**2.1. Back contact layer**

The back contact layer as the back contact electrode of the solar cell is usually deposited on the substrate such as glass or flexible foils. One of the key requirements for the back contact layer is that the contact between the back contact and the absorber layer should be ohmic contact. The back contact layer should have a good conductivity and can be adhered to substrate firmly in order to make the solar cell stable. Molybdenum (Mo) is used in the majority of the CIGS solar cells. The structural and morphological properties of the Mo thin films will greatly affect the sequent growth of the absorber layer.
