Highly-sensitive solution-processed 2,8-difluoro-5,11- bis(triethylsilylethynyl) anthradithiophene (diF-TESADT) phototransistors for optical sensing applications

A highly-sensitive organic phototransistor, based on solution-processed 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TESADT) was fabricated and investigated for an optical sensing element. The phototransistor based on thin crystalline grains of diF-TESADT exhibited a significant threshold voltage (V_TH) shift under a white light illumination in which the response time was estimated to be <0.5 s and a current modulation greater than 10⁶. It was found that the V_TH shift can be further enlarged by an additional gate bias, achieving very high light responsivity >10³ A/W at 0.17 mW/cm² and I_PH/I _DARK ratio higher than 10⁶. Also, by applying an erase gate bias, fast recovering of V_TH to the initial position was possible. This phenomenon can be ascribed to the trapping and de-trapping of photo-generated carriers at the organic channel/dielectric interface, while the amount of trapped carriers can be also modulated simultaneously by the gate bias. This investigation identifies that the solution-processed diF-TESADT phototransistors can be used for large-area and low-cost optical sensors and memory applications. In particular, it can be claimed that performance improvement by a gate bias represents a universal method applicable to the organic phototransistors.