Electrical, Optical and Electrochemical Corrosion Resistance of AZO, GZO, AGZO Films Depending on the Hydrogen Content.

Zinc oxide (ZnO) is a commonly used material for the front contact layer of thin film solar cells based on chalcopyrite CuInGaSe₂ (CIGS), since it satisfies the optical and electrical properties with low cost and is abundantly available. For high-performance, the front contact electrode in the CIGS solar cell should have low resistivity and high transmittance. Hence, efforts to improve ZnO's electrical and optical properties have been widely carried out. The corrosion resistance of the front contact film, however, has not been studied well. So, this paper compared the electrochemical stability of ZnO based transparent conducting oxide (TCO) films such as aluminum zinc oxide (AZO), gallium zinc oxide (GZO) and aluminum gallium zinc oxide (AGZO) grown in H₂. All films predominately grew in the (002) c -axis direction and their crystallites improved with increasing H₂ ratio, reflecting the enhanced electrical properties. Hydrogen acts as a shallow donor in the n -type ZnO semiconductor and increases carrier densities by forming oxygen vacancies by combining with oxygen molecules to form OH ^- functionalities. The electrochemical corrosion resistance of prepared films showed a linear improvement with increasing hydrogen. This change was attributed to the grain morphology and size and resulting grain boundaries. Since corrosion occurs mainly at grain boundaries, corrosion resistance seems to be better with a lower density of grain boundaries, due to larger grains caused by hydrogen. Electric conductivity was better in the order of GZO > AGZO > AZO, while corrosion resistance was in the reverse order. By adding hydrogen, the electrical resistivity of AGZO improved to near GZO, and the corrosion resistance was also enhanced to near AZO.