Developed An Organic EL That Emits Light With A Single Dry Cell With The World's Lowest Voltage (Masahiro Hiramoto Group Et Al.)

• Successfully developed an organic EL that can emit light with the brightness of a display with the electromotive force of one dry cell with the world's lowest voltage.
• By controlling the interaction between organic molecules responsible for the luminescence process at the interface and doping with a fluorescent dye, the luminous efficiency is improved by about 70 times compared to conventional devices with similar structures, and high luminescence at low voltage is achieved. Achieved brightness.
• If this new technology is used, the voltage that drives commercially available organic EL can be significantly reduced, which may lead to energy saving.

The research group of Assistant Professor Seiichiro Izawa, Professor Masahiro Hiramoto, Associate Professor Katsuhiro Morimoto, and Professor Shigeki Naka of Toyama University has the world's lowest voltage that can emit light with the brightness of a display just by connecting one dry battery. We have succeeded in developing a driving organic EL.

Since organic EL has high image quality, it is already on the market in smartphones and large-screen TVs, but its high drive voltage is an issue for energy saving. This time, the research group used a process called up-conversion¹ at the interface between the two types of organic semiconductor materials that are responsible for light emission, and by improving its efficiency, the voltage of one dry cell is as bright as a display. We have succeeded in developing an organic EL that is driven by the world's smallest voltage that can emit light. The developed organic EL element starts emitting light from 1 V or less, which is a voltage in which orange light is much smaller than the energy of the light. Furthermore, deactivation is suppressed by controlling the interaction between organic molecules at the interface, and light emission is promoted by doping with a fluorescent dye, which is about 70 times higher than the devices using up-conversion reported in the past. Achieved luminous efficiency. As a result, it is possible to achieve both low voltage and high luminous efficiency, and the luminous brightness is about the same as that of a display just by connecting one dry battery, which is about 1/3 of the electromotive force for driving a general organic EL element. We have succeeded in developing an organic EL element driven by the world's smallest voltage that can emit light with a brightness of cd / m² or more. With this achievement, the drive voltage of commercially available organic EL can be significantly reduced, which may lead to the realization of energy saving.

This research is a scientific research fund grant project (young research, basic research (C), academic transformation area research (A)), strategic creative research promotion project (PRESTO), nanotechnology platform program, Mazda Foundation research grant, Kao Art. It was published as part of the Research Grant of the Science Foundation and the Research Grant of the Konica Minolta Science and Technology Foundation, and was published on January 6 (Japan time, online version) of the international academic journal "Advanced Optical Materials" published by Wiley-VCH. I did.

Research background
Since organic EL can project high-quality images, it is used in smartphones and large-screen TVs and is already on the market. Furthermore, it is a surface-emitting light source, and it is easy to form a film on a flexible thin film, so it is expected to be used as next-generation lighting.

As for the luminous efficiency of organic EL, the internal quantum yield has already reached 100% because a mechanism has been developed to efficiently emit excitons generated by charge injection. On the other hand, due to the need for multi-layering and the low mobility of electric charges in the material, the problem is that the drive voltage is large, which is an issue for energy saving. For example, in order to emit orange light of about 600 nm at 100 cd / m², which is about the brightness of a display, a voltage of about 4.5 V (three dry batteries) is required.

Research results
This time, the research group used a process called up-conversion at the interface between two types of organic semiconductor materials, and by improving its efficiency, it has the world's lowest voltage that can emit light with the same brightness as a display with the voltage of one dry cell. We have succeeded in developing a driving organic EL.

The luminescence process begins when the injected electrons and holes meet and recombine at the interface between the electron transport layer and the hole transport / light emitting layer. After that, the two triplet excited states generated by recombination collide and emit light through a process called up-conversion that creates one high-energy singlet excited state.

An organic EL device with a structure in which a normal light emitting layer is sandwiched between electron / hole transport layers starts light emission from about 3.5 V. On the other hand, in the organic EL element using the up-conversion process at this interface developed this time, the orange light of 608 nm (2.04 electron volt²) is a voltage much smaller than the energy of the light¹. It was found that light emission starts from V or less. Furthermore, by using highly crystalline peryleneimide instead of fullerene in the electron transport layer, the interaction between organic molecules at the interface was controlled and deactivation was suppressed, and by doping the light emitting layer with a perylene fluorescent substance. By promoting light emission, the light emission brightness is greatly improved, and the luminous efficiency is about 70 times higher than that of the organic EL element using the conventional up-conversion process. As a result, an organic EL element driven by the world's smallest voltage that can emit light with a brightness of 100 cd / m² or more, which is about the brightness of a display, just by connecting one dry cell with an electromotive force of about 1/3 of the conventional one. Was successfully developed.

Future development - Social significance of this research
Through this research, we were able to significantly reduce the drive voltage that causes organic EL to emit light to about 1/3 of the conventional level. In the future, by further improving the conversion efficiency of the light emitting process that has undergone the up-conversion process, both the reduction of the drive voltage of the organic EL and the luminous efficiency will be achieved at a higher level, and the power consumption of the commercially available organic EL will be reduced to save energy. We aim to realize it.

Research group
Institute for Molecular Science, University of Toyama

Research support
Scientific Research Grant Program Young Researcher (18K14115), Fundamental Research C (19K04465), Academic Transformation Area Research A (21H05411), Strategic Creative Research Promotion Project PRESTO (JPMJPR2101), Nanotechnology Platform Program (JPMXP09S21MS0029), Mazda Foundation Research Grant, Kao Arts and Science Foundation Research Grant, Konica Minolta Science and Technology Promotion Foundation Research Grant

¹ Up-conversion
A process in which two excitons collide and one high-energy exciton is generated to increase the energy of light.

² Electronvolt
A unit of energy. One electron volt is the energy gained when one charge accelerates at a voltage of 1 V. Generally, in order to emit light with an energy of 1 electron volt, it is necessary to apply a voltage of 1 V or more to the light emitting element.