Investigation of properties of Molybdenum for CIGS thin film solar cells prepared by DC Sputtering

102

Molybdenum back contact layers of CIGS thin film solar cells were prepared by DC magnetron sputtering in this study. We analyzed the microstructure, electrical, and optical properties for characterization. First, we discuss about the adhesion layers. In order to acquire the thin films which have both porous microstructures and tensile stress, we control the working pressure and sputtering power, and then, characterize the strain, stress, microstructure, and electrical properties for comparison. The proper parameters for Mo adhesion layers are 300 W and 20 mTorr. Then, we prepare bilayer Mo layers by the change of working pressure. The working pressure of bottom layers remains at 20 mTorr, and those of upper layers are varied from 2.5 mTorr to 20 mTorr. The best parameters are 300 W and 2.5 mTorr/20 mTorr. Upper layers have both denser microstructure and lower roughness which are suitable for conduction. Bottom layers are proper for adhesion. This Mo bilayer shows the sheet resistance of 0.330 ohm/sqr with a total thickness about 500 nm which corresponds to a resistivity of 16.49 μohm-cm. The highest reflectivity of 57.5% is obtained in the range of visible light at 300 W and 2.5 mTorr/20 mTorr. Na can easily diffuse into CIGS absorber layers when the microstructures of Mo upper layers are more porous. The highest conversion efficiency of 7.876% is obtained at Mo bilayer of 300 W and 15 mTorr/20 mTorr. Keywords: Mo back contact, Working pressure, Sputtering power, Na diffusion, CIGS thin film solar cells