- Email: [email protected]
Enhanced performance in organic light-emitting diode by utilizing MoO3-doped C60 as effective hole injection layer
Synthetic Metals 161 (2012) 2628–2631
Contents lists available at SciVerse ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Enhanced performance in organic light-emitting diode by utilizing MoO3 -doped C60 as effective hole injection layer Ye Zou a , Zhenbo Deng a,∗ , Denghui Xu b , Jing Xiao c , Maoyang Zhou a , Hailiang Du a , Yongsheng Wang a a b c
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China Department of Mathematics and Physics, Beijing Technology and Business University, Beijing 100037, PR China Physics and Electronic Engineering College, Taishan University, Tai’an, Shandong 271021, PR China
a r t i c l e
i n f o
Article history: Received 7 June 2011 Received in revised form 3 August 2011 Accepted 12 August 2011 Available online 26 December 2011 Keywords: Hole injection Molybdenum oxide p-Doping Charge transfer complex
a b s t r a c t We report an efficient hole injection layer (HIL) composed of MoO3 -doped C60 for organic lightemitting diodes (OLED). The structure of the OLED device is ITO/MoO3 :C60 (5 nm:5 nm)/NPB (45 nm)/Alq3 (55 nm)/LiF (0.5 nm)/Al. Compared with normal device without a HIL, the device using MoO3 -doped C60 as HIL can significantly enhance both hole injection efficiency and electroluminescence. The power efficiency has been increased by approximately 40.7% and 41.7% at the current density of 10 mA/cm2 and 100 mA/cm2 , respectively, for the device using MoO3 -doped C60 as HIL than the control device. The cause for the enhancement was ascribed to the charge transfer complex formed by co-evaporation of MoO3 and C60 . Hole-only devices were fabricated to confirm the hole injection enhancement. Ultraviolet/visible/near-infrared absorption spectra were measured to confirm the formation of the charge transfer complex. © 2011 Published by Elsevier B.V.
1. Introduction Since the last two decades, organic light-emitting diodes (OLEDs) have become a popular research topic as an alternative to inorganic counterparts for solid state lighting and flat-panel display applications. The main advantages of OLEDs include low cost, light weight, mechanically flexible, and available for large area fabrication [1]. Various approaches including new device structures and materials have been adapted for lowering the driving voltage and enhancing the luminance and efficiency of OLEDs. Interfacial study in OLEDs is one of the important topics, among which hole injection and electron injection layers have been attracted a lot of attentions [2–15]. Molybdenum oxide (MoOx ) is a frequently used hole injection layer (HIL) or a p-type dopant hole transport layer for OLEDs [5–9,14]. Ultra-thin C60 neat film within 1.0 nm has also been used as hole buffer layer to enhance the current efficiency of OLEDs [12]. While enhancing the C60 thickness, hole injection current would greatly decrease [13]. The possible reason should be because of bare C60 layer is a n-type material and it has a high highest occupied molecular orbitals (HOMO) energy level (6.2 eV) and a low hole mobility, which are inferior for hole injection and transport in OLED.
∗ Corresponding author. Tel.: +86 10 51684858; fax: +86 10 51683933. E-mail address: [email protected] (Z. Deng). 0379-6779/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.synthmet.2011.08.026
In this study, the electroluminescence performance of OLED by using MoO3 -doped C60 layer as HIL has been investigated. It is found that the presence of MoO3 -doped C60 HIL layer can greatly lower the driving voltage and enhance the luminance and power efficiency when compared to the control device without a HIL. Even compared with the MoO3 HIL device, the brightness and efficiency of OLED with MoO3 -doped C60 as HIL can also obviously increase. We demonstrated that MoO3 doped C60 is an effective HIL for OLEDs.
2. Experiment Cleaned and UV-ozone treated indium tin oxide (ITO) coated glass substrates were transferred into a thermal evaporation chamber. In this study, OLED devices with a configuration of ITO/HIL/N,N -diphenyl-N,N -bis(1-napthyl-phenyl)-1,1 -biphenyl4,4 -diamine (NPB) (45 nm)/tris-(8-hydroxyquinoline)aluminum (Alq3 ) (55 nm)/LiF (0.5 nm)/Al were fabricated under vacuum of ∼2 × 10−4 Pa. Here, HIL represents 5 nm MoO3 , 5 nm C60 , or 5 nm MoO3 doped 5 nm C60 . NPB was used as hole transport layer and Alq3 as electron transport layer and emission layer. The 5 nm MoO3 doped 5 nm C60 HIL was finished via thermal co-deposition from two different independent source boats with their deposition rates and thicknesses independently monitored by two different quartz-crystal oscillator thickness monitor. The control device without a HIL was also prepared. Except for HILs, NPB, Alq3 , LiF and Al were deposited simultaneously for
a
4
3
1200 1000
Control C60 5 nm
2
Current Density (mA/cm )
a
Current Efficiency (cd/A)
Y. Zou et al. / Synthetic Metals 161 (2012) 2628–2631
800
MoO3 5 nm MoO3:C60 5 nm:5 nm
600 400 200
2629
2
Control C60 5 nm MoO3 5 nm
1
MoO3:C60 5 nm:5 nm
0 0
5
10
15
20
0 0
Voltage (V)
200
400
600
800
1000
2
Current Density (mA/cm )
b
20000
b
2
Luminance (cd/m )
Power Efficiency (lm/W)
Control C60 5 nm
15000
MoO3 5 nm MoO3:C60 5 nm:5 nm
10000
5000
1.8
1.5
Control C60 5 nm
1.2
MoO3 5 nm MoO3:C60 5 nm:5 nm
0.9
0.6
0.3
0 0
5
10
15
20
Voltage (V) Fig. 1. Current density vs. voltage (J–V) characteristics and (b) luminance vs. voltage (L–V) characteristics of the OLED devices with 5 nm MoO3 , 5 nm C60 , or 5 nm MoO3 doped 5 nm C60 as HILs and the device without a HIL (control device), respectively.
each four devices without breaking the vacuum. This avoided the uncertainties in comparing devices fabricated with different evaporation processes. Before the experiment of this work, the MoO3 HIL has been firstly optimized by using 1, 2, 3, 4, 5, 7, 10 nm thickness of MoO3 in ITO/MoO3 /NPB/Alq3 /LiF/Al basic OLED structure and 3- to 5-nm-thick were found to be the optimum thickness. So we chose the optimum 5-nm-thick MoO3 in this study. After finishing devices fabrication, the OLEDs were taken out for measurement immediately without encapsulation. The current density–voltage–luminance characteristics of the devices were measured by a Keithley source measurement unit (Keithley 2400) with a silicon photodiode (1830-C) calibrated by PR-650. In our experiment, eight individual samples were prepared simultaneously on one substrate. In order to minimize the experimental error, the data obtained from every substrate have been averaged before discussion. 3. Results and discussion The current density vs. voltage (J–V) and luminance vs. voltage (L–V) characteristics curves of OLEDs with different HILs (5 nm MoO3 , 5 nm C60 , and 5 nm MoO3 doped 5 nm C60 ) and without a HIL (control device) are shown in Fig. 1(a) and (b), respectively. As can be seen from both J–V and L–V curves, the device with 5 nm MoO3 doped 5 nm C60 (MoO3 :C60 ) as HIL shows superior performance than the other three devices (using MoO3 as HIL, using C60
0.0 0
200
400
600
800
1000
2
Current Density (mA/cm ) Fig. 2. (a) Current efficiency vs. current density characteristics and (b) power efficiency vs. current density characteristics of the devices with 5 nm MoO3 , 5 nm C60 , or 5 nm MoO3 doped 5 nm C60 as HILs and the device without a HIL (control device), respectively.
as HIL, and without a HIL). At the same voltage, the device with MoO3 doped C60 as HIL has highest current density and luminance. The device with MoO3 as HIL also shows significant enhancement of performance than control device, but it is inferior to that of the device with MoO3 :C60 as HIL. While the device with C60 as HIL has the lowest current and brightness. From the luminance vs. voltage characteristics of the OLEDs in Fig. 1(b), the highest luminance of OLED device with MoO3 doped C60 as HIL reaches to 18 178 cd/m2 at a driving voltage of 16.2 V, while the luminance of devices with MoO3 as HIL, with C60 as HIL, and without a HIL is only 16,083, 188 and 12,767 cd/m2 at this voltage, respectively. At the luminance of 100 cd/m2 , the driving voltages are 5.73 V, 5.94 V, and 8.86 V for the devices with MoO3 doped C60 as HIL, with MoO3 as HIL, and without a HIL, respectively. The driving voltage of the device with MoO3 doped C60 as HIL has been reduced by 35.3% compare to control device. While the device with C60 as HIL shows a rather high driving voltage of 15.35 V at the luminance of 100 cd/m2 . These indicate that the hole injection efficiency of OLED is enhanced through using the MoO3 -doped C60 as HIL. Figs. 2(a) and (b) depict the current efficiency and power efficiency vs. current density characteristics, respectively. As can be seen from Fig. 2(a), the control device shows the highest current efficiency. Because of the majority carriers of holes injected into emitting layer was enhanced by using MoO3 or MoO3 :C60 as HILs,
2630
Y. Zou et al. / Synthetic Metals 161 (2012) 2628–2631
1000
2
Current Density (mA/cm )
100
C60
5 nm
Intensity (a.u.)
MoO3 5 nm MoO3:C60 5 nm:5 nm
10 1
Control C60 5 nm MoO3 5 nm MoO3:C60 5 nm:5 nm
0.1 0.01 1E-3 1E-4 1E-5 1E-6 0.1
400
800
1200
1600
2000
Wavelength (nm) Fig. 3. UV–vis–NIR absorption spectra of pure 5 nm MoO3 and 5 nm C60 films and composite film of 5 nm MoO3 doped 5 nm C60 .
which will lead to a more unbalanced carrier density between holes and electrons in emitting layer, so the highest current density of the devices with MoO3 or with MoO3 :C60 as HIL were lower than control device. However, the power efficiencies are greatly enhanced when using MoO3 or MoO3 :C60 as HIL. This is because of power efficiency is related to the driving voltage and a lower driving voltage may always bring about higher power efficiency. The power efficiency of the devices with MoO3 or MoO3 :C60 as HIL reaches to 1.51 lm/W and 1.59 lm/W at the current density of 10 mA/cm2 , respectively. In comparison, the power efficiency of the control device (without a HIL) is only 1.13 lm/W at the same current density. The power efficiency has been increased by approximately 40.7% and 41.7% at the current density of 10 mA/cm2 and 100 mA/cm2 , respectively, for the device using MoO3 :C60 as HIL compared to the control device. From both of the current efficiency and power efficiency characteristics, the device using C60 as HIL shows rather poor efficiency, which is corresponding to the poor current density and luminance properties as shown in Fig. 1(a) and (b). It has been reported that charge transfer (CT) complexes can be formed while doping MoOx in NPB, which will contribute to an increase in the cationic charge carrier density in NPB, resulting in lower operating voltage and higher power efficiency of OLED [6,8,9]. MoOx doped with other p-type hole injection or hole transport materials to enhance OLEDs performance, which were also ascribed to the generated CT complexes by doping, has also been reported [7,9]. Recently, Kubo et al. reported that by doping C60 with MoO3 can control the conduction-type of C60 from n- to p-type [16]. Because of MoO3 is likely to extract electrons from the valence band of C60 , the CT complex, i.e. [C60 + :MoO3 − ], could also be formed between C60 and MoO3 as described in Ref. [16]. Ultraviolet/visible/near-infrared (UV–vis–NIR) absorption spectra of 5 nm MoO3 , 5 nm C60 , and 5 nm MoO3 doped 5 nm C60 films deposited on quartz glass were shown in Fig. 3. The absorption spectra were measured with Shimadzu UV-3101 PC spectrometer. As apparent in Fig. 3, a new strong absorption from about 600 nm to 1600 nm can be clearly seen for co-deposited MoO3 :C60 film. This new absorption spectra corresponds to the absorption of charge transfer complex and confirms charge transfer complex formation between MoO3 doped C60 film. The C60 molecule acted as a p-type dopant in the hole transport layer has also been demonstrated in other work [11]. The authors used C60 -doped in
1
10
Voltage (V) Fig. 4. Current density vs. voltage (J–V) characteristics of the “hole-only” devices.
the 1,3,5-tris(N,N-bis-(4,5-methoxyphenyl)-aminophenyl) benzol (TDAPB) hole transport material and obtained nearly one order enhanced of hole mobility of TDAPB. To confirm the hole injection enhancement, hole-only devices with different HILs were fabricated. The devices have the following structures: ITO/HIL/NPB (80 nm)/Al. HIL represents 5 nm MoO3 , 5 nm C60 , or 5 nm MoO3 doped 5 nm C60 . The hole only device without a HIL was also fabricated and set as control device. Fig. 4 shows the current density vs. voltage (J–V) characteristics of the hole only devices. As shown in Fig. 4, the device with 5 nm MoO3 doped 5 nm C60 as HIL shows a dramatically enhanced current density and a considerable reduction in the voltage compared with the control device. The device with 5 nm MoO3 as HIL also shows a higher current density compared with the control device, but lower than the device with 5 nm MoO3 doped 5 nm C60 as HIL. While the device with 5 nm C60 as HIL shows the weakest current density. These J–V characteristics clearly indicate that the high current density is increased by using the MoO3 doped C60 as HIL. 4. Conclusion We have demonstrated an effective HIL made by doping 5 nm MoO3 with 5 nm C60 for an Alq3 -based OLED. By using this kind of HIL, the OLED performance has been greatly enhanced compared with the device without a HIL. Power efficiency has been enhanced by more than 40% at the current density of 10 mA/cm2 and 100 mA/cm2 by using MoO3 -doped C60 as HIL than the control device without a HIL. The OLED device with MoO3 as HIL also shows enhanced performance compared with control device, but it is inferior than the device with MoO3 doped C60 as HIL. While the device with 5 nm thick C60 as HIL shows a very weak performance. The enhancement of hole injection was verified by hole-only devices and was attributed to the charge transfer complex formation by MoO3 doped C60 . We conclude that MoO3 doped C60 is an effective HIL for OLEDs. Acknowledgements This work was supported by National Natural Science Foundation of China under Contract Nos. 60977027, 61007021 and 60825407, the Scientific Research Common Program of Beijing Municipal Commission of Education under Grant No. KM201010011009, Shandong Science & Technology Foundation of
Y. Zou et al. / Synthetic Metals 161 (2012) 2628–2631
China under Contract No. J08LI65 and Shandong Provincial Natural Science Foundation of China under Contract No. ZR2010FQ006. References [1] [2] [3] [4] [5] [6] [7]
S.R. Forrest, Nature (London) 428 (2004) 911. L.S. Hung, C.W. Tang, M.G. Mason, Appl. Phys. Lett. 70 (1997) 152. Z.B. Deng, X.M. Ding, S.T. Lee, W.A. Gambling, Appl. Phys. Lett. 74 (1999) 2227. J. Li, M. Yahiro, K. Ishida, H. Yamada, K. Matsushige, Synth. Met. 151 (2005) 141. H. You, Y.F. Dai, Z.Q. Zhang, D.G. Ma, J. Appl. Phys. 101 (2007) 026105. T. Matsushima, Y. Kinoshita, H. Murata, Appl. Phys. Lett. 91 (2007) 253504. T. Matsushima, C. Adachi, J. Appl. Phys. 103 (2008) 034501.
2631
[8] W.J. Shin, J.Y. Lee, J.C. Kim, T.H. Yoon, T.S. Kim, O.K. Song, Org. Electron. 9 (2008) 333. [9] G.H. Xie, Y.L. Meng, F.M. Wu, C. Tao, D.D. Zhang, M.J. Liu, Q. Xue, W. Chen, Y. Zhao, Appl. Phys. Lett. 92 (2008) 093305. [10] W.F. Xie, Y. Zhao, C.N. Li, S.Y. Liu, Opt. Express 14 (2006) 7954. [11] J.Y. Lee, J.H. Kwon, Appl. Phys. Lett. 88 (2006) 183502. [12] Z.Y. Lv, Z.B. Deng, D.H. Xu, X.F. Li, Y. Jia, Displays 30 (2009) 23. [13] D.D. Song, S.L. Zhao, F.J. Zhang, Z. Xu, J.M. Li, X. Yue, H.N. Zhu, L.F. Lu, Y.S. Wang, J. Lumin. 129 (2009) 1978. [14] Y. Zou, Z.B. Deng, Z.Y. Lv, Z. Chen, D.H. Xu, Y.L. Chen, Y.H. Yin, H.L. Du, Y.S. Wang, J. Lumin. 130 (2010) 959. [15] S.W. Shi, D.G. Ma, Thin Solid Films 518 (2010) 4874. [16] M. Kubo, T. Kai Iketaki, M. Kaji, Hiramoto, Appl. Phys. Lett. 98 (2011) 073311.
Contents lists available at SciVerse ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Enhanced performance in organic light-emitting diode by utilizing MoO3 -doped C60 as effective hole injection layer Ye Zou a , Zhenbo Deng a,∗ , Denghui Xu b , Jing Xiao c , Maoyang Zhou a , Hailiang Du a , Yongsheng Wang a a b c
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China Department of Mathematics and Physics, Beijing Technology and Business University, Beijing 100037, PR China Physics and Electronic Engineering College, Taishan University, Tai’an, Shandong 271021, PR China
a r t i c l e
i n f o
Article history: Received 7 June 2011 Received in revised form 3 August 2011 Accepted 12 August 2011 Available online 26 December 2011 Keywords: Hole injection Molybdenum oxide p-Doping Charge transfer complex
a b s t r a c t We report an efficient hole injection layer (HIL) composed of MoO3 -doped C60 for organic lightemitting diodes (OLED). The structure of the OLED device is ITO/MoO3 :C60 (5 nm:5 nm)/NPB (45 nm)/Alq3 (55 nm)/LiF (0.5 nm)/Al. Compared with normal device without a HIL, the device using MoO3 -doped C60 as HIL can significantly enhance both hole injection efficiency and electroluminescence. The power efficiency has been increased by approximately 40.7% and 41.7% at the current density of 10 mA/cm2 and 100 mA/cm2 , respectively, for the device using MoO3 -doped C60 as HIL than the control device. The cause for the enhancement was ascribed to the charge transfer complex formed by co-evaporation of MoO3 and C60 . Hole-only devices were fabricated to confirm the hole injection enhancement. Ultraviolet/visible/near-infrared absorption spectra were measured to confirm the formation of the charge transfer complex. © 2011 Published by Elsevier B.V.
1. Introduction Since the last two decades, organic light-emitting diodes (OLEDs) have become a popular research topic as an alternative to inorganic counterparts for solid state lighting and flat-panel display applications. The main advantages of OLEDs include low cost, light weight, mechanically flexible, and available for large area fabrication [1]. Various approaches including new device structures and materials have been adapted for lowering the driving voltage and enhancing the luminance and efficiency of OLEDs. Interfacial study in OLEDs is one of the important topics, among which hole injection and electron injection layers have been attracted a lot of attentions [2–15]. Molybdenum oxide (MoOx ) is a frequently used hole injection layer (HIL) or a p-type dopant hole transport layer for OLEDs [5–9,14]. Ultra-thin C60 neat film within 1.0 nm has also been used as hole buffer layer to enhance the current efficiency of OLEDs [12]. While enhancing the C60 thickness, hole injection current would greatly decrease [13]. The possible reason should be because of bare C60 layer is a n-type material and it has a high highest occupied molecular orbitals (HOMO) energy level (6.2 eV) and a low hole mobility, which are inferior for hole injection and transport in OLED.
∗ Corresponding author. Tel.: +86 10 51684858; fax: +86 10 51683933. E-mail address: [email protected] (Z. Deng). 0379-6779/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.synthmet.2011.08.026
In this study, the electroluminescence performance of OLED by using MoO3 -doped C60 layer as HIL has been investigated. It is found that the presence of MoO3 -doped C60 HIL layer can greatly lower the driving voltage and enhance the luminance and power efficiency when compared to the control device without a HIL. Even compared with the MoO3 HIL device, the brightness and efficiency of OLED with MoO3 -doped C60 as HIL can also obviously increase. We demonstrated that MoO3 doped C60 is an effective HIL for OLEDs.
2. Experiment Cleaned and UV-ozone treated indium tin oxide (ITO) coated glass substrates were transferred into a thermal evaporation chamber. In this study, OLED devices with a configuration of ITO/HIL/N,N -diphenyl-N,N -bis(1-napthyl-phenyl)-1,1 -biphenyl4,4 -diamine (NPB) (45 nm)/tris-(8-hydroxyquinoline)aluminum (Alq3 ) (55 nm)/LiF (0.5 nm)/Al were fabricated under vacuum of ∼2 × 10−4 Pa. Here, HIL represents 5 nm MoO3 , 5 nm C60 , or 5 nm MoO3 doped 5 nm C60 . NPB was used as hole transport layer and Alq3 as electron transport layer and emission layer. The 5 nm MoO3 doped 5 nm C60 HIL was finished via thermal co-deposition from two different independent source boats with their deposition rates and thicknesses independently monitored by two different quartz-crystal oscillator thickness monitor. The control device without a HIL was also prepared. Except for HILs, NPB, Alq3 , LiF and Al were deposited simultaneously for
a
4
3
1200 1000
Control C60 5 nm
2
Current Density (mA/cm )
a
Current Efficiency (cd/A)
Y. Zou et al. / Synthetic Metals 161 (2012) 2628–2631
800
MoO3 5 nm MoO3:C60 5 nm:5 nm
600 400 200
2629
2
Control C60 5 nm MoO3 5 nm
1
MoO3:C60 5 nm:5 nm
0 0
5
10
15
20
0 0
Voltage (V)
200
400
600
800
1000
2
Current Density (mA/cm )
b
20000
b
2
Luminance (cd/m )
Power Efficiency (lm/W)
Control C60 5 nm
15000
MoO3 5 nm MoO3:C60 5 nm:5 nm
10000
5000
1.8
1.5
Control C60 5 nm
1.2
MoO3 5 nm MoO3:C60 5 nm:5 nm
0.9
0.6
0.3
0 0
5
10
15
20
Voltage (V) Fig. 1. Current density vs. voltage (J–V) characteristics and (b) luminance vs. voltage (L–V) characteristics of the OLED devices with 5 nm MoO3 , 5 nm C60 , or 5 nm MoO3 doped 5 nm C60 as HILs and the device without a HIL (control device), respectively.
each four devices without breaking the vacuum. This avoided the uncertainties in comparing devices fabricated with different evaporation processes. Before the experiment of this work, the MoO3 HIL has been firstly optimized by using 1, 2, 3, 4, 5, 7, 10 nm thickness of MoO3 in ITO/MoO3 /NPB/Alq3 /LiF/Al basic OLED structure and 3- to 5-nm-thick were found to be the optimum thickness. So we chose the optimum 5-nm-thick MoO3 in this study. After finishing devices fabrication, the OLEDs were taken out for measurement immediately without encapsulation. The current density–voltage–luminance characteristics of the devices were measured by a Keithley source measurement unit (Keithley 2400) with a silicon photodiode (1830-C) calibrated by PR-650. In our experiment, eight individual samples were prepared simultaneously on one substrate. In order to minimize the experimental error, the data obtained from every substrate have been averaged before discussion. 3. Results and discussion The current density vs. voltage (J–V) and luminance vs. voltage (L–V) characteristics curves of OLEDs with different HILs (5 nm MoO3 , 5 nm C60 , and 5 nm MoO3 doped 5 nm C60 ) and without a HIL (control device) are shown in Fig. 1(a) and (b), respectively. As can be seen from both J–V and L–V curves, the device with 5 nm MoO3 doped 5 nm C60 (MoO3 :C60 ) as HIL shows superior performance than the other three devices (using MoO3 as HIL, using C60
0.0 0
200
400
600
800
1000
2
Current Density (mA/cm ) Fig. 2. (a) Current efficiency vs. current density characteristics and (b) power efficiency vs. current density characteristics of the devices with 5 nm MoO3 , 5 nm C60 , or 5 nm MoO3 doped 5 nm C60 as HILs and the device without a HIL (control device), respectively.
as HIL, and without a HIL). At the same voltage, the device with MoO3 doped C60 as HIL has highest current density and luminance. The device with MoO3 as HIL also shows significant enhancement of performance than control device, but it is inferior to that of the device with MoO3 :C60 as HIL. While the device with C60 as HIL has the lowest current and brightness. From the luminance vs. voltage characteristics of the OLEDs in Fig. 1(b), the highest luminance of OLED device with MoO3 doped C60 as HIL reaches to 18 178 cd/m2 at a driving voltage of 16.2 V, while the luminance of devices with MoO3 as HIL, with C60 as HIL, and without a HIL is only 16,083, 188 and 12,767 cd/m2 at this voltage, respectively. At the luminance of 100 cd/m2 , the driving voltages are 5.73 V, 5.94 V, and 8.86 V for the devices with MoO3 doped C60 as HIL, with MoO3 as HIL, and without a HIL, respectively. The driving voltage of the device with MoO3 doped C60 as HIL has been reduced by 35.3% compare to control device. While the device with C60 as HIL shows a rather high driving voltage of 15.35 V at the luminance of 100 cd/m2 . These indicate that the hole injection efficiency of OLED is enhanced through using the MoO3 -doped C60 as HIL. Figs. 2(a) and (b) depict the current efficiency and power efficiency vs. current density characteristics, respectively. As can be seen from Fig. 2(a), the control device shows the highest current efficiency. Because of the majority carriers of holes injected into emitting layer was enhanced by using MoO3 or MoO3 :C60 as HILs,
2630
Y. Zou et al. / Synthetic Metals 161 (2012) 2628–2631
1000
2
Current Density (mA/cm )
100
C60
5 nm
Intensity (a.u.)
MoO3 5 nm MoO3:C60 5 nm:5 nm
10 1
Control C60 5 nm MoO3 5 nm MoO3:C60 5 nm:5 nm
0.1 0.01 1E-3 1E-4 1E-5 1E-6 0.1
400
800
1200
1600
2000
Wavelength (nm) Fig. 3. UV–vis–NIR absorption spectra of pure 5 nm MoO3 and 5 nm C60 films and composite film of 5 nm MoO3 doped 5 nm C60 .
which will lead to a more unbalanced carrier density between holes and electrons in emitting layer, so the highest current density of the devices with MoO3 or with MoO3 :C60 as HIL were lower than control device. However, the power efficiencies are greatly enhanced when using MoO3 or MoO3 :C60 as HIL. This is because of power efficiency is related to the driving voltage and a lower driving voltage may always bring about higher power efficiency. The power efficiency of the devices with MoO3 or MoO3 :C60 as HIL reaches to 1.51 lm/W and 1.59 lm/W at the current density of 10 mA/cm2 , respectively. In comparison, the power efficiency of the control device (without a HIL) is only 1.13 lm/W at the same current density. The power efficiency has been increased by approximately 40.7% and 41.7% at the current density of 10 mA/cm2 and 100 mA/cm2 , respectively, for the device using MoO3 :C60 as HIL compared to the control device. From both of the current efficiency and power efficiency characteristics, the device using C60 as HIL shows rather poor efficiency, which is corresponding to the poor current density and luminance properties as shown in Fig. 1(a) and (b). It has been reported that charge transfer (CT) complexes can be formed while doping MoOx in NPB, which will contribute to an increase in the cationic charge carrier density in NPB, resulting in lower operating voltage and higher power efficiency of OLED [6,8,9]. MoOx doped with other p-type hole injection or hole transport materials to enhance OLEDs performance, which were also ascribed to the generated CT complexes by doping, has also been reported [7,9]. Recently, Kubo et al. reported that by doping C60 with MoO3 can control the conduction-type of C60 from n- to p-type [16]. Because of MoO3 is likely to extract electrons from the valence band of C60 , the CT complex, i.e. [C60 + :MoO3 − ], could also be formed between C60 and MoO3 as described in Ref. [16]. Ultraviolet/visible/near-infrared (UV–vis–NIR) absorption spectra of 5 nm MoO3 , 5 nm C60 , and 5 nm MoO3 doped 5 nm C60 films deposited on quartz glass were shown in Fig. 3. The absorption spectra were measured with Shimadzu UV-3101 PC spectrometer. As apparent in Fig. 3, a new strong absorption from about 600 nm to 1600 nm can be clearly seen for co-deposited MoO3 :C60 film. This new absorption spectra corresponds to the absorption of charge transfer complex and confirms charge transfer complex formation between MoO3 doped C60 film. The C60 molecule acted as a p-type dopant in the hole transport layer has also been demonstrated in other work [11]. The authors used C60 -doped in
1
10
Voltage (V) Fig. 4. Current density vs. voltage (J–V) characteristics of the “hole-only” devices.
the 1,3,5-tris(N,N-bis-(4,5-methoxyphenyl)-aminophenyl) benzol (TDAPB) hole transport material and obtained nearly one order enhanced of hole mobility of TDAPB. To confirm the hole injection enhancement, hole-only devices with different HILs were fabricated. The devices have the following structures: ITO/HIL/NPB (80 nm)/Al. HIL represents 5 nm MoO3 , 5 nm C60 , or 5 nm MoO3 doped 5 nm C60 . The hole only device without a HIL was also fabricated and set as control device. Fig. 4 shows the current density vs. voltage (J–V) characteristics of the hole only devices. As shown in Fig. 4, the device with 5 nm MoO3 doped 5 nm C60 as HIL shows a dramatically enhanced current density and a considerable reduction in the voltage compared with the control device. The device with 5 nm MoO3 as HIL also shows a higher current density compared with the control device, but lower than the device with 5 nm MoO3 doped 5 nm C60 as HIL. While the device with 5 nm C60 as HIL shows the weakest current density. These J–V characteristics clearly indicate that the high current density is increased by using the MoO3 doped C60 as HIL. 4. Conclusion We have demonstrated an effective HIL made by doping 5 nm MoO3 with 5 nm C60 for an Alq3 -based OLED. By using this kind of HIL, the OLED performance has been greatly enhanced compared with the device without a HIL. Power efficiency has been enhanced by more than 40% at the current density of 10 mA/cm2 and 100 mA/cm2 by using MoO3 -doped C60 as HIL than the control device without a HIL. The OLED device with MoO3 as HIL also shows enhanced performance compared with control device, but it is inferior than the device with MoO3 doped C60 as HIL. While the device with 5 nm thick C60 as HIL shows a very weak performance. The enhancement of hole injection was verified by hole-only devices and was attributed to the charge transfer complex formation by MoO3 doped C60 . We conclude that MoO3 doped C60 is an effective HIL for OLEDs. Acknowledgements This work was supported by National Natural Science Foundation of China under Contract Nos. 60977027, 61007021 and 60825407, the Scientific Research Common Program of Beijing Municipal Commission of Education under Grant No. KM201010011009, Shandong Science & Technology Foundation of
Y. Zou et al. / Synthetic Metals 161 (2012) 2628–2631
China under Contract No. J08LI65 and Shandong Provincial Natural Science Foundation of China under Contract No. ZR2010FQ006. References [1] [2] [3] [4] [5] [6] [7]
S.R. Forrest, Nature (London) 428 (2004) 911. L.S. Hung, C.W. Tang, M.G. Mason, Appl. Phys. Lett. 70 (1997) 152. Z.B. Deng, X.M. Ding, S.T. Lee, W.A. Gambling, Appl. Phys. Lett. 74 (1999) 2227. J. Li, M. Yahiro, K. Ishida, H. Yamada, K. Matsushige, Synth. Met. 151 (2005) 141. H. You, Y.F. Dai, Z.Q. Zhang, D.G. Ma, J. Appl. Phys. 101 (2007) 026105. T. Matsushima, Y. Kinoshita, H. Murata, Appl. Phys. Lett. 91 (2007) 253504. T. Matsushima, C. Adachi, J. Appl. Phys. 103 (2008) 034501.
2631
[8] W.J. Shin, J.Y. Lee, J.C. Kim, T.H. Yoon, T.S. Kim, O.K. Song, Org. Electron. 9 (2008) 333. [9] G.H. Xie, Y.L. Meng, F.M. Wu, C. Tao, D.D. Zhang, M.J. Liu, Q. Xue, W. Chen, Y. Zhao, Appl. Phys. Lett. 92 (2008) 093305. [10] W.F. Xie, Y. Zhao, C.N. Li, S.Y. Liu, Opt. Express 14 (2006) 7954. [11] J.Y. Lee, J.H. Kwon, Appl. Phys. Lett. 88 (2006) 183502. [12] Z.Y. Lv, Z.B. Deng, D.H. Xu, X.F. Li, Y. Jia, Displays 30 (2009) 23. [13] D.D. Song, S.L. Zhao, F.J. Zhang, Z. Xu, J.M. Li, X. Yue, H.N. Zhu, L.F. Lu, Y.S. Wang, J. Lumin. 129 (2009) 1978. [14] Y. Zou, Z.B. Deng, Z.Y. Lv, Z. Chen, D.H. Xu, Y.L. Chen, Y.H. Yin, H.L. Du, Y.S. Wang, J. Lumin. 130 (2010) 959. [15] S.W. Shi, D.G. Ma, Thin Solid Films 518 (2010) 4874. [16] M. Kubo, T. Kai Iketaki, M. Kaji, Hiramoto, Appl. Phys. Lett. 98 (2011) 073311.