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)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 [ Handick , E. ; 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 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.