Trap-limited transport in rubrene transistors

The charge carrier mobility in the transport channel of an organic transistor is estimated within the framework of a trap-and-release model. The model accounts for the observed dependence of the mobility on the dielectric constant "epsilon" of the gate insulator. This dependence is attributed to both the effective mass of the carrier and the energetic depth of transport traps due to interface defects being functions of "epsilon". These results are used to describe the critical role of the interface between the organic semiconductor and the dielectric material in governing charge transport in organic transistors.

Approches numériques du transport de charges dans les semiconducteurs organiques

Hocine Houili

Using plastics or dyes like molecular semiconductors to make optoelectronic devices at low cost is a project in industry that is of great interest to the "Information Community" of this beginning of the 21st century. Some applications of this technology have already been realized including flexible flat-panel displays made with organic light-emitting diodes, organic transistors for active matrices, organic solar cells and sensors. In all these applications it is essential to deduce and understand the laws that determine the charge transport and the light emission in these materials. The goal of this thesis is to contribute to the theoretical models that describe these transport processes. In order to predict the behaviour of charge carriers in a given device, one can choose two methods. One can either numerically solve the master equations that describe the general behaviour of charges and currents in each part of the device, or one can simulate the detailed behaviour of each charge in the device by fixing the conditions of its movement using a procedure known as "Monte Carlo". In this thesis I addressed the two approaches. I have been interested successively in the prediction of the electric properties of multilayer light-emitting devices, considered as a whole. Then I was able to understand in detail the processes accruing at the organic-organic interfaces by using the Monte Carlo method. My last project involved the study of the channel of an organic transistor. In each one of these cases I took account of the characteristic features of small molecule organic materials in order to develop models for charge transport for the study and optimization of two important types of organic devices : organic light-emitting diodes and field-effect transistors. Amorphous organic semiconductors are mainly used for the fabrication of organic diodes. They are characterized by an energetically disordered density of states that is assumed to be Gaussian. The transport of charges in these materials occurs via hopping from one molecule to another in this density of states. The correlations between the energies of the molecular sites have important effects on transport ; the dependence of mobility on the applied electric field in particular. If the energetic disorder is due to the random orientations of the permanent dipoles of the molecules, then the correlations between the orientations of these dipoles can profoundly change the spatial configuration of the energetic disorder and consequently the charge transport. Due to the fact that organic diodes often comprise of multiple organic layers, understanding device behaviour at the organic-organic interfaces is of critical importance. Electrons and holes accumulate close to these interfaces and they are therefore more likely to recombine and emit light. The Coulomb interactions between the charge carriers and the energy disorder are at the origin of complex processes taking place at these accumulation regions. I studied these processes through a 3D multi-particle Monte Carlo simulation. The short-range Coulomb repulsion forbids the double-occupancy of the sites and shifts the energy level for injection of carriers across the interface upward, thus increasing the electric current of the diode. The long-range Coulomb interactions increase the electric field and the current at the interface although this increase is less important than suggested by 1D models. Effects related to the hopping process such as the effect of the exact form of the hopping law and backward or return hop at the interface are clearly discussed. The Monte Carlo simulations are cross-compared with an analytical model for hopping transport across the interface. This comparison made it possible to understand other aspects of the physical processes at organic-organic interface and supports the Monte Carlo simulations. The study of the electron-hole recombination showed a decrease of the cross-section with an increase of the applied electric field, but this decrease is less than what can be expected by the classical Langevin theory. In spite of their efficiency, Monte Carlo simulations are not attractive when modelling complex multi-layer diodes. The 1D models are better suited in this case. A 1D model called MOLED, which was developed at Laboratoire d'Optoélectronique des Matériaux Moléculaires (LOMM) for the simulation of the organic diodes, is used and described in detail in this thesis. We used MOLED to explore the effect of the energy correlations on the electric field dependence of the mobility. In fact we simulated the time-of-flight experiment, a common method used to determine of the carrier mobility. The crystalline organic semiconductors achieve mobility performances close or even better than amorphous silicon. Their use for the fabrication of field-effect transistors is thus a good choice. The models for charge transport in crystalline organic materials differ much from their counterparts for amorphous organic materials. Indeed band models are more appropriate to describe charge transport in crystalline organic materials. However it is very important to take into account the effects of electronic polarization in these models. I developed a method of calculation of these effects of polarization in the bulk of the organic semiconductor and I extended it to the case of the interface with a dielectric insulator, a situation encountered in the channel of the organic transistor. These calculations showed that when the polarity of the dielectric increases the inter-molecular transfer integral decreases. This is confirmed by experimental measurements showing increasingly low values of mobility when the permittivity of the insulator increases.

EPFL2006

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

Approches numériques du transport de charges dans les semiconducteurs organiques

Hocine Houili

EPFL2006

Interface issues in organic semiconductor based devices

Mauro Castellani

EPFL2006

Transistors organiques ultraminces à base de pentacène

Adrian von Mühlenen

EPFL2007