Photoabsorption and photoconductivity in C₆₀ layers
A complex investigation of the photoconductivity of fullerene films, prepared by thermal evaporation in vacuum, was carried out. The investigated films contain predominantly C₆₀ in various phases as shown elsewhere. The multiple photocurrent spectra analysis (MPSA method) has been used in the curre...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2006
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irk-123456789-1216282017-06-16T03:03:12Z Photoabsorption and photoconductivity in C₆₀ layers Kanev, St. Nenova, Z. Koprinarov, N. A complex investigation of the photoconductivity of fullerene films, prepared by thermal evaporation in vacuum, was carried out. The investigated films contain predominantly C₆₀ in various phases as shown elsewhere. The multiple photocurrent spectra analysis (MPSA method) has been used in the current study. The MPSA method utilizes a family of routine photocurrent spectra, measured at various intensities generated by the light source. A suitable data processing of such a family allowed several basic characteristics of the films to be obtained – the optical bandgap, subbandgap optical absorption spectra, features of shallow and deep defect states as well as the spectra of the power index in the dependence of the photocurrent on the photon flux. These characteristics were interpreted in terms of structural features of the investigated samples. Conclusions about applications of such C₆₀ films were made. 2006 Article Photoabsorption and photoconductivity in C₆₀ layers / St. Kanev, Z. Nenova, N. Koprinarov // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2006. — Т. 9, № 4. — С. 21-25. — Бібліогр.: 16 назв. — англ. 1560-8034 PACS 72.20.-i, 72.20.-r, 73.50.Pz, 73.61.Wp, 78.40.Ri, 78.66.Tr http://dspace.nbuv.gov.ua/handle/123456789/121628 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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A complex investigation of the photoconductivity of fullerene films, prepared by thermal evaporation in vacuum, was carried out. The investigated films contain predominantly C₆₀ in various phases as shown elsewhere. The multiple photocurrent spectra analysis (MPSA method) has been used in the current study. The MPSA method utilizes a family of routine photocurrent spectra, measured at various intensities generated by the light source. A suitable data processing of such a family allowed several basic characteristics of the films to be obtained – the optical bandgap, subbandgap optical absorption spectra, features of shallow and deep defect states as well as the spectra of the power index in the dependence of the photocurrent on the photon flux. These characteristics were interpreted in terms of structural features of the investigated samples. Conclusions about applications of such C₆₀ films were made. |
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Kanev, St. Nenova, Z. Koprinarov, N. |
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Kanev, St. Nenova, Z. Koprinarov, N. Photoabsorption and photoconductivity in C₆₀ layers Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Kanev, St. Nenova, Z. Koprinarov, N. |
| author_sort |
Kanev, St. |
| title |
Photoabsorption and photoconductivity in C₆₀ layers |
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Photoabsorption and photoconductivity in C₆₀ layers |
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Photoabsorption and photoconductivity in C₆₀ layers |
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Photoabsorption and photoconductivity in C₆₀ layers |
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Photoabsorption and photoconductivity in C₆₀ layers |
| title_sort |
photoabsorption and photoconductivity 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/123456789/121628 |
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Photoabsorption and photoconductivity in C₆₀ layers / St. Kanev, Z. Nenova, N. Koprinarov // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2006. — Т. 9, № 4. — С. 21-25. — Бібліогр.: 16 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
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2025-07-08T20:14:50Z |
| last_indexed |
2025-07-08T20:14:50Z |
| _version_ |
1837111119232106496 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 4. P. 21-25.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
21
PACS 72.20.-i, 72.20.-r, 73.50.Pz, 73.61.Wp, 78.40.Ri, 78.66.Tr
Photoabsorption and photoconductivity in C60 layers
St. Kanev, Z. Nenova, N. Koprinarov
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. A complex investigation of the photoconductivity of fullerene films, prepared
by thermal evaporation in vacuum, was carried out. The investigated films contain
predominantly C60 in various phases as shown elsewhere. The multiple photocurrent
spectra analysis (MPSA method) has been used in the current study. The MPSA method
utilizes a family of routine photocurrent spectra, measured at various intensities
generated by the light source. A suitable data processing of such a family allowed several
basic characteristics of the films to be obtained – the optical bandgap, subbandgap optical
absorption spectra, features of shallow and deep defect states as well as the spectra of the
power index in the dependence of the photocurrent on the photon flux. These
characteristics were interpreted in terms of structural features of the investigated samples.
Conclusions about applications of such C60 films were made.
Keywords: photoconductivity, C60 fullerene, optical absorption.
Manuscript received 11.09.06; accepted for publication 23.10.06.
1. Introduction
In more than a decade, a variety of thin film materials
has been studied as potential candidates for application
in low-cost photovoltaic solar cells. Regardless of the
intensive research in the field, a universal favorite,
satisfying technological and quality demands, is not
established yet. The fullerene films (C60) could be
considered as attractive for photovoltaic solar energy
conversion due to the high photoconductivity of the
material and because of the favorable bandgap width that
can be tailored in the range of 1.6 to 2.3 eV [1-6]. New,
simple and productive preparation methods for C60 are
already developed. They provide opportunity for low
cost roll-to-roll production of flexible solar cells on large
area cheap substrates. Other applications of C60, such as
in thin-film photoelectric devices [7, 8], gas sensors,
electrochromic material [9], doping impurity for
conductive polymer films, superconductors [9] and for
information storage [10] are of interest, too [11-13].
Thus, detailed investigations of the photoelectric
behavior of C60 and its stability under various ambient
conditions are of significant interest.
2. Experimental details
The samples for this study were deposited by using
direct vacuum sublimation from fullerene containing
carbon soot without intermittent enrichment or
separation of the fullerene fractions. This method was
chosen because of its simplicity and low production cost.
The evaporation temperature was kept at 550 ºC, which
results in predominantly C60 containing deposits on glass
substrates at room temperature. Samples of the thickness
between 0.3 and 1.2 μm were prepared. A more detailed
study of the film content and structure is presented in
[14], where it was revealed that the films consist of
microcrystallites, uniformly dispersed in an amorphous
matrix. Coplanar aluminium ohmic contacts, separated
by a 0.8 mm gap, were evaporated onto the films for the
conductivity measurements.
A standard set-up was used for the measurements
of the photocurrent spectra, including a light source
generating variable light intensities (W-halogen lamp
with a variable power supply), a monochromator and a
sample compartment attached to an electrometer,
connected by a computerized data acquisition block, a
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 4. P. 21-25.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
22
spectrally nonselective radiometer checked the light
intensity at the monochromator outlet.
To avoid humidity effects [14] the presented
measurements were carried out at 30 % ambient
humidity. The photocurrent at each measurement point
was recorded after reaching a steady state value (about
30 s after any change of illumination conditions).
3. Measurement of the multiple photocurrent spectra
To perform a complex investigation of the photoelectric
behavior of the available C60 films, we used the multiple
photocurrent spectra analysis (MPSA) method. This
method was proposed and described in more detail in
our previous publication [15].
As a preliminary step to the measurements, the
light source was calibrated at various intensities. A
nonselective radiometer located at the sample holder was
used for the purpose. A family of spectra was measured
− each one obtained by a wavelength scan, performed at
a certain voltage applied to the lamp. The whole family
of spectra was established by applying various
illumination intensities obtained by varying the voltage
at the light source. The family of such baselines is
shown in Fig. 1 in terms of photon flux reaching the
surface of the sample versus wavelength and with the
lamp voltage as a variable parameter.
The major experimental procedure of the MPSA
method consists of measuring a number of photocurrent
spectra of the investigated sample, carried out at several
lamp voltages. Typically, voltages from the same set,
applied to the lamp during the calibration, were used in
this step, too. A family of such spectra (shortly referred to
here as multiple photocurrent spectra (MPS)) is shown in
Fig. 2. The presented MPS were obtained by measuring a
typical 0.8 µm thick fullerene (C60) sample. Further, the
MPS and the lamp calibration spectra were used to derive
several basic characteristics of the investigated sample.
Fig. 1. A family of lamp calibration spectra. Each curve
represents the photon flux, reaching the sample surface, versus
the incident light wavelength for the indicated lamp voltage.
Fig. 2. Multiple photocurrent spectra of a 0.8 µm thick
fullerene film. Each spectrum was obtained with the lamp
voltage kept at the indicated value.
4. Results
Fig. 3 shows a family of dependences of the
photocurrent (Iph) on the incident photon flux (Φ) for
various wavelengths (λ). Each curve is obtained by
presenting the Iph values, corresponding to the certain λ
in MPS, as a function of the Φ values that match the
equivalent lamp voltages at the same λ from the
calibration data. Note that each curve in Fig. 3 is almost
linear in the large range of Φ values as presented in
double logarithmic scales. This indicates the power
dependence between the photocurrent and the incident
photon flux in this range:
Iph = C⋅Φβ. (1)
The power index β varies slightly with λ (change
of the slope at various constant values of λ). As the
power index in this dependence is related to the photo-
carrier recombination processes, the reconstruction of its
spectra shown in Fig. 7 is of interest too and will be
discussed later.
Fig. 3. Dependences between the photocurrent and the photon
flux at various incident light wavelengths for the sample from
Fig. 2. Each curve was obtained by processing the data from
Figs 1 and 2 as explained in the text.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 4. P. 21-25.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
23
Fig. 4. Optical absorption spectrum of the fullerene sample in
the region of weak absorption. The spectrum was obtained by
processing the data from Fig. 3 where the constant photo-
current (Iph = 1.5⋅10−11 A) is kept.
The subbandgap optical absorption is an important
characteristic of the investigated sample that contains
information about the features of the defect states in the
material. Usually, the constant photocurrent method
(CPM) [16] is used for determination of this
characteristic of photoconductive thin films. The essence
of CPM is that the photocurrent in the sample is kept
constant during the wavelength scan by varying (and
measuring) the photon flux Φλ of the incident light. As
shown by Vanecek et al. [16] the following simple
relation is valid in these conditions in the low absorption
region (α(λ)<1):
α(λ) = const/Φ(λ), (2)
where α(λ) is the optical absorption coefficient and
Φ(λ) is the incident photon flux at the same λ. One can
note that the points, obtained by a cross-section of the
family of (Iph – Φ)λ dependences in Fig. 3 at the certain
Iph level (Ic = const indicated by a horizontal line), fulfill
the conditions of CPM. Thus, the subbandgap absorption
spectra of the studied C60 films can be reconstructed by
using the MPSA data as shown in Fig. 4. The data in this
figure are presented in arbitrary units as follows from the
undefined constant in Eq. (2). The complete optical
absorption spectrum of the investigated film is shown in
Fig. 5 in absolute units. As usual, this spectrum has been
obtained by fitting of the weak absorption part from
Fig. 4 to the strong one, obtained by photometric
measurements (a double beam spectrophotometer
“Perkin Elmer 330”).
On the other hand, a cross-section of the data in
Fig. 3 at the constant value of the photon flux (vertical
line) can be used for obtaining the correct spectrum of
sample photoconductivity without the need of any
normalization procedure. These spectra at two different
photon flux values are shown in Fig. 6.
Fig. 5. Complete optical absorption spectrum of the
investigated film as obtained by fitting the spectrum from
Fig. 4 to the data from the photometric measurements.
Fig. 6. Photoconductivity spectra of the investigated fullerene
film at various constant photon fluxes. With the increase of Φ,
the quantum efficiency decreases.
Fig. 7. Variation of the power index β (Eq. (1)) with the photon
energy for the investigated fullerene film.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 4. P. 21-25.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
24
5. Discussion
The complete optical absorption spectrum (Fig. 5) of the
investigated C60 film appears similar to that of other thin
films of interest for application in solar cells (e.g. a-
Si:H). However, several specific features can be
observed. The optical gap of the fullerene films can be
characterized by the value E04 = 2.4 eV (the photon
energy at α = 104 cm−1). This value is higher than the
corresponding values of other materials appropriate for
active absorption layers in solar cells (e.g. a-Si:H with
typical E04 of about 1.6–1.8 eV). On the other hand, the
larger bandgap of C60 films makes them interesting for
application as window layers in hybrid thin-film solar
cells. The low absorption part is rather smooth and falls
down to significantly low values at low energy. This
indicates absence of discrete photoactive electron levels
in the gap and relatively low density of deep defects. An
interesting feature of subbandgap absorption is a small
kink at 1.8-2.0 eV (690–617 nm) that can be observed in
Figs 4-6. A simple exponential decay (Urbach tail),
originating from shallow localized tail states, can be
expected in this region. In contrast, two exponential
decays (lines in semilog. scale) with different
characteristic energies ΔE1 = 155 meV and ΔE2 =
300 meV can be distinguished. This feature can be
explained by the presence of two phases in the
investigated film [14]. The part with lower characteristic
energy could be associated with the crystalline C60
fraction, while that with wider tail with an amorphous
phase. Thus, one can assume that the photons with
energy bellow 1.8 eV are absorbed predominantly by the
crystalline fraction and those with energy above 2.0 eV
by the amorphous one. This assumption is supported
also by the change of the power index in the (Iph –
Φ) dependence at 1.8-2.0 eV that can be clearly seen in
Fig. 7. Such a change obviously originates from a
change in the recombination mechanism of the photo-
generated carriers.
Another interesting feature can be seen clearly
from comparison of Figs 5 and 6. The photoconductivity
spectra has monotonously rising up to an energy as high
as 2.5 eV (Fig. 6). The value of α reaches more than
2⋅104 cm−1 at this energy, which corresponds to light
penetration depth of about 0.5 μm – smaller than the
film thickness. An influence of the surface recom-
bination should be expected at such a strong absorption,
which usually demonstrates in a sharp decrease of
photoconductivity spectra. In contrast, the rise of
photoconductivity continues in this region, indicating
that the surface recombination of the investigated film is
negligible compared to the bulk one. This could be
related to the closed atomic structure of C60.
5. Conclusion
In this work, we have shown that the MPSA method can
be successfully applied for investigation of C60 films.
Information about the optical bandgap, the subbandgap
optical absorption, the features of the shallow and deep
defect states as well as some peculiarities of the carrier
generation-recombination processes in the investigated
material can be obtained by using this method.
The availability of two phases in the C60 films
prepared by direct vacuum sublimation from carbon soot
has found. In support of the availability of the two
phases are the kink in the subbandgap absorption spectra
and the change of the power factor in the dependence of
the photocurrent on the incident photon flux at around
1.8 and 2.0 eV. These two phases exhibit different
effective optical gaps (1.8 eV for “c” phase and 2.0 eV
for “a” phase). This can be used for tailoring the
apparent optical gap of the film material by varying the
relative amount of these two phases.
The investigated films have a large characteristic
optical gap – typically E04 = 2.2…2.4 eV. They also
exhibit a negligible surface recombination. These
electronic characteristics make them interesting
candidates for application in tandem solar cells as well
as window layers in hybrid thin-film solar cells.
Acknowledgements
The sponsorship of the Bulgarian Ministry for Science
and Education under contract F-329 is appreciated.
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