In this paper, different electrical characterization techniques are used to investigate the metastability behavior of the ZnO:Al/ZnMgO/Zn(S,O,OH)/Cu(In,Ga)Se2 solar cell. Whereas this device presents a poor conversion efficiency and metastable behavior just after the deposition stage, Thermal Annealing and Light Soaking considerably improved its conversion efficiency. In order to understand the metastable behavior and Thermal Annealing effect of this device, different electrical and electro-optical characterization measurements were carried out including Sub Gap Modulated Photo-Current Spectroscopy. The application of Sub Gap Modulated Photo-Current Spectroscopy measurements performed on an as-deposited device, revealed two deep defects. These defects became shallower and less efficient for the capture of free carriers after Thermal Annealing. Moreover, the photoluminescence spectra exhibit an increase of the measured signal after Thermal Annealing, which is due to a band-to-band recombination. In addition, capacitance-voltage measurements show a hysteresis behavior that is significantly reduced after Light Soaking or Thermal Annealing. We also performed numerical simulations with SCAPS, a software tool of the University of Gent available to the PV research community, to calculate the current-voltage characteristics under AM1.5G illumination and the capacitance-voltage ones under dark conditions. The evolution of the corresponding experimental characteristics from the as-deposited state to different steps during the Thermal Annealing or Light Soaking posttreatment could be well reproduced by varying a high acceptor-like defect density at the buffer/absorber hetero-interface.