Heterojunction subcells in inverted metamorphic multijunction solar cells

A method of forming a multijunction solar cell comprising:

• forming a heterojunction bottom subcell having a base and emitter and having a bandgap in the range of 0.8 to 1.2 eV, wherein the bandgap of the base of the bottom subcell is narrower than the bandgap of the emitter of the bottom subcell;
• forming a heterojunction middle subcell having a base and emitter, a bandgap greater than the bandgap of the bottom subcell and less than 1.50 eV, and being disposed over and being lattice mismatched to the bottom subcell, wherein the bandgap of the base of the middle subcell is narrower than the bandgap of the emitter of the middle subcell;
• forming a step-graded interlayer comprising a plurality of sublayers composed of InGaAlAs and having a bandgap that remains constant at about 1.50 eV between the middle and bottom subcells by identifying a set of compositions of the formula (InxGa1-x)yAl1-yAs defined by specific values of x and y, wherein 0<x<1 and 0<y<1, each composition having a bandgap of 1.50 eV;
• identifying a lattice constant for one side of the graded interlayer that matches the middle subcell and a lattice constant for an opposing side of the interlayer that matches the bottom subcell;
• identifying a subset of compositions of the formula (InxGa1-x)yAl1-yAs having bandgaps of 1.5 eV that are defined by specific values of x and y, wherein 0<x<1 and 0<y<1, wherein the subset of compositions have lattice constants ranging from the identified lattice constant that matches the middle subcell to the identified lattice constant that matches the bottom subcell;
• and selecting a composition for each sublayer of the plurality of sublayers from the subset of compositions;
• and
• forming a homojunction top subcell having a base and emitter and being disposed over and being lattice matched to the middle subcell,
• wherein said graded interlayer is compositionally graded to lattice match the middle subcell at one side and the bottom subcell at an opposing side,
• wherein forming the graded interlayer further comprises:
• providing a metal organic chemical vapor deposition (MOCVD) system configured to independently control the flow of source gases for gallium, indium, aluminum, and arsenic;
• selecting a reaction time and temperature and a flow rate for each source gas to form each sublayer of the step-graded interlayer disposed on the bottom subcell,
• wherein the source gas for indium is trimethylindium (InMe3), the source gas for gallium is trimethylgallium (GaMe3), the source gas for arsenic is arsine (ASH3), and the source gas for aluminum in trimethylaluminium (Al2Me6).