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An anode with aluminum doped on zinc oxide thin films for organic light emitting devices
Physics Letters A 346 (2005) 148–152 www.elsevier.com/locate/pla
An anode with aluminum doped on zinc oxide thin films for organic light emitting devices Denghui Xu a , Zhenbo Deng a,∗ , Ying Xu a , Jing Xiao a , Chunjun Liang a , Zhiliang Pei b , Chao Sun b a Institute of Optoelectronic Technology, Key Laboratory of Information Storage and Display, Beijing Jiaotong University,
Beijing 100044, PR China b Institute of Metal Research, Chinese Academy of Science, Shenyang 110016, PR China
Received 14 April 2005; accepted 29 July 2005 Available online 9 August 2005 Communicated by A.R. Bishop
Abstract Doped zinc oxides are attractive alternative materials as transparent conducting electrode because they are nontoxic and inexpensive compared with indium tin oxide (ITO). Transparent conducting aluminum-doped zinc oxide (AZO) thin films have been deposited on glass substrates by DC reactive magnetron sputtering method. Films were deposited at a substrate temperature of 150 ◦ C in 0.03 Pa of oxygen pressure. The electrical and optical properties of the film with the Al-doping amount of 2 wt% in the target were investigated. For the 300-nm thick AZO film deposited using a ZnO target with an Al content of 2 wt%, the lowest electrical resistivity was 4 × 10−4 cm and the average transmission in the visible range 400–700 nm was more than 90%. The AZO film was used as an anode contact to fabricate organic light-emitting diodes. The device performance was measured and the current efficiency of 2.9 cd/A was measured at a current density of 100 mA/cm2 . 2005 Elsevier B.V. All rights reserved. Keywords: Aluminum doped zinc oxide (AZO); Transparent conducting oxide (TCO); Organic light emitting diodes (OLEDs)
1. Introduction Organic light-emitting diodes (OLEDs) based on organic small molecule and polymers have been extensively studied for potential applications, especially in * Corresponding author. Tel: +86 10 51688675.
E-mail address: [email protected] (Z. Deng). 0375-9601/$ – see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2005.07.080
the field of flat-panel displays [1–5]. The basic OLED structure consists of one or two organic films between two electrodes, one of which must be transparent. In conventional OLEDs, the work function of the cathode material must be minimized to increase the electron injection efficiency at the cathode/organic interface and the work function of the anode material must be maximized to increase the hole injection efficiency at the anode/organic interface. A large number of transpar-
D. Xu et al. / Physics Letters A 346 (2005) 148–152
ent conducting oxides (TCO), such as In2 O3 , ZnO, SnO2 , and doped oxides, have been widely studied over the years [6–8]. Of all the TCO films, indium tin oxide (ITO) is the most widely used anode material in OLEDs due to its high transparency (∼ 90% at 550 nm), low resistivity (∼ 2 × 10−4 cm), and relatively high work function (∼ 4.8 eV) [9–11]. And ITO has also been the most commonly selected TCO electrode in flat panel displays including liquid crystal displays and plasma displays due to its combined properties of both high visible transmittance and low electrical resistivity. However, since indium is a rare and expensive element, the cost of ITO films is relatively expensive. Recently, zinc oxide or impurity doped zinc oxide films have been actively investigated as alternate materials to ITO because zinc oxide is a nontoxic, inexpensive and abundant material. It is also chemically stable under hydrogen plasma processes that are commonly used for the production of solar cells [12]. Zinc oxide is an n-type semiconductor with an optical band gap of approximately 3.3 eV at room temperature. In the last decade, ZnO films doped with impurities, such as B, Al, Ga, In and Zr, have been actively studied. Among the ZnO films doped with group III elements (B, Al, Ga, In and Zr), aluminum-doped zinc oxide (AZO) films show the lowest electrical resistivity. AZO films are also wide band gap semiconductors (Eg = 3.4–3.9 eV), which show optical transmission in the visible and near-infrared (IR) regions. A number of papers have reported improvements to the electrical conductivity of the AZO thin films grown by conventional vapor deposition techniques such as chemical vapor deposition (CVD) and sputtering in addition to pulsed-laser deposition (PLD). In this Letter, we report a study of the electrical and optical properties of Al doped ZnO film grown by DC reactive magnetron sputtering method on glass substrate. The Al doped ZnO film was used as anode material for OLED devices and the device efficiency was compared with the control device fabricated on commercial ITO. 2. Experimental 2.1. AZO film fabrication and characterization The AZO thin films were produced by a conventional DC reactive magnetron sputtering system from
149
an alloy target. Details of deposition are described elsewhere [13,14]. Based on our experiments, the optimized weight ratio of Al in the Al-doped ZnO (AZO) target is 2.0 percent. Optical grade glasses were used as substrates and ultrasonically cleaned by isopropyl alcohol and blown dry in N2 gas before they were introduced into the chamber. Before each run, the target was pre-sputtered in a pure argon atmosphere for 3 min in order to remove the natural surface oxide layer of the target. The substrate temperature was 150 ◦ C. High purity argon and oxygen were the process gas and reactive gas, respectively. The oxygen partial pressure was 3.0 × 10−2 Pa. The argon and oxygen gases were introduced through a mass flow controller after the vacuum chamber was evacuated to below 3 × 10−3 Pa. The sheet resistance (Rs ) of the film was measured with a four-point probe. By assuming that the thickness of the film was uniform, the resistivity of the film (ρ) was calculated using the simple relation ρ = Rs · d, where d is the film thickness. The optical transmittance and absorb spectra of the films were measured with an ultraviolet-visible spectrophotometer (Shimadzu UV-3101PC photometer, Japan). And the transmittance data were normalized by the transmittance of the bare substrate. The surface morphology of the AZO film was monitored by the atomicforce microscope (AFM, SPA-400, Japan). 2.2. OLED device fabrication and characterization We have used AZO thin film as anode contact for OLED devices. The devices reported here has the configuration of AZO/NPB (45 nm)/AlQ (45 nm)/ LiF (0.2 nm)/Al (150 nm). The hole transport layer (HTL) of 4,4 -bis[N -(1-naphthyl)-N -phenylamino]biphenyl (NPB) and electron transport/emitting layer (ETL/EML) of tris(8-quinolinolato)-aluminum (AlQ) were selected, respectively. A very thin layer of LiF (∼ 0.2 nm) was used as the electron-injecting layer (EIL) on the interface of AlQ and Al cathode. Details of the device fabrication are described elsewhere. The AZO glass plates were cleaned by sonication, in a detergent solution and then in deionized water. Finally the substrate was cleaned by UV ozone treatment for 8 minutes. The organic layers, LiF layer and the Al cathode were sequentially deposited by conventional vacuum vapor deposition in the chamber un-
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der the vacuum of approximately 3 × 10−3 Pa. The rate of deposition was typically 0.2–0.3 nm/s for the NPB and AlQ layers. The deposition rate for LiF was much lower. A quartz-crystal oscillator thickness monitor monitored the thickness and deposition rates of the organic layers and LiF layer. The active emission area of the device is about 3 × 3 mm2 . For comparison, a control device was fabricated on the commercial ITO substrate under the same conditions. The current– voltage–luminescence characteristics of these devices were measured by a keithley 234 source meter and the PR-650 spectrometer. All the measurements were carried out in the air at room temperature. Fig. 1. Optical transmittance spectra for the AZO film (300 nm) and the ITO film.
3. Results and discussion 3.1. Electrical and optical properties of AZO film The thickness of AZO layer was about 300 nm, which is measured by a XP-2 stylus profiler. The electrical sheet resistance of AZO layer was about 25 /sq, which is comparable with the ITO films. Doping of ZnO films with aluminum can improve their electrical properties, such as electrical resistivity and Hall mobility, etc. The Al doped in the ZnO films may supply the free carrier concentrations and reduce the resistivity of the film. When the doping ratio is high (> 2.0 wt%), excess Al molecules will act as carrier traps rather than electron donors. Thus the excess Al doping will increase the resistivity of the film [11,15]. The optical transmission measurements for wavelengths between 300 and 800 nm showed an average transmission of more than 90% for wavelengths in the 400–700 nm range. Fig. 1 shows the transmission curves for the AZO and ITO film, and the ITO substrate was obtained by commercial way. The values of energy gaps Eg can be determined by extrapolations of the straight sections of the square of the absorption coefficients α 2 versus photon energy hν. The absorption coefficient α was determined by the equation, α = (1/d) ln(1/T ), where d is the film thickness and T is the transmittance of the film [15]. The optical band gap values for the AZO is about 3.8 eV. Since the surface properties of the AZO may affect the characteristics of the device, it is very important to investigate the surface morphology of the AZO film. Fig. 2 shows the atomic-force microscope (AFM) im-
Fig. 2. AFM image (1 µm × 1 µm) of the AZO film grown on a glass substrate.
age (1 µm × 1 µm) of AZO film deposited by the DC sputtering method, which was measured by the tap recorder mode. As we can see from Fig. 2, the surface is very flat and no very sharp peak appears in the domain. 3.2. OLED device performance Fig. 3 shows the current density–voltage characteristics of the OLEDs with the AZO anode and an ITO anode for comparison. The device structure used in this study is also shown in the insert of Fig. 3. The device made on an AZO anode shows a typical diode behavior, with the current and power output observed only in the forward bias. The I–V characteristics are described by I ∝ V m , as expected for bulk-limited
D. Xu et al. / Physics Letters A 346 (2005) 148–152
Fig. 3. The Current density–voltage curves of the devices made on the ITO and AZO anode.
conduction, with m varying from 1 at low currents to 7.8 at high-current injection for ITO anode. And the m value for the AZO anode is 1.2 at low currents and 4.3 at high-current injection. As we can see in this figure, when the drive voltage is very low (< 2 V), both devices are ohmic conduction, however, highcurrent regime of about 6 V indicated space-chargelimited conduction (SCLC), when the voltage is 10 V there should consisted with the trap-charge limited conduction [16,17]. And from Fig. 3 we can see that the current density of ITO rises more sharply than that of AZO when the voltage is higher than 1.5 V. This shows that the ITO anode has higher hole injecting ability than AZO, which may come from the lower work function of AZO film compared with ITO film [14]. Fig. 4 shows the brightness–voltage characteristics of the two devices made on the ITO and AZO anodes, respectively. For both ITO and AZO devices, a leakage current of current is observed for the light output less than 1 cd/m2 (as can be seen in Fig. 3), which may cause the cross talk problems between pixels in display applications. For achieving 4000 cd/m2 , less than 8 V was required by the device made with AZO anode. And the brightness of higher than 6000 cd/m2 was achieved by the AZO device at 208 mA/cm2 . Fig. 5 shows the EL efficiency–current density characteristics of the devices. As we can see from this figure, at 100 mA/cm2 , the current efficiency of about 2.9 cd/A and 3.8 cd/A can be achieved for the
151
Fig. 4. Brightness–voltage characteristics of the devices made on the AZO and ITO anode.
Fig. 5. EL efficiency–current density characteristics of the devices made on the ITO and AZO anode.
devices made on the AZO and ITO anode, respectively. When the current densities of the two device are 100 mA/cm2 , the drive voltage for AZO and ITO are 7.2 and 7.3 V, respectively. Both devices almost have the same voltage at 100 mA/cm2 , however, the brightness of the ITO device is higher than that of AZO device at a current density of 100 mA/cm2 . Thus the ITO device has a current efficiency of 3.8 cd/A at 100 mA/cm2 , which is higher than 2.9 cd/A of AZO. The lower current efficiency may come from the work function of AZO anode (∼ 4.0 eV) compared to ITO anode (4.5–4.8 eV) [14]. In OLEDs, hole and electron injections from both anode and cathode affect the luminescence efficiency. The hole injection barrier mainly depends on the energy gap between the
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D. Xu et al. / Physics Letters A 346 (2005) 148–152
work function energy of the anode surface and the highest occupied molecular orbital (HOMO) of the hole transport layer (HTL). In our experiment, both devices used NPB as the hole transport material, the anode material with a higher surface work function leads to a smaller energy gap and so improves the current injection. Based on the discussion, the hole– electron recombination within the emission layer of the device made of conventional ITO may benefit from the enhanced hole injection. As a result, more emissive excitons can be formed and current efficiency was improved.
4. Conclusion In summary, transparent conducting aluminumdoped zinc oxide (AZO) thin films, as potential alternatives to ITO, have been studied for use as the anode contact in OLED device. The AZO film has been deposited on glass substrates by DC reactive magnetron sputtering method. Films were deposited at a substrate temperature of 150 ◦ C in 0.03 Pa of oxygen pressure. The electrical and optical properties of the film with the Al-doping amount of 2.0 wt% in the target were investigated. For the 300-nm thick AZO film deposited using a ZnO target with an Al content of 2.0 wt%, the minimum electrical resistivity was 4 × 10−4 cm and the average transmission in the visible range 400– 700 nm was more than 90%. The AZO film was used as an anode contact to fabricate organic light-emitting diodes. The device performance was measured and the current efficiency of 2.9 cd/A was measured at a current density of 100 mA/cm2 , which is comparable to that of the commercial ITO device. The results suggest that optimized Al-doped ZnO films may serves as an alternate anode material to ITO for OLED applications.
Acknowledgements We gratefully acknowledge the financial support of National Natural Science Foundation of China under contract No. 90201004 and by Beijing Science Foundation under contract No. H0304300204.
References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] J. Kido, K. Hongawa, K. Pkuyama, K. Nagai, Appl. Phys. Lett. 68 (1996) 217. [3] H. Aziz, Z.D. Popovic, N.-X. Hu, A.-M. Hor, G. Xu, Science 283 (1999) 1900. [4] C.D. Muller, A. Faclou, N. Reckefuss, M. Rojahn, V. Wiederhirn, P. Rudati, H. Frohne, Nature 421 (2003) 829. [5] Y. Niu, H. Ma, Q. Xu, et al., Appl. Phys. Lett. 86 (2005) 083504. [6] V. Adamovish, A. Shoustikov, M.E. Thoppson, Adv. Mater. 11 (1999) 727. [7] X. Zhou, J. He, L. Liao, M. Lu, Z. Xiong, X. Ding, X. Hou, F. Tao, C. Zhou, S. Lee, Appl. Phys. Lett. 74 (1999) 609. [8] A. Andersson, N. Johansson, P. Broms, N. Yu, D. Lupo, W.R. Salaneck, Adv. Mater. 10 (1998) 859. [9] H. Kim, A. Pique, J.S. Horwitz, et al., Appl. Phys. Lett. 74 (1999) 3444. [10] S.B. Qadri, H. Kim, J.S. Horwitz, et al., J. Appl. Phys. 88 (2000) 6564. [11] H. Kim, J.S. Horwitz, W.H. Kim, A.J. Makiinen, Z.H. Kafafi, D.B. Chrisey, Thin Solid Films 420–421 (2002) 539. [12] J. Hu, R.G. Gordon, J. Appl. Phys. 71 (1992) 880. [13] X. Jiang, C.L. Jia, B. Szyszka, Appl. Phys. Lett. 80 (2002) 3090. [14] X. Jiang, F.L. Wong, M.K. Fung, S.T. Lee, Appl. Phys. Lett. 83 (2003) 1875. [15] H. Kim, A. Pique, J.S. Horwitz, H. Murata, Z.H. Kafafi, C.M. Gilmore, D.B. Chrisey, Thin Solid Films 377–378 (2000) 798. [16] J.-Y. Kim, M. Kim, J.-H. Choi, Synth. Met. 139 (2003) 565. [17] S.Y. Park, C.H. Lee, W.J. Somg, C. Soul, Curr. Appl. Phys. 1 (2001) 116.
An anode with aluminum doped on zinc oxide thin films for organic light emitting devices Denghui Xu a , Zhenbo Deng a,∗ , Ying Xu a , Jing Xiao a , Chunjun Liang a , Zhiliang Pei b , Chao Sun b a Institute of Optoelectronic Technology, Key Laboratory of Information Storage and Display, Beijing Jiaotong University,
Beijing 100044, PR China b Institute of Metal Research, Chinese Academy of Science, Shenyang 110016, PR China
Received 14 April 2005; accepted 29 July 2005 Available online 9 August 2005 Communicated by A.R. Bishop
Abstract Doped zinc oxides are attractive alternative materials as transparent conducting electrode because they are nontoxic and inexpensive compared with indium tin oxide (ITO). Transparent conducting aluminum-doped zinc oxide (AZO) thin films have been deposited on glass substrates by DC reactive magnetron sputtering method. Films were deposited at a substrate temperature of 150 ◦ C in 0.03 Pa of oxygen pressure. The electrical and optical properties of the film with the Al-doping amount of 2 wt% in the target were investigated. For the 300-nm thick AZO film deposited using a ZnO target with an Al content of 2 wt%, the lowest electrical resistivity was 4 × 10−4 cm and the average transmission in the visible range 400–700 nm was more than 90%. The AZO film was used as an anode contact to fabricate organic light-emitting diodes. The device performance was measured and the current efficiency of 2.9 cd/A was measured at a current density of 100 mA/cm2 . 2005 Elsevier B.V. All rights reserved. Keywords: Aluminum doped zinc oxide (AZO); Transparent conducting oxide (TCO); Organic light emitting diodes (OLEDs)
1. Introduction Organic light-emitting diodes (OLEDs) based on organic small molecule and polymers have been extensively studied for potential applications, especially in * Corresponding author. Tel: +86 10 51688675.
E-mail address: [email protected] (Z. Deng). 0375-9601/$ – see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2005.07.080
the field of flat-panel displays [1–5]. The basic OLED structure consists of one or two organic films between two electrodes, one of which must be transparent. In conventional OLEDs, the work function of the cathode material must be minimized to increase the electron injection efficiency at the cathode/organic interface and the work function of the anode material must be maximized to increase the hole injection efficiency at the anode/organic interface. A large number of transpar-
D. Xu et al. / Physics Letters A 346 (2005) 148–152
ent conducting oxides (TCO), such as In2 O3 , ZnO, SnO2 , and doped oxides, have been widely studied over the years [6–8]. Of all the TCO films, indium tin oxide (ITO) is the most widely used anode material in OLEDs due to its high transparency (∼ 90% at 550 nm), low resistivity (∼ 2 × 10−4 cm), and relatively high work function (∼ 4.8 eV) [9–11]. And ITO has also been the most commonly selected TCO electrode in flat panel displays including liquid crystal displays and plasma displays due to its combined properties of both high visible transmittance and low electrical resistivity. However, since indium is a rare and expensive element, the cost of ITO films is relatively expensive. Recently, zinc oxide or impurity doped zinc oxide films have been actively investigated as alternate materials to ITO because zinc oxide is a nontoxic, inexpensive and abundant material. It is also chemically stable under hydrogen plasma processes that are commonly used for the production of solar cells [12]. Zinc oxide is an n-type semiconductor with an optical band gap of approximately 3.3 eV at room temperature. In the last decade, ZnO films doped with impurities, such as B, Al, Ga, In and Zr, have been actively studied. Among the ZnO films doped with group III elements (B, Al, Ga, In and Zr), aluminum-doped zinc oxide (AZO) films show the lowest electrical resistivity. AZO films are also wide band gap semiconductors (Eg = 3.4–3.9 eV), which show optical transmission in the visible and near-infrared (IR) regions. A number of papers have reported improvements to the electrical conductivity of the AZO thin films grown by conventional vapor deposition techniques such as chemical vapor deposition (CVD) and sputtering in addition to pulsed-laser deposition (PLD). In this Letter, we report a study of the electrical and optical properties of Al doped ZnO film grown by DC reactive magnetron sputtering method on glass substrate. The Al doped ZnO film was used as anode material for OLED devices and the device efficiency was compared with the control device fabricated on commercial ITO. 2. Experimental 2.1. AZO film fabrication and characterization The AZO thin films were produced by a conventional DC reactive magnetron sputtering system from
149
an alloy target. Details of deposition are described elsewhere [13,14]. Based on our experiments, the optimized weight ratio of Al in the Al-doped ZnO (AZO) target is 2.0 percent. Optical grade glasses were used as substrates and ultrasonically cleaned by isopropyl alcohol and blown dry in N2 gas before they were introduced into the chamber. Before each run, the target was pre-sputtered in a pure argon atmosphere for 3 min in order to remove the natural surface oxide layer of the target. The substrate temperature was 150 ◦ C. High purity argon and oxygen were the process gas and reactive gas, respectively. The oxygen partial pressure was 3.0 × 10−2 Pa. The argon and oxygen gases were introduced through a mass flow controller after the vacuum chamber was evacuated to below 3 × 10−3 Pa. The sheet resistance (Rs ) of the film was measured with a four-point probe. By assuming that the thickness of the film was uniform, the resistivity of the film (ρ) was calculated using the simple relation ρ = Rs · d, where d is the film thickness. The optical transmittance and absorb spectra of the films were measured with an ultraviolet-visible spectrophotometer (Shimadzu UV-3101PC photometer, Japan). And the transmittance data were normalized by the transmittance of the bare substrate. The surface morphology of the AZO film was monitored by the atomicforce microscope (AFM, SPA-400, Japan). 2.2. OLED device fabrication and characterization We have used AZO thin film as anode contact for OLED devices. The devices reported here has the configuration of AZO/NPB (45 nm)/AlQ (45 nm)/ LiF (0.2 nm)/Al (150 nm). The hole transport layer (HTL) of 4,4 -bis[N -(1-naphthyl)-N -phenylamino]biphenyl (NPB) and electron transport/emitting layer (ETL/EML) of tris(8-quinolinolato)-aluminum (AlQ) were selected, respectively. A very thin layer of LiF (∼ 0.2 nm) was used as the electron-injecting layer (EIL) on the interface of AlQ and Al cathode. Details of the device fabrication are described elsewhere. The AZO glass plates were cleaned by sonication, in a detergent solution and then in deionized water. Finally the substrate was cleaned by UV ozone treatment for 8 minutes. The organic layers, LiF layer and the Al cathode were sequentially deposited by conventional vacuum vapor deposition in the chamber un-
150
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der the vacuum of approximately 3 × 10−3 Pa. The rate of deposition was typically 0.2–0.3 nm/s for the NPB and AlQ layers. The deposition rate for LiF was much lower. A quartz-crystal oscillator thickness monitor monitored the thickness and deposition rates of the organic layers and LiF layer. The active emission area of the device is about 3 × 3 mm2 . For comparison, a control device was fabricated on the commercial ITO substrate under the same conditions. The current– voltage–luminescence characteristics of these devices were measured by a keithley 234 source meter and the PR-650 spectrometer. All the measurements were carried out in the air at room temperature. Fig. 1. Optical transmittance spectra for the AZO film (300 nm) and the ITO film.
3. Results and discussion 3.1. Electrical and optical properties of AZO film The thickness of AZO layer was about 300 nm, which is measured by a XP-2 stylus profiler. The electrical sheet resistance of AZO layer was about 25 /sq, which is comparable with the ITO films. Doping of ZnO films with aluminum can improve their electrical properties, such as electrical resistivity and Hall mobility, etc. The Al doped in the ZnO films may supply the free carrier concentrations and reduce the resistivity of the film. When the doping ratio is high (> 2.0 wt%), excess Al molecules will act as carrier traps rather than electron donors. Thus the excess Al doping will increase the resistivity of the film [11,15]. The optical transmission measurements for wavelengths between 300 and 800 nm showed an average transmission of more than 90% for wavelengths in the 400–700 nm range. Fig. 1 shows the transmission curves for the AZO and ITO film, and the ITO substrate was obtained by commercial way. The values of energy gaps Eg can be determined by extrapolations of the straight sections of the square of the absorption coefficients α 2 versus photon energy hν. The absorption coefficient α was determined by the equation, α = (1/d) ln(1/T ), where d is the film thickness and T is the transmittance of the film [15]. The optical band gap values for the AZO is about 3.8 eV. Since the surface properties of the AZO may affect the characteristics of the device, it is very important to investigate the surface morphology of the AZO film. Fig. 2 shows the atomic-force microscope (AFM) im-
Fig. 2. AFM image (1 µm × 1 µm) of the AZO film grown on a glass substrate.
age (1 µm × 1 µm) of AZO film deposited by the DC sputtering method, which was measured by the tap recorder mode. As we can see from Fig. 2, the surface is very flat and no very sharp peak appears in the domain. 3.2. OLED device performance Fig. 3 shows the current density–voltage characteristics of the OLEDs with the AZO anode and an ITO anode for comparison. The device structure used in this study is also shown in the insert of Fig. 3. The device made on an AZO anode shows a typical diode behavior, with the current and power output observed only in the forward bias. The I–V characteristics are described by I ∝ V m , as expected for bulk-limited
D. Xu et al. / Physics Letters A 346 (2005) 148–152
Fig. 3. The Current density–voltage curves of the devices made on the ITO and AZO anode.
conduction, with m varying from 1 at low currents to 7.8 at high-current injection for ITO anode. And the m value for the AZO anode is 1.2 at low currents and 4.3 at high-current injection. As we can see in this figure, when the drive voltage is very low (< 2 V), both devices are ohmic conduction, however, highcurrent regime of about 6 V indicated space-chargelimited conduction (SCLC), when the voltage is 10 V there should consisted with the trap-charge limited conduction [16,17]. And from Fig. 3 we can see that the current density of ITO rises more sharply than that of AZO when the voltage is higher than 1.5 V. This shows that the ITO anode has higher hole injecting ability than AZO, which may come from the lower work function of AZO film compared with ITO film [14]. Fig. 4 shows the brightness–voltage characteristics of the two devices made on the ITO and AZO anodes, respectively. For both ITO and AZO devices, a leakage current of current is observed for the light output less than 1 cd/m2 (as can be seen in Fig. 3), which may cause the cross talk problems between pixels in display applications. For achieving 4000 cd/m2 , less than 8 V was required by the device made with AZO anode. And the brightness of higher than 6000 cd/m2 was achieved by the AZO device at 208 mA/cm2 . Fig. 5 shows the EL efficiency–current density characteristics of the devices. As we can see from this figure, at 100 mA/cm2 , the current efficiency of about 2.9 cd/A and 3.8 cd/A can be achieved for the
151
Fig. 4. Brightness–voltage characteristics of the devices made on the AZO and ITO anode.
Fig. 5. EL efficiency–current density characteristics of the devices made on the ITO and AZO anode.
devices made on the AZO and ITO anode, respectively. When the current densities of the two device are 100 mA/cm2 , the drive voltage for AZO and ITO are 7.2 and 7.3 V, respectively. Both devices almost have the same voltage at 100 mA/cm2 , however, the brightness of the ITO device is higher than that of AZO device at a current density of 100 mA/cm2 . Thus the ITO device has a current efficiency of 3.8 cd/A at 100 mA/cm2 , which is higher than 2.9 cd/A of AZO. The lower current efficiency may come from the work function of AZO anode (∼ 4.0 eV) compared to ITO anode (4.5–4.8 eV) [14]. In OLEDs, hole and electron injections from both anode and cathode affect the luminescence efficiency. The hole injection barrier mainly depends on the energy gap between the
152
D. Xu et al. / Physics Letters A 346 (2005) 148–152
work function energy of the anode surface and the highest occupied molecular orbital (HOMO) of the hole transport layer (HTL). In our experiment, both devices used NPB as the hole transport material, the anode material with a higher surface work function leads to a smaller energy gap and so improves the current injection. Based on the discussion, the hole– electron recombination within the emission layer of the device made of conventional ITO may benefit from the enhanced hole injection. As a result, more emissive excitons can be formed and current efficiency was improved.
4. Conclusion In summary, transparent conducting aluminumdoped zinc oxide (AZO) thin films, as potential alternatives to ITO, have been studied for use as the anode contact in OLED device. The AZO film has been deposited on glass substrates by DC reactive magnetron sputtering method. Films were deposited at a substrate temperature of 150 ◦ C in 0.03 Pa of oxygen pressure. The electrical and optical properties of the film with the Al-doping amount of 2.0 wt% in the target were investigated. For the 300-nm thick AZO film deposited using a ZnO target with an Al content of 2.0 wt%, the minimum electrical resistivity was 4 × 10−4 cm and the average transmission in the visible range 400– 700 nm was more than 90%. The AZO film was used as an anode contact to fabricate organic light-emitting diodes. The device performance was measured and the current efficiency of 2.9 cd/A was measured at a current density of 100 mA/cm2 , which is comparable to that of the commercial ITO device. The results suggest that optimized Al-doped ZnO films may serves as an alternate anode material to ITO for OLED applications.
Acknowledgements We gratefully acknowledge the financial support of National Natural Science Foundation of China under contract No. 90201004 and by Beijing Science Foundation under contract No. H0304300204.
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