A large amount of dispersed pBTIT12 nano-fibrils formed easily by adding the additives to its o-dichlorobenzene solution to change the solution aggregation. It is found that a proper solubility parameter difference between polymer and additives (as large as 4J(1/2) cm(-3/2)) and a high boiling point of the additives are two important parameters for choosing the additives. 1-dodecanethiol is a promising additive for pBTTT12 o-dichlorobenzene solution, which can decrease the aggregation size distribution and disentangle unimer chains which are the building blocks of nano-fibrils. Under a slow spin-coating speed, coils tend to form dispersed nano-fibrils in terms of both thermodynamics and kinetics. Moreover, 1-dodecanethiol molecules intercalated into the free volume of adjacent alkyl side-chains of pBTTT molecules and change the crystalline structure. It was found that the distance between (100) crystalline planes became larger when the content of 1-dodecanethiol increased. (C) 2012 Elsevier Ltd. All rights reserved.