On the role of nanoporosity in controlling the performance of moisture permeation barrier layers

Abstract

Satisfactory results in term of moisture permeation barrier performance have been achieved for the encapsulation of organic electronic devices. However, further insight into the correlation between barrier performances and moisture permeation pathways are sought. This contribution focuses on the residual nanoporosity in the inorganic layer and its role in controlling the barrier performance. Inorganic barrier layers (i.e. Al2O3 and SiO2) prepared by plasma-enhanced atomic layer deposition (PE-ALD) and plasma-enhanced chemical vapor deposition (PE-CVD) have been extensively analyzed by means of IR spectroscopy, spectroscopic ellipsometry, Rutherford backscattering spectroscopy, elastic recoil detection and X-ray photoelectron spectroscopy. The calcium test has been performed to determine the intrinsic water vapor transmission rate (WVTR), as well as the effective WVTR values. Ellipsometric porosimetry (EP) has been adopted to determine the open porosity and pore size range in the structure of the layer. Trivinyltrimethyl cyclotrisiloxane (dV3D3=1nm) and water (dH2O=0.3nm) have been chosen as probe molecules. A correlation between the residual nanoporosity and the intrinsic barrier properties has been found, regardless of the chemistry of the layer and deposition method used. Pores larger than 1nm with a relative content above 1% have been found responsible for mediocre barrier layers characterized by a WVTR in the range of 10−2 to 10−3gm−2day−1. Furthermore, the pore size range of 0.3–1nm and its relative content have been found to control the transition in WVTR between the 10−4gm−2day−1 and the 10−6gm−2day−1 regime, highlighting the role of residual nanoporosity in controlling the intrinsic barrier properties.