ELECTRICAL CONDUCTIVITY STUDIES ON ANTHRACENE...

Chapter-4

ELECTRICAL CONDUCTIVITY STUDIES ON ANTHRACENE CARBAZOLE AND OLIGOANILINE THIN FILMS

4.1 Introduction

Studies on the current-voltage-temperature characteristics of polymer

films are very important in understanding the conduction mechanism.

Various mechanisms such as tunneling, Schottky emission, field and thermal

ionization of traps and impurities and avalanche multiplication have been

proposed to explain the electrical transport in thin insulating films. However,

there is no universally accepted theory in the literature that explains the

conductivity of polymers since they are in general a uniform mixture of

polycrystalline and amorphous regions. Many workers engaged in polymer

research have proposed electronic, ionic and protonic conduction for various

polymers. The mechanisms generally discussed for various polymer films are

tunneling, Schottky and Poole-Frenkel emissions and space charge limited

conduction (SCLC) [1, 2]. A detailed description of these various conduction

mechanisms are described in Chapter 3. This chapter deals the studies on d.c.

conductivity in high and low temperature conditions and also a.c.

conductivity.

4.2 Theory

4.2.1 D.C. Conductivity

An ideal crystal has a three-dimensional architecture characterized by

the infinite repetition of identical structure units in space. Its structure can be

described in terms of a lattice characterized by long-range order and strongly

86 Chapter IV

coupled atoms. For silicon or germanium this strong coupling results in the

formation of long-range delocalized energy bands separated by a forbidden

energy gap. In organic crystals, the molecules are held together by weak van

der Waals or London forces. This weak coupling result in a narrow width for

the valence and conduction bands and the band structure can be easily

disrupted by introducing disorder in the system. Although organic molecular

crystals exhibit band conduction, excitations and interactions localized on

individual molecules play a dominant role. By contrast, conjugated polymers

do not have a well-ordered structural configuration as crystals. The

conjugation of the polymer backbone is disrupted by chemical or structural

defects, such as kinks or twists. The band structure model of a periodic lattice

implies sharp edges both for the valence and conduction bands due to the

well defined uniform potential wells of the atoms in crystal lattice resulting

in a uniform density of states N(E) in the band. Anderson [3] envisaged some

fluctuations of the potential well distribution in the crystal lattice due to

perturbation caused by the disorders in the lattice. The configurational

disorder in amorphous materials results from the spatial fluctuation in

potential which will lead to the formation of localized states. As a result there

will be a variation in the density of states from a critical to low value, which

can be treated as localized states. Mott [4] suggested an extended density of

states with a long-range order in phase and these would lead to the formation

of tails in the forbidden gap. The sharp edges of valence and conduction

bands of a crystal will be replaced by their gradual transition to long tails of

localized states extended into the forbidden gap. The extensions outside the

two band edges are known as extended regions. The tail formation is a

consequence of the perturbation of the lattice instead of tight bonding model

for a periodic lattice.

Electrical Conductivity Studies… 87

According to Cohen et al. [5] the tails due to localized states

originating from the conduction and valence bands can some times merge

together depending on the band gap and overlap when the concentration of

disorder is very high. Mott and Davis [6] envisaged that instead of the

overlapping of tails arising from localized states, there will be a defect band

arising near the middle of forbidden energy gap due to the presence of

unsatisfied or dangling bonds leading to some local states at the so-called

pseudo Fermi level regions.

According to Davis and Mott conduction in a semiconducting material

is due to three processes and the total conductivity can be expressed as

Total=Intrinsic+Excitation+Hopping (4.1)

The intrinsic conductivity is related to the generation of carriers

which when thermally excited from Fermi level (EF) to the conduction band

or valence band for electrons and holes respectively. The expression for

conductivity is given as

kT/)EE(exp Fc0Intrinsic , (4.2a) for electrons or

kT/)EE(exp VF0Intrinsic (4.2b) for holes, where 0 is

a constant.

The above equations are generally expressed as,

kTaEIntrinsic /)(exp0 (4.2c) the quantity Ea is the

thermal activation energy for charge carriers and k is Boltzmann’s constant

and T is absolute temperature.

The excitation conductivity is related to the generation of carriers

when electrons excited to the localized states, thus adding to the current.

Conductivity in this case can be expressed as

88 Chapter IV

)/kTΔWE(Eexpσσ 1FA1Excitation (4.3a) for electrons or

)/kTΔWE(Eexpσσ 1BF0Excitation (4.3b) for holes, where

1ΔW is the activation energy for hopping, EA is the localized state nearer to

conduction band and EB is the localized state nearer to conduction band.

Hopping conduction is related to the generation of additional carriers

when charge carriers hopping between neighbours in the localized states. In

the case of a real crystal there are defects found in the periodic potential,

such as due to positional disorder (“wrong position”) or substitutional

disorder (a different chemical species), leads to localization of wave

functions of the electrons. Thus the electron states are localized and are

transported in an electric field by moving across potential barriers separating

the individual localized states. However at finite temperature the electron can

be activated and may hop from one localized position to another causing

‘hopping conduction’ [7]

At a localized site the wave function of the electron can be considered

to decrease exponentially as (-r), where is a constant and r is the distance

from the localized center. Hence the probability of overlapping with another

localized wave function at a distance r is proportional to exp (-2r). By

thermal activation the electron can occupy the higher energy state and this

probability is associated with the difference in energy (E) between these two

states. The conductivity due to hopping is expected to be proportional to the

product of these two probabilities.

exp

B

ΔE-2αr-

k T

(4.4a)

At low temperatures the average energy E may be estimated from the

density of states g(E)rd. The total density of states contained in the volume in


a d-dimensional space with a characteristic length r is g(E) . An average

energy difference E near the Fermi energy is given by

dr)E(gE

(4.4b)

Since E is now expressed as a function of r, one can find a particular value

of r that corresponds to a maximum of. The optimal condition is

0Tkr)E(g

r2dr

d

Bd

(4.4c)

Hence, the conductivity at low temperature vary with temperature as

1d/10

T

Texp (4.4d)

where To is a constant. This formula, due to Mott represents variable range

hopping (VRH) conduction. The temperature dependence varies with

dimension.

For three dimensions d=3 and

4/1

00

T

Texpσσ (4.4e)

The derivation of Mott’s law for the d.c. conductivity is based upon

the assumption that the density of states near the Fermi level is constant.

Pollok [8] and Ambegaokar et al. [9] have pointed out that actually electron

–electron Coulomb interaction should reduce the density of states near the

Fermi level. The Coulomb gap plays an important role in the low

temperature d.c. conductivity. The influence of gap can be neglected when

T>Tc and if T< Tc the states with in the gap are important. The conductivity

in such cases can be described as

90 Chapter IV

1/2

00

T

Texpσσ (4.4f)

The same result is also valid for two- dimensional case. Where T0 is

the characteristic temperature and it is related to the average hopping energy

and average hopping distance under the following equations,

1/ 2

02

kW T T (4.4g)

1

20

4

TRave

T

(4.4h)

where k is the Boltzmann’s constant and is the wave function decay length

(localization length).

4.2.2 A.C. Conductivity

The dielectric phenomena arises form the interaction of

electromagnetic fields with different charged species such as electrons,

protons or ions. The oppositely charged species in a solid will be bound by

electrostatic forces to form the neutral species but separated by a distance to

constitute a dipole with some moments. Interactions of micro level electric

field with these dipoles manifest in the form of some macroscopic behaviour

of the dielectric material such as permittivity, dielectric constant,

capacitance, dipolar relaxation and the resonance. The distribution of dipoles

to align under a field is opposed by the damping forces of the solid, frictional

resistance of the medium and thermal fluctuations.

In a time varying field, polarization of dielectric takes place and the

polarization will lag the applied field by an angle ‘’. Thus dielectric

constant is a complex quantity and can be expressed in terms of real (| )

imaginary parts (|| ) as


||| j (4.5a)

The polarization may not follow the field variation. Thus displacement due to

polarization may persist even when the field is stopped. This gives rise to a

decay time to attain equilibrium and the phenomenon is called Debye’s

relaxation [10].

jj

1

0||| (4.5b)

Now equating real and imaginary parts leads to

22

0|

1

and

22

00

0||

(4.5c)

The above equations are called Debye’s equations. As tends to zero ||

tends to zero but || approaches to a value s which is the static dielectric

constant. When tends to infinity i.e. the frequency is very high say in the

optical region () approaches a constant value which is also known as

dielectric constant at optical frequencies. Thus the real part has two limits

static and optical frequencies.

The frequency dependent conductivity is usually explained by

assuming a multi component system of conductivity has been envisaged by

Webb and Bordie [11] leading to the relation,

relacdctot (4.6a) where

)kT/Eexp( a0dc (4.6b)

)kT/)T,(Eexp(A nac (4.6c)

nKrel , where0

, A and K are constants.

In an ideal parallel plate capacitor no energy losses should occur and

the current should lead the applied voltage exactly 900. In reality the total

92 Chapter IV

current transversing the capacitor produces an inclination against the

applied voltage and is referred as power factor angle (usually <900). The

main reason behind this behaviour is the existence of internal capacitative

element that leads to a dissipation of power. In any dielectric material there

will be some power loss because of the work done to overcome the frictional

damping forces encountered by the dipoles during their rotation. The

imaginary part || is related to the conductivity of the dielectric. The loss

factor is generally expressed as

||

|

tan

(4.7)

In terms of capacitative Cs and resistive R components tan can be expressed as

sRC

1tan

(4.8)

4.3 Experimental details

Spectroscopically pure Anthracene, Carbazole and Oligoaniline in

the powder form are procured from Aldirch (USA). Thoroughly cleaned

micro glass slides are used as substrates. The substrates are cleaned using the

procedure described in the section 2.6 of chapter 2. Evaporation of the

material is carried out using Hind Hivac vacuum (Model 12 4A) coating

plant at a base pressure of 10-5

Torr. Molybdenum boat of dimension

2.91.20.5cm is used for the evaporation. During evaporation the substrates

are kept at a distance of 12cm from the source. Annealing is carried out in a

specially designed furnace equipped with digital temperature controller cum

recorder. A detailed description of furnace and the controllers used in the

present study is given in chapter 2 section 10. Annealing is performed before

the deposition of top electrode. Thicknesses of the films are counter checked

using Tolansky’s multiple beam interference technique [12].


Electrical measurements are performed using Keithley electrometer

(model No.617). The samples are mounted on the sample holder of the

conductivity cell. Electrical contacts are made using copper strands of

diameter 0.6mm and are fixed to the specimen with silver paste. The samples

are heated using a resistive heating filament attached to the sample holder of

the conductivity cell and the temperature in the conductivity cell is measured

using a calibrated chromel-alumel thermocouple. Current to substrate heater

is controlled by a variable voltage transformer. Conductivity measurements

are also made in the temperature range 303 – 127K by cooling the samples

using liquid nitrogen. For conductivity studies longitudinal structure is

preferred. A detailed description of longitudinal thin film structure used in

the present study is given in section 2.9 of Chapter 2.

In order to study the a.c. conductivity we use sandwich structure. A

detailed description of sandwich structure used in the present study is given

in section 2.9. The capacitance, dielectric constant and loss tangent are

directly measured using Hioki3532 LCR Hi-tester in the range 100Hz-

5MHz. Since these semiconductors are photosensitive in nature all the

electrical measurements are performed in darkness. To avoid any possible

contamination all the measurements are carried out in a vacuum of 10-3

Torr.

4.4 Results and Discussion

4.4.1 D.C. conductivity studies at high temperature

Figures 4.1, 4.2 and 4.3 show the variation of d.c. conductivity with

1000/T for as-deposited and annealed Anthracene, Carbazole and

Oligoaniline thin films of thickness 2000Ǻ measured above the room

temperature region. The annealing is done at 750C for 30min, 60min, 90min

and 120min. Form the observed characteristics, it is found that the measured

d.c. conductivity increases with increase in temperature. The dependence of

94 Chapter IV

conductivity on the temperature follows the relation (4.2C). From the slope

of these graphs the activation energies are determined. The determined

activation energies as a function of annealing period are summarized in the

Table 4.1 for Anthracene and Carbazole thin films. The activation energy

determined in this work for as-deposited Anthracene thin film is comparable

with the value 0.55eV reported by Jeena and Xavier [8] from their single

crystal studies. The activation energy determined for as-deposited

Oligoaniline is 0.12eV. The activation energy for Oligoaniline remains

constant with annealing. From the table it is clear that the activation energy

increases with annealing for Anthracene and Carbazole thin films.

It is common practice that in an organic semiconductor, a single

thermally-activated conduction is often explained by using the concept of

charged-defect models. The activation energies determined in this work are

very low and this implies the conduction is an extrinsic process. In a p-type

sample, a possible conduction process gives rise to the high temperature

thermally-activated conductivity of the type described by the equation 4.2b

can be related to excitation of charge carriers (holes) from the Fermi level EF

into the extended states below the valence-band mobility edge EV with the

activation energy Ea=EF-EV. Another conduction mechanism that would be

responsible for the electronic transport in p-type semiconductor is that due to

the excitation of holes from the Fermi level to the energy level EB, the end of

the localized states in the band tail above the valence band, with the

activation energy Ea = (EF - EB) +W1, where W1 is the hopping energy

between the localized states [14].

It may be pointed out that, the activation energy Ea alone does not

provide enough indication to whether the conduction takes place in the

extended states below the mobility edge or by hopping in the localized states.


This is because of the fact that the band-energy structure describing

amorphous materials like organic semiconductors is not known a priori to

give a conclusive evidence for the type of conduction mechanism in these

semiconductors. Thus it is concluded that both these conduction mechanisms

can occur simultaneously, where the band-type conduction occurs at high

temperatures where as the conduction via localized states dominates at low

temperatures [15]. The latter transport process is often known as the variable-

range hopping conduction mechanism due to tunneling between far-distant

defect centers. This hopping conduction often occurs at sufficiently low

temperatures. Detailed discussions of low temperature d.c. conduction in

these films are made in the following sections.

Table 4.1 Variation of activation energy with annealing for Anthracene and

Carbazole thin films

Annealing period

min

Ea (for Anthracene)

eV

Ea (for Carbazole)

eV

As-deposited 0.510.05 0.300.05

30 0.490.05 0.290.05

60 0.490.05 0.270.05

90 0.480.05 0.240.05

120 0.450.05 0.210.05

96 Chapter IV

2.9 3.0 3.1 3.2 3.3

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

As deposited

30min

60min

90min

120min

ln(s

)

1000/T(K-1)

Figure 4.1 Variation of ln(σ) with 1000/T for as-deposited and annealed

Anthracene thin films

2.9 3.0 3.1 3.2 3.3

-6.5

-6.0

-5.5

-5.0

-4.5

As deposited

30min

60min

90min

120min

ln(s

)

1000/T(K-1)



2.9 3.0 3.1 3.2 3.3

-7.00

-6.75

-6.50

-6.25

As deposited

30min

60min

90min

120min

ln(s

)

1000/T(K-1)


Oligoaniline thin films


4.4.2 D.C. conductivity studies in the low temperature region

Figure 4.4 shows the plot of ln() versus 100/T1/2

for as-

deposited and air annealed Anthracene thin film of thickness 2000Å

measured in the low temperature region. The annealing is done at

750C for 30min, 60min, 90min and 120min. The slope of the straight

line region gives T0. The average hopping energy and hopping

distances are determined from the relations 4.4g and 4.4h. The

determined values for as-deposited and annealed samples are listed in

Table 4.2. Figure 4.5 shows the plot of ln() versus 100/T1/2

for as-

deposited and air annealed Carbazole thin film of thickness 2000Å

and Figure 4.6 shows similar plot for Oligoaniline thin film of

thickness 2000Å. All the samples are annealed at 750C for 30min,

60min, 90min and 120min. The hopping parameters obtained for as-

deposited and annealed films of Carbazole are listed in Table 4.3 and

the parameters for Oligoaniline films are collected in Table 4.4. From

the tables it is clear that the average hopping energy and hopping

distances are increased due to annealing. Many authors [16, 17]

suggest that during the process of annealing the nearly amorphous

film crystallizes. Annealing produces a more crystalline film and the

hopping sites are filled with charge carriers. This reduces the energy

sites available for hopping and carriers require higher energy for

hopping between adjacent available sites.

The conduction mechanism in these semiconductors can be

explained as follows. In these semiconductors the interactions

between neighbouring molecules give rise to new states which are

delocalized over the entire molecule. The first step of conduction is

the excitation of π-electrons from the uppermost filled π-orbital to

98 Chapter IV

lowest empty-molecular orbital. In the hopping model the charge

carriers are hop from one localized state to the next. When it falls on

a defect state it is virtually trapped in this state due to the potential

well created by the atomic polarization. Thus the activation energy in

the hopping process is to excite charge carriers from one localized

state to next.

The formation of grain boundaries can also affect the

conductivity in the following way. The charged states at the grain

boundary create depleted regions and potential barriers , which

provide a resistance for the passage of carriers from one grain to the

neighbouring ones. The grain-boundary barrier models have been

successfully used to explain the electrical properties of many

polycrystalline materials. The grain boundaries may be assumed to

have the same structure as that of intrinsic trapping defect states and

they are formed in the film due to defects formed during film

preparation. In such cases the resistivity is due to the deplete space

charge regions and grain-boundary potential barriers that impede

thermionic emission of charge carriers into the grains. The increase in

conductivity upon annealing is thus due to the low energy of potential

barrier heights, and can be enhanced by the improvement in the

grain’s growth. The small variation in activation energy observed in

this study can be attributed to the reduction in extrinsic trapping

states and thus increases the conductivity.


6 7 8 9

-11

-10

-9

-8

-7

-6

As deposited

30min

60min

90min

120min

ln(s

)

100/(T)1/2

(K)-1/2

Figure 4.4 Variation of ln(σ) with 100/(T)

1/2 for as-deposited and annealed

Anthracene thin films

6 7 8 9

-7

-6

-5 As deposited

30min

60min

90min

120mIn

ln(s

)

100/(T)1/2

(K)-1/2

Figure 4.5 Variation of ln(σ) with 100/(T)1/2

for as-deposited and annealed


6 7 8 9

-9

-8

-7

-6

-5 As deposited

30min

60min

90min

120min

ln(s

)

100/T1/2

(K)-1/2

Figure 4.6 Variation of ln(σ) with 100/(T)1/2

for as-deposited and annealed

Oligoaniline thin films

100 Chapter IV

Table 4.2 Variation of hopping parameters as a function of annealing period

for Anthracene film of thickness 2000Å

Hopping parameters Annealing periods

As-deposited 30min 60min 90min 120min

T0(K) 3299 8869 11342 13294 19182

Hopping energy meV 41 67 76 82 99

Hopping distance nm 0.78 1.28 1.45 1.57 1.89

Table 4.3 Variation of hopping parameters as a function of annealing period

for Carbazole film of thickness 2000Å



T0(K) 6416 8217 9122 10000 10506

Hopping energy (meV) 57 65 68 71 73

Hopping distance(nm) 11 12.34 13 13.61 13.95

Table 4.4 Variation of hopping parameters as a function of annealing period for

Oligoaniline film of thickness 2000Å



T0 K 3080 3387 3600 4033 4465

Hopping energy meV 16.39 17.68 18.98 18.38 20.27

Hopping distance nm 0.48 0.50 0.52 0.55 0.58

4.4.3 A.C. Conductivity studies

4.4.3a Capacitance- frequency characteristics

The variation of capacitance with frequency measured in the frequency

range 100Hz – 3.16 MHz for Anthracene films of thickness 5500Å annealed at

750C for different periods of time are shown in Figure 4.7. Figures 4.8 and 4.9

show the variation of capacitance with the frequency for Carbazole and

Oligoaniline thin films of thickness 5500Ǻ annealed for different periods of

time. The capacitance decreases with increase in frequency for all samples. This

effect is believed due to the screening of the electric field across the film by


charge redistribution [21, 22]. At low frequencies, the charges on defects are

more readily redistributed, such that defects closer to the positive side of the

applied field become negatively charged while the defects closer to the negative

side of the field become positively charged. As the frequency is increased, in all

cases the capacitance decreases to the same limit, as the charges on the defects

no longer have time to rearrange in response to the applied voltage.

The decrease in capacitance with increase in frequency can also

explained in terms of equivalent circuit model of Goswami and Goswami [23]

which comprises a frequency and temperature independent capacitive element

C′ with a discreet temperature dependent resistive element R due to the

conducting film parallel with C′ both elements in series with a resistance r due to

connecting leads. According to this model measured series capacitance is given

asCR

1CC

22 (4.9a)

The resistive element is temperature dependent. Form the above expression it is

clear that the measured capacitance should decrease with increase in frequency,

and attains a constant value CC at very high frequencies.

The capacitance of annealed samples is higher than the capacitance of

as-deposited samples. The effect is attributed to the reduction of trapping centers

due to annealing. Due to annealing the structural disorders which can act as

trapping centers are reduced. This in turn increases the conductivity of the film.

From the above mentioned equation it is clear that the reduction in resistance

leads to an increase in the capacitance. The capacitance increases with annealing

in the lower frequency region and their variation is very low in higher frequency

region. This is due to the fact that the contribution of second term in the right

hand side of the equation decreases in the higher frequency region.

102 Chapter IV

2 3 4 5 6 7

15

20

25

30

35

40

45

50

55

As deosited

30min

60min

90min

120min

Ca

pa

cita

nce

(pF

)

log(f) Figure 4.7 Frequency dependence of capacitance for Anthracene thin

films annealed for different periods of time

2 3 4 5 6 7

2

3

4

5

6

7

8

9

As deposited

30min

60min

90min

120min

Cap

acitace

(pF

)

log(f)

Figure 4.8 Frequency dependence of capacitance for Carbazole thin

films annealed for different periods of time

2 3 4 5 6 7

2

3

4

5

6

7 As deposited

30min

60min

90min

120min

Cap

acitan

ce

(pF

)

log(f)

Figure 4.9 Frequency dependence of capacitance for Oligoaniline thin films

annealed for different periods of time


4.4.3b Conductivity-frequency characteristics

Figures 4.10, 4.11 and 4.12 show the variation of conductivity with

frequency for Anthracene, Carbazole and Oligoaniline thin films annealed for

different periods of time. From the figures it is clear that the conductivity

increases linearly with the frequency in a double logarithmic scale. The

conductivity of annealed samples is greater than that of as-deposited samples.

The conductivity generally follows the relation σ ωs where ω is the angular

frequency and s is an index which is temperature dependent. The factor s is

directly determined from the slope of the graphs. The value of s is 0.67 for as-

deposited Anthracene thin film and 0.74 for all annealed Anthracene thin

films. The value of s determined in this work is close to the values reported by

Roberts et al [24]. In the case of as- deposited Carbazole thin film, the slope is

0.68 and for Oligoaniline slope is 0.78.

The experimental results are interpreted in terms of a model initially

proposed by Pollak [25] and modified by Elliot [26]. This model involves a

thermally assisted hopping conduction mechanism between localized states. It

was observed that the a.c. conductivity in all the samples increases with

increase in frequency. Generally a decrease of conductivity with increase in

frequency is associated with band type conduction, while increase indicates

hopping type conduction [27, 28]. The frequency dependence of conductivity

can generally expressed using the following relation,

σa.c. = σtotal – σd.c. = Aωs (4.10)

104 Chapter IV

2 3 4 5 6 7

-8

-7

-6

-5

-4

As deposited

30min

60min

90min

120min

log(s

)

log(f)

Figure 4.10 Variation of conductivity with frequency for as-deposited

and annealed Anthracene thin film

2 3 4 5 6 7

-8

-7

-6

-5

-4

As deposited

30min

60min

90min

120min

log

(s)

log(f)


and annealed Carbazole thin film

2 3 4 5 6 7

-8

-7

-6

-5

As deposited

30min

60min

90min

120min

log(s

)

log(f)


and annealed Oligoaniline thin film


where ω is the angular frequency A is a complex parameter that is weakly

temperature dependent and the value of s depends on whether the effect

considered is an intrinsic (lattice) effect or an extrinsic process like thermal

generation or injection of charge carriers. Values close to unity are measured

when the process involved is dipolar and the values in the range 0.5-0.9 are

normally associated with the higher losses due to charge carrier transport [29,

30]. This effect is generally due to screened hopping of charge carriers where the

charges or dipoles responsible for polarization exhibit many body interactions.

4.4.3c Dielectric constant-frequency characteristics

The frequency dependence of dielectric constant for as-deposited

and annealed Anthracene thin films is shown in Figure 4.13. The dielectric

constant shows a strong dispersion character in the low frequency region.

The dielectric constant falls to low values with increase in frequency. A

relatively high dielectric constant at low frequency and a fast decrease with

the frequency are characteristics of organic semiconductors and are

consistent with other reports [24]. The initial high value of dielectric

constant is due to the faster polarization mechanisms (electronic, atomic)

occurring in the material.

The observed reduction in dielectric constant with increase in

frequency may be due to the tendency of induced dipoles to orient

themselves in the direction of the applied field. When the frequency is

increased, the dipoles are no longer able to rotate sufficiently or rapidly so

that their oscillations begin to lag behind the field by explaining the

observed decrease in the dielectric constant with increasing frequency [31].

In organic molecules, dipoles cannot orient themselves in a rapidly varying

electric field and charge carriers are released slowly from relatively deep

traps in the amorphous state [32]

106 Chapter IV

2 3 4 5 6 7

1.0

1.5

2.0

2.5

3.0

3.5

As deposited

30min

60min

90min

120min

e'

log(f)

Figure 4.13 Variation of dielectric constant as a function of frequency for

Anthracene thin films annealed for different periods of time

2 3 4 5 6 7

1

2

3

4

5 As deposited

30min

60min

90min

120min

e'

log(f)


Carbazole thin films annealed for different periods of time

2 3 4 5 6 7

1

2

3

4

As deposited

30min

60min

90min

120min

e'

log(f)


Oligoaniline thin films annealed for different periods of time


0 20 40 60 80 100 120

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Oligoaniline

Carbazole

Anthracene

die

lectr

ic c

on

sta

nt

Annealing period(min)

Figure 4.16 Variation of dielectric constant as a function of annealing

period for different films

The dielectric constant determined for as-deposited

Anthracene thin film is 2.24 at 100Hz and for an annealing period of

120min, the dielectric constant increases to 3.30. Similar

experimental results are reported elsewhere [33]. Figures 4.14 and

4.15 show the variations of dielectric constant with the frequency for

as-deposited and annealed Carbazole and Oligoaniline thin films

respectively. The variation of dielectric constant with the annealing

period measured at a constant frequency 100Hz for these films is

shown in the Figure 4.16. From the figure it is clear that the dielectric

constant increases with annealing period.

4.4.4d Dielectric loss-frequency characteristics

The dielectric loss factor or loss tangent (tan), for the as-

deposited and annealed Anthracene thin films are shown below

(Figure 4.17). From the figure it is clear that the highest value of loss

factor is at low frequency. This is because the migration of charge

carriers in semiconductor is the main source of dielectric loss at low

frequency and decreases with increase in frequency. Accordingly, the

108 Chapter IV

dielectric loss at low and moderate frequencies characterized by high

values due to the contribution of conduction losses in addition to the

electron polarization loss. The dielectric relaxation phenomena are

associated with the frequency dependence of orientational

polarization and hence with polar dielectrics. In static or slowly

varying fields the permanent dipoles align themselves along the field

acting upon them and thus contribute fully to the polarization of the

dielectric.

Figures 4.18 and 4.19 show the variation of tan as a function

of frequency for as-deposited and annealed thin films of Carbazole

and Oligoaniline respectively. In the case of all samples tan has a

maximum value in the lower applied field and decreases with increase

in frequency. Furthermore the tan increases with annealing in all

these samples. Measuring the tan actually discriminates between real

and imaginary parts of the complex dielectric constant. For 450, the

pure capacitance character is the dominant while the dissipative part

is dominating for >450. Since the values of measured in these

samples are relatively high, the dissipative factor dominates in these

materials.


2 3 4 5 6 7

0.0

0.2

0.4

0.6

As deposited

30min

60min

90min

120min

tan(

)

log(f) Figure 4.17 Variation of tan as a function of frequency for as-deposited

and annealed Anthracene thin films

2 3 4 5 6 7

0.2

0.4

As deposited

30min

60min

90min

120min

tan(

)

log(f) Figure 4.18 Variation of tan as a function of frequency for as- deposited

and annealed Carbazole thin films

2 3 4 5 6 7

0.0

0.2

0.4

0.6 As deposited

30min

60min

90min

120min

tan(

)

log(f) Figure 4.19 Variation of tan as a function of frequency for as- deposited

and annealed Oligoaniline thin films

110 Chapter IV

The dielectric loss arises from two mechanisms: the resistive loss

and relaxation loss of the dipole. In resistive loss mechanism the energy is

consumed by the mobile carriers with in the film. The contribution from

mobile carriers reduces in the high frequency region since they are

immobile in high frequency range. Shen et al. [34] experimentally proved

that annealing increases the density of mobile carriers and the carriers are

accumulated in the interface region between the film and the electrode. In

the case of the materials under present studies the annealing results in a

reduction of trap density and thus an increase in the free carriers. This

effect leads to the observed dielectric relaxation in these materials.

It is found that tan decreases with increase in frequency in the case

of Anthracene and Oligoaniline thin films. But in the case of Carbazole thin

films tan decreases first and show a minimum and then increases. Similar

type of variation is observed for many organic semiconductors like -

Nickel phthalocyanine [35] and cobalt phthalocyanine [28]. This effect can

also be explained with the model of Goswami and Goswami[23].

According to this model loss tangent is given by,

rCRC

R

r1

tan

(4.9b) where

r is the series resistance due to leads (usually frequency and temperature

independent), is angular frequency (=2πf) and C is the frequency

independent inherent capacitative element. From the above equation it is

clear that the tan shows an inverse dependence on frequency and attains a

low value at a particular frequency denoted as min. The minimum

frequency is given as rRC

1min . There after the increase in tan is due


to the supremacy of second term in the right hand side of the equation. In

the higher frequency region the above equation reduces to Crtan .

The min is different for different semiconductors depending on the

resistance and capacitance. Another fact which can be explained with the

above equation is the variation in tan with annealing. As annealing

increases the resistance decreases and the tan increase with annealing and

the variation is comparatively small in higher frequency range since the

dominancy of second term in the equation (4.9b).

4.5 Conclusion

Thin films of Anthracene, Carbazole and Oligoaniline are prepared

and the electrical conductivities in the lower and higher temperature ranges

are studied. In the lower temperature region conduction mechanism is

variable range hopping in a coulomb gap. The annealing increases the

conductivity of the films due to the reduction of delocalized states which

can act as trap centers. The frequency dependent conduction in these films

gives valuable information about the dielectric relaxation. The capacitance

exhibits initial high value which drops to a constant value with the increase

in frequency due to polarization of carriers. In general, the dependence of

capacitance on frequency in these samples could be accounted for using the

equivalent circuit model of Goswami and Goswami [23]. The capacitance

and conductivity increases with annealing due to the reduction of trap

centers. The conductivity follows a power dependence on frequency. The

measurement of power factor yields the information that the conduction

mechanism is hopping type. The variations in capacitance and resistance of

the films are in consistent with the equivalent circuit model. The dielectric

constant and the loss tangent have also high values in the low frequency

112 Chapter IV

regions and drops to constant values with increase in frequency. These

parameters also increase with the changes in conductivity induced by

annealing. Thus in conclusion, electrical properties of these semiconductors

are strongly influenced by annealing and the influence of annealing on the

optical properties due to the variations in surface morphology is studied in

the next chapter.

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