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Blue organic light-emitting diodes with low driving voltage and enhanced power efficiency based on MoO3 as hole injection layer and optimized charge balance
Journal of Non-Crystalline Solids 356 (2010) 1012–1015
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Blue organic light-emitting diodes with low driving voltage and enhanced power efficiency based on MoO3 as hole injection layer and optimized charge balance Khizar-ul-Haq a,*, M.A. Khan a, X.W. Zhang a, Liang Zhang a, X.Y. Jiang a, Z.L. Zhang a,b a b
School of Materials Science and Engineering, Shanghai University, Jiading Shanghai 201800, People’s Republic of China Key Laboratories of Advanced Display and System Applications, Ministry of Education, Shanghai University, Shanghai 200072, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 March 2009 Received in revised form 3 January 2010 Available online 12 February 2010 Keywords: Devices Heterojunctions Optical properties Electroluminescence Luminescence
a b s t r a c t In this letter, blue organic light-emitting diodes (OLEDs) with a different hole injection layer (MoO3 or m-MTDATA), and a different electron transport layer (Alq3 or BPhen) have been investigated. With 9,10-bis(2-naphthyl)-2-t-butylanthracene (TBADN) doped with (3 wt.%) P-bis(P-N,N-diphenyl-aminostyryl)benzene (DSA-Ph) as an emitting layer, the typical device structure is ITO/HIL (5 nm)/NPB (25 nm)/ EML (35 nm)/ETL (15 nm)/LiF (0.8 nm)/Al (100 nm). It has been found that MoO3//BPhen based device shows the lowest driving voltage and highest power efficiency among the referenced devices. At the current density of 20 mA/cm2, its driving voltage and power efficiency is 5.2 V and 4.2 lm/W, which is independently reduced 48% and improved 44% compared with those of the m-MTDATA//Alq3 based one, respectively. The energy level diagram of the devices and single-carrier devices are studied to explain the reasons behind this improvement. The results strongly indicate that carrier injection ability and balance shows significant affects on the performance of OLED. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) have attracted increasing attention in recent years and are considered to hold the promise of the next generation of flat-panel displays due to their wide viewing angle, high contrast, mechanical flexibility, and in particular lowoperating voltage [1,2]. After Tang and VanSlyke first demonstrated new OLEDs with a structure consisting of double-layered organic materials sandwiched between two electrodes, numerous fluorescent materials acting both as host or dopant have been synthesized and developed. A reduction in driving voltage is crucial to improve their power conversion efficiencies and lifetimes [3,4]. Now there are two means frequently used to overcome the driving voltage problem. One is inserting a thin layer as an anode buffer layer between indium tin oxide (ITO) and hole transport layer (HTL), which reduces the energy barrier to enhance charge injection at the interfaces and ultimately reduce the driving voltage and improve the power efficiency of the device. Another method is using strong electron acceptor and donor materials as dopants in organic HTL and electron transport layer (ETL) in OLEDs [3–12]. Moreover, it is well established that such kinds of transport
compounds play electron-vibration trapping levels which can additionally operate the charge transfer. Doping these materials into HTL and ETL induces a charge transfer between the host and dopant materials, leading to a marked increase in free carrier concentration in these doped layers and the formation of an ohmic contact at electrode/organic interfaces [13,14]. It is hard to balance the holes and electrons in the emitting layer because hole mobility is generally faster than the electron mobility in organic materials. In order to solve this problem, several kinds of HTL, ETL, hole block layer, and electron block layer have been studied [6,7]. In this paper, we have demonstrated a simple emitting layer OLED, which is based on co-luminescence of the two components in which the blue material [TBADN] is used as host and [DSA-Ph] as a blue dopant with optimized concentration. Blue emission utilizing incomplete energy transfer from [TBADN] to [DSA-Ph] is achieved. In order to reduce driving voltage and improve efficiency, we use MoO3 as hole injection layer and BPhen as electron transmitting layer, power efficiency and carrier balance having been overwhelmingly improved. 2. Experimental detail
* Corresponding author. Tel.: +86 13482324750; fax: +86 21 39988216. E-mail address: [email protected] ( Khizar-ul-Haq). 0022-3093/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.01.023
The structure of the blue OLEDs fabricated in this work is as follows:
Khizar-ul-Haq et al. / Journal of Non-Crystalline Solids 356 (2010) 1012–1015
CellMA: ITO/m-MTDATA (20 nm)/NPB (10 nm)/[TBADN: DSA-Ph] (35 nm)/Alq3 (15 nm)/LiF (0.8 nm)/Al (100 nm) CellMB: ITO/m-MTDATA (20 nm)/NPB (10 nm)/[TBADN: DSA-Ph] (35 nm)/BPhen (15 nm)/LiF (0.8 nm)/Al (100 nm) Cell3A: ITO/MoO3 (5 nm)/NPB (25 nm)/[TBADN: DSA-Ph] (35 nm)/Alq3 (15 nm)/LiF (0.8 nm)/Al (100 nm) Cell3B: ITO/MoO3 (5 nm)/NPB (25 nm)/[TBADN: DSA-Ph] (35 nm)/BPhen (15 nm)/LiF (0.8 nm)/Al (100 nm). The devices were fabricated as follows. Glass coated with ITO with a sheet resistance of 20 X/h was used as the starting substrate. Prior to the organic films being deposited, the substrates were initially scrubbed in a detergent solution. They were then immersed sequentially in a heated ultrasonic bath of de-ionized water for 15 min. Finally, the substrates were blown dry with nitrogen gas and then treated by UV-ozone for 10 min prior to use. All the layers (organic layers, buffer layer and Al electrode) of the devices were fabricated by conventional thermal evaporation in a high vacuum chamber with a base pressure of 1 106 Torr. In the devices, 4,40 ,40 -tris(N-3-methylpheny-N-phenyi-amino)triphenylamine (m-MTDATA) and MoO3 functioned as buffer layers, 4–7-diphenyl-1,10-phenanlhroline (BPhen) and tri(8-hydroxyquinoline)aluminum (Alq3) as electron-transporting layers. While N,N0 -bis(1-naphthyl)-N,N0 -diphenyl-1,10-biphenyl4,40 ,40 -diamine (NPB) as a hole transporting layer and LiF/Al served as the composite cathode. The composition of EML is as [TBADN: DSA-Ph 3 wt.%]. The deposition rate was controlled by a calibrated quartz crystal oscillator and was maintained at 5–3 Å/s for the organic materials, 0.1 Å/s for LiF and 10–12 Å/s for Al. A shadow mask was used for the deposition of the cathode. The active area of the devices is 5 5 mm2. The electroluminescent (EL) spectra and the Commission International de I’Eclairage (CIE) color coordinates were measured by using a PR650 Spectroscanner, while the current versus voltage (I–V) and luminance characteristics were measured by computer controlled programmable Keithley 2400 dc Source Meter and Minolta LS-110 luminance me-
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ter. All measurements were carried out at room temperature and under ambient conditions without any protective coating. Molecular structures of the organic materials used in this study are shown in Fig. 1.
3. Results Fig. 2(a) and (b) shows current density versus voltage and luminance versus voltage curves for the m-MTDATA//Alq3 (CellMA), mMTDATA//BPhen (CellMB), MoO3//Alq3 (Cell3A) and MoO3//BPhen (Cell3B) based devices, respectively. It can be seen that the two devices with MoO3 as buffer layers have a lower driving voltage than the other two devices. In particular, the device Cell3B shows a much lower driving voltage than all the other devices. At a current den-
Fig. 2. Current density versus voltage characteristics (a) and luminance versus voltage. (b) On the other hand, although the device CellMA has a highest current efficiency than that of the device Cell3B, Cell3A and CellMB, but the device Cell3B has the highest power efficiency among the four devices due to its rather low driving voltage. Fig. 3 shows the power efficiency–current density and current efficiency– current density curves for the four devices. The performances of the four devices are listed in Table 1.
Fig. 1. Molecular structures of organic materials used.
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sity of 20 mA/cm2, the driving voltage is 5.2, 7.3, 7.8, and 9.8 V for Cell3B, Cell3A, CellMB, and CellMA, respectively. It can be seen from Table 1 (or Fig. 3) that the device CellMA has a high current efficiency and a large driving voltage, while the device Cell3B shows rather low driving voltage and higher power efficiency. At a current density of 20 mA/cm2, the driving voltage of device Cell3B is 5.2 V, which reduces by 44% and 39% compared with CellMA (9.8 V) and CellMB (7.8 V), respectively; the power efficiency of Cell3B is 4.2 lm/W, which is enhanced by about 48%, compared with that of CellMA, CellMB and Cell3A (2.34, 2.4, and 2.5 lm/W). Fig. 4(a) and (b) shows the EL spectra and the CIE color coordinate of device CellMA, CellMB, Cell3A, and Cell3B under the current density of 20 mA/cm2. As can be seen that, emission at 460 nm and at around 490 nm can be attributed to [TBADN] and [DSAPh] respectively with an optimized doping concentration of [DSA-Ph 3 wt.%]. At a current density of 20 mA/cm2, the CIE coordinates of device CellMA, and CellMB are of (0.160, 0.252) and (0.157, 0.187), respectively. On the other hand, the device Cell3A and Cell3B show CIE coordinates of (0.204, 0.331) and (0.178, 0.304), respectively. We observed that CIE coordinate deteriorates due to the existence of the Alq3 ETL layer and becomes worse due to insertion of MoO3 as a buffer layer. The better performance of device CellMB (or Cell3B) could be attributed to better hole blocking capability of BPhen as compared to Alq3 based devices. Fig. 5 shows the proposed energy level diagram of the devices studied, with the relative alignment of the HOMO and LUMO levels of each layer. The relative work function of ITO/MoO3 was measured by using contact potential difference measurements re-
Fig. 4. Normalized EL spectra of (a) [CellMA, CellMB] and (b) [Cell3A and Cell3B].
Table 1 Performance of four blue OLEDs with different buffer layers and ETL. V20, L20, gA20, gW20 and CIE20 are the driving voltage, luminance, current efficiency, power efficiency and CIE coordinates at the current density of 20 mA/cm2, respectively. V400, and L400 are the driving voltage and luminance at the current density of 400 mA/ cm2, respectively.
Fig. 5. Energy level diagram of OLEDs studied.
V400 (V)
V20 (V)
L400 (cd/ m2)
L20 (cd/ m2)
gA20
gW20
(cd/ A)
(lm/ w)
CellMA
14.1
9.8
26 655
1403
7.3
2.3
CellMB
12.8
7.8
14 842
1265
6.3
2.4
Cell3A
11.5
7.3
14 629
1206
6.0
2.6
5.2
24 478
1364
6.7
4.2
Devices
Cell3B
9.42
CIE20 (x, y) (0.160, 0.252) (0.157, 0.187) (0.204, 0.331) (0.178, 0.304)
ported by us [15]. The work function of MoO3 is 5.43 eV, which is very close to the HOMO level of NPB (5.4 eV), while the HOMO level of m-MTDATA was reported to be 5.11 eV. Thus, a very small hole injection barrier between MoO3 and NPB leads to a strong hole injection from MoO3 to NPB. On the other hand, the LUMO level of Alq3 is 2.8 eV, which is 0.2 eV higher than that of BPhen. For the study of carrier injection and balance, a series of holeonly devices in which the current consists mainly of the hole-only and electron-only devices in which the current consists mainly of the electrons are fabricated. These devices have the following structures: EAlq3: ITO/Alq3 (30 nm)/LiF (0.8 nm)/Al (100 nm) EBPhen: ITO/BPhen (30 nm)/LiF (0.8 nm)/Al (100 nm)
Fig. 3. Power efficiency versus current density curves of the devices. The inset shows the current efficiency versus current density characteristics.
Fig. 6. Current density versus voltage characteristics of the hole-only devices based on MoO3 and m-MTDATA (a) and electron-only devices based on Alq3 and BPhen (b).
Khizar-ul-Haq et al. / Journal of Non-Crystalline Solids 356 (2010) 1012–1015
HMT: ITO/m-MTDATA (20 nm)/NPB (10 nm)/Al (100 nm) HM3: ITO/MoO3 (20 nm)/NPB (10 nm)/Al (100 nm). Fig. 6 shows the current density versus voltage characteristics of hole-only and electron-only devices. For hole-only devices, at the driving voltage of 6 V, the current density of HM3 is 80.46 mA/cm2, much larger than 20.5 mA/cm2 for HMT. For electron-only devices, the current density of device EBPhen is 20.6 mA/cm2, which is larger than 4.12 mA/cm2 for EAlq3, at the same driving voltage, respectively. It indicates that the conductibility of BPhen is stronger than that of Alq3, and the hole injection ability of MoO3 is stronger than that of m-MTDATA, resulting in low driving voltage. 4. Discussion The blue OLED with MoO3 as a buffer layer presented significant performance. The improved performances of the Cell3B are attributed to high hole injection ability of MoO3 as a buffer layer, the high electron mobility of BPhen as an ETL. Moreover, the electron mobility of BPhen has been estimated by our group using space charge limited current to be between (3.9–5.2) 104 cm2/V s [16], about 200 times more than that of Alq3. Thus, BPhen as an ETL has a stronger electron injection ability, as well as more chance to travel into EML and recombine with hole carriers. It is well known, the power efficiency depends on carrier injection and transport, while current efficiency not only relies on the carrier injection, but also on the carrier balance. From Fig. 6, we can see that the current density of HMT and EAlq3 is close at a low driving voltage, that is to say the electrons and holes have a better balance compared with other devices. This is why device CellMA has the highest current efficiency of the four devices. From Fig. 3 we can see that the current efficiency of device Cell3B is bit lower than CellMA, though it has the best power efficiency. So, not only does the MoO3 have a strong hole injection ability resulting in lowering of the driving voltage, but also its hole injection ability is also much larger than the electron injection ability of ETL. This excess of hole injection results in an imbalance of electrons and holes, which leads to a lower current efficiency. We also observed that emission from device CellMA (and/or Cell3A) is sky blue with the incorporation of Alq3 as an ETL. And it is well established that Alq3 is a strong green emitter, the color purity of blue emission suffers from this green light that is generated by the exciton diffusion from EML into ETL or by direct electron–hole recombination inside the ETL. Hence, a small amount of unwelcoming light emission from Alq3 [17] was unavoidable and may be mixed with blue emission. When HIL of m-MTDATA is replaced by MoO3, the EL spectrum of device Cell3A is broader at longer wavelength (520 nm) [17,18] and CIE coordinates changes due to large amount of emission from Alq3 layer. It is interesting to note that the EL spectrum of device Cell3B is less broad as compared to device Cell3A. The broad spectrum is likely appeared as the hole injection is increased due to MoO3 buffer layer. The change in emission can be attributed to poor hole blocking ability of Alq3 and may be due to the formation of somewhat aggregates (or chromophores). And an aggregate should provide the additional emission (or absorption). Additionally, the relatively wide half
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widths (in case of MoO3 based devices) may indicate on the substantial role of the electron-vibration broadening of line and several contributions of the inter-conformation states. Because different confirmation states substantially change the values of the potential barriers determining the processes of the charge carriers transport including the exciton states [19]. 5. Conclusions In conclusion, an organic light-emitting diodes with MoO3 as buffer layer and BPhen as an ETL, the power efficiency of which is improved by 44% to 4.2 lm/W as compared to the corresponding devices with different ETL and HIL layers. Its driving voltage, at 20 mA/cm2 is 5.2 V, which is independently reduced by 48% than that of the m-MTDATA//Alq3 based device. The superior performances of the MoO3//BPhen based device are attributed to the high hole injection ability of MoO3 and high electron mobility of BPhen, which leads to high power efficiency and low driving voltage. From the energy level diagram of the devices studied, we know a small barrier exists between MoO3 and NPB, which leads to a low driving voltage. A better balance of electrons and holes can contribute to a good current efficiency for the device. Acknowledgments The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (90201034, 60477014, 60577041, 60777018 and 60776040), 973 project (2002CB613400), International Cooperation Foundation of Shanghai Science and Technology Committee (06DZ22013), and Innovation Fund of Shanghai University (A10-0109-07-015). References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] H.K. Lee, J.H. Seo, J.H. Kim, J.R. Koo, K.H. Lee, S.S. Yoon, Y.K. Kim, J. Korean Phys. Soc. 49 (2006) 1052. [3] G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, J. Salbeck, Appl. Phys. Lett. 85 (2004) 3911. [4] R. Meerheim, K. Walzer, M. Pfeiffer, K. Leo, Appl. Phys. Lett. 89 (2006) 061111. [5] D.B. Romero, M. Schaer, L. Zuppiroli, B. Cesar, B. Francois, Appl. Phys. Lett. 67 (1995) 1659. [6] F. Huang, A.G. MacDiarmid, B.R. Hsieh, Appl. Phys. Lett. 71 (1997) 2415. [7] J. Blochwitz, M. Pfeiffer, T. Fritz, K. Leo, Appl. Phys. Lett. 73 (1998) 729. [8] J. Kido, T. Matsumoto, Appl. Phys. Lett. 73 (1998) 2866. [9] A. Yamamori, C. Adachi, T. Koyama, Y. Taniguchi, Appl. Phys. Lett. 72 (1998) 472147. [10] A. Yamamori, C. Adachi, T. Koyama, Y. Taniguchi, J. Appl. Phys. 86 (1999) 4369. [11] V.-E. Choong, S. Shi, J. Curless, F. So, Appl. Phys. Lett. 76 (2000) 958. [12] C. Ganzorig, M. Fujihara, Appl. Phys. Lett. 77 (2000) 4211. [13] M. Pfeiffer, A. Beyer, T. Fritz, K. Leo, Appl. Phys. Lett. 73 (1998) 3202. [14] J. Blochwitz, T. Fritz, M. Pfeiffer, K. Leo, D.M. Alloway, P.A. Lee, N.R. Armstrong, Org. Electron. 2 (2001) 97. [15] Z.L. Zhang, in: S. Miyata, H.S. Nalwa (Eds.), Organic Electrolumies Materials and Devices, Gordon and Breach, New York, 1997, p. 209 (Chapter 5). [16] M.A. Khan, Wei Xu, K.U. Haq, Y. Bai, X. Y Jiang, Z.L. Zhang, W. Q Zhu, J. Appl. Phys. 103 (2008) 014509. [17] J.M. Shi, C.W. Tang, Appl. Phys. Lett. 80 (2002) 3201. [18] S.W. Culligan, A.C.-A. Chen, J.U. Wallace, K.P. Klubek, C.W. Tang, S.H. Chen, Adv. Mater. 16 (2006) 1481. [19] E. Gondek, J. Niziol, A. Danel, I.V. Kityk, M. Pokladko, J. Sanetra, E. Kulig, J. Luminesc. 128 (2008) 1831.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Blue organic light-emitting diodes with low driving voltage and enhanced power efficiency based on MoO3 as hole injection layer and optimized charge balance Khizar-ul-Haq a,*, M.A. Khan a, X.W. Zhang a, Liang Zhang a, X.Y. Jiang a, Z.L. Zhang a,b a b
School of Materials Science and Engineering, Shanghai University, Jiading Shanghai 201800, People’s Republic of China Key Laboratories of Advanced Display and System Applications, Ministry of Education, Shanghai University, Shanghai 200072, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 March 2009 Received in revised form 3 January 2010 Available online 12 February 2010 Keywords: Devices Heterojunctions Optical properties Electroluminescence Luminescence
a b s t r a c t In this letter, blue organic light-emitting diodes (OLEDs) with a different hole injection layer (MoO3 or m-MTDATA), and a different electron transport layer (Alq3 or BPhen) have been investigated. With 9,10-bis(2-naphthyl)-2-t-butylanthracene (TBADN) doped with (3 wt.%) P-bis(P-N,N-diphenyl-aminostyryl)benzene (DSA-Ph) as an emitting layer, the typical device structure is ITO/HIL (5 nm)/NPB (25 nm)/ EML (35 nm)/ETL (15 nm)/LiF (0.8 nm)/Al (100 nm). It has been found that MoO3//BPhen based device shows the lowest driving voltage and highest power efficiency among the referenced devices. At the current density of 20 mA/cm2, its driving voltage and power efficiency is 5.2 V and 4.2 lm/W, which is independently reduced 48% and improved 44% compared with those of the m-MTDATA//Alq3 based one, respectively. The energy level diagram of the devices and single-carrier devices are studied to explain the reasons behind this improvement. The results strongly indicate that carrier injection ability and balance shows significant affects on the performance of OLED. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) have attracted increasing attention in recent years and are considered to hold the promise of the next generation of flat-panel displays due to their wide viewing angle, high contrast, mechanical flexibility, and in particular lowoperating voltage [1,2]. After Tang and VanSlyke first demonstrated new OLEDs with a structure consisting of double-layered organic materials sandwiched between two electrodes, numerous fluorescent materials acting both as host or dopant have been synthesized and developed. A reduction in driving voltage is crucial to improve their power conversion efficiencies and lifetimes [3,4]. Now there are two means frequently used to overcome the driving voltage problem. One is inserting a thin layer as an anode buffer layer between indium tin oxide (ITO) and hole transport layer (HTL), which reduces the energy barrier to enhance charge injection at the interfaces and ultimately reduce the driving voltage and improve the power efficiency of the device. Another method is using strong electron acceptor and donor materials as dopants in organic HTL and electron transport layer (ETL) in OLEDs [3–12]. Moreover, it is well established that such kinds of transport
compounds play electron-vibration trapping levels which can additionally operate the charge transfer. Doping these materials into HTL and ETL induces a charge transfer between the host and dopant materials, leading to a marked increase in free carrier concentration in these doped layers and the formation of an ohmic contact at electrode/organic interfaces [13,14]. It is hard to balance the holes and electrons in the emitting layer because hole mobility is generally faster than the electron mobility in organic materials. In order to solve this problem, several kinds of HTL, ETL, hole block layer, and electron block layer have been studied [6,7]. In this paper, we have demonstrated a simple emitting layer OLED, which is based on co-luminescence of the two components in which the blue material [TBADN] is used as host and [DSA-Ph] as a blue dopant with optimized concentration. Blue emission utilizing incomplete energy transfer from [TBADN] to [DSA-Ph] is achieved. In order to reduce driving voltage and improve efficiency, we use MoO3 as hole injection layer and BPhen as electron transmitting layer, power efficiency and carrier balance having been overwhelmingly improved. 2. Experimental detail
* Corresponding author. Tel.: +86 13482324750; fax: +86 21 39988216. E-mail address: [email protected] ( Khizar-ul-Haq). 0022-3093/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.01.023
The structure of the blue OLEDs fabricated in this work is as follows:
Khizar-ul-Haq et al. / Journal of Non-Crystalline Solids 356 (2010) 1012–1015
CellMA: ITO/m-MTDATA (20 nm)/NPB (10 nm)/[TBADN: DSA-Ph] (35 nm)/Alq3 (15 nm)/LiF (0.8 nm)/Al (100 nm) CellMB: ITO/m-MTDATA (20 nm)/NPB (10 nm)/[TBADN: DSA-Ph] (35 nm)/BPhen (15 nm)/LiF (0.8 nm)/Al (100 nm) Cell3A: ITO/MoO3 (5 nm)/NPB (25 nm)/[TBADN: DSA-Ph] (35 nm)/Alq3 (15 nm)/LiF (0.8 nm)/Al (100 nm) Cell3B: ITO/MoO3 (5 nm)/NPB (25 nm)/[TBADN: DSA-Ph] (35 nm)/BPhen (15 nm)/LiF (0.8 nm)/Al (100 nm). The devices were fabricated as follows. Glass coated with ITO with a sheet resistance of 20 X/h was used as the starting substrate. Prior to the organic films being deposited, the substrates were initially scrubbed in a detergent solution. They were then immersed sequentially in a heated ultrasonic bath of de-ionized water for 15 min. Finally, the substrates were blown dry with nitrogen gas and then treated by UV-ozone for 10 min prior to use. All the layers (organic layers, buffer layer and Al electrode) of the devices were fabricated by conventional thermal evaporation in a high vacuum chamber with a base pressure of 1 106 Torr. In the devices, 4,40 ,40 -tris(N-3-methylpheny-N-phenyi-amino)triphenylamine (m-MTDATA) and MoO3 functioned as buffer layers, 4–7-diphenyl-1,10-phenanlhroline (BPhen) and tri(8-hydroxyquinoline)aluminum (Alq3) as electron-transporting layers. While N,N0 -bis(1-naphthyl)-N,N0 -diphenyl-1,10-biphenyl4,40 ,40 -diamine (NPB) as a hole transporting layer and LiF/Al served as the composite cathode. The composition of EML is as [TBADN: DSA-Ph 3 wt.%]. The deposition rate was controlled by a calibrated quartz crystal oscillator and was maintained at 5–3 Å/s for the organic materials, 0.1 Å/s for LiF and 10–12 Å/s for Al. A shadow mask was used for the deposition of the cathode. The active area of the devices is 5 5 mm2. The electroluminescent (EL) spectra and the Commission International de I’Eclairage (CIE) color coordinates were measured by using a PR650 Spectroscanner, while the current versus voltage (I–V) and luminance characteristics were measured by computer controlled programmable Keithley 2400 dc Source Meter and Minolta LS-110 luminance me-
1013
ter. All measurements were carried out at room temperature and under ambient conditions without any protective coating. Molecular structures of the organic materials used in this study are shown in Fig. 1.
3. Results Fig. 2(a) and (b) shows current density versus voltage and luminance versus voltage curves for the m-MTDATA//Alq3 (CellMA), mMTDATA//BPhen (CellMB), MoO3//Alq3 (Cell3A) and MoO3//BPhen (Cell3B) based devices, respectively. It can be seen that the two devices with MoO3 as buffer layers have a lower driving voltage than the other two devices. In particular, the device Cell3B shows a much lower driving voltage than all the other devices. At a current den-
Fig. 2. Current density versus voltage characteristics (a) and luminance versus voltage. (b) On the other hand, although the device CellMA has a highest current efficiency than that of the device Cell3B, Cell3A and CellMB, but the device Cell3B has the highest power efficiency among the four devices due to its rather low driving voltage. Fig. 3 shows the power efficiency–current density and current efficiency– current density curves for the four devices. The performances of the four devices are listed in Table 1.
Fig. 1. Molecular structures of organic materials used.
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Khizar-ul-Haq et al. / Journal of Non-Crystalline Solids 356 (2010) 1012–1015
sity of 20 mA/cm2, the driving voltage is 5.2, 7.3, 7.8, and 9.8 V for Cell3B, Cell3A, CellMB, and CellMA, respectively. It can be seen from Table 1 (or Fig. 3) that the device CellMA has a high current efficiency and a large driving voltage, while the device Cell3B shows rather low driving voltage and higher power efficiency. At a current density of 20 mA/cm2, the driving voltage of device Cell3B is 5.2 V, which reduces by 44% and 39% compared with CellMA (9.8 V) and CellMB (7.8 V), respectively; the power efficiency of Cell3B is 4.2 lm/W, which is enhanced by about 48%, compared with that of CellMA, CellMB and Cell3A (2.34, 2.4, and 2.5 lm/W). Fig. 4(a) and (b) shows the EL spectra and the CIE color coordinate of device CellMA, CellMB, Cell3A, and Cell3B under the current density of 20 mA/cm2. As can be seen that, emission at 460 nm and at around 490 nm can be attributed to [TBADN] and [DSAPh] respectively with an optimized doping concentration of [DSA-Ph 3 wt.%]. At a current density of 20 mA/cm2, the CIE coordinates of device CellMA, and CellMB are of (0.160, 0.252) and (0.157, 0.187), respectively. On the other hand, the device Cell3A and Cell3B show CIE coordinates of (0.204, 0.331) and (0.178, 0.304), respectively. We observed that CIE coordinate deteriorates due to the existence of the Alq3 ETL layer and becomes worse due to insertion of MoO3 as a buffer layer. The better performance of device CellMB (or Cell3B) could be attributed to better hole blocking capability of BPhen as compared to Alq3 based devices. Fig. 5 shows the proposed energy level diagram of the devices studied, with the relative alignment of the HOMO and LUMO levels of each layer. The relative work function of ITO/MoO3 was measured by using contact potential difference measurements re-
Fig. 4. Normalized EL spectra of (a) [CellMA, CellMB] and (b) [Cell3A and Cell3B].
Table 1 Performance of four blue OLEDs with different buffer layers and ETL. V20, L20, gA20, gW20 and CIE20 are the driving voltage, luminance, current efficiency, power efficiency and CIE coordinates at the current density of 20 mA/cm2, respectively. V400, and L400 are the driving voltage and luminance at the current density of 400 mA/ cm2, respectively.
Fig. 5. Energy level diagram of OLEDs studied.
V400 (V)
V20 (V)
L400 (cd/ m2)
L20 (cd/ m2)
gA20
gW20
(cd/ A)
(lm/ w)
CellMA
14.1
9.8
26 655
1403
7.3
2.3
CellMB
12.8
7.8
14 842
1265
6.3
2.4
Cell3A
11.5
7.3
14 629
1206
6.0
2.6
5.2
24 478
1364
6.7
4.2
Devices
Cell3B
9.42
CIE20 (x, y) (0.160, 0.252) (0.157, 0.187) (0.204, 0.331) (0.178, 0.304)
ported by us [15]. The work function of MoO3 is 5.43 eV, which is very close to the HOMO level of NPB (5.4 eV), while the HOMO level of m-MTDATA was reported to be 5.11 eV. Thus, a very small hole injection barrier between MoO3 and NPB leads to a strong hole injection from MoO3 to NPB. On the other hand, the LUMO level of Alq3 is 2.8 eV, which is 0.2 eV higher than that of BPhen. For the study of carrier injection and balance, a series of holeonly devices in which the current consists mainly of the hole-only and electron-only devices in which the current consists mainly of the electrons are fabricated. These devices have the following structures: EAlq3: ITO/Alq3 (30 nm)/LiF (0.8 nm)/Al (100 nm) EBPhen: ITO/BPhen (30 nm)/LiF (0.8 nm)/Al (100 nm)
Fig. 3. Power efficiency versus current density curves of the devices. The inset shows the current efficiency versus current density characteristics.
Fig. 6. Current density versus voltage characteristics of the hole-only devices based on MoO3 and m-MTDATA (a) and electron-only devices based on Alq3 and BPhen (b).
Khizar-ul-Haq et al. / Journal of Non-Crystalline Solids 356 (2010) 1012–1015
HMT: ITO/m-MTDATA (20 nm)/NPB (10 nm)/Al (100 nm) HM3: ITO/MoO3 (20 nm)/NPB (10 nm)/Al (100 nm). Fig. 6 shows the current density versus voltage characteristics of hole-only and electron-only devices. For hole-only devices, at the driving voltage of 6 V, the current density of HM3 is 80.46 mA/cm2, much larger than 20.5 mA/cm2 for HMT. For electron-only devices, the current density of device EBPhen is 20.6 mA/cm2, which is larger than 4.12 mA/cm2 for EAlq3, at the same driving voltage, respectively. It indicates that the conductibility of BPhen is stronger than that of Alq3, and the hole injection ability of MoO3 is stronger than that of m-MTDATA, resulting in low driving voltage. 4. Discussion The blue OLED with MoO3 as a buffer layer presented significant performance. The improved performances of the Cell3B are attributed to high hole injection ability of MoO3 as a buffer layer, the high electron mobility of BPhen as an ETL. Moreover, the electron mobility of BPhen has been estimated by our group using space charge limited current to be between (3.9–5.2) 104 cm2/V s [16], about 200 times more than that of Alq3. Thus, BPhen as an ETL has a stronger electron injection ability, as well as more chance to travel into EML and recombine with hole carriers. It is well known, the power efficiency depends on carrier injection and transport, while current efficiency not only relies on the carrier injection, but also on the carrier balance. From Fig. 6, we can see that the current density of HMT and EAlq3 is close at a low driving voltage, that is to say the electrons and holes have a better balance compared with other devices. This is why device CellMA has the highest current efficiency of the four devices. From Fig. 3 we can see that the current efficiency of device Cell3B is bit lower than CellMA, though it has the best power efficiency. So, not only does the MoO3 have a strong hole injection ability resulting in lowering of the driving voltage, but also its hole injection ability is also much larger than the electron injection ability of ETL. This excess of hole injection results in an imbalance of electrons and holes, which leads to a lower current efficiency. We also observed that emission from device CellMA (and/or Cell3A) is sky blue with the incorporation of Alq3 as an ETL. And it is well established that Alq3 is a strong green emitter, the color purity of blue emission suffers from this green light that is generated by the exciton diffusion from EML into ETL or by direct electron–hole recombination inside the ETL. Hence, a small amount of unwelcoming light emission from Alq3 [17] was unavoidable and may be mixed with blue emission. When HIL of m-MTDATA is replaced by MoO3, the EL spectrum of device Cell3A is broader at longer wavelength (520 nm) [17,18] and CIE coordinates changes due to large amount of emission from Alq3 layer. It is interesting to note that the EL spectrum of device Cell3B is less broad as compared to device Cell3A. The broad spectrum is likely appeared as the hole injection is increased due to MoO3 buffer layer. The change in emission can be attributed to poor hole blocking ability of Alq3 and may be due to the formation of somewhat aggregates (or chromophores). And an aggregate should provide the additional emission (or absorption). Additionally, the relatively wide half
1015
widths (in case of MoO3 based devices) may indicate on the substantial role of the electron-vibration broadening of line and several contributions of the inter-conformation states. Because different confirmation states substantially change the values of the potential barriers determining the processes of the charge carriers transport including the exciton states [19]. 5. Conclusions In conclusion, an organic light-emitting diodes with MoO3 as buffer layer and BPhen as an ETL, the power efficiency of which is improved by 44% to 4.2 lm/W as compared to the corresponding devices with different ETL and HIL layers. Its driving voltage, at 20 mA/cm2 is 5.2 V, which is independently reduced by 48% than that of the m-MTDATA//Alq3 based device. The superior performances of the MoO3//BPhen based device are attributed to the high hole injection ability of MoO3 and high electron mobility of BPhen, which leads to high power efficiency and low driving voltage. From the energy level diagram of the devices studied, we know a small barrier exists between MoO3 and NPB, which leads to a low driving voltage. A better balance of electrons and holes can contribute to a good current efficiency for the device. Acknowledgments The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (90201034, 60477014, 60577041, 60777018 and 60776040), 973 project (2002CB613400), International Cooperation Foundation of Shanghai Science and Technology Committee (06DZ22013), and Innovation Fund of Shanghai University (A10-0109-07-015). References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] H.K. Lee, J.H. Seo, J.H. Kim, J.R. Koo, K.H. Lee, S.S. Yoon, Y.K. Kim, J. Korean Phys. Soc. 49 (2006) 1052. [3] G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, J. Salbeck, Appl. Phys. Lett. 85 (2004) 3911. [4] R. Meerheim, K. Walzer, M. Pfeiffer, K. Leo, Appl. Phys. Lett. 89 (2006) 061111. [5] D.B. Romero, M. Schaer, L. Zuppiroli, B. Cesar, B. Francois, Appl. Phys. Lett. 67 (1995) 1659. [6] F. Huang, A.G. MacDiarmid, B.R. Hsieh, Appl. Phys. Lett. 71 (1997) 2415. [7] J. Blochwitz, M. Pfeiffer, T. Fritz, K. Leo, Appl. Phys. Lett. 73 (1998) 729. [8] J. Kido, T. Matsumoto, Appl. Phys. Lett. 73 (1998) 2866. [9] A. Yamamori, C. Adachi, T. Koyama, Y. Taniguchi, Appl. Phys. Lett. 72 (1998) 472147. [10] A. Yamamori, C. Adachi, T. Koyama, Y. Taniguchi, J. Appl. Phys. 86 (1999) 4369. [11] V.-E. Choong, S. Shi, J. Curless, F. So, Appl. Phys. Lett. 76 (2000) 958. [12] C. Ganzorig, M. Fujihara, Appl. Phys. Lett. 77 (2000) 4211. [13] M. Pfeiffer, A. Beyer, T. Fritz, K. Leo, Appl. Phys. Lett. 73 (1998) 3202. [14] J. Blochwitz, T. Fritz, M. Pfeiffer, K. Leo, D.M. Alloway, P.A. Lee, N.R. Armstrong, Org. Electron. 2 (2001) 97. [15] Z.L. Zhang, in: S. Miyata, H.S. Nalwa (Eds.), Organic Electrolumies Materials and Devices, Gordon and Breach, New York, 1997, p. 209 (Chapter 5). [16] M.A. Khan, Wei Xu, K.U. Haq, Y. Bai, X. Y Jiang, Z.L. Zhang, W. Q Zhu, J. Appl. Phys. 103 (2008) 014509. [17] J.M. Shi, C.W. Tang, Appl. Phys. Lett. 80 (2002) 3201. [18] S.W. Culligan, A.C.-A. Chen, J.U. Wallace, K.P. Klubek, C.W. Tang, S.H. Chen, Adv. Mater. 16 (2006) 1481. [19] E. Gondek, J. Niziol, A. Danel, I.V. Kityk, M. Pokladko, J. Sanetra, E. Kulig, J. Luminesc. 128 (2008) 1831.