COPPER INDIUM GALLIUM SELENIDE SOLAR
CELLS (CIGS)
ESRA DANIŞAN
KAZIM YALÇIN
TOPICS
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What is CIGS?
Manufacturing?
Positions in the world?
Efficiency
Advantages
Disadvantages
WHAT IS CIGS?
Copper indium gallium selenide (CuIn1-xGaxSe2 or CIGS) is a direct
band gap semiconductor useful for the manufacture of solar cells. The
CIGS absorber is deposited on a glass or plastic backing, along with
electrodes on the front and back to collect current. Because the
material has a high absorption coefficient and strongly absorbs
sunlight, a much thinner film is required than of other semiconductor
materials. Devices made with CIGS belong to the thin-film category of
photovoltaics .
STRUCTURE OF A CIGS THIN-FILM SOLAR CELL
The basic structure of a Cu(In,Ga)Se2 thin-film solar cell is depicted
in the image of Figure 2 to the right. The most common substrate is
soda-lime glass of 1–3 mm thickness. This is coated on one side with
molybdenum (Mo) that serves as metal back contact. The
heterojunction is formed between the semiconductors CIGS and ZnO,
separated by a thin layer of CdS and a layer of intrinsic ZnO. The
CIGS is doped p-type from intrinsic defects, while the ZnO is doped ntype to a much larger extent through the incorporation of aluminum
(Al).
This asymmetric doping causes the space-charge region to extend
much further into the CIGS than into the ZnO. Matched to this
are the layer thicknesses and the bandgaps of the materials: the
wide CIGS layer serves as absorber with a bandgap between 1.02
eV (CuInSe2) and 1.65 eV (CuGaSe2). Absorption is minimized in
the upper layers, called window, by the choice of larger bandgaps:
Eg,ZnO=3.2 eV and Eg,CdS=2.4 eV. The doped ZnO also serves
as front contact for current collection. Laboratory scale devices,
typically 0.5 cm2 in size, are provided with a Ni/Al-grid deposited
onto the front side to contact the ZnO.
MANUFACTURING
MANUFACTURING : The most common vacuum-based
process co-evaporates or co-sputters copper, gallium, and
indium, then anneals the resulting film with a selenide
vapor to form the final CIGS structure. An alternative is to
directly co-evaporate copper, gallium, indium and selenium
onto a heated substrate. Most CIGS solar cells are
produced using a co-evaporation technique that involves
vacuums and can be costly and time-consuming. The
elements are heated and deposited on a surface in a
vacuum.
EFFICIENCY
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CIGS is mainly used in the form of polycrystalline
thin films. The best efficiency achieved as of
December 2005 was 19.5%. In 2013, scientists at
Empa, the Swiss Federal Laboratories for Materials
Science and Technology, developed CIGS cells on
flexible polymer foils with a new record efficiency of
20.4% for converting sunlight into electricity. These
are both the highest efficiency examples as well as
flexible.
Higher efficiencies (around 30%) can be obtained by
using optics to concentrate the incident light. The use
of gallium increases the optical band gap of the CIGS
layer as compared to pure CIS, thus increasing the
open-circuit voltage. In another point of view, gallium
is added to replace as much indium as possible due to
gallium’s relative availability to indium.
Flexible CIGS solar cells
Chirila et al. Nature materials, 10 (2011), 857
Record efficiency: 18.7%
Process: substrate configuration. Co-evaporation of CIGS on polyimide
covered by molybdenum
Other parameters: VOC = 712 mV, JSC = 34.8 mA/cm2, FF = 75.7%, area =
0.582 cm2
PV module production in 2011
Monocrystalline silicon
11.5 GW
Amorphous/microcrystalline silicon
1.26 GW
CdTe
2.05 GW
CIS
0.9 GW
Multicrystalline silicon
21.2 GW
others
0.3 GW
2011 Total Production : 37
GW
PRODUCTİON OF CIGS
CIGS module production is expanding: it reached 0.9 GWp of
production in 2011 and it will increase to about 2 GWp in 2013.
CIGS module efficiencies are approaching those of c-Si modules.
If the predicted cost reduction will be achieved, CIGS could be a strong
competitor.
Direct gap thin film competitor: CIGS & CdTe
Small area solar cells: record efficiency
evolution
Eff=20.3
%
Eff=17.3
%
Cu(In,Ga)Se2
20
CdTe
Efficiency (%)
15
10
5
0
1960
1970
1980
1990
Year
2000
2010
ADVANTAGES OF CIGS
1. The active layer (CIGS) can be deposited in a polycrystalline form directly onto
molybdenum coated glass sheets or steel bands. This uses less energy than
growing large crystals, which is a necessary step in the manufacture of crystalline
silicon solar cells. Also unlike crystalline silicon, these substrates can be flexible.
2. One environmental advantage of CIGS solar cell technologies have over
Cadmium Telluride solar cell panels is that it uses a much lower level of cadmium,
in the form of cadmium sulfide. In some designs, sometimes zinc is used instead
of cadmium sulfide all together.
3. Like Cadmium Telluride panels, CIGS solar cell panels show a better resistance
to heat than silicon based solar panels.
DISADVANTAGES OF CIGS
1. Like all thin film solar panels, CIGS panels are not as efficient as
crystalline silicon solar cells, for which the record efficiency lies at
24.7%. They are however, the most efficient of the thin film
technologies.
2. So far being able to produce solar panels at prices that can compete
with polycrystalline or cadmium telluride panels has not been possible.
There is growing concern by some parties, that the cost of fabricating
the product makes it difficult to be competitive with current grid prices.
It may take several more years to solve the manufacturing problems
and bring the production costs in line with the other leading producers
of solar panels.
CIGS & SILICON
Unlike the silicon cells based on a homojunction, the structure of CIGS
cells is a more complex heterojunction system. CIGS solar cells are not as
efficient as crystalline silicon solar cells, for which the record efficiency lies
at 24.7%.Though some argue that CIGS will be substantially cheaper due to
much lower material costs and potentially lower fabrication costs, in the
second quarter of 2013, Veeco, Inc., a company with expertise in thin film
deposition technology, closed a major CIGS development project due in part
to the continued declining costs of the competing silicon materials and
technology.
A direct bandgap material, CIGS has very strong light absorption and only
1–2 micrometers of CIGS is enough to absorb most of the sunlight. A much
greater thickness of crystalline silicon is required for the same absorption.
The active layer (CIGS) can be deposited in a polycrystalline form directly
onto molybdenum coated glass sheets or steel bands. This uses less energy
than growing large crystals, which is a necessary step in the manufacture of
crystalline silicon solar cells. Also unlike crystalline silicon, these substrates
can be flexible.
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