Novel 9,9′-(1,3-phenylene)bis-9H-carbazole-containing copolymers as hole-transporting and host materials for blue phosphorescent polymer light-emitting diodes

Article abstract of DOI:10.1002/pola.27054

Novel photo-crosslinkable hole-transport and host materials incorporated into multilayer blue phosphorescent polymer light-emitting diodes (Ph-PLEDs) were demonstrated in this study. The oxetane-containing copolymers, which function as hole-transport layers (HTL), could be cured by UV irradiation in the presence of a cationic photoinitiator. The composition of the two monomers was varied to yield three different hole-transporting copolymers, [Poly(9,9′-(5-(((4-(7-(4-(((3-methyloxetan-3-yl)methoxy)methyl)phenyl) octan-3-yl)benzyl)oxy)methyl)-1,3-phenylene)bis(9H-carbazole)) (P(mCP-Ox)-I, -II, and -III)]. In addition, monomer 1 was copolymerized with styrene to produce copolymer P(mCP-Ph) as a host material for bis[2-(4,6-difluorophenyl) pyridinato-C²,N](picolinato)iridium(III) (FIrpic), a blue-emitting dopant. All mCP-based copolymers displayed high glass transition temperatures (T_g) of up to 130-140 °C and triplet energies of up to 3.00 eV. The blue Ph-PLEDs exhibited a maximum external quantum efficiency of 2.55%, in addition to a luminous efficiency of 8.75 cd A^-1 when using the device configuration of indium tin oxide/poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate)/P(mCP-OX)-III/P(mCP-Ph):FIrpic(15 wt %)/3,3′-[5′-[3-(3-pyridinyl)phenyl][1,1′:3′, 1′′-terphenyl]-3,3′′-diyl]bispyridine/LiF/Al. The device bearing P(mCP-Ox)-III HTL, containing the highest composition of mCP unit, exhibited better performance than the other devices, which is attributed to induction of more balanced charge carriers and carrier recombination in the emissive layer. Copyright

Full text of DOI:10.1002/pola.27054

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Novel 9,90-(1,3-Phenylene)bis-9H-carbazole-Containing Copolymers as
Hole-Transporting and Host Materials for Blue Phosphorescent Polymer
Light-Emitting Diodes
Seung Hee Yoon,1 Jicheol Shin,1 Hyun Ah Um,1 Tae Wan Lee,1 Min Ju Cho,1 Yong Jae Kim,2
Young Hoon Son,2 Joong Hwan Yang,3 Geesung Chae,3 Jang Hyuk Kwon,2 Dong Hoon Choi1
1Department of Chemistry, Research Institute for Natural Sciences, Korea University, 5 Anam-dong, Sungbuk-gu, Seoul 136-701,
South Korea
2Department of Information Display, Kyung Hee University, 26 Kyungheedae-ro, Dongdaemoon-gu, Seoul 130-701,
Republic of Korea
3LG Display, Co., LTD. 1007 Deogeon-ri, Wollong-myeon, Paju-si, Gyeonggi-do 413-811, Republic of Korea
Correspondence to: D. H. Choi (E-mail: dhchoi8803@korea.ac.kr); J. H. Kwon (E-mail: jhkwon@khu.ac.kr)
Received 7 September 2013; accepted 29 November 2013; published online 17 December 2013
DOI: 10.1002/pola.27054
ABSTRACT: Novel photo-crosslinkable hole-transport and host
materials incorporated into multilayer blue phosphorescent
polymer light-emitting diodes (Ph-PLEDs) were demonstrated
in this study. The oxetane-containing copolymers, which
function as hole-transport layers (HTL), could be cured by UV
irradiation in the presence of a cationic photoinitiator. The
composition of the two monomers was varied to yield three
different hole-transporting copolymers, [Poly(9,90-(5-(((4-(7-(4-
(((3-methyloxetan-3-yl)methoxy)methyl)phenyl)octan-3-yl)benz
yl)oxy)methyl)21,3-phenylene)bis(9H-carbazole)) (P(mCP-Ox)-I,
-II, and -III)]. In addition, monomer 1 was copolymerized with
styrene to produce copolymer P(mCP-Ph) as a host material for
bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III)
(FIrpic), a blue-emitting dopant. All mCP-based copolymers
displayed high glass transition temperatures (Tg) of up to
130–140 ꢀC and triplet energies of up to 3.00 eV. The blue
Ph-PLEDs exhibited a maximum external quantum efficiency of
2.55%, in addition to a luminous efficiency of 8.75 cd A21
when using the device configuration of indium tin oxide/p-
oly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/P(mCP-
OX)-III/P(mCP-Ph):FIrpic(15 wt %)/3,30-[50-[3-(3-pyridinyl)phenyl]
[1,10:30,100-terphenyl]-3,300-diyl]bispyridine/LiF/Al. The device
bearing P(mCP-Ox)-III HTL, containing the highest composi-
tion of mCP unit, exhibited better performance than the other
devices, which is attributed to induction of more balanced
charge carriers and carrier recombination in the emissive
C
V
layer.
2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
Polym. Chem. 2014, 52, 707–718
KEYWORDS: blue; copolymerization; crosslinking; curing of poly-
mers; light-emitting diodes; luminescence; oxetane; phospho-
rescence; photo-crosslinking; polymer light-emitting diode;
solution process; synthesis
quantum efficiency of 100%.1–3 In particular, cyclometalated
Ir(III) complexes show high phosphorescent efficiencies and
are one of the most important classes of phosphorescent
dyes under investigation at present.4–9
INTRODUCTION Organic light-emitting diodes (OLEDs) have
emerged as a fascinating technology for the development of
advanced flat panel display and flexible displays and flexible
display devices. The maturation of this technology for practi-
cal applications has been made possible by the development
of a number of small organic molecules for use in fabricating
the corresponding display devices. Most conventional OLED
materials show semiconducting properties; fluorescent mole-
cules, in particular, have been utilized for elaborating con-
ventional RGB pixels. As an alternative to fluorescent
molecules, phosphorescent dye molecules are particularly
promising because both singlet and triplet excitons can
generate unique light emissions with a theoretical internal
When preparing phosphorescent OLEDs (Ph-OLEDs), multilay-
ered device configurations are usually designed employing var-
ious auxiliary carrier-transport and blocking layers to improve
device efficiency. These devices are mainly fabricated by vac-
uum processes using a combination of various auxiliary and
emitting layers. Besides practical vacuum deposition-based fab-
rication methods, an alternative solution processing technique
for OLEDs involves solution spin coating. Advantages of the
C
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spin-coating technique over vacuum process include a signifi-
cant reduction in production costs and the ability to coat
larger areas; however, in solution-based preparation of multi-
layer OLEDs, the coated layers tend to be at least partially dis-
solved by the solvent used to apply subsequent layers. A
common method used to overcome this obstacle is to render
the applied polymer layer by crosslinking before the subse-
quent coating step. The crosslinking process for obtaining
phosphorescent polymer light-emitting diodes (Ph-PLED) can
be carried out via a variety of strategies such as thermal,10–12
light,13–22 or chemical treatment.23–25 Polymeric hole-transport
materials and host materials can be designed and synthesized
in this manner to facilitate efficient singlet and triplet energy
transfer in the solid state.
was selected as the side chain of the copolymer to obtain a
polymeric HTL because an oxetane group can undergo fast
cationic polymerization in bulk with high conversion and
low volume shrinkage, thus avoiding cracks in the spin-
coated layers.26,27
Three kinds of P(mCP-Ox) copolymers were successfully
synthesized and applied as HTL coatings on a poly(3,4-ethyl-
enedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)
ionomeric layer, followed by photo-crosslinking phenomenon
under UV light irradiation. In order to fabricate an emitting
layer, a solution of high-ET P(mCP-Ph) as the blue host
material mixed with bis[2-(4,6 difluorophenyl)pyridinato-
C2,N](picolinato) iridium(III) (FIrpic) was spin-coated on top
of the photo-crosslinked HTL polymer layer without any
observable detrimental effects on the underneath layer. The
functionality of each oxetane-containing polymeric HTL was
investigated in terms of the device efficiency for solution-
processed Ph-PLEDs. Solution-processed blue Ph-PLEDs
using photo-crosslinked HTL (P(mCP-Ox)-III) containing
larger proportion hole-transport mCP units (i.e., monomer 1)
showed improved device performance compared to those
containing crosslinked P(mCP-Ox)-I and -II-based HTLs. The
corresponding PLED fabricated with P(mCP-Ox)-III and 15
wt % of FIrpic-doped P(mCP-Ph) showed the highest exter-
nal quantum efficiency of 2.55% and luminous efficiency of
8.75 cd A21 in addition to its low roll-off behavior.
On the other hand, the development of high-efficiency blue
Ph-PLEDs still remains a challenge when compared to red-
and green-emitting materials because of the larger energy
bandgap and higher triplet energy (ET) required for carrier-
transport polymeric materials and blue host polymers. In
addition to high-ET and high carrier mobility with balanced
hole and electron transport, the practical use of blue host
polymers also necessitates the improvement of thermal and
morphological stabilities in order to boost efficiencies. Vari-
ous types of high-ET small molecules have been synthesized
as a component of hole-transport and host polymers for blue
Ph-PLEDs. Among the common large bandgap materials, car-
bazole is the most widely used hole-transport moiety
because of its high ET and rigid molecular framework. The
derivative 1,3-bis(N-carbazolyl)benzene (mCP) is also a well-
known high-ET molecule, which can be utilized as a polymer
side chain. In addition to suitable electronic properties and
charge balance for blue Ph-PLEDs, the thermal properties of
host polymers remain a key consideration in their design,
since long-term device stability is thought to be dependent
on glass transition temperatures (Tg) of the host materials.
EXPERIMENTAL
General Considerations
Commercial reagents were purchased from Sigma-Aldrich,
TCI, or Acros Organics, and used without any purification
unless stated otherwise. The radical initiator, a,a0-azobisiso-
butyronitrile (AIBN) was recrystallized in acetone before
use. HPLC-grade toluene was purchased from Samkyung
Chemical and distilled from CaH2 before use. All reactions
were performed under an argon atmosphere unless other-
wise stated.
In blue Ph-PLEDs, large bandgap host materials such as
mCP-containing polymers with a triplet energy higher than
that of the phosphorescent Ir(III) complex should be
employed to avoid back-energy transfer from the Ir(III)
complex to the host polymer. Because of the high highest
occupied molecular orbital (HOMO) energy level of these
large-bandgap polymers, the efficiency of the hole injection
from an indium tin oxide (ITO) electrode to the emissive
layer (EML) can be suppressed if only a single hole-injection
layer (HIL) is used in the device. Therefore, an additional
hole-transport layer (HTL) or HIL can be combined with a
conventional HIL material to form an optimal cascade energy
profile to achieve efficient hole injection and charge confine-
ment. For the fabrication of multilayered devices solely by
solution processing, either photo- or thermally crosslinked
hole-transporting materials are frequently used.10–22
Synthetic Procedures
Compounds 2, 3, and 9 were synthesized by following the
modified literature method.28–30
Tert-butyl((3,5-dibromobenzyl)oxy)dimethyl silane (4)
Compound 3 (5 g, 18 mmol) and imidazole (1.92 g, 28
mmol) were dissolved in 200 mL dichloromethane (CH2Cl2)
under argon. Tert-butyldimethylsilyl chloride (TBDMS-Cl,
4.25 g, 28 mmol) was added into the mixture and the mix-
ture was allowed to stir at room temperature for 3 h. After
reaction completion, the reaction mixture was extracted with
CH2Cl2 and washed with water. The organic layer was then
dried over Na2SO4 and concentrated under reduced under
pressure. The resulting dark brown oil was purified by silica
gel column chromatography with hexane as an eluent to give
4 in 98% yield (7.00 g).
Here, we demonstrate the fabrication of photo-crosslinkable
copolymers bearing mCP groups (monomer 1) and oxetane
moieties (monomer 2) functioning as hole-transport units in
the side chain, denoted by P(mCP-Ox). An oxetane group
1H NMR (400 MHz, CDCl3): d (ppm) 7.52 (s, 1H), 7.37 (s,
2H), 4.65 (s, 2H), 0.92 (s, 9H) 0.09 (s, 6H); 13C NMR (100
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MHz, CDCl3): d (ppm) 143.82, 136.74, 132.64, 132.48,
124.56, 123.98, 67.32, 25.74, 25.68, 25.66.
under reduced pressure. The oily product was purified by
silica gel column chromatography (CH2Cl2:hexane 5 1:2 v/v)
to give 7 in 72% yield (1.82 g).
9,90-(5-(((Tert-butyldimethylsilyl)oxy)methyl)-1,3-
phenylene)bis(9H-carbazole) (5)
1H NMR (400 MHz, CDCl3): d (ppm) 8.19 (d, J 5 7.58 Hz, 4H)
7.77 (s, 1H), 7.74 (s, 2H), 7.58 (d, J 5 8.22 Hz, 4H), 7.47 (t,
4H), 7.41 (d, J 5 4.68 Hz, 4H), 7.34 (t, 4H), 6.74 (q, 1H), 5.80
(d, J 5 17.58 Hz, 1H), 5.29 (d, J 5 10.58 Hz, 1H), 4.78 (s, 2H),
4.72 (s, 2H); 13C NMR (100 MHz, CDCl3) : d (ppm) 142.43,
142.38, 139.75, 139.72, 139.68, 138.4, 136.44, 128.7, 126.59,
122.54, 122.46, 122.02, 121.58, 121.50, 109.48, 109.43,
109.39, 72.54.
Compound 4 (5 g, 13 mmol) and carbazole (4.83 g, 28
mmol) were dissolved in 30 mL toluene under argon.
Sodium tert-butoxide (NaOt-Bu; 6.32 g, 65 mmol) was
ꢀ
added to the mixture, which was then stirred at 60 C for 1
h.
Next,
tris(dibenzylidene
acetone)dipalladium(0)
(Pd2(dba)3; 0.60 g, 0.65 mmol) and tri-tert-butylphosphine
(P(t-Bu)3, 0.13 g, 0.65 mmol) were added into the mixture,
ꢀ
which was then heated to 110 C. After stirring for 8 h, the
General Procedure for Polymerization
Poly(9,90-(5-(((4-(7-(4-(((3-methyloxetan-3-
yl)methoxy)methyl)phenyl)octan-3-yl)benzyl)oxy)methyl)-
1,3-phenylene)bis(9H-carbazole)) (P(mCP-Ox))
reaction mixture was extracted with CH2Cl2 and washed
with water. The organic layer was then dried over Na2SO4
and concentrated under reduced pressure. The resulting
dark brown oil was purified by silica-gel column chroma-
tography (CH2Cl2: hexane 5 1:3 v/v) to give 5 in 70% yield
(5.08 g).
A solution of monomer 1 (7) and monomer 2 (9) (at a feed-
ing molar ratio of 7:9 5 m:n) in anhydrous toluene was
degassed under nitrogen for 30 min. AIBN (0.41 mg, 0.0025
mmol) was added to the mixture and heated to 80 ꢀC. The
reaction mixture was allowed to stir at 80 ꢀC for 48–60 h.
The resultant polymer was then precipitated in methanol.
The crude polymer was collected by filtration and then puri-
fied by Soxhlet extraction with acetone and chloroform, suc-
cessively. The final product was obtained by precipitation of
the chloroform solution into methanol. The pure copolymer
was dried under vacuum for 24 h.
1H NMR (300 MHz, CDCl3): d (ppm) 8.28 (d, J 5 7.68 Hz,
4H), 7.75 (s, 1H), 7.73 (s, 2H), 7.60 (d, J 5 8.22 Hz, 4H), 7.46
(t, 4H), 7.31(t, 4H), 5.02 (s, 2H), 0.90 (s, 9H), 0.13 (s, 6H);
13C NMR (100 MHz, CDCl3): d (ppm) 142.68, 142.43, 139.72,
139.68, 139.60, 126.65, 122.69, 121.33, 119.74, 110.67,
109.45, 30.59.
(3,5-Di(9H-carbazol-9-yl)phenyl) methanol (6)
Compound 5 (5.0 g, 9.04 mmol) was dissolved in THF under
argon. A 3-M solution of HCl (27 mL) was added dropwise
into the mixture. The reaction mixture was allowed to stir at
room temperature for 4 h. After reaction completion, the
mixture was extracted with CH2Cl2 and washed with water.
The organic layer was then dried over Na2SO4 and concen-
trated under reduced pressure. The product was obtained by
precipitation into hexane to provide 6 in 93% yield (3.68 g).
P(mCP-Ox)-I. Molar feeding ratio (m:n 5 1:1); yield: 47.62%;
1H NMR (400 MHz, CDCl3): d (ppm) 7.90 (br, 2H), 7.34 (br,
8H), 6.80 (br, 2H), 6.27 (br, 2H), 4.22 (br, 4H), 3.22 (br, 1H),
1.11 (br, 3H); anal. calcd.: C 83.9; H 6.85; N 3.48.
P(mCP-Ox)-II. Molar feeding ratio (m:n 5 1:2); yield: 61.97%;
1H NMR (400 MHz, CDCl3): d (ppm) 7.90 (br, 3H), 7.34 (br,
11H), 6.80 (br, 2H), 6.27 (br, 2H), 4.22 (br, 5H), 3.22 (br,
1H), 1.11 (br, 4H); anal. calcd.: C 83.66; H 6.39; N 3.57.
1H NMR (300 MHz, CDCl3) : d (ppm) 8.19 (d, J 5 6.00 Hz,
4H), 7.77 (s, 1H), 7.74 (s, 2H), 7.58 (d, J 5 6.00 Hz, 4H), 7.47
(t, 4H), 7.34 (t, 4H), 4.98 (d, J 5 6.00 Hz, 2H), 2.03 (t, 1H),
13C NMR (100 MHz, CDCl3): d (ppm) 145.01, 140.79, 139.75,
126.51, 126.47, 126.41, 126.40, 126.37, 126.32, 124.46,
124.31, 124.12, 123.97, 123.85, 120.75, 120.66, 120.60,
120.55, 120.54, 110.02, 109.95, 109.86, 104.99, 77.58, 77.26,
76.94, 64.87, 64.68, 64.49.
P(mCP-Ox)-III. Molar feeding ratio (m:n 5 1:4); yield:
46.30%; 1H NMR (400 MHz, CDCl3): d (ppm) 7.90 (br, 6H),
7.34 (br, 18H), 6.80 (br, 2H), 6.27 (br, 3H), 4.22 (br, 5H),
3.22 (br, 1H), 1.11 (br, 6H); anal. calcd.: C 82.80; H 6.08; N
4.24.
Poly(9,90-(5-(((4-(7-phenyloctan-3-yl)benzyl) oxy)methyl)-
1,3-phenylene)bis(9H-carbazole)) (P(mCP-Ph))
9,90-(5-(((4-Vinylbenzyl)oxy)methyl)-1,3-phenylene)bis(9H-
carbazole) (7)
Monomer 1 (0.7 g, 1.26 mmol) and styrene (0.01 g, 0.14
mol) were dissolved in anhydrous toluene and degassed
under nitrogen for 30 min. AIBN (0.41 mg, 0.0025 mmol)
was added to the mixture, which was then heated to 80 ꢀC
for 120 h under argon. The mixture was poured into metha-
nol to collect the precipitate. The crude polymer was col-
lected by filtration and then purified by Soxhlet extraction
with acetone and chloroform, successively. The final product
was obtained by precipitation of chloroform solution into
methanol. Then, the product was dried under vacuum for
24 h to obtain P(mCP-Ph) as a white solid in a yield of
60%.
NaH (0.12 g, 5.01 mmol) was suspended in 35-mL anhy-
ꢀ
drous DMF at 0 C. Compound 5 (2.0 g, 4.56 mmol) in DMF
was added dropwise into the mixture and the mixture was
allowed to stir for 1 h. Next, 1-(chloromethyl)-4-vinylben-
zeneꢀ(0.76 g, 5.01 mmol) in DMF was also added dropwise
at 0 C over a 15-min period; the reaction mixture was then
stirred at room temperature for 12 h. After the reaction was
completed, the reaction mixture was extracted with CH2Cl2
and washed with water; the mixture was extracted again
with diethyl ether and washed with water. The combined
organic layers were dried over Na2SO4 and concentrated
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1H NMR (400 MHz, CDCl3): d (ppm) 7.83 (br, 4H), 7.30 (br,
8H), 7.10 (br, 12H), 4.15 (br, 4H), 1.09 (br, 6H); anal. calcd.:
C 86.66; H 5.58; N 5.19.
then cured with UV light (kmax 5 254 nm) irradiation at 150
ꢀC for 30–60 s. The solubilities of the photo-cured films
were examined by comparing UV spectra of the film samples
before and after rinsing with THF, which is a good solvent
for the pristine polymers.
Instruments
1H NMR spectra were recorded on a Varian Mercury NMR
300 and 400 MHz spectrometer (Varian, Palo Alto, CA) using
deuterated chloroform (CDCl3) purchased from Cambridge
Isotope Laboratories (Andover, MA). Elemental analyses
were performed using an EA1112 (Thermo Electron, West
Chester, PA) elemental analyzer. The molecular weights of
the polymers were determined by gel permeation chroma-
tography (GPC) using polystyrene as the standard and THF
as an eluent at the Korean Polymer Testing and Research
Institute, Seoul, Korea. (Waters GPC, Waters 515 pump,
Waters 410 RI, 23 PLgel Mixed-B).
Measurement of Performances of Blue Ph-PLEDs
When fabricating blue Ph-PLEDs via solution processing,
PEDOT:PSS, FIrpic, and 3,30-[50-[3-(3-pyridinyl)phenyl]
[1,10:30,100-terphenyl]-3,300-diyl]bispyridine (TmPyPB) were
employed as HIL, blue dopant, and electron transport materi-
als, respectively. The fabricated multilayered Ph-PLEDs have
the structure as follows: ITO/PEDOT:PSS (40 nm)/P(mCP-
OX)-I or -II or -III (10 nm)/P(mCP-Ph):FIrpic (20 nm)/
TmPyPB (50 nm)/LiF (1.5 nm)/Al(100 nm). For fabricating
devices A–E, water-soluble conducting PEDOT:PSS was spin-
coated onto the ITO-coated glass under argon, followed by
spin coating the solution of P(mCP-Ox)-I, -II, or -III in chlor-
obenzene (0.5 wt %) onto PEDOT:PSS layer. Each photo-
crosslinkable polymer layer was exposed to the UV light
(kmax 5 254 nm, I 5 40 mW cm22) for 30–60 s at 150 ꢀC.
The emitting layers made of P(mCP-Ph) containing FIrpic in
a solution (0.5 wt %) of chlorobenzene were then spin-
coated onto the thoroughly dried and photocured P(mCP-
Ox)-I, -II, and -III layers.
The thermal properties were studied under a nitrogen
atmosphere on a Mettler differential scanning calorimetry
(DSC) 821e instrument (Mettler, Greifensee, Switzerland).
Thermogravimetric analysis (TGA) was conducted on a Met-
21
ꢀ
tler TGA50 (temperature rate 10 C min under nitrogen).
Absorption spectra of polymers were obtained using a UV–
vis absorption spectrometer (HP 8453, PDA type) in the
wavelength range of 190–1100 nm. In order to prepare the
film sample, the polymers were dissolved in chloroform, and
the solution was spin-coated onto quartz glass and dried
under vacuum thoroughly.
For fabricating hole-blocking and electron-transporting
layers, TmPyPB (50 nm) was vacuum-deposited onto the
emitting polymer layer. Finally, LiF (1.5 nm) and Al (100
nm) electrodes were deposited onto the TmPyPB layer. The
final precise configurations of the fabricated devices are as
follows:
Photoluminescence (PL) spectra were recorded with a
Hitachi F-7000 fluorescence spectrophotometer at room
temperature and a Thermo FA-357 fluorescence spectro-
photometer at 77 K. The triplet energy of the polymers
was determined by the highest energy vibronic sub-band
Device A: ITO/PEDOT:PSS/P(mCP-Ox)-I/P(mCP-Ph):10 wt %
FIrpic/TmPyPB/LiF/Al;
of the phosphorescence spectrum at 77
methyltetrahydrofuran.
K
in 2-
Device B: ITO/PEDOT:PSS/P(mCP-Ox)-II/P(mCP-Ph):10 wt %
FIrpic/TmPyPB/LiF/Al;
The redox properties of polymers were examined using
cyclic voltammetry (CV; Model: EA161 eDAQ). Thin films
were coated on a Pt plate using chloroform (CHCl3) as a sol-
vent. The electrolyte solution was 0.10-M tetrabutylammo-
nium hexafluorophosphate (Bu4NPF6) in freshly dried
acetonitrile. The reference electrode was Ag/AgCl and the
counter electrode was Pt wire (0.5 mm in diameter). The
Device C, D, E: ITO/PEDOT:PSS/P(mCP-Ox)-III/P(mCP-
Ph):10(C), 15(D), 20(E) wt % FIrpic/TmPyPB/LiF/Al.
The current density–voltage (J–V) and (L–V) data of the blue
Ph-PLEDs were measured using Keithley 2635A and Minolta
CS-100A meters, respectively. The PLED emission area was 4
mm2 for all devices studied in this work. Electrolumines-
cence (EL) spectra and CIE coordinates were obtained using
a Minolta CS-1000 spectroradiometer.
scan rate was 50 mV s21
.
Solvent Resistance of Photo-Crosslinked Films
Commercially available microscope slides were used as sub-
strates for examining the solvent resistance of photocured
films. The slides were cut to the size 2.5 3 2.5 cm2 and
cleaned by ultrasonic treatment in distilled water, acetone,
chloroform, and 2-propanol, in sequence. The polymers,
P(mCP-Ox)-I, -II, and -III were dissolved in dry chloroben-
zene (10 mg mL21) and mixed with 2 wt % of the photoini-
tiator [4-[(2-hydroxytetradecyl)oxy]phen-yl]phenyliodonium
hexafluoro antimonate just before spin-coating. 30-nm-thick
polymer layers were fabricated, which were spin-coated on
the glass substrates at ambient conditions. The films were
RESULTS AND DISCUSSION
Synthesis
Monomer Synthesis
Monomers 7 and 9 were first prepared for use as photo-
crosslinkable hole-transporting copolymers and host polymer
for blue dopant (FIrpic). The synthesis of monomer 1 (7)
commenced with the metal–halogen exchange of 1,3,5-tribro-
mobenzene 1 using n-butyllithium in diethyl ether. The ther-
mally unstable monolithiated intermediate was transformed
to aldehyde 2 (76%).28 The 3,5-dibromobenzyl alcohol 3
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SCHEME 1 Synthesis of monomer 1 (7) and monomer 2 (9).
was obtained by the reduction of 2 with sodium borohydride
(93%).29 Subsequent protection of compound 3 by TBDMS-
Cl, followed by CAN coupling using the Buchwald–Hartwig
amination protocol furnished mCP-containing 5 (70%). The
TBDMS protecting group of 5 was deprotected in the pres-
ence of hydrochloric acid in THF. Finally, monomer 1 was
SCHEME 2 Synthesis of hole-transport P(mCP-Ox)s and host polymer P(mCP-Ph).
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TABLE 1 Physical Properties of P(mCP-Ox) and P(mCP-Ph) Copolymers
Feeding Ratio (m:n)
Calculation Ratio (m:n)
Mn (kDa)
PDI
Tg (ꢀC)
Td (ꢀC)
P(mCP-Ox)-I
P(mCP-Ox)-II
P(mCP-Ox)-III
P(mCP-Ph)
1:1
1:2
1:4
1:3
1:0.75a
1:1.45a
1:3.06a
1:2.15b
22
23
23
10
1.86
1.85
1.84
1.76
130
137
140
148
387
387
388
381
a
b
Calculated from the ratio between integrated areas at 3.0–3.4 ppm and
at 8.0 ppm assigned to the protons in 1H NMR spectrum.
Calculated from the ratio between integrated areas at 4.0–4.5 ppm and
at 8.0 ppm in 1H NMR spectrum.
prepared by the reaction of compound 5 with 4-vinyl benzyl
chloride under basic conditions (72%). Oxetane-
functionalized monomer 2 (9) was also synthesized by a
one-step etherification. (3-Methyloxetan-3-yl)methanol (8)
was coupled with 4-vinyl benzyl chloride, yielding the
desired 3-methyl-3-(((4-vinylbenzyl)oxy)methyl)oxetane (9)
in the presence of sodium hydride (Scheme 1).30
integrated areas at 4.0–4.5 ppm assigned to the oxymethy-
lene protons in monomer 1 and at 8.0 ppm assigned to the
protons in aromatic ring.
The molecular weights of polymers were determined by GPC
with polystyrene as a standard and THF as an eluent. The
number average molecular weights (Mn) of P(mCP-Ox)-I, -II,
-III, and P(mCP-Ph) were determined to be 22, 23, 23, and
10 kDa with polydispersites (PDIs) of 1.86, 1.85, 1.84, and
1.76, respectively. The polymers were found to have good
self-film-forming properties and were highly soluble in vari-
ous organic solvents such as chloroform, xylene, dichlorome-
thane, chlorobenzene, and THF.
A simple synthetic route to the photo-crosslinkable hole-con-
ducting P(mCP-Ox) copolymers and host P(mCP-Ph) copoly-
mer are depicted in Scheme 2. Each P(mCP-Ox) copolymer
was prepared by the radical polymerization of monomer 1
and monomer 2 with varied monomer feeding compositions.
After polymerization was complete, the reaction mixture was
precipitated in methanol. The crude polymer was then puri-
fied by Soxhlet extraction with acetone and chloroform, suc-
cessively. The resulting products were obtained as white
powder in 42–65% yield. P(mCP-Ox) copolymers with three
different monomer-feeding ratios were synthesized in order
to investigate the influence of oxetane content on physical
and device performance. The molecular weights and result-
ant copolymer compositions using the two monomers are
shown in Table 1. The resultant molar ratios of the two
monomers incorporated into the P(mCP-Ox) copolymers
were calculated from the ratio between the integrated areas
at 3.0–3.4 ppm assigned to the protons of the oxymethylene
groups in oxetane moiety and at 8.0 ppm assigned to the
protons in aromatic ring. Similarly, the molar ratio of
P(mCP-Ph) was also calculated from the ratio between the
Thermal Analysis
The thermal properties of the polymers were characterized by
means of TGA and DSC; results of the analyses are illustrated in
Figures 1 and 2, respectively. TGA measurements at a heating
rate of 10 ꢀC min21 under nitrogen revealed that the polymers
had good thermal stabilities and an enhanced onset decomposi-
tion temperature (381–388 ꢀC) (Fig. 1). DSC measurements
were performed at a heating (cooling) scan rate of 10 (and
21
ꢀ
210) C min under nitrogen, with the highest temperature
limited to being below the decomposition temperature (Fig. 2).
The P(mCP-Ox) copolymers exhibited no crystalline isotropic
ꢀ
transitions in the range of 25–250 C, whereas glass transition
ꢀ
temperatures (Tg) were observed at 130, 137, and 140 C for
P(mCP-Ox)-I, -II, and -III, respectively. P(mCP-Ox)-III showed
the highest Tg among the three copolymers. The higher
FIGURE 1 TGA curves of (a) P(mCP-Ox)-I (1), -II (2), and -III (3), and (b) P(mCP-Ph).
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FIGURE 2 DSC curves of (a) P(mCP-Ox)-I (1), -II (2), and -III (3), and (b) P(mCP-Ph).
composition of mCP unit in P(mCP-Ox)-III is thought to con-
tribute to the enhancement of polymer rigidity. In addition, the
Tg of P(mCP-Ph) was determined to be 148 ꢀC (Table 1).
and 296 nm, which was attributed to the incorporated carba-
zole units. The large optical bandgaps (Eg) of P(mCP-Ox)-I, -II,
and -III, as determined from the absorption onset wavelengths,
were estimated to be very similar, at 3.56, 3.55, and 3.53 eV,
respectively.
UV–Vis Absorption and PL Spectroscopy
Figure 3 shows UV–vis absorption, PL, and low-temperature
PL spectra of the polymers synthesized in the present. The
absorption spectra of P(mCP-Ox)-I, -II, and -III in the solu-
tion and film states are shown in Figure 3(a,b). The solution
and film samples exhibited absorption maxima between 294
Similarly, the absorption spectra of P(mCP-Ph) in the
solution and film states are shown in Figure 4(b). The solu-
tion and film samples exhibited absorption maxima
between 294 and 395 nm, which were attributed to the
FIGURE 3 UV–vis absorption spectra of (a) P(mCP-Ox)-I (1), -II (2), and -III (3) solutions, and (b) film samples, (c) PL spectra of the
three copolymers at 77 K.
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FIGURE 4 UV–vis absorption and PL spectra of the (a) P(mCP-Ph) solution, and (b) film samples: (i) absorption spectrum; (ii) PL
spectrum at 25 ꢀC; (iii) PL spectrum at 77 K.
incorporated carbazole units and intermolecular interac-
tions. The optical bandgap (Eg) of P(mCP-Ph), as deter-
mined from the absorption onset wavelength, was
estimated to be 3.54 eV.
determined to be 25.78, 25.68, 25.65, and 25.76 eV,
respectively (Table 2). To determine the lowest unoccupied
molecular orbital (LUMO) level, the oxidation potential from
CV was combined with the optical energy bandgap obtained
from the absorption edge in the absorption spectrum. The
LUMO level of P(mCP-Ox)-I, -II, and -III, and P(mCP-Ph)
were determined to be 22.22, 22.13, 22.12, and 22.22 eV,
respectively. The energy levels of P(mCP-Ox)-I, -II, and -III,
and P(mCP-Ph) were thus found to be suitable for utiliza-
tion as ideal hole-transport and host materials for a given
blue-emitting dopant (FIrpic).
PL emission peaks of the P(mCP-Ox) and P(mCP-Ph) films
were observed at 392–413 nm at room temperature. Low-
temperature PL measurements of P(mCP-Ox) [Fig. 3(c)]
and P(mCP-Ph) [Fig. 4(a)] were also carried out at 77 K to
calculate ET from the first phosphorescent emission peak.
The triplet energy was obtained from the first vibronic
transition of the red-shifted phosphorescence spectrum.
The ET values of the copolymers were 3.00 eV (kem 5 413
nm) identically, which is high enough to permit energy
transfer from host polymer to blue phosphorescent FIrpic
dopant. The high ET of mCP was fully maintained despite
being tethered to the styrene polymer backbone. Therefore,
P(mCP-Ox) and P(mCP-Ph) may be suitable for use as the
hole transporting material and host material for blue Ph-
PLEDs, respectively.
Solvent Resistance of the Photo-Crosslinked Polymers
P(mCP-Ox) copolymers are readily soluble in common
organic solvents such as toluene, chloroform, and THF. After
UV-light illumination to induce crosslinking, thin-films solu-
bilities were significantly altered, and the thin films became
insoluble in THF and halogenated hydrocarbon solvents.
Photo-crosslinking of thin films composed of P(mCP-Ox)s
was induced via cationic polymerization, which is commonly
used for the ring-opening reaction of cyclic ethers such as
oxirane and oxetane moieties.
Electrochemical Analysis
Electrochemical behaviors of copolymers P(mCP-Ox)-I, -II, -III,
and P(mCP-Ph) were investigated by CV measurements. The
oxidation potentials were measured relative to ferrocene,
which was used as internal standard. The oxidation poten-
tials of P(mCP-Ox)-I, -II, -III, and P(mCP-Ph) were meas-
ured to be 0.98, 0.88, 0.85, and 0.96 V, respectively. The
HOMO levels of P(mCP-Ox)-I, -II, -III, and P(mCP-Ph) were
The commercially available photoinitiator [4-[(2-hydroxyte-
tradecyl)oxy]phenyl] phenyl-iodonium hexafluoroantimonate,
which can undergo photolysis at 210–254 nm was added to
initiate the crosslinking process. During ultraviolet (UV) illu-
mination, this photoinitiator decomposes via a multiple-step
TABLE 2 Optical and Electrochemical Properties of P(mCP-Ox) and P(mCP-Ph) Copolymers
kabs.
(nm)
Film
kem.
(nm)
Film
Energy Level (eV)
max
max
HOMOc
LUMOd
opt
Sol.
Sol.
ET (eV)
kcut off (nm)
Eg
(eV)
P(mCP-Ox)-I
P(mCP-Ox)-II
P(mCP-Ox)-III
P(mCP-Ph)
294
294
294
294
296
295
296
395
351
351
351
351
392
406
413
368
3.00
3.00
3.00
3.00
348a, 348b
348a, 349b
348a, 351b
348a, 350b
3.56a, 3.56b
3.56a, 3.55b
3.56a, 3.53b
3.56a, 3.54b
25.78b
25.68b
25.65b
25.76b
22.22b
22.13b
22.12b
22.22b
a
c
Absorption measured in CH2Cl2.
Calculated from the oxidation potentials.
b
d
opt
Absorption measured in solid state film.
Calculated from the HOMO energy levels and Eg
.
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FIGURE 5 Absorption spectral analysis of (a) P(mCP-Ox)-I, (b) -II, and (c) -III films: (i) before UV irradiation (254 nm); UV-exposure
time (ii) 5 s, (iii) 30 s, (iv) 60 s, then rinsed in THF; *Inset: Patterned image of P(mCP-Ox)-I film, (d) Solvent resistance of P(mCP-
Ox)-I (1), -II (2), and -III (3) versus UV-exposure time. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
mechanism to generate protons, which open the oxetane ring
and start the polymerization reaction.13–22
Figure 6 illustrates the alignment of the energy levels of all
the layers comprising the PLEDs. PEDOT:PSS was used as a
hole injection layer. P(mCP-Ox) and TmPyPB were used as
hole- and electron- transporting layer.
UV–vis absorption spectroscopy was employed to examine the
efficiency of the crosslinking process (Fig. 5). Changes in solu-
bility of the P(mCP-Ox)-I and -II filꢀms before and after UV
irradiation were investigated at 150 C (Table 3). The photo-
patterned image was illustrated as an inset of Figure 5(a).
The high triplet energy, high-lying HOMO-level good film-
forming ability, and especially fast conversion to insoluble
layer for P(mCP-Ox)-I, -II, and -III make them good candi-
dates for solution-processed hole transport materials for
blue Ph-PLEDs. Figure 7 shows the performances of blue
The films were rinsed with THF after UV irradiation for a
fixed irradiation timed (t 5 5, 30, and 60 s) and the spectra
were recorded again. The polymers P(mCP-Ox)-I and -II
gave similar results; absolute solvent resistivity was
observed for both polymers with 2ꢀwt % of the photoinitia-
tor at a curing temperature of 150 C for 30 s. In contrast, it
was observed that P(mCP-Ox)-III required much longer cur-
ing times (t 5 60 s) at the same temperature as compared to
P(mCP-Ox)-I and -II (t 5 30 s), because of its lower oxetane
monomer concentration.
TABLE 3 Influence of Curing Time on the Solvent Resistivity of
Crosslinked Layer P(mCP-Ox) Copolymers
Solvent Resistivity 5
(Abs1/Abso) 3 100
Polymer
5 s
30 s
60 s
P(mCP-Ox)-I
P(mCP-Ox)-II
P(mCP-Ox)-III
82
81
10
98
96
40
100
100
100
Characterization of Blue Ph-PLEDs
To explore the possibility of the newly synthesized polymers
for hole transport material and host material in blue Ph-
PLEDs, P(mCP-Ox)-I, -II, -III, and P(mCP-Ph) were applied
to multilayered blue Ph-PLED.
Abs0: absorbance at 341 nm after UV exposure.
Abs1: absorbance at 341 nm after rinsing the film in THF.
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FIGURE 6 (a) Energy-level alignment of multilayered blue Ph-PLED; molecular structures of (b) P(mCP-Ox), (c) P(mCP-Ph), (d)
TmPyPB, (e) FIrpic, and (f) cationic photoinitiator. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Ph-PLEDs A, B, and C made of different hole-transport poly-
mers, P(mCP-Ox)s at a fixed concentration of FIrpic (10 wt %
into P(mCP-Ph)).
The current density–voltage–luminance (J–V–L), current effi-
ciency–luminance (CE–L), power efficiency–luminance
(PE–L), and external quantum efficiency–luminance (EQE–L)
FIGURE 7 Device performance of soluble blue Ph-PLED: (a) J–V–L (inset: EL spectra of the three devices), (b) CE–L, (c) PE-L, and
(d) EQE-L characteristics of blue Ph-PLED (i.e., device A: circle, device B: triangle, device C: square). [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
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TABLE 4 Devices Performance of Blue Ph-PLED
HTL Thickness
FIrpic
Vturn on
CEmax
(cd/A)
PEmax
EQEmax
(%)
CIEmax
Device
HTL Polymer
(nm)
(wt %)
(V)
(lm/W)
(x, y)
A
B
C
D
E
P(mCP-Ox)-I
P(mCP-Ox)-II
P(mCP-Ox)-III
P(mCP-Ox)-III
P(mCP-Ox)-III
10
10
10
10
10
10
10
10
15
20
6.0
6.5
6.0
6.0
6.0
1.66
1.71
5.34
8.75
2.69
0.55
0.56
2.24
3.93
1.09
0.96
1.03
2.18
2.55
0.90
0.17, 0.34
0.18, 0.35
0.16, 0.33
0.17, 0.34
0.17, 0.34
values of three devices are well illustrated in Figure 8(a–c),
respectively.
resistance was investigated by UV–vis absorption spectros-
copy. The photo-crosslinkable hole-affine polymers allowed
for the fabrication and investigation of multilayer Ph-PLED
devices by the spin-coating method. The fabricated blue Ph-
PLEDs based on P(mCP-Ox)-I, -II, -III, and P(mCP-Ph)
showed reasonable performances under an applied electric
field. In particular, P(mCP-Ox)-III, which contains the high-
est composition of hole-transport (mCP) unit, exhibited the
best performance as a hole transport material in blue Ph-
PLED, possibly by inducing more balanced charge carriers
and carrier recombination in the EML.
The turn-on voltages for these devices were typical for FIrpic-
doped PLEDs and were in the range of 6.0–6.5 V.31 Moderately
low turn-on voltages were observed even for crosslinked
P(mCP-Ox). The EL spectra of all three devices containing
P(mCP-Ph) host are almost identical without any emission
from the host and or adjacent layers. The EL spectral results
indicate that complete energy transfer from P(mCP-Ph) to
FIrpic took place upon electrical excitation, and charge car-
riers and excitons are well confined in the emitting zone.
ACKNOWLEDGMENTS
Among three devices A, B, and C, the device C exhibited much
better maximum device efficiencies. It might be attributed to
better hole transport property of P(mCP-Ox)-III. As was shown
in Table 4, the improved performance can be attributed to the
charge balance of electron and hole fluxes in emitting zone
because the internal quantum efficiency of the Ph-PLED device
is directly related to the balance of electrons and holes in the
device.32,33 Device C bearing 10 wt % FIrpic, shows a turn-on
voltage of 6.0 V at a luminance of 1.0 cd m22, a maximum CE of
5.34 cd A21, a maximum PE of 2.24 lm W21, and maximum
EQE of 2.18%, with CIE coordinate of (0.16, 0.33). As the device
C showed better performance than the devices A and B,
dopant-concentration dependence of the device efficiency was
investigated further. The concentration of FIrpic dopant was
varied from 10 to 20 wt % into P(mCP-Ph). In comparison, the
device D bearing 15 wt % FIrpic, exhibited the maximum CE of
8.75 cd A21, maximum PE of 3.93 lm W21, and maximum EQE
of 2.55%, with CIE coordinate of (0.17, 0.34). On the while, the
The authors acknowledge the financial support from the Basic Sci-
ence Research Program through the NRF funded by the Ministry of
Education (NRF2012R1A2A1A01008797 and NRF20100020209).
DH Choi particularly thank for the support from LG display Co.
Limited (2013–2014).
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