Open Access Version

Abstract:
The chemical and electronic structure of the interfaces of kesterite-based and related absorber materials in thin-film solar cell device-relevant layer stacks have been investigated with x-ray based spectroscopic techniques (XPS, HAXPES, XAES, XES, and XAS). The analyzed materials are CZTS absorbers grown at Uppsala University with their interfaces to the Mo back contact and to different ZnOxS1-x-buffer layers (x=[0;6 7 ; 1]), SnS absorbers with partly N-doped ZnO buffer layers produced in a collaboration at MIT and Harvard University, and CuSbS2 absorbers with (Cd,Zn)S buffer layers grown at NREL in collaboration with MIT. We find an unchanged CZTS surface in this series which has a profound effect on the ZnOxS1-x growth. Instead of a constant composition throughout the buffer thickness, we find S-rich material growing directly on the CZTS in the ALD process. The composition changes towards the O-rich ZnOxS1-x that forms the main part of the buffer layer. The conduction band offset is therefore found to be around -0.21(0.15) eV. The measured cliff is reduced by chemical interface modification towards a flat band alignment. The interface effect during annealing at the CZTS back contact where MoS2 forms occurs even at low annealing temperatures below 500C. Sn spectra of liftoff CZTS back sides, annealed on the Mo contact, resemble the precursor at all temperatures while Sn spectra of CZTS front sides are almost single phase after standard annealing. We observe the stabilization of SnS at the back contact and different Sn-oxides. A TiN interlayer was effectively introduced to prevent reactions at the CZTS/Mo interface.