Understanding electron transfer control in water splitting dye-sensitized photoelectrochemical cells

The development of renewable energy resources and controlling materials at the level of electrons are pressing scientific challenges. Solar technologies aimed at generating chemical fuels, such as water splitting dye-sensitized photoelectrochemical cells (WS-DSPECs), address both of these challenges. Understanding how to control interfacial electron transfer (IET) at the fundamental level by engineering individual components is one promising avenue for improving the viability of WS-DSPECs. In particular, there are two strategies discussed herein for controlling IET: structural modification of the dye sensitizer and using conduction band tunable mixed metal oxide materials.

Studies were performed on a set of porphyrin dye sensitizers with varying linker and anchoring groups to assay the efficacy of these structural modifications. Using optical-pump THz-probe spectroscopy, we found that IET dynamics were largely independent of the anchoring group and increased linker length slowed down IET, suggesting a through-space IET mechanism. These results suggested that the choice of anchoring group should be based on chemical stability and synthetic convenience rather than perceived benefit to IET and illustrate some of the limitations of controlling IET with modifications to the chromophore.

However, little effort has been placed into improving the metal oxide substrates that act as the primary charge transport pathway and support in WS-DSPECs. Using nanoparticulate Sn_xTi_1-xO₂ (where 0≤x≤1) as a photoanode material alters both the composition of the conduction band (i.e., density of states) and affords approximately 560 mV of tunability in the conduction band minimum. IET dynamics in dye-sensitized Sn_xTi_1-xO₂ were characterized using ultrafast UV-Vis transient absorption and showed complex behavior that is not well described by an intuitive energetic argument. In addition to the potential to improve the performance of WS-DSPECs, these results provide fundamental insight into the key parameters affecting IET behavior at molecule-semiconductor interfaces.