Okechukwu Akpa, Ph.D. (2010)
Dissertation Topic: Chalcopyrite Heterojunctions for Photovoltaic Application
Major Professor: Dr. Kalyan Kumar Das, Associate Professor of Electrical Engineering
M.S.: Electrical Engineering, Tuskegee University
B.S.: Electrical Engineering, Morgan State University, Baltimore, MD
In recent years solar power has become an attractive alternative with the increase in energy demand and prices. Thin film photovoltaic materials are being sought very actively as a replacement for current GaAs solar arrays for space applications and Si based terrestrial technology. Gallium arsenide is fragile, layers are difficult to grow, and the thick films that compose the layers are approaching their limits when it comes to material transparency. The answer to these cost and design concerns might be to employ group II-VI and group I-III-VI2 semiconductors as these materials cover a wide range of bandgaps and many of the materials have high absorption coefficients and are photoconductive.
Thin films of CuInSe2 (CIS), CuGaSe2 (CGS), and several II-VI compound semiconductors, such as ZnTe, ZnSe and ZnO have been grown and characterized for potential photovoltaic applications. In order to enhance the conversion efficiency that could be achieved with a single junction CIS/Si or CGS/Si cell, a tandem structure with an upper ZnTe/ZnSe junction has been considered with a ZnO buffer layer separating the two junctions. However, in the present study CIS/Si, CGS/Si and ZnTe/ZnSe heterojunctions have been studied as single junction devices. The films and devices were characterized to understand microstructure, composition, and photoresponse.