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Enhanced performance of organic electroluminescence diodes with a 2-TNATA:C60 hole injection layer
Journal of Industrial and Engineering Chemistry 15 (2009) 752–757
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Enhanced performance of organic electroluminescence diodes with a 2-TNATA:C60 hole injection layer Hak-Su Kang a, Kyeong-Nam Park a, Young-Rae Cho b, Dae-Won Park a, Youngson Choe a,* a b
Department of Chemical Engineering, Pusan National University, Jangjeon-dong, Geumjeong-gu, Busan 609-735, Republic of Korea Department of Material Engineering, Pusan National University, Busan 609-735, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 13 April 2009 Accepted 23 June 2009
C60-doped 2-TNATA (4,40 ,400 -tris(2-naphthylphenylamino)triphenylamine) as a hole injection material and NPD (4,40 bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl) as a hole transport material are used to fabricate OLEDs via vacuum deposition process in this study. C60-doped 2-TNATA film was treated by means of thermal annealing and in situ electromagnetic field. According to AFM, SEM, XRD, and Raman spectra results, by both thermal annealing and in situ electromagnetic field treatments, the smoothened surface and the closely packed morphology of 2-TNATA:C60 film was obtained without any evidences of crystalline nature after those treatments. The treatments eventually lead to enhancing the current density and efficiency of the multi-layered ITO/2-TNATA:C60 (5% doped) (70 nm)/NPD (30 nm)/Alq3 (50 nm)/LiF (1 nm)/Al (100 nm) devices by facilitating hole injection/transport in the multi-layered organic devices. Consequently, thermal annealing treatment for the 2-TNATA:C60 film is preferred rather than in situ electromagnetic field treatment so as to improve the overall performance of the organic light-emitting diodes in this study. ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: C60 doping 2-TNATA OLED Thermal annealing Current efficiency
1. Introduction Macrocyclic organic materials and conjugated polymers have attracted great attention due to their potential applications in electronic and optoelectronic devices such as organic lightemitting diodes (OLEDs) and photovoltaic cells [1–4]. Many efforts have been made in recent years to improve the performance and stability of OLEDs because of their potential application in flat panel displays [5–10]. For the performance improvement of OLEDs, macrocyclic organic or conjugated polymeric thin films can be used as a light-emitting layer, a hole injection layer, and a hole transport layer material in the devices since charge injection and mobility can be enhanced through the layers [11,12]. In these fields, it is important to control the structural configuration of the devices at a molecular level. This structural configuration, in the devices fabricated with organic or polymeric materials, facilitates injection and balances the transport of electron and holes, and removes the emission region from the metal contacts. This generally results in improved efficiency and higher luminance at low operating voltages [13]. It has been reported that a hole-injecting buffer layer typically plays
* Corresponding author. Tel.: +82 51 510 2396; fax: +82 51 512 8563. E-mail address: [email protected] (Y. Choe).
a key role of improving the performance of the diodes. Enhanced hole injection from ITO into the HTL can lower the operating voltage and, thus, improve performances of the diodes such as the current density–voltage (J–V) and luminescence–voltage (L–V) characteristics [14,15]. Improvements in device performance via thermal annealing highlight the importance of controlling molecular morphology since device performance can be hindered by structural defects in molecular level [16–19]. Thermal annealing can be applied to spin-coated polymers or vacuum-deposited organic materials to achieve morphological tuning. In addition, in situ electromagnetic field treatment can be employed to control the molecular architecture since many macrocyclic organic materials like metal phthalocyanine possess magnetic behavior [20–22]. Magnetic field-induced alignment of molecules in solution is considered as a universal effect, originating from the fact that virtually any molecule features anisotropic magnetic susceptibility. In this reason, the molecular ordering can be achieved by means of high magnetic field typically in organic solutions [13,14]. By a doping process, an interpenetrating network between hole and electron transport is formed and enhances the charge transport between the adjacent small molecules. By mixing planar molecules (i.e. CuPc and 2-TNATA) with a spherically symmetric (i.e. C60) molecules, forming a small-molecule-doped layer such as
1226-086X/$ – see front matter ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2009.09.058
H.-S. Kang et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 752–757
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a CuPc:C60 layer, inter-chain interactions is raised between two different molecules, providing the necessary percolating paths for electron and hole transport. The charge carrier mobility for holes can be also affected by the morphology of the film, determining the degree of inter-chain interactions in conjugated polymer-based organic light-emitting diodes. Therefore, enhancements in current density can be achieved due to the enhanced charge carrier mobility in the small-molecule-doped layer. In the previous work, we investigated the effects of C60 doping in the HIL, resulting in the improved performance of the device [23]. In this study, vacuum-deposited C60-doped 2-TNATA (4,40 ,400 -tris(2-naphthylphenylamino)triphenylamine) as a hole injection material and NPD (4,40 bis[N-(1-naphthyl)-N-phenylamino]biphenyl) as a hole transport material are used to fabricate OLEDs and then the effect of C60 doping into the HIL on the performance of the current devices is intensively investigated. For further investigations, the film morphology has been controlled by means of thermal annealing and in situ electromagnetic field treatment, and then the device performance will be discussed. 2. Experimental 2.1. Materials As a HIL material, 4,40 ,400 -tris(2-naphthylphenylamino)triphenylamine (2-TNATA) (formula: C66H48N4, molecular weight: 897.08, melting point: 245–247 8C, Tg: 110 8C, Tokyo Kasei Kogyo Co. Ltd.) was sublimed to the transparent pre-patterned ITO glass substrate to obtain thin films via vacuum process. Other materials used are C60 (Fullerene) as a dopant (formula: C60, molecular weight: 720.64, melting point: >280 8C, Tg: 174 8C, Sigma–Aldrich, 99.5%), 4,40 bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPD) (formula: C44H32N2, molecular weight: 576.078, melting point: >277 8C, Tg: 96 8C, T H.W. Sands Corp.), Alq3 (tris(8-hydroxyquinoline)aluminum, formula: C27H18AlN3O3, molecular weight: 459.437, melting point: >410 8C, Tg: 174 8C, Aldrich), and lithium fluoride (LiF) (molecular weight: 25.9374, melting point: 870 8C, Tokyo Kasei Kogyo Co. Ltd.). The chemical structures of organic materials are shown in Fig. 1. 2.2. ITO surface treatment and 2-TNATA:C60 film formation Vacuum deposition technique by thermal evaporation was used to obtain a homogeneous layer with well-controlled thickness. Before deposition, the ITO-coated glass was treated by acid solution followed by rinsing with de-ionized water and acetone and dried under nitrogen gas for cleaning the ITO-coated glass. Furthermore, the pre-cleaned ITO surface was treated with O2 + Ar plasma for 15 s, with a power of 150 W at atmospheric pressure. 2-TNATA was deposited onto the plasma-treated ITO layer and the deposition rate was controlled to 1.1 A˚/s to obtain 70 nm thickness of the 2-TNATA film. Also, 5 wt% C60-doped 2-TNATA film was obtained by co-evaporation. Thickness of each layer was measured using well calibrated quartz crystal thickness monitor (CRTM-6000, ULVAC kiko. Ltd.) as used in the previous work [23]. 2.3. Treatment of organic film and characterization of multi-layered devices After vacuum deposition, thermal annealing of all the deposited 2-TNATA and 5% C60-doped 2-TNATA thin films was performed in a cylindrical furnace. During vacuum deposition, electromagnetic field (6mT) was induced in the bell jar vacuum chamber. During thermal annealing, the temperature was maintained at 110 8C for 1 h, which is near the glass transition temperature of 2-TNATA for morphological control. The film formed in electromagnetic field
Fig. 1. Chemical structures of 2-TNATA (4,40 ,400 -tris(N-(2-naphthyl)-Nphenylamino)-triphenylamine) and NPD (N,N0 -di-1-naphthyl-N,N0 -diphenyl-1,10 biphenyl-4,40 -diamine).
was compared with both as-deposited and thermally annealed films. The current density–voltage (J–V) and luminescence–voltage (L–V) characteristics of OLEDs were measured using a multi-source meter (KEITHLEY 2400). XRD (Rigaku Model D/Max 2400, AFM (Nanoscope III-a, Digital Instruments Co. Ltd.), and SEM (HITACHI S-4200) analysis were conducted to investigate the morphology of the prepared films. The multi-layered ITO/2-TNATA:C60 (5% doped) (70 nm)/NPD (30 nm)/Alq3 (50 nm)/LiF (1 nm)/Al (100 nm) device was fabricated as shown in Fig. 2. A light-emitting area of device was made using a shadow mask to be 2 mm 2 mm in dimension. 3. Results and discussion 3.1. Morphology of 2-TNATA:C60 films The plasma treatment may change the surface energy and lead to the improvement in adhesion strength at the interface of layers [19]. Furthermore, the thermal annealing and in situ electromagnetic field treatments can play key roles in improving the interfacial adhesion between the anode and hole transport layer due to the enhanced inter-chain or inter-molecular interactions after those treatments. Cross-sectional views and surface images of 2-TNATA:C60 thin films, deposited on to the glass substrate, before and after the treatments were shown in Figs. 3 and 4. As reported in some works, thermal annealing was performed at near the glass transition temperature [13,14]. After thermal annealing at 110 8C for 1 h, it
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consequently improved by physical-crosslinking effect [22]. It is expected that the shortened inter-molecular distance, obtained by C60 doping, will facilitate the effective transport of charges through the adjacent molecules, and the smoothened surface of the film will provide the easy contact between the layers, leading to the increase of current density in the device layers. For the 2-TNATA:C60 film formed in in situ electromagnetic field, the rms roughness was also decreased from 4.627 to 3.604 nm. In electromagnetic field, enhanced inter-molecular interactions in a planar way can be raised, eventually leading to the closer packing between molecules [20,21]. By this effect, the smoothened surface was obtained compared to the as-deposited film. 3.2. XRD and Raman spectra analysis
Fig. 2. Configuration of a multi-layered OLED.
was found that the surface roughness was substantially decreased and the lowered grains on the surface were observed. The root mean square (rms) roughness was decreased from 4.627 to 2.498 nm. In general, the film stability is improved when organic semiconducting materials are doped with small organic dopants such as C60 and F4-TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) [16–19]. It is expected that the film stability of 2TNATA:C60 can be increased by physical crosslinking that takes place due to the p–p bond interaction between the organic semiconducting materials and C60, causing the inter-molecular interactions in the films [24]. The film density and regularity are
The thin films were characterized by X-ray diffraction analysis and the results are shown in Figs. 5 and 6. For better observation of crystalline properties, the vacuum-deposited C60 and 2-TNATA:C60 thin films of 800 nm thickness were prepared. From the obtained XRD patterns, it is observed that the as-deposited C60 film possesses the crystalline nature as given in Fig. 5(a). The peaks are located at 2u = 20.904, 17.815, 10.9515, and 10.4013 with the plane surfaces of hcp<1 1 2>, hcp<1 1 0>, hcp<1 0 1>, and hcp<0 0 2>, respectively [11]. For the films thermally annealed and formed in in situ electromagnetic field, the crystalline was much more clearly observed as shown in Fig. 5(b) and (c). This indicates that the crystalline nature of the C60 film could be increased by those post- and in situ treatments. The crystalline nature, however, was not found for the 2TNATA:C60 films which are as-deposited, thermally annealed, and formed in in situ electromagnetic field, due to the uniformly dispersed C60 acts as a barrier factor in forming crystallite in those films. By doping of C60 into the 2-TNATA film, in other words, the formation of 2-TNATA crystalline can be effectively interrupted because of the blocking effect by C60 molecules in 2-TNATA:C60 composite films [25].
Fig. 3. 3D AFM images of vacuum-deposited thin films: (a) 2-TNATA:C60 thin film as-deposited (rms = 4.627 nm), (b) 2-TNATA:C60 thin film as-deposited with electromagnetic field (rms = 3.604 nm), and (c) 2-TNATA:C60 thin film treated with thermal annealing (rms = 2.498 nm).
H.-S. Kang et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 752–757
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Fig. 4. SEM images of cross-sectional thin films: (a) 2-TNATA:C60 thin film as-deposited, (b) 2-TNATA:C60 thin film deposited in electromagnetic field, and (c) 2-TNATA:C60 thin film treated with thermal annealing.
The Raman spectra of 2-TNATA:C60 films are shown in Fig. 7. For aromatic ring-containing organic materials, typical Raman bands are observed at 1609, 1594, 1574, 1530, 1466, 1374, 1288, 1222 cm1 [14,15]. These typical bands are attributed to the aromatic ring stretch and inter-ring C–C stretch in macrocyclic organic materials. It has been reported that the width of each band in the amorphous state is broader and lower than that in the crystalline state of macrocyclic organic materials used as hole injection/transport materials in OLEDs as well as photovoltaic cells [14,15]. Due to the cross-section of the Raman band (resonance Raman effect), the intensity of specific band at near 1466 cm1 is more increased for the thermally annealed
2-TNATA:C60 film, indicating that the adjacent organic materials are more closely packed by the overlapping of the p-bond of macrocyclic organic materials after in situ treatment in this study [26]. According to the XRD results, the crystalline nature of C60-doped 2-TNATA films was not observed after both thermal and electromagnetic field treatments. In general, the increase of peak intensities in Raman spectra comes from the overlapping or closer packing of adjacent macrocyclic rings in organic materials. In some works, Raman analysis has been employed to verify the closer packing effect of organic molecules after thermal treatment or after doping of C60 in the organic film [14,15].
Fig. 5. XRD analyses of C60 thin films: (a) as-deposited, (b) deposited in electromagnetic field, and (c) treated with thermal annealing.
Fig. 6. XRD analyses of 2-TNATA:C60 thin film: (a) as-deposited, (b) deposited in electromagnetic field, and (c) treated with thermal annealing.
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Fig. 9. L–V characteristics of multi-layered OLEDs. Fig. 7. Raman spectra of 2-TNATA:C60 thin film: (a) as-deposited, (b) deposited in electromagnetic field, and (c) treated with thermal annealing.
3.3. Performance characteristics of devices The performance of OLEDs are mostly dependent on the hole injection/transport layers. In the optoelectronic devices, modifying the hole injection/transport layers by doping of small molecules such as C60 leads to improving the charge mobility, which substantially facilitates hole injection/transport in the devices. The current density–voltage (J–V), luminance–voltage (L–V), and current efficiency characteristics of the multi-layered ITO/2TNATA:C60 (5% doped) (70 nm)/NPD (30 nm)/Alq3 (50 nm)/LiF (1 nm)/Al (100 nm) device fabricated in this study are shown in Figs. 8–10, respectively. We have treated the C60-doped 2-TNATA layer by means of thermal annealing as well as in situ electromagnetic field induction, expecting to achieve the improved current density and luminance efficiency of the devices. In Figs. 8 and 9, both current density and luminance are increased depending on the treatment methods. Higher device performance was obtained by the thermal annealing treatment rather than the in situ electromagnetic field treatment. This indicates that much more enhanced charge injection and transport could be achieved through the closely packed molecules in the film architecture, obtained by the thermal treatment of the device layers. Furthermore, the same tendency of improvement in current efficiency– current density characteristics was observed as shown in Fig. 10. Increasing charge mobility and current density in hole injection layer plays a key role in improving the performance characteristics
Fig. 8. J–V characteristics of multi-layered OLEDs.
Fig. 10. Current efficiency characteristics of multi-layered OLEDs.
of the current devices. The EL (electroluminescence) spectra are shown in Fig. 11. From the EL spectra analysis, it was found that the thermal annealing and in situ electromagnetic field-induction method did not affect on the absorption features of the fabricated devices in this study.
Fig. 11. EL spectra of multi-layered OLEDs.
H.-S. Kang et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 752–757
4. Conclusions The morphology and performance characteristics of OLEDs from the multi-layered ITO/2-TNATA:C60 (5% doped) (70 nm)/NPD (30 nm)/Alq3 (50 nm)/LiF (1 nm)/Al (100 nm) were substantially improved by the thermal annealing and in situ electromagnetic field treatments for the C60-doped 2-TNATA film as a hole injection layer in this study. From the XRD and Raman spectra analysis, it can be concluded that the organic molecules are more closely packed in a C60-doped hole injection layer by the proper thermal treatment of this layer and thus the efficiency of the devices is improved by facilitating hole injection/transport through the layers in the devices. There were no evidences of crystalline properties in the HIL after treatments. Consequently, the thermal annealing treatment for the C60-doped 2-TNATA film is preferred rather than the in situ electromagnetic field treatment so as to improve the overall performance of the organic light-emitting diodes in this study. Acknowledgments This work was supported by the Brain Korea 21 Project and by grant no. R01-2005-000-0005-0 from the Basic Research Program of the Korea Science & Engineering Foundation, South Korea. References [1] C.J. Brabec, N.S. Sariciftci, J.C. Hummemen, Adv. Funct. Mater. 11 (2001) 15. [2] P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693.
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[3] K. Kim, J. Liu, M.A.G. Namboothiry, D.L. Carroll, Appl. Phys. Lett. 90 (2007) 163511. [4] J.Y. Kim, K. Lee, N.E. Coates, D. Moses, T.Q. Nguyen, M. Dante, A.J. Heeger, Science 317 (2007) 222. [5] Z. Bao, A.J. Lovinger, J. Dodabalapur, Appl. Phys. Lett. 69 (1996) 3066. [6] B. Bialek, I.G. Kim, J.I. Lee, Thin Solid Films 436 (2003) 107. [7] G.E. Collins, V.S. Williams, L.-K. Chau, K.W. Nebesny, C. England, P.A. Lee, T. Lowe, Q. Fernando, N.R. Armstrong, Synth. Met. 54 (1993) 351. [8] M.M. El-Nahass, Z. El-Gohary, H.S. Soliman, Opt. Laser Technol. 35 (2003) 523. [9] K.P. Khrishnakumar, C.S. Menon, Mater. Lett. 48 (2001) 64. [10] S. Yanagiya, S. Nishikata, G. Sazaki, A. Hoshino, K. Nakajima, T. Inoue, J. Cryst. Growth 254 (2003) 244. [11] P.L. Burn, D.D.C. Bradley, R.H. Friend, D.A. Halliday, A.B. Holmes, R.W. Jackson, A. Kraft, J. Chem. Soc. 1 (1992) 3225. [12] S.H. Askari, S.D. Rughooputh, F. Wudl, Synth. Met. 29 (1989) E129. [13] C. Giebeler, H. Antoniadis, D.D.C. Bradley, ShirotaF Y., J. Appl. Phys. 85 (1) (1999) 608. [14] B.J. Chen, X.W. Sun, T.K.S. Wong, X. Hu, A. Uddin, Appl. Phys. Lett. 84 (2005) 063505. [15] T. Sugiyama, Y. Furukawa, H. Fujimura, Chem. Phys. Lett. 405 (2005) 330. [16] B. Maennig, J. Drechsel, D. Gebeyehu, P. Simon, F. Kozlowski, A. Werner, F. Li, S. Grundmann, S. Sonntag, M. Koch, K. Leo, M. Pfeiffer, H. Hoppe, D. Meissner, N.S. Sariciftci, I. Riedel, V. Dyakonov, J. Parisi, Appl. Phys. A 79 (2004) 1. [17] B. Maennig, M. Pfeiffer, A. Nollau, X. Zhou, K. Leo, P. Simon, Phys. Rev. B 64 (2001) 195208. [18] Y. Choe, S.Y. Park, D.W. Park, J. Chem. Eng. Jpn. 38 (8) (2005) 62. [19] Y.F. Zhou, Y.B. Yuan, L.F. Cao, J. Zhang, H.Q. Pang, J.R. Lian, X. Zhou, J. Lumin. 122/ 123 (2007) 602. [20] M.I. Boamfa, P.C.M. Christianen, J.C. Maan, H. Engelkamp, R.J.M. Nolte, Physica B 294/295 (2001) 343. [21] Z.G. Ji, K.W. Wong, P.K. Tse, R.W.M. Kwok, W.M. Lau, Thin Solid Films 402 (2002) 79. [22] C. Ganzorig, K.-J. Kwak, K. Yagi, M. Fujihira, Appl. Phys. Lett. 79 (2001) 272. [23] S.H. Park, D.S. Kang, D.W. Park, Y. Choe, Polymer (Kor.) 32 (4) (2008) 372. [24] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Science 258 (5087) (1992) 1474. [25] J.Y. Lee, H.K. Jang, J. Appl. Phys. 88 (2006) 183502. [26] A.M. Hawkridge, J.E. Pemberton, J. Am. Chem. Soc. 125 (2003) 624.
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Enhanced performance of organic electroluminescence diodes with a 2-TNATA:C60 hole injection layer Hak-Su Kang a, Kyeong-Nam Park a, Young-Rae Cho b, Dae-Won Park a, Youngson Choe a,* a b
Department of Chemical Engineering, Pusan National University, Jangjeon-dong, Geumjeong-gu, Busan 609-735, Republic of Korea Department of Material Engineering, Pusan National University, Busan 609-735, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 13 April 2009 Accepted 23 June 2009
C60-doped 2-TNATA (4,40 ,400 -tris(2-naphthylphenylamino)triphenylamine) as a hole injection material and NPD (4,40 bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl) as a hole transport material are used to fabricate OLEDs via vacuum deposition process in this study. C60-doped 2-TNATA film was treated by means of thermal annealing and in situ electromagnetic field. According to AFM, SEM, XRD, and Raman spectra results, by both thermal annealing and in situ electromagnetic field treatments, the smoothened surface and the closely packed morphology of 2-TNATA:C60 film was obtained without any evidences of crystalline nature after those treatments. The treatments eventually lead to enhancing the current density and efficiency of the multi-layered ITO/2-TNATA:C60 (5% doped) (70 nm)/NPD (30 nm)/Alq3 (50 nm)/LiF (1 nm)/Al (100 nm) devices by facilitating hole injection/transport in the multi-layered organic devices. Consequently, thermal annealing treatment for the 2-TNATA:C60 film is preferred rather than in situ electromagnetic field treatment so as to improve the overall performance of the organic light-emitting diodes in this study. ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: C60 doping 2-TNATA OLED Thermal annealing Current efficiency
1. Introduction Macrocyclic organic materials and conjugated polymers have attracted great attention due to their potential applications in electronic and optoelectronic devices such as organic lightemitting diodes (OLEDs) and photovoltaic cells [1–4]. Many efforts have been made in recent years to improve the performance and stability of OLEDs because of their potential application in flat panel displays [5–10]. For the performance improvement of OLEDs, macrocyclic organic or conjugated polymeric thin films can be used as a light-emitting layer, a hole injection layer, and a hole transport layer material in the devices since charge injection and mobility can be enhanced through the layers [11,12]. In these fields, it is important to control the structural configuration of the devices at a molecular level. This structural configuration, in the devices fabricated with organic or polymeric materials, facilitates injection and balances the transport of electron and holes, and removes the emission region from the metal contacts. This generally results in improved efficiency and higher luminance at low operating voltages [13]. It has been reported that a hole-injecting buffer layer typically plays
* Corresponding author. Tel.: +82 51 510 2396; fax: +82 51 512 8563. E-mail address: [email protected] (Y. Choe).
a key role of improving the performance of the diodes. Enhanced hole injection from ITO into the HTL can lower the operating voltage and, thus, improve performances of the diodes such as the current density–voltage (J–V) and luminescence–voltage (L–V) characteristics [14,15]. Improvements in device performance via thermal annealing highlight the importance of controlling molecular morphology since device performance can be hindered by structural defects in molecular level [16–19]. Thermal annealing can be applied to spin-coated polymers or vacuum-deposited organic materials to achieve morphological tuning. In addition, in situ electromagnetic field treatment can be employed to control the molecular architecture since many macrocyclic organic materials like metal phthalocyanine possess magnetic behavior [20–22]. Magnetic field-induced alignment of molecules in solution is considered as a universal effect, originating from the fact that virtually any molecule features anisotropic magnetic susceptibility. In this reason, the molecular ordering can be achieved by means of high magnetic field typically in organic solutions [13,14]. By a doping process, an interpenetrating network between hole and electron transport is formed and enhances the charge transport between the adjacent small molecules. By mixing planar molecules (i.e. CuPc and 2-TNATA) with a spherically symmetric (i.e. C60) molecules, forming a small-molecule-doped layer such as
1226-086X/$ – see front matter ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2009.09.058
H.-S. Kang et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 752–757
753
a CuPc:C60 layer, inter-chain interactions is raised between two different molecules, providing the necessary percolating paths for electron and hole transport. The charge carrier mobility for holes can be also affected by the morphology of the film, determining the degree of inter-chain interactions in conjugated polymer-based organic light-emitting diodes. Therefore, enhancements in current density can be achieved due to the enhanced charge carrier mobility in the small-molecule-doped layer. In the previous work, we investigated the effects of C60 doping in the HIL, resulting in the improved performance of the device [23]. In this study, vacuum-deposited C60-doped 2-TNATA (4,40 ,400 -tris(2-naphthylphenylamino)triphenylamine) as a hole injection material and NPD (4,40 bis[N-(1-naphthyl)-N-phenylamino]biphenyl) as a hole transport material are used to fabricate OLEDs and then the effect of C60 doping into the HIL on the performance of the current devices is intensively investigated. For further investigations, the film morphology has been controlled by means of thermal annealing and in situ electromagnetic field treatment, and then the device performance will be discussed. 2. Experimental 2.1. Materials As a HIL material, 4,40 ,400 -tris(2-naphthylphenylamino)triphenylamine (2-TNATA) (formula: C66H48N4, molecular weight: 897.08, melting point: 245–247 8C, Tg: 110 8C, Tokyo Kasei Kogyo Co. Ltd.) was sublimed to the transparent pre-patterned ITO glass substrate to obtain thin films via vacuum process. Other materials used are C60 (Fullerene) as a dopant (formula: C60, molecular weight: 720.64, melting point: >280 8C, Tg: 174 8C, Sigma–Aldrich, 99.5%), 4,40 bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPD) (formula: C44H32N2, molecular weight: 576.078, melting point: >277 8C, Tg: 96 8C, T H.W. Sands Corp.), Alq3 (tris(8-hydroxyquinoline)aluminum, formula: C27H18AlN3O3, molecular weight: 459.437, melting point: >410 8C, Tg: 174 8C, Aldrich), and lithium fluoride (LiF) (molecular weight: 25.9374, melting point: 870 8C, Tokyo Kasei Kogyo Co. Ltd.). The chemical structures of organic materials are shown in Fig. 1. 2.2. ITO surface treatment and 2-TNATA:C60 film formation Vacuum deposition technique by thermal evaporation was used to obtain a homogeneous layer with well-controlled thickness. Before deposition, the ITO-coated glass was treated by acid solution followed by rinsing with de-ionized water and acetone and dried under nitrogen gas for cleaning the ITO-coated glass. Furthermore, the pre-cleaned ITO surface was treated with O2 + Ar plasma for 15 s, with a power of 150 W at atmospheric pressure. 2-TNATA was deposited onto the plasma-treated ITO layer and the deposition rate was controlled to 1.1 A˚/s to obtain 70 nm thickness of the 2-TNATA film. Also, 5 wt% C60-doped 2-TNATA film was obtained by co-evaporation. Thickness of each layer was measured using well calibrated quartz crystal thickness monitor (CRTM-6000, ULVAC kiko. Ltd.) as used in the previous work [23]. 2.3. Treatment of organic film and characterization of multi-layered devices After vacuum deposition, thermal annealing of all the deposited 2-TNATA and 5% C60-doped 2-TNATA thin films was performed in a cylindrical furnace. During vacuum deposition, electromagnetic field (6mT) was induced in the bell jar vacuum chamber. During thermal annealing, the temperature was maintained at 110 8C for 1 h, which is near the glass transition temperature of 2-TNATA for morphological control. The film formed in electromagnetic field
Fig. 1. Chemical structures of 2-TNATA (4,40 ,400 -tris(N-(2-naphthyl)-Nphenylamino)-triphenylamine) and NPD (N,N0 -di-1-naphthyl-N,N0 -diphenyl-1,10 biphenyl-4,40 -diamine).
was compared with both as-deposited and thermally annealed films. The current density–voltage (J–V) and luminescence–voltage (L–V) characteristics of OLEDs were measured using a multi-source meter (KEITHLEY 2400). XRD (Rigaku Model D/Max 2400, AFM (Nanoscope III-a, Digital Instruments Co. Ltd.), and SEM (HITACHI S-4200) analysis were conducted to investigate the morphology of the prepared films. The multi-layered ITO/2-TNATA:C60 (5% doped) (70 nm)/NPD (30 nm)/Alq3 (50 nm)/LiF (1 nm)/Al (100 nm) device was fabricated as shown in Fig. 2. A light-emitting area of device was made using a shadow mask to be 2 mm 2 mm in dimension. 3. Results and discussion 3.1. Morphology of 2-TNATA:C60 films The plasma treatment may change the surface energy and lead to the improvement in adhesion strength at the interface of layers [19]. Furthermore, the thermal annealing and in situ electromagnetic field treatments can play key roles in improving the interfacial adhesion between the anode and hole transport layer due to the enhanced inter-chain or inter-molecular interactions after those treatments. Cross-sectional views and surface images of 2-TNATA:C60 thin films, deposited on to the glass substrate, before and after the treatments were shown in Figs. 3 and 4. As reported in some works, thermal annealing was performed at near the glass transition temperature [13,14]. After thermal annealing at 110 8C for 1 h, it
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consequently improved by physical-crosslinking effect [22]. It is expected that the shortened inter-molecular distance, obtained by C60 doping, will facilitate the effective transport of charges through the adjacent molecules, and the smoothened surface of the film will provide the easy contact between the layers, leading to the increase of current density in the device layers. For the 2-TNATA:C60 film formed in in situ electromagnetic field, the rms roughness was also decreased from 4.627 to 3.604 nm. In electromagnetic field, enhanced inter-molecular interactions in a planar way can be raised, eventually leading to the closer packing between molecules [20,21]. By this effect, the smoothened surface was obtained compared to the as-deposited film. 3.2. XRD and Raman spectra analysis
Fig. 2. Configuration of a multi-layered OLED.
was found that the surface roughness was substantially decreased and the lowered grains on the surface were observed. The root mean square (rms) roughness was decreased from 4.627 to 2.498 nm. In general, the film stability is improved when organic semiconducting materials are doped with small organic dopants such as C60 and F4-TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) [16–19]. It is expected that the film stability of 2TNATA:C60 can be increased by physical crosslinking that takes place due to the p–p bond interaction between the organic semiconducting materials and C60, causing the inter-molecular interactions in the films [24]. The film density and regularity are
The thin films were characterized by X-ray diffraction analysis and the results are shown in Figs. 5 and 6. For better observation of crystalline properties, the vacuum-deposited C60 and 2-TNATA:C60 thin films of 800 nm thickness were prepared. From the obtained XRD patterns, it is observed that the as-deposited C60 film possesses the crystalline nature as given in Fig. 5(a). The peaks are located at 2u = 20.904, 17.815, 10.9515, and 10.4013 with the plane surfaces of hcp<1 1 2>, hcp<1 1 0>, hcp<1 0 1>, and hcp<0 0 2>, respectively [11]. For the films thermally annealed and formed in in situ electromagnetic field, the crystalline was much more clearly observed as shown in Fig. 5(b) and (c). This indicates that the crystalline nature of the C60 film could be increased by those post- and in situ treatments. The crystalline nature, however, was not found for the 2TNATA:C60 films which are as-deposited, thermally annealed, and formed in in situ electromagnetic field, due to the uniformly dispersed C60 acts as a barrier factor in forming crystallite in those films. By doping of C60 into the 2-TNATA film, in other words, the formation of 2-TNATA crystalline can be effectively interrupted because of the blocking effect by C60 molecules in 2-TNATA:C60 composite films [25].
Fig. 3. 3D AFM images of vacuum-deposited thin films: (a) 2-TNATA:C60 thin film as-deposited (rms = 4.627 nm), (b) 2-TNATA:C60 thin film as-deposited with electromagnetic field (rms = 3.604 nm), and (c) 2-TNATA:C60 thin film treated with thermal annealing (rms = 2.498 nm).
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Fig. 4. SEM images of cross-sectional thin films: (a) 2-TNATA:C60 thin film as-deposited, (b) 2-TNATA:C60 thin film deposited in electromagnetic field, and (c) 2-TNATA:C60 thin film treated with thermal annealing.
The Raman spectra of 2-TNATA:C60 films are shown in Fig. 7. For aromatic ring-containing organic materials, typical Raman bands are observed at 1609, 1594, 1574, 1530, 1466, 1374, 1288, 1222 cm1 [14,15]. These typical bands are attributed to the aromatic ring stretch and inter-ring C–C stretch in macrocyclic organic materials. It has been reported that the width of each band in the amorphous state is broader and lower than that in the crystalline state of macrocyclic organic materials used as hole injection/transport materials in OLEDs as well as photovoltaic cells [14,15]. Due to the cross-section of the Raman band (resonance Raman effect), the intensity of specific band at near 1466 cm1 is more increased for the thermally annealed
2-TNATA:C60 film, indicating that the adjacent organic materials are more closely packed by the overlapping of the p-bond of macrocyclic organic materials after in situ treatment in this study [26]. According to the XRD results, the crystalline nature of C60-doped 2-TNATA films was not observed after both thermal and electromagnetic field treatments. In general, the increase of peak intensities in Raman spectra comes from the overlapping or closer packing of adjacent macrocyclic rings in organic materials. In some works, Raman analysis has been employed to verify the closer packing effect of organic molecules after thermal treatment or after doping of C60 in the organic film [14,15].
Fig. 5. XRD analyses of C60 thin films: (a) as-deposited, (b) deposited in electromagnetic field, and (c) treated with thermal annealing.
Fig. 6. XRD analyses of 2-TNATA:C60 thin film: (a) as-deposited, (b) deposited in electromagnetic field, and (c) treated with thermal annealing.
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Fig. 9. L–V characteristics of multi-layered OLEDs. Fig. 7. Raman spectra of 2-TNATA:C60 thin film: (a) as-deposited, (b) deposited in electromagnetic field, and (c) treated with thermal annealing.
3.3. Performance characteristics of devices The performance of OLEDs are mostly dependent on the hole injection/transport layers. In the optoelectronic devices, modifying the hole injection/transport layers by doping of small molecules such as C60 leads to improving the charge mobility, which substantially facilitates hole injection/transport in the devices. The current density–voltage (J–V), luminance–voltage (L–V), and current efficiency characteristics of the multi-layered ITO/2TNATA:C60 (5% doped) (70 nm)/NPD (30 nm)/Alq3 (50 nm)/LiF (1 nm)/Al (100 nm) device fabricated in this study are shown in Figs. 8–10, respectively. We have treated the C60-doped 2-TNATA layer by means of thermal annealing as well as in situ electromagnetic field induction, expecting to achieve the improved current density and luminance efficiency of the devices. In Figs. 8 and 9, both current density and luminance are increased depending on the treatment methods. Higher device performance was obtained by the thermal annealing treatment rather than the in situ electromagnetic field treatment. This indicates that much more enhanced charge injection and transport could be achieved through the closely packed molecules in the film architecture, obtained by the thermal treatment of the device layers. Furthermore, the same tendency of improvement in current efficiency– current density characteristics was observed as shown in Fig. 10. Increasing charge mobility and current density in hole injection layer plays a key role in improving the performance characteristics
Fig. 8. J–V characteristics of multi-layered OLEDs.
Fig. 10. Current efficiency characteristics of multi-layered OLEDs.
of the current devices. The EL (electroluminescence) spectra are shown in Fig. 11. From the EL spectra analysis, it was found that the thermal annealing and in situ electromagnetic field-induction method did not affect on the absorption features of the fabricated devices in this study.
Fig. 11. EL spectra of multi-layered OLEDs.
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4. Conclusions The morphology and performance characteristics of OLEDs from the multi-layered ITO/2-TNATA:C60 (5% doped) (70 nm)/NPD (30 nm)/Alq3 (50 nm)/LiF (1 nm)/Al (100 nm) were substantially improved by the thermal annealing and in situ electromagnetic field treatments for the C60-doped 2-TNATA film as a hole injection layer in this study. From the XRD and Raman spectra analysis, it can be concluded that the organic molecules are more closely packed in a C60-doped hole injection layer by the proper thermal treatment of this layer and thus the efficiency of the devices is improved by facilitating hole injection/transport through the layers in the devices. There were no evidences of crystalline properties in the HIL after treatments. Consequently, the thermal annealing treatment for the C60-doped 2-TNATA film is preferred rather than the in situ electromagnetic field treatment so as to improve the overall performance of the organic light-emitting diodes in this study. Acknowledgments This work was supported by the Brain Korea 21 Project and by grant no. R01-2005-000-0005-0 from the Basic Research Program of the Korea Science & Engineering Foundation, South Korea. References [1] C.J. Brabec, N.S. Sariciftci, J.C. Hummemen, Adv. Funct. Mater. 11 (2001) 15. [2] P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693.
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