Preparation, characterization, and application of titanium nano-tube array in dye-sensitized solar cells
© Ho et al; licensee Springer. 2012
Received: 30 November 2011
Accepted: 21 February 2012
Published: 21 February 2012
The vertically orientated TiO2 nanotube array (TNA) decorated with TiO2 nano-particles was successfully fabricated by electrochemically anodizing titanium (Ti) foils followed by Ti-precursor post-treatment and annealing process. The TNA morphology characterized by SEM and TEM was found to be filled with TiO2 nano-particles interior and exterior of the TiO2 nano-tubes after titanium (IV) n-butoxide (TnB) treatment, whereas TiO2 nano-particles were only found inside of TiO2 nano-tubes upon titanium tetrachloride (TiCl4) treatment. The efficiency in TNA-based DSSCs was improved by both TnB and TiCl4 treatment presumably due to the increase of dye adsorption.
KeywordsTiO2 titanium tetrachloride titanium (IV) n-butoxide nano-tube anodization DSSCs
Since O'Regan and Grätzel reported highly efficient TiO2-based dye-sensitized solar cells (DSSCs) in 1991, many attempts have been made to sensitize titanium dioxide (TiO2) nano-scale films. TiO2 nano-particulate films are typically preferred as they provide a high surface area for dye adsorption, leading to high photocurrent conversion efficiency. Due to the three-dimensional transport path, TiO2 nano-particulate films brought higher electron recombination and met larger grain boundary among interconnected nano-particles. In this research, we have fabricated vertically orientated one-dimensional nano-structure TiO2 nano-tube array (TNA) by electrochemical anodization. The TNA-based DSSCs were expected to have a better performance than the nano-particulate-based DSSCs due to the better electron transportation and recombination property. However, due to the less surface area of TiO2 nano-tube array, the efficiency of TNA-based DSSCs is still lower than that of TiO2 nano-particle-based DSSCs. Post-treatment of TNA by Ti precursors to form a TiO2 nano-particulate layers on TNA became a strategy which could increase the TiO2 surface area for more dye adsorption. This research showed that the DSSCs fabricated by TNA after post-treatment by titanium tetrachloride (TiCl4) and TiO2 nano-tubes after titanium (IV) n-butoxide (TnB) raised up the photocurrent conversion efficiency.
DSSCs have aroused intense interest over the past few years because they have been demonstrated to be able to achieve high solar-to-electric energy conversion efficiency with low-cost manufacture process and materials. In DSSCs, the photoelectrodes are made of porous semiconductor layers chemisorbed with an organic sensitizer. When DSSCs are illuminated with sun light, the photoelectron of the sensitizer is ejected into the semiconductor films and sent to the external circuit. The redox pairs in the electrolyte transport the holes from the oxidized dye molecules to the counter electrode to complete the electric cycle . TiO2 is one of the most promising semiconductor materials in preparing the photoanodes for DSSCs due to its wide band gap characteristics and unique photoelectric properties . TiO2 nano-particulate films are preferred as they provide a high surface area for dye adsorption, leading to high photocurrent conversion efficiency. The electron-collecting TiO2 layer in DSSCs is typically 10 to 15 μm thick with a three-dimensional network of interconnected nano-particles. However, TiO2 nano-crystalline films acquire long electron transport path and larger grain boundary between nano-particles [3, 4]. This would hinder the electron collection efficiency and limit the performance of DSSCs. It was proposed that one-dimensional TNA aligned perpendicular to photoanode substrate could enhance the electron transportation and, thus, lower the possibility of electron recombination with redox electrolytes, leading to the higher photo-to-electron conversion efficiency [3–5]. The TNA has been first prepared by Zwilling et al. using the electrochemical anodization method . The TNA morphology, including tube length, hole diameter, and wall thickness, can be systematically controlled by varying the anodization parameters, such as anodization potential, electrolyte, and pH value [7, 8]. Zhu et al. had investigated the dynamics of electron transport and recombination properties of the oriented TiO2 nano-tube structure in DSSCs by frequency-resolved modulated photocurrent/photovoltage spectroscopies and found the higher charge-collection efficiency and slower electron recombination in the TiO2 nano-tube-based DSSCs than the TiO2 nano-particle-based counterparts . One of the reasons for improving the performance of the DSSCs is considered to be due to the increase of the amount of the dye adsorbed onto the TiO2 surface of photoelectrodes in DSSCs. In order to increase the surface area of TiO2 electrodes, post-treatment of TNA to form an extra layer of TiO2 nano-particles has been applied [9–11]. In this work, we compared the effect of post-treatment of anodic TNA by different Ti-precursors on the TNA morphology and the resulting DSSCs performance.
Preparation, modification, and characterization of anodic TNA
Titanium foils with thickness of 0.25 mm (99.5% purity; Alfa Aesar, Ward Hill, MA, USA) were used for anodic growth of TNA. Titanium foils were first polished by sonication in chemical polishing solvent which contained nitric acid, ammonia fluoride, urea, ethanol, and hydrogen peroxide in 12:5:5:3:12 v/v ratio and rinsed subsequently with deionized (DI) water, acetone, and methanol. The anodization reaction was carried out in a two-electrode electrochemical cell with polished Ti foil (2 × 2.5 cm2) which served as the anode-working electrode and Pt foil (thickness 0.025 mm; Alfa Aesar) as the counter electrode. The separation between Ti electrode and Pt electrode was about 3.5 cm. The anodization electrolyte contains 0.3 wt% NH4F and 2 vol% H2O in ethylene glycol solution. The anodization was operated under a constant potential of 60 V at low temperature of 15°C with magnetic stirring. The reaction period controlled the thickness of TiO2 nano-tube arrays. Typically the TNA samples with tube length of approximately 15 μm were obtained after 2 h of anodization process. It is evident that increasing the TNA length leads to the increase of short-circuit photocurrent density due to the higher surface area available for dye adsorption. The TNA foils were then carefully washed with deionized water to remove the surface residual electrolyte in the nano-tube arrays. Such prepared TNA samples were then annealed at 450°C for 3 h with a heating rate of 1°C/min in order to transform the TNA from amorphous to anatase crystalline phase.
The surface morphology and crystal phase of TNA, TNA-TiCl4, and TNA-TnB were investigated by scanning electron microscopy (SEM) (SM6500F, JEOL Ltd., Akishima, Tokyo, Japan) and X-ray diffraction (XRD) (PANalytical X'Pert PRO, Almelo, The Netherlands), respectively. The results were confirmed by high-resolution transmission electron microscopy (Hitachi H-7100, Hitachi Ltd., Chiyoda, Tokyo, Japan).
Dye-sensitized solar cell assemble and performance measurement
To fabricate DSSCs devices, three kinds of TNAs including TNA, TNA-TiCl4, and TNA-TnB, served as photoanodes, were combined with a transparent Pt counter electrode (cathode). The TNAs samples were sensitized by dye molecules (3 × 10-4 M, N719 in a mixed solvent of acetonitrile and tertbutyl acohol (volume ratio = 1:1)) for 24 h. The amount of dye adsorbed on TNA electrodes was determined by desorbing the N719 from TNAs surfaces into a solution of 0.1 M NaOH. The concentration of the adsorbed N719 was analyzed by UV-visible spectrophotometer (V-630, JASCO Corp., Easton, MD, USA). The Pt cathode was made by a 'two-step dip coating' process developed by Wei et al. . We have first prepared the poly-N-vinyl-2-pyrrolidone (PVP)-capped Pt nano-particles by dissolving PVP (M.W. = 8000) and H2PtCl6 (Pt precursor) into deionized water at room temperature and well stirred until a light-yellow solution was obtained. A NaBH4 solution was then added drop by drop to the H2PtCl6-PVP solution, and the solution quickly turned into a black color, indicating the formation of Pt nano-particles (Pt-PVP solution).
FTO glass (8Ω/sq., Solaronix SA, Aubonne VD, Switzerland) was pretreated by 1% ML-371 aqueous solution at room temperature for 1 min in order to increase adhesion between the PVP-capped Pt nano-particles and FTO surface. The ML-371-modified FTO substrate was then dipped into the Pt-PVP solution for 5 min and rinsed with deionized water followed by heat-treatment at 400°C for 5 h to remove completely the organic component and complete the preparation of counter electrode.
To assemble the DSSCs, the liquid electrolyte of 0.1 M lithium iodide, 0.05 M iodine (I2), 0.5 M 4-tert-butylpyridine, 0.5 M 1,2-Dimethyl-3-propylimidazolium iodide in acetonitrile was applied to the above-prepared Pt electrode which was then placed over the N719-coated TNAs electrodes. The edges of the cells sealed with a hot-melt film (Surlyn, 125 μm) and the electrolyte (I-/I2/I3- redox couple) were injected into the space. The active cell area studied in this work is 0.25 cm2 (0.5 cm × 0.5 cm). The photoelectrochemical performance of the resultant solar cells were measured by back illuminated through the Pt counter electrode due to the nonpenetration of light through the photoanode Ti metal substrate.
The current (I)-voltage (V) characteristics were performed using a digital source meter (Keithley model 2400, Keithley Instruments Inc., Cleveland, OH, USA) with the TNA-based DSSCs devices under one-sun AM 1.5 irradiation from a solar simulator (300 W Xe light and filters, Oriel Instruments, Irvine, CA, USA) on a 0.25 cm2 sample area.
Results and discussion
Formation and characterization of anodic titanium oxide nano-tube arrays
Application of anodic TNA electrodes to DSSCs and photoelectrochemical performance study
The I-V characterization of TNAs-based DSSCs
The decorated TNAs were successfully fabricated by anodization method followed by titanium precursor post treatment. The morphology of TNA without post-treatment was observed from SEM and TEM images, typically approximately 15 μm length, approximately 100 nm diameter, and 10 nm wall thickness were achieved after 2 h reaction. TNA with titanium precursor treatment alters the morphology which was confirmed from the SEM and TEM images. In the case of TNA-TnB, TiO2 nano-particles were filled interior and exterior of the TiO2 nano-tubes, whereas TiO2 nano-particles were filled only inside the TiO2 nano-tubes in TNA-TiCl4 upon TiCl4 treatment. An XRD pattern clearly indicates that the TNA, TNA-TiCl4, and TNA-TnB were pure anatase phase after annealing process at 450°C. The photocurrent conversion efficiency of TNA-based, TNA-TiCl4-based, and TNA-TnB-based DSSCs was 1.38%, 1.61%, and 2.40%, respectively. The results showed that the DSSC efficiency in TNAs was enhanced by TiCl4 and TnB precursor post-treatment, presumably due to the increase of dye adsorption. The higher solar efficiency in TnB-doped DSSCs is due to the formation of extra layer of TiO2 nano-particles on TNA, leading to the higher amount of dye adsorption as well as higher photocurrent.
dye-sensitized solar cells
transmission electron microscope
titanium dioxide nano-tube array
titanium (IV) n-butoxide
scanning electron microscopy
This paper is a revised and expanded version of a paper entitled 'Preparation, characterization and application of titanium nano-tube array in dye-sensitized solar cells' presented at IEEE International NanoElectronics Conference, Taiwan, June 21-24, 2011. We acknowledge the financial support from National Science Council of Taiwan, Republic of China (NSC 98-2113-M-027-003-MY3, NSC 100-2113-M-008-004-MY3).
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