NOVELTY - Solar cell comprises: heterostructure on substrate from materials lattice-matched to substrate, which is configured to form top cell, bottom cell, and middle cell(s), each cell has p-n junction (101, 103, 105), is separated from adjacent cell by tunnel junction (104), and material forming each of substrate and cells has band gap; where material for substrate and cells is configured based on evaluation of ability to optimize overall band gap configuration of solar cell and maximize its conversion efficiency while minimizing radiative efficiency.    USE - As solar cell; and as computer-implemented method for determining optimum heterostructure for multijunction solar cell (all claimed).    ADVANTAGE - The solar cell provides lattice-matched multijunction (MJ) solar cell grown on InP substrate using materials that are lattice-matched to InP. The InAlAsSb material can achieve direct band gaps of 1.6-1.8 eV, and InGaAlAs material can achieve band gaps of 0.74-1.4 eV. Further, multiple strain-balanced quantum wells are grown between n and p layers in junction(s) to extend gap to less than or equal to 0.7 eV, and to extend absorption range of junction whilst maintaining lattice match. The MJ solar cell comprises more than 3 junctions, where combined band gaps of junctions are optimized to maximize conversion efficiency of cell, and material compositions is engineered to achieve optimum band gap combination. In solar cell, two junctions can be formed from material of same band gap, to form split cell. Splitting one of junctions in this fashion allows significant voltage increase, and if uppermost junction is thin, current generation can be split equally between two junctions of split cell. Top InAlAsSb junction can be replaced by split InAlAs (1.46 eV) top cell connected via tunnel junction. The alloy having this composition can have direct band gap of up to 1.8 eV and thus is suitable for optimizing performance of 3-junction MJ solar cell. This particular alloy concentration is determined so as to maximize direct (F-valley) band gap while assuming that it is no larger than indirect X-valley gap. In such case, middle cell band gap is much lower than in case of single top cell, while bottom cell band gaps remain same. As band gap of InAlAs is considerably lower than that of lattice-matched InAlAsSb, short-circuit current is only marginally decreased. In solar cell, materials for substrate and top, bottom, and middle cells are configured to maximize conversion efficiency of solar cell for AM1.5D illumination and for AM0 illumination, such as to obtain conversion efficiencies 51% and 38% respectively.    DETAILED DESCRIPTION - A solar cell comprises: heterostructure formed on substrate from materials lattice-matched to substrate, where heterostructure is configured to form top cell, bottom cell, and middle cell(s) situated between top and bottom cells, each cell comprises corresponding p-n junction (101, 103, 105), is separated from adjacent cell by corresponding tunnel junction (104), and material forming each of substrate and top, bottom, and middle cells has associated band gap; where material for substrate and each of top, bottom, and middle cells is configured based on evaluation of its ability to optimize overall band gap configuration of solar cell and maximize its conversion efficiency while minimizing radiative efficiency. The top cell is configured to have band gap of 1.6-1.9 eV, middle cell(s) is configured to have band gap of 1.14-1.35 eV, and bottom cell is configured to have band gap of 0.7-0.74 eV; or heterostructure is configured to include two top cells having band gap of 1.6-1.9 eV, where each of two top cells produces nearly equivalent amount of photocurrent when illuminated, and middle cell is configured to have band gap of 1.05 eV. At least one of top, bottom, and middle cells further comprises several quantum wells (105c) situated between p-type layer and n-type layer of bottom cell p-n junction.    DESCRIPTION OF DRAWING(S) - The figure shows multijunction solar cell formed on InP substrate.    Top p-n junctions (101)    N-type emitter layers (101a, 103a, 105a)    P-type base layers (101b, 103b, 105b)    P-type tunnel diode layer (102a, 104a)    N-type tunnel diode layer (102b, 104b)    Middle p-n junctions (103)    Tunnel junction (104)    Bottom p-n junctions (105)    Quantum well superlattice (105c)