Abstract We report on the fabrication and characterization of ambipolar organic phototransistors based on amorphous thin films of donor/acceptor dyad N -[7-( N , N -diphenylamino)-9,9′-spirobifluoren-2-yl]- N ′-(2,5-di- tert -butylphenyl)-3,4:9,10-perylenetetracarboxylicdiimide. The mechanism of light enhanced photocurrent is based on photoinduced charge transfer between the donor and the acceptor unit. The photoresponsivity of up to 0.2 A/W can be obtained. The drain current is significantly increased by a factor of nearly 100 as the light is used as an additional control parameter, making the device interesting for sensor applications.