The understanding of hydrogen and chemical interfaces in passivating contact stacks is critical for advancing high-efficiency silicon solar cells. This work employs time-of-flight elastic recoil detection analysis (ToF-ERDA) to profile elemental depth distributions, particularly the unambiguous characterisation of hydrogen, in key surface passivation dielectric nanolayers-SiNx, AlOx, and Al-doped ZnO (AZO). By leveraging the high mass resolution and low fluence of ToF-ERDA, combined with Monte Carlo simulations (MCERD), our work resolves hydrogen distributions and distinguishes overlapping signals from Si and Al, enabling precise analysis of interface composition. SiNx layers exhibit hydrogen contents up to 21 at.%, with redistribution and performance degradation observed after high-temperature annealing. In contrast, AlOx shows lower hydrogen content (similar to 2 at.%) but significant lifetime improvements post-annealing, indicating dominant field-effect passivation. For SiNx/AlOx double-layer stacks, enhanced hydrogen diffusion into AlOx was observed, suggesting SiNx as a hydrogen source. AZO/AlOx stacks demonstrated excellent passivation (iV(OC) > 730 mV) post-anneal, with hydrogen predominantly located in the AZO, challenging conventional views of AlOx as the main hydrogen source. These findings underline the value of ToF-ERDA for evaluating passivation mechanisms and demonstrate the tools versatility and usefulness in characterising dielectric nanolayers.

For several classes of crystalline silicon (c-Si) photovoltaic (PV) cells, such as silicon heterojunction (SHJ) or tunnel oxide passivated contact (TOPCon), an obstacle to producing them at the terawatt scale is the use of silver in their contacts. One promising alternative is copper, which is often implemented in combination with a barrier material to prevent degradation via copper diffusion. But because there may be cases where the barrier is locally absent between copper and Si (caused by fabrication imperfections such as dust), it may be desirable to design silicon-based cell layers that are themselves resistant to copper-induced degradation. Here, we study how the crystallinity of n-type polysilicon layers (like those used at the rear of a TOPCon cell) affects their susceptibility to copper-induced degradation. We study simple, symmetric samples consisting of sputtered copper thin films on flat n-type polysilicon / silicon oxide (poly-Si(n) / SiOx) passivating contact stacks. After annealing the samples at 200°C for 30 minutes in nitrogen, the passivation is ruined for samples whose poly-Si crystalline fractions are high, while more amorphous poly-Si(n) show little to no change in passivation. The passivation is also ruined for a sample with highly crystalline poly-Si(n) that is >1.5x thicker than the more amorphous poly-Si(n). This study may aid in the development of poly-Si thin films that, combined with a diffusion barrier layer, provide the highest protection against copper-induced degradation in scalable, copper-metallized PV cells.

The n-type TOPCon technology is currently the leading approach in the industry. Generally, it involves two high-temperature steps that can result in long cycle times and expensive processes. In this context, we propose a lean manufacturing process based on the successive PECVD-deposition of the front and rear doped layers, followed by a co-annealing step in which front emitter formation and rear passivating contact activation are performed simultaneously. We first investigated the influence of the PECVD process parameters and the thermal budget of the co-annealing step on the active boron concentration profile, the passivation quality, and contact resistivity. Then, we investigated the effect of a drive-in step under O2 environment to reduce the surface concentration and increase the depth of the emitter. Finally, we investigated the compatibility of the rear passivating contact with the drive-in step. The introduction of the drive-in step made it possible to obtain active boron concentration profiles with the desired surface concentration and depth. However, even though we obtained promising results regarding the compatibility of the rear passivating contact with the drive-in step, we observed that further optimization is necessary to avoid blistering of the n-type poly-Si layer and improve the uniformity of the rear passivation.

Perovskite–TOPCon 2 tandem solar cells reaching 31.3% efficiency on front-textured devices, enabled by an optimized AlO x -based hydrogenation process that provides high passivation quality.

We investigate rapid thermal processing (RTP) as alternative to the prolonged thermal annealing process used to form tunnel-oxide passivating contacts for silicon solar cells. The thermal treatment is generally followed by hydrogenation to passivate defects at the Si/SiOx interface. Whereas industrial manufacturing generally uses Cz wafers, research is often carried out with FZ wafers. Both types of wafers are prone to the formation of thermal defects in the bulk. To disentangle effects of the interface and the bulk, we assess the lifetime at different steps of the process sequence for both wafer types. We find that the initial bulk lifetime of our p -type FZ material is maintained for RTP up to temperatures of about 450°C, followed by a severe decay and eventually a moderate extent of recovery at temperatures above 800°C. Compared to FZ material, the initial bulk lifetimes in our p -type Cz material are slightly lower, but they are maintained on that level up to about 600°C. Beyond that temperature, the lifetimes also decay, but to a lesser extent than in the FZ material, and there is no curing at higher temperatures. Hydrogenation can partially passivate the bulk defects in FZ material, but the initial state is not recovered. In Cz material, it appears that RTP creates two different types of defects; for those created up to 800°C, the initial state can be recovered by hydrogenation whereas those created at higher temperature cannot be passivated by hydrogenation. We also investigate the formation of n -type passivating contacts by RTP, and we fabricate solar cell precursors with a single RTP step and the same hydrogenation for both contact polarities. After sputtering a transparent conducting ITO layer on the full area and an Ag metallization, we achieve solar cells efficiencies up to 20.5%.

This work investigates the potential of nanocrystalline silicon carbide (nc-SiC) films as transparent passivating contacts for high-efficiency solar cells. A plasma-enhanced chemical vapor deposition process for high hydrogen radical density was developed to fabricate nc-SiC films. The influence of phosphorus (P) doping and thermal treatment on the structural, compositional, and electrical properties of these films was investigated. Increased doping reduced the contact resistance but also negatively affected the open circuit voltage (iV(oc)). We identified a set of parameters that provided a compromise between conductivity and passivation, resulting in a maximum iV(oc) of 708 mV on textured surfaces with a contact resistance of around 100 m Omega cm(2) . In addition, nc-SiC exhibited superior ultraviolet transparency compared to poly silicon (poly-Si) and crystalline silicon (c-Si), with an absorption coefficient of 3 x 10(5) cm(-1) at 350 nm, lower than the typical 1 x 10(6) cm(-1) for poly-Si and c-Si.

A highly efficient hole-selective passivating contact remains the crucial step required to increase the efficiency of polysilicon-based Si solar cells. The future development of solar modules depends on a device structure that can complement the electron-selective tunnel oxide passivating contact with an equivalent hole-selective contact. We investigate plasma enhanced chemical vapor deposited (PECVD) SiN x and atomic layer deposited AlO x as alternative nanolayers for the passivation layer in polysilicon tunnel contacts. We have fabricated p+ poly-Si contacts with resistivities below 100 m Omegacm2 using these alternative metal oxide and nitride nanolayers. Initial passivation tests yielded low levels of passivation; however, a detailed understanding of the nanolayers elucidated the strategies to improve passivation significantly, achieving an implied open-circuit voltage (iV OC) of 698 mV and dark saturation current density (J 0) of 34 fA/cm2 for a p+ poly-Si contact using a PECVD SiN x interlayer. These are among the best reported for nitride-based nanolayer tunneling contacts, with research into nitride-based tunneling contacts being still in its infancy.

Full-area passivating contacts based on SiOx/poly-Si stacks are key for the new generation of industrial silicon solar cells substituting the passivated emitter and rear cell (PERC) technology. Demonstrating a potential efficiency increase of 1 to 2% compared to PERC, the utilization of n-type wafers with an n-type contact at the back and a p-type diffused boron emitter has become the industry standard in 2024. In this work, variations of this technology are explored, considering p-type passivating contacts on p-type Si wafers formed via a rapid thermal processing (RTP) step. These contacts could be useful in conjunction with n-type contacts for realizing solar cells with passivating contacts on both sides. Here, a particular focus is set on investigating the influence of the applied thermal treatment on the interfacial silicon oxide (SiOx) layer. Thin SiOx layers formed via ultraviolet (UV)-O-3 exposure are compared with layers obtained through a plasma treatment with nitrous oxide (N2O). This process is performed in the same plasma enhanced chemical vapor deposition (PECVD) chamber used to grow the Si-based passivating layer, resulting in a streamlined process flow. For both oxide types, the influence of the RTP thermal budget on passivation quality and contact resistivity is investigated. Whereas the UV-O-3 oxide shows a pronounced degradation when using high thermal budget annealing (T > 860 degrees C), the N2O-plasma oxide exhibits instead an excellent passivation quality under these conditions. Simultaneously, the contact resistivity achieved with the N2O-plasma oxide layer is comparable to that yielded by UV-O-3-grown oxides. To unravel the mechanisms behind the improved performance obtained with the N2O-plasma oxide at high thermal budget, characterization by high-resolution (scanning) transmission electron microscopy (HR-(S)TEM), X-ray reflectometry (XRR) and X-ray photoelectron spectroscopy (XPS) is conducted on layer stacks featuring both N2O and UV-O-3 oxides after RTP. A breakup of the UV-O-3 oxide at high thermal budget is observed, whereas the N2O oxide is found to maintain its structural integrity along the interface. Furthermore, chemical analysis reveals that the N2O oxide is richer in oxygen and contains a higher amount of nitrogen compared to the UV-O-3 oxide. These distinguishing characteristics can be directly linked to the enhanced stability exhibited by the N2O oxide under higher annealing temperatures and extended dwell times.

Tandem solar cells are a key technology to exceed the theoretical efficiency limit of single-junction cells. One of the most promising combinations is the silicon-perovskite tandem cells, considering their potential for high efficiency, large-area fabrication, and low cost. Whereas most research focuses on improvements in each subcell, another key challenge relies on the tunnel junction that connects subcells and affects overall cell characteristics. Here, the first demonstration of tunnel junctions using a stack of p/n polysilicon deposited directly on the passivating tunnel oxide are shown to overcome the drawbacks of conventional metal oxide-based tunnel junctions including low tunneling efficiency and sputter damage. Furthermore, using Random Forest analysis, high implied open circuit voltages over 700 mV in the bottom cell with the polysilicon tunnel junction are achieved. Their contact resistivities are as low as 500 mΩ·cm, suggesting FF losses of less than 1 %abs for the operating conditions of a tandem cell.

This work presents a study of p-type passivating contacts based on SiCx formed via a rapid thermal processing (RTP) step, using conditions compatible with the firing used to sinter screen-printed metallization pastes in industry. The contributions of the two interfaces (wafer/contact and contact/metal) to the contact resistivity are first decorrelated, identifying tunnelling at the wafer interface as the main contribution. We then investigate the influence of the active dopant concentration on the contact resistivity and the SiCx sheet resistance and propose strategies to reduce both resistances by increasing the thermal budget applied during RTP. Lastly, we discuss potentials and limitations of implementing the investigated stacks as rear side contacts of p-type devices with localized metallization. We demonstrate that increasing the thermal budget during RTP can effectively mitigate resistive losses and enhance contact performance and we show that an oxide layer that can withstand high thermal budgets is the key factor for obtaining simultaneously high passivation quality and good electrical properties. We investigate three different oxide types grown by HNO3 immersion, UV-O3 exposure and N2O plasma oxidation. The latter is demonstrated to be a promising candidate for an application in devices fabricated with high RTP thermal budget.

Tandem solar cells are a key technology for exceeding the theoretical efficiency limit of single-junction cells. One of the most promising combinations is the silicon–perovskite tandem cells, considering their potential for high efficiency, fabrication on a large scale, and low cost. While most research focuses on improving each subcell, another key challenge lies in the tunnel junction that connects these subcells, significantly impacting the overall cell characteristics. Here, we demonstrate the first use of tunnel junctions using a stack of p+/n+ polysilicon passivating contacts deposited directly on the tunnel oxide to overcome the drawbacks of conventional metal oxide-based tunnel junctions, including low tunneling efficiency and sputter damage. Using Random Forest analysis, we achieved high implied open circuit voltages over 700 mV and low contact resistivities of 500 mΩ cm2, suggesting fill factor losses of less than 1% abs for the operating conditions of a tandem cell.

The recombination of photogenerated charge carriers at metal-semiconductor interfaces remains a major source of efficiency loss in photovoltaic cells. Here, we present SiN x and AlO x nanolayers as promising interface dielectrics to enable high efficiency hole selective passivating contacts. It is demonstrated that SiN x deposited via direct plasma enhanced chemical vapour deposition can be grown controllably at thicknesses of 2 nm. The valence band offsets between crystalline silicon and ultrathin AlO x and SiN x nanolayers are measured as 3.5 and 1.4 eV, respectively. This predicts a larger tunnelling current for holes, compared to SiO x used typically. Resistivity measurements show that SiN x and AlO x nanolayers have lower contact resistivities compared to SiO x , with values as low as 100 mΩ·cm 2 . Analysis of the current transport mechanisms confirmed that tunnelling dominates the conduction through SiN x , while a mixture of tunnelling and pinholes are present in the AlO x structure. Lifetime measurements gave initial indications of the passivation quality of the films, with just 10 cycles of AlO x achieving 260 μ s after annealing and 1.9 ms with extrinsic field effect passivation added. Finally, the intrinsic built-in charge in the dielectrics was determined using surface photovoltage measurements and simulations are used to estimate the influence of nanolayer built-in charge in both poly-Si and dopant-free passivating contacts to enable future high efficiency solar cells.

We present fireable n- and p-type passivating contacts for silicon solar cells, using a tunnelling oxide and a doped SiC x layer. Rapid annealing is used to crystallize the layer and to activate its dopants. Subsequent hydrogenation reveals a fast passivation of interfacial defects and a slow passivation of bulk defects created by the rapid annealing in our floating zone silicon. We apply the contacts in proof-of-concept solar cells with ITO/ Ag contacts, resulting in efficiencies above 20%. Towards an industrially more relevant process using a fireable metallization, we present preliminary results on contacting through the nitride layer with aluminium.

The integration of passivating contacts based on a highly doped polycrystalline silicon (poly-Si) layer on top of a thin silicon oxide (SiOx) layer has been identified as the next step to further increase the conversion efficiency of current mainstream crystalline silicon (c-Si) solar cells. However, the interrelation between the final properties of poly-Si/SiOx contacts and their fabrication process has not yet been fully unraveled, which is mostly due to the challenge of characterizing thin-film stacks with features in the nanometric range. Here, we apply in situ X-ray reflectometry and diffraction to investigate the multiscale (1 Å-100 nm) structural evolution of poly-Si contacts during annealing up to 900 °C. This allows us to quantify the densification and thinning of the poly-Si layer during annealing as well as to monitor the disruption of the thin SiOx layer at high temperature >800 °C. Moreover, results obtained on a broader range of thermal profiles, including firing with dwell times of a few seconds, emphasize the impact of high thermal budgets on poly-Si contacts' final properties and thus the importance of ensuring a good control of such high-temperature processes when fabricating c-Si solar cells integrating such passivating contacts. Overall, this study demonstrates the robustness of combining different X-ray elastic scattering techniques (here XRR and GIXRD), which present the unique advantage of being rapid, nondestructive, and applicable on a large sample area, to unravel the multiscale structural evolution of poly-Si contacts in situ during high-temperature processes.

In this article, the effect of the various processing steps during the fabrication of c-Si/SiO x /SiC x fired passivating contacts on the silicon bulk lifetime is studied, and the kinetics of defect deactivation by hydrogenation is investigated. It is found that the firing step at 800 °C induces shallow bulk defects in float-zone silicon wafers, which can subsequently be passivated with hydrogen provided by an a-SiN x :H/D reservoir layer upon annealing at 450 °C. Experimental results and numerical data treatment indicate a rapid passivation of the surface within less than 1 min, followed by a slower passivation of the shallow bulk defects. In situ lifetime measurements are consistent with a slow bulk lifetime improvement by showing similar lifetime evolutions for both p-type and n-type SiC x layers. The kinetics of the hydrogenation process seems to be limited by the available hydrogen supply at the c-Si/SiO x interface, rather than by its diffusion within the bulk of the wafer. Moreover, it is affected by the bulk doping as well as the SiC x layer thickness. Finally, it is shown that hydrogenation is also possible with an a-SiN x :H/D reservoir layer deposited on one side of the wafer only, although resulting in a lower passivation level ( ${{\boldsymbol{\tau }}_{\mathbf{eff}}}$ ∼700 μ s compared to ${{\boldsymbol{\tau }}_{\mathbf{eff}}}$ ∼1300 μ s for symmetrical samples), and slower kinetics ( ${{\boldsymbol{\tau }}_{\mathbf{reac}}}$ ∼5 min comparedto ${{\boldsymbol{\tau }}_{\mathbf{reac}}}$ ∼0.8 min).

We investigate bulk and interface recombination in crystalline silicon with p-type fired passivating contacts. These consist of a chemical oxide and an in-situ doped layer of SiCx with low carbon content. Following a rapid thermal annealing to activate the dopants in the SiCx layer, we observe the formation of bulk defect states close to one of the band edges at temperatures between 450 and 700 degrees C. At temperatures of 800 degrees C and above, these bulk defects are increasingly cured. At the same time, the surface passivation provided by the interfacial oxide is increasingly deteriorated at temperatures above 800 degrees C, eventually permitting the injection of a recombination current into the SiCx layer. Consequently, there is a trade-off for the effective minority carrier lifetime with an optimum between 800 and 830 degrees C. We develop a formalism that distinguishes recombination through shallow bulk defects from the recombination current at the surface in a plot of the effective lifetime tau(eff) over the ratio n/p.

The aim of this work is to demonstrate the maturity of the TOPCon technology by conducting a round-robin on symmetrically processed lifetime samples in the leading European PV institutes EPFL, ISC, CEA-INES, ISFH, IMEC and Fraunhofer ISE within the H2020 funded project called HighLite. For all layers, dark saturation current-densities ranging between 2 and 10 fA/cm2 can be reported. Simultaneously, no metal induced recombination for the two lower sintering temperatures have been observed pointing towards a true passivated contact. Furthermore, contact resistivities below 10 mΩcm2 have been achieved. It seems that the industrial passivating contact matured to a fully passivated and conducting contact enabling full efficiency potential. The fact that this can be realized using either PECVD or LPCVD from various manufacturer is expected to drive costs down and contribute to the increased adoption of the TOPCon technology.

Polysilicon recombination junctions whose n‐type bottom layers double as a passivating contact to the silicon surface are investigated. Such recombination junctions are a key element in the interconnection of tandem devices with a silicon bottom cell, and they could be used to simplify the processing sequence of single‐junction cells with interdigitated back contacts (IBCs). Polysilicon tunneling junctions require high processing temperatures to crystallize the layers; however, this also facilitates interdiffusion of dopants, whereas tunnelling relies on degenerate doping concentrations in the constituent layers and sharp interfaces between them. Using secondary‐ion mass spectrometry (SIMS) in dynamic mode, it is found that dopants diffuse readily across the interface, thus compromising the junction. The undesired diffusion is suppressed by modifying the interface with C, O, or a combination of these. Moreover, it is found that the modification does not interfere with diffusion of H, an essential element to passivate defects at the surface of the silicon wafer. Thus, implied open‐circuit voltages (iV oc) of up to 740 mV are demonstrated for contact resistivities less than 40 mΩ cm2.