Enhanced performance of room temperature ammonia sensors using morphology-controlled organic field-effect transistors

Developing electronic sensors for ammonia (NH3) is very useful for environmental monitoring and diagnostic purposes. In this work, a highly sensitive, organic field-effect transistor (OFET) based, room temperature sensor for NH3 has been fabricated using dinaphtho [2,3-b:2′,3′-f]thieno [3,2-b]thiophene (DNTT), which showed a fast response to low concentration of the analyte down to 100 ppb. A thin film of solution-processed polymethyl methacrylate (PMMA) has been used as the gate dielectric material and its hydrophobic surface promoted structured growth of organic semiconductor, DNTT, by inducing mass transfer. By controlling the thickness and thereby exploiting the growth dynamics of the semiconductor film, the sensor performance was improved. The sensitivity of the device towards 1 ppm of NH3 was almost doubled with a thinner and porous film of DNTT as compared to that with a thick film. Morphological studies of the sensing layers, using atomic force microscopy (AFM), have established this structure-property relation. The variations in different transistor parameters have been studied with respect to different analyte concentrations. The p-channel devices in the enhancement mode showed depletion upon exposure to NH3. The devices exhibited a fast response and good recovery to the initial state within 2 min.