Metamorphic gaasp and ingap photovoltaic materials on Si for high-efficiency III-V/Si multijunction solar cells

A range of metamorphic GaAsP and InGaP compositions, with direct bandgaps ranging from 1.9 to 2.2 eV, were grown using GaAs_yP_1-y step-graded buffers, which have been previously demonstrated as compatible and integratable with Si substrates, and characterized for optical emission and absorption properties. Such bandgaps are needed for the realization of the highest efficiencies in multijunction photovoltaics, but are currently only available via low-efficiency materials. Photoluminescence measurements of these compositions demonstrate emission spanning a large range of the visible spectrum, from red to green. Transmission/absorption properties were also measured for the GaAsP materials, yielding absorption constants for near-bandgap photons that are similar to that of GaAs, and confirming true optical transparency of the GaAs_yP_1-y buffers for sub-bandgap photons.