Investigations into plasma deposited linalyl acetate thin films for applications in organic electronics

Anderson, Liam James (2013) Investigations into plasma deposited linalyl acetate thin films for applications in organic electronics. PhD thesis, James Cook University.

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Organic electronic device research continues to take place on predominately Si substrates utilising oxide dielectric barriers, which are unable to take advantage of the flexibility and optical transparency of organic semiconducting materials. These organic semiconductors are typically deposited by thermal evaporation or solution processing methods, and substrates must be compatible with exposure to temperatures on the order of 150 °C and insoluble in a variety of organic solvents, in addition to possessing desirable electrical, mechanical, optical and processing (e.g. low cost, large volume) properties. In this work plasma deposited linalyl acetate thin films were considered as a potential candidate for application in organic electronic devices. Linalyl acetate is a component of the essential oil Lavandula angustifolia, which can be obtained non-synthetically. Both the monomer and deposition method are environmentally friendly, low cost and do not result in harmful waste or pollution. Thin films deposited from this monomer were fabricated using plasma enhanced chemical vapour deposition and their properties studied as a function of energy delivered to the reaction chamber.

Films were found to have minimal absorbance at optical wavelengths and were completely transparent. The refractive index of the material was found to be 1.55 – 1.58 at 589 nm and the optical band gap derived from the absorbance profile was 2.95 eV – 3.02 eV. The variation in these parameters was linked directly to variation in the energy used during the deposition. Independent of this energy, the surfaces of the plasma deposited thin films were smooth, free of defects and had an average roughness of 0.20 nm. The hardness of the films was between 0.30 GPa and 0.45 GPa, again correlated with the power density used. The underlying cause of this variation was shown to be related to an increase in carbon content as higher power densities were used.

Electrically, the films were highly insulating, possessing conductivities of less than 10⁻¹⁰ Ω⁻¹ m⁻¹ in all cases and a dielectric breakdown strength of 1.8 MV cm⁻¹. The dielectric function was measured from dc to RF frequencies. The relative permittivity at low frequencies (less than 10 Hz) was found to be 11, while the high frequency dielectric constant was found to be 3 – 3.5 at 100 kHz and 2.4 at optical frequencies. Dielectric loss spectra revealed complex underlying relaxation features present in plasma deposited thin films.

To confirm their applicability to organic electronics, the process compatibility of the thin films was studied. It was found that thermal degradation began at greater than 195 °C, the films were insoluble in a variety of organic solvents including the commonly used dichlorobenzene and chloroform, and possessed a surface free energy of ~45 mJ m⁻². Further, the interaction between the linalyl acetate surface and the n-type organic semiconductor PDI-8CN₂ was investigated and the material shown to influence the growth of the organic semiconductor in a manner similar to common surface treatments of SiO₂. An organic field effect transistor was fabricated, which included a linalyl acetate layer between the SiO₂ and pentacene organic semiconductor. Relative to a similar device without the linalyl acetate layer, improved performance characteristics were observed. In particular, the mobility was increased by two orders of magnitude, the on/off ratio improved, and the threshold voltage reduced.

Item ID: 29890
Item Type: Thesis (PhD)
Keywords: organic electronic devices; thin film; organic electronics; flexible electronics; linalyl acetate
Date Deposited: 24 Oct 2013 02:50
FoR Codes: 09 ENGINEERING > 0912 Materials Engineering > 091208 Organic Semiconductors @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970109 Expanding Knowledge in Engineering @ 100%
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