Dye-sensitized photoelectrochemical cells (DSPECs) have established themselves as the promising solar fuel systems due to the simple preparation and relatively low cost. In DSPECs the reaction of interest is water splitting catalysed by a molecular compound after light absorption by a co-working chromophore, both attached to the mesoporous semiconductor surface, e.g. TiO2 for photoanode . However, the variety of possible reactions can occur and different charge transfer pathways are present during the photocatalytic cycle depending on the conditions in the system, which makes the obtained results complicated to analyse. We looked on different mechanism of charge transfer that are possible when taking into consideration both photoactivity of TiO2 and attached ruthenium complexes (RuP sensitizer and water oxidation catalyst RuOEC). [1] The performance and stability of complete photoelectrochemical cells was compared when different photoanode-solvent interface conditions were maintained: acidic/neutral pH, the presence of proton acceptor bases and Nafion cover film. We optimised our water splitting systems by carefully choosing the proper electrolyte and found that the system with phosphate buffer (among other configurations like acetate buffer, hydrochloric acid, KCl solution, a mixture of hydrochloric acid with phosphate buffer and others) gave the highest photocurrent response. The destructive influence of the low pH environment was investigated, at which additional proton adsorption to the TiO2 induced by the presence of attached dyes and the chemical structure changes of ruthenium catalyst were observed. These phenomena strongly affected the measured photocurrent response. The presence of Nafion layer on the sensitized TiO2 photoanode was found to improve the photocurrent of the DSPEC system. We also investigated the photodynamics of the above mentioned standard ruthenium components in the solar cell configuration to compare the charge separation efficiency in water splitting systems to that in solar cells. [2] Enhanced fast back electron transfer from TiO2 to RuP and partial light absorption by RuOEC were found as possible factors lowering the efficiency of DSPEC. The samples were studied on time scales from femtoseconds to seconds by means of transient absorption, time-resolved emission and electrochemical measurements.

Acknowledgements
This work was supported by NCN (The National Science Centre, Poland) under project 2015/18/E/ST4/00196.

[1] F. Li, K. Fan, B. Xu, E. Gabrielsson, Q. Daniel, L. Li and L. Sun, J. Am. Chem. Soc., 2015, 137, 9153–9159.
[2] I. Grądzka, M. Gierszewski, J. Karolczak and M. Ziółek, Phys. Chem. Chem. Phys., 2018, 20, 7710–7720.