Thiophene and selenophene donor-acceptor polyimides as polymer electrets for nonvolatile transistor memory devices

Article abstract of DOI:10.1021/ma301326r

We report the nonvolatile memory characteristics of n-type N,N'-bis(2-phenylethyl)perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI) based organic field-effect transistors (OFET) using the polyimide electrets of poly[2,5-bis(4-aminophenylenesulfanyl)se

Full text of DOI:10.1021/ma301326r

Article
pubs.acs.org/Macromolecules
Thiophene and Selenophene Donor−Acceptor Polyimides as
Polymer Electrets for Nonvolatile Transistor Memory Devices
Ying-Hsuan Chou,† Nam-Ho You,‡ Tadanori Kurosawa,‡ Wen-Ya Lee,† Tomoya Higashihara,‡
Mitsuru Ueda,‡,* and Wen-Chang Chen†,*
†Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 10617
‡Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8552
Japan
S
* Supporting Information
ABSTRACT: We report the nonvolatile memory character-
istics of n-type N,N′-bis(2-phenylethyl)perylene-3,4:9,10-
tetracarboxylic diimide (BPE-PTCDI) based organic field-
effect transistors (OFET) using the polyimide electrets of
poly[2,5-bis(4-aminophenylenesulfanyl)selenophene−hexa-
fluoroisopropylidenediphthalimide] (PI(APSP-6FDA)), poly-
[2,5-bis(4-aminophenylenesulfanyl)thiophene−hexafluoroiso-
propylidenediphthalimide] (PI(APST-6FDA)), and poly(4,4′-
oxidianiline-4,4′-hexafluoroisopropylidenediphthalic anhy-
dride) (PI(ODA-6FDA)). Among those polymer electrets,
the OFET memory device based on PI(APSP-6FDA) with a strong electron-rich selenophene moiety exhibited the highest field-
effect mobility and Ion/Ioff current ratio of 105 due to the formation of the large grain size of the BPE-PTCDI film. Furthermore,
the device with PI(APSP-6FDA) exhibited the largest memory window of 63 V because the highest HOMO energy level and
largest electric filed facilitated the charges transferring from BPE-PTCDI and trapping in the PI electret. Moreover, the charge
transfer from BPE-PTCDI to the PI(APSP-6FDA) or PI(APST-6FDA) electrets was more efficient than that of PI(ODA-6FDA)
due to the electron-donating heterocyclic ring. The nanowire device with PI(APSP-6FDA) showed a relatively larger memory
window of 82 V, compared to the thin film device. The present study suggested that the donor−acceptor polyimide electrets
could enhance the capabilities for transferring and store the charges and have potential applications for advanced OFET memory
devices.
ferroelectric polymers have a low solubility in common solvents
and require a high annealing temperature for developing the
ferroelectric characteristics, which may degrade the electrical
performance of OFETs. Recently, much attention has been
attracted on the development of single-component polymer
electrets. Kim and co-workers discovered that the pentacene-
based OFET memory device using poly(α-methylstyrene) as
polymer electret displayed fast writing and erasing speeds
around 1 μs.14 The polymer charge trapping ability of the
electret induced memory characteristics and their hydrophilicity
and dielectric polarity significantly affected the device perform-
ance.22 Recently, our group also found that memory voltage
windows of pentacene-based OFET memory devices were
significantly affected by the conjugated lengths of polymer
electrets.23 Donor−acceptor polymers can enhance the charge
storages through the induced intramolecular charge transfer
(ICT) from the donor to acceptor moiety under an applied
electric field. However, such class of polymers has not been
explored as polymer electret for OFET memory devices.
INTRODUCTION
■
Organic memory devices have received extensive research
interest recently due to the advantages of the structural
flexibility, processabiliy, lightweight and simple manufacturing
process compared to those employed in the inorganic silicon
technology.1−5 The proposed organic or polymeric memory
devices include resistor-, capacitor- and transistor-type
memories.3 Among these memory devices, organic field effect
transistor (OFET) type memories showed the most noticeable
application due to easily integrated structure, nondestructive
reading, and multiple-bit storage in a single transistor.6
The configuration of OFET memory devices is a conven-
tional transistor with an additional polymer electret between a
semiconductor layer and dielectric layer. Polymer electrets are
dielectric materials with a long-term charge storing ability or
electrostatic polarization, including ferroelectric polymers,7,8
polymer composites,9−13 and nonconjugated polymer.6,14,15
The electrical switching of the OFET memory devises arose
from charge trapped in chargeable polymer electrets12,16,17 or
polarization of ferroelectric materials.18−20 Naber et al.
demonstrated that OFET type nonvolatile memories using an
organic ferroelectric electret had an on/off ratio of 104, and the
device was stable for more than 1 week.21 However,
Received: June 27, 2012
Revised: August 12, 2012
Published: August 29, 2012
© 2012 American Chemical Society
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Scheme 1. (a) Structure of APSP-6FDA, APST-6FDA, and ODA-6FDA and (b) Synthetic Routes for (I) APSP, (II) APST, (III)
PI(APSP-6FDA), and PI(APST-6FDA)
Donor−acceptor polyimides (PIs) have been explored for
applications in organic electronics, such as light-emitting
diodes,24 photovoltaics,25 and xerography,26 and resistor-type
memory,16,20,27−32 due to their good electrical properties,
thermal stability and chemical resistance.16,28,32−36 Most studies
on the polyimide-based memory devices were mainly resistor-
type devices with a simple sandwich structure of metal/
polymer/metal. However, the memory behaviors of resistor-
type structure are easily affected by filament effects, resulted
from the formation of highly conductive metallic filaments
induced by the migration of metal atoms from electrodes. Thus,
it would be difficult to establish the correlation between
polymer structures and memory characteristics. Therefore, we
explore the OFET memory devices to minimize the filament
effect.
In this study, we report the memory behavior of n-type N,N′-
bis(2-phenylethyl)perylene-3,4:9,10-tetracarboxylic diimide
(BPE-PTCDI) based OFET memory devices using polyimide
electrets. The polyimide electrets, PI(APSP-6FDA) and
PI(APST-6FDA), are consisted of electron-donating 2,5-
bis(4-aminophenylenesulfanyl)selenophene (APSP) or 2,5-
bis(4-aminophenylenesulfanyl)thiophene (APST) and elec-
tron-accepting 4,4′-(hexafluoroisopropylidene)diphthalic anhy-
dride (6FDA), as shown in Scheme 1. For comparison,
poly(4,4′-oxidianiline-4,4′-hexafluoroisopropylidenediphthalic
anhydride) (PI(ODA-6FDA)) (Scheme 1a) is also used as a
polymer electret. The BPE-PTCDI OFET memory device
enables the reversible trapping of hole carriers in gate
dielectrics. The selenophene and thiophene moieties in PIs
are expected to enhance the electron-donating ability. To the
best of our knowledge, this is the first study on n-type OFET
memory devices using donor−acceptor polymer electrets.
EXPERIMENTAL SECTION
■
Materials. Selenophene and 2,5-dichlorothiophene were purchased
from Sigma-Aldrich Corp. Copper, quinoline, and dehydrated N,N-
dimethylacetamide (DMAc) were used as received. p-Aminothiophe-
nol, 1,3-dimethyl-2-imidazolidinone (DMI), oxidianiline (ODA), and
4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) were
purchased from TCI and used as received. All other chemicals were
purchased from TCI Japan. BPE-PTCDI was purchased from
Luminescence Technology Corp (Taiwan).
M o n o m e r S y n t h e s i s . S y n t h e s i s o f 2 , 5 - B i s ( 4 -
aminophenylenesulfanyl)selenophene (APSP). p-Aminothiophenol
(2.61 g, 20.4 mmol), quinoline (15 mL), toluene (15 mL), and
potassium carbonate (3.47 g, 25.1 mmol) were mixed into a 50 mL
round flask. The mixture was heated with stirring at 150 °C for 4 h
under a nitrogen atmosphere to remove water with a Dean−Stark
apparatus. After complete removal of water, residual toluene was
distilled off. Copper powder (0.066 g, 1.04 mmol) and 2,5-
dibromoselenophene (2.95 g, 10.2 mmol) were added to the mixture,
followed by heating at 200 °C for 24 h under a nitrogen atmosphere.
Then, the mixture was cooled to room temperature and filtered to
remove copper powder. The filtrate was extracted with dichloro-
methane and water. The solvent was evaporated, and the crude solid
was purified by column chromatography using dichloromethane and
recrystallized from ethanol to give pale yellow crystals, 1.94 g (51%
yield), mp: 104.2 °C (DSC peak temperature). IR (KBr, cm−1), ν
(cm−1): 3459, 3351, 3050, 1612, 1492, 1299, 825. H NMR (300
1
MHz, DMSO-d6, δ, ppm): 7.15 (d, J = 8.1 Hz, ArH, 4H), 6.99 (s, ArH,
2H), 6.54 (d, J = 8.1 Hz, ArH, 4H), 5.48 (s, N−H, 4H). 13C NMR (75
MHz, DMSO, δ, ppm): 150.8, 146.9, 135.1, 131.5, 119.8, 115.4. Anal.
Calcd For C16H14N2: C, 50.92; H, 3.74; N, 7.42. Found: C, 51.31; H,
3.77; N, 7.39.
Synthesis of 2,5-Bis(4-aminophenylenesulfanyl)thiophene
(APST). In a 250 mL three-necked flask equipped with a magnetic
stirrer, a nitrogen inlet, a Dean−Stark trap, and a condenser were
placed p-aminothiophenol (9.39 g, 0.075 mol), potassium carbonate
(5.39 g, 0.039 mol), DMI (30 mL), and toluene (50 mL). The mixture
was heated with stirring at 140 °C for 4 h under a nitrogen
atmosphere. After complete removal of water, residual toluene was
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distilled off. Then, the mixture was cooled to 120 °C, and 2,5-
dichlorothiophene (4.59 g, 0.030 mol) dissolved in DMI (10 mL) was
added dropwise to the mixture. In the following, the reaction mixture
was poured into cold water (250 mL) to give a brown oil-like product.
The oil solidified several hours later. The upper water layer was
decanted and fresh water was added. The solid was crushed, collected
and washed thoroughly with water to give a crude product. The crude
diamine was recrystallized from ethanol followed by decoloration with
activated carbon. White colored crystals (7.92 g) were obtained in 80%
yield, mp 128.8 °C (DSC peak temperature). IR (KBr), ν (cm−1):
Device Fabrication and Characterization. The transistor-type
memory devices based on a BPE-PTCDI thin film were fabricated on a
wafer with a thermally grown 300-nm thick SiO2 dielectric on highly
doped n-type Si as a gate electrode. The solution of PI(APSP-6FDA)
or PI(APST-6FDA) in chloroform was spin-coated at 1000 rpm for 60
s on a wafer. Thereafter, the polymer thin films were dried under
vacuum (10−6 Torr) at 100 °C for 1 h to remove residue solvents. The
thickness of the prepared thin film was estimated to be 60−65 nm.
The thin film of BPE-PTCDI was prepared by thermally deposition
with a deposition rate of 0.3−0.4 nm s−1 at 90 °C under vacuum (10−7
Torr) to form a 50-nm-thick film. On the other hand, the nanowires of
BPE-PTCDI in solution were spin-coated at 1000 rpm for 60 s onto
the PIs electret. The nanowires were dried under vacuum (10−6 Torr)
at 70 °C for overnight to remove residue solvents. The top-contact
source and drain electrodes were defined by 80-nm thick gold through
a regular shadow mask, and the channel length (L) and width (W)
were 50 and 1000 μm, respectively. The current−voltage (I−V)
characteristics of the devices were measured by using a Keithley 4200-
SCS semiconductor parameter analyzer in a N2-filled glovebox.
Characterization. NMR spectra were recorded on a BRUKER
DPX-300S spectrometer at the resonant frequencies at 300 MHz for
1H and 75 MHz for 13C nuclei using CDCl3 or DMSO-d6 as the
solvent and tetramethylsilane as the reference. FT-IR spectra were
measured by a Horiba FT-120 Fourier transform spectrophotometer.
Mn and Mw values were evaluated by HITACHI SEC with two
polystyrene gel columns (Shodex GPC KD-804, KD-805). N,N-
Dimethylformamide (DMF) containing 0.01 M LiBr was used as an
eluent at a flow rate of 1.0 mL·min−1 calibrated by polystyrene
standard samples. Elemental analyses were performed on a Yanaco
MT-6 CHN recorder elemental analysis instrument. Thermal
properties were estimated from a Seiko TG/DTA 6300 thermal
gravimetric analysis (TGA) system and a thermal analysis instrument
of DSC-Q100 differential scanning calorimetry (DSC) under a
nitrogen atmosphere at a heating rate of 10 and 6 °C/min,
respectively.
1
3451, 3347, 1616, 1592, 1492, 1284, 1176, 829. H NMR (300 MHz,
CDCl3, δ, ppm): 7.11 (d, J = 8.1 Hz, ArH, 4H), 6.91 (s, ArH, 2H),
6.55 (d, J = 8.1 Hz, ArH, 4H), 5.36 (s, N−H, 4H). 13C NMR (75
MHz, CDCl3, δ, ppm): 146.9, 139.8, 133.4, 132.3, 124.3, 116.1. Anal.
Calcd For C16H14N2: C, 58.15; H, 4.27; N, 8.48. Found: C, 58.38; H,
4.35; N, 8.29.
Synthesis of Poly[2,5-bis(4-aminophenylenesulfanyl)-
selenophene−hexafluoroisopropylidenediphthalimide] (PI(APSP-
6FDA)). APSP (0.377 g, 1.00 mmol) and dehydrated DMAc (3.49
mL) were charged into a 20 mL flask equipped with a magnetic stirrer
under a nitrogen atmosphere (Scheme 1b). After APSP was
completely dissolved, 6FDA (0.444 g, 1.00 mmol) was added and
the solution was stirred at room temperature for 24 h to afford a
viscous poly(amic acid) (PAA) solution. A reaction mixture of acetic
anhydride (0.148 g, 1.49 mmol) and pyridine (0.115 g, 1.49 mmol)
was added to the above PAA solution. The reaction mixture was stirred
at room temperature under a nitrogen atmosphere for 24 h. The
resulting solution was poured into methanol, and the precipitate was
collected by filtration and washed with methanol. The final product
was dried at 200 °C for 10 h under vacuum and the polymer yield was
90%. The number-average molecular weight (Mn) and weight-average
molecular weight (Mw) values estimated from size exclusion
chromatography (SEC) were 4.91 × 104 and 9.90 × 104, respectively,
with the polydispersity index (PDI=Mw/Mn) of 2.02. IR (KBr), ν
1
(cm−1): 1786, 1720 (CO stretching), 1365 (C−N stretching). H
Atomic force microscopy (AFM) measurements were obtained with
a NanoScope IIIa AFM at room temperature. Commercial silicon
cantilevers with typical spring constants of 21−78 N m1− was used to
operate the AFM in tapping mode.
The morphology of the nanowires was determined using a field
emission scanning electron microscope (SEM, JEOL JSM-6330F).
The FE-SEM images were taken using a microscope operated at an
accelerating voltage of 10 kV. Before imaging, the samples were
sputtered with Pt.
NMR (DMSO-d6, δ, ppm): 8.13 (d, J = 8.4 Hz, ArH, 2H), 7.93 (d, J =
9.0 Hz, ArH, 2H), 7.74 (s, ArH, 2H), 7.52−7.43 (m, ArH, 10H). Anal.
Calcd For C35H16N2: C, 53.5; H, 2.05; N, 3.57. Found: C, 53.0; H,
2.00; N, 3.35.
Synthesis of Poly[2,5-bis(4-aminophenylenesulfanyl)thiophene−
hexafluoroisopropylidenediphthalimide] (PI(APST-6FDA)). Using
APST as a diamine, PI(APST-6FDA) was synthesized by a similar
procedure as PI(APSP-6FDA) (Scheme 1(b)). The yield of PI(APST-
6FDA) was 92%. Mn, Mw, and PDI values of PI(APST-6FDA) were
4.30 × 104, 7.17 × 104, and 1.66, respectively. IR (KBr), ν (cm−1):
UV−vis absorption spectrum was recorded on a Hitachi U-4100
spectrophotometer. For the thin film spectra, PIs was first dissolved in
chloroform (10 mg/mL), followed with filtering through a 0.45 μm
pore size PTFE membrane syringe filter, and then spin-coated at a
speed rate of 1000 rpm for 60 s onto quartz substrate. Cyclic
voltammetry (CV) was performed with the use of a three-electrode
cell in which ITO (PI films areas were about 0.5 × 0.7 cm2) was used
as a working electrode. A platinum wire was used as an auxiliary
electrode. All cell potentials were taken with the use of a Ag/AgCl,
KCl (sat.) reference electrode. The electrochemical properties of the
PI films were detected under 0.1 M anhydrous acetonitrile solution
containing tetrabutylammonium perchlorate (TBAP) as electrolyte.
The thickness of polymer film was measured with a microfigure
measuring instrument (Surfcorder ET3000, Kosaka Laboratory Ltd.).
The electrical characterization of the memory device was performed by
a Keithley 4200-SCS semiconductor parameter analyzer in a glovebox.
For the capacitance measurement, metal−insulator-semiconductor
(MIS) structure was fabricated by depositing gold electrodes on the
polymer-coated n-type Si(300) wafers. The capacitance of the bilayer
dielectrics was measured on the MIS structure using Keithley 4200-
SCS equipped with a digital capacitance meter (model 4210-CVU).
Computational Methodology. Theoretical molecular simulation
of the PIs was calculated through the Gaussion 03 program package.
Density functional theory (DFT) method, using Becke’s three-
parameter functional with the Lee, Yang, and Parr correlation
functional method (B3LYP) with 6-31G(d), was exploited for the
1
1786, 1720 (CO stretching), 1377 (C−N stretching). H NMR
(DMSO-d6, δ, ppm): 8.13 (d, J = 7.8 Hz, ArH, 2H), 7.93 (d, J = 8.7
Hz, ArH, 2H), 7.74 (s, ArH, 2H), 7.49−7.40 (m, ArH, 10H). Anal.
Calcd For C35H16N2: C, 56.9; H, 2.18; N, 3.79. Found: C, 57.2; H,
2.13; N, 3.75.
Preparation of the PI(ODA-6FDA) Thin Film. The PI(ODA-
6FDA),was prepared according to the literature.37 First, the ODA was
added into a flask and dissolved in DMAc with stirring. After 20 min,
6FDA was added and reacted at room temperature with stirring under
nitrogen. The molar ratio of dianhydried to diamine was 1:1, and the
total concentration of the reaction solution was 15 wt %. After 20 h,
the reaction solution was spin-coated on a silicon wafer at 3000 rpm
for 60 s. The coated film was the cured on a hot plate at 60 °C for 20
min, 100 °C for 20 min, 150 °C for 20 min, and finally at 300 °C in a
furnace for 120 min under nitrogen.
Preparation of the BPE-PTCDI Nanowires. To induce the
nanowire structure, BPE-PTCDI (1 mg) was dissolved in refluxing o-
dichlorobenzene (3 mL) in a round-bottom flask with magnetic
stirring and heated to 160 °C. After the powder had been completely
dissolved in solution at the high temperature of 160 °C, methanol (27
mL) as a nonsolvent was slowly added into the organic solution to
induce crystallization. As the solution cooled, its color changed
gradually from orange to dark green. After several hours, cotton-like
BPE-PTCDI nanowires were observed floating in the solution.38
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optimization of ground-state molecular geometry, and electronic
properties.
are observed at 8.13, 7.93, and 7.74 ppm while the multiple
signals in the range of 7.52−7.43 ppm are assigned to the
protons of selenophene group and aromatic rings. Similar to
PI(APSP-6FDA), the signals of the three protons of
phthalimide moiety of PI(APST-6FDA) are observed at 8.13,
7.93, and 7.74 ppm while the protons signals in the range of
7.49−7.40 ppm are attributed to the thiophene group and
aromatic rings. The complete conversion from PAAs to PIs was
also confirmed by FT-IR spectra. As shown in Figure S3 of
Supporting Information, both PIs exhibit similar IR absorption
peaks which originate from the imide moieties located at 1786
cm−1 (νas,CO), 1720 cm−1 (νs,CO), and 1365 cm−1 (νC−N). In
addition, typical absorption peaks due to the aromatic thioether
(Ar−S−Ar) are observed at 1257 cm−1. The experimental
carbon, hydrogen, and nitrogen contents of the prepared PIs
are in a good agreement with the theoretical contents. The
number-average molecular weights (Mn) of PI(APSP-6FDA)
and PI(APST-6FDA) are 49100 and 43000, respectively, as
determined by SEC.
All polymers are readily soluble in common organic solvents
such as tetrahydrofuran (THF), DMF, DMAc, NMP, and
chloroform. The high solubility may be attributed to the flexible
thioether linkages and hexafluoroisopropyl moieties in the
polymer structure. The thermal stability of these two PIs was
evaluated by TGA and DSC under a nitrogen atmosphere. The
5% weight-loss temperatures (Td5) of PI(APSP-6FDA) and
PI(APST-6FDA) are 423 and 420 °C, respectively. These PIs
not only show high thermal stability but also possess high glass
transition temperatures (Tg) of 250 and 242 °C for PI(APSP-
6FDA) and PI(APT-6FDA), respectively. The excellent
thermal properties of these PIs are expected to satisfy the
requirement of heat resistance in electronic industry.
Optical and Electrochemical Properties. Figure 2a
shows the optical absorption spectra of three PIs in solid
state films on quartz substrates. The absorption peak maximum
wavelengths of PI(APSP-6FDA), PI(APST-6FDA), and PI-
(ODA-6FDA) are observed at 288 nm (4.31 eV), 299 nm (4.15
eV) and 224 nm (5.54 eV), respectively. They are attributed to
the π−π* transition of selenophene (HOMO → LUMO4)
(4.38 eV), thiophene (HOMO → LUMO5) (4.68 eV), and
oxidianiline (HOMO → LUMO5) (5.55 eV) according to the
theoretical analysis shown in Figure 3, Figures S4 (Supporting
Information), and Figure S5 (Supporting Information). The
band gaps of PI(APSP-6FDA), PI(APST-6FDA) and PI(ODA-
6FDA) estimated from the onset of the absorption spectra are
3.26, 3.59, and 3.76 eV, respectively. The order of the band
gaps are consistent with the electron-donating ability and the
resulted intramolecular charge transfer, APSP > APST > ODA.
The electrochemical properties of the PIs were measured by
cyclic voltammetry (CV) obtained in anhydrous acetonitile
using tetrabutylammonium perchlorate as electrolyte. The
highest occupied molecular orbital (HOMO) energy levels of
the PIs were calculated from the onset of oxidation waves in
CV with reference to ferrocence (4.8 eV) by the following
equation: HOMO = −(Eonset + 4.8 − Eferrocene). Besides, the
lowest unoccupied molecular orbital (LUMO) energy levels
were estimated from the difference between the optical band
gap and HOMO level.40 The HOMO levels of PI(APSP-
6FDA), PI(APST-6FDA) and PI(ODA-6FDA) are −5.48,
−5.63, and −5.72 eV, respectively. These results indicate that
the selenophene moiety with the highest HOMO (or smallest
ionization potential) may result in the larger degree of charge
transfer rather than the other two PIs. Besides, PI(APSP-
RESULTS AND DISCUSSION
■
Monomer Synthesis. The synthetic routes for 2,5-bis(4-
aminophenylenesulfanyl)selenophene (APSP) and 2,5-bis(4-
aminophenylenesulfanyl)thiophene (APST) are shown in
Scheme 1b.
APSP was prepared by a two-step procedure with
selenophene as a starting material. Selenophene was then
halogenated with N-bromosuccinimide to give 2,5-dibromose-
lenophene,39 which was further reacted with p-aminothiophe-
nol under the Ullman reaction condition to yield APSP. On the
other hand, APST was synthesized by aromatic nucleophilic
substitution between p-aminothiophenol and 2,5-dichlorothio-
phene. The structure of APSP was characterized by NMR
1
spectroscopy. The H and 13C NMR spectra of APSP are
shown in Figure S1 of Supporting Information with assign-
ments of all peaks. The signal at 5.48 ppm in Figure S1(a) of
the Supporting Information is assigned to the four protons of
the amino groups (Ha). The characteristic protons of the
selenophene group (Hd) are observed at 6.99 ppm in the
spectrum. On the other hand, the protons of the aromatic rings
containing an amino moiety (Hb, Hc) are shown at 6.54 and
7.15 ppm, respectively. In the 13C NMR spectrum (Figure
S1(b) of Supporting Information), the six carbon signals, which
are consistent with the expected structure, are observed. Among
these peaks, three signals are assigned to quaternary carbons by
a nondistored enhancement by polarization transfer-135
(DEPT-135) measurement. In addition, the structure of
APSP was confirmed by elemental analysis. The desired
structure of APST was also confirmed by 1H NMR, 13C
NMR (shown in Figure S2 of Supporting Information), and
elemental analysis.
Synthesis and Characterization of PIs. Both PIs were
prepared by a two-step polycondensation of aromatic
dianhydride such as 4,4′-(hexafluoroisopropylidene)diphthalic
anhydride (6FDA) with APSP or APST via soluble poly(amic
acid) (PAA) precursors, followed by chemical imidization using
acetic anhydride and pyridine as shown in Scheme 1b. The
chemical structures of both PIs were confirmed by 1H NMR, as
shown in Figure 1. For the case of PI(APSP-6FDA), the
characteristic signals of the three protons of phthalimide moiety
1
Figure 1. H NMR spectra of PI(APSP-6FDA) and PI(APST-6FDA)
in DMSO-d6.
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Figure 2. (a) UV−vis absorption spectra of PIs. (b) Energy diagram of
BPE-PTCDI and PIs.
6FDA) and PI(APST-6FDA) apparently exhibit more intense
absorption intensity in the wavelength above 360 nm as
compared to PI(ODA-6FDA). It indicates that the selenophene
and thiophene moieties provide a stronger probability of charge
transferring than oxidianiline moieties. Furthermore, due to the
heavy atom effect, selenophene is the stronger donor than
thiophene, leading to trap the charges deep. Furthermore, the
molecular conformation and electronic properties were
calculated at the B3LYP/6-31G(d). Figure 3 shows HOMO
orbitals are localized at the donor moieties, APSP, while
LUMO orbitals are localized at the acceptor moieties, 6FDA.
This implies that the charge transfer between donor and
acceptor moieties occurs in the excited state of the PIs.
However, the charges can be further segregated under an
electric field and delocalized to the selenophene, thus stabilizing
the charge transfer state.
Surface Structure Characterizations. The water contact
angles for PI(APSP-6FDA), PI(APST-6FDA), and PI(ODA-
6FDA) surfaces are around 88°, 81°, and 71°, respectively,
which exhibit nonpolar and hydrophobic characteristics (see
Figure S6 of Supporting Information). This also indicates that
PI(APSP-6FDA) possess the smallest surface energy among the
three PIs. The morphology of the PIs prepared through spin
coating on bare SiO2 substrates show smooth surface with small
root-mean-square value of 0.2−0.5 nm, as shown in the AFM
image (Figure S7 of Supporting Information). The surface
structure of BPE-PTCDI on the PIs was characterized by AFM,
as shown in Figure 4. The BPE-PTCDI OFET memory device
can be mainly attributed to the chemical nature of the BPE-
Figure 3. Molecular orbitals of the PI(APSP-6FDA).
PTCDI and polymer electrets, since the surface roughness of
the polymer electrets are similar.41 The grain sizes of BPE-
PTCDI grown on the PI(APSP-6FDA), PI(APST-6FDA), and
PI(ODA-6FDA) surfaces are 205, 136, and 73 nm, respectively.
The larger grain size on PI(APSP-6FDA) is probably owing to
the lowest surface energy of PI(APSP-6FDA), allowing better
interconnection between grains during BPE-PTCDI deposi-
tion.42
OFET Performance and Memory Characteristics. The
memory characteristics of PI(APSP-6FDA), PI(APST-6FDA),
and PI(ODA-6FDA) were examined through OFET memory
devices with a bottom-gate/top-contact configuration using n-
type BPE-PTCDI as a charge transport layer, as shown in
Figure 5a. The electric output curves of the devices with PIs as
electrets are shown in Figure 5, which exhibit good current
modulation, well-defined linear and saturation regions. The
transfer characteristics of the BPE-PTCDI OFET devices are
shown in Figure 6a−c. These curves exhibit a typical n-type
accumulation mode. The field-effect mobility is estimated from
the plot of the square root of drain-to-source current (Ids)1/2
versus the gate voltage (Vg) by the following equation in the
saturation regime:43
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Figure 4. AFM topographic images of BPE-PTCDI on different
polymer surface: (a) PI(APSP-6FDA), (b) PI(APST-6FDA), and (c)
PI(ODA-6FDA) on 1 μm × 1 μm area.
WCpoly
μ
Ids =
(V − VTh)2
g
(1)
2L
where μ is the field-effect mobility, Cpoly is the gate dielectric
capacitance per unit area and VTh denotes the threshold voltage.
The capacitances (CTot) of the different gate dielectrics were
measured at 10 kHz. The bare 300-nm-thick SiO2 wafer has the
capacitance of 10.9 nF cm−2. The capacitances of PI(APSP-
6FDA), PI(APST-6FDA), and PI(ODA-6FDA) on the bare
SiO2 are 8.1, 8.63, and 8.7, respectively. The relations between
capacitance (CTot) of the device, SiO2 layer (CSiO ), polymer
2
electrets (Cpoly) and polymer dielectric constant (ε) are defined
as the following:
1
CTot
1
Cpoly
1
C
SiO2
=
+
(2)
(3)
ε ε
d
o
Cpoly
=
Figure 5. (a) Schematic structure of the BPE-PTCDI thin film based
OFET memory device. (b) Output characteristics of the OFET device
with PI(APSP-6FDA) as electret.
Here εo and d are the vacuum permittivity and the insulator
thickness. The dielectric constants of PI(APSP-6FDA), PI-
(APST-6FDA), and PI(ODA-6FDA) estimated from the
measured capacitance are 2.16, 2.85, and 3.24, respectively.
Note that the dielectric constant of PI(ODA-6FDA) is in a
good agreement with that reported in the literature.44 The
order on the mobility of the BPE-PTCDI devices with different
polymer electrets is PI(APSP-6FDA) ((3.6 0.07) × 10−3 cm2
V−1 s−1) > PI(APST-6FDA) ((2.4 0.06) × 10−3 cm2 V−1 s−1)
consistent with the grain size of BPE-PTCDI on the three PIs.
The grain size on the PI(APSP-6FDA), PI(APST-6FDA), and
PI(ODA-6FDA) surfaces are 205, 136, and 73 nm, respectively.
The larger grain sizes and reduced grain boundaries generally
result in higher charge-charier mobilities.45−47 Furthermore, the
interfacial roughness also affect the OFET mobility. The
roughness of the PI spin-coated films on bare SiO2 substrates
> PI(ODA-6FDA) ((1.8
0.11) × 10−3 cm2 V−1 s−1). It is
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threshold voltage observed from the devices with polymer
electrets is probably due to the passivation the silanol groups
through the polymer electrets spin-coated on SiO2 surface.49
To investigate the effects of bias voltages, the bias voltages
ranged from 40 to 100 V were applied on the devices, as
shown in Figure S9. More negative shifts with increasing
writing bias are observed. However, the increase in the negative
shift is obtained with the gate bias ranged from −40 to −80 V.
The memory windows keep near a constant of about −42 V
even though applying a writing bias more than −80 V,
indicating that charge trapping is saturated in the writing bias of
−80 V. Therefore, we applied the gate voltage of −80 V for
controlling memory behaviors of the devices. The drain current
of Figure 6a−c was measured with Vd = 100 V. When applied a
negative gate bias (Vg = −80 V, Vd = 0 V for 1 s), the entire
transfer curves of PI(APSP-6FDA), PI(APST-6FDA), and
PI(ODA-6FDA) are substantially shifted in the negative
direction with threshold voltages (VTh) of −42, −21, and −5
V, respectively, served as the “writing” process. This shift leads
to a high drain current (ON state) at Vg = 0 V. When applying
a reverse gate bias (Vg = 80 V, Vd = 0 V for 1s), the transfer
curves are shifted in the positive direction with threshold
voltages of 21, 18, and 3 V for PI(APSP-6FDA), PI(APST-
6FDA), and PI(ODA-6FDA), respectively, served as the
“erasing” process. Note that the shifting range of the transfer
curves through applying a writing and erasing gate bias is
defined as the memory windows. The memory windows
between writing and erasing process are 63, 42, and 8 V for
PI(APSP-6FDA), PI(APST-6FDA), and PI(ODA-6FDA),
respectively, which are probably related to the electron-
donating characteristics of the PIs. The strong electron-
donating moieties, selenophene and thiophene moieties are
able to provide a higher probability to enhance charge transfer
than oxidianiline moieties. Furthermore, due to the heavy-atom
effect, the selenophene moiety is the stronger donor than
thiophene moiety, leading to the more stabilized charge-
transferred states in PI(APSP-6FDA). As applying a negative
gate pulse (writing process), a large amount of holes are
induced through BPE-PTCDI from the source/drain electrode
and then tend to transfer to the HOMO levels of the PI
electrets. The electrons of BPE-PTCDI are easily accumulated
at the interface between BPE-PTCDI and the PI electrets due
to a built-in electric field induced from retained holes, even
though removing the external gate voltages. Therefore, the
negative shifts in the transfer curves are observed. As compared
to the other polymer electrets, PI(APSP-6FDA) possesses the
highest HOMO energy level may facilitate a large amount of
holes transfer from BPE-PTCDI to PI(APSP-6FDA), leading to
a larger negative VTh shift. PI(APST-6FDA) and PI(ODA-
6FDA) show similar theoretical dipole moment of around 3.00
D, smaller than that of PI(APSP-6FDA) (4.30 D). The larger
polarity of PI(APSP-6FDA) may be attributed to heterocyclic
selenophene with polarizable selenium atoms. The high dipole
moment of polyimide may play important role in the
stabilization of trapped charges in the polymer electrets,
when applying an external gate voltage. Therefore, both energy
levels and molecular polarization may synergistically enhance
the charge storage ability in the OFET device with PI(APSP-
6FDA) as dielectric layer.
Figure 6. Shifts in transfer curves for BPE-PTCDI OFET memory
device with (a)PI(APSP-6FDA), (b) PI(APST-6FDA), and (c)
PI(ODA-6FDA) as polymer electrets.
are 0.247, 0.432, and 0.451 nm for PI(APSP-6FDA), PI(APST-
6FDA), and PI(ODA-6FDA) respectively. Interfacial roughness
may reduce mobility due to charge scattering.48 These results
reveal that the larger grain size of BPE-PTCDI and the small
roughness of polymer electrets surface can produce the high-
mobility OFET-type memory devices. The threshold voltages
(VTh) and the Ion/Ioff current ratios range from 2 to 8 V and 103
to 104, respectively, as summarized in Table 1. The small
On the other hand, Kim et al.18 proposed a model, the
electric field in the insulator layer (Ei) can be expressed by the
following equation:
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Table 1. Characteristics of BPE-PTCI based OFET Memory Devices with Polymer Electrets
a
b
electrets
mobility [cm2 V−1 s−1
]
on/off
memory window [V]
thin film
PI(APSP-6FDA)
PI(APST-6FDA)
PI(ODA-6FDA)
PI(APSP-6FDA)
PI(APST-6FDA)
PI(ODA-6FDA)
(3.6 0.07) × 10−3
(2.4 0.06) × 10−3
(1.8 0.11) × 10−3
(7.5 0.05) × 10−2
(5.6 0.12) × 10−2
(2.1 0.09) × 10−2
8.7 × 103
1.4 × 103
−
63
41
8
nanowires
6.2 × 103
3.7 × 103
1.3 × 103
82
64
43
a
b
On/off drain current ratios of reading at Vg = 0 V. ΔVTh is defined as VTh (erasing) − VTh (writing)
.
Vg
Ei =
⎛ε ⎞
i
⎜ ⎟
di + dj
ε
⎝ ⎠
j
(4)
Here Vg is the applied gate voltage and di, εi and dj, εj are the
thickness and dielectric constant of the polymer electret layer
and SiO2, respectively. The estimation of Ei for the various 60
nm thick polymer electret layers after the application of Vg =
100 V is as follows: 4.24 MV cm−1 (PI(APSP-6FDA)); 3.43
MV cm−1 (PI(APST-6FDA)); 3.09 MV cm−1 (PI(ODA-
6FDA)). The electric field was inversely proportional to the
dielectric constant. Moreover, the higher electric fields in
polymer electrets may affect the VTh shifts in the transfer
curves. After the charges are transferred to the polymer
electrets, most holes would become trapped deep within the
electret, and remain as space charges in the polymer electrets.
Furthermore, the shift on the threshold voltage is enhanced
with the electric filed. PI(APSP-6FDA) reveals the largest
negative shift of the threshold voltages, since PI(APSP-6FDA)
has the smallest dielectric constant, resulting in the highest
electric field. The above result clearly indicates that the
PI(APSP-6FDA) with the smallest ionization potential and
largest electric field lead to more holes transferred from BPE-
PTCDI to PI(APSP-6FDA) among the studied donor−
acceptor PIs.
We prepared the BPE-PTCDI thin film device on a bare
substrate. The OFET mobility of the BPE-PTCDI thin film
device was 1.8 × 10−3 cm2 V−1 s−1. The BPE-PTCDI device on
the bare substrates showed slightly lower charge mobilities,
compared to that on the D−A PIs, probably because the
hydroxyl groups on the bare SiO2, acted as charge traps to
reduce charge transport. Besides, the BPE-PTCDI morphology
on bare SiO2 substrates without PI film showed abundant gain
boundaries (Figure S10 in Supporting Information), which also
acted as charge traps, led to the inferior OFET performance.
The thin-film devices without polymer electrets only exhibited
a memory window of 5 V, much smaller than the devices with
PI(APSP-6FDA) or PI(APST-6FDA). This indicates that the
donor−acceptor PI layer significantly improved charge storing
ability of the devices.
The time during which the stored charge is retained in the
dielectric layer is defined as the retention time. Figure 7 is the
retention time of the BPE-PTCDI thin film with PI(APSP-
6FDA) or PI(APST-6FDA) as electrets. The retention time of
the ON and OFF states of the device at a gate voltage of 0 V
are maintained for 104 s with a high on/off current ratio of
around 103. This long retention time indicates that the OFET
memory device based on donor−acceptor PIs as polymer
electrets is a promising candidate for organic nonvolatile
memories. The multiple switching stability of the device using
PI(APSP-6FDA) and PI(APST-6FDA) as electrets were
evaluated through write−read−erase−read (WRER) cycles, as
Figure 7. Retention time testing of the OFET memory devices based
on BPE-PTCDI thin film with (a) PI(APSP-6FDA) and (b) PI(APST-
6FDA) as electrets.
shown in Figure 8. The operation conditions of WRER cycles
are summarized as follows. The drain current was measured at
Vd =100 V. The writing, reading and erasing were at the gate
voltages of −80, 0, and +80 V, respectively. The ON/OFF
current ratios more than 103 can be achieved in WRER cycles.
The device can be operated over 100 cycles. The good stability
and reversibility reveal that the OFET memories with
polyimide electrets have potential for the applications of
nonvolatile flash-type memories.
BPE-PTCDI Nanowires OFET Memory Performance.
For comparison, we prepared the nanowires of BPE-PTCDI
with a diameter of 200 nm through a solvent-exchanged
method, as shown in Figure 9a. Parts b−d of Figure 9 exhibit
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Figure 8. (a) Gate voltage applied to the WRER cycles. Reversible
current response to the WRER cycles of memory device with (b)
PI(APSP-6FDA) and (c) PI(APST-6FDA) as electrets. The drain
current was measured at Vd = 100 V. The writing, reading, and erasing
were at the gate voltages of −80, 0, and +80 V, respectively.
the transfer curves of the OFET memory devices based on the
BPE-PTCDI nanowires. However, the width (W) and length
(L) of the individual nanowires crossing the source and drain
electrodes were used to estimate the active channel area based
on the SEM images for estimating charge mobility. The OFET
mobility of the BPE-PTCDI nanowires device with PI(APSP-
6FDA) is 7.5 × 10−2 cm2 V−1 s−1, which is an order of
magnitude higher than the thin film device. Because of the thin
film contains abundant gain boundaries as charge traps,
reducing charge transport, and leading to the inferior OFET
performance compared to nanowires. Furthermore, the
Figure 9. (a) SEM images of the BPE-PTCDI nanowires induced by
methanol nonsolvent into o-dichlorobenzene on the PI(APSP-6FDA)
substrate; (b) Shifts in transfer curves of the BPE-PTCDI OFET
memory device with PI(APSP-6FDA), (c) PI(APST-6FDA), and (d)
PI(ODA-6FDA) as polymer electrets.
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memory windows of the nanowires devices using the electret of
PI(APSP-6FDA), PI(APST-6FDA), and PI(ODA-6FDA) are
82, 64, and 43 V, respectively. This is probably due to the
enhanced electrical field of the nanowires in the interface. The
electric field of nanowires was shown to be much higher than
that of thin film because the electric field was inverse
proportional to the diameter.50 Consequently, the high electric
field of nanowires led to the OFET memory device with better
performances compared to the thin film device.
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■
We have successfully synthesized two new donor−acceptor PIs,
PI(APSP-6FDA) and PI(APST-6FDA), as polymer electrets for
BPE-PTCDI-based OFET memory devices. The theoretical
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have potential applications for high performance nonvolatile
OFET memory devices.
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