Provided are methods of fabricating a solar cell and a vacuum deposition apparatus used therefor. The method may include forming a lower electrode on a substrate, forming a light absorption layer on the lower electrode, forming a buffer layer on the light absorption layer, and forming a window electrode layer on the buffer layer. The forming of the buffer layer may include a deposition step of forming a cationic metal material and a diffusion step of diffusing an anionic non-metal material into the cationic metal material.