Interface redox engineering of Cu(In,Ga)Se₂-based solar cells: Oxygen, sodium, and chemical bath effects

The chemical effects of oxygenation of Cu(In,Ga)Se₂ (CIGS) interfaces are analyzed and are shown to involve passivation of Se deficiencies and Cu removal. The former effect is beneficial at grain boundaries, but detrimental at the CdS/CIGS interface. The latter effect is purely detrimental. Na and chemical bath deposition (CBD) treatments are shown to isolate the `good' oxygenation effect from the `bad' ones. Na is shown to promote oxygenation already before the deposition of the buffer and window layers, which allows a maximization of the benefits of Se deficiency passivation and a minimization of Cu removal. Next, the CBD of the CdS buffer layer restores the interface charge, due to creation of Cd_Cu interface donors and possibly a removal of O_Se interface acceptors. This highlights the crucial role that interface redox engineering plays in optimizing the performance of CIGS-based solar cells.