Phototransistors are common, however, phototransistors that turn off when exposed to light are less common. The voltage-controlled transport properties of thin films of poly-D-lysine with and without methylene blue were studied to understand the photo-response both in the presence and absence of illumination and the results suggested these thin films behaves as a polar organic light sensitive material. The devices exhibit an unusual photo-response where the on-state current was higher in dark i.e., under no illumination, hence suggesting an inverse photo transistor can be fabricated from poly-D-lysine. The effect of addition of a photosensitive dye like methylene blue was also studied to understand the optical response of poly-D-lysine devices. The capacitance of both poly-D-lysine thin film with and without methylene blue was observed to be higher in the absence of illumination, which suggests light induced depolarization. This study provides a road map to the fabrication of cost-effective molecular inverse phototransistors.

Voltage controlled organic inverse photo-transistor using poly-D-lysine

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