Semiconducting Divalent Metal Oxides as Blocking Layer Material for SnO₂-Based Dye-Sensitized Solar Cells

This study is to demonstrate that semiconducting materials can be used as the blocking layer material for the electrode of dye-sensitized solar cells (DSSCs) as well as insulating materials studied previously. We modified SnO₂ nanoparticles with various semiconductor divalent metal oxides (CdO, ZnO, NiO, CuO, and PbO) and fabricated DSSCs with the modified SnO₂ nanoparticles. The modifier metal oxides exist as very small nanoparticles, well dispersed on the surfaces of SnO₂ nanoparticles. Except for the case of PbO, all of the modifier materials improve the solar cell efficiency. The detailed mechanisms have been investigated. The basic properties of the modifiers increase the amount of dye adsorbed, increasing the current, and raise the flat-band potentials through deprotonation of the surface to increase the potentials. The coating materials block the recombination reactions between the electrons in the conduction band and the red–ox species in the electrolyte. Therefore, although the narrow band gaps and the d–d transitions of the coating materials may reduce the amount of photons to reach the sensitizer, such negative effects are reduced by the quantum size effects of the modifier metal oxides and are more than offset by the positive effects mentioned above. The case of PbO as the coating material appears to suffer from the high resistance arising from the highly anisotropic crystal structures of PbO. On the basis of the various observations depending on the nature of the modifier materials, a few suggestions are made in selecting good modifier materials.