The influence of potassium post-deposition treatments on grain boundaries and surfaces of Cu(In,Ga)Se2 absorbers

Alkali metal post-deposition treatments (PDTs) have played a pivotal role in the development of high-efficiency Cu(In,Ga)Se2 thin film solar cells. The PDTs impact the near-surface region of the absorbers, the grain boundaries, and the bulk. Since Cu(In,Ga)Se2 solar cells are polycrystalline, the investigations turn out to be very challenging as each grain/grain boundary is affected slightly differently by the PDTs. Therefore, we synthesized Cu(In,Ga)Se2 absorbers on microcrystalline GaAs substrates to form millimeter-sized epitaxial domains with well-defined grain boundaries. This model system allows us to study the same surface orientation and grain boundary before and after PDTs. We find strong preferential (112) facets and Cu depletion on the surface, with the grain boundaries exhibiting very similar Cu-depleted compositions. After K deposition and annealing, Cu is pushed into the absorber, leaving a K-rich compound at the near-surface. Furthermore, we find different diffusion rates of K through the grain interior and via the grain boundaries. The exchange of Cu with K at the surface also occurs at the grain boundaries as measured with atom probe tomography. High-resolution KPFM measurements performed on the same grain boundary before and after PDT treatment show almost negligible grain boundary band bending. Cu-depleted K-rich area at the surface and grain boundaries exhibit very similar compositions, which gives rise to small band bending values. Our results give an unprecedented view of the mechanisms that occur at the surface, grain boundaries, and the bulk of CIGSe solar cells, paving the way to further develop the materials and improve the power conversion efficiency.