Sindhu Seethamraju

Ionomer based blend as water vapor barrier material for organic device encapsulation

Blends of poly (ethylene-co-methacrylic acid) (PEMA) and poly (vinyl alcohol-co-ethylene) (EVOH) were studied for encapsulating Schottky structured organic devices. A calcium degradation test was used to determine water vapor transmission rates and to determine the moisture barrier performance of neat and blend films. Moisture barrier analysis for the neat and blend compositions was discussed concerning the interactions in the blend, diffusivity of water molecules through the unit cell systems, and the occupiable free volumes available in the unit cells using molecular dynamics simulations. The experimental results of water vapor permeation were correlated with diffusion behavior predicted from molecular dynamics simulations results. The effectiveness of the blend as a suitable barrier material in increasing the lifetime of an encapsulated Schottky structured organic device was determined.

Flexible poly(vinyl alcohol-co-ethylene)/modified MMT moisture barrier composite for encapsulating organic devices

Flexible, nano-composite moisture barrier films of poly(vinyl alcohol-co-ethylene) with surface modified montmorillonite fabricated by solution casting were used to encapsulate organic devices. The composite films were characterized by FTIR, UV-visible spectroscopy and SEM imaging. Thermal and mechanical properties of the composite films were studied by DSC and UTM. Calcium degradation test was used to determine the transmission rate of water vapour through the composite films, which showed a gradual reduction from ∼0.1 g m−2 day−1 to 0.0001 g m−2 day−1 with increasing modified montmorillonite loading in the neat copolymer. The increase in moisture barrier performance is attributed to the decreased water vapour diffusivity due to matrix–filler interactions in the composite. The accelerated aging test was carried out for non-encapsulated and encapsulated devices to evaluate the efficiency of the encapsulants. The encapsulated devices exhibited longer lifetimes indicating the efficacy of the encapsulant.

Performance of an ionomer blend-nanocomposite as an effective gas barrier material for organic devices

A new, flexible, gas barrier material has been synthesized by exfoliating organically modified nano-clays (MMT) in the blends of Surlyn (PEMA) using a copolymer of vinyl alcohol (EVOH) and demonstrated as a gas barrier material. The materials were characterized by Fourier transform infra red (FTIR) and UV-visible spectroscopy, differential scanning calorimetry (DSC), thermo-gravimetric analysis (TGA) and tensile studies. The oxygen and water-vapor permeabilities of the fabricated films were determined by calcium degradation test and a novel permeability setup based on cavity ring down spectroscopy, respectively. Hierarchical simulations of these materials helped us to understand the effect of intermolecular interactions on diffusivities of oxygen and water molecules in these materials. Schottky structured poly(3-hexylthiophene) based organic devices were encapsulated with the fabricated films and aging studies were carried under accelerated conditions. Based on permeability test results and accelerated aging studies, the fabricated PEMA/EVOH/MMT composites were found to be effective in decreasing the permeabilities for gases by about two orders of magnitude and maintaining the lifetime of organic devices.

Million-Fold Decrease in Polymer Moisture Permeability by a Graphene Monolayer

Flexible, transparent, and moisture-impermeable materials are critical for packaging applications in electronic, food, and pharmaceutical industries. Here, we report that a single graphene layer embedded in a flexible polymer reduces its water vapor transmission rate (WVTR) by up to a million-fold. Large-area, transparent, graphene-embedded polymers (GEPs) with a WVTR as low as 10(-6) g m(-2) day(-1) are demonstrated. Monolayered graphene, synthesized by chemical vapor deposition, has been transferred onto the polymer substrate directly by a very simple and scalable melt casting process to fabricate the GEPs. The performances of the encapsulated organic photovoltaic (OPV) devices do not vary even after subjecting the GEPs to cyclic bending for 1000 cycles. Accelerated aging studies of working OPV devices encapsulated in the GEPs show a 50% lifetime of equivalent to 1 000 000 min, which satisfies the requirements of organic electronics.

Colorimetric anion sensor based on receptor having indole- and thiourea-binding sites

A new colorimetric sensor L containing nitro-substituted indole and bisthiocarbonohydrazone units for selective fluoride and acetate ions is designed and synthesized. The receptor L shows well-defined color change in the visible region of the spectrum with an absorption band at similar to 515 nm and 506 nm, respectively, for the F- and CH3COO- ions in an acetonitrile solution. Jobs plots indicated the formation of 1 : 1 (L with CH3COO-) and 1 : 2 (L with F-) complexes. The interaction of L with the F- ion undergoes a deprotonation process and release of HX2](-), whereas with the CH3COO- ion, it forms a stable LH2(...)X](-) complex. The relative affinities of the anions with L are rationalized using computational studies.

Layer-by-layer assembly of Nafion on Surlyn with ultrahigh water vapor barrier.

A layer-by-layer approach was used for the fabrication of multilayer films for ultra high gas barrier applications. The ultra high gas barrier material was designed by incorporating Nafion layer in between bilayers of poly(ethylene imine) and poly(acrylic acid) on a Surlyn substrate. When the barrier film with self-assembled Nafion is exposed to the moist environment, Nafion absorbs and desorbs water molecules simultaneously, thereby reducing the ingress of moisture in to the film. In order to study the effect of Nafion, the fabricated barrier materials with and without the presence of Nafion were tested for water vapor barrier properties. The barrier films were further used for encapsulating organic photovoltaic devices and were evaluated for their potential use in barrier applications. The devices encapsulated with the films containing Nafion exhibited better performance when subjected to accelerated aging conditions. Therefore, this study demonstrates the effectiveness of self-assembled Nafion in reducing the water vapor permeability by nearly five orders of magnitude and in increasing the lifetimes of organic devices by ∼22 times under accelerated weathering conditions.

Organic passivation layer on flexible Surlyn substrate for encapsulating organic photovoltaics

Barrier materials are required for encapsulating organic devices. A simple methodology based on organic passivation layer on a flexible substrate has been developed in this work. Stearyl stearate (SS) was directly coated over the flexible Surlyn film. The barrier films with SS passivation layer exhibited much lower water vapor transmission rates compared to the neat Surlyn films. Moreover, the effect of the process of deposition of organic passivation layer on the resultant water vapor properties of the barrier films was evaluated. The accelerated lifetime studies conducted on encapsulated organic photovoltaics showed that the passivation layer improved the device performance by several fold compared to the non-passivated barrier films.

A Surlyn/magnesium oxide nanocomposite as an effective water vapor barrier for organic device encapsulation

A reactive polymer nanocomposite system was proposed as an effective water vapor barrier material for organic device encapsulation. Nanosized magnesium oxide (MgO) was synthesized by the solution combustion technique using two different fuels, lactose and alanine. The purity and crystallite size of MgO were determined from X-ray diffraction studies. The surface areas and porosity measurements were used to determine the water adsorption capacities of MgO. Nanocomposites with various concentrations (wt% = 0.25, 0.5, 1 and 2.5) of MgO were prepared using Surlyn as the base polymer. The permeation rate of moisture through the fabricated films was calculated using calcium degradation test and these rates were further used to calculate the diffusivities. Accelerated aging experiments were conducted to study the performance of organic photovoltaic devices encapsulated with synthesized films under accelerated weathering conditions. The performance of the barrier materials with synthesized MgO was also compared to that obtained with commercial MgO. The films containing MgO obtained from lactose exhibited better barrier properties compared to other films made with commercial MgO and MgO synthesized using alanine as well as other nanocomposites reported in the literature.

Water vapor permeabilities through polymers: diffusivities from experiments and simulations

This study experimentally determines water vapor permeabilities, which are subsequently correlated with the diffusivities obtained from simulations. Molecular dynamics (MD) simulations were used for determining the diffusion of water vapor in various polymeric systems such as polyethylene, polypropylene, poly (vinyl alcohol), poly (vinyl acetate), poly (vinyl butyral), poly (vinylidene chloride), poly (vinyl chloride) and poly (methyl methacrylate). Cavity ring down spectroscopy (CRDS) based methodology has been used to determine the water vapor transmission rates. These values were then used to calculate the diffusion coefficients for water vapor through these polymers. A comparative analysis is provided for diffusivities calculated from CRDS and MD based results by correlating the free volumes.

Encapsulation for Improving the Efficiencies of Solar Cells

Commercialization of solar energy would suffice most of the energy-related demands that vary from domestic to aviation applications. Therefore, there is an increased interest in exploring various solar-based technologies like inorganic and organic semiconductor-based photovoltaics, photoelectrochemical water splitting, and dye-sensitized solar cells. In order to increase the lifetime of these devices, it is essential to encapsulate these devices for protection from water, oxygen, and other detrimental gases. This requires development of materials with high barrier to these gases and the synthesis of these materials is discussed in this chapter. Due to the ultra low permeability, new characterization techniques developed for measuring the barrier properties and these methods are also discussed.