Doctoral Dissertations

Date of Award

5-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Electrical Engineering

Major Professor

Gong Gu

Committee Members

Syed K. Islam, Veerle Keppens, Jie Wu

Abstract

Graphene is a monolayer to a few monolayers of graphite that exhibit distinctive properties from the bulk material. As a two-dimensional (2D) material with unique physical properties, it promises novel applications in optoelectronic and electronic devices. This dissertation focuses on two exemplary applications, one as a passive electrode material in organic solar cells, and one as the active channel material for field-effect transistors in a niche circuit application. A 2D insulating material, hexagonal boron nitride (h-BN) is utilized as the gate dielectric or part thereof for the FETs. A 2D semiconductor, PdSe2, is explored as an alternative to graphene in the circuit application.Replacing the transparent conductive electrode material indium tin oxide (ITO) with graphene in organic optoelectronic devices is an active research field, due to the limited supply of indium as well as the flexibility afforded by graphene. This work, however, explores potential performance improvements that can result from replacing ITO with graphene. In organic solar cells, electron donor materials (e.g. copper phthalocyanine, CuPc) are usually conjugated hydrocarbons. When such molecules are deposited on ITO, the π-π stacking direction, in which electrical transport is optimal, is parallel to the surface, thus perpendicular to the direction of current flow. This work demonstrates that the CuPc molecules on graphene exhibit π-π stacking perpendicular to the surface, allowing for optimization of organic solar cells. The new stacking order is attributed to the π electron system of graphene.Graphene as a FET channel material has a well-known drawback – the zero band gap, making it unsuitable as digital switches, or even for analog circuit applications due to the lack of pinch off. This work explores a niche application of graphene FETs in analog computing, making use of its ambipolar behavior. Graphene FETs exhibiting symmetric, hysteresis-free ambipolar characteristics have been demonstrated, with adequate current modulation ratio for the proposed application. To achieve low power consumption, higher current modulation ratio is desirable, but is limited by the zero band gap of graphene. The 2D semiconductor PdSe2 is therefore explored as an alternative to graphene, and significantly higher current modulation ratios have been achieved.

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