Effects of KF and RbF treatments on Cu(In,Ga)Se2-based solar cells: A combined photoelectron spectroscopy and DFT study

In this work, the alkali-induced chemical and electronic modifications observed at the KF- and RbF-treated Cu(In,Ga)Se-2 (CIGSe)/CdS interfaces are correlated to a Density Functional Theoretical (DFT) model of the alkali metal induced point defects at a CuInSe2/CdS interface. Analysed with hard X-ray photoelectron spectroscopy (HAXPES), the near-interface regions showed a Cu-poor, In-rich and stoichiometric CdS composition for the KF-CIGSe/CdS interface and a Cu-poor, In, S-rich composition for the RbF-CIGSe/CdS interface. The DFT-calculated defect formation energies and valence band offsets (VBO) at the defect-induced interfaces indicate towards possible formation of specific defects at the KF- and RbF-treated CIGSe/CdS interfaces. Cu vacancies indicated by the Cu-poor stoichiometry of the alkali-treated interfaces contribute to an increase in the acceptor densities (N-A). Possible formation of K-Cu and Rb-Cu defects could result in lower N-A at the interfaces because of the Cu vacancies being filled up by K and Rb atoms. Na-Cd and excess Cd-Cu defects at the KF-CIGSe/CdS interface and only Cd-Cu defects at the RbF-CIGSe/CdS interface might have formed that would result in higher donor densities (N-D) at the interfaces. These factors, which showed enhanced type-inversion when applied in device simulations, resulted in fill factor (FF) and open-circuit voltage (V-oc) gains in devices.