Alkali Postdeposition Treatment-Induced Changes of the Chemical and Electronic Structure of Cu(In,Ga)Se₂ Thin-Film Solar Cell Absorbers: A First-Principle Perspective

The effects of alkali postdeposition treatment (PDT) on the valence band structure of Cu(In,Ga)Se₂ (CIGSe) thin-film solar cell absorbers are addressed from a first-principles perspective. In detail, experimentally derived hard X-ray photoelectron spectroscopy (HAXPES) data [ Handick, E.; et al. ACS Appl. Mater. Interfaces 2015, 7, 27414-27420 ] of the valence band structure of alkali-free and NaF/KF-PDT CIGSe are directly compared and fit by calculated density of states (DOS) of CuInSe₂, its Cu-deficient counterpart CuIn₅Se₈, and different potentially formed secondary phases, such as KInSe₂, InSe, and In₂Se₃. The DOSs are based on first-principles electronic structure calculations and weighted according to element-, symmetry-, and energy-dependent photoionization cross sections for the comparison to experimental data. The HAXPES spectra were recorded using photon energies ranging from 2 to 8 keV, allowing extraction of information from different sample depths. The analysis of the alkali-free CIGSe valence band structure reveals that it can best be described by a mixture of the DOS of CuInSe₂ and CuIn₅Se₈, resulting in a stoichiometry slightly more Cu-rich than that of CuIn₃Se₅. The NaF/KF-PDT-induced changes in the HAXPES spectra for different alkali exposures are best reproduced by additional contributions from KInSe₂, with some indications that the formation of a pronounced K-In-Se-type surface species might crucially depend on the amount of K available during PDT.