Invited Paper EN+TF-TuA9
High-Efficiency Multijunction Solar Cells Employing Dilute Nitrides

Tuesday, October 30, 2012, 4:40 pm, Room 15

Concentrating photovoltaic (CPV) systems have the opportunity to provide the lowest cost of electricity in hot, sunny climates. The advantages of CPV are based, in part, from the high performance offered by multijunction solar cells made from group III-V compound semiconductors. Unlike traditional PV, high concentration systems utilize mm-scale solar cells that comprise only 10-15% of the overall system cost. This low cost share means that increasing cell efficiency has significant leverage in driving down upfront capital costs and the levelized cost of electricity of a CPV project.

Production cell efficiencies for triple junction solar cells have reached 40% under concentration (25^oC, AM 1.5D spectrum). We will review a number of exciting approaches for increasing cell efficiency that are being investigated world wide. Solar Junction has developed a set of dilute-nitride compound semiconductors that offer broad bandgap tunability over the infrared while retaining lattice matching to GaAs and Ge substrates. While significant efforts have been undertaken to develop dilute nitrides for multijunction solar cells over the last 15 years, these approaches have resulted in films that exhibited poor minority carrier properties resulting in low current drives and output voltages. Solar Junction has developed a molecular beam epitaxy process utilizing antimony as a surfactant that significantly enhances the minority carrier properties. Triple junction cells utilizing GaInNAsSb bottom junctions have achieved a world record efficiency of 43.5% under concentration. When used in conjunction with well known InAlGaP and AlGaAs compounds, GaInNAsSb films complete a lattice-matched epitaxial platform for enabling 4-, 5-, and 6-junction cells for achieving > 50% efficiencies in the coming years.