Electrochromic tungsten oxide films: Review of progress 1993–1998

Electrochromic (EC) materials are able to change their optical properties, reversibly and persistently, by the application of an electrical voltage. These materials can be integrated in multilayer devices capable of modulating the optical transmittance between widely separated extrema. We first review the recent literature on inorganic EC materials and point out that today's research is focused on tungsten oxide (colouring under charge insertion) and nickel oxide (colouring under charge extraction). The properties of thin films of these materials are then discussed in detail with foci on recent results from two comprehensive investigations in the authors' laboratory. A logical exposition is obtained by covering, in sequence, structural features, thin film deposition (by sputtering), electronic band structure, and ion diffusion. A novel conceptual model is given for structural characteristics of amorphous W oxide films, based on notions of defects in the ideal amorphous state. It is also shown that the conduction band density of states is obtainable from simple electrochemical chronopotentiometry. Ion intercalation causes the charge-compensating electrons to enter localized states, implying that the optical absorption underlying the electrochromism can be described as ensuing from transitions between occupied and empty localized conduction band states. A fully quantitative theory of such transitions is not available, but the optical absorption can be modeled more phenomenologically as due to a superposition of transitions between different charge states of the W ions (6+, 5+, and 4+). The Ni oxide films were found to have a porous structure comprised of small grains. The data are consistent with EC coloration being a surface phenomenon, most likely confined to the outer parts of the grains. Initial electrochemical cycling was found to transform hydrated Ni oxide into hydroxide and oxy-hydroxide phases on the grain surfaces. Electrochromism in thus stabilized films is consistent with reversible changes between Ni hydroxide and oxy-hydroxide, in accordance with the Bode reaction scheme. An extension of this model is put forward to account for changes of NiO to Ni2O3. It was demonstrated that electrochromism is associated solely with proton transfer. Data on chemical diffusion coefficients are interpreted for polycrystalline W oxide and Ni oxide in terms of the lattice gas model with interaction. The later part of this review is of a more technological and applications oriented character and is based on the fact that EC devices with large optical modulation can be accomplished essentially by connecting W-oxide-based and Ni-oxide-based films through a layer serving as a pure ion conductor. Specifically, we treat methods to enhance the bleached-state transmittance by mixing the Ni oxide with other oxides characterized by wide band gaps, and we also discuss pre-assembly charge insertion and extraction by facile gas treatments of the films, as well as practical device manufacturing and device testing. Here the emphasis is on novel flexible polyester-foil-based devices. The final part deals with applications with emphasis on architectural “smart” windows capable of achieving improved indoor comfort jointly with significant energy savings due to lowered demands for space cooling. Eyewear applications are touched upon as well.

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Electrochromics for smart windows: thin films of tungsten oxide and nickel oxide, and devices based on these

Atmospheric vμ ve ratio in the multi-GeV energy range

Superionic conductors are compounds that exhibit exceptionally high values of ionic conductivity within the solid state. Indeed, their conductivities often reach values of the order of 1 Ω−1 cm−1, which are comparable to those observed in the molten state. Following Faraday's first observation of high ionic conductivity within the solids β-PbF2 and Ag2S in 1836, a fundamental understanding of the nature of the superionic state has provided one of the major challenges in the field of condensed matter science. However, experimental and theoretical approaches to their study are often made difficult by the extensive dynamic structural disorder which characterizes superionic conduction and the inapplicability of many of the commonly used approximations in solid state physics. Nevertheless, a clearer picture of the nature of the superionic state at the ionic level has emerged within the past few decades. Many different techniques have contributed to these advances, but the most significant insights have been provided by neutron scattering experiments and molecular dynamics simulations. This review will summarize the state of current knowledge concerning the crystal structures and conduction processes of superionic conductors, beginning with a comparison of the behaviour of two of the most widely studied binary compounds, AgI and β-PbF2. Each can be considered a parent of two larger families of highly conducting compounds which are related by either chemical or structural means. These include perovskite-structured oxides and Li+ containing spinel-structured compounds, which have important commercial applications in fuel cells and lightweight batteries, respectively. In parallel with these discussions, the relative importance of factors such as bonding character and the properties of the mobile and immobile ions (charge, size, polarizability, etc) in promoting the extensive lattice disorder which characterizes superionic behaviour will be assessed and the possibilities for predicting a priori which compounds will display high ionic conductivity discussed.

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Superionics: crystal structures and conduction processes

Dye-sensitized solar cells (DSCs) are well researched globally due to their potential as low-cost photovoltaic (PV) devices especially suited for building and automobile integrated PV (BIPV, AIPV) and portable or indoor light harvesting applications. Since 1991, large monetary and intellectual investments have been made to develop DSCs into deployable technologies, creating a wealth of knowledge about nano-interfaces and devices through an increasing number of research reports. In response to these investments, the dawn of the new millennium witnessed the emergence of a corporate sector of DSC development. Advances in their design, their incorporation on flexible substrates, the development of solid state modules, their enhanced stability in outdoor environments, and their scalable fabrication tools and techniques have allowed DSCs to move from the laboratory to real-life applications. Although photoconversion efficiencies are not on a par with commercially available CIGS or single crystalline silicon solar cells, they possess many features that compel the further development of DSC modules, including transparency, light weight, flexibility, conformability, workability under low-light conditions, and easy integration in buildings as solar windows. In fact, DSC panels have been shown to deliver even more electricity than their silicon and thin film counterparts of similar power ratings when exposed to low light operating conditions due to their workability in such conditions; thus, they are potential market leaders in BIPV and indoor light harvesting photovoltaic technology. However, large area dye-solar modules lack in performance compared to their laboratory scale devices and also suffer from long term stability issues. Herein, we discuss the main factors behind their inferior photovoltaic performance and identify possible opportunities for the design of more efficient DSC modules.

A perspective on the production of dye-sensitized solar modules

We report studies of the electrochemical intercalation of lithium in ${\mathrm{WO}}_{3}$ powder and in polycrystalline evaporated ${\mathrm{WO}}_{3}$ films Electrochemical cells with beryllium x-ray windows were used to study the structure changes in the ${\mathrm{WO}}_{3}$ host in situ Rietveld profile refinement was used to analyze the x-ray-diffraction patterns of ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{WO}}_{3}$ after correcting for the absorption of the beryllium window Upon the intercalation of Li, the monoclinic structure of ${\mathrm{WO}}_{3}$ proceeds to higher and higher symmetry First a first-order phase transition to a tetragonal phase with x\ensuremath{\sim}01 in ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{WO}}_{3}$ occurs Then another first-order phase transition to a cubic phase with x\ensuremath{\sim}036 occurs The cubic phase can accommodate further Li, up to a maximum of about x=05 All the phase transitions can be reversed, although some hysteresis is observed; upon complete deintercalation, monoclinic ${\mathrm{WO}}_{3}$ is recovered We report the structural changes, prove that the transitions are first order, and give a phase diagram for ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{WO}}_{3}$ as a function of x Our results suggest that the formation of the tetragonal phase upon intercalation of Li in ${\mathrm{WO}}_{3}$ is responsible for the electrochromatic properties of crystalline ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{WO}}_{3}$, and that conventional interpretations that ignore the existence of this phase are most likely in error

Lithium intercalation into WO3 and the phase diagram of LixWO3.

This paper shows that the intercalation of Li in WO[sub 3] depends on how the WO[sub 3] host is made. Electrochemical intercalation and deintercalation of Li into WO[sub 3] films caused two reversible phase transitions in a range of 0 [lt] x [lt] 0.5 in Li[sub x] WO[sub 3] films. The same studies on commercial WO[sub 3] powders showed no evidence for these phases and an irreversible phase transition to an amorphous phase instead. The authors prepared WO[sub 3] by heating H[sub 2]WO[sub 4] powders at various temperatures. The reversible Li intercalation seen in the film was reproduced in WO[sub 3] powder made by heating H[sub 2]WO[sub 4] at low temperature (e.g., 400[degrees]C), but WO[sub 3] powder made by heating H[sub 2]WO[sub 4] at high temperatures (e.g., 900[degrees]C) showed the same behavior as the commercial powder. The authors believe the differences in the proton or oxygen content of the materials may be responsible for these differences.

Q. Zhong

Papers

Lithium intercalation into WO3 and the phase diagram of LixWO3.

Dependence of the Intercalation of Li in WO 3 on the Preparation of the WO 3 Host

Dependence of the Intercalation of Li in WO3 on the Preparation of the WO3 Host.