Cadmium sulfide/indium phosphide as a model system for understanding indium related chemical reactivity at CIGS/CdS interface: XPS and ex situ luminescence investigations

In thin film polycrystalline Cu(In, Ga)Se2/CdS/ZnO solar cells, the active CIGS/CdS interface plays a main role to further improvements of their performance. This interface is complex and its formation is under active investigation. Because X-ray Photoelectron Spectroscopy (XPS) features of this interface make it difficult to study unambiguously, we have undertaken a specific study of the interface indium chemical modifications during CdS growth using single crystalline InP wafers and CdS obtained by chemical bath deposition (CBD), as a reference system for the case of the CIGS/CdS interface. We have investigated the behavior of the native oxide layer on the InP surface with respect to the growth of the CdS layer, by means of composition profile from CdS to InP obtained by XPS. The composition of the interface is analyzed before its is touched and damaged by the sputter etching. Attention has been paid to oxygen and carbon impurities in the CdS layer. CdS films are shown by XPS to contain probably Cd(OH)2 or CdO. No evidence of the presence Cd carbonate is observed. The first stages of the CdS nucleation were followed by photoluminescence (PL) and XPS measurements. A critical time in the minute range, before surface chemical modifications lead to surface passivation, was evidenced by PL enhancements and correlated to the evolutions of the XPS signals. These results are relevant to several aspects of to the CIGS/CdS interfaces as shown by parallel experiments.