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Transparent Al-doped ZnO anodes in organic light-emitting diodes investigated using a hole-only device
Applied Surface Science 261 (2012) 360–363
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Transparent Al-doped ZnO anodes in organic light-emitting diodes investigated using a hole-only device Zong-Liang Tseng a , Po-Ching Kao c , Chi-Shin Yang a , Yung-Der Juang d,e , Sheng-Yuan Chu a,b,∗ a
Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan Department of Electrophysics, National Chiayi University, Chiayi 600-83, Taiwan d Department of Material Science, National University of Tainan, Tainan 70005, Taiwan e Department of Greenergy, National University of Tainan, Tainan 70005, Taiwan b c
a r t i c l e
i n f o
Article history: Received 6 May 2011 Received in revised form 9 July 2012 Accepted 5 August 2012 Available online 10 August 2012 Keywords: ZnO OLED AZO TCO
a b s t r a c t Al-doped ZnO (AZO) films with a thickness of ∼400 nm were prepared by sputtering on glass substrates for use as transparent anodes of organic light-emitting diodes (OLED) devices. The operation voltages (at 100 cd/m2 ) of OLED devices with AZO and ITO anode materials were 10.5 and 5.5 V, respectively. The maximum luminance output of the AZO device was 6450 cd/m2 (achieved at 12.5 V) and that of the ITO device was 9830 cd/m2 (achieved at 10.5 V). We demonstrate that a hole-only device method can be used to estimate the suitability of AZO and ITO anodes in the OLED devices and to verify experimental results. The AZO thin films with low price and non-toxicity may be suitable as alternative anodes in OLED devices under high voltage. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) have attracted a lot of interest for use in next generation flat-panel displays and lighting applications [1]. A transparent conductive oxide (TCO) layer is essential in OLEDs in order to inject charges and extract light. Among TCOs, indium tin oxide (ITO) is widely used as the transparent anode of OLEDs due to its high conductivity and transparency. However, indium is an expensive element and the diffusion of indium into the organic layer degrades OLED performance [2]. Therefore, alternative anodes have been proposed for OLEDs [3–5], such as ZnO. ZnO is a wide band gap semiconductor that is considered as a promising material for TCOs, mainly due to its low cost, abundance, and non-toxicity. Low-resistance Al-doped ZnO (AZO) thin films have excellent optical properties and are considered an alternative anode for OLEDs [4,6–10]. Although the properties of AZO are well understood, the hole transportation at the interface between the AZO anode and the organic layer has not been fully investigated. The mechanism responsible for carrier injections from TCO anodes
∗ Corresponding author at: Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan. Tel.: +886 6 275 7575x62381; fax: +886 6 234 5482. E-mail address: [email protected] (S.-Y. Chu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.014
into organic hole transport materials is of great importance for the development of alternative anodes for OLEDs. Hole-only devices are widely used to study hole injection characteristics. They have been used to determine the hole mobility in the bulk of polymer [11] or organic small molecular materials [12], the efficiency of hole-injection and hole-transportation materials [12,13], and the doping effect on hole injecting in organic materials [12,14]. However, the hole-only device method for alternative TCO anode materials in OLED devices has never been reported. In the present study, hole-only devices are used to explore charge behavior at the interface between anode materials and the organic layer. Hole-only devices with AZO film and commercial ITO anodes were used to examine the efficiency of hole injection and its effect on performance of OLEDs. OLED devices with AZO and ITO anode materials, respectively, were also fabricated to verify experimental results. The dependence of hole injection and OLED performance on the anode material was determined.
2. Experiment A conventional radio frequency (RF) magnetron sputtering method was used to prepare AZO thin films on the commercial alkali-free glass substrates (Litefilm Tech. Co., Ltd., Taiwan). Sputtering was carried out in a pure argon gas atmosphere with 3-in. 2 wt% AZO ceramic target. The work pressure, Ar flow rate, and RF power were maintained at 3 mTorr, 4 sccm, and 60 W, respectively.
Z.-L. Tseng et al. / Applied Surface Science 261 (2012) 360–363
361
ITO AZO
100
Transmittance (%)
80
60
40
20
0 200
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. X-ray diffraction patterns of AZO films sputtered on a glass substrate. The inset shows a cross-sectional SEM image of an AZO film with a thickness of about 400 nm.
The substrate holder had no heat treatment. The deposition rate of the AZO films was about 3.9 nm/min. After the AZO thin films were deposited, OLED devices and hole-only devices (2.5 mm × 2.5 mm and 0.5 mm × 0.5 mm, respectively) were formed by thermal evaporation. The base pressure for sputtering and thermal evaporation was lower than 5 × 10−6 mTorr for all depositions. The current-density–luminance–voltage (J–L–V) characteristics and the current efficiency of the OLED devices were measured using a source meter (Keithley-2400) and a luminance meter (LS-100). The hole-only devices were measured using a semiconductor parameter analyzer (Agilent-4155C) and a probe station. The thickness, structural properties, surface roughness, optical transmittance, and electrical properties of TCOs were measured using scanning electron microscopy (SEM) cross-section images, X-ray diffraction (XRD), atomic force microscopy (AFM), a UV-visible spectrophotometer, and Hall measurements, respectively. All spectra were measured at room temperature. 3. Results and discussion Fig. 1 shows the XRD pattern of the sputtered AZO films on a glass substrate. Only hexagonal ZnO (0 0 2) is indexed in the XRD pattern, clearly indicating that the sputtered AZO film was highly caxis oriented. The inset in Fig. 1 shows a cross-sectional SEM image of the grown AZO on a glass substrate. It can be seen that the AZO film was polycrystalline in nature and about 400 nm thick. The inset again confirms the oriented growth of the sputtered AZO film. The Hall measurement results show that the ZnO film was n-type with an electrical resistivity of 4.39 × 10−4 cm, an electron concentration of 6.96 × 1020 cm−3 , and a mobility of 20.46 cm2 V−1 s−1 . Fig. 2 shows the optical transmittance spectra for AZO and ITO films. The film thicknesses of AZO and ITO were about 400 nm and 200 nm, respectively. The average transmittance values in the visible range (400–700 nm) for AZO and ITO films were 92.87% and 90.37%, respectively. The AZO films showed high transmittance and qualification for the OLED application. The absorption coefficient (˛) was determined using T = e−˛t , where t is the film thickness and T is the transmittance of the film. The optical band gap (Eg ) was determined by fitting the linear regions of the square of the absorption coefficient (˛2 ) versus the photon energy (h) [15], as shown in the inset of Fig. 2. The optical band gap values for AZO and ITO films were 3.80 eV and 3.99 eV, respectively.
Fig. 2. Optical transmittance of AZO and ITO films as a function of wavelength. The inset shows the relationship of ˛2 and photon energy for the AZO and ITO films.
The performance of TOC films can be calculated using the figure of merit [16] ˚TC = T10 /R, where T is the transmittance and R is the sheet resistance. The figure of merit is useful for evaluating the electroptical properties of TCO films. The figures of merit for the AZO films and the commercial ITO were 32.98 and 36.11 (10−3 −1 ), respectively, where the sheet resistances of the AZO films and the commercial ITO were 10.98 and 13.21 (), respectively. This indicates that the electroptical properties of the AZO films are comparable to those of ITO films and suitable for transparent electrodes. To investigate AZO films as anodes in OLEDs, OLED devices with AZO films and commercial ITO, respectively, were fabricated for a performance comparison study. The OLED devices fabricated in this study had the following configurations: glass/AZO or ITO/-naphthylphenylbiphenyl diamine (NPB) (40 nm)/tris(8hydroxyquinoline) aluminum (Alq3) (40 nm)/lithium fluoride (LiF) (1 nm)/Al (150 nm), as shown in the inset of Fig. 3(a). Fig. 3(a) shows the current-density–voltage (J–V) characteristics of OLED devices based on AZO and commercial ITO, respectively. As can be seen, both samples exhibited typical diode-like behavior. Under a constant current, the voltages in the ITO device were clearly lower than those of the AZO devices. Fig. 3(b) shows the luminance–voltage (L–V) characteristics of OLED devices based on the AZO with and commercial ITO, respectively. The turn-on voltages (voltage at 1 cd/m2 ) were obtained at 4.5 V and 3.0 V for devices using AZO and ITO as anodes, respectively, and the operation voltages at 100 cd/m2 were 10.5 V and 5.5 V, respectively. The ITO-based device showed the lower operating voltage. The maximum luminance output of the AZO device was 6450 cd/m2 (achieved at 12.5 V) and that of the ITO device was 9830 cd/m2 (achieved at 10.5 V). Fig. 3(c) shows the current efficiency of the AZO and commercial ITO devices. The ITO device exhibited a higher luminance, higher current density, and better current efficiency than those of the AZO device under a low applied voltage. However, the AZO anode showed good performance under a high applied voltage, indicating that AZO films are suitable as anodes of OLED devices under a high applied voltage. The operation voltages and current efficiencies of the OLED devices based AZO films and commercial ITO are summarized in Table 1. The hole-only devices under investigation consist of an NPB layer (40 nm) was sandwiched between two electrodes on a glass substrate. An AZO or ITO bottom electrode was used as a hole injector. MoO3 (10 nm) and Al (150 nm) were used as the top electrode, as shown in the inset of Fig. 4(a). The work function of MoO3
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Z.-L. Tseng et al. / Applied Surface Science 261 (2012) 360–363
Table 1 Compared to the characteristics of OLEDs based on AZOs and commercial ITO anodes. The conductivity and optical properties of the both films are also collected. TCO materials
Thickness (nm)
Sheet resistance ()
Figure of merit (10−3 −1 )
Average transmittance 400 ∼700 nm (%)
Turn-on voltage (V)
Voltage at 100 cd/m2 (V)
Current efficiency at 100 cd/m2 (cd/A)
AZO ITO
∼400 ∼200
13.21 10.98
32.98 36.11
92.87 90.37
3.0 4.5
10.5 5.5
1.79 1.85
Fig. 4. Current-density–voltage (J–V) characteristics of hole-only devices with AZO and commercial ITO films as anodes, respectively. The inset shows the architecture of the hole-only devices.
is 5.43 eV [17,18], which is very close to the HOMO level of NPB (5.4 eV). The difference between the LUMO level of NPB and the work function of MoO3 is thus very large (3.13 eV). Few electrons can pass through the barrier between MoO3 and NPB leading to a weak electron injection from the cathode to the NPB layer. Thus, only holes can inject into the NPB and electrons were blocked. Fig. 4 shows the-current density–voltage (J–V) characteristics of the hole-only devices based on AZO and commercial ITO films, respectively. As can be seen, both samples still exhibited good diode-like behavior. Fig. 5 shows the log–log plot of Fig. 4. As can be seen, the plot is well fitted by two linear segments with different slopes. The slope of the first linear segment is close to 1 for both devices, indicating that the ohmic conduction characteristic of J ∝ V is the dominant conduction mechanism under a low bias. A further increase in the bias gives rise to a steep increase in the current density of both devices. The abrupt increase in the
Fig. 3. (a) Current-density–voltage (J–V), (b) luminance–voltage (L–V), and (c) current efficiency characteristics of OLED devices based on AZO and commercial ITO anodes, respectively. The inset of (a) shows the architecture of the OLED devices. Fig. 5. Log–log plot of the I–V curves of the hole-only devices with AZO and commercial ITO films as anodes, respectively.
Z.-L. Tseng et al. / Applied Surface Science 261 (2012) 360–363
current density is shown by the slops of 2.605 and 3.330 in Fig. 5, demonstrating trap-controlled space charge limited conduction [19] (TC-SCLC). Interestingly, the slope of the ITO device is larger value than that of the AZO device in the TC-SCLC region. This indicates that more interfacial trap states exist at the ITO/NPB interface. This should be due to the contribution of the indium diffusion [2]. On the other hand, observing the voltage switching from the Ohmic conduction to TC-SCLC, The abrupt switching is shown by the voltage of 1.35 and 2.05 V in Fig. 5. It means the ITO device more easily reached the TC-SCLC mode than that of the AZO device. Different switching under positive bias demonstrated different energy barrier conditions at the TCO/NPB interface. This can explain by the different work functions of ITO and AZO films. From the inset of Fig. 5, the magnitude of the work function of ITO film (ITO ) is larger than that of AZO film (AZO ), resulting in a larger barrier height for AZO anodes. The experimental data is in good agreement with the previous study [4], which considers that AZO films have a lower work function than ITO. This should be the reason that the OLED device based on an AZO anode (Fig. 3) had a higher turn-on voltage and was suitable for operation under a high voltage. 4. Conclusion In conclusion, AZO films with a thickness of ∼400 nm were prepared by sputtering on glass substrates for use as transparent anodes of OLED devices. Both OLED devices with AZO and ITO anode materials were fabricated. The ITO device exhibited a higher luminance, higher current density, and better current efficiency than those of the AZO device under a low applied voltage. However, the AZO anode showed good performance under a high applied voltage. Hole-only devices were used to estimate the suitability of both TCO anodes in OLED device. Experimental data from the hole-only devices may explain the diffusion behavior of indium into
363
the organic layer and the magnitude of the work function of TCO films. The results of hole-only devices should be useful for further development of alternative anodes for OLED devices. Acknowledgments We would like to acknowledge the financial support provided by the National Science Council of the Republic of China under grants NSC 98-2622-E-006-044-CC3 and 98-2112-M-415-001-MY3. References [1] J. Shinar, Organic Light-Emitting Devices, Springer-Verlag, New York, 2004. [2] S.T. Lee, Z.Q. Gao, L.S. Hung, Applied Physics Letters 75 (1999) 1404. [3] A. Andersson, N. Johansson, P. Bröms, N. Yu, D. Lupo, W.R. Salaneck, Advanced Materials 10 (1998) 859. [4] H. Kim, C.M. Gilmore, J.S. Horwitz, A. Pique, H. Murata, G.P. Kushto, R. Schlaf, Z.H. Kafafi, D.B. Chrisey, Applied Physics Letters 76 (2000) 259. [5] X. Jiang, F.L. Wong, M.K. Fung, S.T. Lee, Applied Physics Letters 83 (2003) 1875. [6] H. Kim, A. Piqu’e, J.S. Horwitz, H. Murata, Z.H. Kafafi, C.M. Gilmore, D.B. Chrisey, Thin Solid Films 377 (2000) 798. [7] X.T. Hao, L.W. Tan, K.S. Ong, F. Zhu, Journal of Crystal Growth 287 (2006) 44. [8] T.W. Kim, D.C. Choo, Y.S. No, W.K. Choi, E.H. Choi, Applied Surface Science 253 (2006) 1917. [9] X.T. Hao, F.R. Zhu, K.S. Ong, L.W. Tan, Semiconductor Science and Technology 21 (2006) 48. [10] H. Kim, J.S. Horwitz, W.H. Kim, A.J. Makinen, Z.H. Kafafi, D.B. Chrisey, Thin Solid Films 420 (2002) 539. [11] P.W.M. Blom, M.J.M. de Jong, M.G. van Munster, Physical Review B 55 (1997) R656. [12] T.Y. Chu, O.K. Songa, Applied Physics Letters 90 (2007) 203512. [13] C.H. Liao, M.T. Lee, C.H. Tsai, C.H. Chen, Applied Physics Letters 86 (2005) 203507. [14] K.S. Yook, S.O. Jeon, J.Y. Lee, Thin Solid Films 517 (2009) 6109. [15] V. Khranovskyy, U. Grossner, V. Lazorenko, G.V. Lashkarev, B.G. Svensson, R. Yakimova, Superlattices and Microstructures 39 (2006) 275. [16] G. Haacke, Journal of Applied Physics 47 (1976) 4086. [17] T. Matsushima, Y. Kinoshita, H. Murata, Applied Physics Letters 91 (2007) 253504. [18] J.W. Ma, Z. Liang, C. Jin, X.Y. Jiang, Z.L. Zhang, Solid State Communications 149 (2009) 214. [19] M.A. Lampert, P. Mark, Current Injection in Solids, Academic, New York, 1970.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Transparent Al-doped ZnO anodes in organic light-emitting diodes investigated using a hole-only device Zong-Liang Tseng a , Po-Ching Kao c , Chi-Shin Yang a , Yung-Der Juang d,e , Sheng-Yuan Chu a,b,∗ a
Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan Department of Electrophysics, National Chiayi University, Chiayi 600-83, Taiwan d Department of Material Science, National University of Tainan, Tainan 70005, Taiwan e Department of Greenergy, National University of Tainan, Tainan 70005, Taiwan b c
a r t i c l e
i n f o
Article history: Received 6 May 2011 Received in revised form 9 July 2012 Accepted 5 August 2012 Available online 10 August 2012 Keywords: ZnO OLED AZO TCO
a b s t r a c t Al-doped ZnO (AZO) films with a thickness of ∼400 nm were prepared by sputtering on glass substrates for use as transparent anodes of organic light-emitting diodes (OLED) devices. The operation voltages (at 100 cd/m2 ) of OLED devices with AZO and ITO anode materials were 10.5 and 5.5 V, respectively. The maximum luminance output of the AZO device was 6450 cd/m2 (achieved at 12.5 V) and that of the ITO device was 9830 cd/m2 (achieved at 10.5 V). We demonstrate that a hole-only device method can be used to estimate the suitability of AZO and ITO anodes in the OLED devices and to verify experimental results. The AZO thin films with low price and non-toxicity may be suitable as alternative anodes in OLED devices under high voltage. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) have attracted a lot of interest for use in next generation flat-panel displays and lighting applications [1]. A transparent conductive oxide (TCO) layer is essential in OLEDs in order to inject charges and extract light. Among TCOs, indium tin oxide (ITO) is widely used as the transparent anode of OLEDs due to its high conductivity and transparency. However, indium is an expensive element and the diffusion of indium into the organic layer degrades OLED performance [2]. Therefore, alternative anodes have been proposed for OLEDs [3–5], such as ZnO. ZnO is a wide band gap semiconductor that is considered as a promising material for TCOs, mainly due to its low cost, abundance, and non-toxicity. Low-resistance Al-doped ZnO (AZO) thin films have excellent optical properties and are considered an alternative anode for OLEDs [4,6–10]. Although the properties of AZO are well understood, the hole transportation at the interface between the AZO anode and the organic layer has not been fully investigated. The mechanism responsible for carrier injections from TCO anodes
∗ Corresponding author at: Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan. Tel.: +886 6 275 7575x62381; fax: +886 6 234 5482. E-mail address: [email protected] (S.-Y. Chu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.014
into organic hole transport materials is of great importance for the development of alternative anodes for OLEDs. Hole-only devices are widely used to study hole injection characteristics. They have been used to determine the hole mobility in the bulk of polymer [11] or organic small molecular materials [12], the efficiency of hole-injection and hole-transportation materials [12,13], and the doping effect on hole injecting in organic materials [12,14]. However, the hole-only device method for alternative TCO anode materials in OLED devices has never been reported. In the present study, hole-only devices are used to explore charge behavior at the interface between anode materials and the organic layer. Hole-only devices with AZO film and commercial ITO anodes were used to examine the efficiency of hole injection and its effect on performance of OLEDs. OLED devices with AZO and ITO anode materials, respectively, were also fabricated to verify experimental results. The dependence of hole injection and OLED performance on the anode material was determined.
2. Experiment A conventional radio frequency (RF) magnetron sputtering method was used to prepare AZO thin films on the commercial alkali-free glass substrates (Litefilm Tech. Co., Ltd., Taiwan). Sputtering was carried out in a pure argon gas atmosphere with 3-in. 2 wt% AZO ceramic target. The work pressure, Ar flow rate, and RF power were maintained at 3 mTorr, 4 sccm, and 60 W, respectively.
Z.-L. Tseng et al. / Applied Surface Science 261 (2012) 360–363
361
ITO AZO
100
Transmittance (%)
80
60
40
20
0 200
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. X-ray diffraction patterns of AZO films sputtered on a glass substrate. The inset shows a cross-sectional SEM image of an AZO film with a thickness of about 400 nm.
The substrate holder had no heat treatment. The deposition rate of the AZO films was about 3.9 nm/min. After the AZO thin films were deposited, OLED devices and hole-only devices (2.5 mm × 2.5 mm and 0.5 mm × 0.5 mm, respectively) were formed by thermal evaporation. The base pressure for sputtering and thermal evaporation was lower than 5 × 10−6 mTorr for all depositions. The current-density–luminance–voltage (J–L–V) characteristics and the current efficiency of the OLED devices were measured using a source meter (Keithley-2400) and a luminance meter (LS-100). The hole-only devices were measured using a semiconductor parameter analyzer (Agilent-4155C) and a probe station. The thickness, structural properties, surface roughness, optical transmittance, and electrical properties of TCOs were measured using scanning electron microscopy (SEM) cross-section images, X-ray diffraction (XRD), atomic force microscopy (AFM), a UV-visible spectrophotometer, and Hall measurements, respectively. All spectra were measured at room temperature. 3. Results and discussion Fig. 1 shows the XRD pattern of the sputtered AZO films on a glass substrate. Only hexagonal ZnO (0 0 2) is indexed in the XRD pattern, clearly indicating that the sputtered AZO film was highly caxis oriented. The inset in Fig. 1 shows a cross-sectional SEM image of the grown AZO on a glass substrate. It can be seen that the AZO film was polycrystalline in nature and about 400 nm thick. The inset again confirms the oriented growth of the sputtered AZO film. The Hall measurement results show that the ZnO film was n-type with an electrical resistivity of 4.39 × 10−4 cm, an electron concentration of 6.96 × 1020 cm−3 , and a mobility of 20.46 cm2 V−1 s−1 . Fig. 2 shows the optical transmittance spectra for AZO and ITO films. The film thicknesses of AZO and ITO were about 400 nm and 200 nm, respectively. The average transmittance values in the visible range (400–700 nm) for AZO and ITO films were 92.87% and 90.37%, respectively. The AZO films showed high transmittance and qualification for the OLED application. The absorption coefficient (˛) was determined using T = e−˛t , where t is the film thickness and T is the transmittance of the film. The optical band gap (Eg ) was determined by fitting the linear regions of the square of the absorption coefficient (˛2 ) versus the photon energy (h) [15], as shown in the inset of Fig. 2. The optical band gap values for AZO and ITO films were 3.80 eV and 3.99 eV, respectively.
Fig. 2. Optical transmittance of AZO and ITO films as a function of wavelength. The inset shows the relationship of ˛2 and photon energy for the AZO and ITO films.
The performance of TOC films can be calculated using the figure of merit [16] ˚TC = T10 /R, where T is the transmittance and R is the sheet resistance. The figure of merit is useful for evaluating the electroptical properties of TCO films. The figures of merit for the AZO films and the commercial ITO were 32.98 and 36.11 (10−3 −1 ), respectively, where the sheet resistances of the AZO films and the commercial ITO were 10.98 and 13.21 (), respectively. This indicates that the electroptical properties of the AZO films are comparable to those of ITO films and suitable for transparent electrodes. To investigate AZO films as anodes in OLEDs, OLED devices with AZO films and commercial ITO, respectively, were fabricated for a performance comparison study. The OLED devices fabricated in this study had the following configurations: glass/AZO or ITO/-naphthylphenylbiphenyl diamine (NPB) (40 nm)/tris(8hydroxyquinoline) aluminum (Alq3) (40 nm)/lithium fluoride (LiF) (1 nm)/Al (150 nm), as shown in the inset of Fig. 3(a). Fig. 3(a) shows the current-density–voltage (J–V) characteristics of OLED devices based on AZO and commercial ITO, respectively. As can be seen, both samples exhibited typical diode-like behavior. Under a constant current, the voltages in the ITO device were clearly lower than those of the AZO devices. Fig. 3(b) shows the luminance–voltage (L–V) characteristics of OLED devices based on the AZO with and commercial ITO, respectively. The turn-on voltages (voltage at 1 cd/m2 ) were obtained at 4.5 V and 3.0 V for devices using AZO and ITO as anodes, respectively, and the operation voltages at 100 cd/m2 were 10.5 V and 5.5 V, respectively. The ITO-based device showed the lower operating voltage. The maximum luminance output of the AZO device was 6450 cd/m2 (achieved at 12.5 V) and that of the ITO device was 9830 cd/m2 (achieved at 10.5 V). Fig. 3(c) shows the current efficiency of the AZO and commercial ITO devices. The ITO device exhibited a higher luminance, higher current density, and better current efficiency than those of the AZO device under a low applied voltage. However, the AZO anode showed good performance under a high applied voltage, indicating that AZO films are suitable as anodes of OLED devices under a high applied voltage. The operation voltages and current efficiencies of the OLED devices based AZO films and commercial ITO are summarized in Table 1. The hole-only devices under investigation consist of an NPB layer (40 nm) was sandwiched between two electrodes on a glass substrate. An AZO or ITO bottom electrode was used as a hole injector. MoO3 (10 nm) and Al (150 nm) were used as the top electrode, as shown in the inset of Fig. 4(a). The work function of MoO3
362
Z.-L. Tseng et al. / Applied Surface Science 261 (2012) 360–363
Table 1 Compared to the characteristics of OLEDs based on AZOs and commercial ITO anodes. The conductivity and optical properties of the both films are also collected. TCO materials
Thickness (nm)
Sheet resistance ()
Figure of merit (10−3 −1 )
Average transmittance 400 ∼700 nm (%)
Turn-on voltage (V)
Voltage at 100 cd/m2 (V)
Current efficiency at 100 cd/m2 (cd/A)
AZO ITO
∼400 ∼200
13.21 10.98
32.98 36.11
92.87 90.37
3.0 4.5
10.5 5.5
1.79 1.85
Fig. 4. Current-density–voltage (J–V) characteristics of hole-only devices with AZO and commercial ITO films as anodes, respectively. The inset shows the architecture of the hole-only devices.
is 5.43 eV [17,18], which is very close to the HOMO level of NPB (5.4 eV). The difference between the LUMO level of NPB and the work function of MoO3 is thus very large (3.13 eV). Few electrons can pass through the barrier between MoO3 and NPB leading to a weak electron injection from the cathode to the NPB layer. Thus, only holes can inject into the NPB and electrons were blocked. Fig. 4 shows the-current density–voltage (J–V) characteristics of the hole-only devices based on AZO and commercial ITO films, respectively. As can be seen, both samples still exhibited good diode-like behavior. Fig. 5 shows the log–log plot of Fig. 4. As can be seen, the plot is well fitted by two linear segments with different slopes. The slope of the first linear segment is close to 1 for both devices, indicating that the ohmic conduction characteristic of J ∝ V is the dominant conduction mechanism under a low bias. A further increase in the bias gives rise to a steep increase in the current density of both devices. The abrupt increase in the
Fig. 3. (a) Current-density–voltage (J–V), (b) luminance–voltage (L–V), and (c) current efficiency characteristics of OLED devices based on AZO and commercial ITO anodes, respectively. The inset of (a) shows the architecture of the OLED devices. Fig. 5. Log–log plot of the I–V curves of the hole-only devices with AZO and commercial ITO films as anodes, respectively.
Z.-L. Tseng et al. / Applied Surface Science 261 (2012) 360–363
current density is shown by the slops of 2.605 and 3.330 in Fig. 5, demonstrating trap-controlled space charge limited conduction [19] (TC-SCLC). Interestingly, the slope of the ITO device is larger value than that of the AZO device in the TC-SCLC region. This indicates that more interfacial trap states exist at the ITO/NPB interface. This should be due to the contribution of the indium diffusion [2]. On the other hand, observing the voltage switching from the Ohmic conduction to TC-SCLC, The abrupt switching is shown by the voltage of 1.35 and 2.05 V in Fig. 5. It means the ITO device more easily reached the TC-SCLC mode than that of the AZO device. Different switching under positive bias demonstrated different energy barrier conditions at the TCO/NPB interface. This can explain by the different work functions of ITO and AZO films. From the inset of Fig. 5, the magnitude of the work function of ITO film (ITO ) is larger than that of AZO film (AZO ), resulting in a larger barrier height for AZO anodes. The experimental data is in good agreement with the previous study [4], which considers that AZO films have a lower work function than ITO. This should be the reason that the OLED device based on an AZO anode (Fig. 3) had a higher turn-on voltage and was suitable for operation under a high voltage. 4. Conclusion In conclusion, AZO films with a thickness of ∼400 nm were prepared by sputtering on glass substrates for use as transparent anodes of OLED devices. Both OLED devices with AZO and ITO anode materials were fabricated. The ITO device exhibited a higher luminance, higher current density, and better current efficiency than those of the AZO device under a low applied voltage. However, the AZO anode showed good performance under a high applied voltage. Hole-only devices were used to estimate the suitability of both TCO anodes in OLED device. Experimental data from the hole-only devices may explain the diffusion behavior of indium into
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the organic layer and the magnitude of the work function of TCO films. The results of hole-only devices should be useful for further development of alternative anodes for OLED devices. Acknowledgments We would like to acknowledge the financial support provided by the National Science Council of the Republic of China under grants NSC 98-2622-E-006-044-CC3 and 98-2112-M-415-001-MY3. References [1] J. Shinar, Organic Light-Emitting Devices, Springer-Verlag, New York, 2004. [2] S.T. Lee, Z.Q. Gao, L.S. Hung, Applied Physics Letters 75 (1999) 1404. [3] A. Andersson, N. Johansson, P. Bröms, N. Yu, D. Lupo, W.R. Salaneck, Advanced Materials 10 (1998) 859. [4] H. Kim, C.M. Gilmore, J.S. Horwitz, A. Pique, H. Murata, G.P. Kushto, R. Schlaf, Z.H. Kafafi, D.B. Chrisey, Applied Physics Letters 76 (2000) 259. [5] X. Jiang, F.L. Wong, M.K. Fung, S.T. Lee, Applied Physics Letters 83 (2003) 1875. [6] H. Kim, A. Piqu’e, J.S. Horwitz, H. Murata, Z.H. Kafafi, C.M. Gilmore, D.B. Chrisey, Thin Solid Films 377 (2000) 798. [7] X.T. Hao, L.W. Tan, K.S. Ong, F. Zhu, Journal of Crystal Growth 287 (2006) 44. [8] T.W. Kim, D.C. Choo, Y.S. No, W.K. Choi, E.H. Choi, Applied Surface Science 253 (2006) 1917. [9] X.T. Hao, F.R. Zhu, K.S. Ong, L.W. Tan, Semiconductor Science and Technology 21 (2006) 48. [10] H. Kim, J.S. Horwitz, W.H. Kim, A.J. Makinen, Z.H. Kafafi, D.B. Chrisey, Thin Solid Films 420 (2002) 539. [11] P.W.M. Blom, M.J.M. de Jong, M.G. van Munster, Physical Review B 55 (1997) R656. [12] T.Y. Chu, O.K. Songa, Applied Physics Letters 90 (2007) 203512. [13] C.H. Liao, M.T. Lee, C.H. Tsai, C.H. Chen, Applied Physics Letters 86 (2005) 203507. [14] K.S. Yook, S.O. Jeon, J.Y. Lee, Thin Solid Films 517 (2009) 6109. [15] V. Khranovskyy, U. Grossner, V. Lazorenko, G.V. Lashkarev, B.G. Svensson, R. Yakimova, Superlattices and Microstructures 39 (2006) 275. [16] G. Haacke, Journal of Applied Physics 47 (1976) 4086. [17] T. Matsushima, Y. Kinoshita, H. Murata, Applied Physics Letters 91 (2007) 253504. [18] J.W. Ma, Z. Liang, C. Jin, X.Y. Jiang, Z.L. Zhang, Solid State Communications 149 (2009) 214. [19] M.A. Lampert, P. Mark, Current Injection in Solids, Academic, New York, 1970.