ZnO/ZnO Core–Shell Nanowire Array Electrodes: Blocking of Recombination and Impressive Enhancement of Photovoltage in Dye-Sensitized Solar Cells

Abstract

A multistep wet-chemistry route was developed, by combining electrodeposition, colloidal synthesis and spin coating, to obtain arrays of ZnO nanowires (NWs) coated by a ZnO nanocrystalline layer (i.e., ZnO/ZnO core–shell NWs). They were integrated as anodes in dye-sensitized solar cells. With the use of an iodide-based electrolyte, photovoltages as impressive as 870 mV were obtained with an enhancement of more than 250 mV with respect to devices based on bare NWs. A comprehensive device characterization study by means of impedance spectroscopy (EIS) and intensity-modulated photovoltage spectroscopy (IMVS) reveals a significant blockage of recombination upon NW shell deposition. To study the generality of this multistep method, electrodes of core–shell nanostructures based on commercial ZnO nanoparticles were also prepared. A decrease of recombination rate is also detected, although it is much more moderate than the observed for nanowire-based electrodes. The present ZnO synthetic approach allows obtaining nanowire-based dye-sensitized solar cells which exhibit longer electron lifetimes than nanocrystalline analogues. This finding implies a significant improvement of photovoltage with respect to the state of the art ZnO-based dye-sensitized solar cells.