Preparation of a Nanoporous CaCO3-Coated TiO2 Electrode and Its

Oct 10, 2007 - As compared to bare TiO2, nanoporous CaCO3-coated TiO2 provided ..... Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, ...
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Langmuir 2007, 23, 11907-11910

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Preparation of a Nanoporous CaCO3-Coated TiO2 Electrode and Its Application to a Dye-Sensitized Solar Cell Sangwook Lee,† Jin Young Kim,† Sung Hun Youn,† Min Park,† Kug Sun Hong,*,† Hyun Suk Jung,*,‡ Jung-Kun Lee,*,§ and Hyunho Shin⊥ School of Materials Science and Engineering, Seoul National UniVersity, San 56-1, Shillim-dong, Kwanak-gu, Seoul 151-744, South Korea, School of AdVanced Materials Engineering, Kookmin UniVersity, Jeongneung-dong, Seongbuk-gu, Seoul 136-702, South Korea, Materials Science & Technology DiVision, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and Department of Ceramic Engineering, Kangnung National UniVersity, Kangnung, 210-702, South Korea ReceiVed June 20, 2007. In Final Form: August 16, 2007 A nanoporous CaCO3 overlayer-coated TiO2 thick film was prepared by the topotactic thermal decomposition of Ca(OH)2, and its performance as an electrode of a dye-sensitized solar cell was investigated. As compared to bare TiO2, nanoporous CaCO3-coated TiO2 provided higher specific surface area and, subsequently, a larger amount of dye adsorption; this in turn increased short-circuit current (Jsc). Furthermore, the CaCO3 coating demonstrated increased impedance at the TiO2/dye/electrolyte interface and increased the lifetime of the photoelectrons, indicating the improved retardation of the back electron transfer, which increases Jsc, open-circuit voltage (Voc), and fill factor (ff). Thereby, the energy conversion efficiency (η) of the solar cell improved from 7.8 to 9.7% (an improvement of 24.4%) as the nanoporous CaCO3 layer was coated onto TiO2 thick films.

Introduction Dye-sensitized solar cells (DSSCs) have received much attention as a promising alternative to conventional solar energy conversion devices because of their low cost production, environmentally friendly components, and relatively high-energy conversion efficiency.1-4 Previous studies on improving the solar energy conversion efficiencies of DSSCs have shown that one of the key parameters is the surface properties of the photoelectrode consisting of TiO2 nanoparticles. TiO2 nanoparticles have been extensively modified by using various oxide coating layers5 such as ZnO,6 SrO,7 Al2O3,8,9 Nb2O3,10 SrTiO3,11 and MgO.12,13 The improved cell efficiency of DSSCs employing the coating layers has been explained by the retarded back transfer of electrons to the electrolyte solution, because the insulating nature of the wide band gap coating layers increase the surface * Corresponding author. (K.S.H.) Tel: +82-2-880-8316; fax: +82-2886-4156; e-mail address: [email protected]. (H.S.J.) Tel: +822-910-4817; fax: +82-2-910-4320; e-mail address: [email protected]. (J.-K.L.) Tel: +1-505-667-2403; fax: +1-505-665-9030; e-mail address: [email protected]. † Seoul National University. ‡ Kookmin University. § Los Alamos National Laboratory. ⊥ Kangnung National University. (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry, B. R.; Mueller, E.; Vlachopolous, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (3) Gra¨tzel, M. Nature 2001, 414, 338. (4) Gra¨tzel, M. Inorg. Chem. 2005, 44, 6841. (5) Kay, A.; Gra¨tzel, M. Chem. Mater. 2002, 14, 2930. (6) Wang, Z.-S.; Huang, C.-H.; Huang, Y.-Y.; Hou, Y.-J.; Xie, P.-H.; Zhang, B.-W.; Cheng, H.-M. Chem. Mater. 2001, 13, 678. (7) Yang, S.; Huang, C.; Zhao, X. Chem. Mater. 2002, 14, 1500. (8) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. Chem. Commun. 2002, 14, 1464. (9) O’Regan, B. C.; Scully, S.; Mayer, A. C. J. Phys. Chem. B 2005, 109, 4616. (10) Chen, S. G.; Chappel, S.; Diamant, Y.; Zaban, A. Chem. Mater. 2001, 13, 4629. (11) Diamant, Y.; Chen, S. G.; Melamed, O.; Zaban, A. J. Phys. Chem. B 2003, 107, 1977. (12) Kumara, G. R. A.; Okuya, M.; Murakami, K.; Kaneko, S.; Jayaweera, V. V.; Tennakone, K. J. Photochem. Photobiol. A 2004, 164, 183. (13) Taguchi, T.; Zhang, X. T.; Sutanto, I.; Tokuhiro, K.; Rao, T. N.; Watanabe, H.; Nakamori, T.; Uragami, M.; Fujishima, A. Chem. Commun. 2003, 19, 2480.

resistance of TiO2 nanoparticles.14 A recent work15 has shown that the surface layer provides an additional functionality to the TiO2 electrode and thereby improves DSSCs’ energy conversion; when a nanoporous MgO layer was prepared using topotactic decomposition of Mg(OH)2, the amount of adsorbed dye increased as well. This finding indicates that controlling extrinsic parameters such as the specific surface area is desirable in improving the energy conversion efficiency of TiO2-based solar cells. In the present study, a nonmetal oxide-based material, CaCO3, has been selected as a coating layer on the TiO2 film because (1) its band gap energy is sufficiently high (6.0 eV) compared with that of TiO2, (2) its high isoelectric point (IEP) assists dye adsorption,16-18 and (3) nanoporous CaO (being finally converted to CaCO3) can be prepared using thermal topotactic decomposition of Ca(OH)2.19,20 Previous studies on CaCO3-coated TiO2 have focused only on the insulating nature and the high IEP of CaCO3.16-18 Here, we present that the CaCO3-coated TiO2 film with high surface porosity (an extrinsic parameter) can significantly enhance the energy conversion efficiently. Also, the physics underlying the improved efficiency of the surface-modified DSSCs is shown by the optical and electrical investigations on correlation between the CaCO3 coating and the energy conversion efficiency. Experimental Methods A reference TiO2 photoelectrode was prepared using nanocrystalline TiO2 (P25, Degussa, Germany). TiO2 films (0.5 cm × 0.5 cm) were screen-printed on transparent conducting glass (F-doped (14) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. J. Am. Chem. Soc. 2003, 125, 475. (15) Jung, H. S.; Lee, J.-K.; Nastasi, M.; Lee, S.-W.; Kim, J.-Y.; Park, J.-S.; Hong, K. S.; Shin, H. Langmuir 2005, 21, 10332. (16) Lee, S.; Kim, J. Y.; Hong, K. S.; Jung, H. S.; Lee, J.-K.; Shin, H. Sol. Energy Mater. Sol. Cells 2006, 90, 2405. (17) Okada, N.; Karupppuchamy, S.; Kurihara, M. Chem. Lett. 2005, 34, 16. (18) Wang, Z.-S.; Yanagida, M.; Sayama, K.; Sugihara, H. Chem. Mater. 2006, 18, 2912. (19) Glasson, D. R. J. Chem. Soc. 1956, 1506. (20) Beruto, D.; Barco, L.; Searcy, A.; Spinolo, G. J. Am. Ceram. Soc. 1980, 63, 439.

10.1021/la701826v CCC: $37.00 © 2007 American Chemical Society Published on Web 10/10/2007

11908 Langmuir, Vol. 23, No. 23, 2007 tin oxide, Pilkington, England) with a size of 1 cm × 2 cm. The screen-printed films were dried at 80 °C, and then were heat-treated at 450 °C for 1 h. The thickness of the TiO2 electrode was approximately 12 µm as measured by a field emission scanning electron microscope (FESEM, model JSM-6330F, Japan). To coat the CaCO3 layer on the TiO2 photoelectrode, a Ca(NO3)2 solution (0.03 M, in deionized water/ethanol (1/1)) and a NaOH solution (0.06 M, in deionized water/ethanol (1/1)) were spin-coated on the pure TiO2 photoelectrode in sequence. During deposition, the Ca(NO3)2 and NaOH reacted to form a Ca(OH)2 layer. The film was washed with deionized water to remove any residual ionic species. In order to topotactically decompose the Ca(OH)2 to CaO, a Ca(OH)2-coated TiO2 photoelectrode was annealed at 450 °C for 30 min in N2 atmosphere. Additional annealing of the photoelectrode at 450 °C for 30 min in air transformed the CaO layer to a CaCO3 layer via a vigorous reaction between CaO and CO2 in air. In order to investigate the formation of CaCO3, reference powders were also prepared by reacting Ca(NO3)2 and NaOH solutions without TiO2 nanoparticles. CaCO3 nanoparticles produced by the reaction of Ca(NO3)2 and NaOH solutions were treated by the same method used for the CaCO3 coating over TiO2 nanoparticle surfaces. For the fabrication of solar cells, the photoelectrodes (bare and CaCO3-coated TiO2 electrodes) were immersed in a solution of ruthenium dye [ruthenium (2,2′bipyridyl-4,4′-dicarboxilate)2(NCS)2, SOLARONIX, Switzerland, dissolved in ethanol] at 50 °C for 2 h. Then, the dye-adsorbed electrode was assembled with a Pt counterelectrode to form a sandwich-type DSSC. A drop of electrolyte solution (Iodolyte AG-50, SOLARONIX) was infiltrated between the two electrodes of the cell. In order to investigate the degree of dye adsorption onto the electrode, the dye-immersed electrode was soaked in an alkaline alcoholic solution for 2 h to desorb the dye molecules from the electrode. The amount of the desorbed dye molecules was quantified by measuring their optical absorption spectra. The CaCO3 overlayer was observed by high-resolution transmission electron microscopy (HRTEM, model JEM 3000F, Japan). To prepare TEM specimens, the nanocrystalline particles of the films were removed from the electrode, and then dispersed on a TEM grid. The crystalline structures were characterized using an X-ray diffractometer (M18XHF-SRA, MAC-Science Instruments, Japan). The specific surface areas of the bare and CaCO3-coated TiO2 nanoparticles were measured using Brunauer-Emmett-Teller (BET) analysis (ASAP 2400, Micrometrics Instrument Group). Photovoltaic properties of the fabricated solar cells under the illumination of air mass of 1.5 (ORIEL 91193 1000 W xenon lamp; intensity: 100 mW/cm2) were measured with the aid of a potentiostat (CHI 608C, CH Instruments). Impedance of the cells under 100 mW/cm2 illumination and reversely biased open-circuit voltage were also measured by a potentiostat (CHI 608C, CH Instruments).

Results and Discussion Figure 1 shows X-ray diffraction (XRD) of the products from the reaction between Ca(NO3)2 and NaOH. The thermal decomposition of Ca(OH)2 in N2 gas leads to the formation of a CaO phase. When the CaO is further annealed in air, the CaO phase was converted into a CaCO3 phase via the reaction between CaO and a carbonaceous species such as CO2. The TEM micrographs in Figure 2a,b show that a typical topotactic reaction occurs, and thereby a highly nanoporous CaO phase is formed when the reaction products of Ca(NO3)2 and NaOH are thermally annealed. A change in bright-field images and diffraction patterns indicates that a Ca(OH)2 platelet with a size of 0.3 µm (Figure 2a) is decomposed into many CaO nanocrystals (black spots in Figure 2b). Because of the significant change in density between Ca(OH)2 (2.2 g/cm3) and CaO (3.34 g/cm3), the topotactic reaction from Ca(OH)2 to CaO also creates many nanopores with a size of approximately 2-3 nm (white spots). The pore size in Figure 2b is comparable to a previously reported pore size (about 2.7 nm) in CaO that also experienced

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Figure 1. Powder XRD patterns for (a) as-grown products from the reaction between Ca(NO3)2 and NaOH, (b) products annealed at 450 °C in N2 for 30 min, and (c) products annealed at 450 °C in N2 for 30 min and subsequently annealed at 450 °C in air for 30 min.

thermal topotactic decomposition.20 The ring-like diffraction pattern in the inset of Figure 2b indicates that the big hexagonal shape in Figure 2b is not a single crystal and consists of many CaO nanocrystals. A TEM micrograph (Supporting Information) for CaCO3, which was obtained by subsequent annealing of the nanoporous CaO particle in air, shows that the CaCO3 particle retains the nanoporous nature of the CaO particle. The TEM micrograph for the CaCO3-coated TiO2 nanoparticle in Figure 2c clearly confirms that the nanoporous layer was coated onto TiO2 nanoparticles. The thickness of the CaCO3 coating is about 3.5 nm. As a supporting analysis on the presence of the nanoporous layer, the BET surface area was measured, which yielded 55.0 and 47.3 m2/g for CaCO3-coated and bare TiO2 nanoparticles, respectively. Because only 3.5 nm of CaCO3 layer coated on the TiO2 electrode, the increase in the specific surface area indicates that the coated CaCO3 layer has a very high nanoporous structure. The high porosity would be responsible for the absence of the lattice fringe at the CaCO3 shell, as shown in Figure 2c. Figure 3 compares the photocurrent-voltage curve of DSSCs utilizing the CaCO3-coated TiO2 photoelectrode with that of the bare TiO2 counterpart. Important physical parameters governing the efficiency of the DSSCs were determined from the photocurrent-voltage curve, and the results are presented in Table 1. The overall conversion efficiency of the cell increases from 7.8 to 9.7%, corresponding to an improvement of 24.4%. All factors constituting the overall efficiency, that is, short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (ff), significantly increase when the TiO2 electrode was coated by the CaCO3 layer. The 13% increase of Jsc (from 19.4 to 21.9 mA/ cm2) is particularly notable, compared with other surface-modified photoelectrodes.9,11,13,17 This big increase clearly shows the benefit of the nanoporous feature of the CaCO3 coating layer in the current work in improving the DSSC’s performance. In order to investigate the physical origins of the improved efficiency, the influence of the nanoporous CaCO3 overlayer on the dye adsorption of a TiO2 electrode was first measured. The dye molecules on the photoelectrode were desorbed by soaking the photoelectrode in an alkaline alcoholic solution. Figure 4 shows the optical absorption spectra for the desorbed dye molecules. The significant increase in the optical absorption of the CaCO3-coated TiO2 photoelectrode indicates that the nanoporous CaCO3 layer on TiO2 nanoparticles apparently increases the amount of adsorbed dye molecules by increasing the surface

Nanoporous CaCO3-Coated TiO2 Electrodes & DSSCs

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Figure 3. Photocurrent-voltage curves for DSSCs using bare TiO2 (black) and CaCO3-coated TiO2 (red) electrodes.

Figure 4. Absorption spectra of desorbed dye solutions from bare TiO2 (black) and CaCO3-coated (red) TiO2 photoelectrodes.

Figure 2. HRTEM micrographs for (a) as-grown products from the reaction between Ca(NO3)2 and NaOH (Ca(OH)2), (b) products annealed at 450 °C in N2 for 30 min (nanoporous CaO) (the inset shows the selected area diffraction patterns of each powder), and (c) the nanoporous CaCO3-coated TiO2 nanoparticle. Table 1. The Cell Parameters of the DSSCs Based on Bare TiO2 and CaCO3-Coated TiO2 Electrodes sample

Jsc (mA/cm2)

Voc (mV)

ff

η (%)

bare TiO2 CaCO3-coated TiO2

19.39 21.92

654 668

0.618 0.661

7.84 9.68

area. On the basis of the observed optical absorption spectra, the amount of adsorbed dye molecules were calculated by the method shown in reference 4, and the results are 1.69 × 10-7 and 2.60 × 10-7 mol/cm2 for the bare and CaCO3-coated TiO2 photoelectrodes, respectively. The increase of 54% in dye adsorption,

Figure 5. Impedance spectra for DSSCs using bare TiO2 (black) and CaCO3-coated TiO2 (red) solar cells measured under 100 mW/ cm2 illumination. The measurements were achieved with the application of the open-circuit voltage as the bias.

which originates from the high specific surface area of the CaCO3 overlayer, certainly contributes to the increasing Jsc. In addition, the resistance to the back electron transfer was characterized by measuring the impedance spectra (Figure 5) and the lifetime of the photoelectrons (Figure 6). If the back electron transfer is suppressed, Voc, ff, and Jsc can be improved. In Figure 5, three arcs are distinguished in the frequency regime of 103-105 (ω1 or ω2), 1-103 (ω3), and 0.1-1 Hz (ω4), from left to right. These arcs are assigned to resistances at the conducting layer/TiO2 (ω1), Pt/electrolyte (ω2), and TiO2/dye/electrolyte (ω3)

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Figure 6. Electron lifetimes for DSSCs using bare TiO2 (black) and CaCO3-coated TiO2 (red) solar cells as a function of Voc. The inset shows open-circuit voltage decays for each solar cell.

interfaces and diffusion of the I3-/I- redox electrolyte (ω4).21,22 It is noted that the CaCO3 coating layer significantly increased the impedance component corresponding to ω3. The increase in the resistance at the TiO2/dye/electrolyte interface is as much as 20%, while the change in other impedance components was negligible. The increase in the resistance of ω3 indicates that the insulating nature of CaCO3 increases the surface resistance of TiO2 photoelectrodes and suppresses the back transfer of photogenerated electrons from CaCO3-coated TiO2 nanoparticles to the electrolyte at the TiO2/dye/electrolyte interface. Therefore, the photogenerated electrons are extracted more efficiently, and thereby Voc, ff, and Jsc increased together. The lifetimes of electrons in the bare and CaCO3-coated TiO2 photoelectrodes were measured to find further evidence of the correlation between the CaCO3 coating and the retarded back electron transfer. It is well-known that the longer lifetimes of photogenerated carriers for better photovoltaic performance also gain benefits from suppressing the back electron transfer. Figure (21) Hoshikawa, T.; Yamada, M.; Kikuchi, R.; Eguchi, K. J. Electrochem. Soc. 2005, 152, E68. (22) Kern, R.; Sastrawan, R.; Ferber, J.; Stangl, R.; Luther, J. Electrochim. Acta 2002, 47, 4213.

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6 compares the carrier lifetimes for the bare and nanoporous CaCO3-coated TiO2-based DSSCs, which were obtained by measuring the decay of Voc with time (the inset of Figure 6).23 Compared with the bare one, the CaCO3-coated one shows an increase in the carrier lifetime, indicating the higher chance for the photogenerated carriers of the CaCO3-coated one to be extracted to an external load. Although the indicial parameters (Voc, ff, and Jsc) governing the cell efficiency are not individually quantified here, the impedance spectra and the carrier lifetime measurements show that the insulating nature of the nanoporous CaCO3 layer retards the carrier recombination at the surface of the TiO2 nanoparticles and contributes to the increase in the short-circuit current as well as the open-circuit voltage. In summary, a nanoporous CaCO3-coated TiO2 electrode was prepared by thermal topotactic decomposition from Ca(OH)2 and used for DSSCs. The nanoporous nature of CaCO3 significantly enhanced the dye adsorption. Furthermore, the increased impedance at the TiO2/dye/electrolyte interface and the increased lifetime of the photoelectrons show that the insulating property of the CaCO3 nanoporous coating suppressed the back transfer of the photogenerated electrons to the electrolyte. Therefore, the DSSCs employing the nanoporous CaCO3 layer have larger Voc, ff, and Jsc values, which leads to an increase in the energy conversion efficiency by 24.4%. This increase indicates that an extrinsic factor such as the high surface area of the coating layer contributes significantly to improving the photoactive performance of the DSSCs in addition to intrinsic properties such as the highly insulating nature of the coating layer. Acknowledgment. This work was supported by the ERC Program (CMPS, Center for Materials and Processes of SelfAssembly) of MOST/KOSEF (R11-2005-048-00000-0) and by the Core Environmental Technology Development Project for Next Generation (Eco-Technopia-21) funded by the Korea Institute of Environmental Science and Technology under the Ministry of Environment, Republic of Korea. Supporting Information Available: TEM micrograph of CaCO3 obtained by the subsequent annealing of nanoporous CaO in air. This material is available free of charge via the Internet at http://pubs.acs.org. LA701826V (23) Zaban, A.; Greenshtein, M.; Bisquert, J. ChemPhysChem 2003, 4, 859.