Silver nanowire-based transparent electrode as FTO replacement for dye-sensitized solar cell
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In this paper, we present a novel usage of silver nanowire-based transparent electrode instead of fluorine-doped tin oxide for a ZnO–TiO2 core–shell dye-sensitized solar cell. The Ag nanowire-based transparent electrode was fabricated with a simple coating method at room temperature and atmospheric pressure. This method potentially helps to reduce the production cost of dye-sensitized solar cell significantly by eliminating vacuum deposition processes for transparent electrodes and also enables the usage of various flexible substrates as a glass alternative. The Ag nanowire-based electrode is characterized using scanning electron microscope, four-point probe, and UV–Vis methods. With the short-circuit current density, Isc, of 4.3 × 10−4 A/cm2 and the open-circuit voltage, Voc, of 0.63 V, the performance of the dye-sensitized solar cell shows promising preliminary data for a low-temperature ZnO–TiO2 DSSC.
KeywordsSilver nanowire Transparent electrode Core–shell nanoparticles Dye-sensitized solar cell
The first transparent electrode was introduced in 1907 by Karl Badeker . A thin film of cadmium metal was sputter deposited on a substrate before oxidizing it with a heat treatment process to create what we now call a transparent conductive oxide (TCO). Since then, a few other types of TCO have been reported such as SnO2 in 1947  and indium tin oxide in 1954 . Due to their excellent optical properties in addition to the electrical properties, TCO has been a very crucial element of many modern devices used in a wide range of important applications which include deicing plane windows , light-emitting diodes , solar cells , etc. However, all of the TCO materials mentioned above are to be processed under a vacuum environment, which is a costly process and feasible for relatively small-size substrates fitting within the vacuum chamber. Thus, the production cost and time of solar cells has not been significantly reduced, although many former efforts have been made to find improvements in advanced production protocols [7, 8, 9].
Among various solar-energy-harvesting methods, dye-sensitized solar cells (DSSCs) are a newer generation that possesses several advantages such as relatively low production cost, use of wider visible spectrum absorption materials, and simplification of the manufacturing process . Recently, DSSC has been fabricated on various types of flexible substrates with different low-temperature fabrication methods [11, 12]. However, much research remains to be done before DSSC can replace current solid-state p–n junction solar cells. One of its major drawbacks includes the need of high-temperature process for TiO2 and ZnO nanostructures, which require the usage of fluorine-doped tin oxide (FTO) transparent electrodes formed under vacuum, precluding it from manufacturing using a mass productive continuous roll-to-roll method. The fabrication processes of FTO are inherently complicated and typically require high-temperature and vacuum processes [13, 14]. It limits the production capacity and simply increases the production cost. Thus, we have been making efforts to find an alternative transparent electrode produced with a simple and scalable fabrication process to make DSSC available for low-cost industrial production methods. We have learned about a recently introduced silver nanowire electrode  and studied its feasibility for DSSC.
In this paper, we present a silver nanowire-based transparent electrode for dye-sensitized solar cell to replace the typical fluorine-doped tin oxide electrode. Without the need of high-temperature and high-vacuum processes, fabrication process for the silver nanowire-based electrode (SNE) is simply performed by spreading Ag nanowire-contained ink on a transparent substrate. Moreover, the simplicity of the SNE fabrication also makes it possible to use alternative flexible substrates instead of glass substrates in the future. The SNE helps to simplify the overall production process and to reduce the fabrication cost of DSSC. It may advance other DSSC-related technologies and envision alternative options for the current expensive Si-based commercial solar cells.
Materials and methods
ZnO nanoparticles were coated on the thin ZnO layer. Zinc oxide nanoparticles (Alfa Aesar) are ranged from 40 to 100 nm in diameter. The ZnO nanoparticles were mixed with 0.1 M acetic acid solution in ethanol and spread-coated using the blade-coating technique. The nanoparticle-coated samples were baked on a hot plate at 250 °C for 30 min. This step also helps to enhance the bonding strength of the ZnO nanoparticles on the substrate.
The TiO2 shell was coated over the ZnO nanoparticles by dipping in a 0.01 M titanium isopropoxide (TTIP) solution in ethanol for 30 min. The next step was rinsing with ethanol and drying it on a hot plate at 250 °C for 30 min. Then, the samples were sensitized in a 0.15 mM solution of N179 ruthenium dye ((cis-di-(thiocyanato)bis(2,2′-bipryidyl)-4-4′-dicarboxylate) ruthenium-(II) from Solaronix) in ethanol (Sigma Aldrich) for 24 h. The sensitized solution was covered from light to prevent aggregation and degradation of the dye. The counter electrode was prepared by sputtering Pt onto a substrate. Before the sputtering process was initiated, the substrate was cleaned with acetone, methanol, and DI water to remove environmental impurities and organic contaminants. Finally, the samples were assembled, filled with iodide/tri-iodide electrolyte (Iodolyte AN50; Solaronix), and were ready for testing.
Results and discussion
Four-point probe measurement of Ag nanowire-based electrodes and FTO electrode
SNE #1 (Ω/sq)
SNE #2 (Ω/sq)
The XRD patterns suggest that the ZnO film prepared by the sol–gel-coating process at an enough high temperature produces a polycrystalline wurtzite structure with primarily c-axis orientation. Although the quality of the ZnO film improves at a higher annealing temperature, we try to limit the baking temperature lower than 250 °C for the potential use of various alternative flexible substrate materials. Although the crystalline quality of the sol–gel ZnO layer would not be best, it is good enough to electrically isolate the SNE from the electrolyte. Thus, by treating the ZnO layer at a low temperature, we can enable the usage of alternative substrates which has lower melting point such as polymer. The fabrication process will be considerably simplified with possible role-to-role process.
Although the efficiency of 0.12% is not as high as that of other reported ZnO-based DSSCs using an FTO electrode, our results are very intriguing for a device processed at a low temperature. The efficiency of the device can be enhanced by optimizing the device parameters such as the SNE, the size of the core ZnO nanoparticles, and the thickness of the TiO2 shell layer. The fill factor could be further improved by decreasing the series resistance of the testing setup. The use of the Ag nanowire-based transparent electrode eliminates the need for a vacuum environment to fabricate transparent conducting electrodes. Together with low-temperature methods, it may enable the roll-to-roll process for mass production of DSSC at significantly reduced production cost.
The silver nanowire-based transparent electrode for a ZnO–TiO2 core–shell nanoparticle dye-sensitized solar cell has been successfully demonstrated.
Silver nanowire-based transparent electrodes can potentially be used to replace FTO electrodes for DSSCs. The major advantage of Ag nanowire-based electrodes is that devices can be easily fabricated using simple techniques at low temperature without the requirement of a vacuum system for thin-film deposition. In the future, optimizing each individual parameter of the device may significantly improve the efficiency and fill factor of the devices.
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