Analytical model for the optical functions of amorphous semiconductors and its applications for thin film solar cells

• DOI: 10.1016/j.tsf.2003.11.234
• Publication Date: 2004-02-14T05:28:31Z
• Authors: A.S. Ferlauto; G.M. Ferreira; J.M. Pearce; C.R. Wronski; R.W. Collins; X. Deng; G. Ganguly
• Keywords: Photoconductivity; Photon energy; Kramers–Kronig relations

Abstract We have developed a Kramers–Kronig consistent analytical expression to fit the dielectric functions (e1, e2) of hydrogenated amorphous silicon (a-Si:H)-based alloys measured using a combination of photoconductivity, transmission and reflection, and ellipsometric spectroscopies. The alloys of interest include amorphous silicon-germanium (a-Si1−xGex:H) and silicon-carbon (a-Si1−xCx:H), with optical bandgaps ranging from ∼1.30 to 1.95 eV. The fit can be performed simultaneously throughout the following regions: (i) the sub-bandgap (or Urbach tail) region where the absorption coefficient increases exponentially with photon energy, (ii) the band-to-band onset region where transitions are assumed to occur between parabolic bands with constant dipole matrix element, and (iii) the above-bandgap region where a Lorentz oscillator model is applicable. We describe an approach whereby, from a single accessible measure of the optical bandgap, (e1, e2) can be generated for a sample set consisting of optimum electronic quality a-Si:H-based alloys prepared by plasma-enhanced chemical vapor deposition.