Kanagawa Accademy of Science and Technology
Transparent Functional Materials Group

Brief summary

The transparent functional materials group is investigating new functional transparent materials based on the results of a previous project (Hasegawa Nano-magneto-optics Device Project, 2003–2008).
Groop Leader: Dr. Tetsuya Hasegawa (Prof. of University of Tokyo)
Investigation period: 2008-

Summary of the present project

1. Nb-doped TiO2

We found that anatase TiO2 doped with a few percent of Nb (TNO) has a low electrical resistivity of the order of 10–4 Ωcm and a high optical transparency in the visible light region. Such materials are known as transparent conductive oxides (TCOs) and are critical materials for optoelectronics. A widely used TCO, Sn-doped In2O3 (ITO), contains the rare element, indium. It is thus highly desirable to develop alternatives to ITO. We have succeeded in fabricating TNO by sputter deposition, which is widely used for industrial fabrication of TCOs. TNO exhibits unique properties, such as a high refractive index and a high chemical stability. We aim to develop new optoelectronic devices that exploit these properties. Our group is also studying other new materials such as SnO2 that is transparent in the near-infrared region and oxynitride thin films.

(left) Substrate and (right) TNO film on substrate. Resistance and appearance of TNO films on glass substrates.

2. Full-spectrum SnO2

In addition to visible light, sunlight also contains large amounts of invisible radiation that has longer wavelengths than visible light, namely infrared light. Solar cells that utilize infrared light (full-spectrum solar cells) have recently been attracting great interest for enhancing the conversion efficiencies of solar cells.
An important component of solar cells is a transparent electrode that introduces light inside a solar cell without optical loss. However, it is transparent only in the visible light region. Therefore, conventional TCOs are incompatible with full-spectrum solar cells.
We recently succeeded in fabricating SnO2 transparent conducting thin films that have high transparency in the near-infrared region.
To achieve transparency in the near-infrared region, it is necessary to increase the carrier mobility and reduce the carrier density. We grew pseudo-epitaxial SnO2 thin films on various seed layers by pulsed laser deposition. We found that an anatase TiO2 seed layer promotes a-axis orientation of SnO2, dramatically improving the mobility. The obtained film exhibits high transparency (>70%) in the near-infrared region up to wavelengths of 1850 nm.

Structure of SnO2 film with TiO2 seed layer (transmission electron microscopy image) Comparison of transparencies of SnO2 film and a conventional TCO (ITO)

3. Oxynitride (coming soon)