Trifunctional TiO₂ Nanoparticles with Exposed {001} Facets as Additives in Cobalt-Based Porphyrin-Sensitized Solar Cells

In this study, highly mesoporous TiO₂ composite photoanodes composed of functional {001}-faceted TiO₂ nanoparticles (NPs) and commercially available 20 nm TiO₂ NPs are employed in efficient porphyrin-sensitized solar cells together with cobalt polypyridyl-based mediators. Large TiO₂ NPs (approximately 50 nm) with exposed {001} facets are prepared using a fast microwave-assisted hydrothermal (FMAH) method. These unique composite photoanodes favorably mitigate the aggregation of porphyrin on the surface of TiO₂ NPs and strongly facilitate the mass transport of cobalt-polypyridyl-based electrolytes in the mesoporous structure. Linear sweep voltammetry reveals that the transportation of Co(polypyridyl) redox is a diffusion-controlled process, which is highly dependent on the porosity of TiO₂ films. Electrochemical impedance spectroscopy confirms that the FMAH TiO₂ NPs effectively suppress the interfacial charge recombination toward [Co(bpy)₃]³⁺ because of their oxidative {001} facets. In an optimal condition of 40 wt% addition of FMAH TiO₂ NPs in the final formula, the power conversion efficiency of the dye-sensitized cells improves from 8.28% to 9.53% under AM1.5 (1 sun) conditions. A cost-efficient TiO₂ composite photoanode for YD2-oC8 dye-sensitized cells (DSCs) with a Co(polypyridyl) mediator is fabricated by blending exposed {001}-faceted TiO₂ nanoparticles (approximately 50 nm) with commercial 20 nm TiO₂. This photoanode simultaneously overcomes sensitizer aggregation, interfacial recombination, and ionic diffusion in cobalt-mediated, porphyrin-sensitized DSCs.