Introduction
π-Conjugated polymers have attracted increasing attention for their potential
applications in organic electronic devices due to their advantages of low cost and
flexibility.[1 ] Organic field-effect transistors
(OFETs), one of the vital components of organic electronics, have been a hot topic
because of their wide use in integrated circuits, displays, memory storage, skin
sensors and so forth.[2 ] Over the past three decades,
significant progresses have been made in the state-of-the-art p-type semiconducting
polymers in OFETs with impressive hole mobilities of higher than
10 cm2 · V−1 · s−1 .[3 ] However, the evolution of n-type semiconducting polymers still lags
far behind that of their p-type counterparts, which is mainly attributed to the
deficiency of π-conjugated polymers with high electron affinity and electron
mobility. This unbalanced hole and electron mobilities will inevitably become an
obstacle to organic complementary inverters and complementary logic circuits
constructed with organic p–n junctions.[4 ] Therefore, it
is imperative to explore the high-performance n-type semiconducting polymers by
rational molecular designs.
The dominating challenges in achieving effective electron transport in polymer
semiconductors consist in the implementation of sufficiently low-lying LUMO/HOMO
energy levels. The deep LUMO energy levels could enhance the electron injection from
the electrodes and facilitate the stable electron transport, and a low enough HOMO
energy levels can also block the hole injection and accumulation. However, it is
proverbial that a number of high-mobility polymers based on naphthalene diimide
(NDI),[5 ] diketopyrrolopyrrole (DPP)[6 ] and isoindigo (IID)[7 ]
exhibit ambipolar transport characteristics in OFETs, even though they typically
contain electron-withdrawing imide- and amide-substituted groups. Conventionally,
replacing hydrogen atoms in the backbone with strong electron-withdrawing groups
including halogens (fluorine atoms and chlorine atoms)[8 ]
and cyano moiety[9 ] could deepen the LUMO/HOMO energy
levels further, which has been regarded as a facile and promising strategy to design
exclusive n-type polymers. For example, Gao et al.[7a ]
reported that the multifluorination of IID-based polymers shows a conversion of
ambipolar to n-type with a high electron mobility up to
4.97 cm2 · V−1 · s−1 . Kim et al.[10 ] also reported an electron mobility of
1.20 cm2 · V−1 · s−1 for DPP-based polymers
containing cyano groups.
A highly π-extended (E )-1,2-di(thiophen-2-yl)ethene (TVT) unit is a versatile
building block for high-performance polymers because of its good coplanarity to
promote intrachain charge transport.[11 ] On the other
hand, there are a lot of copolymers based on the TVT unit having p-type or ambipolar
character owing to its electron-rich nature.[12 ] In our
previous study,[13 ] we copolymerized the
electron-deficient double B←N-bridged bipyridine (BNBP) with the TVT unit, and the
corresponding copolymer (P-BNBP-TVT ) exhibits ambipolar character due to the
high-lying LUMO/HOMO energy levels. To lower the frontier energy levels, herein, we
introduced cyano groups at the 3,3′-positions of the TVT moiety to successfully
synthesize the copolymer, P-BNBP-2CNTVT . As we expected, the strong
electron-withdrawing nitrile moieties could dramatically enhance the electron
affinity of the copolymers. CV measurements verified that P-BNBP-2CNTVT shows
lower LUMO/HOMO energy levels of −3.80 eV/−5.95 eV, respectively, which are much
lower than those of P-BNBP-TVT . The difference with ambipolar
P-BNBP-TVT is that the OFETs based on P-BNBP-2CNTVT exhibit
exclusive n-type charge transport with a moderate electron mobility of
0.026 cm2 · V−1 · s−1 .
Results and Discussion
Synthesis and Characterizations. The chemical structures of P-BNBP-TVT
and P-BNBP-2CNTVT are shown in [Figure 1 ]. We
decorated the cyano groups at the 3,3′-positions of the TVT unit to construct a
promising n-type π-conjugated polymer based on the BNBP unit. P-BNBP-TVT has
been reported in our previous study.[13 ] The synthetic
route of P-BNBP-2CNTVT is depicted in [Scheme
1 ], and the detailed synthetic procedures are provided in the Supporting
Information. In terms of the syntheses reported, the monomer of 2CNTVT was
successfully prepared under the treatment of lithium diisopropylamine (LDA), then we
performed the Stille-coupling polymerization between the monomer of BNBP and
the monomer of 2CNTVT . The long and branched alkyl chain
2-tetradecyloctadecyl was used to ensure the solubility. The crude product was
purified by sequential Soxhlet extraction with acetone, n-hexane and chloroform to
obtain the desired copolymer P-BNBP-2CNTVT with 93% yield. Expectedly,
P-BNBP-2CNTVT exhibits good solubility in common solvents, such as
chloroform, chlorobenzene (CB), ο -dichlorobenzene and so on, so that we
carried out 1 H NMR to confirm the chemical structure. And the
number-average molecular weight (M
n ) and polydispersity index
(PDI) of the copolymer are estimated by gel permeation chromatography (GPC) using
distribution polystyrene as a standard. The molecular weight (M
n )
of P-BNBP-2CNTVT is 51.5 kDa with a PDI of 1.91 (Figure S2), which is
comparable that of with P-BNBP-TVT .[13 ]
P-BNBP-2CNTVT displays good thermal stability with a thermal decomposition
temperatures (T
d , 5% weight loss) of 334 °C, as measured by
thermogravimetric analysis under a nitrogen atmosphere (Figure S3). The differential
scanning calorimetry scans of P-BNBP-2CNTVT show no melting or glass
transitions in the 25 – 250 °C range (Figure S4).
Figure 1 Molecular design and chemical structures of the polymers
P-BNBP-TVT and P-BNBP-2CNTVT .
Scheme 1 Synthetic route of P-BNBP-2CNTVT .
Molecular Geometry and Electronic Structures. In order to elucidate the
influences on polymer backbone configurations and electronic structures after
incorporating cyano groups, density functional theory (DFT) calculations of two
model tetramers (P-BNBP-TVT and P-BNBP-2CNTVT containing four
repeating units) were performed at the B3LYP/6 – 31 G level of theory. The long
alkyl side chains were replaced with methyl groups to simplify the calculations, and
the results are shown in [Figures 2 ] and S1. For
P-BNBP-TVT , the torsion angle between BNBP and TVT is 22°; however, after
the introduction of cyano groups, the torsion angle between BNBP and 2CNTVT is up to
27°, demonstrating negative influences on the molecular configuration due to the
large steric hindrance of the cyano groups (vide infra). Although with the twisty
backbone, the LUMO of P-BNBP-2CNTVT remains delocalized along the conjugated
backbone, which is the same as that of P-BNBP-TVT. However, the HOMO of
P-BNBP-2CNTVT is mainly localized on the BNBP moiety as compared with
P-BNBP-TVT , in which the HOMO is delocalized along the conjugated
backbone, indicating the strong electron-withdrawing properties of cyano groups.
Further, P-BNBP-2CNTVT based on DFT calculations exhibits deeper LUMO/HOMO
energy levels of −3.58 eV/−5.78 eV than those of P-BNBP-TVT of
−3.00 eV/−5.28 eV. The results demonstrate that cyano‐functionalization on the TVT
moiety dramatically lowers the LUMO/HOMO energy levels, which could effectively
enhance electron injection and block the hole injection to ensure the exclusively
n-type transport character.
Figure 2 Molecular structures, Kohn–Sham LUMO and HOMO and their energy
levels for the tetramer model compounds of a) P-BNBP-TVT and b)
P-BNBP-2CNTVT calculated at the B3LYP/6 – 31 G(d,p) level of
theory.
Photophysical and Electrochemical Properties. The UV-vis absorption spectra of
P-BNBP-TVT and P-BNBP-2CNTVT in diluted CB solutions with the
concentration of 10−5 M and in thin films are shown in [Figure 3 ]. The corresponding photophysical
characteristics are summarized in [Table 1 ]. Both the
absorption spectra of P-BNBP-TVT and P-BNBP-2CNTVT in CB solutions at
room temperature exhibit the similar shape to their absorption spectra of thin
films. In addition, the maximum absorption peaks of the two copolymers are almost
impervious from the solutions to thin films, which demonstrates the strong
pre-aggregation in the solutions. The pre-aggregation characteristics further could
be confirmed by the temperature-dependent absorption spectra shown in Figure S5. The
maximum absorption peaks gradually blue-shift with the increasing temperature of the
solution, along with the reduction of the absorption peak intensity. It is worth
mentioning that the absorption spectra of P-BNBP-2CNTVT attaching cyano
groups display a hypochromic shift in comparison to that of P-BNBP-TVT , and
the maximum absorption peaks (λ
max ) of P-BNBP-TVT and
P-BNBP-2CNTVT are 653 nm and 623 nm, respectively. This could be mainly
attributed to the weak intramolecular charge transfer character after the
cyano‐functionalization leading to a significant decrease of electron-donating
ability of the TVT moiety.[14 ]
Figure 3 Normalized UV-vis absorption spectra of PBNBP-TVT and
P-BNBP-CNTVT in their a) diluted CB solutions (10−5 M)
and b) thin films.
Table 1 Molecular, photophysical and electrochemical
properties of P-BNBP-TVT and P-BNBP-2CNTVT
Polymer
M
n
[a]
[kg · mol−1 ]
PDI
λ
max, sol
[b]
[nm]
λ
max, film
[c]
[nm]
E
g
opt [d]
[eV]
E
onset
red
[V]
E
onset
ox
[V]
E
LUMO
[e]
[eV]
E
HOMO
[e]
[eV]
a
M
n and PDI of the polymers were
determined by GPC using polystyrene standards in TCB at 150 °C.
b Measured in diluted CB solutions
(10−5 M) at 25 °C. c Measured in thin
films. d Calculated from the absorption band edge of
the polymer films,
E
g = 1240/λ
edge .
e LUMO and HOMO energy levels were determined from
the first reduction potential
(E
onset
red ) and oxidation
potential (E
onset
ox ) (vs.
Fc/Fc+ ) with the equations of
E
LUMO /E
HOMO = –(4.80 +
E
onset
red /E
onset
ox )
eV.
P-BNBP-TVT
[13 ]
41.6
4.35
653
652
1.84
−1.40
+0.85
−3.40
−5.65
P-BNBP-2CNTVT
51.5
1.91
628
627
1.88
−1.0
+1.15
−3.80
−5.95
To investigate the electrochemical properties of P-BNBP-TVT and
P-BNBP-2CNTVT , we carried out film CV measurements using
ferrocene/ferrocenium (FC/FC+ ) as the internal standard, and a solution
of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6 ) in acetonitrile
was employed as the electrolyte. The CV curves are displayed in [Figure 4 ]. Both of the two copolymers exhibit obvious
reduction and oxidation processes. On the basis of the onset potentials, the
LUMO/HOMO energy levels are estimated to be −3.40 eV/−5.65 eV for P-BNBP-TVT
and −3.80 eV/−5.95 eV for P-BNBP-2CNTVT , respectively. Obviously, the
LUMO/HOMO energy levels of P-BNBP-2CNTVT dramatically decrease by 0.4 eV and
0.3 eV compared with those of P-BNBP-TVT , respectively, which demonstrates
that the introduction of two strong electron-withdrawing cyano groups in the TVT
moiety effectively lowers the LUMO/HOMO energy levels. The experimental LUMO/HOMO
energy levels of the two copolymers are in good agreement with the theoretical
calculations. The deeper LUMO/HOMO energy levels of P-BNBP-2CNTVT are
expected to enhance the electron injection from the electrodes and generate a hole
injection barrier.[15 ] These results strongly imply that
P-BNBP-2CNTVT is going to be a promising candidate for exclusively
electron transport materials.
Figure 4 a) Cyclic voltammograms of P-BNBP-TVT and
P-BNBP-2CNTVT in thin films
(Fc/Fc+ = ferrocene/ferrocenium); b) Schematic of their LUMO/HOMO
energy level alignments.
OFET Performance. P-BNBP-2CNTVT possesses low-lying LUMO/HOMO energy levels,
which strongly motivates us to investigate the charge-transporting properties of the
copolymer. The solution-processed OFETs with the top-gate/bottom-contact (TGBC)
configuration were fabricated. [Figure 5 ] illustrates
the representative transfer and output characteristics of the OFETs. The detailed
device optimization processes by different annealing temperatures are shown in
Figure S6 and Table S1. Expectedly, in comparison to P-BNBP-TVT showing
ambipolar character, P-BNBP-2CNTVT exhibits typical unipolar n-type transport
characteristic because of the deep LUMO/HOMO energy levels enhancing the electron
injection and restricting hole injection after introducing cyano groups. And the
maximum electron mobility (µ
e ) of P-BNBP-2CNTVT extracted
in the saturation regime is up to
0.026 cm2 · V−1 · s−1 with high
I
on /I
off of 105 after thermal
annealing at 200 °C. In addition, the OFETs based on P-BNBP-2CNTVT show a
lower threshold voltage (V
T ) of about 25 V
(V
T ~50 V for P-BNBP-TVT
[13 ])
and leakage current (I
g ) (Figure S7). All the above observations
verified that cyano‐functionalization could effectively down-shift the LUMO/HOMO
energy levels. However, the causations for moderate electron mobility probably are
attributed to the steric repulsion with neighboring BNBP units after the attachments
of the cyano groups, which gives rise to distortion of the polymer backbone having
been reported in the literatures.[16 ] In a word, we
developed a new n-type semiconductor based on a BNBP unit using
cyano-functionalization.
Figure 5 a) Transfer and b) output curves of the OFETs based on the
P-BNBP-2CNTVT films annealed at 200 °C.
Film Morphology and Microstructural Analysis. The film morphology of
semiconductors plays a crucial role in OFETsʼ performance. We firstly performed
atomic force microscopy (AFM) of P-BNBP-2CNTVT thin films to investigate the
relationship between the film morphology and device performances. [Figures 6a ] and S8 show the AFM height images and phase
images of the P-BNBP-2CNTVT films after thermal annealing at different
temperatures of 100, 200 and 300 °C. The P-BNBP-2CNTVT film annealed at
200 °C shows continuous and smooth film topography with the smallest root mean
square surface roughness value of 0.57 nm, which indicates its enhanced film
ordering, taking an account for the improved electron transport performance in the
OFETs.[17 ]
Figure 6 a) AFM high images and phase images for spin-coated
P-BNBP-2CNTVT films annealed at 200 °C. b) GI-XRD patterns of the
spin-coating films in the out-of-plane direction and the in-plane direction
of P-BNBP-2CNTVT films.
We also utilized grazing incidence X-ray diffraction (GI-XRD) to clarify the polymer
crystallinity and packing structure, which are closely associated with transport
properties. The out-of-plane and in-plane XRDs of P-BNBP-2CNTVT films are
displayed in [Figures 6b ] and S9. The previous study
reported that P-BNBP-TVT exhibits edge-on packing mode in thin films with a
compact π–π stacking distance of 0.36 nm along the in-plane direction, which is
beneficial to the interchain charge transport.[13 ]
However, the P-BNBP-2CNTVT film gives a (100) diffraction peak at
2θ = 4.5° and a (010) diffraction peak at 2θ = 21.2° along the
out-of-plane direction. Lamellar and π–π stacking distance based on the (100) and
(010) diffraction peaks could be calculated to be 1.96 nm and 0.42 nm, respectively.
And there are no obvious diffraction peaks in the in-plane direction. The large π–π
stacking distance is not favorable for electron transport.[18 ] These results demonstrate that the introduction of cyano groups in
polymer backbones causing steric hindrance has undesirable influences on the
molecular packing, which gives the reason why P-BNBP-2CNTVT exhibits moderate
electron mobilities.
Conclusions
In summary, we successfully synthesized a novel unipolar n-type transport polymer
semiconductor based on a BNBP unit, P-BNBP-2CNTVT , where the cyano groups are
decorated in the TVT moiety. In comparison to P-BNBP-TVT without cyano
groups, the LUMO/HOMO energy levels of P-BNBP-2CNTVT dramatically down-shift
by 0.4 eV and 0.3 eV, respectively, demonstrating the high electron affinity which
are very desirable for electron injection. Thus, the OFETs based on
P-BNBP-2CNTVT show unipolar n-type behavior with a moderate electron
mobility of 0.026 cm2 · V−1 · s−1 . Moreover,
P-BNBP-2CNTVT manifests an unsatisfying π–π stacking distance up to
0.42 nm owing to the large steric hindrance after the incorporation of cyano groups,
which gives an explanation for the moderate electron mobility. In a word, this study
demonstrates that organoboron π-conjugated polymers could be regarded as a tool for
constructing exclusive n-type semiconducting polymers used in OFETs. And we are
convinced that the enhanced electron mobility of organoboron π-conjugated polymers
could be realized through the optimizations of chemical structures and thin film
morphology, and the further investigations are in progress in our lab.
Experimental Section
All reagents and solvents were purchased at reagent grade from commercial suppliers
and were used without further purification unless otherwise noted. 1 H and
13 C NMR spectra were measured with a Bruker AV-400 (500 MHz for 1H
and 126 MHz for 13 C) spectrometer in CDCl3 and
C6 D6 at 25 °C or deuterated 1,1,2,2-tetrachloroethane
(C2 D2 Cl4 ) at 100 °C. Chemical shifts are
reported in δ ppm using CDCl3 (7.26 ppm),
C2 D2 Cl4 (5.98 ppm) and
C6 D6 (7.16 ppm) for 1 H NMR, as well as using
CDCl3 (77.16 ppm) for 13 C NMR as an internal standard. The
molecular weights of the polymers were determined by gel permeation GPC on a PL-GPC
220-type at 150 °C. 1,2,4-Trichlorobenzene (TCB) was used as the eluent and
monodisperse polystyrene was used as the standard. UV-vis absorption spectra were
measured with a Shimadzu UV-3600 spectrometer. CV was performed on an CHI660a
electrochemical workstation using Bu4 NClO4 (0.1 M) in
acetonitrile as the electrolyte solution and ferrocene as an internal reference at a
scan rate of 50 mV · s−1 . The CV cell consisted of a glassy carbon
working electrode, a Pt wire counter electrode, and a standard calomel reference
electrode. The polymer was casted on the working electrode for measurements. The
redox potentials were calibrated with ferrocene as the standard. The HOMO and LUMO
energy levels of the materials were estimated by the equations:
E
HOMO /E
LUMO = –(4.80 +
E
onset
ox /E
onset
red ).
Thermal analysis was performed on a Perkin-Elmer 7 instrument at a heating rate of
20 °C · min−1 under a nitrogen flow. AFM was performed with a
SPA300HV (Seiko Instruments, Inc., Japan) in the tapping mode. GI-XRD data were
obtained on a Bruker D8 Discover reflector (Cu Kα, λ = 1.54 056 Å) under 40 kV and
40 mA tube current. The scanning speed was 5 s per step with 0.05° step size
(2θ ). The measurement was obtained in a scanning interval of 2θ
between 2° and 30°. The in-plane XRD profiles were obtained using a Rigaku SmartLab
with an X-ray generation power of 40 kV tube voltage and 30 mA tube current. The
diffraction was recorded in the 2θ – χ mode. The scanning speed was 5 s per
step with 0.02° step size (2θ ). The measurement was obtained in a scanning
interval of 2θ between 2° and 30°.
Procedures
(E )-1,2-Bis(3-bromothiophen-2-yl)ethene
(E )-1,2-Bis(3-bromothiophen-2-yl)ethene was prepared using improved
post-treatment methods with higher yield compared to references.[10 ] TiCl4 (4.38 mL, 39.9 mmol) was
added dropwise to a slurry of 3-bromothiophene-2-carbaldehyde 1 (5 g,
26.2 mmol) in THF (75 mL) with stirring at −18 °C. After stirring at this
temperature for 30 min, Zn powders (5.2 g, 79.5 mmol) were divided into
several equal parts and added in over a period of 30 min. The mixture was
stirred at −18 °C for another 30 min, and then refluxed for 4 h. The
reaction was quenched by adding ice-cold H2 O. The mixture was
extracted with dichloromethane several times and dried over
Na2 SO4 . The solvent was removed and purified by
recrystallization from chloroform to give a pale brown solid. Yield: 57%
(2.7 g). The 1 H NMR spectrum is consistent with the previous
report.[10 ]
1 H NMR (500 MHz, CDCl3 , ppm): δ 7.20 (d,
J = 5.4 Hz, 2 H), 7.12 (s, 2 H), 6.98 (d, J = 5.4 Hz,
2 H).
(E )-1,2-Bis(3-cyanothiophene-2-yl)ethene
A mixture of (E )-1,2-Bis(3-bromothiophen-2-yl)ethene (1.5 g,
4.28 mmol) and CuCN (1.92 g, 21.4 mmol) in anhydrous DMF (65 mL) was stirred
for 24 h at 150 °C. Subsequently, it was allowed to cool to 70 °C. Then,
FeCl3 ·6H2 O (4.65 g, 17.14 mmol) in 2 M aqueous HCl
(9 mL) was added and stirred at 70 °C for 1 h. Next the mixture was
extracted with dichloromethane several times and dried over
Na2 SO4 . The solvent was removed and purified by
silica gel column chromatography using dichloromethane : petroleum
(2 : 1 v/v) to obtain the desired compound as a yellow solid. Yield: 79%
(0.82 g). The 1 H NMR spectrum is consistent with the previous
report.[10 ]
1 H NMR (500 MHz, CDCl3 , ppm): δ 7.39 (s, 2 H), 7.35 (d,
J = 5.1 Hz, 2 H), 7.22 (d, J = 5.1 Hz, 2 H).
(E )-1,2-Bis(5-(trimethylstannyl)-3-cyanothiophene-2-yl)ethene
(2CNTVT)
2CNTVT was prepared according to references.[10 ] The fresh made LDA (1.0 M LDA, 2.42 mL, 2.42 mmol) was added
dropwise at −78 °C to a solution of compound 3 (0.24 g, 1 mmol) in
anhydrous THF (34 mL). After stirring for 1.5 h at −78 °C, trimethyltin
chloride (0.6 g, 3 mmol) was added in one portion to the reaction mixture.
Subsequently, the reaction flask was warmed to room temperature and stirred
another 2 h. After the reaction finished, the mixture was quenched with
distilled cold water and extracted with cold diethyl ether. The organic
layer was dried over Na2 SO4 . After removing the
solvents, the obtained residue was purified by recrystallization from
ethanol to give a yellow product. Yield: 50% (0.28 g). The 1 H NMR
spectrum is consistent with the previous report.[10 ]
1 H NMR (500 MHz, C6 D6 , ppm): δ 7.50 (s,
2 H), 6.72 (s, 2 H),0.11 (s, 18 H).
Polymer P-BNBP-2CNTVT
Starting materials of BNBP (100 mg, 0.075 mmol), 2CNTVT
(42.4 mg, 0.075 mmol), Pd2 (dba)3 •CHCl3
(1.6 mg, 0.02 mmol) and P(o -Tolyl)3 (3.6 mg, 0.16 mmol)
were mixed under argon, and then dried toluene (7.5 mL) was added. The
mixture was stirred at 120 °C for 24 h. After cooling, the solvent was
dispersed in methanol and then the precipitate was collected. The obtained
dark solid was purified by Soxhlet extraction using acetone, hexane and
chloroform. The chloroform fraction was concentrated and poured into
methanol, which were collected and dried in vacuum overnight. Yield:
100.0 mg (93%). GPC (TCB, polystyrene standard, 150 °C):
M
n = 51 523, PDI = 1.91.
1 H NMR (400 MHz, C2 D2 Cl4 , 100 °C,
ppm): δ 9.73 (s, 2 H), 9.05 (s, 2 H), 8.93 (s, 2 H), 8.89 (s, 2 H), 4.98 (s,
4 H), 3.25 (s, 2 H), 2.69 – 2.83 (m, 32 H), 2.60 – 2.64 (m, 80 H), 2.23(t,
12 H).
Device Fabrication and Characterization
TGBC OFETs were fabricated on silicon wafer covered with 300 nm
SiO2 . The substrates were first cleaned with double-distilled
water, acetone and isopropanol in an ultrasonic bath and then dried under a
nitrogen flow. The substrates were heated to 120 °C for 1 hour and finally
treated with a UV-ozone instrument for 15 min. First, Au source/drain
electrodes (~25 nm) were deposited on cleaned bare Si/SiO2 wafer
with W /L = 70 (W = 5.6 mm, L = 80 µm).
P-BNBP-2CNTVT was first dissolved in hot chloroform solutions
(55 °C, 1 mg/mL) by stirring for 2 hours, and then aging for more than 8
hours. The polymer films were spin-coated on the substrates, followed by
thermal annealing at 200 °C for 10 min. Then the solution of 80 mg/mL
poly(methylmethacrylate) (PMMA) (product no. 182 230 from Aldrich,
M
w = 120 kDa) in butyl acetate (~500 nm) as a
dielectric was deposited by spin coating at 2000 rpm for 2 min and then
annealed at 100 °C for 1 h. Finally, Al (~70 nm) was vacuum-deposited as a
gate electrode. Field-effect mobility was extracted in the saturation regime
by using the equation:
I
D
sat = (µC
i
W /2 L )(V
G -V
T )2 ,
where I
D is the drain–source current, µ is the
field-effect mobility, C
i is the capacitance per unit area
of the gate dielectric layer (dielectric constant of PMMA, 500 nm,
5.5 nF/cm2 ), and V
G and
V
T are the gate voltage and threshold voltage,
respectively.
Funding Information
This study was supported by the National Key Research and Development Program of
China (2018YFE0 100 600) funded by MOST and Natural Science Foundation of China
(No. 21 625 403, 21 875 244).
