Germanium solar cells grown by molecular beam epitaxy for lattice-matched, four-junction solar cells

We demonstrate Ge junctions grown by molecular beam epitaxy (MBE) for lattice-matched, four-junction solar cells. Integrating a Ge bottom cell beneath a 1.9 eV InGaP/1.4 eV Ga(In)As/1.0 eV dilute nitride stack could enable ultra-high efficiencies in a single growth step. In this work, we successfully created Ge junctions by growth of homoepitaxial n-Ge emitters using As2 as a dopant on p-Ge wafers within a III-V MBE system. The growth and post anneal conditions are shown to strongly influence open-circuit voltage (Voc) in epitaxial Ge solar cells. Voc = 0.174 V was obtained without a window layer, which is comparable to the Voc of diffused Ge cells fabricated by metal-organic-vapor-phase-epitaxy for triple-junction solar cells.