High-Performance Flexible Gas Sensors Based on Layer-by-Layer Assembled Polythiophene Thin Films

Gas sensors using organic semiconductors (OSCs) as active materials possess many advantages, including tunable selectivity and intrinsic flexibility, but still suffer from slow response and limited sensitivity. Ultrathin OSC films with high surface-to-volume ratios can improve the sensing performance; however, fabrication of continuous and uniform ultrathin OSC films using conventional methods remains challenging. In this work, we introduce a layer-by-layer assembly method, which is derived from the concept of atomic/molecular layer deposition technology, to prepare polythiophene OSC films under precise control. The resulting polythiophene film exhibits a linear growth as a function of the deposition cycle, and the nanoscale ultrathin films present a good thickness uniformity and smooth surface, in contrast to the uneven porous structure of spin-coated films. The assembly mechanism is revealed based on the detailed characterization of the polythiophene films. Further, the fabricated gas sensor using the layer-by-layer assembled polythiophene film as an active layer exhibits a 17%/ppm sensitivity to ammonia and a response time of less than 2 s, which is much superior to its counterpart with the spin-coated polythiophene film and is the best performance compared to previous reports. Flexible gas sensors are further demonstrated by depositing the film on a polymer substrate using the abovementioned method. Therefore, this work provides a widely applicable method to precisely deposit ultrathin and uniform OSC films for advanced applications.