Abstract

We designed a highly uniform core double-shell (CDS) structure consisting of -NaYF₄:Er³⁺,Yb³⁺ crystal as core (~400 nm in diameter and ~470 nm in height), amorphous SiO₂ as inner shell (~10 nm in thickness), and interconnected anatase TiO₂ grains as outer shell (~30 nm in thickness). For device application, various amount of these hexagonal submicroprisms (HSMs) were introduced into TiO₂nanoparticles (20-40 nm in diameter) to form a bifunctional nano-submicron composite (NSMC) layer upon a prior prepared transparent TiO₂ layer. By utilization of these efficient -NaYF₄:Er³⁺,Yb³⁺ upconversion cores, near-infrared irradiation can be absorbed and harvested indirectly by dye molecules to broaden the absorption region and produce more electrons. Owing to the excellent insulating properties, SiO₂ shell creates a perfect electrical isolation for the UCP cores. Through this method, the problem of electron trapping and capture caused by ligands and defects on the surface of NaYF₄:Er³⁺,Yb³⁺ crystals can be thoroughly overcome. However, employment of this submicron sized composites can seriously reduce the internal surface area, leading to a less amount of dye loading. By coating of the outer TiO₂ shell, the disadvantage of SiO₂ shell can be avoided to the utmost extent. Computer simulation results have shown that scattering of 10-25 nm TiO₂particles is negligible and effective Mie scatterers are those particles whose dimensions are comparable to the wavelength of light. With the increasing of mixing amount of these submicron sized HSMs, these bifunctional layers become white gradually, indicating their significant scattering effect of incident light. It is experimentally damostrated that performance of DSCs can be effectively enhanced by employing these bifunctional highly uniform core/double-shell(CDS) structure in the photoelectrodes, an efficiency of 8.65% was obtained, which is 120% higher than the device based on bare NaYF⁴:Er³⁺,Yb³⁺ employed photoelectrode.

© 2013 Optical Society of America