Persistent phenomena in the electrical characteristics of solar cells based on Cu In,Ga S2

Current voltage and capacitance voltage characteristics of ZnO CdS CuInGaS2 solar cells have been measured in order to investigate persistent changes induced by the reverse bias stress or light soaking. Significant metastable fill factor loss after red illumination under reverse bias condition has been observed. We show a correspondence between relaxation times of the capacitance and photovoltaic parameters after all treatments proving that defects in the absorber are responsible or the changes. Similar phenomena have been observed in Cu In,Ga Se2 based devices, therefore we propose similar explanation negative U defects in Cu In,Ga S2 which create a highly negatively charged layer close to the interface. They are responsible also for a difference between current transport in the dark and under illumination. Moreover, these defects seem to be a source of an limitation of the open circuit voltage in these devices

DX like InCu and GaCu defects in chalcopyrite based solar cells

Red illumination of the reverse biased device ROB causes persistent increase of the capacitance of CIGS based solar cells. ROB effect can only be explained in terms of relaxing defects, undergoing shallow deep transition. According to theoretical calculations, IIICu defect is a probable candidate. We demonstrate here that ROB effect is present in all CIGS chalcopyrite compounds including wide band gap CuGaSe2 and CuInS2 and show that photovoltaic parameters of the devices depend on the charge state of the antisite defect. The role of the blue part of the illumination spectrum for establishing favorable charge distribution is demonstrated. The relation between the Fermi level position at interface and the antisite influence on the photovoltaic parameters is discusse