Alkali Postdeposition Treatment-Induced Changes of the Chemical and Electronic Structure of Cu(In,Ga)Se(2) Thin-Film Solar Cell Absorbers: A First-Principle Perspective

[Image: see text] The effects of alkali postdeposition treatment (PDT) on the valence band structure of Cu(In,Ga)Se(2) (CIGSe) thin-film solar cell absorbers are addressed from a first-principles perspective. In detail, experimentally derived hard X-ray photoelectron spectroscopy (HAXPES) data [ E. Handick; ACS Appl. Mater. Interfaces2015, 7, 27414−2742026633568] 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 CuInSe(2), its Cu-deficient counterpart CuIn(5)Se(8), and different potentially formed secondary phases, such as KInSe(2), InSe, and In(2)Se(3). 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 CuInSe(2) and CuIn(5)Se(8), resulting in a stoichiometry slightly more Cu-rich than that of CuIn(3)Se(5). The NaF/KF-PDT-induced changes in the HAXPES spectra for different alkali exposures are best reproduced by additional contributions from KInSe(2), 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.