Solution process ZnO and pentacene bilayer transistor: ambipolar, p-channel and n-channel operation

In this paper we describe ZnO as n-type semiconductor (wide band gap stable inorganic), which has LUMO level at 4.4 eV (much closer to the work function of gold than typical organic semiconductor LUMO) and pentacene as p-type semiconductor (organic), whose HUMO level at 5.0 eV which is close to the gold work function (5.1 eV). ZnO films were prepared via a solution process. For this, we used zinc acetate [Zn(OAc)₂] (15 mM to 40 mM ) and 2-ethanolamine (2 to 5 mM ) in ethanol solvent. The substrate was dip-coated with this solution at a speed approximately 2 mm/sec with an angle 60deg with the horizontal and immediately was placed on a 70degC heater. The zinc acetate coated substrate was inserted in a 500degC furnace and kept for 15 minutes to form a thin layer of continuous polycrystalline ZnO nanoparticle film. Vacuum deposited pentacene film of thickness 100 nm was then grown on the ZnO-coated Si-SiO₂ substrate. Subsequently 50 nm-thick gold electrodes were deposited using a parallel TEM grid (200 mesh) as shadow mask.