Quantifying regeneration in dye-sensitized solar cells

We propose and apply a new experimental pro-tocol for determining the kinetics of the oxidation of iodide (aka dye regeneration) in dye sensitized solar cells (DSSCs) using measurements on full cells under operating conditions. Transient absorption (TA) decays of the oxidized dye after a laser pulse are correlated with the short circuit current (J_sc) and electron concentration in the TiO₂, measured during the TA experiment. Results for a series of cells with N719 [(Bu₄N)₂][Ru(dcbpyH) ₂(NCS)₂] (dcbpy = 4,4′-dicarboxy-2,2′- bipyridyl), a standard electrolyte, and varying iodide concentration allow us to fit for the fundamental regeneration rate constant (k_rg = 7.8 × 10⁵ M^-1 s^-1) and to determine that the order in iodide, in these cells, is near 1 (0.98 ± 0.16) and clearly not 2. The rate and order allow us to rank various reaction mechanisms and to discuss possible rate-limiting steps that could be catalyzed to improve regeneration. The method can also give estimates of the fundamental rate constant for electron dye recombination (EDR) and the apparent reaction order in total electron concentration.