METAMORPHIC III-V MATERIALS, SUBLATTICE DISORDER, AND MULTIJUNCTION SOLAR CELL APPROACHES WITH OVER 37% EFFICIENCY

Studies of the material properties of lattice-mismatched and lattice-matched GaInAs and GaInP, and control of their bandgap by varying composition and sublattice disorder, have allowed terrestrial concentrator solar cells to reach new heights of efficiency. The bandgaps of both GaInP and GaInAs are controlled by varying indium content, up to 35% indium in the GaInAs middle cell, or 2.4% lattice mismatch. The bandgap of lattice-mismatched GaInP is additionally controlled through ordering of Ga and In atoms on the group-III sublattice. Minority-carrier lifetime measurements are made in GaInAs, and in GaInP with different ordering states, as a function of lattice mismatch to the Ge substrate. The lifetimes are observed to be much longer than previously attainable, due to graded buffer growth conditions that inhibit propagation of threading dislocation segments into the active cell regions. In other approaches to high-efficiency multijunction solar cell design, the first 6-junction solar cells have been built and measured. These cells designed for use in space employ an active GaInNAs subcell 5 in a (Al)GaInP/ GaInP/ AlGaInAs/ GaInAs/ GaInNAs/ Ge 6-junction structure, with measured open-circuit voltage over 5.1 V. Terrestrial 3- junction concentrator cells with lattice-matched and metamorphic GaInP/GaInAs/Ge structures have been produced at Spectrolab with 37.3% efficiency measured at NREL, the highest independently verified solar conversion efficiency measured to date for a photovoltaic device (AM1.5 Direct, low-AOD spectrum, 175 suns, 25±1°C).