The present invention relates to the manufacture
of photovoltaic cells, in particular for converting solar
energy to electrical energy. More particularly, the
present invention relates to a method for preparing an
absorber thin film for photovoltaic cells and to a method
for manufacturing a solar cell comprising an absorber
thin film prepared according to the invention.
Photovoltaic cells often have a structure
including a stack of thin films in which at least one of
said thin films has photovoltaic properties.
An example of a structure of photovoltaic cells
is shown in figure 1.
As shown in figure 1, a photovoltaic cell
generally has a stack of films placed on an insulating
support 12. In general, the insulating support 12 is a
layer of glass.
A film of molybdenum 14 having a thickness of
between 0.5 μm and 1 μm is deposited on the insulating
support 12. The molybdenum film 14 is generally
deposited by vacuum evaporation or sputtering.
An absorber film 16 is deposited on the
molybdenum film 14. The absorber film 16 generally has a
thickness of about 2 microns and may be deposited by
vacuum evaporation, or by cathode sputtering.
An interfacial film 18 is deposited on the
absorber film 16. The interfacial film 18, also called
buffer film, may comprise cadmium sulfide or zinc sulfide
deposited chemically in solution. The interfacial film
18 generally has a thickness of between 10 nm and 80 nm.
A slightly doped zinc oxide film 20 is deposited
by cathode sputtering on the interfacial film 18. The
3
slightly doped zinc oxide film 20 has a thickness of
about 50 nm to 100 nm.
An aluminum-doped zinc oxide film 22 is deposited
on the slightly doped zinc oxide film 20. The aluminumdoped
zinc oxide film 22 is conventionally deposited by
vacuum sputtering in order to have a thickness of about
0.5 μm to 1 μm.
The doping of the said zinc oxide film is
intended to make said film n-type conductive and to serve
as an electrode (transparent in the visible) at the front
of the photovoltaic cell.
Photovoltaic cells in which the absorber film 16
consists of CuInSe2 or Cu(Inx,Ga1-x)Se2 compounds have
conversion efficiencies of up to 20%.
The principal method for preparing photovoltaic
cells in thin films makes use of physical methods, for
example co-evaporation or even cathode sputtering.
The photovoltaic properties of the CuInSe2 or
Cu(Inx,Ga1-x)Se2 thin films are considerably dependent on
the composition of the absorber thin film. Thus, it is
important to be able to control the composition of the
absorber thin film as accurately as possible.
Electrodeposition is a method which can serve to
improve the control of the composition of the CuInSe2 or
Cu(Inx,Ga1-x)Se2 thin films.
A method for electrodepositing a CuInSe2 alloy is
described in application US 4,581,108.
The method described in application US 4,581,108
comprises the following successive steps:
- electrodepositing a thin film of copper (Cu)
and indium (In),
- addition of selenium (Se) by selenization in
order to form a CuInSe2 film.
4
The preparation of an absorber thin film by
electrodeposition is complicated to implement, inter
alia, because of:
- the very wide difference in redox potentials of
the various constituent elements of the thin film,
- the low solubility of indium and/or gallium
salts,
- the strong affinity of gallium for oxygen, and
- the great complexity of selenium chemistry.
A need therefore exists for an easily applicable
method for producing absorber films having a well
controlled composition.
The invention thus proposes a method for
preparing an A-B-C2 or A2-(Dx,E1-x)-C4 absorber thin film
for photovoltaic cells where 0≤x≤1, A is an element or a
mixture of elements selected from group 11, B is an
element or a mixture of elements selected from group 13,
C is an element or a mixture of elements selected from
group 16, D is an element or a mixture of elements
selected from group 12 and E is an element or a mixture
of elements selected from group 14, said method
comprising the following successive steps:
 electrodeposition of a thin film of a mixture of
oxides and/or hydroxides comprising, for an A-BC2
film, at least one element from group 11 and
one element from group 13 or, for an A2-(Dx,E1-x)-
C4 film, at least one element from group 11, at
least one element from group 12 if x>0, and at
least one element from group 14 if x<1,
 annealing of the thin film in a reducing
atmosphere,
 supply of at least one element from group 16 in
order to form an A-B-C2 or A2-(Dx,E1-x)-C4 thin
5
film where 0≤x≤1.
Advantageously, the electrodeposition of the
elements of groups 11, 12, 13, and 14 in oxide and/or
hydroxide form is easier to carry out and allows better
control of the final composition than the
electrodeposition of these elements in non-oxidized form.
In particular, the electrolyte solution is more stable
when the oxides are formed in the absence of Se or S
elements in solution, and contrary to the
electrodeposition of the prior art, the chemical
composition of the absorber thin film does not change
during the growth of said film.
Advantageously, it is possible to carry out the
deposition on large areas by a coating technique that is
perfectly established on an industrial level and is
highly advantageous for the production of photovoltaic
panels on a large scale and at low cost.
A method according to the invention may further
comprise one or more of the optional features below,
considered individually or in all possible combinations
thereof:
- the electrodeposition is carried out at a
temperature of at least 5°C and not greater than 95°C;
- the annealing in reducing atmosphere is carried
out at a temperature of at least 300°C and not greater
than 650°C;
- the annealing in reducing atmosphere has a
duration of at least 20 seconds and not greater than 15
minutes;
- prior to the electrodeposition of the thin
film, an aqueous solution is prepared containing a
mixture of salts of A and B for an A-B-C2 film or a
mixture of salt of A, D and/or E for an A2-(Dx,E1-x)-C4
6
film, in the presence of at least one oxygen donor
species;
- the oxygen donor species consists of a nitrate
ion, or of dioxygen, or of hydrogen peroxide or of
hypochlorite ions;
- element A is copper or silver or a mixture of
copper and silver and C is selenium or sulfur or a
mixture of selenium and sulfur;
- the absorber thin film for photovoltaic cells
is the A-B-C2 type and B comprises one or more elements
selected from indium, gallium, aluminum or mixtures
thereof;
- the electrodeposition is carried out by
imposing a voltage of at least -1.8 V, for example at
least -1.0 V, and not greater than -0.5 V, for example
not greater than -0.70 V, in comparison with a saturated
Hg/Hg2SO4/K2SO4 reference electrode, to an electrode
comprising an insulating substrate coated with a film of
molybdenum or a current density of between 1 and 30
mA.cm-2;
- the electrodeposition is carried by imposing a
voltage of
- at least -1.8 V and strictly lower than -
1.0 V, or
- at least -1.0 V and not greater than -0.70
V, or
- strictly greater than -0.70 V and not
greater than -0.5 V,
in comparison with a saturated Hg/Hg2SO4/K2SO4
reference electrode to an electrode comprising an
insulating substrate coated with a film of
molybdenum or a current density of between 1 and 30
mA.cm-2;
7
- the atomic ratio of the elements A and B in the
electrolyte solution is at least 0.2, for example at
least 0.8, and not greater than 1.5, for example not
greater than 1.2;
- the atomic ratio of the elements A and B in the
electrolyte solution is:
- at least 0.2 and strictly lower than 0.8,
or
- at least 0.8 and not greater than 1.2, or
- strictly greater than 1.2 and not greater
than 1.5;
- the absorber thin film for photovoltaic cells
is the A-B-C2 type and B comprises a mixture of indium
and gallium, and wherein the ratio of the indium and
gallium concentrations in the electrolyte solution is at
least 0.2, for example at least 0.8, and not greater than
1.5, for example not greater than 1.2; and
- the absorber thin film for photovoltaic cells
is the A-B-C2 type and B comprises a mixture of indium
and gallium, and wherein the ratio of the indium and
gallium concentrations in the electrolyte solution is:
- at least 0.2 and strictly lower than 0.8,
or
- at least 0.8 and not greater than 1.2, or
- strictly greater than 1.2 and not greater
than 1.5.
The invention also relates to a method for
manufacturing a solar cell, comprising a method for
preparing an absorber thin film for photovoltaic cells
according to the invention.
The invention will be better understood from
reading the description that follows, provided
8
exclusively as an example and with reference to the
appended drawings in which:
- figure 1 schematically shows a stack of thin films
forming a photovoltaic cell,
- figure 2 shows the various steps of a method
according to the invention,
- figure 3 is a graph illustrating the influence of the
gallium content in the absorber thin film on the
conversion efficiency of a photovoltaic cell
comprising such an absorber thin film,
- figure 4 is a graph illustrating the influence of the
annealing time in reducing atmosphere of the absorber
thin film on the conversion efficiency of a
photovoltaic cell comprising such an absorber thin
film,
- figure 5 is a graph illustrating the influence of the
annealing temperature in reducing atmosphere of an
absorber thin film on the conversion efficiency of a
photovoltaic cell comprising such an absorber thin
film,
- figure 6 is an X-ray diffraction diagram of an
absorber thin film after annealing in reducing
atmosphere,
- figures 7 and 8 show temperature profiles of the
annealing in reducing atmosphere and of the annealing
in selenium vapor of an exemplary embodiment of the
invention.
For reasons of clarity, the various elements in
the figures are not necessarily shown to scale.
The invention relates to a method for preparing
an absorber thin film for photovoltaic cells. The
absorber thin film is based on an A-B-C2 or A2-(Dx, E1-x)-
C4 alloy, where 0≤x≤1 and A is an element or a mixture of
9
elements selected from group 11, B is an element or an
element selected from group 13, C is an element or a
mixture of elements selected from group 16, D is an
element or a mixture of elements selected from group 12
and E is an element or mixture of elements selected from
group 14.
According to an embodiment of the invention, A is
an element or a mixture of elements selected from copper
(Cu) and silver (Ag).
According to an embodiment of the invention, B is
an element or a mixture of elements selected from
aluminum (Al), gallium (Ga), and indium (In).
According to an embodiment of the invention, C is
an element or a mixture of elements selected from sulfur
(S) and selenium (Se).
According to an embodiment of the invention, D is
an element or a mixture of elements selected from zinc
(Zn) and cadmium (Cd).
According to an embodiment of the invention, E is
an element or a mixture of elements selected from silicon
(Si), germanium (Ge), tin (Sn) and lead (Pb).
According to an embodiment of the invention, the
absorber thin film is based on a CupAg1-p(InxGayAlz)Se2
alloy where 0≤p≤1, 0≤x≤1, 0≤y≤1, 0≤z≤1 and x+y+z = 1.
According to an embodiment of the invention, the
absorber thin film is based on a Cu(InxGa1-x)Se2 alloy
where 0≤x≤1.
According to an embodiment of the invention, the
absorber thin film is based on a Cu2(SnxZn1-x)(SeyS1-y)4
alloy, where 0≤x≤1 and 0≤y≤1.
As shown in figure 2, the method according to the
invention comprises:
- a step S1 of electrodeposition,
10
- a step S2 of annealing in reducing atmosphere, and
- a step S3 of supplying at least one element from
group 16.
According to an embodiment of the invention,
prior to the electrodeposition step S1, the method
comprises a step of preparation of an electrolyte.
Advantageously, the method according to the
invention uses the deposition of an oxide film of
elements A, B, D or E. This deposition of the oxide
and/or hydroxide film may be carried out by electrolysis
at low temperature, at least 5°C and not greater than
95°C, and does not require costly vacuum or vapor
deposition equipment.
The electrolyte may, for example, be an aqueous
solution containing a mixture of salts of A and B in
order to prepare a follow-up of the preparation of an AB-
C2 film or a mixture of salts of A, D and/or E for a
A2-(DxE1-x)-C4 film. The salts are mixed in the presence
of an oxygen donor species, and according to one
embodiment, the salts of the elements A, B, D and E may
be nitrates. The oxygen donor species may be a nitrate,
or even oxygen gas or hydrogen peroxide or hypochlorite
ions.
The aqueous solution may also comprise a support
electrolyte to improve its conductivity.
Advantageously, the aqueous electrolyte solutions
according to the invention are stable and do not have any
tendency to precipitation.
The electrodeposition can be carried out by
applying a voltage to a deposition electrode in
comparison with a reference electrode or a current
density. The deposition electrode may comprise an
insulating plate, for example, a glass plate coated with
11
a molybdenum film. The reference electrode may be a
saturated calomel electrode or a mercury sulfate
electrode or an Ag/AgCl electrode.
According to an embodiment, the electrodeposition
is carried out at a temperature of at least 5°C and not
greater than 95°C, for example at a temperature of at
least 30°C, preferably at least 60°C and not greater than
83°C, for example substantially equal to 80°C.
According to the embodiment, the element A is
copper and the element B is selected from indium, gallium
and aluminum or mixtures thereof, the electrodeposition
is carried out by imposing on the electrode, in
comparison with a saturated mercury sulfate reference
electrode, a voltage of at least -1.8 V, for example at
least -1 V, and not greater than -0.5 V, for example not
greater than -0.70 V. A current density of between 1.0
and 30 mA.cm-2 may also be imposed.
The oxide and/or hydroxide film is deposited on
the electrode comprising an insulating substrate coated
with a molybdenum film in the form of a thin film of
which the thickness is controlled by the quantity of
electricity exchanged during the reaction, the reaction
temperature, and the reaction time.
The deposition speeds are high, about 3.5 microns
per hour at 25°C and more than 10 microns per hour at
80°C.
Typically, the deposition is carried in aqueous
solution during a period of about 10 to 20 minutes and
leads to the formation of an oxide film having a
thickness of between 600 and 2000 nanometers, for example
between 800 and 1200 nanometers.
The composition of the deposited absorber film is
controlled on the one hand by the salt composition of the
12
aqueous solution and by the voltage or current density
imposed on the deposition electrode.
The inventors found that the photovoltaic cells
obtained with absorber films comprising A-B-C2 alloys
have improved conversion efficiencies when the atomic
ratio of the elements A and B in the electrolyte solution
is at least 0.2, for example greater than 0.8, and not
greater than 1.5, for example not greater than 1.2, for
example not greater than 1, for example substantially
equal to 1.
For example, in the case of an absorber
comprising a Cu(InxGa1-x)Se2 alloy, the inventors observed
that the conversion efficiency is improved when the
Cu/(In+Ga) atomic ratio is close to 1 and the gallium
content: Ga/(In+Ga) is at least 0.2 and not greater than
0.35, for example substantially equal to 0.3.
Figure 3 illustrates the influence of the gallium
content in the Cu(InxGa1-x)S2 absorber film on the
conversion efficiency of the photovoltaic cell comprising
said absorber film.
It appears that the conversion efficiency is
optimal for a gallium Ga(In+Ga) content of between 0.2
and 0.35, preferably substantially equal to 0.3. The
inventors observed by X-ray diffraction that the oxide
and/or hydroxide deposits have an amorphous appearance.
The method according to the invention further
comprises a step of annealing in reducing atmosphere of
the oxide thin film obtained during the
electrodeposition.
Figure 4 illustrates the influence of the
temperature of the annealing step in reducing atmosphere
on the conversion efficiency of a photovoltaic cell
comprising a Cu(InxGa1-x)S2 absorber film, the annealing
13
step being carried out in ethanol atmosphere for 20
seconds.
As shown in figure 4, in order to improve the
conversion efficiency, it is advantageous for the
annealing step in reducing atmosphere to be carried out
at a temperature of at least 300°C and not greater than
650°C, for example at a temperature of at least 500°C and
not greater than 575°C, for example at a temperature
substantially equal to 550°C.
According to an embodiment of the invention, the
step of annealing in reducing atmosphere may last between
20 seconds and 15 minutes, for example between 20 seconds
and 5 minutes.
Figure 5 illustrates the influence of the
duration of the annealing step in reducing atmosphere on
the conversion efficiency of a photovoltaic cell
comprising a Cu(InxGa1-x)S2 absorber film, the annealing
step being carried out in ethanol atmosphere at a
temperature of 550°C.
As shown in figure 5, in order to increase the
conversion efficiency, it is advantageous for the
annealing step in reducing atmosphere to last between 20
seconds and 50 seconds.
The annealing step in reducing atmosphere can be
carried out using a H2/N2 mixture or alcohol vapors,
hydrocarbon vapors or even ammonia vapors.
The annealing step in reducing atmosphere serves
to reduce the oxide film to metal alloy.
Figure 6 is an X-ray diffraction diagram of a
deposit of a mixture of copper, gallium and indium oxide
after the annealing step in reducing atmosphere. The
diffraction diagram shown in figure 6 serves to identify
a Cu9In4 and Cu9Ga4 cubic phase and a large peak
14
corresponding to the molybdenum substrate. The diagram
can be indexed as a mixed Cu9(Inx, Ga1-x)4 phase having a
cubic structure and an intermediate parameter between
that of Cu9In4 and Cu9Ga4.
As shown in the diagram in figure 6, on
completion of the annealing step in reducing atmosphere,
all the oxides have been reduced.
The method according to the invention further
comprises a step of adding at least one element from
group 16 in order to form an A-B-C2 or A2-(Dx, E1-x)-C4
thin film, where 0≤x≤1. For example, the addition step
may be a conventional selenization step that leads to the
formation of a CIS or CIGS compound.
The invention also relates to a method for
manufacturing a solar cell comprising the preparation of
an absorber thin film according to the invention and the
steps of supplementing the cells with a buffer film of
CdS, for example by chemical bath deposition (CBD) and
the deposition of the final transparent conductive oxide
(TCO) film.
The solar cells can then be characterized
conventionally.
Example of preparation of a photovoltaic cell
comprising a Cu(InxGa1-x) Se2 absorber film.
A solution of 15x10-3 mol.l-1 copper nitrate, 8x10-
3 mol.l-1 indium nitrate and 8x10-3 mol.l-1 gallium nitrate
is mixed with a support electrolyte of 0.1 mol.l-1 sodium
nitrate at a pH of 1.86. The temperature of the solution
is set at 80°C.
A deposition electrode consisting of a glass
plate coated with a 500 nm thick molybdenum film is used.
An electrolysis is carried out at a voltage
difference of -0.825 V between the deposition electrode
15
and a saturated mercury sulfate electrode (saturated
Hg/Hg2SO4 / K2SO4) for about 8 minutes. The composition
of the oxide film analyzed by X-ray fluorescence is 44
atomic % copper, 38 atomic % indium and 18 atomic %
gallium, the gallium/ (indium+gallium) ratio is therefore
0.32 and the copper/(indium+gallium) ratio is 0.8. The
thickness of the oxide film is about 1 micron.
A rapid annealing is carried out in ethanol vapor
in a lamp furnace for about 20 seconds at 580°C according
to the temperature profile shown in figure 7.
After this annealing in reducing atmosphere, a
Cu-In-Ga alloy film is obtained having a structure close
to Cu9(In,Ga)4, as confirmed by the diffraction diagram
shown in figure 6.
The reducing treatment is followed by a longer
annealing under selenium vapor pressure according to the
temperature profile shown in figure 8.
After this second heat treatment in selenium
vapor, the thickness of the film is about 2.8 microns,
the film contains about 21 atomic % copper, 54 atomic %
selenium, 19 atomic % indium and 7 atomic % gallium,
corresponding to a (selenium/copper+indium+gallium) ratio
of about 1.2. The film then undergoes a conventional
cyanide pickling treatment followed by the deposition of
a CdS film by chemical deposition.
A final double film of slightly doped zinc oxide
followed by an aluminum-doped zinc oxide film is
deposited by cathode sputtering.
The conversion efficiency of the solar cell
obtained is measured on the solar simulator in standard
conditions (AM 1.5).
The conversion efficiency is about 7.8% on an
area of 0.1 cm², the open circuit voltage is about 0.375
16
V, the open-circuit current density is about 35 mA.cm-²
and the form factor is about 60%. The invention is not
limited to the embodiments described and must be
interpreted as nonlimiting, and encompassing any
equivalent embodiment.
17
I/We claim:
1. A method for preparing an A-B-C2 or A2-(Dx,E1-x)-C4
absorber thin film for photovoltaic cells where 0≤x≤1, A
is an element or a mixture of elements selected from
group 11, B is an element or a mixture of elements
selected from group 13, C is an element or a mixture of
elements selected from group 16, D is an element or a
mixture of elements selected from group 12 and E is an
element or a mixture of elements selected from group 14,
said method comprising the following successive steps:
 electrodeposition of a thin film of a mixture of
oxides and/or hydroxides comprising, for an A-BC2
film, at least one element from group 11 and
one element from group 13 or, for an A2-(Dx,E1-x)-
C4 film, at least one element from group 11, at
least one element from group 12 if x>0, and at
least one element from group 14 if x<1,
 annealing of the thin film in a reducing
atmosphere,
 supply of at least one element from group 16 in
order to form an A-B-C2 or A2-(Dx,E1-x)-C4 thin
film where 0≤x≤1.
2. The method as claimed in claim 1, wherein the
electrodeposition is carried out at a temperature of at
least 5°C and not greater than 95°C.
3. The method as claimed in either of claims 1 and 2,
wherein the annealing in reducing atmosphere is carried
out at a temperature of at least 300°C and not greater
than 650°C.
18
4. The method as claimed in any one of the preceding
claims, wherein the annealing in reducing atmosphere has
a duration of at least 20 seconds and not greater than 15
minutes.
5. The method as claimed in any one of the preceding
claims, wherein, prior to the electrodeposition of the
thin film, an aqueous solution is prepared containing a
mixture of salts of A and B for an A-B-C2 film or a
mixture of salt of A, D and/or E for an A2-(Dx,E1-x)-C4
film, in the presence of at least one oxygen donor
species.
6. The method as claimed in claim 5, wherein the oxygen
donor species consists of a nitrate ion, or of dioxygen,
or of hydrogen peroxide or of hypochlorite ions.
7. The method as claimed in any one of the preceding
claims, wherein A is copper or silver or a mixture of
copper and silver and C is selenium or sulfur or a
mixture of selenium and sulfur.
8. The method as claimed in any one of the preceding
claims, wherein the absorber thin film for photovoltaic
cells is the A-B-C2 type and B comprises one or more
elements selected from indium, gallium, aluminum or
mixtures thereof.
9. The method as claimed in claim 8, wherein the
electrodeposition is carried by imposing a current
density of between 1 and 30 mA.cm-2 or a voltage of at
least -1.8 V and not greater than -0.5 V to an electrode
comprising an insulating substrate coated with a film of
19
molybdenum and a saturated Hg/Hg2SO4/K2SO4 reference
electrode.
10. The method as claimed in either of claims 8 and 9,
wherein the atomic ratio of the elements A and B in the
electrolyte solution is at least 0.2 and not greater than
1.5.
11. The method as claimed in any one of the preceding
claims, wherein the absorber thin film for photovoltaic
cells is the A-B-C2 type and B comprises a mixture of
indium and gallium, and wherein the ratio of the indium
and gallium concentrations in the electrolyte solution is
at least 0.2 and not greater than 1.5.
12. A method for manufacturing a solar cell,
characterized in that it comprises a method for preparing
an absorber thin film for photovoltaic cells as claimed
in one of the preceding claims.