ARTICLES PUBLISHED ONLINE: 8 DECEMBER 2014 | DOI: 10.1038/NMAT4154
Highly efficient blue electroluminescence based on thermally activated delayed fluorescence Shuzo Hirata1, Yumi Sakai1,2, Kensuke Masui1,3, Hiroyuki Tanaka1, Sae Youn Lee1,4, Hiroko Nomura1, Nozomi Nakamura1, Mao Yasumatsu1, Hajime Nakanotani1,4,5, Qisheng Zhang1,4, Katsuyuki Shizu1,4, Hiroshi Miyazaki1,6 and Chihaya Adachi1,4,7* Organic compounds that exhibit highly efficient, stable blue emission are required to realize inexpensive organic light-emitting diodes for future displays and lighting applications. Here, we define the design rules for increasing the electroluminescence efficiency of blue-emitting organic molecules that exhibit thermally activated delayed fluorescence. We show that a large delocalization of the highest occupied molecular orbital and lowest unoccupied molecular orbital in these charge-transfer compounds enhances the rate of radiative decay considerably by inducing a large oscillator strength even when there is a small overlap between the two wavefunctions. A compound based on our design principles exhibited a high rate of fluorescence decay and efficient up-conversion of triplet excitons into singlet excited states, leading to both photoluminescence and internal electroluminescence quantum yields of nearly 100%.
O
rganic light-emitting diodes (OLEDs; ref. 1) are an indispensable technology for flat panel displays and lighting applications because they are highly efficient2 , ultrathin, lightweight and flexible3,4 , and exhibit multicolour emission3 . In particular, an internal electroluminescence quantum efficiency (ηint ) of nearly 100% has been achieved using organometallic emitters such as Ir, Pt and Cu complexes5–8 . However, electroluminescence from metal-free conventional aromatic compounds is desired for low-cost OLEDs because rare metals such as Ir and Pt are expensive and blue-emitting compounds containing these metals have limited operational stability. Generally, ηint from fluorescent materials is limited to 25% because of the singlet–triplet branching ratio. It has been reported that delayed fluorescence can be realized through up-conversion of triplet excitons to singlet excitons by triplet–triplet annihilation (TTA; refs 9–12), although the theoretical maximum ηint obtained using TTA up-conversion cannot reach 100% because the process intrinsically involves the deactivation of triplet excitons. As an alternative to TTA, we recently demonstrated a promising pathway to increase ηint by thermally activated delayed fluorescence (TADF; refs 13–25). In this process, light emission is realized as delayed fluorescence after efficient reverse intersystem crossing (RISC) from the lowest triplet excited state (T1 ) to the lowest singlet excited state (S1 ; ref. 26). To obtain TADF through efficient up-conversion from T1 to S1 , a small energy gap between S1 and T1 , 1EST , is required, which has been achieved by separating the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of a molecule14–25 . By controlling the separation of its HOMO and LUMO, we realized a conventional aromatic compound that emits green electroluminescence with ηint of almost 100% (ref. 21).
Furthermore, while the maximum external electroluminescence quantum efficiency (ηext ) of blue OLEDs using TADF has previously been limited to 11 ± 1% (ref. 16), very recently we realized an ηext value of nearly 20% (ref. 27). Although we determined empirical guidelines for molecular design to achieve TADF in those previous studies, which require satisfying conditions of a small 1EST < 0.2 eV and a certain oscillator strength, detailed molecular design strategies to concurrently achieve small 1EST and high photoluminescence quantum yield (φPL ), which is necessary for high ηint , are still unclear. Here, we explain a molecular design strategy that demonstrates compatibility between low 1EST and high φPL and produce a metal-free blue emitter that realizes both high φPL and low 1EST . Delocalization of the HOMO and LUMO in a charge-transfer compound with a well-separated HOMO and LUMO induces a large transition dipole moment while maintaining a small 1EST (∼ 0.1 eV), resulting in φPL = 100%. An OLED containing the metalfree emitter exhibits blue electroluminescence with an ηint value of ∼100%.
Photoproperty relationships First, we consider the important factors necessary to obtain high ηint in TADF compounds and the various dependences of these factors. ηint can be expressed as follows when the radiative decay rate for fluorescence (kf ) is much faster than the rate constant of internal conversion (kic ) at room temperature (refs 14,16,19,23,28): ηint = nr,S φPF + nr,S φTADF + nr,T
φTADF φT
(1)
where nr,S is the singlet exciton production efficiency and nr,T the triplet exciton production efficiency under electrical excitation, φPF
1 Center
for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan. 2 Research & Development Department, Dyden Corporation, 1-1 Hyakunenkouen, Kurume, Fukuoka 839-0864, Japan. 3 Advanced Core Technology Laboratories, Fujifilm Corporation, 577 Ushijima, Kaisei, Ashigarakami, Kanagawa 258-8577, Japan. 4 JST, ERATO, Adachi Molecular Exciton Engineering Project, 744 Motooka, Nishi, Fukuoka 819-0395, Japan. 5 Innovative Organic Device Laboratory, Institute of Systems, Information Technologies and Nanotechnologies (ISIT), 744 Motooka, Nishi, Fukuoka 819-0395, Japan. 6 Nippon Steel & Sumikin Chemical Co. Ltd., 46-80, Nakabaru Sakinohama, Tobata, Kitakyushu 804-8503, Japan. 7 International Institute for Carbon Neutral Energy Research, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan. *e-mail: [email protected] NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
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1
NATURE MATERIALS DOI: 10.1038/NMAT4154
ARTICLES HOMO
LUMO
N N N N N N
Increasing F, Q and kf
1a
N
N N
N N N
Increasing delocalization of HOMO
N
N N
2b
N N
N
N
Increasing ΔEST
2a
Similar F, Q and kf
Similar ΔEST
Increasing delocalization of HOMO
Chemical structure
N
2c
N
N N N
Figure 1 | Chemical structures and distribution of HOMO and LUMO in 1a, 2a, 2b and 2c. Optimized structures of the HOMO and LUMO at S1 were calculated by TD-DFT (Gaussian09/B3LYP/6-31G(d)).
is the photoluminescence quantum yield of the prompt component of fluorescence, φT is the quantum yield of intersystem crossing from S1 to T1 , and φTADF is the photoluminescence quantum yield of TADF—that is, the fraction of initial singlet excitons that ultimately yield delayed fluorescence. Typically, nr,S and nr,T are 0.25 and 0.75, respectively. On the basis of equation (1), high φPF and φTADF /φT are required to obtain high ηint . φPF is typically expressed as φPF = kf τPF
(2)
τPF = 1/(kf + kic + kisc )
(3)
where τPF is the lifetime of the prompt component of fluorescence and kisc is the rate constant of intersystem crossing from S1 to T1 . In 1962, Strickler and Berg29 reported that the relationship between kf and the absorption coefficient in fluorescent molecules with a large overlap of HOMO and LUMO can be expressed as
−1 kf = 2.88 × 10−9 n2 ν¯ f−3
ν¯ f−3
−1
Z
ε(νa )d ln νa
f (νf )dνf f (νf )νf −3 dνf
(4)
φPF φRISC φTADF = = φT 1 − φT φRISC
R
=R
(5)
where νf is the fluorescence wavenumber, f (νf ) is the fluorescence spectrum at νf , νa is the absorption wavenumber, ε(νa ) is the molar absorption coefficient at νa , and n is the refractive index. The relationships between oscillator strength for absorption (F), transition dipole moment for absorption (Q), and ε(νa ) can be experimentally determined as30 2 Z 8π me chνf i F = 4.32 × 10−9 n−1 ε(νa )dνa = |Q|2 (6) 3he2 2
where me is the electron mass, c is the speed of light, h is Planck’s constant, e is the elementary charge and hνf i is the average wavenumber of fluorescence. We believe that the Strickler–Berg equation (equation (4)) is also applicable to molecules with a well-separated HOMO and LUMO provided that ∫ε(νa )d ln νa calculated from experimental values of φPF and τPF using equation (4) is almost the same as that calculated using ε(νa ) determined from absorption measurements of the molecules. In such cases, the oscillator strength for fluorescence (F) and the transition dipole moment for fluorescence (Q) are similar to those for absorption and can be calculated using equation (6). Since high φPF can be achieved through large kf , based on equations (2) and (3), which in turn requires large F and Q values, based on equations (4)–(6), development of molecular structures with large F and Q is important for obtaining large φPF . On the other hand, when the RISC process follows an Arrhenius model and kic is much lower than kr and kisc , φTADF /φT can be theoretically expressed as28
=
1 knr kRISC φPF
+1
=
φ
PF kRISC +knr kRISC
− φT
1 knr 1 + φPF Aexp(−1E ST / kT )
(7)
where k is the Boltzmann constant, T is the temperature, knr is the rate constant of non-radiative deactivation from T1 to the ground state (S0 ), and A is a constant. On the basis of equation (7), high φTADF /φT is obtained when 1EST is small, so designing molecular structures with a small 1EST is important for achieving high φTADF /φT . From these theoretical considerations, we used the comparison of theoretically calculated and experimentally determined F and 1EST , along with calculated HOMO and LUMO structures for a
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NATURE MATERIALS DOI: 10.1038/NMAT4154
ARTICLES
Table 1 | Photophysical characteristics of 1a and 2a–c in toluene. Calculation∗
Compound F
0.021 0.112 0.130 0.103
1a 2a 2b 2c
Experimental λa † (nm) 403 362 368 367
1EST (eV) 0.098 0.073 0.114 0.134
∫ ε(νa )d ln νa‡ (103 cm3 ) 0.14 1.6 2.2 1.7
τPF (ns) 8.7 6.3 6.0 6.5
φPF (%) 10 74 57 72
kf (107 s−1 ) 1.0 12 9.6 11
∫ ε(νa )d ln νa§ (103 cm3 ) 0.23 2.1 1.7 2.0
Fk 0.010 0.136 0.174 0.128
PLmax ¶ (nm) 495 457 458 458
Qk (D) 0.39 1.4 1.6 1.4
1EST# (eV) 0.09 − 0.12 0.09 0.28 0.32
∗ Results for structure optimized at S0 . † Maximum wavelength of ultraviolet–visible absorption. ‡ Results calculated using the results of ultraviolet–visible absorption measurement shown in Supplementary Fig. 2. § Results calculated using equations (4) and (5) from experimentally obtained values of kf and hνf−1 i−3 . k Results calculated by substituting the results of ultraviolet–visible absorption measurements shown in Supplementary Fig. 2 into equation (6). ¶ Maximum wavelength of photoluminescence. # These values were defined as the energy difference between S1 and T1 obtained from measurements presented in Supplementary Fig. 3.
Excitation energy
c S1 kisc kic
0
kf
T1
φT 81%
φ GT 62%
φ TADF 20% kisc
kic
knr
b
a Excitation energy
a
φ PF 19%
S1
0
φ TADF 19%
kf
φ PF 80%
φT 20%
T1 knr
φ GT 0%
Figure 2 | Photoluminescence processes of 1a (top) and 2a (bottom). kf , kic , kisc , knr , φPF , φT , φTADF and φG T represent fluorescence decay rate, internal conversion decay rate from S1 to S0 , intersystem crossing decay rate from S1 to T1 , non-radiative decay rate from T1 to S0 , quantum efficiency of the prompt component of fluorescence, quantum efficiency of intersystem crossing from S1 to T1 , quantum efficiency of TADF and non-radiative quantum efficiency from T1 to S0 , respectively. See main text for details of the processes.
variety of TADF molecules, to develop a molecular design strategy that achieves both high F and low 1EST for large ηint . For the evaluation of F values, we obtained Q and F from experimentally observed ε(νa ) and kf by using equations (2)–(6). The values are compared with theoretical values of F obtained from density functional theory (DFT) calculations. For the evaluation of 1EST , the values were confirmed experimentally from the difference in the onset energies of fluorescence and phosphorescence18,23 , and they are compared with theoretical 1EST determined by DFT calculations.
Comparison of TADF molecules Compounds 1a (ref. 14) and 2a with comparably small 1EST but different degrees of delocalization of the HOMO were designed using DFT calculations. 1a, which was reported in our early TADF study, possesses a well-separated HOMO and LUMO that are localized on indolocarbazole and triazine moieties, respectively. This effective separation is ascribed to the steric hindrance
between the indolocarbazole and triazine units. Typically, good separation between the HOMO and LUMO decreases electron repulsion, leading to a decrease in 1EST (ref. 14). Consequently, the theoretical 1EST of 1a calculated by time-dependent density functional theory (TD-DFT) is 0.098 eV (ref. 14). For compound 2a, the 3,6-bis(3,6-diphenylcarbazolyl)carbazole (BDPCC) moiety and the triphenyltriazine (TPTA) functional group are largely twisted because of the steric hindrance between them. Therefore, the HOMO and the LUMO are located over the BDPCC and the TPTA units, respectively, and are effectively separated. Consequently, the large separation between the HOMO and LUMO in 2a gives a small 1EST of 0.073 eV according to TD-DFT calculations. For the HOMO on the BDPCC unit, the 3,6-bis(3,6-diphenylcarbazole) (BDPC) components of the BDPCC moiety acts as weak electrondonating units for the carbazole directly attached to the TPTA unit. As a result, the HOMO of 2a is largely delocalized over all of the BDPCC unit while maintaining a high T1 energy for the donor unit through moderate twisting between the BDPC units and the carbazole. The small 1EST values of 1a and 2a were confirmed experimentally from the difference in the onset energies of fluorescence and phosphorescence (Supplementary Fig. 4; refs 18,23). The 1EST values obtained for 1a and 2a in toluene are 0.09–0.12 and 0.09 eV, respectively. Therefore, efficient thermally activated RISC from T1 to S1 is expected to occur in both 1a and 2a. However, Table 1 indicates that the calculated values of F for 1a and 2a differ greatly, which is expected to lead to differences in φPF . Indeed, φPF values of 10% and 74% for 1a and 2a, respectively, were measured, yielding kf values of 1.0 × 107 and 1.2 × 108 s−1 using equation (2) and τPF of 8.7 ns and 6.3 ns for 1a and 2a, respectively. The values of ∫ ε(νa )d ln νa calculated from ultraviolet–visible absorption measurements are similar to those calculated using equations (4) and (5) with the values of kf (Table 1), because optimized structures and distributions of HOMO and LUMO do not show large differences between S0 and S1 (Supplementary Fig. 1). Therefore, the Strickler–Berg equation (equation (4)) is suitable to describe these molecules with a large separation between HOMO and LUMO, and F for absorption and fluorescence can be determined using equation (6). Despite their comparably small 1EST (Table 1), F of 2a is found to be 13.6 times greater than that of 1a, with values of 0.010 and 0.136 for 1a and 2a, respectively Table 1). The large differences in F and kf for 1a and 2a, even though their 1EST are comparable, is tentatively ascribed to differences in the delocalization of the HOMO. This is because 1a and 2a show comparable absorption energies, emission energies and 1EST , so the main difference is only the spatial volume of the HOMO. The HOMO of 2a is delocalized well over its large BDPCC unit compared with the more localized HOMO of 1a found on its indolocarbazole unit. Further, we note that the HOMO slightly seeps into the location of the LUMO, and the large delocalization of the HOMO increases
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NATURE MATERIALS DOI: 10.1038/NMAT4154
ARTICLES 103 102
b
107
1a 2a
106
2b 2c
105
100
104
10−1
103
10−2
L (cd m−2)
J (mA cm−2)
101
1.2
Normalized electroluminescence intensity
a
102
10−3
101
10−4
1a 2a
1.0
2b 2c
0.8
0.6
0.4
0.2
100
10−5 100
0.0
101
400
500 600 Wavelength (nm)
V (V)
c
d
30 1a
2a
2b
2a
2c
25
20
2c
100 cd m−2
1,000 cd m−2
φ TADF/φ T
15
100
φ PF
ηext (%)
2b
100
100 cd m−2
1,000 cd m−2
10
10−1 2a 2b 2c
10−1 5
0 10−2
700
10−2 100 cd 10−1
m−2
100 J (mA cm−2)
1,000 cd
3
101
4
5
6
7
8
1,000/T (K−1)
m−2 102
3
10
30 1,000/T (K−1)
100
300
Figure 3 | Electroluminescence characteristics of OLEDs using emitter layers containing 1a and 2a–c as guests and photoluminescence characteristics of the emitter layers. a, Current density (J)–voltage (V)–luminance (L) characteristics. Open symbols are current density and filled symbols are luminance. b, Electroluminescence spectral characteristics. c, External electroluminescence quantum efficiency (ηext )–J characteristics. In a–c, the device structure was ITO/device (30 nm)/m-CBP (10 nm)/6 wt% guest (1a, 2a, 2b or 2c): DPEPO (15 nm)/DPEPO (10 nm)/TPBi (30 nm)/LiF (0.8 nm)/Al. d, Arrhenius plots of φPF and φTADF /φT (inset) in 6 wt% guest (2a, 2b or 2c): DPEPO films under vacuum. The dashed lines in the inset represent curve fittings based on equation (7).
the product of the HOMO, the LUMO, and the position vector of the two wavefunctions that determines the transition dipole moment for fluorescence. Therefore, this large delocalization of the HOMO in 2a would contribute to the rather large values of F and kf , even when 1EST is small. The influence of HOMO delocalization on F, Q and 1EST was further investigated using compounds 2a–c, in which the acceptor is kept constant and the delocalization of the HOMO is varied by using different donors, as shown in Fig. 1. In 2a–c, the HOMO and LUMO are effectively separated, absorption and emission energies are similar (Table 1), and the twist angle between the donor units and the TPTA acceptor unit is the same in S0 and S1 (Supplementary Fig. 1). Table 1 compares kf , 1EST and F for 2a–c. All three compounds show similarly large F, both theoretically and experimentally. In contrast, 1EST values determined both by DFT calculations and experimentally increase from 2a to 2b to 2c. 4
On the basis of the trends found in previous studies regarding the design of TADF molecules, F and kf would be expected to decrease with decreasing 1EST . However, 2a–c show similar kf and F experimentally (Table 1), and this trend is also found in values calculated by DFT. In the DFT calculations (Fig. 1), the main difference among 2a–c is increasing delocalization of the HOMO from 2c to 2a. Because 2a–c show comparable absorption and emission energies, the suppression of the decrease in F and kf from 2c down to 2a is attributed to the delocalization of the HOMO. The molecules experimentally indicate that extending the molecular orbitals while limiting the overlap between HOMO and LUMO can suppress a decrease in F and kf while lowering 1EST . This is because 1EST is inversely proportional to the position vector between HOMO and LUMO while F and kf is linearly proportional to it. Consequently, both high φPF and low 1EST are achieved in 2a. We note that the kf of 2a (>108 s−1 ) is the largest of molecules with 1EST less than 0.1 eV.
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NATURE MATERIALS DOI: 10.1038/NMAT4154
ARTICLES
Table 2 | Photophysical characteristics of 6 wt% guest:DPEPO films containing 1a and 2a–c. Compound 1a 2a 2b 2c
PLmax ∗ (nm) 492 480 475 475
ELmax† (nm) 506 487 478 477
(x,y)‡ 0.26, 0.43 0.19, 0.35 0.17, 0.27 0.18, 0.28
1EST§
kf
kisc
φPF
φT
φTADF
ηint k
ηint ¶
ηext #
(eV) 0.05 − 0.12 0.11 ± 0.01 0.19 ± 0.02 0.29 ± 0.03
(107 s−1 )
(107 s−1 )
2.3 12 12 11
9.9 2.9 4.2 3.2
(%) 19 80 74 79
(%) 81 20 26 21
(%) 19 20 21 14
(%) 27 100 84 72
(%) 39 ± 7 103 ± 9 84 ± 9 73 ± 5
(%) 7.7 ± 1.3 20.6 ± 1.8 16.8 ± 1.7 14.6 ± 1.0
∗ Maximum wavelength of photoluminescence. † Maximum wavelength of electroluminescence. ‡ Emission colour coordinates. § These values were defined as the energy difference between S1 and T1 obtained from measurements presented in Supplementary Fig. 5. k Results calculated using φPF , φT and φTADF with equation (1). ¶ Results calculated using experimental results for electroluminescence devices assuming a light out-coupling efficiency of 20%. # Experimental results obtained for electroluminescence devices.
The high kf and low 1EST of these compounds are also observed in solid-state films. The φPF of 1a and 2a in 6 wt% guest: bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) films were 19% and 80%, respectively, as summarized in Table 2. Although φPF is determined by kf , kic and kisc , according to equations (2) and (3), kic can be ignored because it is much lower than kf and kisc in 1a and 2a. This is because 1a and 2a did not show temperature dependence of φPF and τPF (Supplementary Fig. 6). Therefore, φPF is determined only by kf and kisc . In 1a, kf is about 23% of kisc , whereas in 2a, kf is about 400% of kisc . Therefore, φPF of 1a was only 19% because of its low kf (2.3 × 107 s−1 ), but that of 2a was 80% because of its high kf (1.2 × 108 s−1 ). Furthermore, the high kf of 2a also increases φTADF through the following process. For 1a, most of the excitons that are thermally up-converted from T1 to S1 go back to T1 again before generation of delayed fluorescence because kisc is higher than kf . Consequently, multiple cycles of intersystem crossing between S1 and T1 will occur (Fig. 2; a in top panel). As a result of this process, 62% of the initially created excitons will eventually be deactivated while in the T1 state (Fig. 2; b in top panel) and only 23% of excitons (φTADF /φT ) contribute to TADF after intersystem crossing of 81% of the initial excitons has occurred (Fig. 2; c in top panel). In contrast, in 2a, after intersystem crossing of 20% of excitons, almost all are thermally upconverted from T1 to S1 and generate delayed fluorescence before the remaining excitons go back to T1 (Fig. 2, a in bottom panel). This is because kf is about four times faster than kisc . Therefore, increasing kf by increasing the delocalization of the HOMO and the LUMO in charge-transfer, molecules can increase not only φPF but also φTADF /φT .
OLED performance
By taking advantage of the high φPF and φTADF of 2a, blue electroluminescence with ηint of ∼100% was achieved in an OLED using 2a as the guest. Figure 3a shows current density (J )–voltage (V )–luminance (L) characteristics of devices with the structure indium tin oxide (ITO)/4,40 -bis[N -(1-naphthyl)-N -phenyl] biphenyl diamine (amine[N -(1-nap(4,4-bis(3-methylcarbazol9-yl)-2,2-biphenyl) (m-CBP, 10 nm)/6 wt% guest: DPEPO (15 nm)/DPEPO (10 nm)/1,3,5-tris(N -phenylbenzimidazol-2yl)benzene (TPBi, 30 nm)/LiF (0.8 nm)/Al, where the guest is 1a, 2a, 2b or 2c. The devices using 2a, 2b and 2c as guests showed similar J –V characteristics, and the luminance of these devices reached 104 cd cm−2 at approximately 12.6, 15.0 and 16.2 V, respectively. Figure 3b presents the electroluminescence spectra of these devices. The electroluminescence spectra of the OLEDs containing 2a–c as guests were blue-shifted compared with those of OLEDs with high ηext using other metal-free guests16,21 . The emission colour coordinates of the OLEDs using 2a–c as guests were similar to those of OLEDs with bis[2-(4,6difluorophenyl)pyridinato-C2 ,N ](picolinato)iridium(III) (FIrpic), a typical blue phosphorescent guest31 . Figure 3c shows ηext –J characteristics of the devices. The maximum ηext of the OLEDs
using 1a, 2a, 2b and 2c as guests were 7.7 ± 1.3%, 20.6 ± 1.8%, 16.8 ± 1.7% and 14.6 ± 1.0% at 10−2 mA cm−2 , respectively. The corresponding ηint of the OLEDs were 39 ± 7%, 103 ± 9%, 84 ± 9% and 73 ± 5%, respectively, assuming a light out-coupling efficiency of 20%. Thus, blue OLEDs with very high ηint , especially for 2a with ηint of ∼100%, were achieved. This maximum ηext is comparable to that of OLEDs using sky-blue Ir complexes as a guest when ηext was measured without special optical modifications and molecular orientations for enhancement of out-coupling efficiency31 . Devices containing 2a, 2b and 2c as guests also exhibited high ηext of 17.1% (0.27 mA cm−2 ), 11.1% (0.49 mA cm−2 ) and 8.2% (0.56 mA cm−2 ) at 102 cd m−2 and 10.4% (4.6 mA cm−2 ), 5.3% (9.2 mA cm−2 ) and 4.3% (12.1 mA cm−2 ) at 103 cd m−2 , respectively. The temperature dependence of the fluorescence from 2a, 2b and 2c provides further evidence that the blue electroluminescence characteristics with high ηint are the result of efficient thermally activated RISC from T1 to S1 . Figure 3d shows Arrhenius plots of φPF and φTADF /φT for 6 wt%-guest: DPEPO films using 2a–c as guests. φTADF /φT corresponds to the efficiency of delayed fluorescence production by thermally activated RISC after the generation of triplet excitons. Although φPF is essentially independent of temperature, φTADF /φT increases with temperature. Furthermore, the values of ηint obtained from the electroluminescence characteristics correlate well with those calculated from φPF , φT and φTADF using photoluminescence measurements (Table 2). Therefore, the delayed fluorescence in 2a–c can be ascribed to singlets generated by RISC from T1 to S1 . The plots of ln(φTADF /φT ) versus 1/T for 2a–c in the inset of Fig. 3d were expressed well using the dashed fitting curves constructed based on equation (7). The values of 1EST estimated by the fitting curves (1Ea TADF ) were 0.055, 0.083 and 0.105 eV, respectively, when 2a, 2b and 2c were used as guests. Although these values are lower than 1EST determined from the difference of the onset energies of fluorescence and phosphorescence (Table 2), and the same tendency has been observed for the molecules in refs 13,15 and 32, the order of the relative values for 2a–c is the same. One possible explanation for this difference is the presence of a 3n−π ∗ intermediate state, as suggested by Monkman and colleagues32 . Our investigation suggests that changes in molecular conformation over time after generation of triplet excited states cause this discrepancy (Supplementary Section 4). The inset in Fig. 3d shows that φTADF /φT in 2b and 2c are less than 1.0 at room temperature, whereas that of 2a is around 1.0 below room temperature. This is because only 1EST of 2a is low enough to realize efficient RISC at room temperature. Therefore, electroluminescence with ηint of ∼100% from the OLED using 2a as a guest is caused by highly efficient TADF in 2a.
Conclusion We designed and synthesized blue metal-free emitting compounds with low 1EST and high φPL . Large delocalization of molecular orbitals in structures with well-separated HOMO and LUMO
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NATURE MATERIALS DOI: 10.1038/NMAT4154
ARTICLES can suppress a decrease in kf while lowering 1EST , because the transition dipole moment is increased. A metal-free blue emitter, 2a, synthesized based on this design, showed both large kf (>108 s−1 ) and small 1EST (
Highly efficient blue electroluminescence based on thermally activated delayed fluorescence Shuzo Hirata1, Yumi Sakai1,2, Kensuke Masui1,3, Hiroyuki Tanaka1, Sae Youn Lee1,4, Hiroko Nomura1, Nozomi Nakamura1, Mao Yasumatsu1, Hajime Nakanotani1,4,5, Qisheng Zhang1,4, Katsuyuki Shizu1,4, Hiroshi Miyazaki1,6 and Chihaya Adachi1,4,7* Organic compounds that exhibit highly efficient, stable blue emission are required to realize inexpensive organic light-emitting diodes for future displays and lighting applications. Here, we define the design rules for increasing the electroluminescence efficiency of blue-emitting organic molecules that exhibit thermally activated delayed fluorescence. We show that a large delocalization of the highest occupied molecular orbital and lowest unoccupied molecular orbital in these charge-transfer compounds enhances the rate of radiative decay considerably by inducing a large oscillator strength even when there is a small overlap between the two wavefunctions. A compound based on our design principles exhibited a high rate of fluorescence decay and efficient up-conversion of triplet excitons into singlet excited states, leading to both photoluminescence and internal electroluminescence quantum yields of nearly 100%.
O
rganic light-emitting diodes (OLEDs; ref. 1) are an indispensable technology for flat panel displays and lighting applications because they are highly efficient2 , ultrathin, lightweight and flexible3,4 , and exhibit multicolour emission3 . In particular, an internal electroluminescence quantum efficiency (ηint ) of nearly 100% has been achieved using organometallic emitters such as Ir, Pt and Cu complexes5–8 . However, electroluminescence from metal-free conventional aromatic compounds is desired for low-cost OLEDs because rare metals such as Ir and Pt are expensive and blue-emitting compounds containing these metals have limited operational stability. Generally, ηint from fluorescent materials is limited to 25% because of the singlet–triplet branching ratio. It has been reported that delayed fluorescence can be realized through up-conversion of triplet excitons to singlet excitons by triplet–triplet annihilation (TTA; refs 9–12), although the theoretical maximum ηint obtained using TTA up-conversion cannot reach 100% because the process intrinsically involves the deactivation of triplet excitons. As an alternative to TTA, we recently demonstrated a promising pathway to increase ηint by thermally activated delayed fluorescence (TADF; refs 13–25). In this process, light emission is realized as delayed fluorescence after efficient reverse intersystem crossing (RISC) from the lowest triplet excited state (T1 ) to the lowest singlet excited state (S1 ; ref. 26). To obtain TADF through efficient up-conversion from T1 to S1 , a small energy gap between S1 and T1 , 1EST , is required, which has been achieved by separating the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of a molecule14–25 . By controlling the separation of its HOMO and LUMO, we realized a conventional aromatic compound that emits green electroluminescence with ηint of almost 100% (ref. 21).
Furthermore, while the maximum external electroluminescence quantum efficiency (ηext ) of blue OLEDs using TADF has previously been limited to 11 ± 1% (ref. 16), very recently we realized an ηext value of nearly 20% (ref. 27). Although we determined empirical guidelines for molecular design to achieve TADF in those previous studies, which require satisfying conditions of a small 1EST < 0.2 eV and a certain oscillator strength, detailed molecular design strategies to concurrently achieve small 1EST and high photoluminescence quantum yield (φPL ), which is necessary for high ηint , are still unclear. Here, we explain a molecular design strategy that demonstrates compatibility between low 1EST and high φPL and produce a metal-free blue emitter that realizes both high φPL and low 1EST . Delocalization of the HOMO and LUMO in a charge-transfer compound with a well-separated HOMO and LUMO induces a large transition dipole moment while maintaining a small 1EST (∼ 0.1 eV), resulting in φPL = 100%. An OLED containing the metalfree emitter exhibits blue electroluminescence with an ηint value of ∼100%.
Photoproperty relationships First, we consider the important factors necessary to obtain high ηint in TADF compounds and the various dependences of these factors. ηint can be expressed as follows when the radiative decay rate for fluorescence (kf ) is much faster than the rate constant of internal conversion (kic ) at room temperature (refs 14,16,19,23,28): ηint = nr,S φPF + nr,S φTADF + nr,T
φTADF φT
(1)
where nr,S is the singlet exciton production efficiency and nr,T the triplet exciton production efficiency under electrical excitation, φPF
1 Center
for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan. 2 Research & Development Department, Dyden Corporation, 1-1 Hyakunenkouen, Kurume, Fukuoka 839-0864, Japan. 3 Advanced Core Technology Laboratories, Fujifilm Corporation, 577 Ushijima, Kaisei, Ashigarakami, Kanagawa 258-8577, Japan. 4 JST, ERATO, Adachi Molecular Exciton Engineering Project, 744 Motooka, Nishi, Fukuoka 819-0395, Japan. 5 Innovative Organic Device Laboratory, Institute of Systems, Information Technologies and Nanotechnologies (ISIT), 744 Motooka, Nishi, Fukuoka 819-0395, Japan. 6 Nippon Steel & Sumikin Chemical Co. Ltd., 46-80, Nakabaru Sakinohama, Tobata, Kitakyushu 804-8503, Japan. 7 International Institute for Carbon Neutral Energy Research, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan. *e-mail: [email protected] NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials
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NATURE MATERIALS DOI: 10.1038/NMAT4154
ARTICLES HOMO
LUMO
N N N N N N
Increasing F, Q and kf
1a
N
N N
N N N
Increasing delocalization of HOMO
N
N N
2b
N N
N
N
Increasing ΔEST
2a
Similar F, Q and kf
Similar ΔEST
Increasing delocalization of HOMO
Chemical structure
N
2c
N
N N N
Figure 1 | Chemical structures and distribution of HOMO and LUMO in 1a, 2a, 2b and 2c. Optimized structures of the HOMO and LUMO at S1 were calculated by TD-DFT (Gaussian09/B3LYP/6-31G(d)).
is the photoluminescence quantum yield of the prompt component of fluorescence, φT is the quantum yield of intersystem crossing from S1 to T1 , and φTADF is the photoluminescence quantum yield of TADF—that is, the fraction of initial singlet excitons that ultimately yield delayed fluorescence. Typically, nr,S and nr,T are 0.25 and 0.75, respectively. On the basis of equation (1), high φPF and φTADF /φT are required to obtain high ηint . φPF is typically expressed as φPF = kf τPF
(2)
τPF = 1/(kf + kic + kisc )
(3)
where τPF is the lifetime of the prompt component of fluorescence and kisc is the rate constant of intersystem crossing from S1 to T1 . In 1962, Strickler and Berg29 reported that the relationship between kf and the absorption coefficient in fluorescent molecules with a large overlap of HOMO and LUMO can be expressed as
−1 kf = 2.88 × 10−9 n2 ν¯ f−3
ν¯ f−3
−1
Z
ε(νa )d ln νa
f (νf )dνf f (νf )νf −3 dνf
(4)
φPF φRISC φTADF = = φT 1 − φT φRISC
R
=R
(5)
where νf is the fluorescence wavenumber, f (νf ) is the fluorescence spectrum at νf , νa is the absorption wavenumber, ε(νa ) is the molar absorption coefficient at νa , and n is the refractive index. The relationships between oscillator strength for absorption (F), transition dipole moment for absorption (Q), and ε(νa ) can be experimentally determined as30 2 Z 8π me chνf i F = 4.32 × 10−9 n−1 ε(νa )dνa = |Q|2 (6) 3he2 2
where me is the electron mass, c is the speed of light, h is Planck’s constant, e is the elementary charge and hνf i is the average wavenumber of fluorescence. We believe that the Strickler–Berg equation (equation (4)) is also applicable to molecules with a well-separated HOMO and LUMO provided that ∫ε(νa )d ln νa calculated from experimental values of φPF and τPF using equation (4) is almost the same as that calculated using ε(νa ) determined from absorption measurements of the molecules. In such cases, the oscillator strength for fluorescence (F) and the transition dipole moment for fluorescence (Q) are similar to those for absorption and can be calculated using equation (6). Since high φPF can be achieved through large kf , based on equations (2) and (3), which in turn requires large F and Q values, based on equations (4)–(6), development of molecular structures with large F and Q is important for obtaining large φPF . On the other hand, when the RISC process follows an Arrhenius model and kic is much lower than kr and kisc , φTADF /φT can be theoretically expressed as28
=
1 knr kRISC φPF
+1
=
φ
PF kRISC +knr kRISC
− φT
1 knr 1 + φPF Aexp(−1E ST / kT )
(7)
where k is the Boltzmann constant, T is the temperature, knr is the rate constant of non-radiative deactivation from T1 to the ground state (S0 ), and A is a constant. On the basis of equation (7), high φTADF /φT is obtained when 1EST is small, so designing molecular structures with a small 1EST is important for achieving high φTADF /φT . From these theoretical considerations, we used the comparison of theoretically calculated and experimentally determined F and 1EST , along with calculated HOMO and LUMO structures for a
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NATURE MATERIALS DOI: 10.1038/NMAT4154
ARTICLES
Table 1 | Photophysical characteristics of 1a and 2a–c in toluene. Calculation∗
Compound F
0.021 0.112 0.130 0.103
1a 2a 2b 2c
Experimental λa † (nm) 403 362 368 367
1EST (eV) 0.098 0.073 0.114 0.134
∫ ε(νa )d ln νa‡ (103 cm3 ) 0.14 1.6 2.2 1.7
τPF (ns) 8.7 6.3 6.0 6.5
φPF (%) 10 74 57 72
kf (107 s−1 ) 1.0 12 9.6 11
∫ ε(νa )d ln νa§ (103 cm3 ) 0.23 2.1 1.7 2.0
Fk 0.010 0.136 0.174 0.128
PLmax ¶ (nm) 495 457 458 458
Qk (D) 0.39 1.4 1.6 1.4
1EST# (eV) 0.09 − 0.12 0.09 0.28 0.32
∗ Results for structure optimized at S0 . † Maximum wavelength of ultraviolet–visible absorption. ‡ Results calculated using the results of ultraviolet–visible absorption measurement shown in Supplementary Fig. 2. § Results calculated using equations (4) and (5) from experimentally obtained values of kf and hνf−1 i−3 . k Results calculated by substituting the results of ultraviolet–visible absorption measurements shown in Supplementary Fig. 2 into equation (6). ¶ Maximum wavelength of photoluminescence. # These values were defined as the energy difference between S1 and T1 obtained from measurements presented in Supplementary Fig. 3.
Excitation energy
c S1 kisc kic
0
kf
T1
φT 81%
φ GT 62%
φ TADF 20% kisc
kic
knr
b
a Excitation energy
a
φ PF 19%
S1
0
φ TADF 19%
kf
φ PF 80%
φT 20%
T1 knr
φ GT 0%
Figure 2 | Photoluminescence processes of 1a (top) and 2a (bottom). kf , kic , kisc , knr , φPF , φT , φTADF and φG T represent fluorescence decay rate, internal conversion decay rate from S1 to S0 , intersystem crossing decay rate from S1 to T1 , non-radiative decay rate from T1 to S0 , quantum efficiency of the prompt component of fluorescence, quantum efficiency of intersystem crossing from S1 to T1 , quantum efficiency of TADF and non-radiative quantum efficiency from T1 to S0 , respectively. See main text for details of the processes.
variety of TADF molecules, to develop a molecular design strategy that achieves both high F and low 1EST for large ηint . For the evaluation of F values, we obtained Q and F from experimentally observed ε(νa ) and kf by using equations (2)–(6). The values are compared with theoretical values of F obtained from density functional theory (DFT) calculations. For the evaluation of 1EST , the values were confirmed experimentally from the difference in the onset energies of fluorescence and phosphorescence18,23 , and they are compared with theoretical 1EST determined by DFT calculations.
Comparison of TADF molecules Compounds 1a (ref. 14) and 2a with comparably small 1EST but different degrees of delocalization of the HOMO were designed using DFT calculations. 1a, which was reported in our early TADF study, possesses a well-separated HOMO and LUMO that are localized on indolocarbazole and triazine moieties, respectively. This effective separation is ascribed to the steric hindrance
between the indolocarbazole and triazine units. Typically, good separation between the HOMO and LUMO decreases electron repulsion, leading to a decrease in 1EST (ref. 14). Consequently, the theoretical 1EST of 1a calculated by time-dependent density functional theory (TD-DFT) is 0.098 eV (ref. 14). For compound 2a, the 3,6-bis(3,6-diphenylcarbazolyl)carbazole (BDPCC) moiety and the triphenyltriazine (TPTA) functional group are largely twisted because of the steric hindrance between them. Therefore, the HOMO and the LUMO are located over the BDPCC and the TPTA units, respectively, and are effectively separated. Consequently, the large separation between the HOMO and LUMO in 2a gives a small 1EST of 0.073 eV according to TD-DFT calculations. For the HOMO on the BDPCC unit, the 3,6-bis(3,6-diphenylcarbazole) (BDPC) components of the BDPCC moiety acts as weak electrondonating units for the carbazole directly attached to the TPTA unit. As a result, the HOMO of 2a is largely delocalized over all of the BDPCC unit while maintaining a high T1 energy for the donor unit through moderate twisting between the BDPC units and the carbazole. The small 1EST values of 1a and 2a were confirmed experimentally from the difference in the onset energies of fluorescence and phosphorescence (Supplementary Fig. 4; refs 18,23). The 1EST values obtained for 1a and 2a in toluene are 0.09–0.12 and 0.09 eV, respectively. Therefore, efficient thermally activated RISC from T1 to S1 is expected to occur in both 1a and 2a. However, Table 1 indicates that the calculated values of F for 1a and 2a differ greatly, which is expected to lead to differences in φPF . Indeed, φPF values of 10% and 74% for 1a and 2a, respectively, were measured, yielding kf values of 1.0 × 107 and 1.2 × 108 s−1 using equation (2) and τPF of 8.7 ns and 6.3 ns for 1a and 2a, respectively. The values of ∫ ε(νa )d ln νa calculated from ultraviolet–visible absorption measurements are similar to those calculated using equations (4) and (5) with the values of kf (Table 1), because optimized structures and distributions of HOMO and LUMO do not show large differences between S0 and S1 (Supplementary Fig. 1). Therefore, the Strickler–Berg equation (equation (4)) is suitable to describe these molecules with a large separation between HOMO and LUMO, and F for absorption and fluorescence can be determined using equation (6). Despite their comparably small 1EST (Table 1), F of 2a is found to be 13.6 times greater than that of 1a, with values of 0.010 and 0.136 for 1a and 2a, respectively Table 1). The large differences in F and kf for 1a and 2a, even though their 1EST are comparable, is tentatively ascribed to differences in the delocalization of the HOMO. This is because 1a and 2a show comparable absorption energies, emission energies and 1EST , so the main difference is only the spatial volume of the HOMO. The HOMO of 2a is delocalized well over its large BDPCC unit compared with the more localized HOMO of 1a found on its indolocarbazole unit. Further, we note that the HOMO slightly seeps into the location of the LUMO, and the large delocalization of the HOMO increases
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NATURE MATERIALS DOI: 10.1038/NMAT4154
ARTICLES 103 102
b
107
1a 2a
106
2b 2c
105
100
104
10−1
103
10−2
L (cd m−2)
J (mA cm−2)
101
1.2
Normalized electroluminescence intensity
a
102
10−3
101
10−4
1a 2a
1.0
2b 2c
0.8
0.6
0.4
0.2
100
10−5 100
0.0
101
400
500 600 Wavelength (nm)
V (V)
c
d
30 1a
2a
2b
2a
2c
25
20
2c
100 cd m−2
1,000 cd m−2
φ TADF/φ T
15
100
φ PF
ηext (%)
2b
100
100 cd m−2
1,000 cd m−2
10
10−1 2a 2b 2c
10−1 5
0 10−2
700
10−2 100 cd 10−1
m−2
100 J (mA cm−2)
1,000 cd
3
101
4
5
6
7
8
1,000/T (K−1)
m−2 102
3
10
30 1,000/T (K−1)
100
300
Figure 3 | Electroluminescence characteristics of OLEDs using emitter layers containing 1a and 2a–c as guests and photoluminescence characteristics of the emitter layers. a, Current density (J)–voltage (V)–luminance (L) characteristics. Open symbols are current density and filled symbols are luminance. b, Electroluminescence spectral characteristics. c, External electroluminescence quantum efficiency (ηext )–J characteristics. In a–c, the device structure was ITO/device (30 nm)/m-CBP (10 nm)/6 wt% guest (1a, 2a, 2b or 2c): DPEPO (15 nm)/DPEPO (10 nm)/TPBi (30 nm)/LiF (0.8 nm)/Al. d, Arrhenius plots of φPF and φTADF /φT (inset) in 6 wt% guest (2a, 2b or 2c): DPEPO films under vacuum. The dashed lines in the inset represent curve fittings based on equation (7).
the product of the HOMO, the LUMO, and the position vector of the two wavefunctions that determines the transition dipole moment for fluorescence. Therefore, this large delocalization of the HOMO in 2a would contribute to the rather large values of F and kf , even when 1EST is small. The influence of HOMO delocalization on F, Q and 1EST was further investigated using compounds 2a–c, in which the acceptor is kept constant and the delocalization of the HOMO is varied by using different donors, as shown in Fig. 1. In 2a–c, the HOMO and LUMO are effectively separated, absorption and emission energies are similar (Table 1), and the twist angle between the donor units and the TPTA acceptor unit is the same in S0 and S1 (Supplementary Fig. 1). Table 1 compares kf , 1EST and F for 2a–c. All three compounds show similarly large F, both theoretically and experimentally. In contrast, 1EST values determined both by DFT calculations and experimentally increase from 2a to 2b to 2c. 4
On the basis of the trends found in previous studies regarding the design of TADF molecules, F and kf would be expected to decrease with decreasing 1EST . However, 2a–c show similar kf and F experimentally (Table 1), and this trend is also found in values calculated by DFT. In the DFT calculations (Fig. 1), the main difference among 2a–c is increasing delocalization of the HOMO from 2c to 2a. Because 2a–c show comparable absorption and emission energies, the suppression of the decrease in F and kf from 2c down to 2a is attributed to the delocalization of the HOMO. The molecules experimentally indicate that extending the molecular orbitals while limiting the overlap between HOMO and LUMO can suppress a decrease in F and kf while lowering 1EST . This is because 1EST is inversely proportional to the position vector between HOMO and LUMO while F and kf is linearly proportional to it. Consequently, both high φPF and low 1EST are achieved in 2a. We note that the kf of 2a (>108 s−1 ) is the largest of molecules with 1EST less than 0.1 eV.
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NATURE MATERIALS DOI: 10.1038/NMAT4154
ARTICLES
Table 2 | Photophysical characteristics of 6 wt% guest:DPEPO films containing 1a and 2a–c. Compound 1a 2a 2b 2c
PLmax ∗ (nm) 492 480 475 475
ELmax† (nm) 506 487 478 477
(x,y)‡ 0.26, 0.43 0.19, 0.35 0.17, 0.27 0.18, 0.28
1EST§
kf
kisc
φPF
φT
φTADF
ηint k
ηint ¶
ηext #
(eV) 0.05 − 0.12 0.11 ± 0.01 0.19 ± 0.02 0.29 ± 0.03
(107 s−1 )
(107 s−1 )
2.3 12 12 11
9.9 2.9 4.2 3.2
(%) 19 80 74 79
(%) 81 20 26 21
(%) 19 20 21 14
(%) 27 100 84 72
(%) 39 ± 7 103 ± 9 84 ± 9 73 ± 5
(%) 7.7 ± 1.3 20.6 ± 1.8 16.8 ± 1.7 14.6 ± 1.0
∗ Maximum wavelength of photoluminescence. † Maximum wavelength of electroluminescence. ‡ Emission colour coordinates. § These values were defined as the energy difference between S1 and T1 obtained from measurements presented in Supplementary Fig. 5. k Results calculated using φPF , φT and φTADF with equation (1). ¶ Results calculated using experimental results for electroluminescence devices assuming a light out-coupling efficiency of 20%. # Experimental results obtained for electroluminescence devices.
The high kf and low 1EST of these compounds are also observed in solid-state films. The φPF of 1a and 2a in 6 wt% guest: bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) films were 19% and 80%, respectively, as summarized in Table 2. Although φPF is determined by kf , kic and kisc , according to equations (2) and (3), kic can be ignored because it is much lower than kf and kisc in 1a and 2a. This is because 1a and 2a did not show temperature dependence of φPF and τPF (Supplementary Fig. 6). Therefore, φPF is determined only by kf and kisc . In 1a, kf is about 23% of kisc , whereas in 2a, kf is about 400% of kisc . Therefore, φPF of 1a was only 19% because of its low kf (2.3 × 107 s−1 ), but that of 2a was 80% because of its high kf (1.2 × 108 s−1 ). Furthermore, the high kf of 2a also increases φTADF through the following process. For 1a, most of the excitons that are thermally up-converted from T1 to S1 go back to T1 again before generation of delayed fluorescence because kisc is higher than kf . Consequently, multiple cycles of intersystem crossing between S1 and T1 will occur (Fig. 2; a in top panel). As a result of this process, 62% of the initially created excitons will eventually be deactivated while in the T1 state (Fig. 2; b in top panel) and only 23% of excitons (φTADF /φT ) contribute to TADF after intersystem crossing of 81% of the initial excitons has occurred (Fig. 2; c in top panel). In contrast, in 2a, after intersystem crossing of 20% of excitons, almost all are thermally upconverted from T1 to S1 and generate delayed fluorescence before the remaining excitons go back to T1 (Fig. 2, a in bottom panel). This is because kf is about four times faster than kisc . Therefore, increasing kf by increasing the delocalization of the HOMO and the LUMO in charge-transfer, molecules can increase not only φPF but also φTADF /φT .
OLED performance
By taking advantage of the high φPF and φTADF of 2a, blue electroluminescence with ηint of ∼100% was achieved in an OLED using 2a as the guest. Figure 3a shows current density (J )–voltage (V )–luminance (L) characteristics of devices with the structure indium tin oxide (ITO)/4,40 -bis[N -(1-naphthyl)-N -phenyl] biphenyl diamine (amine[N -(1-nap(4,4-bis(3-methylcarbazol9-yl)-2,2-biphenyl) (m-CBP, 10 nm)/6 wt% guest: DPEPO (15 nm)/DPEPO (10 nm)/1,3,5-tris(N -phenylbenzimidazol-2yl)benzene (TPBi, 30 nm)/LiF (0.8 nm)/Al, where the guest is 1a, 2a, 2b or 2c. The devices using 2a, 2b and 2c as guests showed similar J –V characteristics, and the luminance of these devices reached 104 cd cm−2 at approximately 12.6, 15.0 and 16.2 V, respectively. Figure 3b presents the electroluminescence spectra of these devices. The electroluminescence spectra of the OLEDs containing 2a–c as guests were blue-shifted compared with those of OLEDs with high ηext using other metal-free guests16,21 . The emission colour coordinates of the OLEDs using 2a–c as guests were similar to those of OLEDs with bis[2-(4,6difluorophenyl)pyridinato-C2 ,N ](picolinato)iridium(III) (FIrpic), a typical blue phosphorescent guest31 . Figure 3c shows ηext –J characteristics of the devices. The maximum ηext of the OLEDs
using 1a, 2a, 2b and 2c as guests were 7.7 ± 1.3%, 20.6 ± 1.8%, 16.8 ± 1.7% and 14.6 ± 1.0% at 10−2 mA cm−2 , respectively. The corresponding ηint of the OLEDs were 39 ± 7%, 103 ± 9%, 84 ± 9% and 73 ± 5%, respectively, assuming a light out-coupling efficiency of 20%. Thus, blue OLEDs with very high ηint , especially for 2a with ηint of ∼100%, were achieved. This maximum ηext is comparable to that of OLEDs using sky-blue Ir complexes as a guest when ηext was measured without special optical modifications and molecular orientations for enhancement of out-coupling efficiency31 . Devices containing 2a, 2b and 2c as guests also exhibited high ηext of 17.1% (0.27 mA cm−2 ), 11.1% (0.49 mA cm−2 ) and 8.2% (0.56 mA cm−2 ) at 102 cd m−2 and 10.4% (4.6 mA cm−2 ), 5.3% (9.2 mA cm−2 ) and 4.3% (12.1 mA cm−2 ) at 103 cd m−2 , respectively. The temperature dependence of the fluorescence from 2a, 2b and 2c provides further evidence that the blue electroluminescence characteristics with high ηint are the result of efficient thermally activated RISC from T1 to S1 . Figure 3d shows Arrhenius plots of φPF and φTADF /φT for 6 wt%-guest: DPEPO films using 2a–c as guests. φTADF /φT corresponds to the efficiency of delayed fluorescence production by thermally activated RISC after the generation of triplet excitons. Although φPF is essentially independent of temperature, φTADF /φT increases with temperature. Furthermore, the values of ηint obtained from the electroluminescence characteristics correlate well with those calculated from φPF , φT and φTADF using photoluminescence measurements (Table 2). Therefore, the delayed fluorescence in 2a–c can be ascribed to singlets generated by RISC from T1 to S1 . The plots of ln(φTADF /φT ) versus 1/T for 2a–c in the inset of Fig. 3d were expressed well using the dashed fitting curves constructed based on equation (7). The values of 1EST estimated by the fitting curves (1Ea TADF ) were 0.055, 0.083 and 0.105 eV, respectively, when 2a, 2b and 2c were used as guests. Although these values are lower than 1EST determined from the difference of the onset energies of fluorescence and phosphorescence (Table 2), and the same tendency has been observed for the molecules in refs 13,15 and 32, the order of the relative values for 2a–c is the same. One possible explanation for this difference is the presence of a 3n−π ∗ intermediate state, as suggested by Monkman and colleagues32 . Our investigation suggests that changes in molecular conformation over time after generation of triplet excited states cause this discrepancy (Supplementary Section 4). The inset in Fig. 3d shows that φTADF /φT in 2b and 2c are less than 1.0 at room temperature, whereas that of 2a is around 1.0 below room temperature. This is because only 1EST of 2a is low enough to realize efficient RISC at room temperature. Therefore, electroluminescence with ηint of ∼100% from the OLED using 2a as a guest is caused by highly efficient TADF in 2a.
Conclusion We designed and synthesized blue metal-free emitting compounds with low 1EST and high φPL . Large delocalization of molecular orbitals in structures with well-separated HOMO and LUMO
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NATURE MATERIALS DOI: 10.1038/NMAT4154
ARTICLES can suppress a decrease in kf while lowering 1EST , because the transition dipole moment is increased. A metal-free blue emitter, 2a, synthesized based on this design, showed both large kf (>108 s−1 ) and small 1EST (
