Thin film encapsulation of DSSCs on plastic substrate

Thin Solid Films 517 (2009) 4207–4210 Contents lists available at ScienceDirect 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 Thin film encapsulation of DSSCs on plastic substrate L.-T. Huang a, M.-C. Lin b, M.-L. Chang a, R.-R. Wang c, H.-C. Lin a,⁎ a b c Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan Department of Materials Engineering, National Chung Hsing University, Taichung, Taiwan Department of Product Development, Taiwan Textile Research Institute, Taipei, Taiwan a r t i c l e i n f o Available online 13 February 2009 Keywords: Aluminum oxynitride film Reactive magnetron sputtering Polyethylene naphthalate Dye-sensitized solar cells Gas permeation a b s t r a c t Aluminum oxynitride (AlOxNy) films were deposited on polyethylene naphthalate (PEN) substrates using a reactive radio frequency (RF) magnetron sputtering system by varying the nitrogen flow rate. Experimental results show that the AlOxNy films deposited on PEN substrate exhibit a pebble-like surface morphology. The deposition rate decreases slightly upon increasing the nitrogen flow rate. The surface roughness of the deposited AlOxNy films also decreases upon increasing the nitrogen flow rate. The AlOxNy film deposited at a nitrogen flow rate of 15 sccm exhibited the lowest water vapor transmission rate of 0.02 g/m2·day. Meanwhile, the passivation of AlOxNy films can effectively improve the long-term stability of plastic DSSC. Their power conversion efficiency can sustain 50% of the initial values even after 300 h. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Recently, there has been a significant interest in light, thin, and flexible electronic devices for portable applications. Plastic substrate plays an important role in developing flexible products. PEN [1] substrates are widely used in many industries, such as organic lightemitting diodes (OLEDs) and dye-sensitized solar cells (DSSC) due to their high transparency, chemical resistance, and high temperature resistance. However, the polymer substrates adsorb moisture limiting their applications. This problem needs to be resolved prior to its successful applications. Among various methods, the film coating is considered as a quite promising technique to improve the properties of polymer substrates, including the abrasive wear, gas permeation, and chemical attack. Aluminum-oxide (Al2O3) and aluminumoxynitride (AlOxNy) films [2] have been widely used as the hardcoating and diffusion barriers for polymer substrates due to their high mechanical property, high chemical stability and good adhesion with polymer substrates. These aluminum-oxide and -oxynitride films could be deposited on polymer substrates by various coating technologies, for example, the magnetron sputtering [3], plasma enhanced chemical vapor deposition [4] and vacuum arc deposition [5]. Among these techniques, the magnetron sputtering is more suitable for film deposition on polymer substrates. By using this method, the film deposition could be carried out at lower temperatures and the film′s quality could be easily controlled by varying the sputtering parameters. In the present study, we deposit the AlOxNy films on PEN substrates by using a reactive radio frequency (RF) magnetron sputtering system. The effects of nitrogen flow rate on the ⁎ Corresponding author. Tel.: +886 2 3366 4532; fax: +886 2 2363 4562. E-mail address: [email protected] (H.-C. Lin). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.02.045 deposition of AlOxNy films are studied. The gas permeation behavior of AlOxNy films and their influence on the performance of DSSC fabricated on PEN substrate are also discussed. 2. Experimental details AlOxNy films were deposited using a reactive magnetron sputtering (made by Junsun Corporation, Taiwan). A 5 cm diameter, 99.999% pure aluminum target was used. The PEN substrates had a thickness of 200 µm. The substrate to target distance was kept at 15 cm and the holder was rotated at a speed of 10 rpm to improve the film homogeneity. The purged N2 gas and the residual oxygen molecules existing in the chamber or coming from the PEN substrate would contribute to form the AlOxNy films during deposition. Table 1 presents the sputtering parameters employed. In essence, the nitrogen flow rate was varied while the other deposition parameters, such as RF power, sputter pressure, and deposition temperature, were held constant. The chemical compositions of AlOxNy films were analyzed by using X-ray photoelectron spectroscopy (XPS, 5000 Versa Probe, ULVACPHI). The surface morphologies of AlOxNy films were studied using a scanning electron microscope (SEM) and atomic force microscope (AFM). The AFM operated in the contact mode to provide the surface images of the AlOxNy films. The film thickness was measured by a step profilometer (Tencor Inc. Alpha-step 500). Transmission rates of water vapor (WVTR) were measured by a Permatran-w 3/61 model system (made by MOCON instruments), using samples with a surface area of 4.5 × 4.5 cm2. The WVTR measurement was carried out at 25 °C and in atmospheric pressure with a relative humidity of 100%. A gelled-type dye-sensitized solar cell (DSSC) was fabricated on PEN substrate. The TiO2 photo-electrode was coated on the PEN substrate by the sol–gel process [6]. The active area with 1.5 × 1.5 cm2 was 4208 L.-T. Huang et al. / Thin Solid Films 517 (2009) 4207–4210 Table 1 Sputtering conditions for deposition of AlOxNy films. Target Substrate RF power Sputter gas Sputter pressure Base pressure Deposition time Al target with diameter of 5 cm PEN 150 W Ar = 30 sccm, N2 = 0–30 sccm 0.001 Torr 1.33 × 10− 3 Pa 40 min immersed in an acetonitrile/tert-butanol mixed solvent for 24 h, and then dried in moisture-free air. The N719 dye, [Ru(Hdcbpy)2(NCS)2]2 − ,2(n-C4H9)4N+,(H2bdcbpy=L = 2,2′-bipyridine-4,4′-dicarboxylic acid) was used as a sensitizer in this study. The concentration of the dye is 3 × 10− 4M and the dipping temperature is about 25 °C. The polymer-gelled electrolytes, i.e., poly-vinylidene fluoride [7], were injected into the aperture between transparent electrode and platinum counter electrode. The two electrodes were then adhered together with epoxy resin. The power conversion efficiency of DSSCs was analyzed by a 500 W Xenon light source (CEP 2000, Bunkoh-Keiki Co.), with 100 W/cm2 light intensity. The irradiation condition is AM1.5 and the specimen's active area is 1.5 × 1.5 cm2. Fig. 2. Deposition rate of AlOxNy films with various nitrogen flow rate. 3. Results and discussion 3.1. Chemical compositions and surface morphology Fig. 1 shows the XPS spectra of AlOxNy film deposited on PEN at a nitrogen flow rate of 15 sccm. The analysis of XPS spectra in Fig. 1 indicates that Al, N and O elements coexist in the deposited AlOxNy films and has a composition of Al:O:N = 1.5:1:3. The appearance of O element is consistent with the reported studies on titanium oxynitride and aluminum oxynitride films by Guillot et al. [8] and Dreer et al. [9]. Their films were also deposited in a sputter chamber with very little or even no oxygen content. This indicates that at a base pressure of 1.3 × 10− 3 Pa or less, there is enough oxygen present to partake in the reaction with Al and N atoms to form AlOxNy films. In this work, the H2O molecules which adhere on the surface or exist in the interior of PEN substrates [10] and the residual O2 impurity in the sputter chamber, are expected to provide the O atoms during the deposition of Fig. 1. XPS spectra of AlOxNy films deposited on PEN substrate at nitrogen flow rate of 15 sccm. Fig. 3. The SEM micrographs of AlOxNy films deposited on PEN substrate at nitrogen flow rate of (a) 5 sccm, (b) 15 sccm and (c) 25 sccm. L.-T. Huang et al. / Thin Solid Films 517 (2009) 4207–4210 AlOxNy films. Besides, the existence of C signal in Fig. 1 should come from the carbon contamination during the thin film experiment. Fig. 2 shows the deposition rates of AlOxNy films at various nitrogen flow rates. The thickness of AlOxNy films can be calculated to be 80–120 nm from these deposition rates. In Fig. 2, it can be seen that the deposition rate decreases slightly upon increasing the nitrogen flow rate. This feature is explained as in the following. It is known that the sputter yield of argon is higher than nitrogen [11,12]. Hence, the higher the nitrogen flow rate, the lower the sputter yield of Al atom is. This feature will reduce the deposition rate of AlOxNy films if there is higher nitrogen concentration (or nitrogen flow rate) in the plasma gas. Fig. 3(a–c) shows the FE-SEM micrographs of AlOxNy films deposited on PEN at various nitrogen flow rates. These micrographs show that the deposited AlOxNy films exhibit a dense pebble-like surface morphology. Carefully examining Fig. 3(a–c), it can be seen that the size of pebble-like particles within the AlOxNy films decreases with increasing the nitrogen flow rates. This phenomenon can be reasonably explained as in the following. As mentioned above, at a lower nitrogen flow rate, more sputtered species (atoms or ions) approach the film's surface. The substrate temperature will be raised due to the intense bombardment of deposited species. Hence, the size of AlOxNy pebble-like particles grows up quickly. On the contrary, less sputtered species come onto the surface at a higher nitrogen flow rate. The substrate temperature is lower and the size of AlOxNy pebble-like particles is finer. This phenomenon is also consistent with the surface roughness of films. As shown in Fig. 4, the film's surface roughness at 15 sccm nitrogen flow rate. Namely, at a higher nitrogen flow rate, the AlOxNy films can exhibit a smother surface morphology with denser pebble-like particles. This feature will effectively improve the gas permeation behavior, as discussed in the next section. 3.2. Water vapor permeation and DSSC performance The water vapor transmission rates (WVTR) of AlOxNy films deposited on PEN under various nitrogen flow rates are plotted as Fig. 5. The WVTR of bare PEN substrate is about 2 g/m2 day. As shown in Fig. 5, the WVTR of the PEN substrates reduce significantly after the deposition of AlOxNy films. The WVTR of AlOxNy film first rapidly Fig. 4. AFM images of AlOxNy films with nitrogen flow rate 15 sccm (rms roughness of 1.1 nm). 4209 Fig. 5. The WVTR of AlOxNy films deposited on PEN substrate at various nitrogen flow rates. decreases, reaches a critical value of 0.02 g/m2 day at a nitrogen flow rate 15 sccm, and then maintains this critical value. This feature is ascribed to the fact that AlOxNy films deposited at higher nitrogen flow rates, say ≥15 sccm in this study, can exhibit a smother and denser surface morphology. These AlOxNy films with good quality will help to inhibit the water vapor permeation through PEN substrate. It is important to understand the DSSC performance using the flexible substrates of PEN with various deposited AlOxNy films. Fig. 6 shows the normalized efficiency η versus storage time of DSSC with polymer gel electrolyte for some flexible substrates prepared in this study. The power conversion efficiency of the initial state is about 1.45% under the standard condition of sun illumination. The power conversion efficiency η is calculated by the equation: η= Voc
Isc
FF Pin ð1Þ where Pin is the incident light power, Voc is open circuit voltage and FF is the Fill Factor. In Fig. 6, it can be seen that the PEN substrates with AlOxNy films can exhibit better DSSC efficiency. For the bare PEN substrate, the DSSC efficiency decreases rapidly to be about 50% in 125 h. However, the substrates of PEN with AlOxNy films, their DSSC efficiency can sustain at 50% of the initial values even after 300 h . This feature is ascribed to the fact that the AlOxNy films can effectively inhibit the water vapor transmission through PEN substrates, and hence increase the long-term stability of plastic DSSC. Fig. 6. The normalized efficiency versus storage time of DSSC encapsulated with bare PEN and with AlOxNy films deposited on PEN substrate at various nitrogen flow rates. 4210 L.-T. Huang et al. / Thin Solid Films 517 (2009) 4207–4210 4. Conclusions Aluminum oxynitride (AlO xNy) films have been successfully deposited on polyethylene naphthalate (PEN) substrates by RF reactive magnetron sputtering. The AlOxNy films deposited on PEN substrate exhibited a pebble-like surface morphology. The deposition rate and surface roughness of the AlOxNy films slightly decreases with increasing the nitrogen flow rate. The AlOxNy film deposited at a nitrogen flow rate of 15 sccm exhibited the lowest water vapor transmission rate of 0.02 g/m2 day. Meanwhile, the passivation of AlOxNy films can effectively improve the long-term stability of plastic DSSC. Their power conversion efficiency can sustain at 50% of the initial values even after 300 h. Acknowledgment This work was financially supported by the Taiwan Textile Research Institute, Taipei, Taiwan. References [1] M. Ikegami, K. Miyoshi, T. Miyasaka, K. Teshima, T.C. Wei, C.C. Wan, Y.Y. Wang, Appl. Phys. Lett. 90 (2007) 153122. [2] A.G. Erlat, B.M. Henry, J.J. Ingram, D.B. Mountain, A. McGuigan, R.P. Howson, C.R.M. Grovenor, G.A.D. Briggs, Y. Tsukahara, Thin Solid Films 388 (2001) 78. [3] A.F. Jankowski, J.P. Hayes, T.E. Felter, C. Evans, A.J. Nelson, Thin Solid Films 420–421 (2002) 43. [4] K. Teshima, Y. Inoue, H. Sugimura, O. Takai, Vacuum 66 (2002) 353. [5] H. Bolt, F. Koch, J.L. Rodet, D. Karpov, S. Menzel, Surf. Coat. Technol. 116–119 (1999) 956. [6] C.J. Barbe, F. Arendse, P. Comte, M. Jirousek, F. Lenzmann, V. Shklover, et al., J. Am. Ceram. Soc. 80 (1997) 3157. [7] Koichi Tsunemi, Hiroyuki Ohno, Eishun Tsuchida, Electrochim. Acta 28 (1983) 833. [8] J. Guillot, A. Jouaiti, L. Imhoff, B. Domenichini, O. Heintz, S. Zerkout, A. Mosser, S. Bourgeois, Surf. Interface Anal. 34 (2002) 577. [9] S. Dreer, R. Krismer, P. Willhartitz, G. Friedbacher, Thin Solid Films 354 (1999) 43. [10] M. Neuhäuser, S. Bärwulf, H. Hilgers, E. Lugscheider, M. Riester, Surf. Coat. Technol. 116–119 (1999) 981. [11] R.S. Houk, Narong Praphairaksit, Spectrochim. Acta Part B 56 (2001) 1069. [12] P.C. Cosby, J. Chem. Phys. 98 (1993) 9544.