Bimodal porous TiO₂ structures templated by graft copolymer/homopolymer blend for dye-sensitized solar cells with polymer electrolyte

Bimodal porous TiO₂ (BP-TiO₂) with large surface area, high porosity, good interconnectivity, and excellent light-scattering ability are synthesized via a facile one-step method using a self-assembled blend template consisting of an amphiphilic poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM) graft copolymer and a hydrophobic poly(vinyl chloride) (PVC) homopolymer. The hydrophilically surface-modified TiO₂ nanoparticles selectively interact with the hydrophilic POEM chains, while the addition of the PVC homopolymer increases the hydrophobic domain size, resulting in the formation of dual pores (i.e., macropores and mesopores). The sizes and numbers of macropores can easily be controlled by changing the molecular weight and amount of the PVC homopolymer. The polymer electrolyte dye-sensitized solar cells (DSSCs) fabricated with BP-TiO₂ photoanodes exhibited energy conversion efficiencies of up to 7.6% at 100 mW cm⁻², which is much higher than those of mesoporous TiO₂ (5.8%) with PVC-g-POEM only and conventional nanocrystalline TiO₂ (4.9%) with commercial Dyesol paste. The enhanced energy conversion efficiencies mostly resulted from the light-scattering effects of the macropores, which increased the light-harvesting efficiencies. The improved light-harvesting and photovoltaic performances of the DSSCs were characterized by UV–vis spectroscopy, incident photon-to-current conversion efficiency analysis, electrochemical impedance spectroscopy, intensity-modulated photocurrent spectroscopy, and intensity-modulated photovoltage spectroscopy.