InGaAs/GaAsP strain-compensated multiple quantum wells (MQW) with an effective bandgap of 1.2 eV are promising candidate materials for solar cells applications. Since high indium content in wells is needed to achieve such a low bandgap, lattice relaxation can easily occur during crystal growth. Here, we propose graded interlayers, consisting of ultra-thin GaAs and InGaAs with lower indium content, inserted between wells and barriers to prevent relaxation during growth and to achieve better crystal quality. Lattice relaxation was detected by in-situ monitoring of surface reflectance including an anisotropic component. Our method allowed the number of MQW periods before detectable lattice relaxation to reach approximately three times that without interlayers. Over 100 periods of In0.26Ga0.74As (7.0 nm)/GaAs0.60P0.40 (9.2 nm) MQW, with an absorption edge at 1.2 eV, were successfully piled up as a result. Lattice relaxation without interlayers was caused dominantly by dislocations due to the large lattice mismatch between a well and a barrier, and graded interlayers made the hetero-interfaces smoother so that the subsequent layers could be more easily grown. Transmission electron microscopy confirmed the existence of the interlayer at both ends of In0.26Ga0.74As layer, with graded indium content and the thickness of 1.2-1.8 nm. (C) 2011 Elsevier B.V. All rights reserved.