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 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 min, 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.

Characterizing doping effects in a conductive polymer and physical diffusion in a passive polymer were performed using a remote-gate field-effect transistor (RG FET) detection system that was able to measure the electrical potential perturbation of a polymer film coupled to the gate of a silicon FET. Poly­(3-hexylthiophene) (P3HT) film doped using various concentrations of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) solutions imposed additional positive potentials on the P3HT RG, resulting in a lower threshold voltage (V th) on the n-channel silicon FET. Changes in V th were related to the induced hole concentrations and hole mobility in P3HT films by using our V th shifting model for the RG FET. We discovered that the electron-donating P3HT and even inorganic materials, indium tin oxide and gold, showed similar electrical potential perturbations dependent on the concentration of F4TCNQ in overlying solutions as the dopant radical anions maximally covered the surfaces. This suggests that there are limited electroactive sites for F4TCNQ binding on electron donor surfaces which results in a similar number of positive charges in film materials forming dipoles with the F4TCNQ radical counteranions. The effect of electron acceptors such as 7,7,8,8-tetracyanoquinodimethane and tetracyanoethylene was compared to that of F4TCNQ in terms of V th shift using our analytical tool, with differences attributed to acceptor size and reduction potential. Meanwhile, this FET analysis tool offered a means of monitoring the physical diffusion of small molecules, exemplified by F4TCNQ, in the passive polymer polystyrene, driven by concentration gradients. The technique allows for nondestructive, nonspectroscopic, ambient characterization of electron donor–acceptor interactions at surfaces.

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Efficient doping of polymer semiconductors is essential for their development as conductors. Although Lewis acids such as B(C 6 F 5 ) 3 have shown promise as dopants for polymers, their doping mechanism is not fully understood. We created 1:1 zwitterionic (including "Wheland-type") complexes of B(C 6 F 5 ) 3 with conjugated molecules difluorobis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT) and didodecylthienothiophene (DTT-12) and characterized them with 1 H NMR, UV−vis spectroscopy, EPR spectroscopy, optical and scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDS), and X-ray diffraction. We employed these complexes as p-dopants for three conjugated polymers and established their doping abilities by conductivity measurements, Seebeck studies, field effect transistor (FET), and remote-gate sensing (RG-FET) measurements. Conductivity changes were dependent on the conjugated molecule adduct component, consistent with the adduct itself serving as the oxidant. The adduct complexes were capable of inducing changes in the surface potential of spun polymer films similar to the behavior shown by conventional dopants. Charge carrier density calculations by remote gate sensing revealed that these adducts can generate holes. We also studied the effect of adding the B(C 6 F 5 ) 3 first, followed by addition of the neat conjugated molecules; the observation of behavior that was different from that using preformed adducts was consistent with the adducts remaining intact during doping. When B(C 6 F 5 ) 3 was added to the polymers, followed by uncomplexed conjugated molecules, the generated hole carrier density is lower than that generated by the B(C 6 F 5 ) 3 dopant but often greater than that generated by the Wheland complexes, indicating a high probability of adduct formation in this case. Density functional theory calculations show that adduct formation between boranes and the conjugated molecules and segments of the polymers is energetically favorable and that some charge transfer between adducts and neutral polymers is plausible if Coulombic and entropic effects are taken into consideration. Thus, such adducts can be considered as possible doping sites for conjugated polymers.

The improvement of conjugated polymer-based gas sensors involves fine tuning the backbone electronic structure and solid-state microstructure to combine high stability and sensitivity. We had previously developed a series of...