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Synthesis and fabrication of graphene-based organic transparent electrodes for flexible optoelectronic devices

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Title
Synthesis and fabrication of graphene-based organic transparent electrodes for flexible optoelectronic devices
Author
Lee, Do Hee
Advisor
Kwon, Soon-Yong
Issue Date
2020-08
Publisher
Graduate School of UNIST
Abstract
With the advance of electronic devices, the continued down-scaling of electronic devices has aroused the thirst for transparent electrodes (TEs) with not only excellent optoelectrical performance but also great mechanical and environmental stabilities. There are various candidates for flexible applications, such as two-dimensional (2D) materials, e.g., graphene (Gr) and transition metal dichalcogenides (TMDs) and one-dimensional (1D) materials, e.g., metal nanowire and carbon nanotube and conducting polymer. However, each candidate material has drawbacks for the use in practical applications. A carbon-based 2D hexagonal structure, graphene has attracted lots of attention, owing to outstanding properties such as, excellent electrical and thermal conductivities, high optical transparency, gas impermeable property and great mechanical properties. However, the graphene films grown by chemical vapor deposition (CVD) are polycrystalline and synthesized on catalytic metal surfaces, leading to deterioration in its unique properties and demands for additional transfer process. As an alternative TE, there is conducting polymer. Among the conducting polymers, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has attracted lots of attentions in organic optoelectronic devices, such as organic light-emitting diodes (OLEDs) and thin film solar cells, due to its solution processability, facile functionalization, flexibility and transparency. The organic materials, however, is vulnerable to oxygen and moisture and its acidic characteristics can damage adjacent metal layers, resulting in a degradation of the device performance. Each representative TE material has huge drawback for the use in optoelectronics. Therefore, it is necessary to design optimal TE structures and establish proper fabrication processes for the use in next-optoelectronic applications. In this thesis, we evaluate that the potential of graphene/PEDOT:PSS organic TE composite for flexible optoelectronic devices. At first, we check the possibility for excellent optoelectrical properties, which is comparable to ITO, and compatibility of graphene/PEDOT:PSS structures via facile functionalization processes. In addition to the property of each materials, clean interfaces is very important for formation of hybrid TE composites and conventional graphene wet transfer process should be revised because the processes hugely affect the surface quality of as-grown graphene. Based on the previous study, we devised two kinds of direct transfer strategies for the fabrication of high-quality and large-area graphene-based composite: (1) Lamination process and (2) transparent and colorless polyimide(TCPI) solution process. The direct fabrication processes without unnecessary organic layer provided excellent optoelectrical properties and better interface conditions between two heterolayers, demonstrating the relevance between optoelectrical properties and interface conditions of the TE composites. Moreover, using hall measurement and Physical Property Measurement System (PPMS), the role of graphene in graphene/functionalized PEDOT:PSS heterostructure was demonstrated in terms of electrical properties. Furthermore, we evaluate the environmental stability of graphene-based organic TE structure under various unfavorable situations, including mechanical stress (e.g., bending and stretching) and exposure to high temperature, humidity, ultraviolet light and so on. Finally, we applied the graphene-based TE to anode for flexible polymer light-emitting diodes (PLEDs). From the harmony of each component in the TE structure, the corresponding PLEDs showed excellent performance with improved current and power efficiencies and mechanical flexibility, exceeding those of indium-tin-oxide anode-based PLEDs. These results suggest that it is important to select and design proper materials and fabrication process for hybrid transparent conducting films in practical optoelectronics.
Description
Department of Materials Science and Engineering
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