Boosting the efficiency of aqueous solar cells by TiCl4 treatment

Aqueous electrolytes represent the new frontier of dye-sensitized solar cells (DSSCs), as they guarantee sustainability, safety and durability at the same time. After the initial concerns on the possibility of replacing nitrile-based organic solvents by water in the electrolyte, today the number of publications focused on the development of highly efficient aqueous DSSCs is rapidly increasing. Post-treatment of the TiO2 photoelectrode that leads to the creation of an extra nanolayer of TiO2 is a well exploited method to improve the performance of standard DSSCs and here, to our knowledge, investigated for the first time in water-based systems. In such a scenario, this work deals with the thorough understanding of the electrochemical and photoelectrochemical effects of the TiCl4 treatment onto TiO2 electrodes, a well-known process for traditional dye-sensitized solar cells. Hence, we propose here a thorough investigation of the procedure (in terms of experimental parameters) for TiCl4 treatment and its effects on the efficiency and long-term stability of laboratory scale assembled cells, thus finally demonstrating why and in which circumstances the TiCl4 treatment is able to greatly increase the performance of these emerging aqueous photovoltaic devices. Regardless of the sensitization process of the photoanode and the composition of the aqueous electrolyte, the sunlight conversion efficiency increased by over 120% when compared to the untreated solar cell counterparts. The electrochemical and photoelectrochemical investigation showed that both the photocurrent and the potential increased as a result of the TiCl4 treatment, and the decrease of the recombination phenomena at the electrode/electrolyte interface is evidenced by an increase of about 40% of the lifetime of photogenerated electrons. It is worth noting that 100% water-based solar cells can be reproducibly fabricated with efficiencies close to 2.5%, without any additive in the electrolyte nor redox pairs based on heavy metals like cobalt. This work represents a solid benchmark for future studies on these truly sustainable novel photoelectrochemical cells, which are under intense investigation from the scientific community worldwide