The microstructure of conjugated polymers film is strongly dependent on factors such as the conformation and the film formation processing. In this article, we show how to induce more planarization conformation of conjugated polymer backbone during film formation processing and finally leading to the fibrils formation of the cast film. The conjugated polymer we used is 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). The main solvent is chloroform (CF), the aliphatic 1,8-diiodooctane (DIO) is used as the additives, which has similar solubility parameter to the conjugated polymer side chain, is a bad solvent to the conjugated backbone and has a lower volatility than CF. Thus, during the film formation, chloroform was evaporated faster than the additive. After the chloroform evaporated completely, the side chain was still dissolved in the additive, which decreases the steric hindrance and improves planarization conformation of the conjugated backbone of PDPPTT-T. Films processed using the aliphatic additives have fibrillar morphology while films cast from the CF solvent were featureless. TEM images reveal that the fibrils were about 30 nm in width and several hundred nanometers in length. The backbones of PDPPTT-T were parallel to the long axis of fibrils. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1079-1086