Effect of compositional gradients on structural defects in Cu(In, Ga)Se2 thin films for solar cells

The objective of this work is to study the influence of compositional gradients on the microstructure of Cu(In, Ga)Se₂ (CIGSe) absorber layers for thin-film solar cells. The deposition of CIGSe films in a multi-stage coevaporation process leads to compositional gradients of gallium and indium, in dependence on process parameters such as the integral gallium content. Transmission electron microscopy (TEM) imaging combined with energy-dispersive X-ray spectroscopy (EDX) enables the determination of structural defects and elemental distributions at identical sample positions. The occurrence of linear (dislocations) and planar defects (stacking faults, microtwins) is studied by means of TEM images along the length of the CIGSe layers. The spatial distribution of the grains is given by electron-backscatter diffraction (EBSD) pattern-quality maps. Elemental distribution maps obtained by EDX exhibit strong Ga/In gradients inside a single large grain along the absorber depth. However, the CIGSe film shows no lateral variations within our measurement accuracy parallel to the substrate inside the grains or in the vicinity of the grain boundaries.