25 April 2016 Review of organic light-emitting diodes with thermally activated delayed fluorescence emitters for energy-efficient sustainable light sources and displays
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Thermally activated delayed fluorescence (TADF) is an emerging hot topic. Even though this photophysical mechanism itself has been described more than 50 years ago and optoelectronic devices with organic matter have been studied, improved, and even commercialized for decades now, the realization of the potential of TADF organic light-emitting diodes (OLEDs) happened only recently. TADF has been proven to be an attractive and very efficient alternative for phosphorescent materials, such as dopants in OLEDs, light-emitting electrochemical cells as well as potent emitters for chemiluminescence. In this review, the TADF concept is introduced in terms that are also understandable for nonchemists. The basic concepts behind this mechanism as well as state-of-the-art examples are discussed. In addition, the future economic impact, especially for the lighting and display market, is addressed here. We conclude that TADF materials are especially helpful to realize efficient, durable deep blue and white displays.




Thermally Activated Delayed Fluorescence as an Emerging Trend in Optoelectronics

The transformation of electrical energy into visible light has remained an important issue for scientists and engineers, even more than a century after the development of the incandescent lightbulb by Swan, Edison, and others.1,2 One of the new emerging topics in this niche of material sciences is emitting materials that feature thermally activated delayed fluorescence (TADF). Although TADF has been known since 1960,3 the potential use of TADF emitters in optoelectronic devices has been proposed independently by several groups led by Adachi et al.45.6 (organic and tin emitters), Deaton et al.,7 Yersin et al.,8 and others.910.11.12 The rise of the TADF-topic is reflected by an exploding number of new peer-reviewed publications dealing with it in general (Fig. 1), peaking close to 200 in 2015. Note that Refs. 48, which propose and show the use of such materials in organic light-emitting diodes (OLEDs) for the first time, have been published between 2009 and 2012, suggesting that the rise of this concept was directly related to a potential (industrial) application in OLEDs. Occasionally, synonyms such as “singlet harvesting”1314.15.16 or “E-type fluorescence”7,17,18 are also being used in the literature.

Fig. 1

TADF-related publications per year according to Scifinder as of February 2016.


The aim of this review article is to briefly introduce this concept and outline its impact on photonics, especially with respect to its potential concerning the realization of energy-efficient solid-state lighting and display devices. The scope of this article does not include any in-depth discussion of the molecular structure of TADF molecules or device architectures, which can be found elsewhere.1920.21.22


What Is Thermally Activated Delayed Fluorescence?

Upon recombination of charge carriers in any electroluminescent device, two different types of excitons are formed: singlet excitons having a spin S=0 and triplet excitons with S=1.23 Due to quantum statistics, the relative probability for the formation of singlet and triplet excitons is 25% to 75%.13,24,25 In order to realize efficient OLEDs, it is crucial to harvest both singlet and triplet excitons for the generation of photons. Currently, there are three main emissive mechanisms available to harvest excitons: fluorescence, phosphorescence as well as TADF. A schematic overview of these mechanisms is shown in Fig. 2.

Fig. 2

Schematic comparison between fluorescence, phosphorescence, and TADF. Straight arrows represent emissive transitions, while dotted arrows indicate up- or down-conversion processes (ISC and rISC).


On a side note, there are also more complex schemes, which usually require at least a bimolecular, often trimolecular emitting mechanism (e.g., multiple species between which energy can be transferred) such as P-type delayed fluorescence or triplet–triplet annihilation,2627.28.29 hyperfluorescence,30 exciplex emission,6 and exciplex-assisted emission.3132.33.34 These mechanisms will not be covered in this article.

Fluorescent molecules have a relatively large energetic difference between the S1 and T1 state. This difference is also known as the singlet–triplet splitting or ΔE (ST).13 In addition, they usually show very weak intersystem crossing (ISC) and reverse intersystem crossing (rISC), meaning that the transformation of singlet (S1) into triplet excitons (T1) and vice versa is quantum-mechanically forbidden, therefore both slow and unlikely.23 The same is true for any transitions between T1 and the singlet ground state S0.35,36 Because of these limitations, triplet excitons cannot be harvested with fluorescence, limiting the overall efficiency of such OLEDs to 25%.13 An archetypical fluorescent OLED emitter is aluminum-tris-(8-hydroxyquinoline) (Alq3; Fig. 3).3738.39.40

Fig. 3

Molecular structure of Alq3, Irppy3, and 4CzIPN.


Phosphorescent molecules41 share a rather larger ΔE (ST) value with their fluorescent counterparts. On the other hand, ISC is very strong, leading to a prompt transformation of any singlet into triplet excitons. Because of the ISC, which is the result of the presence of heavy transition metals such as iridium, platinum, and osmium, the transition between T1 and S0 is possible.35,36 Connected to this is the term triplet harvesting, because all excitons are harvested via the triplet state T1.13 With this, it was first possible to harvest all generated excitons, which resulted in highly efficient OLEDs.42,43 An archetypical phosphorescent OLED emitter is iridium-tris-(2-phenylpyridine) (Irppy3; Fig. 3).4445

TADF molecules have a low ΔE (ST), typically in the order of several hundreds of meV, and mediocre ISC and rISC.21 Although direct transitions between T1 and S0 are still forbidden, it is possible to go from T1 back to S1 by means of rISC, if the thermal energy kbT is sufficient. Compared with normal fluorescence, where the excited-state lifetime is in the order of ns, the excited-state lifetime is longer, often in the order of μs or more—the term TADF accounts for these observations. Occasionally, the term singlet harvesting1415.16,46 is used, because all excitons are harvested via the singlet state S1.13 A typical TADF emitter is tetrakis-N-carbazoyl-isophthalonitrile (4CzIPN; Fig. 3).5,4748.49

In summary, TADF is an alternate concept, which can help to realize efficient OLEDs with maximum efficiency. It competes with phosphorescence, the commercial state-of-the-art solution for efficient OLEDs.


Conceptual Background of Thermally Activated Delayed Fluorescence


Molecular Design Principles

Without going too much into detail, all TADF emitters consist of donor and acceptor moieties. In a way, they are similar to so-called ambipolar host molecules and often even contain similar building blocks, which are either arranged in a different way or used in different combinations in order to realize the large bandgap, which is needed to accommodate dopant molecules.5051. In both hosts and TADF emitters, donor moieties are electron-rich functional units that have a rather deep HOMO, which allow them to be oxidized easily and partake in the transport of holes. Contrarily, acceptor moieties are electron-deficient functional units with a deep LUMO. They can be reduced and thus partake in the transport of electrons. Figure 4 contains the molecular structure of various donor units, while acceptor units are shown in Fig. 5.

Fig. 4

Schematic structures of various donor units.


Fig. 5

Schematic structures of various acceptor units.


It is apparent that—as of today—there is much more variation found on the acceptor part than on the donor part of TADF emitters.19 As indicated in Fig. 4, the vast majority of all published organic TADF emitters contain substituted carbazole-5657.58.59 or arylamine-type donors.6061.62 In most cases, the connection to the acceptors is realized via the aromatic nitrogen. On the acceptor-side (Fig. 5), one can find a plethora of nitrogen-containing heterocycles,63,64 as well as various benzonitriles.4748.49,56,57,59,65,66 A third acceptor-class is sulfones,31,54,58,60,67,68 which seems to be particularly useful to realize deep-blue TADF emission.

The color of the TADF emitter depends on the choice of the donor and acceptor moieties. Approximately, the choice of the donor dictates the HOMO, while the acceptor dictates the LUMO. The energetic difference between HOMO and LUMO is the bandgap, which is closely connected to the emission color of the emitter. Roughly, a bandgap of ca. 2.8 eV (450 nm) relates to blue emission, while 1.9 eV (650 nm) gives red emission.23 In Sec. 1, ΔE (ST) was introduced as the most crucial parameter for TADF. This parameter is highly dependent of the localization of the Frontier orbitals HOMO and LUMO.69,70 The better the separation (or the smaller the overlap between HOMO and LUMO), the smaller ΔE (ST), which facilitates efficient ISC and rISC. From a design point of view, the overlap of HOMO and LUMO can be manipulated by the way in which the donor and acceptor moieties are connected. This is emphasized by analyzing the properties of different combinations of triphenyltriazine-type acceptors (Trz) with carbazole-type donors (Cz and mCz or Me2Cz), which have been published by Lee et al.71,72 This work lead to the TADF emitters DCzTrz, TCzTrz, DCzmCzTrz, and TmCzTrz, which are shown in Fig. 6. Addition of an additional carbazole or dimethylcarbazole to DCzTrz changes the triplet energy ΔE (ST) from 0.23 to 0.16 and 0.07 eV, respectively, in TCtTrz and DCzmCzTrz.

Fig. 6

Impact of the connectivity of donors and acceptors on photophysical properties.71,72


The photoluminescence quantum yield and excited-state lifetime are—in this case—connected to ΔE (ST): it changes from 43% at 31  μs (DCzTrz) to 100% at 14  μs (TCzTrz) and 99% at 14  μs (DCzmCzTrz). Note that the color of the four emitters shown in Fig. 6 depends on the choice of the donor unit. The carbazole-emitters DCzTrz and TCzTrz show peak electroluminescence around 480 nm, while the emitters with dimethylcarbazole, Me2Cz, emit around 500 nm. This can be attributed to a variation of the HOMO energy of carbazole, which is slightly raised when electron-rich methyl groups are added.

When comparing ambipolar host molecules with TADF dopands, the differences are often subtle. Examples are pCzOXD,73 a fluorescent, ambipolar host molecule and 2PXZ-OXD,74 a TADF emitter, which are shown in Fig. 7. They both feature diphenyloxadiazole (OXD) as the acceptor unit (not shown in Fig. 5) as well two carbazole- (Cz) or phenoxazine-donors (PXZ) per molecule. Apart from the different HOMO energy, which leads to a smaller bandgap for 2PXZ-OXD due to the shallower HOMO of phenoxazine, the twist angle between the donor and the phenyl-substituent is different with a reported value based on DFT calculation of 52 deg for pCzOXD73 and 75 deg for 2PXZ-OXD.74 The larger twist breaks the conjugation between the donor and acceptor, which leads to a well-separated HOMO and LUMO and a small ΔE (ST) for 2PXZ-OXD,74 while the HOMO is totally delocalized for 2PXZ-OXD, as suggested by DFT calculations.73

Fig. 7

Differences between ambipolar host molecules73 and TADF emitters74 are often subtle.



Organic Versus Copper Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes

In Sec. 2.1, we highlighted organic, small-molecule TADF emitters. For metal–organic TADF emitters, the same design principles are also essentially true (Fig. 8). In this case, the HOMO is often located on metal (or metal-halide) centered orbitals, while N-heterocyclic ligands (which are also being used in organic TADF emitters, refer to Fig. 5) house the LUMO. The same is also true for polymeric TADF emitters, where the HOMO and LUMO are located on different monomers of the polymer strand.75

Fig. 8

Examples for metal–organic TADF emitters.4,76,77


Both organic66,78,79 and metal–organic TADF emitters77 have been shown to reach the maximum theoretical OLED efficiency, which is in the order of 20% to 25% external quantum efficiency in cases where no light-extraction technology is applied.13 For clarity, the molecular structures of metal–organic TADF emitters are not discussed here. This has already been covered elsewhere.8081.82


Current Strengths and Weaknesses of Thermally Activated Delayed Fluorescence–Organic Light-Emitting Diode

In terms of efficiency, TADF is already directly competing with phosphorescent OLEDs,66,78,79 even though the first TADF OLED was published only 5 years ago.63 In terms of stability, there are some promising results.72,83 With 4CzIPN (see Fig. 2), LT50 values of over 10,000 h (starting luminance of 500  cdm2) have been reported. However, there still exists a gap that needs to be bridged prior to commercialization.

It has been pointed out that TADF emitters partake in the charge transport in the OLED architecture, which are currently used.83 This is a result of the high doping concentrations, ranging from 6% up to neat emitter films that are usually being employed, which facilitate charge transport, as well as the current lack of host and blocking materials with properly aligned energy levels and high-triplet energies.84,85 In another study, it was demonstrated that charge-carrier-induced formation of small (e.g., as a result of unimolecular processes) and large molecular weight (e.g., as a result of multimolecular processes) degradation products occurs, which suggests bond formation as well as bond cleavage needs to be prevented. This is a general problem for all organic semiconductors, but is generally solvable when reaching a certain level of maturity.86 The authors also pointed out that 3,3-Di(9H-carbazol-9-yl)biphenyl, a host that is often used with TADF emitters, is in fact not sufficiently stable upon UV irradiation.87 This further suggests that the development of hosts and supporting materials needs to be investigated in parallel to the actual material development.

A big issue is the excited-state lifetime or emission decay time of materials. Apart from efficiency roll-off,8889.90 the loss of OLED efficiency at high currents, this is also likely to affect the operational stability. Unfortunately, no straightforward structure–property relation has yet been identified to systematically modify the excited-state lifetime of TADF emitters. Especially metal-free TADF emitters often suffer from long excited-state lifetimes up to the millisecond regime,65,90 while ideally, the excited-state lifetime is in the order of 1  μs or even less.13 For the diphenylsulfone acceptor (see Fig. 5), excited-state lifetimes in the order of ms are achieved when donors such as tert-butyl-carbazole are used,68 while in a similar system with dimethoxycarbazole, only several μs are reported.91 This cannot be rationalized or predicted with the current level of understanding. At this point, no general design rules exist on how to specifically design TADF molecules with very low-excited-state lifetime.


Potential Economic Impact of Thermally Activated Delayed Fluorescence: Overcoming the Blue Gap

OLEDs have successfully been introduced in commercial products, such as tablet personal computers, smartwatches, and smartphones in the last few years, thanks to breakthroughs that led to the realization of stable devices with good efficiency. The first commercial successes should not distract from the fact that there is much room for improvement. Currently, the most pressing material issue in OLEDs is the bluegap (Fig. 9): In modern, commercial OLEDs, fluorescent materials92 are used in addition to phosphorescent materials, even though this leads to an increased power consumption. The main reason for this is the unavailability of stable deep-blue phosphorescent emitters.93

Fig. 9

The bluegap. So far, no commercial blue material has been shown to combine both efficiency and stability.


Similar to any other display technology, OLEDs display consists of red, green, and blue pixels. While red and green pixels rely on energy-efficient, phosphorescent materials, blue phosphorescent OLEDs never reached the necessary color purity and stability to allow for commercialization. It needs to be said that academic and industrial institutions tried to solve this issue with blue phosphorescent materials without success for the last 15 years,9495. suggesting that much like the low efficiency of fluorescent OLED materials, the instability of blue phosphorescent materials might be an intrinsic property and that innovative material concepts are required to close the bluegap. This means that as of today, no efficient and stable blue OLED pixels are available. The consequences are drastic, because the human eye is relatively insensitive to blue light, which means that a fairly large amount of blue pixels is needed in any display.

The scope of this problem can be illustrated with a back-of-the-envelope calculation: typical OLED devices have displays with fluorescent blue emitter materials. Due to the aforementioned color sensitivity, 52% of the active display consists of blue pixels (Table 1). With a standard 2600-mAh-capacity battery, such a device plays up to 12.5 h of video.

Table 1

Representative size of AMOLED pixels for smartphones.

Average area in a display (μm2)Relative area (%)
Sum1050Percentage of total active area

Using a deep-blue TADF of phosphorescent emitter will allow for a reduction of the blue pixel area due to the greater light output. Assuming that the energy consumption of blue pixels is decreased by only 66% (corresponding to an efficiency increase by a factor of 3), the energy consumption of the whole display will be decreased by 33% and the battery lifetime of the video mode will be increased from 12.5 to more than 16 h. Note that the theoretical efficiency increase when using blue TADF or phosphorescence technology is even larger than assumed here (factor of 4 instead of 3!).

The first promising steps have already been made to put this into reality: in several cases, efficiency values of more than 20% have been demonstrated in deep-blue TADF OLEDs.64,68,71,72,101,102 The lifetime of blue TADF OLEDs is currently in the order of 50 h (LT80, starting luminance 500  cdm2), which was achieved for derivatives of DDCzTrz (structure, see Fig. 6).72

Given that the issues that were outlined in Sec. 3 are adequately addressed, and TADF technology has the potential to close the bluegap.



Considering that first-world societies use up to 30% of their electric energy either for direct lighting or for the illumination of the displays of large-area TVs or portable devices, the development of stable and efficient deep-blue OLEDs is a problem with great economic relevance. It is evident that after many unsuccessful attempts to realize this with phosphorescent materials, new approaches such as TADF need to be investigated. Apart from aspects such as general chemical stability against dissociative and associative reactions, the focus of future work should be directed toward a more in-depth understanding of the parameters, which can be used to tune the excited-state lifetime. Without it, material development is highly dependent on empiric brute-force approaches and luck, which significantly reduces the probability of success.

In this review, the TADF concept was introduced. With a modular approach, TADF molecules can be designed from donor and acceptor units. Also, the current strengths, especially the very high device efficiency, as well as weaknesses such as the often long excited-state lifetime, have been addressed. The great economic potential of blue TADF technology was outlined, which is one of the main driving forces for future academic and industrial research in this field.


This work has been sponsored by the German Federal Ministry of Education and Research. BMBF in the funding programs “cyCESH” and “cyFLEX.” Also, we acknowledge funding from the European Union in the LEO Project (H2020-ICT-2014-1 call). D.V. thanks the R&D Division of CYNORA and his collaboration partners, among them Stefan Bräse, Clemens Heske, Uli Lemmer, and Christopher Barner-Kowollik (Karlsruhe Institute of Technology), Hartmut Yersin (University of Regensburg), and Franky So (North Carolina State University) for their continued support.


1. R. Friedel and P. Israel, Edison’s Electric Light: Biography of an Invention, Rutgers University Press, New Brunswick, New Jersey (1986). Google Scholar

2. W. H. Meadowcroft, Thomas Alva Edison, Harper Brothers Publishers, New York (1911). Google Scholar

3. C. A. Parker and C. G. Hatchard, “Triplet–singlet emission in fluid solutions. Phosphorescence of eosin,” Trans. Faraday Soc. 57, 1894 (1961).TFSOA40014-7672 http://dx.doi.org/10.1039/tf9615701894 Google Scholar

4. A. Endo et al., “Thermally activated delayed fluorescence from Sn4+-porphyrin complexes and their application to organic light emitting diodes—a novel mechanism for electroluminescence,” Adv. Mater. 21(47), 4802–4806 (2009).ADVMEW0935-9648 http://dx.doi.org/10.1002/adma.200900983 Google Scholar

5. H. Uoyama et al., “Highly efficient organic light-emitting diodes from delayed fluorescence,” Nature 492(7428), 234–238 (2012). http://dx.doi.org/10.1038/nature11687 Google Scholar

6. K. Goushi et al., “Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion,” Nat. Photonics 6(4), 253–258 (2012).NPAHBY1749-4885 http://dx.doi.org/10.1038/nphoton.2012.31 Google Scholar

7. J. C. Deaton et al., “E-type delayed fluorescence of a phosphine-supported Cu2(mu-NAr2)2 diamond core: harvesting singlet and triplet excitons in OLEDs,” J. Am. Chem. Soc. 132(27), 9499–9508 (2010).JACSAT0002-7863 http://dx.doi.org/10.1021/ja1004575 Google Scholar

8. H. Yersin et al., “The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs,” Coord. Chem. Rev. 255(21–22), 2622–2652 (2011).CCHRAM0010-8545 http://dx.doi.org/10.1016/j.ccr.2011.01.042 Google Scholar

9. M. Hashimoto et al., “Highly efficient green organic light-emitting diodes containing luminescent three-coordinate copper(I) complexes,” J. Am. Chem. Soc. 133(27), 10348–10351 (2011).JACSAT0002-7863 http://dx.doi.org/10.1021/ja202965y Google Scholar

10. A. Tsuboyama et al., “Photophysical properties of highly luminescent copper(I) halide complexes chelated with 1,2-bis(diphenylphosphino)benzene,” Inorg. Chem. 46(6), 1992–2001 (2007).INOCAJ0020-1669 http://dx.doi.org/10.1021/ic0608086 Google Scholar

11. D. M. Zink et al., “Synthesis, structure, and characterization of dinuclear copper(I) halide complexes with P^N ligands featuring exciting photoluminescence properties,” Inorg. Chem. 52(5), 2292–2305 (2013).INOCAJ0020-1669 http://dx.doi.org/10.1021/ic300979c Google Scholar

12. C. Bizzarri et al., “Luminescent dinuclear Cu(I) complexes containing rigid tetraphosphine ligands,” Inorg. Chem. 53(20), 10944–10951 (2014).INOCAJ0020-1669 http://dx.doi.org/10.1021/ic5012204 Google Scholar

13. H. Yersin, R. Czerwieniec, “Organometallic emitters for OLEDs. Triplet harvesting, singlet harvesting, case studies, and trends,” in Physics of Organic Semiconductors, , W. Brütting and C. Adachi, Eds., p. 371, Wiley-VCH, Weinheim, Germany (2012). Google Scholar

14. M. J. Leitl et al., “Brightly blue and green emitting Cu(I) dimers for singlet harvesting in OLEDs,” J. Phys. Chem. A 117(46), 11823–11836 (2013).JPCAFH1089-5639 http://dx.doi.org/10.1021/jp402975d Google Scholar

15. R. Czerwieniec, J. Yu and H. Yersin, “Blue-light emission of Cu(I) complexes and singlet harvesting,” Inorg. Chem. 50(17), 8293–8301 (2011).INOCAJ0020-1669 http://dx.doi.org/10.1021/ic200811a Google Scholar

16. D. Volz et al., “High-efficiency OLEDs with fully-bridged PyrPHOS-complexes using singlet harvesting,” Adv. Mater. 27, 2538–2543 (2015). Google Scholar

17. Y. Zhang et al., “Metal-enhanced e-type fluorescence,” Appl. Phys. Lett. 92(1), 013905 (2008).APPLAB0003-6951 http://dx.doi.org/10.1063/1.2829798 Google Scholar

18. J. C. Fetzera and M. Zander, “Fluorescence, phosphorescence, and E-type delayed fluorescence of hexabenzo [bc,ef,hi,kl,no,qr] coronene,” Z. Naturforsch. A Phys. Sci. 45(5), 1–3 (1990). http://dx.doi.org/10.1515/zna-1990-0523 Google Scholar

19. Y. Tao et al., “Thermally activated delayed fluorescence materials towards the breakthrough of organoelectronics,” Adv. Mater. 26(47), 7931–7958 (2014).ADVMEW0935-9648 http://dx.doi.org/10.1002/adma.v26.47 Google Scholar

20. F. Dumur, “Recent advances in organic light-emitting devices comprising copper complexes: a realistic approach for low-cost and highly emissive devices?” Org. Electron. 21, 27–39 (2015).OERLAU1566-1199 http://dx.doi.org/10.1016/j.orgel.2015.02.026 Google Scholar

21. C. Adachi, “Third-generation organic electroluminescence materials,” Jpn. J. Appl. Phys. 53, 060101 (2014). http://dx.doi.org/10.7567/JJAP.53.060101 Google Scholar

22. D. Volz et al., “From iridium and platinum to copper and carbon: new avenues for more sustainability in organic light-emitting diodes,” Green Chem. 17(4), 1988–2011 (2015). http://dx.doi.org/10.1039/C4GC02195A Google Scholar

23. P. W. Atkins and R. Friedman, Molecular Quantum Mechanics, 4th ed., Oxford University Press, New York (2005). Google Scholar

24. J. Shinar, Organic Light-Emitting Devices: A Survey, Springer, New York (2003). Google Scholar

25. K. Sato et al., “Organic luminescent molecule with energetically equivalent singlet and triplet excited states for organic light-emitting diodes,” Phys. Rev. Lett. 110(24), 247401 (2013).PRLTAO0031-9007 http://dx.doi.org/10.1103/PhysRevLett.110.247401 Google Scholar

26. D. Y. Kondakov, “Role of triplet–triplet annihilation in highly efficient fluorescent devices,” J. Soc. Inf. Disp. 17, 137–144 (2009).JSIDE80734-1768 http://dx.doi.org/10.1889/JSID17.2.137 Google Scholar

27. Y. Zhang and S. R. Forrest, “Triplet diffusion leads to triplet–triplet annihilation in organic phosphorescent emitters,” Chem. Phys. Lett. 590, 106–110 (2013).CHPLBC0009-2614 http://dx.doi.org/10.1016/j.cplett.2013.10.048 Google Scholar

28. D. Y. Kondakov et al., “Triplet annihilation exceeding spin statistical limit in highly efficient fluorescent organic light-emitting diodes,” J. Appl. Phys. 106(12), 124510 (2009).JAPIAU0021-8979 http://dx.doi.org/10.1063/1.3273407 Google Scholar

29. P.-Y. Chou et al., “Efficient delayed fluorescence via triplet–triplet annihilation for deep-blue electroluminescence,” Chem. Commun. 50(52), 6869 (2014). http://dx.doi.org/10.1039/c4cc01851f Google Scholar

30. H. Nakanotani, “High-efficiency organic light-emitting diodes with fluorescent emitters,” Nat. Commun. 5, 4016 (2014).NCAOBW2041-1723 http://dx.doi.org/10.1038/ncomms5016 Google Scholar

31. Y. Seino et al., “High-performance blue phosphorescent OLEDs using energy transfer from exciplex,” Adv. Mater. 26(10), 1612–1616 (2014).ADVMEW0935-9648 http://dx.doi.org/10.1002/adma.201304253 Google Scholar

32. J.-H. Lee et al., “An exciplex forming host for highly efficient blue organic light emitting diodes with low driving voltage,” Adv. Funct. Mater. 25(3), 361–366 (2014).AFMDC61616-301X http://dx.doi.org/10.1002/adfm.201402707 Google Scholar

33. Y.-S. Park, K.-H. Kim and J.-J. Kim, “Efficient triplet harvesting by fluorescent molecules through exciplexes for high efficiency organic light-emitting diodes,” Appl. Phys. Lett. 102, 153306 (2013).APPLAB0003-6951 http://dx.doi.org/10.1063/1.4802716 Google Scholar

34. Y.-S. Park et al., “Exciplex-forming co-host for organic light-emitting diodes with ultimate efficiency,” Adv. Funct. Mater. 23(39), 4914–4920 (2013).AFMDC61616-301X http://dx.doi.org/10.1002/adfm.v23.39 Google Scholar

35. M. Montalti et al., Handbook of Photochemistry, CRC Press, Boca Raton (2006). Google Scholar

36. N. J. Turro, Modern Molecular Photochemistry, University Science Books, Sausalito (1991). Google Scholar

37. C. Y. Kwong et al., “Efficiency and stability of different tris(8-hydroxyquinoline) aluminium (Alq3) derivatives in OLED applications,” Mater. Sci. Eng. B 116(1), 75–81 (2005).MSBTEK0921-5107 http://dx.doi.org/10.1016/j.mseb.2004.09.024 Google Scholar

38. F. P. Rosselli et al., “Experimental and theoretical investigation of tris-(8-hydroxy-quinolinate) aluminum (Alq3) photo degradation,” Org. Electron. 10(8), 1417–1423 (2009).OERLAU1566-1199 http://dx.doi.org/10.1016/j.orgel.2009.08.026 Google Scholar

39. H. D. Burrows et al., “Characterization of the triplet state of tris(8-hydroxyquinoline)aluminium(III) in benzene solution,” J. Am. Chem. Soc. 125(50), 15310–15311 (2003).JACSAT0002-7863 http://dx.doi.org/10.1021/ja037254f Google Scholar

40. G. Baldacchini et al., “Emission intensity and degradation processes of Alq3 films,” Electrochem. Solid State Lett. 8(10), J24 (2005).ESLEF61099-0062 http://dx.doi.org/10.1149/1.2035748 Google Scholar

41. B. Minaev, G. Baryshnikov and H. Agren, “Principles of phosphorescent organic light emitting devices,” Phys. Chem. Chem. Phys. 16(5), 1719–1758 (2014).PPCPFQ1463-9076 http://dx.doi.org/10.1039/C3CP53806K Google Scholar

42. M. A. Baldo et al., “Highly efficient phosphorescent emission from organic electroluminescent devices,” Nature 395, 151–154 (1998). http://dx.doi.org/10.1038/25954 Google Scholar

43. M. A. Baldo et al., “Very high-efficiency green organic light-emitting devices based on electrophosphorescence,” Appl. Phys. Lett. 75(1), 4 (1999).APPLAB0003-6951 http://dx.doi.org/10.1063/1.124258 Google Scholar

44. J. C. Ribierre et al., “Temperature dependence of the triplet diffusion and quenching rates in films of an Irppy3-cored dendrimer,” Phys. Rev. B: Condens. Matter 77(8), 085211 (2008). http://dx.doi.org/10.1103/PhysRevB.77.085211 Google Scholar

45. S. Takayasu, T. Suzuki and K. Shinozaki, “Intermolecular interactions and aggregation of fac-tris(2-phenylpyridinato-C2, N)iridium(III) in nonpolar solvents,” J. Phys. Chem. B 117(32), 9449–9456 (2013).JPCBFK1520-6106 http://dx.doi.org/10.1021/jp403974h Google Scholar

46. H. Yersin, R. Czerwieniec and A. Hupfer, “Singlet harvesting with brightly emitting Cu(I) and metal-free organic compounds,” Proc. SPIE 8435, 843508 (2012).PSISDG0277-786X http://dx.doi.org/10.1117/12.921372 Google Scholar

47. R. Ishimatsu et al., “Solvent effect on thermally activated delayed fluorescence by 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene,” J. Phys. Chem. A 117(27), 5607–5612 (2013).JPCAFH1089-5639 http://dx.doi.org/10.1021/jp404120s Google Scholar

48. Y. Im and J. Y. Lee, “Above 20% external quantum efficiency in thermally activated delayed fluorescence device using furodipyridine-type host materials,” Chem. Mater. 26(3), 1413–1419 (2014).CMATEX0897-4756 http://dx.doi.org/10.1021/cm403358h Google Scholar

49. O. Y. Kim, B. S. Kim and J. Y. Lee, “High efficiency thermally activated delayed fluorescent devices using a mixed host of carbazole and phosphine oxide derived host materials,” Synth. Met. 201, 49–53 (2015).SYMEDZ0379-6779 http://dx.doi.org/10.1016/j.synthmet.2014.12.025 Google Scholar

50. M. Leung et al., “Novel ambipolar orthogonal donor–acceptor host for blue organic light emitting diodes,” Org. Lett. 15(18), 4694–4697 (2013).ORLEF71523-7060 http://dx.doi.org/10.1021/ol402001v Google Scholar

51. D. Kim, L. Zhu and J.-L. Brédas, “Electronic structure of carbazole-based phosphine oxides as ambipolar host materials for deep blue electrophosphorescence: a density functional theory study,” Chem. Mater. 24(13), 2604–2610 (2012).CMATEX0897-4756 http://dx.doi.org/10.1021/cm301416n Google Scholar

52. M. S. Park and J. Y. Lee, “9-(Pyridin-3-yl)-9H-carbazole derivatives as host materials for green phosphorescent organic light-emitting diodes,” Org. Electron. 14(5), 1291–1296 (2013).OERLAU1566-1199 http://dx.doi.org/10.1016/j.orgel.2013.02.036 Google Scholar

53. H.-F. Chen et al., “1,3,5-Triazine derivatives as new electron transport-type host materials for highly efficient green phosphorescent OLEDs,” J. Mater. Chem. 19, 8112 (2009).JMACEP0959-9428 http://dx.doi.org/10.1039/b913423a Google Scholar

54. Y. Seino et al., “Novel blue exciplex comprising acridine and sulfone derivatives as a host material for high-efficiency blue phosphorescent OLEDs,” Chem. Lett. 45(3), 283–285 (2016).CMLTAG0366-7022 http://dx.doi.org/10.1246/cl.151094 Google Scholar

55. C. Tang et al., “A versatile efficient one-step approach for carbazole–pyridine hybrid molecules: highly efficient host materials for blue phosphorescent OLEDs,” Chem. Commun. 51, 1650–1653 (2014). http://dx.doi.org/10.1039/C4CC08335K Google Scholar

56. H. Tanaka et al., “Efficient green thermally activated delayed fluorescence (TADF) from a phenoxazine–triphenyltriazine (PXZ–TRZ) derivative,” Chem. Commun. 48(93), 11392–11394 (2012). http://dx.doi.org/10.1039/c2cc36237f Google Scholar

57. D. Zhang et al., “Highly efficient blue thermally activated delayed fluorescent OLEDs with record-low driving voltages utilizing high triplet energy hosts with small singlet–triplet splittings,” Chem. Sci. 25, 1–9 (2016).1478-6524 http://dx.doi.org/10.1039/C5SC04755B Google Scholar

58. F. B. Dias et al., “Triplet harvesting with 100% efficiency by way of thermally activated delayed fluorescence in charge transfer OLED emitters,” Adv. Mater. 25(27), 3707–3714 (2013).ADVMEW0935-9648 http://dx.doi.org/10.1002/adma.v25.27 Google Scholar

59. M. Kim et al., “Highly efficient and color tunable thermally activated delayed fluorescent emitters using a ‘twin emitter’ molecular design,” Chem. Commun. 52, 339–342 (2015). http://dx.doi.org/10.1039/C5CC07999C Google Scholar

60. P. L. Santos et al., “Engineering the singlet–triplet energy splitting in a TADF molecule,” J. Mater. Chem. C (2016). http://dx.doi.org/10.1039/C5TC03849A Google Scholar

61. J. Li et al., “Highly efficient organic light-emitting diode based on a hidden thermally activated delayed fluorescence channel in a heptazine derivative,” Adv. Mater. 25(24), 3319–3323 (2013).ADVMEW0935-9648 http://dx.doi.org/10.1002/adma.v25.24 Google Scholar

62. Y. Sagara et al., “Highly efficient thermally activated delayed fluorescence emitters with a small singlet–triplet energy gap and large oscillator strength,” Chem. Lett. 44(3), 360–362 (2015).CMLTAG0366-7022 http://dx.doi.org/10.1246/cl.141054 Google Scholar

63. A. Endo et al., “Efficient up-conversion of triplet excitons into a singlet state and its application for organic light emitting diodes,” Appl. Phys. Lett. 98(8), 083302 (2011).APPLAB0003-6951 http://dx.doi.org/10.1063/1.3558906 Google Scholar

64. W. Liu et al., “Novel carbazol–pyridine–carbonitrile derivative as excellent blue thermally activated delayed fluorescence emitter for highly efficient organic light-emitting devices,” ACS Appl. Mater. Interfaces 7(34), 18930–18936 (2015).AAMICK1944-8244 http://dx.doi.org/10.1021/acsami.5b05648 Google Scholar

65. K. Masui, H. Nakanotani and C. Adachi, “Analysis of exciton annihilation in high-efficiency sky-blue organic light-emitting diodes with thermally activated delayed fluorescence,” Org. Electron. 14(11), 2721–2726 (2013). http://dx.doi.org/10.1016/j.orgel.2013.07.010 Google Scholar

66. D. R. Lee et al., “Above 30% external quantum efficiency in green delayed fluorescent organic light-emitting diodes,” ACS Appl. Mater. Interfaces 7(18), 9625–9629 (2015).AAMICK1944-8244 http://dx.doi.org/10.1021/acsami.5b01220 Google Scholar

67. Y. Li et al., “Deep blue fluorophores incorporating sulfone-locked triphenylamine: the key for highly efficient fluorescence–phosphorescence hybrid white OLEDs with simplified structure,” J. Mater. Chem. C 3, 6986–6996 (2015). http://dx.doi.org/10.1039/C5TC01373A Google Scholar

68. Q. Zhang et al., “Design of efficient thermally activated delayed fluorescence materials for pure blue organic light emitting diodes,” J. Am. Chem. Soc. 134(36), 14706–14709 (2012).JACSAT0002-7863 http://dx.doi.org/10.1021/ja306538w Google Scholar

69. H. Bässler and A. Köhler, “Charge transport in organic semiconductors,” Top. Curr. Chem. 312, 1–65 (2012).TPCCAQ0340-1022 http://dx.doi.org/10.1007/978-3-642-27284-4 Google Scholar

70. A. Köhler and H. Bäßler, “Triplet states in organic semiconductors,” Mater. Sci. Eng. R 66(4–6), 71–109 (2009). http://dx.doi.org/10.1016/j.mser.2009.09.001 Google Scholar

71. D. R. Lee et al., “Design strategy for 25% external quantum efficiency in green and blue thermally activated delayed fluorescent devices,” Adv. Mater. 27(39), 5861–5867 (2015).ADVMEW0935-9648 http://dx.doi.org/10.1002/adma.201502053 Google Scholar

72. M. Kim et al., “Stable blue thermally activated delayed fluorescent organic light-emitting diodes with three times longer lifetime than phosphorescent organic light-emitting diodes,” Adv. Mater. 27(15), 2515–2520 (2015).ADVMEW0935-9648 http://dx.doi.org/10.1002/adma.201500267 Google Scholar

73. Y. Tao et al., “Tuning the optoelectronic properties of carbazole/oxadiazole hybrids through linkage modes: hosts for highly efficient green electrophosphorescence,” Adv. Funct. Mater. 20, 304–311 (2010).AFMDC61616-301X http://dx.doi.org/10.1002/adfm.v20:2 Google Scholar

74. J. Lee et al., “Oxadiazole- and triazole-based highly-efficient thermally activated delayed fluorescence emitters for organic light-emitting diodes,” J. Mater. Chem. C 1(30), 4599 (2013). http://dx.doi.org/10.1039/c3tc30699b Google Scholar

75. A. E. Nikolaenko et al., “Thermally activated delayed fluorescence in polymers: a new route toward highly efficient solution processable OLEDs,” Adv. Mater. 27(44), 7236–7240 (2015).ADVMEW0935-9648 http://dx.doi.org/10.1002/adma.201501090 Google Scholar

76. Y. Sakai et al., “Zinc complexes exhibiting highly efficient thermally activated delayed fluorescence and their application to organic light-emitting diodes,” Chem. Commun. 51(15), 3181–3184 (2015). http://dx.doi.org/10.1039/C4CC09403D Google Scholar

77. D. Volz et al., “Bridging the efficiency gap: fully bridged dinuclear Cu(I)-complexes for singlet harvesting in high-efficiency OLEDs,” Adv. Mater. 27(15), 2538–2543 (2015).ADVMEW0935-9648 http://dx.doi.org/10.1002/adma.201405897 Google Scholar

78. J. W. Sun et al., “A fluorescent organic light-emitting diode with 30% external quantum efficiency,” Adv. Mater. 26(32), 5684–5688 (2014).ADVMEW0935-9648 http://dx.doi.org/10.1002/adma.201401407 Google Scholar

79. S.-Y. Y. Kim et al., “Organic light-emitting diodes with 30% external quantum efficiency based on a horizontally oriented emitter,” Adv. Funct. Mater. 23(31), 3896–3900 (2013).AFMDC61616-301X http://dx.doi.org/10.1002/adfm.v23.31 Google Scholar

80. M. Wallesch et al., “Bright coppertunities: multinuclear CuI complexes with N–P ligands and their applications,” Chem. Eur. J. 20(22), 6578–6590 (2014). http://dx.doi.org/10.1002/chem.201402060 Google Scholar

81. M. Wallesch et al., “Bright coppertunities: efficient OLED devices with copper(I)iodide-NHetPHOS-emitters,” Proc. SPIE 9183, 918309 (2014).PSISDG0277-786X http://dx.doi.org/10.1117/12.2060499 Google Scholar

82. D. M Zink et al., “Novel oligonuclear copper complexes featuring exciting luminescent characteristics,” Proc. SPIE 8829, 882907 (2013).PSISDG0277-786X http://dx.doi.org/10.1117/12.2028619 Google Scholar

83. H. Nakanotani et al., “Promising operational stability of high-efficiency organic light-emitting diodes based on thermally activated delayed fluorescence,” Sci. Rep. 3, 2127 (2013).SRCEC32045-2322 http://dx.doi.org/10.1038/srep02127 Google Scholar

84. Q. Zhang et al., “Triplet exciton confinement in green organic light-emitting diodes containing luminescent charge-transfer Cu(I) complexes,” Adv. Funct. Mater. 22(11), 2327–2336 (2012).AFMDC61616-301X http://dx.doi.org/10.1002/adfm.v22.11 Google Scholar

85. Y. Noguchi et al., “Charge carrier dynamics and degradation phenomena in organic light-emitting diodes doped by a thermally activated delayed fluorescence emitter,” Org. Electron. 17, 184–191 (2014).OERLAU1566-1199 http://dx.doi.org/10.1016/j.orgel.2014.12.009 Google Scholar

86. F. So and D. Kondakov, “Degradation mechanisms in small-molecule and polymer organic light-emitting diodes,” Adv. Mater. 22(34), 3762–3777 (2010).ADVMEW0935-9648 http://dx.doi.org/10.1002/adma.200902624 Google Scholar

87. A. S. D. Sandanayaka, T. Matsushima and C. Adachi, “Degradation mechanisms of organic light-emitting diodes based on thermally activated delayed fluorescence molecules,” J. Phys. Chem. C 119(42), 23845–23851 (2015).JPCCCK1932-7447 http://dx.doi.org/10.1021/acs.jpcc.5b07084 Google Scholar

88. S. Wang et al., “Achieving high power efficiency and low roll-off OLEDs based on energy transfer from thermally activated delayed excitons to fluorescent dopants,” Chem. Commun. 51, 11972–11975 (2015). http://dx.doi.org/10.1039/C5CC04469C Google Scholar

89. C. Murawski, K. Leo and M. C. Gather, “Efficiency roll-off in organic light-emitting diodes,” Adv. Mater. 25(47), 6801–6827 (2013).ADVMEW0935-9648 http://dx.doi.org/10.1002/adma.v25.47 Google Scholar

90. T. Komino et al., “Suppression of efficiency roll-off characteristics in thermally activated delayed fluorescence based organic light-emitting diodes using randomly oriented host molecules,” Chem. Mater. 25(15), 3038–3047 (2013).CMATEX0897-4756 http://dx.doi.org/10.1021/cm4011597 Google Scholar

91. S. Wu et al., “High-efficiency deep-blue organic light-emitting diodes based on a thermally activated delayed fluorescence emitter,” J. Mater. Chem. C 2(3), 421–424 (2014). http://dx.doi.org/10.1039/C3TC31936A Google Scholar

92. M. Zhu and C. Yang, “Blue fluorescent emitters: design tactics and applications in organic light-emitting diodes,” Chem. Soc. Rev. 42(12), 4963–4976 (2013).CSRVBR0306-0012 http://dx.doi.org/10.1039/c3cs35440g Google Scholar

93. Y. Zhang, J. Lee and S. R. Forrest, “Tenfold increase in the lifetime of blue phosphorescent organic light-emitting diodes,” Nat. Commun. 5, 1–7 (2014).NCAOBW2041-1723 http://dx.doi.org/10.1038/ncomms6008 Google Scholar

94. R. Seifert et al., “Chemical degradation mechanisms of highly efficient blue phosphorescent emitters used for organic light emitting diodes,” Org. Electron. 14(1), 115–123 (2013).OERLAU1566-1199 http://dx.doi.org/10.1016/j.orgel.2012.10.003 Google Scholar

95. S. Schmidbauer, A. Hohenleutner and B. König, “Studies on the photodegradation of red, green and blue phosphorescent OLED emitters,” Beilstein J.Org. Chem. 9, 2088–2096 (2013). http://dx.doi.org/10.3762/bjoc.9.245 Google Scholar

96. R. Seifert et al., “Comparison of ultraviolet- and charge-induced degradation phenomena in blue fluorescent organic light emitting diodes,” Appl. Phys. Lett. 97(1), 013308 (2010).APPLAB0003-6951 http://dx.doi.org/10.1063/1.3460285 Google Scholar

97. Q. Wang and H. Aziz, “Degradation of organic/organic interfaces in organic light-emitting devices due to polaron–exciton interactions,” ACS Appl. Mater. Interfaces 5(17), 8733–8739 (2013).AAMICK1944-8244 http://dx.doi.org/10.1021/am402537j Google Scholar

98. Y. Luo et al., “Similar roles of electrons and holes in luminescence degradation of organic light-emitting devices,” Chem. Mater. 19(8), 2079–2083 (2007).CMATEX0897-4756 http://dx.doi.org/10.1021/cm062621i Google Scholar

99. S. Scholz et al., “Degradation mechanisms and reactions in organic light-emitting devices,” Chem. Rev. 115(16), 8449–8503 (2015).CHREAY0009-2665 http://dx.doi.org/10.1021/cr400704v Google Scholar

100. T. D. Schmidt et al., “Degradation induced decrease of the radiative quantum efficiency in organic light-emitting diodes,” Appl. Phys. Lett. 101(10), 103301 (2012).APPLAB0003-6951 http://dx.doi.org/10.1063/1.4749815 Google Scholar

101. I. Lee and J. Y. Lee, “Molecular design of deep blue fluorescent emitters with 20% external quantum efficiency and narrow emission spectrum,” Org. Electron. 29, 160–164 (2016). http://dx.doi.org/10.1016/j.orgel.2015.12.001 Google Scholar

102. I. H. Lee et al., “High efficiency blue fluorescent organic light-emitting diodes using a conventional blue fluorescent emitter,” J. Mater. Chem. C 3, 8834–8838 (2015). http://dx.doi.org/10.1039/C5TC01626F Google Scholar


Daniel Volz studied chemistry at the KIT and the KSOP in Karlsruhe, Germany, in the group of Professor Stefan Bräse and received his PhD in 2014. He is a team leader and one of the leading scientists at the CYNORA GmbH. He joined CYNORA’s R&D Division in 2009. Since 2015, he has been involved in the company’s R&D strategy. His research interests are aimed toward the development of new, sustainable emitting materials for OLEDs, and exploration of their photophysical and processing properties.

© 2016 Society of Photo-Optical Instrumentation Engineers (SPIE)
Daniel Volz, Daniel Volz, } "Review of organic light-emitting diodes with thermally activated delayed fluorescence emitters for energy-efficient sustainable light sources and displays," Journal of Photonics for Energy 6(2), 020901 (25 April 2016). https://doi.org/10.1117/1.JPE.6.020901 . Submission:

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