## 2 Materials and Methods  
For optical and electrical characterization, aluminum-doped zinc oxide (ZnO:Al) and tin-doped indium oxide (ITO) layers were deposited on low iron soda-line glass (Saint-Gobain planiclear, \(30\times 30\) cm) using a pulsed DC magnetron sputtering system from Leybold Optics (A600V7). For ZnO:Al deposition, a rotatable target (99:1 wt % ZnO:Al\({}_{2}\)O\({}_{3}\)) was used and for the ITO layers, a planar target (97:3 wt % In\({}_{2}\)O\({}_{3}\):SnO\({}_{2}\)). The rotatable target was 600 mm long and had a diameter of between 130 mm (used) and 166 mm (new). The planar target was 600 mm long and 125 mm wide. Both materials were deposited at a nominal substrate temperature of 200\({}^{\circ}\)C with a substrate pre-heating time of 10 minutes before the deposition. During sputtering, the admixture of oxygen to the argon sputtering atmosphere was used to tune the stoichiometry and conductivity of the layers. The O\({}_{2}\) flow ratio r(O\({}_{2}\))=(O\({}_{2}\))/(Ar+O\({}_{2}\)) was varied between 0% and 0.74% for the ZnO:Al and 0% and 5% for the ITO deposition. The exact values of the gas flows were 269 sccm argon for the 0% oxygen sample, 268.2 sccm argon and 0.8 sccm oxygen for the 0.3% oxygen sample, 267.7 sccm argon plus 1.3 sccm oxygen for the 0.48% oxygen sample, and 267 sccm argon plus 2 sccm oxygen for the 0.74% oxygen sample. The ZnO:Al thin films were deposited using a magnetron sputtering power of 2 kW, with a frequency of 40 kHz and a reverse time between pulses of 1.2 us. The substrates were transported in front of the ZnO:Al target with a speed of 0.45 m/min. Charge carrier concentrations and Hall mobility of the layers on glass were determined by Hall measurements in the van der Pauw geometry. Sheet resistance was measured using a four-point probe. Layer thicknesses were obtained fitting a Drude-Lorentz oscillator model to optical spectrometry measurements. All layers deposited on glass had a thickness of 98 \(\pm\) 6 nm which, when deposited on textured substrates, results in an effective thickness of 70-80 nm. The layer thickness was measured with a DETAK profilometer. Figure 1 shows the structure of the SHJ solar cells. An initially 145-um thick n-type Cz-Si wafer was used as the base. After saw-damage etching, texturing, and RCA cleaning, the amorphous intrinsic and doped silicon layers were deposited in an AKT1600 plasma-enhanced chemical vapor deposition (PECVD) cluster tool from Applied Materials. As the front surface field and contact material to the front TCO, a nanocrystalline silicon n-type layer was used [9]. For the back emitter, an amorphous silicon p-type layer was used. TCO sputtering was done through shadow masks to define 4-cm\({}^{2}\) size cells. For all cells, the front contacts were carried out by screen printing of silver contact grids, which covered 2.7% of the cell area. For bifacial cells, the back contacts also consisted of screen-printed silver grids, whereas full area sputtered silver layers were used instead for monofacial cells. Five groups of samples (with two wafers each) were prepared. Each wafer held 14 bifacial SHJ solar cells, three transmission line method (TLM) structures, and three grid-less solar cells for EQE measurements. Four groups featured a ZnO:Al front TCO sputtered with different O\({}_{2}\) concentrations (oxygen content variation), and were named according to the oxygen partial pressure during deposition: ZnO:Al 0%, ZnO:Al 0.3%, ZnO:Al 0.48%, and ZnO:Al 0.74%. One group, with ITO as front TCO, was used as a reference and was named ITO 2.8% according to the oxygen partial pressure under which it was deposited. All of these samples had an 80-nm-thick ITO layer in the back and were processed in bifacial configuration. To test the performance of ITO-free cells, one extra group of two wafers with 14 monofacial solar cells with ZnO:Al as the front and a ZnO:Al/Ag rear contact was prepared and named MF ZnO:Al 0.3%.