Direct growth of III-V/silicon triple-junction solar cells with 19.7% efficiency
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 P 1-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 P 1-y buffer shows a three times higher threading dislocation density on Si (1.4 × 10 8 cm -2 ) than on GaP substrates, and atomic force microscopy shows holes in the GaAs y P 1-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.