Structure and photovoltaic properties of ZnO nanowire for dye-sensitized solar cells
© Kao et al.; licensee Springer. 2012
Received: 29 November 2011
Accepted: 18 May 2012
Published: 18 May 2012
Aligned ZnO nanowires with different lengths (1 to approximately 4 μm) have been deposited on indium titanium oxide-coated glass substrates by using the solution phase deposition method for application as a work electrode in dye-sensitized solar cells (DSSC). From the results, the increases in length of zinc oxide (ZnO) nanowires can increase adsorption of the N3 dye through ZnO nanowires to improve the short-circuit photocurrent (Jsc) and open-circuit voltage (Voc), respectively. However, the Jsc and Voc values of DSSC with ZnO nanowires length of 4.0 μm (4.8 mA/cm2 and 0.58 V) are smaller than those of DSSC with ZnO nanowires length of 3.0 μm (5.6 mA/cm2 and 0.62 V). It could be due to the increased length of ZnO nanowires also resulted in a decrease in the transmittance of ZnO nanowires thus reducing the incident light intensity on the N3 dye. Optimum power conversion efficiency (η) of 1.49% was obtained in a DSSC with the ZnO nanowires length of 3 μm.
Dye-sensitized solar cells (DSSCs) have been studied extensively as a potential alternative to conventional inorganic solid solar cells, by using nanocrystalline TiO2 sensitized with ruthenium polypyridine complexes or metal-free organic dyes as photoelectrodes [1–3]. Solar cells based on TiO2 nanoparticles with a size of 10 to 30 nm have been used as photoanodes with demonstrated 11% photovoltaic conversion efficiency . Zinc oxide (ZnO) is another promising but less explored wide bandgap semiconductor oxide used for DSSC. It has similar energy levels to TiO2. More importantly, its much higher carrier mobility is more favorable for the collection of photo-induced electrons [5, 6]. DSSCs based on TiO2 nanoparticles network usually consist of a porous layer which serves as the photoelectrode. The porous photoelectrode can provide large surface area for anchoring the light-capturing dye molecules to generate electron-hole pairs. However, in this kind of photoelectrode, the photogenerated electrons will interact with a lot of traps when they walk through the random network, which limits the efficiency of the DSSCs. Compared to TiO2, a ZnO semiconductor has a similar band gap energy and conduction band edge, making it a possible candidate for an effective nanorod-based DSSCs. ZnO nanowires as photoelectrodes show a higher conversion efficiency compared to those using the nanostructured films [7, 8]. The single crystalline ZnO nanowires can significantly improve the electron transport in the photoelectrode by providing a direct conduction pathway for photogenerated electrons.
DSSCs based on ZnO nanowires have been prepared by many growth techniques, such as metal-organic chemical vapor deposition, hydrothermal method, vapor deposition and have been employed to grow ZnO nanowires [9–11]. In this paper, we report the controllable growth of ZnO nanowires through a simple and low-temperature solution phase deposition method. The effects of nanowire length on the material and optical properties of ZnO nanowires were investigated in details. Furthermore, ZnO nanowires with optimum material and electrical properties were successfully implemented in DSSCs to improve the photovoltaic performance.
ZnO nanowires were fabricated by solution phase deposition method on the seeded indium titanium oxide (ITO) glass substrate. First, the ZnO thin films were deposited on ITO glass substrate using sol-gel methods. The ZnO thin films were then used as seed layers for growing nanowires. Zn(C2H3O2)2·2H2O and C3H8O2 were employed to synthesize the ZnO precursor. The concentration of ZnO was 0.5 M. Finally, the mixture was stirred at 60°C for 3 h in the water bath to form a transparent homogeneous mixture. After the 24 h aging at room temperature, the mixture keeps clear which can be utilized to deposited ZnO seed layer. The seed layer was deposited by spin coating onto the substrate with the rotation speed of 3,000 rpm for 30 s. Then, samples were performed by rapid thermal processing at 600°C for 2 min and ZnO seed layer was formed on the substrate. Thereafter, ZnO nanowires were grown by a 5 mM zinc acetate (Zn(C2H3O2)2·2H2O) solution mixed with hexamethylenetetramine (C6H12N4) at 90°C. The desired ZnO nanowire lengths of 1 to approximately 4 μm were achieved at growth time of 1, 2, 3 and 4 h, respectively. Finally, the samples were rinsed with deionized water and dried in air at room temperature to remove the solvent.
Results and discussion
Photovoltaic performances of DSSCs fabricated with various lengths of ZnO nanowires
Length of ZnO nanowires (μm)
Fill factor (percent)
Efficiency (η percent)
In this study, the reliability of photocurrent generation concerned with ZnO nanowires-based DSSCs was measured through a chronoamperometry. The photocurrent-time characteristics were measured repeatedly at 60 s light source on and 60 s light source off in the range from 0 to 800 s. From the measurement results, within the scope measurement time of DSSC devices, stable photocurrent density of about 3.2, 4.0, 4.8, and 5.6 mA/cm2 can be obtained for ZnO nanowire lengths of 1, 2, 4, and 3 μm, respectively.
In this study, the influence of ZnO nanowires length on the performance of DSSCs was studied. With increasing the length of ZnO nanowires from 1.0 μm to 3.0 μm, the Jsc and Voc values of DSSC increased from 3.2 to 5.6 mA/cm2 and from 0.56 to 0.62 V, respectively. However, the Jsc and Voc of DSSC with ZnO nanowires length of 4.0 μm (4.8 mA/cm2 and 0.58 V) are smaller than those of DSSC with ZnO nanowires length of 3.0 μm (5.6 mA/cm2 and 0.62 V), respectively. This can be explained by the lower incident light intensity from the lower transmittance of ZnO nanowires with length of 4.0 μm. The corresponding results show that the obtained DSSC with ZnO nanowires length of 3.0 μm exhibited better photovoltaic properties.
This study was supported by the National Science Council (NSC), Republic of China, under contrast NSC 100-2112-M-164-001, NSC 99-2112-M-164-002-MY2 and NSC 99-2112-M-164-001.
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