Solution Deposition of Amorphous IZO Films by Ultrasonic Spray

The successful development of alternative deposition methods for thin-film transparent conducting oxides (TCOs) has important implications for photovoltaic technologies, such as CIGS. This work examines the use of atmospheric-pressure solution processing as an alternative to conventional vacuum-based sputtering for the deposition of a recent archetype TCO material, amorphous indium zinc oxide (a-IZO), which has demonstrated qualitatively better resistance to degradation compared to conventionally used Al-doped ZnO. Solution processing is industrially attractive due to its ease and potential to lower device manufacturing costs. While sputtered IZO shows the highest conductivity in the indium-rich region (∼70 at% In) where the films are amorphous, current TCOs deposited by solution routes have only focused on crystalline, zinc-rich films (3-5 at% In). In nearly all these cases, acetate precursors are used. Here we report on amorphous, indium-rich IZO films prepared by ultrasonic spray deposition from solutions of a novel indium-zinc formate (IZF) precursor. Films were sprayed onto glass at 100-210°C from an IZF-HNO₃-methanol solution and annealed under Ar-4%H₂. Thin films (<200nm) were produced with good optical transmittance (>80%) and conductivities of ∼50 S/cm. Electronic carrier concentrations of the films were consistent with sputtered IZO (∼10²⁰/cm₃). However, the Hall mobility (∼1 cm2/Vs) is an order of magnitude lower than sputtered IZO. Electron microscopy suggests the low mobility was due to porosity and film layering. X-ray diffraction of the sprayed IZO films showed that the amorphous state was successfully obtained after annealing at 300°C but that phase separation of In₂O₃ occurred above 400°C.