Electrical and morphological properties of polystyrene thin films for organic electronic applications
Introduction
Organic electronic and the organic materials that are used in their fabrication have received increasing interest in recent years. This has been in part due to their low cost, ease of processing, and the ability to modify their structure to obtain the desired electrical and mechanical characteristics when compared with conventional inorganic semiconductors [1], [2], [3], [4], [5], [6]. Much of the current work in the field is focused on the development of conducting and semiconducting polymers and improving the quality of those materials. However, insulating materials will also play an important role in any organic device structure. Several insulating polymers have been used successfully in organic electronic devices [7], [8], [9], such as polyvinyl phenol (PVP) [10], polyvinyl acetate (PVAc) [11] and polystyrene (PS) [3]. Out of these materials polystyrene shows greater potential for use in future organic electronic devices, due to it having the advantage of superior material characteristics in terms of water absorption and dielectric strength when compared to PVP or PVAc. Much of the work done to characterise the electrical properties of polystyrene has been performed when polymers were rigidly considered as insulators, with several microns thick polystyrene layers being studied [12], [13], [14]. To date, very few studies have been conducted on films in the nanometre thickness range, showing a lack of the electrical data that is needed for reliable fabrication of organic electronic devices containing polystyrene as a dielectric material.
Dielectric strength and conduction characteristics are important for evaluating the suitability of polystyrene for use in organic electronic devices. Another important characteristic of electrical insulating materials is the levels of trapped charges present, and how trap density is affected by annealing conditions. Trapped charge theory is well established for use with conventional inorganic electronic insulators, such as SiO2 [15], with the field being heavily studied. However, fewer research articles have been published where organic insulators are concerned [16], [17], [18]. This paper aims to address some of these issues with current–voltage (I–V) and breakdown characteristics of metal-insulator-metal (MIM) diodes investigated. Capacitance–voltage (C–V) characteristics of metal-insulator-semiconductor (MIS) capacitors have also been studied as a function of anneal temperature to obtain information on fixed insulator charge densities and mobile insulator charge densities.
Section snippets
Device structures
Polystyrene pellets with an average molecular weight of 280,000, as supplied from Sigma Aldrich were used without further purification, with the structure of the styrene monomer shown in Fig. 1(a). PS pellets were prepared for use by dissolving in dichlorobenzene at a ratio of 25 mg ml−1 PS to dichlorobenzene. The solutions were then ultrasonicated for 30 min to ensure the PS was fully dissolved, and subsequently filtered through chemically resistant 0.7 μm filters to remove any remaining
Morphological properties
The effect of annealing the PS films on both film thickness and surface roughness has been investigated. It was found that increasing the anneal temperature results in a decreased film thickness. Fig. 2 shows the thickness before and after annealing at a range of temperatures, with the change in thickness probably due to residual solvent being evaporated from the PS films.
The average root mean squared (rms) surface roughness varied from 0.18 to 0.22 nm, however no correlation was found between
Conclusions
Thin film polystyrene layers have been studied focusing on characteristics that are important when the polymer is used in organic electronic devices. The dependence of electrical characteristics on the anneal temperature of the polystyrene films has been investigated, with a maximum dielectric strength and minimum leakage current corresponding to an anneal temperature of 90 °C, which is just below the glass transition temperature of polystyrene. Anneals above the Tg value were found to show a
Acknowledgements
D. Prime would like to acknowledge the UK DTI NMS Quantum Metrology Programme and the EPSRC for their funding of his studentship. S. Paul would like thank the EPSRC for the financial support (#EP/E047785/1).
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