Researchers Seek Patent on Electrochromic Smart Windows
February 14, 1992
By Jeffery Kahn, jbkahn@lblL.gov
LBL researchers have applied for a patent on versatile, inexpensive ion-storage materials that will help move "optically switchable"glass out of the laboratory into the marketplace.For several years, Carl Lampert of Energy and Environment's Windows
and Daylighting program has led a research group developing glass devices that can change electronically from transparent to opaque. Performing like an optical shutter, the glass is ideal for use in making "smart windows" -- windows for buildings and cars that could control the amount of light and heat allowed to pass through.
Windows & Daylighting Group Website There is considerable commercial interest in the development of optical switching devices. To date, 1,800 patents have been issued on electrochromic components, one of several types of technology under exploration. Of these patents, 1,500 are owned by Japanese interests.
Lampert says the patent -- for a family of polymers -- would be the first ever issued for one of the few essential elements within an electrochromic device, an improved ion storage layer.
The pending patent is quite broad, covering 10 families of polymer compounds.
Says Lampert, "The patent carves out a major niche, making the commercial development and marketing of electrochromic glass much more attractive to U.S. industry." Currently, LBL's Technology Transfer Office is seeking to license the technology to industry.
The new ion-storage electrodes -- based on polyorganodisulfides -- were the result of a collaborative effort between Lampert and Materials Sciences Division staff scientists Steve Visco, Marca Doeff, and graduate student Yan-Ping Mah. These novel electrode materials are the subject of another, past patent application for lithium polymer batteries, technology which has been licensed and transferred to an East Bay start-up company.
Internationally, a number of alternative approaches to switchable glass are being investigated -- liquid crystal display devices are one example. The solid state electrochromic devices that Lampert is developing consist of thin films that are sandwiched between two glass sheets. They are about 100 times thinner than a human hair.
These films would allow the occupant of a building or the driver of a car to control interior light levels as well as the flow of infrared heat, reducing the energy necessary for lighting, heating, and cooling. Additionally, glare could be controlled, and privacy provided without the use of blinds or drapes.
Lampert says electrochromic devices can be likened to a thin transparent battery. As the electrical field within the window is changed, it optically displays its state of charge, transforming in tint between transparent and opaque.
Switchable glass is multi-layered, with the electrochromic device embedded inside. A window device, for example, might have glass with an interior conductive oxide layer both on the top and bottom. Inside of this sandwich of glass and conductive oxide is the electrochromic device. It includes an electrochromic layer, an ion- storage layer and between these two, an ion conductor.
Lampert says the electrochromic material tints or clears as ions and electrons are shuttled back and forth between the electrochromic layer and the ion-storage layer, which are akin to two battery electrodes separated by an electrolyte. Only a small voltage (1-2 volts DC) is required to inject or eject the ions and electrons.
"These devices have long-term memory, ranging from 12 to 48 hours," says Lampert. "By that I mean you can color it, remove the power, and for that period, it will remain colored."
The first electrochromic products, rear-view auto mirrors that automatically dim bright lights at night, are now on the market, and switchable auto sunroofs may be available soon. But technical obstacles to further commercial applications remain.
One stumbling block has been the ion-storage layer. The LBL researchers believe the new polyorganodisulfide ion-storage polymers resolve several major problems.
The ion-storage and electrochromic layers must be electrochemically compatible with matching charge-storage capacities. Lampert and Visco's patent application covers a family of 10 compounds that have a range of capacities. This range provides manufacturers with a repertoire of compatible materials. As an added bonus, it also allows them to create tintable glass that transmits different amounts of light.
Another advantage of the organodisulfide polymers is that they do not shorten the life of the device.
A number of materials will act as an electrochromic electrode, shuttling ions and electrons back and forth. But as they do so, they gradually form insoluble compounds within the device, and hinder its electrochemical capacity to continue to cycle between transparent and opaque. The organodisulfide polymers can cycle back and forth without significant degradation.
Lampert has filed a second patent for a polymer compound that simplifies and reduces the cost of fabrication of an electrochromic device, making it possible to combine the ion-storage and ion- conductive layers.
The polymer has unusual electrical characteristics. It is electrically insulating but ion conducting. Unlike prior ion- storage materials that were too electrically conductive, the polymer makes it possible to dispense with the adjacent ion- conductor layer. By eliminating this layer, fabrication is simplified and costs reduced.
Lampert says that over the past several years, a succession of other refinements have been made.
"In our laboratory," he says, "we have begun to make progressively better electrochromic layers in terms of the uniformity of coloration. Likewise, we have made improvements in the efficiency of power usage of the devices, which can hold their charge for longer periods than before."
Working with industry, Lampert says he is learning which remaining problems are the most significant in terms of manufacture.
"We are receiving valuable feedback from industry, learning as well as transferring technology we have developed. We now have workable electrochromic components. Next," he says, "we must develop optimal components and learn how to fabricate multilayer polymers uniformly. When we've accomplished that, the day of smart windows will be very near."
