Optical and morphological properties of tetragonal Cu₂ZnSnS₄ thin films grown from sulphide precursors at lower temperatures
Optical constants of Cu₂ZnSnS₄ thin films formed using thermal annealing of pre-deposited layers of copper, zinc and tin sulphides on glass substrates at different temperatures and ambient atmosphere were determined. It has been shown that films grown at lower temperatures have the tetragonal str...
Збережено в:
| Дата: | 2014 |
|---|---|
| Автори: | , , , , , , , , , |
| Формат: | Стаття |
| Мова: | English |
| Опубліковано: |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2014
|
| Назва видання: | Semiconductor Physics Quantum Electronics & Optoelectronics |
| Онлайн доступ: | http://dspace.nbuv.gov.ua/handle/123456789/118499 |
| Теги: |
Додати тег
Немає тегів, Будьте першим, хто поставить тег для цього запису!
|
| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Цитувати: | Optical and morphological properties of tetragonal Cu₂ZnSnS₄ thin films grown from sulphide precursors at lower temperatures / I.S. Babichuk, V.O. Yukhymchuk, M.O. Semenenko, N.I. Klyui, R. Caballero, O.M. Hreshchuk, I.S. Lemishko, I.V. Babichuk, V.O. Ganus, M. Leon // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 3. — С. 284-290. — Бібліогр.: 23 назв. — англ. |
Репозитарії
Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
irk-123456789-118499 |
|---|---|
| record_format |
dspace |
| spelling |
irk-123456789-1184992017-05-31T03:06:08Z Optical and morphological properties of tetragonal Cu₂ZnSnS₄ thin films grown from sulphide precursors at lower temperatures Babichuk, I.S. Yukhymchuk, V.O. Semenenko, M.O. Klyui, N.I. Caballero, R. Hreshchuk, O.M. Lemishko, L.S. Babichuk, L.V. Ganus, V.O. Leon, M. Optical constants of Cu₂ZnSnS₄ thin films formed using thermal annealing of pre-deposited layers of copper, zinc and tin sulphides on glass substrates at different temperatures and ambient atmosphere were determined. It has been shown that films grown at lower temperatures have the tetragonal structure of kesterite, and the corresponding value of the optical band gap is 1.47 eV. 2014 Article Optical and morphological properties of tetragonal Cu₂ZnSnS₄ thin films grown from sulphide precursors at lower temperatures / I.S. Babichuk, V.O. Yukhymchuk, M.O. Semenenko, N.I. Klyui, R. Caballero, O.M. Hreshchuk, I.S. Lemishko, I.V. Babichuk, V.O. Ganus, M. Leon // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 3. — С. 284-290. — Бібліогр.: 23 назв. — англ. 1560-8034 PACS 78.20.-e, 82.80.Jk http://dspace.nbuv.gov.ua/handle/123456789/118499 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| description |
Optical constants of Cu₂ZnSnS₄ thin films formed using thermal annealing of
pre-deposited layers of copper, zinc and tin sulphides on glass substrates at different
temperatures and ambient atmosphere were determined. It has been shown that films
grown at lower temperatures have the tetragonal structure of kesterite, and the
corresponding value of the optical band gap is 1.47 eV. |
| format |
Article |
| author |
Babichuk, I.S. Yukhymchuk, V.O. Semenenko, M.O. Klyui, N.I. Caballero, R. Hreshchuk, O.M. Lemishko, L.S. Babichuk, L.V. Ganus, V.O. Leon, M. |
| spellingShingle |
Babichuk, I.S. Yukhymchuk, V.O. Semenenko, M.O. Klyui, N.I. Caballero, R. Hreshchuk, O.M. Lemishko, L.S. Babichuk, L.V. Ganus, V.O. Leon, M. Optical and morphological properties of tetragonal Cu₂ZnSnS₄ thin films grown from sulphide precursors at lower temperatures Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Babichuk, I.S. Yukhymchuk, V.O. Semenenko, M.O. Klyui, N.I. Caballero, R. Hreshchuk, O.M. Lemishko, L.S. Babichuk, L.V. Ganus, V.O. Leon, M. |
| author_sort |
Babichuk, I.S. |
| title |
Optical and morphological properties of tetragonal Cu₂ZnSnS₄ thin films grown from sulphide precursors at lower temperatures |
| title_short |
Optical and morphological properties of tetragonal Cu₂ZnSnS₄ thin films grown from sulphide precursors at lower temperatures |
| title_full |
Optical and morphological properties of tetragonal Cu₂ZnSnS₄ thin films grown from sulphide precursors at lower temperatures |
| title_fullStr |
Optical and morphological properties of tetragonal Cu₂ZnSnS₄ thin films grown from sulphide precursors at lower temperatures |
| title_full_unstemmed |
Optical and morphological properties of tetragonal Cu₂ZnSnS₄ thin films grown from sulphide precursors at lower temperatures |
| title_sort |
optical and morphological properties of tetragonal cu₂znsns₄ thin films grown from sulphide precursors at lower temperatures |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2014 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/118499 |
| citation_txt |
Optical and morphological properties of tetragonal Cu₂ZnSnS₄ thin films grown from sulphide precursors at lower temperatures / I.S. Babichuk, V.O. Yukhymchuk, M.O. Semenenko, N.I. Klyui, R. Caballero, O.M. Hreshchuk, I.S. Lemishko, I.V. Babichuk, V.O. Ganus, M. Leon // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 3. — С. 284-290. — Бібліогр.: 23 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT babichukis opticalandmorphologicalpropertiesoftetragonalcu2znsns4thinfilmsgrownfromsulphideprecursorsatlowertemperatures AT yukhymchukvo opticalandmorphologicalpropertiesoftetragonalcu2znsns4thinfilmsgrownfromsulphideprecursorsatlowertemperatures AT semenenkomo opticalandmorphologicalpropertiesoftetragonalcu2znsns4thinfilmsgrownfromsulphideprecursorsatlowertemperatures AT klyuini opticalandmorphologicalpropertiesoftetragonalcu2znsns4thinfilmsgrownfromsulphideprecursorsatlowertemperatures AT caballeror opticalandmorphologicalpropertiesoftetragonalcu2znsns4thinfilmsgrownfromsulphideprecursorsatlowertemperatures AT hreshchukom opticalandmorphologicalpropertiesoftetragonalcu2znsns4thinfilmsgrownfromsulphideprecursorsatlowertemperatures AT lemishkols opticalandmorphologicalpropertiesoftetragonalcu2znsns4thinfilmsgrownfromsulphideprecursorsatlowertemperatures AT babichuklv opticalandmorphologicalpropertiesoftetragonalcu2znsns4thinfilmsgrownfromsulphideprecursorsatlowertemperatures AT ganusvo opticalandmorphologicalpropertiesoftetragonalcu2znsns4thinfilmsgrownfromsulphideprecursorsatlowertemperatures AT leonm opticalandmorphologicalpropertiesoftetragonalcu2znsns4thinfilmsgrownfromsulphideprecursorsatlowertemperatures |
| first_indexed |
2025-07-08T14:07:02Z |
| last_indexed |
2025-07-08T14:07:02Z |
| _version_ |
1837087984809148416 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 3. P. 284-290.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
284
PACS 78.20.-e, 82.80.Jk
Optical and morphological properties of tetragonal Cu2ZnSnS4
thin films grown from sulphide precursors at lower temperatures
I.S. Babichuk1,*, V.O. Yukhymchuk1, M.O. Semenenko1, N.I. Klyui1, R. Caballero2, O.M. Hreshchuk1,
I.S. Lemishko3, I.V. Babichuk4, V.O. Ganus1, and M. Leon2
1V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
45, prospect Nauky, 03028 Kyiv, Ukraine;
2Universidad Autónoma de Madrid, Departamento de Física Aplicada,
C/Francisco Tomás y Valiente 7, E-28049 Madrid, Spain;
3National Technical University of Ukraine “Kyiv Polytechnic Institute”,
37, prospect Peremohy, 03056 Kyiv, Ukraine;
4V. Vernadsky Institute of General and Inorganic Chemistry, NAS of Ukraine,
32/34, prospect Akademika Palladina, 03142 Kyiv, Ukraine
*Corresponding author. Phone: 38(044) 525-83-03; e-mail: babichuk@isp.kiev.ua
Abstract. Optical constants of Cu2ZnSnS4 thin films formed using thermal annealing of
pre-deposited layers of copper, zinc and tin sulphides on glass substrates at different
temperatures and ambient atmosphere were determined. It has been shown that films
grown at lower temperatures have the tetragonal structure of kesterite, and the
corresponding value of the optical band gap is 1.47 eV.
Keywords: Raman spectroscopy, SEM, optical constants, kesterite, CZTS.
Manuscript received 25.02.14; revised version received 30.07.14; accepted for
publication 16.09.14; published online 30.09.14.
1. Introduction
Nowadays photovoltaic devices around the world
provide production of about 20 GW of electrical power.
Almost 90% of them are based on silicon solar cells, the
effectiveness of which reaches theoretical limits, and
their cost is still quite high. One of the solutions to
reduce the cost of manufacturing photovoltaic modules
is use of thin-film technology. However, in this respect,
single-crystal silicon is not an effective material, because
it has an indirect energy band structure and,
consequently, it has a relatively low value of absorption
of solar radiation in the visible and near infrared bands
(1-2 eV). Switching to involving the direct energy band
materials can significantly reduce the thickness of
semiconductor layer that efficiently absorbs incident
radiation in this range.
High interest in investigation of quaternary
semiconductor compounds based on chalcogenides and
widespread elements of groups II and IV, for example of
the Cu2B
IICIVS4 type, a typical representative of which is
Cu2ZnSnS4 (CZTS), is caused by search of new
materials for designing solar cells. Their direct energy
band determining the high interband absorption
coefficient for solar radiation in the visible spectral
range is turn-point to make solar cells to be cheaper. The
other two important factors for potential use of CZTS in
solar cell application are widespread in the crust of the
constituent elements and their sustainability in
manufacturing processes and application.
Up to date, the efficiency of CZTS-based solar
cells has reached the level close to 12.6% [1]. To
increase the efficiency, several problems should be
solved. They are collected in series as follows: the
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 3. P. 284-290.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
285
presence of non-stoichiometry elemental composition of
CZTS and the concentration of intrinsic defects in the
crystallographic structure and conformably in the energy
band gap; properties of CZTS by coexistence of
crystallographic phases and by the possible impurities of
secondary binary and ternary compounds formed during
the synthesis.
In this work, optical and vibrational properties of
Cu2ZnSnS4 thin films obtained by thermal annealing the
pre-deposited binary compounds ZnS, CuS and SnS on
glass substrates were investigated. Annealing was
carried out at two different temperatures and in two
different gas atmospheres. To determine optical
constants of this material, the spectra of light reflection
were analyzed at normal incidence, which is convenient
from the viewpoint of conversion of complex-conjugate
functions using dispersion Kramers-Kronig integrals [2].
It is known that, to obtain reliable results, if using the
dispersion Kramers-Kronig integrals, precise measure-
ments of reflection spectra in the whole optical range
should be carried out. In addition, their use for
calculating the optical constants suggests that the
experimental studies are performed with fairly thick
material, for which the reflection spectrum is recorded
between two semi-infinite media.
2. Experimental technique
In this work, Cu2ZnSnS4 thin films were obtained using
deposition of binary compounds ZnS, CuS and SnS on
glass substrates with pre-deposited molybdenum as a
bottom layer. Annealing was carried out at two different
temperatures 370 and 390 ºC in air and in nitrogen
atmosphere. The use of relatively low temperature was
applied to compare with results of deposition of similar
structures on flexible organic polymer films.
Reflection spectra were recorded using the
spectrometer Shimadzu UV-3600, and surface
morphology was investigated applying the scanning
electron microscope (SEM) Tescan Mira 3 LMU.
Component compositions were determined using
energy-dispersive X-ray spectroscopy (EDX) with
Oxford instruments INCA x-sight that was built in the
SEM. To investigate structural properties of Cu2ZnSnS4,
the method of m-Raman spectroscopy (T64000 Horiba
Jobin Yvon) was employed. For excitation of Raman
spectra, radiation of Ar+-laser with the wavelength
514.5 nm was used. The power of laser radiation was
chosen sufficiently minimal (the laser power density was
0.1 mW/μm2) in order not to add changes in the film
structure during these measurements. All the
measurements were performed at room temperature.
Moreover, Raman spectra were recorded in different
parts of each sample, because of morphology and the
phase inhomogeneities on the top of film surface were
visible in the optical microscope. After that, the
collected results were averaged to obtain the average
parameters of the existing medium as well as to establish
the nature of some crystalline phases that segregated.
Spectra of photoluminescence (PL) were recorded at the
temperature of liquid nitrogen. The excitations of spectra
were carried out upon the incidence of light of Nd: YAG
laser with the wavelength 532 nm.
3. Results and discussion
It is known that, during high-temperature annealing,
atoms of sulphur rapidly evaporate from the surface of
CZTS [3]. Being based on this fact, to obtain compounds
with stoichiometric component ratio (2:1:1:4) annealing
should be carried out in atmosphere of sulphur.
Moreover, in the course of annealing in sulphur
atmosphere, zinc first takes all of the sulphur saturation
according to the electrochemical activity of a number of
metals. To clarify this mismatch, it should be first
analyzed the reactions of formation of sulphides by
using standard oxidation-reduction potentials for binary
compounds that were collected in the table [4].
We can see from Table 1 that the first metal will be
reduced to atomic state in the process is copper.
Therefore, observation of the CuS and Cu2S phases in
the synthesized film after sulphurization is the most
probable. Indeed, this assumption is confirmed by the
results of the studies of such structures by Raman
spectroscopy and X-ray diffraction (XRD) in many
works [3, 5-7]. On the other hand, the CuS and Cu2S
phases are less stable in contrast to ZnS and MoS2. As
can be seen from the Table 2, the standard molar
enthalpy of their formation is minimal in its absolute
value for a number of binary compounds [4, 8].
The standard molar enthalpy of formation of any
substance is a measure of its stability. The higher is the
absolute value of the enthalpy for formation of such
compounds, as we assume, the more stable is this
compound. Therefore, during their synthesizing CZTS
films were formed using such compounds as ZnS, MoS2,
CuS, Cu2S.
Table 1. Oxidation-reduction reactions.
Electrode processes Е, V
CuS + 2e– = Cu + S2– –0.76
SnS + 2e– = Sn + S2– –0.87
Cu2S + 2e– = 2Cu + S2– –0.89
ZnS + 2e– = Zn + S2– –1.405
Table 2. Standard molar enthalpy values.
Sulfides ΔНо, kJ/mol
CuS –53.1
Cu2S –53.1
SnS2 –82.4
SnS –110.2
ZnS –205.4
MoS2 –248.1
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 3. P. 284-290.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
286
Fig. 1. SEM images of the surface of CZTS films grown at 370º С (a, b) and 390º С (c, d) in N2 (a, c) and air (b, d).
As can be seen from Fig. 1, surface morphology of
CZTS films strongly depends on technological
conditions during their synthesis: annealing temperature
and atmosphere of processing. The increase in the
temperature of synthesis only by 20 °C from 370 to
390 °C results in significant changes in surface
morphology of the formed CZTS films. To determine
the component structure of formed films and individual
phase inclusions, local-EDRS studies were carried out in
various regions of the film surface. For separating the
bands in the EDRS spectrum obtained from the
individual elements, the energy of electrons used for
sensing was relatively large, resulting in a contribution
to the range of items from all over the film thickness.
However, in this case, it is possible to get information
about the elemental composition inhomogeneities arising
on the surface of the films during its synthesis (Fig. 1).
From these investigations, it was found that the
elemental composition of morphological heterogeneity
(Fig. 1a, region 1) corresponds to the composition
enriched with Cu (Cu-rich). At the same time, in areas
where the film is homogeneous (Fig. 1a, region 2) the
ratio of the components Cu:Zn:Sn:S is closer to the
stoichiometric one, although here we observed a little bit
overstated content of Cu and depletion of Sn.
The Raman spectra of the CZTS films and their
corresponding deconvoluted components (Lorentzians)
are presented in Fig. 2. The films were formed at the
temperature close to 390 °C in air (a) and nitrogen (b)
atmospheres. As it is seen from the spectra shown in
Figs. 2a and 2b, the main bands of the Raman spectra of
CZTS films formed in air and nitrogen (N2) at the
temperature 370 °C were practically indistinguishable.
But in the addition to the main A-symmetry band of
CZTS, a very intense band at the frequency 473 cm–1 is
presented in the spectrum of the film formed in nitrogen
at 370 °C (Fig. 2c, inset), which corresponds to the
vibrations of Cu2S compound [9, 10]. The main
contribution to the obtained spectra is given by A-
symmetry bands of CZTS compound with the
frequencies 288 and 1cm336337 and bands with the
frequencies 254(251), 312(308), 358(360), 1cm372 ,
corresponding to the vibrations of E- and B-symmetry
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 3. P. 284-290.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
287
and discussed in detail in [10-15]. The bands with the
frequencies 412(410) cm-1 correspond to the vibrations
of the MoS2 compound [16]. Raman spectra of the films
that were synthesized in air (Fig. 2a) are slightly
different from the spectra of the films formed in nitrogen
(Fig. 2b). Although, it should be noted that the frequency
position of Raman bands in respect with the films
formed in nitrogen atmosphere is somewhat lower.
During annealing, the presence of oxygen in air can
cause interaction of individual elements in CZTS films
with oxygen, which is reflected in a decrease of the
vibrational frequency of the main modes inherent to
material. Analysis of half-widths of Raman bands in the
films formed in different atmospheres showed that their
values for the bands in the first case are less than that of
the latter one by 4 cm-1. Perhaps, it is caused by partial
oxidation of the binary and ternary phases formed in the
course of the synthesis of CZTS. It results in reduction
of their contribution to the mechanical stress stability of
main material and, therefore, reduction of the half-width
of the bands occurs. The half-widths of the bands
observed in the abovementioned films are larger by
8 cm-1 as compared with the similar films obtained at
higher temperatures of synthesis (475…600 °C) (see
Ref. [13]). The obtained results should be improved to
reach values [13], however, the use of lower
temperatures of annealing is a prerequisite to pronounce
the perspectives of formation of the structures on the
flexible organic polymer films that are not suitable to be
treated at higher temperatures.
Thus, to make the analysis of the Raman spectra of
all the films, we conclude that the annealing temperature
within the range from 370 to 390 °C does not provide a
significant effect on the frequency position and intensity
of bands. But, when the sample is formed at one
temperature, but in different atmospheres, the
corresponding spectra differ quite significantly from
each other (Fig. 2).
Optical reflection spectra R(λ) of the CZTS film
that were obtained under different technological
conditions are shown in Fig. 3. Analysis of the
experimental data showed that the reflections of the
films with increase of the annealing temperature are
significantly reduced, namely, in air by 8% and in N2 –
25%. A significant decrease in the reflectivity of the
films annealed in N2 can be attributed to the
microstructure of the film. SEM images (Figs. 1a and
1b) show that the films annealed at 370 °C have a
relatively lower surface density of microcrystalline
grains as compared with that on the film surface
(Figs. 1c and 1d), when being annealed at 390 °C. The
increase in the annealing temperature by 20 °C results in
a significant decrease of the optical reflectivity. This
result is important for designing the photoelectric
multiplier, because the low reflection coefficient of the
film allows to absorb more solar energy and,
consequently, to increase efficiency.
Fig. 2. Raman spectra of CZTS films grown at 390 °C in air
(а) and N2 (b) with deconvoluted components (c); 370 °C
in N2.
Optical constants of CZTS films were determined
using reflection spectra within the range from 380 to
1600 nm. As well known, it is possible to determine the
complex index of refraction using the Fresnel equations
for the reflection function at the normal incidence of the
light beam on the film, according to the formula:
1~
1~
21
n
n
eRR i , (1)
where, iknn~ is the complex index of
refraction; n, k are real and imaginary components.
The function of the reflection is related to the
Fourier transformation as follows:
iRR ln
2
1
ln . In this case, the phase shift
between the incident and reflected rays can be
determined from the Kramers–Kronig dispersion
integral [17]:
0
2
0
2
00
0
lnln
d
RR
, (2)
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 3. P. 284-290.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
288
Fig. 3. Reflectance spectra of CZTS films grown at 370 °C
(1, 2) and 390 °C (3, 4).
After numerical integration of the dispersion
integrals by using the trapezoidal rule with the step
1 meV, the complex refractive index and its components
can be determined from the following formula:
0
0
2
1
2
1
~
i
L
i
L
eRn
eRn
n (3)
where, nL is the refractive index of air.
It is known that the complex dielectric function and
the absolute value of the refractive index are linked by
equations below:
,~
21
2 in
,22
1 kn
,22 kn
,
2
1
2
2
2
1
n (4)
.
2
1
2
2
2
1
k
Calculation of spectral dependences for optical
constants of CZTS films: the refractive n(hν) and
extinction k(hν) indexes, the real ε1(hν) and imaginary
ε2(hν) parts of the complex dielectric function ε was
performed using packages of the applied MathCad
software. The spectral dependences of n(hν) and k(hν)
are shown in Fig. 4. They were calculated from the latter
two ones by using the equation (4). Fig. 4 shows that the
optical constants n(hν) and k(hν) decrease with
increasing the photon energy hν. Variation of the real
ε1(hν) and imaginary ε2(hν) parts of the dielectric
function is similar to that observed for the optical
constants n(hν) and k(hν).
Fig. 4. Spectra of optical functions: a) refractive index n(hν);
b) extinction coefficient k(hν) of CZTS films obtained at
370 °C (1, 2) and 390 °C (3, 4).
In Fig. 5a the spectra of the absorption coefficient
α(hν) of the films versus photon energy are presented
according to the treatments. These spectra were
transferred from the reflection spectra by using the
dispersion integrals. It should be noted that it does not
lead to the significant errors in calculations, from the
condition that the film thickness CZTS was grown
thicker than 1 μm. It is caused by the high absorption of
light by thicker films and decrease of light interference
inside the glass-CZTS interface. As shown in Fig. 5a, the
increase in absorption coefficient in the low energy
region (hν < 1 eV) is quite sharp. An increase of the
slope of the absorption curve is observed near the
absorption edge for the CZTS film except the film
annealed in nitrogen at 370 °C, for which the absorption
coefficient declines of linearity in this region. The same
situation is observed for the other films in the range
(hν > 1.7 eV).
In Fig. 5b the calculated absorption spectra are
plotted in the Tauc coordinates for direct allowed optical
transitions, which makes it possible to estimate effects of
the atmosphere presences and the temperature of the
annealing on the value of optical energy gap. These
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 3. P. 284-290.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
289
values are slightly less as compared with the data typical
for films with a tetragonal like kesterite structure for
which the optical band gap Eg = 1.5 eV (see Ref. [18]).
This mismatch may appear due to availability of
structural defects composed in the films both on the
surface and in the bulk. In this issue, the inclusion with
the energy lower than the value of Eg can affect on the
optical energy gap. But among them, we can see that
even at low annealing temperatures (390 °C) in both
kinds of atmospheres, in which the films were formed,
values of Eg are very close to that discussed by the
authors in Refs. [18-20]. Moreover, PL spectra of CZTS
films were investigated at the temperatures near 80 K.
They had somewhat asymmetric broad band with the
peak at 1.26 eV. There are several explanations for these
features. One of them, as we assume and suggested in
[21, 22] for structures similar to CZTS at a low
concentration of doping elements, the dominant
radiation was caused by recombination of donor-
acceptor pairs or transition of electrons from the
conduction band to the acceptor level [23].
Fig. 5. Spectra of the absorption coefficient of CZTS films:
а) grown at 370 °C (1, 2) in N2 and at 390 °C (3, 4) in air; b) in
the Tauc coordinates for the case of direct allowed optical
transitions.
4. Conclusion
Analysis of the Raman spectra of the films Cu2ZnSnS4
that were formed by thermal annealing previously
deposited of precursors layers based on copper, zinc and
tin sulphides on glass substrates at different temperatures
and in different surrounding atmospheres showed that
they have a tetragonal structure of kesterite. Thus, it has
been shown that the use of sulphides of copper, tin and
zinc, in contrast to corresponding pure metals during the
synthesis of these compounds, eliminates the need for
the additional annealing in the atmosphere of sulfur to
achieve stoichiometric structure. It is shown that changes
of the temperature of the annealing from 370 to 390 °C
have no significant effect on the features of the Raman
spectra. At the same time, changes in the atmosphere in
the course of the treatments result in the shift of the
frequency of the bands and change in their intensities.
From the reflectance spectra, the optical constants
of the formed films were calculated. It has been shown
that increase of the synthesis temperature by only 20 °C
from 370 to 390 °C leads to a significant increase of the
surface density of microcrystalline grains. It reduces the
reflection coefficient of the film and provides more
efficient light absorption. Therefore, conversion of light
to electricity would be more efficient. From our
calculations of the absorption spectra, the average
estimated energy band gap of films was approximately
1.47 eV.
This research is supported by the People
Programme (Marie Curie Actions) of the European
Union’s Seventh Framework Program FP7 under REA
grant agreement 269167 (PVICOKEST) and grant
OPTEC as well as State Fund for Fundamental
Reasearch of Ukraine (bilateral Belarussian-Ukrainian
project no. 54.1/005).
References
1. W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen,
T.K. Todorov, Y. Zhu, D.B. Mitzi, Device
characteristics of CZTSSe thin-film solar cells with
12.6% efficiency // Adv. Energy Materials, 4(7),
p. 1-5 (2014).
2. Р. Bruzzoni, R.M. Carranza, J.R. Collet Lacoste,
E.A. Crespo, Kramers-Kronig transforms
calculation with a fast convolution algorithm //
Electrochimica Acta, 48, p. 341-347 (2002).
3. M. Espindola-Rodriguez, M. Placidi, O. Vigil-
Galán, V. Izquierdo-Roca, X. Fontané,
A. Fairbrother, D. Sylla, E. Saucedo, A. Pérez-
Rodríguez, Compositional optimization of
photovoltaic grade Cu2ZnSnS4 films grown by
pneumatic spray pyrolysis // Thin Solid Films, 535,
p. 67-72 (2013).
4. V.A. Rabinovich, Z.Ya. Havich, Concise Chemical
Directory. Chemistry, Leningrad, 1991.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 3. P. 284-290.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
290
5. S. Schorr, The crystal structure of kesterite type
compounds: a neutron and x-ray diffraction study //
Solar Energy Materials and Solar Cells, 95,
p. 1482-1488 (2011).
6. I.D. Olekseyuk, E.V. Dudchak, L.V. Piskach,
Phase equilibria in the Cu2S-ZnS-SnS2 system // J.
Alloys and Compounds, 368, p. 135-143 (2004).
7. A. Fairbrother, X. Fontané, V. Izquierdo-Roca, M.
Espindola-Rodriguez, S. Lopez-Marino, M. Placidi,
L. Calvo-Barrio, A. Perez-Rodriguez, E. Saucedo,
On the formation mechanisms of Zn-rich
Cu2ZnSnS4 films prepared by sulfurization of
metallic stacks // Solar Energy Materials & Solar
Cells, 112, p. 97-105 (2013).
8. A.J. Jackson, A. Walsh, Ab initio thermodynamic
model of Cu2ZnSnS4 // J. Mater. Chem. A, 2,
p. 7829-7836 (2014).
9. P.A. Fernandes, P.M.P. Salome, A.F. da Cunha,
Growth and Raman scattering characterization of
Cu2ZnSnS4 thin films // Thin Solid Films, 517,
p. 2519-2523 (2009).
10. І.S. Babichuk, V.О. Yukhymchuk, V.М. Dzhagan,
М.Ya. Valakh, M. Leon, І.B. Yanchuk, E.G. Gule,
О.М. Greshchuk, Thin films of Cu2ZnSnS4 for
solar cells: optical and structural properties //
Functional Materials, 20(2), p. 186-191 (2013).
11. X. Fontane, V. Izquierdo-Rosa, E. Saucedo,
S. Sсhorr, V.O. Yukhymchuk, M.Ya. Valakh, A.
Pеrez-Rodrіguez, J.R. Morante, Vibrational
properties of stannite and kesterite type compounds:
Raman scattering analysis of Cu2(Fe,Zn)SnS4 // J.
Alloys and Compounds, p. 190-194 (2012).
12. A. Khare, B. Himmetoglu, M. Johnson, D.J. Norris,
M. Cococcioni, E.S. Aydil, Calculation of the
lattice dynamics and Raman spectra of copper zinc
tin chalcogenides and comparison to experiments //
J. Appl. Phys. 111(8), 083707-083716 (2012).
13. R. Caballero, E. Garcia-Llamas, J.M. Merino, M.
León, I. Babichuk, V. Dzhagan, V. Strelchuk,
M. Valakh, Non-stoichiometry effect and disorder
in Cu2ZnSnS4 thin films obtained by flash
evaporation: Raman scattering investigation // Acta
Materialia, 65 p. 412-417 (2014).
14. M.Ya. Valakh, O.F. Kolomys, S.S. Ponomaryov,
V.O. Yukhymchuk, I.S. Babichuk, V. Izquierdo-
Rosa, E. Saucedo, A. Pеrez-Rodrіguez,
J.R. Morante, S. Sсhorr, I.V. Bodnar, Raman
scattering and disordering effect in Cu2ZnSnS4 //
Phys. Status Solidi (RRL) – Rapid Research
Letters, 7(4), p. 258-261 (2013).
15. P. Schütz, A.K. Alves, C.P. Bergmann, Effect of
the in-air heat treatment in the phase formation and
morphology of electrospun Cu2ZnSnS4 fibers //
Ceramics Intern. 40(8), p. 11551-11557 (2014).
16. S. Chen, X.G. Gong, Electronic structure and
stability of quaternary chalcogenide
semiconductors derived from cation cross-
substitution of II-VI and I-III-VI2 compounds //
Phys. Rev. B, 79, p. 165211-165221 (2009).
17. M. Cardona, D.L. Greenaway, Optical properties
and band structure of group IV-VI and group V
materials // Phys. Rev. 133(6A), p. A1685-A1697
(1964).
18. M. Ikhlasul Amal, Kyoo Ho Kim, Structural and
optical properties of sulfurized Cu2ZnSnS4 thin
fіlms from Cu–Zn–Sn alloy precursors // J. Mater.
Sci.: Materials in Electronics, 24(2), p. 559-566
(2013).
19. R.B.V. Chalapathy, G.S. Jung, B.T. Ahn, Fabri-
cation of Cu2ZnSnS4 films by sulfurization of
Cu/ZnSn/Cu precursor layers in sulfur atmosphere
for solar cells // Solar Energy Materials and Solar
Cells, 95(12), p. 3216-3221 (2011).
20. P.K. Sarswat, M.L. Free, A study of energy band
gap versus temperature for Cu2ZnSnS4 thin films //
Phys. B: Condens. Matter, 407(1), p. 108-111
(2012).
21. J.P. Leitao, N.M. Santos, P.A. Fernandes,
P.M.P. Salome, A.F. da Cunha, J.C. Gonzalez,
G.M. Ribeiro, F.M. Matinaga, Photoluminescence
and electrical study of fluctuating potentials in
Cu2ZnSnS4-based thin films // Phys. Rev. B, 84,
024120–024128 (2011).
22. A.U. Sheleg, V.G. Hurtavy, A.V. Mudryi, M.Ya.
Valakh, V.O. Yukhymchuk, I.S. Babichuk, M.
Leon, R. Caballero, Study of structural and optical
properties of Cu2ZnSnS4 semiconductor
compounds thin films // Semiconductors, 48(10),
p. 1332-1338 (2014).
23. M. Grossberg, J. Krustok, J. Raudoja, T. Raadik,
The role of structural properties on deep defect
states in Cu2ZnSnS4 studied by photoluminescence
spectroscopy // Appl. Phys. Lett. 101, 102102–
102106 (2012).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 3. P. 284-290.
PACS 78.20.-e, 82.80.Jk
Optical and morphological properties of tetragonal Cu2ZnSnS4
thin films grown from sulphide precursors at lower temperatures
I.S. Babichuk1,*, V.O. Yukhymchuk1, M.O. Semenenko1, N.I. Klyui1, R. Caballero2, O.M. Hreshchuk1,
I.S. Lemishko3, I.V. Babichuk4, V.O. Ganus1, and M. Leon2
1V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
45, prospect Nauky, 03028 Kyiv, Ukraine;
2Universidad Autónoma de Madrid, Departamento de Física Aplicada,
C/Francisco Tomás y Valiente 7, E-28049 Madrid, Spain;
3National Technical University of Ukraine “Kyiv Polytechnic Institute”,
37, prospect Peremohy, 03056 Kyiv, Ukraine;
4V. Vernadsky Institute of General and Inorganic Chemistry, NAS of Ukraine,
32/34, prospect Akademika Palladina, 03142 Kyiv, Ukraine
*Corresponding author. Phone: 38(044) 525-83-03; e-mail: babichuk@isp.kiev.ua
Abstract. Optical constants of Cu2ZnSnS4 thin films formed using thermal annealing of pre-deposited layers of copper, zinc and tin sulphides on glass substrates at different temperatures and ambient atmosphere were determined. It has been shown that films grown at lower temperatures have the tetragonal structure of kesterite, and the corresponding value of the optical band gap is 1.47 eV.
Keywords: Raman spectroscopy, SEM, optical constants, kesterite, CZTS.
Manuscript received 25.02.14; revised version received 30.07.14; accepted for publication 16.09.14; published online 30.09.14.
1. Introduction
Nowadays photovoltaic devices around the world provide production of about 20 GW of electrical power. Almost 90% of them are based on silicon solar cells, the effectiveness of which reaches theoretical limits, and their cost is still quite high. One of the solutions to reduce the cost of manufacturing photovoltaic modules is use of thin-film technology. However, in this respect, single-crystal silicon is not an effective material, because it has an indirect energy band structure and, consequently, it has a relatively low value of absorption of solar radiation in the visible and near infrared bands (1-2 eV). Switching to involving the direct energy band materials can significantly reduce the thickness of semiconductor layer that efficiently absorbs incident radiation in this range.
High interest in investigation of quaternary semiconductor compounds based on chalcogenides and widespread elements of groups II and IV, for example of the Cu2BIICIVS4 type, a typical representative of which is Cu2ZnSnS4 (CZTS), is caused by search of new materials for designing solar cells. Their direct energy band determining the high interband absorption coefficient for solar radiation in the visible spectral range is turn-point to make solar cells to be cheaper. The other two important factors for potential use of CZTS in solar cell application are widespread in the crust of the constituent elements and their sustainability in manufacturing processes and application.
Up to date, the efficiency of CZTS-based solar cells has reached the level close to 12.6% [1]. To increase the efficiency, several problems should be solved. They are collected in series as follows: the presence of non-stoichiometry elemental composition of CZTS and the concentration of intrinsic defects in the crystallographic structure and conformably in the energy band gap; properties of CZTS by coexistence of crystallographic phases and by the possible impurities of secondary binary and ternary compounds formed during the synthesis.
In this work, optical and vibrational properties of Cu2ZnSnS4 thin films obtained by thermal annealing the pre-deposited binary compounds ZnS, CuS and SnS on glass substrates were investigated. Annealing was carried out at two different temperatures and in two different gas atmospheres. To determine optical constants of this material, the spectra of light reflection were analyzed at normal incidence, which is convenient from the viewpoint of conversion of complex-conjugate functions using dispersion Kramers-Kronig integrals [2]. It is known that, to obtain reliable results, if using the dispersion Kramers-Kronig integrals, precise measure-ments of reflection spectra in the whole optical range should be carried out. In addition, their use for calculating the optical constants suggests that the experimental studies are performed with fairly thick material, for which the reflection spectrum is recorded between two semi-infinite media.
2. Experimental technique
In this work, Cu2ZnSnS4 thin films were obtained using deposition of binary compounds ZnS, CuS and SnS on glass substrates with pre-deposited molybdenum as a bottom layer. Annealing was carried out at two different temperatures 370 and 390 ºC in air and in nitrogen atmosphere. The use of relatively low temperature was applied to compare with results of deposition of similar structures on flexible organic polymer films.
Reflection spectra were recorded using the spectrometer Shimadzu UV-3600, and surface morphology was investigated applying the scanning electron microscope (SEM) Tescan Mira 3 LMU. Component compositions were determined using energy-dispersive X-ray spectroscopy (EDX) with Oxford instruments INCA x-sight that was built in the SEM. To investigate structural properties of Cu2ZnSnS4, the method of m-Raman spectroscopy (T64000 Horiba Jobin Yvon) was employed. For excitation of Raman spectra, radiation of Ar+-laser with the wavelength 514.5 nm was used. The power of laser radiation was chosen sufficiently minimal (the laser power density was 0.1 mW/μm2) in order not to add changes in the film structure during these measurements. All the measurements were performed at room temperature. Moreover, Raman spectra were recorded in different parts of each sample, because of morphology and the phase inhomogeneities on the top of film surface were visible in the optical microscope. After that, the collected results were averaged to obtain the average parameters of the existing medium as well as to establish the nature of some crystalline phases that segregated. Spectra of photoluminescence (PL) were recorded at the temperature of liquid nitrogen. The excitations of spectra were carried out upon the incidence of light of Nd: YAG laser with the wavelength 532 nm.
3. Results and discussion
It is known that, during high-temperature annealing, atoms of sulphur rapidly evaporate from the surface of CZTS [3]. Being based on this fact, to obtain compounds with stoichiometric component ratio (2:1:1:4) annealing should be carried out in atmosphere of sulphur. Moreover, in the course of annealing in sulphur atmosphere, zinc first takes all of the sulphur saturation according to the electrochemical activity of a number of metals. To clarify this mismatch, it should be first analyzed the reactions of formation of sulphides by using standard oxidation-reduction potentials for binary compounds that were collected in the table [4].
We can see from Table 1 that the first metal will be reduced to atomic state in the process is copper. Therefore, observation of the CuS and Cu2S phases in the synthesized film after sulphurization is the most probable. Indeed, this assumption is confirmed by the results of the studies of such structures by Raman spectroscopy and X-ray diffraction (XRD) in many works [3, 5-7]. On the other hand, the CuS and Cu2S phases are less stable in contrast to ZnS and MoS2. As can be seen from the Table 2, the standard molar enthalpy of their formation is minimal in its absolute value for a number of binary compounds [4, 8].
The standard molar enthalpy of formation of any substance is a measure of its stability. The higher is the absolute value of the enthalpy for formation of such compounds, as we assume, the more stable is this compound. Therefore, during their synthesizing CZTS films were formed using such compounds as ZnS, MoS2, CuS, Cu2S.
Table 1. Oxidation-reduction reactions.
Electrode processes
Е, V
CuS + 2e– = Cu + S2–
–0.76
SnS + 2e– = Sn + S2–
–0.87
Cu2S + 2e– = 2Cu + S2–
–0.89
ZnS + 2e– = Zn + S2–
–1.405
Table 2. Standard molar enthalpy values.
Sulfides
ΔНо, kJ/mol
CuS
–53.1
Cu2S
–53.1
SnS2
–82.4
SnS
–110.2
ZnS
–205.4
MoS2
–248.1
As can be seen from Fig. 1, surface morphology of CZTS films strongly depends on technological conditions during their synthesis: annealing temperature and atmosphere of processing. The increase in the temperature of synthesis only by 20 °C from 370 to 390 °C results in significant changes in surface morphology of the formed CZTS films. To determine the component structure of formed films and individual phase inclusions, local-EDRS studies were carried out in various regions of the film surface. For separating the bands in the EDRS spectrum obtained from the individual elements, the energy of electrons used for sensing was relatively large, resulting in a contribution to the range of items from all over the film thickness. However, in this case, it is possible to get information about the elemental composition inhomogeneities arising on the surface of the films during its synthesis (Fig. 1). From these investigations, it was found that the elemental composition of morphological heterogeneity (Fig. 1a, region 1) corresponds to the composition enriched with Cu (Cu-rich). At the same time, in areas where the film is homogeneous (Fig. 1a, region 2) the ratio of the components Cu:Zn:Sn:S is closer to the stoichiometric one, although here we observed a little bit overstated content of Cu and depletion of Sn.
The Raman spectra of the CZTS films and their corresponding deconvoluted components (Lorentzians) are presented in Fig. 2. The films were formed at the temperature close to 390 °C in air (a) and nitrogen (b) atmospheres. As it is seen from the spectra shown in Figs. 2a and 2b, the main bands of the Raman spectra of CZTS films formed in air and nitrogen (N2) at the temperature 370 °C were practically indistinguishable. But in the addition to the main A-symmetry band of CZTS, a very intense band at the frequency 473 cm–1 is presented in the spectrum of the film formed in nitrogen at 370 °C (Fig. 2c, inset), which corresponds to the vibrations of Cu2S compound [9, 10]. The main contribution to the obtained spectra is given by A-symmetry bands of CZTS compound with the frequencies 288 and
(
)
1
cm
336
337
-
and bands with the frequencies 254(251), 312(308), 358(360),
1
cm
372
-
, corresponding to the vibrations of E- and B-symmetry and discussed in detail in [10-15]. The bands with the frequencies (412(410) cm-1 correspond to the vibrations of the MoS2 compound [16]. Raman spectra of the films that were synthesized in air (Fig. 2a) are slightly different from the spectra of the films formed in nitrogen (Fig. 2b). Although, it should be noted that the frequency position of Raman bands in respect with the films formed in nitrogen atmosphere is somewhat lower. During annealing, the presence of oxygen in air can cause interaction of individual elements in CZTS films with oxygen, which is reflected in a decrease of the vibrational frequency of the main modes inherent to material. Analysis of half-widths of Raman bands in the films formed in different atmospheres showed that their values for the bands in the first case are less than that of the latter one by 4 cm-1. Perhaps, it is caused by partial oxidation of the binary and ternary phases formed in the course of the synthesis of CZTS. It results in reduction of their contribution to the mechanical stress stability of main material and, therefore, reduction of the half-width of the bands occurs. The half-widths of the bands observed in the abovementioned films are larger by (8 cm-1 as compared with the similar films obtained at higher temperatures of synthesis (475…600 °C) (see Ref. [13]). The obtained results should be improved to reach values [13], however, the use of lower temperatures of annealing is a prerequisite to pronounce the perspectives of formation of the structures on the flexible organic polymer films that are not suitable to be treated at higher temperatures.
Thus, to make the analysis of the Raman spectra of all the films, we conclude that the annealing temperature within the range from 370 to 390 °C does not provide a significant effect on the frequency position and intensity of bands. But, when the sample is formed at one temperature, but in different atmospheres, the corresponding spectra differ quite significantly from each other (Fig. 2).
Optical reflection spectra R(λ) of the CZTS film that were obtained under different technological conditions are shown in Fig. 3. Analysis of the experimental data showed that the reflections of the films with increase of the annealing temperature are significantly reduced, namely, in air by 8% and in N2 – 25%. A significant decrease in the reflectivity of the films annealed in N2 can be attributed to the microstructure of the film. SEM images (Figs. 1a and 1b) show that the films annealed at 370 °C have a relatively lower surface density of microcrystalline grains as compared with that on the film surface (Figs. 1c and 1d), when being annealed at 390 °C. The increase in the annealing temperature by 20 °C results in a significant decrease of the optical reflectivity. This result is important for designing the photoelectric multiplier, because the low reflection coefficient of the film allows to absorb more solar energy and, consequently, to increase efficiency.
Fig. 2. Raman spectra of CZTS films grown at 390 °C in air (а) and N2 (b) with deconvoluted components (c); 370 °C in N2.
Optical constants of CZTS films were determined using reflection spectra within the range from 380 to 1600 nm. As well known, it is possible to determine the complex index of refraction using the Fresnel equations for the reflection function at the normal incidence of the light beam on the film, according to the formula:
(
)
(
)
(
)
(
)
(
)
÷
÷
ø
ö
ç
ç
è
æ
+
w
-
w
=
w
¢
=
w
w
q
1
~
1
~
2
1
n
n
e
R
R
i
,
(1)
where,
(
)
(
)
(
)
w
±
w
=
w
ik
n
n
~
is the complex index of refraction; n, k are real and imaginary components.
The function of the reflection is related to the Fourier transformation as follows:
(
)
(
)
(
)
w
q
±
w
¢
=
w
i
R
R
ln
2
1
ln
. In this case, the phase shift between the incident and reflected rays ( can be determined from the Kramers–Kronig dispersion integral [17]:
(
)
(
)
(
)
ò
¥
w
w
-
w
w
¢
-
w
¢
p
w
=
w
q
0
2
0
2
0
0
0
ln
ln
d
R
R
,
(2)
Fig. 3. Reflectance spectra of CZTS films grown at 370 °C (1, 2) and 390 °C (3, 4).
After numerical integration of the dispersion integrals by using the trapezoidal rule with the step 1 meV, the complex refractive index and its components can be determined from the following formula:
(
)
(
)
(
)
(
)
(
)
0
0
2
1
2
1
~
w
q
w
q
w
¢
-
w
¢
+
=
w
i
L
i
L
e
R
n
e
R
n
n
(3)
where, nL is the refractive index of air.
It is known that the complex dielectric function and the absolute value of the refractive index are linked by equations below:
(
)
(
)
(
)
,
~
2
1
2
w
e
±
w
e
=
=
w
e
i
n
(
)
(
)
(
)
,
2
2
1
w
-
w
=
w
e
k
n
(
)
(
)
(
)
,
2
2
w
w
=
w
e
k
n
(
)
(
)
(
)
(
)
,
2
1
2
2
2
1
w
e
+
w
e
+
w
e
=
w
n
(4)
(
)
(
)
(
)
(
)
.
2
1
2
2
2
1
w
e
-
w
e
+
w
e
=
w
k
Calculation of spectral dependences for optical constants of CZTS films: the refractive n(hν) and extinction k(hν) indexes, the real ε1(hν) and imaginary ε2(hν) parts of the complex dielectric function ε was performed using packages of the applied MathCad software. The spectral dependences of n(hν) and k(hν) are shown in Fig. 4. They were calculated from the latter two ones by using the equation (4). Fig. 4 shows that the optical constants n(hν) and k(hν) decrease with increasing the photon energy hν. Variation of the real ε1(hν) and imaginary ε2(hν) parts of the dielectric function is similar to that observed for the optical constants n(hν) and k(hν).
Fig. 4. Spectra of optical functions: a) refractive index n(hν); b) extinction coefficient k(hν) of CZTS films obtained at 370 °C (1, 2) and 390 °C (3, 4).
In Fig. 5a the spectra of the absorption coefficient α(hν) of the films versus photon energy are presented according to the treatments. These spectra were transferred from the reflection spectra by using the dispersion integrals. It should be noted that it does not lead to the significant errors in calculations, from the condition that the film thickness CZTS was grown thicker than 1 μm. It is caused by the high absorption of light by thicker films and decrease of light interference inside the glass-CZTS interface. As shown in Fig. 5a, the increase in absorption coefficient in the low energy region (hν < 1 eV) is quite sharp. An increase of the slope of the absorption curve is observed near the absorption edge for the CZTS film except the film annealed in nitrogen at 370 °C, for which the absorption coefficient declines of linearity in this region. The same situation is observed for the other films in the range (hν > 1.7 eV).
In Fig. 5b the calculated absorption spectra are plotted in the Tauc coordinates for direct allowed optical transitions, which makes it possible to estimate effects of the atmosphere presences and the temperature of the annealing on the value of optical energy gap. These values are slightly less as compared with the data typical for films with a tetragonal like kesterite structure for which the optical band gap Eg = 1.5 eV (see Ref. [18]). This mismatch may appear due to availability of structural defects composed in the films both on the surface and in the bulk. In this issue, the inclusion with the energy lower than the value of Eg can affect on the optical energy gap. But among them, we can see that even at low annealing temperatures (390 °C) in both kinds of atmospheres, in which the films were formed, values of Eg are very close to that discussed by the authors in Refs. [18-20]. Moreover, PL spectra of CZTS films were investigated at the temperatures near 80 K. They had somewhat asymmetric broad band with the peak at 1.26 eV. There are several explanations for these features. One of them, as we assume and suggested in [21, 22] for structures similar to CZTS at a low concentration of doping elements, the dominant radiation was caused by recombination of donor-acceptor pairs or transition of electrons from the conduction band to the acceptor level [23].
Fig. 5. Spectra of the absorption coefficient of CZTS films: а) grown at 370 °C (1, 2) in N2 and at 390 °C (3, 4) in air; b) in the Tauc coordinates for the case of direct allowed optical transitions.
4. Conclusion
Analysis of the Raman spectra of the films Cu2ZnSnS4 that were formed by thermal annealing previously deposited of precursors layers based on copper, zinc and tin sulphides on glass substrates at different temperatures and in different surrounding atmospheres showed that they have a tetragonal structure of kesterite. Thus, it has been shown that the use of sulphides of copper, tin and zinc, in contrast to corresponding pure metals during the synthesis of these compounds, eliminates the need for the additional annealing in the atmosphere of sulfur to achieve stoichiometric structure. It is shown that changes of the temperature of the annealing from 370 to 390 °C have no significant effect on the features of the Raman spectra. At the same time, changes in the atmosphere in the course of the treatments result in the shift of the frequency of the bands and change in their intensities.
From the reflectance spectra, the optical constants of the formed films were calculated. It has been shown that increase of the synthesis temperature by only 20 °C from 370 to 390 °C leads to a significant increase of the surface density of microcrystalline grains. It reduces the reflection coefficient of the film and provides more efficient light absorption. Therefore, conversion of light to electricity would be more efficient. From our calculations of the absorption spectra, the average estimated energy band gap of films was approximately 1.47 eV.
This research is supported by the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Program FP7 under REA grant agreement 269167 (PVICOKEST) and grant OPTEC as well as State Fund for Fundamental Reasearch of Ukraine (bilateral Belarussian-Ukrainian project no. 54.1/005).
References
1.
W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, D.B. Mitzi, Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency // Adv. Energy Materials, 4(7), p. 1-5 (2014).
2.
Р. Bruzzoni, R.M. Carranza, J.R. Collet Lacoste, E.A. Crespo, Kramers-Kronig transforms calculation with a fast convolution algorithm // Electrochimica Acta, 48, p. 341-347 (2002).
3.
M. Espindola-Rodriguez, M. Placidi, O. Vigil-Galán, V. Izquierdo-Roca, X. Fontané, A. Fairbrother, D. Sylla, E. Saucedo, A. Pérez-Rodríguez, Compositional optimization of photovoltaic grade Cu2ZnSnS4 films grown by pneumatic spray pyrolysis // Thin Solid Films, 535, p. 67-72 (2013).
4.
V.A. Rabinovich, Z.Ya. Havich, Concise Chemical Directory. Chemistry, Leningrad, 1991.
5.
S. Schorr, The crystal structure of kesterite type compounds: a neutron and x-ray diffraction study // Solar Energy Materials and Solar Cells, 95, p. 1482-1488 (2011).
6.
I.D. Olekseyuk, E.V. Dudchak, L.V. Piskach, Phase equilibria in the Cu2S-ZnS-SnS2 system // J. Alloys and Compounds, 368, p. 135-143 (2004).
7.
A. Fairbrother, X. Fontané, V. Izquierdo-Roca, M. Espindola-Rodriguez, S. Lopez-Marino, M. Placidi, L. Calvo-Barrio, A. Perez-Rodriguez, E. Saucedo, On the formation mechanisms of Zn-rich Cu2ZnSnS4 films prepared by sulfurization of metallic stacks // Solar Energy Materials & Solar Cells, 112, p. 97-105 (2013).
8.
A.J. Jackson, A. Walsh, Ab initio thermodynamic model of Cu2ZnSnS4 // J. Mater. Chem. A, 2, p. 7829-7836 (2014).
9.
P.A. Fernandes, P.M.P. Salome, A.F. da Cunha, Growth and Raman scattering characterization of Cu2ZnSnS4 thin films // Thin Solid Films, 517, p. 2519-2523 (2009).
10.
І.S. Babichuk, V.О. Yukhymchuk, V.М. Dzhagan, М.Ya. Valakh, M. Leon, І.B. Yanchuk, E.G. Gule, О.М. Greshchuk, Thin films of Cu2ZnSnS4 for solar cells: optical and structural properties // Functional Materials, 20(2), p. 186-191 (2013).
11.
X. Fontane, V. Izquierdo-Rosa, E. Saucedo, S. Sсhorr, V.O. Yukhymchuk, M.Ya. Valakh, A. Pеrez-Rodrіguez, J.R. Morante, Vibrational properties of stannite and kesterite type compounds: Raman scattering analysis of Cu2(Fe,Zn)SnS4 // J. Alloys and Compounds, p. 190-194 (2012).
12.
A. Khare, B. Himmetoglu, M. Johnson, D.J. Norris, M. Cococcioni, E.S. Aydil, Calculation of the lattice dynamics and Raman spectra of copper zinc tin chalcogenides and comparison to experiments // J. Appl. Phys. 111(8), 083707-083716 (2012).
13.
R. Caballero, E. Garcia-Llamas, J.M. Merino, M. León, I. Babichuk, V. Dzhagan, V. Strelchuk, M. Valakh, Non-stoichiometry effect and disorder in Cu2ZnSnS4 thin films obtained by flash evaporation: Raman scattering investigation // Acta Materialia, 65 p. 412-417 (2014).
14.
M.Ya. Valakh, O.F. Kolomys, S.S. Ponomaryov, V.O. Yukhymchuk, I.S. Babichuk, V. Izquierdo-Rosa, E. Saucedo, A. Pеrez-Rodrіguez, J.R. Morante, S. Sсhorr, I.V. Bodnar, Raman
scattering and disordering effect in Cu2ZnSnS4 // Phys. Status Solidi (RRL) – Rapid Research Letters, 7(4), p. 258-261 (2013).
15.
P. Schütz, A.K. Alves, C.P. Bergmann, Effect of the in-air heat treatment in the phase formation and morphology of electrospun Cu2ZnSnS4 fibers // Ceramics Intern. 40(8), p. 11551-11557 (2014).
16.
S. Chen, X.G. Gong, Electronic structure and stability of quaternary chalcogenide semiconductors derived from cation cross-substitution of II-VI and I-III-VI2 compounds // Phys. Rev. B, 79, p. 165211-165221 (2009).
17.
M. Cardona, D.L. Greenaway, Optical properties and band structure of group IV-VI and group V materials // Phys. Rev. 133(6A), p. A1685-A1697 (1964).
18.
M. Ikhlasul Amal, Kyoo Ho Kim, Structural and optical properties of sulfurized Cu2ZnSnS4 thin fіlms from Cu–Zn–Sn alloy precursors // J. Mater. Sci.: Materials in Electronics, 24(2), p. 559-566 (2013).
19.
R.B.V. Chalapathy, G.S. Jung, B.T. Ahn, Fabri-cation of Cu2ZnSnS4 films by sulfurization of Cu/ZnSn/Cu precursor layers in sulfur atmosphere for solar cells // Solar Energy Materials and Solar Cells, 95(12), p. 3216-3221 (2011).
20.
P.K. Sarswat, M.L. Free, A study of energy band gap versus temperature for Cu2ZnSnS4 thin films // Phys. B: Condens. Matter, 407(1), p. 108-111 (2012).
21.
J.P. Leitao, N.M. Santos, P.A. Fernandes, P.M.P. Salome, A.F. da Cunha, J.C. Gonzalez, G.M. Ribeiro, F.M. Matinaga, Photoluminescence and electrical study of fluctuating potentials in Cu2ZnSnS4-based thin films // Phys. Rev. B, 84, 024120–024128 (2011).
22.
A.U. Sheleg, V.G. Hurtavy, A.V. Mudryi, M.Ya. Valakh, V.O. Yukhymchuk, I.S. Babichuk, M. Leon, R. Caballero, Study of structural and optical properties of Cu2ZnSnS4 semiconductor compounds thin films // Semiconductors, 48(10), p. 1332-1338 (2014).
23.
M. Grossberg, J. Krustok, J. Raudoja, T. Raadik, The role of structural properties on deep defect states in Cu2ZnSnS4 studied by photoluminescence spectroscopy // Appl. Phys. Lett. 101, 102102–102106 (2012).
� �
� �
Fig. 1. SEM images of the surface of CZTS films grown at 370º С (a, b) and 390º С (c, d) in N2 (a, c) and air (b, d).
PAGE
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
284
_1473592264.unknown
_1473592396.unknown
_1473592439.unknown
_1473592458.unknown
_1473592342.unknown
_1471261227.unknown
_1473592185.unknown
_1471067092.unknown
_1471067429.unknown
_1471261224.unknown
_1471067381.unknown
_1471065131.unknown
|