Photoelectric properties of modified C₆₀ films. Maxwell-Vagner type polarization between near-electrode and bulk layers
Within the frequency interval of 10⁻¹ to 10⁶ Hz investigated were the frequency dependences of the capacitance C and resistance R for modified С60 films in darkness and on exposure to the focused white light. A clamping ITO electrode with an intermediate layer of isotropic liquid (glycerin or distil...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2005
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| Zitieren: | Photoelectric properties of modified C₆₀ films. Maxwell-Vagner type polarization between near-electrode and bulk layers / A.V. Koval'chuk, A.F. Shevchuk, D.A. Naiko, T.N. Koval'chuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2005. — Т. 8, № 3. — С. 92-99. — Бібліогр.: 18 назв. — англ. |
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irk-123456789-1209762017-06-14T03:03:56Z Photoelectric properties of modified C₆₀ films. Maxwell-Vagner type polarization between near-electrode and bulk layers Koval'chuk, A.V. Shevchuk, A.F. Naiko, D.A. Koval'chuk, T.N. Within the frequency interval of 10⁻¹ to 10⁶ Hz investigated were the frequency dependences of the capacitance C and resistance R for modified С60 films in darkness and on exposure to the focused white light. A clamping ITO electrode with an intermediate layer of isotropic liquid (glycerin or distilled water) was taken instead of the traditionally used deposited top metal electrode. Found are the conditions when C and R changes are caused by near-electrode (f < 10³ Hz) and bulk (f > 10⁴ Hz) processes. It was shown that the sharp reduction of C and R with growing the frequency corresponds to the transition from one condition to the other, and such process can be described with account of the “classical” Maxwell-Wagner mechanism of interlayer polarization. The relaxation time of such process was found to be equal approximately 10 ns. This time was shown to depend on the manufacturing technology of С₆₀ films. Having analyzed the obtained frequency dependences of C and R, an equivalent circuit of the sample was suggested. We estimated the thicknesses of the liquid layer (≈ 30 µm) and near-electrode layer of С₆₀ films (tens of nanometers). Comparing the frequency dependences of C and R on exposure to light of the bottom and top electrodes, it was assumed that the С₆₀ films under laser UV-irradiation is non-uniform in thickness. 2005 Article Photoelectric properties of modified C₆₀ films. Maxwell-Vagner type polarization between near-electrode and bulk layers / A.V. Koval'chuk, A.F. Shevchuk, D.A. Naiko, T.N. Koval'chuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2005. — Т. 8, № 3. — С. 92-99. — Бібліогр.: 18 назв. — англ. 1560-8034 PACS: 72.80 Rj, 73.61.Wp http://dspace.nbuv.gov.ua/handle/123456789/120976 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Within the frequency interval of 10⁻¹ to 10⁶ Hz investigated were the frequency dependences of the capacitance C and resistance R for modified С60 films in darkness and on exposure to the focused white light. A clamping ITO electrode with an intermediate layer of isotropic liquid (glycerin or distilled water) was taken instead of the traditionally used deposited top metal electrode. Found are the conditions when C and R changes are caused by near-electrode (f < 10³ Hz) and bulk (f > 10⁴ Hz) processes. It was shown that the sharp reduction of C and R with growing the frequency corresponds to the transition from one condition to the other, and such process can be described with account of the “classical” Maxwell-Wagner mechanism of interlayer polarization. The relaxation time of such process was found to be equal approximately 10 ns. This time was shown to depend on the manufacturing technology of С₆₀ films. Having analyzed the obtained frequency dependences of C and R, an equivalent circuit of the sample was suggested. We estimated the thicknesses of the liquid layer (≈ 30 µm) and near-electrode layer of С₆₀ films (tens of nanometers). Comparing the frequency dependences of C and R on exposure to light of the bottom and top electrodes, it was assumed that the С₆₀ films under laser UV-irradiation is non-uniform in thickness. |
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Koval'chuk, A.V. Shevchuk, A.F. Naiko, D.A. Koval'chuk, T.N. |
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Koval'chuk, A.V. Shevchuk, A.F. Naiko, D.A. Koval'chuk, T.N. Photoelectric properties of modified C₆₀ films. Maxwell-Vagner type polarization between near-electrode and bulk layers Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Koval'chuk, A.V. Shevchuk, A.F. Naiko, D.A. Koval'chuk, T.N. |
| author_sort |
Koval'chuk, A.V. |
| title |
Photoelectric properties of modified C₆₀ films. Maxwell-Vagner type polarization between near-electrode and bulk layers |
| title_short |
Photoelectric properties of modified C₆₀ films. Maxwell-Vagner type polarization between near-electrode and bulk layers |
| title_full |
Photoelectric properties of modified C₆₀ films. Maxwell-Vagner type polarization between near-electrode and bulk layers |
| title_fullStr |
Photoelectric properties of modified C₆₀ films. Maxwell-Vagner type polarization between near-electrode and bulk layers |
| title_full_unstemmed |
Photoelectric properties of modified C₆₀ films. Maxwell-Vagner type polarization between near-electrode and bulk layers |
| title_sort |
photoelectric properties of modified c₆₀ films. maxwell-vagner type polarization between near-electrode and bulk layers |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2005 |
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http://dspace.nbuv.gov.ua/handle/123456789/120976 |
| citation_txt |
Photoelectric properties of modified C₆₀ films. Maxwell-Vagner type polarization between near-electrode and bulk layers / A.V. Koval'chuk, A.F. Shevchuk, D.A. Naiko, T.N. Koval'chuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2005. — Т. 8, № 3. — С. 92-99. — Бібліогр.: 18 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
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2025-07-08T18:57:28Z |
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 3. P. 92-99.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
92
PACS: 72.80 Rj, 73.61.Wp
Photodielectrical properties of the modified С60 films.
Maxwell – Vagner-type polarization
between near-electrode and bulk layers
A.V. Koval’chuk1, A.F. Shevchuk2, D.A. Naiko2, T.N. Koval’chuk3
1Institute for Physics, NAS of Ukraine,
46, prospect Nauky, 03028 Kyiv, Ukraine, e-mail: akoval@iop.kiev.ua
2Vinnitsa State Agricultural University,
3, Sonyachna str., 21008 Vinnitsa, Ukraine, e-mail: sld77@svitonline.com
3Institute for Semiconductor Physics, NAS of Ukraine,
45, prospect Nauky, 03028 Kyiv, Ukraine
Abstract. Within the frequency interval of 10–1 to 106 Hz investigated were the
frequency dependences of the capacitance C and resistance R for modified С60 films in
darkness and on exposure to the focused white light. A clamping ITO electrode with an
intermediate layer of isotropic liquid (glycerin or distilled water) was taken instead of the
traditionally used deposited top metal electrode. Found are the conditions when C and R
changes are caused by near-electrode (f < 103 Hz) and bulk (f > 104 Hz) processes. It was
shown that the sharp reduction of C and R with growing the frequency corresponds to the
transition from one condition to the other, and such process can be described with
account of the “classical” Maxwell-Wagner mechanism of interlayer polarization. The
relaxation time of such process was found to be equal approximately 10 ns. This time
was shown to depend on the manufacturing technology of С60 films. Having analyzed the
obtained frequency dependences of C and R, an equivalent circuit of the sample was
suggested. We estimated the thicknesses of the liquid layer (≈ 30 µm) and near-electrode
layer of С60 films (tens of nanometers). Comparing the frequency dependences of C and
R on exposure to light of the bottom and top electrodes, it was assumed that the С60 films
under laser UV-irradiation is non-uniform in thickness.
Keywords: fullerene film, photodielectric properties.
Manuscript received 24.06.05; accepted for publication 25.10.05.
1. Introduction
After it was practically shown that organic materials can
be effective converters of electric energy into the light
one (organic light-emitting diodes have the increasingly
more practical application), the interest to solve the
inverse problem – conversion of light energy to the
electric one [1-3] was rekindled.
As was shown that, in organic semiconductors, the
active layer where generation and divisions of charge
carriers occurs measures by tens of nanometers [4].
Therefore, perspective and multipurpose organic
photosensitive materials can be produced by means of
nanotechnology. In many works, it was shown that
fullerenes can be one of the effective organic
photosensitive nanostructures [5-10]. On researching
these materials, the highest efficiency of
photoconversion has been achieved in a case when the
functional groups are attached to a fullerene due to
chemical reactions of synthesis. Such molecules are able
to form the linked chains of molecules due to
polymerization, which supports more effective transfer
of photogenerated charge carriers. On the other hand, the
fullerene with the attached group can form a complex
with charge transfer. Partial separation of charges in the
molecule itself increases the photoconversion efficiency
(at the stage of their separation). To carry out the film
formation process based on the polymerization of
fullerene molecules with the attached groups, one should
meet certain requirements to cleanness of initial material
(i.e., to synthesis and cleaning of substances with
molecules of a complex structure and complex chemical
composition).
An alternative to polymerization by synthesis of
monomer molecules based on fullerenes can be various
methods of modification and, in particular, the chemical
modification of С60 films [11]. It is important to note that
the modification of fullerene films can be carried out
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 3. P. 92-99.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
93
already at the stage of producing the photoconverters.
From the practical viewpoint, it is rather perspective as
the single starting material provides production of a set
of materials with variable functional properties, being
based on this modification method.
The analysis of the published works showed that the
influence of various modification methods on the
photoelectric properties of fullerenes is scantily studied.
Therefore, it is of interest to investigate how modifi-
cation of the fullerene structure influences on their
photosensitivity. And it was the main task of this work.
One task more in this work concerned the
manufacturing technology of the photosensitive
structures. To increase the efficiency of light conversion
in photoconverters, transparent conducting electrodes
(most commonly ITO) deposited onto glass substrates
are usually used. There are no technical problems to
deposit the photoconducting organic layer onto the ITO
layer. The problem of creation of the top electrode is
more complex. In most cases, for this purpose the Al [6,
8, 10] or Au [7] layers thermally deposited are used.
However, in this case, there is a high probability of
changing the properties of fullerene film due to
penetration of atoms of metal into the bulk of organic
layer. The range of penetration considerably increases
with insignificant disturbances of the technological
process (surface finish and roughness, a constancy of
deposition rate, etc.). On the other hand, it is significant
whether an organic film properties does not change with
varying the thickness, what can be defined basically
through the use of the electrodes identical in their
chemical composition. Therefore, in this work
considered is the opportunity to study the fullerene
photoelectric properties due to application of the
clamping electrodes through an intermediate layer of
liquid. In such case, it is possible to change easily not
only the electrode, but also the chemical composition of
liquids used.
2. Materials and methods
For researches the chemically cleaned С60 fullerene
(concentration of background impurities is less than
1 wt. %) were used. С60 layers were deposited onto the
glass substrate covered by the ITO layer that is
conducting and transparent to the visible spectrum. The
thermal deposition was performed at the room
temperature and pressure 10–6 Тоrr. The thickness of
samples averaged 100 nm. Samples of three types were
studied. The samples of the first type (1) were modified
by means of laser UV-irradiation, the samples of the
second type (2) were nonmodified (referent), the
samples of the third type (3) were chemically modified
in amine vapour. Initial cleaning of the С60 fullerene,
thermal deposition of the films and their modification
were carried out in Universidad Nacional Autonoma de
Mexico (Mexico).
When studying the electric properties, the ITO layer
deposited onto a glass substrate served as the top
electrode. Before measurements, the samples were kept
in air for a long time (about a month). It stabilized their
parameters to exposure to the atmosphere (as shown in
[12], oxygen essentially influenced on the electric
properties of С60).
To create electric contacts, before clamping the top
electrode, the liquid layer was deposited onto the
fullerene film surface. Both isotropic (glycerin, distilled
water) and anisotropic (ferroelectric liquid crystal)
liquids were studied. In what follows, the data obtained
for isotropic liquids (mainly for glycerin) will be
considered.
The available liquid layer between the fullerene and
electrode basically could result in partial passing the С60
molecules into solution and changing the properties of
samples (such effect was observed in the work [13]).
Therefore, it was important to study the stability of
parameters of the obtained structures. Long-time
researches showed, that even under exposure to light the
properties of samples does not change. I.e., the studied
С60 films were reasonably resistant to environmental
influence (including even the available liquid phase).
The photoelectric properties of samples were
analyzed by changing the capacitance and resistance
under exposure to light. The values C and R were
measured within the range of 10–3 to 106 Hz by means of
the oscilloscopic method [14-16]. The analysis of
frequency dependences enabled us to determine where
certainty (in a bulk or near-electrode region) changes of
properties of samples on exposure to light occur. This is
much more complicated to do when analyzing, for
example, the current-voltage characteristics. The
samples were exposed to focused light from a halogen
incandescent lamp with a power of 100 W directly or
through the interferential filter with a transmission
maximum for λ = 485 nm (this wavelength value is close
to a region of the maximal absorption of С60 films). Both
the top electrode (the ITO layer with deposited С60 film)
and the bottom one were exposed to light. The water
filter was used to decrease heating the samples as caused
by the intensive infra-red radiation of the incandescent
lamp.
All measurements were carried out at the temperature
293 K.
3. Experimental results and discussion
In Fig. 1, shown are the frequency dependences of the
capacitance (а) and resistance (b) in darkness (curves d)
and on exposure to the focused white light (curves p) for
the sample 1. In this case, there was a glycerin layer
between the top electrode and С60 film. At first, let’s
analyze the capacitance frequency dependences
measured in darkness. According to our estimations, the
sample capacitance measured at the frequency 0.1 Hz
exceeds by more than one order the fullerene layer one.
From this analysis it may be concluded that, at low
frequencies, the electric field is applied to near-electrode
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 3. P. 92-99.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
94
layers. Such conclusion is confirmed by an essential
increase in the capacitance of structures when exposing
the latter to the focused white light (curve р in Fig. 1а).
In Fig. 2, for three different types of samples,
resulted are the frequency dependences of the relation of
the capacitance Ср, measured on exposure to the focused
light of the bottom electrode, to the capacitance Сd,
measured in darkness. From the obtained data, it follows
that the ratio Ср/Сd has a sharply pronounced peak
within the frequency range of 1 to 102 Hz (most clearly it
is observed for the samples 1 and 3). Thus, the maximal
value for Ср/Сd exceeds 6. Such a large change in
capacitance is not responsible for the changes of
polarizing properties of fullerene molecules on exposure
to light, and it is most likely caused by change of near-
electrode layer parameters (the thickness of near-
electrode layer decreases when exposing to light). It is
also significant to note that both the frequency at which
the Ср/Сd value has its peak and the maximal Ср/Сd
value depends on manufacturing technology of the С60
films. In the samples 2 and 3, with decreasing the
frequency observed is the increase of Ср/Сd (a new peak
appears, probably). Hence, it follows that the suggested
by us technique to analyze photodielectric properties of
the samples allows us to study the influence of various
technological factors on photoelectric properties of
various substances and, in particular, fullerenes.
Since the glycerin does not absorb the light of the
visible spectrum, the change of the capacitance of
structures during their exposure to light is caused by the
change of the parameters of near-electrode regions,
mainly, in the fullerene layer. The presence of С60 and
isotropic layers in the researched structures is essential
for constructing the equivalent circuit of the sample, i.e.,
clarifying the reasons for changing the properties of the
researched structure over all the frequency range.
To estimate how the liquid layer influences on the
studied structure properties, let us analyze the frequency
dependence of capacitance ( f < 103 Hz) for the case
when a layer of distilled water is located between the top
electrode and С60 film. For the definite frequency, the
10-1 100 101 102 103 104 105
10-8
10-7
10-6
ap
d
C, F
f, Hz
10-1 100 101 102 103 104 105
103
104
105
106 b
p
d
R, Ohm
f, Hz
Fig. 1. Frequency dependences of the capacitance (а) and
resistance (b) measured in darkness (d) and on exposure to the
focused white light (р). The data for the sample 1.
Illumination of the bottom electrode at the temperature 293 K.
10-1 100 101 102 103 104 105
0
2
4
6 a
32
1
Cp/Cd
f, Hz
10-1 100 101 102 103 104 105
0
5
10
15
20 b
312
Rd/Rp
f, Hz
Fig. 2. Frequency dependences of Ср /Сd (а) and Rd /Rp (b) for
the researched samples 1, 2, and 3. Exposure to the focused
white light from the bottom electrode side at the temperature
293 K.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 3. P. 92-99.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
95
capacitance of samples with a layer of distilled water
was found to exceed that with a layer of glycerin by a
factor 4 to 5. Like to the samples with a glycerin layer,
the capacitance of samples with water increased with
exposure to white light. Here the changes of Ср/Сd for
the samples with water were much less than those for the
samples with glycerin. For these samples with water, the
maximal value Ср/Сd was equal to 1.5 at the frequency
0.1 Hz, and decreased with increasing the frequency
(i.e., a peak that is characteristic for the samples with a
glycerin layer was not observed).
For constructing the equivalent circuit of the sample,
it is also of interest to analyze the dependence R(f) (first
for f < 103 Hz) as it was made for the C(f) dependence.
These data for samples with a glycerin layer are depicted
in Fig. 1b. At first, it is reasonable to analyze the R(f)
dependence for the case when the sample was in
darkness (curve d). It follows from Fig. 1b that, in the
frequency dependence of R, it is possible to separate the
section where the resistance decreases with increasing
the frequency linearly in the logarithmic scale. It can be
analytically expressed as follows
Rf = kf −m, (1)
where k is a factor of proportionality, and m is an
exponent. The relation of this type is usually adduced for
conductivity (in this case the sign minus before m is
absent as the conductivity increases with growing the
frequency) and characterizes the hopping mechanism of
the charge carrier transport [17]. For the studied
samples, as was shown when analyzing the capacitance
frequency dependence, the considered section of R(f)
(for f < 103 Hz) characterizes the near-electrode region.
Therefore, to proceed from the frequency dependence of
R to that of conductivity, it would be necessary to find a
near-electrode layer thickness that can be only estimated,
as it will be shown below. Taking the above into
account, it was the frequency dependence of R that has
been analyzed. Despite it, the character of this
dependence itself gives foundations to assume that the
transport of charge carriers through the fullerene near-
electrode region can be also described using the
conception of the variable length hopping mechanism.
For this charge transport mechanism, the value m is
an important factor. These values of m for studied
samples are summarized in the table. Close inspection of
these data shows that the m-value depends on conditions
of preparation (modification) of С60 fullerene, for the
samples 2 and 3 the value m being higher than that for
the sample 1. Thus, the different type treatment of С60
films on their manufacturing results in the change of
charge transport conditions, which can be connected
uniquely with changing the film structure.
Let’s consider how the shape of the dependence R (f)
changes after exposure to white light (Fig. 1b, curve p).
In the dependence R (f), there already observed no
section where Eq. (1) would be obeyed. This fact has
been substantiated for all the studied samples. The rea-
son of it can be an essential change of the near-electrode
layer thickness (almost by a factor of 6 according to the
data on the change of capacitance) and, as a consequen-
ce, more essential influence of effects of tunneling. On
the other hand, reduction of the С60 film resistance after
light illumination can result in a new distribution of the
electric field inside the sample in comparison with that
obtained in darkness measurements.
Similarly to the case of the sample capacitance, it is
important to analyze the relative changes of resistance of
samples on exposure to the focused white light in all the
range of frequencies. In Fig. 2b, shown are the frequency
dependences of the ratio of the resistance measured in
darkness, Rd, to that measured in the course of
illumination by white light, Rp. It is well seen that like to
the case of the capacitance ratio, the peaks are observed,
and not only the peak of Rd/Rp is higher (for the sample
3 it practically reaches 20, while the maximal value of
Ср/Сd ≈ 7) but the peak position shifts to the high-
frequency region (in comparison with the corresponding
position for peaks of Ср/Сd). Distinctions between
Fig. 1a and b may be explained on the assumption that
the studied structures consisted of two layers (С60 and
liquids).
Let’s analyze the frequency dependence Ср/Сd when
instead of a glycerin layer we used that of distilled
water. The resistance of structures with water on
exposure to the focused white light appeared to decrease,
but this drop was smaller by several times than that with
glycerin. In the frequency dependence Rd/Rp the peak
also was observed, however, the maximal value Rd/Rp
equaled to 1.6 (i.e., it is almost one order less than that
with glycerin). A peak position by itself was shifted
insignificantly to the more high-frequency region in
comparison with the samples containing glycerin.
Analyzing the frequency dependences of C and R, it
should be pointed out the sharp change of these
parameters for f > 103 Hz as one of the most important
effects. Especially, it is clearly observed in the darkness
measurements (Fig. 1a and b, curves d). For liquids,
such sharp changes of parameters can be associated with
dipole polarization of molecules, while in solids these
changes were not observed. Our researches have shown
that the dipole polarization of glycerin begins since the
frequencies f > 2⋅105 Hz. In the work [16], it was shown
that the sharp change of dielectric properties of glycerin
for f > 102 Hz is caused by change of conditions for
distribution of the electric field in a sample (from a
condition when the electric field is applied to the near-
electrode region for f < 102 Hz to a condition when that
becomes homogeneous at f > 104 Hz).
Therefore, and for the studied samples, it is possible
to assume that the sharp change of C and R at f > 103 Hz
is also caused by transition from a condition when the
field is applied to near-electrode regions to a condition
when the field in a sample is practically homogeneous
(contrary to the samples with glycerin studied in the
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 3. P. 92-99.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
96
work [16], the electric field in our samples should not be
strictly homogeneous as we have a case of contact of
two layers – C60 fullerene and glycerin). In [16], it was
shown that the transition between various conditions can
be considered within the framework of the general
theory of relaxation processes on the basis of Debye’s
equation modified by Cole – Cole (as Cole – Cole’s
diagram has the shape of an arch). Taking into account
the presence of С60 layer and glycerin in studied samples,
it was difficult to find the components of complex
dielectric permeability from the values C and R.
Therefore, instead of the dependence ε'(ε") (Cole –
Cole’s diagrams) we analyzed the dependences
ω−1R−1(С) (where ω = 2πf is the circular frequency).
This dependence for the sample 1 is presented in Fig. 3.
As the analysis showed the dependences ω−1R−1(С)
obtained experimentally may be approximated by
semicircles, which corresponds to the relaxation process
with one relaxation time τ in accord with the theoretical
representations [18]. For the studied samples, the values
τ are summarized in Table. Hence, it follows that
modifying the C60 films tends to change τ. It confirms
that researching the relaxation processes basically make
it possible to estimate the changes of С60 properties under
various influences.
It is important to compare the obtained value τ with
the data obtained for glycerin. In accord to the data [16],
the relaxation process corresponding to the transition
from a condition when the electric field is applied to the
near-electrode regions to that when the field is
homogeneous lasts 1.4⋅10−4 s (i.e., this value is higher by
the order than the data resulted in the table). It once
more confirms a conclusion that the sharp changes of C
and R observed experimentally for f > 103 Hz are caused
not only by the change of properties of liquid, but also
by those of C60 films.
Similarly to the relaxation processes caused by
redistribution of the electric field between two layers of
dielectrics, the observed relaxation process can be
named the polarization of Maxwell−Wagner, too.
However, the relaxation process considered in the work
is more complex for the analysis than the classical
Maxwell−Wagner effect, since not only the bulk
processes but also the near-electrode ones must be taken
into account. From this fact, the considered relaxation
process, strictly speaking, cannot be a full analog of the
Table. The researched structure parameters obtained from
the analysis of frequency dependences C and R, measured
in darkness and on exposure to the focused white light.
Type
of
sample
m τ,
ns
C∞,
F
dg,
μm
C0,
F
WC,
nm
1 0.50 19 2.5·10−10 28 1.8·10−8 43
2 0.66 11 2.2·10−10 32 1.2·10−8 70
3 0.57 9.1 2.2·10−10 32 1.0·10−8 77
classical Maxwell−Wagner effect. Therefore, we called
this effect as the Maxwell−Wagner-type polarization.
Except the relaxation time, the minimal and maximal
values of the capacitance have been estimated from the
analysis of dependences ω−1R−1(С). Using the analysis of
frequency dependences for dielectric permittivities, it is
assumed that Cmin = C∞ and Cmax = C0, where C∞ and C0
are the capacitances of samples at f → ∞ and f → 0,
respectively. The values C and C0 are shown in the table.
Hence, it follows:
1. For the researched samples 1, 2, 3, the values C∞
practically coincide..
2. The values C0 differ appreciablly (the greatest
distinctions take place between the data for the
sample 1 and those for the samples 2 and 3).
On the basis of the abovementioned analysis of
experimental data, an equivalent circuit of the sample
was suggested. Since both the bulk and near-electrode
properties of glycerin and C60 influence on properties of
researched structures, the most simple equivalent circuit
should have the shape shown in Fig. 4. Under the
assumption made earlier, the capacitance C∞ corresponds
to the case when the field is applied to the overall bulk
of liquid and fullerene (here the great values of
capacitances for the near-electrode regions cannot be
taken into account). Then
C∞ = Cbc Cbg / (Cbc+ Cbg), (2)
where Cbc and Cbg are the capacitances of the bulk С60
film and glycerin, respectively. Using the found
geometrical parameters of a glycerin film, we estimated
Cbc ≈ 7.7⋅10−9 F that is more than 20 times greater than
C∞. It immediately follows that C∞ ≈ Cbg. On the basis of
these data, the thickness of glycerin dg in the researched
structures was found to be equal to about 30 µm (these
values dg are summarized in the table) and practically did
not depend on the type of modification of C60 films.
As the capacitance C0 corresponds to a condition
when the electric field is applied to the near-electrode
regions, on the basis of the offered equivalent circuit of
researched structures, it is equal to
0.0 5.0x10-9 1.0x10 -8 1.5x10 -8 2.0x10-8
0.0
5.0x10-9
1.0x10-8
C 0
C
ω
-1R-1, F
C , F
Fig. 3. ω−1R−1(C) is the sample 1 diagram in the darkness
measurements at the temperature 293 К.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 3. P. 92-99.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
97
C0 = Ccs Csg / (Ccs+ Csg), (3)
where Csc and Csg are the capacitances of the near-
electrode regions of С60 and glycerin, respectively. From
the data [16], the thickness of the diffusion region of the
double electric layer in glycerin measures 50 nm, which
corresponds to the capacitance Csg ≈ 1.8⋅10–7 F. From the
table data, the value Csg exceeds the experimentally
obtained value C0 more than one order. Hence, it follows
that C0 ≈ Csc. This conclusion is supported by an
essential capacitance change observed experimentally
(the maximal value is equal to about 6) after exposing
the samples to white light. From the value of the
capacitance of the near-electrode layer, one can estimate
the thickness of this layer:
Wc = d
bcC
C 0 . (4)
The values Wc stemmed from Eq. (4) are listed in the
table. It is significant that the value Wc by itself measures
of nanometers, which is peculiar to the majority of
organic semiconductors [4]. From our analysis of these
data, it follows that after UV-irradiation and treatment of
С60 film in amine vapour (the samples 1 and 3), the value
Wc increases, which, in principle, should increase the
efficiency of phototransformation.
Using the equivalent circuit of the sample, let’s
consider why the ratio Ср/Сd has a peak at a certain
frequency (Fig. 2а). To do that, one should take into
account the frequency dependences of the components
of complex dielectric permeability for glycerin [16]. The
electric field will be applied practically to the near-
electrode layer of С60 only for the frequencies close to
those when C and R sharply reduce (in this case
influence of the near-electrode layer of glycerin is weak
because of “shunting” action of the capacitance of the
double electric layer in liquid. Exchange of charges at
the electrode-liquid interface takes place in a definite
time interval [16], that’s why with increasing the
frequency the electric field will be more and more
applied to the near-electrode layer of liquid, which
results in the decreased ratio Ср/Сd observed expe-
rimentally. The abovementioned analysis can be also
used to explain for the reason of existence of the peak in
the frequency dependence of Rd/Rp. The double electric
layer in liquid consists of dense and diffusion regions,
each of which is corresponded by an intrinsic frequency
dependence of the components of complex dielectric
permittivity. Just this fact based on the offered
equivalent circuit can serve for explaining of the
appearance of more low-frequency peaks that are
observed especially clearly in the frequency dependence
Rd/Rp for the sample 2.
In conclusion, it should be also considered what
changes may appear if glycerin is replaced by distilled
water. The experimental data for the sample 1 have
shown that in the case of distilled water, there was not
observed so sharp change of C and R at the certain
frequency (it is well clearly observed in the frequency
dependence of capacitance), as it follows from Fig. 1.
From these facts, we have the important inference that
for creating the conditions for which, at the certain
frequencies, the field is applied practically to the near-
electrode С60 layer it is necessary to have a liquid with
the definite properties. It is associated not only with
conductivity of liquid but also with its viscosity
specifying the thickness of a liquid layer. Estimations of
the thickness of the water layer by the capacitance C0
have shown, that it is several times less than that of
glycerin one. In the case when there is a water layer
between the top electrode and С60 film, the absence of
sharp transition from a condition when the electric field
is applied to the bulk layer of liquid to that when the
electric field is applied to the near-electrode layer of a
fullerene results in a sharp decrease in the photo-
sensitivity of structures. That’s why the optimization of
parameters of structures with the top clamping electrode
should involve matching not only the organic film, but
also a chemical composition of liquid.
Since the researched samples with two transparent
conducting electrodes (ITO layers) were to be
illuminated from any of two sides, it enabled to analyze
how far the photosensitive properties of researched
structures are tolerant to changing the thickness. As
evident from the analysis of the received data, the
frequency dependences of C and R for the samples 2 and
3 on exposure to white light through the top and bottom
electrodes do not differ. Taking into account that the
significant part of a visible spectra passes through the
С60 layer reaching the opposite electrode, for purity of
experiment the researched structures were illuminated
through the interferential filter with a maximum
transmittance for the wavelength 485 nm (as was already
noted, this fact corresponds to the case of strong
absorption of fullerene). Thus, it was also obtained that
the illumination through the top electrode results in the
same changes as illumination through the bottom one.
In Fig. 5, for the sample 1 shown is the frequency
dependence of the capacitance ratio when illuminating
the bottom electrode СITO to that when exposing the top
Ccs Ccv Cgv Cgs
Rcs Rcv Rgv Rgs
Fig. 4. The equivalent circuit of the sample. Ccs, Rcs are the
capacitance and resistance of the С60 film near-electron region,
Ccv, Rcv are the capacitance and resistance of the С60 film bulk,
Cgv, Rgv are those of the glycerin bulk, Cgs, Rgs are those of the
glycerin near-electrode region.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 3. P. 92-99.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
98
one Cg. As seen from the figure, the greatest changes are
observed in the frequency range where Ср/Сd get to the
greatest values. The same data are obtained for the
frequency dependence of the resistance ratio when
illuminating the bottom electrode RITO to that when
exposing the top one Rg. I.e., this suggests that for the
sample 1 the observable distinctions between СITO and Cg
as well as RITO and Rg can be caused by the change of
film structure with the thickness. These structural
changes can be caused by the modification of the
fullerene structure under the laser UV-irradiation.
4. Conclusions
Thus, it is shown when analyzing the photosensitivity of
organic structures the method for using the clamping
electrodes through a liquid layer is perspective and it
does not effect on an organic film surface.
From the frequency dependences of C and R within
the frequency interval of 10−1 – 106 Hz which were
obtained in darkness and on exposure to focused white
light, we managed to separate the processes caused by
changes in the near-electrode region (f < 103 Hz) from
those caused by the change of bulk properties of samples
(f >105 Hz), as well as to find experimentally the
frequency region where the sharp changes of C and R
with frequency growth were caused by the transition
from the first process to the second one
(103 < f < 105 Hz).
It is shown that, for the frequencies f < 103 Hz, the
resistance of samples when measuring in darkness is
proportional to f -m and the value m being dependent on
manufacturing technology of С60 films.
The frequency dependence of the resistance ratio
measured in darkness, Rd, to that measured on exposure
to focused light, Rp, was found to have a peak. The
position of the peak (f ≈ 103 Hz) and maximal value
Rd/Rp (≈ 20) were experimentally shown to depend on
conditions of preparation of С60 films and a chemical
composition of liquid.
The frequency dependence of the capacitance ratio
measured on exposure to white light, Ср, to that
measured in darkness, Сd, was shown to have a peak,
too. Like to the Rd/Rp case, the maximal value Ср/Сd and
its peak position on the frequency scale depend on
conditions of the manufacturing technology of С60 films.
It is experimentally shown that the transition from
the processes caused by the near-electrode phenomena to
those caused by changes of bulk parameters might be
described basing on the “classical” effect of the
interlayer Maxwell-Wagner polarization.
The equivalent circuit of sample is suggested.
We estimated the thicknesses of the glycerin layer
(≈ 30 µm) and near-electrode layer of С60 film (tens of
nanometers).
Comparing the frequency dependences of C and R on
exposure to light of the top and bottom electrodes, it was
shown the C60 layer exposed to UV-irradiation is non-
uniform in thickness.
Authors express sincere gratitude to E.V. Basiuk for
the availability to use their samples for measurements.
The work was financially supported by the budgetary
theme 1.4.1 В/109 of the Institute for Physics, NAS of
Ukraine.
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