2,7-carbazole-1,4-phenylene copolymers with polar side chains for cathode modifications in polymer light-emitting diodes

Article abstract of DOI:10.1021/ma200191p

Alcohol-soluble 2,7-carbazole-1,4-phenylene copolymers PCP-NOH and PCP-EP, comprising surfactant-like diethanolamino and phosphonate end groups on the side chains, respectively, were synthesized and utilized as electron injection layer (EIL) in combinatio

Full text of DOI:10.1021/ma200191p

ARTICLE
pubs.acs.org/Macromolecules
2,7-Carbazole-1,4-phenylene Copolymers with Polar Side Chains
for Cathode Modifications in Polymer Light-Emitting Diodes
Xiaofeng Xu, Bing Han, Junwu Chen,* Junbiao Peng, Hongbin Wu, and Yong Cao
Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Physics and Chemistry of Luminescence,
South China University of Technology, Guangzhou 510640, China
ABSTRACT: Alcohol-soluble 2,7-carbazole-1,4-phenylene co-
polymers PCP-NOH and PCP-EP, comprising surfactant-like
diethanolamino and phosphonate end groups on the side
chains, respectively, were synthesized and utilized as electron
injection layer (EIL) in combination with high work function
Al electrode in polymer light-emitting diodes (PLEDs). The
UVꢀvis absorption and photoluminescence properties of the
PCP-NOH and PCP-EP are mainly determined by the conju-
gated 2,7-carbazole-1,4-phenylene main chain. The PCP-NOH
and PCP-EP possess comparable HOMO levels of ꢀ5.20 eV and
LUMO levels arround ꢀ2.35 eV. Multilayer PLEDs with a device configuration of ITO/PEDOT:PSS (40 nm)/emissive layer
(70 nm)/EIL (15 nm)/Al (100 nm) were successfully fabricated. With fluorescent PFO-DBT15 as the emissive layer, the PLEDs
using the PCP-NOH and PCP-EP as the EIL displayed luminous efficiency (LE) of 1.01 and 0.88 cd/A, respectively, all obviously
higher than 0.015 cd/A for sole Al cathode and 0.58 cd/A for the well-known Ba/Al cathode. With a phosphorescent Ir(mppy)3-
doped polymer blend (PVK:PBD:Ir(mppy)3 = 100:30:1) as the emissive layer, the PLEDs with the EIL showed high maximum LE
of 39.3 cd/A for PCP-NOH and 42.5 cd/A for PCP-EP, in comparison to 0.35 cd/A for sole Al cathode and 32.2 cd/A for the Ba/Al
cathode. Particularly, the PLED with PCP-EP as EIL exhibited an excellent LE of 33.3 cd/A at a current density of 61.1 mA/cm2
(luminance = 20 300 cd/cm2), showing weak roll-off of efficiency at high current density. Inserting the polar alcohol-soluble
polymers as interlayer can significantly increase built-in potentials in the PLEDs, from which electron injection barrier from the Al
electrode is decreased. The results indicate that the PCP-NOH and PCP-EP are excellent electron injection polymers for high-
performance PLEDs with high work function Al electrode.
’ INTRODUCTION
makes sure the successful fabrications of PLEDs consisting multi-
layer polymers. Ammonium-functionalized cationic polyfluorenes
and their neutral precursors are the early examples.12ꢀ15 They
could be utilized in PLEDs with Al or high work-function Ag
or Au as the cathode.16 For example, with PFN as a cathode
interlayer, 80 and 200 times increases of luminous efficiency for
Al and Ag, respectively, were reported for PLEDs with P-PPV as
the emissive layer. A PLED with Au cathode almost did not emit
detectable light; however, inserting PFN interlayer between
P-PPV and Au could boost luminous efficiency to 11.6 cd/A.
The PFN interlayer also established the feasibility of all-solution
processed PLEDs with silver paste cathode to emit efficient red,
green, and blue lights.17 Using diethanolamino-functionalized
polyfluorenes as cathode interlayer also exhibited excellent
electron injection abilities, from which highly efficient PLEDs
During the past decade, polymer light-emitting diodes (PLEDs)
have undergone significant improvements in efficiency, bright-
ness, and drive voltage toward efficient full-color display panels
and white-color illuminations.1,2 PLEDs are carrier injection
devices, whose balanced hole injection from the anode and elec-
tron injection from the cathode and their fast transports and
recommbination in the emissive layer are the basic require-
ments.3,4 Low work-function metals such as Ba and Ca are the
widely utilized cathode materials to realize efficient electron
injection. However, the low work-function metals are environ-
mentally unstable, which can greatly affect the lifetime of PLEDs.
Thus, it is desirable to use high work-function metals (Al, Ag, Au,
etc.) as the cathode. Tremendous efforts have been paid to
improve electron injection from high-work-function metals into
the emissive layer. Inserting a thin layer of LiF, CsF, or some
surfactants, between Al and the emissive layer, could significantly
improve the electron injection.5ꢀ8 However, all these methods
showed cathode dependence, and they were not effective for
other high-work-function metals such as Ag or Au. Recently,
some alcohol/water-soluble polyfluorenes have been found
excellent electron injection abilities in PLEDs.9ꢀ11 It is the
special solubility of the polyfluorenes in alcohol or water that
for green, blue, and white lights were demonstrated.18ꢀ21
A
phosphonate-functionalized polyfluorene as cathode interlayer
for white PLEDs realized a forward viewing luminous efficiency
of 15.4 cd/A.22 A series of ammonium-functionalized cationic
Received: January 27, 2011
Revised:
April 28, 2011
Published: May 10, 2011
r
2011 American Chemical Society
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polyfluorenes with different counteranions were synthesized to
ellucidate their structure/function relationship.23,24 It was pro-
posed that dipole formation or ionic charge redistribution might
contribute to the distinguished electron injection properties of
the interlayer.13,23 It should be pointed that using the alcohol/
water-soluble polyfluorenes as interlayer in bulk-heterojunc-
tion photovoltaic cells25ꢀ27 and n-channel thin-film transistors
(TFTs)28,29 also improved thier performances, demonstrating
the important roles of the interlayer.
internal reference. Mass (MS) spectrometry was performed on an
Esquire HCT PLUS operating in an atmospheric-pressure chemical
ionization (APCI) mode. Melting points (mp) were obtained by a
Gallenkamp melting point apparatus. Molecular weights of the polymers
were obtained on a Waters GPC 2410 using a calibration curve of
polystyrene standards, with tetrahydrofuran as the eluent. Elemental
analyses were performed on a Vario EL elemental analysis instrument
(Elementar Co.). UVꢀvis absorption spectra were recorded on a HP
8453 spectrophotometer. The PL spectra of the copolymer were ob-
tained on a Jobin Yvon Fluorolog-3 spectrofluorometer. Cyclic voltam-
metry was carried out on a CHI660A electrochemical workstation with
platinum electrodes at a scan rate of 50 mV/s against an Ag/Agþ
reference electrode with a nitrogen-saturated solution of 0.1 M tetra-
butylammonium hexafluorophosphate (Bu4NPF6) in acetonitrile
(CH3CN). The deposition of a copolymer on the electrode was done
by the evaporation of a dilute THF solution.
2,7-Dibromo-9-(60-bromohexyl)carbazole (1). 2,7-Dibromo-
carbazole (1.32 g, 4.1 mmol) in 10 mL of anhydrous THF was mixed
with sodium hydride (0.20 g, 8.2 mmol) in 10 mL of anhydrous THF
under an argon atmosphere. The reaction mixture was stirred at 60 °C
for 1 h and then added to 1,6-dibromohexane (3.00 g, 12.3 mmol) in
50 mL of anhydrous THF. The reaction mixture was refluxed for 18 h.
After cooling to room temperature, the solvent was removed under
reduced pressure, and the oil phase was extracted with dichloromethane
(100 mL), washed successively with water, and then dried over
anhydrous MgSO4. The residue was purified by column chromatogra-
phy with 1:5 (v/v) dichloromethane/petroleum ether as the eluent to
afford the title compound (1.40 g, 72%). White solid, mp: 104ꢀ105 °C.
1H NMR (300 MHz, CDCl3), δ (ppm): 7.88 (d, J = 8.3 Hz, 2H), 7.52
(s, 2H), 7.34 (d, J = 9.9 Hz, 2H), 4.20 (t, J = 7.2 Hz, 2H), 3.51
(t, J = 6.5 Hz, 2H), 1.84 (m, 2H), 1.78 (m, 2H), 1.51 (m, 2H), 1.45
(m, 2H). 13C NMR (75 MHz, CDCl3), δ (ppm): 141.3, 122.6, 121.5,
121.3, 119.7, 112.0, 44.8, 43.2, 32.4, 28.7, 26.6, 26.5. Anal. Calcd for
C18H18Br3N: C, 44.30; H, 3.72; N, 2.87. Found: C, 44.41; H, 3.83; N,
2.89. MS (APCI): 489.2 [(M þ 1)þ].
It is of interest for polymer chemists to develop new alcohol/
water-soluble polymers. Polycarbazoles have an electron-rich main
chain and a large band gap, known as typical p-type semicon-
ductors.30ꢀ35 Compared with the large family of fluorene-based
alcohol/water-soluble conjugated polymers,12,14,15,18,20ꢀ24,27 intro-
ducing a carbazole as a main chain building block to construct
new interlayer polymers for optoelectronic devices receives
much less attention. In this work, two new 2,7-carbazole-1,
4-phenylene copolymers PCP-NOH and PCP-EP, soluble in
alcohol, were synthesized via transformation of a precursor
polymer and one-step polymerization, respectively (Scheme 1).
PCP-NOH and PCP-EP comprise surfactant-like diethanolami-
no and phosphonate end groups on the side chains of the both
main chain blocks, respectively, which impart the alcohol solu-
bilities of the two polymers. UVꢀvis absorption, photolumines-
cence (PL), and electrochemical property of the two polymers
were characterized. Moreover, the two polymers were utilized to
fabricate interlayer for electron injection from Al electrode in
PLEDs with emissive layers based on a fluorescent polymer and a
phosphorescent dye-doped polymer blend. With PFO-DBT15,36
a typical fluoresecent polymer with saturated red-light emis-
sion, as the emissive layer, the PLEDs with PCP-NOH/Al and
PCP-EP/Al bilayer cathodes displayed luminous efficiencies
(LE) of 1.01 and 0.88 cd/A, respectively, all obviously higher
than 0.015 cd/A for sole Al cathode and 0.58 cd/A for the
2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-N-
(6-bromohexyl)carbazole (2). 2,7-Dibromo-9-(60-bromohexyl)
carbazole, 1 (1.40 g, 2.87 mmol), bis(pinacolato)diboron (2.19 g, 8.61
mmol), potassium acetate (2.53 g, 25.83 mmol), and PdCl2(dppf)
(0.07 g, 0.09 mmol) were dissolved in 50 mL of anhydrous dioxane.
The reaction mixture was heated to 80 °C for 8 h under an argon
atmosphere. After cooling to room temperature, the organic layer was
extracted with dichloromethane (100 mL), washed successively with
water, and then dried over anhydrous MgSO4. The residue was
purified by column chromatography with 1:2 (v/v) dichloro-
methane/petroleum ether as the eluent and recrystallized from
THF/ethanol to give the final compound (1.39 g, 83%): white solid;
mp: 203ꢀ204 °C. 1H NMR (300 MHz, CDCl3), δ (ppm): 8.13
(d, J = 7.8 Hz, 2H), 7.88 (s, 2H), 7.69 (d, J = 7.8 Hz, 2H), 4.41
(t, J = 7.1 Hz, 2H), 3.51 (t, J = 6.7 Hz, 2H), 1.92 (m, 2H), 1.73
(m, 2H), 1.56ꢀ1.41 (br, m, 28H). 13C NMR (CDCl3, 75 MHz),
δ (ppm): 140.4, 125.1, 124.9, 120.0, 115.2, 83.8, 45.0, 42.6, 32.5, 29.1,
26.7, 26.4, 24.9. Anal. Calcd (%) for C30H42B2BrNO4: C, 61.89; H,
7.27; N, 2.41. Found: C, 62.19; H, 7.40; N, 2.43. MS (APCI): 583.2
[(M þ 1)þ].
well-known Ba/Al cathode. Iridium tris(2-(4-tolyl)pyridinato-
0
N,C2 ) (Ir(mppy)3),37 a green-emitting phosphorescent dye,
and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(PBD), an electron transport material, were doped in poly
(N-vinylcarbazole) (PVK) host with a blend ratio of PVK:
PBD:Ir(mppy)3 = 100:30:1. The phosphorescent PLEDs with
PCP-NOH/Al and PCP-EP/Al bilayer cathodes showed max-
imum LE values of 39.3 and 42.5 cd/A, respectively, in compar-
ison with 0.35 cd/A for sole Al cathode and 32.2 cd/A for the
Ba/Al cathode. The results clearly demonstrate that PCP-NON
and PCP-EP, synthesized in this work, can realize excellent
electron injection in combination with high work function Al
electrode for PLEDs with different emissive materials.
’ EXPERIMENTAL SECTION
Materials. All reagents and solvents, unless otherwise specified,
were obtained from Aldrich, Acros, and TCI Chemical Co. and were
used as received. Anhydrous tetrahydrofuran was distilled over sodium/
benzophenone under N2 prior to use. All manipulations involving air-
sensitive reagents were performed under an atmosphere of dry argon.
PVK and PBD were purchased from Aldrich, and Ir(mppy)3 was from
America Dye Source, Inc. All of them were used as received. Tetra-
butylammonium hexafluorophosphate (Bu4NPF6) was recrystallized
twice from a 50:50 mixture of methanol/water and dried at 60 °C under
vacuum.
Diethyl 6-(2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)carbazol-9-yl)hexylphosphonate (3). A solution of 2,7-bis-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-N-(6-bromohexyl)carba-
zole, 2 (1.46 g, 2.5 mmol), in triethyl phosphate was heated to 140 °C
under an argon atmosphere for 24 h. Then the excess triethyl phosphate
was distilled in a vacuum, and the crude product was purified by silica
chromatography (ethyl acetate/ether = 1:1) to give the title compound
(1.52 g, 95%): white solid; mp: 94ꢀ95 °C. 1H NMR (CDCl3, 300 MHz),
δ (ppm): 8.12 (d, J = 9.8 Hz, 2H), 7.86 (s, 2H), 7.68 (d, J = 9.7 Hz, 2H),
1
Instrumentations. H and 13C NMR spectra were recorded on
a Bruker AV 300 spectrometer with tetramethylsilane (TMS) as the
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Scheme 1
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4.38 (t, J = 6.3 Hz, 2H), 4.05 (m, 4H), 1.90 (m, 2H), 1.69 (m, 6H), 1.40
(br, s, 26H), 1.29 (t, J = 7.0 Hz, 6H). 13C NMR (CDCl3, 75 MHz),
δ (ppm): 140.4, 125.1, 124.9, 120.0, 115.2, 83.8, 61.4, 42.8, 30.6, 30.4,
29.1, 26.7, 24.9, 22.4, 16.5. Anal. Calcd (%) for C34H52B2NO7P: C,
63.87; H, 8.20; N, 2.19. Found: C, 62.72; H, 8.46; N, 2.16. MS (APCI):
640.5 [(M þ 1)þ].
Poly[(N-(60-bromohexyl)-2,7-carbazole)-alt-(2-hexyloxy-5-(60-bro-
mohexyloxy)-1,4-phenylene)] (PCP-Br). 2,7-Bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-N-(6-bromohexyl)carbazole (0.292 g, 0.5 mmol),
1,4-dibromo-2-(6-bromohexyloxy)-5-hexyloxybenzene (0.258 g, 0.5 mmol),
Pd(PPh3)4 (12 mg), and 3 drops of Aliquat336 were dissolved in a
mixture of 12 mL of toluene and 2 mL of 2 M K2CO3 aqueous solution.
The mixture was refluxed with vigorous stirring for 1 day under an argon
atmosphere. After the mixture was cooled to room temperature, it was
poured into 250 mL of methanol. The precipitated material was
redissolved and filtrated through a funnel and then precipitated again.
The resulting solid material was washed successively with acetone for
24 h and dried in vacuum to afford the title polymer as a white solid
(341 mg, 62%). GPC: Mn = 10.5 kDa; Mw/Mn = 2.3. 1H NMR (CDCl3,
300 MHz), δ (ppm): 8.19 (br, 2H), 7.75 (br, 2H), 7.53 (br, 2H), 7.24
(br, 2H), 4.41 (br, 2H), 4.03 (br, 4H), 3.39ꢀ3.29 (d, 4H), 2.00ꢀ1.28
(m, 24H), 0.85 (br, 3H). Anal. Calcd (%) for (C36H45Br2O2N)n: C,
63.25; 6.59, N, 2.05. Found: C, 64.61; H, 6.33; N, 1.95.
1-(6-Bromohexyloxy)-4-hexyloxybenzene (4). Potassium
hydroxide (0.67 g, 12.0 mmol) in 30 mL of methanol was mixed with
4-hexyloxyphenol (1.94 g, 10.0 mmol) in 30 mL of methanol under
an argon atmosphere. The reaction mixture was stirred for 1 h and
then added to 1,6-dibromohexane (7.32 g, 30.0 mmol) in 50 mL of
acetone. The reaction mixture was refluxed for 12 h. After cooling to
room temperature, the solvent was removed under reduced pres-
sure. The organic phase was extracted with dichloromethane
(100 mL), washed successively with water, and then dried over
anhydrous MgSO4. After removal of the solvent, the residue was
purified by column chromatography (dichloromethane/petroleum
ether = 1:4 as the eluent) to give the title compound (2.5 g, 70%):
white solid, mp: 39ꢀ40 °C. 1H NMR (CDCl3, 300 MHz), δ (ppm):
6.81 (s, 4H), 3.89 (m, 4H), 3.42 (m, 2H), 1.89 (m, 2H), 1.75
(m, 4H), 1.51 (m, 6H), 1.34 (m, 4H), 0.90 (m, 3H). 13C NMR
(CDCl3, 75 MHz), δ (ppm): 153.3, 153.1, 115.43, 115.41, 68.7, 68.4,
33.8, 32.7, 31.6, 29.4, 29.2, 28.0, 25.7, 25.3, 22.6, 14.0. Anal. Calcd
(%) for C18H29BrO2: C, 60.50; H, 8.18. Found: C, 60.62; H, 8.24.
MS (APCI): 358.3 [(M þ 1)þ].
Poly[(9-(N,N-di(200-hydroxyethyl)-60-aminohexyl)-2,7-carbazole)-
alt-(2-hexyloxy-5-(N,N-di(200-hydroxyethyl)-60-aminohexyloxy)-1,4-
phenylene)] (PCP-NOH). Diethanolamine (0.5 g) was added to a
solution of PCP-Br (100 mg) in a mixture of 20 mL of THF and 5 mL of
DMF. The mixture was stirred vigorously for 48 h at 60 °C under an
argon atmosphere. After cooling to room temperature, the mixture was
poured into 250 mL of H2O. The resulting polymer was then collected
and dried in vacuum to afford the final polymer as a white solid (89 mg,
82%). 1H NMR (CDCl3, 300 MHz), δ (ppm): 8.18 (br, 2H), 7.75 (br,
2H), 7.50 (br, 2H), 7.22 (br, 2H), 4.39 (br, 2H), 4.00 (br, 4H),
3.54ꢀ3.51 (br, 8H), 2.58ꢀ2.36 (m, 12H), 1.96ꢀ1.25 (m, 24H), 0.83
(br, 3H). Anal. Calcd (%) for (C44H65O6N3)n: C, 72.23, H, 8.89; N,
5.75. Found: C, 71.05; H, 8.71; N, 5.28.
1,4-Dibromo-2-(6-bromohexyloxy)-5-hexyloxybenzene
(5). 1-(6-Bromohexyloxy)-4-hexyloxybenzene, 4 (2.49 g, 7.0 mmol),
was dissolved in 100 mL of CCl4 and stirred below 5 °C. Bromine
(0.8 mL, 15.5 mmol) was added dropwise to the solution over 30 min.
The resulting solution was stirred overnight. The remaining bromine
was quenched by the addition of saturated aqueous Na2S2O3 (100 mL),
and then the organic layer was extracted with dichloromethane
(100 mL). The organic phase was washed successively with water and
then dried over anhydrous MgSO4. After removal of the solvent, the
residue was purified by column chromatography (dichloromethane/
petroleum ether = 1:3 as the eluent) and recrystallized from ethanol to
give the final compound (3.00 g, 83%): white solid; mp: 57ꢀ58 °C. 1H
NMR (CDCl3, 300 MHz), δ (ppm): 7.08 (s, 2H), 3.94 (m, 4H), 3.42
(m, 2H), 1.90 (m, 2H), 1.80 (m, 4H), 1.52 (m, 6H), 1.35 (m, 4H), 0.91
(m, 3H). 13C NMR (CDCl3, 75 MHz), δ (ppm): 150.2, 150.0, 118.50,
111.2, 70.3, 70.1, 33.7, 32.7, 31.5, 29.1, 28.9, 27.8, 25.6, 25.2, 22.6, 14.0.
Anal. Calcd (%) for C18H27Br3O2: C, 41.97; H, 5.28. Found: C, 42.09;
H, 5.43. MS (APCI): 516.1 [(M þ 1)þ].
Poly[(N-(60-diethylphosphoryl)hexyl-2,7-carbazole)-alt-(2-hexyl-
oxy-5-(60-diethylphosphoryl)hexyloxy-1,4-phenylene)] (PCP-EP). Di-
ethyl 6-(2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)carbazol-
9-yl)hexylphosphonate, 3 (0.320 g, 0.5 mmol), 1,4-dibromo-2-(6-
(diethylphosphoryl)hexyloxy)-5-hexyloxybenzene,6 (0.286g, 0.5 mmol),
Pd(PPh3)4 (12 mg), and several drops of Aliquat336 were dissolved in a
mixture of 12 mL of toluene and 2 mL of 2 M K2CO3 aqueous solution.
The mixture was refluxed with vigorous stirring for 3 days under an
argon atmosphere. After the mixture was cooled to room temperature, it
was poured into 250 mL of hexane. The precipitated material was
redissolved and filtrated through a funnel and then precipitated again.
The resulting solid material was washed successively with hexane for
24 h and dried in vacuum to afford the title polymer as a white solid
(351 mg, 58%). GPC: Mn = 9.3 kDa; Mw/Mn = 2.0. 1H NMR (CDCl3,
300 MHz), δ (ppm): 8.17 (br, 2H), 7.74 (br, 2H), 7.51 (br, 2H), 7.23
(m, 2H), 4.38 (br, 2H), 4.04(m, 12H), 1.96ꢀ1.26 (m, 40H), 0.83
(br, 3H). Anal. Calcd (%) for (C44H65O8P2N)n: 66.25; H, 8.16; N, 1.76.
Found: C, 64.96; H, 8.15; N, 1.51.
PLED Fabrication and Characterization. Patterned indium tin
oxide (ITO)-coated glass with a sheet resistance of 15ꢀ20 ohm/square
were cleaned by a surfactant scrub and then underwent a wet-cleaning
process inside an ultrasonic bath, beginning with deionized water,
followed by acetone and isopropanol. After oxygen plasma cleaning
for 5 min, a 40 nm thick poly(3,4-ethylenedioxythiophene):poly-
(styrenesulfonate) (PEDOT:PSS) (Bayer Baytron 4083) anode buffer
layer was spin-cast onto the ITO substrate and then dried by baking in a
vacuum oven at 80 °C for overnight. For PLEDs with fluorescent PFO-
DBT15 as the emissive layer, PFO-DBT15 film with a thickness of
70 nm was spin-casted from its chlorobenzene solution. For the
phosphorescence devices, 1 wt % of Ir(mppy)3 and 30 wt % of PBD
were doped into a PVK host in chlorobenzene solution, and 70 nm thick
films were formed via spin-coating. A solution of PCP-NOH or PCP-EP
in methanol or methanol/water mixture was spin-coated on the top of
the obtained emissive layer to form a thin interlayer of 15 nm. The
1,4-Dibromo-2-(6-(diethylphosphoryl)hexyloxy)-5-hex-
yloxybenzene (6). A solution of 1,4-dibromo-2-(6-bromohexyloxy)-
5-hexyloxybenzene, 5 (1.64 g, 3.0 mmol), in triethyl phosphate was
heated to 140 °C under an argon atmosphere for 24 h. Then the excess
triethyl phosphate was distilled in a vacuum, and the crude product was
purified by silica chromatography (ethyl acetate/diethyl ether = 1:1 as
the eluent) to give the title compound (1.60 g, 93%): white solid; mp:
48ꢀ49 °C. 1H NMR (CDCl3, 300 MHz), δ (ppm): 7.08 (s, 2H), 4.09
(m, 4H), 3.95 (m, 4H), 1.85ꢀ1.60 (m, 8H), 1.49 (m, 6H), 1.34
(m, 10H), 0.91 (m, 3H). 13C NMR (CDCl3, 75 MHz), δ (ppm):
150.2, 150.0, 118.5, 111.2, 70.3, 70.1, 61.4, 31.5, 30.3, 30.1, 29.0, 26.6,
25.5, 24.7, 22.6, 22.3, 16.5, 14.0. Anal. Calcd (%) for C22H37Br2O5P:
C, 46.17; H, 6.52. Found: C, 46.11; H, 6.69. MS (APCI): 573.3
[(M þ 1)þ].
Polymerization. Polymerizations of PCP-Br and PCP-EP were
carried out by palladium(0)-catalyzed Suzuki coupling reactions with
equivalently molar ratio of a diboronic ester monomer to a dibromo
monomer under argon protection. The purifications of the polymers
were conducted in air. The preparation of PCP-NOH was carried out by
a transformation reaction based on PCP-Br.
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compound 2 with a total yield of 83% after recrystallization in
THF/ethanol mixture was synthesized by Miyaura coupling of 1
and bis(pinacolato)diboron with PdCl2(dppf) as the catalyst.38
A
heating reaction of 1 in triethyl phosphate at 140 °C readily
afforded a carbazole derivative 3 with phosphonate end group in
a high yield of 89.1%. The synthesis of 4 was achieved in a good
yield of 70% by the condensation of 4-hexyloxyphenol with 1,
6-dibromohexane in a basic medium. The 1,4-dibromo com-
pound 5 was synthesized by the bromination of 4 with bromine
in CCl4, with a good yield of 83%. A similar phosphonate
transformation of 5 under heating afforded the target benzene
derivative 6 in a high yield of 93%. The chemical structures of
compounds 1ꢀ6 were confirmed by 1H NMR, 13C NMR, and
elemental analysis (see the Experimental Section for details).
Scheme 1 also shows the polymerization routes for the two
target polymers. The preparations of the PCP-EP and the
intermediate polymer PCP-Br were accomplished via Pd-
(PPh3)4-catalyzed Suzuki reaction of the corresponding mono-
mers. The transformation from PCP-Br to the target polymer
PCP-NOH was achieved by post-treatment of PCP-Br with
diethanolamine in a mixture DMF/THF.18 Some variations of
1
characteristic H NMR peaks could be found. The hydrogen
resonances at 3.39 and 3.29 ppm for the two end CH2-Br groups on
the carbazole and benzene units completely disappeared in the 1H
NMR of PCP-NOH. New resonance peaks at 3.54ꢀ3.51 ppm for
CH2-OH and 2.58ꢀ2.36 ppm for (CH2)3N appeared. Polymers
PCP-NOH and PCP-EP are easily soluble in organic solvents
such as tetrahydrofuran (THF), toluene, chloroform, and di-
methylformamide (DMF). Moreover, the polar diethanolamine
and phosphonate groups on the side chains supply the two
polymers with solubilities in methanol or ethanol to a level of
over 5 mg/mL. The alcohol solutions did not become turbid if a
proper proportion of water was added. Measured by GPC with
calibration of polystyrene standards and THF as the eluent, the
number-averaged molecular weights (Mn) of PCP-EP and the
intermediate polymer PCP-Br are comparable of 9300 and
10 500, respectively, with polydispersity index (Mw/Mn) of 2.0
and 2.3, respectively. The molecular weight of PCP-NOH, a
transformed product, should be comparable to that of its pre-
cursor PCP-Br, based on the high transformation yield of 82%.
The chemical structures of PCP-Br, PCP-NOH, and PCP-EP
Figure 1. Normalized UVꢀvis absorption spectra and PL spectra of
(a) PCP-NOH and (b) PCP-EP in methanol solutions and in their film
states.
thickness of the PEDOT:PSS and the emissive layers were verified by a
surface profilometer (Tencor, Alpha-500). Determination of the thick-
ness of the interlayer followed a previously published paper.13 Finally, a
100 nm aluminum layer was thermally evaporated with a shadow mask at
a base pressure of 3 ꢁ 10ꢀ4 Pa. The overlapping area between the
cathode and anode defined a pixel size of 0.15 cm2. The thickness of the
evaporated cathodes was monitored by a quartz crystal thickness/ratio
monitor (Model: STM-100/MF, Sycon). Except for the deposition of
the PEDOT:PSS layers, all the fabrication processes were carried out
inside a controlled atmosphere of nitrogen drybox (Vacuum Atmo-
sphere Co.) containing less than 10 ppm oxygen and moisture. The
currentꢀluminanceꢀvoltage (IꢀLꢀV) characteristics were recorded
with a Keithley 236 source meter and a calibrated silicon photodiode.
Luminance was calibrated by using a PR-705 SpectraScan spectro-
photometer, and the forward-viewing LE was calculated accordingly.
The external quantum efficiency was verified by measurement in the
integrating sphere (IS-080, Labsphere). The electroluminescent spectra
were collected by a PR-705 photometer. For photocurrent versus
voltage characteristics measurements, the IꢀV characteristics under
illumination were measured with a Keithley 236 source meter. The
photocurrent was measured under solar simulator with AM 1.5G
illumination (100 mW/cm2).
1
were confirmed by H NMR and elemental analysis (see the
Experimental Section for details).
UV Absorption and Photoluminescence. The UVꢀvis
absorption spectra of PCP-NOH and PCP-EP in methanol
solutions and in their film states are shown in Figure 1. The
PCP-NOH solution shows absorption maximum (λabs) at
369 nm (Table 1), pratically identical to λabs = 367 nm of a
copolymer PCMEHP derived from 2,7-carbazole and 2,5-dialk-
oxy-1,4-phenylene in solution.39 The result demonstrates that
the diethanolamino groups on the side chain of PCP-NOH has
very limited influence on the solution absorption of the polymer
main chain. The λabs for the thin solid film of PCP-NOH slightly
red-shifts to 378 nm. Compared with λabs = 368 nm for the film of
PCMEHP, the calculated 10 nm red-shift of PCP-NOH indicates
that the polar diethanolamino groups would have some effect on
the absorption spectrum in the aggregation state. The absorption
edges for the absorption spectra of the solution and the film of
PCP-NOH are at 420 and 426 nm, respectively. The λabs values
for the solution and the film of PCP-EP move to 385 and 389 nm,
respectively, but showing a smaller aggregation effect on the
absorption changing. The absorption edges for the absorption
’ RESULTS AND DISCUSSION
Synthesis of Monomers and Polymers. The synthetic routes
for monomers are shown in Scheme 1. In the presence of
excessive amount of 1,6-dibromohexane, the 2,7-dibromocarba-
zole could be transformed to a carbazole derivative 1 with a
N-(60-bromohexyl) group in a 72% yield. The diboronic ester
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Table 1. Optical Properties and Energy Levels of PCP-NOH and PCP-EP
λabsa (nm)
solution
λemb (nm)
solution
polymer
film
film
Egc (eV)
Eoxd (V)
HOMOe (eV)
LUMOf (eV)
PCP-NOH
PCP-EP
369
385
378
389
417
424
427
428
2.91
2.83
0.53
0.52
ꢀ5.21
ꢀ5.20
ꢀ2.30
ꢀ2.37
a Absorption peak. b Emission peak. c Optical band gap. d Onset voltage of oxidation process during cyclic voltammetry with Ag/Agþ reference electrode.
e Calculated according to HOMO = ꢀe(Eox þ 4.68). f Calculated from HOMO level and the optical band gap.
As Electron Injection Layer (EIL) in Polymer Light-Emit-
ting Diodes. The polar and surfactant-like end groups on the
side chains of PCP-NOH and PCP-EP would have strong
interactions with metals, from which good electron injections
from high work function metal cathodes could be expected. The
special soluble properties of PCP-NOH and PCP-EP in environ-
mental-friendly solvents such as alcohols or alcohol/water mix-
tures could prevent interfacial mixing between the emissive
polymer layer and the conjoint EIL, during multilayer fabrica-
tions by solution processing.11
Multilayer PLEDs with a configuration of ITO/PEDOT:PSS
(40 nm)/PFO-DBT15 (70 nm)/EIL (15 nm)/Al (100 nm)
were fabricated. PFO-DBT15, a typical fluorescent polymer with
saturated red-light emisson, was selected to construct the emis-
sive layer. Then PCP-NOH and PCP-EP solutions with a
concentration of 2 mg/mL in methanol were spin-casted on
top of the PFO-DBT15 layer as the thin EIL. Finally, a 100 nm
thickness of Al was evaporated to form the metal cathode. For a
Figure 2. EL spectra of PLEDs with fluorescent PFO-DBT15 as the
emissive layer by using different cathodes.
comparion study, PLEDs based on sole Al cathode without the
spectra of the solution and the film of PCP-EP are at 425 and
438 nm, respectively. Approximately, the phosphonate pendants
on PCP-EP exhibit some contribution to enhance the conjuga-
tion length of the 2,7-carbazole-1,4-phenylene backbone, possi-
bly due to the side-chain-induced conformational effect.
The PL spectra of PCP-NOH and PCP-EP in methanol
solutions and in their film states are also shown in Figure 1.
The solution and film of PCP-NOH show emission maximum
(λem) at 417 and 427 nm, respectively (Table 1), comparable to
the corresponding 414 and 425 nm for PCMEHP solution and
film.39 The λem values of the solution and film of PCP-EP are at
424 and 428 nm, respectively. The results illustrate that the polar
diethanolamino and phosphonate groups almost do not affect
the PL emissions from the polymer backbone.
Energy Level. The optical band gaps of the films of PCP-NOH
and PCP-EP estimated with the absorption edges of the thin solid
filmsare also listedinTable1. The optical bandgapsof PCP-NOH
and PCP-EP are 2.91 and 2.83 eV, respectively. The HOMO levels
of the polymers were obtained from the onsets of oxidation
potentials (Eox) during cyclic voltammetry (CV) measurements
with an Ag/Agþ electrode as the reference electrode and an energy
level of ferrocene of ꢀ4.80 eV as the internal standard. The
calibrated energy level of the Ag/Agþ electrode is ꢀ4.68 eV. The
HOMO levels of PCP-NOH and PCP-EP are arround ꢀ5.2 eV,
according to their practically identical Eox values of 0.52 V against
the Ag/Agþ reference electrode. The comparable oxidation
processes of PCP-NOH and PCP-EP demonstrate very limited
influences of their pendants on the oxidation of the polymer
backbone. The LUMO levels of the polymers werecalculated from
the HOMO levels and resulting optical band gaps. Thus, the
deduced LUMO levels of PCP-NOH and PCP-EP of ꢀ2.30
and ꢀ2.37 eV, respectively, were obtained.
EIL and a well-known Ba/Al cathode with a low work function
metal were also fabricated so as to reveal the EIL features of PCP-
NOH and PCP-EP.
The PFO-DBT15-based PLEDs with PCP-NOH and PCP-EP
as the EIL emitted saturated red light with practically identical
electroluminescence (EL) spectra (λem = 656 nm) to that of the
Ba/Al cathode (Figure 2), demonstrating that inserting the EIL
does not change the EL spectrum of the PFO-DBT15.36 The
PLED with Al cathode showed a relatively high turn-on voltage
(Von) of 5.9 V, and the Von values of other devices were below 4 V
(Table 2). Performances of the PLEDs at a comparable current
density of ∼39 mA/cm2 are also listed in Table 2. The PLED with
Al cathode only emitted a light with a luminance of 6 cd/cm2 at a
current density of 40.4 mA/cm2, corresponding to an external
quantum efficiency (EQE) of 0.03% and a LE of 0.015 cd/A. The
PLED with low work function barium significantly improved the
performance because of the enhancement of electron injection.
The device displayed a practical luminance of 202 cd/cm2, EQE
of 1.16%, and LE of 0.58 cd/A, at a current density of 37.5 mA/cm2.
The PCP-NOH/Al and PCP-EP/Al bilayer cathodes could
further increase the efficiency. The device with PCP-NOH/Al
bilayer cathode showed EQE of 2.02% and LE of 1.01 cd/A for a
luminance of 405 cd/cm2 at a current density of 40.1 mA/cm2,
while EQE of 1.76% and LE of 0.88 cd/A for a luminance of
307 cd/cm2 at a current density of 38.7 mA/cm2 were realized
with the PCP-EP bilayer cathode. The efficiency data realized by
PCP-NOH/Al and PCP-EP/Al are higher than that of PFO-
DBT15 in the literature.36 During the increasing of current
density up to 100 mA/cm2, the PLEDs with the PCP-NOH/Al
and PCP-EP/Al bilayer cathodes all showed higher efficiency in
comparison to the case of Ba/Al cathode (Figure 3). The results
demonstrate that both the PCP-NOH/Al and PCP-EP/Al
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Table 2. Device Performances of PLEDs with Fluorescent PFO-DBT15 as the Emissive Layer by Using Different Cathodes
ARTICLE
cathode
Vona (V)
voltageb (V)
current densityb (mA/cm2)
luminanceb (cd/m2)
EQEb (%)
LEb (cd/A)
Al
5.9
2.6
3.8
2.7
7.8
3.4
5.2
4.5
40.4
37.5
40.1
38.7
6
202
405
307
0.03
1.16
2.02
1.76
0.015
0.58
1.01
0.88
Ba/Al
PCP-NOH/Al
PCP-EP/Al
a Turn-on voltage defined for a brightness of 1 cd/m2. b Device performance at a current density of ∼39 mA/cm2.
Figure 5. EL spectra of PLEDs with a phosphorescent dye-doped
polymer blend (PVK:PBD:Ir(mppy)3 = 100:30:1) as the emissive layer
by using different cathodes.
Figure 3. Luminous efficiencyꢀcurrent density characteristics of
PLEDs with fluorescent PFO-DBT15 as the emissive layer by using
different cathodes.
device with Al cathode showed an open-circuit voltage (Voc) of
0.95 V. The Voc values increased to 1.5 V for PLEDs with PCP-
NOH/Al and PCP-EP/Al bilayer cathodes, which is even higher
than Voc = 1.2 V for Ba/Al device. The results indicate that
inserting polar interfacial layer between Al electrode and the
PFO-DBT15 emissive layer can decrease the barrier height for
electron injection. Thereby, the PLEDs with PCP-NOH/Al and
PCP-EP/Al bilayer cathodes could significantly improve the
device performances.
Electrophosphorescent light-emitting diodes have been attract-
ing much attention because of their high device efficiency.40,41
Electrophosphorescence can make full use of both singlet and
triplet excitons owing to strong spinꢀorbital coupling of heavy-
metal ions in phosphorescent dyes. As a result, electropho-
sphorescence can theoretically approach 100% internal quantum
efficiency.42 To examine the general applicability of PCP-NOH
and PCP-EP for electron injection, we fabricated electropho-
sphorescent PLEDs with green-emitting Ir(mppy)3 as the phos-
phorescent dye, doped in PVK host, whose device configuration
was ITO/PEDOT:PSS (40 nm)/emissive layer (70 nm)/EIL
(15 nm)/Al (100 nm). PBD was selected to enhance electron
transport in the emissive layer. A blend ratio of PVK:PBD:
Ir(mppy)3 = 100:30:1 was utilized to construct the emissive
layer. PCP-NOH and PCP-EP films were spin-casted from
water/methanol (1:3 by volume) mixtures onto the emissive
layer so as to suppress the erotion of PBD by pure methanol.21
For comparison, PLEDs with sole Al and Ba/Al cathodes were
also fabricated.
Figure 4. Photovoltaic characteristics of PFO-DBT15-based PLEDs
under white light illumination with different cathodes.
bilayer cathodes can establish effective electron injections to the
emissive layer.
It was reported before that the decreased electron injection
barrier from the bilayer cathode to the emissive layer could be
ascribed to the formation of positive interfacial dipole between
the EIL and the Al electrode.13 The polar diethanolamino and
phosphonate groups should generate strong interactions with the
high work function Al electrode. Open-circuit voltage, an im-
portant parameter relevant to the built-in potential across the
junction and the electrodes, would reflect the interfacial char-
acteristics of the different cathodes.13 Figure 4 shows the photo-
voltaic behaviors of the PLEDs with the different cathodes. The
The electrophosphorescent PLED with Al cathode showed
very poor device perforamance, whereas the other three PLEDs
could emit highly efficient green light with same λem at 510 nm
(Figure 5), ascribed to the green emission of Ir(mppy)3.37 Slight
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ARTICLE
Table 3. Device Performances of PLEDs with a Phosphorescent Dye-Doped Polymer Blend (PVK:PBD:Ir(mppy)3 = 100:30:1)
as the Emissive Layer by Using Different Cathodes
cathode
Vona (V) voltageb (V) current densityb (mA/cm2) luminanceb (cd/m2) EQEb (%) LEb (cd/A) LEmaxd (cd/A) Lmaxe (cd/m2)
Al
12.0
4.0
10.5
7.5
2.61
2.27
2.50
2.68
c
732
0.35
32.2
440
26 300
28 300
28 500
Ba/Al
9.6
11.7
12.0
32.2
39.3
40.1
PCP-NOH/Al
5.7
9.6
982
39.3
PCP-EP/Al
5.4
10.2
1078
42.5
a Turn-on voltage defined for a brightness of 1 cd/m2. b Device performance at a current density of ∼2.5 mA/cm2. c No light. d Maximum luminous
efficiency. e Maximum luminance.
displayed higher efficiency than the case of the Ba/Al cathode
again. The device with PCP-NOH/Al bilayer cathode showed
EQE of 11.7% and LE of 39.3 cd/A for a luminance of 982 cd/cm2
at a current density of 2.5 mA/cm2 while EQE of 12.0% and LE of
40.1 cd/A for a luminance of 1078 cd/cm2 at a current density of
2.68 mA/cm2 were found for the PCP-EP bilayer cathode. The
PLED with the PCP-EP bilayer cathode exhibited a maximum LE
(LEmax) of 42.5 cd/A at a higher luminance of 1450 cd/cm2. The
LEmax values of the two PLEDs with the PCP-NOH/Al and
PCP-EP/Al bilayer cathodes are over 100 times better than that
with the sole Al cathode. The maximum luminances (Lmax) of the
two PLEDs with the PCP-NOH/Al and PCP-EP/Al bilayer
cathodes are all over 28 000 cd/m2, which are much better than
the Lmax = 440 cd/m2 for the Al cathode and also better than the
Lmax = 26 300 cd/m2 for the Ba/Al cathode. Except the PLED
with Al cathode, the other three high-performance PLEDs
showed similarly weak roll-off of efficiency at higher current
Figure 6. Luminous efficiencyꢀcurrent density characteristics of
density (Figure 6). Particularly, the PLED with the PCP-EP
bilayer cathode still possessed an excellent LE of 33.3 cd/A at a
current density of 61.1 mA/cm2, corresponding to a much high
luminance of 20 300 cd/cm2. The results clearly demonstrate
that PCP-NOH and PCP-EP are excellent electron injection
polymers for high-performance PLEDs with high work function
Al electrode. It should be noted that the efficiency realized with
the PCP-NOH/Al and PCP-EP/Al bilayer cathodes are also
significantly higher than those of Ir(mppy)3-based electropho-
sphorescent devices (with a similar composition in the emissive
layer) with Ca/Al and CsF/Al cathodes in the literature.37
Photovoltaic characteristics of the electrophosphorescent
PLEDs under white light illumination with different cathodes
are shown in Figure 7. The Voc values for PLEDs with PCP-
NOH/Al and PCP-EP/Al bilayer cathodes are 1.55 and 1.50 V,
respectively, all higher than Voc = 1.25 V for the Ba/Al cathode as
well as Voc = 0.9 V for the Al cathode. The Voc results support the
notable electron injection abilities of PCP-NOH and PCP-EP in
combination with the high work function Al electrode.
PLEDs with a phosphorescent dye-doped polymer blend (PVK:PBD:
Ir(mppy)3 = 100:30:1) as the emissive layer by using different cathodes.
It should be pointed out that the fluorescent PFO-DBT15-
based PLED with PCP-NOH as the EIL showed higher efficiency
than that of PCP-EP as EIL, whereas PCP-EP displayed better
device performance than PCP-NOH in the electrophosphores-
cent PLED. Therefore, the selection of an EIL from PCP-NOH
and PCP-EP needs a consideration of the chemical structure of
an emitting material.
Figure 7. Photovoltaic characteristics of the electrophosphorescent
PLEDs under white light illumination with different cathodes.
deviations of the EL spectra, at wavelength range between 530
and 680 nm, might be ascribed to microcavity effect when
inserting the EIL. The device performances of the electropho-
sphorescent PLEDs at a current density of ∼2.5 mA/cm2 are
summarized in Table 3. No light could be detected for the PLED
with Al cathode even at a current density of 2.61 mA/cm2,
indicating a hole-only current. Using Ba/Al cathode could boost
the device luminance to 732 cd/cm2 at a current density of 2.27
mA/cm2, corresponding to EQE of 9.6% and LE of 32.2 cd/A.
The PLEDs with PCP-NOH/Al and PCP-EP/Al bilayer cathodes
’ CONCLUSIONS
In summary, alcohol-soluble 2,7-carbazole-1,4-phenylene co-
polymers PCP-NOH and PCP-EP, comprising surfactant-like
diethanolamino and phosphonate end groups on the side chains,
respectively, were synthesized and utilized as interlayer for the
4211
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Macromolecules
ARTICLE
modification of electron injection from high work function Al
electrode in PLEDs. The UVꢀvis absorption, photolumines-
cence, and energy levels of the PCP-NOH and PCP-EP were
mainly determined by the conjugated 2,7-carbazole-1,4-phenyl-
ene main chain. With fluorescent PFO-DBT15 as the emissive
layer, the PLEDs using the PCP-NOH and PCP-EP as the EIL
displayed LE of 1.01 and 0.88 cd/A, respectively, all obviously
higher than 0.015 cd/A for sole Al cathode and 0.58 cd/A for the
well-known Ba/Al cathode. With a phosphorescent Ir(mppy)3-
doped polymer blend (PVK:PBD:Ir(mppy)3 = 100:30:1) as the
emissive layer, the PLEDs with the EIL showed outstanding
LEmax of 39.3 cd/A for PCP-NOH and 42.5 cd/A for PCP-EP, in
comparison to 0.35 cd/A for sole Al cathode and 32.2 cd/A for
the Ba/Al cathode. The PLEDs with the EIL also showed weak
roll-off of efficiency at high current density. Inserting the polar
alcohol-soluble polymers as interlayer could significantly increase
built-in potentials in the PLEDs, from which electron injection
barrier from the Al electrode was decreased. The results indicate
that the PCP-NOH and PCP-EP are excellent electron injection
polymers for high-performance PLEDs with high work function
Al electrode.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: psjwchen@scut.edu.cn; Fax: þ8620-8711-0606.
(28) Seo, J. W.; Gutacker, A.; Walker, B.; Cho, S.; Garcia, A.; Yang,
R.; Nguyen, T. Q.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2009,
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’ ACKNOWLEDGMENT
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Synthesis of Conjugated Polymer; Leclerc, M., Morin, J. F., Eds.; Wiley-VCH:
Weinheim, Germany, 2010; pp 205ꢀ226.
We gratefully acknowledge the financial support of National
Natural Science Foundation of China (Nos. 50773023, 50990065,
and 51010003), National Basic Research Program of China (973
program No. 2009CB623600), and SCUT Grant (No. 2009-
ZZ0003).
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