An Al0.14Ga0.86As solar cell with an Al0.92Ga0.08As window has been operated in AM2 sunlight at flux concentrations up to 899 suns with a conversion efficiency at this maximum concentration of 15.5%. Evaluation of the cells shows minority‐carrier diffusion lengths ≳2 &mgr;m and quantum efficiencies ≈0.85. The use of such a cell for spectral‐splitting high‐efficiency solar cell applications is discussed.