Single molecular tuning of the charge balance in blue-emitting iridium dendrimers for efficient nondoped solution-processed phosphorescent OLEDs

Article abstract of DOI:10.1039/c6cc05165k

The single molecular tuning of charge balance has been demonstrated here by integrating a p-type dendron, an n-type dendron and a blue emissive Ir core into one dendritic platform. Compared with the commonly used physical blending, not only can the charge

Full text of DOI:10.1039/c6cc05165k

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Single Molecular Tuning of Charge Balance in Blue-Emitting
Iridium Dendrimers for Efficient Nondoped Solution-Processed
Phosphorescent OLEDs
Received 00th January 20xx,
Accepted 00th January 20xx
Yang Wang,ab Yaoming Lu,c Baoxiang Gao,c Shumeng Wang,a Junqiao Ding,*a Lixiang Wang,*a
Xiabin Jinga and Fosong Wanga
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
single molecular tuning of charge balance has been
At the early stage, the EML in PhOLEDs is usually composed
demonstrated here by integrating the p-type dendron, n-type of a phosphor doped into a hole-transporting (p-type) host,
dendron and a blue emissive Ir core into one dendritic platform. such as 4,4′,4′′-tris(N-carbazolyl)triphenylamine (TCTA),6 4,4′-
Compared with the commonly-used physical blending, not only N,N′-dicarbazole-biphenyl (CBP)7 and N,N′-dicarbazolyl-3,5-
the charge balance can be well tailored, but also the intrinsic benzene (mCP)8 etc. To improve charge balance, several routes
phase separation can be successfully eliminated in such developed have been proposed through a physical blending strategy. One
single molecular systems (B-TCz2TPO1 and B-TCz1TPO2). As a is introducing an additional electron-transporting (n-type) host
consequence, their corresponding nondoped solution-processed to constitute a ternary EML.9 For example, a combination of p-
PhOLEDs achieve more than doubled external quantum type TCTA with n-type 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene
efficiencies accompanied by a negligible efficiency roll-off.
(Tm3PyPB) was used to host the typical blue phosphor
iridium(III) bis(4,6-(di-fluorophenyl)-pyridinato-N,C’) picolinate
(FIrpic) by D. Ma group. A ηEQE of 18.3% at a luminance of 1000
cd/m2 was achieved, superior to the device with only TCTA as
the host.9e The other is adopting a bipolar host instead of a p-
type one to constitute a binary EML. In the case of such bipolar
hosts, p-type and n-type moieties are bound together into one
molecule through some specific linkages so that hole and
electron transporting can be regulated independently.10 For
instance, on the basis of pyridine and carbazole units, J. Kido et
Phosphorescent organic light-emitting diodes (PhOLEDs) have
been taken as one of the most competitive candidates for the
next generation full-color flat-panel displays and energy-
efficient lighting sources.1-3 Their external quantum efficiency
(ηEQE) is well described by the following equation4:
ηEQE = γ × ηr × ηPL × ηout = γ × ηPL × ηout
(1)
where γ is the ratio of the recombined carriers to the injected
carriers; ηr is the efficiency of radiative exciton production, and
is supposed to be 1 because both the generated singlet and
triplet excitons can contribute to the emission; ηPL is the
photoluminescence quantum yield of the phosphor; ηout is the
light-out-coupling constant. According to equation (1), charge
balance between holes and electrons, which is related to the
parameter γ, is necessary to obtain high ηEQE of PhOLEDs. In
addition, the balanced carrier injection and transportation
could broaden exciton recombination zone and avoid the
accumulation of high exciton density in the emitting layer
(EML), resulting in the reduced triplet-triplet annihilation (TTA)
and triplet-polaron annihilation (TPA), and thereby slow
efficiency roll-off at high luminance.5
al. designed
a
bipolar molecule 2,6-bis(3-(carbazol-9-
yl)phenyl)pyridine (26DCzPPy) as the host of FIrpic, and
realized a ηEQE as high as 22.6% at a luminance of 1000
cd/m2.10g We note that these reported promising device
efficiencies were all obtained on vacuum-deposited PhOLEDs
(v-PhOLEDs). That is, the physical blending (ternary or binary
EML) seems to be an effective way to modulate the charge
balance only in v-PhOLEDs.
On the other hand, solution-processed PhOLEDs (s-PhOLEDs)
are believed to be advantageous over v-PhOLEDs in view of the
compatibility with the inherent merits of plastic electronics in
large-area, low-cost, flexible and high-throughput device
fabrication.3,11 However, their performance is found to be far
away from that of v-PhOLEDs, even though the charge balance
has been tuned by utilizing the similar physical blending
method.10b-d Different from v-PhOLEDs, the EML in s-PhOLEDs
is formed via a simple solution process, where the involvement
of solvent may permit the deposited molecules to move
around more easily. If the mixed components are not well
miscible with each other, phase separation would occur
a.State Key Laboratory of Polymer Physics and Chemistry Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences Changchun 130022, P. R. China.
Email: junqiaod@ciac.ac.cn; lixiang@ciac.ac.cn
b.University of the Chinese Academy of Sciences Beijing 100049, P. R. China
c. College of Chemistry and Environmental Science, Hebei University, Baoding
071002, P. R. China
†
Electronic Supplementary Information (ESI) available: Experimental details,
thermal properties, optical properties and EL performance data of single-layer
devices. See DOI: 10.1039/c000000x/
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inevitably, and then deteriorate the performance and long- processed nondoped devices give a promising ηEQE of 11.5%
term stability of s-PhOLEDs.12 In order to avoid the intrinsic and 10.0% for B-TCz2TPO1 and B-TCz1TPO2, respectively,
phase segregation, therefore, much effort should be spent to more than double that of the ternary blending system (4.8%).
develop novel strategies used for charge balance modulation Importantly, benefiting from the well tuned balance between
in s-PhOLEDs.
injected holes and electrons, the ηEQE still remains at 9.8-10.9%
at a luminance of 1000 cd/m2, and only a negligible efficiency
roll-off as low as 2-5% is observed. To the best of our
knowledge, this is the first report on blue-emitting Ir
dendrimers, whose nondoped s-PhOLEDs show both high
efficiency and small efficiency roll-off simultaneously.
A newly-developed post-dendronization route was adopted
for the synthesis of the dendrimers B-TCz2TPO1 and B-
TCz1TPO2 (Scheme S1-S3).13 As reported previously,13c it is a
big challenge to directly prepare the Ir core
4 with three
hydroxyl groups at the para position relative to N atom, since
the possible resonance structure between enol and keto may
weaken the coordination ability of N atom in the C^N ligand.
Therefore, p-toluenesulfonyl (Ts) was introduced as the
protecting group of hydroxyl, which could not only well endure
the conditions of the conventional two-step complexation
reaction, but also be easily removed from the complex to
afford the corresponding reactive Ir core
the constructed second generation p-type carbazole (TCz) and
n-type diphenylphosphine oxide TPO were firstly
demethylated, and then converted to the bromide dendrons
4. At the same time,
(
)
9
and 14, respectively. With these key intermediates in hand,
finally, the target dendrimers B-TCz2TPO1 and B-TCz1TPO2
could be facilely achieved through a Williamson reaction by
simply changing the feed ratio. According to the same method,
the model complex B-G0 without any dendrons was also
synthesized for comparison. The molecular structures of B-
TCz2TPO1 and B-TCz1TPO2 were fully characterized by 1H
NMR, 19F NMR, 31P NMR and MALDI-TOF spectra (Fig. S1-S4).
Moreover, they are thermally stable with decomposition
temperatures higher than 400 oC and glass transition
temperatures higher than 190 oC (Fig. S5).
Fig.1 Experimental design of converting a physical blending into a single
dendritic molecule, both of which could be used for the charge balance
modulation. From up to down, their corresponding schematic diagrams,
chemical structures, AFM (5 μm × 5 μm) surface topographic and phase
images are shown here in sequence.
In this communication, by integrating p-type dendron, n-
type dendron and blue-emitting iridium (Ir) complex into one
dendtritic platform, we demonstrate a single molecular tuning
of charge balance without the appearance of phase
segregation. As shown in Fig. 1, carbazole and
diphenylphosphine oxide are selected as the building blocks
for p- and n-type dendrons, respectively, due to their excellent
hole and electron transporting capabilities as well as higher
The UV vis absorption spectra of the dendrimers in CH2Cl2
−
and their photoluminescence (PL) spectra in toluene at 298 K
are shown in Fig. S6. As for B-TCz2TPO1 and B-TCz1TPO2, the
intense absorption bands below 350 nm can be assigned to the
spin-allowed ligand-centered (LC) transition from the inner
1
C^N ligand14 as well as the π–π* transition from the outer
triplet energy levels than that of the emissive core B-G0
.
dendrons TCz and TPO, while the weak absorption bands in
the region of 350–450 nm are attributed to the metal-to-ligand
charge-transfer (MLCT) transition from the central Ir core.11d
As B-G0 does in toluene solutions, it is found that B-TCz2TPO1
and B-TCz1TPO2 possess very similar PL spectra with a 0-0
emission peak at 467 nm and a 0-1 emission shoulder at 490
nm. And their solution PL quantum yields (PLQYs) are about
Meanwhile, the second generation dendrons are constructed
to ensure the effective encapsulation on the Ir core and inhibit
their intermolecular interactions, which is favorable for the
realization of high performance nondoped s-PhOLEDs. Thirdly,
a non-conjugated alkyl linkage is utilized between dendron and
core to keep the relative independence of each functional
fragment. With these considerations in mind, two novel blue-
emitting Ir dendrimers bearing different ratio of p- to n-type
dendron, namely B-TCz2TPO1 and B-TCz1TPO2, have been
designed and synthesized via a convenient and controllable
post-dendronization strategy. Unlike the ternary blending
system showing obvious phase segregation, both B-TCz2TPO1
and B-TCz1TPO2 could form defect-free and homogeneous
films prepared by spin-coating. Their corresponding solution-
0.53～0.59, close to that of B-G0 (0.57). These observations
suggest that the outer dendrons linked via a non-conjugated
alkyl chain do not influence the property of the lowest excited
state. However, in solid states, a distinct hypsochromic shift is
observed for their PL spectra of B-TCz2TPO1 and B-TCz1TPO2
with respect to B-G0 (Figure S7). And the lifetime significantly
increases from 0.11 μs of B-G0 to 0.66 μs of B-TCz2TPO1 and
0.56 μs of B-TCz1TPO2 (Table S1 and Fig. S8). The blue-shifted
2 | J. Name., 2012, 00, 1-3
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emissions combined with the enhanced lifetimes are dependence of ηEQE are shown in Fig. 2, and the related data
reasonable when considering that the film luminescent are tabulated in Table S2. Both the dendrimers B-TCz2TPO1
quenching among emissive Ir cores can be effectively inhibited and B-TCz1TPO2 emit bright blue luminance mainly from the
due to the encapsulation from the surrounding bulky dendrons. central Ir core, and no emission residue from the peripheral
Cyclic voltammetry (CV) was used to study the dendrons is observed in Fig. 2a. Moreover, their Commission
electrochemical properties of the dendrimers. Upon the International de L’Eclairege (CIE) coordinates are red-shifted to
anodic sweeping in CH2Cl2, B-TCz2TPO1 and B-TCz1TPO2 show (0.18, 0.36-0.37) compared with the control device (CIE: (0.17,
two reversible oxidation processes (Fig. S9). The first oxidation 0.28)). Consistent with the literature,18 the contribution of 0-1
wave is from the central Ir core, whereas the other positive emission is responsible for the observed EL variation, caused
one is from the p-type TCz dendron.11d So their highest by the shift of exciton recombination zone away from the
occupied molecular orbital (HOMO) energy levels are cathode. This is further verified by the improved color purity of
determined to be -5.28 eV, similar to the model complex B-G0 B-TCz2TPO1 when the ETL is deliberately removed (Figure S11).
(-5.23 eV). Upon the cathodic sweeping in DMF, a novel quasi-
reversible reduction, arising from the n-type TPO dendron,
appears at a more positive potential for B-TCz2TPO1 and B-
TCz1TPO2. Compared with B-G0 (-2.26 eV), their lowest
unoccupied molecular orbital (LUMO) energy levels are
lowered to -2.48 eV, meaning that the introduction of TPO can
favor electron injection and transporting.
As we know, the film quality of the solution-processed EML
plays an important role on the performance of s-PhOLEDs.15
Therefore, before device fabrication, the atomic force
microscopy (AFM) measurements were carried out to explore
the morphology of the dendrimer films that are produced by
spin-coating from chlorobenzene solutions. As depicted in Fig.
1, the film of B-TCz2TPO1 displays quite homogeneous and
smooth morphology with
roughness of only 0.28 nm. The variation of the TCz
in dendrimer does not obviously affect the film morphology,
and a RMS roughness of 0.50 nm is observed for B-TCz1TPO2
In contrast, the corresponding ternary blending film, consisting
of B-G0 TCz and TPO at a molar ratio of 1: 2: 1, exhibits
a
root-mean-square (RMS)
/
TPO ratio
.
Fig 2. The EL spectra (a), current density-voltage (b), luminance-voltage (c)
and EQE-luminance characteristics (d) for the undoped s-PhOLEDs of the
dendrimers B-TCz2TPO1 and B-TCz1TPO2 compared with the corresponding
physical blending B-G0: TCz: TPO (1: 2: 1).
,
distinct phase segregation and much coarser surface with a
RMS roughness as high as 9.96 nm. Either the immiscibility
between the p-type TCz dendron and n-type TPO dendron
induced by their different polarity, or the aggregation of the B-
G0 complex by itself might lead to the phase separation for the
physical blending.16 Nevertheless, the observed morphology
difference does illustrate that the modulation of charge
balance at a single molecular level is far superior to the
physical blending system because the intrinsic phase
separation can be successfully eliminated.
As one can see in Fig. 2b, the current density of B-TCz2TPO1
at the same driving voltage seems to be close to that of B-
TCz1TPO2, but much higher than that of B-G0
: TCz: TPO.
Correspondingly, on going from B-G0: TCz: TPO to B-TCz2TPO1
and B-TCz1TPO2, the turn-on voltage is down from 5.2 V to 3.4
and 3.6 V, and the maximum brightness is up from 1600 cd/m2
to 13500 and 24360 cd/m2 (Fig. 2c). As a result, B-TCz2TPO1
and B-TCz1TPO2 give a state-of-art ηEQE as high as 11.5% (26.9
cd/A, 21.2 lm/W) and 10.0% (23.3 cd/A, 16.6 lm/W),
Subsequently, nondoped s-PhOLEDs were fabricated with a
configuration of ITO/PEDOT:PSS (45 nm)/B-TCz2TPO1 or B-
TCz1TPO2 (40 nm)/TPCz (8 nm)/Tm3PyPB (42 nm)/LiF (0.5
nm)/Al (100 nm) (Fig. S10). Here, PEDOT:PSS stands for
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and
is used as the hole-injection layer; TPCz stands for 3,6-
bis(diphenylphosphoryl)-9-(4’-(diphenylphosphoryl)phenyl)-
carbazole and is used as the exciton-blocking layer because of
its high triplet energy (3.0 eV);10b Tm3PyPB is used as the
electron-transporting layer (ETL) because of its excellent
electron mobility.17 For comparison, the control device was
respectively (Fig. S12). In comparison to B-G0: TCz: TPO (4.8%,
10.3 cd/A, 5.4 lm/W), the efficiency is improved by more than
two folds (Fig. 2d). As discussed above, a single molecular
system exhibits better homogeneous film morphology than
the physical blending. Hence some phase separation induced
negative effects, such as luminescence quenching, mobility
reduction and carrier balance deterioration etc.,12,19 can be
attenuated in the case of the single molecular system, thereby
leading to the improvement of device performance.
It is worthy noting that, for these single molecular systems
including B-TCz2TPO1 and B-TCz1TPO2, a low efficiency roll-off is
also achieved. At a brightness of 1000 and 5000 cd/m2, the ηEQE of
B-TCz2TPO1 still remains at 10.9% and 8.8%, respectively, which
correspond to a 5% and 23% reduction compared with the
also prepared based on the physical blending of B-G0
TPO (1: 2: 1).
: TCz:
The electroluminescence (EL) spectra, current density-
voltage-brightness characteristics together with the luminance
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Y. Wong, B. Yao, Z. Xie and L. Wang, J. Mater. Chem., 2008,
18, 1799.
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maximum value. Similarly, the ηEQE of B-TCz1TPO2 decays merely by
about 2% and 12%, respectively, even smaller than that of B-
TCz2TPO1. Given the fact that both p- and n-type dendrons are
simultaneously integrated into one dendritic molecule, the
balanced carrier flux may be responsible for the obtained slow
efficiency roll-off at high luminance. This is proved by the current
density-voltage characteristics of the single carrier devices (Fig. S13).
As can be clearly seen, B-TCz2TPO1 and B-TCz1TPO2 both reveal
considerable hole and electron current, indicative of their bipolar
transporting nature. Meanwhile, with the increasing ratio of
TPO/TCz from B-TCz2TPO1 to B-TCz1TPO2, the electron current is
gradually elevated accompanied by the decreased hole current.
Consequently, a more balanced hole and electron flux is realized in
B-TCz1TPO2 than in B-TCz2TPO1, indicating that charge balance can
be delicately modified at a single molecular level by varying the
ratio between p- and n-type dendrons. Compared with the
previously-reported blue Ir dendrimer B-G2 bearing only p-type
oligocarbazole dendrons (21.1 cd/A, 8.9 lm/W, 10.3%@1000
cd/m2),11d B-TCz2TPO1 and B-TCz1TPO2 show superior high-
brightness performance due to their well-tuned charge balance. That
is, as far as the nondoped s-PhOLEDs of B-TCz2TPO1 and B-
TCz1TPO2 are concerned, both high efficiency and negligible roll-off
have been realized simultaneously for the first time.
8
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In conclusion, by simultaneously introducing the p- and n-
type dendrons into the wedge of a blue-emitting Ir core, two
novel dendrimers (B-TCz2TPO1 and B-TCz1TPO2) have been
reported to well tune the charge balance at a single dendritic
molecule. They can form homogeneous films prepared via
spin-coating, successfully avoiding the phase separation
existed in the physical blending. So their corresponding
nondoped s-PhOLEDs achieve both high efficiency and small
efficiency roll-off at the same time. This work, we believe that,
will shed light on the development of efficient Ir dendrimers
used for solution-processed electrophosphorescent devices
with promising high-brightness performance.
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