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| Book/Dissertation / PhD Thesis | FZJ-2026-01835 |
2026
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-891-9
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Please use a persistent id in citations: doi:10.34734/FZJ-2026-01835
Abstract: Nitrogen-doped hydrogenated nanocrystalline silicon carbide (nc-SiCX:H)(n) emerges as a superior alternative to hydrogenated amorphous silicon (a-Si:H) as the front contact material in silicon heterojunction (SHJ) solar cells. Its enhanced transparency, significantly reduces parasitic absorption in the front contact layers leading to an increased current generation in solar cells. Furthermore, nc-SiCX:H(n) exhibits exceptional electrical properties and passivation capabilities, especially when combined with wet-chemically grown silicon oxide (SiOX). Utilizing the double-layer nc-SiCX:H(n) transparent passivating contact (TPC) approach as a front contact in SHJ solar cells results in power conversion efficiencies of 23.99% and impressive current densities of 40.9 mA cm2 . However, despite the advanced material properties of nc-SiCX:H(n), the TPC approach leads to a reduced Fill Factor (FF) and open-circuit voltage (VOC) compared to standard SHJ cells, although with comparable implied open-circuit voltages (iVOCs). Preliminary findings suggest that the reduced FF may lies in limitations imposed by the applied passivating nc-SiCX:H(n) layer, necessitating further investigation. The contrast between the iVOC of the precursor and the VOC of the solar cell is attributed to the passivation damage induced during the transparent conductive oxide (TCO) sputtering process, an area yet to be fully understood and requiring a deeper analysis. Additionally, methods to restore this induced damage need further exploration. This thesis re-examines the opto-electrical properties of hot wire chemical vapor deposition (HWCVD)-grown nc-SiCX:H(n), focusing on a high-quality material. Detailed analyses reveal that conditions favoring low deposition rates improve both the electrical conductivity and the optical bandgap, attributed to larger crystallites in thin films. Hydrogen radical etching emerges as a critical mechav nism during layer growth. The investigation extends to the passivation capabilities influenced by the filament temperature and their distance to the substrate, and hydrogen content variations due to changes in the filament temperature, hydrogen dilution, and total gas flow rate. Lower filament temperatures enhance passivation and hydrogen content, with the latter peaking at lower hydrogen dilutions and intermediate total gas flow rates. Subsequently, this thesis investigates the effects of varying layer thicknesses of both conducting and passivating nc-SiCX:H(n) layers. Although thicker conducting layers marginally affect the FF beyond a certain threshold, a notable trade-off between iVOC and FF is observed with the passivating nc-SiCX:H(n) layer. To address this, a gradient layer with gradually shifting properties from passivating to conducting is introduced. This approach can significantly reduce the thickness of the passivating layer, enhancing the best solar cell efficiency to 24.20%. The second major challenge for TPC solar cells is the reduced VOC post- TCO deposition. A combination of experimental and simulation methods clarifies the damage mechanisms. The microstructure of the nc-SiCX:H(n) layer is established to remain unaffected during sputtering. Experiments rule out the sputter atmosphere and process heating as sources of degradation. By shielding parts of the sample with filters with varying transmissions, it is determined that plasma luminescence does not deteriorate the passivation quality of the TPC stack. Electron beam simulations and experiments show limited penetration and no degradation impact. Among the ions present during sputtering, oxygen exhibits the deepest penetration into the material, yet is confined to the first nanometers. Various secondary effects of the ion impact are debated. While vacancies are considered to have activation energies too high for low-temperature annealing for passivation restoration, electrons generated from ionization impacts are already excluded as damage sources. The sputter-induced damage is hypothesized to result from a multi-phonon scattering process, displacing hydrogen at the crystalline silicon interface. However, the effect of radiation from ion impacts within the nc-SiCX:H(n) layer remains unexplored. Remarkably, it is discovered that the previously deemed irreparable sputter damage can be fully restored through thermal annealing. Nevertheless, the temperatures required for nc-SiCX:H(n) restoration are too high, compromising the passivation quality of a-Si:H.
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