J. Isidorsson

Towards the smart window : progress in electrochromics

Electrochromic devices have the ability to produce reversible and persistent changes of their optical properties. The phenomenon is associated with joint ion and electron transport into/out of an electrochromic thin film, in most cases being a transition metal oxide. This paper outlines the various applications of such devices in smart windows suitable for energy-conscious architecture, in variable-reflectance mirrors, and in display devices. Critical materials issues and design concepts are discussed. The paper also covers two specific research topics: computed electronic structure of crystalline WO3 incorporating ionic species, showing how reflectance modulation emerges from a first-principles calculation; and Li+ dynamics in heavily disordered Ti oxide, illustrating how diffusion constants derived from impedance spectroscopy can be reconciled with the Anderson—Stuart model.

Highly transparent Ni–Mg and Ni–V–Mg oxide films for electrochromic applications

Abstract Thin films of oxides based on Ni, Ni–V, Ni–Mg, and Ni–V–Mg were made by reactive d.c. magnetron sputtering. Electrochemical cycling showed pronounced anodic electrochromism. The Mg addition was capable of yielding a significantly enhanced optical transparency in the 400

Impedance studies on Li insertion electrodes of Sn oxide and oxyfluoride

Films of Sn oxide and oxyfluoride were made by reactive rf magnetron sputtering onto ITO‐coated glass. We analyzed the composition by Rutherford backscattering spectrometry, the structure by x‐ray diffraction, and the surface topography by atomic force microscopy. Li intercalation from a liquid electrolyte was more facile in the oxide than in the oxyfluoride, as found from cyclic voltammetry. Impedance spectra were taken for a wide range of frequencies and polarizing voltages. Nyqvist diagrams were interpreted from a circuit model with elements representing Li insertion at the electrolyte/film interface and electron insertion at the film/ITO interface. The data were consistent with a fractal surface of the Sn oxide film, with a dimension in excellent agreement with measures obtained through several independent techniques. The chemical diffusion coefficient was ∼10−13 cm2/s and slightly decreasing with increasing potential for all films.

Surface smoothing and roughening in sputtered SnO2 films

Abstract In this paper we study the scaling of the surface roughness in regimes of smoothing and roughening. Transparent SnO2 films are sputtered onto rough glass substrates, and films in the thickness range 15–1200 nm are produced. Surface images are obtained by atomic force microscopy. Specifically we are able to determine the effect on the roughness exponent upon a transition from smoothing to roughening regimes, and it is found that the surface width scales differently in the two regions. To our knowledge this has not been seen before, and we are not aware of a theoretical explanation of this. For thinner films smoothing effects reduces the surface roughness as compared to the substrate, and the correlation length decreases. On the other hand, for thicker films the surface width increases with thickness again. For the thinner films the roughness exponent is in the range 0.5–0.7, and for the thicker films it is approximately 0.9–1. The growth exponent is found to be approximately 0.3.

Electrochromism in lithiated Sn oxide: Mössbauer spectroscopy data on valence state changes

Lithiated Sn oxide films, denoted LixSnO2, were produced by reactive rf magnetron sputtering of Sn and electrochemical post‐treatment. Optical transmittance and Mossbauer spectra were recorded for progressively increased lithiation. Increasing x from zero to ∼0.1 did not have any significant effect on the optical data or Mossbauer spectra, and it is likely that the lithium is located in internal double layers in the film. Further increasing x from ∼0.1 to ∼0.2 yielded significant transmittance drops and Mossbauer spectra unambiguously showing a conversion Sn4+→Sn2+. Hence the optical absorption can be reconciled with intervalency transitions as in other cathodically coloring electrochromic oxides. Electrocrystallization appeared to dominate the electrochemistry at x≳0.2.

Ion transport in porous Sn oxide films: Cyclic voltammograms interpreted in terms of a fractal dimension

Sn oxide films, made by reactive r.f. magnetron sputtering, were studied in a Li+-conducting electrolyte. Cyclic voltammograms taken at different sweep rates were interpreted in terms of a unique structural parameter related to the fractal dimension of a self-affine surface relief.

Cerium-containing counter electrodes for transparent electrochromic devices

Films of Me-Ce oxide (Me: Ti, Zr, Sn, W) and of Ni-Ce hydroxide were produced by reactive magnetron co-sputtering. Li intercalation in Me-Ce oxide, and H exchange in Ni-Ce hydroxide, were accomplished electrochemically. Electrochromism was quenched in proportion with the Ce content in Me-Ce oxide. Films of Zr-Ce (and to some extent Ti-Ce) oxide were able to serve as fully transparent counter electrodes, of much interest for transparent electrochromic devices. In Ni-Ce hydroxide, the Ce addition enhanced the capacity for charge exchange.

Adsorption and Electrodeposition on SnO2 and WO 3 Electrodes in 1 M LiClO4 PC In Situ Light Scattering and In Situ Atomic Force Microscopy Studies

A novel spectroscopic in situ light scattering technique was used with in situ atomic force microscopy (AFM) to study electrode surfaces subjected to adsorption and electrodeposition. Tin dioxide and tungsten trioxide were used as electrodes in a 1 M LiClO 4 /propylene carbonate electrolyte. Both in situ methods showed the same increase in surface roughness immediately after the electrode was immersed in the electrolyte. The onset potential for electrodeposition could be determined; its specific value depended on the film composition as well as on the composition and purity of the electrolyte. A potential step technique revealed a progressive growth of the first electrodeposited layer. The growth mode after fully developed electrodeposition was characterized by a preferential growth of large crystals, evident from light scattering as well as AFM. Our experimental techniques make it possible to determine whether electrodeposition or electrochromism, due to electrochemical insertion of ionic species, dominates the observed modulation of the optical properties. The deposited layer was investigated using infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Although the composition of this layer cannot be stated conclusively, it most likely contains lithium alkyl carbonate species.

Theoretical Analysis of Chronoamperometric Electrochemical Ion Intercalation

We propose that the intercalation of ions from a liquid electrolyte into a solid material can be analyzed in terms of a model [J. Appl. Phys., 55, 3341 (1984)] developed for the release of alkali ions from interface traps in a metal-oxide silicon structur

Surface roughness in sputtered SnO2 films studied by atomic force microscopy and spectroscopic light scattering

In this paper we study the growth of surface roughness during sputtering of transparent SnO2 films onto rough glass substrates. Films in the thickness range 50–1200 nm were produced, and the optical characterization was made with a spectroscopic total integrating scattering instrument in the wavelength range 0.35<λ<1.0 μm. Optical constants for the different SnO2 films were determined. The observed spectral behavior of the diffuse reflectance (transmittance), as compared to the total reflectance (transmittance), could be explained by first-order vector perturbation theory in conjunction with a surface growth model incorporating both smoothing and roughening effects. Good agreement between calculated and measured reflectance and transmittance spectra was found, only by assuming partially correlated interface roughness. The scattering calculations rely on atomic force microscope measurements of the glass substrate and the front surfaces of the films, and a model of the cross correlation that describes the p...