Multi-junction iii-v solar cell

A method comprising:

• obtaining a bottom photovoltaic structure comprising a crystalline silicon base and an emitter layer comprising silicon on the base;
• forming a first buffer layer on the bottom photovoltaic structure, the first buffer layer including a first portion comprising silicon and germanium and a second, germanium-rich portion having a substantially greater percentage of germanium than the first portion;
• forming a tunnel junction over the first buffer layer, and
• forming a top photovoltaic cell having a bandgap between 1.8-2.1 eV over the tunnel junction and lattice matched or pseudomorphic to the germanium-rich portion of the buffer layer, the top photovoltaic cell including a first base layer and a first emitter layer adjoining the first base layer, the first base and emitter layers each being comprised of a III-V semiconductor material.