Electrochromic properties of sputter-deposited rhodium oxide thin films of varying thickness

Amorphous rhodium (Rh) oxide thin films ranging in thickness from 50 to 400 nm were prepared by reactive sputtering at -20 degrees C in either an oxygen (O-2) or water (H2O) vapor atmosphere. Their electrochemical and electrochromic (EC) properties were investigated in an aqueous KOH electrolyte solution. The transferred charge density increased linearly with increasing film thickness, and a 400 nm-thick H2O-deposited film displayed the highest transferred charge density of 60 mC/cm(2). Electrochemically active Rh atoms comprised approximately 20% and 50% of the Rh atoms in the O-2- and H2O-deposited films, respectively. Transmittance by the bleached and colored films decreased with increasing film thickness in accordance with Lambert's law. The 400 nm-thick H2O-deposited film exhibited the largest change in transmittance (57%) at 700 nm. In contrast, the maximum change in transmittance by an O-2-deposited film (16%) was observed at a thickness of 200 nm. The molar absorption coefficients of the bleached H2O-deposited films were much smaller than those of the O-2-deposited films. However, the molar absorption coefficients of the colored H2O- and O-2-deposited films were very similar. The Rh oxide thin films prepared by sputtering in a H2O vapor atmosphere exhibited low molar absorption coefficients in the bleached state and exhibited excellent EC properties and stable transmittance through 25,000 cyclic voltammetry cycles.