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Joohyun KOH*, R. J. KOVAL*, C. R. WRONSKI*, and R. W. COLLINS* * ** ***

Center for Thin Film Devices, The Pennsylvania State University, University Park, PA 16802. Department of Physics, Hanyang University, Ansan, KOREA. Thin Film Silicon Solar Cells Superlaboratory, Electrotechnical Laboratory, Tsulcuba, JAPAN.

ABSTRACT A rotating-compensator multichannel ellipsometer has been used to measure the four spectra (1.44.0 eV) that describe the Stokes vector of the light beam reflected from the surface of an amorphous silicon (a-Si:H) p-i-n solar cell during fabrication on textured tin-oxide (SnO 2 ) coated glass. The Stokes vector elements include the irradiance in the reflected beam (or the reflectance) and the parameters {(Q, z), p} of the reflected beam, where Q and x are the tilt and ellipticity angles of the polarization ellipse and p is the degree of polarization. The value of p deviates from unity in part due to the non-uniform nature of the textured SnO 2 substrate film. An analysis of Q and X that neglects the effects of the texture can provide the time evolution of the thicknesses, microscopic structure, and the optical properties of the component layers of the a-Si:H solar cell. Deviations of the measured reflectance spectra from those predicted on the basis of the (Q, X) analysis provide the thickness dependence of the scattering and the evolution of the macroscopic structure of the solar cell. The measurement and analysis approach is important because of its potential application for real time monitoring of solar cell production. The analysis results also provide realistic inputs for optical modeling of the effects of texture in light trapping for solar cell efficiency enhancement.

INTRODUCTION An understanding of the optics of textured a-Si:H thin film structures is important for two reasons. With such an understanding, optical probes can be applied for the characterization of solar cell structures in the configuration used in production. Furthermore, improved models can be developed to simulate the scattering processes that lead to light trapping and efficiency enhancement in a-Si:H solar cells. The surface roughness associated with the texture exists on a wide range of scales from microscopic to macroscopic. For microscopic roughness (i.e., roughness with correlation lengths L much smaller than the wavelength X.), an effective medium theory (EMT) can be used to model the optical response of the roughness on surfaces; however, the EMT fails as L approaches X [1]. For macroscopic roughness with L-X, numerous theories have been developed of varying complexity from scalar diffraction [2] to first-principles analyses [3]. It is clear that a synthesis of theories, covering the full range of scales, will be needed to fully understand the effect of texture. In this paper, we have addressed the problem of real time optical characterization of textured a-Si:H solar cell structures. We have applied a novel multichannet ellipsometer in the rotatingcompensator configuration [4] to measure spectra from 1.4 to 4.0