The thickness of the CdS:O window layer plays an important role in CdTe-based thin-film solar cell performance. Normally with thinner CdS:O layer, the cells' short wavelength response can be enhanced to achieve higher short-circuit current density (J(sc)), while the open-circuit voltage (V-oc) and fill factor (FF) would degrade. Previous research finds that reducing the CdS:O layer thickness to less than 40 nm would decrease the cells' efficiency (Eff.). In this work, 18% efficiency CdTe thin-film solar cell with only 30 nm CdS:O window layer is demonstrated. A post annealing process is developed to fabricate high efficiency cells with ultrathin CdS:O layers. Our results indicate that the high defect density in CdTe absorber, low built-in potential (V-bi) and high back contact barrier (Phi(b)) are partially responsible for the reduced V-oc and FF of cells with ultrathin CdS:O layers. The post annealing process can increase the carrier concentration (N-cv ) and V-bb and decrease the Phi(b), in which way improves V-oc and FF for higher cell efficiency.