Multi-Functionality of Macroporous TiO₂ Spheres in Dye-Sensitized and Hybrid Heterojunction Solar Cells

Micron-sized macroporous TiO₂ spheres (MAC-TiO₂) were synthesized using a colloidal templating process inside emulsions, which were then coated on a nanocrystalline TiO₂ light absorption film to prepare a bilayered photoanode for liquid-based dye-sensitized solar cells (DSSC) and hybrid heterojunction solid-state solar cells. MAC-TiO₂ layers can enhance light scattering as well as absorption, because their pore size and periodicity are comparable to light wavelength for unique multiple scattering and a porous surface can load dye more. Moreover, due to the bicontinuous nature of macropores and TiO₂ walls, electrolyte could be transported much faster in between the TiO₂ spheres rather than within the small TiO₂ nonporous architectures. Electron transport was also facilitated along the interconnected TiO₂ walls. In DSSCs with these MAC-TiO₂ scattering layers, efficiency was higher than conventional DSSCs incorporating a commercial scattering layer. The unique geometry of MAC-TiO₂ results in strong improvements in light scattering and infiltration of hole-transporting materials, thereby the MAC-TiO₂-based solid-state device showed comparatively higher efficiency than the device with conventional nanocrystalline TiO<sub>2