High Efficiency Deep‐Blue Phosphorescent Organic Light‐Emitting Diodes with CIE x,y (≤ 0.15) and Low Efficiency Roll‐Off by Employing a High Triplet Energy Bipolar Host Material

Recently, bipolar host materials are the most promising candidates for achieving high performance phosphorescent organic light‐emitting diodes (PHOLEDs) in order to maximize recombination efficiency. However, the development of host material with high triplet energy (E _T) is still a great challenge to date to overcome the limitations associated with the present PHOLEDs. Herein, a highly efficient donor‐π‐acceptor (D‐π‐A) type bipolar host (4′‐(9H‐carbazol‐9‐yl)‐2,2′‐dimethyl‐[1,1′‐biphenyl]‐4‐yl)diphenylphosphine oxide (m‐CBPPO) comprising of carbazole, 2,2′‐dimethylbiphenyl and diphenylphosphoryl as D‐π‐A unit, respectively, is developed. Interestingly, a high E _T of 3.02 eV is observed for m‐CBPPO due to highly twisted conformation. Furthermore, the new host material is incorporated in PHOLEDs as emissive layer with a new carbene type Ir(cb)₃ material as a deep‐blue emitter. The optimized devices show an excellent external quantum efficiency (EQE) of 24.8% with a notable Commission internationale de l'éclairage (x, y) ≤ 0.15, (0.136, 0.138) and high electroluminescence performance with extremely low efficiency roll‐off. Overall, the above EQE is the highest reported for deep‐blue PHOLEDs with very low efficiency roll‐off and also indicate the importance of appropriate host for the development of high performance deep‐blue PHOLEDs.     

Towards High Efficiency and Low Roll‐Off Orange Electrophosphorescent Devices by Fine Tuning Singlet and Triplet Energies of Bipolar Hosts Based on Indolocarbazole/1, 3, 5‐Triazine Hybrids

Orange‐emitting phosphorescent organic light‐emitting diodes (PHOLEDs) are drawing more and more attention; however, high‐performance hosts designed for orange PHOLEDs are rare. Here, four indolocarbazole/1, 3, 5‐triazine hybrids are synthesized to optimize the singlet and triplet energies, as well as transporting properties, for ideal orange PHOLEDs. By introducing moieties with different electronegativity, a graded reduction of the singlet and triplet energies is achieved, resulting in minimum injection barrier and minimum energy loss. Besides, the charge transporting abilities are also tuned to be balanced on the basis of the bipolar features of those materials. The optimized orange PHOLED shows a maximum external quantum efficiency (EQE) of 24.5% and a power efficiency of 64 lm W^–1, both of which are among the best values for orange PHOLEDs. What is more, the efficiency roll‐off is extremely small, with an EQE of 24.4% at 1000 cd m^–2 and 23.8% at 10 000 cd m^–2, respectively, which is the lowest efficiency roll‐off for orange PHOLEDs to date, resulting in the highest EQE for orange PHOLEDs when the luminance is above 1000 cd m^–2. Besides the balanced charges, the small roll‐off is also attributed to the wide recombination zone resulting from the bipolar features of the hosts.     

Novel spiro-based host materials for application in blue and white phosphorescent organic light-emitting diodes

Two novel spiro-based host materials, namely 3-(9,9′-spirobi[fluoren]-6-yl)-9-phenyl-9H-carbazole (SF3Cz1) and 9-(3-(9,9′-spirobi[fluoren]-6-yl)phenyl)-9H-carbazole (SF3Cz2) were designed and synthesized. Due to the meta-linkage of spirobifluorene backbone, both SF3Cz1 and SF3Cz2 possess triplet energies over 2.70 eV, indicating they could serve as suitable hosts for blue and even white phosphorescent organic light-emitting diodes (PHOLEDs). The fabricated bis(4,6-(difluorophenyl)-pyridinato -N,C′)picolinate (FIrpic) based PHOLEDs hosted by SF3Cz1 and SF3Cz2 exhibited excellent performance with maximum external quantum efficiencies (EQEs) of 18.1% and 19.7%, respectively. Two-color warm white PHOLEDs fabricated by utilizing SF3Cz1 and SF3Cz2 as hosts also achieved high EQEs and low efficiency roll-offs. The results demonstrate that SF3Cz1 and SF3Cz2 are promising hosts for blue and white PHOLEDs.     

4, 6-Bis[3-(dibenzothiophen-2-yl)phenyl] pyrimidine bipolar host for bright,efficient and low efficiency roll-off phosphorescent organic light-emitting devices

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.     

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.     

Soluble processed low-voltage and high efficiency blue phosphorescent organic light-emitting devices using small molecule host systems

We report low voltage driving and highly efficient blue phosphorescence organic light emitting diodes (PHOLEDs) fabricated by soluble process. A soluble small molecule mixed host system consisting of hole transporting 4,4’,4’’ tris(N-carbazolyl)triphenylamine (TCTA) and bipolar carrier transporting 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) exhibits high solubility with smooth surface properties. Moreover, this small molecule host shows the smoothest morphological property similar to a vacuum deposited amorphous film. A low driving voltage of 5.4 V at 1000 cd/m² and maximum external quantum efficiency 14.6% obtained in the solution processed blue PHOLEDs are useful for large area low cost manufacturing.     

Thermally stable carboline derivative as a host material for blue phosphorescent organic light-emitting diodes

A α-carboline based high triplet energy material, 9,9′-(5′-(carbazol-9-yl)-[1,1′:3′,1″-terphenyl]-3,3″-diyl)di-α-carboline (2CbCzT), was designed and synthesized as the thermally stable host material for blue phosphorescent organic light-emitting diodes (PHOLEDs). The 2CbCzT host showed high glass transition temperature of 149 °C and high decomposition temperature of 518 °C at 5% weight loss. In addition, the 2CbCzT exhibited bipolar charge transport properties due to hole transport type carbazole and electron transport type α-carboline units. Blue PHOLEDs were developed using the high triplet energy 2CbCzT host material and a high quantum efficiency of 22.1% was obtained.     

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.     

Orthogonally substituted aryl derivatives as bipolar hosts for blue phosphorescent organic light-emitting diodes

Four novel bipolar hosts, namely 9,9′-(2-(4,6-diphenylpyrimidin-2-yl)-1,3-phenylene)bis(9H-carbazole) (2CzPm), 9,9′-(2-(4,6-diphenylpyrimidin-2-yl)-1,3-phenylene)bis(3,6-di-tert-butyl-9H-carbazole) (2TCzPm), 5,5′-(2-(4,6-diphenylpyrimidin-2-yl)-1,3-phenylene)bis(5H-benzofuro[3,2-c]carbazole) (2BFCzPm) and 5,5′-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)-1,3-phenylene)bis(5H-benzofuro[3,2-c]carbazole) (2BFCzTrz) were designed and synthesized with diphenylpyrimidine and diphenyltriazine as electron-transporting units and carbazole derivatives as hole-transporting motifs for the application in blue phosphorescent organic light-emitting diodes (PHOLEDs). These electron-accepting and -donating functional groups were attached to the central phenylene bridge in an ortho-substituted fashion, which led to high triplet energies (2.97–3.00 eV) and wide bandgap (3.43–3.55 eV). The effect of modulation of electron-accepting and donating groups on the photophysical properties, frontier orbital energy levels, charge carrier transport properties and device performance of these four hosts has been investigated. 2BFCzPm and 2BFCzTrz featured with large conjugation system exhibited high thermal stability as compared to 2CzPm and 2TCzPm. The bis[2-(4,6-difluorophenyl)-pyridinato-C2,N](picolinato)iridium(III) (FIrpic) based blue PHOLEDs hosted by 2BFCzPm exhibited excellent electroluminescence performance with a peak current efficiency of 38.2 cd/A and a maximum external quantum efficiency of 19.0%, which could be ascribed to the enhanced thermal stability, high triplet energy and good bipolar charge transport properties of the host material.     

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.     

High morphology stability and ambipolar transporting host for use in blue phosphorescent single-layer organic light-emitting diodes

The authors report a small molecule host of 2,7-bis(diphenylphosphoryl)-9-[4-(N,N-diphenylamino)phenyl]-9-phenylfluorene (POAPF) doped with 8 wt% iridium(III)-bis[(4,6-difluorophenyl)pyridinato-N,C^2′]picolinate (FIrpic) for use in efficient and single-layer blue phosphorescent organic light-emitting diodes (PHOLEDs) exhibiting a maximum external quantum efficiency of ∼20.3% at brightness of 100 cd/m². The high performance of such single layer PHOLEDs is attributed to the POAPF host’s high morphological stability, suitable triplet energy level, and equal charge carrier mobilities of hole and electron to form the broad carrier recombination zone in the emitting layer, thus reducing the triplet-triplet annihilation and resulting in a slight efficiency roll off of 0.5% from the brightness of 1 and 1000 cd/m². This work also systematically investigated the arrangement of the POAPF:FIrpic recombination zone for optimizing the performance of the single layer PHOLED.     

Improved efficiency in solution processed green phosphorescent organic light-emitting diodes using a double layer emitting structure fabricated by a stamp transfer printing process

A high efficiency soluble green phosphorescent organic light-emitting diode (PHOLED) was developed using a double layer emitting structure fabricated by a stamp transfer printing process. One green emitting layer with a hole transport type host material was coated on a hole transport layer and the other green emitting layer with an electron transport type host material was stamp transfer printed on the green emitting layer. The efficiency of solution processed green PHOLEDs was more than quadrupled using the double layer emitting structure fabricated by the stamp transfer printing method.     

Carbazole-bridged triphenylamine-bipyridine bipolar hosts for high-efficiency low roll-off multi-color PhOLEDs

Three new bipolar molecules composed of carbazole, triarylamine, and bipyridine were synthesized and utilized as host materials in multi-color phosphorescent OLEDs (PhOLEDs). These carbazole-based materials comprise a hole-transport triarylamine at C3 and an electron-transport 2,4′- or 4,4′-bipyridine at N9. The different bipyridine isomers and linking topology of the bipyridine with respect to carbazole N9 not only allows fine-tuning of physical properties but also imparts conformational change which subsequently affects molecular packing and carrier transport properties in the solid state. PhOLEDs were fabricated using green [(ppy)₂Ir(acac)], yellow [(bt)₂Ir(acac)], and red [(mpq)₂Ir(acac)] as doped emitters, which showed low driving voltage, high external quantum efficiency (EQE), and extremely low efficiency roll-off. Among these new bipolar materials, the 2Cz-44Bpy-hosted device doping with 10% (ppy)₂Ir(acac) as green emitting layer showed a high EQE of 22% (79.8 cd A⁻¹) and power efficiency (PE) of 102.5 lm W⁻¹ at a practical brightness of 100 cd m⁻². In addition, the device showed limited efficiency roll-off (21.6% EQE) and low driving voltage (2.8 V) at a practical brightness of 1000 cd m⁻².     

Orthogonal Molecular Structure for Better Host Material in Blue Phosphorescence and Larger OLED White Lighting Panel

High‐efficiency blue phosphorescence devices with external quantum efficiencies above 25% are developed using a new bipolar host material, diphenyl(10‐phenyl‐10H‐spiro[acridine‐9,9′‐fluoren]‐2′‐yl)phosphine oxide (POSTF), which is constructed in orthogonal molecular structure with a spiro‐coree. The separation of bipolarity from effective spiro‐fluorene‐triphenylamine (STF) structure is elucidated and its versatility in device is evaluated by two kinds of sky‐blue phosphors. Noticeably, large‐size white light‐emitting panel (150 mm × 150 mm) is fabricated with max power efficiency of 75.9 l m W^?1 using this new host.     

9-(Pyridin-3-yl)-9H-carbazole derivatives as host materials for green phosphorescent organic light-emitting diodes

9-(Pyridin-3-yl)-9H-carbazole (PyCz) derivatives were synthesized as bipolar host materials for green phosphorescent organic light-emitting diodes (PHOLEDs) and the device performance was investigated. The PyCz core was modified with carbazole and diphenylamine to prepare the bipolar host materials. Bipolar charge transport properties were observed in the PyCz derivative with the carbazole unit and a high quantum efficiency of 21.3% was obtained in green PHOLEDs.     

Solution‐Processible Carbazole Dendrimers as Host Materials for Highly Efficient Phosphorescent Organic Light‐Emitting Diodes

A group of dendrimers with oligo‐carbazole dendrons appended at 4,4′‐ positions of biphenyl core are synthesized for use as host materials for solution‐processible phosphorescent organic light‐emitting diodes (PHOLEDs). In comparison with the traditional small molecular host 4,4′‐N,N′‐dicarbazolebiphenyl (CBP), the dendritic conformation affords these materials extra merits including amorphous nature with extremely high glass transition temperatures (ca. 376 °C) and solution‐processibility, but inherent the identical triplet energies (2.60–2.62 eV). In comparison with the widely‐used polymeric host polyvinylcarbazole (PVK), these dendrimers possess much higher HOMO levels (–5.61 to –5.42 eV) that facilitate efficient hole injection and are favorable for high power efficiency in OLEDs. The agreeable properties and the solution‐processibility of these dendrimers makes it possible to fabricate highly efficient PHOLEDs by spin coating with the dendimers as phosphorescent hosts. The green PHOLED containing Ir(ppy)₃ (Hppy = 2‐phenyl‐pyridine) dopant exhibits high peak efficiencies of 38.71 cd A^?1 and 15.69 lm W^?1, which far exceed those of the control device with the PVK host (27.70 cd A^?1 and 9.6 lm W^?1) and are among the best results for solution‐processed green PHOLEDs ever reported. The versatility of these dendrimer hosts can be spread to orange PHOLEDs and high efficiencies of 32.22 cd A^?1 and 20.23 lm W^?1 are obtained, among the best ever reported for solution‐processed orange PHOLEDs.     

Highly efficient phosphorescent organic light-emitting diodes using a homoleptic iridium(III) complex as a sky-blue dopant

Homoleptic triscyclometalated iridium(III) complex Ir(dbi)₃ was used as a dopant for sky blue phosphorescent organic light-emitting diodes (PHOLEDs). Its photophysical, thermal, electrochemical properties as well as the device performances were investigated. Ir(dbi)₃ exhibited high quantum yield of 0.52 in solution at room temperature. A maximum current efficiency and external quantum efficiency (EQE) of 61.5 cd A⁻¹ and 23.1% were obtained, which are the highest ever reported for blue homoleptic iridium complexes. High efficiencies of 53.5 cd A⁻¹ and 20.1% EQE were achieved even at the luminance of 1000 cd m⁻².     

High-efficiency phosphorescent organic light-emitting devices with low efficiency roll-off using a thermally activated delayed fluorescence material as host

Phosphorescent organic light-emitting devices (PHOLEDs) with high efficiency and low efficiency roll-off were fabricated. The emissive layer was composed of a thermally activated delayed fluorescence (TADF) material 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN) as host and an orange iridium complex bis(4-tert-butyl-2-phenylbenzothiozolato-N,C^2′)iridium(III)(acetylacetonate) [(tbt)₂Ir(acac)] as dopant. At a low dopant concentration of 1 wt%, a PHOLED without light extraction optimization achieved a maximum power efficiency of 42.1 lm/W, a luminance efficiency of 77.9 cd/A and an external quantum efficiency (EQE) of 26.8%, respectively. Meanwhile, the EQE maintained 26.6% at 1000 cd/m² and 25.8% at 5000 cd/m², respectively. Moreover, a critical current density of 300 mA/cm² was realized, indicating significantly improved efficiency roll-off. The efficient utilization of triplet excitons on 2CzPN for phosphorescence via reverse inter-system crossing of 2CzPN followed by Fӧrster resonance energy transfer from 2CzPN to (tbt)₂Ir(acac) is responsible for the superior performance.     

Silicon-based carbazole and oxadiazole hybrid as a bipolar host material for phosphorescent organic light-emitting diodes

A silicon-based bipolar compound, 2-(4-((4-(9H-carbazol-9-yl)phenyl)dimethylsilyl)phenyl)-5-phenyl-1,3,4-oxadiazole (COHS), was designed and prepared as a host material for phosphorescent organic light-emitting diodes (OLEDs). The conjugated analogue of COHS, 2-(4′-(9H-carbazol-9-yl)biphenyl-4-yl)-5-phenyl-1,3,4-oxadiazole (COH), was also prepared to investigate their structure–property relationships. Thermal-, photophysical- and electrochemical properties as well as their single-crystal X-ray structures were studied for COHS and COH. The central silicon atom in COHS successfully disconnected the electronic communication between the carbazole and oxadiazole groups, resulting in relatively high triplet energy of ca. 2.71 eV, which were capable of hosting green phosphorescent emitters. DFT calculations were conducted to investigate the electronic structures of COHS and COH, and the results showed good correlation to experimental results. Finally, COHS and COH were used as a bipolar host material for a green phosphorescence organic light-emitting diode (PHOLED) devices with Ir(ppy)₃ (tris[2-phenylpyridinato-C²,N]iridium(III)) as a dopant. The resulting device with COHS (device I) showed higher performance than the device with COH (device II), exhibiting high efficiencies and low-efficiency roll-off. Device I achieved maximum external quantum efficiencies (EQE) of 15.8%, whereas device II exhibited a relatively lower EQE of 13.0%.     

m‐Indolocarbazole Derivative as a Universal Host Material for RGB and White Phosphorescent OLEDs

The host materials designed for highly efficient white phosphorescent organic light‐emitting diodes (PhOLEDs) with power efficiency (PE) >50 lm W^‐1 and low efficiency roll‐off are very rare. In this work, three new indolocarbazole‐based materials (ICDP, 4ICPPy, and 4ICDPy) are presented composed of 6,7‐dimethylindolo[3,2‐a]carbazole and phenyl or 4‐pyridyl group for hosting blue, green, and red phosphors. Among this three host materials, 4ICDPy‐based devices reveal the best electroluminescent performance with maximum external quantum efficiencies (EQEs) of 22.1%, 27.0%, and 25.3% for blue (FIrpic), green (fac‐Ir(ppy)₃), and red ((piq)₂Ir(acac)) PhOLEDs. A two‐color and single‐emitting‐layer white organic light‐emitting diode hosted by 4ICDPy with FIrpic and Ir(pq)₃ as dopants achieves high EQE of 20.3% and PE of 50.9 lm W^?1 with good color stability; this performance is among the best for a single‐emitting‐layer white PhOLEDs. All 4ICDPy‐based devices show low efficiency roll‐off probably due to the excellent balanced carrier transport arisen from the bipolar character of 4ICDPy.