Charge recombination in dye-sensitized nanocrystalline TiO2 solar cells

Charge recombination between dye-sensitized nanocrystalline TiO2 electrodes and the I-3(-)/I- couple in nonaqueous solution is described. The sensitizer was [RuL(2)(NCS)(2)] (L = 2,2'-bipyridyl-4,4'-dicarboxylic acid). An apparent inequality between the dark current and the recombination current is ascribed to a voltage shift caused by a potential drop at the SnO2/TiO2 interface, ohmic losses in the SnO2 and TiO2, and an overpotential for the redox reaction at the Pt counter electrode. Treating the dye-coated TiO2 electrodes with pyridine derivatives (4-tert-butylpyridine, 2-vinylpyridine, or poly(2-vinylpyridine)) improves significantly both the open-circuit photovoltage V-oc (from 0.57 to 0.73 V) and the cell conversion efficiency (from 5.8 to 7.5%) at a radiant power of 100 mW/cm(2) (AM 1.5) with respect to the untreated electrode. An analytical expression relating V-oc to the interfacial recombination kinetics is derived, and its limitations are discussed. Analysis of V-oc vs radiant power data with this expression indicates that the pyridine compounds may lower the back-electron-transfer rate constant by 1-2 orders of magnitude. The pyridines are found to have no significant effect on the recombination mechanism and kinetics of electron injection from excited dye molecules to TiO2. Studies of the dye-covered electrodes show that the rate of recombination is second order in I-3(-) concentration, which is attributed to the dismutation reaction 2I(2)(-) --> I-3(-) + I- with I-2 as the electron acceptor in the back-reaction. Mass-transport theory is applied to understand the dependence of the short-circuit photocurrent on the radiant power at low I-3(-) concentration and to calculate the diffusion coefficient of I-3(-) ions (7.6 x 10(-6) cm(2)/s) in the porous TiO2 structure. The dependence of other cell parameters on the I(3)(-)concentration is also investigated.