Thermally Activated Delayed Fluorescence Materials: Towards Realization of High Efficiency through Strategic Small Molecular Design

Abstract

Thermally activated delayed fluorescence (TADF) is one of the most intriguing and promising discoveries towards realization of highly‐efficient organic light emitting diodes (OLED) utilizing small molecules as emitters. It has the capability of manifesting all excitons generated during the electroluminescent processes, consequently achieving 100 % of internal quantum efficiency. Since the report of the first efficient OLED based on a TADF small molecule in 2012 by Adachi et al., the quest for optimal TADF materials for OLED application has never stopped. Various TADF molecules bearing different design concepts and strategies have been designed and produced, with the aim to boost the overall performances of corresponding OLEDs. In this minireview, the general principles of TADF molecular design based on three basic categories of TADF species: twisted intramolecular charge transfer (TICT), through‐space charge transfer (TSCT) and multi‐resonance induced TADF (MR‐TADF) are discussed in detail. Several key aspects with respect to each category, as well as some effective methods to enhance the efficiency of TADF materials and corresponding OLEDs from the molecular engineering perspectives, are summarized and discussed to exhibit a general landscape of TADF molecular design to a wide variety of scientific researchers within this particular disciplinary area.

Electron‐Rich Twistacene‐Modified Arylboron Donor–Acceptor Systems: Synthesis, Photophysics, and Electroluminescence with Hot Exciton Response

“…At present, OLEDsb ased on metal-free thermallya ctivated delayed fluorescence (TADF) materials have received growingi nterest for their possibility to generate 100 %i nternal quantum efficiency by the up-conversionf rom the nonradiative triplet states to the radiatives inglets tates. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Moreover,t he performance of certain TADF materials could compare with those of the phosphorescent OLEDs,a long with low costs and abun-dant resources. As the core building block of the desired compounds is responsible for the optoelectronic properties and molecular stability,t he rational designa nd functionalization of the core building blocks are of great importance.…”

Bipyridine‐Containing Host Materials for High Performance Yellow Thermally Activated Delayed Fluorescence‐Based Organic Light Emitting Diodes with Very Low Efficiency Roll‐Off

“…TADF emitters can harvest singlet excitons and triplet excitons simultaneously resulting in a theoretical maximum internal quantum efficiency (IQE) of 100% and very attractive external quantum efficiency (EQE) . In the past seven years there have been hundreds of TADF emitters reported, most of which based on a donor‐phenyl bridge‐acceptor design paradigm . As TADF emitters usually have long emission lifetimes originating from triplet excitons, OLED efficiency, stability, and efficiency roll‐off can all be adversely affected by a range of deactivation mechanisms including: triplet‐triplet annihilation (TTA), triplet‐polaron quenching (TPQ), and singlet‐triplet annihilation (STA) .…”

Design Strategy of Decorating Phenylcarbazole with a Donor and Acceptor for Blue‐Shifted Emission in Thermally Activated Delayed Fluorescent Emitters

“…Thermally activated delayedf luorescent( TADF) emitters are promisinga sn ext-generation emitterst or eplace low efficiency fluorescent emittersb yf ully harvesting singlet and triplet excitons. [1][2][3][4] In particular,b lue TADF organic light-emitting diodes (OLEDs)a re in demand becausec urrentO LED products use low efficiency fluorescentemitters. [5,6] Severalp ublications already reported high externalq uantum efficiency (EQE) and deep-blue emissionc olor in blue TADF OLEDs,a lthough the highest EQE of blue TADF OLEDs was recorded for sky-blueT ADF devices.…”