A high-performance broadband double-layer antireflection coating (DARC) is designed for GaAsP/SiGe tandem solar cells on a silicon substrate. Optimization of the window layer as part of the ARC design allows lower reflection to be achieved across the full range of the device's spectral response. Previous work has rarely fully optimized the window in DARC designs. An estimated non-zero collection profile from the window layer is used in optimization. Variation of the exact collection profile has minimal impact on the ideal design. DARC designs which result in currents within 1% of similarly optimized triple-layer designs, without the need for increased complexity in fabrication, are achieved. The benefit of an optimized window used with DARC is shown to apply also to 4-junction cells. (C) 2016 Elsevier Ltd. All rights reserved.