Chapter 1
Review of the Electrical, Optical and
Structural Properties in Phthalocyanines
1.1 Introduction
1.1.1 Organic Semiconductors
Organic semiconductors are really useful in conventional electronics
applications, where only inorganic materials, such as silicon, germanium and
compound semiconductors have been used in the past fifty years. Thin films of π conjugated materials are widely used in semiconducting devices such as sensors1-3,
light emitting diodes4, 5, dosimeters6, 7 and solar cells8, 9. Organic semiconductors are
advantageous for the fabrication of electronic devices because of the ease of
processing at low temperature, architectural flexibility, material variety, and
environmental safety. There is a fundamental need to understand the various physical
properties that would open the development of new strategies towards enhanced
mobility of charge carriers in Field Effect Transistors and high electroluminescence
quantum yield in Light Emitting Diodes.
A large number of organic materials have been described, which show
electrical conductivity in the semiconducting range. But, due to the extremely low
intrinsic conductivity, most organic semiconductors should really be designated as
insulators. The use of the name semiconductor is based on the extrinsic
semiconducting properties of organic systems, i.e., the capacity to transport charge
generated by light, injected by electrodes, or provided by chemical dopants. In the
case of organic semiconductors the crystal consists of regular arrays of equally
1
spaced molecules. These molecular units rather than ions - in the case of inorganic
semiconductors, make the conduction mechanism more complex in organic
semiconductors.
1.1.2 Phthalocyanines
Phthalocyanine is an important industrial material used in the production
of inks, colouring for plastics and metal surfaces.These are aromatic
hydrocarbons exhibiting semiconducting properties and hence come under the
class of organic semiconductors. Phthalocyanines play a very important role in
the present day molecular electronics. Phthalocyanines contain conjugated bonding
i.e., alternately double and single bonds. The electrons associated with these bonds
are not localized on a particular atom, but are delocalized with in the entire molecule.
These electrons in the p-state, called the π-electrons are believed to be responsible
for the conduction in organic semiconductors. Conduction would either involve
excitons, hopping and tunneling or band to band transitions. The optical absorption
in phthalocyanines occurs within the molecule rather than within the crystal.
The metal-free phthalocyanine was first detected by Braun and
Tcherniac10 as a minor product in the synthesis of o-cyanobenzamide from
phthalamide and acetic anhydride in 1907, while the first metal complex was
prepared by Diesbach et al.11 in 1927. Linstead used the term ‘Phthalocyanine’
derived from the Greek term ‘naphtha’ (rock oil) and ‘cyanine’ (dark blue) to
describe this particular class of materials. Since then, there have been extensive
studies on the physics and chemistry of phthalocyanines.
2
It is observed that the electrical, optical and structural properties of
phthalocyanine thin films are critically dependent on the film morphology which
in turn is determined by the preparation parameters such as deposition rate,
substrate temperature and post-deposition heat treatment given to the film. The
electrical properties of phthalocyanines are decided by the type of conduction
mechanism involved. For a desired film characterization, an understanding of its
properties at various growth conditions is essential.
The metal-free phthalocyanine (H2Pc) has the general formula C32H18N8
or (C8H4N2)4H2. The molecule is planar consisting of four isoindole molecules
linked together at the corners of the pyrrole ring by four nitrogen atoms. The
space within the four central nitrogen atoms is occupied either by hydrogen
atoms in the case of H2Pc or by a metal atom in metal substituted
phthalocyanines (MPc).
The benzene rings at the four corners of the molecule are found to be
projections of regular plane hexagons. The radius of the benzene ring is 1.39 Å. The
inner system of the molecule consists of a closed system of 16 carbon and nitrogen
atoms, the interatomic distance being 1.34 Å. This inner system is connected to the
four benzene rings by C - C bonds of length 1.49 Å. The carbon links emerging from
the benzene rings are strained about 150 from their normal positions. The molecular
structures of metalllophthalocyanine and fluorinated phthalocyanine are shown in
Figure1.1.2.
Phthalocyanines show an exceptional thermal and chemical stability. In
air PcM undergo no noticeable degradation up to several hundred Kelvin and in
3
vacuum
most
complexes
do
not
decompose
below
1173K12,13.
Metallophthalocyanines are of interest in the fabrication of thin film electronic
molecular devices such as opto-electronic devices, gas sensors, dosimeters, light
emitting diodes and DVD-Rs. The structure, morphology, electronic and optical
properties of the films are very important for their technological applications.
(a)
(b)
Figure1.1.2 (a) Molecular structure of metallophthalocyanine and
(b) Molecular structure of fluorinated phthalocyanine
1.2
Electrical Studies
Phthalocyanines are the most extensively studied material among the
organic semiconductors. Semiconducting behavior was originally observed in
bulk phthalocyanines in 194814. Sadoka et al.15 observed that in the presence of
oxidizing gases, conductance is increased and activation energy is decreased for
H2Pc and ZnPc systems. The dark conductivity in phthalocyanine is due to the
thermal excitation of π-electrons16. It is observed that post deposition annealing
have remarkable influence on the activation energy and electrical conductivity of
4
Pc thin films17-21. Experiments on d.c. electrical conductivity measurements and
activation energy measurements on metal phthalocyanine thin films were
performed by Hassan and Gould22 and they suggested hopping type conduction at
lower temperatures. Saleh et al.23 obtained the activation energy, capacitance and
dielectric loss as function of temperature and frequency on ZnPc film. A.C. and
d.c. conductivity measurements on FePc grafted poly (N-Vinylcarbazole) were
made by Zamora and Gonzalez24. Wagner and Loutfy25 studied the hetrojunction
properties of CdS/MgPc films. Influence of atmospheric pollutants on the
conductance of Pc films was studied by A.de Hann et al.26.
The a.c. electrical behavior of sandwich devices of PbPc with gold
electrode is studied by Azim-Agachi et al.27. Their results gave the relative
importance of the hopping model and band theory in describing the conduction in
films with regard to the operating conditions. Vertical type field effect transistors
(FETs) are fabricated using CuPc and their field effect characteristics were
studied by Kazuhiro Kudo et al.2. Static and dynamic characteristics of an
organic induction transistor (SIT) fabricated by NiPc thin films were studied by
Joseph and Menon28. The d.c. electrical properties of Au/Ni/In thin film
structures were investigated by Shafai and Anthopoulos29. They observed that at
low voltage, current density in the forward direction obey the diode equation,
while for higher voltage levels, conduction is dominated by space-charge limited
conduction mechanism. A Schottky conduction mechanism is demonstrated in
gamma ray irradiated Ag/MnPc/Ag thin films by Arshak et al.30. Many authors
observed ohmic conduction at low voltages and space-charge-limited conduction
5
at higher temperatures in different Pcs31-37. D.C. electrical parameters and
conduction processes in α − ZnPc films are investigated by Saleh et al.38.
Electrical properties of Au/ α − Ni/Al are studied by Anthopoulos and Shafai39.
They observed a rectifying junction between Ni/Al and a transition to exponential
trap distribution mode, in the higher voltage region upon annealing of the sample at
395K. Influence of oxygen doping of Schottky type solar cells based on α − NiPc
is studied by Anthopoulos and Shafai8. Drechsel et al.9 studied the properties of
MIP type organic solar cells incorporating phthalocyanine/fullerene mixed layers
and doped wide gap transport layers. They observed that such a combination can
improve the solar cell parameters. Effect of electrode material on a.c. electrical
conductivity of ZnPc thin films was studied by Abu-Hilal et al.40.
The basic properties of various Pcs, which are relevant to gas sensing,
have been studied by different workers. The conductance response of NO2
sensors based on PbPc has been studied by Cheilmann et al.41. Gas sensing
activity of CuPc and FePc thin films were reported by Zhou and Gould42. Gas
sensing activity of different Pcs were studied by many other workers43-53. Copper
phthalocyanine thin film transistors were fabricated by Puigdollers et al.54 using
polymethyl methacrylate as gate dielectric. They observed p-type electrical
characteristics with field effect mobility and threshold voltage value around
0.2 x 10-4cm2v-1s-1 and 6V, respectively. Xinjun Xu et al.55 fabricated a device
with a structure of ITO/F16CuPc(5.5nm)/Zn-complex/Al, where F16CuPc is
hexadecafluoro copper phthalocyanine. They found that electroluminescent
spectra (EL) varied strongly with the thickness of emissive layer. Multilayered
6
photo detectors were fabricated and their characteristics were studied by
Masamitsu et al.56. Osso et al.57 reported the electronic properties of
hexadecafluoro phthalocyanine (F16CuPc) thin films grown by organic molecular
beam deposition (OMBD) under ultra high vacuum (UHV) conditions on
oxidized Si (001) substrates. Schlettwein et al.58 reported the change in electrical
conductivity of F16ZnPc thin films during its growth on SiO2 surface. They
observed an increase in the thermal activation energy around 473K corresponding
to a loss in spectral fine structure. Yuhong Liu et al.59 reported the tunneling
behaviour of homogeneous molecular junctions using p-type molecules of iron
phthalocyanine (FePc), phthalocyanine (H2Pc), and copper (II) octaalkoxyl
substituted phthalocyanine (CuPcOC) and n-type molecule of copper
hexadecafluoro phthalocyanine (F16CuPc). The measured characteristic tunneling
curves of single component phthalocyanine revealed comparable energy gaps for
homogeneous tunneling junctions using the photoemission method. In contrast,
for the heterogeneous tunnel junctions of mixed phthalocyanine including
fluorinated phthalocyanine a distinctive offset of the energy gaps to the positive
bias voltage direction can be clearly identified.
Accurate radiation dosimetry is essential for the protection of general
public and workers across a wide range of activities including industry, medicine,
radiation processing, research and nuclear power generation. Considerable
research into new sensors is underway, including efforts to enhance the sensors
performance
through
both
the
material
properties
and
manufacturing
technologies. The development of sensors needs to take advantage of the new
7
performances obtained by controlling the physical and chemical properties of the
materials. Khalil Arshak et al.60 reported the dependence of gamma irradiation on
current voltage characteristics of thin and thick Ag/CuPc/Ag sandwich structures.
Arshak et al.6 fabricated MOS-capacitor and MOS transistor using CuPc polymer
thick film and they studied the effect of gamma irradiation on their
characteristics. A. Arshak et al.7 reported the effect of gamma radiation on the
conduction mechanism of CoPc thin films. Arshak et al.61,62 reported the effect
of gamma radiation on CuPc and MnPc thick films.
1.3
Optical Studies
The analysis of the optical properties of thin films enforces the
application of theoretical and experimental skills of thin film optics to the organic
molecular systems. Ahmad and Collins63 reported the optical properties of
phthalocyanine thin films. They observed that Q-band absorption in
phthalocyanines is due to π − π * transition and Soret band absorption is of
electronic origin. Campbell and Collins64 reported that the optical absorption in
phthalocyanines could be used to detect the phase transition in thin films. Optical
characterization of FePc complexes using UV-Vis and Mossbaver spectroscopy
was done by Hanack et al.65. Hoshi et al.66 investigated the effect of substrate,
substrate temperature, thickness of the film and post deposition annealing on the
structure and properties of phthalocyanine thin films. Fejfar et al.67 studied the
optical properties of H2Pc composite films using UV-Vis-NIR and FTIR spectra.
Third order optical properties of FePc were studied by Nalva and Kakuta68.
Freyer and Pragst 69 and Angh et al.70 observed new absorption bands in the green
8
and IR regions of the absorption spectrum in MgPc. The optical and
morphological behaviour of MgPc films in contact with aqueous NaClO4 medium
was studied by Taguchi et al.71 and Khatib et al.72. They measured the absorption
spectra of ZnPc films and the peaks located at around 700nm, 360nm and 210nm
were assigned to be Q band, B band and C band respectively. The investigation
by Gu et al.73 showed that the absorption region is matched with the wave length
of semiconductor diode lasers. The optical properties of both monoclinic and
triclinic forms of ClAlPc thin films were reported by Azim-Araghi and Krier74.
They observed doublet of energies in the absorption spectra of the monoclinic
form at 1.74 and 1.9eV and it is in good agreement with previous findings for
other metal phthalocyanines CuPc, PbPc, NiPc and CoPc. Giovanelli et al.75
investigated the thermal stability of thin film of lead- phthalocyanine deposited
on the InS (100) - (4x2)/c(8x2) surface by synchrotron radiation. Papgeorgious
et al.76 analyzed the core level and valence band spectra of monolayer of PbPc
deposited on the clean InAs(100)- (4x2)/c (8x2) reconstructed surface. Optical
and photoconductive properties of multilayered (CuPc/ZnS) films were studied
by Zhibing He et al.77. Fernandez Alonso et al.78 studied the most probable
factors influencing the non-linear optical response of TiPc2/toluene solution
system. Arshak et al.30 studied the effect of gamma radiation on optical properties
of MnPc thick films. They observed that these films are suitable for the
fabrication of dosimeters. Aleksandra et al.79 reported optical functions of CoPc,
NiPc and FePc calculated from spectroscopic ellipsometry in the spectral range
300-800nm. Salmon et al.80 observed inelastic electron scattering from the lower
energy π − π * transitions and from the C-H stretching vibrational mode with
9
energy dependent structure. Senthilarasu and Sathyamoorthy81 reported the
influence of the film thickness and substrate temperature on optical constants of
ZnPc. They found that lower values of the optical constants lead to the good
quality of ZnPc thin films. Xinjun Xu et al.48 fabricated a device with a structure
of ITO/F16CuPc (5.5nm) Zn-complex/Al. They observed significant change in
the emission spectra with the viewing angles. Masamitsu et al.55 studied the
absorption spectra of F16ZnPc thin films. They observed absorption peaks at 645
nm and 810nm. Osso et al.57 studied the refractive index of F16CuPc thin film
grown by organic molecular beam deposition (OMBD) under ultra high vacuum
conditions on oxidized Si(001) substrates. The optical functions of F16CuPc thin
films grown by organic molecular beam deposition were studied by Alonso
et al.82. Wu Yi-Qun et al.83 studied the absorption and transmission spectra, the
thermal stability and the green light static optical recording properties of NiPc
thin films. They observed that metal phthalocyanines are not only a qualified
material for near infrared optical recording but also a promising recording
medium candidate for green-light DVD-R. Schlettwein et al.58 studied the UVVis absorption spectroscopy of F16ZnPc thin films on SiO2 NaCl, KCl and KBr.
They observed stacks of cofacial parallel molecules for thin films on SiO2 and
NaCl. But in the case of films deposited on KBr and KCl they observed a head to
tail arrangement of molecules. Arshak et al.60studied the effect of gamma
radiation on the optical properties of thin and thick CuPc films. They observed
that organic thin films are sensitive to gamma radiations and they reported that
these films can be used for dosimeter applications. Arshak et al.61 studied the
effect of gamma irradiation on the optical properties of MnPc thick films.
10
1.4
Structural Studies
Phthalocyanines are found to exist in several polymorphic phases, the most
commonly observed ones are α and β phases. The β-form is thermodynamically
stable and the α-form is metastable. Collins and Mohammed 84, 85 studied the phase
behavior of ZnPc and showed that the growth of α -form microcrystallites into
large crystals preceded from lower temperatures. They also showed that ZnPc
remains α -phase up to 523K and is fully converted to β -phase at 613K. Iwatsu
et al.86 studied the phase transition in ZnPc at different alcohol vapour atmosphere
and confirmed the existence of an intermediate phase, α - phase with inter lattice
0
G
spacing of 11.4A0. They obtained lattice vector b = 3.78 A for the α -phase and
0
G
b = 4.85 A for the β - phase. Kajihara et al.87 reported that room temperature
deposited ZnPc film exhibits only one clear diffraction line at 2θ = 6.80 which
corresponds to 1.3nm spacing of the XRD pattern, while powder form showed a
complicated pattern with a large number of peaks. Tada et al.88 observed that two
factors determine the structural arrangement of metal phthalocyanines on alkali
halide substrates. One is the electrostatic interaction between central metal atom of
the molecule and a halogen anion of the surface, and the other is the van der Waals
interaction between each molecule. Debe and Kam89 showed that ZnPc possess
three α -polymorphs and that H2Pc has two α -polymorphs. Mindorff and Brodie90
observed phase changes in H2Pc films. Morphology of metal phthalocyanine thin
films were studied by Schoch et al.91 using TEM and electron diffraction and
observed that films deposited at high substrate temperature of 433K, form β phase. Pizzini et al.92 studied the structure and morphology of ZnPc films in NO2
11
ambient. Kuo-chuan Ho and Yi-Ham Tsou47 studied the XRD pattern of NiPc films
deposited on Al2O3 at substrate temperature 298K. Anthopoulos and Shafai39
studied XRD pattern of α -nickel phthalocyanine/aluminium interface. A strong
reflection peak is observed at 2 θ = 6.920. This due to the reflection from the (200)
crystalline plane of the α phase NiPc. Zhibing He et al.77 conducted X-ray
diffraction studies on CuPc/ZnS films deposited at various substrate temperature,
CuPc (200) peak and β -ZnS(111) peak at diffraction angles 2 θ =6.950 and 28.540
respectively. They observed that the intensity of the peaks increase with the
increasing substrate temperature, indicating the increment of the crystallinity.
Anthopoulos and Shafai34 studied the structural properties of NiPc thin films using
X-ray diffractograms. A strong reflection peak was observed at 2 θ = 6.920.
Kuo-Chuan Ho et al.3 studied the XRD spectra and SEM images of PbPc thin films
post treated with an ethanol vapour. They also studied the response of NO gas both
in adsorption and desorption processes, before and after the post treatment. They
found that XRD intensity increases due to the transformation from amorphous to
α -phase crystal structure (2 θ = 12.5-12.70). In SEM images they observed the
increase in the crystalline formation. Puigdollers et al.53 studied XRD pattern of
CuPc thin film transistors fabricated using polymethyl methacrylate (PMMA) as
gate dielectric. They observed a diffraction peak at 6.90, which correspond to the
(100) diffraction of the α -crystal form at substrate temperatures lower than 473K.
They also studied the SEM image of CuPc films deposited on crystallite silicon.
They found that the film was made of homogeneous small crystal grains with an
average diameter of 40-50nm. Eiji Kawahe et al.93 performed the low energy
diffraction analysis(LEED) and scanning tunneling microscopy (STM) of F16ZnPc
12
and ZnPc monolayer thin film deposited on GeS(001) surface to investigate the
terminal group of phthalocyanine related molecules on the lattice structures of the
monolayer films. From LEED spectra they observed that, the unit cell of F16ZnPc
film is larger than that of ZnPc because the van der Waals radius of the terminal
fluorine atom of a F16ZnPc is larger than that of the terminal hydrogen atom of a
ZnPc. Hipps94 studied the STM images and UPS spectra of sub-monolayer films of
F16CoPc absorbed on Au (111). Gerlach et al.95 studied the absorption geometry of
per fluorinated copper phthalocyanine molecular (F16CuPc) on Cu (111) and Ag
(111) using X-ray standing waves. They observed that on both surfaces the
molecules absorb in a lying down manner but significantly in a distorted
configuration. The non polar absorption structure observed is discussed in terms of
the outer carbon atoms in F16CuPc under going a partial rehybridization
(SP2 → SP3). Osso et al.57 used X-ray diffraction and atomic force microscopy to
characterize
the
structure
and
morphology
of
hexadecafluoro
copper
phthalocyanine (F16CuPc) thin films grown by organic molecular beam deposition
(OMBD) under ultra high vacuum on Si (001) substrates.
13
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