Controlling charge-transfer at the gate interface of organic field-effect transistors

The performance and charge transport characteristics of pentacene transistors with oxide gates such as SiO2 and Al2O3 have been recently shown to largely depend on the density of residual carriers due to electron acceptor defects at the oxide surface. The threshold field in particular is strongly connected to the density of these surface states. We demonstrate that by vapor deposition of self-assembled monolayers on the oxide gate prior to pentacene growth this charge transfer process is inhibited, the residual charge concentration is decreased by up to two orders of magnitude, and the field-effect mobility increases by up to a factor of four.

Interface issues in organic semiconductor based devices

Mauro Castellani

The present doctoral thesis aims to contribute to the field of organic semiconductor physics and technology, both of which have become fast growing disciplines. Two technological applications are emerging from these research efforts: Organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs). OLEDs are already being fabricated on an industrial level today, while some fundamental issues still remain to be resolved for OFETs. During the last ten years, research interest has been continuously shifting from the bulk properties of organic semiconductors to their interfaces. Such issues are in fact addressed in this thesis work and include different interactions of thin organic layers with their environments. In the field of OLEDs, a multitude of issues have been investigated here. Most importantly and in contrast to most publications in this field, OLED's with complex device architectures that fully compete with standard devices performancewise have been fabricated and characterized. An oxygen plasma treatment of indium-tin-oxide anode has been shown to reduce the injection performance while enhancing the quantum efficiency. Two different mechanisms are proposed to explain this. Going a step further, the life-time behavior of such devices has been examined. Different aging mechanisms were identified: in short term and intermediate term aging, an oxygen plasma treatment of anode clearly reduces the operational voltage. Bulk degradation of the organic electroluminescent layer then becomes predominant for long term aging. Another important finding concerns the formation of exciplexes at organic/organic heterojunctions: In devices comprising a high hole injection barrier between the hole-transport and the electron-transport layer, the dominant formation of exciplexes is identified by a red-shifted shoulder in the electroluminescent spectrum and a low quantum yield. In contrast to that, the quantum efficiency is increased markedly by minimizing this hole barrier. In such devices, the formation of exciplexes is beneficial for device performance, since it facilitates hole transfer across the heterojunction. For the first time, the operational currents could be divided into different radiative and non-radiative recombination channels of exciplexes and excitons respectively. This quantification was further developed to simulate current-versus-voltage behavior and quantum efficiency. The work on pentacene based devices was initiated by fabricating ultrathin pentacene films on polished sapphire and by characterizing them with a temperature variable four probe setup. Oxygen plasma treatment of the substrate introduces oxygen radicals onto the surface and subsequent pentacene deposition leads to electron extraction from the pentacene molecules to defects at the oxide surface. Such an increase of charge carrier density within the semiconducting layer enhances its conductivity. Using a thick, aliphatic self-assembled monolayer (SAM) as an intermediate layer, both the activation energy and the film resistance increase by at least an order of magnitude compared to devices without SAM. Hence, charge transfer to the substrate is inhibited. The OFETs fabricated for this thesis work are all based on pentacene as a semiconductor and the gate interface was the focus of interest. Two different device types were fabricated: One device category comprises ultra-thin film OFETs (10nm). In such devices contact resistance is negligible. This gives rise to a dependence of device behavior on the gate interface. For this reason, the dielectric constant of the gate insulator was varied by changing the gate materials: the hole mobility is enhanced four-fold by using an aliphatic SAM at the gate interface instead of an oxygen plasma treated alumina (Al2O3). This effect has been interpreted in terms of self-trapping at the interface and its reduction by a low-k dielectric gate. In addition the on-off ratio was enhanced by a factor of 20, thanks to the SAM-inhibited pentacene doping due to a reduced semiconductor/substrate interaction. The second OFETdevice category was based on thicker (80nm) pentacene films. Their behavior is injection controlled. This finding originates in the application of a top contact configuration which implies that the carriers cross the granular material to reach the channel. Impedance spectroscopic measurements on different metal-insulator-semiconductor diodes unveiled a morphology dependent transveral mobility, which is linked to the degree of structural disorder in the film. In conclusion: Understanding of the various physical processes occurring at the different interfaces (anode and organic/organic interfaces in OLEDs, gate interface in OFETs) has considerably improved the performance of organic semiconductor based devices.

EPFL2006

Transistors organiques ultraminces à base de pentacène

Adrian von Mühlenen

The goal of this thesis is to contribute to the understanding of charge transport in organic field-effect transistors (OFETs) made of pentacene. Organic thin-film transistors (OTFTs) with active layers thicknesses of 5, 10, 20, and 100 nm were fabricated in order to examine their structure and electrical characteristics and identify the key parameters that affect the charge transport in these devices. The conductivity of the pentacene films determined via four probe measurements suggests the presence of extrinsic charge carriers, or residual carriers, which have a large influence on device performance and electrical characteristics. The origin of these carriers is discussed in terms of a charge-transfer process occurring between the organic semiconductor and the electron acceptor states of the gate oxide surface. The gate interface is studied by varying the density of residual carriers via modification of the oxide surface by different plasma treatments with or without using subsequent deposition of various molecular monolayers. The OFETs yielded residual carrier densities ranging from 5 × 109 et 1 × 1013 cm-2 depending on the gate interface modification. The electrical characteristics such as the film conductivity and field-effect mobility are shown to be dependent on the density of residual carriers. Based on the measured density of residual carriers the OFETs are classified into two groups : devices based on doped and on un-doped pentacene. Temperature-dependent measurements of the field-effect mobility and the film conductivity performed on these devices reveal a thermally-activated character. This suggests that charge transport in the examined devices occurs via hopping between localized states. Based on the activation energies of the film conductivity and the field-effect mobility, the difference in energy between the Fermi and the transport level Δε is estimated to be between 10 and 160 meV depending on the density of residual carriers. This range of values is in agreement with thermoelectric power measurements performed on the same devices. The thermoelectric power is discussed in terms of two contributions : one that is dependent on Δε and another constant term that is independent of temperature and gate interface modification. The latter contribution was measured to be 265 ± 40 μV/K and originates from the creation of the phonon cloud associated with local changes in intermolecular interaction. This suggests that the charge carrier in the channel is dressed with a cloud of polarization, an electronic polaron.

EPFL2007

Organic thin-film transistors: the passivation of the dielectric-pentacene interface by dipolar self-assembled monolayers

Franziska Dora Fleischli, Stéphane Suarez, Libero Zuppiroli

In organic thin-film transistors (OTFTs), the conducting channel is located near the interface between the organic semiconductor and the oxide dielectric; this interface is crucial for transistor performance. Self-assembled monolayers (SAMs) on the interface reduce the negative influences of the oxide dielectric surface by decreasing the coupling of the carriers at the gate and the role of the active surface defects on transfer. In this paper, we show that SAMs carrying a dipole moment determine the OTFT performance by controlling the charge transfer between the oxide dielectric and the semiconductor. The charges introduced into the semiconductor by this transfer (i.e., residual carriers) lead to a threshold shift to positive values, as well as a decrease in the contact resistance and an increase in the apparent mobility. In this study, other effects of the SAMs, such as the gate potential shift in the channel or a direct reaction between semiconductor and SAM molecules, can be excluded as dominant processes.