Electrostatic blocking barrier as an effective strategy to inhibit electron recombination in DSSCs

The concept of negatively charged electrostatic blocking barrier is demonstrated using benzotriazole (btzH) as ancillary ligands in a new ruthenium-dicarboxybipyridine photosensitizer, [Ru(dcbpyH)2(btzH)2], which was deprotonated in a stepwise manner generating the (Bu4N)2[Ru(dcbpy)2(btzH)2], (Bu4N)3[Ru(dcbpy)2(btzH)(btz)] and (Bu4N)4[Ru(dcbpy)2(btz)2] acid-base conjugates. All compounds were isolated as solids and used for the preparation of DSSCs with structurally similar TiO2/dye interface but distinct electric charge properties. The results revealed that the negative charges at the TiO2 surface, generated by deprotonation of the dcbpyH ligands, are less effective than those generated by deprotonation of the ancillary ligands located much farther from the surface. This peripheral electrostatic field probably is more effective in inhibiting the approach of the negatively charged hole transporter I3−, and consequently the electron recombination, thus enhancing the overall cell performance. This hypothesis was confirmed by measuring the interfacial resistance and capacitance of running cells, and simulating the respective impedance spectroscopy data using the transmission line model.