A high efficiency pure organic room temperature phosphorescence polymer PPV derivative for OLED

Article abstract of DOI:10.1016/j.orgel.2018.10.012

For the purpose of improving device efficiency and reducing cost, it is necessary to develop pure organic molecules with room temperature phosphorescence (RTP). A high efficiency pure organic RTP polymer, poly-p-phenylene vinylene (PPV) derivative (Br-PPV-CHO), was designed and synthesized, which possesses a blue emission at 496 nm, an emission lifetime of 14.1 μs, and a photoluminescence quantum yield of 12.2%, and the HOMO and LUMO of ?5.35 eV and ?2.75 eV, respectively, showing a great potential application in the emitting layer of OLED. A device fabricated with the PPV derivative as the emitting material layer shows a max luminance of 194 cd/m² when the Br-PPV-CHO content was 0.5 wt %, demonstrating the best RTP performance. The good properties of Br-PPV-CHO come from the substituent groups of bromine and aromatic aldehyde, and the molecular structure design strategy in the PPV derivative supplies a useful guidance for the design and syntheses of organic RTP materials.

Full text of DOI:10.1016/j.orgel.2018.10.012

Accepted Manuscript
A High Efficiency Pure Organic Room Temperature Phosphorescence Polymer
PPV Derivative for OLED
Ying He, Nianhe Cheng, Xin Xu, Jiawei Fu, Jun-an Wang
PII:
S1566-1199(18)30527-5
10.1016/j.orgel.2018.10.012
ORGELE 4928
DOI:
Reference:
To appear in:
Organic Electronics
Received Date:
Accepted Date:
18 May 2018
12 October 2018
Please cite this article as: Ying He, Nianhe Cheng, Xin Xu, Jiawei Fu, Jun-an Wang, A High
Efficiency Pure Organic Room Temperature Phosphorescence Polymer PPV Derivative for OLED,
Organic Electronics (2018), doi: 10.1016/j.orgel.2018.10.012
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ACCEPTED MANUSCRIPT
A High Efficiency Pure Organic Room Temperature Phosphorescence Polymer
PPV Derivative for OLED
a,*
a
a
a
b
Ying He , Nianhe Cheng , Xin Xu , Jiawei Fu , Jun-an Wang
a
Department of Polymer Materials, School of Materials Science and Engineering, Shanghai University,
Shanghai, 200444, China
b
Institute of Materials, School of Materials Science and Engineering, Shanghai University, Shanghai
2
00072, China
ABSTRACT: For the purpose of improving device efficiency and reducing cost, it is necessary to
develop pure organic molecules with room temperature phosphorescence (RTP). A high efficiency pure
organic RTP polymer, poly-p-phenylene vinylene (PPV) derivative (Br-PPV-CHO), was designed and
synthesized, which possesses a blue emission at 496 nm, an emission lifetime of 14.1 μs, and a
photoluminescence quantum yield of 12.2 %, and the HOMO and LUMO of -5.35 eV and -2.75 eV,
respectively, showing a great potential application in the emitting layer of OLED. A device fabricated
2
with the PPV derivative as the emitting material layer shows a max luminance of 194 cd/m when the
Br-PPV-CHO content was 0.5 wt %, demonstrating the best RTP performance. The good properties of
Br-PPV-CHO come from the substituent groups of bromine and aromatic aldehyde, and the molecular
structure design strategy in the PPV derivative supplies a useful guidance for the design and syntheses
of organic RTP materials.
Keywords: Optoelectronic Materials, Room temperature phosphorescence, PPV
derivative, Heavy atom effect, Pure organic molecule
1
. Introduction
The internal quantum efficiency (IQE) of emitting materials has received much
concern because it is essential to improve the device external quantum efficiency (EQE).
According to the quantum spin theory statistics, the excitons generated from electrical
excitation can be divided into singlet excitons and triplet excitons, with a ratio of 1:3.
By this theory, the highest IQE is limited to 25 %, so devices based on the traditional
*
Corresponding author
E-mail address: yinghe@i.shu.edu.cn; yinghe@staff.shu.edu.cn
1

ACCEPTED MANUSCRIPT
fluorescence materials which produce light from the radiative transition of singlet
excitons are hard to use widely. However, the phosphorescence materials may harvest
the energy of both singlet and triplet excitons, and reach a 100 % IQE [1,2]. To
accomplish this quantum efficiency, the traditional phosphorescence materials are
mostly designed to have metal complex [3], where the strong spin-orbit coupling of
transition metal promotes the spin deflection of triplet excitons, and thus contributes to
the radiating transition of triplet excitons [4,5].Due to the high cost and limited
resources, the further developments and practical applications of devices based on
heavy metals are subjected to limitations. So it is extremely attractive about the design
and development of pure organic RTP materials.
However, without heavy atoms, the long lifetime triplet excitons cannot achieve
radiative transition successfully due to the molecular rotation vibration and oxygen
quenching. For this reason, all organic phosphorescence emission only can generally
be observed in the inert and low-temperature environment, such as at 77 K and nitrogen
environment. Recently, some materials that can successfully achieve pure organic RTP
have been reported. Shi et al [6] reported that a series of dibromobenzene derivatives
can be observed efficient phosphorescence emission from their crystals, in which the
highest phosphorescence quantum efficiency can reach 21.9 %. These structures prove
the importance of bromine atoms to the generation of phosphorescence. Adachi et al
[7] reported a host-guest structure, with hydroxysteroid as the host, which provides a
rigid environment for the guest through a strong intermolecular interaction, to suppress
the molecular rotation vibration. The deposited film with this host-guest structure
achieved a quantum efficiency of 39.7 %, and the OLED device fabricated by that
deposited film, with a highest EQE of 1 %, could be observed a continuous green
emission after power off. Kwon and others [8] obtained a cross-linked phosphor-doped
polymer system by Diels–Alder reaction, achieving a phosphorescence quantum
efficiency of 28 %. Lee and co-workers [9] introduced pure organic phosphors, 2,5-
dihexyloxy-4-bromobenzaldehyde (Br6A), into polymethylmethacrylate (PMMA), and
achieved a quantum efficiency of 7.5 %. All this showed that in the organic molecules
containing both bromine and aromatic carbonyl, the RTP can be achieved successfully
2

ACCEPTED MANUSCRIPT
[10,11].
In this work, we designed and synthesized pure organic RTP polymer, poly-p-
phenylene vinylene (PPV) derivative (Br-PPV-CHO), with bromine atoms to enhance
the phosphorescence efficiency via heavy-atom interaction. Based on the photophysical
properties and electrochemical properties of those organics, we have fabricated OLED
devices with different configurations and demonstrated the potential application in the
emitting layer of OLED.
2
. Experiment
By a series of chemical reactions, 2,5-dibromo-1,4-bis(octyloxy)benzene (M1) and
,4-dibromo-2,5-divinylbenzene (M2), were synthesized. Through the halogen-metal
1
exchange reaction of M1 and M2, a high efficiency pure organic RTP polymer, PPV
derivative (Br-PPV-CHO), was synthesized. All the synthesized polymers were
characterized by a Bruker 500 MHz spectrometer. The electrochemical property of the
PPV derivative was measured through a three-electrode system in an anhydrous
acetonitrile solution containing 0.1 M tetrabutylammonium hexafluorophosphate
(
TBAPF ). The photoluminescence performance was characterized by using
6
Fluoromax-4, FLS900, and FLS920 equipped with integrating sphere.
. Results and discussion
3
The synthesis of the pure organic RTP material was performed following the outline
in Fig. 1. In this synthetic scheme, 2,5-dibromo-1,4-bis(octyloxy)benzene (M1) and
1
,4-dibromo-2,5-divinylbenzene (M2) were synthesized separately and the polymer
precursor (Br-PPV-Br) was obtained by the heck reaction of M1 and M2. Subsequently,
through the halogen-metal exchange reaction, PPV derivative (Br-PPV-CHO) was
synthesized.
OH
OH
OC8H17
Br
Br
Br2
C8H17Br, K2CO3
DMF
CH3COOH
Br
Br
OH
OH
OC8H17
M1
（
1（
Br
Br
Br
NBS, BPO
CHCl3
PPh3
KOtBu, Paraformaldehyde
THF
Br
Br2, I2
CH2Cl2
DMF
Br
Br
Br
（
2（
Br
（
3（
M2
3

ACCEPTED MANUSCRIPT
OC8H17
Br
OC8H17
Br
Br
iPrMgCl LiCl
PdCl2
DMF
+
Br
Br
n
OC8H17
OC8H17
Br
DMF
Br
OC8H17
OC8H17
n
CHO
Fig. 1. The synthetic scheme for the Br-PPV-CHO
The photoluminescence spectrum, emission lifetime and luminous efficiency were
measured to judge whether the polymer could meet the requirements for device
preparation. The photoluminescence spectra are showed in Fig. 2. At room temperature
and 77 K, the PPV derivative emits efficient blue-green light with major emission peaks
at 496 nm and 510 nm, respectively; obviously, the emission at 77 K shifts red slightly.
This emission difference between RT and 77 K indicated that under the low temperature
environment, the molecular were bounded, and some molecular aggregation may
enhance the conjugate area, resulting in an emission red shift.
7
RT
7 K
80000
60000
40000
20000
0
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 2. PL spectra of Br-PPV-CHO in CHCl solution. Insert: a photo of the solution emission
3
The luminous life is an important characteristic which could determine whether the
material belongs to fluorescence or phosphorescence materials. For fluorescence
-12
-9
materials, the lifetime of emission is usually 10 10 s, i.e., –ns, while
phosphorescence materials possess a μs - ms lifetime.
The lifetime of emission of this derivative was measured at room temperature and 77
K as shown in Fig. 3. Obviously, at RT or 77 K, the derivative has almost the same
lifetime of emission, i.e., 14.1 μs and 13.9 μs, which differs from the traditional organic
electroluminescent materials. The latter usually possess an ms-s lifetime due to the
triplet excitons generated from π→π*. The lifetime value of the derivative at RT and
7
7 K may induce that the luminescence mechanism is dependent on the aldehyde group
and bromine atom. The aldehyde group reduces the energy gaps between singlet and
4

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triplet, and thus the singlet excitons could successfully convert into triplet excitons by
ISC; meanwhile under the action of bromine atom, a strong spin-orbit coupling
promotes the triplet excitons transit to the ground state and produce light. So the
prepared pure organic polymer material can emit efficient phosphorescence.
10000
77 K
RT
1000
14.1 μs
100
13.9 μs
10
1
0
25
50
75
100
Time (μs)
Fig. 3. Photoluminescence decays of Br-PPV-CHO at RT and 77 K
It is recognized that the IQE of fluorescence materials is essential to the device
efficiency, so excitation line and emission spectra of Br-PPV-CHO were measured by
integrating sphere, as shown in Fig. 4. By simple calculation, it can be obtained that the
photoluminescence quantum yield is 12.2 %.
5000000
4000000
3000000
2000000
1000000
Laser emission
Sample emission
E
R
ES
0
3
60 362 364 366 368 370
Wavelength (nm)
LS
400
500
600
700
Wavelength (nm)
Fig. 4. Excitation line and emission spectra of Br-PPV-CHO collected by using an integrating
sphere. The inset figure shows the magnification of excitation line and emission spectra
Table 1 lists some performance of different electroluminescent polymers. By
comparison to fluorescence polymer materials, i.e., PspiroF, Me-LPyP and
PDHFMPPV, Br-PPV-CHO showed a longer lifetime, which could achieve
phosphorescence emission. Comparing with phosphorescence materials, i.e,
Compound 1, Br-PPV-CHO showed higher photoluminescence quantum yield. All this
data demonstrated that Br-PPV-CHO had a good luminous performance.
Table 1. Photophysical properties of polymer emitting materials
polymer
λ
em
Ф (%)
τ (s)
Ref.
5

ACCEPTED MANUSCRIPT
(
nm)
Br-PPV-
CHO
PspiroF
Me-LPyP
496
12.2
14.1×10-6
-
9
423 17.18±0.07 1.69×10 [12]
-9
440
-
-
4.3×10 [13]
-
PDHFMPPV 475
0.476×10 [14]
9
PFA-OXD
Compound 1 550
449
0.72
5.4
-
[15]
-6
1.3×10 [16]
λem = the emission peak, Ф = quantum yield, τ = lifetime
As an OLED emitting layer, the materials need not only good photophysical
properties but also a match of the energy level with other functional layers. During the
electron injection process, it requires electron across energy barriers to achieve an
effective injection. For this consideration, the electrochemical properties of the
prepared derivative were investigated by cyclic voltammetry. As shown in Fig. 5, the
oxidation potential of Br-PPV-CHO in CH CN solution is 0.946 V, and the reduction
3
potential is -1.655 V. The energy of HOMO and LUMO as well as the energy gap can
be obtained by equations (1) to (3):
OX
onset
HOME   E

 E  4.80 eV

(1)
(2)
ref
red
onset
LUMO   E

 E  4.80 eV

ref
OX
onset
red
onset
E  LUMO  HOMO  E
 E
(3)
g
OX
onset
red
onset
where
E
,
E
are oxidation potential and reduction potential measured by cyclic
voltammetry, respectively; herein the ferrocene/ferrocenium couple potential is
determined to be -0.40 V vs Pt.
According to the equations, the highest occupied molecular orbital (HOMO) is -5.35
eV, the lowest unoccupied molecular orbital (LUMO) is -2.75 eV, and the energy gap
Eg = 2.60 eV.
6

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1
0
-
-
-
-
-
-
-
1
2
3
4
5
6
7
-
2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
Potential (V vs. Ag/AgCl)
Fig. 5. Cyclic voltammogram of Br-PPV-CHO
The photoluminescence performance and electrochemical properties showed that Br-
PPV-CHO could be applied to the OLED devices. Based on those results, we have
fabricated different devices with several guest materials to demonstrate the application.
The device configurations are denoted as Device I, Device II and Device III, as follows,
and, as an example, the energy-level diagram of the device I is showed in Fig. 6.
Device I：ITO / PEDOT:PSS / PVK:PBD:Br-PPV-CHO / TmPyPB / Liq / Al
Device II：ITO / PEDOT:PSS / PVK:NPB:Br-PPV-CHO / TmPyPB / Al
Device III：ITO / PEDOT:PSS / TAPC / PEI / TAPC:Br-PPV-CHO / TmPyPB /
Liq / Al
Fig. 6. Energy-level diagrams of the Device I: ITO / PEDOT:PSS / PVK:PBD:Br-PPV-CHO /
TmPyPB / Liq / Al
Among these three devices, the Device I showed a best performance. The EL
spectrum (Fig. 7) shows two major emission peaks at 464 nm and 504 nm. According
to the PL spectra (Fig. 2), the emission peak at 504 nm belongs to Br-PPV-CHO,
indicating that electroluminescence originated from the RTP material, while the
emission peak at 464 nm may come from PVK:PBD, which is characteristic of an
incomplete energy transfer from host PVK:PBD to Br-PPV-CHO.
7

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32
28
24
20
16
12
8
4
0
PVK:PBD
PVK:PBD:Br-PPV-CHO
a
b
3
50
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 7. EL spectra of the (a) PVK: PBD and (b) PVK: PBD: Br-PPV-CHO based devices. Insert: A
digital photograph of working pixel of device based on PVK: PBD: Br-PPV-CHO
Device II based on NPB did not occur efficient emission from Br-PPV-CHO,
showing that NPB is not appropriate to be a host for Br-PPV-CHO. This phenomenon
comes from that the triplet energy level of NPB is as low as 2.2 eV, by which the energy
cannot transfer to Br-PPV-CHO effectively. In Device III, TAPC was selected as the
base material due to its high triplet energy level (2.75 eV). This device was prepared
by solution process, and water-soluble conducting polymer polyethylenimine (PEI) was
utilized between hole-transporting layer (HTL) and light-emitting layer (EML) to avoid
the mutually soluble between HTL and EML in spin coating. Obviously, the Device III
showed a poor performance, such as high driven voltage (20 V) and fast luminance
decay. This luminous behavior is probably due to the electron transporting of PEI,
especially in the case that PET was placed on the hole-transporting materials.
Based on the luminance-doping concentration curve shown in Fig. 8, device I with
the best performance has been fabricated. The max luminance achieved is 194 cd/m2
when the Br-PPV-CHO content is 0.5 wt %. Obviously, the luminance of device I
reduced with the increase of Br-PPV-CHO, indicating that Br-PPV-CHO has a clear
concentration quenching effect. This phenomenon may be explained as with the
increase of the Br-PPV-CHO, as a phosphorescence guest, the number of triplet
excitons formed after excitation also increases, and the probability of collision between
triplet excitons increases significantly, leading to annihilation. In this case, those triplet
excitons do not emit light but result in the loss of energy, and therefore, the brightness
of the device reduces obviously with increasing doping concentration in the light-
emitting layer.
8

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Fig. 8. Current density–voltage–luminance of the Br-PPV-CHO based device when the Br-PPV-CHO content was
0
.5 wt %. The inset figure shows the luminance variation with different doping concentrations
4
. Conclusions
In summary, we have prepared the pure organic room temperature phosphorescence
material PPV derivate Br-PPV-CHO, and applied it to OLED devices. Br-PPV-CHO
showed a room temperature phosphorescence emission at 496 nm, an emission lifetime
of 14.1 μs, and the photoluminescence quantum yield of 12.2 %. The max luminance
2
of the devices based on Br-PPV-CHO could reach 194 cd/m , while the devices still
need to be optimized to enhance the performance of luminance and device efficiency.
The good properties of Br-PPV-CHO originate from the substituent groups of bromine
and aromatic aldehyde. The molecular structure design strategy in PPV derivate Br-
PPV-CHO supplies a useful guidance for the design and syntheses of all organic RTP
materials for OLED.
Acknowledgement
The authors would like to thank Instrumental Analysis and Research Center of
Shanghai University for experimental assistance.
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1
0

ACCEPTED MANUSCRIPT
A High Efficiency Pure Organic Room Temperature Phosphorescence Polymer
PPV Derivative for OLED
a,*
a
a
a
b
Ying He , Nianhe Cheng , Xin Xu , Jiawei Fu , Jun-an Wang
a
Department of Polymer Materials, School of Materials Science and Engineering, Shanghai
University, Shanghai, 200444, China
b
Institute of Materials, School of Materials Science and Engineering, Shanghai University, Shanghai
2
00072, China
Highlights:
·
High efficiency and low cost pure organic polymer with room temperature phosphorescence,
PPV derivate Br-PPV-CHO, was designed and synthesized.
·
The luminescence control mechanism of the new pure organic polymer material is dependent
on the aldehyde group and bromine atom.
·
The performance of the organic RTP material is suitable to be the emitting layer of OLED.
*
Corresponding author
E-mail address: yinghe@i.shu.edu.cn; yinghe@staff.shu.edu.cn