Hole Drift Research Articles

Well-defined [3,3′]bicarbazolyl-based electroactive compounds for optoelectronics

Well-defined electroactive compounds were synthesized from 9 H,9′ H-[3,3′]bicarbazolyl and different halo-substituted arylamines by the modified Ullmann coupling reaction. Photophysical properties of dilute solutions of the derivatives were investigated in detail. The new materials were found to constitute glasses with glass transition temperatures ranging from 34 to 130 °C as characterized by differential scanning calorimetry. The compounds possess good solubility in common organic solvents and high thermal stability with 5% weight loss temperatures of 359–440 °C. The electron photoemission spectra for amorphous layers of the compounds have been recorded and the ionization potentials of 5.23–5.47 eV have been established. Room temperature hole drift mobility in the layers of some compounds molecularly dispersed in polymeric host exceed 10 −5 cm 2/V s at an electric field of 10 6 V/cm.

Ultrafast transport transient in photoexcited ZnSe

A theoretical study (by using the Nonequilibrium Statistical Operator Method) is performed on the ultrafast transient transport properties of photoexcited carriers in ZnSe subjected to electric fields up to 35 kV/cm. The electron and hole drift velocity evolution towards the steady state present maxima in subpicosecond scale.

Photoconductivity in amorphous selenium blocking structures

AbstractPhotoconductive properties of the first practical a‐Se avalanche photosensor (hole drift mobility, photocurrent response time, quantum yield of charge photogeneration and avalanche multiplication gain) have been studied. It has been shown that hole transport and avalanche multiplication mechanisms in multilayered a‐Se blocking structure used in practical photosensors are similar to those observed in a‐Se monolayers confined between insulating layers and used for the first manifestation of avalanche effect in this material by Juska and Arlauskas, phys. stat. sol. (a) 59, 389 (1980). It is suggested that combination of high avalanche gain (up to 1000) with high quantum efficiency of charge photogeneration (almost 100%) and fast photocurrent time response (∼ 1 ns) makes a‐Se avalanche photosensor a promising candidate to replace vacuum photomultiplier tubes for application in medical radiation imaging. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High Hole Mobilities in the Amorphous Films of 2,7-Di(9-carbazolyl)-9-(2-ethylhexyl)carbazole

Abstract 2,7-Di(9-carbazolyl)-9-(2-ethylhexyl)carbazole was synthesized and investigated as new effective hole-transporting material. The synthesized compound showed high thermal stability with the initial weight loss temperature of 400 °C. It forms glass with glass-transition temperature of 88 °C. Hole drift mobilities in the amorphous films of the newly synthesized compound exceeded 10−2 cm2 V−1 s−1 at high electric field, as characterized by the xerographic time of flight technique.

Synthesis and properties of glass-forming phenothiazine and carbazole adducts

Charge-transporting and photoluminescent phenothiazine and carbazole adducts were synthesized and their thermal, optical, photophysical and photoelectrical properties were studied. The materials synthesized were found to constitute glasses with glass transition temperatures in the range of 63–78 °C. Their initial weight loss temperatures range from 334 to 362 °C. Steady state and time-resolved fluorescence spectrometry techniques have revealed energy transfer in the adducts of carbazole and phenothiazine and the existence of excimer forming sites in the films of 3-(10-phenothiazinyl)-9-ethylcarbazole and 3-(9-carbazolyl)-9-ethylcarbazole. The electron photoemission spectra of the materials were recorded and the ionization potentials of 5.38–5.87 eV were established. Time-of-flight hole drift mobilities of carbazole twin compound molecularly dispersed in bisphenol Z polycarbonate (1:2 by weight) approach 2 × 10 −5 cm 2/V s at high electric field.

Transport Properties of Conversion Materials for Digital Radiography

Applying the moving photo-carrier grating(MPG) technique and time-of-flight(TOF) measurements, we studied the transport properties of stabilized amorphous selenium typical of the material used in direct conversion X-ray imaging devices. For MPG measurement, we obtained electron and hole mobility and the recombination lifetime of <TEX>$\alpha-Se$</TEX> films with arsenic(As) additions. We found an apparent increase in hole drift mobility and recombination lifetime, especially when 0.3 % As was added into <TEX>$\alpha-Se$</TEX> film, whereas electron mobility decreased with the addition of As due to the defect density. For TOF measurement, a laser beam with pulse duration of 5 ns and wavelength of 350 nm was illuminated on the surface of <TEX>$\alpha-Se$</TEX> with a thickness of 400 <TEX>${\mu}m$</TEX>. The measured hole and electron transit times were about 8.73 <TEX>${\mu}s$</TEX> and 229.17 <TEX>${\mu}s$</TEX>, respectively.

Ion beam induced charge imaging of charge transport in CdTe and CdZnTe

Ion beam induced charge (IBIC) imaging is a powerful technique for quantitative mapping of the charge transport performance of wide bandgap semiconductor materials. In this paper we present results from a study of electron and hole mobility–lifetime product and drift mobility in CdTe:Cl and CdZnTe, which are semiconductor materials used for radiation detector applications. IBIC imaging has been used to produce mobility–lifetime product maps in CdTe:Cl and CdZnTe, revealing the influence of extended defects and tellurium inclusions and assessing the large area response uniformity of the materials. The recent extension of this method in the form of digital time-resolved IBIC is also discussed and time of flight maps are presented which give quantitative images of electron and hole drift mobility.

Synthesis and characterization of triphenylamine derivatives by oxidative polymerization

Hole transport polymers consisting of 4-methoxytriphenylamine (P-MOTPA) and 4- n-butyltriphenylamine (P-BTPA) were synthesized by oxidative coupling reaction using FeCl 3 as an oxidant. These polymers had good solubility and their thin films showed sufficient morphological stability. 1H NMR and 13C NMR revealed that the monomers were exclusively connected at the p-positions of unsubstituted phenyl groups. Polymers had UV absorption maximum around 370 nm. Cyclic voltammograms of polymers showed well-defined pairs of reduction and oxidation peaks at 1.1 V versus Ag/AgCl, indicating that the polymers are electrochemically active. P-MOTPA and P-BTPA had hole drift mobility of being 4.04 × 10 −5 and 2.98 × 10 −5 cm 2/V s at the electric field of 50 V/μm, respectively.

Synthesis of Aryl(hetero)methylene-1,3-indandione Based Molecular Glasses

Novel glass-forming 2-(1-phenyl-1,2,3,4-tetrahydroquinoline-6-ylmethylene)-1,3-indandione derivatives were synthesized and their thermal properties were studied. The results of a preliminary investigation of the photoelectric properties of amorphous films of the title compounds are briefly reported. The ionization potential of these molecular glasses is ca. 5.6 eV and the hole drift mobility exceeds 10−8 cm2 V−1 s−1 at strong electric fields.

Charge carrier mobility of mixed-layer organic light-emitting diodes

The authors report the investigation of the charge transport behaviors in mixed thin films of N,N′-diphenyl-N,N′-bis(1-napthyl)-1,1′-biphenyl-4,4′-diamine and tris(8-hydroxyquinoline) aluminum. The extracted electron and hole drift mobility were found to be sensitive to the compositional fraction and interpreted by energy levels, charge mobilities of neat compounds, and microscopic networks within the mixed systems. The carrier conduction characteristics, therefore, were used to illustrate the electrical and optical properties of the organic light emitting devices with a mixed layer and present direct evidences on the role of the mixed layer in these devices.

Nonlinear hole transport and nonequilibrium thermodynamics in group III-nitrides under the influence of electric fields

A theoretical study on the nonlinear transport of holes and of the nonequilibrium thermodynamic characteristics of p-doped wurtzite gallium nitride (GaN), aluminium nitride (AlN), and indium nitride (InN), under the influence of moderate to high electric fields, is presented. It is based on a nonlinear quantum kinetic theory which provides a description of the dissipative phenomena developing in the system. The ultrafast time evolution of the hole drift velocity and of the quasitemperatures of holes and longitudinal optical phonons are obtained. The steady state is analyzed by determining the dependence on the electric field of the nonequilibrium thermodynamic state and of the non-Ohmic mobility. A velocity overshoot is evidenced.

Aniline-based bis(enamines) as new amorphous molecular charge transport materials

A series of new aniline-based bis(enamines) has been synthesized. The full characterization of their structure is presented. The thermal, optical and photoelectrical properties of the newly synthesized enamines are discussed. The reported compounds constitute materials with high thermal stability with 5% weight loss temperatures exceeding 390 °C as characterised by thermogravimetric analysis. They form glasses with the glass transition temperatures ranging from 135 to 154 °C as established by differential scanning calorimetry. Photoelectric properties of the materials were examined by electron photoemission and time of flight technique. The electron photoemission spectra of the layers revealed the ionization potentials of 5.12–5.70 eV. Room temperature hole drift mobility of one newly synthesized derivative molecularly doped in bisphenol Z polycarbonate reached 10 −5 cm 2 V −1 s −1 at high electric fields.

Carbazole-based aromatic amines having oxetanyl groups as materials for hole transporting layers

Several oxetane-functionalized aromatic amines have been synthesized by the multi-step synthetic rout. Full characterization of their structure by nuclear magnetic resonance and mass spectroscopy as well as IR spectrometry is presented. The synthesized materials were examined by various techniques including differential scanning calorimetry, thermogravimetry, UV spectrometry, electron photoemission and time of flight techniques. The electron photoemission spectra of the layers showed the ionisation potentials of 5.24–5.67 eV. Time-of-flight hole drift mobility in amorphous layers of bisphenol Z polycarbonate containing 33 wt. % of the electroactive materials ranges from 10 −8 to 10 −6 cm 2/Vs at high electric fields.

Novel Charge Transporting Materials Containing Phenyl-1,2,3,4-Tetrahydroquinoline Moieties

New synthesized monohydrazone and dihydrazone containing 1-phenyl-1,2,3,4-tetrahydroquinoline moiety were investigated. The materials were examined by various techniques including differential scanning calorimetry (DSC), UV spectrometry, electron photoemission and xerographic time of flight (XTOF) techniques. The highest hole drift mobility 10−5 cm2V−1s−1 at an electric field of 106 Vcm−1 was observed for the dihydrazone.

New Thiophene-Based Glass-Forming Hydrazones for Optoelectronic Applications

A new series of thiophene-containing glass-forming hydrazones is synthesized. The thermal, photoelectrical, and optical properties of these compounds are examined. The synthesized hydrazones exhibit the thermal stability sufficient for their application in optoelectronic devices. The values of ionization potential of amorphous films of the newly synthesized hydrazones are from 4.43 to 5.79 eV. The hole drift mobilities measured by the xerographic time-of-flight method in a molecularly doped polymer containing 50 wt.% of one synthesized hydrazone exceed 10−6 cm2/Vs at an electric field intensity of 6.4 × 10−5 V cm−1.

Condensed Aromatic Amines as Electroactive Materials for Optoelectronic Applications

The synthesis and properties of a series of new glass-forming condensed aromatic amines are reported. The dependence of their thermal and optical propertiess on their chemical structure is discussed. The ionization potentials of the synthesized aromatic amines measured by the electron photoemission technique are 5.30–5.40 eV. Hole drift mobilities of some derivatives molecularly dispersed in bisphenol Z polycarbonate exceed 10−6 cm2 V−1 s−1 at high electric fields (106 V cm−1), at room temperature.

Triazine-Based Aromatic Amines as New Glass-Forming Charge Transport Materials

Various diarylamino-substituted 1,3,5-triazine derivatives have been synthesized by Cu catalyzed Ullmann-type arylamino-aryl coupling reaction. Full characterization of the compounds structures by mass spectrometry, IR and electronic absorption, as well as 1H NMR spectroscopy is presented. Some of the compounds represent amorphous materials with glass transition temperatures exceeding 84°C and with thermal decomposition starting at temperatures > 330°C. The electron photoemission spectra of the materials were recorded and the ionisation potentials of ca. 5.3–5.8 eV were established. Time-of-flight hole drift mobility of some diarylamino-substituted triazines molecularly dispersed in bisphenol Z polycarbonate approached 10−5 cm2/Vs at high electric field.

New Carbazolyl Containing Molecular Glasses for Optoelectronics

The multistep synthesis, characterization and properties of the well defined six carbazolyl and branched aliphatic chain containing organic photosemiconductor is reported. It was prepared by the nucleophilic opening of the oxirane cycle of 1,6-di(carbazol-9-yl)-5-(carbazol-9-methyl)-4-oxa-2-hexanol glycidyl ether with the linking agent 4,4′-thiobisbenzenethiol in the presence of catalyst triethylamine (TEA). The molecular structure of this branched molecular glass prevents its crystallization in the layer and allows stable films to be prepared without binder material. The hole drift mobility, measured by the time-of-flight technique, reaches 10−5 cm2/V · s at an electric field of 106 V · cm−1. The value of ionization potential (5.92 eV) measured by the electron photoemission method in air is close to those reported for the other organic photoconductors containing electronically isolated carbazolyl moieties.

Preparation of polymers having hole and electron transport units by Friedel–Crafts reaction

AbstractPolymers having 2,5‐diphenyl‐1,3,4‐oxadiazole (BCO) or anthracene (BCA) as an electron transport unit and N,N′‐diphenyl‐N,N′‐bis(4‐butylphenyl)‐benzidine (BTPD) as a hole transport unit were prepared by condensation polymerization using Friedel–Crafts reaction. It was found that BCO was less reactive than BCA. The low reactivity of the BCO monomer can be explained by the oxygen atom in the oxadiazole unit, which acts as a Lewis base and reduces the activity of the catalyst. The redox behavior measured by cyclic voltammetry showed for both BTPD‐BCO and BTPD‐BCA almost the same oxidation potential. In addition, the BTPD‐BCO also exhibited a reduction peak. Hole and electron drifts mobility of the polymers were measured by the time‐of‐flight method. The hole drift mobility of both BTPD‐BCO and BTPD‐BCA was 7.4 × 10−5 cm2 V−1 s−1. The electron drift mobilities of BTPD‐BCO and BTPD‐BCA were 6.5 × 10−5 cm2 V−1 s−1 and 5.2 × 10−6 cm2 V−1 s−1, respectively. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3083–3089, 2007

Novel dihydrazone based polymers for electrophotography

The novel family of hole-transporting polymers containing hydrazone moieties is reported. The polymers were prepared in polyaddition reaction of dihydrazone-containing diepoxides with bifunctional nucleophilic linking agents in the presence of catalyst triethylamine. Obtained polymers were found to constitute novel polymeric hole transporting materials characterized by differential scanning calorimetry 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.