Synthesis of a new cross-linkable perfluorocyclobutane-based hole-transport material

Article abstract of DOI:10.1021/ol061642f

(Chemical Equation Presented) A new curable arylamine containing a perfluorocyclobutane (PFCB) structure without an acidic group was synthesized. The material was thermally cured on ITO after spin-coating. The polymer showed excellent solvent resistance,

Full text of DOI:10.1021/ol061642f

ORGANIC
LETTERS
2006
Vol. 8, No. 21
4703-4706
Synthesis of a New Cross-Linkable
Perfluorocyclobutane-Based
Hole-Transport Material
Bogyu Lim,† Jin-Taek Hwang,†,‡ Jin Young Kim,§ Jieun Ghim,† Doojin Vak,†
|
Yong-Young Noh,†, Soo-Hyoung Lee, Kwanghee Lee,§ Alan J. Heeger,§ and
Dong-Yu Kim*,†
Heeger Center for AdVanced Materials, Department of Materials Science and
Engineering, Gwangju Institute of Science and Technology, 1 Oryong-dong, Buk-gu,
Gwangju, Korea, DiVision of EnVironmental and Chemical Engineering, Chonbuk
National UniVersity, Jeonju, 561-756, Korea, and Center for Polymers and Organic
Solids, UniVersity of California, Santa Barbara, California 93106
kimdy@gist.ac.kr
Received July 4, 2006
ABSTRACT
A new curable arylamine containing a perfluorocyclobutane (PFCB) structure without an acidic group was synthesized. The material was
thermally cured on ITO after spin-coating. The polymer showed excellent solvent resistance, high thermal stability, high transparency, and
good surface smoothness.
Polymer light-emitting diodes (PLEDs) have been exten-
sively investigated for possible use in flat panel displays.1
However, the successful development of PLEDs requires
improvements in factors such as lifetime, efficiency, and
stability. To improve the efficiency of PLEDs, the balance
of charge injection and transport from each electrode is an
important factor. For balanced charge injection, multilayer
architectures, for example, the introduction of both hole- and
electron-transporting layers, are required.2 In the case of
PLEDs, multilayer structures can be fabricated using a
solution process such as spin-coating. A number of hole
injection/transporting polymers have been reported. Among
these, the development of poly(styrenesulfonate)-doped poly-
(3,4-ethylenedioxythiophene) (PEDOT-PSS) has indeed led
to a dramatic improvement in lifetime as well as luminous
efficiency.3 Although the PEDOT-PSS shows excellent
† Gwangju Institute of Science and Technology
‡ Present address: R&D Center, e-Polymers Co., Ltd. 217-4, Hwapyung-
ri, Ganam-myun, Yeoju-gun, Gyunggi-do, Korea.
§ University of California, Santa Barbara.
(2) (a) Fuhrmann, T.; Salbeck, J. MRS Bull. 2003, 354. (b) Huang, Q.;
Evmenenko, G. A.; Dutta, P.; Lee, P.; Armstrong, N. R.; Marks, T. J. J.
Am. Chem. Soc. 2005, 127, 10227.
(3) (a) Berntsen, A.; Croonen, Y.; Liedenbaum, C.; Schoo, H.; Visser,
R. J.; Vleggaar, J.; van de Weijer, P. Opt. Mater. 1998, 9, 125. (b) Carter,
J. C.; Grizzi, I.; Heeks, S. K.; Lacey, D. J.; Latham, S. G.; May, P. G.;
Ruiz de los Pan˜os, O.; Pichler, K.; Towns, C. R.; Wittmann, H. F. Appl.
Phys. Lett. 1997, 71, 34. (c) Carter, S. A.; Angelopoulos, M.; Karg, S.;
Brock, P. J.; Scott, J. C. Appl. Phys. Lett. 1997, 70, 2067.
| Present address: OE Group, Cavendish Laboratory, University of
Cambridge, Madingley Road, Cambridge CB3 0HE, U.K.
Chonbuk National University.
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10.1021/ol061642f CCC: $33.50
© 2006 American Chemical Society
Published on Web 09/20/2006

performance, it has some disadvantages, one of which is the
formation of an unstable interface between the ITO and
PEDOT-PSS.4 Moreover, the high conductivity of the
PEDOT-PSS is undesirable, when used in pixilated displays.5
Recently, new hole transporting materials (HTM) for replac-
ing the PEDOT-PSS have been reported, because the long-
term stability of such devices remain an issue for commercial-
ization.5-7
In this paper, we report on the preparation of a new curable
HTM-containing PFCB. When heated, this monomer under-
goes cyclopolymerization, producing PFCB polymers. The
optical, electrochemical, and surface properties of this PFCB-
based HTM were investigated.
The synthetic scheme for the preparation of the TFVE-
type monomer for a new HTM is shown in Scheme 1.
Compound 1 was prepared as described previously8b,9a and
is commercially available from Oak-wood Chemicals, Inc.
Compound 2 was synthesized from 1 as follows. tert-
Butyllithium was added dropwise to a solution of 1 in diethyl
ether at -78 °C. The reaction mixture was stirred at -78
°C for 1 h, and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane was then added. Compound 2 was obtained
as a colorless liquid in 55% yield. Compound 3 was prepared
according to the published procedure.11 TPA-TFVE was
synthesized from 2 and 3 using the palladium-catalyzed
Suzuki coupling method.12 TPA-TFVE was obtained as a
white powder in 35% yield. The TPA-TFVE was soluble
in common organic solvents such as chlorobenzene, THF,
and toluene. Due to the trifunctional TFVE of TPA-TFVE,
it can be thermally cured.8b
Scheme 1
Thermal properties were investigated by means of TGA
and DSC. As shown in Figure 1, the endothermic peak of
TPA-TFVE corresponds to its melting point. The broad
exothermic peak in the DSC curve corresponds to its thermal
polymerization.8b Exothermic peaks were found at about Tonset
) 160 °C and Tmax ) 230 °C. The decomposition temper-
ature (Td) of the TPA-PFCB polymer was found to be 487
°C. Thus, the PFCB polymer has good thermal stability. After
curing, a glass transition temperature was not detected before
Td, implying the existence of a high degree of cross-linking
between the TFVE groups.
An organic solvent resistant cross-linkable hole transport-
ing layer (HTL) would be an alternative to water soluble
PEDOT-PSS.6 Especially, perfluorocyclobutane (PFCB)
based hole transporting polymers have many advantages and
the properties of these polymers are suitable for applications
to PLEDs.5,7 PFCB polymers are synthesized by the radical
mediated thermal cyclopolymerization of trifluorovinyl ethers
(TFVE). These polymers are known to lead to increased
processability, durability, chemical resistance, thermal stabil-
ity, and optical properties.8 Thus, PFCB polymers would be
suitable for use in various applications such as polymer
photonic devices,9 low dielectric coatings,10 and light-
emitting diodes.5,7
The TPA-TFVE was spin-coated onto a ITO substrate
and then cured at 230 °C for 2 h under nitrogen. The resulting
polymer showed high durability in common organic solvents
such as chlorobenzene and toluene. We investigated the
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polymer is transparent over the entire visible region (Figure
S2, Supporting Information). As a result, the output of light
from the emitting layer could be increased. The onset of
absorption of the TPA-PFCB polymer was 395 nm which
corresponds to 3.14 eV of the optical band-gap.
To assess the charge injecting properties and electrochemi-
cal stability of the polymer, cyclic voltammetry (CV)
measurements of the polymer films coated on an ITO
substrate were carried out (Figure S1, Supporting Informa-
tion). The highest occupied molecular orbital (HOMO)
energy was calibrated from the half-wave potential of
ferrocene/ferrocenium (Fc) redox coupling which is 4.8 eV
below vacuum.13 The HOMO level appeared at 5.06 eV, and
the LUMO level was estimated from the optical band-gap.
The LUMO energy level of the polymer was determined to
be 1.92 eV. In addition, the results of CV measurements of
the polymer thin films on ITO substrates showed very good
reversibility, indicating that the TPA-PFCB polymer has
good electrochemical stability.
The surfaces of the TPA-PFCB film were investigated by
atomic force microscopy (AFM). Surface morphology of
polymer film is critical to the device performance.6b Figure
3 shows AFM images of the surface of PEDOT-PSS and
TPA-PFCB film on the ITO. As can be seen, the surface of
the TPA-PFCB film was much smoother than that of the
PEDOT-PSS film. The root-mean-square (rms) roughness
of the TPA-PFCB was 0.47 nm. In contrast, the rms
roughness of PEDOT-PSS was 1.15 nm.
Figure 1. TGA (dash dot line) and DSC (solid line) curves of
TPA-TFVE.
resistance of this new HTM to chlorobenzene via UV/vis
spectroscopy as shown in Figure 2, and the data show that
the cross-linked TPA-PFCB polymer is completely resistant
to chlorobenzene and 2 h of heating is required to ensure
complete curing (Figure 2, a vs b). Therefore, the polymer
could be used to produce a multilayer structure using a
solution process. The absorption spectra also verify that the
Figure 2. Absorption spectra of spin-coated TPA-TFVE films after
curing for (a) 1 h and (b) 2 h before and after rinsing with
chlorobenzene.
Figure 3. Atomic force microscopy images of (a) PEDOT-PSS
and (b) TPA-PFCB thin films on ITO.
Org. Lett., Vol. 8, No. 21, 2006
4705

on voltage of the PEDOT-PSS and the TPA-PFCB were 5
and 7 V, respectively. The TPA-PFCB device as a HTL
showed a higher turn on voltage than a PEDOT-PSS based
device. The reason for this is probably due to the larger
difference in energy barrier between the HOMO level of the
emitting polymer and that of the TPA-PFCB than the
corresponding PEDOT-PSS HOMO level (Figure S3, Sup-
porting Information). However, the maximum luminance
(Lmax) and luminance efficiency (LEmax) were improved,
when TPA-PFCB was used. The Lmax and LEmax of the TPA-
PFCB based device were 1500 cd/m2 and 0.132 cd/A,
respectively, compared with Lmax ) 710 cd/m2 and LEmax
)
0.091 cd/A for the PEDOT-PSS based device. This could
be due to the greater electron blocking properties of the TPA-
PFCB polymer than the PEDOT-PSS.6b
In summary, a newly designed PFCB-based HTM con-
taining an arylamine was successfully synthesized. The
resulting PFCB-based hole transporting polymer exhibits
good thermal properties, solvent resistance, high transpar-
ency, and has good surface smoothness. The TPA-PFCB
device as a HTL showed a higher turn on voltage than the
PEDOT-PSS based device, but Lmax and LEmax were im-
proved when the TPA-PFCB polymer was employed.
Although the performance of the TPA-PFCB based device
was not as good as that of the device fabricated using
PEDOT-PSS, the interface between ITO and TPA-PFCB is
likely to be more stable than PEDOT-PSS due to the no
acidity of TPA-PFCB. In addition, TPA-TFVE should be
amenable for use as a cross-linking agent with various
difunctional TFVE materials. Further improvements in PFCB
polymers and device performance are under investigation,
and other applications of TPA-TFVE as a cross-linking agent
with various difunctional TFVE materials are currently
underway.
Figure 4. (a) Current density-voltage and (b) luminance-voltage
diagrams of PLED of the structure ITO/HTL/PFO/Ba/Al. PEDOT-
PSS (solid line) and TPA-PFCB (dash dot line) were used as a
HTL.
To estimate the device performance of TPA-PFCB as a
HTL, a PLED was fabricated with the device configuration
ITO/TPA-PFCB (30 nm)/PFO (70 nm)/Ba (15 nm)/Al (130
nm). The performance of this device was then compared with
a PEDOT-PSS based device. TPA-TFVE was spin-coated
from a 1 wt % solution in chlorobenzene on an ITO substrate
cleaned by a conventional process and cured at 230 °C for
2 h under nitrogen. Devices based on the PEDOT-PSS and
TPA-PFCB showed the same EL spectra. This indicates that
the emission properties were not affected when TPA-PFCB
was used as a HTL. The current density-voltage-luminance
(I-V-L) characteristics of the PEDOT-PSS based device
and the PFCB based device are shown in Figure 4. The turn
Acknowledgment. We thank the BK21 program, Na-
tional Research Laboratory program of KOSEF, a grant
(F0004021) from the Information Display R&D Center, one
of the 21st Century Frontier R&D Programs funded by the
Ministry of Commerce, Industry and Energy of the Korean
Government, and Heeger Center for Advanced Materials for
their financial support.
Supporting Information Available: Synthetic procedures
and characterization data for the products. This material is
available free of charge via the Internet at http://pubs.acs.org.
(13) Vak, D.; Lim, B.; Lee, S.-Y.; Kim, D.-Y. Org. Lett. 2005, 7, 4229.
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