The effects of adding a copolymer interfacial layer on the performance of Poly (3-hexylthiophene-2, 5-diyl) (P3HT) based organic thin film transistors (OTFT) were investigated. Poly (oligo (ethylene glycol) methyl ether methacrylate- glycidyl methacrylate- lauryl methacrylate), which is referred to as POGL, was used as an interfacial layer between P3HT and the dielectric. OTFTs with and without a POGL interfacial layer were fabricated. The devices were based on the bottom gate bottom contact structure. Current–Voltage (I-V) measurements were performed to characterize the performance of the OTFTs, and atomic force microscopy (AFM) measurements were used to analyze the morphology of the POGL and P3HT surfaces. The field effect mobility and the threshold voltage were extracted for all the devices. The OTFTs with a POGL interfacial layer were observed to have a smaller threshold voltage than the OTFTs without an interfacial layer, which makes the POGL devices attractive for low power applications. The POGL OTFTs were also observed to have much more ideal drain current saturation characteristics with very small I-V curve slope. This is explained by the deep trap states on the POGL surface. However, the POGL OTFTs were observed to have a smaller drain current and mobility than the non-POGL OTFTs. This is explained by the surface roughness of the POGL, which affects the charge transport in the channel of the device. The morphology of the P3HT on POGL surface also showed a higher density of voids and non-uniform coverage than P3HT on the SiO₂ surface, contributing to the smaller drain current and mobility of the POGL devices.