Electrochromic characteristics of fibrous reticulated WO3 thin films prepared by pulsed spray pyrolysis technique

doi:10.1016/j.solmat.2007.03.005

Solar Energy Materials and Solar Cells

Volume 91, Issue 12, 23 July 2007, Pages 1097-1101

https://doi.org/10.1016/j.solmat.2007.03.005 Get rights and content

Abstract

The electrochromic (EC) behavior of fibrous reticulated WO₃ films prepared from ammonium tungstate precursor by pulsed spray pyrolysis method was investigated. All the films were prepared using identical technological parameters and a thorough investigation of the electrochromic properties of the films deposited at 300 °C is reported. The structural properties were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrochromic and optical properties were measured using cyclic voltammetry and ultraviolet (UV)-visible spectrophotometry. The films are amorphous and have a fibrous reticulate-like morphology having micron-size circular rings. The films show high transparency in the visible range and the optical band gap energy is about 3.1 eV. Electrical measurements show that the resistivity monotonically decreases as temperature increases, which indicates thermal hopping transport. The activation energy for hopping transport is of the order 4×10⁻⁴ eV. The electrochromic coloration efficiency (CE) is found to be 34 cm²/C at 630 nm.

Introduction

Interest in the opto-electrical properties of transition metal oxides, has grown rather sharply, since the discovery of the electrochromic property in such compounds [1], [2], [3]. The research on this family of compounds dates back to the 1970s [4], [5]. In an electrochromic material, the optical properties change reversibly and persistently, when voltage is applied to it. Tungsten oxide is the most extensively studied electrochromic material due to its excellent electrochemical stability and its technological implications in smart windows, large-area information displays, rear-view mirrors for automobiles, energy saving, etc. [6], [7], [8], [9]. However, for practical electrochromic devices, further improvement in the EC properties, viz. electrochromic reversibility, stability, etc. is required.

Thin films of WO₃ have been grown by various techniques including physical vapor deposition (PVD) [8], [10], chemical vapor deposition (CVD) [11], [12], and wet chemistry methods [13]. The WO₃ films prepared by spray pyrolysis can offer a cost-effective way to form uniform and large-area EC coatings. However, limited literature is available on WO₃ films prepared by spray pyrolysis. The recent detailed research on WO₃ thin films has, however, shown that much still remains to be understood about the structural, optical and their electrical property correlations in these materials.

In present work, WO₃ thin films were grown from a precursor solution of aqueous ammonium tungstate ((NH₄)₂WO₄) by the pulsed-spray pyrolysis technique. In this spray pyrolysis technique, the spraying solution is adjusted in intervals so that adatoms has sufficient time to decompose. The nozzle atomizers cautiously avoid deposition of a large droplet, which often takes place in conventional spray pyrolysis depositions. We report here the structural, electrical, optical and electrochromic properties of the WO₃ thin films of fibrous reticulated morphology deposited at 300 °C.

Section snippets

Experimental details

The WO₃ thin films used in this study were grown on indium doped tin oxide (ITO) coated glass and microscopic glass substrates by pulsed-spray pyrolysis technique. The precursor solution used for spray pyrolysis deposition was prepared by dissolving pure WO₃ powder in ammonia and distilled water and then heated to 90 °C (prior to spray). At that temperature the formation of ammonium tungstate ((NH₄)₂WO₄) is completed. The substrate temperature was maintained at 300 °C during deposition.

Results and discussion

The thickness of the film calculated using the gravimetric technique is ∼5000 Å. Fig. 1 shows the XRD pattern recorded in the θ–2θ scan mode for a WO₃ film deposited at 300 °C on a glass substrate. XRD pattern shows no distinct diffraction peaks of WO₃, indicating the amorphous nature. The broad hump around 20–30° 2θ values originates from the glass substrate.

The SEM has been used for morphological analysis of WO₃ thin films. Fig. 2 shows a SEM image for a WO₃ thin film prepared at 300 °C on a

Conclusions

The WO₃ thin films synthesized by the pulsed spray pyrolysis technique showed amorphous structure with porous and multi-layered fibrous reticulated surface morphology as confirmed by XRD and SEM. This study has shown that amorphous and fibrous-reticulated WO₃ thin films exhibit the well-known electrochromic properties and that the unique microstructure leads to better electrochemical and optical reversibility with higher rates of coloration/bleaching, compared to crystalline and polycrystalline

Acknowledgments

One of the authors (SRB), gratefully acknowledges the financial support provided by Department of Science and Technology (DST), Government of India, in the form of Woman Scientist Award (Grant No. SR/WOS-A/PS-04/2006). Authors acknowledge financial support through UGC-DRS (SAP) and ASIST programmes sanctioned by the University Grants Commission, New Delhi, India.

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Electrochromic characteristics of fibrous reticulated WO3 thin films prepared by pulsed spray pyrolysis technique

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Conclusions

Acknowledgments

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

J. Eur. Ceram. Soc.

Electrochem. Acta

Appl. Surf. Sci.

Sol. Energy Mater. Sol. Cells

Mater. Res. Bull.

Electrochromic characteristics of fibrous reticulated WO₃ thin films prepared by pulsed spray pyrolysis technique