Functions of self-assembled ultrafine TiO₂ nanocrystals for high efficient dye-sensitized solar cells

In this paper, we demonstrate a simple approach of self-assembled process to form a very smooth and compacted TiO₂ underlayer film from ultrafine titanium oxide (TiO₂) nanocrystals with dimension of 4 nm for improving the electrical properties and device performances of dye-sensitized solar cells (DSSCs). Because the TiO₂ film self-assembles by simply casting the TiO₂ on fluorine-doped tin oxide (FTO) substrate, it can save a lot of materials in the process. As compared with control DSSC without the self-assembled TiO₂ (SA-TiO ₂) layer, short-circuit current density (J_sc) improves from 14.9 mA/cm² for control DSSC to 17.3 mA/cm² for masked DSSC with the SA-TiO₂ layer. With the very smooth SA-TiO ₂ layer, the power conversion efficiency is enhanced from 8.22% (control) to 9.35% for the DSSCs with mask and from 9.79% (control) to 11.87% for the DSSCs without mask. To explain the improvement, we have studied the optical properties, morphology, and workfunction of the SA-TiO₂ layer on FTO substrate as well as the impedance spectrum of DSSCs. Importantly, we find that the SA-TiO₂ layers have better morphology, uniformity, and contact with FTO electrode, increased workfunction and optical transmission, as well as reduced charge recombination at the contact of FTO substrate contributing to the improved device performances. Consequently, our results show that the simple self-assembly of TiO₂ ultrafine nanocrystals forms a very good electron extraction layer with both improved optical and electrical properties for enhancing performances of DSSCs. © 2014 American Chemical Society.