Extremely Low Roll-Off and High Efficiency Achieved by Strategic

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Extremely Low Roll-Off and High Efficiency Achieved by Strategic Exciton Management in Organic Light-Emitting Diodes with Simple Ultrathin Emitting Layer Structure Tianmu Zhang, Changsheng Shi, Chenyang Zhao, Zhongbin Wu, Jiangshan Chen, Zhiyuan Xie, and Dongge Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00513 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

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ACS Applied Materials & Interfaces

Extremely Low Roll-Off and High Efficiency Achieved by Strategic Exciton Management in Organic Light-Emitting Diodes with Simple Ultrathin Emitting Layer Structure Tianmu Zhang, †Changsheng Shi, †Chenyang Zhao, †Zhongbin Wu, †Jiangshan Chen, ‡Zhiyuan Xie* † and Dongge Ma* †,‡ †State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, University of Chinese Academy of Sciences, Changchun, 130022, P. R. China. ‡Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, P. R. China *E-mail: [email protected] (D. M), *E-mail: [email protected] (Z. X.) Keywords: Ultrathin, efficiency roll-off, exciton management, exciton distribution, organic lightemitting diodes

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ABSTRACT

Phosphorescent organic light-emitting diodes (OLEDs) possess the property of high efficiency, but have serious efficiency roll-off at high luminance. Herein, we manufactured high efficiency phosphorescent OLEDs with extremely low roll-off by effectively locating the ultrathin emitting layer (UEML) away from the high concentration exciton formation region. The strategic exciton management in this simple UEML architecture greatly suppressed the exciton annihilation due to the expansion of the exciton diffusion region, thus this efficiency roll-off at high luminance was significantly improved. The resulting green phosphorescent OLEDs exhibited the maximum external quantum efficiency (EQE) of 25.5%, current efficiency (CE) of 98.0 cd A-1 and power efficiency (PE) of 85.4 lm W-1, and still had 25.1%, 94.9 cd A-1 and 55.5 lm W-1 at 5000 cd m-2 luminance, and retained 24.3%, 92.7 cd A-1 and 49.3 lm W-1 at 10000 cd m-2 luminance, respectively. Compared to the usual structures, the improvement demonstrated in this work displays potential value in applications.


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1. INTRODUCTION Organic light-emitting diodes (OLEDs) have generated attention for next generation display and solid-state lighting because of their attractive features including homogeneous large area, color tunability and flexibility.1-5 In past years, phosphorescent OLEDs have been demonstrated to achieve almost 100% internal quantum efficiency (IQE) because they can harvest all generated singlet and triplet excitons, but still exist serious roll-off at high luminance when using a single doping emitting layer (EML) structure.6-10 It is known that, the structural design of multiple doping EMLs could reduce the efficiency roll-off at high luminance to a certain extent while achieving high efficiency in phosphorescent OLEDs.11-15 However, the multiple doping procedure also greatly increases the complexity of device processing, the repeat ability of device performance, and importantly, the efficiency rolloff at high luminance is still requires amelioration. To address these shortcomings, a simplified structure of ultrathin emitting layer (UEML) without doping that generally introduced an ultrathin layer (thickness < 1 nm) at the interface of the carrier transport layers has been experimentally proved to be an effectual way, not only for obtaining high efficiency, but also greatly simplifying the device process.16-23 However, the general UEML structure easily causes the interface charge accumulation, thus quenching the excitons, and the serious efficiency roll-off at high luminance still exists in the devices.22-26 Herein we demonstrate a simple and efficient architecture that greatly improved the efficiency roll-off at high luminance in UEML-based OLEDs. Completely different from general UEML structures (Figure 1a), an ultrathin phosphor EML was inserted to the proper position in electron-transporting layer (EML), away from the exciton recombination interface, as shown in figure 1b. This avoided serious exciton quenching, and significantly minimized the efficiency roll-off of the fabricated OLEDs.


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By optimizing the UEML position, we fabricated a high efficiency green phosphor OLED, exhibiting not only higher efficiency, but also extremely low efficiency roll-off compared to general UEML and doping double EML OLEDs.22,23 The corresponding maximum current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) reached 98.0 cd A-1, 80.59 lm W-1 and 25.6%, respectively and still kept at a high value of 92.7 cd A-1, 49.3 lm W-1 and 24.3% at 10000 cd m-2 luminance. 2. EXPERIMENTAL SECTION The OLEDs studied here were fabricated on indium tin oxide (ITO) which has been fabricated on a glass substrates, and its resistance was 10Ω/□. Before used, the pre-coated glass substrates were bathed in water-based ultrasonic cleanout fluid. Then it was processed in drying cabinet over 105°C for ten minutes, and oxygen plasma treatment for two minutes. Thermal evaporation method was used to fabricate the OLEDs in this work. The vacuum pressure of the evaporate equipment was around 6.0×10-4 pa during processing. Quartz crystal oscillator and Dektak 6M proﬁler was used to monitor and adjust the rate of material evaporation. The material evaporation was optimized to a proper rate in order to attain films of high quality. Owing to the ultrathin thickness, the UEML should not be continuous, and the film is an average thickness measured by the film controlling system. The area of the emission zone was 4 mm × 4 mm overlapped between ITO and Aluminum (Al). By using Keithley 2400 and Keithley 2000, the properties including current, luminance and voltage could be measured. Spectra-scan Spectrophotometer was introduced to realize measurement of the EL spectra. The characteristics of the devices were measured in ambient atmospheric conditions, and all the OLEDs were fabricated without additional out-coupling techniques. 3. RESULTS AND DISSCUSSION


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3.1 Device Structure and Performance. Green phosphorescent OLEDs were designed and fabricated to compare the advantage of UEML structure in this study. In these devices, ITO and Al were chosen as the anode and cathode, respectively. MoO3 and Liq were used as the hole and electron injection layers, respectively. N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) was

selected

for

the

hole-transporting

layer

(HTL),

and

bis(2-[2-

hydroxyphenyl]pyridinato)beryllium (Bepp2) for the electron-transporting layer (ETL). Green phosphorescence bis(2-phenylpyridine)iridium acetylacetonate (Ir(ppy)2(acac)) was used as the UEML.4,4',4''-tri(N-carbazolyl)triphenylamine (TCTA) was used as the host. 1,1'-bis[4-(di-ptolylamino)phenyl]cyclohexane (TAPC) is used as the electron blocking layer. The fabrication of the three devices was as follows. Device D1 (Doping double EML): ITO (180 nm)/MoO3 (10 nm)/MoO3: NPB (25%, 35 nm)/NPB (10 nm)/TAPC (5 nm)/TCTA: Ir(ppy)2(acac) (8%, 5 nm)/Bepp2: Ir(ppy)2(acac) (8%, 5 nm)/Bepp2 (15 nm)/Bepp2: Liq (3%, 25 nm)/Liq (1 nm)/Al (150 nm). Device D2 (UEML-1)： ITO (180 nm)/MoO3 (10nm)/MoO3: NPB (25%, 35 nm)/NPB (10 nm)/TAPC (5 nm)/TCTA (5 nm)/Ir(ppy)2(acac) (0.2 nm)/Bepp2 (20 nm)/Bepp2: Liq (3%, 25 nm)/Liq (1 nm)/Al (150 nm). Device D3 (UEML-2)： ITO (180 nm)/MoO3 (10 nm)/MoO3: NPB (25%, 35 nm)/NPB (10 nm)/TAPC (5 nm)/TCTA (5 nm)/Bepp2 (4 nm)/Ir(ppy)2(acac) (0.2 nm)/Bepp2 (16 nm)/Bepp2: Liq (3%, 25 nm)/Liq (1 nm)/Al (150 nm). The structures of the compared devices are similar except for the EMLs. Device D1 is a green phosphor OLED with double EMLs by doping Ir(ppy)2(acac) in TCTA and Bepp2 by a ratio of


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8%, respectively. Device D2 is a green phosphor OLED with inserting an Ir(ppy)2(acac) UEML of 0.2 nm into the interface of TCTA/Bepp2. Device D3 is a green phosphor OLED with locating the same UEML in Bepp2 4 nm away from the interface of TCTA/Bepp2. The three devices displayed similar turn-on voltage and exhibited high efficiency, indicating good exciton utilization. However, as shown in Figure 2, it remarkably exhibits higher efficiency at high luminance compared to device D1 and D2, indicating the validity of the device structure designed here. The efficiency of device D3, respectively, attained the EQE of 25.5%, CE of 98.0 cd A-1and PE of 85.4 lm W-1 at max. They respectively retained 24.3%, 92.7 cd A-1, 49.3 lm W-1 at 10000 cd m-2 high luminance, higher than those of devices D1 and D2. This indicates the exciton quenching in device D3 was significantly improved. Comparatively, device D2 showed more serious efficiency roll-off. Efficiency was reduced to 19.8%, 75.2 cd A-1 and 38.1 lm W-1 at 10000 cd m-2 luminance from the maximum efficiency of 24.2%, 92.7 cd A-1 and 85.3 lm W-1, respectively. Although the architecture of doping double-EML in device D1 improved the efficiency roll-off compared to device D2, reaching 22.2%, 85.6 cd A-1 and 43.5 lm W-1 at 10000 cd m-2 luminance, it can be seen that the exciton quenching is still serious than that in device D3. Exciton emission from the green phosphor was within the exciton recombination zone in device D1 and D2, easily quenching the radiative excitons, whereas the exciton emission and exciton recombination in device D3 were completely separated. The radiative excitons were far away from the high concentration exciton recombination zone. Therefore, the exciton quenching was greatly suppressed. This should be an important feature of the UEML architecture we designed. Table 1 summarizes the EL performance of device D1, D2 and D3. 3.2 Dependence of Exciton Distribution on the Emissive Layer Structure. To further elucidate the mechanism by which efficiency roll-off at high luminance was improved in device


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D3, we explored the tendency of the exciton distribution in recombination zones of devices D1, D2 and D3. A general method is inserting an ultrathin red or yellow phosphor as a sensing layer in the destination area, and by comparing the relative emission intensity of the red or yellow light to the control emission, information about the spatial distribution of the triplet excitons in OLEDs can be provided well.13,27,28 In this paper, a 0.02 nm PO-01 yellow phosphor layer was placed respectively at distances of -3, -2, -1, 0, 1, 2, 3 nm in device D1 and D2 in sequence, and 1, 0, 1, 2, 3, 5, 6 nm in device D3 from the interface TCTA/Bepp2 (the interface is defined as the position "0 nm"，ranging in the TCTA direction as negative distance, and in the Bepp2 direction as positive distance). The PO-01 sensing layer is extremely thin and its introduction of PO-01 sensing layer should not significantly affect the charge transport in devices (Supporting information, figure S3). To compare the intensity of PO-01 (peak=552 nm) to Ir(ppy)2(acac) (peak=520 nm), the relative intensity of PO-01 spectra could represent the tendency of the exciton distribution in each device well (Supporting information, figure S1, S2). Figure 3 shows the tendency of the exciton distribution in emission zone within devices D1, D2 and D3. Device D3 displayed a wider exciton distribution than that of device D1 and D2. Device D1 illustrates a relative narrow exciton distribution with a peak at 1 nm in ETL, which should be due to the hindering effect of Ir(ppy)2(acac) on electron transport within the doping layer of Bepp2: Ir(ppy)2(acac) (Supporting information, figure S6), and slightly higher hole mobility of TCTA than the electron mobility of Bepp2.29,30 Additionally, the triplet energy level of the host TCTA (2.86 eV) was higher than that of the host Bepp2 (2.6 eV) thus the triplet excitons on the TCTA molecules could diffuse into the Bepp2. This may result in the exciton recombination shifting to the ETL. The energy of generated excitons within recombination zone of TCTA and Bepp2 will ultimately transfer to the Ir(ppy)2(acac), finally leading to green emission (Figure 4a). Because


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the doping of Ir(ppy)2(acac) covered the entire emission layer consisted of TCTA and Bepp2, the generated excitons could attain excellent utilization and effective improvement of the exciton quenching. Therefore the device D1 represented high efficiency and reduced efficiency roll-off. For device D2, due to the energy level structure of TCTA (LUMO=-2.3 eV, HOMO=-5.7 eV and T1=2.86 eV) 29 and Bepp2 (LUMO=-2.6eV, HOMO=-5.7 eV and T1=2.6 eV), 30 the lower triplet energy level of Ir(ppy)2(acac) confined the generated excitons in the interfacial region.22 Therefore, a narrow exciton distribution with a peak at the interface of TCTA/Bepp2 was observed. Similar to device D1, in D2 the generated exciton energy could also effectively transfer to Ir(ppy)2(acac) (LUMO=-2.6 eV, HOMO=-5.0 eV and T1=2.3 eV)31, resulting in high efficiency (figure 4b). However, high exciton concentration at the interface also leads to serious annihilation which manifests as a dramatic efficiency roll-off, as shown in figure 2. In device D2, the UEML was located at the interface of high exciton concentration, leading the high concentration exciton accumulation to severely quench the radiative excitons with the increase of current density. This problem was resolved by locating the UEML away from the high concentration exciton formation region in device D3. As shown in figure 3, the spatial distribution illustrated a sharp reduction of the exciton density at 0 nm to -1 nm, which implying effective confinement of the exciton transfer from the interface to TCTA, due to its higher triplet energy level. Whereas, the exciton distribution showed a gentle slope from 0 nm to 3 nm of a high concentration level, it gradually lowered from 3 nm, suggesting exciton transfer in a single direction, recombining and emitting on the UEML 4 nm from the TCTA/Bepp2 interface. This wide exciton distribution undoubtedly reduces the exciton quenching. As shown in figure 4c, the thin Ir(ppy)2(acac) emission layer would hardly impact the charge carrier transport, therefore the electrons and holes will first be injected to the interface between TCTA/Bepp2 and then


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recombine to form excitons, finally leading to green emission by the long-range energy transfer of excitons across Bepp2. It can be seen that this process is very effective. The formed exciton energy can be 100% utilized with much lower quenching due to the exciton radiation far from the high concentration exciton formation zone. The realization of two discrete channels of exciton radiation and exciton formation provides a promising new avenue to achieve high efficiency OLEDs with extremely low efficiency roll-off. 3.3 Analysis of Quenching Mechanism Models Driven by Exciton-Density. Extensive experiments have shown that triplet-polaron quenching (TPQ) and triplet-triplet annihilation (TTA) are considered as the prime quenching mechanisms on efficiency roll-off in phosphorescence OLEDs. To analyze the quenching mechanism among our OLEDs with UEML architecture, we fitted efficiency properties of device D1, D2 and D3 by a TTA model. As shown in Figure 5, the efficiency degradation properties were fitted well using the TTA model, implying that it is the dominant annihilation process in device D1, D2 and D3. The TTA model refers to the biexcitonic quenching mechanism proposed by Baldo group.32 It can be expressed as follows,

𝜂𝐸𝑄𝐸 𝜂0

=

𝐽0 4𝐽

(√1 +

8𝐽 𝐽0

− 1)

(1)

The maximum EQE in the absence of TTA is represented as η0. The current density is J, and J0 stands for the current density when EQE drops to half of its maximum value, called as the critical density. Efficiency roll-off, and the J0 value are inversely related. The values J0 of device D1, D2 and D3 were 256.1, 137.4 and 324.4, respectively. As D3 had the largest J0 value, this provides additional validity for suppressing exciton quenching in device D3.


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4. CONCLUSION We successfully resolved the problem of efficiency roll-off at high luminance in phosphorescent OLEDs by locating the UEML away from the high concentration exciton formation region. The realization of two discrete channels of exciton radiation and exciton formation greatly suppressed the exciton annihilation due to the expansion of the exciton recombination region, thus not only achieving high efficiency, but also significantly suppressing the efficiency roll-off at high luminance. The resulting green phosphorescence OLEDs exhibited the maximum EQE of 25.5%, CE of 98.0 cd A-1 and PE of 85.4 lm W-1. They retained 25.1%, 94.9 cd A-1 and 55.5 lm W-1 at 5000 cd m-2 luminance, respectively. This novel design provides a promising avenue to fabricate high efficiency phosphorescent OLEDs with extremely low efficiency roll-off.


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Figure 1. Models of exciton emission process in two kinds of UEML architectures. a) OLEDs with general UEML structure. b) OLEDs with improved UEML structure in this work.


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Figure 2. Characteristics of Device D1, D2 and D3: a) Current-voltage-luminance, b) CEluminance, c) PE-luminance, and d) EQE-luminance.


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Figure 3. Exciton distribution of the emission zone in Device D1, D2 and D3.


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Figure 4. Working principles of devices. a) Device D1, b) device D2 and c) device D3.


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Figure 5. Normalized EQE-current density characteristics of devices. a) Device D1, b) device D2, and c) device D3. d) The extracted J0 values are given. The red solid lines are the fitted results by TTA model.


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Table 1. Summary of EL performance of devices D1-D3.

Device

Vona)

EQEb)

CEc)

PEd)

[V]

[%]

[cdA-1]

[lmW-1]

Max.

@5000cd/m2

@10000cd/m2

Max.

@5000cd/m2

@10000cd/m2

Max.

@5000cd/m2

@10000cd/m2

D1

2.8

24.1

23.2

22.2

92.4

87.8

85.6

83.3

51.9

43.5

D2

3

24.2

21.5

19.8

92.7

80.5

75.2

85.3

48.3

38.1

D3

3

25.5

25.1

24.3

98

94.9

92.7

85.4

55.5

49.3

a) Turn-on voltage, b) External quantum efficiency (EQE), c) current efficiency (CE), d) power efficiency (PE).


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXX. Original normalized spectra by inserting a sensing layer of PO-01 into series position in device D1, D2 and D3 for exciton distribution exploring; Current density-Voltage of a 0.02 nm PO-01 sensing layer inserted at different positions to the interface TCTA/Bepp2; Properties of the devices within the optimizing process of the UEML location; Structures and property index of organic materials in this work (PDF)


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (D. M.) *E-mail: [email protected] (Z. X.) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes: The authors declare no competing financial interest. ACKNOWLEDGMENT

The authors gratefully acknowledge the National Key Research and Development Plan of China (2016YFB0400701) for the support of this research.


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REFERENCE (1) Forrest, S. R. The Path to Ubiquitous and Low-Cost Organic Electronic Appliances on Plastic. Nature. 2004, 428, 911-918. (2) Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest,

S.

R.

Highly

Efficient

Phosphorescent

Emission

from

Organic

Electroluminescent Devices. Nature. 1998, 395, 151-154. (3) Tang, C. W.; Vanslyke, S. A. Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913-915. (4) Peng, H. J.; Zhu, X. L.; Sun, J. X.; Yu, X. M.; Wong, M.; Kwok, H. S. Efficiency Improvement of Phosphorescent Organic Light-emitting Diodes Using Semitransparent Ag as Anode. Appl. Phys. Lett. 2006, 88, 033509. (5) Tao, S.; Peng, Z.; Zhang, X.; Wang, P.; Lee, C. S.; Lee, S. T. Highly Efficient NonDoped Blue Organic Light-Emitting Diodes Based on Fluorene Derivatives with High Thermal Stability. Adv. Funct. Mater. 2010, 15, 1716-1721. (6) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nearly 100% Internal Phosphorescence Efficiency in an Organic Light-Emitting Device. J. Appl. Phys. 2001, 90, 5048-5051. (7) Xiao, L.; Su, S. J.; Agata, Y.; Lan, H.; Kido, J. Nearly 100% Internal Quantum Efficiency in an Organic Blue-Light Electrophosphorescent Device Using a Weak Electron Transporting Material with a Wide Energy Gap. Adv. Mater. 2010, 21, 12711274. (8) Zhang, Q.; Tsang, D.; Kuwabara, H.; Hatae, Y.; Li, B.; Takahashi, T.; Lee, S. Y.; Yasuda, T;

Adachi,

C.

Nearly

100%

Internal

Quantum

Efficiency

in

Undoped


19


Page 20 of 24

Electroluminescent Devices Employing Pure Organic Emitters. Adv. Mater. 2015, 27, 2096-2100. (9) Ye, J.; Zheng, C. J.; Ou, X. M.; Zhang, X. H.; Fung, M. K.; Lee, C. S. Management of Singlet and Triplet Excitons in a Single Emission Layer: A Simple Approach for a HighEfficiency Fluorescence/Phosphorescence Hybrid White Organic Light-Emitting Device. Adv. Mater. 2012, 24, 3410-3414. (10) Dias, F. B.; Bourdakos, K. N.; Jankus, V.; Moss, K. C.; Kamtekar, K. T.; Bhalla, V.; Santos J.; Bryce M. R.; Monkman A. P. Triplet Harvesting With 100% Efficiency by Way of Thermally Activated Delayed Fluorescence in Charge Transfer OLED Emitters. Adv. Mater. 2013, 25, 3707-3714. (11) Reineke, S.; Schwartz, G.; Walzer, K.; Leo, K. Reduced Efficiency Roll-Off in Phosphorescent Organic Light Emitting Diodes by Suppression of Triplet-Triplet Annihilation. Appl. Phys. Lett. 2007, 91, 123508. (12) Wu, Z.; Sun, N.; Zhu, L.; Sun, H.; Wang, J.; Yang, D.; Qiao X.; Chen J.; Alshehri, S. M.; Ahamad, T.; Ma, D. Achieving Extreme Utilization of Excitons by an Efficient Sandwich-Type Emissive Layer Architecture for Reduced Efficiency Roll-Off and Improved Operational Stability in Organic Light-Emitting Diodes. ACS Appl. Mat. Interfaces. 2016, 8, 3150-3159. (13) Kim, S. Y.; Jeon, W. S.; Park, T. J.; Pode, R.; Jin, J.; Kwon, J. H. Low Voltage Efficient Simple p-i-n Type Electrophosphorescent Green Organic Light-Emitting Devices. Appl. Phys. Lett. 2009, 94, 133303.


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Page 21 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Lee, J.; Lee, S.; Yoo, S.; Kim, K.; Kim, J. Langevin and Trap-Assisted Recombination in Phosphorescent Organic Light Emitting Diodes. Adv. Funct. Mater. 2014, 24, 46814688. (15) Park, Y. S.; Lee, S.; Kim, K. H.; Kim, S. Y.; Lee, J. H.; Kim, J. J. Exciplex-Forming CoHost for Organic Light-Emitting Diodes with Ultimate Efficiency. Adv. Funct. Mater. 2013, 23, 4914-4920. (16) Xia, D.; Wang, B.; Chen, B.; Wang, S.; Zhang, B.; Ding, J.; Wang, L.X.; Jing, X. B.; Wang, F. S. Self-Host Blue-Emitting Iridium Dendrimer with Carbazole Dendrons: Nondoped Phosphorescent Organic Light-Emitting Diodes. Angew. Chem., 2014, 53, 1048-1052. (17) Zhao, Y.; Chen, J.; Ma, D. Realization of High Efficiency Orange and White Organic Light Emitting Diodes by Introducing an Ultra-Thin Undoped Orange Emitting Layer. Appl. Phys. Lett. 2011, 99, 163303. (18) Tan, T.; Ouyang, S.; Xie, Y.; Wang, D.; Zhu, D.; Xu, X.; Fong, H. H. Balanced White Organic Light-Emitting Diode with Non-Doped Ultra-Thin Emissive Layers Based on Exciton Management. Org. Electron. 2015, 25, 232-236. (19) Zhao, F.; Zhu, L.; Liu, Y.; Wang, Y.; Ma, D. Doping-Free Hybrid White Organic LightEmitting Diodes with Fluorescent Blue, Phosphorescent Green and Red Emission Layers. Org. Electron. 2015, 27, 207-211. (20) Li S. H.; Wu S. F.; Wang Y. K.; Liang J. J.; Sun Q.; Huang C. C.; Wu J.C.; Liao L. S.; Fung M. K. Management of Excitons for Highly Efficient Organic Light-Emitting Diodes with Reduced Triplet Exciton Quenching: Synergistic Effects of Exciplex and Quantum Well Structure. J. Mater. Chem. C. 2018, 6, 342-349.


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(21) Wang, Q.; Oswald, I. W.; Yang, X.; Zhou, G.; Jia, H.; Qiao, Q.; Chen, Y.; HoshikawaHalbert J.; Gnade B. E. A Non-Doped Phosphorescent Organic Light-Emitting Device with above 31% External Quantum Efficiency. Adv. Mater. 2014, 26, 8107-8113. (22) Zhao, Y.; Chen, J.; Ma, D. Ultrathin Nondoped Emissive Layers for Efficient and Simple Monochrome and White Organic Light-Emitting Diodes. ACS Appl. Mat. Interfaces. 2013, 5, 965-971. (23) Wu, S.; Li, S.; Qi, S.; Huang, C.; Fung, M. K. Highly Efficient White Organic LightEmitting Diodes with Ultrathin Emissive Layers and a Spacer-Free Structure. Sci. Rep. 2016, 6, 25821. (24) Liu, B.; Tao, H.; Wang, L.; Gao, D.; Liu, W.; Zou, J.; Xu, M; Ning, H. L.; Peng, J. B; Cao, Y. High-Performance Doping-free Hybrid White Organic Light-Emitting Diodes: the Exploitation of Ultrathin Emitting Nanolayers (

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