Separating the impact of oxygen and water on the long-term stability of n-channel perylene diimide thin-film transistors

A detailed understanding for the mechanisms that control degradation of the electrical performance of organic thin-film transistors (TFTs) during exposure to various environments, such as oxygen and humidity, is still developing. This is particularly true for n-channel organic TFTs. Here we present an investigation of the long-term stability of n-channel TFTs based on the small-molecule organic semiconductor N,N'-bis(2,2,3,3,4,4,4-heptafluorobutyl-1,7-dicyano-perylene-(3,4: 9,10)-tetracarboxylic diimide (PDI-FCN2) during storage in dry nitrogen, dry air, wet nitrogen and ambient air. By monitoring the electrical characteristics of the TFTs over a period of six weeks, we are able to show that the degradation of the electrical parameters (charge-carrier mobility and the simultaneous shift of the threshold voltage) is caused by two distinct mechanisms with different time constants. Exposure to oxygen or nitrogen (in the absence of humidity) causes the carrier mobility to drop by a factor of two and the threshold voltage to shift towards more positive values within 20 days, possibly due to a slight rearrangement of the conjugated molecules within the semiconductor layer. Storing the TFTs in saturated water vapor or in ambient air causes the threshold voltage and the carrier mobility to change much more rapidly, within just one day. The observed degradation in ambient air can be explained by an electrochemical instability of the radical anion of the organic semiconductor. (C) 2015 Elsevier B.V. All rights reserved.

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

Electrical properties of functionalized nanowire field effect transistors

Ralf Thomas Weitz

The utilization of functional organic materials holds great promise for applications in electronic devices. Semiconducting organic molecules are frequently used as channel material in field effect transistors, due to the ease by which they can be assembled as such components, and the ease with which their properties can be specifically tailored. An extension of the use of organic materials in field effect transistors with the potential to substantially improve the performance of such devices is investigated in this thesis. It is the miniaturization of the functional parts of these devices, including their conductive channel or the gate dielectric, down to nanoscopic dimensions, which is important for their integration into complex device architectures. The materials under consideration offer advantages over conventional field effect transistor channels, including improved processibility, high mobility and electrical stability. The first part of the thesis is dedicated to the fabrication and electrochemical functionalization of nanoscopic field effect transistors comprising individual semiconducting single-walled carbon nanotubes. The combination of an ultra-thin gate dielectric based on an organic self-assembled monolayer is used for the fabrication of high-performance field effect transistors that show large on-state transconductance, as well as a large on/off ratio of 107, negligible hysteresis in the drain current, and a subthreshold swing that approaches the room temperature limit of 60 mV/decade. Two different types of self-assembled monolayers are used. The first, a silane-based self-assembled monolayer, is utilized to build field effect transistors in a global back gate geometry. This layout is useful for demonstrating the benefits of the use of a self-assembled monolayer for nanotube field effect transistors. Secondly, a gate dielectric incorporating a self-assembled monolayer composed of an alkane phosphonic-acid is exploited for the fabrication of high-performance field effect transistors with patterned aluminum gate electrodes. This geometry allows for the fabrication of more than one gate electrode per substrate and thus for the realization of logic circuits with all field effect transistors located on the same substrate. The technological relevance of nanotube field-effect transistors is proven by showing that nanotube field effect transistors can be cycled for more than 104 times between their on and off state and stored in ambient air for more than 200 days without degradation of their electrical performance. The electrochemical functionalization of individual single-walled carbon nanotubes with the organic coordination magnet Prussian Blue is investigated in the second part of this thesis. While this modification leaves metallic tubes unaffected, semiconducting SWCNTs become strongly p-doped during the electrochemical process. The temperature-dependent electrical behavior of Prussian Blue-functionalized semiconducting tubes can be understood by the freeze-out of the transfer of holes from the Prussian Blue below 150 K. In the third part of this thesis, novel core-cyanated perylene carboxylic diimides end-functionalized with substituents of varying flourine content and geometry are utilized as active material in air-stable n-channel thin film field- effect transistors. The relation between the thin-film structure and the air stability of the charge carrier mobility is investigated. Contrary to previous reports, it is found that the air stability is not related to the redox potential of the molecules. In addition, one of the compounds is used for the fabrication of field-effect transistors with a nanoribbon channel. These nanoribbons self-assemble from the solution phase, and, owing to their high crystalline order, field-effect transistors with mobilities of up to 0.2 cm2/Vs in air could be realized. The air stability of their mobility is found to be superior to that of thin-film transistors based on the same compound. Since the investigated perylene derivatives offer the possibility of fabricating thin-film as well as nanobelt field-effect transistors, they represent a valuable model system for the investigation of the relation between the crystalline order, mobility, stability and the performance of organic n-channel field-effect transistors. The last part is dedicated to a novel method for the fabrication of nanoscale polymer wires of diameters as small as 25 nm that offer potential as channel material for nanofiber field-effect transistors. These fibers emerge from a polymer solution during a standard spin-coating process. The fiber formation relies upon the Raleigh-Taylor instability of the spin-coated liquid film that arises due to a competition of the centrifugal force and the Laplace force induced by the surface curvature.

EPFL2008

High-mobility organic thin-film transistors based on a small-molecule semiconductor deposited in vacuum and by solution shearing

Klaus Kern, Hagen Klauk

The small-molecule organic semiconductor 2,9-di-decyl-dinaphtho-[2,3-b: 2',3'-f]-thieno[3,2-b]-thiophene (C-10-DNTT) was used to fabricate bottom-gate, top-contact thin-film transistors (TFTs) in which the semiconductor layer was prepared either by vacuum deposition or by solution shearing. The maximum effective charge-carrier mobility of TFTs with vacuum-deposited C-10-DNTT is 8.5 cm(2)/V s for a nominal semiconductor thickness of 10 nm and a substrate temperature during the semiconductor deposition of 80 degrees C. Scanning electron microscopy analysis reveals the growth of small, isolated islands that begin to coalesce into a flat conducting layer when the nominal thickness exceeds 4 nm. The morphology of the vacuum-deposited semiconductor layers is dominated by tall lamellae that are formed during the deposition, except at very high substrate temperatures. Atomic force microscopy and X-ray diffraction measurements indicate that the C-10-DNTT molecules stand approximately upright with respect to the substrate surface, both in the flat conducting layer near the surface and within the lamellae. Using the transmission line method on TFTs with channel lengths ranging from 10 to 100 mu m, a relatively small contact resistance of 0.33 k Omega cm was determined. TFTs with the C-10-DNTT layer prepared by solution shearing exhibit a pronounced anisotropy of the electrical performance: TFTs with the channel oriented parallel to the shearing direction have an average carrier mobility of (2.8 +/- 0.3) cm(2)/V s, while TFTs with the channel oriented perpendicular to the shearing direction have a somewhat smaller average mobility of (1.3 +/- 0.1) cm(2)/V s. (C) 2013 Elsevier B.V. All rights reserved.