An electrochromic material is a material that undergoes reversible changes in its optical absorption properties
when small ions are inserted electrochemically into its crystal structure.
When thin films made of electrochromic materials are applied in several
coatings to glass, for example, they can be used to "intelligently" modulate
the transmission of light through the glass, providing solar heat gain when
needed or screening it out otherwise. Such smart
windows are automatic energy savers, literally darkening when the sun
shines, but lightening on cloudy or overcast days. Combining solar cells
with an electrochromic film results in a self-powered window, which still
has all the intelligence of the smart window.