O2 plasma treatment of mesoporous and compact TiO2 photovoltaic films: Revealing and eliminating effects of Si incorporation

Radio frequency (13.56 MHz) O2 plasmas were used to modify the surface of mesoporous and compact TiO2 films. The effects of substrate location in the plasma, applied rf power, and plasma mode (pulsed or continuous wave) were explored. X-ray photoelectron spectroscopy, contact angle measurements, and scanning electron microscopy were used to characterize changes to the TiO2 films. For mesoporous materials, O2 plasma treatment was found to increase oxygen content in the films, but Si content increased with applied rf power as a result of sputtering and redeposition of Si species from the reactor walls. XPS depth profiling using ion sputtering as well as O2 plasma treatment of dyed materials revealed that Si was deposited throughout the mesoporous network, not as a surface SiO2 layer. Pulsing the plasma with pulse duty cycles < 40% resulted in the elimination of Si and a reduction of damage in the modified films.