Effect of Na on lower open circuit voltage of flexible CIGS thin-film solar cells prepared by the low-temperature process

In this work, we deposit the chalcopyrite semiconductor CuInxGa1-xSe2 (CIGS) thin films using a three-stage co-evaporation process below 450 degrees C. It is observed that when the soda-lime glass substrate is replaced by polyimide, the conversion efficiency of this CIGS device changes from 11.12% down to 9.1%, and this efficiency decrease results mainly from the different open-circuit voltage (V-oc). Temperature-dependent current-voltage measurements are carried out, from which the activation energy is extracted; furthermore, the dominant recombination mechanisms as well as paths are determined, and finally the reason for the different V-oc is analyzed.