The conventional DLTS, deep level transient spectroscopy, reverse-bias DLTS and Laplace transform analysis of capacitance transients have been employed for investigation of defect spectra in the bulk and at the interface of standard ZnO/CdS/Cu(In,Ga)Se-2 devices. The results are reviewed with special attention devoted to the persistent changes induced by illumination or bias voltage. In the discussion a model of mobile copper ions and shallow-deep transition of compensating donors are compared. We show, that defect relaxation model can also account for the observed phenomena, One of its consequences is a dipole layer in the interface region, consisting of positively charged interface states and negatively charged thin layer of absorber with persistently removed compensation.