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White organic light-emitting diodes (WOLEDs) have drawn considerable attention for next-generation lighting and display applications owing to their remarkable advantages. Phosphorescent OLED technology is crucial to realize high-efficiency white OLEDs because phosphorescent emitters enable to achieve almost 100% internal quantum efficiency (IQE) by harvesting all the excitons of 75% of triplets and 25% of singlets. However, an efficiency roll-off at high-brightness and a shift in color under various operation biases remains challenges. With the goal towards commercial applications, it requires WOLEDs should simultaneously realize high efficiency at high-brightness region over 1000 cd/m2 and good color stability over a wide electroluminescent range. In this paper, we first investigated the energy transfer process between the blue-emitting Bis (3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) iridium (III) (Firpic) and the orange emitting Iridium (III) bis(4-(4-tert-butylphenyl)thieno[3,2-c]pyridinato-N,C2')acetylacetonate (PO-01-TB), in addition to the behavior of the carrier trapping in the phosphorescent OLEDs with double emissive layers. Then we successfully fabricated phosphorescent WOLED with multiple emissive layers. The resulting phosphorescent WOLED achieves the maximum forward-viewing current efficiency (CE) of 34.6 cd/A and external quantum efficiency (EQE) of 13.5%, and the CE and the EQE remain 33.9 cd/A and 13.3% at 1000 cd/m2, respectively, indicating that the WOLED exhibits low efficiency roll-off. Furthermore, the WOLED shows very stable white emission with small Commission Internationale de L’Eclairage (CIE) coordinate varying range of (0.016, 0.011) from 1000 to 10000 cd/m2. The results provide a promising avenue to simultaneously achieve high efficiency, lower the efficiency roll-off at high brightness and color-stability for phosphorescent WOLEDs by carefully designing the device architecture to redistribute the charge carriers and excitons in the recombination zone.
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Keywords:
- white organic light-emitting diodes /
- efficiency roll-off /
- color-stability /
- phosphorescence
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图 1 器件中所使用材料的能级示意图及Firpic和PO-01-TB的化学结构
Figure 1. Energy level diagram of materials used in the devices, and the chemical structures of Firpic and PO-01-TB.
图 2 器件A−D的结构示意图
Figure 2. Structure schematic of devices A−D.
图 3 薄膜的紫外-可见光吸收光谱和光致发光光谱
Figure 3. UV-vis absorption and PL spectra of the deposited films.
图 4 器件A−D的归一化电致发光光谱(电流密度为20 mA/cm2)
Figure 4. Normalized EL spectra of devices A−D at 20 mA/cm2
图 5 (a) 单空穴器件H1−H4和单电子器件E1−E4的结构示意图; (b) 单空穴器件H1−H4和(c) 单电子器件E1−E4的电流密度-电压关系特性曲线
Figure 5. (a) Structure schematic of hole-only devices H1−H4 and electron-only devices E1-E4; current density-voltage characteristics of (b) hole-only devices H1−H4 and (c) electron-only devices E1−E4.
图 6 (a)器件A和(b)器件B在不同电压下的归一化电致发光光谱; 插图为波长在540−570 nm之间的光谱放大图
Figure 6. Normalized EL spectra of devices A(a) and B(b) with different voltage. Inset is the corresponding enlarged spectra at 540−570 nm.
图 7 器件A和B的亮度-电压关系特性曲线; 插图为器件A和B的外量子效率-亮度关系特性曲线
Figure 7. Luminance-voltage characteristics of devices A and B. Inset is EQE-luminance characteristic of devices A and B
图 8 白光器件W1和W2的 (a) 能级结构和发光层中激子复合过程示意图, 蓝色虚线框为载流子复合区, S1和S0分别代表单线态能级和基态(○), T1代表三线态能级(△); (b) 亮度-电压和关系特性曲线和归一化电致发光光谱(插图); (c) 电流效率-亮度-外量子效率关系特性曲线; (d) 器件B和W2的外量子效率-电流密度关系特性曲线, 图中红线和蓝线分别为的器件B和W2拟合曲线(TTA模型)
Figure 8. (a) Energy diagram and exciton dynamics of the WOLEDs W1 and W2. S1 and T1 are respectively the singlet (○) and triplet (△) energy levels, and S0 is the ground state (○). The blue dashed box depicts the main region of carrier recombination. Luminance-voltage characteristics and the normalized EL spectra (b), and current efficiency-Luminance-external quantum efficiency characteristics (c) of the WOLEDs W1 and W2; (d) EQE-current density of the OLEDs B and W2. The red and blue lines are corresponding fitting curves based on the TTA model, respectively.
图 9 白光器件W2的亮度从1000 cd/m2增至5000 cd/m2过程中的归一化电致发光光谱和相应CIE色度坐标及显色指数的变化
Figure 9. Normalized EL spectra and the corresponding CIE coordinates, CRI of the device W2 at brightness of 1000−5000 cd/m2.
表 1 器件A, B和器件W1, W2的电致发光性能参数
Table 1. EL performance parameters of the OLEDs in our studies.
Device Max EQE/CE/Luminance/
(%/[cd/A]/[cd/m2])At 1000 cd/m2 At 5000 cd/m2 EQE/CE/(%/[cd/A]) CIE/(x, y) CRI EQE/CE/(%/[cd/A]) CIE/(x, y) CRI A 7.3/16.5/8589 6.1/13.8 0.209, 0.351 44 4.2/9.6 0.215, 0.354 46 B 11.9/31.2/13890 11.7/30.8 0.303, 0.413 56 9.0/23.4 0.294, 0.408 56 W1 11.7/29.4/17260 11.4/28.3 0.320, 0.390 64 9.8/23.7 0.309, 0.383 65 W2 13.5/34.6/18340 13.3/33.9 0.342, 0.403 64 11.7/29.3 0.331, 0.395 65 
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[1] Kido J, Hongawa K, Okuyama K, Nagai K 1994 Appl. Phys. Lett. 64 815
Google Scholar
[2] Liu Y C, Li C S, Ren Z J, Yan S K, Bryce M R 2018 Nat. Rev. Mater. 3 18020
Google Scholar
[3] Ai X, Evans E W, Dong S, Gillett A J, Guo H, Chen Y, Hele T J H, Friend R H, Li F 2018 Nature 563 536
Google Scholar
[4] Uoyama H, Goushi K, Shizu K, Nomura H, Adachi C 2012 Nature 492 234
Google Scholar
[5] Reineke S, Lindner F, Schwartz G, Seidler N, Walzer K, Lüssem B, Leo K 2009 Nature 459 234
Google Scholar
[6] Zhang X, Pan T, Zhang J, Zhang L, Liu S, Xie W 2019 ACS Photonics 6 2350
Google Scholar
[7] Baldo M A, O'Brien D F, You Y, Shoustikov A, Sibley S, Thompson M E, Forrest S R 1998 Nature 395 151
Google Scholar
[8] Zaen R, Park K M, Lee K H, Lee J Y, Kang Y J 2019 Adv. Opt. Mater. 190138 7
[9] Udagawa K, Sasabe H, Igarashi F, Kido J 2016 Adv. Opt. Mater. 4 86
Google Scholar
[10] Liu D, Deng L J, Li W, Yao R J, Li D L, Wang M, Zhang S F 2016 Adv. Opt. Mater. 4 864
Google Scholar
[11] Zhu L P, Wu Z B, Chen J S, Ma D G 2015 J. Mater. Chem. C 3 3304
Google Scholar
[12] Lee C W, Lee J Y 2013 Adv. Mater. 25 5450
Google Scholar
[13] Baldo M A, Adachi C, Forrest S R 2000 Phys. Rev. B 62 10967
Google Scholar
[14] Reineke S, Walzer K, Leo K 2007 Phys. Rev. B 75 125328
Google Scholar
[15] Giebink N C, Forrest S R 2008 Phys. Rev. B 77 235215
Google Scholar
[16] Zhao C Y, Yan D H, Ahamad T, Alshehri S M, Ma D G 2019 J. Appl. Phys. 125 045501
Google Scholar
[17] Yu Z W, Zhang J X, Liu S H, Zhang L T, Zhao Y, Zhao H Y, Xie W F 2019 ACS Appl. Mater. Interaces 11 6292
Google Scholar
[18] Wang F, Zhao Y P, Xu H X, Zhang J, Miao Y Q, Guo K P, Shinar R, Shinar O S, Wang H, Xu B S 2019 Org. Electron. 70 272
Google Scholar
[19] Maheshwaran A, Sree V G, Park H Y, Kim H, Han S H, Lee J Y, Jin S H 2018 Adv. Funct. Mater. 28 1802945
Google Scholar
[20] Lee S, Kim K H, Limbach D, Park Y S, Kim J J 2013 Adv. Funct. Mater. 23 4105
Google Scholar
[21] Gather M C, Alle R, Becker H, Meerholz K 2007 Adv. Mater. 19 4460
Google Scholar
[22] Chen S F, Wu Q, Kong M, Zhao X F, Yu Z, Jia P P, Huang W 2013 J. Mater. Chem. C 1 3508
Google Scholar
[23] Kim S H, Jang J, Lee J Y 2009 Synth. Met. 159 1295
Google Scholar
[24] Liu B Q, Wang L, Xu M, Tao H, Xia X H, Zou J H, Su Y J, Gao D Y, Lan L F, Peng J J 2014 J. Mater. Chem. C 2 5870
[25] Ying S A, Xiao S, Yao J W, Sun Q, Dai Y F, Yang D Z, Qiao X F, Chen J S, Zhu T F, Ma D G 2019 Adv. Opt. Mater. 7 1901291
Google Scholar
[26] 俞浩健, 姚方男, 代旭东, 曹进, 田哲圭 2019 物理学报 68 017202
Google Scholar
Yu H J, Yao F N, Dai X D, Cao J, Jhun C 2019 Acta Phys. Sin. 68 017202
Google Scholar
[27] Tanaka I, Tokito S 2004 Jpn. J. Appl. Phys. 43 7733
Google Scholar
[28] Jou J H, Wang W B, Chen S Z, Shyue J J, Hsu M F, Lin C W, Shen S M, Wang C J, Liu C P, Chen C T, Wu M F, Liu S W 2010 J. Mater. Chem. 20 8411
Google Scholar
[29] Su S J, Chiba T, Takeda T, Kido J 2008 Adv. Mater. 20 2125
Google Scholar
[30] Sun N, Wang Q, Zhao Y B, Chen Y H, Yang D Z, Zhao F C, Chen J S, Ma D G 2014 Adv. Mater. 26 1617
Google Scholar
[31] Xu Z, Gong Y B, Dai Y F, Sun Q, Qiao X F, Yang D Z, Zhan X J, Li Z, Tang B Z, Ma D G 2019 Adv. Opt. Mater. 7 1801539
Google Scholar
[32] Wu Z B, Wang Q, Yu L, Chen J S, Qiao X F, Ahamad T, Alshehri S M, Yang C L, Ma D G 2016 ACS Appl. Mater. Interfaces 8 28780
Google Scholar
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