1. Department of Physics, Bahir Dar University, P. O. Box 79, Bahir Dar , Ethiopia
2. Department of Physics, Addis Ababa University, P. O. Box 1176, Addis Ababa, Ethiopia
3. Florida Solar Energy Center, University of Central Florida, Cocoa, Florida 32922, United States
4. CREOL, the College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, United States
5. c-Si Division, US Photovoltaic Manufacturing Consortium, Orlando, Florida 32826, United States