I. I. Fishchuk

Strain induced anisotropic effect on electron mobility in C60 based organic field effect transistors

The electron mobility was found to increase (decrease) upon applied compressive (tensile) strain, respectively, when a high-performance flexible C60-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 C60 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 C60 films.

Dependence of Meyer–Neldel energy on energetic disorder in organic field effect transistors

Meyer–Neldel rule for charge carrier mobility was studied in C60-based organic field effect transistors (OFETs) fabricated at different growth conditions which changed the degree of disorder in the films. The energetic disorder in the films was found to correlate with a shift in the Meyer–Neldel energy, which is in excellent agreement with the predictions of a hopping-transport model for the temperature dependent OFET mobility in organic semiconductors with a Gaussian density-of-states (DOS). Using this model the width of the DOS was evaluated and it was found to decrease from 88 meV for the films grown at room temperature to 54 meV for films grown at 250 °C.

Effect of source-drain electric field on the Meyer–Neldel energy in organic field effect transistors

We studied the influence of the lateral source-drain electric field on the Meyer–Neldel phenomenon observed for the charge mobility measured in C60-based organic field effect transistors (OFETs). It was found that the characteristic Meyer-Neldel temperature notably shifts with applied source drain electric field. This finding is in excellent agreement with an analytic model recently extended to account also for the field dependence of the charge carrier mobility in materials with a Gaussian density-of-states distribution. As the theoretical model to predict charge carrier mobility is not limited to zero-electric field, it provides a more accurate evaluation of energetic disorder parameters from experimental data measured at arbitrary electric fields.

Charge-carrier and polaron hopping mobility in disordered organic solids: Carrier-concentration and electric-field effects

The effective-medium approximation (EMA) analytical theory is advanced further to describe charge transport at arbitrary charge-carrier concentration in a disordered organic material with superimposed polaron effects. A key point of this model compared to the previous treatment [Phys. Rev. B 76 (2007) 045210] is that it is formulated for arbitrary electric fields and is able to describe consistently both the carrier-concentration and field dependences of charge mobility. The mobilities of both bare charge carriers and polarons were calculated using the Miller–Abrahams and polaron jump rate models, respectively. An excellent quantitative agreement was obtained between the theoretical calculations and the recent numerical simulations of the field- and carrier-density dependences of the mobility for bare charge carriers using the same parameters. The polaronic carrier density effect was also calculated using the complete Marcus jump rate equation and straightforward EMA configurational averaging, and the results compared to that obtained with the use of the symmetrical jump rate model and the effective transport energy concept. This study confirms that a strong dependence of carrier mobility upon increasing carrier density and electric field, which has conventionally been observed in experiment for numerous organic semiconducting materials, is incompatible with the notion of large polaron binding energy in these materials, implying that the energetic disorder plays a dominant role.

Effective Medium Approximation Theory Description of Charge-Carrier Transport in Organic Field-Effect Transistors

In spite of a large amount of work having been done on the description of the charge-carrier transport in organic materials for last decades, the processes that determine charge transport in real organic electronic devices are still not completely understood, but their comprehension is definitely the key for designing materials with improved properties and, thereby, for a further increase in the performance of the devices. In this review, we will present an overview of the current achievements regarding theoretical description of the charge transport in disordered organic semiconductors with emphasis to charge transport behaviors at large carrier concentrations as realized in organic field-effect transistors (OFETs). A particular focus is given to the Effective Medium Approximation (EMA) analytical method, which was applied to describe the carrier-concentration-, electric-field- and temperature-dependent charge transport in organic materials that are used as active layers in OFET devices. In particular, we show that the establishment of the Meyer-Neldel rule (MNR) is a characteristic signature of hopping charge transport in a random system with variable carrier concentration irrespective of their polaronic character. The EMA model provides compact analytical relations which can be readily used for the evaluation of the energetic disorder parameter in organic semiconductor layers from experimentally accessible data on temperature dependent mobility in the OFET devices. The EMA theory is found to be in good agreement with previous computer simulations results and has been applied to describe recent experimental measurements of the temperature dependent electron mobility in a C60-based OFET for different carrier concentrations and different lateral (source-drain) electric fields. Finally, we compare our theory with alternative models suggested before to explain the MNR behavior for the charge transport in organic semiconductors.

Analytic Model of Hopping Transport in Organic Semiconductors Including Both Energetic Disorder and Polaronic Contributions

We developed an analytical model to describe hopping conductivity and mobility in organic semiconductors including both energetic disorder and polaronic contributions. The model is based on the Marcus jump rates with a Gaussian energetic disorder, and it is premised upon a generalized Effective Medium approach yet avoids shortcoming involved in the effective transport energy or percolation concepts. The carrier concentration dependence becomes considerably weaker when the polaron energy increases relative to the disorder energy, indicating the absence of universality that is at variance with recent publications.

Origin of Electric Field Dependence of the Charge Mobility and Spatial Energy Correlations in C60-Based Field Effect Transistors

We report on the influence of the lateral electric field on the charge mobility in organic field-effect transistors (OFET) based on C60 films with multigrain morphology. The experimental data were quantitatively described using a recent analytical model by accounting for the strong local electric fields in a multigrain transistor channel and for the energy correlation effects. To rationalize the presence of a correlated disorder in a non-polar C60 material, we show that randomly oriented permanent dipoles in organic gate dielectric layers can generate a significant dipolar disorder in an adjacent nonpolar semiconductor layer.

Electric field dependence of charge-carrier hopping transport at large carrier concentrations in disordered organic solids: Meyer-Neldel and Gill energies

Effective medium approach has been extended to describe the temperature dependent hopping charge-carrier mobility at arbitrary electric fields in the large carrier density transport regime. We take into account the spatial energy correlations in organic materials with Gaussian disorder. The theory is applied to describe recent experimental measurements of the electron transport properties in a C60-based OFET for different lateral electric fields FDS. Since this model is not limited to zero-field mobility, it allows a more accurate evaluation of important material parameters from experimental data measured at a given electric field. The shift of the Meyer-Neldel energy EMN upon applied lateral electric field FDS and the Gill energy EG upon the gate voltage VG in an OFET is shown to be a consequence of the spatial energy correlation effects in the organic semiconductor film. We showed that both the Meyer-Neldel and Gill energies can be used for estimating the width of the Gaussian density-of-states distribution.

Does the Temperature Dependence of the Charge Carrier Mobility in Disordered Organic Semiconductors at Large Carrier Concentrations Obey the Meyer–Neldel Compensation Law?

The temperature-activated charge transport in disordered organic semiconductors at large carrier concentrations has been thoroughly considered, by using a recent analytical model [Phys.Rev.B 76, 045210 (2007)] assuming a Gaussian density-of-states (DOS) distribution and Miller–Abrahams jump rates. We demonstrate that the apparent Meyer–Neldel compensation rule is recovered with regard for the temperature dependences of the charge carrier mobility upon varying the carrier concentration, but not for varying the DOS distribution width. We show that this phenomenon is entirely due to the evolution of the occupational DOS distribution as a function of the state filling. Predictions of the model are in a quantitative agreement with available experimental results.

Charge Carrier Transport in Disordered Organic Materials in the Presence of Traps

ABSTRACT An effective-medium theory has been developed to describe the nondispersive charge carrier transport in a disordered organic material containing extrinsic traps. The results of calculations are compared to predictions of the Hoesterey and Letson formalism, which has been widely used before to describe trapping. We argue that our theory describes more adequately charge transport in the presence of traps since it accounts for the effects of disorder. Also it was found that both the relaxation of the ensemble of majority charge carriers within the combined intrinsic and extrinsic density of state distribution and the occurrence of trap-to-trap migration alters the temperature dependence of the charge mobility significantly, notably at lower temperature.