Enhanced light harvesting and electron collection in quantum dot sensitized solar cells by TiO 2 passivation on ZnO nanorod arrays

Light capture and electron recombination are the essential processes that determine power conversion efficiency (PCE) in quantum dot sensitized solar cells (QDSCs). It is well known that charges are easily transported in well-built QDSCs based on nanorod arrays. However, this advantage can be drastically weakened by defects located at the zinc oxide (ZnO) array surface which permit faster electron recombination. Hence, we developed a composite nanostructure consisting of ZnO nanorods coated with orthorhombic configuration titanium dioxide (TiO2) nanoparticles, which were synthesized using a solution of H3BO3 and (NH4)2TiF6. This composite nanostructure was designed to take the advantage of the enlarged surface area provided by the nanoparticles and improved electron transport along the nanorods, in order to yield good charge transport and light harvesting. At the same time, the TiO2/ZnO nanorod arrays have fewer recombination centers (hydroxyl groups) after TiO2 modification, which results in fewer electron trapping events at the ZnO nanorod surface; thereby, a reduced charge recombination and longer electron lifetime can be achieved. As a result, the PCE of the QDSCs with TiO2-nanoparticles-decorated ZnO nanorod arrays photoelectrode reaches 4.8%, which is ~78% higher efficiency compared to 2.7% for solar cells without modification.