Electrochromism in copper oxide thin films

Abstract Transparent thin films of copper(I) oxide prepared on conductive SnO 2 /F glass substrates by anodic oxidation of sputtered copper films or by direct electrodeposition of Cu 2 O transformed reversibly to opaque metallic copper films when reduced in alkaline electrolyte. In addition, the same Cu 2 O films transform reversibly to black copper(II) oxide when cycled at more anodic potentials. Copper oxide-to-copper switching covered a large dynamic range, from 85 to 10% photopic transmittance, with a coloration efficiency of about 32 cm 2 /C. Gradual deterioration of the switching range occurred over 20–100 cycles. This is tentatively ascribed to coarsening of the film and contact degradation caused by the 65% volume change on conversion of Cu to Cu 2 O. Switching between the two copper oxides (which have similar volumes) was more stable and more efficient (CE=60 cm 2 /C), but covered a smaller transmittance range (60–44% T ). Due to their large electrochemical storage capacity and tolerance for alkaline electrolytes, these cathodically coloring films may be useful as counter electrodes for anodically coloring electrode films such as nickel oxide or metal hydrides.