Image forming apparatus and image forming method

An image forming apparatus comprising:

• an image bearing member;
• and
• a charging roller configured to charge a circumferential surface of the image bearing member to a positive polarity, wherein
• the image bearing member includes a conductive substrate and a photosensitive layer of a single layer, and satisfies formula (1) shown below,
• the photosensitive layer contains a charge generating material, a hole transport material, an electron transport material, and a first binder resin,
• the charging roller includes a conductive shaft, a base layer covering a surface of the conductive shaft, and a surface layer covering a surface of the base layer, and
• the surface layer has a volume resistivity at a temperature of 32.5° C. and a relative humidity of 80% of at least 13.0 log Ω·cm,
• 0.60
• ≦
• V
• (
• Q
• /
• S
• )
• ×
• (
• d
• /
• ɛ
• r
• ·
• ɛ
• 0
• )
• (
• 1
• )
• where in the formula (1),
• Q represents a charge amount [C] of the circumferential surface of the image bearing member,
• S represents a charge area [m2] of the charged circumferential surface of the image bearing member,
• d represents a film thickness [m] of the photosensitive layer,
• εr represents a specific permittivity of the first binder resin contained in the photosensitive layer,
• ε0 represents vacuum permittivity [F/m],
• V represents a value calculated in accordance with formula V=V0−Vr,
• Vr represents a first potential [V] of the circumferential surface of the image bearing member yet to be charged by the charging roller, and
• V0 represents a second potential [V] of the circumferential surface of the image bearing member charged by the charging roller.