Yellow Phosphorescent Emitter Research Articles

Universal Display acquires OLED patents

Universal Display has acquired Merck KGaA’s intellectual property related to phosphorescent organic light-emitting diodes (OLEDs) for an undisclosed sum. The portfolio includes over 550 issued and pending patents from Merck scientists. At the same time, the two firms have signed a new collaboration pertaining to Universal’s green and yellow phosphorescent emitters and Merck’s transport and host materials for OLEDs.

High-performance single-emitting-layer hybrid white organic light-emitting diodes featuring a stable blue emission component from green thermally activated delayed fluorescence

In this work, we demonstrate single-emitting-layer hybrid white organic light-emitting diodes (WOLEDs) with high-efficiency and low efficiency roll-off. Here, the white emission arises from stable blue emission via electroluminescence (EL) spectral shift from green thermally activated delayed fluorescence (TADF) emitter and yellow phosphorescent emitter. WOLEDs shows a maximum current efficiency of 56.77 cd/A, maximum power efficiency of 50.19 lm/W, and maximum external quantum efficiency of 20.00%. Compared to the devices composed of conventional blue TADF emitters, the device based on EL spectral shift-induced blue emission exhibit higher performances, indicating that efficient white emission can be obtained without a typical blue emitter. Our study provides an alternative way to fabricate high-performance WOLEDs based on EL spectral shift technology.

Neutral Cyclometalated Iridium(III) Complexes Bearing Substituted N-Heterocyclic Carbene (NHC) Ligands for High-Performance Yellow OLED Application.

The synthesis, crystal structure, and photophysics of a series of neutral cyclometalated iridium(III) complexes bearing substituted N-heterocyclic carbene (NHC) ancillary ligands ((C∧N)2Ir(R-NHC), where C∧N and NHC refer to the cyclometalating ligand benzo[h]quinoline and 1-phenylbenzimidazole, respectively) are reported. The NHC ligands were substituted with electron-withdrawing or -donating groups on C4' of the phenyl ring (R = NO2 (Ir1), CN (Ir2), H (Ir3), OCH3 (Ir4), N(CH3)2 (Ir5)) or C5 of the benzimidazole ring (R = NO2 (Ir6), N(CH3)2 (Ir7)). The configuration of Ir1 was confirmed by a single-crystal X-ray diffraction analysis. The ground- and excited-state properties of Ir1-Ir7 were investigated by both spectroscopic methods and time-dependent density functional theory (TDDFT) calculations. All complexes possessed moderately strong structureless absorption bands at ca. 440 nm that originated from the C∧N ligand based 1π,π*/1CT (charge transfer)/1d,d transitions and very weak spin-forbidden 3MLCT (metal-to-ligand charge transfer)/3LLCT (ligand-to-ligand charge transfer) transitions beyond 500 nm. Electron-withdrawing substituents caused a slight blue shift of the 1π,π*/1CT/1d,d band, while electron-donating substituents induced a red shift of this band in comparison to the unsubstituted complex Ir3. Except for the weakly emissive nitro-substituted complexes Ir1 and Ir6 that had much shorter lifetimes (≤160 ns), the other complexes are highly emissive in organic solutions with microsecond lifetimes at ca. 540-550 nm at room temperature, with the emitting states being predominantly assigned to 3π,π*/3MLCT states. Although the effect of the substituents on the emission energy was insignificant, the effects on the emission quantum yields and lifetimes were drastic. All complexes also exhibited broad triplet excited-state absorption at 460-700 nm with similar spectral features, indicating the similar parentage of the lowest triplet excited states. The highly emissive Ir2 was used as a dopant for organic light-emitting diode (OLED) fabrication. The device displayed a yellow emission with a maximum current efficiency (ηc) of 71.29 cd A-1, a maximum luminance (Lmax) of 32747 cd m-2, and a maximum external quantum efficiency (EQE) of 20.6%. These results suggest the potential of utilizing this type of neutral Ir(III) complex as an efficient yellow phosphorescent emitter.

A New Lamellar Gold Thiolate Coordination Polymer, [Au(m-SPhCO2H)]n, for the Formation of Luminescent Polymer Composites.

The photoluminescence of gold thiolate clusters brings about many potential applications, but its origin is still elusive because of its complexity. A strategy in understanding the structure–properties relationship is to study closely related neutral gold thiolate coordination polymers (CPs). Here, a new CP is reported, [Au(m-SPhCO2H)]n. Its structure is lamellar with an inorganic layer made of Au–S–Au–S helical chains, similar to the [Au(p-SPhCO2H)]n analog. An in-depth study of its photophysical properties revealed that it is a bright yellow phosphorescent emitter with a band centered at 615 nm and a quantum yield (QY) of 19% at room temperature and in a solid state. More importantly, a comparison to the para-analog, which has a weak emission, displayed a strong effect of the position of the electron withdrawing group (EWG) on the luminescent properties. In addition, [Au(m-SPhCO2H)]n CPs were mixed with organic polymers to generate transparent and flexible luminescent thin films. The ability to tune the emission position with the appropriate contents makes these nontoxic polymer composites promising materials for lighting devices.

Highly efficient single-stack hybrid cool white OLED utilizing blue thermally activated delayed fluorescent and yellow phosphorescent emitters

Highly efficient single-stack hybrid cool white organic light-emitting diodes (OLEDs) having blue-yellow-blue multiple emitting layers (EMLs) are designed and constructed by utilizing blue thermally activated delayed fluorescent (TADF) and yellow phosphorescent emitters. The out-coupling efficiencies of yellow and blue emissions are maximized by tuning the ITO and total device thickness that satisfies both of antinode positions for yellow and blue emissions in a limited multiple EML thickness. To obtain a cool white emission, the exciton formation ratio in the blue-yellow-blue multiple EML system is controlled by manipulating the recombination zone through charge conductivity variation of host medium in the blue TADF EML. The resulting device exhibits cool white emission with very high maximum external quantum efficiency of 23.1% and CIE color coordinates of (0.324, 0.337). We anticipate that the studied approach will raise the viability of single-stack hybrid cool white OLEDs for high performance display applications.

Design of High‐Efficiency and Long‐Lifetime White Organic Light‐Emitting Diodes by Selective Management of Singlet and Triplet Excitons Using a Triplet Exciton Manager

AbstractA novel device design extending the lifetime of white organic light‐emitting diodes (WOLEDs), comprising a thermally activated delayed fluorescent (TADF) blue emitter and a yellow phosphorescent emitter, is developed using a triplet exciton manager (TEM), which selectively quenches the triplet exciton of the blue TADF emitter while harvesting the yellow phosphorescent emission. The TEM host is designed to have a lower triplet energy than the blue TADF emitter and a higher triplet energy than the yellow phosphorescent emitter. The singlet excitons of the TEM host are used to harvest only the singlet excitons of the blue TADF emitter by energy transfer, while the triplet excitons of the TEM host are utilized to activate the phosphorescent emission of the yellow emitter. The singlet and triplet excitons of the TEM host are thus selectively used to harness the blue singlet emission and yellow triplet emission, respectively. The triplet excitons of the blue TADF emitter can thus be excluded from the emission process, which improves the lifetime of the TEM‐based WOLEDs by five times while achieving similar external quantum efficiency as that of the conventional WOLED with a common high triplet energy host activating the triplet excitons of the blue TADF emitter.

3‐Cyano Imidazopyridine Acceptor‐based Bipolar andn‐type Host Materials for Phosphorescent Organic Light‐Emitting Diodes

AbstractTwo host materials named 8‐(3‐(9H‐carbazol‐9‐yl)phenyl)‐2‐phenylimidazo[1,2‐a]pyridine‐3‐carbonitrile (mCzImPy) and 8,8′‐(1,3‐phenylene)bis(2‐phenylimidazo[1,2‐a]pyridine‐3‐carbonitrile) (mBisImPy) were synthesized as a bipolar host and a n‐type host, respectively, from the herocyclic imidazopyridine unit through its 3‐position CN modification. They worked as the hosts of yellow phosphorescent emitter and the mCzImPy host achieved high quantum efficiency of 18.2% in the yellow phosphorescent devices. Therefore, the 3‐cyanoimidazopyridine showed potential as an electron transport unit of host design.

Peripheral Amplification of Multi-Resonance Induced Thermally Activated Delayed Fluorescence for Highly Efficient OLEDs.

Multi-resonance induced by boron and nitrogen atoms in opposite resonance positions endows a thermally activated delayed fluorescence (MR-TADF) emitter with a strikingly small full width at half maximum of only 26 nm and excellent photoluminescence quantum yield of up to 97.48 %. The introduction of a carbazole unit in the para position of the B-substituted phenyl-ring can significantly boost up the resonance effect without compromising the color fidelity, subsequently enhancing the performances of the corresponding pure blue TADF-OLED, with an outstanding external quantum efficiency (EQE) up to 32.1 % and low efficiency roll-off, making it one of the best TADF-OLEDs in the blue region to date. Furthermore, utilizing this material as host for a yellow phosphorescent emitter, the device also shows a significantly reduced turn-on voltage of 3.2 V and an EQEmax of 22.2 %.

Peripheral Amplification of Multi‐Resonance Induced Thermally Activated Delayed Fluorescence for Highly Efficient OLEDs

AbstractMulti‐resonance induced by boron and nitrogen atoms in opposite resonance positions endows a thermally activated delayed fluorescence (MR‐TADF) emitter with a strikingly small full width at half maximum of only 26 nm and excellent photoluminescence quantum yield of up to 97.48 %. The introduction of a carbazole unit in the para position of the B‐substituted phenyl‐ring can significantly boost up the resonance effect without compromising the color fidelity, subsequently enhancing the performances of the corresponding pure blue TADF‐OLED, with an outstanding external quantum efficiency (EQE) up to 32.1 % and low efficiency roll‐off, making it one of the best TADF‐OLEDs in the blue region to date. Furthermore, utilizing this material as host for a yellow phosphorescent emitter, the device also shows a significantly reduced turn‐on voltage of 3.2 V and an EQEmax of 22.2 %.

P‐207: Late‐News Poster: Efficient Emitter Aggregation Management Using High‐Entropy Non‐crystallizable Hosts

Molecular Glasses Inc. has developed a new class of high‐entropy non‐crystallizable OLED hosts that can be deposited by both vacuum/thermal evaporation and solution‐based processes such as inkjet printing. Benchmark testing demonstrated a 4‐15 times improvement in OLED lifetime using these hosts instead of conventional host 3, 3‐di(9H‐carbazol‐9‐yl) biphenyl (mCBP). Time resolved photoemission spectroscopy on a yellow phosphorescent emitter in a non‐crystallizable host – OLEDIQTM HT‐1700 vs. standard mCBP host shows that aggregation quenching is higher in mCBP. Triplet‐triplet annihilation (TTA) in these hosts is also explored.

Highly efficient blue and warm white organic light-emitting diodes with a simplified structure

Two blue fluorescent emitters were utilized to construct simplified organic light-emitting diodes (OLEDs) and the remarkable difference in device performance was carefully illustrated. A maximum current efficiency of 4.84 cd A−1 (corresponding to a quantum efficiency of 4.29%) with a Commission Internationale de l’Eclairage (CIE) coordinate of (0.144, 0.127) was achieved by using N,N-diphenyl-4″-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1, 1′:4′, 1″-terphenyl]-4-amine (BBPI) as a non-doped emission layer of the simplified blue OLEDs without carrier-transport layers. In addition, simplified fluorescent/phosphorescent (F/P) hybrid warm white OLEDs without carrier-transport layers were fabricated by utilizing BBPI as (1) the blue emitter and (2) the host of a complementary yellow phosphorescent emitter (PO-01). A maximum current efficiency of 36.8 cd A−1 and a maximum power efficiency of 38.6 lm W−1 were achieved as a result of efficient energy transfer from the host to the guest and good triplet exciton confinement on the phosphorescent molecules. The blue and white OLEDs are among the most efficient simplified fluorescent blue and F/P hybrid white devices, and their performance is even comparable to that of most previously reported complicated multi-layer devices with carrier-transport layers.

Novel iridium complexes as yellow phosphorescent emitters for single-layer yellow and white polymer light-emitting diodes

Both yellow and white polymer light-emitting devices are achieved using novel yellow phosphorescent iridium complexes with triphenylamine- and fluorene-functionalized cyclometalating ligands as dopants.

High-power-efficiency hybrid white organic light-emitting diodes with a single emitting layer doped with blue delayed fluorescent and yellow phosphorescent emitters

High-efficiency hybrid white organic light-emitting diodes (HWOLEDs) with a blue thermally activated delayed fluorescent (TADF) emitter and a yellow phosphorescent emitter doped in a single emitting layer were developed. Exciton harvesting by the blue TADF and yellow phosphorescent emitters rendered both singlet and triplet excitons to contribute to the white emission, which leads to a high quantum efficiency of 22.4% and a power efficiency of 60.3 lm W−1 in the HWOLEDs. In addition, the electroluminescence spectra of the HWOLEDs were kept stable from 100 cd m−2 to 5, 000 cd m−2.

Cool and warm hybrid white organic light-emitting diode with blue delayed fluorescent emitter both as blue emitter and triplet host.

A hybrid white organic light-emitting diode (WOLED) with an external quantum efficiency above 20% was developed using a new blue thermally activated delayed fluorescent material, 4,6-di(9H-carbazol-9-yl)isophthalonitrile (DCzIPN), both as a blue emitter and a host for a yellow phosphorescent emitter. DCzIPN showed high quantum efficiency of 16.4% as a blue emitter and 24.9% as a host for a yellow phosphorescent emitter. The hybrid WOLEDs with the DCzIPN host based yellow emitting layer sandwiched between DCzIPN emitter based blue emitting layers exhibited high external quantum efficiency of 22.9% with a warm white color coordinate of (0.39, 0.43) and quantum efficiency of 21.0% with a cool white color coordinate of (0.31, 0.33) by managing the thickness of the yellow emitting layer.

Investigation of triplet harvesting and outcoupling efficiency in highly efficient two-color hybrid white organic light-emitting diodes

We investigate singlet and triplet transfer processes in white triplet harvesting (TH) organic light-emitting diodes (OLEDs), comprising fluorescent and phosphorescent emitter molecules. By analyzing electroluminescence spectra and performing time-resolved measurements, we prove direct TH from the blue fluorescent emitter N,N′-di-1-naphthalenyl-N,N′-diphenyl-[1,1′:4′,1″:4″,1′′′-quaterphenyl]-4,4′′′-diamine (4P-NPD) to the yellow phosphorescent emitter bis(2-(9,9-dihexyluorenyl)-1-pyridine)(acetylacetonate)iridium(III) (Ir(dhfpy)2(acac)). Singlet transfer is identified as a second exciton transfer mechanism in the TH OLEDs under investigation. The CIE coordinates of these devices can be adjusted over a wide range by varying the distance between exciton generation and TH zone. For an OLED with CIE color coordinates of (0.42/0.40), close to the warm white color point A, we obtain nearly perfect Lambertian emission characteristics. This device achieves an external quantum efficiency (EQE) of 9.4% and a luminous efficacy (LE) of 27.1 lm W−1 (at 1000 cd m−2) which can be further increased to 46 lm W−1 using outcoupling enhancement techniques. Optical modeling of the electromagnetic field inside the TH OLED and a comparison to the position of the emitters within the device confirms that the devices operate close to the optical optimum and yields an estimated internal quantum efficiency (IQE) of 48.7% and a singlet/(harvested) triplet ratio of 0.28/0.72 = 0.39.

P‐154: High Efficiency White‐Phosphorescent OLED with Host‐Free Yellow Emitter

AbstractA white PHOLED with high power efficiency of 55 lm/W has been demonstrated by using a host‐free yellow phosphorescent emitter sandwiched by double blue phosphorescent emitters. The white PHOLED with host‐free yellow phosphorescent emitter achieved a comparable device performance to that of using complicated host‐guest doped system. Based on this device concept as well as the molecular engineering of blue host material, highly efficient white PHOLED can be achieved.

Highly efficient and high colour rendering index white organic light-emitting devices using bis(2-(2-fluorphenyl)- 1,3-benzothiozolato-N,C2′) iridium (acetylacetonate) as yellow emitter

Multiple emissive layer (MEML) white organic light-emitting devices (OLEDs) are fabricated. In the MEML, the yellow phosphorescent emitter bis(2-(2-fluorphenyl)-1,3-benzothiozolato-N,C2′) iridium(acetylacetonate) [(F-BT)2Ir(acac)] is introduced to codope with the red one bis(2,4-diphenyl-quinoline) iridium acetylacetonate [Ir(piq)2acac] into 4,4′-N,N′-dicarbazole-biphenyl (CBP), fac tris(2-phenylpyridine) iridium [Ir(ppy)3]-doped CBP is used for green emitting and N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) is used for both hole transporting and blue emitting. By adjusting the devices' structure, we get the OLED which has the maximum luminance of 50 030 cd m−2, maximum efficiencies of 21.6 cd A−1 (at 7 V), 10.3 lm W−1 (at 5 V) and 8.4% (at 7 V) and colour rendering index (CRI) of 85. In addition, its Commission International de L'Eclairage (CIE) coordinates slightly change from (0.3867, 0.4075) to (0.3638, 0.4070) when the drive voltage shifts from 7 V (1862 cd m−2) to 13 V (39 270 cd m−2).