So far, intrinsic hydrogenated amorphous silicon (a-Si:H(i)) has been commonly used below molybdenum oxide (MoO_x) to form a good contact. An a-Si:H(i)/MoO_x stack gives good surface passivation, but often results in poor carrier selectivity after exposure to slightly elevated temperatures >130 °C (Geissbühler et al., 2015) [1]. For this reason, we have investigated an alternative interface layer, a very thin Al₂O₃ tunneling layer (<2 nm), deposited by atomic layer deposition (ALD), that can provide surface passivation, higher transparency and thermal stability without affecting the hole transport across the contact. To demonstrate this new passivating contact a 6” moly-poly cell, with an Al₂O₃/MoO_x stack at the front side and n-type doped polysilicon at the rear side, was made using a high- throughput spatial ALD tool, and E-beam PVD, for the Al₂O₃ and MoO_x layers, respectively. This resulted in an efficiency of 18.2% with a V_oc of 651 mV, a FF of 75.6% and a J_sc of 36.9 mA/cm². A post-deposition anneal (PDA) of the thin Al₂O₃ interlayer has significant effect on the Al₂O₃ thickness, layer stoichiometry, contact selectivity, and sputtering-induced damage. Annealing at higher T_PDA (350–600 °C) results in ineffective hole carrier transport and makes the stack more sensitive to ITO damage. The best performing device was, therefore, made using an Al₂O₃ layer without a PDA treatment. Moreover, we have found that this solar cell structure is thermally stable up to at least 210 °C, and even slightly improves under annealing which makes this device industrially appealing.

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.

High quality passivating contacts can be realized by using the combination of a thin interfacial oxide (SiO_x) and doped polysilicon (polySi). Recombination losses are minimized by providing very good passivation between the thin hydrogenated oxide and the cSi, a high field effect by the highly doped polySi [1-2], combined with the low level penetration of dopants in the wafer [2-3]. To realize this low level in-diffusion of dopants, several interacting options are evaluated in this work: the quality of the thin oxide layer (growth method), combined with a diffusion blocking method (nitridation), doping concentration levels in the polySi and temperature of diffusion. It is shown that for Phosphorus (P)-doped polySi, in-diffusion can be reduced by adding an i-layer in between the oxide and the highly doped polySi, lowering the overall doping level in the system slightly. For Boron (B)-doped polySi, in-diffusion can be blocked by nitridation of the SiO2 layer.

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.

An Ar‑H₂ plasma treatment was applied on an ultrathin RCA oxide to create well-passivated silicon wafers with symmetric c‑Si/SiO_x:H/a‑Si:H passivation layer stacks. The effective lifetime of these samples increased from 10 μs to 4 ms after annealing at 200 °C through Ar‑H₂ plasma treatment of the oxide. The results indicate that the plasma treatment can modify the RCA oxide and this enables atomic hydrogen diffusion at low annealing temperature, leading to a well passivated c‑Si/SiO_x:H interface. This might provide new possibilities to use wet chemical oxides in c-Si solar cells, for example as tunnel contacts.