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Four emitters based on the naphthyridine acceptor moiety and various donor units exhibiting thermally activated delayed fluorescence (TADF) were designed and synthesized. The emitters exhibited excellent TADF properties with a small ΔE ST and a high photoluminescence quantum yield. A green TADF organic light-emitting diode based on 10-(4-(1,8-naphthyridin-2-yl)phenyl)-10H-phenothiazine exhibited a maximum external quantum efficiency of 16.4% with Commission Internationale de L'éclairage coordinates of (0.368, 0.569) as well as a high current and power efficiency of 58.6 cd/A and 57.1 lm/W, respectively. The supreme power efficiency is a record-high value among the reported values of devices with naphthyridine-based emitters. This results from its high photoluminescence quantum yield, efficient TADF, and horizontal molecular orientation. The molecular orientations of the films of the host and the host doped with the naphthyridine emitter were explored by angle-dependent photoluminescence and grazing-incidence small-angle X-ray scattering (GIWAXS). The orientation order parameters (ΘADPL) were found to be 0.37, 0.45, 0.62, and 0.74 for the naphthyridine dopants with dimethylacridan, carbazole, phenoxazine, and phenothiazine donor moieties, respectively. These results were also proven by GIWAXS measurement. The derivative of naphthyridine and phenothiazine was shown to be more flexible to align with the host and to show the favorable horizontal molecular orientation and crystalline domain size, benefiting the outcoupling efficiency and contributing to the device efficiency.

A novel bis-4Ph-substituted 9,10-dipehnylanthracene deep blue [1931 CIE (0.15, 0.08)] fluorescent compound, AnB4Ph, has been synthesized and characterized for organic light-emitting diode (OLED) applications. Our experimental study of AnB4Ph excludes the possibility of triplet-triplet annihilation, hybridized local and charge transfer, or thermally activated delayed fluorescent characteristics of the material. Since the solid-state photoluminescence quantum yield of AnB4Ph was determined to be 48%, assuming a 100% for the charge recombination efficiency, the light outcoupling efficiency (ηout) of an AnB4Ph non-doped OLED achieving an external quantum efficiency (EQE) of 5.3% is at least 44%, which is more than twofold higher than 20% for conventional OLEDs. Both grazing incidence wide-angle X-ray scattering (GIWAXS) and angle-dependent photoluminescence (ADPL) measurements reveal AnB4Ph having a high value of order parameter (SGIWAXS) of 0.61 for a ππ stacking along the normal direction and an orientation order parameter (SADPL) for a horizontal emitting dipole moment of -0.50 or Θ (horizontal-dipole ratios) of 100%, respectively. Otherwise, a refractive index (n) measurement provides a n = 1.80 for AnB4Ph thin films. Based on ηout = 1.2 × n-2, the calculated ηout is 37%, which is also in accordance with the results of GIWAXS and ADPL. We have also fabricated the classical fluorescent DPAVBi-doped AnB4Ph OLEDs, which display a true blue [1931 CIE (0.15 and 0.16)] electroluminescence with a high efficiency (EQE = 6.9%), surpassing the conventional ∼5% EQE. Based on an ηout of 42% for DPAVBi-doped AnB4Ph OLEDs, our studies suggest that the extremely horizontally aligned AnB4Ph host material exerts the same horizontal alignment on the DPAVBi dopant molecules.

35Cbz4BzCN, a novel universal host with a long triplet lifetime, has been developed. The triplet excitons in 35Cbz4BzCN can be effectively harvested by phosphorescence and thermally activated delayed fluorescence emitters. In the 4CzIPN doped device, the best performance of a maximum current efficiency of 111.8 cd A−1, power efficiency of 86.8 lm W−1, and external quantum efficiency of 31.7% with a small efficiency roll-off of 103 cd A−1 at 1000 cd m−2 has been achieved. 35Cbz4BzCN is also compatible with FIrpic to give sky-blue emission up to 46.3 cd A−1 and with TXO-TPA to give orange-red emission up to 60.6 cd A−1 efficiency.

Efficient deep-blue thermally activated delayed fluorescence with emission maximum at 425 nm and a photoluminescence quantum yield of 84% were observed for the synthesised di-tert-butyl-dimethylacridinyl disubstituted oxygafluorene dispersed under a nitrogen atmosphere in an inert polymer. Its TADF intensity decreased by 14 times when the sample was placed under an oxygen environment showing its potential for optical sensing of oxygen. The optical sensor demonstrated oxygen sensitivity of up to ca. 30 000 ppm with a Stern–Volmer constant KSV of 5.26 × 10−5 ppm−1. Electroluminescence spectrum with a peak wavelength at 446 nm and full width at half maximum of 62 nm was observed using the compound as a thermally activated delay fluorescence emitter in organic light-emitting diodes. Maximum external efficiency of the deep-blue device with CIEy of 0.08 reached 9.9%.

Two bipolar hosts, 8-cbzBIFP and 10-cbzBIFP, both with basic skeletons of orthogonally-connected carbazole and benzimidazo〈1,2-f〉phenanthridine (BIFP) have been prepared. The planar structure of the BIFP moiety governs the molecular stacking in the crystal lattice. Electron transport properties, in comparison to the benzimidazole derivatives, are facilitated. Both hosts display fabulous performances in phosphorescent organic light-emitting diodes with bis(2-phenylpyridine)(acetylacetonate)iridium(III) as an emitter. The optimized green PhOLED of 10-cbzBIFP achieves the maximum brightness (Lmax) of 58 810 cd m−2, the maximum current efficiency (ηc) of 87.0 cd A−1 and external quantum efficiency (EQE) of 21.1%. Likewise, 8-cbzBIFP shows an Lmax of 58 870 cd m−2, ηc of 77.3 cd A−1 and EQE of 19.1%.

Two new analogues of a popular host material, meta-bis(N-carbazolyl)phenylene (mCP), bearing either methoxy- (mCP-OMe) or tert-butyl (mCP-t-Bu) substituents at C-3 and C-6 positions of carbazole moieties were synthesized and characterized. These materials exhibit higher glass-transition temperatures (79 and 145 °C) than mCP (67 °C). They show reversibility upon electrochemical oxidation while preserving high enough triplet energy levels (2.86 and 2.97 eV). The vacuum-deposited layer of mCP-t-Bu showed hole mobility of 8 × 10−3 cm2V−1s−1 at the electric field of 1.2 × 105 Vcm−1. This value is more than by one order of magnitude higher than that of mCP. The new compounds were tested as hosts for commercial emitters: blue phosphor, bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic), green and blue thermally-activated delayed fluorescence (TADF) emitters 1,2,3,5-tetrakis(9-carbazolyl)-4,6-dicyanobenzene (4CzIPN) and bis[4-(9,9-dimethyl-9,10-dihydroacridin-10-yl)phenyl]sulfone (DMAC-DPS). Blue phosphorescent OLED based on mCP-t-Bu as the host showed up to 22.0% external quantum efficiency. When doping 4CzIPN into mCP-OMe host, a broad emission with peaks at 388, 514, and 572 nm was observed.

This study presented the effects of carrier-transporting layer (CTL) on electroluminescence (EL) performance of a blue phosphorescent organic light-emitting diodes (PHOLEDs) with electron transporting host based on three kinds of electron-transporting layers (ETLs) including 3-(4-biphenyl-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ), diphenyl-bis[4-(pyridin-3-yl)phenyl]silane (DPPS) and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) and two kinds of hole-transporting layers (HTLs) such as 4,4′-bis[N-1-naphthyl-N-phenyl-amino]biphenyl (NPB), 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC). The carrier recombination and exciton formation zones in blue PHOLEDs strongly depend on the carrier mobility of CTLs and the layer thickness, especially the carrier mobility. Between ETLs and HTLs, the high electron mobility of ETL results in a lower driving voltage in blue PHOLEDs than the high hole mobility of HTL did. In addition, layer thickness modulation is an effective approach to precisely control carriers and restrict carriers within the EML and avoid a leakage emission of CTL. For CTL pairs in OLEDs using the electron transporting host system, ETLs with low mobility and also HTLs with high hole mobility are key points to confine the charge in EML for efficient photon emission. These findings show that appropriate CTL pairs and good layer thickness are essential for efficient OLEDs.

Recently, a new emission mode, triplet-triplet annihilation up conversion (TTAUC), of OLED was proposed with the mechanism as follows: when the carriers recombine at the sensitizer layer with a lower energy bandgap, the generated triplet excitons of sensitizer transfer to the triplet-triplet annihilation (TTA) emitter and emit a higher energy photon. In our experiment, we used tris(8-hydroxyquinolinato)aluminum (Alq3) as sensitizer, 9,10-Bis(2-naphthyl)anthraces (ADN) as TTA emitter and 1,1'-(2,5-Dimethyl-1,4-phenylene)dipyrene (DMPPP) as triplet diffusion singlet blocking (TDSB) layer. To improve the efficiency of TTAUC-OLED, 4,4’ -Bis[4-(di-p-tolylamino)styryl]biphenyl (DPAVBi), was doped in ADN host. With 6% of DPAVBi doping, no obvious change was observed in electrical characteristics, indicating electron traps from dopant is limited. Maximum EQE achieved EQE= 4.66% in the optimized device.

In this study, we report a bistriazoles derivative to be the wide bandgap host for blue emitters. In particular, this bistriazoles derivative possesses a wide bandgap of 4.0 eV and bipolar carrier transportation behavior. In addition, its bipolar behavior is classified to the rare case of electron-favorable bipolar carrier transportation, which was identified by the time-of-flight measurement and unipolar device. The electron mobility is little faster than hole one, which benefits the adjustment of carrier balance in the emitting layer. Employing to be the host of blue phosphorescent OLED, the device exhibits a high-efficiency performance with a 30.2%EQE.

Although phosphorescent and thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLED) exhibit high efficiency and long lifetime for red and green emission, blue OLED is still a bottleneck. In mass production, triplet-triplet emission (TTA) OLED is the main stream for reasonable lifetime together with limited efficiency. To improve the efficiency of blue TTA-OLED, a bilayer emitting layer (EML) was employed. Compared to single-EML device, external quantum efficiency of the bilayer OLED increased from 9.4% to 13.0%, which mainly resulted from the increase of delayed emission from 15.0% to 37% with enhanced TTA process.

The recent progress of efficiency improvement, emission color tuning, and lifetime elongation of blue organic light-emitting diodes (OLEDs) is reviewed. The latter is one of the most important bottlenecks for OLED development. The current status of blue light-emitting material design with emission mechanisms such as fluorescence (F), phosphorescence (Ph), thermally activated delayed fluorescence (TADF), and hybridized local and charge transfer (HLCT) is introduced in the first part of this review. Compared to red and green devices, the long exciton lifetime of the high energy triplet exciton in a blue OLED is the one of the main issues. To avoid the accumulation of high energy triplet excitons in the emitter for blue OLEDs, assisted triplet–triplet fluorescence (TTF) and Hyperfluorescence™ are employed to harvest the triplet excitons. In the second part of this review, we focus on issues from an application viewpoint: what are the requirements of blue OLEDs for display and lighting technologies in terms of efficiency, color, and lifetime? Key performance metrics of blue OLEDs with different technologies over time are summarized. Independent of technology, the trend is similar: the external quantum efficiency improves for the first stage of research, followed by color tuning, and then finally lifetime elongation. The state-of-the-art device performance of blue OLEDs with various emission mechanisms is illustrated. Although Ph- and TADF-emission based devices show satisfactory efficiency and electroluminescence (EL) spectra, despite having a lower efficiency TTF-emission based devices are the mainstream for real applications due to their relatively long operation lifetime. Blue Ph-OLEDs have the potential for lighting applications with suitable material selection and device design. We collected the published results and tried our best to make a fair comparison of the operation lifetime among different technologies. Finally, we discuss the possible future outlook from different viewpoints including new materials, device designs, and applications of blue OLEDs.

The inclusion of a tetraphenylbenzene (4Ph) unit in thermally activated delayed fluorescence emitters is demonstrated as a novel strategy for greatly enhancing the horizontally oriented alignment of the emitters without shifting the emission spectrum to longer wavelengths. Doping of blue-emitting 4PhOXDDMAC or greenish-blue-emitting 4PhOXDPXZ into o-DiCbzBz host layers yielded much higher degrees of horizontally oriented alignment for the emitter (up to 92%) compared to those when the 4Ph unit was excluded (69 and 75%, respectively). The enhanced alignment results in high outcoupling efficiencies of 24 and 35% in organic light-emitting diodes based on 4PhOXDDMAC and 4PhOXDPXZ, respectively, and boosts the external quantum efficiencies to values (8.8 and 29.2%, respectively) that are higher than what would be expected for randomly oriented emitters (outcoupling efficiency of 20%). These enhancements are achieved while avoiding the redshift that often occurs using the common strategy of increasing molecular length and, thereby, conjugation, to increase orientation.

New derivatives of trifluoromethyl benzene and tert-butyl disubstituted acridan or phenothiazine were designed and obtained by single-step synthesis, i.e., by Buchwald-Hartwig cross-coupling reactions of 1,3-dibromo-5-(trifluoromethyl)benzene with 3,7-di-tert-butyldimethylacridan, or 3,7-di-tert-butyl-10H-phenothiazine. The advantages in high thermal and photophysical characteristics were utilized in exciplex emission based OLEDs. In the optimal case, device showed maximum current, power, and external quantum efficiencies of 27.4 cd/A, 28.6 lm/W and 7.6%, respectively. Taking into account that successful employment of acridan or phenothiazine derivatives as donors in exciplex-forming systems remains scarce, the proposed design of donor compounds in this work shows their potential for successful implementation in complex emissive systems of OLEDs.

The efficient conversion of electron-hole (e-h) pairs into triplet excitons is a challenge for blue organic light-emitting diodes (OLEDs) based on triplet–triplet annihilation upconversion (TTAUC). The 25% fraction of e-h pairs that create singlet excitons represents a significant loss channel that can lead to parasitic red-shifted emission. In this study, an intermolecular intersystem crossing that relies on a “dark sensitizer” (DS) layer consisting of tris-(8-hydroxyquinoline)aluminum (Alq3) doped with tris[2-phenylpyridinato-C2,N]Iridium(III) (Ir(ppy)3) is demonstrated to enhance the TTAUC process. Carriers recombination and excitons generation are formed on Alq3 molecules. Alq3 singlet excitons are then quenched by the Ir(ppy)3 triplet state, followed by energy transfer to the Alq3 triplet state. The non-emissive, long-lived Alq3 triplets migrate from the sensitizer layer to an emitter layer where they undergo TTAUC to give blue fluorescence emission. This DS-TTAUC process promises no green emission from the Alq3. The efficiency of a blue OLED utilizing DS-TTAUC was improved by 34.2% compared to a standard TTAUC OLED. In addition, the device exhibited CIE coordinates of (0.15, 0.09). Furthermore, a high photoluminescence quantum yield fluorescence emitter is incorporated to enhance the singlet exciton emission in the emitter layer. A maximum external quantum efficiency of 7.54% can be achieved with recorded-high quantum yield of the TTAUC (ΦTTAUC) = 37.6% in solid state.

Highly efficient organic light emitting diodes based on thermally activated delayed fluorescence were fabricated using 9,9'-(sulfonylbis([1,1':3′,1″-terphenyl]-5′,2′-diyl))bis(9H-carbazole) as a host. The best device demonstrated high maximum external quantum efficiency of 23.3%, maximum current efficiency of 67.7 cd/A, maximum power efficiency of 60.9 lm/W with low efficiency roll-off. These characteristics were achieved due to the utilization of bipolar TADF host with tetraphenyl ornamenting. The molecular design strategy used allowed to obtain OLED host with the required combination of properties, i.e. triplet harvesting ability, perfect host→guest energy transfer, emission not affected by intermolecular interaction (no excimer/exciplex formation), high triplet level (2.98 eV), bipolar charge transporting properties, thermal stability up to sublimation temperature of 467 °C, amorphous forming properties with high glass transition temperature of 153 °C, good charge injecting properties (ionization potential of 5.9 eV and electron affinity of 2.8 eV). When the compound was used as TADF emitter, practically the same electroluminescent spectra peaked at 430 nm were obtained for deep-blue OLEDs with both non-doped and doped emitting layers. Such untypical electroluminescent behaviour of TADF OLEDs is attributed to the restricted by phenyl substituents changes of dihedral angels between donor and acceptor moieties and to reduced posibilities of the formation of the different conformers and aggregates in solid state.

When applying the voltage pulses (6 V) to the organic light-emitting diode based on tris(8-hydroxyquinolinato) aluminium (Alq 3 ) as the electron transporting layer, current density and luminance increased by 16% and 20%, respectively, by providing the reverse bias (−16 V) during the off-period. By using displacement current measurement, we can deduce that such an enhancement resulted from the interfacial positive charges trapped at the Alq 3 /cathode interface, with the relaxation time ∼0.4 ms. By doping the organic material as the carrier trapping sites at Alq 3 /cathode interface, such current density and luminance increase can be further enhanced. 25% and 36% increase in current density and luminance was demonstrated with such driving technique, respectively.

A emission mode based on triplet-triplet annihilation up conversion (TTAUC) was proposed as follows: when the electrons and holes recombined at the sensitizer layer with a lower energy bandgap, the triplet exciton of sensitizer can transfer the energy to the triplet-triplet annihilation (TTA) emitter and emit a higher energy blue photon. TTAUC-OLEDs had proved to have a higher device efficiency with tris(8-hydroxyquinolinato)aluminum (Alq3) as sensitizer and 9,10-Bis(2-naphthyl)anthraces (ADN) as TTA emitter. In this research, we employed Bis(10-hydroxybenzo[h]quinolinato)beryllium (Bebq2) to replace Alq3 which had a higher electron mobility and resulted in a better electrical properties. It showed a higher current density at constant voltages and the turn-on voltage (at 1 cd/m2) decreased 0.3V compared to Alq3 as sensitizer. Bebq2 as sensitizer achieved a higher efficiency with 3.26% in overall EQE, together with 1.91% and 1.35% in blue and green EQE at J=6.71 mA/cm2 compared to Alq3

DG acknowledges funding from the ERDF PostDoc project No. 1.1.1.2/VIAA/1/16/177 . This research is/was funded by the European Regional Development Fund according to the supported activity ‘ Research Projects Implemented by World-class Researcher Groups ’ under Measure No. 01.2.2-LMT-K-718 . Ministry of Science and Technology (MOST), Taiwan , Grant No. MOST 106-2923-E-155-002-MY3 . This work was also supported by the Ministry of Education and Science of Ukraine (projects no. 0117U003908 and 0118U003862 ), and by the Olle Engkvist Byggmästare foundation (contract No. 189-0223 ). The quantum-chemical calculations were performed with computational resources provided by the High Performance Computing Center North (HPC2N) in Umeå, Sweden, through the project ‘‘Multiphysics Modeling of Molecular Materials” SNIC 2018-2-38., Flexible versus rigid molecular structures of donor-acceptor-donor type compounds are investigated with respect to efficiency of thermally activated delayed fluorescence (TADF) by theoretical and experimental approaches. Three highly efficient TADF emitters based on flexible diphenylsulfone and rigid dibenzothiophene-dioxide as acceptor units and di-tert-butyldimethyldihydroacridine as donor moiety were designed and synthesized. Despite they showed similar singlet-triplet splitting (0.01–0.02 eV) and high photoluminescence quantum yields in appropriate hosts, maximum external quantum efficiencies as different as 24.1 and 15.9/19.4% were obtained for organic light emitting devices based on these emitters with, respectively, flexible and rigid molecular structures. The high efficiency of the light-emitting compounds with the flexible molecular structure could be traced to the bi-configurational nature of the lowest singlet and triplet states resulting in higher spin-orbit coupling than for molecules with rigid structures. All derivatives showed bipolar charge transport character. High device efficiency with electron mobility of 3 × 10−5 cm2V−1s−1 and hole mobility of 1.3 × 10−4 cm2V−1s−1 at the electric field of 5 × 105 Vcm−1 was recorded for the layer of para-disubstituted diphenylsulfone with flexible molecular structure. This TADF emitter showed an excellent performance in the organic light emitting device, exhibiting a maximum current efficiency, power efficiency, and external quantum efficiency of 61.1 cdA-1, 64.0 lmW−1, and 24.1%, respectively., Ministry of Education and Science of Ukraine 0117U003908,0118U003862; Stiftelsen Olle Engkvist Byggmästare 189-0223; European Regional Development Fund 1.1.1.2/VIAA/1/16/177; European Regional Development Fund 01.2.2-LMT-K-718; Ministry of Science and Technology (MOST), Taiwan MOST 106-2923-E-155-002-MY3; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

A corrugated indium zinc oxide thin film was fabricated on a glass substrate at room temperature for use in a blue phosphorescent organic light-emitting device (OLED) to facilitate device efficiency in term of the external quantum efficiency (EQE) of the device, which increased from 28.77% to 34.33% after the fabrication of the aforementioned layer. Furthermore, a remarkably high EQE of 52.51% was achieved through external light extraction by attaching a hemispheric macrolens on the substrate of the device. Consequently, the efficiency of the developed blue phosphorescent OLED became 1.83 times higher than that of a planar counterpart. Moreover, this finding was verified through theoretical analyses, that indicated the underlying light extraction physics was attributed to the efficient extraction of trapped light from the surface plasmon polariton, waveguide, and substrate modes. (submitted to Organic Electronics).

1,3,5-(N,N′,N′′)-Tricarbazolyl-2-(benzimidazol-2-yl)benzene (TCbzBz) has been developed as a host for the FIrpic doped blue phosphorescent organic light emitting diodes. The Y-shaped 1,3,5-tricarbazole array networks the host molecules to form continuous hole and electron hopping paths that provide an improved hole mobility of 2.0 × 10−5 cm2 V−1 s−1. Significant breakthroughs on the efficiency of 58.2 cd A−1 and 59.4 lm W−1 with 28% of external quantum efficiency are therefore achieved. Usually aggregation of the emitter molecules will lead to triplet–triplet quenching and significantly hamper the performance of the PhOLEDs. However, the present networked matrix is so spacious that it can accommodate up to 12% of the FIrpic without hampering the device performance. Nano-aggregation in the sublimed film is clearly evidenced by atomic force microscopy, powder X-ray diffractometry, and UV-Vis spectrophotometry.

A new emission mechanism was proposed to enhance the OLED operation lifetime by using exciplex emission to sensitize the triplet-triplet annihilation blue emission. 3-times improvement in operation lifetime was achieved due to the alleviation of electron accumulation at the interface of the hole-transporting layer and the emitting layer.

Organic light emitting diode (OLED) has lots of advantages in display technology such as self-emissive, light-weight, and compatible for flexible substrates. Compared to the red and green OLEDs, blue one has shorter lifetime due to high-energy polaron quenching. Triplet-triplet annihilation up conversion (TTAUC) is a promising way to reduce the driving voltage and improve the operation lifetime for a blue OLED. This system includes two materials, sensitizer and emitter, which has a narrower bandgap and triplet-triplet annihilation (TTA) characteristic, respectively. When the sensitizer is excited, its triplet exciton can transfer the energy to the triplet of emitter with lower triplet energy and fuse into one singlet exciton with a higher energy photon than the sensitizer. In this research, we demonstrate that a convention green fluorescent material, tris(8-hydroxyquinolinato)aluminum (Alq3) can be used as a sensitizer for a blue TTA emitter, 9,10-Bis(2-naphthyl)anthraces (ADN). In a conventional Alq3-based OLED, when the electron and hole coming from cathode and anode recombined at Alq3 as the recombination layer, 25% of singlet and 75% of triplet were generated. All triplet exciton experienced non-radiative recombination, which resulted in low efficiency due to the poor reverse intersystem crossing (RISC) rate. On the other hand, in TTAUC-OLED, triplet exciton of Alq3 (ET=2.0 eV) transferred the energy to the triplet of ADN (ET=1.67 eV) via Dexter energy transfer and two of them fused into one singlet (ES=2.83 eV) with blue emission. This recycled the useless triplet exciton and resulted in a higher external quantum efficiency (EQE) for TTAUC-OLED (2.1%) compared to the Alq3 (1.2%) and ADN (1.67%) control devices. Moreover, the recombination zone was shifted from ADN to Alq3, the operation lifetime of blue component can be increased by 3x times longer than the ADN control device. From transient electroluminescence (TrEL) measurement, Alq3-control device showed a fast decay within 1us which implied only singlet exciton involved. For the TTAUC-OLED, blue emission showed only delayed component which meant the emission came from only TTA process without direct recombination.

Organic light-emitting diodes (OLEDs) have been successfully applied as displays and also recognized as a next-generation lighting technology because their several advantages such as self-emission, high luminous efficiency, full-color capability, wide viewing angle, high contrast, low power consumption, low weight, large area manufacture, transparence and flexibility. Now, the red and green phosphorescent OLEDs are qualified for the commercial products. The bottleneck still is the blue OLED. Currently, the high efficiency (100% internal quantum efficiency, and over 20% external quantum efficiency, EQE) blue OLED can be achieved using phosphorescent and thermal activated delay fluorescent (TADF) emitters, which were the crucial factor to determine color purity, efficiency, and lifetime of device. Beside the emitters, the other important factor is the host material. In this talk, we synthesized several novel carbazole-based, benzimidazole-based, and their combined derivers to be the universal hosts with wide energy bandgap for red, green, blue phosphorescent emitters and also applied for green and blue TADF emitters. These novel host materials could be classified as electron transport, hole transport, and bipolar. The carrier dynamics in OLEDs with these host materials were investigated by probing the position of main recombination zone and optimizing the efficiency performance. With increasing electron mobility of host material, the main carrier recombination zone in emitting layer was moved from the ETL side to the HTL side. Finally, the high EQE of over 30% was achieved in green and blue phosphorescent and TADF OLEDs with our host system. In addition, the operational lifetime was also improved by using our host system, comparing that of OLED with commercial mCP host.

In this study, we demonstrated a blue phosphorescent organic light-emitting diode (BPOLED) based on a host with two carbazole and one trizole (2CbzTAZ) moiety, 9,9′-(2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)-1,3-phenylene)bis(9H-carbazole), that exhibits bipolar transport characteristics. Compared with the devices with a carbazole host (N,N’-dicarbazolyl-3,5-benzene, (mCP)), triazole host (3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole, (TAZ)), or a physical mixture of mCP:TAZ, which exhibit hole, electron, and bipolar transport characteristics, respectively, the BPOLED with the bipolar 2CbzTAZ host exhibited the lowest driving voltage (6.55 V at 10 mA/cm2), the highest efficiencies (maximum current efficiency of 52.25 cd/A and external quantum efficiency of 23.89%), and the lowest efficiency roll-off, when doped with bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) as blue phosphor. From analyses of light leakage of the emission spectra of electroluminescence, transient electroluminescence, and partially doped OLEDs, it was found that the recombination zone was well confined inside the emitting layer and the recombination rate was most efficient in a 2CbzTAZ-based OLED. For the other cases using mCP, TAZ, and mCP:TAZ as hosts, electrons and holes transported with different routes that resulted in carrier accumulation on different organic molecules and lowered the recombination rate.

The novel ambipolar hosts of o-CbzBz and o-DiCbzBz contain carbazole and benzimidazole through an ortho-connection. The orthogonal conformations cause the triplet state to be confined at the carbazole units to secure efficient energy transfer. The phosphorescent organic light-emitting diodes (PhOLEDs) show a high current efficiency, power efficiency, and low efficiency roll-off. o-DiCbzBz can be used as a host for sky-blue, green, and orange-red PhOLEDs, giving 57.5, 78.4, and 60.3 cd/A, respectively.

A high-efficiency blue phosphorescence organic light-emitting diode (OLED) with a new ambipolar organic compound, 9-(2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)-9H-carbazole (CbzTAZ) as the host, structurally engineered the emitting layer (EML) and electron-transporting layer (ETL) to achieve the great carrier balance in the EML. The chemical structure of CbzTAZ contains an electron-transporting 1,2,4-triazole segment and a hole-transporting carbazole ring. By scanning the composite ratio between the CbzTAZ host and the blue emitter iridium(III)bis[4,6-(di-fluorophenyl)-pyridinato-N,C2′]picolinate to be an efficient EML, and then by fine-tuning the layer thicknesses of the ETL and EML, a highly efficient blue PhOLED was obtained because of the improved electron-hole balance and broad recombination zone coverage in the EML. The blue PhOLED exhibited a maximum current efficiency $(\eta _{C})$ of 48 cd/A, a power efficiency $(\eta _{P})$ of 42.2 lm/W, and an external quantum efficiency $(\eta _{{\rm EQE}})$ of 22.9%. In addition, the device demonstrated a satisfactory efficiency roll-off (ηC of 41.1 cd/A, $\eta _{P}$ of 18.1 lm/W, and $\eta _{{\rm EQE}}$ of 19.8%) at a high brightness value of 1000 cd/m2.

In this paper, we demonstrated the changes of electrical and optical characteristics of a phosphorescent organic light-emitting device (OLED) with tris(phenylpyridine)iridium Ir(ppy)3 thin layer (4 nm) slightly codoped (1%) inside the emitting layer (EML) close to the cathode side. Such a thin layer helped for electron injection and transport from the electron transporting layer into the EML, which reduced the driving voltage (0.40 V at 100 mA/cm2). Electroluminescence (EL) spectral shift at different driving voltage was observed in our blue OLED with [(4,6-di-fluoropheny)-pyridinato-N,C2′]picolinate (FIrpic) emitter, which came from the recombination zone shift. With the incorporation of thin-codoped Ir(ppy)3, such EL spectral shift was almost undetectable (color coordinate shift (0.000, 0.001) from 100 to 10,000 cd/m2), due to the compensation of Ir(ppy)3 emission at low driving voltage. Such a methodology can be applied to a white OLED which stabilized the EL spectrum and the color coordinates ((0.012, 0.002) from 100 to 10,000 cd/m2).

An ambipolar material with a high triplet energy of 3.1 eV, 9-(2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)-9H-carbazole (CbzTAZ), was synthesized by conjugating an electron-transporting 1,2,4-triazole and a hole-transporting carbazole and employed as the host doping with blue emitter iridium(III)bis[4,6-(difluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic) to be the emitting layer (EML) of an efficient blue phosphorescent organic light-emitting diode (PhOLED). By adjusting the ratio between CbzTAZ and FIrpic, an efficient OLED performance with a current efficiency (ηC) of 50 cd/A and an external quantum efficiency (ηEQE) of 23.8% can be obtained. The position of the recombination zone located inside the EML close to the electron-transporting layer (ETL) can be detected by partially doping FIrpic in one of the three EML regions. By fine-tuning the thickness of the ETL to balance electrons and holes in the EML, a highly efficient blue PhOLED with a maximum ηC of 52.1 cd/A, a maximum power efficiency of 46...

Blue phosphorescent organic light-emitting diodes (PhOLEDs) was successfully fabricated by using newly imidazole-based host doped with bis [2-(4’,6’-difluoro)phenylpyridinato- N,C2’]iridium(III) picolinate with 57.24 cd/A, 50.42 lm/W and 25.67 % in external quantum efficiency.

Operation lifetime of a blue OLED exhibiting triplet-triplet annihilation increased by 89.3% with the incorporation of 10% assistant host in the emitting layer, which increased diffusion length of singlet exciton, reduced the quenching of triplet exciton, and blocked the triplet pair diffusion.

Strongly medium dependent capability to exhibit not only prompt/delayed thermally-activated fluorescence but also aggregation-induced phosphorescence or exciplex emission is demonstrated either for solutions or for solid samples of acridan-based luminophores. Multicolour emissions of different nature were detected and studied for donor-acceptor derivatives of 2- or 4-(trifluoromethyl)benzene and 2,7-di-tert-butyl-9,9-dimethylacridine containing trifluoromethyl groups in ortho and para positions. For the demonstration of one of possible applications, both compounds were tested in the emissive layers of exciplex-based OLEDs. In the best case, their high maximum current, power and external quantum efficiences of 29.5 cd/A, 29.1 lm/W and 8.2% respectively were achieved for the device based on exciplex electroluminescence resulting from recombination of intermolecular charge transfer states formed on the interface of electron-donating and electron-accepting molecules used.

Two novel bipolar carbzol-triazole derivatives was synthesized by conjugating hole transporting moiety carbazole (Cbz) with electron transporting moiety triazole (TAZ) to achieve the bipolar molecules, which were employed as the hosts of emitting layer doped with blue emitter FIrpic inside blue phosphorescence OLEDs with high current efficiency of 50 and 50.6 cd/A, and EQE of 20% and 21.6%.

Under white ambient illumination and without bias, a reflective organic light-emitting device (ROLED) comprising a microcavity cathode exhibited various colors for static information display applications by means of internal interference and absorption effects. The configuration of this microcavity cathode was a metal/organometallic/metal structure of Al (10 nm)/Ag (15 nm)/Ag nanoparticles doped inside tris(8-hydroxyquinolinato) aluminum (Alq3) (x nm)/Al (100 nm) with excellent conductivity. The thickness of the Ag:Alq3 played a crucial role in determining the reflection color; for example, varying it from 20, 40, 60, 80 and 100 nm yielded the colors light yellow, light orange, reddish purple, greenish blue, and light green, respectively. In the dark, this ROLED can be used to display information with an ultra-high contrast ratio by applying on a small bias, like conventional OLED displays. Hence, this ROLED is a highly promising candidate for applications in energy-saving electronic fixed-pattern signs, logos, indicators, and manual information displays.

The fabrication of a hybrid device architecture fully integrating a transparent organic light-emitting diode (OLED) and a liquid crystal display (LCD) within two glass substrates is reported in this study. The transparent OLED was fabricated on the inner surface of the glass substrate. Twisted nematic liquid crystal (LC) materials were used to fill the space between the two glass substrates. The OLED was driven by an indium-tin oxide (ITO) anode on the glass substrate and a thin bi-metal (Al/Ag) cathode, which also served as the electrode of the LCD. The other electrode for the LCD-mode operation was the ITO on the other glass substrate. A commercially available ultraviolet (UV)-curable resin was spun onto the thin Al/Ag as the passivation layer to protect the OLED from attacks by the following polyimide layer (serving as the alignment layer of the LCD), rubbing process and LC materials. In this device structure, the electrical characteristic of the OLED-mode operation was almost the same as that of the control device. Current efficiency (in terms of cd/A) of the hybrid device from top-emission (towards the LCD) decreased by 26.5% due to optical interference effect, whereas efficiency from bottom-emission remained the same. The driving voltage of the LCD-mode operation increased by 1.6 V due to the insertion of the passivation layer between the two electrodes. The contrast ratio decreased from 150 to 25 due to the reflection of the thin Al/Ag layer. Compared with that of the control device, the storage lifetime of the OLED increased as a result of filling the encapsulated cavity with LC materials, which helped repel ambient water and oxygen.

We have demonstrated a vertically stacked device consisting of organic photovoltaic device (OPV) and organic light-emitting diode (OLED) inside a polymer dispersed liquid crystal (PDLC) cell. In such a device, OLED and PDLC acted as transmissive- (T-) and reflective- (R-) mode, respectively, of a transflective display without the tradeoff of aperture ratio between R- and T-modes in a conventional transflective LC display. OPV functioned as the black background for PDLC operation and increased the contrast ratio of OLED. Electrical characteristics of OLED and OPV in the stacked device were nearly identical to their individual device, which showed the fabrication process had little effect on OLED and OPV performances. However, their optical properties showed a little degradation, coming from the multi-layer structure and hence the suitable optical cavity design was needed. Storage lifetime of OLED increased in the stacked device because LC material helped to prevent the water and oxygen attack. Driving voltage of PDLC increased due to the insertion of passivation layer upon the electrode which was used protect the OLED and OPV underneath.

Ambipolar triplet hosts comprising 1,2,4-triazole and carbazole in ortho-positions have been developed. The blue PHOLED has a high current efficiency of 47.1 cd A(-1), power efficiency of 41.2 lm W(-1), and low efficiency roll-off. The high efficiency was attributed to the successful control of π-conjugation through orthogonal arrangement of the substituents so that a wide T1-S0 gap could be maintained.

In this report, we demonstrated a series of blue phosphorescent organic light-emitting diodes (OLED) using the mixed host as the partial emitting layer to achieve ultrahigh external quantum efficiency over 30%. The blue phosphorescent emitter was FIrpic and these host materials was developed by Professor Leung in Department of Chemistry, National Taiwan University. By well optimizing the doping ratio among hosts and FIrpic, and designing the device structure to balance carrier in the emitting layer, a ultrahigh efficiency blue OLED could be obtained with external quantum efficiency over 30%.

We demonstrated device performances of exciplex organic light emitting diodes (OLEDs) by using 4, 4′, 4”-tris(N-3-methyphenyl-N-phenyl-amino) triphenylamine (m-MTDATA) and 4,7-diphenyl-1,10-phenanthroline (Bphen) as electron donor and acceptor materials, respectively with different emitting layer (EML) thicknesses. The maximum current efficiency and external quantum efficiency (EQE) of the optimized OLEDs are 11.3 cd/A and 4.02%, respectively. Transient electroluminescence (TrEL) was used to understand the carrier and exciton dynamics in the OLEDs. It was found that the thinner EML exhibited a shorter decay time in such devices.

The two novel benzimidazole/carbazole hybrid ambipolar molecules, o-CbzDiBz and m-CbzDiBz, were synthesized as hosts for blue PHOLEDs. Bis[2-(4,6-diiluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic)-based devices can achieve 41.86 and 49.31 cd/A of current efficiency in o-CbzDiBz and m-CbzDiBz matrix, respectively.

We demonstrated a blue phosphorescent organic light-emitting diode with the maximum current efficiency and power efficiency of 57.4 cd/A and 51.6 lm/W, respectively, based on trizaole derivatives as the host of the emitting layer.

We reported a blue phosphorescent organic light-emitting diode (OLED) with the highest current efficiency of 55.2 cd/A, power efficiency of 50.5 lm/W and external quantum efficiency (EQE) of 24.8%, by using a tetraphenylsilane host in the emitting layer doped with blue emitter, bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(in) (FIrpic).

A new organic host material, PCT comprising pyridine, carbazole and triazole moieties, was doped with a blue emitter bis[2-(4,6"difluorophenyl)pyridinato-C2,N](pico]inato)iridium(III) (FIrpic) to be an emitting layer (EML) of a phosphorescent organic light-emitting diodes device (PhOLED). With various FIrpic concentration (12%, 15% and 18% by volume ratio) in the EML, a series of high efficiency blue PhOLED was demonstrated showing the external quantum efficiency over 20%.

The coupling efficiency of OLED can be increased by centered-hollow micro-lens array films with pixel-partition. While the number of pixel-partition increases, the total internal reflection of adjacent centered-hollow arrangement occurs. We provide an additional coupling path through triangle grooves on the substrate. The results show that the luminance efficiency enhancement at normal direction is improved from 63% to 116%.