1. Laboratory of Thermal and Renewable Energies, Department of Physics, Unit of Training and Research in Pure and Applied Sciences, University Joseph KI-ZERBO, Ouagadougou 03 BP 7021, Burkina Faso
2. Department of Physics, University of Bangui, Bangui BP 908, Central African Republic
3. Ecole Normale Supérieure, University of Bangui, Bangui BP 908, Central African Republic