High-performance organic phototransistors (OPTs) have been successfully constructed using bitriisopropylsilylethynyl tetraceno[2,3-b]thiophenes (TIPSEthiotet) or pentacene as a semiconductor layer. Fluorographene (FG) nanosheets were used to modify the interface between an organic semiconductor layer and gate dielectric. The effects of interface modification were investigated. It was found that enhanced photoresponsivity and a boosted photocurrent/dark-current ratio could be easily achieved after the implantation of modification layers. The constructed FG-modified devices based on TIPSEthiotet showed a maximum photoresponsivity of 21.83 A W−1 and a photocurrent/dark-current ratio of 1.85 × 106 under white light irradiation. Meanwhile, for the FG-modified OPT device based on pentacene, a high photoresponsivity of 144 A W−1 was obtained under white light irradiation with an optical power of as low as 25 μW cm−2. This photoresponsivity datum is higher than that of most OPTs based on pentacene reported under the same conditions. In addition, the mobilities of the devices could also be increased distinctly after the introduction of the FG-modified layer. The experimental facts indicate that the strong electron trapping ability of the fluorine atoms in the FG nanosheets and the well-known photovoltaic effect play an important role in these interesting results.