Inkjet printing of semiconducting polymers is desirable for realizing low-cost, large-area printed electronics. However, sequential inkjet printing methods often suffer from nozzle clogging because the solubility of semiconducting polymers in organic solvents is limited. Here, it is demonstrated that the addition of an insulating polymer to a semiconducting polymer ink greatly enhances the solubility and stability of the ink, leading to the stable ejection of ink droplets. This bicomponent blend comprising a liquid-crystalline semiconducting copolymer, poly(didodecylquaterthiophene-alt-didodecylbithiazole) (PQTBTz-C12), and an insulating commodity polymer, polystyrene, is extremely useful as a semiconducting layer in organic field-effect transistors (OFETs), providing fine control over the phase-separated morphology and structure of the inkjet-printed film. Tailoring the solubility-induced phase separation of the two components leads to a bilayer structure consisting of a polystyrene layer on the top and a highly crystalline PQTBTz-C12 layer on the bottom. The blend film is used as the semiconducting layer in OFETs, reducing the semiconductor content to several tens of pictograms in a single device without degrading the device performance. Furthermore, OFETs based on the PQTBTz-C12/polystyrene film exhibit much greater environmental and electrical stabilities compared to the films prepared from homo PQTBTz-C12, mainly due to the self-encapsulated structure of the blend film.