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High-Speed Imaging/Mapping Spectroscopic Ellipsometry for In-Line Analysis of Roll-to-Roll Thin Film Photovoltaics

Ambalanath Shan1, M. Fried2, G. Juhasz2, C. Major2, O. Polgar2, A. Nemeth2, P. Petrik2, Lila R. Dahal1, Jie Chen1, Zhiquan Huang1, N. J. Podraza1, R. W. Collins1 1Department of Physics and Astronomy and Center for Photovoltaics Innovation and Commercialization, University of Toledo, Toledo, Toledo, OH, United States /2Institute for Technical Physics & Materials Science (MFA), Research Centre for Natural Sciences, Budapest, Hungary

We introduce a novel expanded beam spectroscopic ellipsometric  technique capable of mapping large surface areas. Conventional spectroscopic ellipsometers (SE) use a relatively small source beam spot size, on the order of a  few millimeters, to collect the relevant polarization data. For large area measurements, the ellipsometric parameter collection involves a laborious and time consuming point by point re-positioning of the sample space. The expanded beam SE utilizes a narrow strip of polarized incident broadband light and a multichannel  detection system equipped with a fast two dimensional CCD detector for the analyzed light. The resulting data set produces the relevant spectral and spatial components simultaneously in a single pass, for the contiguous portion of  illuminated material.  A practical capability of the expanded beam SE is demonstrated using a sample that consists of a layer of RF magnetron sputtered ZnO deposited onto a silver (Ag) covered polyethylene nephthalate (PEN) substrate. This sample structure forms a standard back-reflector which is used for thin film amorphous silicon (Si:H) solar cells in the n-i-p substrate configuration, whereby the p-layer is the last semiconductor layer in the deposition sequence. The flexible nature of the substrate lends itself to continuous deposition using a roll to roll mechanism. By spreading the incident polarized light over the entire strip of material width-wise, the expanded beam SE technique was used to measure the thickness and optical parameters of the ZnO layer in the 1.24 - 3.54 eV photon energy range as the roll of thin film advanced. The thickness and the complex dielectric optical parameters of the sample (12cm by 40 cm) were extracted using the Cauchy dispersion model.