Novel 2,4-difluorophenyl-functionalized arylamine as hole-injecting/hole- transporting layers in organic light-emitting devices

Article abstract of DOI:10.1016/j.cplett.2011.12.025

Novel 2,4-difluorophenyl-functionalized triphenylamine and its dimer, namely tris(2′,4′-difluorobiphenyl-4-yl)-amine (3FT) and N ⁴,N⁴,N⁴′,N⁴′-tetrakis- (2′,4′-difluorobiphenyl-4-yl)-biphenyl-4,4′-diamine (4FDT), have been synthesized and characterized. The device with the configuration of ITO/4FDT/Alq₃/LiF/Al yields maximum current efficiency of 4.3 cd/A. Moreover, with 4FDT as the hole-injecting layer, the maximum efficiency and luminance of the device was further improved to be 5.0 cd/A and 19 880 cd/m ², respectively. The good performance of devices with 4FDT as HTL or HIL was attributed to the incorporation of the strong electron-withdrawing fluorinated substituents into the arylamine moiety, which reduced holes mobility of HTL and HIL and then balanced the injected carriers in devices.

Full text of DOI:10.1016/j.cplett.2011.12.025

Chemical Physics Letters 527 (2012) 36–41
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Novel 2,4-difluorophenyl-functionalized arylamine as
hole-injecting/hole-transporting layers in organic light-emitting devices
⇑
Zhanfeng Li, Zhaoxin Wu , Bo Jiao, Peng Liu, Dongdong Wang, Xun Hou
Key Laboratory of Photonics Technology for Information of Shaanxi Province, and Key Laboratory for Physical Electronics and Devices of the Ministry of Education,
School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Novel 2,4-difluorophenyl-functionalized triphenylamine and its dimer, namely tris(20,40-difluorobiphenyl-
4-yl)-amine (3FT) and N4,N4,N40,N40-tetrakis-(20,40-difluorobiphenyl-4-yl)-biphenyl-4,40-diamine (4FDT),
have been synthesized and characterized. The device with the configuration of ITO/4FDT/Alq3/LiF/Al yields
maximum current efficiency of 4.3 cd/A. Moreover, with 4FDT as the hole-injecting layer, the maximum
efficiency and luminance of the device was further improved to be 5.0 cd/A and 19880 cd/m2, respectively.
The good performance of devices with 4FDT as HTL or HIL was attributed to the incorporation of the strong
electron-withdrawing fluorinated substituents into the arylamine moiety, which reduced holes mobility of
HTL and HIL and then balanced the injected carriers in devices.
Article history:
Received 3 November 2011
In final form 8 December 2011
Available online 16 December 2011
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
However, fluorine-functionalized arylamine as HTLs in OLEDs is still
scarce, and further studies are still desirable.
Triarylamine and its derivatives have long been exploited as
hole-transport layers (HTLs) for organics-based (opto)electronic de-
vices such as organic light-emitting devices (OLEDs), organic solar
cells (OSCs), or organic thin-film transistors (OTFTs) [1–3], and their
range of application is continuously widening. In many applications,
hole-transport materials are conventionally combined with other
organic molecules or/and with metallic electrodes; it is therefore,
important for substituted triarylamines to tune their electronic
structure in order to control the relative frontier orbital energies,
that is, the energies of the highest occupied molecular orbital
(HOMO) or the lowest unoccupied molecular orbital (LUMO). It is
well-known that the charge balancing control of electrolumines-
cence (EL) devices, namely the reduction of the hole mobility of
the hole-transporting materials (HTMs) in matching the electron
mobility of electron transporting materials (ETMs), is effective in
achieving high EL efficiency [4–7]. Previous studies had shown that
the incorporation of the strong electron-withdrawing fluorinated
substituents into hole-transporting (p-type) semiconductors, which
may contribute to tuning the electronic properties, and altering
charge transport properties [8–14]. In particular, electron-
withdrawing groups, such as –CN, –F and –CF3, in direct conjunction
with the electron-donor N atoms of the well-known hole-transport
material N,N0-bis(tolyl)-N,N0-diphenyl-1,10-biphenyl-4,40-diamine
(TPD), lower the HOMO levels and the hole mobility and tune the
molecular properties, such as solubility or stability [15–18].
In this Letter, we reported the synthesis and characterization of
two novel fluorinated triphenylamine (TPA) and its dimer (DTPA),
3FT and 4FDT being the adducts of TPA and DTPA cores with three
and four 2,4-difluorophenyl peripheral moieties, respectively, in
which electron-withdrawing fluorine atoms were introduced
into the periphery of the aryl substituents of (D)TPA moieties to
lower the HOMO levels and reduce the hole-transport. The new
2,4-difluorophenyl-functionalized (D)TPA, 3FT and 4FDT, exhibited
good thermal stabilities, proper HOMO/LUMO energy levels and
the hole-transporting ability, which were suitable for HIL or HTL in
OLEDs. The typical bilayer devices with the fluorinated (D)TPA as
HTLs yield maximum current efficiency of 3.9 cd/A for 3FT and
4.3 cd/A for 4FDT, which exceed that of the prototypical N,
N0-bis-(1-naphthalenyl)-N,N0-bis-phenyl-(1,10-biphenyl)-4,40-dia-
mine (NPB)-based device (3.8 cd/A) with the same device struc-
ture. Furthermore, by using 4FDT as HIL, high-performance OLED
was achieved with a maximum luminance around 19880 cd/m2
and maximum efficiency above 5.0 cd/A.
2. Experimental section
2.1. Materials and instruments
The manipulation involving air-sensitive reagents was per-
formed under an inert atmosphere of dry nitrogen. Other reagents
in the scheme were used as received from commercial sources.
Absorption (UV) spectra were recorded on a Hitachi UV 3010 spec-
trophotometer. PL spectra were recorded on a Horiba Jobin Yvon
⇑
Corresponding author. Fax: +86 29 82664867.
E-mail address: zhaoxinwu@mail.xjtu.edu.cn (Z. Wu).
0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2011.12.025

Z. Li et al. / Chemical Physics Letters 527 (2012) 36–41
37
Fluoromax-4 spectrophotometer. Decomposition temperatures
2.2.3. N4,N4,N40,N40-tetrakis(20,40-difluorobiphenyl-4-yl)-biphenyl-
4,40-diamine (4FDT)
(Td) were obtained using TG209C thermal gravimetric analysis
(TGA) with a heating rate of 20 °C minꢀ1 and glass transition tem-
peratures (Tg) were determined with a differential scanning calo-
rimeter (DSC, TA instruments DSC200PC) at a heating rate of
10 °C minꢀ1 under a N2 atmosphere. To measure the fluorescence
quantum yields (UPL), degassed solutions of the compounds in
CH2Cl2 were prepared. The concentration was adjusted so that
the absorbance of the solution would be lower than 0.1. The exci-
tation was performed at 334 nm, and quinine sulfate in 1.0 M
H2SO4 solution, which has UPL = 0.56, was used as a standard
[19]. Cyclic voltammetry was performed using a Princeton Applied
Research model 273 A potentiostat at a scan rate of 100 mV sꢀ1. All
experiments were carried out in a three-electrode compartment
cell with a Pt-sheet counter electrode, a glassy carbon working
electrode and a Pt-wire reference electrode. The supporting elec-
trolyte used was 0.1 M tetrabutylammonium perchlorate ([Bu4N]-
ClO4) solution in dry acetonitrile. The cell containing the solution
of the sample (1 mM) and the supporting electrolyte was purged
with a nitrogen gas thoroughly before scanning for its oxidation
and reduction properties. Ferrocene was used for potential calibra-
tion in each measurement. All the potentials were reported relative
to ferrocene-ferrocenium (Fc/Fc+) couple, whose oxidation poten-
tial was +0.22 V relative to the reference electrode. The oxidation
and reduction potentials were determined by taking the average
of the anodic and cathodic peak potentials. The HOMO and LUMO
values were estimated by using the following general equation:
4
4
4
0
4
0
The experimental procedure for N ,N ,N ,N -tetrakis(20,40-
difluorobiphenyl-4-yl)-biphenyl-4,40-diamine (4FDT) was similar
to that for the synthesis of 3FT described above. A white solid
was obtained in 78% yield. 1H NMR (CDCl3, 400 MHz): d 6.91–
7.00 (m, 12H), 7.24–7.28 (t, 8H), 7.41–7.47 (m, 12H), 7.54–7.56
(d, J = 8.8 Hz, 4H). Anal. Calcd for C60H36N2F8:C, 76.92%; H, 3.87%;
N, 2.99%. Found: C, 76.59%; H, 3.42%; N, 3.29%. MS: m/z 936.2759
[M+] (Calcd: 936.9285).
2.3. OLEDs fabrication and characterizations
The devices were fabricated by conventional vacuum deposition
of the organic layers, LiF and Al cathode onto an ITO-coated glass
substrate under a base pressure lower than 1 ꢁ 10ꢀ3 Pa at the rates
of 0.04, 0.025, 0.5 nm/s, respectively. The thickness of each layer
was determined by a quartz thickness monitor. The effective size
of the OLED was 14 mm2. The voltage–current (V–I), voltage–current
density (V–J) and voltage–brightness (V–L) as well as the current
density–current efficiency (J–g) curve characteristics of devices
were measured with a Keithley 2602 and Source Meter. The detailed
devices fabrication can be found in previous paper in our group [22].
3. Results and discussion
3.1. Synthesis
EHOMO ¼ ꢀðqEox þ 4:8Þ eV; ELUMO ¼ EHOMO ꢀ Eopt [20], which were
g
calculated using the internal standard ferrocene value of ꢀ4.8 eV
Scheme 1 shows the chemical structures and the synthetic routes
with respect to the vacuum level [21].
4
4
4
4
0
0
of tris(20,40-difluorobiphenyl-4-yl)-amine (3FT) and N ,N ,N ,N -
tetrakis-(20,40-difluorobiphenyl-4-yl)-biphenyl-4,40-diamine (4FDT)
2.2. Synthesis
4
4
4
4
0
0
examined in this study. The syntheses of N ,N ,N ,N -tetra-
phenylbiphenyl-4,40-diamine (DTPA) was carried out in 89% yield
by using a palladium-catalyzed Buchwald–Hartwig amination reac-
tion between 4,40-dibromobiphenyl and diphenylamine [23,24].
Bromination of 1 (DTPA) by means of NBS in glacial acetic acid at
ambient temperature gave 2 in 88% yield [25]. Suzuki coupling reac-
tions of 2,4-difluorophenylboronic acid with 1 and 2 were catalyzed
by Pd(PPh3)4 [26], giving 3FT and 4FDT, respectively, in good yields
(ca. 78–85%). All the reaction intermediates and final products were
characterized by 1H nuclear magnetic resonance (H NMR), mass
spectrometry (MS) and element analysis. The results are in good
agreement with proposed structures. The synthesized compounds
could dissolve in common organic solvents, such as toluene, chloro-
form and THF, etc.
2.2.1. Tris(20,40-difluorobiphenyl-4-yl)amine (3FT)
Tris(4-bromophenyl)amine (2.41 g, 5 mmol), 2,4-difluorophen-
ylboronic acid (2.84 g, 18 mmol), Pd (PPh3)4 (0.58 g, 0.5 mmol),
aqueous Na2CO3 (2.0 M, 15 mL), ethanol (10 mL) and toluene
(30 mL) were mixed in a flask under nitrogen. The mixture was
heated to reflux and maintained at this temperature overnight.
When the reaction was completed (judging from thin-layer chroma-
tography), water was added to quench the reaction. Then, the
products were extracted with CH2Cl2. The organic layer was
collected, dried over anhydrous MgSO4 and evaporated under
vacuum. The solid was absorbed on silica gel and purified by column
chromatography using light petrol ether/ethyl acetate (20:1) as the
eluent to give 3FT as a white solid (2.44 g, 85%). 1H NMR (CDCl3,
400 MHz): d 6.88–6.97 (m, 6H), 7.22–7.25 (t, J = 8.4 Hz, 6H), 7.39–
7.44 (m, 9H). Anal. Calcd for C36H21NF6: C, 74.35%; H, 3.64%; N,
2.41%. Found: C, 74.31%; H, 3.01%; N, 2.77%. MS: m/z 581.1592
[M+] (Calcd: 581.5491).
3.2. Thermal properties
The thermal properties of 3FT and 4FDT were evaluated by TGA
and DSC. The onset temperatures for 5% weight loss (Td) in nitrogen
of 4FDT was 513 °C, which is significantly higher than that of the
most widely used hole-transporting material (e.g., 410 °C for
NPB). The glass-transition temperature (Tg) of 4FDT was found to
be 113 °C in the second heating scan, after rapid cooling of the
melted sample, while those of the hole-transporting material
NPB and the hole-injecting/transporting material m-MTDATA are
98 and 75 °C, respectively [27]. The thermal transitions Tg, Tm
and Td are 69, 181 and 374 °C, respectively, for 3FT. Therefore, ther-
mal analyses indicate that 4FDT with high Td and Tg would be used
as a good active layer in OLED devices.
4
4
4
0
2.2.2. N ,N ,N ,N40-tetrakis(4-bromophenyl)biphenyl-4,40-diamine (2)
To a 100 mL round-bottom flask were added 2.44 g, 5 mmol
4
4
4
4
0
N ,N ,N ,N -tetraphenylbiphenyl-4,40-diamine (DTPA) and 3.56 g,
20 mmol NBS. Chloroform (30 mL) was added, and the solution
was stirred at room temperature (RT) for 1 h. And then 12.5 mL
of acetic acid were added, and the solution was stirred for further
6.5 h at RT. The product was extracted with diethyl ether, washed
with water and brine twice and then the organic extract dried over
anhydrous sodium sulfate. After solvent evaporation, the crude
product was recrystallized from chloroform and hexane to afford
white solid (3.55 g, 88%). 1H NMR (CDCl3, 400 MHz): d 6.99–7.00
(d, J = 8.8 Hz, 8H), 7.11–7.13 (d, J = 8.4 Hz, 4H), 7.37–7.40 (d,
J = 8.8 Hz, 8H), 7.46–7.49 (d, J = 8.8 Hz, 4H). Anal. Calcd for
0
3.3. Photophysical properties
C
36H24N2Br4:C, 53.77%; H, 3.01%; N, 3.48%. Found: C, 51.71%; H,
Figure 1 shows the normalized absorption and photolumines-
cent (PL) spectra of 3FT and 4FDT dilute solutions in dichlorometh-
2.32%; N, 3.66%.

38
Z. Li et al. / Chemical Physics Letters 527 (2012) 36–41
Scheme 1. Synthesis of compounds 3FT and 4FDT.
Figure 1. Absorption and fluorescence spectra of 3FT (a) and 4FDT (b) in solution and in film.
Table 1
Optical, thermal and electrochemical properties of the compounds.
HOMOd
(eV)
LUMOe
(eV)
Tg/Td
(°C)
a
b
c
Compounds
kabs CH2Cl2
(nm)
kem CH2Cl2
(nm)
kem film
(nm)
UPL CH2Cl2
Eg
EOX
(V)
(eV)
3FT
4FDT
338
349
411
413
399
416
0.49
0.56
3.28
3.06
0.48
0.31, 0.43
ꢀ5.3
ꢀ5.1
ꢀ2.0
ꢀ2.1
69/374
113/513
a
b
c
Determined in CH2Cl2 using quinine sulfate (UPL = 0.56 in 1.0 M H2SO4 solution) as standard.
Calculated from the onset of absorption in thin film.
Measured vs. Fc/Fc+ in CH3CN.
d
e
Estimated from onset oxidation voltages with reference to ferrocene (4.8 eV vs. vacuum).
Calculated from HOMO and optical band gap.
ane and its as-cast thin film at room temperature. In the UV–visible
spectra, the absorption bands observed around 340 nm in solution
can be attributed to
suppresses intimate intermolecular packing in the condensed solid
state, whereas the main characteristics of the spectra remain un-
changed [28].
p–
p⁄ transitions of the conjugated aromatic
rings. The UV–visible absorption maximum in the solution state
was 338 nm for 3FT, while the peak of 4FDT was red-shifted by
11 nm, suggesting that the latter was more conjugated than the
former. From the onset of the film absorption spectrum, the energy
band gap (Egs) of 3FT and 4FDT in thin film were calculated to be
3.28 and 3.06 eV, respectively (Table 1). And the two compounds
could be applied to HTL due to no absorption at visible light region.
3FT and 4FDT in the solid state displayed a bright blue emission
peak with maxima at ca. 399 and 416 nm, respectively (Figure 1). It
is worth noting that the photoluminescence spectra of the com-
pounds in films are almost identical to those in solution, which
indicates the prearrangement of two fluorine atoms at the 2,4-
positions in the phenyl ring of each arm in 3FT and 4FDT efficiently
3.4. Electrochemical properties
The redox behaviors of the compounds 3FT and 4FDT were eval-
uated by cyclic voltammetry (CV) experiments at ambient temper-
ature; see Figure 2. During the oxidation scan in acetonitrile, both
compounds 3FT and 4FDT show reversible oxidation processes. 3FT
displays a reversible oxidation wave, which is a one-electron pro-
cess attributed to the oxidation of the triphenylamine core while
the highly extended 4FDT exhibits two sequential one-electron
processes, corresponding to removal of two electrons from the
DTPA unit. The oxidation potential of 3FT relative to ferrocene is
0.48 V, which is significantly higher than that of tri(biphenyl-4-

Z. Li et al. / Chemical Physics Letters 527 (2012) 36–41
39
Figure 2. Cyclic voltammogram of 3FT (a) and 4FDT (b) in CH3CN, Pt wire used as reference electrode.
yl)amine (TBA) (0.18 V vs. Fc) [29]. The difference between 3FT and
3.5. Quantum chemical calculations
TBA in oxidation potential was attributed to the strongly electron-
withdrawing fluorine substituent on the aryl substituents of TPA
electron donor [15,30]. The DTPA core significantly lowers the oxi-
dation potential, as evident by the 0.31 V value for 4FDT vs. 0.48 V
for 3FT. The reason is that the longer the conjugated aryl core,
which can better stabilize the radical cation(s), the lower the oxi-
dation potential is. Similarly to most other triphenylamine com-
pounds, 3FT and 4FDT have the HOMO (ꢀ5.1 and ꢀ5.3 eV,
respectively) close to the work function of indium tin oxide (ITO)
and thus allows efficient hole injection from the ITO. Thus a
hole-injecting/hole-transport capacity for 3FT and 4FDT was ex-
pected. The LUMO determined by the difference of HOMO and
optical energy gap was tabulated in Table 1. The LUMO energy val-
ues of 3FT and 4FDT were calculated to be ꢀ2.0 and ꢀ2.1 eV,
respectively. And the high-lying LUMO energy level of the com-
pounds 3FT and 4FDT suggested that they possess good electron-
blocking abilities as well.
The geometry and the HOMO and LUMO energy levels of 3FT and
4FDT were optimized using density functional theory (DFT) method
with the B3LYP hybrid functional and the 6-31G(d) on carbon and
hydrogen, and all calculations described here were carried out using
the GAUSSIAN 98 program [31]. As illustrated in Figure 3, the core tri-
phenylamine (TPA) group in 3FT shows the three-dimensional pro-
peller structure, and difluorophenyl groups act as the linear arms.
This geometry could result in good solution processability. The
HOMO distributes in the whole molecule with higher density on
the TPA unit, whereas the LUMO is located predominantly on the
two arms of the molecule. Similarly, the HOMO orbitals of 4FDT
are localized on the amine moieties, that is DTPA unit, and the LUMO
orbital in 4FDT reside on two separated difluorobiphenyl arms of the
molecule. The broad distribution of the HOMO should be good for
the hole transport through the molecules when both 3FT and 4FDT
are used as hole-injecting/hole-transporting materials in the OLEDs.
Figure 3. Calculated spatial distributions of LUMO and HOMO of 3FT and 4FDT.

40
Z. Li et al. / Chemical Physics Letters 527 (2012) 36–41
tune charge transport properties. When the electron-withdrawing
fluorinated substituent to electron-donor moiety ratio is equal to 2,
4FDT exhibits lower mobility than regular DTPA and NPB and the
hole-current of 4FDT-based hole-only device is quit close to elec-
tron-current of the Alq3-based electron-only device at applied volt-
ages lower than 16 V. Moreover, the lowest hole-transport of 3FT
might be due to the fact that there is only one electron-donor moi-
ety and that the ratio of electron-withdrawing fluorinated substi-
tuent to electron-donor moiety is
3 in the molecule [4,5].
Therefore, 4FDT was suitable to be HTL for the more balanceable
carrier-transport in the device.
Based on the unique properties of 3FT and 4FDT including good
thermal stabilities, optical properties, proper HOMO/LUMO energy
levels and the hole-transporting nature of (D)TPA derivatives, both
are expected to be potential hole-transporting materials in electro-
luminescent devices. We fabricated the typical two-layer device
with the structure of ITO/3FT or 4FDT (60 nm)/Alq3 (60 nm)/
LiF(1 nm)/Al (70 nm), where Alq3 is electron transporting and
light-emitting layer. For comparison, a typical and widely used
hole transporter NPB has been chosen and a conventional ITO/
NPB (60 nm)/Alq3 (60 nm)/LiF(1 nm)/Al (70 nm) device was fabri-
cated under the same conditions. In Figure 5, for the 4FDT-based
device, the highest efficiency reached 4.3 cd/A, much higher than
that of the device with NPB as the HTL (3.8 cd/A). The better perfor-
mance of 4FDT-based device was attributed to the more balance-
able injected carriers in the device than that in the NPB-based
device, which was also shown in Figure 4. As for the 3FT-based de-
vice, the current density and luminance are the poorer than those
of NPB-based device, although its maximum current efficiency is
3.9 cd/A, and almost equal to NPB-based device (3.8 cd/A). This
poorer performance of the 3FT-based device may resulted from
the less injected and transported holes than electrons in devices,
as shown in Figure 4.
Figure 4. Current–voltage for the hole-only devices, ITO/NPB, DTPA, 3FT or 4FDT
(100 nm)/Ag (80 nm), and the electron-only device, ITO/Alq3 (100 nm)/LiF (1 nm)/Al
(70 nm).
As shown in Table 1, the experimental HOMO and LUMO are in
excellent agreement with the calculated values (ꢀ5.5 and ꢀ5.3 eV
for the HOMO, and ꢀ1.5 eV for the LUMO of 3FT and 4FDT,
respectively).
3.6. Electroluminescent properties
The incorporation of the strong electron-withdrawing fluori-
nated substituents into hole-transporting triphenylamine and its
dimer moiety in 3FT and 4FDT is expected to lower the hole mobil-
ity. To evaluate the hole-transport of 3FT and 4FDT, we fabricated
and characterized the hole-only devices, ITO/NPB, DTPA, 3FT or
4FDT (100 nm)/Ag (80 nm), and the electron-only device, ITO/
Alq3 (100 nm)/LiF (1 nm)/Al (70 nm). In Figure 4, it was found that
the following sequence for the current at a given applied voltage in
single-layer devices: DTPA > NPB >> 4FDT > Alq3 > 3FT. This result
shows that the incorporation of the strong electron-withdrawing
fluorinated substituents into hole-transporting materials could
Figure 5. Current–density–voltage curves (a), Luminance–voltage curves (b) and Efficiency–current-density curves (c) for the ITO/NPB, 3FT or 4FDT (60 nm)/Alq3 (60 nm)/
LiF(1 nm)/Al (70 nm) devices.
Figure 6. Luminance–voltage curves (a) and Efficiency–current-density (b) curves for the ITO/m-MTDATA, 3FT or 4FDT (40 nm)/NPB(20 nm)/Alq3 (60 nm)/LiF(1 nm)/Al
(70 nm) devices.

Z. Li et al. / Chemical Physics Letters 527 (2012) 36–41
41
Since the HOMO energy levels of 4FDT are similar to those of m-
Acknowledgements
MTDATA (5.1 eV), which is well-known and commonly used hole-
injecting material, both compounds 3FT and 4FDT were introduced
as a hole-injecting layer (HIL). In order to compare the perfor-
mance of the devices, m-MDTATA was selected for control HIL
material. Each device was fabricated with the following structure:
ITO/HIL (40 nm)/NPB (20 nm)/Alq3 (60 nm)/LiF (1 nm)/Al (70 nm),
where NPB is hole-transport layer; Alq3 is electron-transport and
light-emitting layer; and HIL is 3FT, 4FDT or m-MTDATA. Figure 6
shows the voltage–luminance and efficiency–current–density
characteristics for the devices. For the 3FT and 4FDT-based devices,
the current efficiency surpassed those of the m-MTDATA-based de-
vice. Especially for the 4FDT-based device, the highest efficiency
and maximum luminance were 5.0 cd/A and 19880 cd/m2 respec-
tively. The improved EL performance can be attributed to the
greater charge balance and better exciton confinement within
the emission layer.
This work was supported by National Natural Science Founda-
tion of China (Grant 61 106 123).
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4. Conclusions
In summary, two fluorinated (D)TPA molecules 3FT and 4FDT,
have been designed, synthesized and applied as hole-injecting/
hole-transporting materials in organic lighte-mitting devices
(OLEDs). It was found that the introduction of the strong electron-
withdrawing fluorine substituents to the periphery of the aryl sub-
stituents of D(TPA) moieties can reduce the hole-transport and low-
er the HOMO levels, and the hole-transport of 4FDT matched well
with electron-transport of Alq3. The fluorinated compound 4FDT,
used in a typical bilayer OLED, exhibits a better performance with
maximum current efficiency of 4.3 cd/A, which is higher than that
of the NPB-based device (3.8 cd/A) with the same device structure.
In addition, the device performance with 4FDT as the HIL could be
further improved in ITO/4FDT (40 nm)/NPB (20 nm)/Alq3 (60 nm)/
LiF (1 nm)/Al (70 nm) device, and the current efficiency and maxi-
mum luminance were 5.00 cd/A and 19880 cd/m2, respectively. In
sight of its better thermal stability (Tg = 113 °C) than that of NPB
(Tg = 98 °C), and the much better performance of 4FDT-based de-
vice, 4FDT is promising hole-injecting/ hole-transporting material
for applications in organic light-emitting devices.
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