In situ Electrical Characterization of the Thickness Dependence of Organic Field-Effect Transistors with 1−20 Molecular Monolayer of Pentacene

Field-effect mobility (μ) of pentacene-based organic field-effect transistors (OFETs) is studied as a function of the number of molecular monolayer (ML) by in situ electrical characterization, which greatly improves the accuracy and reproducibility of electrical characteristics of OFETs. The hole μ of pentacene OFET with an average 1 ML (∼1.57 nm) thickness has been observed under a vacuum. The μ of pentacene OFET rapidly increases with increasing surface coverage in the region of film thickness less than saturation thickness ( d 0 ), which is about 3.2 ML for pentacene OFETs studied herein. We have observed that pentacene molecular layers beyond d 0 have little contribution to the carrier transport in the semiconducting channel. The threshold voltage ( V T ) of pentacene OFETs has a variable thickness dependence having minimum ∼17 V at pentacene thickness around 3.2 ML. Similar d 0 was verified for both drain current and on/off current ratio of pentacene FETs. The atomic force microscopy (AFM) images of the pentacene layer confirm the layer-plus-island (Stranski-Krastanov mode) pentacene growth mechanism on SiO 2 substrate. Terrace-like stacking structure begins to be disernable around 3.2 ML of pentacene thin film. From AFM images, top few layers of pentacene terrace stacking become smaller in size after reaching the largest domain size around 10 ML. Our experimental results have demonstrated that the growth quality of the first few pentacene MLs on substrate strongly influences the morphology of the thicker film, packing structure, and electrical characteristics of OFET, including change-carrier mobility.