tris(N-carbazolyl)triphenylamine Research Articles

High-performance warm-white OLEDs using interfacial exciplex energy transfer with external quantum efficiency exceeding 30%

To achieve energy-saving and environmentally friendly warm white lighting through the simplification of white organic light-emitting device (WOLED) structure and manufacturing process, we designed and optimized a WOLED by incorporating bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III)(FIrpic)-doped 4,4′,4′-tris(N-carbazolyl)triphenylamine(TCTA) and bis(4-phenylthieno[3,2-c]pyridinato-N,C2')(acetylacetonate)iridium(III)(PO-01)-doped 2,2',2′'-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)(TPBI) thin films as the blue and yellow emissive layer (EML), respectively, and forming an exciplex at TCTA and TPBI interface. By adjusting the doping concentration and position within the EML and the thickness of the transport layer, we achieved the optimal emission balance between the blue and yellow colors in the proposed WOLED. We successfully fabricated a highly efficient and structurally simplified WOLED without the use of light extraction coupling techniques, which exhibited high external quantum efficiency (EQE) of 32.6% and current efficiency (CE) of 80.3 cd/A, along with stable emission color with minimal CIE variation of ΔCIE(0.0027, 0.0023) from 500 cd/m² to 5000 cd/m². The experimental results revealed that the designed device structure with the triple host system (TCTA, TPBI, and interface exciplex) facilitated the widening of the recombination zone (RZ), enhanced exciton confinement and utilization, reduced triplet-triplet annihilation (TTA) effects, and achieved efficient host-guest energy transfer. Furthermore, the designed WOLED emitted warm white light spectrum suitable for healthy lighting applications. Therefore, we believe that our work contributes to the further development of ideal and simplified WOLED structure designs for achieving high-efficiency, simplified manufacturing, low-cost, energy-saving, and environmentally friendly healthy lighting.

Experimental and theoretical studies on charge transfer complex formed by TCTA and TPBi

The studies on charge transfer (CT) complex formed by 4,4,4″-tris(N-carbazolyl)-triphenylamine (TCTA) and 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBi) were carried out by experiment and simulation calculations. Experimentally, exciplex-forming in TCTA:TPBi mixture is observed and electroplex is formed for TCTA/TPBi bi-layer structure. The density functional theory via DMol3 package was performed to explore the geometrical structures, stability, and electronic structures of the complexes. The calculation results demonstrate that geometrically stable structure of the complex is assumed to enable electroplex-forming because the molecular planes of donor and acceptor are separated by 5.5 Å. However, the formation of exciplex requires a small intermolecular separation of below 4 Å, which is relatively unstable for TCTA−TPBi complex and has to be driven by an external force. The requisition can be fulfilled in TCTA:TPBi mixture due to a reduction of intermolecular interval caused by tight molecular packing and strong intermolecular interaction deriving from mixing. The emission of exciplex species is thereof observed in TCTA:TPBi mixture and analogue electroplex in TCTA/TPBi bi-layer system. The calculated frontier molecular orbitals of the complexes exhibit the lowest unoccupied molecular orbital (LUMO) is positioned on TPBi and the highest occupied molecular orbital (HOMO) on TCTA, inferring that the emission of the CT complexes is originated from electron cross transition between the LUMO of TPBi and HOMO of TCTA. Meanwhile, the greater overlap of the molecular orbitals and larger oscillator strength of lowest-lying singlet excited states manifest that the CT complex in TCTA:TPBi mixture can be more efficiently produced, resulting in better electroluminescent performance, compared to TCTA/TPBi system.

Analysis of the characteristics of organic light-emitting diodes with single and mixed-host EML by impedance spectroscopy

Impedance spectroscopy was used to investigate the charge transportation and accumulation mechanisms in a mixed-host emissive layer (EML) of phosphorescent organic light-emitting diodes (OLEDs). 1, 3, 5-tris(1-phenyl-1H-benzimidazole-2-yl)benzene (TPBi) and 4, 4′, 4″-tris(N-carbazolyl)-triphenylamine (TCTA) materials were used as the electron transport and hole transport layers, respectively, to fabricate the mixed-host EML device. The results showed enhanced current and power efficiency owing to the use of the same material as the mixed-host EML, eliminating energy barriers within the device, coupled with the negative interfacial charges in the polarized TPBi material. The hole- and electron-split devices of the mixed-host EML OLED were analyzed to comprehensively understand the enhanced electrical properties within the device. Subsequently, the capacitance-frequency (C–F) characteristics of the devices were simulated with an equivalent circuit to quantitatively determine the capacitance and resistance in each organic layer at specific voltages (0–4 V) representing each characteristic step on the capacitance-voltage (C–V) curve.

A highly-efficient exciplex-based multifunctional fluorescent film probe to aniline and N-methylphenethylamine vapors

Exciplexes are often mentioned in the research of organic light-emitting diode (OLED), but there are nearly no reports using the ready-made exciplexes as fluorescent probes directly. In this work, F1 based on the fluorescent exciplex composed of 4, 4′, 4″- tris (N-carbazolyl) triphenylamine (TCTA) and 1- phenyl- 1H- benzo[d]imidazole −1, 3, 5- triazine (PIM-TRZ) is firstly applied as fluorescent film probe to organic amine vapors. The F1 film can accurately identify aniline (AN) and N-methylphenethylamine (MPEA, a simulant of methamphetamine) vapors, as well as roughly distinguish aliphatic and aromatic amine vapors easily by different output signals: the AN vapor causes an instantaneous and entire fluorescence quenching, MPEA vapor leads to the fluorescence quenching of the F1 film along with obvious red-shifted photoluminescence (PL) spectrum, the common aliphatic amine vapors bring the emergence of emission peak of TCTA, and the common aromatic amine vapors only result in partial fluorescence quenching of the F1 film. To the best of our knowledge, it is the first multifunctional fluorescent probe to highly identify AN and MPEA vapors simultaneously among various amines. Hereby, the study provides both promising accurate fluorescent multifunctional probe to AN and MPEA and new strategy for the construction of fluorescent sensors to organic amine vapors. Moreover, the new application of fluorescent exciplex is explored.

Correlation between the structural morphology and device characteristics of quantum dot based emission layer blended with small molecular hole transport material

Quantum dot-based light-emitting diodes (QD-LED) have advantages over organic-based light-emitting diodes (OLED) with narrow bandwidth and easy tuning of the wavelength range. A blending of the organic hole transport material (HTM) into the quantum dot (QD) emission layer (EML) can improve the efficiency of the QD-LED at an optimized concentration. However, the relationship between structural morphology and efficiency has not been fully understood. In this work, it is found that the efficiency of green QD-LED is improved when we mixed 10 wt% of 4,4′,4′′-tris(N-carbazolyl)triphenylamine (TCTA), a small molecular HTM, to QD EML. We find that the hole transportability is improved as the mixing ratio of TCTA increases up to ∼10 wt%. Above 10 wt% of TCTA, the electrons flowing from the cathode are relatively blocked to prevent exciton formation. There is no significant ordering change for the QD mixed with 10 wt% of TCTA compared to the QD-only film using the grazing incidence small X-ray scattering technique. With increasing TCTA ratio, TCTA induces QDs aligned in 2D hexagonal close-packing. It turns out that the excess TCTA enhances the ordering of QDs but deteriorates the efficiency of the devices due to a carrier leakage of imperfect charge injection into QDs.

Highly efficient yellow organic light-emitting diodes with slow efficiency roll-off by mixing red and green emissions

In this work, we report the design and optimization of yellow organic light-emitting diodes (OLEDs) with tris [2-(p-tolyl)pyridine]iridium (III) (Ir (mppy)3) doped 4,4′,4′-tris(N-carbazolyl)triphenylamine (TCTA) and bis(2-phenylquinoline) (2,2,6,6-tetramethylheptane-3,5-dionate)iridium (III) (PQ2Ir (dpm)) doped 2,6-bis(3-(9H-carbazol-9-yl)phenyl)phyridine (26DCzPPy) films as green and red light-emitting layers (EMLs). Experimental results demonstrated that the designed double EMLs system is beneficial in carriers' distribution, exciton confinement and reducing triplet exciton quenching because of the matched energy levels and triplet energies of hosts and dopants. Finally, the optimal device showed a very maximal current efficiency of 65.51 cd/A, slow roll-off of efficiency with a very high current efficiency of 63.54 cd/A at 5000 cd/m2, and excellent yellow emission with CIE coordinates of (0.468, 0.493) at 10 mA/cm2.

Effect of low-dose doping of small-molecules on the surface potential and carrier balance of TADF OLEDs

Solution-Processed organic light emitting diodes (OLEDs) have been intensively studied due to the simple preparation process. However, the devices made of small molecular luminescent materials always suffer from the reduce of the carrier transport capacity by the influence of molecular agglomeration. In this work, small-molecules material 4,4',4''-tris(N-carbazolyl)-triphenylamine (TCTA) is successfully introduced into the molecular aggregation site formed by traditional host material 1,3-Di-9-carbazolylbenzene (mCP), improving the charge mobility of the emitting layer. Doped mCP film has more compact molecular packing, which makes it has better charge transfer performance as a solid solvent of 2,3,4,6-tetrakis(9H-carbazole-9-yl)−5-fluorobenzonitrile (4CzFCN). Besides, 3 wt% of TCTA is beneficial to optimized electrochemical doping, lowering the surface potential of emitting layer and promoting holes injection. Doped host-guest system device exhibits a maximum current efficiency (CEmax = 29.6 cd/A) which has improved about 60% compared to the device with host of pure mCP (CEmax = 18.5 cd/A). The findings may provide a strategy for the future research on the film structure of solution-processed small molecule OLEDs.

Improved carrier injection and balance in solution-processed blue phosphorescent organic light emitting diodes based on mixed host system and their transient electroluminescence

Solution processed blue phosphorescent organic devices based on mixed host system were fabricated in this paper. A set of blue phosphorescent organic light emitting diodes (PhOLEDs) utilizing two small molecular materials of 3,3-Di(9H-carbazol-9-yl) biphenyl (mCBP) and 4,4′,4′′-tris(N-carbazolyl)-triphenylamine (TCTA) as mixed host doped with phosphor bis[(4,6-difluorophenyl)-pyridina-to-N,C2](picolinate) iridium(III) (FIrpic) are fabricated. The results show that the performance of mixed host system devices is improved compared with two single host devices. The max luminance of 23,340 cd/m2 is achieved in the mCBP:TCTA (2:1) mixed host device while the 14,400 cd/m2 in mCBP single host devices and the 12,968 cd/m2 in TCTA, and the current efficiency is improved from 8.17 cd/A(mCBP), 9.61 cd/A (TCTA) to 13.62 cd/A at the luminance of 1000 cd/m2. The transient electroluminescence measurement was utilized to detect the carrier injection between two kinds of PhOLEDs and penetrate how the trap charges works in the mixed host devices. This presents that the mixed host system contributes to the hole injection and the equilibrium of the carrier transport, reduces trapped charges, thus brings about high efficiency devices.

Novel yellow phosphorescent iridium complexes with cycolmetalated (pyridin-2-yl)dibenzothiophene-S,S-dioxide ligands for singly doped emissive layer hybrid white organic light-emitting diodes

Two novel dibenzothiophene-S,S-dioxide-based iridium complexes with yellow emission were fully synthesized and characterized. The photophysical, electrochemical and thermal properties, as well as electroluminescent properties of the resulting iridium complexes were discussed. Both the iridium complexes possess good thermal stability and high photoluminescence quantum yields. Also, we have successfully fabricated singly doped emissive layer (SDEL) hybrid white organic light-emitting devices (WOLEDs) using (3-PyFSO)2IrPic or (2-PyFSO)2IrPic as yellow-emitting phosphors and 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA) as host and blue-emitting fluorophor. The WOLEDs with (3-PyFSO)2IrPic show a maximal power efficiency (PE) of 4.28 lm/W at a brightness of 3.7 cd/m2 and a Commission Internationale de L'Eclairage (CIE) coordinates of (0.227, 0.260). Compared to (3-PyFSO)2IrPic, the WOLEDs based on (2-PyFSO)2IrPic exhibit a little better device's performances with a maximum PE of 4.70 lm/W at a brightness of 2.6 cd/m2 and a CIE coordinates of (0.244, 0.243).

Thermally activated delayed fluorescence of copper(I) complexes using N, N′-heteroaromatic of 2-(5-phenyl-1,2,3- triazole)pyridine as ligand

Two Cu(I) complexes of [Cu(PPh3)2(pptz)]BF4 and [Cu(POP)(pptz)]BF4 using 2-(5-phenyl-1,2,3-three triazole)pyridine (pptzH), triphenyl phosphine (pph3) and (bis-[2-(diphenyl phosphino) phenyl]ether) (POP) as ligands were synthesized and characterized. The complexes [Cu(PPh3)2(pptz)]BF4 and [Cu(POP)(pptz)]BF4 exhibit thermally activated delayed fluorescence (TADF) features with photoluminescence quantum yields (ϕPL) of 89.87% and 27.82% at room temperature in solid state. The maximum emission peak of [Cu(PPh3)2(pptz)]BF4 is located at 490 nm. The solution-processing organic-light emitting devices (OLEDs) were fabricated by using [Cu(PPh3)2(pptz)]BF4 and [Cu(POP)(pptz)]BF4 as emitters and 4,4′,4′′-tris(N-carbazolyl)-triphenylamine (TCTA) as host. The device based [Cu(POP)(pptz)]BF4 displayed the better electroluminescent performance with the maximum current efficiency of 2.1 cd/A than that of device used [Cu(PPh3)2(pptz)]BF4 as emitter.

Exciplex-Forming Cohost for High Efficiency and High Stability Phosphorescent Organic Light-Emitting Diodes.

An exciplex forming cohost system is employed to achieve a highly efficient organic light-emitting diode (OLED) with good electroluminescent lifetime. The exciplex is formed at the interfacial contact of a conventional star-shaped carbazole hole-transporting material, 4,4',4″-tris(N-carbazolyl)-triphenylamine (TCTA), and a triazine electron-transporting material, 2,4,6-tris[3-(1H-pyrazol-1-yl)phenyl]-1,3,5-triazine (3P-T2T). The excellent combination of TCTA and 3P-T2T is applied as the cohost of a common green phosphorescent emitter with almost zero energy loss. When Ir(ppy)2(acac) is dispersed in such exciplex cohost system, OLED device with maximum external quantum efficiency of 29.6%, the ultrahigh power efficiency of 147.3 lm/W, and current efficiency of 107 cd/A were successfully achieved. More importantly, the OLED device showed a low-efficiency roll-off and an operational lifetime (τ80) of ∼1020 min with the initial brightness of 2000 cd/m2, which is 56 times longer than the reference device. The significant difference of device stability was attributed to the degradation of exciplex system for energy transfer process, which was investigated by the photoluminescence aging measurement at room temperature and 100 K, respectively.

Isomeric N‐Linked Benzoimidazole Containing New Electron Acceptors for Exciplex Forming Hosts in Highly Efficient Blue Phosphorescent OLEDs

Three benzoimidazole containing new electron‐accepting materials of 1,3,5‐tris(2‐(pyridin‐2‐yl)‐1H‐benzo[d]imidazol‐1‐yl)benzene (i TPyBIB), 1,1′,1′′‐(pyridine‐2,4,6‐triyl)tris(2‐phenyl‐1H‐benzo[d]imidazole) (i TPBIPy), and 1,1′,1′′‐(pyridine‐2,4,6‐triyl)tris(2‐(pyridin‐2‐yl)‐1H‐benzo[d]imidazole) (i TPyBIPy) are designed and synthesized. Compared to the commercial electron‐transport 2,2,2‐(1,3,5‐phenylene)‐tris(1‐phenyl‐1H‐benzimidazole) (TPBI) with the C‐atom in benzoimidazole linked to the central phenyl ring, the introduction of isomeric N‐linkage to the central phenyl or pyridine ring in three compounds remarkably simplifies the synthesis to a one‐step CN coupling reaction. iTPyBIB, iTPBIPy, and iTPyBIPy exhibit comparable highest occupied molecular orbital (≈6.0 eV) and lowest unoccupied molecular orbital (≈2.5 eV) energy levels, similar triplet energy levels of ≈2.65 eV with TPBI, whereas reduced electron‐transport properties. All materials can be applied as electron acceptors to form universal exciplex with general hole‐transport electron donors, such as N,N′‐dicarbazolyl‐3,5‐benzene (mCP), 4,4′,4′′‐tris(N‐carbazolyl)‐triphenylamine (TCTA), and 1,1‐bis[4‐[N,N‐di(p‐tolyl)‐amino]phenyl]cyclohexane (TAPC) in the blended films with 1:1 molar ratio. A maximum external quantum efficiency of 17.0%, 19.3%, and 13.4% is achieved in mCP, mCP:iTPBIPy, and mCP:TPBI hosted blue phosphorescent organic light‐emitting diodes, respectively, indicating the superior electron‐acceptor property of the N‐linked iTPBIPy for exciplex forming cohost.

Hole-transporting small molecules as a mixed host for efficient solution processed green phosphorescent organic light emitting diodes

We have demonstrated highly efficient and reduced efficiency roll-off solution processed green phosphorescent organic light emitting diodes (OLEDs) utilizing two hole-transporting small molecular materials of 1,3-bis(carbazol-9-yl)benzene (mCP) and 4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA) as a mixed host. Compared with the corresponding two single host devices, the mixed host one shows a superior overall performance. A low driving voltage of 5.5 V at 1000 cd/m2 with a current efficiency of 39.5 cd/A is achieved in the mCP:TCTA (3:1) mixed host device. Even at the luminance of 10000 cd/m2, the efficiency still reaches 28.6 cd/A. The enhanced charge carrier balance and broadened exciton recombination zone due to the mixed host contribute to the improvement of efficiency and efficiency roll-off.

Use of space interlayer in phosphorescent organic light-emitting diodes to improve efficiency and reduce efficiency roll-off

Typical phosphorescent organic light-emitting diodes (PhOLEDs) encounter efficiency roll-off problems and detrimentally degrades the device performance for practical applications particularity at high-brightness lightening. Here, we report high efficiency and reduced efficiency roll-off PhOLEDs by employing both 4,4′,4″-tris-(N-carbazolyl)-triphenylamine (TCTA) and 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBi) as transporting, hosting and space interlayer (SIL) materials. The use of SIL blending TCTA into TPBi enables the current efficiency roll-up from 57.8 to 60.1 cd A−1 with luminance increasing from 10.0 to 1143.0 cd m−2. In particular, the current efficiency drops <10% from 1143.0 to 5020.0 cd m−2. In addition, the red-emitting sensitized dye further demonstrates that the SIL is dominant in expanding the dual exciton formation zone and suppressing efficiency roll-off. Also, the hole/electron balance can be manipulated by adjusting the doping ratio of the two materials in the SIL.

Study of organic light-emitting diodes with exciplex and non-exciplex forming interfaces consisting of an ultrathin rubrene layer

Fluorescent organic light-emitting diodes (OLEDs) with exciplex and non-exciplex forming interfaces, where an ultrathin 5,6,11,12,-tetraphenylnaphtacene (rubrene) emissive layer was sandwiched, were fabricated. The performances of 4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA)/1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) exciplex-type device and 4,4′-di(9H-carbazol-9-yl)biphenyl (CBP)/TPBi no-exciplex-type device were optimized in terms of the thickness of rubrene ultrathin layer. The results showed that the exciplex-type device yielded high device efficiency, attributing to the utilization of triplet excitons as the triplets of the exciplex can be up-converted into singlets via reverse intersystem crossing. The light emission process in exciplex-type device was dominated by energy transfer, while the non-exciplex type device was dominated by charge trapping emission mechanism, which was verified by the single carrier devices.

High efficiency yellow organic light-emitting diodes with optimized barrier layers

High efficiency Iridium (III) bis (4-phenylthieno [3,2-c] pyridinato-N,C2′) acetylacetonate (PO-01) based yellow organic light-emitting devices are fabricated by employing multiple emission layers. The efficiency of the device using 4,4′,4″-tris(N-carbazolyl) triphenylamine (TCTA) as potential barrier layer (PBL) outperforms those devices based on other PBLs and detailed analysis is carried out to reveal the mechanisms. A forward-viewing current efficiency (CE) of 65.21cd/A, which corresponds to a maximum total CE of 110.85cd/A is achieved at 335.8cd/m2 in the optimized device without any outcoupling enhancement structures.

Highly efficient red phosphorescent organic light-emitting devices based on solution-processed small molecular mixed-host

Highly efficient solution-processed red phosphorescent organic light-emitting devices were developed using 4,4′,4″-tris (N-carbazolyl)-triphenylamine (TCTA) blended with 4,4′-bis-(carbazol-9-yl)biphenyl (CBP) as a mixed-host for the emitting layer. The performances of the fabricated devices made with different mixing ratios of host materials were investigated, and were found to depend on the mixing ratios. Under the optimal TCTA:CBP ratio (3:7), the maximum luminous efficiency of the device reached 19.9cd/A, corresponding to external quantum efficiency of 11.1%. Moreover, this device with the mixed-host structure shows over 50% enhanced efficiency compared with the device using CBP as the single host. These improvements were attributed to the mixed-host structure, which effectively enhanced the hole injection/transport properties and gave a good charge carrier balance.

Study of triplet exciton’s energy transfer in white phosphorescent organic light-emitting diodes with multi-doped single emissive layer

The performance of three color single emissive layer white phosphorescent organic light-emitting diodes (PHOLEDs), with different hole transporting materials of N,N′-diphenyl-N,N′-bis(l-naphthyl-phenyl)-(l,l′-biphenyl)-4,4′-diamine (NPB), 4,4_,4_-tris_N-carbazolyl_tri-Phenylamine (TCTA) and 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), was analyzed. It is found that the luminous efficiency of white PHOLEDs is closely related to the triplet exciton energy level in the hole transporting layer (HTL). White PHOLEDs with a TAPC as HTL, having the highest triplet exciton energy level amongst the three different hole transporting materials, yielded external quantum efficiency of 25.5% at 4.5V and a high luminous efficiency of 51cd/A at 4.5V with CIE color coordinates of (0.34, 0.39) at 10V. The results reveal that the effective confinement of triplet excitons in white PHOLEDs with a high triplet exciton energy level HTL (TAPC) allows improving the luminous efficiency, as compared to the devices made with NPB and TCTA. Additionally, we demonstrated possibility for the loss of intensity in EL spectra of the white PHOLED devices with NPB and TCTA as HTL compared to that of the device with TAPC as HTL.

General application of blade coating to small-molecule hosts for organic light-emitting diode

Blade coating is applied to multi-layer phosphorescent OLED with five small-molecule hosts for the emission layer, including bis[3,5-di (9H-carbazol-9-yl)phenyl]diphenylsilane (SimCP2), 2,6-bis(3-(9H-carbazol-9-yl)phenyl) pyridine (26DCzPPy), 4,4′,4″-tris-(N-carbazolyl)-triphenylamine (TCTA), 9,9-bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl]fluorene (TBCPF), and 2,7-bis(diphenylphosphoryl)-9,9′-spirobi[fluorene] (SPPO13). In general, blade coating gives low surface roughness around 0.2nm without phase separation of the emitter and the host. In the large area of 2cm by 3cm the film thickness distribution is within 10% and uniform light-emission is achieved. 1,3-Bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7) is added to tune the electron transport. Among all the hosts, 26DCzPPy and SimCP2 have by far the best electron–hole balance and consequently show the highest efficiency. For SimCP2, the maximal efficiency is 15.8cd/A for blue and 24.2cd/A for white emission. The order of efficiencies for the hosts is found to be quite different from the order in vacuum evaporation for the same device structures.

Efficient Charge Balance of Green Organic Light Emitting Diodes Using Mixed Charge Transporting Materials as the Host

The mixed layer for an emitting layer (EML) consists of two materials 4,4′,4′′-tris(N-carbazolyl)-triphenylamine (TCTA) and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) without host material. A series of EML structures were fabricated with three types of single mixed layers (mixing ratio 2:1, 1:2) and double mixed layers (mixing ratio 2:1/1:2), and the EML of using double mixed layer was optimized. The maximum luminous efficiency (LE) and quantum efficiency (QE) were 62.07 cd/A, 18.92%, respectively. Moreover, the roll-off ratio of LE was 20.42% at 20,000 cd/m2. Interestingly, this result shows the roll-off ratio was reduced roughly 50% compared to the reference device.