Improved barrier and mechanical properties of Al2O3/acrylic laminates using rugged fluorocarbon layers for flexible encapsulation
Graphical abstract
Introduction
Organic light-emitting diodes (OLEDs) are regarded as a promising display technology due to their great advantages, such as high efficiency, fast response time and wide viewing angle, and have been researched in various aspects in the past three decades [1,2]. The luminous efficiency and lighting quality of OLEDs have been greatly improved to meet commercial needs [3,4]. Furthermore, OLEDs is particular suitable for flexible display, providing potential applications to mobile, wearable and vehicle display where flexible form factor is important [5,6]. For flexible OLEDs, one of the technological difficulties is flexible encapsulation, since organic materials are sensitive to atmospheric moisture vapor and oxygen, the encapsulation performance will influence both the reliability and lifetime of devices. In general, OLED requires an encapsulation with very high degree of protection from molecule permeation, for instance, the water vapor transmission rate (WVTR) of the barriers should be less than 10−5 g/m2/day [7]. Conventional encapsulation methods using glass lid or metal lid have been proved to be effective to achieve long-term durability of OLEDs, which, however, are heavy, fragile and difficult to obtain the flexible performance [8,9].
Among the various approaches to OLED encapsulation, thin film encapsulation (TFE) has attracted a great deal of attention owing to its thinner and lighter form factor, higher flexibility of device form during in-flex use [8]. Barriers based on inorganic thin films, such as silicon oxide (SiO2), silicon nitride (SiNx), aluminum oxide (Al2O3) and titanium oxide (TiO2), were commonly used for OLEDs encapsulation [[10], [11], [12]]. However, defects caused by the deposition processes are inevitably existed in single-layer inorganic barrier films, leading to high permeation rates. Improved barrier performance has been reported by using multi-layered structures which comprised of alternating layer of different inorganic materials with individual layer thickness in nanometer-scale [7,11]. Whereas, cracks often appeared due to stress mismatch between the two inorganic films when the film thickness or the number of layers increased. Besides barrier performance, other critical aspects like mechanical properties, must also be considered in developing encapsulation technology, since the flexible devices are often under the deformation state [13].
Hybrid inorganic/organic multi-layered structure, also called vitex barrier technology, has been extensively investigated, and is considered as the most promising TFE technology. The inserting of organic layers can prolong the diffusion pathways by creating a tortuous path to avoid penetration directly through pinholes and cracks existed in inorganic films [[14], [15], [16]]. Additionally, the organic materials are normally soft compared to dense inorganic materials, and could release a certain degree of thin film stress, leading to improvement of flexibility under mechanical deformation [[17], [18], [19]]. However, the development and integration of high barrier encapsulation films using vitex barrier technology still remain a challenging. It is well known that most of the organic materials are porous with severe permeability, limiting the prolongation of the diffusion pathways. Although the inorganic layers are currently able to be deposited at low temperature using atomic layer deposition (ALD) or plasma-enhanced chemical vapor deposition (PECVD), cracks in the organic films might generate because of severe shrinkage of organic materials, as well as thermal stress mismatch between the inorganic films and the organic films during the growth of inorganic layers, resulting in the deterioration of encapsulation performance [[20], [21], [22]]. The incorporation of nanofillers, such as CaO and SiO2 nanoparticles, in the organic layer might improve its barrier and mechanical properties, but light transmittance will decrease because of light scattering [4,23].
On the other hand, plasma treatment is an efficient alternative for modifying the functional properties of organic materials. During plasma treatment, both the ionization of gas and the heat might induce surface reactions between plasma species and the surface of the polymer, increasing the density of cross-links [[24], [25], [26]]. Consequently, this plasma surface modification might improve the barrier and mechanical properties of organic layers in the inorganic/organic multi-layered structures [27,28].
In this work, Al2O3 thin films were grown using ALD and acrylic films were prepared by ink jet printing (IJP), to create an Al2O3/acrylic multi-layered encapsulation structure. The surface properties of acrylic were modified with SF6 plasma using reactive ion etching (RIE). The barrier and mechanical properties of Al2O3/acrylic multi-layered films were investigated with respect to the intensity and duration of plasma treatments.
Section snippets
Growth of Al2O3 using ALD
The Al2O3 barrier was deposited by an ALD system (Beneq TFS-200) which was carried out in a closed chamber under a constant low temperature of 90 °C. Specifically, trimethylaluminum (TMA) and water (H2O) were used as precursors, high purity nitrogen (N2, 99.999%) was used as both the carrier and purge gas. The reaction was carried out with the chamber pressure below 1 mbar. A single Al2O3 deposition cycle included the four steps: TMA pulse for 0.2 s, N2 purging for 6 s, H2O pulse for 0.15 s and
Effects of SF6 plasma treatments on the morphology of acrylic
Plasma treatment has been widely employed for the surface modification of polymeric materials due to its several advantages, such as selective and controllable surface modification area without affecting bulk properties, easy-tuning of modification by the treatment parameters (power, irradiation time, gas species and gas pressure, etc.) [[24], [25], [26]]. The processes of plasma activation might include etching, cross-linking, branching and surface treatment for the modification of the
Conclusions
In summary, Al2O3/acrylic laminates were successfully fabricated by atomic layer deposition (ALD) and ink-jet printing (IJP). SF6 plasma treatments were introduced for surface modifications of acrylic, comparative WVTR and lifetime measurements were conducted to determine the effects of plasma treatments on the barrier and mechanical properties of Al2O3/acrylic multi-layers. It was found that micro-/nano-structures on acrylic surface occurred because of plasma etching, and could be modulated by
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (No. 61775038 and No. 61904031), and Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2021ZZ130, 2020ZZ111, 2020ZZ113, 2021ZR143). The content of this work is the sole responsibility of the authors.
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Both authors contributed equally.