Ruthenium(II) Complex as Phosphorescent Dopant for Highly Efficient Red Polymers Light-Emitting Diodes

Article abstract of DOI:10.1021/jp0369645

Red electrophosphorescence from light-emitting devices based on a ruthenium(II) complex [Ru(4,7-Ph₂-phen)₃] ²⁺-doped wide-band-gap semiconductive polymers, e.g., poly(vinylcarbazole) (PVK), polydihexyl-fluorene (PF), and ladder-like polyphenylene (LPPP), as the emitting layer are reported. These polymers show the short-wavelength electroluminescence emission peaking ranged from 410 to 490 nm, which overlaps well with the absorption band of [Ru(4,7-Ph ₂-phen)₃]²⁺; however, very efficient energy transfer was investigated in the PVK system, likely due to relative long excited-state lifetimes of PVK than that of PF and LPPP and good chemical compatibility of [Ru(4,7-Ph₂-phen)₃]²⁺ with PVK. The EL spectra show the characteristic spectrum of [Ru(4,7-Ph ₂-phen)₃]²⁺, with a peak at 612 nm and CIE of (0.62, 0.37) which is comparable with standard red color. The optimized device ITO/PVK 5 wt % [Ru(4,7-Ph₂-phen)₃]²⁺/PBD/ Alq₃/LiF/Al shows the maximum luminance efficiency and power efficiency of 8.6 cd/A and 2.1 lm/W, respectively.

Full text of DOI:10.1021/jp0369645

J. Phys. Chem. B 2004, 108, 3185-3190
3185
Ruthenium(II) Complex as Phosphorescent Dopant for Highly Efficient Red Polymers
Light-Emitting Diodes
Hong Xia, Chengbo Zhang, Xiaodong Liu, Song Qiu, Ping Lu, Fangzhong Shen,
Jingying Zhang, and Yuguang Ma*
Key Lab for Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130023, P. R. China
ReceiVed: October 3, 2003; In Final Form: December 26, 2003
2
Red electrophosphorescence from light-emitting devices based on a ruthenium(II) complex [Ru(4,7-Ph -
2
+
3
phen) ] -doped wide-band-gap semiconductive polymers, e.g., poly(vinylcarbazole) (PVK), polydihexyl-
fluorene (PF), and ladder-like polyphenylene (LPPP), as the emitting layer are reported. These polymers
show the short-wavelength electroluminescence emission peaking ranged from 410 to 490 nm, which overlaps
2+
well with the absorption band of [Ru(4,7-Ph
2 3
-phen) ] ; however, very efficient energy transfer was investigated
in the PVK system, likely due to relative long excited-state lifetimes of PVK than that of PF and LPPP and
2
+
good chemical compatibility of [Ru(4,7-Ph
2
-phen)
3
]
with PVK. The EL spectra show the characteristic
2+
spectrum of [Ru(4,7-Ph -phen) ] , with a peak at 612 nm and CIE of (0.62, 0.37) which is comparable with
standard red color. The optimized device ITO/PVK 5 wt % [Ru(4,7-Ph
2 3
2
+
2
3 3
-phen) ] /PBD/Alq /LiF/Al shows
the maximum luminance efficiency and power efficiency of 8.6 cd/A and 2.1 lm/W, respectively.
Introduction
or a heavily doped Ru complex/polymer film, has been
demonstrated to be in a way similar to ECL in solution as
Significant progress has been witnessed since the report on
the organic thin-film heterojunction device reinvigorated the
originally proposed by Tokel and Bard for a liquid cell.19 High-
brightness and high-efficiency emissions with low-driving
voltage of those Ru-complex-based solid-state ECL cells have
research interest and activities in organic light-emitting diodes
1
(
(
LEDs). Extensive research on organic electroluminescence
EL) in recent years is improving both the reliability and
21,23
been achieved.
However, stability of these cells are far away
from the organic EL devices, and the turn-on times to reach
maximum emission are relatively long associated with the low
mobility of the counterion in solid state. In addition, the strong
interaction between Ru complexes in the device may decrease
the emission efficiency due to the self-quenching and triplet-
efficiency of its devices, which realized its commercial use for
mobile applications. However, new materials with better proper-
ties are still needed in order to fully realize the advantages that
organic and polymer LED technology can potentially offer.
Heavy-metal complexes that enable the otherwise spin-forbidden
triplet to ground-state transition (phosphorescence), due to the
heavy atom induced spin-orbit coupling effect, are attractive
for organic light-emitting devices (OLEDs), especially for
improvement in device efficiency.2-15 It has been demonstrated
that efficient electrophosphorescence was obtained from a class
of heavy-metal complexes that feature a metal-to-ligand charge
transfer (MLCT) excited state with short lifetime phosphores-
2
6
triplet annihilation. In principle, a molecular dispersion of a
Ru complex in the semiconductive matrix and using a carrier
injection diode structure may overcome these drawbacks. These
ion-like Ru complexes are not suitable for vacuum deposition
in OLED fabrication due to decomposition in the process of
thermo-evaporation; however, some of them are well soluble
in organic solvents and, thus, can be doped into semiconductive
polymers for LEDs fabrication. In doped OLEDs, the Ru
complexes may be excited through energy transfer from the host
and/or charge trapping-induced direct recombination on the
dopant site, and they differed in their mechanism with the ECL
cell where an electrochemical redox pathway is required.19 In
this paper, we report the photoluminescence and electrolumi-
nescence characteristics of a Ru complex doped semiconductive
polymer films. The results indicate that, by a careful design of
the chemical structure of the Ru complex and device structure
as well as the choice of appropriate host materials, red-emitting
LEDs with efficiencies up to 8.3 cd/A can be achieved.
4
cence. Frequently used electrophosphorescence materials, e.g.,
PtOEP, Ir(ppy)3, and their derivatives, all feature the MLCT
electronic states. High-performance electrophosphorescence
devices are also realized from other heavy-metal ion complexes
2
-8
9
-15
with MLCT state.
Among the many MLCT metal complexes
currently under investigation, tris(bipyridine) ruthium(II) dica-
tion, Ru(bpy)32 , and their derivatives are the most typical ones.
+
The Ru complex has played the pivotal role in the development
of inorganic photochemistry in the past decades,1
6-19
and as
most active materials, it still plays a continuing role in the field
up until today.
2
0-25
The electrochemically generated lumines-
cence (ECL) of these materials in a liquid cell has been
extensively studied, and recently, there are some reports on the
use of these materials as emitters in solid-state ECL devices.
The mechanism of luminescence in these Ru-based solid
devices, which are fabricated by either a Ru complex neat film
Experimental Section
20-26
Materials. PVK, Alq3, PBD, and BCP (all from Aldrich)
were used as received without further purification. PF was
synthesized by the Suzuki coupling process according to ref
2
3
9. The LPPP was synthesized by modifying the method in ref
0, and the details of the synthesis and characteristics will be
*
To whom correspondence should be addressed. Telephone: +86 431
8
498981. Fax: +86 431 8980729. E-mail: ygma@jlu.edu.cn.
published elsewhere.
1
0.1021/jp0369645 CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/12/2004

3186 J. Phys. Chem. B, Vol. 108, No. 10, 2004
Xia et al.
Figure 1. Molecular structures of [Ru(4,7-Ph
2
-phen)
]2+, PVK, PF, LPPP, BCP, and PBD.
3
2+
Synthesis of [Ru(4,7-Ph2-phen)3] . The synthesis of [Ru-
2
+
28
(
4,7-Ph2-phen)3] was done according to the literature.
mixture of the RuCl3 precursor (208 mg) and 4,7-Ph2-phen
1.330 g) in ethanol (25 mL) was refluxed for 48 h under a
A
(
nitrogen atmosphere. The resulting mixture was evaporated to
dryness under reduced pressure. The crude product was purified
by recrystallization from a KClO4 saturated aqueous solution
1
to give bright orange crystalline powder (yield 47%). H NMR
(
(
500 MHz, DCCl3): δ8.835 (d, 2H, J ) 5.5 Hz, H6,6′), δ8.623
S, 2H, H3,3′), δ8.158 (d, 2H, J ) 6 Hz, H5,5′), δ2.55(s, 6H,
CH3). Elemental analysis calcd (%) for: C 66.66, H 3.72, N
6
.48; found: C 65.24, H 3.75, N 6.26.
UV-vis and PL Spectra. UV-vis absorption spectra were
recorded on a UV-3100 spectrophotometer. Fluorescence mea-
surements were carried out with RF-5301PC. The films for
photoluminescence (PL) experiments were formed on a pre-
cleaned quartz plate at air. Polymers were dissolved in
chloroform at a concentration of 20 mg/mL.
Figure 2. Normalized spectra of absorption (solid line) and emission
2+
2
(dash line) of [Ru(4,7-Ph -phen)
3
]
in polycarbonate film, and pho-
toluminescence spectra of PVK (triangle), PF (circle), and LPPP
(square) in neat film.
Device Fabrication. Indium-tin-oxide (ITO)-coated glass
-1
with a sheet resistance of <50 Ω 0 was used as the substrate.
three 4,7-diphenyl-1,10-phenanthroline (4,7-Ph2-phen) and two
The substrate was pre-patterned by photolithography to give
-
ClO4 counteranions was selected in this study due to its high
2
an effective device size of 4 mm . Pretreatment of ITO includes
18
emission efficiency in solution (æ ) 27% in toluene and this
a routine chemical cleaning using detergent and alcohol in
sequence, followed by oxygen plasma cleaning. Active layers
were spin-coated from chloroform solutions containing 20 mg/
mL of x% by weight [Ru(4,7-Ph2-phen)3]2 in polymer on ITO
substrates to give a film thickness of 80-100 nm. The hole
block layer BCP or PBD and electron injecting layer Alq3 were
deposited by thermo-evaporation. The cathode barium (Ba) or
LiF/Al was deposited (50 nm) by thermo-evaporation and
followed by a thick Al capping layer. The electroluminescence
and luminance were recorded on a PR65 spectrometer. Current
voltage and light intensity measurements were made at room
temperature and ambient conditions.
2
value might be increased as doped in polymer matrixes ) as
well as good solubility in organic solvents, e.g., toluene, THF,
2+
and dichloromethane. [Ru(4,7-Ph2-phen)3] shows the MLCT
+
singlet absorption band centered around 460 nm and the MLCT
triplet emission band centered at 600 nm with fresh red color.
Three blue-emitting polymers, polyvinylcarbzole (PVK), poly-
dihexylfluorene (PF), and a modified ladder-type polyphenylene
(
LPPP), were selected as the host to facilitate F o¨ rster energy
27
transfer, because their emission spectra overlap the MLCT
absorption band of [Ru(4,7-Ph2-phen)3] as shown in Figure
2
Meyer et al. PVK was obtained from TCI Co.; PF with
molecular weight of 020000 g/mol and polydispersion index
of 2.5 was synthesized and purified according reference’s
method. LPPP is a novel ladder-type polyphenylene derivative
synthesized by our groups. LPPP has a slight difference in
structure with Me-LPPP reported by Mullen et al.,30 in which
a alkyl substituted fluorene ring replaces the phenylene ring in
2+
2+
. [Ru(4,7-Ph2-phen)3] is synthesized and purified according
28
Results and Discussions
29
Figure 1 shows the chemical structure of the Ru complex
and three emitting polymer hosts used in this study, and their
electronic spectra are shown in Figure 2. The ruthenium
2+
complex, referred to here as [Ru(4,7-Ph2-phen)3] , containing

Ruthenium(II) Complex as Phosphorescent Dopant
J. Phys. Chem. B, Vol. 108, No. 10, 2004 3187
2 3
Figure 4. PLE spectra of PVK film with Ru(4,7-Ph -phen)
]2
+
2+
concentrations of 1 wt % (solid) and Ru(4,7-Ph
bonate film (dash).
2 3
-phen) ]
in polycar-
2
+
PVK in the [Ru(4,7-Ph2-phen)3] -PVK film is completely
quenched, whereas the emission spectra of the Ru(4,7-Ph2-
2
+
phen)3] -PF and -LPPP films do not have a distinctly
observed difference with spectra of 1 wt % doped concentration.
For F o¨ rster energy transfer, the efficiency is proportional to the
overlap integral between the emission spectrum of the donor
2
7
and the absorption spectrum of the acceptor. A calculation
based on spectral overlap for three Ru-doped polymer systems
gives the F o¨ rster radius R0 with the similar value of about 3.0
nm. Therefore, it is indicated that there are other factors for
achieving efficient energy transfer. One possibility is in the
difference of the exciton lifetime of polymer hosts. In three
polymer hosts, PVK showed the longest exciton lifetime of
2
+
+
Figure 3. PL spectra of [Ru(4,7-Ph
PF (b) film with different [Ru(4,7-Ph
2
-phen)
-phen)
3
]
3
doped PVK (a) and
]2 concentrations.
31
several tens of nanoseconds, whereas PF and LPPP showed
2
much shorter lifetimes of typically less than 1 ns.32 Thus, it is
reasonable that a long-lived exciton of PVK increases the
probability of reaching a low energy trap ([Ru(4,7-Ph2-
Me-LPPP. The new LPPP exhibited blue-green photolumines-
cence and electroluminescence, and the single-layer EL device
with barium (Ba) as cathode showed luminance efficiency of
.6 cd/A and maximum brightness up to 6000 cd/m . Three
polymers have been extensively applied as the host materials
for phosphorescence dopants in LEDs.
2
+
phen)3] ). The other possibility is in the difference of the
2
2+
0
chemical compatibility of [Ru(4,7-Ph2-phen)3] with three
33
polymer hosts as suggested by Noh et al. recently. They found
that the formation of aggregates prevents dopant molecules from
being in close proximity with host molecules thereby inhibiting
energy transfer processes, as in the case of Ir(ppy)3 in PF,
whereas Ir(ppy)3:PVK films were homogeneous, efficient energy
transfer from PVK to Ir(ppy)3. The efficient energy transfer in
The electronic structure and photophysic properties of Ru
2
+
complexes including [Ru(4,7-Ph2-phen)3] have been exten-
sively studied.1
6-18
The MLCT excited states are considered to
be generated by the promotion of an electron from the bivalent
Ru(II) ion-centered dπ-orbital to a π* orbital localized on the
ligand, and it has been shown that efficient light emission was
obtained as pumped by light localized at MLCT absorption band.
From this view, it seems that three polymers should transfer
their energy efficiently to [Ru(4,7-Ph2-phen)3]2 due to spectral
overlap as shown in Figure 2; however, from the PL emission
spectrum of [Ru(4,7-Ph2-phen)3]2 in these polymers as shown
in Figure 3, parts a and b, just the [Ru(4,7-Ph2-phen)3] -PVK
film showed efficient energy transfer and distinct red emission
from [Ru(4,7-Ph2-phen)3]2 peaking at 600 nm. The excitation
wavelength for the PL measurements was 345 nm, where the
absorption by [Ru(4,7-Ph2-phen)3]2+ is minimal so that most
of the excitation light is absorbed by the hosts. The energy
2
+
PVK/[Ru(4,7-Ph2-phen)3] films was further unequivocally
confirmed by photoluminescence excitation (PLE) spectra shown
in Figure 4. The PLE spectrum of [Ru(4,7-Ph2-phen)3]2
emission (600 nm) in a doped polycarbonate film showed a
distinct peak at 460 nm originating from a singlet MLCT band
+
+
2
+
of [Ru(4,7-Ph2-phen)3] itself, whereas the PLE spectrum of
+
2+
a [Ru(4,7-Ph2-phen)3] emission in a doped PVK film showed
2+
a peak at 350 nm originating from PVK. Thus, we consider
that the resonant energy transfer from PVK supplied the energy
+
2+
of red light emission from [Ru(4,7-Ph2-phen)3] molecules.
For device characteristics, at first we employed a simple
single-active-layer configuration ITO/Polymer host: x wt %
2+
[Ru(4,7-Ph2-phen)3] (80 nm)/LiF/Al. The emitting layer, [Ru-
2+
2+
transfer in the [Ru(4,7-Ph2-phen)3] -PF and -LPPP films is
(4,7-Ph2-phen)3] doped polymers (PVK, PF and LPPP), was
2+
relatively inefficient. For [Ru(4,7-Ph2-phen)3] -PF in 1 wt %
spin-cast onto the ITO surface from the chloroform solution.
The typical thickness of the [Ru(4,7-Ph2-phen)3]2 doped
polymer film was approximately 80 nm. The [Ru(4,7-Ph2-
+
doping level, only very weak red emission was observed, and
2+
for [Ru(4,7-Ph2-phen)3] -LPPP even, there isn’t any red
emission (no shown). Furthermore, as the [Ru(4,7-Ph2-phen)3]
dopant concentration increases up to 10 wt %, the emission from
2+
2+
phen)3] doping level for device fabrication was limited below
10 wt % in order to rule out the ECL process where the Ru

3188 J. Phys. Chem. B, Vol. 108, No. 10, 2004
Xia et al.
Figure 6. EL spectra of single layer device ITO/PVK: 2 wt % [Ru-
2+
(
2 3
4,7-Ph -phen) ] /LiF/Al at different bias voltages.
Figure 5. EL spectra of single layer device ITO/PVK: x % [Ru(4,7-
2
+
2 3
Ph -phen) ] /LiF/Al.
complexes need to come close to each other. The electrolumi-
nescence of undoped polymer single layer devices showed blue
emission peaking at 416 (PVK), 427 (PF) and 470 nm (LPPP)
(no shown), respectively, which is in agreement with their PL
spectra in the film state. The undoped polymers in single devices
show the luminance efficiency (LE) of LPPP (∼0.5 cd/A) >
PF (∼0.1 cd/A) > PVK (∼0.01 cd/A). However, [Ru(4,7-Ph2-
2+
phen)3] -doped polymer devices showed a reversal order in
2+
efficiency: the [Ru(4,7-Ph2-phen)3] -doped PVK device is the
2+
best, and performance of the [Ru(4,7-Ph2-phen)3] -doped PF
and LPPP devices is very poor, even very instable. It has been
observed that in a platinum(II) porphyrin (PtOX) doped semi-
conductive polymer system electron accumulation in the emit-
ting layer and trapping in PtOX induced a rapid decomposition
3
4
of PtOX due to instability of PtOX in the reduction state.
2
+
However, it may be not a case of [Ru(4,7-Ph2-phen)3] -based
devices because [Ru(4,7-Ph2-phen)3]2 exhibited reversibility
both in oxidation and reduction state in cyclic voltammetry
measurements. This indicated that the efficient energy transfer
plays a very important role for device performance. Due to poor
performance of devices of [Ru(4,7-Ph2-phen)3]2 in PF and
Figure 7. Luminance efficiency as a function of current density of 1
+
2+
wt % Ru(4,7-Ph
2
-phen)
3
)
in PVK in the single layer device.
16
electrophosphorescence can be achieved at a concentration as
low as 0.01 wt %. The absence of a PVK emission peak at
both low and high applied voltages suggests the present of a
process of charge trapping and then recombination with opposite
+
2
+
LPPP, we focus the studies on the [Ru(4,7-Ph2-phen)3] :PVK
system.
2
+
charged carriers in [Ru(4,7-Ph2-phen)3] in the Ru-based EL
device. The charge trapping, and likely combined with F o¨ rster
transfer, is the dominant EL mechanism in the Ru-based LEDs.
Figure 7 is the luminescence efficiency as a function of
The EL spectrum of single layer device changes with doping
concentration as shown in Figure 5. Most of the light was
2+
emitted from [Ru(4,7-Ph2-phen)3] in the EL devices even as
the doping concentration was lower than 0.2%. As the doping
concentration was increased up to 1 wt. %, devices show the
2
+
current density of 1 wt % Ru(4,7-Ph2-phen)3) in PVK in the
2+
single layer device. The efficiency of a [Ru(4,7-Ph2-phen)3] -
PVK single layer device with LiF/Al electrode is relative low
(e.g., LE ) 0.04 cd/A), likely due to lower electron injection
efficiency which induces a poor balance of the electron and
hole within the device. A significant enhancement in device
efficiency was achieved by using a more active metal, e.g., Ba,36
instead of LiF/Al as electrode to increase electron injection. The
2+
pure red emission from [Ru(4,7-Ph2-phen)3] , indicating that
the energy transfer is almost complete at this doping concentra-
tion. The EL spectrum changes with the applied voltage at the
same doping concentration ca. 2 wt % as shown in Figure 6.
No PVK emission was detected from devices as the [Ru(4,7-
Ph2-phen)3]2 doping level of >1 wt % and the driving voltage
ranged from turn-on (about 11 V) to 21 V. Compared to PL in
the same doping level, the triplet emission of [Ru(4,7-Ph2-
phen)3]2 centered at 612 nm is distinctly enhanced in the EL
device. That triplet contribution in total emission was enhanced
in the EL device compared to photoexcitation seems the
common phenomena in electrophosphorescence devices, espe-
cially with polymers as hosts. The investigations by Cleave et
+
2
+
1 wt % [Ru(4,7-Ph2-phen)3] -PVK device of Ba as electrode
shows an efficiency of 0.21 cd/A, which is five times that of
the LiF/Al device. The performance of the present device
changed with the change of electrodes indicating again that the
emission mechanism of the present device is different with the
ECL cell of which the performance is independent of the work
+
2
2,23
function of electrodes.
A measurement of device response
35
36
7
al., Cao et al., and Gong et al. for various phosphorescence
dye doped polymer systems also noted that the efficient
as upon a driving voltage (Figure 8) shows that a response time
to reach a maximum of light emission is less than 0.02 s (an

Ruthenium(II) Complex as Phosphorescent Dopant
J. Phys. Chem. B, Vol. 108, No. 10, 2004 3189
Figure 10. J-L-V characteristics of device ITO/PVK: 5 wt % [Ru-
2+
(4,7-Ph
2
-phen)
3
]
3
(80 nm)/PBD (50 nm)/Alq (45 nm)/LiF (1.8 nm)/
Figure 8. Light emission as a function of time of driving field applied
for device ITO/PVK: 5 wt % [Ru(4,7-Ph -phen) ] /LiF/Al.
2 3
Al (340 nm).
2
+
TABLE 1: Performance of Devices of 5 wt % PBD as Hole
Block Layer with the Different Thickness of Alq
3
and PBD
brightness
PBD
nm)
Alq
(nm)
3
turn-on
(V)
at 20V
(cd/m2)
max LE
(cd/A)
max PE
(lm/W)
(
1
3
4
4
4
4
5
5
3
6
0
0
0
0
0
0
9
18
17
27
35
45
45
60
17
3
41
67
93
53
143
76
60
0.18
1.53
0.93
1.78
1.93
7.10
8.63
8.32
0.033
0.36
0.28
0.37
0.39
1.72
2.08
2.46
11.6
10.6
11
10.5
9.8
11
10.4
device was 2.1 lm/W at 13 V. At the higher brightness of 100
2
cd/m , LE was 2.6 cd/A and the power efficiency was about
Figure 9. Luminance efficiency versus current density of multilayer
0
.39 lm/W. Note that the thickness of Alq3 and PBD are found
to greatly impact on device efficiency, which are ranged from
.18 to 8.6 cd/A (Table 1); however, device performance is
2+
device ITO/PVK:[Ru(4,7-Ph
2 3 3
-phen) ] /PBD/Alq /LiF/Al with Ru(4,7-
2
+
2 3
Ph -phen) ]
concentrations of 0.2%, 1%, 5 wt %.
0
relatively stable as the combination of PBD 40-50 nm and Alq3
40-60 nm. The device gave red emission with the peak
wavelength of 612 nm and the full width at half-maximum was
90 nm. The color coordinates in the Commission Internationale
del’Eclairage (CIE 1931) chromaticity chart are coordinates of
(0.62, 0.37) at 13 V, and the CIE coordinates remained nearly
constant from 13 to 19 V. At a voltage up to 26 V, the weak
emission band ranged from 400 to 550 nm likely from PBD
and Alq3 layers appeared (Figure 11a), which induced a color
shift to the red-orange spectral region (CIE;0.55, 0.38).
up-limitation of minimum time detected by RF-5301PC spec-
trophotometer in time-based determination), demonstrating that
the device operation is most likely according to a diode-like
3
7
mechanism, but a mobility of the counterion in ECL devices.
Furthermore, a great increase in efficiency is achieved in the
multilayer device by inserting an electron injection layer (Alq3)
and hole block layer (PBD) between the cathode and emitting
layer.38 Figure 9 shows LE-current density curves of different
2
+
[
Ru(4,7-Ph2-phen)3] doping concentration in the multilayer
2+
device ITO/PVK: x wt % [Ru(4,7-Ph2-phen)3] (80 nm)/PBD-
50 nm)/Alq3 (45 nm)/LiF (1.8 nm)/Al (340 nm). The device
The other hole-block materials bathocuprine (BCP), which
was frequently used in small molecule electrophosphorescence
devices as hole bock and triplet exciton limiting layers, was
(
2+
with 5 wt % [Ru(4,7-Ph2-phen)3] in PVK as emitting layer
gives the highest EL efficiency than that of other doping
concentrations. The current density (J) and luminance (L) versus
applied voltage (V) characteristics of the 5% [Ru(4,7-Ph2-
phen)3]2 doped PVK multilayer device is shown in Figure 10.
2
+
also tried in the present [Ru(4,7-Ph2-phen)3] -PVK device.
The performance of devices of 5 wt % [Ru(4,7-Ph2-phen)3]2
in PVK using BCP as hole block layer are tabulated in Table
2. The devices show the maximum LE ) 4.1 cd/A and PE )
1.0 lm/W, which was about half of the efficiency of the PBD
device. However, the BCP devices show significant improve-
ment in spectral stability. Figure 11b showed the EL spectra of
+
+
The turn-on voltage of this multilayer device is approximately
2
1
0
0 V. The device has a luminance of 140 cd/m at 21 V and of
2
500 cd/m at 26 V. The efficiency first increases and then
2
+
decreases with increasing current density, similar to the behavior
reported for other polymer based electrophosphorescence de-
5 wt % [Ru(4,7-Ph2-phen)3] in the PVK device using BCP
as a hole block layer. The pure [Ru(4,7-Ph2-phen)3]2 emission
is observed even at high driving voltage indicating a complete
blocking of carriers (hole) and excited energy in [Ru(4,7-Ph2-
+
7
,11,34
vices.
The maximum LE ) 8.6 cd/A, which was among
the highest reported for red OLEDs in the literature, was
achieved at a current density of 0.05 mA/cm and at a brightness
of 4.1 cd/m . The LE value is approximately 6 cd/A at the higher
2
2+
phen)3] doped PVK layers. The CIE coordinates of the BCP
2
device are (CIE; 0.6153, 0.3694) at a low driving voltage (13
V) and (CIE; 0.6036,0.3698) at a high driving voltage (25 V).
current density and brightness. The power efficiency of this

3190 J. Phys. Chem. B, Vol. 108, No. 10, 2004
Xia et al.
Acknowledgment. We are thankful for support by the
National Science Foundation of China. (Grant Nos. 20125421
and 90101026) and by the Ministry of Science and Technology
of China (Grant No. 2002CB6134003).
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