Growth of thin barrier films on flexible polymer substrates by atomic layer deposition

Organic electronics research has received significant attention in recent years. The majority of this research has focused on the development of organic photovoltaic cells (OPVs) and organic light emitting diodes (OLEDs). Polymer substrates are used for organic electronics as they are lightweight, cheap, transparent, printable and flexible. One significant disadvantage of polymers, however, is their high gas/vapour permeability. For both OPVs and OLEDs to have sufficient lifetimes for commercial applications, barrier films are required to reduce the degradation resulting from exposure to water vapour and oxygen. Atomic layer deposition (ALD) is a thin film deposition technique ideally suited to the deposition of inorganic films for barrier applications as it produces conformal pinhole-free coatings. Inorganic films produced by ALD have achieved lower water permeation rates with thinner layers than other techniques due to film integrity. ALD alumina has been the most frequently studied barrier coating on polymer substrates in work to date. While single ALD films layers have been successfully used to reduce the water vapour transmission rate (WVTR) of polymer substrates, the lowest WVTRs have been achieved using multilayer films. Inorganic multilayer barrier films have been produced with alternate layers of two ALD metal oxides such as alumina and titania. In addition, alternating layers of ALD metal oxide and a polymer have also been combined to form inorganic/organic multilayer barrier films. The use of a multilayer structure reduces diffusion pathways through the entire film thickness, thus producing lower WVTR values. This review highlights the effectiveness of ALD barrier films in reducing water permeation through polymer substrates, which is an essential requirement for the longevity of organic electronic devices.