ZnSnO thin films were deposited by atomic layer deposition (ALD) process using diethyl zinc (Zn(C2H5)2) and tetrakis (dimethylamino) tin (Sn(C2H6N)4) as metal precursors and water vapor as a reactant. ALD process has several advantages over other
deposition methods such as precise thickness control, good conformality, and good uniformity for large area. The composition of ZnSnO thin films was controlled by varying the ratio of ZnO and SnO2 ALD cycles. The ALD ZnSnO film was an amorphous state. The band gap of ZnSnO thin films increased as the Sn content increased. The CIGS solar cell using ZnSnO buffer layer showed about 18% energy conversion efficiency. With such a high efficiency with the ALD ZnSnO buffer and no light soaking effect, AlD ZnSnO buffer mighty be a good candidate to replace Zn(S,O) buffer in CIGSsolar cells.