(A) study on the interface phenomena in $TiO_2-nanotube-based$ dye-sensitized solar cells

Abstract

Recently, dye-sensitized solar cells (DSCs) have been received great attention due to their low cost, easy fabrication and high efficiency up to 12%. TiO2 nanoparticle structure in photoanodes of DSCs gives the best efficiency. However, structural disorder between crystalline nanoparticles inhibits the charge transport and also 3-D network structure gives an inefficient path for charge percolation. Several nanostructures such as nanowires, nanofibers, nanotubes, hollow hemisphere, etc. were studied to overcome the limitation of nanoparticle structure. Among these structures, TiO2 nanotube structure was given considerable interests due to its 1-D structure which has superior path for charge percolation and charge collection-efficiency. In this study, surface passivation method and transplanting process were applied to improve the efficiency of $TiO_2$-nanotube-based DSCs. Passivating the surface of photoanodes with high band gap materials such as MgO, ZnO, $WO_3$, $SrTiO_3$, $Nb_2O_5$, $SnO_2$, etc. retards back transfer of electrons into the electrolyte and minimizes the charge recombination. Moreover, dye attachment becomes more favorable because the surface area is increased due to the surface-passivating layers and its basicity, and consequently, a light harvesting efficiency can be improved. The concept of isoelectric point was introduced to select the surface-passivating materials for $TiO_2$-nanotube-based DSCs. Two basic materials (MgO, ZnO) and one acidic material ($WO_3$) were selected to find the effect of isoelectric point of surface-passivating materials on the performance of $TiO_2$-nanotube-based DSCs. On the other hand, a fullerene derivative called $PC_{61}BM$ was used as a surface-passivating material for $TiO_2$-nanotube-based DSCs. PC61BM is an n-type organic semiconductor which is widely used in P3HT/$PC_{61}BM$ bulk heterojunction solar cells. The LUMO level of $PC_{61}BM$ (3.7 eV) is higher than the position of $TiO_2$ conduction band (4...