Progress toward a Si-plus architecture: epitaxially-integrable Si sub-cells for III-V/Si multijunction photovoltaics

GaP/active-Si junctions were grown by metalorganic chemical vapor deposition via a previously developed process that yields GaP-on-Si integration free of heterovalent-related defects. N-type Si emitter layers were grown on p-type (100)-oriented Si substrates, followed by the growth of n-type GaP window layers, to form fully-active sub-cell structures compatible with integration into monolithic III-V/Si multijunction solar cells. Si bulk minority carrier lifetime was found to track the epitaxial process, with initial degradation followed by full recovery. Fabricated test devices from in-situ (all-epitaxial) GaP/Si structures yielded good preliminary performance characteristics and demonstrate great promise for the epitaxial sub-cell approach. Additional test structures based on ex-situ diffusion processed solar wafers demonstrate the impact and importance of back surface field layers for such sub-cells.