OSA-Direct
Friday, 20 Oct 2017

Kyushu University develops world's first demo of persistent luminescence from organic materials

This is expected to create a new wave of glow-in-the-dark materials based on organics look poised to invigorate the area and expand their applications

Kyushu University - A circular film of glow-in-the-dark organic materials is shown

2 Oct 2017 | Editor

Researchers at Kyushu University's Center for Organic Photonics and Electronics Research (OPERA) have improved the flexibility and transparency of *glow-in-the-dark paints while also making them easier to manufacture and lowering their costs.

So in a groundbreaking demonstration, light emission lasting more than one hour was achieved from organic materials, which are also promising for unlocking new applications such as in bio-imaging.

Based on a process called persistent luminescence, glow-in-the-dark materials work by slowly releasing energy absorbed from ambient light. Used in watches and emergency signs, commercial glow-in-the-dark materials are based on inorganic compounds and include rare metals such as europium and dysprosium.

However, these materials are expensive, require high temperatures to manufacture, and scatter light—as opposed to being transparent—when ground into powders for paints.

Carbon-based organic materials—similar to those used in plastics and pigments—can overcome many of these disadvantages. They can be excellent emitters and are already widely used in organic light-emitting diodes (OLEDs). But achieving long-lived emission has been difficult, and the longest emission from organics under indoor lighting at room temperature was, until now, only a few minutes.

The researchers at Kyushu University's Center for Organic Photonics and Electronics Research (OPERA) have now broken through this limit using simple mixtures of two appropriate molecules. In films formed by melting together molecules that donate electrons and ones that accept electrons, emission lasting for over an hour was demonstrated for the first time from organic materials without the need for intense light sources or low temperatures.

In the mixtures, absorption of light by an electron-accepting molecule, or acceptor, gives the molecule extra energy that it can use to remove an electron from an electron-donating molecule, or donor. This transfer of an electron is effectively the same as a positive charge being transferred from the acceptor to the donor.

The extra electron on the acceptor can then hop to other acceptors and move away from the positively charged donor, resulting in separation of the charges. The separated charges gradually come back together—some slowly and some more quickly—and release their energy as light over the span of almost an hour.

The mixtures and processes are similar to what are found in organic solar cells and OLEDs. After building up separated charges like in a solar cell, the charges have nowhere to escape, so they eventually comeback together to emit light like an OLED. The key difference in the newly developed mixtures is that the charges can exist in a separated state for very long periods of times.

The first challenge to tackle on the road to practical use is the sensitivity of the process to oxygen and water. Protective barriers are already used in organic electronics and inorganic glow-in-the-dark materials, so the researchers are confident that a solution can be found. Concurrently, they are also looking into new molecular structures to increase the emission duration and efficiency as well as to change the colour.

With efforts to solve these remaining issues underway, a new wave of glow-in-the-dark materials based on organics look poised to invigorate the area and expand their applications

Kyushu University - Chihaya Adachi (left) and Ryota Kabe (right) of Kyushu University's Center for Organic Photonics and Electronics Research (OPERA)

Figure: Kyushu University - Chihaya Adachi (left) and Ryota Kabe (right) of Kyushu University's Center for Organic Photonics and Electronics Research (OPERA)

"Many organic materials can use energy absorbed from light to emit light of a different colour, but this emission is generally fast because the energy is stored directly on the molecule that produces the emission."


"By contrast, our mixtures store the energy in electrical charges separated over a longer distance. This additional step allows us to greatly slow down the release of the energy as light, thereby achieving the glow-in-the-dark effect."


Ryota Kabe, lead author on the paper

"With organics, we have a great opportunity to reduce the cost of glow-in-the-dark materials, so the first place we expect to see an impact is large-area applications, such as glowing corridors or roadways for added safety."


"After that, we can start thinking about exploiting the versatility of organic materials to develop glow-in-the-dark fabrics and windows, or even bio-compatible probes for medical imaging."


Chihaya Adachi, Director of OPERA.

Organic long persistent luminescence

Ryota Kabe | Chihaya Adachi

Nature (2017) | doi:10.1038/nature24010

Received 29 May 2017 | Accepted 07 August 2017 | Published online 02 October 2017

Long persistent luminescence (LPL) materials—widely commercialized as 'glow-in-the-dark' paints—store excitation energy in excited states that slowly release this energy as light. At present, most LPL materials are based on an inorganic system of strontium aluminium oxide (SrAl2O4) doped with europium and dysprosium, and exhibit emission for more than ten hours. However, this system requires rare elements and temperatures higher than 1,000 degrees Celsius during fabrication, and light scattering by SrAl2O4 powders limits the transparency of LPL paints. Here we show that an organic LPL (OLPL) system of two simple organic molecules that is free from rare elements and easy to fabricate can generate emission that lasts for more than one hour at room temperature. Previous organic systems, which were based on two-photon ionization, required high excitation intensities and low temperatures. By contrast, our OLPL system—which is based on emission from excited complexes (exciplexes) upon the recombination of long-lived charge-separated states—can be excited by a standard white LED light source and generate long emission even at temperatures above 100 degrees Celsius. This OLPL system is transparent, soluble, and potentially flexible and colour-tunable, opening new applications for LPL in large-area and flexible paints, biomarkers, fabrics, and windows. Moreover, the study of long-lived charge separation in this system should advance understanding of a wide variety of organic semiconductor devices.

www.kyushu-u.ac.jp    www.cstf.kyushu-u.ac.jp/~adachilab/opera/index_e.html   


About OPERA

Source: OPERA

About Kyushu University

Source: Kyushu University


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