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Color stable and low driving voltage white organic light-emitting diodes with low efficiency roll-off achieved by selective hole transport buffer layers
Organic Electronics 13 (2012) 2296–2300
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Color stable and low driving voltage white organic light-emitting diodes with low efficiency roll-off achieved by selective hole transport buffer layers Zhensong Zhang a, Guohua Xie b,1, Shouzhen Yue a, Qingyang Wu a, Yu Chen a, Shiming Zhang a, Li Zhao a, Yang Luo a, Yi Zhao a,⇑, Shiyong Liu a a State Key Laboratory on Integrated Optoelectronics, College of Electronics Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China b Institut für Angewandte Photophysik, Technische Universtität Dresden, 01062 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 18 January 2012 Received in revised form 28 February 2012 Accepted 1 July 2012 Available online 25 July 2012 Keywords: White light Low operating voltage High color stability Low efficiency roll-off Hole transport buffer layers
a b s t r a c t White organic light-emitting diodes (WOLEDs) showing high color stability, low operating voltage, high efficiency and low efficiency roll-off by adopting different hole transport buffer layers which also behaves as electron/exciton blocking layers (EBL) have been developed. The characteristics of WOLEDs based on blue–green and orange phosphors could be easily manipulated by hole transport buffer layer, which tailors charge carrier transportation and energy transfer. Our WOLEDs show low operating voltages, 100 cd/m2 at 3.2 V, 1000 cd/m2 at 3.7 V and 10000 cd/m2 at 4.8 V, respectively, and achieve a current efficiency of 35.0 cd/A, a power efficiency of 29.0 lm/W at a brightness of 1000 cd/m2, and a low efficiency roll-off 8.7% calculated from the maximum efficiency value to that of 5000 cd/m2. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Since the pioneering works by Tang and Vanslyke [1], organic light-emitting diodes (OLEDs) have attracted intensive attention due to their bright future of practical and commercialization. OLEDs have many advantages, such as low driving voltage, low power consumption, wide viewing angle, high efficiency, fast response speed, flexibility and low cost, so they have potential application in flat displays and solid state lighting [2–5]. Due to their intrinsic advantages and excellent characteristics, OLED technology becomes one of the world’s hottest research spots. It is worth mentioning that reducing the operating voltage is crucially important to improve the power efficiency and
⇑ Corresponding authors. Tel.: +86 431 85168242 8301. 1
E-mail address: [email protected] (Y. Zhao). Current address.
ensure the compatibility with the common low voltage active matrix driver circuitry [6]. Charge carriers balance and triplet excitons confinement are two pivotal factors to obtain high performance phosphor-based OLEDs [7–10]. In general, the observed efficiency roll-off is obvious with increased current density for phosphor-based OLEDs. The imbalance distribution of electrons and holes in the emitting layer (EML) is one of the key factors which impact and shape the efficiency roll-off at low current density, which is reported by Forrest’s group [11]. While the accelerated efficiency roll-off of phosphorescent devices at high current densities can be ascribed to be twofold: the imbalance distribution of electrons and holes in EML and nonradiative excitons quenching processes, such as triplet–triplet excitons annihilation (TTA) [11] and triplet-polaron annihilation (TPA) [12] processes, and/or field-induced quenching [13]. It is considered that WOLEDs based on complementary blue and yellow phosphors is one of the most cost-effective
1566-1199/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.07.001
Z. Zhang et al. / Organic Electronics 13 (2012) 2296–2300
methods to achieve white light emitters aiming for mass production, as the simplified WOLEDs configurations would be of significant advantage for high volume fabrication associated with potential general lighting and display applications. Toward this end, we fabricated WOLEDs by combining blue–green with orange phosphors. The materials iridium(III)bis[(4,6-difluo-rophenyl)-pyridinato-N,C2] (FIrpic) and bis(2-phenylbenzothiazolato) (acetylacetonate)iridium(III) (Ir(BT)2(acac)) were selected to display broad emission covering as much of the visible light spectrum as possible and generate white light, and 2,6-bis[(3carbazol-9-yl)phenyl)]pyridine (26DCzPPy) was selected as a common bipolar host [14]. We tailored the characteristics of WOLEDs by simply adopting different buffer layers between hole transport layer (HTL) and EML. The dynamic influences of the buffer layers on charges and excitons are investigated. We used N,N0 -dicarbazolyl-3,5-benzene (mCP), 4,40 ,400 -tris(Ncarbazolyl)triphenylamine (TcTa), and fac-tris(1-phenylpyrazolato,N,C2) (Ir(ppz)3) as buffer layers, respectively, which are inserted between the HTL and EML. We demonstrated that the efficiency as well as the spectra of WOLEDs could be easily tailored by the buffer layers.
2. Experimental The primary architecture of our devices is ITO/TAPC:MoOx (10 nm, 15 wt.%)/TAPC (30 nm)/buffer layer (10 nm)/26DCz PPy:FIrpic (5 nm, 15 wt.%)/26DCzPPy: Ir(BT)2(acac) (5 nm, 4 wt.%)/BPhen (40 nm)/Cs2CO3 (1 nm)/Al (100 nm), where buffer layer denotes mCP for Device A, TcTa for Device B, and Ir(ppz)3 for Device C, respectively. As compared, Device D with a 10 nm TAPC which replaces the buffer layer is used to investigate the roles of the buffer layers. Furthermore, we also utilize an additional orange EML to enlarge the excitons formation zone and harvest high energy triplets for improving the efficiency and reducing the driving voltage. The WOLEDs composed of ITO/TAPC:MoOx (10 nm, 15 wt.%)/TAPC (35 nm)/mCP:Ir(BT)2(acac) (5 nm, 4 wt.%)/ 26DCzPPy:FIrpic (5 nm, 15 wt.%)/26DCzPPy:Ir(BT)2(acac) (5 nm, 4 wt.%)/BPhen (40 nm)/Cs2CO3 (1 nm)/Al (100 nm) (Device E) and ITO/TAPC:MoOx (10 nm, 15 wt.%)/TAPC (35 nm)/TcTa:Ir(BT)2(acac) (5 nm, 4 wt.%)/26DCzPPy:FIrpic (5 nm, 15 wt.%)/26DCzPPy:Ir(BT)2(acac) (5 nm, 4 wt.%)/ BPhen (40 nm)/Cs2CO3 (1 nm)/Al (100 nm) (Device F) structures were fabricated. Here, Ir(BT)2(acac)-doped mCP/TcTa served as orange EML as well as EBL. TAPC is 1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane served as HTL, MoOx-doped TAPC served as hole injection layer (HIL), and BPhen is 4,7-diphenyl-1,10-phenanthroline served as electron transport layer (ETL), respectively. The lowest unoccupied molecular orbital (LUMO) positions, highest occupied molecular orbital (HOMO) positions, and triplet levels of the materials we used are plotted in Fig. 1. The electroluminescence (EL) spectra are measured by a PR650 spectroscan spectrometer. The luminance–current density–voltage characteristics are recorded simultaneously with the measurement of the EL spectra by combining the spectrometer with a Keithley model 2400
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Fig. 1. (a) Triplet energy levels of some materials used in this paper. (b) Schematic diagram of HOMO–LUMO values of the materials. The values are obtained from the literature [15–19].
programmable voltage–current source. All measurements are carried out in ambient environments. 3. Results and discussion The luminance–current density–voltage characteristics are plotted in Fig. 2. Device D without a buffer layer shows obvious emission from TAPC (k 425 nm) [20,21] (shown in the inset of Fig. 2) which suggests that the electron leakage into the HTL is rather significant in Device D. To eliminate the undesirable inefficiency TAPC emission and improve the performance of our WOLEDs, we insert different buffer layers between HTL and EML. Device A with mCP as a buffer layer has a relative low current density at the same voltage owing to its lower intrinsic hole mobility compared with that of other buffer layers [16,22,23].
Fig. 2. Luminance–current density–voltage characteristics of Device A, B, C and D. The inset shows the normalized EL spectra of the devices at 10 mA/cm2.
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Fig. 4. Luminance–current density–voltage characteristics of Device E and F. The inset shows the normalized EL spectra of the devices at 10 mA/ cm2.
Fig. 3. Current efficiency (a), power efficiency (b) and External quantum efficiency (c) versus current density characteristics of Device A, B, C and D, respectively.
However, there is a discernable emission from TAPC in Device A when the driving voltage above 5 V (shown in Fig. S3(a)), it suggests that there exists electrons leaked into HTL, i.e., a number of electrons flow through mCP and arrive at HTL, which is in accordance with Lee’s report [24] in which state that the electron mobility of mCP rapidly increases as the driving voltage rising. Here, leakage of the electrons into HTL in Device A and D accounts for the lower efficiencies (shown in Fig. 3). The emissions in Device B and C are almost originated from FIrpic and Ir(BT)2(acac), and these two devices show similar low operating voltages, 100 cd/m2 at 3.4 V, 1000 cd/m2 at 4.1 V and 10000 cd/m2 at 5.4 V, respectively. The energy barriers between the LUMO of EML and the ETL are very small in our devices. Therefore, electrons can be easily injected into the EML, which is one of the main reasons for the low operating voltage. However, the energy barriers between the HOMO of buffer layers and the EML are quite different in Device B and C. The former one with a buffer layer of TcTa has a smaller energy barrier, holes can be easily injected into the EML, and as such the carrier balance in EML is emerging as a possibility, the excitons formation in FIrpic:26DCzPPy layer and Ir(BT)2(acac):26DCzPPy layer are equivalent, and subsequently a balance blue and orange emissions were observed (shown in Fig. S3(b)). Although the later one with a buffer layer of Ir(ppz)3 has a barrier as high as 0.95 eV, Ir(ppz)3 exhibits not only superior electron blocking ability but also excellent triplet excitons confinement capability. Due to the strong electronblocking effect of Ir(ppz)3 the concentration of electrons at the interface between Ir(ppz)3 and EML increased remarkably while the driving voltage raising, which enhanced holes injection and facilitated the excitons formation in FIrpic:26DCzPPy layer, and subsequently an
increase of blue emission was observed (shown in Fig. S3(c)). Furthermore, TcTa and Ir(ppz)3 have higher T1 than that of 26DCzPPy and FIrpic, excitons could be effectively confined in EML. Therefore, more balance distribution of electrons and holes in the EML as well as effective confinement of triplet excitons in Device B and C come true, and then they show higher efficiencies than Device A and D, at last they gave almost the same performances. Fig. 4 shows the luminance–current density–voltage characteristics of Device E and F. As previously explained, TcTa has a superior hole mobility than mCP [22,23], thus Device F has a higher brightness at the same operating voltage. Device F shows lower operating voltages, e.g.
Fig. 5. Current efficiency (a), power efficiency (b) and external quantum efficiency (c) versus current density characteristics of Device E and F, respectively.
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Z. Zhang et al. / Organic Electronics 13 (2012) 2296–2300 Table 1 Summary of the performances of the Devices.
a b c
Device
Voltage at 100, 1000 and 10000 cd/m2 (V)
CIEa (x, y) over 103–104 cd/m2
CRIb at 1000 cd/m2
A B C D E F
3.5, 3.4, 3.4, 3.6, 3.3, 3.2,
(0.34 ± 0.005, 0.39 ± 0.007) (0.37 ± 0.005, 0.41 ± 0.005) (0.32 ± 0.005, 0.39 ± 0.007) (0.42 ± 0.008, 0.42 ± 0.009) (0.41 ± 0.012, 0.44 ± 0.005) (0.44 ± 0.005, 0.45 ± 0.005)
68 71 67 61 60 58
4.2 4.1 4.1 4.5 3.8 3.7
and and and and and and
5.8 5.4 5.4 6.2 5.0 4.8
EQEc (%) 1 mA/cm2
10 mA/cm2
9.6 10.2 12.3 3.5 11.5 12.5
9.0 10.5 11.3 3.1 11.1 12.5
The Commission Internationale de l’Eclairage 1931 coordinates. Color rendering index. External quantum efficiency.
100 cd/m2 at 3.2 V, 1000 cd/m2 at 3.7 V and 10000 cd/m2 at 4.8 V. There is no discernable emission from TAPC is observed in Device E and F, where direct electron trapping and excitons formation on guest play a role in Ir(BT)2(acac)-doped mCP or TcTa layer, so the additional orange EML could effectively block the electrons and excitons from transiting or diffusing into HTL, thus the higher efficiencies are reasonable. Here, Device E achieved a current efficiency of 31.6 cd/A and a power efficiency of 26.1 lm/ W, compared to 35.0 cd/A and 29.0 lm/W of Device F at a brightness of 1000 cd/m2 (shown in Fig. 5(a) and (b)). While the efficiency roll-offs concerning the maximum efficiency value and that of 5000 cd/m2 are only 13.8% for Device E, and 8.7% for Device F (shown in Fig. 5(c)), respectively, which could be ascribed to the additional EML where the electron trapping sites not only enlarge the excitons formation zone but also reduce the electron leakage current to HTL. The balanced carriers injection and enlarged excitons formation zone further suppressed the notorious TTA process at high current density. As listed in Table 1, our WOLEDs with different buffer layers showed low operating voltage without n-doping ETL, and extremely stable CIEs ranged over 1000 cd/m2 to 10000 cd/m2. Device A with mCP buffer layer exhibits lower efficiencies which are caused by the electron leakage into the HTL. Device B with TcTa buffer layer expresses superior hole injection from HTL to EML and electron blocking ability, thus it realizes high efficiency and retards the efficiency roll-off. Device C with Ir(ppz)3 buffer layer manifests distinctive electron and triplet excitons blocking abilities which are propitious to achieve high efficiency. Device E and F with an additional 5 nm Ir(BT)2(acac)-doped mCP or TcTa served as orange EML as well as electron trapping sites not only enlarge the excitons formation zone but also reduce the electron leakage current to HTL, which is favorable to improve the efficiency and retard the efficiency roll-off. 4. Conclusions In conclusion, by modulating the hole transportation and electron blocking characteristics of the hole transport buffer layer, it is easy to demonstrate color stable WOLEDs. In this contribution, we investigated the effects of charge carrier transport and confinement on the performances of WOLEDs by adopting different hole transport buffer layers,
which tailored the distribution of charge carriers and excitons. We also demonstrated WOLEDs with an additional orange phosphor-doped buffer layer, which served as orange EML as well as electron trapping layer, can effectively improve the radiative recombination. Here, we have achieved WOLEDs with low operating voltage (2290 cd/ m2 at 4.0 V), high efficiencies (35.0 cd/A, 30.6 lm/W and 13.2% for the maximum current efficiency, power efficiency and external quantum efficiency, respectively), and low efficiency roll-off (8.7%). In prospect, the efficiency can be 2–3 times increased by out-coupling schemes as well as air-stable n-doping strategies to meet the requirements for solid-state lighting sources. Acknowledgements We acknowledge funding for this research from the National Key Basic Research and Development Program of China under Grant No. 2010CB327701, and the National Science Foundation of China (Grant Nos. 60977024 and 60906021). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2012.07.001. References [1] C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (1987) 913. [2] M. Pfeiffer, S.R. Forrest, K. Leo, M.E. Thompson, Adv. Mater. 14 (2002) 1633. [3] B.W. D’ Andrade, S.R. Forrest, Adv. Mater. 16 (2004) 1585. [4] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, K. Leo, Nature 459 (2009) 234. [5] R. Meerheim, K. Walzer, G. He, M. Pfeiffer, K. Leo, SPIE 6192 (2006) 61920P. [6] D.L. Mathine, H.S. Woo, W. He, T.W. Kim, B. Kippelen, N. Peyghambarian, Appl. Phys. Lett. 76 (2000) 3849. [7] Y. Sun, N.C. Giebink, H. Kanno, B. Ma, M.E. Thompson, S.R. Forrest, Nature 440 (2006) 908. [8] Q. Wang, J.Q. Ding, D.G. Ma, Y.X. Cheng, L.X. Wang, X.B. Jing, F.S. Wang, Adv. Funct. Mater. 19 (2009) 84. [9] M. Lebental, H. Choukri, S. Chenais, S. Forget, A. Siove, B. Geffroy, E. Tutis, Phys. Rev. B 79 (2009) 165318. [10] J. Wünsche, S. Reineke, B. Lüssem, K. Leo, Phys. Rev. B 81 (2010) 245201. [11] N. Giebink, S.R. Forrest, Phys. Rev. B 77 (2008) 235215. [12] S. Reineke, K. Walzer, K. Leo, Phys. Rev. B 75 (2007) 125328.
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[13] J. Kalinowski, W. Stampor, J. Szmytkowski, D. Virgili, M. Cocchi, V. Fattori, C. Sabatini, Phys. Rev. B 74 (2006) 085316. [14] S.J. Su, H. Sasabe, T. Takeda, J. Kido, Chem. Mater. 20 (2008) 1691– 1693. [15] F.S. Juang, L.A. Hong, S.H. Wang, Y.S. Tsai, M.H. Gao, Y. Chi, H.P. Shieh, J.S. Hsu, Jpn. J. Appl. Phys. 50 (2011) 04DK04. [16] J.H. Lee, C.L. Huang, C.H. Hsiao, M.K. Leung, C.C. Yang, C.C. Chao, Appl. Phys. Lett. 94 (2009) 223301. [17] T. Peng, H. Bi, Y. Liu, Y. Fan, H.Z. Gao, Y. Wang, Z.M. Hou, J. Mater. Chem. 19 (2009) 8072–8074. [18] Y. Zheng, S.-H. Eom, N. Chopra, J. Lee, F. So, J.G. Xue, Appl. Phys. Lett. 92 (2008) 223301.
[19] V.I. Adamovich, S.R. Cordero, P.I. Djurovich, A. Tamayo, M.E. Thompson, B.W. D’Andrade, S.R. Forrest, Org. Electron. 4 (2003) 77. [20] A.P. Kulkarni, S.A. Jenekhe, J. Phys. Chem. C 112 (2008) 5174–5184. [21] T.-Y. Kim, D.-G. Moon, Trans. Electron Mater. 12 (2) (2011) 83. [22] G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, J. Salbeck, Appl. Phys. Lett. 85 (2004) 3911. [23] V. Adamovich, J. Brooks, A. Tamayo, A.M. Alexander, P.I. Djurovich, B.W. D’Andrade, C. Adachi, S.R. Forrest, M.E. Thompson, New J. Chem. 26 (2002) 1171–1178. [24] C.-H. Hisiao, S.-W. Liu, C.-T. Chen, J.-H. Lee, Org. Electron. 11 (2010) 1500–1506.
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Color stable and low driving voltage white organic light-emitting diodes with low efficiency roll-off achieved by selective hole transport buffer layers Zhensong Zhang a, Guohua Xie b,1, Shouzhen Yue a, Qingyang Wu a, Yu Chen a, Shiming Zhang a, Li Zhao a, Yang Luo a, Yi Zhao a,⇑, Shiyong Liu a a State Key Laboratory on Integrated Optoelectronics, College of Electronics Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China b Institut für Angewandte Photophysik, Technische Universtität Dresden, 01062 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 18 January 2012 Received in revised form 28 February 2012 Accepted 1 July 2012 Available online 25 July 2012 Keywords: White light Low operating voltage High color stability Low efficiency roll-off Hole transport buffer layers
a b s t r a c t White organic light-emitting diodes (WOLEDs) showing high color stability, low operating voltage, high efficiency and low efficiency roll-off by adopting different hole transport buffer layers which also behaves as electron/exciton blocking layers (EBL) have been developed. The characteristics of WOLEDs based on blue–green and orange phosphors could be easily manipulated by hole transport buffer layer, which tailors charge carrier transportation and energy transfer. Our WOLEDs show low operating voltages, 100 cd/m2 at 3.2 V, 1000 cd/m2 at 3.7 V and 10000 cd/m2 at 4.8 V, respectively, and achieve a current efficiency of 35.0 cd/A, a power efficiency of 29.0 lm/W at a brightness of 1000 cd/m2, and a low efficiency roll-off 8.7% calculated from the maximum efficiency value to that of 5000 cd/m2. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Since the pioneering works by Tang and Vanslyke [1], organic light-emitting diodes (OLEDs) have attracted intensive attention due to their bright future of practical and commercialization. OLEDs have many advantages, such as low driving voltage, low power consumption, wide viewing angle, high efficiency, fast response speed, flexibility and low cost, so they have potential application in flat displays and solid state lighting [2–5]. Due to their intrinsic advantages and excellent characteristics, OLED technology becomes one of the world’s hottest research spots. It is worth mentioning that reducing the operating voltage is crucially important to improve the power efficiency and
⇑ Corresponding authors. Tel.: +86 431 85168242 8301. 1
E-mail address: [email protected] (Y. Zhao). Current address.
ensure the compatibility with the common low voltage active matrix driver circuitry [6]. Charge carriers balance and triplet excitons confinement are two pivotal factors to obtain high performance phosphor-based OLEDs [7–10]. In general, the observed efficiency roll-off is obvious with increased current density for phosphor-based OLEDs. The imbalance distribution of electrons and holes in the emitting layer (EML) is one of the key factors which impact and shape the efficiency roll-off at low current density, which is reported by Forrest’s group [11]. While the accelerated efficiency roll-off of phosphorescent devices at high current densities can be ascribed to be twofold: the imbalance distribution of electrons and holes in EML and nonradiative excitons quenching processes, such as triplet–triplet excitons annihilation (TTA) [11] and triplet-polaron annihilation (TPA) [12] processes, and/or field-induced quenching [13]. It is considered that WOLEDs based on complementary blue and yellow phosphors is one of the most cost-effective
1566-1199/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.07.001
Z. Zhang et al. / Organic Electronics 13 (2012) 2296–2300
methods to achieve white light emitters aiming for mass production, as the simplified WOLEDs configurations would be of significant advantage for high volume fabrication associated with potential general lighting and display applications. Toward this end, we fabricated WOLEDs by combining blue–green with orange phosphors. The materials iridium(III)bis[(4,6-difluo-rophenyl)-pyridinato-N,C2] (FIrpic) and bis(2-phenylbenzothiazolato) (acetylacetonate)iridium(III) (Ir(BT)2(acac)) were selected to display broad emission covering as much of the visible light spectrum as possible and generate white light, and 2,6-bis[(3carbazol-9-yl)phenyl)]pyridine (26DCzPPy) was selected as a common bipolar host [14]. We tailored the characteristics of WOLEDs by simply adopting different buffer layers between hole transport layer (HTL) and EML. The dynamic influences of the buffer layers on charges and excitons are investigated. We used N,N0 -dicarbazolyl-3,5-benzene (mCP), 4,40 ,400 -tris(Ncarbazolyl)triphenylamine (TcTa), and fac-tris(1-phenylpyrazolato,N,C2) (Ir(ppz)3) as buffer layers, respectively, which are inserted between the HTL and EML. We demonstrated that the efficiency as well as the spectra of WOLEDs could be easily tailored by the buffer layers.
2. Experimental The primary architecture of our devices is ITO/TAPC:MoOx (10 nm, 15 wt.%)/TAPC (30 nm)/buffer layer (10 nm)/26DCz PPy:FIrpic (5 nm, 15 wt.%)/26DCzPPy: Ir(BT)2(acac) (5 nm, 4 wt.%)/BPhen (40 nm)/Cs2CO3 (1 nm)/Al (100 nm), where buffer layer denotes mCP for Device A, TcTa for Device B, and Ir(ppz)3 for Device C, respectively. As compared, Device D with a 10 nm TAPC which replaces the buffer layer is used to investigate the roles of the buffer layers. Furthermore, we also utilize an additional orange EML to enlarge the excitons formation zone and harvest high energy triplets for improving the efficiency and reducing the driving voltage. The WOLEDs composed of ITO/TAPC:MoOx (10 nm, 15 wt.%)/TAPC (35 nm)/mCP:Ir(BT)2(acac) (5 nm, 4 wt.%)/ 26DCzPPy:FIrpic (5 nm, 15 wt.%)/26DCzPPy:Ir(BT)2(acac) (5 nm, 4 wt.%)/BPhen (40 nm)/Cs2CO3 (1 nm)/Al (100 nm) (Device E) and ITO/TAPC:MoOx (10 nm, 15 wt.%)/TAPC (35 nm)/TcTa:Ir(BT)2(acac) (5 nm, 4 wt.%)/26DCzPPy:FIrpic (5 nm, 15 wt.%)/26DCzPPy:Ir(BT)2(acac) (5 nm, 4 wt.%)/ BPhen (40 nm)/Cs2CO3 (1 nm)/Al (100 nm) (Device F) structures were fabricated. Here, Ir(BT)2(acac)-doped mCP/TcTa served as orange EML as well as EBL. TAPC is 1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane served as HTL, MoOx-doped TAPC served as hole injection layer (HIL), and BPhen is 4,7-diphenyl-1,10-phenanthroline served as electron transport layer (ETL), respectively. The lowest unoccupied molecular orbital (LUMO) positions, highest occupied molecular orbital (HOMO) positions, and triplet levels of the materials we used are plotted in Fig. 1. The electroluminescence (EL) spectra are measured by a PR650 spectroscan spectrometer. The luminance–current density–voltage characteristics are recorded simultaneously with the measurement of the EL spectra by combining the spectrometer with a Keithley model 2400
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Fig. 1. (a) Triplet energy levels of some materials used in this paper. (b) Schematic diagram of HOMO–LUMO values of the materials. The values are obtained from the literature [15–19].
programmable voltage–current source. All measurements are carried out in ambient environments. 3. Results and discussion The luminance–current density–voltage characteristics are plotted in Fig. 2. Device D without a buffer layer shows obvious emission from TAPC (k 425 nm) [20,21] (shown in the inset of Fig. 2) which suggests that the electron leakage into the HTL is rather significant in Device D. To eliminate the undesirable inefficiency TAPC emission and improve the performance of our WOLEDs, we insert different buffer layers between HTL and EML. Device A with mCP as a buffer layer has a relative low current density at the same voltage owing to its lower intrinsic hole mobility compared with that of other buffer layers [16,22,23].
Fig. 2. Luminance–current density–voltage characteristics of Device A, B, C and D. The inset shows the normalized EL spectra of the devices at 10 mA/cm2.
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Z. Zhang et al. / Organic Electronics 13 (2012) 2296–2300
Fig. 4. Luminance–current density–voltage characteristics of Device E and F. The inset shows the normalized EL spectra of the devices at 10 mA/ cm2.
Fig. 3. Current efficiency (a), power efficiency (b) and External quantum efficiency (c) versus current density characteristics of Device A, B, C and D, respectively.
However, there is a discernable emission from TAPC in Device A when the driving voltage above 5 V (shown in Fig. S3(a)), it suggests that there exists electrons leaked into HTL, i.e., a number of electrons flow through mCP and arrive at HTL, which is in accordance with Lee’s report [24] in which state that the electron mobility of mCP rapidly increases as the driving voltage rising. Here, leakage of the electrons into HTL in Device A and D accounts for the lower efficiencies (shown in Fig. 3). The emissions in Device B and C are almost originated from FIrpic and Ir(BT)2(acac), and these two devices show similar low operating voltages, 100 cd/m2 at 3.4 V, 1000 cd/m2 at 4.1 V and 10000 cd/m2 at 5.4 V, respectively. The energy barriers between the LUMO of EML and the ETL are very small in our devices. Therefore, electrons can be easily injected into the EML, which is one of the main reasons for the low operating voltage. However, the energy barriers between the HOMO of buffer layers and the EML are quite different in Device B and C. The former one with a buffer layer of TcTa has a smaller energy barrier, holes can be easily injected into the EML, and as such the carrier balance in EML is emerging as a possibility, the excitons formation in FIrpic:26DCzPPy layer and Ir(BT)2(acac):26DCzPPy layer are equivalent, and subsequently a balance blue and orange emissions were observed (shown in Fig. S3(b)). Although the later one with a buffer layer of Ir(ppz)3 has a barrier as high as 0.95 eV, Ir(ppz)3 exhibits not only superior electron blocking ability but also excellent triplet excitons confinement capability. Due to the strong electronblocking effect of Ir(ppz)3 the concentration of electrons at the interface between Ir(ppz)3 and EML increased remarkably while the driving voltage raising, which enhanced holes injection and facilitated the excitons formation in FIrpic:26DCzPPy layer, and subsequently an
increase of blue emission was observed (shown in Fig. S3(c)). Furthermore, TcTa and Ir(ppz)3 have higher T1 than that of 26DCzPPy and FIrpic, excitons could be effectively confined in EML. Therefore, more balance distribution of electrons and holes in the EML as well as effective confinement of triplet excitons in Device B and C come true, and then they show higher efficiencies than Device A and D, at last they gave almost the same performances. Fig. 4 shows the luminance–current density–voltage characteristics of Device E and F. As previously explained, TcTa has a superior hole mobility than mCP [22,23], thus Device F has a higher brightness at the same operating voltage. Device F shows lower operating voltages, e.g.
Fig. 5. Current efficiency (a), power efficiency (b) and external quantum efficiency (c) versus current density characteristics of Device E and F, respectively.
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Z. Zhang et al. / Organic Electronics 13 (2012) 2296–2300 Table 1 Summary of the performances of the Devices.
a b c
Device
Voltage at 100, 1000 and 10000 cd/m2 (V)
CIEa (x, y) over 103–104 cd/m2
CRIb at 1000 cd/m2
A B C D E F
3.5, 3.4, 3.4, 3.6, 3.3, 3.2,
(0.34 ± 0.005, 0.39 ± 0.007) (0.37 ± 0.005, 0.41 ± 0.005) (0.32 ± 0.005, 0.39 ± 0.007) (0.42 ± 0.008, 0.42 ± 0.009) (0.41 ± 0.012, 0.44 ± 0.005) (0.44 ± 0.005, 0.45 ± 0.005)
68 71 67 61 60 58
4.2 4.1 4.1 4.5 3.8 3.7
and and and and and and
5.8 5.4 5.4 6.2 5.0 4.8
EQEc (%) 1 mA/cm2
10 mA/cm2
9.6 10.2 12.3 3.5 11.5 12.5
9.0 10.5 11.3 3.1 11.1 12.5
The Commission Internationale de l’Eclairage 1931 coordinates. Color rendering index. External quantum efficiency.
100 cd/m2 at 3.2 V, 1000 cd/m2 at 3.7 V and 10000 cd/m2 at 4.8 V. There is no discernable emission from TAPC is observed in Device E and F, where direct electron trapping and excitons formation on guest play a role in Ir(BT)2(acac)-doped mCP or TcTa layer, so the additional orange EML could effectively block the electrons and excitons from transiting or diffusing into HTL, thus the higher efficiencies are reasonable. Here, Device E achieved a current efficiency of 31.6 cd/A and a power efficiency of 26.1 lm/ W, compared to 35.0 cd/A and 29.0 lm/W of Device F at a brightness of 1000 cd/m2 (shown in Fig. 5(a) and (b)). While the efficiency roll-offs concerning the maximum efficiency value and that of 5000 cd/m2 are only 13.8% for Device E, and 8.7% for Device F (shown in Fig. 5(c)), respectively, which could be ascribed to the additional EML where the electron trapping sites not only enlarge the excitons formation zone but also reduce the electron leakage current to HTL. The balanced carriers injection and enlarged excitons formation zone further suppressed the notorious TTA process at high current density. As listed in Table 1, our WOLEDs with different buffer layers showed low operating voltage without n-doping ETL, and extremely stable CIEs ranged over 1000 cd/m2 to 10000 cd/m2. Device A with mCP buffer layer exhibits lower efficiencies which are caused by the electron leakage into the HTL. Device B with TcTa buffer layer expresses superior hole injection from HTL to EML and electron blocking ability, thus it realizes high efficiency and retards the efficiency roll-off. Device C with Ir(ppz)3 buffer layer manifests distinctive electron and triplet excitons blocking abilities which are propitious to achieve high efficiency. Device E and F with an additional 5 nm Ir(BT)2(acac)-doped mCP or TcTa served as orange EML as well as electron trapping sites not only enlarge the excitons formation zone but also reduce the electron leakage current to HTL, which is favorable to improve the efficiency and retard the efficiency roll-off. 4. Conclusions In conclusion, by modulating the hole transportation and electron blocking characteristics of the hole transport buffer layer, it is easy to demonstrate color stable WOLEDs. In this contribution, we investigated the effects of charge carrier transport and confinement on the performances of WOLEDs by adopting different hole transport buffer layers,
which tailored the distribution of charge carriers and excitons. We also demonstrated WOLEDs with an additional orange phosphor-doped buffer layer, which served as orange EML as well as electron trapping layer, can effectively improve the radiative recombination. Here, we have achieved WOLEDs with low operating voltage (2290 cd/ m2 at 4.0 V), high efficiencies (35.0 cd/A, 30.6 lm/W and 13.2% for the maximum current efficiency, power efficiency and external quantum efficiency, respectively), and low efficiency roll-off (8.7%). In prospect, the efficiency can be 2–3 times increased by out-coupling schemes as well as air-stable n-doping strategies to meet the requirements for solid-state lighting sources. Acknowledgements We acknowledge funding for this research from the National Key Basic Research and Development Program of China under Grant No. 2010CB327701, and the National Science Foundation of China (Grant Nos. 60977024 and 60906021). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2012.07.001. References [1] C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (1987) 913. [2] M. Pfeiffer, S.R. Forrest, K. Leo, M.E. Thompson, Adv. Mater. 14 (2002) 1633. [3] B.W. D’ Andrade, S.R. Forrest, Adv. Mater. 16 (2004) 1585. [4] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, K. Leo, Nature 459 (2009) 234. [5] R. Meerheim, K. Walzer, G. He, M. Pfeiffer, K. Leo, SPIE 6192 (2006) 61920P. [6] D.L. Mathine, H.S. Woo, W. He, T.W. Kim, B. Kippelen, N. Peyghambarian, Appl. Phys. Lett. 76 (2000) 3849. [7] Y. Sun, N.C. Giebink, H. Kanno, B. Ma, M.E. Thompson, S.R. Forrest, Nature 440 (2006) 908. [8] Q. Wang, J.Q. Ding, D.G. Ma, Y.X. Cheng, L.X. Wang, X.B. Jing, F.S. Wang, Adv. Funct. Mater. 19 (2009) 84. [9] M. Lebental, H. Choukri, S. Chenais, S. Forget, A. Siove, B. Geffroy, E. Tutis, Phys. Rev. B 79 (2009) 165318. [10] J. Wünsche, S. Reineke, B. Lüssem, K. Leo, Phys. Rev. B 81 (2010) 245201. [11] N. Giebink, S.R. Forrest, Phys. Rev. B 77 (2008) 235215. [12] S. Reineke, K. Walzer, K. Leo, Phys. Rev. B 75 (2007) 125328.
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