Conductivity Of Organic Semiconductors Research Articles

Doping-related broadening of the density of states governs integer-charge transfer in P3HT

Molecular p-doping allows for an increase in the conductivity of organic semiconductors, which is regularly exploited in thermoelectric devices. Upon doping, integer and fractional charge transfer have been identified as the two competing mechanisms to occur, where the former is desired to promote the generation of mobile holes in the semiconductor host. In general, high dopant electron affinity is expected to promote integer-charge transfer, while strong coupling between the frontier molecular orbitals of dopant and host promotes fractional charge transfer instead. Here, we investigate the role that the width of the density of states (DOS) plays in the doping process by doping the conjugated polymer poly(3-hexylthiophene) (P3HT) with tetracyanoquinodimethane (TCNQ) derivatives of different electron affinities at a 2% dopant ratio. Cyclic voltammetry confirms that only the electron affinity of F4TCNQ (tetrafluorotetracyanoquinodimethane) exceeds the ionization energy of P3HT, while TCNQ and FTCNQ (2-fluoro-7,7,8,8-tetracyanoquinodimethane) turn out to have significantly lower but essentially identical electron affinities. From infrared spectroscopy, we learn, however, that ca. 88% of FTCNQ is ionized while all of TCNQ is not. This translates into P3HT conductivities that are increased for F4TCNQ and FTCNQ doping, but surprisingly even reduced for TCNQ doping. To understand the remarkable discrepancy between TCNQ and FTCNQ, we calculated the percentage of ionized dopants and the hole densities in the P3HT matrix resulting from varied widths of the P3HT highest occupied molecular orbital (HOMO)-DOS via a semi-classical computational approach. We find that broadening of the DOS can yield the expected ionization percentages only if the dopants have significantly different tendencies to cause energetic disorder in the host matrix. In particular, while for TCNQ the doping behavior is well reproduced if the recently reported width of the P3HT HOMO-DOS is used, it must be broadened by almost one order of magnitude to comply with the ionization ratio determined for FTCNQ. Possible reasons for this discrepancy lie in the presence of a permanent dipole in FTCNQ, which highlights that electron affinities alone are not sufficient to define the strength of molecular dopants and their capability to perform integer-charge transfer with organic semiconductors.

Hybrid Light-Matter States in a Molecular and Material Science Perspective.

The notion that light and matter states can be hybridized the way s and p orbitals are mixed is a concept that is not familiar to most chemists and material scientists. Yet it has much potential for molecular and material sciences that is just beginning to be explored. For instance, it has already been demonstrated that the rate and yield of chemical reactions can be modified and that the conductivity of organic semiconductors and nonradiative energy transfer can be enhanced through the hybridization of electronic transitions. The hybridization is not limited to electronic transitions; it can be applied for instance to vibrational transitions to selectively perturb a given bond, opening new possibilities to change the chemical reactivity landscape and to use it as a tool in (bio)molecular science and spectroscopy. Such results are not only the consequence of the new eigenstates and energies generated by the hybridization. The hybrid light-matter states also have unusual properties: they can be delocalized over a very large number of molecules (up to ca. 105), and they become dispersive or momentum-sensitive. Importantly, the hybridization occurs even in the absence of light because it is the zero-point energies of the molecular and optical transitions that generate the new light-matter states. The present work is not a review but rather an Account from the author's point of view that first introduces the reader to the underlying concepts and details of the features of hybrid light-matter states. It is shown that light-matter hybridization is quite easy to achieve: all that is needed is to place molecules or a material in a resonant optical cavity (e.g., between two parallel mirrors) under the right conditions. For vibrational strong coupling, microfluidic IR cells can be used to study the consequences for chemistry in the liquid phase. Examples of modified properties are given to demonstrate the full potential for the molecular and material sciences. Finally an outlook of future directions for this emerging subject is given.

Conductivity in organic semiconductors hybridized with the vacuum field.

Much effort over the past decades has been focused on improving carrier mobility in organic thin-film transistors by optimizing the organization of the material or the device architecture. Here we take a different path to solving this problem, by injecting carriers into states that are hybridized to the vacuum electromagnetic field. To test this idea, organic semiconductors were strongly coupled to plasmonic modes to form coherent states that can extend over as many as 10(5) molecules and should thereby favour conductivity. Experiments show that indeed the current does increase by an order of magnitude at resonance in the coupled state, reflecting mostly a change in field-effect mobility. A theoretical quantum model confirms the delocalization of the wavefunctions of the hybridized states and its effect on the conductivity. Our findings illustrate the potential of engineering the vacuum electromagnetic environment to modify and to improve properties of materials.

Calculation of electronic coupling matrix elements in 4,4′-bis(9-carbazolyl)-biphenyl amorphous material

The electronic coupling matrix element responsible for positive charge carrier (hole) transport has been calculated for a variety of 4,4′-bis(9-carbazolyl)-biphenyl dimers corresponding to a disordered solid matrix. A procedure applying the computational scheme to a real system (organic molecules with a large molecular mass) is proposed. The consistency of the results with the existing model concept of hopping conductivity in organic semiconductors is discussed.

Protic Ionic Liquids as p-Dopant for Organic Hole Transporting Materials and Their Application in High Efficiency Hybrid Solar Cells

Chemical doping is a powerful method to improve the charge transport and to control the conductivity in organic semiconductors (OSs) for a wide range of electronic devices. We demonstrate protic ionic liquids (PILs) as effective p-dopant in both polymeric and small molecule OSs. In particular, we show that PILs promote single electron oxidation, which increases the hole concentration in the semiconducting film. The illustrated PIL-doping mechanism is compatible with materials processed by solution and is stable in air. We report the use of PIL-doping in hybrid solar cells based on triarylamine hole transporting materials, such as 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9'-spirobifluorene (spiro-OMeTAD). We show improved power conversion efficiency by replacing lithium salts, typical p-dopants for spiro-OMeTAD, with PILs. We use photovoltage-photocurrent decay and photoinduced absorption spectroscopy to establish that significantly improved device performance is mainly due to reduced charge transport resistance in the hole-transporting layer, as potentiated by PIL-doping.

Spin-dependent dynamics of polaron pairs in organic semiconductors

We present a theoretical investigation of the effect of spin manipulation of polaron pairs (PPs) on the conductivity of organic semiconductors. Control of the PP spin state is achieved using pulsed electron-spin resonance. We demonstrate that manipulation of PPs will result in changes in the free-polaron density in the material, with corresponding changes in the conductivity due to the contribution of PP dissociation to the free-carrier density. The time-dependent form of this conductivity change following spin resonant perturbation is determined, and the effect of a number of experimental variables investigated. We find that, under certain conditions, these time-dependent current measurements reveal the dynamics of PP intersystem crossing. We compare these predictions with previous experiments on organic light-emitting diodes made of poly[2-methoxy-5-(${2}^{\ensuremath{'}}$-ethyl-hexyloxy)-1,4-phenylene vinylene] and conclude that PP intersystem crossing times ${\ensuremath{\tau}}_{isc}$ in this material may exceed $10\text{ }\ensuremath{\mu}\text{s}$ at low temperatures.

Electric field-dependent charge transport in organic semiconductors

An analytical description is elaborated for the variable range hopping conduction mechanism in the presence of temperature and electric fields for quasi-three-dimensional organic semiconductor systems. In the proposed description, it is assumed that the localized states are randomly distributed in energy and space coordinates. The expression for the hopping conductivity is obtained for the Gaussian density of states. The model is applied to the analysis of both temperature and electric field-dependent hopping transport in organic semiconductors. It is shown that the Poole–Frenkel behavior is only valid in medium electric field regime. Moreover, we conclude that the electric field determines whether the temperature dependence of conductivity in organic semiconductors obeys the Arrhenius law.

Temperature and field-dependence of hopping conduction in organic semiconductors

Electrical characteristics of the hopping transport in organic semiconductors are studied theoretically. Based on percolation theory of hopping between localized states, an analytical mobility model is obtained. This model is applied to the analysis of both the electric field dependence and the temperature dependence of the mobility. The results agree quantitatively with recent experimental data.

Electric-field-induced charge injection or exhaustion in organic thin film transistor

The conductivity of organic semiconductors is measured in situ and continuously with a bottom contact configuration, as a function of film thickness at various gate voltages. The depletion layer thickness can be directly determined as a shift of the threshold thickness at which electric current began to flow. The in situ and continuous measurement can also determine qualitatively the accumulation layer thickness together with the distribution function of injected carriers. The accumulation layer thickness is a few monolayers and it does not depend on gate voltages. Rather it depends on the chemical species.

Relation between the activation energy of the electrical conduction in organic semiconductors and their first excited singlet state energies

An examination of the darkconductivity of seven substituted anthraquinones show that these compounds are high resistance semiconductors. There is a significant increase in the conductivity of the compounds by the introduction of polar groups to the anthraquinone molecule. There exists a correspondence between the thermal activation energy of the darkconductivity (ΔE D) and the energy of the first excited singlet state (1 E). The search for a correlation between the thermal activation energy of the darkconductivity and the energy of the triplet state (3 E) was unsuccessful.

Electronic Structure and Conductivity of Organic Semiconductors

Electronic Structure and Conductivity of Organic Semiconductors

On the Meyer-Neldel rule in the electrical conductivity of organic semiconductors

On the Meyer-Neldel rule in the electrical conductivity of organic semiconductors

The contact and bulk conductivity in organic semiconductors

The contact and bulk conductivity in organic semiconductors

On the interpretation of thermal activation energies for conduction in organic semiconductors

Different thermal activation energies of dark conduction in the ohmic and square-law regions have been measured for anthracene. The ohmic activation energy has been interpreted in terms of localised dominant exponentially distributed hole and dominant electron levels within the band-gap. The square-law activation energy is attributed to shallow trapping.

Über den Leitungsmechanismus des organischen Festkörpers

AbstractA new model for electrical conduction in organic semiconductors is given. Conclusions from the model are compared with experimental results.

Some problems concerning electrical conductivity of organic semiconductors

Some problems concerning electrical conductivity of organic semiconductors

Electrical conduction in the phthalocyanines. I. Optical properties

The polarized crystal and vapour spectra of metal-free, nickel, copper, zinc, chromium, cobalt, and iron phthalocyanines are reported and the results compared with published data. Comparison between these optical data and activation energies for dark conduction raises doubts as to the ability of the "singlet" or "triplet" theories to explain satisfactorily electrical conduction in organic semiconductors.