## 1 Introduction  
Transition metal oxides are a multifaceted and diversified class of inorganic materials whose chemico-physical, structural and functional properties encompass a very wide range of different features [1, 2, 3, 4]. From the structural point of view, also depending on their stoichiometry, they display very different structures, such as, for instance, (the most common) NaCl, rutile, corundum, fluorite, spinel and cuprite structures. As far as their electric behavior is concerned, it ranges from superconductors (e.g., high-T\({}_{\text{c}}\) copper oxides), to good metallic conductors (e.g., V\({}_{2}\)O\({}_{3}\), FeO\({}_{3}\)) through to semiconductors (e.g., NiO, ZnO,TiO\({}_{2}\), VO\({}_{2}\)) and they can also display interesting magnetic (e.g., Fe\({}_{3}\)O\({}_{4}\)), and electrochromic (e.g., WO\({}_{3}\)) characters [2; 3]. From the chemical point of view, their properties span the full range, from acidic through amphoteric to basic. These chameleonic features allow these materials to be widely used in commercial products combined to their thermal and chemical stability, ease of processing, and excellent mechanical robustness [5; 6; 7; 8; 9]. Focusing on the semiconducting materials, there is flourishing research in the optimization of electronic properties of metal oxides in order to fulfill the stringent requirements of next generation clean energy conversion devices such as solar cells [10]. This is the case, for example, of MoO\({}_{3}\) and Co\({}_{3}\)O\({}_{4}\), that show different electrical properties [11; 12; 13; 14; 15] but present one issue: the low mobility of charge carriers. In fact, both oxides are used in photovoltaics for different reasons. First, MoO\({}_{3}\) is a n-type semiconductor; however, due to the high band gap (about 3 eV [16]) and a work function similar to that of Au (5.1 eV or higher depending on conditions in which it is measured [17; 18]), it is commonly used as hole transporting material (HTM) and/or as blocking layer for hampering photocharges recombination together with other more conductive HTM. This feature increases the overall cost of device production in terms of money and time consumption. In addition, the charge mobility that affects the actual conductivity of the material is strongly dependent on the exact stoichiometry: the electron conductivity in MoO\({}_{3}\) is almost zero (10\({}^{-7}\) S cm\({}^{-1}\)), whereas in the oxygen-poor MoO\({}_{3\times}\) species, the conductivity can be pushed up by several orders of magnitude, reaching 10\({}^{4}\) S cm\({}^{-1}\) in pure MoO\({}_{2}\)[19; 20]. These results are in agreement with the fact that the atmospheric conditions (mainly either vacuum or ambient) dramatically change the valence/conduction band (VB/CB) values estimation measured with conventional spectroscopic techniques, with a remarkable variation found in the literature that inevitably affects the design of device architectures [17; 18]. On the other hand, the spinel Co\({}_{3}\)O\({}_{4}\) shows a similar low hole mobility (order of 10\({}^{-5}\) S cm\({}^{-1}\)), slightly higher due to the presence of a double electronic direct band gap that allows the oxide to have the proper band gap value (about 1.5 eV) to be directly used as a light absorber and HTM in a pn junction-based next generation all-oxide solar cell [21; 22; 23; 24]. As previously mentioned, the charge conductivity is the bottleneck for the real implementation of these oxides in high-performance energy-conversion devices. There are two main strategies used for improving the conductivity of thin films based on these materials: (i) improvement in the quality of the film morphology (i.e., by grain boundaries engineering) and (ii) chemical doping [25] of the metal oxide (MO) constituting the film with heteroatoms to accomplish band engineering instead.