Red Phosphorescent Organic Light-emitting Diodes Research Articles

1,2,4]Triazolo[1,5-a]pyridine as Building Blocks for Universal Host Materials for High-Performance Red, Green, Blue and White Phosphorescent Organic Light-Emitting Devices.

The electron-accepting [1,2,4]triazolo[1,5-a]pyridine (TP) moiety was introduced to build bipolar host materials for the first time, and two host materials based on this TP acceptor and carbazole donor, namely, 9,9'-(2-([1,2,4]triazolo[1,5-a]pyridin-2-yl)-1,3-phenylene)bis(9H-carbazole) (o-CzTP) and 9,9'-(5-([1,2,4]triazolo[1,5-a]pyridin-2-yl)-1,3-phenylene)bis(9H-carbazole) (m-CzTP), were designed and synthesized. These two TP-based host materials possess a high triplet energy (>2.9 eV) and appropriate highest occupied molecular orbital/lowest unoccupied molecular orbital levels as well as the bipolar transporting feature, which permits their applicability as universal host materials in multicolor phosphorescent organic light-emitting devices (PhOLEDs). Blue, green, and red PhOLEDs based on o-CzTP and m-CzTP with the same device configuration all show high efficiencies and low efficiency roll-off. The devices hosted by o-CzTP exhibit maximum external quantum efficiencies (ηext) of 27.1, 25.0, and 15.8% for blue, green, and red light emitting, respectively, which are comparable with the best electroluminescene performance reported for FIrpic-based blue, Ir(ppy)3-based green, and Ir(pq)2(acac)-based red PhOLEDs equipped with a single-component host. The white PhOLEDs based on the o-CzTP host and three lumophors containing red, green, and blue emitting layers were fabricated with the same device structure, which exhibit a maximum current efficiency and ηc of 40.4 cd/A and 17.8%, respectively, with the color rendering index value of 75.

Assistant dopant system in red phosphorescent OLEDs and its mechanism reveal

We have constructed an assistant dopant system in red phosphorescent OLEDs. In the system, we chose the 4,4′-Bis(carbazol-9-yl)biphenyl (CBP) as the host material, a thermally activated delayed fluorescence (TADF) 4CzIPN as the assistant dopant and a red phosphor Hex-Ir(phq)2(acac) as the guest. The assistant dopant system can remarkably reduce the turn-on voltage and increase device brightness. Meanwhile, devices with assist dopant system show negligible efficiency roll-off even in high luminance and current density. The current efficiency of optimized device with 15 wt% 4CzIPN only drops to 12.7 cd/A at 74,532 cd/m2 from maximum 13.7 cd/A. By further investigation on the exciton generation mechanism and the energy transfer process in the system, the main cause of the improvement has been unveiled.

9,9′-bis(2,2-diphenylvinyl)[3,3′]bicarbazole as low cost efficient hole transporting material for application in red PhOLEDs

Low-cost bicarbazole-based hole transporting derivative with diphenylvinyl substitutions was synthesized by simple procedure and characterized. The respective glass transition temperature of the material was estimated to be 81 °C, which can provide morphologically-stable amorphous films for applications in organic light emitting diodes. The compound possesses an adequate ionization potential, suitable triplet energy and high hole drift mobility of 5.54 eV, 2,61 eV and of 2 × 10−3 cm2/V·s, respectively, which make it a suitable hole transporting material for use in red phosphorescent organic light-emitting diodes. The respective peak efficiency of the red device with the p-type dopant was recorded at 8.3% (5.2 cd/A and 3.3 lm/W) and at 7.8% (4.9 cd/A and 1.9 lm/W) for 100 cd/m [2] brightness, demonstrating high potential of the material for applications in light emitting diodes. The characteristics indicated that device with the bicarbazole-based compound exhibited better performance than those of widely used 1,1-bis [(di-4-tolylamino)phenyl]cyclohexane (TAPC) -based device.

Power-efficient and solution-processed red phosphorescent organic light-emitting diodes by choosing combinations of small molecular materials to form a well-dispersed exciplex co-host

Intrinsic compatibility of exciplex couple determines the EL performance of the resultant solution-processed phosphorescent OLEDs, particularly driving voltage behaviours.

9-Silafluorene and 9-germafluorene: novel platforms for highly efficient red phosphorescent organic light-emitting diodes

De novo molecular platforms have been applied to build host materials for highly efficient PHOLEDs. A 9-silafluorene derivative was studied for the first time as a host and an external quantum efficiency of over 28% was achieved successfully in red PHOLEDs.

Naphthalene-based host materials for highly efficient red phosphorescent OLEDs at low doping ratios

Abstract Four naphthalene-based host materials, 4′-[8-(4-phenoxazin-10-yl-phenyl)-naphthalen-1-yl]-biphenyl-4-carbonitrile (POZ), 4′-[8-(4-phenothiazin-10-yl-phenyl)-naphthalen-1-yl]-biphenyl-4-carbonitrile (POT), 4′-[8-(9-phenyl-9H-carbazol-3-yl)-naphthalen-1-yl]-biphenyl-4-carbonitrile (3CZ), and 4′-[8-(4-carbazol-9-yl-phenyl)-naphthalen-1-yl]-biphenyl-4-carbonitrile (9CZ), were designed and synthesized, where one electron-donating group and one electron-withdrawing group were covalently linked to the 1,8-positions of each naphthalene unit, respectively. Single crystal X-ray diffraction analyses of the four hosts revealed that the host molecules are held together by intensive non-covalent interactions, such as π⋯π and hydrogen bonding interaction at solid state, which can facilitate carrier transport and migration of excitons and improve the ability of thin film formation. The photophysical properties, thermal stability, and electrochemical behaviors of these hosts were systematically investigated. Red phosphorescent organic light-emitting diodes (PHOLED) based on these host materials with Ir(MDQ)2(acac) as dopant were fabricated and exhibited excellent device performance at low doping concentrations of 1–2 wt% due to the efficient energy transfer and reduced concentration quenching. Remarkably, POT based OLED at the doping ratio of 1 wt% achieved a maximum efficiency of 39.5 cd A−1, 37.6 lm W−1 and 20.3% with Commission Internationale de l'Eclairage (CIE) coordinates of (0.60, 0.40) without an external out-coupling structure. This result is comparable to the best red PHOLEDs with such a low doping ratio, indicating their potential utility as a host for cost-effective and highly efficient red PHOLEDs.

Structure effect of carbazole-oxadiazole based organic small molecule hosts on the solution-processed phosphorescent OLEDs performance

A group of solution-processed bipolar small molecular host materials, namely 1,4-bis(5-(9-(2-ethylhexyl)-9H-carbazol-3-yl)-1,3,4-oxadiazol-2-yl)benzene (p-BCOB), 1,3-bis(5-(9-(2-ethylhexyl)-9H-carbazol-3-yl)-1,3,4-oxadiazol-2-yl)benzene (m-BCOB) and 1,3,5-tris(5-(9-(2-ethylhexyl)-9H-carbazol-3-yl)-1,3,4-oxadiazol-2-yl)benzene (TCOB) were synthesized by means of connecting carbazole-oxadiazole substituents on the para-, meta- and 1,3,5-meta- positions of benzene ring respectively. The influence of different linking positions of carbazole-oxadiazole substituents on the thermal, photophysical, electrochemical and charge transport properties of the compounds is comprehensively studied. For verifying the host configuration effect on the performance of phosphorescent organic light-emitting diodes (PHOLEDs), solution-processed green and red PHOLEDs with fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) and bis(1-phenylisoquinolinato)(acetylacetonate) iridium (Ir(piq)2acac) as phosphorescent dopants were fabricated. The m-BCOB:Ir(ppy)3 and TOCB:Ir(ppy)3 PHOLEDs show excellent device performance with the maximum luminance efficiencies of 15.0cdA−1 and 13.7cdA−1, respectively, which are increased by 9.8 and 9.0 times compared with the p-BCOB:Ir(ppy)3 PHOLED because of effective Förster and Dexter energy transfer. However, the p-BCOB:Ir(piq)2acac PHOLED is the best among three hosts with a maximum external quantum efficiency of 7.9% due to the effective charge trapping. This work demonstrates that different host molecular structures have a big effect on the PHOLEDs performance.

Diphenanthroline Electron Transport Materials for the Efficient Charge Generation Unit in Tandem Organic Light-Emitting Diodes

In this paper, we report two new phenanthroline-based compounds, 1,4-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene (p-bPPhenB) and 1,3-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene (m-bPPhenB), for the charge generation unit of tandem organic light-emitting diodes (OLEDs). These two compounds exhibited high electron mobility of (5.8–4.4) × 10–3 cm2/(V s), a very small injection barrier at the p–n junction interface, a high glass transition temperature of 123.9–182.1 °C, and exceptionally good operational stability. Because of such excellent characteristics, a single-stack red phosphorescent OLED (PhOLED) with p-bPPhenB showed a low driving voltage (2.7 V) and significantly improved maximum power efficiency (56.8 lm/W), external quantum efficiency (30.8%), and device lifetime (LT95, 130 h) compared to those of the control device using bathophenanthroline (Bphen) (3.7 V, 39 lm/W, 27.1%, and 13 h). Furthermore, a two-stack (tandem) red PhOLED using p-bPPhenB in the charge generation unit exhibited superior cha...

Versatile Donor-π-Acceptor-Type Aggregation-Enhanced Emission Active Fluorophores as Both Highly Efficient Nondoped Emitter and Excellent Host.

By employing triphenylamine (TPA) and/or carbazole as donor and cyano group as acceptor, three donor-π-acceptor (D-π-A)-type fluorophores, 4,6-di(4-(diphenylamino)phenyl)isophthalonitrile (DTPAIPN), 4-(4-(diphenylamino)phenyl)-6-(3,5-(9,9'-dicarbazolyl)phenyl)isophthalonitrile (TPAmCPIPN), and 4,6-di(3,5-(9,9'-dicarbazolyl)phenyl)isophthalonitrile (DmCPIPN), were designed, synthesized, and characterized for their thermal, photophysical, electrochemical, and electroluminescent properties. All three compounds exhibited excellent thermal and morphological stabilities with thermal decomposition (Td) and glass transition temperature (Tg) values ranging from 396 to 516 °C and from 114 to 211 °C, respectively. The investigation into the photophysical properties revealed the aggregation-enhanced emission (AEE) feature of these chromophores. The performances of the nondoped devices were significantly improved as compared to those of the doped devices, further confirming the AEE nature. In particular, DmCPIPN-based nondoped device rendered the maximum current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) values of 26.6 cd A-1, 22.4 lm W-1, and 8.3%, respectively, in comparison with 9.1 cd A-1, 4.9 lm W-1, and 4.2% for the doped device. The versatility of these new luminogens was demonstrated through the utilization of them as host for highly efficient red phosphorescent organic light-emitting diodes (PHOLEDs). The EQE of the DmCPIPN-hosted device reached as high as 19.3% with a reduced efficiency roll-off of 10.3% at 1000 cd m-2. These results demonstrate that the new luminogens of this work can be used as both nondoped emitter and host for high-performance OLEDs, which facilitates the development of multifunctional materials for OLEDs applications.

BiPh-m-BiDPO as a Hole-Blocking Layer for Organic Light-Emitting Diodes: Revealing Molecular Structure-Properties Relationship**Supported by the National Natural Science Foundation of China under Grant No 11690044.

We report a simple hole-blocking material (biphenyl-3,3’-diyl)bis(diphenylphosphine oxide) (BiPh-m-BiDPO) based on our recent advance. The bis(phosphine oxide) compound shows HOMO/LUMO levels of ∼−6.71/−2.51 eV. Its phosphorescent spectrum in a solid film features two major emission bands peaking at 2.69 and 2.4 eV, corresponding to 0–0 and 0–1 vibronic transitions, respectively. The measurement of the electron-only devices reveals that BiPh-m-BiDPO possesses electron mobility of – cmVs at E = 2– V/cm. The characterization of the sky blue fluorescent and red phosphorescent pin organic light-emitting diodes (OLEDs) utilizing BiPh-m-BiDPO as the hole blocker shows that its shallow LUMO level as well as the low electron mobility affects significantly the power efficiency and hence operational stability, relative to the luminous efficiency, especially at high luminance. In combination with our recent results, the present study provides an indepth insight on the molecular structure-property correlation in the organic phosphinyl-containing hole-blocking materials.

Dibenzo[g,p]chrysene: A new platform for highly efficient red phosphorescent organic light-emitting diodes

One polycyclic aromatic hydrocarbon compound, 3,6,11,14-tetraphenyldibenzo[g,p]chrysene (TPDBC), was designed, synthesized, and fabricated in a red phosphorescent organic light-emitting diode (PHOLED) with a maximum external quantum efficiency (EQE) of 14.4%, which represented the first report of a dibenzo[g,p]chrysene motif as the building block for host materials. It was conjectured that dibenzo[g,p]chrysene may serve as a next generation molecular platform which is readily functionalizable for the preparation of electroactive materials for applications in the emerging areas of molecular electronics.

Influence of the linkage mode and D/A ratio of carbazole/oxadiazole based host materials on phosphorescent organic light-emitting diodes

In the present study, by combining mCP with oxadiazole through tuning the linkage mode and donor/acceptor (D/A) ratio, six different bipolar host materials, o-mCPOXD, m-mCPOXD, p-mCPOXD, o−2mCPOXD, m−2mCPOXD, and p−2mCPOXD were designed and synthesized. Their thermal, photophysical, and electrochemical properties were investigated to reveal the relationship between molecular structure and properties. To evaluate the electroluminescent properties of the six host materials, blue, green, and red phosphorescent organic light-emitting diodes (PhOLEDs) were fabricated and characterized. The compounds with ortho-linkage, o-mCPOXD and o−2mCPOXD, exhibit the highest triplet energy (ET) value of 2.8eV, which is sufficiently high for blue phosphorescent emitters. The PhOLEDs hosted by hybrids with para-linkage, p-mCPOXD and p−2mCPOXD, show highest current densities but lowest efficiencies due to the relatively low ET value and unbalanced charge flux. The increase of mCP unit leads to lower current densities and device efficiencies. These results demonstrate that bipolar host materials for high-performance PhOLEDs can be constructed by considering the influence of both the linkage mode and D/A ratio.

Novel Bipolar Indole-Based Solution-Processed Host Material for Efficient Green and Red Phosphorescent OLEDs.

A new bipolar host material 2-(2'-(5-(dibenzo[b,d]furan-4-yl)-1H-indol-1-yl)-1,1'-biphenyl]-4-yl)-1-phenyl-1H-benzo[d]imidazole (INDY) has been designed and synthesized for solution-processed PHOLEDs. A hole-transporting indole conjugated dibenzofuran scaffold and an electron-transporting benzimidazole unit were orthogonally connected with incorporation of a 2,4'-biphenyl bridge, which endows a balanced bipolar charge transfer distribution and appropriate triplet energy level (>2.6 eV). The twisted conformation of INDY also ensures good thermal/morphological stability with high Tg (148 °C). The solution-processed green PHOLEDs achieved great device efficiencies with 27.33 cd/A and 12.26 lm/W at 7 V. Meanwhile, the red solution-processed PHOLEDs gained excellent performance with the maximum efficiencies of 17.20 cd/A, 9.82 lm/W, and 12.61%. In addition, all of the red devices presented excellent color stability with the same CIE coordinate as (0.65, 0.35) under varying dopant contents and operating voltages.

Electrolyte-Gated Red, Green, and Blue Organic Light-Emitting Diodes.

We report vertical electrolyte-gated red, green, and blue phosphorescent small-molecule organic light-emitting diodes (OLED), in which light emission was modified by tuning the electron injection via electrochemical doping of the electron injection layer 4,4-bis(N-carbazolyl)-1,1-biphenyl (CBP) under the assistance of a polymer electrolyte. These devices comprise an electrolyte capacitor on the top of a conventional OLED, with the interfacial contact between the electrolyte and electron injection layer CBP of OLEDs achieved through a porous cathode. These phosphorescent OLEDs exhibit the tunable luminance between 0.1 and 10 000 cd m-2, controlled by an applied bias at the gate electrode. This simple device architecture with gate-modulated luminance provides an innovative way for full-color OLED displays.

Novel bipolar host for highly efficient green, yellow, orange, red and deep-red phosphorescent organic light-emitting devices

A novel bipolar host tris(4-(pyrimidin-5-yl)phenyl)amine (TPMTPA) constructed by incorporating triphenylamine as the electron-donating core and pyrimidine as the electron-accepting peripheries was designed and synthesized. TPMTPA achieves excellent bipolar charge transport properties and has high enough triplet energy level to sensitize green, yellow, orange, red and deep-red phosphors. By using TPMTPA as a host, high performance green, yellow, orange, red and deep-red phosphorescent organic light-emitting devices (PhOLEDs) were demonstrated with maximum external quantum efficiencies of 20.4%, 17.6%, 15.1%, 15.3% and 15.7% respectively. These results suggested that TPMTPA is a versatile high performance host for PhOLEDs of different emission colors.

Colour stability of Blue–Green and white phosphorescent organic light-emitting diode employing a 9-(2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)-9H-carbazole host

An ambipolar compound, 9-(2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)-9H-carbazole, is employed as a universal single host for efficient sky-blue (B), green (G), orange–red (R), BG, and white (W) phosphorescent organic light-emitting diodes (PhOLEDs) with an external quantum efficiency (ηEQE) of more than 20%. By integrating B and G emitters into the emitting layer (EML), an efficient BG PhOLED exhibits a maximum current efficiency (ηC) of 81.7 cd/A, power efficiency (ηP) of 73.2 lm/W, and ηEQE of 23.1%. Another BG PhOLED exhibits excellent colour stability with a slight shift (Δx of 0.0006, Δy of 0.0014) in CIE1931 colour coordinates while its luminance varies from 3.5 to 38360 cd/m2. Furthermore, a 1 nm R EML is employed as a probe inserted at three distinct positions to investigate the main recombination zone; the main recombination zone is detected near the interface between B/G EML. At the same time, a W spectrum combines individual R, G, B emission peaks. Besides, a W PhOLED simply comprises of a B and R EML, exhibiting good colour stability (Δx of 0.0004, Δy of 0.0023 between 4 and 12 V) and high efficiency (a maximum ηC, ηP, ηEQE of 58.4 cd/A, 50.7 lm/W, 21.9%). In addition, transient electroluminance signals described green emitters have less trapped charges and a longer exciton lifetime which benefit the device lifetime.

Exciplex-Forming Co-Host-Based Red Phosphorescent Organic Light-Emitting Diodes with Long Operational Stability and High Efficiency.

The use of exciplex forming cohosts and phosphors incredibly boosts the efficiency of organic light-emitting diodes (OLEDs) by providing a barrier-free charge injection into an emitting layer and a broad recombination zone. However, most of the efficient OLEDs based on the exciplex forming cohosts has suffered from the short operational lifetime. Here, we demonstrated phosphorescent OLEDs (PhOLEDs) having both high efficiency and long lifetime by using a new exciplex forming cohost composed of N,N'-diphenyl-N,N'-bis(1,1'-biphenyl)-4,4'-diamine (NPB) and (1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) (PO-T2T). The red-emitting PhOLEDs using the exciplex forming cohost achieved a maximum external quantum efficiency (EQE) of 34.1% and power efficiency of 62.2 lm W1- with low operating voltages and low efficiency roll-offs. More importantly, the device demonstrated a long lifetime around 2249 h from 1000 cd m-2 to 900 cd m-2 (LT90) under a continuous flow of constant current. The efficiencies of the devices are the highest for red OLEDs with an LT90 > 1000 h.

Benzoquinoline-based fluoranthene derivatives as electron transport materials for solution-processed red phosphorescent organic light-emitting diodes

A new electron transport material (ETM) with two fluoranthene and a benzoquinoline moiety was synthesized for the fabrication of solution-processed phosphorescent organic light-emitting diodes (PHOLEDs).

Phenanthro[9,10-d]imidazole based new host materials for efficient red phosphorescent OLEDs

We report on the synthesis and characterization of a new series of bipolar phenanthro[9,10-d]imidazole based compounds. The derivatives are highly thermally stable materials as demonstrated by thermogravimetric analysis. Electron photoemission spectra of thin layers of the materials show ionization potentials in the range of 5.1–5.4 eV. Spectroscopic properties of the phenanthro[9,10-d]imidazole based derivatives were determined by absorption and photoluminescence measurements. All the developed materials were used as hosts in red phosphorescent organic light-emitting diodes and their properties were compared with commercially available host materials. Results indicate that a device with 2-[4-(N,N-diphenylamino)phenyl]-1-phenylphenanthro[9,10-d]imidazole exhibited superior performance with peak efficiency of 15.9% (21.5 cd/A and 29.9 l m/W) and very low turn on voltage of 2.8 V. Efficiency of the device is about 35–67% higher than those of devices containing commercial host materials.

Naphthyl or pyrenyl substituted 2-phenylcarbazoles as hole transporting materials for organic light-emitting diodes

A new series of 9-hexylcarbazole-based hole transporting materials (HTM) with naphthyl or pyrenyl substitutions were synthesized and characterized. The respective glass transition temperatures of the new HTM were estimated to be 58 and 61 °C, which can provide morphologically-stable films for electroluminescence (EL) applications. The pyrenyl substituted analogue possesses an adequate ionization potential and triplet energy gap of 5.54 eV and 2.48 eV, respectively, which makes it a suitable HTM for use in red phosphorescent organic light-emitting diodes (OLEDs). In contrast, the ionization potential of the naphthyl substituted analogues was estimated to be 5.72 eV, thus the p-type conducting dopant should be introduced for device fabrication. The respective peak efficiencies of the naphthyl and pyrenyl based OLEDs with the p-type dopant were recorded at 15.4% (26.0 cd/A and 24.2 lm/W) and 17.3% (26.1 cd/A and 19.1 lm/W), demonstrating their high potential for EL applications.