POLYMER THIN FILMS FOR BENDABLE ELECTRONICS

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Context

This independent research project was performed as a part of my M.S. degree at Texas A&M university. The motivation for this research was impending demand for wearable electronic gadgets and miniaturization of electronics which are based on advances in thin and flexible film technology. There is a need for not only spot cooling of crammed transistors in thin wearable electronics, but also for re-utilization of the low-grade waste heat generated to produce additional power. Here, thin film organic thermoelectrics offer a viable solution to convert heat to electric power and cool the surface simultaneously. However, their low conversion efficiency compared to that of inorganic materials prevents them from being used for practical applications. I investigated ways to modulate the efficiency of organic thermoelectric materials through novel fabrication methods.

 

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Aim

Engineer methods for fabrication of flexible thin film thermoelectric substrates with high power factor (ZT).

 

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Results

I developed a novel fabrication method that produced free-standing organic thermoelectric and carbon nanotube composite bendable thin films with ~100 times improvement in electrical conductivity (and thereby ZT) compared to those reported previously. 

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CONTRIBUTIONS

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Liquid crystal template fabrication

Thermal conductivity characterization

Electrical conductivity characterization

Nanoparticle characterization

In-house instrument design

Polarized Microscopy

Experimental design and protocols

Data analysis

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Approach

The tradeoff between Seebeck coefficient and electrical conductivity restricts the improvement of power factor through increase in number of charge carriers. However, controlled modulation of the mobility of charge carriers has a potential to increase the electrical conductivity without adversely affecting the Seebeck coefficient. Here I developed a novel way to fabricate organic thermoelectric thin films with high power factor by modulating the morphology of the conducting polymer poly-(3,4-ethylenedioxythiophene) (PEDOT) and creating a composite with carbon nanotubes to control the mobility and hence the electrical conductivity of the thin films. Aligned PEDOT-carbon nanotube composite thin films were fabricated and characterized to study both the alignment of the polymer chains and change in their electrical conductivity.

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I utilized the bottom up technology with self organized molecular system templates to control the nano structure and ordering of the polymer-carbon nanotube composite. Using a Liquid crystal template, the monomer EDOT (3,4-ethylenedioxythiophene) molecules were captured within the cylindrical cores of hexagonal mesophase oriented in effective net direction within domains, and the monomers were electro-polymerized to obtain aligned polymer chains. This aligned structure rendered better anisotropic electrical conductivity along the polymer chain direction. A non percolated dispersion of carbon nanotubes and dopants were incorporated into the aligned PEDOT thin film by spraying as well as internally dispersing within the liquid crystal network before polymerization.

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This research has been conducted under the supervision of my advisor Dr. Choongho Yu and collaborators Dr. Jui-Hung Hsu and Dr. Hong Wang.