Transparent Conducting Oxide Sculptured Thin Films for Photovoltaic Applications

Abstract

Optical losses in thin film solar cells arise due to reflections at the top interfaces where dielectric discontinuities may be significant, e.g., between the glass and transparent conducting oxide (TCO) contact and between the TCO and semiconductor structure. Advanced optical engineering approaches are needed to minimize such losses. One approach is to incorporate multilayered or graded-index TCO films designed to act as broad-band anti-reflectors. Thus, it is important to be able to modulate the near-infrared/visible index of refraction of the TCO over a relatively wide range (e.g., 1.3 < n < 2.0) without increasing its extinction coefficient k or significantly degrading its electrical conductance. Here we report an investigation of SnO₂ and ZnO sculptured thin films (STFs) under development for this purpose Sculptured thin films are deposited under low surface mobility conditions using stepwise or continuous variations in the polar and/or azimuthal angles of the deposition flux impinging on the surface. Deposition at a glancing polar angle leads to columnar growth, optical anisotropy, and low ordinary indices of refraction, whereas normal incidence deposition under the same conditions leads to dense isotropic films and high indices. In this study, we explore the dependence of the optical properties of SnO₂ and ZnO, including index and birefringence spectra, on the polar deposition angle. Optical modeling reported here assesses the ability of the STF concept to provide tailored TCOs for advanced optical engineering of CdTe solar cell structures.