Design and Synthesis of Air-Stable p-channel Conjugated Polymers for High Signal-to-Drift Nitrogen Dioxide and Ammonia Sensing.

The development of high-performance conjugated polymer-based gas sensors involves detailed structural tailoring such that high sensitivities are achieved without compromising the stability of the fabricated devices. In this work, we systematically developed a series of diketopyrrolopyrrole (DPP)-based polymer semiconductors by modifying the polymer backbone to achieve and rationalize enhancements in gas sensitivities and electronic stability in air. NO2 and NH3-responsive polymer-based organic field-effect transistors (OFETs) are described with improved air stability compared to all-thiophene conjugated polymers. Five DPP-fluorene based polymers were synthesized and compared to two control commercial polymers and used as active layers to detect a concentration of NO2 at least as low as 0.5 ppm. The hypothesis that the less electron-donating fluorene main chain subunit would lead to increased signal/drift compared to thiophene and carbazole subunits was tested. The sensitivities exhibited a bias-voltage dependent behavior. The proportional on-current change of OFETs using a dithienyl DPP-fluorene polymer reached ~614% for an exposure to 20 ppm of NO2 for 5 minutes, testing at a bias voltage of -33 V, among the higher reported NO2 sensitivities for conjugated polymers. Electronic and morphological studies reveal that introduction of the fluorene unit in the DPP backbone decreases the ease of backbone oxidation and induces traps in the thin films. The combination of thin film morphology and oxidation potentials governs the gas-absorbing properties of these materials. The ratio of responses on exposure to NO2 and NH3 compared to drifts while taking the device through repeated gate voltage sweeps is the highest for two polymers incorporating electron-donating linkers connecting the DPP and thiophene units in the backbone, in this category of organic semiconductors. The responses to NO2 were much larger than that to NH3, indicating increased susceptibility to oxidizing vs reducing gases, and that the capability of oxidizing gases to induce additional charge density has a more dramatic electronic effect than when reducing gases create traps. This work demonstrates the capability of achieving improved stability with the retention of high sensitivity in conjugated polymer-based OFET sensors by modulating redox and morphological properties of polymer semiconductors by structural control.