Paper TF+EN-MoA8
Ultra-thin TiO₂ Blocking Layer by Atomic Layer Deposition for Dye-Sensitized Solar Cells

Monday, October 29, 2012, 4:20 pm, Room 11

In dye-sensitized solar cells (DSSCs), one of major recombination routes occurs at the interface of fluorine-doped tin oxide (FTO) glass and electrolyte solution. Typically, a thin and compact blocking layer (B/L) on the FTO-glass has been introduced to reduce electron loss before mesoporous TiO₂ layer integration. A variety of deposition methods have been tried and studied to make efficient B/L on the FTO-glass. Optimal thickness that suppresses the recombination on the interface of FTO glass and electrolyte, is typically 25 to 450 nm, depending on deposition methods and conditions. Compared to other methods, atomic layer deposition (ALD) with TiO₂ is not well established and investigated for blocking layers even though it is a valuable process in making pin-hole, crack-free, and dense TiO₂ films.

In this study, ALD TiO₂ was performed on FTO-glass with titanium isopropoxide and H₂O as precursors, producing B/L thickness from 5 to 100 nm. Cells without mesoporous titania but with ALD TiO₂ B/L were also made. We find the optimal thickness of the ALD TiO₂ blocking layer is 10 nm. This blocking layer thickness significantly reduces recombination, resulting in an average overall efficiency of 8.5%, compared to 7.1% for similar cells without the blocking layer present. We also find that a blocking layer of 4.3 nm effectively prevents electrons of FTO surface from recombining with I₃^- in the electrolyte. On the other hand, a thick ALD TiO₂ blocking layer in excess of 10 nm tended to reduce the overall efficiency because the thick ALD TiO₂ film increases the charge transfer resistance and hinders the electron transport to FTO-glass. This work contributes to understand effective blocking layer from TiO₂ ALD process for DSSCs and other high-performance electrical devices