CHARGE RECOMBINATION IN DYE-SENSITIZED NANOCRYSTALLINE TIO2 SOLAR-CELLS

Charge recombination between dye-sensitized nanocrystalline TiO2 elect rodes and the I-3(-)/I- couple in nonaqueous solution is described. Th e sensitizer was [RuL(2)(NCS)(2)] (L = 2,2'-bipyridyl-4,4'-dicarboxyli c acid). An apparent inequality between the dark current and the recom bination current is ascribed to a voltage shift caused by a potential drop at the SnO2/TiO2 interface, ohmic losses in the SnO2 and TiO2, an d 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)) improve s 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 rad iant power of 100 mW/cm(2) (AM 1.5) with respect to the untreated elec trode. An analytical expression relating V-oc to the interfacial recom bination kinetics is derived, and its limitations are discussed. Analy sis of V-oc vs radiant power data with this expression indicates that the pyridine compounds may lower the back-electron-transfer rate const ant by 1-2 orders of magnitude. The pyridines are found to have no sig nificant effect on the recombination mechanism and kinetics of electro n injection from excited dye molecules to TiO2. Studies of the dye-cov ered 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 bac k-reaction. Mass-transport theory is applied to understand the depende nce 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 dependen ce of other cell parameters on the I(3)(-)concentration is also invest igated.