We have investigated the photoresponse of organic thin film transistors (OTFTs) based on evaporated films of dinaphtho [2,3-b:2′,3′-f] thieno[3,2-b'] thiophene (DNNT) as the active semiconductor and spin-coated polystyrene as the gate insulator. Both during illumination and in subsequent measurements in the dark after long periods under illumination, transfer characteristics shift to more positive gate voltages. The greatest photoresponse was achieved at 460 nm, near the absorption maximum of DNTT. The maximum photosensitivity and photoresponsivity measured were ∼104 and 1.6 A/W respectively. The latter is the highest reported for an organic semiconductor on a polymeric gate insulator and by suitable adjustments to device geometry could be increased to match the highest reported, ∼105 A/W, for organic semiconductors. Weaker responses were also obtained when exposed to light from the long-wavelength tail in the absorption spectrum. At these longer wavelengths, the response arises entirely from a shift in flatband voltage caused by deep interface trapping of photo-generated electrons. At 460 nm, however, the positive shift, ΔVON, in turn-on voltage is much greater than the shift, ΔVT, in threshold voltage suggesting that ∼3.5 × 1011 electrons/cm2 are trapped at the interface at the start of the gate voltage sweep, but ∼60% are neutralised by holes from the channel as the device begins to turn on. While the resulting change in subthreshold slope could be interpreted as a change in the density of states (DoS) in the DNTT, this is discounted. Gate bias stress measurements made under illumination, reveal that positive bias enhances interface electron trapping while negative bias reduces the effect owing to the simultaneous trapping of holes from the accumulation channel