## 3 Results and Discussion
### Transmission Line Method (TLM) Measurements
A direct comparison to the series resistance of the solar cells obtained when comparing the IV curve with the SunSV\({}_{\rm oc}\) curve is also shown in Table 1. This series resistance is calculated following the method shown in [15]. The simple model used for estimating the R\({}_{\rm s}\) contributions of the wafer/TCO/silver grid system uses the data extracted from the TLM measurements and takes into account two main contributions: one due to lateral transport through the TCO to the silver grid and the other due to the contact between TCO and the silver grid (calculated using the \(\rho_{\rm c}\) extracted from the TLM measurements and dividing it through the contact area between the TCO and the silver grid). Table 1 shows the calculated contact resistance (R\({}_{\rm s}\)\({}^{\rm C}\)), TCO resistance (R\({}_{\rm s}\)\({}^{\rm TCO}\)), and the sum of those two quantities. The last column shows the R\({}_{\rm s}\) measured from the comparison of the SunSV\({}_{\rm oc}\) and the sun simulator IV curve (R\({}_{\rm s}\)\({}^{\rm SunSVoc-IV}\)). The estimated R\({}_{\rm s}\) using a simple model is of the same order of magnitude as that found when comparing the IV and the SunSV\({}_{\rm oc}\) measured curves. However, the changes in R\({}_{\rm s}\)\({}^{\rm SunSVoc-IV}\) are less pronounced than R\({}_{\rm s}\)\({}^{\rm C}\) + R\({}_{\rm s}\)\({}^{\rm TCO}\) expected from the model. It can be observed that the measured values and the calculated values are very similar for those samples deposited under r(O\({}_{2}\)) = 0.3% and r(O\({}_{2}\)) = 0.48%, and deviates more significantly for the samples deposited under r(O\({}_{2}\)) = 0% and r(O\({}_{2}\)) = 0.74%, which are the extremes of the oxygen variation. The layer deposited under r(O\({}_{2}\)) = 0% possess both the lowest R\({}_{\rm sh}\) and the highest absorption (and therefore, smaller j\({}_{\rm sc}\)), while for the layer deposited under r(O\({}_{2}\)) = 0.74%, the opposite is true. It is possible that the combination of these factors (low Rsh and low j\({}_{\rm sc}\) for one of the samples, high R\({}_{\rm sh}\) and high j\({}_{\rm sc}\) for the other) accounts for the deviation between the measured and the calculated values for these samples. This is also confirmed by the TLM measurements where it can be seen that the contact resistivity of the layer deposited under r(O\({}_{2}\)) = 0.74% to the silver grid is almost twice the value of those deposited under r(O\({}_{2}\)) = 0.3% and r(O\({}_{2}\)) = 0.48%. Furthermore, it is important to note that the R\({}_{\rm s}\) contribution from the lateral transport in the TCO behaves differently than R\({}_{\rm s}\) contribution from the TCO/Ag contact resistance: The R\({}_{\rm s}\) due to the transport stays almost constant for all the ZnO:Al samples and is similar to that of the ITO reference, while the R\({}_{\rm s}\) due to the contact is higher for all the ZnO:Al samples than that of the ITO reference. This result was expected already from the TLM measurement analysis. For a more accurate analysis of the R\({}_{\rm s}\) difference between the measured values and the calculated ones, it would be useful to repeat the TLM measurements under illumination since the conductivity of the system could be different when photocurrent generation is taking place. Under illumination, \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**TCO Variation**} & **R\({}_{\rm s}\)** & **R\({}_{\rm s}\)\({}^{\rm TCO}\)** & **R\({}_{\rm s}\)\({}^{\rm c}\) + R\({}_{\rm s}\)\({}^{\rm TCO}\)** & **R\({}_{\rm s}\)\({}^{\rm SunSVoc-IV}\)** \\ \cline{2-5}  & **[\(\Omega\)]** & **[\(\Omega\)]** & **[\(\Omega\)]** & **[\(\Omega\)]** \\ \hline
0\% & 0.5 & 0.3 & 0.8 & 1.1 \\
0.3\% & 0.8 & 0.4 & 1.2 & 1.1 \\
0.48\% & 0.8 & 0.4 & 1.2 & 1.1 \\
0.74\% & 1.3 & 0.5 & 1.8 & 1.2 \\ ITO & 0.3 & 0.4 & 0.7 & 1.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Calculated contact resistance (R\({}_{\rm s}\)\({}^{\rm c}\)), transparent conductive oxide (TCO) resistance (R\({}_{\rm s}\)\({}^{\rm TCO}\)) and its addition.