Electrophotographic photoreceptor, process cartridge, and image forming apparatus

1. An electrophotographic photoreceptor comprising:

a conductive substrate;

a photosensitive layer including a charge-generating layer containing a charge-generating material and a charge-transporting layer containing a first charge-transporting material, the charge-transporting layer having a hole mobility of 5.0×10 −6 (cm 2 /V·sec) or less at an electric field of 2 V/cm; and

an overcoat layer that is provided on the photosensitive layer and contains a cured material of at least one compound and at least one second charge-transporting material, the at least one compound being selected from a group consisting of guanamine compounds and melamine compounds, the at least one second charge-transporting material having at least one substituent selected from a group consisting of —OH, —OCH 3 , —NH 2 , —SH, and —COOH, a content of the at least one compound in the overcoat layer being in a range from 0.1% by weight to 5% by weight, and a content of the at least one second charge-transporting material in the overcoat layer being in a range from 90% by weight to less than 99.9% by weight;

wherein the following relational expression (1) is satisfied

Ip(OCL)>Ip(CTL)>Ip(CGL)  (1)

wherein Ip(CGL) represents an ionization potential (eV) of the charge-generating material; Ip(CTL) represents an ionization potential (eV) of the first charge-transporting material; and Ip(OCL) represents an ionization potential (eV) of the at least one second charge-transporting material.

2. The electrophotographic photoreceptor according to claim 1 , wherein the following relational expressions (2) and (3) are satisfied

|Ip(OCL)−Ip(CTL)≦|≦0.4 (eV)  (2)

|Ip(CTL)−Ip(CGL)|≦0.3 (eV)  (3).

3. The electrophotographic photoreceptor according to claim 1 , wherein the content of the at least one compound in the overcoat layer is in a range from 1% by weight to 3% by weight.

4. The electrophotographic photoreceptor according to claim 1 , wherein the at least one compound includes a guanamine compound, and the guanamine compound is a compound represented by the following formula (A) or an oligomer thereof

wherein R 1 represents a linear or branched alkyl group having from 1 to 10 carbon atoms, a substituted or unsubstituted phenyl group having from 6 to 10 carbon atoms, or a substituted or unsubstituted alicyclic hydrocarbon group having from 4 to 10 carbon atoms; R 2 to R 5 each independently represent a hydrogen atom, —CH 2 —OH, or —CH 2 —O—R 6 ; and R 6 represents a linear or branched alkyl group having from 1 to 10 carbon atoms.

5. The electrophotographic photoreceptor according to claim 1 , wherein the at least one compound includes a melamine compound, and the melamine compound is a compound represented by the following formula (B) or an oligomer thereof

wherein R 7 to R 12 each independently represent a hydrogen atom, —CH 2 —OH, —CH 2 —O—R 13 , or —O—R 14 ; and R 13 and R 14 each independently represent an alkyl group that has from 1 to 5 carbon atoms and may be branched.

6. The electrophotographic photoreceptor according to claim 1 , wherein the at least one second charge-transporting material includes a compound represented by the following formula (I)

F(—(R 15 —X) n1 —(R 16 ) n2 —Y) n3   (I)

wherein F represents an organic group derived from a compound having a hole transporting ability; R 15 and R 16 each independently represent a linear or branched alkylene group having from 1 to 5 carbon atoms; n1 represents 0 or 1; n2 represents 0 or 1; n3 represents an integer from 1 to 4; X represents an oxygen atom, NH, or a sulfur atom; and Y represents —OH, —OCH 3 , —NH 2 , —SH, or —COOH.

7. A process cartridge comprising the electrophotographic photoreceptor according to claim 1 , wherein the process cartridge is attachable to and detachable from an image forming apparatus for forming an image on a recording medium.

8. The process cartridge according to claim 7 , wherein the following relational expressions (2) and (3) are satisfied in the electrophotographic photoreceptor

|Ip(OCL)−Ip(CTL)|≦0.4 (eV)  (2)

|Ip(CTL)−Ip(CGL)|≦0.3 (eV).  (3)

9. The process cartridge according to claim 7 , wherein the content of the at least one compound in the overcoat layer is in a range from 1% by weight to 3% by weight.

10. An image forming apparatus comprising:

the electrophotographic photoreceptor according to claim 1 ;

a charging unit that charges the electrophotographic photoreceptor;

a latent image forming unit that forms an electrostatic latent image on a surface of the charged electrophotographic photoreceptor;

a development unit that develops the electrostatic latent image formed on the surface of the electrophotographic photoreceptor with a toner to form a toner image; and

a transfer unit that transfers the toner image formed on the surface of the electrophotographic photoreceptor onto a recording medium.

11. The image forming apparatus according to claim 10 , wherein the following relational expressions (2) and (3) are satisfied in the electrophotographic photoreceptor

|Ip(OCL)−Ip(CTL)|≦0.4 (eV)  (2)

|Ip(CTL)−Ip(CGL)|≦0.3 (eV).  (3)

12. The image forming apparatus according to claim 10 , wherein the content of the at least one compound in the overcoat layer is in a range from 1% by weight to 3% by weight.