We have fabricated CdTe solar cells on commercial SnO 2 /SnO 2 :F coated soda-lime glass substrates using ZnS/CdS window layers with various thickness combinations. With our standard fabrication method, CdTe solar cells with ZnS/CdS window layers have exhibited efficiencies as high as 12.3% under AM1.5 illumination. However, the efficiencies of the cells with ZnS/CdS window layers are still lower than the reference CdTe cells with CdS window layer (13%). We found that optimizing the thickness combination and heat treatment of the ZnS/CdS window layers prior to CdTe deposition may be a viable approach to optimize the performance of CdTe solar cells with ZnS/CdS window layers.