Transport Properties Of Organic Semiconductors Research Articles

Impact of the halogenated substituent on electronic and charge transport properties of organic semiconductors: A theoretical study

The electronic and charge transport properties of the derivatives based on tetracene with aryl and halogenated aryl substituents have been investigated theoretically. This kind of functionalization is demonstrated to have a significant effect to stabilize the molecular orbital, densify the molecular packing, enhance the electronic coupling, and further lead to a high mobility, though it would also cause some increases in the reorganization energy. The packing modes of FPPT and PPT crystals are analyzed in details and effective coupling projected areas are put forward to understand the intermolecular interactions. Interestingly, FPPT is found to have a well-ordered packing as well as the improved hole mobility of 4.67cm2V−1s−1. In addition, the contributions of different frequencies of vibration to the total reorganization energies are also discussed with the normal mode analysis. This study clarifies the halogenated substituent effect on transport properties and provides the guide for molecular design of novel functional materials.

Characterization of transport properties of organic semiconductors using impedance spectroscopy

Methods for the determination of drift mobility, localized-state distribution and deep trapping lifetime in organic semiconductors using impedance spectroscopy are reviewed. The theoretical basis is single-injection space-charge-limited current under small sinusoidal voltage perturbation. Major advantages of impedance spectroscopy are: full automatic measurements and simultaneous measurements of these physical quantities. Information on these physical quantities are essential for the understanding of transport properties in organic semiconductors and for the design of organic devices such as organic light-emitting diodes and organic solar cells using a device simulator. The determination of drift mobility, localized-state distribution and deep trapping lifetime from impedance spectra is demonstrated in a molecularly doped polymer. A molecularly doped polymer is a prototypical organic semiconductor and is a good example for the demonstration of simultaneous determination of these quantities. The methods presented here are applicable to insulating semiconductors and thus to inorganic disordered semiconductors such as hydrogenated amorphous silicon and amorphous oxide semiconductors, as well.

Numerical modeling of current-voltage characteristics to extract transport properties of organic semiconductors

The current vs. voltage (I-V) characteristics of single crystal rubrene Organic Field-Effect Transistors (OFETs) and polycrystalline poly(p-phenylenevinylene) (PPV) films are modeled using the polaron transport theory presented in a previous work [A. F. Basile et al., J. Appl. Phys. 115, 244505 (2014)]. The model is first applied to rubrene OFETs, where transport is two-dimensional and is confined near the interface between the insulator and the organic semiconductor. By considering the effect of image charges in the insulator and by assuming a constant intrinsic mobility, we reproduce both the positive and the negative temperature dependences of the channel mobilities measured on OFETs having a gate dielectric and an air-gap insulator, respectively. In addition, we adapt this model to the three-dimensional transport in PPV films, characterized by effective mobilities which depend on temperature, charge density, and electric field. We show that the I-V characteristics of these materials can be matched by the numerical solution of the Poisson and drift-diffusion equations assuming a constant intrinsic mobility. The polaron binding energy can account for the thermally activated behavior of the I-V characteristics and for the increase of the effective mobility at high applied voltages. Therefore, this model enables to extract the intrinsic transport parameters of organic semiconductors, independent of the device structure, and of the measurement conditions.

Trap healing and ultralow-noise Hall effect at the surface of organic semiconductors

Fundamental studies of intrinsic charge transport properties of organic semiconductors are often hindered by charge traps associated with static disorder present even in optimized single-crystal devices. Here, we report a method of surface functionalization using an inert non-conjugated polymer, perfluoropolyether (PFPE), deposited at the surface of organic molecular crystals, which results in accumulation of mobile holes and a 'trap healing' effect at the crystal/PFPE interface. As a consequence, a remarkable ultralow-noise, trp-free conduction regime characterized by intrinsic mobility and transport anisotropy emerges in organic single crystals, and Hall effect measurements with an unprecedented signal-to-noise ratio are demonstrated. This general method to convert trap-dominated organic semiconductors to intrinsic systems may enable the determination of intrinsic transport parameters with high accuracy and make Hall effect measurements in molecular crystals ubiquitous.

The Influence of the Vinyl Terminal Group on the Poly(Para-Phenylenevinylene) Charge Transfer Integrals

The charge transport properties of organic semiconductors are one of the foremost limiting factors in technological applications of these materials, which are becoming important competitors with respect to the inorganic semiconductors. In fact, conjugated organic molecules are used at present as active materials in different types of devices. For this reason, the theoretical study of the electron and hole mobility, carried out in order to give hints for the design of new molecules or for the optimization of their supramolecular organization, is a task of great interest. Here, we present the results of a quantum chemical study, in the framework of the Marcus and density functional theories, on the effects of terminal groups (when they directly interact with the pi-conjugated system of the organic semiconductors) on the charge carriers mobility of organic semiconductors. In particular, using a representative oligomer of poly(para-phenylenevinylene) as a model system, we have found that strong effects on the predicted values of the intramolecular transfer integrals as well as on their dependence on the supramolecular organizations occur, when the vinyl moiety (as ending group) is taken into account.

Progress in organic single-crystal field-effect transistors

Abstract

Interplay between electron-phonon coupling and disorder strength on the transport properties of organic semiconductors

The combined effect of bulk and interface electron-phonon couplings on the transport properties is investigated in a model for organic semiconductors gated with polarizable dielectrics. While the bulk electron-phonon interaction affects the behavior of mobility in the coherent regime below room temperature, the interface coupling is dominant for the activated high $T$ contribution of localized polarons. In order to improve the description of the transport properties, the presence of disorder is needed in addition to electron-phonon couplings. The effects of a weak disorder largely enhance the activation energies of mobility and induce the small polaron formation at lower values of electron-phonon couplings in the experimentally relevant window $150 K<T<300 K$. The results are discussed in connection with experimental data of rubrene organic field-effect transistors.

Study of the transport properties of organic semiconductors based on europium diphthalocyanine and bi-tris-phthalocyanine complexes with ortho-bis(oxymethyl)phenyl bridge and based on erbium and europium dinaphthalocyanine complexes

The transport properties of organic semiconductors based on europium diphthalocyanine and bitris-phthalocyanine complexes with ortho-bis(oxymethyl)phenyl bridge and based on europium and erbium dinaphthalocyanine are studied. The temperature dependences of the dc conductivity for all types of the structures under study are obtained; it is shown that all dependences include two activation portions. For high-temperature portions, the activation energies are determined as 0.85 eV for europium diphthalocyanine with the ortho-bis(oxymethyl)phenyl bridge, 1.135 eV for europium bi-tris-phthalocyanine with the orthobis(oxymethyl)phenyl bridge, 0.98 eV for europium dinaphthalocyanine, and 1.18 eV for erbium dinaphthalocyanine. For the low-temperature activation portion, it is shown that lanthanide ions and their bond with a ligand make the dominant contribution to the conductivity of the structures under study.

Modeling of charge transport in polycrystalline sexithiophene from quantum charge transfer rate theory beyond the first-order perturbation

Charge mobility in polycrystalline organic semiconductors is often thermally activated, so a semiclassical Marcus charge transfer rate theory has long been used to investigate the charge transport properties of organic semiconductors. However, the classical treatment for the nuclear degrees of freedom and the first-order perturbative nature of electronic coupling in the semiclassical Marcus charge transfer rate theory is often invalid in organic semiconductors. Furthermore, traps in polycrystalline organic semiconductors are not considered during the simulations with the semiclassical Marcus charge transfer rate theory. In the present work, we propose a model to study charge transport properties in polycrystalline organic semiconductors which consist of trap-free crystallitic grains separated by boundaries between them. The charge transfer rate in grains is evaluated with a quantum charge transfer rate theory without weak electronic coupling approximation while the charge transport at grain boundaries is limited by energy barriers there. We find that a thermally activated mobility can be obtained from the quantum charge transfer rate theory when traps at grain boundaries are considered. Meanwhile, a roughly linear dependence of mobility on grain size is shown for large grain size while a rapid variation of mobility with grain size is observed when the grain size is small, which reconciles the discrepancy of the mobility versus grain size in experiments. In addition, the different mobilities for sexithiophene crystal structures in high- and low-temperature phases show that the mobility in polycrystalline organic semiconductors not only depends on the boundary property between grains but also the molecular packing in grains.

Ultrathin organic single crystals: fabrication, field-effect transistors and thickness dependence of charge carrier mobility

Organic single crystals with thickness ranging from a few monolayers to micrometres were fabricated by an “Organic Crystal Cleavage” method. The mobility slightly increased with decreased thickness and rose sharply when the crystal thickness was below some critical thickness. The gate induced charges in the field-effect transistor (FET) channel and in the vicinity of the metal–semiconductor interface reducing the contact barrier. The values of mobility measured on very thin crystals without the contact barrier precisely reflect the transport properties of organic semiconductors.

Physicochemical, self-assembly and field-effect transistor properties of anti- and syn- thienoacene isomers

The relationships between the molecular structure, self-assembly and charge transport properties of two fused-ring thienoacene isomers, a sickle-like syn isomer and a linear anti isomer, were studied from both an experimental and a theoretical perspective. Under the same self-assembly conditions, the syn isomer formed one-dimensional (1D) micro- and nanoribbons, while the anti isomer formed two-dimensional (2D) nanoplates (all were single crystals). The differences were assigned to the change in intermolecular interactions due to the tiny tuning of the molecular structures. The field-effect mobility in the single crystals of the syn isomer was about one-third as high as that of the anti isomer. The different transfer integrals of the isomers explained the observed different mobilities. This study opens a vivid window to examine the self-assembly and charge transport properties of organic semiconductors by using configurational isomers.

Influences of molecular packing on the charge mobility of organic semiconductors: from quantum charge transfer rate theory beyond the first-order perturbation

The electronic coupling between adjacent molecules is an important parameter for the charge transport properties of organic semiconductors. In a previous paper, a semiclassical generalized nonadiabatic transition state theory was used to investigate the nonperturbative effect of the electronic coupling on the charge transport properties, but it is not applicable at low temperatures due to the presence of high-frequency modes from the intramolecular conjugated carbon-carbon stretching vibrations [G. J. Nan et al., J. Chem. Phys., 2009, 130, 024704]. In the present paper, we apply a quantum charge transfer rate formula based on the imaginary-time flux-flux correlation function without the weak electronic coupling approximation. The imaginary-time flux-flux correlation function is then expressed in terms of the vibrational-mode path average and is evaluated by the path integral approach. All parameters are computed by quantum chemical approaches, and the mobility is obtained by kinetic Monte-Carlo simulation. We evaluate the intra-layer mobility of sexithiophene crystal structures in high- and low-temperature phases for a wide range of temperatures. In the case of strong coupling, the quantum charge transfer rates were found to be significantly smaller than those calculated using the weak electronic coupling approximation, which leads to reduced mobility especially at low temperatures. As a consequence, the mobility becomes less dependent on temperature when the molecular packing leads to strong electronic coupling in some charge transport directions. The temperature-independent charge mobility in organic thin-film transistors from experimental measurements may be explained from the present model with the grain boundaries considered. In addition, we point out that the widely used Marcus equation is invalid in calculating charge carrier transfer rates in sexithiophene crystals.

Charge transfer rates in organic semiconductors beyond first-order perturbation: From weak to strong coupling regimes

Semiclassical Marcus electron transfer theory is often employed to investigate the charge transport properties of organic semiconductors. However, quite often the electronic couplings vary several orders of magnitude in organic crystals, which goes beyond the application scope of semiclassical Marcus theory with the first-order perturbative nature. In this work, we employ a generalized nonadiabatic transition state theory (GNTST) [Zhao et al., J. Phys. Chem. A 110, 8204 (2004)], which can evaluate the charge transfer rates from weak to strong couplings, to study charge transport properties in prototypical organic semiconductors: quaterthiophene and sexithiophene single crystals. By comparing with GNTST results, we find that the semiclassical Marcus theory is valid for the case of the coupling <10 meV for quaterthiophene and <5 meV for sexithiophene. It is shown that the present approach can be applied to design organic semiconductors with general electronic coupling terms. Taking oligothiophenes as examples, we find that our GNTST-calculated hole mobility is about three times as large as that from the semiclassical Marcus theory. The difference arises from the quantum nuclear tunneling and the nonperturbative effects.

Cooperative CH···π Interactions in the Crystal Structure of 2,5-Di(3-biphenyl)-1,1-dimethyl-3,4-diphenyl-silole and Its Effect on Its Electronic Properties

In order to develop a fundamental understanding for the effect of molecular packing on the carrier transport properties of organic semiconductors, single crystals of 2,5-di-(3-biphenyl)-1,1-dimethy...

Numerical simulation of impedance and admittance of OLEDs

AbstractIn this issue's Editor's Choice [1], the electrical characteristics of organic light‐emitting diodes (OLEDs) are calculated for the dc and ac regimes by numerically solving the basic semiconductor equations. The cover picture is taken from a diagram showing the steady‐state electron concentration n0 and the hole concentration p0 for an applied voltage V0 = 3 V as well as the real part of the electron amplitude $ \tilde n $ and the hole amplitude $ \tilde p $ for frequencies of 1 Hz and 105 Hz. α‐NPD and Alq3 molecules being used as active hole‐ and electron‐transporting materials, respectively, are inserted.The first author is working as a postdoctoral fellow for an European project on OLEDs. His current research interests are the transport properties of organic semiconductors, where the conduction mechanisms and the role of traps are studied.This issue also contains a second Editor's Choice entitled “AlGaN/AlN distributed bragg reflectors for deep ultraviolet wavelengths” by Craig G. Moe et al. [2] as well as the Feature Article “p‐type transparent conducting oxides” by Su Sheng, Guojia Fang et al. [3].

Anomalous room temperature magnetoresistance in organic semiconductors

We report the substantial change in the large room temperature (∼8% at 100 Oe, up to 15% at 1000 Oe) magnetoresistance of thin organic semiconductor films of tris-(8-hydroxyquinoline) aluminum (Alq 3) upon doping with PtOEP and Ir(ppy) 3 complexes. The origin of magnetic field effects on charge transport properties of organic semiconductors until now has remained obscure. We propose a model for the anomalous magnetoresistance and its change with doping based on the charge transport in these semiconductors being electron–hole (e–h) recombination limited. The process of e–h recombination includes formation of correlated e–h pairs and the subsequent annihilation of e–h pairs with different rates for the singlet and triplet spin states. The e–h pairs may also dissociate back into free charge carriers. We suggest that a magnetic field controls spin interconversion of e–h pairs. In the absence of field the singlet mixes with the entire triplet manifold by hyperfine interaction. The magnetic field lifts the triplet degeneracy, and for strong field, the mixing remains only between the singlet and the T 0 component of the triplet, thus changing the e–h recombination rate and hence the current. The experimental results are consistent with the model.

Electrical characterization of organic semiconductors by transient current methods

This paper presents a method for characterizing the transport properties of organic semiconductors by means of large-signal electrical transients. For validating the method, poly(phenylene-vinylene) (PPV) was chosen as a test material, thanks to its widely investigated properties. Large voltage steps are applied to Au/PPV/Au test structures and, from the resulting current transient, the carrier mobility is measured. Transient phenomena are shown to give insight on the physical properties of the material and on the technological characteristics of the device, while requiring a simpler test system in comparison to other methods, like impedance spectroscopy and time-of-flight measurements.

Transport Properties of Organic Semiconductors

Transport Properties of Organic Semiconductors