Center for Organic Photonics and Electronics Research

Nishi, Japan

Center for Organic Photonics and Electronics Research

Nishi, Japan
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News Article | May 1, 2017
Site: www.rdmag.com

New research could make lasers emitting a wide range of colors more accessible and open new applications from communications and sensing to displays. Researchers at Kyushu University's Center for Organic Photonics and Electronics Research (OPERA) reported an optically pumped organic thin-film laser that can continuously emit light for 30 ms, which is more than 100 times longer than previous devices. Unlike the inorganic lasers commonly found in CD drives and laser pointers, organic thin-film lasers use a thin layer of organic molecules as the laser medium, which is the material in the device that actually produces lasing by emitting and amplifying light when excited with an energy source. In this case, the energy source was intense ultraviolet light from an inorganic laser. A very promising feature of organic thin-film lasers is the possibility to more easily achieve colors that are difficult with inorganic lasers. By designing and synthesizing molecules with new structures, emission of any color of the rainbow is possible. "People have been studying organic thin-film lasers for a long time, but degradation and loss processes have greatly limited the duration of emission," says Atula S. D. Sandanayaka, lead author of the paper in Science Advances reporting the new results. The researchers were able to reduce these problems and extend the duration of the lasing by combining three strategies. To reduce major losses originating from the absorption of laser emission by packets of energy - called triplet excitons - that build up in the organic laser medium during operation, the researchers found an organic laser medium with triplet excitons that absorb a different color of light than that emitted by the laser. Thermal degradation caused by heating of the lasers during operation was reduced by building the devices on a crystalline silicon wafer and gluing a piece of sapphire glass on top of the organic laser medium with a special polymer. The silicon and sapphire, which are good heat conductors, help to quickly remove heat from the devices while at the same time encapsulating them. Finally, through optimization of a frequently used grating structure - called a mixed-order distributed feedback structure - placed under the organic laser medium to provide optical feedback, the input energy needed to operate the lasers was reduced to new lows, further lessening the heating. "These devices really operate at the extreme, so we have to keep finding new tricks to eliminate any inefficiencies and prevent the devices from burning themselves out," says Professor Chihaya Adachi, director of OPERA. Using these simple devices in conjunction with inorganic lasers is promising for more easily achieving colors that are difficult to produce using common lasers, with applications in spectroscopy, communications, displays, and sensors. Development is still ongoing to sustain the emission for even longer durations, but as for what is next? "Our ultimate goal is realizing organic thin-film lasers that directly use electricity as the energy source, and this is an important step in that direction," says Adachi.


News Article | April 28, 2017
Site: phys.org

Researchers at Kyushu University's Center for Organic Photonics and Electronics Research (OPERA) reported an optically pumped organic thin-film laser that can continuously emit light for 30 ms, which is more than 100 times longer than previous devices. Unlike the inorganic lasers commonly found in CD drives and laser pointers, organic thin-film lasers use a thin layer of organic molecules as the laser medium, which is the material in the device that actually produces lasing by emitting and amplifying light when excited with an energy source. In this case, the energy source was intense ultraviolet light from an inorganic laser. A very promising feature of organic thin-film lasers is the possibility to more easily achieve colors that are difficult with inorganic lasers. By designing and synthesizing molecules with new structures, emission of any color of the rainbow is possible. "People have been studying organic thin-film lasers for a long time, but degradation and loss processes have greatly limited the duration of emission," says Atula S. D. Sandanayaka, lead author of the paper in Science Advances reporting the new results. The researchers were able to reduce these problems and extend the duration of the lasing by combining three strategies. To reduce major losses originating from the absorption of laser emission by packets of energy - called triplet excitons - that build up in the organic laser medium during operation, the researchers found an organic laser medium with triplet excitons that absorb a different color of light than that emitted by the laser. Thermal degradation caused by heating of the lasers during operation was reduced by building the devices on a crystalline silicon wafer and gluing a piece of sapphire glass on top of the organic laser medium with a special polymer. The silicon and sapphire, which are good heat conductors, help to quickly remove heat from the devices while at the same time encapsulating them. Finally, through optimization of a frequently used grating structure - called a mixed-order distributed feedback structure - placed under the organic laser medium to provide optical feedback, the input energy needed to operate the lasers was reduced to new lows, further lessening the heating. "These devices really operate at the extreme, so we have to keep finding new tricks to eliminate any inefficiencies and prevent the devices from burning themselves out," says Professor Chihaya Adachi, director of OPERA. Using these simple devices in conjunction with inorganic lasers is promising for more easily achieving colors that are difficult to produce using common lasers, with applications in spectroscopy, communications, displays, and sensors. Development is still ongoing to sustain the emission for even longer durations, but as for what is next? "Our ultimate goal is realizing organic thin-film lasers that directly use electricity as the energy source, and this is an important step in that direction," says Adachi. Explore further: Researchers present new solution for miniaturized organic lasers More information: "Towards continuous-wave operation of organic semiconductor lasers," Atula S. D. Sandanayaka et al., Science Advances 3, e1602570 (2017). DOI: 10.1126/sciadv.1602570


Tanaka H.,Center for Organic Photonics and Electronics Research | Shizu K.,Center for Organic Photonics and Electronics Research | Adachi C.,Center for Organic Photonics and Electronics Research | Adachi C.,Kyushu University
Journal of Physical Chemistry C | Year: 2015

Radiationless transition between the lowest singlet (S1) and triplet (T1) excited states in the thermally activated delayed fluorescence (TADF) were investigated with respect to molecular design. The photophysical, transient photoluminescence and electroluminescence (EL) characteristics of two chalcogenodiazole-containing TADF emitters were compared. These contained 1,3,4-oxadiazole or 1,3,4-thiadiazole. The effect of substituting oxygen with sulfur on TADF was caused by an electron-pair-accepting conjugative effect. This effect resulted from the vacant 3d-orbitals of divalent sulfur in the thiadiazole heteroring. Atom substitution narrowed the gap between the highest occupied and lowest unoccupied molecular orbital energy levels, and enhanced S1 → T1 intersystem crossing. These effects resulted from the enhanced acceptor strength and orbital angular momentum by the vacant 3d-orbitals of sulfur. Atom substitution increased the contribution of the delayed fluorescence component to the total EL efficiency (65.1% → 78.0%). This resulted from enhanced reverse intersystem crossing, because of the reduced energy gap between S1 and T1. © 2015 American Chemical Society.


News Article | December 28, 2016
Site: www.eurekalert.org

Reproducibility is a necessity for science but has often eluded researchers studying the lifetime of organic light-emitting diodes (OLEDs). Recent research from Japan sheds new light on why: impurities present in the vacuum chamber during fabrication but in amounts so small that they are easily overlooked. Organic light-emitting diodes use a stack of organic layers to convert electricity into light, and these organic layers are most commonly fabricated by heating source materials in vacuum to evaporate and deposit them onto a lower temperature substrate. While issues affecting the efficiency of OLEDs are already well understood, a complete picture of exactly how and why OLEDs degrade and lose brightness over time is still missing. Complicating matters is that devices fabricated with seemingly the same procedures and conditions but by different research groups often degrade at vastly different rates even when the initial performance is the same. Unable to attribute these reproducibility issues to known sources such as the amount of residual water in the chamber and the purity of the starting materials, a report published online in Scientific Reports on December 13, 2016, adds a new piece to the puzzle by focusing on the analysis of the environment in the vacuum chamber. "Although we often idealize vacuums as being clean environments, we detected many impurities floating in the vacuum even when the deposition chamber is at room temperature," says lead author Hiroshi Fujimoto, chief researcher at Fukuoka i3-Center for Organic Photonics and Electronics Research (i3-OPERA) and visiting associate professor of Kyushu University. Because of these impurities in the deposition chamber, the researchers found that the time until an OLED under operation dims by a given amount because of degradation, known as the lifetime, sharply increased for OLEDs that spent a shorter time in the deposition chamber during fabrication. This trend remained even after considering changes in residual water and source material purity, indicating the importance of controlling and minimizing the device fabrication time, a rarely discussed parameter. Research partners at Sumika Chemical Analysis Service Ltd. (SCAS) confirmed an increase of accumulated impurities with time by analyzing the materials that deposited on extremely clean silicon wafers that were stored in the deposition chamber when OLED materials were not being evaporated. Using a technique called liquid chromatography-mass spectrometry, the researchers found that many of the impurities could be traced to previously deposited materials and plasticizers from the vacuum chamber components. "Really small amounts of these impurities get incorporated into the fabricated devices and are causing large changes in the lifetime," says Professor Chihaya Adachi, director of Kyushu University's Center for Organic Photonics and Electronics Research (OPERA), which also took part in the study. In fact, the new results suggest that the impurities amount to less than even a single molecular layer. To improve lifetime reproducibility, a practice often adopted in industry is the use of dedicated deposition chambers for specific materials, but this can be difficult in academic labs, where often only a limited number of deposition systems are available for testing a wide variety of new materials. In these cases, deposition chamber design and cleaning in addition to control of the deposition time are especially important. "This is an excellent reminder of just how careful we need to be to do good, reproducible science," comments Professor Adachi. For more information, see "Influence of vacuum chamber impurities on the lifetime of organic light-emitting diodes," Scientific Reports 6, 38482 (2016); doi: 10.1038/srep38482. This work was performed by research groups at Kyushu University's Center for Organic Photonics and Electronics Research (OPERA), the Fukuoka i3-Center for Organic Photonics and Electronics Research (i3-OPERA), and the Institute of System, Information Technology and Nanotechnology (ISIT) in cooperation with Sumika Chemical Analysis Service Ltd. (SCAS). This research is ongoing in part under the Adachi Molecular Exciton Engineering Project funded by the Exploratory Research for Advanced Technology (ERATO) program of the Japan Science and Technology Agency (JST).


News Article | December 28, 2016
Site: phys.org

Organic light-emitting diodes use a stack of organic layers to convert electricity into light, and these organic layers are most commonly fabricated by heating source materials in vacuum to evaporate and deposit them onto a lower temperature substrate. While issues affecting the efficiency of OLEDs are already well understood, a complete picture of exactly how and why OLEDs degrade and lose brightness over time is still missing. Complicating matters is that devices fabricated with seemingly the same procedures and conditions but by different research groups often degrade at vastly different rates even when the initial performance is the same. Unable to attribute these reproducibility issues to known sources such as the amount of residual water in the chamber and the purity of the starting materials, a report published online in Scientific Reports on December 13, 2016, adds a new piece to the puzzle by focusing on the analysis of the environment in the vacuum chamber. "Although we often idealize vacuums as being clean environments, we detected many impurities floating in the vacuum even when the deposition chamber is at room temperature," says lead author Hiroshi Fujimoto, chief researcher at Fukuoka i3-Center for Organic Photonics and Electronics Research (i3-OPERA) and visiting associate professor of Kyushu University. Because of these impurities in the deposition chamber, the researchers found that the time until an OLED under operation dims by a given amount because of degradation, known as the lifetime, sharply increased for OLEDs that spent a shorter time in the deposition chamber during fabrication. This trend remained even after considering changes in residual water and source material purity, indicating the importance of controlling and minimizing the device fabrication time, a rarely discussed parameter. Research partners at Sumika Chemical Analysis Service Ltd. (SCAS) confirmed an increase of accumulated impurities with time by analyzing the materials that deposited on extremely clean silicon wafers that were stored in the deposition chamber when OLED materials were not being evaporated. Using a technique called liquid chromatography-mass spectrometry, the researchers found that many of the impurities could be traced to previously deposited materials and plasticizers from the vacuum chamber components. "Really small amounts of these impurities get incorporated into the fabricated devices and are causing large changes in the lifetime," says Professor Chihaya Adachi, director of Kyushu University's Center for Organic Photonics and Electronics Research (OPERA), which also took part in the study. In fact, the new results suggest that the impurities amount to less than even a single molecular layer. To improve lifetime reproducibility, a practice often adopted in industry is the use of dedicated deposition chambers for specific materials, but this can be difficult in academic labs, where often only a limited number of deposition systems are available for testing a wide variety of new materials. In these cases, deposition chamber design and cleaning in addition to control of the deposition time are especially important. "This is an excellent reminder of just how careful we need to be to do good, reproducible science," comments Professor Adachi. Explore further: Lifetime breakthrough promising for low-cost and efficient OLED displays and lights More information: Hiroshi Fujimoto et al, Influence of vacuum chamber impurities on the lifetime of organic light-emitting diodes, Scientific Reports (2016). DOI: 10.1038/srep38482

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