Blue Dopant Research Articles

Blue organic light-emitting diodes based on spiro[fluorene-indeno]pyridine derivatives

ABSTRACTWe designed and synthesized two blue fluorescent materials based on spiro[9H-fluorene-9, 5′-[5H] indeno[1, 2-b] pyridine via Buchwald-Hartwig amination and Suzuki coupling reaction. To characterize their electroluminescent properties, multilayered organic light emitting diodes (OLEDs) were fabricated with the following device structures: ITO(180 nm)/2-TNATA(60 nm)/NPB(20 nm)/α,β-ADN: 3, 5, or 10 wt% blue dopants (30 nm)/Bphen(30 nm)/Liq(2 nm)/Al(100 nm). All the devices showed blue emissions with high electroluminescent efficiencies. Particularly, a device using 7′-((4-(N,N-diphenyl)-styryl)-4-phenyl)-spiro[9H-fluorene-9, 5′-[5H] indeno[1, 2-b] pyridine] as a dopant material with 10 wt% doping concentration exhibited highly efficient blue emission with an external quantum efficiency of 4.46% at 20 mA/cm2 and the CIE coordinates of (0.18, 0.30) at 1000 cd/m2. In this device, the twisted structural feature of emitter originated from spiro-type core prevented self-quenching and lead to the improved efficiencies even at high doping concentration in the device.

Efficient, color-stable and high color-rendering-index white organic light-emitting diodes employing full thermally activated delayed fluorescence system

Abstract High efficiency, high color rendering index (CRI) and excellent color-stability are important requirements for high-performance white organic light emitting diodes (WOLEDs). To realize these issues for the WOLEDs based on the full thermally activated delayed fluorescence (TADF) system, we constructed WOLED devices by employing CDBP:PO-T2T exciplex as the host, while 2CzPN and AnbTPA as blue and red dopants, respectively. We carefully optimize the device to realize balanced carrier transporting property of each emitting layer and good exciton confinement in the device. As a result, the WOLED achieves stable white emission with a high CRI of 82, a CIE coordinate of (0.33, 0.38) at a luminance of 1000 cd m −2 , and a small CIE variation value of (0.00, 0.02) in the luminance range of 100–3000 cd m −2 . It also demonstrates high maximum forward-viewing external quantum efficiency of 19.2%, current efficiency of 36.7 cd A −1 and power efficiency of 46.2 lm W −1 . Our work presents a novel and useful approach to develop highly efficient, color-stable and high-CRI WOLEDs through the full TADF mechanism.

Blue Phosphorescent Organic Light‐emitting Diode Using a Novel Starburst‐type Donor and Acceptor Material as a Co‐host

Starburst‐type hole‐transporting and electron‐transporting materials, tris(3‐(9H‐carbazol‐9‐yl)phenyl)amine ( TmCTA ), 2,4,6‐tri([1,1′‐biphenyl]‐3‐yl)‐1,3,5‐triazine (1), 2,4,6‐tris(3′‐methyl‐[1,1′‐biphenyl]‐3‐yl)‐1,3,5‐triazine (2), and 2,4,6‐tris(3′,5′‐dimethyl‐[1,1′‐biphenyl]‐3‐yl)‐1,3,5‐triazine (3) having high triplet energies and excellent thermal stabilities were synthesized. Deposited films of TmCTA , with 1, 2, or 3 as hosts, were doped with the blue phosphorescent dopant bis[2‐(4,6‐difluorophenyl)pyridinato‐C2 ,N](picolinato)iridium(III) (FIrpic) and their device characteristics were evaluated. In co‐deposited films, the molecular weight ratio of TmCTA to 1, 2, or 3 was 1: 1, and these co‐hosts formed exciplexes. By comparison with those of single‐host systems, the PL spectra of the co‐deposited films were red‐shifted, suggesting that exciplexes are formed by the two constituent molecules. When doped with 10% FIrpic, TmCTA :1 device showed an external quantum efficiency (EQE) of 14.56% (maximum 16.46%) at 1000 cd/m2; similar results were obtained for the remaining films. The EQE roll‐offs in the co‐host systems are much smaller than those of single‐host device using TmCTA .

Synthesis and properties of polymeric host materials constructed by silane-carbazole backbone and electron-affinitive cyanohexyl substituent for blue phosphorescence dopant

A series of blue phosphor hosts, using 3,6-linked carbazole with δ-π tetraphenylsilane segment as the main chain and modified by peripheral cyanohexyl group with different ratios, was designed and synthesized. These polymers show good solubility in common organic solvents. They all possess wide bandgap of 3.40 eV, high thermal stabilities and good film forming abilities. The peripheral cyanohexyl substituents have no effect on the polymer backbones but enhance the electron transporting properties of the polymers in devices. Among them, doped devices using poly[(tetraphenylsilicon/9-hexyl-9H-3,6-carbazole)50-co-(tetraphenylsilicon/9-hexanenitrile-9H-3,6-carbazole)50 (PCNCzSi50) as the host and bis[(4,6-difluorophenyl)pyridinato-N,C2]-(picolinato)iridium(III)(FIrpic) as the guest show the best performance with the maximum current efficiency and power efficiency of 2.00 cd/A and 1.40 lm/W, respectively.

Improved efficiency in blue phosphorescent organic light-emitting diodes by the stepwise doping structure

The electro-optical properties of the blue phosphorescent organic light-emitting diodes (PHOLEDs) can be affected by the stepwise doping structure in the emitting layer (EML). A series of multi-EML devices with different doping concentration of blue dopant (FIrpic) are fabricated. The effect of the stepwise doping structure close to the electron transport layer is more obvious than that close to the hole transport layer. When the doping concentration increases gradually from the hole injection side to the electron injection side, the maximum values of the luminance, current and power efficiency can reach to 9745 cd/m2 (at 9 V), 32.0 cd/A and 25.1 lm/W in the device with the asymmetric tri-EML structure, which is improved by about 10% compared with that in the bi-EML device. When the number of the EML is four, the performance of the device becomes worse because of the interface effect resulting from different concentration of dopant.

Carrier transfer and luminescence characteristics of thickness‐dependent organic light‐emitting diodes using transporting material as the host of emitting layer

We have systematically investigated hole/electron distribution and exciton dynamics in blue fluorescent and yellow phosphorescent organic light‐emitting diodes (OLEDs). We demonstrate that the importance of appropriate charge carrier confinement and exciton management for the realization of highly efficient and stable OLEDs. For yellow phosphorescent device based on Ir(MePQ)2(acac), we have studied the behavior of various intrinsic emission zones on the characteristics of OLEDs with the hole/electron‐transporting matrix. The current efficiency increases from 15.7 to 31.7 cd A−1 with the rise in electron‐transporting matrix from 20 to 80 nm. We attributed the improved performance to the strong hole‐transfer capabilities and relatively weak electron‐donating abilities of Ir‐complex doped transporting matrix. In contrast, the efficiencies of blue devices decreased with increasing emitting layer (EML), suggesting that the rate of hole‐electron pair recombination continuously decrease with the increasing EML thickness due to blue fluorescent dopant hindering hole transfer. In addition, the complementary‐color white OLEDs (WOLEDs) with various EML thickness were also demonstrated. Based on their performance, we have found that the use of thin EML in WOLEDs can significantly improve optoelectronic conversion efficiency, whereas deteriorated device properties reversely.

Synthesis and characterization of diphenylamine derivative containing malononitrile for thermally activated delayed fluorescent emitter

To increase the efficiency of blue organic light-emitting diodes (OLEDs), new blue dopants, 2-(bis(4-(diphenylamino)phenyl)methylene)malononitrile (4-DPM) and 2-(bis(3-(diphenylamino)phenyl) methylene)malononitrile (3-DPM), with thermally activated delayed fluorescence were synthesized. 4-DPM with a p-phenyl linker between the electron-donating diphenylamine unit and the electron-withdrawing malonitrile unit showed a large overlap in the phenyl linker as well as the malonitrile unit, while 3-DPM with an m-phenyl linker showed a weak overlap in the phenyl linker. The OLED with the 3-DPM dopant showed higher efficiencies of 12.62 cd/A and 8.81 lm/W compared to that with the 4-DPM dopant (1.38 cd/A and 0.70 lm/W). This result was attributed to the small overlap between the electron-donating and electron-withdrawing units, the small charge-transfer singlet-triplet state splitting below 0.1 eV, and the well-aligned structure in the horizontal direction, as determined by grazing incidence X-ray diffraction.

Effect of various red phosphorescent dopants in single emissive white phosphorescent organic light-emitting devices

In order to realize single emissive white phosphorescent organic light-emitting devices (PHOLEDs) with three color phosphorescent dopants (red, green, and blue), the energy transfer between the host material and the three dopants, as well as the among the three dopants themselves, should be considered and optimized. To explore the effect of red phosphorescent dopant on the color rendering index (CRI), the authors investigate the wavelength position of the maximum emission peak from three phosphorescent dopants. The CRI and luminous efficiency of white PHOLED in which Ir(pq)2(acac) acts as the red phosphorescent dopant are found to be greater than those of devices prepared using Ir(piq)3 and Ir(btp)2(acac) as the emission spectrum has a relatively high intensity near the human perception of blue, red, and green wavelengths. Furthermore, we demonstrate that the performance of the three dopants is related to the absorption characteristics of the red phosphorescent dopant. With a maximum emission peak at 600 nm, Ir(pq)2(acac) has a higher intensity in the concave section between 550 and 600 nm seen for red and blue dopants. In addition, the long metal-to-ligand charge transfer (MLCT) absorption tail of Ir(pq)2(acac) overlaps with the emission spectra of the green dopant, enhancing emission. Such energy transfer mechanisms are confirmed to optimize white emission in the single emissive white PHOLEDs.

Correction: Thermally activated delayed fluorescence blue dopants and hosts: from the design strategy to organic light-emitting diode applications

Correction for ‘Thermally activated delayed fluorescence blue dopants and hosts: from the design strategy to organic light-emitting diode applications’ by Mallesham Godumala et al., J. Mater. Chem. C, 2016, DOI: 10.1039/c6tc04377a.

Design and fabrication of two-stack tandem-type all-phosphorescent white organic light-emitting diode for achieving high color rendering index and luminous efficacy.

White organic light-emitting diodes (WOLEDs) are regarded as the general lighting source. Although color rendering index (CRI) and luminous efficacy are usually in trade-off relation, we will discuss about the optimization of both characteristics, particularly focusing on the spectrum of a blue emitter. The emission at a shorter wavelength is substantially important for achieving very high CRI (> 90). The luminous efficacy of a phosphorescent blue emitter is low as its color falls in the deeper blue range; however, that does not show any significant influence on the WOLEDs. WOLEDs with different blue dopants are compared to confirm the calculation of the CRI and luminous efficacy, and the optimized WOLEDs exhibit luminous efficacy of 38.3 lm/W and CRI of 90.9.

SimCP3-An Advanced Homologue of SimCP2 as a Solution-Processed Small Molecular Host Material for Blue Phosphorescence Organic Light-Emitting Diodes.

We have overcome the synthetic difficulty of 9,9′,9′′,9′′′,9′′′′,9′′′′′-((phenylsilanetriyl)tris(benzene-5,3,1-triyl))hexakis(9H-carbazole) (SimCP3) an advanced homologue of previously known SimCP2 as a solution-processed, high triplet gap energy host material for a blue phosphorescence dopant. A series of organic light-emitting diodes based on blue phosphorescence dopant iridium (III) bis(4,6-difluorophenylpyridinato)picolate, FIrpic, were fabricated and tested to demonstrate the validity of solution-processed SimCP3 in the device fabrication.

Fully Solution Processed Organic Light-emitting Electrochemical Cells (OLEC) with ZnO Interlayer for Lab-on-Chip Applications

Microfluidic lab-on-a-chip devices can be used for chemical and biological analyses such as DNA tests or environmental monitoring. In order to make a monolithic and cost-efficient/disposable sensing device, direct integration of excitation light source for fluorescent sensing is often required. A manufacturing process for fully solution-processed blue organic light-emitting electrochemical cells (OLECs) is presented, which consist of pre-patterned ITO, spin-coated ZnO buffer layer and blue light-emitting polymer plus dopants and an inkjet-printed PEDOT:PSS transparent top anode. Furthermore, the fully transparent blue OLEC is able to emit > 2000 cd/m2 light under pulsed driving mode, which fulfils requirements for simple fluorescent on-chip sensing applications. Furthermore, ITO electrodes can be replace by PEDOT:PSS transparent electrodes when a ZnO interlayer is solution processed on top, which enable the mask-free and fully solution processing integration on chips.

ChemInform Abstract: Pd‐Catalyzed Intramolecular Heck Reaction, C(sp2)—H Activation, 1,4‐Pd Migration, and Aminopalladation: Chemoselective Synthesis of Dihydroindeno[1,2,3‐kl]acridines and 3‐Arylindoles.

Palladium-catalyzed intramolecular Heck reaction and aminopalladation of N-(2-(1-phenylvinyl)phenyl)aniline for the efficient synthesis of dihydroindeno[1,2,3-kl]acridines and 3-arylindoles via tuning of the phosphine ligands and solvents under two optimized conditions are reported. The reaction follows a 1,4-Pd migration, aminopalladation, C(sp2)–H activation, as well as five- and six-membered-ring fusion to form different products. The dihydroindeno[1,2,3-kl]acridine derivatives showed higher triplet energy (ET) levels than common blue phosphorescent dopant and may serve as good host candidates for blue triplet emitters.

High efficiency and simplified white organic light-emitting diode based on a single-host emission layer

Efficient white organic light-emitting diodes (WOLEDs) with high stability are highly desired nowadays to produce devices that meet the commercial illumination requirement. WOLEDs based on single-host emission layer, is a preferred route to simplify device structures and thus lower the production cost. In this work, we report on single-host WOLEDs based on bipolar material, 2,6-bis[(3-carbazol-9-yl)phenyl)]pyridine (26DCzPPy), as the host, and Iridium (III)bis[(4,6-difluorophenyl)-pyridinato-N,C2′] (FIrpic), [3,2-c]pyridinato-N,C2′) acetylacetonate (PO-01), as blue and yellow dopants, respectively. The devices exhibit not only higher efficiency, but also superior stability compared to the conventional devices and doping-free devices. Detailed analysis is conducted to clarify the underlying reasons. This work enriches the mechanism understanding of single-host devices and paves the way for development of high performance WOLEDs.

Pd-Catalyzed Intramolecular Heck Reaction, C(sp2)–H Activation, 1,4-Pd Migration, and Aminopalladation: Chemoselective Synthesis of Dihydroindeno[1,2,3-kl]acridines and 3-Arylindoles

Palladium-catalyzed intramolecular Heck reaction and aminopalladation of N-(2-(1-phenylvinyl)phenyl)aniline for the efficient synthesis of dihydroindeno[1,2,3-kl]acridines and 3-arylindoles via tuning of the phosphine ligands and solvents under two optimized conditions are reported. The reaction follows a 1,4-Pd migration, aminopalladation, C(sp(2))-H activation, as well as five- and six-membered-ring fusion to form different products. The dihydroindeno[1,2,3-kl]acridine derivatives showed higher triplet energy (ET) levels than common blue phosphorescent dopant and may serve as good host candidates for blue triplet emitters.

24-4: Investigation of Effect of Triplet-Triplet Annihilation and Molecular Orientation on External Quantum Efficiency of Ultrahigh-Efficiency Blue Fluorescent Device

The effects of triplet-triplet annihilation (TTA) and molecular orientation were quantitatively investigated for a high-efficiency blue fluorescent organic light-emitting diode. Both the efficient TTA generation by improving carrier balance and the horizontal orientation of the transition dipole moment of the blue dopant realized ultrahigh external quantum efficiency (approximately 12%).

Stable Blue Phosphorescence Iridium(III) Cyclometalated Complexes Prompted by Intramolecular Hydrogen Bond in Ancillary Ligand.

Improvement of the stability of blue phosphorescent dopant material is one of the key factors for real application of organic light-emitting diodes (OLEDs). In this study, we found that the intramolecular hydrogen bonding in an ancillary ligand from a heteroleptic Ir(III) complex can play an important role in the stability of blue phosphorescence. To rationalize the role of intramolecular hydrogen bonding, a series of Ir(III) complexes is designed and prepared: Ir(dfppy)2(pic-OH) (1a), Ir(dfppy)2(pic-OMe) (1b), Ir(ppy)2(pic-OH) (2a), and Ir(ppy)2(pic-OMe) (2b). The emission lifetime of Ir(dfppy)2(pic-OH) (1a) (τem = 3.19 μs) in dichloromethane solution was found to be significantly longer than that of Ir(dfppy)2(pic-OMe) (1b) (τem = 0.94 μs), because of a substantial difference in the nonradiative decay rate (knr = 0.28 × 10(5) s(-1) for (1a) vs 2.99 × 10(5) s(-1) for (1b)). These results were attributed to the intramolecular OH···O═C hydrogen bond of the 3-hydroxy-picolinato ligand. Finally, device lifetime was significantly improved when 1a was used as the dopant compared to FIrpic, a well-known blue dopant. Device III (1a as dopant) achieved an operational lifetime of 34.3 h for an initial luminance of 400 nits compared to that of device IV (FIrpic as dopant), a value of 20.1 h, indicating that the intramolecular hydrogen bond in ancillary ligand is playing an important role in device stability.

Water‐Soluble Lacunary Polyoxometalates with Excellent Electron Mobilities and Hole Blocking Capabilities for High Efficiency Fluorescent and Phosphorescent Organic Light Emitting Diodes

High performance solution‐processed fluorescent and phosphorescent organic light emitting diodes (OLEDs) are achieved by water solution processing of lacunary polyoxometalates used as novel electron injection/transport materials with excellent electron mobilities and hole blocking capabilities. Green fluorescent OLEDs using poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐(1,4‐benzo‐{2,1′,3}‐thiadiazole)] (F8BT) as the emissive layer and our polyoxometalates as electron transport/hole blocking layers give a luminous efficiency up to 6.7 lm W−1 and a current efficiency up to 14.0 cd A−1 which remained nearly stable for about 500 h of operation. In addition, blue phosphorescent OLEDs (PHOLEDs) using poly(9‐vinylcarbazole) (PVK):1,3‐bis[2‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazo‐5‐yl]benzene (OXD‐7) as a host and 10.0 wt% FIrpic as the blue dopant in the emissive layer and a polyoxometalate as electron transport material give 12.5 lm W−1 and 30.0 cd A−1 power and luminous efficiency, respectively, which are among the best performance values observed to date for all‐solution processed blue PHOLEDs. The lacunary polyoxometalates exhibit unique properties such as low electron affinity and high ionization energy (of about 3.0 and 7.5 eV, respectively) which render them as efficient electron injection/hole blocking layers and, most importantly, exceptionally high electron mobility of up to 10−2 cm2 V−1 s−1.

Solution Processable White Organic Light-Emitting Diodes Using New Blue Host Material Including Substituent Group

New host material of T-TATa isomer substituted t-butyl group was investigated in solution process WOLED device compared with 4-(10-(3',5'-diphenylbiphenyl-4-yl)anthracen-9-yl)-N,N-diphenylaniline [TATa]. A two-color WOLED of a co-host system using solution process method was demonstrated. The device configuration was ITO/PEDOT:PSS (40 nm)/emitting layer (50 nm)/TPBi (20 nm)/LiF (1 nm)/AI. The emitting layer consisted of TATa or T-TATa isomer, NPB, DPAVBi (blue dopant), and rubrene (yellow dopant). NPB was used as not only blue host but also helping hole carrier transport. The device using T-TATa compound as a co-host exhibited a luminance efficiency of 3.39 cd/A, which is about twice higher than TATa device of 1.58 cd/A at 10 mA/cm2.

Correlation of Molecular Structure with Photophysical Properties and Device Performances of Thermally Activated Delayed Fluorescent Emitters

Thermally activated delayed fluorescent (TADF) emitters having bicarbazole as a donor unit based on a twin emitting core design were developed by substituting the bicarbazole donor with a diphenyltriazine acceptor. Three bicarbazole derived delayed fluorescent emitters linked via 2,3′-, 3,3′-, and 3,4′-positions of carbazole were synthesized and investigated as blue dopants of the TADF devices. A twin emitting core based TADF emitter derived from a 3,3′-bicarbazole donor exhibited a high quantum efficiency of 25.0% in the greenish blue TADF device. A 3,3′-linkage induced red shift of the emission wavelength, a small singlet–triplet energy gap, and a high quantum efficiency were revealed.