Hierarchical ZnO aggregates assembled by orderly aligned nanorods for dye-sensitized solar cells

Hierarchical ZnO aggregates assembled by orderly aligned nanorods were prepared via a facile solvothermal method using diethylene glycol as solvent and zinc acetate dehydrate as precursor. Time dependent trails evidenced that the formation of the hierarchical aggregates experienced a multistep self-assembly process. Moreover, it was found that the reaction medium plays an important role in the assembling process and the diameter of the product could be tuned by simply altering the precursor dosage. The hierarchical product was further applied as scattering layer in bi-layered dye-sensitized solar cell (DSSC), and a high conversion efficiency of 5.2% was demonstrated, indicating a substantial improvement compared with the nanoparticle cell of 2.7%. Based on the optical and electrochemical investigations, the high conversion efficiency was mainly ascribed to the unique hierarchical structure of the ZnO aggregates. The rough surface of the nanorod subunits could enhance the dye loading capacity and the aggregates with micrometer sized diameter could improve the scattering effect. Moreover, the orderly aligned nanorods could minimize the grain boundaries, suppress the surface recombination and provide a direct pathway for fast electron transport, which therefore enhance the collection efficiency of the photoelectrons.