An Understanding of the Band Gap Shrinkage in Sn-Doped ZnO for Dye-Sensitized Solar Cells

Sn-doped ZnO (TZO) films were deposited onto glass substrates by a spray pyrolysis technique. The temperature-dependent conductivity measurements and room-temperature Hall effect measurements were carried out, which indicated that the films exhibited a degenerate semiconductor behavior. Band gap energy of the films was studied by transmission measurements. With increasing Sn content, band gap shrinkage was observed. It was determined that this shrinkage was associated with the competition between many body interactions and the Burstein-Moss effect. We concluded that there is a good agreement between experimental results and theoretical calculations in terms of the shift in band gap. Furthermore, the effective mass value based on parabolic band considerations needed to be replaced by one based on nonparabolic band structure of ZnO for higher content of Sn (> 3% at.) to correlate the calculations with the results.