## 3 Results and Discussion  
### Transition Metal Oxide Characterization  
Transition metal oxides are wide bandgap semiconductors that have been employed as hole and electron transport layers in organic solar cells and light emitting diodes due to their wide range of work functions varying from 3 eV (ZrO2) to 7 eV (V2O5) [16]. Their doping character is determined by defect vacancies in their atomic structure, resulting in either p-type doping for metal vacancies or in n-type doping for oxygen vacancies, as is the case in the four n-type oxides under study (Figure 1(b)) [13,14]. Likewise, the conductivity and work function vary with oxygen content in an inverse manner, shifting from high work function insulators (MoO3) to semiconductors (MoO3-x) to low work function metallic-like conductors (MoO2) as the cation oxidation state decreases (Mo^+6 \(\rightarrow\) Mo^+5 \(\rightarrow\) Mo^+4) [16]. In order to determine the stoichiometry of the prepared samples, Figure 2 shows the elemental content of Mo^+6 and Mo^+5 cations calculated from the integrated areas of the fitted Mo 3d XPS peaks (inset) for different MoO3 thicknesses and two different post-deposition treatments (air-exposure and UV-ozone, 15 min). For the air-exposed films, it is observed that the Mo^+6 content increases with film thickness and stabilizes at a value of \(\sim\)94%, while the Mo^+5 content inversely decreases to \(\sim\)6%. This phenomenon has been thoroughly investigated for several metallic substrates and has been attributed to thermodynamically driven redox reactions at the MoO/substrate interface [17]. Similarly, the UV-ozone treated samples follow the same trend but with a larger Mo^+6 content due to ozone oxidation, stabilizing at a value of \(\sim\)96% (\(\sim\)4% for Mo^+5). By integrating the fitted O 2p peaks, average oxygen to metal (O/M) ratios were calculated at MoO2.5 (air-exposed) and MoO2.6 (UV-ozone). Oxygen deficiency has also been reported for thermally evaporated V2O5 [18], WO3 [7] and ReO3 [13], which allows for fine-tuning of the desired work function by means of different process conditions and post-deposition treatments [19,20]. Figure 2: Relative content of Mo^+6 and Mo^+5 oxidation states for different MoO3 film thicknesses (2.5, 5, 8 and 10 nm) and post-deposition treatments (air exposure and ultraviolet light ozone—UVO). Inset shows the fitted XPS spectra for both Mo 3d states identified (8 nm). Lines are a guide to the eye. Even if variations in composition allow for enhancements in conductivity, transition metal oxides are in general poor conductors when compared to doped silicon or transparent conductive oxides. To determine the electrical properties of the evaporated MoO\({}_{3}\) films, the Transfer Length Method (TLM) results were analyzed to extract the sheet resistance (\(R_{sh}\)), both in glass and n-Si substrates, according to the relation [21]:  
\[R_{T}=2R_{C}+(\frac{R_{Sh}}{W})d \tag{1}\]  
where \(R\tau\) is the total resistance measured between two electrodes of width \(W\) separated by a distance \(d\), while 2\(R_{C}\) is the transversal contact resistance between Au/MoO\({}_{3}\) (glass sample) and Au/MoO\({}_{3}\)/n-Si (n-Si sample). Figure 3 plots the dark \(R_{T}\) measurements at increasing \(d\) intervals for both samples, obtaining a sheet resistance of \(\sim\)5 \(\times\) 10\({}^{12}\)\(\Omega\)/sq for MoO\({}_{3}\)/glass and \(\sim\)1 \(\times\) 10\({}^{4}\)\(\Omega\)/sq for MoO\({}_{3}\)/n-Si. The large difference between both measurements is explained by the formation of an inversion layer on silicon's surface, acting as p-type contacts in the n-Si substrate [10,22]. Note also that \(R_{sh}\) for the n-Si substrate (with \(\sim\)2.5 \(\Omega\)cm resistivity) is \(\sim\)65 \(\Omega\)/sq and does not contribute significantly to the inversion layer resistance. Similar results were obtained for V\({}_{2}\)O\({}_{5}\) and WO\({}_{3}\) deposited on glass and n-Si substrates (Table 1), indicating the formation of an inversion layer.