Atmospheric deposition process for enhanced hybrid organic–inorganic multilayer barrier thin films for surface protection

Highlights

• A hybrid organic-inorganic multilayer barrier thin film is reported for the protection of electronic devices.

• The organic thin films of PVA were developed by using roll to roll electrohydrodynamic atomization (R2R-EHDA).

• Inorganic thin film of Al₂O₃ was deposited by using roll to roll spatial atmospheric atomic layer deposition (R2R-SAALD).

• Use of these two technologies together is very useful for the cost efficient and mass production of such protective layers.

• Uniform thin films of Al₂O₃ reduced the roughness of PVA thin film while PVA elongated the delay time for water vapors.

Abstract

In this study, organic polymer poly-vinyl acetate (PVA) and inorganic aluminum oxide (Al₂O₃) have been used together to fabricate a hybrid barrier thin film for the protection of PET substrate. The organic thin films of PVA were developed through roll to roll electrohydrodynamic atomization (R2R-EHDA) whereas the inorganic thin films of Al₂O₃ were grown by roll to roll spatial atmospheric atomic layer deposition (R2R-SAALD) for mass production. The use of these two technologies together to develop a multilayer hybrid organic-inorganic barrier thin films under atmospheric conditions is reported for the first time. These multilayer hybrid barrier thin films are fabricated on flexible PET substrate. Each layer of Al₂O₃ and PVA in barrier thin film exhibited excellent morphological, chemical and optical properties. Extremely uniform and atomically thin films of Al₂O₃ with average arithmetic roughness (Ra) of 1.64 nm and 1.94 nm respectively concealed the non-uniformity and irregularities in PVA thin films with Ra of 2.9 nm and 3.6 nm respectively. The optical transmittance of each layer was ∼ 80-90% while the water vapor transmission rate (WVTR) of hybrid barrier was in the range of ∼ 2.3 × 10⁻² g m⁻² day⁻¹ with a total film thickness of ∼ 200 nm. Development of such hybrid barrier thin films with mass production and low cost will allow various flexible electronic devices to operate in atmospheric conditions without degradation of their properties.

Introduction

The development of electronic devices such as solar cells, organic light emitting diodes (OLEDs), organic light emitting transistors (OLETs), organic thin film transistors (OTFTs) and memristors on flexible substrates would result in great advantages such as reduced weight, low cost, and high flexibility [1], [2], [3], [4], [5]. However, a few severe challenges need to be resolved before fabricating these devices on polymeric substrates such as high permeability of flexible substrates to water vapors and oxygen that can disintegrate metallic electrodes and sensitive functional materials [6]. Therefore, the polymeric substrates must be passivized with additional barrier coatings such as atomically thin films of Al₂O₃ to improve the efficiency and life time of these devices [7]. Encapsulation of electronic devices made of organic materials is especially important because they are often very sensitive to atmospheric species such as H₂O and O₂ [8]. The atmospheric H₂O and O₂ can significantly degrade the performance of electronic devices by oxidizing their metallic electrodes and highly sensitive functional electronic materials that will ultimately result in the malfunctioning of such devices. The permeation of H₂O through the surface defects into the active layer triggers chemical reactions that results in volumetric expansions thus creating the delamination. Furthermore, the penetration of moisture may also deteriorate the functionality of a molecular device through hydrolysis [9]. Therefore, such devices must be encapsulated with thin films of barrier material (Al₂O₃) to protect them against atmospheric H₂O and O₂.

The Al₂O₃ is one of the most efficient and largely reported gas barrier material used for the development of barrier thin films. The Al₂O₃ thin films can be efficiently deposited on a wide variety of substrates through state of the art technology of ALD. The excellent dielectric properties of Al₂O₃ are consistent with defect-free thin films that are desirable for high quality permeation barriers. Unlike the crystalline films, the ALD deposited Al₂O₃ films grown under moderately low temperatures have amorphous nature, free of grain boundaries. These excellent conformal and amorphous yet atomically thin films of Al₂O₃ offer the promising advantages such as blocking the migration of ions, gas molecules and moisture, thus making it a potential candidate for barrier applications [10]. Thin Al₂O₃ films with the order of less than or equal to 30 nm have already shown a great decrease in water vapor and oxygen transmission rates [11], [12]. These films have proved to enhance the functionality of electronic devices for a longer time period [13]. However, these barrier films alone are not the final solution of the problem of permeation. The performance of these atomically thin inorganic films deteriorate with the passage of time resulting in the infusion of H₂O and O₂ through them. Therefore, these inorganic thin films must be coupled with additional support to enhance device protection and further improve its life time by preventing transmission of H₂O and O₂ into the sensitive functional materials.

Several research groups have reported different strategies to develop barrier films using wide variety of fabrication technologies and coating materials. The performance of barrier films greatly depends upon the processing technology and used material. The latest barrier thin film approaches include; organic/inorganic multilayers, inorganic multilayers, and organic modified inorganic thick single layer [6], [14]. Although these approaches have shown improved barrier properties yet they have limitations such as poor heat resistance, poor flexibility and most importantly lack of being processed through roll-to-roll fabrication technology that is a key to mass production and cost reduction [6], [15], [16], [17], [18].

In this study, we have reported the fabrication of high performance hybrid organic-inorganic multilayer barrier thin films on a flexible PET substrate through a unique technology of R2R-SAALD accompanied with a single nozzle R2R-EHDA printing technique. Unlike, the conventional ALD, the flexible substrate is continuously moving relative to the precursor sources in R2R-SAALD, and therefore, the deposition rate is determined by the speed of the substrate rather than the cycle time of the precursor exposure sequence. Since there is no requirement for pulsing and purging of precursors from a common volume; therefore, the growth rates of the as deposited thin films are limited only by the surface kinetics and the reaction rates of each half cycle. Such unique features make R2R-SAALD a potential candidate for mass production. The R2R-EHDA printing technique has also made its landmark in deposition of highly uniform thin films of wide variety of materials under atmospheric conditions and has a promising future for mass production. As both R2R-ALD and R2R-EHDA are capable of operating under atmospheric conditions; therefore, this combination of coating technologies is an excellent and optimum choice for the fabrication of hybrid multilayer barrier films to protect electronic devices. The objective of this research work has been accomplished in such a way that the layers of Al₂O₃ have been fabricated through R2R-SAALD by using Trimethylaluminum [Al (CH₃)₃, TMA] and water (H₂O) as the precursors at a temperature of 100 °C and the organic layers of PVA are fabricated through R2R-EHDA. This approach of involving the combination of these two technologies has never been reported before and has a great potential to be used for the development of barrier films in various industries, especially in the field of printed electronics which involves the development of electronic devices under atmospheric conditions [2], [3], [4], [6]. The fabricated films have been investigated as gas barrier films and have shown promising results to be used in the field of flexible electronics. The conditions for deposition process are investigated in terms of layer characterizations including thickness, growth rates, surface morphology, chemical compositions, electrical characterization and optical properties.