Chromogenic coatings for smart windows

Smart windows make it possible to regulate the throughput of visible light and solar energy and can provide energy efficiency for buildings and at the same time give indoor comfort. We work with two approaches: electrochromic (EC) materials and devices, which allow electrical transmittance control, and thermochromic (TC) materials which alter their transmittance when the temperature is changed.

Electrochromic devices can be constructed as shown below. A typical device uses two EC thin films connected by an electrolyte layer. When an electrical voltage is applied between two transparent electrical contacts, ions and compensating electrons (via the outer circuit) will be transported between the EC films. One of the EC films should darken when the ions are transported into it by the electric field and the other should darken when the ions are withdrawn. Thus, both films darken and brighten simultaneously as shown in the figure. In most practical applications, tungsten oxide and nickel oxide are used as EC layers. The entire structure can be based on flexible plastic sheets (shown at the top of the left part figure) which in turn can be used to laminate window glass. In 2002, the company ChromoGenics AB was founded as a result of our research on EC devices and windows. The third figure below shows full-scale EC windows that have been manufactured and sold commercially by ChromoGenics AB for a few years.

Thermochromic materials can consist of thin films or nanoparticles, in both cases based on vanadium dioxide. TC materials change their optical and electrical properties at a temperature in the vicinity of room temperature. The change occurs in the near infrared wavelength range and the visible transmittance is only slightly affected. The figure below shows schematic data of wavelength dependent transmittance and reflectance. The properties are very different at room temperature (where the material is semiconducting) and at higher temperatures (where the material is metallic). Layers of nanoparticles have higher visible transmittance and larger variation in solar transmittance than homogeneous films (see the second figure). TC materials are not yet commercial and face major challenges in terms of performance, i.e. achieved variation of solar radiation and in terms of durability.

Our overall aim is to develop improved EC and TC materials and to make improved EC devices (which means that electrolytes and transparent electrical conductors are of interest too). In recent years, we have focused on durability issues of EC materials during accelerated ageing by electrochemical cycling between light and dark states. We have found that aging kinetics can be described with simple mathematical relations, which opens the possibility for facile lifetime estimates of EC devices. Electrochemical treatments of the layers can provide a greatly improved durability and can also restore aged films to almost their original properties. We are now seeking to apply this knowledge to EC devices.

Our research encompasses thin film deposition and characterization with physical and (electro)chemical techniques, optical measurements, device integration and testing. Both experimental and theoretical/computational work is performed. The research has evolved during many years and has a high international profile.

References: 
G.B. Smith and C.G. Granqvist, Green Nanotechnology: Solutions for Sustainability and Energy in the Built Environment, CRC Press, Boca Raton, USA.
G.A. Niklasson and C.G. Granqvist, J. Mater. Chem. 17 (2007) 127.
C.G. Granqvist, Thin Solid Films 564, 1 (2014).
S.-Y. Li, G.A. Niklasson, and C.G. Granqvist, J. Appl. Phys. 115 (2014) 053513.

Optical properties of thin films, nanoparticles and composites are an integral part of many of our projects on energy efficiency and environmental applications. In particular, many of the materials used for applications in energy-efficient buildings and solar energy utilization employ optical functionality. Fundamental studies of optical properties are necessary in order to improve and understand the functional properties of these materials. We have a top-quality optical measurement laboratory with spectrophotometers for the ultraviolet, visible and infrared wavelength ranges, a spectroscopic ellipsometer and a photoluminescence setup with cryogenic cooling capability. The instruments enable us to measure spectral and angular light scattering and include a range of integrating sphere detectors. We participate in international collaborations to develop superior techniques and methods for accurate optical characterization. Additionally, we have large experience in analysis of optical properties of thin films and nanoparticles, in order to determine their optical constants. This includes development of our own software.

Some projects of current interest within this broad field are briefly summarized below.

1. Determination of the optical constants of thin films of functional optical materials. We use thin film optics to determine the complex refractive index of the materials from measurements of transmittance and reflectance. This methodology is used to study the optical properties of, for example, electrochromic and thermochromic thin films, photocatalytic layers of TiO₂ and transparent electrically conductive oxide materials.
2. We also determine effective optical constants of inhomogeneous materials consisting of metallic nanoparticles in an oxide matrix and use effective-medium theory to increase our understanding of optical phenomena in such materials. This type of materials is used in the interesting field of plasmonics. They are also of interest as surface coatings on solar collectors for hot water production or heating. This application needs spectrally selective coatings with high solar absorption, while their radiance of heat must be low. Detailed understanding of their optical properties is of importance for (a) the optimization of solar absorptance and thermal emittance by model computations, and (b) to establish which compositional and structural changes during high temperature ageing that lead to degradation of the optical properties, and hence limit the performance of the coatings. Another issue of current interest is to produce solar collectors with architecturally desirable colours. Some examples of optical reflectance spectra are shown in the figure below.
    

   

3. Light scattering should be avoided in for example window coatings but can also add functionality to a material or a thin film. In both cases, it is important to characterize and understand the scattering. We are primarily studying light scattering from small particles as a component in inhomogeneous materials. We have recently developed methods to characterize light-scattering materials by determining their scattering and absorption efficiencies from measurements of direct and diffuse transmittance as well as specular and diffuse reflectance. The “inverse problem” for light scattering is very complicated and our current solutions contain approximations that need to be validated. The materials we study inside include paint layers for solar collectors and "cool roofs", materials for radiative cooling, materials that can switch between transparent and light-scattering states and sunscreen lotions. Light scattering can also be an aesthetically pleasing phenomenon as illustrated below.

   

References:
E. Wäckelgård, G.A. Niklasson and C.G. Granqvist: Selectively solar-absorbing coatings, in Solar Energy: The State of the Art. ISES Position Papers, edited by J. Gordon (James & James, London, 2001), Ch. 3, pp. 109-144.
J-X. Wang, C. Xu, A.M. Nilsson, D.L.A. Fernandes, M. Strömberg, J-F. Wang and G.A. Niklasson, Adv. Opt. Mater., 2018, 1801315