Efficient violet non-doped organic light-emitting device based on a pyrene derivative with novel molecular structure

Article abstract of DOI:10.1016/j.orgel.2015.04.024

A novel pyrene derivative 1,6-bis[2-(3,5-diphenylphenyl)phenyl]pyrene (DPPP) was successfully designed and synthesized. X-ray analysis shows the pyrene core in this compound is fully protected by the introduced 3,5-diphenylphenyl groups, resulting no π-π

Full text of DOI:10.1016/j.orgel.2015.04.024

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Organic Electronics xxx (2015) xxx–xxx
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Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
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Efficient violet non-doped organic light-emitting device based
on a pyrene derivative with novel molecular structure
a,1
b,d,1
, Cai-Jun Zheng , Wei-Zhou Wang , Chen Xu , Mei Zhu a,
⇑
b
a
a,
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Zhi-Qiang Wang , Chuan-Lin Liu
a
b
b,c,
⇑
Bao-Ming Ji , Fan Li , Xiao-Hong Zhang
a
9
0
1
2
3
4
College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, PR China
Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences, Beijing 100190, PR China
Functional Nano & Soft Materials Laboratory (FUNSOM) and Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu
b
1
1
1
1
1
c
215123, PR China
d
Materials Science & Engineering School, BeiHang University, Beijing 100191, PR China
1
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5
1
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a r t i c l e i n f o
a b s t r a c t
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0
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Article history:
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A
novel pyrene derivative 1,6-bis[2-(3,5-diphenylphenyl)phenyl]pyrene (DPPP) was successfully
designed and synthesized. X-ray analysis shows the pyrene core in this compound is fully protected by
the introduced 3,5-diphenylphenyl groups, resulting no stacking between pyrene units, and the
2
2
2
Received 29 January 2015
Received in revised form 20 April 2015
Accepted 27 April 2015
Available online xxxx
p–p
dihedral angle between pyrene ring and adjacent benzene ring is as large as 80.1°. This structure charac-
ter leads to DPPP achieving a violet emission both in solution and as a thin solid film. Furthermore, DPPP
exhibits high thermal properties due to its non-coplanar structure and large molecular size. The non-
doped electroluminescence device employing DPPP as emitting layer shows a stable and efficient violet
emission with a maximum external quantum efficiency of 2.2% and a CIE coordinate of (0.16, 0.04), which
is remarkable in reported violet devices.
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Ó 2015 Published by Elsevier B.V.
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1. Introduction
confined, which in turn would usually impose constraints in
molecular size. However, the resultant limited molecular size nor-
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Organic light-emitting devices (OLEDs) have attracted much
attention because of their applications in flat-panel displays and
solid-state lightings [1]. Efficient violet emitting materials play
an important role in OLEDs since violet emitters can be utilized
to generate light emission over the whole visible region and white
light emission [2–9]. Moreover, high-efficiency short-wavelength
emission can effectively reduce the power consumption of full-
color OLEDs [10]. In fact, violet OLEDs are also of great importance
in other fields, such as high-density information storage [11].
Although many short-wavelength (blue-violet/violet) OLEDs have
been reported in recent years [12–25], violet OLEDs with emission
peak below 400 nm and high color purity are still very rare to date
[22–24]. The molecular design of organic materials capable of effi-
cient violet emission in OLEDs is highly challenging. To achieve a
violet emission, the extent of conjugation in molecule must be
mally reduce the thermal stability of organic molecules and ren-
ders it more difficult to form stable and uniform amorphous thin
films, which are detrimental for device operation. Furthermore,
the confined conjugation length will cause decrease of charge-
transporting ability and fluorescent quantum yield for organic
molecules [26,27].
As a large conjugated aromatic system, pyrene not only has
highly efficient violet emission around 390 nm, but also has high
carrier mobility and excellent hole injection ability [28–30].
Nevertheless, pyrene itself is not suitable to act as violet-emitter
in OLEDs due to its strong tendency to crystallize and form exci-
mers by p–p stacking, which leads to a red-shifted emission above
480 nm as well as the decrease of fluorescence efficiency and make
it incapable of forming stable amorphous thin film [28,29]. To sup-
press the
p–p stacking, bulky groups generally are introduced to
pyrene backbone at various positions, such as 1- and 6-positions,
2- and 7-positions, or 1-, 3-, 6-, and 8-positions [25,28,31–36]. In
addition, incorporating pyrene with other chromophores also is a
common strategy to design light-emitting materials for OLEDs
⇑
Corresponding authors at: Nano-organic Photoelectronic Laboratory and Key
Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical
Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR
China (X.-H. Zhang).
[
28,37–41]. As far as we know, numerous pyrene based light-
E-mail addresses: xubohan@163.com (C. Xu), xhzhang@mail.ipc.ac.cn
emitting materials have been prepared through above mentioned
methods, but only one violet OLED using pyrene derivative as
(
X.-H. Zhang).
These authors contributed equally to this work.
1
http://dx.doi.org/10.1016/j.orgel.2015.04.024
566-1199/Ó 2015 Published by Elsevier B.V.
1
Please cite this article in press as: Z.-Q. Wang et al., Efficient violet non-doped organic light-emitting device based on a pyrene derivative with novel molec-
ular structure, Org. Electron. (2015), http://dx.doi.org/10.1016/j.orgel.2015.04.024

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emitting layer was reported by Kaafarani and co-workers, and this
device exhibits a low efficiency of 0.04 cd/A [12]. The important
reasons may be (i) introducing bulky rigid groups to pyrene back-
bone increases the extent of conjugation in molecule resulting in a
longer emission wavelength, (ii) interactions between pyrene units
have not been fully suppressed in solid state, which leads to pyrene
derivatives exhibiting a obviously red-shifted emission in solid
state compared with in solution.
In this paper, we designed and synthesized a novel violet-light-
emitting material 1,6-bis[2-(3,5-diphenylphenyl)phenyl]pyrene
(DPPP) employing pyrene as the chromophore (Scheme 1). In this
compound, two 3,5-diphenylbenzene groups are connected with
pyrene backbone by two benzene rings, respectively, and the bulky
3,5-diphenylbenzene group and pyrene unit locate at the adjacent
positions of benzene ring. The structural characteristic of DPPP has
significant advantages. Firstly, it can increase the dihedral angles
between pyrene ring and adjacent benzene rings to confine the
extent of conjugation in molecule through improving steric hin-
drance. Secondly, the bulky 3,5-diphenylbenzene groups can cap
the pyrene core from both sides to prohibit the strong interactions
between pyrene units. In addition, the bulky substituents at pyrene
backbone can increase the molecular size to improve the high
thermal stability. We characterized the structure and physical
properties of DPPP, as we desired, it exhibits high-efficiency violet
emission and high thermal stability. The non-doped electrolumi-
nescence device using DPPP as emitting layer achieved an efficient
and stable violet emission (kmax = 396 nm, EQEmax = 2.2%).
with a rotary evaporator. The crude product was puried by column
139
140
141
142
143
chromatography (eluent = petroleum ether 60–90 °C) to give a
1
white powder (yield: 1.56 g, 82%). H NMR (400 MHz, CDCl ) d
3
(ppm): 7.83 (1H, s), 7.72–7.69 (5H, m), 7.63 (2H, s), 7.48–7.44 (5H,
t, J = 8.0), 7.35–7.40 (3H, q, J = 6.7), 7.24 (1H, s). TOF-MS: 384.126.
2.2.2. Compound 4
144
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147
148
149
150
151
152
153
154
155
156
157
158
n-Butyl lithium (1.6 M, 1.5 mL) was added dropwise to the solu-
tion of compound 3 (1.5 g, 4 mmol) in anhydrous THF (20 mL) at
ꢀ80 °C over half an hour, and the mixture was stirred for another
1 h at this temperature. Then trimethylborate (0.6 mL, 5.2 mmol)
was added to the reaction mixture at ꢀ80 °C, and stirred for 12 h
at room temperature. Then HCl (2 M, 10 mL) was added, and stir-
red for 1 h. The resulting solution was extracted with CH Cl three
2
2
times, then the combined organic solution was dried over
anhydrous MgSO4 and evaporated with a rotary evaporator.
The crude product was puried by column chromatography
(eluent = dichloromethane/petroleum ether, 1:3 v/v) to give a
1
white powder (yield: 0.82 g, 58%). H NMR (400 MHz, CDCl ) d
3
(ppm): 7.94 (1H, d, J = 8.0), 7.85 (1H, s), 7.65–7.68 (4H, m), 7.62
(2H, s), 7.28–7.52 (9H, m), 4.66 (2H, s).
1
1
1
1
1
1
00
01
02
03
04
05
2.2.3. DPPP
159
160
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162
163
164
165
166
167
168
169
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171
172
173
174
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177
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179
1,6-Dibromopyrene (0.36 g, 1 mmol), compound 3 (0.77 g,
2.2 mmol) and tetrakis(triphenylphosphine)palladium [Pd(PPh ) ]
3
4
(25 mg) were mixed in toluene (15 ml), then Na CO (2 M, 3 mL)
2
3
and ethanol (5 mL) were added under the nitrogen atmosphere.
The mixture reacted at 110 °C under the nitrogen atmosphere over
night. After cooling, the resulting solution was extracted with
1
1
06
07
2. Experimental
CH
2
Cl
2
three times, then the combined organic solution was dried
and evaporated with a rotary evaporator.
over anhydrous MgSO
4
2.1. General information
The crude product was puried by column chromatography (elu-
ent = dichloromethane/petroleum ether, 1:8 v/v) to give a white
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
08
09
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13
14
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16
17
18
19
20
21
22
23
24
25
26
27
28
Commercially available reagents were used without purifica-
1
powder (yield: 0.37 g, 46%). H NMR (400 MHz, CDCl
3
) d (ppm):
1
13
tion unless otherwise stated. The H NMR spectra and C NMR
spectrum were recorded on Bruker Avance 400 spectrometer.
Elemental analysis was performed on a Vario III elemental
analyzer. Mass spectra were obtained on a Bruker Microflex spec-
trometer. The absorption and photoluminescence spectra were
recorded on a Hitachi U-3010 UV–Vis spectrophotometer and a
Hitachi F-4500 fluorescence spectrophotometer, respectively.
Photoluminescence quantum yield of solid film was measured by
8
(
.20 (1H, d, J = 4), 8.15–8.07 (4H, m), 8.04–7.98 (3H, m), 7.81
1H, d, J = 4), 7.76 (1H, d, J = 8), 7.68–7.65 (3H, m), 7.61–7.56 (3H,
m), 7.51–7.48 (3H, m), 7.42 (1H, d, J = 8), 7.33 (1H, S), 7.24 (4H,
13
s), 7.17–7.13 (7H, m), 7.08–7.04 (4H, m), 6.96–6.94 (6H, m).
C
NMR (100 MHz, CDCl ) d (ppm): 142.12, 141.97, 141.09, 141.05,
3
1
1
1
C
41.02, 140.09, 140.00, 137.60, 132.08, 130.49, 130.21, 129.91,
28.82, 128.57, 128.48, 128.27, 127.72, 127.22, 127.15, 127.07,
27.03, 125.62, 124.55, 124,27. TOF-MS: 811.232. Anal. Calcd. for
a
combined measurement system for infrared fluorescence
64
H42: C, 94.78; H, 5.22. Found: C, 94.62; H, 5.26.
R
(Nanolog FluoroLog-3-2-iHR320) equipped with F-3018 integrat-
ing sphere. Cyclic voltammetry was performed on a CHI620C elec-
trochemical analyzer, and the electrolytic cell is a conventional
three-electrode cell consisting of a Pt working electrode, a Pt wire
counter electrode and an Ag/AgCl reference electrode. Thermal
gravimetric analysis (TGA) and differential scanning calorimetry
(DSC) were performed on a TA instrument TGA2050 and a TA
2.3. X-ray structure determination
180
The single crystal suitable for X-ray diffraction analysis was
181
182
183
184
185
186
187
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192
193
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195
obtained by slowly evaporating the CDCl
Crystal data were collected on a Bruker SMART APEX-II CCD
diffractometer with Mo K radiation (k = 0.071073 Å) at 296 K.
3
solution of DPPP.
a
instrument DSC2910, respectively, with
a heating rate of
The data were corrected for Lorentz-polarization factors as well
10 °C/min under the nitrogen atmosphere. DFT calculations
were performed using Gaussian 03 with the B3LYP/6-31G(d)
method [42].
as for absorption. Structures were solved by direct methods and
2
refined by full-matrix least squares methods on F with the
SHELX-97 program. All nonhydrogen atoms were refined
anisotropically, while hydrogen atoms were placed in geometri-
1
29
2.2. Synthesis
cally calculated positions. Crystal data for DPPP: C64H42
,
M = 810.98, Monoclinic, P2(1)/c, a = 9.201(4) Å, b = 11.487(5) Å,
3
1
1
1
1
1
1
1
1
1
30
31
32
33
34
35
36
37
38
2.2.1. Compound 3
c = 21.142(10) Å,
a = c = 90°, b = 94.410(6)°. V = 2228.1(18) Å ,
3
1-Iodo-2-bromobenzene 2 (1.41 g, 5.0 mmol), 3,5-diphenylphe-
nyl boronic acid 1 (1.37 g, 5.0 mmol), and tetrakis(triphenylphos-
phine)palladium [Pd(PPh ) ] (80 mg) were mixed in toluene
Z = 2, DCalc = 1.209 Mg/m , 16535 reflections measured, 4128
1 2
unique reflections with I > 2r(I), R = 0.0459, wR = 0.1049. CCDC
1008121.
3
4
(35 ml), then Na CO (2 M, 10 mL) and ethanol (10 mL) were added
2
3
under the nitrogen atmosphere. The mixture reacted at 80 °C under
the nitrogen atmosphere over night. After cooling, the resulting
solution was extracted with CH Cl three times, then the combined
2.4. OLED fabrication and measurements
196
Indium-tin-oxide (ITO) coated glass substrates were cleaned
with isopropyl alcohol and deionized water, then dried in an oven
197
198
2
2
organic solution was dried over anhydrous MgSO and evaporated
4
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Scheme 1. Synthesis and structure of DPPP.
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203
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207
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at 120 °C, and finally treated with UV-ozone. The devices were
ꢀ6
prepared in a vacuum at a pressure of about 1 ꢁ 10 Torr.
Organic layers were deposited on to the substrate at a rate of
0.1–0.2 nm/s. After the organic film deposition, LiF and aluminum
were thermally evaporated onto the surface of organic layer.
Electroluminescence (EL) spectra, luminance and 1931 CIE color
coordinates were measured with a PR650 spectrascan photometer
and the current–voltage characteristic was measured with a
computer-controlled Keithley 2400 SourceMeter under ambient
atmosphere.
2
2
09
10
3. Results and discussion
3.1. Synthesis and crystal structure
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Synthetic route for DPPP is shown in Scheme 1. Firstly, the
Suzuki coupling reaction of 3,5-diphenylphenyl boronic acid 1
and 1-iodo-2-bromobenzene 2 afforded 1-(3,5-diphenylphenyl)-
2-bromobenzene 3. This Suzuki coupling reaction was carried out
at a lower temperature of about 80 °C to protect the bromine atom
in compound 2. Then compound 3 was translated to 1-(3,5-diphe-
nylphenyl)phenyl-2-boronic acid 4 with the aids of n-butyl
lithium, trimethylborate and HCl. Finally, the designed material
DPPP was obtained by Suzuki coupling reaction between
compound 4 and 1,6-dibromopyrene 5. The structure of DPPP
Fig. 1. Single crystal structure of DPPP (H atoms are omitted for clarity).
molecular structure, the important bond lengths and dihedral
angles are listed in Table 1. The bond lengths of C4–C9 and C14–
C15 are longer than those of C17–C21 and C19–C27, which should
be caused by steric hindrance from the bulky substituents at 1- and
6-positions of pyrene backbone. The adjacent aromatic rings are
twisted each other to a larger angle from 37.0° to 80.1°, which indi-
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229
230
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232
233
1
13
was characterized by H NMR, C NMR, TOF-MS, and elemental
analysis (EA).
The molecular structure of DPPP was further confirmed by X-ray
diffraction analysis (Fig. 1). This compound has a symmetrical
cates DPPP has
a highly non-coplanar molecular structure.
Especially the two benzene rings at 1- and 6-positions of pyrene
backbone are highly twisted toward the pyrene ring to an angle
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Table 1
The important bond lengths and dihedral angles.
Bond lengths (Å)
Dihedral anglesa (°)
C4–C9
1.501(2)
1.500(2)
1.484(3)
1.484(3)
Pr-B1
80.1
66.5
37.0
42.4
C14–C15
C17–C21
C17–C21
B1–B2
B2–B3
B2–B4
a
Pr denotes the pyrene ring, B1, B2, B3 and B4 denote the benzene rings C9–C14,
C15–C20, C21–C26 and C27–C32 orderly (Fig. 1).
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of 80.1°, such large dihedral angle can strictly confine the extent of
conjugation in DPPP to reduce the emission wavelength. Fig. 1
shows the pyrene core is capped from both sides by two
3,5-diphenylbenzene groups, this structure arrangement can pro-
hibit the strong
p–p interactions between pyrene units, which is
confirmed by the packing diagram of DPPP (Fig. 2). It can be found
that the pyrene units in adjacent DPPP molecules do not overlap
each other, it suggests there is no
units which usually leads to red-shifted emission [43]. As shown
interac-
p–p stacking between pyrene
2 2
Fig. 3. Absorption and photoluminescence of DPPP in CH Cl solution and solid
film, photoluminescence of pyrene in CH Cl2 solution.
2
in Fig. 2, the intramolecular and intermolecular C–Hꢂ ꢂ ꢂ
p
tions exit in DPPP, the distances are measured to be 2.783 (Å) and
2.746 (Å), respectively, which should affect the photophysical
properties of DPPP in solid state.
Table 2
2 2
Photophysical properties of DPPP in CH Cl solution and solid film.
Absorption
nm)
Emission
(nm)
FWHM
(nm)
Quantum
yield
(
2
47
3.2. Photophysical properties
0.68 a
Solution 250, 358
Film 254, 368
398
410
41
50
0.37 b
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
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The UV–vis absorption and photoluminescence (PL) spectra of
a
Measured using DPA as the reference standard.
Absolute quantum yield.
DPPP were measured in dilute CH Cl2 solution. The absorption
2
b
spectrum shows two prominent bands at 250 nm and 358 nm,
respectively (Fig. 3 and Table 2), which are the characteristic vibra-
tion pattern of pyrene derivatives [32,36,43–46]. When the dilute
CH Cl solution of DPPP is excited at either 250 nm or 358 nm,
2
2
the same PL spectrum with a structureless emission peak at
398 nm is observed, which also shows a very narrow FWHM
(full-width at half-maximum) of about 41 nm (Table 2). Quantum
chemical calculations show that the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) mainly locate on pyrene core, which implies that the
absorption and emission of DPPP were controlled mostly by pyrene
unit (Fig. 4). In addition, The HOMO and LUMO contain a certain
amount of contributions from the substituents at 1- and 6-posi-
tions of pyrene backbone, which make the PL spectrum of DPPP
lost vibronic structure and red-shift about 15 nm with respect to
that of pyrene monomer (Fig. 3). Fortunately, the red-shift makes
Fig. 4. Calculated HOMO and LUMO of DPPP.
most of the PL spectrum of DPPP fall in the visible violet light
region above 380 nm. The fluorescence quantum yield ( ) of
DPPP in dilute CH Cl solution was measured to be 0.68 by using
,10-diphenylanthracene (DPA) as the reference standard.
The solid film of DPPP was fabricated by vacuum vapor deposi-
tion on a quartz disc. The shape of the absorption spectrum of DPPP
in thin film is almost identical to that of in CH Cl solution except a
red-shift <10 nm. The PL spectrum of DPPP in thin film exhibits an
emission peak at 410 nm with a FWHM of about 50 nm (Fig. 3 and
Table 2). It can be found that the absorption and PL spectra mea-
sured from solid film only shows slight difference compared to
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U
f
2
2
9
2
2
2 2
those acquired from CH Cl solution. This suggests that the strong
interactions between pyrene units do not occur in thin solid film,
which is in accord with the results of single crystal analysis. In
addition, the C–Hꢂ ꢂ ꢂ
p interactions is probably another important
reason for the slight change besides the difference of dielectric
constant of the environment [47].
3.3. Thermal properties and electrochemical properties
283
Thermal gravimetric analysis (TGA) and differential scanning
calorimetry (DSC) show that DPPP has an excellent thermal
284
285
Fig. 2. Packing diagram of DPPP (All H atoms except H1 and H22 are omitted for
clarity).
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stability. The decomposition temperature (T ) of DPPP was mea-
sured to be 442 °C (Fig. 5). DSC measurements were performed
from 30 to 375 °C under the nitrogen atmosphere (Fig. 6). DPPP
d
melted at 312 °C (T ) upon the first heating process, and then
m
the liquid sample was rapidly cooled to form an amorphous glassy
state. When the amorphous glassy sample was heated again, a
glass transition occurred at 148 °C (T ). The high glass transition
g
temperature is presumably due to the non-coplanar structure
and large molecular weight, and implies DPPP can form stable
amorphous film in the devices. Upon further heating beyond Tg,
an exothermic peak due to crystallization was observed at around
204 °C.
The electrochemical property of DPPP was investigated by
cyclic voltammetry in CH Cl solution containing tetrabutylammo-
2
2
nium hexauorophosphate (0.1 M) with a scan rate of 50 mV/s. As
shown in Fig. 7, DPPP exhibits a reversible oxidation process with
half-wave oxidation potential of 1.25 V, and shows a quasi-
reversible oxidation process at a higher potential. The HOMO
energy level was calculated to be ꢀ5.65 eV by using the energy
level value of ꢀ4.8 eV for ferrocene (Fc) with respect to zero vac-
uum level. The LUMO energy level was estimated to be ꢀ2.55 eV
according to the HOMO energy level value in combination with
the band gap derived from the absorption band edge.
Fig. 7. Cyclic voltammogram recorded for DPPP.
Fig. 8. Energy level diagram of the DPPP based device.
Fig. 5. TGA curve of DPPP.
Fig. 9. EL spectra of the DPPP based device under different luminance. Inset:
comparison of EL and PL spectra of DPPP solid film.
3.4. Electroluminescence
309
To investigate the electroluminescence (EL) performance of
DPPP, the non-doped OLED was fabricated with a configuration of
ITO/NPB (25 nm)/TCTA (5 nm)/DPPP (30 nm)/TPBI (40 nm)/LiF
310
311
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(
1.5 nm)/Al (100 nm). In this device, ITO (indium tin oxide) and
LiF/Al are the anode and the cathode, respectively; NPB
0
(
4,4 -bis[N-(1-naphthyl)-N-phenylamino]biphenyl) is the hole
0
00
Fig. 6. DSC curve of DPPP.
transporting layer; TCTA (4,4 ,4 -tris(N-carbazolyl)triphenylamine)
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ular structure, Org. Electron. (2015), http://dx.doi.org/10.1016/j.orgel.2015.04.024

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is used as the exciton blocking layer; TPBI (1,3,5-tris(phenyl-2-ben-
zimidazolyl)benzene) is the electron transporting layer and exciton
blocking layer simultaneously; DPPP is used as the emitting layer.
Relative energy level alignment of the device is shown in Fig. 8.
Fig. 9 shows the EL spectra of the device under different lumi-
nance, which are not fully measured since the ultraviolet emission
below 380 nm cannot be detected by PR650 spectrascan photome-
ter. From the EL spectra, the DPPP based device shows a violet
emission with an emission peak at 396 nm and a narrow FWHM
of about 50 nm, similar to the PL spectra of DPPP. The 1931 CIE
coordinates of the device under different luminance are all about
(0.16, 0.04), which is located in the violet area of the CIE chro-
maticity diagram and rarely reported in OLEDs to our knowledge
[13,17,22]. Furthermore, the high color purity and stable violet
emissions also confirm that the non-coplanar molecular structure
of DPPP can effectually hinder intermolecular aggregation.
Table 3
EL performance of the device based on DPPP.
Vona
(V)
Max CE
(cd/A)
Max EQE
(%)
kmax
(nm)
Max luminance
(cd/m )
CIE (x, y)b
2
4.0
0.74
2.2
396
1890
(0.160,
0.041)
a
Recorded at 1 cd/m2.
Recorded at 100 cd/m .
b
2
efficiency of the device is 0.74 cd/A at a luminance of 69.3 cd/m2,
corresponding a maximum external quantum efficiency (EQE) of
2.2%. Moreover, this violet device exhibits a lower efficiency roll-
off, remaining a high EQE of 1.56% at the practical luminance of
342
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344
345
346
347
348
349
350
2
1000 cd/m , which is highly desirable for OLEDs. EL performance
of the device was summarized in Table 3. As far as we know, the
comprehensive performance of the device is the best result for
violet OLEDs with kmax < 400 nm considering the FWHM of EL
spectrum, color purity, EL efficiency and stability [22–24].
The current density–voltage–luminance characteristic of the
device is shown in Fig. 10. The device has a low turn-on voltage
2
below 4 V at a brightness of 1 cd/m , and exhibits a maximum
luminance of 1890 cd/m2 at 9 V with a current density of
2
446 mA/cm . Such low turn-on voltage is due to the high carrier
4
. Conclusions
351
mobility and excellent hole injection ability of the pyrene deriva-
tives and the small injection barriers in the devices (HOMO:
ꢀ5.7/ꢀ5.65 eV at TCTA/DPPP interface; LUMO: ꢀ2.55/ꢀ2.7 eV at
DPPP/TPBI interface). As shown in Fig. 11, the maximum current
In summary,
a
novel high-efficiency violet-light-emitting
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material DPPP has been successfully developed employing pyrene
as the chromophore. In this compound, the introduced functional
groups play three important roles, (i) protect pyrene unit to
prohibit
p–p stacking, (ii) confine the extent of conjugation in
molecule utilizing the steric hindrance, (iii) enlarge the molecular
size to improve the thermal stabilities. The non-doped violet
OLED based on DPPP achieved an excellent performance: high effi-
ciency (EQEmax = 2.2%), high color purity (CIE, x = 0.16, y = 0.04),
lower efficiency roll-off, which indicates DPPP is a promising violet
material for OLEDs.
Acknowledgements
363
We are grateful to the National Natural Science Foundation of
China (No. U1404209, 21173113, 21202078, 21372112,
364
365
366
367
368
369
5
1373190, and 51103169), the Instrument Developing Project of
the Chinese Academy of Sciences (Grant No. YE201133), and the
Natural Science Foundation of Henan Education Department,
China (No. 13A150804) for financial support of this work.
Fig. 10. Current density–voltage–luminance characteristic of the DPPP based
device.
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ular structure, Org. Electron. (2015), http://dx.doi.org/10.1016/j.orgel.2015.04.024