| AVS 61st International Symposium & Exhibition | |
| Thin Film | Thursday Sessions |
| Session TF-ThA |
| Session: | Thin Film for Permeation Barriers and Membranes |
| Presenter: | Alberto Perrotta, Eindhoven University of Technology; Dutch Polymer Institute (DPI), Netherlands |
| Authors: | A. Perrotta, Eindhoven University of Technology; Dutch Polymer Institute (DPI), Netherlands S.J. García, Delft University of Technology, Netherlands J.J. Michels, Holst Centre / TNO, Netherlands W.M.M. Kessels, Eindhoven University of Technology, Netherlands M. Creatore, Eindhoven University of Technology, Netherlands |
| Correspondent: | Click to Email |
In engineering 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 moisture permeation barriers is generally validated by means of water vapor transmission rate (WVTR, gm-2day-1) measurements as well as visual inspection/identification of the local defects (e.g. pinholes) acting as unhindered pathways for water molecules. Furthermore, it has been demonstrated that the water permeation through the nanoporosity - or free volume - of the bulk of the barrier layer can be 15-20 times higher than the one through local 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 a recent work [2], ellipsometric porosimetry has been demonstrated to be a valuable technique for nanopore characterization. Adopting different probing molecules (i.e trivinyltrimethyl cyclotrisiloxane, dV3D3 = 1 nm, and water, dH2O = 0.3 nm), a correlation has been found between the (residual) nanoporosity in PE-CVD and (PE-)ALD barriers and their intrinsic barrier properties. The pore size range of 0.3–1 nm and its relative content have been found to control the transition in WVTR in the regime of 10-4 - 10-6 gm-2 day-1. In order to further investigate this range, electrochemical impedance spectroscopy (EIS) has been adopted for the first time in the study of moisture permeation barriers. EIS allows to follow the diffusion of electrolytes through the barrier and cations having different hydrated shell sizes in the range 0.5-0.7 nm, i.e. Na+, Li+, K+ and Cs+, have been selected. Changes in the barrier layer resistance have been attributed to the formation of conductive pathways due to the ion diffusion and allow to study differences in layer porosity in the above-mentioned pore diameter/cation size range. Moreover, it is possible to investigate the water permeation as a function of the variation of the barrier capacitance value upon immersion in the electrolyte solution. In this way, the water uptake (φ, the volume fraction of water in a coating) and diffusivity coefficient (D) of different moisture barriers are determined. φ and D values in the range of 0.8-4% and 10-13-10-15 cm2s-1 have been found, respectively. This demonstrates that EIS is a versatile tool for the characterization of moisture permeation barriers.
[1] J. Affinito et al. 47th Annual Technical Conference Proceedings (2004) 563
[2] A. Perrotta et al., Microporous and Mesoporous Materials (2014) 163