FTIR spectroscopy is a versatile and non-destructive optical characterization method for many materials, including a-Si:H and nc-Si:H, and structural material properties can be derived with relative ease. The ratio of the FTIR absorption in the hydrogen-silicon stretching modes at 2090 and 2000 cm-1was correlated early in the history of a- Si:H solar cells to light-induced degradation. However, the stretching modes were predominantly attributed to the number of hydrogen atoms bonded to a silicon atom and only recently a more adequate model based on the a-Si:H nanostructure has been established, which accounts for the influence that vacancies and voids have on the material properties. Hydrogenated amorphous silicon stands out from other semiconductors by great tunability in a wide deposition parameter space. This allows for the synthesis of different layers with unique properties, and the IR absorptance spectra have proven to be useful as a tool to select the right materials for the right application: • For the archetypical application as a PV absorber layer, a-Si:H material is optimized for high mass density, low defect density, and a low microstructure factor. The combination of a moderately narrow bandgap with minimized light-induced degradation yields high-efficiency devices [10, 51, 83-87]. • Narrow-bandgap a-Si:H can be used as bottom-cell absorber in multi-junction solar cells, yielding high currents . Alloying with Ge reduces the bandgap further. • Wide-bandgap a-Si:H can be used as top-cell absorber, yielding high voltages [14, 88-93]. Alloying with C or O widens the bandgap further. • Few nanometer thick a-Si:H layers are optimized for the surface passivation of crystalline silicon in heterojunction solar cells, in which case not only low microstructure material performs well, but also more porous a-Si:H can be suitable . • Stress-controlled a-Si:H is required to grow thick a-Si:H for detector applications . • For optical applications, a-Si:H can be used in waveguides [34, 35] and is also useful for programmable applications due to the tunability of the complex optical response [28, 32]. For the latter application, a-Si:H with a somewhat elevated microstructure factor seems to be preferred to realize a larger difference between two switchable values of the refractive index, owing to the more pronounced Staebler-Wronski effect in such a-Si:H material in comparison to the type of a-Si:H that is typically preferred as a PV absorber layer. • Porous a-Si:H can serve as solid matrix or reservoir to embed other materials such as lithium for battery applications . When a-Si:H is utilized in each of these applications, the particular nanostructure, hydrogen content, and the way hydrogen is configured in the material all impact the final material and device functionality.
|Title of host publication||The World Scientific Reference of Amorphous Marterials|
|Subtitle of host publication||Structure, Properties, Modeling and Main Applications : Volume 3 Structure, Properties, and Applications of Tetrahedrally Bonded Thin-Film Amorphous Semiconductors|
|Editors||Nikolas J. Podraza, Robert W. Collins|
|Number of pages||44|
|Publication status||Published - 2021|
|Name||Materials and Energy|
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