Oligosiloxane functionalized with pendant (1,3-Bis(9-carbazolyl)benzene) (mCP) for solution-processed organic electronics

Article abstract of DOI:10.1002/chem.201402374

A new oligosiloxane derivative (ODCzMSi) functionalized with the well-known 1,3-bis(9-carbazolyl)benzene (mCP) pendant moiety, directly linked to the silicon atom of the oligosiloxane backbone, has been synthesized and characterized. Compared to mCP, the attachment of the oligosiloxane chain significantly improves the thermal and morphological stabilities with a high decomposition temperature (T_d = 540°C) and glass transition temperature (T_g = 142°C). The silicon-oxygen linkage of ODCzMSi disrupts the backbone conjugation and maintains a high triplet energy level (E_T=3.0eV). A phosphorescent organic light-emitting diode (PhOLED) using iridium bis(4,6-difluorophenyl)pyridinato-N,C² picolinate (FIrpic) as the emitter and ODCzMSi as the host shows a relatively low turn-on voltage of 5.0 V for solution-processed PhOLEDs, maximum external quantum efficiency of 9.2%, and maximum current efficiency of 17.7 cdA^-1. The overall performance of this device is competitive with the best reported solution-processed blue PhOLEDs. Memory devices using ODCzMSi as an active layer exhibit non-volatile write-once read-many-times (WORM) characteristics with high stability in retention time up to 10⁴s and a low switch on voltage. This switching behaviour is explained by different stable conformations of ODCzMSi with high or low conductivity states which are obtained under the action of electric field through a π - π stacking alignment of the pendant aromatic groups. These results with both PhOLEDs and memory devices demonstrate that this oligosi-loxane-mCP hybrid structure is promising and versatile for high performance solution-processed optoelectronic applications.

Full text of DOI:10.1002/chem.201402374

DOI: 10.1002/chem.201402374
Full Paper
&
Organic Electronics
Oligosiloxane Functionalized with Pendant (1,3-Bis(9-
carbazolyl)benzene) (mCP) for Solution-Processed Organic
Electronics
Dianming Sun,[a] Zhaomin Yang,[a] Zhongjie Ren,*[a] Huihui Li,[a] Martin R. Bryce,*[b]
Dongge Ma,[c] and Shouke Yan*[a]
Abstract: A new oligosiloxane derivative (ODCzMSi) func-
tionalized with the well-known 1,3-bis(9-carbazolyl)benzene
(mCP) pendant moiety, directly linked to the silicon atom of
the oligosiloxane backbone, has been synthesized and char-
acterized. Compared to mCP, the attachment of the oligosi-
loxane chain significantly improves the thermal and morpho-
logical stabilities with a high decomposition temperature
(Td =5408C) and glass transition temperature (Tg =1428C).
The silicon–oxygen linkage of ODCzMSi disrupts the back-
bone conjugation and maintains a high triplet energy level
(ET =3.0 eV). A phosphorescent organic light-emitting diode
(PhOLED) using iridium bis(4,6-difluorophenyl)pyridinato-
N,C2 picolinate (FIrpic) as the emitter and ODCzMSi as the
host shows a relatively low turn-on voltage of 5.0 V for solu-
tion-processed PhOLEDs, maximum external quantum effi-
ciency of 9.2%, and maximum current efficiency of
17.7 cdAꢀ1. The overall performance of this device is com-
petitive with the best reported solution-processed blue PhO-
LEDs. Memory devices using ODCzMSi as an active layer ex-
hibit non-volatile write-once read-many-times (WORM) char-
acteristics with high stability in retention time up to 104 s
and a low switch on voltage. This switching behaviour is ex-
plained by different stable conformations of ODCzMSi with
high or low conductivity states which are obtained under
the action of electric field through a p–p stacking alignment
of the pendant aromatic groups. These results with both
PhOLEDs and memory devices demonstrate that this oligosi-
loxane–mCP hybrid structure is promising and versatile for
high performance solution-processed optoelectronic applica-
tions.
Introduction
of PhOLEDs and the precise triplet level of PVK is ambigu-
ous;[19] moreover, its low-lying HOMO energy level (ꢀ5.9 eV)
and high resistivity lead to a high operating voltage, so it is far
from an ideal host material.[20] As a conductance switching ma-
terial, PVK has an aliphatic hydrocarbon backbone that exhibits
low dimensional stability. For another example, 1,3-bis(9-carba-
zolyl)benzene (mCP) is one of the major milestones as a small-
molecule host of choice for vacuum deposited blue PhO-
LEDs.[21,22] mCP has the benefits of a simple molecular structure
and a triplet energy of 3.0 eV. However, mCP has the draw-
backs of low thermal and morphological stability (with a glass
transition temperature Tg of only 608C) which limit its use in
high-performance blue PhOLEDs for real applications and com-
mercialization. Thus, it remains a great challenge to develop
new materials with high thermal, electrochemical, and mor-
phological stability, and good film-forming ability as hosts for
PhOLEDs and as active layers in non-volatile memory devices.
From this viewpoint, functionalized PVK derivatives show im-
proved electron-transporting ability, thermal and morphologi-
cal stabilities as compared to PVK, such as poly(N-vinyl-3-(9-
phenylfluoren-9-yl)carbazole (PVPFK).[17,23,24] Efforts have also
been made to improve the thermal stability of mCP while still
maintaining the molecules’ desirable optoelectronic proper-
ties.[21,25–27] Organic–inorganic hybrid materials are one of the
best solutions for ideal host materials because the functionality
Carbazole-based optoelectronic materials continue to attract
widespread attention for their hole-transporting properties,
good thermal stability and versatile chemical modification.[1–7]
Typically, the semiconductor poly(N-vinylcarbazole) (PVK) the
good hole-transporting properties of which are attributed to
the intra-chain p–p stacking alignment, has been widely used
in organic photoconductive materials,[8] as the host material
for green OLEDs and blue phosphorescent OLEDs,[9–14] and con-
ductance switching materials.[15–18] However, PVK forms exci-
plexes and triplet trap states which can quench blue emission
[a] Dr. D. Sun, Dr. Z. Yang, Dr. Z. Ren, Dr. H. Li, Prof. S. Yan
State Key Laboratory of Chemical Resource Engineering
Beijing University of Chemical Technology, Beijing 100029 (P.R. China)
E-mail: renzj@mail.buct.edu.cn
skyan@mail.buct.edu.cn
[b] Prof. M. R. Bryce
Department of Chemistry, Durham University, Durham, DH1 3LE (UK)
E-mail: m.r.bryce@durham.ac.uk
[c] Prof. D. Ma
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences
Changchun, 130022 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402374.
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and structural strength are easily achieved by design of the or-
ganic and the inorganic parts, respectively.[28–33] For instance,
3,5-di(N-carbazolyl)tetraphenylsilane (SimCP) is a good host for
blue PhOLEDs: by linking the bulky triphenylsilyl group to the
carbazole moiety in a non-conjugated way, the thermal stabili-
ty is improved and steric hindrance favourably affects the mo-
lecular packing in the condensed phase.[35,36]
(methyl)silyl-derivative 2 was achieved by a Barbier–Grignard
reaction with methyldiethyoxylchlorosilane. (3,5-Di(9H-carba-
zol-9-yl)phenyl)(methyl)silanediol (3) was obtained by subse-
quent acid hydrolysis in dilute THF/HCl solution at 08C. The
desired oligomer 4 was easily synthesized from the corre-
sponding diethoxy monomer (OEt-Si) and dihydroxy monomer
(OH-Si) under tetrabutyl titanate (TBOT) catalysed polyconden-
sation as described in the Experimental Section. The end
groups were blocked with trimethylchlorosilane to stabilize the
resultant oligosiloxane. After completion of the reaction, the
solution was simply concentrated and the product 4 precipitat-
ed in methanol as a white powder, which was characterized by
1H, 29Si NMR spectroscopies and gel permeation chromatogra-
phy (GPC). The weight average molecular weight was deter-
mined to be 2.13 kDa with a narrow polydispersity index of
1.13 by GPC in THF using polystyrene as a standard. Product 4
is highly soluble in common organic solvents such as chloro-
form, tetrahydrofuran, toluene and chlorobenzene. Therefore,
it can be easily fabricated into films by solution casting, spin-
coating and dipping techniques which are advantageous for
organic electronics applications.[40]
Polysiloxanes are a fascinating class of organosilicon poly-
mers with good solubility in common organic solvents, good
film-forming ability, fair adhesion to various substrates and ex-
cellent resistance to thermal, chemical and irradiation degrada-
tion. To improve thermal, electrochemical, and morphological
stability and film-forming ability of organic semiconductors, in-
corporation of siloxanes into organic molecular structures has
been successful.[30] This strategy does not significantly affect
the electronic properties of the organic semiconductor units.
Strohriegl synthesized a side-chain carbazole polymer with a si-
loxane main chain (PSX), which displays better performance
than PVK, including greater conformational freedom, good so-
lution processability and minimized excimer formation upon
photoexcitation.[36] However, this method adopted a common
synthetic pathway to post-polymerization functionalization
which gave a low content of carbazole groups in the polymer
chains, which would not be applicable in high performance
conductance switching materials.
Thermal analysis
Herein, we successfully incorporate 1,3-bis(9-carbazolyl)ben-
zene (mCP) moieties into oligosiloxane chains as pendant
groups to obtain the new mCP modified oligosiloxane
(ODCzMSi). ODCzMSi possesses a sufficiently high ET of 3.0 eV
because the silicon-oxygen bond interrupts p-conjugation in
the polymer main chain.[37–39] This material discourages self-
quenching of triplet excitons by preventing phosphorescent
dyes from aggregating. In addition, ODCzMSi possesses a high
degradation temperature, excellent film-forming ability and
good compatibility with the standard blue phosphorescent
emitter FIrpic. An efficient blue PhOLED with a solution-pro-
cessed emitting layer of ODCzMSi host and FIrpic (10% by
weight) displayed a maximum current efficiency of 17.7 cdAꢀ1
The thermal properties of OCzMSi were characterized by ther-
mal gravimetric analysis (TGA) and differential scanning calo-
(7.7 lmWꢀ1, external quantum efficiency 9.2%) at 1306 cdmꢀ2
.
Additionally, memory devices using ODCzMSi as the active
layer exhibit non-volatile write-once read-many-times (WORM)
characteristics with low turn-on threshold voltages. ODCzMSi
has, therefore, the very attractive properties of both a solution-
processable host for blue PhOLEDs and an active material for
memory devices, indicating its considerable versatility in or-
ganic electronics.
Results and Discussion
Synthesis and characterization
The synthetic route of ODCzMSi (4) is depicted in Scheme 1.
The starting material 9,9’-(5-bromo-1,3-phenylene)bis(9H-carba-
zole) (1) was synthesized by a modified Ullmann condensation
of carbazole with 1,3,5-tribromobenzene in the presence of
copper(I) iodide as a catalyst and 18-crown-6 as a ligand in o-
dichlorobenzene. Transformation of
1
into the diethoxy-
Scheme 1. Synthetic route for ODCzMSi (4).
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smooth and homogeneous morphology with a small value of
root-mean-square (RMS) roughness of 0.33 nm. It is free of par-
ticle aggregation or phase separation, suggesting both good
film-forming ability and good miscibility with the FIrpic. To fur-
ther investigate the thermal stability, the film was then an-
nealed at 1408C, which is almost the same temperature as
with the corresponding Tg of ODCzMSi, for 12 h. As shown in
Figure 2b, the surface roughness changed a little from 0.33 to
0.48 nm, however, with increasing time (up to 48 h) at the
same temperature, no further change can be observed (Fig-
ure 2c and d). The excellent stability of the film morphology,
which is due to the siloxane backbone, means that films
should retain their integrity throughout the fabrication and
device-operation processes.
Table 1. Physical properties of ODCzMSi.
ET
[eV][a]
HOMO
[eV][b]
LUMO
[eV][c]
Tg
[8C][d]
Td
[8C][e]
3.0
ꢀ5.65
ꢀ2.09
142
540
[a] Triplet energy level. [b] Highest occupied molecular orbital. [c] Lowest
unoccupied molecular orbital. [d] Glass-transition temperature deter-
mined by DSC. [e] Decomposition temperature determined by TGA at 5%
weight loss.
rimetry (DSC) under a nitrogen atmosphere and are summar-
ized in Table 1.
As shown in Figure 1, the decomposition temperature (Td),
which corresponds to a weight loss of 5% at a heating rate of
108Cminꢀ1 during TGA, was measured as 5408C for ODCzMSi,
which implies that the material has very good thermal stability.
Meanwhile, a higher glass transition temperature (Tg) of 1428C
(compared to 608C for mCP) was determined by DSC during
the second heating cycle. In addition, no exothermic crystalli-
zation or endothermic melting peaks were observed from 40
to 2008C, suggesting that amorphous ODCzMSi should exhibit
high morphological stability in a device. The excellent thermal
stability of ODCzMSi is ascribed to the oligosiloxane backbone,
which should be capable of enduring the inevitable Joule
heating that occurs during device operation.
Figure 2. AFM topographical images of the solution-processed films of
ODCzMSi doped with 10% FIrpic: a) unannealed, b) annealed at 1408C for
12 h, c) annealed at 1408C for 24 h, and d) annealed at 1408C for 48 h.
Photophysical properties
UV/Vis absorption, photoluminescence (PL; recorded at room
temperature) and phosphorescence spectra (recorded at 77 K)
of ODCzMSi in dilute dichloromethane solution are presented
in Figure 3a. The absorption peaks in the UV/Vis spectrum at
around 293 nm can be assigned to p!p* transitions of the
carbazole moiety and weaker absorption peaks at longer wave-
lengths of 326 and 339 nm are attributed to the n!p* transi-
tions of extended conjugation of the carbazole moiety.[25] In
addition, a relatively wide energy gap (Eg) of 3.56 eV is ob-
tained from the onset of absorption. The photoluminescence
(PL) spectrum of displays intense UV emission peaks at 346
and 363 nm. The absorption and emission spectra of ODCzMSi
are nearly identical to those previously reported for phenylcar-
bazole structure and mCP,[38,39,41–43] suggesting that the oligosi-
loxane backbone does not significantly influence the photo-
physical properties of the organic chromophore. The phos-
phorescence spectrum in a frozen dichloromethane solution at
77 K revealed a well-structured band in the region of 400–
500 nm. An ET of 3.0 eV was calculated from the highest
energy vibronic sub-band of the phosphorescence spectrum;
Figure 1. a) TGA trace of ODCzMSi recorded at a heating rate of 108Cminꢀ1
.
b) DSC measurement recorded at a heating rate of 108Cminꢀ1
.
Morphology properties
The film-forming ability, morphological stability of ODCzMSi
and its compatibility with the dopant FIrpic were investigated
by atomic force microscopy (AFM). As shown in Figure 2a, the
AFM image of 10 wt% FIrpic doped into ODCzMSi film displays
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Figure 4. Calculated spatial distributions of the HOMO and LUMO energy
densities of ODCzMSi.
desirable properties of mCP while significantly enhancing the
thermal and morphological stability of the material.
Blue phosphorescent organic light emitting diode
To evaluate the ability of ODCzMSi to act as a host material for
solution-processed blue PhOLEDs, we chose FIrpic (ET 2.6 eV)[45]
as the standard blue phosphor. Devices were fabricated with
the following configuration: indium tin oxide (ITO)/PEDOT:PSS
(40 nm)/host/10 wt% FIrpic (40 nm)/1,3,5-tri(m-pyrid-3-yl-phe-
nyl)benzene (Tm3PyPB, 5 nm)/1,3,5-tris(1-phenyl-1H-benzimida-
zol-2-yl)benzene (TPBi, 30 nm)/LiF (1 nm)/Al (100 nm). In this
architecture, PEDOT:PSS acts as a hole-injection layer, the host
doped with 10 wt% FIrpic is spin-coated to form an emitting
layer, and Tm3PyPB[46] and TPBi function as the hole/exciton-
blocking and electron transporting layers, respectively. The rel-
ative energy levels of each layer are presented in Figure 5a.
The ET values of ODCzMSi and Tm3PyPB are sufficiently high to
prevent luminescence quenching by the carrier transport layer
and to confine triplet excitons in the emitting layer. As shown
in Figure S1 in the Supporting Information, the electrolumines-
cent (EL) spectra of the blue PhOLED shows emission exclu-
sively from FIrpic (lmax =472 nm), without any contribution
from ODCzMSi. This indicates that reverse transfer of triplet ex-
citons from phosphor to host is prevented in this device. Im-
portantly, a stable emission spectrum is observed throughout
the operating voltage (6–10 V). The intensity of the emission
shoulder around 500 nm increases when the charge recombi-
nation zone shifts to the side of the hole-transporting layer. As
the operating voltage increased, the intensity of this shoulder
is enhanced, which indicates that the emission zone is shifted
away from the electron-transporting layer because of the im-
proved electron flow.
Figure 3. a) Absorption, photoluminescence (at room temperature) and
phosphorescence spectra (at 77 K) of ODCzMSi in dichloromethane. b) Cyclic
voltammogram of ODCzMSi in CH2Cl2 for oxidation scan.
this is the same value as mCP and is consistent with the dis-
ruption of p conjugation caused by the siliconꢀoxygen bond
between the mCP groups.
Electrochemical analysis
The electrochemical behaviour of ODCzMSi was investigated
by cyclic voltammetry (CV) using tetra(n-butyl)ammonium hex-
afluorophosphate (0.1m) as a supporting electrolyte in anhy-
drous acetonitrile under a nitrogen atmosphere. In Figure 3b,
the CV shows an irreversible oxidation feature similar to those
of other carbazole derivatives with unprotected 3,6-posi-
tions.[16,28] The highest occupied molecular orbital (HOMO)
energy level of ODCzMSi was calculated according to the for-
mula EHOMO =ꢀ(4.8+EooxnꢀEFc) eV.[29] The lowest unoccupied mo-
lecular orbital (LUMO) energy level was obtained by adding
the optical Eg to the HOMO level (as shown in Table 1). The
HOMO level of ODCzMSi is ꢀ5.65 eV, which is very similar to
that of mCP and 4,4’-N,N’-dicarbazolylbiphenyl (CBP).[44] This in-
dicates that the HOMO of ODCzMSi is mainly located at the
electron-rich carbazole units. Quantum chemical calculations
on a dimer model of ODCzMSi at the B3LYP/6-31G* level gave
the contour plots depicted in Figure 4. The HOMO is localized
on the carbazole group, while the LUMO is mainly localized on
the phenyl group attached to the carbazole. There is no fron-
tier molecular orbital distribution on the siliconꢀoxygen bond.
All the above data establish that the presence of the siloxane
chain has a negligible effect on the electronic transition ener-
gies and energy levels of the mCP units, thus maintaining the
It is notable that the highest external quantum efficiency for
the device is 9.2% as shown in Figure 5c. Power and current
efficiencies plotted with respect to current density are depict-
ed in Figure 5d, and all the device data are summarized in
Table 2. The device shows a current efficiency of 17.7 cdAꢀ1
and maximum power efficiency of 7.7 lmWꢀ1, which are ob-
served over a wide range of high luminance. The overall per-
formance of this device is excellent and is competitive with
the best blue PhOLEDs using solution processed host materi-
als.[47,48] The efficiency is considerably enhanced compared to
analogous optimized FIrpic devices with PVK as the host,
which gave maximum values of 1640 cdmꢀ2
3.3 lmWꢀ1; hext 3.3%.[14]
; ;
6.3 cdAꢀ1
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ode and Au as anode) was ap-
plied. Initially a low-conductivity
(OFF) state (current density level
~10ꢀ6 mAcmꢀ2) was observed.
As the negative bias increased,
a sharp transition from the low-
conductivity (OFF) state to
a high-conductivity (ON) state
was observed at ꢀ0.91 V (the
switch-ON voltage), as indicated
by the abrupt increase in the
current density (sweep 1). For
a memory device, this transition
from the OFF- to the ON-state
could serve as the “writing” pro-
cess for practical data storage
applications. Once the device
has reached its ON-state, it could
retain its ON-state when the
voltage sweep was repeated
from 0 to ꢀ3 V (sweep 2). Even
when the power was shut off or
during the reverse voltage
sweep (sweep 3), the ON-state
was still retained (sweeps 4 and
Figure 5. a) Relative energy levels of the materials used in the 10 wt% FIrpic-doped PhOLED; b) J–V–L characteris-
tics; c) external quantum efficiency versus current density; d) power efficiency and current efficiency versus cur-
rent density.
Table 2. Performance of the FIrpic-ODCzMSi-based PhOLED device.
Von
[V][a]
Lmax
[cdmꢀ2
hc,max
[cdAꢀ1
hp,max
[lmWꢀ1
hext,max
[%][e]
CIE(x,y)[f]
[b]
[c]
[d]
]
]
]
5.0
4730
17.7
7.7
9.2
0.15,0.33
[a] Turn-on voltage, measured at a brightness of 1 cdmꢀ2. [b] Maximum
luminance. [c] Maximum luminous efficiency. [d] Maximum power efficien-
cy. [e] External quantum efficiency. [f] Commission Internationale de
l’ꢁclairage coordinates measured at 8 V.
To further verify the exciton confinement property of the
host, transient photoluminescence decays were measured at
a wavelength of 475 nm for a thin film (formed on quartz sub-
strates) with 10 wt% FIrpic dispersed in ODCzMSi. As shown in
Figure 6, the emission displays nearly mono-exponential decay
curves, indicating that the triplet energy transfer from FIrpic to
ODCzMSi is completely suppressed and the energy is well con-
fined in the emission layer. Meanwhile, the decay curve of the
corresponding film annealed at 1408C for 24 h was essentially
unchanged which is consistent with high thermal stability of
the FIrpic-ODCzMSi film.
Figure 6. Transient photoluminescence decay (excited at 343 nm) curves at
room temperature monitored at 475 nm for the unannealed 10 wt% FIrpic
co-deposited with ODCzMSi, and the corresponding film annealed at 1408C
for 24 h.
5), meeting the features of a WORM (write-once-read-many-
times) device. Further experiments on duplicated devices re-
vealed that the J–V characteristics were repeatable with good
accuracy, and device degradation was not observed. Further-
more, the almost linear current–area dependence of both the
OFF- and ON-states when the active device area was reduced
(Au electrode)—giving almost constant current densities—indi-
cates the absence of sample degradation and dielectric break-
down. The observed conductance switching can thus be fully
attributed to the change in the material properties of the
ODCzMSi layer upon applying an external bias. It is noteworthy
that the on-switching voltage was only ꢀ0.91 V which facilities
Memory device characteristics
To investigate the applicability of ODCzMSi in non-volatile
memory devices, we fabricated a sandwich device with a con-
figuration of ITO/ODCzMSi/Au. Figure 7 shows the typical cur-
rent density–voltage (J–V) characteristics and stability tests of
this device. As depicted in Figure 7a, the device could be
switched on when a negative voltage sweep (with ITO as cath-
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low-power consumption. The electrical stability performance
was also carried out for the memory device under ambient air
conditions. Figure 7b shows the retention time test under
a constant voltage for the OFF- and ON-states. Under a con-
stant bias of ꢀ0.5 V, no significant degradation in current is ob-
served for both the ON- and OFF-states with longer operating
time (8000 s); the device maintained an ON/OFF current ratio
of about 2.5ꢂ103. Moreover, this non-volatile irreversible elec-
trical switching behaviour was also observed when scanning
the device first with positive bias (with ITO as anode and Au as
cathode), indicating its independence on the voltage polarity.
However, the switching voltage is slightly increased to around
1.26 V and the ON/OFF current density ratio is reduced to
about 300, as shown in Figure 7c and d.
conductivity states. The regular p–p stacking conformation of
the high-conductivity states should be converted into a disor-
dered state by thermal annealing. Thus, we chose 1408C,
which is close to the Tg of ODCzMSi where its chain segments
should have higher flexibility. When the device with high-con-
ductivity states was annealed at 1408C for 12 h, the previous
sustained high-conductivity (ON) state disappeared as shown
in Figure 8. Instead its initial low-conductivity (OFF) state from
0 V to ꢀ1.01/1.59 V is observed, and then there is a sharp in-
crease, indicating another transition of the device from the
low-conductivity to a high-conductivity state. Once the an-
nealed device is switched on, it remains there and cannot be
returned to its initial low conductivity state. This suggests that
the stored data can be erased by thermal treatment. The rever-
sibility of the process confirms
the proposed conformation
change mechanism. In addition,
comparing this non-volatile elec-
trical switching behaviour of the
annealed device with that of the
non-written device, some differ-
ence was observed. With a nega-
tive bias sweep, the switching
voltage is slightly increased to
around ꢀ1.01 V and the ON/OFF
current density ratio is increased
to about 7ꢂ103 as shown in Fig-
ure 8a and b. With a positive
bias sweep, the switching volt-
age is slightly increased to
around 1.59 V and the ON/OFF
current density ratio is almost
the same as shown in Figure 8c
and d. The different switching
voltages and ON/OFF current
density ratios may be attributed
Figure 7. Typical J–V curves of the ITO/ODCzMSi/Au memory device. The sweep sequence and direction are indi-
cated by the number and arrow, respectively. a) Sweep numbers 1, 2, 4: 0 to ꢀ3 V; sweep numbers 3, 5: 0 to
+3 V; c) sweep numbers 1, 2, 4: 0 to +3 V; sweep numbers 3, 5: 0 to ꢀ3 V. The effect of operation time on the
current density of the device at the ON- and OFF-states tested at different biases under ambient conditions:
b) ꢀ0.5 V; d) 0.5 V.
to the different initial states of
the annealed device and the
non-written device, which have
different regio-regularity of the
PDCzMSi chains.
Conformation induced electrical switching mechanism can
satisfactorily explain the electrical switching behaviour of
Conclusion
ODCzMSi. There are literature precedents for the electrical bist-
ability of some non-conjugated polymers with pendant active
groups.[49–51] For example, memory effects of PVK derivatives
with flexible spacers between the carbazole pendant group
and the polymer backbone,[16,17] in which regio-random and
regio-regular alignments correspond to the low- and high-con-
ductivity states, showed only WORM or volatile features. Simi-
larly, different stable conformations of ODCzMSi with high or
low conductivity states could be obtained under the action of
electric field through the p–p stacking alignment of its pend-
ent aromatic groups inducing the electrical switching per-
formance.
In summary, a solution-processable oligosiloxane derivative has
been designed and synthesized by attaching the mCP moiety
to the oligosiloxane backbone. Compared to mCP, ODCzMSi
exhibits excellent thermal and morphological stability. Because
the mCP segments are connected through siliconꢀoxygen
bonds that disrupt the p conjugation, ODCzMSi maintains
a high ET of 3.0 eV. A highly efficient blue PhOLED with maxi-
mum external quantum, current and power efficiencies of
9.2%, 17.7 cdAꢀ1, and 7.7 lmWꢀ1, respectively, was fabricated
using ODCzMSi as the host material and FIrpic as the triplet
emitter. Moreover, memory devices using PDCzMSi as an active
layer exhibited non-volatile memory performance with high
stability in retention time up to 8000 s and low switch on volt-
The proposed mechanism of conformation change is sup-
ported by annealing the ITO/ODCzMSi/Au device with high-
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tion material PEDOT/PSS, and elec-
tron-transporting and hole-block-
ing material TPBi, were used from
commercial sources. ITO-coated
glass with a sheet resistance of
10 Wsquareꢀ1 was used as the
substrate. Before device fabrica-
tion, the ITO-coated glass substrate
was pre-cleaned carefully and ex-
posed to UV-ozone for 2 min. After
that, PEDOT/PSS was spin-coated
onto the clean ITO substrate as
a hole-injection layer. Then, the
host ODCzMSi doped with 10%
(by weight) FIrpic was spin-coated
to form a 40 nm thick emissive
layer (EML) and annealed at 1008C
for 30 min to remove residual sol-
vent. Finally a 5 nm thick hole-
blocking layer (HBL) of Tm3PyPB
and a 30 nm-thick electron-trans-
porting layer (ETL) of TPBi were
vacuum deposited, and for the
cathode of a 1 nm thick layer of
lithium fluoride (LiF) and aluminum
(100 nm) were sequentially depos-
ited onto the substrate through
shadow masking with a pressure
of 10ꢀ6 Torr. The current density–
voltage–luminance (J–V–L) charac-
teristics of the device was mea-
sured using a Keithley 2400 Source
meter and a Keithley 2000 Source
Figure 8. The ITO/ODCzMSi/Au memory device in its high-conductivity states is annealed at 1408C for 12 h and
then the device properties are determined again. Typical J–V curves of the annealed memory device, the sweep
sequence and direction are indicated by the number and arrow, respectively. a) Sweep numbers 1, 2, 4 :0 to ꢀ3 V;
sweep numbers 3, 5: 0 to +3 V; c) sweep numbers 1, 2, 4: 0 to +3 V; sweep numbers 3, 5: 0 to ꢀ3 V. The effect
of operation time on the current density of the device at the ON- and OFF-states tested at different biases under
ambient conditions: b) ꢀ0.5 V; d) 0.5 V.
age. These results demonstrate the great promise and versatili-
multimeter equipped with a calibrated Si-photodiode in a glove
box. The EL spectra were measured using a JY SPEX CCD 3000
spectrometer. The EQE values were calculated from the luminance,
current density, and electroluminescence spectrum according to
previously reported methods.[54] All measurements were performed
at room temperature under ambient conditions.
ty of organic–inorganic hybrid materials for organic electronics
applications.
Experimental Section
Materials
Memory device
The intermediate methyldiethyoxylchlorosilane was prepared ac-
cording to our previous report.[52] All reactants (Adamas-beta) were
purchased from Adamas Reagent, Ltd. without further purification,
and all solvents were supplied by Beijing Chemical Reagent Co.,
Ltd. Anhydrous and deoxygenated solvents were obtained by dis-
tillation over sodium benzophenone complex.
The memory devices with the configuration of ITO/ODCzMSi/Au
were fabricated as follows. ITO-coated glass with a sheet resistance
of 10 Wsquareꢀ1 was used as the substrate. Before device fabrica-
tion, the ITO-coated glass substrate was pre-cleaned carefully. After
that ODCzMSi was spin-coated onto the ITO glass substrate. Finally,
an Au top electrode was sputter coated onto the active layers
through a shadow mask at a system pressure of 10ꢀ3 bar. The cur-
rent density–voltage (J–V) characteristics of the sandwich devices
were recorded with a Keithley 4200 SCS semiconductor parameter
analyser (Keithley, Cleveland, OH) equipped with a Micromanipula-
tor 6150 probe station in a clean and metallically shielded box in
an ambient environment.
Molecular simulation
The geometrical and electronic calculations were carried out on
the dimer of PDCzMSi with the Gaussian09 program package.[53]
The molecular orbitals were calculated by the density functional
theory (DFT) method at the B3LYP level with the 6–31G(d) atomic
basis set.
Characterization
1H and 13C NMR spectra were recorded on a Bruker AV400
(400 MHz) spectrometer. Chemical shifts (d) are given in parts per
million (ppm) relative to tetramethylsilane (TMS; d=0) as the inter-
Device fabrication
PhOLEDs
1
The configuration used is: indium tin oxide (ITO)/PEDOT/PSS
(40 nm)/ODCzMSi/10 wt% FIrpic (40 nm)/1,3,5-tri(m-pyrid-3-yl-phe-
nyl)benzene (Tm3PyPB, 5 nm)/1,3,5-tris(1-phenyl-1H-benzimidazol-
2-yl)benzene (TPBi, 30 nm)/LiF (1 nm)/Al (100 nm). The hole-injec-
nal reference. H NMR spectral data are reported as chemical shift,
relative integral, multiplicity (s=singlet, d=doublet, m=multiplet),
coupling constant (J in Hz) and assignment. Elemental analyses of
carbon, hydrogen, and nitrogen were performed on a Vario EL
Chem. Eur. J. 2014, 20, 1 – 10
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1
cube. UV/Vis absorption spectra were recorded on a Shimadzu UV-
3600 spectrophotometer. PL spectra were recorded on a Hitachi F-
7000 fluorescence spectrophotometer. Differential scanning calo-
rimetry (DSC) was performed on a TA Q2000 Differential Scanning
Calorimeter at a heating rate of 108Cminꢀ1 from 25 to 2008C
under nitrogen atmosphere. The glass transition temperature (Tg)
was determined from the second heating scan. Thermogravimetric
analysis (TGA) was undertaken with a METTLER TOLEDO TGA/DSC
1/1100SF instrument. The thermal stability of the samples under
a nitrogen atmosphere was determined by measuring their weight
loss while heating at a rate of 108Cminꢀ1 from 25 to 8008C. Cyclic
voltammetry (CV) was carried out in nitrogen-purged dichlorome-
thane (oxidation scan) at room temperature with a CHI voltammet-
ric analyser. Tetrabutylammonium hexafluorophosphate (TBAPF6;
0.1m) was used as the supporting electrolyte. The conventional
three-electrode configuration consists of a glassy carbon working
electrode, a platinum wire auxiliary electrode, and an Ag/AgNO3
pseudo-reference electrode with ferrocenium–ferrocene (Fc+/Fc) as
the internal standard. Cyclic voltammograms were obtained at
scan rate of 100 mVsꢀ1. The onset potential was determined from
the intersection of two tangents drawn at the rising and back-
ground current of the cyclic voltammogram. Gel permeation chro-
matography (GPC) analysis was carried out on a Waters 515-2410
system using polystyrene standards as molecular weight references
and tetrahydrofuran (THF) as the eluent. The morphologies and
thickness of the oligomer films coated on the ITO substrate were
measured using atomic force microscopy (Agilent-5500 AFM)
under tapping mode.
oil (3.46 g, 64%). H NMR (400 MHz, [D8]THF): d=0.37 (s, 3H, -CH3),
1.73 (t, J=7 Hz, 6H; -OCH2CH3), 3.84 (q, J=7 Hz, 4H; -OCH2), 7.25
(t, J=8 Hz, 4H; Ar H), 7.42 (t, 4H; Ar H), 7.56 (d, J=8 Hz, 4H; Ar H),
7.95 (s, 1H; Ar H), 8.0 (s, 2H; Ar H), 8.14 ppm (d, J=8 Hz, 4H; Ar
H); 13C NMR (400 MHz, [D8]THF): d=ꢀ8.10, 16.01, 56.62, 107.62,
118.23, 121.81, 124.06, 129.08, 137.22, 138.26, 138.84 ppm;
29Si NMR (400 MHz, [D8]THF): d=ꢀ22.17 ppm; elemental analysis
calcd for C35H32N2O2Si: C 77.74, H 5.96, N 5.18; found: C 77.78, H
5.91, N 5.20.
ODCzMSi
To a mixture of 9,9’-(5-(diethoxy(methyl)silyl)-1,3-phenylene)bis(9H-
carbazole) (1.08 g, 2 mmol) and THF (100 mL), water (2 mL) and
1m HCl (2 drops) was added. The mixture was stirred at 08C for
24 h. After the reaction was complete, the mixture was extracted
with Et2O and washed with distilled water three times and dried
over anhydrous magnesium sulfate. The filtrate was concentrated
by reduced pressure to give OH-Si monomer to which was added
a
solution of 9,9’-(5-(diethoxy(methyl)silyl)-1,3-phenylene)bis(9H-
carbazole) (1.08 g, 2.0 mmol) in THF (50 mL) and tetra-n-butyl tita-
nate (2 drops) as catalyst. The mixture was stirred at 808C for
5 days while tracking the progress of the reaction with a Fourier
transform infrared (FTIR) spectrometer. After the reaction was com-
plete, trimethylchlorosilane (0.5 mL) was added and stirred for an-
other 12 h. Afterwards, the solvent was evaporated in vacuum to
give a sticky liquid, which was dissolved in toluene and precipitat-
1
ed with methanol to obtain the product as white powder. H NMR
(400 MHz, CDCl3): d=7.02–7.52 (m, 13H; ArH), 7.71–8.23 (m, 6H;
ArH), 0.33 ppm (s, 3H; -CH3); GPC (RI, polystyrene calibration)
Mw=2.13ꢂ103, Mw/Mn=1.13.
Synthetic procedures
9,9’-(5-Bromo-1,3-phenylene)bis(9H-carbazole)
A mixture of carbazole (16.7 g, 100 mmol), 1,3,5-tribromobenzene
(15.7 g, 50 mmol), CuI (0.95 g, 5 mmol), 18-crown-6 (2.0 g,
7.5 mmol), potassium carbonate (36.6 g, 150 mmol), and o-dichlor-
obenzene (200 mL) was de-gassed and stirred at 1808C for 24 h
under argon. After cooling to room temperature, the mixture was
quenched with saturated (NH4)2CO3 solution and extracted with
chloroform; the combined organic layer was washed with distilled
water three times and dried over anhydrous magnesium sulfate.
Then the solvent was evaporated in vacuum to give the crude
product, which was purified by column chromatography on silica
gel using (petroleum ether/dichloromethane=60:1 v/v) as the
Acknowledgements
The financial supports of NSFC (21104002 & 51221002) and Bei-
jing Higher Education Young Elite Teacher Project (YETP0491)
and the CHEMCLOUDCOMPUTING of the Beijing University of
Chemical Technology are gratefully acknowledged. Z.R. thanks
the China Scholarship Council for funding a visit to Durham
University.
1
Keywords: OLEDs · organic electronics · organic–inorganic
hybrid composites · polymers · worm memory device
eluent to obtain the product as a white solid (11.9 g, 49%). H NMR
(400 MHz, [D8]THF): d=7.36 (t, J=7 Hz, 4H; Ar H), 7.49 (t, J=7 Hz,
4H; Ar H), 7.57 (d, J=8 Hz, 4H; Ar H), 7.82 (s, 1H; Ar H), 7.89 (s,
2H; Ar H), 8.17 ppm (d, J=8 Hz, 4H; Ar H); 13C NMR (400 MHz,
[D8]THF): d=109.54, 120.47, 123.81, 124.23, 126.12, 128.72,
140.43 ppm.
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Received: February 26, 2014
Revised: August 27, 2014
Published online on && &&, 0000
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Chem. Eur. J. 2014, 20, 1 – 10
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FULL PAPER
Won’t forget: Oligosiloxane (ODCzMSi)
functionalized with mCP has been syn-
thesized and shown to have outstand-
ing properties for solution-processed
electronics applications. The hybrid ma-
terial has high thermal and morphologi-
cal stability and high triplet energy. A
phosphorescent OLED with an emissive
layer of ODCzMSi–FIrpic shows very
high efficiency blue emission. Efficient
memory devices using ODCzMSi as the
active layer have also been demonstrat-
ed.
&
Organic Electronics
D. Sun, Z. Yang, Z. Ren,* H. Li,
M. R. Bryce,* D. Ma, S. Yan*
&& – &&
Oligosiloxane Functionalized with
Pendant (1,3-Bis(9-carbazolyl)benzene)
(mCP) for Solution-Processed Organic
Electronics
&
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Chem. Eur. J. 2014, 20, 1 – 10
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!