Direct synthesis of vertically aligned ZnO nanowires on FTO substrates using a CVD method and the improvement of photovoltaic performance
© Lu et al.; licensee Springer. 2012
Received: 10 April 2012
Accepted: 6 June 2012
Published: 6 June 2012
In this work, we report a direct synthesis of vertically aligned ZnO nanowires on fluorine-doped tin oxide-coated substrates using the chemical vapor deposition (CVD) method. ZnO nanowires with a length of more than 30 μm were synthesized, and dye-sensitized solar cells (DSSCs) based on the as-grown nanowires were fabricated, which showed improvement of the device performance compared to those fabricated using transferred ZnO nanowires. Dependence of the cell performance on nanowire length and annealing temperature was also examined. This synthesis method provided a straightforward, one-step CVD process to grow relatively long ZnO nanowires and avoided subsequent nanowire transfer process, which simplified DSSC fabrication and improved cell performance.
Keywordsnanowires ZnO direct growth FTO-coated glass substrate dye-sensitized solar cell
Dye-sensitized solar cells (DSSCs) have attracted significant research interest due to their promising power conversion efficiency and low fabrication cost . Typical photoanodes of DSSCs are layers of nanoparticles of wide band gap semiconductors such as TiO2 or ZnO, and the substrates are usually fluorine-doped tin oxide (FTO)-coated glasses . However, in these nanoparticle-DSSCs, photo-generated electrons have to percolate through the nanoparticle network before they reach the collection electrode, which increases charge recombination possibility and limits cell performance. One approach to improving charge collection efficiency in DSSCs is to replace the nanoparticle network with one-dimensional structures such as semiconductor nanowires that can provide direct transport pathway for the carriers. Due to the enhanced diffusion length, longer wires and thus thicker photoanode films can be incorporated into DSSCs, which could lead to better quantum efficiency in the long-wavelength region of the solar spectrum . In addition, recent studies also show that the open-circuit voltage of DSSCs can be improved by employing nanowire-photoanodes, which is attributed to a suppressed back electron transfer reaction that occurs at the photoanode/redox electrolyte solution interface, highlighting the importance of exploring nanowire-based photoanodes for DSSC applications [4, 5].
There has been a significant amount of reports on DSSCs based on nanowires, where the semiconductor nanowires are mainly synthesized using solution-based hydrothermal method [6–8]. Using this method, the nanowires can be directly grown on FTO-coated substrates, which make subsequent solar cell fabrication straightforward. However, solution-based synthesis is usually slow and involves multiple processes, and post-growth annealing is necessary to remove the unwanted chemicals from the nanowire surface and ensure good electrical contact between the wires and the substrate . Furthermore, it is generally difficult to produce long wires using the hydrothermal approach. Due to the multiple steps involved and the low growth rate, it is very time consuming to synthesize nanowires with a length of more than 10 μm [10–12]. Another popular nanowire synthesis approach is the chemical vapor deposition (CVD) method that is based on vapor–liquid–solid (VLS) growth mechanism. Nanowires with very long length can be synthesized this way; however, the substrates used are typically silicon or sapphire other than FTO-coated glasses since the high CVD growth temperature can easily damage the transparent conducting oxide [13–15]. In addition, since the nanowires are not directly synthesized on FTO-coated substrates, a nanowire transfer process is needed in the subsequent solar cell fabrication. Such a transfer process causes contact issues between the nanowires and the substrate as well as broken wires in the device structure that creates additional transport barriers and recombination possibilities for photo-generated electrons, which all could limit solar cell performance.
Thus far, there is only limited research on direct synthesis of nanowires on FTO-coated substrates using the CVD method, and the reported nanowires were not aimed at DSSC applications and had low density and random morphology [16, 17]. In this work, we investigated a controlled CVD synthesis of nanowires directly on FTO-coated glass substrates. Long, vertically aligned ZnO nanowires were fabricated at a relatively low temperature of 550 °C, and they formed dense arrays with length of tens of microns in a one-step vapor deposition process. DSSCs were fabricated using these directly grown nanowires, and the performance was compared to those fabricated using transferred ZnO nanowires. The effects of nanowire length and annealing temperature on device performance were also examined.
Direct growth of ZnO nanowires on FTO substrates by the CVD method
Solar cell fabrication using ZnO nanowires directly synthesized on FTO substrates
The as-grown ZnO nanowires were ready for solar cell fabrication without any further processing. To sensitize the nanowires, the FTO substrate with the ZnO nanowires was soaked in a 0.05-mM solution of N719 dye (dissolved in dry ethanol; SOLARONIX, Aubonne, Switzerland) at 50 °C for 2 h. Another FTO substrate coated with 25 nm Pt was used as the counter electrode and was bonded together with the nanowire/FTO substrate through a hot-melt spacer (75 μm; Bynel, Dupont, Wilmington, DE, USA). A drop of electrolyte (0.5 M LiI (Aldrich, St. Louis, MO, USA), 50 mM I2 (Alfa Aesar), and 0.5 M 4-tertbutylpyridine (Aldrich) in 3-methoxypropionitrile (Aldrich)) was injected into the space between the two electrodes of the cell. Current density-voltage (J-V) curves were acquired by a source measurement unit (Agilent 4156 Semiconductor Parameter Analyzer, Agilent Technologies, Santa Clara, CA, USA) under a simulated sunlight (100 mW/cm2, calibrated by a KG-5 filtered silicon photodiode) using a setup with a Xenon lamp. The optical absorption of the dye solution was characterized by an ultraviolet–visible (UV–vis) spectrophotometer (Lamda 950, PerkinElmer, Waltham, MA, USA).
Solar cell fabrication using ZnO nanowires transferred onto FTO substrates
DSSCs based on transferred ZnO nanowires were also fabricated and tested in this research for the purpose of a comparison study. Since it was very difficult to remove the directly synthesized nanowires from the FTO substrates, the transferred ZnO nanowires were those grown on silicon substrates. The solar cell fabrication procedure was almost identical, except that a nanowire transfer process was involved. To transfer the ZnO nanowires, a polydimethylsiloxane (PDMS) solution was first spin-coated on the silicon substrate with the ZnO nanowires, which, after annealing, would form a flexible but solid film that holds the nanowires in position [20, 21]. After being annealed at 150 °C on a hot plate in the air, the nanowire film was peeled off by a sharp razor blade. The nanowire film was then soaked in the N719 dye solution for 2 h at 50 °C, which was the same sensitization condition for the DSSCs based on directly grown ZnO nanowires. After dye sensitization, the film was transferred onto an FTO substrate and was glued down using a thin layer of silver paste. Since the silver paste was easy to dissolve in a dye solution, dye sensitization was performed before the nanowire film attachment, which was different from the previously reported procedure .
Results and discussion
Direct synthesis of vertically aligned ZnO nanowires on FTO substrates by the CVD method
DSSCs based on directly synthesized and transferred ZnO nanowires
DSSCs based on directly synthesized ZnO nanowires with different lengths
Effect of annealing on the performance of DSSCs
In this research, we demonstrated a method to directly synthesize vertically aligned long ZnO nanowires on FTO-coated glass substrates. The synthesis is based on a straightforward, one-step CVD approach, which avoided the wet chemical processing in typical hydrothermal growth and eliminated the nanowire transfer process for DSSC fabrication. DSSCs based on these directly grown ZnO nanowires showed improved performance compared to those fabricated using transferred nanowires. The performance of the DSSCs could be further improved when longer nanowires were used. The relatively long nanowires provided an alternative for hybrid nanowire/composite solar cells in efficiency enhancement [31, 32]. The effect of the annealing temperature was also examined, and it was observed that high annealing temperature caused a substantial increase in the cell's series resistance and lowered the device performance. The reported direct synthesis approach could be further improved and applied for the growth of other types of nanowires and could benefit the fabrication of dye- or quantum dot-sensitized solar structures.
This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award DE-FG02-10ER46728 (materials synthesis), and by the NASA EPSCoR under award NNX10AR90A (device characterization).
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