Efficiencies of 25% have been obtained with 1‐cm‐diam AlGaAs/GaAs heteroface concentrator solar cells utilizing an ultrathin AlGaAs window layer design. A low specific resistance (<0.005 &OHgr; cm2) Ohmic contact is achieved by direct contact to thep‐GaAs active layer. Liquid phase epitaxy has been developed to grow <500‐A˚ thick window layers on large‐area (3.3×3.3 cm) GaAs substrates. Four 1‐cm‐diam cells are produced from each wafer and demonstrate the potential for larger‐scale production.