Hole Drift Mobility Research Articles

Indolyl-substituted carbazole derivatives as amorphous electro-active materials for optoelectronics

Various indolyl-substituted carbazole derivatives were synthesized and found to constitute glass-forming materials with glass transition temperatures in the range of 40–95 °C as characterised by differential scanning calorimetry. The electron photoemission spectra of the materials were recorded and the ionisation potentials of 5.2–5.7 eV were established. Time-of-flight hole drift mobility of the indolyl-substituted carbazole dihydrazone molecularly dispersed in bisphenol Z polycarbonate approached 4 × 10 −6 cm 2/V s at high electric field.

Electrical Transport and Defect Spectroscopy of Free Standing Single Crystal CVD Diamond Prepared from Methane Rich Mixtures

ABSTRACTTransient photocurrent measurements in the Time-of-Flight (TOF) configuration were used to study the electrical transport in single crystalline diamond layers using a Nd:YAG - pumped OPO (2.7 nsec) laser pulse excitation source working at a wavelength of ∼ 218 nm. The amount of collected charge was measured and the hole and electron drift mobilities were determined at room temperature for natural IIa diamond and intrinsic single crystalline CVD diamond samples. A variation of the laser intensity over several orders of magnitude enabled switching between the so called “small signal TOF” and “space charge limited current” (SCLC) modes. Experiments were done using electrical fields in the range of 0.05-1.2 V/μm.

Hole transporting 3,4-ethylenedioxythiophene-based hydrazones

Hole transporting glass-forming low-molar-mass hydrazones containing 3,4-ethylenedioxythiophene moiety were synthesized and characterized by nuclear magnetic resonance, infrared spectroscopy, and mass spectrometry. The thermal, optical and photoelectrical properties of the synthesized compounds are reported. Four synthesized compounds di(4-(((3,4-ethylenedioxythiophene)-2-methylene 1-phenylhydrazino)-2-hydroxypropyl)-thiophenyl)sulphur, 1,3-bis(((3,4-ethylenedioxythiophene)-2-methylene 1-phenylhydrazino)(3-hydroxy)thiabutyl)benzene, 3,4-ethylenedioxythiophene-2-carbaldehyde N, N-diphenylhydrazone and 3,4-ethylenedioxythiophene-2-carbaldehyde N-(2,3-epoxypropyl)- N-phenylhydrazone form glasses with glass transition temperatures of 68, 64, 37 and 26 °C, respectively as established by differential scanning calorimetry. The ionisation potentials of the synthesized compounds measured by electron photoemission technique in air range from 5.45 to 5.6 eV. Hole drift mobilities in the films of the solid solutions of some of the synthesized hydrazones in bisphenol Z polycarbonate measured by the time-of-flight technique exceed 10 -5 cm 2 V -1 s -1 at high electric fields.

The Moving Photocarrier Grating (MPG) Technique for the Transport Properties of α-Se:As Films

The moving photocarrier grating (MPG) technique for the determination of the carrier mobilities and the recombination lifetime of <TEX>$\alpha$</TEX>-Se:As films has been studied. The electron and hole drift mobility and the recombination lifetime of <TEX>$\alpha$</TEX>-Se films with arsenic (As) additions have been obtained from measurement of the short circuit current density <TEX>$j_{sc}$</TEX> as a function of grating velocity and spatial period. The hole mobility decreases due to defect density of hole traps when x exceeds 0.003, whereas the hole mobility increases for the case of low As addition (x<TEX>$\le$</TEX>0.003). We have found an increase in hole drift mobility and recombination lifetime, especially when As with (x = 0.003) is added into the <TEX>$\alpha$</TEX>-Se film.

Drift mobility and mobility-lifetime products in CdTe:Cl grown by the travelling heater method

We report the electron and hole charge transport properties of semi insulating CdTe:Cl grown by the Travelling Heater Method (THM). An alpha-particle Time of Flight (TOF) method was used to measure electron and hole drift mobility, with room temperature values of 880 cm2 /Vs for electrons and 90 cm2/Vs for holes. The variation in mobility was also investigated as a function of temperature, with electron and hole mobilities at 190 K of 1150 cm2 /Vs and 20 cm2/Vs respectively. Using a Hecht analysis the electron and hole mobility-lifetime products were also measured over the same temperature range, with values at room temperature of 8times10-4 cm2/V and 7times10-5 cm 2/V respectively. Time-resolved ion beam induced charge (IBIC) imaging was used to produce micrometer resolution maps of electron drift mobility and signal amplitude, which showed excellent spatial uniformity

Novel hydrazone moieties containing polymers for optoelectronics

The novel family of hole transporting polymers containing hydrazone moieties is reported. The polymers were prepared in polyaddition reaction of hydrazone-containing diepoxydes with 2,5-dimercapto-1,3,4-thiadiazole in the presence of catalyst triethylamine (TEA). Obtained polymers were found to constitute novel polymeric hole transporting materials (TM) characterized by differential scanning calorimetry (DSC) and time of flight method. The highest hole-drift mobility in the newly synthesized polymers exceed 10 −5 cm 2 V −1 s −1 at an electric field of 10 6 V cm −1 was observed in the TM with triphenylamine moieties. The electron photoemission spectra of the obtained polymers have been recorded and the ionization potentials of 5.24–5.33 eV have been determined.

Synthesis and properties of glass-forming condensed aromatic amines with reactive functional groups

A series of new glass-forming condensed aromatic amines containing functional groups was synthesized by stepwise route and characterized by nuclear magnetic resonance, infrared and mass spectrometry. The thermal, optical and photoelectrical properties of the synthesized compounds are reported. The synthesized aromatic amines form glasses with the glass transition temperatures in the range of 73–104 °C as characterized by differential scanning calorimetry. The ionisation potentials of these compounds range from 5.25 to 5.36 eV as determined by electron photoemission method. The best charge transport properties were shown by di(4-(4-(3,6-di( N, N-diphenylamino)carbazole-9)-phenyloxy-2-hydroxypropylthio) phenyl)sulphide. Room temperature hole-drift mobilities in its 50% solid solutions in bisphenol Z polycarbonate established by the xerographic time-of-flight technique were found to be 2 × 10 −6 cm 2 V −1 s −1 at electric field of 6.4 × 10 5 V cm −1.

New hole-transporting dihydrazones

Synthesis and photoelectric properties of new glass-forming diphenylsulfone dihydrazones with the glass transition temperatures ranging from 22 to 123 °C are reported. The ionization potentials of the films of diphenylsulfone dihydrazones measured by the electron photoemission technique range from 5.00 to 5.33 eV. The hole-drift mobilities in the amorphous films of the diphenylsulfone dihydrazones bearing alkyl substituents doped in bisphenol Z polycarbonate (PC-Z) approach 10 −5 cm 2/(Vs) at high-applied electric fields as characterized by the time-of-flight method.

Temperature dependence of hole drift mobility in high‐purity single‐crystal CVD diamond

AbstractHole transport properties in high‐purity single crystal CVD diamond samples were studied using the time of flight technique with optical excitation of the carriers. The measurements were taken at different temperatures in the interval 80–470 K. By varying the intensity of the optical excitation over several orders of magnitude, measurements at different carrier concentrations have been performed. In this way, measurements have been made both in the space charge limited and non space charge limited regimes, with consistent results. The temperature dependence of the low‐field hole drift mobility shows a Tα dependence with α ≈ –1.5, below 350 K. This indicates that acoustic phonon scattering is the dominant scattering mechanism and a very low concentration of ionized impurities in this material. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Novel hydrazone based polymers as hole transporting materials

The novel family of hole transporting polymers containing hydrazone moieties is reported. The polymers were prepared in polyaddition reaction of hydrazone-containing diepoxydes with 4,4′-thiobisbenzenethiol in the presence of catalyst triethylamine (TEA). Obtained polymers were found to constitute novel polymeric hole transporting materials (TM) characterized by differential scanning calorimetry and time of flight method. The highest hole drift mobility in the newly synthesized polymers exceed 10−4cm2V−1s−1 at an electric field of 106Vcm−1 was observed in the TM with triphenylamine moieties.

Charge-carrier transport and triplet exciton diffusion in a blue electrophosphorescent emitting layer

Charge-carrier transport and triplet exciton diffusion in iridium(III) bis[(4,6-di-fluoropheny)-pyridinato-N,C2′]picolinate (FIrpic) doped in 4,4′-N, N′-dicarbazole-biphenyl (CBP) thin films have been studied by time-of-flight (TOF) transient photocurrent and steady-state photocurrent measurements. With increasing FIrpic concentration, hole TOF transients become highly dispersive and the hole drift mobility is decreased. In contrast, the electron transit signals become unobservable. The TOF results are found to be consistent with those of steady-state photocurrent measurements of FIrpic-doped CBP thin films; the photocurrent spectra are symbatic and antibatic when the illuminated electrode is positively and negatively biased, respectively. The diffusion lengths of triplet excitons in FIrpic-doped CBP thin films are determined from the symbatic photocurrent spectra.

The study of elementary steps of the photolysis of silver halides by the method of microwave photoconductivity

The major part of inconsistencies in the description of the mechanism of formation of nonhalide silver clusters under exposure to light is due to the lack of quantitative data on electronic-ionic processes in microdispersed silver halide systems. The use of the microwave photoconductivity technique partially fills the gap. The drift mobility of holes and electrons in powdered AgBr (0.8 and 60 cm2 V−1 s−1, respectively) and of electrons in powdered AgCl (40 cm2 V−1 s−1) and the rate constants of the recombination of localized electrons and free holes in powder AgBr (1.4 × 10−7 cm3 s−1) and of free electrons and holes in AgBr and AgCl (1.5 ± 0.5) × 10−11 and 2 × 10−12 cm3 s− 1, respectively) were determined. The range of the depth of traps formed by treatment with aqueous solutions of sodium thiosulfate in AgCl (0.45–0.63 eV) and by preliminary exposure of AgCl and AgBr grains (0.4 and 0.25–0.36 eV, respectively) was estimated. The rate constant of the hole capture by the centers formed by introducing AgI (7 × 10−9 cm3 s−1) was determined, and the lifetime of the Ag atom (10−4 s) was estimated. The paper summarizes the results obtained.

Charge Carrier Transport Properties in Polymer Liquid Crystals Containing Oxadiazole and Amine Moieties in the Same Side Chain

Steady-state and transient photocurrent measurements were carried out to study the charge carrier transport properties of polymer liquid crystal (LC) containing oxadiazole (OXD) and amine moieties in the same side chain. The steady-state photocurrent measurement with asymmetric electrodes of ITO and Al and a short penetration depth of the illumination light indicated that both electrons and holes can be transported in this film. The transient hole photocurrent observed by time-of-flight (TOF) experiments was dispersive at room temperature. The hole drift mobility significantly depended on temperature and electric field and was determined to be 6.1 x 10(-8) cm2/Vs at a field of 9.1 x 10(5) V/cm. According to the disorder formalism, the Gaussian width of the density of states was determined to be 170 meV for holes. Despite the indication of possible electron transport in this film, we could not determine the electron mobility by TOF experiments due to strong dispersive photocurrent. We discuss the present charge transport properties of the film in relation to a large dipole attributed to an electrical push-pull structure of p-dimethylaminophenyl-substitited OXD moiety in polymer LC and its electroluminescent properties.

Phenothiazinyl-containing aromatic amines as novel amorphous molecular materials for optoelectronics

Novel aromatic amines have been synthesized by the modified Ullmann reaction of 10H-phenothiazine and di(4-iodophenyl)ethylamine or 3,6-diiodo-9-hexylcarbazole. The full characterization of their structure is presented. The compounds possess high thermal stability with glass transition temperatures of 87–103 °C and onset decomposition temperatures of 346–377 °C. Room temperature hole drift mobilities in amorphous films of the materials were, respectively, 1.2 × 10 −6 and 2 × 10 −5 cm 2/V s at an applied electric field of 3.6 × 10 5 V/cm. The electron photoemission spectra of the molecular glasses have been recorded and the ionisation potentials of 5.5–5.54 eV have been established. The values obtained have been compared with the values of ionisation potential of photoconductive oligomers containing unsubstituted carbazolyl or diphenylamino groups.

Hole-Transporting Glass-Forming 3,3′-Dicarbazyl-Based Hydrazones

ABSTRACT 3,3′-Di(9-(4-butylphenyl)carbazyl) and 3,3′-di(9-ethylcarbazyl)-based hydrazones were synthesized by the multi-step synthetic route including oxidative dimerization of 9-(4-butylphenyl)carbazole and 9-ethylcarbazole, formylation of the dimmers obtained and the reactions of the formyl derivatives with different hydrazines. The chemical structure of the compounds was confirmed by 1HNMR, IR and mass spectroscopy. The hydrazones synthesized form stable glasses with the glass transition temperatures exceeding 140°C. The values of ionisation potentials measured by electron photoemission technique are in the range of 5,21–5,4 eV. The hole drift mobilities in the films of the 50% solid solutions of the hydrazones in bisphenol Z polycarbonate established by the xerographic time-of-flight technique exceed 10−5 cm2/Vs at high electric fields.

9-(4-Methoxyphenyl) Carbazolyl-Containing Hydrazones for Optoelectronic Applications

ABSTRACT 9-(4-methoxyphenyl)carbazolyl-containing hydrazones were prepared by multi-step synthetic route including Ullmann coupling of carbazole with 4-iodoanisole, formylation of 9-(4-methoxyphenyl)carbazole by Vilsmeier reaction and the condensation of mono- or dicarbaldehydes obtainexd with differently substituted hydrazines. All the hydrazones synthesized form glasses with the glass transition temperatures ranging from 56 to 110°C. Their ionization potentials established by electron photoemission technique are in the range 5.3–5.47 eV. Hole drift mobilities of 50% solid solutions of the hydrazones in polycarbonate established by the xerographic time-of-flight technique exceed 10−5 cm2/Vs at high electric fields.

A High Tg Carbazole-Based Hole-Transporting Material for Organic Light-Emitting Devices

A new branched carbazole derivative, 1,3,5-tris(2-(9-ethylcarbazyl-3)ethylene)benzene (TECEB), was prepared as a hole-transporting material for organic light-emitting devices (OLEDs). TECEB is comparable to 1,4-bis(1-naphthylphenylamino)biphenyl (NPB) in terms of highest-occupied molecular orbital/lowest-unoccupied molecular orbial energy levels, hole-drift mobility, as well as device performance (maximum luminence of about 10 000 cd m-2 and current efficiency of 3.27 cd A-1) in a standard hole-transporting layer/tris-(8-hydroxyquinoline)aluminum double-layer device, but is superior to NPB in terms of its higher glass-transition temperature (Tg, 130 °C) and ease of synthesis. The latter features suggest that TECEB can be a potential alternative material to NPB especially for high-temperature applications in OLEDs and other organic electronic devices.

Electric field-induced retardation of the drift mobility of the photocarriers in SrTiO [formula omitted] crystal

Transient photocurrent of SrTiO 3 induced by excitation in the fundamental absorption has been measured by the time-of-flight method at low temperatures. The drift mobilities of electrons and holes at 4 K were evaluated to be 1.10 × 10 4 and 7.0 × 10 3 mm 2 s - 1 V - 1 , respectively. The result indicates that the hole as well as the electron is highly mobile in SrTiO 3 crystal and that the photo-generated carriers relax into large polarons. We have found that the drift mobilities of the carriers are retarded with increasing the electric field. The field dependence of the drift mobility may arise from the polarization induced by the carrier relaxation with the external electric field.

Ultrahigh room-temperature hole mobility in a SiGe quantum well

We report an ultrahigh room-temperature hole drift mobility obtained in a Si0.2Ge0.8 quantum well with a parabolic-like Ge profile, which has at its core a p-type modulation-doped (MOD) Si∕Si0.2Ge0.8∕Si0.65Ge0.35∕Si(001) heterostructure. High-conductivity holes at 293 K with a drift mobility of 3600cm2V−1s−1 at a sheet carrier density of 4.94×1012cm−2 were obtained in the Si0.2Ge0.8 quantum well after optimum annealing at 750 °C for 30 min. Hall mobility and sheet carrier density of this heterostructure are 1776cm2V−1s−1 and 2.37×1013cm−2, respectively. Structural characterization of the as-grown and the annealed samples revealed that the annealing had caused Si0.2Ge0.8 channel broadening, smearing of interfaces, and formation of a parabolic-like Ge profile that significantly improved room-temperature hole transport properties. The reported values of hole mobility are much higher than in the bulk Ge.

Mobility of electrons and holes in a liquid crystalline perylene diimide thin film with time of flight technique

When N, N′-diheptyl-3,4,9,10-perylenebiscarboximide (PTCDI-C7) was evaporated on an ITO glass substrate, a vapor-deposited polycrystalline film was normally obtained. It was then thermally treated on its liquid crystalline phase to get an ordered intermolecular structure. Then an ITO/PTCDI-C7/Al device was fabricated to study the drift mobility of electrons and holes by means of time-of-flight techniques. The negative charges were measured faster than the positive ones, as expected for an n-type organic semiconductor: the electron and the hole mobilities were 0.0139 and 0.0089 cm 2/(V s), respectively, at an electric field of 186 (V/cm) 1/2.