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Towards an all-polymer cathode for dye sensitized photovoltaic cells
Thin Solid Films 518 (2010) 2871–2875
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
Towards an all-polymer cathode for dye sensitized photovoltaic cells P.M. Sirimanne, B. Winther-Jensen, H.C. Weerasinghe, Yi-Bing Cheng ⁎ Department of Materials Engineering, Faculty of Engineering, Monash University, Welligton Road, Clayton 3800, Victoria, Australia
a r t i c l e
i n f o
Article history: Received 1 April 2009 Received in revised form 23 September 2009 Accepted 25 September 2009 Available online 7 October 2009 Keywords: Conjugated polymer composites Poly(3,4-ethylenedioxythiophene: para-toluenesulfonate Power conversion efficiency
a b s t r a c t Highly conducting conjugated polymer composite was deposited by vapour phase polymerization. Poly[3,4ethylenedioxythiophene:para-toluenesulfonate] (PEDOT:PTS) itself was used as the counter electrode in a dye sensitized solar cell. The maximum photocurrent of 9.7 mA/cm2, open circuit voltage of 759 mV, fill factor of 0.71 with a power conversion efficiency of 5.25% were observed for glass based wet type dye sensitized solar cell, under illumination of 100 mW/cm2. It was observed that the resistance, during operation of the dye sensitized solar cells, due to the I− 3 conversion was less with PEDOT:PTS coated cathodes than with standard platinum coated fluorine doped tin oxide and was confirmed by steady state electrochemical measurements. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Dye sensitized solar cells (DSSCs) have aroused much attention as a cheap and next generation solar cell. The maximum conversion efficiency of over 10% was achieved for this type of solar cells composed with sensitized TiO2 electrodes from ruthenium complexes [1]. It is an important strategy to improve the performance of both working and counter electrodes to achieve maximum efficiency for the cell. The commercial potential of DSSCs is dependent on the development of low cost materials for the photovoltaic device as well. Recently, several attempts have been made to replace the high cost Pt counter electrode with conjugated polymers, such as poly[3,4-ethylenedioxythiophene] (PEDOT) derivatives in DSSCs, due to their higher catalytic properties (reduction of I− 3 ions at the counter electrode). PEDOT films can be deposited on various substrates using inexpensive low temperature deposition techniques, which is another advantage of polymer based counter electrodes. Photovoltaic performance of DSSCs consisting of PEDOT and PEDOT based composites as counter electrodes have been demonstrated recently [2–5]. In these studies, PEDOT was used to form films on transparent conducting oxide coated glass due to insufficient conductivity of the poly[3,4-ethylenedioxythiophene]:poly[styrenesulfonate] (PEDOT:PSS) or poly(3,4-ethylenedioxythiophene:para-toluenesulfonate) (PEDOT:PTS). It is clear, therefore, that the role of PEDOT counter electrodes in these devices was chiefly of a single function for − catalysing the I− redox reaction in the electrolyte. Vapour phase 3/I polymerization technique produces thicker PEDOT:PTS films on plain glass and plastic substrates [without conductive fluorine doped tin oxide (FTO) or tin doped indium oxide (ITO) coatings] as the counter electrode in DSSC devices [6,7]. We have demonstrated that the
⁎ Corresponding author. E-mail address: [email protected] (Y.-B. Cheng). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.114
conducting PEDOT:PTS polymer films alone can be very effective counter electrodes and perform dual functions of redox catalysis and electron conduction in glass and plastic based DSSCs. 2. Experimental details 2.1. Materials Nanocrystalline TiO2 paste (PST-18NR, JGC Catalysts and Chemicals Ltd., Japan), and colloidal TiO2 suspensions (PASOL HPW-18NR, PASOL HPW-400C, JGC Catalysts and Chemicals Ltd., Japan) were used for making the working electrodes for glass and Ti foil based devices. P-25 TiO2 powder (Degusa, Japan) was used to make the plastic based cell. Ru centred dye cis-bis (isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium (II) bis-tetrabutyl-ammonium (N719, Solaronicx SA, Switzerland) was used the sensitizer. FeIII-para-toluenesulfonate (FeIIIPTS) (H.C. Starck) and PEDOT monomer (Sigma) were used as received from suppliers without any further purification. FTO coated glass substrates (Nippon sheet glass, sheet resistance 13Ω/□) and ITO-coated polyethelene naphthalate (PEN) (known as ITO|PEN) substrates (Peccells Technologies, Japan) were ultrasonically cleaned with a detergent prior to use. The conducting side of ITO|PEN films was used to construct the working electrode and the PEDOT:PTS counter electrode was built on the non-ITO coated side of the PEN for the plastic DSSC devices. 2.2. Preparation of TiO2 on FTO coated glass substrates A highly compact TiO2 blocking layer with a thickness of 300 nm was deposited on well cleaned FTO coated glass substrates (2× 2.5 cm2) by spraying a solution of titanium di-isopropoxide bis(acetyl-acetonete) in ethanol with a volume ratio of 1:9. The spray deposition was applied through a rectangular opener (1× 1 cm2) in a metal mask placed on FTO
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substrates at 450 °C. Nanocrystalline TiO2 films (7× 7 mm2) were then deposited on the blocking layer by screen printing of the nanocrystalline TiO2 paste and drying at 125 °C. The thickness of nanocrystalline TiO2 film was controlled by repeating coating and drying procedure. A TiO2 scattering layer was then deposited on top of the nanocrystalline TiO2 film by screen printing. The scattering layer was made with a paste consisting of 20% TiO2 of 18 nm particles and 80% TiO2 of 400 nm particles. TiO2 electrodes with the scattering layer were sintered at 450 °C for 30 min. TiO2 electrodes were then wetted by 0.05 M TiCl4 in 20% HCl (aq) and placed in humidity bath and were kept in an oven at 70°C, for 30min. Electrodes were subsequently rinsed thoroughly with distilled water and ethanol and were allowed to dry. Finally, TiO2 coated FTO substrates were again heated at 450 °C for 30 min. 2.3. Preparation of TiO2 films on ITO|PEN substrates Nanocrystalline TiO2 films on ITO|PEN substrates were deposited as follows, 7 g of P-25 TiO2 powder and 14 ml of moisture free ethanol were placed in a dried agate ball milling jar with different sizes of agate balls. The mixture was then milled continuously at 250 rpm speed for 10h. Ball milled slurry was coated on ITO|PEN substrates by spin-coating (spin coater — Laurell WS-400B-6NPP). TiO2 coated ITO|PEN substrates were dried at room temperature for several minutes and then heated at 150 °C for 30 min, on a hotplate, in the atmospheric conditions. 2.4. Preparation, electochemical and swelling measurements of PEDOT: PTS films The oxidant FeIIIPTS (40% solution in butanol) was coated on substrates (glass, PEN film, and Ti foil) by spin-coating and the thicker ones were then made by spraying multiple layers of the oxidant solution onto the substrate using a conventional air-brush. Three different thicknesses of films were deposited under different selected conditions. The oxidant coated substrates were exposed to PEDOT monomer vapour at 70 °C for 1 h, followed by drying. The samples were washed in ethanol for 2 h for removing excess FeII and PTS from the PEDOT films and then dried at room temperature. The thickness of PEDOT:PTS films were measured by a profilomertry (Dektak 500). The steady state measurements of conversion current on different electrodes were made in a − 10 ml sealed three-electrode cell, in the presence of I− standard 3 |I electrolyte for DSSCs). A platinum sheet was used for counter as well as the reference electrode. A constant distance between the electrodes was secured by mounting the electrodes in the lid of the cell. A conventional magnetic stirrer was used to maintain stirring at 450 rpm during the experiments. After 5 min at a given potential the correlating conversion current was measured. The swelling measurements of PEDOT:PTS films were made by Quarts Crystal Microbalance (QCM200 from Stanford Research Systems). PEDOT:PTS was coated on the QCM crystal and then exposed to acetonitrile using a micro-flow setup. The frequency changes were monitored as a direct out-put from the instrument and the percentage of swelling was calculated from these values.
the DSSCs from mono-chromatic and white light (100 mW/cm2), respectively. The interfacial properties of the cells were studied by applying negative 0.7 V on working electrode under a two electrode configuration, at the dark, in the same electrolyte, by using a multichannelled potentiostat (Princeton Applied Research) coupled with a computer. The charge transfer resistance at the electrolyte–electrode interface was evaluated from EClab software. 3. Results and discussion Standard working electrodes consisted of a nano-porous TiO2 layer with thickness of 5 µm on the top of a 300 nm dense TiO2 blocking layer deposited on FTO glass substrates. The TiO2 blocking layer beneath the nanocrystalline TiO2 film exhibits a highly compact array of grains that prevents direct contact of electrolyte with back contact of the cell. The nanocrystalline TiO2 layer shows a high degree of porosity, resulting in maximum dye absorption on the working electrode, essential for higher conversion efficiency of the cell. The top surface of the nano-porous TiO2 film was covered with a light scattering TiO2 layer made of 400 nm particles to achieve the maximum performance for the cell, as reported in a previous work [8]. The outer most TiO2 scattering layer increases the optical length of incident light in the film, thus improving the light harvesting. The N719 dye was coated on TiO2 electrodes to bring their absorption to the visible region. PEDOT:PTS films with the maximum thickness of 20 µm were obtained by vapour phase polymerization. Resistance of such film was 4Ω/□ and resistance increased with decreasing the film thickness. A thinner PEDOT:PTS film (0.5 µm) exhibited resistance of 60Ω/□, at room temperature. As is proposed the counter electrode must possess high − conductivity and pronounced catalytic properties to reduce I− 3 ions to I ions for, optimum performance of the cell. Catalytic capability of PEDOT: PTS has been shown earlier, [3] but further understanding of the electrocatalytic mechanism has recently been identified [9]. We have studied variation of Faradic current for the conversion of I−/I− 3 under steady state condition in stirred electrolyte used for solar cells. Variation of Faradic current with applied potential for (a) PEDOT:PTS|Ti and (b) Pt|FTO electrodes is shown in Fig. 1. It is notable that there is apparently no over potential for both electrodes [seen by the straight line through (0,0)] and that PEDOT:PTS and Pt have same equilibrium potential (both curves are going through zero V vs. Pt). The I− 3 reduction current on PEDOT:PTS is
2.5. Preparation of solar cells TiO2 electrodes were immersed overnight in ethanolic solutions of N719 dye at room temperature. Dye coated electrodes were then washed in ethanol and dried in a nitrogen steam. Photovoltaic cells were completed by inserting electrolyte (a mixture of iodine—0.03 M, 4tertbutylpyridine—0.5 M, 1-butyl-3-methyl imidazolium iodide—0.6 M, and guanidinium thiocyanate—0.1 M in acetonitrile:valeronitrile—85:15 by vol.%) to the well attached dye coated TiO2 working electrode and PEDOT:PTS counter electrode. A spacer with the thickness of 64 µm was used to separate the electrodes. Incident photon to current conversion efficiency (IPCE) spectra and current–voltage (IV) characteristics were ascertained for each thickness group of the PEDOT:PTS film by exciting
− Fig. 1. Steady state measurement of the I− 3 /I conversion current at different potentials vs. Pt reference electrode for (a) PEDOT:PTS|Ti-foil and (b) Pt|FTO in standard DSSC electrolyte under continuous stirring.
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seen to reach a diffusion limit at about 9.5 mA/cm2 below approximately −0.2 V vs. Pt under the given conditions (concentration, electrode distance and stirring speed). The slope of the curves at zero current reflects 1/resistance (1/R) for the entire cell, where R is the total of resistance of electrode (Relectrode), reaction (Riodide reaction), electrolyte (Relectrolyte) and the resistance of other parts of the cell (Rcell). The resistance of Pt electrode (RPt) and PEDOT:PTS electrode (RPEDOT:PTS) can be obtained from the gradient at zero current by inter-changing Pt and PEDOT electrodes and keeping other conditions as constant. For PEDOT:PTS|Ti electrode PEDOT:PTS
R
PEDOT:PTS
= 20 Ω = ðRelectrode + Riodide reaction Þ
+ Rcell + Relectrolyte :
ð1Þ For Pt|FTO electrode R
Pt
Pt
= 168 Ω = ðRelectrode + Riodide reaction Þ
+ Rcell + Relectrolyte :
ð2Þ
According to Eq. (1), Rcell + Relectrolyte must be smaller than 20 Ω as (Relectrode + Riodide reaction)PEDOT:PTS cannot be negative. Therefore, (Relectrode +Riodide reaction)Pt must be 148Ω or higher. A 1×3cm2 FTO substrate with the surface resistance of 15Ω/□ was used for the Pt|FTO electrode. Thus, (Relectrode)Pt and (Riodide reaction)Pt should be 45Ω and 103Ω (or higher), respectively. On the other hand (Riodide reaction)PEDOT:PTS cannot be higher than 20Ω, which was obtained for the entire cell. This means that the resistance of the iodide reaction on the Pt electrode is about five times (103/20) higher than that on the PEDOT electrode. This result shows the real potential in using PEDOT:PTS as an electro-catalyst in DSSC. The mechanism of the electro-catalytic reduction of I− 3 on PEDOT: PTS is shown in Scheme 1. I− ions inject electrons into the normally oxidized PEDOT:PTS creating an intermediate state of reduced PEDOT: PTS. The reduced PEDOT:PTS is not stable in the presence of I− 3 and − therefore readily re-oxidized by reducing I− 3 to 3I . PEDOT is well-known to be in its oxidized state at potentials where the I− 3 reduction is taking place [7] and measurements of the conductivity of PEDOT during the I− 3 reduction show decreases of conductivity, indicating that PEDOT remains partly oxidized. This observation fits with the proposed reaction scheme, where an average oxidation state between reduced and oxidized PEDOT segments is predicted. We have studied the photo-performance of the cell. IPCE of the cell is a factor determining photovoltaic performance of a solar cell and given by IPCE = 1240 × iph / (λIs), where iph is short circuit photocurrent density in µA/cm− 2, λ is wavelength in nm and Is is the intensity of the incident light in µW/cm− 2 [10]. IPCE spectra of (a) glass|FTO| TiO2|dye|electrolyte|PEDOT:PTS|glass, and (b) PEN|ITO|TiO2|dye|elec-
Scheme 1. The mechanism for the reduction of I− 3 on PEDOT:PTS.
Fig. 2. IPCE spectra of (a) glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS|glass cell, (b) PEN| ITO|TiO2|dye|electrolyte|PEDOT:PTS|PEN cell, (c) glass|FTO|TiO2|dye|electrolyte|Pt| FTO|glass cell and (d) transmittance of ITO|PEN film.
trolyte|PEDOT:PTS|PEN cells are shown in Fig. 2. The maximum IPCE of 55% was observed for glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS| glass cell. The PEN|ITO|TiO2|dye|electrolyte|PEDOT:PTS|PEN cells exhibit maximum IPCE of 18% at wavelength of 525 nm for the same electrolyte. The maximum IPCE of 65% was observed for the glass|FTO| TiO2|dye|electrolyte|Pt|FTO|glass cells under same condition (curve c). A more efficient electron transfer process in titania films sintered at high temperature is one of the reasons for the better IPCE observed in glass based devices compared to that of flexible substrates. Lack of a dense blocking layer would also result in increased charge recombination at the interface between the electrolyte and the ITO|PEN substrate. IPCE spectrum of the plastic based cell has shifted toward the longer wavelengths compared to that of glass based cells at shorter wavelength range. This might be due to the cut-off of incident light by PEN substrates. The transmittance spectrum of the PEN substrate is shown in Fig. 2 (curve d). The variation of current–voltage characteristics of glass|FTO|TiO2| dye|electrolyte|PEDOT:PTS|glass cells for the different thicknesses of PEDOT:PTS electrodes (a) 0. 44 µm, b). 10.2 µm and (c) 18.6 µm are shown in Fig. 3. The maximum photovoltage of 781 mV was observed for the thinnest PEDOT:PTS electrode. This cell shows lowest fill factor (51%) due to relatively high series resistance of the PEDOT:PTS counter electrode. The maximum photovoltage of the cells gradually decreases with increasing the PEDOT:PTS film thickness. By contrast, a gradual increment of fill factor is observed with thicker PEDOT:PTS films, probably due to the decrease of the resistance of the film. However, the photocurrent of the cell initially increases with thickness of PEDOT:PTS films and reaches to a maximum of ~ 10 mA/cm2 for the cell having a (PEDOT:PTS film of 10.2 µm) and then gradually decreases by further increasing the thickness of the polymer film. A similar pattern of variation was observed for the efficiency of the cell as efficiency is given by product of photovoltage and photocurrent. The performances of those cells are summarized in Table 1. The reason for the decrease in the photocurrent higher PEDOT:PTS film thickness and thereby lower electrical resistance of the cathode is quite intriguing, and may be due to the decreasing in the diffusion rate of the iodide|iodine ions with the PEDOT:PTS layer when it swells in the presence of the electrolyte. The swelling behaviour of conducting polymer is a well-known phenomena [11] and swelling in acetonitrile is no exemption. Using a Quartz Crystal
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Fig. 3. Current–voltage characteristics of glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS| glass cells with different thicknesses of polymer counter electrodes (a) 0. 44 µm, (b) 10.2 µm and (c) 18.6 µm. Dark current–voltage characteristics of the cells are also in the figure, with similar codes.
Microbalance the swelling of the PEDOT:PTS film in acetonitrile was determined to be 15% after 1 h and it reached a maximum of 36% after 16 h. This degree of swelling corresponds to 7 μm in the assembled DSSC for a 20 μm PEDOT:PTS counter-electrode film, which may indeed give rise to changes in the overall diffusion co-efficient of iodides. Swelling and aggregation of the PEDOT can improve the conductivity by forming a more conductive network when the film is thin (for example 50 nm in thickness [12], but could reduce the catalytic surface area and cause higher resistance of the electrolyte diffusion when the film is thick). This effect will become more apparent by increasing the thickness of the PEDOT:PTS layer and will limit the overall efficiency of the cell although the electrical resistance of the PEDOT:PTS is decreasing. We have observed physical defects such as partial peeling off and difference in the roughness on PEDOT: PTS when film thickness is greater than 10 µm. This may be another reason for the decrease of efficiency of the cell. To overcome this problem the PEDOT:PTS cathode has to be structured on the micro| nanometre scale allowing access of iodine|iodide to the catalytic sites without limiting the diffusion speed. Further, we have compared the performance of dye sensitized solar cells composed of different types of counter electrodes. Current–voltage characteristics of (a) glass|FTO| TiO2|dye|electrolyte|Pt|FTO|glass, (b) glass|FTO|TiO2|dye| electrolyte| PEDOT:PTS|Ti-foil, (c) glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS|glass
Fig. 4. Current–voltage characteristics for (a) glass|FTO|TiO2|dye|electrolyte|Pt|FTO|glass, (b) glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS|Ti-foil, (c) glass|FTO|TiO2|dye|electrolyte| PEDOT:PTS|glass and (d) PEN|ITO|TiO2|dye|electrolyte|PEDOT:PTS|PEN cells. Dark current–voltage characteristics of the cells are also in the same figure, with a similar code.
cells and (d) PEN|ITO|TiO2|dye|electrolyte|PEDOT:PTS|PEN are shown in Fig. 4 and the characteristics of each cell are also summarized in Table 1. Similar open circuit voltage is observed for devices (a–c) at optimum conditions. The replacement of PEDOT:PTS on plain glass for Pt|FTO as a counter electrode has produced comparable conversion efficiency between the two cells and thus demonstrated the significant potential in the application of PEDOT:PTS polymers for high efficiency, low cost DSSCs. However, higher photocurrent is expected for glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS|Ti cell compared to that of other cells due to fast catalytic properties of PEDOT: PTS on Ti foil (curve a, Fig. 1) and lower resistance of the substrate. We have studied electrical impedance of TiO2|dye|electrolyte cells for better understanding of these devices. Electrical impedance of (a) glass| FTO|TiO2|dye| electrolyte|Pt|FTO|glass and (b) glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS|glass cells are shown in Fig. 5. The decrease in Z1 (the diameter of the first semicircle from the high frequency side; normally recognised as the contribution from the iodine reduction on the cathode) [13] for the cell composed with PEDOT:PTS counter electrode is in line − with our electrochemical measurements of Pt and PEDOT:PTS|Ti in I− 3 /I electrolyte (Fig. 1), where higher conversion currents were measured on PEDOT:PTS|Ti than on Pt at same potential. This confirms the great potential in using PEDOT:PTS as a catalyst for the cathode reaction. It can be seen that the cell resistance (Z2, the diameter of second semicircle of the impedance spectrum from the high frequency side) for the PEDOT:
Table 1 The performances of the different types of dye sensitized cells. Type of the cell
VOC [mV]
ISC [mA/cm− 2]
ff (%)
Glass|FTO|TiO2|dye|electrolyte|Pt|FTO|glass Glass|FTO|TiO2|dye|electrolyte|PEDOT: PTS|Ti-foil Glass|FTO|TiO2|dye|electrolyte|PEDOT: PTSd = 0.44 μm|glass Glass|FTO|TiO2|dye|electrolyte|PEDOT: PTSd = 10.2 μm|glass Glass|FTO|TiO2|dye|electrolyte|PEDOT: PTSd = 18.6 μm|glass PEN|ITO|TiO2|dye|electrolyte|PEDOT: PTSd = 10.2 mm|PEN
746 750
11.36 10.35
65 75
781
6.9
51
758
9.71
71
699
4.9
72
670
6.17
61
Fig. 5. Normalized electrical impedance spectra of (a) glass|FTO|TiO2|dye|electrolyte|Pt| glass and (b) glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS|glass cells.
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PTS device (curve b) is slightly higher than the standard TiO2|dye| electrolyte|Pt|FTO system indicating that optimisation of the PEDOTelectrolyte interface layer is still needed. 4. Conclusion Highly conducting conjugated PEDOT:PTS can be made as allpolymer counter electrodes in DSSCs to perform dual functions of redox catalyst and electrical conductor. The efficiency of these devices is comparable to that of conventional DSSCs with Pt|FTO as counter electrodes. It is also possible to use the PEDOT:PTS polymer to replace Pt| ITO as counter electrodes for plastic based DSSCs. PEDOT:PTS seems to − be a more efficient electro-catalyst for the I− 3 /I conversion than Pt, but the construction of PEDOT:PTS counter electrodes has to be optimised to take full advantage of this phenomena for high efficiency, low cost DSSCs. Acknowledgements The generous gift of nanocrystalline TiO2 paste (18 nm, PST-18NR) and TiO2 suspensions (PASOL HPW-400C and PASOL HPW-18NR) by JGC Catalysts and Chemicals, Japan is highly appreciated by the authors.
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References [1] B. O'Regan, M. Gratzel, Nature 353 (1999) 737. [2] Y. Saito, W. Kubo, T. Kitamura, Y. Wada, S. Yanagida, J. Photochem. Photobiol., A Chem. 164 (2004) 153. [3] L. Bay, K. West, B. Winther-Jensen, T. Jacobsen, Sol. Energy Mater. Sol. Cells 90 (2006) 341. [4] J.G. Chen, H.Y. Wei, K.C. Ho, Sol. Energy Mater. Sol. Cells 91 (2007) 1472. [5] W. Hong, Y. Xu, G. Lu, C. Li, G. Shi, Electrochem. Commun. 10 (2008) 1555. [6] B. Winther-Jensen, K. West, Macromolecules 37/12 (2004) 4538. [7] B. Winther-Jensen, K. West, React. Funct. Polym. 66/5 (2006) 479. [8] H. Arakawa, T. Yamaguchi, A. Takeuchi, S. Agatsuma, IEEE 2006, IEEE 4th world conference on photovoltaic conversion, Hawaii, U.S.A, May 7–12, 2006, p. 36. [9] B. Winther-Jensen, O. Winther-Jensen, M. Forsyth, D.R. MacFarlane, Science 321 (2008) 671. [10] T.A. Heimer, E.J. Heilweil, C.A. Bignozzi, G.J. Meyer, J. Phys. Chem., A 104 (2000) 4256. [11] Y.F. Li, Electrochem. Acta 42 (1997) 203. [12] H.J. Snaith, H. Kenrick, M. Chiesa, R.H. Friend, Polymer 46 (2005) 2573. [13] D. Kuang, C. Klein, Z. Zhang, S. Ito, J-E. Moser, S.M. Zakeeruddin, M. Gratzel, Small 3/12 (2007) 2049.
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
Towards an all-polymer cathode for dye sensitized photovoltaic cells P.M. Sirimanne, B. Winther-Jensen, H.C. Weerasinghe, Yi-Bing Cheng ⁎ Department of Materials Engineering, Faculty of Engineering, Monash University, Welligton Road, Clayton 3800, Victoria, Australia
a r t i c l e
i n f o
Article history: Received 1 April 2009 Received in revised form 23 September 2009 Accepted 25 September 2009 Available online 7 October 2009 Keywords: Conjugated polymer composites Poly(3,4-ethylenedioxythiophene: para-toluenesulfonate Power conversion efficiency
a b s t r a c t Highly conducting conjugated polymer composite was deposited by vapour phase polymerization. Poly[3,4ethylenedioxythiophene:para-toluenesulfonate] (PEDOT:PTS) itself was used as the counter electrode in a dye sensitized solar cell. The maximum photocurrent of 9.7 mA/cm2, open circuit voltage of 759 mV, fill factor of 0.71 with a power conversion efficiency of 5.25% were observed for glass based wet type dye sensitized solar cell, under illumination of 100 mW/cm2. It was observed that the resistance, during operation of the dye sensitized solar cells, due to the I− 3 conversion was less with PEDOT:PTS coated cathodes than with standard platinum coated fluorine doped tin oxide and was confirmed by steady state electrochemical measurements. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Dye sensitized solar cells (DSSCs) have aroused much attention as a cheap and next generation solar cell. The maximum conversion efficiency of over 10% was achieved for this type of solar cells composed with sensitized TiO2 electrodes from ruthenium complexes [1]. It is an important strategy to improve the performance of both working and counter electrodes to achieve maximum efficiency for the cell. The commercial potential of DSSCs is dependent on the development of low cost materials for the photovoltaic device as well. Recently, several attempts have been made to replace the high cost Pt counter electrode with conjugated polymers, such as poly[3,4-ethylenedioxythiophene] (PEDOT) derivatives in DSSCs, due to their higher catalytic properties (reduction of I− 3 ions at the counter electrode). PEDOT films can be deposited on various substrates using inexpensive low temperature deposition techniques, which is another advantage of polymer based counter electrodes. Photovoltaic performance of DSSCs consisting of PEDOT and PEDOT based composites as counter electrodes have been demonstrated recently [2–5]. In these studies, PEDOT was used to form films on transparent conducting oxide coated glass due to insufficient conductivity of the poly[3,4-ethylenedioxythiophene]:poly[styrenesulfonate] (PEDOT:PSS) or poly(3,4-ethylenedioxythiophene:para-toluenesulfonate) (PEDOT:PTS). It is clear, therefore, that the role of PEDOT counter electrodes in these devices was chiefly of a single function for − catalysing the I− redox reaction in the electrolyte. Vapour phase 3/I polymerization technique produces thicker PEDOT:PTS films on plain glass and plastic substrates [without conductive fluorine doped tin oxide (FTO) or tin doped indium oxide (ITO) coatings] as the counter electrode in DSSC devices [6,7]. We have demonstrated that the
⁎ Corresponding author. E-mail address: [email protected] (Y.-B. Cheng). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.114
conducting PEDOT:PTS polymer films alone can be very effective counter electrodes and perform dual functions of redox catalysis and electron conduction in glass and plastic based DSSCs. 2. Experimental details 2.1. Materials Nanocrystalline TiO2 paste (PST-18NR, JGC Catalysts and Chemicals Ltd., Japan), and colloidal TiO2 suspensions (PASOL HPW-18NR, PASOL HPW-400C, JGC Catalysts and Chemicals Ltd., Japan) were used for making the working electrodes for glass and Ti foil based devices. P-25 TiO2 powder (Degusa, Japan) was used to make the plastic based cell. Ru centred dye cis-bis (isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium (II) bis-tetrabutyl-ammonium (N719, Solaronicx SA, Switzerland) was used the sensitizer. FeIII-para-toluenesulfonate (FeIIIPTS) (H.C. Starck) and PEDOT monomer (Sigma) were used as received from suppliers without any further purification. FTO coated glass substrates (Nippon sheet glass, sheet resistance 13Ω/□) and ITO-coated polyethelene naphthalate (PEN) (known as ITO|PEN) substrates (Peccells Technologies, Japan) were ultrasonically cleaned with a detergent prior to use. The conducting side of ITO|PEN films was used to construct the working electrode and the PEDOT:PTS counter electrode was built on the non-ITO coated side of the PEN for the plastic DSSC devices. 2.2. Preparation of TiO2 on FTO coated glass substrates A highly compact TiO2 blocking layer with a thickness of 300 nm was deposited on well cleaned FTO coated glass substrates (2× 2.5 cm2) by spraying a solution of titanium di-isopropoxide bis(acetyl-acetonete) in ethanol with a volume ratio of 1:9. The spray deposition was applied through a rectangular opener (1× 1 cm2) in a metal mask placed on FTO
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substrates at 450 °C. Nanocrystalline TiO2 films (7× 7 mm2) were then deposited on the blocking layer by screen printing of the nanocrystalline TiO2 paste and drying at 125 °C. The thickness of nanocrystalline TiO2 film was controlled by repeating coating and drying procedure. A TiO2 scattering layer was then deposited on top of the nanocrystalline TiO2 film by screen printing. The scattering layer was made with a paste consisting of 20% TiO2 of 18 nm particles and 80% TiO2 of 400 nm particles. TiO2 electrodes with the scattering layer were sintered at 450 °C for 30 min. TiO2 electrodes were then wetted by 0.05 M TiCl4 in 20% HCl (aq) and placed in humidity bath and were kept in an oven at 70°C, for 30min. Electrodes were subsequently rinsed thoroughly with distilled water and ethanol and were allowed to dry. Finally, TiO2 coated FTO substrates were again heated at 450 °C for 30 min. 2.3. Preparation of TiO2 films on ITO|PEN substrates Nanocrystalline TiO2 films on ITO|PEN substrates were deposited as follows, 7 g of P-25 TiO2 powder and 14 ml of moisture free ethanol were placed in a dried agate ball milling jar with different sizes of agate balls. The mixture was then milled continuously at 250 rpm speed for 10h. Ball milled slurry was coated on ITO|PEN substrates by spin-coating (spin coater — Laurell WS-400B-6NPP). TiO2 coated ITO|PEN substrates were dried at room temperature for several minutes and then heated at 150 °C for 30 min, on a hotplate, in the atmospheric conditions. 2.4. Preparation, electochemical and swelling measurements of PEDOT: PTS films The oxidant FeIIIPTS (40% solution in butanol) was coated on substrates (glass, PEN film, and Ti foil) by spin-coating and the thicker ones were then made by spraying multiple layers of the oxidant solution onto the substrate using a conventional air-brush. Three different thicknesses of films were deposited under different selected conditions. The oxidant coated substrates were exposed to PEDOT monomer vapour at 70 °C for 1 h, followed by drying. The samples were washed in ethanol for 2 h for removing excess FeII and PTS from the PEDOT films and then dried at room temperature. The thickness of PEDOT:PTS films were measured by a profilomertry (Dektak 500). The steady state measurements of conversion current on different electrodes were made in a − 10 ml sealed three-electrode cell, in the presence of I− standard 3 |I electrolyte for DSSCs). A platinum sheet was used for counter as well as the reference electrode. A constant distance between the electrodes was secured by mounting the electrodes in the lid of the cell. A conventional magnetic stirrer was used to maintain stirring at 450 rpm during the experiments. After 5 min at a given potential the correlating conversion current was measured. The swelling measurements of PEDOT:PTS films were made by Quarts Crystal Microbalance (QCM200 from Stanford Research Systems). PEDOT:PTS was coated on the QCM crystal and then exposed to acetonitrile using a micro-flow setup. The frequency changes were monitored as a direct out-put from the instrument and the percentage of swelling was calculated from these values.
the DSSCs from mono-chromatic and white light (100 mW/cm2), respectively. The interfacial properties of the cells were studied by applying negative 0.7 V on working electrode under a two electrode configuration, at the dark, in the same electrolyte, by using a multichannelled potentiostat (Princeton Applied Research) coupled with a computer. The charge transfer resistance at the electrolyte–electrode interface was evaluated from EClab software. 3. Results and discussion Standard working electrodes consisted of a nano-porous TiO2 layer with thickness of 5 µm on the top of a 300 nm dense TiO2 blocking layer deposited on FTO glass substrates. The TiO2 blocking layer beneath the nanocrystalline TiO2 film exhibits a highly compact array of grains that prevents direct contact of electrolyte with back contact of the cell. The nanocrystalline TiO2 layer shows a high degree of porosity, resulting in maximum dye absorption on the working electrode, essential for higher conversion efficiency of the cell. The top surface of the nano-porous TiO2 film was covered with a light scattering TiO2 layer made of 400 nm particles to achieve the maximum performance for the cell, as reported in a previous work [8]. The outer most TiO2 scattering layer increases the optical length of incident light in the film, thus improving the light harvesting. The N719 dye was coated on TiO2 electrodes to bring their absorption to the visible region. PEDOT:PTS films with the maximum thickness of 20 µm were obtained by vapour phase polymerization. Resistance of such film was 4Ω/□ and resistance increased with decreasing the film thickness. A thinner PEDOT:PTS film (0.5 µm) exhibited resistance of 60Ω/□, at room temperature. As is proposed the counter electrode must possess high − conductivity and pronounced catalytic properties to reduce I− 3 ions to I ions for, optimum performance of the cell. Catalytic capability of PEDOT: PTS has been shown earlier, [3] but further understanding of the electrocatalytic mechanism has recently been identified [9]. We have studied variation of Faradic current for the conversion of I−/I− 3 under steady state condition in stirred electrolyte used for solar cells. Variation of Faradic current with applied potential for (a) PEDOT:PTS|Ti and (b) Pt|FTO electrodes is shown in Fig. 1. It is notable that there is apparently no over potential for both electrodes [seen by the straight line through (0,0)] and that PEDOT:PTS and Pt have same equilibrium potential (both curves are going through zero V vs. Pt). The I− 3 reduction current on PEDOT:PTS is
2.5. Preparation of solar cells TiO2 electrodes were immersed overnight in ethanolic solutions of N719 dye at room temperature. Dye coated electrodes were then washed in ethanol and dried in a nitrogen steam. Photovoltaic cells were completed by inserting electrolyte (a mixture of iodine—0.03 M, 4tertbutylpyridine—0.5 M, 1-butyl-3-methyl imidazolium iodide—0.6 M, and guanidinium thiocyanate—0.1 M in acetonitrile:valeronitrile—85:15 by vol.%) to the well attached dye coated TiO2 working electrode and PEDOT:PTS counter electrode. A spacer with the thickness of 64 µm was used to separate the electrodes. Incident photon to current conversion efficiency (IPCE) spectra and current–voltage (IV) characteristics were ascertained for each thickness group of the PEDOT:PTS film by exciting
− Fig. 1. Steady state measurement of the I− 3 /I conversion current at different potentials vs. Pt reference electrode for (a) PEDOT:PTS|Ti-foil and (b) Pt|FTO in standard DSSC electrolyte under continuous stirring.
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seen to reach a diffusion limit at about 9.5 mA/cm2 below approximately −0.2 V vs. Pt under the given conditions (concentration, electrode distance and stirring speed). The slope of the curves at zero current reflects 1/resistance (1/R) for the entire cell, where R is the total of resistance of electrode (Relectrode), reaction (Riodide reaction), electrolyte (Relectrolyte) and the resistance of other parts of the cell (Rcell). The resistance of Pt electrode (RPt) and PEDOT:PTS electrode (RPEDOT:PTS) can be obtained from the gradient at zero current by inter-changing Pt and PEDOT electrodes and keeping other conditions as constant. For PEDOT:PTS|Ti electrode PEDOT:PTS
R
PEDOT:PTS
= 20 Ω = ðRelectrode + Riodide reaction Þ
+ Rcell + Relectrolyte :
ð1Þ For Pt|FTO electrode R
Pt
Pt
= 168 Ω = ðRelectrode + Riodide reaction Þ
+ Rcell + Relectrolyte :
ð2Þ
According to Eq. (1), Rcell + Relectrolyte must be smaller than 20 Ω as (Relectrode + Riodide reaction)PEDOT:PTS cannot be negative. Therefore, (Relectrode +Riodide reaction)Pt must be 148Ω or higher. A 1×3cm2 FTO substrate with the surface resistance of 15Ω/□ was used for the Pt|FTO electrode. Thus, (Relectrode)Pt and (Riodide reaction)Pt should be 45Ω and 103Ω (or higher), respectively. On the other hand (Riodide reaction)PEDOT:PTS cannot be higher than 20Ω, which was obtained for the entire cell. This means that the resistance of the iodide reaction on the Pt electrode is about five times (103/20) higher than that on the PEDOT electrode. This result shows the real potential in using PEDOT:PTS as an electro-catalyst in DSSC. The mechanism of the electro-catalytic reduction of I− 3 on PEDOT: PTS is shown in Scheme 1. I− ions inject electrons into the normally oxidized PEDOT:PTS creating an intermediate state of reduced PEDOT: PTS. The reduced PEDOT:PTS is not stable in the presence of I− 3 and − therefore readily re-oxidized by reducing I− 3 to 3I . PEDOT is well-known to be in its oxidized state at potentials where the I− 3 reduction is taking place [7] and measurements of the conductivity of PEDOT during the I− 3 reduction show decreases of conductivity, indicating that PEDOT remains partly oxidized. This observation fits with the proposed reaction scheme, where an average oxidation state between reduced and oxidized PEDOT segments is predicted. We have studied the photo-performance of the cell. IPCE of the cell is a factor determining photovoltaic performance of a solar cell and given by IPCE = 1240 × iph / (λIs), where iph is short circuit photocurrent density in µA/cm− 2, λ is wavelength in nm and Is is the intensity of the incident light in µW/cm− 2 [10]. IPCE spectra of (a) glass|FTO| TiO2|dye|electrolyte|PEDOT:PTS|glass, and (b) PEN|ITO|TiO2|dye|elec-
Scheme 1. The mechanism for the reduction of I− 3 on PEDOT:PTS.
Fig. 2. IPCE spectra of (a) glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS|glass cell, (b) PEN| ITO|TiO2|dye|electrolyte|PEDOT:PTS|PEN cell, (c) glass|FTO|TiO2|dye|electrolyte|Pt| FTO|glass cell and (d) transmittance of ITO|PEN film.
trolyte|PEDOT:PTS|PEN cells are shown in Fig. 2. The maximum IPCE of 55% was observed for glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS| glass cell. The PEN|ITO|TiO2|dye|electrolyte|PEDOT:PTS|PEN cells exhibit maximum IPCE of 18% at wavelength of 525 nm for the same electrolyte. The maximum IPCE of 65% was observed for the glass|FTO| TiO2|dye|electrolyte|Pt|FTO|glass cells under same condition (curve c). A more efficient electron transfer process in titania films sintered at high temperature is one of the reasons for the better IPCE observed in glass based devices compared to that of flexible substrates. Lack of a dense blocking layer would also result in increased charge recombination at the interface between the electrolyte and the ITO|PEN substrate. IPCE spectrum of the plastic based cell has shifted toward the longer wavelengths compared to that of glass based cells at shorter wavelength range. This might be due to the cut-off of incident light by PEN substrates. The transmittance spectrum of the PEN substrate is shown in Fig. 2 (curve d). The variation of current–voltage characteristics of glass|FTO|TiO2| dye|electrolyte|PEDOT:PTS|glass cells for the different thicknesses of PEDOT:PTS electrodes (a) 0. 44 µm, b). 10.2 µm and (c) 18.6 µm are shown in Fig. 3. The maximum photovoltage of 781 mV was observed for the thinnest PEDOT:PTS electrode. This cell shows lowest fill factor (51%) due to relatively high series resistance of the PEDOT:PTS counter electrode. The maximum photovoltage of the cells gradually decreases with increasing the PEDOT:PTS film thickness. By contrast, a gradual increment of fill factor is observed with thicker PEDOT:PTS films, probably due to the decrease of the resistance of the film. However, the photocurrent of the cell initially increases with thickness of PEDOT:PTS films and reaches to a maximum of ~ 10 mA/cm2 for the cell having a (PEDOT:PTS film of 10.2 µm) and then gradually decreases by further increasing the thickness of the polymer film. A similar pattern of variation was observed for the efficiency of the cell as efficiency is given by product of photovoltage and photocurrent. The performances of those cells are summarized in Table 1. The reason for the decrease in the photocurrent higher PEDOT:PTS film thickness and thereby lower electrical resistance of the cathode is quite intriguing, and may be due to the decreasing in the diffusion rate of the iodide|iodine ions with the PEDOT:PTS layer when it swells in the presence of the electrolyte. The swelling behaviour of conducting polymer is a well-known phenomena [11] and swelling in acetonitrile is no exemption. Using a Quartz Crystal
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Fig. 3. Current–voltage characteristics of glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS| glass cells with different thicknesses of polymer counter electrodes (a) 0. 44 µm, (b) 10.2 µm and (c) 18.6 µm. Dark current–voltage characteristics of the cells are also in the figure, with similar codes.
Microbalance the swelling of the PEDOT:PTS film in acetonitrile was determined to be 15% after 1 h and it reached a maximum of 36% after 16 h. This degree of swelling corresponds to 7 μm in the assembled DSSC for a 20 μm PEDOT:PTS counter-electrode film, which may indeed give rise to changes in the overall diffusion co-efficient of iodides. Swelling and aggregation of the PEDOT can improve the conductivity by forming a more conductive network when the film is thin (for example 50 nm in thickness [12], but could reduce the catalytic surface area and cause higher resistance of the electrolyte diffusion when the film is thick). This effect will become more apparent by increasing the thickness of the PEDOT:PTS layer and will limit the overall efficiency of the cell although the electrical resistance of the PEDOT:PTS is decreasing. We have observed physical defects such as partial peeling off and difference in the roughness on PEDOT: PTS when film thickness is greater than 10 µm. This may be another reason for the decrease of efficiency of the cell. To overcome this problem the PEDOT:PTS cathode has to be structured on the micro| nanometre scale allowing access of iodine|iodide to the catalytic sites without limiting the diffusion speed. Further, we have compared the performance of dye sensitized solar cells composed of different types of counter electrodes. Current–voltage characteristics of (a) glass|FTO| TiO2|dye|electrolyte|Pt|FTO|glass, (b) glass|FTO|TiO2|dye| electrolyte| PEDOT:PTS|Ti-foil, (c) glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS|glass
Fig. 4. Current–voltage characteristics for (a) glass|FTO|TiO2|dye|electrolyte|Pt|FTO|glass, (b) glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS|Ti-foil, (c) glass|FTO|TiO2|dye|electrolyte| PEDOT:PTS|glass and (d) PEN|ITO|TiO2|dye|electrolyte|PEDOT:PTS|PEN cells. Dark current–voltage characteristics of the cells are also in the same figure, with a similar code.
cells and (d) PEN|ITO|TiO2|dye|electrolyte|PEDOT:PTS|PEN are shown in Fig. 4 and the characteristics of each cell are also summarized in Table 1. Similar open circuit voltage is observed for devices (a–c) at optimum conditions. The replacement of PEDOT:PTS on plain glass for Pt|FTO as a counter electrode has produced comparable conversion efficiency between the two cells and thus demonstrated the significant potential in the application of PEDOT:PTS polymers for high efficiency, low cost DSSCs. However, higher photocurrent is expected for glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS|Ti cell compared to that of other cells due to fast catalytic properties of PEDOT: PTS on Ti foil (curve a, Fig. 1) and lower resistance of the substrate. We have studied electrical impedance of TiO2|dye|electrolyte cells for better understanding of these devices. Electrical impedance of (a) glass| FTO|TiO2|dye| electrolyte|Pt|FTO|glass and (b) glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS|glass cells are shown in Fig. 5. The decrease in Z1 (the diameter of the first semicircle from the high frequency side; normally recognised as the contribution from the iodine reduction on the cathode) [13] for the cell composed with PEDOT:PTS counter electrode is in line − with our electrochemical measurements of Pt and PEDOT:PTS|Ti in I− 3 /I electrolyte (Fig. 1), where higher conversion currents were measured on PEDOT:PTS|Ti than on Pt at same potential. This confirms the great potential in using PEDOT:PTS as a catalyst for the cathode reaction. It can be seen that the cell resistance (Z2, the diameter of second semicircle of the impedance spectrum from the high frequency side) for the PEDOT:
Table 1 The performances of the different types of dye sensitized cells. Type of the cell
VOC [mV]
ISC [mA/cm− 2]
ff (%)
Glass|FTO|TiO2|dye|electrolyte|Pt|FTO|glass Glass|FTO|TiO2|dye|electrolyte|PEDOT: PTS|Ti-foil Glass|FTO|TiO2|dye|electrolyte|PEDOT: PTSd = 0.44 μm|glass Glass|FTO|TiO2|dye|electrolyte|PEDOT: PTSd = 10.2 μm|glass Glass|FTO|TiO2|dye|electrolyte|PEDOT: PTSd = 18.6 μm|glass PEN|ITO|TiO2|dye|electrolyte|PEDOT: PTSd = 10.2 mm|PEN
746 750
11.36 10.35
65 75
781
6.9
51
758
9.71
71
699
4.9
72
670
6.17
61
Fig. 5. Normalized electrical impedance spectra of (a) glass|FTO|TiO2|dye|electrolyte|Pt| glass and (b) glass|FTO|TiO2|dye|electrolyte|PEDOT:PTS|glass cells.
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PTS device (curve b) is slightly higher than the standard TiO2|dye| electrolyte|Pt|FTO system indicating that optimisation of the PEDOTelectrolyte interface layer is still needed. 4. Conclusion Highly conducting conjugated PEDOT:PTS can be made as allpolymer counter electrodes in DSSCs to perform dual functions of redox catalyst and electrical conductor. The efficiency of these devices is comparable to that of conventional DSSCs with Pt|FTO as counter electrodes. It is also possible to use the PEDOT:PTS polymer to replace Pt| ITO as counter electrodes for plastic based DSSCs. PEDOT:PTS seems to − be a more efficient electro-catalyst for the I− 3 /I conversion than Pt, but the construction of PEDOT:PTS counter electrodes has to be optimised to take full advantage of this phenomena for high efficiency, low cost DSSCs. Acknowledgements The generous gift of nanocrystalline TiO2 paste (18 nm, PST-18NR) and TiO2 suspensions (PASOL HPW-400C and PASOL HPW-18NR) by JGC Catalysts and Chemicals, Japan is highly appreciated by the authors.
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