Functionalized terfluorene for solution-processed high efficiency blue fluorescence OLED and electrophosphorescent devices

A new multifunctional blue-emitting terfluorene derivative (TFDPA) featured with triphenylamine groups for hole-transportation and long alkyl chains for solution processability on the conjugation inert bridge centers was reported. TFDPA can give homogeneous thin film by solution process and exhibits high hole mobility (μ_h ≈ 10^?3 cm² V^?1 s^?1) and suitable HOMO for hole injection. Particularly, TFDPA performs efficient deep-blue emission with high quantum yield (～100% in solution, 43% in thin film) and suitable triplet energy (E_T = 2.28 eV), making solution-processed OLED devices of using TFDPA as blue emitter and as host for iridium-containing phosphorescent dopants feasible. The solution-processed nondoped blue OLED device gives saturated deep-blue electroluminescence [CIE = (0.17, 0.07)] with EQE of 2.7%. TFDPA-hosted electrophosphorescent devices performed with EQE of 6.5% for yellow [(Bt)₂Ir(acac)], 9.3% of orange [Ir(2–phq)₃], and 6.9% of red [(Mpq)₂Ir(acac)], respectively. In addition, with careful control on the doping concentration of [(Bt)₂Ir(acac)], a solution-processed fluorescence–phosphorescence hybrided two-color-based WOLED with EQE of 3.6% and CIE coordinate of (0.38, 0.33) was successfully achieved.     

Highly efficient two component phosphorescent organic light-emitting diodes based on direct hole injection into dopant and gradient doping

A series of two component phosphorescent organic light-emitting diodes (PHOLEDs) combing the direct hole injection into dopant strategy with a gradient doping profile were demonstrated. The dopant, host, as well as molybdenum oxide (MoO₃)-modified indium tin oxide (ITO) anode were investigated. It is found that the devices ITO/MoO₃ (0 or 1 nm)/fac-tris(2-phenylpyridine)iridium [Ir(ppy)₃]:1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) (30 → 0 wt%, 105 nm)/LiF (1 nm)/Al (100 nm) show maximum external quantum efficiency (EQE) over 20%, which are comparable to multi-layered PHOLEDs. Moreover, the systematic variation of the host from TPBi to 4,7-diphenyl-1,10-phenanthroline (Bphen), dopant from Ir(ppy)₃ to bis(2-phenylpyridine)(acetylacetonate)iridium [Ir(ppy)₂(acac)], and anodes between ITO and ITO/MoO₃ indicates that balancing the charge as well as controlling the charge recombination zone play critical roles in the design of highly efficient two component PHOLEDs.     

Concentration-insensitive and low-driving-voltage OLEDs with high efficiency and little efficiency roll-off using a bipolar phosphorescent emitter

A series of simplified trilayer phosphorescent organic light-emitting diodes (PHOLEDs) with high efficiency and little efficiency roll-off based on a bipolar iridium emitter Iridium(III) bis(2-phenylpyridinato)-N,N′-diisopropyl-diisopropyl-guanidinate (ppy)₂Ir(dipig) has been demonstrated. They are dominated by the efficient direct-exciton-formation mechanism and show gratifying concentration-insensitive and low-driving-voltage features. In particular, very high and stable electroluminescence (EL) efficiencies (maximum power efficiency and external quantum efficiency >98 lm W^?1 and 25% respectively, and external quantum efficiency >20% over a wide luminance range of 1–15,000 cd m^?2) are achieved in the PHOLEDs based on emitting layers (EMLs) consisting of (ppy)₂Ir(dipig) codeposited with common host CBP in an easily controlled doping concentration range (15–30 wt%). The EL performance of the PHOLEDs is comparable to the highest PHOLEDs reported in scientific literature.     

Enhancing the efficiency of simplified red phosphorescent organic light emitting diodes by exciton harvesting

High efficiency red phosphorescent organic light emitting diode (PHOLED) employing co-doped green emitting molecule bis(2-phenylpyridine)(acetylacetonate)iridium(III) [Ir(ppy)₂(acac)] and red emitting molecule bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) [Ir(MDQ)₂(acac)] into 4,4′-bis(carbazol-9-yl)biphenyl (CBP) host in a simplified wide-bandgap platform is demonstrated. The green molecule is shown to function as an exciton harvester that traps carriers to form excitons that are then efficiently transferred to the Ir(MDQ)₂(acac) by triplet-to-triplet Dexter energy transfer, thereby significantly enhancing red emission. In particular, a maximum current efficiency of 37.0 cd/A and external quantum efficiency (EQE) of 24.8% have been achieved without additional out-coupling enhancements. Moreover, a low efficiency roll-off with the EQE remaining as high as 20.8% at a high luminance of 5000 cd/m² is observed.     

Improved hole-transporting properties of Ir complex-doped organic layer for high-efficiency organic light-emitting diodes

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.     

Pyridoindole modified carbazole compounds as high triplet energy host materials of imidazole derived blue triplet emitters for high quantum efficiency

Carbazole compounds modified with a pyridoindole moiety were examined as thermally stable high triplet energy host materials for tris[1-(2,4-diisopropyldibenzo[b,d]furan-3-yl)-2-phenylimidazole] (Ir(dbi)₃) based blue phosphorescent organic light-emitting diodes. A well-known carbazole compound, N,N′-dicarbazolyl-3,5-benzene, was substituted with one or two pyridoindole moieties to develop the thermally stable host materials for Ir(dbi)₃ blue triplet emitters. Remarkably high glass transition temperature of 196 °C and thermal decomposition temperature of 486 °C in addition to high triplet energy of 2.89 eV were achieved by the pyridoindole modification. The pyridoindole modified carbazole compounds also delivered high quantum efficiency of 25.4% in the blue phosphorescent devices by doping Ir(dbi)₃.     

A rational molecular design on choosing suitable spacer for better host materials in highly efficient blue and white phosphorescent organic light-emitting diodes

A series of host materials, 3,3′-linked carbazole-based molecules have been designed with phenyl and biphenyl spacers. Their optical and electrical properties can be fine-tuning by the spacers. Their HOMO energy levels depend on HOMO distributions within the range of −5.64 to −5.96 eV. On the other hand, the three compounds have similar LUMO energy levels and triplet energies. Their thermal, photophysical, electrochemical and carrier mobilities properties were also systematically investigated. The relationship between the molecular structures and optoelectronic properties are discussed. A blue PHOLED device incorporating PBCz achieved a maximum external quantum efficiency, current efficiency, and power efficiency of 19.5%, 45.5 cd/A and 43.8 lm/W, respectively. Moreover a two-color, all-phosphor and single-emitting-layer WOLED hosted by PBCz was also achieved with a maximum external quantum efficiency, current efficiency and power efficiency of 24.6%, 76.3 cd/A and 69.4 lm/W respectively. Furthermore, we also utilized this versatile host for three-component RGB white PHOLEDs and show excellent performance. For example, combination of PBCz with FIrpic, Ir(ppy)₂(acac) and Ir(MDQ)₂(acac) in the active layer, the resulting WOLEDs showed three evenly separated peaks and gave a high efficiency of 49.2 cd/A. The efficient PHOLEDs demonstrated that the versatile host PBCz has great potential for applications in the solid-state lighting.     

An efficient bis(2-phenylquinoline) (acetylacetonate) iridium(III)-based red organic light-emitting diode with alternating guest:host emitting layers

We report a highly efficient electrophosphorescent bis(2-phenylquinoline) (acetylacetonate) iridium(III) [Ir(2-phq)₂(acac)]-based red organic light-emitting diode. The emission layer consists of a periodic thin layer of guest material of Ir(2-phq)₂(acac) separated by host material of 4,4′-Bis(carbazol-9-yl)biphenyl. The guest and host thicknesses were optimized independently to obtain the best performance. The current efficiency reaches to a maximum of 16.2 cd/A then drops to 15 and 11 cd/A at brightness of 10 and 100 cd/m², respectively. By reducing the thickness of the host layer, the power efficiency was further improved. Device with a maximum power efficiency of 8.3 lm/W was obtained. We also found that the concentration quenching in Ir(2-phq)₂(acac) is dominated by molecular aggregation. Excitonic quenching by radiationless Förster process is miniscule.     

Indenocarbazole based bipolar host materials for highly efficient yellow phosphorescent organic light emitting diodes

We report bipolar host materials with robust indenocarbazole and biphenyl moiety as hole-electron-transporting unit for phosphorescent yellow organic light-emitting diodes (OLEDs). New host materials demonstrated an excellent morphological stability with high glass transition temperature of 207 °C. Simultaneously, it also revealed appropriate triplet energy of about 2.6 eV for ideal triplet energy transfer to yellow phosphorescent dopant. A phosphorescent yellow OLED with new host ICBP1 (and ICBP2) and conventional yellow dopant iridium(III)bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C2′)acetylacetonate (Ir(tptpy)₂acac) shows a low driving voltage of 3.4 (and 3.6 V) at 1000 cd/m², and maximum external quantum efficiency as high as 26.4%. Such efficient performance of phosphorescent yellow OLEDs is attributed to a good charge balance and high electron transport properties of host materials.     

Charge conduction study of phosphorescent iridium compounds for organic light-emitting diodes application

The charge conduction properties of a series of iridium-based compounds for phosphorescent organic light-emitting diodes (OLEDs) have been investigated by thin-film transistor (TFT) technique. These compounds include four homoleptic compounds: Ir(ppy)₃, Ir(piq)₃, Ir(Tpa-py)₃, Ir(Cz-py)₃, and two heteroleptic compounds Ir(Cz-py)₂(acac) and FIrpic. Ir(ppy)₃, Ir(piq)₃ and FIrpic are commercially available compounds, while Ir(Tpa-py)₃, Ir(Cz-py)₃ and Ir(Cz-py)₂(acac) are specially designed to test their conductivities with respect to the commercial compounds. In neat films, with the exception of FIrpic, all Ir-compounds possess significant hole transporting capabilities, with hole mobilities in the range of about 5 × 10⁻⁶–2 × 10⁻⁵ cm² V⁻¹ s⁻¹. FIrpic, however, is non-conducting as revealed by TFT measurements. We further investigate how Ir-compounds modify carrier transport as dopants when they are doped into a phosphorescent host material CBP. The commercial compounds are chosen for the investigation. Small amounts of Ir(ppy)₃ and Ir(piq)₃ (<10%) behave as hole traps when they are doped into CBP. The hole conduction of the doped CBP films can be reduced by as much as 4 orders of magnitude. Percolating conduction of Ir-compounds occurs when the doping concentrations of the Ir-compounds exceed 10%, and the hole mobilities gradually increase as their values reach those of the neat Ir films. In contrast to Ir(ppy)₃ and Ir(piq)₃, FIrpic does not participate in hole conduction when it is doped into CBP. The hole mobility decreases monotonically as the concentration of FIrpic increases due to the increase of the average charge hopping distance in CBP.     

Efficient hole-transporter for phosphorescent organic light emitting diodes with a simple molecular structure

N,N-diphenyl-4-(quinolin-8-yl)aniline (SQTPA), which composes a triphenylamine group and a quinoline group, has been synthesized and employed as a hole-transporter in phosphorescent OLEDs. It has been proved that SQTPA has efficient hole-transport property with a hole-mobility of 3.60 × 10⁻⁵ cm²/V s at the electric field of 800 (V/cm)^1/2, which is higher than that of NPB (1.93 × 10⁻⁵ cm²/V s). Blue, orange and green phosphorescent OLEDs have been fabricated based on FIrpic, Ir(2-phq)₃, Ir(ppy)₃ with typical structures by using SQTPA as the hole-transporter. The SQTPA-based devices show maximum external quantum efficiencies and power efficiencies of 17.5%, 32.5 lm/W for blue, 12.3%, 20.5 lm/W for orange and 20.3%, 64.5 lm/W for green. The performances of SQTPA-based devices are much better than that of NPB-based phosphorescent OLEDs with similar structures. Thought of its very simple molecular structure and easy synthetic route, SQTPA should be an efficient hole-transporter for phosphorescent OLEDs.     

High quantum efficiency in blue phosphorescent organic light emitting diodes using ortho-substituted high triplet energy host materials

A high triplet energy material derived from carbazole and ortho terphenyl, 3,3′′-di(9H-carbazole-9-yl)-1,1′:2′,1′′-terphenyl (33DCTP), was synthesized as the host material for blue phosphorescent organic light-emitting diodes (PHOLEDs). The 33DCTP host showed high glass transition temperature of 110 °C, high triplet energy of 2.77 eV, the highest occupied molecular orbital of ?6.12 eV and the lowest unoccupied molecular orbital of ?2.52 eV. High efficiency blue PHOLEDs were developed using the 33DCTP host and bis((3,5-difluorophenyl)pyridine) iridium picolinate dopant material, and high quantum efficiency of 23.7% was achieved with a color coordinate of (0.14, 0.28).     

Highly efficient single-layer organic light-emitting devices using cationic iridium complex as host

Highly efficient single-layer organic light-emitting devices (OLEDs) based on blended cationic Ir complexes as emitting layer have been demonstrated using narrow band gap cationic Ir complex [Ir(Meppy)₂(pybm)](PF₆) (C1) as guest and wide band gap cationic Ir complex [Ir(dfppy)₂(tzpy-cn)](PF₆) (C2) as host. As compared with single cationic Ir complex emitting layer, these host–guest systems exhibit highly enhanced efficiencies, with maximum luminous efficiency of 25.7 cd/A, external quantum efficiency of 8.6%, which are nearly 3-folds of those of pure C1-based device. Compared with a multilayer host-free device containing C1 as emitting layer and TPBI as electron-transporting and hole-blocking layer, the above single-layer devices also show 2-folds enhancement efficiencies. The high efficiencies achieved in these host–guest systems are among the highest values reported for ionic Ir complexes-based solid-state light-emitting devices. In addition, a white-similar emission with CIE of (0.36, 0.47) has also been achieved with luminous efficiency of 4.2 cd/A as the C1 concentration is 0.1 wt.%. The results demonstrate that the ionic Ir complexes-based host–guest system provides a new approach to achieve highly efficient OLEDs upon single-layer device structure and solution-processing technique.     

Effect of electron injection and transport materials on efficiency of deep-blue phosphorescent organic light-emitting devices

We investigate the performance of FIr6-based deep-blue phosphorescent organic light-emitting devices (PHOLEDs) with three different electron transport materials, bathocuproine (BCP), 4,7-diphenyl-1,10-phenanthroline (BPhen), and tris[3-(3-pyridyl)mesityl]borane (3TPYMB), and study the effect of doping alkaline metals (Li and Cs) into these charge transport materials. External quantum efficiency (η_EQE) of (20 ± 1)% and peak power efficiency (η_P) of (36 ± 2) lm/W were achieved maintaining Commission Internationale de L’Eclairage (CIE) coordinates of (x = 0.16, y = 0.28) in p-i-n dual-emissive-layer (D-EML) deep-blue PHOLEDs with 3TPYMB as the electron transport material and 3TPYMB:Cs as the electron injection layer. The high efficiencies are attributed to the high triplet energy of 3TPYMB as well as the increased conductivity of 3TPYMB:Cs.     

Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet electrofluorescence,blue and green phosphorescent OLEDs

A high efficient UV–violet emission type material bis[4-(9,9′-spirobifluorene-2-yl)phenyl] sulfone (SF-DPSO) has been synthesized by incorporating electron deficient sulfone and morphologically stable spirobifluorene into one molecule. The steric and bulky compound SF-DPSO exhibits an excellent solid state photoluminescence quantum yield (Φ_PL = 92%), high glass transition temperature (T_g = 211 °C) and high triplet energy (E_T = 2.85 eV). In addition, the uniform amorphous thin film could be formed by spin-coating from its solution. These promising physical properties of the material made it suitable for using as UV–violet emitter in non-doped device and appropriate host in phosphorescent OLEDs. With SF-DPSO as an emitter, the non-doped solution processed device achieved an efficient UV–violet emission with the EL peak around 400 nm. By using SF-DPSO as a host, solution processed blue and green phosphorescent organic light emitting diodes showed a high luminous efficiency of 13.7 and 30.2 cd A⁻¹, respectively.     

Synthesis of phenylcarbazole–thiophene-based structural isomers as unipolar host materials for blue PHOLEDs and their device performance

New high triplet-energy host materials, which are symmetrically or asymmetrically designed by using phenylcarbazole and thiophene moieties, were synthesized by Suzuki–Miyaura cross-coupling reactions and their device performances of blue phosphorescent organic light-emitting diodes were also investigated. The synthesized compounds showed a high triplet energy (>2.84 eV) and good thermal stability. Highly efficient blue PHOLEDs were obtained when employing the symmetric compounds having C2 symmetry as the host material and bis[2-(4,6-difluorophenyl)pyridinato-C²,N](picolinato)iridium(III) (FIrpic) as the guest material. Their maximum external quantum efficiency of the device reached as high as 18.9% with blue color coordinate of (0.15, 0.35).     

Green and blue-green phosphorescent heteroleptic iridium complexes containing carbazole-functionalized β-diketonate for non-doped organic light-emitting diodes

Two novel iridium complexes both containing carbazole-functionalized β-diketonate, Ir(ppy)₂(CBDK) [bis(2-phenylpyridinato-N,C²)iridium(1-(carbazol-9-yl)-5,5-dimethylhexane-2,4-diketonate)], Ir(dfppy)₂(CBDK) [bis(2-(2,4-difluorophenyl)pyridinato-N,C²)iridium(1-(carbazol-9-yl)-5,5-dimethylhexane-2,4-diketonate)] and two reported complexes, Ir(ppy)₂(acac) (acac = acetylacetonate), Ir(dfppy)₂(acac) were synthesized and characterized. The electrophosphorescent properties of non-doped device using the four complexes as emitter, respectively, with a configuration of 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)aluminum (AlQ) (45 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) were examined. In addition, a most simplest device, ITO/Ir(ppy)₂(CBDK) (80 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm), and two double-layer devices with configurations of ITO/NPB (30 nm)/Ir(ppy)₂(CBDK) (30 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) and ITO/Ir(ppy)₂(CBDK) (30 nm)/AlQ (30 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) were also fabricated and examined. The results show that the non-doped four-layer device for Ir(ppy)₂(CBDK) achieves maximum lumen efficiency of 4.54 lm/W and which is far higher than that of Ir(ppy)₂(acac), 0.53 lm/W, the device for Ir(dfppy)₂(CBDK) achieves maximum lumen efficiency of 0.51 lm/W and which is also far higher than that of Ir(dfppy)₂(acac), 0.06 lm/W. The results of simple devices involved Ir(ppy)₂(CBDK) show that the designed complex not only has a good hole transporting ability, but also has a good electron transporting ability. The improved performance of Ir(ppy)₂(CBDK) and Ir(dfppy)₂(CBDK) can be attributed to that the bulky carbazole-functionalized β-diketonate was introduced, therefore the carrier transporting property was improved and the triplet–triplet annihilation was reduced.     

Highly efficient phosphorescent organic light-emitting diodes using a beryllium metal–chelate complex as electron-transporting host material

A classical fluorescent metal–chelate complex bis(2-(2-hydroxyphenyl)-pyridine)beryllium (Bepp₂) has been used as an efficient electron-transporting host material to construct highly efficient phosphorescent organic light-emitting diodes (PHOLEDs) with an orange-emitting phosphorescent guest bis(7,8-benzoquinolinato) iridium (III) (N,N′-diisopropyl-benzamidine) ((bzq)₂Ir(dipba)). Due to the well-matched energy levels of Bepp₂ with the corresponding hole-/electron- transporting (HT/ET) materials and the high-efficiency and complete energy transfer of this host–guest system, the Bepp₂-based PHOLEDs exhibit rather low driving voltage (2.8 V) and high peak EL efficiencies of over 70 cd A⁻¹ for luminous efficiency, 55 lm W⁻¹ for power efficiency, and 23% for external quantum efficiency, a performance significantly better than that using CBP as the host.     

Spiro-fused N-phenylcarbazole-based host materials for blue phosphorescent organic light-emitting diodes

Two host materials, SFCA and SFCC, consist of a diphenylamine or carbazole unit linking to spiro-fused phenyl carbazole (SFC) backbone, were designed and synthesized. By choosing the meta linkage way between diphenylamine/carbazole units and SFC ring, higher triplet energies could be easily achieved for the two new materials, which mean that they could be used as effective host material for popular blue phosphorescent material Iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C^2′] picolinate (FIrpic, E_T = 2.65). Besides that, the steric SFC structure could guarantee their good thermal stabilities. Their thermal, photophysical and electroluminescent properties were systematically investigated. The blue phosphorescent OLEDs with the two materials as hosts and FIrpic as a dopant exhibited excellent performance with maximum current efficiencies of 33.9 and 40.8 cd/A, respectively.     

High efficiency yellowish green phosphorescent emitter derived from phenylbenzothienopyridine ligand

A yellowish green phosphorescent dopant derived from phenylbenzothienopyridine ligand, iridium (III) [bis(1-phenylbenzo[4,5]thieno[2,3-c]pyridinato-N,C²]picolinate. (Ir(DTNP)₂pic) was synthesized and the device performances of the Ir(DTNP)₂pic was studied. The Ir(DTNP)₂pic dopant exhibited yellowish green emission at 548 nm and showed a high quantum efficiency of 22.4% at 1000 cd/m² with a color coordinate of (0.43, 0.57) in yellowish green phosphorescent organic light-emitting diodes.