Sae Youn Lee

Highly efficient blue electroluminescence based on thermally activated delayed fluorescence

Organic compounds that exhibit highly efficient, stable blue emission are required to realize inexpensive organic light-emitting diodes for future displays and lighting applications. Here, we define the design rules for increasing the electroluminescence efficiency of blue-emitting organic molecules that exhibit thermally activated delayed fluorescence. We show that a large delocalization of the highest occupied molecular orbital and lowest unoccupied molecular orbital in these charge-transfer compounds enhances the rate of radiative decay considerably by inducing a large oscillator strength even when there is a small overlap between the two wavefunctions. A compound based on our design principles exhibited a high rate of fluorescence decay and efficient up-conversion of triplet excitons into singlet excited states, leading to both photoluminescence and internal electroluminescence quantum yields of nearly 100%.

Luminous Butterflies: Efficient Exciton Harvesting by Benzophenone Derivatives for Full‐Color Delayed Fluorescence OLEDs

Butterfly-shaped luminescent benzophenone derivatives with small energy gaps between their singlet and triplet excited states are used to achieve efficient full-color delayed fluorescence. Organic light-emitting diodes (OLEDs) with these benzophenone derivatives doped in the emissive layer can generate electroluminescence ranging from blue to orange-red and white, with maximum external quantum efficiencies of up to 14.3%. Triplet excitons are efficiently harvested through delayed fluorescence channels.

High-efficiency organic light-emitting diodes utilizing thermally activated delayed fluorescence from triazine-based donor-acceptor hybrid molecules

We have designed and synthesized a high-efficiency purely organic luminescent material, 2,4-bis{3-(9 H-carbazol-9-yl)-9 H-carbazol-9-yl}-6-phenyl-1,3,5-triazine (CC2TA) comprising the bicarbazole donor and phenyltriazine acceptor units, which is capable of emitting thermally activated delayed fluorescence. The molecular design of CC2TA allows spatial separation of HOMO and LUMO on the donor and acceptor fragments, respectively, leading to an exceptionally small singlet–triplet exchange energy (ΔEST = 0.06 eV) together with a high triplet energy. Furthermore, a high external electroluminescence quantum efficiency as high as 11% ± 1% has been achieved in the sky-blue organic light-emitting diodes employing CC2TA as an emitter.

Nearly 100% Internal Quantum Efficiency in Undoped Electroluminescent Devices Employing Pure Organic Emitters

The design of efficient and concentration-insensitive metal-free thermally activateddelayed fluorescence (TADF) materials is reported. Blue and green organic light-emitting diodes (OLEDs) containing a hole-transport layer, an undoped TADF emissive layer, and an electron-transport layer achieve maximum external quantum efficiencies of 19%, which is comparable to the best doped OLEDs.

Thermally Activated Delayed Fluorescence Polymers for Efficient Solution-Processed Organic Light-Emitting Diodes.

Thermally activated delayed fluorescence (TADF) π-conjugated polymers are developed for solution-processed TADF-OLEDs. Benzophenone-based alternating donor-acceptor structures contribute to the small ∆EST , enabling efficient exciton-harvesting through TADF. Solution-processed OLEDs using the TADF polymers as emitters can achieve high maximum external electroluminescence efficiencies of up to 9.3%.

High-Efficiency Blue Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence from Phenoxaphosphine and Phenoxathiin Derivatives.

High-efficiency blue thermally activated delayed fluorescence (TADF) molecules, consisting of phenoxaphosphine oxide and phenoxathiin dioxide as acceptor units and 9,9-dimethylacridan as a donor unit, are reported. Maximum external electroluminescence quantum efficiencies of up to 20.5% are achieved in blue organic light-emitting diodes (OLEDs) by employing these materials as TADF emitters.

A six-carbazole-decorated cyclophosphazene as a host with high triplet energy to realize efficient delayed-fluorescence OLEDs

A high triplet energy (ET) host material, hexakis(9H-carbazol-9-yl)cyclotriphosphazene (PzCz), is used in high-efficiency organic light-emitting diodes (OLEDs). PzCz (ET = 3.00 eV) functions as an effective host for thermally activated delayed fluorescence (TADF) molecules. Highest external electroluminescence quantum efficiencies over 15% and 18% are achieved for blue-green and green TADF-OLEDs, respectively.

Blue Oleds: High-Efficiency Blue Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence from Phenoxaphosphine and Phenoxathiin Derivatives (Adv. Mater. 23/2016)

High-performance blue thermally activated delayed fluorescence (TADF) emitters containing a phenoxaphosphine oxide or phenoxathiin dioxide acceptor unit coupled with a dimethylacridan donor unit are developed by T. Yasuda and co-workers, as desribed on page 4626. These emitters can allow efficient up-conversion of triplet excitons into singlet excitons, leading to both photoluminescence and internal electroluminescence quantum efficiencies of up to nearly 100%.