A method of fabricating a thin-film compound solar cell having an n-type buffer layer formed therein for providing a heterojunction with a p-type compound semiconductor light absorbing layer formed on a back electrode by spreading a powder of a sulfur compound containing n-type doping element over the light absorbing layer surface or applying a coat of a solution of a sulfur compound containing n-type doping element onto the light absorbing layer surface and then fusing the powder or the coat thereon by heat. This process can produce a high-quality n-type buffer layer tightly adhered to a p-type compound semiconductor light absorbing layer to achieve stable characteristic of heterojunction therewith.