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RETRACTED: Passivation properties of OLEDs with aluminum cathodes prepared by ion-beam-assisted deposition process
Applied Surface Science 241 (2005) 352–361 www.elsevier.com/locate/apsusc
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Passivation properties of OLEDs with aluminum cathodes prepared by ion-beam-assisted deposition process Soon Moon Jeonga, Won Hoi Kooa, Sang Hun Choia, Sung Jin Joa, Hong Koo Baika,*, Seong Min Leeb b
a Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Republic of Korea Department of Materials Science and Engineering, Incheon University, Incheon 402-736, Republic of Korea
Received in revised form 16 July 2004; accepted 17 July 2004 Available online 25 September 2004
Abstract
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A long-lived organic light emitting diode (OLED) was fabricated using a dense aluminum cathode prepared by the ion-beamassisted deposition (IBAD) process. We investigated the passivation properties of ion-beam-assisted and thermal evaporationinduced aluminum cathodes mounted on Ph-PPV. The dense and highly packed Al cathode effectively prevents the permeation of H2O and O2 through pinhole defects, which results in retarding dark spot growth. Employing thin Al buffer layer diminished Ar+ ion-induced damages in Ph-PPV and limited permeation against H2O and O2. The interface between Al and Ph-PPV may be modified in IBAD case, even though buffered Al layer was deposited to 30 nm by thermal evaporation prior to Ar+ ion beam irradiation. It is believed that the buffered Al film cannot screen the Ar+ ions or Al atoms wholly due to the existence of pinholes or non-deposited regions among the columnar structures. # 2004 Elsevier B.V. All rights reserved.
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Keywords: OLED; Ion-beam-assisted deposition; Passivation; Lifetime
1. Introduction
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Since efficient electroluminescence (EL) from an organic light emitting device (OLED) was first reported, there has been considerable interest in utilizing OLEDs for full color flat panel display applications [1]. However, the OLED devices still * Corresponding author. Tel.: +82 2 2123 2838; fax: +82 2 312 5375. E-mail address: [email protected] (H.K. Baik).
have a limitation of short lifetime due to degradation resulting from crystallization of organic materials [2,3], chemical reactions including oxidation [4] and gas evolution [5,6]. Nevertheless, there has been little mention of passivation layer that protect OLED devices from water and oxygen in a room ambient. In the food packaging or medical device industries it is desirable to manufacture thin, flexible and transparent films that act as a protector against oxygen and moisture. Some researchers proposed a quantitative model for SiOx/polymer films which estimate the
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.07.048
S.M. Jeong et al. / Applied Surface Science 241 (2005) 352–361
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degree of oxygen permeation through the lattice (‘pore’ size < 0.3 nm), nano-defects (0.3–1.0 nm), and macro-defects (>1 nm) at a given oxide layer. The existence of nano-defects in silica glasses has been previously found based on observed decreases in the activation energy of permeation at low temperatures [7]. The role of nano-defects was reported to be effective for relatively large molecules (e.g. O2 and H2O) since their lattice permeabilities at room temperature are virtually negligible. So note that the passivation layer should not contain nano-defects to protect the OLED device from O2 and H2O. However, there have been few reports about enhancing passivation property through cathode modification. To minimize damage in organic materials, the cathode has been deposited by the thermal evaporation process with low adatom mobility. This process allows the porous thin film cathode having many pinholes, even though damage is not produced. In this paper, we develop the highly packed aluminum cathode using the IBAD technique, leading to an increase lifetime of the OLED device through the improvement of protection ability against O2 and H2O.
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Indium tin oxide (ITO) coated glass was used as the substrate for OLEDs which was about 200 nm thick with a sheet resistance of approximately 15 V/square. ITO was patterned using a gas mixture of hydrochloric acid (HCl) and nitric acid (HNO3) with same volume quantity and then 3 mm-sized ITO stripes were obtained. After ITO patterning, the standard cleaning procedure included the sonication in a detergent, acetone, and isopropyl alcohol successively, and then rinsing in deionized water. Poly-3,4-ethylenedioxythiophene (PEDOT) doped with poly(styrenesulfonate) (PSS) was spin-coated up to a thickness of 50 nm as a hole transport layer (HTL). A soluble phenylsubstituted poly-p-phenylene-vinylene (Ph-PPVs) obtained from Covion was spin-coated to have a thickness of 80 nm as an emissive layer on the HTL layer successively. Then, Al cathodes were deposited up to about 100 nm in thickness by the conventional thermal evaporation method and IBAD process, respectively. The schematic diagram of an IBAD
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Fig. 1. Schematic diagram of (a) IBAD system and (b) device structure.
system and the device are shown in Fig. 1. The Kaufman ion gun was used to increase aluminum adatom mobility. Ar+ ion energy and current density were 150 eV and 70 mA/cm2, and Al deposition rate ˚ /s. In all cases of ion beam irradiation was 1–2 A conditions, the incident angle of the ion beam was 458. To avoid Ph-PPV damages by Ar+ ion bombardment, thermally evaporated Al is deposited up to 30 nm in thickness prior to ion-beam-assisted deposition.
3. Results and discussion It was reported that smoother barrier coatings have low values of diffusivity, whereas for rougher coatings the diffusivity is much worse [8]. In order to investigate the effects of deposition rate on the roughness, we manipulated aluminum deposition rate ˚ . It has been generally accepted that the from 1 to 50 A surface morphology is flat at slow deposition rate because adatom moving time is longer than that of higher deposition speed. Fig. 2 shows the SEM images of aluminum thin films on Ph-PPV having different deposition rates. The
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˚ /s, respectively and (e) ionFig. 2. The SEM images of (a) bare PC substrate, (b)–(d) thermally evaporated Al with deposition rate of 50, 10, 1 A ˚ /s. beam-assisted Al with deposition rate of 1 A
˚ and aluminum deposition rate varies from 1 to 50 A the speed of spin coating is fixed at 4000 rpm spin. In Fig. 2(b)–(d), the surface of aluminum thin films obtained at low deposition speed is smoother than that
at high deposition speed. It might be due to prolonged adatom moving time. The impingement of energetic ions or atoms upon a solid surface produces a wide variety of effects, and
S.M. Jeong et al. / Applied Surface Science 241 (2005) 352–361
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there has been a vast amount of experimental and theoretical work on the subject. Many of these effects are beneficial to thin film deposition, thus ion bombardment is one of the most important thin film process parameters. Careful studies of atomic trajectories during ion impact onto a porous microcolumnar surface structure clarified the essential mechanisms which contribute to an improvement in layer growth: (i) ion bombardment during growth removes overhanging atoms and causes void regions to remain open until filled by new depositing atoms; (ii) sputtered atoms are redeposited mainly in voids; (iii) ions induce surface diffusion (diffusion distance is a few interatomic spacings), local heating, collapse of voids and recrystallization [9]. The ion-beam-assisted deposition process was adopted to obtain a highly flat and packed aluminum structure. Fig. 2 shows the SEM data of the Al cathodes deposited by thermal evaporation and IBAD process. In the thermally evaporated Al cathode, islands and voids are very large. On the other hand, the Al cathode deposited by IBAD contains the fine islands and voids that may restrict the permeation of H2O and O2. The Al atoms bombarded by energetic Ar+ ion produce the cathode having the highly packed structure due to their high surface adatom mobility. The degree of surface roughness of the Al films deposited by IBAD is lowered remarkably for Ar+ ion energy of 150 eV. To study the effect of ion-beam-assisted Al on the protection against H2O, the water vapor transmission rate (WVTR) of bare PC (polycarbonate) and cathodes obtained from different deposition processes on PC substrate was measured using a MOCON instrument at 37.8 8C. All WVTR values are reported in units of g/ m2 per day. In Fig. 3(a), it can be seen that the Al layers from thermal evaporation and IBAD have much lower WVTR value than the bare PC substrate does. Further, the WVTR value of the Al layer from IBAD is even lower than that from thermal evaporation. But these values are somewhat higher than those reported for the most commonly used coatings on PET (polyethylene terephthalate), Al [10,11], SiO [12–14] and AlOx [15]. This might be due to the macro-defects existed in the bare PC substrate as shown in Fig. 3(b). These macrodefects induce the pinholes in Al films that act as a pathway of H2O due to the shadowing effect.
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Fig. 3. (a) Water vapor transmission rate (WVTR) and (b) optical image of bare PC substrate.
However, the shadowing effect can be minimized in the ion-beam-assisted Al layer because of its relative high adatom mobility, resulting in a decrease in the value. The lowest value of WVTR may be caused by not only macro-defects existed in bare PC substrate but also nano-defects or lattice interstices. Roberts et al. reported that macro-defects (>1 nm), nano-defects (0.3–0.4 nm) and the lattice interstices (<0.3 nm) all contribute to total permeation [7]. The relative contribution of each component depends on the size of the permeant molecule (or atom), and the number and size of each class of defect. Although the relative
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luminescent features were captured using optical microscopy to investigate the growth of dark spots. The corresponding devices consisted of multi-layers, ITO/PEDOT/Ph-PPV/LiF/Al, without any encapsulation or passivation layer. Fig. 4 shows the optical images of both Al cathodes. Dark spots induced by dust particles were formed inevitably since the devices were fabricated at an ordinary laboratory without filtering process from the air. In the thermally evaporated Al device, the dark spots were growing gradually with time whereas in the ion-beam-assisted Al device remained almost steady. It was believed that the formation of pinholes resulting
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contribution of each factor were not clearly examined in this experiment, it is obvious that films prepared with ion bombardment have packed grain structure and flat grain morphology resulting in smaller size of nano-defects or lattice interstices [16]. These may induce the lower WVTR value in ion-beam-assisted Al sample. In order to compare the passivation properties of the thermally evaporated Al and ion-beam-assisted Al cathode, the storage lifetime test was performed. Substrates were fabricated identically for the comparison of two different cathodes. The resulting specimens were stored in air condition and electro-
Fig. 4. Optical images of the electroluminescence with time in Al cathodes prepared by thermal evaporation (a) and ion-beam-assisted deposition (b).
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Fig. 5. AFM images of Al films prepared by thermal evaporation and ion-beam-assisted deposition: (a) phase (2D) images and (b) oblique (3D) images.
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from dust particles was minimized due to high surface adatom mobility for the IBAD-related device. Moreover, this enhanced passivation property can be explained by dense Al structure using Ar+ ion irradiation resulting in reduced pinholes. Atomic force microscopy has been employed to identify differences arising in surface morphology and roughness of samples prepared by thermal evaporation and IBAD exhibiting barrier efficacies. To ascertain if morphological features are at least partly responsible for the differences in H2O, higher resolution 2 mm–2 mm phase and oblique (3D) images are presented in Fig. 5. These images reveal that the surface of Al films can be described as columnar or granular structure. The phase images displayed in Fig. 5(a) show that the Al film surface prepared by IBAD consists of densely packed small spherical clusters (grains) with well-defined boundaries. Such fine uniform features are responsible for the lower WVTR value because the penetrant molecules locate and diffuse through the relatively few and small defect
sites (due to the dense packing of particles) within the Al film. In case of Al film prepared by thermal evaporation, a poorer barrier coating is produced. An increase in particle size is accompanied by an increase in grain boundary area. When this effect is combined with the irregular grain texture, it is anticipated that a greater number of penetrant molecules will diffuse through intergranular pores. When compared on a fixed vertical scale (z-range of 100 nm), the topography of the IBAD sample (Fig. 5(b)) appears to be composed of uniformly distributed, small columnar features, whereas large domes create valleys that may not be completely covered with Al, thereby forming diffusive pathways in coatings that exhibit poor barrier quality in thermal sample. Such megalithic structures increase the rms surface roughness, which is shown in Fig. 5(b). In addition, the cell even degraded due to the permeation of H2O and O2 at its edge. This is because the present device was a passive matrix having the structure exposed to air at the interface between Al and
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Fig. 6. Optical images of edge permeation in electroluminescent devices of thermal evaporated Al (a) during luminescence and (b) comparison with off-luminescence.
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Ph-PPV, and so we failed to show the optical image of electroluminescence only after 3 h for the thermal evaporated Al device. However, edge permeation in the IBAD device is much smaller than the thermal device due to its high packed Al structure. Fig. 6(a) shows H2O or O2 permeation progress in thermal evaporated Al device through the edge structure with increasing time. When the device was stored in air condition, dark spots were growing with increasing time. However, the degradation of edge was too fast to observe growth of dark spots. After storing the device in air condition for 3 h, electroluminescent region was shrunk by edge permeation as shown in Fig. 6(a). This Al cathode oxidation by edge permeation of H2O and O2 can be observed even when bias is not applied to the device as shown in Fig. 6(b). Gas bubbles were formed in the oxidation region and
these bubbles have dome-like structure filled with gases (mostly oxygen) presumably evolved during electrochemical and photoelectrochemical processes in the presence of water [17]. Gases stimulated by significant heating during the device operation form these bubbles, and then cause cathode delamination at metal/organic material interface [18]. On the other hand, in case of IBAD sample, edge permeation is reduced compared than thermal case even though an Al buffer layer is deposited to 30 nm in thickness by thermal evaporation prior to ion-beamassisted deposition to avoid ion bombardment induced damages in Ph-PPV. This indicates that IBAD process may influence the interfacial state because edge permeation is slower than thermal case. Although an Al buffer layer is deposited to 30 nm in thickness by thermal evaporation prior to ion-beam-
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stack, when organic films are subjected to irradiation through pinholes [19]. The formation of pinholes is possibly related to the island-like growth of thin films of MgAg on Alq [20]. Fig. 7 illustrates the deposition process of Al to make the buffered IBAD device. As thermally evaporated Al is deposited to 30 nm in thickness prior to Ar+ ion irradiation, the buffered Al film may have many pinholes and porous structure due to the low surface adatom mobility. When the Ar+ ions or their activated Al atoms are irradiated to the buffered Al film surface, they cannot be screened by the buffered Al film wholly because they can transmit to Ph-PPV through the pinholes or non-deposited regions among the columnar structures. This idea can be supported by TEM data indirectly. Fig. 8 is TEM images showing the plan-view images of Al specimens prepared by three different processes. In this experiment, the thickness of the Al film is fixed at 30 nm for transmitting the electron
Fig. 7. Schematic diagram of ion-beam-assisted deposition process in buffered Al layer.
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assisted deposition to avoid ion bombardment induced damages in Ph-PPV, IBAD process may influence the interfacial state because edge permeation is slower than thermal case. Hung and Madathil reported that catastrophic failure still occurs even though 10 nm of MgAg is sufficient thick to protect the organic layer
Fig. 8. TEM images (100,000) of Al prepared by (a) thermal evaporation (30 nm), (b) thermal (20 nm) + IBAD (10 nm) and (c) IBAD Al (30 nm).
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beam through it in a given transmission electron microscopy. Here, the conventional TEM sample preparation method cannot be used for the present organic samples. Fig. 8(a) is a TEM image of the Al film (30 nm) deposited by thermal evaporation. The resulting buffered Al film consists of thermally evaporated Al (20 nm) prior to the Al film (10 nm), which is obtained by only Ar+ ion-beam-assisted deposition, as shown in Fig. 8(b) and (c). As seen in Fig. 8(a), the grain size of the evaporated film was in ˚ . In contrast, Fig. 8(c) the range of 800–1000 A revealed that the grain size was much smaller, merely ˚ , for the films prepared by IBAD. As may 200–300 A be seen from these results, the concurrent ion bombardment of the depositing film has led to a considerable grain refinement. Note that the buffered Al film has partly small grain size compared than that of thermal evaporation. Although they are different from the previous buffered Al film which is deposited up to 30 nm just by thermal evaporation, TEM data show the decreased average grain size in buffered Al film (Fig. 8(b)) compared with thermally evaporated Al (Fig. 8(a)). That might be caused by transmitting Ar+ ions or activated Al atoms through the pinholes resulting in breakup of 3D structure (Fig. 7). So the significant enhancement in the passivation properties of the OLEDs prepared with ion bombardment result from not only densification of the Al structure but also interface modification between Al and Ph-PPV retarding H2O and O2 permeation at edge structure. In order to observe whether Ar+ ions or activated Al atoms transmit through the pinholes or non-deposited regions, we investigated the electrical characteristics of OLED device to measure damages in buffered IBAD device. Fig. 9 shows the current–voltage (I–V) characteristics of the OLEDs with ion-beam-assisted Al cathodes were measured and compared with that of a conventional Ph-PPV based device. The Al cathode depositions by IBAD were made in two different conditions, one with the thermally evaporated Al ˚ before using IBAD process to buffer layer about 300 A avoid Ph-PPV damages by Ar+ ion bombardment and another without Al buffer layer. The measurements have been carried out in a glove box filled with argon gas of 99.9999% purity to exclude the permeation effect of H2O and O2 because dark spots formed by the cathode oxidation restrain the current injection.
Fig. 9. I–V plot of OLED device with Al cathode deposited by thermal evaporation and ion-beam-assisted deposition: (a) voltage– current and (b) magnified voltage–current.
There is some leakage current even in buffered IBAD device though that has a little smaller value compared than IBAD device without buffer layer as shown in Fig. 9. Although above I–V data cannot be related directly with the generation of damages through pinholes in buffered Al film, it is certain that the leakage current is induced by Ar+ ion-induced damages because the leakage current must not be generated at reverse bias regime due to large barrier for electrons and holes injection.
4. Conclusion It was investigated in the present work that the Al cathode generated by IBAD has the more improved
S.M. Jeong et al. / Applied Surface Science 241 (2005) 352–361
References
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[1] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913. [2] Y. Sato, H. Kanai, Mol. Cryst. Liq. Cryst. 253 (1994) 143. [3] H. Aziz, Z. Popovic, S. Xie, A. Hor, N. Hu, C. Tripp, G. Xu, Appl. Phys. Lett. 72 (1998) 756. [4] P.E. Burrows, V. Bulovic, S.R. Forrest, L.S. Sapochak, D.M. McCarty, M.E. Thompson, Appl. Phys. Lett. 65 (1994) 2922. [5] L.M. Do, E.M. Han, Y. Niidome, M. Fujihira, T. Kanno, S. Yoshida, A. Maeda, A.J. Ikushidma, J. Appl. Phys. 76 (1994) 5118. [6] H. Aziz, G. Xu, Synth. Met. 80 (1996) 7. [7] A.P. Roberts, B.M. Henry, A.P. Sutton, C.R.M. Grovenor, G.A.D. Briggs, T. Miyamoto, M. Kano, Y. Tsukahara, M. Yanaka, J. Membr. Sci. 208 (2002) 75.
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[8] G. Garcia-Ayuso, L. Vazquez, J.M. Martinez-Duart, Surf. Coat. Tech. 80 (1996) 203. [9] J.J. Cuomo, S.M. Rossnagel, H.R. Kaufman, Handbook of Ion Beam Processing Technology, Noyes Publications, 1989. [10] B.M. Henry, F. Dinelli, K.-Y. Zhao, C.R.M. Grovenor, O.V. Kolosov, G.A.D. Briggs, A.P. Roberts, R.S. Kumar, R.P. Howson, Thin Solid Films 355–356 (1999) 500. [11] E.H.H. Jamieson, A.H. Windle, J. Mater. Sci. 18 (1983) 64. [12] Y.G. Tropsha, N.G. Harvey, J. Phys. Chem. B 101 (1997) 2259. [13] A.G. Erlat, R.J. Spontak, R.P. Clarke, T.C. Robinson, P.D. Haaland, Y. Tropsha, N.G. Harvey, E.A. Vogler, J. Phys. Chem. B 103 (1999) 6047. [14] A.S. da Silva Sobrinho, G. Czeremuszkin, M. Latreche, M.R. Wertheimer, J. Vac. Sci. Tech. A 18 (2000) 149. [15] B.M. Henry, F. Dinelli, K.-Y. Zhao, A.G. Erlat, C.R.M. Grovenor, G.A.D. Briggs, in: Proceedings of the 42nd Annual Technical Conference of Society of Vacuum Coaters Society, Chicago, USA, April 17–22, 1999, p. 403. [16] J.S. Kwak, H.K. Baik, J.H. Kim, S.M. Lee, Appl. Phys. Lett. 71 (1997) 2451. [17] L.M. Do, M. Oyamada, A. Koike, E.M. Han, N. Yamamoto, M. Fujira, Thin Solid Films 273 (1996) 209. [18] V.N. Savvateev, A.V. Yakimov, D. Davidov, R.M. Pogreb, R. Neumann, Y. Avny, Appl. Phys. Lett. 71 (1997) 3344. [19] L.S. Hung, J. Madathil, Thin Solid Films 410 (2002) 101–106. [20] P.E. Burrows, G. Gu, S.R. Forrest, E.P. Vicenzi, T.X. Zhou, J. Appl. Phys. 87 (2000) 3080.
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passivation properties than that by thermal evaporation by allowing the more dense structure that restricts the permeation of H2O and O2. Employing thin Al buffer layer diminished Ar+ ion-induced damages in Ph-PPV and limited permeation against H2O and O2 at the edge structure exposed to air. It might result from transmitting Ar+ ions through porous buffered Al film, leading to prolonged lifetime.
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Passivation properties of OLEDs with aluminum cathodes prepared by ion-beam-assisted deposition process Soon Moon Jeonga, Won Hoi Kooa, Sang Hun Choia, Sung Jin Joa, Hong Koo Baika,*, Seong Min Leeb b
a Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Republic of Korea Department of Materials Science and Engineering, Incheon University, Incheon 402-736, Republic of Korea
Received in revised form 16 July 2004; accepted 17 July 2004 Available online 25 September 2004
Abstract
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A long-lived organic light emitting diode (OLED) was fabricated using a dense aluminum cathode prepared by the ion-beamassisted deposition (IBAD) process. We investigated the passivation properties of ion-beam-assisted and thermal evaporationinduced aluminum cathodes mounted on Ph-PPV. The dense and highly packed Al cathode effectively prevents the permeation of H2O and O2 through pinhole defects, which results in retarding dark spot growth. Employing thin Al buffer layer diminished Ar+ ion-induced damages in Ph-PPV and limited permeation against H2O and O2. The interface between Al and Ph-PPV may be modified in IBAD case, even though buffered Al layer was deposited to 30 nm by thermal evaporation prior to Ar+ ion beam irradiation. It is believed that the buffered Al film cannot screen the Ar+ ions or Al atoms wholly due to the existence of pinholes or non-deposited regions among the columnar structures. # 2004 Elsevier B.V. All rights reserved.
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Keywords: OLED; Ion-beam-assisted deposition; Passivation; Lifetime
1. Introduction
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Since efficient electroluminescence (EL) from an organic light emitting device (OLED) was first reported, there has been considerable interest in utilizing OLEDs for full color flat panel display applications [1]. However, the OLED devices still * Corresponding author. Tel.: +82 2 2123 2838; fax: +82 2 312 5375. E-mail address: [email protected] (H.K. Baik).
have a limitation of short lifetime due to degradation resulting from crystallization of organic materials [2,3], chemical reactions including oxidation [4] and gas evolution [5,6]. Nevertheless, there has been little mention of passivation layer that protect OLED devices from water and oxygen in a room ambient. In the food packaging or medical device industries it is desirable to manufacture thin, flexible and transparent films that act as a protector against oxygen and moisture. Some researchers proposed a quantitative model for SiOx/polymer films which estimate the
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.07.048
S.M. Jeong et al. / Applied Surface Science 241 (2005) 352–361
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degree of oxygen permeation through the lattice (‘pore’ size < 0.3 nm), nano-defects (0.3–1.0 nm), and macro-defects (>1 nm) at a given oxide layer. The existence of nano-defects in silica glasses has been previously found based on observed decreases in the activation energy of permeation at low temperatures [7]. The role of nano-defects was reported to be effective for relatively large molecules (e.g. O2 and H2O) since their lattice permeabilities at room temperature are virtually negligible. So note that the passivation layer should not contain nano-defects to protect the OLED device from O2 and H2O. However, there have been few reports about enhancing passivation property through cathode modification. To minimize damage in organic materials, the cathode has been deposited by the thermal evaporation process with low adatom mobility. This process allows the porous thin film cathode having many pinholes, even though damage is not produced. In this paper, we develop the highly packed aluminum cathode using the IBAD technique, leading to an increase lifetime of the OLED device through the improvement of protection ability against O2 and H2O.
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Indium tin oxide (ITO) coated glass was used as the substrate for OLEDs which was about 200 nm thick with a sheet resistance of approximately 15 V/square. ITO was patterned using a gas mixture of hydrochloric acid (HCl) and nitric acid (HNO3) with same volume quantity and then 3 mm-sized ITO stripes were obtained. After ITO patterning, the standard cleaning procedure included the sonication in a detergent, acetone, and isopropyl alcohol successively, and then rinsing in deionized water. Poly-3,4-ethylenedioxythiophene (PEDOT) doped with poly(styrenesulfonate) (PSS) was spin-coated up to a thickness of 50 nm as a hole transport layer (HTL). A soluble phenylsubstituted poly-p-phenylene-vinylene (Ph-PPVs) obtained from Covion was spin-coated to have a thickness of 80 nm as an emissive layer on the HTL layer successively. Then, Al cathodes were deposited up to about 100 nm in thickness by the conventional thermal evaporation method and IBAD process, respectively. The schematic diagram of an IBAD
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Fig. 1. Schematic diagram of (a) IBAD system and (b) device structure.
system and the device are shown in Fig. 1. The Kaufman ion gun was used to increase aluminum adatom mobility. Ar+ ion energy and current density were 150 eV and 70 mA/cm2, and Al deposition rate ˚ /s. In all cases of ion beam irradiation was 1–2 A conditions, the incident angle of the ion beam was 458. To avoid Ph-PPV damages by Ar+ ion bombardment, thermally evaporated Al is deposited up to 30 nm in thickness prior to ion-beam-assisted deposition.
3. Results and discussion It was reported that smoother barrier coatings have low values of diffusivity, whereas for rougher coatings the diffusivity is much worse [8]. In order to investigate the effects of deposition rate on the roughness, we manipulated aluminum deposition rate ˚ . It has been generally accepted that the from 1 to 50 A surface morphology is flat at slow deposition rate because adatom moving time is longer than that of higher deposition speed. Fig. 2 shows the SEM images of aluminum thin films on Ph-PPV having different deposition rates. The
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˚ /s, respectively and (e) ionFig. 2. The SEM images of (a) bare PC substrate, (b)–(d) thermally evaporated Al with deposition rate of 50, 10, 1 A ˚ /s. beam-assisted Al with deposition rate of 1 A
˚ and aluminum deposition rate varies from 1 to 50 A the speed of spin coating is fixed at 4000 rpm spin. In Fig. 2(b)–(d), the surface of aluminum thin films obtained at low deposition speed is smoother than that
at high deposition speed. It might be due to prolonged adatom moving time. The impingement of energetic ions or atoms upon a solid surface produces a wide variety of effects, and
S.M. Jeong et al. / Applied Surface Science 241 (2005) 352–361
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there has been a vast amount of experimental and theoretical work on the subject. Many of these effects are beneficial to thin film deposition, thus ion bombardment is one of the most important thin film process parameters. Careful studies of atomic trajectories during ion impact onto a porous microcolumnar surface structure clarified the essential mechanisms which contribute to an improvement in layer growth: (i) ion bombardment during growth removes overhanging atoms and causes void regions to remain open until filled by new depositing atoms; (ii) sputtered atoms are redeposited mainly in voids; (iii) ions induce surface diffusion (diffusion distance is a few interatomic spacings), local heating, collapse of voids and recrystallization [9]. The ion-beam-assisted deposition process was adopted to obtain a highly flat and packed aluminum structure. Fig. 2 shows the SEM data of the Al cathodes deposited by thermal evaporation and IBAD process. In the thermally evaporated Al cathode, islands and voids are very large. On the other hand, the Al cathode deposited by IBAD contains the fine islands and voids that may restrict the permeation of H2O and O2. The Al atoms bombarded by energetic Ar+ ion produce the cathode having the highly packed structure due to their high surface adatom mobility. The degree of surface roughness of the Al films deposited by IBAD is lowered remarkably for Ar+ ion energy of 150 eV. To study the effect of ion-beam-assisted Al on the protection against H2O, the water vapor transmission rate (WVTR) of bare PC (polycarbonate) and cathodes obtained from different deposition processes on PC substrate was measured using a MOCON instrument at 37.8 8C. All WVTR values are reported in units of g/ m2 per day. In Fig. 3(a), it can be seen that the Al layers from thermal evaporation and IBAD have much lower WVTR value than the bare PC substrate does. Further, the WVTR value of the Al layer from IBAD is even lower than that from thermal evaporation. But these values are somewhat higher than those reported for the most commonly used coatings on PET (polyethylene terephthalate), Al [10,11], SiO [12–14] and AlOx [15]. This might be due to the macro-defects existed in the bare PC substrate as shown in Fig. 3(b). These macrodefects induce the pinholes in Al films that act as a pathway of H2O due to the shadowing effect.
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Fig. 3. (a) Water vapor transmission rate (WVTR) and (b) optical image of bare PC substrate.
However, the shadowing effect can be minimized in the ion-beam-assisted Al layer because of its relative high adatom mobility, resulting in a decrease in the value. The lowest value of WVTR may be caused by not only macro-defects existed in bare PC substrate but also nano-defects or lattice interstices. Roberts et al. reported that macro-defects (>1 nm), nano-defects (0.3–0.4 nm) and the lattice interstices (<0.3 nm) all contribute to total permeation [7]. The relative contribution of each component depends on the size of the permeant molecule (or atom), and the number and size of each class of defect. Although the relative
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luminescent features were captured using optical microscopy to investigate the growth of dark spots. The corresponding devices consisted of multi-layers, ITO/PEDOT/Ph-PPV/LiF/Al, without any encapsulation or passivation layer. Fig. 4 shows the optical images of both Al cathodes. Dark spots induced by dust particles were formed inevitably since the devices were fabricated at an ordinary laboratory without filtering process from the air. In the thermally evaporated Al device, the dark spots were growing gradually with time whereas in the ion-beam-assisted Al device remained almost steady. It was believed that the formation of pinholes resulting
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contribution of each factor were not clearly examined in this experiment, it is obvious that films prepared with ion bombardment have packed grain structure and flat grain morphology resulting in smaller size of nano-defects or lattice interstices [16]. These may induce the lower WVTR value in ion-beam-assisted Al sample. In order to compare the passivation properties of the thermally evaporated Al and ion-beam-assisted Al cathode, the storage lifetime test was performed. Substrates were fabricated identically for the comparison of two different cathodes. The resulting specimens were stored in air condition and electro-
Fig. 4. Optical images of the electroluminescence with time in Al cathodes prepared by thermal evaporation (a) and ion-beam-assisted deposition (b).
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Fig. 5. AFM images of Al films prepared by thermal evaporation and ion-beam-assisted deposition: (a) phase (2D) images and (b) oblique (3D) images.
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from dust particles was minimized due to high surface adatom mobility for the IBAD-related device. Moreover, this enhanced passivation property can be explained by dense Al structure using Ar+ ion irradiation resulting in reduced pinholes. Atomic force microscopy has been employed to identify differences arising in surface morphology and roughness of samples prepared by thermal evaporation and IBAD exhibiting barrier efficacies. To ascertain if morphological features are at least partly responsible for the differences in H2O, higher resolution 2 mm–2 mm phase and oblique (3D) images are presented in Fig. 5. These images reveal that the surface of Al films can be described as columnar or granular structure. The phase images displayed in Fig. 5(a) show that the Al film surface prepared by IBAD consists of densely packed small spherical clusters (grains) with well-defined boundaries. Such fine uniform features are responsible for the lower WVTR value because the penetrant molecules locate and diffuse through the relatively few and small defect
sites (due to the dense packing of particles) within the Al film. In case of Al film prepared by thermal evaporation, a poorer barrier coating is produced. An increase in particle size is accompanied by an increase in grain boundary area. When this effect is combined with the irregular grain texture, it is anticipated that a greater number of penetrant molecules will diffuse through intergranular pores. When compared on a fixed vertical scale (z-range of 100 nm), the topography of the IBAD sample (Fig. 5(b)) appears to be composed of uniformly distributed, small columnar features, whereas large domes create valleys that may not be completely covered with Al, thereby forming diffusive pathways in coatings that exhibit poor barrier quality in thermal sample. Such megalithic structures increase the rms surface roughness, which is shown in Fig. 5(b). In addition, the cell even degraded due to the permeation of H2O and O2 at its edge. This is because the present device was a passive matrix having the structure exposed to air at the interface between Al and
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Fig. 6. Optical images of edge permeation in electroluminescent devices of thermal evaporated Al (a) during luminescence and (b) comparison with off-luminescence.
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Ph-PPV, and so we failed to show the optical image of electroluminescence only after 3 h for the thermal evaporated Al device. However, edge permeation in the IBAD device is much smaller than the thermal device due to its high packed Al structure. Fig. 6(a) shows H2O or O2 permeation progress in thermal evaporated Al device through the edge structure with increasing time. When the device was stored in air condition, dark spots were growing with increasing time. However, the degradation of edge was too fast to observe growth of dark spots. After storing the device in air condition for 3 h, electroluminescent region was shrunk by edge permeation as shown in Fig. 6(a). This Al cathode oxidation by edge permeation of H2O and O2 can be observed even when bias is not applied to the device as shown in Fig. 6(b). Gas bubbles were formed in the oxidation region and
these bubbles have dome-like structure filled with gases (mostly oxygen) presumably evolved during electrochemical and photoelectrochemical processes in the presence of water [17]. Gases stimulated by significant heating during the device operation form these bubbles, and then cause cathode delamination at metal/organic material interface [18]. On the other hand, in case of IBAD sample, edge permeation is reduced compared than thermal case even though an Al buffer layer is deposited to 30 nm in thickness by thermal evaporation prior to ion-beamassisted deposition to avoid ion bombardment induced damages in Ph-PPV. This indicates that IBAD process may influence the interfacial state because edge permeation is slower than thermal case. Although an Al buffer layer is deposited to 30 nm in thickness by thermal evaporation prior to ion-beam-
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stack, when organic films are subjected to irradiation through pinholes [19]. The formation of pinholes is possibly related to the island-like growth of thin films of MgAg on Alq [20]. Fig. 7 illustrates the deposition process of Al to make the buffered IBAD device. As thermally evaporated Al is deposited to 30 nm in thickness prior to Ar+ ion irradiation, the buffered Al film may have many pinholes and porous structure due to the low surface adatom mobility. When the Ar+ ions or their activated Al atoms are irradiated to the buffered Al film surface, they cannot be screened by the buffered Al film wholly because they can transmit to Ph-PPV through the pinholes or non-deposited regions among the columnar structures. This idea can be supported by TEM data indirectly. Fig. 8 is TEM images showing the plan-view images of Al specimens prepared by three different processes. In this experiment, the thickness of the Al film is fixed at 30 nm for transmitting the electron
Fig. 7. Schematic diagram of ion-beam-assisted deposition process in buffered Al layer.
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assisted deposition to avoid ion bombardment induced damages in Ph-PPV, IBAD process may influence the interfacial state because edge permeation is slower than thermal case. Hung and Madathil reported that catastrophic failure still occurs even though 10 nm of MgAg is sufficient thick to protect the organic layer
Fig. 8. TEM images (100,000) of Al prepared by (a) thermal evaporation (30 nm), (b) thermal (20 nm) + IBAD (10 nm) and (c) IBAD Al (30 nm).
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beam through it in a given transmission electron microscopy. Here, the conventional TEM sample preparation method cannot be used for the present organic samples. Fig. 8(a) is a TEM image of the Al film (30 nm) deposited by thermal evaporation. The resulting buffered Al film consists of thermally evaporated Al (20 nm) prior to the Al film (10 nm), which is obtained by only Ar+ ion-beam-assisted deposition, as shown in Fig. 8(b) and (c). As seen in Fig. 8(a), the grain size of the evaporated film was in ˚ . In contrast, Fig. 8(c) the range of 800–1000 A revealed that the grain size was much smaller, merely ˚ , for the films prepared by IBAD. As may 200–300 A be seen from these results, the concurrent ion bombardment of the depositing film has led to a considerable grain refinement. Note that the buffered Al film has partly small grain size compared than that of thermal evaporation. Although they are different from the previous buffered Al film which is deposited up to 30 nm just by thermal evaporation, TEM data show the decreased average grain size in buffered Al film (Fig. 8(b)) compared with thermally evaporated Al (Fig. 8(a)). That might be caused by transmitting Ar+ ions or activated Al atoms through the pinholes resulting in breakup of 3D structure (Fig. 7). So the significant enhancement in the passivation properties of the OLEDs prepared with ion bombardment result from not only densification of the Al structure but also interface modification between Al and Ph-PPV retarding H2O and O2 permeation at edge structure. In order to observe whether Ar+ ions or activated Al atoms transmit through the pinholes or non-deposited regions, we investigated the electrical characteristics of OLED device to measure damages in buffered IBAD device. Fig. 9 shows the current–voltage (I–V) characteristics of the OLEDs with ion-beam-assisted Al cathodes were measured and compared with that of a conventional Ph-PPV based device. The Al cathode depositions by IBAD were made in two different conditions, one with the thermally evaporated Al ˚ before using IBAD process to buffer layer about 300 A avoid Ph-PPV damages by Ar+ ion bombardment and another without Al buffer layer. The measurements have been carried out in a glove box filled with argon gas of 99.9999% purity to exclude the permeation effect of H2O and O2 because dark spots formed by the cathode oxidation restrain the current injection.
Fig. 9. I–V plot of OLED device with Al cathode deposited by thermal evaporation and ion-beam-assisted deposition: (a) voltage– current and (b) magnified voltage–current.
There is some leakage current even in buffered IBAD device though that has a little smaller value compared than IBAD device without buffer layer as shown in Fig. 9. Although above I–V data cannot be related directly with the generation of damages through pinholes in buffered Al film, it is certain that the leakage current is induced by Ar+ ion-induced damages because the leakage current must not be generated at reverse bias regime due to large barrier for electrons and holes injection.
4. Conclusion It was investigated in the present work that the Al cathode generated by IBAD has the more improved
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passivation properties than that by thermal evaporation by allowing the more dense structure that restricts the permeation of H2O and O2. Employing thin Al buffer layer diminished Ar+ ion-induced damages in Ph-PPV and limited permeation against H2O and O2 at the edge structure exposed to air. It might result from transmitting Ar+ ions through porous buffered Al film, leading to prolonged lifetime.
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