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Electrical and optical characteristics of polymer light-emitting devices with surface-treated indium-tin-oxide electrodes
ARTICLE IN PRESS
Microelectronics Journal 38 (2007) 108–113 www.elsevier.com/locate/mejo
Electrical and optical characteristics of polymer light-emitting devices with surface-treated indium-tin-oxide electrodes Zhong Zhi Youa,, Jiang Ya Dongb a
College of Electronic Information Engineering, South-Central University for Nationalities, Wuhan 430074, People’s Republic of China State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China
b
Received 6 July 2006; accepted 24 September 2006 Available online 15 November 2006
Abstract Effects of differently surface-treated indium-tin-oxide (ITO) electrodes in poly[2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV)-based polymer light-emitting electrochemical cells (LECs) were investigated. It is found that the surface properties of ITO substrates are more effectively improved by the oxygen plasma compared with other treatments. Atomic force microscopy (AFM) measurements show that the oxygen plasma treatment reduces the roughness of the ITO surface and yields more smooth and homogeneous surface. X-ray photoelectron spectroscopy (XPS) analyses reveal that the oxygen plasma treatment increases the oxygen content and decreases the carbon content on the ITO surface. Contact angle and surface energy results indicate that the oxygen plasma treatment enhances the wettability of the ITO surface. The LECs with the oxygen plasma-treated ITO substrates exhibit the higher injection current, luminance and efficiency than that of the devices based on the ITO substrates treated in other different ways, due to the improvement of interface formation and electrical contact of the ITO electrode with the polymer blend in the LECs. r 2006 Elsevier Ltd. All rights reserved. PACS: 85.60.q; 68.47.Gh; 81.65.b Keywords: Optoelectronic devices; Indium-tin-oxide electrodes; Surface treatments
1. Introduction Polymer electronics has been developed very fast in recent years since conducting polymers were discovered in 1977 by Chiang et al. [1]. Conducting polymer materials have been used as an active medium in several optoelectronic devices, such as light-emitting diodes (LEDs), field effect transistors, photodiodes and solar cells [2–12]. As a new type of organic LEDs, polymer light-emitting electrochemical cells (LECs) have attracted increasing scientific and industrial interest due to their high electroluminescence efficiency, low operating voltage, good mechanical flexibility and ease of fabrication since the first report by Pei and co-workers [13]. In general, a typical LEC device consists of a light-emitting active blend layer of conjugated Corresponding author. Tel.: +86 27 6784 3890.
E-mail address: [email protected] (Z.Z. You). 0026-2692/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2006.09.019
luminescent polymer and solid electrolyte with supporting salt sandwiched between an indium-tin-oxide (ITO) electrode and a metal electrode [13–16]. When a sufficiently high bias is applied onto the electrodes, electrochemical doping takes place accompanied by the redistribution of the ions in the polymer blend film. As a result, the contact at the polymer electrode interface is ohmic, and the conjugated polymer is doped p-type near the anode and n-type near the cathode, respectively, and a dynamic p–i–n junction is created in situ. Hence, light is emitted from the intrinsic region near the center. Since the polymer blend layer is in direct contact with the ITO electrode, the surface characteristics of ITO and the interfacial properties between the polymer blend layer and the ITO electrode play an important role in the operation of LECs. In this work, different surface treatments were performed on the ITO substrates for LECs. Then, we investigated the effect of surface treatments upon LECs
ARTICLE IN PRESS Z.Z. You, J.Y. Dong / Microelectronics Journal 38 (2007) 108–113
by measuring the electrical and optical performance of the devices and by characterizing the surface-treated ITO substrates through the measurements of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), contact angle and surface energy.
2. Experimental methods Commercial ITO-coated glass substrates with nominal sheet resistance of 11 O/square were purchased from CSG Holding Company Limited. The ITO substrates were firstly etched by concentrated hydrochloric acid solution to the required pattern. Prior to their use, the ITO substrates were routinely cleaned by rubbing in a detergent, rinsing in deionized (DI) water, successive ultrasonification with acetone and isopropanol each for 10 min, and finally dried in a flow of nitrogen. Then, the ITO substrates were treated in different processing techniques. For solvent cleaning, the substrates were successively washed in an ultrasonic bath of acetone and alcohol each for 20 min, then rinsed in DI water, and dried with nitrogen. For acid treatment, the substrates were dipped into 10% hydrochloric acid solution at room temperature for 15 min, and then rinsed in DI water and finally dried in a flow of nitrogen. For oxygen plasma treatment, the substrates were exposed to the oxygen plasma in a radio frequency (RF) plasma generator for 5 min at room temperature under pressure 20 Pa, plasma power 30 W and gas flow rate 20 ml/min. The ITO substrates without any treatment are referred to untreated ITO. Then LECs were fabricated using the polymer blend of poly[2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly(ethylene oxide) (PEO) complexed with lithium trifluoromethanesulfonate (LiCF3SO3) as the
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active light-emitting layer, and the differently surfacetreated ITO substrates as device electrodes. The weight ratio of the MEH-PPV:PEO:LiCF3SO3 in the solution was 12:5:2. The polymer blend film of thickness of about 200 nm was prepared by spin-coating cyclohexanone solution onto the ITO substrate. The silver layer (150 nm) was then deposited on the top of the blend film by the vacuum sublimation technique (about 104 Pa) to form ITO/{MEHPPV+PEO+LiCF3SO3}/Ag sandwich structure. The organic materials were purchased from Sigma-Aldrich Chemical Company and used as received. The device configuration and molecular structures are shown in Fig. 1. After the fabrication, the bias voltage was applied with the ITO electrode as anode and the Ag electrode as
Fig. 2. (a) Absorption spectrum, and (b) Tauc plot of electroluminescent polymer MEH-PPV thin film. Dotted line indicates the linear part of the curve. E g presents the optical bandgap of MEH-PPV thin film.
OCH2CH(C2H5)C4H9 O CH2
CH2
Li
O n
O
F S
O
n
C
F
F
OCH3 { MEH-PPV + PEO + LiCF3SO3}
Ag
Ag Light-emitting layer ITO
Glass
Fig. 1. Molecular structures (top part) and device configuration schematic diagram (bottom part).
Fig. 3. (a) Current–voltage characteristics, and (b) luminance–voltage characteristics of the devices fabricated with different ITO treatments. V Jon and V Lon present the threshold voltages of current injection and light emission, respectively.
ARTICLE IN PRESS Z.Z. You, J.Y. Dong / Microelectronics Journal 38 (2007) 108–113
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cathode. The current–voltage-luminance characteristics of the devices were measured at a scan rate of 0.1 V/s with a KEITHLEY-4200 sourcemeter and a ST-86LA luminance meter. All measurements were performed at room temperature in ambient air. The surface morphology of the ITO was characterized by AFM with a Seiko Instruments SPA-400. The chemical composition of the ITO was analyzed by XPS using a XSAM-800 XPS. The contact angles of distilled water (H2O) and diiodomethane (CH2I2, from Beijing Chemical Reagents Company) on the ITO were measured using a JY-82 contact angle analyzer by the sessile drop technique [17] at 20 1C. The ITO surface energy ðgS Þ as the sum of the
Table 1 Changes in chemical composition and surface roughness of ITO under different surface treatments ITO treatment
Untreated Solvent cleaning Acid treatment Oxygen plasma
Chemical composition (%)
Surface roughness (nm)
O
Sn
In
C
Rrms
Ra
15.1 14.9 13.6 16.5
10.5 8.4 9.0 10.4
68.8 67.5 72.5 68.5
5.6 9.2 4.9 4.6
3.0 3.9 7.6 2.8
2.5 3.2 6.1 2.2
polar ðgpS Þ and dispersion ðgdS Þ components was evaluated from the measured contact angles using the harmonicmean method [17]. 3. Experimental results and discussion 3.1. Electrical and optical characteristics of the devices with different ITO treatments Fig. 2(a) presents the optical absorption spectrum of a 100 nm MEH-PPV film as a function of wavelength measured by a Perkin Elmer Lambda 900 UV/VIS/NIR spectrophotometer at room temperature. The absorption coefficient a was calculated using [18,19] a ¼ 2:303
A , d
(1)
where A is the absorbance, and d is the film thickness. The optical bandgap E g can be obtained by fitting the absorption coefficient a to Tauc’s relation [19,20]: ðaEÞ2 ¼ BðE E g Þ,
(2)
where E ¼ hn is the photon energy, and B is an energyindependent constant. Fig. 2(b) shows the Tauc plot: ðaEÞ2 versus E for the MEH-PPV film. The value of E g can be
Fig. 4. AFM images of ITO substrates with different surface treatments: (a) untreated, (b) solvent cleaning, (c) hydrochloric acid treatment, (d) oxygen plasma treatment.
ARTICLE IN PRESS Z.Z. You, J.Y. Dong / Microelectronics Journal 38 (2007) 108–113
readily obtained by extrapolation of the linear portion of the graph to ðaEÞ2 ¼ 0, which is about 2.18 eV. Fig. 3 illustrates the semilogarithmic plots of the measured current–voltage ðJ V Þ and luminance–voltage ðL V Þ characteristics of the devices with different ITO treatments. As shown in the Fig. 3(a), the threshold voltages of current injection ðV Jon Þ for all the devices share almost the same value (about 2.2 V), which is in agreement with the measured E g -value of MEH-PPV (i.e., V Jon ¼ E g =q, q is the electronic charge). Note from Fig. 3(b) that the values of threshold voltage of light emission ðV Lon Þ for the devices with the untreated, solvent cleaned, acid treated, and oxygen plasma-treated ITO substrates are about 2.7, 2.6, 2.9, and 2.4 V, respectively, and the difference between V Lon and V Jon exists among all the devices. Each V Lon is observed to be slightly higher than its corresponding V Jon , indicating that the threshold voltage of light emission lags behind that of current injection. Similar results have been reported by several research groups [21–23]. There may be two reasons for this voltage lag of light emission. One is related to the sensitivity of the light intensity detector, which is not sensitive enough for the weak luminance at the voltage lower than V Lon value. Another reason is the relatively slow response of the LECs, which also make the light emission lag behind the current injection. Note also from Fig. 3 that the injection current ðJÞ, luminance ðLÞ and efficiency ðZÞ of the LECs depend on the ITO surface treatments. At V ¼ 7 V for devices fabricated with the untreated, solvent cleaned, acid treated, and oxygen plasma-treated ITO substrates, the values of J are 752.8, 811.1, 594.4, and 1355.1 A/m2; and L are 25.5, 40.3, 6.9, and 98.2 cd/m2, respectively. And the corresponding Z values can be readily calculated out to be about 0.034, 0.050, 0.012, and 0.073 cd/A, respectively.
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AFM images shown in Fig. 4 display that the oxygen plasma treatment makes ITO substrate exhibit more smooth and homogeneous surface, compared with other treatments. Fig. 5 gives the measured contact angles, surface energies and polarities of all the ITO substrates. As can be seen, the oxygen plasma treatment brings about the maximum reduction in H2O contact angle yH2 O and CH2I2 contact angle yCH2 I2 , from 85.11 and 46.21 to 30.51 and 32.11, respectively, implying that the oxygen plasma-treated ITO surface appears to be highly polar. Additionally, the oxygen plasma treatment yields the highest surface energy gS (66.3 mJ/m2) and the highest surface polarity wp (0.56), mainly due to the significantly enhanced polar component gpS (from 8.2 to 36.4 mJ/m2). We propose that the increase of the surface energy and polarity is due to several reasons [28–30]. One reason is the improvement of the surface stoichiometry, since oxygen plasma treatment introduces oxygen and increases the oxygen content, and meanwhile effectively removes the hydrocarbon contaminants from the ITO surface so as to reduce the carbon content. The second reason is the increase of water concentration that is physi- and/or chemisorbed on the surface. The third possible reason is that organic contamination may become more hydrophilic after oxygen plasma treatment. Table 2 summarizes the electrical and optical performance of the devices with different ITO treatments. As can be seen, the oxygen plasma-treated device in our present work yields the higher injection current, luminance and
3.2. Surface properties of the ITO with different treatments Table 1 shows the changes in chemical composition and surface roughness of ITO under different surface treatments. Note that besides containing the elements of O, Sn, and In, the carbon element also exists on each ITO surface, indicating that carbon is the only major contaminant. Note also that for the untreated, solvent cleaned, acid treated, and oxygen plasma-treated ITO surfaces, the oxygen contents are 15.1%, 14.9%, 13.6%, 16.5%, and the carbon contents are 5.6%, 9.2%, 4.9%, 4.6%, respectively. It suggests that the oxygen plasma treatment largely increases the oxygen content and decreases the carbon content, and improves the surface stoichiometry, and thereby more effectively enhances the work function of the ITO, since the ITO work function is determined by the oxygen content and the carbon content [24–27]. Furthermore, we observe from Table 1 that the surface roughness values of ITO substrates are also subjected to the surface treatments. Obviously, only the oxygen plasma treatment decreases the surface roughness, while other treatments more or less increase the surface roughness of ITO substrates. Also, the
Fig. 5. Changes in contact angle, surface energy and polarity of ITO under different surface treatments.
ARTICLE IN PRESS Z.Z. You, J.Y. Dong / Microelectronics Journal 38 (2007) 108–113
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Table 2 Device performance of LECs fabricated with different ITO treatments ITO treatment
Untreated Solvent cleaning Acid treatment Oxygen plasma
ITO surface properties
Device electrical and optical performance
O (%)
C (%)
gS (mJ/cm2)
wp
V Jon (V)
V Lon (V)
J (A/m2)
L (cd/m2)
Z (cd/A)
15.1 14.9 13.6 16.5
5.6 9.2 4.9 4.6
37.9 47.5 38.6 66.3
0.213 0.386 0.254 0.556
2.2 2.2 2.2 2.2
2.7 2.6 2.9 2.4
752.8 811.1 594.4 1355.1
25.5 40.3 6.9 98.2
0.034 0.050 0.012 0.073
The values of J, L and Z are measured at V ¼ 7 V.
efficiency, exhibiting the relatively better performance, while the hydrochloric acid-treated device the poorer. Such an experimental result suggests that the device performance of LECs is closely related to the characteristics of electrical contact at the ITO/polymer interface. The Oxygen plasma treatment results not only in higher work function, lower surface roughness, but also higher surface energy and polarity of the ITO substrate which is highly important to improve the surface morphological properties of polymer thin film and to optimize the interface characteristics of ITO/polymer. Firstly, the increase of surface polarity of ITO substrate leads to a more uniform spreading of the polymer solution on the ITO surface, reduces the voids and pinholes, and thereby obtains the homogeneous and compact polymer thin film. Secondly, the increase of the surface energy of ITO substrate provides a better adhesion of the polymer film and reduces the interfacial energy between the polymer film and ITO substrate, and therefore increases the effective area available between these two materials. These two effects above-stated can significantly improve the interface formation and electrical contact of the ITO substrate with the polymer blend in the LECs, so as to facilitate the electrochemical doping of the electroluminescent polymer when sufficiently large bias was applied, and therefore yield the relatively better performance of the LECs. Contrary to the situation in the oxygen plasma treatment, yet, the hydrochloric acid treatment results in a poor electrical contact to the emissive layer in the LECs and hinders charge injection through the electrochemical doping, and thereby produces the poorer performance of LECs. 4. Conclusion In this study, the different surface treatments were carried out on the ITO substrates and the surface properties of treated ITO were investigated through the measurements of AFM, XPS, contact angle and surface energy. Experimental results indicate that the surface properties of the ITO substrates strongly depend on the surface treatments. The oxygen plasma more effectively improves the ITO surface properties compared with other treatments, since plasma decreases the surface roughness, enhances the surface energy and polarity, and improves the surface stoichiometry. Furthermore, the LECs with the
differently treated ITO substrates as device electrodes were fabricated and characterized. It is observed that the surface treatments on ITO substrates have a certain degree of influence upon the injection current, luminance and efficiency, but hardly upon the threshold voltages of current injection and light emission. The oxygen plasma treatment on the ITO substrate gives the better performance of the LECs, due to the improvement of interface formation and electrical contact between the ITO substrate and the polymer blend in the LECs. Acknowledgments The authors would like to acknowledge financial supports from the National Natural Science Foundation of China (Grant no. 60425101). References [1] C.K. Chiang, C.R. Fincher Jr., Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis, S.C. Gau, A.G. MacDiarmid, Phys. Rev. Lett. 39 (1977) 1098. [2] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 (1990) 539. [3] M. Berggern, O. Inganas, G. Guslafsson, J. Ramusson, M.R. Anderson, T. Hjertberg, O. Wennerstrom, Nature 372 (1994) 444. [4] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 (1999) 121. [5] J.S. Kim, R.H. Friend, F. Cacialli, Appl. Phys. Lett. 74 (1999) 3084. [6] Z. Zhong, Y. Zhong, C. Liu, S. Yin, W. Zhang, D. Shi, Phys. Status Solidi A 198 (2003) 197. [7] L. Ke, R.S. Kumar, K. Zhang, S.-J. Chua, A.T.S. Wee, Microelectron. J. 35 (2004) 325. [8] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789. [9] M.C. Tanese, D. Fine, A. Dodabalapur, L. Torsi, Microelectron. J. 37 (2006) 837. [10] F. Zhang, M. Jonforsen, D.M. Johansson, M.R. Andersson, O. Ingana¨s, Synth. Met. 138 (2003) 555. [11] A.J. Breeze, Z. Schlesinger, S.A. Carter, P.J. Brock, Phys. Rev. B 64 (2001) 125205. [12] F.M. Nodari, M. Koehler, M.G.E. da Luz, L.S. Roman, Microelectron. J. 36 (2005) 995. [13] Q. Pei, G. Yu, C. Zhang, Y. Yang, A.J. Heeger, Science 269 (1995) 1086. [14] G. Yu, Q. Pei, A.J. Heeger, Appl. Phys. Lett. 70 (1997) 934. [15] Y. Li, J. Gao, G. Yu, Y. Cao, A.J. Heeger, Chem. Phys. Lett. 287 (1998) 83.
ARTICLE IN PRESS Z.Z. You, J.Y. Dong / Microelectronics Journal 38 (2007) 108–113 [16] F.-C. Chen, Y. Yang, Appl. Phys. Lett. 81 (2002) 4278. [17] S. Wu, Polymer Interface and Adhesion, Marcel Dekker, New York, 1982. [18] J.E.A.M van den Meerakker, E.A. Meulenkamp, M. Scholten, J. Appl. Phys. 74 (1993) 3282. [19] T.S. Moss, G.J. Burrell, B. Ellis, Semiconductor Opto-Electronics, Butterworths, London, 1973. [20] J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi 15 (1966) 627. [21] Y. Yang, Q. Pei, Appl. Phys. Lett. 68 (1996) 2708. [22] Y. Chuai, D.N. Lee, C. Zhen, J.H. Min, B.H. Kim, D. Zou, Synth. Met. 145 (2004) 259. [23] H.C. Ko, D.K. Lim, S.-H. Kim, W. Choi, H. Lee, Synth. Met. 144 (2004) 177.
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[24] C.C. Wu, C.I. Wu, J.C. Sturm, A. Kahn, Appl. Phys. Lett. 70 (1997) 1348. [25] J.S. Kim, M. Granstro¨m, R.H. Friend, N. Johansson, W.R. Salaneck, R. Daik, W.J. Feast, F. Cacialli, J. Appl. Phys. 84 (1998) 6859. [26] M. Ishii, T. Mori, H. Fujikawa, S. Tokito, Y. Taga, J. Lumin. 87–89 (2000) 1165. [27] Z.Z. You, J.Y. Dong, Appl. Surf. Sci. 249 (2005) 271. [28] J.S. Kim, P.K.H. Ho, D.S. Thomas, R.H. Friend, F. Cacialli, G.W. Bao, S.F.Y. Li, Chem. Phys. Lett. 315 (1999) 307. [29] S.K. So, W.K. Choi, C.H. Cheng, L.M. Leung, C.F. Kwong, Appl. Phys. A 68 (1999) 447. [30] C.H. Yi, C.H. Jeong, Y.H. Lee, Y.W. Ko, G.Y. Yeom, Surf. Coat. Technol. 177–178 (2004) 711.
Microelectronics Journal 38 (2007) 108–113 www.elsevier.com/locate/mejo
Electrical and optical characteristics of polymer light-emitting devices with surface-treated indium-tin-oxide electrodes Zhong Zhi Youa,, Jiang Ya Dongb a
College of Electronic Information Engineering, South-Central University for Nationalities, Wuhan 430074, People’s Republic of China State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China
b
Received 6 July 2006; accepted 24 September 2006 Available online 15 November 2006
Abstract Effects of differently surface-treated indium-tin-oxide (ITO) electrodes in poly[2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV)-based polymer light-emitting electrochemical cells (LECs) were investigated. It is found that the surface properties of ITO substrates are more effectively improved by the oxygen plasma compared with other treatments. Atomic force microscopy (AFM) measurements show that the oxygen plasma treatment reduces the roughness of the ITO surface and yields more smooth and homogeneous surface. X-ray photoelectron spectroscopy (XPS) analyses reveal that the oxygen plasma treatment increases the oxygen content and decreases the carbon content on the ITO surface. Contact angle and surface energy results indicate that the oxygen plasma treatment enhances the wettability of the ITO surface. The LECs with the oxygen plasma-treated ITO substrates exhibit the higher injection current, luminance and efficiency than that of the devices based on the ITO substrates treated in other different ways, due to the improvement of interface formation and electrical contact of the ITO electrode with the polymer blend in the LECs. r 2006 Elsevier Ltd. All rights reserved. PACS: 85.60.q; 68.47.Gh; 81.65.b Keywords: Optoelectronic devices; Indium-tin-oxide electrodes; Surface treatments
1. Introduction Polymer electronics has been developed very fast in recent years since conducting polymers were discovered in 1977 by Chiang et al. [1]. Conducting polymer materials have been used as an active medium in several optoelectronic devices, such as light-emitting diodes (LEDs), field effect transistors, photodiodes and solar cells [2–12]. As a new type of organic LEDs, polymer light-emitting electrochemical cells (LECs) have attracted increasing scientific and industrial interest due to their high electroluminescence efficiency, low operating voltage, good mechanical flexibility and ease of fabrication since the first report by Pei and co-workers [13]. In general, a typical LEC device consists of a light-emitting active blend layer of conjugated Corresponding author. Tel.: +86 27 6784 3890.
E-mail address: [email protected] (Z.Z. You). 0026-2692/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2006.09.019
luminescent polymer and solid electrolyte with supporting salt sandwiched between an indium-tin-oxide (ITO) electrode and a metal electrode [13–16]. When a sufficiently high bias is applied onto the electrodes, electrochemical doping takes place accompanied by the redistribution of the ions in the polymer blend film. As a result, the contact at the polymer electrode interface is ohmic, and the conjugated polymer is doped p-type near the anode and n-type near the cathode, respectively, and a dynamic p–i–n junction is created in situ. Hence, light is emitted from the intrinsic region near the center. Since the polymer blend layer is in direct contact with the ITO electrode, the surface characteristics of ITO and the interfacial properties between the polymer blend layer and the ITO electrode play an important role in the operation of LECs. In this work, different surface treatments were performed on the ITO substrates for LECs. Then, we investigated the effect of surface treatments upon LECs
ARTICLE IN PRESS Z.Z. You, J.Y. Dong / Microelectronics Journal 38 (2007) 108–113
by measuring the electrical and optical performance of the devices and by characterizing the surface-treated ITO substrates through the measurements of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), contact angle and surface energy.
2. Experimental methods Commercial ITO-coated glass substrates with nominal sheet resistance of 11 O/square were purchased from CSG Holding Company Limited. The ITO substrates were firstly etched by concentrated hydrochloric acid solution to the required pattern. Prior to their use, the ITO substrates were routinely cleaned by rubbing in a detergent, rinsing in deionized (DI) water, successive ultrasonification with acetone and isopropanol each for 10 min, and finally dried in a flow of nitrogen. Then, the ITO substrates were treated in different processing techniques. For solvent cleaning, the substrates were successively washed in an ultrasonic bath of acetone and alcohol each for 20 min, then rinsed in DI water, and dried with nitrogen. For acid treatment, the substrates were dipped into 10% hydrochloric acid solution at room temperature for 15 min, and then rinsed in DI water and finally dried in a flow of nitrogen. For oxygen plasma treatment, the substrates were exposed to the oxygen plasma in a radio frequency (RF) plasma generator for 5 min at room temperature under pressure 20 Pa, plasma power 30 W and gas flow rate 20 ml/min. The ITO substrates without any treatment are referred to untreated ITO. Then LECs were fabricated using the polymer blend of poly[2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly(ethylene oxide) (PEO) complexed with lithium trifluoromethanesulfonate (LiCF3SO3) as the
109
active light-emitting layer, and the differently surfacetreated ITO substrates as device electrodes. The weight ratio of the MEH-PPV:PEO:LiCF3SO3 in the solution was 12:5:2. The polymer blend film of thickness of about 200 nm was prepared by spin-coating cyclohexanone solution onto the ITO substrate. The silver layer (150 nm) was then deposited on the top of the blend film by the vacuum sublimation technique (about 104 Pa) to form ITO/{MEHPPV+PEO+LiCF3SO3}/Ag sandwich structure. The organic materials were purchased from Sigma-Aldrich Chemical Company and used as received. The device configuration and molecular structures are shown in Fig. 1. After the fabrication, the bias voltage was applied with the ITO electrode as anode and the Ag electrode as
Fig. 2. (a) Absorption spectrum, and (b) Tauc plot of electroluminescent polymer MEH-PPV thin film. Dotted line indicates the linear part of the curve. E g presents the optical bandgap of MEH-PPV thin film.
OCH2CH(C2H5)C4H9 O CH2
CH2
Li
O n
O
F S
O
n
C
F
F
OCH3 { MEH-PPV + PEO + LiCF3SO3}
Ag
Ag Light-emitting layer ITO
Glass
Fig. 1. Molecular structures (top part) and device configuration schematic diagram (bottom part).
Fig. 3. (a) Current–voltage characteristics, and (b) luminance–voltage characteristics of the devices fabricated with different ITO treatments. V Jon and V Lon present the threshold voltages of current injection and light emission, respectively.
ARTICLE IN PRESS Z.Z. You, J.Y. Dong / Microelectronics Journal 38 (2007) 108–113
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cathode. The current–voltage-luminance characteristics of the devices were measured at a scan rate of 0.1 V/s with a KEITHLEY-4200 sourcemeter and a ST-86LA luminance meter. All measurements were performed at room temperature in ambient air. The surface morphology of the ITO was characterized by AFM with a Seiko Instruments SPA-400. The chemical composition of the ITO was analyzed by XPS using a XSAM-800 XPS. The contact angles of distilled water (H2O) and diiodomethane (CH2I2, from Beijing Chemical Reagents Company) on the ITO were measured using a JY-82 contact angle analyzer by the sessile drop technique [17] at 20 1C. The ITO surface energy ðgS Þ as the sum of the
Table 1 Changes in chemical composition and surface roughness of ITO under different surface treatments ITO treatment
Untreated Solvent cleaning Acid treatment Oxygen plasma
Chemical composition (%)
Surface roughness (nm)
O
Sn
In
C
Rrms
Ra
15.1 14.9 13.6 16.5
10.5 8.4 9.0 10.4
68.8 67.5 72.5 68.5
5.6 9.2 4.9 4.6
3.0 3.9 7.6 2.8
2.5 3.2 6.1 2.2
polar ðgpS Þ and dispersion ðgdS Þ components was evaluated from the measured contact angles using the harmonicmean method [17]. 3. Experimental results and discussion 3.1. Electrical and optical characteristics of the devices with different ITO treatments Fig. 2(a) presents the optical absorption spectrum of a 100 nm MEH-PPV film as a function of wavelength measured by a Perkin Elmer Lambda 900 UV/VIS/NIR spectrophotometer at room temperature. The absorption coefficient a was calculated using [18,19] a ¼ 2:303
A , d
(1)
where A is the absorbance, and d is the film thickness. The optical bandgap E g can be obtained by fitting the absorption coefficient a to Tauc’s relation [19,20]: ðaEÞ2 ¼ BðE E g Þ,
(2)
where E ¼ hn is the photon energy, and B is an energyindependent constant. Fig. 2(b) shows the Tauc plot: ðaEÞ2 versus E for the MEH-PPV film. The value of E g can be
Fig. 4. AFM images of ITO substrates with different surface treatments: (a) untreated, (b) solvent cleaning, (c) hydrochloric acid treatment, (d) oxygen plasma treatment.
ARTICLE IN PRESS Z.Z. You, J.Y. Dong / Microelectronics Journal 38 (2007) 108–113
readily obtained by extrapolation of the linear portion of the graph to ðaEÞ2 ¼ 0, which is about 2.18 eV. Fig. 3 illustrates the semilogarithmic plots of the measured current–voltage ðJ V Þ and luminance–voltage ðL V Þ characteristics of the devices with different ITO treatments. As shown in the Fig. 3(a), the threshold voltages of current injection ðV Jon Þ for all the devices share almost the same value (about 2.2 V), which is in agreement with the measured E g -value of MEH-PPV (i.e., V Jon ¼ E g =q, q is the electronic charge). Note from Fig. 3(b) that the values of threshold voltage of light emission ðV Lon Þ for the devices with the untreated, solvent cleaned, acid treated, and oxygen plasma-treated ITO substrates are about 2.7, 2.6, 2.9, and 2.4 V, respectively, and the difference between V Lon and V Jon exists among all the devices. Each V Lon is observed to be slightly higher than its corresponding V Jon , indicating that the threshold voltage of light emission lags behind that of current injection. Similar results have been reported by several research groups [21–23]. There may be two reasons for this voltage lag of light emission. One is related to the sensitivity of the light intensity detector, which is not sensitive enough for the weak luminance at the voltage lower than V Lon value. Another reason is the relatively slow response of the LECs, which also make the light emission lag behind the current injection. Note also from Fig. 3 that the injection current ðJÞ, luminance ðLÞ and efficiency ðZÞ of the LECs depend on the ITO surface treatments. At V ¼ 7 V for devices fabricated with the untreated, solvent cleaned, acid treated, and oxygen plasma-treated ITO substrates, the values of J are 752.8, 811.1, 594.4, and 1355.1 A/m2; and L are 25.5, 40.3, 6.9, and 98.2 cd/m2, respectively. And the corresponding Z values can be readily calculated out to be about 0.034, 0.050, 0.012, and 0.073 cd/A, respectively.
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AFM images shown in Fig. 4 display that the oxygen plasma treatment makes ITO substrate exhibit more smooth and homogeneous surface, compared with other treatments. Fig. 5 gives the measured contact angles, surface energies and polarities of all the ITO substrates. As can be seen, the oxygen plasma treatment brings about the maximum reduction in H2O contact angle yH2 O and CH2I2 contact angle yCH2 I2 , from 85.11 and 46.21 to 30.51 and 32.11, respectively, implying that the oxygen plasma-treated ITO surface appears to be highly polar. Additionally, the oxygen plasma treatment yields the highest surface energy gS (66.3 mJ/m2) and the highest surface polarity wp (0.56), mainly due to the significantly enhanced polar component gpS (from 8.2 to 36.4 mJ/m2). We propose that the increase of the surface energy and polarity is due to several reasons [28–30]. One reason is the improvement of the surface stoichiometry, since oxygen plasma treatment introduces oxygen and increases the oxygen content, and meanwhile effectively removes the hydrocarbon contaminants from the ITO surface so as to reduce the carbon content. The second reason is the increase of water concentration that is physi- and/or chemisorbed on the surface. The third possible reason is that organic contamination may become more hydrophilic after oxygen plasma treatment. Table 2 summarizes the electrical and optical performance of the devices with different ITO treatments. As can be seen, the oxygen plasma-treated device in our present work yields the higher injection current, luminance and
3.2. Surface properties of the ITO with different treatments Table 1 shows the changes in chemical composition and surface roughness of ITO under different surface treatments. Note that besides containing the elements of O, Sn, and In, the carbon element also exists on each ITO surface, indicating that carbon is the only major contaminant. Note also that for the untreated, solvent cleaned, acid treated, and oxygen plasma-treated ITO surfaces, the oxygen contents are 15.1%, 14.9%, 13.6%, 16.5%, and the carbon contents are 5.6%, 9.2%, 4.9%, 4.6%, respectively. It suggests that the oxygen plasma treatment largely increases the oxygen content and decreases the carbon content, and improves the surface stoichiometry, and thereby more effectively enhances the work function of the ITO, since the ITO work function is determined by the oxygen content and the carbon content [24–27]. Furthermore, we observe from Table 1 that the surface roughness values of ITO substrates are also subjected to the surface treatments. Obviously, only the oxygen plasma treatment decreases the surface roughness, while other treatments more or less increase the surface roughness of ITO substrates. Also, the
Fig. 5. Changes in contact angle, surface energy and polarity of ITO under different surface treatments.
ARTICLE IN PRESS Z.Z. You, J.Y. Dong / Microelectronics Journal 38 (2007) 108–113
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Table 2 Device performance of LECs fabricated with different ITO treatments ITO treatment
Untreated Solvent cleaning Acid treatment Oxygen plasma
ITO surface properties
Device electrical and optical performance
O (%)
C (%)
gS (mJ/cm2)
wp
V Jon (V)
V Lon (V)
J (A/m2)
L (cd/m2)
Z (cd/A)
15.1 14.9 13.6 16.5
5.6 9.2 4.9 4.6
37.9 47.5 38.6 66.3
0.213 0.386 0.254 0.556
2.2 2.2 2.2 2.2
2.7 2.6 2.9 2.4
752.8 811.1 594.4 1355.1
25.5 40.3 6.9 98.2
0.034 0.050 0.012 0.073
The values of J, L and Z are measured at V ¼ 7 V.
efficiency, exhibiting the relatively better performance, while the hydrochloric acid-treated device the poorer. Such an experimental result suggests that the device performance of LECs is closely related to the characteristics of electrical contact at the ITO/polymer interface. The Oxygen plasma treatment results not only in higher work function, lower surface roughness, but also higher surface energy and polarity of the ITO substrate which is highly important to improve the surface morphological properties of polymer thin film and to optimize the interface characteristics of ITO/polymer. Firstly, the increase of surface polarity of ITO substrate leads to a more uniform spreading of the polymer solution on the ITO surface, reduces the voids and pinholes, and thereby obtains the homogeneous and compact polymer thin film. Secondly, the increase of the surface energy of ITO substrate provides a better adhesion of the polymer film and reduces the interfacial energy between the polymer film and ITO substrate, and therefore increases the effective area available between these two materials. These two effects above-stated can significantly improve the interface formation and electrical contact of the ITO substrate with the polymer blend in the LECs, so as to facilitate the electrochemical doping of the electroluminescent polymer when sufficiently large bias was applied, and therefore yield the relatively better performance of the LECs. Contrary to the situation in the oxygen plasma treatment, yet, the hydrochloric acid treatment results in a poor electrical contact to the emissive layer in the LECs and hinders charge injection through the electrochemical doping, and thereby produces the poorer performance of LECs. 4. Conclusion In this study, the different surface treatments were carried out on the ITO substrates and the surface properties of treated ITO were investigated through the measurements of AFM, XPS, contact angle and surface energy. Experimental results indicate that the surface properties of the ITO substrates strongly depend on the surface treatments. The oxygen plasma more effectively improves the ITO surface properties compared with other treatments, since plasma decreases the surface roughness, enhances the surface energy and polarity, and improves the surface stoichiometry. Furthermore, the LECs with the
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