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

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.

Quantitative microstructural characterisation of concrete cured under realistic temperature conditions

Xinyu Zhang

The curing temperature is known to influence the strength and durability of concrete. In general, curing at elevated temperature results in high early strength gain, but impairs the long-term strength and transport properties. Previous investigations have not provided a complete understanding of these effects; in particular, no quantitative relation between microstructural and macroscopic changes has been developed. This work has adopted a combination of macro and micro characterisation to further a comprehensive and quantitative understanding of the property development under different curing conditions, through approaches which include both qualitative and quantitative microstructural analysis with SEM, EDS, XRD, NMR and TG. A systematic study on several mix designs and curing regimes has been carried out, to relate microstructural properties such as degree of hydration, C-S-H composition, C-S-H relative density and capillary porosity to compressive strength and water sorptivity. The results show that the Ca/(Si+Al) ratio of C-S-H is constant with temperature, and the increase in relative C-S-H brightness is thus attributed to the higher C-S-H density and lower chemically bound water at higher temperatures. The early hydration of concretes is accelerated by high temperatures, but the ultimate value is not limited, as opposed to widely reported lower values in the literature. This discrepancy is explained by the fact that previous workers have assumed that the bound water is constant for different curing temperatures whereas this is not the case. Linear relationships were established between the degree of hydration and the compressive strength of concretes, which depend on curing temperatures and mix design. Through their influence on the density and distribution of hydration products, elevated temperatures increase the ultimate capillary porosity of concretes. The relationship between capillary porosity and compressive strength seems independent of temperature to a first approximation, but no unique relationship is shown between sorptivity and compressive strength. Materials which were exposed to higher temperatures for only short periods of time, such as might be experienced in prefabrication or in the curing of large masses, showed some recovery in both microstructure and mechanical properties from the detrimental effects of high temperature.

EPFL2007

Quantum coherence and carriers mobility in organic semiconductors

Marie-Noëlle Bussac, Jean-David Picon, Libero Zuppiroli

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.

2007

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

Transistors organiques ultraminces à base de pentacène

Adrian von Mühlenen

EPFL2007

Quantitative microstructural characterisation of concrete cured under realistic temperature conditions

Xinyu Zhang

EPFL2007