Ultrathin Al2O3 layers, deposited using atomic layer deposition (ALD), have been utilized as the primary barrier layers for encapsulation of organic solar cells. This work shows that the encapsulation characteristics of a barrier layer can be accentuated by replacing H2O with O3 as the ALD oxidant. The Al2O3 layers deposited using O3 offered superior device encapsulation compared to the films deposited using H2O. The organic solar cell efficiency has also been studied as a function of Al2O3 thickness and effective encapsulation has been monitored for two different ALD temperatures.