Influence of Mo-N as diffusion barrier in Mo back contacts for Cu(In,Ga)Se2 solar cells

In a recent article, we have shown that a high selenium amount (selenium partial pressure) during the stacked elemental layer process is beneficial for the Cu(In,Ga) Se-2 absorber layer in terms of Ga/In interdiffusion and Ga enhancement at near-surface regions. For highly efficient Cu(In,Ga) Se-2 thin film solar cells in the stacked elemental layer process a certain MoSe2 thickness at the interface between Mo and Cu(In,Ga) Se-2 is necessary. However, the high selenium supply leads to back contact corrosion, which results in adhesion problems of the film and an increase of the series resistance of the solar cell device. Introduction of a thin diffusion barrier against selenium penetration at the interface between Cu(In,Ga) Se-2 absorber layer and Mo back contact is a possible solution which may ensure homogeneous back contact properties while allowing high selenium supply during the stacked elemental layer process. In this work, a new approach for an alternative back contact system is presented which includes a functional Mo-N layer. During the Mo sputtering process, the implementation of a reactive N-2 gas process is easily possible. According to structural analysis just a thin self-limited MoSe2 layer with a constant thickness is formed during the stacked elemental layer process at the back contact. It is further demonstrated that a thin Mo-N barrier layer leads to an improved fill factor in the current-voltage characteristic of the cell, whereas thicker barrier layers negatively affect the series resistance of the cell. (C) 2016 Elsevier B.V. All rights reserved.