Real Time Optics of Amorphous Silicon Solar Cellfabrication on Textured Tin-Oxide-Coated Glass

A rotating-compensator multichannel ellipsometer has been used to measure the four spectra (1.4-4.0 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 (SnO2) coated glass. The Stokes vector elements include the irradiance in the reflected beam (or the reflectance) and the parameters {(Q, χ), p} of the reflected beam, where Q and χ 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 SnO2 substrate film. An analysis of Q and χ 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, χ) 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.