We synthesized highly branched and electron-donating side chain subunits and attached them to polystyrene (PS) used as a dielectric layer in a pentacene field-effect transistor. The influence of these groups on dielectric function, charge retention, and threshold voltage shifts (ΔVth) depending on their positions in dielectric multilayers was determined. We compared the observations made on an N-perphenylated iminobisaniline side chain with those from the same side chains modified with ZnO nanoparticles and with an adduct formed from tetracyanoethylene (TCNE). We also synthesized an analogue in which six methoxy groups are present instead of two amine nitrogens. At 6 mol % side chain, hopping transport was sufficient to cause shorting of the gate, while at 2 mol %, charge trapping was observable as transistor threshold voltage shifts (ΔVth). We created three types of devices: with the substituted PS layer as single-layer dielectric, on top of a cross-linked PS layer but in contact with the pentacene (bilayers), and sandwiched between two PS layers in trilayers. Especially large bias stress effects and ΔVth, larger than those in the case of the hexamethoxy and previously studied dimethoxy analogues, were observed in the second case, and the effects increased with the increasing electron-donating properties of the modified side chains. The highest ΔVth was consistent with a majority of the side chains stabilizing the trapped charge. Trilayer devices showed decreased charge storage capability compared to previous work in which we used less donating side chains but in higher concentrations. The ZnO and TCNE modifications resulted in slightly more and less negative ΔVth, respectively, when the side chain polystyrene was not in contact with the pentacene and isolated from the gate electrode. The results indicate a likely maximum combination of molecular charge stabilizing activity and side chain concentration that still allows gate dielectric function.
Keywords: gate dielectric; highly branched phenylenediamine; multilayer dielectric; organic transistor; phenylenediamine side chain; static charge storage; threshold voltage shift.