Researchers at Institute for Molecular Science and at Univer­sity of Toyama in Japan report that an efficient organic light emitting diode (OLED), operable by a 1.5-V battery, produces bright emission equivalent to luminance of a typical display. The OLED is based on upcon­version transition associated with triplet–triplet anni­hilation that doubles the energy of excited states.

Various properties of OLEDs have been optimized, including current to photon conversion effi­ciency. An external quantum effi­ciency exceeding 20 %, with internal quantum effi­ciency of 100 %, has been achieved owing to the development of thermally activated delayed fluorescent and phos­phorescent materials that manage the spin of excited states. However, the operating voltage of common types of OLEDs is very high; that of an OLED emitting approxi­mately 600-nm light at a luminance of 100 cd/m², which is a general display condition, is as high as 4.5 V.

The group of Seiichiro Izawa and Masahiro Hiramoto at Institute for Molecular Science in Japan, and Masahiro Morimoto and Shigeki Naka at University of Toyama in Japan report that an efficient OLED, operable by a 1.5-V battery, that produces bright emission equivalent to luminance of a typical display. The OLED has a smaller turn-on voltage at 0.97 V than the optical energy of emitted photons at 2.04 eV (608 nm), because the OLED is based on upconversion transition associated with triplet–triplet anni­hilation that doubles the energy of excited states.

The researchers reveal that the charac­teristics of charge transfer state at the interface are key to efficient upcon­version, and the percentage of excited states deactivated by parasitic loss processes during the upcon­version transition is significantly reduced from over 90 % to approxi­mately 10 % by introducing a highly crystalline acceptor material and an emissive dopant.

Consequently, the upconversion-OLED reaches 100 cd/m² at a voltage and emission wavelength of 1.33 V and 608 nm (2.04 eV), respec­tively. This is the lowest operating voltage reported for an OLED that achieves a luminance of 100 cd/m². It was further demons­trated that the UC-OLED can be operated by a 1.5-V battery to realize luminance of 177 cd/m². (Source: NINS)

Reference: S. Izawa et al.: Efficient Interfacial Upconversion Enabling Bright Emission at an Extremely Low Driving Voltage in Organic Light-Emitting Diodes, Adv. Opt. Mat. 2101710 (2022); DOI: 10.1002/adom.202101710

Link: Hiramoto Group, Institute for Molecular Science, National Institutes of Natural Sciences, Okazaki, Japan