Advanced high-efficiency concentrator tandem solar cells

Computer modeling studies of two-junction concentrator tandem solar cells show that infrared (IR)-responsive bottom cells are essential to achieve the highest performance levels in both terrestrial and space applications. These studies also show that medium-bandgap/low-bandgap tandem pairs hold a clear performance advantage under concentration when compared to high-bandgap/medium-bandgap pairs, even at high operating temperatures (up to 100°C). Consequently, two novel concentrator tandem designs that utilize low-bandgap bottom cells have been investigated. These include mechanically stacked, four-terminal GaAs-0.95-eV-GaInAsP tandem, and monolithic, lattice-matched. three-terminal InP-0.75-eV-GaInAs tandem. In preliminary experiments, terrestrial concentrator efficiencies exceeding 30% have been achieved with each of these designs. Methods for improving the efficiency of each tandem are discussed