Doping Ratio Control of Fluorine-Doped Tin Oxide for a High-Quality Transparent Conducting Electrode

Transparent conducting oxides (TCOs) are very important materials for application in flat panel displays, thin-film solar cells, and touch panels. These days, the low-temperature synthesis of TCO thin films is being actively researched for realizing flexible electronics such as displays and solar cells. TCO thin films, mainly tin-doped indium oxide, are normally fabricated by high-vacuum sputtering. However, fluorine-doped tin oxide (FTO) has been in use for thin-film solar cells from the beginning. Our study attempts to lower the deposition temperature of FTO to 250 degrees C, which is suitable for deposition on polyimide substrates. We varied the F/Sn precursor ratio at different deposition temperatures, and found that a low F/Sn ratio of about 0.15 produces the lowest electrical resistivity below 350 degrees C, whereas a F/Sn ratio of 1.5 is obtained at 450 degrees C. At lower temperatures, crystal growth is very slow, and therefore, we should conduct very slow deposition to obtain high-quality TCO thin films.