Deep centers and fill factor losses in the CIGS devices

Abstract

ABSTRACTThe influence of non-equilibrium space charge distributions on the fill factor of ZnO/CdS/Cu(In,Ga)Se2 photovoltaic devices has been investigated. Metastable changes of the amount of charge captured in deep levels have been produced by red or white illumination. Subsequent characterization by current-voltage, capacitance-voltage and admittance spectroscopy has been conducted at low temperature. The fill factor increases after a white light soaking and decreases after illumination of the reverse-biased device with red light at low temperature. This effect has been attributed to the metastable change of net doping concentration in the p+ layer of absorber close to interface. Blue light, absorbed in CdS provides holes to that region neutralizing the negative charge and thus improving the fill factor. The defects responsible for the interface-related metastability are in our opinion the same as those causing the persistent increase of the net doping concentration in the bulk of Cu(In,Ga)Se2 and seem to be related to selenium vacancies.