Depositing Calcium Fluoride Template Layers for Solar Cells

Abstract

A biaxially textured crystalline layer formed on a substrate is provided. The biaxially textured crystalline layer includes an oriented CaF2 crystalline layer having crystalline grains oriented in both in-plane and out-of-plane directions, where the out-of-plane orientation is a (111) out-of-plane orientation. The oriented CaF2 crystalline layer is disposed for growth of a subsequent epitaxial layer and the CaF2 crystalline layer comprises an IBAD CaF2 layer. The biaxially textured CaF2 layer can be used in a photovoltaic cell, an electronic or optoelectronic device, an integrated circuit, an optical sensor, or a magnetic device.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application claims priority from U.S. Provisional Patent Application 61 / 516,834 filed Apr. 8, 2011, which is incorporated herein by reference.FIELD OF THE INVENTION[0002]The present invention relates generally to biaxially oriented, polycrystalline thin films. More particularly, the invention relates to a method for forming biaxially oriented CaF2 with a (111) biaxial texture for crystallographically oriented silicon solar cells.BACKGROUND OF THE INVENTION[0003]Solar cell efficiency is a strong function of minority carrier lifetime, since photo-generated carriers that recombine before reaching the p-n junction do not contribute to photocurrent. Grain boundaries in polycrystalline silicon films provide electron traps that act as recombination centers that reduce minority carrier lifetimes. This recombination is a function of the grain boundary structure. In particular, the high dislocation density of high-angle grain boundaries result...

AI Technical Summary

Problems solved by technology

In particular, the high dislocation density of high-angle grain boundaries result in a higher recombination rate than low angle grain boundaries.

Both types of misorientation result in defect densities that lead to recombination.

However, silicon wafer fabrication is expensive and energy intensive, limiting potential silicon PV cost reductions.

Such inexpensive substrates (e.g. display glass) are generally amorphous or polycrystalline and cannot sustain high temperatures for long periods; therefore, it is difficult to grow high quality c-Si on them.

Furthermore, grain boundaries parallel to the surface may impede hydrogen diffusion into the film, reducing the efficacy of post-growth hydrogenation treatments.

Although solar cells made from silicon wafers dominate the existing photovoltaic (PV) market, the wafer fabrication process is energy-intensive and expensive, comprising about half of the typical module price.

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