Charles B. Greenberg

Undoped and doped VO2 films grown from VO(OC3H7)3

Abstract Thin VO2-containing films have been grown on glass from vanadyl tri- isopropoxide by both chemical vapor deposition (CVD) and gel hydration. Undoped films grown by CVD were used to characterize the near-IR transmittance switching at 68°C. Spectral and resistive switching are known to accompany this monoclinic-tetragonal symmetry transition. Relatively large spectral switching occurred within 0.8–2.2 μm even when resistive switching was poor, i.e. less than twofold. The largest resistive switching observed was about 103-fold, which is similar to the best reported elsewhere for VO2 films prepared on glass in other ways. In the case of gel preparation, the films were doped with tungsten, molybdenum or niobium to demonstrate shifting of the transition below 68°C. The depression achieved with these films is qualitatively consistent with the conclusions of other researchers that are based on doped VO2 crystals and powders. By comparison with undoped CVD films, however, there is no indication that doping has any unusual effect on IR switching.

Optically switchable thin films: a review

Abstract Reversible transitions in molecular structure, symmetry and energy banding are very common in solids and liquids. Such transitions are known to occur both gradually and discontinuosly, but they are of greatest interest when they occur relatively abruptly. The transitions are induced electrically, thermally, with pressure or by exposure to UV-visible radiation. Materials that respond to these excitations with spectral changes, notably coloration in the visible range, are electrochromic, thermochromic, piezochromic or photochromic respectively. Some of the materials in these classes have been prepared as thin films, but they are relatively few in number. Thin films that have been shown to switch dramatically in the visible range, as well as the near-IR, and their properties, are reviewed here. Spectral switching of these films has been of interest to us because of the potential to control solar loading dynamically through transparencies in buildings and vehicles.

Tungsten Oxide‐Prussian Blue Electrochromic System Based on a Proton‐Conducting Polymer Electrolyte

A new solid-state electrochromic system is presented. It is transparent and is comprised of a tungsten oxide and Prussian blue (PB) thin film couple in combination with a proton-conducting, solid polymer electrolyte. This electrochromic system exhibits rapid and deep optical switching; characteristics of a complementary configuration, both electrochromic films color and bleach in phase. Complementary electrochromic cells with the tungsten oxide-PB couple have previously been based on Li[sup +] or K[sup +]-conducting electrolytes. A repetitively cycling cell has not previously been reported with a proton-conducting solid polymer electrolyte. The devices were operated at low applied voltages, +1.2 V to darken and [minus]0.6 V to bleach. Repeated reduction and oxidation of the current system over 20,000 cycles has been demonstrated, indicating a large number of switchings without great degradation or irreversible side reactions. The sustained, high overall coloration efficiency of the devices suggests the insertion/extraction of protons into and out of both WO[sub 3] and PB films. The effects of cell size and operating temperature on the switching response are discussed.

The Influence of Terminal Effect on the Performance of Electrochromic Windows

A simple theory of potential and current distributions developed elsewhere, based on the parallel-plate electrochemical cell under linear kinetics, is applied to electrochromic windows. In our case, only the working electrode has appreciable resistance. The model permits a predictive analysis of the effects of operating current density, cell length, cell length, and sheet resistance of this transparent electrode on the devices performance in the darkening process

Enabling Thin Films for Solar Control Transparencies: A Review

The science of thin films has virtually become a field of study in itself in the second half of the 20th century. One outcome of this is the large-scale commercial application of transparent, inorganic films onto soda-lime-silica flat glass. The various reflecting and absorbing surface properties have modified the authors perception of what a window is. Thin films deposited onto glass by pyrolytic, electroless, vacuum, and chemical vapor deposition techniques have led to new aesthetic effects and some ability to manage solar energy for buildings and automobiles. The strategy has been to shade some of the visible, while maximizing attenuation of the solar infrared, and do both by reflection if possible. Improved attenuation of solar energy is calculated from spectral properties under standard conditions and expressed as a desirably low shading coefficient. A few important enabling thin film materials, ones exhibiting acceptable durability, have driven these developments. Some are discussed herein in context with present and future energy budgets. Possibilities for advancing the solar control strategy with switchable transparencies are also considered.

Morphological clue to a pyrolytic thin film growth mechanism

Methods for forming thin, transition metal oxide films pyrolytically from liquid spray vehicle are well known. On the one hand, there are the uniform and finely crystallized spinel-structured films grown from organic solutions of acetylacetonates; one of these is herein reviewed. A spectrally similar film grown from aqueous solution, of another spinel composition, is not so well formed. This films relatively gross morphology is a clue to its growth mechanism. The morphology can be explained qualitatively by invoking a model for closely associated phenomena characteristic of spray drying. By this model, solid or partially molten metal-organic material approaches the hot substrate in the form of roughly spherical, porous, hollow shells. These remain after solvent evaporation from the droplets of the liquid spray. The hollow shells vaporize on the approach to the substrate, and the film grows by a CVD reaction at the substrate. Constituents which do not readily vaporize, such as impurities, impact the substrate and imprint a morphological pattern of collapsed shells. These comprise the rare clue for the analysis and for modeling film growth.