Date of Award

12-23-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Applied Science

First Advisor

Tar-Pin Chen

Second Advisor

Jingbiao Cui

Abstract

The main goal of this research is to fabricate carbon nanostructured materials, such as carbon nanotubes, graphene, etc., to apply in the groundbreaking third generation photovoltaic devices. I investigated the electric and optical transport on: MWNTs/Si hybrid solar cells, acid-enhanced MWNTs/n-Si heterojunction for light harvesting, photovoltaic properties of CdS/MWNTs/n-Si, n-p-n transistor structure devices, NiOx/graphene/Si double Schottky diodes for enhanced photo-conversion, Cu2O nanorods for hybrid solar cell applications. The desired outcome in researching these novel carbon nanostructured materials is the decrease in fabrication costs. The MWNTs/n-Si junction showed a strong rectifying behavior, while the MWNTs/p-Si junction formed on ohmic-like contact. Photovoltaic effects were observed in the MWNTs/n-Si junction when the device is illuminated. We also investigated the influence of chemical functionalization of MWNTs networks on its electronic and optical properties, and found that nitric and sulfuric acid can significantly downshifts the fermi level of MWNTs, and can significantly reduce the internal resistance of MWNTs film. A new type of n-p-n transistor photovoltaic devices was developed based on CdS/MWNTs/n-Si configuration by using a simple and efficient process. I-V characteristic measurements show that the efficiency increases with increasing the thickness of the CdS thin film. This technique creates an interface between the CdS/MWNTs and the MWNTs/n-Si. The preliminary results may provide information on improving the efficiency of the promising transistor solar cell. I developed a new method to optimize the photovoltaic conversion efficiency for the SWNTs/n-Si configuration by introducing Cu2O nanoparticles, a p-type wide bandgap semiconductor, into the void space of the SWNTs network. The Cu2O nanoparticles increase light absorption in the original void space. Charge transport within the device has also been improved through the Cu2O/SWNTs interface. The p-n heterojunctions formed at the Cu2O/n-Si interface can also help photon absorption and exciton separation and the short circuit current of the Cu2O-SWNTs/n-Si devices increase with increasing the Cu2O thickness. We employed the CVD approach to the synthesis of graphene at large scale and low cost, which makes the wide applications of graphene possible. By comparing different graphene sources, it was found that the supernatant graphene is the most desirable material for the fabrication of Graphene/n-Si junction with excellent rectifying capability and good photovoltaic conversion efficiency. Although Graphene/p-Si also exhibit rectification behavior, they demonstrate poor photovoltaics conversion efficiency due to the weak p-type conductivity of our graphene flakes. A simple and effective method, the electrodeposition method, was used to fabricate a NiOx/graphene bilayer Schottky junction. Thus this double Schottky combination contains an n-type Si/graphene Schottky junction and a p-type NiOx-graphene Schottky junction, an overall p-n junction. The I-V characteristic curves show that the power conversion efficiency improved when electrodeposition time was increased, i.e., the thickness of NiOx thin film increased. To improve the light-harvesting efficiency of the promising graphene/silicon solar cells, a thin layer of NiOx thin film was deposited on graphene/silicon junction for making a double Schottky diode.

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