Hole Drift Mobility Research Articles

Temperature dependence of charge carrier ranges in stabilized a-Se photoconductors

Using time-of-flight (TOF) and interrupted-field TOF we have measured the electron and hole drift mobilities, μe and μh, lifetimes, τe and τh, and hence carrier ranges, μeτe and μhτh, in stabilized a-Se as a function of temperature from −25 to 45 °C. Above room temperature, μeτe and μhτh show a slight increase with temperature and peak around 31 and 35 °C, respectively. The μeτe at 35 °C and μhτh at 40 °C are approximately the same as room temperature values. Below room temperature, down to −25 °C, both μe and μh decrease with decreasing temperature with activation energies Eμe and Eμh that are 0.38 and 0.21 eV. The electron and hole lifetimes, τe and τh, are also thermally activated but they increase with decreasing temperature with activation energies Eτe and Eτh that are 0.31 and 0.18 eV. These are quite close to corresponding activation energies for the drift mobilities within experimental errors. The electron and hole ranges, μeτe and μhτh, therefore exhibit only a weak temperature dependence, both increase slightly with temperature. These observations are consistent with shallow trap controlled transport in the presence of a set of deep traps. The X-ray sensitivity of a photoconductor, among other factors, depends on the charge collection efficiency, which depends on electron and hole ranges, μeτe and μhτh. Because these carrier ranges do not show any significant temperature dependence, one can conclude that the X-ray sensitivity of stabilized a-Se over the temperature range from −25 to 40 °C should not be affected by changes in the collection efficiency.

Temperature and field dependent effective hole mobility and impact ionization at extremely high fields in amorphous selenium

An analytical model for the electric field and temperature dependent effective drift mobility of holes in amorphous selenium (a-Se) has been developed by considering density of states distribution near the valence band, field enhancement release rate from the shallow traps, and carrier heating. The models for the field-dependent microscopic mobility and momentum relaxation mean free path considering carrier heating are also proposed. The models are fitted with the published experimental results on effective hole mobility and impact ionization with wide variations of applied electric fields and temperatures. The fittings of the model with the published experimental data on the effective hole mobility reveal that, while the effective hole drift mobility increases with increasing temperature and field, the microscopic mobility and momentum relaxation mean free path in a-Se decreases with increasing electric field. A better fitting considering thermally activated tunneling for the field-enhancement release rate indicates that the shallow hole traps in a-Se are neutral defects.

Synthesis and properties of hole-transporting indolo[3,2-b]carbazole-based hydrazones with reactive functional groups

The synthesis of indolo[3,2-b]carbazole-based mono- and dihydrazones with reactive vinylbenzyl or vinyloxyethyl functional groups at the nitrogen atom of hydrazine moiety is reported. The thermal, optical, photophysical, electrochemical and photoelectrical properties of the synthesized compounds are described. They exhibit moderate thermal stability with 5% weight loss temperatures ranging from 273 to 343 °C. Self-polymerization of the monomers with vinylbenzyl functional groups starts at around 180 °C. The values of ionization potentials of the compounds measured by cyclic voltammetry are in the range of 4.94–5.00 eV and those estimated by electron photoemission in air range from 4.99 to 5.21 eV. Electron affinities range from −2.28 to −2.23 eV. The molecular structure of these hydrazones allows stable amorphous films to be prepared and the hole drift mobilities of the films exceed 10−3 cm2/V s at high electric fields.

Hole-transporting phenothiazine-based hydrazones with reactive vinylbenzyl groups

Synthesis and optical, thermal, electrochemical, photoelectrical properties of new phenothiazine-based hydrazones with reactive vinyl groups are reported. The synthesized compounds were characterized by 1H NMR, IR, UV/VIS, fluorescence, mass spectrometries, and cyclic voltammetry. The oxidation potentials of the phenothiazine-based mono- and dihydrazones are 0.17V and 0.19V, respectively. Solid-state ionization potentials of these compounds are 4.97eV and 4.99eV, respectively. Hole-drift mobilities in the layer of the phenothiazine-based monohydrazone exceed 10−5cm2/Vs at the electric field of 1×106V/cm, at 25°C. The self-polymerization in the solid state of the synthesized monomers was demonstrated by differential scanning calorimetry.

Synthesis and Properties of Triindole-Based Monomers and Polymers

New star-shaped derivatives of 10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (triindole) with reactive functional groups are synthesized. Triindole monomers were found to have similar profiles of absorption and fluorescence spectra due to the presence of the same chromophore. The ionization potentials of the monomers are found to be comparable (5.37–5.53 eV). Hole drift mobilities of the amorphous layers of oxetane and vinyl ether molecularly dispersed in bisphenol Z polycarbonate (50%) exceed 10−5cm2/Vs at high electric fields. Cationic photocross-linking of triindole containing oxirane, oxetane and vinyl ether initiated with diphenyliodoniun tetrafluorborate is carried out.

Structure-properties relationship of carbazole and fluorene hybrid trimers: experimental and theoretical approaches.

Synthesis and properties of fluorene and carbazole derivatives having three electrophores per molecule with different architectures are reported. The synthesized compounds possess high thermal stabilities with 5% weight loss temperatures exceeding 350 °C. They form glasses with glass transition temperatures ranging from 60 to 68 °C. Cyclovoltammetric experiments revealed the high electrochemical stability of the fluorene trimer. In contrast, 2- and 2,7-fluorenyl substituted carbazole derivatives show irreversible oxidation in the CV experiments. The electron photoemission spectra of the films of the synthesized compounds revealed ionization potentials of 5.65-5.89 eV. Hole drift mobilities in the amorphous layers of the synthesized compounds reach 10(-2) cm(2) V(-1) s(-1) at high electric fields, as established by a xerographic time-of-flight technique. DFT calculations show that HOMO and LUMO orbitals of the compounds are very similar in energy and shape. The similar hole mobilities observed for the three compounds are discussed in the frame of the Marcus theory. An important influence of the alkyl groups on the ionization potentials and on the hole mobilities was also observed and its origin is discussed.

Synthesis, properties, and self-polymerization of hole-transporting carbazole- and triphenylamine-based hydrazone monomers

Synthesis, optical, thermal, electrochemical, and photoelectrical properties of new carbazole- and triphenylamine-based hydrazones with reactive vinyl groups are reported. Self-polymerization in the solid state of the synthesized monomers was studied by differential scanning calorimetry (DSC) and IR spectrometry. Molecular weights of the soluble polymers were determined by gel chromatography. The synthesized compounds were characterized by 1H and 13C NMR, IR, UV/VIS, fluorescence, and mass spectrometries, as well as by cyclic voltammetry. The synthesized carbazole- and triphenylamine-based hydrazones form glasses with the glass transition temperatures ranging from 83 to 118 °C, as was elucidated by DSC. Their thermal degradation starts above 270 °C, as was revealed by thermogravimetric analysis. The ionization potentials of the films of carbazole- and triphenylamine-based hydrazones, measured by the electron photoemission in air technique, range from 5.32 to 5.65 eV. Hole-drift mobilities in amorphous layers of the monomers, estimated by xerographic time of flight technique, approach 10−3 cm2/Vs at high electric fields.

Demonstration of determination of electron and hole drift-mobilities in organic thin films by means of impedance spectroscopy measurements

The electron and hole drift-mobilities of tris(8-hydroxy-quinolinato) aluminium (Alq3) thin films have been determined using impedance spectroscopy (IS) measurements. The theoretical basis of drift-mobility measurement with IS rests on a theory for single-injection space-charge limited current. The electron drift-mobilities in Alq3 which measured from the frequency dependence of both capacitance and conductance had the same electric-field dependence and were identical to those measured by a time-of-flight (TOF) transient photocurrent technique. To estimate the hole drift-mobility in Alq3, a 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl thin film was inserted as an injection layer to reduce the hole injection barrier between an injection electrode and the Alq3 thin film. The hole drift-mobilities of Alq3 thin films measured with IS were identical to those measured by the TOF method. These results are essential for clarifying the mechanism of the carrier transport of organic light emitting diodes (OLEDs) with the aim of improving OLED performance.

Synthesis of electroactive hydrazones derived from carbazolyl-based 2-propenals for optoelectronics

Herein, we report the synthesis of 2,3-unsaturated carbazolyl and 3,3′-bicarbazolyl aldehydes by a Heck coupling methodology and the further condensation of these aldehydes with N-methyl-N-phenylhydrazine and N,N-diphenylhydrazine, resulting in mono- and dihydrazones that act as effective hole transporting materials. The thermal, optical and photophysical properties of the newly synthesized organic electroactive derivatives have been investigated. Room temperature hole-drift mobility values ranged from 10−4 to 10−3cm2/Vs in strong electric fields. The ionization potentials (Ip) of the investigated compounds were very close and ranged from 5.15 to 5.28eV, which can be attributed to a very weak conjugation of the carbazole segments of the 3,3′-bicarbazolyl moiety of the synthesized dihydrazones.

Structure–Properties Relationship of Phenylethenyl-Substituted Triphenylamines

Diversity of the (opto)electronic devices in which the derivatives of triphenylamine are used requires thorough knowledge of the structure–properties relationship of this class of organic semiconductors. Diphenylethenyl- or methylphenylethenyl-substituted triphenylamines were prepared by condensation of methoxy-substituted triphenylamines with 2,2-diphenylacetaldehyde or 2-phenylpropionaldehyde, respectively. Diphenylethenyl-substituted triphenylamines form molecular glasses with glass transition temperatures up to 50 °C. Increasing the number of phenylethenyl substituents resulted in suppression of intramolecular motions of the triphenylamine core, thereby significantly (up to 48%) enhancing fluorescence quantum yield of the derivatives in solutions. The enhancement was more pronounced for diphenylethenyl-substituted triphenylamines. The ionization potentials of the synthesized materials do not depend on the number and the structure of substituents and are ca. 5.3 eV. Hole drift mobility of the glassy layer of bis 4-(2,2-diphenylethenyl)phenyl](4-methoxyphenyl)amine reach 1.8 × 10–2 cm2/V·s at an electric field of 1 × 106 V/cm.

Truxene discotic liquid crystals with two different ring substituents: synthesis, mesomorphism and high charged carrier mobility

Discotic liquid crystals as a kind of promising organic electronic materials show self-assembly property as well as high charged carrier mobility. New truxene discogens, six 3,8,13-tri(alkoxy)-2,7,12-tri(methoxy)truxenes and two 3,8,13-tri(alkoxy)-2,7,12-tri(hydroxy)truxenes, have been synthesised with improved yields, were fully characterised and exhibited good thermal stability. The mesomorphism has been investigated by using polarised optical microscopy, differential scanning calorimetry and X-ray diffraction, and they exhibited ordered hexagonal columnar mesophase. More importantly, the charge carrier mobilities of 6d and 7f have been measured by using time-of-flight technique and 7f displayed hole drift mobility rate of 4.2 × 10−2 cm2 V−1 s−1 in the Colho phase and 0.1 cm2 V−1 s−1 at a lower temperature M mesophase.

Hole drift mobility in anthrone and antrachinone layers

Drift mobility of holes in antrachinone and anthrone polycrystalline thin films evaporated in the vacuum of the order of 10-5 Torr was measured with time-of-flight- method. The one order difference in mobility values for both –four ring acenes purified with zone melting before vaporization and identical in crystallization structure but with different molecule symmetry may have the origin in the presence of the difference in permanent dipole moment for both molecules.

1,3-Bisdiphenylethenyl-substituted Carbazolyl Derivatives as Charge Transporting Materials

Synthesis of 1,3-diphenylethenylcarbazolyl-based charge transporting materials involving electron donating hydrazone moieties and an electron withdrawing 1,3-indandione moiety is reported. The obtained materials were examined by various techniques, including differential scanning calorimetry, UV-Vis spectroscopy, xerographic time of flight technique and the electron photoemission in air method. Photoemission spectra of the amorphous films of the investigated compounds showed ionization potentials of 5.54–5.90 eV. The hole drift mobility was measured by the xerographic time of flight technique. The highest hole drift mobility, exceeding 10−5 cm2/V·s at 6.4 × 105 V/cm electric field, was observed for the 1,3-diphenylethenylcarbazolyl derivative molecularly doped with a N,N-diphenylhydrazone moiety in the polymeric host bisphenol-Z polycarbonate (PC-Z).

Simulation of impedance spectra of double‐layer organic light‐emitting diodes for the determination of hole drift mobility of NPB/Alq3 diodes by means of impedance spectroscopy

AbstractDetermination of hole drift mobility of 4,4′‐bis[N‐(1‐naphthyl)‐N‐phenyl‐amino]‐biphenyl (NPB)/ tris(8‐hydroxy‐quinolinato) aluminium (Alq3) double‐layer diodes has been examined from the simulation of impedance spectra. The theoretical basis for the determination of carrier drift mobility rests on a theory for single‐injection space‐charge limited current. The simulation for NPB/Alq3 double‐layer hole‐only devices shows that the transit‐time effect seen in the impedance spectra is due to hole transit in Alq3 layer of NPB/Alq3 double‐layer hole‐only devices. The experimentally obtained hole drift mobilities of the NPB/Alq3 double‐layer hole‐only devices are in good agreement with those of Alq3 in literature, consistent with the simulation results. The results are useful for determination of charge‐carrier drift mobility in multilayer organic light‐emitting diodes. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Diphenylamino-substituted derivatives of 9-phenylcarbazole as glass-forming hole-transporting materials for solid state dye sensitized solar cells

The synthesis and properties of glass-forming diphenylamino-substituted derivatives of 9-phenylcarbazole with methoxy groups in the different position of diphenylamino moieties are reported. A comparative study on their thermal, optical, photoelectrical and electrochemical properties is presented. The synthesized compounds exhibit high thermal stability with 5% weight loss temperatures ranging from 344 to 475°C. The derivatives absorb electromagnetic irradiation in the range of 225–425nm with the band gaps of 2.94–3.08eV. The ionization energies of the synthesized compounds range from 5.04 to 5.56eV. The lowest ionization energies and band gaps are observed for compounds containing para methoxy-substituted phenyl rings of diphenylamino moieties and for disubstituted carbazole derivatives. Charge-transporting properties of the selected compounds were tested by time-of-flight technique. Hole drift mobilities in the amorphous layers of the materials reach 10−3cm2/Vs at high electric fields. The derivatives were tested as hole transport materials in solid-state dye sensitized solar cells and showed conversion efficiency up to 0.54%.

Performance of blue fluorescence and red phosphorescent organic light‐emitting diodes using a molecular material with high hole drift mobility

AbstractThis study focuses on the carrier balance factor and transient characteristics of organic light‐emitting diodes (OLEDs) utilizing tris[4‐(5‐phenylthiophen‐2‐yl)phenyl] amine (TPTPA) as a blue emissive material or a host material of a red phosphorescent dye, which exhibits the highest level of hole drift mobility (µ = 1.0×10‐2 cm2V‐1s‐1) among those reported for organic disordered systems. We investigated the doping effect of a red phos‐phorescent material in the TPTPA. The peak wavelengths and the shape of the electroluminescent (EL) spectra agree with those of the photoluminescent spectra, which indicate that the EL originates from the excited state of the TPTPA and dopant. The maximum luminance of approximately 11,000 and 2,000 cd/m2 were achieved for devices without and with dopant. While TPTPA device achieved good carrier balance, the hole current was decreased by doping a red phosphorescent material. Compared with the short response times of TPTPA device, relatively long response times of doped device was observed owing to phosphorescent recombination lifetime of a dopant. The TPTPA device exhibited the relatively high light outcoupling efficiency of approximately 34% (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dimethyldiphenylamino-substituted carbazoles as electronically active molecular materials

Abstract Synthesis and properties of new dimethyldiphenylamino-substituted carbazoles are reported. A comparative experimental analysis of 2-, 2,7-substituted carbazole derivatives and 3-, 3,6-substituted analogues is performed. Disubstituted carbazole derivatives exhibit more than 2-fold higher glass transition temperatures (99–100 °C) as compared to the monosubstituted compounds (38–40 °C). Dilute solutions of the dimethyldiphenylamino-substituted carbazole compounds are found to emit in the blue region with the fluorescence quantum yield of up to 0.45. More symmetrical molecular geometry of the 2-, 2,7-substituted carbazole compounds favors closer molecular packing in the neat films thereby facilitating exciton migration and migration induced exciton quenching, which results in the significantly reduced excited state decay times (down to sub-nanoseconds) and fluorescence quantum yields (down to 0.04). The 2-, 2,7-substituted carbazole compounds are shown to demonstrate the superiority over 3-, 3,6-substituted compounds by giving rise to extended π-conjugation and expressing over one order of magnitude higher radiative decay rates. The synthesized compounds are electrochemically stable: their cyclic voltammograms show reversible oxidation behavior. The ionization energies of the synthesized compounds are in the range of 5.16–5.28 eV. Hole drift mobilities of the molecular mixtures of dimethyldiphenylamino-substituted carbazoles with bisphenol Z polycarbonate reach 10 −5 cm 2 V −1 s −1 at high electric fields.

Drift mobility of thermalized and highly energetic holes in thin layers of amorphous dielectric SiC

AbstractThe development of new technology in the electronics industry requires new dielectric materials. It is also important to understand the charge‐carrier transport mechanism in these materials. We examined the hole drift mobility in amorphous SiC dielectric thin films using the time‐of‐flight (TOF) method. Charge carriers were generated using an electron gun. The generated holes gave a dispersive TOF signal and the mobility was low. For electric field strengths above 4 × 105 V cm−1 the drift mobility shows a very strong dependence on the electric field and a weak temperature dependence (transport of “high‐energy” charge carriers). At lower electric fields and for thermalized charge carriers the mobility is practically field independent and thermally activated. The observed phenomenon was attributed to the changes in the effective energy of the generated carriers moving in the high electric fields and consequently in the density of localized states taking part in the transport.

Synthesis and Properties of Methoxyphenyl-Substituted Derivatives of Indolo[3,2-b]carbazole

The synthesis and full characterization of new derivatives of indolo[3,2-b]carbazole with differently substituted phenyl groups at nitrogen atoms is reported. Comparative study on their thermal, optical electrochemical, and photoelectrical properties is presented. The synthesized compounds are electrochemically stable. Their highest occupied molecular orbital energy values range from -5.14 to -5.07 eV. The electron photoemission spectra of the films of synthesized materials revealed the ionization potentials of 5.31-5.47 eV. Hole drift mobility of the amorphous film of 5,11-bis(3-methoxyphenyl)-6-pentyl-5,11-dihydroindolo[3,2-b]carbazole exceed 10(-3) cm(2)/V·s at high electric fields, as it was established by xerographic time-of-flight technique. In contrast to diphenylamino substituted derivatives of carbazole, no effect of the position of methoxy groups on the photoelectrical properties was observed for the synthesized methoxyphenyl-substituted derivatives of indolo[3,2-b]carbazole. The indolo[3,2-b]carbazole core has a larger resonance structure that includes 3 phenyl rings, and thus the energy gap of the HOMO and LUMO π orbitals is lower as compared to that of carbazoles. With a larger energy difference between the phenyl substituents and the core moiety, the indolo[3,2-b]carbazole derivatives studied all have a weaker coupling between the phenyl group and a much weaker dependence of the molecular properties on the position of substituents on the phenyl groups as compared to those observed in substituted carbazoles.

Carrier mobility enhancement versus channel direction in highly-doped Si

Carrier mobilities (μ) versus Si channel directions have been studied for boron-, arsenic-, and phosphorus-implanted highly-doped Si (100) which is equivalent to a source and drain (SD) region of MOSFET by using a continuous anodic oxidation technique/differential Hall effect (CAOT/DHE) technique. The results for B-doped Si show that the hole drift mobility along the Si <100> channel direction can be enhanced relative to the Si <110> channel direction by an average of 12%, and nearly 115% near the surface. For As-doped Si the electron drift mobility along the Si <100> channel direction has little to no enhancement compared to the Si <110> channel direction. For P-doped Si, the electron drift mobility along the Si <100> channel direction can be enhanced relative to the Si <110> channel direction by an average of 10%, and as much as 18% near the surface. This study shows for the first time that electron mobility in P-doped Si is enhanced in the Si <100> channel direction, although the enhancement is not as significant as in B-doped Si. The results are qualitatively and semi-quantitatively consistent with the implant species-dependent effective mobility (μeff) behaviors of CMOS device I–V characteristics.