The lifetime of a full-color OLED display is mainly determined by the lifetime of the red, green, and blue color subpixels. Specifically, the lifetime is predominantly.
23.3 / L.-S. Liao
23.3: Distinguished Paper: High-Efficiency Tandem Blue OLEDs Liang-Sheng Liao, Kevin P. Klubek, Margaret J. Helber, Lelia Cosimbescu, and Dustin L. Comfort Research & Development, Eastman Kodak Company, 1999 Lake Avenue, Rochester, NY 14650-2110, USA
Abstract Tandem blue OLEDs using different fluorescent dopants have achieved 20 cd/A (CIEx,y = 0.14, 0.18) and 38 cd/A (CIEx,y = 0.15, 0.42) with external quantum efficiencies higher than 11%. The tandem blue OLEDs include two electroluminescent units that are connected in series with an organic intermediate connector. At an initial brightness of 1,000 cd/m2, the drive voltage of the devices can be less than 6.5 V, and the operational lifetime (T50) of the devices is estimated to be greater than 10,000 h. We also demonstrate that the selection of electron-transporting material and an intermediate connector have significant impact on the electroluminescence performance of the tandem blue OLEDs.
1.
Introduction
Long operational lifetime (T50) and high electroluminescence (EL) efficiency of a full-color OLED display are very important for many applications. The lifetime of a full-color OLED display is mainly determined by the lifetime of the red, green, and blue color subpixels. Specifically, the lifetime is predominantly determined by the subpixel that has the shortest lifetime among the three colored subpixels. Obviously, the blue subpixel is currently a bottleneck for lifetime improvement and luminescence improvement in OLED displays. Therefore, improving the lifetime and luminance of the blue OLED will have a large impact on display applications as well as on organic solid-state lighting applications. From an operational point of view, the lifetime of an OLED is dependent on the drive current density. A higher drive current density translates to a shorter lifetime. Therefore, increasing luminous efficiency (cd/A) to reduce drive current density is an effective approach to achieve an improved lifetime in OLEDs.
to reduce drive voltage while maintaining high luminous efficiency so that low-cost drive circuitry could be utilized. In this work, we will report high efficiency with relatively low drive voltage achieved from blue OLEDs using a tandem structure.
2.
Experimental
Tandem blue OLEDs, along with conventional blue OLEDs, were fabricated on ~1.1-mm-thick glass substrates precoated with a transparent indium-tin oxide (ITO) conductive layer having a thickness of ~25 nm and a sheet resistance of ~70 Ω/square. The substrates were cleaned and dried using a commercial glass scrubber tool. The ITO surface was subsequently treated with oxygen plasma, and then a ~1-nm-thick layer of CFx was deposited on the clean ITO surface to condition the ITO as a modified anode by decomposing CHF3 gas in a plasma treatment chamber. The substrates in the experiments were then transferred into a vacuum chamber for sequential deposition of all organic and metal layers on top of the substrates by a thermal evaporation method under a vacuum of approximately 10–6 Torr. During the fabrication of the OLEDs, the layer thicknesses and the doping concentrations were controlled and measured in situ using calibrated thickness monitors. After the deposition of all the layers, the devices were transferred from the deposition chamber into a dry box for encapsulation. Each OLED has an emission area of 10 mm2. The EL characteristics of all the fabricated devices were evaluated using a constant current source and a photometer at room temperature. The color was reported using Commission Internationale de l’Eclairage (CIE) coordinates. Operational lifetime of the devices was measured at room temperature under dc drive conditions.
Recently, improving blue emission for OLEDs has drawn much attention. Universal Display Corporation made a breakthrough in the research of highly efficient blue phosphorescent OLEDs [1]. However, the color and lifetime are not yet satisfactory for display applications. Hosokawa et al. achieved improved luminous efficiency and lifetime by using Idemitsu Kosan Co.’s BD1 fluorescent dopant [2]. Helber et al. also achieved improved luminous efficiency and lifetime by using Kodak’s new OLED fluorescent blue dopant and host system [3]. Idemitsu further reported a fluorescent blue OLED having a luminous efficiency of 9 cd/A with good blue color and a long operational lifetime [4]. However, further improvement on the EL performance of blue OLEDs is still needed for OLED display applications.
The schematic device structures of both conventional and tandem blue OLEDs are shown in Figure 1. The organic layers, i.e., holeinjecting layer (HIL)/hole-transporting layer (HTL)/light-emitting layer (LEL)/electron-transporting layer (ETL)/electron-injecting layer (EIL), between the modified ITO anode and the aluminum cathode in the conventional OLEDs in Figure 1(a), are defined as an EL unit. Two EL units connected in series by an intermediate connector and sandwiched between the anode and cathode form a tandem OLED as shown in Figure 1(b).
Using a tandem or stacked structure to form an LED is a useful way to achieve higher efficiency and a longer operational lifetime [5–11]. However, the reported tandem OLEDs [6–11] have a drive voltage higher than 10 V at 1,000 cd/m2, which may not be suitable for use in active-matrix OLED displays because of the voltage limitation of the backplane drive circuitry. In order to take advantage of tandem OLEDs for display applications, it is critical
Type II: ITO/optional HIL/NPB/EK-BH109:3 vol% OP31/
Four types of blue OLEDs have been made in this work: Type I:
ITO/optional HIL/NPB/BH3:7 vol% EK9/ ETL/EIL/Al (or ITO/EL-Unit-1/Al); ETL/EIL/Al (or ITO/EL-Unit-2/Al);
Type III: ITO/EL-Unit-1/intermediate connector/EL-Unit-1/Al; Type IV: ITO/EL-Unit-2/intermediate connector/EL-Unit-2/Al. Types I and II are conventional blue OLEDs with different fluorescent dopants, and Types III and IV are tandem blue OLEDs
ISSN0006-0966X/06/3702-1197-$1.00+.00 © 2006 SID
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23.3 / L.-S. Liao using Type I EL and Type II EL units, respectively. Each of the four types of blue OLEDs may include several different devices containing different ETLs or EILs.
the LEL in the devices resulting in quite different EL performance. Table 1: EL performance of Type I and Type II devices.
(a)
Al Cathode EIL
Device ID
CIE
ETL
(ETL)
x,y
LEL
Type I-1
0.143
Optional HIL
(Alq3)
0.168
M odified ITO Anode
Type I-2
0.142
(phenanthroline deriv.)
0.170
Al Cathode EL Unit Organic Intermediate Connector EL Unit M odified ITO Anode
Figure 1: Schematic structure of (a) a conventional blue OLED and (b) a tandem blue OLED. KODAK OLED Materials HT1 (NPB), GH1 (Alq3), BH3, EKBH109, EK9, and EK-ET902 were used in this work. N,N’di(naphthalene-1-yl)-N,N’-diphenyl-benzidine (NPB) is a holetranporting material, tris(8-hydroxyquinoline)aluminum (Alq3) is an electron-transporting material, BH3 and EK-BH109 are new host materials, EK9 is a new blue dopant material, and EK-ET902 is an electron-injecting material. 4,4'-(1,4-phenylenedi-2,1ethenediyl)bis[N,N-bis(4-methylphenyl)benzenamine] (OP31) is a greenish blue dopant material that is available elsewhere. The ETL used in this work can be Alq3, EK-BH109, or phenanthroline derivatives such as 4,7-diphenyl-1,10-phenanthroline (Bphen). The EIL can be LiF, EK-ET902, or an alkali metal-doped ETL such as Li-doped phenanthroline derivatives.
3. 3.1
Results and Discussion Optimization of Blue EL Unit
In order to fabricate high-efficiency tandem blue OLEDs, we must first optimize the EL unit in conventional blue OLEDs. We found that different ETLs in the EL unit have significant effects on drive voltage and luminous efficiency. For example, three Type I devices, Type I-1, Type I-2, and Type I-3, have been made. The three devices have the same layer structure except that their ETL is different. The comparison of the EL performance (both at 20 mA/cm2 and at 1,000 cd/m2) among the three devices is shown in Table 1. The external quantum efficiencies (EQE) reported in this paper (Tables 1 and 2) have been corrected by considering nonLambertian distribution of the EL emission from the devices based on angular EL data. Varying the ETL material changes electron injection from the EIL to the ETL and from the ETL to
1198 • SID 06 DIGEST
cd/A
EQE (%)
@ 20 mA/cm2 / @ ~1000 cd/m2
EL Unit
HTL
(b)
Volts
6.2/
7.7/
4.8/
5.9
7.4
~4.6
4.3/
10.2/
6.3/
3.9
10.5
~6.5
Type I-3
0.142
3.6/
11.3/
6.6/
(EK-BH109)
0.185
3.3
11.2
~6.6
Type II-1
0.169
4.0/
20.0/
7.0/
(EK-BH109)
0.417
3.3
20.4
~7.1
In terms of operational lifetime, it has been found that by using Alq3 as an ETL in a Type I device, a lifetime greater than 10,000 h at 1,000 cd/m2 can be achieved. However, Type I EL units having Alq3 as an ETL cannot be used for constructing low voltage tandem blue OLEDs because each of the EL units will have a voltage drop higher than 5 V. Therefore, EK-BH109 and some phenanthroline derivatives are better suited materials for use as an ETL in the EL units. We also fabricated a Type II device (Type II-1) using EK-BH109 as the ETL, and the EL performance is shown in Table 1 as well. Therefore, we have chosen to optimize our blue EL units using EK-BH109 as the ETL in order to achieve both low drive voltage and high luminous efficiency.
3.2
Selection of Intermediate Connector
How to properly connect each EL unit is critical for making good tandem OLEDs. Intermediate connectors for use in tandem OLEDs can be divided into three types, i.e., metallic intermediate connector [10], organic intermediate connector [7], and hybrid intermediate connector comprising an organic layer and an inorganic compound layer or a metallic layer [6,8,9,11]. Generally, all three types of intermediate connectors work well in tandem OLEDs containing only two EL units. However, in order to achieve the highest luminous efficiency possible, especially for a tandem OLED having more than two EL units or having blue emission, use of a high transparency intermediate connector must be seriously considered. Therefore, in this work, we prefer to use organic intermediate connectors or hybrid intermediate connectors that offer improved transparency compared to the metallic intermediate connectors. Two Type III devices have been fabricated with the first containing a hybrid intermediate connector (Type III-1) and the second containing an organic intermediate connector (Type III-2). The EL performance of these two tandem blue OLEDs is listed in Table 2. The hybrid intermediate connector includes a 1-nm-thick metallic layer. The organic intermediate connector has an ntype/p-type structure with a total thickness of more than 20 nm. As indicated from Table 2, both intermediate connectors work well but the hybrid exhibits lower light output, which may be due
23.3 / L.-S. Liao to its higher optical absorption. Therefore, use of an organic intermediate connector in this case has an advantage for achieving higher luminous efficiency.
22.0 20.0 18.0
Device ID
CIE
(Intermediate Connector)
Volts
cd/A
Luminous Efficiency (cd/A)
Table 2: EL performance of Type III and Type IV devices. EQE (%)
@ 20 mA/cm2 /
x,y
@ ~1000 cd/m2
Type III-1
0.136
8.4/
14.3/
9.8/
16.0 14.0 12.0 10.0 8.0 6.0
(Hybrid)
0.143
6.5
15.9
~10.6
Type III-2
0.141
8.9/
20.0/
11.4/
2.0
(Organic)
0.180
7.8
20.1
~11.4
0.0
Type IV-1
0.152
7.2/
38.3/
12.9/
(Organic)
0.427
6.2
36.0
~12.1
4.0
Type I-2
0
Type III-2
20 40 60 Current Density (m A/cm 2)
80
Figure 3: Luminous efficiecy vs. current density
We also fabricated a Type IV device (Type IV-1) using an organic intermediate connector, and the EL performance is listed in Table 2 as well. The data in Table 2 demonstrate that we have achieved both low voltage and high efficiency from tandem blue OLEDs using an organic intermediate connector to connect two optimized blue EL units.
0.45 EL @ 20 mA/cm2
0.40
Type I-2
Radiance (W/Sr/m 2/nm)
0.35
We also compared EL data between two devices, Type I-2 and Type III-2. These comparisons are illustrated in Figures 2, 3, and 4, where we plotted current density vs. voltage characteristics, luminous efficiency vs. current density characteristics, and electroluminescence spectra. It is shown that the tandem blue OLED (Type III-2) has almost doubled the luminous efficiency (Figure 3) and doubled the drive voltage (Figure 2) while the EL spectrum (Figure 4) is almost unchanged compared to the optimized conventional blue OLED (Type I-2). If the intermediate connector is not properly selected, it will be difficult to achieve this luminous efficiency as is shown in our tandem blue OLEDs.
Type III-2
0.30 0.25 0.20 0.15 0.10 0.05 0.00 350
450
550
650
750
Wavelength (nm )
90
Type I-2
Figure 4: Electroluminescence spectra
Type III-2
Current Density (mA/cm 2)
80
3.3
70 60 50 40 30 20 10 0 0
2
4
6
8
10
12
Voltage (V)
Figure 2: Current density vs. voltage
Operational Lifetime
The operational lifetime of the tandem blue OLEDs varies from about 5,000 h to greater than 50,000 h at an initial brighness of 1,000 cd/m2 and at room temperature depending on the selection of the luminescent dopant (color purity), ETL, EIL, and intermediate connector. Generally speaking, the bluer the color, the shorter the lifetime. For example, shown in Figure 5 are the operational stability behaviors (normalized luminance vs. operational time) of two tandem blue OLEDs selected from Type III and Type IV devices. The Type III device having a luminous efficiency of 20 cd/A (CIEx,y = 0.14, 0.17) at 20 mA/cm2 has a lifetime of 4,200 h when tested at an initial brightness of 2,000 cd/m2. Based on our experimental observations, the lifetime can be converted to greater than 10,000 h at an initial brightness of 1,000 cd/m2. The Type IV device having a luminous efficiency of 38 cd/A (CIEx,y = 0.15, 0.42) at 20 mA/cm2 is still under testing at an initial brightness of 5,000 cd/m2, yet the lifetime is estimated to be greater than 50,000 h at an initial brightness of 1,000 cd/m2. Although the color purity of the Type IV device is not sufficient
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23.3 / L.-S. Liao for most display applications, if one were to take into consideration the high efficiency and long lifetime, there is still potential to achieve pure blue emission by using a color filter.
Mr. D. Arnold, and Mr. D. Neill for their technical support. We also thank Drs. C. W. Tang, D. Preuss, and D. Kondakov for their valuable suggestions and discussions.
6.
References
[1] http://www.physorg.com/news4887.html.
Normalized Luminance
1.0
Type III, Initial Brightness: 2000 cd/m^2
0.9
Type IV, Initial Brightness: 5000 cd/m^2
0.8
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[3] M. J. Helber, M. Ricks, and L. Cosimbescu, "Highly Efficient and Stable Blue OLED Systems", SID Symposium Digest, Vol. 36. pp. 863-865, 2005.
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0.7
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0.6
0.5 0
1000
2000
3000
4000
Operational Tim e (h)
Figure 5: Normalized luminance vs. operational time
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4.
Summary
We have demonstrated that both low voltage and high efficiency can be achieved from tandem blue OLEDs by properly selecting the electron-transporting material and intermediate connector. Two types of tandem blue OLEDs have been fabricated. One type has color coordinates of CIEx,y = 0.14, 0.18 and exhibits 20 cd/A with an external quantum efficiency higher than 11%. The drive voltage can be lower than 8 V at 1,000 cd/m2 while the operational lifetime can reach up to 10,000 h at an initial brightness of 1,000 cd/m2. The other type of device has color coordinates of CIEx,y = 0.15, 0.42 and exhibits 38 cd/A with an external quantum efficiency higher than 12.5%. The drive voltage can be lower than 6.5 V at 1,000 cd/m2 while the operational lifetime is estimated to be greater than 50,000 h at an initial brightness of 1,000 cd/m2.
5.
Acknowledgements
The authors are grateful to Drs. S. Conley, W. Begley, and W. Slusarek for their OLED material research contributions. We are grateful to Ms. R. Miller, Ms. M. Wojcik-Cross, Mr. M. Culver,
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[10] J. X. Sun, X. L. Zhu, H. J. Peng, M. Wong, and H. S. Kwok, "Effective Intermediate Layers for Highly Efficienct Stacked Organic Light-Emitting Devices", Appl. Phys. Lett., Vol. 87, pp. 093504-1-093504-3, 2005.
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