Photovoltaics will play a pivotal role in achieving a low-carbon-emission society. Remarkable advancements in the efficiency of crystalline Si (c-Si) solar cells, combined with standardized processes along the whole value chain, have enabled cost-competitive solar electricity production. In order to further decrease the cost of photovoltaic, significant efforts must be dedicated to further enhancing the efficiency of solar cells. The implementation passivating and carrier-selective contacts in recent years has led to a remarkable increase in the conversion efficiency of c-Si solar cells. Solar cells, such as silicon heterojunction (SHJ) and poly-Si/SiOx-based technologies are prime examples of the efficacy of these contacts, as cell efficiencies above 26% have been demonstrated. These achievements can be attributed to the excellent surface passivation properties of the intrinsic amorphous hydrogenated silicon (a-Si:H(i )) and the ultra-thin SiOx interlayers, coupled with the high carrier-selectivity exhibited by the doped Si-based layers. However, a significant limitation associated with these passivating and carrier-selective contacts is related to their optical parasitic absorption losses within the layers. These losses stem from the free-carrier absorption in a-Si:H layers of SHJ solar cells and poly-Si layers, which possess relatively narrow bandgaps, making them prone to absorbing the ultraviolet (UV) portion of sunlight. Consequently, these parasitic absorption losses diminish the amount of light reaching the c-Si absorber, ultimately restricting the short-circuit current (J sc) output of the solar cell. To mitigate these losses, wide-bandgap metal oxide layers, such as MoOx and TiOx, have been proposed as promising alternatives to replace these highly doped Si-based contacts. These metal oxides possess distinct carrier-selective characteristics, primarily due to their differences in work function (WF) with respect to the c-Si, leading to induced band bending within the absorber. Despite significant progress in recent years, several challenges still exist, as metal oxide contacts often suffer from carrier-selectivity issues due to material instability and interface reactions with adjacent layers. This thesis explores several possible strategies to minimize the parasitic absorption losses in passivating and carrier-selective contacts. A considerable portion of the research is devoted to understanding and enhancing the contact properties of the MoOx layer. The focus on MoOx is driven by the challenges it faces with carrier-selectivity, stemming from its low thermal stability and its susceptibility to band alignment issues when combined with various passivating interlayers. An innovative MoOx contact is introduced, incorporating ultra-thin surface passivating interlayers based on Al2O3 films. These ultra-thin interlayers offer substantial advantages, including enhanced surface passivation, minimal parasitic absorption, and improved transport of majority carriers. Additionally, MoOx and TiOx contacts, deposited by pulsed laser deposition (PLD) are also explored for c-Si solar cells...@en

In this work, the applicability of pulsed laser deposition (PLD) of transparent conductive oxides (TCOs) on high-quality ultra-thin poly-Si based passivating contacts is explored. Parasitic absorption caused by poly-Si layers can be minimized by reducing the poly-Si layer thickness. However, TCO deposition on poly-Si contacts, commonly by sputtering, results in severe deposition-induced damage and further aggravates the surface passivation for thinner poly-Si layers (<20 nm). Although a thermal treatment at elevated temperature (∼350 °C) can be used to partially repair the surface passivation quality, the contact resistivity severely increases due to the formation of a parasitic oxide layer at the poly-Si/ITO interface. Alternatively, we show that PLD TCOs can be used to mitigate the damage on ultra-thin (∼10 nm) poly-Si layers. Further improvement in poly-Si contact passivation can be achieved by increasing the deposition pressure while low contact resistivities (∼45 mΩ cm²) and good thermal stability (up to 350 °C) are achieved with a PLD indium-doped tin oxide (ITO) layer on high-quality ultra-thin poly-Si(n⁺) contacts. This allows for the application of a highly transparent front side contact by combining the excellent opto-electrical properties of a PLD ITO film with a 10 nm thin poly-Si contact.

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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.

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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.

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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.

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