Deposition, Characterization and Process Optimization of Molybdenum Thin Films using DC-Plasma Magnetron Sputtering for Back Contact Application in CIGS based Thin Films Solar Cells

Abstract

Molybdenum (Mo) is a commonly used back contact material for CIGS-based Cu(In,Ga)Se2 thin films solar cells. Numerous properties are required for Mo to play better role as back contact material including chemical and mechanical inertness during deposition process, high conductivity, an appropriate thermal expansion coefficient with contact layers and low contact resistance with the Cu(In,Ga)Se2 layer. Mo thin films were deposited on well-cleaned soda-lime glass substrates using DC-plasma magnetron sputtering. In the design of experiment deposition was optimized for maximum beneficial characteristics by monitoring effect of process variables such as deposition power (100-200W), Ar pressure (0.002-0.0123mbar) and substrate temperature (23-4000C). Their electrical, structural and morphological properties were analyzed to study the effect of these variables. The electrical resistivity of Mo thin films could be reduced either by increasing the deposition power and substrate temperature or by decreasing the Ar pressure. Within the range of analyzed deposition parameters, Mo thin films showed a mono-crystalline nature and the crystallites were found to have an orientation along [110] direction. The surface morphology of thin films showed that a highly dense microstructure has been obtained. The surface roughness of films increased with substrate temperature and deposition power. The adhesion of Mo thin films could be improved by increasing the Ar pressure and deposition power. Atomic force microscopy was used for the topographical study of the films and to determine the roughness of the films. X-ray diffractrometer (XRD) and scanning electron microscopy (SEM) analysis were used to investigate the crystallinity and surface morphology of the films. Hall Effect Measurement System was used to find resistivity, carrier mobility and carrier density of deposited films. The adhesion test was performed using scotch hatch tape adhesion test. Mo thin films prepared at deposition power of 200W, substrate temperature of 4000C and Ar pressure of 0.002mbar exhibiting a mono-crystalline structure with an orientation along (110) direction, thickness equal to 1μm and electrical resistivity equal to 7.02×10-05Ωcm was obtained.