Direct and inverse photoemission were used to study the impact of alkali fluoride postdeposition treatments on the chemical and electronic surface structure of Cu(In,Ga)Se2 (CIGSe) thin films used for high-efficiency flexible solar cells. We find a large surface band gap (E(g)(Surf), up to 2.52 eV) for a NaF/KF-postdeposition treated (PDT) absorber significantly increases compared to the CIGSe bulk band gap and to the Eg(Surf) of 1.61 eV found for an absorber treated with NaF only. Both the valence band maximum (VBM) and the conduction band minimum shift away from the Fermi level. Depth-dependent photoemission measurements reveal that the VBM decreases with increasing surface sensitivity for both samples; this effect is more pronounced for the NaF/KF-PDT CIGSe sample. The observed electronic structure changes can be linked to the recent breakthroughs in CIGSe device efficiencies.

The chemical structure of the CdS/Cu2ZnSnSe4 (CZTSe) interface was studied by a combination of electron and X-ray spectroscopies with varying surface sensitivity. We find the CdS chemical bath deposition causes a “redistribution” of elements in the proximity of the CdS/CZTSe interface. In detail, our data suggest that Zn and Se from the Zn-terminated CZTSe absorber and Cd and S from the buffer layer form a Zn–Se–Cd–S interlayer. We find direct indications for the presence of Cd–S, Cd–Se, and Cd–Se–Zn bonds at the buffer/absorber interface. Thus, we propose the formation of a mixed Cd­(S,Se)–(Cd,Zn)­Se interlayer. We suggest the underlying chemical mechanism is an ion exchange mediated by the amine complexes present in the chemical bath.

The chemical and electronic interface structure (including energy level alignment) between wet-chemical deposited CdS and low-band-gap Cu2ZnSn­(S,Se)4 (CZTSSe) thin-film solar cell absorbers was studied using direct and inverse photoemission. Complementarily, the activation energy (E a) of the dominant charge carrier recombination process of related solar cell devices was derived by temperature-dependent current–voltage [I(V,T)] measurements. We find the CZTSSe surface to be free of any significant amount of sulfur and the formation of Cd–Se bonds at the interface. A small, positive (“spike”-like) conduction band offset of 0.21 ± 0.28 eV between CZTSSe and CdS was measured. In conjunction with the I(V,T) derived E a of 1.09 ± 0.07 eV, which is in excellent agreement with the CZTSSe bulk band-gap energy of 1.07 eV, this reveals that high-rate charge carrier recombination at the CdS/CZTSSe interface can mainly be excluded as the performance-limiting factor in corresponding solar cells.

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