Characteristics of molybdenum bilayer back contacts for Cu(In,Ga)Se2 solar cells on Ti foils

Abstract Molybdenum back contact properties are critical for Cu(In,Ga)Se 2 (CIGS) solar cell performance on metallic substrates. In this work, we investigated the properties of sputter-deposited Mo bilayer back contacts on Ti foils. The morphology, electrical resistivity, optical reflectance and residual mechanical stress of the bottom Mo layer were modified by varying the working pressure during its deposition. Working pressures ranging from 0.27 Pa to 4.00 Pa were used. The top Mo layer was deposited using constant conditions at a pressure of 0.13 Pa. It was demonstrated that unlike a Mo monolayer, the use of a Mo bilayer allows controlling the mechanical stress at the Mo/CIGS interface without degrading the optical reflectance and the electrical resistance of the back contact. It was also found that the morphology of the bottom Mo layer affects the growth of the top Mo layer, resulting in a modified back contact surface morphology. This induces changes in the crystalline orientation of the CIGS layer. The resulting solar cell characteristics strongly vary as a function of the bottom Mo layer deposition pressure. A bottom Mo layer growth at 2.93 Pa allows improving the solar cell conversion efficiency by 1.5 times compared to a bottom Mo layer deposited at 0.27 Pa. Using the improved Mo bilayer back contact, a maximum solar cell efficiency of 10.0% was obtained without sodium addition nor anti-reflection coating.