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

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


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.


Electroluminescence Lighting devices OLEDs Organic emitters Thermally activated delayed fluorescence 



Singlet-triplet energy gap


Acceptor moiety


Charge transfer




Donor moiety




Delayed fluorescence




External quantum efficiency


Excited-state intramolecular proton transfer


Förster resonance energy transfer


Highest occupied molecular orbital


Internal quantum efficiency


Intersystem crossing




Lowest unoccupied molecular orbital


Metal center


Metal-to-ligand charge transfer


Organic light-emitting diode


Prompt fluorescence




Phosphorescence-based organic light-emitting diode


Photoluminescence quantum yield




Reverse intersystem crossing


Lowest excited singlet state


Spin-orbit coupling


Lowest triplet excited state


Thermally activated delayed fluorescence


Triplet-triplet annihilation



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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