The effects of white light on the electrical performance of polymer thin-film transistors (PTFTs) based on regioregular poly(3-octylthiophene) (P3OT) are investigated. Upon illumination, a significant increase in the PFET's drain current is observed with a maximum photosensitivity of 10/sup 4/ in the subthreshold operation and a broad-band responsivity with a maximum value of 160 mA/W at irradiance of 1.7 mW/cm/sup 2/ and at low gate biases. The photosensitivity decreases with the increase in the absolute gate bias. The simultaneous control of the device with both the gate voltage and illumination is possible at low irradiances of <0.7 mW/cm/sup 2/. It is found that the illumination effectively decreases the threshold voltage of the device, but it does not change the field-effect mobility. Using a trap model, it is shown that the narrow layers close to the drain and source contacts with high concentrations of defects are two possible regions for photogeneration of excitons and separation of charges. Using the theory of space-charge limited conduction, the extracted band mobility for P3OT is 0.08 cm/sup 2//V/spl middot/s, while a mobility of 8/spl times/10/sup -5/cm/sup 2//V/spl middot/s is found for the regions next to the source and drain contacts. The PTFT's high photosensitivity at zero gate voltage suggests a simple design of low-voltage, high-sensitivity two-terminal photodetectors for applications in large-area flexible optoelectronics.