Towards Conduction Band Mediated Multi-electron Transfer: Spectroscopic Studies of Photosensitizer-semiconductor-acceptor Assemblies

Licentiate thesis, 2018

Converting solar energy into a storable form in chemical bonds, i.e., generating solar fuels, is a promising strategy for producing renewable fuels. This however comes with several challenges that need to be solved before generation of solar fuels can become a reality. One major challenge that has been the focus of this thesis is to achieve multiple electron transfer in solar fuel assemblies. Multiple electron transfer is a requirement for solar fuel forming reactions, and for that long-lived charge separation is typically necessary. In order to achieve long-lived charge separation and multiple electron transfer, hybrid systems consisting of dye-sensitized semiconductors together with molecular catalysts have been designed and studied in this work. Two main designs have been prepared, patterned combinations of two different semiconductors and co-sensitized TiO2 films with dye and catalyst molecules co-attached to the same film. In the patterned films, a pattern of TiO2 was deposited on a substrate of SnO2. By dye-sensitization of only the TiO2-SnO2 parts, the charge separation lifetime was increased by a factor of 55 compared to evenly dye-sensitized films due to the created energy barrier for back-electron transfer. Furthermore, conduction band mediated electron transfer to a catalyst attached to the SnO2 areas was demonstrated. In the co-sensitized films, the charge separation lifetime between the dye and TiO2 was increased by one order of magnitude for samples with the highest concentration of catalyst. By tuning the relative ratio of dye and catalyst to a large excess of dye, efficient conduction band mediated two-electron transfer was demonstrated by visible light excitation of the dye.