Methods of forming inverted multijunction solar cells with distributed Bragg reflector

A method of forming an inverted metamorphic multijunction solar cell including an upper solar subcell, a middle solar subcell, and a lower solar subcell comprising:

• providing a first substrate for the epitaxial growth of semiconductor material;
• forming an upper first solar subcell on said first substrate, wherein said upper first solar subcell has a first band gap;
• forming a middle second solar subcell comprising in order an emitter layer and a base layer over said first solar subcell, wherein said middle second solar subcell has a second band gap smaller than said first band gap;
• forming a first back surface field (BSF) layer directly adjacent the base layer of said middle second solar subcell;
• forming a first distributed Bragg reflector (DBR) layer directly adjacent the first back surface field (BSF) layer that is directly adjacent the base layer of said middle second solar subcell;
• forming a graded interlayer over the first distributed Bragg reflector layer using an MOCVD process, wherein a band gap of the interlayer remains constant throughout its thickness;
• forming a lower third solar subcell over said graded interlayer having a fourth band gap smaller than said second band gap such that said third solar subcell is lattice mismatched with respect to said second solar subcell;
• forming a second back surface field (BSF) layer directly adjacent a base layer of said lower third solar subcell;
• forming a second distributed Bragg reflector layer directly adjacent the second back surface field (BSF) layer that is directly adjacent the base layer of said lower third solar subcell;
• forming a spacer layer composed of p+ InGaAlAs directly adjacent the second distributed Bragg reflector layer;
• forming a high band gap contact layer composed of InGaAlAs directly adjacent the spacer layer;
• mounting a surrogate second substrate over said contact layer;
• and
• removing said first substrate to provide a top surface of the inverted metamorphic multijunction solar cell;
• wherein light can enter the top solar subcell, pass through the middle solar subcell and the directly adjacent back surface field (BSF) layer, and be reflected by the first directly adjacent distributed Bragg reflector layer back into the middle solar subcell;
• and
• wherein light can enter the top solar subcell, pass through the middle solar subcell, the lower third solar subcell, and the directly adjacent back surface field (BSF) layer, and be reflected by the second directly adjacent distributed Bragg reflector layer back into the lower third solar subcell.