Hydrothermal fabrication of hierarchically macroporous Zn2SnO4 for highly efficient dye-sensitized solar cells

Hierarchical macroporous Zn2SnO4 consisting of nanoparticles has been synthesized for the first time through an in situ hydrothermal and a following annealing process in the presence of a polystyrene (PS) template. Zn2SnO4 macropore sizes are tuned in the range of 180–650 nm by selecting the appropriate size of PS spheres, and the building unit size of the Zn2SnO4 macropore is 4.2 nm regardless of the PS sizes. The photovoltaic performances of the dye-sensitized solar cell based on hierarchical macroporous Zn2SnO4 with 200, 400, 600 and 750 nm PS spheres are 5.01, 4.76, 4.39 and 3.92%, respectively. The smaller pore size of Zn2SnO4 exhibits higher photovoltaic performance, which is ascribed to the higher dye loading, faster electron transport rate and slower electron recombination rate. These are confirmed by UV-vis absorption spectroscopy, intensity-modulated photocurrent spectroscopy, intensity-modulated photovoltage spectroscopy and electrochemical impedance spectroscopy. The double layered photoelectrode based on a Zn2SnO4 nanoparticles dye adsorption layer (4.2 nm in particle size, 15 μm in film thickness) and a macroporous light scattering layer (180 nm in macropore size, 4.0 μm in thickness) shows a remarkable enhancement in power conversion efficiency (6.10%) compared to that of Zn2SnO4 nanoparticles photoelectrode (5.36%) because of its superior light scattering ability.