The electrical performance of organic thin-film transistors (OTFTs) continues to improve, but the effect of electromagnetic radiation on the device performance is still unclear. OTFTs made with solution-processed 5, 11 bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT) in a bottom-gated bottom-contact configuration were fabricated on SiO2 gate dielectric and the interaction of visible light with the semiconducting layer was studied. Monochromatic illumination (λ = 532 nm) that matches the highest absorption band of crystalline diF-TES ADT was used to generate a large number of carriers during device operation. We observe that the OTFTs showed an efficient photocurrent response when incident light of intensity ranging from 1 to 11 µW/µm2 was focused at the center of the channel. Over this range, transfer characteristic curves shifted by up to +14 V as illumination was increased. At an intensity of 1 µW/µm2, the ratio of the number of photons absorbed (and thus excitons generated) to the number of holes measured at the electrode was approximately equal to one. With a five-fold increase of the illumination intensity, we found that the ratio of the excitons generated to the measured charge carriers was an order of magnitude less indicating that the effects of trapping in OFETs has a stronger impact at higher incident power.