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 N'Djamena, P.O. Box 1027, N'Djamena, Chad
3. Carnot Energy Laboratory, Department of Physics, Faculty of Science, University of Bangui, P.O. Box 908, Bangui, Central African Republic