Electronic properties of CuGaSe2-based heterojunction solar cells. Part II. Defect spectroscopy

CuGaSe2/CdS/ZnO heterostructures with different CuGaSe2 stoichiometry devia tions, glass substrates with different Na content and varying CdS buffer de position procedures are analyzed with admittance spectroscopy, deep level t ransient spectroscopy, and capacitance-voltage measurements. Cu-rich CuGaSe 2 exhibits two acceptor-like bulk traps with activation energies of 240 and 375 meV. The density of both defect states is reduced by air annealing at 200 degrees C. Ga-rich CuGaSe2 material displays a tail-like energetic dist ribution of acceptor defects. The maximum of this distribution is at an ene rgy of 250 meV. Defect densities and doping concentrations of Ga-rich mater ial are considerably lower than in Cu-rich material. The different defect a nd doping densities found in the present investigation fully explain the ef ficiency gain which has recently been made by changing the material stoichi ometry, the glass substrate and the CdS-deposition method for CuGaSe2-based thin film solar cells. (C) 2000 American Institute of Physics. [S0021- 897 9(00)05901-6].