Photoinduced reactivity of titanium dioxide

Transparent electrodes are a necessary component in many modern devices such as touch screens, LCDs, OLEDs, and solar cells, all of which are growing in demand. Traditionally, this role has been well served by doped metal oxides, the most common of which is indium tin oxide, or ITO. Recently, advances in nano-materials research have opened the door for other transparent conductive materials, each with unique properties. These include CNTs, graphene, metal nanowires, and printable metal grids. This review will explore the materials properties of transparent conductors, covering traditional metal oxides and conductive polymers initially, but with a focus on current developments in nano-material coatings. Electronic, optical, and mechanical properties of each material will be discussed, as well as suitability for various applications.

Emerging Transparent Electrodes Based on Thin Films of Carbon Nanotubes, Graphene, and Metallic Nanostructures

Transparent conductors as solar energy materials: A panoramic review

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

Electrochromics for smart windows: Oxide-based thin films and devices

Electrical transport and optical properties were investigated in porous thin films consisting of ${\mathrm{In}}_{2}{\mathrm{O}}_{3}:\mathrm{Sn}$ (indium tin oxide, ITO) nanoparticles with an initial crystallite size of $\ensuremath{\sim}16 \mathrm{nm}$ and a narrow size distribution. Temperature dependent resistivity was measured in the $77ltl300\mathrm{K}$ temperature interval for samples annealed at a temperature in the $573l~{t}_{\mathrm{A}}l~1073\mathrm{K}$ range. Samples annealed at $573l~{t}_{\mathrm{A}}l~923\mathrm{K}$ exhibited a semiconducting behavior with a negative temperature coefficient of the resistivity (TCR). These data were successfully fitted to a fluctuation induced tunneling model, indicating that the samples comprised large conducting clusters of nanoparticles separated by insulating barriers. Samples annealed at ${t}_{\mathrm{A}}=1073\mathrm{K}$ displayed a metallic behavior with no signs of insulating barriers; then the TCR was positive at $tg130\mathrm{K}$ and negative at $tl130\mathrm{K}.$ Effects of annealing on the ITO nanoparticles were investigated by analyzing the spectral optical reflectance and transmittance using effective medium theory and accounting for ionized impurity scattering. Annealing was found to increase both charge carrier concentration and mobility. The ITO nanoparticles were found to have a resistivity as low as $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\ensuremath{\Omega}\mathrm{cm},$ which is comparable to the resistivity of dense high quality ${\mathrm{In}}_{2}{\mathrm{O}}_{3}:\mathrm{Sn}$ films. Particulate samples with a luminous transmittance exceeding 90% and a resistivity of $\ensuremath{\sim}{10}^{\ensuremath{-}2}\ensuremath{\Omega}\mathrm{cm}$ were obtained.

Electrical and optical properties of thin films consisting of tin-doped indium oxide nanoparticles

Indium tin oxide films made from nanoparticles: models for the optical and electrical properties

Thickness Dependence of the Optical Properties of Sputter Deposited Ti Oxide Films

Porous tungsten oxide nanoparticle films were prepared by reactive gas evaporation. The structure was studied by x-ray diffraction and scanning electron microscopy, and the oxygen nonstoichiometry was inferred by x-ray photoelectron spectroscopy, elastic recoil detection analysis, and neutron scattering. Specifically, the films consisted of WO3−x with 0.25<x<0.4. The optical and electrical data were consistent with the formation of small polarons having a radius of 5–6A. The infrared optical data, used to extract information on phonon energies, were instrumental to reach this conclusion. The polaron hopping energy was about half the polaron binding energy, as expected from the theory.

Small polaron formation in porous WO3−x nanoparticle films

Electrical and optical properties were investigated in porous thin films consisting of In2O3:Sn (indium tin oxide; ITO) nanoparticles. The temperature-dependent resistivity was successfully described by a fluctuation-induced tunneling model, indicating a sample morphology dominated by clusters of ITO nanoparticles separated by insulating barriers. An effective-medium model, including the effect of ionized impurity scattering, was successfully fitted to measured reflectance and transmittance. Post-deposition treatments were carried out at 773 K for 2 h in both air and vacuum. It is shown that vacuum annealing increases either the barrier width or the area between two conducting clusters in the samples and, furthermore, an extra optical absorption occurs close to the band gap. A subsequent air annealing then reduces the effect of the barriers on the electrical properties and diminishes the absorption close to the band gap.

J. Ederth

Papers

Electrical and optical properties of thin films consisting of tin-doped indium oxide nanoparticles

Indium tin oxide films made from nanoparticles: models for the optical and electrical properties

Thickness Dependence of the Optical Properties of Sputter Deposited Ti Oxide Films

Small polaron formation in porous WO3−x nanoparticle films

Thin porous indium tin oxide nanoparticle films: effects of annealing in vacuum and air