Tuning the Rheology of Conducting Polymer Inks for Various Deposition Processes

Organic semiconducting polymers are attractive because of their competitive price and low processability requirements. Aqueous solutions of these polymers can be deposited with a variety of processes to make thin, flexible, and transparent films. Such films can be used as transparent electrodes in organic light-emitting diodes or organic photovoltaic cells. Poly(3,4-ethylene dioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) is the only PEDOT:polyanion system that is commercially available. However, other counterions such as poly(4-styrene trifluoromethyl(bissulfonylimide)) (PSTFSI) have been shown to stabilize PEDOT in aqueous solvents and give comparable conductivity and transparency properties to the films. We show that the rheological properties of these two aqueous systems are especially different. For example, PEDOT:PSTFSI inks can form a physical gel even at low concentrations, making them attractive for processing purposes, while PEDOT:PSS shows only minor shear thinning behavior for concentrations as high as 1 wt %. This difference in behavior is in part due to structural differences between the polyanions: PSTFSI favors hydrogen bonds while PSS does not. Here, we present a systematic study of the rheological properties and the film properties of PEDOT:PSTFSI inks for different concentrations and using various deposition processes such as doctor blade, screen-printing, inkjet, and soft blade deposition. We show that such inks can be adapted to each of these processes by simply tuning the concentration, making their formulation simple while maintaining their comparable optoelectronic properties as commercial inks.