Electrochromic Research Activities

For electrochromic windows to be a practical addition to homes and commercial buildings, they have to be capable of coloring and bleaching thousands of times without any loss of performance. Because they're most likely to be used to block out direct sunlight, the windows are expected to be exposed continually to the radiation of the sun while reaching temperatures as high as 90°C or more.

NREL's research focuses on exposing test samples of electrochromic windows to the harshest environment they are likely to experience and cycling them between their light and dark modes, over and over, to determine if their performance remains constant for long periods of time. NREL is also working to develop and standardize these durability test methods, so that a standard benchmark for the performance of electrochromic windows can be obtained.

NREL's work isn't finished when a window fails; in fact, it has just begun. NREL performs destructive analysis on failed windows to advance the understanding of their failure mechanisms. The knowledge gained from these analyses is helping to improve the performance of electrochromic windows, bringing them closer to commercial reality.

In addition to this work, NREL performs research and development (R&D) on electrochromic materials and degradation mechanisms, and is working to develop advanced prototype electrochromic windows. NREL also works to educate people about electrochromic window technology and to find new markets and applications for electrochromic windows.

• Durability Testing
• Materials and Degradation Mechanisms
• Advanced Prototypes
• Education and New Markets
• Standards and Ratings
• Insulating Glass

Durability Testing of Electrochromic Windows

NREL tests the durability of electrochromic windows under the following conditions:

Test Conditions

To perform durability testing of electrochromic windows, the test samples are placed in one of NREL's controlled temperature and humidity chambers. These include two XR-260 chambers and one Atlas 1600. The chambers are outfitted with Xenon arc or alkali halide lamps to provide a uniform irradiance equal to one sun.

Samples are typically held at 85°C and a low relative humidity, while the windows are automatically cycled between their colored and bleached modes. The windows are exposed to thousands of cycles, typically over a period of about three weeks. Their optical properties are then recharacterized at room temperature, they are visually inspected in their colored and bleached states, then they are re-inserted into the test chamber for further durability testing.

Performance Characterization

Prior to testing, photographs are taken of the windows. These can later be compared to "after" photos, particularly if the samples fail. The samples' optical transmittance is also measured, over a spectrum from 300 to 1100 nanometers, using a fiber-optic based photodiode array spectrophotometer. A complete spectrum is measured every 4 to 10 seconds as the samples are cycled.

The samples are also studied using electrochemical impedance spectroscopy, measuring their percent transmittance as they color and as they bleach. These tests are repeated after the durability testing to compare before and after performances.

Ideally, a sample should maintain its physical integrity and performance after at least 20,000 cycles. It should maintain a photopic (400 nm to 720 nm) contrast ratio of more than four, that is, the transmittance in the colored state should be one fourth of the transmittance in the bleached state.

Development and Standardization of Durability Test Methods

NREL staff are serving on a variety of committees to standardize the test methods developed at NREL so that a consistent approach to testing electrochromic windows can be established within the window industry. NREL serves on special committees with the American Society for the Testing of Materials, the National Fenestration Rating Council, the Institute of Electrical and Electronics Engineers and the International Energy Agency.

For more information about durability testing or standards, see Publications.

Electrochromic Materials and Degradation Mechanisms

NREL performs fundamental research to support electrochromic windows, through such projects as X-ray photoelectron spectroscopic (XPS) and Raman spectroscopic examination of the fundamental properties of tungsten oxide films. NREL has also used XPS to identify the ratios of W⁴⁺, W⁵⁺, and W⁶⁺ in WO₂ and WO₃ powders.

NREL is investigating potential weaknesses at organic/metal or metal oxide interfaces in certain types of electrochromic window designs. Two papers examining these weaknesses include "Penetration of Deposited Au, Cu, and Ag Overlayers through Alkanethiol Self-assembled Monolayers on Gold or Silver," by G. C. Herdt, A. W. Czanderna, and D. King, and "Interactions and Reactions at Metal/Self-assembled Organic Monolayer Interfaces," by D. R. Jung and A. W. Czanderna, both of which were also presented at a Workshop on Self-assembling and Biomimitic Materials in Los Alamos, New Mexico, in December 1997.

NREL is also investigating gasochromic reactions in electrochromic windows. Using FTIR infrared spectra of Pd/WO₃ samples in a controlled environmental exposure chamber, NREL found indications that trace water is generated during the coloration process. To further investigate this reaction, NREL is using isotopically labeled oxygen to find the source of oxygen in the evolved water vapor. The oxygen may come from "sorbed" water in the electrochromic materials, or from the tungsten oxide itself.

Since the discovery of the solid-state, thin film electrochromic phenomena in WO₃ by S. Deb more than twenty five years ago, various models have been formulated to explain the electrochromic mechanism in this material. NREL researchers have recently proposed a new model for the chromic mechanism in amorphous WO3. Descriptions of the mechanism are based on the small polaron transition between the charge-induced W⁵⁺ state and the original W⁴⁺ state instead of the W⁵⁺ and W⁶⁺ states as suggested in previous models. This model not only explains a variety of seemingly conflicting experimental results reported in the literature, but also has practical implications with respect to improving the coloring efficiency and durability of electrochromic devices. See "Chromic Mechanism in Amorphous WO₃ films," by J. G. Zhang, D. K. Benson, C. E. Tracy, S. K. Deb, and A. W. Czanderna.

NREL also performs research using surface characterization techniques such as XPS, AES, ISS, SIMS, and STM.

One goal of this research is to ultimately be able to predict the service lifetime of electrochromic windows. The NREL Electrochromics Team has concluded that with a substantial research and development effort to understand the factors that limit the durability of electrochromic windows, it would be possible to predict their service lifetimes. For more information, see "Durability Issues and Service Lifetime Prediction of Electrochromic Windows for Building Applications." A.W. Czanderna, D. K. Benson, G. J. Jorgensen, J.-G. Zhang, C. E. Tracy, and S. K. Deb.

For more information, see Publications.

Advanced Prototypes for Electrochromic Windows

NREL continues to advance the technology of electrochromic windows through basic research and development (R&D) of advanced prototypes. Through work such as this, NREL hopes to address fundamental and supporting R&D for next-generation industrially-fabricated devices or for next-generation electrochromic windows.

Photovoltaic-Powered Electrochromic Devices

PV-EC devices mount a thin-film photovoltaic device onto the ion storage layer of the electrochromic device. The photovoltaic layer generates a voltage which causes the electrochromic layer to darken. With an additional middle contact layer inserted between the photovoltaic and electrochromic devices, the voltage generated by the photovoltaic layer could also be used to charge an external battery. The battery could then be used to lighten the electrochromic layer. See the following references:

"First a-SiC:H photovoltaic-powered monolithic tandem electrochromic smart window device," W. Gao, S. H. Lee, J. Bullock, Y. Xu, D. K. Benson, S. Morrison and H. M. Branz.

"Monolithic self-powered photovoltaic-electrochromic coating for windows," S.-H. Lee, W. Gao, C. E. Tracy, H. M. Branz, and D. K. Benson.

"Low voltage electrochromic device for photovoltaic-powered smart windows," C. Bechinger, J. N. Bullock, J.-G. Zhang, C. E. Tracy, D.K. Benson, S. K. Deb, and H. M. Branz.

"Semi-transparent a-SiC:H solar cells for self-powered photovoltaic-electrochromic devices," J. N. Bullock, C. Bechinger, D. K. Benson, and H. M. Branz.

"Design goals and challenges for a photovoltaic-powered electrochromic window covering," D.K. Benson and H. M. Branz, Solar Energy Materials and Solar Cells, 39 (1995), 204-211.

Photoelectrochromic Devices

Photoelectrochromic (PEC) devices use a dye-sensitized electrode to generate electrons, thereby creating the voltage necessary to drive lithium ions into the electrochromic layer and color it.

The essential aspect of PEC devices is the use of a dye-impregnated layer of titanium dioxide. A low concentration of dye is used to maximize the transparency of the window. Between the titanium dioxide and the electrochromic layer is a layer of either lithium iodide solution or a solid polymer containing lithium iodide. This entire device is sandwiched between two layers of transparent conducting oxide material.

When sunlight strikes this device, the dye absorbs some of the sunlight and releases electrons, which are injected into the titanium dioxide. The electrons are then conducted to the adjacent conducting oxide layer, and pass through an external circuit to the conducting layer adjacent to the electrochromic layer, on the other side of the device. This electron flow, in turn, causes iodide ions to migrate through the solution or solid polymer toward the titanium dioxide, and causes lithium ions to migrate into the electrochromic layer. As in a standard electrochromic device, the injection of lithium ions into the electrochromic layer causes it to color.

When sunlight stops hitting the device, the charge stored in the electrochromic layer drives the process in reverse, ejecting lithium ions from the electrochromic layer and causing it to bleach. Thus, with no external controls, the window will color in sunlight and bleach in its absence.

The external circuit can also be used as a control device: The disconnection of the circuit will cause the window to remain in its current state regardless of the presence or absence of sunlight. In addition, an external voltage can be applied to the device through this circuit to drive the device to either the bleached or colored state.

For more information, see "Photoelectrochromic Cells and Their Applications," B. A. Gregg.

Other Advances

NREL's expertise in thin-film photovoltaics has been applied to the fabrication processes for electrochromics, through the development of a novel plasma-enhanced chemical vapor deposition method of depositing thin films of electrochromic materials. U.S. Patent No. 4,487,560 was awarded to NREL for this work, and the process has been licensed to Eclipse Incorporated.

NREL is also investigating the application of a novel inorganic lithium ionic conductor for use in electrochromic devices.

For more information, see Publications.

Education and New Markets for Electrochromic Windows

NREL works to make the public aware of advanced window technologies such as electrochromic windows. NREL provided an electrochromic prototype window for the Roofus' Solar Home exhibit, which was displayed before thousands of people beginning in March 1998 at Walt Disney's Epcot Center and later at the U.S. Department of Energy headquarters in Washington D.C. The exhibit now resides permanently at the NREL Visitor Center in Golden, Colorado.

In addition to this work, NREL explores new markets and applications for electrochromic windows. A prototype electrochromic window from OCLI was incorporated in the sunroof of an NREL-developed "cool car"-a concept car that demonstrated how NREL advanced technologies can be used to reduce the energy used to heat and cool the interiors of cars.

Standards and Ratings

NREL researchers work with several organizations to set the performance standards for electrochromic windows.

American Society for Testing and Materials (ASTM)

NREL actively participates in the American Society for Testing and Materials (ASTM), Committee E06 on Performance of Buildings, Subcommittee 22, entitled Durability Performance. A. W. Czanderna is a member of ASTM and the Chair of Task Group E06.22.07, Durability of Chromogenic Glazings; the purpose of this Task Group is to prepare, ballot, and have ASTM issue documents related to electrochromic windows and other chromogenic glazings. Czanderna is also an active member of E06.22.02, Durability Performance; and E06.22.05, Sealed Insulating Glass Units. Subcommittee meetings are typically held in April and October.

National Fenestration Rating Council (NFRC)

NREL is a member of the National Fenestration Rating Council (NFRC), a non-profit, public/private organization created by the window, door and skylight industry. It is comprised of manufacturers, suppliers, builders, architects, and designers, specifiers, code officials, utilities and government agencies. NFRC's primary goal is to provide accurate information to measure and compare the energy performance of window, door or skylight products. A. W. Czanderna is NREL's voting member and D. Balcomb is the designated voting alternate. Czanderna is primarily active on the Long-Term Energy Performance Subcommittee, but also attends meetings of the Technical Committee, Annual Energy Performance Subcommittee, Optical Properties Subcommittee, Condensation Resistance Subcommittee, and Solar Heat Gain Subcommittee. Meetings of these subcommittees are typically held in January, April, July, and September, but some only meet in April and September.

For more information, see Publications.

Insulating Glass

Insulating glass is comprised of two or more transparent glazing layers, separated by at least dead air spaces to reduce the conduction of heat through the unit. These units are standard for modern commercial glass applications and a requirement for most new construction. The full potential for electrochromic windows will be realized only with this type of construction for buildings. Electrochromic coatings will be assembled on side 2 (or the inside of the outside pane) of the insulating glass unit (IGU).

We have begun looking at issues that affect the energy efficiency and cost of IGU's and have held discussions with a number of interested manufacturers. There is general agreement that there is a role for government in defining realistic, accelerated tests for IGU seals and the relationship to real-time weathering as a function of geographic location and the related environmental factors. Dr. A. W. Czanderna issued a report on Dec. 4, 2000 detailing the need for work in this area. The executive summary and the list of Appendices are appended.

The first action item recommended by the report was to convene a panel of experts to define a list of tasks to address issues in IGU Seal Durability. This panel meeting was held in Cincinnati (June 26, 2001), with a follow-up meeting of part of the panel at La Malbai, Quebec (Aug. 12, 2001). Summary reports of these panel meetings, the list of attendees, and the recommendation derived from these meetings were drawn up by A. William Lingnell, PE, and appended here as well.

For more information, see Publications.