Quantum coherence and carriers mobility in organic semiconductors

We present a model of charge transport in organic molecular semiconductors based on the effects of lattice fluctuations on the quantum coherence of the electronic state of the charge carrier. Thermal intermolecular phonons and librations tend to localize pure coherent states and to assist the motion of less coherent ones. Decoherence is thus the primary mechanism by which conduction occurs. It is driven by the coupling of the carrier to the molecular lattice through polarization and transfer integral fluctuations as described by the hamiltonian of Gosar and Choi. Localization effects in the quantum coherent regime are modeled via the Anderson hamiltonian with correlated diagonal and non-diagonal disorder leading to the determination of the carrier localization length. This length defines the coherent extension of the ground state and determines, in turn, the diffusion range in the incoherent regime and thus the mobility. The transfer integral disorder of Troisi and Orlandi can also be incorporated. This model, based on the idea of decoherence, allowed us to predict the value and temperature dependence of the carrier mobility in prototypical organic semiconductors that are in qualitative accord with experiments.

De l'effet de la polarisation électronique sur le transport de charge dans les semi-conducteurs moléculaires

Jean-David Picon

This work aims at a better understanding of transport mechanisms which take place in organic semiconductors such as pentacene. More accurately, we believe that the electronic polarization plays a leading part, which we try to identify in the particular case of the acenes. These semiconductors are very polarisable molecules that can crystalise easily enough into ordered structures. Moreover, there is quite an important transfer integral between neighbouring molecules, which is very uncommon for such an organic. After some general remarks regarding quantum transport in chapter 1, we present the main features of this family of semiconductors (chapter 2). The question that arises then is : how the characteristic properties of the acenes can influence the ground state of the charge carrier in the cristal (chapter 3)? In order to build this ground state, we take into account the electronic polarisability and the coupling to the lattice, which dress the charge with a cloud of polarization and a cloud of phonons, and also the overlap and the thermal disorder. The time scales related to these different phenomenons play an essential part. By comparig them with the characteristic time of transfer between molecules, we classify those phenomenons into slow and fast effects. The fast ones characterize the effective mass of the charge carrier, whereas the slow ones determine the localization of the charge in a more or less disordered crystal. We will see that the slowest of these effects act in the same way as the static disorder and are able to induce electronic localization. Then comes a quantitative survey of the electronic polaron (chapter 4). We show that, on the one hand, electronic polaron leads to a renormalisation of the transfer integral whereas, on the other hand, it amplifies the low geometric disorder. We finally achieve a modelisation of the system by an Anderson hamiltonian, in which the energetic disorder –of which we study the statistic distribution– is the translation of the former geometric one. Before the quantitative study of the eigenstates of this hamiltonian, we take a glance at the different models already developped in order to understand the experimental data from organic semiconductors (chapter 5). There is one important feature which is rarely well taken into account : slow disorder. Yet, such a disorder can disturb the building of the Bloch states and thus lead to electronic localization that we study quantitatively (chapter 6). Finally, we gather both the fast and slow effects in a single picture : a model providing an expression of mobility according to the microscopic parameters (chapter 7). As a conclusion, we take a further step towards the devices. We leave the bulk out to consider a charge in the canal of a transistor, i. e. layers of pentacene on a dielectric medium, in order to take into account the interface effects (chapter 8).

EPFL2006

Scrutinizing thermally stimulated current transients originating from trapped charges in organic semiconductors: A drift-diffusion study

Beat Ruhstaller

Thermally stimulated current (TSC) is a widely used technique to assess trap states and extract their density, energy, and capture rate using analytical expressions. In many cases, the latter are derived from physical models pertaining to inorganic semiconductors stipulating the absence of space charge or constant lifetime of free charge carriers. Especially for organic semiconductors, the validity of these equations can, therefore, be argued. Here, we investigate the validity range of this approach by fitting the classical equations to synthetic TSC data obtained from drift-diffusion simulation using representative input parameters for organic semiconductors. We find that the equation derived for slow recapture rate as well as the initial rise method provide excellent trap parameter predictions. On the other hand, the equation using the temperature of the peak current as well as the one derived for fast retrapping have a limited range of validity. An important merit of drift-diffusion modeling is the possibility to access local variables such as charge carrier density, electric field, and recombinaton. We unravel that a small fraction of traps nearby the electrode cannot be emptied even at high temperature due to the diffusion of charge carriers from the electrode into the semiconductor. Additionally, we find that an important electrostatic factor relates the extracted charge carriers measured by the external circuit and the input trap density. For the homogeneously distributed trap states used here, this factor is precisely two. Finally, extensions of the model are analyzed by implementing temperature and field dependent mobility into the drift-diffusion model. Published under an exclusive license by AIP Publishing.

AIP Publishing2022

Contact Effects in Organic Transistors

Frederik Sebastian Ante

Organic thin-film transistors (TFTs) are of interest for applications that require electronic functionality with low or medium complexity distributed over large areas on unconventional substrates, such as flexible polymeric films. Generally these are applications in which the use of silicon devices and circuits is technically or economically not feasible. Examples include flexible displays and large-area sensor arrays. The static and dynamic performance of state-of-the-art organic p-channel transistors is already sufficient for applications in which the transistors operate with frequencies of a few tens of kilohertz. Strategies to improve the performance of organic TFTs include improvements in the field-effect mobility of the charge carriers in the organic semiconductor layer and more aggressive scaling of the transistor dimensions. Additional advances are also required in the environmental and operational stability of organic TFTs and in the application of controlled contact doping for the reduction of the contact resistance of organic TFTs. In this thesis, high-resolution silicon stencil masks are introduced to pattern top-contact organic TFTs with lateral dimensions as small as 0.8 µm. Taking advantage of the recently synthesized high-mobility air-stable organic semiconductor dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]thiophene (DNTT), polycrystalline organic p-channel TFTs with excellent air-stability and an intrinsic mobility of 3.4 cm2/Vs were fabricated. The transistors employ an ultra-thin gate dielectric based on a plasma-grown oxide layer in combination with an organic self-assembled monolayer (gate capacitance ∼1 µF/cm2), allowing the TFTs to operate with low voltages of about 3 V. Transistors with dimensions of less than 100 nm were manufactured using polymer resist shadow masks that have been fabricated by electron-beam lithography. Thanks to the 5 nm thick gate dielectric the TFTs show no short-channel effects. The subthreshold swing, the on/off current ratio, and the threshold voltage are comparable to long-channel TFTs with lateral dimensions of ∼100 µm. However, organic transistors with reduced dimensions are often limited by the energy barrier at the interface between the semiconductor and the source/drain contacts. One approach to reduce this energy barrier is the introduction of a strong electron acceptor into the contact area that acts as a dopant. With contact doping, the contact resistance is reduced and the effective mobility of short-channel TFTs increases by more than 200%. Transistors (without doping) with lateral dimensions of ∼1 µm have a transconductance of about 1 S/m, so that the TFTs operate at frequencies of several megahertz. The influence of strongly reduced lateral dimensions on the contact resistance and the maximum operation frequency is investigated and scaling limitations are discussed.