Effect of Se-Free Annealing on Cesium Fluoride-Treated Cu(In,Ga)Se-2 Thin Films and Corresponding Solar Cells

The effect of selenium-free annealing on cesium fluoride (CsF)-treated Cu(In,Ga)Se-2 (CIGS) thin films is investigated and their solar cell performance is evaluated. Annealing of CsF-treated CIGS thin films changes the surface morphology and the chemical composition, and reduces the Urbach energy. However, the full benefit of the reduced Urbach energies of the annealed samples is not obtained, because of an increased buffer/CIGS interface recombination as a consequence of re-evaporation of the alkali-containing layer from the surface region upon annealing. On the other hand, CsF-treated CIGS thin films without any annealing after the post-deposition treatment (PDT) preserve the alkali-containing layer at the surface, thereby leading to an improved buffer/CIGS interface. Consequently, the open-circuit voltage (V (OC)) and fill factor improve significantly in these devices. The Urbach energies of both annealed and nonannealed solar cells are calculated from external quantum efficiency measurement to understand their impact on V (OC) and V (OC) deficit. No correlation between the V (OC) deficit and the Urbach energies is observed.