Printed electronics is a rapidly developing field of research which covers any electronic devices or circuits that can be processed using direct printing techniques. Among those printing techniques, inkjet printing is a technique of increasing interest for organic field-effect transistors (FETs) due to its fully data driven and direct patterning. In this work, the morphology of semi-conducting polymer/insulating polymer blends from inkjet printing and their FET properties have been investigated. We attempted to optimize the morphology of the blends by the addition of a co-solvent to the blend solution prior to film deposition. By varying the boiling temperature of the co-solvent, blend films are fabricated with varying domain purity and different degree of semi-conducting polymer ordering. The morphologies of all the as-cast samples from inkjet printing and subsequently thermally annealed samples are characterized by grazing incidence wide angle x-ray scattering and small angle neutron scattering. The results indicate that the sample where a low boiling temperature cosolvent is used exhibits a lower degree of semi-conducting polymer ordering and less pure domains, resulting in a decrease of hole mobility. The morphologies that are formed when high boiling temperature co-solvent is used, however, give a higher degree of semi-conducting polymer ordering along with higher domain purity, significantly improving hole mobility up to 1.44 cm(-2) V-1 s(-1) at V-DS = 40 V. More importantly, with thermal annealing, all the samples exhibit similar semi-conducting polymer ordering and domain sizes while the domain purity significantly varies. This work is a unique example that demonstrates the importance of domain purity in the optimization of morphology and FET performance, which is previous unavailable. It also provides a novel process that can efficiently control the morphology of semi-conducting polymer/insulating polymer mixtures during deposition to maximize FET performance from inkjet printing. (C) 2016 Wiley Periodicals,