An organic image sensor may be configured to obtain a color signal associated with a particular wavelength spectrum of light absorbed by the organic image sensor may omit a color filter. The organic image sensor may include an organic photoelectric conversion layer including a first material and a second material. The first material may absorb a first wavelength spectrum of light, and the second material may absorb a second wavelength spectrum of light. The organic photoelectric conversion layer may include stacked upper and lower layers, and the respective material compositions of the lower and upper layers may be first and second mixtures of the first and second materials. A ratio of the first material to the second material in the first mixture may be greater than 1/1, and a ratio of the first material to the second material in the second mixture may be less than 1/1.