A multijunction solar cell comprising: an upper first solar sub-cell comprising an emitter and a base both having a first bandgap, and being composed of InGa(Al)P. a middle second solar sub-cell adjacent to said upper first solar sub-cell and having a heterojunction InGaP emitter and a GaAs or In 0.015 GaAs base, having a band gap in the range of 1.2 to 1.6 eV, the emitter of the middle solar sub-cell having also a first bandgap equal to the first bandgap of the emitter of the upper first sub-cell, and wherein the base of the middle second subcell has a second bandgap that is narrower than the first bandgap of the emitter of the middle subcell such that said middle subcell is lattice matched with respect to said upper subcell, a graded interlayer adjacent to said middle solar subcell, said graded interlayer having a third bandgap greater than the second bandgap of the base of said middle subcell; and wherein the graded interlayer is composed of (In)(Ga 1-x) y Al 1-y As and has a constant bandgap, wherein the upper subcell is disposed over and being lattice matched to the middle subcell, and has a band gap in the range of 1.8 to 2.1 eV; and a lower third solar subcell adjacent to said graded interlayer having a heterojunction emitter and base, the emitter of the lower subcell having a fourth bandgap greater than the second bandgap of the base of the middle subcell, wherein said lower solar subcell being composed of an InGaAs base and an InGaP emitter lattice matched to the base, wherein the base of the lower subcell has a bandgap in the range of 0.8 to 1.2 eV, the base of the middle subcell has a band gap in the range of 1.2 to 1.6 eV, and the upper subcell is disposed over and being lattice matched to the middle subcell, and has a band gap in the range of 1.8 to 2.1 eV. wherein the fifth bandgap of the base of the lower subcell is a) narrower than the fourth bandgap of the emitter of the lower subcell, and b) narrower than the second bandgap of the base of the middle subcell such that said lower subcell is lattice mismatched with respect to said middle subcell. an upper first solar sub-cell comprising an emitter and a base both having a first bandgap, and being composed of InGa(Al)P. a middle second solar sub-cell adjacent to said upper first solar sub-cell and having a heterojunction InGaP emitter and a GaAs or In 0.015 GaAs base, having a band gap in the range of 1.2 to 1.6 eV, the emitter of the middle solar sub-cell having also a first bandgap equal to the first bandgap of the emitter of the upper first sub-cell, and wherein the base of the middle second subcell has a second bandgap that is narrower than the first bandgap of the emitter of the middle subcell such that said middle subcell is lattice matched with respect to said upper subcell, a graded interlayer adjacent to said middle solar subcell, said graded interlayer having a third bandgap greater than the second bandgap of the base of said middle subcell; and wherein the graded interlayer is composed of (In)(Ga 1-x) y Al 1-y As and has a constant bandgap, wherein the upper subcell is disposed over and being lattice matched to the middle subcell, and has a band gap in the range of 1.8 to 2.1 eV; and a lower third solar subcell adjacent to said graded interlayer having a heterojunction emitter and base, the emitter of the lower subcell having a fourth bandgap greater than the second bandgap of the base of the middle subcell, wherein said lower solar subcell being composed of an InGaAs base and an InGaP emitter lattice matched to the base, the base of the middle subcell has a band gap in the range of 1.2 to 1.6 eV, and the upper subcell is disposed over and being lattice matched to the middle subcell, and has a band gap in the range of 1.8 to 2.1 eV. wherein the fifth bandgap of the base of the lower subcell is a) narrower than the fourth bandgap of the emitter of the lower subcell, and b) narrower than the second bandgap of the base of the middle subcell such that said lower subcell is lattice mismatched with respect to said middle subcell. a) narrower than the fourth bandgap of the emitter of the lower subcell, and b) narrower than the second bandgap of the base of the middle subcell such that said lower subcell is lattice mismatched with respect to said middle subcell.