## 3 Results and Discussion
### Transition Metal Oxide Characterization
Also, MoO\({}_{3}\) samples yielded a 3% higher \(R_{sh}\) when treated with UV-ozone, as expected from a higher Mo\({}^{+6}\) content. Due to the high sheet resistance values discussed above, a transparent conductive oxide is required for current collection, so that the main contribution of the inversion layer (IL) to the cell series resistance comes from the transversal current flow across the IL/oxide/ITO interfaces, which can be approximated from the contact resistivity \(\rho_{C}=Rc\,A\tau\) (where \(A\tau\) is the effective current flow area). However, the obtained contact resistivities were overestimated due to the large errors inherent to the TLM itself [21], being inconsistent with the series resistances and fill factors measured in the finished solar cells and needing further analysis. Figure 3: Transfer length method measurements of 15 nm thick MoO\({}_{3}\) films deposited on (**a**) glass and (**b**) n-Si, measured with 40 nm Au electrodes. Insets show the current-voltage measurements at different distance intervals \(d\). From the optical perspective, the thicknesses of both ITO and transition metal oxide layers need to be adjusted for an optimum anti-reflection quality. Figure 4(a) shows the maximum photocurrent density \(J_{ph}\) for the MoO\({}_{3}\)/n-Si solar cell simulated with the _Wafer Ray Tracer_ program (AM1.5g solar spectrum), showing an optimum window across the 0-100 nm thickness range. The simulation parameters (refractive index and extinction coefficient) where wavelength dependent and taken from the program material database [15], including MoO\({}_{3}\) (alpha-phase) from [23]. Since the sputtered ITO has a moderately high \(R_{sh}\)\(\sim\)130 \(\Omega\)/sq, a minimum thickness of 80 nm was fixed in order to limit the resistance losses, yielding a maximum photocurrent of 35.7 mA/cm\({}^{2}\) for a MoO\({}_{3}\) thickness of 15 nm. This oxide thickness also ensures a uniform coverage of the random pyramids. Furthermore, the large bandgap energy of transition metal oxides (\(E_{gap}\)\(\sim\)3 eV) reduces the absorption losses in comparison to standard a-Si:H (\(\sim\)1.7 eV). Figure 4(b) shows the distribution of the incident current density for both MoO\({}_{3}\) and a-Si:H as front window layers, showing the optical losses by reflection and by parasitic absorption (front and rear layers). The larger MoO\({}_{3}\)\(E_{gap}\) represents a potential gain in photocurrent of \(\sim\)1.8 mA/cm\({}^{2}\), although reflection losses are considerably high and could be lowered to \(<\)2.7 mA/cm\({}^{2}\) if a texturized wafer is used. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Oxide**} & \multicolumn{2}{c}{**Sheet Resistance (\(R_{sh}\)), \(\Omega\)/sq**} \\ \cline{2-3}  & **Glass substrate** & **Silicon substrate** \\ \hline V\({}_{2}\)O\({}_{x}\) & \(3.3\times 10^{9}\) & \(5.8\times 10^{3}\) \\ MoO\({}_{x}\) & \(5\times 10^{12}\) & \(1.0\times 10^{4}\) \\ WO\({}_{x}\) & \(1.1\times 10^{9}\) & \(1.2\times 10^{5}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Sheet resistance (\(R_{sh}\)) of the oxides calculated from the transfer length method (TLM). The ReO\({}_{3}\) device was not measured due to film instabilities. Figure 4: **(a)** Simulated photocurrent density (\(J_{ph}\)) of the planar solar cell as a function of MoO\({}_{3}\) and Indium-Tin Oxide (ITO) layer thickness, achieving a maximum photocurrent of 35.7 mA/cm\({}^{2}\) for 80 nm ITO and 15 nm MoO\({}_{3}\). **(b)** Simulated optical losses (in mA/cm\({}^{2}\)) for MoO\({}_{x}\) and a-Si:H (15 nm each).