A Comparative Study of Nanostructured TiO2, ZnO and Bilayer TiO2/ZnO Dye-Sensitized Solar Cells

Titanium dioxide (TiO2), Zinc oxide (ZnO) and bilayer TiO2/ZnO (TZO) based cells have been developed and sensitized with five organic dyes and one cocktail dye composed of five dyes. Photovoltaic performance of TiO2 and ZnO solar cell sensitized with six dyes is compared to that of bilayer TZO cells. The forward current is found to increase with applied voltage in the range V ≤ 0.4 V, which is dominated by thermionic emission, whereas in 0.4 ≤ V ≤ 0.7 V, the current transport is due to space charge-limited current controlled by exponential trap distribution in all devices. The combined properties of the materials enhance the efficiency of composite TZO cells. TiO2 permits the formation of an energy barrier at the ZnO electrode/electrolyte interface, which reduces the back electron transfer from the conduction band of ZnO to I3− in the electrolyte. Also, due to the TiO2 layer on the ZnO, the latter forms a compact layer between flourine-doped tin oxide (FTO)/TiO2 which benefits the fast electron transfer from TiO2 to ZnO to FTO glass. This reduces the charge recombination occurring at the ZnO/FTO interface leading to higher open circuit voltage (Voc), higher short circuit current (Jsc), lower series resistance (Rs), and in turn higher efficiency in TZO solar cells as compared to ZnO cells. Among the six dyes, Eosin-Y and Rose Bengal dye gave the best performance as sensitizers with TZO.