We have calculated the performance of indium phosphide solar cells with lattice matched wide band‐gap In0.52Al0.48As window layers using the PC‐1D computer code. The conversion efficiency ofp+nInP solar cells is improved significantly by the window layer. No improvement is seen forn+pstructures. The improvement in InP cell efficiency was studied as a function of In0.52Al0.48As layer thickness. The use of the window layer improves both the open circuit voltage and short circuit current. For a typical In0.52Al0.48As window layer thickness of 20 nm, the cell efficiency improves in excess of 27% to a value of 18.74%.