From nanowires to hierarchical structures of template-free electrodeposited ZnO for efficient dye-sensitized solar cells

A new and original method for the electrochemical growth of ZnO nanocrystalline porous layers and multiscale hierarchical structures is described. The structures are designed by simply playing with the growth conditions and without any use of template or additive in the aqueous deposition bath. Two types of hierarchical structures are described combining electrodeposited ZnO nanowire arrays and a nanoporous layer: nanowire arrays covered by a conformal nanoporous layer and nanowire arrays embedded in a nanoporous layer. The global performances of dye sensitized solar cells (DSSCs) fabricated using the hierarchical structures are higher than those found for nanoparticulate sol–gel ZnO films and for the two basic electrodeposited structures. Films made of nanowires embedded in a nanocrystalline matrix show a maximum energy conversion efficiency of ∼4.1%. The wires play several important beneficial roles in the presented structures since they permit the electrodeposition of thick nanoporous ZnO films which immobilize a large amount of dye, they act as preferential electron pathways for efficient charge collection and, due to their size, they enhance the light trapping in the photoanode and hence increase the light diffusion length before its harvesting by the dye. Another interest of the proposed ZnO hierarchical structures is a synthesis as well as an applied post-growth thermal treatment performed below 150 °C in soft environments which are then perfectly compatible with lightweight plastic flexible and other fragile substrates.