CdS layer using chemical bath deposition (CBD) is widely used as the main process of the buffer layer which compensates the difference of the bandgap between the absorption layer and the window layer among the components of the solar cell. Zn (O, S) with atomic layer deposition (ALD) is being studied as an alternative to this problem due to the difficulty in tuning the bandgap and strong toxicity. With ALD method, it is possible to control the atomic unit deposition so that a very sophisticated process can be performed and the ratio of the most important oxygen and sulfur in the experiment can be precisely controlled. Zn (O, S) is a material that has the advantage of not using toxic Cd and the bandgap tuning according to the ratio of ZnO and ZnS. We are working on the development of this compound as an alternative buffer layer for copper chalcogenide-based thin film solar cells