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The development of solar cells with a low ecological footprint requires the optimization of semiconductor thin films having different and complementary optoelectronic properties: transparent electrode (TCO: transparent conductive oxide), window, buffer and absorber layers. Oxide semiconductors have two crucial advantages for this purpose: they can be developed using low-cost techniques such as ultrasonic spray pyrolysis (USP), and their optoelectronic properties can be extensively modulated through the chemical composition to adapt them to each of the layers. ZnMgAlO thin films have been developed by USP by optimizing the most impacting growth parameters: concentrations and pH of the precursors solution, substrate temperature, flow rate, nozzle speed, dwell time and shaping air pressure. In addition, a post-deposition annealing process has been developed by optimizing the annealing temperature, duration and nitrogen flow. The thickness can be easily adjusted by the deposition parameters from 50 nm to 700 nm and films investigated in the present study have a thickness of 200 nm. X-ray diffraction (XRD) analysis evidenced single-phase wurtzite structure with a strong preferred <002> orientation. A grain size varying from 40 nm to 70 nm was achieved, thus being in the upper range of the state of the art. Optical transmission results are well correlated with XRD data and indicate the bandgap increases linearly with the magnesium composition, an Urbach energy of about 70 meV, and a transparency of more than 90 % in the visible region. A Mg-modulable electrical resistivity of 10−2 Ω · cm to 102 Ω · cm was obtained with a mobility of approximately 5 cm2V−1s−1. The incorporation of aluminum into ZnMgO was achieved for the first time by USP and making possible to adjust the resistivity for application as a TCO. These results show the full potential of ZnMgAlO for application in all-oxide solar cells.
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