A Combined Microstructure Characterization of Moisture Permeation Barriers Layers by Means of Electrochemical Impedance Spectroscopy and Ellipsometric Porosimetry

In engineering and manufacturing organic electronic devices, encapsulation layers are mandatory due to the sensitivity of active layers and low work function cathodes to moisture. The quality of the barrier is generally validated by means of water vapor transmission rate (WVTR, gm^-2day^-1) measurements as well as in terms of conformal control on the density of local defects (e.g. pinholes) that act as pathways for the water molecules. Furthermore, it has been demonstrated that the water permeation through the barrier nanoporosity (i.e. the intrinsic barrier), responsible for the homogeneous water permeation through the bulk of the barrier, can be 15-20 times higher than through large defects [1]. While several methods allow the identification of pinholes/defects, novel techniques able to characterize the barrier microstructure in the broad range of nano- and meso-porosity are sought. In this work, electrochemical impedance spectroscopy (EIS) and ellipsometric porosimetry (EP) have been adopted to study the residual (open) porosity and water permeation in PE-CVD and (PE-) ALD single inorganic barrier layers and the results have been correlated to the intrinsic barrier performances as well as to their opto-chemical properties. Trivinyltrimethyl cyclotrisiloxane (d_V3D3 = 1 nm) and water (d_H2O = 0.3 nm) have been adopted as adsorptives in EP and the barrier microstructure has been studied. The pore size range of 0.3–1 nm and its relative content have been found to control the intrinsic WVTR transition towards the 10^-4-10^-6 gm^-2day^-1 regime, highlighting the role of residual nanoporosity in controlling the intrinsic barrier properties [ref]. Furthermore, EIS has been employed for the first time to study the water uptake (φ, the volume fraction of water) and diffusivity coefficient (D) of different moisture barrier layers. The layer capacitance has been determined by modelling the impedance data with the proper equivalent circuit and the change of the capacitance upon water permeation has been followed. The Brasher-Kingsbury equation has been successfully applied and φ and D values in the range of 0.8-4% and 10^-13-10^-15 cm²sec^-1 have been found, respectively. In addition, the diffusion of cations having different hydrated shell sizes, i.e. Na⁺, Li⁺, K⁺ and Cs⁺, through the barriers has been studied. Changes in the resistance of the layer have been attributed to the formation of conductive pathways due to the ion diffusion. With this method, the relative pore size distribution in the range 0.3-1 nm has been characterized in detail.

[1] J. Affinito, D. Hilliard, in: 47th Annual Technical Conference Proceedings (2004) 563.

[2] A. Perrotta, E.R.J. van Beekum, G. Aresta, A. Jagia, W. Keuning, M.C.M. van de Sanden, W.M.M. Kessels, M. Creatore, Microporous and Mesoporous Materials, 188 (2014), 163-171.