The crucial role of molecular weight of the polymer dielectric in regulating the electrical performance and photo-response of TIPS-pentacene OFETs has been explored using poly(vinyl alcohol) (PVA) as the polymer dielectric layer. With increasing molecular weight of PVA, a degradation in the electrical and photo-response was observed. Atomic-force microscopy (AFM), Fourier-transform infrared spectroscopy (FTIR) and X-ray Diffraction (XRD) studies revealed that with increase in PVA molecular weight, polarizability of PVA dielectric layer and thus strength of the OFET channel reduced due to decreased hydroxyl group concentration oriented perpendicular to the PVA-semiconductor interface. Further, due to rising surface roughness of underlying PVA film with its molecular weight, macroscopic irregularities and therefore trap density in the active layer grown over it also increases. Therefore, both of these molecular weight dependent factors influence overall device performance and photo-response of OFETs in a critical way.