This article presents a study on defects in CuGaSe2/CdS/ZnO heterojunction solar cells by the use of admittance spectroscopy and deep level transient spectroscopy. We investigate devices that differ in the CuGaSe2 stoichiometry, the type of substrate and the CdS preparation method. The most important difference is made up by the Cu/(Cu + Ga)-ratio of the absorber material with Cu-rich composition exhibiting doping levels exceeding 10(17) cm(-3) and similarly high concentrations of deep traps. Absorbers with Ga-rich composition have considerably lower doping densities. The energetic distribution of the defects in this material has an exponential tail. The density of these defects depends on the CdS buffer layer preparation and on the sodium content of the substrate glass. Recombination in the space charge region via these defects limits the open circuit voltage of solar cells with Ga-rich absorbers.