Kunhee Han

Graduation Semester and Year

2009

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Electrical Engineering

Department

Electrical Engineering

First Advisor

Meng Tao

Abstract

The electrochemically deposited cuprous oxide (Cu2O) as a photovoltaic absorber material is introduced in this work. The electrical, structural, and optical characteristics of electrodeposited Cu2O are well investigated. The pH of the deposition solution controls the flat band voltage of Cu2O. The variation of the flat band voltage by solution pH is related to oxygen incorporation in the film, which is controlled by the amount of oxygen precursor, OH-, in the solution. Hence, the high pH in the solution is preferable for producing a p-type Cu2O with copper vacancy, while the low pH in the solution is preferable for producing an n-type Cu2O with oxygen vacancy or copper interstitial. The resistivity of Cu2O is controlled by the deposition solution pH. As the pH of a solution increases, the resistivity of Cu2O decreases. The crystal orientation and the grain size have significant roles in controlling the resistivity of p-type Cu2O. Cu2O with a crystal orientation of (111) shows lower resistivity than Cu2O with a crystal orientation of (100). A possible reason for this is that Cu2O with (111) orientation grows in the solution of high pH, and it leads to a more intrinsic point defect, i.e, Cu vacancies that determine the carrier concentration in the film. Electrodeposited Cu2O shows a direct band gap of ~2.06 eV, which is independent of the deposition condition. Cu2O p-n homojunction solar cells with two different structures are successfully fabricated by a two-step electrodeposition method. The highest efficiency of 0.15% is achieved in an electrodeposited Cu2O substrate solar cell. The solution bath pH of n-Cu2O controls the open circuit voltage of the device. Several pin holes which were observed in the device contribute to the formation of shunt paths and cause the degradation of solar cell performance in the large superstrate solar cells. The thermal reliability test indicates that the device performance of the Cu2O solar cell is not stable above the temperature of 150 degree C. The Cu2O superstrate solar cell exhibits better device performance than the Cu2O substrate cell. The thickness of the p-Cu2O layer influences the open circuit voltage of superstrate cells. Low efficiency of Cu2O solar cells is mainly influenced by the high resistivity of both the p and the n-Cu2O. In order to control the electrical properties of Cu2O, two different n-type doping techniques, thermal diffusion doping and electro doping, are introduced. The resistivity of Cu2O is successfully controlled by Cu thermal diffusion. Also, the Fermi energy of Cu2O is shifted and the n type property of Cu2O is enhanced by Cu diffusion. This confirms that the intrinsic point defects in Cu2O, such as copper vacancy and oxygen vacancy, are controlled by external copper diffusion. It is speculated that Cu diffusion in Cu2O takes place mainly along grain boundaries (100) at the beginning of the annealing process, and then lattice diffusion follows along the (111) planes, due to its lower lattice distortion. N type electrodoping in Cu2O is successfully achieved with Br ion. Br concentration plays a significant role in the crystal growth. The formation of a dendritic crystal of Cu2O is caused by Br in the solution. It is believed that the mechanism of electrodoping is the co-deposition of the n-type dopant with Cu2O.

Disciplines

Electrical and Computer Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

Download

Share

COinS