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 (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– → 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+ → 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.