A key challenge to the development of polymer-based organic solar cells is the issue of long-term stability, which is mainly caused by the unstable time-dependent morphology of active layers. In this study, poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C60-butyric acid methyl ester (PCBM) blend is used as a model system to demonstrate that the long-term stability of power conversion efficiency can be significantly improved by the addition of a small amount of amorphous regiorandom P3HT into semicrystalline regioregular one. The optical properties measured by UV-vis absorption and photoluminescence reveal that regiorandom P3HT can intimately mix with PCBM and prevent the segregation of PCBM. In addition, X-ray scattering techniques were adopted to evidence the retardation of phase separation between P3HT and PCBM when regiorandom P3HT is added, which is further confirmed by optical microscopy that shows a reduction of large PCBM crystals after annealing at high temperature in the presence of regiorandom P3HT. The improvement of the long-term stability is attributed to the capability of amorphous P3HT to be thermodynamically miscible with PCBM, which allows the active layer to form a more stable structure that evolves slower and hence decelerates the device decay. (C) 2016 Wiley Periodicals, Inc.