A novel bipolar hosting material, 11-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole (DPDDC), was designed, synthesized, and characterized for green phosphorescent organic light-emitting diodes (PhOLEDs). The DPDDC exhibits excellent hole and electron transport properties, superior thermal stability, a high glass-transition temperature and a small singlet-triplet energy gap for efficient reverse intersystem crossing from triplet to singlet, reducing the triplet density of the host for PhOLEDs. The electrophosphorescence properties of the devices using DPDDC as the host and three green phosphorescent iridium(III) complexes, bis(2-(4-tolyl)pyridinato-N,C2′)iridium(III) acetylacetonate, bis(2-phenylpyridine)iridium(III) acetylacetonate, and bis(4-methyl-2,5-diphenylpyridine)iridium(III) acetylacetonate [(mdppy)2Iracac] as the emitter were investigated. The green PhOLED with 5 wt% (mdppy)2Iracac presents an excellent performance, including a high power efficiency of 92.3 lm W−1, high external quantum efficiency of 23.6%, current efficiency roll-off as low as 5.5% at 5000 cd m−2 and a twentyfold lifetime improvement (time to 90% of the 5000 cd m−2 initial luminance) over the reference electrophosphorescent device.

Two carboline derivatives 5-(3′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-3-yl)-5H-pyrido[3,2-b]indole (CzBPDCb) and 9-(3′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-3-yl)-9H-pyrido[2,3-b]indole (CzBPCb) were employed as both a ligand and a host and co-deposited with CuI to form emissive Cu(I) complexes. Their structure–property relationships were studied to demonstrate the strategy of achieving highly efficient organic light emitting diodes (OLEDs) with thermally activated delayed fluorescence (TADF) characteristics. Co-deposited CzBPDCb:CuI films showed photoluminescence at 520 nm with TADF features of a delayed decay component (τ, 1.05 μs) and a prompt decay component (τ, 5.11 ns). Their OLEDs exhibited a maximum external quantum efficiency (EQE) of 17.5%, a current efficiency (CE) of 53.84 cd A−1, and a power efficiency (PE) of 48.31 lm W−1. The device also showed a low efficiency roll-off of 48.1 cd A−1 at 100 cd m−2, 41.8 cd A−1 at 1000 cd m−2, and 38.5 cd A−1 at 2000 cd m−2, respectively. While as a comparison, the CzBPCb:CuI-based OLED only showed a maximum EQE of 3.24%, a CE of 8.58 cd A−1, and a PE of 6.24 lm W−1. CzBPDCb has a δ-carboline unit with the benefit of the nitrogen atom locating outside of the molecule, which could efficiently coordinate with CuI resulting in higher efficiencies. This promising result gives us a unique strategy to develop a series of TADF emitters by co-depositing a normal organic ligand and a copper source, resulting in cost-effective OLED fabrication.

The effect of different linkages on thermal, electrochemical, photophysical, and optoelectronic properties was discussed in detail to illuminate the structure–property relationship of four δ-carboline derivatives, 5-(3′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-3-yl)-5H-pyrido[3,2-b]indole (CzBPDCb), 3,3′-bis(5H-pyrido[3,2-b]indol-5-yl)-1,1′-biphenyl (BDCbBP), 5-(5-(3-(9H-carbazol-9-yl)phenyl)pyridin-3-yl)-5H-pyrido[3,2-b]indole (Cz35PyDCb), and 5-(6-(3-(9H-carbazol-9-yl)phenyl)pyridin-2-yl)-5H-pyrido[3,2-b]indole (Cz26PyDCb). Their triplet energies (ET = 2.78–2.96 eV) and the lowest unoccupied molecular orbital (LUMO) energy could be manipulated by the linkage and the number of δ-carboline units. These materials exhibited bipolar features with good hole- and electron-transporting properties. These host materials when doped with bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III)) (FIrpic) demonstrated efficient blue phosphorescent organic light emitting diode (OLED) with reduced driving voltage, high efficiency, and negligible roll-off. Among these four materials, Cz35PyDCb performed as a host material for blue OLED with ideal current and power efficiencies (CEmax at 44.7 cd A−1, and PEmax at 40.2 lm W−1), as well as good external quantum efficiency (EQEmax at 22.3%). Most importantly, at the normal 1000 cd m−2 operation brightness, the efficiencies remained at 42.6 cd A−1 for CE (95% of CEmax), 31.7 lm W−1 for PE (79% of PEmax), and 20.1% for EQE (90% of EQEmax), which indicated well-balanced carriers transporting and good confinement of the triplet excitons in the emitting layer, even at high current density. Moreover, introduction of the δ-carboline moiety could improve the thermal and morphological stabilities.

•2,4-substituted-quinazoline dyes were synthesized and characterized.•DFT calculation, electrochemical, and photophysical properties were discussed.•Efficient red PhOLEDs with low turn-on voltage were demonstrated by using them as host materials.•Bipolar or electron-only host materials showed potential utilizations in efficient PhOLEDs.Quinazoline-centered derivatives with benzoimidazole, carbazole, and triphenylene moieties, were synthesized. Their relationships between electrochemical, photophysical, and optoelectronic properties and structure were discussed in detail. Efficient red phosphorescent organic light-emitting diodes with low turn-on voltage were demonstrated by using them as host materials, and achieved maximum external quantum efficiencies, current efficiencies, and power efficiencies of 19.2%, 18.3 cd/A, 21.7 lm/W for 4-[4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl]-2-[3-(tri-phenylen-2-yl)phen-3-yl]quinazoline, of 18.4%, 17.6 cd/A, 19.3 lm/W for 4-(9-phenyl-9H-carbazol-3-yl)-2-[3-(triphenylen-2-yl)phenyl]quinazoline, of 15.6%, 14.4 cd/A, 16.7 lm/W for 2,4-bis[4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl]quinazoline, and of 17.4%, 16.7 cd/A, 15.7 lm/W for 2,4-bis(9-phenyl-9H-carbazol-3-yl)quinazoline, respectively. Moreover, improving the electron-injection/transport abilities of host materials could ideally improve the performance of organic light-emitting diodes under low operation voltage, while enhancement of hole-transporting abilities by using bipolar materials could balance the carriers to maintain high efficiency under high operating voltage. These materials exhibited high glass-transition temperature of 146–154 °C and decomposition temperature of 400–447 °C.Highly thermal stable quinazoline-centered derivatives with tunable electron-only or bipolar nature were employed as host materials to achieve high efficient red phosphorescent organic light-emitting diodes.Figure optionsDownload full-size imageDownload as PowerPoint slide

•Trifluoromethyl-pyrazolo[3, 4-b]quinolines (PQs) were synthesized and characterized.•DFT calculation, electrochemical, and photophysical properties were discussed.•A linear relationship of dipole moments versus Stokes shifts was demonstrated.•These PQs showed potential applications in organic light-emitting diodes (OLED).Organic dyes containing trifluoromethylated-pyrazolo[3, 4-b]quinoline were synthesized by using condensation reaction between substituted pyrazolo-4-formaldehyde and aryl amines in moderate yields. Quantum calculation, electrochemical, photophysical, and electroluminescent properties were discussed in detail. Electron-withdrawing groups at 6-position of dyes stabilized the LUMO energy levels significantly resulting in red-shifted absorption/emission spectra. Addition of electron-donating group destabilized the HOMO strongly and the LUMO energy levels slightly resulting in red-shifted absorption maxima, but in blue-shifted emission maxima. Experimental and quantum calculation results are presented here to explain this unusual phenomenon. A linear relationship of dipole moments versus Stokes shifts is obtained for these fluorophores, where the increase of dipole moment is accompanied by a decrease in Stokes shift. Organic light-emitting diodes based on selected dyes showed emission at 456 nm, maximum external quantum efficiency at 1.43%, and current efficiency at 1.95 cd/A.A linear relationship of dipole moments versus Stokes shifts is obtained for Pyrazolo[3, 4-b]Quinoline derivatives, where the increase of dipole moment is accompanied by a decrease in Stokes shift.

We have investigated the hole-transporting properties of three different Ir complexes doped 4,4′,4″-tri (N-carbazolyl) triphenylamine (TCTA) using a series of hole-only devices. The improvement of hole-transporting ability was depended on the species of Ir complexes and their doping concentrations. We attributed the improved performance to their strong electron-accepting abilities or hole-transfer capabilities. Yellow organic light-emitting diodes (OLEDs) based on bis(2-phenylbenzothiazolato)(acetylacetonate)iridium bt2Ir(acac) were fabricated by utilizing this method with optimized doping concentration. The best electroluminescent (EL) performance of maximum 83.6 lm/W was obtained for the yellowing-emitting OLED by doping of Firpic into TCTA hole transport layer, compared with the cases of doping of Ir(ppy)3 into TCTA and doping of Ir(bpiq)2acac into TCTA. Moreover, the turn-on voltage of device decreased to 2.2 V, which was corresponding to the optical band gap of the emitter.Graphical abstractThe hole-transporting properties of three different Ir complexes doped 4,4′,4″-tri (N-carbazolyl) triphenylamine (TCTA) using a series of hole-only devices have been investigated. The improvement of hole-transporting ability was attributed the improved performance to their strong electron-accepting abilities or hole-transfer capabilities.Highlights► Ir complex-doped hole transport layer can greatly improve the performance of OLEDs. ► The improved hole-transporting ability depended on the doping concentrate and species of Ir complex. ► The strong electron-accepting or hole-transfer capability contributes to the improvement of hole-transporting. ► A maximum 83.6 lm/W has been obtained for the yellow-emitting OLED by doping of Firpic into TCTA.

Applications of platinum complexes as phosphorescent emitters in high efficiency organic light-emitting diodes (OLEDs) were shortly discussed in this paper. Key recent studies on highly efficient blue, green, red and white-phosphorescent OLEDs based on Pt complexes are presented in terms of efficiency and color quality.

A novel bipolar hosting material, 11-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole (DPDDC), was designed, synthesized, and characterized for green phosphorescent organic light-emitting diodes (PhOLEDs). The DPDDC exhibits excellent hole and electron transport properties, superior thermal stability, a high glass-transition temperature and a small singlet-triplet energy gap for efficient reverse intersystem crossing from triplet to singlet, reducing the triplet density of the host for PhOLEDs. The electrophosphorescence properties of the devices using DPDDC as the host and three green phosphorescent iridium(III) complexes, bis(2-(4-tolyl)pyridinato-N,C2′)iridium(III) acetylacetonate, bis(2-phenylpyridine)iridium(III) acetylacetonate, and bis(4-methyl-2,5-diphenylpyridine)iridium(III) acetylacetonate [(mdppy)2Iracac] as the emitter were investigated. The green PhOLED with 5 wt% (mdppy)2Iracac presents an excellent performance, including a high power efficiency of 92.3 lm W−1, high external quantum efficiency of 23.6%, current efficiency roll-off as low as 5.5% at 5000 cd m−2 and a twentyfold lifetime improvement (time to 90% of the 5000 cd m−2 initial luminance) over the reference electrophosphorescent device.

The effect of different linkages on thermal, electrochemical, photophysical, and optoelectronic properties was discussed in detail to illuminate the structure–property relationship of four δ-carboline derivatives, 5-(3′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-3-yl)-5H-pyrido[3,2-b]indole (CzBPDCb), 3,3′-bis(5H-pyrido[3,2-b]indol-5-yl)-1,1′-biphenyl (BDCbBP), 5-(5-(3-(9H-carbazol-9-yl)phenyl)pyridin-3-yl)-5H-pyrido[3,2-b]indole (Cz35PyDCb), and 5-(6-(3-(9H-carbazol-9-yl)phenyl)pyridin-2-yl)-5H-pyrido[3,2-b]indole (Cz26PyDCb). Their triplet energies (ET = 2.78–2.96 eV) and the lowest unoccupied molecular orbital (LUMO) energy could be manipulated by the linkage and the number of δ-carboline units. These materials exhibited bipolar features with good hole- and electron-transporting properties. These host materials when doped with bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III)) (FIrpic) demonstrated efficient blue phosphorescent organic light emitting diode (OLED) with reduced driving voltage, high efficiency, and negligible roll-off. Among these four materials, Cz35PyDCb performed as a host material for blue OLED with ideal current and power efficiencies (CEmax at 44.7 cd A−1, and PEmax at 40.2 lm W−1), as well as good external quantum efficiency (EQEmax at 22.3%). Most importantly, at the normal 1000 cd m−2 operation brightness, the efficiencies remained at 42.6 cd A−1 for CE (95% of CEmax), 31.7 lm W−1 for PE (79% of PEmax), and 20.1% for EQE (90% of EQEmax), which indicated well-balanced carriers transporting and good confinement of the triplet excitons in the emitting layer, even at high current density. Moreover, introduction of the δ-carboline moiety could improve the thermal and morphological stabilities.

Two carboline derivatives 5-(3′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-3-yl)-5H-pyrido[3,2-b]indole (CzBPDCb) and 9-(3′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-3-yl)-9H-pyrido[2,3-b]indole (CzBPCb) were employed as both a ligand and a host and co-deposited with CuI to form emissive Cu(I) complexes. Their structure–property relationships were studied to demonstrate the strategy of achieving highly efficient organic light emitting diodes (OLEDs) with thermally activated delayed fluorescence (TADF) characteristics. Co-deposited CzBPDCb:CuI films showed photoluminescence at 520 nm with TADF features of a delayed decay component (τ, 1.05 μs) and a prompt decay component (τ, 5.11 ns). Their OLEDs exhibited a maximum external quantum efficiency (EQE) of 17.5%, a current efficiency (CE) of 53.84 cd A−1, and a power efficiency (PE) of 48.31 lm W−1. The device also showed a low efficiency roll-off of 48.1 cd A−1 at 100 cd m−2, 41.8 cd A−1 at 1000 cd m−2, and 38.5 cd A−1 at 2000 cd m−2, respectively. While as a comparison, the CzBPCb:CuI-based OLED only showed a maximum EQE of 3.24%, a CE of 8.58 cd A−1, and a PE of 6.24 lm W−1. CzBPDCb has a δ-carboline unit with the benefit of the nitrogen atom locating outside of the molecule, which could efficiently coordinate with CuI resulting in higher efficiencies. This promising result gives us a unique strategy to develop a series of TADF emitters by co-depositing a normal organic ligand and a copper source, resulting in cost-effective OLED fabrication.