Hole Drift Mobility Research Articles

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.

Highly transparent microcrystalline silicon carbide grown with hot wire chemical vapor deposition as window layers in n-i-p microcrystalline silicon solar cells

Microcrystalline silicon carbide (μc-SiC) films were prepared using hot wire chemical vapor deposition at low substrate temperature. The μc-SiC films were employed as window layers in microcrystalline silicon (μc-Si:H) n-i-p solar cells. Quantum efficiency (QE) and short circuit current density (JSC) in these n-side illuminated n-i-p cells were significantly higher than in standard p-i-n cells. A high QE current density of 26.7mA∕cm2 was achieved in an absorber layer thickness of 2μm. The enhanced JSC was attributed to the wide band gap of the μc-SiC layer and a sufficiently high hole drift mobility in μc-Si:H absorber layer.

Glass-Forming Hole-Transporting Triphenylamine-Based Hydrazones with Reactive Functional Groups

Synthesis and the properties of a series of triphenylamine-based hydrazones are reported. The dependence of their thermal and glass-forming properties on their chemical structure is discussed. The ionization potentials of the synthesized hydrazones measured by the electron photoemission technique range from 5.16 to 5.32 eV. Hole-drift mobilities of some derivatives molecularly dispersed in bisphenol Z polycarbonate exceed 10−4 cm2V−1s−1 at high electric fields (106 V/cm),at room temperature.

Carbazolyl- and diphenylamino substituted fluorenes as hole transport materials

Four fluorene-containing aromatic amines with carbazole and diphenylamine moieties were synthesized by Ullmann coupling. Comparative study on their thermal, optical and photoelectrical properties is presented. One synthesized compound is found to form glass with the glass transition temperature 58 °C as characterized by differential scanning calorimetry. The ionization potentials established by electron photoemission technique are in the range 5.9–6.1 eV. Hole-drift mobilities of 50% solid solution of 2,7-dicarbazolyl-9,9-dihexylfluorene in bisphenol Z polycarbonate established by the xerographic time-of-flight technique exceed 10 −4 cm 2 V −1 s −1 at electric field of 10 6 V cm −1.

Synthesis and Photophysical Properties of Ferrocene Containing Monomer and Polymer

Synthesis of a new monomer and polymer containing both ferrocene and hydrazone moieties are reported. The obtained materials were examined by various techniques including differential scanning calorimetry, UV, IR, NMR spectroscopy, and time of flight method. These materials may be of particular interest for the development of future electrophotographic photoreceptors as electron photoemission spectra of the layers showed ionization potentials of 5.35–5.41 eV. The hole drift mobility values in compositions of the designed structures with bisphenol Z polycarbonate exceeded 10−8 cm2/Vs at strong electric fields.

Molecular glasses possessing a phenyl-1,2,3,4-tetrahydroquinoline moiety as hole transporting materials for electrophotography

The new hole transporting materials which comprise molecules consisting of two hydrazone branches linked with flexible thiobiphenyl or thiophenyl sulphide and two hydroxyl groups containing central bridge were synthesized and investigated. These transporting materials possessing phenyl-1,2,3,4-tetrahydroquinoline moieties are low molecular glasses and allow preparing stable to crystallization layers. The ionization potentials of these compounds are ca. 5.3 eV as determined by the electron photoemission method. These low molecular glasses can be used as hole transporting materials with or, in the case of a solid substrate, without binder polymer. The hole drift mobility of the newly synthesized compounds, measured by xerographic time of flight method, exceeds 10 −5 cm 2/V s at an electric field of 10 6 V/cm.

An unpaired electron-based hole-transporting molecule: Triarylamine-combined nitroxide radicals

A durable nitroxide radical combined with a triarylamine moiety exhibited a hole-drift mobility of 6 x 10(-3) cm(2) V(-1) s(-1), to which the aminophenyl nitroxide structure contributed.

Analysis of field dependent steady-state photocarrier grating measurements on polymorphous and microcrystalline silicon films

The steady-state photocarrier grating (SSPG) technique has been employed to investigate the field dependence of different polymorphous and microcrystalline silicon samples prepared by plasma enhanced chemical vapor deposition technique. The field-dependent experimental data at different temperatures are analyzed using two different approaches based on the small-signal photocurrent to extract more information on the electronic properties (like small signal mobility life time product, average drift length for holes and electrons,... etc.). The quality of the fits is tested by the χ2 indicator and the best choice of the transport parameters is obtained in each approach. The values of electron and hole drift mobilities obtained from one approach are compared to those found by others. The trapped charge density can be determined, for all samples, and correlated to minority-carrier diffusion length. It is also found that the decrease in temperature in a microcrystalline sample may result in an increase of the trapped charge density. This increase in trapped carrier density may be consistent with the increase in sub-gap absorption. Such behavior is found in agreement with that reported in the literature. In addition, the polymorphous samples that have values of diffusion lengths comparable to those of microcrystalline samples exhibit higher values of trapped carrier density.

Hydrazones Possessing a Phenyl-1,2,3,4-tetrahydroquinoline Moiety as Hole Transporting Materials

Hydrazones containing 1-phenyl-1,2,3,4-tetrahydroquinoline units were synthesized starting from diphenylamine. These compounds were found to constitute novel hole transporting materials and were characterized by the time of flight method. The hole drift mobility in these compounds exceeds 10−6 cm2 V−1 s−1 at an electric field of 106 V cm−1.

Charge‐transporting polymers and molecular glasses for optoelectronic applications

AbstractOrganic charge transport materials used in optoelectronic devices such as electrophotographic photoreceptors, light emitting diodes, solar cells, photorefractive materials can be devided into polymers, molecularly doped polymers and molecular glasses.Among charge‐transporting polymers hole‐transporting polymers are the most widely used and studied. An aromatic amino group is a common feature of many known charge‐transporting polymers. Almost all practical systems used in electrophotography belong to this category.Organic low‐molar‐mass compounds with stable solid amorphous phase above room temperature are called molecular glasses, amorphous molecular materials or low‐molar‐mass glasses. A real explosion of interest in organic electronically active low‐molar‐mass glass‐forming materials occurred after the discovery of high performance multilayer electroluminescent devices prepared from vacuum‐sublimed organic dye amorphous films.In this presentation the recent results of the work on the synthesis and properties of charge‐transporting polymers and molecular glasses performed in the author's laboratories are reviewed. The main attention is paid to the synthesis and studies of hole‐transporting polymers and molecular glasses. In the design and synthesis of these materials aromatic amino groups such as carbazolyl, tripehenylamino, phenothiazinyl and hydrazone moieties, have been used as the main building blocks. The properties of the charge‐transporting compounds synthesized are reported. Ionization potentials established by the electron photoemission in air technique range from 5.00 to 5.60 eV. Hole drift mobilities in the amorphous films of some hydrazones and aromatic amines approach 10−2 cm2/V sec at high electric fields as revealed by the time‐of‐flight technique. Copyright © 2006 John Wiley & Sons, Ltd.

Influence of tellurium content and chlorine concentration on carrier drift mobility of Se 100–x Te x :Cl glasses

Carrier drift mobility in undoped and doped with chlorine Se100-xTex (0 ≤ x ≤ 10) glassy alloys has been investigated as a function of Te content and Cl concentration. It was found that the drift mobilities of both electrons and holes were decreased with Te content increasing. The dependence of the carriers drift mobilities on Cl concentration followed a complicated behavior and was resulted in rapid decrease of the hole mobility when chlorine concentration approached 0.1 at.%. The concentrations and energies of localized states controlling the charge transport in the material have been found depending on Te content and Cl concentration. The observed charge-transport features are considered to attribute to changes in the concentrations of intrinsic charged defects and explained in terms of VAP model. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Hole-transporting [3,3′]bicarbazolyl-based polymers and well-defined model compounds

Polymers containing bicarbazolyl moieties in the main chain have been synthesized by the modified Ullmann coupling reaction from 9H,9′H-[3,3′]bicarbazolyl and different dihalo derivatives. The number-average molecular weights of the polymers synthesized were in the range of 2500–6200 with a molecular weight distribution of 1.6–3.1. Well-defined model compounds for the polymers have been synthesized by stepwise reactions. All these compounds have been found to form glasses with glass transition temperatures in the range of 57–119 °C as characterised by differential scanning calorimetry. The electron photoemission spectra of the compounds have been recorded and the ionisation potentials of 5.35–5.4 eV have been established. Room temperature hole drift mobility of the synthesized compounds molecularly dispersed in a polymer host range from 10 −6 to 3 × 10 −5 cm 2/V s at an electric field of 10 6 V/cm at the room temperature.

Soluble polyphenylene homopolymers with controllable microstructure and properties: Optical and electrical characteristics of completely dehydrogenated poly(1,3-cyclohexadiene) as a π-conjugated polymer semiconductor

The optical and electrical characteristics of soluble polyphenylene (PPH) homopolymers obtained by the complete dehydrogenation of poly(1,3-cyclohexadiene) (PCHD) are reported for the first time. The optical properties were strongly affected by the molar ratios of 1,2-/1,4-phenylene (Ph) units. The HOMO and LUMO energy levels were approximately −5.2 and −2.0 eV, and the microstructure did not influence those energy levels or the band gap energies. The generation of carriers (both electron and hole) in the polymer films was observed, and the drift mobility of electrons and holes was affected by the molar ratios of 1,2-/1,4-Ph units in the polymer chain. The drift mobility of electrons in PPH homopolymers with a high content of 1,4-Ph units was in the order of 10 −4 to 10 −5 (cm 2/Vs). The I– V characteristics of soluble PPH homopolymers were controllable by the microstructure of PPH. The 1,2-Ph unit imparted appropriate solubility and toughness to the PPH homopolymers.

Di(9-alkylcarbazol-3-yl)arylamines as Electroactive Amorphous Materials for Optoelectronics

Star-shaped molecules of di(9-alkylcarbazol-3-yl)arylamines were synthesized and found to constitute new glass-forming materials with glass transition temperatures ranging from 40 to 147°C. The electron photoemission spectra of the molecular glasses were recorded and ionisation potentials of 4.9–5.0 eV were established. Room temperature time of flight hole drift mobilities of the di(9-alkylcarbazol-3-yl)phenylamines molecularly dispersed in polycarbonate-Z approached 5 · 10−6 cm2/Vs at high electric fields. Some of the compounds were converted to cross-linkable derivatives, which are potential components for insoluble charge transport layers.

Phenotiazinyl-based hydrazones as new hole-transporting materials for electrophotographic photoreceptors

Various phenothiazinyl-based hydrazones including cross-linkable derivatives have been synthesized by the multi-step synthetic route. Full characterization of their structure is presented. The materials were examined by various techniques including differential scanning calorimetry, UV spectrometry, electron photoemission and time of flight techniques. The electron photoemission spectra of the layers showed the ionisation potentials of 5.3–5.4 eV. Room temperature hole drift mobility of some derivatives molecularly dispersed in bisphenol Z polycarbonate approached 5 × 10 −6 cm 2/Vs at high electric fields.

N, N-Bis(2,2-diphenylvinyl)anilines as new effective glass-forming hole transport materials for electrophotographic photoreceptors

A series of N, N-bis(2,2-diphenylvinyl)anilines was synthesized by a stepwise route and characterized by nuclear magnetic resonance, infrared and mass spectrometries. The thermal, optical and photoelectrical properties of the newly synthesized compounds are reported. The synthesized compounds form glasses with the glass transition temperatures ranging 71–76 °C as characterized by differential scanning calorimetry. The electron photoemission spectra of the synthesized compounds have been recorded and the ionization potentials of 5.45–5.55 eV have been determined. The best charge transport properties were shown by N, N-bis(2,2-diphenylvinyl)-4-methoxyaniline. Room temperature hole-drift mobilities in its 50% solid solutions in bisphenol Z polycarbonate (PC-Z) established by the xerographic time-of-flight technique were found to exceed 10 −4 cm 2 V −1 s −1 at electric field of 10 6 V cm −1.

Density of states of a-Se near the valence band

The density of states (DOS) distribution in the vicinity of the valence band mobility edge in vacuum coated a-Se films has been constructed by studying the dependence of the shape of time-of-flight (TOF) transient photocurrent on the electric field and temperature, and also the dependence of the drift mobility on the temperature and field. We provide not only new experimental TOF data, but also re-examine previous TOF data, and show that, analogous to electron transport in a-Si:H, the DOS near Ev is a featureless, monotonically decreasing distribution in energy up to Ev+0.4eV, without the 0.28eV peak near the valence band which was thought to control the hole drift mobility. Such a DOS was able to account for hole TOF data reported previously by several authors.

Hole mobilities and the physics of amorphous silicon solar cells

The effects of low hole mobilities in the intrinsic layer of pin solar cells are illustrated using general computer modeling; in these models electron mobilities are assumed to be much larger than hole values. The models reveal that a low hole mobility can be the most important photocarrier transport parameter in determining the output power of the cell, and that the effects of recombination parameters are much weaker. Recent hole drift-mobility measurements in a-Si:H are compared. While hole drift mobilities in intrinsic a-Si:H are now up to tenfold larger than two decades ago, even with recent materials a-Si:H cells are low-mobility cells. Computer modeling of solar cells with parameters that are consistent with drift-mobility measurements give a good account for the published initial power output of cells from United Solar Ovonic Corp.; deep levels (dangling bonds) in the intrinsic layer were not included in this calculation. Light-soaking creates a sufficient density of dangling bonds to lower the power from cells below the mobility limit, but in contemporary a-Si:H solar cells degradation is not large. We discuss the speculation that light-soaking is ‘self-limiting’ in such cells.

Electron and hole transport in microcrystalline silicon solar cells studied by time-of-flight photocurrent spectroscopy

A photocurrent time-of-flight study of carrier transport in microcrystalline silicon pin diodes prepared over a range of crystallinities is presented. Electron and hole drift mobilities at a crystalline volume fraction >0.35 are typically 3.8 and 1.3cm2/(Vs) respectively at 300K and a thickness to electric field ratio of 1.8×10−7cm2/V. A factor of five enhancement in hole mobility over amorphous silicon persists at a crystalline volume fraction as low as 0.1. Current decays are dispersive and mobilities are thermally activated, although detailed field-dependence is still under investigation. Evidence for a sharp fall in the density of states at 0.13eV above the valence band edge is presented. Similarities in behaviour with certain amorphous and polymorphous silicon samples are identified.