Cation control of energetics on dye-sensitized nanocrystalline TiO2 for solar cells

Regenerative solar cells based on nanocrystalline TiO₂ (anatase) and the dye Ru(deeb)(bpy)₂(PF₆)₂, where deeb is 4,4′-(CO₂CH₂CH₃)2-2,2 ′-bipyridine and bpy is 2,2′-bipyridine, have increased efficiency when in the presence of a high concentration of cations with a large charge-to-radius ratio. Concentration-dependent photoluminescence (PL) quenching and increased quantum yield for interfacial charge separation have been explored for mono- and divalent cations by absorbance, time-resolved and steady-state PL. Cation adsorption stabilizes TiO₂ acceptor states resulting in energetically favorable electron transfer from the dye into the semiconductor conduction band. Quenching of the PL of excited states is reversible.; A new luminescence approach for sensing alkali and alkaline earth metal cations utilizes the surface-adsorption/desorption induced energetic shifts of a semiconductor conduction band to alter the electron transfer quenching efficiency of a photoluminescent dye such as Ru(deeb)(bpy)₂(PF ₆)₂ anchored to TiO₂ nanoparticles. This approach yields intensity, lifetime, and wavelength-ratiometric calcium ion sensors that are sensitive to 5 × 10−4 M concentrations.; In situ photoluminescence of a regenerative solar cell has been demonstrated as a probe of injection and efficiencies. The smaller the alkali cation, the higher the photocurrent and the more quenched the photoluminescence. The extent of quenching in 0.1 M iodide/0.01 M iodine electrolytes was 10-fold with LiI and 3-fold with NaI. A millimolar threshold concentration is observed for Li+ at which point a red shift in absorbance and photoluminescence spectra concomitant with significant static and dynamic quenching occurs. For Na+, the threshold concentration for observable red shift is more than an order of magnitude higher than for Li+.; Cation adsorption was also observed on planar TiO₂ surfaces in the absence of dye. The flat band potentials of single crystal TiO ₂ (rutile) with cations in propylene carbonate and protons in H ₂O were quantified by Mott-Schottky analysis of capacitance data. A difference of 230 mV is reported between extracted flat band potentials of Li+ and TBA+. The flat band potential for Li + is 150 mV more positive than for Na+. Nearly Nernstian behavior was observed on single crystal TiO₂, with measured flat band shifts of 51 mV/pH unit, and on thermally oxidized titanium with shifts of 69 mV/pH unit.