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Zinc oxide-based dye-sensitized solar cells with a ruthenium dye containing an alkyl bithiophene group
Accepted Manuscript Zinc oxide-based dye-sensitized solar cells with a ruthenium dye containing an alkyl bithiophene group Chuan-Pei Lee, Chen-Yu Chou, Chia-Yuan Chen, Min-Hsin Yeh, Lu-Yin Lin, R. Vittal, Chun-Guey Wu, Kuo-Chuan Ho PII:
S0378-7753(13)00891-4
DOI:
10.1016/j.jpowsour.2013.05.101
Reference:
POWER 17429
To appear in:
Journal of Power Sources
Received Date: 15 December 2012 Revised Date:
13 May 2013
Accepted Date: 14 May 2013
Please cite this article as: C.-P. Lee, C.-Y. Chou, C.-Y. Chen, M.-H. Yeh, L.-Y. Lin, R. Vittal, C.-G. Wu, K.-C. Ho, Zinc oxide-based dye-sensitized solar cells with a ruthenium dye containing an alkyl bithiophene group, Journal of Power Sources (2013), doi: 10.1016/j.jpowsour.2013.05.101. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Highlights (for review)
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Highlights Zinc oxide-based dye-sensitized solar cells with a ruthenium dye containing an alkyl bithiophene group
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Chuan-Pei Leea, Chen-Yu Choua, Chia-Yuan Chenb, Min-Hsin Yeha, Lu-Yin Lina,
a
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R. Vittala, Chun-Guey Wub,﹢, and Kuo-Chuan Hoa,c,*
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan b
c
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Department of Chemistry, National Central University, Jhong-Li 32001, Taiwan.
Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
1. CYC-B1 dye is used in a zinc oxide (ZnO)-based dye-sensitized solar cell (DSSC).
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2. The CYC-B1/ZnO film is almost free from Zn2+/dye-agglomerations. 3. Efficiency of a CYC-B1/ZnO DSSC is 94% higher than that of an N3 dye/ZnO cell.
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4. PMMA spheres are synthesized to template the ZnO film as the scattering layer.
Graphical Absrtact (for review)
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Graphical abstract
alkyl bithiophene group
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Zinc oxide-based dye-sensitized solar cells with a ruthenium dye containing an
Chuan-Pei Leea, Chen-Yu Choua, Chia-Yuan Chenb, Min-Hsin Yeha, Lu-Yin Lina,
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R. Vittala, Chun-Guey Wub,﹢, and Kuo-Chuan Hoa,c,* a
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan b
Department of Chemistry, National Central University, Jhong-Li 32001, Taiwan.
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Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
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c
1
*Manuscript text (double-spaced) Click here to view linked References
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Zinc oxide-based dye-sensitized solar cells with a ruthenium dye containing an
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alkyl bithiophene group
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Chuan-Pei Leea, Chen-Yu Choua, Chia-Yuan Chenb, Min-Hsin Yeha, Lu-Yin Lina,
a
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R. Vittala, Chun-Guey Wub,﹢, and Kuo-Chuan Hoa,c,*
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
Department of Chemistry, National Central University, Jhong-Li 32001, Taiwan. c
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b
Institute of Polymer Science and Engineering, National Taiwan University,
Corresponding author: Tel: +886-2-2366-0739; Fax: +886-2-2362-3040
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*
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Taipei 10617, Taiwan
﹢Corresponding
E-mail: [email protected]
author: Tel: +886-3-422-7151 x 65903; Fax: +886-3-422-7664 E-mail: [email protected]
Revised manuscript (POWER-D-12-03954R1) prepared for the consideration for publication in Journal of Power Sources.
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Zinc oxide-based dye-sensitized solar cells with a
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ruthenium dye containing an alkyl bithiophene group Chuan-Pei Leea, Chen-Yu Choua, Chia-Yuan Chenb, Min-Hsin Yeha, Lu-Yin Lina,
a
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R. Vittala, Chun-Guey Wub,﹢, and Kuo-Chuan Hoa,c,*
Department of Chemical Engineering, National Taiwan University,
b
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Taipei 10617, Taiwan
Department of Chemistry, National Central University, Jhong-Li 32001, Taiwan. c
Institute of Polymer Science and Engineering, National Taiwan University,
Abstract
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Taipei 10617, Taiwan
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An improved light-harvesting ruthenium dye with an alkyl bithiophene group, designated as CYC-B1, is employed as the photosensitizer for a zinc oxide (ZnO)-based
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dye-sensitized solar cell (DSSC). ZnO films with N3 and CYC-B1 dyes exhibit different surface appearances. Interestingly, the ZnO nanocrystal electrode surface with CYC-B1 dye is found to be almost free from Zn2+/dye-agglomerations, usually observed in the case of other commercial ruthenium dyes for such an electrode surface, e.g., N3 dye. Owing to this favorable effect of the CYC-B1 dye, the DSSC with a ZnO film (designated as ZN20) sensitized with this dye exhibited an average efficiency (η) of 1
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analyzing the electrochemical impedance spectra (EIS) and laser-induced photo-current transients. Further, polymethylmethacrylate (PMMA) spheres with uniform sizes of ca.
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300 nm are synthesized and incorporated to template the ZN20 film (designated as PMMA-ZN20); this PMMA-ZN20 film is used as the scattering layer and it is coated on
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the underlayer ZN20 film to make the photoanode film (ZN20/PMMA-ZN20) of a DSSC. The thus fabricated DSSC shows an η of 5.40±0.03%.
Dye-sensitized
solar
cells;
Electrochemical
impedance
spectra;
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Keywords:
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Laser-induced photo-current transients; Polymethylmethacrylate; Zinc oxide
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1. Introduction
Zinc oxide (ZnO) has been investigated as a promising alternative photoanode
material for dye-sensitized solar cells (DSSCs). Since the inception of research on TiO2-based DSSCs, ZnO has been examined, since TiO2 and ZnO have similar electron affinities and almost the same band gap energies, i.e., ~3.2 eV and ~3.3 eV, respectively. ZnO, however, has much higher electron diffusivity than TiO2 [1, 2]. The
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diverse morphologies of ZnO are probably richer than those of any known materials [3-11], which would be expected to result in a range of ZnO photoanode designs
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[12-24]. ZnO has a large excitation binding energy (60 eV), is inexpensive, and is stable against photocorrosion. However, the power conversion efficiency (η) of ZnO-based
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DSSCs is still lower than that of TiO2-based DSSCs, which is most likely due to the instability of the ZnO film in an acidic dye solution. Strategies to improve the efficiency
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of ZnO-based DSSCs also include designing of new dyes that are suitable for the ZnO photoanode. Therefore, the ZnO-based DSSCs with novel dye molecules have been continuously investigated.
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The use of organic dyes for ZnO-based DSSCs in some researches has been explored. Otsuka et al. [25] have developed an efficient, cyan-colored, and novel
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squaraine-dye, namely Sq3, for a ZnO-based DSSC, which produced η=1.5%. Guillén
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et al. [26] reported a comparative study on the photovoltaic performances of ZnO-based DSSCs with xanthene dyes (such as Eosin Y, Eosin B, and Mercurochrome) and with the frequently-used ruthenium-ligand N719 dye; an η of 1.2% was achieved for a ZnO-based DSSC by using Eosin Y. This study shows that, at least for ZnO-based DSSCs, xanthene dyes can be considered as suitable candidates to replace common ruthenium photosensitizers (such as N3, N719, and so-called "black dye"). Wang et al.
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have been developed for ZnO-based DSSCs. Their molar extinction coefficients are significantly higher than those of the common ruthenium photosensitizers [28].
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Specifically, the indoline derivative dye, D149, with an absorption coefficient five times larger than that of N719 at the maximum of the solar spectrum, was reported to have
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shown a remarkable efficiency for a DSSC with mesoporous TiO2 [29]. Wu et al. [23] reported an efficiency of 3.74% for a ZnO-based DSSC with D149. Yoshida and co-workers have reported efficiencies of above 5% for ZnO-based DSSCs [28]. Cheng
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and Hsieh obtained an η of 4.95% for a cell with a photoanode of ZnO nanoparticles, sensitized with D149 dye [30]. Higashijima et al. [31] have recently reported that two
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novel indoline dyes, DN319 and DN350, have been developed for ZnO-based DSSCs,
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which possess cell efficiencies of 5.01% and 5.55%, respectively.
On the other hand, DSSCs based on mesoporous ZnO sensitized with common
ruthenium photosensitizers have usually reached efficiencies of 0.5–5.4% under 1 sun illumination, depending on preparation conditions and the electrolyte composition [32, 33]. The low conversion efficiency of a ZnO-based DSSC with a ruthenium photosensitizer is most likely due to the dissolution of the ZnO to Zn2+ by the adsorbed
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acidic dye, followed by the formation of agglomerates (giving an insulating layer of Zn2+ and ruthenium photosensitizer molecules), and eventually followed by the
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blocking of the injected electrons from the dye molecules to the semiconductor by the insulating layer [34-36]. The agglomerates are thus detrimental to the performance of a
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ZnO-based DSSC, because they fill up nanopores of the ZnO electrode and reduce the electron injection efficiency of the dye adsorbed to the ZnO layer. In order to prepare
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efficient ZnO-based DSSCs with ruthenium photosensitizers, Nguyen et al. [37] have found that a ruthenium photosensitizer molecule with a hydrophobic side chain could prevent agglomerate formation by hindering coordination of the dye with Zn2+; they applied
a
ruthenium-based
dye,
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have
namely
Ru(H2dcbpy)(4-(4-(N,N-di(p-
hexyloxyphenyl)amino)styryl)-4'-methyl-2,2'-bipyridine)(NCS)2
(HMP-2,
extinction
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coefficient, ε, 3.33×104 M–1cm–1 at 429 nm; 2.00×104 M–1cm–1 at 534 nm), to a ZnO-based DSSC; the cell achieved a higher η of 4.03% as compared to that with N3
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dye (0.11%). Besides, with this dye, the synergic impact on ZnO nanocrystals was practically negligible even after 24 h of immersion time, where the routinely preferred N3 dye practically failed to a great extent [37, 38]. Since there are limited reports on the exploration of ruthenium-based dyes for ZnO-based DSSCs, exploitative studies are
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thus necessary to understand the influence of ruthenium-based dyes on the performance of a ZnO-based DSSC.
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An efficient ruthenium photosensitizer with an alkyl bithiophene group, designated as CYC-B1, was reported previously [39,40]; to prepare this dye, one of the
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dcbpy ligands (4,4'-dicarboxylic acid-2,2'-bipyridine) in N3 was replaced with abtpy(4,4'-di-2-octyl-5-(thiophen-2-yl)thiophen-2-yl-2,2-bipyridine), a bipyridine ligand
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substituted with alkyl bithiophene groups (Fig. 1). The CYC-B1 dye has a superior extinction coefficient (4.64×104 M–1cm–1 at 400 nm; 2.12×104 M–1cm–1 at 553 nm) among ruthenium-based dyes (N3: 1.52×104 M–1cm–1 at 385 nm; 1.45×104 M–1cm–1 at
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530 nm); the TiO2-based DSSC with this CYC-B1 dye thus possesses good efficiency.
In this study, we used the CYC-B1 dye in all-ZnO-based DSSCs; an average
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efficiency of 4.85±0.03% was achieved for such a cell by adjusting the dye-exposure
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time. The effects of the CYC-B1 dye on the surfaces of ZnO particles were compared to those of N3 dye on the surfaces of the same ZnO particles. Moreover, spheres of poly(methyl methacrylate), PMMA with uniform sizes of ca. 300 nm were synthesized and incorporated to template a ZnO (ZN20) film; this PMMA incorporated ZnO film was used as a scattering layer, and coated on the underlayer ZN20 film to prepare the photoanode film of a DSSC (this composite film is designated as ZN20/PMMA-ZN20);
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understand the kinetics of the photoelectrochemical processes for the ZnO-based DSSC with CYC-B1 dye.
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2. Experimental Section 2.1 Materials
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Lithium iodide (LiI, synthetical grade) and iodine (I2, synthetical grade) were obtained from Merck (Darmstadt, Germany); 4-tert-butylpyridine (TBP, 96%) and tert-butyl alcohol (tBA, 96%) were obtained from Acros Organics (Geel, Belgium);
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acetonitrile (ACN, 99.99%), dimethyl sulfoxide (DMSO, ≧99.6%), ethanol (EtOH, 99.5%), methyl methacrylate (MMA, 99%), methacrylic acid (MAA, 99%), and
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Mo).
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isopropyl alcohol (IPA, 99.5%) were obtained from Aldrich (Sigma-Aldrich, St. Louis,
2.2 Synthesis of PMMA spheres A pH = 3.35 buffer solution (320 g), consisting of 0.05 M glycine (C2H5NO2, >
99%, Riedel-de Haën, Honeywell International Inc., Seelze, Germany) and 0.0168 M HCl was purged with N2 for 30 min and the temperature of the solution was raised to 80 o
C. 2,2-azobis(2-amino-propane) dihydrochloride (1.60 g, AAPH > 95%, Wako Pure
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Chemical Industries, Ltd., Osaka, Japan) was added into the buffer solution as the initializer for the polymerization of methyl methacrylate (MMA). Methyl methacrylate
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(35 g, MMA) and methacrylic acid (0.35 g, MAA) were gently injected into the mixing solution at the same rate, keeping the whole solution stirred. The reaction was allowed
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to take place at 80 oC for 2 h to finish the polymerization. The solution was then cooled to room temperature, washed with DI water, and centrifuged to obtain spherical PMMA
2.3 Preparation of ZnO pastes
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powder.
Commercial ZnO powder (4.23 g, Degussa VP AdNano®ZnO20, Evonik Degussa
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Taiwan Ltd., Taipei, Taiwan) with an approximate nanoparticle size of 20 nm was thoroughly mixed with 20 mL solution of EtOH and DI-water (v/v=70/30). This
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colloidal solution was stirred for 3 days to obtain a well-dispersed suspension of 20
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wt% ZnO; this ZnO paste was used for preparing the ZnO film (designated as ZN20). In addition, we were able to prepare a PMMA-templated ZnO film (designated as PMMA-ZN20) by incorporating 30 wt% of PMMA spheres into the ZN20 paste.
2.4 Assembly of DSSCs A fluorine-doped SnO2 conducting glass (FTO, 7 Ω sq. –1, visible transmittance ≧ 80%, NSG, Tokyo, Japan) was first cleaned with a neutral cleaner, and then washed
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technique. The above synthesized PMMA spheres were incorporated into the ZN20 film; this film (designated as PMMA-ZN20) was used as an overlayer on the underlayer ZN20 film to make the composite film (ZN20/PMMA-ZN20) for a DSSC. A 0.4×0.4
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cm2 square was selected on the coated FTO glass as the active area by removing the side
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portions by scraping. The ZN20 or ZN20/PMMA-ZN20 film was gradually heated to 450 oC in an oxygen atmosphere, and subsequently sintered at that temperature for 30 min. After sintering at 450 oC and cooling to 80 oC, the film was immersed in a 1×10–4
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M solution of CYC-B1 [39] in ACN, tBA, and DMSO (volume ratio of 1:1:1), at room temperature for 3, 6, 12, 24 and 48 h. The film was also immersed in a 1×10–4 M
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solution of N3 (Solaronix S.A., Aubonne, Switzerland) in EtOH, at room temperature for 24 h. The thus dye-coated electrode was placed on a platinum-sputtered conducting –1
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glass electrode (indium tin oxide,
, visible transmittance ≧ 85%, Ritek
Corporation, Hsinchu, Taiwan), keeping the two electrodes separated by a 25 μm-thick Surlyn® layer (SX1170-25, Solaronix S.A., Aubonne, Switzerland). The two electrodes were then sealed by heating. A mixture of 0.1 M LiI, 0.6 M DMPII (Solaronix S.A., Aubonne, Switzerland), 0.05 M I2, and 0.5 M TBP in 3-methoxypropionitrile (MPN,
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Fluka)/ACN (volume ratio of 1:1) was used as the electrolyte. The electrolyte was injected into the gap between the two electrodes by capillarity; the electrolyte-injecting
sealed with hot-melt glue after the injection of the electrolyte.
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2.5 Instrumentation and measurements
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hole was previously made in the counter electrode with a drilling machine; the hole was
Surface of the DSSC was illuminated by a class A quality solar simulator
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(XES-3003S-100, AM1.5G, San-Ei Electric Co., Ltd., Osaka, Japan) and the incident light intensity (100 mW cm–2) was calibrated with a standard Si cell (PECSI01, Peccell Technologies, Inc., Yokohama, Japan). Photoelectrochemical characteristics of the
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DSSC were recorded with a potentiostat/galvanostat (PGSTAT 30, Autolab, Eco-Chemie, Utrecht, the Netherlands). All the photovoltaic parameters are the average
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values obtained with three separate devices for each condition.
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Film thickness was determined using a surface profilometer (Sloan Dektak 3030, CAE, Redwood City, CA). Morphologies of the ZnO films were observed using a scanning electron microscopy (SEM, Nova NanoSEM 230, FEI Ultra-High Resolution FE-SEM with low vacuum mode, FEI, Hillsboro, OR). The thermal property of the PMMA spheres was analyzed by a thermogravimetric analyzer (TGA, TGA-7, PerkinElmer, Waltham, MA).
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Electrochemical impedance spectra (EIS) were obtained by the above-mentioned potentiostat/galvanostat equipped with an FRA2 module, under a constant light
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illumination of 100 mW cm–2. The frequency range explored was 10 mHz to 65 kHz. Applied bias voltage was set at the open-circuit voltage of the DSSC between the
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ITO-Pt counter electrode and the FTO-ZnO-dye working electrode, starting from the short-circuit condition; the corresponding AC amplitude was 10 mV. The impedance
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spectra were analyzed using an equivalent circuit model [41, 42]. Photocurrent transients of the assembled devices were recorded with a digital oscilloscope (model LT322, Teledyne LeCroy, Chestnut Ridge, NY). The transients were measured under a
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continuous bias illumination of white light and weak pulsed laser. Pulsed laser excitation was applied by a frequency-doubled Q-switched Nd:YAG laser (model
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Quanta-Ray GCR-3-10, Spectra-Physics laser, Newport Corporation, Irvine, CA) with a 2 Hz repetition rate at 532 nm, and a 7 ns pulse width at half-height. The diffusion
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coefficient of electrons (De) was estimated by fitting a decay of the transient photocurrent with exp (–t τc–1), which was derived from the equation of continuity for electrons in the conduction band [43], where t and τc are the time and average time constant, respectively. The apparent diffusion coefficient of electrons may be calculated by using the following equation,
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(1)
where w is the film thickness and the factor 2.35 arises from the geometry of the
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diffusion problem. Incident photon-to-current conversion efficiency (IPCE) curves were obtained at short-circuit condition. The light source was a quality solar simulator
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(PEC-L11, AM1.5G, Peccell Technologies, Inc., Yokohama, Japan); light was focused through a monochromator (Oriel Instrument, model 74100, Newport, Irvine, CA) onto
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the photovoltaic cell. The monochromator was incremented through the visible spectrum to generate the IPCE (λ) as defined below,
IPCE (λ) = 1240(JSC/λφ)
(2)
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where λ is the wavelength, JSC is short-circuit photocurrent density (mA cm–2) recorded with a potentiostat/galvanostat, and φ is the incident radiative flux (W m–2) measured
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with an optical detector (Oriel Instrument, model 71580, Newport) and power meter
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(Oriel Instrument, model 70310, Newport).
3. Results and Discussion 3.1 Characterization of the ZnO films after dipping in N3 and CYC-B1 dye solutions Although ZnO has been investigated as a promising alternative photoanode material for DSSCs, the conversion efficiency of a ZnO-based DSSC with a common
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ruthenium photosensitizer is still low, attributable to the formation of agglomerates of Zn2+ ions and ruthenium photosensitizer molecules [34-36]. Horiuchi et al. [35]
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found that the formation of aggregates consisting of N 3 dye molecules and Zn 2+ ions occurs in the ZnO film for immersion times of the film greater than 10 min. The
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aggregates exist in the pores of the ZnO film and on the surface of the film as micrometer-sized particles. These aggregates have the potential to change the quality
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of the film and its surface structure. Fig. 2a and 2b show SEM images of the surface of the bare ZnO film at different magnifications; the porous film in 1a and 1b shows ZnO nanoparticles of sizes between 20~100 nm; Fig. 2c and 2d show SEM images of
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the same ZnO film at different magnifications after its dipping in N3 dye for 24 h. Careful observation clearly indicates the change in the surface of the film due to its
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dipping in the N3 dye. The surface of ZnO nanoparticles in the dye is smooth and there are fewer islands in the film before it was immersed in the dye. Several small
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crystallites form some kind of diffusion barrier layer in the film after immersion; this type of change in the morphology of the ZnO film is assumed to have arisen due to the formation of aggregations of N3 dye molecules with Zn 2+ ions on the surface of the film, caused by the immersion of the ZnO film in N3 dye solution for a prolonged time [37-38]. In a similar way, the CYC-B1 dye with a high extinction
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the CYC-B1 dye; the film was dipped in CYC-B1 dye solution for 24 and 48 h. Fig. 3a and 3b show SEM images of the ZnO film at different magnifications, after its
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dipping in CYC-B1 dye solution for 24 h; Fig. 3c and 3d show SEM images of the ZnO film at different magnifications, after immersion in CYC-B1 dye solution for 48
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h. The surfaces of the ZnO nanoparticles, after immersion in CYC-B1 dye solution are physically stable, and the islands in the film before and after the immersion are undisturbed (compare Fig. 2b with Fig. 3b). In other words, the surface modification
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of the ZnO nanocrystals in CYC-B1 dye is insignificant, even after the dipping of the ZnO film for 24 or 48 h in the dye solution. The presence of a hydrophobic
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side-chain is assumed to have prevented the coordination between Zn 2+ ions and dye molecules to form the respective agglomerates [37]. Encouraged by this finding, we
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have employed the CYC-B1 dye to sensitize the ZnO film for a DSSC.
3.2 Photovoltaic characteristics of the DSSCs sensitized with N3 and CYC-B1 dyes It has been reported that the immersion time of a ZnO film in a sensitizing dye is a key factor for the functioning of the ZnO-based DSSC [26]. Fig. 4 shows the
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photocurrent density-voltage curves of the DSSCs sensitized with CYC-B1 dye for different periods of time; Table 1 summarizes the corresponding photovoltaic
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parameters. All the photovoltaic parameters are the average values obtained with three separate devices for each condition. Table 1 shows that the highest power
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conversion efficiency (η) is achieved at an immersion time of 24 h; at 48 h of immersion, the power conversion efficiency shows a decrease. The ZnO-based
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DSSC with the optimal immersion time of 24 h for dye CYC-B1 shows an η value of 4.85±0.03%, the open-circuit voltage (VOC), the short-circuit current density (JSC), and the fill factor (FF) being 562±4 mV, 13.75±0.05 mA cm–2, and 0.63±0.00,
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respectively. Based on these results, the immersion time was fixed to be 24 h for the CYC-B1 dye. Fig. 5a shows the current density-voltage curves of the DSSCs,
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obtained both at 100 mW cm –2 light intensity and in the dark, in which the photoanodes of the cells were sensitized with CYC-B1 dye and N3 dye for 24 h. The
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corresponding photovoltaic parameters under illumination are listed in Table 2. The DSSC with N3 dye shows a lower η value of 2.50±0.12%, as compared to that of the cell with CYC-B1 dye (4.85±0.03%). The low performance of the DSSC with N3 dye can be explained as follows. The chemical instability of ZnO nanocrystals, as seen in the SEM images of Fig. 2c and 2d, due to the formtion of agglomerates of
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reduces the JSC of the affected cell. The formed Zn2+-dye complex can form a barrier layer on the surface of the ZnO particels, thereby preventing the injected electrons to reach the ZnO conduction band. This favors recombination of charges between the
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ZnO particles and the I–/I3– electrolyte, thereby decreasing the Voc of the relevant
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DSSC. The formation of agglomerates and the associated barrier layer increases the internal resistance in the ZnO matrix, thus reducing the cell FF. Lower values of JSC, VOC, and FF result in a lower cell power conversion [35,44-46].
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The higher VOC value of the CYC-B1 cell was thought to be due to the lower dark currents, with reference to those in the case of the cell with N3; this assumption
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was verified by measuring the I-V characteristics of the cells under dark conditions. It is well known that the dark currents is resulted from the recombination of charges
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at the interface between ZnO particles and I–/I3–. Fig. 5a also demonstrates the current density-voltage characteristics of the DSSCs with CYC-B1 and N3 dyes, measured in the dark. The onsets of the dark currents of the cells with CYC-B1 and with N3 dyes occur at 550, and 535 mV, respectively. These values were obtained by drawing tangents to the curves starting from the voltage scale and a tangent at the
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zero current of the curves (zero-current line) and measuring the voltages corresponding to the intersection points of the tangents at the zero current line. The
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onset of the dark current of the cell with CYC-B1 occurs at the highest forward bias, which confirms that the cell with CYC-B1 has less recombination of charges at the
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interface between ZnO particles and I–/I3–. FF values for DSSCs are usually limited by series resistance losses, light-intensity dependent recombination, and dark
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currents. Apparently reduction in dark currents, as discussed above, should have caused a higher FF value for the cell with CYC-B1 than that for the cell with N3
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(Table 2).
3.3 EIS measurements and photocurrent transients of the DSSCs sensitized with N3
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and CYC-B1 dyes
In order to show further evidence to substantiate the higher FF in the case of
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the cell with CYC-B1, electrochemical impedance spectra (EIS) were obtained at 1 sun illumination. Fig. 5b shows Nyquist plots of EIS of the DSSCs with CYC-B1 and N3 dyes. The equivalent circuit is shown as the inset in Fig. 5b. The Nyquist plots essentially consist of two sets of semicircles. The smaller semicircle at higher frequencies corresponds to the charge-transfer resistances at the counter
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electrode/electrolyte interface (Rct1) [41,47]. The larger circle at lower frequencies (10~100 Hz range) represents the resistances of the charge-transfer process at the
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ZnO/dye/electrolyte interface (Rct2), and that of ion diffusion within the electrolyte (Rdiff) [41,47,48]. Rdiff is virtually overlapped by Rct2 due to the short length for I–
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diffusion available with the thin spacer used (25 μm thick), and owing to the low viscosity of the solvents used in our electrolyte (viscosities of ACN and MPN are
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0.37 cp and 1.60 cp, respectively). It can be seen in Fig. 5b that Rct2 value decreases for the cell with CYC-B1 dye compared with that of the cell with N3 dye; this decrease of Rct2 value is consistent with the larger JSC and FF values of the cell with
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CYC-B1 dye, compared to those of the cell with N3 dye.
In addition, laser-induced photo-current transients of the two ZnO-based
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DSSCs with CYC-B1 and N3 dyes were obtained, in order to confirm the higher electron transport in the ZnO film with CYC-B1 dye, as compared to that in the ZnO
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film with N3 dye (Fig. 6). The characteristic time constant τc could be fitted by exponential decay process, and the electron diffusion coefficient De could be calculated from Eq (1). The fitted τc values are 0.19 and 0.26 ms, and the calculated De values are 2.24×10–3 and 1.64×10–3 cm2 s–1 for the ZnO electrodes with CYC-B1 dye and N3 dye, respectively. The higher electron transport in the ZnO film in the
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case of the cell with CYC-B1dye thus explains the higher JSC of this DSSC.
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3.4 Characterization of the synthesized PMMA spheres and their double-layer ZnO film
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It has been shown that larger pores (~300 nm) in the TiO 2 electrode can increase the light-scattering distance, giving higher JSC values [49, 50]. In order to
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further enhance the efficiency of the cell with CYC-B1dye, incorporation of PMMA spheres into the ZnO paste to form the micro-pore after sintering was considered. Thus, poly (methyl methacrylate), PMMA spheres were synthesized and
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incorporated into the ZN20 paste (PMMA-ZN20 paste); this PMMA-ZN20 paste was coated as a sacttering layer, and coated on the ZN20 film for the DSSC with
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CYC-B1 dye. Fig. 7a shows SEM image of the PMMA spheres; the image shows PMMA spheres of uniform size, i.e., ca. 300 nm. The inset in Fig. 7a shows a highly
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magnified version of the spheres. Fig. 7b shows the thermogravimetric trace of the PMMA spheres; the trace shows that the decomposition temperature of the PMMA spheres is about 300 oC; this decomposition temperature explains the fact that the PMMA-ZN20 overlayer can show micro-pores after its sintering at 450 oC. Fig. 8a shows the cross-sectional SEM image of the ZN20 film with the overlayer of
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show the top view images for the underlayer and overlayer, respectively. A flat surface with nano-pores and a rough surface with micro-pores can be clearly seen for
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the underlayer (ZN20 film) and scattering layer (PMMA-ZN20 film), respectively.
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3.5 Photovoltaic characteristics and IPCE measurements of the DSSCs with or without PMMA-templated ZnO films
To further study the application of the double layer film for a DSSC, we
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fabricated three types of ZnO photoanodes, one, consisting only of a 10 μm underlayer, the second only of a 15 μm underlayer, and the third consisting of a
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double layer with 10 μm of underlayer (ZN20 film) and 5 μm of scattering layer (PMMA-ZN20 film). Fig. 9a shows the photocurrent density-voltage curves of the
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DSSCs with the three photoanodes described above, and Table 3 gives their corresponding photovoltaic parameters. It can be seen in Table 3 that the optimal film with the double layer, with 10 μm of underlayer and 5 μm of scattering layer, shows the highest η of 5.40±0.03%; this highest η is due to the significant improvement in JSC, with reference to those of the cells with only underlayers. The
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enhancement in JSC for the cell with the double layer could further be supported by incident photo-to-current conversion efficiency (IPCE) spectra of the cells. Fig. 9b
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shows that the DSSC with the double layer has the highest IPCE value of about 70%. The higher IPCE values of the cell with PMMA-ZN20 in the higher wavelength
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range, i.e., from 600 to 750 nm, are attributable to PMMA-ZN20. It is assumed that relatively large pores in the film with PMMA-ZN20 extends the travel distance for
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light scattering and thereby leads to a higher JSC value for this DSSC [50]. The JSC values of the cells with 10 and 15 µm of underlayers are also consistent with their corresponding IPCE values.
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Despite that the cell efficiency of 100% ZnO-based DSSCs are still lower than those of TiO2-based DSSCs, strategies to improve the efficiency of a ZnO-based
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DSSC are being investigated worldwide [12-38]. Systematic work is in progress in our laboratory for further increasing the performance of the ZnO-based DSSCs with
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CYC-B1 dye, by incorporating well-aligned and single-crystalline ZnO nanowires into the ZnO-particle-based photoanode, which may facilitate more rapid electron transport [23].
4. Conclusions
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In summary, an efficient dye, CYC-B1, with an alkyl bithiophene group, high extinction coefficient and absorbance has proven its potential to enable an
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aggregation-free ZnO surface morphology even after 24 h of dying the ZnO surface with it, where the routinely used N3 dye failed completely. The ZnO-based DSSC,
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sensitized by the CYC-B1 dye showed an η of 4.85±0.03%. The dye-immersion time of the ZnO film (ZN20) to achieve this efficiency was optimized at 24 h. Impedance
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measurements under open-circuit voltage have provided evidence that the charge injection kinetics are faster in the case of a DSSC with a ZN20 film sensitized with CYC-B1 dye, compared with one sensitized by N3 dye. Transient photo-current curves
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have shown faster electron transport in the ZN20 film with CYC-B1 dye, compared to that in the ZN20 film with N3 dye. PMMA spheres with uniform sizes of ca. 300 nm
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were synthesized and incorporated to template the ZN20 film (PMMA-ZN20 film). The PMMA-ZN20 film was coated as a scattering layer, and coated on the ZN20 (underlayer)
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film (this composite film is designated as ZN20/PMMA-ZN20). Thermogravimetric results for the PMMA spheres and the cross-sectional SEM image support the fact that the ZN20/PMMA-ZN20 film consists of micro-pores after sintering at 450 oC. The DSSC with the film of ZN20/PMMA-ZN20 showed a further improvement of the η (5.40±0.03%), with reference to the η of the cell without this PMMA (4.85±0.03%).
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This enhancement is due to a significant improvement in JSC, which is substantiated by
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IPCE measurements.
5. Acknowledgements
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This work was supported in part by the National Science Council of Taiwan. Some of the instruments used in this study were made available through the Academia Sinica,
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Taipei, Taiwan.
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References [1]
M. Law, L. E. Greene, J. C. Johnson, R. Saykally and P. D. Yang, Nature
[2]
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Materials, 2005, 4, 455. P. S. Archana, R. Jose, C. Vijila and S. Ramakrishna, J. Phys. Chem. C, 2009, 113,
SC
21538.
E. Hosono, S. Fujihara and T. Kimura, Electrochim. Acta, 2004, 49, 2287.
[4]
C. L. Kuo, T. J. Kuo and M. H. Huang, J. Phys. Chem. B, 2005, 109, 20115.
[5]
A. L. Pan, R. C. Yu, S. S. Xie, Z. B. Zhang, C. Q. Jin and B. S. Zou, J. Crystal Growth, 2005, 282, 165.
H. Zhang, D. R. Yang, D. S. Li, X. Y. Ma, S. Z. Li and D. L. Que, Cryst. Growth
TE D
[6]
M AN U
[3]
Des., 2005, 5, 547.
S. C. Hu, Z. W. Zhou and Z. R. Zhou, J. Crystal Growth, 2006, 287, 54.
[8]
S. Music, D. Dragcevic and S. Popovic, J. Alloys Compds., 2007, 429, 242.
[9]
S. Yang, C. Ge, Z. Liu, Y. Fang, Z. Li, D. Kuang and C. Su,
AC C
EP
[7]
RSC Adv., 2011, 1,
1691.
[10] M. Chen, Y. Wang, L. Song, P. Gunawan, Z. Zhong, X. She and F. Su, RSC Adv., 2012, 2, 4164. [11] Q. Tang, L. Lin, Z. Mao, Z. Tang and J. Wu, RSC Adv., 2012, 2, 2211.
24
ACCEPTED MANUSCRIPT
[12] A. B. F. Martinson, J. E. McGarrah, M. O. K. Parpia and J. T. Hupp, Phys. Chem. Chem. Phys., 2006, 8, 4655.
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[13] J. B. Baxter and E. S. Aydil, Sol. Energy Mater. Sol. Cells, 2006, 90, 607. [14] M. Vafaee and M. S. Ghamsari, Mater. Lett., 2007, 61, 3265.
SC
[15] K. Kakiuchi, M. Saito and S. Fujihara, Thin Solid Films, 2008, 516, 2026.
[16] A. B. F. Martinson, M. S. Góes, F. Fabregat-Santiago, J. Bisquert, M. J. Pellin and
M AN U
J. T. Hupp, J. Phys. Chem. A, 2009, 113, 4015.
[17] Y. H. Lai, C. Y. Lin, H. W. Chen, J. G. Chen, C. W. Kung, R. Vittal and K. C. Ho, J. Mater. Chem., 2010, 20, 9379.
TE D
[18] C. P. Lee, J. C. Lin, Y. C. Wang, C. Y. Chou, M. H. Yeh, R. Vittal and K. C. Ho, Phys. Chem. Chem. Phys., 2011, 13, 20999.
EP
[19] J. C. Lin, C. P. Lee and K. C. Ho, J. Mater. Chem. 2012, 22, 1270. [20] Q. Tang, L. Lin, Z. Mao and J. Wu, RSC Adv., 2012, 2, 1863.
AC C
[21] Z. Yang, T. Xu, Y. Ito, U. Welp and W. K. Kwok, J. Phys. Chem. C, 2009, 113, 20521.
[22] Y. Xie, P. Joshi, S. B. Darling, Q. Chen, T. Zhang, D. Galipeau and Q. Qiao, J. Phys. Chem. C, 2010, 114, 17880. [23] C. T. Wu, W. P. Liao and J. J. Wu, J. Mater. Chem. 2011, 21, 2871.
25
ACCEPTED MANUSCRIPT
[24] Y. Z. Zheng, X. Tao, Q. Hou, D. T. Wang, W. L. Zhou and J. F. Chen, Chem. Mater., 2011, 23, 3.
RI PT
[25] A. Otsuka, K. Funabiki, N. Sugiyama, T. Yoshida, H. Minoura and M. Matsui, Chem. Lett., 2006, 35, 666.
SC
[26] E. Guillén, F. Casanueva, J. A. Anta, A. Vega-Poot, G. Oskam, R. Alcántara, C. Fernández-Lorenzo and J. Martín-Calleja, J. Photochem. Photobiol. A: Chem.,
M AN U
2008, 200, 364.
[27] X. F. Wang, O. Kitao, E. Hosono, H. Zhou, S. Sasaki and H. Tamiaki, J. Photochem. Photobiol. A: Chem., 2010, 210, 145.
TE D
[28] T. Yoshida, J. Zhang, D. Komatsu, S. Sawatani, H. Minoura, T. Pauporte, D. Lincot, T. Oekermann, D. Schlettwein, H. Tada, D. Wohrle, K. Funabiki, M.
EP
Matsui, H. Miura and H. Yanagi, Adv. Funct. Mater., 2009, 19, 17. [29] S. Ito, S. Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet, P. Comte, M.
AC C
Nazeeruddin, P. Pechy, M. Takata, H. Miura, S. Uchida and M. Grätzel, Adv.
Mater., 2006, 18, 1202.
[30] H. Cheng and W. Hsieh, Energy Environ. Sci., 2010, 3, 442. [31] (a) S. Higashijima, H. Miura, T. Fujita, Y. Kubota, K. Funabiki, T. Yoshida and M. Matsui, Tetrahedron, 2011, 67, 6289. (b) S. Higashijima, Y. Inoue, H. Miura, Y.
26
ACCEPTED MANUSCRIPT
Kubota, K. Funabiki, T. Yoshida and M. Matsui, RSC Adv., 2012, 2, 2721. [32] Q. F. Zhang, C. S. Dandeneau, X. Y. Zhou and G. Z. Cao, Adv. Mater., 2009, 21,
[33] F. Xu and L. Sun, Energy Environ. Sci. 2011, 4, 818.
RI PT
4087.
SC
[34] I. Bedja, P. V. Kamat, X. Hua, P. G. Lappin and S. Hotchandani, Langmuir, 1997, 13, 2398.
M AN U
[35] H. Horiuchi, R. Kaoh, K. Hara, M. Yanagida, S. Murata, H. Arakawa and M. Tachiya, J. Phys. Chem. B, 2003, 107, 2570.
[36] K. Keis, J. Lindgren, S. E. Lindquist and A. Hagfeldt, Langmuir, 2006, 16, 4688.
2009, 113, 9206.
TE D
[37] H. M. Nguyen, R. S. Mane, T. Ganesh, S. H. Han and N. Kim, J. Phys. Chem. C,
EP
[38] T. P. Chuo, Q. F. Zhang and G. Z. Cao, J. Phys. Chem. C, 2007, 111, 18804. [39] C. Y. Chen, S. J. Wu, C. G. Wu, J. G. Chen and K. C. Ho, Angew. Chem. Int. Ed.,
AC C
2006, 45, 5822.
[40] C. G. Chen, C. Y. Chen, S. J. Wu, J. Y. Li, J. G. Wu and K. C. Ho, Sol. Energy Mater. Sol. Cells, 2006, 45, 5822.
[41] L. Han, N. Koide, Y. Chiba and T. Mitate, Appl. Phys. Lett., 2004, 84, 2433. [42] L. Han, N. Koide, Y. Chiba, A. Islam and T. Mitate, C. R. Chimie., 2006, 9, 645.
27
ACCEPTED MANUSCRIPT
[43] J. van de Lagemaat and A. J. Frank, J. Phys. Chem. B, 2001, 105, 11194. [44] N. A. Anderson, X. Ai and T. J. Lian, J. Phys. Chem. B, 2003, 107, 14414.
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[45] G. Redmond, D. Fitzmaurize and M. Grätzel, Chem. Mater., 1994, 6, 686. [46] K. Keis, C. Bauer, G. Boschloo, A. Hagfeldt, K. Westermark, H. Rensmo and H. Seigbahn, J. Photochem. Photobiol., A: Chem., 2002, 57, 148.
SC
[47] Z. Zhang, S. M. Zakeeruddin, B. C. O’ Regan, R. Humphry-Baker and M. Grätzel,
M AN U
J. Phys. Chem. B, 2005, 109, 21818.
[48] R. Kern, R. Sastrawan, J. Ferber, R. Stangl and J. Luther, Electrochim. Acta, 2002, 47, 4213.
2005, 2011.
TE D
[49] S. Hore, P. Nitz, C. Vetter, C. Prahl, M. Niggemann and R. Kern, Chem. Commun.,
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[50] (a) K. M. Lee, C. Y. Hsu, W. H. Chiu, M. C. Tsui, Y. L. Tung, S. Y. Tsai and K. C. Ho, Sol. Energy Mater. Sol. Cells, 2009, 93, 2003. (b) J. G. Chen, C. Y. Chen, C.
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G. Wu and K. C. Ho, J. Phys. Chem. C, 2010, 114, 13832. (c) C. Y. Chou, C. P.
Lee, R. Vittal and K. C. Ho, J. Power Sources, 2011, 196, 6595.
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List of Tables Table 1. Photovoltaic parameters of the DSSCs sensitized with CYC-B1 dye for
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different periods of time, obtained at 100 mW cm–2 light intensity. Table 2. Photovoltaic parameters of the DSSCs, obtained at 100 mW cm–2 light
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intensity, in which the photoanodes of the cells were sensitized with CYC-B1 dye and N3 dye for 24 h. The table also shows both Rct2 and De obtained from
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electrochemical impedance spectroscopic and transient photo-current curves, respectively.
Table 3. Photovoltaic parameters of the DSSCs with photoanodes consisting of 10 μm
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of ZN20 (underlayer), 15 μm of Zn20 (underlayer), and 10 μm of ZN20 (underlayer) with 5μm of PMMA-ZN20 (overlayer), measured at 100 mW
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cm–2 light intensity. All the photoanodes of the cells were sensitized with
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CYC-B1 dye for 24 h.
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ACCEPTED MANUSCRIPT Figure Captions Fig. 1
The structure of CYC-B1 [39].
Fig. 2
(a) and (b) SEM images of bare ZnO film at different magnifications; (c) and
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(d) SEM images of the same ZnO film at different magnifications after its dipping in N3 dye for 24 h.
(a) and (b) SEM images of the ZnO film at different magnifications, after its
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Fig. 3
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dipping in CYC-B1 dye solution for 24 h; (c) and (d) SEM images of the ZnO film at different magnifications, after its dipping in CYC-B1 dye solution for 48 h. Fig. 4
Photocurrent density-voltage curves of the DSSCs sensitized with CYC-B1
Fig. 5
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dye for different periods of time, obtained at 100 mW cm–2 light intensity. (a) Current density-voltage curves of the DSSCs, obtained both at 100 mW
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cm–2 light intensity and in the dark, in which the photoanodes of the cells were
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sensitized with CYC-B1 and N3 dyes for 24 h; (b) electrochemical impedance spectra of the cells in (a), measured at 100 mW cm–2 light intensity under the open-circuit voltage. The inset shows the equivalent circuit.
Fig. 6
Laser-induced photocurrent transients of the DSSCs with CYC-B1 and N3 dyes, for the sensitization time of 24 h.
Fig. 7
(a) SEM image of the PMMA spheres. The inset shows highly magnified
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version of the spheres; (b) thermogravimetric trace of the PMMA spheres. Fig. 8
(a) Cross-sectional SEM image of the composite film with the underlayer of
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ZN20 and overlayer of PMMA-ZN20, (b) top view of ZN20 film, and (c) top view of PMMA-ZN20 film. Fig. 9
(a) Photocurrent density-voltage curves of the DSSCs with photoanodes
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consisting of 10 μm of ZN20 (underlayer), 15 μm of Zn20 (underlayer), and
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10 μm of Zn20 (underlayer) with 5μm of PMMA-ZN20 (overlayer), measured at 100 mW cm–2 light intensity; (b) corresponding incident photo-to-current
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conversion efficiency (IPCE) curves.
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Table 1
VOC (mV)
JSC (mA cm–2)
FF
η (%)
3
507±7
10.78±0.21
0.52±0.01
2.84±0.07
6
524±4
12.85±0.20
12
539±3
13.07±0.07
24
562±4
13.75±0.05
48
547±4
12.62±0.10
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Dye-loading time (h)
3.49±0.08
0.56±0.01
3.97±0.06
0.63±0.00
4.85±0.03
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0.52±0.01
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0.53±0.00
3.63±0.06
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Table 2
VOC (mV)
JSC (mA cm–2)
FF
η (%)
Rct2 (ohm)
De (10–3 cm2s–1)
CYC-B1
562±4
13.75±0.05
0.63±0.00
4.85±0.03
25.61
2.24
N3
549±7
10.17±0.38
0.45±0.01
2.50±0.12
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Dye
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36.43
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1.64
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Table 3
VOC (mV)
JSC (mA cm–2)
FF
η (%)
10 μm ZN20
562±4
13.75±0.05
0.63±0.00
4.85±0.03
15 μm ZN20
550±2
14.57±0.03
10 μm ZN20/5 μm PMMA-ZN20
565±3
16.09±0.22
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ZnO film of the photoanode
4.44±0.04
0.59±0.01
5.40±0.03
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0.55±0.00
34
Fig. 1
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(a)
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100 nm
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100 nm
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0
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100
200
300
Fig. 4
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38
400
500
600
(201)
8
(c)
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CYC-B1 3h 6h 12 h 24 h 48 h
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Photocurrent density (mA cm )
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(a) 16
-2
8
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24 h dying time CYC-B1 N3
4 0
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Current density (mA cm )
Under 100 mW cm
Under dark
0
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200
300
400
500
600
Voltage (mV)
(b)
30
RC R ct1 t1
ZZW R w diff
RCct2t2 R
RSs R
20
CPE2 CPE2
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CPE1 CPE1
10
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24 h dying time CYC-B1 N3
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10
20
30 Z' (ohm)
Fig. 5
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40
50
60
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Photocurent (a.u.)
24 h dying time CYC-B1 N3
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τc= 0.19 ms, De= 2.24 × 10-3 cm2 s–1
τc= 0.26 ms, De= 1.64 × 10-3 cm2 s–1
0.0
0.5
1.0
1.5
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80
PMMA
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200
300
400 o
Temperature ( C)
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(a) -2
Photocurrent density (mA cm )
18
12 9
CYC-B1, 24 h dying time 10 m ZN20 15 m ZN20 10 m ZN20/5 m PMMA-ZN20
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100
200
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500
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CYC-B1, 24 h dying time 10 m ZN20 20 15 m ZN20 10 m ZN20/5 m PMMA-ZN20
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60
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400
500
600 Wavelength (nm)
Fig. 9
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S0378-7753(13)00891-4
DOI:
10.1016/j.jpowsour.2013.05.101
Reference:
POWER 17429
To appear in:
Journal of Power Sources
Received Date: 15 December 2012 Revised Date:
13 May 2013
Accepted Date: 14 May 2013
Please cite this article as: C.-P. Lee, C.-Y. Chou, C.-Y. Chen, M.-H. Yeh, L.-Y. Lin, R. Vittal, C.-G. Wu, K.-C. Ho, Zinc oxide-based dye-sensitized solar cells with a ruthenium dye containing an alkyl bithiophene group, Journal of Power Sources (2013), doi: 10.1016/j.jpowsour.2013.05.101. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Highlights (for review)
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Highlights Zinc oxide-based dye-sensitized solar cells with a ruthenium dye containing an alkyl bithiophene group
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Chuan-Pei Leea, Chen-Yu Choua, Chia-Yuan Chenb, Min-Hsin Yeha, Lu-Yin Lina,
a
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R. Vittala, Chun-Guey Wub,﹢, and Kuo-Chuan Hoa,c,*
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan b
c
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Department of Chemistry, National Central University, Jhong-Li 32001, Taiwan.
Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
1. CYC-B1 dye is used in a zinc oxide (ZnO)-based dye-sensitized solar cell (DSSC).
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2. The CYC-B1/ZnO film is almost free from Zn2+/dye-agglomerations. 3. Efficiency of a CYC-B1/ZnO DSSC is 94% higher than that of an N3 dye/ZnO cell.
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4. PMMA spheres are synthesized to template the ZnO film as the scattering layer.
Graphical Absrtact (for review)
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Graphical abstract
alkyl bithiophene group
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Zinc oxide-based dye-sensitized solar cells with a ruthenium dye containing an
Chuan-Pei Leea, Chen-Yu Choua, Chia-Yuan Chenb, Min-Hsin Yeha, Lu-Yin Lina,
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R. Vittala, Chun-Guey Wub,﹢, and Kuo-Chuan Hoa,c,* a
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan b
Department of Chemistry, National Central University, Jhong-Li 32001, Taiwan.
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Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
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c
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*Manuscript text (double-spaced) Click here to view linked References
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Zinc oxide-based dye-sensitized solar cells with a ruthenium dye containing an
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alkyl bithiophene group
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Chuan-Pei Leea, Chen-Yu Choua, Chia-Yuan Chenb, Min-Hsin Yeha, Lu-Yin Lina,
a
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R. Vittala, Chun-Guey Wub,﹢, and Kuo-Chuan Hoa,c,*
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
Department of Chemistry, National Central University, Jhong-Li 32001, Taiwan. c
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b
Institute of Polymer Science and Engineering, National Taiwan University,
Corresponding author: Tel: +886-2-2366-0739; Fax: +886-2-2362-3040
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*
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Taipei 10617, Taiwan
﹢Corresponding
E-mail: [email protected]
author: Tel: +886-3-422-7151 x 65903; Fax: +886-3-422-7664 E-mail: [email protected]
Revised manuscript (POWER-D-12-03954R1) prepared for the consideration for publication in Journal of Power Sources.
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Zinc oxide-based dye-sensitized solar cells with a
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ruthenium dye containing an alkyl bithiophene group Chuan-Pei Leea, Chen-Yu Choua, Chia-Yuan Chenb, Min-Hsin Yeha, Lu-Yin Lina,
a
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R. Vittala, Chun-Guey Wub,﹢, and Kuo-Chuan Hoa,c,*
Department of Chemical Engineering, National Taiwan University,
b
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Taipei 10617, Taiwan
Department of Chemistry, National Central University, Jhong-Li 32001, Taiwan. c
Institute of Polymer Science and Engineering, National Taiwan University,
Abstract
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Taipei 10617, Taiwan
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An improved light-harvesting ruthenium dye with an alkyl bithiophene group, designated as CYC-B1, is employed as the photosensitizer for a zinc oxide (ZnO)-based
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dye-sensitized solar cell (DSSC). ZnO films with N3 and CYC-B1 dyes exhibit different surface appearances. Interestingly, the ZnO nanocrystal electrode surface with CYC-B1 dye is found to be almost free from Zn2+/dye-agglomerations, usually observed in the case of other commercial ruthenium dyes for such an electrode surface, e.g., N3 dye. Owing to this favorable effect of the CYC-B1 dye, the DSSC with a ZnO film (designated as ZN20) sensitized with this dye exhibited an average efficiency (η) of 1
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analyzing the electrochemical impedance spectra (EIS) and laser-induced photo-current transients. Further, polymethylmethacrylate (PMMA) spheres with uniform sizes of ca.
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300 nm are synthesized and incorporated to template the ZN20 film (designated as PMMA-ZN20); this PMMA-ZN20 film is used as the scattering layer and it is coated on
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the underlayer ZN20 film to make the photoanode film (ZN20/PMMA-ZN20) of a DSSC. The thus fabricated DSSC shows an η of 5.40±0.03%.
Dye-sensitized
solar
cells;
Electrochemical
impedance
spectra;
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Keywords:
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Laser-induced photo-current transients; Polymethylmethacrylate; Zinc oxide
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1. Introduction
Zinc oxide (ZnO) has been investigated as a promising alternative photoanode
material for dye-sensitized solar cells (DSSCs). Since the inception of research on TiO2-based DSSCs, ZnO has been examined, since TiO2 and ZnO have similar electron affinities and almost the same band gap energies, i.e., ~3.2 eV and ~3.3 eV, respectively. ZnO, however, has much higher electron diffusivity than TiO2 [1, 2]. The
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diverse morphologies of ZnO are probably richer than those of any known materials [3-11], which would be expected to result in a range of ZnO photoanode designs
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[12-24]. ZnO has a large excitation binding energy (60 eV), is inexpensive, and is stable against photocorrosion. However, the power conversion efficiency (η) of ZnO-based
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DSSCs is still lower than that of TiO2-based DSSCs, which is most likely due to the instability of the ZnO film in an acidic dye solution. Strategies to improve the efficiency
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of ZnO-based DSSCs also include designing of new dyes that are suitable for the ZnO photoanode. Therefore, the ZnO-based DSSCs with novel dye molecules have been continuously investigated.
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The use of organic dyes for ZnO-based DSSCs in some researches has been explored. Otsuka et al. [25] have developed an efficient, cyan-colored, and novel
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squaraine-dye, namely Sq3, for a ZnO-based DSSC, which produced η=1.5%. Guillén
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et al. [26] reported a comparative study on the photovoltaic performances of ZnO-based DSSCs with xanthene dyes (such as Eosin Y, Eosin B, and Mercurochrome) and with the frequently-used ruthenium-ligand N719 dye; an η of 1.2% was achieved for a ZnO-based DSSC by using Eosin Y. This study shows that, at least for ZnO-based DSSCs, xanthene dyes can be considered as suitable candidates to replace common ruthenium photosensitizers (such as N3, N719, and so-called "black dye"). Wang et al.
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have been developed for ZnO-based DSSCs. Their molar extinction coefficients are significantly higher than those of the common ruthenium photosensitizers [28].
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Specifically, the indoline derivative dye, D149, with an absorption coefficient five times larger than that of N719 at the maximum of the solar spectrum, was reported to have
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shown a remarkable efficiency for a DSSC with mesoporous TiO2 [29]. Wu et al. [23] reported an efficiency of 3.74% for a ZnO-based DSSC with D149. Yoshida and co-workers have reported efficiencies of above 5% for ZnO-based DSSCs [28]. Cheng
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and Hsieh obtained an η of 4.95% for a cell with a photoanode of ZnO nanoparticles, sensitized with D149 dye [30]. Higashijima et al. [31] have recently reported that two
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novel indoline dyes, DN319 and DN350, have been developed for ZnO-based DSSCs,
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which possess cell efficiencies of 5.01% and 5.55%, respectively.
On the other hand, DSSCs based on mesoporous ZnO sensitized with common
ruthenium photosensitizers have usually reached efficiencies of 0.5–5.4% under 1 sun illumination, depending on preparation conditions and the electrolyte composition [32, 33]. The low conversion efficiency of a ZnO-based DSSC with a ruthenium photosensitizer is most likely due to the dissolution of the ZnO to Zn2+ by the adsorbed
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acidic dye, followed by the formation of agglomerates (giving an insulating layer of Zn2+ and ruthenium photosensitizer molecules), and eventually followed by the
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blocking of the injected electrons from the dye molecules to the semiconductor by the insulating layer [34-36]. The agglomerates are thus detrimental to the performance of a
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ZnO-based DSSC, because they fill up nanopores of the ZnO electrode and reduce the electron injection efficiency of the dye adsorbed to the ZnO layer. In order to prepare
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efficient ZnO-based DSSCs with ruthenium photosensitizers, Nguyen et al. [37] have found that a ruthenium photosensitizer molecule with a hydrophobic side chain could prevent agglomerate formation by hindering coordination of the dye with Zn2+; they applied
a
ruthenium-based
dye,
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have
namely
Ru(H2dcbpy)(4-(4-(N,N-di(p-
hexyloxyphenyl)amino)styryl)-4'-methyl-2,2'-bipyridine)(NCS)2
(HMP-2,
extinction
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coefficient, ε, 3.33×104 M–1cm–1 at 429 nm; 2.00×104 M–1cm–1 at 534 nm), to a ZnO-based DSSC; the cell achieved a higher η of 4.03% as compared to that with N3
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dye (0.11%). Besides, with this dye, the synergic impact on ZnO nanocrystals was practically negligible even after 24 h of immersion time, where the routinely preferred N3 dye practically failed to a great extent [37, 38]. Since there are limited reports on the exploration of ruthenium-based dyes for ZnO-based DSSCs, exploitative studies are
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thus necessary to understand the influence of ruthenium-based dyes on the performance of a ZnO-based DSSC.
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An efficient ruthenium photosensitizer with an alkyl bithiophene group, designated as CYC-B1, was reported previously [39,40]; to prepare this dye, one of the
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dcbpy ligands (4,4'-dicarboxylic acid-2,2'-bipyridine) in N3 was replaced with abtpy(4,4'-di-2-octyl-5-(thiophen-2-yl)thiophen-2-yl-2,2-bipyridine), a bipyridine ligand
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substituted with alkyl bithiophene groups (Fig. 1). The CYC-B1 dye has a superior extinction coefficient (4.64×104 M–1cm–1 at 400 nm; 2.12×104 M–1cm–1 at 553 nm) among ruthenium-based dyes (N3: 1.52×104 M–1cm–1 at 385 nm; 1.45×104 M–1cm–1 at
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530 nm); the TiO2-based DSSC with this CYC-B1 dye thus possesses good efficiency.
In this study, we used the CYC-B1 dye in all-ZnO-based DSSCs; an average
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efficiency of 4.85±0.03% was achieved for such a cell by adjusting the dye-exposure
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time. The effects of the CYC-B1 dye on the surfaces of ZnO particles were compared to those of N3 dye on the surfaces of the same ZnO particles. Moreover, spheres of poly(methyl methacrylate), PMMA with uniform sizes of ca. 300 nm were synthesized and incorporated to template a ZnO (ZN20) film; this PMMA incorporated ZnO film was used as a scattering layer, and coated on the underlayer ZN20 film to prepare the photoanode film of a DSSC (this composite film is designated as ZN20/PMMA-ZN20);
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understand the kinetics of the photoelectrochemical processes for the ZnO-based DSSC with CYC-B1 dye.
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2. Experimental Section 2.1 Materials
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Lithium iodide (LiI, synthetical grade) and iodine (I2, synthetical grade) were obtained from Merck (Darmstadt, Germany); 4-tert-butylpyridine (TBP, 96%) and tert-butyl alcohol (tBA, 96%) were obtained from Acros Organics (Geel, Belgium);
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acetonitrile (ACN, 99.99%), dimethyl sulfoxide (DMSO, ≧99.6%), ethanol (EtOH, 99.5%), methyl methacrylate (MMA, 99%), methacrylic acid (MAA, 99%), and
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Mo).
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isopropyl alcohol (IPA, 99.5%) were obtained from Aldrich (Sigma-Aldrich, St. Louis,
2.2 Synthesis of PMMA spheres A pH = 3.35 buffer solution (320 g), consisting of 0.05 M glycine (C2H5NO2, >
99%, Riedel-de Haën, Honeywell International Inc., Seelze, Germany) and 0.0168 M HCl was purged with N2 for 30 min and the temperature of the solution was raised to 80 o
C. 2,2-azobis(2-amino-propane) dihydrochloride (1.60 g, AAPH > 95%, Wako Pure
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Chemical Industries, Ltd., Osaka, Japan) was added into the buffer solution as the initializer for the polymerization of methyl methacrylate (MMA). Methyl methacrylate
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(35 g, MMA) and methacrylic acid (0.35 g, MAA) were gently injected into the mixing solution at the same rate, keeping the whole solution stirred. The reaction was allowed
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to take place at 80 oC for 2 h to finish the polymerization. The solution was then cooled to room temperature, washed with DI water, and centrifuged to obtain spherical PMMA
2.3 Preparation of ZnO pastes
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powder.
Commercial ZnO powder (4.23 g, Degussa VP AdNano®ZnO20, Evonik Degussa
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Taiwan Ltd., Taipei, Taiwan) with an approximate nanoparticle size of 20 nm was thoroughly mixed with 20 mL solution of EtOH and DI-water (v/v=70/30). This
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colloidal solution was stirred for 3 days to obtain a well-dispersed suspension of 20
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wt% ZnO; this ZnO paste was used for preparing the ZnO film (designated as ZN20). In addition, we were able to prepare a PMMA-templated ZnO film (designated as PMMA-ZN20) by incorporating 30 wt% of PMMA spheres into the ZN20 paste.
2.4 Assembly of DSSCs A fluorine-doped SnO2 conducting glass (FTO, 7 Ω sq. –1, visible transmittance ≧ 80%, NSG, Tokyo, Japan) was first cleaned with a neutral cleaner, and then washed
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technique. The above synthesized PMMA spheres were incorporated into the ZN20 film; this film (designated as PMMA-ZN20) was used as an overlayer on the underlayer ZN20 film to make the composite film (ZN20/PMMA-ZN20) for a DSSC. A 0.4×0.4
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cm2 square was selected on the coated FTO glass as the active area by removing the side
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portions by scraping. The ZN20 or ZN20/PMMA-ZN20 film was gradually heated to 450 oC in an oxygen atmosphere, and subsequently sintered at that temperature for 30 min. After sintering at 450 oC and cooling to 80 oC, the film was immersed in a 1×10–4
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M solution of CYC-B1 [39] in ACN, tBA, and DMSO (volume ratio of 1:1:1), at room temperature for 3, 6, 12, 24 and 48 h. The film was also immersed in a 1×10–4 M
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solution of N3 (Solaronix S.A., Aubonne, Switzerland) in EtOH, at room temperature for 24 h. The thus dye-coated electrode was placed on a platinum-sputtered conducting –1
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glass electrode (indium tin oxide,
, visible transmittance ≧ 85%, Ritek
Corporation, Hsinchu, Taiwan), keeping the two electrodes separated by a 25 μm-thick Surlyn® layer (SX1170-25, Solaronix S.A., Aubonne, Switzerland). The two electrodes were then sealed by heating. A mixture of 0.1 M LiI, 0.6 M DMPII (Solaronix S.A., Aubonne, Switzerland), 0.05 M I2, and 0.5 M TBP in 3-methoxypropionitrile (MPN,
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Fluka)/ACN (volume ratio of 1:1) was used as the electrolyte. The electrolyte was injected into the gap between the two electrodes by capillarity; the electrolyte-injecting
sealed with hot-melt glue after the injection of the electrolyte.
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2.5 Instrumentation and measurements
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hole was previously made in the counter electrode with a drilling machine; the hole was
Surface of the DSSC was illuminated by a class A quality solar simulator
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(XES-3003S-100, AM1.5G, San-Ei Electric Co., Ltd., Osaka, Japan) and the incident light intensity (100 mW cm–2) was calibrated with a standard Si cell (PECSI01, Peccell Technologies, Inc., Yokohama, Japan). Photoelectrochemical characteristics of the
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DSSC were recorded with a potentiostat/galvanostat (PGSTAT 30, Autolab, Eco-Chemie, Utrecht, the Netherlands). All the photovoltaic parameters are the average
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values obtained with three separate devices for each condition.
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Film thickness was determined using a surface profilometer (Sloan Dektak 3030, CAE, Redwood City, CA). Morphologies of the ZnO films were observed using a scanning electron microscopy (SEM, Nova NanoSEM 230, FEI Ultra-High Resolution FE-SEM with low vacuum mode, FEI, Hillsboro, OR). The thermal property of the PMMA spheres was analyzed by a thermogravimetric analyzer (TGA, TGA-7, PerkinElmer, Waltham, MA).
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Electrochemical impedance spectra (EIS) were obtained by the above-mentioned potentiostat/galvanostat equipped with an FRA2 module, under a constant light
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illumination of 100 mW cm–2. The frequency range explored was 10 mHz to 65 kHz. Applied bias voltage was set at the open-circuit voltage of the DSSC between the
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ITO-Pt counter electrode and the FTO-ZnO-dye working electrode, starting from the short-circuit condition; the corresponding AC amplitude was 10 mV. The impedance
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spectra were analyzed using an equivalent circuit model [41, 42]. Photocurrent transients of the assembled devices were recorded with a digital oscilloscope (model LT322, Teledyne LeCroy, Chestnut Ridge, NY). The transients were measured under a
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continuous bias illumination of white light and weak pulsed laser. Pulsed laser excitation was applied by a frequency-doubled Q-switched Nd:YAG laser (model
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Quanta-Ray GCR-3-10, Spectra-Physics laser, Newport Corporation, Irvine, CA) with a 2 Hz repetition rate at 532 nm, and a 7 ns pulse width at half-height. The diffusion
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coefficient of electrons (De) was estimated by fitting a decay of the transient photocurrent with exp (–t τc–1), which was derived from the equation of continuity for electrons in the conduction band [43], where t and τc are the time and average time constant, respectively. The apparent diffusion coefficient of electrons may be calculated by using the following equation,
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(1)
where w is the film thickness and the factor 2.35 arises from the geometry of the
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diffusion problem. Incident photon-to-current conversion efficiency (IPCE) curves were obtained at short-circuit condition. The light source was a quality solar simulator
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(PEC-L11, AM1.5G, Peccell Technologies, Inc., Yokohama, Japan); light was focused through a monochromator (Oriel Instrument, model 74100, Newport, Irvine, CA) onto
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the photovoltaic cell. The monochromator was incremented through the visible spectrum to generate the IPCE (λ) as defined below,
IPCE (λ) = 1240(JSC/λφ)
(2)
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where λ is the wavelength, JSC is short-circuit photocurrent density (mA cm–2) recorded with a potentiostat/galvanostat, and φ is the incident radiative flux (W m–2) measured
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with an optical detector (Oriel Instrument, model 71580, Newport) and power meter
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(Oriel Instrument, model 70310, Newport).
3. Results and Discussion 3.1 Characterization of the ZnO films after dipping in N3 and CYC-B1 dye solutions Although ZnO has been investigated as a promising alternative photoanode material for DSSCs, the conversion efficiency of a ZnO-based DSSC with a common
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ruthenium photosensitizer is still low, attributable to the formation of agglomerates of Zn2+ ions and ruthenium photosensitizer molecules [34-36]. Horiuchi et al. [35]
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found that the formation of aggregates consisting of N 3 dye molecules and Zn 2+ ions occurs in the ZnO film for immersion times of the film greater than 10 min. The
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aggregates exist in the pores of the ZnO film and on the surface of the film as micrometer-sized particles. These aggregates have the potential to change the quality
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of the film and its surface structure. Fig. 2a and 2b show SEM images of the surface of the bare ZnO film at different magnifications; the porous film in 1a and 1b shows ZnO nanoparticles of sizes between 20~100 nm; Fig. 2c and 2d show SEM images of
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the same ZnO film at different magnifications after its dipping in N3 dye for 24 h. Careful observation clearly indicates the change in the surface of the film due to its
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dipping in the N3 dye. The surface of ZnO nanoparticles in the dye is smooth and there are fewer islands in the film before it was immersed in the dye. Several small
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crystallites form some kind of diffusion barrier layer in the film after immersion; this type of change in the morphology of the ZnO film is assumed to have arisen due to the formation of aggregations of N3 dye molecules with Zn 2+ ions on the surface of the film, caused by the immersion of the ZnO film in N3 dye solution for a prolonged time [37-38]. In a similar way, the CYC-B1 dye with a high extinction
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the CYC-B1 dye; the film was dipped in CYC-B1 dye solution for 24 and 48 h. Fig. 3a and 3b show SEM images of the ZnO film at different magnifications, after its
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dipping in CYC-B1 dye solution for 24 h; Fig. 3c and 3d show SEM images of the ZnO film at different magnifications, after immersion in CYC-B1 dye solution for 48
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h. The surfaces of the ZnO nanoparticles, after immersion in CYC-B1 dye solution are physically stable, and the islands in the film before and after the immersion are undisturbed (compare Fig. 2b with Fig. 3b). In other words, the surface modification
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of the ZnO nanocrystals in CYC-B1 dye is insignificant, even after the dipping of the ZnO film for 24 or 48 h in the dye solution. The presence of a hydrophobic
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side-chain is assumed to have prevented the coordination between Zn 2+ ions and dye molecules to form the respective agglomerates [37]. Encouraged by this finding, we
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have employed the CYC-B1 dye to sensitize the ZnO film for a DSSC.
3.2 Photovoltaic characteristics of the DSSCs sensitized with N3 and CYC-B1 dyes It has been reported that the immersion time of a ZnO film in a sensitizing dye is a key factor for the functioning of the ZnO-based DSSC [26]. Fig. 4 shows the
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photocurrent density-voltage curves of the DSSCs sensitized with CYC-B1 dye for different periods of time; Table 1 summarizes the corresponding photovoltaic
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parameters. All the photovoltaic parameters are the average values obtained with three separate devices for each condition. Table 1 shows that the highest power
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conversion efficiency (η) is achieved at an immersion time of 24 h; at 48 h of immersion, the power conversion efficiency shows a decrease. The ZnO-based
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DSSC with the optimal immersion time of 24 h for dye CYC-B1 shows an η value of 4.85±0.03%, the open-circuit voltage (VOC), the short-circuit current density (JSC), and the fill factor (FF) being 562±4 mV, 13.75±0.05 mA cm–2, and 0.63±0.00,
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respectively. Based on these results, the immersion time was fixed to be 24 h for the CYC-B1 dye. Fig. 5a shows the current density-voltage curves of the DSSCs,
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obtained both at 100 mW cm –2 light intensity and in the dark, in which the photoanodes of the cells were sensitized with CYC-B1 dye and N3 dye for 24 h. The
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corresponding photovoltaic parameters under illumination are listed in Table 2. The DSSC with N3 dye shows a lower η value of 2.50±0.12%, as compared to that of the cell with CYC-B1 dye (4.85±0.03%). The low performance of the DSSC with N3 dye can be explained as follows. The chemical instability of ZnO nanocrystals, as seen in the SEM images of Fig. 2c and 2d, due to the formtion of agglomerates of
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reduces the JSC of the affected cell. The formed Zn2+-dye complex can form a barrier layer on the surface of the ZnO particels, thereby preventing the injected electrons to reach the ZnO conduction band. This favors recombination of charges between the
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ZnO particles and the I–/I3– electrolyte, thereby decreasing the Voc of the relevant
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DSSC. The formation of agglomerates and the associated barrier layer increases the internal resistance in the ZnO matrix, thus reducing the cell FF. Lower values of JSC, VOC, and FF result in a lower cell power conversion [35,44-46].
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The higher VOC value of the CYC-B1 cell was thought to be due to the lower dark currents, with reference to those in the case of the cell with N3; this assumption
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was verified by measuring the I-V characteristics of the cells under dark conditions. It is well known that the dark currents is resulted from the recombination of charges
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at the interface between ZnO particles and I–/I3–. Fig. 5a also demonstrates the current density-voltage characteristics of the DSSCs with CYC-B1 and N3 dyes, measured in the dark. The onsets of the dark currents of the cells with CYC-B1 and with N3 dyes occur at 550, and 535 mV, respectively. These values were obtained by drawing tangents to the curves starting from the voltage scale and a tangent at the
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zero current of the curves (zero-current line) and measuring the voltages corresponding to the intersection points of the tangents at the zero current line. The
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onset of the dark current of the cell with CYC-B1 occurs at the highest forward bias, which confirms that the cell with CYC-B1 has less recombination of charges at the
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interface between ZnO particles and I–/I3–. FF values for DSSCs are usually limited by series resistance losses, light-intensity dependent recombination, and dark
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currents. Apparently reduction in dark currents, as discussed above, should have caused a higher FF value for the cell with CYC-B1 than that for the cell with N3
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(Table 2).
3.3 EIS measurements and photocurrent transients of the DSSCs sensitized with N3
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and CYC-B1 dyes
In order to show further evidence to substantiate the higher FF in the case of
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the cell with CYC-B1, electrochemical impedance spectra (EIS) were obtained at 1 sun illumination. Fig. 5b shows Nyquist plots of EIS of the DSSCs with CYC-B1 and N3 dyes. The equivalent circuit is shown as the inset in Fig. 5b. The Nyquist plots essentially consist of two sets of semicircles. The smaller semicircle at higher frequencies corresponds to the charge-transfer resistances at the counter
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electrode/electrolyte interface (Rct1) [41,47]. The larger circle at lower frequencies (10~100 Hz range) represents the resistances of the charge-transfer process at the
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ZnO/dye/electrolyte interface (Rct2), and that of ion diffusion within the electrolyte (Rdiff) [41,47,48]. Rdiff is virtually overlapped by Rct2 due to the short length for I–
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diffusion available with the thin spacer used (25 μm thick), and owing to the low viscosity of the solvents used in our electrolyte (viscosities of ACN and MPN are
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0.37 cp and 1.60 cp, respectively). It can be seen in Fig. 5b that Rct2 value decreases for the cell with CYC-B1 dye compared with that of the cell with N3 dye; this decrease of Rct2 value is consistent with the larger JSC and FF values of the cell with
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CYC-B1 dye, compared to those of the cell with N3 dye.
In addition, laser-induced photo-current transients of the two ZnO-based
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DSSCs with CYC-B1 and N3 dyes were obtained, in order to confirm the higher electron transport in the ZnO film with CYC-B1 dye, as compared to that in the ZnO
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film with N3 dye (Fig. 6). The characteristic time constant τc could be fitted by exponential decay process, and the electron diffusion coefficient De could be calculated from Eq (1). The fitted τc values are 0.19 and 0.26 ms, and the calculated De values are 2.24×10–3 and 1.64×10–3 cm2 s–1 for the ZnO electrodes with CYC-B1 dye and N3 dye, respectively. The higher electron transport in the ZnO film in the
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case of the cell with CYC-B1dye thus explains the higher JSC of this DSSC.
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3.4 Characterization of the synthesized PMMA spheres and their double-layer ZnO film
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It has been shown that larger pores (~300 nm) in the TiO 2 electrode can increase the light-scattering distance, giving higher JSC values [49, 50]. In order to
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further enhance the efficiency of the cell with CYC-B1dye, incorporation of PMMA spheres into the ZnO paste to form the micro-pore after sintering was considered. Thus, poly (methyl methacrylate), PMMA spheres were synthesized and
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incorporated into the ZN20 paste (PMMA-ZN20 paste); this PMMA-ZN20 paste was coated as a sacttering layer, and coated on the ZN20 film for the DSSC with
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CYC-B1 dye. Fig. 7a shows SEM image of the PMMA spheres; the image shows PMMA spheres of uniform size, i.e., ca. 300 nm. The inset in Fig. 7a shows a highly
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magnified version of the spheres. Fig. 7b shows the thermogravimetric trace of the PMMA spheres; the trace shows that the decomposition temperature of the PMMA spheres is about 300 oC; this decomposition temperature explains the fact that the PMMA-ZN20 overlayer can show micro-pores after its sintering at 450 oC. Fig. 8a shows the cross-sectional SEM image of the ZN20 film with the overlayer of
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show the top view images for the underlayer and overlayer, respectively. A flat surface with nano-pores and a rough surface with micro-pores can be clearly seen for
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the underlayer (ZN20 film) and scattering layer (PMMA-ZN20 film), respectively.
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3.5 Photovoltaic characteristics and IPCE measurements of the DSSCs with or without PMMA-templated ZnO films
To further study the application of the double layer film for a DSSC, we
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fabricated three types of ZnO photoanodes, one, consisting only of a 10 μm underlayer, the second only of a 15 μm underlayer, and the third consisting of a
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double layer with 10 μm of underlayer (ZN20 film) and 5 μm of scattering layer (PMMA-ZN20 film). Fig. 9a shows the photocurrent density-voltage curves of the
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DSSCs with the three photoanodes described above, and Table 3 gives their corresponding photovoltaic parameters. It can be seen in Table 3 that the optimal film with the double layer, with 10 μm of underlayer and 5 μm of scattering layer, shows the highest η of 5.40±0.03%; this highest η is due to the significant improvement in JSC, with reference to those of the cells with only underlayers. The
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enhancement in JSC for the cell with the double layer could further be supported by incident photo-to-current conversion efficiency (IPCE) spectra of the cells. Fig. 9b
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shows that the DSSC with the double layer has the highest IPCE value of about 70%. The higher IPCE values of the cell with PMMA-ZN20 in the higher wavelength
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range, i.e., from 600 to 750 nm, are attributable to PMMA-ZN20. It is assumed that relatively large pores in the film with PMMA-ZN20 extends the travel distance for
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light scattering and thereby leads to a higher JSC value for this DSSC [50]. The JSC values of the cells with 10 and 15 µm of underlayers are also consistent with their corresponding IPCE values.
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Despite that the cell efficiency of 100% ZnO-based DSSCs are still lower than those of TiO2-based DSSCs, strategies to improve the efficiency of a ZnO-based
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DSSC are being investigated worldwide [12-38]. Systematic work is in progress in our laboratory for further increasing the performance of the ZnO-based DSSCs with
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CYC-B1 dye, by incorporating well-aligned and single-crystalline ZnO nanowires into the ZnO-particle-based photoanode, which may facilitate more rapid electron transport [23].
4. Conclusions
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In summary, an efficient dye, CYC-B1, with an alkyl bithiophene group, high extinction coefficient and absorbance has proven its potential to enable an
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aggregation-free ZnO surface morphology even after 24 h of dying the ZnO surface with it, where the routinely used N3 dye failed completely. The ZnO-based DSSC,
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sensitized by the CYC-B1 dye showed an η of 4.85±0.03%. The dye-immersion time of the ZnO film (ZN20) to achieve this efficiency was optimized at 24 h. Impedance
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measurements under open-circuit voltage have provided evidence that the charge injection kinetics are faster in the case of a DSSC with a ZN20 film sensitized with CYC-B1 dye, compared with one sensitized by N3 dye. Transient photo-current curves
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have shown faster electron transport in the ZN20 film with CYC-B1 dye, compared to that in the ZN20 film with N3 dye. PMMA spheres with uniform sizes of ca. 300 nm
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were synthesized and incorporated to template the ZN20 film (PMMA-ZN20 film). The PMMA-ZN20 film was coated as a scattering layer, and coated on the ZN20 (underlayer)
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film (this composite film is designated as ZN20/PMMA-ZN20). Thermogravimetric results for the PMMA spheres and the cross-sectional SEM image support the fact that the ZN20/PMMA-ZN20 film consists of micro-pores after sintering at 450 oC. The DSSC with the film of ZN20/PMMA-ZN20 showed a further improvement of the η (5.40±0.03%), with reference to the η of the cell without this PMMA (4.85±0.03%).
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This enhancement is due to a significant improvement in JSC, which is substantiated by
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IPCE measurements.
5. Acknowledgements
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This work was supported in part by the National Science Council of Taiwan. Some of the instruments used in this study were made available through the Academia Sinica,
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Taipei, Taiwan.
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References [1]
M. Law, L. E. Greene, J. C. Johnson, R. Saykally and P. D. Yang, Nature
[2]
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Materials, 2005, 4, 455. P. S. Archana, R. Jose, C. Vijila and S. Ramakrishna, J. Phys. Chem. C, 2009, 113,
SC
21538.
E. Hosono, S. Fujihara and T. Kimura, Electrochim. Acta, 2004, 49, 2287.
[4]
C. L. Kuo, T. J. Kuo and M. H. Huang, J. Phys. Chem. B, 2005, 109, 20115.
[5]
A. L. Pan, R. C. Yu, S. S. Xie, Z. B. Zhang, C. Q. Jin and B. S. Zou, J. Crystal Growth, 2005, 282, 165.
H. Zhang, D. R. Yang, D. S. Li, X. Y. Ma, S. Z. Li and D. L. Que, Cryst. Growth
TE D
[6]
M AN U
[3]
Des., 2005, 5, 547.
S. C. Hu, Z. W. Zhou and Z. R. Zhou, J. Crystal Growth, 2006, 287, 54.
[8]
S. Music, D. Dragcevic and S. Popovic, J. Alloys Compds., 2007, 429, 242.
[9]
S. Yang, C. Ge, Z. Liu, Y. Fang, Z. Li, D. Kuang and C. Su,
AC C
EP
[7]
RSC Adv., 2011, 1,
1691.
[10] M. Chen, Y. Wang, L. Song, P. Gunawan, Z. Zhong, X. She and F. Su, RSC Adv., 2012, 2, 4164. [11] Q. Tang, L. Lin, Z. Mao, Z. Tang and J. Wu, RSC Adv., 2012, 2, 2211.
24
ACCEPTED MANUSCRIPT
[12] A. B. F. Martinson, J. E. McGarrah, M. O. K. Parpia and J. T. Hupp, Phys. Chem. Chem. Phys., 2006, 8, 4655.
RI PT
[13] J. B. Baxter and E. S. Aydil, Sol. Energy Mater. Sol. Cells, 2006, 90, 607. [14] M. Vafaee and M. S. Ghamsari, Mater. Lett., 2007, 61, 3265.
SC
[15] K. Kakiuchi, M. Saito and S. Fujihara, Thin Solid Films, 2008, 516, 2026.
[16] A. B. F. Martinson, M. S. Góes, F. Fabregat-Santiago, J. Bisquert, M. J. Pellin and
M AN U
J. T. Hupp, J. Phys. Chem. A, 2009, 113, 4015.
[17] Y. H. Lai, C. Y. Lin, H. W. Chen, J. G. Chen, C. W. Kung, R. Vittal and K. C. Ho, J. Mater. Chem., 2010, 20, 9379.
TE D
[18] C. P. Lee, J. C. Lin, Y. C. Wang, C. Y. Chou, M. H. Yeh, R. Vittal and K. C. Ho, Phys. Chem. Chem. Phys., 2011, 13, 20999.
EP
[19] J. C. Lin, C. P. Lee and K. C. Ho, J. Mater. Chem. 2012, 22, 1270. [20] Q. Tang, L. Lin, Z. Mao and J. Wu, RSC Adv., 2012, 2, 1863.
AC C
[21] Z. Yang, T. Xu, Y. Ito, U. Welp and W. K. Kwok, J. Phys. Chem. C, 2009, 113, 20521.
[22] Y. Xie, P. Joshi, S. B. Darling, Q. Chen, T. Zhang, D. Galipeau and Q. Qiao, J. Phys. Chem. C, 2010, 114, 17880. [23] C. T. Wu, W. P. Liao and J. J. Wu, J. Mater. Chem. 2011, 21, 2871.
25
ACCEPTED MANUSCRIPT
[24] Y. Z. Zheng, X. Tao, Q. Hou, D. T. Wang, W. L. Zhou and J. F. Chen, Chem. Mater., 2011, 23, 3.
RI PT
[25] A. Otsuka, K. Funabiki, N. Sugiyama, T. Yoshida, H. Minoura and M. Matsui, Chem. Lett., 2006, 35, 666.
SC
[26] E. Guillén, F. Casanueva, J. A. Anta, A. Vega-Poot, G. Oskam, R. Alcántara, C. Fernández-Lorenzo and J. Martín-Calleja, J. Photochem. Photobiol. A: Chem.,
M AN U
2008, 200, 364.
[27] X. F. Wang, O. Kitao, E. Hosono, H. Zhou, S. Sasaki and H. Tamiaki, J. Photochem. Photobiol. A: Chem., 2010, 210, 145.
TE D
[28] T. Yoshida, J. Zhang, D. Komatsu, S. Sawatani, H. Minoura, T. Pauporte, D. Lincot, T. Oekermann, D. Schlettwein, H. Tada, D. Wohrle, K. Funabiki, M.
EP
Matsui, H. Miura and H. Yanagi, Adv. Funct. Mater., 2009, 19, 17. [29] S. Ito, S. Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet, P. Comte, M.
AC C
Nazeeruddin, P. Pechy, M. Takata, H. Miura, S. Uchida and M. Grätzel, Adv.
Mater., 2006, 18, 1202.
[30] H. Cheng and W. Hsieh, Energy Environ. Sci., 2010, 3, 442. [31] (a) S. Higashijima, H. Miura, T. Fujita, Y. Kubota, K. Funabiki, T. Yoshida and M. Matsui, Tetrahedron, 2011, 67, 6289. (b) S. Higashijima, Y. Inoue, H. Miura, Y.
26
ACCEPTED MANUSCRIPT
Kubota, K. Funabiki, T. Yoshida and M. Matsui, RSC Adv., 2012, 2, 2721. [32] Q. F. Zhang, C. S. Dandeneau, X. Y. Zhou and G. Z. Cao, Adv. Mater., 2009, 21,
[33] F. Xu and L. Sun, Energy Environ. Sci. 2011, 4, 818.
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4087.
SC
[34] I. Bedja, P. V. Kamat, X. Hua, P. G. Lappin and S. Hotchandani, Langmuir, 1997, 13, 2398.
M AN U
[35] H. Horiuchi, R. Kaoh, K. Hara, M. Yanagida, S. Murata, H. Arakawa and M. Tachiya, J. Phys. Chem. B, 2003, 107, 2570.
[36] K. Keis, J. Lindgren, S. E. Lindquist and A. Hagfeldt, Langmuir, 2006, 16, 4688.
2009, 113, 9206.
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[37] H. M. Nguyen, R. S. Mane, T. Ganesh, S. H. Han and N. Kim, J. Phys. Chem. C,
EP
[38] T. P. Chuo, Q. F. Zhang and G. Z. Cao, J. Phys. Chem. C, 2007, 111, 18804. [39] C. Y. Chen, S. J. Wu, C. G. Wu, J. G. Chen and K. C. Ho, Angew. Chem. Int. Ed.,
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2006, 45, 5822.
[40] C. G. Chen, C. Y. Chen, S. J. Wu, J. Y. Li, J. G. Wu and K. C. Ho, Sol. Energy Mater. Sol. Cells, 2006, 45, 5822.
[41] L. Han, N. Koide, Y. Chiba and T. Mitate, Appl. Phys. Lett., 2004, 84, 2433. [42] L. Han, N. Koide, Y. Chiba, A. Islam and T. Mitate, C. R. Chimie., 2006, 9, 645.
27
ACCEPTED MANUSCRIPT
[43] J. van de Lagemaat and A. J. Frank, J. Phys. Chem. B, 2001, 105, 11194. [44] N. A. Anderson, X. Ai and T. J. Lian, J. Phys. Chem. B, 2003, 107, 14414.
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[45] G. Redmond, D. Fitzmaurize and M. Grätzel, Chem. Mater., 1994, 6, 686. [46] K. Keis, C. Bauer, G. Boschloo, A. Hagfeldt, K. Westermark, H. Rensmo and H. Seigbahn, J. Photochem. Photobiol., A: Chem., 2002, 57, 148.
SC
[47] Z. Zhang, S. M. Zakeeruddin, B. C. O’ Regan, R. Humphry-Baker and M. Grätzel,
M AN U
J. Phys. Chem. B, 2005, 109, 21818.
[48] R. Kern, R. Sastrawan, J. Ferber, R. Stangl and J. Luther, Electrochim. Acta, 2002, 47, 4213.
2005, 2011.
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[49] S. Hore, P. Nitz, C. Vetter, C. Prahl, M. Niggemann and R. Kern, Chem. Commun.,
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[50] (a) K. M. Lee, C. Y. Hsu, W. H. Chiu, M. C. Tsui, Y. L. Tung, S. Y. Tsai and K. C. Ho, Sol. Energy Mater. Sol. Cells, 2009, 93, 2003. (b) J. G. Chen, C. Y. Chen, C.
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G. Wu and K. C. Ho, J. Phys. Chem. C, 2010, 114, 13832. (c) C. Y. Chou, C. P.
Lee, R. Vittal and K. C. Ho, J. Power Sources, 2011, 196, 6595.
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List of Tables Table 1. Photovoltaic parameters of the DSSCs sensitized with CYC-B1 dye for
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different periods of time, obtained at 100 mW cm–2 light intensity. Table 2. Photovoltaic parameters of the DSSCs, obtained at 100 mW cm–2 light
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intensity, in which the photoanodes of the cells were sensitized with CYC-B1 dye and N3 dye for 24 h. The table also shows both Rct2 and De obtained from
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electrochemical impedance spectroscopic and transient photo-current curves, respectively.
Table 3. Photovoltaic parameters of the DSSCs with photoanodes consisting of 10 μm
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of ZN20 (underlayer), 15 μm of Zn20 (underlayer), and 10 μm of ZN20 (underlayer) with 5μm of PMMA-ZN20 (overlayer), measured at 100 mW
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cm–2 light intensity. All the photoanodes of the cells were sensitized with
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CYC-B1 dye for 24 h.
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The structure of CYC-B1 [39].
Fig. 2
(a) and (b) SEM images of bare ZnO film at different magnifications; (c) and
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(d) SEM images of the same ZnO film at different magnifications after its dipping in N3 dye for 24 h.
(a) and (b) SEM images of the ZnO film at different magnifications, after its
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Fig. 3
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dipping in CYC-B1 dye solution for 24 h; (c) and (d) SEM images of the ZnO film at different magnifications, after its dipping in CYC-B1 dye solution for 48 h. Fig. 4
Photocurrent density-voltage curves of the DSSCs sensitized with CYC-B1
Fig. 5
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dye for different periods of time, obtained at 100 mW cm–2 light intensity. (a) Current density-voltage curves of the DSSCs, obtained both at 100 mW
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cm–2 light intensity and in the dark, in which the photoanodes of the cells were
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sensitized with CYC-B1 and N3 dyes for 24 h; (b) electrochemical impedance spectra of the cells in (a), measured at 100 mW cm–2 light intensity under the open-circuit voltage. The inset shows the equivalent circuit.
Fig. 6
Laser-induced photocurrent transients of the DSSCs with CYC-B1 and N3 dyes, for the sensitization time of 24 h.
Fig. 7
(a) SEM image of the PMMA spheres. The inset shows highly magnified
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version of the spheres; (b) thermogravimetric trace of the PMMA spheres. Fig. 8
(a) Cross-sectional SEM image of the composite film with the underlayer of
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ZN20 and overlayer of PMMA-ZN20, (b) top view of ZN20 film, and (c) top view of PMMA-ZN20 film. Fig. 9
(a) Photocurrent density-voltage curves of the DSSCs with photoanodes
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consisting of 10 μm of ZN20 (underlayer), 15 μm of Zn20 (underlayer), and
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10 μm of Zn20 (underlayer) with 5μm of PMMA-ZN20 (overlayer), measured at 100 mW cm–2 light intensity; (b) corresponding incident photo-to-current
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conversion efficiency (IPCE) curves.
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Table 1
VOC (mV)
JSC (mA cm–2)
FF
η (%)
3
507±7
10.78±0.21
0.52±0.01
2.84±0.07
6
524±4
12.85±0.20
12
539±3
13.07±0.07
24
562±4
13.75±0.05
48
547±4
12.62±0.10
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Dye-loading time (h)
3.49±0.08
0.56±0.01
3.97±0.06
0.63±0.00
4.85±0.03
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0.52±0.01
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0.53±0.00
3.63±0.06
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Table 2
VOC (mV)
JSC (mA cm–2)
FF
η (%)
Rct2 (ohm)
De (10–3 cm2s–1)
CYC-B1
562±4
13.75±0.05
0.63±0.00
4.85±0.03
25.61
2.24
N3
549±7
10.17±0.38
0.45±0.01
2.50±0.12
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Dye
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36.43
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1.64
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Table 3
VOC (mV)
JSC (mA cm–2)
FF
η (%)
10 μm ZN20
562±4
13.75±0.05
0.63±0.00
4.85±0.03
15 μm ZN20
550±2
14.57±0.03
10 μm ZN20/5 μm PMMA-ZN20
565±3
16.09±0.22
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ZnO film of the photoanode
4.44±0.04
0.59±0.01
5.40±0.03
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0.55±0.00
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Fig. 1
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(a)
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(b)
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100 nm
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400 nm
(d)
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Fig. 2
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100 nm
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(b)
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(c)
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Fig. 3
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100 nm
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0
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4
100
200
300
Fig. 4
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38
400
500
600
(201)
8
(c)
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CYC-B1 3h 6h 12 h 24 h 48 h
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Photocurrent density (mA cm )
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(a) 16
-2
8
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24 h dying time CYC-B1 N3
4 0
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Current density (mA cm )
Under 100 mW cm
Under dark
0
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200
300
400
500
600
Voltage (mV)
(b)
30
RC R ct1 t1
ZZW R w diff
RCct2t2 R
RSs R
20
CPE2 CPE2
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CPE1 CPE1
10
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-Z'' (ohm)
24 h dying time CYC-B1 N3
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0
10
20
30 Z' (ohm)
Fig. 5
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40
50
60
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Photocurent (a.u.)
24 h dying time CYC-B1 N3
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τc= 0.19 ms, De= 2.24 × 10-3 cm2 s–1
τc= 0.26 ms, De= 1.64 × 10-3 cm2 s–1
0.0
0.5
1.0
1.5
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2.5
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(b)
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Mass loss (%)
80
PMMA
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100
4 μm
0 100
200
300
400 o
Temperature ( C)
Fig. 7
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500
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5 μm
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Fig. 8
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(a) -2
Photocurrent density (mA cm )
18
12 9
CYC-B1, 24 h dying time 10 m ZN20 15 m ZN20 10 m ZN20/5 m PMMA-ZN20
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100
200
300
400
500
600
Voltage (mV)
(b)
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CYC-B1, 24 h dying time 10 m ZN20 20 15 m ZN20 10 m ZN20/5 m PMMA-ZN20
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IPCE (%)
60
0
400
500
600 Wavelength (nm)
Fig. 9
43
700
800