The close pi-pi stacking and the high J-aggregation during the formation of fibrillar morphology in films of the poly[[2,5-bis(2-octyldodecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c] pyrrole-1,4-diyl]-alt-[[2,2'-(2,5-thiophene) bis-thieno [3,2-b] thiophen]-5,5'-diyl]] (PDPPTT-T) are demonstrated via blending with polystyrene (PS). The hydrodynamic radius (R-h) of PDPPTT-T is decreased from 16.7 nm in the neat solution to 12.7 nm in the blend solution at the ratio of 1/20(PDPPTT-T/PS). This phenomenon suggests that blending PS is beneficial for the disentanglement of PDPPTT-T. The disentanglement of PDPPTT-T facilitates the formation of fibrillar morphology. The growth of the fibrils occurs along the molecular backbones and the width of the fibrils is parallel to the p-p stacking direction. The disentanglement of PDPPTT-T helps the molecules adjust conformation to improve J-aggregation and decrease the pi-pi stacking distance. The maximum absorption is red-shifted from 825 nm to 849 nm and the relative intensity of J-aggregation (the 0-0/0-1 ratio) is increased from 1.19 to 1.60. The pi-pi stacking distance decreases from 3.57 to 3.52 angstrom. The charge-carrier mobility will be improved in the fibrillar morphology with close p-p stacking and high J-aggregation. (C) 2016 Wiley Periodicals, Inc.