It is reported on drain current modulation by UV light, in an organic thin‐film phototransistor, based on a small molecule semiconductor 2,7‐dipentylbenzo[b]benzo[4,5]thieno[2,3‐d]thiophene (BTBT‐C5) blended with a linear unsaturated polyester (L‐upe) to form the active channel material. The electrical properties of the phototransistor (L‐upe+BTBT‐C5) and physical properties of thin‐films are evaluated by means of two‐ and three‐terminal current–voltage characteristics (in the dark and under UV light) and structure–morphology analysis, respectively. Upon illumination with UV light, a dramatic change in the electrical properties of the L‐upe+BTBT‐C5 transistors is observed, compared to low responsive control transistors based on a blend of poly(methyl methacrylate) and BTBT‐C5. Drain current is increased by more than six orders of magnitude with maximum photosensitivity and responsivity of 4.0 × 106 and 11.1 A W−1, respectively, using a VG = 0 V. Modulated photoelectrical properties are explained by structure–morphology characteristics and efficient charge trapping at the polymer/semiconductor interface due to the presence of electron‐withdrawing groups in the polymer. Fast rise time of 1.80 s and slow relaxation time of 24.27 s, estimated using bi‐exponential fitting models, confirm a charge trapping/releasing mechanism and reveal sensing‐memory properties of the devices.
