Efficient blue organic light-emitting diodes employing ...

ARTICLES PUBLISHED ONLINE: 2 MARCH 2014 | DOI: 10.1038/NPHOTON.2014.12

Efficient blue organic light-emitting diodes employing thermally activated delayed fluorescence Qisheng Zhang1†, Bo Li1†, Shuping Huang1, Hiroko Nomura1, Hiroyuki Tanaka1 and Chihaya Adachi1,2 * Organic light-emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) have emerged as cheaper alternatives to high-performance phosphorescent OLEDs with noble-metal-based dopants. However, the efficiencies of blue TADF OLEDs are still low at high luminance, limiting full-colour display. Here, we report a blue OLED containing a 9,10-dihydroacridine/diphenylsulphone derivative that has a comparable performance to today’s best phosphorescent OLEDs. The device offers an external quantum efficiency of 19.5% and reduced efficiency roll-off characteristics at high luminance. Through computational simulation, we identified six pretwisted intramolecular chargetransfer (CT) molecules with small singlet–triplet CT state splitting but different energy relationships between 3CT and locally excited triplet (3LE) states. Systematic comparison of their excited-state dynamics revealed that CT molecules with a large twist angle can emit efficient and short-lifetime (a few microseconds) TADF when the emission peak energy is high enough and the 3LE state is higher than the 3CT state.

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rganic light-emitting diodes (OLEDs) are a promising solution for large-area, high-resolution flat panel displays and lighting sources. In the past 20 years, research has successively led to fluorescence-based OLEDs as the first generation of these devices and phosphorescence-based OLEDs (PHOLEDs) as the second. In OLEDs using only fluorescent emitters, the radiative decay of triplet excitons (75%) is spin forbidden, so only the singlet excitons (25%) can emit light. Conversely, noble-metal-based organometallic phosphors exhibit emissive triplet states because of singlet–triplet mixing via effective spin–orbit coupling, so PHOLEDs containing them can exhibit near-unity internal electroluminescence quantum efficiency1,2. However, the noble metals are expensive and reserves of them are limited, and the reliability of blue PHOLEDs needs to be improved. Therefore, new conceptual mechanisms for OLEDs, such as triplet–triplet annihilation, have been considered3–5. Efficient OLEDs based on charge-transfer (CT) Cu(I) complexes have also attracted much interest in the last decade6–11. Although radiative decay of the 3CT state of first-row transition-metal complexes is inefficient, the nearby 1CT state provides a thermal activation pathway that facilitates emissive decay7,8,11–14, as their gap can be remarkably small (0.1 eV) because of the spatial separation of the frontier orbitals and some singlet–triplet mixing14,15. However, the CT transitions in Cu(I) complexes are always associated with partial oxidation of the metal centre, resulting in large structural deformation and relaxation energies13. This makes the triplet excitons in OLEDs containing green-emitting Cu(I) complexes difficult to confine11, while the use of high triplet energy host and exciton-blocking layers results in devices with poor reliability. High-performance blue OLEDs based on Cu(I) complexes have not been reported, partly for this reason14. In 2009, our group discovered a new conceptual emission mechanism for OLEDs: highly efficient thermally activated delayed

fluorescence (TADF) based on conventional organic aromatic compounds, which are the third generation of OLED emitters16–26. TADF is observed from CT systems with a thermally accessible gap between the lowest singlet (S1) and triplet (T1) excited states, which enables the harvesting of both singlet and triplet excitons under electrical excitation to achieve efficiencies that are comparable with those of high-efficiency PHOLEDs21,23,24. Recent TADF emitters characterized by pretwisted intramolecular CT27,28 have a relatively small relaxation energy because of the steric hindrance between donor (D) and acceptor (A) moieties21–26. This facilitates triplet exciton confinement in OLEDs, making efficient blue OLEDs possible21. However, reported blue TADF-based OLEDs suffer from efficiency roll-off at high current density, which is primarily caused by the relatively large energy difference between the S1 and T1 states (DEST) and the long excited-state lifetime of the emitters21,22,25,26. Solving this roll-off problem through a deep understanding of the structure–property relationships in TADF emitters is essential for developing all-TADF OLEDs for fullcolour display and white lighting applications. In reported blue TADF emitters, some strongly localized states with 3pp* or 3np* character are lower in energy than the 3CT state21,26,29,30, suggesting that DEST can be minimized by adjusting the energy levels of the 1CT state and the lowest locally excited triplet state (3LE). Increasing the twist angle between D and A moieties limits their electronic interaction, which stabilizes the CT state and increases the energy of 3LE (refs 27,28), although it is insufficient to ensure that the 3CT state is lower than 3LE. Herein, by using a new computational method to predict the 1CT, 3CT and 3 LE levels of molecules30, we successfully design a high-efficiency blue TADF molecule with a short excited-state transient lifetime. We realize high-efficiency blue OLEDs with low roll-off using an organic emitter for the first time and open up the possibility of achieving low-cost, high-efficiency, full-colour and white OLEDs.

1

Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan, 2 International Institute for Carbon Neutral Energy Research, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan; † These authors contributed equally to this work. * e-mail: [email protected]

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DOI: 10.1038/NPHOTON.2014.12

Molecular design

the highest value of 3.00 eV for DMAC–DPS implies a possible blue emission band with an onset at 414 nm. The calculations reveal that the gap between E0–0(1CT) and E0–0(3CT) is ,0.08 eV for all compounds. For the three DPS molecules and PPZ–DPO, the E0–0(3LE) are equal to or higher than E0–0(1CT), whereas those of PPZ–3TPT and PPZ–4TPT are lower than E0–0(1CT) by 0.32 and 0.42 eV, respectively, suggesting their T1 state has a 3LE nature. Because all PPZ molecules have an approximate calculated E0–0(3LE) of 2.38+0.02 eV, this triplet state is localized over the PPZ moieties.

The molecules designed in this work are presented in Fig. 1a. D–Atype molecules PPZ–DPO, PPZ–3TPT and PPZ–4TPT all have a 5-phenyl-5,10-dihydrophenazine (PPZ) donor unit, but different acceptor units of 2,5-diphenyl-1,3,4-oxadiazole (DPO) and 3,4,5triphenyl-1,2,4-triazole (TPT). The D–A–D-type molecules PPZ– DPS, PXZ–DPS and DMAC–DPS all have a diphenylsulphone (DPS) acceptor but different donor units of PPZ, phenoxazine (PXZ) and 9,9-dimethyl-9,10-dihydroacridine (DMAC). First, we used density functional theory (DFT) with the most popular functional, B3LYP, to simulate the ground-state geometries of these six compounds31. Very large dihedral angles of 808, 878 and 898 were found between the planes of PXZ, PPZ and DMAC and their connected phenyl rings, respectively, resulting in small overlap between their highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) (Fig. 1b, Supplementary Fig. 1). According to calculations at the timedependent (TD)-DFT/B3LYP level, the DEST values for all six molecules are less than 0.1 eV (Supplementary Table 1). However, we recently found that B3LYP generally underestimates the DEST of CT molecules with localized T1 states because transitions with different CT amounts should be calculated by functionals containing different percentages of Hartree–Fock exchange (HF%)30. Using the optimal HF exchange method presented in our recent paper30, the zero–zero energies (E0–0) of the 1CT, 3CT and 3LE states of these six compounds were calculated in toluene based on the geometries optimized by B3LYP (Fig. 1c, Supplementary Table 1). The calculated E0–0(1CT) vary from 2.31 eV to 3.00 eV, in which

Photophysical properties The low-lying energy levels and photophysical properties of these molecules were investigated in toluene and 10 wt%-doped m-bis(N-carbazolyl)benzene (mCP) films. Broad unstructured CT absorption bands (1 ¼ 2.0–3.5 mM21 cm21) were observed at low energy in the toluene solutions of the DPS molecules and PPZ– DPO (Fig. 2a,b). In contrast, the CT band appears only as a shoulder below the PPZ centred absorption (lmax ¼ 370 nm) in the spectra of PPZ–3TPT and PPZ–4TPT solutions. The onset of their broad unstructured photoluminescence spectra in toluene at room temperature (Supplementary Fig. 4) provides E0-0(1CT) values that match the calculated ones well (error ¼+0.04 eV)11,30. As expected, DMAC–DPS exhibits a blue emission band with a maximum at 460 nm in toluene. The fluorescence spectra of these compounds doped into mCP films are almost temperature independent and are identical to those in toluene at room temperature. The fluorescence (,0.1 ms) and phosphorescence (.0.1 ms) spectra of the doped films at 10 K are shown in Fig. 2a,b.

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Figure 1 | Molecular structures and TD-DFT results. a, Molecular structures of the investigated compounds. b, HOMO (lower image) and LUMO (upper image) of PPZ–4TPT and DMAC–DPS calculated at the B3LYP/6-31G* level. c, Jablonski diagram of the energy levels of PPZ–4TPT and DMAC–DPS calculated in toluene. F, fluorescence; P, phosphorescence; GS, ground state; IC, internal conversion; RIC, reverse internal conversion; ISC, intersystem crossing; RISC, reverse intersystem crossing. NATURE PHOTONICS | VOL 8 | APRIL 2014 | www.nature.com/naturephotonics


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Figure 2 | Absorption, emission and transient decay spectra. a,b, Absorption spectra of the TADF emitters in toluene at room temperature, and fluorescence (,0.1 ms) and phosphorescence (.0.1 ms) spectra of the TADF emitters doped into mCP films (10 wt%) at 10 K, measured by a streak camera. c,d, Transient decay spectra of the TADF emitters doped into mCP films (10 wt%) at room temperature.

The profiles of the phosphorescence spectra of the three DPS molecules are similar to their fluorescence spectra with a small energy difference of 0.08 eV between emission onsets, confirming the expected 3CT character of their T1 states. Compared with PPZ–DPS, the electron-withdrawing ability of the acceptor in PPZ–DPO, PPZ–3TPT and PPZ–4TPT decreases in that order, enhancing their CT energy levels. Conversely, the energy of the 3 pp* transition centred on the PPZ moiety is almost constant. Therefore, phosphorescence from both 3CT and 3LE states can be observed from a PPZ–DPO film at 10 K because of their similar energy11, whereas only 3LE emission with a well-defined vibronic structure is observed from PPZ–3TPT and PPZ–4TPT films. The E0–0 of this low-lying 3LE state was determined from the highest energy peak of its emission band to be 2.38 eV, which is also consistent with the computational prediction. The small DEST of 0.08 eV meant that delayed fluorescence with a short lifetime (tTADF) of 1–3 ms was readily observed from the doped films of the DPS molecules and PPZ–DPO at room temperature, together with a fluorescence component with a lifetime (tF) of the order of nanoseconds (Fig. 2c). Although the values of DEST are as high as 0.30 and 0.43 eV for PPZ–3TPT and PPZ–4TPT in the doped films, respectively, TADF emission can also be observed with tTADF of 4.9 and 28 ms, respectively (Fig. 2d). In addition to the room-temperature phosphorescence from some crystalline organic compounds32, millisecond-scale lifetimes have only been observed for several Cu(I) and Re(I) complexes at room temperature as a result of the thermal equilibrium formed between the emissive state and a lower-energy ligand-centred triplet state33,34. In oxygen-free toluene at room temperature, the delayed components can also be clearly observed 328

from these CT compounds except for PPZ–4TPT (Supplementary Table 2); its long-lived TADF is completely quenched by nonradiative decay in the fluid solution. In oxygen-free toluene at room temperature, the photoluminescence quantum yields (PLQYs) of DMAC–DPS (lmax¼ 460 nm) and PXZ–DPS (lmax ¼ 507 nm) are as high as 0.80, while those of PPZ–DPS (lmax ¼ 577 nm) and PPZ–DPO (lmax ¼ 577 nm) are only 0.03 and 0.12, respectively. The energy gap law can be used to explain the different PLQYs of these closely related molecules with small DEST (Supplementary Fig. 6)15, as well as the higher PLQY of PXZ–DPS than that of PXZ–TRZ (lmax ¼ 545 nm, F ¼ 0.30, in toluene), another PXZ-based TADF compound reported previously22. Although the emission energies of PPZ– 3TPT and PPZ–4TPT are comparable with that of PXZ–DPS, their low-lying 3LE states prevent efficient repopulation of the emissive 1CT state (see below). Instead, the energy from 3LE states dissipates by non-radiative processes, leading to a low PLQY of 0.07 for PPZ–3TPT and 0.04 for PPZ–4TPT in toluene. Because a rigid matrix suppresses collision-induced intramolecular radiationless transitions (Fig. 1e) and bimolecular processes15, the PLQYs of PPZ–DPS, PPZ–DPO, PPZ–3TPT and PPZ–4TPT increase to 0.20, 0.45, 0.44 and 0.12, respectively, in mCP films at room temperature. Unlike most green-emitting Cu(I) complexes, changing the host of PXZ–DPS from mCP (T1 ¼ 2.90 eV) to 4,4′ -bis(carbazol-9-yl)biphenyl (CBP, T1 ¼ 2.64 eV) does not decrease its PLQY (0.90). Conversely, the PLQY of blue-emitting DMAC–DPS can be increased from 0.80 to 0.90 by changing the host from mCP to bis(2-(diphenylphosphino)phenyl)ether oxide (DPEPO), which has an ultrahigh T1 level of 3.30 eV (Supplementary Fig. 7). NATURE PHOTONICS | VOL 8 | APRIL 2014 | www.nature.com/naturephotonics


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As reported previously, the individual PLQYs of fluorescence (FF) and TADF (FTADF) can be distinguished from the total PLQY by comparing the integrated intensity of the prompt and delayed components in the transient photoluminescence spectra18. Then, the rate constants of fluorescence (kF) and TADF (kTADF) can be calculated using equations (1) and (2): kF = FF /tF

(1)

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where FF is related to the rate constants of fluorescence, internal conversion (kIC) and intersystem crossing (kISC) through equation (3)15: FF = kF /(kF + kIC + kISC )

(3)

For emitters with a high quantum yield and large FTADF/FF ratio, such as PXZ–DPS and DMAC–DPS, kISC . kF ≫kIC is expected, so equation (3) can be simplified to FF = kF /(kF + kISC )

(4)

Thus, the kISC of PXZ–DPS and DMAC–DPS are estimated to be 5.4 × 107 and 3.7 × 107 s21, respectively. Although these rates are markedly lower than those of transition-metal complexes (101021013 s21) (refs 2,10) this does not seem to prevent organic TADF emitters from realizing efficient kTADF. Considering that tTADF (.1 ms) is generally long enough to reach thermal equilibrium of the relaxed excited states at room temperature, McMillin’s two-state model established for CT Cu(I) complexes can be used to interpret the delayed emission of TADF emitters with only one triplet state (that is, 3CT) below 1 CT (refs 11,12). The observed rate of delayed emission (kobs) of such two-state systems is then expressed as a kISC-independent Boltzmann average: kobs =

kF K + kP,CT 1+K

(5a)

K = 1/3 exp(−DEST /RT)

(5b)

where kP,CT , R, T and 1/3 denote the rate constant of phosphorescence from the 3CT state, the ideal gas constant, absolute temperature, and the ratio of the degeneracies of S1 to T1 states, respectively. For aromatic compounds, kF and kp are generally 106–109 s21 and 1022–1 s21, respectively35. Therefore, kP,CT ≪ kFK is expected at room temperature when DEST is smaller than 0.3 eV (K . 3.0 × 1026), consistent with the observation that TADF dominates the delayed emission at room temperature for emitters with small DEST. Moreover, given that DEST is generally larger than 0.05 eV in CT molecules (that is, K , 0.05 at 300 K)16–26, equation (5a) can be simplified to kobs = kTADF = kF K

(6)

Because the 3LE state of DMAC–DPS is calculated to be higher than its 1CT state in mCP host, one can predict a kF/kTADF ratio of 67 at 300 K from equation (6) using DEST ¼ 0.08 eV, which is consistent with our experimental result (Supplementary Table 2). Doped into a solid film, DMAC–DPS has a typical kF of 107 s21. This limits the natural lifetime of its TADF (1/kTADF) to a microsecond timescale that is comparable to those of heavy metal complexes. However, if there is an additional 3LE state below 1CT and in thermal equilibrium with it and 3CT, kobs is given by equation (7)2:

kobs =

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where kP,LE is the rate constant of phosphorescence from the 3LE state, and DETT is the energy difference between the two triplet states. Similar to the two-state system discussed above, in the case of 0.05 , DEST , 0.3 eV and T ¼ 300 K, equation (7a) can be simplified to kobs = kTADF = kF K/(1 + K ′ )

(8)

Comparison of equations (6) and (8) indicates that when DETT , 0.05 eV (0.15 , K′ ≤ 1), the influence of the middle state on kTADF cannot be neglected. This explains why kF and DEST of PPZ–DPO and PXZ–DPS are similar, whereas kTADF of PPZ– DPO (DETT ¼ 0) is nearly half that of PXZ–DPS (DETT ≈ 0.05 eV, Supplementary Table 2). Additionally, equation (8) reveals that kTADF decreases considerably with increasing DETT when the 3LE state lies below 3CT. Both kF and 1CT–3CT splitting of PPZ–3TPT are similar to those of PPZ– DPO. However, unlike the energetically equivalent 3LE and 3CT states of PPZ–DPO in mCP, 3LE is lower than 3CT by 0.22 eV in PPZ–3TPT. From equation (8), kTADF of PPZ–3TPT in mCP is calculated to be 1/2,500 that of PPZ–DPO, in accordance with the experimental result. Thus, for TADF emitters with small frontier overlap but lifetimes of 1025–1022 s, one or more low-lying locally excited triplet state(s) is expected to be in thermal equilibrium with upper CT states21,26,29.

Device characterization Finally, we examined the electroluminescence properties of all six TADF emitters using two device structures: (I) ITO/a-NPD (40 nm)/emitter layer (EML) (20 nm)/TPBI (60 nm)/LiF (1 nm)/Al and (II) ITO/a-NPD (30 nm)/TCTA (20 nm)/CzSi (10 nm)/EML (20 nm)/DPEPO (10 nm)/TPBI (30 nm)/LiF (1 nm)/Al, where a-NPD, TCTA, CzSi and TPBI are N,N′ -diphe4,4′ ,4′′ nyl-N,N′ -bis(1-naphthyl)-1,10-biphenyl-4,4′ -diamine, tris(N-carbazolyl)triphenylamine, 9-(4-tert-butyl phenyl)-3,6-bis (triphenylsilyl)-9H-carbazole and 1,3,5-tris(N-phenylbenzimidazol2-yl)benzene, respectively (Fig. 3a). Blue emitters DMAC–DPS and PPZ–4TPT were doped into a DPEPO host (EML) in device structure II, while the others were doped into CBP (EML) in structure I. Current density–voltage–luminance characteristics of the above OLEDs are presented in Fig. 3b,c. The devices with structure I display significantly higher current densities than the devices with structure II, mainly because of the utilization of DPEPO as host and exciton-blocking layer in the latter. DPEPO is noted to have a shallow LUMO level of 22.0 eV and a deep HOMO level of 26.1 eV (ref. 11), which are unfavourable for the injection of both electrons and holes. Moreover, the weak intermolecular interactions in DPEPO also lead to low charge mobility36, although they can avoid the formation of low triplet energy dimers and allow efficient confinement of the triplet energy on the dopant11. The DMAC–DPS-based device turned on at 3.7 V and achieved a luminance of 10,000 cd m22 at 11.0 V. In contrast, by using a CBP host, which exhibits excellent hole mobility, the corresponding voltages for the PXZ–DPS-based device are only 2.7 V and 5.5 V, respectively. As shown in Fig. 3d,e, the electroluminescence spectra of all of the emitters coincide with their photoluminescence spectra in mCP films, while their maximum external electroluminescence



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Figure 3 | Structures and performance of the TADF OLEDs. a, Structures of OLED devices and the compounds used in them. The electroluminescence properties of PPZ–DPO, PPZ–3TPT, PPZ–DPS and PXZ–DPS were tested using device structure I with a doping concentration of 10 wt% in CBP host. The electroluminescence properties of PPZ–4TPT and DMAC–DPS were tested using device structure II with a doping concentration of 10 wt% in DPEPO host. b–e, Current density–voltage characteristics (b), luminance–voltage characteristics (c), EQE–current density characteristics (d) and electroluminescence spectra (e, at 10 mA cm22) of the above OLEDs.

quantum efficiencies (EQEs) are almost in proportion to their PLQYs in doped films. The devices with PXZ–DPS and DMAC–DPS have the highest EQEs of 17.5% and 19.5%, respectively, and maintain EQEs of 15.5% and 16.0%, respectively, at a luminance of 1,000 cd m22. The devices containing PPZ–DPO and PPZ–DPS with short tTADF also achieve small efficiency rolloff characteristics. Conversely, the devices with long-lived TADF emitters PPZ–3TPT and PPZ–4TPT exhibit considerable efficiency roll-off, as expected. 330

For comparison, the well-known blue phosphorescent emitter bis[(4,6-difluorophenyl) pyridinato-N,C 2](picolinato) iridium (FIrpic) was included in a device with structure II. This device exhibits sky-blue emission with Commission Internationale de L’Eclairage (CIE) coordinates of (0.16, 0.34), in contrast to the (0.16, 0.20) for the DMAC–DPS-based device. Although the emission maxima of both devices are at 470 nm, the DMAC–DPS-based device offers better colour purity because of a deep blue component at 400–450 nm in its electroluminescence spectrum (Supplementary NATURE PHOTONICS | VOL 8 | APRIL 2014 | www.nature.com/naturephotonics


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Fig. 8a). The maximum EQE of the FIrpic-based OLED is close to that of the DMAC–DPS-based one (Supplementary Fig. 8b), consistent with their similar PLQYs in a DPEPO host11. However, the FIrpic-based OLED shows relatively low efficiency at high current density in our control experiment, together with higher turn-on and driving voltages than the DMAC–DPS-based OLED (Supplementary Figs 8c,d). Because the excited-state lifetime of FIrpic (1 ms) is not longer than that of DMAC–DPS in doped films11, the more serious efficiency roll-off for the FIrpic-based OLED cannot be explained in terms of triplet–triplet annihilation and is tentatively assigned to loss of charge balance at high current density37. In comparison with the 26.1 eV for FIrpic, DMAC–DPS has a relatively shallow HOMO level of 25.9 eV (Supplementary Fig. 9), which provides a better match for hole injection. Although the above electroluminescence properties of the DMAC–DPS-based OLED are comparable with those of the best deep blue PHOLEDs reported recently38–41, its reliability is unsatisfactory as a common fault of the devices containing a DPEPO layer11,20,21,26. The half-life of both FIrpic- and DMAC–DPS-based devices is 1 h at an initial luminescence of 500 cd m22. Analogous to blue PHOLEDs, the development of a high triplet energy host material with moderate bandgap and high charge mobility could be an approach to achieve stable blue TADF OLEDs39–42. Alternatively, our latest studies reveal that DMAC–DPS exhibits concentration-insensitive properties that are similar to some CT Cu(I) and Ir(III) complexes6,43, partly due to the relatively short TADF lifetime. Hence, a high-performance non-doped blue OLED based on this bipolar TADF emitter might be possible. Device optimization is under way and will be presented in a later publication.

Quantaurus-QY measurement system (C11347-11, Hamamatsu Photonics) and all samples were excited at 380 nm. The PLQYs of the film samples were measured under nitrogen flow. The transient photoluminescence decay characteristics of both solution and film samples were recorded using a Quantaurus-Tau fluorescence lifetime measurement system (C11367-03, Hamamatsu Photonics). The fast decay component was recorded in TCC900 mode in conjunction with LED excitation, while the slow decay component was recorded in M9003-01 mode with excitation by a 340 nm flash lamp. The emission decay of the fast component for both solution and film samples was well fitted by a single exponential. For all solution samples and PXZ–DPS- and DMAC–DPS-doped mCP films, the emission decay of the slow components was best fitted by a single exponential. For other TADF emitter-doped mCP films, the emission decay of the slow component was best fitted with three exponentials. The average lifetime (tav) can be calculated using tav ¼ SAiti2/SAiti , where Ai is the pre-exponential for lifetime ti. Two-dimensional transient decay (streak) images of the transient photoluminescence of the film samples were investigated under vacuum conditions using a streak camera (C4334, Hamamatsu Photonics) equipped with a Nd:YAG pulsed laser (l ¼ 266 nm, pulse width ≈ 10 ns, repetition rate ¼ 10 Hz) as the excitation source. The HOMO energy levels of the compounds in the films were determined by atmospheric ultraviolet photoelectron spectroscopy using a photoelectron emission spectrometer (Riken Keiki AC-3). Device fabrication and measurements. After the precleaned ITO-coated glass substrates were treated with ozone for 15 min, the organic layers were deposited consecutively on the substrates in an inert chamber under a pressure of ,4 × 1024 Pa. Next, the cathode was fabricated by thermal evaporation of a LiF layer (1.0 nm), followed by an Al layer (100 nm). The deposition rates of the organic and Al layers were 0.1–0.2 nm s21, while that of the LiF layer was 0.01 nm s21. The intersection of the ITO and metal electrodes gave an active device area of 4 mm2. The current density J, voltage V and luminance L characteristics of the OLEDs were measured in ambient air with a semiconductor parameter analyser (E5273A, Agilent) and optical power meter (1930C, Newport). Electroluminescence spectra were recorded using a multichannel spectrometer (PMA12, Hamamatsu Photonics).

Received 5 August 2013; accepted 14 January 2014; published online 2 March 2014

References

Conclusion In summary, we have achieved efficient deep-blue TADF from an organic aromatic molecule with pretwisted intramolecular CT character. The overall electroluminescence performance of an OLED containing this TADF molecule is comparable to that of today’s best blue PHOLEDs, confirming that TADF materials can realize low-cost, high-performance blue OLEDs. The successful design of blue OLEDs based on TADF emitters involves the formation of a small energy gap between 1CT and 3CT states and higher energy of the 3LE state than the 3CT one. Fortunately, an optimal HF exchange method based on TD-DFT can now be used to accurately predict the energy levels of these three excited states.

Methods Quantum chemical calculations. All calculations were performed using the Gaussian 09 program package. The geometries in the ground state were optimized via DFT calculations at the B3LYP/6-31G* level. Vertical absorption energies (EVA) were calculated based on TD-DFT with the B3LYP, PBE0, MPW1B95, BMK, M062X and M06-HF functionals using 6-31G* basis sets. The calculated EVA(S1) corresponding to LE or CT transitions were distinguished by orbital transition analyses. Based on the relationship between q and the optimal HF% (OHF), that is, OHF ¼ 42q, EVA(1CT, OHF) was read from the fitted straight line of the EVA(1CT)– HF% points plotted on a log–log scale (Supplementary Fig. 2), and E0–0(1CT) was obtained using a common gap of 0.24 eV between E0–0(1CT) and EVA(1CT, OHF). Furthermore, by looking at the change in EVA(S1)–EVA(T1) with HF%, we distinguished the EVA(T1) points corresponding to the 3CT or 3LE transitions and calculate E0–0(3CT) and E0–0(3LE) from them, respectively (Supplementary Fig. 2 and Table 1). For more details on the calculations of CT amount (q), optimal HF%, E0–0(1CT), E0–0(3CT) and E0–0(3LE), see our previous report30. Photoluminescence measurements. Solutions of the samples (0.1 mM) for luminescence studies were degassed with nitrogen for several minutes before use. Thin-film samples (100 nm) for luminescence and photoelectron spectroscopy studies were deposited on quartz glass substrates by vacuum evaporation (pressure, ,4 × 1024 Pa; rate, 0.2 nm s21). UV–vis absorption spectra of the compounds in toluene were measured on a Perkin–Elmer Lambda 950-PKA UV–vis spectrophotometer in the range of 280–600 nm. Photoluminescence spectra were recorded on a Jasco FP-6500 spectrofluorometer equipped with a liquid nitrogen attachment at room temperature and 77 K. Absolute PLQYs were obtained using a

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Acknowledgements This work was supported by a Grant-in-Aid from the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST) and the International Institute for Carbon Neutral Energy Research (WPI-I2CNER) sponsored by MEXT. The authors thank J.-L. Bre´das, M. Kotani and K. Tokumaru for stimulating discussions regarding this work. The authors also thank W. J. Potscavage Jr for assistance with preparation of this manuscript.

Author contributions Q.Z. designed the molecules. B.L. measured photoluminescence and electroluminescence characteristics. S.H. performed the computational experiments. Q.Z and H.N. synthesized the compounds. H.T. provided experimental support and suggestions. Q.Z. and C.A. wrote the manuscript.

Additional information Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to C.A.

Competing financial interests The authors declare no competing financial interests.

