Fabrication of hierarchically assembled microspheres consisting of nanoporous ZnO nanosheets for high-efficiency dye-sensitized solar cells

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Abstract
Porous nanosheet-assembled ZnO microspheres (PNMSs) were synthesized via one-pot hydrothermal treatment followed by calcination and were used as photoanodes for dye-sensitized solar cells (DSSCs). An overall light conversion efficiency of up to 5.16% has been achieved with this unitary material and structure. Superior to the referenced porous nanosheet-assembled ZnO microflowers (PNMFs) and porous dispersed ZnO nanosheets (PDNs), as well as precedent nanostructures, the present PNMS fully accommodates all indispensable characteristics for a high-performance DSSC through: (1) the hierarchical ZnO microsphere generates a prominent aggregation-induced light scattering center, which is in favor of enhancing the light absorption and the light propagation; (2) the single crystal nature of the nanosheet enhances the transport of injected electrons along the nanosheet, providing a direct electron pathway throughout the film and lowering the hole–electron recombination; (3) the thin thickness and porous character of the nanosheets results in a large surface area for dye adsorption; (4) intersectional contact with one another of the nanosheets of neighboring spheres increases the transport channel of the injected electrons through the adjacent spheres, avoiding the high resistance existing in the nanoparticle-based sphere aggregates due to the relatively small contact area among the aggregates; (5) these spherical assemblies form large external pores in the photoelectrode film, thus providing a “main trunk” for the quick electrolyte diffusion throughout the ZnO layer in the film. Simultaneously, the nanopores embedded in the nanosheet act as “branch lines” for more efficient electrolyte diffusion into the interstices of the densely packing nanosheets in the sphere.

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