Organic thin-film transistors for display applications

status: publishe

Design of an organic pixel addressing circuit for an active-matrix OLED display

In this paper, we present a design of a flat-panel display (FPD) based on organic light-emitting diodes (OLEDs) and on organic thin-film transistors (OTFTs). Addressing mode, circuit topology, layout, and drive scheme are developed in order to reach the desired frame rate and to control the gray levels against the threshold voltage dispersions of OTFTs and OLEDs. Our design shows that the current OLED and OTFT technology are suitable for FPD technology, though setting serious constraints on driver design.status: publishe

Modeling of transport in polycrystalline organic semiconductor films

A grain boundary barrier model with an energy distribution of interfacial traps to describe charge transport in polycrystalline organic semiconductor films was presented. The model was applied to the interpretation of charge transport in unintentionally doped pentacene films. The concomitant increase in the threshold voltage and mobility in different films were elaborated, allowing an understanding of the observed difference between the dopant-concentration and gate-voltage dependences of the mobility.status: publishe

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

The properties of intra-grain and oligo-grain transistors without influencing film growth and with little contact effects were studied. Intra-grain devices enabled to extract the intrinsic charge-carrier mobility. Results of organic thin film transistors (OTFT) showed that for small channel lengths, grain boundary barriers were found to dominate the output characteristics of OTFT.status: publishe

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, the gate dielectric is thermal SiO2, treated with an OTS self-assembled monolayer, and the source and drain electrodes are gold, treated with thiols. The source and drain are defined by photolithography, and the channel length is 100 microns. 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 to measure the voltage drop in the intrinsic transistor channel separately from the voltage drop over the source and the drain contact regions. When applying a constant (DC) gate-source bias (VGS) corresponding to a field in the gate insulator of 1 MV/cm, we observe two degradation mechanisms: one part of the degradation is permanent, the other part recovers. The permanent degradation could be traced back to the drain side. It typically results in the current halving after 2 hours of stress at a vertical (gate insulator) field of 1 MV/cm. The degradation measured during stress includes both this permanent degradation and the recoverable pan. It is significantly larger in magnitude than the permanent degradation, resulting in typically a factor of 10 or more in drain current. The magnitude of the recoverable degradation not only depends on the gate voltage, but also on the drain voltage. It is therefore a consequence of the lateral field at the drain side of the channel. This phenomenological study is a first step towards a comprehensive model for degradation of bias stress in organic field-effect transistors. © 2004 IEEE.status: publishe

Pentacene organic field-effect phototransistor with memory-effect

Now organic thin film transistor technology gains growing maturity, high performance organic photodetectors are the missing link towards full organic photosensitive sensor arrays, needed for the realization of applications like organic scanners and organic cameras. In the borderline of the research in organic solar cells, research in organic photodetectors has mainly been limited to organic photodiodes. However, phototransistors offer the possibility to reach higher sensitivities, thanks to the internal current gain of a transistor structure. This document focuses on organic field-effect phototransistors (organic photoFETs) where illumination can be used as an optical gate signal in addition to the electrical gate signal. An experimental description of pentacene photoFETs during broadband illumination is given. A distinction between the direct photocurrent and the additional current enhancement by a thresholdshift can be made, and a relation for this thresholdshift in function of time is revealed. Consequently, the difficulty to compare different organic photoFETs without any information about the illumination time is highlighted and a comparison based on this power law is proposed. Subsequently it is pointed out that by the stability of this thresholdshift after illumination, the pentacene photoFET does not only act as a light detector but also as a memory element, capable to store information about illumination power and duration for an extended amount of time.status: publishe

Nucleation of organic semiconductors on inert substrates

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 (2D) 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. © 2003 The American Physical Society.status: publishe

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. © 2006 American Institute of Physics.status: publishe

Correlation between bias stress instability and phototransistor operation of pentacene thin-film transistors

The authors study the use of pentacene thin-film transistors as phototransistors. The shift in turn-on voltage (V-on), responsible for the high photosensitivity of these devices, is shown to be strongly dependent on illumination time and applied gate voltage. The time dependence of this process is similar to the shift in V-on during bias stress experiments in the dark, and illumination can simply be accounted for as an acceleration factor for bias stress instability. By comparing the characteristics of devices with different gate dielectrics, trapping of electrons by OH groups at the gate dielectric interface is indicated as a main origin for these shifts. (C) 2007 American Institute of Physics.status: publishe

An organic light-emitting diode with field-effect electron transport

We describe an organic light-emitting diode (OLED) using field-effect to transport electrons. The device is a hybrid between a diode and a field-effect transistor. Compared to conventional OLEDs, the metallic cathode is displaced by one to several micrometers from the light-emitting zone. This micrometer-sized distance can be bridged by electrons with enhanced field-effect mobility. The device is fabricated using poly(triarylamine) (PTAA) as the hole-transport material, tris(8-hydroxyquinoline) aluminum (Alq(3)) doped with 4-(dicyanomethylene)-2-methyl-6-julolindin-4-yl-vinyl)-4H-pyran (DCM2) as the active light-emitting layer, and N,N'-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13H27), as the electron -transport material. The obtained external quantum efficiencies are as high as for conventional OLEDs comprising the same materials. The quantum efficiencies of the new devices are remarkably independent of the current, up to current densities of more than 10 A cm(-2). In addition, the absence of a metallic cathode covering the light-emission zone permits top-emission and could reduce optical absorption losses in waveguide structures. These properties may be useful in the future for the fabrication of solid-state high-brightness organic light sources.status: publishe