We reported on photovoltaic and photoconductivity effects as the underlying mechanism in thin-film phototransistors based on 2,2′,6,6′-Tetraphenyl-4,4′-spirobi[cyclopenta[2,1-b;3,4-b′]dithiophene]. The responsivity and the ratio of photocurrent to dark current were 25 A/W and 290, respectively. Our devices also exhibited a field-effect mobility of (1−2)×10−4 cm2∕V s and an ON/OFF ratio of ∼103. Hysteresis in the transfer curve was observed after the curves were measured from the ON-state to the OFF-state and vice versa. In this case, the gate voltage (at drain bias −20 V and drain current −1 nA) shifted by 8 V at an optical power of ∼20 μW.

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