Two-step sol-gel method-based TiO₂ nanoparticles with uniform morphology and size for efficient photo-energy conversion devices

The performance of dye-sensitized solar cells (DSSCs) consisting of anatase TiO₂ nanoparticles that were synthesized via a two-step sol-gel process was investigated using electron transport and optical characterizations. Spherical nanoparticles with the average diameter of 20 nm, elongated nanorods with an aspect ratio (AR) of 5, and nanowires with AR = 10 were synthesized. The synthesized nanoparticles possess narrow size distribution, high crystallinity, and negligible surface defects and residual organics, which is very suitable for achieving highly efficient photovoltaic devices. The effect of particle size distribution on the performance of DSSC was characterized by comparing the synthesized TiO₂ nanoparticles and commercial TiO₂ nanoparticles (P25). In comparison with P25, the two-step sol-gel-grown nanoparticles significantly improved the photovoltaic conversion efficiency by 32.5%, because of a larger specific surface area, higher electrolyte penetration ability, and lower optical reflectance. Therefore, the photoelectrode of the two-step sol-gel-derived TiO₂ nanoparticles enhanced the adsorption of dye sensitizers (N719), promotes the transfer of photogenerated carriers, and decreases the ratio of reflected solar spectrum that is not harnessed. As a result, the energy conversion efficiency of DSSCs increased to 6.72% without the use of a scattering layer and coadsorbants. We also investigated the effect of aspect ratio of TiO₂ particles on photovoltaic characteristic. An increase in the aspect ratio of the synthesized nanomaterials resulted in an increase in carrier lifetime. A decrease in the density of grain boundaries suppresses the trapping of carriers and the subsequent recombination of electron-hole pairs. This study demonstrates that the two-step sol-gel-derived nanomaterials provide a way to achieve appreciable efficiency of photoconversion devices.