In this paper Al2O3 films are prepared with a method of atomic layer deposition (ALD) as the thin film encapsulation technology for top-emitting organic light-emitting diodes (TE-OLED). Time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-Ray Reflectometry (XRR) and X-ray photoelectron spectroscopy (XPS) are used to analyze the effect of different chemical precursors and behavioral factors on the performance of the Al2O3 thin films. The analyses disclosed that Al2O3 films prepared with a trimethylaluminum (TMA) and H2O (TMA + H2O) process contained more unreacted -CH3 groups, and the films with a TMA + O3 process show a large number of carbon-based impurities. However, the Al2O3 films prepared using H2O and O3 in turn in a deposition cycle as oxygen sources exhibited higher density and purity, leading to a superior water vapor transmission rate (WVTR) as low as 5.43 × 10-5 g per m2 per day estimated with the calcium (Ca) corrosion method at 40 °C/100%. The TE-OLED with Al2O3 (TMA + H2O + O3) thin film as an encapsulated layer has longer lifetimes, and produces no black spots under operational times up to 400 hours.