Transparent thermally stable poly(etherimide) film as

Thin Solid Films 518 (2009) 1419–1423

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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Transparent thermally stable poly(etherimide) film as flexible substrate for OLEDs V.L. Calil a,b, C. Legnani a, G.F. Moreira a, C. Vilani a, K.C. Teixeira a,b, W.G. Quirino a, R. Machado a, C.A. Achete a, M. Cremona a,b,⁎ a b

Dimat, Divisão de Metrologia em Materiais, Inmetro, Duque de Caxias, RJ, Brazil LOEM, Laboratório de Optoeletrônica Molecular, Departamento de Física, PUC-Rio, Rio de Janeiro, RJ, Brazil

a r t i c l e

i n f o

Available online 19 September 2009 Keywords: ITO Flexible substrate Flexible organics electronics Poly(etherimide) Thermal stable

a b s t r a c t In this work, ITO thin films were deposited onto poly(etherimide) (PEI) substrates at room temperature using r.f. magnetron sputtering and successively they were annealed in the 423–523 K (150–250 °C) temperature range. PEI/ITO substrates were structurally, optically and electrically characterized in order to verify the quality of the deposited ITO films and the PEI thermal stability during the ITO annealing process. A transmittance of about 80% was measured in the visible range. The best electrical properties achieved were: 3.04 × 10− 4 Ω cm, 12.07 × 1021cm2/V.s, 16.8 × 1021 cm− 3, for resistivity, carrier concentration and mobility, respectively. Small molecule Flexible Organic Light Emitting Diodes (FOLED) were then fabricated and characterized onto ITO functionalized PEI substrates. These preliminary results show clearly that PEI can be successfully used as substrate in flexible optoelectronic devices either operating in high temperature or when the process needs high temperatures. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, several efforts are employed in order to investigate new polymeric substrates to be used in the so-called flexible electronics [1,2]. For certain applications such as touch screens, PDAs, mobile phones, among others, flexibility is desirable and the use of glass is not possible. The development of Flexible Organic Light Emitting Diodes (FOLEDs) is one of the hottest topics in organic optoelectronic field [3,4] also contributing to the development of new applications [5,6]. However, for these purpose, polymeric substrates have to be functionalized depositing transparent conductive oxides (TCOs), as for example Indium Tin Oxide (ITO) [7,8], to achieve the necessary conducting properties. It is well known that TCOs with good properties are frequently obtained by heat treatment [9] or with deposition at high temperature [10]. Commonly used polymers like polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polymethylmethacrylate (PMMA), polycarbonate (PC), metallocene olefin copolymer (m-COC) and polyethersulfone (PES), present low thermal stability temperatures (around 423–493 K) [11,12]. High performance polyimide polymers can offer a suitable solution for this problem. These polymers are well known for their excellent thermal and chemical stability and good mechanical, optical and electrical properties [13]. Their strength, heat and chemical resistance are so good these materials often replace glass and metals in many demanding industrial appli-

⁎ Corresponding author. Departamento de Física, PUC-Rio, Rio de Janeiro, RJ, Brazil. E-mail address: cremona@fis.puc-rio.br (M. Cremona). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.055

cations [6,14]. In particular, poly(etherimide) (PEI) is a commercial polyimide which combines high temperature resistance, low moisture absorption, and excellent dielectric properties [15,16]. Moreover, its degradation temperature is about 818 K and the glass transition temperature (Tg) is 491 K, allowing its application in hostile environments [17,18]. Due to its properties and together with flexibility and good transparency in the visible range, this polymer can offer a promising alternative for the development of plastic optoelectronic devices. In this work, ITO thin films were deposited onto PEI substrates at room temperature using r.f. magnetron sputtering and successively they were annealed in the 423–523 K (150–200 °C) temperature range. PEI/ITO substrates were structurally, optically and electrically characterized in order to verify the quality of the deposited ITO films and the PEI thermal stability during the ITO annealing process. Finally, small molecule OLEDs were successfully fabricated and characterized onto this substrate. 2. Experimental The poly(etherimide) (Ultem 1000) used in this study was kindly supplied by Saudi Basic Industries Corporation (SABIC). Before use, PEI pellets were dried in oven at 333 K (60 °C) for several days to remove any water content. The casting solution was prepared by dissolution of PEI in n-methyl pyrrolidone (NMP) solvent by stirring for one day at room temperature in the proportion of 10.6 wt.% PEI. The solution was then poured into a silicon wafer and the polymeric film was obtained by inversion phase by solvent evaporation in an oven at 343 K with nitrogen flux. The formed film was maintained in

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the oven at 373 K for one day, to remove any solvent residue, and then treated at 523 K in vacuum to obtain the final dimensions. The obtained PEI films were transparent with an average thickness of ~0.1 mm. ITO thin films were deposited in Åmod Series Angstrom Engineering Coating Systems by r.f. magnetron sputtering using a 3 inch diameter ceramic target 90 wt.% In2O3 and 10 wt.% SnO2, purchased from Kurt J. Lesker. The r.f. power (13.56 MHz) was supplied by an r.f. generator matched to the target by a tuning network, both from RF VII Inc. ITO films with a thickness of 150 nm were deposited at room temperature varying the deposition power and pressure in order to optimize the properties of ITO films onto this specific substrate. The vacuum chamber was evacuated below 1 mPa before filling with Ar gas. ITO annealing was performed in vacuum (1 mPa), with temperatures in the 423–523 K range, for 60 min. The Tg of the polymeric films was measured with Differential Scanning Calorimetry (DSC) in a DSC Q1000 TA equipment. The weight loss was obtained by thermogravimetric analyses (TGA) in a Pyris 1 TGA from Perkin-Elmer. A JPK NanoWizard Atomic force microscopy (AFM) was used for surface morphology analysis. Optical absorption measurements were performed in a Perkin-Elmer Lambda 950 spectrophotometer. The electrical properties of the ITO films onto PEI substrates were measured using four-point probe technique, in an ECOPIA Hall Effect Measurement System HS 3000. The crystal structure was characterized by X-ray grazing incidence (2°) diffraction (XRD), using a Bruker D8 Discovery diffractometer with a Sol-X detector (Baltic scientific instrument), and using the Cu-Kα radiation. The ITO film thickness was measured using a Dektak6M Stylus Profile from Veeco. OLED devices were fabricated depositing copper phthalocyanine (CuPc), N,N′-bis(1-naphtyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), tris(8-hydroxyquinoline) aluminum (Alq3) and aluminum (Al) by thermal evaporation onto PEI/ITO substrates in high vacuum environment and without vacuum breaking. The base pressure was 1.0 mPa (~8 × 10− 6 Torr) and during evaporation the pressure was between 1.3 and 6.7 mPa. The deposition rates for organic and Al films were 0.2 and 1.0 nm/min, respectively. The device active area was about 5 mm2. In order to verify the influence of the PEI substrate on the device performance, a reference OLED was fabricated using a commercial ITO/glass substrate provided from Delta Technologies.

Fig. 2. Transmittance as a function of r.f. power deposition.

DSC and TGA analyses of PEI films confirmed their high thermal stability. The Tg value obtained was 491 K and the TGA analysis shows that the PEI degradation only occurs above 818 K, furthermore at

1073 K the PEI lost only 40% in weight. These analyses indicated that it is possible to perform annealing treatment on PEI substrate at high temperatures. Polymeric shrinkage was avoided performing a thermal treatment above Tg (523 K). The electrical characteristics of ITO films as a function of the r.f. power deposition are given in Fig. 1. All the 150 nm ITO thick films were deposited at 2 × 10− 1 Pa. The resistivity presents an abrupt decrease when the power deposition changes from 40 W to 50 W due to the increase of carrier mobility and concentration. Above 50 W there is no strong variation in resistivity, despite of the mobility and carrier concentration behavior. However, the optical transmittance (Fig. 2) decreases due to the higher sputtering power which causes an oxygen deficient material [19] resulting in the darkening of the films. In the ITO crystalline structure some oxygen sites are not occupied (oxygen vacancies, Vo) and Sn4+ ions replacing In3+ ions (SnIn) are responsible for the high conductivity [20,21]. Then, the ITO conductivity depends directly upon the Sn valence state and on the concentration of oxygen vacancies. Therefore, the carrier concentration increasing observed may be related with the enhancement of oxygen vacancies resulting in darker films. Fig. 3 shows resistivity, carrier mobility and concentration behavior as a function of the deposition pressure. The results indicate that low deposition pressures lead to the formation of ITO films with low resistivity. This could be due to the increase of carrier concentrations at lower pressures. Increasing of carrier density results in a decrease in the mobility [22]. The transmittance of the films does not exhibit large changes and the transmittance was about 80% at 550 nm. In order to obtain the best compromise between optical and electrical properties, pressure and power deposition values were fixed at 0.13 Pa and 50 W respectively. The electrical characteristics of the ITO films deposited in these conditions were: 3.28 × 10− 4 Ω cm, 13.78 cm2/V s and 1.37 × 1021/cm3 for resistivity, carrier mobility and

Fig. 1. Resistivity as a function of the r.f. power deposition.

Fig. 3. Resistivity as a function of deposition pressure.

3. Results and discussion


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Fig. 4. Transmittance spectra of PEI and PEI/ITO substrates.

Fig. 7. AFM image of ITO/PEI annealed at 473 K.

concentration, respectively. In Fig. 4 the optical transmittance of PEI and PEI/ITO substrates is showed. The ITO/PEI transmittance is limited by the PEI transmittance (~80%). In order to improve the electrical properties of ITO thin films deposited at room temperature several annealing processes were performed on the ITO thin films deposited onto PEI substrate. Fig. 5 presents the resistivity, carrier mobility and concentration as a function of thermal annealing. The best optical and electrical properties were obtained for films annealed at 473 K and this can

be understood as a function of the improved crystallization existing in these films. Indeed, morphology analysis shows the grain growth with the increasing of annealing temperature (Figs. 6–8). Fig. 6 shows AFM images of ITO surface annealed at 423 K where it is not possible to view any structure formation, but small holes (1.4 nm) are observed. These holes can be formed during the sputtering deposition, but they are not sufficiently large to modify ITO electrical properties. These surface structures were confirmed by AFM height analysis (not reported here). At this temperature, the root mean square roughness (Rrms) found was very small (Rrms = 0.3 nm) and the maximum height measured was 1.4 nm. With the annealing at 473 K some grains begin to appear, (Fig. 7) and the Rrms and maximum height increased to 0.4 nm and 4.5 nm, respectively. In Fig. 8 the AFM image of ITO films annealed at 523 K shows well defined grains on the surface, leading to a larger increase of Rrms and maximum height of 2.7 nm and 20.3 nm, respectively. This behavior is probably due to the aggregation of the native grains into larger clusters upon annealing [23]. The surface of the films annealed at 473 K resembles a “cauliflower”: it is rough, composed of tiny grains. At 523 K, the number and the size of these agglomerates decrease considerably and the surface morphology is



Fig. 5. Resistivity as a function of annealing temperature.

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change in ordinary crystalline orientation is observed. The literature [24] has also shown a relationship between the presence of the (400) crystalline orientation in ITO films (achieved at high deposition rates) and oxygen deficiency. Far from stoichiometry, polycrystalline films can be obtained even at room temperature, always showing a (400) crystal orientation. In our case the increase in the annealing temperature caused reorientations in the structural films. PEI/ITO substrates were successfully used to fabricate small molecule flexible OLEDs with the architecture: CuPc (15 nm)/NPB (45 nm)/Alq3 (50 nm)/Al (120 nm). The devices were operated under forward bias voltage, in nitrogen atmosphere with no device encapsulation, with ITO as positive electrode and Al as the negative one. Fig. 10 shows the (J vs. V–L) behavior of such a device and the inset shows its picture. The measured luminance was about 2200 cd/ m2 at 100 mA/cm2 with an efficiency of 2.2 cd/A. The reference OLED fabricated onto glass in the same conditions exhibited the same characteristics. Fig. 9. XRD patterns of ITO films annealed at: 453 K, 473 K and 523 K.

completely changed: the grain size is roughly 30 nm and they are more uniform. The increase of the annealing temperature was very efficient for the disappearance of the agglomerates, however, it promoted only a small increase in the average grain size of ITO films. Therefore, the crystallization is the preponderant phenomenon governing the decrease in the sheet resistance after annealing above 473 K. The grains growth with the raise of the annealing temperature is possibly related to the increase of the carrier mobility. However, high temperatures increase carrier concentration decreasing the carrier mobility. The X-rays characterization of ITO thin films (Fig. 9) confirms the results obtained with morphology analyses. It is possible to observe that below 473 K the films do not exhibit significant crystallization. Starting at 473 K all ITO crystalline peaks are observed, confirming the crystallization of the ITO films. At this temperature the optical and electrical properties of the ITO films were 85% of optical transmittance at 550 nm and 3.04 × 10− 4 Ω cm, 12.07 cm2/V.s, 16.8 × 1021 cm− 3, for resistivity, carrier concentration and mobility respectively. With the increase of the temperature a

4. Conclusions Highly conducting ITO films have been deposited by r.f. sputtering onto poly(etherimide) flexible substrates and annealing cycles were performed in vacuum (1 mPa), in the 423–523 K range. After thermal treatment, the PEI substrate did not show thermal degradation allowing the annealing of PEI/ITO substrate at high temperatures. The ITO thin films deposited at room temperature exhibited amorphous structure. After annealing at 473 K, the ITO films revealed a polycrystalline structure with predominant (222) and (400) orientations. Moreover, after the treatment at this temperature the films presented the lowest resistivity: 3.04 × 10− 4 Ω cm (for a thickness of 150 nm). Furthermore, PEI substrates presented optical transmittance in the visible spectra of about 85%. Flexible OLEDs with 2.2 cd/A efficiency were successfully fabricated indicating PEI as a promising substrate to be used in flexible advanced optoelectronic devices. Acknowledgements The authors would like to thank the Saudi Basic Industries Corporation of Brazil for the PEI pellets and the Brazilian agencies CNPq, FAPERJ and the Brazilian Nanotechnology Network RENAMI, for the financial support.

Fig. 10. Current density (J) and Luminance (L) as a function of applied voltage (V) for the PEI based FOLED. In the inset a picture of the device.


References [1] Z.W. Yang, S.H. Han, T.L. Yang, Ye Lina, D.H. Zhang, H.L. Ma, C.F. Cheng, Thin Solid Films 366 (2000) 4. [2] J. van den Brand, J. de Baets, T. van Mol, A. Dietzel, Microelectron. Reliab. 48 (2008) 1123. [3] P.E. Burrows, G.L. Graff, M.E. Gross, P.M. Martin, M.K. Shi, M. Hall, E. Mast, C. Bonham, W. Bennett, M.B. Sullivan, Displays 22 (2001) 65. [4] H. Klauk, Nat. Matters 6 (2007) 397. [5] C. Legnani, C. Vilani, V.L. Calil, H.S. Barud, W.G. Quirino, C.A. Achete, S.J.L. Ribeiro, M. Cremona, Thin Solid Films 517 (2008) 1016. [6] D.J. Gundlach, Nat. Matters 6 (2007) 173. [7] J.W. Yoon, T. Sasaki, N. Koshizaki, Appl. Surf. Sci. 197 (2002) 684. [8] C. Legnani, S.A.M. Lima, H.H.S. Oliveira, W.G. Quirino, R. Machado, R.M.B. Santos, M.R. Davolos, C.A. Achete, M. Cremona, Thin Solid Films 516 (2007) 193. [9] L.R. Cruz, C. Legnani, I.G. Matoso, C.L. Ferreira, H.R. Moutinho, Mater. Res. Bull. 39 (2004) 993. [10] L. Meng, M.P. dos Santos, Thin Solid Films 322 (1998) 56.

1423

[11] M. Yan, T.W. Kim, A.G. Erlat, M. Pellow, D.F. Foust, J. Liu, et al., Proc IEEE 93 (2005) 1468. [12] S.K. Park, J.I. Han, W.K. Kim, M.G. Kwak, Thin Solid Film 397 (2001) 49. [13] Chen Bor-Kuan, J.-U. Du, C.-W. Hou, IEEE Trans. Dielectr. Electr. Insul. 15 (2008) 127. [14] T. Ahn, Y. Choi, H.M. Jung, M. Yi, Org. Electron. 10 (2009) 12. [15] Y. Kato, S. Iba, R. Teramoto, T. Sekitani, T. Someya, Appl. Phys. Lett. 84 (2004) 3789. [16] T. Liu, Y. Tong, W.-D. Zhang, Compos. Sci. Technol. 67 (2007) 406. [17] C.-P. Yang, Y.-Y. Su, Y.-C. Chen, Eur. Polymer J. 42 (2006) 721. [18] E. Barbosa-Coutinho, V.M.M. Salim, C.P. Borges, Carbon 41 (2003) 1707. [19] Wu Wen-Fa, Chiou Bi-Shiou, Hsieh Shu-Ta, Semicond. Sci. Technol. 9 (1994) 1242. [20] Y. Shigesato, D.C. Paine, Appl. Phys. Lett. 62 (1993) 1268. [21] D.C. Paine, T. Whitson, D. Janiac, R. Beresford, C.O. Yang, J. Appl. Phys. 85 (1999) 8445. [22] Yang Chih-Hao, Lee Shih-Chin, Chen Suz-Cheng, Lin Tien-Chai, Mater. Sci. Eng., B 129 (2006) 154. [23] F. Matino, L. Persano, V. Arima, D. Pisignano, R.I.R. Blyth, R. Cingolani, Ross Rinaldi, Phys. Rev., B 72 (2005) 085437. [24] E. Terzini, P. Thilakan, C. Minarini, Mater. Sci. Eng., B 77 (2000) 110.