Sodium induced grain growth, defect passivation and enhancement in the photovoltaic properties of Cu2ZnSnS4 thin film solar cell

• Reactive sputtering with reduced annealing time have been used for the growth of CZTS thin film.
• NaF has been deposited over precursor film before annealing.
• Na addition resulted in grain growth, improved compactness and reduction in band gap.
• An enhancement in the photovoltaic characteristics have been observed with addition of Na.

Sodium diffusion from soda lime glass (SLG) during high temperature annealing is known to play a crucial role in affecting the grain growth and defect passivation in chalocogenide/kesterite solar cells. Additional sodium is required when low temperature or short term annealing is used. Although this fact is known, a systematic comparative study for kesterite films is seldom reported. In the present study, Cu2ZnSnS4 thin films were deposited on SLG and Mo coated SLG using stacked layer reactive sputtering. Na was deposited over the CZTS thin film and the film was annealed in N2 atmosphere in order to enhance the grain growth. This resulted in the shift in the XRD peak towards lower diffraction angle. The optical bandgap shifted from 1.45 eV to 1.38 eV with Na addition. Significant grain growth from hundreds of nanometer to micrometer was observed in samples with Na. Device fabricated in SLG/Mo/CZTS/CdS/ZnO/ITO configuration with Al front contact shows increase in efficiencies values from 1.50% to 2.84%.