TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Multijunction Solar Cell: The first graded interlayer is compositionally graded to lattice match the second subcell on one side and the third subcell on the other side. The second graded interlayer is compositionally graded to lattice match the third subcell on one side and the bottom fourth subcell on the other side. The first graded interlayer is composed of any one of arsenic (As), phosphorus (P), nitrogen (N), antimony (Sb) based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter greater or equal to that of the second subcell and less than or equal to that of the third subcell, and having a band gap energy greater than that of the second subcell and of the third subcell. The second graded interlayer is composed of any of the As, P, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter greater or equal to that of the third subcell and less than or equal to that of the bottom fourth subcell, and having a band gap energy greater than that of the third subcell and of the fourth subcell. The first and second graded interlayers are composed of (InxGa1-x)Al1-yAs with x and y selected so that the band gap of each interlayer remains constant throughout its thickness. The band gap of the first graded interlayer remains constant at 1.5 eV and the band gap of the second graded interlayer remains constant at 1.1 eV. The upper subcell is composed of an InGaP emitter layer and an InGaP base layer, where the second subcell is composed of InGaP emitter layer and a GaAs base layer, the third subcell is composed of an InGaP emitter layer and an InGaAs base layer, and the bottom fourth subcell is composed of an In-GaAs base layer and an InGaAs emitter layer lattice matched to the base. The lower fourth subcell has a band gap of approximately 0.6 to 0.8 eV, where the third subcell has a band gap of approximately 0.9 to 1.1 eV, where the second subcell has a band gap of approximately 1.35 to 1.45 eV, and the upper subcell has a band gap of 1.8 to 2.1 eV.