Highly efficient green and white fluorescent organic electroluminescent devices with co-doped electron transport material as both supplementary host and blue emitter

Author’s Accepted Manuscript Highly efficient green and white fluorescent organic electroluminescent devices with co-doped electron transport material as both supplementary host and blue emitter Rongzhen Cui, Liang Zhou, Yanan Li, Yunlong Jiang, Xuesen Zhao, Liping Lu www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(18)31263-8 https://doi.org/10.1016/j.jlumin.2018.08.075 LUMIN15873

To appear in: Journal of Luminescence Received date: 12 July 2018 Revised date: 10 August 2018 Accepted date: 24 August 2018 Cite this article as: Rongzhen Cui, Liang Zhou, Yanan Li, Yunlong Jiang, Xuesen Zhao and Liping Lu, Highly efficient green and white fluorescent organic electroluminescent devices with co-doped electron transport material as both supplementary host and blue emitter, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.08.075 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highly efficient green and white fluorescent organic electroluminescent devices with co-doped electron transport material as both supplementary host and blue emitter

Rongzhen Cui a, b, Liang Zhou b*, Yanan Li b, Yunlong Jiang b, Xuesen Zhao b, and Liping Lu a*

a

Material Science and Engineering College, Changchun University of Science and

Technology, Changchun 130022, People’s Republic of China b

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of

Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China [email protected] (L. Zhou) [email protected] (L. Lu). 

Correspondence to: Material Science and Engineering College, Changchun University of Science and Technology, Changchun 130022, People’s Republic of China. Tel.: +86 431 85583147.

Abstract An alternative approach was exploited to achieve high performance fluorescent organic light-emitting diodes (OLEDs) by co-doping traditional electron transport

1

material into hole dominant light-emitting layer as supplementary host and blue emitter. Experimental results demonstrated that in this device structure, the co-doping of supplementary host was beneficial in broadening recombination zone, improving carriers trapping and facilitating the balance of holes and electrons, due to its well matched energy levels and high electron mobility. Based on this device design strategy, the optimized green electroluminescent (EL) device obtained the maximum brightness and external quantum efficiency up to 21176 cd/m2 and 5.4%, respectively, which are higher than those of reference devices. In addition, bright white OLED with the maximum brightness, current efficiency and external quantum efficiency up to 17207 cd/m2, 10.47 cd/A and 4.4%, respectively, was realized by modulating the doping

concentrations

of

blue-emitting

electron

transport

material

and

orange-emitting material. Thus, our investigation provides a chance to construct high performance white OLEDs with simplified device structure.

We presented an effective method to realize high performance green and white fluorescent organic light-emitting diodes (OLEDs) by co-doping traditional electron transport material into hole dominant light-emitting layer as supplementary host and blue emitter.

2

Keywords: electron transport material; co-doped devices; supplementary host; white organic light-emitting diodes

1. Introduction White organic light-emitting diodes (WOLEDs) are considered as strong candidates for the next-generation displays and solid-state lighting applications due to their promising features of high-efficiency, low-cost and easy-flexibility [1-6]. In this field, the development of new functional materials and novel device structure is crucial to realize high-performance WOLEDs [7-11]. Currently, red and green organic light-emitting diodes (OLEDs) with high efficiencies and high color purity have been realized by utilizing phosphorescent metal complexes as emitters due to their ability of harvesting both singlet and triplet excitons to achieve theoretical 100% internal quantum efficiency [12-18]. However, developing efficient blue phosphorescent OLEDs with high operation stability remains a significant challenge due to the relatively inferior stability of most blue phosphorescent materials [19-23]. In addition, phosphorescent OLEDs suffer from high manufacture cost and serious efficiency roll-off especially at high brightness [24-26]. As a result, the development of phosphorescent

3

WOLEDs is seriously obstructed by these deficiencies to some degree. Therefore, many groups devoted to developing efficient fluorescent emitters and corresponding devices to realize high performances WOLEDs with high stability. Generally, dye doping method is efficient in avoiding self-quenching caused by molecule aggregation in the fluorescent OLEDs, thus realizing high performances. As we know, high doping concentration favors efficient energy transfer from host to dye, but results in molecules aggregation and thus serious self-quenching. On the contrary, low doping concentration limits not only the direct radioactive recombination of holes and electrons on dye molecules but also the utilization of exciton energy on host molecules [27-31]. As a result, EL efficiencies of fluorescent devices in both cases are rarely satisfactory. Recently, many groups have paid their attentions to improve the electroluminescent (EL) performances of fluorescent OLEDs by utilizing dye co-doping method. Research results have demonstrated that co-doped emitter system can not only reduce self-quenching but also facilitate energy transfer, which caused the enhanced EL efficiency. For example, Su et al. have once reported the fabrication and investigation of green EL device with the current efficiency (CE) of 9.33 cd/A by co-doping N,N'-dimethyl-quinacridone (DMQA) and Coumarin6 (C6) into tris-(8-hydroxyquinoline) aluminum (Alq3) [30]. Previously, Tang et al. have realized efficient green EL device with the CE of 10.8 cd/A by utilizing 2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)benzopyropyrano(6,7-8-ij)quinolizin-11-one (C545T) to sensitize DMQA [31]. It was notable that these optimized co-doped devices displayed higher EL performances than reference devices. In the past decade, we have devoted to the design and fabrication of OLEDs based

4

on fluorescent emitters, and certain progresses have been obtained. In previous, we have demonstrated a high-brightness, broad-spectrum WOLED with two EMLs consisting of

only

three

organic

layers

[32].

Alq3

was

doped

into

N,N'-diphenyl-N,N'-bis(1-napthyl)-1,1'-diphenyl-4,4'-diamine (NPB) as hole transport layer and EML1, which provides green and blue emissions. Meanwhile, 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran

(DCJTB)

was doped into Alq3 as electron transport layer and EML2, which provides red and green emissions. Finally, WOLED with the maximum brightness, CE and PE as high as 35788 cd/m2, 8.02 cd/A and 9.33 lm/W, respectively, was obtained by inserting a thin 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as hole block layer between EML1 and EML2. Recently, high performance pure blue EL devices were realized by doping electron transport material 10-bis[4-(6-methylbenzothiazol-2-yl)phenyl]anthracene (DBzA) into hole dominant host material 4,4',4"-tris(carbazole-9-yl)triphenylamine (TcTa) as emitter [33]. Research results revealed that the well matched energy levels of DBzA and TcTa helped to improve carriers trapping, broaden recombination zone and facilitate the balance of holes and electron on emitter molecules. Finally, pure blue single light-emitting layer (EML) EL device gave the maximum brightness, CE and power efficiency (PE) up to 10384 cd/m2, 6.41 cd/A and 6.71 lm/W, respectively. In this paper, enlightened by our recent investigation mentioned above, we presented an effective co-doped device structure to further improve the EL performances of fluorescent emitters by introducing electron transport material DBzA into EML as supplementary host. Consequently, C545T and DMQA based devices realized the maximum CE (external

5

quantum efficiency (EQE)) up to 7.23 (EQE=2.7%) and 17.04 (EQE=5.5%), respectively. Inspired by the emergence of efficient blue DBzA emission in co-doped devices, high performances WOLEDs were constructed by co-doping DBzA and orange-emitting material DCJTB into host materials. Consequently, the optimized WOLED achieved the maximum brightness, CE and EQE of 17207 cd/m2, 10.47 cd/A and 4.4%, respectively. Even at the certain brightness of 1000 cd/m2 (5.5 V), inspiring current efficiency and external quantum efficiency as high as 9.88 cd/A and 4.2%, respectively, can still be retained. 2. Experiments All the organic materials used in this study were obtained commercially and used as received without further purification. Indium-tin-oxide (ITO) coated glass with sheet resistance of 10 Ω/sq was used as the anode substrate. Prior to film deposition, patterned ITO substrates were cleaned with detergent, rinsed in de-ionized water and finally dried in an oven. All the organic layers were deposited with the rate of 0.1 nm/s under high vacuum (≤ 3.0×10-5 Pa). The doped and co-doped EMLs were prepared by co-evaporating dopant(s) and host material from two or three individual sources, and the doping concentrations were modulated by controlling the evaporation rate of dopant(s). MoO3, LiF and Al were deposited in another vacuum chamber (≤ 8.0×10-5 Pa) with the rates of 0.01, 0.01 and 1.0 nm/s, respectively, without being exposed to the atmosphere. The thicknesses of these deposited layers and the evaporation rate of individual materials were monitored in vacuum with quartz crystal monitors. A shadow mask was used to define the cathode and to make ten emitting dots

with

the

active

area

of

mm2

9

on

each

substrate.

Current

density-brightness-voltage (J-B-V) characteristics were measured by using a

6

programmable Keithley source measurement unit (Keithley 2400 and Keithley 2000) with a silicon photodiode. The EL spectra were measured with a calibrated Hitachi F−7000 fluorescence spectrophotometer. EQE of EL device was calculated based on the photo energy measured by the photodiode, the EL spectrum, and the current pass through the device. 3. Results and discussion 3.1 Design of device structure and the selection of used materials The device structure, energy levels diagram and molecular structures of used materials are shown in Figure 1. In this case, MoO3 functions as hole injection layer. TAPC is di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane,

while

TmPyPB

is

1,3,5-tri(m-pyrid-3-yl-phenyl)benzene [34-36]. Within C545T or DMQA based devices, p-type material TcTa was used as the predominant host material, while DBzA was co-doped into TcTa as the supplementary host [33,37-39]. Within the WOLEDs, TcTa and n-type material 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) were chosen as host materials of EML1 and EML2, respectively, while DBzA was co-doped into TcTa as both blue emitter and supplementary host material aiming to balance carriers distribution and broaden recombination zone [33,34,38]. Within the C545T or DCJTB doped TcTa EML, both holes and electrons will be preferentially trapped by C545T or DCJTB molecules because the energy levels (the highest occupied molecular orbital (HOMO)/the lowest unoccupied molecular orbital (LUMO) levels) of emitters are within those of TcTa [32,37,38]. Within the DMQA doped TcTa EML, electrons will also be preferentially trapped by DMQA molecules, while few holes will be directly trapped by DMQA molecules due to its relatively lower HOMO level (-5.8 eV) than that (-5.7 eV) of TcTa [37,38]. 3.2 Performances of the devices based on C545T and DBzA co-doped system

7

To optimize the co-doping concentration of C545T, a series of co-doped single-EML devices with the structure of ITO/MoO3 (3 nm)/TAPC (50 nm)/C545T (x wt%):DBzA (15 wt%):TcTa (10 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm) were manufactured and examined. According to our previous investigations, the co-doping concentration of DBzA was determined as 15 wt%. The dependence of EL efficiency on co-doping concentration of C545T was depicted in Figure 2(a), while the current density-brightness-voltage characteristics of these co-doped devices were shown in the insert of Figure 2(a). As shown in Figure 2(a) and Table 1, the 0.8 wt% co-doped device (defined as S1-0.8 wt%) obtained the maximum brightness, CE, PE and EQE up to 12202 cd/m2, 7.23 cd/A, 6.48 lm/W and 2.7%, respectively. At the brightness of 1000 cd/m2 (5.1 V), CE and EQE up to 5.40 cd/A and 2.0%, respectively, were obtained. For comparison, 0.8 wt% C545T doped device without the co-doping of DBzA was also fabricated (defined as reference device, RD-1) and compared. Compared with RD-1, S1-0.8 wt% displayed higher EL performances, demonstrating the advantage of this device design strategy. As we know, it is crucial to sufficiently utilize the singlet excitons generated in the EML for realizing high-performance fluorescent OLEDs. As depicted in Figure 2(b), S1-0.8 wt% displayed weaker TcTa emission (peaked at 396 nm) than RD-1, which indicates the sufficient utilization of excitons resulting in superior EL performances. In addition, with increasing co-doping concentration of C545T, the relative intensity of DBzA emission (peak at 455 nm) decreases rapidly, which indicates the improved carriers trapping. 3.3 Performances of the devices based on DMQA and DBzA co-doped system To further validate the feasibility of this novel device designing strategy, a series of co-doped single-EML devices following the structure of ITO/MoO3 (3 nm)/TAPC (40 nm)/DMQA (y wt%):DBzA (15 wt%):TcTa (10 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm) were designed and fabricated. The detailed EL performances of these co-doped devices

8

were summarized in Table 2. As shown in Table 2 and Figure 3(a), the 1.0 wt% co-doped device (defined as S2-1.0 wt%) gave the maximum brightness, CE, PE and EQE as high as 21176 cd/m2, 17.04 cd/A, 17.20 lm/W and 5.4%, respectively. The high EQE of 5.4% is higher than the theoretical EQE (5%). Even at the brightness of 1000 cd/m2 (4.6 V), CE and EQE up to 10.13 cd/A and 3.2%, respectively, can be retained. For comparison, 1.0 wt% DMQA doped device without the co-doping of DBzA was also fabricated (defined as reference device, RD-2). Compared with RD-2, S2-1.0 wt% achieved higher brightness and efficiency. For comparison, co-doped double-EMLs device (D-1.0 wt%) was also fabricated by inserting 1.0 wt% DMQA doped 26DCzPPy film between EML and electron transport layer (ETL, TmPyPB) of S2-1.0 wt%. As shown in Figure S1 in the Supplementary Data, D-1.0 wt% displayed relatively lower EL performances than those of S2-1.0 wt%, due to the unbalanced carriers distribution within EMLs. Inspired by the enhanced EL performances of co-doped devices, normalized EL spectra of these co-doped devices were subsequently measured and investigated. As shown in Figure 3(b), RD-2 showed obvious TcTa emission, while the co-doped devices displayed nearly invisible TcTa emission, which demonstrated there were only few carriers recombining on TcTa molecules in the co-doped devices. In addition, the relative intensity of DBzA emission decreased rapidly with increasing co-doping concentration of DMQA, corresponding to the results of C545T co-doped devices. As shown in Figure 3(c), normalized EL spectra of S2-1.0 wt% operating at different current densities were also investigated. When the current density was low, S2-1.0 wt% showed weak DBzA emission, demonstrating carriers were trapped preferentially by DMQA molecules. With increasing current densities, the intensity of DBzA emission became higher revealing that besides DMQA molecules, many carriers were trapped subsequently by DBzA molecules. These research results demonstrate it is an

9

effective and promising method to utilize excitons and improve the EL performances of fluorescent devices by co-doping DBzA into TcTa as the supplementary host. To better understand the mechanisms of improved EL performances in the DMQA co-doped devices, carriers distribution within EMLs was analyzed in detail. For RD-2, as depicted in Figure 4(a), carriers distribution on emitter molecules will be unbalanced due to the hole dominant transporting characteristic of TcTa [34]. Moreover, some holes and electrons situate on TcTa molecules due to the lower HOMO level (-5.8 eV) of DMQA than that (-5.7 eV) of TcTa and low doping concentration of DMQA. As a result, obvious TcTa emission and poor EL performances were observed in RD-2. According to previous investigation, co-doping DBzA with high electron mobility and low LUMO level into hole dominant EML is helpful in facilitating the injection and transport of electrons, broadening recombination zone and facilitating carriers balance in the EML [33]. For co-doped devices, as depicted in Figure 4(b) and 4(c), electrons within EML are trapped preferentially by DMQA molecules and subsequently by DBzA molecules due to their lower LUMO levels (-3.5 and -2.8 eV, respectively), thus decreasing the accumulation of electrons on TcTa molecules. With increasing the co-doping concentration of DMQA, as shown in Figure 4(b) (S2-1.0 wt%), more and more carriers will be trapped by DMQA molecules, which results in improved carriers trapping and superior utilization of excitons. When the co-doping concentration was higher than the optimal value, as shown in Figure 4(c) (S2-1.2 wt%), the expected balanced carriers distribution within the EML of S2-1.0 wt% was broken. Consequently, S2-1.0 wt% showed high EL performances as well as weak TcTa emission. 3.4 Performances of the WOLED based on DCJTB and DBzA co-doped system As demonstrated above, besides improved EL performances, DBzA emission was observed in all co-doped devices and the intensity of DBzA emission could be modulated by altering the co-doping concentrations of emitters. Enlightened by these experimental results,

10

we aimed to construct efficient WOLEDs by co-doping DBzA and orange-emitting material DCJTB into TcTa. Then, a series of co-doped single-EML devices with the structure of ITO/MoO3 (3 nm)/TAPC (40 nm)/DCJTB (z1 wt%):DBzA (15 wt%):TcTa (10 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm) were fabricated and examined to optimize the co-doping concentration of DCJTB. As shown in Figure 5(a) and Table 2, the 0.1 wt% co-doped single-EML WOLED (here defined as W1) gave the highest brightness, CE, PE and EQE of 14326 cd/m2, 5.25 cd/A, 3.36 lm/W and 2.9%, respectively. However, as depicted in the insert of Figure 5(b), the normalized EL spectra of W1 showed low DCJTB emission (peaked at 572 nm). To enhance the intensity of DCJTB emission and EL performances, W2 was fabricated by inserting 0.1 wt% DCJTB doped 26DCzPPy EML2 between EML1 and ETL of W1. For comparison with W2, reference device (defined as RD-3) without the co-doping of DBzA were also fabricated. As shown in Figure 5(b), for W2, enhanced DCJTB emission was achieved and the intensity of DBzA emission increased gradually with increasing the current density, indicating DCJTB molecules have preferentially trapped sufficient carriers and residual carriers were utilized by DBzA molecules within EML1. Finally, as shown in Figure 5(a) and Table 2, W2 gave the maximum brightness, CE and EQE up to 17207 cd/m2, 10.47 cd/A and 4.4%, respectively. Even at practical brightness of 1000 cd/m2 (5.5 V), CE, EQE and Commission Internationale de l'Eclairage coordinates (CIE) as well as 9.88 cd/A, 4.2% and (0.27, 0.33), respectively, were retained. 4. Conclusions In summary, we have demonstrated the feasibility and efficacy of co-doping traditional electron transport material DBzA into TcTa as light-emitting layer in realizing high performances fluorescent OLEDs. Compared with reference devices, co-doped devices

11

displayed higher EL performances ascribed to the improved injection and transport of electrons, more balanced carriers distribution, wider recombination zone and superior utilization of excitons within EML resulting from high electron mobility and lower LUMO level of the co-doped DBzA molecules within hole dominant EML. Subsequently, high-performance fluorescent WOLEDs were designed and fabricated by utilizing DBzA as supplementary host and blue emitter. Finally, the optimized WOLED realized the maximum brightness, CE and EQE up to 17207 cd/m2, 10.47 cd/A and 4.4%, respectively. It was notable that even at the practical brightness of 1000 cd/m2, CE and EQE as high as 9.88 cd/A and 4.2%, respectively, can still be retained.

Acknowledgements The authors are grateful to the financial aid from Research Equipment Development project of Chinese Academy of Sciences (YZ201562), Youth Innovation Promotion Association Chinese Academy of Sciences (Y72011), Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000), National Natural Science Foundation of China (61307118, 21771172, 21521092, 21590794 and 21210001), Jilin Provincial Science and Technology Development Program of China (20170519006JH), and National Key Basic Research Program of China (No. 2014CB643802), and the Key Research Program of Frontier Sciences, CAS (Grant No. YZDY-SSW-JSC018).

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using

4,4',4''-tri(N-carbazolyl)triphenylamine

novel

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(TCTA)

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18

Captions Fig. 1. Proposed energy levels diagram of the devices used in this work and the molecular structures of DBzA, C545T, DMQA and DCJTB. Fig. 2. (a) EL efficiency-current density (η-J) characteristics of the C545T co-doped devices and RD-1. Insert: Brightness-voltage-current density (B-V-J) characteristics of the C545T co-doped devices and RD-1. (b) Normalized EL spectra of C545T co-doped devices and RD-1 operating at 10 mA/cm2. Fig. 3. (a) EL efficiency-current density (η-J) characteristics of the DMQA co-doped devices and RD-2. Insert: Brightness-voltage-current density (B-V-J) characteristics of the DMQA co-doped devices and RD-2. (b) Normalized EL spectra of DMQA co-doped devices and RD-2 operating at 10 mA/cm2. (c) Normalized EL spectra of S2-1.0 wt% operating at different current densities. 19

Fig. 4. Schematic representation of carriers distribution within the EMLs of RD-2 (a), S2-1.0 wt% (b) and S2-1.2 wt% (c). The dash dot and dot lines represent the energy levels of DBzA and DMQA, respectively. The symbols '+' and '-' represent holes and electrons, respectively. Fig. 5. (a) EL efficiency-current density (η-J) characteristics of W1 and W2. Insert: Brightness-voltage-current density (B-V-J) characteristics of W1 and W2. (b) Normalized EL spectra of W2 operating at different current densities. Insert: Normalized EL spectra of W1, W2 and RD-3 operating at 10 mA/cm2.

Table 1 Key properties of C545T co-doped devices with different co-doping concentrations and RD-1. Device

Vturn-on

Ba

ηc b (EQE c)

ηp d

ηc e (cd/A) (EQE f)

(V)

(cd/m2)

(cd/A)

(lm/W)

(1000 cd/m2)

CIEx, y g

S1-0.4 wt% 3.2

8389

8.21 (3.4%)

7.58

5.30 (2.2%, 5.9 V) (0.18, 0.37)

S1-0.6 wt% 3.2

8332

8.56 (3.4%)

7.75

5.62 (2.2%, 5.8 V) (0.18, 0.41)

S1-0.8 wt% 3.1

12202

7.23 (2.7%)

6.48

5.41 (2.0%, 5.1 V) (0.18, 0.46)

S1-1.0 wt% 3.1

10341

6.90 (2.3%)

5.24

5.93 (2.0%, 5.0 V) (0.19, 0.52)

RD-1

2753

10.82 (4.7%) 10.93

3.1

3.41 (1.5%, 6.3 V) (0.20, 0.50)

Table 2 Key properties of DMQA co-doped devices with different co-doping concentrations, RD-2, RD-3, W1 and W2.

20

Device

a

Vturn-on

Ba

ηc b (EQE c)

ηp d

ηc e (cd/A) (EQE f)

(V)

(cd/m2)

(cd/A)

(lm/W)

(1000 cd/m2)

CIEx, y g

S2-0.6 wt% 2.9

19049

13.40 (5.1%) 12.95

8.93 (3.4%, 4.4 V)

(0.21, 0.35)

S2-0.8 wt% 2.9

19778

14.00 (4.7%) 13.40

8.92 (3.0%, 4.6 V)

(0.23, 0.40)

S2-1.0 wt% 2.9

21176

17.04 (5.4%) 17.20

10.13 (3.2%, 4.6 V) (0.23, 0.43)

S2-1.2 wt% 2.9

20713

13.01 (4.1%) 11.78

9.80 (3.1%, 4.6 V)

RD-2

3.0

9492

15.16 (4.4%) 14.69

10.74 (3.1%, 4.5 V) (0.28, 0.58)

RD-3

3.5

14211

12.26 (4.2%) 11.15

9.42 (3.2%, 5.6 V)

(0.42, 0.43)

W1

3.0

14326

5.25 (2.9%)

3.39

5.01 (2.7%, 5.3 V)

(0.20, 0.23)

W2

3.4

17207

10.47 (4.4%) 6.93

9.88 (4.2%, 5.5 V)

(0.27, 0.32)

The data for maximum brightness (B),

b

(0.24, 0.45)

maximum current efficiency (ηc),

c

maximum

external quantum efficiency (EQE), d maximum power efficiency (ηp), e current efficiency (ηc) at the certain brightness of 1000 cd/m2, f external quantum efficiency (EQE) at the certain brightness of 1000 cd/m2, g Commission Internationale de l'Eclairage coordinates (CIEx, y) at 10 mA/cm2.

21

Figure 1

22

23

Figure 2

24

Figure 3

Figure 4 25

26

Figure 5

27