Thermally Activated Delayed Fluorescence from Azasiline Based Intramolecular Charge-Transfer Emitter (DTPDDA) and a Highly Efficient Blue Light Emitting Diode

Article abstract of DOI:10.1021/acs.chemmater.5b02515

For electroluminescence with delayed fluorescence, the azasiline unit has been introduced for the first time as a donor in a thermally activated delayed fluorescence (TADF) material. The TADF material (DTPDDA) shows strong intramolecular charge transfer (CT) character with large spatial separation with the acceptor of triazine leading to narrow splitting of singlet and triplet excited states for the efficient reverse intersystem crossing (RISC). A blue organic light emitting diode (OLED) based on DTPDDA not only displays deep blue in the Commission Internationale de L'Eclairage (CIE) coordinates of (0.149, 0.197) but also exhibits a high external quantum efficiency (EQE) of 22.3% which is the highest value ever reported for a blue fluorescent OLED. Theoretical prediction based on transient photoluminescence (PL) and optical simulation result agrees well with the achieved EQE indicating the successful conversion of triplet excitons to singlet in the blue fluorescent OLED by using DTPDDA.

Full text of DOI:10.1021/acs.chemmater.5b02515

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Article
Thermally Activated Delayed Fluorescence from
Azasiline Based Intramolecular Charge-Transfer Emitter
(DTPDDA) and a Highly Efficient Blue Light Emitting Diode
Jin Won Sun, Jang Yeol Baek, Kwon-Hyeon Kim, Chang-Ki Moon,
Jeong-Hwan Lee, Soon-Ki Kwon, Yun-Hi Kim, and Jang-Joo Kim
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02515 • Publication Date (Web): 14 Sep 2015
Downloaded from http://pubs.acs.org on September 16, 2015
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Thermally Activated Delayed Fluorescence from Azasiline
Based Intramolecular Charge-Transfer Emitter (DTPDDA) and
a Highly Efficient Blue Light Emitting Diode
Jin Won Sun‡, Jang Yeol Baek†, Kwon-Hyeon Kim‡, Chang-Ki Moon‡, Jeong-Hwan Lee‡, Soon-Ki
Kwon†, Yun-Hi Kim¶* and Jang-Joo Kim‡*
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‡Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National
University, Seoul 151-744, South Korea
†School of Materials Science and Engineering, Gyeongsang National University, Jinju, 660-701, South Korea
¶Department of Chemistry and Research Institute for Green Energy Convergence Technology (RIGET), Gyeongsang
National University, Jinju, 660-701, South Korea.
ABSTRACT: For electroluminescence with delayed fluorescence, the azasiline unit has been introduced for the first time
as a donor in a thermally activated delayed fluorescence (TADF) material. The TADF material (DTPDDA) shows strong
intramolecular charge transfer (CT) character with large spatial separation with the acceptor of triazine leading to narrow
splitting of singlet and triplet excited states for the efficient reverse intersystem crossing (RISC). A blue organic light
emitting diode (OLED) based on DTPDDA not only displays deep blue in the Commission Internationale de L’Eclairage
(CIE) coordinates of (0.149, 0.197) but also exhibits a high external quantum efficiency (EQE) of 22.3% which is the high-
est value ever reported for a blue fluorescent OLED. Theoretical prediction based on transient photoluminescence (PL)
and optical simulation result agrees well with the achieved EQE indicating the successful conversion of triplet excitons to
singlet in the blue fluorescent OLED by using DTPDDA.
clearing stability issues. Especially, blue dyes among the
three primary colors have been considered as the most
■ INTRODUCTION
Enhancing electroluminescence (EL) efficiency might be
crucial ones due to their important role in generating
never ending pursuit from displays to lightings where
white light with good color purity. So far, blue fluorescent
organic light emitting diodes (OLEDs) are being used. For
materials are being used for commercial OLEDs in spite
past years, phosphorescent OLEDs (PhOLEDs) based on
of their low efficiency due to short device lifetime of
heavy metal complexes have been considered as an only
phosphorescent blue OLEDs. Even with such importance,
solution to realize high efficiencies by harvesting both
there are only a few reports on efficient blue TADF
singlet and triplet excitons as light.1-7 However, recently
materials which meet both the high EL efficiency and
reported OLEDs utilizing delayed fluorescence challenged
color purity up to now. TADF materials are charge
the conventional idea of achieving high EL efficiencies
transfer (CT) type emitters which are composed of
and has demonstrated an equal performance to
electron donating and accepting units to have small but a
PhOLEDs by achieving 30% external quantum efficiency
certain degree of overlap between the highest occupied
(EQE).8-11 There are two different mechanisms that drive
molecular orbital (HOMO) and the lowest unoccupied
delayed
fluorescence,
which
are
triplet-triplet
molecular orbital (LUMO) to minimize the singlet-triplet
energy gap, yet still to have large enough radiative
transition rate. Carbazole and dimethyl dihydroacridine
have been mainly used as the donor moieties and sulfone
oxide and triazine as the acceptor unit in the blue TADF
molecules. However, it seems quite challenging to search
for an appropriate combination of donor and acceptor
moieties to achieve good color purity and high EL
efficiency for blue fluorescent OLEDs at the same time.
Even a highly efficient blue fluorescent OLED of 20.6%
annihilation (TTA) and thermally activated delayed
fluorescence (TADF).12-13 Fluorescent materials showing
TADF enables additional harvest of triplet excitons as well
as singlet excitons to light via thermally activated reverse
intersystem crossing (RISC) from triplet (T1) to singlet (S1)
state due to small charge transfer singlet-triplet state
splitting (∆EST ).
Replacing phosphorescent materials by efficient TADF
materials will eventually lower cost with the potential of
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was reported recently yet the OLED displayed sky-blue
light with the CIE coordinates of (0.19, 0.35).
anti-bonding lobes located on the butadiene fragments
adjacent to the silicon bridge are separated further from
each other, which ultimately should lead to a reduction of
the HOMO energy level.22 The reduction of HOMO
energy level can increase band gap, leading to deep blue
emission.
Prior to synthesis of the blue fluorescent material with
the chemical structure described in Figure 1a, the
quantum chemical properties of the molecule have been
investigated through the density functional theory (DFT)
calculation. Firstly, the geometry optimization (Figure 1b)
and the energy levels of the compound and Kohn-Sham
orbitals of the HOMO (Figure 1c) and the LUMO states
(Figure 1d) were calculated through the DFT calculation.
Then, Singlet and triplet energies of DTPDDA were
calculated with optimized geometry using the time-
dependent(TD)-DFT. The details of calculation method
were explained in experimetal section. The result showed
that the HOMO and LUMO of DTPDDA are separated
distinctively on two different units: the phenyl-dibenzo
azasiline and the triphenyl triazine, respectively, by
distorting 92~93° of dihedral angle. The TD-DFT
calculation predicted the ∆EST of 0.01 eV, indicating that
the separation of Kohn-Sham orbitals of HOMO and
LUMO lowered the exchange energy to enhance the RISC.
However, the HOMO and LUMO levels were weakly
overlapped in the phenyl unit, which is known to increase
radiative transition rate.8 As the HOMO of DTPDDA is
distributed over the azasiline unit while the LUMO
resides at the triphenyl triazine unit at the other side of
DTPDDA molecule, the replacement of a carbon bridge
with silicon in the phenyl-dibenzo azasiline resulted in
deepening the HOMO level of DTPDDA consequently
enlarging the bandgap of DTPDDA, which is desired for
blue emission. To further demonstrate the benefit of hav-
ing azasiline unit, HOMO and LUMO levels of DTPDDA
and 2-phenoxazine-4,6-diphenyl-1,3,5-triazine (PXZ-TRZ)
were compared through DFT calculation.23 DTPDDA and
PXZ-TRZ have same acceptor unit as diphenyl-triazine,
but different donor units. DTPDDA with an azasiline unit
has 0.5 eV lower HOMO level than PXZ-TRZ with phe-
noxazine unit owing to a silicon bridge in the moiety.
(Table S1)
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Figure 1. (a) Chemical structure of DTPDDA. DFT
calculation data: (b) Molecular structure of DTPDDA. (c)
HOMO level at the phenyl-dibenzo azasiline unit. (d)
LUMO level at the triphenyl triazine unit.
The highest EQE ever reported in deep blue fluorescent
OLEDs, in terms of CIE Y value below 0.2, was ~19.5%,
which is still lower than the achieved EQE in this work.14-
19
In this article, we have utilized a new donor unit of
azasiline group to synthesize a deep blue TADF dye, 5-(4-
(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10,10-diphenyl-
5,10-dihydrodibenzo[b,e][1,4]azasiline (DTPDDA). An
efficient blue fluorescent OLED with an unprecedented
high EQE of 22.3% was fabricated by doping the TADF
material in a mixed co-host system. The device emitted
deep blue with the CIE coordinates of (0.149, 0.197). The
comparison with the theoretical prediction of the EQE
indicated that the RISC is efficient in the molecule to
convert almost all the triplet excitons to the singlet
excitons and the device has little electrical loss.
■ RESULTS AND DISCUSSION
The azasiline has 6-membered heterocyclic structure
which is composed of a nitrogen atom, two aromatic
carbon-carbon π-bonds and a silicon atom. The azasiline
unit was introduced for the first time in this article as a
donor in the donor-acceptor type molecule to lower
HOMO energy level by using silicon connection instead
of carbon connection, thereby to enlarge the energy gap
between the HOMO and LUMO states for blue
emission.20 Also, the azasiline unit is spatially separated
from the triazine unit, leading to the low ∆EST.
Overall, from the DFT calculation, DTPDDA was
expected to be an efficient CT type emitter for blue
fluorescent OLEDs.
The introduction of silicon atom in a bridge between
adjacent aromatic groups has several beneficial effects.
Unlike carbazole unit, the azasiline unit has more rigidity
and bulkiness resulting in large dihedral angle due to its
sp3 bridge. Rigid tetravalent silicon bridge structure offers
morphological stability and reduces the conformation
disorder, which can increase efficiency due to decrease in
vibronic transitions.21 Moreover, since silicon-carbon
bond is slightly longer than a carbon-carbon bond, the
The synthetic scheme of the new blue emitting material is
shown in Scheme 1. The Buchwald-Hartwig amination
was carried out to obtain bis(2-bromophenylamine).24-26
The proton of amine group was blocked by 4-
methoxybenzyl group, and the cyclization for azasiline
was carried out by nucleophilic substitution of dilithiated
compound.
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Scheme 1. Synthetic Scheme of DTPDDA.
After deblocking reaction, the new emitting material, 5-
(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10,10-diphenyl-
5,10-dihydrodibenzo[b,e][1,4]azasiline (DTPDDA), was
obtained by the Buchwald-Hartwig amination between 2-
(4-bromophenyl)-4,6-diphenyl-1,3,5-triazine and 10,10-
diphenyl-5,10-dihydrodibenzo[b,e][1,4]azasiline.
and TSPO1.28 The PL spectra of the DTPDDA doped films
The basic synthetic scheme of azasiline unit as well as the
material itself has never been introduced nor considered
for
an
application
for
OLED
materials
or
electroluminescence up to now to our best knowledge.
The phenomenon of cyclization forming diphenylsilynene
through desulfurization was reported back in 1960s yet
none of them demonstrated the same unit or the same
synthetic scheme of the blocking and deblocking
reaction.27 Furthermore, the spatial separation of the two
different moieties of the triazine and the azasiline units is
firstly reported in this study. DTPDDA displays good
thermal stability with the decomposition temperature (Td:
temperature at 5% weight loss to initial weight) close to
404°C and the high glass transition (Tg) of 325°C
determined from the thermal gravimetric analysis (TGA)
and differential scanning calorimetry (DSC) measurement
under nitrogen atmosphere (Figure S5-6). HOMO and
LUMO levels were calculated to be -5.57 eV and -2.80 eV
(HOMO = -[Eoxonset+4.43 eV], LUMO = -[Eredonset+4.43 eV] )
by considering oxidation onset potential (Eoxonset, +1.14 V)
and reduction onset potential (Eredonset, -1.63 V) (Figure S7,
Table S2). The PL spectra of DTPDDA exhibited
solvatochromic shifts as shown in Figure 2a assuring CT
nature of the material.
Figure 2. (a) The absorption spectrum of DTPDDA
measured in toluene and PL spectra of DTPDDA in
various solvents. Numbers in the brackets are the
dielectric constants of the solvents. (b) The PL spectra of
DTPDDA and TSPO1 measured in toluene and methylene
chloride, espectively. The rest of the PL spectra of mCP,
mCP:TSPO1, mCP:DTPDDA, TSPO1:DTPDDA and
mCP:TSPO1:16 wt% DTPDDA were measured with 50 nm
The mixed co-host of N,N′-dicarbazolyl-3,5-benzene
(mCP) and Diphenyl-4-triphenylsilylphenyl-phosphine
oxide (TSPO1) with molar ratio of 1:1 was selected as an
emitting layer (EML) for DTPDDA. Figure 2b shows the
photoluminescence (PL) spectra of the materials used in
this work. The resemblance of spectrum of mCP:TSPO1
film to mCP film can be understood from the mixed CT
and locally excited (LE) states of exciplexes between mCP
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thick films on 1 mm thick fused silicas. Inset: the chemical
structures of two hosts used for EML, mCP and TSPO1
that the delayed emission is coming from a thermally
activiated process consistent with the nature of TADF
material.
of mCP:DTPDDA, TSPO1:DTPDDA and mCP:TSPO1:16 wt%
DTPDDA showed the emission solely from DTPDDA
without the emission from the hosts, indicating that
energy transfer from the host molecules to the dopant is
efficient in this system. The photoluminescent quantum
yield (PLQY) of the mCP:TSPO1:16 wt% DTPDDA film
was 74 ± 2%.
The transient PL decays showed multiexponential decays
fitted with the prompt lifetime of 11.8 ns and three
delayed lifetimes of 100 ns, 2.3 μs and 25.4 μs at. The ratio
of the prompt to delayed emission intensity in PL
calculated from the decay pattern at RT is 0.13:0.87.
Emission dipoles of DTPDDA in the mCP:TSPO1:16 wt%
DTPDDA film are randomly oriented with the horizontal
transition dipole ratio (Θ) of 0.66 characterized by
theoretical fitting of the angle dependent p-polarized PL
spectrum shown in Figure S1.2,30-33
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Figure 3. (a) Prompt and delayed PL spectra of
mCP:TSPO1: 16 wt% DTPDDA at 35 K. Prompt and
delayed components were collected at 20 ns and 300 μs,
respectively. (b) Transient PL decays of mCP:TSPO1:16 wt%
DTPDDA film measured at 450 nm at temperature of 300
K and 35 K. Inset: Transient PL within the range of 0 to 50
ns after excitation. Red dotted line is the single
exponential fitting with decay time of 11.8 ns for the
prompt fluorescence.
Figure 4. (a) Schematic diagram of the device structure
and energy levels (eV) of the device. (b) Contour plot of
the predicted maximum EQEs as function of the
thickness of HIL and EIL for blue fluorescent OLEDs. A
star indicates the achieved EQE in this work.
TADF OLEDs were fabricated using mCP:TSPO1:16 wt%
DPTDDA as the emitting layer. The device structure was
ITO (70 nm) / 4 wt% ReO3:mCP (y nm) / mCP (15 nm) /
mCP:TSPO1:16 wt% DPTDDA (15 nm) (0.42:0.42:0.16 in
wt%) / TSPO1 (15 nm) / 4 wt% Rb2CO3:TSPO1 (x nm) / Al
(100 nm) shown in Figure 4a. The hole injection layer (p-
doped mCP, HIL) and electron injection layer (n-doped
TSPO1, EIL) were inserted in the device structure to
enhance charge injection, but assumed to have the same
refractive indices as the un-doped layers.34-39 Optical
simulation was performed using the classical dipole
Experimentally achieved ∆EST of DTPDDA was 0.14 eV
with S1 and T1 levels of 2.79 and 2.65 eV measured from
the prompt and delayed PL spectra (Figure 3a) at 35 K,
respectively. However, from the TD-DFT calculation, S1
and T1 levels were estimated to be 2.65 and 2.64,
respectively, resulting in ∆EST of 0.01 eV. The gap between
the achieved and calculated ∆EST can be caused by the
selection of DFT calculation scheme.29 The transient PL
decays of the film measured at the wavelength of 450 nm
showed larger delayed emission and faster decay rate at
room temperature (RT) than 35 K (Figure 3b), indicating
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model with the measured Θ = 0.66 and PLQY = 0.74 of
fluorescent OLEDs when the thicknesses of the hole and
electron injection layers are 45 nm and 50 nm,
respectively.
DTPDDA to predict the maximum achievable EQE of the
blue fluorescent OLEDs as functions of the thickness of
the HIL and EIL under the assumption of no electrical
loss and all the triplet excitons are converted to the
singlet excitons at RT in the device. Details of the
simulation procedure was described before.2
The fabricated OLED with the optimized structure
emitted blue fluorescence which is 13 nm red shifted from
the PL spectrum due to the cavity effect as shown in
Figure 5a. The CIE coordinates of the blue fluorescence
from the device is (0.149, 0.197) (inset of Figure 5a). Figure
5b and 5c show the current density voltage luminance
(J V L) characterstics and the power efficiency of
optimized OLED, respectively. The factor of 1.07 was used
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for the calibration of efficincy obtained from the broader
angular distribution of the EL intensity of the OLED than
the Lambertian as shown in the inset of Figure 5b. The
turn-on voltage of the OLED was 3.0 V and the driving
voltage was 5.7 V and6.5 V at 1,000 cd/m2 and 2,000
cd/m2, respectively. Maximum power efficiency of the
OLED was 30.4 lm/W. In comparison with the previously
reported efficient blue fluorescent OLED with 20.6% EQE,
the device using DTPDDA demonstrated not only higher
EL efficiency but also deeper blue emission with lower
turn-on voltage.17
The device exhibited the maximum EQE of 22.3%, which
is by far the highest efficiency reported in blue fluorescent
OLEDs with high color purity. The experimentally
obtained EQE matches very well with the theoretically
predicted maximum EQE indicating that the device was
well optimized with little electrical loss and all the triplet
excitons are converted to singlet excitons via RISC. The
efficiency roll off was not so large compared to other blue
TADF devices.14-19 The voltages, current efficiencies, EQEs
and power efficiencies of the OLED at different
luminances are summarized in Table 1.
■ CONCLUSION
In this article, azasiline unit was introduced for the first
time as the donor moiety to synthesize a new deep blue
TADF dye, 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-
10,10-diphenyl-5,10-dihydrodibenzo[b,e][1,4]azasiline
(DTPDDA). The azasiline unit was separated spatially
from the acceptor of triazine owing to large dihedral
angle formed at nitrogen-carbon bridge. Replacing carbon
connection by silicon connection widens the bandgap
suitable for blue emission. An efficient blue fluorescent
OLED with the CIE coordinates of (0.149, 0.197) was
fabricated with an unprecedented high EQE of 22.3% and
low efficiency roll-off by doping DTPDDA in a mixed co-
host system of mCP:TSPO1. Theoretical prediction
assuming the fraction of radiative exciton as unity agrees
well with EQE indicating efficient harvest of triplets
occuring in DTPDDA through RISC.
Figure 5. (a) EL spectrum of the blue fluorescent OLED
Inset : (top) the image of blue emission from the OLED.
(bottom) A star denotes the CIE coordinate of the EL
spectrum at (0.149, 0.197). (b) Current density−voltage−
luminance characteristics of the OLED. Inset: Angular
distribution of the EL intensity of the OLED. The solid
line represents the Lambertian distribution. (c) EQE and
power efficiency of the blue fluorescent OLED to current
densities.
The simulation results displayed in Figure 4b indicates
that maximum EQE of 22% is achievable in the blue
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Voltage [V]
Current Efficiency [cd/A]
EQE [%]
Max.
Power Efficiency [lm/W]
Turn
on
1,000
cd/m2
2,000
cd/m2
1,000
cd/m2
2,000
cd/m2
2,000
1,000
2,000
1,000
Max.
Max.
30.4
cd/m2
cd/m2
cd/m2
cd/m2
3.0
5.7
6.5
35.6
17.9
13
22.3
10.6
8
9.8
5.7
8
Table 1. The voltage, current efficiency, EQE, and power efficiency of the OLED.
9
(Keithley 2400) and a spectrophotometer (Spectrascan
PR650, Photo Research). The angular distribution of the
EL was measured with a programmable source meter
(Keithley 2400), goniometer, and fiber optic spectrometer
(Ocean Optics S2000). The EQE and the power efficiency
of the OLEDs were calculated from J-V-L characteristics,
EL spectra, and the angular distribution of the EL
intensity.
■ EXPERIMENTAL SECTION
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Geometry optimization and energy level of HOMO and
LUMO were calculated through DFT calculations with
B3LYP level of theory, the 6-31G(d) basis set for all the
atoms without solvent model and singlet and triplet ener-
gies which were performed with TD-DFT calculations
with Gaussian09.40
Detailed information of the optimized molecular
geometry was addressed in Table S3.
■ ASSOCIATED CONTENT
UV-vis absorption spectra and photoluminescence (PL)
spectra were measured with a Shimadzu UV-1650PC
Supporting Information Supplemental tables and figures;
experimental details; synthesis and computational
geometries of the investigated compounds.
spectrophotometer
and
LS-50B
luminescence
spectrophotometer, respectively. Cyclic voltammetry (CV)
was performed using an EG&G Parc model 273
potentiostat/galvanostat system with a three-electrode
cell in a solution containing Bu4NClO4 (0.1 M) in
chloroform at a scan rate of 100 mV/s. A Pt wire was used
as the counter electrode, and an Ag/AgNO3 (0.1 M)
electrode as the reference electrode.
■ AUTHOR INFORMATION
Corresponding Author
jjkim@snu.ac.kr
ykim@gnu.ac.kr
Organic films for the measurement of the PLQY and PL
spectra were fabricated by thermal evaporation on
precleaned quartz substrates at a base pressure of < 5 ×
10−7 Torr. The PLQY was measured by using an
integrating sphere. A continuous-wave He/Cd laser (325
nm) was used as an excitation light source and a
monochromator-attached photomultiplier tube (PMT)
was used as an optical detection system.41 The PL spectra
was also measured by PMT with a xenon lamp (260 nm)
as an excitation light source.
Author Contributions
J. W. Sun and J. Y. Baek equally contributed to this work.
Notes
The authors declare no competing financial interest.
Funding Source
This work was supported by the Industrial Strategic
Technology Development Program [10048317] funded by
MKE/KEIT.
Orientation of transition dipole moments was measured
using a continuous wave diode laser (405 nm, Edmund
optics Inc.). The incident angle of the excitation light was
fixed at 45º from the plane normal direction of substrate
and the p-polarized emitted light was detected at 465 nm
that is close to the peak wavelength of the PL spectrum of
the fluorescent dye.
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