Growth of Comb-like ZnO Nanostructures for Dye-sensitized Solar Cells Applications
© to the authors 2009
Received: 20 April 2009
Accepted: 14 May 2009
Published: 29 May 2009
Dye-sensitized solar cells (DSSCs) were fabricated by using well-crystallized ZnO nanocombs directly grown onto the fluorine-doped tin oxide (FTO) via noncatalytic thermal evaporation process. The thin films of as-grown ZnO nanocombs were used as photoanode materials to fabricate the DSSCs, which exhibited an overall light to electricity conversion efficiency of 0.68% with a fill factor of 34%, short-circuit current of 3.14 mA/cm2, and open-circuit voltage of 0.671 V. To the best of our knowledge, this is first report in which thin film of ZnO nanocombs was used as photoanode materials to fabricate the DSSCs.
The II-VI semiconductor ZnO is one of the most important multifunctional materials due to its various exotic properties such as direct wide band gap (3.37 eV) and high optical gain of 300 cm−1 (100 cm−1 for GaN) at room temperature, large saturation velocity (3.2 × 107 cm/s), high breakdown voltage, large exciton binding energy (60 meV), piezoelectric, biocompatibility, and so on [1–12]. ZnO can be used in variety of high-technological practical applications such as ultraviolet (UV) lasers, light-emitting diodes, photodetectors, piezoelectric transducers and actuators, hydrogen storage, chemical and biosensors, surface acoustic wave guides, solar cells, photocatalysts, etc. [1–24]. Among various applications, the use of ZnO nanomaterials as photoelectrodes for the fabrication of dye-sensitized solar cells (DSSCs) has received a great attention due to its compatibility and higher electronic mobility with TiO2 nanomaterials and similar electron affinity and band gap (3.37 eV at 298 K) . Therefore, some ZnO nanostructures have been used as photoelectrode materials for the fabrication of DSSCs and reported in the literature [15–21]. Hsu et al.  reported the ZnO nanorods-based DSSC with the electricity conversion efficiency (ECE) of 0.22%. Branched ZnO nanowires based DSSCs, grown by thermal evaporation process at 800–1,000 °C, with an ECE of ~0.46% have been reported by Suh et al. . In another report, by using branched ZnO nanowires grown by MOCVD process, the fabricated DSSCs exhibited an ECE of ~0.5% . Cheng et al.  also demonstrated the thermally grown ZnO nanorods-based DSSC with the ECE of 0.6%.
In this paper, we report the direct synthesis of well-crystallized ZnO nanocombs on FTO substrates and their DSSCs application. To fabricate the DSSCs, the thin films of as-grown ZnO nanocombs on FTO substrates were used as photoanode materials, which exhibited an overall light to electricity conversion efficiency of 0.68%. To the best of our knowledge, the use of ZnO nanocombs for the fabrication of DSSCs is not reported yet in the literature.
ZnO nanocombs were grown in a horizontal quartz tube furnace on the FTO substrate. The high purity metallic zinc powder (99.999%) and oxygen gas were used as source materials. In a typical reaction process, about 1.5 g of metallic zinc powder was put into a ceramic boat and placed at the center of the quartz tube. The furnace temperature was raised up to the desired temperature, and oxygen and nitrogen were fed continuously into the quartz tube furnace with the flow rates of 60 and 240 sccm, respectively. The temperature of the substrate, placed 8-cm away from the source boat, was 570 °C. The reaction lasted for 60 min. During this period, the metallic zinc was vaporized and oxidized with O2, and finally deposited onto the FTO substrate.
For DSSC fabrication, the prepared ZnO nanocomb thin-film electrodes was immersed in the ethanolic solution of 0.3 mM cis-bis (isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium (II) bis-tetrabutylammonium (N719, Solaronix) at room temperature for 6 h. The dye-adsorbed ZnO nanocombs thin-film electrodes were then rinsed with acetonitrile and dried under a nitrogen stream. Pt counter electrode was prepared by electron beam deposition of a thin layer of Pt (~ 60 nm) on the top of ITO glass. The Pt electrode was placed over the dye-adsorbed ZnO nanocombs electrode, and the edges of the cell were sealed with 60-μm thick sealing sheet (SX 1170-60, Solaronix). Sealing was accomplished by pressing the two electrodes together on a double hot-plate at a temperature of about 70 °C. The electrolyte, consisting of 0.5 M LiI, 0.05 mM I2, and 0.2 M tert-butyl pyridine in acetonitrile, was introduced into the cell through one of two small holes drilled in the counter-electrode. The holes were then covered and sealed with a small square of sealing sheet and microscope objective glass. The resulting cell had an active area of about 0.25 cm2. Photocurrent–Voltage (I–V) curve was measured by using computerized digital multimeters. The light source was 1000-W metal halide lamp, and its radiant power was adjusted with respect to Si reference solar cell to about one-sun-light intensity (100 mW/cm2).
Results and Discussion
Structural and Optical Properties of As-grown ZnO Nanocombs
As a wurtzite hexagonal-phase ZnO possesses a positively charged Zn-(0001) surfaces that are catalytically active, the negatively charged O-(0001) surfaces are chemically inert . The comb stem grows along the [2ī ī0] direction, while the top and bottom surfaces are zinc and oxygen terminated (0001) respectively. It is reported that the catalytically active Zn-terminated (0001) surfaces tend to have tiny Zn clusters and other Zn particles at the growth front, which could provide an active site for the further growth process, and hence comb teeth can grow in front of zinc-terminated (0001) surfaces . Due to higher growth velocity in  direction of ZnO crystals, the comb teeth were also grown in  directions .
Photovoltaic Properties of As-grown ZnO Nanocombs
In summary, well-crystallized ZnO nanocombs were directly grown onto the FTO substrate via noncatalytic simple thermal evaporation process and utilized as photoanode materials to fabricate the DSSCs. The fabricated DSSCs demonstrated an overall light to electricity conversion efficiency of ~0.68% with a fill factor of 34%, short-circuit current of 3.14 mA/cm2and open-circuit voltage of 0.671 V. This research opens a new way to utilize various kinds of ZnO nanostructures as photoanode material for the fabrication of efficient DSSCs.
This work has been done through the service contract between Najran University, Saudi Arabia and Chonbuk National University, South Korea. Author would like to thank Professor Yoon-Bong Hahn, School of Semiconductor and Chemical Engineering, Chonbuk National University and Dr. D. H. Kim, Hanyang University, South Korea for useful discussions and helps to carry out the experiments. This work was partially supported by the research project funded by Najran University, Najran, Saudi Arabia.
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