High performance p-channel and ambipolar OFETs based on imidazo[4,5-: F] -1,10-phenanthroline-triarylamines

Article abstract of DOI:10.1039/d0ra00210k

A series of phenanthroline functionalized triarylamines (TAA) has been designed and synthesised to evaluate their OFET characteristics. Solution processed OFET devices have exhibited p-channel/ambipolar behaviour with respect to the substituents, in parti

Full text of DOI:10.1039/d0ra00210k

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High performance p-channel and ambipolar OFETs
based on imidazo[4,5-f]-1,10-phenanthroline-
triarylamines†
Cite this: RSC Adv., 2020, 10, 13043
a
a
Ramachandran Dheepika, Ramakrishnan Abhijnakrishna,
Predhanekar Mohamed Imran and Samuthira Nagarajan *a
b
A series of phenanthroline functionalized triarylamines (TAA) has been designed and synthesised to evaluate
their OFET characteristics. Solution processed OFET devices have exhibited p-channel/ambipolar
behaviour with respect to the substituents, in particular methoxyphenyl substitution resulted with highest
Received 8th January 2020
Accepted 24th March 2020
DOI: 10.1039/d0ra00210k
2
ꢀ1 ꢀ1
6
mobility (m
h
) up to 1.1 cm
V
s
with good Ion/off (10 ) ratio. These compounds can be potentially
rsc.li/rsc-advances
utilized for the fabrication of electronic devices.
circuits and inverters. Although, the unipolar devices are more
appealing, the covalent interaction between the donor and
Introduction
Organic eld effect transistors (OFETs) unquestionably are the acceptor leads to optimized band gap which allows them to
candidates of future electronics owing to their exibility which unveil their balanced p and n channel behaviour.
is very essential for a variety of applications including articial
A few combinations of TAA and phenanthroline have been
1
9
human skin. In the past decade OFETs have been exploited for analysed for their application in OLEDs, linear and non-linear
10
radio frequency identication tags, sensors, integrated circuits, optics and guest–host systems. In addition, phenanthrolines
2
logic switches, and larger exible display applications. Solution were investigated by coordination chemists due to its perfectly
processing based small molecule are studied due to their facile placed N atoms. Imidazole fragment, in organic frame work is
and economic methodologies which enables the commerciali- acknowledged to exert substantial inspiration in inter and
11
zation of organic electronic devices. Among the molecules intramolecular interactions. Although, TAAs are investigated
studied, pentacene, rubrene and polythiophene have exhibited for their application in OFETs and resulted in good mobility
3
12,13
astonishingly good transistor characteristics. However, the and ON/OFF ratio,
aforementioned molecules are classic p-channel materials; n- based on phenanthrolines have not been reported yet.
channel molecules face problems in terms of stability because
Herein, we have designed and synthesised a series of new
to the best of our knowledge, OFETs
of their low lying LUMO levels and are lagging behind in the fused D–A systems for OFET application with TAA served as
4
race. Ambipolar devices will integrate electronic properties of donor and phenanthrolines as acceptor. The OFET character-
5
both p and n channel molecules in a single device. From istics with respect to the substituents are scrutinised. In one
a performance perspective complementary logic is also crucial arm of TAA electron accepting, planar phenanthroline is
to achieve high speed with low power dissipation to ensure low attached through an azole ring. This substitution tunes the
noise margin and robust operation of devices such as inverters bandgap by altering the frontier molecular orbitals and can
and memory elements.
facilitate charge carrier movement. Solution processing from
The ambipolar devices can be obtained from a fused D–A a chloroform solution assisted the molecule to self-assemble
system or D and A co-systems. Donor–acceptor hybrids have efficiently to improve the communication among the mole-
gained importance owing to their intramolecular electron cules. Post thermal annealing gave highly crystalline thin lm.
transfer nature which can be tuned/altered by external stim- This investigation can give an insight about tuning the OFET
6,7
ulus. D–A systems, in terms of OFET have grabbed enormous behaviour of TAAs and the effect of substituents.
8
attention in order to make ambipolar OFETs for integrated
Experimental
a
Department of Chemistry, Central University of Tamil Nadu, Thiruvarur-610 005, Materials and methods
India. E-mail: snagarajan@cutn.ac.in
1
,10-Phenanthroline, triphenylamine, CH COONH , NaBr, and
b
3 4
Department of Chemistry, Islamiah College, Vaniyambadi-635 752, India
Electronic supplementary information (ESI) available: Detailed synthetic
the solvents used for synthesis are purchased from commercial
resources and used as received. ACS grade solvents were used
for analysis. Nuclear magnetic spectroscopic (NMR)
†
methodology, NMR and HRMS data, thermal and computational studies. See
DOI: 10.1039/d0ra00210k
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characterisation of the compounds were done in a Bruker 400 wafer was used as substrate with thermally grown silicon
6
MHz spectrometer. TMS was used as standard in d -DMSO/ dioxide layer as dielectric layer. Si wafer was pre-cleaned with
CDCl . The molecules were further analysed by Thermo Exactive standard cleaning procedures before the fabrication. The Si
3
Plus mass spectrometric technique. UV-visible absorption wafer acts as gate electrode. The device architecture is shown
spectra were recorded in JASCO UV-NIR spectrophotometer. Fig. S1.† The compounds were dissolved in chloroform (5 mg
ꢀ1
Emission behaviour was investigated using PerkinElmer Spec- mL ) and sonicated for 20 minutes. The prepared sample
trouorimeter LS 55. Electrochemical work station (CHI 6035D) solution was spun over the dielectric layer in 1600 rpm for 60 s.
was utilised to study the electrochemistry of the molecules. Then the device was so baked and followed by annealing at
ꢁ
PerkinElmer TGA 4000 instrument was utilised to study the 80 C for 40 minutes. To complete the fabrication source and
thermal behaviour of the compounds by thermogravimetric drain contacts were placed through a mask with channel length
analysis.
of 150 mm and width of 5 mm. Transistor characteristics were
Mass spectra was recorded in positive scan mode in methanol acquired using a Keithley 4200 SCS semiconductor parameter
as solvent. Photophysical studies were carried out in dichloro- analyzer system.
methane at room temperature. The concentration was xed at
ꢀ
5
ꢀ7
10
M for absorption and 10 M for emission studies. For
Results and discussion
electrochemical characterizations, classic three electrode set up
was used with glassy carbon working electrode and standard The target molecules are synthesised via microwave assisted
calomel electrode as reference electrode. To complete the circuit green synthesis with high purity and yield. The synthetic
platinum wire was used as the counter electrode. The electrode sequence and the molecular structure are shown in Scheme 1.
was polished before each experiment with alumina slurry and The compounds are characterised well by various techniques. In
1
ferrocene was used as standard. Tetrabutylammonium hexa-
H NMR, the N–H proton is observed as high de-shielded proton
uorophosphate was used as supporting electrolyte. The solution at around 13 ppm and the aromatic protons are resonating in 7
2
was purged with N before each experiment to remove oxygen to 10 ppm region. In compound 3, methoxy proton is observed
inference. Thermal analysis were done in nitrogen atmosphere at at 3.865 ppm and in compound 6 t-butyl group protons are
ꢁ
the heating rate of 10 C per minute. DFT and TD-DFT methods resonating at 1.321 ppm. ESI mass analysis in positive scan
+
have been used to understand the theoretical aspects.
mode has resulted in [M + H] molecular ion peak as base peak
for all the molecules. Thermal analysis, device fabrication, thin
lm characterizations and computational analysis are dis-
cussed in detail in ESI.†
Synthetic procedure
Sulphuric acid (20 mL) was stirred with 1,10-phenanthroline
(
1.01 mmol) in an ice bath for 10 min. To the stirring mixture
Photophysical studies
NaBr (11.08 mmol) was added in several small portions. The
reaction mixture was stirred for 5 min in ice and followed by To investigate the photophysical behaviour of the new mole-
slow addition of HNO (10 mL) and continued to stir for 20 min cules, UV-vis absorption and emission spectra were analysed.
3
ꢁ
ꢀ5
in ice bath. Temperature was increased to reach 100 C. Aer 3 Spectra were recorded in dichloromethane (DCM) at 10
hours, the reaction was quenched and poured into crushed ice. concentration, given in the Fig. 1(a) and the data are repre-
Neutralization with NaHCO to pH: 6–7 yielded the product and sented in Table 1. The two peaks are attributed to n–pi* and pi–
extracted using CH Cl . The organic layer was dried over
M
3
2
2
sodium sulphate and concentrated to give a yellow orange solid
of 89% yield aer recrystallization from ethanol.
Triarylamine aldehydes were synthesised according to the
13
procedure reported in our previous work. A mixture of 1,10-
phenanthroline-5,6-dione (5 mmol), ammonium acetate (100
mmol) and corresponding TAA aldehydes (6 mmol) was dis-
ꢁ
solved in glacial acetic acid (4 mL), heated at 100 C for 20 min
in CEM microwave synthesizer (power: 150 W, pressure: 150 psi,
pre-stirring: 2 min) and monitored. Aer completion the reac-
tion mixture was poured into ice to get a yellow precipitate and
collected by ltration (Whatmann 40). The precipitate was
neutralised with concentrated ammonia solution and followed
by hot water wash. The precipitate was dried to get a yellow solid
in 95–98% yield. The crude product was recrystallized from
ethanol and DMF mixture.
Device fabrication
Bottom gate top contact architecture has been chosen for robust
+
+
device fabrication by solution processing. n doped silicon Scheme 1 Synthetic route and the structures of compounds 1–6.
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Fig. 1 (a) Absorption and (b) emission spectra of compounds 1–6.
Table 1 Photophysical properties of compounds 1–6
Fig. 2 Cyclic voltammogram of compounds 1–6.
interference of dissolved oxygen in the system. HOMO levels
Comp.
no.
l
abs
l
em
Stokes' shi
Absorption co-efficient were calculated with reference to ferrocene using the formula
ꢀ1
4
ꢀ1
ꢀ1
(nm)
(nm)
(cm
)
3 (10 M cm
)
fc/fc+
15
EHOMO ¼ ꢀ(E + 4.8 ꢀ E
). The values fall in the range of
ox
ꢀ
5.2 to ꢀ5.6 eV which is matching with widely used hole
1
2
3
4
5
6
282, 368
288, 366
274, 369
270, 370
370
489
452
459
454
433
463
6724
5199
5171
5001
3933
5356
2.7
2.9
3.9
3.2
3.2
5.1
16
transporting and electron blocking layers. This HOMO values
ensure that these molecules can be effectively employed in
various electronic devices including solar cells, light emitting
g
diodes and transistors. Band gap (optical E ) was calculated
277, 371
from absorption onset and corresponding LUMO values were
obtained. Compounds 1 and 2 have band gap at 3.0 eV may be
due to the insignicant role of –H and –I substituents in altering
the FMOs position. Introducing electron rich thiophene ()4 and
methoxyphenyl ()3 groups in TAA moiety have tuned the band
gap to 2.95 and 2.96 eV respectively. The irreversible redox peak
observed in the range of 0.8 to 1.2 V corresponds to the TAA
moiety (Table 2 and Fig. 2).
pi* transitions. Compounds 1 and 2 with relatively less conju-
gation than the other compounds, have lower absorbance (3 ¼
4
ꢀ1
ꢀ1
2.7 and 2.9 ꢂ 10
M
cm ). Compounds 3 and 4 have
exhibited red shi in their lower energy absorption. This may be
due to the electron rich nature of the OMe-phenyl and thio-
phene substituents attached to TAA arms. Molecule 6 exhibited
4
ꢀ1
ꢀ1
the highest absorbance (3 ¼ 5.1 ꢂ 10 M cm ). In addition,
Thermal behaviour
TD-DFT calculations informs that the S –S transition is most
0
1
Thermal stability of a semiconducting molecule is an impera-
feasible for all the compounds with regard to the oscillator
strength (Table S6†). The emission spectra were recorded in
DCM at 10 M concentration and the spectra are represented
tive parameter to decide upon the lifetime and efficiency of the
ꢀ
7
in Fig. 1(b). All the molecules exhibited broad peaks covering Table 2 HOMO, LUMO and band gap of compounds 1–6
from 400–600 nm. The shape of the emission spectra are not
mirror images of the absorption spectra, which implies the
a
Experimental (eV)
Computational (eV)
geometrical changes in the excited state of the molecule. In C. no. HOMO LUMO Band gap HOMO LUMO Band gap
polar medium, excited state of the molecules gain more dipole
1
4
1
2
3
4
5
6
ꢀ5.347 ꢀ2.307 3.040
ꢀ5.450 ꢀ2.387 3.063
ꢀ5.282 ꢀ2.328 2.954
ꢀ5.604 ꢀ2.604 2.960
ꢀ5.345 ꢀ2.356 2.989
ꢀ5.647 ꢀ2.676 2.971
ꢀ5.083 ꢀ1.433 3.649
ꢀ5.504 ꢀ1.893 3.610
ꢀ4.917 ꢀ1.435 3.482
ꢀ5.075 ꢀ1.527 3.548
ꢀ5.035 ꢀ1.489 3.546
ꢀ4.968 ꢀ1.452 3.515
moment due to the charge separation within the molecule. In
addition, all the molecules have signicant Stokes shi values
and are given in Table 1. Compound 3 exhibited the higher
intensity, may be due to the chromophoric effect of OCH
3
group. From DFT studies, with respect to the substituents the
planarity of the molecule changes from ground state to excited
state. This observation is consistent with regard to the dihedral
angle of two nitrogen centres (TAA and azole ring) (Fig. S2 and
Table S7†).
a
Obtained from DFT-(B3LYP 6-31(d)-G) method.
Electrochemistry
Electrochemical behaviour of the new molecules were investi-
gated by cyclic voltammetry technique in a classic three elec-
trode cell set up. Tetrabutylammonium hexauoro phosphate
0
was used as supporting electrolyte in N,N dimethyl formamide
ꢀ
1
(
DMF) at the scan rate of 50 mV s . Irreversible redox peak was
observed for all the compounds in positive potential region. The
solution was purged with nitrogen for 15 min to eliminate the Fig. 3 TGA curves of compounds 1–6.
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devices. All the new phenanthroline-TAAs were analysed by
thermogravimetric (TGA) method to study their thermal char-
ꢁ
acteristics. The experiments were carried out at 10 C per
minute heating rate in nitrogen atmosphere. The TGA curves
are given in Fig. 3 and data are represented in Table 3.
Introducing a planar phenanthroline moiety in one arm of
the TAA have noticeably improved the thermal stability and
ensures efficiency. The melting point ranging from 295 to
ꢁ
ꢁ
3
3
76 C and for compounds 3 and 5 the higher T
m
(374 and
76 C respectively) have been observed. Similarly, Td corre-
Fig. 5 Thin film XRD analysis.
ꢁ
sponding to 10% weight loss is in the range of 375 to 560 C.
The decomposition temperature of these phenanthroline
substituted TAAs are upgraded from previously reported TAAs.
19
phenanthroline unit in altering the self-assembling nature of
the TAAs. Interestingly, compound shows petal like
morphology may be due to the t-butyl group present in TAA side.
6
Thin lm characterization
Thin lms of bottom gate top contact devices for compounds 1– t-Butyl group is known to play a vital role in supramolecular self-
2
0
6
are characterized by SEM and XRD analysis. Films are spin assembly of organic frameworks.
coated from chloroform solution and annealed at 80 C for 40
ꢁ
Thin lm X-ray diffraction analysis have been carried out to
minutes. Fig. 4 shows the morphology of the active thin lm by study the lm morphology. All the thin lms were coated from
SEM analysis. All the compounds have given good self- a chloroform solution, spun in 1600 rpm for 60 seconds and
ꢁ
assembled crystalline thin lm. However, in case of thermally annealed at 80 C to remove the residual solvents and
compound 2, blend granule like arrangements was observed to improve the self-assembly. The thin lm XRD patterns are
but the assembly is not well connected may be due the presence given in Fig. 5. All the compounds have produced highly crys-
of heavy atom. Thus this compound failed to produce any talline lm. In thin lm, the compounds exhibited a diffraction
ꢁ
ꢁ
transistor characteristics. Compounds 1, and 3–5 shows peak around 2q ¼ 37 and 43 corresponding to d-spacing of 24
˚
˚
22
ꢁ
uniform packing and almost similar arrangements with well- A and 20 A respectively. The peak at 37 was much sharper
connected network. On comparison with reported TAA deriva- with a stronger intensity than the other peaks indicates that the
ꢁ
23
tives, self-assembly has changed signicantly. In case of orientation of the molecule was almost in the way of 37 . In
compound 3, the micro cluster is observed where the corre- addition, compounds 1 and 4 have exhibited an additional weak
sponding TAA aldehyde have exhibited spindle brous diffraction peak with d-spacing of 19.5 A which indicates long
˚
13
24
network.
This observation describes the role of range order. Crystallinity is one of the important criteria to
21
attain good charge carrier mobility in thin lm transistors.
The average crystal size of the lms were calculated to be in the
range of 200–500 nm. The compound 4 has exhibited the low
Table 3 Thermal behaviour of compounds 1–6
ꢁ
reection at 2q ¼ 37 , may be due to the relatively small crystal
ꢁ
ꢁ
Comp. no
T
m
( C)
T
d
( C)
ꢁ
size. Thermal annealing at 80 C have changed the crystallinity
and resulted in less intense diffraction peaks. The crystalline
size must be uniform with less grain boundaries to avoid trap-
ping of charges which deviates the charge carrier movement
and ultimately reduces the performance of the device. Here,
compounds have resulted in good crystalline lm aer
1
2
3
4
5
6
343
330
374
295
376
356
475
375
427
392
560
450
ꢁ
annealing at 80 C. It is important to note that without thermal
annealing uniform lm was not obtained. From SEM and XRD
analysis, it is ensured that the constant microstructural
construction of active thin lm renders good charge carrier
mobility in OFET device.
OFET characteristics
The bottom gate top contact devices are analysed for their
transistor behaviour in ambient conditions. Among the
compounds analysed, ve compounds have exhibited transistor
characteristics. Chloroform allows the molecule to self-
assemble efficiently and the residual solvent can be removed
easily by thermal annealing. Compounds 1 and 5 have shown
bipolar charge carrier mobility whereas, compounds 3, 4 and 6
exhibited only p-channel behaviour (Fig. 6, Table 4). This
Fig. 4 Thin film morphology SEM analysis.
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Table 4 OFET characteristics of compounds 1 and 3–6
2
ꢀ1 ꢀ1
Comp. no
Channel
Mobility, m (cm V
s
)
I
on/off
VTH (V)
ꢀ
ꢀ
2
2
3
1
p
n
p
p
p
n
P
2.4 ꢂ10
1.5 ꢂ10
1.1
10
10
10
10
10
10
10
ꢀ3
ꢀ2
ꢀ2
4
6
5
3
4
4
a
3
4
5
ꢀ
2
2
5.9 ꢂ10
ꢀ11
ꢀ3
ꢀ
2.7 ꢂ10
ꢀ2
a
13 ꢂ10
5.6 ꢂ10
ꢀ1
ꢀ2
6
ꢀ1
a
17
Mobility calculated considering the kink observed in transfer curve.
2
L
2
m ¼
ðslopeÞ
CW
where, C is capacitance of the dielectric layer per unit area, L
and W are length and width of the channel, respectively.
Threshold voltage (VTH) is calculated from the plot of square
root of IDS vs. gate voltage (V
G
). All the compounds exhibited
(expect 1) magnitude and good on/
off current ratio with minimized threshold voltage. Compounds
and 5 have resulted in ambipolar behaviour may be due to the
ꢀ2
2
ꢀ1 ꢀ1
mobility in 10 cm V
s
1
substitutions (TAA and phenyl extended TAA) with a balanced
hole and electron transport, which is very crucial in comple-
18
mentary circuits. Surprisingly, compound 3 has exhibited the
2
ꢀ1 ꢀ1
highest hole mobility of 1.1 cm V
s
with good on/off ratio
6
of 10 . In addition the threshold voltage is also observed as ꢀ2 V
with no kink in the transfer curve. Good I
ratio ensures
on/off
minimum leakage and improved device performance. These
12
values are ameliorated from many of the previous reports.
The methoxy group present in the TAA alter the electron
distribution in the triarylamine system. It can assist the
molecular self-assembly through pi–pi stacking and improve
the communication among the molecules. From cyclic voltam-
metry analysis, it is observed that among the compounds
studied, compound 3 has the lowest band gap (2.954 eV). In
compounds 3, 4, and 6, although the phenanthroline moiety is
clearly electron decient, n-channel behaviour was not
1,2
observed. Compounds 4 and 6 (thiophene and t-butyl) possess
same HOMO levels at ꢀ5.6 eV and exhibited almost similar hole
ꢀ
2
2
ꢀ1 ꢀ1
mobilities up to 5.9 and 5.6 ꢂ 10 cm V
s , respectively. In
compound 3, the methoxy substitution may help the molecule
to self-assemble in an efficient way to improve the communi-
cation among the molecules and resulted in highest mobility.
Fig. 6 OFET characteristics of compounds 1 ((a–d), ambipolar) 3 ((e
and f), p-channel), 4 ((g and h), p-channel) 5 ((i–l), ambipolar) and 6 ((m
Conclusions
1/2
and n), p-channel) in transfer curves, log I
D
and (I
D
)
curves are
In summary, a set of phenanthroline-TAAs has been designed,
synthesized and investigated for their OFET behaviour. The
compounds possessed HOMO levels in the range of ꢀ5.2 to
represented in green and blue colour, respectively.
observations can be accounted for the nature of substituents;
electron rich methoxyphenyl, thiophene and tert-butylphenyl
groups which improves the donating nature of the TAA moiety,
thus p-channel behaviour is prominent and no n-channel
behaviour is observed. The mobility is extracted from the
saturation region using the formula,
ꢀ
5.6 eV ensures less energy barrier for charge injection with
ꢁ
thermal stability up to 560 C. Solution processing from
a chloroform solution has produced uniform lm with high
coverage area and good crystallinity as witnessed from SEM and
thin lm XRD analysis. The bottom gate top contact devices
ꢀ
2
2
ꢀ1
exhibited ambipolar behaviour with mobility of 10 cm V
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ꢀ
1
3
4
s
and Ion/off 10 /10 . Especially, compound 3 with methox-
J. Smith, S. Rossbauer, A. J. P. White, T. D. Anthopoulos
and M. Heeney, Adv. Mater., 2012, 24, 3205.
5 (a) Y. Zhao, Y. Guo and Y. Liu, Adv. Mater., 2013, 25, 5372; (b)
J. Mei, Y. Diao, A. L. Appleton, L. Fang and Z. Bao, J. Am.
Chem. Soc., 2013, 135, 6724.
yphenyl substitution in TAA side had resulted in p-channel
behaviour with the highest mobility up to 1.1 cm V
good Ion/off of 10 . This investigation gives an insight about
effect of substituents in tuning the electronic behaviour of TAAs
in OFET application.
2
ꢀ1 ꢀ1
s
and
6
6 (a) J. J. Bergkamp, S. Decurtins and S. X. Liu, Chem. Soc. Rev.,
2015, 44, 863; (b) R. Pfattner, E. Pavlica, M. Jaggi, S. X. Liu,
S. Decurtins, G. Bratina, J. Veciana, M. M. Torrent and
C. Rovira, J. Mater. Chem. C, 2013, 1, 3985.
Conflicts of interest
7
(a) J. Zhang, J. Jin, H. Xu, Q. Zhang and W. Huang, J. Mater.
Chem. C, 2018, 6, 3485; (b) A. Wadsworth, M. Moser,
A. Marks, M. S. Little, N. Gasparini, C. J. Brabec, D. Baran
and I. McCulloch, Chem. Soc. Rev., 2019, 48, 1596; (c) S. Bi,
Z. Ouyang, S. Shaik and D. Li, Sci. Rep., 2018, 8, 9574; (d)
J. M. Mativetsky, M. Kastler, R. C. Savage, D. Gentilini,
M. Palma, W. Pisula, K. Mullen and P. Samorı, Adv. Funct.
Mater., 2009, 19, 2486.
(a) E. C. P. Smits, T. D. Anthopoulos, S. Setayesh,
E. V. Veenendaal, R. Coehoorn, P. W. M. Blom, B. d. Boer
and D. M. deLeeuw, Phys. Rev. B: Condens. Matter Mater.
Phys., 2006, 73, 205316; (b) E. J. Meijer, D. M. DeLeeuw,
S. Setayesh, E. VanVeenendaal, B. H. Huisman,
P. W. M. Blom, J. C. Hummelen, U. Scherf and
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There are no conicts to declare.
Acknowledgements
Financial assistance provided by the Council of Scientic &
Industrial Research, New Delhi (02(0222)/14/EMR-II) is grate-
fully acknowledged. One of the authors (DR) thanks the Central
University of Tamil Nadu for a research fellowship.
8
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