Electroluminescence based on thermally activated delayed fluorescence generated by a spirobifluorene donor-acceptor structure

Article abstract of DOI:10.1039/c2cc31468a

An organic light emitting diode based on thermally activated delayed fluorescence (TADF) has been produced using a spirobifluorene derivative (Spiro-CN) having the donor-acceptor moieties as an emitter.

Full text of DOI:10.1039/c2cc31468a

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Showcasing research from Prof. Chihaya Adachi’s
laboratory in the Center for Organic Photonics and
Electronics Research (OPERA), Kyushu University,
Japan and Prof. Ken-Tsung Wong’s laboratory in the
As featured in:
Department of Chemistry, National Taiwan University,
Taiwan
Electroluminescence based on thermally activated delayed
fluorescence generated by a spirobifluorene donor−acceptor
structure
An organic light emitting diode based on thermally activated
delayed fluorescence (TADF) is produced using a spirobifluorene
derivative (Spiro-CN) having the donor-acceptor units as an emitter.
See Ken-Tsung Wong,
Chihaya Adachi et al.,
Chem. Commun., 2012, 48, 9580.
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COMMUNICATION
Electroluminescence based on thermally activated delayed fluorescence
generated by a spirobifluorene donor–acceptor structurew
Tetsuya Nakagawa,a Sung-Yu Ku,b Ken-Tsung Wong*b and Chihaya Adachi*a
Received 28th February 2012, Accepted 16th April 2012
DOI: 10.1039/c2cc31468a
An organic light emitting diode based on thermally activated
delayed fluorescence (TADF) has been produced using a spiro-
bifluorene derivative (Spiro-CN) having the donor–acceptor moieties
as an emitter.
In this communication, we report the observation of TADF
based on a spirobifluorene derivative and its application in
OLEDs. Spirobifluorene derivatives have become promising
candidates for OLEDs since these compounds exhibit a high
glass transition temperature,13 excellent thermal stability and
color stability.14 Although a large number of spirobifluorene
derivatives have been reported as emitters15 as well as host
materials,16 no TADF property has been reported for them.
In our recent study, we found that a fluorescent molecule
containing a sufficiently separated highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) shows efficient thermally activated delayed fluorescence.17
In the case of this molecule, spatially separated HOMO and
LUMO orbitals lead to efficient TADF due to the small energy
gap (DEST) between the lowest excited singlet (S1) state and the
lowest excited triplet (T1) state. This finding prompted us to
expand our methodology to spirobifluorene derivatives. Here,
we focus on a spirobifluorene derivative with electron donating
and electron accepting units in the same molecular structure.
Fig. 1 shows the molecular structure of 20,70-bis(di-p-tolylamino)-
9,90-spirobifluorene-2,7-dicarbonitrile (Spiro-CN) containing two
di-p-tolylamino electron donating units and two cyano electron
accepting units. Details of the synthetic scheme are given in the
ESI.w Calculations to determine the HOMO and LUMO of
Spiro-CN were carried out with the Gaussian 09 package18 at
the B3LYP/6-31G(d) level. Since the donor and acceptor units
are orthogonally connected via the spirobifluorene moiety, the
HOMO and LUMO are located on the donor and acceptor
units, respectively.
Organic light emitting diodes (OLEDs) have attracted significant
attention because of their promising applications in flat-panel
displays1 and in general lighting applications.2 Their characteristic
organic semiconducting material features will realize flexibility over
a large-area, low cost of fabrication and high-performance optical
and electrical properties. In particular, advances in the molecular
design of modern optoelectronics have contributed significantly
toward the commercialization of highly efficient OLEDs.
Phosphorescent materials such as Ir(III) complexes3 and
Pt(II) complexes4 are possible solutions for highly efficient
OLEDs because of their highly intrinsic exciton generation
efficiency. On the other hand, fluorescence materials have an
intrinsic limitation with regard to electroluminescence efficiency
because of their low exciton generation efficiency. However, a
recent study revealed that highly efficient OLEDs are potentially
feasible by using delayed fluorescence materials that enhance the
exciton generation efficiency because of the upconversion of a
triplet exciton into a singlet excited state.5–7 Until now, two
mechanisms have been identified for delayed fluorescence. One is
a triplet–triplet annihilation: TTA (P-type delayed fluorescence),8
and the other is thermally activated delayed fluorescence:
TADF (E-type delayed fluorescence),9 which thermally converts
triplet excitons into singlet excited states. Among them, the
TADF process is more promising because the additional singlet
exciton generation efficiency (75%) is potentially higher than
that of TTA (37.5%). Although fluorescent materials have
been widely studied, TADF materials remain quite rare and
this behaviour has been observed in some dyes,10 fullerenes11
and metal complexes.12
The photoluminescence (PL) spectra and transient PL character-
istics of a 6 wt% Spiro-CN:1,3-bis(carbazole-9-yl) benzene (mCP)
co-deposited film were measured as shown in Fig. 2. Spiro-CN
showed yellow emission with a PL quantum efficiency of FPL
=
27%. The transient PL characteristics of 6 wt% Spiro-CN:m-CP
co-deposited film showed prompt and delayed PL spectra at 300 K.
A prompt component with a transient decay time of 24 ns and a
a Center for Organic Photonics and Electronics Research (OPERA),
Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan.
E-mail: adachi@cstf.kyushu-u.ac.jp; Fax: +81-92-802-6921;
Tel: +81-92-802-6920
b Department of Chemistry, National Taiwan University,
Taipei 10617, Taiwan. E-mail: kenwong@ntu.edu.tw
w Electronic supplementary information (ESI) available: Synthesis
scheme for Spiro-CN, experimental details, additional photophysical
data and complete ref. 18. See DOI: 10.1039/c2cc31468a
Fig. 1 Molecular structure of Spiro-CN and its HOMO and LUMO
calculated at the B3LYP/6-31G(d) level.
c
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Based on these results, the thermal activation energy
was estimated using a Berberan-Santos plot and the energy
difference (DEST) between the lowest excited singlet (S1) state
and the lowest excited triplet (T1) state can, in principle, be
obtained from the temperature dependence of the emission
intensity ratio (Iprompt/Idelayed = Fprompt/Fdelayed) from the
following equation,11
ꢂ
ꢀ
ꢁꢃ
ꢀ
ꢁ
1
kp þ knr
DEST
RT
Iprompt
ln
ꢀ
ꢀ 1
¼ ln
þ
ð1Þ
Idelayed
FT
kRISC
where FT is the triplet formation efficiency, kp is the phos-
phorescence rate constant, knr is the non-radiative rate constant
from T1, kRISC is the T1 - S1 reverse intersystem crossing rate
constant and R is the gas constant. The Berberan-Santos plot
gave a straight line as shown in Fig. 3(b) and from the slope we
obtained DEST = 5.5 kJ molꢀ1 (57 meV). Thus, Spiro-CN shows
an efficient TADF (Fdelayed d Fprompt) at room temperature
and a far smaller DEST compared with those of common TADF
Fig. 2 Transient PL of a 6 wt% Spiro-CN:mCP co-deposited film.
The inset shows PL spectra of a 6 wt% Spiro-CN:mCP co-deposited
film: prompt component (fluorescence, red line), delayed component
(delayed fluorescence, black line).
delayed component of 14 ms were clearly observed. Since both
spectra are nearly coincident, the delayed component can be
attributed to the TADF process. In order to further clarify the
origin of the delayed component, we measured the transient
PL characteristics of Spiro-CN in a toluene solution with and
without oxygen. As shown in Fig. S1 (ESIw), the emission
decay of the nitrogen-purged solution showed both prompt
and delayed components. In contrast, the delayed component
disappeared in a non-degassed solution. This oxygen sensitive
behaviour of the emission decay provides a good indication
that the delayed component is caused by the TADF that
occurs via a triplet excited state. Similar triplet quenching
behaviour was also observed in a 6 wt% Spiro-CN:a-NPD
co-deposited film as shown in Fig S2 (ESIw). Furthermore, the
PL efficiency (5.4%) of a 6 wt% Spiro-CN:a-NPD co-deposited
film was found to be significantly smaller than that of a 6 wt%
Spiro-CN:mCP co-deposited film.
compounds such as C70 (B25 kJ molꢀ1 11
and tin(IV) fluoride–
)
porphyrin (B39 kJ molꢀ1).7 Furthermore, the triplet formation
efficiency (FT) was estimated to be 78% (for the determination
method of FT, see Fig. S3, ESIw).
To study the electroluminescence properties of Spiro-CN, a
multilayer device with a configuration of glass/ITO/a-NPD
(60 nm)/6 wt% Spiro-CN:mCP (20 nm)/Bphen (40 nm)/
MgAg (100 nm)/Ag (20 nm) was fabricated, where 4,4-bis
[N-(1-naphthyl)-N-phenylamino]]biphenyl (a-NPD) was used
as a hole-transport layer and 4,7-diphenyl-1,10-phenanthroline
(Bphen) as an electron-transport layer. This device gives yellow
electroluminescence with maximum current and power efficiencies
of 13.5 cd Aꢀ1 and 13.0 lm Wꢀ1 and a maximum external
quantum efficiency of 4.4% (Fig. 4(a)), which are the highest
values reported to date for a device containing spirobifluorene as
an emitter. Furthermore, a high luminance of 12 000 cd mꢀ2 was
observed at 15 V, as shown in Fig. 4(b).
To investigate the TADF property, a temperature dependent
It is noteworthy that the external quantum efficiency observed
at low current density (o1 mA cmꢀ2) in the Spiro-CN based
device is about 3 times higher than the theoretical maximum of
about 1.4% for common fluorescent-type molecular materials
with a PL efficiency of 27%. As mentioned above, a recent study
proposed that two possible mechanisms for such an unusual
increase in external quantum efficiency are the TTA and the
TADF. For a discussion of the mechanism we examined the
luminescence measurement was obtained. The overall FPL
,
prompt fluorescence efficiency (Fprompt) and delayed fluorescence
efficiency (Fdelayed) are shown in Fig. 3(a), the prompt
components showed almost no temperature dependence (for the
determination method of Fprompt and Fdelayed, see ESIw). On the
other hand, an increase in the delayed fluorescence intensity with
an increase in temperature was clearly observed. These results
indicate the presence of thermal activation energy during delayed
fluorescence.
Fig. 4 (a) External quantum efficiency versus current density for
glass/ITO/a-NPD (60 nm)/6 wt% Spiro-CN:mCP (20 nm)/Bphen
(40 nm)/MgAg (100 nm)/Ag (20 nm). The inset shows an electro-
luminescence spectrum. (b) Current density (black closed squares)–
luminance (blue closed squares)–voltage characteristics for glass/ITO/
a-NPD (60 nm)/6 wt% Spiro-CN:mCP (20 nm)/Bphen (40 nm)/MgAg
(100 nm)/Ag (20 nm).
Fig. 3 (a) Temperature dependence of the total photoluminescence
(FPL): open circles, prompt fluorescence (Fprompt): open squares and
delayed fluorescence (Fdelayed): open triangles. (b) Log plot of the
intensity ratio of prompt fluorescence to delayed fluorescence vs. 1/T.
c
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Chem. Commun., 2012, 48, 9580–9582 9581

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current density dependence of the luminance. For the TTA,
the luminance is known to increase more than linearly with an
increase in current density.19 On the other hand, we observed a
linear increase in the luminance with an increase in current
density, as shown in Fig. S4 (ESIw), suggesting that the high
external quantum efficiency is not due to the TTA.
This research is granted by the Japan Society for the
Promotion of Science (JSPS) through the ‘‘Funding Program
for World-Leading Innovative R&D on Science and Technology
(FIRST Program),’’ initiated by the Council for Science and
Technology Policy (CSTP).
Next, we estimated the theoretical maxima of the internal
and external electroluminescence efficiency for the TADF.
Under electrical excitation, triplet excitons formed directly
by carrier recombination and they were up-converted to the
S1 level leading to the total internal electroluminescence
efficiency. The theoretical maxima of internal electroluminescence
efficiency (FEL(int)) and external electroluminescence efficiency
(FEL(ext)) are given by the following equations,
Notes and references
1 C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.
2 S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer,
B. Lussem and K. Leo, Nature, 2009, 459, 234.
3 C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest,
J. Appl. Phys., 2001, 90, 5048.
4 D. F. O’Brien, M. A. Baldo, M. E. Thompson and S. R. Forrest,
Appl. Phys. Lett., 1999, 74, 442.
5 C. T. Brown and D. J. Kondakov, J. Soc. Inf. Disp., 2004, 12, 323.
6 D. Kondakov, T. D. Pawlik, T. K. Hatwer and J. P. Spindler,
J. Appl. Phys., 2009, 106, 124510.
FEL(int) = Zr(S1)Zf + Zr(S1)ZTADF + Zr(T1)ZTADF
FEL(ext) = FEL(int)Zout
(2)
(3)
7 A. Endo, M. Ogasawara, A. Takahashi, D. Yokoyama, Y. Kato
and C. Adachi, Adv. Mater., 2009, 21, 4802.
8 J. B. Birks, Photophysics of Aromatic Molecules, Wiley, New York,
1970, p. 378.
9 B. Valeur, Molecular Fluorescence, Wiley-VCH, Weinheim, 2002,
p. 41.
10 C. A. Parker, Photoluminescence of Solutions, Elsevier, Amsterdam,
1968, p. 44.
11 M. N. Berberan-Santos and J. M. M. Garcia, J. Am. Chem. Soc.,
1996, 118, 9391.
where Zr(S1) is the branching ratio of singlet exciton formation
(0.25), Zr(T1) is the branching ratio of a triplet exciton (0.75),
Zf is the fluorescence efficiency (B0.05), ZTADF is the TADF
efficiency (B0.22) and Zout is the light out-coupling efficiency
(B0.20). Therefore, FEL(int) is estimated to be 23% and the
theoretical maximum of the external quantum efficiency is,
therefore, FEL(ext) = 4.6% assuming a light out-coupling
efficiency of 20%. The experimental external quantum efficiency
maximum of 4.4% agrees well with the theoretical external
quantum efficiency maximum. Therefore, we conclude that the
unusual increase in the external quantum efficiency observed in
this study can be attributed to the TADF.
12 (a) J. C. Deaton, S. C. Switalski, D. Y. Kondakov, R. H. Young,
T. D. Pawlik, D. J. Giesen, S. B. Harkins, A. J. M. Miller,
S. F. Mickenberg and J. C. Peters, J. Am. Chem. Soc., 2010,
132, 9499; (b) R. Czerwienic, J. Yu and H. Yersin, Inorg. Chem.,
2011, 50, 8293.
13 T.-C. Chao, Y.-T. Lin, C.-Y. Yang, T. S. Hung, H.-C. Chou,
C.-C. Wu and K.-T. Wong, Adv. Mater., 2005, 17, 992.
14 Y. Wu, J. Li, Y. Fu and Z. Bo, Org. Lett., 2004, 6, 3485.
15 (a) C.-L. Chiang, M.-F. Wu, D.-C. Dai, Y.-S. Wen and
J.-K. Wang, Adv. Funct. Mater., 2005, 15, 231; (b) J. Luo,
Y. Zhou, Z.-Q. Niu, Q.-F. Zhou, Y. Ma and J. Pei, J. Am. Chem.
Soc., 2007, 129, 11314; (c) Y. T. Lee, C.-L. Chiang and C.-T. Chen,
Chem. Commun., 2008, 217.
16 (a) W.-Y. Hung, T.-C. Tsai, S.-Y. Ku, L.-C. Chi and K.-T. Wong,
Phys. Chem. Chem. Phys., 2008, 10, 5822; (b) Z. Jiang, H. Yao,
Z. Zhang, C. Yang, Z. Liu, Y. Tao, J. Qin and D. Ma, Org. Lett.,
2009, 11, 2607; (c) Y.-Y. Lyu, J. Kwak, W. S. Jeon, Y. Byun,
H. S. Lee, D. Kim, C. Lee and K. Char, Adv. Funct. Mater., 2009,
19, 420.
17 A. Endo, K. Sato, K. Yoshimura, T. Kai, A. Kawada,
H. Miyazaki and C. Adachi, Appl. Phys. Lett., 2011, 98, 083302.
18 M. J. Frisch, et al., Gaussian 09, Rev. A. 02, Gaussian, Inc,
Wallingford CT, 2009.
In summary, efficient thermally activated delayed fluorescence
as well as efficient electroluminescence based on a spirobifluorene
derivative are reported. The Spiro-CN based device shows an
unusual increase in the external quantum efficiency. Although
the external quantum efficiency of the present system is almost
comparable to that of efficient yellow fluorescent materials,20 the
external quantum efficiency of the present system is still lower
than that of yellow phosphorescent materials.21 However, it is
expected that external quantum efficiency can be enhanced by
developing spiro-based TADF materials with high PL efficiency
by suppression of nonradiative transition. Furthermore, it is also
expected that emission color of spiro-based TADF materials can
be controlled from blue to red by controlling the p-conjugation
of the electron donor and acceptor units, respectively. We believe
that these findings are of fundamental interest for the development
of highly efficient OLEDs based on fluorescent materials.
19 C. Ganzorig and M. Fujihira, Appl. Phys. Lett., 2002, 81, 3137.
20 L. Zhang, S. Hu, J. Chen, Z. Chen, H. Wu, J. Peng and Y. Cao,
Adv. Funct. Mater., 2011, 21, 3760.
21 H. Wu, G. Zhou, J. Zou, C.-L. Ho, W.-Y. Wong, W. Yang, J. Peng
and Y. Cao, Adv. Mater., 2009, 21, 4181.
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9582 Chem. Commun., 2012, 48, 9580–9582
This journal is The Royal Society of Chemistry 2012