Extraordinary Field Enhancement of TiO 2 Porous Layer up to 500‐Fold

Titanium dioxide (TiO 2 ) is known as a very important material for photocatalysts, the photoelectrode for hydrogen evolution reaction, and the porous layer of perovskite solar cells. Their properties as well as device performances such as photoconversion efficiencies are significantly enhanced if integration of TiO 2 providing a large field enhancement of light is made possible. However, the field enhancement has not been revealed for TiO 2 materials, and then increased device performances have not been reported either so far. Here, extraordinary field enhancement of a porous TiO 2 layer, mesoscopic film, prepared by a facile wet process in conjunction with ball milling and drop casting is shown. The field enhancement is investigated with respect to the fluorescence intensity of a dye molecule and an enhancement factor (EF) of up to 500 is achieved, which corresponds to the largest EF for a semiconductor. Furthermore, EF is up to 30 000 after numerical corrections. The large EF is realized for a porous TiO 2 layer composed of a specific particle size of 550 nm, which is consistent with the results of fluorescence intensity, scattering intensity, and two different theoretical calculations based on Mie scattering theory, with respect to particle size. A special advantage of the current system is the mesoscopic porous layer giving many hot spots among TiO 2 particles of the specific size.