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

Abstract Amorphous rhodium (Rh) oxide thin films ranging in thickness from 50 to 400 nm were prepared by reactive sputtering at −20 °C in either an oxygen (O2) 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/cm2. Electrochemically active Rh atoms comprised approximately 20% and 50% of the Rh atoms in the O2- 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 O2-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 O2-deposited films. However, the molar absorption coefficients of the colored H2O- and O2-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.