1

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

2

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.

3

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

4

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

5

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

6

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

7

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

8

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 *ππ − 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

9

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 *ππ − transitions and from the C-H stretching vibrational mode with

10

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 UV-

Vis 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.

11

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

spacing of 11.4A0. They obtained lattice vector b = 3.78 0A for the α -phase and

b = 4.850A 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

12

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 2θ = 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 2θ =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 2θ = 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 (2θ = 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

13

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.

14

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