Conductivity and photoconductivity peculiarities observed in C₆₀ layers
Thin layers of various thickness prepared from C60 with traces of C70 were studied. They were deposited by thermal evaporation on quartz, glass, p-Si or n-Si substrates. An apparatus fixing current values every 3 ms was used to measure and register the kinetics of layer conductivity and photoconduct...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2006
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Цитувати: | Conductivity and photoconductivity peculiarities observed in C₆₀ layers / St. Kanev, Z. Nenova, N. Koprinarov, K. Ivanova // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2006. — Т. 9, № 4. — С. 17-20. — Бібліогр.: 16 назв. — англ. |
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irk-123456789-1216272017-06-16T03:03:10Z Conductivity and photoconductivity peculiarities observed in C₆₀ layers Kanev, St. Nenova, Z. Koprinarov, N. Ivanova, K. Thin layers of various thickness prepared from C60 with traces of C70 were studied. They were deposited by thermal evaporation on quartz, glass, p-Si or n-Si substrates. An apparatus fixing current values every 3 ms was used to measure and register the kinetics of layer conductivity and photoconductivity. Series of peculiarities were observed in the kinetics of the current when an electrical field was applied to illuminated or darkened samples. For example, when voltage is applied to the sample the current immediately rises to a certain value and then falls to a different quasi-stationary value. These peculiarities depend on the particular state of the samples. After analyzing the phenomena, a scheme explaining these peculiarities was proposed which relates mainly to intrinsic polarization. The influence, which these processes might exert on the photoconductivity and data accuracy, was discussed. A substantial influence of ambient humidity was determined. Our study of the observed changes showed that humidity did not substantially affect the generation-recombination processes in the bulk material, but predominantly the carrier transport mechanism. 2006 Article Conductivity and photoconductivity peculiarities observed in C₆₀ layers / St. Kanev, Z. Nenova, N. Koprinarov, K. Ivanova // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2006. — Т. 9, № 4. — С. 17-20. — Бібліогр.: 16 назв. — англ. 1560-8034 PACS 72.20.-i, 72.80.Rj, 73.61.Wp, 73.50.Pz http://dspace.nbuv.gov.ua/handle/123456789/121627 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Thin layers of various thickness prepared from C60 with traces of C70 were studied. They were deposited by thermal evaporation on quartz, glass, p-Si or n-Si substrates. An apparatus fixing current values every 3 ms was used to measure and register the kinetics of layer conductivity and photoconductivity. Series of peculiarities were observed in the kinetics of the current when an electrical field was applied to illuminated or darkened samples. For example, when voltage is applied to the sample the current immediately rises to a certain value and then falls to a different quasi-stationary value. These peculiarities depend on the particular state of the samples. After analyzing the phenomena, a scheme explaining these peculiarities was proposed which relates mainly to intrinsic polarization. The influence, which these processes might exert on the photoconductivity and data accuracy, was discussed. A substantial influence of ambient humidity was determined. Our study of the observed changes showed that humidity did not substantially affect the generation-recombination processes in the bulk material, but predominantly the carrier transport mechanism. |
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Kanev, St. Nenova, Z. Koprinarov, N. Ivanova, K. |
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Kanev, St. Nenova, Z. Koprinarov, N. Ivanova, K. Conductivity and photoconductivity peculiarities observed in C₆₀ layers Semiconductor Physics Quantum Electronics & Optoelectronics |
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Kanev, St. Nenova, Z. Koprinarov, N. Ivanova, K. |
author_sort |
Kanev, St. |
title |
Conductivity and photoconductivity peculiarities observed in C₆₀ layers |
title_short |
Conductivity and photoconductivity peculiarities observed in C₆₀ layers |
title_full |
Conductivity and photoconductivity peculiarities observed in C₆₀ layers |
title_fullStr |
Conductivity and photoconductivity peculiarities observed in C₆₀ layers |
title_full_unstemmed |
Conductivity and photoconductivity peculiarities observed in C₆₀ layers |
title_sort |
conductivity and photoconductivity peculiarities observed in c₆₀ layers |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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2006 |
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http://dspace.nbuv.gov.ua/handle/123456789/121627 |
citation_txt |
Conductivity and photoconductivity peculiarities observed in C₆₀ layers / St. Kanev, Z. Nenova, N. Koprinarov, K. Ivanova // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2006. — Т. 9, № 4. — С. 17-20. — Бібліогр.: 16 назв. — англ. |
series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
work_keys_str_mv |
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first_indexed |
2025-07-08T20:14:45Z |
last_indexed |
2025-07-08T20:14:45Z |
_version_ |
1837111114118201344 |
fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 4. P. 17-20.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
17
PACS 72.20.-i, 72.80.Rj, 73.61.Wp, 73.50.Pz
Conductivity and photoconductivity
peculiarities observed in C60 layers
St. Kanev, Z. Nenova, N. Koprinarov, K. Ivanova
Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences
72, “Tzarigradsko chaussee” blvd, 01784 Sofia, Bulgaria
Phone: +3592778448, fax: +35928754016
E-mail: kanevstefan@yahoo.com; znenova@yahoo.com
Abstract. Thin layers of various thickness prepared from C60 with traces of C70 were
studied. They were deposited by thermal evaporation on quartz, glass, p-Si or n-Si
substrates. An apparatus fixing current values every 3 ms was used to measure and
register the kinetics of layer conductivity and photoconductivity. Series of peculiarities
were observed in the kinetics of the current when an electrical field was applied to
illuminated or darkened samples. For example, when voltage is applied to the sample the
current immediately rises to a certain value and then falls to a different quasi-stationary
value. These peculiarities depend on the particular state of the samples. After analyzing
the phenomena, a scheme explaining these peculiarities was proposed which relates
mainly to intrinsic polarization. The influence, which these processes might exert on the
photoconductivity and data accuracy, was discussed. A substantial influence of ambient
humidity was determined. Our study of the observed changes showed that humidity did
not substantially affect the generation-recombination processes in the bulk material, but
predominantly the carrier transport mechanism.
Keywords: photoconductivity, C60, electrical properties and measurements.
Manuscript received 11.09.06; accepted for publication 23.10.06.
1. Introduction
Upon determination of the energetic characteristics of
the electronic shell of the C60 molecule [1, 2], it is
acceptable to use the forbidden band of C60 deposited as
pure or as a hybrid compound with polymers [3-6] for
the purposes of photoelectrical conversion of solar
energy [7-10] and for the creation of photoelectrical
sensors. The overwhelming part of the following studies
was about pure fullerene material (C60 or C70), deposited
alone or in combination with other materials as a thin
film. It was necessary to gather knowledge both of
problems concerning the generation-recombination
processes in the fullerene molecule itself and the
mechanisms, by which the generated carriers could be
separated and transported inside the bulk of the layer.
The present study concerns certain peculiarities of the
conductivity and photoconductivity in thin layers of the
fullerene material (C60 with traces of C70), obtained by
direct sublimation from the fullerene containing soot.
2. Experimental details
Thin films were obtained by direct sublimation of soot
heated to a certain temperature favors C60 vacuum
evaporation. The method for soot production is the
classical one [11] – arc discharge in an inert atmosphere
between carbon electrodes. The inert gas was Ar, the
working pressure – 8·104 Pa, and the current of the AC
discharge – 75 A. More details about the method used
and the experimental set-up could be found elsewhere
[12]. The soot was not treated for enrichment or
separation of the various fullerene or cluster fractions,
contained. This was the simplest possible method for the
production of this type of thin films. The substrate
material was glass, quartz, p-Si and n-Si. No additional
heating of the substrate has performed. The layer
thickness varied from 0.06 to 0.6 μm.
The conductivity measurement set-up is a routine
technique. The measurements were performed using a
vibrating-capacitor electrometer (VA-J-51) connected to
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 4. P. 17-20.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
18
Fig. 1. Fullerene layer morphology.
Fig. 2. SEM - photo illustrating the presence of amorphous
and microcrystal phases in the fullerene layer.
analog-digital converter to allow for computer control
of the measurement. The electrometer has an input
resistance of 108 Ohm and input capacitance of 25 pF.
The time constant of the measurement set-up was thus
approximately 3 ms, while the observed current peaks
were of the order of seconds. The measurement set-up
and its time response was additionally fixed using
commercial resistors with the resistance within the range
of that of the C60 layers. The same measurement set-up
has been used in comparable investigations performed
previously by us on a-Si:H, CdS and other thin layer
samples [13], but no peaks have been observed in the
photocurrent kinetics behavior.
3. Results and discussion
The obtained layers are uniform and have very good
adhesion to the substrate. A grainy, morphology of the
layers is seen under lower SEM power (Fig. 1).
Diffractograms from various regions show the presence
of microcrystals of different lattice types.
When using magnification of under higher SEM
power (Fig. 2) the crystals are seen to be dispersed in an
amorphous phase. The microcrystals are surrounded by
the amorphous phase, and it can be assumed at this point
that the both phases participate in the transport of the
charge carriers.
For studying the electrical behavior of the layers,
aluminum strip contacts were deposited on the surface of
the layers at the distance 0.8 mm. We have also
established the excellence of the ohmic behavior of those
contacts. The current values and respective kinetics were
registered by the apparatus fixing these values with the
frequency 330 s-1. Upon applying the voltage, the current
increases until reached a maximum value and then
slowly diminishes, converging towards an equilibrium
value. Apart from the applied voltage, the appearance of
the mentioned above peculiarities also depends strongly
upon the starting state of the material, i.e. upon the pre-
vious treatment (e.g., light, electric pulses, variations in
the humidity of the ambient air). If the samples were
kept without voltage, in the dark, and humidity 30-45 %
before each experiment, the observed peculiarities
always remain the same. All the results below have
obtained, using this standard initial state of the sample.
Fig. 3 shows the dependence I = I(t) recorded at the
field 100 V/cm applied to the sample, kept in dark or
after illumination with the excitatory light. The shorter
time between measuring lowers the peaks of current
measured (Fig. 4). The decrease was observed in the
maximum value of the current peaks as well as in the
interval of returning to a quasi-stationary state.
For both illuminated and non-illuminated samples
(at 45% humidity of the ambient air) (see Fig. 3) the
shape of the curve I = I(t) is preserved, but in the
illuminated samples the curve is shifted towards the
higher values of current.
According to our previous investigations, it turns
out that the fullerene layers tend to actively absorb and
desorb large quantities of water molecules and other
chemical compounds [14, 15], which also strongly
changes the fullerene layer electrical conductivity.
Fig. 5 illustrates the dependence between the
amount of absorbed water molecules as measured by a
quartz microbalance and the percentage of ambient
humidity. The frequency variation Δf at 95 % humidity
corresponds to 0.35×10–6 g/cm2 of water molecules
absorbed in the sample.
For 80 % humidity a considerable change of the
dependence I = I(t) takes place (Fig. 6), both in the dark
and upon illumination. The current increases more than
one order of magnitude. An interesting conclusion is
that, with the increase in amount of the absorbed water
molecules, the photocurrent (i.e. the photosensitivity of
the samples) rises steeply.
Taking into account the peculiarities of the layer
morphology as well as our other studies, a scheme of the
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 4. P. 17-20.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
19
Fig. 3. Current kinetics when the voltage applying in the dark
and on illumination.
Fig. 4. Current kinetics I = I(t) after a different time of
relaxation (the voltage to the specimen is switched off).
Fig. 5. Dependence between the frequency of a quartz
resonator with a fullerene layer and the relative air humidity at
the temperature 30 °C.
Fig. 6. Current kinetics at 80 % humidity.
processes explaining the dependences described above
can be proposed. The thin fullerene layer is a hetero-
geneous system consisting of two phases. As concluded
above, one of the phases is microcrystalline or made of
other monolithic formations and is dispersed almost
uniformly, the second phase is amorphous fullerene
structure. Most probable a barrier type mechanism of
current flow exists. The amorphous phase is low
conducting, less photosensitive and of greater absorbing
ability for water molecules. The basic photoelectrical
processes (photogeneration and recombination) take place
in the micrograins. Upon applying the constant voltage to
the layer, fast polarization takes place, most probably
connected with the interphase barriers. The obtained
current describes with time a characteristic peak, the
height of which depends on the depolarization time as
well as on the amount of electric charge carriers, created
by light or the absorbed water molecules at high humidity
(for example 80 % – Fig. 6). At low humidity (for
example 45 % – Fig. 3), the maximal peak value of the
current remains almost equal both in the dark and under
illumination. Consequently, the barriers and the
corresponding polarization under illumination do not
substantially influence on the stationary measured photo-
current. Тhe increase of the photocurrent at higher levels
of humidity is explained by the corresponding enhanced
dark conductivity of the amorphous phase. This improves
the transport between the electrodes of photocarriers
generated mainly in the crystals.
On the base of the results presented here and the
scheme for their explanation, a more accurate picture of
the processes of photoexcitation and charge carrier
transport can be drawn, if the spectral curve
characteristics would be determined at its equilibrium
value and low humidity.
4. Conclusion
The investigations of the conductivity and photo-
conductivity of thin fullerene layers, prepared by
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 4. P. 17-20.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
20
vacuum sublimation of fullerene containing soot, show
that the measured values are strongly dependent on the
preliminaries and the ambient conditions (illumination,
humidity, electrostatic field etc.). The accuracy of the
investigations requires the knowledge of the exact point
in time when each value of the conductivity and
photoconductivity has measured. In the investigated
processing peculiarities of the current kinetics occur in
illuminated and non-illuminated samples. If those
peculiarities are not taken into account and no steps are
made to suppress their effect, the obtained results of
electrical and photoelectrical measurements of this type
of layer may differ from the real ones. Using the
proposed scheme, we can explain the observed
phenomena being based on the polarizing processes that
occur under the influence of an external voltage. The
experimental results enable optimization of the
measuring methods.
Acknowledgements
The sponsorship of the Bulgarian Ministry of Science
and Education under contract F-329 is appreciated.
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