From precursor towards incorporated Al in ZnO TCO films

Abstract

As both Zn and Al are abundantly available, Al-doped ZnO (AZO) has become an attractive candidate to replace indium tin oxide (ITO; 90 % In2O3 and 10 % SnO2), the most commercialized transparent conductive oxide (TCO) material. Although the conductivity as well as the transparency of ITO is superior, indium is scarce and costly. An Al and Zn aqueous solution-gel precursor is developed here, with a high ecological character and a low health risk, certainly in comparison with organic solvents like 2-methoxyethanol, which is commonly used. As it is possible to obtain spin coated conductive and transparent films of these aqueous precursors, it is of interest to understand the chemistry behind this system. An in-depth study is performed of the complexation of citric acid with Zn by different Nuclear Magnetic Resonance (NMR) Spectroscopy techniques like 1H NMR, Nuclear Overhauser Effect and in combination with Fourier Transform Infrared Spectroscopy. The actual performances of these films are explained by association to the powders, which are made by thermal treatment of the same precursors. These powders are intensively studied with 27Al Magic Angle Spinning NMR Spectroscopy, giving an indication for the exact incorporation mechanism of Al inside the ZnO lattice. These new insights provide a basis for understanding the high conductivity of the obtained AZO films.