This research topics is based on the studies published in “Scientific Reports“on June 10, 2015.

Assist Prof. Masai, H.,¹ Program-Specific Assoc Prof. Yamada, Y.,² Prof Kanemitsu, Y.,³ et al.

¹Inorganic Photonics Materials, Division of Materials Chemistry
²Nano-Interface Photonics (SEI Group CSR Foundation)
³Photonic Elements Science, International Research Center for Elements Science

Although inorganic crystalline phosphors can exhibit high quantum efficiency, their use in phosphor films has been limited by a reliance on organic binders that have poor durability when exposed to high-power and/or high excitation energy light sources. To this end, the group reports on a method for producing Sn²⁺-doped transparent phosphor films through the heat-treatment and dip-coating of a precursor melt. The obtained monolithic inorganic amorphous film exhibits an internal quantum efficiency of over 60% and can potentially utilize transmitted light. The emitting color can be tuned by using the energy transfer process between Sn²⁺+ and Mn²⁺. It is therefore concluded that amorphous films containing such emission centers can provide a novel emitting films, which is quite different from conventional films consisting of crystalline phosphors and organic binders.

The high durability and good emissivity afforded by thin films of monolithic inorganic materials makes them an ideal alternative to the powdered phosphors presently used in light-emitting devices such as white LEDs or solar cells. In established techniques, the conventional wisdom is that the covering glass functions as a passive material, a notion we challenge by suggesting that they possess the potential to improve the device performance. Since this requires an active light conversion mechanism suitable for existing device structures, there is a clear need for a thin-film preparation method that can be adapted to large-area devices. In particular, monolithic inorganic (and ideally amorphous) materials possessing good emissivity will be required for industrial applications of large-area devices.
By using conventional sol-gel technique, both the flatness and transparency of the amorphous film were insufficient due to the evaporation of the solvent or water. Thus, in order to improve the film condition, our group have since focused on a solvent-free acid–base reaction between phosphoric acid and metal chloride. This method is demonstrated to be capable of producing films several microns in thickness in just one coating procedure, which is a notable improvement over the submicron thickness attained using conventional sol-gel techniques insofar as industrial applications are concerned.
Analysis of the emissivity confirms the feasibility of this method for large-area devices. In particular, the colour of the emitted light can be readily controlled by varying the amount of Mn²⁺. Furthermore, the absence of grain boundaries leads to an internal quantum efficiency that is comparable to that of a conventional MgWO₄ crystalline phosphor. Unlike the matrixes obtained using conventional techniques, however, the random matrix leads to broad emission that is attractive from both scientific and industrial perspectives. This study therefore provides a valuable benchmark for novel transparent and highly photoluminescent inorganic films produced using an industrially viable fabrication technique.

Figure 1. Photographs of Sn²⁺-doped amorphous phosphor film with and
without UV irradiation (left) and Sn²⁺-Mn²⁺ co-doped amorphous films (right).

This work was partially supported by the Yazaki Memorial Foundation for Science and Technology, the Asahi Glass Foundation, the Research Institute for Production Development, a Collaborative Research Program of ICR Kyoto University (Grant #2013-62, #2014-31), the SPRITS program of Kyoto University, and a Grant-in-Aid for Young Scientists (A; Number 26709048). The work was also supported by ICR Grants for Young Scientists, and the Sumitomo Electric Industries Group CSR Foundation (to Y.Y. and Y.K.).

Masai, H.; Miyata, H.; Yamada, Y.; Okumura, S.; Yanagida, T.; Kanemitsu, Y., Tin-Doped Inorganic Amorphous Films for Use as Transparent Monolithic Phosphors, Scientific Reports, 5, 11224 (2015).