Chemically doped perylene diimide lamellae based field effect transistor with low operating voltage and high charge carrier mobility

Article abstract of DOI:10.1039/c3cc45391j

Chemical doping of an electron transporter results in the formation of a radical anion containing semiconductor which showed high electron mobility (13 cm² V^-1 s^-1) at low operating voltage (1 V).

Full text of DOI:10.1039/c3cc45391j

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Chemically doped perylene diimide lamellae
based field effect transistor with low operating
voltage and high charge carrier mobility†
Cite this: Chem. Commun., 2014,
50, 326
Received 16th July 2013,
Accepted 25th October 2013
Arulraj Arulkashmir,‡ Bhanprakash Jain,‡ Jino C. John, Kanak Roy and
Kothandam Krishnamoorthy*
DOI: 10.1039/c3cc45391j
www.rsc.org/chemcomm
Chemical doping of an electron transporter results in the formation
of a radical anion containing semiconductor which showed high
electron mobility (13 cm2 VÀ1
s
À1) at low operating voltage (1 V).
Scheme 1 Synthesis and chemical structure of 1.
Molecular doping by either strong electron donors or acceptors has
been demonstrated to modulate the Fermi level (EF) of organic demonstrate OFETs comprising PDIs as semiconductors and hydra-
semiconductors (OSC) and impact device efficiency. In thin film zine as a dopant with the objective of modulating EF and OFETs
devices, n-type doping has been found to increase the charge carrier device efficiency.
mobility and improve the air stability of the devices.1 To donate
PDIs with four carboxylic acid functionalities (1) were designed
electrons to the lowest unoccupied molecular orbital (LUMO) of an and synthesised (Scheme 1). These molecules spontaneously form
OSC, the highest occupied molecular orbital (HOMO) of the dopant nanowires, but they disintegrate into small assemblies upon
must be at higher energy. Such a HOMO energy level is vulnerable to hydrazine treatment. To impart structural integrity, 1 was reacted
oxidation due to the low lying oxygen energy level (À5.2 eV).1b This with Zn2+ to get 2 by following the procedure used for preparation
complicates the device fabrication process and limits the use of of metal organic systems using pyromellitic and naphthalene
dopants in organic field effect transistors (OFETs). Tetrathianaphtha- bismides.8 The hydrazine doped 2 shows high electron carrier
zene and cationic salt precursors have been explored as dopants with mobility (m), low threshold voltage (VT) and a sub-threshold slope
limited success.2 Another approach is the use of organometallic (S) under atmospheric conditions.
sandwich dimers as n-type dopants, wherein the dimer cleaves
upon heat treatment and injects electrons into the OSC.3 Recently, The thermogram of 1 showed about 5% weight loss at 90 1C
4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyldimethyl indicating the removal of water. Significant weight loss was observed
Molecule 1 was synthesized by following the reported procedure.9
amine (N-DMBI) and its cationic analog have been employed to n at 400 1C revealing the initiation of thermal degradation of 1 with
dope [6,6]-phenyl C61 butyric acid methyl ester (PCBM) and C60, complete weight loss at 700 1C (ESI,† Fig. S3). The absorption and
respectively.4 This doping increased the conductivity, but the field self-assembly characteristics of 1 were studied by UV-vis absorption
induced charge carrier mobility was not altered significantly.4b
spectroscopy. The UV-vis absorption spectrum of 1 dissolved in DMF
Self-assembly and charge transport properties of perylene diimides shows three peaks at 458, 490 and 525 nm. These features corre-
(PDIs) have been well studied.5 PDIs (ER0ed of À0.59 V vs. SCE) can be spond to H type aggregates of PDIs in solution.9 To study the impact
reduced to their radical anions by hydrazine (E0Ox + 0.43 V vs. SCE).6 It of temperature, a solution of 1 was gradually heated from 0 to
is necessary to note that the OFETs based on radical containing 100 1C. During this the intensity of the peak at 458 nm decreases
semiconductors have been found to work at low operating voltage with concurrent increase in the peak intensity at 490 and 525 nm
even while using a few hundred nanometers thick SiO2.7 Thus, it is (ESI,† Fig. S4). This indicates the disassembly of H type aggregates.10
reasonable to expect the radical containing PDI based devices to work One of the objectives of this work was to use hydrazine as a dopant
at low operating voltages. Considering these factors, we attempted to to produce radical anions of 1 and use them in OFET fabrication.
Therefore, it is necessary to study the impact of hydrazine reduction
Polymer science and engineering division, CSIR-National Chemical Laboratory,
on the aggregation properties of 1. Upon heating 1 from 0 to 100 1C
in the presence of hydrazine, three broad peaks evolve at 502, 620
CSIR-Network Institutes of Solar Energy, Dr Homi Bhabha Road, Pune-411008,
India. E-mail: k.krishnamoorthy@ncl.res.in; Tel: +91-20-25903075
and 720 nm. The former peaks are attributed to a change in the type
† Electronic supplementary information (ESI) available: Synthetic procedure,
of aggregates from H to J11 and the latter is attributed to the
characterization and discussion. See DOI: 10.1039/c3cc45391j
‡ These authors have contributed equally to the work.
formation of radical anions (Fig. 1a).12 The evolution and existence
326 | Chem. Commun., 2014, 50, 326--328
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Fig. 2 (a) TEM image of 2 before, and (b) after hydrazine addition.
(c) HRTEM image showing the lamellae in the nanowires of 2.
the formation of stable radical anions of 2 under atmospheric
conditions. SEM and TEM images of 2 were recorded before and
after addition of hydrazine. The TEM images showed the presence of
a nanowire network with individual wire diameter ranging between
70 and 100 nm (Fig. 2a). Interestingly, the nanowires did not
disintegrate upon addition of hydrazine (Fig. 2b). This confirms our
hypothesis of connecting individual 1 with zinc atoms to impart
structural integrity. Furthermore, the nanowires were subjected to
high resolution TEM (HRTEM) to identify the presence of any smaller
features in the nanowires. In the HRTEM images, presence of
lamellae in the nanowires of 2 was observed (Fig. 2c). The width of
an individual lamella was found to be 2 nm (see ESI† for lattice
fringes, Fig. S10), which is indeed the length of 1 (determined by
density functional theory calculations, ESI,† Fig. S12). From these
experiments, we can conclude that the carboxylic acid functionalities
of molecule 1 have been connected by the zinc ions to form the
lamellae. These lamellae are likely to assist charge hopping and
increase the charge carrier mobility in field effect transistors. Please
note that the lamellae were not found in the nanowires of 1 (ESI,†
Fig. S5). In order to study the effect of hydrazine doping on the
frontier orbital energy levels of 1 and 2, Ultra Violet Photoelectron
Spectroscopy (UVPES) was employed.13 From these experiments, it
was found that the hydrazine injects electrons into the partially filled
HOMO/shallow LUMO levels. More details are provided in the ESI.†
With an understanding of orbital energy levels and self-assembly
properties of 1 and 2 in solution and in the solid state, we proceeded
to fabricate OFET devices. The pre-fabricated OFET substrates were
coated with 1 from a DMF solution. The output characteristic IV
measurements were carried out by sweeping the drain voltage (VD)
between 0 and 1 V while keeping the gate voltage (VG) constant. The
output characteristics showed only linear regimes, with very little
change in ID as a function of VG (Fig. 3a). Such a characteristic of a
charge transport material in an OFET device has been attributed to
electron transport in the bulk.4a The conductance of 1 was calculated
using Ohm’s law and found to be 1 Â 10À6 S. Reliable IV curves could
not be obtained upon treatment of 1 with hydrazine, presumably due
Fig. 1 (a) UV-vis absorption spectra of 1 as a function of temperature in
the presence of hydrazine. (b) TEM image of 1 before hydrazine addition,
and (c) after hydrazine addition. (d) ESR spectra of 2.
of the peak at 720 nm under atmospheric conditions indicate the
formation of stable radical anions in the presence of oxygen and
moisture. The radical anion formation was further confirmed by ESR
spectroscopy with a g-tensor value of 2.0014 (ESI,† Fig. S6).
PDIs self-assemble into nanostructures with very high aspect
ratios.5a,b,d To study the morphology of 1 and its stability upon
addition of hydrazine, electron microscopy imaging was employed.
The scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) images showed the presence of nanowires with
diameters ranging between 70 and 100 nm (Fig. 1b). Upon addition
of hydrazine, these nanowires are disintegrated into small assem-
blies and the same was manifested in the TEM images shown in
Fig. 1c. The disintegration of nanowires is attributed to electrostatic
repulsion between the negative charges generated by hydrazine.
Although the hydrazine addition damaged the nanowires, they still
remain as small assemblies. The UV-vis absorption studies showed
the presence of J type aggregates upon addition of hydrazine because
of the presence of these small assemblies.
In order to impart integrity to the assemblies of 1, compound 2
was prepared by reacting 1 with Zn(NO3)2. Synthetic details and
spectroscopic characterization are provided in the ESI.† Inductively
Coupled Plasma Atomic Emission Spectroscopy (ICPAES) was used to
further confirm the presence of zinc in 2. ICPAES is an effective and
sensitive tool to quantitatively determine the presence of metals in
organic compounds. The presence of 3.54 mg lÀ1 of zinc in 2 further
confirms the reaction between zinc and 1. In the thermogram of 2,
complete degradation did not occur at 900 1C contrary to complete
degradation at 700 1C in the case of 1.This is likely due to the
presence of zinc in 2. In the UV-vis absorption spectra, peaks are
observed at 450, 490 and 527 nm with a small hump at 568 nm.
Upon gradual heating, the peak at 450 nm decreased to a hump and
the intensity of the peaks at 490 and 527 nm increased indicating the
change in the aggregation type (ESI,† Fig. S8). Upon hydrazine
addition, the peak at 450 nm disappeared and new peaks evolved
at 507, 620 and 720 nm. The peak at 720 nm is due to the formation
of radical anions that are stable under atmospheric conditions. To
further corroborate the formation of radical anions, ESR spectra were
recorded. We were gratified to find the g-tensor value of 2.0016, which
is close to that observed for 1 (Fig. 1d). The ESR experiment confirms
Fig. 3 (a) Output characteristic I–V curves of 1 and (b) 2 after hydrazine
doping.
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to the disintegration of nanowires upon addition of hydrazine. These nanowires with B2 nm lamellae are stable upon hydrazine
small assemblies were not sufficient to connect the source and drain treatment. The doped semiconductor shows electron mobility
electrodes, which hinders the IV measurement. OFET performance of as high as 13 cm2 VÀ1 sÀ1 with a threshold voltage of À0.1 V.
2 was studied by following the procedure employed for compound 1. Furthermore, the device works at a low operating voltage of
The output characteristic IV curves exhibited only linear regimes. The 1 V under atmospheric conditions with a subthreshold slope of
conductance was found to be 1.2 Â 10À6 S. Contrary to this, well 160 mV decÀ1
.
defined linear and saturation regimes were observed for the hydra-
Funding from CSIR (TAPSUN NWP 54) is greatly acknowledged.
zine treated devices of 2. Interestingly, while sweeping the VD between AA, BJ, JCJ and KR thank CSIR for scholarship. We thank Dr
0 and 1 V, a profound increase in ID was observed as a function of Gopinath and Dr Sathyanarayana for UVPES measurements and
increasing VG (Fig. 3b). The low operating voltage is due to the ICPAES measurements, respectively. Dr Asha is acknowledged
presence of radical anions in doped 2 despite the presence of 210 nm for UV-vis measurements. Professor Anil Kumar and Ms Sreelekha
thick SiO2. It is necessary to note that low operating voltage OFETs are (IIT-Bombay, Mumbai) are acknowledged for ESR measurements.
usually fabricated using thin and modified gate dielectric materials.14
The m calculated in the saturation regime15 was found to be
Notes and references
13 cm2 VÀ1 sÀ1. The VT and Ion/Ioff were À0.1 V and 40, respectively.
´
Such low Ion/Ioff has been reported for poly(3-hexyl thiophene), which
is used as standard.16 The low operating voltage and threshold
voltage (VT) should result in a low sub-threshold slope (S).7 The
sub-threshold slope (S) is the voltage swing required to switch the
transistor ‘‘on’’ from the ‘‘off’’ state.17 Thus, a low S is desirable. The
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.
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Environmental stability of the devices was studied by measuring
device metrics at various time intervals. There was no change in
device efficiency over a period of 24 h, while the device was exposed
to atmospheric conditions. One order decrease in m was observed
when measured at 150 h (Table 1). After this, the output character-
istics of the device resemble the undoped devices fabricated using 1
and 2. The reason for this change can be explained by the electrode
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levels of 1 and 2. The work function of Au is À5.1 eV,7 which is
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HOMO orbital energy (À5 eV) level of 2. Therefore, the electrons in
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´
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Table 1 Organic field effect transistor metrics for 2
Compound
VT (V) m (cm2 VÀ1 sÀ1
)
S (mV decÀ1
)
gm (S)
2 + Hyd
2 + Hyd (48 h)
2 + Hyd (96 h)
2 + Hyd (150 h) À0.3
À0.1
À0.1
0.3
13
2.5
2.7
1.2
160
—
245
4405
1 Â 10À4
7 Â 10À5
4 Â 10À6
3 Â 10À7
328 | Chem. Commun., 2014, 50, 326--328
This journal is ©The Royal Society of Chemistry 2014