Novel dibenzothiophene based host materials incorporating spirobifluorene for high-efficiency white phosphorescent organic light-emitting diodes

Two host materials, DBTSF2 and DBTSF4, were designed and synthesized, incorporating dibenzothiophene (DBT) and spirobifluorene (SF) blocks. Their thermal, electrochemical and photo-physical properties were fully characterized. DBTSF4, which adopted an ortho-linkage between DBT and SF moieties, showed a significantly higher T₁ energy of 2.82 eV as compared to its para-linkage analogue DBTSF2 (2.49 eV). Their applications as host for green, blue and white phosphorescent organic light-emitting diodes (PHOLEDs) were explored. The DBTSF4 based blue PHOLED has a highest current efficiency of 23.5 cd A^?1. And using DBTSF4 as a single host, two-color based white PHOLEDs were achieved from cold white emission with CIE coordinate of (0.31, 0.43) to yellowish warm white emission (0.44, 0.49) with maximum current efficiencies varying from 35.8 to 52.3 cd A^?1 and maximum external quantum efficiencies from 13.1% to 16.9% respectively. The white PHOLED devices also showed a low efficiency roll-off even at 10,000 cd m^?2.     

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).     

Efficient blue organic light-emitting diodes using newly-developed pyrene-based electron transport materials

We synthesized new kinds of pyrene-based electron transport materials: 1,6-di(pyridin-3-yl)-3,8-di(naphthalen-1-yl)pyrene (N1PP) and 1,6-di(pyridin-3-yl)-3,8-di(naphthalen-2-yl)pyrene (N2PP). The external quantum efficiencies of the device with these electron transport materials increase by more than 50% at 1 mA cm^?2 compared with those of the device with representative Alq₃ as an electron transport material. The enhanced quantum efficiency is due to a balanced charge recombination in an emissive layer. Electron mobilities in N1PP and N2PP films are three times higher than that in Alq₃. Highly enhanced power efficiency is achieved due to a low electron injection barrier and a high electron mobility. We find that the luminance degradation in the blue OLEDs is correlated with the HOMO energy levels of electron transport materials.     

Controlling singlet–triplet splitting in carbazole–oxadiazole based bipolar phosphorescent host materials

A rational molecular design strategy for carbazole–oxadiazole based bipolar host materials was developed to improve the device efficiency of blue phosphorescent organic light-emitting diodes (PHOLED). Steric effects of strategically placed methyl groups led to an increase of triplet energies (o-2MPCzPOXD: 2.66 eV and o-3MPCzPOXD: 2.73 eV versus the initial host material o-PczPOXD: 2.62 eV) while less pronouncedly affecting singlet energies and, therefore, retaining low driving voltages, high power efficiencies and remarkably low efficiency roll-offs in PHOLEDs. The maximum quantum efficiencies (EQE) for blue devices (FIrpic) were significantly raised for o-2MPCzPOXD (13.6%) and o-3MPCzPOXD (11.5%) versus o-PCzPOXD (9.0%) although yielding comparable values for green devices (Ir(ppy)₃; 12.9% and 15.4% versus 13.2%). Supported by theoretical calculations a structure–property relationship was established from photo-physical properties, PHOLED performance measurements and structural characterization from single crystal data.     

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.     

Orange phosphorescent organic light-emitting diodes with high operational stability

In this article we report on the performances of phosphorescent orange organic light-emitting diodes (OLEDs) having a high operational stability. The fabricated devices all consist of a “hybrid” structure, where the hole-injection layer was processed from solution, while the rest of the organic materials were deposited by vacuum thermal evaporation. A device stack having an emissive layer comprising a carbazole-based host TCzMe doped with the orange phosphor tris(2-phenylquinoline)iridium(III) [Ir(2-phq)₃] shows improved efficiencies compared to a the same device with the standard N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) as host material. External quantum efficiency (EQE) up to 7.4% and a power efficiency of 16 lm/W were demonstrated using TCzMe. Most importantly, the operational stability of the device was largely improved, resulting in extrapolated values reaching lifetimes well above 100,000 h at initial luminance of 1000 Cd/m².     

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.     

Thermally cross-linkable host materials for enabling solution-processed multilayer stacks in organic light-emitting devices

A thermally cross-linkable host material, i.e., two vinylbenzyl ether groups containing a carbazole derivative (DV-CBP), was developed for solution-processed multilayer organic light-emitting devices (OLEDs). DV-CBP was thermally cross-linked at styrene end-groups through curing at approximately 180 °C in the absence of a polymerization initiator. This cross-linking reaction rendered the emissive layer insoluble and enabled the subsequent solution deposition of an upper electron-transporting layer. Furthermore, photoluminescence quantum efficiencies of the emissive layer were maintained at greater than 75% throughout the cross-linking reaction. A solution-processed small-molecule electron-transporting layer on top of the cross-linked emissive layer led to lower driving voltages and higher efficiencies in the OLEDs compared to those of a device with a vacuum-deposited Ca electrode on the emissive layer.     

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).     

High-efficiency low color temperature organic light emitting diodes with solution-processed emissive layer

Low color temperature (CT) lighting provides a warm and comfortable atmosphere and shows mild effect on melatonin suppression. A high-efficiency low CT organic light emitting diode can be easily fabricated by spin coating a single white emission layer. The resultant white device shows an external quantum efficiency (EQE) of 22.8% (34.9 lm/W) with CT 2860 K at 100 cd/m², while is shown 18.8% (24.5 lm/W) at 1000 cd/m². The high efficiency may be attributed to the use of electroluminescence efficient materials and the ambipolar-transport host. Besides, proper device architecture design enables excitons to form on the host and allows effective energy transfer from host to guest or from high triplet guest to low counterparts. By decreasing the doping concentration of blue dye in the white emission layer, the device exhibited an orange emission with a CT of 2280 K. An EQE improvement was observed for the device, whose EQE was 27.4% (38.8 lm/W) at 100 cd/m² and 20.4% (24.6 lm/W) at 1000 cd/m².     

Simple-structured efficient white organic light emitting diode via solution process

High-efficiency white emission is crucial to the design of energy-saving display and lighting panels, whereas solution-process feasibility is highly desirable for large area-size and cost-effective roll-to-roll manufacturing. In this study, we demonstrate highly-efficient, bright and chromaticity stable white organic light emitting diodes (OLEDs) with solution-processed single emissive layer. The resultant best white OLED shows excellent electroluminescence performance with forward-viewing external quantum efficiency, current efficiency and power efficiency of 22.7%, 48.8 cd A^− 1 and 27.8 lm W^− 1 at 100 cd m^− 2, respectively, with a maximum luminance of 19,590 cd m^− 2. Furthermore, we also observed an increment of 112% in the power efficiency, 86.9% in the current efficiency and a decrement of 39.2% in the external quantum efficiency at 100 cd m^− 2 as the doping concentration of blue dye was increased from 10 wt% to 25 wt% in the devices. The better efficiency performance may be attributed to the effective exciton-confining device architecture and low-energy barrier for electrons to inject from the hole-blocking electron-transport layer to the host layer.     

Molecular design of large-bandgap host materials and their application to blue phosphorescent organic light-emitting diodes

New large-bandgap host materials with carbazole and carboline moieties were designed and synthesized for high-performance blue phosphorescent organic light-emitting diodes (PhOLEDs). The two kinds of host materials, 9-(4-(9H-carbazol-9-yl)phenyl)-6-(9H-carbazol-9-yl)-9H-pyrido[2,3-b]indole (pP2CZCB) and 9-(3-(9H-carbazol-9-yl)phenyl)-6-(9H-carbazol-9-yl)-9H-pyrido[2,3-b]indole (mP2CZCB), displayed promisingly high triplet energies of ∼2.92–2.93 eV for enhancing the exothermic energy transfer to bis[2-(4,6-difluorophenyl)pyridinato-C²,N](picolinato)iridium(III) (FIrpic) in PhOLED devices. It was found that the blue PhOLEDs bearing the new host materials and the FIrpic dopant exhibited markedly higher external quantum efficiencies (EQEs) than a device made with 1,3-bis(N-carbazolyl)benzene (mCP) as the host. In particular, the PhOLED device made with 3 wt% FIrpic as the dopant and mP2CZCB as the host material displayed a low driving voltage of 4.13 V and the high EQE of 25.3% at 1000 cd m⁻².     

Efficient solution-processed blue phosphorescent organic light-emitting diodes with halogen-free solvent to optimize the emissive layer morphology

Efficient solution-processed blue phosphorescent organic light-emitting diodes (OLEDs) featuring with halogen-free solvent processing are fabricated in this study. The organic molecule 3,6-bis(diphenylphosphoryl)-9-(4′-(diphenylphosphoryl) phenyl)-carbazole (TPCz) that possesses good solubility in halogen-free polar solvents is selected to serve as the host of blue phosphorescent iridium(III) [bis(4,6-difluorophenyl)-pyridinato-N,C²]-picolinate (FIrpic) dopant. The morphology of the TPCz:FIrpic emissive layer prepared with different polar solvents including chlorobenzene (CB), n-butanol (ButA) and isopropanol (IPA) and the effect on their electroluminescent performance have been investigated in detail. It is found that the more polar halogen-free solvent IPA restrains the FIrpic aggregation and renders a more densely packed emissive layer as compared to the CB-processed counterpart, which results in the enhanced electroluminescent performance. The luminous efficiency and power efficiency of the blue phosphorescent OLEDs prepared with CB are merely 5.7 cd/A and 3.3 lm/W, respectively. When using more polar halogen-free solvent IPA, the efficiencies are enhanced to 22.3 cd/A and 15.6 lm/W, about 2.9 and 3.7-time increment, respectively. This work provides an approach to fabricate efficient solution-processed phosphorescent OLEDs with environmental-friendly solvents, which is highly required in large-scale solution-processed manufacturing.     

Synthesis of dibenzothiophene-based host materials and their blue phosphorescent device performances

Two structural isomeric host materials, 9-(4-(dibenzo[b,d]thiophen-4-yl)phenyl)-9H-pyrido-[2,3-b]indole (pDBTCb) and 9-(3-(dibenzo[b,d]thiophen-4-yl)phenyl)-9H-pyrido-[2,3-b]indole (mDBTCb), were designed and synthesized, incorporating dibenzothiophene (DBT) and α-carboline moieties via phenyl linkages and their device performances of phosphorescent organic light-emitting diodes (PHOLEDs) were also investigated. The different linkages between DBT and α-carboline on central phenyl spacer play an important role in the structure–property correlations. Although their photophysical properties were similar regardless of different linkage positions, the bis[2-(4,6-difluorophenyl)pyridinato-C²,N](picolinato)iridium(III)-based blue device with mDBTCb, which adopted a meta-linkage showed a significantly higher maximum quantum efficiency of 19.8% as compared to its para-linkage analog, pDBTCb (16.2%). A high quantum efficiency of 19.8% and only ca. 10% reduction of quantum efficiency at 1000 cd/m² were demonstrated from the blue PHOLEDs with the mDBTCb host material.     

White emission polymer light-emitting devices with efficient electron injection from alcohol/water-soluble polymer/Al bilayer cathode

We demonstrate highly efficient white emission polymer light-emitting diodes (WPLEDs) from multilayer structure formed by solution processed technique, in which alcohol/water-soluble polymer, poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) was incorporated as electron-injection layer and Al as cathode. It was found that the device performance was very sensitive to the solvents from solution of which the PFN electron-injection layer was cast. Devices with electron-injection layer cast from methanol solution show degraded performance while the best device performance was obtained when mixed solvent of water and methanol with ratio of 1:3 was used. We attribute the variation in device performance to washing out the electron transport material in the emissive layer due to rinse effect. As a result of alleviative loss of electron transport material in the emissive layer, the optimized device with a peak luminous efficiency of 18.5 cd A^?1 for forward-viewing was achieved, which is comparable to that of the device with same emissive layer but with low work-function metal Ba cathode (16.6 cd A^?1). White emission color with Commission International de I’Eclairage coordinates of (0.321, 0.345) at current 10 mA cm^?2 was observed.     

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.     

Efficient blue organic light-emitting diodes based on triphenylimidazole substituted anthracene derivatives

A series of new blue emissive materials based on the conjugates of highly fluorescent diaryl anthracene and electron-transporting triphenylimidazole moieties: 2-(4-(anthracen-9-yl)phenyl)-1,4,5-triphenyl-1H-imidazole (ACBI), 2-(4-(10-(naphthalen-1-yl)anthracen-9-yl)phenyl)-1,4,5-triphenyl-1H-imidazole (1-NaCBI), 2-(4-(10-(naphthalen-2-yl)anthracen-9-yl)phenyl)-1,4,5-triphenyl-1H-imidazole (2-NaCBI) were designed and synthesized successfully. These materials exhibit good film-forming properties and excellent thermal stabilities. Meanwhile, the decreased π-conjugation in these compounds compared with phenanthroimidazole derivatives leads to obvious hypsochromic shift. To explore the electroluminescence properties of these materials, typical three-layer organic light-emitting devices were fabricated. With respect to the three layer device 2 using 1-NaCBI as the emitting layer, its maximum current efficiency reaches 3.06 cd A⁻¹ with Commission Internationale del’Eclairage (CIE) coordinates of (0.149, 0.092). More interestingly, sky blue doped device 5 based on 1-NaCBI achieved a maximum current efficiency of 15.53 cd A⁻¹ and a maximum external quantum efficiency of 8.15%, high EQE has been proved to be induced by the up-conversion of a triplet excited state.     

Ideal host and guest system in phosphorescent OLEDs

Ideal host-guest system for emission in phosphorescent OLEDs with only 1% guest doping condition for efficient energy transfer have been demonstrated in the present investigation. Using a narrow band-gap fluorescent host material, bis(10-hydroxybenzo[h] quinolinato)beryllium complex (Bebq₂), and red dopant bis(2-phenylquinoline)(acetylacetonate)iridium (Ir(phq)₂acac), highly efficient red phosphorescent OLEDs (PHOLEDs) exhibiting excellent energy transfer characteristics with a doping concentration of 1% were developed. Fabricated PHOLEDs show a driving voltage of 3.7 V, maximum current and power efficiencies of 26.53 cd/A and 29.58 lm/W, and a maximum external quantum efficiency of 21%. Minimized electron or hole trapping at the phosphorescent guest molecules and efficient Förster and Dexter energy transfers from the Bebq₂ host singlet and triplet states to the emitting triplet of Ir(phq)₂acac guest appear to be the key mechanism for ideal phosphorescence emission.     

High-efficiency hybrid organic–inorganic light-emitting devices

We report efficient red, orange, green and blue organic–inorganic light emitting devices using light emitting polymers and polyethylenimine ethoxylated (PEIE) interlayer with the respective luminance efficiency of 1.3, 2.7, 10 and 4.1 cd A⁻¹, which is comparable to that of the analogous conventional devices using a low work-function metal cathode. This is enabled by the enhanced electron injection due to the effective reduction of the ZnO work-function by PEIE, as well as hole/exciton-blocking function of PEIE layer. Due to the benign compatibility between PEIE and the neighboring organic layer, the novel phosphorescent organic–inorganic devices using solution-processed small molecule emissive layer show the maximum luminance efficiency of 87.6 cd A⁻¹ and external quantum efficiency of 20.9% at 1000 cd m⁻².     

High-performance azure blue quantum dot light-emitting diodes via doping PVK in emitting layer

Highly bright and efficient azure blue quantum dot-based light-emitting diodes (QD-LEDs) have been demonstrated by employing ZnCdSe core/multishell QDs as emitters and the crucial development we report here is the ability to dramatically enhance the efficiency and brightness through doping poly vinyl(N-carbazole) (PVK) in the emissive layer to balance the charge injection. The best device displays remarkable features like maximum luminance of 13,800 cd/m², luminous efficiency of 6.41 cd/A, and external quantum efficiency (EQE) of 8.76%, without detectable red-shift and broadening in electroluminescence (EL) spectra with increasing voltage as well as good spectral matching between photoluminescence (PL) and EL. Such azure blue quantum-dot LEDs show a 140% increase in external quantum efficiency compared with QD-LEDs without PVK. More important, the peak efficiency of the QD-LEDs with PVK dopant is achieved at luminance of about 1000 cd/m², and high efficiency (EQE > 8%) can be maintained with brightness ranging from 200 to 2400 cd/m². There are two main aspects of the role of PVK in the proposed system. Firstly, the lower HOMO of PVK than (poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) can reduce the potential barrier for 0.4 eV at the interface of QDs and hole transport layer which could result in higher hole injection efficiency along with good EQE as compared to TFB-only HTLs. Secondly, with PVK acting as buffer layer of TFB and QDs, the exciton energy transfer from the organic host to the QDs can be effectively improved.