DOI: 10.1002/chem.201402250
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& Solar Cells
Hierarchically Structured ZnO Nanorods as an Efficient Photoanode for Dye-Sensitized Solar Cells Wenqin Peng,[a] Liyuan Han ,*[a, b] and Zhengming Wang[c]
Abstract: Hierarchical ZnO nanorods composed of interconnected nanoparticles, which were synthesized by controlling precursor concentrations in a solvothermally assisted process, were exploited as photoanodes in dye-sensitized solar cells (DSCs). The as-prepared hierarchical nanorods showed greatly enhanced light scattering compared to ZnO nanoparticles for boosting light harvesting while maintaining sufficient dye-adsorption capability. The charge-transfer charac-
Introduction Zinc oxide (ZnO) has been extensively studied for its applications in optoelectronic devices, photocatalysis, photochemical cells, sensing, and so forth.[1] Recently, ZnO has been of special interest as a promising photoanode material for dye-sensitized solar cells (DSCs).[2–5] The main driving forces are its high electron mobility, which is favorable for electron diffusion with lower recombination loss, and a suitable conduction bandedge position for electron injection.[6] Furthermore, crystallization and morphological tailoring of ZnO nanostructures can be easily realized under mild conditions. The size and morphology of photoanode materials play important roles in improving the photovoltaic performance of DSCs. In DSCs, porous films made from 10–50 nm nanoparticles have been routinely employed to provide high surface areas for loading dye molecules. However, loosely packed nanoparticles impede electron transport and decrease electron lifetime.[7, 8] In contrast, 1D nanostructures, such as nanowires, nanotubes, and nanorods, have been shown to facilitate electron transport by providing a direct electron-transport pathway.[9–13] For example, Law et al. demonstrated that electron transport in a nanowire-based ZnO electrode was several hun[a] Dr. W. Peng, Dr. L. Han Photovoltaic Materials Unit National Institute for Materials Science Tsukuba, Ibaraki 305-0047 (Japan) E-mail: [email protected] [b] Dr. L. Han State Key Laboratory of Metal Matrix Composites Shanghai Jiaotong University Shanghai 200240 (P. R. China) [c] Dr. Z. Wang Environmental Management Technology Research Institute National Institute of Advanced Industrial Science and Technology Tsukuba, Ibaraki 305-8569 (Japan) Chem. Eur. J. 2014, 20, 8483 – 8487
teristics were studied by electrochemical impedance measurements, and reduced electron recombination and longer electron lifetime were observed for the ZnO nanorods. Photovoltaic characterization demonstrated that DSCs utilizing the hierarchical nanorods significantly improved the overall conversion efficiency by 34 % compared to nanoparticlebased DSCs.
dred times faster than in a nanoparticle electrode.[11] Improved electron lifetime was achieved by using 1D nanostructures.[14, 15] In addition, 1D structures with large sizes have superior lightscattering ability compared to nanoparticles and thus can enhance light-harvesting efficiency. Unfortunately, such 1D structures have much lower surface area than nanoparticles, which leads to inferior photocurrent and conversion efficiencies of DSCs. To address this issue, construction of hybrid systems by mixing nanoparticles and 1D nanomaterials has been proposed. Different materials, including ZnO nanoparticles, layered basic zinc acetate nanoparticles, and TiO2 nanoparticles, were introduced into the matrix of ZnO 1D nanostructures, and enhanced photovoltaic performance was reported for these hybrid photoanodes.[16–20] However, the total internal surface area of such hybrid materials is still unavoidably decreased compared to that of nanoparticles. Moreover, fabricating the hybrid photoanode requires a multistep process and thus higher cost compared to a single-composition photoanode. Herein, we report a multifunctional photoanode for DSCs consisting of ZnO nanorods prepared by a facile solvothermally assisted process. The ZnO nanorods are composed of interconnected 30 nm nanoparticles. Such a photoanode can not only enhance light harvesting by superior light scattering and good dye loading, but also facilitates electron transport. Photovoltaic characteristics of DSCs based on the ZnO hierarchical nanorods were investigated, and a 34 % enhancement of conversion efficiency was achieved compared to DSCs with ZnO nanoparticles prepared at low concentrations.
Results and Discussion Hierarchical ZnO nanorods were obtained at a zinc acetate concentration of 0.6 m (see Experimental Section). The morphology and size of the ZnO products were characterized by field-emission (FE) SEM. As shown in Figure 1 a, the products 8483
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Figure 3. XRD patterns of a) hierarchical nanorods and b) nanoparticles.
Figure 1. a) FESEM image, b) TEM image, and c) magnified TEM image of the rectangular area in b) of ZnO hierarchical nanorods. The inset of b) shows the corresponding SAED pattern.
have a 1D rod shape with diameters of 100–400 nm and lengths of 1–2 mm. The low- and high-magnification TEM images (Figure 1 b and c) demonstrate that ZnO nanorods are composed of 20–50 nm particles. These nanoparticles are tightly connected, which is beneficial for electron transport. In addition, mesopores can be observed in the nanorods. The selected-area electron diffraction (SAED) pattern (inset of Figure 1 b) revealed that the hierarchical nanorods are polycrystalline and have wurtzite ZnO structure. To investigate the effect of morphology, we also prepared ZnO nanoparticles at a lower zinc acetate concentration of 0.03 m. The FESEM and TEM images of ZnO nanoparticles are shown in Figure 2. The wurtzite structure of the ZnO nanoparticles was confirmed by the SAED pattern (inset of Figure 2 b). Crystal structures of ZnO hierarchical nanorods and nanoparticles were investigated by XRD (Figure 3). All diffraction peaks could be indexed to the hexagonal wurtzite structure of ZnO, in agreement with the results of SAED analysis (insets of Figures 1 b and 2 b). The sharp peaks show that both ZnO products are well crystallized. By applying the Debye–Scherrer equation to the (100), (002), and (101) peaks, the average crystallite sizes of ZnO nanorods and nanoparticles were estimated to be 32
Figure 2. a) FESEM image and b) TEM image of ZnO nanoparticles. The inset of b) shows the corresponding SAED pattern. Chem. Eur. J. 2014, 20, 8483 – 8487
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and 29 nm, respectively. Thus, XRD also showed that the ZnO nanorods are composed of small particles. The as-prepared ZnO hierarchical nanorods and nanoparticles were applied as photoanodes in DSCs. The cross-sectional SEM images of the two photoelectrodes (Figure 4) revealed
Figure 4. Cross-sectional SEM images of a) hierarchical nanorod film and b) nanoparticle film on FTO glass. The inset of a) shows a magnified image.
film thickness of about 20 mm. Figure 5 a shows the photocurrent density–voltage (J–V) curves of DSCs based on hierarchical nanorods and nanoparticles under illumination with standard AM 1.5 sunlight (100 mW cm 2). The ZnO photoanode made from hierarchical nanorods exhibits a short-circuit current JSC of 9.07 mA cm 2, an open-circuit voltage VOC of 0.664 V, and an overall conversion efficiency h of 4.13 %. The photovoltaic performance was greatly enhanced compared to the nanoparticle photoanode, which gave a JSC of 7.03 mA cm 2, a VOC of 0.643 V, and an efficiency of 3.08 % (Table 1). Clearly, the improved conversion efficiency of the nanorod-based photoanode is mainly attributable to the increased JSC value. Measurement of the incident photon-to-current conversion efficiency (IPCE) provided further evidence for the variation in JSC. Figure 5 b shows the IPCE spectra of the nanorod and nanoparticle photoanodes. Compared to the nanoparticle film, the nanorod film has a higher IPCE in the wavelength range from 400 to 700 nm. The peak of the IPCE spectrum at 530 nm, which corresponds to the absorption characteristic of the N719 dye, is 62 % for the nanorod film. In the case of the nanoparticle film, the maximum IPCE decreased to 50 %. These results indicate that light harvesting is increased for the nanorod film compared to the nanoparticle film.
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Figure 6. Reflectance spectra of ZnO nanoparticle and hierarchical nanorod films.
red region, which suggests superior light scattering of the nanorods. The nanorods can lengthen the optical pathway within the photoanode due to superior light scattering and thus increase the probability of photon capture by dye molecules, which leads to enhanced JSC. Electrochemical impedance spectroscopy (EIS) is a useful technique to study charge transfer and recombination in DSCs.[24–26] Figure 7 shows the EI spectra of the nanorod- and nanoparticle-based DSCs obtained in the dark at an applied bias of 0.62 V in the frequency range from 100 kHz to 0.1 Hz. The Nyquist plot of each device is composed of two semicircles (Figure 7 a). The right semicircle in the medium-frequency region (0.1–1000 Hz) corresponds to the charge-transfer process at the oxide/dye/electrolyte interface, whereas the left
Figure 5. J–V curves (a) and IPCE spectra (b) of DSCs based on hierarchical nanorods and nanoparticles.
Table 1. Photovoltaic performance of DSCs based on nanorods and nanoparticles.
nanorods nanoparticles
JSC [mA cm 2]
VOC [V]
FF[a]
h [%]
9.07 7.03
0.664 0.643
0.686 0.683
4.13 3.08
[a] Fill factor.
To understand the effect of different ZnO nanostructures on JSC, the dye-adsorption capability was evaluated. The amounts of adsorbed dye for the hierarchical nanorod film and the nanoparticle film were 1.13 10 7 and 1.21 10 7 mol cm 2, respectively. The comparable dye loadings of the two films can be attributed to the similar surface areas of nanorods (24 m2 g 1) and nanoparticles (25 m2 g 1). Thus, the increased JSC for the nanorod photoanode relative to the nanoparticle electrode should be caused by the light scattering of the hierarchical nanorods. SEM and TEM images show that the hierarchical nanorods have submicrometer sizes, which are comparable to the wavelengths of visible light and thus expected to promote light scattering.[21–23] The reflectance spectra of the films were measured to estimate the light-scattering properties. Figure 6 compares the UV/Vis reflectance spectra of the nanorod and nanoparticle films. The nanorod film exhibits much higher light reflectance than the nanoparticle film in the entire visible wavelength range. The reflectance of the nanoparticle film quickly declines to about 18 % in the wavelength range of 500–900 nm. On the contrary, the nanorod film maintains a high reflectance of greater than 62 % even in the infraChem. Eur. J. 2014, 20, 8483 – 8487
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Figure 7. EI spectra of DSCs based on ZnO nanoparticles and hierarchical nanorods. a) Nyquist and b) Bode phase plots.
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Full Paper semicircle in the high-frequency region (> 1000 Hz) reflects the redox reaction at the counter electrode/electrolyte interface.[24] Compared to the nanoparticle cell, the nanorod-based device has a larger radius of the right semicircle, which indicates increased charge-recombination resistance (i.e., longer electron lifetime), which implies that photoexcited electrons can travel a longer distance in the hierarchical nanorods composed of interconnected nanoparticles than in the nanoparticles. The electron lifetime te can be evaluated from the peak frequency fp in the medium-frequency region according to the equation te = 1/2pfp. The Bode phase plots (Figure 7 b) show that the peak frequency in the medium-frequency region shifts from 25.1 Hz for the nanoparticle cell to 15.8 Hz for the nanorod cell. This suggests that the nanorod photoanode has a longer electron lifetime than the nanoparticle photoanode. Figure 4 shows that the nanorod photoanode achieves a higher VOC than the nanoparticle photoanode. The increased VOC may be explained by the longer electron lifetime of the nanorods. Similar characteristics have also been observed in nanotubes and nanowires.[27, 28]
Conclusion ZnO hierarchical nanorods with diameters of 100–400 nm and lengths of 1–2 mm were synthesized at a precursor concentration of 0.6 m. TEM observations confirmed that the hierarchical nanorods were composed of interconnected 30 nm nanoparticles. The ZnO nanorods were applied as photoanodes for DSCs and achieved an overall conversion efficiency of 4.13 %, that is, a 34 % enhancement in comparison with the ZnO nanoparticles obtained at lower concentration (0.03 m). The improved performance was ascribed to superior light scattering, sufficient dye loading, and enhanced electron-transport properties of the hierarchical nanorods.
(1/1 v/v) for 1 h in the dark. The photoanode and counter electrode (platinum-coated FTO glass) were sealed together with a Surlyn hot-melt film. Finally, assembly of a cell was completed by injecting the electrolyte into the cell through a hole on the back of the counter electrode. The electrolyte compositions were 0.6 m methylpropylimidazolium iodide, 0.05 m I2, and 0.1 m LiI in acetonitrile with tert-butylpyridine, as described in our previous work.[31]
Characterization XRD patterns were recorded on a Rigaku Ultima3 X-ray diffractometer at 40 kV and 40 mA. Morphologies and sizes were characterized with a JEM-2100F transmission electron microscope and a JSM-7001F field-emission scanning electron microscope. Specimens for TEM observation were prepared by dropping a small amount of an aqueous suspension onto carbon-coated copper grids. UV/Vis spectra of the film samples were recorded on a Shimadzu UV-3600 spectrophotometer equipped with an integrating sphere. BET surface area was measured with a volumetric N2 adsorption/desorption apparatus (Belsorp 18A, BEL Japan) at 77 K. Dye desorption experiments were carried out by soaking the dyesensitized films in 0.1 m solution of NaOH in water/ethanol (1/1 v/v). Photovoltaic performance of the DSC devices was measured by using a Keithley 2400 source meter under 1 Sun illumination (AM 1.5, 100 mW cm 2, WXS-90S-L2, Wacom). The active area of each device was 0.25 cm2. IPCE measurements were performed with monochromatic incident light at 100 mW cm 2 (CEP-2000 BX, Bunkoh-Keiki). EIS was performed on an electrochemical workstation (Solartron 1287 and 1255B) in the dark under a bias voltage at frequencies ranging from 100 kHz to 0.1 Hz.
Acknowledgements This work was financially supported by Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Agency. Keywords: energy conversion · nanostructures · solar cells · zinc oxide
Experimental Section
nanoparticles
·
Preparation of ZnO hierarchical nanorods and nanoparticles ZnO nanostructures were synthesized by a solvothermally assisted method. For the synthesis of ZnO hierarchical nanorods, equimolar ethanolic solutions (0.6 m) of zinc acetate and oxalic acid were first prepared. When these solutions were mixed under stirring, a white gel was formed.[29, 30] The gel was heated in an autoclave at 80 8C for 6 h and then allowed to cool to room temperature. The obtained precipitate was collected by centrifugation and washed with ethanol. After calcination in air at 450 8C for 1 h, hierarchical nanorods were obtained. For comparison, ZnO nanoparticles were prepared by a similar procedure, except for lower concentrations (0.03 m) of zinc acetate and oxalic acid.
Fabrication of DSCs To prepare the paste, ZnO products were dispersed in a mixture of ethanol and deionized water by mechanical grinding and ultrasonic treatment. Then, the ZnO paste was coated onto fluorine-doped tin oxide (FTO) glass and sintered in an oven at 200 8C to give a 20 mm-thick film. The resulting ZnO photoanode film was dipped into a 0.5 mm solution of N719 dye in acetonitrile/tert-butanol Chem. Eur. J. 2014, 20, 8483 – 8487
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Received: February 19, 2014 Published online on May 30, 2014
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& Solar Cells
Hierarchically Structured ZnO Nanorods as an Efficient Photoanode for Dye-Sensitized Solar Cells Wenqin Peng,[a] Liyuan Han ,*[a, b] and Zhengming Wang[c]
Abstract: Hierarchical ZnO nanorods composed of interconnected nanoparticles, which were synthesized by controlling precursor concentrations in a solvothermally assisted process, were exploited as photoanodes in dye-sensitized solar cells (DSCs). The as-prepared hierarchical nanorods showed greatly enhanced light scattering compared to ZnO nanoparticles for boosting light harvesting while maintaining sufficient dye-adsorption capability. The charge-transfer charac-
Introduction Zinc oxide (ZnO) has been extensively studied for its applications in optoelectronic devices, photocatalysis, photochemical cells, sensing, and so forth.[1] Recently, ZnO has been of special interest as a promising photoanode material for dye-sensitized solar cells (DSCs).[2–5] The main driving forces are its high electron mobility, which is favorable for electron diffusion with lower recombination loss, and a suitable conduction bandedge position for electron injection.[6] Furthermore, crystallization and morphological tailoring of ZnO nanostructures can be easily realized under mild conditions. The size and morphology of photoanode materials play important roles in improving the photovoltaic performance of DSCs. In DSCs, porous films made from 10–50 nm nanoparticles have been routinely employed to provide high surface areas for loading dye molecules. However, loosely packed nanoparticles impede electron transport and decrease electron lifetime.[7, 8] In contrast, 1D nanostructures, such as nanowires, nanotubes, and nanorods, have been shown to facilitate electron transport by providing a direct electron-transport pathway.[9–13] For example, Law et al. demonstrated that electron transport in a nanowire-based ZnO electrode was several hun[a] Dr. W. Peng, Dr. L. Han Photovoltaic Materials Unit National Institute for Materials Science Tsukuba, Ibaraki 305-0047 (Japan) E-mail: [email protected] [b] Dr. L. Han State Key Laboratory of Metal Matrix Composites Shanghai Jiaotong University Shanghai 200240 (P. R. China) [c] Dr. Z. Wang Environmental Management Technology Research Institute National Institute of Advanced Industrial Science and Technology Tsukuba, Ibaraki 305-8569 (Japan) Chem. Eur. J. 2014, 20, 8483 – 8487
teristics were studied by electrochemical impedance measurements, and reduced electron recombination and longer electron lifetime were observed for the ZnO nanorods. Photovoltaic characterization demonstrated that DSCs utilizing the hierarchical nanorods significantly improved the overall conversion efficiency by 34 % compared to nanoparticlebased DSCs.
dred times faster than in a nanoparticle electrode.[11] Improved electron lifetime was achieved by using 1D nanostructures.[14, 15] In addition, 1D structures with large sizes have superior lightscattering ability compared to nanoparticles and thus can enhance light-harvesting efficiency. Unfortunately, such 1D structures have much lower surface area than nanoparticles, which leads to inferior photocurrent and conversion efficiencies of DSCs. To address this issue, construction of hybrid systems by mixing nanoparticles and 1D nanomaterials has been proposed. Different materials, including ZnO nanoparticles, layered basic zinc acetate nanoparticles, and TiO2 nanoparticles, were introduced into the matrix of ZnO 1D nanostructures, and enhanced photovoltaic performance was reported for these hybrid photoanodes.[16–20] However, the total internal surface area of such hybrid materials is still unavoidably decreased compared to that of nanoparticles. Moreover, fabricating the hybrid photoanode requires a multistep process and thus higher cost compared to a single-composition photoanode. Herein, we report a multifunctional photoanode for DSCs consisting of ZnO nanorods prepared by a facile solvothermally assisted process. The ZnO nanorods are composed of interconnected 30 nm nanoparticles. Such a photoanode can not only enhance light harvesting by superior light scattering and good dye loading, but also facilitates electron transport. Photovoltaic characteristics of DSCs based on the ZnO hierarchical nanorods were investigated, and a 34 % enhancement of conversion efficiency was achieved compared to DSCs with ZnO nanoparticles prepared at low concentrations.
Results and Discussion Hierarchical ZnO nanorods were obtained at a zinc acetate concentration of 0.6 m (see Experimental Section). The morphology and size of the ZnO products were characterized by field-emission (FE) SEM. As shown in Figure 1 a, the products 8483
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Figure 3. XRD patterns of a) hierarchical nanorods and b) nanoparticles.
Figure 1. a) FESEM image, b) TEM image, and c) magnified TEM image of the rectangular area in b) of ZnO hierarchical nanorods. The inset of b) shows the corresponding SAED pattern.
have a 1D rod shape with diameters of 100–400 nm and lengths of 1–2 mm. The low- and high-magnification TEM images (Figure 1 b and c) demonstrate that ZnO nanorods are composed of 20–50 nm particles. These nanoparticles are tightly connected, which is beneficial for electron transport. In addition, mesopores can be observed in the nanorods. The selected-area electron diffraction (SAED) pattern (inset of Figure 1 b) revealed that the hierarchical nanorods are polycrystalline and have wurtzite ZnO structure. To investigate the effect of morphology, we also prepared ZnO nanoparticles at a lower zinc acetate concentration of 0.03 m. The FESEM and TEM images of ZnO nanoparticles are shown in Figure 2. The wurtzite structure of the ZnO nanoparticles was confirmed by the SAED pattern (inset of Figure 2 b). Crystal structures of ZnO hierarchical nanorods and nanoparticles were investigated by XRD (Figure 3). All diffraction peaks could be indexed to the hexagonal wurtzite structure of ZnO, in agreement with the results of SAED analysis (insets of Figures 1 b and 2 b). The sharp peaks show that both ZnO products are well crystallized. By applying the Debye–Scherrer equation to the (100), (002), and (101) peaks, the average crystallite sizes of ZnO nanorods and nanoparticles were estimated to be 32
Figure 2. a) FESEM image and b) TEM image of ZnO nanoparticles. The inset of b) shows the corresponding SAED pattern. Chem. Eur. J. 2014, 20, 8483 – 8487
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and 29 nm, respectively. Thus, XRD also showed that the ZnO nanorods are composed of small particles. The as-prepared ZnO hierarchical nanorods and nanoparticles were applied as photoanodes in DSCs. The cross-sectional SEM images of the two photoelectrodes (Figure 4) revealed
Figure 4. Cross-sectional SEM images of a) hierarchical nanorod film and b) nanoparticle film on FTO glass. The inset of a) shows a magnified image.
film thickness of about 20 mm. Figure 5 a shows the photocurrent density–voltage (J–V) curves of DSCs based on hierarchical nanorods and nanoparticles under illumination with standard AM 1.5 sunlight (100 mW cm 2). The ZnO photoanode made from hierarchical nanorods exhibits a short-circuit current JSC of 9.07 mA cm 2, an open-circuit voltage VOC of 0.664 V, and an overall conversion efficiency h of 4.13 %. The photovoltaic performance was greatly enhanced compared to the nanoparticle photoanode, which gave a JSC of 7.03 mA cm 2, a VOC of 0.643 V, and an efficiency of 3.08 % (Table 1). Clearly, the improved conversion efficiency of the nanorod-based photoanode is mainly attributable to the increased JSC value. Measurement of the incident photon-to-current conversion efficiency (IPCE) provided further evidence for the variation in JSC. Figure 5 b shows the IPCE spectra of the nanorod and nanoparticle photoanodes. Compared to the nanoparticle film, the nanorod film has a higher IPCE in the wavelength range from 400 to 700 nm. The peak of the IPCE spectrum at 530 nm, which corresponds to the absorption characteristic of the N719 dye, is 62 % for the nanorod film. In the case of the nanoparticle film, the maximum IPCE decreased to 50 %. These results indicate that light harvesting is increased for the nanorod film compared to the nanoparticle film.
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Figure 6. Reflectance spectra of ZnO nanoparticle and hierarchical nanorod films.
red region, which suggests superior light scattering of the nanorods. The nanorods can lengthen the optical pathway within the photoanode due to superior light scattering and thus increase the probability of photon capture by dye molecules, which leads to enhanced JSC. Electrochemical impedance spectroscopy (EIS) is a useful technique to study charge transfer and recombination in DSCs.[24–26] Figure 7 shows the EI spectra of the nanorod- and nanoparticle-based DSCs obtained in the dark at an applied bias of 0.62 V in the frequency range from 100 kHz to 0.1 Hz. The Nyquist plot of each device is composed of two semicircles (Figure 7 a). The right semicircle in the medium-frequency region (0.1–1000 Hz) corresponds to the charge-transfer process at the oxide/dye/electrolyte interface, whereas the left
Figure 5. J–V curves (a) and IPCE spectra (b) of DSCs based on hierarchical nanorods and nanoparticles.
Table 1. Photovoltaic performance of DSCs based on nanorods and nanoparticles.
nanorods nanoparticles
JSC [mA cm 2]
VOC [V]
FF[a]
h [%]
9.07 7.03
0.664 0.643
0.686 0.683
4.13 3.08
[a] Fill factor.
To understand the effect of different ZnO nanostructures on JSC, the dye-adsorption capability was evaluated. The amounts of adsorbed dye for the hierarchical nanorod film and the nanoparticle film were 1.13 10 7 and 1.21 10 7 mol cm 2, respectively. The comparable dye loadings of the two films can be attributed to the similar surface areas of nanorods (24 m2 g 1) and nanoparticles (25 m2 g 1). Thus, the increased JSC for the nanorod photoanode relative to the nanoparticle electrode should be caused by the light scattering of the hierarchical nanorods. SEM and TEM images show that the hierarchical nanorods have submicrometer sizes, which are comparable to the wavelengths of visible light and thus expected to promote light scattering.[21–23] The reflectance spectra of the films were measured to estimate the light-scattering properties. Figure 6 compares the UV/Vis reflectance spectra of the nanorod and nanoparticle films. The nanorod film exhibits much higher light reflectance than the nanoparticle film in the entire visible wavelength range. The reflectance of the nanoparticle film quickly declines to about 18 % in the wavelength range of 500–900 nm. On the contrary, the nanorod film maintains a high reflectance of greater than 62 % even in the infraChem. Eur. J. 2014, 20, 8483 – 8487
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Figure 7. EI spectra of DSCs based on ZnO nanoparticles and hierarchical nanorods. a) Nyquist and b) Bode phase plots.
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Full Paper semicircle in the high-frequency region (> 1000 Hz) reflects the redox reaction at the counter electrode/electrolyte interface.[24] Compared to the nanoparticle cell, the nanorod-based device has a larger radius of the right semicircle, which indicates increased charge-recombination resistance (i.e., longer electron lifetime), which implies that photoexcited electrons can travel a longer distance in the hierarchical nanorods composed of interconnected nanoparticles than in the nanoparticles. The electron lifetime te can be evaluated from the peak frequency fp in the medium-frequency region according to the equation te = 1/2pfp. The Bode phase plots (Figure 7 b) show that the peak frequency in the medium-frequency region shifts from 25.1 Hz for the nanoparticle cell to 15.8 Hz for the nanorod cell. This suggests that the nanorod photoanode has a longer electron lifetime than the nanoparticle photoanode. Figure 4 shows that the nanorod photoanode achieves a higher VOC than the nanoparticle photoanode. The increased VOC may be explained by the longer electron lifetime of the nanorods. Similar characteristics have also been observed in nanotubes and nanowires.[27, 28]
Conclusion ZnO hierarchical nanorods with diameters of 100–400 nm and lengths of 1–2 mm were synthesized at a precursor concentration of 0.6 m. TEM observations confirmed that the hierarchical nanorods were composed of interconnected 30 nm nanoparticles. The ZnO nanorods were applied as photoanodes for DSCs and achieved an overall conversion efficiency of 4.13 %, that is, a 34 % enhancement in comparison with the ZnO nanoparticles obtained at lower concentration (0.03 m). The improved performance was ascribed to superior light scattering, sufficient dye loading, and enhanced electron-transport properties of the hierarchical nanorods.
(1/1 v/v) for 1 h in the dark. The photoanode and counter electrode (platinum-coated FTO glass) were sealed together with a Surlyn hot-melt film. Finally, assembly of a cell was completed by injecting the electrolyte into the cell through a hole on the back of the counter electrode. The electrolyte compositions were 0.6 m methylpropylimidazolium iodide, 0.05 m I2, and 0.1 m LiI in acetonitrile with tert-butylpyridine, as described in our previous work.[31]
Characterization XRD patterns were recorded on a Rigaku Ultima3 X-ray diffractometer at 40 kV and 40 mA. Morphologies and sizes were characterized with a JEM-2100F transmission electron microscope and a JSM-7001F field-emission scanning electron microscope. Specimens for TEM observation were prepared by dropping a small amount of an aqueous suspension onto carbon-coated copper grids. UV/Vis spectra of the film samples were recorded on a Shimadzu UV-3600 spectrophotometer equipped with an integrating sphere. BET surface area was measured with a volumetric N2 adsorption/desorption apparatus (Belsorp 18A, BEL Japan) at 77 K. Dye desorption experiments were carried out by soaking the dyesensitized films in 0.1 m solution of NaOH in water/ethanol (1/1 v/v). Photovoltaic performance of the DSC devices was measured by using a Keithley 2400 source meter under 1 Sun illumination (AM 1.5, 100 mW cm 2, WXS-90S-L2, Wacom). The active area of each device was 0.25 cm2. IPCE measurements were performed with monochromatic incident light at 100 mW cm 2 (CEP-2000 BX, Bunkoh-Keiki). EIS was performed on an electrochemical workstation (Solartron 1287 and 1255B) in the dark under a bias voltage at frequencies ranging from 100 kHz to 0.1 Hz.
Acknowledgements This work was financially supported by Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Agency. Keywords: energy conversion · nanostructures · solar cells · zinc oxide
Experimental Section
nanoparticles
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Preparation of ZnO hierarchical nanorods and nanoparticles ZnO nanostructures were synthesized by a solvothermally assisted method. For the synthesis of ZnO hierarchical nanorods, equimolar ethanolic solutions (0.6 m) of zinc acetate and oxalic acid were first prepared. When these solutions were mixed under stirring, a white gel was formed.[29, 30] The gel was heated in an autoclave at 80 8C for 6 h and then allowed to cool to room temperature. The obtained precipitate was collected by centrifugation and washed with ethanol. After calcination in air at 450 8C for 1 h, hierarchical nanorods were obtained. For comparison, ZnO nanoparticles were prepared by a similar procedure, except for lower concentrations (0.03 m) of zinc acetate and oxalic acid.
Fabrication of DSCs To prepare the paste, ZnO products were dispersed in a mixture of ethanol and deionized water by mechanical grinding and ultrasonic treatment. Then, the ZnO paste was coated onto fluorine-doped tin oxide (FTO) glass and sintered in an oven at 200 8C to give a 20 mm-thick film. The resulting ZnO photoanode film was dipped into a 0.5 mm solution of N719 dye in acetonitrile/tert-butanol Chem. Eur. J. 2014, 20, 8483 – 8487
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Received: February 19, 2014 Published online on May 30, 2014
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