Optics and calculated efficiencies of mechanically stacked two‐junction solar cells

Mechanically stacked two‐junction solar cells avoid the multijunction problems of interfering growth conditions, shorting layers, and current matching. They also allow the use of well‐developed Si and GaAs junctions. Doping the top junction substrates at 2(1017) cm−3 reduces measured free‐carrier absorption by up to a factor of 5 compared to doping at 8(1017) cm−3. Separately measured quantum‐yield spectra, open‐circuit voltages, and fill factors provide the basis to calculate stack efficiencies of 24%–28% for Si stacked under GaAsP and GaAs top junctions for a direct air mass 1.5 (AM1.5D) terrestrial sunlight spectra and a 400× light concentration. The GaAs is a preferred top junction because it has a direct gap, operates at near its theoretical limits, and minimizes transmission loss effects by contributing over 70% of the total stack output without compromising potential stack performance. The open‐circuit voltages of GaAs and Si cells are measured to vary with light intensity as predicted by the stand...