Synergistic Effect of Alkyl Chain Barriers on Heteroleptic Ruthenium Dyes and Co3+/2+ Complex Mediators for Reduced Charge Recombination in Dye-Sensitized Solar Cells

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The performance of photoelectrochemical solar cells using heteroleptic Ru dyes paired with Co 3+/2+ electrolytes is worse than their organic-dye-based analogues. Previous studies point to the importance of minimizing intermolecular interactions between the Ru dyes and the Co 3+/2+ complexes by bulky groups on the dyes or the redox mediators. However, the performance boosting additive 4- tert -butylpyridine (t-BPy) is often added to the electrolyte for electron lifetime studies, which masks the intrinsic effect of the molecular structure of the redox-active molecules. Here, electron lifetime studies including four cobalt mediators decorated with alkyl chains and two Ru dyes with/without nonyl chains have been performed in the absence and presence of t-BPy. A synergistic effect of alkyl chain substitution on Ru dyes and Co 3+/2+ complex mediators is revealed in the absence of t-BPy. The electron lifetime increases dramatically when both the dyes and the mediators are decorated with alkyl chains. The synergistic effect is explained by the size of the free space between the dyes compared to the size of the redox mediators. Adding alkyl chains to the Ru dyes decreases the free space between the dyes, so that the bulky Co 3+/2+ complexes cannot approach the TiO 2 surface, increasing the recombination distance. Although alkyl chains successfully diminish intermolecular interactions leading to higher charge separation efficiency and increased electron lifetime, the fill factor is limited by the slow diffusion of the bulky mediators, especially in the presence of t-BPy. The exceptionally long electron lifetime achieved due to the synergistic effect of alkyl chain barriers paves the way for t-BPy free photoelectrochemical solar cells.