Light-blue thermally activated delayed fluorescent emitters realizing a high external quantum efficiency of 25% and unprecedented low drive voltages in OLEDs

Article abstract of DOI:10.1039/c5tc04057d

Thermally activated delayed fluorescent (TADF) emitters are one of the most promising candidates for low-cost and high efficiency organic light-emitting devices (OLEDs) to realize an internal quantum efficiency of unity. However, the power efficiency (η

Full text of DOI:10.1039/c5tc04057d

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DOI: 10.1039/C5TC04057D
Journal of Materials Chemistry Cꢀ
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Light-Blue Thermally Activated Delayed Fluorescent Emitters
Realizing a High External Quantum Efficiency of 25% and
Receivedꢀ00thꢀJanuaryꢀ20xx,ꢀ
Acceptedꢀ00thꢀJanuaryꢀ20xxꢀ
Unprecedented Low Drive Voltages in OLEDsꢀ
a
a,b
a
a
a,b
Ryutaro Komatsu, Hisahiro Sasabe,* Yuki Seino, Kohei Nakao, and Junji Kido*
DOI:ꢀ10.1039/x0xx00000xꢀ
www.rsc.org/ꢀ
Thermally activated delayed fluorescent (TADF) emitters are high-performance OLEDs to realize 100% internal quantum
one of the most promising candidates for low-cost and high efficiencies (IQE) because these platinum heavy metals can harvest
efficiency organic light-emitting devices (OLEDs) to realize an all the electrogenerated molecular excitons. Phosphorescent OLEDs
internal quantum efficiency of unity. However, the power have already achieved a high external quantum efficiency (ηext) of
p
efficiency (η ), which is inversely related to drive voltage, is >30% and low drive voltages for the three primary colors, leading to
[9–15]
significantly lower than that of the phosphorescent counterparts, high power efficiencies (ηp).
especially for blue devices. Here, we developed a series of TADF
Recently, thermally activated delayed fluorescent (TADF) emitters
consisting of pure organic compounds have been considered as an
alternate technology to harvest all the molecular excitons without the
use of expensive platinum group metals and are thus able to realize
emitters,
2-functionalized-4,6-bis[4-(9,9-dimethyl-9,10-
dihydroacridine)phenyl]pyrimidine called Ac–RPM. We
introduced a phenylacridine moiety into the 4,6-position of
pyrimidine core to induce a twisted structure leading to a high
photoluminescent quantum yield of ~80%, and a small singlet
and triplet excited energy difference of <0.20 eV. The optimized
[
16]
an IQE of 100%.
achieved a high ηext of 30% and/or high η
However, the corresponding blue TADF devices still require
development to reach the levels of state-of-the-art blue
phosphorescent devices. Recently, a few highly efficient blue TADF
Indeed, several green TADF devices have
−
1 [17, 18]
p
of ~90 lm W .
−
1
p
device realized an η of 62 lm W , a high external quantum
η
p
efficiency of 25%, light-blue emissions with the Commission
Internationale de l’Eclairage chromaticity coordinates of (0.19,
[19–24]
devices with ηext of over 20% have been reported.
Adachi and
0
.37) and a low turn-on voltage of <3.0 V.
co-workers developed bis[4-(9,9-dimethyl-9,10-
Highly efficient organic light-emitting devices (OLEDs) are one of
the most promising green technologies for future lighting and flat-
dihydroacridine)phenyl]sulfone composed of two acridine (Ac)
moieties as donors and one diphenylsulfone moiety as an
[1–8]
panel display applications.
Unlike conventional light sources,
[19]
acceptor.
The same group developed a carbazole/triazine-based
OLEDs have outstanding advantages, such as area lighting, because
they are ultra-thin, lightweight, and flexible when a flexible substrate
is used. Until very recently, phosphorescent emitters containing
platinum heavy metals, such as Ir, Os, and Pt, were required for
light-blue TADF emitter exbihiting a high ηext of 20% with a
[20]
relatively low turn-on voltage (Von) of 4.0 V. More recently, Lee
and co-workers developed a light-blue TADF emitter, TCzTrz,
composed of one triphenyltriazine moiety and three carbazole
moieties showing a maximum external quantum efficiency (ηext,max
)
−
1
of 25% and a maximum power efficiencies of (ηp,max) of 43 lm W
[23]
with a Von of 4.0 V.
As mentioned above, blue TADF devices
have not reached the efficiency levels of state-of-the-art blue
KAKENHI Grant Numbers (26620202) and
(
25-7551).
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phosphorescent devices; however, they should be able to achieve a and Table S-1). All Ac–RPM derivatives showed a very small ΔE
View Article Onli nS eT
DOI: 10.1039/C5TC04057D
high η
p
without any loss of ηext
.
of 0.01 eV and a wide energy gap of >3.3 eV; thus, a blue TADF
emission was expected (Table S-1). The electron cloud distribution
in Figure 2 shows that the highest occupied molecular orbital
In this report, we developed a novel series of TADF emitters, 2-
functionalized-4,6-bis[4-(9,9-dimethyl-9,10-
(
HOMO) is located on the Ac moiety, whereas the lowest
dihydroacridine)phenyl]pyrimidine called Ac–RPM composed of
one pyrimidine (PM) and two Ac moieties. This series of Ac–RPM
unoccupied molecular orbital (LUMO) is located on the 4,6-
diphenylpyrimidine skeleton. The frontier molecular orbitals, in
other words, HOMO and LUMO, are almost completely separated,
with a very-small overlap.
derivatives exhibited a high photoluminescent quantum yield (ηPL
)
of 77%–80% and delayed fluorescence lifetime (τ ) of 21–26 µs in a
d
solid host matrix. The optimized OLED showed light-blue emissions
with Commission Internationale de l’Eclairage (CIE) chromaticity
−
1
coordinates of (0.19, 0.37), and realized a high ηp,max of 62 lm W , a
high ηext,max of 25%, and an unprecedentedly low Von of 2.8 V, which
is the lowest Von to date.
–
2.09 eV
–
2.11 eV
–2.19 eV
.31 eV
LUMO
HOMO
3
.37 eV
3.38 eV
3
ΔEST = 0.01 eV
ΔEST = 0.01 eV
f = 0.0003
ΔEST = 0.01 eV
Although conventional fluorescent blue emitters composed of PM
and donor moieties, such as alkyl amines, carbazoles, and thiophenes,
are well known throughout the past decade as efficient fluorescent
emitters with high ηPL values, the PM moiety has not been used as a
f = 0.0003
f = 0.0003
–
5.50 eV
–5.48 eV
–5.47 eV
[25, 26]
component of TADF emitters.
A large variety of TADF
emitters could be developed using PM because the 2,4,6-positions of
the PM moiety is easily functionalized. In the development of TADF
emitters, the energy difference between the singlet and triplet excited
states (ΔEST) must be small (<0.2 eV) to obtain the TADF
Ac-HPM
Ac-PPM
Ac-MPM
Figure 2. HOMO and LUMO distribution, energy levels, optical
energy gap (E ), the energy difference between the singlet and triplet
g
[20]
character.
For this, a twisting π-conjugated system between the
excited states (ΔEST), and oscillator strength (f) of Ac–RPM
donor and accepter moieties in the molecule is a promising design
strategy. Here, for the donor, we introduced a phenylacridine moiety
derivatives.
into the 4,6-position of the PM moiety to induce a twisted structure The synthetic routes of Ac–RPM are shown in Figure 1. The
in the molecule. Three different Ac–RPM derivatives (Ac–HPM, precursor boronate ester, Ac–Bpin, can be easily prepared according
[
27]
Ac–PPM, and Ac–MPM, where R = H, phenyl, and CH
respectively) were introduced at the 2-position of PM (Figure 1).
3
,
to the literature. A Suzuki–Miyaura coupling reaction of Ac–Bpin
with 2-functionalized-4,6-dichloropyrimidine afforded Ac–RPM in
a reasonable yield. This simple, one-step synthesis allows a large
amount of Ac–RPM to be prepared on a multigram scale.
R
R
N
N
N
N
Cl
Cl
1
Characterization was performed using H-NMR, mass spectroscopy,
N
Bpin
N
N
Pd
.35 M K
,4-dioxane, reflux
2
(dba)
3
/ S–phos
and elemental analysis. The product was purified by train
sublimation before device fabrication.
1
3
PO aq
4
1
R=H : Ac–HPM
R=Ph : Ac–PPM
R=Me : Ac–MPM
The thermal properties were estimated by thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC) (Figure
S-2). A weight loss of 5% (Td5) was observed at temperatures over
Figure 1. Synthetic routes and chemical structures of Ac–RPM
derivatives.
4
30 ˚C, which indicates high thermal stability and is applicable for
Prior to the preparation of the materials, we conducted density thermal evaporation, whereas a glass-transition temperature was not
functional theory calculations to estimate their geometrical structures, observed between 50 °C and 380 °C.
g
ΔEST, and energy gap (E ) of the Ac–RPM (Figure S-1, Figure 2
ꢀ
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Table 1. Thermal and photophysical properties of Ac–RPM derivatives.
ꢀCOMMUNICATIONꢀ
View Article Online
[a]
[b]
[c]
D O[ dI ]: 10.1039/C5TC04 0[ e5] 7D
Compound
Ac–HPM
Ac–PPM
Ac–MPM
T
g
/T
m
/Td5 [˚C]
I
p
/E
a
/E
g
[eV]
E
S
/E
T
/ΔEST [eV]
τ
d
[µs]
21.4
20.7
26.2
η
PL [%]
n.d./325/430
n.d./388/442
n.d./348/432
5.65/2.84/2.81
5.65/2.85/2.80
5.66/2.85/2.81
2.84/2.66/0.18
2.84/2.65/0.19
2.89/2.70/0.19
77
79
80
[
a] T
of the normalized absorption spectra; E
DPEPO film were measured by using a streak camera. ΔEST = E
g
and T
m
were measured by DSC; Td5 was measured by TGA. [b] I
was calculated by using I
–E
p
was measured by PYS; The E
and E . [c] Onset of phosphorescence of 10 wt% Ac–RPM-doped
T
. [d] Delayed fluorescent lifetime of 10 wt% Ac–RPM-doped DPEPO
g
was taken as the point of intersection
a
p
g
S
film. [e] Photoluminescent quantum yield of 10 wt% Ac–RPM-doped DPEPO film.
–
5
The optical properties were evaluated in dilute toluene solution. The Figure 3. UV–vis and PL spectra of Ac–RPM in toluene (1.0 × 10
ultraviolet-visible (UV-vis) absorption and photoluminescent (PL) M).
spectroscopy spectra of Ac–RPM measured at room temperature are
The PL spectra were then investigated in a host matrix of bis[2–
shown in Figure 3. The weak absorption of approximately 380 nm
can be attributed to the intramolecular charge transfer state from the
Ac moiety to the PM moiety. The absorption coefficient was
[19]
(
diphenylphosphino) phenyl]ether oxide (DPEPO) , which has
high triple energy (E ). As shown in Figure S-3, the emission peaks
T
of 10 wt% Ac–HPM-, Ac–PPM-, and Ac–MPM-doped DPEPO
films are located at 498, 498, and 489 nm, respectively. The
emission peaks of these films show an approximate 10-nm red shift
compared with the solution states.
3
−1
−1
calculated as approximately 4.0 × 10 M cm . The emission peaks
of Ac–HPM, Ac–PPM, and Ac–MPM are very similar, located at
4
89, 484, and 478 nm, respectively (Table S-2). These emission
peaks are well consistent with the results of E estimated by DFT
calculation. The electron-donating property of methyl group makes
the LUMO level shallower thus leading to wider E of Ac–MPM.
g
The ηPL values of the Ac–RPM derivatives were then evaluated.
The ηPLs of the 10 wt% Ac–RPM-doped DPEPO films were
g
Ac–RPM derivatives show light-blue emissions in dilute toluene
measured under N
2
flow using an integrating sphere with an
solution. The physical properties of the material were evaluated in
excitation light at 290 nm, with a multichannel spectrometer as the
optical detector. The ηPL values of 10 wt% Ac–HPM-, Ac–PPM-,
and Ac–MPM-doped DPEPO films are 77%, 79%, and 80%,
respectively. The TADF character is confirmed by the means of the
transient PL decay curve at a high temperature of 300 K (Figure 4).
The delayed PL intensities of the three materials increase at 300 K,
which indicates the presence of TADF. Transient PL decay curves of
10 wt% Ac–RPM-doped DPEPO films exhibit a triple-exponential
the solid state. The ionization potential (I
measured by photoelectron yield spectroscopy (PYS), were observed
at approximately 5.7 eV. The electron affinity (E ) levels were
estimated at approximately 2.9 eV by the subtraction of the optical
energy gap (E ) from the I levels.
p
) levels, which were
a
g
p
1
.2
1
Ac-HPM_UV
Ac-PPM_UV
Ac-MPM_UV
Ac-HPM_PL
Ac-PPM_PL
Ac-MPM_PL
d
decay with a delayed lifetime (τ ) of 21–26 µs. ΔEST was estimated
from the onset of the prompt and delayed emission components to be
approximately 0.20 eV at 5 K (Figure S-4). All photophysical
properties of Ac–RPM derivatives are summerized in Table 1.
0
0
0
0
.8
.6
.4
.2
0
3
00 350 400 450 500 550 600 650
Wavelength (nm)
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green emission region, whereas the Ac–MPM-based dVeievwicAerticslheoOwnlsinea
0
0
Ac-HPM_300 K
Ac-HPM_5 K
Ac-PPM_300 K
Ac-PPM_5 K
DOI: 10.1039/C5TC04057D
light-blue emission with the CIE coordinates of (0.19, 0.37). All the
1
p
devices show very-high ηext of approximately 20% and η of over 50
Ac-MPM_300 K
Ac-MPM_5 K
−1
lm W with an unprecedented Von of 2.9 V (Figure S-7 and Table
-
1
2
3
2
) To obtain a better understanding of the operation mechanism in
1
0
the Ac-MPM-based device, we compared three types of devices
with emission layers (EMLs) of (A) 10 wt% Ac–MPM-doped
DPEPO (20 nm); (B) 10 wt% Ac–MPM-doped DPEPO (10 nm)/
DPEPO (10 nm); and (C) DPEPO (10 nm)/ 10 wt% Ac–MPM-
doped DPEPO (10 nm). Figure S-8 shows the energy diagrams of
these devices, and all the device performances are summarized in
Table S-3. The EL spectra of devices (A) and (B) are the same and
show emissions from only Ac–MPM (Figure S-9), whereas the
spectrum of device (C) shows an emission from Ac–MPM along
with an emission at λmax = 580 nm, which is identical to the emission
-
1
1
0
0
-
0
10 20 30 40 50 60 70 80
Time (µs)
Figure 4. Transient PL decay curves of Ac–RPM at 5 K and 300 K.
[29]
from the electromer of TAPC. Devices (A) and (B) show similar
J–V characteristics (Figure S-9), indicating that their carrier
recombination pathways are similar, whereas device (C) is
completely different, probably because of the poor hole injection
ability of DPEPO. Thus, it can be considered that the carrier
recombination zone is located near the HTL/EML interface.
Next, we evaluated OLED performances. In our TADF devices, we
used a carrier- and exciton-confining device structure to maximize
the OLED performance. We used di-[4-(N,N-ditolyl-amino)-
phenyl]cyclohexane (TAPC) as a hole transport layer (HTL),
DPEPO as a host material, and 3,3’’,5,5’-tetra(3-pyridyl)-1,1’;3’,1’’-
[28]
terphenyl (B3PyPB)
triplet energy (E ) levels of TAPC, DPEPO, and B3PyPB are 2.98,
.30, and 2.77 eV, respectively. All the materials have a higher E
as an electron transport layer (ETL). The
T
3
T
than that of Ac–RPM derivatives (E
T
= 2.65–2.67 eV). Therefore,
the triplet excitons of the emitter can be completely suppressed. All
the chemical structures of materials used in this study are shown in
Figure S-5. Three types of OLEDs with a structure of [ITO/TAPC
(
(
30 nm)/10 wt% Ac–RPM:DPEPO (20 nm) /B3PyPB (50 nm)/LiF
0.5 nm)/Al (100 nm)] were fabricated. Figure S-6a shows the
energy diagrams of these devices. All the EL spectra show an
emission only from Ac–RPM with no emission from neighboring
materials. The CIE chromaticity coordinates (x, y) for Ac–HPM and
Ac–PPM were evaluated as (0.21, 0.44), which is located in the
Table 2. Summary of OLED performances.
V
on/ ηp, on/ηc, on/ηext, on
[a]
–1 –1
V
100/ ηp, 100/ηc, 100/ηext, 100
[b]
–1 –1
V
1000/ηp, 1000/ηc, 1000/ηext, 1000
[c]
–1 –1
[d]
Emitter
Host
CIE (x, y)
[
V/ lm W / cd A / %]
[V/ lm W / cd A / %]
[V/ lm W / cd A / %]
Ac–HPM
Ac–PPM
Ac–MPM
Ac–MPM
DPEPO
DPEPO
2.85 / 60.3 / 54.7 / 20.9
2.93 / 52.8 / 49.2 / 19.0
2.93 / 49.9 / 46.6 / 20.4
2.80 / 61.6 / 54.9 / 24.5
3.79 / 38.2 / 46.0 / 17.6
4.01 / 33.8 / 43.2 / 16.6
3.96 / 29.2 / 36.8 / 16.1
3.57 / 34.0 / 38.5 / 17.2
4.82 / 20.0 / 30.6 / 11.7
5.25 / 17.4 / 29.0 / 11.2
5.27 / 13.2 / 22.1 / 9.7
4.57 / 15.5 / 22.5 / 10.1
(0.21, 0.44)
(0.21, 0.44)
(0.19, 0.37)
DPEPO
mCP/DPEPO
(0.19, 0.37)
–
2
[
a] Voltge (V), power efficiency (η
p
), current efficiency (η
c
) and external quantum efficiency (ηext) at 1 cd m . [b] V, η
p
, η
c
, and ηext at 100
–
2
–2
–2
p c
cd m . [c] V, η , η , and ηext at 1000 cd m . [d] CIE at 100 cd m .
4
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Considering the above results, there is still much room to improve In summary, we have developed a novel series of TVAiewDAFrticelme Oitntleinres
the carrier balance in the EML by introducing a hole transporting called Ac–RPM derivatives using a PM/ DA Oc I: c1 0o .m1 0 b3 i 9n /aCt i5 oT nC .0 4T 0 h5 e7 sDe
material that is superior to DPEPO between the HTL and the EML. derivatives exhibited a light-blue emission in a toluene solution.
Therefore, we introduced N,N-dicarbazoyl-3,5-benzene (mCP) with Among these, Ac–MPM showed an intense emission with a high
a shallower I
p
d
level (6.0 eV) than that of DPEPO (6.7 eV). In other ηPL of 80%, and a τ of 26 µs in a DPEPO host matrix. By a carrier-
words, we used a double emission layer (DEML) strategy using and exciton-confining structure, the optimized OLED showed a
−
1
mCP/DPEPO host materials. We fabricated a DEML device with a
structure of [ITO/TAPC (30 nm)/10 wt% Ac–MPM-doped mCP (10
nm)/10 wt% Ac–MPM-doped DPEPO (10 nm) /B3PyPB (50 nm)/
LiF (0.5 nm)/Al (100 nm)]. Figure S-6b shows the energy diagrams
of these devices. The J–V–L and the PE–L–EQE characteristics are
shown in Figure 5. This light-blue device with λEL = 487 nm
light-blue emission and realized a high ηp,max of 62 lm W and a
high ηext,max of 25% with an unprecedentedly low Von of 2.8 V,
[19–24]
which is the lowest Von so far (Table S-4).
For the practical
application of blue TADF devices, there are still several challenges,
such as pure-blue emission, reduced efficiency roll-off at high
current density, and low drive voltage at high brightness. Further
design and synthesis of novel blue TADF emitters are currently
−
1
achieves an ηext,max of 24.5% and ηp,max of 61.6 lm W with a low
V
on of 2.80 V. The Von is very close to the energy-gap-voltage values being developed in our laboratory. We believe that our results can
−
2
of Ac–MPM (2.54 eV). Even at 100 cd m , a ηext,100 of 17.2%, accelerate the development of TADF OLED technology for future
−
1
lighting applications.
η
p,100 of 34 lm W , and V100 of 3.57 V are realized.
[
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Figure 5. a) J–V–L characteristics, inset: EL spectra at 0.5 mA, and
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Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C5TC04057D
Graphical abstract
LightꢀBlue Thermally Activated Delayed Fluorescent Emitters Realizing a High
External Quantum Efficiency of 25% and Unprecedented Low Drive Voltages in
OLEDs
By Ryutaro Komatsu, Hisahiro Sasabe*, Yuki Seino, Kohei Nakao, and Junji Kido*
N
N
N
N
Max PE = 62 lm/W
Max EQE = 25 %
Von = 2.8 V
A series of thermally activated delayed fluorescent (TADF) emitters called Ac–RPM,
composed of one pyrimidine and two acridine moieties are developed. The optimized
−1
device realized its highest power efficiency of 62 lm W , a high external quantum
efficiency of 25%, lightꢀblue emissions with the Commission Internationale de
l’Eclairage chromaticity coordinates of (0.19, 0.37) and an unprecedentedly low turnꢀon
voltage of <3.0 V.