Understanding the mechanism of interfacial charge recombination has been a pivotal issue in optimizing DSSCs for some time, giving rise to theoretical and experimental investigations. This work adds to the body of knowledge by investigating the rate law by adapting the Ostwald isolation method, a kinetic tool typically used for solution chemistry. The experimental results are discussed in terms of several theoretical models of the recombination reaction. Experimental conditions for the utilization of Ostwald isolation at a TiO2-molecular sensitizer interface were established, allowing rigorous determination of the reaction order of both electrons, abbrev. TiO2(e-), and surface-anchored oxidized sensitizers, abbrev. S+, in the TiO2(e-)|S+? TiO2|S charge recombination reaction. A known, constant number of S+ or TiO2(e-), was maintained by an external bias. Pulsed laser excitation resulted in injection of electrons into TiO2 and the subsequent equilibration through interfacial charge recombination was quantified spectroscopically. The data provide compelling evidence for the rate law rate = k[S+]1[TiO2(e-)]1 where k = 5.0 x 10-16 cm3 s-1 with excess S+. The analogous data with excess TiO2(e-) were more complex. To account for the data, a pre-equilibrium diffusional encounter of TiO2(e-) and S+ is incorporated into the proposed recombination mechanism. Along with previously reported results, these data strongly suggests that TiO2(e-) diffusion limits recombination when the [TiO2(e-)] is small, but under conditions of higher [TiO2(e-)] interfacial electron transfer predominates.

Determining the reaction order for TiO2(e-)|S+? TiO2|S charge recombination at sensitized TiO2 interfaces with Ostwald Isolation

248th National Meeting (2014)

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