Factors Influencing the Conductivity of Aqueous Sol(ution)–Gel-Processed Al-Doped ZnO Films

Abstract

Solution processing of Al-doped ZnO (AZO) is interesting from an economical point of view for reducing synthesis and deposition costs in comparison to that for vacuum methods. Several (aqueous) chemical solution deposition routes have been explored for AZO, but the question that has never been answered is how state-of-the-art conductivity is achieved. Here, we fine tune an aqueous solution precursor for AZO, resulting in resistivities within the 10−3 Ohm cm range after a reductive treatment. Profound insights are gained through the study of the density of the film, the crystal phase, the optimum Al doping, and the effect of Al positioning in the ZnO lattice, as determined by 27Al magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy in combination with 1 H NMR, in order to understand the conductivity mechanism. As the conductivity of the AZO films drops as a function of time, the position of Al is studied with respect to the observance of charge carriers using Fourier transform infrared spectroscopy. The influences of all of these different factors on conductivity are summarized in a general overview.