Electronic and compositional properties of the rear‐side of stoichiometric CuInSe 2 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 MoSe 2 /CuInSe 2 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 CuGaSe 2 , 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 Cu x Se secondary phase is present at the MoSe 2 /CuInSe 2 interface in contrast to reports on Cu‐poor material. Our findings shed new light into the complex interface formation of CuInSe 2 ‐based thin film solar cells grown under Cu‐rich conditions.