NOVELTY - Forming multijunction solar cell involves forming a bottom subcell (T1) having a bandgap of 0.8-1.2 eV; forming a heterojunction middle subcell (T2) having a base and emitter, a bandgap greater than the bandgap of the bottom subcell and less than 1.50 eV, and is disposed over and being lattice mismatched to the bottom subcell; selecting a bandgap for an interlayer and forming a continuously-graded interlayer; and forming a homojunction top subcell (T3) having a base and emitter and being disposed over and being lattice matched to the middle subcell.    USE - For forming a multijunction solar cell (claimed) which is used in the applications such as concentrator terrestrial power systems and satellites used in data communications; and used in photodetectors and light emitting devices.    ADVANTAGE - The method forms a commercially viable and energy efficient multijunction solar cell having improved power and energy conversion characteristics, with low cost and improved safety.    DETAILED DESCRIPTION - Forming a multijunction solar cell involves forming a bottom subcell (T1) having a bandgap of 0.8-1.2 eV; forming a heterojunction middle subcell (T2) having a base and emitter, a bandgap greater than the bandgap of the bottom subcell and less than 1.50 eV, and is disposed over and being lattice mismatched to the bottom subcell; selecting a bandgap for an interlayer and forming a continuously-graded interlayer, where the bandgap that remains constant at 1.50 eV between the middle and bottom subcells; and forming a homojunction top subcell (T3) having a base and emitter and being disposed over and being lattice matched to the middle subcell, where the graded interlayer is compositionally graded to lattice match the middle subcell at one side and the bottom subcell at an opposing side. An INDEPENDENT CLAIM is included for a multijunction solar cell comprising the bottom subcell (T1); the middle subcell (T2) having a base and an emitter, where the base and the emitter of the middle subcell forms a heterojunction; the top subcell (T3) having a base and emitter, where the base and the emitter of the top subcell forms a homojunction; and a graded interlayer having a bandgap that remains constant at 1.50 eV and a composition (C1) defined by the formula (InxGa1-x)yAl1-yAs, with the values for x and y selected such that the compositionally graded interlayer has a bandgap of 1.50 eV, and lattice matches the middle subcell at one side and the bottom subcell at an opposing side.    x and y=0 or 1.