Simultaneous improvement of emission color, singlet-triplet energy gap, and quantum efficiency of blue thermally activated delayed fluorescent emitters using a 1-carbazolylcarbazole based donor

Article abstract of DOI:10.1039/c6cc04516b

Blue thermally activated delayed fluorescent (TADF) emitters having 1-carbazolylcarbazole based donor moieties were developed to resolve the low quantum efficiency and large singlet-triplet energy splitting issues of the linker free TADF emitters. Investigation of the 1-carbazolylcarbazole derived donors as the donor units of two blue TADF emitters in comparison with 3-carbazolylcarbazole demonstrated that the 1-carbazolylcarbazole based donors increased the triplet energy, decreased the singlet-triplet energy gap, blue-shifted the emission color, and enhanced the quantum efficiency of the blue TADF devices.

Full text of DOI:10.1039/c6cc04516b

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Simultaneous improvement of emission color,
singlet–triplet energy gap, and quantum
efficiency of blue thermally activated delayed
fluorescent emitters using a 1-carbazolylcarbazole
based donor†
Cite this:DOI: 10.1039/c6cc04516b
Received 30th May 2016,
Accepted 12th July 2016
DOI: 10.1039/c6cc04516b
Mounggon Kim,a Jeong Min Choib and Jun Yeob Lee*b
www.rsc.org/chemcomm
Blue thermally activated delayed fluorescent (TADF) emitters having popular design is to adopt a basic backbone structure containing
1-carbazolylcarbazole based donor moieties were developed to diphenyl triazine, a phenyl linker, and carbazole derivatives.12–20
resolve the low quantum efficiency and large singlet–triplet energy Each unit has a triplet energy higher than 2.7 eV and the merged
splitting issues of the linker free TADF emitters. Investigation of the molecule can show a triplet energy higher than 2.6 eV when the
1-carbazolylcarbazole derived donors as the donor units of two carbazole derived unit is properly selected. The diphenyltriazine
blue TADF emitters in comparison with 3-carbazolylcarbazole demon- and carbazole derived units are introduced to separate the
strated that the 1-carbazolylcarbazole based donors increased the HOMO/LUMO for the TADF process itself and the phenyl linker
triplet energy, decreased the singlet–triplet energy gap, blue-shifted is inserted to manage the HOMO/LUMO separation for high
the emission color, and enhanced the quantum efficiency of the blue efficiency from the TADF process. However, the phenyl linker has a
TADF devices.
problem of increasing singlet energy–triplet energy splitting (DEST)
due to reduced HOMO/LUMO isolation and the accompanying
Delayed fluorescence type organic light-emitting diodes (OLEDs) triplet excited state lifetime prior to reverse intersystem crossing.
are emerging as third generation light-emitting materials resolving However, high QE cannot be anticipated from the diphenyltriazine
the technical challenges of conventional fluorescent and and carbazole based TADF emitters in the absence of the phenyl
phosphorescent OLEDs.1 Although the delayed fluorescence linker in spite of the challenging large DEST issue. Therefore, an
technology is not matured yet for commercial application, it is advanced molecular design platform to obtain high QE without
a potential candidate for high efficiency OLED technology. In the phenyl linker is desired to avoid the large DEST issue of blue
particular, it is a promising methodology in blue OLEDs because TADF emitters derived from the diphenyltriazine and carbazole
no high efficiency and long lifetime blue OLEDs are in mass- derived units.
production line.
There are two different delayed fluorescence processes, and an advanced design platform adopting 1-carbazolylcarbazole
thermally activated delayed fluorescence (TADF) is preferred to based donor units was proposed for high QE and small DEST
To resolve the low QE issue of the linker free TADF emitters,
.
triplet–triplet fusion (TTF) because high theoretical quantum Two donor units, 1CzCz and 13CzCz, were 1-carbazolylcarbazole
efficiency (QE) is expected from the TADF process.1–3 The blue based donor units and they were included in the linker free blue
emitting materials for TADF and TTF are totally different, and TADF molecular structure. Investigation of the new donors as
the basic design principle of the TADF materials is to make a the donor unit of two blue TADF emitters, 9-(4,6-diphenyl-1,3,5-
high triplet energy emitter with a singlet energy close to the triazin-2-yl)-9H-1,90-bicarbazole (1CzCzTrz) and 90-(4,6-diphenyl-
triplet energy.4–14 This means that a high triplet energy back- 1,3,5-triazin-2-yl)-90H-9,10:30,900-tercarbazole (13CzCzTrz), in com-
bone structure having a separated highest occupied molecular parison with 3-carbazolylcarbazole demonstrated that the 1CzCz
orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) and 13CzCz donors increased the triplet energy, decreased DEST
,
is the basic design platform of the blue TADF emitters. Up to blue-shifted the emission color, and enhanced the QE of the blue
now, only a few design platforms are available and the most TADF OLEDs.
The basic platform of the 1CzCzTrz and 13CzCzTrz emitters
was 1-carbazolylcarbazole with a carbazole substituent at the
1-position of carbazole. The 1-position modification differs from
the common 3-position modification in that it can distort the
donor moiety from the acceptor plane and increase the charge
a Department of Polymer Science and Engineering, Dankook University, Yongin,
Gyeonggi, 448-701, Republic of Korea
b School of Chemical Engineering, Sungkyunkwan University, Suwon, Gyeonggi,
446-740, Republic of Korea. E-mail: leej17@skku.edu; Tel: +82 31 8005 3585
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cc04516b transfer (CT) character of the TADF emitters. Strong CT character
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at the 1-position. The calculated dihedral angles between
diphenyltriazine and the carbazole plane were 501, 501, and
181 in the optimized 1CzCzTrz, 13CzCzTrz and 3CzCzTrz struc-
ture, respectively. It is apparent from the geometrical structure
that the 1-position substitution of carbazole twists the carbazole
from the diphenyltriazine plane by steric hindrance. The geo-
metrical twisting of the donor structure from the acceptor plane
did not change the molecular orbital distribution itself, but it
may change the singlet and triplet energy of the TADF emitters.
The fluorescence emission spectrum at room temperature
and the phosphorescence emission spectrum at low temperature
after delay time are plotted together in Fig. 2 to clarify the effect
of geometrical distortion on the PL emission. Relative positions
of the fluorescence and phosphorescence emission peaks were
shifted to short wavelength by the 1CzCz donor moiety.
Fluorescence emission peak wavelengths of the 1CzCzTrz and
13CzCzTrz emitters were 460 and 465 nm, respectively, while that
of the 3CzCzTrz device was 502 nm. The same trend was observed
in the phosphorescence emission wavelength. From the PL
emission wavelength, it can be provisionally concluded that the
carbazole modification at the 1-position of carbazole increases
the singlet and triplet energy of the TADF emitters because of the
shortening of the conjugation length of the molecule by the
distorted geometrical structure. As the origin of the fluorescence
and phosphorescence emission is intramolecular CT emission,
DEST of the three emitters was estimated using the singlet and
triplet energy calculated from the edge wavelength of the fluores-
cence and phosphorescence spectra. DESTs of 1CzCzTrz, 13CzCzTrz
and 3CzCzTrz were 0.03, 0.01, and 0.12, respectively. The 1-position
modification of carbazole decreased DEST by increased intra-
molecular CT character and HOMO–LUMO separation due to
distortion of the donor structure from the acceptor moiety.
The effect of the geometrical twisting of the three TADF
emitters on emission behaviour was experimentally studied by
measuring the photoluminescence (PL) emission spectra of the
TADF emitters in toluene and tetrahydrofuran. Fig. S1 (ESI†)
shows solvent dependent PL spectra of 1CzCzTrz, 13CzCzTrz
and 3CzCzTrz. In all TADF emitters, the gradual shift of the PL
emission spectra from short wavelength to long wavelength was
commonly observed according to the increase of solvent polarity,
Scheme 1 Synthetic scheme of TADF emitters.
is desirable for the light emission of the TADF emitters because
it can decrease DEST for better triplet up-conversion. Therefore,
the 1CzCz and 13CzCz donors were compared with 3-carbazolyl-
carbazole to examine the effect of 1-position modification of
carbazole.
The synthesis of 13CzCzTrz was carried out through ring
closing reaction with two carbazole units at the meta-position of
benzene as shown in Scheme 1. The synthetic yield obtained in
the preparation of the 13CzCz donor was about 80%. N-Arylation
of carbazole derivatives was simply performed using chloro-
diphenyltriazine and sodium hydride. Vacuum train sublimation
was carried out for each TADF material before characterization
and device fabrication for high purity.
The effect of 1-position modification of carbazole on the
electronic molecular orbital distribution of the 1CzCzTrz and
13CzCzTrz emitters was investigated by comparing the HOMO
and LUMO of 1CzCzTrz, 13CzCzTrz and 9-(4,6-diphenyl-1,3,5-
triazin-2-yl)-9H-3,90-bicarbazole (3CzCzTrz) in Fig. 1. Funda-
mentally, the HOMO and LUMO calculation results of the three
emitters had little difference. The direct connection of the
diphenyltriazine donor and the carbazolylcarbazole acceptors
localized the HOMO on the carbazole substituent at the 1- or
3-position of carbazole and the LUMO on the diphenyltriazine.
However, the optimized geometrical structure of the three TADF
emitters was significantly different by the carbazole substituent
Fig. 1 HOMO and LUMO distribution and dihedral angles of 1CzCzTrz,
3CzCzTrz, and 13CzCzTrz calculated by using the B3LYP 6-31G* basis set
of the Gaussian 09 program.
Fig. 2 Solution (in toluene) and low temperature photoluminescence
spectra of TADF emitters.
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Table 1 Photophysical and electrochemical properties of TADF emitters
B.Ga (eV) S1/T1 (eV) ec
3.00/2.97 1.00 11.7
td (ms) FPL (%)
b
e
Emitters
IP/EA (eV)
1CzCzTrz ꢁ6.08/ꢁ3.32 3.60
3CzCzTrz ꢁ6.08/ꢁ3.27 3.50
13CzCzTrz ꢁ6.09/ꢁ3.32 3.43
76 (82)
57 (62)
72 (85)
2.92/2.80 0.92 19.4
2.98/2.97 1.13 11.5
a
The band gap was calculated from the edge of the absorption spectrum
in tetrahydrofuran. S1 and T1 were calculated from the edge of fluores-
b
c
cence and phosphorescence PL spectra. The extinction coefficient was
d
calculated in tetrahydrofuran solution (105 Mꢁ1 cmꢁ1). Delayed lifetime
e
of TADF materials. Quantum yields of TADF materials without and with
(in parenthesis) nitrogen bubbling in films.
Fig. 3 UV-vis absorption spectra of TADF emitters.
demonstrating that intramolecular CT emission is the origin of
the PL emission.
Ultraviolet–visible (UV-vis) absorption spectra of the 1CzCzTrz,
13CzCzTrz and 3CzCzTrz emitters were also collected to examine
the donor effect on the light absorption. Fig. 3 shows UV-vis
absorption spectra of the 1CzCzTrz, 13CzCzTrz and 3CzCzTrz
emitters. The absorption coefficient of the 1.0 ꢀ 10ꢁ5 M tetra-
hydrofuran solution of each emitter had little difference even
though a slight increase of the absorption coefficient was
observed in the 13CzCzTrz emitter. As the main chromophore
Fig. 4 Transient PL decay curves of (a) 1CzCzTrz, (b) 3CzCzTrz, and
(c) 13CzCzTrz.
in the three TADF emitters is the carbazole based donor moiety, lifetime of the 1CzCzTrz and 13CzCzTrz TADF emitters. There-
the absorption coefficients were comparable to each other. The fore, the 1-position modification of carbazole was favorable for
slightly high absorption coefficient of 13CzCzTrz is due to the the fast up-conversion of the triplet excitons.
additional carbazole unit in the 13CzCz donor. The UV-vis
In addition to the photophysical analysis using PL measure-
absorption edge was red-shifted in 13CzCzTrz and 3CzCzTrz ments, the HOMO and LUMO analysis of the TADF emitters was
by extension of the conjugation caused by the 3-position performed to optimize the device structure for maximum QE.
carbazole modification. The absence of the 3-position substi- The similarity of the donor and acceptor moieties resulted in
tuted carbazole resulted in a relatively short UV-vis absorption similar HOMO and LUMO levels in 1CzCzTrz, 13CzCzTrz, and
edge of 361 nm.
3CzCzTrz as summarized in Table 1 and Fig. S2 (ESI†). In the
The relationship between the donor structure and photo- oxidation and reduction scans of the TADF emitters, the onset
physical properties was studied in more detail by analyzing the voltage for the oxidation (ionization potential) and reduction
PL quantum yield (PLQY) of the emitters doped in the bis[2- (electron affinity) was measured in a similar voltage range.
(diphenylphosphino)phenyl]ether oxide (DPEPO) matrix. PLQYs
Devices were fabricated on poly(3,4-ethylenedioxythiophene):
of the emitters measured under oxygen and under nitrogen are poly(styrenesulfonate) PEDOT:PSS by using a thermal evaporator
presented in Table 1. All three emitters showed typical PL at a vacuum pressure of 1.0 ꢀ 10ꢁ6 torr. PEDOT:PSS (60 nm)
quenching behavior of TADF emitters. The PLQYs of 1CzCzTrz, was spin coated on an indium tin oxide substrate (120 nm).
13CzCzTrz and 3CzCzTrz under nitrogen were 0.82, 0.85, and After that, tris(4-carbazol-9-ylphenyl)amine (TAPC, 10 nm), 1,3-
0.62, respectively. The 1CzCzTrz and 13CzCzTrz emitters with bis(N-carbazolyl)benzene (mCP, 10 nm), DPEPO:TADF emitters
a carbazole moiety at the 1-position of the central carbazole (10%, 25 nm), diphenylphosphine oxide-4-(triphenylsilyl)phenyl
exhibited a higher PLQY than the 3CzCzTrz emitter without the (TSPO1, 5 nm), 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene
carbazole moiety at the 1-position.
(TPBi, 30 nm), LiF (1 nm) and Al (200 nm) were fabricated step
Transient PL decay characteristics of 1CzCzTrz, 13CzCzTrz by step. Encapsulation of the devices was finished inside a glove
and 3CzCzTrz were also different in the three TADF emitters as box after device fabrication.
shown in Fig. 4. The delayed fluorescence intensity was rather
Based on the material characteristics of the three emitters,
strong and the excited state lifetime for delayed fluorescence optimized TADF devices were developed and fabricated. The three
was short in 1CzCzTrz and 13CzCzTrz. The delayed fluorescence TADF emitters were doped in the DPEPO host and optimized at 10%
lifetimes of 1CzCzTrz and 13CzCzTrz were 11.7 and 11.5 ms, while doping concentration. Basic current density–voltage–luminance data
that of 3CzCzTrz was 19.5 ms. A good correlation between DEST are shown in Fig. S3 of the ESI† and the current density was high in
and the excited state lifetime can be found in the three TADF the 1CzCzTrz device possibly because of improved hole hopping due
emitters and small DEST shortened the delayed fluorescence to a less extended molecular structure. The quantum efficiency of
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two blue TADF emitters, 1CzCzTrz and 13CzCzTrz, compared to
3-carbazolylcarbazole demonstrated that 1CzCzTrz and 13CzCzTrz
increased the triplet energy, decreased DEST, blue-shifted the
emission color, and enhanced the QE of the blue TADF OLEDs
by inducing distortion between the donor and acceptor. Therefore,
the concept of inducing distortion between the donor and acceptor
can be suitable for increased device performances with a blue-
shifted emission color.
This work was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by Ministry of Science, ICT, and future Planning
(2013R1A2A2A01067447, 2016R1A2B3008845).
Fig. 5 (a) External quantum efficiency and (b) electroluminescence spectra
of TADF emitters.
Table 2 Summarized device performances of TADF materials
Emitters
Max QEa
Max PEb
Max CEc
FWHMd
CIE
Notes and references
1 H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature,
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1CzCzTrz
3CzCzTrz
13CzCzTrz
15.7
12.4
15.7
21.0
21.8
19.1
27.5
28.4
27.7
74
93
78
0.17, 0.24
0.22, 0.36
0.17, 0.25
2 N. J. Turro, Modern Molecular Photochemistry, Benjamin Cummings,
1978, p. 98.
3 C. Bohne, E. B. Abuin and J. C. Scaiano, J. Am. Chem. Soc., 1990,
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a
b
c
QE: quantum efficiency (%). PE: power efficiency (lm Wꢁ1). CE:
current efficiency (cd Aꢁ1). FWHM: full width at half maximum (nm).
d
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1CzCzTrz, 3CzCzTrz, and 13CzCzTrz shown in Fig. 5 proposed that
the 1-position substitution of carbazole, 1CzCzTrz (15.7%) and
13CzCzTrz (15.7%), enhanced the quantum efficiency compared
to 3CzCzTrz (12.4%). The much faster delayed fluorescence life-
times and higher PLQY of 1CzCzTrz and 13CzCzTrz than those of
3CzCzTrz resulted in a better quantum efficiency. Three TADF
device performances are summarized and shown in Table 2.
Electroluminescence (EL) spectra of TADF devices in Fig. 5
show that the EL peaks of 1CzCzTrz, 3CzCzTrz, and 13CzCzTrz
were at 473, 488, and 476 nm and the color coordinates were
(0.17, 0.24), (0.22, 0.36), and (0.17, 0.25) at 10% doping concen-
tration, respectively. The reason for blue-shifted emission by
the 1-position substitution of carbazole is that a more distorted
geometrical structure causes shortening of the conjugation length
of the molecule, as mentioned above. Additionally, the distortion
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In conclusion, we investigated an approach to resolve the
low QE issue of the linker free TADF emitters by applying
1-carbazolylcarbazole based donor units. The new donors of
Chem. Commun.
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