Two step wet surface treatment influence on the electronic properties of Cu(In,Ga)Se₂ solar cells

In this work we investigate the electronic properties of Glass/Mo/Cu(In,Ga)Se₂ (CIGS)/CdS/ZnO solar cells where the co-evaporated CIGS surfaces are subjected to a two steps chemical treatment. This two steps treatment consisting of a strong oxidative etch (bromine) of the absorber surface followed by a cyanide treatment leads to a significant (by 30%) improvement of the photovoltaic performances. The bromine etching time is used as variable in the study, while admittance spectroscopy is employed for electrically active defects characterization. Particular attention was given to the interpretation of the origin of the defects probed in the samples. One type of defects with activation energy ranging between 0.2 eV to 0.5 eV, commonly probed in the CIGS based devices and labelled N₂, are passivated by the cyanide treatment. This allowed us to propose a new interpretation concerning the nature of this type of defect.