Efficiency Considerations for SnO-Based Dye-Sensitized Solar Cells.

A comparative study of mesoporous thin films based on SnO (rutile) and TiO (anatase) nanocrystallites sensitized to visible light with [Ru(dtb)(dcb)](PF), where dtb = 4,4'-(-butyl)-2,2'-bipyridine and dcb = 4,4'-(COH)-2,2'-bipyridine, in CHCN electrolyte solutions is reported to identify the reason(s) for the low efficiency of SnO-based dye-sensitized solar cells (DSSCs). Pulsed laser excitation resulted in rapid excited state injection ( > 10 s) followed by sensitizer regeneration through iodide oxidation to yield an interfacial charge separated state abbreviated as MO(e)|Ru + I. Spectral features associated with I and the injected electron MO(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 (Mg or Li) as well as the oxide material. Average MO(e) + I → recombination rate constants quantified spectroscopically were about 25 times smaller for SnO (6.0 ± 0.14 s) than for TiO (160 ± 10 s). Transient photovoltage measurements of operational DSSCs indicated a 78 ms lifetime for electrons injected into SnO compared to 27 ms for TiO; behavior that is at odds with the view that recombination with I underlies the low efficiencies of nanocrystalline SnO-based DSSCs. In contrast, the average rate constant for charge recombination with the oxidized sensitizer, MO(e)|-S → MO|-S, was about 2 orders of magnitude larger for SnO ( = 9.8 × 10 s) than for TiO ( = 1.6 × 10 s). Sensitizer regeneration through iodide oxidation were similar for both oxide materials ( = 6 ± 1 × 10 M s). The data indicate that enhanced efficiency from SnO-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.