## 3 Results and Discussion
### Transmission Line Method (TLM) Measurements  
The electrical properties of TCO layers implemented in solar cells were investigated using dedicated TLM structures co-processed together with the SHJ solar cells on the same wafer. An explanation of the TLM method can be found in [14]. Figure 4 shows R\({}_{\rm sh}\) and the contact resistance \(\rho_{\rm c}\) between the TCO layer and the silver grid. This data was extracted from TLM measurements for the front TCOs used in the SHJ structures with ZnO:Al and ITO, varying the O\({}_{2}\) concentration during the sputter deposition. The TLM measurements show an increase in R\({}_{\rm sh}\) with O\({}_{2}\) partial flow for the ZnO:Al front TCOs, which correlates with the increasing R\({}_{\rm sh}\) values measured on the layers on the glass with the four-point probe (Figure 2a). It can be observed from Figure 4 that R\({}_{\rm sh}\) and \(\rho_{\rm c}\) of the ZnO:Al TLM structures are higher than those of the ITO reference. While R\({}_{\rm sh}\) of the ZnO:Al is only 5-20% higher than R\({}_{\rm sh}\) of the ITO, the contact resistance \(\rho_{\rm c}\) is approximately three times higher. Furthermore, comparing the TCO layers on glass vs. those on the solar cells, the R\({}_{\rm sh}\) value of the TLM structures increases from 94 \(\Omega\)/sq to 142 \(\Omega\)/sq for ZnO:Al with an O\({}_{2}\) partial flow ratio variation from r(O\({}_{2}\)) = 0% to r(O\({}_{2}\)) = 0.74%, and for the ZnO:Al layers on glass it increases from 120 \(\Omega\)/sq to 226 \(\Omega\)/sq. This difference can be explained by the lateral current transport through the Si wafer. The contact resistance \(\rho_{\rm c}\) of all the ZnO:Al samples with the silver grid is higher than that of the ITO reference sample, increasing from 16 m\(\Omega\) cm\({}^{2}\) to 40 m\(\Omega\) cm\({}^{2}\) for a partial O\({}_{2}\) flow of r(O\({}_{2}\)) = 0% to r(O\({}_{2}\)) = 0.74%. For the ZnO:Al samples deposited under r(O\({}_{2}\)) = 0.3% to r(O\({}_{2}\)) = 0.48%, the value is similar (around 23 m\(\Omega\) cm\({}^{2}\)), while the sample deposited under r(O\({}_{2}\)) = 0.74% presents a value almost twice as large and approximately four times larger than the \(\rho_{\rm c}\) of the ITO reference. Taking into account the measured R\({}_{\rm sh}\) values of the ZnO:Al layers on the glass, and the wafer having a resistivity of \(\approx\) 5 \(\Omega\) cm at 125 \(\mu\)m thickness (R\({}_{\rm sh,cSi}\) = 400 \(\Omega\)/sq), the equivalent sheet resistance of the system can be calculated assuming a simple model of two resistors (the TCO and the cSi) connected in parallel: R\({}_{\rm sh,tot}\) = [(R\({}_{\rm sh,cSi}\))\({}^{-1}\)+(R\({}_{\rm sh,TCO}\))\({}^{-1}\)]\({}^{-1}\). According to this model, the overall  
Figure 4: \(\rho_{\rm c}\) and R\({}_{\rm sh}\) obtained from transmission line method (TLM) measurements on the front side of rear emitter SHJ structures with ITO 2.8% as a reference and ZnO:Al as the front TCO deposited under different oxygen partial flows. Each item on the horizontal axis represents a group of samples, with the material for the front TCO and the oxygen partial pressure under which it was deposited. resistance of the system then ranges from 92 \(\Omega\) for a sample deposited under r(O\({}_{2}\)) = 0% to 144 \(\Omega\) for a sample deposited under r(O\({}_{2}\)) = 0.74%. Note that this model neglects TCO/Si contact resistivity and changes in charge carrier density in the c-Si, which occur both upon contact formation and under illumination, i.e., excess charge carrier generation. Interestingly, the calculated values are in good accordance with the R\({}_{\rm sh}\) values determined from the TLM measurements. This indicates that the assumption of a negligible TCO/Si contact resistivity is probably justified. Using the data obtained from the TLM measurements, an estimation of the different contributions of the R\({}_{\rm s}\) of the solar cells was made (Table 1).