Quasi-Rheotaxial growth of large crystalline grain thin films on low cost substrates for photovoltaic applications

Abstract

A simple method, suitable to grow large (> 30 μm in size) grain thin films on amorphous or microcrystalline substrates like glass or steel, is described. This method, which has been called Quasi-Rheotaxy, consists in the deposition of a given material on a thin layer (2–3 μm) of another material usually but not necessarily metallic, which is kept at a temperature lower but not far from its melting point. Due to the high surface mobility exhibited by such a substrate, a material deposited on it tends to grow with large crystalline grains with an almost unique orientation. Recently, this method has been greatly improved after the discovery than a thin layer deposited on an amorphous or microcrystalline substrate can be easily crystallized by a suitable thermal treatment. By using such a crystallized alloy layer as a substrate for the growth of the desired photovoltaic semiconducting material, it has been found that the film can be grown, by taking advantage of Quasi-Rheotaxy, with crystalline grains as large as 30–500 μm, with a unique orientation and at a low substrate temperature. Films grown in this way exhibit structural qualities better than those required for the fabrication of high efficiency solar cells.