Facile synthesis of size-tunable mesoporous anatase TiO2 beads using a graft copolymer for quasi-solid and all-solid dye-sensitized solar cells

Abstract

Multi-functional mesoporous TiO2 (M-TiO2) 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 TiO2 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-TiO2 beads showed a high surface area (86.5 m2 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-TiO2 beads effectively offered better pore infiltration of the polymer electrolyte. Furthermore, the improved interconnectivity of M-TiO2 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-TiO2 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-TiO2 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.