We report on solution processed, highly light sensitive thin film transistors (TFTs) based on poly(9,9-dioctylfluorene-co-bithiophene) (F8T2). Transistors without heat treatment showed the highest saturation mobility, while devices annealed at 280 °C showed the highest drain current. The latter annealed transistors were found to give highly stable and reproducible performance over many light cycles. Measurements were carried out using an inorganic light emitting diode (LED) light source with a peak wavelength of 465 nm and 19 nm bandwidth from 0 to 400 μW/ cm-2 light intensity on TFTs with an F8T2 film thickness of 30 nm. The TFT OFF current was found to increase both with light intensity and gate bias. The bulk photogenerated carrier density was calculated to change from 5× 1011 to 1× 1013cm-3 over the measured light intensity range. The TFT saturation mobility did not change with light intensity, remaining constant at 1.2× 10-4 cm -2 /V s. The TFT ON current instead increased due to a shift in the turn-on voltage VT. This changed from -27 to -20 V over the measured light intensity range, initially changing rapidly but then saturating at higher intensity values. Contact resistance RC measurements showed large values in the dark. RC rapidly decreases with increasing light intensity, again saturating at higher values. From these results, we propose a phototransistor model in which illumination varies the device performance by effecting injection. By considering this shift in RC as photoassisted barrier lowering which additionally varies the width of the region depleted of carriers between the injecting interface and the channel, it is possible to explain the observed shift in VT as a change in the fraction of the gate bias dropped across the contact capacitance CC. By operating the phototransistor at a value of Vg =-5 V (below VT), it was possible to achieve a highly linear response of the photocurrent with light intensity. Alternatively, by operating at a value of Vg =-40 V (above VT), it was possible to maximize the photoresponsivity within the measured range. A photoresponsivity of 18.5 A/W at 5 μW/ cm-2 light intensity was achieved. © 2010 American Institute of Physics.