Metal–Organic Framework as a Functional Analyte Channel of Organic-Transistor-Based Air Pollution Sensors

Air pollution sensors based on organic transistors have attracted much interest recently; however, the devices suffer from low responsivity and slow response and recovery rates for gas analytes. These shortcomings are attributed to the low charge-carrier mobility of organic semiconductors and to a structural limitation resulting from the use of a thick and continuous active layer. In the present work, we investigated the material properties of a multiscale porous zeolitic imidazolate framework, [Zn(2-methylimidazole)2]n (ZIF-8), and examined its potential as an analyte channel material inserted at an organic-transistor active layer. A series of carbonized zeolitic imidazolate frameworks (ZIFs) were prepared by thermal conversion of ZIF-8 and also studied for comparison. The microstructures, morphologies, and optical/electrical characteristics of polythiophene/ZIF-8 hybrid films were systematically investigated. Organic-transistor-type nitrogen dioxide sensors based on the polythiophene/ZIF-8 hybrid films showed substantially improved sensing properties, including responsivity, response rate, and recovery rate. The electrical conductivity of the carbonized ZIF-8s enhanced the field-effect mobility of the organic transistors; however, the sensing performance was not improved, because of the closed pore structures resulting from the carbonization. These results provide invaluable information and useful insights into the design of transistor-type gas sensors based on organic semiconductor/metal–organic framework hybrid films.