Deep blue fluorophores incorporating sulfone-locked triphenylamine: The key for highly efficient fluorescence-phosphorescence hybrid white OLEDs with simplified structure

Article abstract of DOI:10.1039/c5tc01373a

Two novel bipolar isomeric blue fluorophores, PPI-TPA-SO2-1 and PPI-TPA-SO2-2, consisting of electron-withdrawing phenanthro[9,10-d]imidazole and sulfone-locked electron-donating triphenylamine, were designed and synthesized. The sulfone lock induces a more twisted molecular conformation, and thus a higher triplet energy level and better triplet exciton confining ability compared with the analogue TPA-PPI without the sulfone lock. In addition, the introduced sulfone lock also offers the developed materials improved electron affinities and an electron dominant transporting ability. They were utilized as the blue emitter and the host for a yellow phosphorescent emitter to fabricate fluorescence-phosphorescence (F-P) hybrid white organic light-emitting diodes (WOLEDs) in a single-emissive-layer architecture, giving forward-viewing maximum current efficiencies of 44.2 and 47.6 cd A^-1, power efficiencies of 49.5 and 53.4 lm W^-1, and external quantum efficiencies of 14.4% and 15.6%, respectively, which are much higher than those of the devices based on TPA-PPI (29.5 cd A^-1, 33.1 lm W^-1, and 9.6%) due to their superior singlet and triplet exciton separation and utilization ability over TPA-PPI. These efficiencies are also the highest values ever reported for the F-P hybrid WOLEDs in a similar architecture, and their power efficiencies are even comparable with most reported highly efficient all phosphorescent WOLEDs without using any out-coupling technology.

Full text of DOI:10.1039/c5tc01373a

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DOI: 10.1039/C5TC01373A
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ARTICLE
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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Deep Blue Fluorophors Incorporated Sulfone-Locked
Triphenylamine: the Key for Highly Efficient
Fluorescence/Phosphorescence Hybrid White OLEDs
with Simplified Structure
Yunchuan Lia, Xiang-Long Lia, Xinyi Caia, Dongcheng Chena, Xin Liua, Gaozhan Xiea, Zhiheng
Wanga, Yuan-Chun Wub, Chang-Cheng Lob, A. Lienb, Junbiao Penga, Yong Caoa, and Shi-Jian
Sua*
Two novel bipolar isomeric blue fluorophors, PPI-TPA-SO2-1 and PPI-TPA-SO2-2, consisting of electron-withdrawing
phenanthro[9,10-d]imidazole and sulfone-locked electron-donating triphenylamine, were designed and synthesized. The
sulfone lock induces more twisted molecular conformation, thus higher triplet energy level and better triplet exciton
confining ability compared with the analogue TPA-PPI without the sulfone lock. In addition, the introduced sulfone lock
also offers the developed materials improved electron affinities and electron dominant transporting ability. They were
utilized as the blue emitter and the host for a yellow phosphorescent emitter to fabricate fluorescence/phosphorescence (F/P)
hybrid white organic light-emitting diodes (WOLEDs) in a single-emissive-layer architecture, giving forward-viewing
maximum current efficiencies of 44.2 and 47.6 cd A-1, power efficiencies of 49.5 and 53.4 lm W-1, and external quantum
efficiencies of 14.4% and 15.6% , respectively, which are much higher than those of the devices based on TPA-PPI (29.5 cd
A-1, 33.1 lm W-1, and 9.6%) due to their superior singlet and triplet exciton separation and utilization ability over TPA-PPI.
These efficiencies are also the highest values ever reported for the F/P hybrid WOLEDs in a similar architecture, and their
power efficiencies are even comparable with most reported highly efficient all phosphorescent WOLEDs without using any
out-coupling technology.
complementary colors is considered to be an ideal solution to realize
high efficiency and long lifetime fluorescence/phosphorescence (F/P)
Introduction
hybrid WOLEDs.5a,b Besides, a blue emitter with high enough triplet
energy level can give blue emission and function as the host of
green/red phosphorescent emitters to realize potential 100% exciton
utilization in F/P hybrid WOLEDs.6 In such kind of F/P hybrid
WOLEDs, the key point is to utilize the singlet and triplet excitons
respectively. As triplet excitons have a much longer diffusion length
(100 nm) than singlet excitons (4 nm), a spacer is generally
introduced between the fluorescent and phosphorescent emission
layers (EMLs) to separate the singlet and triplet excitons.5,6 So these
F/P hybrid WOLEDs usually consist of complicated multi-EML
structures, which raise the difficulty in reproducibility and
fabrication cost. Such complicated multi-EML structure is also
difficult to be achieved via solution processing, which is considered
to be an important process for future low-cost and large-area
manufacturing.7 To further simplify the F/P hybrid WOLEDs, a
complementary phosphor could be doped into a blue fluorophor to
form a single EML directly. Similarly, a superior singlet and triplet
exciton separation and utilization ability is also requested in this case
to achieve efficient utilization of the singlet and triplet excitons.
Since the pioneering work by Tang et al. in 1987, extensive efforts
have been devoted to promoting organic light-emitting diodes
(OLEDs) into commercial applications such as flat-panel displays
and lighting.1 Tremendous developments of red, green, and blue
OLEDs have paved the way for the recent advancements of white
OLEDs (WOLEDs).
During the past two decades, various fabrication strategies based
on different material systems were promoted to produce white light
emission. Fully phosphorescent WOLEDs produced the highest
reported efficiencies owing to the potential up to 100% internal
quantum efficiency of the phosphorescent materials.2 However, the
lifetime of the blue phosphorescent materials hinders their
commercial application in many fields.3 In addition, iridium(III)
bis[(4,6-difluorophenyl)-pyridinato-N,C2’]picolinate (FIrpic) remains
the best and most widely used blue phosphorescent emitter, which is
actually a greenish blue emitter exhibiting two emission peaks
centered at 472 and 500 nm.4 In contrast to the phosphorescent
emitter, blue fluorescent emitters can produce deep blue emission
and are more stable with much longer lifetime. Therefore, a
combination of blue fluorescence and phosphorescence of other
The blue emitter should not only give efficient blue fluorescence,
but also has to satisfy the requirements for a phosphorescent host
including high triplet energy, narrow singlet/triplet split energy
(ΔEST), bipolar property, suitable energy levels and good thermal
stability.8 Much efforts have been devoted to the single-EML F/P
hybrid WOLEDs. Leo et al. developed a blue fluorescent host 4P-
NPD, and the WOLED based on it was reported to have a power
efficiency (PE) of 10.5 lm W−1 at a practical brightness of 1000 cd
m−2 and a maximum external quantum efficiency (EQE) of 5.5%.
The limited performance was probably due to the hole dominant
transporting property of 4P-NPD.9a Tao et al. demonstrated a bipolar
aState Key Laboratory of Luminescent Materials and Devices and Institute of Polymer
Optoelectronic Materials and Devices, South China University of Technology,
Guangzhou 510640, China Fax: +86 20 87110606; Tel: +86 20 22237098; E-mail:
mssjsu@scut.edu.cn
bShenzhen China Star Optoelectronics Technology Co., Ltd., Shenzhen 518132, China
Electronic Supplementary Information (ESI) available: Thermal analysis curves,
Lippert−Mataga calculations, time-resolved phosphorescent spectra, cyclic
voltammograms, calculated singlet and triplet excited energies, energy level
diagrams of the devices, device characteristics, and optimized molecular
coordinates. See DOI: 10.1039/x0xx00000x
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blue fluorescent emitter TPA-o-OXD doped with
a
red emitter
iridium
(III)
bis(4-phenylthieno[3,2-c]pyridinato-
phosphorescent emitter to give a single-EML WOLED with a N,C2’)acetylacetonat (PO-01)14 in a single-EML archiVteiecwtuArretic,legOivnilninge
DOI: 10.1039/C5TC01373A
maximum current efficiency (CE) of 7.9 cd A−1 (a corresponding forward-viewing CE values of 44.2, 47.6 cd A–1, PE values of 49.5,
EQE of 5.2%).9b Hung et al. achieved
a
deep-blue 53.4 lm W–1, and EQE values of 14.4% and 15.6%, respectively. The
fluorescence/yellow-green phosphorescence single-EML WOLED current values are hitherto one of the highest efficiencies ever
with a maximum EQE of 7% and a maximum PE of 12.8 lm W−1.9c reported for the F/P hybrid WOLEDs in a similar architecture and
Nevertheless, these efficiencies are still far from the theoretically are much higher than those of the devices based on the analogous
maximum EQE of 20%.10 This situation was changed until Zhang et compound TPA-PPI without the sulfone lock (29.5 cd A–1, 33.1 lm
al adopted DADBT as the host realizing a maximum EQE of W-1, and 9.6%) due to their superior singlet and triplet exciton
16.6%.11 Therefore, developments of high-performance blue separation and utilization ability over TPA-PPI.
fluorophors as emitters as well as hosts have been considered as the
RESULTS AND DISCUSSION
current key challenges for progressing single-EML F/P hybrid
WOLEDs. And all the aforementioned requirements could be
satisfied only by judicious molecular design of blue emitters.
As shown in Figure S1 and Table 1, PPI-TPA-SO2-1 and PPI-
TPA-SO2-2 exhibit excellent thermally stability with decomposition
temperatures (Td, corresponding to 5% weight loss) of 429 and
483°C , respectively. In addition, a glass transition temperature (Tg)
of 189°C was observed for PPI-TPA-SO2-2 which is much higher
than that of TPA-PPI (130°C ).12b The sulfone-locked TPA offers
PPI-TPA-SO2-2 more rigid molecular conformation and thus higher
Tg. In contrast, no glass transition or other phase transition was
found for PPI-TPA-SO2-1 in the same temperature range. Their
good thermally stability and high glass transition temperature are a
prerequisite for their application in OLEDs, indicating their high
morphologic stability of amorphous phase in a deposited film.
Scheme 1. Synthetic routes and chemical structures of the target
materials PPI-TPA-SO2-1 and PPI-TPA-SO2-2. (a) 1,4-
bromoiodobenzene or iodobenzene, N,N-dimethylformamide (DMF),
phenothiazine, copper powder, reflux; (b) dichloromethane (DCM),
acetic acid, 30% hydrogen peroxide, 80°C; (c) DCM, bromine, room
temperature; (d) boronic acid ester (1.08 equiv), K2CO3 (5.0 equiv, 2
M aq), Pd(PPh3)4 (5 mol%), toluene/EtOH = 3/1, reflux.
(a)
solution
Abs
PL
A series of donor-acceptor (D-A) compounds consisting of
imidazole and triphenylamine (TPA) were reported as blue
fluorophors for nondoped blue OLEDs.12 In this work, we
introduced a new strategy of sulfone lock to develop two isomeric
blue fluorophors combining phenanthro[9,10-d]imidazole (PPI) and
sulfone-locked TPA units (Scheme 1). As the sulfone unit has a
strong electron affinity to offer the compound good electron
injection and transporting ability,13 more balanced carrier
recombination may be expected compared with the analogous
compound without such a sulfone lock. In addition, attaching the
sulfone unit between the two phenyls of TPA may give a specific
molecular conformation and thus different electronic characteristics.
Theoretical calculation reveals that the sulfone lock induces a more
twisted molecular conformation and thus increased triplet energy
level. Maximum CE values of 3.56 and 4.80 cd A–1, corresponding
to EQE values of 3.50% and 4.62%, are respectively achieved for the
blue OLEDs with PPI-TPA-SO2-1 and PPI-TPA-SO2-2 as a
nondoped EML. Moreover, F/P hybrid WOLEDs were also
fabricated by utilizing PPI-TPA-SO2-1 and PPI-TPA-SO2-2 as the
blue emitter as well as the host of the yellow phosphorescent
(b)
film
250 300 350 400 450 500 550 600
Wavelength (nm)
Figure 1. Normalized UV-vis absorption (open symbol) and PL
spectra (solid symbol) of PPI-TPA-SO2-1 (○●) and PPI-TPA-
SO2-2 (□■) in chloroform solution (a) and in thin solid film (b).
Table 1. Physical properties of PPI-TPA-SO2-1 and PPI-TPA-SO2-2
a
b
c
d
c
d
optf
λabs
λabs
λem
λem
IPe
EAe
Eg
Tg
Td
g
h
compound
ФPL
ФPL
(nm)
(nm)
(nm)
510
508
(nm)
439
446
(eV)
-5.62
-5.67
(eV)
-2.72
-2.67
(eV)
2.98
2.93
(℃)
--
(℃)
429
483
PPI-TPA-SO2-1
PPI-TPA-SO2-2
262.2, 341.3, 365.5
264.3, 335.0, 363.5
270.1, 353.5, 370.8
0.90
0.86
0.71
0.68
185
259.3, 339.4, 368.6
b
aGlass transition temperature (Tg) obtained from DSC measurements. Decomposition temperature (Td) obtained from TGA measurements.
d
cUV-vis absorption and PL spectra measured in chloroform solution. UV-vis absorption and PL spectra measured in thin solid film.
eIonization potential (IP) and electron affinity (EA) estimated from the onset of the oxidation and reduction potentials [vs
ferrocene/ferrocenium (Fc/Fc+)]. fOptical energy band gaps (Egopt) estimated from the film absorption edge. gPL quantum yields of the DCM
h
solutions measured using an integrating sphere (10-5 mol L-1). PL quantum yields of the thin solid films (50 nm) deposited on the quartz
substrates measured using an integrating sphere.
2 | J. Name., 2012, 00, 1-3
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UV-Vis absorption and photoluminescence (PL) spectra of PPI- values were achieved for the sulfone-locked compounds due to the
TPA-SO2-1 and PPI-TPA-SO2-2 in dilute CHCl3 solutions and in strong electron affinity of the sulfone uniDt.OTI:h1e0.1l0o3Vw9iee/wCr-A5lrTytiCcinl0eg1O3n7Eli3nAAe
thin solid films are shown in Figure 1. The absorption features of the values and thus lower electron injection barrier may offer easy
two compounds are similar to each other due to their homologous electron injection from the electron transport layer. In addition, the
molecular skeleton. The absorption bands at around 270–300 nm deeper IP values may improve the hole injection barrier and thus
could be associated with the π–π* transitions of the sulfone-locked reduced hole current.
TPA units, and the absorption bands in the range from 345 to 362
nm can be assigned to the π–π* transitions of the PPI moiety. Optical
energy band gaps (Egopt) are estimated to be 2.98 and 2.93 eV for
PPI-TPA-SO2-1 and PPI-TPA-SO2-2, respectively, which are
calculated from the threshold of the absorption spectra in films.
LUMO+1
-0.29
-0.79
Photoluminescence
quantum
yield
(PLQY)
in
dilute
dichloromethane (DCM) solution (10-5 mol L–1) is 0.90 for PPI-
TPA-SO2-1 and 0.86 for PPI-TPA-SO2-2. In neat films (~50 nm), it
is 0.71 for PPI-TPA-SO2-1 and 0.68 for PPI-TPA-SO2-2 as
measured by an integrating sphere under air condition. The slightly
reduced PLQY in thin solid film may be benefited from the twisted
angle between the donor and the acceptor, which reduces
intermolecular π-π stacking efficiently to reduce the aggregation-
caused quenching problem.
-1.00
-1.62
-1.05
-1.39
LUMO
TPA-PPI
PPI-TPA-SO2-1 PPI-TPA-SO2-2
To distinguish the excited states of the two emitters, the PL
spectra of the two emitters in various solvents with different
polarities were also measured, as shown in Electronic
Supplementary Information (ESI) (Figure S2). PPI-TPA-SO2-1 and
PPI-TPA-SO2-2 display obvious solvatochromaticity such that the
emission spectrum exhibits a clear bathochromic shift (121 and 90
nm) from nonpolar solvent hexane to high polar solvent N,N-
dimethylformamide (DMF). The increase of dipole moment from the
ground state to the excited state was calculated from the
Lipper−Mataga calculation. PPI-TPA-SO2-1 and PPI-TPA-SO2-2
exhibit a large slope of 21694 (R = 0.98) and 19171 (R = 0.98),
respectively (Figure S3). The increase of dipole moment of PPI-
TPA-SO2-1 (28.27 D) and PPI-TPA-SO2-2 (28.20 D) indicates they
are in a strong charge-transfer (CT) state. A strong CT state may be
of benefit to reduced S1 and T1 split energy (ΔEST) and exchange
energy loss as a host.5 In addition, an EML based on them with a
small ΔEST is extremely important to acquire a low driving voltage
and a high PE in single-EML F/P hybrid WOLEDs.16
HOMO
-4.89
-5.24
-5.14
-5.34
-5.74
-5.80
HOMO-1
Figure 2. Calculated spacial distributions of HOMOs and LUMOs
of TPA-PPI, PPI-TPA-SO2-1 and PPI-TPA-SO2-2.
Table 2. Summary of the theoretical calculations (DFT, B3LYP/6-
31G (d, p) and TD-DFT, M06-2X/6-31G (d, p), Gaussian 03W)
HOMOa LUMOa
Eg
S1
T1
a
b
b
a
g
compound
fb
(eV)
-4.89
-5.34
-5.14
(eV)
-0.79
-1.63
-1.39
(eV)
4.10
3.71
3.75
(eV) (eV)
3.84 3.06
3.95 3.08
3.91 3.06
(D)
4.15
7.49
TPA-PPI
1.72
1.23
As aforementioned, higher triplet energy level (T1) than the
phosphor is generally requested if the current blue fluorophors are
used as the emitter as well as the host of the phosphor. Time-
resolved phosphorescence spectra of these blue fluorophors
dissolved in 2-methyltetrahydrofuran were measured at 77 K at
different delayed times (Figure S4, see ESI). From the highest
energy peaks of the phosphorescence spectra, T1 values are
estimated to be 2.23, 2.30, and 2.30 eV for TPA-PPI, PPI-TPA-SO2-
1, and PPI-TPA-SO2-2, respectively, which are equal to or higher
than that of PO-01 (2.21 eV, corresponding to the emission peak).
Their singlet levels are 2.82, 2.82 and 2.78 eV derived from their
film emission peak. So a narrower S1/T1 split energy (0.52 and 0.48
eV) was acquired for PPI-TPA-SO2-1 and PPI-TPA-SO2-2.
PPI-TPA-SO2-1
PPI-TPA-SO2-2
10.44 1.59
aCalculated results of the ground state. bCalculated results of the first
excited state.
To gain further insight into the structure-property relationships of
the developed materials at the molecular level, quantum chemical
calculations were also performed at the B3LYP/6-31G(d,p) level to
investigate their ground states.18 As shown in Figure 2, for TPA-PPI,
the highest occupied molecular orbital (HOMO) locates on the TPA
moiety, while the lowest unoccupied molecular orbital (LUMO)
mostly locates on the PPI unit. Its HOMO and LUMO overlap at a
long range which gives the biggest oscillator strength (f = 1.72)
among the three compounds (Table 2). The introduction of the
sulfone unit endows tremendous effect on the distribution of
HOMOs and LUMOs of the sulfone-locked analogues. For PPI-
TPA-SO2-1, its HOMO and LUMO are obviously separated that the
HOMO locates on the sulfone-locked diphenylamine moiety while
the LUMO mostly located on the PPI unit. Due to the electron-
withdrawing ability of the sulfone lock, the HOMO of PPI-TPA-
SO2-1 was pulled towards the sulfone lock side and concentrated at
the diphenylamine part. So the overlap of the HOMO and LUMO of
PPI-TPA-SO2-1 was reduced and the oscillator strength of its first
excited state is the smallest among the three compounds (f = 1.23).
In contrast to TPA-PPI and PPI-TPA-SO2-1, the HOMO of PPI-
TPA-SO2-2 mainly locates on the PPI unit which is generally
As shown in Figure S5 (see ESI), the oxidization potentials (Eox)
of PPI-TPA-SO2-1 and PPI-TPA-SO2-2 were found to be 1.69 and
1.64 V. Under the same condition, ferrocene/ferrocenium (Fc/Fc+)
exhibits a redox potential of 0.25 V. Note that the redox potential of
Fc/Fc+ has an absolute energy level of -4.8 eV to vacuum, ionization
potentials (IPs) of PPI-TPA-SO2-1 and PPI-TPA-SO2-2 were
estimated to be -5.62 and -5.67 eV, respectively, according to IP = -e
(Eox + 4.55).17 Similarly, the reduction potentials (Ered) for PPI-TPA-
SO2-1 and PPI-TPA-SO2-2 were found to be -1.83 and -1.88 V, and
their electron affinities (EAs) were estimated to be -2.72 and -2.67
eV, respectively, according to EA = -e (Ered + 4.55). Compared with
the analogous compound TPA-PPI without the sulfone lock (IP = -
5.22 eV, EA = -2.27 eV),12b simultaneously reduced IP and EA
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ARTICLE
regarded as an electron-withdrawing component while the LUMO
Journal Name
To evaluate their electroluminescent performance, a series of blue
View Article Online
mostly locates on the two middle benzene rings. Because the sulfone fluorescent OLEDs were fabricated with PPDI-OTI:P1A0.-1S0O392/-C15TaCn0d13P7P3IA-
unit accesses to the entire conjugated system and has much stronger TPA-SO2-2 as the nondoped EML in a structure of ITO (95 nm)/
electron-withdrawing ability than imidazole, then its LUMO occurs HATCN (5 nm)/ NPB (40 nm)/ TCTA (5 nm)/ EML (20 nm)/ TPBI
at the place close to the sulfone unit giving a moderate oscillator (40 nm)/ LiF (1 nm)/ Al (80 nm). In this device, ITO (indium tin
strength (f = 1.59). As their similar molecular structure, the triplet oxide) and LiF/Al are the anode and the cathode, respectively; NPB
energy levels of these compounds could be affected by their dihedral (4,4’-bis[N-(1-naphthyl)-N-phenyl amino]biphenyl) is the hole-
angles between PPI and the sulfone-locked TPA. As shown in Figure transporting
layer
(HTL);
TCTA
(4,4’,4’’-tris(N-
3, the twisted angles between the central phenyls are 35.49º for carbazolyl)triphenylamine) is the electron-blocking layer; TPBI
TPA-PPI, 36.97º for PPI-TPA-SO2-1, and ‒34.53º for PPI-TPA- serves as the electron-transporting layer (ETL) and the hole-blocking
SO2-2, indicating the dihedral angles of PPI-TPA-SO2-1 and PPI- layer;
and
HATCN
(dipyrazino(2,3-f:2',3'-h)-quinoxaline-
TPA-SO2-2 increased or reversed compared with TPA-PPI. The 2,3,6,7,10,11-hexacarbonitrile) was introduced as an anode buffer
more twisted angles of the newly developed compounds ensure them layer to improve the hole injection to NPB (Figure S7, see ESI).
sufficient space resistance to inhibit aggregation-caused quenching Current density and luminance versus driving voltage and current
problem. Moreover, the more twisted angles may also restrain the efficiency versus current density characteristics are shown in Figure
conjugation of the backbone to give improved singlet and triplet S8 (see ESI). All the devices exhibited a pretty low turn-on voltage
energy levels.8 Their singlet (S1) and triplet energy levels (T1) are (~2.8 V), and the current density of the devices based on PPI-TPA-
also calculated, as shown in Table 2. The current sulfone-locked SO2-1 and PPI-TPA-SO2-2 are both improved compared with that
compounds have slightly higher triplet excited state energies than of the device based on TPA-PPI.
TPA-PPI which give positive effect as a host to fabricate F/P hybrid
As shown in Figure S9 (see ESI), all the devices exhibit quite
WOLEDs. It is also well consistent with the experimental results by
similar electroluminescence (EL) spectra which are well consistent
the phosphorescence measurement.
with their corresponding PL spectra in the thin film state and are
very stable in a wide driving current density (not shown here). As
shown in Table S2, at the practical application luminance of 100-
Ө1 = 35.49°
1000 cd m-2, both PPI-TPA-SO2-1 and PPI-TPA-SO2-2 realized
efficient deep blue emission, which is defined as having a
Commission Internationale de l’Eclairage (CIE) coordinate y ＜0.15
TPA-PPI
along with x+y ＜ 0.30. Besides, the EQE values are 3.12% and 4.67%
for the devices based on PPI-TPA-SO2-1 and PPI-TPA-SO2-2,
respectively, which are among the best of ever reported deep blue
Ө3 = ‒34.53°
emitters.12
Ө2 = 36.82°
The device based on PPI-TPA-SO2-2 exhibited the highest
efficiency among the devices, and it is suspected that the carrier
injection and balance could play an important role.
PPI-TPA-SO2-1
PPI-TPA-SO2-2
As well-known, the upper limit of EQE is:
Figure 3. Twisted angles between the central phenyls of TPA-PPI,
PPI-TPA-SO2-1, and PPI-TPA-SO2-2 based on their optimized
ground states.
EQEmax = ηop × φfl × ηr × γ ≈ ηop × φfl × 0.25 × γ
(1),
where the optical out-coupling factor (ηop) is about 0.25-0.3, the
possibility of a singlet exciton formed (ηr) is 25%, Φfl is PLQY (0.84,
0.71, and 0.68 for TPA-PPI, PPI-TPA-SO2-1, and PPI-TPA-SO2-2,
respectively), and γ is the carrier balance factor. Thus γ could be
calculated by equation (2):
The excited state energies of PPI-TPA-SO2-1 and PPI-TPA-SO2-
2 were calculated by M06-2X, including the first eight singlet and
triplet excited states (Figure S6). The calculated exchange energy
(∆EST) between S1 and T1 is 0.87 and 0.85 eV for PPI-TPA-SO2-1
and PPI-TPA-SO2-2, respectively. Such a huge energy barrier
between S1 and T1 is the main obstacle for up-conversion of the
triplet excited state to the singlet excited state to give thermal
activated delayed fluorescence (TADF).19 However, the energy
barriers between S1 and T6 for these two compounds are as narrow as
zero. So in an electroluminescent device, it might be possible that
the initially generated triplet excitons convert to the singlet excitons
through reversed intersystem crossing.20
γ = 4 EQEmax / (ηop × φfl)
(2),
and the calculated γ values are 0.65-0.78, 0.66-0.79, and 0.91-
1.09 for TPA-PPI, PPI-TPA-SO2-1, and PPI-TPA-SO2-2,
respectively, indicating the device based on PPI-TPA-SO2-2 exhibits
the most balanced carrier.
In order to evaluate its superior bipolar transporting property over
TPA-PPI, hole-only and electron-only devices were also fabricated.
Table 3. Summary of the device performance of the F/P hybrid WOLEDs with TPA-PPI, PPI-TPA-SO2-1, and PPI-TPA-SO2-2 as the blue
emitter as well as the host of PO-01 at 0.5 wt% doping concentration. CRI: color rendering index
at 100 cd m-2
at 1000 cd m-2
PE CIE
(%) (V) (cd A-1) (lm W-1)
(x, y)
Von
(V)
CEmax
PEmax
EQEmax
(%)
Host
V
CE
PE
CIE
EQE
V
CE
EQE CRI
(%)
(cd A-1) (lm W-1)
(V) (cd A-1) (lm W-1)
(x, y)
TPA-PPI
2.6
2.8
2.6
29.5
44.2
47.6
33.1
49.5
53.4
9.67
14.4
15.6
2.9
3.5
3.2
27.1
34.7
43.2
28.3
35.8
46.9
(0.42, 0.46) 9.02 3.2
(0.43, 0.48) 11.3 4.6
(0.43, 0.48) 14.2 4.1
18.2
23.7
30.3
17.8 (0.40, 0.43) 6.3
20.8 (0.40, 0.43) 8.2
43
44
PPI-TPA-SO2-1
PPI-TPA-SO2-2
28.9 (0.40, 0.43) 10.4 44
4 | J. Name., 2012, 00, 1-3
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Page 5 of 11
Journal of Materials Chemistry C
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Journal Name
ARTICLE
The configuration of the hole-only devices is ITO (95 nm)/ HATCN current density. At the practical application luminanceVoiefw1A0r0tic0lecOdnlmine–
(5 nm)/ NPB (40 nm)/ TCTA (5 nm)/ PPI-TPA, PPI-TPA-SO2-1 or 2, the forward-viewing CE and PE values of the PPI-TPA-SO2-2
DOI: 10.1039/C5TC01373A
PPI-TPA-SO2-2 (20 nm)/ NPB (40 nm)/ Al (80 nm), and the based device remain as high as 30.3 cd A–1 and 28.9 lm W−1 due to
electron-only devices have a structure of ITO (95 nm)/ TPBI (40 the reduced efficiency roll-off.
nm)/ TPA-PPI, PPI-TPA-SO2-1 or PPI-TPA-SO2-2 (20 nm) / TPBI
(40 nm)/ LiF (1 nm)/ Al (80 nm). As shown in Figure S10 (see ESI),
500
(a)
104
though the devices based on TPA-PPI exhibit the highest hole and
electron current density, the hole current density is higher than its
400
electron current density. It could be ascribed to its large electron
103
102
101
300
injection barrier from TPBI, small hole injection barrier and hole
dominant transporting property. In contrast, even though the hole
and electron current are both reduced for PPI-TPA-SO2-1 and PPI-
TPA-SO2-2, their electron transporting ability is stronger than their
corresponding hole transporting ability after TPA is locked by
sulfone, indicating their electron dominant transporting property. As
there is nearly no electron injection barrier from the ETL of TPBI to
the EML and improved hole injection barrier, improved carrier
balance could be achieved for both PPI-TPA-SO2-1 and PPI-TPA-
SO2-2 especially for PPI-TPA-SO2-2.
200
100
0
2
3
4
5
6
7
8
9
Voltage (V)
100
10
1
100
80
60
40
20
0
(b)
To fabricate F/P hybrid WOLEDs, PPI-TPA-SO2-1 and PPI-TPA-
SO2-2 were utilized as the blue emitter, and PO-01 with an emission
peak at 560 nm was utilized as the complementary phosphor.
Considering their high enough triplet energy, TPA-PPI, PPI-TPA-
SO2-1, and PPI-TPA-SO2-2 were adopted as the blue emitter,
meanwhile, as the host of the yellow phosphorescent emitter PO-01
to fabricate F/P hybrid WOLEDs with a single-EML configuration
of ITO (95 nm)/ HATCN (5 nm)/ NPB (40 nm)/ TCTA (5 nm)/
EML (20 nm)/ TPBI (40 nm)/ LiF (1 nm)/ Al. Different from the
nondoped blue OLEDs, PO-01 was doped into TPA-PPI, PPI-TPA-
SO2-1 or PPI-TPA-SO2-2 in a low concentration of 0.5 wt% to give
a single EML, and the 0.5 wt% PO-01 doping concentration is based
on our previous work without further optimization. In this structure,
singlet excitons are utilized for blue emission, and triplet excitons
with much longer diffusing length excite the highly decentralized
phosphorescent dopant PO-01 to give yellow emission. In an ideal
situation, both 25% singlet excitons and 75% triplet excitons can be
utilized in the F/P hybrid WOLEDs, leading to an unit exciton
utilization and an excellent efficiency. All the devices exhibit a low
turn-on voltage (<2.8 V) which is remarkable without using a p-i-n
device structure and is favorable for achieving high power efficiency
(Figure 4). As a result shown in Table 3, maximum forward-viewing
current efficiencies of 29.5, 44.2, 47.6 cd A–1, power efficiencies of
33.1, 49.5, 53.4 lm W–1, and EQE values of 9.67%, 14.4%, 15.6%
were respectively achieved for the F/P hybrid WOLEDs based on
TPA-PPI, PPI-TPA-SO2-1, and PPI-TPA-SO2-2 without employing
any light out-coupling technology. As illumination sources are
generally characterized by their total efficiency, the maximum total
PE values may reach 84.2 and 90.8 lm W-1 with a general factor of
1.7 for the devices based on PPI-TPA-SO2-1 and PPI-TPA-SO2-2,5
and they are hitherto among the most efficient F/P hybrid WOLEDs
in a single-EML architecture, as summarized in Table S3. Besides,
the power efficiencies of the F/P hybrid WOLEDs based on PPI-
TPA-SO2-1 and PPI-TPA-SO2-2 are even unexpectedly higher than
most previously reported highly efficient fully phosphorescent
WOLEDs, as summarized in Table S4. It is worth noting that
although the nondoped blue device based on PPI-TPA-SO2-1
exhibits lower efficiency than the device based on TPA-PPI, the CE
value of the F/P hybrid white device based on PPI-TPA-SO2-1 is 1.5
times higher than that of the device based on TPA-PPI. Further
higher CE value which is 1.6 times higher than the device based on
TPA-PPI is achieved for the device based on PPI-TPA-SO2-2, and it
could be attributed to the improved triplet exciton utilization in
WOLEDs. In addition, the doping concentration of 0.5 wt% is so
low that the triplet-triplet annihilation of the devices is minimized
and the efficiency roll-off is efficiently suppressed even at high
101
102
103
104
Luminance (cd/m2)
Figure 4. Current density and luminance versus driving voltage (a)
and current efficiency and power efficiency versus luminance (b)
characteristics of the F/P hybrid WOLEDs with TPA-PPI (○●),
PPI-TPA-SO2-1(△▲), and PPI-TPA-SO2-2( ◇ ◆ ) as the blue
emitter as well as the host of PO-01 at 0.5 wt% doping concentration.
The typical EL spectra of the F/P hybrid WOLEDs at a current
density of 1 mA cm-2 are shown in Figure 5, and the EL spectra at
different luminances are shown in Figure S11. All the EL spectra
could be divided into their blue emission corresponding to the
fluorophor host and yellow emission corresponding to the phosphor
guest. The phosphorescence part contributes mostly to the emission
for all the WOLEDs, indicating the efficiency of the
phosphorescence emission should undoubtedly be the decisive factor
for the F/P hybrid WOLEDs.
Transient PL spectra of the films doped with 0.5 wt% PO-01 were
measured by time-correlated single photon counting method under
the excitation of a nanosecond lamp (λ = 340 nm) (Figure 6). The
emission decay curves from the host detected at around 450 nm
exhibit quite similar lifetimes in nanosecond level, indicating
efficient singlet exciton radiative transition according to the kinetics
function (kr=Ф/. The emission decay curves from the phosphor
dopant detected at 560 nm resemble typical phosphorescence
emission with long lifetimes in microsecond level. Except for the co-
deposited film of PPI-TPA-SO2-1:PO-01 that gives a single
exponential decay of phosphorescence with a lifetime of 1.22 s,
TPA-PPI:PO-01 and PPI-TPA-SO2-2:PO-01 give double
exponential decays of phosphorescence with lifetimes of 0.719/2.27
and 0.798/1.94 s, respectively. As T1 of TPA-PPI (2.23 eV) is only
0.02 eV higher than that of PO-01 (2.21 eV), the longest decay time
(2.27 s) could originate from more easily reverse energy transfer
from the dopant to the host. Longer lifetime impairs the device
performance due to the bigger possibility of non-radiative transitions
of the triplet excitons.21 On the contrary, PPI-TPA-SO2-1 and PPI-
This journal is © The Royal Society of Chemistry 20xx
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Journal of Materials Chemistry C
Page 6 of 11
ARTICLE
Journal Name
TPA-SO2-2 both with T1 of 2.30 eV exhibit shorter decay times and
As the single-EML architecture is free of spacer tVoiewseAprtaicrlaetOentlihnee
thus much better triplet exciton confining ability, and it could be the singlet and triplet excitons, the singlet and triplet exciton separation
DOI: 10.1039/C5TC01373A
most important decisive factor of their F/P white device superiority.
and utilization efficiency of the host itself would surely be even
more important. The PL spectra of the neat films of TPA-PPI, PPI-
TPA-SO2-1, and PPI-TPA-SO2-2 and their co-deposited films with
0.5 wt% PO-01 were utilized to investigate the energy-transfer
mechanisms between the host and the dopant (Figure 7). As
blue OLED
WOLED
(a) TPA-PPI
f
: 0.135
TPA-PPI
f
: 0.865
PO-01
aforementioned, PLQY values of the neat films (Ф푏푙푢푒
푛푒푎푡 ) are 0.84,
0.71, and 0.68 for TPA-PPI, PPI-TPA-SO2-1, and PPI-TPA-SO2-2,
respectively. It is supposed only the host could be excited due to the
extremely low doping concentration (0.5 wt%) of the dopant and all
the generated excitons are singlet excitons under the photo exciting
process. On the one hand, one part of the generated singlet excitons
on the host would decay directly to give blue emission. On the other
hand, the other part of the singlet exciton reaches PO-01 to give
yellow emission. Similarly, PLQY values of the doped films
(Ф푤푑표ℎ푝ꢀ푡푒푒푑) are measured to be 0.86, 0.94, and 0.97 for TPA-PPI, PPI-
TPA-SO2-1, and PPI-TPA-SO2-2, respectively. As shown in Figure
7, the respective blue and yellow emission proportions were
calculated according to the integrations of the fitted corresponding
emission spectra for the host and the guest. The contribution of the
blue OLED
WOLED
(b) PPI-TPA-SO2-1
f
: 0.175
PPI-TPA-SO2-1
f
: 0.825
PO-01
blue OLED
WOLED
(c) PPI-TPA-SO2-2
blue fluorescence (Ф푏푙푢푒
푑표푝푒푑) for PLQYs of the doped films are 0.104,
f
: 0.162
PPI-TPA-SO2-2
f
: 0.838
0.162, and 0.147 for the doped films of TPA-PPI, PPI-TPA-SO2-1,
and PPI-TPA-SO2-2, respectively, as estimated by the following
equation (3):
PO-01
푏푙푢푒
푤ℎꢀ푡푒
푑표푝푒푑
Ф
= Ф
× 푓
(3),
푏푙푢푒
푑표푝푒푑
where 푓
is the contents of the blue emission from the host.
푏푙푢푒
400
450
500
550
600
650
700
750
푦푒푙푙표푤
Similarly, the contribution of the yellow phosphorescence (Ф
)
Wavelength (nm)
푑표푝푒푑
for PLQYs of the doped films are estimated to be 0.756, 0.778, and
Figure 5. EL spectra of the blue OLEDs and the F/P hybrid
WOLEDs based on TPA-PPI, PPI-TPA-SO2-1, and PPI-TPA-SO2-2
at a current density of 1 mA cm-2. Also shown are the fitted
emissions corresponding to the blue fluorophor (dashed blue lines)
and the yellow phosphor PO-01 (dashed yellow lines).
(a) TPA-PPI:0.5%PO-01
TPA-PPI
TPA-PPI:PO-01
f
: 0.121
TPA-PPI
f
: 0.879
PO-01
(a)
103
(b) PPI-TPA-SO2-1:0.5%PO-01
PPI-TPA-SO2-1
PPI-TPA-SO2-1:PO-01
102
f
: 0.172
TPA-PPI:0.5%PO-01
PPI-TPA-SO2-1:0.5%PO-01
PPI-TPA-SO2-2:0.5%PO-01
PPI-TPA-SO2-1
f
: 0.828
PO-01
101
100
0
20
40
60
80
100
 (ns)
(c) PPI-TPA-SO2-2:0.5%PO-01
PPI-TPA-SO2-2
PPI-TPA-SO2-2:PO-01
(b)
103
102
101
100
f
: 0.152
PPI-TPA-SO2-2
f
: 0.848
TPA-PPI:0.5%PO-01
PO-01
PPI-TPA-SO2-1:0.5%PO-01
PPI-TPA-SO2-2:0.5%PO-01
400
450
500
550
600
650
700
750
Wavelength (nm)
6
8
10
12
14
Figure 7. PL spectra of the neat films of TPA-PPI, PPI-TPA-SO2-
1, and PPI-TPA-SO2-2 and their co-deposited films with PO-01 at
a doping concentration of 0.5 wt% excited at 340 nm. Also shown
are the fitted emissions corresponding to the host (blue dashed
lines) and the phosphor (yellow dashed lines).
(us)
Figure 6. Transient PL decays of the co-deposited films of TPA-
PPI:PO-1, PPI-TPA-SO2-1:PO-01, and PPI-TPA-SO2-2:PO-01
detected at 450 nm (a) and 560 nm (b).
6 | J. Name., 2012, 00, 1-3
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Page 7 of 11
Journal of Materials Chemistry C
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Journal Name
ARTICLE
Table 4. Summary of the experimental and fitted photoluminescent emissions of the co-deposited films for TPA-PPI:PO-1, PPI-TPA-
View Article Online
DOI: 10.1039/C5TC01373A
SO2-1:PO-01, and PPI-TPA-SO2-2:PO-01.
푦푒푙푙표푤
푑표푝푒푑
푝ℎ표푡표
푥
푡푟푎푛푠
푤ℎꢀ푡푒
푑표푝푒푑
푏푙푢푒
푏푙푢푒
푏푙푢푒
푏푙푢푒
⁄
Ф푛푒푎푡
푑표푝푒푑
host
Ф
Ф
Ф
Ф
Ф
푑표푝푒푑
푛푒푎푡
TPA-PPI
0.84
0.71
0.68
0.86
0.94
0.97
0.104
0.162
0.147
0.756
0.778
0.823
0.124
0.228
0.216
0.876
0.772
0.784
PPI-TPA-SO2-1
PPI-TPA-SO2-2
0.823 for the doped films of TPA-PPI, PPI-TPA-SO2-2, and PPI- corresponding PLQY values.
TPA-SO2-1, respectively according to equation (4):
Meanwhile, the singlet and triplet excitons could be transferred to
the dopant, and the carriers might also be captured by the dopant to
generate excitons directly in the electroluminescent process. The
EQE variation of the WOLEDs could be affected by such process as
well. As shown in Table 5, at a typical current density of 1 mA cm–2,
the EQE values of the F/P hybrid WOLEDs (휂푊ℎꢀ푡푒 ) are 7.84%,
10.5%, and 12.8% for the devices based on TPA-PPI, PPI-TPA-
SO2-1, and PPI-TPA-SO2-2, respectively. The EL spectra of the
WOLEDs can also be divided into those from the fluorophor host
and the phosphor guest (Figure 5). It is of interest the contents of the
blue and yellow emissions in EL spectra are almost the same as
those in PL spectra (Figure 7), indicating most of the carriers are
recombined on the host molecules and the yellow electroluminescent
emission could be attributed to the energy transfer from the host to
the guest. According to the fitted results, the contributions of the
푦푒푙푙표푤
푑표푝푒푑
푤ℎꢀ푡푒
푑표푝푒푑
Ф
= Ф
× 푓
(4),
푦푒푙푙표푤
where 푓
is the contents of the yellow emission from the
푦푒푙푙표푤
guest. The fraction of the singlet excitons transferred to the dopant
(푥푡푝푟ℎ푎표푛푡푠표) under the photo exciting process could be defined as the
following equation (6):
푝ℎ표푡표
푡푟푎푛푠
푏푙푢푒
푏푙푢푒
Ф
= Ф
(1 − 푥
)
(5),
(6).
푛푒푎푡
푑표푝푒푑
푝ℎ표푡표
푡푟푎푛푠
푏푙푢푒
푏푙푢푒
⁄
Ф푛푒푎푡
푑표푝푒푑
푥
= 1 − Ф
푝ℎ표푡표
푡푟푎푛푠
The 푥
values calculated for the doped films of TPA-PPI,
PPI-TPA-SO2-1, and PPI-TPA-SO2-2 are 0.876, 0.772, and 0.784,
respectively, as shown in Table 4. It indicates a larger amount of
singlet excitons is transferred to the dopant to suppress the blue
emission of the host TPA-PPI itself compared with PPI-TPA-SO2-1
and PPI-TPA-SO2-2. However, the captured excitons by the
phosphor dopant could not decay efficiently as they contribute the
smallest yellow phosphorescence content for PLQY. The doped
films of PPI-TPA-SO2-1 and PPI-TPA-SO2-2 exhibit higher
efficiency than the doped film of TPA-PPI for both the blue and
blue fluorescence emission ( 휂퐹−푃
WOLEDs are 1.06%, 1.84%, and 2.07%, and the contributions of the
yellow phosphorescence emission (휂퐹−푃
푌푒푙푙표푤) are 6.78%, 8.66%, and
퐵푙푢푒 ) to the EQE values of the
10.73% for the devices based on TPA-PPI, PPI-TPA-SO2-1, and
PPI-TPA-SO2-2, respectively.
yellow emission parts even though their neat film PLQY values are
푦푒푙푙표푤
푑표푝푒푑
inferior to TPA-PPI. Especially for PPI-TPA-SO2-2, the Ф
푝ℎ표푡표
value is even larger than the 푥
value, indicating some of the
푡푟푎푛푠
exciton energy which nonradiatively decays in the neat film of PPI-
TPA-SO2-2 could even be transfered to the phosphor dopant in the
co-deposited film to give radiative decay (Figure 8). Benefited from
better triplet exciton confining ability, although less amount of
singlet excitons transfers to PO-01, more photons can be released for
the systems of PPI-TPA-SO2-1 and PPI-TPA-SO2-2, indicating both
PPI-TPA-SO2-1 and PPI-TPA-SO2-2 have superior singlet and
triplet exciton separation and utilization ability as a host. In
comparison, the defect of TPA-PPI as a host could even be amplified
due to a larger portion of 75% triplet excitons initially generated in
an electroluminescent process. Lower singlet and triplet exciton
separation and utilization ability would cause greater exciton waste
and lower efficiency for the F/P hybrid WOLEDs based on TPA-PPI
compared with PPI-TPA-SO2-1 and PPI-TPA-SO2-2. And the
performance difference between the devices based on PPI-TPA-
SO2-1 and PPI-TPA-SO2-2 are in accordance to the trend of their
Figure 8. A schematic diagram for the photoluminescence process
of (a) the neat films of PPI-TPA-SO2-1 and PPI-TPA-SO2-2 and (b)
the co-deposited films with PO-01 at a doping concentration of 0.5
wt% excited at 340 nm. Part of the singlet excited energy that is
nonradiatively decayed in the neat film is transferred to the PO-01
dopant to give phosphorescence and reduced nonradiative transition.
A: absorption, F: fluorescence, IC: internal conversion, P:
phosphorescence, and ISC: intersystem conversion.
On the assumption that the WOLEDs have the same optical out-
coupling factor (ηop) and carrier balance factor (γ) as the nondoped
blue OLEDs, the singlet excitons contributed to the blue
Table 5. Summary of the experimental and fitted electroluminescent emissions of the F/P hybrid WOLEDs based on TPA-PPI, PPI-TPA-
SO2-1, and PPI-TPA-SO2-2 at 1 mA cm-2.
휂퐵푙푢푒
(%)
휂푊ℎꢀ푡푒
(%)
휂
휂
휂퐵푆푙푢푒
(%)
휂
휂
푌푒푙푙표푤
퐹−푃
퐹−푃
푆+푇
푆+푇
퐵푙푢푒
푌푒푙푙표푤
퐵푙푢푒
host
(%)
(%)
(%)
(%)
TPA-PPI
4.11
3.34
4.54
7.84
1.06
1.84
2.07
6.78
8.66
10.73
25.8
54.8
45.8
6.45
13.7
11.45
93.55
86.3
PPI-TPA-SO2-1
PPI-TPA-SO2-2
10.5
12.8
88.55
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Journal of Materials Chemistry C
Page 8 of 11
ARTICLE
Journal Name
electroluminescence among all the electro-generated singlet excitons and 53.4 lm W-1, and EQE values of 14.4% andV1ie5w.6Ar%tic.le BOnolitnhe
(휂퐵푆푙푢푒) is only 25.8% for the WOLED based on TPA-PPI (Table 5),
are significantly improved because of theiDrOsIu: 1p0e.1r0io3r9/sCi5nTgCle0t13a7n3dA
triplet exciton separation and utilization ability over TPA-PPI,
as proved by the energy-transfer in both the photoluminescent
and electroluminescent processes. The current values are also
hitherto the highest efficiency for the F/P hybrid WOLEDs in a
single-EML architecture. The current findings also provide
instructive insight into the molecular design of blue fluorophor
and host for F/P hybrid WOLED applications.
as estimated by the following equation (7):
휂퐵푆푙푢푒 = 휂
휂퐵푙푢푒
(7),
퐹−푃
⁄
퐵푙푢푒
where 휂퐵푙푢푒 is the EQE value of the nondoped blue fluorescent
device. Considering 25% of the excitons are singlet excitons in the
electroluminescent process, the excitons contributed to the blue
푆+푇
electroluminescence ( 휂
) among all the electro-generated
퐵푙푢푒
excitons is only 6.45%. The rest of the excitons (휂푌푆푒+푙푇푙표푤=93.55%)
EXPERIMENTAL SECTION
may contribute to the yellow electroluminescent emission or
Measurements and characterization
푆+푇
nonradiatively decays to the ground state. It is of interest the 휂
퐵푙푢푒
1H and 13C NMR spectra were measured on a Bruker NMR
spectrometer operating at 600 and 150 MHz, respectively, in
deuterated chloroform (CDCl3) solution at room temperature.
Differential scanning calorimetry (DSC) measurements were
operated on a Netzsch DSC 209 under a N2 flow at a heating and
cooling rate of 10 °C min–1. Thermogravimetric analyses (TGA)
were recorded on a Netzsch TG 209 under a N2 flow at a heating rate
of 10 °C min–1. UV-vis absorption spectra were recorded on a HP
8453 spectrophotometer. Photoluminescence (PL) spectra were
values for the WOLEDs based on PPI-TPA-SO2-1 (13.7%) and PPI-
TPA-SO2-2 (11.45%) are almost two times larger than that of the
푆+푇
device based on TPA-PPI (6.45%). The relatively lower 휂
value
퐵푙푢푒
for the device based on TPA-PPI further indicates the
phosphorescent dopant in TPA-PPI may suppress the blue emission
from TPA-PPI through energy transfer and/or nonradiative
푆+푇
quenching. Although the 휂
values for the devices based on
푌푒푙푙표푤
PPI-TPA-SO2-1 (86.3%) and PPI-TPA-SO2-2 (88.55%) are lower
than that of the device based on TPA-PPI (93.55%), their
contributions to the yellow phosphorescence emission (휂퐹−푃
푌푒푙푙표푤) are
measured using
a
Jobin-Yvon spectrofluorometer. Cyclic
voltammetry (CV) was performed on a CHI600D electrochemical
work station with a Pt working electrode and a Pt wire counter
electrode at a scanning rate of 100 mV s–1 against a Ag/Ag+ (0.1 M
of AgNO3 in acetonitrile) reference electrode with a nitrogen-
saturated anhydrous acetonitrile and dichloromethane (DCM)
solution of 0.1 mol L–1 tetrabutylammonium hexafluorophosphate.
PL quantum yields (PLQYs) in solution and film were measured by
using an integrating sphere on a HAMAMATSU absolute PL
quantum yield spectrometer C11347. Transient PL was measured
with an Edinburgh FL920 fluorescence spectrophotometer. The thin
solid films used for absorption and PL spectral measurement were
vacuum vapor deposited on quartz substrates. Matrix-assisted laser-
desorption/ionization time-of-flight (MALDI-TOF) mass spectra
were recorded on a Bruker BIFLEXIII TOF mass spectrometer.
much higher than the device based on TPA-PPI. It further proves
their superior singlet and triplet exciton separation and utilization
ability over TPA-PPI.
Last but not the least, the carrier injection and transporting
ability would unavoidably affect the carrier recombination efficiency
and thus the device performance. In order to investigate the potential
effect of the phosphor dopant on the carrier transporting ability,
hole-only and electron-only devices were also fabricated at a doping
concentration of 0.5 wt%. The configuration of the hole-only devices
is ITO (95 nm)/ HATCN (5 nm)/ NPB (40 nm)/ TCTA (5 nm)/ TPA-
PPI, PPI-TPA-SO2-1 or PPI-TPA-SO2-2: 0.5 wt% PO-01 (20 nm)/
NPB (30 nm)/ Au (80 nm), and the electron-only devices have a
structure of ITO (95 nm)/ TPBI (45 nm)/ TPA-PPI, PPI-TPA-SO2-1
or PPI-TPA-SO2-2: 0.5 wt% PO-01 ( 20 nm)/ TPBI (40 nm)/ LiF (1
nm)/ Au (80 nm). As shown in Figure S12, similar to the nondoped
carrier-only devices, the TPA-PPI based devices show the biggest
hole current, and its hole current surpasses its electron current,
indicating its hole dominant transporting property. In contrast, the
electron current of the devices based on PPI-TPA-SO2-1 and PPI-
TPA-SO2-2 is larger than the hole current due to the lower electron
injection barrier from the ETL of TPBI to the active layer. The
similar tendency in hole and electron current of the nondoped and
doped devices further indicates most of the carriers should be
transported and recombined on the host molecules and then part of
the exciton energy could be transferred to the phosphor guest to give
complementary yellow emission.
Theoretical Calculation
Density functional theory (DFT) calculations were performed on
B3LYP/6-31G(d, p) basis set based on Gaussian suite of programs
(Gaussian 03W).17 Spacial distributions of the HOMOs and LUMOs
of the compounds were obtained from the optimized ground state
structures. The M06-2X functional, which is well-known for
intermediate description of the charge-transfer systems, was utilized
to gain further insight into the character of the excited states. Based
on the optimized structures of the ground states, the excited singlet
energy (ES) and triplet energy (ET) were further calculated to
investigate the split energy (ΔEST) between ES and ET.
Device Fabrication and Characterization
Conclusions
Glass substrates pre-coated with a 95 nm thin layer of indium tin
oxide (ITO) with a sheet resistance of 10 Ω per square were
carefully cleaned by acetone, isopropyl alcohol, detergent, deionized
water, and isopropyl alcohol under ultrasonic bath and treated with
O2 plasma for 20 min in sequence. Organic layers were deposited
onto the ITO-coated glass substrates by thermal evaporation under
high vacuum (<5×10−4 Pa). Cathode, consisting of a 1 nm thin layer
of LiF followed by a 90 nm thin layer of Al, was patterned using a
shadow mask with an array of 3 mm×3 mm openings. Deposition
rates are kept at 1~2 Å/s for organic materials, 0.1 Å/s for LiF, and 6
Å/s for Al. Electroluminescence (EL) spectra were measured by an
optical analyzer, Photo Research PR705. The current density and
luminance versus driving voltage were detected by Keithley 2420
and Konica Minolta chromameter CS-200. EQE was calculated from
In summary, two novel isomeric deep blue fluorophors of PPI-
TPA-SO2-1 and PPI-TPA-SO2-2 combined with PPI and
sulfone-locked TPA were designed and synthesized. The
sulfone lock induces larger twisted angles between PPI and
sulfone-locked TPA, thus higher triplet energy levels and better
triplet exciton confining ability compared with their analogue
TPA-PPI without the sulfone lock. In addition, different from
the hole dominant transporting property of TPA-PPI, the
sulfone-locked
analogues
exhibit
electron
dominant
transporting property to give more balanced carriers. Thus, the
F/P hybrid WOLEDs based on PPI-TPA-SO2-1 and PPI-TPA-
SO2-2 as the host of yellow phosphor as well as the blue
emitter achieved maximum forward-viewing current
efficiencies of 44.2 and 47.6 cd A-1, power efficiencies of 49.5
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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Page 9 of 11
Journal of Materials Chemistry C
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Journal Name
ARTICLE
the luminance, current density, and EL spectrum on the premise of a 2H), 7.90 (d, J = 8.4 Hz, 2H), 7.84-7.49 (m, 11H), V7ie.w52A-rt7ic.l3e4On(lmine,
Lambertian distribution.
4H), 7.33-7.23 (m, 6H), 6.70 (d, J = 8.6 HDzO, I2:H10)..10133C9/CN5MTCR013(1753A0
MHz, CDCl3, δ, ppm): 140.73, 132.80, 130.90, 130.35, 130.08,
129.81, 129.12, 127.01, 124.20, 123.47, 123.17, 122.69, 122.13,
120.91, 117.21, 77.23, 77.02, 76.81. MS (MALDI-TOF): m/z calcd
for C45H29N3O2S: 675.2; found: 676.188. Anal. Calcd. for
C45H29N3O2S: C, 79.98; H, 4.33; N, 6.22; O, 4.73; S, 4.74. Found: C,
79.88; H, 4.53; N, 6.14; S, 4.46
Materials
All solvents and regents were used as received from commercial
suppliers without further purification. 10-Phenyl-10H-phenothiazine
(4)15a and 1-phenyl-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)phenyl)-1H-phenanthro[9,10-d]imidazole (7)15b were synthesized
according to the literature procedures. Synthetic routes of the
intermediates and the target compounds PPI-TPA-SO2-1 and PPI-
TPA-SO2-2 are outlined in Scheme 1. TPA-PPI was synthesized
according to the reported procedures.12b The developed target
materials were further purified by repeated temperature gradient
vacuum sublimation.
3-2’-(1’-phenyl)-1’-phenyl-1’H-phenanthro[9’,10’-d]imidazole-10-
phenyl-10H-phenothiazine-S,S-dioxide (PPI-TPA-SO2-2). PPI-
TPA-SO2-2 (1.02 g, yield 76%) was synthesized as a white solid in
1
a similar manner of PPI-TPA-SO2-1 with 6 instead of 3. H NMR
(600 MHz, CDCl3, , ppm): 8.89 (d, J = 7.8 Hz, 1H), 8.74 (dd, J =
37.9 and 8.4 Hz, 2H), 8.36 (d, J = 2.2 Hz, 1H), 8.19 (dd, J = 7.9 and
1.4 Hz, 1H), 7.77-7.60 (m, 11H), 7.53 (m, 5H), 7.39 (m, 3H), 7.26-
7.19 (m, 3H), 6.65 (dd, J = 27.1 and 8.8 Hz, 2H). 13C NMR (151
MHz, CDCl3, δ, ppm):140.59, 139.97, 138.82, 134.26, 132.84,
131.40, 131.15, 130.34, 129.90, 129.31, 129.10, 128.29, 127.30,
126.34, 125.65, 124.93, 124.11, 123.50, 123.11, 122.92, 122.76,
122.19, 121.35, 120.90, 117.87, 117.26, 77.22, 77.01, 76.80. MS
(MALDI-TOF): m/z calcd for C45H29N3O2S: 675.2; found: 676.178.
Anal. Calcd. for C45H29N3O2S: C, 79.98; H, 4.33; N, 6.22; O, 4.73;
S, 4.74. Found: C, 79.85; H, 4.51; N, 6.13; S, 4.58
10-(4-bromophenyl)-10H-phenothiazine (2). Phenothiazine (3.98 g,
20 mmol) in dry N,N-dimethylformamide (DMF) (60 mL) was
stirred under argon. 1,4-Bromoiodobenzene (8.46 g, 30 mmol, 1.5
equiv) was added with vigorous stirring. Then copper powder (1.3 g,
20 mmol) was added. The reaction mixture was stirred at 160°C
under the protection of nitrogen gas. After stirred for 48 h, the
reaction mixture was extracted with dichloromethane (DCM) and
further purified by column chromatography to afford 2 (3.82 g, yield
54%) as a white solid. 1H NMR (600 MHz, CDCl3, , ppm): 7.71 (d,
J = 7.3 Hz, 2H), 7.26 (s, 4H), 7.04 (d, J = 8.9 Hz, 2H), 6.88 (s, 2H),
6.25 (s, 2H).
Acknowledgements
The authors greatly appreciate the financial support from the
Ministry of Science and Technology (2015CB655003 and
2014DFA52030), the National Natural Science Foundation of
China (91233116 and 51073057), the Ministry of Education
(NCET-11-0159), and the Guangdong Natural Science
Foundation (S2012030006232). The authors are grateful to Liang
Yao for his help in low temperature phosphorescence
measurements.
10-(4-bromophenyl)-10H-phenothiazine–S,S-dioxide (3). A mixture
of 2 (7.06 g, 20 mmol), 90 mL of DCM, 45 mL of AcOH, 5.2 mL of
H2O2 (100 mmol, 2.5 equiv) was stirred under atmosphere at 80°C
for 24 h. The reaction mixture was extracted with DCM and further
purified by column chromatography to afford 3 (6.21 g, yield 88%)
as a white solid. H NMR (600 MHz, CDCl3, , ppm): 8.20 (dd, J =
7.9 and 1.3 Hz, 2H), 7.86 (d, J = 8.5 Hz, 2H), 7.51-7.35 (m, 2H),
7.35-7.20 (m, 4H), 6.64 (d, J = 8.6 Hz, 2H).
1
Notes and references
3-bromo-10-phenyl-10H-phenothiazine
(5).
10-Phenyl-10H-
phenothiazine (4) (5.5 g, 20 mmol) was dissolved in 80 mL of DCM.
Bromine (4.8 g, 30 mmol, 1.5 equiv) was added slowly with ice
bath, and the ice bath was removed after the addition of bromine.
The reaction mixture was stirred at room temperature for 5 h. After
the reaction, the residual bromine was quenched by NaHSO3, and the
reaction mixture was extracted by DCM. The solvent was removed
by vacuum distillation to give a brown oil (6.2 g). The product is a
mixture containing 5 and could not be purified and was used directly
for the next step.
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10 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5TC01373A
Table of Contents entry:
Deep blue fluorophors incorporated sulfone-locked triphenylamine were developed
for highly efficient fluorescence/phosphorescence hybrid white OLEDs.