Correlation of structure parameters of absorber layer with efficiency of Cu(In, Ga)Se2 solar cell

Polycrystalline Cu(In, Ga)Se-2 with Ga-content x=Ga/(In+Ga) ranging from 0.0 (CuInSe2) to 1.0 (CuGaSe2) in heterojunction thin film solar cells were grown by multi-source evaporation. Solar cells with a highest efficiency of eta=15.3% need a composition of x approximate to 0.2. At this composition, the c/a ratio of the lattice constants for the tetragonal lattice equals c/a=2, indicating ideal tetragonality. These results suggest that low electronic defect densities occur at x approximate to 0.2, due to the smallest possible crystallographic distortion of the tetragonal lattice at this composition. Cells with high efficiencies require grain sizes above 145 nm and a high preferred orientation (P-204/P-220 pole density ratio) for the grains.