Hydrogen Could Provide Path to High-Efficiency OLEDs

A molecule that changes its chemical structure slightly before and after emission could provide a novel, stable approach to achieving high efficiency in OLEDs.

The mechanism explored by the researchers involves the reversible transfer of a hydrogen atom — i.e., the transfer of its positive nucleus — from one atom in the emitting molecule to another in the same molecule, to create an environment conducive to thermally activated delayed fluorescence (TADF).

Current molecular designs of TADF materials focus on combining donor and acceptor units. Researchers from Kyushu University’s Center for Organic Photonics and Electronics Research (OPERA) have presented a system based on the use of excited-state intramolecular proton transfer (ESIPT) to achieve efficient TADF without relying on the donor-acceptor scheme.

ESIPT was found to occur spontaneously when the emitting molecule was excited with optical or electrical energy. Quantum chemical calculations made by the researchers indicated that TADF was not possible before transfer of the hydrogen.

After the hydrogen transferred to a different atom in the molecule, this led to a molecular structure capable of TADF. The hydrogen transferred back to its initial atom after the molecule emitted light. The molecule was then ready to repeat the process.

ESIPT led to separation of the highest occupied and lowest unoccupied molecular orbitals, resulting in TADF emission with a photoluminescence quantum yield of nearly 60 percent. High external electroluminescence quantum efficiencies of up to 14 percent in OLEDs using this emitter indicate that efficient triplet harvesting is possible with ESIPT-based TADF materials.

Excited-state intramolecular proton transfer (ESIPT) makes possible OLEDs that are highly efficient by creating the necessary conditions to enable thermally activated delayed fluorescence (TADF). After excitation of the emitting molecule, a hydrogen atom — technically, just its nucleus — is transferred to a different atom in the same molecule through a process called ESIPT. The reconfigured molecule can then undergo TADF to convert a high fraction of the excitations into light. Following emission, the molecule returns to its original state. This mechanism increases the molecular design strategies available for the creation of novel and improved light-emitting materials. Courtesy of William J. Potscavage Jr.

Although TADF from an ESIPT molecule has been reported previously, according to the researchers this is the first demonstration of highly efficient TADF observed inside and outside of a device.

“Many new TADF molecules are being reported each month, but we keep seeing the same underlying design with electron-donating groups connected to electron-accepting groups,” said researcher Masashi Mamada.

“Finding fundamentally different molecular designs that also exhibit efficient TADF is a key to unlocking new properties, and in this case, we found one by looking at the past with a new perspective.”

The molecule used in the research was originally synthesized for use in the creation of a light-absorbing pigment.

"Organic molecules never cease to amaze me," said professor Chihaya Adachi. “Many paths with different advantages and disadvantages exist for achieving the same goal, and we have still only scratched the surface of what is possible.”

The work could expand and accelerate the development of a wide variety of TADF materials for high performance OLEDs.

The advantages of the design strategy are just beginning to be explored, but one particularly promising area is related to stability. Molecules similar to the one investigated are known to be highly resistant to degradation, so researchers hope that these kinds of molecules might help to improve the lifetime of OLEDs. To see if this is the case, tests are now underway.

The research was published in ACS Central Science (doi: 10.1021/acscentsci.7b00183).