Barrier Encapsulation for Flexible Photovoltaics
High barrier materials are required for the encapsulation of all types of flexible thin film photovoltaics (PV). The level of barrier necessary varies depending on the thin film PV technology employed, the environmental conditions and the required lifetime of the devices.
Back sheet encapsulation is relatively straightforwardly accomplished, with the incorporation of sufficiently thick metal foils in the polymer laminate structures. However, the clear barrier front sheet and edge seal with sufficient barrier performance are more difficult to achieve, especially for outdoor applications at acceptable cost points for grid parity BIPV targeted a $0.5/Wp.
Moisture Barrier Permeation Measurement
CPI characterises the moisture permeation of clear barrier materials using the standard MOCON ‘Aquatran I’ operating at 38oC and 90% RH for large area measurement (50cm2), which is referred to as the extrinsic barrier permeation. For intrinsic measurements, the Ca Test is used; this can measure the small area permeation and also highlight defects in the barrier coating.
Ultra Barrier Deposition Systems
CPI have deposited experimental ultra-barrier single and multi-layer stacks by sputtering techniques in a 400mm-wide roll-to-roll (R2R) sputter coater, depositing AlOx PET and PEN substrates. In addition, single layer of AlOx on similar substrates have been deposited by temporal conventional batch atomic layer deposition (ALD) system.
Ultra barrier levels are only achieved with multiple sputtered layers; with single layer ALD deposition ultra barrier extrinsic performance can be achieved with layers as thin as 5nm.
The intrinsic performance of the ALD barrier is shown in the photographs above and the table below them. The photographs show the ultra barrier samples after exposure to damp-heat conditions (80oC, 80%RH) and the table shows the calibrated moisture permeation levels. The abrupt change in intrinsic permeation is caused by a nearby defect causing the Ca degradation rather than the overall degradation of the ALD barrier film.
The key development required is overcoming the effect of defects, rather than issues with the intrinsic barrier performance of the ALD deposited layers; which will be addressed in CPI’s R2R ALD ultra-barrier programme (see below).
Future R2R ALD Ultra Barrier Deposition Development
A 500mm-wide R2R spatial ALD coater from BENEQ (Finland) to be installed in Q2 2014. CPI aims to produce clear flexible ultra-barrier by R2R processing (including pre- and post-processing). The work will be carried out in collaboration with the supplier and other interested parties. Further, CPI will develop direct encapsulation technology for PV and other flexible electronic applications. CPI believes R2R ALD deposition systems will play an important role not only in ultra-barrier and encapsulation technologies but also in conformal highly-dense films for other PV applications.
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