Electron Microscopy and Electrochemistry of Nickel Oxide Films for Electrochromic Devices Produced by Different Techniques

In this study we report on our investigation of the microstructure of nickel oxide films produced by e-beam evaporation, sol-gel deposition, and sputtering techniques. We give characteristic cyclic voltammetry, current-voltage relationships, and optical transmission data for films made by each technique. Data is shown for electrodes, both uncycled and cycled, for 17-20 hrs. We found all samples have at least one phase corresponding to cubic nickel oxide (NaC1 structure). Other phases, such as nickel hydroxide, may exist but are not immediately identifiable. The structure of the films ranges from fine polycrystalline to amorphous and varies over the surface of the sample. Films that were cycled for 17-20 hours all tended to have improved transmittance, as high as as20% change. The highest transmission range from bleached to colored was for the evaporated films, which showed delta T=60%. The sol-gel films showed a large residual coloration in the bleached state after cycling (about a 40% decline was noted). Overall, after cycling the films appeared to be slightly more crystalline. In all films, after cycling there were increases in the coloration and bleaching current. Also, peak shifts were noted after cycling; the coloration peak tended to shift to higher and the bleaching peak shifted to lower potentials. The overall coloration efficiency (550 nm) for these films ranged from 26-36 cm²/C.