Effects of amorphous poly(3‐hexylthiophene) on active‐layer structure and solar cells performance

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. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 975–985