Chemical vapour deposition (CVD) has emerged as the dominant technique to combine high quality with large scale production of graphene. The key challenge for CVD graphene remains the transfer of the film from the growth substrate to the target substrate while preserving the quality of the material. Avoiding the transfer process of single or multi-layered graphene (SLG-MLG) has recently garnered much more interest. Here we report an original method to obtain a 4-inch wafer fully covered by MLG without any transfer step from the growth substrate. We prove that the MLG is completely released on the oxidized silicon wafer. A hydrogen peroxide solution is used to etch the molybdenum layer, used as a catalyst for the MLG growth via CVD. X-ray photoelectron spectroscopy proves that the layer of Mo is etched away and no residues of Mo are trapped beneath MLG. Terahertz transmission near-field imaging as well as Raman spectroscopy and atomic force microscopy show the homogeneity of the MLG film on the entire wafer after the Mo layer etch. These results mark a significant step forward for numerous applications of SLG-MLG on wafer scale, ranging from micro/nano-fabrication to solar cells technology.

Electron beam (E-beam) deposited molybdenum oxide (MoO_x) has been investigated for its potential to replace p-type hydrogenated amorphous silicon (a-Si:H) in Si heterojunction (SHJ) solar cells. Excellent passivation was achieved for our best MoO_x/c-Si junction based device, reaching an average implied V_oc (iV_oc) of 734 mV on textured, commercially available 6-inch Cz wafers. This confirms the compatibility of MoO_x as a hole selective layer with industrial SHJ cell processing. A hole barrier was, however, observed for our MoO_x-based solar cells due to inefficient hole extraction. The formation of this hole barrier can be related to annealing of MoO_x and the presence of a native oxide grown on the intrinsic a-Si:H interface layer below. Pre-annealing, followed by an HF treatment on the a-Si:H(i) layer prior to MoO_x deposition, proved to be useful to mitigate the formed barrier, while making it more stable under standard SHJ annealing conditions.

We present large-area "moly-poly" cells, with a front side MoO_x/a-Si:H(i) passivating contact and a rear-side poly-Si/SiO_x stack, and we have demonstrated that MoO_x based c-Si solar cell technology can be scaled to industrial wafer size. Excellent surface passivation was achieved using MoO_x and poly-Si, leading to implied V_oc values above 700 mV, and a final cell V_oc of 687 mV. However, some care needs to be taken to avoid parasitic optical losses in the infra-red (IR) spectral range due to free-carrier absorption (FCA). These losses were investigated by comparing poly-Si layers of different thicknesses, deposited by low-pressure or plasma-enhanced chemical vapor deposition (LPCVD or PECVD), at the rear side of moly-poly cells. We found that ultra-thin PECVD layers are most suitable for solar cell applications due to a very good trade-off between surface passivation and reduced FCA. Based on this result, a 18.1% efficient 9.2 × 9.2 cm² moly-poly cell was made, which is the highest reported efficiency so far for moly-poly cells. Finally, we present a preliminary study of the parasitic IR losses in the MoO_x layer itself, when deposited on either a-Si:H or SiO_x passivation layers.