Nonlinear optical properties and memory effects of the azo polymers carrying different substituents

Article abstract of DOI:10.1016/j.dyepig.2010.04.010

The third-order nonlinear optical properties and electric memory behavior of azobenzene polymers were influenced by different substituents. The nonlinear optical coefficients of polymer films measured using the Z-scan technique revealed that the polymers with different substituents exhibited different nonlinear absorption. The reversibility of sandwich memory devices based on the azobenzene polymers was dependant on the terminal substituent on the azobenzene chromophore. Polymers with electron-accepting terminal moieties in the azobenzene pendant showed write-once-read-many-times memory whereas the polymer that contained electron-donating moieties displayed a FLASH-type (rewritable) memory.

Full text of DOI:10.1016/j.dyepig.2010.04.010

Dyes and Pigments 88 (2011) 18e24
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Nonlinear optical properties and memory effects of the azo
polymers carrying different substituents
,
a
b,
Najun Li a, Jianmei Lu a , Hua Li , En-Tang Kang
*
**
a Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Material Science, Suzhou University, Suzhou, Jiangsu 215123, China
b Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge, 119260 Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
The third-order nonlinear optical properties and electric memory behavior of azobenzene polymers were
influenced by different substituents. The nonlinear optical coefficients of polymer films measured using
the Z-scan technique revealed that the polymers with different substituents exhibited different nonlinear
absorption. The reversibility of sandwich memory devices based on the azobenzene polymers was
dependant on the terminal substituent on the azobenzene chromophore. Polymers with electron-
accepting terminal moieties in the azobenzene pendant showed write-once-read-many-times memory
whereas the polymer that contained electron-donating moieties displayed a FLASH-type (rewritable)
memory.
Received 5 January 2010
Received in revised form
26 April 2010
Accepted 26 April 2010
Available online 20 May 2010
Keywords:
Azo
Polymer
Ó 2010 Elsevier Ltd. All rights reserved.
Substituent
Nonlinear optical
Electric memory
Device
1. Introduction
2. Experimental
In recent years, azobenzene-containing polymers have received
much attention owing to their potential application in optical data
storage, nonlinear optical (NLO) materials, holographic memories
and waveguide switches [1e8]. Besides optical manipulation, some
reports have demonstrated the potential of azobenzene molecules
to function as molecular switches at the nanoscale level [9e11]. In
addition, electric memories based on voltage-induced electric
bistability of polymers containing an azobenzene main chain group
have also been described [12,13]. The current research group has
reported the NLO properties of azo polymers [14e17]. In this work,
a series of azo monomers containing different electron donor or
acceptor groups and their polymers (PAzos) were synthesized. The
third-order NLO coefficients of the polymers were measured using
the Z-scan technique; the voltage-induced electrical switching
behavior and associated memory behavior of devices comprising
PAzo films sandwiched between an indium-tin-oxide (ITO) bottom
electrode and an Al top electrode are also reported.
2.1. Materials
N-ethyl-N-(2-hydroxyethyl) aniline (Tokyo Kasei Kogyo Co. Ltd.
in Japan, 98%) and ethyl 2-bromoisobutyrate (EBiB) (Acros, 99%)
were used as received. 4-Nitroaniline, 4-bromoaniline, 4-methox-
yaniline and triethylamine were all purchased from Shanghai
chemical reagent Co. Ltd. as analytical regents and used without
further purification (declared purity grade, ꢀ99%). Methacryloyl
chloride was produced by Haimen Best Fine Chemical Industry
Co. Ltd. and used after redistillation. N,N,N0,N00,N00-pentam-
ethyldiethylenetriamine (PMDETA) (Jiangsu Liyang Jiangdian
Chemical Factory, 98%) was dried with molecular sieve and dis-
tillated. Copper (I) bromide (CuBr) (A.R., Shanghai Zhenxin Chem-
ical Reagent Factory) was purified with sodium sulfite and glacial
acetic acid and stored under argon atmosphere at room tempera-
ture. Tetrahydrofuran (THF), cyclohexanone and N,N-dimethyl
formamide (DMF) were purified by reduced pressure distillation.
Other reagents were commercially available and used as received.
2.2. Instruments for characterization
* Corresponding author. Tel./fax: þ86 512 65882875.
** Corresponding author. Tel./fax: þ65 6516 2189.
FTIR spectra were recorded on a PerkineElmer 577 FTIR spec-
trophotometer in KBr disks. Elemental analysis was performed by
E-mail addresses: lujm@suda.edu.cn (J. Lu), cheket@nus.edu.sg (E.-T. Kang).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.04.010

N. Li et al. / Dyes and Pigments 88 (2011) 18e24
19
Italian 1106FT analyzer. 1H NMR spectra of monomers and polymers
in CDCl3 were obtained on an Inova 400 MHz FT-NMR spectrometer
at ambient temperature. The UVeVis absorption spectra were
carried out at room temperature in the region of 190e700 nm with
a UV-1601 spectrophotometer using cell path lengths of 0.5 cm.
Molecular weights (Mn) and polydispersity (Mw/Mn) were
measured on a gel permeation chromatography (GPC) utilizing
Waters 515 pump and differential refractometer. THF was used as
a mobile phase at a flow rate of 1.0 M minꢁ1. The glass transition
temperature (Tg) values of the azo polymers were determined on
a 2010 DSC TA Instrument at a heating rate of 10 K minꢁ1 under
nitrogen atmosphere. The surface morphology of the films was
examined by atomic force microscopy (AFM) using an SPA 400
system from Seiko Instruments Inc., Japan. Scanning electron
microscope (SEM) cross-section images of the memory device were
taken on a Hitachi S-4700 equipped with an energy-dispersive
X-ray spectrum (EDS).
62.816, H 5.7983, N 14.651, Found C 62.636, H 5.9426, N 14.516.
UVeVis (DMF): lmax
(3
) ¼ 465 nm (1300 L molꢁ1 cmꢁ1).
2.3.2. BAzo
1H NMR (CDCl3):
d
(ppm) ¼ 7.88e7.73 (d, 4H, ArH), 7.60 (d, 2H,
ArHeBr), 6.82 (d, 2H, ArH), 6.11 (s, 1H, ]CH2), 5.58 (s, 1H, ]CH2),
4.38 (t, 2H, eOCH2), 3.71 (t, 2H, H2CeN), 3.50 (m, 2H, eCH2), 1.94 (s,
3H, eCH3), 1.25 (t, 3H, eCH3). (C20H22N3BrO2) (416.3): Calcd. C
57.701, H 5.3261, N 10.093, Found C 57.588, H 5.4812, N 9.8862.
UVeVis (DMF): lmax
(3
) ¼ 416 nm (1750 L molꢁ1 cmꢁ1).
2.3.3. MAzo
1H NMR (CDCl3):
d
(ppm) ¼ 7.83e7.81 (d, 4H, ArH), 6.98 (d, 2H,
ArHeOCH3), 6.80 (d, 2H, ArH), 6.09 (s, 1H, ]CH2), 5.57 (s, 1H, ]
CH2), 4.34 (t, 2H, eOCH2), 3.70 (s, 3H, eOCH3), 3.66 (t, 2H, H2CeN),
3.48 (m, 2H, eCH2), 1.93 (s, 3H, eCH3), 1.23 (t, 3H, eCH3).
(C21H25N3O3) (367.4): Calcd. C 68.644, H 6.857, N 11.436, Found C
68.987, H 6.8526, N 11.524. UVeVis (DMF): lmax
(3) ¼ 405 nm
(1200 L molꢁ1 cmꢁ1).
2.3. Synthesis of azo monomers and their homopolymers
The three monomers namely, 40-[N-(2-methacryloyloxy ethyl)-
N-ethyl] amino-4-nitro azobenzene (NAzo), 40-[N-(2-meth-
acryloyloxy ethyl)-N-ethyl] amino-4-bromo azobenzene (BAzo)
and 40-[N-(2-methacryloyloxy ethyl)-N-ethyl] amino-4-methoxy
azobenzene (MAzo) were synthesized following a procedure that
was modified from that described in the literature [18,19], as
shown in Scheme 1. Atom transfer radical polymerization (ATRP)
was successfully used to prepare the homopolymers with
controlled number-average Mr and low polydispersity. Each poly-
mer was prepared using ethyl 2-bromoisobutyrate (EBiB), CuBr,
and N,N,N0,N00,N00-pentamethyldiethylenetriamine (PMDETA) as
initiator, catalyst and ligand, respectively.
2.3.4. PNAzo
Mn ¼ 4500, Mw/Mn ¼ 1.27 (GPC, polystylene calibration). 1H
NMR (CDCl3):
d
(ppm) ¼ 8.4e8.2 (ArHeNO2), 7.9e7.7 (ArH), 6.8e6.7
(ArH), 4.3e3.9 (eOCH2), 3.8e3.6 (eCH2), 2.0e1.4 (eCH3), 1.2e0.8
(eCH3 and eCH2). UVeVis (DMF): lmax
(3)
¼
470 nm
(1450 L molꢁ1 cmꢁ1). Tg ¼ 142 ꢂC.
2.3.5. PBAzo
Mn ¼ 5350, Mw/Mn ¼ 1.26 (GPC, polystylene calibration). 1H
NMR (CDCl3):
d(ppm) ¼ 7.9e7.6 (ArH), 7.6e7.45 (BreArH), 6.8e6.6
(ArH), 4.2e3.8 (eOCH2), 3.6e3.2 (eCH2), 2.0e1.4 (eCH3), 1.2e0.8
(eCH3 and eCH2). UVeVis (DMF): lmax
(3
)
¼
420 nm
(1800 L molꢁ1 cmꢁ1). Tg ¼ 120 ꢂC.
2.3.1. NAzo
1H NMR (CDCl3):
4H, ArH), 6.81 (d, 2H, ArH), 6.11 (s, 1H, ]CH2), 5.59 (s, 1H, ]CH2),
4.39 (t, 2H, eOCH2), 3.72 (t, 2H, H2CeN), 3.53 (m, 2H, eCH2), 1.94 (s,
3H, eOCH3), 1.27 (t, 3H, eCH3). (C20H22N4O4) (382.4): Calcd. C
d
(ppm) ¼ 8.31 (d, 2H, ArHeNO2), 7.94e7.90 (d,
2.3.6. PMAzo
Mn ¼ 5500, Mw/Mn ¼ 1.27 (GPC, polystylene calibration). 1H
NMR (CDCl3):
d
(ppm) ¼ 7.8e7.6 (ArH), 6.9e6.75 (ArHeOCH3),
6.75e6.6 (ArH), 4.3e4.1 (eOCH2), 3.9e3.7 (eOCH3), 3.6e3.2
Scheme 1. Synthetic route of the azo monomers and homopolymers containing different substituted azobenzene side chains.

20
N. Li et al. / Dyes and Pigments 88 (2011) 18e24
ITO-coated glass and the solvent was removed in a vacuum
chamber at 10ꢁ5 torr at 60 ꢂC for 10 h. Finally, a layer of Al was
thermally evaporated and deposited onto the polymer surface at
about 10ꢁ7 torr through a shadow mask to form the top electrode
and so as to define an active cross-area of 0.4 ꢃ 0.4 mm2. All
electrical measurements of the devices were characterized, under
ambient condition without any encapsulation, using a Hew-
lettePackard 4156A semiconductor parameter analyzer equipped
with an Agilent 16440A SMU/pulse generator and a Keithley 4200-
SCS semiconductor characterization system connected to a Cascade
probe station with a microchamber and hot chuck. Cyclic voltam-
metry was carried out by Chi 604c electrochemical analysis to
evaluate the reduction and oxidation (redox) behavior of each azo
polymer.
Fig. 1. UVeVis spectra of azo homopolymers containing different substituted azo-
3. Results and discussion
benzene side chains.
3.1. Material properties
(eCH2), 2.0e1.6 (eCH3), 1.3e0.7 (eCH3 and eCH2). UVeVis (DMF):
lmax
(3
) ¼ 409 nm (1400 L molꢁ1 cmꢁ1). Tg ¼ 112 ꢂC.
The good solubility in a wide range of organic solvents and the Tg
value of each polymer above 100 ꢂC indicates that these azobenzne-
containing homopolymers are promising for solid-state applica-
tions in NLO and electric memory devices. As shown in Fig. 1, the
UVeVis spectrum of each azo polymer exhibits two major
absorption peaks. The absorption peak at the shorter wavelength is
2.4. Preparation of polymer films and the third-order
NLO measurement
Appropriate amounts of polymer were dissolved in freshly
distilled DMF to give 25% w/w solutions. After filtration through
attributed to the
the peak at the longer wavelength is due to the
pe
p* electronic transition of aromatic ring while
p* electronic
0.2
m filter, films were prepared by spin-coating the solution on
pe
quartz glass. The prepared glass slides were placed in a vacuum
transfer of the azo bond. The polymers with different terminal
electron donor and acceptor groups in the azobenzene side chains
exhibit significant positive solvatochromic shift for the lowest
chamber at 60 ꢂC for 24 h to remove residual solvent, resulting in
a film thickness of w0.5
mm, as measured using an alpha-step
energy pe
p* transition.
profilometer. The third-order NLO coefficients of the azo polymer
films were measured using the Z-scan technique [20e22] using
a
Continuum Q-switched Nd:YAG laser system from which
3.2. Third-order NLO properties
produces 4 ns (Full width at half-maximum) laser pulses at 532 nm
with a repetition rate of 1 Hz. The single pulse incident energy is
In our experiment, the third-order NLO coefficients of the
synthetic azo polymer films were investigated utilizing Z-scan
technique with a single-beam method, which can obtain the
nonlinear absorption and nonlinear refraction simultaneously
[23e26]. It can be seen from Fig. 1 that PBAzo and PMAzo have
hardly linear absorbance at 532 nm, which promises low intensity
loss and little temperature change by photon absorption during the
NLO measurements. But PNAzo has a little linear absorbance at
532 nm. In order to avoid the accumulative intensity loss and
heating effect by linear absorption of PNAzo, all the azo polymer
about 1.0
mJ. The input and output energies of the pulses were
measured simultaneously with two energy detectors (laser Preci-
sion Rjp-765) controlled by computer. Before measurement for
PAzo films, carbon disulfide (CS2) is taken as a reference to rectify
the Z-san optical path.
2.5. Fabrication and measurement of memory
devices based on PAzo
The memory devices consisting of the PAzo films sandwiched
between an indium-tin-oxide (ITO) bottom electrode and an Al top
electrode were fabricated. The N,N0-dimethylacetamide (DMAc)
solution of PAzo (10 mg mLꢁ1) was spin-coated (2000 rps) onto
films were measured with a low input energy (1.0 mJ) and a repe-
tition rate of 1 Hz.
The results of Z-scan with and without an aperture showed that
all the azo polymers have both nonlinear absorption and nonlinear
Fig. 2. (a) Normalized open-aperture Z-scan curves and (b) normalized transmittance curves of the azobenzene-containing acrylates and their polymers.

N. Li et al. / Dyes and Pigments 88 (2011) 18e24
21
Table 1
Third-order NLO coefficients of azo-containing polymers in film state.
(ꢃ10ꢁ8 m Wꢁ1
)
n2 (ꢃ10ꢁ17 m2 Wꢁ1
)
a
PAzo
T0
b
PNAzo
PBAzo
PMAzo
0.56
0.61
0.63
ꢁ4.75
ꢁ15.1
ꢁ9.8
ꢁ9.3
3.24
3.01
a
T0 is the linear transmittance of the samples.
refraction. The nonlinear absorption coefficient
b can be deter-
mined through the fitting of the experimental data based on
Eq. (1)e(2) [20],
Fig. 4. Cross-section SEM image of the device ITO/PNAzo/Al device.
N
X
m
½ꢁq0ðzÞꢄ
Tðz; s ¼ 1Þ ¼
; for jq0j < 1
(1)
(2)
m ¼ 0 ðm þ 1Þ3=2
a negative sign for the nonlinear refraction and exhibited a strong
self-defocusing behavior. The third-order NLO coefficients of PAzos
were calculated by equations and listed in Table 1. The nitro-
substituted polymer (PNAzo) film has the largest coefficients of the
ꢀ
ꢁ
q0ðzÞ ¼ bI0ðtÞLeff = 1 þ Z2=Z02
nonlinear absorption (
b) and nonlinear refraction (n2) values
Here
b
is the nonlinear absorption coefficient, I0(t) is the intensity of
because a pushepull (De
peA) electron system is formed by the
laser beam at focus (z ¼ 0), Leff ¼ [1 ꢁ exp(ꢁa0L)/a0] is the effective
thickness with a0, the linear absorption coefficient and L, the
sample thickness, z0 is the diffraction length of the beam, and z is
the sample position.
electron withdrawing nitro-substituent with dimethylamino-
group in the azobenzene side chain, which enhances the delocal-
ization effect and is favorable to increase the NLO effect.
The nonlinear refractive component of the sample was obtained
by dividing the normalized Z-scan measured under a closed aper-
ture configuration by the normalized Z-scan data obtained under
the open-aperture configuration. Thus, the refractive curves were
obtained. The third-order nonlinear refractive index, n2, could be
got by fitting the refractive curves using Eq. (3):
3.3. Electrical characterization of PAzo-based memory devices
Electrical transitions in the ITO/PAzo/Al devices were observed
by measuring the current response to an external applied voltage.
ITO was maintained as the ground electrode in all electrical
measurements. The surfaces of the fabricated azo polymer films
were all observed under AFM. A typical AFM image of polymer film
and the schematic diagram of the memory devices are shown in
Fig. 3. It showed that the surface of the film is flat and clean, no sign
of any phase separation was observed. In order to confirm the
polymer film continuity across the interface in the sandwiched
device, a SEM cross-section image of ITO/PAzo/Al was investigated.
As shown in Fig. 4, each layer of the memory device (ITO/PNAzo/Al)
is clear and the polymer film continuity across the interface is good.
In addition, we can make an estimate of the thickness of each layer
(ITO film on glass, polymer film and Al layer). The polymer film is
about 50e70 nm and the aluminum top layer is about 80e100 nm.
The current densityevoltage (JeV) curves of the ITO/PNAzo/Al
in Fig. 5(a) show two distinct conductivity states. The device is
initially in the low-conductivity (OFF) state with a current density
of about 10ꢁ6 A cm2 when a voltage sweep was applied starting
from 0 to 1.0 V. When the negative voltage was increased further
(from 0 to 1.5 V), the device switches from the OFF-state to the
high-conductivity (ON) state at a threshold voltage of about ꢁ1.5 V,
as indicated by the abrupt increase of the current density to about
a0lDTpev
0:812pI0ð1 ꢁ sÞ0:25 1 ꢁ eꢁ
n2
¼
(3)
ꢂ
ꢃ
a0
L
where
D
Tpevis the measured peakevalley transmittance difference
from a normalized Z-scan curve, I0 is the on-axis peak intensity at
the focus (z ¼ 0), s is the transmittance of the aperture ahead the
detector in absence of a sample. Here, s is 0.13.
The open-aperture experimental Z-scan curves of azo polymers
with different substituents are shown in Fig. 2(a). The normalized
energy transmittance of PNAzo increases at the focus indicating
a saturated absorption (SA) (
display reverse saturated absorption (RSA) (
b
< 0), while PBAzo and PMAzo
> 0). It is caused by
b
the different electronic effects of the substituent on the azobenzene
chromophore. As electron withdrawing nitro-group substituted in
the azobenzene system, the
p-electron conjugation is enhanced
obviously and the absorption cross-section of its excited state is
smaller than that of ground state under the irradiation of 532 nm
laser, which caused saturated absorption [27,28]. The peakevalley
pattern of the normalized transmittance curves in Fig. 2(b) indi-
cated that each of the azobenzene-containing polymers had
Fig. 3. (a) Topographical AFM image of a PNAzo film spin-coated on ITO-glass substrate and (b) the schematic diagram of the memory device consisting of a thin film of azo polymer
sandwiched between ITO and Al electrodes.

22
N. Li et al. / Dyes and Pigments 88 (2011) 18e24
Fig. 5. (a) Current densityevoltage (JeV) characteristics, (b) effect of operation time (at ꢁ1.0 V) on the device current density in the OFF- and ON-states of ITO/PNAzo/Al device, (c)
current densityevoltage (JeV) characteristics of ITO/PBAzo/Al WORM device and (d) current densityevoltage (JeV) characteristics of ITO/PMAzo/Al FLASH device.
10ꢁ1 A cm2 (Sweep 1). The high current density observed during
the subsequent voltage sweep indicates retention of the ON-state
in the device (Sweep 2). The ON-state JeV curves under negative
and opposite biases were symmetrical. The transition from the OFF-
state to the ON-state serves as the “writing” process for the
memory device. The ON-state of the device could be distinguished
from the OFF-state by an ON/OFF current ratio of about 104 when
read at ꢁ1.0 V. After undergoing the OFF-to-ON transition, ITO/
PNAzo/Al remain in its ON-state even after electrical power has
been turned off and cannot be returned to the low-conductivity
state by applying a reverse bias. This behavior is characteristic of
a write-once-read-many-times (WORM) memory device. The ON-
and OFF-states of the device is stable for up to 108 continuous read
increase in the current density was observed at about ꢁ1.7 V when
a voltage sweep from 0 to ꢁ3.0 V was applied (Sweep 1), as shown in
Fig. 5(d). The high-conductivity ON-state was retained when the
voltage sweep was applied again (Sweep 2). In a voltage sweep of
opposite bias from 0 to 3.0 V (Sweep 3), an abrupt decrease in the
current density was observed at a threshold voltage of about 2.0 V,
indicating the device transition from the ON-state back to the OFF-
state. This electrical transition represents the “erase” process for
the memory device. The device remains in the OFF-state during the
following positive sweep (Sweep 4). The “erased” state can be
further converted to the “written” state when the switching
threshold voltage was re-applied, indicating that the memory
device was rewritable (Sweep 5). Both the low- and the high-
conductivity (OFF and ON) states of the device were stable under
power-off conditions and under constant voltage stress conditions
of ꢁ1.0 V, as well as maintaining an ON/OFF current ratio of about
104 when read at ꢁ1.0 V. The ability of the device to undergo write,
read and erase cycles fulfills the functionality of a FLASH-type
memory device.
pulses of ꢁ1.0 V (pulse period ¼ 2
ms, pulse width ¼ 1
ms) without
any resistance degradation. The retention ability at the OFF- and
ON-states of this device is tested by first subjecting a device in
the OFF-state to a constant voltage stress of ꢁ1.0 V and recording
the current passing through the device at regular intervals. The
device is then programmed by a voltage sweep and the ON-state is
evaluated under the same stress condition. Fig. 5(b) shows that the
ITO/PNAzo/Al device has both stable OFF- and ON-state currents
within 3 h, with the high ON/OFF current ratio sustained.
From the above electrical characteristics of PAzos, we can see
that the memory type of the ITO/PAzo/Al device is affected by the
electron accepting or donating of the terminal substitutent on
azobenzene chromophore. We did some experiments to discuss the
physical meaning of the electrical behaviors of the ITO/PAzo/Al
devices. The UVeVis spectroscopy and cyclic voltammetry of three
The electrical switching behavior and the retention ability of
another azo polymer device, ITO/PBAzo/Al, is also investigated. The
bromo group also possesses electron-accepting properties but is
a weaker acceptor in comparison with the nitro moiety. As shown
in Fig. 5(c), the behavior of ITO/PBAzo/Al is of WORM type, with
a switching threshold voltage of ꢁ1.9 V and an ON/OFF current ratio
of about 104. The stability of the two conductivity states (OFF-state
and ON-state) of the WORM device is demonstrated by the almost
constant current densities observed when the device was placed
under constant voltage stress for a period exceeding 3 h.
Table 2
High-occupied-molecular-orbital (HOMO) and lowest-unoccupied-molecular-
orbital (LUMO) energy levels obtained from cyclic voltammetry and UVeVis
spectroscopy.
PAzo
HOMO
(eV)
LUMO
(eV)
HOMO / ITO
(eV)
Al / LUMO
(eV)
Threshold
voltage (V)
PNAzo
PBAzo
PMAzo
5.53
5.54
5.45
3.30
2.96
2.77
0.73
0.74
0.65
0.98
1.32
1.51
ꢁ1.5
ꢁ1.9
ꢁ1.7
When the nitro terminal moiety in the azobenzene side chain of
the polymer was replaced by the methoxy group, different electrical
behaviors were observed. As the ITO/PMAzo/Al device, an abrupt

N. Li et al. / Dyes and Pigments 88 (2011) 18e24
23
Fig. 6. Schematic energy diagram of ITO/PAzo/Al devices.
azo polymers were investigated to obtain the experimental values
References
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[29,30]. Based on an analysis of the barrier heights from charge
injection to the electrodes (work function of ITO ¼ 4.8 eV, work
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ITO electrode is smaller compared to the barrier to hole injection
from the Al electrode. So charge injection into the polymer layer via
hole injection from ITO to the HOMO of each polymer is more
favorable under the applied negative bias, as shown in Fig. 6. The
azo polymers become p-doped under the induction of the electric
field and switches to the high-conductivity state (ON-state).
Correspondingly, the devices based on azo polymers exhibit OFF-
to-ON transitions when a negative bias is applied. The threshold
voltage value of each polymer is coincident with their Eg value.
Bigger the Eg of the azo polymer is, higher its threshold voltage is
(PBAzo> PMAzo> PNAzo). It can be considered that bigger the Eg
is, more charges are requisite to be injected into the polymer layer
to overcome the energy barrier, so the OFF-to-ON threshold voltage
is in a higher negative value.
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This work is supported by the National Natural Science Foun-
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Science Foundation of the Jiangsu Higher Education Institutions of
China (Grant No. 09KJB430010).

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