Spatial separation of a TADF sensitizer and fluorescent emitter with a core-dendron system to block the energy loss in deep blue organic light-emitting diodes

Article abstract of DOI:10.1039/c9tc03334c

An optimized, thermally activated delayed fluorescence-(TADF)-sensitized fluorescence (TSF) strategy is proposed to boost the electroluminescence efficiency of solution-processed deep blue organic light-emitting diodes (OLED). The TADF sensitizer and the

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PAPER
Nosang V. Myung, Yong-Ho Choa et al.
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JPolueransael odfoMnaotetraiadljsuCsht emmairsgtriynsC
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DOI: 10.1039/C9TC03334C
ARTICLE
Spatial separation of TADF sensitizer and fluorescent emitter with
core-dendron system to block the energy loss in deep blue organic
light-emitting diodes
Received 00th January 20xx,
Accepted 00th January 20xx
Dan Liu,a Kaiyong Sun, ab Guimin Zhao, a Jingyi Wei, a Jiaxin Duan, a Minghui Xia, a Wei Jiang* a and
Yueming Sun a
DOI: 10.1039/x0xx00000x
An optimized thermally activated delayed fluorescence-(TADF)-sensitized fluorescence (TSF) strategy is proposed to boost
the electroluminescent efficiency of the solution-processed deep blue organic light-emitting diodes(OLED). The TADF
sensitizer and the fluorescent emitter with core-dendron structure emit mostly by a Förster energy transfer process while
the Dexter energy transfer is suppressed by spatial separation. In addition, high-bandgap carbazole terminal substituents
are introduced to protect the hyper-fluorescent core by a cascade energy transfer channel. Fluorescent OLEDs based on
core-dendron system achieved a deep-blue emission with Commission Internationale de L’Eclairage (CIE) coordinates of
(0.15, 0.14) and a high external quantum efficiency of 10.16%. The new TSF system can be used in high efficiency fluorescent
OLEDs with 100% exciton utilization, thereby promoting the development of solution-processed blue OLEDs.
can be used.18,19 Tang et al. designed molecules with
aggregation-induced emission (AIE). AIE uniquely increases the
emission efficiency in film state compared to solution state.20-
1. Introduction
Organic luminescent materials have attracted much attention
due to their wide application potential for organic light
emitting transistor and sensors, especially organic light-
emitting diodes (OLEDs).1-6 According to common equation
휂퐸푄퐸 = (훾 × 휂훾 × Φ푃퐿) × 휂표푢푡 , the photoluminescence (PL)
quantum yield (ɸPL) of the emitter molecules in film state
greatly influences the external quantum efficiency (ηEQE) of the
OLEDs. However, most fluorescent chromophores have good
luminescence properties at low concentrations, but the
fluorescence is significantly attenuated at high concentrations
or in thin film. This is called aggregation-caused quenching
(ACQ).7-9 Highly efficient organic light-emitting materials have
been developed to avoid the concentration quenching effect
and improve the ɸPL of fluorophores in aggregation states.10-15
First, the host-guest doping technology can be employed to
reduce molecular collisions and further suppress fluorescence
25 Moreover, the modification of the core chromophores is an
important way to optimize the properties of the molecules.
Performance of the final material is adjusted by substituting
side groups in the core to limit emission quenching.26-32 Most
emissive materials undergo ACQ and AIE remains difficult.
Therefore, core-dendron system (CDS) is a promising route to
modify the core chromophores and fabricate solution-
processed OLEDs.33-35 The process can be controlled by several
parameters. (1) Reducing accumulation of molecules when
using bulky side groups to limit emission quenching. (2)
Controlling the electron/hole injection and the transport
properties, when using dendritic groups with specific electron-
donating and electron withdrawing properties. (3) Influencing
molecular orientation to increase outcoupling effect. (4)
Increasing solubility when introducing alkyl chains.
Many dendrimers have been synthesized with this chemical
modification and the performance of the OLED device was
improved. 36-37 However, blue and deep blue dendrimers are
still a challenge. Compared to phosphorescent and TADF cores,
traditional fluorophores are still ideal blue light materials to
develop blue dendrimers, such as anthracene, oligomeric
fluorene, bis(styryl) arylene and pyrene derivatives. These
molecules have high fluorescence quantum yield and good
color purity. By applying CDS and encapsulating with bulky
groups, the ɸPL of the fluorescent dendrimers is expected to
improve in film state. However, the efficiency of radiative
exciton production (ηγ) still severely limits ηEQE in fluorescent
quenching by diluting the guest concentration.16,
Additionally, a series of chemical engineering methods to
inhibit the aggregation quenching of luminescent molecules
17
a
Jiangsu Province Hi-Tech Key Laboratory for Bio-Medical Research, Jiangsu
Engineering Laboratory of Smart Carbon-Rich Materials and Device, School of
Chemistry and Chemical Engineering, Southeast University, Nanjing, 211189, China
b School of Chemistry and Chemical Engineering, Yancheng Institute of Technology,
Yancheng, Jiangsu 224051, China
Email: jiangw@seu.edu.cn (Wei Jiang)
†Electronic Supplementary Information (ESI) available: Synthesis and
characterization details, NMR spectra of compounds, CV curves, TGA curves, XRD
spectra, AFM images. photophysical properties and supplementary device
performance of PY and PY-Cz. See DOI: 10.1039/x0xx00000x
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OLEDs if the 75% of electrically generated excitons formed in
the triplet state are not harvested under electrical excitation.
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DOI: 10.1039/C9TC03334C
38, 39
Therefore, many studies were focused on developing
singlet-exciton-harvesting fluorescent OLEDs using energy
transfer processes to address the 25% internal quantum
efficiency threshold.40-46 TADF-sensitized fluorescence (TSF)
with traditional fluorescent emitter is a favorable method. A
schematic diagram of TSF system is presented in Figure 1.
Ideally, all electrically generated singlet-and triplet-excitons of
the TADF host are directly transferred into the singlet state of
the fluorescent emitter through a Förster energy transfer,
which results in efficient radiative decay. However, the triplets
of the host may be transferred to the triplet state of the
dopant via Dexter transfer and non-radiative decay can occur
(Figure 1a). This serious eciton quenching is not only caused by
the host-host interaction, but also by host-guest interaction. It
inevitably presents in traditional host-guest systems and was
usually ignored in the previous studies. Therefore, to optimize
THS, the quenching of the triplet excitons must be suppressed,
which means Dexter transfer from electron exchange
interaction must be prevented.
Figure 1 A schematic diagram describing the singlet-exciton-harvesting methods of a)
traditional TADF host and traditional fluorescent guest and b) TADF dendrimer host
and fluorescent dendrimer guest.
synthesized and studied to highlight the properties of
dendrimers.
Herein, we propose a novel TSF system by applying CDS. The
short-range Dexter energy transfer can be effectively blocked
by increasing the intermolecular distance.40 In CDS, the
encapsulation of host and guest with the alkyl chain linked
dendrons restricts the intermolecular interactions between
the emissive cores and makes the molecules more suitable for
solution processing.47 Because of the increased distance
between the cores, potential exciton quenching pathways,
such as triplet-triplet annihilation (TTA) and triplet-polaron
annihilation (TPA), were blocked, as shown in Figure 1b.
Consequently, more triplet excitons can be up-converted for
the radiative transition. Based on this energy transfer, we
designed and synthesized 2,4,6-tris(3-(6-(9H-carbazol-9-
2. Results and Discussion
2.1 Synthesis and Electrochemical Properties
The synthetic routes of the TADF hosts (3CzBN and 3CzBN-Cz)
and the fluorescent guests (PY, PY-Cz) are illustrated in Scheme
1 and in the supplementary information. The fluorescent
guests PY and PY-Cz are synthesized through Suzuki aryl-
coupling reaction with a yield around 70%. 3CzBN-Cz is
synthesized by lithium-halogen exchange reaction and
aromatic nucleophilic substitution reactions, that avoid the
addition of oxygen and change of the energy gap of the core.
Before device fabrication, the target product was separated
and purified by silica gel column chromatography and
1
recrystallization. H NMR, 13C NMR, mass spectrometry, and
yl)hexyl)-9H-carbazol-9-yl)benzonitrile
(3CzBN-Cz)
TADF
elemental analysis were used to confirm the chemical
structure of the new compounds. Subsequently, the
electrochemical properties of the materials were examined by
cyclic voltammetry (CV). The HOMO energy levels of PY, PY-Cz,
3CzBN and 3CzBN-Cz are obtained from the onset of oxidation
molecules as the host and 1,3,6,8-tetrakis(4-((6-(9H-carbazol-
9-yl)hexyl)oxy)phenyl)pyrene (PY-Cz) as the fluorescent
emitter dopants. We subsequently demonstrate an efficient
deep-blue OLED with the maximum external quantum
efficiency of 10.16%. For comparison, the corresponding small
molecules of 2,4,6-tri(9H-carbazol-9-yl)benzonitrile (3CzBN)
and 1,3,6,8-tetrakis(4-methoxyphenyl)pyrene (PY) were also
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(Figure S1). the LUMO energy levels are calculated using the
following equation: ELUMO = EHOMO + Eg. The experimental datas
of HOMO and LUMO levels were listed in Table 1, which can
match well with the theoretical simulations described later.
slightly more delocalized than the LUMOsViewaAnrdticle hOanlvinee
appreciable amplitude on the oxyphDeOnIy:l1e0n.1e039m/Co9ieTCti0e3s3,34aCs
shown in Figure 2. The large orbital overlap of the HOMO and
LUMO are beneficial to the radiative transition. In addition, the
HOMOs and LUMOs of 3CzBN-Cz were in agreement with
those of 3CzBN. The LUMOs are distributed primarily over the
benzonitrile groups, whereas, the HOMOs are delocalized on
the carbazole units. 3CzBN and 3CzBN-Cz have a separated
HOMO/LUMO distribution and a small energy gap (ΔEst)
between the lowest excited singlet state (S1) and the lowest
excited triplet state (T1), which can produce reverse
intersystem crossing (RISC). The theoretical calculated
HOMO/LUMO energy levels of PY-Cz and 3CzBN-Cz are -4.76
eV/-1.52 eV and -5.29/-1.98 eV (Table S1). This is similar to the
values for PY (-4.74/-1.48eV) and 3CzBN (-5.65/-1.92 eV). Then
the calculated S1/T1/ΔEst values of PY, PY-Cz, 3CzBN and
3CzBN-Cz are 3.03/1.87/1.16 eV, 3.00/1.86/1.14 eV,
3.09/2.78/0.31 eV and 3.02/2.81/0.21eV, respectively, which
are close to the values estimated from the onset of
fluorescence and phosphorescence spectra. Notably, small
ΔEst of 3CzBN and 3CzBN-Cz ensures the efficient RISC and,
subsequently, efficient TADF emission.
2.2 Thermal Properties and Film Forming Abilities
To investigate the influence of their thermal properties on the
morphological stability and device performance, thermal
gravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were carried out at a heating rate of 10 °C
min-1 under a nitrogen atmosphere. The good thermal stability
of dendrimers is indicated by the high decomposition
temperatures (Figure S2), and the values of Td corresponding
to 5% weight loss are 413°C and 432°C for PY-Cz and 3CzBN-Cz,
respectively. These values are consistent with and slightly
higher than the ones for small molecules PY (385°C) and 3CzBN
(401°C).48 Moreover, the values of Td and Tg are listed in Table
1. The Tg of PY-Cz and 3CzBN-Cz are reduced due to the
introduction of flexible chains. The high Td and Tg of the
dendrimers ensure the high quality film through the wet
process and guarantee the long term reliability of the devices.
In addition, the film forming ability of the encapsulated
materials is also important for the performance of solution-
processed OLEDs. The solution-processed thin films were
analysed by atomic force microscope (AFM). Figure S3
indicates that the PY-Cz and 3CzBN-Cz films are greatly smooth
with root-mean-square (RMS) values of 0.37 nm and 0.36 nm.
which are obviously better than PY (0.49 nm) and 3CzBN (0.50
nm). It can be attributed to the introducing flexible linkers.
Furthermore, X-ray diffraction (XRD) analysis of PY, PY-Cz,
3CzBN and 3CzBN-Cz was carried out. The sharp peaks in the
diffraction pattern of PY and 3CzBN (Figure S4) prove the
existence of crystalline forms. In contrast, no peaks were
observed in the XRD spectra of PY-Cz and 3CzBN-Cz, indicating
that the flexible chain-linked carbazole units reduce the
tendency to crystallize and inhibit molecular aggregation.
Figure 3 The molecular packing morphologies of mixtures of host and guest by Molclus
program simulations and the curve of number of neighbors versus distance of center of
mass.
In our optimized TSF system, by blending the fluorescent
emitters and the TADF materials, the ΔEst of the host and the
PL of the dopant are no longer linked factors and can be
optimized separately. In addition, Dexter transfer from T1 of
host to T1 of guest is suppressed due to the increased
distance.49 Dexter energy transfer rate constant KDEX is
expressed as:
2.3 Theoretical Calculations and analysis
The electronic properties of synthesized compounds were
investigated using density functional theory (DFT) calculations
at the B3LYP level of the theory with 6-31g(d) as the basis set.
The LUMOs are localized on the central pyrene core group and
partially on the oxyphenylene moieties and the HOMOs are
퐾퐷푒푥푡 푒푥푐ℎ푎푛ꢀ푒 = 퐾 ∙ 퐽퐷퐴 ∙ 푒푥푝 (− 2푅
)
퐷퐴
(
)
퐿
Where K is the interaction between related electron orbitals,
JDA is the integrated spectral overlap between the donor
emission spectrum and acceptor absorption spectrum, RDA is
the distance between the donor and the acceptor, and L is the
Van der Waals radius between them. According to the
equation, KDEX is inversely proportional to the distance of
donor and acceptor. And in general, Dexter energy transfer
distance (RDA) is small, within 10-15Å.50 Using this theoretical
foundation, we constructed host-guest molecular clusters.
More than 100 structures were generated using the Molclus
program and adopted as initial structures for the semi-
empirical quantum mechanical optimization at PM7 level using
MOPAC2016.51,52 Then we calculated the distance between
the centers of mass of the host and guest. 51 Figure 3 indicates
Figure 2 HOMO and LUMO electronic distributions for (a) PY, (b) PY-Cz, (c) 3CzBN and
(d) 3CzBN-Cz at the B3LYP/6-31G(d) level.
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128휋5푛4푁퐴
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Figure 4 Absorption, fluorescence and phosphorescence spectra of PY (a), PY-Cz (b), 3CzBN (c) and 3CzBN-Cz (d) in toluene, measured in dilute solution with solutes
concentration of 1×10-5 M, and set 10 ms delayed time for the Phosphorescence spectrum.
that the distance between the centers of mass (rmin = 16.3Å) of
3CzBN-Cz and that of PY-Cz is significantly larger than those
(rmin = 10.6 Å) between 3CzBN and PY. This means that the
Dexter energy transfer is most likely very reduced in the
3CzBN-Cz:PY-Cz blending system. In addition, the number of
neighbors in core-dendron blending system is lower for a given
distance, indicating that the probability of intermolecular
collisions and electron exchange is remarkably reduced.
Therefore, the alkyl linked carbazole dendron makes it
possible to achieve spatial separation (increased RDA) and
reduce the collision probability between the triplet excitons
(reduced KDext).
The Förster radius (R0) can be used to measure the transfer
efficiency as:
푅06 =
Φ
휅2퐽퐷퐴
푃퐿
Therefore, the ΦPL of the TADF host and the spectral overlap
JDA should be high to achieve effective Förster energy transfer.
In conclusion, our calculations suggest that the use of TADF
host-sensitized fluorescent emitters and the encapsulation of
cores can break the 25% IQE and maximally utilize excitons via
efficient Förster transfer rather than Dexter transfer.
2.4 Photophysics Properties
In addition, the spectral overlap JDA affects the Dexter
energy transfer efficiency and the Förster energy efficiency.53
Table1 Basic photophysical and electrochemical parameters of PY, PY-Cz, 3CzBN and 3CzBN-Cz.
Materials
PY
Td, Tg [℃]
385,113
413,82
λabs [nm]a)
346, 391
346, 392
331, 348
333, 354
λem [nm]b)
431
Eg [eV]c)
2.94
S1/T1 [eV]d,e)
3.08/1.95
3.06/1.96
3.20/2.87
3.18/2.88
ΔEst [eV]f)
1.13
HOMO [eV]g)
-5.47
LUMO [eV]h)
-2.53
PY-Cz
431
2.94
1.10
-5.50
-2.56
3CzBN
3CzBN-Cz
401,117
432,85
415
3.19
0.23
-5.75
-2.56
423
3.10
0.30
-5.74
-2.64
a) Measured in toluene solution at 300 K. b) Measured in toluene solution at 300 K. c) Estimated from the absorption edges in toluene. d) Calculated from the onset of the
fluorescence spectra in toluene at 300 K. e) Estimated from onset of the phosphorescence spectra in toluene at 77 K. f) The difference between S1 energy and T1 energy. g)
Determined by the CV measurement. h) Calculated from the energy gap and HOMO.
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The UV-Visible absorption and photoluminescence (PL)
spectra of the materials were measured (Figure 4 and Figure
S5). Intense absorption bands were observed in toluene for all
materials. The lowest absorption bands of PY and PY-Cz are
located at 395 nm, which is ascribed to the π-π* transitions of
pyrene. The emission band peaks at 431 nm are attributed to
vibronic structures. The absorption and emission spectra show
a good mirror-image relationship. 3CzBN and 3CzBN-Cz have a
intensity when water fraction increased graduallVyiefwroArmticle0O%nlitnoe
100%. The decrease is due to the formatDioOnI:o10f .a10g3g9r/eCg9aTtCe0s3f3r3o4mC
ACQ. When PY was encapsulated with alkyl chain linked
carbazole (PY-Cz), the ACQ was suppressed to a certain extent.
At the same concentration of 1×10-5M, PY-Cz shows a stronger
photoluminescence intensity than PY, as shown in Figure 5b.
Therefore, the dendrimer structure can suppress molecular
stacking and emission quenching because of the peripheral
Figure 5 Fluorescence spectra of PY (a) and PY-Cz (b) (10-5 M) in tetrahydrofuran and water mixtures with the volume fractions of water are 0%, 10%, 20%, 30%, 40%, 50%, 60%,
70%, 80%, 90%, 100% (Fluorescence images of PY upon excitation with 365 nm light source). (c) Schematic diagram of film emission in the presence and absence of stacking. PL
spectra of PY (left) and PY-Cz (right) in solution (10-5mol L-1 in CH2Cl2) and in neat film.
lowest absorption band around 360 nm due to the π–π*
transitions of carbazoles. The phosphorescence spectra at 77K
were measured in toluene to obtain T1 energy. For PY and PY-
Cz, no long-lived phosphorescence emission was directly
observed after 10 ms. This suggests that the intersystem
crossing (ISC) rate from S1 to T1 may be much lower than the
radiative rate of S1, which leads to an invisible
phosphorescence spectrum. We further attempted to
sensitize the T1 state using the PO-01 phosphorescent dye with
a low ET1 of 2.30 eV. New emission peaks at 636 nm and 632
nm were observed in the delayed phosphorescence spectrum.
They were assigned to the T1 state of PY and PY-Cz. The
measurements clearly showed the existence of much lower T1
energy levels of 1.95 eV and 1.96 eV for PY and PY-Cz,
respectively. Then ΔEst values were calculated as 1.13 eV and
1.10 eV. In addition, traditional blue fluorescent materials
knowingly always suffer from serious aggregation-caused
quenching (ACQ) due to conjugated structure and π-π*
stacking. Figure 5 shows that PY also had a strong emission in
diluted tetrahydrofuran (THF) with a significantly decreasing
carbazole shield. It is even more pronounced in the film state
(Figure 5c). PY showed PL quenching caused by intermolecular
interactions in the film state, and PY-Cz showed improved PL
intensity because of the disruption of intermolecular stacking
from the substitution of the bulky side groups on the PY core.
The photoluminescence quantum yield (PLQY) measured
provided further evidence of the enhanced PL. In the film
states, PY-Cz had a high PLQY of 86% significantly higher than
for PY (32%). Apart from the improved PLQY, the spectral
overlap between the host and these molecules is also
important for the TSF system. The TADF host materials 3CzBN
and 3CzBN-Cz were designed and synthesized to sensitize PY
and PY-Cz, respectively. Figure S5 shows that 3CzBN-Cz had a
deeper emission peak (435 nm) than 3CzBN (450nm) due to
restricted molecular aggregation in neat film state. It was also
reported adding the carbazole dendrons enhances the PLQY of
the TADF materials. The PLQY of the 3CzBN and 3CzBN-Cz films
were 45% and 63%, respectively. In addition, UV-Visible
absorption spectra of the PY-Cz film and the PL spectra of the
3CzBN-Cz film exhibit a larger spectral overlap between the
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Figure 6 (a) The PL spectra of the 3CzBN-Cz:PY-Cz films. (b) The transient PL decay curves of the 3CzBN-Cz:PY-Cz films observed at 439 nm.
donor emission spectrum and the acceptor absorption
spectrum. From the integration of the emission and
absorption spectra, 3CzBN-Cz had the larger JDA (0.18) than
3CzBN (0.12), along with a deeper emission peak at 439 nm
(shown in Figure S5). It confirms that encapsulation of
fluorescence cores improved the PLQY and the energy Förster
transfer efficiency.
enhancement of the Forster energy transfer and demonstrate
the TADF properties of 3CzBN-Cz. In the presence of oxygen,
3CzBN and 3CzBN-Cz possess short lifetimes at the scale of
tens of nanoseconds. However, after deoxygenation of the
solutions by bubbling nitrogen, the delayed PL decay
components are observed for these compounds, which imply
that the triplet excitons of the emitters are involved in the
radiative decay process. The values of PLQY, the prompt and
delayed lifetimes and kinetic parameters including the
The time-resolved photoluminescence spectra and the PLQY
in the film state were measured to further confirm the
Figure 7 (a) Energy-level arrangement of the organic layers in the OLEDs devices. (b) Chemical structures of the organic materials used in the OLED devices (c) The EL spectra of
Device A from 7V to 18V (Inset: the CIE of Device A in different voltage). (d) The EL spectra of Device B from 7V to 18V (Inset: the CIE of Device A in different voltage).
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Figure 8 (a) Current density-voltage-luminance (J-V-L) characteristics; (d) Current efficiencies and power efficiencies versus luminance plots. (c) The EL spectra of Device
B in different voltage. (d) The EL spectra of Device B in different voltage.
radiative decay rate are summarized in Table S2. Because of
the encapsulation effect by the peripheral carbazole, the
2.5 Electroluminescent Properties
lifetime of 3CzBN-Cz is 2.3
μs and is larger than that of 3CzBN
Given the efficient Förster energy transfer obtained, the
performance of devices with self-encapsulation materials was
measured. The solution-processed OLED devices were
(1.8 s). Figure 6 and Figure S6 shows the evolution of PL
μ
spectra of 3CzBN: PY and 3CzBN-Cz:PY-Cz with the doping
concentration. When the doping concentration increases, the
PL emission intensity of 3CzBN-Cz gradually decreases and the
PL emission of PY-Cz with the vibrational structure appears.
This is especially clear for a doping concentration of 8% where
the fluorescent shoulder peak of PY-Cz increased, which
indicates the efficient energy transfer from the host to the
dopant. A similar trend was also observed in the 3CzBN: PY film
(Figure S6). The PLQY of the 3CzBN:PY film first increases from
0.30 to 0.52, then decrease to 0.43 with the increasing doping
rate. The optimal doping concentration is 10%. Similarly, by
applying CDS, the best doping concentration for 3CzBN-Cz:PY-
Cz is 10%, which corresponds to a maximum PLQY value of 0.87.
The prompt and delayed photoluminescence spectra of the
doped films were also measured to investigate the energy
transfer process. The delay component of the 3CzBN: PY and
3CzBN-Cz: PY-Cz photoluminescence decay curves gradually
when the doping concentration increased. This indicates that
the energy can efficiently transfer from the TADF hosts to the
fluorescent dopants by Förster energy transfer.45 As a result,
the encapsulation of the host and the guest facilitates the
efficient energy-transfer and yields a high PLQY, which is useful
for the optimization of the device efficiency.
fabricated
using
the
device
structure
of
ITO/PEDOT:PSS(40nm)/EML(60nm)/TPBI(40nm)/Cs2CO3(2nm)
/Al, as shown in Figure 7. First, non-doped devices using
pristine films were fabricated and characterized. Figure 7c
shows the electroluminescent (EL) spectra of devices using PY
that exhibit additional emissions at longer wavelengths, which
were attributed to the excimer formed by the excited and
ground states of the PY molecule. The intensity of emission at
longer wavelength gradually increased with the driving voltage.
The emission color depends on the driving voltage and
changes from deep blue to yellow. After encapsulation by alkyl
chained carbazoles, the EL spectra of PY-Cz only showed a
slight emission at longer wavelengths (Figure 7d). Although the
excimer still exists, the intensity of the excimer emission was
effectively suppressed, which fully demonstrates that the
molecular aggregation and the formation of excimers were
significantly reduced in the encapsulated system. Furthermore,
the performance of the device using PY-Cz was also improved
with an increased luminance and a higher efficiency, as shown
in Figure S7. It indicates that exciton utilization efficiency was
improved and that exciton quenching was suppressed from
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DOI: 10.1039/C9TC03334C
Table 2 Device performances of the solution-processed OLEDs based on PY, PY-Cz, 3CzBN:PY, 3CzBN-Cz:PY-Cz emissive layer.
a
b
c
d
f
Device
A
emitter
PY
Von
EL(nm)
CEmax
PEmax
EQEmax
CEe
Lmax
CIE(x,y)g
6.3
450, 532
0.26
0.07
0.23
0.14/0.20
1000
(0.21,0.28)
B
C
PY-Cz
3CzBN:PY
4.0
4.5
472
460
1.05
2.71
0.52
1.70
0.96
2.02
0.99/0.85
2.40/2.12
1500
3200
(0.16,0.17)
(0.16,0.13)
D
3CzBN-Cz:PY-Cz
4.1
468
13.92
7.60
10.16
12.40/8.61
6200
(0.15,0.14)
aVon = turn-on voltage at 1cd m−2
,
bCEmax = maximum current efficiency, cPEmax = maximum power efficiency, dEQEmax = maximum external quantum efficiency, eCurrent
efficiency at the luminance of 100 cd m−2, 1000 cd m−2, fLmax = maximum luminance, gCIE = the Commission Internationale de L’Eclairage coordinates at 10 V
the effect of the peripheral carbazole branches. Even so, the
exciton utilization is only 25% under electric excitation
because the other 75% excitons are injected into the triplet
state of the traditional fluorophore and radiation transition is
forbidden.
In summary, we have demonstrated a novel technique to
improve the efficiency of fluorescent OLEDs. By combining
core-dendron system and TADF-sensitized fluorescence
system, Förster energy-transfer from the host to the guest was
efficiently enhanced whereas Dexter energy transfer was
suppressed by spatially separating the fluorescent guest and
the TADF host. Because of the suppressed short-range
electron exchange, the triplet-triplet annihilation of the
fluorescent guest is blocked and the resulting deep-blue OLEDs
had high external quantum efficiency of 10.16% with a stable
color purity (FWHM = 65 nm). Our work provides a practical
strategy to develop high efficient deep blue emitters, and
highly-efficient deep blue OLEDs when TADF dendrimers with
To verify the better performance of the TADF dendrimer in
the doped device, two types of devices with different emitting
layers (EMLs) were compared: (C) 3CzBN: PY and (D) 3CzBN-
Cz:PY-Cz. The optimized doping concentrations of the EML was
10 wt%. Figure 8 shows the emission from the PY-Cz (PY)
dopant in the EL spectra and the low hardly visible emission
from 3CzBN-Cz (3CzBN), which indicates the complete energy
transfer from the host to the guest. In addition, excimer
formation is prevented and the emission at longer wavelength
disappears in the TSF system, especially in the optimized
system using encapsulated materials. The 3CzBN-Cz:PY-Cz
based device exhibited an improved color purity with
Commission International de I’Eclarirage (CIE) coordinates of
(0.15, 0.14) due to the reduced host-host and host-guest
a
smaller ΔEST are combined with hyper-fluorescent
dendrimers.
Conflicts of interest
There are no conflicts to declare.
intermolecular interactions. Figure
8 shows that the
performance of Device C based on 3CzBN:PY emissive layer
and Device D using the 3CzBN-Cz:PY-Cz emissive layer. Device
C exhibits a maximum luminance of current efficiency (CE) of
2.71 cd/A and external quantum efficiency (EQE) of 2.06%.
They are higher than for Device A due to triplet harvesting
through the TADF process. Encouragingly, by applying CDS, the
EQE of Device D is above 5-fold higher than those of Device C.
This further confirms the efficient Forster energy transfer
through the spatial separation of the TADF hosts and the
fluorescent guests, and that the energy loss via TTA, TPA of
host-host interaction, and host-guest interaction quenching is
efficiently blocked. A cascade energy transfer channel formed
in Device D due to high-bandgap carbazole branches, and
excitons can flow to the low-energy fluorescent cores and
radiate light. Meanwhile the FWHM of the devices based on
optimized TSF system is only 65 nm for driving voltage within
7-18 V. It is narrower than for the devices using TADF materials
(typically 80-120 nm).54 This shows the superiority of the
optimized TADF-sensitized fluorescence system.
Acknowledgements
We are grateful for the grants from the National Natural
Science Foundation of China (21875036 and 51103023), the
Fundamental Research Funds for the Central Universities
(2242016K41082), We are also thankful for the National Basic
Research Program (973 program, 2013CB932902), the Priority
Academic Program Development of Jiangsu Higher Education
Institutions, Nanjing science and technology committee (2014-
030002).
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Spatial separation of TADF sensitizer and fluorescent emitter improve the energy transfer efVfiiecwiAertniclceyOnline
DOI: 10.1039/C9TC03334C
by suppressing energy loss and further enhance electroluminescent performance.
TOC
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