Electric Field Dependence Of Mobility Research Articles

Effects of Gaussian disorder on charge carrier transport and recombination in organic semiconductors

AbstractIn this review, we discuss recent advances in our understanding of charge transport and exciton generation in disordered organic semiconductors with a Gaussian DOS, with a focus on applications to organic light‐emitting diodes (OLEDs). Three‐dimensional (3D) modeling shows that the actual current density in OLEDs based on materials with a Gaussian electron and hole DOS is filamentary. However, it is possible to accurately calculate the average current density by solving a one‐dimensional (1D) drift‐diffusion equation, making use of compact expressions for the temperature, electric field, and carrier density dependent mobility which have been derived from 3D‐modeling. For the cases of spatially uncorrelated energetic disorder and spatially correlated disorder due to random dipole fields, these models are called the extended Gaussian disorder model (EGDM) and extended correlated disorder model (ECDM), respectively. We discuss how the effects of trapping on guest molecules can be included, and how exciton generation is described. The application of these models to hole and electron transporting polymer and small molecule materials is discussed, with an emphasis on the modeling of the transport and emission of blue‐emitting OLEDs based on a polyfluorene‐derivative.

Characterization of the charge transport and electrical properties in solution-processed semiconducting polymers

The conventional charge transport models based on density- and field-dependent mobility, only having a non-Arrhenius temperature dependence, cannot give good current-voltage characteristics of poly (2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) hole-only devices. In this paper, we demonstrate that the current-voltage characteristics can give a good unified description of the temperature, carrier density and electric field dependence of mobility based on both the Arrhenius temperature dependence and the non-Arrhenius temperature dependence. Furthermore, we perform a systematic study of charge transport and electrical properties for MEH-PPV. It is shown that the boundary carrier density has an important effect on the current-voltage characteristics. Too large or too small values of boundary carrier density will lead to incorrect current-voltage characteristics. The numerically calculated carrier density is a decreasing function of the distance to the interface, and the numerically calculated electric field is an increasing function of the distance. Both the maximum of carrier density and the minimum of electric field appear near the interface.

Polaron effects and electric field dependence of the charge carrier mobility in conjugated polymers

Charge transport in conjugated polymers has been investigated using Monte Carlo simulations implemented on top of the Marcus theory for donor-acceptor transition rates. In particular, polaron effects and the dependency of the mobility on the temperature and the applied electric field have been studied. The conclusions are that while the qualitative temperature dependence is similar to that predicted by Miller-Abrahams theory in the Gaussian disorder model (GDM), the electric field dependence is characterized by a crossover into the Marcus inverted region, not present in the GDM. Furthermore, available analytical approximations to describe the electric field dependence of the mobility in Marcus theory fail to fit the simulation data and hence cannot be used to directly draw conclusions about the importance of polaron effects for charge transport in conjugated polymers.

Temperature and electric field dependent hole mobility in a polyfluorene copolymer

Transport of holes in thin films of a low-bandgap alternating polyfluorene copolymer, APFO-Green5, was investigated by means of admittance spectroscopy as a function of field and temperature. The values of hole mobility were evaluated from the position of the maxima in the plots of the negative differential susceptance as a function of frequency. Hole mobility was found to be strongly field- and temperature-dependent. The charge transport parameters were extracted by analyzing the mobility data by the uncorrelated and the correlated Gaussian Disorder Models.

Multiple steady state current–voltage characteristics in drift–diffusion modelisation of N type and semi-insulating GaAs Gunn structures

Theoretical and numerical investigations of carriers transport in N–Semi-Insulating (SI)–N and P–SI–P diodes is extended to the case of extrinsic (N type) or SI samples with Gunn like electric field dependent mobilities. The results obtained in a preceding publication [1] are valid as long as the bulk electric field does not increase above a threshold field E th associated with the beginning of negative electron differential mobility values: μ n,diff = ( dv n / dE) < 0, v n being the electron drift velocity. Convergence and stability problems occur only, for the steady state numerical simulation, in long N +–N–N + or N +–SI(N −)–N + diodes. SI(N −) characterizes a SI layer which keeps, under applied bias, a free electron concentration close to its thermal equilibrium value up to the beginning of electron space charge injection. A systematic study has been made by varying the contact boundary properties: flat band, metallic, N + or P +; the length of the sample and the electric parameters of the deep compensating trap of the SI layers. We show that these steady state numerical instabilities are related to the existence of multiple current–voltage solutions when numerical modelisation is made using the drift–diffusion model.

Mobility with negative coefficient in Poole–Frenkel field dependence in conjugated polymers: Role of injected hot electrons

In this work we study the electric field dependence of mobility extracted from Electroluminescence Transient (ELT) measurements in both single carrier and double carrier Polymer Light emitting Diode (PLED) configurations by varying the cathode. We have chosen two different families of conjugated light emitting polymers (structurally different hole transporting materials). The Poole–Frenkel (P–F) plot of mobility μ as a function electric field F ( log μ ∝ F ) shows negative slope ( β < 0) in the case of double carrier injection, while positive slope is observed as expected in the case of dominantly single carrier injection. The mobility values and their corresponding field dependent parameters for both the carriers are derived. It is shown that electrons injected to higher lying transport states can have high mobilities behaving as hot electrons. We propose a phenomenological model based on alignment of Gaussian local density of states (DOS) to explain the occurrence of reduction of mobility with increasing electric field for electrons, which do not fully relax into equilibrium distribution within the DOS in the timescales of transit time.

Temperature and electric field dependent mobility in poly(3-hexylthiophene) diodes

Current-voltage (I-V) curves of poly(3-hexylthiophene) (P3HT) diodes have been collected to investigate the polymer hole-dominated charge transport. At room temperature and at low electric fields the I-V characteristic is purely Ohmic whereas at medium-high electric fields, experimental data shows that the hole transport is trap dominated in the space charge limited current (SCLC) regime. In this regime, it is possible to extract the I-V characteristic of the P3HT/Al junction showing the ideal Schottky diode behavior over five orders of magnitude. At high-applied electric fields, holes’ transport is found to be in the trap free SCLC regime. We have measured and modeled in this regime the holes’ mobility to evaluate its dependence from the electric field applied and the temperature of the device.

Analysis of charge-injection characteristics at electrode-organic interfaces: Case study of transition-metal oxides

The formation of resistance-free or Ohmic contacts at metal/organic interfaces remains a significant challenge for achieving high-performance organic electronic devices such as organic light-emitting diodes. Several oxides have recently been reported to yield extremely low-voltage devices and thus have excited a renewed interest in developing the next generation of contacting electrodes. In this paper, major metal oxides, CuO, ${\text{Cu}}_{2}\text{O}$, ${\text{Ni}}_{2}{\text{O}}_{3}$, ${\text{Co}}_{3}{\text{O}}_{4}$, ${\text{WO}}_{3}$, ${\text{MoO}}_{3}$, ${\text{V}}_{2}{\text{O}}_{5}$, and indium tin oxide, have been systematically studied to compare their relative performance as hole injection anodes, as well as to provide an experimental database for theoretical analysis of current-voltage $(IV)$ characteristics with a diverse range of injection barrier heights. Contrary to previous reports in the literature, none of the oxides studied in this work were found to form a true Ohmic contact with commonly used hole transport layers, such as N,N-diphenyl-N, N-bis-1-naphthyl-1--1-biphenyl-4,4-diamine ($\ensuremath{\alpha}$-NPD). This discrepancy is attributed to incorrect $IV$ data analysis of the quasi-Ohmic injection regime---the region in between space-charge limited current (SCLC) and injection limited current (ILC)---in previous studies. It is found that the quasi-Ohmic regime is much larger (i.e., covers a greater range of injection barrier height) than has previously been expected. A criterion that defines Ohmic, quasi-Ohmic, and injection limited contacts has been quantified based on a time-domain simulation of charge transport across $\ensuremath{\alpha}$-NPD single-carrier devices. This criterion includes the effects of the electric field dependent mobility, organic layer thickness, and charge-injection barrier height. The effects of the built-in potential on the $IV$ characteristics are also evaluated. A barrier-thickness-voltage ``phase'' diagram that defines the regions of SCLC, quasi-Ohmic, and ILC for $\ensuremath{\alpha}$-NPD is presented.

Negative electric field dependence of mobility in TPD doped Polystyrene

A total negative field dependence of hole mobility down to low temperature was observed in N, N′-diphenyl- N, N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) doped in Polystyrene. The observed field dependence of mobility is explained on the basis of low values of energetic and positional disorder present in the sample. The low value of disorder is attributed to different morphology of the sample due to aggregation of TPD. Monte Carlo simulations were also performed to understand the influence of aggregates on charge transport in disordered medium with correlated site energies. The simulation supports our experimental observations and justification on the basis of low values of disorder parameters.

Thickness dependence of space charge limited current and injection limited current in organic molecular semiconductors

We report the experimental investigations on space charge limited current (SCLC) and injection limited current (ILC) in copper phthalocyanine (CuPc), sandwiched between two metal electrodes. Thickness dependence of current-voltage characteristics of SCLC and ILC is accurately reproduced by the electric field and temperature dependent charge carrier mobility, without invoking charge density dependent mobility. These results are interpreted using a consistent description of SCLC and ILC, based on a unified model of hopping transport within Gaussian density of states in CuPc.

Electric field and temperature dependence of the hole mobility in a bis-fluorene cored dendrimer

Time-of-flight mobility measurements on a first generation, bis(fluorene)-cored dendrimer are reported. A charge generation layer was used enabling measurements to be performed on spin-coated films, comparable to those used in devices such as organic light-emitting diodes. The results are compared with spin-coated polyfluorene films. The temperature and electric field dependence of the mobility of the dendrimer was studied and found to be in excellent agreement with the Gaussian disorder model, with an energetic disorder parameter, σ, of 74 meV and a positional disorder parameter, Σ, of 2.6.

Ambipolar charge transport in bulk heterojunction of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene)∕C60 composite

Using the time-of-flight technique (TOF), the influence of C60 concentrations on ambipolar charge transport in poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV)∕C60 composite has been investigated. Adding C60 to the blend increases both electron and hole mobilities, compared to the pristine polymer. In the pristine MEH-PPV, the mobility of electrons is below the measurable range. However, electron mobility (10−7 cm2 V−1 s−1) could be calculated when the weight ratio of C60: MEH-PPV increased to 1:18. The temperature and electric field dependence of mobility is analyzed in the disorder formalism. It is observed that above ∼290 K the slope of the electric field dependence of the electron mobility becomes negative. The occurrence of negative field dependence in composite samples is attributed to the large positional disorder (Σ=4).

Theoretical study of electric field-dependent polaron-type mobility in conjugated polymers

We have used a self-consistent quantum molecular dynamics approach to calculate the mobility of both positive and negative polaron-type carriers on isolated chains of poly(p-phenylene vinylene) (PPV) and some of its derivatives and the dependence of their mobility on the applied electric field. Our results suggest that polaron-type mobility along most of these polymer chains has a clear dependence on the electric field which is quite different from the result derived for bulk PPV-based materials.

Photoinduced Transient Stark Spectroscopy in Organic Semiconductors: A Method for Charge Mobility Determination in the Picosecond Regime

Subpicosecond photoinduced Stark spectroscopy experiments are carried out for measuring charge carrier mobility in organic semiconductors. The technique is demonstrated in state-of-art devices based on methanofullerene. The transient mobility of photogenerated charge carriers is measured in the picosecond time domain. Electric field dependent mobility is observed from the earliest time scales. In addition, two distinct transport regimes are revealed: a short-lived state, approximately 10 ps, of high mobility and a transient towards the trap limited transport, associated with the mesoscopic structure of the medium.

Hall Effect in Organic Single-crystal Field-effect Transistors

ABSTRACTWe report Hall effect of charge carriers accumulated in organic field-effect transistors. Rubrene (C42H28) single crystals are shaped in to the Hall-bar congiguration in the devices so that the Hall signal is appropriately detected in external magnetic fields. It turned out that inverse Hall coefficient, having a positive sign, is close to the amount of electric-field induced charge upon the hole accumulation. The observation of the normal Hall effect means that the electromagnetic character of the surface charge is not of hopping carriers but resembles that of a two-dimensional hole-gas system. The direct access to the density of mobile charge carriers provides a tool to understand nontrivial features of organic field-effect transistors such as gate electric field dependent mobility.

Photoconduction in Alq3

Photoelectronic properties of Alq3 were studied by photoconductivity (PC) measurements in thin film, sandwich (indium-tin-oxide/Alq3∕LiF∕Al) devices. We find that the photocurrent is dominated by bulk generation of carriers for incident photon energies greater than 2.75eV. The quantum efficiency of photocarrier generation has been measured from carrier collection measurements to be about 10%. The quantum efficiency is largely independent of electric field. This enables a direct measurement of the electric field dependence of mobility using photoconductivity measurements, which is used for quantitative analysis of the dark forward current in these devices. PC measurements were also used to obtain (μ0nτn) product which can be used as a measure of material quality. For Alq3, we find that the value of (μ0nτn) product was between 3×10−15cm2∕V to 8×10−15cm2∕V for different samples. In forward bias, at high field the photocurrent shows saturation accompanied by a phase shift. These effects are attributed to space charge effects in the device.

Charge transport and recombination in bulk heterojunction solar cells studied by the photoinduced charge extraction in linearly increasing voltage technique

Charge carrier mobility and recombination in a bulk heterojunction solar cell based on the mixture of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-phenylene vinylene] (MDMO-PPV) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)-C61 (PCBM) has been studied using the novel technique of photoinduced charge carrier extraction in a linearly increasing voltage (Photo-CELIV). In this technique, charge carriers are photogenerated by a short laser flash, and extracted under a reverse bias voltage ramp after an adjustable delay time (tdel). The Photo-CELIV mobility at room temperature is found to be μ=2×10−4cm2V−1s−1, which is almost independent on charge carrier density, but slightly dependent on tdel. Furthermore, determination of charge carrier lifetime and demonstration of an electric field dependent mobility is presented.

Negative electric field dependence of charge carrier drift mobility in conjugated, semiconducting polymers

The temperature and electric field dependence of the hole mobility in regioregular poly(3-hexylthiophene) (P3HT) is studied by time-of-flight (TOF) technique. It is observed that above ∼250 K the slope of the electric field dependence of mobility becomes negative. Such phenomenon has been reported previously for a variety of amorphous charge transport materials, but not clearly observed in conjugated semiconducting polymers. The temperature and electric field dependence of mobility is analyzed both in the framework of Gill’s phenomenological data analysis and disorder formalism. The occurrence of negative field dependence in the measured regioregular P3HT samples is attributed to the small energetic ( σ=70 meV), but large spatial disorder ( Σ=3.4).

Theoretical Investigations of Polymer Based Solar Cells

AbstractWe investigate the behavior of a polymer blend (M3EH-PPV:CN-ether-PPV) bulk heterojunction solar cell using a numeric model that self-consistently solves Poisson's equation and the charge continuity equation while incorporating electric field dependent mobilities. We obtain good quantitative agreement with present experimental data for J-V curves and photocurrent action spectra. To reproduce experimental photocurrent action spectra, our model predicts 36% exciton dissociation efficiencies in the bulk of the polymer. We also study the limiting conditions of polymer solar cell development by simulating an ideal solar cell using an AM1.5 global spectrum and assuming all absorbed photons hitting a M3EH-PPV:CN-ether-PPV polymer blend (band gap ∼2.0 eV) based solar cell at normal incidence contribute to current. If such a solar cell has 100 nm length, open circuit voltage=0.6 V and 50% fill factor, then the maximum theoretical power conversion efficiency is ηp=5.6%. A similar analysis for a M3EH-PPV:PCBM bulk heterojunction cell yields, ηp=3.5%. These results further highlight the need to develop smaller band gap materials and help explain why the best polymer based solar cells have power conversion efficiencies that remain stuck at about 3%. Our model is used to investigate the important increase in power conversion efficiencies we can expect as lower band gap polymers become available.

Trap Distribution and Field Effect Transistor (FET) of Perylene by Organic Molecular Beam Deposition (OMBD)

Organic semiconducting perylene molecule was deposited as active layer on the FET pattern by using organic molecular beam deposition (OMBD) method. For the morphology and structure, we performed the atomic force microscopy, scanning electron microscopy, and X-ray diffraction experiments. From the I-V characteristics, bulk trap density, trap distribution, and electric field dependent mobility were estimated. Thin film field effect transistor based on perylene molecule was fabricated. From the results of the current modulation with different gate voltages of the FET, gate induced carrier concentration and field effect mobility were obtained. We observed Gaussian-like deep trap distribution, and that mobility was 3×10 -7 cm 2 /Vs.