Infrared optical properties: Hydrogen bonding and stability

Jimmy Melskens, Nikolas J. Podraza, Michael E. Stuckelberger

Research output: Chapter in Book/Report/Conference proceedingChapterAcademicpeer-review


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 [14]. 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 [16]. • Stress-controlled a-Si:H is required to grow thick a-Si:H for detector applications [94]. • 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 [95]. 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.

Original languageEnglish
Title of host publicationThe World Scientific Reference of Amorphous Marterials
Subtitle of host publicationStructure, Properties, Modeling and Main Applications : Volume 3 Structure, Properties, and Applications of Tetrahedrally Bonded Thin-Film Amorphous Semiconductors
EditorsNikolas J. Podraza, Robert W. Collins
PublisherWorld Scientific
Number of pages44
ISBN (Electronic)978-981-121-594-0
ISBN (Print)978-981-121-555-1
Publication statusPublished - 2021

Publication series

NameMaterials and Energy
ISSN (Print)2335-6596
ISSN (Electronic)2335-660X

Bibliographical note

Publisher Copyright:
© 2021 World Scientific Publishing Co. Pte Ltd. All rights reserved.

Copyright 2021 Elsevier B.V., All rights reserved.


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