A flexible transparent gas barrier film employing the method of mixing ALD/MLD-grown Al2O3 and alucone layers

Xiao et al. Nanoscale Research Letters A flexible transparent gas barrier film employing the method of mixing ALD/MLD-grown Al O 2 3 and alucone layers Wang Xiao 0 Duan Ya Hui 0 Chen Zheng 0 Duan Yu 0 Yang Yong Qiang 0 Chen Ping 0 Chen Li Xiang 0 Zhao Yi 0 0 State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University , Jilin 130012 , China Atomic layer deposition (ALD) has been widely reported as a novel method for thin film encapsulation (TFE) of organic light-emitting diodes and organic photovoltaic cells. Both organic and inorganic thin films can be deposited by ALD with a variety of precursors. In this work, the performances of Al2O3 thin films and Al2O3/alucone hybrid films have been investigated. The samples with a 50 nm Al2O3 inorganic layer deposited by ALD at a low temperature of 80C showed higher surface roughness (0.503 0.011 nm), higher water vapor transmission rate (WVTR) values (3.77 104 g/m2/day), and lower transmittance values (61%) when compared with the Al2O3 (inorganic)/alucone (organic) hybrid structure under same conditions. Furthermore, a bending test upon single Al2O3 layers showed an increased WVTR of 1.59 103 g/m2/day. However, the film with a 4 nm alucone organic layer inserted into the center displayed improved surface roughness, barrier performance, and transmittance. After the bending test, the hybrid film with 4 nm equally distributed alucone maintained better surface roughness (0.339 0.014 nm) and barrier properties (9.94 105 g/m2/day). This interesting phenomenon reveals that multilayer thin films consisting of inorganic layers and decentralized alucone organic components have the potential to be useful in TFE applications on flexible optical electronics. Thin film encapsulation; Water vapor transmission rate; Molecular layer deposition; Low-temperature atomic layer deposition - Background Organic electronics is an emerging technology that has potential uses in highly efficient lighting, super-bright displays, novel photovoltaic devices, and integrated smart systems [1-3]. Furthermore, it offers promising opportunities for the development of new products that utilize the special features of organic electronics such as flexibility, bendability, and transparency [4-6]. However, one major impediment to the mass production of organic devices is insufficient product lifetimes caused by their inclination to stop functioning when exposed to water vapor, oxygen, and other detrimental compounds present in air. Encapsulation layer, also known as barrier film, is a necessary and often overlooked part of the organic device architecture. Furthermore, polymer substrates, often used in flexible devices, provide better flexibility and toughness properties, but possess insufficient barrier properties against water vapor and oxygen permeation [7]. Since oxide films have to be of high quality to provide superior barrier performance, atomic layer deposition (ALD) is being pursued as an alternative to traditional chemical and physical vapor deposition methods. Reducing the number of defects (pinholes, grain boundaries, etc.) can reduce the layer thickness and/or number of layers required to achieve the required water vapor transmission rates (WVTR, g/m2/day). Recently, this type of thin film encapsulation (TFE) has attracted great attention in order to overcome the airsensitive issue [8-10]. The inorganic/organic encapsulation method based on ALD and molecular layer deposition (MLD), respectively, has demonstrated better barrier performance and mechanical properties than single inorganic layers [11-13]. On the one hand, the organic layer could potentially decouple any defects and prolong the permeation path, leading to lower WVTR values [14,15]. On Figure 1 A schematic diagram of prepared TFE structures. (a) Film A: Al2O3 50 nm. (b) Film B: Al2O3/alucone/Al2O3: 23/4/23 nm. (c) Film C: Al2O3/ alucone/Al2O3/alucone/Al2O3/alucone/Al2O3/alucone/Al2O3 9/1/9/1/9/1/9/1/9 nm. the other hand, single inorganic encapsulation films are brittle in general, but the hybrid inorganic/organic structure reduces the internal stress of inorganic films generally improving flexibility [16,17]. It is therefore important to consider the development of high-barrier functionalities as well as the mechanical properties of TFE samples. In this study, samples with Al2O3 (ALD) or alucone (MLD) layers were grown and characterized. All encapsulation films were deposited at a low temperature of 80C [18,19]. We investigated single Al2O3 films with Al2O3/alucone hybrid laminate before and after a bending test. The gas barrier and mechanism performances were both optimized [20] upon Al2O3 samples incorporating a 4-nm transparent organic component of the same nominal thickness. From this analysis, some important insights were determined, demonstrating that the performance of TFE with hybrid inorganic-organic structure could be optimized by prudent selection of certain design parameters. Methods In the experiments, we fabricated a group TFE consisting of three different thin films. All films have nominal thicknesses of approximately 50 nm. As shown in Figure 1, film A was a 50 nm Al2O3 inorganic film. Films B and C consisted of approximately 46 nm Al2O3 and 4 nm alucone. For film B, 4 nm alucone was in the center of the hybrid film (23/4/23 nm). However, the alucone layer was divided into four equal parts in film C (9/1/9/1/9/1/9/1/ 9 nm). Both Al2O3 and alucone thin films were deposited by a LabNano 9100 ALD system (Ensure Nanotech Inc., Beijing, China) at 80C, and all pipes were heated to 120C, while the pressure in the reaction chamber was 1.5 100 Pa. Table 1 summarizes the film deposition parameters during the ALD process. Al(CH3)3 (trimethylaluminum or TMA, Sigma Aldrich, St. Louis, MO, USA) and deionized water were prepared as precursors for Al2O3 inorganic layer. During the growth process, high-purity N2 (flow rate = 20 sccm) was used as carrier gas for these precursors. One reaction cycle included the following: 0.02 s TMA dose, 30 s nitrogen purge, 0.02 s H2O dose, and 30 s nitrogen purge. This sequence was repeated to obtain the desired thicknesses. For alucone organic layer, TMA and HO-(CH2)2-OH (ethylene glycol or EG, Sigma Aldrich) were reactants grown under identical conditions. Before the deposition process, EG was preheated to 95C to increase its vapor pressure [21]. The timing sequence was as follows: 0.02 s TMA dose, 30 s nitrogen purge, 0.07 s EG dose, and 120 s nitrogen purge. The growth mechanism for each type of film has been described previously [22]. WVTR measurements were Table 1 The thin film deposition parameters for the ALD process N2 purge time (s) Temperature (C) Pressure (Pa) Carrier gas 30 80 1.5 100 N2 0.07 (preheated to 95C) 120 Table 2 A summary of the surface film characteristics after deposition by ALD/MLD carried out to test the barrier performance of the films through the calcium (Ca) (...truncated)