nicoleas

• Søndenå, Rune; Aßmann, Nicole; Mosel, Frank & Hansen, Per-Anders (2026). Observation of an Injection Dependent Lifetime Effect in Highly Hydrogenated Boron Doped Wafers. SiliconPV Conference Proceedings. 3. doi: 10.52825/siliconpv.v3i.2669. Full text in Research Archive Show summary

  The correlation between hydrogen content and boron-oxygen related degradation is under debate. In this study surface passivated boron doped silicon wafers with different hydrogen content were illuminated at room temperature with three different light intensities. Their lifetime degradation was monitored with quasi-steady state photoconductance. A characteristic degradation that is separated into a fast and a slow decay characteristic for all light intensities is expected for such wafers. However, with increasing hydrogen content, we observe that the initial lifetime prior to illumination decreases, reducing the fast decay part of the degradation curves. For low intensity illumination the samples with reduced initial lifetimes follow the predicted degradation curve and show only the slow decay. For higher light intensities the degradation curves differ considerably; there is first a temporary lifetime increase before the decay sets in.

• Aßmann, Nicole; Hammann, Benjamin; Frøland, Mariann Carlsen; Kwapil, Wolfram & Monakhov, Eduard (2025). The differential impact of light soaking and dark annealing on hydrogen species during a LeTID cycle in boron-doped FZ silicon: A quantitative FT-IR study. Solar Energy Materials and Solar Cells. ISSN 0927-0248. 293. doi: 10.1016/j.solmat.2025.113853. Full text in Research Archive Show summary

  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 , 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, is formed, HB and H2 increase in concentration, while VH4 and V2H6 disappear. Light soaking at 75 °C and 1 sun-equivalent and dark annealing at 175 °C both reduce H2 and 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 and VH4 in the hydrogen balance in a LeTID cycle is demonstrated. Finally, several unidentified peaks are observed in the wavenumber range of 3000cm−1 to 3300cm−1, showing sensitivity to different treatment conditions. Specifically, peaks at 3029.1cm−1, 3102.1cm−1, and 3238.9cm−1 grow only when light soaking took place as final step. Conversely, a peak at 3025.8cm−1 increases under dark annealing conditions and decreases with prolonged light soaking

• Mehler, Melanie; Weinert, Nicolas; Aßmann, Nicole; Herguth, Axel; Hahn, Giso & Geml, Fabian (2025). Impact of high-temperature processing steps on the long-term stability of charge carrier lifetime in n -type FZ-silicon. EPJ Photovoltaics. 16. doi: 10.1051/epjpv/2025006. Full text in Research Archive Show summary

  In this work, the long-term behavior of excess charge carrier lifetime in n-type FZ-Si wafers without presence of highly doped layers is investigated during illumination at elevated temperatures. Thereby, the influence of high-temperature processing steps such as short high temperature thermal treatment, often referred to as tabula rasa (TR), and phosphorus diffusion gettering is considered. Degradation of effective lifetime is observed as well as a very strong regeneration effect that significantly exceeds the initial effective lifetime value. The phosphorus gettering step appears to have no significant effect on the degradation and regeneration behavior. Samples subjected to a TR treatment show only bulk-related regeneration. Investigations in darkness showed that the bulk-related regeneration is carrier-induced and not necessarily light-induced. FT-IR measurements at 5 K showed that the samples with a TR step have a higher Oi concentration in the Si bulk and that the hydrogen-passivated vacancies (VH4) are dissolved. The regeneration behavior of the samples with a TR step could be correlated with a reduced vacancy concentration. Such a thermal processing step appears to offer a strong mitigation strategy.

• Hammann, Benjamin; Rodrigues, Pedro Vieira; Aßmann, Nicole; Kwapil, Wolfram; Schindler, Florian & Schubert, Martin C. [Show all 7 contributors for this article] (2024). Deciphering the Role of Hydrogen in the Degradation of Silicon Solar Cells under Light and Elevated Temperature. Solar RRL. 8(20). doi: 10.1002/solr.202400457. Full text in Research Archive Show summary

  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 (H2), 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.

• Mehler, Melanie; Weinert, Nicolas; Aßmann, Nicole; Herguth, Axel; Hahn, Giso & Geml, Fabian (2024). Long-term carrier lifetime instabilities in n-type FZ- and Cz-silicon under illumination at elevated temperature. Solar Energy Materials and Solar Cells. ISSN 0927-0248. 278. doi: 10.1016/j.solmat.2024.113169. Full text in Research Archive Show summary

  Even though n-type Si wafers show a lower extent of light-induced degradation, similar degradation and regeneration behavior as in p-type Si materials has been observed for various n-type Si monocrystalline materials. In this work, the long-term behavior of minority charge carrier lifetime in non-diffused n-type FZ-Si and Cz-Si wafers during illumination at elevated temperatures is investigated. Thereby, various influencing factors such as peak firing temperature and treatment temperature are considered. Degradation and a very strong regeneration effect, which exceeds the initial effective lifetime value, can be observed in the samples. Since surface-related saturation current density remains almost constant during degradation and regeneration, the effect seems to be bulk related. The extent of degradation and regeneration could be shown to be firing temperature-dependent. It could also be shown for n-type FZ-Si wafers that higher firing temperatures result in an increased H2 content within the silicon bulk. Investigations of the injection-dependent defect density revealed that the defect formed during degradation is not a deep level Shockley-Read-Hall defect. Observations at different treatment temperatures and constant illumination of 0.9(1) sun photon flux equivalent have shown that a higher treatment temperature leads to faster degradation and regeneration kinetics. Similarly, higher injection rates also speed up regeneration kinetics. Since degradation also occurs in darkness, the bulk related degradation effect in n-type FZ-Si is charge carrier-induced and not necessarily light-induced.

• Aßmann, Nicole; Søndenå, Rune; Hammann, Benjamin; Kwapil, Wolfram & Monakhov, Eduard (2024). Reducing Time and Costs of FT-IR Studies of the Effect of SiNx, Dopants, and Emitter on Hydrogen Species in Si Wafers and Solar Cell Structures. SiliconPV Conference Proceedings. doi: 10.52825/siliconpv.v1i.840. Full text in Research Archive Show summary

  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.

• Rodrigues, Rodrigues Vieira; Hammann, B.; Aßmann, Nicole; Schön, J.; Kwapil, W. & Niewelt, T. [Show all 9 contributors for this article] (2024). Doping dependence of boron-hydrogen dynamics in crystalline silicon. Journal of Applied Physics. ISSN 0021-8979. 136(8). doi: 10.1063/5.0215089. Full text in Research Archive Show summary

  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 (Al 2O 3/SiN x: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, H 2, 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 H 0 binding to a sink. Additionally, we investigate the temperature dependence of the reaction rates and find that the H 2 dissociation process has an activation energy of (1.11 ± 0.05) eV, which is in close agreement with theoretical predictions.

• Søndenå, Rune; Weiser, Philip Michael; Mosel, Frank; Aßmann, Nicole; Hansen, Per-Anders Stensby & Monakhov, Eduard (2023). Controlling the hydrogen concentration in boron- and gallium-doped silicon wafers. AIP Conference Proceedings. ISSN 0094-243X. 2826(1), p. 110008-1–110008-6. doi: 10.1063/5.0141155. Full text in Research Archive Show summary

  The influence of the thickness of the hydrogen-rich silicon nitride layer on the amount of hydrogen introduced into the bulk of silicon wafers, with boron or gallium as the acceptor dopant species, is investigated using cryogenic Fourier Transform-infrared spectroscopy. Nitride layers with comparable refractive indices are deposited on Czochralski wafers and subjected to a simulated contact firing process. Thus, hydrogenation of wafers is performed with different thicknesses of the respective hydrogen sources. Fourier Transform-infrared spectroscopy at 5.0 K show that the hydrogen concentrations can be varied by altering the film thickness. The effect of a subsequent passivation process, i.e. deposition of a second hydrogen rich dielectric film at an elevated temperature, is also investigated. We observe that the passivation may alter the states of hydrogen in the bulk silicon or cause unintentional introduction of hydrogen. This passivation process also revealed considerable differences between boron and gallium doped wafers; The hydrogen-boron concentration grew more than the corresponding hydrogen-gallium concentration during the passivation process.

• Hammann, Benjamin; Assmann, Nicole; Weiser, Philip Michael; Kwapil, Wolfram; Niewelt, Tim & Schindler, Florian [Show all 9 contributors for this article] (2023). The Impact of Different Hydrogen Configurations on Light- and Elevated-Temperature- Induced Degradation. IEEE Journal of Photovoltaics. ISSN 2156-3381. 13(2), p. 224–235. doi: 10.1109/JPHOTOV.2023.3236185. Full text in Research Archive Show summary

  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.

• Hammann, Benjamin; Aßmann, Nicole; Schön, Jonas; Kwapil, Wolfram; Schindler, Florian & Roder, Sebastian [Show all 8 contributors for this article] (2023). Understanding the impact of the cooling ramp of the fast-firing process on light- and elevated-temperature-induced degradation. Solar Energy Materials and Solar Cells. ISSN 0927-0248. 259, p. 1–9. doi: 10.1016/j.solmat.2023.112462. Full text in Research Archive
• Hammann, Benjamin; Aßmann, Nicole; Weiser, Philip Michael; Kwapil, Wolfram; Niewelt, Tim & Schindler, Florian [Show all 9 contributors for this article] (2022). Hydrogen complexes present after different firing profiles and their influence on LeTID degradation. Conference record of the Photovoltaic Specialists Conference. ISSN 0160-8371. doi: 10.1109/PVSC48317.2022.9938782. Full text in Research Archive
• Assmann, Nicole; Persson, Clas; Kuznetsov, Andrej & Monakhov, Eduard (2021). Fine structure in electronic transitions attributed to nitrogen donor in silicon carbide. Applied Physics Letters. ISSN 0003-6951. 119(26). doi: 10.1063/5.0074046. Full text in Research Archive Show summary

  Nitrogen in group-IV semiconductors has become a well-established element of qubits capable of room-temperature operation. In silicon carbide, nitrogen can occupy different nonequivalent lattice sites, giving rise to different shallow donor states. We report a triplet fine structure in electronic transitions of nitrogen donors on the quasi-cubic carbon site in 4H silicon carbide with activation enthalpies of around 100 meV. The intensities of triplet components have a prominent dependence on the voltage bias. The activation enthalpies of the transitions exhibit the Poole–Frenkel effect, while no bias dependence is observed for the magnitude of splitting. A tentative explanation of the fine structure involves local symmetry changes due to stacking faults.

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• Aßmann, Nicole; Sondenå, Rune; Hammann, Benjamin; Kwapil, Wolfram & Monakhov, Eduard (2023). Reducing Time and Costs of FT-IR Studies of the Ef-fect of SiNx, Dopants, and Emitter on Hydrogen Spe-cies in Si Wafers and Solar Cell Structures. Full text in Research Archive
• Aßmann, Nicole (2026). Hydrogen Centers and Light- and Elevated Temperature-Induced Degradation in Silicon. Universitetet i Oslo. Full text in Research Archive Show summary

  Hydrogen is both a key ingredient and a potential challenge in modern silicon solar cells. It is essential for achieving today’s high efficiencies. Yet, growing evidence suggests that hydrogen-related processes may contribute to a puzzling degradation phenomenon known as light- and elevated temperature-induced degradation (LeTID), which affects the lifetime of charge carriers and, ultimately, solar cell performance. This thesis uses Fourier-transform infrared (FTIR) spectroscopy, which enables probing of hydrogen centers in silicon. Combined with carrier lifetime measurements, the work tracks the evolution of hydrogen centers and links these changes to degradation. The results show that the amount of hydrogen introduced during processing strongly influences degradation, with lower hydrogen concentrations leading to weaker degradation. The study also reveals that different dopants favor different dominant hydrogen species and identifies several previously overlooked hydrogen complexes that contribute to the overall hydrogen balance. In addition, the measured evolution of hydrogen centers over time was quantitatively compared with a model. This provides further insight into the mechanisms behind degradation. These findings provide new insight into the role of hydrogen in silicon and support the development of silicon solar cells that maintain their performance more reliably over time.

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