pubs.acs.org/Langmuir © 2010 American Chemical Society
Incorporating Hierarchical Nanostructured Carbon Counter Electrode into Metal-Free Organic Dye-Sensitized Solar Cell Baizeng Fang,† Sheng-Qiang Fan,†,‡ Jung Ho Kim,† Min-Sik Kim,† Minwoo Kim,† Nitin K. Chaudhari,† Jaejung Ko,*,† and Jong-Sung Yu*,† †
Department of Advanced Materials Chemistry, BK21 Reasearch Team, Korea University, 208 Seochang, Jochiwon, ChungNam 339-700, Republic of Korea, and ‡Department of Applied Chemistry, School of Science, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China Received February 6, 2010. Revised Manuscript Received March 11, 2010
Hierarchical nanostructured carbon with a hollow macroporous core of ca. 60 nm in diameter in combination with mesoporous shell of ca. 30 nm in thickness has been explored as counter electrode in metal-free organic dye-sensitized solar cell. Compared with other porous carbon counterparts such as activated carbon and ordered mesoporous carbon CMK-3 and Pt counter electrode, the superior structural characteristics including large specific surface area and mesoporous volume and particularly the unique hierarchical core/shell nanostructure along with 3D large interconnected interstitial volume guarantee fast mass transport in hollow macroporous core/mesoporous shell carbon (HCMSC), and enable HCMSC to have highly enhanced catalytic activity toward the reduction of I3-, and accordingly considerably improved photovoltaic performance. HCMSC exhibits a Voc of 0.74 V, which is 20 mV higher than that (i.e., 0.72 V) of Pt. In addition, it also demonstrates a fill factor of 0.67 and an energy conversion efficiency of 7.56%, which are markedly higher than those of its carbon counterparts and comparable to that of Pt (i.e., fill factor of 0.70 and conversion efficiency of 7.79%). Furthermore, HCMSC possesses excellent chemical stability in the liquid electrolyte containing I-/I3- redox couples, namely, after 60 days of aging, ca. 87% of its initial efficiency is still achieved by the solar cell based on HCMSC counter electrode.
1. Introduction Green and renewable energies are considered to be technological drivers of the future economy. In the past few decades, significant progress has been made in the development of renewable energy technologies such as solar cells and fuel cells. Photovoltaic technology is one of the most favorable ways to convert solar energy into electricity and photovoltaic cells such as dye-sensitized solar cells (DSSCs) have attracted much attention as the main energy source in the future.1 DSSCs, which incorporate a porousstructured oxide film with adsorbed dye molecules as the photosensitized anode, a platinized fluorine-doped tin oxide (FTO) glass as the counter electrode, and a liquid electrolyte traditionally containing I-/I3- redox couples as a conductor,2-4 have some advantages over traditional Si-solar cells such as low production cost and relatively high energy conversion efficiency.5-7 However, further enhancement in conversion efficiency and decrease in production cost are highly desired for commercialization of DSSCs.8,9 To lower the production costs, DSSCs based on oxide semiconductors and organic dyes or metallorganic-complex dyes have *To whom correspondence should be addressed. E-mail: jsyu212@ korea.ac.kr (J.-S.Y.);
[email protected] (J.K.).
(1) Dresselhaus, M. S.; Thomas, I. L. Nature 2001, 414, 332. (2) Gr€atzel, M. Nature 2001, 414, 338. (3) Bisquert, J.; Cahen, D.; Hodes, G.; Ruhle, S.; Zaban, A. J. Phys. Chem. B 2004, 108, 8106. (4) Martinson, A. B. F.; Hamann, T. W.; Pellin, M. J.; Hupp, J. T. Chem.;Eur. J. 2008, 14, 4458. (5) Nazeeruddin, M. K.; Angelis, F. De; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gr€atzel, M. J. Am. Chem. Soc. 2005, 127, 16835. (6) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Y. Jpn. J. Appl. Phys. 2006, 45, L638. (7) Cao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gr€atzel, M. J. Am. Chem. Soc. 2008, 130, 10720. (8) Green, M. A. Sol. Energy 2004, 76, 3. (9) Bagnall, D. M.; Boreland, M. Energy Polym. 2008, 36, 4390.
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recently emerged as promising approaches to efficient solar-energy conversion. Compared with solar cells based on metallorganiccomplex dyes, metal-free organic photovoltaic systems hold the promise for a cost-effective,10-12 and lightweight solar energy conversion platform,13-15 which could benefit from simple solution processing of the active layer. In addition, metal-free organic photovoltaic systems offer the potential to change our energy landscape due to their other advantages such as the versatility of organic materials design.16,17 Furthermore, metal-free organic sensitizers are an attractive choice because the molar extinction coefficients of organic dyes are typically 2 to 3 times higher than those of the ruthenium complexes, and the photovoltaic conversion efficiencies of organic dye-based DSCs are getting closer to the performance of ruthenium-based DSCs with ion liquid-based electrolytes.18,19 The counter electrode is an equally important component of DSSC.20 FTO loaded with platinum has been frequently used as the counter electrode in DSSCs for its high catalytic activity (10) Hara, H.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. J. Phys. Chem. B 2003, 107, 597. (11) Hagberg, D. P.; Edvinsson, T.; Marinado, T.; Boschloo, G.; Hagfeldt, A.; Sun, L. Chem. Commun. 2006, 21, 2245. (12) Qin, H.; Wenger, S.; Xu, M.; Gao, F.; Wang, P.; Gr€atzel, M. J. Am. Chem. Soc. 2008, 130, 9202. (13) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183. (14) Hiramoto, H.; Fujiwara, H.; Yokoyama, M. Appl. Phys. Lett. 1991, 58, 1062. (15) Sariciftci, N. S.; Braun, D.; Zhang, C.; Srdanov, V. I.; Heeger, A. J.; Stucky, G.; Wudl, F. Appl. Phys. Lett. 1993, 62, 585. (16) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864. (17) Brabec, C. J. Sol. Energy Mater. Sol. Cells 2004, 83, 273. (18) Zhang, G.; Bai, Y.; Li, R.; Shi, D.; Wenger, S.; Zakeeruddin, S. M.; Gr€atzel, M.; Wang, P. Energy Environ. Sci. 2009, 2, 92. (19) Xu, M.; Wenger, S.; Bala, H.; Shi, D.; Li, R.; Zhou, Y.; Zakeeruddin, S. M.; Gr€atzel, M.; Wang, P. J. Phys. Chem. C 2009, 113, 2966. (20) Kay, A.; Gr€atzel, M. Sol. Energy Mater. Sol. Cells 1996, 44, 99.
Published on Web 03/24/2010
Langmuir 2010, 26(13), 11238–11243
Fang et al. Scheme 1. Typical Scheme for Synthesis of HCMSC Capsule
toward triiodide reduction. However, cost consideration necessitates the development of alternative material. In addition, there are also some reports of corrosion of Pt in triiodide-containing solutions.20,21 Therefore, it is highly desirable to develop alternative cheap materials for the counter electrodes. Recently, much attention has been paid to porous carbon materials as potential alternative. To date, various carbon materials have been investigated as the counter electrode such as graphite,22 carbon nanotubes,23,24 activated carbon,22 carbon black,25 hard carbon spherule,26 nanocarbon,27,28 and mesoporous carbon.29 However, energy conversion efficiency (η) is still very low for the metal-free organic DSSCs, ca. 6.18% for the solar cell using N3 dye and mesoporous carbon as the counter electrode.29 Therefore, further enhancement in η is necessary for practical application of DSSCs. In this study, hollow macroporous core/mesoporous shell carbon (HCMSC) with a hierarchical nanostructure was explored as a counter electrode in DSSC. For comparison, ordered mesoporous carbon CMK-3 and commercially available activated carbon (AC) (Duksan Pharm. Co., Korea) were also investigated. Because of its superior structural characteristics such as large specific surface area and mesoporous volume and particularly unique hierarchical nanostructure consisting of hollow macroporous core and mesoporous shell, the HCMSC outperforms other carbon counterparts and demonstrates a photovoltaic performance comparable to that of the Pt electrode.
2. Experimental Section 2.1. Fabrication of HCMSC. HCMSC was fabricated by replication through nanocasting of solid core/mesoporous shell (SCMS) silica.30,31 A representative scheme for synthesis of HCMSC capsule is shown in Scheme 1. Typically, 5 mL of aqueous ammonia (32 wt %) was added to a solution containing 125 mL of ethanol and 10 mL of deionized water. After stirring at 30 °C for ca. 15 min, 0.5-1.5 mL of tetraethyl orthosilicate (TEOS, 98%, ACROS) was added to the above-prepared mixture, and the reaction mixture was stirred for about 4-6 h to yield uniform silica spheres. A mixture solution containing 1.2 mL of TEOS and 0.7 mL of n-octadecyltrimethoxysilane (C18-TMS, 90% tech., Aldrich) was added to the colloidal solution containing the silica spheres and further reacted for 1 h. The resulting (21) Olsen, E.; Hagen, G.; Lindquist, S. E. Sol. Energy Mater. Sol. Cells 2000, 63, 267. (22) Imoto, K.; Takahashi, K.; Yamaguchi, T.; Komura, T.; Nakamura, J.; Murata, K. Sol. Energy Mater. Sol. Cells 2003, 79, 459. (23) Jang, S.-R.; Vittal, R.; Kim, K. Langmuir 2004, 20, 9807. (24) Lee, W. J.; Ramasamy, E.; Lee, D.-Y.; Song, J.-S. ACS Appl. Mater. Interface 2009, 1, 1145. (25) Murakami, T. N.; Ito, S.; Wang, Q.; Nazeeruddin, M. K.; Bessho, T.; Cesar, I.; Liska, P.; Humphry-Baker, R.; Comte, P.; Pechy, P.; Gr€atzel, M. J. Electrochem. Soc. 2006, 153, A2255. (26) Huang, Z.; Liu, X.; Li, K.; Li, D.; Luo, Y.; Li, H.; Song, W.; Chen, L.; Meng, Q. Electrochem. Commun. 2007, 9, 596. (27) Lee, W.-J.; Ramasamy, E.; Lee, D.-Y.; Song, J.-S. Sol. Energy Mater. Sol. Cells 2008, 92, 814. (28) Ramasamy, E.; Lee, W.-J.; Lee, D.-Y.; Song, J.-S. Appl. Phys. Lett. 2007, 90, 173103. (29) Wang, G.; Xing, W.; Zhuo, S. J. Power Sources 2009, 194, 568. (30) Fang, B.; Kim, M.-S.; Kim, J.-H.; Yu, J.-S. Langmuir 2008, 24, 12068. (31) Fang, B.; Kim, J.-H.; Kim, M.-S.; Kim, M.-W.; Yu, J.-S. Phys. Chem. Chem. Phys. 2009, 11, 1380.
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Article octadecyl group incorporated silica shell/solid core nanocomposite was retrieved by centrifugation, dried at room temperature and further calcined at 823 K for 6 h under an oxygen atmosphere to produce the final SCMS silica material. Aluminum was incorporated into the silicate framework through an impregnation method to produce acidic points on the surface of SCMS silica, which will catalyze polymerization of phenol and paraformaldehyde. A total of 1.0 g of SCMS silica was added to an aqueous solution containing 0.27 g of AlCl3 3 6H2O in 0.3 mL of water and the resulting slurry was stirred for 30 min. The powder was dried in air at 353 K. Finally, the Al-impregnated SCMS silica was calcined at 823 K for 5 h in air to yield SCMS aluminosilicate. A typical synthesis route for HCMSC capsules is as follows.32 First, 0.374 g of phenol was incorporated into the mesopores of 1.0 g of SCMS template by heating at 100 °C for 12 h under vacuum. The resulting phenol-incorporated SCMS template was reacted with paraformaldehyde (0.238 g) under vacuum at 130 °C for 24 h to yield a phenol-resin/SCMS aluminosilicate composite. The composite was heated to 160 °C at 1 K/min and held for 5 h under a nitrogen flow. The temperature was then raised to 950 °C at 3 K/min and held for 7 h to carbonize the cross-linked phenol resin inside the mesopores of the SCMS structure. The SCMS silica template was dissolved by using 2.0 N NaOH at 80 °C overnight, and the slurry washed with ethanol-H2O solution, and the as-prepared HCMSC dried at 80 °C overnight. 2.2. Fabrication of CMK-3. CMK-3 was fabricated by replication through nanocasting of SBA-15 silica and using phenol as the carbon source.33 Rod type SBA-15 silica ca. 1.2 um in length and ca. 800 nm in diameter was synthesized according to the procedures reported elsewhere.34 Aluminum was also incorporated into the silicate framework through the same impregnation method as in SCMS silica to produce acidic sites on the surface of SBA-15 silica. A typical synthesis route for CMK-3 is as follows. The same method was followed using phenol-paraformaldehyde to form phenol-resin/SBA-15 like in the synthesis of HCMSC. Excess resin was removed by vacuum at ambient temperature. The resultant polymer/SBA-15 silica composite was heated under N2 gas flow to 950 °C at a ramping rate of 3 K/min, and then carbonized at 950 °C for 7 h to produce carbon/SBA-15 silica composite. The silica template was dissolved by soaking the composite in a 2.0 M NaOH solution for 10 min followed by heating in an oven at 80 °C overnight. The template-free carbon product thus obtained was filtered, washed with ethanol and dried at 393 K overnight. 2.3. Surface Characterization of Carbon Materials. The morphologies of the carbon materials (i.e., HCMSC, CMK-3, and AC) were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images were obtained using a Hitachi S-4700 microscope operated at an acceleration voltage of 10 kV, and TEM analysis was operated on EM 912 Omega at 120 kV. N2 adsorption and desorption isotherms were measured at 77 K on a KICT SPA-3000 Gas Adsorption Analyzer after the carbon was degassed at 423 K to 20 μTorr for 12 h. The specific surface areas were determined from nitrogen adsorption using the Brunauer-Emmett-Teller (BET) equation. Total pore volume was determined from the amount of gas adsorbed at the relative pressure of 0.99. Micropore volumes of the porous carbons were calculated from the analysis of the adsorption isotherms using the Horvath-Kawzoe method. Pore size distribution (PSD) was derived from the analysis of the adsorption branch using the Barrett-Joyner-Halenda (BJH) method.
2.4. Electrode Preparation, Cell Configuration, and Photovoltaic Performance Tests. Carbon counter electrodes were fabricated as follows. First, 100 mg of HCMSC (or CMK-3, (32) Fang, B.; Kim, J.-H.; Lee, C.-G.; Yu, J.-S. J. Phys. Chem. C 2008, 112, 639. (33) Yoon, S. B.; Kim, J. H.; Kooli, F.; Lee, C. W.; Yu, J, -S. Chem. Commun. 2003, 14, 1740. (34) Kang, S.; Chae, Y, B.; Yu, , J.-S. J. Nanosci. Nanotechnol. 2009, 9, 527.
DOI: 10.1021/la100564c
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Article AC) was dispersed in 10 mL of ethanol and ultrasonicated for 30 min followed by overnight magnetic stirring. The carbon slurry was then coated onto an FTO glass substrate (Pilkington TEC Glass-TEC 8, Solar 2.3 mm thickness) with a carbon loading of ca. 120 μg cm-2 by using a doctor blade. Finally, the electrodes were heat-treated at 400 °C in air for 5 min using a hot-wind gun (Leister CH6060 Sarnen, Switzerland). For comparison, the Pt counter electrode was prepared with an optimized Pt loading of ca. 50 μg cm-2.35 Thickness and thickness distribution of each counter electrode were examined with a surface profilometer (alpha-step 200, Tencor Instruments, San Jose, CA). The thickness was 9 ( 2 μm for HCMSC and AC, 8 ( 2 μm for CMK-3, and 3 ( 1 μm for Pt. The TiO2 anode was prepared as reported elsewhere.35 Typically, first, TiO2 films of 10 μm in thickness were fabricated on FTO glass plates by using a doctor blade printing TiO2 paste (Solaronix, Ti-Nanoxide T/SP). Scattering layers were deposited by doctor blade printing a paste containing 400 nm-sized anatase particles (CCIC, PST-400C). After being sintered at 500 °C for 30 min, the films were treated with a 40 mM of TiCl4 aqueous solution followed by heat treatment at 500 °C for 30 min. TiO2 electrodes were fabricated by immersing TiO2 films into a metalfree organic dye 3-{50 -[N,N-bis(9,9-dimethylfluorene-2-yl)phenyl]2,20 -bisthiophene-5-yl}-2-cyanoacrylic acid- (JK2-) ethanol solution (0.3 mM) containing 3a,7a-dihydroxy-5b-cholic acid (10 mM). The dye-adsorbed TiO2 electrode (surface area: ca. 0.2 cm2) and the counter electrode (surface area: ca. 0.2 cm2) were assembled into a sealed sandwich-type cell, which were separated by using a 50 μm-thick hot-melt ionomer film (Surlyn) as a spacer. The electrolyte contained 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPII), 0.05 M I2, 0.1 M LiI, and 0.5 M 4-tert-butylpyridine in acetonitrile. The J-V curves of solar cells were measured under illumination of 100 mW/cm2 using 1000 W xenon light source, whose power of an AM 1.5 Oriel solar simulator was calibrated by using KG5 filtered Si reference solar cell. The incident photon-to-current conversion efficiency (IPCE) spectra for the cells were measured on an IPCE measuring system. For long-term chemical stability tests, continuous illumination with light of 100 mW cm-2 was conducted during the testing period.
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Figure 1. Representative SEM (a) and TEM (b) images for the HCMSC (inset in part b: HRTEM and HRSEM images).
2.5. Electrochemical Characteristics of Various Electrode Materials. Various electrochemical techniques such as cyclic voltammetric (CV) and electrochemical impedance spectroscopy (EIS) measurements were employed to characterize the various counter electrode materials. CV measurements were carried out in a N2-purged acetonitrile solution containing 0.1 M LiClO4, 0.45 mM LiI, and 0.05 mM I2. Various carbon materials or Pt was used as the working electrode, Pt coil as the counter electrode and Ag/AgCl as the reference electrode. After the measurement, ferrocene was added as the internal reference for potential calibration. All the potentials shown in CV plots were transformed to ones versus NHE. EIS measurements were carried out with a sandwich cell composed of two identical counter electrodes with an apparent surface area of 0.5 cm2, a 100-μm-thick Surlyn film as a spacer and an electrolyte consisted of 0.6 M DMPII, 0.05 M I2, 0.1 M LiI, and 0.5 M 4-tert-butylpyridine in acetonitrile. For EIS measurements, an impedance analyzer (Parstat 2273, Princeton) was employed, and a zero-bias potential and 10 mV of amplitude were applied over the frequency range of 100 kHz to 0.1 Hz.
3. Results and Discussion 3.1. Characteristics of the HCMSC. Figure 1 shows representative SEM and TEM images of the HCMSC. From the SEM image, it is evident that the HCMSC capsules are generated (35) Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J. H.; Fantacci, S.; De Angelis, F.; Censo, D. Di; Nazeeruddin, M. K.; Gr€atzel, M. J. Am. Chem. Soc. 2006, 128, 16701.
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Figure 2. Typical nitrogen adsorption-desorption isotherms at 77 K and the derived PSD (the inset) for the HCMSC.
as uniform individual discrete particles with a particle size of 120 ( 10 nm. The TEM image reveals that the HCMSC has a hollow macroporous core of ca. 60 nm in diameter and shell thickness of ca. 30 nm. High resolution TEM and SEM images shown in the inset of Figure 1b reveal the mesoporous disordered channels and mesopores with diameters of ca. 3 nm in the shell of the HCMSC. The nitrogen adsorption-desorption isotherms shown in Figure 2 can be classified as a type IV isotherm with a type H2 hysteresis, according to IUPAC nomenclature and reveals a narrow PSD for the HCMSC, proving again that the HCSMC has mesopores in the shell. The pore size was estimated to be ca. 3.5 nm from the PSD maximum. The HCMSC capsules show a BET surface area of 980 m2/g and a total pore volume Langmuir 2010, 26(13), 11238–11243
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Table 1. Structural Parameters Based on N2 Adsorption-Desorption Isotherms of the Studied Porous Carbon Materials
Table 2. Photovoltaic parameters of DSSCs based on various counter electrode materials
material
SBET/m2 g-1
Vtotal/cm3 g-1
Vmeso/cm3 g-1
pore/nm
material
Jsc/mA cm-2
Voc/V
FF
η/%
HCMSC AC CMK-3
980 ( 50 1084 ( 40 1228 ( 50
1.10 0.81 1.49
0.86 0.38 1.04
3.5 ( 0.5 0.6 ( 0.2 3.5 ( 0.5
Pt HCMSC CMK-3 AC
15.95 ( 0.28 15.81 ( 0.49 15.65 ( 0.57 15.00 ( 0.65
0.72 ( 0.01 0.74 ( 0.01 0.73 ( 0.01 0.73 ( 0.01
0.70 ( 0.02 0.67 ( 0.03 0.61 ( 0.03 0.54 ( 0.03
7.79 ( 0.22 7.56 ( 0.23 7.03 ( 0.28 6.24 ( 0.34
Figure 3. Typical photovoltaic performance of DSSCs using various counter electrode materials.
of 1.10 cm3/g, which are mainly attributable to the presence of the mesopores (2 nm < pore size
