Constructing nanorod–nanoparticles hierarchical structure at low temperature as photoanodes for dye-sensitized solar cells: combining relatively fast electron transport and high dye-loading together

Abstract

ZnO nanorod–nanoparticles (NR–NPs) hierarchical structure was prepared via a two-step hydrothermal process at 70 °C. In the hierarchical structure, ZnO nanorods prepared at step one served as the backbone for direct electron transport while ZnO nanoparticles synthesized at step two offered large surface area for dye-loading. Both reaction temperature and reaction time at step two had a significant influence on the morphology of the product. At a higher temperature, microspheres appeared above the nanorod film instead of nanoparticles surrounding the nanorods. Prolonging the reaction time to 24 h, the NR–NPs structure would transform to nanorod–nanoplants. Intensity-modulated photocurrent spectroscopy results showed that the photoanode composed of the NR–NPs hierarchical structure had an electron diffusion coefficient (D_n) much higher than that of the nanoparticles. The dye desorption results showed that the dye adsorption amount for the NR–NPs structure was as much as 250% of that for the nanorods. Compared with dye-sensitized solar cells (DSCs) based on nanorods, the incident photon-to-electron conversion efficiency of the DSCs based on NR–NPs hierarchical structure improved remarkably. Under AM 1.5G illumination (100 mW cm⁻²), the power conversion efficiency of DSCs based on photoanodes composed of NR–NPs hierarchical structure exhibited a significant improvement (more than 120%) compared with that of ZnO nanorods.