Direct conversion of two-dimensional ZIF-L film to porous ZnO nano-sheet film and its performance as photoanode in dye-sensitized solar cell

A zeolitic imidazolate framework-L (ZIF-L) film with a leaf-like morphology was successfully fabricated on FTO substrate in aqueous solution at room temperature on a large-scale. This ZIF-L film with a thickness of ∼4 μm was composed of highly uniform nanosheets of 100-150 nm in thickness, which stood nearly perpendicularly on the substrate. A porous ZnO nanosheet film was subsequently obtained by simply calcining the ZIF-L film at 550 °C for 0.5 h. The resulting ZnO nanosheets presented a curved petal-like morphology, which was composed of interconnected nanoparticles. The ZnO film with a thickness of only about 4 μm showed highly crystalline and well-defined porous features. An overall light conversion efficiency of 2.52% was achieved by using this thin porous ZnO nanosheet film as a photoanode in dye-sensitized solar cell (DSSC), significantly better than that derived from the film of ZnO nanorods with similar thickness (1.27%). The superior performance of the DSSC with porous ZnO nanosheet is attributed to the following: (1) the high surface area leads to a higher dye loading; (2) the nanopores enable more efficient electrolyte diffusion and rapid adsorption of the dye, thereby reducing the chances of formation of Zn²⁺/dye aggregates; and (3) increased light-harvesting efficiency.