(405e) Surface Chemical Modification to Systematically Vary the Dielectric Interface in Semiconducting Polymer Field Effect Transistors

Conjugated organic semiconductors such as poly(3-hexylthiophene) (P3HT) have shown promise for use in applications such as flexible display technology and radio frequency identification tags. Unlike inorganic materials, organic device performance is strongly dependent on the structure and properties of the interfaces between materials. Previous research has shown that P3HT films spin-coated onto modified dielectric interfaces can show an increase in the charge carrier mobility presumably due to changes in microstructure that result in improved p-p interactions in the plane of charge transport. A clear understanding of the effect of interfacial properties on the structure and electrical characteristics of the polymer, however, remains incomplete.

Here, we systematically investigate the influence of interfacial chemistries on the electronic properties of conjugated semiconducting polymers. Specifically, the chemical functionality at the polymer/monolayer interface was modified using a versatile monolayer chemistry based on isocyanate-amine chemistry. Our previous results showed that surface bound isocyanates will not react with amines at the solid-liquid interface. However, by coupling the amine and isocyanate in solution and then depositing, silicon surfaces were prepared with functionalities such as thiophene, fluorene, pyrene, pyridine and octadecane. This strategy allowed for the production of monolayers with tailored functionality to systematically quantify, for example, the effect of a strong dipole and the interfacial chemical composition on the electronic properties of the semiconducting polymer. In one study, octadecyl triethoxy silane and 1-(3-(triethoxysilyl)propyl)-3-octadecylurea were used as dielectric modifications to evaluate the effect of a buried dipole on organic transistor performance. These chemistries provide identical polymer/ monolayer interfaces; however, the urea monolayer has a buried dipole that should affect the electrical properties of the transistor. We measure the relative surface coverage of the modified interface using NEXAFS and characterize the monolayer before coating with polymer. Transistor measurements were used to evaluate the effect of the two monolayer chemistries on the electronic properties of the semiconducting polymer. Using this chemistry, we find that strong dipoles buried in the dielectric layer can strongly influence the device properties.