U.S. patent application number 15/103628 was filed with the patent office on 2016-10-27 for persulfate bath and method for chemically depositing a layer.
The applicant listed for this patent is CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE- CNRS, ELECTRICITE DE FRANCE. Invention is credited to Thibaud Hildebrandt, Nicolas Loones, Negar Naghavi, Nathanaelle Schneider.
Application Number | 20160312346 15/103628 |
Document ID | / |
Family ID | 50289928 |
Filed Date | 2016-10-27 |
United States Patent
Application |
20160312346 |
Kind Code |
A1 |
Hildebrandt; Thibaud ; et
al. |
October 27, 2016 |
PERSULFATE BATH AND METHOD FOR CHEMICALLY DEPOSITING A LAYER
Abstract
A chemical bath for depositing a layer made from at least metal
and sulphur is described. A method for depositing such a layer is
also presented. The bath comprises, in solution: a metal salt
comprising a metal chosen from at least one of the elements from
groups BIB and MA of the periodic table; and a sulphur precursor.
The bath further comprises a persulfate compound.
Inventors: |
Hildebrandt; Thibaud;
(Tallard, FR) ; Naghavi; Negar; (Paris, FR)
; Loones; Nicolas; (Nanterre, FR) ; Schneider;
Nathanaelle; (Paris, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ELECTRICITE DE FRANCE
CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE- CNRS |
Paris
Paris |
|
FR
FR |
|
|
Family ID: |
50289928 |
Appl. No.: |
15/103628 |
Filed: |
December 9, 2014 |
PCT Filed: |
December 9, 2014 |
PCT NO: |
PCT/FR2014/053248 |
371 Date: |
June 10, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 18/1204 20130101;
C23C 2/04 20130101; Y02P 70/50 20151101; Y02E 10/543 20130101; C23C
18/1216 20130101; H01L 31/1828 20130101 |
International
Class: |
C23C 2/04 20060101
C23C002/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 12, 2013 |
FR |
13 62537 |
Claims
1. A chemical bath for depositing a layer based on at least metal
and sulfur, the chemical bath comprising, in solution: a metal salt
comprising a metal selected from among at least one of the elements
of groups IIB and IIIA of the periodic table; and a sulfur
precursor; wherein the chemical bath further comprises a persulfate
compound.
2. The chemical bath according to claim 1, wherein the persulfate
compound is from a group consisting of ammonium persulfate of
chemical formula (NH.sub.4).sub.2S.sub.2O.sub.8, sodium persulfate
of chemical formula Na.sub.2S.sub.2O.sub.8, and potassium
persulfate of chemical formula K.sub.2S.sub.2O.sub.8.
3. The chemical bath according to claim 1, wherein a concentration
between 10.sup.-5 mol/L and 10 mol/L of persulfate is provided in
the chemical bath, for a concentration between 0.05 mol/L and 1
mol/L of sulfur precursor.
4. The chemical bath according to claim 3, wherein the metal salt
is in a solution selected from among: zinc sulfate, zinc acetate,
and zinc chloride, at a concentration between 0.01 mol/L and 1
mol/L.
5. The chemical bath according to claim 3, further comprising an
ammonia solution at a concentration between 0.1 mol/L and 10
mol/L.
6. The chemical bath according to claim 1, wherein the sulfur
precursor is in a solution of thiourea CS(NH.sub.2).sub.2.
7. The chemical bath according to claim 1, wherein the metal is an
element from column IIB.
8. The chemical bath according to claim 7, wherein the metal is
zinc.
9. A method for chemically depositing a layer based on at least
metal and sulfur, the method comprising: depositing, in a solution
in a chemical bath, a metal salt comprising a metal selected from
among at least one of the elements of groups IIB and IIIA of the
periodic table; and depositing, in the solution in the chemical
bath, a sulfur precursor; wherein a persulfate compound is further
provided in said chemical bath.
10. The method of claim 9, wherein the layer is based on a metal
sulfide.
11. The method of claim 9, wherein the layer is based on a metal
oxysulfide.
12. The method of claim 9, wherein the temperature of the chemical
bath during deposition is between 40.degree. C. and 100.degree.
C.
13. The method of claim 9, wherein the layer based on metal and
sulfur is deposited on a layer having photovoltaic properties, said
layer having photovoltaic properties forming an absorber of a
thin-film solar cell.
14. The method of claim 13, wherein the absorber is based on a
chalcopyrite compound among Cu(In,Ga)(S,Se).sub.2, Cu.sub.2(Zn,
Sn)(S, Se).sub.4, and their derivatives.
Description
TECHNICAL FIELD
[0001] The invention relates to the field of chemical deposition
baths for the deposition of layers based on sulfur and metal. It
also relates to methods for chemically depositing a layer based on
metal and sulfur.
TECHNOLOGICAL BACKGROUND
[0002] Methods for Chemical Bath Deposition (CBD) are commonly used
in industry, for example to fabricate alloys in thin layers ("thin
films"). These methods are particularly suitable for large-scale
deposition to cover large areas exceeding 60.times.30 cm.sup.2. In
addition, this technique is widely used in industry due to its low
cost and its technical simplicity.
[0003] Chemical bath deposition can thus be considered in the
fabrication of certain thin layers of photosensitive devices. More
particularly, the absorber layers in some photosensitive devices
are generally covered by a so-called "buffer" layer made of an
alloy comprising metal and sulfur. Generally, this buffer layer
consists of an alloy of cadmium sulfur CdS.
[0004] The toxicity of cadmium prompts us to look for alternative
materials for the buffer layer, such as zinc sulfide ZnS and its
derivatives. However, the deposition rate of these alloys on any
surface, and particularly on other thin layers such as light
absorbers, is not optimal. Obtaining a chemical deposition that
meets industry requirements can involve times considered to be
long, exceeding 15 minutes or even an hour. Such lengthening of the
deposition time adds to manufacturing costs and penalizes the
entire chain of production of a device.
[0005] For this reason, special attention is being paid to finding
technical means for achieving the deposition of a layer based on at
least sulfur and a metal, by CBD.
[0006] To reduce the deposition time of such a layer, it is known
to increase either the concentration of reagents in the chemical
bath or the temperature, or to preheat the reagents. However, these
three means have the disadvantage of causing an increase in the
consumption of materials and energy. They may also present the risk
of damage to the deposition surface, or may involve additional
steps in the chain of production of a type that incurs additional
costs.
[0007] Another solution for accelerating the deposition rate is to
use a specific sulfur precursor in a chemical bath. Generally, the
chemical baths for depositing a layer of metal and sulfur make use
of a thiourea solution as sulfur precursor. Document US2013/0084401
proposes replacing thiourea by thioacetamide. This alternative,
however, has the disadvantage that thioacetamide is a highly toxic
compound and is therefore not very suitable for an industrial
application.
[0008] Another alternative suggested in document DE102009015063A1
consists in adding hydrogen peroxide H.sub.2O.sub.2 to a chemical
bath provided for deposition of a thin layer based on sulfur and
metal. This solution is not suitable for industrial applications
where the deposition surface is fragile, given the corrosive nature
of the H.sub.2O.sub.2 additive.
[0009] For these reasons, we seek a means for increasing the
deposition rate in a chemical bath of a layer comprising at least
sulfur and a metal, and which is compatible with a wide range of
industrial applications.
DISCLOSURE OF INVENTION
[0010] To address the problems described above, the present
invention provides a chemical bath for depositing a layer based on
at least metal and sulfur. This bath comprises, in solution: [0011]
a metal salt comprising a metal selected from among at least one of
the elements of groups JIB and IIIA of the periodic table; and
[0012] a sulfur precursor.
[0013] This bath further comprises a persulfate compound.
[0014] A bath for the chemical deposition of a layer comprising
sulfur and a metal from groups IIB or IIIA of the periodic table is
thus optimized by adding a persulfate-type compound into the bath
solution mixture. The applicant has found that the addition of
persulfate produces in the mixture an effect comparable to that of
a reaction accelerator. The persulfate appears to interact with the
sulfur precursor and accelerates the formation of the sulfur- and
metal-based alloy on the deposition surface. This effect seems to
be particularly pronounced when the metal belongs to groups IIIA or
IIB, such as zinc Zn or indium In for example.
[0015] In addition to increasing the deposition rate, the applicant
has found that the addition of persulfate allows obtaining
homogeneous metal- and sulfur-based layers on large surfaces. The
addition of persulfate to the chemical bath thus allows obtaining
more homogeneous depositions over a large surface area compared to
CBD depositions involving baths without this additive.
[0016] Another advantageous effect obtained by the addition of
persulfate lies in the fact that the deposition can be done without
preheating the bath reagents. By eliminating a prior preheating
step, it is possible to increase the production rate at the
industrial scale. However, it is also possible to preheat the
reagents, enabling the bath described above to easily be prepared
in existing production lines.
[0017] These different effects are independent of the surface on
which the deposit is made. In particular, the optimized bath
described above allows for example making deposits on glass, metal
substrates, or semiconductors, as well as on compounds having
photovoltaic properties such as absorbers.
[0018] These effects have been observed in different experimental
configurations, and seem to occur regardless of the concentration
of reagents in the chemical bath or the temperature of the
bath.
[0019] According to one advantageous embodiment, the compound may
be chosen from the group consisting of ammonium persulfate of
chemical formula (NH.sub.4).sub.2S.sub.2O.sub.8, sodium persulfate
of chemical formula Na.sub.2S.sub.2O.sub.8, and potassium
persulfate of chemical formula K.sub.2S.sub.2O.sub.8.
[0020] These compounds may advantageously be in a solution that is
miscible with the other reagents of the chemical bath. It has been
noted that the deposition rate and the homogeneity of the obtained
layer can be optimized in a particularly visible manner when at
least one of these additives is used. However, the active component
of the inorganic additive seems to be found in persulfate, and
other persulfate-based compounds could therefore be envisaged.
[0021] According to one embodiment, a concentration between
10.sup.-5 mol.L.sup.-1 and 1 mol/L of persulfate may be provided in
the bath, for a concentration between 0.05 mol/L and 1 mol/L of
sulfur compound.
[0022] Such a reaction mixture in the bath corresponds to a
compromise between quantity of reagents used and rapidity of
deposition. By keeping the amount of additive and of sulfur
compound low, it is possible to achieve significant savings during
production at an industrial scale. This is due to less waste of
reagents.
[0023] Advantageously, the solution containing the metal salt may
be a solution selected from among: zinc sulfate, zinc acetate, and
zinc chloride, at a concentration between 0.01 mol/L and 1
mol/L.
[0024] The metal salt may include other metals from groups IIB and
IIIA of the periodic table. However, a zinc metal salt in the bath
provided a particularly pronounced reduction of the deposition
time. In comparison to CBD deposition in a bath without persulfate
and using a zinc metal salt, the invention achieves a deposition
rate which is up to eight times higher.
[0025] A concentration between 0.01 mol/L and 1 mol/L of metal salt
allows reducing the amount of raw material used to deposit the
layer.
[0026] Advantageously, the bath may further comprise an ammonia
solution at a concentration of between 0.1 mol/L and 10 mol/L.
[0027] The use of an ammonia solution gives a basic pH to the
chemical bath, in order to initiate hydrolysis of the sulfur
precursor so that it reacts with the metal salt.
[0028] According to one embodiment, the solution containing the
sulfur compound may be a solution of thiourea
CS(NH.sub.2).sub.2.
[0029] Thiourea is a sulfur precursor that is particularly suitable
for the deposition of layers comprising sulfide. It is commonly
used in the photosensitive devices industry, for example.
[0030] Thiourea enables particularly rapid deposition in the
presence of persulfate and of a metal salt. The deposition rates
obtained when thiourea is used can thus be less than 5 minutes for
a layer of metal sulfide or metal oxysulfide that is 20 nm
thick.
[0031] More particularly, the metal may be an element from column
IIB.
[0032] Metals of column IIB are of particular interest to the
photosensitive devices industry. In these devices, metals such as
cadmium or zinc may be present in the buffer layer between the
absorber and the front electrical contact of a photosensitive cell.
Elements of column IIB are therefore particularly suitable for a
chemical bath intended for the creation of buffer layers.
[0033] In one particular embodiment, the metal may be zinc.
[0034] Deposition of a zinc sulfide or oxysulfide layer by CBD is
of interest for example in the photosensitive devices industry due
to its optical properties and non-toxicity. A layer made of such an
alloy is an effective and non-toxic alternative to buffer layers of
CdS, while allowing the transmission of more radiation of
wavelengths below 500 nm.
[0035] A layer of zinc sulfide or oxysulfide has a higher energy
band gap than a CdS layer, therefore transmitting more light at
wavelengths below 500 nm than a CdS layer.
[0036] In addition, a layer of zinc sulfide or oxysulfide has
optical transmission properties equivalent to those of layers of
zinc oxide ZnO, which are often used to form the front electrical
contacts of photosensitive devices.
[0037] The invention also relates to a method for chemically
depositing a layer based on at least metal and sulfur, in a bath
comprising, in solution: [0038] a metal salt comprising a metal
selected from among at least one of the elements of groups IIB and
IIIA of the periodic table; and [0039] a sulfur precursor.
[0040] In addition, a persulfate compound is provided in the
bath.
[0041] CBD deposition of a layer based on at least metal and sulfur
with the addition of persulfate in the chemical bath offers several
advantages, described above. The addition of persulfate in the bath
increases the deposition rate, allows achieving more homogeneous
layers, and can save preparation time due to the possibility of
eliminating an earlier step of preheating the chemical bath
reagents.
[0042] In one particular embodiment, the layer may be based on a
metal sulfide.
[0043] The metal sulfide, for example ZnS, may be an alloy
particularly suitable for applications in photosensitive devices.
For example, it may be used as a buffer layer on absorbers of
photosensitive devices.
[0044] In another embodiment, the layer may be based on a metal
oxysulfide.
[0045] Metal oxysulfides, for example Zn(S,O), Zn(S,O,OH), or
In.sub.x(S,O).sub.y, In.sub.x(S,O,OH).sub.y, where 0<x<2 and
0<y<3, have optical properties that are particularly suitable
for the requirements of the photosensitive devices industry. They
may also be suitable for use as buffer layers on a photosensitive
layer.
[0046] Advantageously, the bath temperature during deposition may
be between 40.degree. C. and 100.degree. C.
[0047] A deposition temperature below 100.degree. C. and in
particular below 70.degree. C. allows a work environment that is
less damaging to devices comprising alloys having a low melting
point, without being penalized by an increase in the deposition
duration. In addition, by reducing the temperature of the reaction
medium, it is possible to save energy due to less heating of the
CBD deposition bath. These savings increase with the size of the
bath, which can reach several square meters at the industrial
scale.
[0048] According to one embodiment, the layer based on metal and
sulfur may be deposited on a layer having photovoltaic properties,
said layer having photovoltaic properties forming the absorber of a
thin-film solar cell.
[0049] In this manner, the layer based on metal and sulfur can
represent a buffer layer deposited on an absorber of a thin-film
photovoltaic cell, to interface the absorber with a front
electrical contact. The quality of the interface between the
absorber of a photosensitive cell and the buffer layer is crucial
to achieving high conversion efficiency in the resulting device.
Applying the chemical deposition method described above to the
deposition of a buffer layer on an absorber yields homogeneous
layers, having half as many defects as buffer layers deposited in a
bath containing no persulfate, deposited in less than 10 minutes
and without damaging the absorber itself.
[0050] Due to the quality of the buffer layer obtained by
implementing the method, the resulting photovoltaic device can have
a conversion efficiency exceeding 14%.
[0051] It should be noted that using persulfate as an additive in a
chemical bath for the deposition of a buffer layer on a
photovoltaic cell absorber is counterintuitive. Persulfate, in
particular ammonium persulfate, is generally used in industry as a
cleaning and etching agent due to its highly oxidizing character.
The applicant has noted that the addition of persulfate to a
chemical bath as described above does not give the bath corrosive
properties, which protects the surface on which the deposition
takes place from chemical attack.
[0052] The absorber may be based on a chalcopyrite compound among
Cu(In,Ga)(S,Se).sub.2, Cu.sub.2(Zn,Sn)(S,Se).sub.4, and their
derivatives.
[0053] These compounds may include, for example, Cu(In,Ga)Se.sub.2,
CuInSe.sub.2, CuInS.sub.2, CuGaSe.sub.2, Cu.sub.2(Zn,Sn)S.sub.4,
and Cu.sub.2(Zn,Sn)Se.sub.4. When these absorbers contain zinc and
tin, these compounds are sometimes called CZTS.
[0054] The solar cell absorbers listed above correspond to
absorbers of thin-film cells of CIGS and CZTS type and their
derivatives having conversion efficiencies that can exceed 20%.
Making use of the method described above for depositing a buffer
layer on these absorbers is particularly advantageous given the
performance gains this provides. For example, by thus depositing a
buffer layer of ZnS, Zn(S,O), or Zn(S,O,OH) on an absorber based on
a chalcopyrite compound, it is possible to obtain a conversion
efficiency exceeding 14% and an open circuit voltage and
short-circuit current of the end device that are greater than those
observed in devices obtained by other deposition methods.
DESCRIPTION OF FIGURES
[0055] The method of the invention will be better understood by
reading the following description of some exemplary embodiments
presented for illustrative purposes but in no way limiting, and
from observing the following drawings in which:
[0056] FIG. 1 is a schematic representation of a sample that is to
receive the deposition of a layer of metal and sulfur; and
[0057] FIG. 2 is a schematic representation of the procedure for
preparing a chemical bath; and
[0058] FIG. 3 is a graph comparing the measured deposition times to
obtain different layer thicknesses by three different processes;
and
[0059] FIG. 4 is a schematic representation of a photosensitive
device; and
[0060] FIG. 5 is a graph comparing the quantum efficiencies of two
layers made of different materials, as a function of the wavelength
received.
[0061] For the sake of clarity, the dimensions of the various
elements represented in these figures are not necessarily in
proportion to their actual dimensions. In the figures, identical
references correspond to identical elements.
DETAILED DESCRIPTION
[0062] The invention relates to an improved chemical deposition
bath and an improved CBD deposition method. The improvement aims in
particular to significantly increase the deposition rate. Other
advantageous effects have also been obtained in the context of the
invention, such as increased deposition quality for example.
[0063] In the embodiments described below by way of example, the
particular case of deposition by CBD of a buffer layer on a
photovoltaic cell absorber will be described. However, the
invention can also be applied to deposition on any other type of
surface, as will be restated further below.
[0064] In the context of depositing a thin layer comprising at
least sulfur and a metal, FIG. 1 illustrates an example of an
initial sample 100, comprising a substrate 101, a rear metal
contact 102, and an absorber layer 103. The initial sample 100
shown therefore represents an unfinished portion of a thin-film
photosensitive device. As an example, the absorber 103 intended to
convert radiation into current may be a chalcopyrite compound such
as one of the compounds among Cu(In,Ga)(S,Se).sub.2,
Cu.sub.2(Zn,Sn)(S,Se).sub.4, and their derivatives. These
derivatives may include, for example,
[0065] Cu(In,Ga)Se.sub.2, CuInSe.sub.2, CuInS.sub.2, CuGaSe.sub.2,
Cu.sub.2(Zn,Sn)S.sub.4, or Cu.sub.2(Zn,Sn)Se.sub.4, more commonly
called CIGS and CZTS.
[0066] In order to complete the fabrication of this photosensitive
device, the invention proposes a chamber 200 for the chemical bath,
schematically represented in FIG. 2. As in most chemical baths for
CBD deposition, the chamber 200 may be closed by a cover 220. This
chamber 200 contains a solution 50 consisting of a mixture of
reagents in the chosen concentrations. The sample 100 rests in this
solution 50. Means for heating this reaction medium may be
provided. In FIG. 2, such a means is represented by a water bath
210 surrounding the chamber containing the reaction medium. A motor
230 may also be used to drive a stirring mechanism that stirs the
solution 50. FIG. 2 also shows a summary of the steps for obtaining
the solution forming the reaction mixture 50.
[0067] In the example illustrated in FIG. 2, the chemical bath is
configured for deposition of a buffer layer of a photovoltaic
device. For this reason, it is prepared from a first aqueous
solution comprising a metal salt 10, represented as being zinc
sulfate ZnSO.sub.4. A second aqueous solution comprising a sulfur
precursor 20 is also provided. This second solution is represented
as being thiourea, of chemical formula CS(NH.sub.2).sub.2. Ammonia
30, to give the reaction mixture a basic pH, may also be provided.
A medium that is basic due to the presence of ammonia promotes the
reaction of the precursor with the metal salt. Finally, a fourth
aqueous solution comprising a persulfate-based inorganic additive
40 is prepared. This fourth solution 40 is represented as
comprising ammonium peroxydisulfate of chemical formula
(NH.sub.4).sub.2S.sub.2O.sub.8.
[0068] Alternatives to the first three solutions can be envisaged,
as will be described below.
[0069] These four solutions 10, 20, 30, 40, are then mixed to
create a reaction mixture 50. The reaction mixture 50 is the
solution into which the sample 100 is dipped.
[0070] Advantageously, the addition of peroxydisulfate
significantly reduces the time required to achieve deposition of
ZnS on the absorber. To illustrate the time saved, the graph of
FIG. 3 compares the rates of deposition of a thin layer by CBD,
measured under three different conditions.
[0071] The deposited ZnS layer may include oxygen and form a layer
of Zn(S,O) or Zn(S,O,OH) type. "ZnS layer" will be used hereinafter
to refer to a layer of pure ZnS as well as to a layer of Zn(S,O) or
Zn(S,O,OH).
[0072] Curve 301 shows the time required to deposit ZnS layers of
different thicknesses, when the reaction mixture 50 is in a
conventional configuration. "Conventional" is understood to mean a
thiourea concentration of 0.65 mol/L, a ZnSO.sub.4 concentration of
0.15 mol/L, and an ammonia concentration of 2 mol/L. The reagents
were all preheated to a temperature of 80.degree. C. before being
placed in a chamber brought to the same temperature of 80.degree.
C.
[0073] Curve 302 represents the time required to deposit ZnS layers
of different thicknesses, when the reaction mixture 50 comprises a
laboratory-tested configuration corresponding to a particularly
advantageous embodiment. It is characterized by a thiourea
concentration of 0.4 mol/L, a ZnSO.sub.4 concentration of 0.1
mol/L, and an ammonia concentration of 2 mol/L. No preheating of
the reagents is provided and the deposition temperature is
70.degree. C.
[0074] Curve 303 represents the time required to deposit ZnS layers
of different thicknesses, when the reaction mixture 50 comprises
the same characteristics as those associated with curve 302, but
with the addition of a concentration of 0.001 mol/L
peroxydisulfate. Table 1 below summarizes the three configurations
described above.
TABLE-US-00001 TABLE 1 Summary of the three deposition
configurations represented in FIG. 3 Thiourea ZnSO.sub.4 NH.sub.3
(NH.sub.4).sub.2S.sub.2O.sub.8 Deposition ZnS layer (mol/L) (mol/L)
(mol/L) (mol/L) T (.degree. C.) Conventional 0.65 0.15 2 80
deposition With 0.4 0.1 2 0.001 70 additive Without 0.4 0.1 2 70
additive
[0075] It is apparent from the evolution of the three curves 301,
302, and 303 of FIG. 3, that the addition of
(NH.sub.4).sub.2S.sub.2O.sub.8 in a chemical bath significantly
increases the rate of deposition of a ZnS layer. In particular, to
obtain a layer 20 nm thick, the bath optimized according to the
invention divides the deposition time by 2.5 compared to
conventional techniques, and by more than 5 compared to a
deposition conducted under the same conditions without the
additive.
[0076] In addition, it should be noted that the deposition
conditions in the applicant's chemical bath are more efficient in
materials saving and energy saving. This results from the lower
concentrations of reagents, lower deposition temperature, and no
preheating of the reagents.
[0077] The example described above can advantageously result in the
creation of a complete photovoltaic device as shown in FIG. 4.
[0078] FIG. 4 schematically illustrates a thin-film solar cell
comprising the same structural elements as those of FIG. 1. The
represented device 400 further comprises a buffer layer 104
deposited on the absorber by CBD, as described above. Over the
buffer layer 104 a first window layer 105, for example of intrinsic
zinc oxide or ZnMgO, can be deposited by known techniques such as
reactive sputtering, chemical vapor deposition, electrodeposition,
CBD deposition, or ILGAR.RTM. deposition. A front electrical
contact 106 can then be deposited. For example, it may be a layer
of aluminum-doped zinc oxide ZnO.
[0079] Other advantages inherent to using the chemical bath
described above for depositing a buffer layer are reflected in the
performance of the photovoltaic devices obtained.
[0080] Table 2 compares technical characteristics of solar cells
such as the solar cell of FIG. 4, comprising a chalcopyrite-type
CIGS absorber. A first cell comprises a ZnS buffer layer obtained
under conventional deposition conditions as described above in
relation to curve 301 of FIG. 3. A second cell comprises a ZnS
buffer layer obtained by a CBD deposition process involving the
bath of the invention, under conditions identical to those
described in relation to curve 303 of FIG. 3. A third cell
comprises a CdS buffer layer obtained by CBD under conventional
deposition conditions.
TABLE-US-00002 TABLE 2 Performance comparison of three solar cells.
Efficiency Form factor Voc Buffer layer (%) (%) (mV) Jsc
(mA/cm.sup.2) ZnS 13.7 71.8 611 31.3 Conventional CBD Standard
deviation +/-0.41 +/-1.9 +/-3.3 +/-0.52 ZnS 14.2 73.7 622 31.4 CBD
with additive Standard deviation +/-0.18 +/-0.99 +/-1.5 +/-0.18 CdS
13.8 73.7 619 30.1 Conventional CBD Standard deviation +/-0.13
+/-0.36 +/-4.6 +/-0.15
[0081] Each cell of Table 2 has a surface area of 5.times.5
cm.sup.2 and a buffer layer 20 nm thick.
[0082] The columns in Table 2 represent four parameters for each of
the three solar cells. The first column represents the conversion
efficiency of the solar cell. The second column represents the form
factor of each cell, providing an indication of the quality of the
interface between buffer layer and absorber. The third column
represents an open circuit voltage Voc. The higher this voltage,
the better the electrical properties of the cell. The fourth column
represents the short-circuit current Jsc. The higher this current,
the better the electrical properties of the cell.
[0083] For each cell of Table 2, and for each parameter, the
standard deviation of the corresponding value is indicated. This
information provides an estimate of the homogeneity of the cell.
The more a parameter varies within the cell, the higher the
associated standard deviation. Such instability is indicative of
structural defects in the cell, and all the more so in the buffer
layer which is the only layer presenting substantial differences
between the three compared cells.
[0084] It is apparent from the values of Table 2 that the cell
having a ZnS buffer layer formed by the CBD deposition method
developed in the context of the invention has a homogeneity that is
superior to the other cells. In particular, it has a higher
homogeneity than the cell having a ZnS layer formed by conventional
CBD deposition.
[0085] Moreover, the cell produced by the method of the invention
has a conversion efficiency that is greater than that of the other
cells, as well as improved electrical properties.
[0086] The form factor of the cell obtained by the method of the
invention is comparable to that of cells having a CdS buffer layer.
Nevertheless, the overall homogeneity of the cell created by CBD
deposition with the addition of persulfate is better. Therefore the
deposition method with the addition of persulfate is well suited to
the creation of photosensitive cells of large surface area, or more
generally to the creation of layers comprising metal and sulfur in
industrial settings.
[0087] Observation by electron microscope has confirmed these
observations concerning the structural quality of the deposition
obtained by implementing the method of the invention.
[0088] The chemical bath developed, and the method that uses it to
create a buffer layer, allow obtaining under adapted deposition
conditions a buffer layer of a non-toxic material, for example
cadmium-free.
[0089] The improved electrical and optical properties of the zinc
sulfide and oxysulfide buffer layers were analyzed by calculating
the quantum efficiency of the layer, in comparison to that of a CdS
buffer layer.
[0090] FIG. 5 shows a graph comparing the quantum efficiency
between 300 nm and 1100 nm of a ZnS buffer layer, represented by
curve 502, with that of a CdS buffer layer, represented by curve
501. Quantum efficiency is a parameter that represents the ratio
between the amount of electrons produced and the amount of photons
received by the photosensitive device.
[0091] It is apparent in FIG. 5 that a ZnS buffer layer allows
better light conversion at wavelengths below 500 nm. This gain in
current can be explained by a greater light transmission
coefficient in ZnS at these wavelengths in comparison to CdS. This
optical property is itself the result of the band structure of the
material, which has a higher energy band gap than CdS.
[0092] The present invention is not limited to the embodiments
described above by way of example; it also extends to other
variants. Indeed, the bath described above and the method using
this bath to produce a thin layer comprising metal and sulfur can
be implemented in different configurations which all benefit from
the gains in deposition rate and in quality of the obtained layer
that are described above.
[0093] The concentrations of the various components of the reaction
mixture 50 are therefore adjustable. For the sake of economy, it is
preferable to reduce the concentration of reagents. However,
reduced concentrations tend to increase the time required to
produce a thin layer of a given thickness. The examples described
above correspond to a compromise between concentration and reaction
rate. It is possible to use other concentrations and other
temperatures to meet different specifications. It may be of
interest, for example, to adjust the concentrations and
temperatures to deposit a thin layer of a given thickness within a
fixed time constraint. Indeed, due to the generally increased
deposition rate, it is possible to use CBD deposition with
persulfate as an additive to achieve layers more than 150 nm thick
within a reasonable time, for example under an hour.
[0094] By increasing the deposition rate, it is possible to have
the reaction mixture at a low temperature. Compared to conventional
deposition techniques which generally involve temperatures of
around 70.degree. C., the invention allows obtaining a deposit in
less than 15 minutes even when the temperature is below 60.degree.
C., for example down to 40.degree. C.
[0095] The compromise between deposition rate and concentration of
the reagents can be considered to be satisfactory for a
concentration of metal salt between 0.01 mol/L and 1 mol/L, a
concentration of sulfur precursor between 0.05 mol/L and 1 mol/L, a
persulfate concentration between 10.sup.-5 mol/L and 1 mol/L, and
an ammonia concentration between 0.1 mol/L and 10 mol/L.
[0096] However, the bath and the method that uses it can be
envisaged without the addition of ammonia. It is possible, for
example, to substitute another compound of basic pH, or a compound
of pKa greater than 7. The ammonia can be eliminated for example by
using potassium hydroxide KOH with a citrate-type complexing agent.
At a minimum, the reaction mixture of the chemical bath may contain
only persulfate, sulfur precursor, and a metal salt. Beyond these
basic components, the composition of the reaction mixture may
differ from what is detailed above.
[0097] In particular, it is possible to use other persulfate-based
compounds such as sodium persulfate of chemical formula
Na.sub.2S.sub.2O.sub.8 or potassium persulfate of chemical formula
K.sub.2S.sub.2O.sub.8, with equivalent performance.
[0098] The thiourea may also be replaced by other sulfur
precursors, preferably having equivalent chemical properties. Aside
from zinc sulfate, the metal salt may be replaced by zinc chloride
or acetate for the same applications as those described above. The
zinc acetate may be anhydrous or hydrated, for example of formula
Zn[CH.sub.3COOH].sub.2. An indium- or cadmium-based salt may also
be suitable for the metal salt when creating buffer layers on
photosensitive devices.
[0099] Furthermore, as the deposition of a layer of sulfur and zinc
occurs in an aqueous medium, oxygen may be incorporated into the
deposited layers to form a zinc oxysulfide of Zn(S,O) or Zn(S,O,OH)
type. Similarly, it is possible to incorporate oxygen into a layer
comprising another metal element such as indium, to form an indium
oxysulfide of In.sub.x(S,O).sub.y or In.sub.x(S,O,OH).sub.y type.
Other elements of groups IIB and IIIA of the periodic table may
also be considered for the metal, however, due to their chemical
properties similar to those of zinc, indium, or cadmium.
[0100] As mentioned above, it is possible to apply the method
described above in contexts other than deposition of a buffer layer
on a photosensitive cell absorber.
[0101] Indeed, the invention has also been tested successfully on
other deposition surfaces such as glass, a semiconductor substrate,
and a metal.
[0102] More generally, the invention described above optimizes a
chemical bath for the deposition of a thin layer comprising sulfur
and a metal. This optimization increases the deposition rate while
improving the structural quality of the layer obtained, and saves
materials and energy. In addition, the invention has the advantage
of being compatible with existing chemical baths for CBD chemical
deposition, and offers an advantageous solution for industrial
scale CBD deposition on large surface areas.
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