Efficiency Considerations for SnO2-Based Dye-Sensitized Solar Cells

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 (k(inj) > 10(8) s(-1)) followed by sensitizer regeneration through iodide oxidation to yield an interfacial charge separated state abbreviated as MO2(e(-))vertical bar Ru + I-3(-). Spectral features associated with I-3(-) 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(-)) + I-3(-) -> 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 I-3(-) 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+ -> MO2 vertical bar-S, was about 2 orders of magnitude larger for SnO2 (k = 9.8 x 10(4) s(-1)) than for TiO2 (k = 1.6 x 10(3) s(-1)). Sensitizer regeneration through iodide oxidation were similar for both oxide materials (k(reg) = 6 +/- 1 x 10(10) 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.