Hedgehog-like hierarchical ZnO needle-clusters with superior electron transfer kinetics for dye-sensitized solar cells

Hedgehog-like hierarchical ZnO needle-clusters, three-dimensional (3-D) ZnO flowers and one-dimensional (1-D) ZnO needles have been synthesised via a facile hydrothermal method. These samples with different morphologies and microstructures were used to fabricate photoelectrodes for dye-sensitized solar cells (DSSCs). Out of the three samples, current-voltage (I-V) curve measurements show that DSSCs with hedgehog-like ZnO needle-clusters display the best photoelectrochemical performance which can be attributed to enhanced light harvesting and faster reaction kinetics resulting from the unique morphology. The UV-vis absorption and diffused reflectance spectra indicate that hedgehog-like ZnO needle-clusters show higher light harvesting abilities due to high UV absorption, stronger light scattering, as well as a high surface area. Electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS), and intensity-modulated voltage spectroscopy (IMVS) further demonstrate that hedgehog-like ZnO needle-clusters provide superior electron transfer kinetics: fast electron transfer and long electron lifetimes with suppressed recombination. The ZnO needle-clusters obtained not only provide a 3-D matrix but also avoid extensive grain boundary formation. The discoveries from this work are important for the design of efficient photoanode materials with optimum structures for DSSCs.