Growth control of molybdenum thin films with simultaneously improved adhesion and conductivity via sputtering for thin film solar cell application

Abstract

Thin molybdenum (Mo) films have been recognized as an effective bottom contact in Cu2ZnSnS4 (CZTS) based thin film solar cells. In typical deposition settings, the Mo films are required to have a favourable microstructure that facilitates sodium diffusion from glass substrates to the absorber layer in addition to the requirement of very high conductivity for efficient charge transfer, and chemical inertness and strong adhesion to the substrate. We have fabricated a bilayer structured Mo film by direct current (DC) magnetron sputtering and carried out a comparative study to elucidate the effects of thickness of bilayers and the sputter power on the microstructure, adhesion and electrical property of the Mo films. The optimized bilayer-structured Mo film of ∼1 μm thickness showed a crack-free microstructure, strong adhesion to the substrate and a resistivity as low as 1.8 × 10−4 Ω-cm. Considering the importance of Na out-diffusion from the substrates to the CZTS in the solar cells, Na concentration profile is established using time of flight secondary ion mass spectroscopy for the bi-layered Mo films grown on different glass substrates including one of the in-house prepared glasses.