High efficiency, monolithic multijunction solar cells containing lattice-mismatched materials and methods of forming same

A multijunction solar cell for converting solar energy into electrical power, said multijunction solar cell comprising substantially unstrained subcells and substantially lattice-mismatched and strained subcells, said multijunction solar cell comprising:

• a support structure having lateral spatial extent and sufficient rigidity for providing structural support for a plurality of layers of material forming said solar cell, said support structure including a metal surface across said lateral spatial extent for reflecting light that passes through said layers of material and reaches said support structure;
• at least one lower subcell comprising semiconductor material disposed over said metal surface of said support structure, said lower subcell having a first lower band gap;
• a middle subcell comprising semiconductor material disposed over said lower subcell, said middle subcell having a second middle band gap, said first lower band gap being smaller than said second middle band gap;
• and
• an upper subcell comprising semiconductor material disposed over said middle subcell, said upper subcell having a third, upper band gap, said third upper band gap being larger than said second middle band gap,
• wherein said upper subcell and said at least one lower subcell are substantially lattice-mismatched and said lower subcell is more strained than said upper subcell.

A method of forming a multijunction solar cell comprising a high band gap substantially unstrained subcell and at least one lattice-mismatched low band gap subcell, the method comprising:

• providing a growth semiconductor substrate for the epitaxial growth of semiconductor material;
• forming the high band gap subcell over the growth semiconductor substrate, the high band gap subcell having a first high band gap.
• forming the at least one low band gap subcell over the high band gap subcell, the low band gap subcell having a second low band gap, the first high band gap being larger than the second low band gap, said growth substrate and said low band gap subcell having substantial lattice mismatch;
• providing a support substrate over the low band gap subcell;
• and
• removing the growth semiconductor substrate.

A multijunction solar cell for efficiently converting solar radiation into electrical power having lattice-mismatched subcells, the multijunction solar cell comprising:

• a support structure having lateral spatial extent and sufficient rigidity for providing structural support for a plurality of layers of material forming the solar cell, the support structure including a metal surface across the lateral spatial extent of the support structure for reflecting light that passes through the layers of material and reaches the support structure;
• at least one low band gap lattice-mismatched subcell comprising semiconductor material disposed over the metal surface of the support structure, the low band gap lattice-mismatched subcell having a low band gap;
• and
• a high band gap subcell comprising semiconductor material disposed over the low band gap lattice-mismatched subcell, the high band gap subcell having a high band gap, the high band gap being larger than the low band gap,
• wherein said low band gap lattice-mismatched subcell is substantially lattice-mismatched with respect to said high band gap subcell.

A multijunction solar cell having a high AMO conversion efficiency at 28° C. and both lattice-mismatched subcells and substantially unstrained subcells, the multijunction solar cell comprising:

• a support structure having lateral spatial extent and sufficient rigidity for providing structural support for a plurality of layers of material forming the solar cell;
• at least one lower lattice-mismatched subcell comprising semiconductor material disposed over the support structure, the lower lattice-mismatched subcell having a lower band gap;
• and
• a substantially unstrained upper subcell comprising semiconductor material disposed over the lower subcell, the upper substantially unstrained subcell having an upper band gap, the upper band gap being larger than the lower band gap,
• wherein said lattice-mismatched subcell is substantially lattice-mismatched with respect to said substantially unstrained upper subcell and said substantially unstrained upper subcell has sufficiently low strain such that said AM0 efficiency is at least about 30%.