Evidence for First-Order Charge Recombination in Dye-Sensitized Solar Cells

Charge recombination between electrons injected into TiO 2 (TiO 2 (e – )s) and acceptors at the dye-sensitized electrolyte interface have been quantified by measurement of the open-circuit photovoltage, V OC, as a function of the incident photon flux. Literature reports indicate that the order of the reaction with respect to TiO 2 (e – )s is less than unity, typically 0.5–0.85. Herein, an alternative model is proposed and tested that incorporates a characteristic temperature T 0 to model the density of acceptor states and photon flux-dependent TiO 2 (e – ) lifetimes to account for shorter lifetimes at higher concentrations. Tests of this model with standard dye-sensitized solar cells based on the ditetrabutylammonium salt of cis -Ru(dcb) 2 (NCS) 2 (N719, where dcb is 4,4′-(CO 2 H) 2 -2,2′-bipyridine) sensitizers in iodide/iodine acetonitrile electrolytes under 0.1–5 sun illumination revealed a reaction that is first-order in TiO 2 (e – )s with T 0 = 1150 K. The first-order reactivity is consistent with an underlying Ti IV/III redox reaction, and the kinetic data under 1 sun illumination suggests recombination to molecular iodine, I 2 . Other implications for solar energy conversion and quantitative analysis of dye-sensitized solar cells are discussed.