Tandem Si-Ge solar cell with improved conversion efficiency

A method of forming a solar cell, comprising the following steps:

• (a) providing an n-type Si substrate having a first light-receiving surface and a second surface;
• (b) forming in said substrate adjacent said second surface regions of n-type dopant and regions of p-type dopant;
• (c) forming over said second surface a layer of Si—Ge epitaxial material such that forming said layer diffuses said n-type dopant and said p-type dopant into said layer of epitaxial material and into said substrate;
• and
• (d) collecting electrical current generated by said solar cell in response to photon energy entering said first light-receiving surface of said substrate.

A solar cell, comprising:

• a substrate having a first light receiving surface and a second, opposite, surface;
• n-type dopant regions and p-type dopant regions formed adjacent said second surface of said substrate;
• a layer of epitaxial material formed on said second surface, wherein forming said layer of epitaxial material diffuses said n-type dopant and said p-type dopant into said substrate and into said layer of epitaxial material, said epitaxial material having a bandgap smaller than a bandgap of said substrate material;
• wherein n-type and p-type junctions are formed in said substrate and in said layer of epitaxial material;
• and
• means for collecting current from at least some junctions in said substrate material and from at least some junctions in said layer of epitaxial material.