1. a Central Laboratory for Solar Energy and New Energy Sources , Tzarigradsko Chaussee 72, 1784 , Sofia , Bulgaria
2. b Institute of Solid State Physics , Tzarigradsko Chaussee 72, 1784 , Sofia , Bulgaria
3. c Faculty of Physics , Sofia University , J. Bouchier 5, 1126f , Sofia , Bulgaria
4. d A. F. Ioffe Physico-Technical Institute , Politekhnicheskaya 26, 194021, St Petersburg , Russia