Multi-junction III-V photovoltaics on lattice-engineered Si substrates

Dual junction (DJ) In_0.49Ga_0.51P/GaAs solar cells were grown on compositionally graded Ge/Se_1-xGe_x/Si (SiGe), fabricated and characterized. The DJ solar cells exhibited open-circuit voltage (V_OC) values in excess of 2 V for both AM0 and AM1.5 illumination. The high V_OC values result from maintaining very low defect densities in these highly lattice-mismatched structures by using SiGe graded layers and monolayer-scale control over the III-V/Ge interface formation. Comparisons made with identical cells grown on GaAs substrates reveal that the DJ solar cell on SiGe retained 91% of the V_OC and 99% of the short circuit current density achieved by the homoepitaxial DJ cell, demonstrating the potential for high efficiency multi-junction solar cells grown on SiGe. In addition, modeling shows that In_0.49Ga_0.51P/GaAs DJ cells should be more tolerant of the low residual dislocation densities characteristic of lattice-engineered SiGe substrates than single junction GaAs cells, indicating great promise for achieving a high efficiency III-V multijunction cell technology on Si.