Ray Tracing Analysis of Light Scattering Properties of Randomly Nano-Textured ZnO Films

We investigate the scattering behavior of nano-textured ZnO-Air and ZnO-Silicon interfaces for the application in thin film silicon solar cells. Contrary to the common approach, the numerical solution of the Maxwell's equations, we introduce a ray tracing approach based on geometric optics and the measured interface topography. The validity of this model is discussed by means of SNOM measurements and numerical solutions of the Maxwell's equations. We show, that the ray tracing model can qualitatively describe the formation of micro lenses, which are the dominant feature of the local scattering properties of the investigated interfaces. A quantitative analysis for the ZnO-Silicon interface at lambda=488 nm shows that the ray tracing model corresponds well to the numerical solution of the Maxwell's equations. At lambda=780 nm, a good agreement up to distance of approximately 1.5 mu m from the topography minimum is achieved. The reduced effective wavelength in silicon leads to a better description of the ZnO-Silicon interface with respect to the ZnO-Air interface by the ray tracing model.