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High-efficiency organic light-emitting diodes with fluorescent emitters

High-efficiency organic light-emitting diodes with fluorescent emitters ARTICLE Received 13 Feb 2014 | Accepted 30 Apr 2014 | Published 30 May 2014 DOI: 10.1038/ncomms5016 High-efficiency organic light-emitting diodes with fluorescent emitters 1,2 1 1 1,3 1 Hajime Nakanotani , Takahiro Higuchi , Taro Furukawa , Kensuke Masui , Kei Morimoto , 1 1 1 1,4 1,2,4 Masaki Numata , Hiroyuki Tanaka , Yuta Sagara , Takuma Yasuda & Chihaya Adachi Fluorescence-based organic light-emitting diodes have continued to attract interest because of their long operational lifetimes, high colour purity of electroluminescence and potential to be manufactured at low cost in next-generation full-colour display and lighting applications. In fluorescent molecules, however, the exciton production efficiency is limited to 25% due to the deactivation of triplet excitons. Here we report fluorescence-based organic light-emitting diodes that realize external quantum efficiencies as high as 13.4–18% for blue, green, yellow and red emission, indicating that the exciton production efficiency reached nearly 100%. The high performance is enabled by utilization of thermally activated delayed fluorescence molecules as assistant dopants that permit efficient transfer of all electrically generated singlet and triplet excitons from the assistant dopants to the fluorescent emitters. Organic light-emitting diodes employing this exciton harvesting process provide freedom for the selection of emitters from a wide variety of conventional fluorescent molecules. 1 2 Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan. Innovative Organic Device Laboratory, Institute of Systems, Information Technologies and Nanotechnologies (ISIT), 744 Motooka, Nishi, Fukuoka 819-0395, Japan. Advanced Research Laboratories, Fujifilm Co., 577 Ushijima, Kaisei, Ashigarakami, Kanagawa 258-8577, Japan. International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan. Correspondence and requests for materials should be addressed to C.A. (email: adachi@cstf.kyushu-u.ac.jp). NATURE COMMUNICATIONS | 5:4016 | DOI: 10.1038/ncomms5016 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5016 pin statistics states that one singlet exciton is generated for we demonstrated an alternative route for increasing Z by using int 19–23 every three triplet excitons after the recombination of a thermally activated delayed fluorescence (TADF) process . Sholes and electrons in organic semiconductor materials . In the TADF process, light emission can be extracted as Light emission can be extracted through fluorescence or delayed fluorescence after intersystem crossing (ISC) from the phosphorescence by the decay of these excitons from a singlet T to the S states in TADF emitters, resulting in efficient 1 1 (S ) or triplet (T ) excited state to a ground state. By utilizing radiative decay from the S state. In fact, after comprehensive 1 1 1 efficient radiative decay of electrically generated excitons, organic development of TADF materials, an Z of nearly 100% was int light-emitting diodes (OLEDs) are attracting intense attention for realized in green OLEDs . use as advanced displays and lighting sources. In addition to the use of the TADF process for emitters in The external electroluminescence (EL) quantum efficiency OLEDs, we propose a promising route for triplet harvesting by (Z ) of OLEDs is a key parameter and is described by the well- applying TADF molecules as an assistant dopant in OLEDs. EQE known equation Thus, the OLED is composed of a double-dopant system—that is, a wide-energy-gap host, a TADF-assistant dopant and a Z ¼ Z Z ¼ðgZ F ÞZ ð1Þ EQE int out g PL out fluorescent emitter dopant—that leads to Z ¼ 100%. In this where Z is the internal EL quantum efficiency and Z is the system, triplet excitons created on an assistant TADF molecule int out light-out-coupling efficiency. According to equation (1), Z is (T ) by electrical excitation are upconverted to the S state of the int 1 A A limited by the following three factors: (i) charge balance of TADF molecule (S ), and all S excitons are transferred to the S 1 1 injected holes and electrons (g), (ii) efficiency of radiative exciton state of a fluorescent emitter molecule (S ) via a FRET process, production (Z ) and (iii) photoluminescence (PL) quantum yield which results in efficient radiative decay from S of the of the emitter molecules (F ). The ideal g can be achieved by fluorescent emitter. PL circumspect design of OLED structures with the appropriate On the basis of this cascade energy transfer, we demonstrate selection of charge transport layers, host–guest system, and highly efficient OLEDs with Z as high as 13.5, 15.8, 18 and EQE anode and cathode materials. In addition, based on a proper 17.5% for blue, green, yellow and red colours, respectively. We molecular design for light emission, F of nearly 100% has been use the TADF molecules 10-phenyl-10H, 10 H-spiro[acridine-9, PL 0 0 23 demonstrated in a wide variety of fluorescent and phosphorescent 9 -anthracen]-10 -one (ACRSA) , 3-(9,9-dimethylacridin-10 materials. However, Z can severely limit Z if the 75% of (9H)-yl)-9H-xanthen-9-one (ACRXTN), 2-phenoxazine-4, g int electrically generated excitons formed in triplet states are not 6-diphenyl-1,3,5-triazine (PXZ-TRX) and 2,4,6-tri(4-(10H- harvested. phenoxazin-10H-yl)phenyl)-1,3,5-triazine (tri-PXZ-TRZ) as Several routes have been proposed to obtain a high Z through assistant dopants and 2,5,8,11-tetra-tert-butylperylene (TBPe), int the efficient harvesting of triplet excitons in OLEDs. While the 9,10-Bis[N,N-di-(p-tolyl)-amino]anthracene (TTPA), 2,8-di- very first trials of ketone derivatives showing intense phosphor- tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene escence at low temperature opened a new method for triplet (TBRb) and tetraphenyldibenzoperiflanthene (DBP) as fluo- harvesting, the Z was limited to a low value . Rare metal rescent emitter dopants (Fig. 1a). These fluorescent molecules EQE complexes containing Eu and Tb established intramolecular have been widely used as conventional fluorescent emitters in 24–27 cascade energy transfer as another route to harvest triplet excitons OLEDs and are commercially available . As host materials, 3–5 but did not show promising Z . Later, a successful strategy we use bis-(2-(diphenylphosphino)phenyl)ether oxide (DPEPO), EQE was realized using room temperature phosphorescent emitters 1,3-Bis(N-carbazolyl)benzene (mCP), 3,3-di(9H-carbazol-9- 0 0 such as platinum and iridium complexes. In this case, according yl)biphenyl (mCBP) and 4,4 -bis(9-carbazolyl)-1,1 -biphenyl to the mixing of the spin orbitals of S and T states due to the (CBP) for the blue, green, yellow and red OLEDs, respectively. 1 1 presence of a heavy atom, the radiative decay rate from a T state In this double-dopant system, the assistant dopant does not itself to a ground state is significantly accelerated, resulting in the emit light but passes all of the electrically generated excitons to 6–10 radiative decay of nearly 100% of triplet excitons . In addition, fluorescent emitter molecules for radiative decay. the utilization of phosphorescence emitters as a triplet sensitizer 11,12 has been proposed . Using this process, triplet harvesting realized by energy transfer from the T state of a phosphorescent Results emitter such as an iridium 2-phenylpyridine complex to the S Energy transfer process. Figure 1a shows an energy transfer state of a fluorescent emitter via dipole–dipole coupling (that is, diagram for the emitter layers (EMLs) in our cascade-type EL Fo¨rster energy transfer, FRET) resulted in an Z of B45% devices. The emitter and assistant dopant molecule combinations int (ref. 12). However, the rather limited Z is due to the presence of and concentrations studied as EMLs here are listed in Table 1. int the competitive deactivation process of triplet–triplet energy In the case of an EML without any assistant dopant, the injected transfer. carriers are transported on the host molecules, and the carriers Although OLEDs based on fluorescent molecules, which are are eventually trapped on an emitter dopant due to its shallower composed of simple aromatic compounds, have continued to highest occupied molecular orbital and deeper lowest unoccupied attract interest because of their longer operational lifetimes in molecular orbital compared with those of the host material. This blue OLEDs, higher colour purity (narrow spectral width) EL and results in direct carrier recombination dominantly on the emitter broader freedom of molecular design compared with phosphor- dopants. Therefore, no triplet excitons contribute to the total 13–15 escence-based OLEDs , the Z of traditional fluorescence- EL efficiency. int based OLEDs is limited to less than 25% even in the ideal case. On the other hand, when assistant dopants are doped into Therefore, the enhancement of Z in OLEDs using conventional these EMLs, exciton formation mainly on the assistant molecules int fluorescence-based emitters is still obviously a major concern for is desired. Here to reduce the effect of direct carrier trapping on the development of future OLEDs. the emitter molecules in the EMLs, the doping concentration of To achieve this, singlet exciton generation via a triplet–triplet the emitter dopants was held at 1 wt%. Conversely, assistant 16,17 annihilation (TTA) process is one possible route . However, dopants were doped into the EMLs in doping concentrations the theoretical upper limit of the singlet exciton production ratio ranging between 15 and 50 wt%, which ensures that the assistant when the TTA process is included is still less than 62.5%, dopants act as the main carrier recombination centres in the 18 A A corresponding to an Z of 12.5% in the ideal case . Recently, EMLs. Thus, after both singlet (S ) and triplet (T ) exciton EQE 1 1 2 NATURE COMMUNICATIONS | 5:4016 | DOI: 10.1038/ncomms5016 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5016 ARTICLE a + – Exciton formation e 25% 75% FRET S ISC S A 1 E S E 1 E TBPe TTPA TBRb DBP Host Assistant dopant Assistant dopant N N N ON N ON N N N Tri-PXZ- ACRSA ACRXTN PXZ-TRZ O TRZ ACRSA ACRXTN TTPA TBPe 400 500 600 700 800 400 500 600 700 800 Wavelength (nm) Wavelength (nm) de tri-PXZ- PXZ-TRZ TRZ DBP TBRb 400 500 600 700 800 400 500 600 700 800 Wavelength (nm) Wavelength (nm) Figure 1 | Energy transfer mechanism. (a) Schematic illustration of proposed energy transfer mechanism in the emitter dopant:assistant dopant:host matrix under electrical excitation and chemical structures of the assistant dopants used in this study. (b–e) Fluorescence spectra of assistant dopant:host 5  1 co-deposited film (upper), and absorption (dashed line) and fluorescence (solid line) spectra of emitter dopant in solution (10 mol l in CH Cl ) 2 2 (bottom). Rather large Fo¨rster transfer radii of B2.2, B7.3, B6.9 and B10 nm were estimated for blue, green, yellow and red EML matrices based on the spectral overlap between the absorption spectrum of the acceptor and the PL spectrum of the donor, suggesting that efficient FRET is possible. formation on the assistant dopants, the formed triplet excitons because of the large DE of conventional fluorescence molecules ST are upconverted to the S state through ISC because of the rather as shown in Fig. 1a. Thus, the created triplet excitons would not small energy gap between the S and T levels (DE ), as contribute to the energy transfer processes if the assistant dopant ST 1 1 summarized in Table 1. Then, according to the spectral overlap is a conventional fluorescence material. between the absorption spectra of the emitter molecules and the The proposed influence of concentration on carrier trapping PL spectra of the assistant dopants (Fig. 1b–e), the S exciton and recombination is well supported by the dependence of Z EQE energies are resonantly transferred to the S states of emitter on the concentration of the assistant dopant as shown in molecules based on a FRET process. Finally, light can be emitted Supplementary Figs 1–3 and Supplementary Note 1. Here we as delayed fluorescence from the S state of the emitter molecules. observed the best Z -luminance (L) characteristics with the EQE If conventional fluorescence molecules were used as the assistant assistant dopant concentration of 15%. In the case of assistant dopant, triplet excitons would non-radiatively decay from T to dopant concentrations less than 15%, carrier recombination may the ground state and would not contribute to light emission not perfectly occur on the assistant dopants, while Dexter energy NATURE COMMUNICATIONS | 5:4016 | DOI: 10.1038/ncomms5016 | www.nature.com/naturecommunications 3 & 2014 Macmillan Publishers Limited. All rights reserved. Emission intensity Emission intensity and absorbance (a.u.) and absorbance (a.u.) Emission intensity Emission intensity and absorbance (a.u.) and absorbance (a.u.) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5016 Table 1 | Components of the emitter layers of the four colour OLEDs. EL colour Host Assistant Assistant dopant DE Emitter Emitter dopant U (%) ST PL dopant concentration (wt%) (eV) dopant concentration (wt%) H H * A A * E* (S , T ) (eV) (S , T ) (eV) S (eV) 1 1 1 1 1 Blue DPEPO ACRSA 15 0.03 TBPe 180 2 (3.50, 3.00) (2.55, 2.52) (2.69) Green mCP ACRXTN 50 0.06 TTPA 181 2 (3.40, 2.90) (2.53, 2.47) (2.34) Yellow mCBP PXZ-TRZ 25 0.07 TBRb 190 2 (3.37, 2.90) (2.30, 2.23) (2.18) Red CBP tri-PXZ-TRZ 15 0.11 DBP 188 2 (3.36, 2.55) (2.27, 2,16) (2.03) 0 0 0 0 0 ACRSA, 10-phenyl-10H,10 H-spiro[acridine-9,9 -anthracen]-10 -one; ACRXTN, 3-(9,9-dimethylacridin-10(9H)-yl)-9H-xanthen-9-one; CBP, 4,4 -bis(9-carbazolyl)-1,1 -biphenyl; DBP, tetraphenyldibenzoperiflanthene; DPEPO, bis-(2-(diphenylphosphino)phenyl)ether oxide; EL, electroluminescence; mCBP, 3,3-di(9H-carbazol-9-yl)biphenyl; mCP, 1,3-bis(N-carbazolyl)benzene; PXZ- TRZ, 2-phenoxazine-4,6-diphenyl-1,3,5-triazine; OLED, organic light-emitting diode; TBPe, 2,5,8,11-tetra-tert-butylperylene; TBRb, 2,8-di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene; tri- PXZ-TRZ, tri-PXZ-TRZ, 2,4,6-tri(4-(10H-phenoxazin-10H-yl)phenyl)-1,3,5-triazine; TTPA, 9,10-bis[N,N-di-(p-tolyl)-amino]anthracene. *The S and T energies were estimated from the peak wavelengths of fluorescence and phosphorescence emission, respectively. 1 1 transfer from the assistant dopants to the triplet level of a 66–99%, respectively, assuming a charge carrier balance factor of fluorescent emitter (T ) or concentration quenching of the 1 and a light-out-coupling efficiency of 20–30% (refs 28,29). assistant dopants may happen in the case of the concentrations These results clearly indicate that the devices overcame the over 15%. The optimum concentration would change for different theoretical limit of 25% for the singlet exciton production combinations of host and guest molecules having different carrier efficiency that is assumed for fluorescence-based OLEDs and even transport and photophysical characteristics. We note that the host that of 62.5% in the case of TTA . H H layers have both higher singlet (S ) and triplet (T ) energy levels 1 1 A A than those of the assistant molecules, S and T . Therefore, back 1 1 energy transfer from the S state of the assistant dopants to the Transient EL analysis. In order to confirm the contribution of singlet exciton generation via energy transfer from the assistant S state of the host layers is inhibited. In addition, dispersing the assistant dopants in a host matrix can additionally prevent direct dopants to the emitter dopants after harvesting triplet excitons on A E the assistant dopants under electrical excitation, transient EL was energy transfer from T to T —that is, minimize a Dexter energy 1 1 transfer process that would result in losses . measured with an electrical excitation pulse width of 1 msat 300 K. Figure 3a shows the streak image and the transient EL decay curve obtained from the red OLED with an EML of 1 OLED characteristics. On the basis of the optimized configura- wt% DBP and 15 wt% tri-PXZ-TRZ in a CBP host matrix. tion of EMLs for blue, green, yellow and red EL, full-colour After turning off the electrical pulse excitation, a clear delayed OLEDs were fabricated to demonstrate the impact of this triplet component with emission bands centred at 610 nm was observed. harvesting process on OLED performance. The Z -L curves of In addition, the emission spectrum of the delayed fluorescence is EQE the four OLEDs are shown in Fig. 2a–d, and Table 2 summarizes consistent with that of the prompt component, which their device performance. Although the OLEDs without assistant is consistent with the described mechanism of triplet to singlet dopants, which use only conventional single doping by fluor- exciton upconversion in tri-PXZ-TRZ followed by successive escent dopants, show low device performance as shown by the resonant singlet energy transfer to the S states of DBP open circles (Z o5%), remarkably high Z of 13.4, 15.8, 18 emitters. EQE EQE and 17.5% for blue, green, yellow and red EL were achieved by Here we describe the exciton formation process in the double- including the assistant dopants. In addition, the magnitudes of dopant system under either optical or electrical excitation. In the the slopes of the Z -L curves of the double-dopant systems case of optical excitation, singlet excitons are mainly generated in EQE are much lower than those of the corresponding single-dopant the S state of a CBP host molecule by photoabsorption, and systems at similar luminance, indicating that the efficiency roll- nearly all of the singlets are then resonantly transferred to the S off characteristics are well suppressed in the double-dopant state of tri-PXZ-TRZ molecules because of the large concentra- system. The reduced roll-off is probably because of the expansion tion of the assistant dopants. In addition, a number of singlet of the carrier recombination site or the reduction of the inter- excitons in tri-PXZ-TRZ would also be formed by direct action between excitons and polarons (that is, exciton–polaron absorption. Next, a fraction of the singlet excitons is transferred annihilation) as a result of rapid energy transfer from the carrier into the S state of DBP through a FRET process, producing the recombination centre (TADF assistant dopants) to the emission prompt fluorescence decay of DBP. Simultaneously, singlet centre (fluorescent emitter dopants). excitons on tri-PXZ-TRZ also decay to the T state internally The EL spectra of the OLEDs are presented in Fig. 2e, through ICS and are successively upconverted into the S state Supplementary Fig. 4 and Supplementary Note 2. These OLEDs again by thermal conversion, followed by energy transfer via emit a full range of visible colours from blue (462 nm) to red FRET to the S state of DBP that produces the delayed emission. (610 nm). Although a weak emission originating from the Here we can ignore the direct formation of excitons on DBP assistant dopant is observed in the red OLED, other OLEDs because of the dilute concentration of 1 wt%. Under electrical showed pure emission originating from the emitter dopants that excitation, on the other hand, since the singlet and triplet excitons A A is in good accordance with the PL spectrum of the emitter are directly created on the S and T states of the tri-PXZ-TRZ 1 1 ± ± ± dopants in solution. Here F of 80 2, 81 2, 90 2 and molecules based on the exciton branching ratio of 25–75%, PL 88 2% were obtained for the blue, green, yellow and red emitter respectively, the contribution of the delayed fluorescence in dopants when doped with assistant dopants in films (Table 1). the total emission is significantly larger compared with that Thus, we can estimate Z of 55–84%, 65–97%, 66–100% and of the optical excitation, as shown in Fig. 3b. 4 NATURE COMMUNICATIONS | 5:4016 | DOI: 10.1038/ncomms5016 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5016 ARTICLE ab 1 1 10 10 Without assistant dopant NN Without 0 0 10 10 assistant dopant 0 1 2 3 4 0 1 2 3 4 10 10 10 10 10 10 10 10 10 10 –2 –2 Luminance (cd m ) Luminance (cd m ) cd 1 1 10 10 Without 0 0 10 10 Without assistant dopant assistant dopant 0 1 2 3 4 0 1 2 3 4 10 10 10 10 10 10 10 10 10 10 –2 –2 Luminance (cd m ) Luminance (cd m ) 1.0 0.8 0.6 0.4 0.2 0.0 400 500 600 700 Wavelength (nm) Figure 2 | Device performance of OLEDs. (a–d) External EL quantum efficiency as a function of luminance for the blue, green, yellow and red OLEDs. The external EL quantum efficiency for OLEDs without an assistant dopant is plotted as open symbols. Inset: chemical structures of emitter dopants used in this study. (e) EL spectra of the devices at a luminance of 100 cd m . Table 2 | Device performance of the four colour OLEDs with assistant dopants for triplet harvesting. Device Turn on Max EQE Max CE Max PE CIE Performance at 1,000 cd m 1  1 voltage (V) (%) (cd A ) (lm W ) 1  1 Voltage (V) EQE (%) CE (cd A ) PE (lm W ) Blue 4.7 13.4 27 18 (0.17, 0.30) 7.8 8.7 18 7 Green 3.0 15.8 45 47 (0.29, 0.59) 4.1 11.7 38 30 Yellow 3.2 18.0 60 58 (0.45, 0.53) 5.2 17.2 56 33 Red 3.0 17.5 25 28 (0.61, 0.39) 6.4 10.9 20 10 OLED, organic light-emitting diode; CE, current efficiency; CIE, Commission Internationale de l‘Eclairage; EQE, external electroluminescence quantum efficiency; PE, power efficiency. Device operational stability. The introduction of assistant The driving voltages of the devices are also rather stable, dis- dopants into a host–guest system provides not only a significant playing a rise of less than 0.5 V after 100 h of operation. More enhancement of Z but also an enhancement of device opera- interestingly, the operational lifetime was longer than that of the EQE tional stability under electrical excitation. For example, the nor- OLED with PXZ-TRZ molecules as the emitter dopant (device C) malized luminance of the yellow OLEDs as a function of while the voltage rise curves are almost the same, indicating operation time at a constant current density of 10 mA cm are that the combination of fluorescent emitter dopants and presented in Fig. 4. Although a rapid decrease in luminance TADF assistant dopants provides improved device performance. was observed in the OLED without assistant dopants (device B), We note that the nearly identical film morphology for each of the the OLED with the assistant dopants (device A) showed co-deposited films compared with that of the host-only films improved luminance decay characteristics compared with that of suggests that device stability is not influenced by a morphology the OLED without the assistant dopants, resulting in an improved change caused by the dopant molecules as shown in operational lifetime, defined as the elapsed operation time at Supplementary Fig. 5 and Supplementary Note 3. Since it is well which the luminance drops to 50% of the initial value, of 194 h. established that TBRb is a stable emitter electrochemically , the NATURE COMMUNICATIONS | 5:4016 | DOI: 10.1038/ncomms5016 | www.nature.com/naturecommunications 5 & 2014 Macmillan Publishers Limited. All rights reserved. External EL quantum External EL quantum efficiency (%) efficiency (%) Normalized EL intensity (a.u.) External EL quantum External EL quantum efficiency (%) efficiency (%) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5016 Discussion a 0 Our results suggest that both a higher Z and an enhancement EQE of operational stability can be expected with an optimum EML design using a wide variety of TADF and fluorescent materials. As numerous electrochemically stable fluorescent emitters Delayed have been widely developed in the past two decades, the triplet EL harvesting mechanism realized here can provide a greater Prompt EL flexibility in the design of OLED architectures. In addition, since Delayed EL 40 Z can exceed 20% by using emitter molecules with horizontally out oriented dipoles , the flexibility of simple aromatic compound design can further boost the Z by enhancement of Z without EQE out special light-out-coupling structures. In summary, we presume that the cascade energy transfer scheme using TADF assistant Wavelength (nm) EL intensity (a.u.) dopants and fluorescent emitter dopants will be the most promising device architecture for OLEDs with ultimate Electrical excitation performance. Optical excitation Methods Materials. mCP, 4,4 -cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] 0 00 (TAPC), 2,2 ,2 -(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole (TPBi), TBPe, TTPA, TBRb and DBP were purchased from Luminescence Technology Corp. mCBP was purchased from NARD Institute Ltd. CBP, 4,4 -bis(N-phenyl- 1-naphthylamino)biphenyl (a-NPB) and tris(8-hydroxyquinolinato)aluminum (Alq3) were used as received from the Nippon Steel Chemical Co., Ltd. ACRSA, DPEPO , 2-phenoxazine-4,6-diphenyl-1,3,5-triazine (PXZ-TRX), tri-PXZ-TRZ 0 10 20 30 40 50 and 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T) were synthesized according Time (µs) to the reported procedures. Figure 3 | Transient EL characteristics. (a) Time-resolved electroluminescence image for the red OLED. The streak image (left) and Synthesis of ACRXTN. We synthesized ACRXTN according to the general procedure. The complete synthetic route for ACRXTN and the 1H NMR spectrum time-resolved EL decay curve (right) are for the red OLED under electrical of ACRXTN are included in the Supplementary Methods section and excitation with a pulse of 1 ms at 300 K. A delayed EL spectrum was Supplementary Fig. 6, respectively. NMR spectra were obtained with a Bruker collected from 3 to 50 ms after excitation. (b) Time-resolved PL and EL Biospin Avance-III 500 NMR spectrometer at ambient temperature. High-resolu- response of a 1 wt%-DBP:15 wt%-tri-PXZ-TRZ:CBP film (red line) and tion mass spectrometry by fast atom bombardment was performed using a JEOL JMS-700 spectrometer. 1 wt%-DBP:15 wt%-tri-PXZ-TRZ:CBP-based OLED (black line), respectively. 3-bromo-9H-xanthen-9-one 1.38 g (5 mmol), 9,9-dimethyl-9,10- dihydroacridine 1.15 g (5.5 mmol), tert-BuONa 0.96 g (10 mmol), tri-tert- butylphosphonium tetrafluoroborate 145 mg (0.5 mmol) and palladium acetate 56 mg (0.25 mmol) were put into a flask and purged three times by nitrogen/ 1.0 vacuum cycle. Then, anhydrous toluene was added and refluxed for 8 h. After cooling to room temperature, the resulting solution was filtered through celite and concentrated. The crude product was purified using a silica gel chromatography (CH Cl :hexane ¼ 8:2) and recrystallized twice from mixed solvent of 2 2 0.8 hexane:AcOEt ¼ 9:1 ml g . The desired product was obtained as yellow powder (2.06 g, 85%). H-NMR (500 MHz, CDCl ): d (p.p.m.) ¼ 8.53 (d, 1H, 8.5 Hz), 8.38 (dd, 1H, 8 Hz), 7.74 (ddd, 1H, 8.6, 7.2, 1.8 Hz), 7.53 (d, 1H, 1.9 Hz), 7.52–7.46 0 50 100 150 200 (m, 3H), 7.42 (ddd, 1H, 8.1, 7.2, 1 Hz), 7.38 (dd, 1H, 8.5, 2 Hz), 7.08–6.98 (m, 4H), Time (h) 0.6 13 6.53 (dd, 2H, 7.8, 1.6 Hz). C-NMR (125 MHz, CDCl ): d (p.p.m.) ¼ 176.47, 157.83, 156.25, 148.09, 140.28, 134.94, 132.55, 129.32, 126.79, 126.45, 125.27, 124.59, 124.20, 121.99, 121.96, 120.52, 117.95, 117.57, 115.88, 36.38, 30.53. High- Device B Device C Device A resolution mass spectrometry (m/z): [M þ H] þ calculated for C28H22NO2, –2 –2 –2 L =677 cd m L =2,791 cd m L =3,225 cd m 0 0 0 0.4 404.1651; found, 404.1651. 0 50 100 150 200 Optical characterization of organic thin films. PL quantum efficiency was measured by an absolute PL quantum yield measurement system (C11347-01, Time (h) Hamamatsu Photonics) under the flow of nitrogen gas with an excitation wave- Figure 4 | Device stability of yellow OLEDs. For operational lifetime length of 337 nm. Low-temperature PL intensity and emission lifetimes were measured using a streak camera (C4334, Hamamatsu Photonics) and cryostat measurement of the double-dopant system, an OLED with a device (Iwatani Industrial Gases Co.) with a nitrogen gas laser (MNL200, Laser Technik) structure of ITO/a-NPD (35 nm)/1 wt%-TBRb:25 wt%-PXZ-TRZ:mCBP as an excitation light source under a pressure of about 3 Pa. (30 nm)/T2T (10 nm)/Alq (55 nm)/LiF (0.8 nm)/Al (100 nm) (device A) was used. To confirm the effect of the assistant dopant, OLEDs with an EML Fabrication of OLEDs. Glass substrates with a pre-patterned, 100-nm-thick, of either 1 wt%-TBRb:mCBP (device B) or 25 wt%-PXZ-TRZ:mCBP (device 100 Ohm sq tin-doped indium oxide (ITO) coating were used as anodes. Sub- C) were also measured at a constant current density of 10 mA cm . Initial strates were washed by sequential ultrasonication in neutral detergent, distilled luminances (L ) are 3,225, 677 and 2,791 cd m for devices A, B and C, water, acetone and isopropanol, and then exposed to ultraviolet–ozone (NL- UV253, Nippon Laser & Electronics Lab) to remove adsorbed organic species. respectively. Inset: voltage rise curves for devices A, B and C (coloured After pre-cleaning of the substrates, effective device areas of 1 mm were defined accordingly). on the patterned-ITO substrates by a polyimide insulation layer using a conven- tional photolithography technique. Substrates were treated with ultraviolet–ozone for 25 min and immediately transferred into the evaporation chamber. Organic layers were formed by thermal evaporation. Doped emitting layers differences in reliability would be because of the change of carrier were deposited by co-evaporation. Deposition was performed under vacuum at recombination and exciton formation area with and without the pressures of o5  10 Pa. Devices were exposed to nitrogen gas once after presence of the assistant dopants, in addition to the formation of the organic layers to apply a metal mask that defines the cathode area. electrochemical stability of emitter molecules. After device fabrication, devices were immediately encapsulated with glass lids 6 NATURE COMMUNICATIONS | 5:4016 | DOI: 10.1038/ncomms5016 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. –3 –2 –1 EL intensity (a.u.) Intensity (a.u.) Normalized EL intensity (a.u.) Voltage (V) Time (µs) NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5016 ARTICLE using epoxy glue in a nitrogen-filled glove box (O o0.1 p.p.m., H Oo0.1 p.p.m.). 2 2 16. Helfrich, W. & Schneider, W. G. Transients of volume-controlled current and Commercial calcium oxide desiccant (Dynic Co.) was included in each of recombination radiation in anthracene. J. Chem. Phys. 44, 2902–2909 (1966). encapsulated package. 17. Pope, M. & Swenberg, C. E. Electronic Processes in Organic Crystals 64 (Oxford Blue OLEDs with the structure ITO/a-NPB (35 nm)/mCP (10 nm)/1 wt%- University Press, 1982). TBPe: 15 wt%-ACRSA: DPEPO (15 nm)/DPEPO (8 nm)/TPBi (57 nm)/LiF 18. Kondakov, D. Y., Pawlik, T. D., Hatwar, T. K. & Spindler, J. P. Triplet (0.8 nm)/Al (100 nm) were fabricated. As a control device, an OLED with an annihilation exceeding spin statistical limit in highly efficient fluorescent EML that consisted of 1 wt%-TBPe: DPEPO was also made. organic light-emitting diodes. J. Appl. Phys. 106, 124510–124516 (2009). Green OLEDs with the structure ITO/TAPC (35 nm)/1 wt%-TTPA: 50 wt%- 4 þ 19. Endo, A. et al. Thermally activated delayed fluorescence from Sn –porphyrin ACRXTN: mCP (15 nm)/TPBi (65 nm)/LiF (0.8 nm)/Al (100 nm) were fabricated. complexes and their application to organic light emitting diodes—a novel As a control device, an OLED with an EML consisting of 1 wt%-TTPA: mCP mechanism for electroluminescence. Adv. Mater. 21, 4802–4806 (2009). was also made. 20. Tanaka, H., Shizu, K., Miyazaki, H. & Adachi, C. Efficient green thermally Yellow OLEDs with the structure ITO/TAPC (35 nm)/1 wt%-TBRb:25 wt%- activated delayed fluorescence (TADF) from a phenoxazine–triphenyltriazine PXZ-TRX:mCBP (30 nm)/T2T (10 nm)/Alq (55 nm)/LiF (0.8 nm)/Al (100 nm) (PXZ–TRZ) derivative. Chem. Commun. 48, 11392–11394 (2012). were fabricated. As a control device, an OLED with an EML consisting of 21. Uoyama, H., Goushi, K., Shizu, K., Nomura, H. & Adachi, C. Highly efficient 1 wt%-TBRb: mCBP was also made. organic light-emitting diodes from delayed fluorescence. Nature 492, 234–238 Red OLEDs with the structure ITO/TAPC (35 nm)/1 wt%-DBP:15 wt%-tri- (2012). PXZ-TRZ:CBP (15 nm)/TPBi (65 nm)/LiF (0.8 nm)/Al (100 nm) were fabricated. 22. Tanaka, H., Shizu, K., Nakanotani, H. & Adachi, C. Twisted intramolecular As a control device, an OLED with an EML consisting of 1 wt%-DBP: CBP charge transfer state for long-wavelength thermally activated delayed was also made. Schematic diagrams of the energy levels of the fabricated devices and the fluorescence. Chem. Mater. 25, 3766–3771 (2013). chemical structures of the assistant dopant materials used in them are presented 23. Nasu, K. et al. A highly luminescent spiro-anthracenone-based organic in Supplementary Fig. 7. light-emitting diode through thermally activated delayed fluorescence. Chem. Commun. 49, 10385–10387 (2013). 24. Mi, B.-X. et al. Reduction of molecular aggregation and its application to the Characterization of OLEDs. The current density–voltage–luminance character- high-performance blue perylene-doped organic electroluminescent device. istics of the OLEDs were evaluated using a source meter (Keithley 2400, Keithley Appl. Phys. Lett. 75, 4055–4057 (1999). Instruments Inc.) and an absolute external quantum efficiency measurement 25. Yu, Y.-H., Huang, C.-H., Yeh, J.-M. & Huang, P.-T. Effect of methyl system (C9920-12, Hamamatsu Photonics). The OLEDs were mounted to the substituents on the N-diaryl rings of anthracene-9,10-diamine derivatives for entrance port of the measurement system’s integrating sphere to collect the pho- OLEDs applications. Org. Electron 12, 694–702 (2011). tons emitted from the front face of the devices. Each EL spectrum was collected 26. Wu, Y.-S., Liu, T.-H., Chen, H.-H. & Chen, C.-H. A new yellow fluorescent by an optical fiber connected to a spectrometer (PMA-12, Hamamatsu Photonics). dopant for high-efficiency organic light-emitting devices. Thin Solid Films 496, The repeatability of device performances of the present devices was confirmed 626–630 (2006). by four different samples. To confirm the validity of our Z measurements, we EQE 27. Okumoto, K., Kanno, H., Hamada, Y., Takahashi, H. & Shibata, K. High also measured Z using another independent measurement system based EQE efficiency red organic light-emitting devices using on a luminance meter (Supplementary Fig. 8 and Supplementary Note 4). tetraphenyldibenzoperiflanthene-doped rubrene as an emitting layer. Appl. Time-resolved EL decay curves were obtained using a streak camera (C4334, Phys. Lett. 89, 013502–013504 (2006). Hamamatsu Photonics) with a pulse generator (81101A, Agilent) as an electrical 28. Smith, L. H., Wasey, J. A. E. & Barnes, W. L. Light out coupling efficiency excitation source. The operational lifetime was measured using a luminance meter (SR-3AR, TOPCON) at a constant DC current. 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Chen, H.–F. et al. 1,3,5-Triazine derivatives as new electron transport–type host 3. Kido, J., Nagai, K. & Ohashi, Y. Electroluminescence in a terbium complex. materials for highly efficient green phosphorescent OLEDs. J. Mater. Chem. 19, Chem. Lett. 19, 657–660 (1990). 8112–8118 (2009). 4. Kido, J., Hayase, H., Hongawa, K., Nagai, K. & Okuyama, K. Bright red light- emitting organic electroluminescent devices having a europium complex as an emitter. Appl. Phys. Lett. 65, 2124 (1994). Acknowledgements 5. Hong, Z. R. et al. Rare earth complex as a high-efficiency emitter in an This work was supported in part by the Funding Program for World-Leading Innovative electroluminescent device. Adv. Mater. 13, 1241–1245 (2001). R&D on Science and Technology (FIRST) and the International Institute for Carbon 6. Baldo, M. A. et al. Highly efficient phosphorescent emission from organic Neutral Energy Research (WPI-I2CNER) sponsored by the Ministry of Education, electroluminescent devices. Nature 395, 151–154 (1998). Culture, Sports, Science and Technology (MEXT). We thank Ms Nozomi Nakamura and 7. Baldo, M. A., Lamansky, S., Burrows, P. E., Thompson, M. E. & Forrest, Mr Hiroshi Miyazaki for synthesis and purification of organic materials. We also thank S. R. Very high-efficiency green organic light-emitting devices based on W. Potscavage for his assistance with preparation of this manuscript. electrophosphorescence. Appl. Phys. Lett. 75, 4–6 (1999). 8. Adachi, C., Baldo, M. A., Thompson, M. E. & Forrest, S. R. Nearly 100% internal phosphorescence efficiency in an organic light-emitting device. Author contributions J. Appl. Phys. 90, 5048–5050 (2001). The experiments were conceived and designed by H.N. and K.M. and carried out by 9. Watanabe, S., Ide, N. & Kido, J. High-efficiency green phosphorescent organic H.N., T.H., T.F. and K.M. M.N., H.T., Y.S. and T.Y. performed the synthetic work. light-emitting devices with chemically doped layers. Jpn J. Appl. Phys. 46, 1186– H.N. and C.A. wrote the manuscript. The project was supervised by C.A. All the authors 1188 (2007). discussed the results and contributed to the article. 10. Reineke, S. et al. White organic light-emitting diodes with fluorescent tube efficiency. Nature 459, 234–238 (2009). 11. Baldo, M. A., Thompson, M. E. & Forrest, S. R. High-efficiency fluorescent Additional information organic light-emitting devices using a phosphorescent sensitizer. Nature 403, 750–753 (2000). Supplementary Information accompanies this paper at http://www.nature.com/ 12. D’Andrade, B. W. et al. High-efficiency yellow double-doped organic naturecommunications light-emitting devices based on phosphor-sensitized fluorescence. Appl. Phys. Competing financial interests: The authors declare no competing financial interests. Lett. 79, 1045–1047 (2001). 13. Iwakuma, T. et al. Red and white EL materials based on a new fused aromatic Reprints and permission information is available online at http://npg.nature.com/ ring. SID Int. Symp. Digest Tech. Papers 33, 598–601 (2002). reprintsandpermissions/ 14. Hosokawa, C. et al. Improvement of lifetime in organic electroluminescence. SID Int. Symp. Digest Tech. Papers 35, 780–783 (2004). How to cite this article: Nakanotani, H. et al. High-efficiency organic light-emitting 15. Kawamura, M. et al. Highly efficient fluorescent blue OLEDs with efficiency- diodes with fluorescent emitters. Nat. Commun. 5:4016 doi: 10.1038/ncomms5016 enhancement layer. SID Int. Symp. Digest Tech. Papers 41, 560–563 (2010). (2014). NATURE COMMUNICATIONS | 5:4016 | DOI: 10.1038/ncomms5016 | www.nature.com/naturecommunications 7 & 2014 Macmillan Publishers Limited. All rights reserved. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nature Communications Springer Journals

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Abstract

ARTICLE Received 13 Feb 2014 | Accepted 30 Apr 2014 | Published 30 May 2014 DOI: 10.1038/ncomms5016 High-efficiency organic light-emitting diodes with fluorescent emitters 1,2 1 1 1,3 1 Hajime Nakanotani , Takahiro Higuchi , Taro Furukawa , Kensuke Masui , Kei Morimoto , 1 1 1 1,4 1,2,4 Masaki Numata , Hiroyuki Tanaka , Yuta Sagara , Takuma Yasuda & Chihaya Adachi Fluorescence-based organic light-emitting diodes have continued to attract interest because of their long operational lifetimes, high colour purity of electroluminescence and potential to be manufactured at low cost in next-generation full-colour display and lighting applications. In fluorescent molecules, however, the exciton production efficiency is limited to 25% due to the deactivation of triplet excitons. Here we report fluorescence-based organic light-emitting diodes that realize external quantum efficiencies as high as 13.4–18% for blue, green, yellow and red emission, indicating that the exciton production efficiency reached nearly 100%. The high performance is enabled by utilization of thermally activated delayed fluorescence molecules as assistant dopants that permit efficient transfer of all electrically generated singlet and triplet excitons from the assistant dopants to the fluorescent emitters. Organic light-emitting diodes employing this exciton harvesting process provide freedom for the selection of emitters from a wide variety of conventional fluorescent molecules. 1 2 Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan. Innovative Organic Device Laboratory, Institute of Systems, Information Technologies and Nanotechnologies (ISIT), 744 Motooka, Nishi, Fukuoka 819-0395, Japan. Advanced Research Laboratories, Fujifilm Co., 577 Ushijima, Kaisei, Ashigarakami, Kanagawa 258-8577, Japan. International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan. Correspondence and requests for materials should be addressed to C.A. (email: adachi@cstf.kyushu-u.ac.jp). NATURE COMMUNICATIONS | 5:4016 | DOI: 10.1038/ncomms5016 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5016 pin statistics states that one singlet exciton is generated for we demonstrated an alternative route for increasing Z by using int 19–23 every three triplet excitons after the recombination of a thermally activated delayed fluorescence (TADF) process . Sholes and electrons in organic semiconductor materials . In the TADF process, light emission can be extracted as Light emission can be extracted through fluorescence or delayed fluorescence after intersystem crossing (ISC) from the phosphorescence by the decay of these excitons from a singlet T to the S states in TADF emitters, resulting in efficient 1 1 (S ) or triplet (T ) excited state to a ground state. By utilizing radiative decay from the S state. In fact, after comprehensive 1 1 1 efficient radiative decay of electrically generated excitons, organic development of TADF materials, an Z of nearly 100% was int light-emitting diodes (OLEDs) are attracting intense attention for realized in green OLEDs . use as advanced displays and lighting sources. In addition to the use of the TADF process for emitters in The external electroluminescence (EL) quantum efficiency OLEDs, we propose a promising route for triplet harvesting by (Z ) of OLEDs is a key parameter and is described by the well- applying TADF molecules as an assistant dopant in OLEDs. EQE known equation Thus, the OLED is composed of a double-dopant system—that is, a wide-energy-gap host, a TADF-assistant dopant and a Z ¼ Z Z ¼ðgZ F ÞZ ð1Þ EQE int out g PL out fluorescent emitter dopant—that leads to Z ¼ 100%. In this where Z is the internal EL quantum efficiency and Z is the system, triplet excitons created on an assistant TADF molecule int out light-out-coupling efficiency. According to equation (1), Z is (T ) by electrical excitation are upconverted to the S state of the int 1 A A limited by the following three factors: (i) charge balance of TADF molecule (S ), and all S excitons are transferred to the S 1 1 injected holes and electrons (g), (ii) efficiency of radiative exciton state of a fluorescent emitter molecule (S ) via a FRET process, production (Z ) and (iii) photoluminescence (PL) quantum yield which results in efficient radiative decay from S of the of the emitter molecules (F ). The ideal g can be achieved by fluorescent emitter. PL circumspect design of OLED structures with the appropriate On the basis of this cascade energy transfer, we demonstrate selection of charge transport layers, host–guest system, and highly efficient OLEDs with Z as high as 13.5, 15.8, 18 and EQE anode and cathode materials. In addition, based on a proper 17.5% for blue, green, yellow and red colours, respectively. We molecular design for light emission, F of nearly 100% has been use the TADF molecules 10-phenyl-10H, 10 H-spiro[acridine-9, PL 0 0 23 demonstrated in a wide variety of fluorescent and phosphorescent 9 -anthracen]-10 -one (ACRSA) , 3-(9,9-dimethylacridin-10 materials. However, Z can severely limit Z if the 75% of (9H)-yl)-9H-xanthen-9-one (ACRXTN), 2-phenoxazine-4, g int electrically generated excitons formed in triplet states are not 6-diphenyl-1,3,5-triazine (PXZ-TRX) and 2,4,6-tri(4-(10H- harvested. phenoxazin-10H-yl)phenyl)-1,3,5-triazine (tri-PXZ-TRZ) as Several routes have been proposed to obtain a high Z through assistant dopants and 2,5,8,11-tetra-tert-butylperylene (TBPe), int the efficient harvesting of triplet excitons in OLEDs. While the 9,10-Bis[N,N-di-(p-tolyl)-amino]anthracene (TTPA), 2,8-di- very first trials of ketone derivatives showing intense phosphor- tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene escence at low temperature opened a new method for triplet (TBRb) and tetraphenyldibenzoperiflanthene (DBP) as fluo- harvesting, the Z was limited to a low value . Rare metal rescent emitter dopants (Fig. 1a). These fluorescent molecules EQE complexes containing Eu and Tb established intramolecular have been widely used as conventional fluorescent emitters in 24–27 cascade energy transfer as another route to harvest triplet excitons OLEDs and are commercially available . As host materials, 3–5 but did not show promising Z . Later, a successful strategy we use bis-(2-(diphenylphosphino)phenyl)ether oxide (DPEPO), EQE was realized using room temperature phosphorescent emitters 1,3-Bis(N-carbazolyl)benzene (mCP), 3,3-di(9H-carbazol-9- 0 0 such as platinum and iridium complexes. In this case, according yl)biphenyl (mCBP) and 4,4 -bis(9-carbazolyl)-1,1 -biphenyl to the mixing of the spin orbitals of S and T states due to the (CBP) for the blue, green, yellow and red OLEDs, respectively. 1 1 presence of a heavy atom, the radiative decay rate from a T state In this double-dopant system, the assistant dopant does not itself to a ground state is significantly accelerated, resulting in the emit light but passes all of the electrically generated excitons to 6–10 radiative decay of nearly 100% of triplet excitons . In addition, fluorescent emitter molecules for radiative decay. the utilization of phosphorescence emitters as a triplet sensitizer 11,12 has been proposed . Using this process, triplet harvesting realized by energy transfer from the T state of a phosphorescent Results emitter such as an iridium 2-phenylpyridine complex to the S Energy transfer process. Figure 1a shows an energy transfer state of a fluorescent emitter via dipole–dipole coupling (that is, diagram for the emitter layers (EMLs) in our cascade-type EL Fo¨rster energy transfer, FRET) resulted in an Z of B45% devices. The emitter and assistant dopant molecule combinations int (ref. 12). However, the rather limited Z is due to the presence of and concentrations studied as EMLs here are listed in Table 1. int the competitive deactivation process of triplet–triplet energy In the case of an EML without any assistant dopant, the injected transfer. carriers are transported on the host molecules, and the carriers Although OLEDs based on fluorescent molecules, which are are eventually trapped on an emitter dopant due to its shallower composed of simple aromatic compounds, have continued to highest occupied molecular orbital and deeper lowest unoccupied attract interest because of their longer operational lifetimes in molecular orbital compared with those of the host material. This blue OLEDs, higher colour purity (narrow spectral width) EL and results in direct carrier recombination dominantly on the emitter broader freedom of molecular design compared with phosphor- dopants. Therefore, no triplet excitons contribute to the total 13–15 escence-based OLEDs , the Z of traditional fluorescence- EL efficiency. int based OLEDs is limited to less than 25% even in the ideal case. On the other hand, when assistant dopants are doped into Therefore, the enhancement of Z in OLEDs using conventional these EMLs, exciton formation mainly on the assistant molecules int fluorescence-based emitters is still obviously a major concern for is desired. Here to reduce the effect of direct carrier trapping on the development of future OLEDs. the emitter molecules in the EMLs, the doping concentration of To achieve this, singlet exciton generation via a triplet–triplet the emitter dopants was held at 1 wt%. Conversely, assistant 16,17 annihilation (TTA) process is one possible route . However, dopants were doped into the EMLs in doping concentrations the theoretical upper limit of the singlet exciton production ratio ranging between 15 and 50 wt%, which ensures that the assistant when the TTA process is included is still less than 62.5%, dopants act as the main carrier recombination centres in the 18 A A corresponding to an Z of 12.5% in the ideal case . Recently, EMLs. Thus, after both singlet (S ) and triplet (T ) exciton EQE 1 1 2 NATURE COMMUNICATIONS | 5:4016 | DOI: 10.1038/ncomms5016 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5016 ARTICLE a + – Exciton formation e 25% 75% FRET S ISC S A 1 E S E 1 E TBPe TTPA TBRb DBP Host Assistant dopant Assistant dopant N N N ON N ON N N N Tri-PXZ- ACRSA ACRXTN PXZ-TRZ O TRZ ACRSA ACRXTN TTPA TBPe 400 500 600 700 800 400 500 600 700 800 Wavelength (nm) Wavelength (nm) de tri-PXZ- PXZ-TRZ TRZ DBP TBRb 400 500 600 700 800 400 500 600 700 800 Wavelength (nm) Wavelength (nm) Figure 1 | Energy transfer mechanism. (a) Schematic illustration of proposed energy transfer mechanism in the emitter dopant:assistant dopant:host matrix under electrical excitation and chemical structures of the assistant dopants used in this study. (b–e) Fluorescence spectra of assistant dopant:host 5  1 co-deposited film (upper), and absorption (dashed line) and fluorescence (solid line) spectra of emitter dopant in solution (10 mol l in CH Cl ) 2 2 (bottom). Rather large Fo¨rster transfer radii of B2.2, B7.3, B6.9 and B10 nm were estimated for blue, green, yellow and red EML matrices based on the spectral overlap between the absorption spectrum of the acceptor and the PL spectrum of the donor, suggesting that efficient FRET is possible. formation on the assistant dopants, the formed triplet excitons because of the large DE of conventional fluorescence molecules ST are upconverted to the S state through ISC because of the rather as shown in Fig. 1a. Thus, the created triplet excitons would not small energy gap between the S and T levels (DE ), as contribute to the energy transfer processes if the assistant dopant ST 1 1 summarized in Table 1. Then, according to the spectral overlap is a conventional fluorescence material. between the absorption spectra of the emitter molecules and the The proposed influence of concentration on carrier trapping PL spectra of the assistant dopants (Fig. 1b–e), the S exciton and recombination is well supported by the dependence of Z EQE energies are resonantly transferred to the S states of emitter on the concentration of the assistant dopant as shown in molecules based on a FRET process. Finally, light can be emitted Supplementary Figs 1–3 and Supplementary Note 1. Here we as delayed fluorescence from the S state of the emitter molecules. observed the best Z -luminance (L) characteristics with the EQE If conventional fluorescence molecules were used as the assistant assistant dopant concentration of 15%. In the case of assistant dopant, triplet excitons would non-radiatively decay from T to dopant concentrations less than 15%, carrier recombination may the ground state and would not contribute to light emission not perfectly occur on the assistant dopants, while Dexter energy NATURE COMMUNICATIONS | 5:4016 | DOI: 10.1038/ncomms5016 | www.nature.com/naturecommunications 3 & 2014 Macmillan Publishers Limited. All rights reserved. Emission intensity Emission intensity and absorbance (a.u.) and absorbance (a.u.) Emission intensity Emission intensity and absorbance (a.u.) and absorbance (a.u.) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5016 Table 1 | Components of the emitter layers of the four colour OLEDs. EL colour Host Assistant Assistant dopant DE Emitter Emitter dopant U (%) ST PL dopant concentration (wt%) (eV) dopant concentration (wt%) H H * A A * E* (S , T ) (eV) (S , T ) (eV) S (eV) 1 1 1 1 1 Blue DPEPO ACRSA 15 0.03 TBPe 180 2 (3.50, 3.00) (2.55, 2.52) (2.69) Green mCP ACRXTN 50 0.06 TTPA 181 2 (3.40, 2.90) (2.53, 2.47) (2.34) Yellow mCBP PXZ-TRZ 25 0.07 TBRb 190 2 (3.37, 2.90) (2.30, 2.23) (2.18) Red CBP tri-PXZ-TRZ 15 0.11 DBP 188 2 (3.36, 2.55) (2.27, 2,16) (2.03) 0 0 0 0 0 ACRSA, 10-phenyl-10H,10 H-spiro[acridine-9,9 -anthracen]-10 -one; ACRXTN, 3-(9,9-dimethylacridin-10(9H)-yl)-9H-xanthen-9-one; CBP, 4,4 -bis(9-carbazolyl)-1,1 -biphenyl; DBP, tetraphenyldibenzoperiflanthene; DPEPO, bis-(2-(diphenylphosphino)phenyl)ether oxide; EL, electroluminescence; mCBP, 3,3-di(9H-carbazol-9-yl)biphenyl; mCP, 1,3-bis(N-carbazolyl)benzene; PXZ- TRZ, 2-phenoxazine-4,6-diphenyl-1,3,5-triazine; OLED, organic light-emitting diode; TBPe, 2,5,8,11-tetra-tert-butylperylene; TBRb, 2,8-di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene; tri- PXZ-TRZ, tri-PXZ-TRZ, 2,4,6-tri(4-(10H-phenoxazin-10H-yl)phenyl)-1,3,5-triazine; TTPA, 9,10-bis[N,N-di-(p-tolyl)-amino]anthracene. *The S and T energies were estimated from the peak wavelengths of fluorescence and phosphorescence emission, respectively. 1 1 transfer from the assistant dopants to the triplet level of a 66–99%, respectively, assuming a charge carrier balance factor of fluorescent emitter (T ) or concentration quenching of the 1 and a light-out-coupling efficiency of 20–30% (refs 28,29). assistant dopants may happen in the case of the concentrations These results clearly indicate that the devices overcame the over 15%. The optimum concentration would change for different theoretical limit of 25% for the singlet exciton production combinations of host and guest molecules having different carrier efficiency that is assumed for fluorescence-based OLEDs and even transport and photophysical characteristics. We note that the host that of 62.5% in the case of TTA . H H layers have both higher singlet (S ) and triplet (T ) energy levels 1 1 A A than those of the assistant molecules, S and T . Therefore, back 1 1 energy transfer from the S state of the assistant dopants to the Transient EL analysis. In order to confirm the contribution of singlet exciton generation via energy transfer from the assistant S state of the host layers is inhibited. In addition, dispersing the assistant dopants in a host matrix can additionally prevent direct dopants to the emitter dopants after harvesting triplet excitons on A E the assistant dopants under electrical excitation, transient EL was energy transfer from T to T —that is, minimize a Dexter energy 1 1 transfer process that would result in losses . measured with an electrical excitation pulse width of 1 msat 300 K. Figure 3a shows the streak image and the transient EL decay curve obtained from the red OLED with an EML of 1 OLED characteristics. On the basis of the optimized configura- wt% DBP and 15 wt% tri-PXZ-TRZ in a CBP host matrix. tion of EMLs for blue, green, yellow and red EL, full-colour After turning off the electrical pulse excitation, a clear delayed OLEDs were fabricated to demonstrate the impact of this triplet component with emission bands centred at 610 nm was observed. harvesting process on OLED performance. The Z -L curves of In addition, the emission spectrum of the delayed fluorescence is EQE the four OLEDs are shown in Fig. 2a–d, and Table 2 summarizes consistent with that of the prompt component, which their device performance. Although the OLEDs without assistant is consistent with the described mechanism of triplet to singlet dopants, which use only conventional single doping by fluor- exciton upconversion in tri-PXZ-TRZ followed by successive escent dopants, show low device performance as shown by the resonant singlet energy transfer to the S states of DBP open circles (Z o5%), remarkably high Z of 13.4, 15.8, 18 emitters. EQE EQE and 17.5% for blue, green, yellow and red EL were achieved by Here we describe the exciton formation process in the double- including the assistant dopants. In addition, the magnitudes of dopant system under either optical or electrical excitation. In the the slopes of the Z -L curves of the double-dopant systems case of optical excitation, singlet excitons are mainly generated in EQE are much lower than those of the corresponding single-dopant the S state of a CBP host molecule by photoabsorption, and systems at similar luminance, indicating that the efficiency roll- nearly all of the singlets are then resonantly transferred to the S off characteristics are well suppressed in the double-dopant state of tri-PXZ-TRZ molecules because of the large concentra- system. The reduced roll-off is probably because of the expansion tion of the assistant dopants. In addition, a number of singlet of the carrier recombination site or the reduction of the inter- excitons in tri-PXZ-TRZ would also be formed by direct action between excitons and polarons (that is, exciton–polaron absorption. Next, a fraction of the singlet excitons is transferred annihilation) as a result of rapid energy transfer from the carrier into the S state of DBP through a FRET process, producing the recombination centre (TADF assistant dopants) to the emission prompt fluorescence decay of DBP. Simultaneously, singlet centre (fluorescent emitter dopants). excitons on tri-PXZ-TRZ also decay to the T state internally The EL spectra of the OLEDs are presented in Fig. 2e, through ICS and are successively upconverted into the S state Supplementary Fig. 4 and Supplementary Note 2. These OLEDs again by thermal conversion, followed by energy transfer via emit a full range of visible colours from blue (462 nm) to red FRET to the S state of DBP that produces the delayed emission. (610 nm). Although a weak emission originating from the Here we can ignore the direct formation of excitons on DBP assistant dopant is observed in the red OLED, other OLEDs because of the dilute concentration of 1 wt%. Under electrical showed pure emission originating from the emitter dopants that excitation, on the other hand, since the singlet and triplet excitons A A is in good accordance with the PL spectrum of the emitter are directly created on the S and T states of the tri-PXZ-TRZ 1 1 ± ± ± dopants in solution. Here F of 80 2, 81 2, 90 2 and molecules based on the exciton branching ratio of 25–75%, PL 88 2% were obtained for the blue, green, yellow and red emitter respectively, the contribution of the delayed fluorescence in dopants when doped with assistant dopants in films (Table 1). the total emission is significantly larger compared with that Thus, we can estimate Z of 55–84%, 65–97%, 66–100% and of the optical excitation, as shown in Fig. 3b. 4 NATURE COMMUNICATIONS | 5:4016 | DOI: 10.1038/ncomms5016 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5016 ARTICLE ab 1 1 10 10 Without assistant dopant NN Without 0 0 10 10 assistant dopant 0 1 2 3 4 0 1 2 3 4 10 10 10 10 10 10 10 10 10 10 –2 –2 Luminance (cd m ) Luminance (cd m ) cd 1 1 10 10 Without 0 0 10 10 Without assistant dopant assistant dopant 0 1 2 3 4 0 1 2 3 4 10 10 10 10 10 10 10 10 10 10 –2 –2 Luminance (cd m ) Luminance (cd m ) 1.0 0.8 0.6 0.4 0.2 0.0 400 500 600 700 Wavelength (nm) Figure 2 | Device performance of OLEDs. (a–d) External EL quantum efficiency as a function of luminance for the blue, green, yellow and red OLEDs. The external EL quantum efficiency for OLEDs without an assistant dopant is plotted as open symbols. Inset: chemical structures of emitter dopants used in this study. (e) EL spectra of the devices at a luminance of 100 cd m . Table 2 | Device performance of the four colour OLEDs with assistant dopants for triplet harvesting. Device Turn on Max EQE Max CE Max PE CIE Performance at 1,000 cd m 1  1 voltage (V) (%) (cd A ) (lm W ) 1  1 Voltage (V) EQE (%) CE (cd A ) PE (lm W ) Blue 4.7 13.4 27 18 (0.17, 0.30) 7.8 8.7 18 7 Green 3.0 15.8 45 47 (0.29, 0.59) 4.1 11.7 38 30 Yellow 3.2 18.0 60 58 (0.45, 0.53) 5.2 17.2 56 33 Red 3.0 17.5 25 28 (0.61, 0.39) 6.4 10.9 20 10 OLED, organic light-emitting diode; CE, current efficiency; CIE, Commission Internationale de l‘Eclairage; EQE, external electroluminescence quantum efficiency; PE, power efficiency. Device operational stability. The introduction of assistant The driving voltages of the devices are also rather stable, dis- dopants into a host–guest system provides not only a significant playing a rise of less than 0.5 V after 100 h of operation. More enhancement of Z but also an enhancement of device opera- interestingly, the operational lifetime was longer than that of the EQE tional stability under electrical excitation. For example, the nor- OLED with PXZ-TRZ molecules as the emitter dopant (device C) malized luminance of the yellow OLEDs as a function of while the voltage rise curves are almost the same, indicating operation time at a constant current density of 10 mA cm are that the combination of fluorescent emitter dopants and presented in Fig. 4. Although a rapid decrease in luminance TADF assistant dopants provides improved device performance. was observed in the OLED without assistant dopants (device B), We note that the nearly identical film morphology for each of the the OLED with the assistant dopants (device A) showed co-deposited films compared with that of the host-only films improved luminance decay characteristics compared with that of suggests that device stability is not influenced by a morphology the OLED without the assistant dopants, resulting in an improved change caused by the dopant molecules as shown in operational lifetime, defined as the elapsed operation time at Supplementary Fig. 5 and Supplementary Note 3. Since it is well which the luminance drops to 50% of the initial value, of 194 h. established that TBRb is a stable emitter electrochemically , the NATURE COMMUNICATIONS | 5:4016 | DOI: 10.1038/ncomms5016 | www.nature.com/naturecommunications 5 & 2014 Macmillan Publishers Limited. All rights reserved. External EL quantum External EL quantum efficiency (%) efficiency (%) Normalized EL intensity (a.u.) External EL quantum External EL quantum efficiency (%) efficiency (%) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5016 Discussion a 0 Our results suggest that both a higher Z and an enhancement EQE of operational stability can be expected with an optimum EML design using a wide variety of TADF and fluorescent materials. As numerous electrochemically stable fluorescent emitters Delayed have been widely developed in the past two decades, the triplet EL harvesting mechanism realized here can provide a greater Prompt EL flexibility in the design of OLED architectures. In addition, since Delayed EL 40 Z can exceed 20% by using emitter molecules with horizontally out oriented dipoles , the flexibility of simple aromatic compound design can further boost the Z by enhancement of Z without EQE out special light-out-coupling structures. In summary, we presume that the cascade energy transfer scheme using TADF assistant Wavelength (nm) EL intensity (a.u.) dopants and fluorescent emitter dopants will be the most promising device architecture for OLEDs with ultimate Electrical excitation performance. Optical excitation Methods Materials. mCP, 4,4 -cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] 0 00 (TAPC), 2,2 ,2 -(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole (TPBi), TBPe, TTPA, TBRb and DBP were purchased from Luminescence Technology Corp. mCBP was purchased from NARD Institute Ltd. CBP, 4,4 -bis(N-phenyl- 1-naphthylamino)biphenyl (a-NPB) and tris(8-hydroxyquinolinato)aluminum (Alq3) were used as received from the Nippon Steel Chemical Co., Ltd. ACRSA, DPEPO , 2-phenoxazine-4,6-diphenyl-1,3,5-triazine (PXZ-TRX), tri-PXZ-TRZ 0 10 20 30 40 50 and 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T) were synthesized according Time (µs) to the reported procedures. Figure 3 | Transient EL characteristics. (a) Time-resolved electroluminescence image for the red OLED. The streak image (left) and Synthesis of ACRXTN. We synthesized ACRXTN according to the general procedure. The complete synthetic route for ACRXTN and the 1H NMR spectrum time-resolved EL decay curve (right) are for the red OLED under electrical of ACRXTN are included in the Supplementary Methods section and excitation with a pulse of 1 ms at 300 K. A delayed EL spectrum was Supplementary Fig. 6, respectively. NMR spectra were obtained with a Bruker collected from 3 to 50 ms after excitation. (b) Time-resolved PL and EL Biospin Avance-III 500 NMR spectrometer at ambient temperature. High-resolu- response of a 1 wt%-DBP:15 wt%-tri-PXZ-TRZ:CBP film (red line) and tion mass spectrometry by fast atom bombardment was performed using a JEOL JMS-700 spectrometer. 1 wt%-DBP:15 wt%-tri-PXZ-TRZ:CBP-based OLED (black line), respectively. 3-bromo-9H-xanthen-9-one 1.38 g (5 mmol), 9,9-dimethyl-9,10- dihydroacridine 1.15 g (5.5 mmol), tert-BuONa 0.96 g (10 mmol), tri-tert- butylphosphonium tetrafluoroborate 145 mg (0.5 mmol) and palladium acetate 56 mg (0.25 mmol) were put into a flask and purged three times by nitrogen/ 1.0 vacuum cycle. Then, anhydrous toluene was added and refluxed for 8 h. After cooling to room temperature, the resulting solution was filtered through celite and concentrated. The crude product was purified using a silica gel chromatography (CH Cl :hexane ¼ 8:2) and recrystallized twice from mixed solvent of 2 2 0.8 hexane:AcOEt ¼ 9:1 ml g . The desired product was obtained as yellow powder (2.06 g, 85%). H-NMR (500 MHz, CDCl ): d (p.p.m.) ¼ 8.53 (d, 1H, 8.5 Hz), 8.38 (dd, 1H, 8 Hz), 7.74 (ddd, 1H, 8.6, 7.2, 1.8 Hz), 7.53 (d, 1H, 1.9 Hz), 7.52–7.46 0 50 100 150 200 (m, 3H), 7.42 (ddd, 1H, 8.1, 7.2, 1 Hz), 7.38 (dd, 1H, 8.5, 2 Hz), 7.08–6.98 (m, 4H), Time (h) 0.6 13 6.53 (dd, 2H, 7.8, 1.6 Hz). C-NMR (125 MHz, CDCl ): d (p.p.m.) ¼ 176.47, 157.83, 156.25, 148.09, 140.28, 134.94, 132.55, 129.32, 126.79, 126.45, 125.27, 124.59, 124.20, 121.99, 121.96, 120.52, 117.95, 117.57, 115.88, 36.38, 30.53. High- Device B Device C Device A resolution mass spectrometry (m/z): [M þ H] þ calculated for C28H22NO2, –2 –2 –2 L =677 cd m L =2,791 cd m L =3,225 cd m 0 0 0 0.4 404.1651; found, 404.1651. 0 50 100 150 200 Optical characterization of organic thin films. PL quantum efficiency was measured by an absolute PL quantum yield measurement system (C11347-01, Time (h) Hamamatsu Photonics) under the flow of nitrogen gas with an excitation wave- Figure 4 | Device stability of yellow OLEDs. For operational lifetime length of 337 nm. Low-temperature PL intensity and emission lifetimes were measured using a streak camera (C4334, Hamamatsu Photonics) and cryostat measurement of the double-dopant system, an OLED with a device (Iwatani Industrial Gases Co.) with a nitrogen gas laser (MNL200, Laser Technik) structure of ITO/a-NPD (35 nm)/1 wt%-TBRb:25 wt%-PXZ-TRZ:mCBP as an excitation light source under a pressure of about 3 Pa. (30 nm)/T2T (10 nm)/Alq (55 nm)/LiF (0.8 nm)/Al (100 nm) (device A) was used. To confirm the effect of the assistant dopant, OLEDs with an EML Fabrication of OLEDs. Glass substrates with a pre-patterned, 100-nm-thick, of either 1 wt%-TBRb:mCBP (device B) or 25 wt%-PXZ-TRZ:mCBP (device 100 Ohm sq tin-doped indium oxide (ITO) coating were used as anodes. Sub- C) were also measured at a constant current density of 10 mA cm . Initial strates were washed by sequential ultrasonication in neutral detergent, distilled luminances (L ) are 3,225, 677 and 2,791 cd m for devices A, B and C, water, acetone and isopropanol, and then exposed to ultraviolet–ozone (NL- UV253, Nippon Laser & Electronics Lab) to remove adsorbed organic species. respectively. Inset: voltage rise curves for devices A, B and C (coloured After pre-cleaning of the substrates, effective device areas of 1 mm were defined accordingly). on the patterned-ITO substrates by a polyimide insulation layer using a conven- tional photolithography technique. Substrates were treated with ultraviolet–ozone for 25 min and immediately transferred into the evaporation chamber. Organic layers were formed by thermal evaporation. Doped emitting layers differences in reliability would be because of the change of carrier were deposited by co-evaporation. Deposition was performed under vacuum at recombination and exciton formation area with and without the pressures of o5  10 Pa. Devices were exposed to nitrogen gas once after presence of the assistant dopants, in addition to the formation of the organic layers to apply a metal mask that defines the cathode area. electrochemical stability of emitter molecules. After device fabrication, devices were immediately encapsulated with glass lids 6 NATURE COMMUNICATIONS | 5:4016 | DOI: 10.1038/ncomms5016 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. –3 –2 –1 EL intensity (a.u.) Intensity (a.u.) Normalized EL intensity (a.u.) Voltage (V) Time (µs) NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5016 ARTICLE using epoxy glue in a nitrogen-filled glove box (O o0.1 p.p.m., H Oo0.1 p.p.m.). 2 2 16. Helfrich, W. & Schneider, W. G. Transients of volume-controlled current and Commercial calcium oxide desiccant (Dynic Co.) was included in each of recombination radiation in anthracene. J. Chem. Phys. 44, 2902–2909 (1966). encapsulated package. 17. Pope, M. & Swenberg, C. E. Electronic Processes in Organic Crystals 64 (Oxford Blue OLEDs with the structure ITO/a-NPB (35 nm)/mCP (10 nm)/1 wt%- University Press, 1982). TBPe: 15 wt%-ACRSA: DPEPO (15 nm)/DPEPO (8 nm)/TPBi (57 nm)/LiF 18. Kondakov, D. Y., Pawlik, T. D., Hatwar, T. K. & Spindler, J. P. Triplet (0.8 nm)/Al (100 nm) were fabricated. As a control device, an OLED with an annihilation exceeding spin statistical limit in highly efficient fluorescent EML that consisted of 1 wt%-TBPe: DPEPO was also made. organic light-emitting diodes. J. Appl. Phys. 106, 124510–124516 (2009). Green OLEDs with the structure ITO/TAPC (35 nm)/1 wt%-TTPA: 50 wt%- 4 þ 19. Endo, A. et al. Thermally activated delayed fluorescence from Sn –porphyrin ACRXTN: mCP (15 nm)/TPBi (65 nm)/LiF (0.8 nm)/Al (100 nm) were fabricated. complexes and their application to organic light emitting diodes—a novel As a control device, an OLED with an EML consisting of 1 wt%-TTPA: mCP mechanism for electroluminescence. Adv. Mater. 21, 4802–4806 (2009). was also made. 20. Tanaka, H., Shizu, K., Miyazaki, H. & Adachi, C. Efficient green thermally Yellow OLEDs with the structure ITO/TAPC (35 nm)/1 wt%-TBRb:25 wt%- activated delayed fluorescence (TADF) from a phenoxazine–triphenyltriazine PXZ-TRX:mCBP (30 nm)/T2T (10 nm)/Alq (55 nm)/LiF (0.8 nm)/Al (100 nm) (PXZ–TRZ) derivative. Chem. Commun. 48, 11392–11394 (2012). were fabricated. As a control device, an OLED with an EML consisting of 21. Uoyama, H., Goushi, K., Shizu, K., Nomura, H. & Adachi, C. Highly efficient 1 wt%-TBRb: mCBP was also made. organic light-emitting diodes from delayed fluorescence. Nature 492, 234–238 Red OLEDs with the structure ITO/TAPC (35 nm)/1 wt%-DBP:15 wt%-tri- (2012). PXZ-TRZ:CBP (15 nm)/TPBi (65 nm)/LiF (0.8 nm)/Al (100 nm) were fabricated. 22. Tanaka, H., Shizu, K., Nakanotani, H. & Adachi, C. Twisted intramolecular As a control device, an OLED with an EML consisting of 1 wt%-DBP: CBP charge transfer state for long-wavelength thermally activated delayed was also made. Schematic diagrams of the energy levels of the fabricated devices and the fluorescence. Chem. Mater. 25, 3766–3771 (2013). chemical structures of the assistant dopant materials used in them are presented 23. Nasu, K. et al. A highly luminescent spiro-anthracenone-based organic in Supplementary Fig. 7. light-emitting diode through thermally activated delayed fluorescence. Chem. Commun. 49, 10385–10387 (2013). 24. Mi, B.-X. et al. Reduction of molecular aggregation and its application to the Characterization of OLEDs. 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Red and white EL materials based on a new fused aromatic Reprints and permission information is available online at http://npg.nature.com/ ring. SID Int. Symp. Digest Tech. Papers 33, 598–601 (2002). reprintsandpermissions/ 14. Hosokawa, C. et al. Improvement of lifetime in organic electroluminescence. SID Int. Symp. Digest Tech. Papers 35, 780–783 (2004). How to cite this article: Nakanotani, H. et al. High-efficiency organic light-emitting 15. Kawamura, M. et al. Highly efficient fluorescent blue OLEDs with efficiency- diodes with fluorescent emitters. Nat. Commun. 5:4016 doi: 10.1038/ncomms5016 enhancement layer. SID Int. Symp. Digest Tech. Papers 41, 560–563 (2010). (2014). NATURE COMMUNICATIONS | 5:4016 | DOI: 10.1038/ncomms5016 | www.nature.com/naturecommunications 7 & 2014 Macmillan Publishers Limited. All rights reserved.

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