Isoindigo based small molecules have attracted much attention in the field of optoelectronic devices due to their broad absorbance and high charge carrier mobilities. Herein, we have investigated the field-effect transistor (FET) characteristics of a series of isoindigo based donor–acceptor–donor (D–A–D) small molecules containing one, two or three thiophene moieties (named IDT, ID2T, and ID3T, respectively) or an ethylenedioxythiophene moiety (IDED). In order to improve the carrier mobility, 1-chloronaphthalene (CN) and 1,8-diiodooctane (DIO) as solvent additives were used. Films were processed with various concentrations of solvent additives and their morphology was examined using atomic force microscopy (AFM). This study provides an understanding of the influence of volatile solvent additives and processing conditions on the morphology of small molecule systems, and serves as a guide to aid in the use of solvent additives in small molecules FET devices.