Mesoporous TiO₂ Bragg stack templated by graft copolymer for dye-sensitized solar cells.

Organized mesoporous TiO₂ Bragg stacks (om-TiO₂ BS) consisting of alternating high and low refractive index organized mesoporous TiO₂ (om-TiO₂) films were prepared to enhance dye loading, light harvesting, electron transport, and electrolyte pore-infiltration in dye-sensitized solar cells (DSSCs). The om-TiO₂ films were synthesized via a sol-gel reaction using amphiphilic graft copolymers consisting of poly(vinyl chloride) backbones and poly(oxyethylene methacrylate) side chains, i.e., PVC-g-POEM as templates. To generate high and low index films, the refractive index of om-TiO₂ film was tuned by controlling the grafting ratio of PVC-g-POEM via atomic transfer radical polymerization (ATRP). A polymerized ionic liquid (PIL)-based DSSC fabricated with a 1.2-μm-thick om-TiO₂ BS-based photoanode exhibited an efficiency of 4.3%, which is much higher than that of conventional DSSCs with a nanocrystalline TiO₂ layer (nc-TiO₂ layer) (1.7%). A PIL-based DSSC with a heterostructured photoanode consisting of 400-nm-thick organized mesoporous TiO₂ interfacial (om-TiO₂ IF) layer, 7-μm-thick nc-TiO₂, and 1.2-μm-thick om-TiO₂ BS as the bottom, middle and top layers, respectively, exhibited an excellent efficiency of 7.5%, which is much higher than that of nanocrystaline TiO₂ photoanode (3.5%).