Experimental scheme for a stable molybdenum bilayer back contacts for photovoltaic applications

Abstract

Molybdenum (Mo) back contacts have been widely used for thin film based solar cells, especially for CuIn1-xGaxSe2 (CIGS) and Cu2ZnSnS4 (CZTS) based devices. Traditionally, a bilayer approach at two different sputtering pressures is used to produce highly conducting and adhesive films on glass substrates. Mo bilayers undergo a harsh heat treatment during the absorber layer (CIGS/CZTS) deposition or formation process, which in turn changes the residual stress and the morphological properties of the back contact layers and can cause poor adhesion of the absorber layers. This is a key layer for the photovoltaic industry because the entire device is deposited on top of this layer. Adhesion issues can degrade device performance and can affect the long-term stability of the solar cell. This work reports in-situ study of Mo stress variation during anneals ranging from 20 °C to 500 °C and compares the film’s morphological and electrical properties before and after heating to develop a thermally stable Mo bilayer. For bilayer films with a total thickness of 1 μm, this study revealed that the optimum pressure and thickness are 5 mTorr and 150 nm, and 1 mTorr and 850 nm for the bottom and top layers, respectively.