Dimitri Janssen

Influence of the dielectric roughness on the performance of pentacene transistors

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 dielectric–semiconductor interface as the limiting step for the roughness-dependent mobility in the transistor channel.

Ambipolar injection in a submicron-channel light-emitting tetracene transistor with distinct source and drain contacts

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.

Patterned growth of pentacene

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.

Self-aligned surface treatment for thin-film organic transistors

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.

Electroactive dipyrromethene-Cu(II) self-assembled monolayers: complexation reaction on the surface of gold electrodes.

In the work presented, thiol- and COOH-terminated dipyrromethene derivatives have been applied for gold electrode modification. Dipyrromethene deposited onto a solid support, after binding Cu2+, can act as a redox active monolayer. The complexation of Cu(II) ions has been performed on the surface of gold electrodes modified with dipyrromethene. The characterization of dipyrromethene-Cu(II) self-assembled monolayers (SAMs) has been done by cyclic voltammetry (CV), wettability contact angle measurements, and atomic force microscopy (AFM). The new electroactive monolayer could be applied for the immobilization of proteins and ssDNA or for electrochemical anion sensing without redox markers in the solution.

Multiscale modeling of the electrostatic impact of self-assembled monolayers used as gate dielectric treatment in organic thin-film transistors.

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.

Bias stress in pentacene transistors measured by four probe transistor structures

This paper deals with operational lifetime measurements of organic field-effect transistors. The organic semiconductor under study is pentacene. We apply DC stress conditions on these structures, and monitor the output characteristics of the TFTs during stress as well as during recovery after stress. The transistor structures have been modified to incorporate two voltage-measurement probes in the channel in addition to the source and drain contacts. This results in a 4-probe configuration, that allows us to measure the voltage drop in the intrinsic transistor channel separately from the voltage drop over the source and the drain contact regions. This phenomenological study is a first step towards a comprehensive model for degradation of bias stress in organic field-effect transistors.

Organic circuit components for pentacene RF-ID tags

Summary form only given. RF-ID tags with organic semiconductors require the development of several types of devices and components. In particular, apart from logic gates, a tag also requires memory and power management. In a passive tag, that draws energy from the RF field of the reader coil, the power management includes a rectifier to generate the DC supply voltage for the circuit. In this presentation, we discuss our progress in logic, memory and rectifiers using organic semiconductors. For logic and rectifying diodes, we employ evaporated pentacene. Memory devices are based the cross-point concept, with impedance switching of the charge-transfer complex material CuTCNQ.

Intra-grain and oligo-grain top-contact organic thin film transistors

We describe and demonstrate a micromachined shadowmask that allows the realization of intra-grain and oligo-grain top-contact organic thin film transistors (OTFTs). First experimental results of OTFTs show that for small channel lengths, grain boundary barriers indeed appear to dominate the output characteristics of OTFTs.