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Design and Fabrication of Thermochromic Energy-Harvesting Windows

National Renewable Energy Laboratory

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PDF Document PublicationPatent Application 15/279,062 (528 KB)

Technology Marketing Summary

Smart windows, such as electrochromic or thermochromic windows, can intelligently regulate their transparency, allowing for control of transmitted light or heat into building interiors.  In turn, control of transmitted light and heat allows for improved energy efficiency and occupant comfort.  While electrochromic windows are user-controlled and change from a resting transparent state to a light-absorbing state when an external voltage is applied, thermochromic windows avoid the added expense of electrical switching by changing from a transparent state to a tinted state when heated by sunlight. As such, thermochromic technologies are projected to be both more energy efficient and cost-effective.

Today’s conceptual thermochromic technologies utilize a ligand exchange process to effectuate the phase change of vanadium dioxide. However, these technologies must inconveniently avoid moisture during manufacturing, maintain inert conditions for continued functionality, and are unable to convert sunlight into energy. To improve upon these limitations, NREL scientists have developed a novel design and fabrication of a window insert that combines the functionality of a thermochromic window with a photovoltaic (PV) cell.


This novel thermochromic window insert is processed from aqueous solutions and is composed of non-toxic materials for scalable solution processing. When heated by incident sunlight, the device switches from its resting, transparent state to a tinted, light-absorbing state by a phase-change process wherein a volatile ligand bonded to perovskite film undergoes a reversible phase-change into a gaseous phase, allowing the perovskite film to adopt a tinted crystalline structure that is photovoltaically active. When in this tinted state, the device can convert incident light into electricity through leveraging the known properties of perovskite films. Not only does this allow for cooling of a building interior by absorption of incident sunlight and heat, power is also generated that may be funneled back into the building to power air conditioning or similar units.

This window insert does not require a fixed temperature for phase changes, allowing this design to be tunable for any environment, is easily incorporated into existing window fabrications, and has demonstrated switchable absorption in the near infrared and visible spectrum. In addition, the insert does not require an inert environment, which can ultimately reduce the cost of production.

Alternatively, this device can be fabricated with CsPbI2Br, where the CsPbI2Br film is deposited onto a coated glass substrate and is then coated with a hole-transport layer and gold contacts. The resulting CsPbI2Br device is transparent at room temperature and switches to a tinted state when heated by incident sunlight. This device is also simpler than the aforementioned insert as the device bypasses the use of a volatile ligand to switch between its transparent state and its tinted state. Furthermore, the resulting device is photovoltaically active in both its tinted and transparent states, with power conversion efficiencies ranging from 0.1% in its transparent state to 7% in its tinted state.

This technology is within the Device Architecture category of NREL’s perovskite portfolio. For further information regarding NREL's broader perovskite portfolio, please visit NREL's Perovskite Patent Portfolio website.

The Device Architecture category comprises new perovskite solar cell device layouts, such as interdigitated back-contact perovskite solar cell devices, that capitalize on the unique properties of the perovskite layer to create low-cost devices with improved efficiency and reliability.

  • First example of a “switchable” energy-harvesting window
  • Reduces production costs by allowing manufacturing in atmospheric conditions
  • Can be inserted into existing window fabrications
  • Switching temperature is tunable for any climate
  • Switchable absorption in the near infrared and visible spectrum
Applications and Industries
  • Windows
  • Automobile Industry
  • Commercial and Residential Buildings
Patents and Patent Applications
ID Number
Title and Abstract
Primary Lab
Application 20170089128
An aspect of the present disclosure is a device that includes a switchable material and an intercalating species, such that when a first condition is met, at least a portion of the intercalating species is associated with the switchable material and the switchable material is substantially transparent and substantially colorless, and when a second condition is met, at least a fraction of the portion of the intercalating species is transferred from the switchable material and the switchable material is substantially transparent and substantially colored.
National Renewable Energy Laboratory 09/28/2016
Application 20180252028
The present disclosure relates to devices that include a perovskite, where, when a first condition is met, at least a portion of the perovskite is in a first phase that substantially transmits light, when a second condition is met, at least a portion of the perovskite is in a second phase that substantially absorbs light, and the perovskite is reversibly switchable between the first phase and the second phase by reversibly switching between the first condition and the second condition.
National Renewable Energy Laboratory 02/27/2018
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
ROI 15-108, ROI 17-09PrototypeAvailable10/05/201610/05/2016

Contact NREL About This Technology

To: Bill Hadley<>