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

Electrochromic (EC) materials and polymer electrolytes are the most imperative and active components in an electrochromic device (ECD). EC materials are able to reversibly change their light absorption properties in a certain wavelength range via redox reactions stimulated by low direct current (dc) potentials of the order of a fraction of volts to a few volts. The redox switching may result in a change in color of the EC materials owing to the generation of new or changes in absorption band in visible region, infrared or even microwave region. In ECDs the electrochromic layers need to be incorporated with supportive components such as electrical contacts and ion conducting electrolytes. The electrolytes play an indispensable role as the prime ionic conduction medium between the electrodes of the EC materials. The expected applications of the electrochromism in numerous fields such as reflective-type display and smart windows/mirrors make these materials of prime importance. In this article we have reviewed several examples from our research work as well as from other researchers’ work, describing the recent advancements on the materials that exhibit visible electrochromism and polymer electrolytes for electrochromic devices. The first part of the review is centered on nanostructured inorganic and conjugated polymer-based organic-inorganic hybrid EC materials. The emphasis has been to correlate the structures, morphologies and interfacial interactions of the EC materials to their electronic and ionic properties that influence the EC properties with unique advantages. The second part illustrates the perspectives of polymer electrolytes in electrochromic applications with emphasis on poly (ethylene oxide) (PEO), poly (methyl methacrylate) (PMMA) and polyvinylidene difluoride (PVDF) based polymer electrolytes. The requirements and approaches to optimize the formulation of electrolytes for feasible electrochromic devices have been delineated.

Hybrid Materials and Polymer Electrolytes for Electrochromic Device Applications

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China; Department of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200233, People’s Republic of China; and National Engineering Research Center for Nanotechnology, 28 East Jiangchuan Road, Shanghai 200241, People’s Republic of China

On the synergetic catalytic effect in heterogeneous nanocomposite catalysts.

Inverse opal structures are desirable for fuel cell electrodes, but application of such structures in polymer electrolyte membrane fuel cells is yet to be realised. Kim et al. report fabrication of a platinum catalyst layer with an inverse opal structure, and show improved fuel cell performance.

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Ordered macroporous platinum electrode and enhanced mass transfer in fuel cells using inverse opal structure.

The authors prepared uniformly shaped WO2.72 nanowire bundles using the solvothermal synthesis method. They investigated the potential of the WO2.72 nanowire bundles to be used as a cathode electrode for electrochromic devices and the effect of the Li+ insertion (or extraction) kinetics and diffusion of Li+. An electrode consisting of arrays of WO2.72 nanowire bundles was formed and used in an experiment using the Langmuir-Blodgett technique. The one-dimensional nanostructure of WO2.72 has a high Li-ion diffusion coefficient (∼5.2×10−11cm2∕s) and low charge transfer resistance (∼28.6Ω), which result in its having a fast electrochromic response time (coloring time 55cm2∕C).

http://mse2.kaist.ac.kr/~ncrl/pub/2007APL.elchrom.yhJung.pdf

Ju Wan Lim

Papers

Ordered macroporous platinum electrode and enhanced mass transfer in fuel cells using inverse opal structure.

Fast switchable electrochromic properties of tungsten oxide nanowire bundles

Improved electrochromic devices with an inorganic solid electrolyte protective layer

High contrast ratio and fast switching polymeric electrochromic films based on water-dispersible polyaniline-poly(4-styrenesulfonate) nanoparticles

Electrochromic properties of one-dimensional tungsten oxide nanobundles