AVS 61st International Symposium & Exhibition
    Spectroscopic Ellipsometry Focus Topic Thursday Sessions
       Session EL-ThP

Paper EL-ThP5
Spectroscopic Ellipsometry Studies of Amorphous Silicon Based Photovoltaic Devices

Thursday, November 13, 2014, 6:00 pm, Room Hall D

Session: Spectroscopic Ellipsometry Poster Session
Presenter: Maxwell Junda, University of Toledo
Authors: M.M. Junda, University of Toledo
L. Karki Gautam, University of Toledo
R.W. Collins, University of Toledo
N.J. Podraza, University of Toledo
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Strategies for improving thin film photovoltaics (PV) are largely dependent on the ability to effectively characterize the opto-electronic and structural properties of each layer and correlate these properties with electrical performance. The common approach of growing and characterizing each layer individually, outside of a complete device, is hindered by the fact that individually grown layers are often not representative of the same layer in a complete device due to substrate dependent growth processes. We have applied spectroscopic ellipsometry (SE) to extract layer thicknesses, interface composition, and optical response in the form of complex dielectric spectra (ε = ε1 + iε2) for all hydrogenated amorphous silicon (a-Si:H) layers grown via plasma enhanced chemical vapor deposition onto rough transparent conducting oxide (TCO) coated glass. These samples are processed into single junction p-i-n PV devices and electrically characterized. Real time SE (RTSE) is used in situ, during deposition to monitor the growth evolution of each a-Si:H layer. To accurately model the initial TCO/p-layer interface, a parameterized description of the TCO structure, morphology, and ε was developed. From RTSE collected in the early stages of a-Si:H growth, changes in structural and optical properties of the TCO due to plasma exposure are tracked. Characterizing these changes to the substrate material has proved essential to accurately modeling the overlying a-Si:H layers. Leveraging previous studies that determined functional relationships for ε of a-Si:H in terms of only the band gap, physically realistic optical and structural properties for each layer are determined by allowing a minimal number of free parameters to fit models to RTSE data. This technique is effective in providing sensitivity to otherwise inaccessible material properties such as ε of the thin p-layer and subtle band gap gradients within the i-layer. Models generated from RTSE have been applied to ex situ SE collected over a spectral range of 0.04 – 5.88 eV. This combination of RTSE and infrared extended SE utilized on PV devices enables study of free carrier absorption in the TCO layers, silicon-hydrogen vibrational modes, higher energy electronic transitions in each material, and identification of spectral ranges with enhanced sensitivity to different layers and interfaces.