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

Abstract In this work, the alkali-induced chemical and electronic modifications observed at the KF- and RbF-treated Cu(In,Ga)Se2 (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 (NA). Possible formation of KCu and RbCu defects could result in lower NA at the interfaces because of the Cu vacancies being filled up by K and Rb atoms. NaCd and excess CdCu defects at the KF-CIGSe/CdS interface and only CdCu defects at the RbF-CIGSe/CdS interface might have formed that would result in higher donor densities (ND) at the interfaces. These factors, which showed enhanced type-inversion when applied in device simulations, resulted in fill factor (FF) and open-circuit voltage (Voc) gains in devices.