Room temperature synthesis of compact TiO2 thin films for 3-D solar cells by chemical arrested route

https://doi.org/10.1016/j.apsusc.2004.11.023Get rights and content

Abstract

Essential requirement of compact TiO2 thin films for 3-D solar cells prefers high temperature techniques (≥400 °C) such as spray pyrolysis or sputtering. Under optimized preparative conditions, compact, uniform, adherent and pinhole free, TiO2 thin films were synthesized at room temperature by using arrested precipitation technique on ITO substrates. As-deposited and heat-treated TiO2 films were amorphous with small enlargement in grain size as evidenced from XRD and SEM studies. Small blue shift was detected due to annealing and attributed to change in grain size. As deposited and heat-treated TiO2 films were used in this study showed water contact angles 66.14° and 66.44°, respectively. Efforts were also taken to use these films in dye-sensitized solar cells after introducing cis-dithiocyanato (4,4′-dicarboxylic acid-2,2′-bipyridide) ruthenium (II) (N3) dye but no significant improvement due to low contact angle in photo-electrochemical cell performance was observed due to high compactness.

Introduction

Nanoclusters of metals and semiconductors are more and more considered as building blocks of the future modern technologies. This is due to the size dependent electronic properties of these particles. TiO2 has been one of the most extensively studied oxides because of its remarkable optical and electrical properties. TiO2 film in anatase phase could accomplish the photo-catalytic degradation of organic compounds under the radiation of UV. It has many application prospects in the field of environmental protection such sterilization and sewage disposal. A recent interest is focused on an amphiphilic TiO2 surface induced by UV irradiation, which is expected to be applicable to a windshield and a mirror for vehicles [1]. On the other hand, in the field of alternate energy, a dye-sensitized solar cell is now a hot topic due to its high conversion efficiency produced with a porous TiO2 electrode that is composed of few tenths of nanometer-sized particles [2], [3]. The specific advantage of nanoparticles of TiO2 with special reference to their applications in paints is that the finer sized particles tend to disperse in the medium homogeneously. In addition, it is also observed that this size of the pigment particles gives a much brighter color due to enhanced scattering and better hiding effect once dye is adsorbed. TiO2 film with rutile phase is known as good blood compatibility from the point of view of practical use, the fixation of TiO2 onto a substrate is very important and some methods to fix the TiO2 particles have been developed. Consequently, a low cost preparation and fixation of the TiO2 photo-catalyst with nano-sized particle is necessary for practical applications. Since crystalline titania (anatase), is well-known material having noticeable photo-catalytic properties, its various applications are actively studied for antifouling, anti-microbial, deodorizing and photovoltaic effects. For example, titania thin films are applied to antibacterial coating, deodorization disinfection sheet, soil proofing household furnishing, ant algal oil proofing plate, antifogging coating and deodorant fiber [4]. Many deposition methods such as thermal and anodic oxidation of titanium, electron beam evaporation, chemical vapor deposition, reactive sputtering, sol–gel, spray pyrolysis, etc. have been reported to prepare nanometer-sized particle powder or thin films [5]. Recently, much emphasis has been put on the soft solution chemical processes for the preparation of advanced inorganic materials such as pervoskite-type oxides, spinel type oxides, and nanodots with quantum size effects. These low cost processes have been used environmentally being conditions. Therefore, these soft chemical processes are important for the preparation and fixation of TiO2 particles. Such processes include sol–gel, atomic layer deposition, electro-deposition, spray pyrolysis, solution hydrolysis, etc. However, spray pyrolysis is found to be expensive and working temperature is high (≥400 °C). The TiO2 nanopaticles have been prepared by chemical method using hydrolysis of TiCl4, Ti(SO4)2, and TiI4 [6], [7], [8]. The TiCl4 was slowly dropped in deionized water at 273 K to get TiO2 particles. Whereas, Ti(SO4)2 solution was dropped in aqueous ammonia solution under stirring to get TiO2 powder. Barringer and Bowen [9] prepared sub-micron TiO2 powders by controlled hydrolysis of an alcoholic solution of Ti(OC2H5) or Ti(i-OC3H7)4, and showed that these powders gave sintered bodies with fine grained microstructure and high density of a low sintering temperature. Kato et al. [10] synthesized spherical TiO2 powder from a aqueous solution of TiO(SO4) by homogeneous precipitation using urea as the precipitating agent at 343–363 K. The reports to produce TiO2 films from chemical hydrolysis method are sanity in the literature. Transparent nanocrystalline anatase TiO2 films were deposited onto conducting glass substrates from titanium tetraisopropoxide colloidal solution [11], [12]. In order to increase film thickness, the anatase TiO2 films were treated to TiCl4 solution for few hours and annealed at high temperatures. The TiO2 coated glass was heated at 623 K for 30 min. Vigil et al. [13] have deposited TiO2 films using microwave heating on glass and fluorine doped tin oxide coated glass substrates. Among the various methods used for the production of porous as well as compact titanium dioxide thin films, the chemical bath deposition referred as arrested precipitation technique appears to be a simple and low cost method. However, in applications, which require very thin films with high uniformity on large-area substrates, arrested precipitation method has some clear advantages compared with other chemical deposition techniques. An example of this kind of applications is deposition of gate dielectrics for metal–oxide–semiconductor devices. In these devices, the dielectric layers must be of very uniform thickness and it should be possible to reduce the thickness of oxide down to few nanometers. Deposition of optical and protective coatings and growing active layers for chemical sensors can be considered as other applications, where this method might have significant advantages. The main advantage is the easy control of film thickness, morphology, composition, etc. through quantities such as pH, bath composition, temperature of deposition bath, volume ratio, etc.

In the present investigation, chemical deposition of compact and uniform TiO2 thin film at room temperature was carried out from titanium tri-chloride on ITO substrates. As-deposited TiO2 films were annealed at 450 °C for 1 h and characterized for their structural, optical properties by using X-ray diffraction, scanning electron microscopy (SEM) and UV–vis spectroscopy techniques, respectively. The quantitative stoichiometry of the TiO2 films was obtained from electron dispersion X-ray analysis (EDAX). Water contact angle for deposited and heat-treated TiO2 films were measured and reported. Attempt was also made to prepare dye-sensitized solar cells by using cis-dithiocyanato (4,4′-dicarboxylic acid-2,2′-bipyridide) ruthenium (II) (N3) dye.

Section snippets

Experimental details

In the present research work, chemical deposition of TiO2 thin films at room temperature was carried out titanium tri-chloride, TiCl3 solution. Twenty to 30 wt.% HCl solution of titanium tri-chloride was taken into the beaker of capacity 50 mL. It was complexed with appropriate amount suitable complexing agent such as sodium salt of ethylenediaminetetetraacitic acid (Na2EDTA) under constant stirring the solution for 30 min. The pH of resultant solution was adjusted between 4–6 using liquid ammonia

Growth process and reaction mechanism

In chemical arrested method, reaction takes place between the dissolve precursors generally in aqueous solution at low temperature. When the solution is saturated, the ionic product of anion and cation is equal to solubility product of metal chalcogenide/oxide and when it exceeds, precipitation occurs and ions combine on the substrate and in the solution to form nuclei. Generally metal ions are complexed and chalcogenide ions are chosen in such a way that reaction take place between slowly

Conclusions

Highly compact, uniform, amorphous, and pin hole free thin films of TiO2 thin films, one of the alternatives to spray pyrolysis, sputtering (high temperature processes), and application potential to 3-D solar cells, were deposited at room temperature by arrested chemical route on ITO substrates. Films were stoichiometric with Ti:O ratio as 33.2:66.8. After heat treatment at 450° for 1 h in air, films exhibits little grain growth without alteration in crystallographic structure, except slight

Acknowledgements

One of the authors (RSM) wishes to thank Brain Korea 21 project and National R&D project for Nano-Science and Technology, Korea, for the award of POST-DOC fellowship.

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