No abstract

We have adapted the microscopic theory of nucleation for the epitaxial growth of inorganic materials to the nucleation of organic small molecules on an inert substrate like the gate dielectric of an organic thin-film transistor. The parameters required to explore the model were calculated with the standard MM3 force field and also include experimentally determined vapor pressure data, as well as film growth data. Sufficient agreement is found between the experimentally determined equilibrium crystal shape and heats of sublimation on the one hand and the calculated parameters on the other hand. The growth of pentacene, tetracene, and perylene on inert substrates has been studied in terms of this theory, especially focusing on the two-dimensional ͑2D͒ to 3D nucleation transition. It is demonstrated that 3D nucleation leads to ill-connected grains, while 2D nucleated grains form continuous films suitable for charge transport. The analysis of this transition allows for the experimental determination of the molecule-substrate interactions for a given molecule on a given surface. It was found that the deposition conditions for 2D growth shift to less favorable substrate temperatures and deposition rates as the difference between interlayer interactions and molecule-substrate interactions increase and the intralayer interactions decrease. Moreover, those interactions affect the nucleation rate and therefore the ultimate 2D grain size that can be obtained.

The properties of the dielectric strongly influence the performance of organic thin-film transistors. In this letter, we show experimental results that quantify the influence of the roughness of the dielectric on the mobility of pentacene transistors and discuss the cause of it. We consider the movement of charge carriers out of the "roughness valleys" or across those valleys at the dielectricsemiconductor interface as the limiting step for the roughness-dependent mobility in the transistor channel.

Discotic liquid crystals can self-align to form one-dimensional semiconducting wires, many tens of microns long. In this letter, we describe the preparation of semiconducting films where the stacking direction of the disc-like molecules is perpendicular to the substrate surface. We present measurements of the charge carrier mobility, applying temperature-dependent time-of-flight transient photoconductivity, space-charge limited current measurements, and field-effect mobility measurements. We provide experimental verification of the highly anisotropic nature of semiconducting films of discotic liquid crystals, with charge carrier mobilities of up to 2.8 × 10 −3 cm 2 /Vs. These properties make discotics an interesting choice for applications such as organic photovoltaics.

This study sheds light on the microscopic mechanisms by which self-assembled monolayers (SAMs) determine the onset voltage in organic thin-film transistors (OTFTs). Experiments and modeling are combined to investigate the self-assembly and electrostatic interaction processes in prototypical OTFT structures (SiO2/SAM/pentacene), where alkylated and fluoroalkylated silane SAMs are compared. The results highlight the coverage-dependent impact of the SAM on the density of semiconductor states and enable the rationalization and the control of the OTFT characteristics.

We propose a way of patterning organic small molecule thin films without requiring a hardmask and therefore more compatible with printing technologies. Active and passive areas for transistors are predefined by different surface chemistries. The subsequent growth takes place under conditions that cause the formation of a high mobility two-dimensional film in the active area and a disconnected three-dimensional film or no film in the passive area. This concept is founded on the basic theory of nucleation of organic small molecules on inert substrates and applied to the growth of patterned pentacene layers.

We report on organic light-emitting transistors with a submicron-channel length, gold source, and calcium drain contacts. The respective contact metals allow efficient injection of holes and electrons in the tetracene channel material. Transistor characteristics were measured in parallel with electroluminescence being recorded by a digital camera focused on the transistor channel. In the case of submicron-channel lengths, the transistor source-drain current at higher gate voltages was determined by the source-drain voltage. At larger channel lengths, the source-drain current was limited by the injection of electrons from the calcium contact, as hole ejection to this contact was fully blocked. The hole blocking is explained in terms of a chemical reaction occurring at the Ca/tetracene interface.

For organic thin-film transistors where source-drain contacts are defined on the gate dielectric prior to the deposition of the semiconductor (“bottom-contact” configuration), the gate dielectric is often treated with a self-assembled molecular monolayer prior to deposition of the organic semiconductor. In this letter, we describe a method to apply an ultrathin solution-processed polymer layer as surface treatment. Our method is compatible with the use of the bottom-contact configuration, despite the fact that the polymeric surface treatment does not stand a photolithographic step. Furthermore, we show that our surface treatment results in superior transistor performance.