Theoretical models for III-V compound multijunction solar cells show that solar cells with bandgaps of 1.95-2.3 eV are needed to create ideal optical partitioning of the solar spectrum for device architectures containing three, four and more junctions. For III-V solar cells integrated with an active Si sub-cell, GaInP alloys in the Ga-rich regime are ideal since direct bandgaps of up to ~ 2.25 eV are achieved at lattice constants that can be integrated with appropriate GaAsP, SiGe and Si materials, with efficiencies of almost 50% being predicted using practical solar cell models under concentrated sunlight. Here we report on Ga-rich, lattice-mismatched Ga0.57In0.43P sub-cell prototypes with a bandgap of 1.95 eV grown on tensile step-graded metamorphic GaAsyP1-y buffers on GaAs substrates. The goal is to create a high bandgap top cell for integration with Si-based III-V/Si triple-junction devices. Excellent carrier collection efficiency was measured via internal quantum efficiency measurements and with their design being targeted for multijunction implementation (i.e. they are too thin for single junction cells), initial cell results are encouraging. The first generation of identical 1.95 eV cells on Si were fabricated as well, with efficiencies for these large bandgap, thin single junction cells ranging from 7% on Si to 11% on GaAs without antireflection coatings, systematically tracking the change in defect density as a function of growth substrate.
A III-V/Si metamorphic epitaxy approach to achieve multi-junction solar cells having nearly ideal optical partitioning of
the solar spectrum is described. Following our previously-established methodology for the growth of defect-free GaP on
Si(100) substrates and demonstrations of heteroepitaxially integrated III-V-on-Si photovoltaics via GaAsyP1-y
metamorphic buffers, we discuss work undertaken on the further development and refinement of these processes and
materials, with the goal of minimization of threading dislocation densities in order to enable high-performance solar
cells. A substantial, non-trivial increase in growth temperature and general improvement of growth conditions and
designs has been achieved for both the heterovalent GaP/Si epitaxial integration process and the GaAsyP1-y compositional
grading. Improved dislocation glide and significantly more efficient epitaxial relaxation is found for the GaP/Si system,
while enhanced dislocation glide dynamics in the metamorphic GaAsyP1-y buffer system is demonstrated by the evolution
of new epitaxial tilt characteristics.