Red Phosphorescent Dopants Research Articles

Thermally activated delayed fluorescence dendrimers with AIE property and functional dendrons for high-efficiency solution-processed white OLEDs

Thermally activated delayed fluorescence (TADF) materials with encapsulation dendrons have exhibited superior advantages in the solution-processed OLEDs. Herein, we utilized electron-donating carbazole and electron-withdraw phenylbenzimidazole as the peripheral wrapping unit to design and synthesize two TADF materials, TRZCz-Cz and TRZCz-Bz, to discuss the distinction about different encapsulations for one identical emissive core. The photophysical investigation demonstrate that both compounds have small ΔE ST and clearly aggregation-induced enhancement (AIE) properties, which are benefit for enhancing the exciton unitization of electroluminescence devices. Despite similar characters in photophysical property, the blue phosphorescence OLEDs host with TRZCz-Cz obtain the maximum current efficiency and external quantum efficiency of 38.6 cd A −1 and 18.4%, respectively, which far exceed TRZCz-Bz based one. Thanks to the high performance of blue device, the fully phosphorescent white OLEDs with Ir(MDQ) 2 (acac) as red phosphorescent dopant achieves the maximum luminescence, current efficiency, external quantum efficiency as high as 12874 cd m −2 , 33.8 cd A −1 and 16.5%, respectively, which are among the highest efficiency of solution-processed white OLEDs. Moreover, the hybrid white OLEDs with TADF as blue host and red phosphor as dopant was also achieved with a low turn-on voltage of 3.2 V, and more standard white emission with CIE coordinate of (0.34,0.33). Thus, this work significantly extends the range of AIE TADF materials and paves the useful way for the fabrication of high efficiency white OLEDs through solution-process. • The optimized wrapping unit enhance the device performance almost two times higher. • The TADF host ensure the solution-processed blue phosphorescence OLEDs with the EQE max of 18.4%. • Both the fully phosphorescent and hybrid white OLEDs achieves high luminescence and stable color purity.

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.

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.

High-performance red organic light-emitting devices based on an exciplex system with thermally activated delayed fluorescence characteristic

To realize high-performance red organic light-emitting diodes (OLEDs), a novel exciplex system Tris-PCz:B4PyPPM with thermally activated delayed fluorescence (TADF) characteristic was developed as the host of both red fluorescent and phosphorescent emitters. Due to the blue-green PL spectrum and suitable triplet energy level of 2.53 eV, the exciplex system matches the requirements of both red fluorescent and phosphorescent dopants. With an optimized mixing molar ratio of 5:5, Tris-PCz:B4PyPPM exciplex system shows outstanding host performance in both the red fluorescent and phosphorescent devices. The red fluorescent device based on DCJTB exhibits a low turn-on voltage of 2.3 V and an extremely high maximum external quantum efficiency (EQE) of 9.3%. And the red phosphorescent device with the Ir(MDQ)2acac as the emitter realizes a maximum EQE of 20.3%. These remarkable results prove the feasibility of TADF exciplex systems simultaneously as the hosts of both high-performance fluorescent and phosphorescent red devices.

28.3: Invited Paper: Progress on Phosphorescent OLED Materials

In this paper, we present the development of phosphorescent host materials, having triplet energy more than 2.5 eV, to sensitize red phosphorescent dopants. Some of the new phosphorescent hosts exhibited lifetimes (T70) of about 40,000h at an initial luminance of 1000 cd/m2, with efficiencies as high as 28 cd/A at a driving voltage of about 4.2 V.

Wide bandgap materials design toward tunable bandgap and increased carries injection property: silane interrupting π-conjugation together with peripheralcarbazolyl substituents

Universal host for blue, green and red phosphorescent dopants is achieved by rational molecule design with silane interrupting π-conjugation and non-conjugated peripheralcarbazolyl substituents.

Highly efficient, color-stable white top-emitting organic light-emitting diodes

Top-emitting white organic light-emitting diodes (WOLEDs) have potential applications in lighting and full color dis- plays due to the potential realization of a large aperture ratio and a high resolution when recombining top-emitting WOLEDs with active-matrix driving circuits. In the present paper, the authors fabricated top-emitting WOLEDs based on the red/blue dual-phosphorescent emitting layers and improved luminous efficiency and color stability in WOLEDs through inserting an tris(pheny-pyrazole)iridium (Ir(ppz)3) thin film as an electron-blocking layer between the red layer and the blue one. The insertion of an Ir(ppz)3 thin film can easily generate white emission through the enhancement of doping concentration of the red dopant, thus reducing the serious requirements of manufacture technology and improving the reproducibility of process technology. In addition, the authors analyzed the mechanism of color stability, optimized the concentrations of the red and blue phosphorescent dopants, and realized a luminous efficiency of as high as 7.9 cd · A(-1) in the authors' top-emitting WOLED. The white emission lies in the warm-white region with a very small chromaticity change of (0.006, 0.01) under a wide brightness range of 87-2,403 cd · m(-2).

Organic Light Emitting Diodes: Low Roll‐Off and High Efficiency Orange Organic Light Emitting Diodes with Controlled Co‐Doping of Green and Red Phosphorescent Dopants in an Exciplex Forming Co‐Host (Adv. Funct. Mater. 33/2013)

Organic Light Emitting Diodes: Low Roll‐Off and High Efficiency Orange Organic Light Emitting Diodes with Controlled Co‐Doping of Green and Red Phosphorescent Dopants in an Exciplex Forming Co‐Host (Adv. Funct. Mater. 33/2013)

Multifunctional electron-transporting indolizine derivatives for highly efficient blue fluorescence, orange phosphorescence host and two-color based white OLEDs

In this work, derivatives of indolizine are first used as electron-transporting host materials for hybrid fluorescence/phosphorescence white organic light-emitting devices (F/P-WOLED). Of the indolizine derivatives, a blue fluorescent material BPPI (3-(4,4′-biphenyl)-2-diphenylindolizine) was found to have: (1) blue emission with high quantum yields, (2) good morphological and thermal stabilities, (3) electron-transporting properties, and (4) a sufficiently high triplet energy level to act as a host for red or yellow-orange phosphorescent dopants. The multifunctional BPPI enables adaptation of several simplified device configurations. For example, a non-doped blue fluorescent device exhibits good performance with an external quantum efficiency of 3.16% and Commission Internationale de l'Eclairage coordinates of (0.15, 0.07). A high-performance orange phosphorescent device was found to have a high current efficiency of 23.9 cd A−1. Using BPPI, we also demonstrate a F/P-WOLED with a simplified structure, stable emissions and respectable performance (current and external quantum efficiencies of 17.8 cd A−1 and 10.7%, respectively).

High‐Efficiency Tris(8‐hydroxyquinoline)aluminum (Alq3) Complexes for Organic White‐Light‐Emitting Diodes and Solid‐State Lighting

Combinations of electron-withdrawing and -donating substituents on the 8-hydroxyquinoline ligand of the tris(8-hydroxyquinoline)aluminum (Alq(3)) complexes allow for control of the HOMO and LUMO energies and the HOMO-LUMO gap responsible for emission from the complexes. Here, we present a systematic study on tuning the emission and electroluminescence (EL) from Alq(3) complexes from the green to blue region. In this study, we explored the combination of electron-donating substituents on C4 and C6. Compounds 1-6 displayed the emission tuning between 478 and 526 nm, and fluorescence quantum yield between 0.15 and 0.57. The compounds 2-6 were used as emitters and hosts in organic light-emitting diodes (OLEDs). The highest OLED external quantum efficiency (EQE) observed was 4.6%, which is among the highest observed for Alq(3) complexes. Also, the compounds 3-5 were used as hosts for red phosphorescent dopants to obtain white light-emitting diodes (WOLED). The WOLEDs displayed high efficiency (EQE up to 19%) and high white color purity (color rendering index (CRI≈85).

Simple-structure white organic light emitting diodes with high color temperature

We present high color temperature white organic light emitting diodes with a simple p-i-n structure. A sky blue phosphorescent dopant of iridium(III) bis[4,6-(difluorophenyl)-pyridinato-N,C2’] picolinate and a red phosphorescent dopant of bis(2-phenylquinoline)(acetylacetonate)iridium(III) in the emissive layers is employed to make high color temperature devices. Very stable color variation under ▵0.02 until a 5000 cd/m2 brightness value is realized by efficient carrier control in a multi stacked emitting layer of blue/red/blue colors. Maximum current and power efficiencies of 23.8 cd/A and 22.9 lm/W in forward direction are obtained. With balanced emissions from the two emitters, the white light emission with very high correlated color temperature of 7308 K as well as CIE coordinates of (0.30, 0.33) is achieved.

Power Efficiency Improvement of White Phosphorescent Organic Light-Emitting Diode with Thin Double-Emitting Layers and Hole-Trapping Mechanism

This study is carried out to discuss how to reduce the driving voltage of blue phosphorescent organic light-emitting diodes (PHOLEDs) by using a thin double-emission layer. A hole transport-type host (TCTA) is inserted between the hole transport layer (TAPC) and the emitting layer (EML), constituting a buffer layer between them with the aim of improving charge carrier balance. Furthermore, in this study, we also utilize the interface between double light-emitting layers of devices by codoping them with a red phosphorescent dopant [Os(bpftz)2(PPh2Me)2]. An Os complex with a high-lying highest occupied molecular orbital (HOMO) energy level (trapping holes) is codoped at the interface between emitting layers and an exciton-formation zone is expanded to obtain a white PHOLED with high efficiency. From the results, the optimal structure of the white device exhibits a yield of 35 cd A-1, a power efficiency of 22 lm W-1, and CIE coordinates of (0.33,0.38) at a luminance of 1000 cd m-2. Furthermore, the power efficiency can be improved to 30 lm W-1 by attaching the outcoupling enhancement film.

Efficient carrier- and exciton-confining device structure that enhances blue PhOLED efficiency and reduces efficiency roll-off

In this study we used a known host material diphenylbis[4-(9-carbazoyl)phenyl]silane (SiCa) to realize a highly efficient bis[4,6-(difluorophenyl)-pyridinato-N,C 2′]picolinate (FIrpic)–based blue phosphorescent device exhibiting high efficiencies of up to 18.3%, 41.5 cd/A, and 31 lm/W in a carrier- and exciton-confined device structure. At a practical brightness of 1000 cd/m 2, the device performance ( η ext) remained as high as 16.7%. The enhanced efficiency and reduced roll-off resulted mainly from the introduction of (a) DTAF as an HTL that efficiently blocked and confined excitons within the FIrpic-doped emissive layer, and (b) a thin layer of DPPS between the EML and TAZ, serving as an exciton- and hole-blocking layer to suppress exciton quenching by FIrpic at the EML–ETL interface. In addition, we incorporated a red phosphorescent dopant (Mpq 2Iracac) into the blue device to fabricate a white phosphorescent organic light-emitting device, which exhibited satisfactory efficiencies (11.6%, 21.2 cd/A, 14.2 lm/W) with the Commission Internationale de L’éclairage (CIE) coordinates of (0.35, 0.38).

Influence of the Performance of Organic Light Emitting Devices by Chemical Reaction at the Interface between Electron Transport Material and Ba Cathode

Red electrophosphorescent organic light-emitting devices using red phosphorescent dopant, and tris-(8-hydroxyquinoline)-aluminum (Alq3) and 2,9-dimethyl-4,7 diphenyl-1,10-phenanthroline (BCP) electron injecting have been fabricated. A device with Alq3 as electron transport layer shows high turn-on voltage (@ 1 cd/m 2 ) of 6.25 V, but a device with BCP does lower turn-on voltage of 5.0 V. Alq3 and BCP have silimar lowest unoccupied molucular obital, 3.0 eV and 2.9 eV, which are the higher values than work function of Ba (2.7 eV). So, Ba cathode does not form energy barrier to inject electron to organic layer from simple energy band of view. However, these devices represent different injection properties, therefore we analyze the interface between organic layer and cathode. X-ray photoelectron spectroscopy depth profiling measurements reveal the strong chemical reaction and bonding of Ba with the C and O present in the Alq3 molecules. Results show that the performance of the electrophosphorescent devices is mainly governed by the structure and the chemical reaction at the interface between Ba metal and electron-injecting material. [doi:10.2320/matertrans.M2010381]

Emitting layer thickness dependence of color stability in phosphorescent organic light-emitting devices

We investigated the strong influence of the thickness of iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C 2′] picolinate (FIrpic) doped N, N′-dicarbazolyl-3, 5-benzene (mCP) blue-emitting layer (B-EML) on color stability. The large voltage drop across the B-EML resulted in a higher sensitivity of the carrier transport and injection properties to the applied external voltage. According to carrier mobility measurements by the time-of-flight method, the electron mobility of the mCP exhibited a strong dependence on the electric field. Therefore, at a higher driving voltage, the more rapidly increasing electron mobility of the mCP and the decreasing energy barrier height on the electron transport path would extend the recombination zone from the B-EML to the tris(phenylpyridine)iridium (Ir(ppy) 3) doped mCP green-emitting layer (G-EML) in devices with thinner B-EMLs. Coupled with the fluctuations of the recombination zone, stronger triplet–triplet exciton annihilation occurring in the thinner B-EMLs led to an even more evident deterioration of the color stability. After circumventing these two negative factors, a green–blue organic light-emitting device (OLED) with ultra-high color stability was demonstrated, with the CIE coordinates slightly shifted from (0.256, 0.465) to (0.259, 0.467) with increased luminance from 48.7 to 12,700 cd/m 2. Further adding a red phosphorescent dopant into this green–blue EML backbone, we successfully fabricated a white OLED with high color stability, which exhibited a nearly invariant CIE coordinate throughout the practical luminance range from 1050 (0.310, 0.441) to 9120 cd/m 2 (0.318, 0.446) and maximum efficiencies of 26.4 cd/A and 19.8 lm/W.

Lifetime study of red phosphorescent organic light-emitting diodes with a double doping structure

The lifetime of red phosphorescent organic light-emitting diodes with a double doping structure was studied. Green and red phosphorescent dopants were co-doped in the emitting layer and the lifetime of the red devices with the doubly doped emitting structure was dependent on the doping concentration of the green dopant due to the energy transfer from the green dopant to the red dopant.

Pure red electrophosphorescence from polymer light-emitting diodes doped with highly emissive bis-cyclometalated iridium(III) complexes

In order to develop highly emissive red phosphorescent materials for OLED application, novel bis-cyclometalated iridium(III) complexes were developed using the 1-(dibenzo[ b, d]furan-4-yl)isoquinolinato- N, C 3′ ( dbfiq) cyclometalating ligand. When 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate ( bdbp) is employed as an ancillary ligand, Ir( dbfiq) 2( bdbp) 1 exhibits red photoluminescence (PL) at 640 nm with a quantum yield ( Φ PL) of 0.61 (in toluene, 298 K). Replacement of bdbp to dipivaloylmethanate ( dpm) and acetylacetonate ( acac) (Ir( dbfiq) 2( dpm) 2 and Ir( dbfiq) 2( acac) 3, respectively) does not affect the PL spectrum, but reduces Φ PL to 0.55 and 0.49 for 2 and 3, respectively. Similar tendency is also found in the doped poly(methyl methacrylate) (PMMA) film, and 1 is more emissive ( Φ PL = 0.17) than 2 and 3 ( Φ PL = 0.08 and 0.06, respectively). Using 1 as a phosphorescent dopant, polymer light-emitting diodes (PLEDs) were fabricated, of which structure was ITO/PEDOT:PSS (40 nm)/PVCz: 1:PBD (100 nm)/CsF (1 nm)/Al (250 nm). Pure red electroluminescence (EL) is obtained from the fabricated PLEDs, affording a CIE chromaticity coordinate of (0.68, 0.31). When 0.51 mol% of 1 is incorporated in the PVCz-based emitting layer, the PLED shows maximum luminance of 7270 cd m −2 at 16.5 V, power efficiency of 1.4 lm W −1 at 7.5 V, and external quantum efficiency of 6.4% at 9.0 V. PLEDs with the same structure and components were also fabricated using 2 and 3, and their device characteristics were investigated. In proportion to the PL quantum yields, 1 affords better device performance than 2 and 3. Owing to four butoxy groups introduced to the bdbp ligand, 1 exhibits high solubility in organic solvents such as chloroform and toluene, and thus, is an excellent red phosphorescent dopant for solution-processed OLEDs.

Synthesis of Carbazole-Based Polymers with 1,3,4-Oxadiazole Pendant Group and Their Application in Organic Light-Emitting Diodes

Structurally well-defined copolymers with high solubility were prepared via palladium-catalyzed polycondensation of N-(2-ethylhexyl)-3,6-dibromocarbazole with 2-aryl-5-(4-aminophenyl)-1,3,4-oxadiazole (aryl = phenyl, p-methylphenyl, p-methoxyphenyl). Copolymers consisted of alkylcarbazole groups in the main chain and 1,3,4-oxadiazole pendants in the side chain. The influence of their aryl substituents on physical, optical, band gaps and electroluminescent characteristics of the copolymers was investigated. Both UV-Visible absorption and photoluminescence emission peaks of the copolymers were similar to each other. The band gap energy of the polymers was measured in the range of 2.84 to approximately 2.88 eV, and HOMO energy in the range of -5.13 to approximately -5.18 eV. These copolymers were used as hole-transporting layers (HTL) in the light-emitting diodes with Alq3 as an emitting layer. Compared to devices with P-H and P-OCH3 used as HTL, it should be noted that device with P-CH3 used as HTL showed higher luminescence. Maximum luminescence of devices was measured to be 276 cd/m2 at 14 V with P-H, 625 cd/m2 at 15 V with P-CH3, and 471 cd/m2 at 14 V with P-OCH3. This might be due to effect of subtle changes in HOMO energy level of polymers with changing substituent groups. Phosphorescent polymer light emitting diodes were also fabricated with an emitting layer consisting of P-CH3 matrix and a red phosphorescent dopant (lr-PIQCH).

Synthesis and Photoluminescent Properties of Red Phosphorescent Random Copolymers with Photopatternable Moiety

Two types of photopatternable blue emitting random copolymers, poly(VK-ran-CEMA) ( 1 ) and poly(VK-ran-CEMA-ran-MAE) ( 2 ), containing UV curable cinnamoyl moieties capable of generating negative type patterned images were synthesized and characterized. In addition, their potential possibility to be used as a photopatternable polymeric host matrix for a red phosphorescent dopant was studied by using UV-Visible absorption and PL emission spectroscopy. The λmax,PL value of poly(VK-ran-CEMA) ( 1 ) doped with a red phosphorescent dopant, (bsn)2Ir(acac), was shifted from 343 nm of poly(VK-ran-CEMA) ( 1 ) to 610 nm, with increasing the content of (bsn)2Ir(acac). Poly(VK-ran-CEMA-ran-(bsn)2IrMAE) ( 3 ), prepared by the reaction of [(bsn)2IrCl]2 with poly(VK-ran-CEMA-ran-MAE) ( 2 ), exhibited λmax,PL at 610 nm in the range of red emission. These results implied efficient energy transfer from photopatternable blue emitting random copolymers to a red phosphorescent dopant. It was also found upon UV exposure on the synthesized copolymers that negative patterned images with high resolution were produced by standard photolithographic process.

P‐164: Highly Efficient White Organic Light‐Emitting Diodes Using Two Emitting Materials for Three Primary Colors (Red, Green and Blue)

AbstractThese results have demonstrated the highly efficient white organic light‐emitting diodes (WOLEDs) with two separated emissive materials for the three primary colors, red, green, and blue, using blue fluorescent and red phosphorescent dopants. The two devices with the optimum structure showed a maximum luminance of 30500 and 20400 cd/m2 and a luminous efficiency of 38.0 and 32.7 cd/A. Furthermore, device B showed better CIEx,y coordinates of (x=0.36, y=0.35) at 10V. We expect that the WOLEDs fabricated using device structures and materials described here may be applicable for backlights in liquid crystal displays.