NOVELTY - The photovoltaic cell comprises distinct subcells having an emitter layer that includes a first material having a first intrinsic material lattice constant. The distinct subcells further have a window layer that is disposed adjacent to the emitter layer and includes a second material having a second intrinsic material lattice constant differing by 1% from the first intrinsic material lattice constant. The window layer is a fully strained pseudomorphic layer having a thickness of 15 nm, where the fully strained pseudomorphic layer has an internal lattice constant.    USE - Used as a multijunction photovoltaic solar cell.    ADVANTAGE - The high-efficiency photovoltaic solar cell has improved external and internal quantum efficiencies and excellent current collection efficiency, and maximizes the band gap of the window layer of the top subcell.    DETAILED DESCRIPTION - The photovoltaic cell comprises distinct subcells having an emitter layer that includes a first material having a first intrinsic material lattice constant. The distinct subcells further have a window layer that is disposed adjacent to the emitter layer and includes a second material having a second intrinsic material lattice constant differing by 1% from the first intrinsic material lattice constant. The window layer is a fully strained pseudomorphic layer having a thickness of 15 nm, where the fully strained pseudomorphic layer has an internal lattice constant that corresponds to the first intrinsic material lattice constant for its entire layer in a plane perpendicular to the direction of epitaxial growth. The second intrinsic material lattice constant is greater than or less than the first intrinsic material lattice constant. The photovoltaic cell is grown on a substrate. Each subcells has a base layer with a different bandgap. An INDEPENDENT CLAIM is included for a method for forming a fully strained window layer that is adjacent to an emitter in a subcell of a photovoltaic cell.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic cross-sectional view of a photovoltaic cell.    Top contact (20)    Cap (21)    Anti-reflection coating (22)    Window (23, 28, 33)    Emitter (24, 29, 34)    Base (25, 30, 35)    Buffer surface field layer. (26, 31, 36)