We investigate the effect of the interfacial properties between a polymer dielectric and an organic semiconductor (OS) layers on the photoresponse properties of an organic phototransistor (OPT). Three different polymer materials having different interfacial properties, poly(methyl methacrylate), amorphous fluoropolymer CYTOP, and poly(4-vinylphenol), were used as gate insulators for the OPTs in a bottom-gate top-contact configuration. For the trapping and detrapping processes of minority carriers, the physicochemical nature and the density of functional group of the dielectric material were found to more dominantly govern the photoresponsive properties of the OPT than the morphological effect. Our molecular level description provides a useful guideline to select a proper dielectric material, being used as a gate insulator, for optical applications of the OPTs such as optical memory devices and optical sensors. (C) 2016 Elsevier B.V. All rights reserved.