The role of copper in bifacial CdTe based solar cells
We present an innovative back contact for CdTe solar cell by the application of a transparent conducting oxide, typically ITO, as a back electrical contact on CdTe/CdS photovoltaic devices that acts as a free-Cu stable back contact and at the same time allows to realize bifacial CdTe solar cells,...
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Khrypunov, G. Meriuts, A. Shelest, T. Deineko, N. Klyui, N. Avksentyeva, L. Gorbulik, V. 2017-05-26T16:29:52Z 2017-05-26T16:29:52Z 2011 The role of copper in bifacial CdTe based solar cells / G. Khrypunov, A. Meriuts, T. Shelest, N. Deineko, N. Klyui, L. Avksentyeva, V. Gorbulik // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 3. — С. 308-312. — Бібліогр.: 7 назв. — англ. 1560-8034 PACS 88.40.jm https://nasplib.isofts.kiev.ua/handle/123456789/117765 We present an innovative back contact for CdTe solar cell by the application of a transparent conducting oxide, typically ITO, as a back electrical contact on CdTe/CdS photovoltaic devices that acts as a free-Cu stable back contact and at the same time allows to realize bifacial CdTe solar cells, which can be illuminated from either or both sides. The controlled insertion of a very limited amount of copper into the ITO back contact permits to have reproducible devices with high efficiencies still keeping the bifacial configuration. Thin CdTe layer solar cells with ITO back contact have been realized with efficiencies exceeding 10%, the reduced thickness of CdTe allows to have a better performance on the back-side illumination and reduces the amount of CdTe material. This work was supported by STCU Project 4301. en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics The role of copper in bifacial CdTe based solar cells Article published earlier |
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The role of copper in bifacial CdTe based solar cells |
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The role of copper in bifacial CdTe based solar cells Khrypunov, G. Meriuts, A. Shelest, T. Deineko, N. Klyui, N. Avksentyeva, L. Gorbulik, V. |
| title_short |
The role of copper in bifacial CdTe based solar cells |
| title_full |
The role of copper in bifacial CdTe based solar cells |
| title_fullStr |
The role of copper in bifacial CdTe based solar cells |
| title_full_unstemmed |
The role of copper in bifacial CdTe based solar cells |
| title_sort |
role of copper in bifacial cdte based solar cells |
| author |
Khrypunov, G. Meriuts, A. Shelest, T. Deineko, N. Klyui, N. Avksentyeva, L. Gorbulik, V. |
| author_facet |
Khrypunov, G. Meriuts, A. Shelest, T. Deineko, N. Klyui, N. Avksentyeva, L. Gorbulik, V. |
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2011 |
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English |
| container_title |
Semiconductor Physics Quantum Electronics & Optoelectronics |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| format |
Article |
| description |
We present an innovative back contact for CdTe solar cell by the application
of a transparent conducting oxide, typically ITO, as a back electrical contact on
CdTe/CdS photovoltaic devices that acts as a free-Cu stable back contact and at the same
time allows to realize bifacial CdTe solar cells, which can be illuminated from either or
both sides. The controlled insertion of a very limited amount of copper into the ITO back
contact permits to have reproducible devices with high efficiencies still keeping the
bifacial configuration. Thin CdTe layer solar cells with ITO back contact have been
realized with efficiencies exceeding 10%, the reduced thickness of CdTe allows to have a
better performance on the back-side illumination and reduces the amount of CdTe
material.
|
| issn |
1560-8034 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/117765 |
| citation_txt |
The role of copper in bifacial CdTe based solar cells / G. Khrypunov, A. Meriuts, T. Shelest, N. Deineko, N. Klyui, L. Avksentyeva, V. Gorbulik // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 3. — С. 308-312. — Бібліогр.: 7 назв. — англ. |
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| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 3. P. 308-312.
PACS 88.40.jm
The role of copper in bifacial CdTe based solar cells
G. Khrypunov1, A. Meriuts1, T. Shelest1, N. Deineko1, N. Klyui2, L. Avksentyeva2, V. Gorbulik3
1Kharkov State Polytechnic University, Kharkov 61002, Ukraine; e-mail: khrip@ukr.net
2V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Kyiv, Ukraine
3Kharkov State Polytechnic University, Chernivtsi Department, Chernivtsi, Ukraine
Abstract. We present an innovative back contact for CdTe solar cell by the application
of a transparent conducting oxide, typically ITO, as a back electrical contact on
CdTe/CdS photovoltaic devices that acts as a free-Cu stable back contact and at the same
time allows to realize bifacial CdTe solar cells, which can be illuminated from either or
both sides. The controlled insertion of a very limited amount of copper into the ITO back
contact permits to have reproducible devices with high efficiencies still keeping the
bifacial configuration. Thin CdTe layer solar cells with ITO back contact have been
realized with efficiencies exceeding 10%, the reduced thickness of CdTe allows to have a
better performance on the back-side illumination and reduces the amount of CdTe
material.
Keywords: bifacial solar cells, front illumination, back illumination, efficiency, thin
film.
Manuscript received 28.03.11; accepted for publication 14.09.11; published online 21.09.11.
1. Introduction
At the moment First Solar company has realized a
75 MW production plant in USA and is about to set up a
production plant of 100 MW in Germany. At the same
time, the investigations directed to development of new
types solar cells (SCs) on the CdTe film basis are
continued. Among such SCs are the so-called bifacial
solar cells. One of the main problems in this direction is
connected with development of a transparent and stable
electrical contact for p-CdTe. The development of an
efficient and long-term stable electrical contact to p-type
CdTe is difficult because of both the high electron
affinity and high energy band gap of CdTe. Typically, a
quasi-ohmic contact to p-type CdTe is obtained by
chemical etching the CdTe surface in either bromine-
methanol or nitric-phosphoric (NPH) acid etchant
followed by the deposition of very thin buffer metal (Cu,
Ni, Sb, etc.) or metal-chalcogenide (CuxTe, Sb2Te3,
ZnTe, HgTe, etc.) layer. Finally, a metal layer (Mo, Au,
Al, etc.) is deposited [1]. The best results with “non-Cu
containing” contacts were obtained for Sb2Te3/Mo
system [2]. The effect of copper on the SCs properties is
complicated. On the one hand, the cooper forms a quasi-
ohmic contact with an increasing concentration of
charge carriers. On the other hand, diffusion of copper
into CdTe leads to degradation of the solar cell
parameters (see, for example [3, 4]).
In this paper, we present the development of
bifacial thin film solar cells on a glass substrate with rear
cooper-containing contacts of new type. The properties
of the contacts and parameters of the SCs with these
contacts were studied.
2. Fabrication and testing of solar cells
All the layers were grown by PVD methods.
Commercially available soda-lime glass coated with
fluorine doped tin oxide (FTO) was used as a substrate.
CdS layer was grown in a high-vacuum evaporation
chamber at the substrate temperature 150 °C. CdTe layer
was then deposited at the substrate temperature close to
300 °C in the same chamber without breaking the
vacuum. After that, the so-called “chloride” treatment
was performed. The procedure included deposition of
CdCl2 layer with the thickness 400 to 600 nm onto CdTe
surface followed by thermal annealing in air at 430 °C
for 30 min. The standard back contact was made by
evaporating Cu/Au after bromine-methanol treatment of
the CdTe surface, followed by a short annealing at
200 °C in air. Standard solar cells have a typical
efficiency value from 11 to 12% [5]. Some of the cells
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
308
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 3. P. 308-312.
were subjected to light illumination in a special
chamber. The treatment of the SCs was performed under
conditions of open circuit. The SCs were illuminated by
the 500-W tungsten lamp, and the SC temperature was
80 °C. This accelerated treatment simulates operation of
SCs during long time, and the time of measurements
corresponded to 0.81, 1.62, 2.43, 3.78, 4.56, 5.4, 5.94,
and 8 years. The measurements were carried out in
Switzerland technological institute (Zürich). Therefore,
in Figs 1 and 2 there are two scales and the bottom one
corresponds to real time of testing, while the top one to
estimated time of the SC operation under illuminating
intensities 100 mW/cm2.
3. Copper free bifacial cells
The bifacial devices are fabricated exactly in the way as
the standard devices are made, the difference is
exclusively in the back contact. We have introduced a
new approach to apply a transparent back contact on
CdTe solar cells: after bromine-methanol etching the
CdTe surface a thin layer of transparent and conducting
ITO is sputtered and short annealing treatment in air is
applied in the end. The conductivity of the ITO layers on
glass has been measured as being about 15 Ohm/ , the
layer has a sufficient transparency. Due to the
transparency of the ITO back contact and FTO front
contact, the solar cell can be illuminated both from the
front and rear sides like a bifacial solar cell. As-
deposited solar cells with a pure ITO back contact
perform typically very low efficiencies around 2.5%
against regular 12.5% efficiency with traditional back
contact, in particular, Voc and fill factor (FF) are
extremely low. The change of these SC output
parameters after the back contact post-deposition
annealing is shown in Table 1. As a result of an
annealing after ITO deposition, the Voc, FF and short
circuit current density Jsc increases, however, efficiency
and after an annealing does not exceed 5%.
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Fig. 1. Accelerated lifetime stability tests of an
ITO/CdTe/CdS/FTO solar cell (before and after annealing at
350 °C).
0 5 10 15 20
3
4
5
6
7
8
0 1 2 3 4 5 6
days in light exposure
η, %
t, years
1
2
Fig. 2. Accelerated lifetime stability test of
ITO/Cu/CdTe/CdS/FTO solar cells with 3 nm copper
deposition (1) and less than 0.5 nm copper deposition (2).
It is logical to assume that, as the SC has a different
only back contact, the difference of performance can be
due to missing the buffer layer that allows tunneling of
the carriers through the contact. To realize the dramatic
difference between the traditional Cu-Au contact and the
ITO contact properties, we have analyzed the fabrication
process of the first one (Table 2). The main observation
is the change of the device parameters before and after
the back contact post-deposition annealing, the
efficiency of the solar cells jumps from a poor value up
to 10% efficiency as shown in Table 2. It can be
stipulated by crystallization of tellurium containing
layers on a back contact. The layers may appear after
etching the base layer and (or) diffusion of copper into
CdTe, which gives in its additional doping.
Table 1. Typical output parameters for SC with the ITO
back contact.
SC output
parameters As-deposited Annealed at
200 °C
Voc (mV) 367 502
Jsc (mA/cm2) 18.4 21.1
FF (%) 36.2 44
η (%) 2.4 4.6
Table 2. Typical output parameters of SC with the Cu/Au
back contact.
SC output
parameters
Voc
(mV)
Jsc
(mA/cm2)
FF
(%)
η
(%)
As-deposited 322 12.2 40.45 1.6
Annealed
200 °C 723 22.2 54.4 8.8
Light-exposed 825 22 65 11.8
309
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 3. P. 308-312.
Various cells were prepared and after ITO RF
sputtering, annealing in air annealed at different tem-
peratures from 200 up to 400 °C, best results were given
with annealing around 350 °C. The results are that from a
starting efficiency of 1-2% and after annealing jumps to
4-5% that can still rise to an upper efficiency above 6%
under light exposure. The increase in the efficiency is not
the same as in the case of Cu/Au back contact, but the
annealing is however improving the back contact,
suggesting that the heating of the device after bromine-
methanol etching re-crystallizes the Te rich layer.
The interesting feature of these solar cells is the
stability under accelerated lifetime test, we have
performed preliminary test also on these devices (solar
cells were kept under AM1.5 illumination and at 80 °C
temperature under open circuit conditions) and they
presented an excellent stability and the performance
actually improves instead of degrading (see Fig. 1).
The light exposure effect is different in cases of as-
deposited and annealed devices: in the as-deposited
device, the improvement is of the order of 200% for the
first days (from about 1% efficiency up to above 3%
efficiency) and successively tends to stabilize. In the
annealed devices, the improvement is lower but still
there is a stable increase in an almost ten year estimated
operation time. It is clear that the defects at the junction
between ITO and CdTe are clearly more in the as-
deposited case and the light exposure fills the
recombination centers at the junction, while in case of
the annealed device the hetero-junction has a better
quality and probably the crystallized Te layer helps to
inject the carriers across the junction.
To reach typical efficiencies of a CdTe cell above
10% seems that the influence of copper is then
fundamental as mentioned also by [6].
4. Copper containing bifacial cells
To have a complete panorama and in order to compare
the SCs with ITO contacts with the standard copper-gold
ones, we have analyzed the possibility of inserting
copper into the back contact and to study the effects in
the performance and in the stability. The basic principle
is that a small amount of the copper would still leave the
back contact transparent and, at the same time, might not
affect the stability of the solar cell.
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Several different devices have been made with a
standard fabrication process but with different thickness
of the copper film deposited onto SC prior to ITO
deposition. Every cell has been processed with half area
shadowed from the copper deposition in order to compare
the different cells. Fig. 2 shows results of accelerated
lifetime stability tests of cells with different amount of
copper. One can see that initial efficiencies for both SCs
are practically the same but the stability trend is opposite.
The 3-nm Cu cell has a degradation that tends to stabilize
after 4 years hypothetical time, is similar to the one
observed for Cu/Au [7] but still the stability is higher,
most probably due to the one order of magnitude less
amount of copper as compared to the standard cells
(80 nm Cu). On the other hand, the <0.5nm-Cu cell starts
from the same efficiency, but instead of degrading it
improves slightly in the performance at the beginning of
the light exposure and a successive stabilization as a very
similar behavior of Sb2Te3/Mo cell [2].
For the back-side illumination, efficiencies have
been in the order of 0.5% with Voc of about 300 mV, Jsc
of 3-4 mA/cm2 and fill factor of 45%. The low current in
the back-side illumination is due to the long distance
between the hole-electron pair generation and the
junction. We believe that if a stable compound with Cu
is formed, then the back contact could be very stable, in
particular what could happen is that after the creation of
a thin layer of tellurium due to the bromine-methanol
etching the copper reacts with the tellurium giving place
to copper telluride that is a stable compound.
The proof for stability comes also from the
annealing test made successively on the light exposed
cells, annealing on these devices has been performed for
25 min at 300 °C. The annealed cells did not actually
degrade, but instead their efficiency was improved in
general by about 20% from a starting efficiency of 7.5%
the efficiency of 9.2% was reached. The calculation of
diode parameters has shown that the increase in
efficiency is related to increase of shunting resistance
and lowering the diode saturation current, at the same
time, it is necessary to mark that after annealing the
series resistance is grown.
5. Bifacial cells with a thin base layer
In order to improve the performance in the back-side
illumination, we tried to reduce the distance between the
junction and the electron-hole pair generation zone by
reducing the thickness of the CdTe layer. The solar cell
fabrication process has been adjusted to the different
CdTe thicknesses, in particular the CdCl2 treatment has
been processed with only 20% of the standard CdCl2
quantity and by reducing the annealing time.
The first cells were made with about 2.7 μm CdTe
thickness and the most performing quantity of copper
(0.5 nm). The solar cells started with a very good
performance from the beginning as shown in Fig. 3a and
increased after light exposure to reach the efficiency of
10% with Voc = 713 mV, Jsc = 19.3 mA/cm2, FF = 73%.
The back-side illumination performance improved
considerably compared to the standard cell, as shown in
Fig. 3b after 3-day light exposure we have 2.1% with
Voc = 659 mV, Jsc = 8.8 mA/cm2 and FF = 36%, i.e.,
slight degradation follows. The stability of these cells is
shown in Fig. 4, the cells are practically stable having a
strong increase at the beginning and a successive slow
degradation afterwards.
6. Bifacial cells with an ultra-thin base layer
Further lowering the CdTe layer thickness could open up
different possibilities. First of all, there should be a
310
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 3. P. 308-312.
further increase in the back-side illumination
performance, second, the solar cell becomes semi-
transparent and could be used for tandem-configuration
cells, and third, a mirror on rear side could be applied in
order to absorb the light with energy close to the
material band gap. Besides, in this case, a very low
amount of material is used.
One micrometer CdTe thick solar cells were
prepared using even more reduced CdCl2 treatment, 30-
nm thick CdCl2 and 15 min annealing in air at standard
annealing temperatures, copper insertion was reduced to
less than 0.3 nm.
а
b
Fig. 3. I-V characteristic front-illuminated (а) and back-
illuminated (b) CdTe. μm-7.2
Fig. 4. Stability of thin absorber solar cells.
Fig. 5. I-V characteristic CdTe solar cell with 1-μm thick
absorber: (1) front (Voc = 676 mV, Jsc = 19.7 mA/cm2,
FF = 60.5%, η = 8%) and (2) back (Voc = 622 mV, Jsc =
12.2 mA/cm2, FF = 43%, η = 3.2%) illuminated ones.
As expected, the short circuit current density is
reduced, and the efficiency drops to 7-8% under
illumination of the SC from the front side. However, the
SC efficiency under the back-side illumination improves
up to an exceeding 3% (see Fig. 5). The maximum
efficiency is reached after some days under light
exposure, and then there is slight degradation, similar to
the behavior shown in Fig. 4 for the thicker cell.
7. Conclusion
Application of a novel back contact, based on
transparent conductive oxide, on p-CdTe opens a variety
of new applications of CdTe solar cells. They can work
as bifacial cells, illuminating the back and the front
surfaces simultaneously or they can be used in tandem
solar cells. The controlled insertion of a very limited
amount of copper into the ITO back contact permits to
have reproducible devices with high efficiencies still
keeping the bifacial configuration.
Thin CdTe layer solar cells have been realized with
efficiencies exceeding 10%, the reduced thickness of
CdTe allows to have a better performance (efficiencies
exceeding 3.5%) on the back-side illumination and
reduces the amount of CdTe material.
This work was supported by STCU Project 4301.
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