Optimized Grafting Density of End-Functionalized Polymers to Polar Dielectric Surfaces for Solution-Processed Organic Field-Effect Transistors

Polystyrene (PS) grafted to silicon oxide (SiO₂, referred to as <i>g</i>PS-SiO₂) bilayers generated via a polymer grafting method were used as organic-oxide hybrid gate dielectrics to fabricate solution-processed triethylsilylethynyl anthradithiophene (TES-ADT) organic field-effect transistors (OFETs). The dielectric surface properties were significantly altered by the areal grafting densities of different molecular weight (<i>M</i>_w) PS chains with end-functionalized dimethylchlorosilane attached to the SiO₂ surfaces. Lesser grafting densities of longer PS chains increased the surface roughness of the treated SiO₂ surfaces from 0.2 to 1.5 nm, as well as the water contact angles from 94° to 88°. Below a critical <i>M</i>_w of end-functionalized PS, the <i>g</i>PS chains on the SiO₂ surfaces appeared to form a brush-like conformation with an areal density value greater than 0.1 chains nm^–2, but other high-<i>M</i>_w <i>g</i>PS chains formed pancake structures in which the polymeric layers were easily incorporated with solution-processed TES-ADT as a solute. These findings indicate that low-density <i>g</i>PS layers interfered with the self-assembly of TES-ADT in cast films, causing great decreases in crystal grain size and π-conjugated orientation. The presence of compact <i>g</i>PS chains on the SiO₂ surface could yield high electrical performance of TES-ADT OFETs with a field-effect mobility of 2.1 cm² V^–1 s^–1, threshold voltage of −2.0 V, and on/off current ratio of greater than 10⁷ when compared to those developed using less-concentrated <i>g</i>PS-SiO₂ surfaces