Direct Growth of III–V/Silicon Triple-Junction Solar Cells With 19.7% Efficiency

Abstract

Monolithic multi-junction solar cells made on active silicon substrates are a promising pathway for low-cost high-efficiency devices. We present results of GaInP/GaAs/Si triple-junction solar cells, fabricated by direct growth on silicon in a metal–organic vapor phase epitaxy reactor using a GaAs y P1- y buffer structure to overcome the lattice mismatch between Si and GaAs. A low-temperature (750 °C) Si surface preparation process and a SiN x diffusion barrier at the rear side have been implemented to maintain the minority carrier lifetime in the Si bottom cell. Conversion efficiencies up to 19.7% have been achieved under AM 1.5g spectral conditions. The cells are compared with identical GaInP/GaAs dual-junction solar cells grown on bulk GaP and GaAs substrates to identify loss mechanisms. Subcell electrical characterization using electroluminescence reveals a significant voltage loss of the III–V subcells on Si, compared with the same structures grown on GaP or GaAs. Electron channeling contrast imaging of the metamorphic GaAs y P1- y buffer shows a three times higher threading dislocation density on Si (1.4 × 108 cm−2) than on GaP substrates, and atomic force microscopy shows holes in the GaAs y P1- y buffer on Si that are not observed on GaP. Approaches to reach lower defect densities for the III–V layers on silicon are discussed.