Transparent Conducting Oxide-Free Dye-Sensitized Solar Cells

Research Note pubs.acs.org/IECR

Transparent Conducting Oxide-Free Dye-Sensitized Solar Cells Based Solely on Flexible Foils Caio Bonilha,† Joaõ E. Benedetti,‡ Ana F. Nogueira,‡ and Agnaldo de Souza Gonçalves*,† †

Tezca Research and Development of Solar Cells, Ltd., 13087-548 Campinas, SP, Brazil Laboratory of Nanotechnology and Solar Energy, Institute of Chemistry, University of Campinas − UNICAMP, P.O. Box 6154, 13083-970, Campinas, SP, Brazil

‡

ABSTRACT: Flexible, lightweight transparent conducting oxide-free dye-sensitized solar cells were assembled by using metal foils for constructing both electrodes. Because of the opacity of metal foils, a different architecture was employed to allow the illumination of the photoelectrode (PE). Thousands of micrometer-sized through holes were laser-drilled into one of the metal foils employed as PE substrate, providing an ionic pathway to the counter-electrode (CE). Larger hole diameters provided higher fill factor (FF) values. Higher performances were achieved by improving porous TiO2 layer deposition and electrolyte injection, along with a TiO2 blocking layer (BL) and metal foil surface treatment.

1. INTRODUCTION Dye-sensitized solar cells (DSCs) have attracted worldwide attention, because of their potential low production cost, power conversion efficiency (>10%),1 and ability to work at low light intensities. Moreover, they are Cd-free and do not use scarce elements (Ga, Se, Te, In). Nowadays, there are various Pt-free catalytic materials for CE,2−4 Ru-free dyes,5,6 and iodide-free electrolytes.7−9 Since relatively inexpensive materials are employed in small amounts for assembling DSCs, the transparent conducting oxide (TCO) glass substrate contribution to the bill of materials becomes relatively high. Earlier calculations estimate that the substrate could account for 24%10 or even up to 64%11 of the total materials costs for DSCs. Therefore, substrates should be carefully chosen in order to have low-cost DSCs. Relatively inexpensive substrates are available to replace indium tin oxide (ITO, In2O3:Sn). Among them, metal foils are interesting, in light of their low cost, high electrical conductivity, and high-temperature processing ability,12 being effective barriers against O2 and water vapor diffusion, and presenting flexibility, proper mechanical stability, and roll-toroll (R2R) compatibility. Ultralow-cost DSCs could be achieved by constructing both PE and CE on inexpensive metal foils, without any TCO glass substrate. Because of the opacity of metal foils, one way to circumvent illumination issues can be an architecture that employs a substrate endowed with thousands of through holes (via holes or vias), which would somewhat resemble the emitter wrap-through (EWT) approach in terms of the number of vias.13 The metallization wrap-through (MWT)14 approach employs a smaller number of vias, but this could provide electrolyte mass transport problems when employing highly viscous iodide-free electrolytes. Vias allow metallization for bus bars to the back of some Si-based solar cells.15 In this work, vias allow an ionic pathway between PE and CE through the electrolyte. By employing thousands of holes in at least one of the metal foils, it is possible to assemble DSCs based on these opaque © 2012 American Chemical Society

substrates. This architecture can provide working devices, as long as requirements such as PE substrate thickness, number, pitch, and diameter of through holes are matched with mechanical properties of the porous TiO2 layer. However, as the number of holes increases to provide better ionic pathways between electrodes, the mechanical properties of PE might be impaired due to smaller substrate area for adhesion of the porous TiO2 layer. Even though this configuration resembles mesh-based DSCs,16−21 it might provide some advantages, such as better mechanical properties of the PE, allowing highly efficient, truly flexible devices. The objective of this work is to improve the performance of DSCs based on metal foils for constructing both PE and CE. The influence of hole diameter on DSC performance was studied, and preliminary optimization is also reported.

2. EXPERIMENTAL SECTION Stainless steel (SS) foils (AISI 301) were used as substrates for both the PE and CE. The proposed device architecture in this work (see inset in Figure 2, presented later in this paper) was composed of 5628, 4888 and 4128 through holes with diameters of 0.1, 0.12, or 0.15 mm, respectively, which were laser-drilled into an area of 4.8 cm2. An SS foil with a thickness of 0.05 mm was used as the PE substrate. In this work, the hole diameter was changed, whereas pitch varied slightly (0.3, 0.32, and 0.35 mm for hole diameters of 0.1, 0.12, and 0.15 mm, respectively). A TiO2 slurry was prepared according to a procedure reported elsewhere.22 TiO2 paste was applied by doctor blading onto perforated SS foils and the resulting layer was sintered at 450 °C for 30 min. Electrodes were sensitized in a 0.25 mmol L−1 ethanolic solution of the complex N719 (cisbis(isothiocyanate)bis(2,2′-bipyridyl-4,4′-dicarboxylate)-Ru(II) bis-tetrabutylammonium, Everlight Chemical Ind. Corp.) for ca. Received: Revised: Accepted: Published: 9700

March 30, 2012 May 31, 2012 July 5, 2012 July 5, 2012 dx.doi.org/10.1021/ie300842v | Ind. Eng. Chem. Res. 2012, 51, 9700−9703

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Research Note

Figure 1. SEM images of a perforated SS foil with through hole diameters of 0.15 mm. (a) Side view of the porous nanostructured TiO2 layer deposited onto the perforated SS foil; (b) morphological details of the porous nanostructured TiO2 layer at the vicinity of the through hole; and (c) presence of cracks in the porous TiO2 layer when film thickness was larger than 6 μm. All SEM images were performed after the porous TiO2 layers had been heat-treated at 450 °C for 30 min.

16 h. The electrolyte was composed of 0.1 mol L−1 NaI, 0.8 mol L−1 tetrabutylammonium iodide, and 0.05 mol L−1 I2 in methoxypropionitrile:acetonitrile (50:50 v/v). CE was a platinized, nonperforated SS foil with a thickness of 0.15 mm. A metal oxide film was spray-coated over the metal foil, which serves as a Pt underlayer, to avoid short-circuiting. This metal oxide film (thickness of ca. 200 nm) was deposited by using an Airbrush A3205 and a diluted titanium isopropoxide in isopropanol solution. The distance between the CE foil and the nozzle was kept at 30 cm. After drying at room temperature, the resulting film was submitted to a heat treatment of 450 °C for 30 min. The CE was then prepared by the polyol method23 by using hexachloroplatinic acid. A 60-μm-thick double-faced adhesive was used as spacer between SS foils. A Li-ion battery separator was employed between the PE and the CE to avoid short-circuiting. A 12-μm-thick M824 microporous trilayer (polypropylene−polyethylene−polypropylene) membrane with a medium porosity was kindly provided by Celgard. A polyethylene terephthalate (PET) transparent foil was used as a window layer, which was attached to the PE by using a 60μm-thick double-faced adhesive. In the optimization studies, a ca. 200-nm-thick TiO2 BL was spray-coated onto the perforated foil by using the same experimental procedure described for the Pt underlayer at the CE. A commercially available gel chemical remover, whose composition according to the supplier contains HF, HNO3, pH 0−1, thickeners and surfactants, was used for the surface treatment of the PE substrate. Only the side of the PE foil that would support the porous TiO2 layer was submitted to this surface treatment. A small amount of the gel chemical remover was deposited over one side of the precleaned PE foil (the other side was protected) and they were kept in contact for ca. 30 min at room temperature. Afterward, the PE foils were cleaned again prior to the porous TiO2 layer deposition. I−V curves of the DSCs were measured under AM 1.5 illumination using a solar simulator (Sciencetech, Inc., Model 550.5 KW, class A) and a Keithley electrometer (Model 2410-C), at the Centro de Tecnologia da Informaçaõ (CTI Campinas, Brazil). A field-emission-gun scanning electron microscopy (FEG-SEM) system (Jeol, Model JSM 6340F) operating at 5 kV and current of 12 μA was used for morphology characterization at the Brazilian National Nanotechnology Laboratory (LNNano), Campinas.

Laser-drilled holes have tilted walls (see Figure 1a). Reproducible, ca. 6-μm-thick TiO2 layers were successfully deposited, without obstructing through holes (Figure 1b), although, since the TiO2 layer is porous, it would not severely impair electrolyte mass transport properties if it covered the holes. However, when trying to achieve thicker TiO2 porous layers similar to that in optimized devices,25 holes were covered and many cracks appeared (Figure 1c). Therefore, only ca. 6μm-thick porous TiO2 layers were used in this study. Thicker TiO2 layers could provide a higher dye-loading and, this way, higher performance. Alternative deposition techniques, such as spray coating, along with an optimized TiO 2 slurry composition, are under testing to obtain thicker, crack-free porous TiO2 layers. I−V curves of DSCs based on flexible foils are shown in Figure 2.

Figure 2. I−V curves of DSCs based on flexible foils for the construction of both electrodes. DSCs were based on 6-μm-thick porous TiO2 layers on perforated SS foils with through hole diameters of 0.1 mm (DSC 0.10), 0.12 mm (DSC 0.12), or 0.15 mm (DSC 0.15) and liquid electrolyte. Measurements were carried out under 100 mW cm−2 (AM1.5) and the geometrical active area was 4.8 cm2. The I−V curves represent the average behavior for at least three devices. The inset schematically shows the DSC architecture used in this work.

Open-circuit voltage (Voc) values were practically the same (ca. 0.62 V), as well as the photocurrent densities (Jsc). The main differences were observed in the fill factor (FF; see Table 1) and thus, energy conversion efficiency (η). As the hole diameter increased from 0.1 mm to 0.15 mm, FF was improved significantly, probably due to better electrolyte mass transport properties (Table 1). Indeed, a higher electrochemical impedance due to the limitation of electrolyte flow through the perforated metal foil has already been observed.26 In the referred work, the size of the second

3. RESULTS AND DISCUSSION The choice of SS foils as the substrate was based on previous studies, which have shown its relatively higher corrosion stability in iodide-based electrolytes compared to other metal foils.24 SEM images of the porous TiO2 layer deposited onto a perforated SS foil are shown in Figure 1. 9701

dx.doi.org/10.1021/ie300842v | Ind. Eng. Chem. Res. 2012, 51, 9700−9703

Industrial & Engineering Chemistry Research

Research Note

Table 1. Electrical Parameters from I−V curves (AM1.5, 100 mW cm−2) of DSCs Based on Flexible Foils for the Construction of Both Electrodesa

device DSC 0.10 DSC 0.12 DSC 0.15 DSC 0.15A DSC 0.15B

hole diameter (μm)

photocurrent density, Jsc (mA cm−2)

opencircuit voltage, Voc (V)

fill factor, FF

energy conversion efficiency, η (%)

100

1.30

0.63

0.35

0.29

120

1.38

0.62

0.37

0.32

150

1.33

0.63

0.51

0.43

150

3.21

0.67

0.40

0.86

150

5.55

0.67

0.36

1.34

Figure 3. I−V curves of optimized DSCs based on flexible foils for the construction of both electrodes. DSCs were based on ca. 6-μm-thick TiO2 layers on perforated SS foils with through hole diameters of 0.15 mm as PE substrate. A thin TiO2 BL was used. Measurements were carried out under 100 mW cm−2 (AM1.5) and the active area was estimated as 4.07 cm2, since the porous TiO2 layer does not cover through holes after deposition optimization. I−V curves represent the average behavior for at least 3 devices. The inset at the upper right corner (a) shows the suitable mechanical stability by means of a digital picture of the porous TiO2 layer when the perforated metal foil is highly bent. The inset at the bottom left corner (b) shows a digital picture of the working device.

a

DSCs were based on 6-μm-thick TiO2 layers on perforated SS foils with through hole diameters of 0.1 (DSC 0.10), 0.12 (DSC 0.12), and 0.15 (DSC 0.15) mm as PE substrate. An iodide/tri-iodide liquid electrolyte and platinized SS foil were used. DSC 0.15A and DSC 0.15B represent DSCs subjected to a preliminary performance optimization.

semicircle observed in the electrochemical impedance spectrum from DSCs based on an analogous architecture increased with longer pitches. This behavior was assigned, among other factors, to the limited diffusion of the charged electrolyte to the dye-sensitized PE brought about by the nonperforated area.26 It is interesting to notice that, in the present work, as the hole diameter increased from 0.1 mm to 0.15 mm, the total number of holes decreased from 5628 to 4128, and the nonperforated area decreased from 4.36 cm2 to 4.07 cm2, respectively. Therefore, as the hole diameter increased in this work, the nonperforated area decreased, somewhat enhancing FF. Despite the slightly smaller Jsc for DSC 0.15 compared to DSC 0.12 (Table 1), the better performance justified the choice of the former for a preliminary optimization. The performance of DSCs was improved by (i) treating the surface of the perforated SS foil with a gel chemical remover prior to porous TiO2 layer deposition, (ii) using a TiO2 BL, and (iii) optimizing porous TiO2 layer deposition and electrolyte injection. I−V curves of DSCs based on perforated SS foils with hole diameters of 0.15 mm as PE substrate are shown in Figure 3. Figure 3 shows that surface treatment of the perforated SS foil with a gel chemical remover and a TiO2 BL (DSC 0.15A) provided a ca. 2-fold increase in performance (Table 1). The gel chemical remover can strip oxide layers present on the surface of the SS foil, significantly improving charge collection at the substrate. The barrier effect of some oxide layers on metal foils has already been reported.27−29 Additional performance improvement was observed by optimizing porous TiO2 layer deposition and electrolyte injection (DSC 0.15B, Figure 3), Table 1. As shown in the inset of Figure 3 (upper right corner), the porous TiO2 layer has suitable mechanical stability even when the perforated metal foil is highly bent. The optimized porous TiO2 layer does not cover the through holes and the DSC active area was estimated to be equal to the nonperforated area (4.07 cm2). This preliminary optimization study provided DSCs (DSC 0.15B, Table 1) with a ca. 3-fold increase in performance compared to the DSC 0.15 (see Figure 2). An analogous DSC architecture employing an SS foil as the PE substrate with hole diameter of ca. 50 μm for the lower surface and ca. 20 μm for the upper surface, pitch of 0.12 mm, a

platinized Ti foil as the CE and a TCO-free glass as the window layer provided an overall conversion efficiency of 1.45% under an illumination of 100 mW cm−2.26 This conversion efficiency is very similar to that achieved herein (DSC 0.15B, Table 1), even though the DSCs in the present work have a considerably larger active area. An overall conversion efficiency of 2.25% under an illumination of 100 mW cm−2 and FF = 0.676 were achieved for DSCs, based on an analogous architecture with a pitch of 60 μm and a photoactive area of 0.2 cm2.26 In this work, FF is still rather poor (ca. 0.4). Experiments to improve FF include a thinner spacer between metal foils, optimization of hole diameter and pitch, thinner PE substrate foil, better electrolyte composition, use of an insulating oxide layer at the back of the perforated foil, more porous membrane separators, among others. The use of protection layers (ITO and SiO2, for example) might also lead to higher performances.27−29 Even using a relatively thin porous TiO2 layer, photocurrent densities higher than 5 mA cm−2 could be successfully achieved in this initial study. The use of less-expensive via drilling techniques, compared to laser micromachining (chemical etching, for example), could make perforated metal foils very competitive, compared to TCO glass.

4. CONCLUSIONS In summary, the use of metal foils to assemble both electrodes of dye-sensitized solar cells (DSCs) was demonstrated, providing lightweight, thin, and truly flexible devices. Even though the performance from such a DSC configuration has not yet been fully optimized, it was observed that the treatment of the perforated stainless steel (SS) foil surface provided a significant improvement in photocurrent. Preliminary optimization in TiO2 layer deposition and electrolyte injection provided further enhancement of performance, even though FF still needs to be improved significantly. The use of iodidefree, noncorrosive electrolytes will open up several choices for relatively inexpensive metal foils, providing further reduction in 9702

dx.doi.org/10.1021/ie300842v | Ind. Eng. Chem. Res. 2012, 51, 9700−9703

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(13) Mingirulli, N.; Stüwe, D.; Specht, J.; Fallisch, A.; Biro, D. Screen-printed emitter-wrap-through solar cell with single step side selective emitter with 18.8% efficiency. Prog. Photovoltaics 2010, 19, 366. (14) Clement, F.; Menkoe, M.; Erath, D.; Kubera, T.; Hoenig, R.; Kwapil, W.; Wolke, W.; Biro, D.; Preu, R. High throughput viametallization technique for multi-crystalline metal wrap through (MWT) silicon solar cells exceeding 16% efficiency. Sol. Energy Mater. Sol. Cells 2010, 94, 51. (15) Fellmeth, T.; Menkoe, M.; Clement, F.; Biro, D.; Preu, R. Highly efficient industrially feasible metal wrap through (MWT) silicon solar cells. Sol. Energy Mater. Sol. Cells 2010, 94, 1996. (16) Yoshida, Y.; Pandey, S. S.; Uzaki, K.; Hayase, S.; Kono, M.; Yamaguchi, Y. Transparent conductive oxide layer-less dye-sensitized solar cells consisting of floating electrode with gradient TiOx blocking layer. Appl. Phys. Lett. 2009, 94, 093301. (17) Wang, Y.; Yang, H.; Lu, L. Three-dimensional double deck meshlike dye-sensitized solar cells. J. Appl. Phys. 2010, 108, 064510. (18) Fan, X.; Wang, F.; Chu, Z.; Chen, L.; Zhang, C.; Zou, D. Conductive mesh based flexible dye-sensitized solar cells. Appl. Phys. Lett. 2007, 90, 073501. (19) Wang, Y.; Yang, H.; Liu, Y.; Wang, H.; Shen, H.; Yan, J.; Xu, H. The use of Ti meshes with self-organized TiO2 nanotubes as photoanodes of all-Ti dye-sensitized solar cells. Prog. Photovoltaics 2010, 18, 285. (20) Liu, Z.; Subramania, V.; Misra, M. Vertically oriented TiO2 nanotube arrays grown on Ti meshes for flexible dye-sensitized solar cells. J. Phys. Chem. C 2009, 113, 14028. (21) Huang, X.; Shen, P.; Zhao, B.; Feng, X.; Jiang, S.; Chen, H.; Li, H.; Tan, S. Stainless steel mesh-based flexible quasi-solid dyesensitized solar cells. Sol. Energy Mater. Sol. Cells 2010, 94, 1005. (22) Avellaneda, C. O.; Gonçalves, A. D.; Benedetti, J. E.; Nogueira, A. F. Preparation and characterization of core-shell electrodes for application in gel electrolyte-based dye-sensitized solar cells. Electrochim. Acta 2010, 55, 1468. (23) Sun, K.; Fan, B.; Ouyang, J. Nanostructured platinum films deposited by polyol reduction of a platinum precursor and their application as counter electrode of dye-sensitized solar cells. J. Phys. Chem. C 2010, 114, 4237. (24) Toivola, M.; Ahlskog, F.; Lund, P. Industrial sheet metals for nanocrystalline dye-sensitized solar cell structures. Sol. Energy Mater. Sol. Cells 2006, 90, 2881. (25) Ito, S.; Chen, P.; Comte, P.; Nazeeruddin, M. K.; Liska, P.; Péchy, P.; Grätzel, M. Fabrication of screen-printing pastes from TiO2 powders for dye-sensitised solar cells. Prog. Photovoltaics 2007, 15, 603. (26) Yun, H.-G.; Kim, M.; Kang, M. G.; Lee, I.-H. Cost-effective dyesensitized solar cells consisting of two metal foils instead of transparent conductive oxide glass. Phys. Chem. Chem. Phys. 2012, 14, 6448. (27) Kang, M. G.; Park, N.-G.; Ryu, K. S.; Chang, S. H.; Kim, K.-J. Flexible metallic substrates for TiO2 film of dye-sensitized solar cells. Chem. Lett. 2005, 34, 804. (28) Kang, M. G.; Park, N.-G.; Ryu, K. S.; Chang, S. H.; Kim, K.-J. A 4.2% efficient flexible dye-sensitized TiO2 solar cells using stainless steel substrate. Sol. Energy Mater. Sol. Cells 2006, 90, 574. (29) Jun, Y.; Kang, M. G. The characterization of nanocrystalline dyesensitized solar cells with flexible metal substrates by electrochemical impedance spectroscopy. J. Electrochem. Soc. 2007, 154, B68.

production costs. The use of metal foils for constructing both electrodes is interesting in order to take advantage of the economy of scale provided by the roll-to-roll (R2R) production process.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +55 19 3307 7047. E-mail: agnaldo.goncalves@ tezca.com.br. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank FAPESP for financial support (No. 09/ 53726-3). A.S.G. (No. 10/1682-3) and J.E.B. (11/080304-6) thank FAPESP for scholarships. Prof. Carol H. Collins is gratefully acknowledged for English revision. Two of the authors (C.B., A.S.G.) thank the company Celgard for providing Li-ion battery separator samples and CTI for technical support. J.E.B. and A.F.N. thank LNNano for technical support.

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REFERENCES

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dx.doi.org/10.1021/ie300842v | Ind. Eng. Chem. Res. 2012, 51, 9700−9703