Synthesis and characterization of plasma polymer thin films from γ-terpinene for encapsulation of electronic devices

Ahmad, Jakaria (2015) Synthesis and characterization of plasma polymer thin films from γ-terpinene for encapsulation of electronic devices. PhD thesis, James Cook University.

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Abstract

The rapid development of polymers and polymer based materials is leading to a number of promising organic devices e.g., solar cells and solid-state lighting, advancing display technology, sensors, and thin-film transistors. One obstacle to this development is the susceptibility of these devices to water vapor and oxygen that causes rapid degradation in many organic electronic devices. In order to maintain the efficiency and guarantee the minimum lifetime needed for various applications, high barrier performance encapsulation materials and structures must be developed. In this work, radio frequency (RF) plasma deposited γ–terpinene thin films were considered as a potential candidate for OPVs, specifically as encapsulation coatings, and as insulating layers in flexible electronics.

γ-terpinene is a non-synthetic isomeric hydrocarbon derived from Melaleuca alternifolia essential oil. Thin films from this monomer were fabricated using plasma enhanced chemical vapor deposition in this research work under varied process conditions. The resultant polymer, plasma polymerized γ-terpinene (pp–GT) thin films were found to be optically transparent, with refractive indices in a range of 1.57–1.58 (500 nm). The optical band gap (Eᵍ) of pp–GT thin films were ~3 eV that fell into the insulating Eᵍ region. Independent of deposition conditions, the surfaces were smooth and defect-free, with uniformly distributed morphological features. Films fabricated at higher deposition power displayed enhanced resistance to delamination and wear, and improved hardness, from 0.40 GPa (10 W) to 0.58 GPa (75 W).

Investigations on the wetting, solubility and chemical composition of pp–GT thin films revealed that the polymers were structurally dissimilar to the original monomer and highly cross-linked. The polymer surfaces were hydrocarbon-rich, with oxygen present in the form of O–H and C=O functional groups. The oxygen content decreased with deposition power, with films becoming more hydrophobic and, thus, less wettable. The polymers were determined to resist solubilisation in solvents commonly used in the deposition of organic semiconducting layers, including chloroform and chlorobenzene, with higher stability observed in films fabricated at higher RF power.

Electrically, pp–GT thin films were highly insulating, possessing decreasing conductivity from 1.39 × 10⁻¹²S/cm (10W) to 1.02 × 10⁻¹³S/cm (75W), attributed to the change in the chemical composition and structure of the polymer. At a frequency of 100 kHz, the dielectric constant varied from 3.69 (10 W) to 3.24 (75 W). The current density–voltage (J−V) characteristics revealed that at higher applied voltage region, Richardson Schottky conduction was the dominant DC conduction mechanism.

pp–GT thin films were demonstrated to be optically, physically and chemically stable under the ambient conditions. The bulk of aging occurred after fabrication was attributed to oxidation and volumetric relaxation. Photostability experiments showed that photooxidation takes place under UV irradiation in oxygen–rich conditions. With UV–C light, photodegradation occurred by additional cleavage pathways. Photostability could be improved (UV-C) or completely achieved (UV-A) in an oxygen-poor environment. Finally, fully characterized pp–GT encapsulation layers were integrated with organic PCPDTBT:PC₇₀BM solar cell to validate the effectiveness of the barrier layers. The encapsulated solar cell exhibited very slow decrease in efficiency compared to the nonencapsulated device.

Item ID: 39960
Item Type: Thesis (PhD)
Keywords: electronics; engineering; gamma-terpinene; plasma polymers; plastics; polymer engineering; polymerisation; polymerization; polymers; thin films; γ-terpinene
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Ahmad, Jakaria, Bazaka, Kateryna, Anderson, Liam J., White, Ronald D., and Jacob, Mohan V. (2013) Materials and methods for encapsulation of OPV: a review. Renewable and Sustainable Energy Reviews, 27. pp. 104-117. Item availability may be restricted.

Ahmad, Jakaria, Bazaka, Kateryna, and Jacob, Mohan V. (2014) Optical and surface characterization of radio frequency plasma polymerized 1-isopropyl-4-methyl-1,4-cyclohexadiene thin films. Electronics, 3 (2). pp. 266-281.

Ahmad, Jakaria, Bazaka, Katia, Oelgemöller, Michael, and Jacob, Mohan V. (2014) Wetting, solubility and chemical characteristics of plasma-polymerized 1-isopropyl-4-methyl-1,4-cyclohexadiene thin films. Coatings, 4 (3). pp. 527-552.

Date Deposited: 12 Aug 2015 04:26
FoR Codes: 09 ENGINEERING > 0912 Materials Engineering > 091209 Polymers and Plastics @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970109 Expanding Knowledge in Engineering @ 100%
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