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In a previous paper, we reported that thin films of ZnO:Al [aluminum-zinc oxide (AZO)] deposited after achieving a very low base pressure [from 4.0×10–7 Torr (5.6×10–5 Pa) to 5.7×10–7 Torr (7.6×10–5 Pa)] result dark yellow in color and are resistive. These are undesirable characteristics for the application of AZO thin films as front electrodes in solar cells. However, given the increasingly tendency in the acquisition of equipment that allow us to reach excellent vacuum levels, it is necessary to find the deposition conditions that lead to an improving of transmittance without greatly impacting the electrical properties of materials deposited after achieving these levels of vacuum. In this way, the present work is focused on AZO thin films deposited after achieving a very low base pressure value: 4.2×10–7 Torr (5.6×10–5 Pa). For this, we studied the effect of the variation of the oxygen volume percent in the argon/oxygen mixture (by maintaining the deposition pressure constant) and the effect of deposition pressure with only argon gas on the main properties of AZO thin films. The depositions were done at room temperature on glass substrates by direct-current magnetron sputtering with a power of 120 W (corresponding to a power density of 2.63 W/cm2). As results, we found that the variation of deposition pressure with only argon gas is a good option for the control of optical and electrical properties, since the addition of oxygen, although improves transmittance, greatly impacts on the electrical properties. Furthermore, an interesting correlation was found between the optical and electrical properties and the chemical composition of the AZO films, the latter depending on the argon pressure (for this, a careful X-ray photoelectron spectroscopy analysis was performed). Also, the inverse relationship between crystallinity and deposition rate was confirmed, in which deposition rate inversely depends on argon pressure.

The acid−base chemistry at the interface of zinc oxide (ZnO) and methylammonium lead tri-iodide (perovskite) leads to a proton transfer reaction that results in perovskite degradation. In perovskite solar cells (PSCs), this reaction produces low efficiency and reduces the long-term stability. In this work, an aluminum (Al) layer of 1−2 nm thickness is thermally evaporated on top of ZnO or Al 3+ -doped ZnO (ZnO:Al) thin films and then annealed at 450 °C for 30 min. Thermal annealing converts the surface aluminum film into a transparent and approximately 2 nm thick aluminum oxide (AlO x ) layer. Also, a larger concentration of oxygen vacancies is obtained by the annealing of Al and attributed to the diffusion of Al into the ZnO surface, and the ZnO underlayer results in a more conductive material. As a result, the chemical stability of perovskite coatings on top of AlO x -coated ZnO films is significantly enhanced, and the flat-band level of ZnO shifts 0.09 eV upwards, which improves the energetic level alignment in PSCs. This allows us to obtain ZnO:Al/AlO x -based planar PSCs that show a maximum efficiency of 16.56% with the perovskite layer prepared in ambient conditions under a relative humidity of 40−50%. After continuous illumination of about 30 min in air, ZnObased PSCs without AlO x layer retain only 34.5% of their original efficiency, whereas those with AlO x retain about 92.5%. It is demonstrated that thermal evaporation−oxidation is an efficient method to modify the surface properties of inorganic semiconductor thin films and improves both the performance and stability of PSCs.