Charge Generation Material Research Articles

Time-Resolved Absorption Study on the Photocarrier Generation Process in Layered Organic Photoreceptors: A Role of Delocalized Holes in Photocarrier Generation

We have applied time-resolved absorption spectroscopy to the study on the photocarrier generation process in layered organic photoreceptors. The photoreceptors are comprised of azo-pigments as charge generation materials (CGMs) and charge transport materials (CTMs). Two or three distinct transient absorptions ranging from visible to mid-infrared wavelength region (600 – 2500 nm) were observed after photoexcitation in these photoreceptors. We ascribed all these transients to the cationic species of CTMs from the results of the measurements in various conditions. The transient absorption spectrum showed remarkable dependence on the CTM concentration. From this observation, we concluded that the cationic species of CTMs were monomer, dimer and larger aggregate cations. The latter two cationic species are regarded as the holes delocalized within some CTM molecules. The role of delocalized holes in photocarrier generation is discussed from the decay dynamics.

Single-layered organic photoreceptors based on chlorodiane blue/TiOPc/BAH three component composites: I. Device fabrication and photoconductivity

By using an amphiphilic organic solvent 1,4-dioxane, both charge generation materials (CGMs) of chlorodiane blue (CDB) and titanium oxide phthalocyanine (TiOPc) as well as charge transport material (CTM) N, N′-diethyl-4-amino-benzaldehyde-1-phenyl-1′-α-naphthyl-hydrozone (BAH) were uniformly dispersed in a polycarbonate (PC) matrix. The fabricating conditions, such as solvents, components, and device configurations that affected the photoconductive property, were examined. The single-layered photoreceptors based on these composites showed higher photosensitivity in comparison with those of dual-layered photoreceptors. Experimental results indicated that TiOPc pigment was significantly sensitized by CDB, and gave rise to synergetic enhancement in photosensitivity.

Photoconductivity Study of Modified Carbon Nanotube/Oxotitanium Phthalocyanine Composites

Carbon nanotubes (CNTs) bonded to dodecylamine groups were obtained by chemical modification. The modified CNTs showed improved solubility in organic solvents and miscibility with polymer matrix. The photoconductivity of oxotitanium phthalocyanine (TiOPc) doped with modified CNTs was investigated by the xerographic photoinduced discharge method. The results showed that the photosensitivity of the dual-layer photoreceptor (P/R) with modified CNT/TiOPc composite as charge generation material was higher than that with pristine TiOPc and increased with increasing the content of modified CNTs in the composites. It was explained that, as demonstrated by X-ray photoelectron spectroscopy (XPS), after doping with modified CNTs, the intramolecular charge transfer occurred in TiOPc and the charge-transfer exciton was prone to be produced, which was favorable to charge carrier generation. The efficiency of charge carrier separation increased as well because of the photoinduced charge transfer between TiOPc and CNTs, which might also contribute to the improved photosensitivity of the modified CNT/TiOPc composites.

Crystal structures and photoconductivity of charge generation materials

Crystal structures and photoconductivity of charge generation materials

Investigation on the photoconductive composites: TiOPc with various AZOs

The relationship between photoconductivity and content of azo has been studied in double-layered photoreceptor devices where azo/titanium oxide phthalocyanine (TiOPc) composites were used as the charge generation materials (CGMs) and N, N′-diethyl- p-phenyl aldehyde biphenyl hydrazone (DENH) served as the charge transporting material (CTM). Composites of 3,3′-dichloro-4,4′-bis(1″-azo-2″-hydroxy-3″-naphthoyl- o-chloroanide) biphenyl (DBCL) and TiOPc change photosensitivity with the content of DBCL, and two maximums were observed at 5 and 60% (by weight), respectively. Composites of 4,4′-bis (1″-azo-2″-hydroxy-3″-naphthoyl- o-chloroanide)biphenyl ether (DBO) and TiOPc show two maximums at 25 and 68%, respectively, while two maximums at 8 and 70% were recorded for composites of TiOPc with 4,4′-bis(1″-azo-2″-hydroxy-3″-naphthoyl- o-chloroanide) biphenyl (AZO-AS). Results from ESR measurements indicate that there exists a charge transfer from azos to TiOPc. Different contents of TiOPc change the degree of the charge transfer, resulting in various xerographic properties of the composites.

Industrial applications of phthalocyanines

Phthalocyanines are the second most important class of colorant, and copper phthalocyanine is the single largest-volume colorant sold. Traditional uses of phthalocyanine colorants are as blue and green pigments for automotive paints and printing inks and as blue/cyan dyes for textiles and paper. Phthalocyanines have also found extensive use in many of the modern high technologies, e.g. as cyan dyes for ink jet printing, in electrophotography as charge generation materials for laser printers and as colorants for cyan toners. In the visible region, phthalocyanines are limited to blue, cyan and green colours. However, their absorption may be extended into the near infrared and by suitable chemical engineering it is possible to fingerprint the 700-1000 nm region. The properties and effects of these infrared-absorbing phthalocyanines are diverse and cover many important hi-tech applications, including photodynamic therapy, optical data storage, reverse saturable absorbers and solar screens.

Novel Crystals of Titanium Oxide Phthalocyanine with Submicron Diameter

Reported here are novel submicron diameter crystals of titanium oxide phthalocyanine (TiOPc) and the electrophotographic properties of photoreceptors that employ them. The use of triether solvents such as diethylene glycol dimethylether and dimethoxy tetrahydrofuran produces five times smaller TiOPc crystals than the phase 3 TiOPc crystals produced in tetrahydrofuran. The photosensitivity of these smaller TiOPcs is as high as that of the tetrahydrofuran produced phase 3 TiOPc crystals, which have been a source for one of the most photosensitive charge generation materials used in electrophotographic photoreceptors.

Investigation of the photoconductive AZO-AS/TiOPc composites

The relationship between photoconductivity and composition has been studied in double-layered photoreceptor devices where AZO-AS/TiOPc composites were used as the charge generation materials (CGMs) and DENH served as the charge transporting material (CTM). It was found that AZO-AS/TiOPc composites have high photosensitivity not only in the visible region but also in the near-IR region and their xerographic properties vary with the ratio of AZO-AS to TiOPc. The composites exhibit better sensitivity at ∼500 nm than at other wavelengths at the ratio of 6 : 4 (by weight) while the best sensitivity was found at ∼570 nm and in the near-IR region at the ratio of 1 : 9 (by weight). The absorption spectra and X-ray powder diffraction patterns were investigated and some changes were found in the composites. For the first time, a charge transfer from AZO-AS to TiOPc was revealed by the measurements of XPS and ESR. Various xerographic properties of the composites result from different degrees of charge transfer as the content of TiOPc varies.

Xerographic property of azo/TiOPc composites in double-layered photoreceptor

The xerographic property has been studied in the double-layered photoreceptor device where azo/TiOPc composite is used as the charge-generation material (CGM) and p-diethylaminobenzaldehyde-α-naphthalenylphenylhydrazone (DENPH) or p-dimethylaminobenzaldehyde diphenylhydrazone (DMDPH) is used as the charge-transportation material (CTM) under the exposure of various spectral regions. The results show that the azo/TiOPc composites exhibit high photosensitivity not only in the visible and near-infrared regions, but also in the entire spectral ranges, indicating the co-enhancement effect. The UV–VIS spectra suggest that the absorption spectra of the azo/TiOPc composites are broadened. The composite mechanism is investigated through X-ray diffraction and ESR.

Xerographic property, optical absorption, and X-Ray diffraction study of azo/TiOPc composites

The photoconductivities of bilayer photoreceptor devices, containing azo pigments or titanium oxide phthalocyanine (TiOPc) or azo/TiOPc composites as the charge-generation materials (CGMs), and p-diethylaminobenzaldehyde- α-naphthalenyl-phenylhydrazone (DENPH) or p-dimethylaminobenzaldehyde diphenylhydrazone (DMDPH) as the charge-transportation materials (CTMs), have been studied. The results show that the photosensitivities of azo/TiOPc composites are higher than that of pure azo pigments or pure TiOPc. UV/Vis spectra indicate that the absorption spectra of the azo pigments can be broadened by blending with TiOPc. The X-ray diffraction patterns of the blended composites are also studied.

Absorption Spectra and Electrophotographic Properties of Squarylium Dyes Containing a Nitro group

Squarylium dye containing a nitro group, absorbs at 676 nm in chloroform and showed a bathochromic shift of 22 nm in comparision to the unsubstituted analogue. The reaction between squaric acid and 2,3,3-trimethyl-5-nitroindolenine proceeded readily giving a product in 97% yield. Electrophotographic characteristic of negatively charged dual-layered photoreceptors with squarylium dyes for charge generation materials and five charge transport materials have been investigated. Photoreceptors that used squarylium dyes containing a nitro group, exhibited high sensitivity under light exposure.

Synthesis and photoconductivity study of phthalocyanine polymers. VI. Ethylenediamine bridged polymeric MnPc

A novel polymer with quasi-one-dimensional structure of ethylenediamine bridged polymeric manganese phthalocyanine [PcMn(eda)]n was synthesized by complexing of manganese phthalocyanine (MnPc) and ethylenediamine (eda). The structure of the polymer was identified by infrared, ultraviolet–visible, and differential scanning calorimetric analysis. It was found that the polymer [PcMn(eda)]n showed much better photoconductive property than MnPc monomer when [PcMn(eda)]n or MnPc was used as the charge-generation material (CGM)-matched hydrazone charge-transportation material (CTM). Furthermore, the photoconductivities of [PcMn(eda)]n and MnPc were improved if the charge-generation layers (CGLs) were made by electric field poling. All these observations are ascribed to the effects of the one-dimensional molecular structure and the ordered material structure, respectively. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 69: 2609–2613, 1998

Synthesis and photoconductivity study of phthalocyanine polymers. VI. Ethylenediamine bridged polymeric MnPc

A novel polymer with quasi-one-dimensional structure of ethylenediamine bridged polymeric manganese phthalocyanine [PcMn(eda)]n was synthesized by complexing of manganese phthalocyanine (MnPc) and ethylenediamine (eda). The structure of the polymer was identified by infrared, ultraviolet–visible, and differential scanning calorimetric analysis. It was found that the polymer [PcMn(eda)]n showed much better photoconductive property than MnPc monomer when [PcMn(eda)]n or MnPc was used as the charge-generation material (CGM)-matched hydrazone charge-transportation material (CTM). Furthermore, the photoconductivities of [PcMn(eda)]n and MnPc were improved if the charge-generation layers (CGLs) were made by electric field poling. All these observations are ascribed to the effects of the one-dimensional molecular structure and the ordered material structure, respectively. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 69: 2609–2613, 1998

Syntheses, characteristics and electrophotographic properties of new dithiosquarylium dyes

A new class of dithiosquarylium dyes (DTSQ) has been synthesized by the reaction of squarylium dye (SQ) with Lawesson's reagent or P 4S 10. The λ max , of DTSQ dyes undergoes a bathochromic shift of about 25 nm compared with corresponding SQ dyes. The visible absorption spectra of DTSQ dyes are well accounted for by Pariser-Parr-Pople Molecular Orbital (PPP-MO) calculations. Electrophotographic characteristics of negatively charged dual layered photoreceptors with DTSQ for charge generation materials (CGM) and 1-phenyl-1,2,3,4-tetrahydroquinoline-6-carboxyaldehyde-1′,1′-diphenylhydrazone as a charge transport material (CTM) have been investigated. The photoreceptor that used DTSQs exhibited high photosensitivity at white light.

Preparation and photoconductivity study of TiOPc nanometer particles

Oxotitanium phthalocyanine (TiOPc) nanometer particles dispersed in polycarbonate (PC) resin are obtained successfully by the method of mechanical sand grinder. The mean grain sizes of TiOPc particles and the particle morphology are measured by particle size analyzer and TEM. The xerographic properties of TiOPc nanometer particles are investigated in function-separated single-layer photoreceptors that consist of TiOPc nanometer particles as the charge-generation material (CGM) and α-naphthalic hydrazone ( α-NH) as the charge-transportation material (CTM). It is found that the photosensitivity of the function-separated single-layer photoreceptor is higher than that of the double layer device (same CGM and CTM). The photoconductivity–particle size relationship study shows that the smaller the particle size of TiOPc, the better the photoconductivity, which may result from the large specific surface area of the TiOPc nanometer particles.

The effect of laser wavelength on the photorefractive characteristics of PMDA-DR19 based photorefractive polymeric materials

The photoconductivity and photorefractive characteristics of PMDA-DR19-based photoconducting nonlinear optical polymeric films with different compositions were studied using red and green lasers. Contributions of dopants and the host to the photoconductive and photorefractive properties are systematically discussed. The virgin nonlinear optical material PMDA-DR19 was neither photoconductive nor photorefractive in nature. However, it became photoconductive and photorefractive once the charge transport material was incorporated into the PMDA-DR19/PVB polymer film. Films containing more nonlinear optical chromophore exhibited higher diffraction efficiency. Response speed was further elevated when an additional charge generation material was doped in. Diffraction grating was observed under zero electric field and was confirmed due to a photorefractive effect by the two-beam coupling effect. In addition to composition modification, the laser light source was also an important factor in changing the PR response speed. The photorefractive response speed was tremendously improved when films were written and read by two different laser beams, either red/green lasers or green/red lasers.

Soluble alkyl‐ and alkoxy‐substituted titaniumoxo phthalocyanines: Synthesis and photoconductivity

The high photosensitivity of titaniumoxo phthalocyanines makes them suitable for application as charge generation materials, e.g., in photocopiers. The synthesis, photophysical, and photochemical properties of soluble titaniumoxo phthalocyanines containing alkyl and alkoxy substitutents (see Figure) in the periphery of the phthalocyanine ring are reported, the photoconductivity and photooxidation behavior being stressed. magnified image

A New Polymorph of Hydroxygallium Phthalocyanine and Its Application in a Photoreceptor

In recent polymorphism studies of gallium phthalocyanines, a new crystal form of hydroxygallium phthalocyanine (Type V HOGaPc) was obtained. It shows an extremely high photosensitivity (E1/2 = 1.3 mJ/m+2 at 780 nm) and excellent thermal stability. Furthermore, its photosensitivity is not affected by changing humidity. Hence, it is an useful charge generation material for an IR photoreceptor. From the TGA and Rietveld analysis of the Type V crystal, it is indicated that the highly photosensitive characteristic of this pigment does not depend on sensitization by water. The high sensitivity is attributed to the specific crystal structure and to the presence of the hydroxyl group, both of which are favorable for charge generation.

Inter-layer mixing effect on the transport properties of a dual-layer organic photoreceptor interface

Dual-layer organic photoreceptors, with applications in electrophotography, are widely fabricated using solvent-casting techniques. The presence of solvent when overcoating the charge generation layer with the charge transport layer suggests that the interface between the two layers will not be sharp and may consist of a mixed layer containing both charge transport and charge generation materials. In this work we have used transient photoconductivity techniques to examine the effects on hole transport of adding small amounts (up to a few percent by weight) of charge generation material (x-form metal-free phthalocyanine) to a standard charge transport layer ( p-diethylaminobenzaldehyde-1,1′-diphenylhydrazone in a polyester binder). The addition of x-form metal-free phthalocyanine is found to have no effect on the hole mobility of the transport layers but greatly affects the observed time-of-flight pulse shapes. It is proposed that the addition of the x-form metal-free phthalocyanine creates deep trapping states and this is supported by simple computer simulations.

Crystal structures and light absorption spectra of 1,4-dithioketo-3,6-diphenyl-pyrrolo-[3,4-c]-pyrrole

1,4-dithioketo-3,6-diphenyl-pyrrolo-[3,4-c]-pyrrole, which is a good charge generation material for a near-infrared electrophotographic photoreceptor, has three polymorphs called the I, II, and III forms. The I and II forms are transformed into the III form after having been kept in suitable organic solvents. On the other hand, the III form is changed to the I and/or II forms by heat treatment above 320 °C. Among the three polymorphs, only the highly ordered III form shows the necessary and characteristic absorption in the near-infrared region.