Synthesis and characterization of solution-processable highly branched iridium (III) complex cored dendrimer based on tetraphenylsilane dendron for host-free green phosphorescent organic light emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2010.10.005

Solution processable highly branched iridium(III) complex fac-tris(2-phenylpyridyl)iridium-cored dendrimer based on tetraphenylsilane dendron and 2-ethylhexyloxy surface groups was prepared. The structure of dendrimer was confirmed by nuclear magnetic resonance, mass and infrared studies. Thermogravimetric analysis and differential scanning calorimetry studies show the thermal stability (T_5%) of 625 K with high glass transition temperature of 423 K. Photoluminescence studies of dendrimer showed the core emission at 516 nm in solution and at 521 nm in film, indicating the highly branched non-conjugated tetrahedral tetraphenylsilane dendrons around the core highly inhibit the intermolecular interactions between the cores in the film. The solution processed green emitting phosphorescent organic light emitting diodes based on dendrimer were fabricated and characterized. The device using host free dendrimer as emitter showed the external quantum efficiency of 0.4%. The results show the potential of this new type of dendritic emitting structure in highly efficient solution processed host-free green phosphorescent organic light emitting diodes.

Full text of DOI:10.1016/j.dyepig.2010.10.005

Dyes and Pigments 90 (2011) 139e145
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and characterization of solution-processable highly branched
iridium (III) complex cored dendrimer based on tetraphenylsilane dendron
for host-free green phosphorescent organic light emitting diodes
,
a,
Seul-Ong Kim a, Qinghua Zhao a, K. Thangaraju a, Jang Joo Kim c, Yun-Hi Kim b , Soon-Ki Kwon
*
*
a School of Materials Science and Engineering, Engineering Research Institute (ERI), Gyeongsang National University, Jinju 660-701, Republic of Korea
b Department of Chemistry, and Research Institute of Natural Science (RINS), Gyeongsang National University, 900 Gajwa Jinju 660-701, Republic of Korea
c Department of Materials Sciences and Engineering, OLED Center, Seoul National University, Seoul 151-744, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Solution processable highly branched iridium(III) complex fac-tris(2-phenylpyridyl)iridium-cored den-
drimer based on tetraphenylsilane dendron and 2-ethylhexyloxy surface groups was prepared. The
structure of dendrimer was confirmed by nuclear magnetic resonance, mass and infrared studies.
Thermogravimetric analysis and differential scanning calorimetry studies show the thermal stability
(ΔT5%) of 625 K with high glass transition temperature of 423 K. Photoluminescence studies of dendrimer
showed the core emission at 516 nm in solution and at 521 nm in film, indicating the highly branched
non-conjugated tetrahedral tetraphenylsilane dendrons around the core highly inhibit the intermolec-
ular interactions between the cores in the film. The solution processed green emitting phosphorescent
organic light emitting diodes based on dendrimer were fabricated and characterized. The device using
host free dendrimer as emitter showed the external quantum efficiency of 0.4%. The results show the
potential of this new type of dendritic emitting structure in highly efficient solution processed host-free
green phosphorescent organic light emitting diodes.
Received 22 July 2010
Received in revised form
6 October 2010
Accepted 14 October 2010
Available online 19 November 2010
Keywords:
Highly branched dendrimer
Iridium(III) complex
Tetraphenylsilane dendron
Green emitter
Solution processed host-free
phosphorescent OLEDs
Inhibited intermolecular interactions
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
intermolecular interactions play a crucial role in the performances
of such devices [14e18]. A number of methods such as most widely
Organic light emitting diodes (OLEDs) have attracted a lot of
attention due to their potential in full color flat panel displays as an
efficient and low cost alternative to the widely used liquid-crystal
display. When compared to fluorescent OLEDs where only singlet
states emit the light and the efficiency is reduced because of triplet
formation, phosphorescent OLEDs (PHOLEDs) are more efficient
because of both singlet and triplets can be harvested for the light
emission with close to 100% internal quantum efficiency [1e13].
Most of the phosphorescent devices were prepared by thermal
evaporation process, making the fabrication process relatively
complicated and expensive compared with the solution processed
devices. In this context, solution-processable materials, which offer
simple and cost-effective device fabrication process by spin-coating
or ink-jet printing, could provide a better manufacturing process if
simple device structures can also give high efficiencies. Although
the efficiencies of phosphorescent small molecule based OLEDs are
reasonably high, like in other organic light emitting materials,
used method being blended the emissive material with a suitable
host have been employed to control the intermolecular interactions
[5,14,19e23]. However, in a blended system, whether evaporated or
solution processed, there is always the issue of how evenly the
guest is distributed in the host.
Intermolecular interactions can also be controlled very efficiently at
the molecular level by using dendrimers [24e29]. Light emitting
dendrimers are consisting of three parts: (i) a light emitting core,
which is at the centre of the molecule and determines the light
emitting properties, (ii) dendrons, which would generally be attached
to the core chromophore and act as spacer that controls the interac-
tions between cores in the solid state by increasing the distance
between them, and (iii) surface groups, which are generally placed at
the distal ends of the dendrons and control the processing properties
of the molecules. In order to control the intermolecular interactions
effectively in dendrimers, a strategy of increasing generation number
has been utilized [25,26]. It has been reported that the intermolecular
interactions have been significantly controlled for the dendrimers with
highly efficient fac-tris(2-phenylpyridyl)iridium (III) [Ir(ppy)3] cores,
highly branched dendrons, and 2-ethylhexyloxy surface groups [30].
* Corresponding authors. Tel.: þ82 55 751 5296; fax: þ82 55 753 6311.
E-mail addresses: ykim@gnu.ac.kr (Y.-H. Kim), skwon@gnu.ac.kr (S.-K. Kwon).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.10.005

140
S.-O. Kim et al. / Dyes and Pigments 90 (2011) 139e145
Lo et al. reported that the increasing conjugation length of the ligand
associated with the emissive metal-to-ligand charge transfer state
resulted in the red-shifted emission of the core chromophore [27].
The purpose of the present work is to investigate an approach for
controlling the intermolecular interactions between the core units in
film state with reduced red-shift of the core emission, leading to be
emitter for host-free device. In this approach, a highly branched well-
known non-conjugated tetrahedral tetraphenylsilane dendron is
attached to the each ligand of emissive core in the dendrimer (G1) to
provide reduced interactions between core units in the film state
with decreased conjugation length to the core as well as high band
gap host [31]. PHOLEDs based on solution processed dendrimer (G1)
consisting of (i) core: fac-tris(2-phenylpyridine)iridium [Ir(ppy)3],
(ii) dendrons: tetraphenylsilane, and (iii) surface groups: 2-
ethylhexyloxy groups, which lead to high solubility, were fabricated
and characterized. Photoluminescence and electroluminescence
characteristics of the dendrimer (G1) and its devices were presented.
using hexane: EA (20:1) as eluent. Yield: 14 g (71%). 1H NMR (CDCl3,
ppm): 8.62 (d, 1H); 8.13 (s, 1H); 7.83 (d, 1H); 7.64 (t, 1H); 7.60 (t, 1H);
7.46 (d, 1H); 7.24 (t,1H); 7.15 (t, 1H). MS (EI) m/z: 233 (Mþ). Anal. Calcd
for C11H8BrN: C, 56.44; H, 3.44; Found: C, 56.41; H, 3.47.
2.3.2. 2-[3-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-
phenyl]-pyridine (3)
N-butyllithium (19.54 g, 28 mL) was added to a solution of 2 (15 g,
64 mmol) in anhydrous tetrahydrofuran (THF) that had been cooled
down using a dry ice/acetone bath under argon. The mixture was
stirred at 195 K for 2 h and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (13 g, 70 mmol) was added rapidly to the cold mixture.
The reaction mixture was further stirred up at 195 K for 2 h, allowed
to room temperature and stirred for 20 h. The reaction was quenched
with water and extracted with EA. Then the product was purified by
column using hexane: EA (20:1) as eluent. Yield: 6 g (33%). 1H NMR
(CDCl3, ppm): 8.71 (m, 1H); 8.40 (m, 1H); 8.14 (m, 1H); 7.87 (m, 1H);
7.80 (m, 1H); 7.76 (m, 1H); 7.51 (m, 1H); 7.23 (m, 1H); 1.37 (s, 12H).
2. Experimental
2.3.3. 1-Bromo-4-(2-ethylhexyloxy)benzene (4)
2.1. Materials
1- p-Bromophenol (17.3 g, 0.10 mol), 1-bromo-2-ethylhexane
(44.4 g, 0.23 mol) and potassium carbonate (27.6 g, 0.20 mol) were
mixed in acetone (100 mL). After refluxed for 24 h, the reaction
mixture was allowed to cool to room temperature. Then the reac-
tion mixture was poured into water and extracted with diethyl
ether. After the combined organic material was washed with
aqueous sodium hydroxide solution (10%, 100 mL) and water, the
residue was distilled under reduced pressure to afford colorless oil.
Yield: 23 g (75%). B.p. 131.8 ꢀC/2 mmHg 1H NMR (CDCl3): 7.36 (d,
2H); 6.78 (d, 2H); 3.80 (d, 2H); 1.72 (m, 1H); 1.22 w 1.58 (m, 8H);
0.90 (m, 6H). MS (EI) m/z: 284 (Mþ). Anal. Calcd for C14H21BrO: C,
58.95; H, 7.42; Found: C, 58.91; H, 7.44.
Dibromobenzene, n-butyllithium, and 2-isopropoxy-3,3,4,4-
tetramethyl-1,3,2-dioxaborolane were purchased from Aldrich. 2-Tri-
n-butylstannylpyridine and 4-bromophenol was purchased from
Lancaster. Tetrakis(triphenylphosphine)palladium was purchased
from Strem. All reagents purchased commercially were used without
further purification. Tetrahydrofuran (THF) and diethyl ether were
dried over sodium/benzophenone.
2.2. Measurements
A Genesis II FT-IR spectrometer was used to record infrared (IR)
spectra. 1H-nuclear magnetic resonance (NMR) and 13C-NMR
spectra were recorded using Avance 300 and DRX 500 MHz NMR
Bruker spectrometers and chemical shifts were reported in ppm
units with tetramethylsilane as internal standard. Matrix-assisted
laser desorption-ionization time-of-flight (MALDI-TOF) analysis
was performed on an Applied Biosystems Voyager DE-STR MALDI-
ToF (matrix; dithranol) mass spectrometer. Thermogravimetric
analysis (TGA) was performed under nitrogen using a TA instru-
ment 2050 thermogravimetric analyzer and the sample was heated
at a heating rate of 283 K/min from 323 K to 1073 K. Differential
scanning calorimeter (DSC) analysis was carried out under nitrogen
environment using a TA instrument 2100 differential scanning
calorimeter. The sample was heated at a heating rate of 283 K/min
from 303 K to 573 K. UVeVisible (UVeVis) absorption and photo-
luminescence (PL) spectra were recorded using Perkin Elmer
LAMBDA-900 UV/VIS/NIR spectrophotometer and LS-50B lumi-
nescence spectrophotometer respectively. Cyclic voltammograms
of the polymer films were recorded using an epsilon E3 cyclic
voltammeter at room temperature in a 0.1 mol solution of tetra-
butylammonium perchlorate (Bu4NClO4) in acetonitrile under
nitrogen at a scan rate of 50 mV/s. A Pt wire was used as the counter
electrode and an Ag/AgNO3 electrode as reference electrode.
2.3.4. 2-[4-(2-Ethylhexyloxy)phenyl]-4,4,5,5-tetramethyl-[1,3,2]
dioxaborolane (5)
N-butyllithium (3.8 mL, 5.66 mmol) was added to a solution of 4
(1.01 g, 3.54 mmol) in anhydrous tetrahydrofuran (7 mL) that was
cooled bya dry ice/acetone bath under argon. The mixturewasstirred
at 195 K for 1 h. 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxabor-
olane (0.87 mL, 4.26 mmol) was added rapidly to the cold mixture
and the mixture was stirred at 195 K for 2 h. Then, the mixture was
allowed to room temperature and stirred for 8 h. Then the reaction
was quenched with water and extracted with ethylacetate. The
product was obtained by recrystallization in hexane. Yied: 0.8 g
(69%). 1H NMR (CDCl3, ppm): 7.76 (d, 2H); 6.91 (d, 2H); 3.88 (m, 2H);
1.70e1.78 (m, 1H); 1.30e1.58 (m, 20H); 0.87e0.97 (m, 6H).
2.3.5. Tetrakis(4-bromophenyl)-silane (6)
Dibromobenzene (10 g, 40 mmol) and 100 mL of freshly dried
diethyl ether were added to a 250 mL 2-necked flask. When the
solutionwascooled to195Kusingdry ice/acetone bath, n-BuLi (11.7g,
40 mmol) was injected slowly and stirred for 2 h at 195 K. After the
reaction mixturewasmaintainedat 195K, silicon tetrachloride (SiCl4)
(1.13 g, 6.7 mmol) wasrapidly injected. Thenthe reactionmixturewas
stirred for 10 h at room temperature. After the reaction mixture was
poured into water, the product was extracted with diethyl ether and
dried over MgSO4. The pure product was obtained by colum-
nchromatography using hexane as eluent. Yield: 3 g (67%). 1H NMR
(CDCl3, ppm): 7.55 (d, 8H); 7.38 (d, 8H). MS (EI) m/z: 651 (Mþ). Anal.
Calcd for C24H16Br4Si: C, 44.21; H, 2.47; Found: C, 44.17; H, 2.47.
2.3. Synthesis
2.3.1. 2-(3-Bromophenyl)pyridine (2)
2-(Tri-n-butyl)stannylpyridine (31 g, 84 mmol) and 1,3-dibromo-
benzene (22 g, 93 mmol) were added to a 250 mL 2-neck flask with
anhydrous toluene (200 mL) under nitrogen environment. After Pd
(pph3)4 (0.48 g, 0.4 mmol) was added, the reaction mixture was
refluxed for 24 h. Then the reaction mixture was poured into water and
extracted with ethylacetate (EA). The product was purified by column
2.3.6. 2-{40-[Tris-(4-bromophenyl)-silanyl]-biphenyl-3-yl}-
pyridine (7)
3 (1.72 g, 6.1 mmol) and 6 (8 g, 12.3 mmol) were mixed in THF
and K2CO3 (2 mol, 15 mL). After tetrakis(triphenylphosphine)

S.-O. Kim et al. / Dyes and Pigments 90 (2011) 139e145
141
palladium (0) (Pd(pph3)4) (0.15 g, 0.13 mmol) was added, the
3. Results and discussion
reaction mixture was refluxed for 24 h. Then, the reaction mixture
was poured into 2 N HCl and extracted with ethylacetate. The pure
product was obtained by column using hexane:EA as eluent. Yield:
2 g (45%). 1H NMR (CDCl3, ppm): 8.75 (d, 1H); 8.35 (s, 1H); 8.0 (d,
1H); 7.9 (d, 2H); 7.75 (m, 3H); 7.5 (m, 9H); 7.4 (m, 6H); 7.32 (m, 1H).
MS (EI) m/z: 726 (Mþ). Anal. Calcd for C35H24Br3NSi: C, 57.87; H,
3.33; Found: C, 57.81; H, 3.37.
The dendrimer (G1) was prepared as depicted in Fig.1. The ligand
(8) was prepared via manifold chemical reaction such as alkylation,
Suzuki coupling reaction, etc as described below. 2-(3-Bromo-
phenyl)pyridine was obtained from 2-tri-n-butylstannylpyridine
and 1,3-dibromobenzene in the presence of Pd(PPh3)4 and toluene.
Then, n-butyllithium and 2-isopropxy-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane were injected slowly to a solution of 2-(3-bromo-
phenyl)pyridine and THF at 195 K to get the compound 3. The
compound 5 was prepared using the previous procedure [32].
Tetrachlorosilane and dibromobenzene were used to synthesize
tetrakis(4-bromophenyl)-silane, it was further reacted with pyri-
dine borate to give compound 7. Similarly, the compound 7 was
obtained by Suzuki coupling reaction with compound 5 to get the
ligand 8. The dendrimer (G1) with an iridium core was prepared by
a modified, one-step approach. The iridium chloride trihydrate
(IrCl3) was treated with an excess of 1,4-diazabicyclo[2,2,2]octane
(DABCO) and ligand 8 at 473 K for 72 h and served nitrogen atmo-
sphere to give dendrimer (G1). Purification of the mixture by silica
chromatography provided dendrimer (G1) as air-stable yellow
powder. The obtained dendrimer (G1) showed high solubility due
to the presence of 2-ethylhexyloxy surface groups. The structure of
the dendrimer (G1) was confirmed by 1H NMR, Mass and FT-IR
spectroscopic studies. The thermo-gravimetric analysis (TGA) of
dendrimer (G1) reveals that 5% weight-reduction temperatures
(ΔT5%) is 625 K. Especially, the differential scanning calorimetry
(DSC) of dendrimer (G1) exhibits a distinct glass transition (Tg)
around 423 K.
2.3.7. 2-(40-{Tris-[40-(2-ethylhexyloxy)-biphenyl-4-yl]-silanyl}-
biphenyl-3-yl)-pyridine (8)
5 (1.8 g, 5.5 mmol) and 7 (1 g, 1.3 mmol) were mixed in THF with
K2CO3 (2 mol, 15 mL). After tetrakis(triphenylphosphine)palladium
(0) (Pd(pph3)4) (0.06 g, 0.07 mmol) was added, the reaction mixture
was refluxed for 24 h. Then, the reaction mixture was poured into
2 N HCl and extracted with ethylacetate. The pure product was
obtained by column using hexane: EA as eluent. Yield: 1.5 g (88%).
1H NMR (CDCl3, ppm): 8.8 (d, 1H); 8.38 (s, 1H); 8.1 (d, 1H);
7.92 w 7.72 (m, 14H); 7.6e7.7 (m, 12H); 7.3 (m, 1H); 7 (d, 6H). MS
(FABþ) m/z: 1101. Anal. Calcd for C77H87NO3Si: C, 83.88; H, 7.95.
Found: C, 83.81; H, 7.99.
2.3.8. Dimer
A mixture of iridium chloride trihydrate (IrCl3) (0.13 g, 0.43 mmol)
and ligand (8) (1.5 g, 1.29 mmol) was added to a 3-neck 100 mL flask
with 2-ethoxyethanol (21 mL) and water (7 mL) under nitrogen
atmosphere. After the mixture was refluxed for 24 h, poured into
water and extracted with dichoromethane. The pure product was
obtained by column using hexane, hexane: EA, hexane: MC.
Fig. 2 shows UVevisible absorption and photoluminescence (PL)
spectra of dendrimer (G1) and fac-tris(2-phenylpyridine)iridium [Ir
(ppy)3] in CH2Cl2 solution (10ꢁ5 mol solution) at the excitation
wavelength of 381 nm. In the UVevis absorption spectrum of
2.3.9. Dendrimer (G1)
A
mixture of dimer, 1,4-diazabicyclo[2,2,2]octane (DABCO)
(1.3 g, 12 mmol), ligand (8) (0.5 g, 0.5 mmol) in glycerol (25 mL) and
tetra(ethylene glycol) (6 mL) were refluxed for 72 h. After the
reaction mixture was poured into water and extracted with
dichloromethane, the product was obtained by column using
hexane: MC (1:1). Then, the yellow solid of dendrimer (G1) was
obtained by PPT in MeOH. Yield: < 10%. 1H-NMR (CDCl3, ppm):
7.97 w 8.02 (m, 2H); 7.7 w 7.8 (m, 10H); 7.62 w 7.5 (m, 16H); 7.2 (m,
1H); 6.95 (d, 6H); 3.96 (d, 6H); 1.8 w 1.9 (m, 3H); 1.3 w 1.7 (m, 24H);
0.9 w 1.1 (m, 18H). FT-IR (KBr, cmꢁ1): 3065e3040 (aromatic C ¼ C);
2994e2877 (aliphatic CeH); 1242 (CeOeC). MALDI-ToF MS (m/z):
Calcd. For C231H258IrN3O9Si3: 3496.8827, Found: 3496.7610.
dendrimer (G1), the absorption at around 280 nm assigned to p-p*
transitions of the core ligands is increased relative to that of Ir
(ppy)3 due to the biphenyl units within the dendron. The absorp-
tion at around 331 nm is due to the absorption of tetraphenylsilane
dendron around the iridium complex core [33,24]. The absorption
band with shoulders in the lower energy region spanning from
378 nm to 500 nm is attributed to spin-allowed and spin-forbidden
metal-ligand charge transfer (MLCT) transitions of the Ir(III)
complex [34e36]. From the PL spectra of dendrimer (G1) and Ir
(ppy)3, no changes were observed in the PL emission peak (emis-
sion wavelength ¼ 516 nm) of dendrimer (G1) with slightly reduced
full width at half maximum (FWHM) of 65 nm compared with that
of Ir(ppy)3 (emission wavelength ¼ 516 nm, FWHM ¼ 72 nm). This
confirms the highly branched non-conjugated tetrahedral tetra-
phenylsilane dendron around the core provides the reduced
conjugation length to core ligand in the dendrimer (G1) and
inhibited intermolecular interactions between the core units [37].
The slightly reduced FWHM of emission spectrum of dendrimer
(G1) compared with that of Ir(ppy)3 also reveals the reduced
interactions between the emissive core units in dendrimer (G1) in
the solution. In Fig. 3, it is observed that the PL intensity (at
emission wavelength of 516 nm) of dendrimer (G1) in solution is
increased by two times compared with that of Ir(ppy)3 when the
excited wavelength was 331 nm, assigned to the dendron absorp-
tion (Fig. 2). This confirms that the energy absorbed by the den-
drons could be efficiently transferred to the emissive core in the
dendrimer (G1), leading to increased core emission. Fig. 4 shows
the PL emission of dendrimer (G1) at 516 nm in solution and at
521 nm in film without the tail emission at higher wavelengths,
which indicates that the coreecore interactions are effectively
controlled to a certain extent in the film by the highly branched
non-conjugated tetrahedral tetraphenylsilane denrons around the
2.4. Device fabrication
PHOLEDs based on synthesized dendrimer (G1) were fabricated
using the configuration: indium-tin-oxide (ITO)/Poly(3,4-ethyl-
enedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS) (40 nm)/
emissive layer (EML) (40 nm)/bathocuproine (BCP) (10 nm)/tris-
(8-hydroxyquinoline)aluminum (Alq3) (40 nm)/LiF (1 nm)/Al
(100 nm). Prior to the deposition of organic layers, ITO substrates
(anode) were degreased in acetone and IPA followed by the
UV eozone flux for 10 min. EML of Poly(N-vinylcarbazole) (PVK)
(host) blended with dendrimer (G1) (guest emitter) with the
concentration of 0.045 mmol/g for device A, 0.060 mmol/g for
device B, 0.090 mmol/g for device C and EML of G1 neat film for
device D were spin-coated on PEDOT:PSS thin film on ITO. BCP as
hole blocking layer (HBL), Alq3 as electron transport layer (ETL), LiF
as electron injection layer and Al as cathode were deposited by
thermal evaporation process under a vacuum of 6.6 ꢂ 10ꢁ6 Pa. The
electroluminescence spectra were measured at a driving current of
1 mA cmꢁ2. The current density-voltage-luminescence (J-V-L)
characteristics of PHOLEDs were carried out using Keithley 2400
source meter and Spectra Colorimeter PR650.

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S.-O. Kim et al. / Dyes and Pigments 90 (2011) 139e145
Fig. 1. Synthetic scheme of fac-tris(2-phenylpyridyl)iridium (III) cored dendrimer (G1).
core [38,39]. The PL emission of dendrimer (G1) film showed
a slight red shift (5 nm) compared to that of solution. In general,
a red shift of PL emission from solution to film was often observed,
which is attributed either to the difference in the energy transfer
processes between the film and the solution due to the presence of
processed PHOLED using the emissive layer (EML) of host-free
dendrimer (G1) neat film was fabricated using the device config-
uration of ITO/PEDOT:PSS (40 nm)/emissive layer (EML) (40 nm)/
BCP (10 nm)/tris-(8-hydroxyquinoline)aluminum (Alq3) (40 nm)/
LiF (1 nm)/Al (100 nm) (device D). The emissive layer was spin-
coated from chloroform for the film thickness of 40 nm. The
PEDOT:PSS layer was spin-coated for the thickness of 40 nm on ITO
as hole injection and transport layer. The BCP and Alq3 were used as
the effective hole blocking layer and electron transporting layer,
respectively. Indium tin oxide (ITO) was used as anode, while LiF/Al
as cathode in the devices. The current density-voltage-lumines-
cence (J-V-L) characteristics of the devices are shown in Fig. 5.
rotational conformers in the solution reducing the p-conjugation of
the chromophores, or to the effect of the packing and local geom-
etry of the materials [40]. The electrochemical properties of the
dendrimer (G1) were investigated by cyclic-voltammogram (CV)
studies. HOMO and LUMO energy levels were found to be 5.36 eV
and 2.85 eV, respectively. The bandgap (2.51 eV) for the dendrimer
(G1) was determined from the UVevisible absorption edge
(495 nm) and this is slightly higher than that of Ir(ppy)3 (2.46 eV) so
as to be used as host as well as emitter in host-free devices [41].
The dendrimer (G1) has very good film and glass-forming
properties for evaluating its electrophosphorescent ability. Solution
Luminance efficiency (hc)-current density (J)-power efficiency (hp
)
characteristics of the devices are shown in Fig. 6. The device D
showed the turn-on voltage of 4.7 V and maximum luminescence
of 67 cd/m2 at the operating voltage of 12.5V. The maximum device

S.-O. Kim et al. / Dyes and Pigments 90 (2011) 139e145
143
Fig. 5. Current density-voltage-luminescence (J-V-L) characteristics of devices A, B, C,
Fig. 2. UVevisible absorption and Photoluminescence spectra of dendrimer (G1) and Ir
and D.
(ppy)3 in CH2Cl2 at the excitation wavelength of 381 nm.
efficiencies of 0.4%, 1.2 cd/A and 0.3 lm/W were observed for the
device D with host-free dendrimer neat film as emitting layer. The
electroluminescence (EL) spectra of the devices are shown in Fig. 7.
The device D shows the electroluminescence peak at 522 nm,
which is consistent with the PL emission of dendrimer (G1) in film,
indicating EL and PL spectra are from the same excited state.
In order to compare the device performances of PHOLED using
the EML of host-free dendrimer (G1) neat film, we fabricated
similar structured-PHOLEDs using PVK: dendrimer (G1) blend as
EML. The structure was ITO/PEDOT:PSS (40 nm)/PVK: dendrimer
(G1) blend as EML (40 nm)/BCP (10 nm)/tris-(8-hydroxyquinoline)
aluminum (Alq3) (40 nm)/LiF (1 nm)/Al (100 nm). The dendrimer
(G1) concentration of 0.045, 0.060 and 0.090 mmol/g of PVK was
used for devices A, B and C, respectively. All three devices showed
the similar performances. The devices based on PVK host showed
the lower turn-on voltage (around 3.4 V) and higher current
density compared with that of device D (4.7V), indicating the
effective hole injection and transport properties of PVK host based
devices. The maximum luminescence of 1237 cd/m2 was reached
for the device A. The device A showed the EL emission peak at
516 nm, which is consistent with the PL emission of dendrimer (G1)
Fig. 3. Photoluminescence spectra of dendrimer (G1) and Ir(ppy)3 in CH2Cl2 at the
excitation wavelength of 331 nm.
Fig. 4. Photoluminescence spectra of dendrimer (G1) in CH2Cl2 and film state at the
Fig. 6. Luminescence efficiency-current density-power efficiency characteristics of
excitation wavelength of 381 nm.
devices A, B, C and D.

144
S.-O. Kim et al. / Dyes and Pigments 90 (2011) 139e145
4. Conclusion
We prepared solution processable, highly branched fac-tris(2-
phenylpyridyl)iridium (III) cored dendrimer (G1) and its structure
was confirmed by 1H NMR, mass and infrared spectroscopic studies.
The dendrimer (G1) showed the high thermal stability (ΔT5 %) of
625 K with high Tg of 423 K. The highly branched non-conjugated
tetrahedral tetraphenylsilane dendrons around the core unit
effectively inhibited the intermolecular interactions between the
core units in the dendrimer (G1) film. The device efficiencies of
0.4%, 1.2 cd/A and 0.3 lm/W were observed for the solution pro-
cessed green emitting PHOLEDs using host-free dendrimer (G1) as
emitter in the device structure of ITO/PEDOT:PSS/emissive layer/
BCP/Alq3/LiF/Al, while the device using PVK(host):dendrimer (G1)
blend as emissive layer showed the maximum efficiencies of 0.7%,
2.1 cd/A and 1.3 lm/W. The results show that this new type of
dendritic emitting structure has the potential in the highly efficient
solution processed host-free green PHOLEDs.
Acknowledgement
Fig. 7. Electroluminescence spectra of devices A, B, C and D.
This research was financially supported by MKE and KIAT
through the Workforce Development Program in Strategic Tech-
nology, by Strategic Technology Under Ministry of Knowledge
Economy of Korea and by Basic Science Research Program through
the National Research Foundation of Korea (NRF) funded by the
Ministry of Education, Science and Technology (2010-000826).
in solution. This is slightly blue-shifted by 6 nm and narrowed
compared with that (522 nm) of device D using the host-free
dendrimer neat film as EML. This can be explained by the slightly
different environment of dendrimer (G1) in PVK host compared
with that of dendrimer (G1) in film state (40). It is observed that
luminescence is increased at higher current densities for the
devices A, B, and C compared with that of device D with host-free
dendrimer neat film. The maximum device efficiencies of 0.7%,
2.1 cd/A and 1.3 lm/W were observed for the device A with den-
drimer (G1) concentration of 0.045 mmol/g of PVK, while the
device efficiencies of 0.3%, 0.5 cd/A and 0.9 lm/W were observed for
device C with higher dendrimer concentration of 0.090 mmol/g of
PVK. The device performances are decreasing upon increasing the
dendrimer (G1) concentrations in the EML of devices B and C
(Table 1tbl1tbl1) compared with that of device A without any
change in the EL emission peak position with a small increase in the
EL emission tail at higher wavelengths. This can be attributed to the
fact that upon increasing dendrimer (G1) concentration in EML,
aggregates seem to be increasing, which cause the long emission
tail and act as trapping sites [42]. However, it has also been
reported that the aggregates apparently are not operating as trap-
ping sites as the current density is rather high and the local nano-
phase crystallization can occur in solid films of small amorphous
molecule [42e44]. It is observed that the device efficiencies of the
devices A, B, and C were unusually increasing at the higher current
densities from 3 ꢂ 102 mA/cm2, which can be related to the
structural issues of the dendrimer (G1) in EML, according to the
previous reports [42,43]. This could also in our case be the origin of
such increase in the device performances of the devices A, B and C
at higher current densities.
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