The search for efficient materials for organic light emitting diodes is one of the most imperative research area. The focus is to obtain a bright large area emitter, limited by the low internal quantum efficiency of conventional organic emitters. Recently, a new generation of the organic materials (TADF) with a theoretical internal quantum efficiency up to 100%, opened new frameworks. However, significant challenges persist to achieve full understanding of the TADF mechanism and to improve the OLEDs stability. Starting from the photo-physical analysis, we show the relationship with the molecular electrical carrier dynamics and internal quantum efficiencies. The OLED structure, fabrication, and characterization are also discussed. Several examples for the full color emitters are given. Special emphasis on experimental results is made, showing the major milestones already achieved in this field.
- organic light emitting diodes
- thermally activated delayed fluorescence
- triplet harvesting
Since the first invention of the organic light emitting diodes (LEDs) in 1987 by Tang and Van Slyke  that represented an advancement in display and lightening technologies, Organic light-emitting diodes (OLEDs) have emerged as an extensive active field in the both scientific as well technological aspects. Organic light-emitting diodes (OLEDs) are cheap, flexible, and cheap like a movie projector screen. They have attracted considerable attention due to their promising applications in cheap, energy-saving, eco-friendly and solid-state lighting [2, 3, 4].
The current research in OLEDs is emerging technology which is also a growing market and expected to cross 20 billion by 2030 . OLEDs are used in several flat and roll displays, also in white EDs for the lightning. OLEDs give freedom of taking advantage of emission in different colors, color modulation (color coordinates, temperature and color rendering—white lighting), diffused light—(light from flat panels (large area) and high viewing angle), Freedom design (thin, lightweight, flexible transparent—easy incorporation into 3D surfaces), etc. Along with these characteristics, there is some difficulty in getting a large homogeneous emitter area, where the organic materials rapidly degrade in the presence of oxygen and/or humidity. Although solved with rigid OLEDs, flexible ones have low lifetime (no efficient encapsulation system has been developed). The main principle behind OLED technology is electroluminescence and such devices offer brighter, thinner, high contrast, and flexibility.
In the conventional OLEDs, the materials used are π-electron-rich molecules, which helps in the fast charge transfer at the interface. But in these OLEDs, the internal quantum efficiency (IQE) is lower which results to the lower external quantum efficiency (EQE) of 5% and limits the OLEDs development because of the nonradiative triplet exciton non-harnessing. Usually, materials used for OLEDs are phosphorescent emitters such as iridium [6, 7, 8] or platinum complexes [9, 10] that are used to achieve the electroluminescence efficiency. In such systems, both 25% singlet excitons and 75% triplet excitons can be used for harnessing the electroluminescence. In phosphorescent OLEDs, the internal quantum efficiency was reported close to 100% [11, 12, 13, 14, 15, 16], but the disadvantage in such phosphorescent material is their high cost and poor stability. Along with phosphorescent material harnessing phosphorescence [7, 17], triplet-triplet annihilation  were also used. Therefore, to achieve 100% low-cost IQE, the development of an alternative to harvest the 75% triplet exciton is important for the future of OLEDs. In this context, response to this need, the development of the thermally activated delayed fluorescence (TADF) materials with the most promising exciton harvesting mechanism used in OLED devices, which was firstly reported by Adachi et al. in  received tremendous attention, and in recent years, considerable efforts have been devoted towards the fabrication of OLEDs based on TADF materials where the IQE can be easily achieved up to 100% [20, 21].
In this chapter, we summarize the fundamentals of thermally activated delayed fluorescence process, their optoelectronic behavior linking with the device performance and recent experimental studies of the introduction of TADF emitters used as the doping/guest material for OLED fabrication. Along with, a summary of the best TADF emitters used for fabrication of orange-red, blue and green-yellow OLEDs is provided. In addition, a correlation is provided between the structure and doping percentage of TADF emitters and their optoelectronic properties.
2. Theory and concepts of thermally activated delayed fluorescence
The starting point to understand the TADF principle in organic luminescent materials is to consider the fundamental
The analysis of this process can be based on the exciton formation (electron-hole pair) in a conjugated organic material. An electronic charge can be transferred between both entities in a two molecules system (or also in different parts of the same molecule). This process is called of charge-transfer (donor-acceptor complex) leading origin to the CT energy levels. The primary effect of these levels is to provide an electrostatic attraction, stabilizing the molecule. But, interestingly, this CT state is spin selective and is supposed to be able to change the triplet / singlet balance, allowing a conversion of triplet excitons to singlet ones. Although being still an unclear mechanism, was the fundamental starting point to the TADF materials. The Figure 1 shows, in a simple scheme, the fundamental process involved in the excitation / de-excitation of an organic molecule.
An efficient TADF emission needs to enhance the transition probability
The immediate conclusion is that the minimization of
The physical model to explain the
Besides the usual D-A molecule separated structure, some new molecules based on D-A-D (so-called “butterfly-shaped” structure) also exhibits TADF emission. Surprisingly, in several of such molecules, the energy gap between the lowest 1CT state and the lowest 3LE state (with π → π* transition) are much higher than those found between 1CT and 3CT energy states in the conventional D-A molecules. The explanation was the two-step model above referred. This model appears to be the most interesting and well supported by experimental results.
Particularly important in this model, is the ability to modulate the energy of the excited 1CT state via the environment polarity . In solution, the photophysics analysis can help in revealing the main process involved, in turn, dependent on the solvent polarity. On film (solid state), this possibility opens a wide range of choice for the organic host material in order to significantly improve the efficiency of an OLED based on a TADF material. In a simple scheme, we can, therefore, represent the excited state of the TADF molecule as shown in Figure 2.
It must be noted that, according to this model, and following several experimental data (see  and references therein) the energy transfer SOC-ISC is more efficient in a D-A perpendicular geometry, in a transition
3. Fundamental photophysics of an organic TADF emitter
The starting point for developing an efficient OLED using TADF emitter is based on the luminescence properties of the emitter itself. As an earlier point, the physical process involved are not really straightforward, but leaving aside the pure photophysics process studies, the important figures of merit regarding efficiency can be easily obtained.
From Figure 1, we can formally consider two different kinds of radiative emissions arising in S1 state: from its own electrons population (25% of excited ones) and from the population via
According , if the ratio
is near (or above) four, the yield of the
This relationship can be useful to determine the ratio of that is a fundamental key for the material characterization. In simple but practical ways, two different approaches can be used for such determination. Both are related to the fact that almost know TADF materials exhibit very poor or none DF in the presence of oxygen. Thus, measuring the luminescence emission parameters under a normal or degassed environment, we can achieve either PF or PF + DF. Under steady state photo physics, the direct measurement of the luminescence spectra in both environment conditions will give only (with great certainty) the PF (normal environment) and PF + DF (degassed environment). The direct ratio of the integrated spectra (intensity) further gives a very precise value for . Naturally this simple calculation is possible (quantum yield ratio from integrated intensity) because the intensity is proportional to the quantum yield and, in the case of TADF materials, the values of the proportionality constants for both emissions (DF and PF) are the same due to the fact that both arise from the same excited energy level . The exact calculation is the performed considering the simple relationship is given by . Figure 3 shows a simple example of this behavior.
By another hand, the higher transition probability associated with PF compared to the DF probability (that in a crude way depends on the
Finally, and by TRP is possible to estimate the transition probability of the
Due to several different kinds of triplet harvesting in an organic molecule, sometimes is not simple to attribute an enhanced luminescence to a TADF process. For instance, triplet-triplet annihilation (TTA) is also a wide investigated process for emission efficiency improvement. Distinguish both process is important. Due to the competition between the triplet quenching and decay of triplet states, usually the DF from TTA is non-linear on excitation energy; on the contrary, and because TADF process is purely intramolecular, its DF must follow a linear relationship with excitation energy. Figure 4 shows an example of the 2PX-TAZ emitter.
Besides the excitation energy dependence, the TADF emission is also strongly dependent on temperature. As the DF is thermally activated, we expect that its intensity decreases strongly with temperature and eventually vanishes at very low temperature. On the contrary, PF must be unaffected by thermal variations. This means that under TRP we must observe a decrease of the high lifetime emission as temperature decreases until remaining only the fast component.
The full understanding of the photo physics properties of the TADF emitter is naturally of extreme importance for further OLED development.
4. OLEDs based on TADF emitters
In the contest of finding best organic emitters for the lightening industry, in 2011 Adachi et al.  reported the very first purely organic TADF emitter
The application of TADF emitters is generally focused on OLEDs applications. As we have discussed in photo physics behavior of TADFs, it requires a solid host to disperse TADF emitters and this host material has a strong influence on the photo physical properties of these emitters . To encounter this, the design and optimization of TADF emitter is a key factor for the fabrication of OLEDs, and this requires the photo physical characterization of TADF in the host molecule which used in the device. Some of the most used hosts are DPEPO, CBP, mCP, mCBP, TPBi, TCTA and TAPC. The OLEDs are usually fabricated by thermally vacuum deposition, but several reports have been focused on fabrication via spin coating solution processed methods which is more suitable for large area OLEDs.
Many groups reported various green TADF based on different donor and acceptor molecules, due to less space it is difficult to discuss all of them. Herein, we will discuss some of the TADF red, green and blue emitters based on their donor and acceptor groups, photo-physical characteristics, and device performance.
4.1. Red-orange TADFs
Herein, we present red-orange TADF emitters which exhibit an electroluminescence peak at wavelength (ELmax) > 580 nm. The first reported red TADF emitter,
In 2013, Li et al.  synthesized orange-red emitter,
In another study, Wang et al.  demonstrated the effect of the twist angle during the designing strategy of TADF emitters. This twist angle can be reduced by increasing the D-A distance which gives an orbital overlap to increase kt. They synthesized the first near-infrared (NIR) TADF emitter
4.2. Blue TADF emitters
To synthesize efficient blue TADF emitter and their use in the device, it is important to take account of the π-conjugation length and the redox potential of the donor and acceptor moieties and in DPS derivatives the advantage is that the oxygens of the sulfonyl group have significant electronegativity, which gives the sulfonyl group electron-withdrawing properties and sulfonyl group exhibit tetrahedral geometry which limit the conjugation . The device fabricated with 10 wt% emitter showed good results, the device ITO/α-NPD/TCTA/CzSi/10 wt%
Adachi et al.  reported deep blue emitters
It is very important to design the geometry of TADF molecule to induce TADF process, to counter this, Rajmalli et al.  reported novel blue TADF emitters based on benzoylpyridine acceptor
In 2015, Kim et al.  synthesized two new blue TADF emitters
In another study, Lin et al. demonstrated  a novel triazine-based blue TADF emitter named
Among various D-A and D-A-D structured TADF molecules, the main chemical moiety plays an important role for the exhibition of the TADF behavior, and as we have discussed various acceptors have been used for the synthesis of TADF materials, among them Cyano-based acceptors are used as most usual building blocks for the synthesis of deep-blue TADFs. The first Cyano based blue TADF
Solution-processed TADF materials was reported by Cho et al. , they synthesized two blue TADF emitters named
In another study, very recently, Hatakeyama et al.  demonstrated synthesis of boron-based acceptor TADFs. They synthesized TADF emitters
4.3. Green-yellow TADFs
Among various TADF emitters, plenty of them are the green-yellow emitter and most those green to yellow emitters are based on cyano-based acceptors. The molecular design of these cyano-based emitters is based “on the presence of a twisted conformation of donor carbazoles with respect to phthalonitrile plane” to confer the HOMO-LUMO separation and result to lower ΔEST. These cyano-based green TADF emitters are classified in three categories: (a) monomeric series with orthosteric hindrance, (b) homoconjugation series, and (3) dimeric emitters. In monomeric emitters, ΔEST is very small and high PLQY yield. In homoconjugation series, the HOMO and LUMO separation is easily achievable but lower PLQY yield and in dimeric series the ΔEST is very higher i.e. 0.11–0.21 eV. The first green emitter was reported by Adachi et al.  in 2012,
In the similar contest, Taneda and co-workers  synthesized a highly efficient green TADF emitter
Tang et al. revealed the strategy to synthesized solution processed green TADF emitters . They synthesized emitter
Apart from Cyano-based green TADF emitters, many researchers reported TADF emitters based on 1,2,5-triazine acceptor (
In 2014, Wang et al. reported  sulfone-based acceptor green TADF emitters. They synthesized two green TADF emitter named
1,3,4-Oxadiazole was used to synthesized green TADF emitter and these emitters are most commonly used for the applications. Three new green TADF emitters were synthesized by Lee et al. . The three emitters were
Many groups reported various green TADF based on different donor and acceptor molecules. The more significant are focused here.
5. Conclusions and outlook
Past few years have witnessed tremendous development in the field of organic electronics and especially in synthesis of organic light emitting materials which helped to boost the cost reduction of OLEDs and performance enhancement. The potential inexpensive synthesis methods, OLEDs fabrication process, flexibility and lightweight make them one of the promising materials for energy devices. A large number of phosphorescent and fluorescent organic materials have been already synthesized for their use in OLEDs. Recently, TADF materials have been introduced in OLEDs fabrication to achieve 100% IQE and high EQE, which can be achieved near 50%.
The actual development of organic TADF emitters achieves a status that surpasses a simple curiosity. The new OLED framework based on highly efficient TADF materials opens a new field of applications for display and lightening. These materials, based on separated donor-acceptor moieties in one molecule, leads to a unique luminescent process of a triplet harvesting in the excited state with a further huge increase of internal efficiency up to the 100% limit. This simple way to tailored pure organic emitters is really one of the most recent advances in chemistry and OLEDs filed. Achieving very high external efficiencies without using an expensive and rare transition metals, puts the organic emitters towards new possibilities. Particularly important is the application in lighting from large area emitters, a field that is until now, a technological problem. The possibility to have a full range color emitters with high efficiency (as shown, and particularly in blue) is also an outstanding achievement towards a new device design and application, particularly in white emission. Nevertheless, improving the efficiency towards the theoretical efficiency limit, can only be achieved with a deep understand of the TADF process in these organic molecules. This is due to the fact that the interactions between energy levels of TADF emitter and the host can condition the emission due to the TADF process itself as referred before. Thus, the next steps must be focused on the physical models for this
TADFs have been studied and used for OLEDs so far, and this field is relatively young but it has developed significantly during the past 5 years. By the incorporation of TADFs 40–50% EQE can be achieved. However, at the present stage despite of numerous characterization techniques to understand the TADF behavior, more rigorous efforts are required for their understanding and use for commercially production. The fabrication cost of the OLEDs based on TADF emitter should be less to make them major candidate in both display and lightening industry and the cost is critical. To reduce the cost, it is necessary to develop solution processed OLED fabrication methods and synthesis of polymeric or dendrimeric TADF emitters. These TADFs have already been found a significant role in next-generation displays and lightning materials and their use can only be realized as their synthesis characterization and device fabrication progresses.
The authors would like to acknowledge the i3N support from UID/CTM/50025/2013 and the EXCILIGHT Project from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 674990.
Tang CW, Van Slyke SA. Organic electroluminescent diodes. Applied Physics Letters. 1987; 51(12):913-915. DOI: 0.1063/1.98799
Yersin H. Highly Efficient OLEDs with Phosphorescent Materials. 1st ed. Verlag GmbH & Co. KGraA: Wiley-VCH; 2008
Franky S, editor. Organic Electronics Materials, Processing, Devices and Applications. 1st ed. Boca Raton: CRC Press; 2010
Gaspar DJ, Polikarpov E, editors. OLED Fundamentals: Materials, Devices, and Processing of Organic Light-Emitting Diodes. 1st ed. Boca Raton: CRC Press; 2015
Sasabe H, Kido J. Recent progress in phosphorescent organic light-emitting devices. European Journal of Organic Chemistry. 2013; 2013(14):7653-7663. DOI: 10.1002/ejoc.201300544
Baldo M, Lamansky S, Burrows P, Thompson M, Forrest S. Very high-efficiency green organic light-emitting devices based on electrophosphorescence. Applied Physics Letters. 1999; 75(1):4-6. DOI: 10.1063/1.124258
Adachi C, Baldo MA, Thompson ME, Forrest SR. Nearly 100% internal phosphorescence efficiency in an organic light-emitting device. Journal of Applied Physics. 2001; 90(10):5048-5051. DOI: 10.1063/1.1409582
Tsuzuki T, Nakayama Y, Nakamura J, Iwata T, Tokito S. Efficient organic light-emitting devices using an iridium complex as a phosphorescent host and a platinum complex as a red phosphorescent guest. Applied Physics Letters. 2006; 88(24):243511. DOI: 10.1063/1.2213017
Kwong RC, Sibley S, Dubovoy T, Baldo M, Forrest SR, Thompson ME. Efficient, saturated red organic light emitting devices based on phosphorescent platinum(II) porphyrins. Chemistry of Materials. 1999; 11(12):3709-3713. DOI: 10.1021/cm9906248
Kalinowski J, Fattori V, Cocchi M, Williams JG. Light-emitting devices based on organometallic platinum complexes as emitters. Coordination Chemistry Reviews. 2011; 255(21):2401-2425. DOI: 10.1016/j.ccr.2011.01.049
Sun Y, Giebink NC, Kanno H, Ma B, Thompson ME, Forrest SR. Management of singlet and triplet excitons for efficient white organic light-emitting devices. Nature. 2006; 440(7086):908-912. DOI: 10.1038/nature04645
Tanaka D, Sasabe H, Li YJ, Su SJ, Takeda T, Kido J. Ultra high efficiency green organic light-emitting devices. Japanese Journal of Applied Physics. 2006; 46(1L):L10. DOI: 10.1143/JJAP.46.L10
Reineke S, Lindner F, Schwartz G, Seidler N, Walzer K, Lüssem B, Leo K. White organic light-emitting diodes with fluorescent tube efficiency. Nature. 2009; 459(7244):234-238. DOI: 10.1038/nature08003
Helander MG, Wang ZB, Qiu J, Greiner MT, Puzzo DP, Liu ZW, Lu ZH. Chlorinated indium tin oxide electrodes with high work function for organic device compatibility. Science. 2011; 332(6032):944-947. DOI: 10.1126/science.1202992
Tao Y, Wang Q, Yang C, Zhong C, Qin J, Ma D. Multifunctional triphenylamine/oxadiazole hybrid as host and exciton-blocking material: High efficiency green phosphorescent OLEDs using easily available and common materials. Advanced Functional Materials. 2010; 20(17):2923-2929. DOI: 10.1002/adfm.201000669
Lee CW, Lee JY. Above 30% external quantum efficiency in blue phosphorescent organic light-emitting diodes using Pyrido [2, 3-b] indole derivatives as host materials. Advanced Materials. 2013; 25(38):5450-5454. DOI: 10.1002/adma.201301091
Baldo MA, O’brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, Forrest SR. Highly efficient phosphorescent emission from organic electroluminescent devices. Nature. 1998; 395(6698):151-154. DOI: 10.1038/25954
Kondakov DY, Pawlik TD, Hatwar TK, Spindler JP. Triplet annihilation exceeding spin statistical limit in highly efficient fluorescent organic light-emitting diodes. Journal of Applied Physics. 2009; 106(12):124510. DOI: 10.1063/1.3273407
Endo A, Sato K, Yoshimura K, Kai T, Kawada A, Miyazaki H, Adachi C. Efficient up-conversion of triplet excitons into a singlet state and its application for organic light emitting diodes. Applied Physics Letters. 2011; 98(8):42. DOI: 10.1063/1.3558906
Tao Y, Yuan K, Chen T, Xu P, Li H, Chen R, Zheng C, Zhang L, Huang W. Thermally activated delayed fluorescence materials towards the breakthrough of organoelectronics. Advanced Materials. 2014; 26(47):7931-7958. DOI: 10.1002/adma.201402532
Adachi C. Third-generation organic electroluminescence materials. Japanese Journal of Applied Physics. 2014; 53(6):060101. DOI: 10.7567/jjap.53.060101
Pereira L. Organic Light Emitting Diodes - The Use of Rare Earth and Transition Metals. Singapore: Pan Stanford Publishing; 2012. 348p. ISBN: 978-9814267298
Dias Fernando B, Penfold Thomas J, Monkman AP. Photophysics of thermally activated delayed fluorescence molecules. Methods and Applications in Fluorescence. 2017; 5:012001. DOI: 10.1088/2050-6120/aa537e
Turro NJ, Scaiano JC, Ramamurty V. Principles of Molecular Photochemistry: An Introduction. 1st ed. Mill Valleri, CA, USA: University Science Books; 2010
Wong Michael Y, Eli Z-C. Purely organic thermally activated delayed fluorescence materials for organic light-emitting diodes. Advanced Materials. 2017; 29:1605444. DOI: 10.1002/adma.201605444
Milián-Medina B, Gierschner J. Computational design of low singlet–triplet gap all-organic molecules for OLED application. Organic Electronics. 2012; 13(6):985-991. DOI: 10.1016/j.orgel.2012.02.010
Zhang Q, Li J, Shizu K, Huang S, Hirata S, Miyazaki H, Adachi C. Design of efficient thermally activated delayed fluorescence materials for pure blue organic light emitting diodes. Journal of the American Chemical Society. 2012; 134(36):14706-14709. DOI: 10.1021/ja306538w
Lim BT, Okajima S, Chandra AK, Lim EC. Radiationless transitions in electron donor-acceptor complexes: Selection rules for S1 → T intersystem crossing and efficiency of S1 → S0 internal conversion. Chemical Physics Letters. 1981; 79(1):22-27. DOI: 10.1016/0009-2614(81)85280-3
Nobuyasu RS, Ren Z, Griffiths GC, Batsanov AS, Data P, Yan S, Monkman AP, Bryce MR, Dias FB. Rational design of TADF polymers using a donor–acceptor monomer with enhanced TADF efficiency induced by the energy alignment of charge transfer and local triplet excited states. Advanced Optical Materials. 2016; 4(4):597-607. DOI: 10.1002/adom.201500689
Ogiwara T, Wakikawa Y, Ikoma T. Mechanism of intersystem crossing of thermally activated delayed fluorescence molecules. The Journal of Physical Chemistry. A. 2015; 119(14):3415-3418. DOI: 10.1021/acs.jpca.5b02253
Gibson J, Monkman AP, Penfold TJ. The importance of vibronic coupling for efficient reverse intersystem crossing in thermally activated delayed fluorescence molecules. ChemPhysChem. 2016; 17(19):2956-2961. DOI: 10.1002/cphc.201600662
Meches G, Goushi K, Potscavage WJ, Adachi C. Influence of host matrix on thermally-activated delayed fluorescence: Effects on emission lifetime, photoluminescence quantum yield, and device performance. Organic Electronics. 2014; 15(9):2027-2037. DOI: 10.1016/j.orgel.2014.05.027
Dias FB, Santos J, Graves DR, Data P, Nobuyasu RS, Fox MA, Batsanov AS, Palmeira T, Berberan-Santos MN, Bryce MR, Monkman AP. The role of local triplet excited states and D-A relative orientation in thermally activated delayed fluorescence: Photophysics and devices. Advancement of Science. 2016; 3(12):1600080. DOI: 10.1002/advs.201600080
Kaji H, Suzuki H, Fukushima T, Shizu K, Suzuki K, Kubo S, Komino T, Oiwa H, Suzuki F, Wakamiya A, Murata Y. Purely organic electroluminescent material realizing 100% conversion from electricity to light. Nature Communications. 2015; 19(6):8476. DOI: 10.1038/ncomms9476
Yang Z, Mao Z, Xie Z, Zhang Y, Liu S, Zhao J, Xu J, Chi Z, Aldred MP. Recent advances in organic thermally activated delayed fluorescence materials. Chemical Society Reviews. 2017; 46(3):915-1016. DOI: 10.1039/C6CS00368K
Masui K, Nakanotani H, Adachi C. Analysis of exciton annihilation in high-efficiency sky-blue organic light-emitting diodes with thermally activated delayed fluorescence. Organic Electronics. 2013; 14(11):2721-2726. DOI: 10.1016/j.orgel.2013.07.010
Komino T, Nomura H, Koyanagi T, Adachi C. Suppression of efficiency roll-off characteristics in thermally activated delayed fluorescence based organic light-emitting diodes using randomly oriented host molecules. Chemistry of Materials. 2013; 25(15):3038-3047. DOI: 10.1021/cm4011597
Uoyama H, Goushi K, Shizu K, Nomura H, Adachi C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature. 2012; 492(7428):234-238. DOI: 10.1038/nature11687
Zhang Q, Kuwabara H, Potscavage WJ Jr, Huang S, Hatae Y, Shibata T, Adachi C. Anthraquinone-based intramolecular charge-transfer compounds: Computational molecular design, thermally activated delayed fluorescence, and highly efficient red electroluminescence. Journal of the American Chemical Society. 2014; 136(52):18070-18081. DOI: 10.1021/ja510144h
Li J, Nakagawa T, Macdonald J, Zhang Q, Nomura H, Miyazaki H, Adachi C. Highly efficient organic light-emitting diode based on a hidden thermally activated delayed fluorescence channel in a heptazine derivative. Advanced Materials. 2013; 25(24):3319-3323. DOI: 10.1002/adma.201300575
Wang S, Yan X, Cheng Z, Zhang H, Liu Y, Wang Y. Highly efficient near-infrared delayed fluorescence organic light emitting diodes using a phenanthrene-based charge-transfer compound. Angewandte Chemie, International Edition. 2015; 54(44):13068-13072. DOI: 10.1002/anie.201506687
Chen P, Wang LP, Tan WY, Peng QM, Zhang ST, Zhu XH, Li F. Delayed fluorescence in a solution-processable pure red molecular organic emitter based on dithienylbenzothiadiazole: A joint optical, electroluminescence, and magnetoelectroluminescence study. ACS Applied Materials & Interfaces. 2015; 7(4):2972-2978. DOI: 10.1021/am508574m
Data P, Pander P, Okazaki M, Takeda Y, Minakata S, Monkman AP. Dibenzo [a, j] phenazine-cored donor–acceptor–donor compounds as green-to-red/NIR thermally activated delayed fluorescence organic light emitters. Angewandte Chemie, International Edition. 2016; 55(19):5739-5744. DOI: 10.1002/anie.201600113
Zhang Q, Li B, Huang S, Nomura H, Tanaka H, Adachi C. Efficient blue organic light-emitting diodes employing thermally activated delayed fluorescence. Nature Photonics. 2014; 8(4):326-332. DOI: 10.1038/nphoton.2014.12
Dias FB, Bourdakos KN, Jankus V, Moss KC, Kamtekar KT, Bhalla V, Santos J, Bryce MR, Monkman AP. Triplet harvesting with 100% efficiency by way of thermally activated delayed fluorescence in charge transfer OLED emitters. Advanced Materials. 2013; 25(27):3707-3714. DOI: 10.1002/adma.201300753
Chen XK, Zhang SF, Fan JX, Ren AM. Nature of highly efficient thermally activated delayed fluorescence in organic light-emitting diode emitters: Nonadiabatic effect between excited states. The Journal of Physical Chemistry C. 2015; 119(18):9728-9733. DOI: 10.1021/acs.jpcc.5b00276
Etherington MK, Gibson J, Higginbotham HF, Penfold TJ, Monkman AP. Revealing the spin–vibronic coupling mechanism of thermally activated delayed fluorescence. Nature Communications. 2016; 7. DOI: 10.1038/ncomms13680
Wu S, Aonuma M, Zhang Q, Huang S, Nakagawa T, Kuwabara K, Adachi C. High-efficiency deep-blue organic light-emitting diodes based on a thermally activated delayed fluorescence emitter. Journal of Materials Chemistry C. 2014; 2(3):421-424. DOI: 10.1039/C3TC31936A
Rajamalli P, Senthilkumar N, Gandeepan P, Huang PY, Huang MJ, Ren-Wu CZ, Yang CY, Chiu MJ, Chu LK, Lin HW, Cheng CH. A new molecular design based on thermally activated delayed fluorescence for highly efficient organic light emitting diodes. Journal of the American Chemical Society. 2016; 138(2):628-634. DOI: 10.1021/jacs.5b10950
Kim M, Jeon SK, Hwang SH, Lee JY. Stable blue thermally activated delayed fluorescent organic light-emitting diodes with three times longer lifetime than phosphorescent organic light-emitting diodes. Advanced Materials. 2015; 27(15):2515-2520. DOI: 10.1002/adma.201500267
Lee DR, Kim M, Jeon SK, Hwang SH, Lee CW, Lee JY. Design strategy for 25% external quantum efficiency in green and blue thermally activated delayed fluorescent devices. Advanced Materials. 2015; 27(39):5861-5867. DOI: 10.1002/adma.201502053
Lin TA, Chatterjee T, Tsai WL, Lee WK, Wu MJ, Jiao M, Pan KC, Yi CL, Chung CL, Wong KT, Wu CC. Sky-blue organic light emitting diode with 37% external quantum efficiency using thermally activated delayed fluorescence from spiroacridine-triazine hybrid. Advanced Materials. 2016; 28(32):6976-6983. DOI: 10.1002/adma.201601675
Komatsu R, Sasabe H, Seino Y, Nakao K, Kido J. Light-blue thermally activated delayed fluorescent emitters realizing a high external quantum efficiency of 25% and unprecedented low drive voltages in OLEDs. Journal of Materials Chemistry C. 2016; 4(12):2274-2278. DOI: 10.1039/C5TC04057D
Sun JW, Kim KH, Moon CK, Lee JH, Kim JJ. Highly efficient sky-blue fluorescent organic light emitting diode based on mixed cohost system for thermally activated delayed fluorescence emitter (2CzPN). ACS Applied Materials & Interfaces. 2016; 8(15):9806-9810. DOI: 10.1021/acsami.6b00286
Cho YJ, Chin BD, Jeon SK, Lee JY. 20% external quantum efficiency in solution-processed blue thermally activated delayed fluorescent devices. Advanced Functional Materials. 2015; 25(43):6786-6792. DOI: 10.1002/adfm.201502995
Hatakeyama T, Shiren K, Nakajima K, Nomura S, Nakatsuka S, Kinoshita K, Ni J, Ono Y, Ikuta T. Ultrapure blue thermally activated delayed fluorescence molecules: Efficient HOMO–LUMO separation by the multiple resonance effect. Advanced Materials. 2016; 28(14):2777-2781. DOI: 10.1002/adma.201505491
Taneda M, Shizu K, Tanaka H, Adachi C. High efficiency thermally activated delayed fluorescence based on 1, 3, 5-tris (4-(diphenylamino) phenyl)-2, 4, 6-tricyanobenzene. Chemical Communications. 2015; 51(24):5028-5031. DOI: 10.1039/C5CC00511F
Xiang Y, Gong S, Zhao Y, Yin X, Luo J, Wu K, Lu ZH, Yang C. Asymmetric-triazine-cored triads as thermally activated delayed fluorescence emitters for high-efficiency yellow OLEDs with slow efficiency roll-off. Journal of Materials Chemistry C. 2016; 4(42):9998-10004. DOI: 10.1039/C6TC02702D
Tang C, Yang T, Cao X, Tao Y, Wang F, Zhong C, Qian Y, Zhang X, Huang W. Tuning a weak emissive blue host to highly efficient green dopant by a CN in tetracarbazolepyridines for solution-processed thermally activated delayed fluorescence devices. Advanced Optical Materials. 2015; 3(6):786-790. DOI: 10.1002/adom.201500016
Tanaka H, Shizu K, Miyazaki H, Adachi C. Efficient green thermally activated delayed fluorescence (TADF) from a phenoxazine–triphenyltriazine (PXZ–TRZ) derivative. Chemical Communications. 2012; 48(93):11392-11394. DOI: 10.1039/C2CC36237F
Wada Y, Shizu K, Kubo S, Suzuki K, Tanaka H, Adachi C, Kaji H. Highly efficient electroluminescence from a solution-processable thermally activated delayed fluorescence emitter. Applied Physics Letters. 2015; 107(18):105_1. DOI: 10.1063/1.4935237
Wang H, Xie L, Peng Q, Meng L, Wang Y, Yi Y, Wang P. Novel thermally activated delayed fluorescence materials–thioxanthone derivatives and their applications for highly efficient OLEDs. Advanced Materials. 2014; 26(30):5198-5204. DOI: 10.1002/adma.201401393
Lee J, Shizu K, Tanaka H, Nomura H, Yasuda T, Adachi C. Oxadiazole-and triazole-based highly-efficient thermally activated delayed fluorescence emitters for organic light-emitting diodes. Journal of Materials Chemistry C. 2013; 1(30):4599-4604. DOI: 10.1039/C3TC30699B