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Thermally Activated Delayed Fluorescence Emitters for Light-Emitting Diodes and Sensing Applications

  • João Avó
  • Tiago Palmeira
  • Fernando B. DiasEmail author
Chapter
Part of the Springer Series on Fluorescence book series (SS FLUOR, volume 18)

Abstract

Thermally activated delayed fluorescence (TADF) has revamped the scientific and technological interest in metal-free organic fluorescent compounds in recent years. The application of TADF emitters in organic light-emitting diodes (OLEDs) resulted in highly energy-efficient devices that promise to replace metal-complex systems based on iridium(III) and platinum(II) in a near future.

Three quarters of the excitons that are created by the electrical current driving an OLED are non-emissive triplet states, therefore unable to generate electroluminescence. The maximum device efficiency is thus limited to 25%. OLED emitters based on metal complexes respond to this problem by promoting emission directly from the triplet state, which is induced by the presence of the heavy metal that enhances spin-orbit coupling interactions. Remarkably, OLEDs with internal quantum efficiency of nearly 100% have been fabricated with metal complexes, owning to the fast intersystem crossing (ISC) and room-temperature phosphorescent properties of these materials. However, while the heavy-metal complexes have many advantages, they also show significant problems when applied in light-emitting diodes. These are scarce and expensive materials that create environmental challenges and are affected by strong degradation in the blue spectral region. These issues, therefore, may create difficulties for the utilization of metal complexes in areas that require high-volume manufacturing, such as in lighting and display technologies, and alternative materials free of heavy metals are needed. TADF molecules allow for efficient triplet harvesting with no use of heavy-metal atoms and appear to improve device stability in the blue region. In addition, they display interesting properties that grant sensitivity to several parameters of the surrounding media, making it an ideal tool for optical sensing applications. TADF research toward application in lighting devices started in 2012 and had not yet entered in commercial applications, as of mid-2018. This chapter covers the principles governing the mechanism behind the TADF process, the recent developments on TADF emitter design, and their planned applications in commercial devices.

Keywords

Electroluminescence Lighting devices OLEDs Organic emitters Thermally activated delayed fluorescence 

Abbreviations

ΔEST

Singlet-triplet energy gap

A

Acceptor moiety

CT

Charge transfer

Cz

Carbazole

D

Donor moiety

DBTO2

Dibenzothiophene-S,S-dioxide

DF

Delayed fluorescence

DPSO2

Diphenylsulfoxide

EQE

External quantum efficiency

ESIPT

Excited-state intramolecular proton transfer

FRET

Förster resonance energy transfer

HOMO

Highest occupied molecular orbital

IQE

Internal quantum efficiency

ISC

Intersystem crossing

L

Ligand

LUMO

Lowest unoccupied molecular orbital

M

Metal center

MLCT

Metal-to-ligand charge transfer

OLED

Organic light-emitting diode

PF

Prompt fluorescence

PH

Phosphorescence

PhOLED

Phosphorescence-based organic light-emitting diode

PLQY

Photoluminescence quantum yield

PN

Phthalonitrile

RISC

Reverse intersystem crossing

S1

Lowest excited singlet state

SOC

Spin-orbit coupling

T1

Lowest triplet excited state

TADF

Thermally activated delayed fluorescence

TTA

Triplet-triplet annihilation

Notes

Acknowledgements

Fundação para a Ciência e a Tecnologia (FCT) is acknowledged for funding fellowships SFRH/BPD/120599/2016 (JA) and SFRH/BD/118525/2016 (TP) and project PTDC/QUIQFI/32007/2017.

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© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.CQFM-IN and iBB-Institute for Bioengineering and Biosciences, Instituto Superior Técnico, Universidade de LisboaLisboaPortugal
  2. 2.Department of PhysicsDurham UniversityDurhamUK

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