Electrochemical studies of the electrodeposition of copper-zinc-tin alloys from pyrophosphate electrolytes followed by selenization for CZTSe photovoltaic cells

• Electrodeposition of copper-zinc-tin from a pyrophosphate electrolyte was studied.
• A tin(II) salt instead of a tin(IV) salt was used as the tin precursor.
• Dense, closed, homogeneous copper-zinc-tin coatings were deposited and selenized.
• The compact CZTSe thin films can be used as absorber material in solar cells.

Ternary copper-zinc-tin coatings have been prepared by a one-step electrodeposition on molybdenum sputtered soda-lime glass (SLG) (20××20 mm2) from an aqueous pyrophosphate electrolyte at room temperature. The plating bath contains copper(II), zinc(II), tin(II) and potassium pyrophosphate at pH = 10. The influence of pyrophosphate as a complexing agent is characterized by speciation simulation, cyclic voltammetry and linear sweep voltammetry. The electrolyte and deposition conditions were adjusted to deposit ternary coatings with the appropriate copper-poor and zinc-rich composition (Cu/(Zn + Sn) = 0.89 and Zn/Sn = 1.04), making them suitable for preparing kesterite absorber films for photovoltaic applications. The composition is influenced by variation of the deposition potential, pH, temperature, composition of the electrolyte and control of the mass transport. The morphology, composition and structure of the as-deposited and selenized films were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), inductively coupled plasma optical emission spectroscopy (ICP-OES) and X-ray diffraction (XRD). Dense, closed, homogeneous films of ternary copper-zinc-tin alloys with a mirror-bright gray appearance were deposited from the pyrophosphate electrolyte. After selenization, the coatings were matte gray and consisted of micrometer sized grains of pure CZTSe reaching over the complete thickness with only a few voids near the molybdenum back contact.