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

Abstract

${\mathrm{CuGaSe}}_{\mathrm{2}}\mathrm{/CdS/ZnO}$ heterostructures with different ${\mathrm{CuGaSe}}_2$ stoichiometry deviations, glass substrates with different Na content and varying CdS buffer deposition procedures are analyzed with admittance spectroscopy, deep level transient spectroscopy, and capacitance–voltage measurements. Cu-rich ${\mathrm{CuGaSe}}_2$ 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 °C. Ga-rich ${\mathrm{CuGaSe}}_2$ material displays a tail-like energetic distribution of acceptor defects. The maximum of this distribution is at an energy of 250 meV. Defect densities and doping concentrations of Ga-rich material are considerably lower than in Cu-rich material. The different defect and doping densities found in the present investigation fully explain the efficiency gain which has recently been made by changing the material stoichiometry, the glass substrate and the CdS-deposition method for ${\mathrm{CuGaSe}}_2$ -based thin film solar cells.