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Design and Fabrication of Thermochromic Energy-Harvesting Windows

Stage: Development

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.

Today’s conceptual thermochromic technologies must avoid moisture during manufacturing, maintain inert conditions for continued functionality, and cannot convert sunlight into energy. To improve upon these limitations, NREL scientists have fabricated a window insert that combines the functionality of a thermochromic window with a 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, photovoltaically active, crystalline structure. When in this tinted state, the device converts incident light into electricity by leveraging known properties of perovskite films. Not only does this allow for cooling of a building interior by absorption of incident sunlight and heat, but energy is also produced to power a building’s air conditioning or similar units.

This window insert does not require a fixed temperature for phase changes and allows for tunability in any environment. It is easily incorporated into existing window fabrications and has demonstrated switchable absorption in the near infrared and visible spectrum.

Alternatively, this device can be fabricated with a CsPbI2Br film that is deposited onto a coated glass substrate and 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 it bypasses the use of a volatile ligand to switch between transparent and tinted states. Furthermore, the resulting device is photovoltaically active in both such states with power conversion efficiencies ranging from 0.1% in its transparent state to 7% in its tinted state.

For further information about this technology or advances in NREL's broader perovskite portfolio, please visit NREL's Perovskite Patent Portfolio Website or contact Bill Hadley at bill.hadley@nrel.gov.

ROIs 15-108 and 18-23.

Applications and Industries

  • Windows
  • Automobile Industry
  • Commercial and Residential Buildings


  • 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