Method for making electrochromic films having improved etch resistance

A method for making an electrochromic display having electrochromic material with improved resistance to the degrading effects, such as etching, resulting from the contact with electrolyte associated with electrochromic displays, comprising the steps of:

• (a) providing a first substrate having transparent electrodes thereon;
• (b) depositing a thin amorphous, water-bearing film of electrochromic material upon each electrode and subjecting each film to a heat treatment at a selected high temperature at least equivalent to the crystallization temperature of the electrochromic material for a selected short time of sufficient duration so as to cause crystallization through at least the free surface portion of the film but of insufficient duration to cause substantial loss of water therefrom;
• (c) providing a second substrate having a counter electrode, the first and second substrates being positioned in spaced working relation;
• (d) filling the space between the substrates with electrolyte;
• and
• (e) sealing the first and second substrates together with adhesive means, whereby crystallization of at least said free surface portion of each electrochromic film in step (b) substantially increases resistance of the film to etching by said electrolyte, thereby increasing the useful life of said electrochromic display, while retention of water in said films preserves the electrochromic properties thereof for display purposes.