E. Avendano, A. Azens, J. Backholm, G. Gustavsson*, R. Karmhag*, G. A. Niklasson and C. G. Granqvist

Department of Engineering Sciences, The Angstrom Laboratory, Uppsala University,

P. O. Box 534, SE-75121 Uppsala, Sweden

*Also at Chromogenics Sweden AB, Uppsala University Holding, SE-75183 Uppsala, Sweden

This paper presents a detailed study of the optical properties of a number of electrochromic nickel-oxide-based and iridium-oxide-based films. Chromaticity is analyzed with regard to a set of illuminants pertinent to different natural and artificial lighting conditions. In particular, it is shown that additions of Mg, Al, Si, Zr, Nb, and Ta can improve the transmittance of nickel-oxide-based films, and that Mg, Al, and Ta can have the same effect for iridium-oxide-based films.

Introduction

Electrochromic materials are able to reversibly change their optical properties upon charge insertion-extraction induced by an external voltage [1-3]. The materials can be integrated in electrochromic devices of several different types and can be all-solid — state constructions as well as polymer laminated ones, with or without self-powering by solar cells [4]. These devices open a number of technologically interesting possibilities to modulate optical transmittance, reflectance, absorptance, and emittance. Recently, special attention has been devoted to designs incorporating electrochromic hydrated nickel oxide films operating in conjunction with electrochromic tungsten oxide; this combination of materials makes it possible to attain a neutral gray color in the dark state. Optical scattering can be essentially nil

[5] . Rigid (usually glass-based) devices [2,3] as well as flexible, polyester-based foil devices [6,7] have been investigated during the last decade.

Among the numerous applications of electrochromism, we note architectural “smart windows”, which are able to combine improved indoor comfort (less glare and thermal stress) with good energy efficiency (especially lowered air conditioning loads in cooled buildings) as apparent from order-of magnitude estimations [6] as well as buildings simulations [8]. The use of “smart windows” has been discussed in detail in literature on innovative architecture [9,10]. Other applications concern non-emissive displays, variable-reflectance mirrors, variable-transmittance eyewear of different kinds, and variable-emittance surfaces for temperature stabilization of space vehicles. For these and other uses, however, there is a long-standing problem with hydrated nickel oxide, which tends to show residual optical absorption in the 400 < A < 500 nm wavelength interval, thereby precluding a fully transparent state.

This paper contains an investigation of the optical properties of hydrated nickel oxide with several different additives introduced with the objective of reducing the

absorption in the visible range without compromising the electrochromic properties. This investigation is complemented by a study of iridium-oxide-based electrochromic materials. The films were made by reactive magnetron sputtering, which is an industrially viable technology with proven upscaling capability.