Improving device performance of p-type organic field-effect transistor using butterfly like triarylamines

Article abstract of DOI:10.1016/j.orgel.2019.105568

Facile and economic strategy to create a controlled self-structured architecture of semiconducting molecules is very crucial in the accomplishment of carbon electronic devices. A series of new unsymmetrically functionalized butterfly type triarylamines with electron donating/withdrawing substituents were synthesized. Simple and efficient solution based method was used as a successful strategy for OFET with improved characteristics. Binary solvent system with chloroform and toluene (7:3) has improved inter/intra molecular communication in the active layer. The morphology of the films was analyzed by SEM reveals the uniform packing. In addition post thermal annealing has enhanced the crystallinity with minimal grain boundaries. HOMO values -5.1 to -5.4 eV obtained from cyclic voltammetry ensure that these molecules can efficiently transport holes. OFET devices were fabricated in bottom gate top contact (BGTC) architecture by spin coating. Molecules exhibited typical p-channel transistor behavior with mobility up to 1 cm² V^?1s^?1 with 10⁶ I_on/off current ratio. Density functional theory (DFT) visualized the packing and interaction of molecules and supported the efficient mobility. Our investigation gives insight into the role of solvents to control the film architecture in solution processing OFET device construction.

Full text of DOI:10.1016/j.orgel.2019.105568

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Improving device performance of p-type organic field-effect transistor using butterfly
like triarylamines
Ramachandran Dheepika, Anisha Shaji, Predhanekar Mohamed Imran, Samuthira
Nagarajan
PII:
S1566-1199(19)30595-6
DOI:
https://doi.org/10.1016/j.orgel.2019.105568
ORGELE 105568
Reference:
To appear in: Organic Electronics
Received Date: 5 June 2019
Revised Date: 28 August 2019
Accepted Date: 17 November 2019
Please cite this article as: R. Dheepika, A. Shaji, P.M. Imran, S. Nagarajan, Improving device
performance of p-type organic field-effect transistor using butterfly like triarylamines, Organic Electronics
(2019), doi: https://doi.org/10.1016/j.orgel.2019.105568.
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Improving Device Performance of p-type Organic
Field-effect Transistor using Butterfly like Triarylamines
Ramachandran Dheepika, a Anisha Shaji, a Predhanekar Mohamed Imran, b
Samuthira Nagarajan*a
5
aDepartment of Chemistry, Central University of Tamil Nadu, Thiruvarur, India, 600 005.
bDepartment of Chemistry, Islamiah College, Vaniyambadi, India, 635 752.
*Corresponding Author: snagarajan@cutn.ac.in, +91-94430 46272
10
Abstract
Facile and economic strategy to create a controlled self-structured architecture of
semiconducting molecules is very crucial in the accomplishment of carbon electronic
devices. A series of new unsymmetrically functionalized butterfly type triarylamines
with electron donating/withdrawing substituents were synthesized. Simple and
efficient solution based method was used as a successful strategy for OFET with
improved characteristics. Binary solvent system with chloroform and toluene (7:3)
has improved inter/intra molecular communication in the active layer. The
morphology of the films was analysed by SEM reveals the uniform packing. In
addition post thermal annealing has enhanced the crystallinity with minimal grain
boundaries. HOMO values 5.1 to 5.4 eV obtained from cyclic voltammetry ensure
that these molecules can efficiently transport holes. OFET devices were fabricated in
bottom gate top contact (BGTC) architecture by spin coating. Molecules exhibited
typical p-channel transistor behaviour with mobility up to 1 cm2 V-1s-1 with 106 Ion/off
current ratio. Density functional theory (DFT) visualised the packing and interaction
of molecules and supported the efficient mobility. Our investigation gives insight into
the role of solvents to control the film architecture in solution processing OFET
device construction.
15
20
25
Keywords: Solvent engineering, binary solvent system, triarylamines, OFET
30

1. Introduction
Organic field-effect transistors (OFETs) are being actively studied by organic
electronics community due to their facile, cost-effective production [1-2] and their
potential application in flexible large area display devices, smart cards and sensors [3-
7]. Extraordinary beliefs are placed on solution processable organic small molecules
as a gateway towards the real world applications. Because polymer molecules have a
large distribution of molecular weight their devices suffers with batch to batch
variations [8]. Among many small molecules investigated rubrene, pentacene, and
thiophene have excellent OFET characteristics [9-12]. In addition triarylamines
(TAAs) are also studied for their applicability in OFET owing to their structure and
electronic properties [13-17]. TAA based molecules have inherent advantages
including a peculiar geometry, high electron donating and hole transporting ability
[18]. Their optoelectronic applications are studied extensively in the past years.
However their application towards OFETs are restricted because of their amorphous
film forming tendency [19]. Understanding of structure-property relationship remains
as the challenge of now-a-days research to outperform their transistor characteristics.
The real life application of OFETs relies on their high performance in terms of
mobility, on-off ratio and operational stability. Therefore, recent research focuses on
developing the impressive molecular design to improve the molecular assembly and
to optimise the film morphology. The aspects related to device performance are
scrutinised by a few factors such as molecular self-assembly, thin film morphology,
and crystallinity which are regulated by inter and intramolecular interaction. These
can be altered and/or improved by different processing methods [20-22]. A few layers
of organic semiconductor near dielectric/active layer interface is significantly
responsible for carrier movement and influenced by crystallinity. The grain
boundaries, which will trap or scatter the charge carriers can be minimised by
choosing a suitable fabrication style [23, 24]. Solution processing techniques, such as
spin coating, drop casting, inkjet printing are unquestionably easy and economical
way to produce crystalline films.
35
40
45
50
55
60
However, controlling the crystallisation to reach ordered thin film remains as the
question of research. The anisotropy of the films lead to variation in performance

[25]. Consequently, choice of solvent is an impressive way to regulate the film
morphology. Use of additives with solvents for active layer deposition helps to
control the crystallization [26]. The crystalline order can be improved by post
deposition methods such as solvent vapour annealing and thermal annealing [27-29].
Combination of a low and high boiling solvents result in favourable molecular
assembly due to the control in evaporation rate. Low volatile solvents provide
sufficient time for supramolecular self-assembly but the residual solvents must be
removed by further process. In case of highly volatile solvents the residual solvents
can be removed easily but time available for self-assembly is insufficient [25].
Specific proportion of high and low boiling solvents will be the perfect strategy to
produce favourable assembly without any further complicated processes.
65
70
75
80
85
In this work, we designed and synthesised a series of new TAA molecules with an
ethylene spacer and electron withdrawing/donating group to explore their self-
assembling tendency towards high performing p-channel OFET application. The
molecules are designed with an expectation that the unsymmetrical substitution of
TAA provides better molecular packing due to unequal electron distribution assisted
pi-pi stacking. A simple and proficient solvent engineering strategy to produce
crystalline thin film by spin coating method is investigated. The binary solvent system
assists the molecule to communicate via inter and intramolecular interactions and
produce uniform film. Whereas, the proportion of the solvent helps to change the
alignment of the molecules in solution. And a post deposition thermal annealing
process to improve the order of the thin film is also scrutinised for high charge carrier
mobility. This investigation can give an idea about solvent engineering as a platform
to understand solution processing and eventually guides to high performing OFETs.
2. Experimental Section
2.1 Materials
Triphenylamine, dimethyl formamide (DMF), POCl3, potassium iodide, potassium
iodate, palladium (II) acetate, and styrenes were purchased from commercial sources

90
and used as received. Solvents used for synthesis were obtained from Merck; for
analysis anhydrous solvents were used.
2.2 Methods
Molecules were characterized by 1H and 13C NMR in a Bruker 400 MHz
instrument using CDCl3 as solvent and TMS as internal standard. Thermo
Exactiveplus UPHLC – HRMS was used for further confirmation of the molecules.
Photophysical properties were analyzed in JASCO UV-NIR spectrophotometer and
Perkin Elmer Spectrofluorimeter LS55. CHI electrochemical workstation was utilized
to scrutinize the electrochemical behavior (cyclic voltammetry, CV) in a three
electrode electrochemical cell. The electrochemical cell was composed of three
electrodes; glassy carbon served as working electrode, platinum wire as counter
electrode and SCE as reference electrode. The glassy carbon electrode was polished
with alumina (1, 0.5 and 0.03 micron successively) before each experiment. TA
thermal analyzer was used to study their thermal behavior. Density functional theory,
DFT was employed to get insight into geometrical and electronic features of the
molecules in ground state. And extended to time dependent density functional theory
(TD-DFT) to analyze their excited state. OFET devices were characterized in Keithley
4200 SCS. The topography of the thin films coated from a binary solvent system by
spin coating were investigated using Jeol JSM 6390 scanning electron microscope
(SEM).
95
100
105
110
2.3 Synthesis
Precursors 1 and 2 were synthesized according to the literature methods [30-31].
Commercially available triphenylamine underwent Vilsmeier-Haack formylation and
followed by iodination. The formyl group was converted into nitrile group by
hydroxylamine hydrochloride in presence of DMSO. The precursors were subjected
to Heck coupling in presence of palladium catalyst to yield the ethylene bridged
triarylamines in good yield. Anhydrous solvents were used for synthesis and purged
115

with nitrogen for 10 minutes. Detailed spectra of the compounds are given in
supplementary information.
2.4 General procedure of Heck coupling reaction
120
125
130
Precursor 1 or 2 (0.95 mmol, 1 equiv.) was dissolved in a mixture of CH3CN and
triethylamine (1:1 ratio) in a flask and purged with nitrogen for 10 minutes. Palladium
acetate (10 mol %) was added under nitrogen and stirred at room temperature for 15
minutes. Finally corresponding styrene (2.09 mmol, 2.2 Equiv.) was introduced and
temperature raised to reflux, continued for 12 hours. The reaction progress was
monitored by thin layer chromatography. After completion of the reaction, filtered
through celite and the crude was washed with DCM and extracted. The organic layer
was washed twice with water and once with brine solution, dried over Na2SO4. The
product obtained was purified by column chromatography (SiO2, 100-200 mesh size)
with hexane: ethyl acetate gradient solvent system. Yellow solid was obtained in
moderate to good yields.
2.5 Device fabrication and characterization
Bottom gate top contact OFETs are fabricated using heavily n++ doped silicon
wafer. The wafer was ultrasonically cleaned in acetone and methanol. And followed
by a mild piranha solution wash. Then the cleaned wafer was heated up to 1200 °C for
80 minutes to grow silicon dioxide dielectric layer. This thermally grown dielectric
layer thickness was ~300 nm. Si wafer served as gate. The compounds were dissolved
in a binary solvent mixture composed of chloroform and toluene in 7:3 ratio and
sonicated for 20 minutes. Then this solution was spin coated on the SiO2 dielectric
layer at 3200 rpm speed for 45 seconds. The device was then soft baked over a hot
plate at 60 °C to get rid of residual solvent. As a post deposition treatment it is
thermally annealed at 90 °C for 30 minutes. Finally source and drain contact were
made by conductive silver paste to finish the fabrication. The channel length and
width were 150 mm and 5 mm respectively. BGTC devices were characterized using
135
140

Keithley 4200 SCS analyzer. Typical p-channel behavior was observed in the BGTC
145
devices fabricated.
3. Results and Discussion
TAA based new small molecules were designed due to their unusual geometry;
where, the unsymmetrical design improves the self-assembly on substrate and
consequently upgrade the transistor characteristics. Distinct crystalline alignment of
the film produces inspiring π-stacking between the molecules to attain better
electronic coupling and promotes mobility. Transistor characteristics greatly depends
on the processing methods. Here, the binary solvent system used for active layer
deposition is responsible for the morphology. The solution was sonicated, which lead
150
Scheme 1. Sythetic route and structure of compounds 1-8

to formation of an alignment of molecules in solution and on spin coating process it
can produce ordered structure [32]. As post deposition process thermal annealing was
performed to improve the order of the crystallinity of the film. Molecules were
constructed by Heck coupling reactions under nitrogen atmosphere. Synthetic route
and the molecular structure is given in Scheme 1.
155
Figure 1. (a) Absorption (b) emission spectra of compounds 3-8 in anhydrous
dichloromethane
3.1 Photophysical properties
160
To understand the optoelectronic properties of the synthesized molecules (1-8),
absorption and emission behavior was analyzed in anhydrous dichloromethane
(DCM) at 10-5 M concentration in room temperature. Corresponding spectral
parameters are represented in Table 1 and UV-vis absorption spectra is given in
Figure 1 (a). These compounds exhibit two distinct bands: higher energy band
corresponds to π-π* transition and lower energy band is attributed to n-π* transitions.
According to quantum chemical calculations, this lower energy transitions involves
the entire molecule and originated from S0-S1 transitions [33]. However, the result
reveals that increasing the conjugation improves the overall electron density and
lowers the energy of the system leads to red shift irrespective of electron withdrawing
165

170
and donating substituents [34]. Except compound 6, all other nitrile substituted
molecules exhibited relatively high absorbance than the corresponding aldehydes. The
low absorbing tendency of the compound 6 may be due to the twist created by
fluorine with the nitrile in the other arm [33].
175
Table 1. Photophysical properties of compounds 1-8
Compd.
No.
λabs (nm)
λem
(nm)
Stokes
shift
(nm)
Absorption Quantum
coefficient yield,
(ε) (104 M-1 (ϕ)a
cm-1)
λ1
λmax
273
277
280
283
294
296
382
375
385
340
367
378
444
452
449
435
450
452
69
77
63
94
84
54
4.9
5.5
5.0
1.6
4.9
6.0
0.77
0.93
0.66
0.59
0.71
0.68
3
4
5
6
7
8
a Quantum yield calculated using quinine sulphate as standard.
Emission spectra of the compounds were also analyzed in anhydrous DCM at 10-8
M and are shown in Figure 1 (b). The compounds were excited at corresponding
absorption λmax and invariably all the compounds exhibited blue emission in the broad
region of 380-580 nm [35]. Following the trend of absorption, compounds 3 and 4
exhibited higher intensity among the molecules studied. The electron donating and
withdrawing groups may change the excited state geometry of the molecules in polar
medium thus the molecule gains twist and rupture the conjugation [36]. Although, for
all the compounds emission wavelength is observed to be the same, intensity of
emission varies significantly. Quantum yield (ϕ) is determined using quinine as
standard. All the compounds have very high quantum yield, especially 3 and 4 have
0.93 and 0.77 respectively. This is evidently by the incorporation of highly conjugated
naphthalene units. Compounds 7 and 8 with OMe phenyl exhibits 0.71 and 0.68 ϕ
value. Pentafluoro phenyl substitution resulted in relatively lower quantum yield. In
electronically excited state of the molecule charge separation over the molecules is
180
185
190

high; dipole moment is high and thus results in significant Stokes shift. In addition,
the excitons are localized in any one of the three arms of TAA due to the
conformational changes occurs in high polar medium [37]. They gain remarkable
dipole moment in locally excited state.
195
3.2 Electrochemistry
To evaluate electrochemical behavior of the synthesized molecules cyclic
voltammetry technique was used. Samples at 10-5 M concentration in anhydrous
acetonitrile was purged with nitrogen for 10 minutes before the experiment: this
reduces the interference of oxygen on redox process [38]. Experiments were carried
out in 50 mV/s with tetrabutylammonium hexafluorophosphate (TBAPF6) as
supporting electrolyte. The instrument was calibrated with ferrocene as external
standard. Using this Fc/Fc+ redox potential, HOMO and LUMO values were
calculated using the formula, EHOMO = - (Eox + 4.8 - EFc/Fc+) [39]. Electrochemistry of
TAA is interesting because, it can transfer positive charge via radical cation [40]. All
the compounds have exhibited irreversible redox peak in positive potential which
corresponds to triarylamine moiety and no peaks were observed in negative potential.
HOMO values lie in the range of -5.1 to -5.5 eV: compounds 3-6. HOMO values are
matching with most widely used hole transporting materials such as PEDOT:PSS (-
5.3 eV) [41, 39] and ensures very low barrier for hole injection [42]. Peng and co-
200
205
Figure 2. Cyclic voltammogram of compounds 1-8 in acetonitrile at 50 mV/s
scan rate

210
workers have also reported TAA based polymers with shallow HOMO values [42].
Poly(4-butylphenyl)diphenylamine (Poly-TPD) with HOMO -5.4 eV exhibited
Table 2. HOMO, LUMO and band gap values
Experimental
Computational
Comp.
No
Bandgap (eV)c
Gaussian VASP
Band gap
(eV)b
HOMOa
LUMO
HOMO
LUMO
-5.52
-5.53
-2.50
-2.25
3.05
3.28
-5.79
-5.64
-2.03
-2.49
3.76
3.15
3.21
3.31
1
2
mobility up to 10-4 cm2 V-1 s-1 [43, 44]. Interestingly compounds 7 and 8 have HOMO
level at -5.53 and -5.41 eV ensures that these compounds can be used for easy hole
transport. LUMO values are obtained from the absorption edge: and band gap values
are calculated [44].
215

-5.30
-5.38
-2.47
-2.46
2.83
2.92
-5.01
-5.06
-1.75
-1.78
3.25
3.28
3.46
3.31
3
4
-5.22
-2.36
2.86
-5.37
-2.14
3.23
3.67
5
-5.18
-5.53
-5.41
-1.89
-2.55
-2.55
3.29
2.98
2.86
-5.45
-4.85
-4.90
-2.19
-1.57
-1.51
3.25
3.27
3.39
3.72
3.21
3.17
6
7
8
a Calculated from Eox of cyclic voltammetry
b Optical band gap
c Gaussian (B3LYP 6-31G*) & VASP (VASP GGA-PBA)
3.3 Thermal Behaviour
Thermal stability of active semiconducting material is also an important
characteristics for better device performance and durability. The thermal behavior of
the new compounds were studied by differential scanning calorimetry (DSC). And
220
225
230
thermogravimetric analysis (TGA). Thermograms are given in Figures 3 and 4.
Experiments were carried out in nitrogen atmosphere at a heating rate of 5 °C per
minute. In our previous works [45], we have reported formyl/nitrile substituted
triarylamines with no spacer; they possess good thermal stability. Introducing an
ethylene spacer improved the thermal behavior and ensures these molecules can
withstand post deposition thermal annealing process to improve the film morphology.
After post deposition annealing, the effectiveness in the packing of molecule in film
has been found to be noticeable due to extensive pi-pi stacking.
In compound 3, the structural change induced by the spacer has reduced the rigidity in
structure and lowered the melting point. Compounds 7 and 8 exhibited the highest
melting points among the compounds studied. Compounds 5 and 6 with
pentafluorophenyl groups have moderate melting points. Thermogravimetric analysis
have been carried out to support the thermal properties of these molecules. 10 %
weight loss was observed in the range of 299-373 °C.

5
1
2
3
6
7
4
8
100
Temperature
200
300
100
200
300
Temperature
(°C)
(°C)
Figure 3. DSC thermogram of compounds 1-8
235
Table 3. Thermal properties of compounds 1-8
Compd. No.
T °C
T °C
m
d
127
165
177
178
205
224
299
373
343
315
304
366
3
4
5
6
7
8
100
3
4
5
6
7
8
75
50
25
0
200
400
600
800
Temperature °C
Figure 4. TGA curves of compounds 3-8

3.4 Morphological analysis of thin films
The control in the self-assembly of films casted via any solution processing technique
is an important challenge for mono/multi-layered organic electronic devices. The
morphology of films can be tuned by various techniques such as solvent choice, use
of additives and post deposition methods [25-29].
240
Figure 5. Topographic images obtained by SEM for compound 3 (a), 4 (b), 7 (c), and
8 (d)
For SEM analysis, samples were dissolved in a binary mixture of toluene and
chloroform (3:7) and spin coated at 3200 rpm for 45 seconds and heated to remove
the residual solvent. Topography of active layer of the devices (four devices with
compounds 3, 4, 7, and 8) was analyzed by SEM and shown in Figure 5. Compounds
3 and 4 with naphthalene substitution have shown featureless random sized clusters.
Although compounds 7 and 8 are structurally similar their film topographic structures
245

250
are non-identical. Compound 7 has well defined rod like structures which are tightly
packed in the defined area. Compound 8 exhibited clustered flakes of molecules. This
topographical alignment of the molecules in thin film strongly supports uniform
packing and ultimately ensures the higher mobility and improved charge injection
[46].
255
3.5 Thin film X-ray diffraction analysis
Spin coated thin films are analyzed by XRD technique to support the crystallinity
induced by the binary solvent system with post thermal annealing process. The
optimized ratio (7:3, chloroform and toluene) only gives uniform film, whereas the
other ratios and mono solvent systems produces disconnected patches.
3
4
5
6
35
40
45
50
55
2θ (°)
260
Figure 6. Thin film XRD for the compounds 3, 4, 7, and 8.
The XRD of the active layer of the four devices are given in Figure 6. Films made in
mono solvent system produced amorphous nature. Similarly before thermal annealing
process there was no significant peaks were observed. The sharp peaks at 31, 37, 43
and 47 revealed the crystalline nature of the films. Highly ordered arrangement of the
donor molecules are expected to result in high hole mobility.
265

3.6 Computational basis
For better understanding of electronic and spectral properties, the new TAA
molecules are studied through computational methods. Calculations of DFT are
executed using VASP ab-initio software under MedeA Package. The Density of states
(DOS) were computed by PBE functional theory to get insight into the Fermi levels.
Band gap values were calculated by Gaussian and VASP methods and are given in
Table 2. The negligible variation can be accounted for the additional Fermi level in
crystalline structure. However, these band gap values are well in agreement with
experimentally obtained values (Table 2).
270
275
280
285
290
Frontier molecular orbitals (FMOs) are visualized in Figure 7. HOMO is
located over main triphenylamine moiety and LUMO is located over acceptor
substituted arms. In molecules 3 & 4 and 7 & 8 (comparison with respect to
substituents) only CHO group holds LUMO distribution. Molecules 5 & 6 shows
LUMO only over aryl substitution side because of the pentafluoro groups. The DOS
band gap values predicted by MedeA range from 1.41 eV to 2.55 eV and represented
as graphs in Figure S3. The Fermi regions near the FMOs have shown good spread
and signifies the possible states the charge carrier can move. In this context molecule
7 and 8 have highest possible charge carrier movement. Consequently the molecule 8
is arranged in the orthorhombic P212121 pattern in best alignment with feasible
hopping distance. The shortest distance was observed along the edges of the molecule
where the HOMO and LUMO were located. The DOS studies was used to calculate
energy per unit volume and the calculated data are given in Table S1. The highest
volume is obtained for molecule 8 and it signifies the denser packing per unit. In
molecules 7 and 8, swapping of CHO and CN group results in almost same
appearance of LUMO, but 8 has more states than any other molecule in the series.
The smallest gaps are found in 1 and 2, obviously suggesting that extension of
conjugation has enhanced the DOS levels.

Figure 7. Frontier molecular orbitals visualized by Gaussian
295
3.7 OFET characteristics
Charge transport properties of the new unsymmetrical TAA were studied by OFET
devices. All the solution processed BGTC devices exhibited p channel behavior with
good transistor characteristics as shown in Figure 8. The pre-alligned solution of
compounds in the binary solvent was spin coated over thermally grown silicon
dioxide layer. The solubility of compounds and misciblity of the solvents are very
crucial as they define the crystallization of active layer and hole mobility. Among the
six compounds studied 3, 4, 7, and 8 have shown FET behaviour. Nevertheless,
compounds 3 and 4 seems to be promising to possess good mobility by structure, the
ethylene spacer changes the self-organzing ability of naphthalene units and they result
in modest performance. The molecular packing pattern obtained from computational
studies helped us to ratinalise this behaviour of compounds 3 and 4 (Figure S4). From
the linear plot of the square root of drain to source current (IDS1/2) versus the gate
voltage, threshold voltage (VTH) and charge carrier mobility (µ) were calculated.
300
305

Mobility is extracted from the saturation regime of the transfer curve using the
310
formula
2ꢁ
ꢀ =
(ꢄꢅꢆꢇꢈ)ꢉ
ꢂꢃ
Compounds 5 and 6 failed to give FET behaviuor may be due to electron withdrawing
nature of pentafluorophenyl substitution which hampers the hole flow in the molecule.
Compounds 3 and 4 exhibited mobilties up to 0.01 annd 0.07 cm2 V-1s-1 respectively,
with ION/OFF ratio of 106 and 105. Ultimately, methoxy substituion resulted in good
OFET behaviour; might be due to the electronic effect of donating methoxy group and
their mode of interactions. The mobility obtained was 1 cm2 V-1s-1, with 106 on/off
ratio. All the transistor charateristics are greately improved from the previously
reported values for TAA based molecules [13-17]. Molecular glasses of TAA have
been analysed by Sonntag and co-workers exihibited only 10-4 magnitude of mobility
[15]. Cranivo et al [20] have reported good mobility with TAA-thiophene hybrids but
Ion/off was very low. Molecule 8 possess the best crystalline packing with 2.1 Å, 2.1 Å
and 5.1 Å interatomic distance (DFT) and exhibited the higher charge carrier mobility
among the molecules studied. The new TAA molecules can be utilized for various
electronic application.
315
320
325
330
335
Table 4. OFET characteristics of ethylene bridged TAAs
Compd.
No.
µ (cm2V-1s-1)
VTH (V)
ION/OFF
0.01±0.09
0.07±0.02
0.11±0.01
1.0±0.05
1
-16
-4
106
105
106
106
3
4
7
8
-1
*Standard deviation is calculated from five devices

Figure 8. Output and transfer curves of compounds 3 (a, b), 4 (c, d), 7 (e, f), and 8 (g,
h)

340
Conclusion
In summary, new butterfly type unsymmetrical triarylamines were designed and
synthesised successfully by Heck coupling reaction. All the molecules exhibited
HOMO values in the range of -5.1 to -5.4 eV and ensures they can be potentially
employed as hole transporting and electron blocking layer for various organic
electronic devices. The molecules showed good thermal stability to withstand the post
deposition process at 90 °C. Thermal annealing improved the crystallinity of the thin
film coated from a binary solvent system comprised of chloroform and toluene in 7:3
proportion. Among many solvents tried out, this specific ratio produced uniform film
with improved the crystallinity after post thermal annealing process. Devices
fabricated in bottom gated top contact (BGTC) architecture showed typical p-channel
behaviour. Among the molecules, methoxy substituted compound 8 showed high
carrier mobility up to 1 cm2 V-1 s-1 with 106 ION/OFF current ratio. This investigation
signifies the role of new organic semiconducting p - channel material towards high
performing OFETs and encourages their application in various electronic devices.
345
350
355
Acknowledgements
Financial assistance provided by the Council of Scientific & Industrial Research,
New Delhi (02(0222)/14/EMR-II) is gratefully acknowledged. One of the authors
(DR) thank the Central University of Tamil Nadu for a research fellowship.

Appendix I Supplementary data
1
360
Supplementary data contains detailed H NMR, 13C NMR, and HRMS spectral data
and spectra of compounds; Computational studies-Density functional theory (DFT):
molecule geometrical parameters at ground state, density of states graph, crystal
parameters, packing pattern; Time dependent-density functional theory (TD-DFT):
Electronic absorption behaviour.
365
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535

Highlights
‹ New functionalized triarylamines are designed, synthesised and scrutinised for OFET
applications.
‹ HOMO level (5.2 to 5.4 eV) ensures efficient hole injection and transport.
‹ Band gap tuned with respect to the electron donating and withdrawing substituents.
‹ OFETs with bottom gate top contact architecture is fabricated by spin coating from a
binary solvent mixture.
‹ Exhibited higher melting points ensures the device durability with higher mobility up
to 1 cm2 V-1 s-1 with 106 Ion/off current ratio.