Showing papers in "Solar Energy Materials and Solar Cells in 2003"

Solar photocatalytic degradation of azo dye: comparison of photocatalytic efficiency of ZnO and TiO2

Abstract: The photocatalytic activity of commercial ZnO powder has been investigated and compared with that of Degussa P25 TiO2. Laboratory experiments with acid brown 14 as the model pollutant have been carried out to evaluate the performance of both ZnO and TiO2 catalysts. Solar light was used as the energy source for the photocatalytic experiments. These catalysts were examined for surface area, particle size and crystallinity. The effect of initial dye concentration, catalyst loading, irradiation time, pH, adsorption of acid brown 14 on ZnO and TiO2, intensity of light and comparison of photocatalytic activity with different commercial catalysts were studied. The progress of photocatalytic degradation of the acid brown 14 has been observed by monitoring the change in substrate concentration of the model compound employing HPLC and measuring the absorbance in UV–Visible spectrophotometer for decolourisation. The photodegradation rate was determined for each experiment and the highest values were observed for ZnO suggesting that it absorbs large fraction of the solar spectrum and absorption of more light quanta than TiO2. The complete mineralisation was confirmed by total organic carbon (TOC) analysis, COD measurement and estimation of the formation of inorganic ions such as NH4+, NO3−, Cl− and SO42−.

High-performance carbon counter electrode for dye-sensitized solar cells

Abstract: Here, we reported that a new carbon electrode prepared with an activated carbon was superior to a Pt sputtered electrode as the counter electrode ofdye-sensitized solar cells. The photovoltaic performance was largely influenced by the roughness factor of carbon electrode. The open-circuit voltage increased by about 60 mV using the carbon counter electrode compared to the Pt counter electrode because of positive shift of the formal potential for I � =I �

Electrical and optical properties of Cu2ZnSnS4 thin films prepared by rf magnetron sputtering process

Abstract: Cu 2 ZnSnS 4 thin films were deposited on corning 7059 glass substrates without substrates heating by rf magnetron sputtering. The Cu/(Zn+Sn) ratio of the thin film sputtered at 75 W was close to the stoichiometry of Cu 2 ZnSnS 4 . However, the S/(Cu+Zn+Sn) ratio was less than the stoichiometry. The as-deposited films were amorphous and annealed in the atmosphere of Ar+S 2 (g). The annealed (1 1 2), (2 0 0), (2 2 0), (3 1 2) planes were conformed to all the reflection of a kesterite structure. A preferred (1 1 2) orientation was observed with the increase of the annealing temperature. The optical absorption coefficient of the thin film was about 1.0×10 4 cm −1 . The optical band energy was derived to be 1.51 eV. The optical absorption coefficient of the sputtered Cu 2 ZnSnS 4 thin films was less than that of CuInS 2 thin film, however, the band gap energy was more appropriate for photovoltaic materials.

Material and solar cell research in microcrystalline silicon

Abstract: This contribution describes the introduction of hydrogenated microcrystalline silicon (μc-Si:H) as novel absorber material for thin-film silicon solar cells. Work done at IMT Neuchatel in connection with deposition of μc-Si:H layers by very high frequency glow discharge deposition is related in detail. Corresponding layer properties w.r.t. material microstructure, hydrogen content, stability and electronic transport are referred to. Basic properties of single-junction, entirely microcrystalline, thin-film silicon solar cells are related: Spectral response, stability w.r.t. light-induced degradation, basic solar cell parameters (Voc, Jsc and FF) obtained by IMT Neuchatel and by other laboratories are listed and commented; the deposition rate issue is addressed. Finally, microcrystalline/amorphous, i.e. "micromorph" silicon tandem solar cells, are described, together with recent developments on the research and industrial front. © 2002 Elsevier Science B.V. All rights reserved.

A Cu2O/TiO2 heterojunction thin film cathode for photoelectrocatalysis

Abstract: A thin film heterojunction photocathode was developed consisting of 100 nm of n-type titanium dioxide (TiO2) cathode surface deposited on p-type cuprous oxide (Cu2O). The cuprous oxide was deposited electrochemically on Ti foil. A photocurrent of 0.7 mA / cm 2 (at −1 V bias) and an open circuit photovoltage of 460 mV were obtained under an illumination of 700 W / m 2 . The photoresponse as a function of pH demonstrated that the TiO2 film protected the Cu2O underlayer against corrosion. These results suggest that using a simple and inexpensive heterostucture configuration, the corrosion limitations of Cu2O alone may be overcome while maintaining a relatively high efficiency for photoelectrolysis.

Evolution of microstructure and phase in amorphous, protocrystalline, and microcrystalline silicon studied by real time spectroscopic ellipsometry

Abstract: Real time spectroscopic ellipsometry has been applied to develop deposition phase diagrams that can guide the fabrication of hydrogenated silicon (Si:H) thin films at low temperatures (

Efficient sensitization of nanocrystalline TiO2 films with cyanine and merocyanine organic dyes

Abstract: Various kinds of cyanine and merocyanine organic dyes having short anchoring groups as sensitizers on nanocrystalline TiO2 electrodes were investigated to promote the short-circuit photocurrent (Jsc) and the solar light-to-power conversion efficiency (ηsun). The Jsc and ηsun improved when the three different three dyes (yellow and red cyanine dyes, and blue squarylium cyanine dye) were adsorbed simultaneously on a TiO2 electrode, as compared with the Jsc and ηsun of the TiO2 electrodes adsorbed by each single dye. The maximum ηsun was 3.1% (AM-1.5, 100 mW/cm2). The Jsc and ηsun were influenced by the solvents for the dye adsorption on the TiO2 electrode, and the efficiencies were improved by the addition of some cholic acids into the dye solution for adsorption. The electron transfer and/or the energy transfer from the red cyanine dye to the blue cyanine dye was observed on a SiO2 film using emission spectroscopy, suggesting a strong interaction between two dyes. The J-like aggregates of the blue cyanine dyes hardly showed sensitization efficiency.

Review of surface photovoltage spectra of nano- sized semiconductor and its applications in heterogeneous photocatalysis

Abstract: Heterogeneous photocatalysis is a promising technique valuable for environmental purification. Nano-sized semiconductors such as ZnO and TiO 2 , which is one of the most basic functional materials, have emerged as effective photocatalyst materials. The surface photovoltage spectra (SPS) can be an effective method for quickly evaluating the photocatalytic activity of semiconductor materials since it can provide a rapid, non-destructive monitor of the semiconductor surface properties such as surface band bending, surface and bulk carrier recombination and surface states, mainly showing the carrier separation and transfer behavior with the aid of light, especially the electric-field-induced surface photovoltage spectra (EFISPS), in which SPS is combined with the electric-field-modified technique. In this review, the basic principles, measurement and applications of the SPS and EFISPS are mainly discussed together with some fundamental aspects like the electric properties of semiconductor surface and the principle of electric field effect. In particular, the applications of SPS to nano-sized semiconductors such as ZnO and TiO 2 in heterogeneous photocatalysis are emphasized, which involve mainly evaluating the photocatalytic activity by analyzing semiconductor surface properties such as the separation efficiency of photoinduced carriers under illumination by the SPS measurement, highlighting our own contributions. The results show that the weaker the surface photovoltage signal is, the higher the photocatalytic activity is in the case of nano-sized semiconductor photocatalysts.

Large-Area Smart Glass And Integrated Photovoltaics

Abstract: The field of switchable materials is ever expanding, with many types of new market and the technology [1, 2, 3, 4]. Switchable windows can be used for many applications including architectural and vehicle windows, aircraft windows, skylights and sunroofs. Related to glazing are applications for low-information-content displays for the presentation of information. Switchable windows rely on a variety of processes and materials. Several companies through out the world are developing dynamic glazing. University and National Laboratory groups are researching new materials and processes to improve these products. Switchable glazing for building and vehicle application is very attractive. Conventional glazing only offers fixed transmittance and control of energy passing through it. Given the wide range of illumination conditions and glare, dynamic glazing with adjustable transmittance offers the best solution.

III–V compound multi-junction solar cells: present and future

Abstract: As a result of top cell material quality improvement, development of optically and electrically low-loss double-hetero structure tunnel junction, photon and carrier confinements, and lattice-matching between active cell layers and substrate, the last 15 years have seen large improvements in III–V compound multi-junction (MJ) solar cells. In this paper, present status of R&D program for super-high-efficiency MJ cells in the New Sunshine Project in Japan is presented. InGaP/InGaAs/Ge monolithic cascade 3-junction cells with newly recorded efficiency of 31.7% at AM1.5 (1-sun) were achieved on Ge substrates, in addition to InGaP/GaAs//InGaAs mechanically stacked 3-junction cells with world-record efficiency of 33.3%. Future prospects for realizing super-high-efficiency and low-cost MJ solar cells are also discussed.

Influence of the thickness on structural, optical and electrical properties of chemical bath deposited CdS thin films

Abstract: Cadmium sulfide films of different thicknesses were deposited by chemical bath deposition (CBD) from a bath containing cadmium acetate, ammonium acetate, thiourea, and ammonium hydroxide. The XRD patterns show that the films are of hexagonal phase with preferred (0 0 2) orientation and the grain size increases with the thickness of the film. The band gap of the films was calculated from the transmittance data and it was found that the band gap decreases as the film grows in thickness. The photo-response studies indicate that the film thickness has an influence on the current decay under dark. The observed opto-electronic properties were attributed to the crystallite size and internal microstrain.

Dye-sensitized nanocrystalline TiO2 solar cells based on novel coumarin dyes

Abstract: We have developed dye-sensitized nanocrystalline TiO2 solar cells (DSSCs) based on novel coumarin-dye photosensitizers. The absorption spectra of these novel dyes are red-shifted remarkably in the visible region relative to the spectrum of C343, a conventional coumarin dye. Introduction of a methine unit (–CHCH–) connecting the cyano (–CN) and carboxyl (–COOH) groups into the coumarin framework expanded the π-conjugation in the dye and thus resulted in a wide absorption in the visible region. These novel dyes performed as efficient photosensitizers for DSSCs. A DSSC based on 2-cyano-5-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro-1H,4H,10H-11-oxa-3a-aza-benzo[de]anthracen-9-yl)-penta-2,4-dienoic acid (NKX-2311), produced a 6.0% solar energy-to-electricity conversion efficiency (η), the highest performance among DSSCs based on organic-dye photosensitizers, under AM 1.5 irradiation (100 mW cm–2) with a short-circuit current density (Jsc) of 14.0 mA cm–2, an open-circuit voltage (Voc) of 0.60 V, and a fill factor of 0.71. Our results suggests that the structure of NKX-2311 whose carboxyl group is directly connected to the –CHCH– unit, is advantageous for effective electron injection from the dye into the conduction band of TiO2. In addition, the cyano group, owing to its strong electron-withdrawing ability, might play an important role in electron injection in addition to a red shift in the absorption region. On a long-term stability test under continuous irradiation with white light (80 mW cm–2), stable performance was attained with a solar cell based on the NKX-2311 dye with a turnover number of 2.6×107 per one molecule.

Simulation of I–V characteristics of a PV module with shaded PV cells

Abstract: The authors are studying a diagnostic method of a PV power generating system We consider that the change of I–V characteristics can be utilized for the diagnosis However, the report on the change of I–V characteristics is very little In this paper, we investigate the relation between the output lowering due to shaded PV cells and the change of I–V characteristics, utilizing the computer simulation It was proven from the simulation that I–V characteristics are changed by the condition of the shadow, which covered the module The change of I–V characteristics of a PV module with shaded PV cells is discussed by the shift of the avalanche breakdown voltage of shaded PV cells

Single-material TiO2 double-layer antireflection coatings

Abstract: This paper demonstrates that a double-layer antireflection (DLAR) coating can be fabricated using a single material, titanium dioxide (TiO2) The optical properties of the top and bottom TiO2 layers were controlled by varying the deposition and sintering conditions, resulting in a range of refractive indices, n=173–263 at 600 nm Weighted average reflectances of 65% (measured) and 70% (calculated) were achieved for TiO2 DLAR coatings in air and under glass, respectively When implemented in a high-efficiency silicon solar cell, a short-circuit current density increase of Δ J sc =25 mA/cm 2 can be expected for an optimised TiO2 DLAR coating when compared to a commercial TiO2 single-layer antireflection coating

Low temperature polycrystalline silicon: a review on deposition, physical properties and solar cell applications

Abstract: This review article gives a comprehensive compilation of recent developments in low temperature deposited poly Si films, also known as microcrystalline silicon. Important aspects such as the effect of ions and the frequency of the plasma ignition are discussed in relation to a high deposition rate and the desired crystallinity and structure. The development of various ion energy suppression techniques for plasma enhanced chemical vapour deposition and ion-less depositions such as HWCVD and expanding thermal plasma, and their effect on the material and solar cell efficiencies are described. The recent understanding of several important physical properties, such as the type of electronic defects, structural effects on enhanced optical absorption, electronic transport and impurity incorporation are discussed. For optimum solar cell efficiency, structural considerations and predictions using computer modelling are analysed. A correlation between efficiency and the two most important process parameters, i.e., growth rate and process temperature is carried out. Finally, the application of these poly Si cells in multijunction cell structures and the best efficiencies worldwide by various deposition techniques are discussed.

Optical and photoconductive properties of SnS thin films prepared by electron beam evaporation

Abstract: Thin films of SnS have been prepared by electron beam evaporation The films represent Herzbergite orthorhombic structure, established by their XRD patterns The band gap energy and type of optical transitions were determined from transmission spectra and an optical band gap of E g (tr) =123 eV for indirect transitions and E g (tr) =138 eV for direct transitions were estimated Using the dependence of photoconductivity from wavelength, a band gap of E g (ph) =12 eV was determined as well A thermal band gap of E g (T) =129 eV was evaluated from the temperature dependence of the dark resistivity, and admixture level with activation energies (025 and 036 eV) were found Roughness of the surface of SnS thin films was evaluated using atomic force microscopy

Bulk-heterojunction photovoltaic devices based on donor-acceptor organic small molecule blends

Abstract: We present a systematic study on photovoltaic devices that combine an organic small molecule photoactive donor–acceptor bulk heterojunction system with controlled doping of the charge transport layers. The doped transport layers are formed using high vacuum co-evaporation deposition technique (i.e. co-sublimation of matrix and dopant). Solar cell devices have been fabricated based on zinc-phthalocyanine (ZnPc) as donor (D) and fullerene (C 60 ) as electron acceptor (A) with doped charge transport layers. The cells show a short circuit current, I sc =1.5 mA/cm 2 , an open circuit voltage, V oc =450 mV, a fill factor, FF=0.5, and a power conversion efficiency, η e =3.37% under 1 10 sun (10 mW/cm 2 ) white light illumination. In addition, these bulk-heterojunction photovoltaic devices were characterized under 1 sun (100 mW/cm 2 ) white light illumination showing I sc =6.3 mA/cm 2 , V oc =500 mV, and η e =1.04%. We have observed that the performance of such ‘bulk-heterojunction’ photovoltaic devices is critically dependent on the transport properties of the interpenetrating network D/A system and doped charge transport layers.

Preparation, physical property and thermal physical property of phase change microcapsule slurry and phase change emulsion☆

Abstract: Phase change microcapsule slurry and phase change emulsion are two novel two-phase heat transfer fluids. Compared with a conventional single-phase heat transfer fluid such as water, their apparent specific heats in the phase change temperature range are greatly increased. Due to this, the heat transfer ability and energy transport ability can be obviously improved. Therefore, they have many potentially important applications in fields such as heating, ventilating, air-conditioning, refrigeration and heat exchangers. In this paper, a phase change emulsion was prepared by mixing film synthesis, and a phase change microcapsule slurry was prepared by in situ polymerization with polystyrene, polymethyl methacrylate, polyethyl methacrylate as encapsulation material, respectively. Physical properties, such as viscosity, diameter and its distribution of microcapsule and emulsion were investigated. The relationship between the concentration of tetradecane and physical properties have been discussed in detail. Meanwhile, the thermal physical properties of these two fluids were determined by DSC. Also, the influence of tetradecane concentration on phase change temperature and phase change heat has been discussed.

Modeling the optical absorption within conjugated polymer/fullerene-based bulk-heterojunction organic solar cells

Abstract: In this paper, we report our results on the modeling of the optical properties of the bulk-heterojunction “plastic solar cells”, consisting of a solid-state blend of the conjugated polymer poly-[2-(3,7-dimethyloctyloxy)-5-methyloxy]-para-phenylene-vinylene and the fullerene C 60 derivative 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C 61 . Upon illuminating these cells with the standard AM 1.5 solar spectrum, the short circuit current can be determined for any given internal quantum efficiency as a function of the active layer thickness. In addition, the depth profiles of photoinduced charge generation rates are calculated. Based on the agreement of this modeling with experimentally determined efficiencies of these solar cells, an internal quantum efficiency of about 80% has been estimated.

A technique to compare polythiophene solid-state dye sensitized TiO2 solar cells to liquid junction devices

Abstract: In this communication, we report on a technique to fabricate solid-state polythiophenebased dye sensitized solar cells (DSSCs) that can be directly compared to analogous liquid junction devices. The device configuration is based on non-porous TiO2 thin films and one of the three undoped polythiophene hole conductors: poly[3-(11 diethylphosphorylundecyl) thiophene], P3PUT, poly(4-undecyl-2,2 0 -bithiophene), P4UBT, or poly(3-undecyl-2,2 0 -bithiophene), P3UBT. These polymers were spin coated and cast from organic solutions onto the TiO2 films. The dense TiO2 thin films (ca. 30 nm) were deposited on conductive glass via facile spray pyrolysis and sol–gel techniques. After that, cis-(SCN)2 Bis(2,2 0 bipyridyl-4,4 0 dicarboxylate) ruthenium(II) (a.k.a. Ru N3 dye) was adsorbed on the TiO2 surface, and the polythiophenes were utilized as hole conductors in a simplified solar cell geometry. The results were compared to the control DSSC device made with dense TiO2 and a liquid electrolyte, or

Recombination processes in dye-sensitized solid-state solar cells with CuI as the hole collector

Abstract: Construction of dye-sensitized solid-state solar cells requires identification of hole collectors and understanding of the dissipative processes that limit the energy conversion efficiency. One of the hole collectors fairly well studied and giving a reasonably high efficiency is CuI. In this note we show that stoichiometrically excess iodine molecules adsorbed at the CuI surface acts as hole trapping sites (located at∼0.2 eV above the conduction band edge) that mediate recombination, affecting the performance of the cell. Fluorescence measurements reveal that exposure of CuI films to iodine vapor generates surface traps and as iodine diffuses to the bulk, surface trap density is greatly reduced. Methods by which the recombination originating from this effect may be circumvented are also discussed.

Optical properties of chemical vapor deposited thin films of molybdenum and tungsten based metal oxides

Abstract: Thin films of tungsten oxide, molybdenum oxide and mixed MoO3–WO3 oxides were obtained by atmospheric pressure chemical vapor deposition (CVD). All the films were prepared using identical technological parameters and through investigation of the optical properties of as deposited and annealed at 400°C a comparative study is reported. Raman, IR and VIS spectrophotometry and spectral ellipsometry methods were used. The mixed MoO3–WO3 films have higher optical absorption with maxima at a closer position with respect to the human eye sensitivity peak at 2.5 eV. The observed electrochromic effect is better expressed in the mixed films; the electrical charge inserted is higher.

Silica antireflective films on glass produced by the sol-gel method

Abstract: Up to now antireflective silica thin films deposited on glass do not have enough strength and display poor adhesion to the substrate. Three methods to obtain silica antireflective films on glass were surveyed to identify the best way to produce the antireflective effect on glass by introducing a certain degree of porosity. Porous silica layers, obtained from the polymeric and colloidal methods, permit a considerable reduction of these light reflections compared with uncoated glasses in all the cases studied, but the degree of reduction as well as the adhesive properties is different depending on the method used to achieve the precursor solution. The AM1.5 solar transmittance increased from 0.915 for the bare slide up to 0.970 for the best-made sample corresponding to the Triton-doped silica.

Experimental investigation and analysis on a thermoelectric refrigerator driven by solar cells

Abstract: Experimental investigation and performance analysis on a solar cell driven, thermoelectric refrigerator has been conducted. Research interest focused on testing the system performance under sunshine. Experiment results demonstrated that the unit could maintain the temperature in the refrigerator at 5–10°C, and have a COP about 0.3. Further analysis indicated that the performance of the system is strongly dependent on intensity of solar insolation and temperature difference of hot and cold sides for the thermoelectric module, etc. There exist optimum solar insolation rates, which let the cooling production and COP achieve maximum value, respectively. It was expected that the refrigerator would be potential for cold storage of vaccine, food and drink in remote area, or outdoor conditions where electric power supply is absent.

Field-test analysis of PV system output characteristics focusing on module temperature

Abstract: Field-test data from a 50 kW photovoltaic (PV) system installed at The Nara Institute of Science and Technology (NAIST) were analyzed in detail. We found that the PV system operated in a wide temperature range and was strongly affected by the temperature coefficient of conversion efficiency when the module temperature became high. The temperature coefficient dependence of the system performance was analyzed in order to estimate the annual output of the system in an actual operating environment. As a result, it was found that the annual output energy of the PV system increased about 1% by an improvement of 0.1%/°C in the temperature coefficient. This result indicates that it is very important to consider the temperature characteristics in solar cell development.

Structural and optical properties of hot wall deposited CdSe thin films

Abstract: Cadmium selenide (CdSe) films were prepared by hot wall deposition technique using optimized tube length under a vacuum of 6 mPa on to well-cleaned glass and ITO substrates. The X-ray diffraction analysis revealed that the films are polycrystalline in nature for lower thickness and at lower substrate temperatures, but with increasing thickness and increasing substrate temperature a more preferred orientation along (0 0 2) direction was observed. The crystallite size (D), dislocation density (δ) and strain (e) were calculated. An analysis of optical measurements revealed a sharp absorption around 700 nm and a direct allowed transition. The band gap was found to be around 1.7 eV. The effect of thickness and substrate temperature on the fundamental optical parameters like band gap, refractive index and extinction coefficient are studied.

Diffusion and evolution of hydrogen in hydrogenated amorphous and microcrystalline silicon

Abstract: Hydrogen (H) diffusion and evolution effects in hydrogenated amorphous and microcrystalline silicon (Si) films are discussed. Numerous diffusion and evolution related effects have been discovered and various models for H motion, trapping and evolution effects have been proposed. Microstructure and H diffusion and evolution effects are found to be strongly interdependent. Various aspects of H diffusion in compact material are consistent with a trapping/detrapping motion of atomic H, restricted by H solubility effects and describable in terms of an equilibrium band model.

Facile fabrication of mesoporous TiO2 electrodes for dye solar cells: chemical modification and repetitive coating

Abstract: A chemical dispersing technique for preparing a coating paste of TiO2 nanoparticles is disclosed to fabricate mesoporous electrodes for dye-sensitized TiO2 solar cells. The suspension of TiO2 (P-25) powder was stirred in aqueous nitric acid at 80°C, and then evaporated to dryness, giving the nitric acid-adsorbed P-25 powder. The coating paste was obtained by mixing the nitric acid-adsorbed P-25 with PEG (Mw 20,000) as a porosity-controlling agent and cellulosic polymer as a thickener. The mesoporous TiO2 films were fabricated on conducting glasses by repetitive coating and calcined at 500°C (30 min). The TiO2 film obtained by the five times repetitive coating (20 μm thickness) resulted in the 1.4 times higher energy conversion efficiency of the dye-sensitized solar cells than that of the one time coating TiO2 film (Voc=690 mV, Jsc=12.2 mA/cm2, the fill factor=0.71 and η=6.0%).

Experimental study of variations of the solar spectrum of relevance to thin film solar cells

Abstract: The influence of variations in the incident solar spectrum on solar cells is often neglected. This paper investigates the magnitude of this variation and its potential influence on the performance of thin film solar cells in a maritime climate. The investigation centres on the analysis of a large number of measurements carried out in Loughborough, UK, at 10 min intervals over a period of 30 months. The magnitude of the spectral variation is presented both on a daily and a seasonal basis. Of the different thin film materials studied, amorphous silicon is shown to be the most susceptible to changes in the spectral distribution, with the “useful fraction” of the light varying in the range +6% to −9% of the annual average, with the maximum occurring in summer time.

Comparison of electrochromic amorphous and crystalline tungsten oxide films

Abstract: A detailed systematic study of the tungsten oxide thin films has been carried out using WO 3 films after they were annealed at progressively increasing temperatures ranging from 350°C to 450°C in oxygen environments. The structural properties of the films were characterized using X-ray diffraction and Raman spectroscopy. The amorphous WO 3 films remain as an amorphous phase up to 385°C and begin to crystallize at 390°C and then are completely crystallized at 450°C. Absorption peaks of the films are found to shift to a higher energy side with increasing annealing temperature up to 385°C and then shift abruptly to a lower energy as the films begin to crystallize at 390°C. Deconvolution of the absorption spectra shows that there are two different polaron transitions in the amorphous WO 3 films.