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Al as an efficient cathode in organic light-emitting devices
Applied Surface Science 252 (2006) 6337–6341 www.elsevier.com/locate/apsusc
NaCl/Ca/Al as an efficient cathode in organic light-emitting devices Shengwei Shi, Dongge Ma * State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Changchun 130022, PR China Received 30 May 2005; received in revised form 3 August 2005; accepted 23 August 2005 Available online 6 October 2005
Abstract An efficient cathode NaCl/Ca/Al used to improve the performance of organic light-emitting devices (OLEDs) was reported. Standard N,N0 -bis(1-naphthyl)-N,N0 -diphenyl-1,10 biphenyl 4,40 -dimaine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq3) devices with NaCl/Ca/Al cathode showed dramatically enhanced electroluminescent (EL) efficiency. A power efficiency of 4.6 lm/W was obtained for OLEDs with 2 nm of NaCl and 10 nm of Ca, which is much higher than 2.0 lm/W, 3.1 lm/W, 2.1 lm/ W and 3.6 lm/W in devices using, respectively, the LiF (1 nm)/Al, LiF (1 nm)/Ca (10 nm)/Al, Ca (10 nm)/Al and NaCl (2 nm)/ Al cathodes. The investigation of the electron injection in electron-only devices indicates that the utilization of the NaCl/Ca/Al cathode substantially enhances the electron injection current, which in case of OLEDs leads to the improvement of the brightness and efficiency. # 2005 Elsevier B.V. All rights reserved. PACS: 85.60.Jb; 85.30.Mn Keywords: Organic semiconductors; Optoelectronic devices; Electrical properties and measurements; Alkali metals
1. Introduction Since the first demonstration of the organic lightemitting devices (OLEDs) in 1987 [1], a large research effort is focused on understanding the device physics and improving the performance of OLEDs. Upon * Corresponding author. Tel.: +86 431 5262357; fax: +86 431 5262873. E-mail address: [email protected] (D. Ma).
application of forward bias electrons and holes are injected from the cathode and the anode, respectively, and recombine inside the organic layers, producing light emission. The processes of charge injection are of fundamental importance, as it can control the electrical characteristics and the efficiency of devices [2]. A great deal of research addresses various aspects of metal/organic interfaces, such as morphology [3,4], energetics [5–10] and charge transport [11– 13]. Modification of interfaces has also received
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.08.036
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considerable attention. On the anode side of devices, indium tin oxide (ITO) has been widely used in conventional OLEDs because of its good conductivity and transparence. Modification of ITO with selfassembled monolayers and conducting polymers has been shown to increase OLED efficiency [14–18], and with inserted insulating layer at ITO/organic has also been reported [19,20]. Modification of interfaces has been more applied on the cathode side, such as using low work functional metals or composites [21,22], metal doping in organic layer near to cathode [23] or inserting an insulating layer at the metal/organic interface. As an example, typically a thin lithium fluoride (LiF) insulating layer inserted between the tris-(8-hydroxyquinoline) aluminum (Alq3) layer and the metal cathode has shown the most drastic performance improvement of both driving voltage and luminance of the OLEDs [24]. Since then many insulating materials, such as CsF, CaF2, MgF2, NaF, NaCl and MgO, have been used in OLEDs [24–36]. In addition to the different interface materials, recent experiment found that the used cathode metals on top of the insulating layer also showed significant effect on the device efficiency and operational stability [22,31,32,37–39]. Therefore, reasonable choose of the buffer layers and cathode metals is greatly important in achieving high performance OLEDs. In this paper, we report the application of a thin NaCl overlapped by the Ca as efficient cathode in OLEDs. Compared to NaCl/Al, LiF/Al, LiF/Ca/Al and Ca/Al cathodes, NaCl/Ca/Al cathode improves significantly the electroluminescent (EL) efficiency, and the power efficiency is enhanced up to 4.6 lm/W with respect to 2.0 lm/W for LiF/Al cathode, 3.1 lm/W for LiF/Ca/Al cathode, 2.1 lm/W for a Ca/Al cathode and 3.6 lm/W for NaCl/Al cathode, and found that the turn-on voltage was also reduced from 4.2 V for devices with LiF buffer layer to 3.3 V for devices with NaCl buffer layer.
2. Experimental Patterned indium tin oxide (ITO) (80 V/&) glasses were used as substrates. Prior to film deposition, the substrates were cleaned with TFD 7, rinsed in deionized water, then dried in an oven, and finally positioned in a thermal evaporation chamber. Devices
with a configuration of ITO/N,N0 -bis(1-naphthyl)-N,N0 diphenyl-1,10 biphenyl 4,40 dimaine (NPB) (50 nm)/ tris-(8-hydroxyquinoline) aluminum (Alq3) (60 nm)/ NaCl (x nm) or LiF (x nm)/Ca (y nm)/Al (150 nm) were prepared by thermal evaporation under vacuum of 5 104 Pa. The Al was used as a metallic cap to protect oxidation of the reactive Ca underneath. The NPB was thermally evaporated (thickness 50 nm, ˚ /s) on the cleaned ITO glasses deposition rate 2–4 A as a hole-transporting layer. A 60-nm thick Alq3 was ˚ /s as an electron-transporting evaporated at a rate 2–4 A and light-emitting layer. The electron-injection media of thin NaCl or LiF were deposited on Alq3/NPB/ITO ˚ /s without breaking the glass substrates at a rate 0.1 A vacuum, and then 10 nm thick Ca and 150 nm Al ˚ /s. The electrodes were deposited at a rate of 1–2 A deposition rates were controlled by quartz oscillating thickness monitor. The luminance–current–voltage characteristics were measured using a Keithley (2400 and 2000) source with a calibrated silicon photodiode. All measurements were performed at ambient atmosphere. The active area of the devices was 9 mm2.
3. Results and discussion It was found experimentally that not only the thickness of the buffer layer affects significantly on the EL performance, but the utilization of the Ca layer also plays significant role on the improvement of the EL performance. We first optimized the thicknesses of the buffer and Ca layers. For the case of NaCl buffer layer, the device with 2 nm NaCl and 10 nm Ca exhibits the best results in the EL efficiency, whereas for the case of LiF buffer layer, the device with 1 nm LiF and 10 nm Ca shows the highest EL efficiency. Fig. 1(a and b) shows the current–voltage (I–V) and the brightness–voltage (B–V) characteristics, respectively, for device of ITO/NPB (50 nm)/Alq3 (60 nm)/ NaCl (2 nm)/Ca (10 nm)/Al (150 nm). For comparison, the I–V and B–V characteristics of the optimizing devices of ITO/NPB (50 nm)/Alq3 (60 nm)/ NaCl (2 nm)//Al (150 nm), ITO/NPB (50 nm)/Alq3 (60 nm)/Ca (10 nm)/Al (150 nm), ITO/NPB (50 nm)/ Alq3 (60 nm)/LiF (1 nm)/Ca (10 nm)/Al (150 nm) and ITO/NPB (50 nm)/Alq3 (60 nm)/LiF (1 nm)/Al (150 nm) are also given in Fig. 1. It can be seen that the introduction of a thin NaCl layer reduces the
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Fig. 1. Current–voltage (a) and luminance–voltage (b) characteristics of ITO/NPB/Alq3/metal devices with NaCl/Ca/Al, NaCl/Al, Ca/Al, LiF/ Al and LiF/Ca/Al cathode.
turn-on voltage, and enhances the brightness compared with Ca/Al, LiF/Al and LiF/Ca/Al cathodes. The turn-on voltage is decreased from 4.2 V for devices with Ca/Al, LiF/Al and LiF/Ca/Al cathodes to 3.3 V for devices with NaCl/Ca/Al and NaCl/Al cathodes. The brightness is enhanced to 5000 cd/m2 for devices with NaCl/Ca/Al and NaCl/Al cathodes from 960 cd/m2 for devices with Ca/Al and LiF/Al cathodes and 2000 cd/m2 for device with LiF/Ca/Al cathode at a voltage of 10 V. Importantly, The insertion of a thin Ca layer between buffer layer and Al greatly improves the EL efficiency compared with the cases of the cathode without Ca thin layer. Fig. 2 shows the brightness–current characteristics of the devices with NaCl/Ca/Al, NaCl/Al, Ca/Al, LiF/Al and LiF/Ca/Al. The power efficiency as a function of current for the devices with different cathodes is shown in the inset of Fig. 2. It is clearly seen that the insertion of Ca thin layer between buffer layer and Al enhances the EL efficiency, and the NaCl/Ca/Al cathode shows the highest EL efficiency with respect to NaCl/Al, Ca/Al, LiF/Al, even the similar LiF/Ca/Al cathode. The maximum power efficiency reaches 4.6 lm/W for the case of NaCl/Ca/Al cathode, which is higher than the power efficiency of 3.6 lm/W for NaCl/Al cathode and 3.1 lm/W for LiF/Ca/AL cathode, and is over two factors higher of magnitude than devices with Ca/Al and LiF/Al cathodes. The enhancement of the power efficiency is great
important in the reduction of power assumption in practical applications. NaCl as interface layer in OLEDs has been studied and shows better EL performance than LiF interface layer when using Al as the cathode [40]. The same results have been obtained in our paper. However, the introduction of a thin Ca layer between buffer layer and Al further improves the EL performance, as given in our experiment. To elucidate the enhancement mechanism,
Fig. 2. Luminance–current characteristics of ITO/NPB/Alq3/metal devices with NaCl/Ca/Al, NaCl/Al, Ca/Al, LiF/Al and LiF/Ca/Al cathode, respectively. The inset shows the power efficiency as function of current for different cathode cases.
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electron-only devices of Al/Alq3/NaCl/Ca/Al, Al/Alq3/ NaCl/Al, Al/Alq3/Ca/Al, Al/Alq3/LiF/Al, Al/Alq3/LiF/ Ca/Al and Al/Alq3/Al devices were studied. Fig. 3 shows the comparison of the current–voltage characteristics of the different devices. It can be seen that the utilization of NaCl/Ca/Al and LiF/Ca/Al as the cathodes enhances the electron injection current with respect to NaCl/Al and LiF/Al, and comparing with LiF buffer layer, the NaCl buffer layer shows higher injection current and the insert of the thin Ca layer further enhances the injection current. This clearly indicates that the improvement of the EL efficiency in devices with NaCl/Ca cathode is due to the improvement of the electron injection current. Obviously, The NaCl/Ca/Al is promising cathode in OLEDs. Different mechanisms have been assumed to explain the enhancement of electron injection current by interface layer [41–47]. The debate on the mechanism has mainly focused on two models. One is tunneling probability enhancement resulting from buffer-induced energy level realignment; the other is the chemical reaction model, by which it is through alkali atoms or alkali earth atoms in the compounds adopted are liberated and improve the electron injection due to their low work functions. Recently, Brown et al. [37] measured the built-up potential across the organic layer for the case of different cathodes by electroabsorption, and found that the
magnitude of the built-up potential is strongly dependent on the used buffer layer and cap metals. LiF/Ca/Al cathode showed the highest built-up potential (2.71 eV) compared with Al (1.22 eV), LiF (2.31 eV) and Ca/Al (2.43 eV) cathodes. The high built-up potential facilitates the more electrons to inject into the emissive layer, resulting in high EL efficiency [37]. Because the built-up potential is directly related to the work function of metal ion contained in buffer material, and the low work function will produce high built-up potential. It was well known that Na in NaCl possesses lower work function than Li in LiF [38]. This means that using NaCl as the buffer layer should produce more high built-up potential with respect to LiF buffer layer, and the built-up potential is further increased when inserting Ca thin layer between NaCl and Al due to lower work function of Ca.
4. Conclusions We have shown that the utilization of the NaCl/Ca/ Al cathode improves significantly the EL efficiency in OLEDs. The improvement is attributed to the production of the higher built-up potential within the emissive layer in the case of NaCl/Ca/Al cathode, resulting in more electron current injection from cathode to emissive layer. Our results indicate that NaCl/Ca/Al is promising cathode in OLEDs.
Acknowledgements The authors thank the support of Hundreds Talents Program, Chinese Academy of Sciences and the National Science Fund for Distinguished Young Scholars of China (50325312).
References [1] [2] [3] [4] Fig. 3. Current–voltage characteristics of Al/Alq3/NaCl/Ca/Al, Al/ Alq3/NaCl/Al, Al/Alq3/Ca/Al, Al/Alq3/LiF/Al, Al/Alq3/LiF/Ca/Al and Al/Alq3/Al.
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NaCl/Ca/Al as an efficient cathode in organic light-emitting devices Shengwei Shi, Dongge Ma * State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Changchun 130022, PR China Received 30 May 2005; received in revised form 3 August 2005; accepted 23 August 2005 Available online 6 October 2005
Abstract An efficient cathode NaCl/Ca/Al used to improve the performance of organic light-emitting devices (OLEDs) was reported. Standard N,N0 -bis(1-naphthyl)-N,N0 -diphenyl-1,10 biphenyl 4,40 -dimaine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq3) devices with NaCl/Ca/Al cathode showed dramatically enhanced electroluminescent (EL) efficiency. A power efficiency of 4.6 lm/W was obtained for OLEDs with 2 nm of NaCl and 10 nm of Ca, which is much higher than 2.0 lm/W, 3.1 lm/W, 2.1 lm/ W and 3.6 lm/W in devices using, respectively, the LiF (1 nm)/Al, LiF (1 nm)/Ca (10 nm)/Al, Ca (10 nm)/Al and NaCl (2 nm)/ Al cathodes. The investigation of the electron injection in electron-only devices indicates that the utilization of the NaCl/Ca/Al cathode substantially enhances the electron injection current, which in case of OLEDs leads to the improvement of the brightness and efficiency. # 2005 Elsevier B.V. All rights reserved. PACS: 85.60.Jb; 85.30.Mn Keywords: Organic semiconductors; Optoelectronic devices; Electrical properties and measurements; Alkali metals
1. Introduction Since the first demonstration of the organic lightemitting devices (OLEDs) in 1987 [1], a large research effort is focused on understanding the device physics and improving the performance of OLEDs. Upon * Corresponding author. Tel.: +86 431 5262357; fax: +86 431 5262873. E-mail address: [email protected] (D. Ma).
application of forward bias electrons and holes are injected from the cathode and the anode, respectively, and recombine inside the organic layers, producing light emission. The processes of charge injection are of fundamental importance, as it can control the electrical characteristics and the efficiency of devices [2]. A great deal of research addresses various aspects of metal/organic interfaces, such as morphology [3,4], energetics [5–10] and charge transport [11– 13]. Modification of interfaces has also received
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.08.036
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considerable attention. On the anode side of devices, indium tin oxide (ITO) has been widely used in conventional OLEDs because of its good conductivity and transparence. Modification of ITO with selfassembled monolayers and conducting polymers has been shown to increase OLED efficiency [14–18], and with inserted insulating layer at ITO/organic has also been reported [19,20]. Modification of interfaces has been more applied on the cathode side, such as using low work functional metals or composites [21,22], metal doping in organic layer near to cathode [23] or inserting an insulating layer at the metal/organic interface. As an example, typically a thin lithium fluoride (LiF) insulating layer inserted between the tris-(8-hydroxyquinoline) aluminum (Alq3) layer and the metal cathode has shown the most drastic performance improvement of both driving voltage and luminance of the OLEDs [24]. Since then many insulating materials, such as CsF, CaF2, MgF2, NaF, NaCl and MgO, have been used in OLEDs [24–36]. In addition to the different interface materials, recent experiment found that the used cathode metals on top of the insulating layer also showed significant effect on the device efficiency and operational stability [22,31,32,37–39]. Therefore, reasonable choose of the buffer layers and cathode metals is greatly important in achieving high performance OLEDs. In this paper, we report the application of a thin NaCl overlapped by the Ca as efficient cathode in OLEDs. Compared to NaCl/Al, LiF/Al, LiF/Ca/Al and Ca/Al cathodes, NaCl/Ca/Al cathode improves significantly the electroluminescent (EL) efficiency, and the power efficiency is enhanced up to 4.6 lm/W with respect to 2.0 lm/W for LiF/Al cathode, 3.1 lm/W for LiF/Ca/Al cathode, 2.1 lm/W for a Ca/Al cathode and 3.6 lm/W for NaCl/Al cathode, and found that the turn-on voltage was also reduced from 4.2 V for devices with LiF buffer layer to 3.3 V for devices with NaCl buffer layer.
2. Experimental Patterned indium tin oxide (ITO) (80 V/&) glasses were used as substrates. Prior to film deposition, the substrates were cleaned with TFD 7, rinsed in deionized water, then dried in an oven, and finally positioned in a thermal evaporation chamber. Devices
with a configuration of ITO/N,N0 -bis(1-naphthyl)-N,N0 diphenyl-1,10 biphenyl 4,40 dimaine (NPB) (50 nm)/ tris-(8-hydroxyquinoline) aluminum (Alq3) (60 nm)/ NaCl (x nm) or LiF (x nm)/Ca (y nm)/Al (150 nm) were prepared by thermal evaporation under vacuum of 5 104 Pa. The Al was used as a metallic cap to protect oxidation of the reactive Ca underneath. The NPB was thermally evaporated (thickness 50 nm, ˚ /s) on the cleaned ITO glasses deposition rate 2–4 A as a hole-transporting layer. A 60-nm thick Alq3 was ˚ /s as an electron-transporting evaporated at a rate 2–4 A and light-emitting layer. The electron-injection media of thin NaCl or LiF were deposited on Alq3/NPB/ITO ˚ /s without breaking the glass substrates at a rate 0.1 A vacuum, and then 10 nm thick Ca and 150 nm Al ˚ /s. The electrodes were deposited at a rate of 1–2 A deposition rates were controlled by quartz oscillating thickness monitor. The luminance–current–voltage characteristics were measured using a Keithley (2400 and 2000) source with a calibrated silicon photodiode. All measurements were performed at ambient atmosphere. The active area of the devices was 9 mm2.
3. Results and discussion It was found experimentally that not only the thickness of the buffer layer affects significantly on the EL performance, but the utilization of the Ca layer also plays significant role on the improvement of the EL performance. We first optimized the thicknesses of the buffer and Ca layers. For the case of NaCl buffer layer, the device with 2 nm NaCl and 10 nm Ca exhibits the best results in the EL efficiency, whereas for the case of LiF buffer layer, the device with 1 nm LiF and 10 nm Ca shows the highest EL efficiency. Fig. 1(a and b) shows the current–voltage (I–V) and the brightness–voltage (B–V) characteristics, respectively, for device of ITO/NPB (50 nm)/Alq3 (60 nm)/ NaCl (2 nm)/Ca (10 nm)/Al (150 nm). For comparison, the I–V and B–V characteristics of the optimizing devices of ITO/NPB (50 nm)/Alq3 (60 nm)/ NaCl (2 nm)//Al (150 nm), ITO/NPB (50 nm)/Alq3 (60 nm)/Ca (10 nm)/Al (150 nm), ITO/NPB (50 nm)/ Alq3 (60 nm)/LiF (1 nm)/Ca (10 nm)/Al (150 nm) and ITO/NPB (50 nm)/Alq3 (60 nm)/LiF (1 nm)/Al (150 nm) are also given in Fig. 1. It can be seen that the introduction of a thin NaCl layer reduces the
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Fig. 1. Current–voltage (a) and luminance–voltage (b) characteristics of ITO/NPB/Alq3/metal devices with NaCl/Ca/Al, NaCl/Al, Ca/Al, LiF/ Al and LiF/Ca/Al cathode.
turn-on voltage, and enhances the brightness compared with Ca/Al, LiF/Al and LiF/Ca/Al cathodes. The turn-on voltage is decreased from 4.2 V for devices with Ca/Al, LiF/Al and LiF/Ca/Al cathodes to 3.3 V for devices with NaCl/Ca/Al and NaCl/Al cathodes. The brightness is enhanced to 5000 cd/m2 for devices with NaCl/Ca/Al and NaCl/Al cathodes from 960 cd/m2 for devices with Ca/Al and LiF/Al cathodes and 2000 cd/m2 for device with LiF/Ca/Al cathode at a voltage of 10 V. Importantly, The insertion of a thin Ca layer between buffer layer and Al greatly improves the EL efficiency compared with the cases of the cathode without Ca thin layer. Fig. 2 shows the brightness–current characteristics of the devices with NaCl/Ca/Al, NaCl/Al, Ca/Al, LiF/Al and LiF/Ca/Al. The power efficiency as a function of current for the devices with different cathodes is shown in the inset of Fig. 2. It is clearly seen that the insertion of Ca thin layer between buffer layer and Al enhances the EL efficiency, and the NaCl/Ca/Al cathode shows the highest EL efficiency with respect to NaCl/Al, Ca/Al, LiF/Al, even the similar LiF/Ca/Al cathode. The maximum power efficiency reaches 4.6 lm/W for the case of NaCl/Ca/Al cathode, which is higher than the power efficiency of 3.6 lm/W for NaCl/Al cathode and 3.1 lm/W for LiF/Ca/AL cathode, and is over two factors higher of magnitude than devices with Ca/Al and LiF/Al cathodes. The enhancement of the power efficiency is great
important in the reduction of power assumption in practical applications. NaCl as interface layer in OLEDs has been studied and shows better EL performance than LiF interface layer when using Al as the cathode [40]. The same results have been obtained in our paper. However, the introduction of a thin Ca layer between buffer layer and Al further improves the EL performance, as given in our experiment. To elucidate the enhancement mechanism,
Fig. 2. Luminance–current characteristics of ITO/NPB/Alq3/metal devices with NaCl/Ca/Al, NaCl/Al, Ca/Al, LiF/Al and LiF/Ca/Al cathode, respectively. The inset shows the power efficiency as function of current for different cathode cases.
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electron-only devices of Al/Alq3/NaCl/Ca/Al, Al/Alq3/ NaCl/Al, Al/Alq3/Ca/Al, Al/Alq3/LiF/Al, Al/Alq3/LiF/ Ca/Al and Al/Alq3/Al devices were studied. Fig. 3 shows the comparison of the current–voltage characteristics of the different devices. It can be seen that the utilization of NaCl/Ca/Al and LiF/Ca/Al as the cathodes enhances the electron injection current with respect to NaCl/Al and LiF/Al, and comparing with LiF buffer layer, the NaCl buffer layer shows higher injection current and the insert of the thin Ca layer further enhances the injection current. This clearly indicates that the improvement of the EL efficiency in devices with NaCl/Ca cathode is due to the improvement of the electron injection current. Obviously, The NaCl/Ca/Al is promising cathode in OLEDs. Different mechanisms have been assumed to explain the enhancement of electron injection current by interface layer [41–47]. The debate on the mechanism has mainly focused on two models. One is tunneling probability enhancement resulting from buffer-induced energy level realignment; the other is the chemical reaction model, by which it is through alkali atoms or alkali earth atoms in the compounds adopted are liberated and improve the electron injection due to their low work functions. Recently, Brown et al. [37] measured the built-up potential across the organic layer for the case of different cathodes by electroabsorption, and found that the
magnitude of the built-up potential is strongly dependent on the used buffer layer and cap metals. LiF/Ca/Al cathode showed the highest built-up potential (2.71 eV) compared with Al (1.22 eV), LiF (2.31 eV) and Ca/Al (2.43 eV) cathodes. The high built-up potential facilitates the more electrons to inject into the emissive layer, resulting in high EL efficiency [37]. Because the built-up potential is directly related to the work function of metal ion contained in buffer material, and the low work function will produce high built-up potential. It was well known that Na in NaCl possesses lower work function than Li in LiF [38]. This means that using NaCl as the buffer layer should produce more high built-up potential with respect to LiF buffer layer, and the built-up potential is further increased when inserting Ca thin layer between NaCl and Al due to lower work function of Ca.
4. Conclusions We have shown that the utilization of the NaCl/Ca/ Al cathode improves significantly the EL efficiency in OLEDs. The improvement is attributed to the production of the higher built-up potential within the emissive layer in the case of NaCl/Ca/Al cathode, resulting in more electron current injection from cathode to emissive layer. Our results indicate that NaCl/Ca/Al is promising cathode in OLEDs.
Acknowledgements The authors thank the support of Hundreds Talents Program, Chinese Academy of Sciences and the National Science Fund for Distinguished Young Scholars of China (50325312).
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