Electronic and compositional properties of the rear-side of stoichiometric CuInSe2 absorbers

In-depth understanding and subsequent optimization of the contact layers in thin
film solar cells are of high importance in order to reduce the amount of nonradiative
recombination and thereby improve device performance. In this work, the buried
MoSe2/CuInSe2 interface of stoichiometric absorbers is investigated with scanning
tunneling spectroscopy and Kelvin probe force microscopy combined with compositional
measurements acquired via photo-electron spectroscopy after a mechanical
lift-off process. We find that the local density of states, as measured with scanning
tunneling spectroscopy, is similar to the front-side of ultra-high vacuum annealed
CISe absorbers. The grain boundaries exhibit a weak upward band bending, opposite
to Cu-poor CuGaSe2, and we measure an increased Cu accumulation at the rear CISe
surface compared to the bulk composition and a non-zero concentration of Cu on
the Mo-side. Grazing incidence X-ray diffraction measurements corroborate that a
small amount of a CuxSe secondary phase is present at the MoSe2/CuInSe2 interface
in contrast to reports on Cu-poor material. Our findings shed new light into the complex
interface formation of CuInSe2-based thin film solar cells grown under Cu-rich
conditions.