Hole injection/transport materials derived from heck and sol-gel chemistry for application in solution-processed organic electronic devices

Article abstract of DOI:10.1021/ja1061517

An organosilicate polymer, based on N,N′-diphenyl-N,N′-bis(4- ((E)-2-(triethoxysilyl)vinyl)phenyl)biphenyl-4,4′-diamine (TEVS-TPD) with extended conjugation between the Si atom and the aromatic amine, was prepared under mild conditions via sequential Heck and sol-gel chemistry and used as an alternative to poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), the most widely used planarizing hole injection/transport layer in solution-processed organic electronic devices. Spin-coating TEVS-TPD polymer solutions yield defect-free, uniform, thin films with excellent adhesion to the ITO electrode. Upon thermal cross-linking at 180 C, the cross-linked polymer exhibits excellent solvent resistance and electrochemical stability. Solution-processed organic light emitting diode (OLED) devices using iridium-based triplet emitting layers and cross-linked TEVS-TPD films as a hole injection/transport layer show significantly improved performance including lower leakage current, lower turn-on voltage, higher luminance, and stability at high current density, as compared to the control device prepared with PEDOT:PSS.

Full text of DOI:10.1021/ja1061517

ARTICLE
pubs.acs.org/JACS
Hole Injection/Transport Materials Derived from Heck and
Sol-Gel Chemistry for Application in Solution-Processed
Organic Electronic Devices
Younhee Lim,† Young-Seo Park,‡ Yerang Kang,† Do Young Jang,† Joo Hyun Kim,§ Jang-Joo Kim,*,‡
Alan Sellinger,*, and Do Y. Yoon*,†
†Department of Chemistry, Seoul National University, Seoul 151-747, Korea
‡Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea
§Department of Polymer Engineering, Pukyong National University, Busan 608-739, Korea
Department of Material Science and Engineering and the Center for Advanced Molecular Photovoltaics (CAMP), Stanford University,
Stanford, California 94305, United States
S Supporting Information
b
ABSTRACT: An organosilicate polymer, based on N,N0-diphenyl-
N,N0-bis(4-((E)-2-(triethoxysilyl)vinyl)phenyl)biphenyl-4,40-diamine
(TEVS-TPD) with extended conjugation between the Si atom
and the aromatic amine, was prepared under mild conditions via
sequential Heck and sol-gel chemistry and used as an alterna-
tive to poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS), the most widely used planarizing hole injection/
transport layer in solution-processed organic electronic devices.
Spin-coating TEVS-TPD polymer solutions yield defect-free,
uniform, thin films with excellent adhesion to the ITO electrode. Upon thermal cross-linking at 180 °C, the cross-linked polymer
exhibits excellent solvent resistance and electrochemical stability. Solution-processed organic light emitting diode (OLED) devices
using iridium-based triplet emitting layers and cross-linked TEVS-TPD films as a hole injection/transport layer show significantly
improved performance including lower leakage current, lower turn-on voltage, higher luminance, and stability at high current
density, as compared to the control device prepared with PEDOT:PSS.
mainly focused on cross-linkable polymers, because insoluble layers
are easily obtained via in situ cross-linking of solution-deposited
films. In this regard, photocross-linkable polymers containing
oxetane,7,8 acrylate,9,10 and cinnamic-functionalized groups11 have
been shown to be effective to a certain extent. In addition, ther-
mally cross-linkable polymers containing trifluorovinylether,12-15
cyclobutene,16 vinyl groups,17-19 and siloxanes20,21 have been
investigated to prepare all solution-processed multilayer PLEDs.
In particular, organosilicate derivatives have been shown to render
good adhesion properties to the inorganic ITO anode surface and
good compatibility with the adjacent organic layer due to their
organic-inorganic hybrid nature.20-27 Recently, Marks et al.
demonstrated that self-assembling monomers of trichlorosilyl-
alkyl/trimethoxysilylalkyl functionalized aromatic amines such
as N,N,N0,N0-tetraphenyl-1,10-biphenyl-4,40-diamine (TPD) ex-
hibited good hole injection and transport performance in small
molecule-based OLEDs22-27 and PLEDs.20,21 Despite the significant
advances resulting from such extensive efforts, there is still a strong
need for an improved material that can outperform the currently
used PEDOT:PSS with regard to important device properties
I. INTRODUCTION
Organic light emitting diodes (OLEDs) and organic photo-
voltaic solar cells (OPVs) critically depend on the hole injection/
transport layers which, in addition to their main function to
transport charges, also serve to planarize the often relatively rough
surfaces of indium-tin-oxide (ITO) electrodes. Although not
originally designed for this purpose, water-dispersed poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)
has been widely used for this application because it has an appro-
priate energy level for hole injection and transport character-
istics, as well as the planarization capability. Moreover, the depos-
ited PEDOT:PSS film is completely insoluble in organic solvents,
which is essential to the solution processing of multilayer organic
electronic devices such as polymer light emitting diodes (PLEDs)
and OPVs. However, PEDOT:PSS has some shortcomings such
as exciton quenching, anode corrosion, degradation of adjacent
active polymeric layers, and weak interface adhesion,1-6 which
raise serious questions concerning the long-term device stability.
Furthermore, the corrosive nature of the water-based PEDOT:
PSS solutions are known to degrade print-head cartridges and inkjet
nozzles when depositing by inkjet print methods. To overcome
such problems, there have been extensive studies to prepare
alternative planarizing hole injection/transport layers (HITLs)
Received: July 12, 2010
Published: December 30, 2010
r
2010 American Chemical Society
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Scheme 1. Synthesis of TEVS-TPD Monomer and Initial Polymera
a (a) Pd2(dba)3, dppf, NaOtBu, toluene, 90 °C, 10 h, 68% yield; (b) vinyltriethoxysilane, Pd[P(tBu)3]2, dicyclohexylmethylamine, dioxane, 80 °C, 10 h,
63% yield; (c) HCl, H2O, THF, 60 °C, 2 h, >95% yield.
such as turn-on voltage, efficiency (cd/A, lm/W), brightness levels
(cd/m2), and most importantly device lifetime. Furthermore, the
process to prepare such new materials must be straightforward
involving simple chemistry for scalable processes and provide
materials with a long shelf life and easy use.
and differential scanning calorimetry (DSC) measurements were carried
out on a TGA Q50 and DSC 2910 (TA Instruments), respectively, with
a heating rate of 5 °C/min. UV-vis absorption and photoluminescence
(PL) spectra of the initial and the thermally cured films spin-coated on
quartz plates were recorded with a JASCO V-530 UV-vis spectro-
photometer and a JASCO FP-6500 spectrofluorometer, respectively.
FT-IR spectra of the initial polymer and the cured polymer films spin-
coated on silicon wafers were obtained using a JASCO 660 Plus FT-IR
spectrometer. Cyclic voltammetry was performed using indium tin oxide
(ITO) as the working electrode, Ag/Agþ as the reference electrode, and
Pt wire as the counter electrode at a scan rate of 100 mV/s. 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF) in acetonitrile was
the electrolyte. The redox potentials (Eox and Ered) were calibrated by
using ferrocene (Fc) as the standard, whose HOMO energy level is -4.8 eV.
Atomic force microscopy (AFM) images were obtained on a NanoScope
III AFM in the tapping mode. Thickness of the layers were measured
with Alpha step IQ (KLA Tencor). The OLED device characteristics
were investigated using a Keithley 2400 programmable source meter and
a SpectraScan PR650. The current-voltage data of organic thin film
transistor (OTFT) devices were obtained using a Keithley 4200-SCS
semiconductor parameter analyzer in ambient air at room temperature.
Synthesis of N,N0-Diphenyl-N,N0-bis(4-bromophenyl)-
biphenyl-4,40-diamine (TPD-Br2, 1). Tris(dibenzylideneacetone)-
dipalladium (Pd2(dba)3, 0.687 g, 0.750 mmol) and diphenylphosphino-
ferrocene (dppf, 0.626 g, 1.13 mmol) were dissolved in dry toluene
under argon atmosphere. After 1,4-dibromobenzene (23.4 g, 0.100 mol)
was added, the reaction mixture was stirred for 10 min. To the mixture
were added sodium-tert-butoxide (6.01 g, 62.5 mmol) and N,N0-diphenyl-
benzidine (8.41 g, 25.0 mmol). The reaction mixture was heated at
90 °C overnight and deemed complete via thin layer chromatography
(TLC) analysis. After being cooled to room temperature, the mixture was
filtered through Celite and washed with toluene and water. The organic
layer was extracted with toluene (100 mL ꢀ 3) and dried over MgSO4.
The solvent was evaporated on a rotary evaporator, and the dark brown
mixture was purified by column chromatography on silica gel using
methylene chloride/n-hexane (1:6) as eluent to give a white solid (Rf:
0.3). Yield: 11 g (68%). 1H NMR (CDCl3, δ): 7.45 (d, 4H, J = 8.53 Hz),
7.34 (d, 4H, J = 8.76 Hz), 7.27 (t, 4H, J = 8.20 Hz), 7.12-7.03 (m, 10H),
6.99 (d, 4H, J = 8.63 Hz). 13C NMR (CDCl3, δ): 147.23, 146.87, 146.36,
135.13, 132.22, 129.44, 127.49, 125.32, 124.56, 124.32, 123.41, 114.99.
In this Article, we report the preparation and device perfor-
mance of a thermally cross-linkable organosilicate polymer with
aromatic amine substituents as an improved alternative to PEDOT:
PSS for application in OLEDs and OPVs. In comparison with
previous studies that emphasized the self-assembly of monomer-
type aromatic amine organosilicates, our work emphasizes soluble,
branched organosilicate polymers for preparing defect-free, uniform,
thin films by simple solution processing. Most importantly,
triethoxysilane groups are attached straightforwardly by a mild
Heck reaction28,29 instead of traditional methods that use organo-
magnesium or butyllithium.30 Moreover, this procedure to attach
triethoxysilane groups introduces extended conjugation between
the Si atom and the aromatic amine portion, instead of the insu-
lating alkylsiloxane linkage in previous reports.20-27 This extended
conjugation will improve the hole injection/transport by de-
creasing the content and size of charge blocking moieties that
impede efficient hopping of charges.
II. EXPERIMENTAL SECTION
Materials. Vinyltriethoxysilane was purchased from TCI. All other
reagents were purchased from Sigma-Aldrich and used as received unless
otherwise indicated. All the reagents were used without further purifica-
tion. Toluene and 1,4-dioxane were distilled over sodium before use.
Experimental Methods. 1H NMR and 13C NMR spectra were
recorded using a Bruker Advanced 300 MHz spectrometer. The molar
masses of TEVS-TPD monomer and the polymer (see Scheme 1) were
determined by a MALDI-TOF mass spectrometer (from Bruker Daltonics)
and gel permeation chromatography (GPC) calibrated with polystyrene
standards (Polymer Standard Service USA, Inc.), respectively. The GPC
setup was comprised of two Agilent PLgel Mixed-D GPC columns, a
Waters 2410 differential refractometer, and a Viscotek 270 viscometer/
refractive index dual detector. Tetrahydrofuran (THF) was used as an
eluent at a flow rate of 1.0 mL/min. Thermogravimetric analysis (TGA)
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Scheme 2. Structure of TEPCS-TPD Monomer
to measure its charge mobility. They were fabricated in bottom-gate/
bottom-contact type, where heavily doped silicon substrates were
employed as substrate and gate electrodes, thermally grown SiO2, 300
nm, as gate dielectric, and Cr/Au (2 nm/50 nm) as source and drain
electrodes. The source and drain electrodes were thermally evaporated
and patterned by a lift-off process. Channel lengths of 10 μm were
employed in bottom-contact type devices. The active layer was depos-
ited by spin-coating TEVS-TPD polymer solution in chlorobenzene (5
mg/mL, 3000 rpm) followed by in situ thermal curing at 180 °C for 1 h
under nitrogen atmosphere. The current-voltage data of the devices
were obtained in ambient condition, and the field-effect mobility was
1/2
estimated from the slope of ID versus VG in the saturation regime
(transfer characteristic).
OLED Device Fabrication. The solution-processed phosphores-
cent devices were prepared to investigate TEVS-TPD polymer as a
replacement for PEDOT:PSS, as follows. Indium-tin-oxide (ITO)
coated glass substrates were cleaned with acetone and isopropyl alcohol
and dried in an oven at 200 °C for 5 min. Next, they were treated with a
UV-ozone procedure before device fabrication. The organosilicate
HITLs were obtained by spin-coating the solutions onto the ITO
substrates and subsequently thermally curing at 180 °C for 1 h. The
emissive layer (40 nm) was formed via spin-coating a blended solution of
poly(vinylcarbazole) (PVK) and fac-tris(2-phenylpyridine) iridium(III)
(Ir(ppy)3) in chlorobenzene (total 1.5 wt % in chlorobenzene, 6 wt %
Ir(ppy)3:PVK). 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP,
10 nm), tris(8-hydroxyquinolinato)aluminum (Alq3, 40 nm), LiF
(1 nm), and Al (100 nm) were deposited as the hole blocking layer,
electron transporting layer, and cathode by thermal evaporation at a base
pressure of <5 ꢀ 10-7 Torr. Next, the device was encapsulated with a
glass cap. The device based on PEDOT:PSS was fabricated as the control
device, with 40 nm thick PEDOT:PSS (Bayer, Baytron P VP AI 4083)
layer spin-coated and subsequently annealed at 200 °C for 5 min.
Synthesis of N,N0-Diphenyl-N,N0-bis(4-((E)-2-(triethoxysilyl)-
vinyl)phenyl)biphenyl-4,40-diamine (TEVS-TPD, 2). To a flame-
dried round-bottom flask were added 1 (4.01 g, 6.20 mmol), bis(tritert-
butylphosphine)palladium (Pd[P(tBu)3]2, 0.063 g, 0.12 mmol), and dry
dioxane (30 mL). To the reaction mixture, vinyltriethoxysilane (3.25 mL,
15.4 mmol) and argon degassed dicyclohexylmethylamine (3.3 mL, 15.4
mmol) were added. The mixture was stirred at 80 °C overnight under
argon atmosphere and deemed complete by TLC analysis. After being
cooled to room temperature, the reaction mixture was filtered and the
filtrate concentrated on a rotary evaporator. The crude mixture was sub-
sequently dissolved in ethyl acetate and washed with cold 5% HCl three
times and with water twice. The organic layer was dried over Na2SO4, and
the solvent was evaporated on a rotary evaporator. The mixture was puri-
fied by column chromatography on silica gel eluting with ethyl acetate/
n-hexane (1:8) to give a bright yellow solid (Rf: 0.2). Yield: 3.4 g (63%). 1H
NMR (CDCl3, δ): 7.47 (d, 4H, J = 8.53 Hz), 7.37 (d, 4H, J = 8.54 Hz), 7.30
(d, 4H, J= 8.05 Hz), 7.17 (d, 2H, J= 11.7 Hz), 7.15-7.05 (m, 12H), 6.07 (s,
1H), 6.00 (s, 1H), 3.88 (q, 12H, 7.00 Hz), 1.27 (t, 18H, 7.00 Hz). 13CNMR
(CDCl3, δ): 148.50, 148.24, 147.30, 146.40, 135.19, 131.84, 129.37, 127.77,
127.43, 124.84, 124.61, 123.41, 123.13, 115.29, 58.57, 18.30. MS (MALDI-
TOF): m/z 864.33 (100%) calcd for C52H60N2O6Si2 864.40 (100%).
Synthesis of TEVS-TPD Polymer. To a solution of TEVS-TPD
(2) (0.300 g, 0.347 mmol) in THF (2.7 g) were added HCl (35%,
0.38 mg, 10.4 μmol) and water (62.5 mg, 3.47 mmol). The mixture was
stirred at 60 °C for 2 h. After being cooled to room temperature, the
reaction mixture was diluted with chloroform (20 mL) and washed with
water. The organic layer was dried over MgSO4, and the solvent was
removed on a rotary evaporator. The product was dried as a yellow
powder under a vacuum and stored below 4 °C. Yield: 0.225 g (>95%).
Mn = 10 400, Mw = 26 500 (polystyrene standards).
Preparation of Spin-Coated Organosilicate Films on ITO.
TEVS-TPD polymer films were obtained by spin-coating the initial
polymer solution in chlorobenzene (5 mg/mL, 3500 rpm for the 10 nm
thick film; 20 mg/mL, 4000 rpm for the 40 nm thick film) onto clean and
UV-ozone treated ITO substrates and subsequent thermal curing at 180
°C for 1 h under argon atmosphere.
For comparison, TEVS-TPD monomer films were prepared from
aqueous alcohol solutions adjusted to pH of 4.5 with acetic acid to
facilitate cross-linking, as described previously.22 A solvent for the mono-
mer solutions was comprised of 80% ethanol, 10% H2O, and 10%
chlorobenzene. The monomer solutions of 10 mg/mL (30 mg/mL)
were spin-coated at 3000 rpm (2500 rpm) for the films of 10 nm (40
nm) thickness in ambient air followed by in situ thermal curing at 180 °C
for 1 h under argon atmosphere. Cross-linked TEPCS-TPD polymer
films (see Scheme 2) were prepared from the polymer solutions in the
same way (5 mg/mL, 3000 rpm for the 10 nm thick film; 15 mg/mL,
2500 rpm for the 40 nm thick film).
III. RESULTS AND DISCUSSION
1. Synthesis of TEVS-TPD Monomer and Polymer. N,N0-
Diphenyl-N,N0-bis(4-bromophenyl)biphenyl-4,40-diamine (TPD-
Br2, 1) was prepared in 68% yield by the Buchwald-Hartwig
reaction of 1,4-dibromobenzene and N,N0-diphenylbenzidine.
The aryl-Br moiety was then directly substituted with vinyl-
triethoxysilane using mild Heck chemistry to obtain the desired
monomer, N,N0-diphenyl-N,N0-bis(4-((E)-2-(triethoxysilyl)vinyl)-
phenyl) biphenyl-4,40-diamine (TEVS-TPD, 2)28 (Scheme 1).
This procedure to attach triethoxysilane groups is very straight-
forward, introduces extended conjugation between Si and the
aromatic portion, and tolerates many functional groups unlike
traditional approaches that use organomagnesium or butyl-
lithium chemistry.30 Furthermore, the reaction can be performed
at room temperature by adjusting the catalyst Pd:P ratio to be
1.0:1.1.29 The TEVS-TPD polymer was prepared by an acid-
catalyzed sol-gel reaction as described previously in >95%
yield.31,32 The molar mass of the synthesized polymer could be
carefully adjusted by varying the acid and water content. The
number-average molar mass (Mn) and weight-average molar
mass (Mw) of TEVS-TPD polymer were 10 400 and 26 500,
respectively, as determined by gel permeation chromatography
(GPC) calibrated with polystyrene standards. The molar masses
of the polymer in chlorobenzene (20 mg/mL) stored for one
month in ambient condition were measured to check the stability
of the polymer. The values of Mn and Mw of 10 900 and 31 500,
respectively, and the GPC curves shown in the Supporting
Information demonstrate a good stability of TEVS-TPD polymer
in ambient condition.
Organic Thin Film Transistor (OTFT) Preparation. The
OTFT devices were prepared with the HITL film as the active layer
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Figure 2. FT-IR spectra of the initial TEVS-TPD polymer film and the
cross-linked film prepared by thermal curing at 180 °C for 1 h.
Figure 1. (a) DSC and (b) TGA results of TEVS-TPD polymer
obtained with 5 °C/min heating rate.
The synthetic protocol is simplified to allow for scaling the
material synthesis to high volumes as shown in Scheme 1. The
key to our process is avoiding pyrophoric chemistry such as
n-butyl lithium, which is known to be very dangerous for bench
chemistry as well as scale-up processes. That is, the final material
was prepared in three simple steps from commercially available
materials to form a stable polymer powder that can be exposed to
ambient conditions without degradation. For example, aliquots
can be taken of the solid polymer to prepare solutions as needed,
or stock solutions can be prepared and used over extended time
periods without detrimental changes. Thus, we anticipate our
HITLs to have a much longer shelf life than PEDOT:PSS
dispersions that are known to aggregate over time.
2. Characterization of TEVS-TPD Polymer. DSC results of
spin-coated and dried TEVS-TPD polymer exhibit a broad
exothermic peak between 125 and 175 °C in the first heating,
as shown in Figure 1a, which corresponds to the secondary con-
densation (cross-linking) involving the silanol moieties, and
the TGA data show a corresponding weight loss characteristic
(Figure 1b). The DSC and TGA results indicate that solvent-
resistant cross-linked films can be obtained by carrying out the
curing reaction above ca. 175 °C. Therefore, we prepared highly
cross-linked TEVS-TPD films by spin-coating the polymer solu-
tion in chlorobenzene, followed by in situ thermal cross-linking
(curing) at 180 °C for 1 h under nitrogen atmosphere. The
secondary condensation reaction of TEVS-TPD polymer films
was monitored by FT-IR measurements before and after thermal
curing (Figure 2). The absorption signals of the asymmetric
Si-O-Si stretch near 1100 cm-1 increased after thermal curing
due to the secondary condensation reaction involving silanol
Figure 3. UV-vis absorption spectra of TEVS-TPD polymer films
thermally cured at 180 °C for 1 h before (solid) and after rinsing (dash)
with chlorobenzene.
groups. The thermally cured films showed no visible damage nor
any discernible change in the UV-vis spectrum upon exhaustive
rinsing with chlorobenzene, a common spin-coating solvent for
the emissive layers (Figure 3).
UV-vis absorption and photoluminescence (PL) spectra
showed a red-shift after thermal curing, most likely due to the
increased film density and closer packing of the chromophores as
shown in Figure 4a. The optical band gap of cross-linked TEVS-
TPD thin films is 3.0 eV, as determined from the absorption edge
of the UV-vis spectrum. Electrochemical characteristics of
cross-linked TEVS-TPD on an ITO electrode, investigated by
cyclic voltammetry, showed fully reversible electrochemical
behavior with no change in the cathodic and anodic peak current
upon repeated scans (Figure 4b). This shows that cross-linked TEVS-
TPD films have excellent electrochemical stability. The highest
occupied molecular orbital (HOMO) energy level is -5.4 eV,
and the lowest unoccupied molecular orbital (LUMO) energy
level is -2.4 eV, as determined from the cyclic voltammetry and
optical band gap. Such a high-lying LUMO energy level is a
desired attribute, indicating the capability of TEVS-TPD film to
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function also as an electron blocking layer, which helps to
enhance the efficiency of OLEDs and OPVs.
The surface morphology study of cross-linked TEVS-TPD
films of 10 and 40 nm thickness, respectively, on ITO substrate
was carried out using tapping mode AFM. In addition, a dual
layer film of 10 nm thick cross-linked TEVS-TPD layer on top
of 40 nm thick PEDOT:PSS film was investigated. The rms
roughnesses of the 10 nm and the 40 nm thick cross-linked TEVS-
TPD, the dual layer, and PEDOT:PSS films were 2.9, 1.1, 1.8, and
1.5 nm, respectively. After being immersed in chlorobenzene and
subsequent drying, the 10 nm thick and the 40 nm thick cross-
linked TEVS-TPD and the dual layer films showed decreased rms
roughnesses of 1.6, 0.7, and 1.1 nm, respectively (see the Support-
ing Information). Therefore, the films with cross-linked TEVS-
TPD are very smooth as compared to the ITO substrate, whose
rms roughness was determined to be 3.0-3.7 nm by AFM,
indicating the effective planarization of the ITO surface.
Field-effect charge mobility of cross-linked TEVS-TPD was
investigated by OTFT measurements. The OTFTs were fabri-
cated with gold bottom contacts, n-doped Si gate, and thermally
grown SiO2 dielectric. Figure 5 shows the transfer and output
characteristics of the OTFT device. Cross-linked TEVS-TPD
devices exhibited p-channel transistor characteristics with an
average field-effect hole mobility of 1.2 ꢀ 10-6 cm2/(V s). This
3
value is substantially lower than the reported mobility of PEDOT:
PSS (10-2-10-3 cm2/(V s)),33,34 but it is somewhat higher
3
than the reported hole mobility of other triphenylamine (TPA)
based, similarly conjugated, vinyl cross-linked polymers (∼10-7
cm2/(V s)).17,34 The mobility is even comparable to TPA-based,
3
fluorene-containing, fully conjugated polymers (∼10-6 cm2/
(V s)).34,35 In comparison, inserting nonconjugated moiety
3
between the silicon and the aromatic amine resulted in a signif-
icantly decreased mobility too small to be measured by the
OTFT characteristics (see below). Therefore, the measured hole
mobility of cross-linked TEVS-TPD clearly indicates the impor-
tance of having extended conjugation between the silicon atom
and the aromatic amine portion in TEVS-TPD.
3. OLED Device Characteristics. In this section, perfor-
mance characteristics of OLEDs with TEVS-TPD polymer layers
of different thicknesses as HITLs are presented. In addition,
devices based on the TEVS-TPD monomer and a different
organosilicate polymer containing insulating moieties between
the Si atom and the aromatic amine moiety are also presented for
comparison.
3.1. Characteristics of Devices Based on Cross-linked TEVS-
TPD Polymer HITL Layers. To test the performance of the organo-
silicate polymer as HITLs, solution-processed phosphorescent
OLED devices were fabricated using the following device structure:
ITO/HITL/6 wt % fac-tris(2-phenylpyridine) iridium(III) (Ir-
(ppy)3):poly(vinyl carbazole) (PVK)/2,9-dimethyl-4,7-diphenyl-
1,10-phenanthroline (BCP)/tris(8-hydroxyquinolinato)aluminum
Figure 4. (a) UV-vis absorption (thick) and PL (thin) spectra of the
initial TEVS-TPD polymer film (dash) and the cross-linked film (solid)
by thermal curing at 180 °C for 1 h. The cured film shows a red-shift in
both absorption and emission spectra. (b) Cyclic voltammogram of
cross-linked TEVS-TPD on ITO scanned three times to show enhanced
electrochemical stability.
Figure 5. (a) Transfer and (b) output characteristics of OTFT with cross-linked TEVS-TPD polymer film as the active layer. The field-effect mobility
calculated in the saturation regime is 1.2 ꢀ 10-6 cm2/V s.
3
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(Alq3)/LiF/Al. Figure 6 shows the device structures and the
corresponding energy diagram schemes.36-38 The cross-linked
TEVS-TPD layers of two different thicknesses, 10 and 40 nm,
were employed as HITL layers in preparing the phosphorescent
OLED devices, denoted as OS10 and OS40, respectively. For
comparison, the control devices were prepared using 40 nm thick
PEDOT:PSS as the HITL layer. In addition, devices using a dual
HITL layer of 50 nm thickness, comprised of cross-linked TEVS-
TPD (10 nm) on top of PEDOT:PSS (40 nm) layer in contact
with ITO, were also tested and denoted OSPP50.
The electroluminescence (EL) spectra of the OLED devices
exhibit green emission without any emission from the HITLs
(Figure 7a, inset), showing that hole/electron recombination
occurs in the emissive layer (EML). The important device
characteristics are summarized in Table 1. Figure 7 shows the
current density-voltage-luminance characteristics (Figure 7a)
and the external quantum efficiency (EQE) versus current
density (Figure 7b), respectively. As shown in Figure 7a, the
devices based on cross-linked TEVS-TPD exhibit a lower leakage
current. The turn-on voltages (voltage at 1.0 cd/m2) of OS10
and OS40 are 5.0 and 5.5 V, respectively, which are lower than
the corresponding value, 6.0 V, for the control device with
PEDOT:PSS. This is the first time that a single cross-linked
polymer HITL layer is shown to successfully exhibit a lower turn-
on voltage with lower leakage current than that obtained with
PEDOT:PSS. Moreover, the maximum luminance of 12 100
cd/m2 for OS10 device is greater than 8740 cd/m2 obtained
for the control PEDOT:PSS device. As shown in Table 1, the
maximum luminance efficiency (LE), maximum power efficiency
(PE), and maximum external quantum efficiency (EQE) of
OS10, as compared to the corresponding values for PEDOT:
PSS in parentheses, are 22.3 cd/A (22.4 cd/V), 13.5 lm/W (10.1
lm/W), and 6.4% (6.4%) at the current density of 0.10 mA/cm2
(0.43 mA/cm2), respectively. The maximum efficiencies of
OS10-based devices are comparable to the PEDOT:PSS device.
At 1000 cd/m2, the values of LE and EQE are 16.3 cd/A and 4.7%
for OS10, as compared to 14.9 cd/A and 4.2% for the PEDOT:
PSS device. Experimental results in Figure 7 and Table 1 show
that OS40 devices with thicker (40 nm) TEVS-TPD layer exhibit
slightly lower performance characteristics than OS10 (10 nm
thickness). It is possibly due to relatively low hole transport in
thicker cross-linked TEVS-TPD layer. In comparison, the results
for dual HITL layer show results of slightly lower performance
than PEDOT:PSS at low current density. Overall, a single thin
layer of TEVS-TPD of 10-40 nm thickness seems to provide
device performance characteristics superior to those of the dual
layer (OSPP50).
Figure 6. Device structures [ITO/HITL (OS10, OS40, OSPP50, PEDOT:
PSS)/Ir(ppy)3:PVK emissive layer (40 nm)/BCP (10 nm)/Alq3 (40 nm)/
LiF (1 nm)/Al (100 nm)] and the corresponding energy level diagram.
The energy levels of the materials except cross-linked TEVS-TPD are
literature values.36-38
3.2. Characteristics of Devices Based on TEVS-TPD Monomer.
To investigate the benefits of employing TEVS-TPD polymer, as
compared to the corresponding monomer, for use as HITL, we
fabricated devices employing TEVS-TPD monomer with the
Figure 7. (a) Current density-voltage-luminance characteristics of
the OLED devices prepared with cross-linked TEVS-TPD polymer
HITLs. The inset is the normalized EL spectra. (b) External quantum
efficiency (EQE) versus the current density of the devices.
Table 1. Comparison of Device Characteristics of OLEDs Prepared with Various Cross-linked TEVS-TPD Polymer HITLs
Von (V)a
max L (cd/m2)b
VB (V)c
LEmax (cd/A)d
PEmax (lm/W)e
EQEmax (%)f
EQE1000 (%)g
OS10
5.0
5.5
5.0
6.0
12 090
8350
5500
8740
15.0
16.5
14.0
13.5
22.3
20.0
18.8
22.4
13.5
11.4
10.3
10.1
6.4
6.2
5.5
6.4
4.7
4.3
4.5
4.3
OS40
OSPP50
PEDOT:PSS
a Turn-on voltage. b Maximum luminance. c Voltage at maximum luminance. d Maximum luminance efficiency. e Maximum power efficiency. f Maximum
external quantum efficiency (EQE). g EQE at 1000 cd/m2.
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Table 2. Device Characteristics of OLEDs Prepared with Various Cross-linked TEVS-TPD Monomer HITLs
Von (V)a
max L (cd/m2)b
VB (V)c
LEmax (cd/A)d
PEmax (lm/W)e
EQEmax (%)f
EQE1000 (%)g
(OS10)
(5.0)
6.0
(12 090)
11 580
10 970
7890
(15.0)
14.0
(22.3)
15.1
(13.5)
6.6
(6.4)
4.3
(4.7)
3.8
OSM10
OSM40
OSMPP50
6.0
14.0
13.8
5.9
4.0
3.6
5.0
12.0
11.4
5.1
3.4
2.2
a Turn-on voltage. b Maximum luminance. c Voltage at maximum luminance. d Maximum luminance efficiency. e Maximum power efficiency. f Maximum
external quantum efficiency (EQE). g EQE at 1000 cd/m2.
Table 3. Device Characteristics of OLEDs Prepared with Various Cross-linked TEPCS-TPD Polymer HITLs
Von (V)a
max L (cd/m2)b
VB (V)c
LEmax (cd/A)d
PEmax (lm/W)e
EQEmax (%)f
EQE1000 (%)g
(OS10)
OSI10
(5.0)
7.5
(12 090)
6200
(15.0)
15.0
(22.3)
14.1
(13.5)
4.6
(6.4)
4.1
(4.7)
3.4
OSI40
8.5
2070
17.0
20.1
7.1
5.8
3.0
OSPPI50
6.5
7440
16.5
15.3
6.9
4.5
2.9
a Turn-on voltage. b Maximum luminance. c Voltage at maximum luminance. d Maximum luminance efficiency. e Maximum power efficiency. f Maximum
external quantum efficiency (EQE). g EQE at 1000 cd/m2.
same device structure. TEVS-TPD monomer layers were pre-
pared from aqueous alcohol solutions with pH = 4.5 in a solvent
of 80% ethanol, 10% H2O, and 10% chlorobenzene to facilitate
cross-linking.22 The characteristics of solvent resistance and film
morphology of thermally cured films (180 °C for 1 h) are inferior
to those of the corresponding films prepared from the TEVS-
TPD polymer (Supporting Information). HITLs of TEVS-TPD
monomer of 10 and 40 nm thickness were deposited by spin-
coating followed by thermal curing at 180 °C for 1 h, denoted as
OSM10 and OSM40. Devices employing a dual layer of 10 nm
thick TEVS-TPD monomer layer on PEDOT:PSS as HITL were
also prepared and denoted as OSMPP50. Table 2 summarizes
their device characteristics as compared to those of the device
based on TEVS-TPD polymer layer of 10 nm thickness (OS10);
the full details are shown in the Supporting Information. The
devices employing TEVS-TPD monomer HITLs (OSM) show
significantly deteriorated device performances and higher leak-
age current as compared to the devices using cross-linked TEVS-
TPD polymer HITLs (OS10), most likely due to the inferior
solvent resistance and film morphology.
3.3. Characteristics of Devices Based on an Organosilicate
Polymer Containing Insulating Moiety between Si and Aro-
matic Amine. To investigate the effects of having extended
conjugation between the Si atom and the aromatic amine moiety
in TEVS-TPD polymer due to the vinyl link, a different organo-
silicate containing the aromatic amine TPD and insulating moiety,
N,N0-bis(4-n-butylphenyl)-N,N0-bis[4-{3-(triethoxysilyl)propyl}-
carbamoyloxymethylphenyl]-1,10-biphenyl-4,40-diamine(TEPCS-
TPD), was prepared. Scheme 2 shows the structure of the TEPCS-
TPD monomer, and its synthesis and polymerization is described
in the Supporting Information. The thermally cured (at 180 °C
for 1 h) TEPCS-TPD polymer samples showed excellent solvent
resistance (i.e., no change in UV-vis spectrum after rinsing in
various solvents) and good film morphology as shown in the Sup-
porting Information. OLED devices were prepared with cross-
linked TEPCS-TPD polymer layers of 10 and 40 nm thickness,
and a dual layer of 10 nm thick cross-linked TEPCS-TPD poly-
mer layer on PEDOT:PSS as HITLs, denoted as OSI10, OSI40,
and OSIPP50, respectively. The device performance character-
istics are summarized in Table 3 as compared to those of 10 nm
thick TEVS-TPD polymer (OS10). The turn-on voltages are
higher, and the efficiencies of the devices are significantly lower
than those of the TEVS-TPD polymer HITLs. This result is most
likely due to a very low hole mobility of TEPCS-TPD polymer
films, because the OTFT devices with cross-linked TEPCS-TPD as
the active layer did not exhibit any measurable field-effect mobility.
IV. CONCLUSIONS
A thermally cross-linkable, TEVS-TPD organosilicate poly-
mer with aromatic amine functionality was prepared under mild
conditions via sequential Heck and sol-gel chemistry to obtain a
highly cross-linked HITL thin film by spin-coating, followed by
in situ thermal curing. In addition to the mild synthetic process,
important to our process is the extended conjugation introduced
between the silicon atom and the aromatic amine portion of the
material by employing a vinylsiloxane linkage, which leads to an
enhanced hole mobility as compared to the traditional alkylsil-
oxane linkage. In initial tests with solution-processed phospho-
rescent OLEDs, the devices with cross-linked TEVS-TPD HITL
(10 nm thickness) exhibited lower turn-on voltage with lower
leakage current, higher luminance and stability at high current
density, and similar maximum efficiencies at low voltages as com-
pared to those based on PEDOT:PSS HITL. In addition, the devices
based on cross-linked TEVS-TPD exhibited device performances
superior to those of the devices prepared with TEVS-TPD mono-
mer or TEPCS-TPD polymer that contains insulating moieties
between the silicon atom and the aromatic amine. To test the general
applicability of the highly conjugated organosilicate polymers in
organic electronic devices, further tests with other OLED
structures and OPVs employing cross-linked TEVS-TPD type
HITLs are currently under investigation. It is anticipated that the
interface between the active organic layer and our new HITLs (in
OLEDs and OPVs) will be much more mild than with the
corresponding corrosive PEDOT:PSS, leading to devices with
improved device performance and lifetime.6,39 Last, we anticipate
our HITLs to have a much longer shelf life than PEDOT:PSS
dispersions that are known to aggregate over time.
’ ASSOCIATED CONTENT
S
Supporting Information. Description of synthesis and
b
characterization of the materials, AFM images of HITL films, and
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Journal of the American Chemical Society
ARTICLE
additional device results. This material is available free of charge
via the Internet at http://pubs.acs.org.
(25) Huang, Q.; Evmenenko, G. A.; Dutta, P.; Lee, P.; Armstrong,
N. R.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 10227.
(26) Huang, Q.; Evmenenko, G.; Dutta, P.; Marks, T. J. J. Am. Chem.
Soc. 2003, 125, 14704.
(27) Cui, J.; Huang, Q.; Veinot, J. C. G.; Yan, H.; Wang, Q.;
Hutchison, G. R.; Richter, A. G.; Evmenenko, G.; Dutta, P.; Marks,
T. J. Langmuir 2002, 18, 9958.
’ AUTHOR INFORMATION
Corresponding Author
dyyoon@snu.ac.kr; jjkim@snu.ac.kr; aselli@stanford.edu
(28) Wahab, M. A.; Sudhakar, S.; Yeo, E.; Sellinger, A. Chem. Mater.
2008, 20, 1855.
(29) Littke, A. F.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 6989.
(30) Shea, K. J.; Loy, D. A. Chem. Mater. 2001, 13, 3306.
(31) Lee, J,-K.; Char, K.; Rhee, H.-W.; Ro, H. W.; Yoo, D. Y.; Yoon,
D. Y. Polymer 2001, 42, 9085.
(32) Dong, H.; Brook, M. A.; Brennan, J. D. Chem. Mater. 2005, 17,
2807.
’ ACKNOWLEDGMENT
This research was supported by the Division of Chemistry and
Molecular Engineering of Brain Korea 21 Program and the
OLED Center at Seoul National University. It was in part sup-
ported by the Korea-Germany International Research Training
(IRTG) grant from the National Research Foundation of Korea.
(33) Bernards, D. A.; Malliaras, G. G. Adv. Funct. Mater. 2007, 17,
3538.
(34) The mobilities in refs 17, 33, and 35 were obtained from space
charge limited current (SCLC) hole-only devices, which yield mobility
values somewhat different from the values obtained from thin film
transistor (TFT) characteristics as employed in this study. The relation-
ship between the SCLC mobility and the TFT mobility can be found
from a reference: Pasveer, W. F.; Cottaar, J.; Tanase, C.; Coehoorn, R.;
Bobbert, P. A.; Blom, P. W. M.; de Leeuw, D. M.; Michels, M. A. J. Phys.
Rev. Lett. 2005, 94, 206601.
(35) Craciun, N. I.; Wildeman, J.; Blom, P. W. M. J. Phys. Chem. C
2010, 114, 10559.
(36) Xia, H.; Li, M.; Lu, D.; Zhang, C.; Xie, W.; Liu, W.; Yang, B.;
Ma, Y. Adv. Funct. Mater. 2007, 17, 1757.
(37) Shi, M.-M.; Lin, J.-J.; Shi, Y.-W.; Ouyang, M.; Wang, M.; Chen,
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