Facile synthesis of size-tunable mesoporous anatase TiO ₂ beads using a graft copolymer for quasi-solid and all-solid dye-sensitized solar cells

Multi-functional mesoporous TiO ₂ (M-TiO ₂) beads with high porosity and good interconnectivity in the anatase phase were synthesized via a solvothermal reaction at low temperature (100 °C) using a graft copolymer, i.e., poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM), as a structure-directing agent. Field-emission scanning electron microscopy (FE-SEM), energy-filtering transmission electron microscopy (EF-TEM) and X-ray diffraction (XRD) revealed that the TiO ₂ beads consisted of 13 nm interconnected nanocrystallites and were monodisperse with tunable sizes of approximately 120, 250, 500 and 750 nm. The photoelectrodes fabricated with M-TiO ₂ beads showed a high surface area (86.5 m ² g ^-1) and a stronger light scattering effect, as confirmed by Brunauer-Emmett-Teller (BET) and incident photon-to-electron conversion efficiency (IPCE) measurements. The structures of M-TiO ₂ beads effectively offered better pore infiltration of the polymer electrolyte. Furthermore, the improved interconnectivity of M-TiO ₂ beads improved the electron diffusion coefficient and electron lifetime, resulting in an improvement in the light harvesting efficiency. Thus, quasi-solid-state polymer electrolyte dye-sensitized solar cells (DSSCs) with M-TiO ₂ beads showed a higher efficiency (4.8% at 100 mW cm ^-2) than those with conventional P25 (3.8%). A structure-property relation among M-TiO ₂ beads was investigated in terms of surface area and light scattering. Upon utilizing double layer structures and a solid polymerized ionic liquid (PIL), the efficiency was increased up to 6.7% at 100 mW cm ^-2, one of the highest values for all-solid-state DSSCs.