Role of planar defects in Cu(In,Ga)Se2 thin-film solar cells

Thin-film solar cells with polycrystalline Cu(In,Ga)Se-2 absorber layers exhibit record power-conversion efficiencies of currently 22.6%. Such performance is impressive in view of the rather small average grain sizes of 0.5-1.5 mu m with Cu(In,Ga)Se-2 layer thicknesses of 2-3 mu m. The present work gives insight to the chemistry at planar defects (stacking faults and grain boundaries) in Cu(In,Ga)Se-2 absorber with and without Na doping by using correlative microscopy approach. Surprisingly, the planar defects in the Na-free absorber show strong In, Se-enrichment and Cu-depletion. This is contrary to what have been observed for the planar defects in Na-containing absorber, i.e. very weak In-enrichment and Cu-depletion accompanied by Na segregation. This work clearly proves that Na does not only act as a dopant, but also as a passivator inside the Cu(In,Ga)Se-2 absorber layer, explaining in this way his undoubtful beneficial effect on the cell performance.