JPH05210247A - Organic electrophotographic sensitive body - Google Patents

Abstract

(57) [Summary] (Modified) [Purpose] Highly sensitive organic electrophotography capable of high-sensitivity photoresponse by defining the electrical properties of the carrier generation layer from the energy levels of the photoconductive material and the doping material. Provide a photoreceptor. In a negatively charged organic electrophotographic photosensitive member having an electrode layer and an organic photoconductive laminated structure forming a carrier generation layer and a carrier transport layer, the energy from the vacuum level of the photoconductive layer to the upper end of the valence band The difference E _{V is} the energy difference E _A from the vacuum level when the acceptor is doped in the photoconductive layer to the acceptor level created in the band gap of the photoconductive layer, E _A is
Defining the electrical properties of the charge generation layer so as to satisfy _A <E _{V -4kT} / q (where k In the formula is the Boltzmann constant, T is the absolute temperature, q is the elementary charge).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electrophotographic photosensitive member having a highly sensitive photoresponsiveness by defining the electrical properties of a carrier generation layer from the energy levels of a photoconductive material and a doping material. Regarding the body

[0002]

2. Description of the Related Art In recent years, research and development of organic electrophotographic photoconductors applied to copying machines, facsimile machines, laser beam printers, etc. have been actively conducted.

The early organic electrophotographic photoreceptors were reported by JW Weigl et al. In the early 1970s (Current Problem in Electrop
As shown in the hotography), an organic photoconductive pigment such as phthalocyanine is dispersed as a photoconductive material in a polymer, and as shown in a cross section in FIG. 14, a film thickness of 10 to 20 μm is formed on one surface of an electrode. It had a single layer structure formed by film formation.
In FIG. 14, 2 indicates a photoconductive material dispersed in the polymer 3. The operation mechanism in this single-layer structure is that electric charges are generated in the photoconductive material 2 on the surface of the photoconductor 4 by light irradiation, and the carriers having the same sign as the surface charge among the generated free electrons and free holes are generated by the electric field. According to the direction of the photoconductive material 2, the photoconductive material 2 is conducted and is recombined with the electric charge on the substrate (electrode) 1. This structure is very simple in terms of process, but the carrier transport efficiency is extremely low, and the sensitivity is considerably inferior to that of inorganic type such as silicon type or chalcogen type.

On the other hand, from 1976 to 1978, FIG.
As shown in a sectional view in FIG. 1, a negative charge type organic material having a laminated structure in which a carrier generation layer (film thickness about 1 μm) 5 and a carrier (hole) transport layer (film thickness 10 to 20 μm) 6 are provided on the surface of an Al electrode. Electrophotographic photoreceptors have been proposed and reported to have achieved practical performance (eg PLNelz et al., Phot
ogr.Sci.Eng.21,73 (1977)). The operation mechanism in this laminated structure is that the carrier generation layer 5 generates charges by light irradiation, and holes among the generated free electrons and free holes are selectively injected into and transported into the carrier transport layer 6 to generate surface charges. To rejoin with. In this structure, the respective roles of carrier generation and carrier transport are shared by the layers 5 and 6, and the low carrier transport efficiency, which has been a problem with the single-layer photoconductor 4 described above, is greatly improved.

From this, it has been clarified that the photosensitivity of the photoconductor is determined by the carrier generation efficiency of the photoconductive material used for the carrier generation layer 5. However, the design of an organic material having a high carrier generation efficiency is determined. At present, there is no guideline, and only a few carrier generation materials have been obtained by comprehensive molecular synthesis and screening thereof. It has also been pointed out that the loss of carrier injection at the interface between the carrier generation layer 5 and the carrier transport layer 6 affects the photosensitivity, but at present there is no definitive solution, and it is At present, the carrier (hole) transfer material having a small ionization potential is selected to improve the carrier injection efficiency.

[0006]

The organic electrophotographic photosensitive member is almost satisfying the performance for a low speed machine due to long-term development, but due to the downsizing and speeding up of the electrophotographic apparatus,
There is a demand for a photoconductor having a photosensitivity with higher sensitivity. In order to realize such a requirement, the charge characteristic of the photoconductor is improved by suppressing the injection of thermal carriers from the electrode into the photoconductive layer, and the carrier generation efficiency in the carrier generation layer, which can be said to be the heart of the photoconductor, In addition, it is necessary to improve the carrier injection efficiency from the carrier generation layer to the carrier transport layer. However, compared with inorganic materials such as amorphous silicon, these performances are inferior in the case of organic materials, and there are almost no guidelines for improving carrier generation / carrier injection, which remains an unsolved problem. There is.

It is generally known that the carrier generation efficiency of organic electrophotographic photosensitive materials is at most 20 to 30%, usually 1% or less. This situation will be described using a semiconductor band model with reference to FIG. First, when a photon having energy higher than the band gap is incident on a solid, its absorption occurs. That is, the electrons in the valence band are excited in the conduction band. Here, if the speed at which the excited electrons 7 become free carriers due to an external field such as an electric field is K _G and the speed at which they recombine (deactivate) with the holes 8 in the valence band is K _R , the probability of becoming free carriers ( Quantum efficiency) is Φ = K _G / K _G
Expressed as + K _R. In a general organic electrophotographic photosensitive material, since the value of K _G is considerably smaller than the value of K _R , the carrier generation efficiency shows a small value. On the other hand, in inorganic materials such as silicon, the value of K _G is large and the efficiency is close to 1. Therefore, it is necessary to select and set a material having a large K _G and a small K _R as the carrier generating material of the photoconductor. In other words, it is necessary to optimize the electrical properties of the organic electrophotographic photosensitive material to be used in order to satisfy the required characteristics.

The present invention has been made in consideration of the above circumstances, and enables high-sensitivity photoresponsiveness by defining the electrical properties of the carrier generation layer from the energy levels of the photoconductive material and the doping material. The purpose of the present invention is to provide an organic electrophotographic photoreceptor.

[0009]

According to the present invention, a band model is applied to an organic electrophotographic photosensitive layer to determine the electrical properties of a carrier generation layer such as the conduction band, Fermi level and valence band of a photoconductive material. Of the doping material and the acceptor level or the donor level of the doping material, the main point is to realize highly sensitive photoresponsiveness.

That is, according to the present invention, in the organic electrophotographic photosensitive member formed by laminating the organic layers of the carrier generating layer and the carrier transporting layer on the electrode surface, the carrier generating layer comprises the photoconductive material and the acceptor material. contained, the energy difference between the vacuum level of the photoconductive material to the upper end of the valence band (valence band level) and E _V, photoconductive material from the vacuum level by doping the acceptor material to the photoconductive material Let E _A be the energy difference (A level) to the acceptor level created in the band gap of
_A conditional _{expression, E A <E V -4kT /} q (q is an elementary charge, k
Where B is the Boltzmann constant and T is an absolute temperature), or the carrier generation layer contains a photoconductive material and an acceptor material, and the energy from the vacuum level of the photoconductive material to the lower end of the conduction band is satisfied. the difference (conduction band level) as the E _C, the photoconductive material doped with a donor material energy difference from the vacuum level to donor levels to create within the band gap of the photoconductive material (D level) E _D , E _D is a conditional expression, E _D > E _C +
It is characterized by satisfying 4 kT / q (q: elementary charge, k: Boltzmann constant, T: absolute temperature).

[0011]

In the organic electrophotographic photosensitive member according to the present invention, the photoconductive layer is doped with an electrically neutral acceptor material or donor material, and the electrical properties of the carrier generation layer are regulated from the energy level. ing. However, the charging property in the dark is almost unchanged as compared with the case where no doping is performed, and the A level (D level) acts as an effective electron trap level (hole trap level). Due to this, excited electrons (excited holes) are quickly captured at the A level (D level), and the generation efficiency of free holes (free electrons) is greatly increased. Therefore, high-sensitivity photoresponsiveness is easily exhibited, and the function improvement as an organic electrophotographic photoreceptor is achieved.

[0012]

EXAMPLES Examples of the present invention will be described below.

The organic electrophotographic photosensitive member according to the present invention having the above structure is based on the following principle. First, as schematically shown in Fig. 1 (b), the energy difference from the vacuum level to the Fermi level of the photoconductive material (hereinafter, abbreviated as Fermi level) is E _F , and from the vacuum level to the lower end of the conduction band. Energy difference (hereinafter, abbreviated as conduction band level) to E _C , and energy difference from the vacuum level to the upper end of the valence band (hereinafter,
The valence band level and abbreviation) is E _V. The energy difference (hereinafter referred to as A level) from the vacuum level to the acceptor level when the acceptor material is doped in the photoconductive material is E _A , and the donor material is in the photoconductive material. The energy difference from the vacuum level to the donor level (hereinafter, abbreviated as D level) in the doped state is E _D.

Here, considering the case where the photoconductive layer is doped with an acceptor material or a donor material, the electronegativity x _HOST in the monomolecular state of the photoconductive material,
The magnitude relationship of the electronegativity x _{GEST in} the single molecule state of the doping material has a great influence on the electrical properties in the solid state. The electronegativity is obtained by dividing the sum of the electron affinity and the ionization potential in a single molecule state placed in a vacuum by 2, and corresponds to the Fermi level E _F in the solid state. Then, as schematically shown in FIG. _2A , x _HOST > x _GEST in the donor material, and the photoconductive material exhibits the property of attracting electrons as compared with the doping material. Therefore, in the solid state, the D level acts as a hole trap level. On the other hand, as schematically shown in FIG. 2B, the acceptor material has x _HOST <x _GEST , and the doping material has a property of attracting electrons as compared with the photoconductive material. Therefore, in the solid state, the A level acts as an electron trap level. In addition,
2 (a) and 2 (b), I _P is the ionization potential, E
_a is the electron affinity.

On the other hand, considering a photoconductive layer doped with a donor material in a dark state at room temperature and in a thermal equilibrium state, the D level formed in the bandgap of the photoconductive layer has a thermal energy kT / q (k is Boltzmann). Constant, T is absolute temperature, q is elementary charge, and depends on the magnitude of energy difference ΔE (= E _D −E _C ) between the D level and the conduction band level,
The rate at which it is positively charged changes. That is, FIG. 3 (a)
As schematically shown in FIG. 3, the density of the positively charged D level is [D ⁺ ] and the density of the electrically neutral D level is [D ⁰ ].
And the charge ratio of D level at this time, [D ⁺ ] /
Assuming that [D ⁰ ] follows the Boltzmann distribution, ΔE =
With kT / q, [D ⁺ ] / [D ⁰ ] to 0.5, ΔE = 4 kT
/ Q is D ⁺ / D ^{0 to} 0.02, and ΔE = 10 kT / q is [D ⁺ ] / [D ⁰ ] to 0.00001. The larger ΔE is compared with the thermal energy, the more rapidly [D ⁰ ] becomes The rate increases. Further, as ΔE becomes smaller than thermal energy, thermal excitation of electrons from the D level to the conduction band becomes easier. Therefore, free carriers inside the photoconductive layer (in this case, free electrons in the conduction band) decrease as ΔE increases. Further, the probability that the D level traps free holes in the valence band increases as ΔE increases.

Further, considering the case of irradiating the photoconductive layer with light having energy higher than the band gap, the molecular density of the donor material is set to the molecular density of the photoconductive material.
When it is 1% or less, most of the optical absorption is performed by the photoconductive material. And the behavior of holes generated in the valence band is
As shown schematically in FIG. 4 (a), it is divided into the following three processes. (1) A process of dissociating from electrons to become free electrons and free holes (carrier generation rate K _G ). (2) The process of recombining with electrons that have fallen into the valence band (recombination rate K _R ). (3) Process of being trapped at D level (hole trapping rate K _HT ).

Here, the process (3) is a process of trapping excited holes to generate free electrons in the conduction band. Then, the probability of becoming a free carrier when the D level is involved is Φ = (K _G + K _HT ) / (K _G + K _R +
Represented by K _HT), the proportion of recombination as K _HT approaches increased and K _R is greatly reduced. Therefore, the carrier efficiency Φ is greatly increased as compared with the case without the process (3), and in particular, a large number of free electrons are generated as compared with free holes.

Next, considering a photoconductive layer doped with an acceptor material in a dark state, at room temperature and in a thermal equilibrium state, the A level formed in the band cap of the photoconductive layer has a thermal energy kT / q (k: The Boltzmann constant, T is the absolute temperature, q is the elementary charge, and the energy difference ΔE (= E _V −E _A ) between the A level and the valence band level depends on the size of Change. That is,
As schematically shown in FIG. 3B, the density of the negatively charged A level is the [A ⁻ ] level, and the electrically neutral A level is the [A ⁰ ] level. Assuming that the charging rate at the A level at this time follows the Boltzmann distribution, ΔE = kT / q
In ^{[A -] / [A 0} ] ~0.6, △ E = 4kT / q in ^{[A -] / [A 0} ] ~0.02, and △ E = 10kT / q in ^{[A -] / [A 0} ] ~ The value becomes 0.00005, and the ratio of [A ⁰ ] increases rapidly as ΔE becomes larger than the thermal energy. Further, as ΔE becomes smaller as compared with the thermal energy, thermal excitation of holes from the A level to the valence band becomes easier, so that free carriers inside the photoconductive layer (in this case, free positive holes in the valence band) are excited. The hole) decreases as ΔE increases. Further, the probability of capturing free electrons in the conduction band at the A level increases as ΔE increases.

Here, the case where the photoconductive layer is irradiated with light having energy larger than the band gap will be considered. At this time, if the molecular density of the acceptor material is 1% or less with respect to the molecular density of the photoconductive material, most of the optical absorption is performed by the photoconductive material, and the behavior of the electrons hit into the conduction band is As shown schematically in Fig. 4 (b), the following 3
Divided into processes.

(4) A process of dissociating from holes to form free electrons and free holes (carrier generation rate K _G ). (5) Process of falling into the valence band and recombining with holes (recombination rate K _R ).

(6) Process of being captured at A level (electron capture rate K _ET ).

Here, the process (6) is a process of capturing free electrons in the valence band by trapping the excited electrons. And the probability of becoming a free carrier when the A level is involved is Φ = (K _G + K _ET ) / (K _G + K _R + K _ET ).
The recombination rate decreases greatly as K _ET increases and approaches K _R. Therefore, the carrier efficiency Φ is
The number is greatly increased as compared with the case without the above process (6), and in particular, a large number of free holes are generated as compared with free electrons.

The electrical properties required for the carrier generation layer of the negatively chargeable organic electrophotographic photosensitive member are: (7) The density of free holes is low in darkness (improvement of charging property).

(8) The density of free holes is high during light irradiation (improvement of photosensitivity).

It is required to satisfy For this reason,
An acceptor material is doped as a carrier generation layer, the energy position of the A-level at that time, is satisfied _{E A <E V -4kT / q} (1), A levels mostly electrically neutral level Becomes Therefore, an electrically neutral [A ⁰ ]
Acts as a trapping level for excited electrons, and is free positive compared to when [A ⁰ ] is smaller than [A ⁻ ] or when the donor material is doped or not doped. It is expected that the efficiency of pore generation will be greatly improved. Further, as described above, most of the A levels are neutral, so that the density of free holes in the dark can be suppressed to a very small level, and the charging property of the photoconductor is expected to be improved.

A photoconductive material, an acceptor material,
Since E _C , E _V , E _A , and E _D are different for each donor material, in order to determine the material that satisfies the conditional expression (1), the energy level (E _C , E _V ) and the energy level (E _A , E
A method for determining _D ) is needed. Then, in order to actually confirm the doping effect, a method for determining the carrier generation efficiency of the thin film is required. This can be obtained according to the method described below.

First, in order to determine E _C and E _V of the photoconductive material, a metal electrode 10a, a photoconductive layer (photoconductive material film) 11, a silicon oxide film 12 and a silicon 13 as shown in FIG. An element in which the respective layers of the metal electrode (back surface electrode) 10b are sequentially laminated is formed. Then, a triangular wave voltage is applied to this element, and the displacement current flowing in the element at that time is measured. Assuming that the electric capacitance of the device is C, the displacement current I is I = C · dV / dt (2).

Here, when the photoconductive layer (organic layer) 11 is not provided, the element is usually a well-known MOS element, as shown in FIG.
current value I as shown in (b) is determined by the capacitance C _{Si O} of the silicon oxide film 12. On the other hand, when the organic layer 11 is present, the following displacement current is observed depending on the magnitude relationship between the work function Φ _M of the electrode 10a and the Fermi level E _F of the photoconductive layer (organic layer) 11.

(A) Φ _M <E _{F In} this case, the electrical characteristics of the interface formed between the electrode 10a and the photoconductive layer (organic layer) 11 are ohmic junction for electron injection and blocking junction for holes. Becomes Therefore,
When a triangular wave voltage is applied to the device, electrons are injected from the electrode 10a into the photoconductive layer 11 when the electrode 10a side is negative, and the electrons are accumulated at the interface between the photoconductive layer 11 and the silicon oxide film 12. At this time, the current value I flowing through the element is the capacitance C _{SiO of the} oxide film 12.
It becomes a value determined by and becomes equal to the current level of the MOS element. On the other hand, when the electrode 10a side is positive, the electrons in the photoconductive layer 11 flow away to the metal electrode 10a and holes are not injected, so that the photoconductive layer 11 becomes an insulator. Therefore, the capacitance of the capacitor C _{Si O} and the photoconductive layer 11 of the current value I oxide film 12 C _Org
Becomes a value determined by the series capacitance of, and decreases to a smaller value than the current level of the MOS element. FIG. 6 (a) schematically shows this state.

Next, in order to determine E _{C of the} photoconductive layer 11,
_{Note the} rising voltage V _{Inj at} which the displacement current begins to increase. If the work function of the electrode 10a and the energy barrier at the conduction band level of the photoconductive layer 11 are E _BH = Φ _M −E _C , then E _BH ≦ 0
Then, since there is no energy barrier, the displacement current starts to increase from 0 volt, and V _Inj = 0. On the other hand, when E _BH > 0, there is an energy barrier for electron injection, so the rising voltage is V _Inj = −E _BH , and the larger the energy barrier, the more negative the voltage V _Inj at which the displacement current starts to increase. Shift to the higher voltage side. FIG. 6 (b) schematically shows this state. Such behavior, select the electrodes 10a serving as the E _BH> 0, by determining the V _Inj, it is possible to determine the conduction band level of the photoconductive layer 11 from the E _C = Φ _M -V _Inj.

(B) Φ _M > E _{F In} this case, the electrical characteristics of the interface between the electrode 10a and the photoconductive layer (organic layer) 11 are ohmic junction for hole injection and blocking junction for electrons. Becomes Therefore,
When a triangular wave voltage is applied to the element, holes are injected from the electrode 10a into the photoconductive layer 11 when the electrode 10a side is positive, and the holes are accumulated at the interface between the photoconductive layer 11 and the silicon oxide film 12.
At this time, the current value I flowing through the element is the capacitance C of the oxide film 12.
_The value is determined by _SiO , and is equal to the current level of the MOS element. On the other hand, when the electrode 10a side becomes negative, the photoconductive layer
The holes in 11 flow away to the metal electrode 10a and no electrons are injected, so that the photoconductive layer 11 becomes an insulator. For this reason,
The current value I is the capacitance C _SiO of the oxide film 12 and the capacitance C _SiO of the photoconductive layer.
It becomes a value determined by the series capacitance of _Org , and decreases to a small value as compared with the current level of the MOS element. FIG. 7 (a) schematically shows this state.

Next, in order to determine the E _V of the photoconductive layer, attention is paid to the rising voltage V _{Inj at} which the displacement current starts to increase. Letting E _BH = E _V −Φ _M be the work function of the electrode 10 a and the energy barrier at the valence band level of the photoconductive layer 11, E
_Since there is no energy barrier at _BH ≤ 0, the displacement current starts to increase from 0 volt and V _Inj = 0. On the other hand, when E _BH > 0, there is an energy barrier for hole injection, so the rising voltage becomes V _Inj = E _BH ,
As the energy barrier is larger, the voltage V _Inj at which the displacement current starts to increase shifts to the positive high voltage side. FIG. 7 (b) schematically shows this state. Therefore, E
Select _BH> 0 become electrode 10a, by obtaining the V _Inj, it is possible to determine the valence band level of the photoconductive layer from _{E C = Φ M + V Inj} .

The donor property and the acceptor property of the doping material can be determined as follows. That is, FIG.
An element having the same structure as that shown in (a) is formed (however, the photoconductive layer 11 contains a doping material), and the same displacement current measurement as described above is performed. Doping material E _C , E
_V is determined and the Fermi level E _F is E _F = (E _C +
Obtained from the E _V) / 2. At this time, E _F (photoconductive material)
> If E _F (doping material), doping material acts as a donor. Also, E _F (photoconductive material) <E
_{If F} (doping material), the doping material acts as an acceptor.

Next, in order to determine E _D and E _A of the doping material, the displacement current is similarly measured using the element formed for determining the donor property and the acceptor property of the doping material. Here, when the measurement is performed while irradiating near infrared light or infrared light whose energy is smaller than the band gap of the photoconductive layer, the following displacement current is observed depending on the energy position of the doping material. ..

[0035] (c) when D ⁺ level D level is close to the conduction band, i.e. △ E = E _D -E
_{When C} is small, the D level is positively charged. When near-infrared light enters here, light having a wavelength corresponding to hc / λ = E _V −E _D (h is Planck's constant, c is the speed of light, λ is the wavelength of light) is selectively absorbed, and the valence band To D level, electronic excitation (D ⁺ + e → D ⁰ ) occurs and free holes are generated.
When a metal that does not cause injection of both electrons and holes in the photoconductive layer is selected as the electrode metal, and the current value of the displacement current is observed while changing the wavelength of infrared light, when the wavelength corresponding to the above electronic excitation is obtained, The displacement current greatly increases. FIG. 8 (a) schematically shows this state. From such a phenomenon, from the wavelength λ and the E _{V of the} photoconductive material, the D level E
_D can be determined.

[0036] (d) If D ⁰ level D level is far from the conduction band, ie, △ E = E _D -
If E _C is large, the D level is electrically neutral. In this state, when near infrared light or infrared light is incident, hc /
λ = E _D -E _C (h is Planck's constant, c is the speed of light, lambda is the wavelength of the light) the light of the wavelength corresponding to is selectively absorbed, electron excitation (D ⁰ -e from D levels to the conduction band → D ⁺ ) occurs and free electrons are generated. When a metal that does not cause injection of electrons or holes into the photoconductive layer is selected as the electrode metal, and the current value of the displacement current is observed while changing the wavelength of infrared light,
At the wavelength corresponding to the above electronic excitation, the displacement current greatly increases. FIG. 8 (b) schematically shows this state. From such a phenomenon, the D level E _D can be determined from the wavelength λ and the E _{C of the} photoconductive material.
(e) A ^- If the level A level is close to the valence band, i.e. △ E = E _V -
When E _A is small, the A level is negatively charged. When near-infrared light enters in this state, hc / λ = E _A −E
Light having a wavelength corresponding to _C (h is Planck's constant, c is the speed of light, λ is the wavelength of light) is selectively absorbed, and electronic excitation (A ⁻ −e → A ⁰ ) occurs from the A level to the conduction band, Free electrons are generated. When a metal that does not cause injection of both electrons and holes in the photoconductive layer is selected as the electrode metal, and the current value of the displacement current is observed while changing the wavelength of infrared light, when the wavelength corresponding to the above electronic excitation is obtained, The displacement current greatly increases. FIG. 9 (a) schematically shows this state. From such a phenomenon, from the wavelength λ and E _{C of the} photoconductive material, A
You can determine the level E _A.

(F) A ⁰ level When the A level is far from the valence band, that is, ΔE = E _V −
If E _A is large, the A level is electrically neutral. When near-infrared light enters in this state, hc / λ = E _V −E
Light having a wavelength corresponding to _A (h is Planck's constant, c is the speed of light, λ is the wavelength of light) is selectively absorbed, and electronic excitation (A ⁰ + e → A ⁻ ) occurs from the valence band to the A level, Free holes are generated. When a metal that does not cause injection of both electrons and holes into the photoconductive layer is selected as the electrode metal, and the current value of the displacement current is observed while changing the wavelength of infrared light, when the wavelength corresponding to the above electronic excitation is obtained, The displacement current greatly increases. FIG. 9 (b) schematically shows this state. From such a phenomenon, from the wavelength λ and the E _{V of the} photoconductive material, A
You can determine the level E _A.

Next, each energy level (E _C , E _V ,
_A method for obtaining the photocarrier generation efficiency Φ of the photoconductive layer in which E _D , E _A ) has been determined will be described.

Similar to the above, metal electrode 10a / photoconductive layer (photoconductive material + doping material) 11 / silicon oxide film 12
/ Silicon 13 / Metal electrode 10b (Fig. 5
(see (a)) is formed, and as the electrode metal at this time, a material that does not cause carrier injection from the electrode to the photoconductive layer 11 is selected.
Here, when light having a wavelength corresponding to the absorption band of the photoconductive material in the device is irradiated, free carriers are generated inside the photoconductive layer 11, and the displacement current increases as schematically shown in FIG. In the dark, at φ volt, the free carrier density generated per unit volt in the photoconductive layer 11 is n _d , the total carrier density induced from 0 to φ volt is N _d , and the carrier density at the time of light irradiation is similarly n. , N. From this carrier density n _ph generated in excess by the light irradiation, N _ph is expressed as _{n ph = n-n d (} 3a) N ph = N-N d (3b). Displacement currents at the time of darkness and light irradiation are I _d , I _{ph
When, n ph = (e · dV} / dt) 1 (I ph -I d) (4a)

[0040]

[Equation 1]

[Mathematical formula-see original document] n _ph is easily obtained from the difference in displacement current between light irradiation and darkness, and N _ph is easily obtained from the difference in integrated value of displacement current. Is required. Here, _Nph is obtained, and the efficiency Φ derived from this is adopted as the generation efficiency. Φ = N _ph / P (5) Based on the above measurement, the photocarrier generation efficiency Φ of the photoconductive layer obtained by combining the photoconductive material and various acceptor materials and changing the A level energy E _A thereof , A level becomes electrically neutral, that is, the conditional expression (1) is satisfied, an acceptor material that does not satisfy the conditional expression (1) is used, a donor material is used, or doping is performed. It was confirmed that the efficiency of generation of free holes was improved as compared with the case of not performing. Due to these facts, the doping of the electrically neutral acceptor material into the carrier generation layer of the negatively chargeable organic electrophotographic photosensitive member also has the same effect as described above.

The same applies to the case where the photoreceptor is doped with a donor material in the carrier generation layer in the positive charge type element structure, for example, the organic laminated structure composed of the electrode / carrier generation layer / carrier transport layer. That is, the energy E _D from the vacuum level to the donor level created in the band cap of the photoconductive material by doping the photoconductive material with the donor material satisfies the following conditional expression E _D > E _C +4 kT / q (6) Satisfactory electrically neutral [D ⁰ ] is lower than [D ⁰ ] as compared to [D ⁺ ] or doped with an acceptor material or not doped. It is expected that the generation efficiency of free electrons will be improved and the charging characteristics in the dark will be improved.

Then, based on the above measurement, a photoconductive material and various donor materials are combined, and the photocarrier generation efficiency Φ of the photoconductive layer in which the D level energy E _D is changed.
As a result, when D level becomes electrically neutral, that is, when conditional expression (6) is satisfied, when a donor material that does not satisfy conditional expression (6) is used, and when an acceptor material is used , Or an improvement in the generation efficiency for free electrons was observed as compared with the case where no doping was performed. That is, the doping of an electrically neutral donor material into the carrier generation layer of an actual positively chargeable organic electrophotographic photosensitive member can obtain the same action and effect as when the acceptor is doped with pink.

Next, a specific example of the present invention will be described.

FIG. 11 is a sectional view showing a negatively chargeable organic electrophotographic photosensitive member according to the present invention.
And an organic layer (carrier generation layer) 5 and a second organic layer (carrier transport layer) 6. Light is emitted from the carrier transport layer 6 side, and the carrier generation layer 5 has a structure having a film thickness of about 1/100 to 1/3 as compared with the carrier transport layer 6.

FIG. 12 (a) schematically shows a band state in an independent state before the bonding of the respective layers constituting the negatively charged organic electrophotographic photosensitive member having the above-mentioned constitution, and the work function of the electrode 1 Is Φ _N , the Fermi level of the carrier generation layer 5 of the first organic layer is E _F1 , the conduction band level is E _C1 , and the valence band level is E _V1 , and the acceptor material is within the band gap of the carrier generation layer 5. The A level formed is E _A. Assuming that the Fermi level of the carrier transporting layer 6 forming the second organic layer is E _F2 , the conduction band level is E _C2 , and the valence band level is E _V2 , E _C1 > E _{C2 from Eq} . E _V1 ~ E
_V2 , E _G1 = E _V1 −E _C1 (band gap of carrier generation layer 5) <E _G1 = E _V2 −E _C2 (band gap of carrier transport layer 6), and holes at the carrier generation layer / carrier transport layer interface A material that is easy to inject is selected.

FIG. 12 (b) schematically shows the band state under the thermal equilibrium state of each layer after the joining. In the thermal equilibrium state, the Fermi level of each layer is the same, and the electrode 1 and the carrier generation are generated. The A level is negatively charged in the immediate vicinity of the interface of the layer 5, and the electrical neutrality is maintained in the other regions. Further, at the interface between the carrier generation layer 5 and the carrier transport layer 6, a junction is formed in which holes are easily injected.

Here, even if a positive voltage is applied to the electrode 1 of the photoconductor and a negative voltage is applied to the surface of the carrier transport layer 6, since the A level is energetically separated from the valence band, the photoconductor is newly charged by charging. The density at the A level at which negative holes are negatively charged to generate free holes thermally is extremely small. The surface of the carrier transport layer 6 functions to prevent injection of electrons into the photoconductor, so that the photoconductor maintains a stable charged state for a long time. When the photoconductor is subsequently irradiated with light, the electrons excited in the carrier generation layer 5 are quickly captured at the A level, and free holes are generated in the valence band. These free holes are swiftly swept out to the carrier transport layer 6 according to the direction of the electric field, and compared with the case where a large number of [A ⁻ ] levels are present in the carrier generation layer 5 or the case where no doping is carried out, the free positive holes are It is possible to increase the efficiency of generating holes.

Example 1 In the negative charge type organic electrophotographic photoreceptor having the structure shown in FIG. 12 (a), the electrode 1 was made of aluminum and the carrier generation layer 5 was made.
With aluminum chlorophthalocyanine (photoconductive material) and bisanthanthrone (acceptor material)

[0050]

[Chemical 1]

[0051]

[Chemical 2]

Further, the carrier (hole) transport layer 6 is made of tetraphenylbenzidine represented by the following formula:

[0053]

[Chemical 3]

Configured. This configuration system, the above-mentioned FIG. 12 (a),
It was confirmed by the various displacement current methods described above that the condition (b) was satisfied.

Next, the process for producing the photoconductor will be described.
Here, the vacuum sublimation method (~ 10
^-6 torr) to co-evaporate aluminum chlorophthalocyanine and bisanthanthrone at a weight ratio of 100: 1 to obtain a film thickness of about 100n.
A carrier generation layer 5 of m (1000 angstrom) was formed. The method of forming the carrier generation layer 5 in the actual manufacturing process may be a film formed by a vacuum deposition method, a vacuum sublimation method, or the like, or a photoconductive material doped with an acceptor material used for the carrier generation layer 5 may be used. It may be formed into a film by dispersing it in a polymer that is a binder. In the former case, the film thickness is 50 to 500 nm.
(500 to 5000 angstroms), the latter case
A film is formed on the electrode 1 to a thickness of about 0.1 to 3 μm. Finally, tetraphenylbenzidine was dispersed in polycarbonate to form a film having a thickness of about 20 μm, which was used as a photoreceptor. The method for forming the carrier transport layer 6 in the actual manufacturing process is as follows.
A carrier (hole) transfer agent is dispersed in a polymer as a binder to form a film having a thickness of about 10 to 30 μm on the carrier generation layer 5. The type of polymer used for the carrier generation layer 5 and the carrier transport layer 6 is not particularly limited. (Below blank) For comparison, a carrier generation layer doped with tetracyanoquinodimethane represented by the following formula as an acceptor material (Comparative Example 1) and a carrier generation layer of aluminum chlorophthalocyanine alone without doping (comparison) An organic electrophotographic photosensitive member having a constitution using Example 2) was prepared. The A level formed by tetracyanoquinodimethane is
It was confirmed from the previous displacement current that it was very close to the valence band.

[0056]

[Chemical 4]

In the case of Comparative Example 1, aluminum chlorophthalocyanine and tetracyanoquinodimethane are used in a weight ratio.
Co-evaporation was performed at a ratio of 100: 1 to form a carrier generation layer having a film thickness of about 100 nm (1000 angstrom). In the case of Comparative Example 2, aluminum chlorophthalocyanine was vacuum-deposited alone,
A carrier generation layer having a film thickness of about 100 nm (1000 angstrom) was formed.

When a corona charging of about -6 kV was performed on the photoconductor according to the present invention having the above-mentioned structure, a charging potential of -1580 V was obtained.
After 1 minute, the potential was hardly attenuated. Then, it showed a photosensitivity of 3.6 erg / cm ² by light irradiation. On the other hand, in the case of Comparative Example 1, only −80 V was charged, and the potential decreased to about 1/2 after 1 minute. And it showed almost no photosensitivity. In the case of Comparative Example 2, a good charging potential of -1810 V was shown, but the photosensitivity of 10.3 erg / cm ² was shown by the light irradiation.

Example 2 In the case of Example 1 above, the carrier generation layer 5 was made of titanyl phthalocyanine and bisanthanthalone (acceptor material) represented by the following formula, and the carrier transport layer 6 was made of tetraphenylbenzidine. did.

[0060]

[Chemical 5]

It was confirmed by the various displacement current methods described above that this structural system satisfies the conditions shown in FIGS. 12 (a) and 12 (b).

For comparison, a carrier generation layer obtained by doping titanyl phthalocyanine with tetracyanoquinodimethane (acceptor material) (Comparative Example 3) and a carrier generation layer containing no titanyl phthalocyanine alone (Comparative Example 4) An organic electrophotographic photosensitive member having a structure using was prepared.

In each of Comparative Examples 3 and 4, the carrier generation layer was formed under the same conditions as in Comparative Examples 1 and 2.

When a corona charging of about -6 kV was performed on the photoconductor of Example 2 having the above-mentioned structure, a charging potential of -1750 V was obtained.
After a minute, the potential was hardly attenuated. Then, it showed a photosensitivity of 5.4 erg / cm ² by light irradiation. On the other hand, in the case of Comparative Example 3, only −100 V was charged, and the potential decreased to 2/3 after 1 minute. And it showed almost no photosensitivity. In the case of Comparative Example 4, the charge potential was as good as -1850 V, but the photosensitivity was only 11.6 erg / cm ² when irradiated with light.

Example 3 Next, the case of a positive charging type organic electrophotographic photosensitive member will be described.

The photoconductor has a structure basically similar to that shown in FIG. 11, and includes an electrode 1, a first organic layer (carrier generation layer) 5, and a second organic layer (carrier transport layer). )
It is composed of 6. Light is emitted from the carrier transport layer side 6, and the carrier generation layer 5 has a film thickness of about 1/100 to 1/3 as compared with the carrier transport layer 6.

FIG. 13 (a) schematically shows a band in an independent state before the bonding of the respective layers constituting the photoreceptor, in which the work function of the electrode 1 is Φ _M and the first organic layer is E Fermi level of the carrier generation layer 5 constituting the _F1, the conduction band level E _C1, and valence electron band level and E _V1, the D level donor material is formed within the band gap of the charge generation layer 5 E _D And Further, the Fermi level of the carrier transport layer 6 of the second organic layer is E _F2 , the conduction band level is E _C2 , and the valence band level is EV2. From Fig. 13 (a), E _C1
-E _C2 , E _V1 <E _V2 , E _C1 = E _V1 −E _C1 (bandgap of carrier generation layer) <E _C2 = E _V2 −E _C2 (bandgap of carrier transport layer), and carrier generation layer 5 and A material that facilitates electron injection at the interface of the carrier transport layer 6 is selected.

FIG. 13 (b) schematically shows the bands of the respective layers under the thermal equilibrium state after joining, and the Fermi levels of the respective layers are the same in the thermal equilibrium state. Then, the D level is positively charged in the very vicinity of the interface between the electrode 1 and the carrier generation layer 5, and the electrical neutrality is maintained in the other regions. Further, at the interface between the carrier generation layer 5 and the carrier transport layer 6, a bond is formed in which electrons are easily injected.

Here, even if a negative voltage is applied to the electrode 1 of the photoconductor and a positive voltage is applied to the surface of the carrier transport layer 6, since the D level is energetically separated from the conduction band, the D level is newly charged by the photoconductor. Which is positively charged to generate a free electron thermally D
Level density is extremely low. And carrier transport layer 6
Since the surface of (1) functions to prevent the injection of holes into the photoconductor, the photoconductor maintains a stable charged state for a long time. Then,
When the photoconductor is irradiated with light, the holes excited in the valence band are quickly captured at the D level, and free electrons are generated in the conduction band. The free electrons are swiftly swept to the carrier transport layer 6 according to the direction of the electric field, and the free electron generation efficiency is higher than that in the case where a large number of D ⁺ levels are present in the carrier generation layer 5 or when no doping is performed. Can be increased.

In the positively charged organic electrophotographic photosensitive member, the electrode 1 is made of aluminum, the carrier generation layer 5 is made of the above aluminum chlorophthalocyanine (photoconductive material) and an aluminum phthalocyanine dimer represented by the following formula (donor material).

[0071]

[Chemical 6]

Further, the carrier transport layer 6 is formed of tetrapropyldiphenoquinone represented by the following formula.

[0073]

[Chemical 7]

It is composed of This configuration system is shown in FIG.
It was confirmed by the various displacement current methods described above that the conditions shown in (a) and (b) were satisfied.

The photoconductor having the above-mentioned structure was prepared by the following process. That is, aluminum chlorophthalocyanine and aluminum phthalocyan dimer in a weight ratio of 100: are formed on an Al substrate 1 as an electrode by a vacuum sublimation method (up to 10 ⁻⁶ torr).
Co-deposited on 1 to a film thickness of about 100 nm (1000 angstrom)
The carrier generation layer 5 was formed. The carrier generation layer 5 may be formed in an actual manufacturing process by a film formed by a vacuum deposition method, a vacuum sublimation method, or a photoconductive material doped with a donor material used for the carrier generation layer 5. It may be formed into a film by dispersing it in a polymer that is a binder. In the former case, the film thickness is 50
Approximately 500 nm (500 to 5,000 angstroms), and in the latter case 0.1 to 3 μm is formed on the electrode. Finally, tetrapropyldiphenoquinone was dispersed in polycarbonate to form a film having a thickness of about 20 μm, which was used as a photoreceptor. The carrier transport layer 6 is formed in the actual manufacturing process by dispersing a carrier (electron) transport agent in a polymer as a binder to form a film having a thickness of about 10 to 30 μm on the carrier generation layer 5. To film. The type of polymer used for the carrier generation layer 5 and the carrier transport layer 6 is not particularly limited.

For comparison, tetrathiafulvalene represented by the following formula is used as a donor material.

[0077]

[Chemical 8]

Photoconductive layer doped with (Comparative Example 5)
Then, a photoconductor having the same structure as that of the above case was prepared except that a photoconductive layer containing only aluminum chlorophthalocyanine without doping (Comparative Example 6) was used. It was confirmed by the above displacement current that the D level formed by tetrathiafulvalene was very close to the conduction band. Further, in the case of Comparative Example 5, aluminum chlorophthalocyanine and tetrathiafulvalene were co-evaporated at a weight ratio of 100: 1 to form a carrier generation layer 5 having a film thickness of about 100 nm (1000 Å). In this case, aluminum chlorophthalocyanine alone was vacuum-deposited to form a carrier generation layer 5 having a film thickness of about 100 nm (1000 angstrom).

When the photoreceptor of Example 3 was corona-charged at about 6 kV, it showed a charging potential of 1380 V, and the potential was hardly attenuated even after 1 minute. Then, it showed a photosensitivity of 3.8 erg / cm ² by light irradiation. On the other hand, in the case of Comparative Example 5, only 100V was charged, and the potential was about 1/3 after 1 minute.
Decreased to. And it showed almost no photosensitivity. Further, Comparative Example 6 showed a good charging potential of 1440 V, but showed only a photosensitivity of 12.6 erg / cm ² by light irradiation.

Example 4 In the case of Example 3 described above, the carrier generation layer 5 was made of titanyl phthalocyanine (photoconductive material) and bisquinacridone (donor material) represented by the following formula, and the carrier transport layer 6 was made of tetrapropyldiphenoxy. Configured with each non.

[0081]

[Chemical 9]

It was confirmed by the various displacement current methods described above that this system satisfies the conditions shown in FIGS. 13 (a) and 13 (b).

For comparison, a carrier generation layer in which titanyl phthalocyanine was doped with tetrathiafulvalene (donor material) (Comparative Example 7) and a carrier generation layer containing no titanyl phthalocyanine alone (Comparative Example 8) were used. An organic electrophotographic photoreceptor having the above structure was prepared.

In each of Comparative Examples 7 and 8, the carrier generation layer was formed under the same conditions as in Comparative Examples 5 and 6.

When a corona charging of about 6 kV was carried out on the photoconductor of Example 4 having the above-mentioned structure, a charging potential of 1420 V was obtained.
After a minute, the potential was hardly attenuated. Then, it showed a photosensitivity of 5.1 erg / cm ² by light irradiation. On the other hand, in the case of Comparative Example 7, only 80 V was charged, and the potential decreased to 1/2 after 1 minute. And it showed almost no photosensitivity. In the case of Comparative Example 8, the charge potential was as good as 1470 V, but the photosensitivity was only 14.7 erg / cm ² when irradiated with light.

[0086]

As described above, according to the present invention, when the photoconductive layer is doped with an electrically neutral acceptor material or donor material, the dark charging property is the same as when no doping is performed. There is almost no change in comparison. In addition, since the A level (D level) acts as an effective electron trap level (hole trap level), the excited electrons (excited holes) are rapidly trapped at the A level (D level) by light irradiation and are free. It is understood that the hole (free electron) generation efficiency is greatly increased, which contributes to the improvement of the photoreceptor characteristics.

[Brief description of drawings]

FIG. 1A is a qualitative band diagram for explaining a photocarrier generation process in an organic electrophotographic photosensitive member according to the present invention, and FIG. 1B is a band diagram similarly showing each energy level of a photoconductive layer.

2A is a qualitative energy diagram for explaining a donor property of a doping material to a photoconductive layer in the organic electrophotographic photosensitive member according to the present invention, and FIG. 2B is a qualitative energy diagram for explaining an acceptor property of the doping material. Qualitative energy diagram.

FIG. 3 (a) is a qualitative band diagram for explaining a change in a charging ratio of D level depending on an energy position by a donor material doped in a photoconductive layer in the organic electrophotographic photosensitive member according to the present invention. , (B) are qualitative band diagrams explaining the change in the charging rate of the A level depending on the energy position due to the acceptor material that is also doped.

FIG. 4 (a) is a band diagram qualitatively explaining the behavior of excited holes due to a donor material with which a photoconductive layer is doped in the organic electrophotographic photoreceptor according to the present invention, and (b) is an acceptor with the same doping. FIG. 6 is a band diagram qualitatively explaining the behavior of excited electrons due to a conductive material.

5A is an explanatory view of a displacement current measuring method for obtaining a conduction band level and a valence band level of a photoconductive layer in the organic electrophotographic photosensitive member according to the present invention, and FIG. 5B is the same photoconductive layer. Displacement current-voltage characteristic diagram when there is no.

6A is a qualitative current-voltage characteristic diagram of a displacement current element when electrons are injected into a photoconductive layer in an organic electrophotographic photoreceptor according to the present invention, and FIG. 6B is also an energy barrier taken into consideration. A qualitative current-voltage characteristic diagram in the case of performing.

FIG. 7 (a) is a qualitative current-voltage characteristic diagram of a displacement current element when holes are injected into a photoconductive layer in the organic electrophotographic photosensitive member according to the present invention, and (b) is also an energy barrier. A qualitative current-voltage characteristic diagram when considering.

8A and 8B show current-voltage characteristics when an organic electrophotographic photosensitive member according to the present invention has an excitation wavelength of near infrared light or infrared light and a D ⁺ level and a D ⁰ level. Qualitative explanatory diagram showing the relationship of

9 (a) and 9 (b) are graphs showing the excitation wavelength of near-infrared light or infrared light and A in the organic electrophotographic photosensitive member according to the present invention.
^- level, current when there is A ⁰ level - qualitative explanatory view showing a relationship between the voltage characteristic.

FIG. 10 is an explanatory diagram of a light irradiation displacement current method for obtaining the photocarrier generation efficiency of the photoconductive layer in the organic electrophotographic photosensitive member according to the present invention.

FIG. 11 is a cross-sectional view showing a configuration example of a main part of an organic electrophotographic photosensitive member according to the present invention.

12A is a band diagram showing the energy level of each layer before bonding in the negatively chargeable organic electrophotographic photosensitive member according to the present invention, and FIG. 12B is a band diagram of the same thermal equilibrium state after bonding.

13A is a band diagram showing the energy level of each layer before bonding in the positively chargeable organic electrophotographic photosensitive member according to the present invention, and FIG. 13B is a band diagram of the same thermal equilibrium state after bonding.

FIG. 14 is a cross-sectional view showing the structure of a single-layer photoconductor in which a photoconductive material is dispersed in a polymer in a conventional organic electrophotographic photoconductor.

FIG. 15 is a cross-sectional view showing the structure of a conventional negatively chargeable organic laminated photoreceptor including an Al electrode / carrier generation layer / carrier transport layer.

[Explanation of symbols]

1 ... Al (electrode) 2 ... Photoconductive material 3 ... Polymer
4 ... Photosensitive member 5 ... Carrier generation layer 6 ... Carrier transport layer 7 ... Electrons 8 ... Holes 10a, 10b ... Electrodes (front and back) 11 ... Photoconductive layer 12 ... SiO ₂ 13 ... Si

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[Procedure amendment]

[Submission date] January 11, 1993

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Correction target item name] Figure 9

[Correction method] Change

[Correction content]

[Figure 9]

Claims (2)

[Claims] 1. An organic electrophotographic photosensitive member comprising a carrier generating layer and an organic layer of a carrier transporting layer laminated on an electrode surface, wherein the carrier generating layer contains a photoconductive material and an acceptor material, The energy difference (valence band level) from the vacuum level of the photoconductive material to the top of the valence band is E _V ,
When this was in the photoconductive material an acceptor material doped, when the energy difference between the vacuum level to the acceptor level to create in the band gap of the photoconductive material (A level) was E _A, E _A conditional expression, _{E a <E V -4kT / q} (q is an elementary charge,
k is a Boltzmann's constant and T is an absolute temperature), which is a negatively charged organic electrophotographic photosensitive member. 2. An organic electrophotographic photosensitive member comprising an organic layer of a carrier generation layer and a carrier transport layer laminated on an electrode surface, wherein the carrier generation layer contains a photoconductive material and an acceptor material, The energy difference (conduction band level) from the vacuum level of the photoconductive material to the lower end of the conduction band is E _C , and when this photoconductive material is doped with a donor material, within the band gap of the photoconductive material from the vacuum level. to produce energy difference to donor levels when the (D level) was E _D, E _D is the conditional _{expression, E D> E C + 4kT} / q (q is an elementary charge,
A positively chargeable organic electrophotographic photosensitive member characterized in that k satisfies Boltzmann's constant and T satisfies absolute temperature.