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 > 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 (k_reg = 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.