ZnMgO Thin Films by Ultrasonic Spray Pyrolysis: Modulation of Optical and Electrical Properties by Post-annealing and Magnesium Composition

ZnMgO semiconductor is of major interest in sustainable thin-film solar cells in order to replace buffer layers based on nonabundant or toxic elements. In addition, it has the potential 1) to allow optimal band alignment with absorbers such as CIGS, CZTS, and Cu2O; 2) to have high transparency; 3) to have adjustable properties for use as a window or buffer layer; and 4) to be deposited using low-cost and energy-efficient techniques such as ultrasonic spray pyrolysis. This study focuses on modulating ZnMgO properties with the optimization of the ultrasonic spray pyrolysis and post-annealing parameters. Optical transmission, X-ray diffraction, and van der Pauw/Hall effect measurements are used to study the material properties. Thin films with a strong <002> preferred orientation are obtained with large crystallite size (42-80 nm), high transparency (>90%), a bandgap increase from 3.28 to 3.34 eV with magnesium composition and annealing, an optimal Urbach energy of 0.068 eV, and electrical properties modulated by magnesium composition and annealing with resistivity varying from 6.520 Omega cm down to 0.022 Omega cm and a carrier concentration from 4.8 x 10(17) up to 1.4 x 10(20) cm(-3).