Impact of Na on MoSe2 formation at the CIGSe/Mo interface in thin-film solar cells on polyimide foil at low process temperatures

The aim of this work is to study the effect of Na on the formation of MoSe2 at the absorber/Mo back contact interface of Cu(In,Ga)Se2 (CIGSe) thin-film solar cells at low process temperatures using polyimide foil as substrate material. As reported previously, the presence of Na has been observed to modify the formation of the back interface, which may in part explain the different electronic properties of the completed device, as was determined by admittance spectroscopy and I–V–T measurements. In order to further study this interface formation, break-off experiments are performed and a lift-off technique is used to enable investigation of the different surfaces via X-ray photoelectron spectroscopy and Raman scattering. Both techniques confirm the dependence of the MoSe2 layer formation at the back interface on the presence of Na. The experiments also reveal the relevance of the composition of the absorber layer to the development of the MoSe2 layer during the Cu(In,Ga)Se2 deposition process. Hence this work describes routines that may be employed to develop an “appropriate” CIGSe/Mo back interface for high-efficiency solar cells.