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Thermally Activated Delayed Fluorescence Materials Towards the Breakthrough of Organoelectronics Ye Tao, Kai Yuan, Ting Chen, Peng Xu, Huanhuan Li, Runfeng Chen,* Chao Zheng, Lei Zhang, and Wei Huang*
emitting diodes (OLEDs),[1–3] organic photovoltaic cells (OPVs),[4–6] organic field-effect transistors (OFETs),[7–9] bio-/ chemo-/photo-sensors,[10–12] memories,[13–15] and even organic lasers.[16,17] Unlike conventional inorganic conductors and semiconductors, organic electronic small molecules and polymers can be synthesized using different synthetic strategies in organic and polymer chemistry. As a result, a large variety of organic semiconductors have been prepared, which significantly promote the progress of organoelectronics with advanced device performance. The prospect of using organic materials for OLEDs has flourished for decades since the pioneering work of Tang and Vanslyke[18] in 1980s, due to the enormous potential of OLEDs for smart phones, flatpanel displays, and solid-light emitting applications. The extensive research in this field to gain insight into the fundamental processes that determine the operation of the devices provides numerous interesting challenges to material design and fundamental questions to generate and transport charges and excitons. It is well known that exciton formation under electrical excitation typically results in 25% singlet excitons and 75% triplet excitons. However, 75% of the electrically generated energy is dissipated as heat by triplet excitons in the fluorescence materials, leading to the theoretically highest external quantum efficiency (EQE) of 5% after considering a light outcoupling efficiency of ∼20% in device. To increase the efficiency of the OLEDs, many efforts to utilize the non-emissive triplet excitons have been devoted to breaking through the 5% limitation of the OLED device. The most successful one is by incorporating heavy metals into the organic aromatic frameworks to increase spin–orbit interactions, which facilitate the lowest triplet excited state (T1) to the ground state (S0) transition (T1→S0) for phosphorescence luminescence.[19,20] This approach harvests light from both triplet and singlet excitons, allowing the internal quantum efficiency of the device to reach nearly 100%. After delicately dispersed in carefully selected host materials, the phosphorescent metal complex exhibits very high external quantum efficiency (EQE), which has been over 30% in doped phosphorescent OLEDs
The design and characterization of thermally activated delayed fluorescence (TADF) materials for optoelectronic applications represents an active area of recent research in organoelectronics. Noble metal-free TADF molecules offer unique optical and electronic properties arising from the efficient transition and interconversion between the lowest singlet (S1) and triplet (T1) excited states. Their ability to harvest triplet excitons for fluorescence through facilitated reverse intersystem crossing (T1→S1) could directly impact their properties and performances, which is attractive for a wide variety of low-cost optoelectronic devices. TADF-based organic light-emitting diodes, oxygen, and temperature sensors show significantly upgraded device performances that are comparable to the ones of traditional rare-metal complexes. Here we present an overview of the quick development in TADF mechanisms, materials, and applications. Fundamental principles on design strategies of TADF materials and the common relationship between the molecular structures and optoelectronic properties for diverse research topics and a survey of recent progress in the development of TADF materials, with a particular emphasis on their different types of metal-organic complexes, D-A molecules, and fullerenes, are highlighted. The success in the breakthrough of the theoretical and technical challenges that arise in developing high-performance TADF materials may pave the way to shape the future of organoelectronics.
1. Introduction Organic electronic materials, including not only organic semiconductors but also organic dielectrics, conductors, and light emitters, have shown controllable optical and electronic properties for a wide range of applications including organic light Y. Tao, K. Yuan, T. Chen, P. Xu, H. H. Li, Dr. R. F. Chen, Dr. C. Zheng, Dr. L. Zhang, Prof. W. Huang Key Laboratory for Organic Electronics and Information Displays (KLOEID) Institute of Advanced Materials (IAM) Nanjing University of Posts and Telecommunications Nanjing 210023, China E-mail: [email protected]; [email protected] Prof. W. Huang Jiangsu-Singapore Joint Research Center for Organic/ Bio-Electronics & Information Displays and Institute of Advanced Materials Nanjing University of Technology Nanjing 210003, China
DOI: 10.1002/adma.201402532
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(PhOLEDs) recently.[21,22] However, the generally used heavy metals for phosphorescence are confined to Iridium (Ir) and Platinum (Pt), which are rather expensive and dependent on limited global resources. In order to avoid the use of expensive metals in practical applications, several other strategies such as triplet–triplet annihilation (TTA),[23] tuning spin–orbit coupling by side-stepping Kasha’s rule,[24] hybridized local and chargetransfer (HLCT)[25–29] and thermally activated delayed fluorescence (TADF)[30–32] have also been proposed to harvest the 75% triplet excitons for luminescence. Among them, TADF was found to have the most rapid progress in recent investigations. Figure 1 shows the important milestones in the development of TADF materials. The first discovery of TADF, which is previously named as E-type delayed fluorescence, can be dated back to 1961,[33] when the first purely organic TADF material was believed to be found by Parker and Hatchard in the eosin dye. The first metal-containing TADF material was observed in a Cu(I)-complex by Blasse and co-workers in 1980.[34] In the late 1990s, the efficient delayed fluorescence was further verified in fullerenes by Berberan-Santos and co-workers, and was firstly used in the detection of oxygen and temperature.[35] The attempt to apply TADF materials in OLEDs appeared in 2009.[36] The device required a rather high onset of current injection around 10 V and 29 V for 100 mA cm−2. Research on TADF OLEDs culminated in 2012 after the work of Adachi and co-workers.[31] With the newly synthesized highly efficient TADF molecules, the EQE of the OLED device reaches up to 30%, which clearly breaks the efficiency limitation of fluorescent OLEDs and is comparable to the rare metal-complex PhOLEDs. The emission colors of TADF materials have been shifted from blue to red; the EQE of OLED devices have reached 19.5% for blue and 30.0% for green TADF emitters, showing their great potential in replacing the common noble metal-based complexes. The oxygen and temperature sensitive nature of the TADF phenomenon enables the application of TADF molecules in highly efficient optical sensors for oxygen with a lower detection limit in the ppbv range and for temperatures in a broad sensoring range from −75°C to 105°C. These remarkable discoveries have truly revolutionized our understandings of organic semiconductors and optoelectronics. In light of the rapid progress in this area, it is necessary to systematically summarize and discuss the TADF mechanism, design principles, and device applications and to point out future developments of TADF materials and devices. In this review, we will focus on the molecular design and properties of TADF materials as well as their recent advances in organoelectronics. We start with a description of the basic principles of TADF in harvesting triplet excitons for photoluminescence, which is a major theoretical breakthrough in photophysics for OLEDs. Next, TADF materials divided into three systems according to their molecular structure design considerations will be presented, followed by thorough coverage of the latest research on OLEDs and sensors involving TADF phenomena. The emphasis will be placed on the illustration of different types of TADF emitters, and on the discussion of recent efforts in device applications such as high-efficiency OLEDs to take advantage of TADF effects. A major goal of this review is to provide illustrative accounts on the recent progress and to systematize our knowledge in this subject, extracting fundamental
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Runfeng Chen received his BE degree in Polymer Science and Engineering and MS degree in Material Science from Tongji University. He did his PhD in Fudan University with Professor Wei Huang and his postdoctoral work at the National University of Singapore with Professor Xiaogang Liu. He joined the faculty of Nanjing University of Posts and Telecommunications in 2006. His present interests are in the development of optoelectronic materials and devices.
Wei Huang received his Ph.D. degree from Peking University in 1992. In 1993, he began his postdoctoral research in National University of Singapore. In 2001, he was appointed as a chair professor with Fudan University, where he founded and chaired the Institute of Advanced Material. In 2006, he was appointed vice president of Nanjing University of Posts and Telecommunications. Now, he is the president of Nanjing University of Technology. He was elected to Chinese Academy of Sciences in 2011. His research interests include organic/plastic materials and devices, nanomaterials and nanotechnology, etc.
principles on design strategies of TADF materials and the common relationship between the molecular structures of TADF compounds and their optoelectronic properties for diverse research topics.
2. Basic Understandings in the TADF Phenomenon The quantum mechanically allowed transition of singlet excitons with an antisymmetric spin and a total spin quantum number of zero (S = 0) to the ground state results in fluorescence within nanoseconds. Conversely, the quantum mechanically forbidden transition of the triplet state with even symmetry and S = 1 to the singlet ground state results in phosphorescence with lifetimes in the microsecond to second regime. Due to the corresponding multiplicities of the angular momentum states (i.e., mS = 0 for S = 0 and mS = -1, 0, 1 for S = 1) and
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REVIEW Figure 1. Milestones in the development of TADF materials. Parker et al. firstly demonstrated the TADF phenomenon in eosin in 1961.[33] The first TADF Cu-complex was reported by Blasse and co-workers in 1980.[34] Berberan-Santos et al. found TADF in fullerenes.[35] Endo and co-workers in 2009 reported a TADF OLED using a Sn(IV)-complex.[36] In 2012, the TADF exciplex was applied in OLED by Goushi et al.[37] The remarkable progress was made by Adachi and co-workers in 2012 with EQE up to 19.3% in TADF OLED.[31] In 2014, Adachi et al. reported a blue TADF OLED with EQE of 19.5%.[38]
the random nature of spin production in OLED devices, simple statistics predict that only 25% of the injected charges result in fluorescence (S1→S0 transition) whereas 75% emit light in the form of phosphorescence (T1→S0 transition). Although recent studies on spin statistics suggest variations in the singlet-totriplet-ratios, the ground states of most luminescent materials are in the singlet state and the majority of luminescent compounds exhibit only weak spin-orbit couplings, rendering negligible radiative deactivation rates from triplet states. In addition to competing non-radiative processes (e.g., triplet-triplet annihilation or vibronic relaxation), the phosphorescence of the associated excited states is effectively quenched, limiting the maximum EQE of non-doped OLED devices of fluorescent small molecules and polymeric materials to ca. 8%.[39,40] 2.1. Fundamental Aspects in Harvesting Excitons The first breakthrough in harvesting triplet excitons for luminescence was made by Forrest et al. in 1998, through incorporating transition metals into organic ligands. The prepared organometallic compounds show strong phosphorescence, due to the fast and efficient intersystem crossing (ISC) from singlet excited states to light-emitting triplet states caused by strong spin-orbit couplings as well as the ability of triplet state emission, allowing theoretically internal quantum efficiencies up to 100% (Figure 2a). Up to now, considerable scientific efforts on
Adv. Mater. 2014, DOI: 10.1002/adma.201402532
these phosphorescent dyes have provided numerous new electrophosphorescent materials with varied transition metals such as iridium, platinum, osmium, ruthenium, etc. for PhOLEDs, exhibiting very desirable material properties such as emission wavelengths covering the entire visible spectrum, high quantum yields, and long lifetimes. However, the use of expensive rare metals inevitably harms their practical applications in display and lighting products, let alone their difficult synthesis and complex device fabrication. Another attempt to utilize the triplet exciton is via the triplet–triplet annihilation (TTA) process, which can convert two triplet excitons into one singlet exciton when there is a large energy gap (ΔEST) between the lowest excited singlet (S1) and triplet (T1) states (2T1 > S1),[41] as shown in Figure 2b. Two triplet excitons will produce one single exciton via an up-conversional TTA process, when the up-converted triplet state (Tm) is close to that of the singlet one (Sn), resulting in p-type delayed fluorescence. With the additional singlet excitons generated by the triplet excitons, the luminescent efficiency can be increased from 15 to 37.5% depending on the up-conversion ratio.[42] Thus, the maximum EQE of TTA OLED devices can be further improved to a maximum 62.5%. In addition, the ability to harvest triplet excitons of TTA molecules renders them efficient host materials for fluorescent guests to break through the singlet production limit (25%) of fluorescent OLEDs.[43] However, in order to promote the non-linear up-conversion TTA process, high driving voltages or concentrated sensitizers are generally
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Figure 2. Summary of processes in harvesting the triplet excitons for luminescence in OLED devices. Rare-metal incorporation (a); Triplet–triplet annihilation (TTA) (b); Hybridized local and charge-transfer (HLCT) excited state structure (c); Thermally activated delayed fluorescence (TADF) (d). F, P, IC, ISC, RISC, VR, NR, ΔEST represent fluorescence (F), phosphorescence (P), internal conversional (IC), intersystem crossing (ISC), reverse intersystem crossing (RISC), vibrational relaxation (VR), non-radiative relaxation (NR) and the singlet-triplet energy splitting (ΔEST), respectively.
essential, accompanied by proportionally increased device efficiency roll-off.[44] The intersystem crossing of excitons between Tm and Sn with close energy levels may also happen in hybridized local and charge-transfer (HLCT) excited state (Figure 2c). The HLCT state combines both local excited (LE) and charge transfer (CT) states into a special one, possessing two combined and compatible characteristics: a large transition moment from the LE state (cold exciton) and a weakly bound exciton from the CT state (hot exciton).[25–29] The LE state contributes to a high-efficiency fluorescence radiative decay, while the CT state ensures the generation of singlet excitons in high yield through the reverse intersystem crossing (RISC) from high-lying CT-based triplet excited state (TCT) back to the CT-based singlet excited state (SCT). Theoretically, 100% non-luminescent triplet excitons can be transferred to singlet ones via RISC, when the internal conversion (IC) from TCT to TLE is blocked, leading to increased SCT and SLE (after IC process) as well as enhanced fluorescence from SLE→S0 with a comparable EQE of the OLED device to that of the conventional metal-complex device. However, according to Kasha's rule, the blocking of the internal conversion process is rather challenging, and the majority of the molecules tend to occupy the lowest singlet or triplet states instead of the high-lying Sn or Tn excited states; consequently, the construction and synthesis of this molecule system with a HLCT excited state structure is a great challenge.[25–29] A more efficient way to harvest triplet excitons via the RISC process is from T1 to S1 as illustrated in Figure 2d. According to Hund's rule, T1 is always lower than S1. This kind of RISC process should be stimulated or activated. When T1 and S1 are close in energy i.e., the singlet-triplet energy splitting (ΔEST) is small, the endothermic RISC process can be overcome by the thermal motions of the molecule atoms. As a result, the nonradiative triplet excitons, due to a spin-forbidden T1→S0 transition, are transformed to singlet excitons via RISC in TADF
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molecules, leading to significantly enhanced luminescence from delayed fluorescence (S1→S0). Thanks to the continuous efforts of investigators, especially Adachi, the thermally activated delayed fluorescence (TADF) has made a great breakthrough from the exciton statistical limit of 25% in fluorescent materials to 100% in a quite large number of rare metal-free organic molecular fluorescent systems.
2.2. Key TADF Processes Two distinct unimolecular mechanisms exist for thermally activated delayed fluorescence (Figure 3): the promote fluorescence (PF) and the delayed fluorescence (DF).[45,46] In the PF mechanism of singlet excitons, the emission occurs almost immediately (within several nanoseconds) after the excitation with a fast decay from the S1 to S0 state. Whereas in the DF mechanism, triplet excitons have to be converted into luminescent singlet excitons for the PF process via a reverse intersystem crossing (RISC) mechanism; the additional process before fluorescence emission delays the luminescent process, leading to an increased fluorescent lifetime up to several microseconds. When TADF materials are optically excited (Figure 3a), triplet excitons are formed after an efficient inter-system crossing (ISC) process and both PF and DF can be observed with two different fluorescent lifetimes although they have the same spectral distribution as normal fluorescence. In OLEDs, four important processes for TADF emission should be mentioned (Figure 3b): (1) singlet and triplet excitons are formed after electron/ hole recombination in a singlet-to-triplet ratio of 1:3; (2) the high exciton states are transferred to the lowest exciton states (S1 or T1) via quick vibrational relaxation (VR) and internal conversion (IC); (3) the accumulated triplet excitons at T1 are back transferred to S1 via RISC process with the aid of thermal activation; (4) the singlet excitons at S1 formed either initially after
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[S1 ] = C1 exp (kPF t ) + C2 exp (kDF t )
(3)
where, kPF , kDF =
S krS + knr + kISC + knrT + kRISC 2 S ⎛ 4 (krS + knr + kISC )(knrT + kRISC ) − 4kISCkRISC × ⎜1 ± 1− 2 ⎜⎝ (krS + knrS + kISC + knrT + kRISC )
⎞ ⎟ ⎟⎠ (4)
When there are negligible deactivation channels from T1, the values of krS, knrS and kISC are significantly larger than those of knrT and kRISC. As a result, kPF and kDF can be approximated by the following formulas: S kPF = krS + knr + kISC
(5)
kISC ⎛ ⎞ kDF = knrT + ⎜ 1 − S S kRISC ⎝ kr + knr + kISC ⎟⎠
(6)
The PL quantum efficiencies of PF and DF components (ΦPF and ΦDF) are written as: Figure 3. Photoluminescence (a) and electroluminescence (b) processes in TADF molecules. k rS and kDF are the rate constants of PF and DF process, respectively; kISC and kRISC are the ISC and RISC rate constants, respectively; knrS and knrT are the non-radiative decay constants of S1 and T1 respectively; ΦPF, ΦDF, ΦRISC, and ΦISC represent the prompt fluorescence efficiency, TADF efficiency, reverse intersystem crossing efficiency, and intersystem crossing efficiency, respectively.
electronic excitation or back-transferred from T1 are radiatively deactivated to S0 following the PF mechanism for fluorescent emissions with different luminescence lifetimes of PF and DF. In these processes, the efficient RISC from T1 to S1 is the key to harvest triplet excitons to enhance the fluorescent luminescence. In order to facilitate the RISC process, small energy level difference between T1 and S1 (singlet-triplet energy splitting, ΔEST) is essential for efficient transition (T1→S1) stimulated via thermal activities of molecular atoms. However, when sustaining such an efficient RISC, the intersystem crossing (ISC) is also, even more, efficient due to the higher energy level of S1 relative to T1. As a result, there is a balance of exciton distribution on S1 and T1 in TADF molecules and the majority of the excitons are located at T1 which is relatively lower in energy. Consequently, T1 should be stable enough for the RISC process from T1 to S1 for fluorescence emission. Some key parameters, such as rate constant of ISC (kISC) and RISC (kRISC) as well as ΔEST, should be taken into strict consideration. Theoretically, the decay rate of S1 and T1 after removing the excitation source can be written in Equation (1) and (2) under consideration of all of their decay channels as illustrated in Figure 3. d [S1 ] S = – (krS + knr + kISC ) [S1 ] + kRISC [ T1 ] d [t]
(1)
d [ T1 ] T = – (knr +kRISC ) [ T1 ] +kISC [S1 ] d [t]
(2)
The solution of these differential equations can be expressed in a biexponential form as
Adv. Mater. 2014, DOI: 10.1002/adma.201402532
krS krS = S k + knr + kISC kPF
ΦPF =
(7)
S r
∞
ΦDF = ∑ ( ΦISC ΦRISC ) ΦPF = k
k =1
ΦISC ΦRISC ΦPF 1 − ΦISC ΦRISC
(8)
where ΦISC and ΦRISC are the intersystem crossing efficiency and reverse intersystem crossing efficiency, respectively, which can be expressed by the follow equations ΦISC =
kISC kISC = S krS + knr + kISC kPF
ΦRISC =
kRISC kRISC + knrT
(9) (10)
Experimentally, the fluorescence efficiency of PF (ΦPF) and DF (ΦDF) are distinguished from the total photoluminescence quantum yields (PLQY) by comparing the integrated intensity of their components in the transient photoluminescence spectra, according to their different luminescent lifetime.[47] The two fluorescent lifetimes (τPF and τDF) can be revealed by fitting the decay curve of the time-resolved PL spectrum. Then their rate constants (kPF and kDF) can be obtained experimentally using Equation (11) and (12) kPF =
ΦPF τ PF
(11)
kDF =
ΦDF τ DF
(12)
2.2.1. The Intersystem Crossing From Equation (5)∼(9), both PF and DF processes are strongly related to the intersystem crossing (ISC), which is a
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non-radiative transition between two iso-energetic vibrational levels belonging to electronic states of different multiplicities.[46] Assuming that the non-radiative rate constants of the T triplet state (k nr ) are significantly lower than kRISC when T1 is relatively stable, the quantum yield of the reverse intersystem crossing is almost 100% (ΦRISC ≈ 1), and thus, the kRISC can be obtained from Equation (8) and (9) kISC
ΦDF = kPF ΦDF + ΦPF
(13)
where the values of ΦDF, ΦPF and kPF can be experimentally measured according to Equation (11). The rate of the intersystem crossing (kISC) was reported to be in the order of 106–1011 s−1 in TADF molecules.[31,48–50]
2.2.2. The Reverse Intersystem Crossing The key process in the TADF phenomenon is the facilitated RISC (T1→S1) when the energy difference between S1 and T1 is small and T1 is stable enough. Because reverse intersystem crossing (RISC) is a non-radiative transition from the lower vibrational level of T1 to the higher one of S1 in an excited molecule, the DF efficiency (ΦDF) and its transient lifetime (τDF) are highly dependent on the RISC rate constant (kRISC). According to Equation (6), (9) and (10), kRISC can be determined as follow: kRISC =
kDF ΦRISC 1 − ΦISC ΦRISC
(14)
With Equation (8) and (9), kRISC can be further transformed
temperature effects on the photoluminescence strength suggest the existence of an optimized temperature for the strongest luminescence of the TADF emission.
2.2.3. The Singlet-Triplet Energy Splitting (ΔEST) For an efficient TADF emission facilitated by the fast RISC process, a small ΔEST is particularly important according to Equation (16). However, S1 is considerably higher than T1 by 0.5–1.0 eV in most light-emitting molecules.[31] To obtain small ΔEST in optical active molecules, special approaches for construction of TADF molecules need to be considered. In principle, the molecular energy of the lowest singlet (ES) and triplet (ET) excited state can be decided by the orbital energy (E), electron repulsion energy (K) and exchange energy (J) of the two unpaired electrons at the excited states, as shown in Equation (17) and (18). Because of the same electron arrangement of the singlet and triplet in one molecule, E, K, and J at the two excited states are the same with each other. However, due to the same spin states of the unpaired electrons in T1, the ET is reduced (Equation (18)) in comparison with increased ES in S1 (Equation (17)). Hence, ΔEST which is the difference between ES and ET, is equal to the twice of J (Equation (19)).[48] ES = E + K + J
(17)
E T = E +K -J
(18)
ΔEST = ES − E T = 2 J
(19)
to: kRISC =
kDFkPF ΦDF kISC ΦPF
(15)
where kDF, kP and kISC can be estimated from the experimentally observable rate constants and the PL quantum efficiencies of the prompt and delayed components (Equation (11) and (12)). From Equation (15), the rate of the reverse intersystem crossing (kRISC) in TADF molecules is generally in the order of 103–106 s−1 in TADF molecules, which is usually lower than kISC.[31,48–50] RISC is a temperature sensitive process. High temperature facilitates this endothermic transition. At low temperature (< 100 K), the PLQYs of TADF emitters are largely suppressed and strong photoexcitation intensity dependence of the emission appears, which are regarded as characteristic features of TADF molecules.[51] The dependence of kRISC on temperature can be expressed in a Boltzmann distribution relation: ⎛ ΔEST ⎞ kRISC ∝ exp ⎜ ⎝ kB T ⎟⎠
(16)
where kB is the Boltzmann constant, T is the temperature, and ΔEST is the singlet-triplet energy splitting. It needs to be pointed out that although the kRISC is enhanced at high temperature, the ΦDF may be decreased due to the simultaneously enhanced non-radiative deactivation processes. This contradictory
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At S1 or T1, the unpaired two electrons are mainly distributed on the frontier orbitals of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively, with the same J value regardless of the different spin states. Therefore, the exchange energy (J) of these two electrons at HOMO and LUMO can be calculated by Equation (20):[52] ⎛ e2 ⎞ J = ∫ ∫ φL (1)φH (2) ⎜ φL (2)φH (1)dr1dr2 ⎝ r1 − r2 ⎟⎠
(20)
where φH and φL represent the HOMO and LUMO wave functions, respectively; e is the electron charge. From Equation (20), it is clear that a small ΔEST can be resulted via a small overlap integral of , i.e., via spatial wave function separation of HOMO and LUMO.[53,54] The general strategy adopted to obtain the spatially separated HOMO and LUMO orbitals is to introduce a large steric hindrance structure[47,52,55] or a donor-acceptor system with twist/spiro/bulky connection which can reduce the overlap between the HOMO and LUMO to enhance the charge transfer state.[56–58] To address this point, various systems consisting of metal-organic complexes, donor-acceptor systems involving the intramolecular and intermolecular D-A systems and fullerenes are reported to have the ability to generate efficient TADF emission, which will be discussed later.
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Since the TADF emitters can realize highly efficient luminescence through utilizing the delay fluorescence (DF), most of their optoelectronic applications were found in OLEDs and sensors. Just like the phosphorescent materials, the TADF emitters also have serious concentration quenching and/or triplet–triplet annihilation at a high concentration of triplet excitons. The emitters also need to be dispersed or doped in a suitable host matrix, which should have the following characteristics:[59–61] (i) sufficiently higher triplet energy (ET) than the TADF emitter to prevent reverse energy transfer from the guest to the host, (ii) suitably aligned HOMO and LUMO levels to ensure effective charge injection from the adjacent layers, (iii) larger HOMO – LUMO energy gap (Eg) than that of the guest to facilitate charge trapping on the dopant emitter, (iv) bipolar charge transport properties for balanced hole and electron densities in the emitting layer, and (v) high morphological stability and film-forming ability. Moreover, the dipole interactions between the host and guest molecules will lead to a time-varying ΔEST for large variations in the PL quantum yield, TADF spectrum, and DF transient lifetime of TADF molecules.[62] Under the electrical excitation in OLEDs, the theoretical maximum external quantum efficiency (EQE) of the TADF OLEDs can be estimated by Equation (21)
as low as 1% but with EQEs up to 21.2%;[64] the EQE can further increase to 30% in co-host system.[65] The device stability of TADF OLEDs is comparable to that of Ir complex-based OLEDs with lifetimes of 2800 h at an initial luminance of 1000 cd m−2 and of over 10 000 h at 500 cd m−2.[66] The utilization of TADF molecules as host materials was also reported to take advantage of the balanced charge transporting/injection properties.[67] In light of the highly dynamic exciton transitions between singlet and triplet states, TADF molecules were further used as singlet exciton sensitizers to harvest triplet excitons in OLEDs for fluorescent emitters; the devices showed a significantly increased EQE and an enhanced device operational stability.[68,69] Triplet excitons are very sensitive to oxygen, in which the two oxygen atoms are chemically bonded to each other with a triplet electron spin configuration at ground state. The quick quenching of triplet exciton upon triplet oxygen makes TADF emitters highly efficient oxygen sensors.[70] Its limit of detection can be as low as 0.25 ppmv.[71] Whereas, the temperature dependent nature of TADF emission opens a door to probe the temperature.[72] Owing to the long-lived luminescence of the DF component of the TADF emission, TADF molecules were also applicable for time-resolved fluorescence imaging in living cells.[73] The great success of TADF materials in the application of OLEDs, sensors, and bio-imaging is just the beginning for their prosperity in organoelectronics when the general concepts in fluorescence were extended with a variety of TADF molecules that can be designed and synthesized rationally.
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2.3. Features of TADF Materials for Optoelectronic Devices
EQE = ηintηout =γηrηPLηout ⎡∞ k k ⎤ = γ ⎢ ∑ 0.75ΦPF ΦRISC ( ΦISC ΦRISC ) +0.25ΦPF ( ΦISC ΦRISC ) ⎥ ηout ⎣ k=0 ⎦ 0.75 + 0.25(1-ΦPF ) ⎡ ⎤ = γ ⎢0.25ΦPF + ΦDF ⎥ ηout 1-ΦPF ⎣ ⎦ ΦDF ⎤ ⎡ ηout = γ ⎢0.25( ΦPF + ΦDF ) +0.75 1 − ΦPF ⎥⎦ ⎣ ⎡ ⎤ ΦDF = γ ⎢0.25ηPL +0.75 ⎥ ηout η 1 ( ) − − Φ PL DF ⎦ ⎣
(
)
(21) where ηint is the internal quantum efficiency; ηout is the outcoupling constant; γ is the ratio of the charge combination to the electron and hole transportation; ηr is the excitation–production singlet-to-triplet ratio; ηPL is the photoluminescence efficiency; ΦPF is the photoluminescence quantum yield of the prompt component; and ΦDF is the photoluminescent quantum yield of the delayed component. If ΦPF + ΦDF = ηPL = 1, then internal quantum efficiency can be 100% according to Equation (21). It should be noted that a high degree of the orientation of the transition dipole moment of TADF emitters will effectively increase the ηout, leading to a high EQE of the device.[63] Since the first report of the TADF phenomenon that was initially called as E-type delayed emission in 1961,[33] significant progress has been made to develop highly efficient TADF emitters involving metal (Cu(I), Ag(I), Au(I), and Sn(IV))-organic complexes, D-A molecule systems, and fullerenes for organoelectronics. The doping concentration of TADF emitters can be
Adv. Mater. 2014, DOI: 10.1002/adma.201402532
3. Metal-Organic Complexes for TADF Emission Previous studies on luminescent metal-organic complexes were principally focused on third-row (Ir, Pt, or Os) and second-row (Ru) rare metal-containing phosphors that have large spin-orbit coupling (SOC) interactions to overcome the spin-forbidden nature of the triplet to singlet ground state transition (T1→S0) for phosphorescence. In contrast to these d6 and d8 metal complexes, the first-row d10 metal complexes often have much weaker spin-orbit coupling and the T1→S0 transition is largely forbidden. Fortunately, they generally have a small ΔEST and a stable T1, which are two key features of the TADF phenomenon. The small ΔEST values of d10 metal-organic complexes composed of d10 metal (M) moiety and large steric hindrance ligands (L), can be rationalized by a distinct charge transfer from the metal center to the ligand (MLCT),[74] due to a pronounced charge separation between the metal and ligand upon excitation, which can be regarded as a formal oxidation of M(I) to M(II) in the excited state. The stable T1 is related to the rigid metal complex structure with a suppressed non-radiative deactivation process and the weak phosphorescence (T1→S0) with low SOC interactions. Furthermore, the close shell d10 metal complexes possess full d-orbitals and the internal quenching of excited states by low-lying d–d* states cannot occur. These characteristics of d10 metal complexes with low cost and toxicity are advantageous for the design of highly emissive TADF complexes.[75–78] The physical properties and electroluminescence performance of the representative TADF complexes are summarized in Table 1 and 2.
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Table 1. PL peak (λ), PL quantmum yield (Φ), lifetime at room temperature (τ), singlet-triplet energy gap (ΔEST) of TADF metal complexes. Complex
λ/nm
Φ/%
τ / µs
ΔEST/eV
Complex
λ/nm
Φ/%
τ / µs
[34]
20
523a)
68
[30]
21
521a)
52
22
d)
520a) 527d)
47
5.4
514a)
71
5.5
Ref.
1
543
a)
14
8.1
0.12
2
509a)
32
18.1
–
3
504a)
57
32.9
–
[30]
4
555a)
16
4.6
–
[30]
5
527a)
49
13.2
–
[30]
6
519a)
69
20.3
–
[75]
11
12
13
523b)
–
–
–
530c)
80
14
0.12
535a)
65
20
558d)
40
10
490c)
56
20.4
590d)
2.1
1.6
465c)
87
12.2
536d) 14
492c) 540d)
15
436c) a)
462
535d) 16
17
45 75 30 45 35 9
24 [84] 25 [79]
26
8.2
0.18
[87]
2.1
0.06
[74]
43
4.9
–
[88]
68
6.1 0.07
[88]
–
[88]
0.09
[89]
0.08
[89]
517d)
60
6.5
504a)
57
3.2
488b)
95
6.6
d)
24
1.4
500b)
95
5.0
592 0.18
Ref.
d)
15
1.0
27
524a)
57
11.3
0.10
[76]
28
539
c)
80
6.5
0.05
[90]
29
506c)
45
6.6
0.06
[91]
30
c)
490
65
4.1
0.05
[91]
31
464c)
65
4.6
0.07
[91]
1.3
32
c)
465
65
5.6
0.08
[91]
33
505c)
32
2.8
0.02
[74]
34
610c)
12
2.1
0.05
[74]
35
571
c)
–
–
0.41
[36]
36
570c)
–
–
0.40
[36]
c)
0.17
22.8
0.18
[79]
13.3 20
0.16
[85,86]
22
447
90
22
457a)
30
24
500d)
2
0.5
464c)
90
13
0.12
0.10
546
[79]
11.9
c)
a)
23
534
ΔEST/eV
[85,86]
[85,86]
466
41
23
37
576
–
–
0.38
[36]
498d)
8
1.8
38
579c)
–
–
0.39
[36]
18
545a)
50
3.8
0.16
[87]
39
c)
571
–
–
0.40
[36]
19
534a)
63
3.6
0.20
[87]
40
569c)
–
–
0.41
[36]
a)
Measured in doped film; b)Measured in pristine film; c)Measured in powder; d)Measured in solution.
3.1. Cu(I) Complexes As a typical d10 metal, Cu(I) is the most widely investigated one used to construct d10 metal-organic complexes that have lowlying MLCT excited states with small singlet-triplet energy gap (ΔEST) to efficiently harvest triplet excitons for TADF emission. Cu(I) complexes usually have a distorted tetrahedral geometry around the Cu(I) atom connected with a diimine ligand and an ancillary diphosphine ligand (Figure 4). The pseudo tetrahedral structures, unfortunately, often exhibit a bit low quantum yield due to increased non-radiative decay caused by the distortion of excited states. To overcome this problem, the most notable way in the design of highly luminescent Cu(I) complexes for TADF emission is to provide a firm framework of bulky chromophoric ligands around Cu(I), suppressing the molecular geometry changes and shielding the metal center from the environment (Figure 4a). Other ways include using the trigonal-planar complex structure with rigid ligands to hamper the Jahn-Teller distortion (Figure 4b) and adopting the multinuclear Cu(I)
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complex structure, especially dinuclear one (Figure 4c), to profit from the strong covalency of Cu2N2 core.
3.1.1. Four-Coordinate Mononuclear Cu(I) Complexes Four-coordinate mononuclear Cu(I) complexes have a tetrahedral geometry around the Cu(I) atom connected with a diimine ligand and an ancillary diphosphine ligand (Figure 4a). As mentioned previously, the tetrahedral d10 metal complexes with two bidentate reducible aromatic ligands, i.e., with energetically low-lying π* orbitals, may undergo efficient non-radiative transitions caused by a structural change from tetrahedral to planar geometry because of the MLCT character of their excited states.[79] Such structural rearrangement is especially distinct in non-rigid environments. To achieve more separated orbital overlap and highly efficient TADF, the enhancement of structural rigidity of both complex and ligand must be taken into consideration. A distorted tetrahedral geometry by chromophoric
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Table 2. The photophysical properties and OLED performance of representative TADF materials. HOMO
LUMO
λ/nm
ΔEST/eV
Host
ηext (%)
Ref.
4
–
–
555
–
PVK
ITO/PEDOT:PSS/1 wt% 4: PVK/BCP/Alq3/LiF/Al
–
[30]
6
–
–
519
–
PVK
ITO/PEDOT:PSS /16 wt% 6: PVK/BCP/Alq3/LiF/Al
–
[30]
12
5.41
2.23
590
0.18
26mCPy
ITO/PEDOT:PSS/20 wt% 12: 26mCPy /DPEPO /LiF/Al
3.18
[79]
13
5.39
2.16
536
0.17
26mCPy
ITO/PEDOT:PSS/20 wt% 13: 26mCPy /DPEPO /LiF/Al
1.59
[79]
14
5.53
2.49
540
0.18
26mCPy
ITO/PEDOT:PSS/20 wt% 14: 26mCPy /DPEPO /LiF/Al
8.47
[79]
18
5.13
1.17
545
0.16
mCP
ITO/TAPC/10 wt% 18: mCP/3TPYMB/LiF/Al
11.9
[87]
19
5.61
1.60
534
0.20
mCP
ITO/TAPC/10 wt% 19: mCP/3TPYMB/LiF/Al
16.0
[87]
20
5.87
2.06
523
0.18
mCP
ITO/TAPC/10 wt% 20: mCP/3TPYMB/LiF/Al
17.7
[87]
TADF
21
4.7
1.9
521
0.06
mCP: TAPC
23
–
–
527
0.07
mCP
27
–
–
524
0.10
CBP: TAPC
Device Structure
ITO/PEDOT: PSS/PVK/10 wt% 21: 30 wt%TAPC: mCP/3TPYMB/Al
7.8
[74]
ITO/TAPC/10 wt% 23: mCP/3TPYMB /LiF/Al
21.3
[88]
ITO/CFx/TAPC/25 wt% TAPC: CBP/25 wt% TAPC: 0.2 wt% 27: CBP/CBP/ BAlq-13/LiF/Al
16.1
[76]
39
–
–
571
0.40
PVCz
ITO/PEDOT:PSS/2 wt%39: PVCz/MgAg/Ag
41
–
–
–
0.057
mCP
ITO/α-NPD/6 wt% 41: mCP/Bphen/MgAg/Ag
–
[36]
4.4
[47]
42
6.07
2.53
530
0.028
TPSiF
ITO/TAPC/mCP/6 wt% 42: TPSiF/TmPyPB/LiF/Al
44
5.8
3.0
473
0.09
PPT
ITO/α-NPD/5 wt% 44: PPT/PPT/LiF/Al
10.1
[52]
8.0 ± 1
45
5.8
3.4
507
0.083
CBP
ITO/α-NPD/5 wt% 45: CBP/TPBI/LiF/Al
19.3 ± 1.5
[31] [31]
ITO/α-NPD/5 wt% 48: CBP/TPBI/LiF/Al
48
–
–
577
–
–
11.2 ± 1
[31]
49
6.2
3.2
470
0.14
DPEPO
ITO/α-NPD/mCP/10 wt% 49: DPEPO/TPBi/LiF/Al
9.2
[59]
50
6.05
3.15
488
0.06
DPEPO
ITO/α-NPD/mCP/10 wt% 50: DPEPO/TPBi/LiF/Al
9.6
[106]
51
6.2
3.5
∼490
0.08
PzCz
ITO/α-NPD/mCP/3 wt% 51: PzCz/PPT/LiF/Al
15.0
[106]
52
5.37
2.7
466
0.11
mCP
ITO/α-NPD/mCP/6 wt% 52: mCP/BP4mPy/LiF/Al
5.3
[67,109]
54
–
–
506
0.02
PYD2
ITO/TAPC/6 wt% 54 : PYD2/DPEPO/DPEPO/LiF/Al
14 ± 1
[110]
55
5.9
2.6
435/513
0.06
mCP: DPEPPO
56
5.49
2.77
526
0.085/0.008
DPEPO
ITO/α-NPD/6 wt% 55: mCP/6 wt% 55: DPEPO/DPEPO/TPBI/LiF/Al
11
[58]
ITO/α-NPD/TCTA/CzSi/3 wt% 56: DPEPO/DPEPO/TPBI/LiF/Al
6
[111,112]
ITO/α-NPD/6 wt% 57: CBP/TPBi/LiF/Al
57
5.5
3.1
545
0.07
CBP
12.5
[113]
58
5.7
3.4
560
0.054
mCBP
ITO/α-NPD/6 wt% 58: mCBP/TPBi/LiF/Al
9.1 ± 0.5
[115]
59
5.7
3.4
568
0.065
mCBP
ITO/α-NPD/6 wt% 59: mCBP/TPBi/LiF/Al
13.3 ± 0.5
[115]
62
–
–
502
0.15
DPEPO
ITO/α-NPD/mCP/6 wt% 62: DPEPO/DPEPO/TPBi/LiF/Al
14.9
[119]
64
–
–
462
0.23
DPEPO
ITO/α-NPD/mCP/6 wt% 64: DPEPO /DPEPO/TPBi/LiF/Al
6.4
[119]
65
5.6
3.4
560
0.17
26mCPy
ITO/α-NPD/6 wt% 65: 26mCPy/Bphen/MgAg/Ag
17.5
[120]
66
5.6
3.1
529
0.09
CBP
ITO/α-NPD /6wt% 66: CBP /TPBi /LiF /Al
11.7
[121]
67
5.8
3.0
492
0.16
mCP
ITO/α-NPD/mCP/6wt% 67: mCP /PPT/TPBi/LiF/Al
7.5
[121]
68
5.7
3.1
524
0.04
mCP
ITO/α-NPD /6wt% 68: mCP /TPBi /LiF /Al
12.6
[121]
69
5.9
3.1
483
0.13
mCBP
ITO/α-NPD/mCP/6wt% 69: mCBP /PPT/TPBi/LiF/Al
8.7
[121]
70
5.7
3.1
499
0.26
mCP
ITO/α-NPD/mCP/6wt% 70: mCP /PPT/TPBi/LiF/Al
7.5
[121]
71
5.89
2.62
402
0.54
DPEPO
ITO/α-NPD/TCTA/CzSi/10 wt% 71: DPEPO/DPEPO/TPBi/LiF/Al
2.9
[95]
72
5.65
2.46
419
0.45
DPEPO
ITO/α-NPD/TCTA/CzSi/10 wt% 72: DPEPO /DPEPO/TPBi/LiF/Al
5.6
[95]
73
5.81
2.52
404
0.32
DPEPO
ITO/α-NPD/TCTA/CzSi/10 wt% 73: DPEPO /DPEPO/TPBi/LiF/Al
9.9
[95]
74
5.55
2.43
445
0.21
DPEPO
ITO/α-NPD/TCTA/CzSi/10 wt% 74: DPEPO /DPEPO/TPBi/LiF/Al
14.5
[123]
79
5.10
2.69
577
0.09
CBP
ITO/α-NPD/TCTA/CzSi/79: DPEPO/DPEPO/TPBi/LiF/Al
REVIEW
Thermally Activated Delayed Fluorescence Materials Towards the Breakthrough of Organoelectronics Ye Tao, Kai Yuan, Ting Chen, Peng Xu, Huanhuan Li, Runfeng Chen,* Chao Zheng, Lei Zhang, and Wei Huang*
emitting diodes (OLEDs),[1–3] organic photovoltaic cells (OPVs),[4–6] organic field-effect transistors (OFETs),[7–9] bio-/ chemo-/photo-sensors,[10–12] memories,[13–15] and even organic lasers.[16,17] Unlike conventional inorganic conductors and semiconductors, organic electronic small molecules and polymers can be synthesized using different synthetic strategies in organic and polymer chemistry. As a result, a large variety of organic semiconductors have been prepared, which significantly promote the progress of organoelectronics with advanced device performance. The prospect of using organic materials for OLEDs has flourished for decades since the pioneering work of Tang and Vanslyke[18] in 1980s, due to the enormous potential of OLEDs for smart phones, flatpanel displays, and solid-light emitting applications. The extensive research in this field to gain insight into the fundamental processes that determine the operation of the devices provides numerous interesting challenges to material design and fundamental questions to generate and transport charges and excitons. It is well known that exciton formation under electrical excitation typically results in 25% singlet excitons and 75% triplet excitons. However, 75% of the electrically generated energy is dissipated as heat by triplet excitons in the fluorescence materials, leading to the theoretically highest external quantum efficiency (EQE) of 5% after considering a light outcoupling efficiency of ∼20% in device. To increase the efficiency of the OLEDs, many efforts to utilize the non-emissive triplet excitons have been devoted to breaking through the 5% limitation of the OLED device. The most successful one is by incorporating heavy metals into the organic aromatic frameworks to increase spin–orbit interactions, which facilitate the lowest triplet excited state (T1) to the ground state (S0) transition (T1→S0) for phosphorescence luminescence.[19,20] This approach harvests light from both triplet and singlet excitons, allowing the internal quantum efficiency of the device to reach nearly 100%. After delicately dispersed in carefully selected host materials, the phosphorescent metal complex exhibits very high external quantum efficiency (EQE), which has been over 30% in doped phosphorescent OLEDs
The design and characterization of thermally activated delayed fluorescence (TADF) materials for optoelectronic applications represents an active area of recent research in organoelectronics. Noble metal-free TADF molecules offer unique optical and electronic properties arising from the efficient transition and interconversion between the lowest singlet (S1) and triplet (T1) excited states. Their ability to harvest triplet excitons for fluorescence through facilitated reverse intersystem crossing (T1→S1) could directly impact their properties and performances, which is attractive for a wide variety of low-cost optoelectronic devices. TADF-based organic light-emitting diodes, oxygen, and temperature sensors show significantly upgraded device performances that are comparable to the ones of traditional rare-metal complexes. Here we present an overview of the quick development in TADF mechanisms, materials, and applications. Fundamental principles on design strategies of TADF materials and the common relationship between the molecular structures and optoelectronic properties for diverse research topics and a survey of recent progress in the development of TADF materials, with a particular emphasis on their different types of metal-organic complexes, D-A molecules, and fullerenes, are highlighted. The success in the breakthrough of the theoretical and technical challenges that arise in developing high-performance TADF materials may pave the way to shape the future of organoelectronics.
1. Introduction Organic electronic materials, including not only organic semiconductors but also organic dielectrics, conductors, and light emitters, have shown controllable optical and electronic properties for a wide range of applications including organic light Y. Tao, K. Yuan, T. Chen, P. Xu, H. H. Li, Dr. R. F. Chen, Dr. C. Zheng, Dr. L. Zhang, Prof. W. Huang Key Laboratory for Organic Electronics and Information Displays (KLOEID) Institute of Advanced Materials (IAM) Nanjing University of Posts and Telecommunications Nanjing 210023, China E-mail: [email protected]; [email protected] Prof. W. Huang Jiangsu-Singapore Joint Research Center for Organic/ Bio-Electronics & Information Displays and Institute of Advanced Materials Nanjing University of Technology Nanjing 210003, China
DOI: 10.1002/adma.201402532
Adv. Mater. 2014, DOI: 10.1002/adma.201402532
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(PhOLEDs) recently.[21,22] However, the generally used heavy metals for phosphorescence are confined to Iridium (Ir) and Platinum (Pt), which are rather expensive and dependent on limited global resources. In order to avoid the use of expensive metals in practical applications, several other strategies such as triplet–triplet annihilation (TTA),[23] tuning spin–orbit coupling by side-stepping Kasha’s rule,[24] hybridized local and chargetransfer (HLCT)[25–29] and thermally activated delayed fluorescence (TADF)[30–32] have also been proposed to harvest the 75% triplet excitons for luminescence. Among them, TADF was found to have the most rapid progress in recent investigations. Figure 1 shows the important milestones in the development of TADF materials. The first discovery of TADF, which is previously named as E-type delayed fluorescence, can be dated back to 1961,[33] when the first purely organic TADF material was believed to be found by Parker and Hatchard in the eosin dye. The first metal-containing TADF material was observed in a Cu(I)-complex by Blasse and co-workers in 1980.[34] In the late 1990s, the efficient delayed fluorescence was further verified in fullerenes by Berberan-Santos and co-workers, and was firstly used in the detection of oxygen and temperature.[35] The attempt to apply TADF materials in OLEDs appeared in 2009.[36] The device required a rather high onset of current injection around 10 V and 29 V for 100 mA cm−2. Research on TADF OLEDs culminated in 2012 after the work of Adachi and co-workers.[31] With the newly synthesized highly efficient TADF molecules, the EQE of the OLED device reaches up to 30%, which clearly breaks the efficiency limitation of fluorescent OLEDs and is comparable to the rare metal-complex PhOLEDs. The emission colors of TADF materials have been shifted from blue to red; the EQE of OLED devices have reached 19.5% for blue and 30.0% for green TADF emitters, showing their great potential in replacing the common noble metal-based complexes. The oxygen and temperature sensitive nature of the TADF phenomenon enables the application of TADF molecules in highly efficient optical sensors for oxygen with a lower detection limit in the ppbv range and for temperatures in a broad sensoring range from −75°C to 105°C. These remarkable discoveries have truly revolutionized our understandings of organic semiconductors and optoelectronics. In light of the rapid progress in this area, it is necessary to systematically summarize and discuss the TADF mechanism, design principles, and device applications and to point out future developments of TADF materials and devices. In this review, we will focus on the molecular design and properties of TADF materials as well as their recent advances in organoelectronics. We start with a description of the basic principles of TADF in harvesting triplet excitons for photoluminescence, which is a major theoretical breakthrough in photophysics for OLEDs. Next, TADF materials divided into three systems according to their molecular structure design considerations will be presented, followed by thorough coverage of the latest research on OLEDs and sensors involving TADF phenomena. The emphasis will be placed on the illustration of different types of TADF emitters, and on the discussion of recent efforts in device applications such as high-efficiency OLEDs to take advantage of TADF effects. A major goal of this review is to provide illustrative accounts on the recent progress and to systematize our knowledge in this subject, extracting fundamental
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Runfeng Chen received his BE degree in Polymer Science and Engineering and MS degree in Material Science from Tongji University. He did his PhD in Fudan University with Professor Wei Huang and his postdoctoral work at the National University of Singapore with Professor Xiaogang Liu. He joined the faculty of Nanjing University of Posts and Telecommunications in 2006. His present interests are in the development of optoelectronic materials and devices.
Wei Huang received his Ph.D. degree from Peking University in 1992. In 1993, he began his postdoctoral research in National University of Singapore. In 2001, he was appointed as a chair professor with Fudan University, where he founded and chaired the Institute of Advanced Material. In 2006, he was appointed vice president of Nanjing University of Posts and Telecommunications. Now, he is the president of Nanjing University of Technology. He was elected to Chinese Academy of Sciences in 2011. His research interests include organic/plastic materials and devices, nanomaterials and nanotechnology, etc.
principles on design strategies of TADF materials and the common relationship between the molecular structures of TADF compounds and their optoelectronic properties for diverse research topics.
2. Basic Understandings in the TADF Phenomenon The quantum mechanically allowed transition of singlet excitons with an antisymmetric spin and a total spin quantum number of zero (S = 0) to the ground state results in fluorescence within nanoseconds. Conversely, the quantum mechanically forbidden transition of the triplet state with even symmetry and S = 1 to the singlet ground state results in phosphorescence with lifetimes in the microsecond to second regime. Due to the corresponding multiplicities of the angular momentum states (i.e., mS = 0 for S = 0 and mS = -1, 0, 1 for S = 1) and
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REVIEW Figure 1. Milestones in the development of TADF materials. Parker et al. firstly demonstrated the TADF phenomenon in eosin in 1961.[33] The first TADF Cu-complex was reported by Blasse and co-workers in 1980.[34] Berberan-Santos et al. found TADF in fullerenes.[35] Endo and co-workers in 2009 reported a TADF OLED using a Sn(IV)-complex.[36] In 2012, the TADF exciplex was applied in OLED by Goushi et al.[37] The remarkable progress was made by Adachi and co-workers in 2012 with EQE up to 19.3% in TADF OLED.[31] In 2014, Adachi et al. reported a blue TADF OLED with EQE of 19.5%.[38]
the random nature of spin production in OLED devices, simple statistics predict that only 25% of the injected charges result in fluorescence (S1→S0 transition) whereas 75% emit light in the form of phosphorescence (T1→S0 transition). Although recent studies on spin statistics suggest variations in the singlet-totriplet-ratios, the ground states of most luminescent materials are in the singlet state and the majority of luminescent compounds exhibit only weak spin-orbit couplings, rendering negligible radiative deactivation rates from triplet states. In addition to competing non-radiative processes (e.g., triplet-triplet annihilation or vibronic relaxation), the phosphorescence of the associated excited states is effectively quenched, limiting the maximum EQE of non-doped OLED devices of fluorescent small molecules and polymeric materials to ca. 8%.[39,40] 2.1. Fundamental Aspects in Harvesting Excitons The first breakthrough in harvesting triplet excitons for luminescence was made by Forrest et al. in 1998, through incorporating transition metals into organic ligands. The prepared organometallic compounds show strong phosphorescence, due to the fast and efficient intersystem crossing (ISC) from singlet excited states to light-emitting triplet states caused by strong spin-orbit couplings as well as the ability of triplet state emission, allowing theoretically internal quantum efficiencies up to 100% (Figure 2a). Up to now, considerable scientific efforts on
Adv. Mater. 2014, DOI: 10.1002/adma.201402532
these phosphorescent dyes have provided numerous new electrophosphorescent materials with varied transition metals such as iridium, platinum, osmium, ruthenium, etc. for PhOLEDs, exhibiting very desirable material properties such as emission wavelengths covering the entire visible spectrum, high quantum yields, and long lifetimes. However, the use of expensive rare metals inevitably harms their practical applications in display and lighting products, let alone their difficult synthesis and complex device fabrication. Another attempt to utilize the triplet exciton is via the triplet–triplet annihilation (TTA) process, which can convert two triplet excitons into one singlet exciton when there is a large energy gap (ΔEST) between the lowest excited singlet (S1) and triplet (T1) states (2T1 > S1),[41] as shown in Figure 2b. Two triplet excitons will produce one single exciton via an up-conversional TTA process, when the up-converted triplet state (Tm) is close to that of the singlet one (Sn), resulting in p-type delayed fluorescence. With the additional singlet excitons generated by the triplet excitons, the luminescent efficiency can be increased from 15 to 37.5% depending on the up-conversion ratio.[42] Thus, the maximum EQE of TTA OLED devices can be further improved to a maximum 62.5%. In addition, the ability to harvest triplet excitons of TTA molecules renders them efficient host materials for fluorescent guests to break through the singlet production limit (25%) of fluorescent OLEDs.[43] However, in order to promote the non-linear up-conversion TTA process, high driving voltages or concentrated sensitizers are generally
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Figure 2. Summary of processes in harvesting the triplet excitons for luminescence in OLED devices. Rare-metal incorporation (a); Triplet–triplet annihilation (TTA) (b); Hybridized local and charge-transfer (HLCT) excited state structure (c); Thermally activated delayed fluorescence (TADF) (d). F, P, IC, ISC, RISC, VR, NR, ΔEST represent fluorescence (F), phosphorescence (P), internal conversional (IC), intersystem crossing (ISC), reverse intersystem crossing (RISC), vibrational relaxation (VR), non-radiative relaxation (NR) and the singlet-triplet energy splitting (ΔEST), respectively.
essential, accompanied by proportionally increased device efficiency roll-off.[44] The intersystem crossing of excitons between Tm and Sn with close energy levels may also happen in hybridized local and charge-transfer (HLCT) excited state (Figure 2c). The HLCT state combines both local excited (LE) and charge transfer (CT) states into a special one, possessing two combined and compatible characteristics: a large transition moment from the LE state (cold exciton) and a weakly bound exciton from the CT state (hot exciton).[25–29] The LE state contributes to a high-efficiency fluorescence radiative decay, while the CT state ensures the generation of singlet excitons in high yield through the reverse intersystem crossing (RISC) from high-lying CT-based triplet excited state (TCT) back to the CT-based singlet excited state (SCT). Theoretically, 100% non-luminescent triplet excitons can be transferred to singlet ones via RISC, when the internal conversion (IC) from TCT to TLE is blocked, leading to increased SCT and SLE (after IC process) as well as enhanced fluorescence from SLE→S0 with a comparable EQE of the OLED device to that of the conventional metal-complex device. However, according to Kasha's rule, the blocking of the internal conversion process is rather challenging, and the majority of the molecules tend to occupy the lowest singlet or triplet states instead of the high-lying Sn or Tn excited states; consequently, the construction and synthesis of this molecule system with a HLCT excited state structure is a great challenge.[25–29] A more efficient way to harvest triplet excitons via the RISC process is from T1 to S1 as illustrated in Figure 2d. According to Hund's rule, T1 is always lower than S1. This kind of RISC process should be stimulated or activated. When T1 and S1 are close in energy i.e., the singlet-triplet energy splitting (ΔEST) is small, the endothermic RISC process can be overcome by the thermal motions of the molecule atoms. As a result, the nonradiative triplet excitons, due to a spin-forbidden T1→S0 transition, are transformed to singlet excitons via RISC in TADF
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molecules, leading to significantly enhanced luminescence from delayed fluorescence (S1→S0). Thanks to the continuous efforts of investigators, especially Adachi, the thermally activated delayed fluorescence (TADF) has made a great breakthrough from the exciton statistical limit of 25% in fluorescent materials to 100% in a quite large number of rare metal-free organic molecular fluorescent systems.
2.2. Key TADF Processes Two distinct unimolecular mechanisms exist for thermally activated delayed fluorescence (Figure 3): the promote fluorescence (PF) and the delayed fluorescence (DF).[45,46] In the PF mechanism of singlet excitons, the emission occurs almost immediately (within several nanoseconds) after the excitation with a fast decay from the S1 to S0 state. Whereas in the DF mechanism, triplet excitons have to be converted into luminescent singlet excitons for the PF process via a reverse intersystem crossing (RISC) mechanism; the additional process before fluorescence emission delays the luminescent process, leading to an increased fluorescent lifetime up to several microseconds. When TADF materials are optically excited (Figure 3a), triplet excitons are formed after an efficient inter-system crossing (ISC) process and both PF and DF can be observed with two different fluorescent lifetimes although they have the same spectral distribution as normal fluorescence. In OLEDs, four important processes for TADF emission should be mentioned (Figure 3b): (1) singlet and triplet excitons are formed after electron/ hole recombination in a singlet-to-triplet ratio of 1:3; (2) the high exciton states are transferred to the lowest exciton states (S1 or T1) via quick vibrational relaxation (VR) and internal conversion (IC); (3) the accumulated triplet excitons at T1 are back transferred to S1 via RISC process with the aid of thermal activation; (4) the singlet excitons at S1 formed either initially after
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[S1 ] = C1 exp (kPF t ) + C2 exp (kDF t )
(3)
where, kPF , kDF =
S krS + knr + kISC + knrT + kRISC 2 S ⎛ 4 (krS + knr + kISC )(knrT + kRISC ) − 4kISCkRISC × ⎜1 ± 1− 2 ⎜⎝ (krS + knrS + kISC + knrT + kRISC )
⎞ ⎟ ⎟⎠ (4)
When there are negligible deactivation channels from T1, the values of krS, knrS and kISC are significantly larger than those of knrT and kRISC. As a result, kPF and kDF can be approximated by the following formulas: S kPF = krS + knr + kISC
(5)
kISC ⎛ ⎞ kDF = knrT + ⎜ 1 − S S kRISC ⎝ kr + knr + kISC ⎟⎠
(6)
The PL quantum efficiencies of PF and DF components (ΦPF and ΦDF) are written as: Figure 3. Photoluminescence (a) and electroluminescence (b) processes in TADF molecules. k rS and kDF are the rate constants of PF and DF process, respectively; kISC and kRISC are the ISC and RISC rate constants, respectively; knrS and knrT are the non-radiative decay constants of S1 and T1 respectively; ΦPF, ΦDF, ΦRISC, and ΦISC represent the prompt fluorescence efficiency, TADF efficiency, reverse intersystem crossing efficiency, and intersystem crossing efficiency, respectively.
electronic excitation or back-transferred from T1 are radiatively deactivated to S0 following the PF mechanism for fluorescent emissions with different luminescence lifetimes of PF and DF. In these processes, the efficient RISC from T1 to S1 is the key to harvest triplet excitons to enhance the fluorescent luminescence. In order to facilitate the RISC process, small energy level difference between T1 and S1 (singlet-triplet energy splitting, ΔEST) is essential for efficient transition (T1→S1) stimulated via thermal activities of molecular atoms. However, when sustaining such an efficient RISC, the intersystem crossing (ISC) is also, even more, efficient due to the higher energy level of S1 relative to T1. As a result, there is a balance of exciton distribution on S1 and T1 in TADF molecules and the majority of the excitons are located at T1 which is relatively lower in energy. Consequently, T1 should be stable enough for the RISC process from T1 to S1 for fluorescence emission. Some key parameters, such as rate constant of ISC (kISC) and RISC (kRISC) as well as ΔEST, should be taken into strict consideration. Theoretically, the decay rate of S1 and T1 after removing the excitation source can be written in Equation (1) and (2) under consideration of all of their decay channels as illustrated in Figure 3. d [S1 ] S = – (krS + knr + kISC ) [S1 ] + kRISC [ T1 ] d [t]
(1)
d [ T1 ] T = – (knr +kRISC ) [ T1 ] +kISC [S1 ] d [t]
(2)
The solution of these differential equations can be expressed in a biexponential form as
Adv. Mater. 2014, DOI: 10.1002/adma.201402532
krS krS = S k + knr + kISC kPF
ΦPF =
(7)
S r
∞
ΦDF = ∑ ( ΦISC ΦRISC ) ΦPF = k
k =1
ΦISC ΦRISC ΦPF 1 − ΦISC ΦRISC
(8)
where ΦISC and ΦRISC are the intersystem crossing efficiency and reverse intersystem crossing efficiency, respectively, which can be expressed by the follow equations ΦISC =
kISC kISC = S krS + knr + kISC kPF
ΦRISC =
kRISC kRISC + knrT
(9) (10)
Experimentally, the fluorescence efficiency of PF (ΦPF) and DF (ΦDF) are distinguished from the total photoluminescence quantum yields (PLQY) by comparing the integrated intensity of their components in the transient photoluminescence spectra, according to their different luminescent lifetime.[47] The two fluorescent lifetimes (τPF and τDF) can be revealed by fitting the decay curve of the time-resolved PL spectrum. Then their rate constants (kPF and kDF) can be obtained experimentally using Equation (11) and (12) kPF =
ΦPF τ PF
(11)
kDF =
ΦDF τ DF
(12)
2.2.1. The Intersystem Crossing From Equation (5)∼(9), both PF and DF processes are strongly related to the intersystem crossing (ISC), which is a
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non-radiative transition between two iso-energetic vibrational levels belonging to electronic states of different multiplicities.[46] Assuming that the non-radiative rate constants of the T triplet state (k nr ) are significantly lower than kRISC when T1 is relatively stable, the quantum yield of the reverse intersystem crossing is almost 100% (ΦRISC ≈ 1), and thus, the kRISC can be obtained from Equation (8) and (9) kISC
ΦDF = kPF ΦDF + ΦPF
(13)
where the values of ΦDF, ΦPF and kPF can be experimentally measured according to Equation (11). The rate of the intersystem crossing (kISC) was reported to be in the order of 106–1011 s−1 in TADF molecules.[31,48–50]
2.2.2. The Reverse Intersystem Crossing The key process in the TADF phenomenon is the facilitated RISC (T1→S1) when the energy difference between S1 and T1 is small and T1 is stable enough. Because reverse intersystem crossing (RISC) is a non-radiative transition from the lower vibrational level of T1 to the higher one of S1 in an excited molecule, the DF efficiency (ΦDF) and its transient lifetime (τDF) are highly dependent on the RISC rate constant (kRISC). According to Equation (6), (9) and (10), kRISC can be determined as follow: kRISC =
kDF ΦRISC 1 − ΦISC ΦRISC
(14)
With Equation (8) and (9), kRISC can be further transformed
temperature effects on the photoluminescence strength suggest the existence of an optimized temperature for the strongest luminescence of the TADF emission.
2.2.3. The Singlet-Triplet Energy Splitting (ΔEST) For an efficient TADF emission facilitated by the fast RISC process, a small ΔEST is particularly important according to Equation (16). However, S1 is considerably higher than T1 by 0.5–1.0 eV in most light-emitting molecules.[31] To obtain small ΔEST in optical active molecules, special approaches for construction of TADF molecules need to be considered. In principle, the molecular energy of the lowest singlet (ES) and triplet (ET) excited state can be decided by the orbital energy (E), electron repulsion energy (K) and exchange energy (J) of the two unpaired electrons at the excited states, as shown in Equation (17) and (18). Because of the same electron arrangement of the singlet and triplet in one molecule, E, K, and J at the two excited states are the same with each other. However, due to the same spin states of the unpaired electrons in T1, the ET is reduced (Equation (18)) in comparison with increased ES in S1 (Equation (17)). Hence, ΔEST which is the difference between ES and ET, is equal to the twice of J (Equation (19)).[48] ES = E + K + J
(17)
E T = E +K -J
(18)
ΔEST = ES − E T = 2 J
(19)
to: kRISC =
kDFkPF ΦDF kISC ΦPF
(15)
where kDF, kP and kISC can be estimated from the experimentally observable rate constants and the PL quantum efficiencies of the prompt and delayed components (Equation (11) and (12)). From Equation (15), the rate of the reverse intersystem crossing (kRISC) in TADF molecules is generally in the order of 103–106 s−1 in TADF molecules, which is usually lower than kISC.[31,48–50] RISC is a temperature sensitive process. High temperature facilitates this endothermic transition. At low temperature (< 100 K), the PLQYs of TADF emitters are largely suppressed and strong photoexcitation intensity dependence of the emission appears, which are regarded as characteristic features of TADF molecules.[51] The dependence of kRISC on temperature can be expressed in a Boltzmann distribution relation: ⎛ ΔEST ⎞ kRISC ∝ exp ⎜ ⎝ kB T ⎟⎠
(16)
where kB is the Boltzmann constant, T is the temperature, and ΔEST is the singlet-triplet energy splitting. It needs to be pointed out that although the kRISC is enhanced at high temperature, the ΦDF may be decreased due to the simultaneously enhanced non-radiative deactivation processes. This contradictory
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At S1 or T1, the unpaired two electrons are mainly distributed on the frontier orbitals of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively, with the same J value regardless of the different spin states. Therefore, the exchange energy (J) of these two electrons at HOMO and LUMO can be calculated by Equation (20):[52] ⎛ e2 ⎞ J = ∫ ∫ φL (1)φH (2) ⎜ φL (2)φH (1)dr1dr2 ⎝ r1 − r2 ⎟⎠
(20)
where φH and φL represent the HOMO and LUMO wave functions, respectively; e is the electron charge. From Equation (20), it is clear that a small ΔEST can be resulted via a small overlap integral of , i.e., via spatial wave function separation of HOMO and LUMO.[53,54] The general strategy adopted to obtain the spatially separated HOMO and LUMO orbitals is to introduce a large steric hindrance structure[47,52,55] or a donor-acceptor system with twist/spiro/bulky connection which can reduce the overlap between the HOMO and LUMO to enhance the charge transfer state.[56–58] To address this point, various systems consisting of metal-organic complexes, donor-acceptor systems involving the intramolecular and intermolecular D-A systems and fullerenes are reported to have the ability to generate efficient TADF emission, which will be discussed later.
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Since the TADF emitters can realize highly efficient luminescence through utilizing the delay fluorescence (DF), most of their optoelectronic applications were found in OLEDs and sensors. Just like the phosphorescent materials, the TADF emitters also have serious concentration quenching and/or triplet–triplet annihilation at a high concentration of triplet excitons. The emitters also need to be dispersed or doped in a suitable host matrix, which should have the following characteristics:[59–61] (i) sufficiently higher triplet energy (ET) than the TADF emitter to prevent reverse energy transfer from the guest to the host, (ii) suitably aligned HOMO and LUMO levels to ensure effective charge injection from the adjacent layers, (iii) larger HOMO – LUMO energy gap (Eg) than that of the guest to facilitate charge trapping on the dopant emitter, (iv) bipolar charge transport properties for balanced hole and electron densities in the emitting layer, and (v) high morphological stability and film-forming ability. Moreover, the dipole interactions between the host and guest molecules will lead to a time-varying ΔEST for large variations in the PL quantum yield, TADF spectrum, and DF transient lifetime of TADF molecules.[62] Under the electrical excitation in OLEDs, the theoretical maximum external quantum efficiency (EQE) of the TADF OLEDs can be estimated by Equation (21)
as low as 1% but with EQEs up to 21.2%;[64] the EQE can further increase to 30% in co-host system.[65] The device stability of TADF OLEDs is comparable to that of Ir complex-based OLEDs with lifetimes of 2800 h at an initial luminance of 1000 cd m−2 and of over 10 000 h at 500 cd m−2.[66] The utilization of TADF molecules as host materials was also reported to take advantage of the balanced charge transporting/injection properties.[67] In light of the highly dynamic exciton transitions between singlet and triplet states, TADF molecules were further used as singlet exciton sensitizers to harvest triplet excitons in OLEDs for fluorescent emitters; the devices showed a significantly increased EQE and an enhanced device operational stability.[68,69] Triplet excitons are very sensitive to oxygen, in which the two oxygen atoms are chemically bonded to each other with a triplet electron spin configuration at ground state. The quick quenching of triplet exciton upon triplet oxygen makes TADF emitters highly efficient oxygen sensors.[70] Its limit of detection can be as low as 0.25 ppmv.[71] Whereas, the temperature dependent nature of TADF emission opens a door to probe the temperature.[72] Owing to the long-lived luminescence of the DF component of the TADF emission, TADF molecules were also applicable for time-resolved fluorescence imaging in living cells.[73] The great success of TADF materials in the application of OLEDs, sensors, and bio-imaging is just the beginning for their prosperity in organoelectronics when the general concepts in fluorescence were extended with a variety of TADF molecules that can be designed and synthesized rationally.
REVIEW
2.3. Features of TADF Materials for Optoelectronic Devices
EQE = ηintηout =γηrηPLηout ⎡∞ k k ⎤ = γ ⎢ ∑ 0.75ΦPF ΦRISC ( ΦISC ΦRISC ) +0.25ΦPF ( ΦISC ΦRISC ) ⎥ ηout ⎣ k=0 ⎦ 0.75 + 0.25(1-ΦPF ) ⎡ ⎤ = γ ⎢0.25ΦPF + ΦDF ⎥ ηout 1-ΦPF ⎣ ⎦ ΦDF ⎤ ⎡ ηout = γ ⎢0.25( ΦPF + ΦDF ) +0.75 1 − ΦPF ⎥⎦ ⎣ ⎡ ⎤ ΦDF = γ ⎢0.25ηPL +0.75 ⎥ ηout η 1 ( ) − − Φ PL DF ⎦ ⎣
(
)
(21) where ηint is the internal quantum efficiency; ηout is the outcoupling constant; γ is the ratio of the charge combination to the electron and hole transportation; ηr is the excitation–production singlet-to-triplet ratio; ηPL is the photoluminescence efficiency; ΦPF is the photoluminescence quantum yield of the prompt component; and ΦDF is the photoluminescent quantum yield of the delayed component. If ΦPF + ΦDF = ηPL = 1, then internal quantum efficiency can be 100% according to Equation (21). It should be noted that a high degree of the orientation of the transition dipole moment of TADF emitters will effectively increase the ηout, leading to a high EQE of the device.[63] Since the first report of the TADF phenomenon that was initially called as E-type delayed emission in 1961,[33] significant progress has been made to develop highly efficient TADF emitters involving metal (Cu(I), Ag(I), Au(I), and Sn(IV))-organic complexes, D-A molecule systems, and fullerenes for organoelectronics. The doping concentration of TADF emitters can be
Adv. Mater. 2014, DOI: 10.1002/adma.201402532
3. Metal-Organic Complexes for TADF Emission Previous studies on luminescent metal-organic complexes were principally focused on third-row (Ir, Pt, or Os) and second-row (Ru) rare metal-containing phosphors that have large spin-orbit coupling (SOC) interactions to overcome the spin-forbidden nature of the triplet to singlet ground state transition (T1→S0) for phosphorescence. In contrast to these d6 and d8 metal complexes, the first-row d10 metal complexes often have much weaker spin-orbit coupling and the T1→S0 transition is largely forbidden. Fortunately, they generally have a small ΔEST and a stable T1, which are two key features of the TADF phenomenon. The small ΔEST values of d10 metal-organic complexes composed of d10 metal (M) moiety and large steric hindrance ligands (L), can be rationalized by a distinct charge transfer from the metal center to the ligand (MLCT),[74] due to a pronounced charge separation between the metal and ligand upon excitation, which can be regarded as a formal oxidation of M(I) to M(II) in the excited state. The stable T1 is related to the rigid metal complex structure with a suppressed non-radiative deactivation process and the weak phosphorescence (T1→S0) with low SOC interactions. Furthermore, the close shell d10 metal complexes possess full d-orbitals and the internal quenching of excited states by low-lying d–d* states cannot occur. These characteristics of d10 metal complexes with low cost and toxicity are advantageous for the design of highly emissive TADF complexes.[75–78] The physical properties and electroluminescence performance of the representative TADF complexes are summarized in Table 1 and 2.
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Table 1. PL peak (λ), PL quantmum yield (Φ), lifetime at room temperature (τ), singlet-triplet energy gap (ΔEST) of TADF metal complexes. Complex
λ/nm
Φ/%
τ / µs
ΔEST/eV
Complex
λ/nm
Φ/%
τ / µs
[34]
20
523a)
68
[30]
21
521a)
52
22
d)
520a) 527d)
47
5.4
514a)
71
5.5
Ref.
1
543
a)
14
8.1
0.12
2
509a)
32
18.1
–
3
504a)
57
32.9
–
[30]
4
555a)
16
4.6
–
[30]
5
527a)
49
13.2
–
[30]
6
519a)
69
20.3
–
[75]
11
12
13
523b)
–
–
–
530c)
80
14
0.12
535a)
65
20
558d)
40
10
490c)
56
20.4
590d)
2.1
1.6
465c)
87
12.2
536d) 14
492c) 540d)
15
436c) a)
462
535d) 16
17
45 75 30 45 35 9
24 [84] 25 [79]
26
8.2
0.18
[87]
2.1
0.06
[74]
43
4.9
–
[88]
68
6.1 0.07
[88]
–
[88]
0.09
[89]
0.08
[89]
517d)
60
6.5
504a)
57
3.2
488b)
95
6.6
d)
24
1.4
500b)
95
5.0
592 0.18
Ref.
d)
15
1.0
27
524a)
57
11.3
0.10
[76]
28
539
c)
80
6.5
0.05
[90]
29
506c)
45
6.6
0.06
[91]
30
c)
490
65
4.1
0.05
[91]
31
464c)
65
4.6
0.07
[91]
1.3
32
c)
465
65
5.6
0.08
[91]
33
505c)
32
2.8
0.02
[74]
34
610c)
12
2.1
0.05
[74]
35
571
c)
–
–
0.41
[36]
36
570c)
–
–
0.40
[36]
c)
0.17
22.8
0.18
[79]
13.3 20
0.16
[85,86]
22
447
90
22
457a)
30
24
500d)
2
0.5
464c)
90
13
0.12
0.10
546
[79]
11.9
c)
a)
23
534
ΔEST/eV
[85,86]
[85,86]
466
41
23
37
576
–
–
0.38
[36]
498d)
8
1.8
38
579c)
–
–
0.39
[36]
18
545a)
50
3.8
0.16
[87]
39
c)
571
–
–
0.40
[36]
19
534a)
63
3.6
0.20
[87]
40
569c)
–
–
0.41
[36]
a)
Measured in doped film; b)Measured in pristine film; c)Measured in powder; d)Measured in solution.
3.1. Cu(I) Complexes As a typical d10 metal, Cu(I) is the most widely investigated one used to construct d10 metal-organic complexes that have lowlying MLCT excited states with small singlet-triplet energy gap (ΔEST) to efficiently harvest triplet excitons for TADF emission. Cu(I) complexes usually have a distorted tetrahedral geometry around the Cu(I) atom connected with a diimine ligand and an ancillary diphosphine ligand (Figure 4). The pseudo tetrahedral structures, unfortunately, often exhibit a bit low quantum yield due to increased non-radiative decay caused by the distortion of excited states. To overcome this problem, the most notable way in the design of highly luminescent Cu(I) complexes for TADF emission is to provide a firm framework of bulky chromophoric ligands around Cu(I), suppressing the molecular geometry changes and shielding the metal center from the environment (Figure 4a). Other ways include using the trigonal-planar complex structure with rigid ligands to hamper the Jahn-Teller distortion (Figure 4b) and adopting the multinuclear Cu(I)
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complex structure, especially dinuclear one (Figure 4c), to profit from the strong covalency of Cu2N2 core.
3.1.1. Four-Coordinate Mononuclear Cu(I) Complexes Four-coordinate mononuclear Cu(I) complexes have a tetrahedral geometry around the Cu(I) atom connected with a diimine ligand and an ancillary diphosphine ligand (Figure 4a). As mentioned previously, the tetrahedral d10 metal complexes with two bidentate reducible aromatic ligands, i.e., with energetically low-lying π* orbitals, may undergo efficient non-radiative transitions caused by a structural change from tetrahedral to planar geometry because of the MLCT character of their excited states.[79] Such structural rearrangement is especially distinct in non-rigid environments. To achieve more separated orbital overlap and highly efficient TADF, the enhancement of structural rigidity of both complex and ligand must be taken into consideration. A distorted tetrahedral geometry by chromophoric
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Table 2. The photophysical properties and OLED performance of representative TADF materials. HOMO
LUMO
λ/nm
ΔEST/eV
Host
ηext (%)
Ref.
4
–
–
555
–
PVK
ITO/PEDOT:PSS/1 wt% 4: PVK/BCP/Alq3/LiF/Al
–
[30]
6
–
–
519
–
PVK
ITO/PEDOT:PSS /16 wt% 6: PVK/BCP/Alq3/LiF/Al
–
[30]
12
5.41
2.23
590
0.18
26mCPy
ITO/PEDOT:PSS/20 wt% 12: 26mCPy /DPEPO /LiF/Al
3.18
[79]
13
5.39
2.16
536
0.17
26mCPy
ITO/PEDOT:PSS/20 wt% 13: 26mCPy /DPEPO /LiF/Al
1.59
[79]
14
5.53
2.49
540
0.18
26mCPy
ITO/PEDOT:PSS/20 wt% 14: 26mCPy /DPEPO /LiF/Al
8.47
[79]
18
5.13
1.17
545
0.16
mCP
ITO/TAPC/10 wt% 18: mCP/3TPYMB/LiF/Al
11.9
[87]
19
5.61
1.60
534
0.20
mCP
ITO/TAPC/10 wt% 19: mCP/3TPYMB/LiF/Al
16.0
[87]
20
5.87
2.06
523
0.18
mCP
ITO/TAPC/10 wt% 20: mCP/3TPYMB/LiF/Al
17.7
[87]
TADF
21
4.7
1.9
521
0.06
mCP: TAPC
23
–
–
527
0.07
mCP
27
–
–
524
0.10
CBP: TAPC
Device Structure
ITO/PEDOT: PSS/PVK/10 wt% 21: 30 wt%TAPC: mCP/3TPYMB/Al
7.8
[74]
ITO/TAPC/10 wt% 23: mCP/3TPYMB /LiF/Al
21.3
[88]
ITO/CFx/TAPC/25 wt% TAPC: CBP/25 wt% TAPC: 0.2 wt% 27: CBP/CBP/ BAlq-13/LiF/Al
16.1
[76]
39
–
–
571
0.40
PVCz
ITO/PEDOT:PSS/2 wt%39: PVCz/MgAg/Ag
41
–
–
–
0.057
mCP
ITO/α-NPD/6 wt% 41: mCP/Bphen/MgAg/Ag
–
[36]
4.4
[47]
42
6.07
2.53
530
0.028
TPSiF
ITO/TAPC/mCP/6 wt% 42: TPSiF/TmPyPB/LiF/Al
44
5.8
3.0
473
0.09
PPT
ITO/α-NPD/5 wt% 44: PPT/PPT/LiF/Al
10.1
[52]
8.0 ± 1
45
5.8
3.4
507
0.083
CBP
ITO/α-NPD/5 wt% 45: CBP/TPBI/LiF/Al
19.3 ± 1.5
[31] [31]
ITO/α-NPD/5 wt% 48: CBP/TPBI/LiF/Al
48
–
–
577
–
–
11.2 ± 1
[31]
49
6.2
3.2
470
0.14
DPEPO
ITO/α-NPD/mCP/10 wt% 49: DPEPO/TPBi/LiF/Al
9.2
[59]
50
6.05
3.15
488
0.06
DPEPO
ITO/α-NPD/mCP/10 wt% 50: DPEPO/TPBi/LiF/Al
9.6
[106]
51
6.2
3.5
∼490
0.08
PzCz
ITO/α-NPD/mCP/3 wt% 51: PzCz/PPT/LiF/Al
15.0
[106]
52
5.37
2.7
466
0.11
mCP
ITO/α-NPD/mCP/6 wt% 52: mCP/BP4mPy/LiF/Al
5.3
[67,109]
54
–
–
506
0.02
PYD2
ITO/TAPC/6 wt% 54 : PYD2/DPEPO/DPEPO/LiF/Al
14 ± 1
[110]
55
5.9
2.6
435/513
0.06
mCP: DPEPPO
56
5.49
2.77
526
0.085/0.008
DPEPO
ITO/α-NPD/6 wt% 55: mCP/6 wt% 55: DPEPO/DPEPO/TPBI/LiF/Al
11
[58]
ITO/α-NPD/TCTA/CzSi/3 wt% 56: DPEPO/DPEPO/TPBI/LiF/Al
6
[111,112]
ITO/α-NPD/6 wt% 57: CBP/TPBi/LiF/Al
57
5.5
3.1
545
0.07
CBP
12.5
[113]
58
5.7
3.4
560
0.054
mCBP
ITO/α-NPD/6 wt% 58: mCBP/TPBi/LiF/Al
9.1 ± 0.5
[115]
59
5.7
3.4
568
0.065
mCBP
ITO/α-NPD/6 wt% 59: mCBP/TPBi/LiF/Al
13.3 ± 0.5
[115]
62
–
–
502
0.15
DPEPO
ITO/α-NPD/mCP/6 wt% 62: DPEPO/DPEPO/TPBi/LiF/Al
14.9
[119]
64
–
–
462
0.23
DPEPO
ITO/α-NPD/mCP/6 wt% 64: DPEPO /DPEPO/TPBi/LiF/Al
6.4
[119]
65
5.6
3.4
560
0.17
26mCPy
ITO/α-NPD/6 wt% 65: 26mCPy/Bphen/MgAg/Ag
17.5
[120]
66
5.6
3.1
529
0.09
CBP
ITO/α-NPD /6wt% 66: CBP /TPBi /LiF /Al
11.7
[121]
67
5.8
3.0
492
0.16
mCP
ITO/α-NPD/mCP/6wt% 67: mCP /PPT/TPBi/LiF/Al
7.5
[121]
68
5.7
3.1
524
0.04
mCP
ITO/α-NPD /6wt% 68: mCP /TPBi /LiF /Al
12.6
[121]
69
5.9
3.1
483
0.13
mCBP
ITO/α-NPD/mCP/6wt% 69: mCBP /PPT/TPBi/LiF/Al
8.7
[121]
70
5.7
3.1
499
0.26
mCP
ITO/α-NPD/mCP/6wt% 70: mCP /PPT/TPBi/LiF/Al
7.5
[121]
71
5.89
2.62
402
0.54
DPEPO
ITO/α-NPD/TCTA/CzSi/10 wt% 71: DPEPO/DPEPO/TPBi/LiF/Al
2.9
[95]
72
5.65
2.46
419
0.45
DPEPO
ITO/α-NPD/TCTA/CzSi/10 wt% 72: DPEPO /DPEPO/TPBi/LiF/Al
5.6
[95]
73
5.81
2.52
404
0.32
DPEPO
ITO/α-NPD/TCTA/CzSi/10 wt% 73: DPEPO /DPEPO/TPBi/LiF/Al
9.9
[95]
74
5.55
2.43
445
0.21
DPEPO
ITO/α-NPD/TCTA/CzSi/10 wt% 74: DPEPO /DPEPO/TPBi/LiF/Al
14.5
[123]
79
5.10
2.69
577
0.09
CBP
ITO/α-NPD/TCTA/CzSi/79: DPEPO/DPEPO/TPBi/LiF/Al
