New insights into the Mo/Cu(In,Ga)Se₂ interface in thin film solar cells: Formation and properties of the MoSe₂...

Being at the origin of an ohmic contact, the MoSe₂ interfacial layer at the Mo/Cu(In,Ga)Se₂ interface in CIGS (Cu(In,Ga)Se₂ and related compounds) based solar cells has allowed for very high light-to-electricity conversion efficiencies up to 22.3%. This article gives new insights into the formation and the structural properties of this interfacial layer. Different selenization-steps of a Mo covered glass substrate prior to the CIGS deposition by co-evaporation led to MoSe₂ interfacial layers with varying thickness and orientation, as observed by x-ray diffraction and atomic resolution transmission electron microscopy. A novel model based on the anisotropy of the Se diffusion coefficient in MoSe₂ is proposed to explain the results. While the series resistance of finished CIGS solar cells is found to correlate with the MoSe₂ orientation, the adhesion forces between the CIGS absorber layer and the Mo substrate stay constant. Their counter-intuitive non-correlation with the configuration of the MoSe₂ interfacial layer is discussed and related to work from the literature.