLED exhibits high luminance of 2529 cd/m2 and power efficiency of 7.86 lm/W at 5 V, and shows ultra-high general CRI and R9 of 96 and 91. The average of all 14 CRIs is up to 95 at 5 V. Besides, the current efficiency roll-off of the WOLED is reduced due to the balance of carrier injection and transportation in emissive layers. The results indicated that the deep red emission is very important for an ultra-high CRI WOLED.

•In a tolerable Duv, a white OLED with high CRI and GAI is developed by using a bipolar homojunction emitting layers (BHELs).•At 4 V, the general CRI, special CRI R9, an average of R9–R12, and GAI are 82, 75, 69 and 95.•The BHELs structure induces a stable color coordinates and reduced efficiency roll-off.High general and special color rendering index (CRI) together with gamut area index (GAI) is demanded for lighting source. In a tolerable Duv, an efficient warm white organic light-emitting device (WOLED) with a high CRI and GAI is developed by using a bipolar homojunction emitting layers, i.e., junction between different emitting layers with the same bipolar host material. At a low drive voltage of 4 V, the WOLED shows a high luminance of 1594 cd/m2 and a power efficiency of 12.95 lm/W, and, it shows a high general CRI of 82, a special CRI R9 of 75, an average of R9 to R12 of 69 and a GAI of 95. Besides, the WOLED shows relative stable color coordinates due to the elimination of the charges accumulation at interface between different emitting layers. And the current efficiency roll-off of the WOLED is also reduced due to the balance of carrier injection and transport in emitting layers.

High-efficiency blue and white organic light-emitting devices (OLEDs) combined fluorescent and phosphorescent blue emitters were reported. The hybrid blue OLED showed better color purity than that of all phosphorescent device without sacrificing efficiency. The maximum power efficiency of the blue device could reach 23.5 lm/W with the CIE coordinates of (0.163, 0.325). High-efficiency white OLED with maximum power efficiency of 50.6 lm/W was obtained by combined such hybrid blue device and ultrathin phosphorescent yellow emitter. At the practical brightness of 1000 cd/m2, the power efficiency of the white device was 28.3 lm/W with a low voltage of 3.37 V and CIE coordinates of (0.40, 0.44). The excitons recombination zone was adjusted by the introduction of the fluorescent blue emitter which resulting a relative high color rendering index and power efficiency of the white device.Graphical abstractHighlights► Blue and white organic light-emitting devices combined fluorescent and phosphorescent blue emitters are fabricated. ► The hybrid blue and white OLED show good color purity and high efficiency. ► The maximum power efficiency of the blue and white device can reach 23.5 and 50.6 lm/W. ► The power efficiency of the white device was 28.3 lm/W @1000 cd/m2 with a low voltage of 3.37 V.

We present a simulation of the optical properties of ITO-free top-emitting white organic light-emitting devices (TEWOLEDs). Metals (Al, Ag, Mo, Cu, Au, Sm) are used as anode or cathode depending on their work functions. Besides, devices with 1D metallic–dielectric photonic crystal anode are also demonstrated. The influence of the capping layer or the metallic–dielectric multilayer structure on the optical properties (such as efficiency, angular emission characteristics, color gamut and color rendering index) of the devices has been investigated. The results indicated that TEWOLEDs can be used as lighting with high color rendering index or as a display with wide color gamut by carefully designing the structure of the devices.Graphical abstractHighlights► Optical properties of ITO-free top-emitting white organic light-emitting devices (TEWOLED) were simulated and optimized. ► Metal–metal microcavity TEWOLED could be used as lighting with high CRI. ► TEWOLED with 1D metallic–dielectric photonic crystal anode could realize a near-to-eye display with a wide color gamut.

A semitransparent white-light organic light-emitting device (SWOLED) with an (Ag/Alq3)2 cathode and (Alq3/Ag)2 anode was fabricated. The light emitted from the emitters subjecting to the same propagation process because of the symmetrical electrode structure. The device showed few differences in luminance, power distribution of electroluminescence (EL) spectra, efficiency and chromaticity coordinates from both sides. The maximum total current efficiency of the SWOLED (8.46 cd/A) is comparable to that of the corresponding bottom-emitting OLED (9.2 cd/A). The SWOLEDs have potential uses as tinted thin-film coatings on architectural surfaces, such as windows and walls. In addition, this kind of electrode can be used in flexible OLEDs.Graphical abstractHighlights► Semitransparent white-light organic light-emitting device with symmetrical electrode structure was fabricated. ► (Ag/Alq3)2 were used as cathode and anode. ► The device showed few differences in performances from both sides. ► The maximum total current efficiency of the device is comparable to that of the corresponding bottom-emitting device.

A top-emitting warm-white organic light emitting diode (TEWOLED) was fabricated with a conductive transparent MAM cathode [MAM = MoO3 (40 nm)/Ag (17 nm)/MoO3 (40 nm)], which had a higher color rendering index (CRI) than that of corresponding ITO-based bottom-emitting organic light-emitting device (OLED). We measured and calculated the optical transmittance of multilayer MAM fabricated on PET substrate by vacuum thermal evaporation. The average transmittance in visible range is above 84%, which is similar to the conventional indium tin oxide (ITO). The weak microcavity effect on the device performance was also studied. MAM multilayer has the potential for using as transparent conductor electrodes for white OLEDs, especially for flexible devices due to its unique optical and electrical properties.Graphical abstractA top-emitting white organic light-emitting device (TEWOLED) was fabricated with a conductive transparent MAM cathode [MAM = MoO3 (40 nm)/Ag (17 nm)/MoO3 (40 nm)]. A warm-white light emission was obtained with a correlated color temperature of Tc=3736K and the TEWOLED had a higher color rendering index (CRI=84) than that of corresponding ITO-based bottom-emitting organic light-emitting device (OLED, CRI=75 at Tc=8224K)Highlights► A conductive transparent MoOx/Ag/MoOx (MAM) cathode was proposed. ► The top-emitting white OLED with MAM cathode was fabricated. ► Weak microcavity effect on the performance of OLED was studied. ► The TEWOLEDpossessed a higher CRI than that of bottom-emitting OLED.

Efficient hybrid white organic light-emitting devices (WOLEDs) were developed using an ambipolar blue fluorescent emitter 2-diphenylamino-7-(2,2′′-diphenylvinyl)-9,9′-spirobifluorene (DPV) which has a relatively electric-field independent hole and electron mobilities. The effects of the triplet energies and charge transporting properties of the blue materials on the performance of the device are discussed. By using such a blue emitter in the device, a broader charge recombination zone is formed, and the energy loss is reduced. WOLEDs with a maximum current efficiency of 25.1 cd/A which shift to 19.5 cd/A at 10000 cd/m2 have been achieved. The power efficiency and the Commission Internationale de l'Eclairage coordinates of the device are 14.1 lm/W and (0.41, 0.41) at 1000 cd/m2. By attaching a microlens array (MLA) on the backside of the substrate, the outcoupling of electroluminescence in the forward direction is enhanced, resulting in elevated power efficiency 18.6 lm/W at 1000 cd/m2.

Microcavity top-emitting white organic light-emitting devices (TEWOLEDs) with M/(Alq3/Ag)2 anode (M = Ag, Au or Al, Alq3 is tris (8-hydroxyquinoline) aluminum) were investigated. All devices show white emission and the optical properties of the anode play an important effect on the efficiency and contrast of the devices. The maximum efficiencies of the top-emitting devices are comparable to the corresponding bottom one and the contrasts of top-emitting devices are much higher than that of the bottom one. The results indicated that such TEWOLEDs are compatible with complementary metal oxide semiconductor or organic thin film transistor techniques which using Au or Al as electrode.

Nondoped blue organic light-emitting devices with a structure of ITO/hole transporting layer (HTL)/2-diphenylamino-7-(2,2′′-diphenylvinyl)-9,9′- spirobifluorene/electronic transporting layer (ETL)/LiF/Al are fabricated. The performances of the devices are dependent on the charge mobility, charge injection, and energy level characteristics of HTL and ETL. The device with 4,4′,4′′-tris(3-methylphenylphenylamino)triphenylamine and 4,7-diphenyl-1,10- phenanthroline as HTL and ETL shows high efficiency and brightness at low voltage (5.8 cd/A and 1000 cd/m2 at 4.6 V). Furthermore, the device shows a slight efficiency roll-off of 16% from the brightness at maximum current efficiency to 10 000 cd/m2. We attributed this to the formation of a broader carrier recombination zone and relative charge-balancing in the device.

High efficiency electrophosphorescent red organic light-emitting devices with double-emission layers (DELs) have been fabricated with a changing of the hole blocking layer. Bis(1-(phenyl)isoquinoline) iridium (III) acetylanetonate [Ir(piq)2(acac)] as the red emitting dopant is doped into both 4,4′-bis(N-carbazolyl)biphenyl (CBP) host and hole blocking layers. The DELs devices show significantly improved efficiency compared to the conventional devices with a single emitting layer. The maximum power efficiency of 4.05 lm/W at 5 V, which is about 25% greater than that of the device with a single emitting layer, is obtained in a device with 2,2′,2″-(1,3,5-phenylene) tris(1-phenyl-1H-benzimidazole) (TPBI) as hole blocking layer. We attribute it to the excitons-collected effect of the doped TPBI layer and the excellent electron transport performance of TPBI.

Mechanical resilience is important for future organic light-emitting devices (OLEDs) in lighting and displays. A key constraint is the electrode used in OLEDs. Here we demonstrate ultra-thin silver/germanium/silver (AGA)-based flexible organic light-emitting devices, with comparable efficiency to their ITO-based counterparts. The fabrication process of AGA electrodes is compatible with that of the OLEDs, and they show excellent optical, electrical and mechanical properties. They can be used as transparent cathodes and anodes for top-emitting, bottom-emitting and transparent flexible OLEDs.

Two novel iridium(III) complexes (pbi)2Ir(mtpy) (1) and (pbi)2Ir(pbim) (2) adopting 1,2-diphenyl-1H-benzoimidazole (Hpbi) as cyclometalated ligands were successfully synthesized and characterized. Strong emissions at 501 and 536 nm with high photoluminescence quantum yields of 48% and 91% in CH2Cl2 at 298 K were obtained for 1 and 2, respectively. The quantum chemical calculations and the photophysical properties indicated that the dominant 3MLCT (metal-to-ligand charge-transfer) state mixed with 3LLCT (ligand-to-ligand charge-transfer) and 3LC (ligand-centered 3π–π*) characters contributed to their phosphorescence emissions. Doped organic light-emitting diodes (OLEDs) based on 1 and 2 showed a peak current efficiency of 45.0 cd A−1 and power efficiency of 47.9 lm W−1 accompanied by very low efficiency roll-off values. In their non-doped OLEDs, high efficiencies of 24.4 cd A−1 and 26.3 lm W−1 were achieved as well. These appealing results reveal that complexes 1 and 2 open interesting perspectives for the development of high-performance OLEDs in the future.

Construction of donor–acceptor (D–A) molecules with a highly hybridized local and charge-transfer (HLCT) excited state has been shown to be an effective strategy to achieve the maximum electroluminescence (EL) efficiency through the synchronous harvest of high photoluminescence (PL) efficiency and exciton utilization. Herein, two novel D–A-structured bifunctional (emissive and hole-transporting) materials, PPI-2TPA and PPI-2NPA, have been designed and synthesized for application in deep-blue OLEDs. As revealed by theoretical calculations and comprehensive photophysical experiments, both of them exhibit significant HLCT excited-state characteristics and ambipolar properties. Using them as emitting layers (EML) in multilayer non-doped devices presents true deep-blue Commission Internationale de l’Eclairage (CIE) coordinates of ca. (0.15, 0.06), accompanied by record-setting performance with maximum external quantum efficiencies (EQEs) of 7.20% for PPI-2TPA and 6.33% for PPI-2NPA. Remarkably, the simple bilayer devices fabricated using them as non-dopant EML and hole-transporting layers (HTLs) still deliver EQEs as high as 4.69% and 4.10% with little changes in color purity (PPI-2TPA: CIE (0.150, 0.063) and PPI-2NPA: (0.152, 0.063)). To the best of our knowledge, this performance is the highest among the reported non-doped devices in this color gamut, irrespective of whether the two newly formed molecules functioned as EML or EML and HTL simultaneously.

A novel iridium(III) (pbi)2Ir(biq) with a strong orange emission at 573 nm was synthesized, which showed a high photoluminescence quantum yield (PLQY) of 45% in CH2Cl2 at 298 K. The quantum chemical calculations together with the photophysical properties indicated that the transition incorporation of 3MLCT (metal-to-ligand charge transfer), 3LLCT (ligand-to-ligand charge transfer) mixed with 3LC (ligand-centered 3π–π*) characters contributed to the emission of (pbi)2Ir(biq). Cyclic voltammetry (CV), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were then performed to investigate its electrochemical and thermal properties for fabrication of electroluminescent devices. Interestingly, complex (pbi)2Ir(biq) exhibited wide-bandwidth in electroluminescence (EL) of its orange electroluminescent devices, which made it effectively available to fabricate high-quality two-element white organic light-emitting diodes (WOLEDs). The resultant WOLED possessed a high current efficiency (ηc) of 22.1 cd A−1 and power efficiency (ηp) of 25.5 lm W−1 with a high color rendering index (CRI) of 80. Especially, the WOLED retained a favorable ηp of 10.9 lm W−1 and ηc of 16 cd A−1 at a high luminance of 1000 cd m−2. Moreover, it was worthwhile to note that this WOLED exhibited a qualified R9 of 13 and Duv of −0.0013, and these values together with the high CRI conformed to the required standard for lamp illumination of the ENERGY STAR.