Al2O3/ZrO2 Nanolaminates as Ultrahigh Gas‐Diffusion Barriers—A Strategy for Reliable Encapsulation of Organic Electronics

Abstract

Adv. Mater. 2009, 21, 1845–1849 2009 WILEY-VCH Verlag G Gas-diffusion barriers play an important role in many applications today, such as flat-panel displays or solar cells grown on plastic substrates. In particular, organic light-emitting diodes (OLEDs) need efficient encapsulation because of the high sensitivity of many organic or electrode materials to moisture and oxygen, which has been demonstrated to cause device degradation and limited lifetime. Maximum allowable permeation rates for a viable encapsulant to be used in organic optoelectronics are still under debate, but it is common sense that the requirements are far more demanding than those for typical barrier films used in food or pharmacy packaging. A very commonly quoted figure for the upper limit of the water-vaportransmission rate (WVTR) in order to reach a minimum OLED lifetime of 10000 h is 10 6 gm 2 day . This value originated from an estimate of the amount of water needed to degrade the reactive cathode material. For the oxygen-transmission rate (OTR), maximum values for similar OLED lifetimes have been reported in the region of 10 –10 3 cmm 2 day . Nowadays, a very common technique is the encapsulation of organic devices with a glass or metal lid, which is applied under nitrogen atmosphere using, for example, an epoxy resin as glue. In addition, the residual moisture in the cavity between the organic device and lid is further minimized by an opaque getter material. For large-area, flexible, or transparent applications, glass-lid encapsulation is not suitable. A promising alternative is thin-film barriers of metal oxides or nitrides, such as Al2O3, SiO2, TiO2, or SiN. These materials can be deposited by several techniques. Plasma-assisted processes, such as sputtering or plasma-enhanced chemical vapor deposition (PECVD), provide high quality films, but their typically reported OTR and WVTR values of about 0.5 cmm 2 day 1 and 0.3 gm 2 day , respectively, limited by imperfections in the films, are not sufficient for OLED applications. Moreover, efficient step coverage and conformal coating are not straightforward with these techniques. As a workaround, hybrid organic/inorganic multilayer structures have been introduced. In these, the polymeric intermediate layer levels the inorganic layer so that diffusion pathways through the entire stack of the barrier can be reduced. However, the combination of alternating nonvacuum-based polymer deposition processes and vacuum-based deposition processes is timeconsuming and costly, and therefore not really practical for production. An alternative technique that promises highly uniform thin-film coatings is atomic-layer deposition (ALD). ALD relies on the sequential exposure of the surface to be coated to a metal–organic precursor and a reactant (H2O, O3, NH3, etc.). The sequential dosing of precursors leads to a self-limiting process with concomitant precise control over thickness and homogeneity. The technique is well-established in the processing of insulating films for gate dielectrics and capacitors. ALD has the advantage of allowing the deposition of very dense films at low temperatures (<100 8C), and thus appears to be a promising technique suitable for preparing encapsulation layers on top of organic electronic devices. There are a few reports dealing with ALD-grown thin films for OLED encapsulation. Very-low permeation rates of 6.5 10 5 gm 2 day 1 at 60 8C have been reported for Al2O3 films that were grown at 120 8C. [13] However, deposition temperatures above 100 8C might be critical, because glass-transition temperatures for many functional OLED materials have been reported in this very temperature range. In this Communication, we report on thin encapsulation layers prepared by ALD at a temperature of 80 8C. Specifically, we study neat Al2O3 films as a reference, and introduce novel highly efficient permeation barriers based on so-called nanolaminate structures. A nanolaminate, in this case, is a composite film consisting of repetitively deposited ultrathin (a few nanometers thick) alternating layers of Al2O3 and ZrO2. The alternating nanolayer structure suppresses the formation of both microscopic voids and nanocrystals. As a result, the probability of the occurrence of statistical defects in the barrier structure related to permeation paths along connected voids or grain boundaries is significantly reduced.We demonstrate that nanolaminates render pinhole-free thin-film encapsulation layers suitable for large-area organic devices possible. First, the quantitative assessment of permeation rates on the order of 10 6 gm 2 day 1 for water and 10 3 cmm 2 day 1 for oxygen is very challenging and, as yet, no commercially available method can be applied in this range. Therefore, we used a very sensitive permeation-measurement technique that was introduced by Paetzold et al. The method allows the measurement