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Aggregation-Free ZnO Nanocrystals Coupled HMP-2 Dye of Higher Extinction Coefficient for Enhancing Energy Conversion Efficiency Hong-Minh Nguyen, Rajaram S. Mane,† T. Ganesh, Sung-Hwan Han,* and Nakjoong Kim* Inorganic Nanomaterials, and Organic Photonic and Electronic Materials Laboratory, Department of Chemistry, Hanyang UniVersity, Seongdong-Gu, Haengdang-dong 17, Seoul, 133-791 Republic of Korea ReceiVed: February 25, 2009; ReVised Manuscript ReceiVed: April 6, 2009
A novel and highly efficient dye, Ru(H2dcbpy)(4-(4-(N,N-di(p-hexyloxyphenyl)amino)styryl)-4′-methyl-2,2′bipyridine)(NCS)2, HMP-2, with extinction coefficients of about 33 260 L · mol-1 · cm-1 at 534 nm and about 20 000 L · mol-1 · cm-1 at 490 nm, is designed and further applied onto ZnO nanocrystals for dye-sensitized solar cells application. ZnO nanocrystals of a few nanometers are spin coated onto an indium-tin oxide substrate for forming about 2.5 µm uniform film thickness. Change in the surface appearance of ZnO in the presence of N3 and in newly designed HMP-2 dye is illustrated. Structural elucidation of the newly developed HMP-2 dye is presented in depth. A ZnO nanoparticle electrode is dipped in HMP-2 dye and in commercially used N3 dyes for 24 h to see the influence of dye structure on the surface aggregation effect due to insulating Zn+/dye complex layer formation. Interestingly, the ZnO nanocrystal electrode surface with HMP-2 dye is almost free of this efficiency-diminishing effect, and a solar-to-electrical conversion efficiency of 4.03%, which is about 40-fold times higher than that of N3 dye, is obtained. Introduction Zinc oxide (ZnO), a wide band gap semiconductor (Eg ) 3.37 eV), is one of the most versatile and technologically important materials due to its high optical transmittance coupled with IR reflectance, diverse and abundant configurations, absence of toxicity, large exciton binding energy (60 meV), etc.1-3 Because of rapidly diminishing energy sources and higher energy production costs, dye-sensitized solar cells (DSSCs) have attracted greatly a special line of research in addition to TiO2, SnO2, Nb2O4, CeO2, etc.4-6 In general, such cells using ZnO as an active host material, in particular, have increased dramatically in recent years.4,7,8 The morphologies of ZnO including nanoparticles, core-shell nanoparticles, tetrapods, hierarchical structures, nanoflowers, nanosheets, nanowires, nanorods, nanotips, nanotubes, and branched nanostructures have been proven to be the richest among inorganic semiconductors, and its electrical and optical properties depend sensitively on both the morphology and size.7-10 Said nanoforms of ZnO have been synthesized via a variety of techniques and proposed for enhancing the conversion efficiency; however, solar cell performances based on ZnO n-type material are not satisfactory. Conversion efficiencies of e2.4, 1.5, and 1.6% for ZnO nanocrystals,9 nanowires,7,10 and nanotubes4,8 are documented. The effect of light scattering on light-harvesting efficiency and hence on conversion efficiency (3.5%) is explored.11 Using the polydispersed aggregates of ZnO, 4.4 and 5.4% conversion efficiencies are documented due to light scattering enhancement.12,13 In recent reviews, the importance of one-dimensional ZnO nanostructures as an electron transport material in electronic devices is well reviewed.14,15 In general, the low conversion efficiency in ZnO-based systems is most likely caused by the dissolution of Zn2+ by the adsorption of acidic dye followed by the * Corresponding authors. Telephone: + 822 2292 5212. Fax: +822 2299 0762. E-mail:
[email protected] (S.-H.H.);
[email protected] (N.K.). † Also at Department of Physics, Clarendon Laboratory, Oxford University, Oxford, U.K.
formation of agglomerates (insulating layer) with dye molecules, which eventually blocks the injection of electrons from the dye molecules to the semiconducting electrode16-18 as the ZnO nanoparticle surface is chemically unstable in acidic conditions. In the case of ZnO n-type material, the presence of carboxylic groups increases the dissolution of Zn2+ and, further, the formation of Zn2+/dye agglomerates with four carboxylic groups. The efficient dyes so far developed have carboxylic groups on bipyridine ligands coordinated to a Ru metal center, which improves the adsorption in the visible region of solar light. Only the nanostructure form of ZnO is not adequate to achieve higher and compatible to TiO2 electrode conversion efficiency, but there is a need to design new sensitizers with enhanced molar extinction coefficients that allow a high light-harvesting yield because of reduced transport losses in the nanopores by avoiding this synergic efficiency-diminishing effect. Therefore, modifying the electron-donating group of a dye structure has been considered as one strategy. On account of extensively untouched literature, it is necessary to have (i) the excited state redox potential close to the energy of the conduction band edge of the oxide and (ii) a strong conjugation across the chromophore and anchoring groups for a effective electronic coupling between the lowest unoccupied molecular orbital of the dye and the metal oxide conduction band that not only provides fast charge transport and thereby higher conversion efficiency but also would facilitate an extended device life for practical application; therefore, research is really waiting for this ideal breakthrough, especially for ZnO-based DSSC systems. In order to prepare efficient ZnO-based DSSCs, in continuation of our ongoing ZnO-based DSSCs investigations,19,20 we here have applied a different design strategy for the dye molecular structure: first, the optimum number of carboxylic groups; second, the presence of hydrophobic side chain that hinder the coordination to Zn2+ forming agglomerates. Herein, we report a novel dye design whose synergic impact on ZnO nanocrystals is practically negligible even after 24 h of dipping time, where the routinely preferred N3 dye practically fails to
10.1021/jp901736w CCC: $40.75 2009 American Chemical Society Published on Web 05/01/2009
ZnO Nanocrystals Coupled HMP-2 Dye SCHEME 1: Chemical Route Presenting a Facile and Easy Synthesis of HMP-2 Dye
J. Phys. Chem. C, Vol. 113, No. 21, 2009 9207 100 CONC spectrophotometer. Electrochemical impedance spectroscopy (EIS) is a useful technique that has been widely employed to investigate the charge transport kinetic processes occurring in DSSCs, mostly in the dark wherein the observed current response to a harmonically modulated voltage is superimposed on a constant applied bias voltage and the resulting frequency analysis typically shows three well-separated semicircles in the Nyquist diagram. For EIS measurements, the ac potential amplitude was applied at 10 mV, while its frequency region was maintained from 0.01 Hz to 100 kHz. The electrolyte, discussed below, is the same as that used in the DSSCs measurements. The applied potential for EIS measurement was taken from the open circuit voltage of the DSSCs. The dyeimmobilized ZnO nanocrystal electrode and the 100-nm-thick Pt-sputtered ITO were sandwiched together using a cell holder, into which an electrolyte solution (0.6 M 1-hexyl-2,3-dimethylimidazolium iodide (C6DMI), 0.1 M lithium iodide (LiI), 0.05 M iodide (I2), and 0.5 M 4-tert-butylpyridine (t-BPy) in 15 mL of methoxyacetonitrile (98%)) was infiltrated using a fine 10 mL nontoxic Kovax syringe. DSSC measurements of ZnO/N3 and ZnO/HMP-2 electrodes were performed using a 1-kW xenon lamp with a photointensity of 100 mW/cm2 and an effective electrode area of 0.28 cm2. In all experiments, the sample was illuminated from the conducting glass substrate side. Results and Discussion
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a great extent. With this motivation, in this paper, we demonstrate the design of a novel dye, Ru(H2dcbpy)(4-(4-(N,Ndi(p-hexyloxyphenyl)amino)styryl)-4′-methyl-2,2′-bipyridine)(NCS)2, HMP-2. For comparison, we also present the observed agglomeration effect of ZnO in the presence of a routinely preferred dye, ruthenium(II) cis-di(thiocyanato)bis(2,2′bipyridyl-4,4′-dicarboxylic acid (N3). We also report a highly efficient dye of large molar extinction coefficient and facile design and synthesis which exhibits about 4% overall solar-toelectrical conversion efficiency. The electronic conjugation of HMP-2 dye is sufficiently stronger than that of N3 dye to ZnO nanocrystals. Experimental Section To prepare a ZnO nanoparticle (NP) electrode, ZnO (purchased from Alfa Aesar), NanoShield ZN-5060, 50% in H2O, colloidal dispersion with dispersant was preferred as received. Scheme 1 presents a flow chart of the HMP-2 dye synthesis route, which is different from the N3 structure,22 composed of two carboxylic groups at the end of the pyridyl rings and two NCS ligands connected to Ru(II). Using a drop-cast method, the ZnO NP electrode was prepared on a precleaned indium-tin oxide glass substrate of 2 × 2 cm2 area. The fixing of indium-tin oxide substrate was achieved using a rotary pump by creating a vacuum of 10-3 Torr. The speed of the motor was adjusted by using a variable controller and fixed at 2000 rpm. Films were then annealed at 300 °C for 1 h in air to remove the residual solvents and organic chemicals on ZnO nanocrystallites if any and, finally, sensitized by immersing into N3 and HMP-2 dyes maintained at 45 °C for 24 h. The ZnO nanocrystals were examined with a Philips Japan MPD 1880 X-ray powder diffractometer using Cu KR radiation (V ) 40 kV and I ) 100 mA), along with scanning electron microscopy (SEM; JEOL). UV-vis solution optical spectra of N3 and HMP-2 dyes and those of ZnO, ZnO/N3, and ZnO/ HMP-2 electrodes were recorded using a Cary Japan Model
Scheme 1 presents a simple and facile synthesis route followed while designing the novel HMP-2 sensitizer dye, whose analytical data can be collected from the Supporting Information. Figure 1a presents the X-ray diffraction pattern (XRD) of the ITO substrate-supported ZnO nanoparticles within a 2θ range of 20-80° at room temperature with a scan step of 0.03 deg/s. It is noted that the reflection peak intensities of (110) and (101) are relatively stronger than that of (002). The intensity of the (002) peak in the XRD pattern is supposed to be sensitive to the concentration of oxygen vacancies, i.e., indirectly related to the electrode resistivity. The higher the number of oxygen vacancies, the lower the resistivity, and therefore, the best conducting films should ideally have a zero peak intensity of (002) when used in electronic devices.23 This result thus indicates that the present ZnO nanoparticle electrode can be used as a working electrode in DSSCs. Figure 1b shows the surface and cross-sectional (inset) SEM images, obtained to elucidate the surface morphology and thickness evolution. Irregular dimensions of ZnO porous nanoparticles of 30-300 nm are clearly seen in the surface SEM scan. We believe that this type of nanocrystal will be meritorious in the present work as large particles are effective for light scattering and small particles are effective for dye adsorption.24 The inset crosssectional SEM scan image confirms ∼2.5 µm uniform thickness of the ZnO nanoparticle electrode. Figure 1c,d presents the SEM surface images of ZnO nanocrystals after dipping in N3 and HMP-2 dyes for 24 h. The change in ZnO surface images in the two dyes is clearly visible. The surface of ZnO nanoparticles in N3 dye is smooth and island-free, in contrast to Figure 1b, with several small crystallites forming some kind of diffusion barrier layer, whereas, in HMP-2 dye, the surface is physically stable with undisturbed islands. Therefore, the ZnO nanocrystal surface modification in HMP-2 dye even after dipping for 24 h is practically negligible. To compare the difference in the optical absorption character between these two kinds of dyes, UV-vis optical spectra were measured. In Figure 2a, the UV-vis solution electronic absorption spectra of the N3 and HMP-2 dyes in ethanol (1.5 × 10-5
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Figure 1. (a) XRD pattern of ZnO nanocrystals onto a precleaned ITO substrate. SEM morphologies of ZnO nanocrystals of irregular dimensions, (b) as-formed, and after 24 h dipping in (c) N3 and (d) HMP-2 dyes, respectively.
Figure 2. (a) Optical absorption spectra of N3 and HMP-2 dyes. (b) Photocurrent density versus applied voltage characteristics, (c) optical density versus light wavelength variation (electrode), and (d) Nyquist plots of ZnO-based NPs DSSCs in N3 and HMP-2 dyes, respectively.
M) are presented. The two absorption peaks observed are due to metal-to-ligand charge transfer transitions. To compare the intensity of light absorption of the dyes, the molar extinction coefficients were determined by measuring the optical density of the individual dyes of definite concentration. At 429 and 393 nm, 33 260 and 14 930 L · mol-1 · cm-1 and, at 534 nm, 20 000 and 14 800 L · mol-1 · cm-1 values of extinction coefficients for HMP-2 and N3 dyes, respectively, are obtained. These extinction coefficients are higher than that recently reported for the TAST-CA dye (31 600 M-1 cm-1 at 386 nm).25 The increase in extinction coefficient results from the extension of π-conjugation in the complex due to the introduction of two hexyloxyphenyl moieties, which can alter electron-donating properties. The dye with more donor hydrophobic groups exhibits much stronger absorption than the one without,26 supporting the fact that the π-electrons are more delocalized in the HMP-2 dye than in the
N3 dye. Due to a higher extinction coefficient, favorable energy levels, two carboxylic groups (less acidic), and long alkyl chains, the HMP-2 dye would contribute a higher solar-to-electrical performance than the N3 dye. Figure 2b shows the absorbance change in the presence of N3 and HMP-2 dyes compared to a bare ZnO electrode. This absorption is comprised of the intrinsic absorption of the ZnO semiconductor at a wavelength close to 390 nm. The HMP-2 involved ZnO nanocrystal electrode presents more absorption than N3 and bare ZnO electrodes due to its higher extinction coefficient. Figure 2c shows the photocurrent density versus applied voltage variation of ZnO NPs in N3 and HMP-2 dyes under the conditions stated in the Experimental Section. The plot corresponding to the N3 dye indicates a short circuit current density (Jsc) of 1.01 mA/cm2, open circuit voltage (Voc) of 0.44 V, fill factor (ff) of 0.20, and conversion efficiency (η%) of 0.11%. The chemical instability of ZnO nanocrystals as seen in the SEM image lead to the formation of aggregates of ZnO against the acidic dye molecules due to the formation of a metal ion/dye complex caused by the carboxylic acid groups, which is a function of the dye pH reducing the dye concentration on the ZnO surface, slow electron injection kinetics, and the recombination of charges at the interface between ZnO platelets and I-/I3- electrolyte, contributing to a poor device performance, especially the fill factor.17,27-29 The Zn2+/dye complex layer, in general, is nonconductive and can suppress the electron injection from the dye to ZnO nanocrystals.30 As seen in Figure 2c, the ZnO nanocrystal electrode cell shows the higher solar-toelectrical conversion performance in HMP-2 dye than in N3 dye. This curve indicates Jsc ) 17.35 mA/cm2, Voc ) 0.51 V, ff ) 0.36, and η ) 4.03%, which is higher than recently reported ZnO nanoplate-based DSSCs and is attributed to a larger surface area of ZnO NPs than that of nanoplates.30 In the presence of HMP-2 dye, DSSC performance is remarkable. This dye contains more donor groups with an extended π-conjugation and seems to increase the charge separation by intramolecular charge transfer. Electron injection into an ITO conducting substrate seems to be easy and efficient due to the absence of
ZnO Nanocrystals Coupled HMP-2 Dye surface aggregation, which is also reflected from the impedance studies discussed later. The device with HMP-2 sensitizer shows a substantial increase in Jsc due to the higher molecular extinction coefficient and the aggregate-free ZnO surface morphology. The optimization of various parameters including film thickness, dyeing time, annealing, etc., would be of great interest for obtaining an optimum conversion efficiency value because the value of the fill factor reported here is considerably lower than that reported earlier,12,13,31-34 which is underway. Figure 2d shows the Nyquist plots of the ZnO electrode with the N3 and HMP-2 dyes. Different Nyquist plots are attributed to the Nernst diffusion of the redox mediator within the electrolyte, the electron transfer at the oxide/electrolyte interface as well as their diffusion in the nanoparticle network, and the electrochemical reaction at the counter electrode. The impedance measured in dye-sensitized ZnO/N3 and ZnO/HMP-2 DSSCs is interpreted in terms of charge transfer resistance, which is found to be a crucial feature for describing this dynamic operation.35 The sheet resistance offered by the transparent conducting oxide, i.e., indium-tin oxide in the present case, contributed the initial lag in the high frequency region. The semicircles at high frequency and intermediate frequency regions were decreased in the HMP-2 dye, indicating the fast redox activity of the electrolyte in the counter electrode and working electrode interface, respectively.36 Systematic efforts are also in progress to increase the performance of ZnO-based DSSCs by designing dyes of still higher extinction coefficients. Making solid state solar cells, similar to the recent report by Snaith et al.,37 will be also one of the great issues in future research. Conclusion In conclusion, a novel and highly effective dye, Ru(H2dcbpy)(4(4-(N,N-di(p-hexyloxyphenyl)amino)styryl)-4′-methyl-2,2′-bipyridine)(NCS)2, of higher extinction coefficient and absorbance has proven its potentiality in ZnO-based dye-sensitized solar cells by maintaining an aggregation-free ZnO surface morphology even after 24 h dipping time, where the routinely used N3 dye is practically unsuccessful. Dye-sensitized solar cells with ZnO nanocrystallites and the HMP-2 dye showed 4.03% solarto-electrical conversion efficiency. The impedance measurement under open circuit voltage has provided evidence that charge injection kinetics is faster in ZnO nanocrystals when conjugated with the HMP-2 dye on account of its smaller charge transport resistance compared to the N3 dye. This indicates that the dye molecular structure with an optimum number of carboxylic groups and hydrophobic alkyl chains is equally important along with the electrode structure/morphology for increasing the DSSC performance of ZnO-based systems. In-depth research of such variables as dye-loading time and electrode thickness is needed to achieve an optimal performance. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean government (MEST) (No. R11-2008-088-03001-0). Supporting Information Available: Details of the syntheses of 4-(N,N-di-p-hexyloxyphenylamino)benzaldehyde (1), 4-(4(N,N-di(p-hexyloxyphenyl)amino)phenyl-2-hydroxyethyl)-4′methyl-2,2′-bipyridine (2), 4-(4-(N,N-di(p-hexyloxyphenyl)amino)styryl)-4′-methyl-2,2′-bipyridine (ligand-2), and cis-[Ru-
J. Phys. Chem. C, Vol. 113, No. 21, 2009 9209 (H2dcbpy)(ligand-2)(NCS)2] (HMP-2). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Choy, J. H.; Jang, E. S.; Won, J. H.; Chung, J. H.; Chang, D. J. AdV. Mater. 2003, 15, 1911. (2) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, K.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (3) Wang, Z. L.; Song, J. Science 2006, 312, 242. (4) Gratzel, M. J. Photochem. Photobiol., C: Chem. 2003, 4, 145. (5) Bergeron, B. V.; Marton, A.; Oskam, G.; Meyer, G. J. J. Phys. Chem. B 2005, 109, 937. (6) Kayama, K.; Sugihara, H. Mater. Sci. (P) 2007, 25, 137. (7) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (8) Martinson, A. B. F.; Elam, J. W.; Hupp, J. T.; Pellin, M. J. Nano Lett. 2007, 7, 2183. (9) Otsuka, A.; Funabika, K.; Sugiyama, N.; Yoshida, T. Chem. Lett. 2006, 35, 666. (10) Baxter, J. B.; Aydil, E. S. Appl. Phys. Lett. 2005, 86, 53114. (11) Chou, T. P.; Zhang, Q. F.; Fryxell, G. E.; Cao, G. Z. AdV. Mater. 2008, 19, 2588. (12) Zhang, Q.; Chou, T. P.; Russo, B.; Jenekhe, S. A.; Cao, G. AdV. Funct. Mater. 2008, 18, 1654. (13) Zhang, Q.; Chou, T. P.; Russo, B.; Jenekhe, S. A.; Cao, G. Angew. Chem. 2008, 120, 2436. (14) Gonzalez-Valls, I.; Lira-Cantu, M. Energy EnViron. Sci. 2009, 2, 19. (15) Yaing, C. Y.; Sun, X. W.; Tan, K. W.; Lo, G. Q.; Kyat, A. K. K.; Kwong, D. L. Appl. Phys. Lett. 2008, 92, 143101. (16) Bedja, I.; Kamat, P. V.; Hua, X.; Lappin, P. G.; Hotchandani, S. Langmuir 1997, 13, 2398. (17) Horiuchi, H.; Kaoh, R.; Hara, K.; Yanagida, M.; Murata, S.; Arakawa, H.; Tachiya, M. J. Phys. Chem. B 2003, 107, 2570. (18) Keis, K.; Lindgren, J.; Lindquist, S. E.; Hagfeldt, A. Langmuir 2006, 16, 4688. (19) Mane, R. S.; Pathan, H. M.; Lee, W. J.; Han, S. H. J. Phys. Chem. B 2005, 109, 24254. (20) Roh, S. J.; Mane, R, S.; Min, S. K.; Lee, W. J.; Lokhande, C. D.; Han, S. H. Appl. Phys. Lett. 2006, 89, 53512. (21) Chuo, T. P.; Zhang, Q. F.; Cao, G. Z. J. Phys. Chem. C 2007, 111, 18804. (22) Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000, 33, 269. (23) Asadov, A.; Gao, W.; Li, Z.; Lee, J.; Hodgson, M. Thin Solid Films 2005, 476, 201. (24) Koo, H. J.; Kim, Y. J.; Lee, Y. H.; Lee, W. I.; Kim, K.; Park, N. G. AdV. Mater. 2008, 20, 195. (25) Hwang, S.; Lee, J. H.; Park, C.; Lee, H.; Kim, C.; Park, C.; Lee, M.; Lee, W.; Park, J.; Kim, K.; Park, N.; Kim, C. Chem. Commun. 2007, 4887. (26) Karthikeyan, C. S.; Peter, K.; Wietasch, H.; Thelakkat, M. Sol. Energy Mater. Sol. Cells 2007, 91, 432. (27) Anderson, N. A.; Ai, X.; Lian, T. J. J. Phys. Chem. B 2003, 107, 14414. (28) Redmond, G.; Fitzmaurize, D.; Gratzel, M. Chem. Mater. 1994, 6, 686. (29) Keis, K.; Bauer, C.; Boschloo, G.; Hagfeldt, A.; Westermark, K.; Rensmo, H.; Seigbahn, H. J. Photochem. Photobiol., A: Chem. 2002, 57, 148. (30) Mane, R. S.; Nguyen, H. M.; Ganesh, T.; Kim, N. J.; Ambade, S. B.; Han, S. H. Electrochem. Commun. 2009, 11, 752. (31) Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H. AdV. Mater. 2005, 17, 2091. (32) Matsui, M.; Ito, A.; Kotani, M.; Kubota, Y.; Funabiki, K.; Jin, J.; Yoshida, T.; Minoura, H.; Miura, H. Dyes Pigm. 2009, 80, 233. (33) Saito, M.; Fujihara, S. Energy EnViron. Sci. 2008, 1, 280. (34) Keis, K.; Magnusson, E.; Lindstrom, H.; Lindquist, S.; Hagfeldt, A. Sol. Energy Mater. Sol. Cells 2002, 73, 51. (35) Wang, M.; Chen, P.; Humphry-Baker, R.; Zakeeruddin, M. S.; Gratzel, M. ChemPhysChem 2009, 10, 290. (36) Wang, Q.; Moser, J. E.; Gratzel, M. J. Phys. Chem. B 2005, 109, 14945. (37) Snaith, H. J.; Humphry-Baker, R.; Chen, P.; Cesar, I.; Zakeeruddin, S. M.; Gratzel, M. Nanotechnology 2008, 19, 424003.
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