Charge Recombination in Dye-Sensitized Nanocrystalline TiO2Solar Cells
Charge recombination between dye-sensitized nanocrystalline TiO 2 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 SnO 2 /TiO 2 interface, ohmic losses in the SnO 2 and TiO 2, and an overpotential for the redox reaction at the Pt counter electrode. Treating the dye-coated TiO 2 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 TiO 2 . 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 × 10 -6 cm 2 /s) in the porous TiO 2 structure. The dependence of other cell parameters on the I 3 - concentration is also investigated.