Dye Aggregation Effect on Interfacial Electron-Transfer Dynamics in Zinc Phthalocyanine-Sensitized Solar Cells

Aggregation of adsorbed dye molecules on TiO2 electrode typically decreases the yield of photoinduced charge separation at the dye/TiO2 interface. The decreased yield could be caused by the alternations of energy levels and/or adsorption geometry of sensitizers by the aggregation. We investigated the origin of the decreased yield for the aggregated sensitizers by employing zinc phthalocyanine-sensitized TiO2 electrode in redox-containing electrolytes. The degree of aggregation was controlled by the amount of coadsorbent, the addition of bulky molecular unit to phthalocyanine cores, and the alternation of the adsorption angle by changing the position of adsorption site. Femtosecond transient absorption measurements showed that injection yield was not significantly influenced by the aggregation but by dye adsorption angle and by the amount of dye. On the other hand, aggregation induced subnanosecond charge recombination, and the recombination seemed independent of the adsorption angle. These results appear to be not consistent with an interpretation where flat adsorption geometry enhances fast recombination. Here we interpreted the results with the dye-adsorption-density-dependent tunneling barrier height.