Influence of TiO2 Particle Size on Dye-Sensitized Solar Cells Employing an Organic Sensitizer and a Cobalt(III/II) Redox Electrolyte

Dye-sensitized solar cells (DSSCs) are highly efficient and reliable photovoltaic devices that are based on nanostructured semiconductor photoelectrodes. From their inception in 1991, colloidal TiO2 nanoparticles (NPs) with the large surface area have manifested the highest performances and the particle size of around 20 nm is generally regarded as the optimized condition. However, though there have been reports on the influences of particle sizes in conventional DSSCs employing iodide redox electrolyte, the size effects in DSSCs with the state-of-the-art cobalt electrolyte have not been investigated. In this research, systematic analyses on DSSCs with cobalt electrolytes are carried out by using various sizes of NPs (20-30 nm), and the highest performance is obtained in the case of 30 nm sized TiO2 NPs, indicating that there is a reversed power conversion efficiency trend when compared with those with the iodide counterpart. Detailed investigations on various factors-light harvesting, charge injection, dye regeneration, and charge collection-reveal that TiO2 particles with a size range of 20-30 nm do not have a notable difference in charge injection, dye regeneration, and even in light-harvesting efficiency. It is experimentally verified that the superior charge collection property is the sole origin of the higher performance, suggesting that charge collection should be prioritized for designing nanostructured TiO2 photoelectrodes for DSSCs employing cobalt redox electrolytes.