An organic p-type dopant with high thermal stability for an organic semiconductor

Article abstract of DOI:10.1039/b713566a

To overcome the thermal instability of a p-doped organic hole transporting layer using the state-of-the-art p-type dopant, 2,3,5,6-tetrafluoro-7,7,8,8- tetracyanoquinodimethane, a potent electron accepter, 3,6-difluoro-2,5,7,7,8,8- hexacyanoquinodimethane, has been found to possess superior thermal stability and proved to be an excellent p-type dopant. The Royal Society of Chemistry.

Full text of DOI:10.1039/b713566a

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An organic p-type dopant with high thermal stability for an organic
semiconductor{
Zhi Qiang Gao,a Bao Xiu Mi,*ae Gui Zhen Xu,c Yi Qian Wan,c Meng Lian Gong,c Kok Wai Cheahab and
Chin H. Chend
Received (in Cambridge, UK) 5th September 2007, Accepted 8th October 2007
First published as an Advance Article on the web 17th October 2007
DOI: 10.1039/b713566a
These and other problematic issues concerning the use of F4-
TCNQ in low driving-voltage OLEDs spurred our interest in
pursuing alternative applicable p-type dopants with high thermal
stability. According to the literature,16 when the hydrogen atoms
in TCNQ (7,7,8,8-tetracyanoquinodimethane, Ered is 0.17 V) were
replaced by strong electron withdrawing groups such as two cyano
groups [TCNQ(CN)2] or four fluorine atoms [F4-TCNQ], the
reduction potentials of the resulting materials increased dramati-
cally (Ered is 0.65 V and 0.53 V for TCNQ(CN)2 and F4-TCNQ,
respectively). Cyano is a much stronger electron withdrawing
group than fluorine which can be predicted from their Hammett
constants (CN: sp = 0.66; F: sp = 0.06217). We envisage that if two
fluorine atoms in F4-TCNQ are further replaced by cyano groups,
the products will have even stronger reduction potential.
Furthermore, it is anticipated that the resulting asymmetrical
structure will be likely to improve thermal stability.18 Thus, 3,6-
difluoro-2,5,7,7,8,8-hexacyanoquinodimethane (F2-HCNQ) was
synthesized according to Scheme 1 and characterized by elemental
analysis, high resolution mass spectra, UV–visible and IR spectra.{
The NMR spectrum could not be obtained because of its low
solubility and ready radical anion formation in most deuterated
solvents. F2-HCNQ, as a powerful oxidant, tends to deteriorate in
the solid state under air and changes color from brown yellow to
black within one month. However, if it is sealed with paraffin film
and put into a desiccator, there will be no problems with storage.
Fig. 1 compares the absorption spectra of F2-HCNQ and F4-
TCNQ along with their cyclic voltammograms (CVs) as depicted
in the inset. The shape of the optical absorption of F2-HCNQ
(peak at 403 nm) is very similar to that of F4-TCNQ (peak at
391 nm), and both show a pronounced shoulder on the short
wavelength side (located at 379 nm and 371 nm, respectively). The
subtle bathochromic shift observed from F4-TCNQ to F2-HCNQ
is expected because of the slightly extended conjugation resulting
from replacement of fluorine with cyano.
To overcome the thermal instability of a p-doped organic hole
transporting layer using the state-of-the-art p-type dopant,
2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, a potent
electron accepter, 3,6-difluoro-2,5,7,7,8,8-hexacyanoquinodi-
methane, has been found to possess superior thermal stability
and proved to be an excellent p-type dopant.
Highly reductive electron acceptors are a group of molecules that
play an important role in organic electro-active materials. They
help to improve the conductivity of organic semiconducting
molecules by forming a p-complex and/or an anion radical
complex.1–5 For example, they have been doped into wide gap hole
transporting materials (HTM) or hole injecting materials (HIM) to
provide good ohmic contact with an indium tin oxide (ITO) anode
and to provide high conductivity in organic light emitting diodes
(OLEDs)6 from which OLEDs with low driving voltage and high
power efficiency can be realized. For instance, Huang et al.
reported that an OLED with both p-doped and n-doped carrier
transporting layers (p-i-n OLED) has been able to achieve a
luminance of 1000 cd m22 at a voltage of 2.9 V.7
Among existing electron acceptors, 2,3,5,6-tetrafluoro-7,7,8,8-
tetracyanoquinodimethane (F4-TCNQ) has been widely accepted
as a state-of-the-art p-type dopant in organic optoelectronic
devices.8–12 Nevertheless, highly volatile F4-TCNQ molecules and
an unstable F4-TCNQ doped hole injecting layer (HIL)/hole
transporting layer (HTL) at elevated temperatures raised serious
concerns over issues of cross-contamination, reproducibility and
thermal stability of the organic device.13–15 Wellmann et al.
reported a long lifetime p-i-n OLED using a high temperature (low
volatile) p-dopant of NDP2 coupled with Novaled’s proprietary
host HT1.14 Unfortunately, no useful structural information has
been disclosed.
aCentre for Advanced Luminescence Materials (CALM), Hong Kong
Baptist University, Kowloon, Hong Kong, P. R. China
bDepartment of Physics, Hong Kong Baptist University, Kowloon, Hong
Kong, P. R. China
As expected, F2-HCNQ is a much stronger electron acceptor
than F4-TCNQ, exhibiting two reversible one-electron reduction
waves corresponding to the anion-radicals and dianions. The first
cState Key Laboratory of Optoelectronic Materials and Technologies,
School of Chemistry and Chemical Engineering, Sun Yat-Sen University,
Guangzhou 510275, P. R. China
dDisplay Institute, Microelectronics and Information Systems Research
Center, National Chiao Tung University, Hsinchu, Taiwan 300, ROC
eInstitute of Advanced Materials (IAM), Nanjing University of Posts
and Telecommunications, Nanjing, Jiangsu 210003, P. R. China.
E-mail: iambxmi@njupt.edu.cn; Fax: 86-25-83492039;
Tel: 86-25-52997096
{ Electronic supplementary information (ESI) available: Synthesis proce-
dures and characterization for F2-HCNQ; devices fabrication details. See
DOI: 10.1039/b713566a
Scheme 1 Synthesis of F2-HCNQ.
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Fig. 1 Absorption spectra of F2-HCNQ and F4-TCNQ. Inset: their CV
Fig. 2 Deposition temperature of F2-HCNQ and F4-TCNQ vs.
data.
deposition rate.
0.05 A s21. Those of F4-TCNQ are in the range of 131 to 154 uC.
reduction potential, determined as the mean values between the
cathodic current peak voltages and their corresponding anodic
peaks, is 0.87 V for F2-HCNQ and 0.61 V for F4-TCNQ. Based
on CV data and optical bandgap (Fig. 1), the lowest unoccupied
molecular orbital (LUMO) and the highest occupied molecular
orbital (HOMO) energy levels can be estimated according to the
relationship: EHOMO/LUMO = Eref 2 Eox/red, where Eref is the
potential of reference to the vacuum level, where for Ag/AgCl, Eref
= 24.72 eV; Eox/red refers to oxidation/reduction potential of
molecules relative to the reference elctrode used in measure-
ment.19–22 Thus, HOMO/LUMO of F4-TCNQ and F2-HCNQ
can be estimated as 28.25/25.33 eV and 28.39/25.59 eV,
respectively. Our LUMO value for F4-TCNQ (25.33 eV) by the
CV method is quite consistent with that (25.24 eV) by the
ultraviolet photoemission spectroscopy (UPS) method,23 merely
differing by 0.09 eV; this indicates the accuracy of our CV
measurement. For a p-type dopant/matrix system, doping occurs
via charge transfer from the HOMO of the host to the LUMO of
the dopant. It will be energetically favorable when there is a close
energy match between the host HOMO and the dopant LUMO. It
is clear that the choice of host matrix for F2-HCNQ is greater than
that for F4-TCNQ because its LUMO at 25.59 eV is significantly
larger than that of F4-TCNQ (25.33 eV). The wider choice of
HIM/HTMs for F2-HCNQ can be advantageous in the design of
a p-doped organic hole transporting layer in organic optoelectronic
devices.
˚
It is evident that F2-HCNQ is much more thermally tolerant than
F4-TCNQ. Hence, the disadvantages of F4-TCNQ as a p-type
dopant, such as difficult control of evaporation rate and
manufacturing process, chamber pollution as well as an undesir-
able aging effect are expected to be avoided by using F2-HCNQ.
For the study of hole-injection and transporting ability of F2-
HCNQ doped HIM/HTM films, as well as to demonstrate the
advantages of these films, three types of hole only devices with the
structure of ITO/2-TNATA/Au (Type I), ITO/2-TNATA: 2% F4-
TCNQ/Au (Type II), and ITO/2-TNATA: 2% F2-HCNQ/Au
(Type III) were fabricated (2-TNATA
= 4,49,40-tris(N-(2-
naphthyl)-N-phenyl-amino)-triphenylamine), as shown in Fig. 3.
The thickness of the organic layer and the Au layer in all three
˚
˚
types of device is 600 A and 150 A respectively. There are three
pieces in the device for each type: one piece worked as a control
device that was measured immediately after having been freshly
prepared; the other two pieces were annealed for one hour under
1 6 1026 Torr at two different temperatures of 65 uC and 85 uC,
respectively. The I–V characteristics of these samples are shown in
Fig. 4. The currents of the F4-TCNQ or F2-HCNQ doped devices
are nearly two orders of magnitude larger than those of the
corresponding undoped ones. The dramatic enlargement of hole
current can be attributed to an increase in film conductivity or in
carrier injection via tunneling, or both.24 When the F2-HCNQ
As thermal stability of materials is an essential pre-requisite for a
long lifetime of an organic optoelectronic device, the thermal
behavior of F2-HCNQ was compared to that of F4-TCNQ by
thermal gravimetric analysis (TGA, see ESI{). It reveals that the
temperature for 5% weight loss of F4-TCNQ is 256.8 uC, while
that for our newly synthesized F2-HCNQ is 372.2 uC, which is
over 115 uC higher. The main weakness of F4-TCNQ is its high
volatility,13 which makes it difficult to handle as a dopant, and
may result in undesirable heterogeneities in the dopant/host layer.
Concomitantly, the risk of cross contamination during the
fabrication process is also high. Accordingly, the deposition
temperatures of F4-TCNQ and F2-HCNQ as a function of
deposition rates were studied under the same conditions; the
results are shown in Fig. 2. Consistent with the TGA results, the
deposition temperatures of F2-HCNQ are higher, in the range of
187 to 212 uC when the deposition rate increases from 0.01 to
Fig. 3 Materials used for p-doped hole only device, and the device
structure. Type I: no dopant, Type II: X = F4-TCNQ, Type III: X = F2-
HCNQ.
118 | Chem. Commun., 2008, 117–119
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Support Grant, ITC/05-06/02 and e-Ray Optoelectronics, Ltd.,
HK, for providing some of the organic materials for this work.
Notes and references
{ Characterization data for F2-HCNQ: mp: 309 uC decomp.; high
resolution MS: m/z 290.0159 (M2, theoretical value: 290.0158, error:
0.3448 ppm); Calcd. (C14F2N6), C: 57.95, H: 0, N: 28.96; Found, C: 57.39,
N: 28.55, H: 0.564%; FT IR (KBr, cm21): 2202, 1630 (br), 1462, 1335, 919;
UV–vis (CH2Cl2, l/log e): 402 nm/4.23.
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doped 2-TNATA devices with or without annealing.
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microscope. The I–V characteristic remains unchanged (see ESI{),
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optoelectronic device. Compared to the state-of-the-art F4-TCNQ,
F2-HCNQ is a stronger electron acceptor which has higher
thermal stability and lower volatility. The deposition temperature
of F2-HCNQ is much higher than that of F4-TCNQ. A F2-
HCNQ doped 2-TNATA layer shows high hole transporting
ability up to 85 uC. All these results demonstrate that F2-HCNQ is
a promising high temperature p-type dopant material for organic
semiconductor application. Based on the preliminary study on F2-
HCNQ, we expect that an OLED device with F2-HCNQ as a
p-type dopant will have considerably improved performance, such
as low driving voltage, long lifetime, high thermal stability.
We are grateful for the support of Hong Kong Innovation and
Technology Commission Guangzhou-Hong Kong Industrial
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