Organic Field-Effect Transistors (OFETs) are used to study how insulating polymers influence charge transport at the semiconductor–dielectric interface. The molecular weight and chain-end groups of polymers are linked to interfacial morphology and mobility, but their roles remain unclear. In this study, Bottom-Gate, Top-Contact (BGTC) OFETs were fabricated using Polystyrene (PS) as the dielectric to examine the effect of molecular weight on interface and transport. Electrical measurements showed that mobility increased with molecular weight. Atomic Force Microscopy (AFM) revealed that as PS molecular weight increased, the initial growth of Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) shifted from a compact layered structure to a more separated island-like morphology. These findings demonstrate that the molecular weight and chain-end groups of insulating polymers significantly affect semiconductor film growth and charge transport at the interface.