Mechanical stack structure for multi-junction photovoltaic devices and manufacturing method thereof

A multijunction photovoltaic structure includes a first photovoltaic sub-cell having at least one junction, a second photovoltaic sub-cell having at least one junction and having a band gap which is smaller than the smallest band gap of the first photovoltaic sub-cell, and an intermediate layer which provides an optical coupling between the first and second photovoltaic cells, wherein the intermediate layer has a physical layer thickness that is substantially equal to or smaller than a free space wavelength of light that corresponds with the smallest band gap of the second photovoltaic sub-cell.