Red Phosphorescent Organic Light-emitting Diodes Research Articles

Twisted vibration of Aza-azulene promotes ultra-fast triplet-singlet up-conversion for extremely stable red electroluminescence

The fast triplet-singlet up-conversion based on thermally activated delayed fluorescence (TADF) molecules is a unique effort to improve the utilization of triplet excitons. We construct two TADF molecules using nonbenzenoid aromatic aza-azulene as donor and triazine as acceptor. The distortion structural resonance of aza-azulene enhances the reverse intersystem crossing (RISC) process, resulting in efficient inter/intramolecular charge transfer (CT) emission. The rate constant of RISC (kRISC) is 9.0 × 106 and 8.4 × 106 s−1 in solid states. Efficient Förster Energy Transfer (FRET) and/or Dexter Energy Transfer (DET) between these molecules and dopants make them suitable host materials to improve the performances of red phosphorescent organic light-emitting diodes (OLEDs). Especially, the top-emitting device achieves an external quantum efficiency (EQE) of 51.3% and a current efficiency of 68.2 cd/A at 1000 cd m−2 with a long device lifetime of LT95 = 52,350 and LT50 = 536,200 h.

Highly efficient phosphorescent organic light-emitting diodes with low turn-on voltages using N-phenylcarbazole/pyrimidine-based bipolar host materials

Herein, three novel bipolar host materials with D-π-A structure, termed 2CzPm, 3CzPm, and 9CzPm, were successfully synthesized by incorporating a pyrimidine moiety as an accepter and N-phenylcarbazole group as a donor through different connection sites. The three hosts exhibited good thermal stability with a glass transition temperature (Tg) of 104, 103, and 102 °C, respectively. They also possessed high enough triplet energy level (ET = 2.58–2.75 eV) to be efficient hosts for red and green phosphorescent organic light-emitting diodes (PhOLEDs) (ETdopants <2.5 eV). Moreover, considerable hole and electron current density in the conventional voltage range were observed, suggesting their good bipolar transport properties. When utilized in green and red PhOLEDs, the devices hosted by 9CzPm exhibited the best electroluminescence (EL) performance with low turn-on voltages (Von) of 2.6 and 2.5 V, and maximum external quantum efficiency (EQEmax) of 21.79% and 25.03%, respectively, indicating that pyrimidine moiety is a promising building block for bipolar host materials.

Novel bipolar host materials including carbazole and dioxy[2,3-b] pyrazine units for red phosphorescent organic light-emitting diodes

In this work, two novel bipolar host materials, 3,5-bis(9H-carbazol-9-yl) phenyl) benzo [1,4,5,6] dioxy [2,3-b] pyrazine (a-CzDp) and 7-(9-phenyl-9H-[3,9′- bicarbazole]-3′-yl) benzo [1,4,5,6] dioxy [2,3-b] pyrazine (b-CzDp) were designed and synthesized. In these two host materials, carbazole and dioxy [2,3-b] pyrazine act as the hole and electron transport unit, respectively. The two dipolar hosts both exhibit good thermal stability and deep highest occupied molecular orbital (HOMO) energy levels of -5.31 eV and −5.39 eV. The phosphorescent organic light-emitting diodes (PhOLEDs) including a-CzDp and b-CzDp as host and red phosphorescent material bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) (Ir(piq)2acac) as guest in the emitting layer. The corresponding PhOLED devices with a-CzDp and b-CzDp as host exhibit the maximum brightness of 8932 cd m−2 and 12782 cd m−2 at 624 nm, together with the maximum external quantum efficiency (EQE) of 10.32% and 13.82%, respectively. B-CzDp based red PhOLED device shows higher EQE and lower turn-on voltage due to the more balanced hole and electron transport. The results demonstrate the potential of a-CzDp and b-CzDp as efficient host for red PhOLEDs.

Intrinsically Stretchable Emissive Layer for Green and Red Phosphorescent OLEDs: Small Molecules Blended with SEBS Elastomer

In the field of stretchable electronics, intrinsically stretchable organic light‐emitting diodes (IS‐OLEDs) are considered to be essential units. However, research on stretchable electroluminescent layers for light‐emitting devices is limited. The most common method to obtain an effective stretchable emitting layer is blending a conjugated polymer with an elastomer. In this study, blending small molecules with a thermoplastic styrene‐ethylene‐butylene‐styrene (SEBS) elastomer to fabricate a stretchable emitting layer is proposed. The mechanical, electrical, and morphological properties of the blended films are studied. The results show that the blended films can be stretched up to 200% without any morphological change such as cracking or particle shedding. Additionally, an OLED is fabricated with a stretchable electroluminescent layer and demonstrated its functionality.

Highly thermal stable and efficient carbazole/pyridine/dibenzothiophene based bipolar host material for red phosphorescent light-emitting diodes

In this work, we designed and synthesized two bipolar host materials, DBTPC1 and DBTPC2 through different linkage ways between carbazole, pyridine and dibenzothiophene. The photophysical properties and thermal properties of these hosts were systematically investigated. Their rigid molecular structures result in high glass transition temperatures, contributing to excellent thermal stability of molecules. Employing DBTPC1 and DBTPC2 as hosts, red phosphorescent organic light-emitting devices (PHOLEDs) with Ir(piq)2acac as an emitter display maximum external quantum efficiencies (EQEs) of 18.8 and 13.2%, respectively. The experimental results reveal that DBTPC1 shows better performance than commercial host 4,4’-bis(carbazol-9-yl)biphenyl (CBP), thus, would be a promising host material for practical application in red PHOLEDs and for the future exploration in multi-color PHOLEDs.

High performance solution-processed red phosphorescent organic light-emitting diodes by co-doping europium complex as sensitizer

In this work, efficient red phosphorescent OLEDs (PHOLEDs) were designed and fabricated by solution processing method. Based on well optimized doping concentrations of emitter and ratio of mixed host materials, Eu(DBM) 3 Phen (DBM = 1,3-diphenylpropane-1,3-dion, Phen = 1,10-phenonthroline) was doped into the light-emitting layer (EML) which plays the role of sensitizer. Experimental results indicated that the incorporation of Eu(DBM) 3 Phen molecules can effectively trap the superfluous electrons within EML, which is helpful for balancing charges’ distribution and widening the recombination zone. Besides, the presence of Eu(DBM) 3 Phen molecules can accelerate the transfer of triplet energy from hosts to red emitters, therefore remarkably enhances device performance. By optimizing the doping concentrations of emitter and sensitizer, ratio of co-hosts and the functional layer thickness, recombination center was accurately adjusted to further broaden recombination zone, therefore causing decreased excitons density and suppressed excitons quenching. Eventually, the optimal solution-processed red PHOLED obtained the maximum current efficiency (47.12 cd/A) and external quantum efficiency (27.3%). High performance solution-processed red phosphorescent organic light-emitting diodes by incorporating trivalent rare earth europium complex Eu(DBM) 3 Phen (DBM = 1,3-diphenylpropane-1,3-dion, Phen = 1,10-phenonthroline) into light-emitting layer as sensitizer. • High performance solution-processed red phosphorescent OLED was designed by utilizing trivalent rare earth europium complex Eu(DBM) 3 Phen into light-emitting layer as sensitizer. • Co-doped sensitizer molecules function as electron trappers, which is helpful in balancing carriers' distribution and broadening recombination zone. • The optimal solution-processed red PHOLED obtained the maximum current efficiency up to 47.12 cd/A and external quantum efficiency up to 27.3%. • This work firstly demonstrated the efficacy of utilizing rare earth complex as sensitizer in realizing high performance solution-processed OLEDs.

A Comparative Study of Ir(dmpq)2(acac) Doped CBP, mCP, TAPC and TCTA for Phosphorescent OLEDs

In this work, we present the fabrication and characterization of solution-processable red Phosphorescent Organic Light-Emitting Diodes (PhOLEDs). The proposed approach is based on Ir(III) complex, namely Bis(2-(3,5-dimethylphenyl)quinoline-C,N)(acetylacetonato)Iridium(III), also known as Ir(dmpq)2(acac), which was doped in four different host materials: (a) 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP), (b) 1,3-Bis(N-carbazolyl)benzene (mCP), (c) 1,1-Bis[(di-4-tolylamino) phenyl]cyclohexane (TAPC), and (d) tris(4-carbazoyl-9-ylphenyl)amine (TCTA). The metal–organic complex offers unique optical and electronic properties arising from the interplay between the inorganic metal and the organic material. The optical and photophysical properties of the produced thin films are investigated in detail using spectroscopic ellipsometry and photoluminescence, whereas the structural characteristics are examined by atomic force microscopy. This comparative study of the four different Host:Ir-complex systems provides valuable information to evaluate the emission characteristics in order to achieve pure red light. Finally, these materials were applied as a single-emissive layer in PhOLED devices, and the electroluminescence characteristics were studied.

Pinpointing the origin of the increased driving voltage during prolonged operation in a phosphorescent OLED based on an exciplex host

We report on the origin of the reduced power efficiency in a red phosphorescent OLED with an exciplex host after prolonged operation. The power efficiency is reduced solely by an increased driving voltage while the radiant flux remains constant. An electrical model describing the driving voltage increase is, thus, sufficient to explain the reduced power efficiency. The electrical model of the fresh OLED and at different stages of degradation was devised from four different measurement methods. Using multiple measurement methods to determine the model parameters results in a rather unique set of model parameters, despite the large number of model parameters (38) as revealed by a correlation analysis. The increase in driving voltage could be reproduced by modifying only 7 out of the 38 model parameters. A sensitivity analysis identified the parameters with the largest effect (66%) on the driving voltage increase to be the trap density and the mobility of the employed hole transporting layer. This work highlights the benefit of using multiple measurement methods to derive reliable model parameters and the use of a sensitivity analysis to pinpoint the origin of the investigated property. • Increased driving voltage after prolonged operation. • Radiant flux remained constant, so an electrical model is sufficient. • Determination of reliable model parameters by using multiple measurement methods. • Sensitivity analysis reveals origin of the increased driving voltage. • Hole traps in TAPC are the main cause of the increased driving voltage.

High-performance non-doped near ultraviolet OLEDs with the EQE ∼ 6 % and CIEy ∼ 0.03 from high-lying reverse intersystem crossing

Developing high-performance organic fluorophors with short-wavelength emission is both significant and challenging subject in the application of organic light-emitting diodes (OLEDs). In this work, we proposes a “coupling hardness with distortion” design concept to construct an efficient near ultraviolet (NUV) fluorophor mP2MPC with 1,4-dimethyl-2,5-diphenylbenzene as a rotatable bridge via the meta-positons linking rigid and bulky phenanthroimidazole and carbazole groups. As a result, the non-doped OLED with it as emitter not only exhibits an emission peak at 395 nm with the Commission Internationale de l’éclairage coordinates of (0.163, 0.028), but also realizes a high forward-viewing external quantum efficiency of 6.09 % due to the high-lying reverse intersystem crossing. The value still keeps as high as 5.56 % at 1000 cd m−2, showing a small efficiency roll-off. This is one of the highest efficiencies among the non-doped devices with the electroluminescence peak below 400 nm. Moreover, the green and red phosphorescent OLEDs with mP2MPC as host can also show excellent electroluminescence performance with the forward-viewing external quantum efficiencies over 21.5 %, illustrating that mP2MPC should be a prospective material for OLEDs.

New bipolar host materials based on isoquinoline and phenylcarbazole for red PhOLEDs

Three new bipolar host materials (BPCzB, p-BPBCz and m-BPBCz) which consist of carbazoles as electron donors (D) and an isoquinoline as an electron acceptor (A) were synthesized and characterized. They all exhibited excellent thermal stability and appropriate triplet energy levels (ET). Hence, red phosphorescent organic light-emitting diodes (PhOLEDs) based on the three hosts were fabricated. The device hosted by m-BPBCz with isoquinoline linked to the meta position of phenylcarbazole unit showed the best electroluminescent (EL) performance with a turn-on voltage of 3.4 V, a maximum brightness of 52840 cd/m2, a maximum current efficiency (ηc,max) of 29.60 cd/A, a maximum power efficiency (ηp,max) of 22.58 lm/W and an external quantum efficiency (EQE) of 17.2%, demonstrating its potential application as a host for practical applications in red PhOLEDs.

Structurally modified [1,2,4]triazolo[1,5‑a]pyridine derivatives as promising materials for highly efficient blue fluorescent organic light-emitting diodes

Here, we designed and synthesized three donor–acceptor (D-A) emitters, 1-phenyl-2-(4-(2-phenyl-[1,2,4]triazolo[1,5-a]pyridin-6-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (m-PPI-TP), 1-phenyl-2-(4-(2-phenyl-[1,2,4]triazolo[1,5-a]pyridin-7-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (p-PPI-TP) and 1-phenyl-2-(4-(2-phenyl-[1,2,4]triazolo[1,5-a]pyridin-8-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (o'-PPI-TP), based on [1,2,4]triazolo[1,5-a]pyridine (TP) as electron-transporting unit and 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole (PPI) as hole-transporting moiety serving as emitting block concurrently. All three compounds were demonstrated to be thermally stable with decomposition temperatures (Td) in the range of 454–476 °C and high photoluminescence quantum yields over 86%. Besides, high exciton utilization efficiency (EUE) via “hot exciton” process was proved according to photophysical experiments and theoretical calculations analysis. What’s more, the structure–property relationships between different substituted sites of the TP building block and PPI unit were also unveiled. Finally, the non-doped blue and doped deep-blue OLEDs were fabricated with high external quantum efficiency (EQE) approaching 8%. The yellow and red phosphorescent OLEDs (PHOLEDs) based on m-PPI-TP as the host material achieved the best EL performance with EQE of 26.55% and 20.35%, respectively.

High-performance red phosphorescent light-emitting diodes built on bipolar host materials obtained from dibenzo[F,H]quinoxaline and dibenzothiophene

DTBDQ, a bipolar host material with an acceptor unit dibenzo[F,H]quinoxaline and a donor unit dibenzothiophene connected by two interstitial benzenes, was employed. DTBDQ exhibits deep LUMO and HOMO energy levels, along with a triplet energy of 2.75 eV, owing to rational molecular design. Red phosphorescent organic light-emitting diodes (PhOLEDs) devices with DTBDQ here as the host and Ir(piq)2acac as the guest layer have an external quantum efficiency(EQE) of 19.79% with a limited roll-off and an EQE of 18.08% at 1000 cd/m2, with CIE coordinates of (0.68,0.32), which is quite comparable to the National Television Systems Committee (NTSC) CIE coordinates of (0.67,0.33). The device performance is significantly better than that of devices using hole-type host CBP, electron-type host TPBI, and electron and hole-type hosts together, indicating that the performance of bipolar host materials transferring both electrons and holes is superior to that of simply two hosts as a double light-emitting layer separately, which is beneficial to carrier transport balancing and avoiding carrier buildup. DTBDQ demonstrates that it is an excellent bipolar host candidate.

Vacuum deposited WO3/Al/Al:Ag anode for efficient red organic light-emitting diodes

Vacuum deposited multilayered electrode with ultra-smooth surface morphology and suitable work function for efficient electron or hole injection at the interface between the organic and metal layers is attracting more attentions recently. Following the trend, a simplified structure of WO 3 /Al/Al:Ag anode with the sheet resistance of 1.93 Ω/sq, work function of ∼5.11 eV, and surface roughness of 0.72 nm was fabricated to develop a red organic light-emitting diode (OLED) with the external quantum efficiency (EQE) of ∼16.18% at 1000 cd m −2 , which is much higher than that of an red OLED based on standard ITO electrode (EQE ∼ 12.32%). In addition, the red OLED with our proposed ITO-free anode exhibit a bright emission with luminance of >10,000 cd m −2 at 5 V as well as could be fabricated on a large active area of 4 cm 2 without any breakdown issue. This is attributed to the ultra-smooth multilayer with a suitable workfunction reducing the contact barrier energy in the interface of organic and metal layer. Impedance spectroscopy, transient electroluminescence, and optical simulation were carried out to understand the device physics and carrier recombination behaviors. In the present work, we have developed the ITO-free, red, and exciplex organic light-emitting diodes (OLEDs) demonstrating remarkably high EL efficiency and low turn-on voltage. The simplified structure of WO 3 /Al/Al:Ag anode with the sheet resistance of 1.93 Ω/sq, work function of ∼5.11 eV, and surface roughness of 0.72 nm was fabricated to develop a red organic light-emitting diode (OLED) with the external quantum efficiency (EQE) of ∼16.18% at 1000 cd/m 2 . • The ITO-free red phosphorescent organic light-emitting diodes (OLEDs) with a metal-alloy electrode was demonstrated. • The WO 3 /Al/Al:Ag anode with low resistance of 1.93 Ω/sq, work function of 5.11 eV, and smooth surface roughness of 0.72 nm. • The ITO-free OLEDs exhibit a bright emission with luminance of >10,000 cd m −2 at 5 V. • The external quantum efficiency (EQE) of ITO-free OLEDs achieved 16.18% at 1000 cd m −2 .

Improved Efficiency and Stability of Red Phosphorescent Organic Light-Emitting Diodes via Selective Deuteration

The secondary kinetic isotope effects (KIEs) of red phosphorescent Ir(III) complexes and the corresponding devices have been investigated. The selective deuterated red emitters show negligible influence on the luminescent spectra, but have positive effects on the quantum efficiencies and stabilities in the devices. As secondary KIEs predicted, the photolysis coinciding with the electrolysis of the deuterated complexes in the devices, measured via decreasing of luminescent intensity, are reduced in rate. An about 33% increase of the device working lifetime could be readily obtained with the strategy of selective deuteration on the emitter complexes. The findings demonstrate the importance of the isotopic effect on the performance improvement of organic light emitting devices and will also trigger the study on organic optoelectronic materials via isotopic tools.

High Performance Solution-Processed Red Phosphorescent Organic Light-Emitting Diodes by Co-Doping Europium Complex as Sensitizer

High Performance Solution-Processed Red Phosphorescent Organic Light-Emitting Diodes by Co-Doping Europium Complex as Sensitizer

Highly stable and efficient deep-red phosphorescent organic light-emitting devices using a phenanthroline derivative as an n-type exciplex host partner

A new entry of nBPhen-based n-type exciplex host partner realizes high efficiency and highly stable deep red phosphorescent organic light-emitting devices with an emission peak wavelength of 670 nm, and maximum external quantum efficiency of 17%.

Imidazolyl-Phenylcarbazole-Based Host Materials and Their Use for Co-host Designs in Phosphorescent OLEDs.

In recent years, owing to the demand for high-efficiency phosphorescent organic light-emitting devices (PhOLEDs), many studies have been conducted on the development of bipolar host materials. A series of imidazolyl-phenylcarbazole-based host materials, i. e., im-CzP, im-CzPCz, im-CzPtBu, and im-OCzP, were synthesized to obtain high-efficiency green and red-emitting PhOLEDs. With im-OCzP as the host, satisfactory peak efficiencies of 22.2 (77.0 cd A-1 and 93.1 lm W-1 ) and 14.1 % (9.0 cd A-1 and 10.1 lm W-1 ) could be obtained, respectively. To further improve the performance of the devices, an electron transport material, bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PyMPM) was selected to construct a co-hosted system. The efficiency of im-OCzP combined with B3PyMPM forming co-hosts could also achieve high values of 23.0 (80.0 cd A-1 and 98.8 lm W-1 ) and 16.5 % (10.2 cd A-1 and 13.4 lm W-1 ) for green and red PhOLEDs, respectively. These results exhibited that the proposed bipolar hosts have great flexibility in adjusting the carrier balance of EML in OLEDs, demonstrating their ingenious design and high potential.

Carbazole-pyridine pyrroloquinoxaline/benzothiadiazine 1,1-dioxide based bipolar hosts for efficient red PhOLEDs

Two novel bipolar hosts Cbz-Py-PQ and Cbz-Py-SA have been designed, synthesized, and eventually successfully used for fabrication of red phosphorescent organic light-emitting diodes (PhOLEDs). Considering higher hole mobility than that of electron mobility in most of the bipolar host with 1:1 donor: acceptor ratio, herein we have made it 1:2 by linking carbazole (donor core) to pyrroloquinoxaline/benzothiadiazine 1,1-dioxide (acceptor core) through pyridine (acceptor core) featuring donor-acceptor-acceptor (D-A-A) architecture. Structure-property-performance relationship have been realized through evaluation of thermal, photophysical and electrochemical properties of both the molecules. Cbz-Py-PQ hosted red PhOLED revealed maximum efficiencies of 16.4%, 9.6 cd A−1 and 9.4 lm W−1 with maximum luminance of 20753 cd m−2 at 11.0 V.

Universally applicable small-molecule co-host ink formulation for inkjet printing red, green, and blue phosphorescent organic light-emitting diodes

For organic light-emitting diodes (OLEDs), inkjet printing technology is being developed as an alternative to the traditional vacuum evaporation, because of its precise patterning, high-efficient material utilization, large-area compatibility and low-cost. In this work, we report a universal ink formulation of small-molecule co-host and binary solvents for red, green and blue phosphorescent OLEDs. Moreover, the effect of hole-transporting layers on the ink spreading, film uniformity and exciton confinement ability is investigated. Furthermore, a large-area (170 mm × 170 mm) and homogeneous light-emitting film is inkjet-printed. Finally, red, green and blue OLEDs are successfully constructed using these optimized ink formulations on the solvent resistance hole-transporting layer. This work can reduce the complexity to adjust the host materials and solvents for different color inks, and could be applied in large-area and low-cost OLED displays with high resolution. ● A small-molecule co-host system of TCTA and 26DCzPPy is proposed for red, green and blue phosphorescent dopants. ● The binary solvents of o -dichlorobenzene and 1-chloronaphthalene are formulated to optimize the ink properties. ● The large-area light-emitting film was inkjet-printed, and OLEDs were constructed using these red, green and blue inks.

Tuning the position of methyl groups to realize highly efficient deep-red PhOLEDs

As the promising candidates for red phosphorescent organic light-emitting diodes (PhOLEDs), aryl quinoline Ir(III) complexes have attracted enormous and continual attention. The molecular design strategy of introducing methyl group to aryl quinoline unit to tune their optoelectronic properties is reported here. The synthesis, structures, electrochemical and photophysical properties of a series of methyl-substituted aryl quinoline Ir(III) complexes were systematically studied. Red and deep-rep phosphorescent emission with the maximum wavelength from 610 nm to 646 nm were accessed by combing the electron-donating effect of the 3-methyl group to centered Ir atom and increased twist angle between phenyl unit and quinoline group induced by the 2-methyl group. Deep-red PhOLEDs with the emission peak at 620 nm, CIE coordinates of (0.68, 0.32), luminescence of 23720 cd/m2, and an EQE of 40.5% were realized.