Deep-blue Organic Light-emitting Diodes Research Articles

Spatial separation of a TADF sensitizer and fluorescent emitter with a core-dendron system to block the energy loss in deep blue organic light-emitting diodes

Spatial separation of the TADF sensitizer and the fluorescent emitter improve the energy transfer efficiency by suppressing the energy loss and further enhance the electroluminescent performance.

Highly efficient deep-blue organic light-emitting diodes based on pyreno[4,5-d]imidazole-anthracene structural isomers

By integrating PyI with anthracene, two high-efficiency blue emitters C-BPyIA and N-BPyIA are obtained. Especially, N-BPyIA exhibits decent device performance with an EQEmax of 7.67% in the deep blue region.

A novel bipolar carbazole/ phenanthroimidazole derivative for high efficiency nondoped deep-blue organic light-emitting diodes

Abstract It is still a matter of worldwide continuous concern about how to obtain efficient, stable, pure blue light emitting materials and apply them in industrialized OLED devices. Here, a novel deep blue fluorescent material 2-(4'-(9H-carbazol-9-yl)-[1,1′-biphenyl]-4-yl)-1-(4-methoxyphenyl)-1H-phenanthro [9,10-d]imidazole(CzB-MOPPI) was firstly designed, synthesized and employed as emitters for deep-blue OLEDs. CzB-MOPPI, with carbazole as donor, twisted biphenyl bonded phenanthroimidazole which contains rigid plane structure as acceptor, exhibits high thermal stability with decomposition temperature (Td) of 480 °C. Weak donor carbazole unit and twist biphenyl both make the compound emit less red shift meanwhile maintain in deep blue region. The non-doped OLEDs based on the CzB-MOPPI as emitter show maximum emission at 435 nm, maximum luminance of 6450 cd/m2, maximum current efficiency (CE) of 3.34 cd/A. The chromaticity coordinate is (0.16, 0.08) which is extremely close to the NTSC standard blue. In Particular, the devices show an impressive external quantum efficiency (EQE) as high as 5.97%.

Polarity tuning of fluorene derivatives by chromophores to achieve efficient blue electroluminescent materials

Abstract Highly efficient deep-blue organic light-emitting diode is indispensable to fabricate energy efficient high quality displays and solid-state lightings. In this report, we report a series of fluorene-based emitters, which exhibit tunable polarity dependent on the choice of chromophore. The absorption and emission spectra of the compounds are highly dependent on the nature of chromophore and its contribution to the conjugation or donor-acceptor interaction. The solvatochromism of the dyes in the fluorescence is indicative of the realization of polar excited state, which is dependent on the donor strength of chromophore. Similarly, the oxidation potentials of the dyes are reminiscent of the electron-richness of the chromophores. The dyes are thermally robust comparable to the prominent electroluminescent materials and possess thermal decomposition temperatures greater than 340 °C which increases with the conjugation. The OLED device containing an emitter derived from triphenylamine showed a maximum power efficiency of 5.6 lm/W, maximum external quantum efficiency of 4.2%, and maximum luminance of 4150 cd/m2 with Commission Internationale de l’Eclairage (CIE) coordinates of (0.15, 0.11). The work demonstrates the use of simple chromophores to achieve efficient electroluminescent materials.

Enabling a 6.5% External Quantum Efficiency Deep-Blue Organic Light-Emitting Diode with a Solution-Processable Carbazole-Based Emitter

Highly efficient deep-blue emission is crucial to realize energy-efficient and high-quality display and solid-state lighting applications. A solution-processable deep-blue emitter is essential for producing cost-effective large-area devices via roll-to-roll fabrication. Here, we demonstrate a highly efficient solution-processable deep-blue organic light-emitting diode by utilizing a carbazole-based fluorescent emitter 6-((9,9-dibutyl-7-((7-cyano-9-(2-ethylhexyl)-9H-carbazol-2-yl)ethynyl)-9H-fluoren-2-yl)ethynyl)-9-(2-ethylhexyl)-9H-carbazole-2-carbonitrile (JV55). The resultant device showed a maximum external quantum efficiency (EQE) of 6.5% and an EQE of 5.5% at 100 cd/m2 with CIE coordinates of (0.16, 0.06). The optimized device showed a maximum EQE of 6.5%. Additionally, the deep-blue emission also realized a 101% color saturation according to the NTSC standard. The high efficiency may be attributed to engineering appropriate device architecture and enabling balanced carrier injection, effective host-...

Tetraphenylethylene-substituted phenothiazine-based AIEgens for non-doped deep-blue organic light-emitting diodes with negligible efficiency roll-off

Deep-blue organic light-emitting diodes (OLEDs) with low efficiency roll-off and high color purity are critical for the tetraphenylethylene-based AIEgens derivative. In this work, the novel blue emitter, 3-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)-10-(4-(1,2,2-triphenylvinyl)phenyl)-10H-phenothiazine (TPEPPI), was designed and synthesized to develop a new material possessing aggregation-induced emission (AIE) characteristics and deep-blue emission. The TPEPPI hardly emits fluorescence in tetrahydrofuran (THF), the luminescence intensity increased by 5.5 times when the water content reaches 95%, showing strong fluorescence in aggregated state. Three different non-doped device structures were fabricated, all of the OLEDs showed deep-blue emission (∼460 nm). The optimized device Ⅲ exhibited an emission peak at 467 nm, a maximum current efficiency of 4.25 cd A−1, a maximum power efficiency of 3.35 lm W−1, a maximum luminance of 16750 cd m−2, a maximum external quantum efficiency of 2.36%, and the essentially negligible efficiency roll-off of 3.3%, which is the first observation of deep-blue emission using a tetraphenylethylene-substituted phenothiazine-based AIEgens derivative in OLEDs.

Benzofuropyridine-Based Highly Efficient Thermally Activated Delayed Fluorescence Emitters.

Novel thermally activated delayed fluorescence (TADF) materials with benzofuro[2,3-b]pyridine (BFPd) as an electron acceptor and carbazole derivatives [carbazole (Cz), 1,3,6,8-tetramethyl-9H-carbazole (tmCz), and 9'H-9,3':6',9''-tercarbazole (terCz)] as electron donors are designed, and their electronic and optical properties are investigated theoretically for a deep blue organic light-emitting diode (OLED). We obtain the energies of the first singlet (S1) and first triplet (T1) excited states of TADF materials by performing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the ground state by using the dependence on charge transfer amounts for the optimal Hartree-Fock percentage in the exchange-correlation of TD-DFT. We show that terCz-BFPd would be a suitable blue OLED emitter because it has sufficiently small ΔEST value (0.048 eV), which is favorable for a reverse intersystem crossing process from the T1 to S1 states and an emission wavelength of 432.8 nm with sufficiently large oscillator strength (F) value.

Study of Phenoxaborin Derivatives for Thermally Activated Delayed Fluorescence Emitters.

Novel thermally activated delayed fluorescence (TADF) materials including 10H-phenoxaborin (PXB) as an electron acceptor and carbazole derivatives [carbazole (Cz), 1-methyl-carbazole (1-m-Cz), and 3,6-bis(3,6-diphenylcarbazolyl)carbazole (BDPCC)] as electron donors (Cz-PXB, 1-m-Cz-PXB, and BDPCC-PXB) were designed and investigated theoretically for deep blue organic light-emitting diode (OLED). Using density functional theory (DFT) and time-dependent DFT calculations, we obtained the electron distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and the energy of the lowest singlet (S1) and the lowest triplet (T1) excited states. Values for the calculated energy difference between the S1 state and the T1 state (ΔEST) of 1-m-Cz-PXB (0.158 eV) and BDPCC-PXB (0.058 eV) were smaller than for Cz-PXB (0.265 eV). Both materials had sufficiently small ΔEST values, which is favorable for a reverse intersystem crossing process (RISC) from the T1 to the S1 states. 1-m-Cz-PXB (0.1067) and BDPCC-PXB (0.0857) also have larger oscillator strengths (F) than reference material DMAC-PXB (0.0001). Our results showed that 1-m-Cz-PXB and BDPCC-PXB would have highly efficient TADF properties in terms of a small-enough energy difference (ΔEST) between the S1 state and the T1 state and a large F for the OLED.

Highly efficient non-doped deep-blue organic light-emitting diodes by employing a highly rigid skeleton

Two novel deep-blue emitters based on a highly rigid unit, IDC-PA and IDC-Py, were prepared by respectively introducing the 7,7-dimethyl-5-phenyl-5,7-dihydro indeno [2,1-b] carbazole (IDC) unit with anthracene and pyrene derivatives. The emitters exhibit high quantum efficiency, excellent thermal stability, narrow full width at half maximum and deep-blue emission. Moreover, the IDC-PA and IDC-Py-based devices demonstrated maximum EQEs of 4.41% and 6.08% and maintain 4.35% and 5.35% as applied in non-doped devices even at the brightness of 5000 cd m−2 with CIE coordinates of (0.15, 0.10) and (0.15, 0.08), respectively.

Acquiring High-Performance Deep-Blue OLED Emitters through an Unexpected Blueshift Color-Tuning Effect Induced by Electron-Donating -OMe Substituents.

A series of blue-emissive 7-(diphenylamino)-4-phenoxycoumarin derivatives bearing -CF3 , -OMe, or -N(Me)2 substituents on the phenoxy subunit were synthesized. Although both the -CF3 and -N(Me)2 modifications were found to trigger redshifted fluorescence, the -OMe substitution was demonstrated to exert an unexpected blueshift color-tuning effect toward the deep-blue region. The reason is that the moderate electron-donating -OMe group can endow coumarins with unaltered HOMO but elevated LUMO energy levels. Moreover, the -OMe substitution was found to be beneficial to the thermal stability of these coumarins. Therefore, the trimethoxy-substituted objective compound can act as a high-performance deep-blue organic light-emitting diode (OLED) emitter, and OLED based on it emits deep-blue light with CIE coordinates of (0.148, 0.084), maximum luminance of 7800 cd m-2 , and maximum external quantum efficiency of 5.1 %. These results not only shed light on the molecular design strategy for high-performance deep-blue OLED emitters through color-tuning, but also show the perspective of coumarin derivatives as deep-blue OLED emitters.

A wide-energy-gap naphthalene-based liquid organic semiconductor host for liquid deep-blue organic light-emitting diodes

We developed a novel naphthalene derivative to function as a wide-energy-gap liquid organic semiconductor (LOS) host material for the limited range of liquid deep-blue light-emitting materials that have been developed to date. The naphthalene derivative 1-naphthaleneacetic acid 2-ethylhexyl ester (NLQ), which shows a low viscosity of 20 mPa·s at 25 °C, was synthesized as a LOS by introducing an ethylhexyl group into naphthalene. We doped 9,10-diphenylanthracene (DPA) into NLQ as a guest deep-blue dye. The highest occupied molecular orbital (HOMO) energy level of NLQ was estimated to be − 6.40 eV from photoelectron spectroscopy measurements in air. The energy gap of NLQ was estimated to be 4.08 eV from its absorption spectrum, indicating that NLQ has the widest energy gap of any such host material to date. The lowest unoccupied molecular orbital energy level of NLQ was calculated to be − 2.31 eV. Deep-blue electroluminescence emission in a liquid state was obtained by doping DPA into NLQ. Light emission could be achieved by a combination of Förster resonance energy transfer and direct recombination of trapped holes and electrons because the energy gap of DPA is straddled by the wider energy gap of NLQ.

Molecular engineering of pyrimidine-containing thermally activated delayed fluorescence emitters for highly efficient deep-blue (CIEy < 0.06) organic light-emitting diodes

The development of efficient and robust, deep-blue, thermally activated delayed fluorescence (TADF) emitters, especially those matching the European Broadcasting Union (EBU) standard with Commission International de L'Eclairage (CIE) coordinates of (0.15, 0.06), is one of the challenging issues in organic light-emitting devices (OLEDs). Through sophisticated molecular engineering, two novel pyrimidine derivatives with D-π-A configuration, 5-(4-(2,6-diphenylpyrimidin-4-yl)phenyl)-5H-benzofuro[3,2-c]carbazole (pBFCz-26DPPM) and 5-(4-(2,6-diphenylpyrimidin-4-yl)phenyl)-5H-benzo [4,5]thieno[3,2-c]carbazole (pBTCz-26DPPM) have been designed and synthesized. There are only negligible differences in the molecular configuration and thermal, electrochemical and photophysical properties for the two compounds. However, TADF properties can be tuned by changing the heteroatom in the donor moiety. Compound pBFCz-26DPPM offers a shorter delayed fluorescence lifetime (τd) than that of pBTCz-26DPPM. The optimized device serving pBFCz-26DPPM as dopant presents deep-blue Commission Internationale de l’Eclairage (CIE) coordinates of (0.154, 0.054) and maximum external quantum efficiencies (EQEmax) of 6.20%. Remarkably, no-doped device based on pBFCz--26DPPM delivers CIE coordinates of (0.153, 0.067) and EQEs as high as 5.80%. These device performances are amongst some of those of the best performing deep blue TADF OLEDs with similar color gamut.

Highly Efficient Deep Blue Fluorescent Organic Light-Emitting Diodes Boosted by Thermally Activated Delayed Fluorescence Sensitization.

Highly efficient deep blue fluorescent material and various thermally activated delayed fluorescent (TADF) blue sensitization materials were synthesized for fluorescent deep blue organic light-emitting diodes (OLEDs). These materials were designed and selected by considering efficient energy transfer conditions (i.e., spectral overlap and quantum efficiency) between sensitizer and acceptor. Energy transfer process from TADF host sensitizers to deep blue fluorescent emitter has been investigated by measuring the energy transfer rate. Measured energy transfer rate was to be 1.24 × 1010 s-1 (mol/dm3)-1 for a prompt decay of fluorescence and 2.61 × 108 s-1 (mol/dm3)-1 for delayed fluorescence, which demonstrated the efficient energy transfer. Indeed, highly efficient deep blue fluorescent OLEDs boosted by the TADF host-sensitization process were successfully fabricated. The maximum external quantum efficiency was 19.0% with color coordinates of (0.14, 0.15) and 15.5% with color coordinates of (0.15, 0.11) in the different host system. The efficiency roll-off characteristic and device operating lifetime were also improved by this efficient sensitization process.

Sulfur-bridged tetraphenylethylene AIEgens for deep-blue organic light-emitting diodes

Pure color deep-blue organic light-emitting diodes based on sulfur-bridged tetraphenylethylene AIEgens show efficient device performances.

Cyclometalated Iridium(III) Carbene Phosphors for Highly Efficient Blue Organic Light-Emitting Diodes.

Five deep blue carbene-based iridium(III) phosphors were synthesized and characterized. Interestingly, one of them can be fabricated into deep blue, sky blue and white organic light-emitting diodes (OLEDs) through changing the host materials and exciton blocking layers. These deep and sky blue devices exhibit Commission Internationale de l'Éclairage (CIE) coordinates of (0.145, 0.186) and (0.152, 0.277) with external quantum efficiency (EQE) of 15.2% and 9.6%, respectively. The EQE of the deep blue device can be further improved up to 19.0% by choosing a host with suitable energy level of its lowest unoccupied molecular orbital (LUMO).

Bis-Tridentate Ir(III) Metal Phosphors for Efficient Deep-Blue Organic Light-Emitting Diodes.

Emissive Ir(III) metal complexes possessing two tridentate chelates (bis-tridentate) are known to be more robust compared to those with three bidentate chelates (tris-bidentate). Here, the deep-blue-emitting, bis-tridentate Ir(III) metal phosphors bearing both the dicarbene pincer ancillary such as 2,6-diimidazolylidene benzene and the 6-pyrazolyl-2-phenoxylpyridine chromophoric chelate are synthesized. A deep-blue organic light-emitting diode from one phosphor exhibits Commission Internationale de l'Eclairage (CIE(x,y) ) coordinates of (0.15, 0.17) with maximum external quantum efficiency (max. EQE) of 20.7% and EQE = 14.6% at the practical brightness of 100 cd m-2 .

High triplet energy exciplex hosts for deep blue phosphorescent organic light-emitting diodes

High triplet energy exciplex hosts for deep blue phosphorescent organic light-emitting diodes were developed by synthesizing a high triplet energy hole transport type host material designed for exciplex formation with a high triplet energy electron transport type host material derived from a diphenyltriazine.

A novel bipolar D–π–A type phenanthroimidazole/carbazole hybrid material for high efficiency nondoped deep-blue organic light-emitting diodes with NTSC CIEy and low efficiency roll-off

Nondoped OLEDs utilizing a new emitter exhibited EL with NTSC CIEy and high EQEmax (5.80%) with low efficiency roll-off.

Highly Efficient Deep Blue Organic Light-Emitting Diodes Based on Imidazole: Significantly Enhanced Performance by Effective Energy Transfer with Negligible Efficiency Roll-off.

Great efforts have been devoted to develop efficient deep blue organic light-emitting diodes (OLEDs) materials meeting the standards of European Broadcasting Union (EBU) standard with Commission International de L'Eclairage (CIE) coordinates of (0.15, 0.06) for flat-panel displays and solid-state lightings. However, high-performance deep blue OLEDs are still rare for applications. Herein, two efficient deep blue emitters, PIMNA and PyINA, are designed and synthesized by coupling naphthalene with phenanthreneimidazole and pyreneimidazole, respectively. The balanced ambipolar transporting natures of them are demonstrated by single-carrier devices. Their nondoped OLEDs show deep blue emissions with extremely small CIEy of 0.034 for PIMNA and 0.084 for PyINA, with negligible efficiency roll-off. To take advantage of high photoluminescence quantum efficiency of PIMNA and large fraction of singlet exciton formation of PyINA, doped devices are fabricated by dispersing PyINA into PIMNA. A significantly improved maximum external quantum efficiency (EQE) of 5.05% is obtained through very effective energy transfer with CIE coordinates of (0.156, 0.060), and the EQE remains 4.67% at 1000 cd m-2, which is among the best of deep blue OLEDs reported matching stringent EBU standard well.

Structure-simplified and highly efficient deep blue organic light-emitting diodes with reduced efficiency roll-off at extremely high luminance.

Based on a series of new fluorescent emitters, deep blue non-doped multilayer OLEDs with EQEs exceeding 5.10% and single layer devices excluding any charge carrier transporting materials with an EQE of 4.22% were obtained at an extremely high luminance of 10 000 cd m-2.