60.3: Two-stacked White Organic Light-emitting Diodes Consisting ... Sung-Hoon Pieh, Myung-Seop Kim, Chang-Je Sung, Jeong-Dae Seo, Hong-Seok Choi,.
60.3 / S.-H. Pieh
60.3: Two-stacked White Organic Light-emitting Diodes Consisting of Fluorescent and Phosphorescent Hybrid Structure with High Efficiency and Good color Characteristics Sung-Hoon Pieh, Myung-Seop Kim, Chang-Je Sung, Jeong-Dae Seo, Hong-Seok Choi, Chang-Wook Han, Yoon-Heung Tak LG Display Co., Ltd., 1007, Deogeun-ri, Wollong-myeon, Paju-si, Gyeongki-do 431-811, Korea
Abstract We report an improved bottom-emission two-stacked white OLED structure consisting of one unit emitting fluorescent blue light and the other unit emitting phosphorescent yellow in one-host and two-dopant (R+G) system. By searching EL condition optimizing both phosphorescent green and red dopants within just one lightemitting layer, we can get WOLEDs suitable for a display application with very high current and power efficiencies, and excellent color characteristics. Our device exhibits high external quantum efficiency of 28.8%, current efficiency of 56.5 cd/A, and power efficiency of 30 lm/W at 5.9 V and 1000 nit where color point is (0.368, 0.385) in x-y color space (CIE 1931). Halflifetime (LT50) of the device at 1000 nit is estimated more than 31,000 hrs and color variation by aging is suppressed within less than 0.02 in u’-v’color space (CIE 1976). Color filter developed for our WOLED enables color gamut to become wider than the 100% NTSC in uniform color space (u’, v’).
1.
Introduction
White organic light emitting diode (WOLED) has been attracting researchers’ interests in view point of lighting sources which are eco-friend since no mercury is involved, and in view point of a strong candidate for OLED display for large-sized substrate being larger than Gen. 4 (730 x 920 mm2) with a combination of color filter (CF).[1,2] Though WOLED+CF patterning method needs no fine metal mask (FMM), it has been challenged due to its low efficiency,[3] since WOLED has low current efficiency corresponding average value of each efficiency of RGB monocolor OLED, and since CF of each subpixel absorbs about 2/3 of the white light. To overcome the issue, two methods were proposed, i.e., one is 4-subpixel color filter of RGBW,[2] and the other is tandem structure in which a charge generation layer (CGL) connects two independent OLED units. [4,5] There have been many studies about the development of high efficiency WOLEDs. In the late-news paper at SID’08 [6], Liao et al. reported their tandem WOLEDs with 56 cd/A, 30 lm/W, CIE (0.34, 0.40), and 5.9 V at 1000 nit by “combining fluorescent and phosphorescent emission”. In order to increase the efficiency, they applied an orange dopant into a light-emitting layer, instead of a deep red dopant, but they have showed broad electroluminescent (EL) spectrum in the phosphorescent emitting portion of the white spectrum. In this work, we focus on improving efficiency and color characteristics of the phosphorescent emitting portion which is strongly affected by energy transfer from phosphorescent host to red dopant via green dopant. We demonstrate two-stacked WOLEDs in which a fluorescent blue stack and a phosphorescent stack are connected by the charge generation layer. The latter stack whose emission layer was co-deposited with phosphorescent red and green dopants was optimized by controlling doping ratio of the two dopants. And then, we find it is possible to get relatively sharp
emission peaks in red, green, and blue wavelength ranges for display application, while still maintaining the high efficiency of WOLED.
2.
Experimental
OLEDs were fabricated on ITO of 150 nm thickness for anode. After cleaning of substrates, the ITO film was exposed to plasmas of 50 sccm oxygen gas in inductively coupled-type reactor with a RF power of 150W. The plasma treatment before the deposition of first organic layer reduces the energy barrier height for hole injection from anode, removes surface contaminations, and also improves adhesion between anode and organic layer. Figure 1 shows the schematic structure of our two-stacked WOLED. We deposited in sequence the first unit emitting fluorescent blue light and the second unit emitting phosphorescent yellow lights in one-host and two-dopant (R+G) system, followed by aluminum cathode. In detail, the organic layers were prepared on ITO anode in the following structures: hole-injection layer (HIL1), hole-transport layer (HTL), fluorescent blue emitting layer (B EML), electron-transport layer (ETL1), charge generation layer (CGL), hole-injection layer (HIL2), holetransport layers (HTL), exciton block layer (EBL), phosphorescent red and green emitting layer (R+G EML), electron-transport layer (ETL2) and electron-injection layer (EIL) next to the metal electrode. The two units are connected with a Li CGL. All of organic and metal layers were deposited by thermal evaporation method under vacuum at approximately 5x10–7 torr. The emitting area was 0.2X0.2 cm2, and J-V-L characteristics were measured with Keithely 238 source meter and PR655 luminance meter at room temperature.
2.1.
Red, Green, and Blue Simple Devices
Figure 2 shows schematic diagram of simple devices of red, green, and blue, and their EL spectra. Theses devices were fabricated to understand device performances of each light-
Device A: W
Figure 1. Schematics of bottom emission 2 stack WOELD
ISSN/009-0966X/09/3902-0903-$1.00 © 2009 SID
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60.3 / S.-H. Pieh emitting layer. Device configuration of the simple mono-color is “ITO/HIL/HTL/EML/ETL1 or ETL2/EIL/Al”. Device B-1 has an emitting layer of Host1: red dopant (4 vol% doping), Device B-2 had an emitting layer of Host1: green dopant (10 vol% doping) and Device B-3 had an emitting layer of Host2 : blue dopant (4 vol%). We used common host material i.e., Host1, to lightemitting layer in the phosphorescent red and green devices. Table 1 shows J-V-L data & color coordinates at each device. The green Device B-2 has high efficiency, while the red Device B-1 shows 50% decrease in efficiency compared to Device B-0 with the red host adopted by LGD for mass production. It seems that energy transfer from Host1 to green dopant is efficiently occurred but that from Host1 to red dopant is not working effectively.
2.2.
Basic Characteristics of Phosphorescent Device
Before advancing study on two-stacked WOLED, we studied maximizing efficiency and color tunability of the phosphorescent stack with one host-two doped layer emitting red and green, by changing the concentration of the dopants. To examine the influence of the red and green doping ratio, we fabricated the phosphorescent device with the structure of “ITO/HIL/HTL/ Host1: x vol% green dopant: y vol% red dopant/ETL2/EIL/Al. J-
V-L data & color coordinates at each device are summarized in Table 2. Figure 3(a) illustrates EL spectra when changing the green dopant ratio with the red dopant kept at 0.4% concentration. Intensity in green region is little changed, comparing between the devices of 5 % and 10 % green dopants. Interestingly, intensity in green region is drastically decreased as the green dopant is increased into 20 %. Moreover, the increase of green dopant results in the enhancement in intensity of red region. It is concluded that Device C-2 with 10% green dopant concentration shows the best efficiency. Figure 3(b) displays EL spectra when changing the red dopant ratio from 0.1 % to 0.4 %, with the green dopant fixed at 10 %. As red dopant is increased, it is found that the intensity in green region is reduced, while the intensity in red region is increased. These phenomena can be explained by the increase of the exciton energy transfer from green dopant to red dopant. In other words, green dopant acts as a host of red dopant. Considering the result of simple Device B-1 showing low efficiency at 4% concentration of the red dopant, the exciton transfer from the host to the red dopant at less than 0.4% concentration seems to have little contribution. We found that Device D-2 with 0.2 % red dopant and 10 % green dopant concentration shows best result, in respect of color coordinate and efficiency. In addition, when we insert an exciton block layer (EBL) having hole transporting characteristics between HTL and EML (Device D4), current efficiency is enhanced by about 10%, compared with device without EBL. We believe that this enhancement comes from confining triplet 0.12
Intensity [a.u]
0.1
0.14 Device B-0: R Device B-1: R Device B-2: G Device B-3: B
0.1 0.08
Device C-3
0.08 0.06 0.04
0 450
0.06
500
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800
Wavelength [nm]
0.04 0.02
0.12
0
0.1
380
Device C-2
0.02
430
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530
580
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680
730
780
Wavelength [nm]
Figure 2. (a) Schematics and (b) EL spectra of simple device in bottom emission. ID
V
Cd/A
lm/W
Q.E(%)
CIEx
CIEy
B-0
3.7
18.2
15.3
16.9
0.662
0.338
B-1
3.6
9.7
8.5
8.4%
0.655
0.345
B-2
3.9
47.1
37.7
15.2%
0.371
0.598
B-3
3.7
6.2
5.3
6.4%
0.133
0.140
Table 1. EL performance of Devices B-0, 1, 2, and 3 at current density of 10mA/cm2
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Intensity [a.u]
Intensity [a.u]
0.12
Device C-1
(a)
Device D-1 Device D-2 Device D-3 Device C-2 Device D-4
(b)
0.08 0.06 0.04 0.02 0 450
500
550
600
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800
Wavelength [nm]
Figure 3. EL spectra of (a) the various ratio of green doping at same red dopant ratio and (b) the various ratio of red doping at same green dopant ratio in one host-two doping system
60.3 / S.-H. Pieh
Q.E(%)
CIEx
CIEy
C-1
X
R(0.4%)+G(5%)
3.6
30.0
13.5%
0.477
0.504
C-2
X
R(0.4%)+G(10%)
3.6
36.4
17.8%
0.507
0.478
C-3
X
R(0.4%)+G(20%)
3.7
27.8
16.6%
0.565
0.419
D-1
X
R(0.1%)+G(10%)
3.5
44.8
16.2%
0.411
0.562
D-2
X
R(0.2%)+G(10%)
3.4
39.9
16.0%
0.445
0.532
D-3
X
R(0.3%)+G(10%)
3.6
36.5
17.4%
0.501
0.482
D-4
O
R(0.2%)+G(10%)
3.4
44.8
18.1%
0.449
0.529
Table 2. Experimental conditions and performance of the phosphorescent devices at current density 10 mA/cm2
2.3.
Device characteristics of two-stacked WOLED
Figure 4 shows EL spectra and J-V-L characteristics of the blue stack, R+G stack, and two-stacked WOLED. As total organic layer thickness in two-stacked WOLED is varied, we considered micro-cavity effect coming from interference among original light and lights reflected by interfaces like as between OLED and Al cathode. In designing layer structure in WOLED of bottom emission type though, it is critical to position where light is emitting, as well as to satisfy total thickness between anode and cathode into micro-cavity condition. As shown in Fig. 4(a), the EL spectrum of Device A adopting optimum micro-cavity condition has distinctive emission peaks in red, green, blue range suitable for display, while the EL spectrum is comparable to the sum of EL spectra of the blue stack (Device B-3) and R+G stack (Device D-4). Interestingly, it is found that blue intensity of Device A is rather increased than that of Device B-3. Such a result can be explained in two aspects of optical and electrical effects, i.e., organic layer structure in Device A may have advantage in extracting blue light due to micro-cavity effect, or ETL1 and CGL in Device A may result in enhancing charge balance in B-EML. Regarding driving voltage of two-stacked WOLED, the sum of driving voltage of each stack is comparable to that of Device A, as shown in Fig. 4(b). Figure 4(c) and (d) show the current efficiency and external quantum efficiency (EQE) of Device A, respectively, with comparison of Device B-3 and Device D-4. Efficiency and EQE of Device A are 56 cd/A, 28.8 % at 5.9 V and 1000 nit. The efficiency decrease of both Device A and Device D-4 at high current density is due to the triplet-triplet annihilation phenomena commonly observed in phosphorescent emitters[7]. But, the efficiency and EQE are flat over the operation range for a display. Regarding variation of color by aging, there was a little change of relative color spectrum, as shown in Fig. 5. Half-lifetime (LT50) of the device at 1000 nit is estimated more than 31,000 hrs. The key to the high efficiency and color stability within less than 0.02 in u’-v’ color space (CIE 1976) of the two-stacked
0.08 0.06 0.04 0.02 0 380 420 460 500 540 580 620 660 700 740 780
Wavelength [nm] 50
(b) Device B-3: B Device D-4: R+G Device A: W
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Voltage [V]
(c)
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Device B-3: B Device D-4: R+G
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Device A: W
Efficiency [cd/A]
Cd/A
Intensity [a.u]
V
Current Density [mA/cm2]
Dopant ratio
Device B-3: B Device D-4: R+G Device A : W
(a)
0.1
Current Efficiency [cd/A]
EBL
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Device A: W
40 30 0
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Luminance [cd/m2]
10 0 0
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Current Density [mA/cm2] 35%
(d)
30%
Device B-3: B Device D-4: R+G Device A: W
25%
EQE [%]
excitons to the phosphorescent EML and preventing loss of triplets to the HTL due to its high LUMO level and triplet energy level. The highest quantum efficiency was observed in the Device D-4 adopting EBL.[6]
20% 15% 10% 5% 0% 0
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Current Density [mA/cm2]
Figure 4. Comparison of operational characteristics of three devices A, B-3, and D-4. (a) EL spectra of the blue stack and R+G stack, two-stacked WOLED, (b) driving voltage vs current density, (c) current density vs current efficiency (d) current density vs EQE.
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60.3 / S.-H. Pieh 3.
0.12 No Aging After 36% aging After 52% aging 0.51
0.08
0.50
v'
Intensity [a.u]
0.1
0.49
0.06
0.48 0.19 0.20 0.21 0.22
0.04
u'
0.02 0 380
4. 430
480
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Wavelength [nm]
Figure 5. EL spectrum variation by aging for Device D WOLED is to make the phosphorescent stack optimized in one host-two doped system. Recently, we developed new color filters optimized to our twostacked WOLED, whose transmittance is displayed in Fig. 6. If the color filter is incorporated with our WOLED, we can get excellent color gamut exceeding 100% NTSC in uniform color space (u’, v’), because we separated white emission spectrum into distinct red, green, and blue peaks.
Normalized Intensity
White Spectrum + CF
0.8
Green CF 0.6
Blue CF White EL
0.4 0.2 0 430
480
530
580
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680
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780
Wavelength (nm)
Figure 6. Transmittance spectra of new color filters and twostacked WOLED EL spectra
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[2] J. W. Hamer, A. D. Arnold, M. L. Boroson, M. Itoh, T. K. Hatwar, M. J. Helber, K. Miwa, C. I. Levey, M. E. Long, J. E. Ludwicki, D. C. Scheier, J. P. Spindler, and S. A. Van Slyke, “System Design for a High Color-Gamut TV-sized AMOLED Display”, Journal of the SID, vol. 16, p.3 (2008), [3] T. Urabe, T. Sasaoka, K. Tatsuki, and J. Takaki , ”Technological Evolution for Large Screen Size Active Matrix OLED Display”, in Proc. the International Meeting on Information Display, p.161 (2007),.
[5] Sungsoo Lee, Changwoong Chu, Jinkoo Chung, Joohyeon Lee, Junho Choi, Jaekook Ha, Seongmin Kim, Joonhoo Choi, Jaehoon Jung, Chiwoo Kim, and Jinseok Lee, " Achieving High Color Gamut with Microcavity on White OLED", SID Symposium Digest, Vol. 39, pp. 1042-1045, (2008).
Red CF
380
References
[1] C. Chu, J Ha, J. Choi, S. Lee, J. Rhee, D. Lee, J. Chung, H. Kim, and K. Chung, “Advences and Issues in White OLED and Color Filter Architecture”, in Proc. the International Meeting on Information Display, pp 1118-1121 (2007),.
[4] Jeffrey P. Spindler and Tukaram K. Hatwar, “Development of Tandem White Architecture for Large-Sized AMOLED Displays with Wide Color Gamut”, in Proc. the International Meeting on Information Display, p.89 (2007),
1.2 1
Conclusions
Our two-stacked WOLED with high current efficiency and distinctive emission peaks in red, green, and blue wavelengths result from optimized phosphorescent EL unit. The current efficiency and quantum efficiency at 1000 nit were 56.5 cd/A, 28.8% with CIExy (0.368, 0.385). Half-lifetime (LT50) of the device at 1000 nit is estimated more than 31,000 hrs. Our highly efficient WOLED will be able to realize low power consumption enough to compete LCD panel, even though employing color filter with high color gamut more than 100%.
[6] Liang-Sheng Liao, Xiaofan Ren, William J. Begley, YuanSheng Tyan, and Cynthia A. Pellow, " Tandem White OLEDs Combining Fluorescent and Phosphorescent Emission", SID Symposium Digest, Vol. 39, pp. 818-821, (2008). [7] M. A. Baldo, C. Adachi, and S. R. Forrest " Transient analysis of organic electrophosphorescence. II. Transient analysis of triplet-triplet annihilation", Phys. Rev. B 62, 10 967 (2000).
