1. Environmental Energy Technologies Laboratory (EETL), Department of Physics, Faculty of Science, University of Yaounde I, P.O. Box 812, Yaounde, Cameroon
2. Department of Physics, Faculty of Science, University of Yaounde I, P.O. Box 812, Yaounde, Cameroon
3. Department of Physics, University of Dschang, P.O. Box 96 Dschang, Cameroun Colline de Foto, Cameroon