Improving the electroluminescence performance of donor-acceptor molecules by fine-tuning the torsion angle and distance between donor and acceptor moieties

Article abstract of DOI:10.1039/c6tc01269h

The torsion angle and distance between the donor (D) and the acceptor (A) are two important factors in determining the photoluminescence and electroluminescence properties of twisted D-A type organic molecules. Here, two new D-A compounds, 2-(10-butyl-10H-phenothiazin-3-yl)-5-phenyl-1,3,4-oxadiazole (PO) and 2-(4-(10-butyl-10H-phenothiazin-3-yl)phenyl)-5-phenyl-1,3,4-oxadiazole (PPO), were designed and synthesized to tune the torsion angle and distance between D and A moieties, and their photophysical and electroluminescence properties were investigated. The D-A type molecule PO has a planar conformation, whereas the D-π-A type molecule PPO has a twisted conformation because of the insertion of the phenyl bridge between the donor and the acceptor. Therefore, the charge transfer (CT) of PPO is much stronger than that of PO, and the singlet exciton yield of PPO may be higher than that of PO. On the other hand, the introduction of a phenyl unit can also improve the photoluminescence quantum efficiency (doped film Φ_PL ≈ 70%). As a result, the PPO-doped device showed better device performance than PO. The device based on PPO as an emitter exhibited stable and high luminous efficiency (15.2 cd A^-1, corresponding to an external quantum efficiency of 5.4%), which is increased by about 1.05 fold as compared to the device based on PO as an emitter (luminous efficiency 7.4 cd A^-1 and external quantum efficiency 2.9%).

Full text of DOI:10.1039/c6tc01269h

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Improving the Electroluminescence Performance of
Donor−Acceptor Molecule by Fine Tuning the Torsion Angle
Received 00th January 20xx,
Accepted 00th January 20xx
and Distance Between Donor and Acceptor Moieties
†
Xu Qiu, Shanfeng Xue*, Yongjin Wu, Mingshuai Chen, Qikun Sun, Wenjun Yang*
DOI: 10.1039/x0xx00000x
The torsion angle and distance between donor (D) and acceptor (A) are two important factors in determining the
photoluminescence and electroluminescence properties of twisted D–A type organic molecules. Here, two new D−A
compounds, 2ꢀ(10ꢀbutylꢀ10Hꢀphenothiazinꢀ3ꢀyl)ꢀ5ꢀphenylꢀ1,3,4ꢀoxadiazole (PO) and 2ꢀ(4ꢀ(10ꢀbutylꢀ10Hꢀphenothiazinꢀ3ꢀ
yl)phenyl)ꢀ5ꢀphenylꢀ1,3,4ꢀoxadiazole (PPO) were designed and synthesized to tune the torsion angle and distance between
the DꢀA, and their photophysical and electroluminescent properties were investigated. The DꢀA type molecule PO has a
planar conformation, whereas the DꢀπꢀA type molecule PPO has a twisted conformation because of the insertion of the
phenyl bridge between donor and acceptor. Therefore, the charge transfer (CT) of PPO is much stronger than PO, and the
singlet exciton yield of PPO may be higher than that of PO. On the other hand, the introduction of phenyl unit can also
improve the photoluminescence quantum efficiency (doped film ΦPL ≈ 70%). As a result, the PPOꢀdoped device showed
better device performance than PO. The device based on the PPO as emitter exhibited stable and high luminous efficiency
(15.2 cd Aꢀ1, corresponding external quantum efficiency of 5.4%), which is increased by about 1.05 fold as compared to the
device based on the PO as emitter (the luminous efficiency of 7.4 cd Aꢀ1 and the external quantum efficiency of 2.9%).
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higher than the normally accepted value of 25% for
conventional fluorescent materials.38‒40 However, the CT state
is harmful for achieving high photoluminescence quantum
Introduction
Since the pioneering work by C. W. Tang and his co-workers,
Organic light-emitting diodes (OLEDs) have been hot
efficiency (ΦPL) because of the forbidden electronic transition
between the spatially separated orbitals.41, 42 In contrast, the
locally excited (LE) state usually endows fluorescent molecule
with high photoluminescence quantum efficiency because of
the large transition moment with almost complete orbital
overlap,28 but the spin flip from triplet state to singlet state is
limited to the strongly coulomb bounded hole−electron pair of
a
research topic because of their potential applications in
lighting and display.1 Despite the optoelectronic properties of
OLEDs have been investigated meticulously and great progress
have been made, there are still many challenges to improve
their stability, brightness and luminous efficiency from both
materials and devices aspects.2‒14 Recently, some novel types
of organic fluorescent materials have aroused great interest
because the triplet excitons utilization through a reverse
intersystem crossing (RISC) process,15‒22 triplet–triplet
annihilation (TTA),23‒27 and hybridized local and charge
transfer (HLCT) states.28–37 The characteristics of the molecular
structure are mainly concentrated in donor (D)‒acceptor (A)
motif with unique excited state properties. The D‒A structure
often results in the formation of a charge transfer (CT) excited
state with a weak coulomb bounded hole−electron pair, and
thus the singlet exciton yield under electrical injection may be
LE exciton and lead to
a
low singlet exciton yield.30
Consequently, for D−A molecules, regulaꢀng the LE and CT
state properties for achieving simultaneously high
photoluminescence quantum efficiency and singlet exciton
formation are still a challenge for acquiring high device
efficiency.
As for D‒A structure molecular design, the donor and acceptor
units should be carefully chosen to control the CT state energy
and LE state energy. 10-Butyl-10H-phenothiazine is a non-
planar heterocyclic compound with butterfly conformation
and electron-rich sulfur and nitrogen atoms. Comparing with
only nitrogen atom containing donor such as carbazole and
diphenylamine, etc. the additional electron-rich sulfur atom in
10-butyl-10H-phenothiazine could further enhance the
electron-donating ability. Meanwhile, its non-planar
“butterfly” structure could hinder intermolecular π-stacking
aggregation and excimer formation.43‒46 On the other hand, 2-
phenyl-1,3,4-oxadiazole is an electron-deficient unit easy-to
forming planar structure and has been widely used building
Key Laboratory of Rubber-plastics of Ministry of Education/Shandong Province
(QUST), School of Polymer Science & Engineering, Qingdao University of Science
&Technology, 53-Zhengzhou Road, Qingdao, 266042, P. R. China. E-mail:
sfxue@qust.edu.cn, ywjph2004@qust.edu.cn
†
Electronic supplementary information (ESI) available: The corresponding
synthetic routes and molecular structures characterization of the intermediate
products; the UVꢀvis absorption spectra and PL spectra in THF solution and in thin
film of PO and PPO; and lifetime measurement of the PO and PPO in hexane,
isopropylether, tetrahydrofuran and acetonitrice solution. See DOI:
10.1039/x0xx00000x
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ARTICLE
blocks to construct electron-transporting materials for containing
Journal Name
0.1
M
tetrakis(n-butyl)ammonium
OLEDs.47‒49 In view of this, in this work, 10-butyl-10H- hexafluorophosphate (NBu4PF6) as the supporting electrolyte
phenothiazine is selected as the donor and 2-phenyl-1,3,4- were used as solvents under a nitrogen atmosphere. All
oxadiazole is used as the acceptor to construct novel D‒A solutions were purged with a nitrogen stream for 10 min
molecules. The D and A moieties are direct linked or indirectly before measurement. The procedure was performed at room
by a phenyl ring, and the corresponding asymmetric D‒A and temperature, and a nitrogen atmosphere was maintained over
−A structure molecules PO and PPO are constructed.
D−π
We the solution during measurements. A scan rate of 50 mV s-1
hope that the introduction of phenyl ring could tune the was applied.
torsion angle and distance between D and A moieties to
Theoretical Calculation
regulate the singlet exciton yield and CT state characters. It is
found that the PPO affords higher fluorescence efficiency than The ground-state geometries were optimized at the level of
PO, and the device based on the PPO as emitter exhibits fairly B3LYP/6-31G(d, p), spatial distributions of the HOMOs and
stability and higher luminous efficiency (15.2 cd A-1), which is LUMOs of the compounds were obtained from the optimized
increased by about 105%, as compared to that based on the ground state structures. On the basis of the optimized
PO as emitter. The results show that fine tuning the torsion configuration of the ground state (S0), the high excitation
angle and distance between the donor and acceptor could energy levels of singlet and triplet states were evaluated using
indeed improve CT state properties. This is helpful for deeply TD-M06-2X/6-31G(d, p). In order to gain further insight into
understanding the molecular structure-property relationships the character of excited states, natural transition orbitals
and developing novel fluorescent materials applicable in (NTOs) were evaluated for S1 states.
OLEDs.
Synthesis
Scheme
1 illustrates the synthetic routes and chemical
Experiment section
General information
Materials
structures of the target compounds PO and PPO. The
corresponding synthetic routes and molecular structures
characterization of the intermediate products were shown in
support information-1. To begin with, 10-butyl-10H-
phenothiazine-3-carbaldehyde and benzoyl hydrazine in
CH3CH2OH was refluxed to generate (E)-N'-benzylidene-10-
butyl-10H-phenothiazine-3-carbohydrazide, and then followed
by addition of potassium carbonate, iodine in sequence. The
reaction mixture was stirred at 100 °C for 4 h. Finally, PO
compound was obtained and the corresponding yield is 76%.
PPO was obtained through Suzuki coupling reaction between
3-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-10-n-butyl-
10H-phenothiazine and 2-(4-bromophenyl)-5-phenyl-1,3,4-
oxadiazole, and the corresponding yield is 82%. The molecular
structures of PO and PPO were confirmed by 1H and 13C, MS.
All raw materials were commercially purchased from Aldrich
Chemical Co. or Energy Chemical Co., China (10-butyl-10H-
phenothiazine, benzohydrazide, 4-bromobenzoyl chloride,
catalysts and charge-transporting materials). Tetrahydrofuran
(THF)
was
distilled
over
metallic
sodium
and
dimethylformamide (DMF) over calcium hydride before use.
The orther organic solvents and reagents were all
commercially available analytical-grade products and used as
received without further purification.
Measurements
1H and 13C NMR spectra were recorded on a Bruker AC500
spectrometer at 500 and 125 MHz, respectively, when the
compounds were dissolved in deuterated chloroform (CDCl3)
with tetramethylsilane (TMS) as the internal standard. The
MALDI-TOF-MS mass spectra were measured using an AXIMA-
CFRTM plus instrument. UV−vis absorpꢀon and fluorescence
spectra of solution and films were measured by the Hitachi U-
4100
spectrophotometer
and
Hitachi
F-4600
Scheme 1 Synthetic routes and chemical structures of the target materials PO and PPO.
(a) CH3CH2OH, 80 oC, 11 h; (b) K2CO3, I2, DMSO, 100 oC, 4 h; (c) Pd(pph3)4, K2CO3,
THF/H2O, 70 oC, 24 h.
spectrophotometer,
respectively.
Photoluminescence
quantum yield, the low temperature fluorescence spectrum
and low temperature phosphorescence spectrum were carried
out with an FLS980 spectrometer. The lifetimes of doped films
were measured on an Edinburgh FLS-980 with an EPL-375
optical laser. Cyclic voltammetry (CV) was performed with a
BAS 100W Bioanalytical Systems, using a glass carbon disk
(diameter = 3 mm) as the working electrode, a platinum wire
with a porous ceramic wick as the auxiliary electrode, and
Ag/Ag+ as the reference electrode standardized by the redox
Synthesis of 2-(10-butyl-10H-phenothiazin-3-yl)-5-phenyl-1,3,4-
oxadiazole (PO)
A solution of M1 (1.00 g, 3.53 mmol) and benzoyl hydrazine
(0.58 g, 4.23 mmol) in CH3CH2OH (40 mL) was refluxed 11 h,
and then the solvent was evaporated under reduced pressure,
and the resulting residue was redissolved in dimethyl sulfoxide
(DMSO, 25 mL), followed by addition of potassium carbonate
(1.46 g, 10.59 mmol), iodine (1.07 g, 4.23 mmol) in sequence.
The reaction mixture was stirred at 100 °C for 4 h. After being
couple
ferrocenium/ferrocene.
Anhydrous
N,
N-
dimethylformamide (DMF) and dichloromethane (CH2Cl2)
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cooled to room temperature, it was treated with 5% Na2S2O3 thickness/ratio monitor (STM-100/MF, Sycon). The device
(20 mL), extracted with ethyl acetate. The combined organic structures with ITO/PEDOT:PSS(40 nm)/ TAPC (50 nm)/ 20%PO
layer was washed with brine, dried over anhydrous or PPO: MADN (20 nm)/ TPBi (40 nm)/ LiF (0.5 nm)/ Al (120
magnesium sulfate, and concentrated. The given residue was nm). Electroluminescence (EL) spectra were measured by a
purified through silica gel column chromatography using ethyl PR650 fluorescence spectrophotometer. Luminance−voltage
acetate/ petroleum ether (1:10; v/v) as eluent to give the and current density−voltage characteristics were recorded
product as yellow green solid (1.33 g, 76.0%). 1H NMR (500 simultaneously by combining the spectrometer with a Keithley
MHz, CDCl3): δ 8.13 (dd, J = 7.4, 2.0 Hz, 2H), 7.92 (dd, J = 8.5, model 2400 programmable voltage−current source. EQE was
1.9 Hz, 1H), 7.84 (d, J = 1.9 Hz, 1H), 7.58 – 7.50 (m, 3H), 7.17 calculated from the luminance, current density, and EL
(m, 2H), 6.96 (dd, J = 14.1, 7.9 Hz, 2H), 6.90 (d, J = 8.1 Hz, 1H), spectrum according to the literature.
3.91 (t, J = 7.2 Hz, 2H), 1.88 – 1.77 (m, 2H), 1.53 – 1.43 (m, 2H),
0.96 (t, J = 7.4 Hz, 3H). 13C NMR (125 MHz, CDCl3): δ 164.17,
Results and Discussion
164.08, 148.32, 144.08, 131.54, 129.03, 127.54, 126.85, 126.36,
Thermal Properties
125.58, 124.08, 123.92, 123.19, 117.78, 115.77, 115.29, 47.46,
28.90, 20.10, 13.78. MALDI-TOF MS (mass m/z): 400.3 [M+].
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were carried out under nitrogen atmosphere
to investigate the thermal properties of PO and PPO. PPO
Synthesis of 2-(4-(10-butyl-10H-phenothiazin-3-yl)phenyl)-5-
phenyl-1,3,4-oxadiazole (PPO)
exhibited
a
high decomposition temperature (Td,
A mixture of M2 (1.30 g, 3.41 mmol), M3 (0.85 g, 2.84 mmol),
potassium carbonate (0.59 g, 4.26 mmol), tetrahydrofuran (30
mL) and deionized water (6 mL), with Pd(PPh3)4 (98 mg, 84.8
μmol) acting as catalyst was refluxed at 70 °C for 24 h under
nitrogen. After the mixture was cooled down, 40 mL deionized
water was added to the resulting solution and the mixture was
extracted with dichloromethane for several times. The organic
phase was dried over anhydrous magnesium sulphate. After
filtration and solvent evaporation, the given residue was
purified through silica gel column chromatography using ethyl
acetate/ petroleum ether (1:6; v/v) as eluent to give the
product as yellow green solid (1.11 g, 82.0%). 1H NMR (500
MHz, CDCl3): δ 8.20 – 8.13 (m, 4H), 7.69 (d, J = 8.3 Hz, 2H), 7.59
– 7.52 (m, 3H), 7.44 (dt, J = 5.6, 1.9 Hz, 2H), 7.20 – 7.13 (m, 2H),
6.97 – 6.91 (m, 2H), 6.89 (d, J = 8.1 Hz, 1H), 3.90 (t, J = 7.1 Hz,
2H), 1.83 (dt, J = 14.8, 7.5 Hz, 2H), 1.53 – 1.44 (m, 2H), 0.97 (t, J
= 7.4 Hz, 3H). 13C NMR (125 MHz, CDCl3): δ 164.51, 145.37,
144.81, 143.21, 133.75, 131.69, 129.07, 127.48, 127.39, 126.91,
125.95, 125.74, 125.49, 124.18, 123.94, 122.61, 122.18, 115.56,
115.44, 47.19, 28.92, 20.15, 13.80. MALDI-TOF MS (mass m/z):
476.5 [M+].
corresponding to 5% weight loss) of 365 °C (Fig. 1), compared
to the PO (Td of 324 °C). It indicated that the phenyl ring was
inserted between the donor and acceptor, which offers the
PPO molecule a more rigid molecular conformation and higher
thermal stability. Meanwhile, the PPO molecule showed the
endothermic melting temperatures of 158
℃
, which is
). The
significantly higher than that of the PO molecule (107
℃
good thermal stability is a prerequisite for their application in
OLEDs, indicating the high morphologic stability of their
amorphous phase in a deposited film
.
Fig.1 TGA and DSC (the inset) curves for PO and PPO.
Device Fabrication
Indium−ꢀn oxide (ITO) coated glass with a sheet resistance of
15‒20 Ω/cm2 was used as the substrate. Before device
fabrication, the ITO glass substrates were cleaned with
acetone, detergent, deionized water, and isopropanol as the
clean agent, dried in an oven at 120 °C, and treated with UV-
zone for 20 min. And then 40 nm of PEDOT:PSS layer was spin-
coated and baked at 120 °C for 30 min. On the top of above
active
layer,
di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexan
(TAPC) as hole-transporting layer, 20% PO or PPO doped in
host material 2-methyl-9,10-di(naphthalen-2-yl)anthracene
(MADN) as emitting layer (EML), and 1,3,5-tri(phenyl-2-
benzimidazolyl)benzene (TPBi) as electron-transporting layer
were vacuum deposited in a sequence. Finally, 0.5 nm of LiF
film and 120 nm of Al film were vacuum deposited to form the
anode at a base pressure of 5 × 10−6 mbar. The thickness of
each deposition layer was monitored using a quartz crystal
Fig.2 Cyclic voltammograms of PO (red) and PPO (blue).
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Electrochemical Properties
phenyl ring that totally contributes to the increased LE
component in S1 state.
Based on the electrochemical measurement (CV), the highest
occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) energy levels were
obtained for PO and PPO, respectively (Fig. 2). The HOMO and
LUMO energy levels of the PO were different from those of
PPO. The HOMO level of -5.08 eV in PPO is higher than -5.18
eV of PO, the LUMO level of -2.67 eV in PPO is lower than -2.52
eV of PO. The increased HOMO level and the decreased LUMO
level in PPO can be attributed to the introduction of phenyl
ring unit, which is beneficial to enlarge the torsion angle and
the distance between the 10-butyl-10H-phenothiazine donor
and 2-phenyl-1,3,4-oxadiazole acceptor, and improve the
charge-transfer (CT) state characteristic.
Fig. 3 (a) The optimized molecular geometry and (b) the natural transition orbitals
(NTOs) of S1 → S0 of PO and PPO.
Theoretical Calculations
Photophysical Properties
Density functional theory (DFT) calculations (B3LYP) were
performed for PO and PPO with a 6-31G(d) basis set by using
the Gaussian 09 program. The optimized molecular geometry
and the calculated torsion angle and distance of donor
(D)−acceptor (A) are shown in Fig. 3a. From the opꢀmized
molecular geometry, the twist angle θ1 and θ2 are 0˚ and 34.4˚,
the distance d1 and d2 are 1.45 Å and 5.76 Å between the D‒A
for PO and PPO, respectively. Compared with PO, the larger θ2
of PPO can be ascribed to the stronger repulsion between the
two adjacent hydrogen atoms in 10-butyl-10H-phenothiazine
and bridging phenyl ring, as a result of stronger rigidity of PPO
than PO, meanwhile, an additional phenyl ring is inserted
between 10-butyl-10H-phenothiazine and 2-phenyl-1,3,4-
oxadiazole with the extended π-conjugation to increase LE
component, for the purpose of the improvement of ΦPL and
simultaneously enhance the CT component of donor and
acceptor to the best extent. For the purpose of examining the
nature of excited state, natural transition orbitals (NTOs) of S1
→ S0 were calculated using the TD-M062X/6-31G(d, p) method
based on the optimized lowest singlet excited state
configuration. As showed in Fig. 3b. For the S1 state, NTO hole
and particle of PO are different from the PPO molecule. For
the PO molecule, hole and particle were mainly distributed on
the donor and the oxadiazole ring of the acceptor, which
showed that the PO possesses the strong LE state. In contrast,
for the PPO molecule, the hole was mainly distributed on the
donor and adjacent phenyl ring with little residual on
oxadiazole ring, while the particles was mainly localized on the
donor and adjacent phenyl and the acceptor oxadiazole ring.
The overlap indicates a LE like transition of S1 → S0 and is
beneficial for improving the ΦPL. Whereas the separation of
hole and particle distributions demonstrates a CT featured
transition of S1 → S0 due to the insertion of an extra phenyl
ring in PPO, accordingly, the electronic transition of S1 → S0
should be comprised of both LE and CT like transitions and
should possess HLCT property.28, 35 On the other hand, the
oscillator strength of PPO (0.9529) in S1 state is higher than
that of PO (0.2685), which indicates that the higher ΦPL could
be expected in PPO than in PO, as a result of an additional
To better understand the intramolecular charge-transfer
character in excited states, the solvent effect was examined on
PO and PPO, respectively.50, 51 The UV-vis absorption spectra
and PL spectra of PO and PPO in dilute THF solution were
investigated (support information-2). Compared with the
monomer, the absorption peak of PO and PPO at around 370
nm becomes very strong, which can be ascribed to a newly
generated charge-transfer (CT) transition from D to A. This CT
character of PO and PPO were observed more obviously in the
fluorescent measurements. As shown in Fig. 4a and Fig. 4b.
From low-polarity hexane to high-polarity acetonitrile, the
fluorescence of PPO exhibited obvious solvatochromic effect,
and the total redshift of 100 nm is much larger than that of PO
(49 nm), indicating that the low-lying excited state S1 of these
two materials possesses a certain CT state characters.52
Moreover, these large redshift suggest that the proportion of
CT character in S1 state of PPO is surely higher than that in PO.
Especially, in low-polarity solvents (such as hexane, butyl
ether), PO exactly demonstrates a LE-like character because of
the existence of the vibrational fine structure in PL spectrum.
While PPO only exists the vibrational fine structure in hexane
solvent, it suggests that the proportion of LE character of the
S1 state in PPO should be much lower than that in PO.
In order to estimate the dipole moments (μe) of the excited
state, the slope of a plot of the Stokes shift (ʋa‒ʋf) versus the
solvent polarity function f was fitted for PO and PPO, according
to the Lippert–Mataga equation. As shown in Fig. 4c and 4d,
for PO and PPO, they both show linear relation as the solvent
polarity increases. The dipole moment μe of PPO is calculated
to be ≈15 D (r = 0.93). This dipole moment is much larger than
that of the CT section in PO (≈ 9 D, r = 0.95).34 Thus, the S1
state of PPO may consist of a highly mixed or hybrid local and
charge transfer (HLCT) state when the phenyl ring was inserted
between the D‒A. In low polar solvents, the LE energy of the
HLCT state is a little lower than the CT energy; therefore, the
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Fig. 4 a), b) Solvatochromic PL spectra of PO and PPO. c), d) Linear correlation of orientation polarization (f) of solvent media with the Stokes shift (ʋa – ʋf; a: absorbed light; f:
fluorescence) for PO and PPO.
molecule has LE dominated emission with fine vibrational
structure. The solvatochromism behavior of PPO agrees well
with the NTOs calculations as shown in Fig. 3b and also in
accordance with the recent report.29
In Fig. 5 shows the transient PL decay of the doped PO and
PPO film under the excitation of a 375 nm laser source with a
68.9 ps pulse width. The result revealed the two lifetimes of
3.69 ns and 10.06 ns in doped PO film. However, for doped
PPO film, the transient PL decay curve gave one lifetime of
5.89 ns. The one lifetime indicated that the LE and CT states in
the S1 are mixed or hybridized state (as called a hybridized
local and charge-transfer state HLCT), not two separated state.
Thus, the phenyl ring was inserted between the D‒A for PPO,
Fig. 5 Lifetime measurement of PO and PPO film (5% doped in PMMA) using time-
correlated single photon counting method under the excitation of a laser (375 nm) with
which is beneficial for forming the special exciton states and
achieving good device performance as we expected.
68.9 ps pulse width.
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Fig. 6 (a) The current density-luminance-voltage (J-L-V), (b) the luminous efficiency-power efficiency-luminance (LE-PE-J), (c) the external quantum efficiency -luminance (EQE-L), (d)
the luminance-current density (L-J) characteristics of the ITO/PEDOT:PSS (40 nm)/ TAPC (50 nm)/ 20% PO or PPO: MADN (20 nm)/ TPBi (40 nm)/ LiF (0.5 nm)/ Al (120 nm).
performances of the devices show that the maximum
Electroluminescence Properties
luminous efficiency of the PPO-based device was increased by
To understand the structure and EL performance relationship about 105%, compared to the PO-based device. In addition,
for PO and PPO, we constructed doped device with the PO and based on the external quantum efficiency (EQE)‒luminance
PPO (host material: MADN) as the emissive material, characteristics of the two devices (Fig. 6c), the PPO-based
respectively. The PPO-based device exhibited good device also harvests a maximum EQE of 5.4%, which is
performance.
density‒luminance‒voltage (J‒L‒V) and (b) the luminous the meantime, the PPO device exhibits the more excellent
efficiency‒power efficiency‒luminance (LE‒PE‒L) operational stability without roll-off with increasing luminance.
Fig.
6
shows
(a)
the
current significantly higher than that of the PO-based device (2.9%). In
characteristics of the PO-based and PPO-based devices. And Furthermore, the radiative exciton yield can be calculated
detailed data are listed in Table 1. The EL spectrum of the PO- according to the following equation:
based and PPO-based devices at 7 V is shown in Fig. 7a. The EQE = γ × ΦPL × ηs × ηout
inset exhibits the Commission Internationale de I'Eclairage (CIE) where EQE is the external quantum efficiency; γ is the carrier
coordinates of (0.25, 0.44) and (0.25, 0.50) at a voltage of 7 V. recombination efficiency, which in the ideal case is supposed
The PO-based device shows a maximum L of 16000 cd m‒2 at to be unity if the injected holes and electrons are fully
12.4 V, a maximum LE of 7.4 cd A‒1 and a maximum PE of 3.7 recombined and degrade to excitons in the emissive layer, ΦPL
lm W‒1 at a current density of 4.2 mA cm‒2, respectively. The is photoluminescence efficiency of the emission layer (
PPO-based device exhibits significantly improved performance for doped PO film, 70% for doped PPO film); ηs is the
~ 60%
~
with a maximum L of 25000 cd m‒2 at 12.4 V, a maximum LE of radiative exciton ratio; and ηout is the light out-coupling
15.2 cd A‒1 and a maximum PE of 6.4 lm W‒1 at a current efficiency. Therefore, the ηs value of the PO-based device is
density of 1.7 mA cm‒2, respectively. Remarkably, the 24%, while the ηs value of the PPO-based device reaches 39%,
This journal is © The Royal Society of Chemistry 20xx
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Table 1. Electroluminescence Characteristics of the Devices
The maximums of quantum efficiency
LEmax
[mA cm‒2] [cd m‒2] [cd A‒1]a
Lmax
Vturn-on
EL
Device
Voltage
[V]
J
L
PEmax
EQEmax
[%]c
[cd m‒2]d
[V]e
[nm]f
[lm W‒1]b
PO
6.2
4.2
310
7.4
3.7
2.9
16000
25000
3.8
3.8
492
500
PPO
7.4
1.7
250
15.2
6.4
5.4
‒2
aMaximum luminous efficiency, bMaximum power efficiency, cExternal quantum efficiency, dMaximum luminance, eDefined as the bias voltage at the luminance of 1 cd m
fMaximum peak of EL spectra at 7 V. Device structure: ITO/PEDOT:PSS (40 nm)/ TAPC (50 nm)/ 20% PO or PPO: MADN (20 nm)/ TPBi (40 nm)/ LiF (0.5 nm)/ Al (120 nm)
.
which greatly exceeds the limit of 25% for fluorescent OLEDs
and promote a large ratio of singlet exciton formation. For the
Conclusions
high ηs of PPO-based device, the several different mechanisms
are taken into account, such as TADF, TTA and HLCT. Further,
we tried to explore the reason for the high ηs of the PPO-based
In summary, two new donor−acceptor (D−A) compounds (PO
and PPO) were designed and synthesized by using 10ꢀbutylꢀ
10Hꢀphenothiazine as donor and 2ꢀphenylꢀ1,3,4ꢀoxadiazole as
acceptor. The experimental results demonstrated that the only
one phenyl ring was inserted into the D−A molecule, which
increases the D−A the torsion angle and the distance, and
improves the chargeꢀtransfer (CT) state characteristic for PPO.
The dipole moments (ꢁe) of the excited state and Timeꢀresolved
PL decay with one lifetime indicate the PPO forming a special
exciton states (HLCT), which is possible beneficial for
device. Firstly, in Fig. 6d, the luminance versus current density
curve of PPO behaves a linear relationship, the high ηs could
not be attributed to the TTA process.23‒27 Secondly, the S1/T1
energy gap was determined by the fluorescence spectra and
phosphorescence spectra of the compounds. As shown in Fig.
7b, we calculate the S1/T1 energy gap (∆EST ≈ 0.34 eV) between
S1 and T1 from the maximum peak of the low temperature
fluorescence spectrum and the first vibronic peak of the low-
temperature phosphorescence spectrum. These large S1/T1 achieving good device performance. As a result, the maximum
energy gaps cannot allow the RISC process from the T1 state to
efficiency (15.2 cd A‒1) of the PPOꢀbased device has improved
the S1 state.16‒22 So, TADF mechanism can also be excluded.
by about 105% compared that of the POꢀbased device (7.4 cd
A‒1). Our results not only facilitate an understanding of the
We tentatively explained the high ηs that is possible having
relationship between structure and EL performance relationship
both the LE and CT emissions simultaneously in the device,
in D−A system but also this study could provide a useful
strategy in the design of new fluorescent materials to satisfy
arising from the intercrossed excited-state (LE and CT) in the
PPO-based device.
special criteria required for efficient fluorescent OLED via fine
tuning molecule structure.
Acknowledgements
We thank the NSFC of P. R. China (No. 51573081, 51303091,
51173092, and 51473084), Specialized Research Fund for the
Doctoral
Program
of
Higher
Education
(No.
201333719120005), China Postdoctoral Science Fund
(2015M582061, 2014M 560537), NSF of Shandong Province
(No. ZR2014EMQ013), and Open Project of State Key
Laboratory of Supramoleꢀcular Structure and Materials of Jilin
University (SKLSSMꢀ201627).
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Journal of Materials Chemistry C
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DOI: 10.1039/C6TC01269H
Improving the Electroluminescence Performance of
Donor−Acceptor Molecule by Fine Tuning the Torsion Angle
and Distance Between Donor and Acceptor Moieties
Xu Qiu, Shanfeng Xue,* Yongjin Wu, Mingshuai Chen, Qikun Sun, Wenjun Yang*
Key Laboratory of Rubberꢀplastics of Ministry of Education/Shandong Province (QUST), School
of Polymer Science & Engineering, Qingdao University of Science &Technology, 53ꢀZhengzhou
Road, Qingdao, 266042, P. R. China. Eꢀmail: sfxue@qust.edu.cn, ywjph2004@qust.edu.cn
Two new compounds were designed and synthesized to tune the torsion angle and
distance between the donor (D)−acceptor (A), and their photophysics and EL
properties were investigated.