ABSTRACT
A comparative study of mesoporous thin films based on SnO2 (rutile) and TiO2 (anatase) nanocrystallites sensitized to visible light with [Ru(dtb)2(dcb)](PF6)2, where dtb = 4,4'-(tert-butyl)2-2,2'-bipyridine and dcb = 4,4'-(CO2H)2-2,2'-bipyridine, in CH3CN electrolyte solutions is reported to identify the reason(s) for the low efficiency of SnO2-based dye-sensitized solar cells (DSSCs). Pulsed laser excitation resulted in rapid excited state injection (kinj > 108 s-1) followed by sensitizer regeneration through iodide oxidation to yield an interfacial charge separated state abbreviated as MO2(e-)|Ru + I3-. Spectral features associated with I3- and the injected electron MO2(e-) were observed as well as a hypsochromic shift of the metal-to-ligand charge-transfer absorption of the sensitizer attributed to an electric field. The field magnitude ranged from 0.008 to 0.39 MV/cm and was dependent on the electrolyte cation (Mg2+ or Li+) as well as the oxide material. Average MO2(e-) + I3- â recombination rate constants quantified spectroscopically were about 25 times smaller for SnO2 (6.0 ± 0.14 s-1) than for TiO2 (160 ± 10 s-1). Transient photovoltage measurements of operational DSSCs indicated a 78 ms lifetime for electrons injected into SnO2 compared to 27 ms for TiO2; behavior that is at odds with the view that recombination with I3- underlies the low efficiencies of nanocrystalline SnO2-based DSSCs. In contrast, the average rate constant for charge recombination with the oxidized sensitizer, MO2(e-)|-S+ â MO2|-S, was about 2 orders of magnitude larger for SnO2 (k = 9.8 × 104 s-1) than for TiO2 (k = 1.6 × 103 s-1). Sensitizer regeneration through iodide oxidation were similar for both oxide materials (kreg = 6 ± 1 × 1010 M-1 s-1). The data indicate that enhanced efficiency from SnO2-based DSSCs can be achieved by identifying alternative redox mediators that enable rapid sensitizer regeneration and by inhibiting recombination of the injected electron with the oxidized sensitizer.
ABSTRACT
A study of SnO2/TiO2 core/shell films was undertaken to investigate the influences of shell thickness and post deposition sintering on electron localization and transport properties. Electrochemical reduction of the materials resulted in the appearance of a broad visible-near IR absorbance that provided insights into the electronic state(s) within the core/shell structures. As the shell thickness was increased from 0.5 to 5 nm, evidence for the presence of a SnxTi1-xO2 interfacial state emerged that was physically located between the core and the shell. The lifetime of photoinjected electrons increased with the shell thickness. Electron transport occurred through the SnO2 core; however, when materials with shell thicknesses ≥2 nm were annealed at 450 °C, a new electron transport pathway through the shell was evident. The data indicate that these materials are best described as SnO2/SnxTi1-xO2/TiO2 where electrons preferentially localize in a SnxTi1-xO2 interfacial state and transport through SnO2 and annealed TiO2 (if present). The implications of these results for applications in solar energy conversion are discussed.