Design of TiO₂ dye-sensitized solar cell photoanode electrodes with different microstructures and arrangement modes of the layers

Different structures of TiO₂ photoelectrodes were fabricated with various arrangement modes of the layers. TiO₂ nanoparticles were synthesized by a combination of sol–gel and solvothermal methods and were employed as the active layer of solar cells. Hierarchical TiO₂ spheres (HTS) and spherical TiO₂ particles (STP) were also prepared by hydrothermal and sol–gel processes, respectively, and used as the scattering layer of solar cells. Field emission scanning electron microscope and X-ray diffraction analyses revealed that all synthesized morphologies showed anatase structure with particle size around 20, 200–400 nm and 1.5–2.0 µm for the nanoparticles, spherical particles and hierarchical spheres, respectively. Moreover, the nanoparticles, spherical particles and hierarchical spheres had surface areas of 131.1, 5.7 and 253.5 m²/g, respectively. The optical properties of these morphologies were studied through diffuse reflectance spectroscopy analysis. The indirect optical band gap energy of the nanoparticles, spherical particles and hierarchical spheres was calculated 3.10, 3.14 and 3.13 eV, respectively. Furthermore, the light scattering particles (i.e., HTS and STP) exhibited high diffuse reflectance because of their large particle size. The improvement of power conversion efficiency of monolayer cells was achieved by light harvesting mechanism aided by controlling the thickness of the film. The monolayer solar cell made of TiO₂ nanoparticles with 20 μm thickness showed the highest efficiency of 7.21 %. Further enhancement of photovoltaic performance was obtained by light scattering mechanism aided by fabrication of double-layer solar cells with different arrangement modes of the layers. The enhancement was attributed to higher incident photon-to-current conversion yield due to greater fraction of light scattering as well as less recombination of photogenerated electrons. Hierarchical spheres had the highest amount of dye adsorption, whereas spherical particles showed the greatest light scattering property. In addition, incorporation of nanoparticles into hierarchical spheres and spherical particles would lead to better connection between the particles and retard charge recombination in the photoanode. It was found that the influence of electron transport rate on cell efficiency improvement is more dominant than that of light scattering. Therefore, the double-layer solar cell made of TiO₂ nanoparticles as the active layer and mixtures of nanoparticles and hierarchical particles as the scattering layer showed the highest efficiency of 8.21 % among all fabricated solar cells. Graphical Abstract: Different structures of TiO2 photoelectrodes were fabricated with various arrangement modes of the layers containing TiO2 nanoparticles, hierarchical TiO₂ spheres and spherical TiO2 particles for dye-sensitized solar cell applications.[Figure not available: see fulltext.]