Focus: Designing Better Materials for LEDs
Organic LEDs are similar to traditional, crystalline LEDs, but organics can be engineered to emit a specific color or to give flexibility to a display screen. To improve these LEDs, a research team reporting in Physical Review Letters has designed an organic molecule with the smallest measured energy difference between two low-lying excited states. This near zero energy gap leads to a boost in the light emitting efficiency, while also opening the door to other uses, such as sensors and organic lasers.
Organic light emitting diodes, or OLEDs, can be made cheaper, lighter, and more flexible than traditional LEDs, but OLEDs are often inefficient at converting electrical energy into photons. In a typical OLED, electrons and holes (the positive charge carriers) flow in opposite directions, essentially hopping from one molecule to the next. When an electron and a hole meet, they form an excited state, called an exciton, in which the two spins are either oppositely directed (spin , a “singlet”) or aligned (spin , a “triplet”). From spin statistics, the triplet state is three times more likely, but only the singlet can directly transition to the ground state through the release of a photon, in a process called fluorescence. By contrast, the triplet-to-ground transition, called phosphorescence, is much slower. In most cases, the triplet energy is lost to heat before any photon can be emitted.
Researchers have found ways to recover the triplet energy, for example, by adding metallic compounds (or complexes) that help drive phosphorescence, but the metals used, such as iridium, are often rare and expensive. So Chihaya Adachi of Kyushu University in Fukuoka, Japan, and his colleagues have been working over the past several years on “all-organic” OLED designs that take advantage of a process called thermally activated delayed fluorescence (TADF). This form of light emission occurs when the singlet and triplet states in a molecule are so close in energy that temperature fluctuations can drive transitions to the singlet state, where fluorescence can occur.
Adachi and his team have designed several organic molecules with small singlet-triplet gaps. This gap depends in part on the spatial arrangement of the molecular orbitals through which the electrons and holes travel. Designing a molecule with decreased overlap between the orbital that accepts electrons and the orbital that accepts holes reduces the singlet-triplet energy gap. Recently, the team synthesized a complex molecule with a gap of electron volts . When this molecule was incorporated into an OLED, the overall efficiency (converting electrical energy into light) was percent, which is nearly as high as the efficiencies of iridium-based OLEDs.
In their latest effort, the team has aimed to reduce the energy gap even further. They tinkered with a previously studied molecule, PIC-TRZ , which is a combination of triazine ( ), a hexagonal ring, and two indolocarbazole units, which each contain three hexagonal and two pentagonal rings. Using density-functional theory, the team located the electron-accepting orbital on the indolocarbazole units and the hole-accepting orbital on the triazine and then designed a new molecule—named PIC-TRZ2—with just one indolocarbazole unit. This simplification reduced the overlap between the orbitals. The researchers synthesized PIC-TRZ2 and then determined the singlet-triplet gap using temperature-dependent emission measurements. The observed gap was just electron volts, the smallest ever measured for an OLED molecule and less than the typical thermal energy of a molecule at room temperature, electron volts.
To determine OLED performance, Adachi’s team incorporated the PIC-TRZ2 molecule into different organic host materials and observed the light emission properties. After some optimization, they found efficiencies as high as percent. Even though this efficiency is lower than that of other TADF molecules, Adachi says that the PIC-TRZ2 result “is more meaningful from the aspect of molecular design.” The near zero gap could prove useful in oxygen sensors or organic semiconductor lasers, which require a short lifetime of the triplet state, he says.
“This is a breakthrough in the field,” says Franky So of the University of Florida in Gainesville. He believes the new work could lead to stable blue OLEDs, which are essential for displays but have thus far eluded developers. However, Andrew Monkman of Durham University in the UK believes the electronic structure of TADF emitters may be more complicated than previously assumed . “There are a lot of questions that when answered will change our understanding of organic emitters very profoundly,” Monkman says. But whether these new molecules will be useful in OLEDs, “we will have to wait and see.”
Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.
- H. Uoyama, K. Goushi, K. Shizu, H. Nomura, and C. Adachi, “Highly Efficient Organic Light-Emitting Diodes from Delayed Fluorescence,” Nature 492, 234 (2012)
- A. Endo, K. Sato, K. Yoshimura, T. Kai, A. Kawada, H. Miyazaki, and C. Adachi, “Efficient Up-Conversion of Triplet Excitons into a Singlet State and its Application for Organic Light Emitting Diodes,” Appl. Phys. Lett. 98, 083302 (2011)
- F. B. Dias, K. N. Bourdakos, V. Jankus, K. C. Moss, K. T. Kamtekar, V. Bhalla, J. Santos, M. R. Bryce, and A. P. Monkman, “Triplet Harvesting with 100% Efficiency by Way of Thermally Activated Delayed Fluorescence in Charge Transfer OLED Emitters,” Adv. Mater. (2013)