A double layered TiO2 photoanode consisting of hierarchical flowers and nanoparticles for high-efficiency dye-sensitized solar cells

We report the innovative development of a double layered photoanode made of hierarchical TiO2 flowers (HTFs) as the overlayer and TiO2 nanoparticles (TNPs) as the underlayer, for dye-sensitized solar cells (DSSCs). They were prepared via a mild and simple one-step hydrothermal reaction of TiO2 nanoparticles/FTO glass substrate in an alkaline solution. The underlayer made of TNPs with a small size (20 nm in diameter) serves as a transparent photoanode for efficient dye adsorption. The overlayer consisting of HTFs (3-5 mu m in diameter) embedded by TiO2 nanosheets plays multiple roles in enhancing light-scattering and fast electron transport. DSSCs based on this novel double layered photoanode (5 mu m TNPs + 5 mu m HTFs) exhibit greater than 7.4% power conversion efficiency (PCE), which is higher than that of single layer TNP based photoanodes (6.59%) with similar thickness (similar to 10 mu m), and this is mainly attributed to the superior light scattering ability and fast electron transport of the former. Meanwhile, the thickness of the TNP underlayer has been optimized to further improve the PCE and an excellent PCE of over 9% has been achieved based on a 15 mu m TNP + a 5 mu m HTF double layered photoanode, accompanied by a short-circuit photocurrent density of 17.85 mA cm(-2), an open-circuit voltage of 763 mV and a fill factor of 0.67.