## 1 Introduction  
Zinc oxide (ZnO) belongs to the II-VI semiconductor group and exhibits high exciton binding energy (-60 meV) and high transmittance in the visible and NIR (near-infrared) spectral ranges due to its wide optical bandgap of about 3.3-3.4 eV [1, 2]. ZnO is nontoxic and biocompatible with UV emission and piezoelectricity, and has relatively low conductivity due to its low electron concentration [1, 2]. ZnO is regarded as a suitable platform for doping with different dopants to achieve the appropriate properties for diverse applications such as transparent conductive electrodes [3], thin film photocatalysts [4, 5, 6], active media for sensing [7, 8, 9], antibacterial agents [4, 10], and photoanodes for efficient perovskite solar cells [11]. Although the research on ZnO goes back several decades, there has been renewed interest in recent years. One of the sources of this scientific interest is the theoretical prediction of room-temperature ferromagnetism in transition-metal-doped ZnO [12] and its experimental confirmation in Co-doped ZnO [13, 14, 15]. Thus, along with the applications in optoelectronic devices, ZnO has also been studied as a dilute magnetic semiconductor (DMS) through doping with transition metals (Co, Ni, Mn, etc.) [16, 17, 18]. DMSs are a class of materials in which a small portion of the host lattice cations is replaced with magnetic impurities. If optically transparent DMSs can be made, they would be very attractive for optospintronic applications [19]. In addition to ferromagnetism, cobalt as a dopant in ZnO is also beneficial for enhancing the photocatalytic performance of ZnO because it suppresses charge carrier recombination, which improves photocatalytic performance [4, 20]. A large number of studies have been reported on cobalt-doped ZnO, and various techniques have been implemented for deposition of Co-doped ZnO thin films, including atomic layer deposition [18], pulsed laser deposition [21,22], magnetron sputtering [16,23], chemical bath deposition [24,25], sol-gel combined with spin coating or dip-coating [4,26,27,28] and spray pyrolysis [5,29]. However, simple, versatile, and low-cost deposition techniques that can be performed in an ambient environment are still needed. These techniques could attract wide interest and application if they can produce high-quality thin films with comparable properties to those produced by other means. We have selected the electrospray (ES) method for film deposition because we have already demonstrated that it offers a simple, scalable, and inexpensive method of deposition of ZnO films and Al-doped ZnO thin films with reproducible properties [30,31]. Structure, morphology, and optical properties can be easily tuned through the variation of deposition parameters [30]. Moreover, there is no need for complicated equipment because ES is performed in air and uses nontoxic and inexpensive precursors and solvents [30]. Unlike the sol-gel method, in the electrospray method postdeposition annealing is not required because both the moderate substrate temperatures (200-300 \({}^{\circ}\)C) and small size of the droplets ensure precursor decomposition during film deposition [30]. It is also worth exploring the possibility of implementing the ES method for co-doping using a mixture of appropriate amounts and types of precursors. We have selected Al for co-doping of cobalt-doped ZnO films because it has been shown that co-doping of ZnO with Co and Al enables the synthesis of transparent and conductive materials with specific magnetic properties [32,33]. Furthermore, [PERSON] et al. [34] have recently theoretically studied the effect of Co/Al co-doping of ZnO and have shown that additional electrons introduced by Al change the magnetic ground state of Co-doped ZnO from an antiferromagnetic to a ferromagnetic state, and the estimated Curie temperature is expected to be higher than room temperature.