Article pubs.acs.org/IECR
Water Vapor Barrier Material by Covalent Self-Assembly for Organic Device Encapsulation Gayathri N. Kopanati,† Sindhu Seethamraju,‡ Praveen C. Ramamurthy,‡,§ and Giridhar Madras*,† †
Department of Chemical Engineering, ‡Centre for Nanoscience and Engineering, §Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India ABSTRACT: Development of barrier materials for organic device encapsulation is of key interest for the commercialization of organic electronics. In this work, we have fabricated barrier films with ultralow water vapor permeabilities by reactive layer-bylayer approach. Using this technique, alternative layers of polyethylene imine and stearic acid were covalently bonded on a Surlyn film. The roughness, transparency and thickness of the films were determined by atomic force microscopy, UV−visible spectroscopy and scanning electron microscopy, respectively. Water vapor transmission rates through these films and the ability of these films to protect the organic photovoltaic devices was investigated. The films with covalently assembled bilayers exhibited lower water vapor transmission rates and maintained higher organic photovoltaic device efficiencies compared to the neat Surlyn film. additional advantage of ease of processing ultrathin films. This process involves self-assembling of molecular layers by the adsorption of oppositely charged species on to the substrate from solution. The alternate adsorption of cationic and anionic species results in the formation of subsequent cationic and anionic layers due to electrostatic interactions. The thickness of the assembled layers can be tailored by small modifications in pH of the medium,16 temperature17 and time of immersion.18 These multilayers obtained from conventional noncovalent LBL techniques are either due to electrostatic or hydrogen bonding interactions among the layers.19 However, covalent bonding of the oppositely charged layers further helps increasing the stability of the deposited layers. Few studies20,21 deal with reactive LBL processing, wherein one layer is physically adsorbed onto the film and the assembly of other layers onto the substrate is driven by chemical reactions. Reactive LBL has the potential advantage of stability and mechanical robustness over traditional noncovalent LBL. This process results in the formation of a cross-linked network of interlayers by the formation of covalent bonds, which is not possible in conventional LBL.22 Moreover, reactive LBL has proved to be promising for the fabrication of thin films for applications such as drug delivery and sensing applications due to its mechanical robustness.23 Therefore, the process of reactive LBL deposition has been chosen in this work to fabricate barrier films with selfassembled dense organic layers on a flexible polymer substrate. Poly(ethylene-co-methacrylic acid) (Surlyn) was used as the substrate. Surlyn is an ionomer that has been previously used in blends and composites for gas barrier applications24 because of its lower permeability (∼0.1 g m−2 day−1) when compared to other polymers such as poly(ethylene terephthalate) (∼10 g
1. INTRODUCTION Organic devices have a wide range of desirable properties such as flexibility, lightweight and ease of processing.1 This has resulted in various applications such as photovoltaics, light emitting diodes, sensors, etc.2 However, their instability to withstand environmental factors such as oxygen and water vapor limit their commercial applications.3 Therefore, the development of ultrahigh barrier materials is needed to restrict the permeation of oxygen and moisture. The permeation rates of water vapor through the barrier materials are quantified by water vapor transmission rates (WVTRs), which are required to be less than 10−6 g m−2 day−1 for potential applications of these devices.4,5 Glass is the conventional encapsulant with ultrahigh barrier properties. However, it compromises the fundamental advantage of mechanical flexibility and the possibility of roll-to-roll processing of organic devices.6 Thus, the development of polymer based ultrahigh barrier materials could be a solution for increasing the lifetimes of organic devices. Polymer films with a WVTR of 0.1 g m−2 day−1 used in food packaging cannot suffice the requirement of ultralow permeability required for organic device encapsulation.7 Therefore, fabrication of polymer based composites/blends with ultralow WVTRs is one of the key challenges. Various researchers have studied the possibility of depositing inorganic layers such as SiO2, Al2O3, ZrO2, etc.8,9onto various polymers to improve the gas barrier. However, pinhole mediated permeation is prominent in such inorganic/organic multilayered architectures.10 This further affects the performance of organic devices due to the formation of dark spots.11 Moreover, the process of deposition of inorganic layers is rather complicated and expensive.12 To overcome the issues associated with pinholes and crack formation in inorganic layers, organic based layers have been investigated.13,14 Wet processing techniques for thin film coatings are preferred over conventional dry processing methods due to their simplicity and convenient handling. Layer-by-layer (LBL) deposition,15 a versatile wet processing technique, has an © 2014 American Chemical Society
Received: Revised: Accepted: Published: 17894
September 18, 2014 October 21, 2014 October 22, 2014 October 22, 2014 dx.doi.org/10.1021/ie5036995 | Ind. Eng. Chem. Res. 2014, 53, 17894−17900
Industrial & Engineering Chemistry Research
Article
m−2 day−1). In addition, it also possesses the property of chemical inertness25 and self-healing.26 The choice of the materials used in this work is based on the hydrophobic nature of stearic acid (SA) and good moisture barrier exhibited by Surlyn. Poly(ethylene imine) (PEI) has been previously used for depositing barrier layers by LBL using other anionic polyelectrolytes.27 Branched polymers are more suitable for cross-linking and are easier to cross-link than linear polymers. Moreover, in the presence of N′,N-dicyclohexylcarbodiimide (DCC), branched PEI and stearic acid (SA) react to form a cross-linked network. Sequential adsorption of PEI and SA layers on to the Surlyn substrate with chemical interactions using a dehydrating agent was carried out and the process was optimized for the fabrication of barrier materials. The deposited layers were characterized by Fourier transform infrared (FTIR) spectroscopy and gravimetric analysis. The interactions between the layers were obtained from theoretical density functional theory (DFT) calculations. The thermal, transparency, hydrophobic, surface and barrier properties of the fabricated films were determined from thermogravimetric analysis (TGA), UV− visible, contact angle, atomic force microscopy (AFM)and calcium degradation studies, respectively. Further, these materials were tested for their ability to encapsulate organic devices from accelerated aging studies.
sensitivity of 0.1 mg. A PerkinElmer FTIR/FIR frontier spectrometer was used to characterize all the samples in the range of 4000 to 400 cm−1 by accumulating eight scans at a resolution of 4 cm−1. UV−visible characterization was carried out on a PerkinElmer (Lambda 35) UV−visible spectroscope over the range of 230−1100 nm. The thermal studies of the neat polymer film and film with bilayers were studied using thermogravimetric analysis (TGA, PerkinElmer) at a heating rate of 5 °C min−1. The thickness of the deposited bilayers was determined from scanning electron microscopy (SEM) analysis using an ULTRA 55, field emission scanning electron microscopy (FESEM) instrument (Carl Zeiss) with energydispersive X-ray spectroscopy (EDS). The samples were cryofractured and gold sputtered prior to SEM analysis. To determine the roughness and surface morphology of all the samples, atomic force microscopy (AFM, Bruker) was used. An Holmarc contact angle instrument with a goniometer HO-IADCAM-01A was used to calculate the contact angle (CA) of water on all the samples. 2.4. Computational Studies. Density functional theory (DFT) calculations were performed using DMol3 module of Accelrys, Materials Studio 6.0, to determine the interaction energies between the individual layers. The geometries for PEI, SA and Surlyn were optimized following the gradient-corrected functional GGA-BLYP with double nuclear polarization and a 4.4 atomic orbital basis set. The difference in relative energies was used for calculating the interaction energies between the layer components, Surlyn−PEI and PEI−SA layers. 2.5. Determination of WVTR. Calcium degradation test is the commonly used industrial method to measure permeation rates over a wide range of WVTR. This uses the basic chemistry of calcium and permeant H2O molecules. Calcium, when exposed to moisture, reacts to form calcium hydroxide. While calcium is metallic and conducting, oxidized calcium is nonconducting. Therefore, the permeation rates are calculated using the change of conductivity of calcium thin films on a glass slide sealed with the barrier film when exposed to high humid conditions (95% RH). 2 × 2 cm glass slides were cut and cleaned. The centers of glass slides with dimensions 1 × 1 cm (l × b) were deposited with calcium of thickness 200 nm in a thermal evaporator. Aluminum, which acts as an electrode, was thermally evaporated on either side of calcium. The deposited calcium was sealed by the fabricated sample films using epoxy glue. All the depositions were carried out under inert conditions (
