Small Efficiency Roll-off Research Articles

Solution-processable cyclometalated gold(III) complexes for high-brightness phosphorescent white organic light-emitting devices

We report two yellow-emitting cyclometalated gold(III) complexes serving as phosphorescent dopants for high-brightness solution-processed white organic light-emitting devices (WOLEDs). Dichromatic WOLEDs consisting of our newly synthesized Au–Y or Au–wY for yellow emission in combination with FIrpic for blue emission have been studied. Notably, white emissions in terms of Commission Internationale de L’Eclairage coordinates of (0.31, 0.39) and (0.40, 0.47) and color rendering indices of 66 and 69 have been accomplished with Au–Y- and Au–wY-based WOLEDs. In addition, current efficiencies of 26.0 and 25.7 cd/A corresponding to external quantum efficiencies of 11.0% and 10.5% are achieved and can be maintained at 19.8 and 25.3 cd/A at a high brightness level of 10,000 cd/m2 for these devices. Such high-performance WOLEDs in terms of small efficiency roll-offs over a wide brightness range are believed to be due to the use of gold(III) complexes with very short-lived emission lifetimes of ~ 1 μs, high photoluminescence quantum yields of up to 86% and broad yellow emission bandwidths of up to 107 nm. This study opens up the possibility of using solution-processable cyclometalated organogold(III) complexes with short-lived emissive excited states for the fabrication of WOLEDs, especially for high-brightness applications.

High-efficiency near-infrared fluorescent organic light-emitting diodes with small efficiency roll-off based on AIE-active phenanthro[9,10-d]imidazole derivatives

PIBz-3-PTZ exhibits strong fluorescence with a photoluminescence quantum yield of 35% in films. The nondoped device achieves a maximum external quantum efficiency of 2.02% peaking at 672 nm with small efficiency roll-off.

Efficient Nondoped Pure Blue Organic Light-Emitting Diodes Based on an Anthracene and 9,9-Diphenyl-9,10-dihydroacridine Derivative.

Organic light-emitting diodes (OLEDs) have been greatly developed in recent years owing to their abundant advantages for full-color displays and general-purpose lightings. Blue emitters not only provide one of the primary colors of the RGB (red, green and blue) display system to reduce the power consumption of OLEDs, but are able able to generate light of all colors, including blue, green, red, and white by energy transfer processes in devices. However, it remains a challenge to achieve high-performance blue electroluminescence, especially for nondoped devices. In this paper, we report a blue light emitting molecule, DPAC-AnPCN, which consists of 9,9-diphenyl-9,10-dihydroacridine and p-benzonitrile substituted anthracene moieties. The asymmetrically decoration on anthracene with different groups on its 9 and 10 positions combines the merits of the respective constructing units and endows DPAC-AnPCN with pure blue emission, high solid-state efficiency, good thermal stability and appropriate HOMO and LUMO energy levels. Furthermore, DPAC-AnPCN can be applied in a nondoped device to effectively reduce the fabrication complexity and cost. The nondoped device exhibits pure blue electroluminescence (EL) locating at 464 nm with CIE coordinates of (0.15, 0.15). Moreover, it maintains high efficiency at relatively high luminescence. The maximum external quantum efficiency (EQE) reaches 6.04 % and still remains 5.31 % at the luminance of 1000 cd m-2 showing a very small efficiency roll-off.

Novel blue fluorescent materials for high-performance nondoped blue OLEDs and hybrid pure white OLEDs with ultrahigh color rendering index

Pure blue luminogens that can be applied in high-efficiency nondoped blue organic light-emitting diodes (OLEDs) and act as main component to generate white OLEDs with high color rendering index (CRI) simultaneously are rarely reported. Herein, two blue emitters, TPAATPE and PPIATPE, consisting of triphenylamine/phenanthroimidazole and tetraphenylethene-substituted anthracene with asymmetric structures are designed and synthesized. The nondoped OLED using TPAATPE as the emitter exhibits pure blue emission with the maximum external quantum efficiency (EQE) of 6.97% and Commission International de L'Eclairage (CIE) coordinates of (0.15, 0.16). Employing TPAATPE as the blue-emitting component, combined with a thermally activated delayed fluorescent (TADF) molecule PTZMes2B, which is adopted as the green emitter and the host for phosphors to modulate the long wavelength emission, a series of highly efficient hybrid white OLEDs are successfully achieved. Among them, the two-color white OLED exhibits eye-friendly warm white light with a maximum forward-viewing EQE of 25.2%. In particular, the three-color white OLED achieves pure white emission with CIE coordinates of (0.34, 0.38), a maximum forward-viewing EQE of 25.3% and an ultrahigh CRI of 92. All of nondoped blue OLEDs and hybrid white OLEDs exhibit very small efficiency roll-offs with excellent color stabilities. To the best of our knowledge, these results are among the best outcomes for white OLEDs reported so far.

Organometallic Ir(III) Phosphors Decorated by Carbazole/Diphenylphosphoryl Units for Efficient Solution-Processable OLEDs with Low Efficiency Roll-Offs.

Recently, solution-processable PhOLEDs have been attracting great interest for their low cost and high productivity relative to the vacuum-deposited devices. Similar to vacuum-deposited OLEDs, however, they usually suffer from serious efficiency roll-offs, especially in high brightness. Finding a feasible way and/or designing novel materials to increase efficiencies and reduce roll-offs simultaneously are highly desired. Herein, a new family of solution-processable cyclometalated iridium(III) phosphors with carbazole (Cz) and/or diphenylphosphoryl (Ph2PO) units functionalized main ligands has been designed. Owing to Cz and Ph2PO moieties possessing bulky steric effects, they can suppress the intermolecular strong packing and then decrease TTA effects and emission quenching. Meanwhile, the resulting OLEDs based on the designed phosphors exhibit considerable efficiencies and relatively small efficiency roll-offs. The device based on 4 containing both Cz and Ph2PO units realized a maximum current efficiency of 21.3 cd A-1, accompanied by a small roll-off. By optimization of the configuration of OLEDs, the device performance can be further enhanced, demonstrating their potential for high-performance solution-processable PhOLEDs.

Phthalonitrile-based bipolar host for efficient green to red phosphorescent and TADF OLEDs

A bipolar host material named 4-(9H-carbazol-9-yl)phthalonitrile (CzPN) was developed by combining carbazole and phthalonitrile as donor and acceptor units, respectively. The introduction of the phthalonitrile unit realized a relatively small energy gap of 3.00 eV as well as a deep lowest unoccupied molecular orbital level of −2.72 eV. The characterization of carrier-only devices demonstrated the bipolar charge transporting property of CzPN. With a triplet energy of 2.70 eV, CzPN was investigated as host in green, yellow, and red phosphorescent organic light-emitting diodes (PhOLEDs) and green thermally activated delayed fluorescence (TADF) devices. A low turn-on voltage of 2.8 V was achieved by all the four diodes. Both PhOLEDs and TADF devices exhibited high external quantum efficiency values over 20% with relatively small efficiency roll-offs. These findings demonstrate that CzPN is an excellent host material for both phosphorescent and TADF emitters, and phthalonitrile is a promising unit to develop universal host materials.

Realization of Highly Efficient Red Phosphorescence from Bis-Tridentate Iridium(III) Phosphors.

Bis-tridentate Ir(III) metal complexes bring forth interesting photophysical properties, among which the orthogonal arranged, planar tridentate chelates could increase the emission efficiency due to the greater rigidity and, in the meantime, allow strong interligand stacking that could deteriorate the emission efficiency. We bypassed this hurdle by design of five bis-tridentate Ir(III) complexes (1-5), to which both of their monoanionic ancillary and dianionic chromophoric chelate were functionalized derivative of 2-pyrazolyl-6-phenylpyridine, i.e. pzpyphH2 parent chelate. Hence, addition of phenyl substituent to the pyrazolyl fragment of pzpyphH2 gave rise to the precursors of monoanionic chelate (A1H-A3H), on which the additional tert-butyl and/or methoxy groups were introduced at the selected positions for tuning their steric and electronic properties, while precursors of dianionic chelates was judiciously prepared with an isoquniolinyl central unit on pziqphH2 in giving the red-shifted emission (cf. L1H2 and L2H2). Factors affected their photophysical properties were discussed by theoretical methods based on DFT and TD-DFT calculation, confirming that the T1 excited state of all investigated Ir(III) complexes shows a mixed metal-to-ligand charge transfer (MLCT), intraligand charge transfer (ILCT), ligand-to-ligand charge transfer (LLCT), and ligand-centered (LC) transition character. In contrast, the poor quantum yield of 3 is due to the facilitation of the nonradiative decay in comparison to the radiative process. As for potential OLED applications, Ir(III) complex 2 gives superior performance with max. efficiencies of 28.17%, 41.25 cd·A-1 and 37.03 lm·W-1, CIEx,y = 0.63, 0.37 at 50 mA cm-2, and small efficiency roll-off.

Universal Bipolar Host Materials for Blue, Green, and Red Phosphorescent OLEDs with Excellent Efficiencies and Small-Efficiency Roll-Off.

Host materials are indispensable for the fabrication of organic light-emitting diodes (OLEDs) with phosphorescent emitters, but high-quality host materials that can efficiently and simultaneously function in blue, green, and red phosphorescent OLEDs (PHOLEDs) are much rare. In this work, four bipolar materials are developed using carbazole and 9,9-dimethyl-9,10-dihydroacridine as hole-transporting groups, pyridine as electron-transporting groups, and biphenyl and m-methylbiphenyl as π-spacers. The crystal and electronic structures indicate that these materials have highly twisted conformations, which endow them with aggregation-induced emission features, intramolecular charge transfer processes, wide energy band gaps, and high triplet energies. The carrier transport ability and energy transfer property analyses show that these materials are able to achieve balanced hole and electron transports and can serve as bipolar host materials for PHOLEDs. A series of monochromatic PHOLEDs with different phosphorescent dopants, including blue-emissive FIrpic, green-emissive Ir(ppy)2(acac), and red-emissive Ir(piq)2(acac), are fabricated by employing these four host materials. The green PHOLEDs can provide an impressive luminance of up to 230 200 cd m-2. Based on an identical host material, excellent external quantum efficiencies as high as 25.12, 24.73, and 19.71%, as well as minor efficiency roll-off, are attained for blue, green, and red PHOLEDs, respectively, clearly demonstrating the promising applications as universal bipolar host materials in PHOLEDs with monochromatic light and white light.

Phenothiazinen-Dimesitylarylborane-Based Thermally Activated Delayed Fluorescence: High-Performance Non-doped OLEDs With Reduced Efficiency Roll-Off at High Luminescence.

We report a phenothiazinen-dimesitylarylborane thermally activated delayed fluorescence (TADF) molecule that exhibits high external quantum efficiency (EQE) in non-doped organic light-emitting diodes (OLEDs) at high luminescence. The non-doped device shows green electroluminescence with an emission peak of 540 nm and a maximum EQE of 19.66% obtained at a luminescence of ~170 cd m−2. The EQE is still as high as 17.31% at a high luminescence of 1,500 cd m−2 with small efficiency roll-off.

Thiophene Disubstituted Benzothiadiazole Derivatives: An Effective Planarization Strategy Toward Deep-Red to Near-Infrared (NIR) Organic Light-Emitting Diodes.

As one of the three primary colors that are indispensable in full-color displays, the development of red emitters is far behind the blue and green ones. Here, three novel orange-yellow to near-infrared (NIR) emitters based on 5,6-difluorobenzo[c][1,2,5]thiadiazole (BTDF) namely BTDF-TPA, BTDF-TTPA, and BTDF-TtTPA were designed and synthesized. Density functional theory analysis and photophysical characterization reveal that these three materials possess hybridized local and charge-transfer (HLCT) state feature and a feasible reverse intersystem crossing (RISC) from the high-lying triplet state to the singlet state may conduce to an exciton utilization exceeding the limit of 25% of traditional fluorescence materials under electrical excitation. The insertion of thiophene with small steric hindrance as π-bridge between the electron-donating (D) moiety triphenylamine (TPA) and the electron-accepting (A) moiety BTDF not only results in a remarkable 67 nm red-shift of the emission peak but also brings about a large overlap of frontier molecular orbitals to guarantee high radiative transition rate that is of great significance to obtain high photoluminescence quantum yield (PLQY) in the “energy-gap law” dominated long-wavelength emission region. Consequently, an attractive high maximum external quantum efficiency (EQE) of 5.75% was achieved for the doped devices based on these thiophene π-bridged emitters, giving a deep-red emission with small efficiency roll-off. Remarkably, NIR emission could be obtained for the non-doped devices, achieving an excellent maximum EQE of 1.44% and Commission Internationale de l'Éclairage (CIE) coordinates of (0.71, 0.29). These results are among the highest efficiencies in the reported deep-red to NIR fluorescent OLEDs and offer a new π-bridge design strategy in D-π-A and D-π-A-π-D red emitter design.

New Aggregation-Induced Delayed Fluorescence Luminogens With Through-Space Charge Transfer for Efficient Non-doped OLEDs.

In this work, two tailor-made luminogens comprising of electron donors (acridine and phenoxazine) and acceptor (triazine) bridged by the through-space conjugated hexaphenylbenzene (HPB) are synthesized and characterized. Their thermal stability, electrochemical behaviors, crystal, and electronic structures, and photophysical properties are systematically investigated. The crystal and electronic structures reveal that the peripheral phenyls in HPB are closely aligned in a propeller-like fashion, rendering efficient through-space charge transfer between donor and electron moieties. These molecules display weak fluorescence with negligible delayed component in solutions but strong fluorescence with greatly increased delayed component upon aggregate formation, namely aggregation-induced delayed fluorescence (AIDF). Their neat films exhibit high photoluminescence quantum yields (PLQY), and prominent delayed fluorescence. The non-doped organic light-emitting diodes (OLEDs) based on these new luminogens exhibit excellent performance with maximum external quantum efficiency of 12.7% and very small efficiency roll-off of 2.7% at 1,000 cd m−2. Designing AIDF molecules with through-space charge transfer could be a promising strategy to explore robust luminescent materials for efficient non-doped OLEDs.

Nondoped deep-blue fluorescent organic electroluminescent device with CIEy = 0.06 and low efficiency roll-off based on carbazole/oxadiazole derivatives

Abstract Deep-blue organic light-emitting diodes (OLEDs) with the Commission Internationale de L'e'clairage (CIE) coordinates matching the European Broadcasting Union (EBU) standard blue CIE (x, y) coordinates of (0.15, 0.06) are very limited and in enormous demands. Here, we designed and synthesized four blue-emitting compounds with D-A structures based on carbazole and oxadiazole units. After the systematic studies of their photophysical, thermal and electrochemical properties, it is found that their photoluminescent properties were tuned successfully via changing the electronic structures of Ar group connecting with the oxadiazole and they all have high photoluminescent quantum yields (PLQYs). Then we selected the outstanding one as the emitter to prepare nondoped deep-blue OLEDs. The devices have maximum brightness up to 4406 cd m−2 and high maximum external quantum efficiency (EQE) up to 4.0% along with very small efficiency roll-off. Furthermore, the CIE coordinates of (0.16, 0.06) almost reaches the EBU standard blue. The comprehensive performance of the device is competitive among the best reported nondoped deep-blue fluorescent OLEDs with EBU standard blue CIE coordinates.

Efficient Non-doped Blue Fluorescent Organic Light-Emitting Diodes Based on Anthracene-Triphenylethylene Derivatives.

The development of efficient blue materials has been a continuous research topic in the field of organic light-emitting diodes (OLEDs). In this paper, three aggregation-induced emission enhancement active blue emitters, PIAnTPE, TPAAnTPE and CzAnTPE, are successfully synthesized by attaching a triphenylethylene unit and phenanthroimidazole/triphenylamine/carbazole moieties to the 9,10-positions of anthracene, respectively. The three compounds exhibit good thermal stabilities, appropriate for the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels and display high photoluminescence quantum yields (PLQYs) of 65, 70 and 46 % in the solid state. Non-doped blue devices using PIAnTPE, TPAAnTPE and CzAnTPE as the emitting layers show good electroluminescent performances, with the maximum external quantum efficiencies (EQEs) of 4.46, 4.13 and 4.04 %, respectively. More importantly, EQEs of all the three devices can be still retained when the luminescence reaches 1000 cd m-2 , exhibiting quite small efficiency roll-offs in the non-doped OLEDs.

Robust luminescent small molecules with aggregation-induced delayed fluorescence for efficient solution-processed OLEDs

New luminescent materials with aggregation-induced delayed fluorescence are developed and used to fabricate efficient solution-processed OLEDs with an extremely small efficiency roll-off.

Aggregation-Induced Delayed Fluorescence Luminogens for Efficient Organic Light-Emitting Diodes.

Aggregation-induced delayed fluorescence (AIDF) can be regarded as a special case of aggregation-induced emission (AIE). Luminogens with AIDF can simultaneously emit strongly in solid state and fully utilize the singlet and triplet excitons in organic light-emitting diodes (OLEDs). In this work, two new AIDF luminogens, DMF-BP-DMAC and DPF-BP-DMAC, with an asymmetric D-A-D' structure, are designed and synthesized. The characteristics of both luminogens are systematically investigated, including single crystal structures, theoretical calculations, photophysical properties and thermal stabilities. Inspired by their AIDF nature, the green-emission non-doped OLEDs based on them are fabricated, which afford good electroluminescence performances, with low turn-on voltages of 2.8 V, high luminance of 52560 cd m-2 , high efficiencies of up to 14.4 %, 42.3 cd A-1 and 30.2 lm W-1 , and very small efficiency roll-off. The results strongly indicate the bright future of non-doped OLEDs on the basis of robust AIDF luminogens.

In Situ Generation of Red-Emissive AIEgens from Commercial Sources for Nondoped OLEDs.

Luminescent materials with red emission are promising materials for optoelectronic devices and biological sciences. However, their synthesis is often complicated. In this work, we developed a facile approach to generating red-emissive luminogens with aggregation-induced emission (AIE) characteristics from commercially available reactants. The new compounds, abbreviated PN-BTZ-Cz and DP-BTZ-Cz, were prepared by coupling 4,7-dibromobenzo[c][1,2,5]thiadiazole with (1-naphehyl)phenylamine or diphenylamine followed by the reaction of the monobromo-substituted compounds with carbazole. Although all of the reactants were non-AIE-active, PN-BTZ-Cz and DP-BTZ-Cz showed AIE characteristics. This suggested that the AIE chromophores were generated in situ through the reactions. PN-BTZ-Cz and DP-BTZ-Cz were soluble in common organic solvents and showed red emission with high fluorescence quantum yields of 42.2 and 38.3% in the film, respectively. They were thermally and morphologically stable, as revealed by their high decomposition temperature (up to 327 °C) and glass-transition temperature (up to 120 °C). Nondoped organic light-emitting diodes with a configuration of ITO/HATCN/TAPC/PN-BTZ-Cz or DP-BTZ-Cz/Bphen/Liq/Al were fabricated using these compounds as emitting layers, which emitted red electroluminescence at a low turn-on voltage (down to 2.8 V) with a maximum external quantum yield of up to 2.7% and a small efficiency roll-off.

Dipole Moment in the Excited State: An Important Property for TADF Hosts

In this issue of Chem, Xu and co-workers report highly efficient blue organic light-emitting diodes with small-efficiency roll-off by reducing the dipole moment of the host materials in the excited state. Their simple but effective molecular design introduces a σ-bridge into ambipolar host materials as a separator of π-conjugation.

Highly efficient blue, orange and red PhOLEDs with low roll-off of efficiency using a carbazole dendritic thermally activated delayed fluorescence (TADF) material as host

Numerous efforts have been devoted to improve the device efficiency of phosphorescent organic light-emitting diodes (PhOLEDs), however, strategies to suppress the device efficiency roll-off are still in urgent need. Here, a general and effective approach to boost the efficiency and suppress the efficiency roll-off of PhOLEDs is proposed, that is, utilizing TADF material with high triplet energy (ET) as the host for blue, orange and red phosphorescent dopants is proposed. In this work, we investigate the performance of these PhOLEDs based a new TADF host phenylbis(4-(3,3″,6,6″-tetra-tert-butyl-9′H-[9,3′:6′,9″-tercarbazol]-9′-yl)phenyl)phosphineoxide (TPPOCz). The blue device based on the TPPOCz host exhibit efficiencies of 43.6 cd A−1 and 20.4%, and small efficiency roll-off of 2.1% and 2.9% for CE and EQE from the peak efficiencies to these at 1000 cd m−2. We compared the performances with that reported in literature, and confirmed these devices in this work are among the best of PhOLEDs. These results can be ascribed to the rapid reverse intersystem crossing (RISC) process and high triplet energy (ET). These excellent performances indicating the potential of TADF host for the realization of highly efficient and stable devices in future.

High-Efficiency Organic Light-Emitting Diodes Based on Sublimable Cationic Iridium(III) Complexes with Sterically Hindered Spacers

Cationic iridium(III) complexes show great promise as phosphorescent materials, while their utilization in organic light-emitting diodes is severely hindered by the inferior sublimability and low device performance. Here we devise a judicious strategy to develop high-efficiency sublimable cationic iridium(III) complexes by simultaneously introducing sterically hindered spacers into the negative counterions, major and ancillary ligands. We have exploited a novel series of yellow and red emitters, investigated their photophysical properties, electrochemical behaviors, and thermal stabilities, and finally fabricated organic light-emitting diodes by vacuum evaporation deposition. Record-high device performance has been achieved with a superior external quantum efficiency of 16%, excellent power efficiency of 49 lm/W, maximum brightness over 27.3 × 103 cd/m2, very low turn-on voltage below 2.5 V, and quite small efficiency roll-off. Our study represents a significant advance in the development of sublimable ca...

Highly luminescent phosphine oxide-containing bipolar alkynylgold(iii) complexes for solution-processable organic light-emitting devices with small efficiency roll-offs.

We report the synthesis of alkynylgold(iii) complexes with an electron-transporting phosphine oxide moiety in the tridentate ligand and hole-transporting triarylamine moieties as auxiliary ligands to generate a new class of phosphine oxide-containing bipolar gold(iii) complexes for the first time. Such gold(iii) complexes feature high photoluminescence quantum yields of over 70% in 1,3-bis(N-carbazolyl)benzene thin films with relatively short excited-state lifetimes of less than 3.9 μs at a 20 wt% dopant concentration. Highly efficient solution-processable organic light-emitting devices have been prepared with superior current efficiencies of up to 51.6 cd A-1 and external quantum efficiencies of up to 15.3%. Notably, triplet-triplet annihilation has been significantly reduced, as exemplified by a very small efficiency roll-off of ∼1% at a practical brightness of 500 cd m-2.