Charge Transport Mobility Research Articles

Charge transport in organic semiconductors: Assessment of the mean field theory in the hopping regime

The performance of the mean field theory to account for charge transfer rate in molecular dimers and charge transport mobility in molecular stacks with small intermolecular electronic coupling and large local electron-phonon coupling (i.e., in the hopping regime) is carefully investigated against various other approaches. Using Marcus formula as a reference, it is found that mean field theory with system-bath interaction and surface hopping approaches yield fully consistent charge transfer rates in dimers. However, in contrast to the dimer case, incorporating system-bath interaction in the mean field approach results in a completely wrong temperature dependence of charge carrier mobility in larger aggregates. Although the mean field simulation starting from the relaxed geometry of a charged molecule and neglecting system-bath interaction can reproduce thermally activated transport, it is not able to characterize properly the role of additional nonlocal electron-phonon couplings. Our study reveals that the mean field theory must be used with caution when studying charge transport in the hopping regime of organic semiconductors, where the surface hopping approach is generally superior.

Study of the Hole and Electron Transport in Amorphous 9,10-Di-(2′-naphthyl)anthracene: The First-Principles Approach

An accurate description of charge transport in amorphous organic semiconductors is challenging. Many previously reported methods largely involve empirical parameters, which may hinder the understanding of the charge transport process in a specific material. In this paper, Born–Oppenheimer molecular dynamics (BOMD) is used to simulate the amorphous structure of a widely used small molecule 9,10-di-(2′-naphthyl)anthracene (ADN). Its hole and electron mobilities are calculated using an ab initio method. It is found that the inaccuracy in the calculation of the nonadiabatic couplings caused by the periodicity of the cell in the BOMD simulation can be greatly reduced by taking into account the mirror states in the surrounding cells. The calculated hole and electron mobilities both have the same order of magnitude with their corresponding experimental results, demonstrating the possibility to obtain reasonable charge transport mobilities for amorphous small-molecule semiconductors via the first-principles approach. Our work may shed light on the understanding of the charge transport process in amorphous organic semiconductors and the design of new charge transport materials.

Critical Role of Alkyl Chain Branching of Organic Semiconductors in Enabling Solution-Processed N-Channel Organic Thin-Film Transistors with Mobility of up to 3.50 cm2 V–1 s–1

Substituted side chains are fundamental units in solution processable organic semiconductors in order to achieve a balance of close intermolecular stacking, high crystallinity, and good compatibility with different wet techniques. Based on four air-stable solution-processed naphthalene diimides fused with 2-(1,3-dithiol-2-ylidene)malononitrile groups (NDI-DTYM2) that bear branched alkyl chains with varied side-chain length and different branching position, we have carried out systematic studies on the relationship between film microstructure and charge transport in their organic thin-film transistors (OTFTs). In particular synchrotron measurements (grazing incidence X-ray diffraction and near-edge X-ray absorption fine structure) are combined with device optimization studies to probe the interplay between molecular structure, molecular packing, and OTFT mobility. It is found that the side-chain length has a moderate influence on thin-film microstructure but leads to only limited changes in OTFT performance. In contrast, the position of branching point results in subtle, yet critical changes in molecular packing and leads to dramatic differences in electron mobility ranging from ~0.001 to >3.0 cm(2) V(-1) s(-1). Incorporating a NDI-DTYM2 core with three-branched N-alkyl substituents of C(11,6) results in a dense in-plane molecular packing with an unit cell area of 127 Å(2), larger domain sizes of up to 1000 × 3000 nm(2), and an electron mobility of up to 3.50 cm(2) V(-1) s(-1), which is an unprecedented value for ambient stable n-channel solution-processed OTFTs reported to date. These results demonstrate that variation of the alkyl chain branching point is a powerful strategy for tuning of molecular packing to enable high charge transport mobilities.

Controllably tunable phenanthroimidazole–carbazole hybrid bipolar host materials for efficient green electrophosphorescent devices

A series of phenanthroimidazole–carbazole (1 : 2) hybrids as bipolar host materials have been designed and synthesized through facile typical Ullmann reactions. These compounds with rigid configurations exhibit excellent thermal and morphological stabilities with high glass transition temperatures (Tg) (143–282 °C). Their photoelectronic properties, energy levels, charge transport mobility, and film morphologies can be controllably tuned through judicious engineering of the linkage modes between the two carbazole groups and the 2,5-diphenyl-1,3,4-phenanthroimidazole (para and meta). The promising physical properties of these new compounds make them suitable for use as hosts doped with Ir-based phosphor for realizing highly efficient phosphorescent organic light emitting diodes (PhOLEDs). A green device hosted by compound PhBIDmpCP shows a maximum current efficiency of 74.3 cd A−1 and a maximum power efficiency of 74.4 lm W−1 (corresponding EQEmax = 20.2%).

Impact of the arrangement of functional moieties within small molecular systems for solution processable bulk heterojunction solar cells

A series of molecules based on thienofluorene derivatives as electron donating (D) and benzothiadiazole as electron accepting (A) moieties have been assembled into DAD and ADA architectures in order to investigate the influence of the way of assembling the D and A units along the conjugated backbone, on the photovoltaic performances of bulk heterojunction solar cells. It was found that the major difference in going from ADA to DAD architecture is the change in molecular organization (nematic to crystalline), which increases the charge transport mobility and probably also affects the structural organization in blends with PCBM that ultimately leads to higher photovoltaic performances.

Electronic Transport and Raman Scattering in Size-Controlled Nanoperforated Graphene

We demonstrate the fabrication and study of the structure-property relationships of large-area (>1 cm(2)) semiconducting nanoperforated (NP) graphene with tunable constriction width (w = 7.5-14 nm), derived from CVD graphene using block copolymer lithography. Size-tunable constrictions were created while minimizing unintentional doping by using a dual buffer layer pattern-transfer method. An easily removable polymeric layer was sandwiched between an overlying silicon oxide layer and the underlying graphene. Perforation-size was controlled by overetching holes in the oxide prior to pattern transfer into graphene while the polymer protected the graphene from harsh conditions during oxide etching and lift off. The processing materials were removed using relatively mild solvents yielding the clean isolation of NP graphene and thereby facilitating Raman and electrical characterization. We correlate the D to G ratio as a function of w and show three regimes depending on w relative to the characteristic Raman relaxation length. Edge phonon peaks were also observed at 1450 and 1530 cm(-1) in the spectra, without the use of enhancement methods, due to high density of nanoconstricted graphene in the probe area. The resulting NP graphene exhibited semiconducting behavior with increasing ON/OFF conductance modulation with decreasing w at room temperature. The charge transport mobility decreases with increasing top-down reactive ion etching. From these comprehensive studies, we show that both electronic transport and Raman characteristics change in a concerted manner as w shrinks.

White-light bias external quantum efficiency measurements of standard and inverted P3HT : PCBM photovoltaic cells

We have investigated the behaviour of inverted poly(3-hexylthiophene) : [6,6]-phenyl- C61-butyric acid methyl ester (P3HT : PCBM) solar cells with different active layer thickness upon changing light intensity. Using white-light bias external quantum efficiency (EQE) measurements and photocurrent transient measurements we explain the different thickness dependence of device performance of inverted (ITO/ZnO/P3HT : PCBM/WO3/Ag) and standard (ITO/PEDOT : PSS/P3HT : PCBM/Ca/Al) cells. Whereas for inverted devices where high EQEs of up to 68% are measured under low light intensities (∼3.5 mW cm−2), a dramatic reduction in EQE is observed with increasing white-light bias (up to ∼141.5 mW cm−2) accompanied by a severe distortion of the EQE spectrum. For the inverted device this spectral distortion is characterized by a dip in the EQE spectrum for wavelengths corresponding to maximum light absorption and becomes more prominent with increasing active layer thickness. For regular P3HT : PCBM devices, in contrast, a less dramatic reduction in EQE with increasing light intensity and only a mild change in EQE spectral shape are observed. The change in EQE spectral shape is also different for standard devices with a relative reduction in EQE for spectral regions where light is absorbed less strongly. This asymmetry in device behaviour is attributed to unbalanced charge transport with the lower mobility carrier having to travel further on average in the inverted device structure. Thus at high light intensities charge recombination is more pronounced at the front half of the device (close to the transparent electrode) for inverted cells where most of the light is absorbed, and more pronounced at the back half of the device for standard cells. Our results therefore indicate that bulk charge transport mobilities rather than vertical composition gradients are the dominant factor in determining the performance of standard and inverted P3HT : PCBM cells.

Experimental and theoretical study of the charge transport property of 4,4′-N,N′-dicarbazole-biphenyl

The hole and electron mobilities of the amorphous films of the organic semiconductor 4,4′-N,N′-dicarbazole-biphenyl (CBP) at different electric fields were measured through the time of flight (TOF) method. Based on its crystalline structure, the hole and electron mobilities of CBP were calculated. A detailed comparison between experimental and theoretical results is necessary for further understanding its charge transport properties. In order to do this, charge mobilities at zero electric field, μ(0), were deduced from experimental data as a link between experimental and theoretical data. It was found that the electron transport of CBP is less affected by traps compared with its hole transport. This unusual phenomenon can be understood through the distributions of frontier molecular orbitals. We showed that designing materials with frontier molecular orbitals localized at the center of the molecule has the potency to reduce the influence of traps on charge transport and provide new insights into designing high mobility charge transport materials.

Investigation of VOPcPhO as an acceptor material for bulk heterojunction solar cells

In this study, we have successfully demonstrated a new system of donor–acceptor blend for bulk heterojunction solar cells of poly(3-hexylthiophene) (P3HT) by using vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine (VOPcPhO) as acceptor material. A broad absorption over the whole visible range (450–750nm) is achieved. Utilizing this blend system in solar cell fabrication, ITO/PEDOT:PSS/P3HT:VOPcPhO/Al solar cells have been fabricated and characterized in open air. A maximum power conversation efficiency up to 1.09% has been recorded. To confirm the charge transport, the electron and hole mobility of VoPcPhO has been measured. The results show that the VoPcPhO has bipolar transport and can act as an electron as well as hole transporting material. The electron mobility is comparable with hole mobility.

Lattice Defect-Enhanced Hydrogen Production in Nanostructured Hematite-Based Photoelectrochemical Device

Nanostructured hematite photoanodes have been intensively studied in photoelectrochemical (PEC) water splitting for sustainable hydrogen production. Whereas many previous efforts have been focused on doping elements in nanostructured hematite (α-Fe(2)O(3)), we herein demonstrated an alternative approach to enhance the PEC performance by exploiting intrinsic nanostructuring properties of hematite. We found that the introduction of lattice defects effectively decreased the flatband potential and increased the charge transport mobility of nanostructured hematite, hence enhance the light harvest for more efficient hydrogen production via PEC. The nanostructured hematite photoanodes with lattice defects yielded water-splitting photocurrent density of 1.2 mA/cm(2) at 1.6 V vs reversible hydrogen electrode (RHE), which excelled defect-free ones by approximately 1.5 folds. This study thus provides a new strategy for finely tuning properties of nanostructured hematite photoanodes and enhancing the water-splitting ability of PEC.

Directed crystallization of poly(3-hexylthiophene) in micrometre channels under confinement and in electric fields

We present two methods for controlling the in-plane alignment of polymer chains in poly(3-hexylthiophene) (P3HT) films within the channel of a field effect transistor device. Solvent-induced dewetting into the transistor channels of a prepatterned, bottom-gate bottom-contact transistor channel followed by controlled, low nucleation density recrystallization under confinement resulted in a preferential orientation of the π-stacked nanocrystalline lamellae parallel to the resulting P3HT micrometre-sized lines. The contrasting alignment, of perpendicular lamellae, was induced by application of an electric field during recrystallization. Preliminary measurements of the dependence of charge transport mobility on the global orientation of the polymer chain direction are consistent with faster charge transport perpendicular to the π-stacked lamellae (direction parallel to the polymer chains) compared to along the π-stacking direction in the common edge-on oriented morphology.

Photocrosslinkable π-conjugated cruciform molecules for electronic/optoelectronic applications

The development of organic field-effect transistors (OFETs) and organic photovoltaic cells (OPV) has seen much progress using solution-processable organic semiconductors, which can combine high charge transport mobility, stability and patternability. In this work, we report on the design and synthesis of a new type of photopatternable π-conjugated cruciform molecule. These molecules are capable of photopatterning by virtue of photopolymerization of the reactive end-groups (pentadien or acrylate). Their solubility is very good for solution processing. Transistor devices using these molecules provided a maximum field-effect mobility of 3.7×10−3 cm 2 V −1 s −1 as well as a high current on/off ratio. Remarkably, it was found that their charge carrier mobilities were well-maintained, even after the photocrosslinking process.

Charge Transport in a Quantum Dot Supercrystal

Colloidal semiconductor quantum dots connected by organic or inorganic molecules can form periodic supercrystals. These supercrystals can be used for various types of electronic and optical applications with properties superior to those of random quantum dots and organic polymer mixtures. We have used ab initio calculations to study the charge transport and carrier mobility in such supercrystals. Among the different possible charge transport mechanisms, we found that the phonon-assisted hopping is the most likely mechanism. The calculated carrier mobility agrees well with the experimentally measured results. Our predictions of the size and temperature dependences on the mobility are awaiting experimental confirmation.

Electronic Structures and Charge Transport of Stacked Annelated β-Trithiophenes

Density functional theory (DFT) calculations were performed using a MPWB1K functional to interpret the charge transport (CT) properties of stacked annelated β-trithiophene molecules. The goal was to help understand how the chemical composition, face-to-face stacking, intra- and intermolecular correlations, and the applied electric field affect the CT properties of this organic semiconductor compared with those of the edge-to-face "herringbone" motif. The variations in the frontier molecular orbitals, energy gap, nonadiabatic electron attachment energies (EAE), and vertical detachment energies (VDE) were investigated under an external electric field as a function of the number of stacked layers (n). Two possible CT pathways, monomer-to-monomer (MM) and dimer-to-dimer (DD) transports, were postulated to determine the charge carrier conductivities of these molecules. The results highlight that the intermolecular electronic couplings and electrostatic interactions can significantly affect the stacking geometry, even in more extended structures; displaced stacks with smaller interlayer spacing resulted in more compact stacking and, thus, higher CT efficiency; face-to-face stacking geometries can help to reduce the energy gap and VDE. Despite the fact that common thiophene-based oligomers adopting edge-to-face herringbone motifs exhibited p-type (hole-transporting) characters, the face-to-face stacked models based on annelated β-trithiophenes exhibited remarkably increased n-type (electron-transporting) performances. The electric field in the z-direction produced a small influence on the DD charge transport, whereas both electron and hole mobilities decreased dramatically in the MM case. More importantly, in MM charge transport, the electric field increases the hole mobility so that it is higher than that of the electron.

Synthesis and properties of 6,13‐Bis(4‐propylphenyl)pentacene

In this study, 6,13‐Bis(4‐propylphenyl)pentacene was synthesized and characterized. The structures of the products that were obtained during the reaction steps were identified via FT‐IR and NMR spectroscopy and elemental analysis. The compound was found to be soluble in organic solvents like chloroform, dichloromethane, THF, and toluene. The charge transport mobility was 10‐4 cm2V‐1s‐1, and the on/off current ratio was 102, while the compound was 2.5 times more stable under oxidation in a solution compared with pentacene.

Preface

Preface

High charge density and mobility in poly(3-hexylthiophene) using a polarizable gate dielectric

Organic field-effect transistors (OFETs) typically exhibit either a high charge transport mobility or a high charge density. Here we demonstrate an OFET in which both the mobility and the charge density have high values of 0.1 cm(2)/V s and 28 mC/m(2), respectively. The high charge density is induced by the ferroelectric polarization of the gate dielectric poly(vinylidene fluoride/trifluoroethylene). The high mobility is achieved in a regioregular poly(3-hexylthiophene) semiconductor using a transistor with a top-gate layout that inherently exhibits a smooth semiconductor-dielectric interface. The combination of high mobility and charge density yields a record conductance value for polymer-based FETs of 0.3 mu S. (c) 2005 Elsevier B.V. All rights reserved.

The influence of the time-of-flight mobility on the efficiency of solid-state dye-sensitized TiO2 solar cells

The dependence of zero field charge carrier mobilities obtained from time-of-flight (TOF) measurements for a series of doped triphenyldiamines on trap depth and concentration were determined. The influence of these mobilities on the solar cell characteristics have been studied by measuring I–V characteristics and IPCE values using the same doped hole conductors in solid-state dye-sensitized TiO2 solar cells. The traps exhibit only a weak influence on the solar cell characteristics, because the traps which are responsible for low charge transport mobilities in the TOF experiment are filled under steady state illumination conditions. In consequence the effective mobility is raised almost to that of the pristine hole conductor, thus allowing the use of doped hole conductor systems in solar cells without any adverse effects.

Ambipolar Charge Transport in Air‐Stable Polymer Blend Thin‐Film Transistors

AbstractAmbipolar thin‐film transistors based on a series of air‐stable, solution‐processed blends of an n‐type polymer poly(benzobisimidazobenzophenanthroline) (BBL) and a p‐type small molecule, copper phthalocyanine (CuPc) are demonstrated, where all fabrication and measurements are performed under ambient conditions. The hole mobilities are in the range of 6.0 × 10–6 to 2.0 × 10–4 cm2 V–1 s–1 and electron mobilities are in the range of 2.0 × 10–6 to 3.0 × 10–5 cm2 V–1 s–1, depending on the blend composition. UV‐vis spectroscopy and electron diffraction show crystallization of CuPc in the metastable α‐crystal form within the semicrystalline BBL matrix. These CuPc domains develop into elongated ribbon‐like crystalline nanostructures when the blend films are processed in methanol, but not when they are processed in water. On methylene chloride vapor annealing of the blend films, a phase transformation of CuPc from the α‐form to the β‐form is observed, as shown by optical absorption spectroscopy and electron diffraction. Ambipolar charge transport is only observed in the blend films where CuPc crystallized in the elongated ribbon‐like nanostructures (α‐form). Ambipolar behavior is not observed with CuPc in the β‐polymorph. Unipolar hole mobilities as high as 2.0 × 10–3 cm2 V–1 s–1 are observed in these solution‐processed blend field‐effect transistors (FETs) on prolonged treatment in methanol, comparable to previously reported hole mobilities in thermally evaporated CuPc FETs. These results show that ambipolar charge transport and carrier mobilities in multicomponent organic semiconductors are intricately related to the phase‐separated nanoscale and crystalline morphology.

Low molecular weight and polymeric heterocyclics as electron transport/hole-blocking materials in organic light-emitting diodes

The use of low molecular weight, oligomeric and polymeric heterocyclics as electron transport/hole-blocking layers in organic light-emitting diodes is reviewed. The most widely applied materials are π-electron deficient heterocyclics carrying imine nitrogen atoms in the aromatic ring, such as 1,3,4-oxadiazoles, 1,2,4-triazoles, 1,3,5-triazines, and 1,4-quinoxalines. Properties such as redox potentials, ionization potential, electron affinity and charge transport mobility of the materials, if known, are taken into consideration to support the electron injection/transport and hole-blocking effectiveness. It can be generalized that heterocyclic moieties with high reduction potential reduce the interface barriers caused by the band offset between organic material and cathode and are most suitable materials for electron injection in organic electroluminescent devices. These materials are generally characterized by high ionization potential values that contribute towards the hole-blocking property. A general comparison of devices and materials is only possible with limitations owing to the variations in device structure, fabrication, electrode materials, emitter materials, etc. © 1998 John Wiley & Sons, Ltd.