Carl-Gustaf Ribbing

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.

Solar optical properties of thin films of Cu, Ag, Au, Cr, Fe, Co, Ni and Al

Abstract The optical properties of metal coated glass substrates have been investigated. Thin films of various thicknessesof the noble metals: Cu, Ag, Au, the transition metals: Cr, Fe, Co, Ni and the free electron-like metal Al were thermally evaporated onto glass substrates. The front and backside reflectance and the transmittance between 0.35 and 15 μ m were measured. The obtained data were used to calculate the integrated values of solar reflection and transmission as a function of metal film thickness. The application of metal films on domestic windows as sun-screens and heat-mirrors are discussed. It is concluded that Cu is the best coating in a window system if good heat insulating properties are desired. This is due to its ability to remain continuous at very thin film thicknesses. An infra-red reflectance of 86 per cent combined with a solar transmittance of 55 per cent was obtained for a 70film. For solar heat-protection Au-films are found to be superior owing to their transmittance peak in the middle of the visible wavelength region. The transition metals are less selective than the noble metals, but due to their flat response-curves in the visible range they cause a smaller change in colour of the transmitted and reflected light.

Optical constants of thin TiN films: thickness and preparation effects.

The optical properties of reactively sputtered TiN films 5–80 nm thick are reported. The stoichiometry and purity of the films were verified with x-ray diffraction and Auger spectroscopy, respectively. The optical constants have been determined by combined R and T measurements at oblique incidence. It is found that deposition onto 400°C substrates significantly improves the optical quality of the films. In particular, the optical selectivity for heat mirror applications is increased, the refractive index in the visible is lowered, and the thickness variation of the optical constants is reduced. It is argued that a low refractive index in the visible is a sensitive measure of film quality. The Drude relaxation time was ~25% longer for the films prepared on heated substrates. Electrical measurements confirm the high quality of the films.

Optical constants and Drude analysis of sputtered zirconium nitride films

Opaque and semitransparent dc magnetron-sputtered ZrN films on glass and silicon have been optically characterized with spectral reflectance measurements and ellipsometry. High rate sputtered ZrN has good optical selectivity, i.e., higher than 90% infrared reflectance and a pronounced reflectance step in the visible to a reflectance minimum of less than 10% at 350 nm. The results are comparable with those obtained for single crystalline samples and those prepared by chemical vapor deposition. The complex optical constant (N = n v ik) for opaque films has been determined in the 0.23-25-µm wavelength range with Kramers-Kronig integration of bulk reflectance combined with oblique incidence reflectance for p-polarized light. A variable angle of incidence spectroscopic ellipsometer has been used for determination of the optical constants in the 0.28-1.0-µm wavelength region. The results of the two methods show excellent agreement. The results indicate that ZrN is free electronlike and the Drude model can be applied. The best opaque films had Drude plasma energies (ħω(p) between 6.6 and 7.5 eV and relaxation energies (ħ/τ) between 0.29 and 0.36 eV. Ellipsometer data for the semitransparent films show that the refractive index (n) in the visible increases with decreasing film thickness whereas the extinction coefficient (k) is essentially unchanged. The optical properties are improved by deposition upon a heated substrate.

Optical characterisation of titanium-nitride-based solar control coatings

Abstract Results on the optical properties of titanium-nitride-based multilayers are reported. Single layer TiO2 and TiO2/TiN/TiO2 triple layers were prepared by DC magnetron sputtering. The refractive index n and the extinction coefficient k have been calculated for the TiO2. The transmittance and reflectance spectra as well as the solar optical parameters of the triple layers are presented. The plasma current used to sputter the TiN is noted to affect the performance of the triple layer. The triple layer is shown to be an option as a solar control coating.

Influence of spray conditions and dopants on highly conducting tin dioxide films

Abstract Transmitting and IR reflicting films of doped tin dioxide have been prepared by pyrolytic spraying on glass. The influence of substrate temperature and dopants on the optical selectivity and electrical conductivity has been studied. A higher substrate temperature gives a higher deposition rate. The IR-reflectance increases with doping, but is also correlated to the crystallization of the films as for the undoped case. Fluorine doping gave the best performance both in the visible and in the infrared regions. Such films have a good technological potential for heat mirror applications on energy conserving windows. Experimental curves for the absorption in the NIR region are obtained from measured reflectance and transmission spectra and compared with experimental and calculated results of others.

Materials for solar-transmitting heat-reflecting coatings

Abstract A coating for solar energy applications which combines heat reflection with transparency to solar radiation may be of four different types: a metallic film which is sufficiently thin to be transparent; a metal-based multilayer coating; a wide band gap heavily doped semiconductor such as SnO2 or In2O3; a conducting microgrid. We prepared such coatings on glass by evaporating thin films of silver, copper, gold, aluminium, cobalt, iron, chromium and nickel of various thicknesses and by spraying SnO2 films. The spectral variations in the transmittance, and the front side and back side reflectances were measured in the wavelength range 0.4–15 μm. The properties of a three-layer coating of the dielectric/metal/dielectric type were calculated with a multilayer program using known bulk optical constants. The effect of these films when coated onto a domestic window was demonstrated with a heat transfer calculation using an equivalent thermal net. When a large transmittance over a broad range of the solar spectrum is required, gold is an equally good, or a slightly better, choice than silver as the metal in a three-layer coating. In general, an SnO2 film exhibits a higher solar transmittance as well as a higher emittance than a coating containing metals. This implies that the oxide is to be preferred as a coating on a window when the maximum passive solar heating is sought. However, a metal-based coating could be better when a very low UL value is the most important requirement.

International round-robin experiment to test the International Organization for Standardization total-scattering draft standard

An international round-robin experiment has been conducted among laboratories in different countries to test the measurement and the data-analysis procedures in the International Organization for Standardization draft standard ISO/DIS 13696 for measuring total scattering from low-scatter laser optics. Ten laboratories measured total backscattering from high-reflectance mirrors, 50% beam splitters, and antireflection-coated windows. Results were sent to the Laser Zentrum Hannover, which acted as coordinator and analyzed all the backscattering data. The results showed that the procedure in the draft standard was useful for measuring and reporting backscattering for low-scatter optics. Problems encountered in the round-robin experiment included the accumulation of particles on the surfaces, particularly on the high-reflectance mirrors.

Optical constants of sputtered hafnium nitride films. Intra- and interband contributions

Abstract Transparent and opaque films of hafnium nitride have been prepared by reactive magnetron sputtering in an argon-nitrogen atmosphere. The films were deposited upon heated Coming glass substrates with a deposition rate of about 2.1 nm/s. The optical constants of the films were calculated using Kramers-Kronig integration and ellipsometry for the opaque samples and a combined R T method for transparent samples. Detailed observation of the thickness variation in the optical constants revealed a clear trend: larger n-values and smaller k-values in the VIS-and NIR-range for thinner films. This was analysed within the framework of the classical Drude model and found to be an effect of shorter relaxation time for thinner films. The effect is as large as a factor of three in the film thickness interval 15–380 nm. The optical effective mass of the d-electrons was found to be in the interval 0.82–0.95 of the free electron mass, which is significantly lower than in TiN but similar to ZrN. The interband contribution to ϵ2(ω) was obtained by subtraction of the Drude part from the experimental dielectric function. It exhibits a sharp increase for λ

Transition-metal-nitride films for optical applications

The three transition metal nitrides TiN, ZrN and HfN have remarkably high stability due to their bonding: a mixture of covalent and ionic contributions. The optical properties of these nitride compounds are free-electron like to a surprisingly large extent, in particular in comparison with the corresponding carbides. It is argued that the interband optical excitations of the d-electrons are restricted by selection rules, resulting in a Drude like behavior of these d-electron compounds. Hitherto, one of the main optical applications has been as opaque, wear-resistant replacements for gold-coatings. This review includes the efforts to study, understand and enhance the optical selectivity of group IVB transition metal nitrides as the selective surface in high temperature thermal solar absorbers, as the metal layer in LE-coatings on energy-efficient or solar control windows and as a Langmuir probe coating. The reflectance edge is not in the optimum position for absorber applications and attempts to shift it with alloying were unsuccessful. Recent development of inhomogeneous, cermet- type nitrides hold some promise. The LE-coatings will not reach as high selectivity as the current noble metal based multilayers, but many find use in aggressive environments because of their excellent stability.