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

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

Polarons in wide-band-gap molecular materials: Polysilanes

Ozáření polysilanů vede k formování volných nosišů náboje. Vlivem nábojově- fononové interakci a molekulární deformaci jsou vytvářeny polarony. Jejich existence a vznik může být detekována termostimulovanou luminiscencí (TSL). Vazební energie polaronu Ep může být stanovena fitováním experimentálníích závislostí pohyblivostí nosičů na elektrickém poli při různých teplotách. Hodnota Ep pro poly[methyl(phenyl)silylene] filmy byla zjištěna 0,29 eV.Illumination of polysilanes leads to the formation of free charge carriers. Due to the charge-phonon coupling and molecule deformation, polarons are formed. Their occurrence can be detected by thermostimulated luminescence (TSL). Polaron binding energy E-p can be determined by fitting experimental dependences of charge carrier mobility vs. applied electric field measured at different temperatures. E-p for poly[methyl(phenyl)silylene] films was found to be 0.29 eV. (c) 2007 Elsevier B.V. All rights reserved

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

Charge-carrier mobility has been investigated by time-of-flight (TOF) transient photocurrent in a lateral transport con- figuration in highly crystalline thin films of 2,7-dioctyl[1]benzothieno [3,2-b][1] benzothiophene (C8-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 ≥ 295 K. Moreover, the mobility was surprisingly found to decrease with electric field in the high-temperature region. These observations are not compat- ible 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

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

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

Conduction mechanism in amorphous InGaZnO thin film transistors

We validate a model which is a combination of multiple trapping and release and percolation model for describing the conduction mechanism in amorphous indium gallium zinc oxide (a-IGZO) thin film transistors (TFT). We show that using just multiple trapping and release or percolation model is insufficient to explain TFT behavior as a function of temperature. We also show the intrinsic mobility is dependent on temperature due to scattering by ionic impurities or lattice. In solving the Poisson equation to find the surface potential and back potential as a function of gate voltage, we explicitly allow for the back surface to be floating, as is the case for a-IGZO transistors. The parameters for gap states, percolation barriers and intrinsic mobility at room temperature that we extract with this comprehensive model are in good agreement with those extracted in literature with partially-complete models.journal_title: Japanese Journal of Applied Physics article_type: paper article_title: Conduction mechanism in amorphous InGaZnO thin film transistors copyright_information: © 2016 The Japan Society of Applied Physics date_received: 2015-05-28 date_accepted: 2015-10-13 date_epub: 2015-12-04status: publishe

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

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

© 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