Enhancing the performance of solid-state dye-sensitized solar cells using a mesoporous interfacial titania layer with a bragg stack

High efficiency dye-sensitized solar cells (DSSCs) are fabricated with a heterostructured photoanode that consists of a 500-nm-thick organized mesoporous TiO₂ (om-TiO₂) interfacial layer (IF layer), a 7 or 10-μm thick nanocrystalline TiO₂ layer (NC layer), and a 2-μm-thick mesoporous Bragg stack (meso-BS layer) as the bottom, middle and top layers, respectively. An om-TiO₂ layer with a high porosity, transmittance, and interconnectivity is prepared via a sol-gel process, in which a poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM) graft copolymer is used as a structure-directing agent. The meso-BS layer with large pores is prepared via alternating deposition of om-TiO₂ and colloidal SiO₂ (col-SiO₂) layers. Structure and optical properties (refractive index) of the om-TiO₂ and meso-BS layers are studied and the morphology of the heterostructured photoanode is characterized. DSSCs fabricated with the heterostructured IF/NC/BS photoanode and combined with a polymerized ionic liquid (PIL) exhibit an energy conversion efficiencies of 6.6% at 100 mW/cm², one of the highest reported for solid-state DSSCs and much larger than cells prepared with only a IF/NC layer (6.0%) or a NC layer (4.5%). Improvements in energy conversion efficiency are attributed to the combination of improved light harvesting, decreased resistance at the electrode/electrolyte interface, and excellent electrolyte infiltration.