Vertically Aligned ZnO Nanorod Arrays Sentisized with Gold

Jul 7, 2009 - City UniVersity of Hong Kong, Hong Kong Special AdministratiVe Region, People's Republic of China. ReceiVed: April 6, 2009; ReVised ...
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J. Phys. Chem. C 2009, 113, 13433–13437

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Vertically Aligned ZnO Nanorod Arrays Sentisized with Gold Nanoparticles for Schottky Barrier Photovoltaic Cells Z. H. Chen, Y. B. Tang, C. P. Liu, Y. H. Leung, G. D. Yuan, L. M. Chen, Y. Q. Wang, I. Bello,* J. A. Zapien,* W. J. Zhang, C. S. Lee, and S. T. Lee Center of Super-Diamond and AdVanced Films (COSDAF) and Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong Special AdministratiVe Region, People’s Republic of China ReceiVed: April 6, 2009; ReVised Manuscript ReceiVed: May 26, 2009

Vertically aligned zinc oxide (ZnO) nanorod arrays coated with gold nanoparticles have been used in Schottky barrier solar cells. The nanoparticles enhance the optical absorption in the range of visible light due to the surface plasmon resonance. In charge separations, photoexcited electrons are transferred from gold nanoparticles to the ZnO conduction band while electrons from donor (I-) in the electrolyte compensate the holes left on the gold nanoparticles. The fill factors of the dye-free Schottky barrier cell reach a value of ∼0.50. However, after incorporation of N719 sensitizing dye, the open circuit voltage increases to 0.63 V from 0.5 V being measured for dye-sensitized solar cells based on the bare ZnO nanorods. The Schottky barrier at the ZnO/Au interface blocks the electron transfer back from ZnO to the dye and electrolyte, and thus increases the electron density at the ZnO conduction band. The efficiency of the gold-coated ZnO nanorod dye-sensitized solar cells is thus increased from 0.7% to 1.2%. 1. Introduction Quantum dots and nanoparticles provide new approaches for effective energy harvesting in the visible and infrared regions of the solar spectra.1-4 Gold nanoparticles (NPs) are one of the nanosystems in which light absorption in the visible spectrum depends on the NPs size due to the collective electron oscillations at the surface.5 Recently, gold nanoparticles have been investigated for their photocatalytic,6 optical,7 biosensing,8 and potential photovoltaic9 applications. In the structure of a solar cell device, gold nanoparticles are employed to interface semiconductors and serve as a Schottky barrier thus reducing the electron-hole recombination. Tetsu et al.10 studied the enhancement of anodic photocurrents induced by visible light irradiation in a device structure based on gold nanoparticles deposited on TiO2 films. Their data indicate that using gold Schottky contacts in photovoltaic cells may indeed yield higher device performance. Both TiO2 and ZnO are inherently n-type semiconducting materials for which the n-type conductivity is usually caused by oxygen vacancies.11-13 Although the conduction band minimum of ZnO is nearly aligned with that of TiO2,14 the electron mobility of ZnO is greater than that of TiO2.15 However, the most important advantage of ZnO over TiO2 is the fact that ZnO can be prepared by many deposition techniques and in a variety of morphologies comprising nanowires/nanorods,16 nanotubes,17 tetrapods,18 etc. Thus, ZnO electrodes can be tailored for extremely large surfaces, for example, using vertically aligned nanowires/nanorods.19,20 Nanostructuring the electrodes and interfaces in return can enhance the performance of many electronic devices including solar cells.21 Some methodologies of structuring the electrodes are easily scalable and suitable for various substrates.22,23 These arguments make ZnO a very versatile and attractive material for fabrication of solar cell devices. This is particularly true when ZnO nanostructures can * To whom correspondence should be addressed. E-mail: apibello@ cityu.edu.hk and [email protected].

be prepared at low temperature and low cost by solution techniques and applied to, for instance, dye-sensitized solar cells (DSSCs) comprising ruthenium complex dye and iodide electrolyte in addition to front and back electrodes.24 The efficiency of such DSSCs is partly limited by the reverse recombination reactions of photoinjected electrons with triiodide ions in the electrolyte, and partly by the presence of electron acceptors, such as oxygen and iodine. These processes are then responsible for losing photogenerated electrons during their transport via the electrolyte and nanostructured semiconducting interface to the back electrode. Consequently both the short circuit photocurrent (Isc) and overall efficiency (η)25 are reduced. One of the approaches to suppress the recombination processes is modification of the ZnO nanorod (NR) surfaces by incorporation of quantum dots to block the transfer of electrons from the ZnO conduction band back to the dye and/or electrolyte.26 In the present work, we use gold nanoparticles deposited on the surface of aligned ZnO nanorods to construct the Schottky barrier photovoltaic cells comprising ruthenium dye N719 [bis(tetrabutylammonium)-cis-(dithiocyanato)-N,N′bis(4-carboxylate-4′-carboxylic acid-2,2′-bipyridine)ruthenium(II)]. We show that the Schottky barrier at the ZnO/Au interface results in improved efficiency compared with bare ZnO nanorod based DSSCs. 2. Experimental Section We prepared ZnO nanorod arrays on ITO coated glass substrates by a low-temperature chemical seeding method.13,14 The ZnO nanorods were then immersed in a chloroauric acid (HAuCl4, 1 mM) solution with a pH value between 8 and 9 being adjusted by sodium citrate (Na3C6H5O7 · 2H2O). The Au ions (Au3+) were reduced to neutral atoms attached to ZnO nanorod surfaces. The morphology and structure of the asprepared and gold nanoparticles coated ZnO nanorods were analyzed with a Philips XL30 FEG scanning electron microscope (SEM) and a Philips CM20 transmission electron mi-

10.1021/jp903153w CCC: $40.75  2009 American Chemical Society Published on Web 07/07/2009

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Figure 1. (a) A representative SEM image of bare ZnO nanorods. (b) SEM image of ZnO nanorods coated with Au nanoparticles. (c) TEM image of ZnO nanorods with precipitated Au nanoparticles. The inset is the HRTEM image at the ZnO-Au interface. (d) XRD patterns acquired from bare ZnO nanorods and ZnO nanorods with gold nanoparticles (red line) on ITO substrates. The inset is the scaled-up XRD patterns over the 2-θ scan angles ranging from 30° to 80°.

croscope (TEM). High-resolution transmission electron microscopic (HRTEM) images were obtained with a Philips CM200 FEG TEM operated at 200 kV. X-ray diffraction (XRD) spectra were recorded with a Siemens D500 diffractometer. Ultravioletvisible spectroscopy (UV-vis) spectra were acquired at room temperature with a LAMBDA 750 UV-vis spectrophotometer. The substrate with ZnO nanorod arrays coated by gold nanoparticles, and with or without application of the dye N719, was sandwiched between platinum (Pt) and ITO counter electrodes. The two electrodes were separated by a ∼20 µm polypropylene spacer and bonded with metal clips. The internal space of the cells was filled with a liquid electrolyte and sealed by capillary forces. The electrolyte consisted of 0.1 M LiI, 50 mM I2, and 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII) dissolved in acetonitrile. Current density-voltage (J-V) characteristics were measured with assistance of air mass 1.5 global (AM1.5G) illumination, a calibrated solar simulator, with an intensity of 100 mW/cm2 (equivalent to 1 sun). 3. Results and Discussion Vertically aligned ZnO nanorod arrays were synthesized on an underlying ITO film on glass. The diameter of the ZnO nanorods varies from ∼50 to ∼130 nm, while their length ranges from 3 to 5 µm. The nanorods have uniform diameters and smooth surfaces along their whole lengths as demonstrated by the SEM image in Figure 1a. The vertically aligned ZnO nanorods grown on an ITO film and coated by gold nanoparticles, used in the DSSC architecture, are depicted in the SEM image in Figure 1b. The distribution of the nanoparticles over individual nanowires is fairly uniform as is apparent from the SEM image (Figure 1b) and with a closer inspection as shown in the bright field TEM image in Figure 1c. The diameter of the precipitated Au nanoparticles on the ZnO nanorod surface ranges from 20 to 30 nm. The inset in Figure 1c shows a HRTEM image taken near the edge of the ZnO nanorod surface and illustrates its single crystalline structure. The coherent interface between the ZnO nanorod surface and gold nanoparticles demonstrates their good attachment.

Figure 2. (a) Attenuance spectra of ZnO nanorods coated with Au nanoparticles, bare ZnO nanorod, and ITO film. (b) J-V characteristics of a solar cell device with bare ZnO nanorods and ZnO nanorods coated with gold nanoparticles under simulated white light illumination of 100 mW/cm2. (c) A schematic of a band diagram corresponding to the ZnO/ Au/electrolyte cell structure.

The XRD patterns of the prepared bare ZnO nanorods and ZnO nanorods with precipitated gold nanoparticles (red line) are shown in Figure 1d. The intense ZnO (002) and (004) peaks indicate that the prepared ZnO nanorods are crystalline. The inset in Figure 1b presents additional XRD detail on the 30° to 80° 2-θ scan range and reveals smaller peaks at 37°, 44°, and 65° which correspond to the gold nanoparticles anchored on the surfaces of ZnO nanorods. Attenuance studies over the 400-800 nm range of the glass/ ITO electrode with and without ZnO nanorods and gold nanoparticles have been evaluated as illustrated in Figure 2a. The attenuance is the lowest for the ITO substrate as is expected for the transparent electrode; the spectral attenuance increases slightly for the ITO/ZnO nanorods structure resulting mostly from light scattering. Finally, the absorption spectrum of the ITO/ZnO nanorods structure with precipitated gold nanoparticles is characterized by a large attenuance over the measured spectral range and a clear peak close to 490 nm that is attributed to the plasmon resonance absorption peak arising from the gold nanoparticles. The J-V characteristics of the sandwiched PV structures with both bare ZnO nanorods and gold coated ZnO nanorods were examined. These cells are dye free and use an iodine electrolyte. Upon illumination with simulated sunlight, the device with ZnO nanorods coated with gold nanoparticles can deliver photocurrent while the device with bare ZnO nanorods cannot supply measurable photocurrent. Under the illumination intensity of 100 mW/cm2, the J-V characteristic in Figure 2b shows the short circuit current density and open circuit voltage are JSC ) 1.72 mA/cm2 and VOC ) 0.37 V, respectively. The fill factor

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(FF) and power conversion efficiency (η) can be determined from the following equations

FF ) VMJM/VOCJSC

(1)

η ) VOCJSC /FF

(2)

where VOC is the open-circuit voltage, JSC is the short circuit current, FF is the fill factor, and VM and JM are the voltage and current density at the maximum power output, respectively.27,28 The calculation using the J-V characteristic obtained under the simulated illumination of 100 mW/cm2 yields the fill factor FF ) ∼0.46 and power conversion efficiency η ) ∼0.30%, respectively. The key point for discussion of the observed photovoltaic effect is in separation of induced electric carriers in the presented structures. First of all, the structures of ZnO nanorods with anchored nanoparticles enhance the optical absorption in the visible light range due to the surface plasmon resonance of gold nanoparticles, which is evident in Figure 2a. Then the induced charge can be separated at plasmon excited gold nanoparticles without degradation. The electrons excited at the gold nanoparticle surfaces can be transported to the conduction band of ZnO nanorods, and drift to the bottom ITO electrode (Figure 2c) via the following elementary process. First, the Schottky Au-ZnO contact enables the injection of electrons from gold to ZnO nanorods but transport of electrons in the reverse direction is restricted by the formed Schottky barrier. Second, the Au ions capture the electrons donated by the redox species (I-) of the electrolyte to compensate for their lost electrons. Last, the triiodide (I3-) then obtains electrons at the counter electrode and regenerates oxidized iodide (I-). The electrontransfer process of the redox couple should be fast and their redox potential should be sufficiently negative to provide electrons to gold continuously. The whole process of generating photocurrent within the dye-free Schottky barrier solar cell is demonstrated by following reactions:

Anode:

Au + hν f Au++e-(Au)

2Au+ + 3I- f 2Au + I3Cathode:

I3- + 2e-(counter electrode) f 3I-

The charge transport kinetics is further clarified by the dependence of the short-circuit photocurrents on the redox potential of the donor/acceptor couples. Obviously there is an optimum potential for inducing the donor/acceptor redox couples. The donor potential should be more negative than that of the gold nanoparticles with holes. Otherwise the donor cannot provide electrons to the gold nanoparticles. On the other hand, the potential should be more positive than that of the ZnO conduction band to prevent the acceptor receiving electrons from the nanoparticles transported from ZnO and the counter electrode. This rectifying provision facilitates the ZnO-Au interface working as a Schottky contact. Among the examined donors we use the I- donor electrolyte providing a large fill factor FF and good cell performance. The effect of light intensity on the solar cell performance has also been investigated. Figure 3a shows the J-V curves and the variation of solar cell performance characteristics under

Figure 3. Characteristics of a measured solar cell device based on ZnO nanorods coated with gold nanoparticles at different light intensities (40, 60, 80, and 100 mW/cm2). (a) J-V curves. (b) Plot of short-circuit current density (JSC, black circles) and open-circuit voltage (VOC, blue squares). (c) Plot of fill factor (FF, black circles) and power-conversion efficiency (η, blue squares).

different illumination intensities. The short circuit current JSC follows an approximately linear relationship with varying the illumination intensity, whereas the open voltage circuit VOC is merely constant (Figure 3b). The saturation of the open circuit voltage might be caused by suppression of redox potential with decelerated electron transfer from the donor to the hole in the gold particle and accelerated reverse electron transfer from the gold or ZnO conduction band to the oxidized donor. Figure 3c shows that the energy conversion efficiency of these devices increases when the illumination intensity increases from 40 to 100 mW/cm2. At the same time, the fill factor generally decreases with increasing the light intensity, which is probably caused by an increase in the resistive loss.27,28 To further improve the device performance, the commonly used N719 dye has been incorporated into the device architecture. The hybrid ZnO/Au/dye structures with identical iodine electrolyte have been fabricated and investigated. It was found (see Figure 4a, red circles) that upon illumination of the front surface with simulated sunlight, the cell with ZnO nanorods coated by Au nanoparticles yields open circuit voltage and short circuit current (∼0.62 V and 4.2 mA/cm2) which are significantly higher than those of the cell consisting of only bare ZnO nanorods (∼0.5 V and 3.8 mA/cm2). As a result, the power conversion efficiency (η) was increased from 0.7% to 1.2%. Incorporation of dye N719 into the device architecture facilitates better matching band structures with the absorption spectrum of dye.24 As a result the excited singlet states of the dye participate in inducing sensitized photocurrent. The electron transfer from the excited dye into the ZnO nanorods occurs probably via a combination of two possible mechanisms. In the first mechanism the electrons from the lowest unoccupied molecular orbital (LUMO) band of the dye (electrolyte in

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Chen et al. in the solution. The use of a thick layer of gold nanoparticles results in a decrease in the current density and fill factor FF as illustrated by the blue line in Figure 4a. In this case, the fill factor FF decreases from 0.36 to 0.29, while the power conversion efficiency decrease from 0.70% to 0.58%. The thicker monolayer of gold nanoparticles reduces the electron transition by tunneling from the excited dye to ZnO nanorods. Therefore, controlling the gold nanoparticles coating is vital for optimizing the performance of the Shottky barrier solar cells. 4. Conclusion

Figure 4. (a) J-V characteristics of solar cells obtained under simulated AM1.5G illumination with intensity of 100 mW/cm2 based on (i) the bare ZnO nanorods (black sqares), (ii) ZnO nanorods coated with a thick gold layer with 5 mM HAuCl4 (blue triangles), and (iii) ZnO nanorods coated with a thin layer gold of nanoparticles with 1 mM HAuCl4 (red circles). (b) Energy level diagram and mechanism of photocurrent generation in the photoelectrochemical cell with ITO/ ZnO/Au/Dye as the photoanode. A detailed description can be found in the text.

Figure 4) are transported to the conduction band (C.B.) of ZnO nanorods via tunneling effect across a thin layer of gold nanoparticles. The second mechanism involves two steps: (i) First the ITO/ZnO/Au/dye electrode, at a higher energy level upon excitation with visible light being predominantly absorbed by the dye (electrolyte), injects electrons into the gold nanoparticles, as illustrated in Figure 4b. It is further believed that the second mechanism leads to electron accumulation in the gold nanoparticles raising their Fermi levels to more negative potentials until matching the Fermi level of ZnO, whereupon a quick transport of electrons from Au to ZnO takes place.29 The electrons transferred to ZnO nanorods are collected at the bottom ITO electrode, and thereby generate an anodic photocurrent. The Schottky barrier at the Au-ZnO interface reduces charge recombination by blocking the transfer of electrons from the ZnO conduction band to the dye and/or electrolyte thus improving the efficiency of DSSCs. The simplified sequence of generation of sensitized photocurrent in an ITO/ZnO/Au/ dye electrode in electrolyte can be illustrated as follows:

Finally we observe that a large density of multilayer gold nanoparticles is in fact counter productive. We have prepared a thick layer of gold nanoparticles covering the ZnO nanorods (not shown) by increasing the concentration of chloroauric acid from 1 mM (see Section 2) to 5 mM while, as previously, keeping the pH value of the solution between 8 and 9 by adjusting the amount of sodium citrate (Na3C6H5O7´2H2O)

In summary, ZnO nanorod arrays coated with gold nanoparticles have been incorporated in the architecture of solar cells. The charge separation is obtained by electron transfer from the excited gold nanoparticles to ZnO and compensation of the hole left on gold nanoparticles by an electrolyte donor. The reverse electron transfer from ZnO to gold nanoparticles is blocked by the interfacial ZnO-Au Schottky barrier. The fill factor of the Schottky barrier solar cell reaches the value of 0.50. Incorporation of N719 dye into the device architecture results in increased open circuit voltage from 0.5 to 0.65 V with respect to the DSSC based on bare ZnO nanorods. The enhancement of the performance is probably associated with the injection of electrons from gold nanoparticles into ZnO and simultaneous tunneling electrons from dye into ZnO. The efficiency of these hybrid gold nanoparticle-coated ZnO nanorod DSSCs can be improved from 0.7% to 1.2%. Acknowledgment. This work was fully supported by the Research Grant Council of Hong Kong under grant no. CityU110208. References and Notes (1) Murry, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (2) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877. (3) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (4) Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2008, 130, 4007. (5) Hu, X. P.; Blackwood, D. J. J. Electroceram. 2006, 16, 593. (6) Lia, D.; McCanna, J. T.; Gratta, M.; Xia, Y. N. Chem. Phys. Lett. 2004, 394, 387. (7) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (8) Haes, A. J.; Duyne, R. P. V. J. Am. Chem. Soc. 2002, 124, 10596. (9) Su, Y. H.; Lai, W. H.; Teoh, L. G.; Hon, M. H.; Huang, J. L. Appl. Phys. A: Mater. Sci. Process. 2007, 88, 173. (10) Yang, T.; Tetsu, T. J. Am. Chem. Soc. 2005, 127, 7632. (11) Yuan, G. D.; Zhang, W.-J.; Jie, J. S.; Fan, X.; Tang, J. X.; Shafiq, I.; Ye, Z. Z.; Lee, C. S.; Lee, S. T. AdV. Mater. 2008, 20, 168. (12) Kim, Y. S.; Park, C. H. Phys. ReV. Lett. 2009, 102, 086403. (13) Diebold, U.; Anderson, J. F.; Ng, K. O.; Vanderbilt, D. Phys. ReV. Lett. 1996, 77, 1322. (14) Ramaneti, R.; Lodder, J. C.; Jansen, R. Phys. ReV. B 2007, 76, 195207. (15) Quintana, M.; Edvinsson, T.; Hagfeld, A.; Boschloo, G. J. Phys. Chem. C 2007, 111, 1035. (16) Gao, P. X.; Ding, Y.; Wang, Z. L. Nano Lett. 2003, 3, 1315. (17) Sun, Y.; Fuge, G. M.; Fox, N. A.; Riley, D. J.; Ashfold, M. N. R. AdV. Mater. 2005, 17, 2477. (18) Liu, Y.; Chen, Z. H.; Kang, Z. H.; Bello, I.; Fan, X.; Shafiq, I.; Zhang, W. J.; Lee, S. T. J. Phys. Chem. C 2008, 112, 9214. (19) Greene, L.; Law, M.; Tan, D. H.; Goldberger, J.; Yang, P. Nano Lett. 2005, 5, 1231. (20) Chung, T. F.; Luo, L. B.; He, Z. B.; Leung, Y. H.; Shafiq, I.; Yao, Z. Q.; Lee, S. T. Appl. Phys. Lett. 2007, 91, 233112. (21) Fan, Y.; Max, S.; Forrest, S. R. Nat. Mater. 2004, 4, 37. (22) Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y.; Saykally, R. J.; Yang, P. Angew. Chem., Int. Ed. Engl. 2003, 42, 3031. (23) Wang, Y. D.; Zang, K. Y.; Chua, S. J.; Fonstad, C. G. Appl. Phys. Lett. 2006, 89, 263116.

Vertically Aligned ZnO Nanorod Arrays (24) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (25) Barazzouk, S.; Hotchandani, S. J. Appl. Phys. 2004, 96, 12. (26) Li, C. C.; Li, L. M.; Du, Z. H.; Yu, H. C.; Xiang, Y. Y.; Li, Y.; Cai, Y.; Wang, T. H. Nanotechnology 2008, 19, 035501. (27) Tang, Y. B.; Chen, Z. H.; Song, H. S.; Lee, C. S.; Cong, H. T.; Cheng, H. M.; Zhang, W. J.; Bello, I.; Lee, S. T. Nano Lett. 2008, 8, 12.

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