Nonvolatile memory devices based on polyimides bearing noncoplanar twisted biphenyl units containing carbazole and triphenylamine side-chain groups

Article abstract of DOI:10.1039/c1jm12453f

Two novel polyimides, PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA), consisting of alternating electron-donating 2,2′-bis[4-(9H-carbazol-9-yl)phenyl]- or 2,2′-bis[4-(diphenylamino)phenyl]-substituted biphenyl moieties and electron-accepting phthalimide moieties were synthesized and characterized. These polyimides are thermally stable with 5% weight loss over 500 °C and the glass transition temperatures of the polyimides were found to be 293 °C. The optical band gaps of PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA) were 3.42 and 3.30 eV, respectively, indicating the significance of the linkage groups. The estimated energy levels (HOMO, LUMO) of PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA) were (-5.51, -2.10) and (-5.22, -2.02) eV, respectively. Resistive switching devices with the configuration of Al/polymer/ITO were constructed from these polyimides by using the conventional solution coating process. The as-fabricated PI(CzBD-BTFBPDA) film exhibited a nonvolatile bipolar write-once-read-many times (WORM) memory character, whereas devices with the PI(TPABD-BTFBPDA) film showed "write-read-erase" flash type memory capability. The ON/OFF current ratios of the devices were both around 10⁶ in the ambient atmosphere. The mechanisms associated with the memory effect were further elucidated from the density functional theory (DFT) method at the B3LYP level with the 6-31G(d) basis set. The present study suggested that the tunable switching behavior could be achieved through the appropriate design of the donor-acceptor PIs structure to have potential applications for memory devices.

Full text of DOI:10.1039/c1jm12453f

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PAPER
Nonvolatile memory devices based on polyimides bearing noncoplanar twisted
biphenyl units containing carbazole and triphenylamine side-chain groups†
Yue-Qin Li,ab Run-Chen Fang,b An-Min Zheng,c Yue-Ying Chu,c Xian Tao,a Hui-Hua Xu,a Shi-Jin Dingb
ab
*
and Ying-Zhong Shen
Received 31st May 2011, Accepted 28th July 2011
DOI: 10.1039/c1jm12453f
Two novel polyimides, PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA), consisting of alternating
electron-donating 2,20-bis[4-(9H-carbazol-9-yl)phenyl]- or 2,20-bis[4-(diphenylamino)phenyl]-
substituted biphenyl moieties and electron-accepting phthalimide moieties were synthesized and
ꢀ
characterized. These polyimides are thermally stable with 5% weight loss over 500 C and the glass
transition temperatures of the polyimides were found to be 293 ꢀC. The optical band gaps of PI(CzBD-
BTFBPDA) and PI(TPABD-BTFBPDA) were 3.42 and 3.30 eV, respectively, indicating the
significance of the linkage groups. The estimated energy levels (HOMO, LUMO) of PI(CzBD-
BTFBPDA) and PI(TPABD-BTFBPDA) were (ꢁ5.51, ꢁ2.10) and (ꢁ5.22, ꢁ2.02) eV, respectively.
Resistive switching devices with the configuration of Al/polymer/ITO were constructed from these
polyimides by using the conventional solution coating process. The as-fabricated PI(CzBD-
BTFBPDA) film exhibited a nonvolatile bipolar write-once–read-many times (WORM) memory
character, whereas devices with the PI(TPABD-BTFBPDA) film showed ‘‘write–read–erase’’ flash type
memory capability. The ON/OFF current ratios of the devices were both around 106 in the ambient
atmosphere. The mechanisms associated with the memory effect were further elucidated from the
density functional theory (DFT) method at the B3LYP level with the 6-31G(d) basis set. The present
study suggested that the tunable switching behavior could be achieved through the appropriate design
of the donor–acceptor PIs structure to have potential applications for memory devices.
Introduction
As a new family of organic electronic devices, organic- or poly-
mer-based resistive memory devices have become an active
research topic in recent years, because they are likely to be an
alternative or supplementary technology to the conventional
memory technology facing the problems and challenges in
miniaturizing from microscale to nanoscale.1,2 However, small
organic molecules are susceptible to damage during the pro-
cessing involved in the fabrication of memory devices because of
their low boiling points and low chemical resistance, and they
also require more elaborate and expensive processing such as
vacuum evaporation and deposition.3 In contrast, polymer
materials exhibit easy processability, flexibility, high mechanical
strength and good scalability. Further, they can be processed at
low cost, and with their use the three-dimensional (3D) multi-
stack layer structures required for high density memory devices
can easily be fabricated.1,3 As a result, significant research effort
is currently being invested in the development of polymer
switching materials with properties and processability that meet
the requirements of memory devices. A number of polymer
materials, including conjugated polymers,3–15 dopants,16–18 and
complex systems,19–26 have been investigated for electronic
memory applications. The materials, devices, and mechanistic
aApplied Chemistry Department, School of Material Science & Engineering
Nanjing University of Aeronautics &Astronautics, Nanjing, 210016, P.R.
China. E-mail: yz_shen@nuaa.edu.cn; Fax: +8625-52112626; Tel:
+8625-52112906-84765
bState Key Laboratory of ASCI and System, School of Microelectronics,
Fudan University, Shanghai, China. E-mail: sjding@fudan.edu.cn; Fax:
+8621-55664845; Tel: +8621-55664845
cState Key Laboratory Magnetic Resonance and Atomic Molecular
Physics, Wuhan Centre for Magnetic Resonance, Wuhan Institute of
Physics and Mathematics, Chinese Academy of Science, Wuhan, China.
E-mail: zhenganm@wipm.ac.cn; Fax: +8627-87199291; Tel: +8627-
87199291
† Electronic supplementary information (ESI) available: AFM images of
PI(CzBD-BTFFBPDA) and PI(TPABD-BTFFBPDA); FT-IR spectra of
CzBD and TPABD; 1H NMR and 13C NMR spectra of CzBD and
TPABD;
DSC
curves
of
PI(CzBD-BTFBPDA)
and
PI
(TPABD-BTFBPDA); I–V curves of ten Al/PI(CzBD-BTFBPDA)/ITO
cells at the SET and RESET operation and the resistance distribution
of the high and low conductivity states; I–V curves of ten Al/PI
(TPABD-BTFBPDA)/ITO cells at the SET and RESET operation and
the resistance distribution of the high and low conductivity states; the
endurance characteristic of the Al/PI(TPABD-BTFBPDA)/ITO cell.
CCDC reference number 778136. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1jm12453f
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aspects of polymer electronic memories have been reviewed
recently.1,20,27 The design and synthesis of thermally stable and
processable polymers that can provide the required electronic
properties within a single polymer system for memory device
applications are thus highly desirable.
exact switching mechanism of PI memory devices is still a subject
of debate. Thus, the development of PI-based memory devices
remains in the exploration stage.
In this study, two PI monomers, 2,20-bis[4-(9H-carbazol-9-yl)
phenyl]-4,40-biphenyl diamine (CzBD) and 2,20-bis[4-(diphenyl-
amino)phenyl]-4,40-biphenyl diamine (TPABD), were synthe-
sized and characterized. Two functional aromatic PIs containing
carbazole and triphenylamine moieties were also synthesized
from these diamines reacting with 2,20-bis[40-(300,400,500-tri-
fluorophenyl)phenyl]-4,40,5,50-biphenyltetracarboxylic dianhy-
dride (BTFBPDA), respectively. These PIs were characterized by
FT-IR, elemental analysis, thermogravimetric analysis (TGA)
and differential scanning calorimetric (DSC) analysis, gel
permeation chromatography (GPC), intrinsic viscosity
measurement, UV–vis absorption, fluorescence spectra and
cyclic voltammogram (CV). Electronic switching devices were
fabricated based on the solution-cast film of PIs, sandwiched
between the indium–tin oxide (ITO) and Al electrodes. The
devices exhibit nonvolatile electrical switching and fulfil the
requirements of WORM and flash memory characteristics.
Molecular simulations of the PI repeat unit in ground states were
carried out to better understand the mechanism underlying the
electrical switching phenomena.
As nonvolatile memory that is capable of holding data
permanently and being read from repeatedly, write-once–read-
many-times (WORM) memory and flash-type memory are very
desirable for ultralow-cost permanent storage of digital images,
eliminating the need for slow, bulky, and expensive mechanical
drives used in conventional magnetic and optical memories.1,4,8
Basically, the WORM memory functions as conventional CD-R,
DVD-R, or programmable read-only memories (PROMs).
Flash-type memory is widely used in portable systems, such as
mobile personal computers (PC), personal digital assistants
(PDA), and digital cameras,27 because it does not need power to
keep the stored information and could be written repeatedly. Ree
et al.8 reported hydroxytriphenylamine moieties of a PI polymer
with thicknesses less than 77 nm exhibit excellent unipolar
WORM memory behavior with a high ON/OFF current ratio of
up to 106, a long retention time and a low power consumption of
less than 3.0 V. Furthermore, these WORM characteristics were
found to persist even at high temperatures of up to 150 ꢀC.
Triarylamine moieties have attracted considerable interest as
a hole-transport group for use in multilayer organic electrolu-
minescence devices due to their relatively high mobilities and
their low ionization potentials. For example, triphenylamine
(TPA) moiety can act as a strong donor with a strong charge-
transfer effect with acceptor in different PI systems. Kang and
coworkers first reported the dynamic random access memory
(DRAM) characteristics of TPA-functionalized PIs.6,28 Ree and
coworkers showed that di-TPA-based PI memory devices had
stable digital nonvolatile WORM and volatile DRAM charac-
teristics depending on the film thickness.3 Recently, Chen and
coworkers also reported TPA-based PIs containing mono- or
dual-mediated phenoxy linkages that exhibited the DRAM and
static random access memory (SRAM) behaviors, respectively.14
Song et al. reported that TPA-containing PI exhibited the
distinct electronic switching behaviors of memories with different
bottom electrode materials (ITO or Al).29 Meanwhile, the
Experimental
Materials
4-(9H-Carbazol-9-yl)phenylboronic acid and 4-(diphenylamino)
phenylboronic acid (99%, Hebei Delongtai Chemicals Co. Ltd.,
China) were used as received. 2,20-Dibromobiphenyl-4,40-
diamine was synthesized on the basis of the procedure in ref. 37.
2,20-Bis[40-(300,400,500-trifluorophenyl)phenyl]-4,40,5,50-biphenyl-
tetracarboxylic dianhydride (BTFBPDA) was prepared as
described in previous work.38 All the other reagents and solvents
were of analytical grade and were used without further
purification.
Measurements
The FT-IR measurements (KBr pellets) were recorded in the
range of 400–4000 cmꢁ1 using a Thermo Nicolet Nexus 670
infrared spectrometer. NMR spectra were recorded on a Bruker
Advance 300 spectrometer at resonant frequencies of 300 MHz
for 1H and 75 MHz for 13C nuclei using DMSO-d6 as the solvent
and tetramethylsilane as the reference. The weight average
molecular weight (Mw) and the number average molecular
weight (Mn) were determined by GPC on a Water GPC system
equipped with four Waters Ultrastyragel columns (300 ꢂ
7.5 mm, guarded and packed with 1 ꢂ 105, 1 ꢂ 104, 1 ꢂ 103, and
carbazole moiety is
a well-known hole-transporting and
electroluminescent unit, which has demonstrated its unique
properties in various optoelectronic applications such as
photoconductive, electroluminescent, and photorefractive
materials.30 The hole-transporting characteristic of the carbazole
moiety results from the electron-donating capabilities associated
with the nitrogen in the carbazole.31,32 Ree and coworkers
reported novel programmable ‘‘write–read–erase’’ flash memory
devices based on PI containing carbazole moieties in its side
groups.33 Recently, Qi and coworkers presented a PI containing
carbazole moiety in the main chain exhibiting a DRAM effect.34
These PIs exhibit ON–OFF switching and WORM, flash
memory or DRAM behaviors, depending on the polymer
backbone’s chemical structure. Moreover, functional polymers
containing pull–push carbazole and triphenylamine moieties
exhibit the typical bistable electrical switching and nonvolatile
rewritable memory effect.35,36 Therefore, aromatic PIs containing
carbazole and triphenylamine moieties are potential materials for
thermally stable polymer electronic memories. However, the
500 A gels) in series. Tetrahydrofuran (THF, 1 mL minꢁ1) was
ꢀ
used as the eluent and the signal was monitored by a UV detector
at the wavelength of 254 nm. Monodispersed polystyrene was
used as the molecular weight standard. Elemental analysis was
performed on a Perkin-Elmer 240C elemental analyzer. TGA
and DSC studies were carried out with a NETZSCH STA 449C
with a constant heating rate of 10 ꢀC minꢁ1 under argon (1.0 cm3
minꢁ1). The single-crystal X-ray structure measurement was
performed on a Rigaku Satum 724 CCD single-crystal X-ray
diffractometer. The detector was equipped with graphite
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ꢀ
monochromatic Mo Ka radiation (k ¼ 0.71073 A) at 293 K.‡
Synthesis of 2,20-bis[40-(9H-carbazol-9-yl)phenyl]-4,40-biphenyl
diamine (CzBD)
UV–vis absorption spectra were obtained on a UV-2550 UV-vis
spectrophotometer and fluorescence spectra were measured with
an Eclipse fluorescence spectrometer. Melting points were
measured by a WBR-1B Melting Point Digital Apparatus. Cyclic
voltammetry was performed at room temperature using
a working electrode (ITO, polymer film area of about 10 ꢂ
30 mm2), a reference electrode (Ag/AgCl in 3.8 M KCl solution)
and a counter electrode (Pt plate) at a sweep rate of 0.05 V sꢁ1
(CHI 660B electrochemical analyzer). A 0.1 M solution of tetra-
butylammonium perchlorate (TBAP) in anhydrous acetonitrile
A
solution of 2,20-dibromobiphenyl-4,40-diamine (3.42 g,
0.01 mol), 4-(9H-carbazol-9-yl)phenylboronic acid (6.32 g,
0.022 mol), Pd(PPh3)4 (0.2 g, 0.174 mmol), Na2CO3 (4.24 g, 0.04
mol) in ethanol (40 mL) and water (20 mL) was heated at 80 ꢀC
under an nitrogen atmosphere for 4 h. When the reaction was
complete, the reaction mixture was cooled and filtered under
nitrogen. The filtrate was condensed and extracted twice with
1,2-dichloroethane (2 ꢂ 20 mL). The combined organic layer was
washed twice with saturated NaHCO3 solution (40 mL) and
water, respectively, and then dried over anhydrous MgSO4. The
solvent was evaporated to obtain brown powders. The filter cake
and the powders were combined and recrystallized from
1,2-dichlorethane to give a white product. Yield: 4.33 g (65.0%).
Mp: 170 ꢀC (dec.). FT-IR: nmax/cmꢁ1 3450 and 3380 (N–H
asymmetric and symmetric stretching), 1316 (C–N stretching).
1H NMR (300 MHz, DMSO-d6, d, ppm): 8.22 (d, 2H, J ¼ 7.7 Hz,
Ar–H), 7.23–7.38 (m, 8H, Ar–H), 7.15 (d, 1H, J ¼ 8.1 Hz, Ar–H),
6.98 (d, 2H, J ¼ 8.2 Hz, Ar–H), 6.70 (dd, 1H, J ¼ 8.1, 1.6 Hz, Ar–
H), 6.56 (d, 1H, J ¼ 1.6 Hz, Ar–H), 5.17 (s, 2H, NH2). 13C NMR
(75 MHz, DMSO-d6, d, ppm): 147.7, 141.4, 140.2, 139.8, 134.4,
132.4, 130.1, 127.6, 126.2, 125.7, 122.6, 120.5, 119.9, 115.5, 113.8,
109.4. Anal. Calcd for C48H34N4 (666.81) C, 86.46; H, 5.14; N,
8.40; found: C, 86.29; H, 5.10; N, 8.68%.
was used as an electrolyte. The energy level of the HOMO was
ox
determined from the onset oxidation (E ) based on the refer-
onset
ence energy level of ferrocene (4.8 eV below the vacuum level)
ox
according to the following relation: HOMO ¼ ꢁe(Eonset
ꢁ
Eferrocene1/2 + 4.8) eV. The LUMO level was calculated from the
HOMO and the value of the optical band gap was according to
opt
the relation LUMO ¼ HOMO + Eg (eV). Atomic force
microscopy (AFM) images of the polymer films on the device
surface were obtained with a NanoScope IIIa AFM operated in
the tapping mode at room temperature. Images were taken
continuously with the scan rate of 1.0 Hz.
Fabrication and measurement of the memory device
The indium–tin oxide (ITO)/glass was precleaned with water,
acetone, alcohol, and 2-propanol (in that order), in an ultrasonic
bath for 20 min, respectively. The PI solutions were prepared in
DMAc (9 mg mLꢁ1) and filtered through microfilters with
a pinhole of 0.22 mm. The PI films were formed under the
revolving speed of the spin-coating controlled at 3000 rpm for
30 s. The morphology of the polymer films was investigated using
tapping-mode AFM (ESI†, Fig. S1). The film thicknesses were
about 50 nm, as determined by the Microfigure Measuring
Instrument (Surforder ET-3000). Finally, an Al top electrode
with different areas of 0.5024, 0.1256 and 0.0314 mm2 and
a thickness of about 110 nm was formed by thermal evaporation
onto the polymer surface through a shadow mask at a pressure of
about 10ꢁ6 Torr. Electrical measurements of the devices were
characterized under ambient conditions, without any encapsu-
lation, using an Agilent B1500A Semiconductor Device
Analyzer. ITO was maintained as the ground electrode during
the electrical measurements and Al was set as the top electrode
during the voltage sweep.
Synthesis of 2,20-bis[40-(diphenylamino)phenyl]-4,40-biphenyl
diamine (TPABD)
A solution of 2,20-dibromobiphenyl-4,40-diamine (3.42 g, 0.01
mol), 4-(diphenylamino)phenylboronic acid (6.36 g, 0.022 mol),
Pd(PPh3)4 (0.2 g, 0.174 mmol), Na2CO3 (4.24 g, 0.04 mol) in
ethanol (40 mL) and water (20 mL) was heated at 80 ꢀC under an
nitrogen atmosphere for 4 h. When the reaction was complete,
the reaction mixture was cooled and filtered under nitrogen. The
filtrate was condensed and extracted twice with dichloromethane
(2 ꢂ 20 mL). The combined organic layer was washed twice with
saturated NaHCO3 solution (40 mL) and water, respectively, and
then dried over anhydrous MgSO4. The solvent was evaporated
to obtain brown powders. The filter cake and the powders were
combined and recrystallized from 1,2-dichlorethane to give
a white product. Yield: 3.92 g (58.4%). Mp: 203.5–204.2 ꢀC. FT-
IR: nmax/cmꢁ1 3438, 3377 (N–H asymmetric and symmetric
stretching), 1317 (C–N stretching). 1H NMR (300 MHz, DMSO-
d6, d, ppm): 7.21 (t, 4H, J ¼ 7.8 Hz, Ar–H), 7.00–6.91 (m, 7H,
Ar–H), 6.65 (d, 2H, J ¼ 8.14 Hz, Ar–H), 6.60–6.51 (m, 3H, Ar–
H), 6.38 (d, 1H, J ¼ 2.2 Hz, Ar–H), 5.02 (s, 2H, NH2). 13C NMR
(75 MHz, DMSO-d6, d, ppm): 147.8, 145.7, 145.4, 141.3, 136.1,
132.8, 130.9, 129.8, 129.2, 124.1, 123.2, 122.6, 116.4, 114.2, 77.6,
77.2, 76.7. Anal. Calcd for C48H34N4: C, 85.94; H, 5.71; N, 8.35;
found: C, 85.79; H, 5.70; N, 8.48%.
Theoretical analysis
Molecular calculations studied in this work have been performed
with the Gaussian 09 program package.39 The ground state
geometry and electron structures of the basic unit of CzBD-
BTFBPDA and TPABD-BTFBPDA were optimized by means
of the density functional theory (DFT) method at the B3LYP
level of the theory with the 6-31G(d) basic set.
Synthesis of PI(CzBD-BTFBPDA)
A three-necked flask was charged with diamine CzBD (0.6668 g,
1 mmol) and 5 mL of DMAc. The solution was stirred under
a nitrogen atmosphere until diamine dissolved completely, and
then BTFBPDA (0.7065 g, 1 mmol) was added. The reaction
mixture was stirred at ambient temperature for 24 h under
‡ Crystal structure analysis of CzBD. C48H34N4, 666.79, crystal system:
ꢀ
ꢀ
triclinic, space group P1, a ¼ 11.209(2) A, b ¼ 12.600(3) A, c ¼ 14.572
ꢀ
ꢀ
ꢀ
3
ꢀ
ꢀ
(3) A, a ¼ 91.81(3) , b ¼ 111.78(3) , g ¼ 107.79(3) , V ¼ 1795.3(6) A ,
293(2) K, 2, (reflections) 0.0825 (4036), wR2
(reflections) ¼ 0.2118 (6913).
T
¼
Z
¼
R
¼
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a nitrogen atmosphere to form poly(amic acid). A mixture of
equimolar amounts of acetic anhydride and pyridine was added
to the poly(amic acid) solution with stirring at ambient temper-
ature for 1 h, and then heated to 100 ꢀC and maintained at 100 ꢀC
for 3 h. After cooling, the viscous polymer solution was poured
into methanol and collected by filtration. The solid was further
purified by dissolving in DMAc and reprecipitated into meth-
anol. The resulting PI was thoroughly washed with methanol,
and then dried at 60 ꢀC under vacuum. Yield: 92%. The number
average molecular weight (Mn) and weight average molecular
weight (Mw) values tested by GPC were 2.6 ꢂ 104 and 4.7 ꢂ
104, respectively, with the dispersity (Mw/Mn) of 1.8. FT-IR:
nmax/cmꢁ1 3056, 1778 and 1724 (C]O asymmetric and symmetric
stretching), 1615, 1537, 1509, 1478, 1451, 1363 (C–N stretching),
1229, 1045 (C–F stretching), 833, 750, 724. Anal. Calcd for
(C88H46F6N4O4)n: C, 78.62; H, 3.21; N, 4.48; found: C, 79.03; H,
3.47; N, 4.39%.
of the aromatic ring appeared at 109.4–147.7 ppm. For TPABD,
the proton peaks of the aromatic ring appeared at 6.37–7.21 ppm
and the carbon peaks of the aromatic ring appeared at 114.2–
147.8 ppm. All the spectroscopic data were in good agreement
with the expected structure.
The molecular structure of diamine CzBD was also confirmed
by single-crystal X-ray diffraction. A single crystal of CzBD
could be grown by evaporation of a saturated 1,2-dichloroethane
solution. The molecular structure of CzBD is demonstrated in
Fig. 1. The dihedral angle of the two central nitrogen-connected
phenyl rings (plane 1: C19–C24 and plane 2: C25–C30) was
64.15ꢀ. And the dihedral angle between the 1,4-disubstituted
phenyl ring (plane 3: C13–C18) and the central nitrogen-con-
nected phenyl ring (plane 1) was 23.17ꢀ, while a delicate differ-
ence dihedral angle between plane 2 and plane 5 (C31–C35) was
44.05ꢀ on the other side. Meanwhile, the dihedral angle between
plane 3 and plane 4 (C1–C12) was 52.24ꢀ, which is much smaller
than that of plane 5 and plane 6 (C37–C48) 85.55ꢀ. Most of all,
the dihedral angle between plane 3 and plane 5 was 23.17ꢀ. As
shown in Fig. 1, compound CzBD displayed a rod-like twisted
geometry configuration. This three-dimensional structure
together with the bulky pendant groups in the tetracarboxylic
dianhydride will hinder the close packing of the polymer chain
and enhance the solubility of formed PIs.
Synthesis of PI(TPABD-BTFBPDA)
Using TPABD as a diamine, PI(TPABD-BTFBPDA) was
synthesized by a procedure similar to that of PI(CzBD-
BTFBPDA). The yield of PI(TPABD-BTFBPDA) was 89%. The
number average molecular weight (Mn) and weight average
molecular weight (Mw) values tested by GPC were 2.9 ꢂ 104 and
4.3 ꢂ 104, respectively, with the dispersity (Mw/Mn) of 1.5. FT-
IR: nmax/cmꢁ1 3033, 1778 and 1724 (C]O asymmetric and
symmetric stretching), 1590, 1537, 1509, 1491, 1364 (C–N
stretching), 1276, 1045 (C–F stretching), 832, 751, 696. Anal.
Calcd for (C88H50F6N4O4)n: C, 78.80; H, 3.76; N, 4.18; found: C,
78.53; H, 3.57; N, 4.46%.
The functional PIs in Scheme 2 were prepared via the
conventional two-step method by reacting equimolar amounts of
the diamines CzBD and TPABD with BTFBPDA to form the
poly(amic-acid)s at ambient temperature, followed by chemical
cyclodehydration. Chemical imidization of the poly(amic-acid)s
with a dehydrating agent, such as a mixture of acetic anhydride
and pyridine, was also effective in preparing the PIs. The polymer
structures were confirmed by FT-IR spectroscopy (Fig. 2) and
elemental analysis. Both of the PIs exhibited characteristic
absorption bands of the imide ring near 1778 and 1724 cmꢁ1 due
to C]O asymmetric and symmetric stretching, and 1364 cmꢁ1
ascribing to C–N stretching. Furthermore, the intrinsic viscosi-
ties of PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA) were
of 0.55 and 0.64 dL gꢁ1 in DMAc, respectively. The GPC analysis
referenced to polystyrene standards showed the number average
molecular weights (Mn) and the dispersity (Mw/Mn) of PI(CzBD-
BTFBPDA) were of 2.6 ꢂ 104 and 1.8, respectively, while those
Results and discussion
Synthesis and characterization of monomers and polyimides
In this study, new functional compounds, CzBD and TPABD,
were successfully synthesized by Suzuki reaction with moderate
yields (Scheme 1). The FT-IR spectra (ESI†, Fig. S2) of the
obtained compound present the characteristic absorption of N–
H asymmetric and symmetric stretching located at 3450 and
3380 cmꢁ1 and the characteristic absorption due to the C–N
1
stretching at 1316 cmꢁ1. H and 13C NMR spectra of CzBD are
shown in Fig. S3 and S4 (ESI†), respectively, with the assignment
of the observed resonance. For CzBD, the proton peaks of the
aromatic ring appeared at 6.56–8.22 ppm and the carbon peaks
Fig. 1 ORTEP drawing of CzBD, showing 30% probability displace-
Scheme 1 Synthesis of CzBD and TPABD.
ment ellipsoids and the hydrogen atoms are omitted for clarity.
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Scheme 2 Synthesis of PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA).
of PI(TPABD-BTFBPDA) were of 2.9
respectively.
ꢂ
104 and 1.5,
Thermal, optical and electrochemical properties of the polymers
The thermal properties of the PIs were investigated by TGA
(Fig. 3) and DSC (ESI†, Fig. S5). The 5% weight loss tempera-
tures (T5%
)
of PI(CzBD-BTFBPDA) and PI(TPABD-
BTFBPDA) were 526 ꢀC and 547 ꢀC, respectively. These
polymers afforded a high anaerobic char yield of around 75% at
900 ꢀC in an argon atmosphere. From the DSC curves, both PIs
showed the glass transition temperature39 at 293 ꢀC. Good
thermal stability and high values of Tg are important parameters
for polymers incorporated in memory devices, because they
provide resistance against the deformation or degradation of the
active layers.
Fig.
2 FT-IR spectra of PI(CzBD-BTFBPDA) and PI(TPABD-
BTFBPDA).
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in dilute solution. All the emission spectra were obtained with the
excitation wavelength of 300 nm. As shown in Fig. 4a, the CzBD
and TPABD exhibit intense emission peaks at 451 and 444 nm,
respectively, consistent with the fact that the carbazole and tri-
phenylamine derivatives are efficient fluorescence chromo-
phores.41,42 In comparison with the diamine monomers, the
fluorescence of PI(CzBD-BTFBPDA) and PI(TPABD-
BTFBPDA) is significantly quenched, indicating considerable
decay of the excited singlet state of carbazole and triphenylamine
moieties, either by charge or energy transfer between these side
groups and phthalimide moieties.43
The cyclic voltammetry curves of PI(CzBD-BTFBPDA) and
PI(TPABD-BTFBPDA) are shown in Fig. 4b, with the PI films
coated on ITO glass in anhydrous acetonitrile containing 0.1 M
tetrabutylammonium perchlorate as supporting electrolyte. Both
Fig.
3 TGA curves of PI(CzBD-BTFBPDA) and PI(TPABD-
BTFBPDA) at a scan rate of 10 ꢀC minꢁ1 under argon.
the polymers showed one oxidation peak, which could be due to
ox
The UV-vis and fluorescence spectra of CzBD, TPABD and
the studied polymers in THF solution are shown in Fig. 4a. The
UV-vis spectrum of CzBD shows the absorption peak maximum
(lmax) at 293 nm and two shoulder peaks at 329 and 341 nm
characteristic of the p–p* transition in carbazole and phenyl
nuclei, while the absorption peak maximum of TPABD is at 306
nm corresponding to the p–p* transition among the phenyl
rings. PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA) films
coated on quartz glass exhibit a maximum absorption peak at
312 and 337 nm, respectively. These were assignable to the p–p*
transition, which was attributed to the twisted structure between
side chain phenyl or carbazole and the main chain biphenyl
aromatic rings. In addition, from Fig. 4, the absorption edges of
PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA) extend to
about 362 nm and 388 nm, from which the optical band gaps (i.e.,
the difference between the highest occupied molecular orbital
(HOMO) level and the lowest unoccupied molecular orbital
(LUMO) level) of PI(CzBD-BTFBPDA) and PI(TPABD-
BTFBPDA) are estimated to be 3.42 and 3.20 eV, respectively.
Fig. 4a shows the photoluminescence40 spectrum of PIs, as well as
those of the two diamine monomers in THF solution. The
concentrations of PI solution for the PL measurement were of
about 10ꢁ7 mol Lꢁ1. The concentrations of CzBD and TPABD
were adjusted to be comparable to that of the repeat units of PIs
the nitrogen-containing structure. The onset oxidation E
for
onset
PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA) was deter-
mined to be 1.15 V and 0.86 V versus Ag/AgCl, respectively,
which indicates that the TPA-containing PI is more easily
oxidized than that of carbazole-containing PI. The external
ferrocene/ferrocenium (Fc/Fc+) redox standard potential,
1/2
ferrocene
E
, was measured to be 0.44 V vs. Ag/AgCl in acetonitrile.
Assuming that the HOMO level for the Fc/Fc+ standard is
ꢁ4.80 eV with respect to the zero vacuum level, the HOMO levels
for PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA) are
determined to be ꢁ5.51 eV and ꢁ5.22 eV, respectively. There-
fore, the LUMO levels of PI(CzBD-BTFBPDA) and PI
(TPABD-BTFBPDA) are estimated to be ꢁ2.10 eV and
ꢁ2.02 eV, respectively. In the Al/PI(CzBD-BTFBPDA)/ITO
device, the energy barrier to hole injection from the electrode to
the active layer was estimated to be 0.71 eV from the work
function (F) of ITO (ꢁ4.80 eV) and the HOMO of the active PI
layer, and the energy barrier to electron injection from the elec-
trode to the active layer was estimated to be 2.10 eV from the F
of Al (ꢁ4.20 eV) and the LUMO of the active layer. This indi-
cates that hole injection from ITO into the HOMO of PI(CzBD-
BTFBPDA) is easier than electron injection from Al into the
LUMO of PI(CzBD-BTFBPDA). Therefore, PI(CzBD-
BTFBPDA) is a p-type material and holes predominate the
Fig. 4 (a) UV/vis absorption and photoluminescence spectra (lex ¼ 300 nm, respectively) of CzBD and TPABD in dilute THF solution as well as PI
(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA) films coated on quartz glass. (b) Cyclic voltammograms of PI(CzBD-BTFBPDA) and PI(TPABD-
BTFBPDA) on an ITO electrode in 0.1 M TBAP/acetonitrile solution with Ag/AgCl as reference electrode and Pt plate as counter electrode.
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conduction process. In the Al/PI(TPABD-BTFBPDA)/ITO
device, the energy barrier to hole injection from the electrode to
the active layer was estimated to be 0.42 eV and the energy
barrier to electron injection from the electrode to the active layer
was estimated to be 2.18 eV, indicating that PI(TPABD-
BTFBPDA) is also a p-type material and the conduction process
in the device is dominated by hole injection as well. Comparing
the energy barrier to hole injection from the electrode to the
active layer of the two PIs, it can be concluded that hole injection
in the Al/PI(TPABD-BTFBPDA)/ITO device could be more
easier than that in the Al/PI(CzBD-BTFBPDA)/ITO device.
in the data storage) with a current in the range of 10ꢁ12 to 10ꢁ7
A
as the voltage swept from 0 to about ꢁ2.7 V. When the voltage
increases further, it induces a sharp increase in current at
a threshold voltage of about ꢁ2.7 V (sweep 1), as indicated by the
transition from the OFF state to the ON state (‘‘1’’ signal in the
data storage) of the current from 10ꢁ7 to 10ꢁ2 A. The electronic
transition from the OFF state to the ON state in the first sweep
serves as the ‘‘writing’’ process (defined as the SET operation).
Furthermore, on reaching the ON state, the device maintains the
state under both the negative (sweep 2) and positive voltage scans
(sweep 3), and its current–voltage curves were almost symmet-
rical. Sweeps 4, 5 and 6 were the measurements of another cell of
the device. Over the voltage range of 0 to +4 V, it was observed
an abrupt transition from a low conductivity state to a high
conductivity state at a switching threshold voltage of about +2.6
V (sweep 4). After undergoing the OFF-to-ON transition, the
device remains in the ON state even after turning off the electrical
power for 10 min (sweep 5 and sweep 6). This memory device
could not be returned to the OFF state by turning off the power
and application of a reverse bias of the same magnitude after it
had been switched on. Thus, this device exhibits the WORM
memory behavior. Moreover, I–V curves of ten Al/PI(CzBD-
BTFBPDA)/ITO cells at the SET operation are shown
in Fig. S6a (ESI†), indicating the SET current in the range of
Memory device characteristics of the polymers
The memory effects of PI(CzBD-BTFBPDA) and PI(TPABD-
BTFBPDA) are demonstrated by the current–voltage (I–V)
characteristics of an Al/polymer/ITO device. The polymeric
memory devices store data based on the high (ON) and low44
conductivity responses to the external applied voltages.20 Fig. 5
exhibits the typical I–V curves of the memory devices fabricated
with PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA). For
the case of PI(CzBD-BTFBPDA) (Fig. 5a), the I–V character-
istics were measured by scanning the voltage from 0 to ꢃ4 V with
grounded ITO. The device is initially in the OFF state (‘‘0’’ signal
Fig. 5 (a) Current–voltage (I–V) characteristics of Al/PI(CzBD-BTFBPDA)/ITO devices with an electrode area of 0.0314 mm2. (b) Retention char-
acteristics of both ON and OFF states for the Al/PI(CzBD-BTFBPDA)/ITO device under a constant stress (ꢁ1.0 V) at room temperature. (c) Current–
voltage (I–V) characteristics of the Al/PI(TPABD-BTFBPDA)/ITO device with an electrode area of 0.0314 mm2. (d) Retention characteristics of both
ON and OFF states for the Al/PI(TPABD-BTFBPDA)/ITO device under a constant stress (ꢁ1.0 V) at room temperature.
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2.5–2.8 V. Besides the switching capability, stability is always an
important issue in memory device performance. Fig. 5b shows
the retention time and stress tests of both the ON and OFF states
of PI(CzBD-BTFBPDA). Under a constant stress of ꢁ1.0 V, no
obvious degradation in current is observed in both ON and OFF
states for at least 104 s during the readout test. And the resistance
distribution of the high and low conductivity states is shown in
Fig. S6b (ESI†), indicating the resistance of the high and low
conductivity state were about 109 and 103 U.
carbazole, triphenylamine, phthalimide and 30,40,50-tri-
fluorobiphenyl moieties. During the calculations, the BUs were
first optimized at the B3LYP/6-31G(d) level with the Gaussian 09
program package, and then the molecular orbitals of HOMO
and LUMO were predicted at the same level. The optimized
structures of the BUs of CzBD-BTFBPDA and TPABD-
BTFBPDA are plotted in the right part of Fig. 6. It has been
demonstrated that the DFT calculations for the carbazole unit
can provide the reliable parameters as evidenced by the X-ray
results in our work. For example, the theoretical dihedral angle
between the 1,4-disubstituted phenyl ring (plane 3) and carbazole
ring (plane 4) is 54ꢀ, which is in a good agreement with the
experimental result of 52ꢀ. Besides, the noncoplanar geometry is
shown between the nitrogen-connected phenyl ring (such as
plane 1) and the 1,4-disubstituted phenyl ring with the dihedral
angle at 37ꢀ predicted by the theoretical calculation, which is also
much closer to the experimental value (44ꢀ). These factors
contribute to the twist geometries of CzBD-BTFBPDA. While
for TPABD-BTFBPDA, the DFT optimized geometries of the
triphenylamine moiety are also in good agreement with data
from X-ray analysis of triphenylamine,45 which has a non-planar
structure with the N atom deviating from the plane of the bonded
C atoms and a dihedral angle of about 75ꢀ between any two
phenyl rings of the triphenylamine molecule. Therefore, it
suggests that the DFT method at the B3LYP/6-31G(d) energy
level is a reliable method to optimize the geometry and electronic
structures of nitrogen-containing imide compounds. The better
coplanarity of the CzBD-BTFBPDA than that of TPABD-
BTFBPDA could be due to the structure difference on the
nitrogen-containing moiety. The molecular electrostatic poten-
tial (ESP) surfaces of CzBD-BTFBPDA and TPABD-
BTFBPDA are also shown in the right part of Fig. 6. The ESP
surfaces show that both polymers consist mainly of the positive
ESP region (red color as shown in Fig. 6), with large negative
regions (blue color as shown in Fig. 6) arising from the sp2
hybridized O atoms in the phthalimide group and the minor
negative region also arising from the sp2 hybridized N atoms in
the carbazole and triphenylamine moieties. These negative ESP
regions suggest that concentrated local electron densities near the
sp2 hybridized atoms possess a certain electron-withdrawing
ability, indicating that, under excitation, electrons will transfer
from the N-containing pendent moieties to the phthalimide
moieties.
The electrical behavior of the Al/PI(TPABD-BTFBPDA)/ITO
device was also investigated as shown in Fig. 5c. During voltage
sweeping from 0 to ꢁ1.6 V, the current is very low and in the
order of 10ꢁ10 to 10ꢁ8 A, which indicates that the device is in low
conductivity state. However, the continuous sweeping voltage
induces an abrupt increase in current from 10ꢁ8 to 10ꢁ3 A at
around ꢁ1.6 V, which serves as the ‘‘writing’’ process for the
memory device. The device remains in the ON state during the
subsequent second sweep from 0 to ꢁ4.0 V and does not relax to
the OFF state even after the power is turned off for 10 min. As
the voltage sweeps positively from 0 to +3.0 V, an abrupt
decrease in current is observed at a threshold voltage of about
+2.2 V, corresponding to the ‘‘erasing’’ process for the memory
device (defined as the RESET operation). Similarly, the device
remains in a low conductivity (OFF state) during the subsequent
fourth sweep from 0 to +3.0 V and even after the power is turned
off. The OFF state can be further turned on to the ON state by
reapplying the switching threshold voltages, as the fifth to the
eighth sweep in Fig. 5c. Moreover, I–V curves of ten Al/PI
(TPABD-BTFBPDA)/ITO cells at the SET and RESET opera-
tion are shown in Fig. S7a (ESI†), indicating the SET current in
the range of ꢁ1.4 to ꢁ1.8 V and the RESET current in the range
of +2.2 to +2.7 V. Therefore, this memory device could be used
as a flash type memory. Further, the retention time characteris-
tics of the ON/OFF states for the Al/PI(TPABD-BTFBPDA)/
ITO memory cell are shown in Fig. 5d. It is found that the
memory cell exhibits two stable states of ON and OFF within 104
s, and the resulting ON/OFF ratio is as large as 106 under ꢁ1.0 V.
No degradation was observed during the test, suggesting good
device stability. The resistance distribution of the high and low
conductivity state is shown in Fig. S7b (ESI†), indicating the
resistance of the high and low conductivity state was also of
about 109 and 103 U. And the endurance characteristic of the Al/
PI(TPABD-BTFBPDA)/ITO cell is presented in Fig. S7c (ESI†).
The resistance ratios of the high resistance state to the low
resistance state are more than 105 during the 1000 cycles of the
fatigued test.
The calculated energy levels (HOMO, LUMO) of PI(CzBD-
BTFBPDA) and PI(TPABD-BTFBPDA) were of (ꢁ5.30, ꢁ2.52)
and (ꢁ4.92, ꢁ2.43) eV, respectively, which are comparable to
those determined from the CV measurement ((HOMO, LUMO)
energy levels of (ꢁ5.51, ꢁ2.10) and (ꢁ5.22, ꢁ2.02) eV, respec-
tively). Although the molecular computation only considers the
single basic unit of the polymer which does not include the effect
of the solid-state conjugated network, the calculated energy
levels of the polymers are a bit far from the experimental value. It
has been demonstrated that the relative ordering of the occupied
and virtual molecular orbitals gives a reasonable indication of
the excited properties and charge transport ability.46 On the basis
of the optimized structures, the charge density isosurfaces of the
basic unit of PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA)
can also be obtained. As shown in the left part of Fig. 6, the
HOMOs of CzBD-BTFBPDA and TPABD-BTFBPDA are
Theoretical study on geometry and electronic properties
It has been demonstrated that quantum chemical calculations are
capable of predicting the structures and relevant energy and
electronic orbital parameters associated with charge transfer of
photoelectric materials. In order to obtain a better understanding
of the switching behavior of the PI-based memory devices, the
structure and electronic properties of the PI were investigated by
density functional theory (DFT) theoretical calculations. In the
calculations, the basic units (BU) of CzBD-BTFBPDA and
TPABD-BTFBPDA were used, and some models have taken all
of the component functional moieties into account, i.e., the
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Fig. 6 Electronic density contours of molecular orbitals, the corresponding energy levels and electrostatic potential (ESP) surface of the basic units:
CzBD-BTFBPDA and TPABD-BTFBPDA.
located mainly on the carbazole and triphenylamine moieties,
respectively, while the first LUMOs of both BUs are located on
the phthalimide moieties, indicating that the carbazole and tri-
phenylamine moieties act as the electron donors and the phtha-
limide moieties act as the electron acceptors in the copolymer. In
addition, the fourth LUMOs are located on the nitrogen-con-
nected phenyl ring and the1,4-disubstituted phenyl ring of the
amine parts. The two intermediate LUMOs (LUMO + 1 and
LUMO + 2) are distributed on the 30,40,50-trifluorobiphenyl unit
on CzBD-BTFBPDA and TPABD-BTFBPDA. Based on the
molecular electrostatic potential surfaces, both of the HOMO
and LUMO + 3 isosurface tend to be located on the donor while
the LUMO, LUMO + 1 and LUMO + 2 are located on the
acceptor. Under excitation with sufficient energy, electrons in the
ground state of PIs can transit to the various excited states. This
indicates that the electron transfer, subsequent to the excitation
of the CzBD or TPABD moiety, leads to the charge transfer
state. The polarized charge transfer mechanism is partially
similar to that of PYTPA-PI under the condition of applied
field.28 It is noteworthy that the enhanced dihedral angle between
the ground and excited states was also verified by theoretical
calculation,12 which agrees with the increase in torsional angle
based on the action of a static electric field perpendicular to the
ring–ring bond.47 The electrical switching under the applied
negative and positive threshold voltages is suggested to form
a stable environment temporarily and store the transferred
charges. Therefore, the charge transfer state can be sustained
steadily, and the ON state remains for a period of time.
To further understand the electrical switching characteristics
and the current conduction mechanism of our devices, the
measurement I–V data in both states are analyzed by using
various theoretical conduction models reported in the litera-
ture.19,48–53 For the Al/PI(CzBD-BTFBPDA)/ITO device, under
a low positive bias (for example, 0 to +2.6 V in the fourth sweep
in Fig. 5a, with Al as the anode), a Schottky barrier forms at the
Al interface due to the difference in work functions between Al
and the HOMO of PI(CzBD-BTFBPDA). Thus, the current
density is limited by hole injection and can be fitted by the
Schottky emission model48 (Fig. 7a) of the form:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðI=AÞ ¼ T2exp ½ꢁqðfB ꢁ qV=4p3idÞ=kTꢄ, where A is the
effective Richardson constant, T is room temperature (298 K),
fB is the barrier height, 3i is the dynamic permittivity of the
insulator and d is the film thickness. At the threshold voltage, the
electric field-induced charge transfer occurs, resulting in
the generation of holes in the HOMO, which can delocalize to the
conjugated carbazole-substituted biphenyl groups. Thus, hole
migrations form a pathway via hopping, leading to a surge in
current and the transition of the device from the OFF state to the
ON state. In the ON state, the I–V curve follows the ohmic
conduction model48 (Fig. 7b) of the form: J ¼ qnmV/d, where q is
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Fig. 7 (a) I–V plot of the Al/PI(CzBD-BTFBPDA)/ITO device in the OFF state: the symbols are the measured data and the solid line is the data fitted
with the Schottky emission model. (b) I–V plot of the Al/PI(CzBD-BTFBPDA)/ITO device in the ON state: the symbols are the measured data and the
solid line is the data fitted with the ohmic conduction model. (c) Log–log I–V plot of the Al/PI(TPABD-BTFBPDA)/ITO device in the OFF state:
the symbols are the measured data and the solid line is the data fitted with the SCLC model. (d) I–V plot of the Al/PI(TPABD-BTFBPDA)/ITO device in
the ON state: the symbols are the measured data and the solid line is the data fitted with the ohmic conduction model.
the electronic charge, n is the charge barrier density, m is the
carrier mobility and d is the film thickness. For the Al/PI
(TPABD-BPDA)/ITO device, as shown in Fig. 7c, the loga-
rithmic plot of the I–V data for the OFF state shows a linear
region with a slope of 1.3; namely, the I–V data for the OFF state
is satisfactorily fitted by a trap-limited space-charge conduction
(SCLC) model54 following the equation: I ¼ 9A330mV2/8d3, where
330 is the absolute permittivity of the active layer, d is the
thickness of the polymer film, and m is the carrier mobility. This
result indicates that a trap limited SCLC mechanism is dominant
when the device is in the OFF state. On the other hand, the
logarithmic plot of the I–V data for the ON state shows a linear
region with a slope of 1.0 (Fig. 7d), indicating that ohmic current
conduction is dominant when the device is in the ON state.
Moreover, the current level of our devices in the ON state was
found not to be dependent on the device cell size, which is
indicative of heterogeneously local filament formation.
mechanisms of charge transfer between the electron donor and
acceptor moieties, as well as electric field induced conformational
ordering of the polymer side chains/backbones, are proposed for
the polymer electronic memories. However, switching behav-
iours of PI(CzBD-BTFBPDA) and PI(TPABD-BTFBPDA) are
different. The reason why the device with the PI(CzBD-
BTFBPDA) film exhibits very stable WORM memory behav-
iour, whereas that with the PI(TPABD-BTFBPDA) film exhibits
rewritable flash memory features, may be attributed to the
difference in the pendent moieties in the polymer repeat units.
For Al/PI(CzBD-BTFBPDA)/ITO devices, the fact that the
electrical switching arises from the electric field-induced charge
transfer, rather than the hole injection process, is supported by
the symmetrical bidirectional switching, with comparable posi-
tive and negative threshold voltages from the first and the fourth
sweeps in Fig. 5a. If the hole injection process plays the dominant
role, the device should exhibit an asymmetrical switching or
unidirectional switching phenomenon55 due to the difference in
energy barriers for hole injection under different applied biases
(1.31 eV for hole injection into the HOMO from Al under
forward bias and 0.71 eV for hole injection into the HOMO from
ITO under the reverse bias). When comparing properties of
carbazole and triphenylamine, the carbazole units possess
Due to the electron donating and hole transporting nature of
the carbazole and triphenylamine molecules, both WORM and
rewritable bistable memory characteristics have been demon-
strated in Al/polymer/ITO sandwich structure devices. Based on
the unique molecular structures and electronic properties of these
carbazole and triphenylamine-containing polymers, switching
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a larger band gap and more planarity improved by the bridging
nitrogen atom. Once the free path for electrons along the whole
molecular sweeping by the threshold voltages, the more planarity
of carbazole moieties can prevent the charge from going back to
the initial state even after the application of the reverse bias, thus
the ON state of the device can be sustained. TPA units exhibit
easier oxidizability of the nitrogen center, and transportability of
positive charge centers via the radical cation species.56 For Al/PI
(TPABD-BTFBPDA)/ITO devices, the ON state of the device
cannot be sustained due to the limited delocalization of charges
on the triphenylamine substituted biphenyl moieties.28 The PI
(TPABD-BTFBPDA) relaxes to the initial conformation after
the removal of the applied electric field for a short period of time.
The dihedral angles between the ring moieties reduce to the
original state, and, also, the potential barriers of back charge
transfer disappear. Hence, the back charge transfer occurs from
the phthalimide to TPA (reset to the OFF state) and confirms
the bistable switching behavior. This characteristic is similar
to the rapid decay of photocurrent in aromatic PIs, resulting
from the back electron transfer or the germinating recombina-
tion process57,58 after the irradiation had been turned off.
Moreover, the calculated dipole moment of CzBD-BTFBPDA is
only 3.55 D while that of TPABD-BTFBPDA is 2.05 D, indi-
cating that the polarity of the PI(TPABD-BTFBPDA) is not
strong enough to retain the charge transfer state, thus accounting
for the short retention time of the ON state. Also, a lower
HOMO level of CzBD-BTFBPDA than TPABD-BTFBPDA
results in a higher turn-on voltage of the former.
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Conclusions
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may offer an opportunity to reduce the chain interactions,
thereby improving the electric, optoelectronic, and photonic
properties of the resulting polymers. In this study, PI(CzBD-
BTFBPDA) and PI(TPABD-BTFBPDA) containing carbazole
and triphenylamine pendent groups were prepared by respective
reaction of 2,20-bis[4-(9H-carbazol-9-yl)phenyl]-4,40-biphenyl
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voltages around ꢃ2.7 V, while Al/PI(CzBD-BTFBPDA)/ITO
devices presented flash memory behavior with low turn-on and
turn-off threshold voltages at ꢁ1.6 and +2.2 V, respectively.
Both of the devices exhibited high ON/OFF current ratios
around 106 and long retention times of 104 s even in the ambient
atmosphere. The different turn-on threshold voltages apparently
resulted from the two different low-lying HOMO energy levels.
The theoretical analysis suggested that the charge transfer
mechanism could be used to explain the memory characteristics.
The relatively higher dipole moments of the carbazole-containing
PI compared to the triphenylamine-based PI provided a stable
charge transfer complex for the WORM memory devices. The
present study suggests that these new carbazole and triphenyl-
amine-containing PIs have potential applications for memory
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