The effect of dye-sensitized solar cell based on the composite layer by anodic TiO2 nanotubes
© Yang et al.; licensee Springer. 2014
Received: 18 July 2014
Accepted: 1 December 2014
Published: 12 December 2014
TiO2 nanotube arrays are very attractive for dye-sensitized solar cells (DSSCs) owing to their superior charge percolation and slower charge recombination. Highly ordered, vertically aligned TiO2 nanotube arrays have been fabricated by a three-step anodization process. Although the use of a one-dimensional structure provides an enhanced photoelectrical performance, the smaller surface area reduces the adsorption of dye on the TiO2 surface. To overcome this problem, we investigated the effect of DSSCs constructed with a multilayer photoelectrode made of TiO2 nanoparticles and TiO2 nanotube arrays. We fabricated the novel multilayer photoelectrode via a layer-by-layer assembly process and thoroughly investigated the effect of various structures on the sample efficiency. The DSSC with a four-layer photoelectrode exhibited a maximum conversion efficiency of 7.22% because of effective electron transport and enhanced adsorption of dye on the TiO2 surface.
KeywordsDSSCs Anodic oxidation Photoelectrode TiO2 nanotube array Composite layer
Dye-sensitized solar cells (DSSCs) have attracted great interest in scientific and industrial fields during the past two decades because of their low cost, impressive power conversion efficiency, and easy fabrication compared to conventional p-n junction solar cells. Despite these advantages, the low efficiency of DSSCs compared to that of silicon-based cells has limited their commercial implementation [1–4]. Consequently, there is a critical need to improve the efficiency of state-of-the-art DSSCs in order to realize next-generation solar cells. In principle, DSSCs have four components: (1) a TiO2 electrode film layer covered by a monolayer of dye molecules that absorbs solar energy, (2) a transparent conductive oxide layer that facilitates charge transfer from the electrode layer, (3) a counter electrode layer made of Pt or C, and (4) a redox electrolyte layer that reduces the amount of energy transferred from dye molecules [5, 6]. Thus, research efforts to increase the efficiency of DSSCs have been primarily focused on improvements in the aforementioned DSSC components . One of the important features of DSSCs is the mesoporous film of interconnected TiO2 nanoparticles (TNPs), which can supply a large surface area for the adsorption of dye molecules. However, the performance of DSSCs is limited by electron transport in the nanocrystal boundaries and recombination of electrons with the electrolyte during migration. Many researchers have reported that one-dimensional nanostructures can be used in DSSCs in place of nanoparticles to facilitate the electron transfer [8–14]. In addition to their unique electron properties, one-dimensional TiO2 nanostructures also function as light-scattering materials with minimal sacrifice of the surface area. On the other hand, the small specific surface area of one-dimensional nanostructures is a serious flaw as it causes insufficient dye adsorption. Achieving a balance between the two conflicting desirable features of DSSCs, a large specific surface area and an efficient electron transfer, remains a challenge.
Preparation of TiO2 nanotube array layers
Preparation of TiO2 layer
TiO2 paste was prepared from TiO2 powder (anatase, 99.9% purity, Sigma-Aldrich, St. Louis, MO) and used as the reference [15, 16]. TiO2 photoelectrodes of various structures were coated on fluoride-doped tin oxide (FTO) glass using a doctor blade method (single-layer). TNAs were then transferred onto the coated photoelectrode (two-layer). The three-layer (TNP/TNA/TNP), four-layer (TNP/TNA/TNP/TNA), and five-layer (TNP/TNA/TNP/TNA/TNP) photoelectrodes were prepared by the same process. The prepared TNP/TNA photoelectrode was sintered at 450°C for 1 h in air. The Pt catalyst electrode was prepared by mixing 5 mM of H2PtCl6 in isopropyl alcohol, followed by an ultrasonic treatment. A counter electrode, which facilitates the redox reaction of the electrolyte, was fabricated by spin coating the prepared H2PtCl6 solution at 1,000 rpm for 30 s, followed by heat treatment at 450°C for 30 min.
Assembly of dye-sensitized solar cell
The dye solution to be adsorbed on the TNP/TNA photoelectrode film was prepared by mixing 0.5 mM of Ru dye (N-719, Solaronix) with ethanol. Adsorption of the dye molecules was accomplished by placing the photoelectrode film in the dye solution and allowing it to stand in dark for 24 h. Finally, the DSSCs were fabricated by fusing the TNP/TNA photoelectrode film and the counter electrode together at 120°C for 10 min using a hot-melt sealant (60°C). The electrolyte (I-/I3-) was injected between the two electrodes through the inlet and then sealed with a cover glass.
The phases of the TNAs prepared by anodization, as well as that of TNPs, were examined by X-ray diffraction (XRD) using a Rigaku D/MAX-2200 diffractometer (Rigaku, Shibuya-Ku, Tokyo) with a Cu Kα radiation source. The morphology of the prepared TNP/TNA photoelectrode film was investigated by field-emission scanning electron microscopy (FE-SEM, S-4700, Hitachi, Chiyoda, Tokyo). The absorbance of the TNP/TNA photoelectrode film was measured using a UV-Vis spectrometer (Lambda 750, Perkin Elmer, Waltham, MA). The conversion efficiency and electrochemical impedance spectroscopy (EIS) of the fabricated DSSCs were measured using an I-V solar simulator (K3400, K3000, McScience, Youngtong, Suwon). The active area of the cell exposed to light was approximately 0.25 cm2 (0.5 × 0.5 cm).
Results and discussion
Integral photocurrent density ( J SC ), open-circuit voltage ( V OC ), fill factor (FF), and efficiency (η) of DSSCs fabricated using multilayer photoelectrodes
In this work, improvement in the performance of DSSCs by using a TNP/TNA multilayer photoelectrode was proposed. The DSSCs were constructed with TiO2 films made of TNAs fabricated from an anodization process and TNPs. The multilayer photoelectrode DSSCs have higher efficiencies than the single-layer or bare DSSCs. A single-layer photoelectrode DSSC with a light-to-electric energy conversion efficiency of 5.04% was achieved under a simulated solar light irradiation of 100 mW · cm2 (AM 1.5). The DSSCs based on a TNP/TNA multilayer photoelectrode showed a better photovoltaic performance (i.e., higher JSC and FF) than the cell made purely of TiO2 nanoparticles. The conversion efficiency of DSSCs was significantly affected by the properties of TNAs. The TNP/TNA four-layer photoelectrode provided a large surface area for dye adsorption. The DSSC based on this photoelectrode was measured to have a maximum conversion efficiency of 7.22% because of effective electron transport. Thus, the use of TNAs and TNP/TNA multilayer photoelectrodes was found to be an effective method to improve the efficiency of TiO2 film-based DSSCs.
This work was supported by the Human Resources Development program (No. 20124030200010) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Knowledge Economy. And this work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MEST) (No. 2012R1A1A2044472).
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