We investigate the degradation of n- and p-type tunnel oxide passivating contact (TOPCon) under illumination and elevated temperatures to elucidate the degradation mechanism and identify mitigation strategies. Because the understanding regarding surface-related degradation (SRD) differs between n- and p-type TOPCon, we address different aspects for each layer type. Regarding p-type TOPCon, the focus is on the impact of the fast-firing peak temperature, whose correlation has already been investigated for n-type TOPCon but not for p-type TOPCon. It is found that the degradation extent of p-type TOPCon is inversely correlated with the peak temperature of the fast-firing step. This is consistent with previous reports on n-type TOPCon, making a common degradation mechanism for n- and p-type TOPCon likely. For n-type TOPCon, samples processed by plasma-enhanced chemical vapor deposition and low-pressure chemical vapor deposition (LPCVD) exhibit markedly different degradation extents. We attribute the lower degradation extent observed for LPCVD layers to its higher phosphorus concentration. We propose a unified mechanism for the degradation of both, n- and p-type TOPCon, that explains the various experimental data: hydrogen diffuses from a hydrogen containing dielectric layer (e.g., silicon nitride, SiNx) through the highly doped poly-Si layer toward the Si/SiO2 interface, where interface defects are formed. Based on this pathway, the degradation can be mitigated either by reducing the hydrogen content within the dielectric layers or by hindering the diffusion of hydrogen to the Si/SiOx interface—for example, by increasing the poly-Si dopant concentration.

This study investigates the dynamics of hydrogen in p-type silicon semiconductors, with a particular focus on the influence of excess minority carriers on hydrogen reactions. Our findings reveal that the presence of these carriers significantly increases atomic hydrogen concentration due to enhanced acceptor-hydrogen dissociation. We propose a unified model that effectively describes the behavior of hydrogen species under both thermal equilibrium and non-equilibrium conditions in both B- and Ga-doped Si. This model provides a coherent explanation for phenomena associated with Light- and Elevated Temperature-Induced Degradation (LeTID) during dark annealing and light soaking processes. We believe that any light soaking process in the photovoltaic industry causes hydrogen-related effects, even if unintentional. This understanding is pivotal for optimizing light soaking processes, ultimately improving defect and surface passivation and enhancing the efficiency of silicon solar cells.

Hydrogen is recognized as the key species responsible for light-and elevated-temperature-induced degradation (LeTID) in silicon. The simplest hydrogen species detectable is acceptor-hydrogen, HB in boron-doped material. Many studies track HB trough resistivity measurements and infer the evolution of the hydrogen molecule, H2, often excluding other potentially relevant hydrogen-related species from consideration. This study presents a Fourier transform-infrared analysis of the evolution of the hydrogen species, HB, H2, the hydrogen dimer H*2, and the vacancy-hydrogen complexes VH4 and V2H6, during dark annealing and light soaking. The findings are compared to changes in lifetime-equivalent defect density observed over a LeTID cycle. Before solar cell processing the hydrogen species found are HB, H2, V2H6, and VH4. After processing, H*2 is formed, HB and H2 increase in concentration, while VH4 and V2H6 disappear. Light soaking at 75 degrees C and 1 sun-equivalent and dark annealing at 175 degrees C both reduce H2 and H*2 concentrations, while HB concentration increases with dark annealing but decreases during light soaking. Extended dark annealing results in a re-appearance of VH4. A considerable role of H*2 and VH4 in the hydrogen balance in a LeTID cycle is demonstrated. Finally, several unidentified peaks are observed in the wavenumber range of 3000 cm-1 to 3300 cm-1, showing sensitivity to different treatment conditions. Specifically, peaks at 3029.1 cm-1, 3102.1 cm-1, and 3238.9 cm-1 grow only when light soaking took place as final step. Conversely, a peak at 3025.8 cm-1 increases under dark annealing conditions and decreases with prolonged light soaking.

Light- and elevated-temperature-induced degradation (LeTID) has been the most discussed degradation phenomenon during the late p-type passivated emitter and rear cell (PERC) era. It is important to investigate whether similar issues affect the new generation of n-type tunnel oxide passivating contacts (TOPCon) solar cells. This study focuses on the temporary recovery (TR), the reverse reaction to the degradation reaction. TR has been key to understanding LeTID in p-type Si. Its existence is demonstrated and reported in n-type silicon solar cells for the first time. Further, the repeatability of the degradation-TR-cycle is shown and other effects unique to TR are demonstrated, such as a negative activation energy, and the TR rate constant scaling superlinearly with Delta n. With TR as a tool, LeTID defects are identified to be present already after the fast-firing process. Such defects, if initially present, can have a multitude of effects. First, they lower the initial efficiency. Second, this might lead to the development of processes that are indirectly based on TR, enhancing the efficiency but concurrently leading to stronger degradation. Furthermore, it is imperative to exercise caution and to include the potential for TR when devising stability tests. Finally, it is demonstrated that strategies to mitigate LeTID in p-type also work for n-type.

In this work, we present an overview of the current understanding of hydrogen in modern silicon solar cells. The ambivalent nature of hydrogen poses a significant challenge for solar cells: While hydrogen is highly beneficial due to the passivation of bulk and surface defects, it is also detrimental to long-term stability, being associated with two degradation phenomena. Specifically, we examine the relation between hydrogen and light- and elevated-temperature-induced degradation (LeTID) and surface-related degradation (SRD). Our findings indicate that LeTID is mitigated when total hydrogen concentrations are below 5 × 1014 cm−3. For the surface degradation of an aluminium oxide/silicon nitride (Al2O3/SiNx:H) passivation layer stack, our data indicate the existence of a similar upper tolerance limit. Thus, managing hydrogen content is key to reducing these degradation phenomena. Therefore, we discuss various strategies to control the hydrogen content. One important factor is the hydrogen source, typically hydrogen-rich silicon nitride. Furthermore, the hydrogen diffusion process is discussed that occurs mainly during the fast-firing step, including both in-diffusion at around the peak temperature and out-diffusion during subsequent cool-down. Additionally, we consider the effects of other interlayers, such as Al2O3 or highly-doped surface-near layers, on the diffusion process. Thus, depending on the cell process, the most suitable adjustments can be employed to achieve optimum hydrogen management.

A model for hydrogen in silicon is presented, which accounts for both in-diffusion and out-diffusion from a passivation layer (e.g., SiNx), as well as the known hydrogen reactions within the silicon matrix. The model is used to simulate hydrogen diffusion and reactions during contact firing in a solar cell process, with a particular focus on variations in the cooling process, the sample thickness, and boron doping levels. The model reproduces the measured differences in hydrogen concentration due to these variations and thus helps to understand hydrogen-induced surface degradation and the dependencies of light and elevated temperature-induced degradation (LeTID) on the cooling process due to the close relation of LeTID and hydrogen. The same model and parameters are utilized to simulate the subsequent annealing of the fired samples at temperatures ranging from 160 to 290 degrees C. By successfully modeling the development of boron-hydrogen pairs during dark annealing processes across varying temperatures and doping levels, it is demonstrated that diffusion toward the Si/SiNx interface explains the observed decrease in resistivity and reductions in boron-hydrogen concentrations over extended dark annealing durations. Our simulations show the necessity of considering the depth-dependent hydrogen distributions after the firing process for analyzing the dark annealing.

In this contribution, we investigate the formation and dissociation of boron–hydrogen (BH) pairs in crystalline silicon under thermal equilibrium conditions. Our samples span doping concentrations of nearly two orders of magnitude and are passivated with a layer stack consisting of thin aluminum oxide and hydrogen-rich silicon nitride (Al2O3/SiNx:H). This layer stack acts as a hydrogen source during a following rapid thermal annealing. We characterize the samples using low-temperature Fourier-transform infrared spectroscopy and four-point-probe resistivity measurements. Our findings show that the proportion of hydrogen atoms initially bound to boron (BH pairs) rises with increasing boron concentration. Upon isothermal dark annealing at (163 ± 2) °C, hydrogen present in molecular form, H2, dissociates at a rate directly proportional to the concentration of boron atoms, ∝ [B−], leading to the formation of BH pairs. With prolonged annealing, an unknown hydrogen complex is formed at a rate that is inversely proportional to the square of the boron concentration, ∝ 1/[B−]2, resulting in the disappearance of BH pairs. Based on experimental observations, we derive a kinetic model in which we describe the formation of the unknown complex through neutral hydrogen H0 binding to a sink. Additionally, we investigate the temperature dependence of the reaction rates and find that the H2 dissociation process has an activation energy of (1.11 ± 0.05) eV, which is in close agreement with theoretical predictions.

Accurately measuring the hydrogen content in silicon (Si) solar cells is essential due to its connection to surface degradation and light and elevated temperature induced degradation (LeTID). Fourier Transform-Infrared (FT-IR) spectroscopy provides a quantitative technique for determining the content of various hydrogen species in Si wafers that have undergone various process steps. In this study, we examine both the effect of a silicon nitride (SiNx:H) layer during FT-IR spectroscopic measurements on hydrogen species, as well as the impact of an emitter present during firing on the amount of hydrogen introduced into Si wafers. We find that the presence of SiNx:H during measurements has negligible effects on the measured hydrogen species, potentially simplifying the preparation steps for FT-IR. For the emitter investigation we analyze boron (B)- and gallium (Ga)-doped p-type wafers to detect H-B, H-Ga, Oi-H2, and H2. We observe that hydrogen species initially present in B- and Ga-doped Si wafers differ significantly. Only H-Ga is detected in Ga-doped wafers, while H-B, Oi-H2, and H2 signals are measured in B-doped wafers. Moreover, we cannot confirm an increased release of H through the emitter into the bulk during the firing process. Finally, we conduct measurements at different temperatures and confirm that cryogenic temperatures are more effective for detecting H-B and H2 with concentrations in the 1014 cm-3 range. Nevertheless, useful spectra can still be obtained at liquid nitrogen (N2) temperatures.

In recent years, significant attention has been paid to the research of light‐ and elevated‐temperature‐induced degradation (LeTID) in silicon solar cells due to the substantial power loss and instability it causes. It has been discovered that the presence of hydrogen is closely linked to the occurrence of LeTID. In this study, a thorough review and re‐assessment of previously published results is conducted and connected with newly obtained data. The findings indicate a complex interaction between different hydrogen complexes and the LeTID defect states. The precursor of LeTID is connected to molecular hydrogen (H 2 ), while the LeTID degradation and regeneration are related to the binding of atomic hydrogen to the precursor and defect, respectively. A detailed description of the various reactions that occur under illumination and in the dark is provided. Additionally, explanation is given on how pre‐annealing can significantly affect the kinetics of LeTID during subsequent light soaking. Furthermore, a comprehensive hydrogen model that incorporates these various reactions and demonstrates an agreement between simulation and experimental results is developed. Finally, the implications of the findings on strategies for mitigating LeTID are discussed.

The recombination parameter J 0 s provides an important metric to characterize surface recombination. For its calculation, numerous methods and models have to be applied. Since the models for the Auger and radiative recombination in crystalline silicon were recently revised, it is important to investigate the influence of these changes on J 0 s . The origin and possible ways of obtaining J 0 s from effective lifetime measurements as well as simulations are described in detail, including the potential to fit the full lifetime curve and a new approach that is based upon the reparameterization of the excess charge carrier density Δ n . Using the effective lifetime measurements, we find that J 0 s values determined with the older parameterization by Richter et al. will result in erroneous values up to 5 fA/cm 2 , depending on the chosen conditions. By simulating the recombination parameter J 0 s in near surface, highly doped structures, such as emitters, it is shown that these errors can even go up to 50%. If used in a simulation, we highlight the importance of having the parameterizations of surface recombination being determined with the corresponding parameterization of intrinsic recombination. Therefore, an update for the recombination at oxide-passivated and phosphorous doped surfaces is given that can be used with the new intrinsic recombination models. Finally, we give some best-practice examples on how recent improvements in effective lifetime measurements affect J 0 s values as well as possible pitfalls.

In current silicon solar cell technologies, hydrogen is incorporated into the solar cell during the fast-firing process. It passivates defects at the surface and in the bulk, but also leads to light- and elevated-temperature-induced degradation (LeTID). Although it is known that the hydrogen content and the LeTID extent can be reduced by employing a slower cooling ramp during the fast-firing process, the exact mechanism behind this phenomenon remains unclear. This study aims at closing this gap by investigating the impact of cooling ramps with different temperature plateaus on hydrogen (complexes) in B-doped FZ-Si wafers. The fired wafers are analyzed with FT-IR spectroscopy, four-point-probe resistivity measurements, and LeTID tests via effective lifetime measurements. Our findings provide evidence that hydrogen not only diffuses into the silicon bulk but can also effuse out of it during the cooling ramp. A one-dimensional hydrogen model is built in Sentaurus TCAD to simulate the in- and out-diffusion of hydrogen and to compare it with the experimental results. The experimentally determined hydrogen concentrations align with our simulation, with the diffusion being dominated by the fast-diffusing neutral hydrogen H0. Moreover, we find stronger out-diffusion at higher temperatures, resulting in a lower total hydrogen concentration for slower cooling ramps. The extent of LeTID and surface-related degradation (SRD) are found to scale with the final total hydrogen concentration in the bulk. Therefore, modifying the cooling ramp can be an effective tool to optimize the hydrogen content and minimize the impact of degradation phenomena on silicon solar cells.

Herein, effects related to reactions involving hydrogen during carrier injection at room temperature in boron‐doped Czochralski‐grown silicon wafers are investigated. It is shown that these conditions lead to boron–oxygen defect regeneration. Under these conditions, bulk material quality degradation induced by a dark annealing can be temporarily recovered in the same way as light and elevated temperature‐induced degradation. Dissociation of boron–hydrogen pairs by carrier injection at room temperature is observed in parallel. These observations are discussed within the framework of known hydrogen reactions. With this study, significant aspects of hydrogen‐related meta‐stabilities at temperatures that are most relevant for solar modules in moderate climates are covered.

In this article, the impact of different hydrogen configurations and their evolution on the extent and kinetics of light- and elevated-temperature-induced degradation (LeTID) is investigated in float-zone silicon via charge carrier lifetime measurements, low-temperature Fourier-transform infrared spectroscopy, and four-point-probe resistance measurements. Degradation conditions were light soaking at 77 °C and 1 sun-equivalent illumination intensity and dark anneal at 175 °C. The initial configuration of hydrogen is manipulated by varying the wafer thickness, the cooling ramp of the fast-firing process, and the dopant type (B- or P-doped). We find lower hydrogen concentrations in thinner samples and samples with a slower cooling ramp. This suggests that hydrogen diffuses out of the sample during the cool-down, which strongly affects the final concentration of hydrogen molecules H 2 , and to a smaller degree the concentration of boron-hydrogen (BH) pairs. A regeneration of potential LeTID defects and a presumed LeTID degradation during dark annealing is found in n-type Si. In p-type Si, the LeTID extent was found to scale with H 2 , suggesting a direct link between both. The temporal evolution of BH pairs, LeTID degradation/regeneration, and surface degradation depends on wafer thickness and the cooling ramp of the fast-firing process. Based upon these findings, we formulate a theory of the hydrogen-related mechanism behind LeTID: Hydrogen originating from H 2 moves between different temporary traps. First, hydrogen binds to LeTID precursors and acceptor atoms in the silicon bulk, later moving toward the surface. This leads first to the LeTID degradation and regeneration and then to the degradation of surface passivation.

Characterisation and optimization of next-generation silicon solar cell concepts rely on an accurate knowledge of intrinsic charge carrier recombination in crystalline silicon. Reports of measured lifetimes exceeding the previous accepted parameterisation of intrinsic recombination indicate an overestimation of this recombination in certain injection regimes and hence the need for revision. In this work, twelve high-quality silicon sample sets covering a wide doping range are fabricated using state-of-the-art processing routes in order to permit an accurate assessment of intrinsic recombination based on wafer thickness variation. Special care is taken to mitigate extrinsic recombination due to bulk contamination or at the wafer surfaces. The combination of the high-quality samples with refined sample characterisation and lifetime measurements enables a much higher level of accuracy to be achieved compared to previous studies. We observe that reabsorption of luminescence photons inside the sample must be accounted for to achieve a precise description of radiative recombination. With this effect taken into account, we extract the lifetime limitation due to Auger recombination. We find that the extracted Auger recombination rate can accurately be parameterized using a physically motivated equation based on Coulomb-enhanced Auger recombination for all doping and injection conditions relevant for silicon-based photovoltaics. The improved accuracy of data description obtained with the model suggests that our new parameterisation is more consistent with the actual recombination process than previous models. Due to notable changes in Auger recombination predicted for moderate injection, we further revise the fundamental limiting power conversion efficiency for a single-junction crystalline silicon solar cell to 29.4%, which is within 0.1%abs compared to other recent assessments.

We investigate possible correlations between the temporal evolution of Light and elevated Temperature Induced Degradation (LeTID) and of boron-hydrogen pairs. For this, samples were treated under dark annealing conditions at 160 degrees C and the formation and annihilation of the LeTID defect and the formation of BH-pairs were studied simultaneously. Simulations were used to accompany the experiment. A linear correlation between the formation of the LeTID defect formation with the BH-pair formation was found, while no correlation was found for the annihilation of the LeTID defect and the degradation of surface passivation with the reactions revolving around BH-pairs. This correlation can be explained by assuming the LeTID defect formation to be a hydrogen-related dissociation process: Hydrogen dissociates from the LeTID precursor, binds to boron and the remains of the LeTID precursor are a recombination active defect responsible for LeTID. This assumption leads to a good agreement between simulation and experimental results.

The influence of the cooling rate during the fast-firing process and of the sample thickness on the initial hydrogen (complex) distribution in p- and n-type silicon wafers is investigated using low-temperature Fourier Transform-Infrared (FT -IR) spectroscopy. The impact of the introduced hydrogen on the formation of defects during dark annealing and light soaking is then studied by resistivity and charge carrier lifetime measurements. We observe a lower overall hydrogen concentration for thinner wafers or slower cooling rates. This is especially pronounced for the concentration of the hydrogen molecule H2A. We observe a weak signature of light- and elevated-temperature-induced degradation (LeTID) during dark annealing accompanied by a significant increase in BH pair concentration. Interestingly, the extent of degradation does not correlate with the chosen process variations. Regeneration of the carrier lifetime occurs earlier in thinner wafers and in fast-fired samples. During light soaking, the LeTID extent clearly correlates with the initial hydrogen (H2A) content, while the BH pair formation appears to be suppressed. In addition to H2A and BH-pairs, the dark annealing experiments indicate that at least one more source of hydrogen is present in the initial wafers.

By investigating the formation of light‐ and elevated‐temperature‐induced degradation (LeTID) defects under dark annealing conditions alongside the formation of the boron–hydrogen complex, it is found that both formations are limited by presumably the same reaction. Starting with this observation, two possible mechanisms of LeTID defect formation, a hydrogen‐related association and dissociation process, are discussed. Including the current knowledge on hydrogen in silicon as well as on the reverse reaction to the defect formation, the dissociation mechanism seems to be more likely: the LeTID precursor is composed of hydrogen being bound to another complex. During dark annealing, the hydrogen dissociates and binds to its sink, which herein is boron. The remaining complex is recombination active and leads thus to the observed degradation.

Hydrogenation of poly-Si based passivating contacts (TOPCon) is an essential process to achieve a very high level of surface passivation, especially on textured surface. This contribution is dedicated to improve the understanding of the hydrogenation mechanism. We compare different hydrogen sources with regard to their ability to chemically passivate defects at the Si/SiOx interface and presumably in the doped poly-Si layer as well as their thermal stability in a low and high temperature range. To this end, hotplate annealing series were performed on textured n-type TOPCon structures and Al2O3/SiNx multi-layer stacks were exposed to fast-firing processes. Very distinct activation characteristics were detected. It was also observed that the Al2O3 capping layers enable a higher level of surface passivation and higher thermal stability compared to SiNx. When implemented in multi-layer stacks, Al2O3 acts as a hydrogen diffusion barrier und prevents effusion from the TOPCon structure that deteriorates the passivation quality irreversibly.

The interest in cast mono silicon is increasing due to its lower energy consumption and resulting smaller carbon footprint, lower oxygen content and resulting less oxygen-related defects as well as easy scalability to large wafer formats like 210 x 210 mm(2) full square. As a cast silicon alternative to high performance multicrystalline (hpm) silicon, which rapidly lost market share, we analyze the cell efficiency potential of cast mono silicon in a TOPCon cell structure. We show how the absence of grain boundaries and the exceptional tolerance of the material quality towards high temperature processing enable this significant increase of the cell efficiency potential compared to hpm silicon. The very effective suppression of crystal defects by the Seed Manipulation for ARtificially controlled defect Technique (SMART) results in a very low lateral variation of the high material quality. We present certified cell efficiencies of 23.3% on n-type material crystallized in our labs, which demonstrates the high efficiency potential even for our lab-type G2 crystallization. An additional crystallization experiment for 210 x 210 mm(2) wafers demonstrates that SMART mono is compatible to large wafer sizes. A significant difference of the crystallization costs for Cz and cast mono crystallization as a function of electricity costs is discussed.

Light and elevated temperature induced degradation (LeTID) kinetics in float-zone silicon are investigated by varying the initial sample state, composed of different base material, base doping, SiNx:H films, and subsequent firing, and/or annealing steps. The approach of deliberately changing the initial sample state is shown to allow for specific studies of influences of LeTID kinetics. Bulk- and surface-related degradations are examined separately and the influence on the kinetics of bulk- and surface-related degradation is illustrated by a four-state and three-state model, respectively. In case of bulk-related degradation, an increase in defect density because of the firing step is shown, whereas the annealing step has an inverse effect. Both temperature steps—individually and combined—influence the transition rates of bulk-related degradation and regeneration by presumably changing the distribution of a defect precursor. For surface-related degradation, the firing step reduces the transition rate from the initial to the degraded state. In addition, the influence of a comparably humid atmosphere and the absence of UV light are found to be negligible.