Patent ID: 8766087

Claim:
A photovoltaic cell comprising: at least one subcell with a window structure, said at least one subcell comprising an emitter layer comprising InGaP with a first lattice spacing in the plane perpendicular to the direction of epitaxial growth; said window structure being positioned above said emitter layer and in direct contact with said emitter layer; said window structure having at least two window layers, a bottom window layer having an intrinsic material lattice spacing that is substantially the same as the intrinsic material lattice spacing of said emitter layer in the plane perpendicular to the direction of epitaxial growth, and at least one upper window layer overlying the bottom window layer and having a lattice spacing in the plane perpendicular to the direction of epitaxial growth that is substantially different from that of the emitter layer, wherein, the bottom window layer is characterized by a thickness from 1 nm to 15 nm; and the bottom window layer comprises Al x In 1-x P, wherein x is 0.52; the at least one upper window layer comprises Al x In 1-x P, wherein x ≧0.60; and the lattice spacing and in-plane strain of the emitter layer and the lattice spacing and in-plane strain of the bottom window layer differ by less than 0.3%; and said at least one upper window layer having a band gap greater than a band gap of the bottom window layer.