a b s t r a c t We report significant enhancement in the electronic properties of nickel phthalocyanine (NiPc) by attaching a potassium-tetrasulpho functional group to synthesize its water soluble derivative nickel (II)4,4 0 ,4 00 ,4&apos; 00 potassium-tetrasulphophthalocyanine (K 4 NiTSPc). To study the potential of this organic compound for electronics applications, Au/K4NiTSPc/Ag diodes have been fabricated and their electronic parameters have been calculated. The mobility and conductivity of the device have been found to be 1.5 Â 10 À 4 cm 2 V À 2 S À 1 and 2.5 Â 10 NiTSPc has shown much better electronic properties as compared to NiPc reported in the literature, which makes it a promising candidate for its potential use in electronics applications

F Wahab

Fakhra Aziz

J A Chaudry

Khaulah Sulaiman

M A Munawar

M H Sayyad

M Shahid

Zubair Ahmad

CiteSeerX

Physica B 413 (2013) 21–23Contents lists available at SciVerse ScienceDirectPhysica B0921-45
http://d
n Corr
E-mjournal homepage: www.elsevier.com/locate/physbEnhancement of electronic and charge transport properties of NiPc
by potassium-tetrasulpho groupZubair Ahmad a,n, M.H. Sayyad b, F. Wahab b, Khaulah Sulaiman a, M. Shahid c, J.A. Chaudry c,
M.A. Munawar c, Fakhra Aziz d
a Low Dimensional Materials Research Centre, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia
b Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi 23640, Pakistan
c Institute of Chemistry, University of the Punjab, Lahore 54000, Pakistan
d Department of Electronics, Jinnah College for Women, University of Peshawar, Peshawar 25120, Pakistana r t i c l e i n f o
Article history:
Received 27 November 2012
Received in revised form
21 December 2012
Accepted 24 December 2012
Available online 3 January 2013
Keywords:
K4NiTSPc
Current–voltage characteristics
Electronic properties26/$ - see front matter & 2012 Elsevier B.V. A
x.doi.org/10.1016/j.physb.2012.12.034
esponding author. Tel.: þ60 3 79674147; fax
ail address: zubairtarar@gmail.com (Z. Ahmada b s t r a c t
We report significant enhancement in the electronic properties of nickel phthalocyanine (NiPc) by
attaching a potassium-tetrasulpho functional group to synthesize its water soluble derivative nickel
(II)4,40 ,400 ,4’00potassium-tetrasulphophthalocyanine (K4NiTSPc). To study the potential of this organic
compound for electronics applications, Au/K4NiTSPc/Ag diodes have been fabricated and their
electronic parameters have been calculated. The mobility and conductivity of the device have been
found to be 1.5104 cm2 V2 S1 and 2.5104 O1 cm1. The K4NiTSPc has shown much better
electronic properties as compared to NiPc reported in the literature, which makes it a promising
candidate for its potential use in electronics applications.
& 2012 Elsevier B.V. All rights reserved.1. Introduction
Researchers in the field of electronics have been focused to
address the issues of high production cost, complex fabrication
techniques and performance of the organic devices [1–3]. To
overcome these problems a large number of solution process-
able organic semiconductors have proven viable materials for
the fabrication of electronic devices [4,5]. By employing these
materials, electronic devices can be fabricated by simple and
cost effective fabrication techniques which include spin coating,
printing, etc. Among the organic compounds, metallophthalocyanines
(MPcs) and their soluble derivatives have acquired great importance
due to their thermal and chemical stability [6]. Thin films of these
compounds can be deposited by thermal evaporation without dis-
sociation [7–9]. Previous investigations on phthalocyanines per-
formed by our research group have shown that there is a need to
improve the electrical properties in order to make the phthalocya-
nines suitable for practical application [7,10–12]. Most of the phtha-
locyanines compounds have shown high ideality factor, greater
barrier height and large series resistance. For instance, the electrical
properties of NiPc (without any functional group attached at the
periphery) in surface-type Schottky diodes have been studied by
Shah et al. [7,10,11] and various electrical parameters of metal/NiPcll rights reserved.
: þ60 3 79674146.
).junctions from current–voltage (I–V), capacitance–voltage (C–V) and
capacitance–frequency (C–F) characteristics have been determined.
These studies have shown high ideality factor, greater barrier height
and large series resistance. On the other hand low mobility and
conductivity and small rectification ratio in these NiPc based diodes
have also been observed. Very recently, Ahmad et al. [13] have
investigated the NiPc derivative, Nickel (II) phthalocyaninetetra-
sulfonic acid tetrasodium salt (NiTSPc), for sensors application which
indicate enhancement of the electrical properties of NiPc by attaching
tetrasulfonic functional group. Keeping in view these facts, in this
study, a derivative of NiPc, Nickel (II) 4, 40, 40 0, 40 0 0potassium-tetra-
sulphophthalocyanine (K4NiPTS), has been synthesized by attaching a
potassium-tetrasulpho functional group to NiPc. The K4NiTSPc is
employed to fabricate surface-type Au/K4NiTSPc/Ag Schottky diodes
to determine its electronic properties. The current–voltage (I–V)
characteristics of the device have been investigated in air at room
temperature. The electronic parameters such as rectification ratio,
mobility and conductivity have been significantly improved.2. Experimental
To synthesize Nickel (II) 4,40,40 0,40 0 0 potassium-tetrasulpho-
phthalocyanine (K4NiTSPc), all reagents and solvents were pur-
chased from Sigma Aldrich and used as received. K4NiTSPc was
synthesized by a modification of the method of Webar and Busch
[14]. The ammonium salt of 4-sulphophthalic acid was reacted
-4 -2 0 2 4
e-15
e-14
e-13
e-12
e-11
e-10
e-9 
C
ur
re
nt
 (A
)
Voltage (V)
Fig. 3. Current–Voltage (I–V) characteristics of a surface type Au/K4NiTSPc/Ag
device.
Z. Ahmad et al. / Physica B 413 (2013) 21–2322with urea and NiCl2 in nitrobenzene as solvent at 180 1C. After
cooling, dilution with water and addition of KOH to make the
sample alkaline, the solution was filtered to remove inorganic
and low molecular mass impurities. The product was isolated
by vacuum evaporation. Molecular structure of nickel (II) 4,
40,40 0,40 0 0 potassium-tetrasulphophthalocyanine (K4NiTSPc) is
shown in Fig. 1.
A solution (1 wt%) of the K4NiTSPc was prepared in distill water
at room temperature. With a little shaking, a homogeneous solution
was obtained. To determine the electronic properties of K4NiTSPc,
the surface-type Au/K4NiTSPc/Ag Schottky diodes were fabricated.
First, the substrates were cleaned for 10 min using distill water in
the ultrasonic cleaner and after drying, the substrate was plasma
cleaned for 5 min followed by the thermal deposition of Au and Ag
electrodes using a shadow mask. During thermal deposition the
chamber pressure was 5.5105 mbar while the deposition rate
was kept at 0.1 nm/s. The thickness of the electrodes was 100 nm
and the gap between the electrodes was 50 mm. The length of the
gap was 3 mm. Afterwards the 100 nm thin films of K4NiTSPc
were deposited by drop casting. The fabricated devices were left at
room temperature for 24 h to let the moisture evaporate. Cross-
sectional view of the fabricated Au/K4NiTSPc/Ag surface type
Schottky diode is shown in Fig. 2. The current–voltage (I–V)
characteristics of the diodes were measured at room temperature
using Keithley 236 Source Measurement Unit.3. Results and discussion
The forward bias and reverse bias I–V characteristics for the
Au/K4NiTSPc/Ag are shown in Fig. 3. The forward bias corresponds
to the positive potential to the Au with respect to Ag. The samples
display nonlinear, asymmetric and rectification behavior, which
indicates the formation of the depletion regions at interfaces. The
rectification ratio (RR) is determined as the ratio of the forward
current to the reverse current at a certain applied voltage (IF/IR).
The rectification ratio depending on the applied voltage reflects
increased charge injection into the K4NiTSPc. The value ofNi-SulfonatedPhthalocyanine
N
N
N
N
N
N
N
NNi
SO3K 
KO3S
SO3K
KO3S
Fig. 1. Molecular structure of nickel (II) 4,40 ,40 0 ,40 0 0 potassium-tetrasulphophtha-
locyanine (K4NiTSPc).
Ag
Glass Substrate
K4NiTSPc
Fig. 2. Cross-sectional view of the Au/K4NiTSPc/Ag diode.rectification ratio is found as 19.8 at71.4 V. This behavior is
due to the fact that a space charge layer is formed at the interface.
The I-V characteristics of the samples show an exponential
increase in current due to decrease in the depletion layer width
at the interface. Calculated values of ideality factor, barrier height
and series resistance are given in Table 1. The thermionic
emission theory is taken into consideration for the extraction of
these diode parameters. According to the thermionic emission
theory the current in Schottky barrier diodes can be given by [15]:
I¼ Io exp
q VIRsð Þ
nkT
 
1
 
ð1Þ
where
Io ¼ AA
nT2 exp
qfb
kT
 
ð2Þ
Io is the saturation current, V is the applied voltage, Rs is the
series resistance, An is the effective Richardson constant equal to
102 A cm2 K2 for organic material [16], A is the effective
diode area (3105 mm2 in this case), T is the temperature
(300 K in this case), k is the Boltzmann constant and fb is the
barrier height. The reverse saturation current Io, determined from
the semi-log I–V curve of Fig. 3, is equal to 2.59108 A. Using
the value of reverse saturation current Io, the effective barrier
height of the junction fb, which is the contact potential barrier
that exists at the interface of the metal and semiconductor, has
been calculated using Eq. (2). Its value is found as 0.72 eV.
The quality of a diode is measured by the ideality or quality
factor ‘n’ using the I–V characteristics. The value of n is unity for
an ideal diode [15] but it deviates from the ideal value and usually
it is observed greater than unity. The value of the ideality factor
for the diode was calculated from the slope of the linear region of
the forward bias semi-logarithmic current–voltage characteristics.
The value of n has been determined from the linear region of the I–V
curves (Fig. 3) using the following relation [17]:
n¼
q
kT
dV
d lnIð Þ
ð3Þ
Table 1
Comparison of the electronic parameters of the Au/K4NiTSPc/Ag and Au/NiPc/Ag junction diodes.
Ideality factor Rectification ratio (RR) Barrier height (eV) Mobility (cm2 V 1 S1) Conductivity (O1 cm1) Rs
Au/K4NiTSPc/Ag [present work] 11 19.8 at71.4 V 0.72 1.510
4 2.5104 47 KO
Au/NiPc/Ag [11] 70 3 at714 V 1.11 9.3109 3.42107 12 MO
11.0
1E-7
1E-6
1E-5
1E-4
I α V~2
I α V4.8
C
ur
re
nt
 (A
)
Voltage (V)
I α V~1
Fig. 4. Double logarithmic current voltage characteristics (log I–log V) of a surface
type Au/K4NiTSPc/Ag diode.
Z. Ahmad et al. / Physica B 413 (2013) 21–23 23High values of ideality factors can be attributed to the non-
homogenous barrier. It may also be due to the other effects, such
as non-homogeneous thickness of organic film and organic layer
effect, etc. [18–20]. The effect of the bias voltage drop across the
interface layer could also be one of the factors for such high
values of n. According to [21], the greater values of n can result
from the analysis of I–V data in regions where the organic bulk
properties are dominant as compared to interfacial processes.
Therefore, the large deviation of the ideality factor from unity
suggests that the transport mechanisms of the device cannot be
well modeled by the thermionic emission only as the value of ‘n’
reaching 11. The higher values of ideality factor can also be
attributed to secondary mechanisms and defects at the interface
[15,21,22].
The I–V characteristics of the Au/K4NiTSPc/Ag diode are
depicted in a log I–log V scale in Fig. 4. As it can be seen from
the Fig. 4, three major regions have been found. At low voltages,
from 0 to 1.0 V, the current increases linearly with an increase in
the bias voltage. It can also be observed that for voltage between
1.0 and 2.0 V the current is directly proportional to the V4.8. With
further increase of bias voltage, the current shows V2 dependence
for the applied bias voltage. The transport through K4NiTSPc layer
in the second and third regions with slope Z2, is like space-
charge-limited-current (SCLC). The second region having a slope
equal to 4.8 indicates space-charge-limited-current (SCLS) with
exponential distribution of deep traps in the band gap of organic
semiconductor (K4NiTSPc) [23]. The third region has a slope of
2 which can be same to SCLC with all traps being filled. The
presence of traps might be due to impurities and various defects
in the chemical structure of organic thin film. By using theequation for traps filled region mentioned in [11], the values of
free carrier mobility m and conductivity for Au/K4NiTSPc/Ag
junction diodes are found 1.5104 cm/(V s) and 2.5
104 O1 cm1, respectively.4. Conclusion
In conclusion, the comparison of the electronic parameters of
Au/K4NiTSPc/Ag Schottky diode and Au/NiPc/Ag shows that the
electronic properties of nickel thalocyanine (NiPc) have been
remarkably improved by attaching a potassium-tetrasulpho func-
tional group to form a water soluble derivative Nickel (II) 4, 40, 40 0,
40 0 0 potassium-tetrasulphophthalocyanine (K4NiTSPc). The conduc-
tivity and mobility of the device have been increased by 3–5 orders
of magnitude than the NiPc based devices previously reported. The
work reported here, can be helpful for the fabrication of future
organic electronic devices such as sensors, transistors and solar cells
using solution processable metal phthalocyanines.References
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Enhancement of electronic and charge transport properties of NiPc by potassium-tetrasulpho group

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Enhancement of electronic and charge transport properties of NiPc by potassium-tetrasulpho group

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