Optical properties of zirconium oxide thin films for semitransparent solar cell applications

We prepared zirconium oxide thin films using reactive sputter deposition, and investigated their optical properties as a dielectric layer constituting semitransparent silicon thin film solar cells. The transmittance and reflectance of zirconium oxide thin films, deposited at various oxygen flow rates, are distinctly divided into three different ranges depending on the oxygen flow rate. Highly transparent zirconium oxide films are grown at relatively high flow rates, but they become nearly opaque at very low flow rates. Microstructural observation and refractive index measurement support that the transition from metallic zirconium-rich to dielectric zirconium oxide film proceeds as the oxygen flow rate increases. X-ray diffraction analysis indicates that the highly transparent films have a monoclinic phase of stoichiometric ZrO₂. While the transmittance of the monoclinic ZrO₂ films gradually decreases with an increase of film thickness, the reflectance exhibits a strong dependence on the film’s antireflection characteristic. The simulation results are in close agreement with the measured transmittance and reflectance. The simulated back reflectance of semitransparent silicon thin film solar cells demonstrates that the color of the semitransparent cells can be adjusted with the addition of the ZrO₂ thin film.