Electrodeposition and selenization of brass/tin/germanium multilayers for Cu₂Zn(Sn_1-xGe_x)Se₄ thin film photovoltaic devices

Dense, closed, homogeneous brass/tin/germanium multilayers have been prepared by electrochemical methods. The stacks were electrodeposited on molybdenum-sputtered soda-lime glass. Brass was deposited first from an aqueous pyrophosphate electrolyte. Then, tin was deposited from a tin(II) pyrophosphate electrolyte. Germanium was deposited as the top film from 5 vol% GeCl₄ in propylene glycol. The composition of the brass/tin/germanium stack could easily be controlled by adjustment of the charge of the deposition of each coating and with special attention to exchange reactions. The variation in film thickness was minimized by using a rotating disk electrode (RDE) sample holder with a current thief. Soft-annealing under a nitrogen atmosphere and selenization with elemental selenium resulted in dense, closed copper-poor and zinc-rich CZTGSe absorber material. P-type conductivity was confirmed by photoelectrochemistry and a first photovoltaic device showed a power conversion efficiency of 0.6% and a band gap near 1.2 eV (with Ge/(Sn + Ge) = 0.38).