Multi-junction solar cells combine multiple junctions made of materials with different band gaps that are carefully selected to optimize power conversion efficiency. Current state of the art triple-junction devices achieve power conversion efficiencies over 40% under concentrated sunlight. As the industry drives for higher efficiency with additional junctions, lattice matching becomes a limiting factor due to specific band gap requirements for each junction. This work describes a bonding method that was developed to enable the creation of multi-junction cells using sub-cells, which cannot be grown monolithically due to lattice constant constrains.

A four-junction (4J) configuration is proposed in which an upper GaInP/GaAs 2J tandem cell is bonded to a lower GaInAsP/GaInAs 2J tandem cell. In this 4J configuration, the upper tandem is grown inverted and lattice-matched to a GaAs substrate and the lower tandem is grown upright and lattice-matched to an InP substrate. A five-junction (5J) configuration with a GaN-based wide-band gap material integrated with the four-junction structure described above is also possible, and the expected efficiency for this 5J device exceeds 50%. A wide variety of other device configurations using similar bonding methods can be explored. Devices have been fabricated using Au-Au bonded interconnects and either SiO2 or GaInP2 as a transparent filler for the remainder of the interfacial volume. Several different sub-cell configurations have been successfully fabricated, using both SiO2 and GaInP 2 fillers. So far, the most complex configuration fabricated is a GaInP/GaAs two-junction cell bonded to an InGaAs cell using a GaInP2 optical coupling layer. This configuration results in Voc= 2.70 V, J sc = 12.66 mA/cm2, FF = 83.0%, and efficiency =28.39% under the 1-sun ASTM G173 direct spectrum.

This dissertation will discuss the design of the bonded III-V multi-junction device, such as band gap selection, metal topology design and optical design. The fabrication development process is also discussed and both device characterization and results will be presented.