The Optical Properties Characterization of Hydrogenated Silicon by Spectroscopic Ellipsometry for Solar Cell Applications

Abstract

Amorphous and microcrystalline silicon play a critical role as layers in solar cell design, specifically in the absorption process layer which is crucial for the efficiency of sunlight conversion. This study explores the creation of hydrogenated amorphous silicon films (a-Si:H) via very high-frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) and analyzes their optical properties and crystal structure. We determine the optical band gap of amorphous silicon using a Tauc plot and Spectroscopic Ellipsometry (SE) analysis of transmittance results. By comparing the data, we gain insights into the material's optical properties, aiding our understanding of the findings and enabling a swift evaluation of its optical band gap. Furthermore, we characterize the film's crystal structure using grazing X-ray diffraction. Our results reveal that a-Si:H on glass, produced by VHF-PECVD at a substrate temperature of 200°C with various hydrogen dilutions (RH=H2/SiH4) ranging from 1.0 to 5.0, remains in the amorphous phase. The SE modeling provides the optical band gap of the a-Si:H film, with the lowest and highest optical band gap values occurring at RH 1.0 (1.79 eV) and RH 3.0 (1.84 eV), respectively. We employed the Tauc-Lorentz model with five fitting parameters to extract optical properties and the band gap of amorphous silicon, including the real part of the dielectric function. This model incorporates three layers: the interface layer between the glass substrate and a-Si:H, the a-Si:H film, and surface roughness, yielding the most accurate model. This comprehensive analysis unveils the exact optical properties of the fabricated films, offering valuable insights for solar cell design and manufacturing. This research confirms that Spectroscopic Ellipsometry, a rapid and non-destructive method for measuring the optical band gap, can significantly benefit the solar cell fabrication industry.