Coprocessing of F₄TCNQ Dopant and Poly(3-alkylthiophene) in a Two-Dimensional Bilayer

Doping organic semiconductors to modify and enhance their electronic properties is a crucial step in the development of these devices. In this study, a liquid surface is used to organize poly(3-hexylthiophene) (P3HT) in a homogeneous highly crystalline nanometer-thick bilayer that can subsequently be transferred onto a chosen solid substrate without impacting its structure. Coprocessing of various amounts of F4TCNQ as dopant and P3HT was investigated to obtain, for the first time to our knowledge, an F4TCNQ-doped polymer layer preorganized on a liquid substrate. The optical and electrical properties, as well as the structure and morphology of the doped layer, were determined on liquid and solid substrates using a large range of experimental techniques adapted to each substrate. Our findings revealed that optimal doping is obtained when the molar ratio of the F4TCNQ: thiophene unit is 1:4. A 2D-organized F4TCNQ-doped P3HT bilayer with an edge-on orientation was formed on both substrates. The bilayer thickness is close to 4 nm, and the F4TCNQ dopant was found to be intercalated within the alkyl side-chain interlayer. In addition, two in-plane crystalline structures were observed to coexist, the major one corresponding to F4TCNQ-doped P3HT and the minority one to neutral polymer. The in-plane lattice associated with each phase was completely determined, emphasizing the high order achieved through such a coprocessing method on a liquid substrate compared with usual drop-casting techniques. This resulted in the bilayer in-plane electronic conductivity of about 0.1 S/cm reaching 5 S/cm for the 5-bilayer film due to the contribution of out-of-plane charge transport.