6 research outputs found

    Unraveling the Role of Multiphonon Excitations and Disorder Concerning the Meyer-Neldel Type Compensation Effect in Organic Semiconductors

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    The Meyer-Neldel (MN) compensation rule, implying an exponential increase in the prefactor with increasing activation energy in a thermally activated process, is naturally emerging in two-site transition rates as a result of multiphonon excitation processes. However, it has been recently demonstrated [Phys. Rev. B. 90, 245201 (2014)] that the experimentally observed compensation behavior for the temperature activated charge transport in thin-film organic field-effect transistors (OFETs) is not a genuine phenomenon, but rather it is an apparent extrapolated effect that arises as a consequence of the partial filling of the Gaussian density-of-state (DOS) distribution. To resolve the contradiction, we investigate the impact of different jump rate models on macroscopic hopping charge transport in a random organic system using an Effective Medium analytic approach. The principal result of this study is that the averaging over the individual jump rates in a conventional Gaussian disordered system erodes the genuine thermodynamically-determined MN compensation effect, and therefore the macroscopic transport does no longer reflect the microscopic rates. The apparent compensation behavior observed for OFET mobilities upon varying the carrier concentrations can be reproduced irrespective to the single-phonon or multi-phonon character of activated transitions. Another remarkable finding is that the disorder formalism does predict a genuine MN compensation effect using multi-phonon rates if a disordered semiconductor contains a significant concentration of deep traps, so the cumulative DOS features a double-peak Gaussian. Thus, this study bridges the gap between Gaussian disorder and multi-excitation entropy (MEE) models concerning the MN effect, and has important implications for the interpretation of the isokinetic MN-temperature in disordered organic semiconductors.accepted october 19 urldate: 2018-11-29status: Published onlin

    Origin of Meyer-Neldel-type compensation behavior in organic semiconductors at large carrier concentrations: Disorder vs. thermodynamic description

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    We have extended an effective medium approximation theory [Fishchuk, Kadashchuk, Genoe, Ullah, Sitter, Singh, Sariciftci, and Bässler, Phys. Rev. B 81, 045202 (2010)] to investigate how polaron formation affects the Meyer-Neldel (MN) compensation behavior observed for temperature-dependent charge-carrier transport in disordered organic semiconductors at large carrier concentrations, as realized in organic field-effect transistors (OFETs). We show that the compensation behavior in organic semiconductor thin films can be consistently described for both nonpolaronic and polaronic hopping transport in the framework of the disorder formalism using either Miller-Abrahams or polaron Marcus rates, respectively, provided that the polaron binding energy is small compared to the width of the density of states (DOS) distribution in the system. We argue that alternative models based on thermodynamic reasoning, like the multiexcitation entropy (MEE) model, which assumes charge transport dominated by polarons with multiphonon processes and ignores the energy disorder, are inherently not applicable to describe adequately the charge-carrier transport in disordered organic semiconductors. We have suggested and realized a test experiment based on measurements of the compensation behavior for the temperature-dependent conductivity and mobility in OFET devices to check the applicability of these models. We point out that the MN behavior observed in thin-film OFETs has nothing to do with the genuine MN rule predicted by the MEE approach, but rather it is an apparent effect arising as a consequence of the functional dependence of the partial filling of the DOS in a disordered system with hopping transport. This fact is fully supported by experimental results. The apparent MN energy was found to depend also on the shape of the DOS distribution and polaron binding energy.status: publishe

    Role of transport band edge variation on delocalized charge transport in high-mobility crystalline organic semiconductors

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    We demonstrate that the degree of charge delocalization has a strong impact on polarization energy and thereby on the position of the transport band edge in organic semiconductors. This gives rise to long-range potential fluctuations, which govern the electronic transport through delocalized states in organic crystalline layers. This concept is employed to formulate an analytic model that explains a negative field dependence coupled with a positive temperature dependence of the charge mobility observed by a lateral time-of-flight technique in a high-mobility crystalline organic layer. This has important implications for the further understanding of the charge transport via delocalized states in organic semiconductors.status: publishe

    Negative field-dependent charge mobility in crystalline organic semiconductors with delocalized transport

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    © 2018, Institute of Chemistry, Slovak Academy of Sciences. Charge-carrier mobility has been investigated by time-of-flight (TOF) transient photocurrent in a lateral transport configuration in highly crystalline thin films of 2,7-dioctyl[1]benzothieno [3,2-b][1] benzothiophene (C 8 -BTBT) grown by a zone-casting alignment technique. High TOF mobility has been revealed that it is consistent with the delocalized nature of the charge transport in this material, yet it featured a positive temperature dependence at T≥295K. Moreover, the mobility was surprisingly found to decrease with electric field in the high-temperature region. These observations are not compatible with the conventional band-transport mechanism. We have elaborated an analytic model based on effective-medium approximation to rationalize the puzzling findings. The model considers the delocalized charge transport within the energy landscape formed by long-range transport band-edge variations in imperfect organic crystalline materials and accounts for the field-dependent effective dimensionality of charge transport percolative paths. The results of the model calculations are found to be in good agreement with experimental data.status: publishe

    Strain induced anisotropic effect on electron mobility in C-60 based organic field effect transistors

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    The electron mobility was found to increase (decrease) upon applied compressive (tensile) strain, respectively, when a high-performance flexible C-60-based organic field-effect transistor (OFET) was subjected to different bending radii. The observed almost twofold relative change in the electron mobility is considerably larger than that reported before for pentacene-based OFETs. Moreover, the strain dependency of electron mobility in C-60 films is strongly anisotropic with respect to the strain direction measured relative to the current flow. Analysis within a hopping-transport model for OFET mobility suggests that the observed strain dependency on electron transport is dominated mostly by the change of inter-grain coupling in polycrystalline C-60 films. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4747451
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