The effect of sulphur pressure on the depth distribution of elements in Cu(In,Ga)S2 films

We have studied the deposition of Cu(In,Ga)S2 films for thin-film solar cells. In this study, Cu(In,Ga)S2 films have been prepared with varied sulphur pressure. A sequential multi-source evaporation process has been employed. Deposition of an In–Ga–S precursor layer was followed by the diffusion of Cu and S into the precursor layer. The depth profiles of the impurities O and C, and constituent elements In and Ga were investigated by secondary-ion mass spectroscopy (SIMS). Coevaporated films were measured for comparison and the concentrations of O, C, In, and Ga were almost constant throughout the film. In case of our sequential process, a higher Ga-concentration inside the Cu(In,Ga)S2 layer and a lower one towards the surface has been confirmed. X-ray diffraction (XRD) data also confirm the existence of two Cu(In,Ga)S2 layers with different Ga content. It has been found that the O-concentration was constant in the bulk and maximum at the interface between Ga-rich bottom layers and In-rich surface layers with a decreased concentration next to the surface. On the other hand, C was also nearly constant in the bulk, but tended to increase in a layer close to the surface. A model of the incorporation of impurities will be proposed. The changes of concentrations of O, C, In, and Ga become less pronounced with increasing S-pressure. This indicates that higher S-partial pressure may promote the diffusion of the constituent elements and impurities in the film. A solar cell made from one of our Cu(In,Ga)S2 films showed an efficiency of 9.3% (total area, no antireflection coating, effective band gap≌1.52 eV).