An average household spends over 40 percent of its annual energy budget on heating and cooling costs. Office buildings now account for about one-third of all the energy used in the U.S., a quarter of which is lost through the inefficiency of standard windows to retain heat in the winter or deflect heat in the summer. ‘Smart’ window has the capability to be tuned or dimmed, to permit desired amount of light to pass through it. Therefore, it will allow more sunlight into the rooms during the winter and block most of it during the summer. As a result, ‘Smart’ windows that can significantly minimize the energy consumption of residence and office buildings are in great demand. It is estimated that $11 billion to $20 billion dollars a year savings in heating, lighting and air-conditioning costs can be realized if the “smart windows” replace the regular windows. “Smart windows” also boast other benefits, such as increased comfort, light and view, and decreased condensation. Residents are given control over their privacy and environment, and harmful ultraviolet rays are blocked, thereby minimizing the fading of furniture, carpets, drapes, artwork and other valuables. The cost of blinds, curtains and drapes are also significantly reduced or, in many cases, even eliminated.
Presently, three distinct ‘smart’ window technologies are positioning themselves for this endeavor, competing for shares of a global architectural glass market that produces an estimated 20 billion square feet of flat glass each year. Three ‘smart’ window technologies are liquid crystal, electrochromic, and suspended particle devices.
Switchable windows using Polymer Dispersed Liquid Crystals (PDLCs) can change from clear to opaque with the flip of a switch. In the opaque state, the glass diffuses direct sunlight and eliminates 99 percent of the ultraviolet rays responsible for the fading of carpets and curtains. Most uses of PDLCs, however, are confined to privacy applications, where popular uses are found in glass walls for offices, conference rooms, lobbies, and store-fronts. Privacy glass also provides unique opportunities for use by homebuilders in bathrooms, entryways, family rooms, bedrooms, and skylights. Liquid crystal technology has not been a commercial success. The windows are hazy because they scatter rather than absorb light, so there is a fog factor even when the device is in the transparent state. Also, while liquid crystals work well for interior privacy control, the technology only provides two statuses: clear or opaque, nothing between—it can't be used as a shading device. In addition, it also tends to be a little expensive for most popular applications, running between $85 and $150 per square foot.
Suspended Particle Device (SPD) technology works in such a way that there are millions of black, light-absorbing, suspended-particle devices (SPD) within a film placed between glass layers. When an increased voltage of electricity is applied to the film, the SPDs line up and become perpendicular to the window, which allows more light to pass through and increases visibility until the window is completely clear. As the amount of voltage is decreased, the window becomes darker until it reaches a bluish-black color that allows no light to pass through it. Therefore, a user has complete control over the amount of transmitted light from the glass or plastic walls. Windows in homes and office buildings, skylights and sun roofs, automobile dashboard displays and bright, high-contrast digital displays for laptop and other electronic instruments made with this new SPD technology can now be dimmed or brightened with electronic precision to suit individual needs, allowing an infinite range of adjustment between completely dark and completely clear. SPD, which produces little or no haze in the transparent state, can be controlled either automatically by means of a photocell or other sensing or control device, or adjusted manually with a rheostat or remote control by the user. In spite of all the activity in this field, SPD windows have yet to appear on the market. Developing the technology and manufacturing processes has been long and difficult.
Electrochromic technology may attract most attention for smart windows and a larger number of companies and research organizations are trying electrochromics. Electrochromic windows can be adjusted to control the amount of light and heat passing through them. Electrochromic windows generally comprise up to seven layers of material. Three central layers (ion storage layer, ion conducting layer and electrochromic layer) are sandwiched between two layers of a transparent conductor, all of which are further sandwiched between two layers of glass or plastic. All seven layers are, of course, transparent to visible light. These windows function as the result of transport of charged ions from an ion storage layer, through an ion conducting layer into an electrochromic layer by applying certain voltage. The presence of the ions in the electrochromic layer changes its optical properties, causing it to absorb visible light, the result of which is to darken (“unbleach”) the window. To reverse the process, the voltage is reversed, driving the ions in the opposite direction, out of the electrochromic layer, through the ion conducting layer, and back into the ion storage layer. As the ions migrate out of the electrochromic layer, it brightens (or “bleaches”), and the window becomes transparent again.
Electrochromic windows can also be used to help keep cars cool. An electrochromic sunroof could darken in the direct sunlight but lighten at other times, providing sunroof function while keeping the car cool. Conceivably, electrochromic rear or side windows in a vehicle could darken while the car is parked, keeping the car cool, and then lighten again once the car is started. So far the technology is used only in self-dimming rear-view mirrors that change from light to dark to prevent eyestrain and temporary blindness from the glare of headlights approaching from the rear, and then reversing when conditions permit.
In general, the electrochromic (EC) devices can be divided into two groups depending on the type of electrolyte employed in the device: lithium conducting medium or proton conducting medium of either inorganic solid-state or liquid/polymer gel type. Liquid/polymer gel type media encompass liquid type and polymer gel type media.
Existing EC devices based on solid state lithium conducting medium, unfortunately, are quite slow. It can take six seconds for something as small as an automobile's rear-view mirror to go from clear to dark, and it may take 10 seconds to return to clear. But for something the size of a window it may take six to 10 minutes to change between coloring and bleaching. Most people want instant feedback to adjust their window properly.
Since ionic diffusivity of protons is two orders of magnitude higher than that of lithium ions, much more rapid coloring and bleaching processes occur in proton conducting medium than lithium conducting medium. It is therefore more desirable to use proton conducting electrolyte for achieving fast response time.
Although inorganic solid-state EC devices can achieve better durability, its response time is relatively slow since the diffusivity of protons and lithium in solid medium is significantly lower than in liquid/polymer gel type media. However, durability is a significant concern for proton conducting liquid/polymer gel type EC devices, since the solid ion insertion layers (EC and counter electrode layer) can exhibit long-term degradation due to the contact with liquid/polymer gel type electrolytes. For example, WO3, by far the most common cathodic electrochromic material, has a tendency to slowly dissolve in proton-based electrolyte, which limits its use in proton conducting liquid/polymer gel type electrolytes.
An electrochromic compound based on a mixed oxide of tungsten and tantalum has been previously reported (Yang, D. et al., Thin Solid Films, 469-470 (2004) 54-58). This compound has the formula W0.9Ta0.1Ox, where x≦2.95. While electrochromic properties for this compound are reported, there is no indication of cyclic durability, especially in a proton-based environment.
Therefore, there is a need for novel electrochromic materials that have increased cyclic durability and/or do not suffer from long-term degradation effects when in contact with a proton-based environment in electrochromic devices, especially in liquid/polymer gel type electrochromic devices.
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