• Pierson, Jean-François
• Boulet, Pascal
• Horwat, David
• Hamady, Sidi Ould Saad
• Berjali, Wafae El
• Fressengeas, Nicolas

Abstract

Metal-oxide materials based on Earth-abundant elements such as Zn, Mg and Al are crucial for next-generation materials of optoelectronic devices. For instance, the co-incorporation of various elements in ZnO gives the ability to tune the optical and electrical properties for the targeted application by using a simple and cost-effective technique such as Ultrasonic Spray Pyrolysis (USP) using only water-based precursors solution and sodalime glass substrates. In particular, for transparent conductive oxide (TCO) films and buffer layers in solar cells, it is necessary to modulate the conductivity, for TCO, and the bandgap for optimal band alignment for buffer films.Therefore, the incorporation of Al in ZnO can increase the conductivity while the incorporation of Mg can increase the bandgap. However, the co-incorporation of Al and Mg in ZnO is particularly challenging. First, Mg can decrease the conductivity and, second, the poor miscibility of Al precursor in water and the high-density of defects degrade the electrical and optical properties of thin films deposited by USP.We have therefore developed a two-step process to address these issues. The first step consists in the optimization of the precursors solution parameters, mainly the concentrations and the pH, and the USP deposition parameters: the substrate temperature, the flow rate, the nozzle speed and the shaping air pressure. The second step of this process is based on a post-annealing treatment for which we optimized the temperature, the duration and the controlled atmosphere. The structural, optical and electrical properties of the deposited ZnMgAlO thin films, with Mg compositions up to 10% and Al compositions up to 5%, have been investigated using high-resolution X-Ray diffraction, UV-Vis. transmission spectroscopy and Hall Effect measurements, in addition to profilometry and microscopy for the surfaces analysis. The XRD results show that with using this process, the films exhibit a controlled single wurtzite phase when increasing Mg composition with a high preferential orientation along c-axis. The surface morphology of the films is uniform and homogeneous with a low roughness and an increase in the crystallite size for the optimized set of deposition and annealing parameters using the developed process, which well correlated to the XRD results. The transparency of the films increases up to almost 90%. The bandgap increases as expected when the Al and Mg compositions were increased. The electrical measurements show an increase in carrier concentration upon Al incorporation. However, this increase of free carrier concentration is balanced by the incorporation of Mg, which induces an increase in the band gap and passivation mechanism of the shallow donors. The developed process can be used to tune the optoelectronic properties of Zn(Mg,Al)O thin films for application as window and buffer layers in the next generation all-oxide solar cells based on Earth-abundant elements.

Topics

• Deposition
• density
• impedance spectroscopy
• surface
• phase
• x-ray diffraction
• thin film
• glass
• glass
• defect
• ultrasonic
• annealing
• microscopy
• profilometry
• spray pyrolysis