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Efficient blue organic light-emitting diodes with low operation voltage by improving the injection and transport of holes
Optical Materials 97 (2019) 109383
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Efficient blue organic light-emitting diodes with low operation voltage by improving the injection and transport of holes
T
Xuesen Zhaoa,b, Liang Zhoub,∗, Qi Zhub, Wang Yujiac,∗∗, Rongzhen Cuib, Yingjie Cuib, Weiqiang Liub, Mi Xiaoyuna,∗∗∗ a
School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun, 130022, People's Republic of China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, People's Republic of China c Institute of Modern Optics, Nankai University, Tianjin, 300350, People's Republic of China b
ARTICLE INFO
ABSTRACT
Keywords: Electroluminescence p-type dopant Operation voltage Efficiency roll-off
In this work, we reported the highly efficient organic electroluminescent (EL) devices based on blue phosphorescent material FIr6. By utilizing p-type dopant HAT-CN as hole injection layer material, the injection of holes from the anode was significantly improved. In addition, doping HAT-CN into hole transport layer (HTL) and inserting thin TcTa film between HTL and light-emitting layer (EML) improved the transport of holes. The stepwise energy levels between functional layers reduced the injection barriers of carriers, thus improving the balance of carriers within EML. Finally, highly efficient blue EL device realized the maximum current efficiency, power efficiency, external quantum efficiency (EQE) and brightness up to 39.14 cd/A, 38.68 lm/W, 24.6% and 17201 cd/m2, respectively. Even at the high brightness of 1000 cd/m2 (3.7 V), current efficiency and EQE as high as 36.73 cd/A and 23.1%, respectively, can still be retained by the same device. Our investigation provides an idea for the design of high-performance devices with low operation voltage and slow efficiency roll-off.
1. Introduction
is particularly important because they have been the most important factor limiting the development of white OLEDs. Since Tang et al. reported the first vacuum-deposited double-layers OLEDs, which contains a hole transport layer (HTL) and an emissive electron transport layer [11], great progresses in improving the electroluminescent (EL) efficiency and lifetime have been obtained. For example, Ivaniuk et al. reported a sky-blue emitter with good hole transport property, high thermal and electrochemical stability [12]. Many groups devoted to facilitating carriers' transport and balancing carriers' distribution by developing bipolar materials [13–16]. Despite great progresses in material synthesis have been achieved, the design of device structure aim to resolve above problems is still scarce. In the past years, our group have reported a series of novel devices and achieved significant achievements by optimizing device structure and fabrication technologies [17–22]. Efficient blue fluorescent devices were realized by constructing supplementary light-emitting layer, which is helpful in broadening the recombination zone, improving the trapping of carriers
Organic light-emitting diodes (OLEDs) have attracted great interest throughout the world owing to their potential applications in solid-state lighting and full-color flat panel displays [1–4]. For commercial application, three primary colors of blue, green and red are basically required [5]. Up to now, efficiency and brightness of green and red OLEDs are high enough for general applications [6,7]. However, further improvements of blue OLEDs in the context of color purity, efficiency and brightness are still necessary for practical applications. For blue emitters, the wider energy gap makes the selection of proper host materials more difficult, which results in high electron or hole injection barrier and unbalanced carriers' distribution within light-emitting layer (EML) [8,9]. In addition, most blue emitters transport only holes or only electrons, which results in narrow recombination zone, thus causing the rapid roll-off of EL efficiency due to exciton quenching [10]. Therefore, further investigation on blue electroluminescent materials and devices
∗ Corresponding author. State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, People's Republic of China. ∗∗ Corresponding author. Institute of Modern Optics, Nankai University, Tianjin, 300350, People's Republic of China. ∗∗∗ Corresponding author. School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun, 130022, People's Republic of China. E-mail addresses: [email protected] (L. Zhou), [email protected] (Y. Wang), [email protected] (X. Mi).
https://doi.org/10.1016/j.optmat.2019.109383 Received 25 July 2019; Received in revised form 4 September 2019; Accepted 11 September 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Proposed energy level diagram of the devices used in this work and the molecular structures of HAT-CN, FIr6 and CzSi. (b) Injection and transport processes of holes and electrons in the designed devices. The dash dot and solid lines within EML represent the HOMO and LUMO levels of FIr6 and CzSi, respectively.
as well as balancing the distribution of holes and electrons on emitter molecules [22]. Consequently, the optimized device exhibits high efficiency and slow roll-off. Even so, under the premise of ensuring high efficiency and brightness, how to further reduce the operation voltage is still a major problem. Recently, Kido and co-workers achieved reduction in operation voltage while increasing device efficiency by synthesizing novel materials [23], utilizing a carbazole-based host material [24], blue emitter with a phenylimidazole-based ligand [25] and a mixed-host system [26]. Hole transport materials, which are critical for device performances, were widely utilized in most multilayer OLEDs. As reported previously, improving hole injection from anode into HTL is efficient in lowing
operation voltage and improving device efficiency [27]. For example, 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) has been extensively utilized for high efficiency OLEDs due to its high hole mobility (1 × 10−2 cm2 V−1 s−1) and high triplet energy (2.87 eV) [28], which are beneficial for improving hole transport efficiency and limiting carriers' recombination region. Zheng et al. reported efficient blue devices based on bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) with TAPC as the HTL, the device with EQE of 19% was obtained [29]. However, EL devices with TAPC as HTL was generally persecuted by some disadvantages such as low operation stability originating from the build-up of trapped charges at the TAPC/EML interface, which leads to increased operation voltage and decreased luminance [30]. 2
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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 (≤2.0 × 10−5 Pa). The EMLs were prepared by co-evaporating FIr6 and host material from two individual sources, and the doping concentration was modulated by controlling the evaporation rate of FIr6. MoO3, LiF and Al were deposited in another vacuum chamber (≤8.0 × 10−5 Pa) with the rate of 0.01, 0.01 and 1 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 eight emitting dots with the active area of 9 mm2 on each substrate. Current density-brightness-voltage (J-BV) characteristics were measured by using a programmable Keithley source measurement unit (Keithley 2400 and Keithley 2000) with a silicon photodiode. The EL spectra were measured with a calibrated Hitachi F7000 fluorescence spectrophotometer. The external quantum efficiency (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 Device structure and HOMO/LUMO levels diagram of the designed OLEDs are depicted in Fig. 1(a). In this case, FIr6 was utilized as the blue emitter because of its pure blue emission characteristic, high luminescent efficiency and excellent thermal stability. We utilized p-type material HAT-CN, which possesses deep energy level, as the buffer layer material [31]. And HAT-CN was doped into traditional material TAPC as hole transport/electron block layer (HTL/EBL) due to the strong electron withdrawing ability, which helps to generate free holes. Meanwhile, 1,3,5-tris(6-(3-(pyridine-3-yl)phenyl)pyridine-2-yl) (Tm3PyP26PyB) was utilized as electron transport/hole block layer (ETL/HBL) material due to its low-lying HOMO level (−6.5 eV) and excellent electron transport ability [32]. To prevent the reverse transfer of energy, 9-(4-tert-Butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) was utilized as the host material due to its high triplet energy (3.0 eV) and wide energy gap [33]. The molecular structures of HAT-CN, FIr6 and CzSi were also shown in Fig. 1(a). As shown in Fig. 1(b), holes and electrons inject from anode and cathode into EML via HTL and ETL, respectively. Previous investigations have reported and explained that the presence of MoO3 layer can effectively improve the injection of holes [34]. According to the previous reports, the stepwise HOMO levels of TAPC (−5.5 eV), TcTa (−5.7 eV), and CzSi (−6.0 eV) are beneficial for the injection and transport of holes, while the stepwise LUMO levels of Tm3PyP26PyB (−2.9 eV), CzSi (−2.5 eV) are beneficial for the injection and transport of electrons. From this perspective, balanced distribution of holes and electrons and wide recombination zone could be expected. In addition, the LUMO level of TAPC is 0.6 eV higher than that of TcTa, while the HOMO level of Tm3PyP26PyB is 0.5 eV lower than that of CzSi. As a result, holes and electrons are well confined within EML. In this case, most holes within EML should be preferentially trapped by CzSi molecules due to it higher HOMO level (−6.0 eV) than that of FIr6 (−6.1 eV). On the other hand, the low-lying LUMO level of FIr6 (−3.1 eV) makes it easy to trap electrons within EML. As a result, holes are less than electrons on FIr6 molecules. Therefore, improving the injection and transport of holes to achieve balanced distribution of carriers is the focus of optimization. Firstly, a series of single-EML devices with the structure of ITO/MoO3 (3 nm)/TAPC (50 nm)/FIr6 (x wt%):CzSi (10 nm)/Tm3PyP26PyB (50 nm)/ LiF (1 nm)/Al (100 nm) were fabricated and examined. Fig. 2(a) depicts the doping concentration dependence of EL efficiency of these devices, and the current density-brightness-voltage characteristics of these devices are depicted in the insert of Fig. 2(a). As listed in Table 1, the 30 wt% doped device (defined as device A) gave the highest brightness, current efficiency, and power efficiency of 4812 cd/m2, 17.59 cd/A (EQE = 11.4%), and 17.83 lm/W, respectively. At the certain brightness of 1000 cd/m2 (5.1 V),
Fig. 2. (a) EL efficiency-current density (η-J) characteristics of the single-EML devices with FIr6 at different doping concentrations. Inset: Current densitybrightness-voltage (J-B-V) characteristics of the single-EML devices with FIr6 at different doping concentrations. (b) Normalized EL spectra of single-EML devices with FIr6 at different doping concentrations operating at 10 mA/cm2.
In this work, we aim to further enhance the EL performances of blue emitter FIr6 by optimizing device structure reasonably. In this case, ptype material 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HATCN) was doped into TAPC film as the HTL, while HAT-CN was selected as buffer layer material between anode and HTL instead of MoO3. To reduce the hole transport barrier, 4,4′,4″-Tri(9-carbazoyl)triphenylamine (TcTa) film was inserted between HTL and EML as interlayer. Eventually, balance of holes and electrons on emitter molecules was significantly improved, while the operation voltage was also greatly reduced. Finally, highly efficient blue EL device with the maximum current efficiency, power efficiency, external quantum efficiency (EQE) and brightness up to 39.14 cd/A, 38.68 lm/W, 24.6% and 17201 cd/m2, respectively, was realized. Even at the high brightness of 1000 cd/m2, current efficiency and EQE as high as 36.73 cd/A and 23.1%, respectively, can still be retained by the same device. More importantly, operation voltage of the device at the brightness of 1000 cd/m2 was only 3.7 V. 2. Experimental 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 a sheet resistance of 10 Ω/sq was used as the anode substrate. Prior to film deposition, patterned ITO substrates were 3
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Table 1 The key properties of devices A, B, C, D and E. Device
Vturn-on (V)
Ba (cd/m2)
ηcb (EQEc) (cd/A)
ηpd (lm/W)
ηce(cd/A) (EQEf) (1000 cd/m2)
CIEx,yg
A B C D E
2.9 3.1 3.1 3.1 2.8
4812 18235 12580 9795 17201
17.59 24.63 23.17 26.40 39.14
17.83 23.22 23.47 25.91 38.68
10.82 (7.0%, 5.1 V) 16.82 (10.5%, 5.3 V) 11.95(8.0%, 5.0 V) 12.77 (8.6%, 4.6 V) 36.73 (23.1%, 3.7V)
(0.137, 0.221) (0.128, 0.245) (0.132, 0.213) (0.130, 0.216) (0.126,0.244)
a b c d e f g
(11.4%) (15.3%) (15.5%) (17.8%) (24.6%)
The data for maximum brightness (B). Maximum current efficiency (ηc). Maximum external quantum efficiency (EQE). Maximum power efficiency (ηp). Current efficiency (ηc) at the certain brightness of 1000 cd/m2. External quantum efficiency (EQE) at the certain brightness of 1000 cd/m2. Commission Internationale de l'Eclairage coordinates (CIEx, y) at 10 mA/cm2.
Fig. 3. (a) EL efficiency-current density (η-J) characteristics of the double-EMLs devices with FIr6 at different doping concentrations. Inset: Current densitybrightness-voltage (J-B-V) characteristics of the double-EMLs devices with FIr6 at different doping concentrations. (b) Normalized EL spectra of double-EMLs devices with FIr6 at different doping concentrations operating at 10 mA/cm2.
Fig. 4. (a) EL efficiency-current density (η-J) characteristics of the single-EML devices with CzSi at different thicknesses. Inset: Current density-brightnessvoltage (J-B-V) characteristics of the single-EML devices with CzSi at different thicknesses. (b) Normalized EL spectra of single-EML devices with CzSi at different thicknesses operating at 10 mA/cm2.
the current efficiency of device A was only 10.82 cd/A due to the rapid rolloff of efficiency, which eventually caused the low brightness. Normalized EL spectra of these devices operating at the current density of 10 mA/cm2 were measured and shown in Fig. 2(b). Pure FIr6 emission was observed in all these devices with FIr6 at different doping concentrations. As discussed above, the inherent energy levels of the materials result in unbalanced distribution of holes and electrons on FIr6 molecules, which is the main reason for the low performances of device A.
For comparison, a series of double-EMLs devices with the structure of ITO/MoO3 (3 nm)/TAPC (50 nm)/FIr6 (x wt%):TcTa (10 nm)/FIr6 (x wt%):CzSi (10 nm)/Tm3PyP26PyB (50 nm)/LiF (1 nm)/Al (100 nm) were also fabricated and examined. As shown in Fig. 3(a) and Table 1, the optimal doping concentration of FIr6 in these double-EMLs devices was also 30 wt%, the corresponding device (defined as device B) obtained the highest brightness, current efficiency, and power efficiency 4
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Fig. 5. (a) EL efficiency-current density (η-J) characteristics of the single-EML devices with HAT-CN at different thicknesses. Inset: Current density-brightnessvoltage (J-B-V) characteristics of the single-EML devices with HAT-CN at different thicknesses. (b) Normalized EL spectra of single-EML devices with HATCN at different thicknesses operating at 10 mA/cm2.
Fig. 6. (a) EL efficiency-current density (η-J) characteristics of the single-EML devices with HAT-CN at different doping concentrations within TAPC layer. Inset: Current density-brightness-voltage (J-B-V) characteristics of the singleEML devices with HAT-CN at different doping concentrations within TAPC layer. (b) Normalized EL spectra of single-EML devices with HAT-CN at different doping concentrations within TAPC layer operating at 10 mA/cm2. Inset: real-life working photograph at 4 V and CIE diagram at 10 mA/cm2 of the device E.
of 18235 cd/m2, 24.63 cd/A (EQE = 15.3%), and 23.22 lm/W, respectively. Even at the certain brightness of 1000 cd/m2 (5.3 V), device B retained the current efficiency up to 16.82 cd/A (EQE = 10.5%). Compared with single-EML devices, double-EMLs devices displayed obvious improved performances, which were attributed to the improved carriers' balance within EMLs. EL spectra of these double-EMLs devices operating at the current density of 10 mA/cm2 were measured and shown in Fig. 3(b), pure FIr6 emission was observed in all these devices with FIr6 at different doping concentrations. As listed in Table 1, although device B possessed the relatively higher brightness and efficiencies, device A displayed relatively lower operation voltage. The turn-on voltage of device A was 2.9 V, and the operation voltage of this device at certain brightness of 1000 cd/m2 was 5.1 V. For practical application, high performance OLEDs with simple device structure and low operation voltage are very important. Therefore, the following optimization was performed based on device A. Based on the above discussion, HOMO level of host material is even higher than that of emitter, which leads to insufficient hole trapping on emitter molecules. To facilitate the balance of holes and electrons within EML, thin TcTa film was inserted between HTL and EML as interlayer to accelerate the transport of holes. Devices with the structure of ITO/MoO3 (3 nm)/TAPC (50 nm)/TcTa (5 nm)/FIr6 (30 wt%):CzSi (x nm)/Tm3PyP26PyB (50 nm)/LiF (1 nm)/Al (100 nm) were fabricated and examined. The optimal device (defined as device C) was obtained
by controlling the thickness of EML to be 10 nm. As shown in Fig. 4(a) and Table 1, this device displayed the maximum current efficiency, power efficiency, external quantum efficiency and brightness up to 23.17 cd/A, 23.47 lm/W, 15.5% and 12580 cd/m2, respectively. The operation voltage at the brightness of 1000 cd/m2 was 5.0 V. Normalized EL spectra of these devices operating at the current density of 10 mA/cm2 were observed in Fig. 4(b). Besides the emission of FIr6, extremely weak emission peaked at about 414 nm was also observed. With increasing thickness of EML, the relative intensity of this emission decreased gradually. When the thickness was thicker than 10 nm, this emission was almost invisible. So, we infer that this emission originates from the interface between TcTa layer and EML. Based on device C, MoO3 layer was replaced by HAT-CN, which possesses deep energy levels, to further decrease the operation voltage and enhance the performances. To optimize the thickness of HAT-CN, a series of devices with the structure of ITO/HAT-CN (x nm)/TAPC (50 nm)/TcTa (5 nm)/FIr6 (30 wt%):CzSi (10 nm)/Tm3PyP26PyB (50 nm)/LiF (1 nm)/Al (100 nm) were fabricated and examined. As shown in Fig. 5(a) and Table 1, the device with 6 nm HAT-CN (defined as device D) obtained the maximum current efficiency, power efficiency, external quantum efficiency and brightness up to 26.40 cd/A, 5
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Fig. 7. Schematic representation of carriers' distribution in devices A, C and E. The dash dot and solid lines within EML represent the HOMO and LUMO levels of FIr6 and CzSi, respectively. Symbols - and + represent electrons and holes, respectively.
25.91 lm/W, 17.8% and 9795 cd/m2, respectively. At the certain brightness of 1000 cd/m2 (4.6 V), current efficiency reduced to 12.77 cd/A (EQE = 8.6%). The relatively lower operation voltage and higher performances of device D compared with device C confirms the efficacy of this device design strategy. The normalized EL spectra were measured and shown in Fig. 5(b), pure FIr6 emission was observed. Although the total performances have been significantly improved, the roll-off of EL efficiency was still rapid, so we believe that there is still certain room to further improve the device performances. To slow down the roll-off of EL efficiency and further decrease the operation voltage, we doped HAT-CN into TAPC to construct p-type doped HTL, and a series of devices with the structure of ITO/HAT-CN (6 nm)/HAT-CN (x wt%):TAPC (50 nm)/TcTa (5 nm)/FIr6 (30 wt %):CzSi (10 nm)/Tm3PyP26PyB (50 nm)/LiF (1 nm)/Al (100 nm) were fabricated and examined. As depicted in Fig. 6(a) and Table 1, the 0.2 wt% doped device (defined as device E) gave the maximum brightness, current efficiency, power efficiency and EQE of 17201 cd/ m2, 39.14 cd/A, 38.68 lm/W and 24.6%, respectively. More importantly, at the certain brightness of 1000 cd/m2, current efficiency up to 36.73 cd/A (EQE = 23.1%) was obtained at the very low operation voltage of only 3.7 V. These results were attributed to the fact that ptype material doped HTL generated a large number of free holes, which greatly improves the injection and transport of holes, thus results in the reduce of operation voltage eventually. As shown in Fig. 6(b), pure FIr6 emission was observed, real-life working photograph at 4 V and CIE diagram at 10 mA/cm2 of device E are depicted in the inset of Fig. 6(b). In addition, the optimal thicknesses of TAPC, CzSi and Tm3PyP26PyB were concluded to be 50, 10, and 50 nm, respectively. To better understand the EL mechanisms of these devices, distribution of holes and electrons in devices A, C and E were analyzed in detail. Within EML of all these devices, as shown in Fig. 7, most electrons were preferentially trapped by FIr6 molecules due to the low-lying LUMO level of FIr6, while most holes situate on CzSi molecules due to the high-lying HOMO level of CzSi. Therefore, no discernible CzSi emission was observed in the EL spectra. For device A, holes and electrons preferentially situate on CzSi and FIr6 molecules, respectively, some holes can transfer onto FIr6 molecules with the help of holes accumulation within EML. Compared with device A, device C was expected to possess the improved
balance of holes and electrons on FIr6 molecules due to the inserting of TcTa interlayer, which can facilitate the injection of holes from HTL into EML. For device E, the replacement of MoO3 with HAT-CN further reduces the energy barrier of hole injection from anode into HTL. The strong electron withdrawing characteristic of HAT-CN facilitates the generation of free holes on TAPC molecules, thus further accelerates the transport of holes within HTL. In this case, the introduction of TcTa film as interlayer helps to reduce the resistance of hole transporting into EML. In addition, the smaller HOMO level barrier between TcTa, FIr6 and CzSi makes it easier for holes to situate on FIr6 molecules, thus greatly improving the balance of carriers' distribution. 4. Conclusions In summary, two device design strategies have been performed to improve the performances and reduce the operation voltage. One is selecting HAT-CN, which has low-lying levels, as buffer layer between anode and HTL, which was confirmed to be efficient in reducing the energy barrier of hole injection. The other is doping HAT-CN into HTL with appropriate doping concentration to generate free holes in the doping film because the strong electron withdrawing characteristic of HAT-CN facilitates accepting electrons from TAPC molecules. In addition, the insertion of 5 nm TcTa interlayer between HTL and EML further facilitated the injection of holes from HTL into EML, thus improved the balance of holes and electrons on FIr6 molecules. Finally, high performance blue device with the highest brightness, current efficiency and power efficiency up to 17201 cd/m2, 39.14 cd/A (EQE = 24.6%) and 38.68 lm/W, respectively, was obtained. Even at the certain brightness of 1000 cd/m2, current efficiency and EQE as high as 36.73 cd/A and 23.1%, respectively, can still be retained by the same device. More importantly, the operation voltage of the device was significantly reduced. At the brightness of 1000 cd/m2, operation voltage of the device was only 3.7 V. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 6
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Acknowledgements The authors are grateful to the financial aid from Youth Innovation Promotion Association of Chinese Academy of Sciences (2013150), Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000), and National Natural Science Foundation of China (Grant Nos. 21771172, 21521092, 21590794 and 21210001).
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J.V. Grazulevicius, A.P. Monkman, Deep-blue high-efficiency TTA OLED using Paraand Meta- conjugated cyanotriphenylbenzene and carbazole derivatives as emitter and host, J. Phys. Chem. Lett. 8 (2017) 6199–6205. L. Zhou, L.J. Li, Y.L. Jiang, R.Z. Cui, Y.N. Li, X.S. Zhao, H.J. Zhang, Rare earth complex as electron trapper and energy transfer ladder for efficient red iridium complex based electroluminescent devices, ACS Appl. Mater. Interfaces 7 (2015) 16046–16053. X.S. Zhao, L. Zhou, Y.L. Jiang, R.Z. Cui, Y.N. Li, Y.X. Zheng, J.L. Zuo, H.J. Zhang, Efficient green electroluminescent devices based on iridium complex with wide energy gap complexes as sensitizers, Org. Electron. 37 (2016) 85–92. R.Z. Cui, W.Q. Liu, L. Zhou, Y.N. Li, Y.L. Jiang, X.S. Zhao, Y.J. Cui, Q. Zhu, Y.X. Zheng, R.P. Deng, H.J. Zhang, Highly efficient green phosphorescent organic electroluminescent devices with a terbium complex as the sensitizer, Dyes Pigments 136 (2017) 361–367. R.Z. Cui, W.Q. Liu, L. Zhou, X.S. Zhao, Y.L. Jiang, Y.X. Zheng, H.J. Zhang, High performance red phosphorescent organic electroluminescent devices with characteristic mechanisms by utilizing terbium or gadolinium complex as sensitizer, J. Mater. Chem. C. 5 (2017) 2066–2073. Y.L. Jiang, L. Zhou, R.Z. Cui, Y.N. Li, X.S. Zhao, H.J. Zhang, Highly efficient pure red organic light-emitting devices based on tris(1-phenyl-isoquinoline) iridium(Ⅲ) with another wide gap iridium(Ⅲ) complex as sensitizer, Dyes Pigments 128 (2016) 26–32. X.S. Zhao, L. Zhou, Y.L. Jiang, R.Z. Cui, Y.N. Li, H.J. Zhang, Efficient organic blue fluorescent light-emitting devices with improved carriers' balance on emitter molecules by constructing supplementary light-emitting layer, Dyes Pigments 130 (2016) 148–153. R. Komatsu, H. Sasabe, Y. Seino, K. Nakao, J. Kido, Light-blue thermally activated delayed fluorescent emitters realizing a high external quantum efficiency of 25% and unprecedented low drive voltages in OLEDs, J. Mater. Chem. C. 4 (2016) 2274–2278. H. Sasabe, N. Toyota, H. Nakanishi, T. Ishizaka, Y.-J. Pu, J. Kido, 3,3'-BicarbazoleBased host materials for high-efficiency blue phosphorescent OLEDs with extremely low driving voltage, Adv. Mater. 24 (2012) 3212–3217. K. Udagawa, H. Sasabe, C. Cai, J. Kido, Low-driving-voltage blue phosphorescent organic light-emitting devices with external quantum efficiency of 30%, Adv. Mater. 26 (2014) 5062–5066. K. Udagawa, H. Sasabe, F. Igarashi, J. Kido, Simultaneous realization of high EQE of 30%, low drive voltage, and low efficiency roll-off at high brightness in blue phosphorescent OLEDs, Adv. Optical. Mater. 4 (2016) 86–90. C. Giebeler, H. Antoniadis, D.D.C. Bradley, Y. Shirota, Influence of the hole transport layer on the performance of organic light-emitting diodes, J. Appl. Phys. 85 (1999) 608–615. J. Lee, N. Chopra, S.-H. Eom, Y. Zheng, J.G. Xue, F. So, J.M. Shi, Effects of triplet energies and transporting properties of carrier transporting materials on blue phosphorescent organic light emitting devices, Appl. Phys. Lett. 93 (2008) 123306. Y. Zheng, S.-H. Eom, N. Chopra, J. Lee, F. So, J.G. Xue, Efficient deep-blue phosphorescent organic light-emitting device with improved electron and exciton confinement, Appl. Phys. Lett. 92 (2008) 223301. J.H. Kim, Y. Chen, R. Liu, F. So, Phosphorescent dye-doped hole transporting layer for organic light-emitting diodes, Org. Electron. 15 (2014) 2381–2386. Y.-K. Kim, J.W. Kim, Y. Park, Energy level alignment at a charge generation interface between 4,4'-bis(N-phenyl-1-naphthylamino)biphenyl and 1,4,5,8,9,11hexaazatriphenylene-hexacarbonitrile, Appl. Phys. Lett. 94 (2009) 063305. H. Ye, D.C. Chen, M. Liu, S.-J. Su, Y.-F. Wang, C.-C. Lo, A. Lien, J. Kido, Pyridinecontaining electron-transport materials for highly efficient blue phosphorescent OLEDs with ultralow operating voltage and reduced efficiency roll-off, Adv. Funct. Mater. 24 (2014) 3268–3275. M.-H. Tsai, H.-W. Lin, H.-C. Su, T.-H. Ke, C.-C. Wu, F.-C. Fang, Y.-L. Liao, K.T. Wong, C.-I. Wu, Highly efficient organic blue electrophosphorescent devices based on 3,6-Bis(triphenylsilyl)carbazole as the host material, Adv. Mater. 18 (2006) 1216–1220. M. Kroger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, A. Kahn, Role of the deeplying electronic states of MoO3 in the enhancement of hole-injection in organic thin films, Appl. Phys. Lett. 95 (2009) 123301.
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Efficient blue organic light-emitting diodes with low operation voltage by improving the injection and transport of holes
T
Xuesen Zhaoa,b, Liang Zhoub,∗, Qi Zhub, Wang Yujiac,∗∗, Rongzhen Cuib, Yingjie Cuib, Weiqiang Liub, Mi Xiaoyuna,∗∗∗ a
School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun, 130022, People's Republic of China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, People's Republic of China c Institute of Modern Optics, Nankai University, Tianjin, 300350, People's Republic of China b
ARTICLE INFO
ABSTRACT
Keywords: Electroluminescence p-type dopant Operation voltage Efficiency roll-off
In this work, we reported the highly efficient organic electroluminescent (EL) devices based on blue phosphorescent material FIr6. By utilizing p-type dopant HAT-CN as hole injection layer material, the injection of holes from the anode was significantly improved. In addition, doping HAT-CN into hole transport layer (HTL) and inserting thin TcTa film between HTL and light-emitting layer (EML) improved the transport of holes. The stepwise energy levels between functional layers reduced the injection barriers of carriers, thus improving the balance of carriers within EML. Finally, highly efficient blue EL device realized the maximum current efficiency, power efficiency, external quantum efficiency (EQE) and brightness up to 39.14 cd/A, 38.68 lm/W, 24.6% and 17201 cd/m2, respectively. Even at the high brightness of 1000 cd/m2 (3.7 V), current efficiency and EQE as high as 36.73 cd/A and 23.1%, respectively, can still be retained by the same device. Our investigation provides an idea for the design of high-performance devices with low operation voltage and slow efficiency roll-off.
1. Introduction
is particularly important because they have been the most important factor limiting the development of white OLEDs. Since Tang et al. reported the first vacuum-deposited double-layers OLEDs, which contains a hole transport layer (HTL) and an emissive electron transport layer [11], great progresses in improving the electroluminescent (EL) efficiency and lifetime have been obtained. For example, Ivaniuk et al. reported a sky-blue emitter with good hole transport property, high thermal and electrochemical stability [12]. Many groups devoted to facilitating carriers' transport and balancing carriers' distribution by developing bipolar materials [13–16]. Despite great progresses in material synthesis have been achieved, the design of device structure aim to resolve above problems is still scarce. In the past years, our group have reported a series of novel devices and achieved significant achievements by optimizing device structure and fabrication technologies [17–22]. Efficient blue fluorescent devices were realized by constructing supplementary light-emitting layer, which is helpful in broadening the recombination zone, improving the trapping of carriers
Organic light-emitting diodes (OLEDs) have attracted great interest throughout the world owing to their potential applications in solid-state lighting and full-color flat panel displays [1–4]. For commercial application, three primary colors of blue, green and red are basically required [5]. Up to now, efficiency and brightness of green and red OLEDs are high enough for general applications [6,7]. However, further improvements of blue OLEDs in the context of color purity, efficiency and brightness are still necessary for practical applications. For blue emitters, the wider energy gap makes the selection of proper host materials more difficult, which results in high electron or hole injection barrier and unbalanced carriers' distribution within light-emitting layer (EML) [8,9]. In addition, most blue emitters transport only holes or only electrons, which results in narrow recombination zone, thus causing the rapid roll-off of EL efficiency due to exciton quenching [10]. Therefore, further investigation on blue electroluminescent materials and devices
∗ Corresponding author. State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, People's Republic of China. ∗∗ Corresponding author. Institute of Modern Optics, Nankai University, Tianjin, 300350, People's Republic of China. ∗∗∗ Corresponding author. School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun, 130022, People's Republic of China. E-mail addresses: [email protected] (L. Zhou), [email protected] (Y. Wang), [email protected] (X. Mi).
https://doi.org/10.1016/j.optmat.2019.109383 Received 25 July 2019; Received in revised form 4 September 2019; Accepted 11 September 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Proposed energy level diagram of the devices used in this work and the molecular structures of HAT-CN, FIr6 and CzSi. (b) Injection and transport processes of holes and electrons in the designed devices. The dash dot and solid lines within EML represent the HOMO and LUMO levels of FIr6 and CzSi, respectively.
as well as balancing the distribution of holes and electrons on emitter molecules [22]. Consequently, the optimized device exhibits high efficiency and slow roll-off. Even so, under the premise of ensuring high efficiency and brightness, how to further reduce the operation voltage is still a major problem. Recently, Kido and co-workers achieved reduction in operation voltage while increasing device efficiency by synthesizing novel materials [23], utilizing a carbazole-based host material [24], blue emitter with a phenylimidazole-based ligand [25] and a mixed-host system [26]. Hole transport materials, which are critical for device performances, were widely utilized in most multilayer OLEDs. As reported previously, improving hole injection from anode into HTL is efficient in lowing
operation voltage and improving device efficiency [27]. For example, 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) has been extensively utilized for high efficiency OLEDs due to its high hole mobility (1 × 10−2 cm2 V−1 s−1) and high triplet energy (2.87 eV) [28], which are beneficial for improving hole transport efficiency and limiting carriers' recombination region. Zheng et al. reported efficient blue devices based on bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) with TAPC as the HTL, the device with EQE of 19% was obtained [29]. However, EL devices with TAPC as HTL was generally persecuted by some disadvantages such as low operation stability originating from the build-up of trapped charges at the TAPC/EML interface, which leads to increased operation voltage and decreased luminance [30]. 2
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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 (≤2.0 × 10−5 Pa). The EMLs were prepared by co-evaporating FIr6 and host material from two individual sources, and the doping concentration was modulated by controlling the evaporation rate of FIr6. MoO3, LiF and Al were deposited in another vacuum chamber (≤8.0 × 10−5 Pa) with the rate of 0.01, 0.01 and 1 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 eight emitting dots with the active area of 9 mm2 on each substrate. Current density-brightness-voltage (J-BV) characteristics were measured by using a programmable Keithley source measurement unit (Keithley 2400 and Keithley 2000) with a silicon photodiode. The EL spectra were measured with a calibrated Hitachi F7000 fluorescence spectrophotometer. The external quantum efficiency (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 Device structure and HOMO/LUMO levels diagram of the designed OLEDs are depicted in Fig. 1(a). In this case, FIr6 was utilized as the blue emitter because of its pure blue emission characteristic, high luminescent efficiency and excellent thermal stability. We utilized p-type material HAT-CN, which possesses deep energy level, as the buffer layer material [31]. And HAT-CN was doped into traditional material TAPC as hole transport/electron block layer (HTL/EBL) due to the strong electron withdrawing ability, which helps to generate free holes. Meanwhile, 1,3,5-tris(6-(3-(pyridine-3-yl)phenyl)pyridine-2-yl) (Tm3PyP26PyB) was utilized as electron transport/hole block layer (ETL/HBL) material due to its low-lying HOMO level (−6.5 eV) and excellent electron transport ability [32]. To prevent the reverse transfer of energy, 9-(4-tert-Butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) was utilized as the host material due to its high triplet energy (3.0 eV) and wide energy gap [33]. The molecular structures of HAT-CN, FIr6 and CzSi were also shown in Fig. 1(a). As shown in Fig. 1(b), holes and electrons inject from anode and cathode into EML via HTL and ETL, respectively. Previous investigations have reported and explained that the presence of MoO3 layer can effectively improve the injection of holes [34]. According to the previous reports, the stepwise HOMO levels of TAPC (−5.5 eV), TcTa (−5.7 eV), and CzSi (−6.0 eV) are beneficial for the injection and transport of holes, while the stepwise LUMO levels of Tm3PyP26PyB (−2.9 eV), CzSi (−2.5 eV) are beneficial for the injection and transport of electrons. From this perspective, balanced distribution of holes and electrons and wide recombination zone could be expected. In addition, the LUMO level of TAPC is 0.6 eV higher than that of TcTa, while the HOMO level of Tm3PyP26PyB is 0.5 eV lower than that of CzSi. As a result, holes and electrons are well confined within EML. In this case, most holes within EML should be preferentially trapped by CzSi molecules due to it higher HOMO level (−6.0 eV) than that of FIr6 (−6.1 eV). On the other hand, the low-lying LUMO level of FIr6 (−3.1 eV) makes it easy to trap electrons within EML. As a result, holes are less than electrons on FIr6 molecules. Therefore, improving the injection and transport of holes to achieve balanced distribution of carriers is the focus of optimization. Firstly, a series of single-EML devices with the structure of ITO/MoO3 (3 nm)/TAPC (50 nm)/FIr6 (x wt%):CzSi (10 nm)/Tm3PyP26PyB (50 nm)/ LiF (1 nm)/Al (100 nm) were fabricated and examined. Fig. 2(a) depicts the doping concentration dependence of EL efficiency of these devices, and the current density-brightness-voltage characteristics of these devices are depicted in the insert of Fig. 2(a). As listed in Table 1, the 30 wt% doped device (defined as device A) gave the highest brightness, current efficiency, and power efficiency of 4812 cd/m2, 17.59 cd/A (EQE = 11.4%), and 17.83 lm/W, respectively. At the certain brightness of 1000 cd/m2 (5.1 V),
Fig. 2. (a) EL efficiency-current density (η-J) characteristics of the single-EML devices with FIr6 at different doping concentrations. Inset: Current densitybrightness-voltage (J-B-V) characteristics of the single-EML devices with FIr6 at different doping concentrations. (b) Normalized EL spectra of single-EML devices with FIr6 at different doping concentrations operating at 10 mA/cm2.
In this work, we aim to further enhance the EL performances of blue emitter FIr6 by optimizing device structure reasonably. In this case, ptype material 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HATCN) was doped into TAPC film as the HTL, while HAT-CN was selected as buffer layer material between anode and HTL instead of MoO3. To reduce the hole transport barrier, 4,4′,4″-Tri(9-carbazoyl)triphenylamine (TcTa) film was inserted between HTL and EML as interlayer. Eventually, balance of holes and electrons on emitter molecules was significantly improved, while the operation voltage was also greatly reduced. Finally, highly efficient blue EL device with the maximum current efficiency, power efficiency, external quantum efficiency (EQE) and brightness up to 39.14 cd/A, 38.68 lm/W, 24.6% and 17201 cd/m2, respectively, was realized. Even at the high brightness of 1000 cd/m2, current efficiency and EQE as high as 36.73 cd/A and 23.1%, respectively, can still be retained by the same device. More importantly, operation voltage of the device at the brightness of 1000 cd/m2 was only 3.7 V. 2. Experimental 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 a sheet resistance of 10 Ω/sq was used as the anode substrate. Prior to film deposition, patterned ITO substrates were 3
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Table 1 The key properties of devices A, B, C, D and E. Device
Vturn-on (V)
Ba (cd/m2)
ηcb (EQEc) (cd/A)
ηpd (lm/W)
ηce(cd/A) (EQEf) (1000 cd/m2)
CIEx,yg
A B C D E
2.9 3.1 3.1 3.1 2.8
4812 18235 12580 9795 17201
17.59 24.63 23.17 26.40 39.14
17.83 23.22 23.47 25.91 38.68
10.82 (7.0%, 5.1 V) 16.82 (10.5%, 5.3 V) 11.95(8.0%, 5.0 V) 12.77 (8.6%, 4.6 V) 36.73 (23.1%, 3.7V)
(0.137, 0.221) (0.128, 0.245) (0.132, 0.213) (0.130, 0.216) (0.126,0.244)
a b c d e f g
(11.4%) (15.3%) (15.5%) (17.8%) (24.6%)
The data for maximum brightness (B). Maximum current efficiency (ηc). Maximum external quantum efficiency (EQE). Maximum power efficiency (ηp). Current efficiency (ηc) at the certain brightness of 1000 cd/m2. External quantum efficiency (EQE) at the certain brightness of 1000 cd/m2. Commission Internationale de l'Eclairage coordinates (CIEx, y) at 10 mA/cm2.
Fig. 3. (a) EL efficiency-current density (η-J) characteristics of the double-EMLs devices with FIr6 at different doping concentrations. Inset: Current densitybrightness-voltage (J-B-V) characteristics of the double-EMLs devices with FIr6 at different doping concentrations. (b) Normalized EL spectra of double-EMLs devices with FIr6 at different doping concentrations operating at 10 mA/cm2.
Fig. 4. (a) EL efficiency-current density (η-J) characteristics of the single-EML devices with CzSi at different thicknesses. Inset: Current density-brightnessvoltage (J-B-V) characteristics of the single-EML devices with CzSi at different thicknesses. (b) Normalized EL spectra of single-EML devices with CzSi at different thicknesses operating at 10 mA/cm2.
the current efficiency of device A was only 10.82 cd/A due to the rapid rolloff of efficiency, which eventually caused the low brightness. Normalized EL spectra of these devices operating at the current density of 10 mA/cm2 were measured and shown in Fig. 2(b). Pure FIr6 emission was observed in all these devices with FIr6 at different doping concentrations. As discussed above, the inherent energy levels of the materials result in unbalanced distribution of holes and electrons on FIr6 molecules, which is the main reason for the low performances of device A.
For comparison, a series of double-EMLs devices with the structure of ITO/MoO3 (3 nm)/TAPC (50 nm)/FIr6 (x wt%):TcTa (10 nm)/FIr6 (x wt%):CzSi (10 nm)/Tm3PyP26PyB (50 nm)/LiF (1 nm)/Al (100 nm) were also fabricated and examined. As shown in Fig. 3(a) and Table 1, the optimal doping concentration of FIr6 in these double-EMLs devices was also 30 wt%, the corresponding device (defined as device B) obtained the highest brightness, current efficiency, and power efficiency 4
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Fig. 5. (a) EL efficiency-current density (η-J) characteristics of the single-EML devices with HAT-CN at different thicknesses. Inset: Current density-brightnessvoltage (J-B-V) characteristics of the single-EML devices with HAT-CN at different thicknesses. (b) Normalized EL spectra of single-EML devices with HATCN at different thicknesses operating at 10 mA/cm2.
Fig. 6. (a) EL efficiency-current density (η-J) characteristics of the single-EML devices with HAT-CN at different doping concentrations within TAPC layer. Inset: Current density-brightness-voltage (J-B-V) characteristics of the singleEML devices with HAT-CN at different doping concentrations within TAPC layer. (b) Normalized EL spectra of single-EML devices with HAT-CN at different doping concentrations within TAPC layer operating at 10 mA/cm2. Inset: real-life working photograph at 4 V and CIE diagram at 10 mA/cm2 of the device E.
of 18235 cd/m2, 24.63 cd/A (EQE = 15.3%), and 23.22 lm/W, respectively. Even at the certain brightness of 1000 cd/m2 (5.3 V), device B retained the current efficiency up to 16.82 cd/A (EQE = 10.5%). Compared with single-EML devices, double-EMLs devices displayed obvious improved performances, which were attributed to the improved carriers' balance within EMLs. EL spectra of these double-EMLs devices operating at the current density of 10 mA/cm2 were measured and shown in Fig. 3(b), pure FIr6 emission was observed in all these devices with FIr6 at different doping concentrations. As listed in Table 1, although device B possessed the relatively higher brightness and efficiencies, device A displayed relatively lower operation voltage. The turn-on voltage of device A was 2.9 V, and the operation voltage of this device at certain brightness of 1000 cd/m2 was 5.1 V. For practical application, high performance OLEDs with simple device structure and low operation voltage are very important. Therefore, the following optimization was performed based on device A. Based on the above discussion, HOMO level of host material is even higher than that of emitter, which leads to insufficient hole trapping on emitter molecules. To facilitate the balance of holes and electrons within EML, thin TcTa film was inserted between HTL and EML as interlayer to accelerate the transport of holes. Devices with the structure of ITO/MoO3 (3 nm)/TAPC (50 nm)/TcTa (5 nm)/FIr6 (30 wt%):CzSi (x nm)/Tm3PyP26PyB (50 nm)/LiF (1 nm)/Al (100 nm) were fabricated and examined. The optimal device (defined as device C) was obtained
by controlling the thickness of EML to be 10 nm. As shown in Fig. 4(a) and Table 1, this device displayed the maximum current efficiency, power efficiency, external quantum efficiency and brightness up to 23.17 cd/A, 23.47 lm/W, 15.5% and 12580 cd/m2, respectively. The operation voltage at the brightness of 1000 cd/m2 was 5.0 V. Normalized EL spectra of these devices operating at the current density of 10 mA/cm2 were observed in Fig. 4(b). Besides the emission of FIr6, extremely weak emission peaked at about 414 nm was also observed. With increasing thickness of EML, the relative intensity of this emission decreased gradually. When the thickness was thicker than 10 nm, this emission was almost invisible. So, we infer that this emission originates from the interface between TcTa layer and EML. Based on device C, MoO3 layer was replaced by HAT-CN, which possesses deep energy levels, to further decrease the operation voltage and enhance the performances. To optimize the thickness of HAT-CN, a series of devices with the structure of ITO/HAT-CN (x nm)/TAPC (50 nm)/TcTa (5 nm)/FIr6 (30 wt%):CzSi (10 nm)/Tm3PyP26PyB (50 nm)/LiF (1 nm)/Al (100 nm) were fabricated and examined. As shown in Fig. 5(a) and Table 1, the device with 6 nm HAT-CN (defined as device D) obtained the maximum current efficiency, power efficiency, external quantum efficiency and brightness up to 26.40 cd/A, 5
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Fig. 7. Schematic representation of carriers' distribution in devices A, C and E. The dash dot and solid lines within EML represent the HOMO and LUMO levels of FIr6 and CzSi, respectively. Symbols - and + represent electrons and holes, respectively.
25.91 lm/W, 17.8% and 9795 cd/m2, respectively. At the certain brightness of 1000 cd/m2 (4.6 V), current efficiency reduced to 12.77 cd/A (EQE = 8.6%). The relatively lower operation voltage and higher performances of device D compared with device C confirms the efficacy of this device design strategy. The normalized EL spectra were measured and shown in Fig. 5(b), pure FIr6 emission was observed. Although the total performances have been significantly improved, the roll-off of EL efficiency was still rapid, so we believe that there is still certain room to further improve the device performances. To slow down the roll-off of EL efficiency and further decrease the operation voltage, we doped HAT-CN into TAPC to construct p-type doped HTL, and a series of devices with the structure of ITO/HAT-CN (6 nm)/HAT-CN (x wt%):TAPC (50 nm)/TcTa (5 nm)/FIr6 (30 wt %):CzSi (10 nm)/Tm3PyP26PyB (50 nm)/LiF (1 nm)/Al (100 nm) were fabricated and examined. As depicted in Fig. 6(a) and Table 1, the 0.2 wt% doped device (defined as device E) gave the maximum brightness, current efficiency, power efficiency and EQE of 17201 cd/ m2, 39.14 cd/A, 38.68 lm/W and 24.6%, respectively. More importantly, at the certain brightness of 1000 cd/m2, current efficiency up to 36.73 cd/A (EQE = 23.1%) was obtained at the very low operation voltage of only 3.7 V. These results were attributed to the fact that ptype material doped HTL generated a large number of free holes, which greatly improves the injection and transport of holes, thus results in the reduce of operation voltage eventually. As shown in Fig. 6(b), pure FIr6 emission was observed, real-life working photograph at 4 V and CIE diagram at 10 mA/cm2 of device E are depicted in the inset of Fig. 6(b). In addition, the optimal thicknesses of TAPC, CzSi and Tm3PyP26PyB were concluded to be 50, 10, and 50 nm, respectively. To better understand the EL mechanisms of these devices, distribution of holes and electrons in devices A, C and E were analyzed in detail. Within EML of all these devices, as shown in Fig. 7, most electrons were preferentially trapped by FIr6 molecules due to the low-lying LUMO level of FIr6, while most holes situate on CzSi molecules due to the high-lying HOMO level of CzSi. Therefore, no discernible CzSi emission was observed in the EL spectra. For device A, holes and electrons preferentially situate on CzSi and FIr6 molecules, respectively, some holes can transfer onto FIr6 molecules with the help of holes accumulation within EML. Compared with device A, device C was expected to possess the improved
balance of holes and electrons on FIr6 molecules due to the inserting of TcTa interlayer, which can facilitate the injection of holes from HTL into EML. For device E, the replacement of MoO3 with HAT-CN further reduces the energy barrier of hole injection from anode into HTL. The strong electron withdrawing characteristic of HAT-CN facilitates the generation of free holes on TAPC molecules, thus further accelerates the transport of holes within HTL. In this case, the introduction of TcTa film as interlayer helps to reduce the resistance of hole transporting into EML. In addition, the smaller HOMO level barrier between TcTa, FIr6 and CzSi makes it easier for holes to situate on FIr6 molecules, thus greatly improving the balance of carriers' distribution. 4. Conclusions In summary, two device design strategies have been performed to improve the performances and reduce the operation voltage. One is selecting HAT-CN, which has low-lying levels, as buffer layer between anode and HTL, which was confirmed to be efficient in reducing the energy barrier of hole injection. The other is doping HAT-CN into HTL with appropriate doping concentration to generate free holes in the doping film because the strong electron withdrawing characteristic of HAT-CN facilitates accepting electrons from TAPC molecules. In addition, the insertion of 5 nm TcTa interlayer between HTL and EML further facilitated the injection of holes from HTL into EML, thus improved the balance of holes and electrons on FIr6 molecules. Finally, high performance blue device with the highest brightness, current efficiency and power efficiency up to 17201 cd/m2, 39.14 cd/A (EQE = 24.6%) and 38.68 lm/W, respectively, was obtained. Even at the certain brightness of 1000 cd/m2, current efficiency and EQE as high as 36.73 cd/A and 23.1%, respectively, can still be retained by the same device. More importantly, the operation voltage of the device was significantly reduced. At the brightness of 1000 cd/m2, operation voltage of the device was only 3.7 V. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 6
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Acknowledgements The authors are grateful to the financial aid from Youth Innovation Promotion Association of Chinese Academy of Sciences (2013150), Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000), and National Natural Science Foundation of China (Grant Nos. 21771172, 21521092, 21590794 and 21210001).
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