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When an organic semiconductor (OSC) is blended with an electron acceptor molecule that can act as a p-type dopant, there should ideally be complete (integer) transfer of charge from the OSC to the dopant. However, some dopant–OSC blends instead form charge transfer complexes (CTCs), characterized by fractional charge transfer (CT) and strong orbital hybridization between the two molecules. Fractional CT doping does not efficiently generate free charge carriers, but it is unclear what conditions lead to incomplete charge transfer. Here we show that by modifying film processing conditions in the semiconductor–dopant couple poly(3-hexylthiophene):2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (P3HT:F4TCNQ), we can selectively obtain nearly pure integer or fractional CT phases. Fractional CT films show electrical conductivities approximately 2 orders of magnitude lower than corresponding integer CT films, and remarkably different optical absorption spectra. Grazing incidence wide-angle X-ray diffraction (GIXD) reveals that fractional CT films display an unusually dense and well-ordered crystal structure. These films show lower paracrystallinity and shorter lamellar and π-stacking distances than undoped films processed under similar conditions. Using plane-wave DFT we obtain a structure with unit cell parameters closely matching those observed by GIXD. This first-ever observation of both fractional and integer CT in a single OSC–dopant system demonstrates the importance of structural effects on OSC doping and opens the door to further studies. 

Recent development of dopant induced solubility control (DISC) patterning of polymer semiconductors has enabled direct‐write optical patterning of poly‐3‐hexylthiophene (P3HT) with diffraction limited resolution. Here, the optical DISC patterning technique to the most simple circuit element, a wire, is applied. Optical patterning of P3HT and P3HT doped with the molecular dopant 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) wires with dimensions of 20–70 nm thickness, 200–900 nm width, and 40 μm length is demonstrated. In addition, optical patterning of wire patterns like “L” bends and “T” junctions without changing the diameter or thickness of the wires at the junctions is demonstrated. The wires themselves show up to 0.034 S cm^‐1conductance when sequentially doped. It is also demonstrated that a P3HT nanowire can be doped, de‐doped, and re‐doped from solution without changing the dimension of the wire. The combined abilities to optically pattern and reversibly dope a polymer semiconductor represents a full suite of patterning steps equivalent to photolithography for inorganic semiconductors.

 

Organic mixed conductors are increasingly employed in electrochemical devices operating in aqueous solutions that leverage simultaneous transport of ions and electrons. Indeed, their mode of operation relies on changing their doping (oxidation) state by the migration of ions to compensate for electronic charges. Nevertheless, the structural and morphological changes that organic mixed conductors experience when ions and water penetrate the material are not fully understood. Through a combination of electrochemical, gravimetric, and structural characterization, the effects of water and anions with a hydrophilic conjugated polymer are elucidated. Using a series of sodium‐ion aqueous salts of varying anion size, hydration shells, and acidity, the links between the nature of the anion and the transport and structural properties of the polymer are systematically studied. Upon doping, ions intercalate in the crystallites, permanently modifying the lattice spacings, and residual water swells the film. The polymer, however, maintains electrochemical reversibility. The performance of electrochemical transistors reveals that doping with larger, less hydrated, anions increases their transconductance but decreases switching speed. This study highlights the complexity of electrolyte‐mixed conductor interactions and advances materials design, emphasizing the coupled role of polymer and electrolyte (solvent and ion) in device performance.

 

Avoiding faradaic side reactions during the operation of electrochemical devices is important to enhance the device stability, to achieve low power consumption, and to prevent the formation of reactive side‐products. This is particularly important for bioelectronic devices, which are designed to operate in biological systems. While redox‐active materials based on conducting and semiconducting polymers represent an exciting class of materials for bioelectronic devices, they are susceptible to electrochemical side‐reactions with molecular oxygen during device operation. Here, electrochemical side reactions with molecular oxygen are shown to occur during organic electrochemical transistor (OECT) operation using high‐performance, state‐of‐the‐art OECT materials. Depending on the choice of the active material, such reactions yield hydrogen peroxide (H₂O₂), a reactive side‐product, which may be harmful to the local biological environment and may also accelerate device degradation. A design strategy is reported for the development of redox‐active organic semiconductors based on donor–acceptor copolymers that prevents the formation of H₂O₂during device operation. This study elucidates the previously overlooked side‐reactions between redox‐active conjugated polymers and molecular oxygen in electrochemical devices for bioelectronics, which is critical for the operation of electrolyte‐gated devices in application‐relevant environments.