## 3 Results and Discussion
### Solar Cell Results  
Figure 5 shows the IV parameters for the bifacial SHJ solar cells with ZnO:Al and ITO as front TCOs. The TCO on the rear side of all the cells was kept constant and the reference ITO (r(O\({}_{2}\)) = 2.8%) was used. With the exception of the ZnO:Al sample deposited under 0% oxygen partial pressure, the solar cells with ZnO:Al front TCO show a slightly higher j\({}_{\mathrm{sc}}\) compared to those of the ITO reference cells. The j\({}_{\mathrm{sc}}\) of these solar cells increases with increasing O\({}_{2}\) partial flow. This will be further explained in the next section where the characteristics of the single layers on glass are discussed. The fill factors of all ZnO:Al samples are lower than that of the ITO sample, and they show a decreasing trend with increasing O\({}_{2}\) partial flow (r(O\({}_{2}\)) = 0... 0.74%). This result agrees qualitatively with the increase of the R\({}_{\mathrm{sh}}\) value of ZnO:Al layers deposited under an increasing oxygen ratio in the process gas. The average FF of the ITO bifacial samples (78.4%) is higher than those of the ZnO:Al solar cells. This is the main reason for the better performance of the ITO solar cells, since their j\({}_{\mathrm{sc}}\) and V\({}_{\mathrm{oc}}\) are in the same range as those of the ZnO:Al solar cells, excluding the ZnO:Al solar cell deposited under 0% oxygen flow, which presents both a lower j\({}_{\mathrm{sc}}\) and V\({}_{\mathrm{oc}}\) than all other samples. The V\({}_{\mathrm{oc}}\) of all these cells is very similar and in the range of approx. 728 mV.  
The smaller FF of the ZnO:Al solar cells is explained in Section 3.3 in terms of a higher value of the series resistance of the solar cells, measured using the TLM structures.  
Figure 6 shows the external quantum efficiency (EQE) measurements of the solar cells with the highest power conversion efficiency of three groups: a bifacial ITO cell, and a bifacial and a monofacial  
Figure 5: (**a**) j\({}_{\mathrm{sc}}\), (**b**) V\({}_{\mathrm{oc}}\), (**c**) FF and (**d**) \(\eta\) for the bifacial ZnO:Al solar cells and the ITO reference. The box plots represent the statistics of the measurements made on the two wafers with 14 cells each. The horizontal axis of each graph shows the material used in that group as a front TCO and the partial oxygen flow under which this material was deposited.  
ZnO:Al solar cell both deposited under r(O\({}_{2}\)) = 0.3%, as well as 1-R\({}_{\rm tot}\), the fraction of non-reflected light from the UV-VIS measurements, normalized to 1.  
The monofacial (MF) ZnO:Al solar cell has a higher EQE for long wavelengths, mainly due to the silver back contact, which reflects a fraction of the light back into the solar cell. In the case of the bifacial solar cells, a fraction of light is transmitted, lowering the j\({}_{\rm sc}\).  
The EQE response of both bifacial cells could suggest that the ITO cell performs worse than the ZnO:Al cell in the short wavelength region. Nevertheless, when analyzed in detail, it can be seen that the reflection minimum is different for ITO and ZnO:Al, which could be caused by a thickness deviation of the ITO layer. The j\({}_{\rm sc}\) of the EQE measurements agree qualitatively with the IV measurements.