Ultra-barrier Encapsulation by ALD R2R Processing for Flexible Large Area DevicesAlf Smith *, David Bird, Andrew Cook, Steve Edge, Phil Hollis, Paolo Melgari, Steve Spruce

Abstract

High barrier materials are required for the encapsulation of a wide variety of flexible large area devices; these include printable electronics, OLED devices, OPV, thin-film inorganic PV to name a few. The level of barrier necessary varies depending on the type of device, the required life-time, the materials used in fabrication and the environmental conditions the device will be used in. Barrier encapsulation for devices not requiring any transparency; for light in (e.g. PV applications) or out (e.g. OLED applications), can be protected relatively straightforwardly with the incorporation of sufficiently thick metal foils (15-25micron) in polymer laminate structures. However, the clear barrier front sheet or back sheet with edge seal, with sufficient barrier performance are much more difficult to achieve, especially for outdoor applications (e.g. PV), where acceptable cost points and high levels of robustness are required.

Introduction

CPI is a Technology Innovation Centre located in the UK and part of the High Value Manufacturing Catapult. The UK’s Catapult Innovation centres work in the space between the discovery of an idea and the delivery of a product or service to the commercial market. This is often referred to as the “valley of death”. CPI works with clients and collaborators to bring new products and processes to market quickly and efficiently, by offering facilities and expertise that help reduce R&D risk and capital investment.

CPI’s National Centre for Printable Electronics is focusing on the development, scale-up and commercialisation of printable and large area electronic type applications such as OLED displays and lighting and PV applications. CPI is equipped with an extensive range of open access assets specifically chosen and developed to allow clients to understand how their products and process perform under pilot manufacturing conditions.

As part of the overall challenge to develop flexible larger area devices a robust cost-effective flexible clear ultra-barrier technology is required and CPI has undertaken a programme of development to investigate and ultimately demonstrate and provided pilot production facilities.

For example, a typical flexible solar cell module is shown in the diagram below. This shows a high efficiency monolithically interconnected thin film CIGS solar cell with the key encapsulation of a top clear barrier sheet and a back barrier sheet. Additionally edge sealing is required to form the fully weather-able UV-protected flexible solar cells for outdoor PV applications, including building integrated PV. Also shown below is a flexible OLED light element produced on CPI’s LACE line and encapsulated with flexible clear barrier.

Top Clear Encapsulation

Diagram 1: Schematic Flexible CIGS Solar Cell Module

Bus-bar for Contacting

Back Side Encapsulation

Monolithically Interconnected CIGS Active Layer

Low Barrier

Medium Barrier

High Barrier

Time

Single Barrier

H2O

(g/m2)Polymer

Low Barrier

Medium Barrier

Lag Time

High Barrier

Time

Double Barrier

H2O

(g/m2)Polymer

Lag Time

Inter-layer

Single ‘Perfect’ Barrier Layer Approach Multi Barrier Layer (Vitex Approach)

Barrier Layer

Plastic Film

Single ‘Perfect’ Layer

Plastic Film

Ultra Barrier Deposition Systems

CPI have deposited ultra-barrier single and multi-layer stacks by sputtering techniques in a 400mm-wide roll-to-roll (R2R) sputter coater, depositing AlOx on PET and PEN substrates. In addition, single layer of AlOx on similar substrates have been deposited by conventional (temporal) atomic layer deposition (ALD) in a batch system. The deposition systems are shown below.

CPI, National Printable Electronics Centre, Thomas Wright Way, NETPark, Sedgefield, County Durham, TS21 3FG, UNITED KINGDOM

For grid parity flexible PV applications, the price point is particularly demanding. Assuming 100 mW/cm2 solar irradiance and 15% module efficiency with a target of 0.5 €/Watt peak, a flexible solar cell would not be more expensive than 60 €/m2 (allowing for balance of systems costs). Solar cell modules require encapsulation; the cost of them is estimated to be 25% of the total device cost, with an estimated flexible clear barrier (including substrate) to be in the range of 5-8 €/m2. This is very challenging for all barrier technologies, especially the multilayer-barrier approach. For flexible OLED applications the price point is not so challenging (10s €/m2) but the barrier performance requirement is very high (10-6 g/m2/day moisture permeation or lower!).

The schematic moisture permeation diagram shown below indicates that the multi layer approach only introduces a ‘lag time’ for equivalent thick barrier layers. With requirements of 25 years for BIPV applications, the ‘lag time’ effect may not be sufficient; the preferred solution being an ultra-barrier single layer, ideally as thin as possible to allow flexibility without cracking and allowing higher line-speed production to be achieved.

Moisture Barrier Permeation Measurement

CPI characterises the moisture permeation of clear barrier materials using the standard MOCON ‘Aquatran 1’ operating at 38°C 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.

The results for barrier layers of 30-40nm for sputtering and 5-40nm for ALD systems are shown in the table to the right (the sputtered AlOx layers are deposited by reactive sputtering from dual rotatable magnetron cathodes; the ALD layers are deposited by sequential introduction of Tri-Methyl Aluminium (TMA) and oxygen plasma).

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 (although less consistency in production is achieved at present).

The intrinsic performance of an ALD barrier layer showing defects is shown below.

Sputtered Barrier Stacks Moisture Barrier (Extrinsic) g.m2.day-1 (Mocon, 38C, 90%RH)

Single AIOX Layer 5.10-2 - 5.10-1

Double AIOX Layer 4 - 6.10-3

Triple AIOX Layer <5.10-4

ALD Single Barrier

AIOX (40nm thick) <5.10-4

AIOX (5nm thick) <5.10-4 (But less consistancy)

Substrate

PET 195 AI2O

31.4x10-6

3.7x10-2

480 hours

640 hours

Thickness(Micons)

ALD LayerWVTR

(g/m2/day)Time in Damp Heat Test

(80°C, 80%RH)

The intrinsic performance of the ALD barrier is shown in the photographs and table shown above-right. The photographs show the ultra barrier samples after exposure to damp-heat conditions (80°C, 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 (note bottom left area of unaffected Ca)

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).

The pictures below show Ca-Test data, formed from transmitted-light stitched-microscopy images taken during aging in 60°C, 90%RH damp heat aging trials. It can be seen that the rate of moisture permeation varies in a large range for the defects present. Initial estimates suggest that 10% or less of the defects account for the majority of the moisture permeation in most ‘large area’ samples. Control of these types of defects are key to the production of ultra-barrier films with the chance of achieving ultra-barrier performance.

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Initial trials for the encapsulation of flexible OLED devices produced on the CPI ‘LACE’ line have shown promising results using single layer ALD ultra-barrier film encapsulants.

Future R2R ALD Ultra Barrier Deposition Development

CPI will be installing a 500mm-wide R2R spatial ALD coater from BENEQ (Finland) in the next few weeks. The first CPI aim is to produce clear flexible ultra-barrier by R2R-ALD processing (including pre-and post-processing). The work will be carried out in collaboration with the BENEQ and other interested parties. Further, CPI will develop R2R-ALD direct encapsulation technology for a variety of flexible electronic applications. CPI believes R2R ALD deposition systems will play an important role not only in ultra-barrier and encapsulation technologies, but also for conformal highly-dense films for incorporation in a wide range of larger area ‘plastic’ electronic devices.

Initial coating trials have been carried out with promising results already achieved.

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