Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF MANUFACTURING PHOTOVOLTAIC CELLS, PHOTOVOLTAIC
CELLS PRODUCED THEREBY AND USES THEREOF
FIELD AND BACKGROUND OF THE INVENTION
The present invention, in some embodiments thereof, relates to energy
conversion, and, more particularly, but not exclusively, to a photovoltaic
cell comprising
a doped semi-conductive substrate, and to methods of producing same.
Photovoltaic cells are capable of converting light directly into electricity.
There
is considerable hope that conversion of sunlight into electricity by
photovoltaic cells will
provide a significant source of renewable energy in the future, thereby
enabling a
reduction in the use of non-renewable sources of energy, such as fossil fuels.
However,
despite world-wide demand for environmentally friendly renewable energy
sources, the
high cost of manufacture of photovoltaic cells, as well as their limited
efficiency of
conversion of sunlight to electricity, has so far limited their use as a
commercial source
of electricity. There is therefore a strong demand for photovoltaic cells
which are
relatively inexpensive to produce, yet have a high efficiency.
Photovoltaic cells commonly comprise a p-type silicon substrate doped on one
side thereof with an n-dopant (e.g., phosphorus) so as to form a n+ layer, and
doped on
the other side thereof with a p-dopant (e.g., boron) so as to form a p+ layer,
thereby
forming a n+-p-p+ structure. If an n-type silicon substrate is used, an n+-n-
p+ structure is
formed.
Electrical contacts are then applied to each side. Electrical contacts must
cover
only a small fraction of the surface area in order to avoid impeding the
passage of light.
Electrical contacts are typically applied in a grid pattern in order to avoid
covering much
of the surface area. Monofacial photovoltaic cells have such a grid pattern on
one side
of the photovoltaic cell, whereas bifacial photovoltaic cells have such a
pattern on both
sides of the photovoltaic cell, and can therefore collect light from any
direction.
Efficiency may be improved by reducing reflectance of light from the surface
of
the photovoltaic cell. Methods for achieving this include texturing the
surface and
applying an antireflective coating. Titanium dioxide (Ti02), Zr02, Ta205 and
silicon
nitride are examples of antireflective coatings that are currently in
practice.
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An exemplary process of producing a photovoltaic cell with a silica/silicon
nitride stack system on the rear side is described in Kranzel et al., in a
paper submitted
for the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion in
Hawaii.
In addition, attempts to improve efficiency include producing photovoltaic
cells
with a selective emitter, in which the n+ layer is more heavily doped in
regions
underlying electrical contacts, in order to decrease contact resistance.
German Patent No. 102007036921 is illustrative of such an approach, disclosing
a method of producing a solar cell with an n+-p-p+ structure, in which a
masking layer
having openings corresponding to the pattern of the contact grid is used while
doping
with phosphorus, so that the concentration of phosphorus will be highest under
the
contact grid.
U.S. Patent No. 6,277,667 discloses a method of manufacturing a solar cell
using
screen printing to apply an n-dopant source to form n+ regions, while a low
dose n-
dopant source is used to form shallowly doped n regions. Electrodes are then
screen-
printed onto the n+ regions.
U.S. Patent No. 5,871,591 discloses diffusing phosphorus into a surface of a
silicon substrate, metallizing a patterned grid onto the phosphorus-doped
surface, and
plasma etching the phosphorus-doped surface, such that the substrate below the
electrical contacts is masked and material that is not masked is selectively
removed.
Another approach to achieving an n+ layer that is more heavily doped in
regions
underlying electrical contacts is the use of self-doping electrodes.
For example, U.S. Patent No. 6,180,869 discloses a self-doping electrode to
silicon formed primarily from a metal alloyed with a dopant. When the alloy is
heated
with a silicon substrate, dopant is incorporated into molten silicon.
Russian Patent No. 2139601 discloses a method of manufacturing a solar cell
with an n+-p-p+ structure, by high-temperature processing of a silicon
substrate with a
borosilicate film applied to the back side thereof and a phosphosilicate film
applied to
the front side thereof. Removal of a layer of silicon from the front side of
the substrate
and texturing of the front side is then performed in one procedure. An n+
layer is then
formed on the front side, followed by formation of contacts.
Additional background art includes U.S. Patent No. 6,825,104, U.S. Patent No.
6,552,414, European Patent No. 1738402 and U.S. Patent No. 4,989,059.
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SUMMARY OF THE INVENTION
According to an aspect of some embodiments of the present invention there is
provided a method of producing a photovoltaic cell, the method comprising:
a) doping a first surface of a semiconductive substrate with an n-dopant so
as to form a first n+ layer in the substrate;
b) doping a second surface of the substrate with a p-dopant so as to form a
p+ layer in the substrate;
c) applying an antireflective coating to the second surface, the
antireflective
coating comprising a substance selected from the group consisting of silicon
nitride and
silicon oxynitride;
d) removing a portion of the first n+ layer from the first surface of the
substrate, such that a concentration of the n-dopant remaining in the first
surface of the
substrate is variable throughout the first surface;
e) doping the first surface of the substrate with an n-dopant so as to form a
second n+ layer, such that a concentration of the n-dopant in the second n+
layer is
variable throughout the first surface; and
f) forming electrical contacts on each of the first surface and the second
surface,
thereby producing the photovoltaic cell,
wherein applying the antireflective coating is performed prior to or
subsequent to
removing the portion of the first n+ layer from the first surface, and prior
to doping the
first surface of the substrate with the n-dopant so as to form the second n+
layer.
According to an aspect of some embodiments of the present invention there is
provided a photovoltaic cell produced according to a method described herein.
According to an aspect of some embodiments of the present invention there is
provided a photovoltaic cell comprising a semiconductive substrate, the
substrate
comprising an n+ layer on a first surface thereof and a p+ layer on a second
surface
thereof, the n+ layer comprising an n-dopant and the p+ layer comprising a p-
dopant, the
second surface being coated by an antireflective coating which comprises a
substance
selected from the group consisting of silicon nitride and silicon oxynitride,
and electrical
contacts attached to each of the first surface and the second surface,
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wherein the first surface is textured so as to comprise peaks and troughs, and
wherein a concentration of the n-dopant in the n+ layer is greater in the
peaks of
the first surface than in the troughs of the first surface.
According to an aspect of some embodiments of the present invention there is
provided a photovoltaic array comprising a plurality of photovoltaic cells
described
herein, the plurality of photovoltaic cells being interconnected to one
another.
According to an aspect of some embodiments of the present invention there is
provided a power plant comprising a photovoltaic array described herein.
According to an aspect of some embodiments of the present invention there is
provided an electric device comprising a photovoltaic cell described herein.
According to an aspect of some embodiments of the present invention there is
provided a detector of electromagnetic radiation, the detector comprising a
photovoltaic
cell described herein, wherein the electromagnetic radiation is selected from
the group
consisting of ultraviolet, visible and infrared radiation.
According to some embodiments of the invention, the first n+ layer is
characterized by a sheet resistance of less than 30 ohms.
According to some embodiments of the invention, the first n+ layer has a depth
in
a range of 0.4-2 m.
According to some embodiments of the invention, the second n+ layer is
characterized by a sheet resistance in a range of 30-100 ohms.
According to some embodiments of the invention, the n+ layer of the
photovoltaic cell is characterized by a sheet resistance in a range of 30-100
ohms.
According to some embodiments of the invention, the second n+ layer has a
depth in a range of 0.2-0.7 m.
According to some embodiments of the invention, the n+ layer of the
photovoltaic cell has a depth in a range of 0.2-0.7 m.
According to some embodiments of the invention, removing the portion of the
first n+ layer from the first surface comprises texturing the first surface.
According to some embodiments of the invention, the texturing generates peaks
and troughs in the first surface, wherein a concentration of the n-dopant
remaining in the
first surface following texturing is greater in the peaks than in the troughs.
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According to some embodiments of the invention, a concentration of the n-
dopant in the second n+ layer is greater in the peaks than in the troughs.
According to some embodiments of the invention, a concentration of the n-
dopant in the peaks in the second n+ layer is at least twice a concentration
of the n-
5 dopant in the troughs in the second n+ layer.
According to some embodiments of the invention, a concentration of the n-
dopant in the peaks in the photovoltaic cell is at least twice a concentration
of the n-
dopant in the troughs in the photovoltaic cell.
According to some embodiments of the invention, a concentration of the n-
dopant in the peaks in the second n+ layer is at least 5x1020 atoms/cm3.
According to some embodiments of the invention, a concentration of the n-
dopant in the peaks in the photovoltaic cell is at least 5x1020 atoms/cm3.
According to some embodiments of the invention, a concentration of the n-
dopant in the troughs in the second n+ layer is less than 1021 atoms/cm3.
According to some embodiments of the invention, a concentration of the n-
dopant in the troughs in the photovoltaic cell is less than 1021 atoms/cm3.
According to some embodiments of the invention, removing the portion of the n+
layer from the first surface comprises etching the first surface to an average
depth in a
range of from 4 m to 12 m.
According to some embodiments of the invention, etching is by an alkaline
solution.
According to some embodiments of the invention, the first n+ layer and the p+
layer are formed simultaneously.
According to some embodiments of the invention, the doping with the n-dopant
so as to form the first n+ layer and the doping with the p-dopant so as to
form the p+
layer is effected by:
(i) applying a film comprising the p-dopant to the second surface;
(ii) applying a film comprising the n-dopant to the first surface; and
(iii) heating the substrate,
thereby simultaneously forming the first n+ layer and the p+ layer.
According to some embodiments of the invention, the film comprising the p-
dopant and the film comprising the n-dopant each comprise silicon dioxide.
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According to some embodiments of the invention, the film comprising the p-
dopant comprises boron oxide.
According to some embodiments of the invention, the film comprising the n-
dopant comprises phosphorus pentoxide.
According to some embodiments of the invention, the film comprising the n-
dopant comprises at least 20 weight percents phosphorus pentoxide.
According to some embodiments, the method further comprises subjecting the
antireflective coating on the second surface to a thermal treatment.
According to some embodiments, the thermal treatment increases a refractive
index of the antireflective coating.
According to some embodiments, the thermal treatment increases a refractive
index of at least a portion of the antireflective coating by at least 0.05.
According to some embodiments, the thermal treatment simultaneously dopes
the first surface of the substrate with the n-dopant so as to form the second
n+ layer.
According to some embodiments of the invention, the method comprises
applying an antireflective coating characterized by a refractive index in a
range of from
2.1 to 2.2.
According to some embodiments, the antireflective coating on the second
surface
of the photovoltaic cell is characterized by a refractive index in a range of
from 2.1 to
2.4.
According to some embodiments, the antireflective coating on the second
surface
is characterized by a graded refractive index which decreases from the
direction of an
interface with the substrate.
According to some embodiments, the method comprises applying an
antireflective coating with a graded refractive index within a range of from
1.7 to 2.25.
According to some embodiments, the graded refractive index of the
antireflective
coating of the photovoltaic cell is within a range of from 1.7 to 2.45.
According to some embodiments, the antireflective coating applied to the
second
surface inhibits doping by the n-dopant of a surface coated by the
antireflective coating.
According to some embodiments of the invention, the method further comprises
applying an antireflective coating to the first surface subsequent to forming
the second
n+ layer.
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According to some embodiments, the photovoltaic cell further comprises an
antireflective coating on the first surface.
According to some embodiments of the invention, the semiconductive substrate
is an n-type semiconductor prior to said doping.
According to some embodiments of the invention, the semiconductive substrate
is a p-type semiconductor prior to said doping.
According to some embodiments of the invention, the semiconductive substrate
comprises silicon.
According to some embodiments of the invention, the n-dopant comprises
phosphorus.
According to some embodiments of the invention, the p-dopant comprises boron.
According to some embodiments of the invention, the photovoltaic cell is
characterized by a short circuit current density of at least 0.033
amperes/cm2.
According to some embodiments of the invention, the photovoltaic cell is
characterized by a fill factor of at least 75.5 %.
According to some embodiments of the invention, the photovoltaic cell is
characterized by an efficiency of at least 16.7 %.
According to some embodiments of the invention, the photovoltaic cell is a
bifacial photovoltaic cell.
According to some embodiments of the invention, the photovoltaic cell
comprises an n+-n-p+ structure.
According to some embodiments of the invention, the photovoltaic cell
comprises an n+-p-p+ structure.
Unless otherwise defined, all technical and/or scientific terms used herein
have
the same meaning as commonly understood by one of ordinary skill in the art to
which
the invention pertains. Although methods and materials similar or equivalent
to those
described herein can be used in the practice or testing of embodiments of the
invention,
exemplary methods and/or materials are described below. In case of conflict,
the patent
specification, including definitions, will control. In addition, the
materials, methods, and
examples are illustrative only and are not intended to be necessarily
limiting.
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DESCRIPTION OF THE DRAWINGS
Some embodiments of the invention are herein described, by way of example
only, with reference to the accompanying drawings. With specific reference now
to the
drawings in detail, it is stressed that the particulars shown are by way of
example and for
purposes of illustrative discussion of embodiments of the invention. In this
regard, the
description taken with the drawings makes apparent to those skilled in the art
how
embodiments of the invention may be practiced.
In the drawings:
FIG. 1 is a scheme depicting an exemplary method for producing a photovoltaic
cell according to some embodiments of the invention;
FIG. 2 is a scheme depicting another exemplary method for producing a
photovoltaic cell according to some embodiments of the invention;
FIG. 3 is a graph showing the dependence of short circuit current density,
Jsc, (in
mA/cm2) on etching depth (in micrometers) in photovoltaic cells produced
according to
an embodiment of the invention, wherein the sheet resistance of the first n+
layer of the
cells was 13, 17 or 25 ohm;
FIG. 4 is a graph showing the dependence of fill factor (FF) on etching depth
(in
micrometers) in photovoltaic cells produced according to an embodiment of the
invention, wherein the sheet resistance of the first n+ layer of the cells was
13, 17 or 25
ohm;
FIG. 5 is a graph showing the dependence of efficiency on etching depth (in
micrometers) for photovoltaic cells produced according to an embodiment of the
invention, wherein the sheet resistance of the first n+ layer of the cells was
13, 17 or 25
ohm;
FIG. 6 is a graph showing the measured effective minority carrier lifetime (in
microseconds) in silicon wafers with a p+-p-p+ structure after p+-p-p+
structure
formation by boron doping (1), after silicon nitride deposition (2) and after
thermal
treatment of the wafer (3); and
FIG. 7 is a graph showing calculated short circuit current density (in
milliamperes/cm2) of a silicon photovoltaic cell with theoretically maximal
internal
quantum efficiency in a medium with a refractive index of 1.45 as a function
of the
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refractive index of the lowermost layer of a 1-layer or 2-layer antireflective
coating of
the photovoltaic cell.
DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION
The present invention, in some embodiments thereof, relates to energy
conversion, and, more particularly, but not exclusively, to a photovoltaic
(PV) cell
comprising a doped semi-conductive substrate, and to methods of producing
same.
In a search for efficient, yet relatively inexpensive, photovoltaic cells for
converting light energy to electrical energy, the present inventors have
uncovered that a
photovoltaic cell with an n-doped layer characterized by a variable
concentration of an
n-dopant exhibits improved efficiency.
In addition, the present inventors have conceived that when doping of a
substrate
to produce a photovoltaic cell is performed by doping one side of the
substrate with a p-
dopant followed by doping of another side with an n-dopant, efficiency of the
photovoltaic cell can be enhanced by introducing a simple, inexpensive
procedure for
applying an antireflective coating to the p-doped surface prior to doping of
the other side
with an n-dopant. The antireflective coating can prevent contact of the n-
dopant with
the p-doped surface, thereby advantageously separating the two types of
dopant.
Moreover, optionally, the antireflective properties of the coating may be
optimized by
the same thermal treatment used to introduce the n-dopant, thereby making the
process
more efficient.
The present inventors have therefore devised and successfully practiced a
novel
methodology for producing a photovoltaic cell, which involves a reduced number
of
procedures in comparison with other methodologies, and is hence cost-efficient
and
yield-efficient, resulting in less defects during the manufacturing process.
This novel
methodology further results in photovoltaic cells with performance parameters
that
surpass many other PV cells.
While reducing the present invention to practice, the present inventors have
produced a photovoltaic (PV) cell with an n+-p-p+ structure and a variable
concentration
of an n-dopant in the n+ layer, using a relatively simple, and hence
relatively
inexpensive, procedure. A first n+ layer is formed by doping and is then
removed to a
varying degree at different regions of the photovoltaic cell, such that the
remaining n-
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dopant is present in a variable concentration. A second n+ layer is then
formed by
doping, and the concentration of n-dopant throughout the second n+ layer is
variable, due
to the variable nature of the removal of the first n+ layer.
Without being bound to any particular theory, it is believed that a variable
5 concentration of an n-dopant in the n+ layer provides a combination of
advantages of a
high concentration of n-dopant and advantages of a low concentration of n-
dopant.
Thus, it is believed that the presence of randomly distributed local regions
of a high
concentration reduces series resistance of the photovoltaic cell, thereby
increasing fill
factor and efficiency of the photovoltaic cell, and that presence of regions
of a low
10 concentration increases efficiency by preventing the decrease in short
circuit current
which is characteristic of high dopant concentrations.
Before explaining at least one embodiment of the invention in detail, it is to
be
understood that the invention is not necessarily limited in its application to
the details of
construction and the arrangement of the components and/or methods set forth in
the
following description and/or illustrated in the drawings and/or the Examples.
The
invention is capable of other embodiments or of being practiced or carried out
in various
ways.
Referring now to the drawings, Figure 1 illustrates an exemplary method for
producing a photovoltaic cell according to some embodiments of the invention.
A semiconducting substrate 1 is coated on one side by a p-dopant-containing
film 2. Substrate 1 is then coated with an n-dopant-containing film 3 on the
side of the
substrate opposite from p-dopant-containing film 2. Diffusion of dopants from
the films
is induced (e.g., by heating), thereby resulting in simultaneous formation of
a first n+
layer 4 and a p+ layer 5. Films 2 and 3 are then removed. Substrate 1 is then
textured at
the surface thereof by an etching solution, resulting in peaks and troughs at
the surface
of the substrate (except at p+ layer 5, which resists texturing). First n+
layer 4 remains
only at the peaks of the textured surface. Substrate 1 is then coated by a
rear
antireflection coating 6. A second n+ layer 7 is formed and then coated by a
front
antireflection coating 8. Rear antireflection coating 6 prevents second n+
layer 7 from
contacting p+ layer 5 at an edge of substrate 1. Electrical contacts 9 are
then formed on
both sides of the substrate, to form a photovoltaic cell.
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Figure 2 illustrates another exemplary method for producing a photovoltaic
cell
according to some embodiments of the invention.
A semiconducting substrate 1 is coated on one side by a p-dopant-containing
film 2. Substrate 1 is then coated with an n-dopant-containing film 3 on the
side of the
substrate opposite to p-dopant-containing film 2. Diffusion of dopants from
the films is
induced (e.g., by heating), thereby resulting in simultaneous formation of a
first n+ layer
4 and a p+ layer 5. Films 2 and 3 are then removed. p+ layer 5 is then coated
by a rear
antireflection coating 6. Substrate 1 is then textured at the surface thereof
by an etching
solution, resulting in peaks and troughs at the surface of the substrate
(except at rear
antireflection coating 6, which resists texturing). First n+ layer 4 remains
only at the
peaks of the textured surface. A second n+ layer 7 is formed and then coated
by a front
antireflection coating 8. Rear antireflection coating 6 prevents second n+
layer 7 from
contacting p+ layer 5 at an edge of substrate 1. Electrical contacts 9 are
then formed on
both sides of the substrate, to form a photovoltaic cell.
The above-described exemplary methods achieve a variable concentration of n-
dopant, as the concentration of n-dopant is higher at the peaks of the
textured surface,
where n-dopant originating from formation of the second n+ layer 7 is present
along with
n-dopant remaining from the first n+ layer 4.
The above-described exemplary methods also result in no overlap between the p+
layer and n+ layer because the p+ layer is protected by the rear
antireflection coating
when the second n+ layer is formed.
In addition, the above methods are particularly advantageous in that they
utilize
procedures which improve efficiency of a photovoltaic cell by more than one
mechanism. Thus, texturing improves efficiency of photovoltaic cells both by
reducing
the percentage of light wasted by reflectance from the surface of the cell and
by creating
a variable concentration of n-dopant. Formation and removal of a first n+
layer improves
efficiency both by facilitating the creation of a variable concentration of n-
dopant and by
beneficially preventing formation of p+ regions within the n+ layer, which
would
detrimentally increase shunting. The rear antireflective coating both reduces
reflectance
and protects the p+ layer when forming the second n+ layer.
Thus, these methods do not require excessive procedures, and in fact involve
less
procedures than commonly utilized for producing PV cells, and none of the
procedures
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included in these methods are particularly complex. As a result, the methods
are
relatively simple and inexpensive to perform. The reduced number of procedures
reduces the chances of defects formation, thus render the entire process more
efficient.
Figure 3 shows that the short circuit current density of photovoltaic cells
prepared according to embodiments of the invention is reduced when etching
during
texturing is relatively shallow (e.g., less than about 4 m on average).
Figure 4 shows
that the fill factor of photovoltaic cells prepared according to methods
described herein
is reduced when etching during texturing is relatively deep (e.g., more than
about 12 m
on average). Figure 5 shows that the efficiency (which is correlated to both
fill factor
and short circuit current) of photovoltaic cells prepared according to methods
described
herein is greatest when etching is at an intermediate depth (e.g., about 4-12
m on
average).
Without being bound to any particular theory, it is believed that shallow
texturing does not create the desired variable concentration of n-dopant
because not
enough of the n-dopant of the first n+ layer is removed, while relatively deep
etching
does not create a variable concentration of n-dopant because virtually all of
the n-dopant
of the first n+ layer is removed. Thus, it is believed that an intermediate
average depth
of etching is optimal for producing a variable concentration of n-dopant, as
an
intermediate average depth comprises both regions with relatively deep etching
(troughs)
and regions with relatively shallow etching (peaks).
The above-described exemplary methods also form a non-symmetrical structure
in which one side is textured and the other side is smooth (non-textured).
Without being
bound by any particular theory, it is believed that such a structure is
advantageous when
radiation is incident on the textured surface, as the textured surface
decreases reflection,
and the smooth, non-textured surface enhances internal reflection of long-
wavelength
radiation reaching the back of the cell, thereby increasing the contribution
of long-
wavelength radiation to the generated current. In addition, the effective
surface
recombination of the smooth p+ surface is lower than that of a textured
surface, resulting
in lower losses of efficiency.
However, it is further believed that the smooth rear surface is
disadvantageous
for bifacial photovoltaic cells in that the relatively high reflectance of the
rear surface
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reduces the efficiency when the rear surface of the cell is illuminated. It is
therefore
advantageous to provide for an effective antireflective coating for the rear
surface.
As described herein, silicon nitride and/or silicon oxynitride may be
deposited on
the substrate so as to form a coating with a controllable refractive index. As
shown in
FIG. 7, a high refractive index (e.g., 2.3 or higher) improves the
effectiveness of an
antireflective coating. However, silicon nitride layers with a refractive
index exceeding
2.2 are characterized by increased light absorption at short wavelengths [Opto-
Electronics Rev. 2004, 12:41-44], a feature which may reduce the efficiency of
a
photovoltaic cell. Hence, it is advantageous to deposit a coating with a
refractive index
of less than about 2.2 (e.g., in a range of from 2.1 to 2.2), despite the sub-
optimal
antireflective properties of such coatings.
Without being bound by any particular theory, it is believed that the thermal
treatment of an antireflective coating described herein at least partially
overcomes the
above-described problem by increasing a refractive index of an antireflective
coating to
a more optimal level, without sacrificing the low absorption at short
wavelengths which
is characteristic of coatings with low refractive indices.
Moreover, the present inventors have uncovered that the thermal treatment of
silicon nitride and/or silicon oxynitride coatings maintains a low level of
surface
recombination, as shown in FIG. 6, thereby enhancing the efficiency of
photovoltaic
cells. Thus, it is to be understood that application of an antireflective
coating according
to embodiments described herein can increase efficiency by mechanisms which
are not
necessarily related to reducing reflection.
Without being bound by any particular theory, it is believed that deposition
of
silicon nitride on the surface increases levels of p+ layer surface
recombination due to
the introduction of electrical charge and/or hydrogen atoms, and that thermal
treatment
eliminates such electrical charge and/or hydrogen atoms, thereby decreasing
levels of
surface recombination.
Hence, according to an aspect of some embodiments of the invention, there is
provided a method of producing a photovoltaic cell, the method comprising:
a) doping a first surface of a semiconductive substrate with an n-dopant so
as to form a first n+ layer in the substrate;
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b) doping a second surface of the substrate with a p-dopant so as to form a
p+ layer in the substrate;
c) applying an antireflective coating (e.g., comprising silicon nitride and/or
silicon oxynitride) to the second surface;
d) removing a portion of the first n+ layer from the first surface of the
substrate, such that a concentration of the n-dopant remaining in the first
surface of the
substrate is variable throughout the first surface;
e) doping the first surface of the substrate with an n-dopant so as to form a
second n+ layer, such that a concentration of the n-dopant in the second n+
layer is
variable throughout the first surface; and
f) forming electrical contacts on each of the first surface and the second
surface.
The application of the antireflective coating (step c) may be performed prior
to or
subsequent to the removal of a portion of the first n+ layer (step d), but in
any case is
performed prior to the doping of the first surface to form a second n+ layer
(step e). As
discussed herein, such an application of an antireflective coating may be
useful in
preventing overlap between the p+ layer and the second n+ layer, when the
antireflective
coating is resistant, at least to some extent, to diffusion of the n-dopant.
Hence, in some embodiments, the antireflective coating on the second surface
is
impermeable, at least to some extent, to n-dopant (e.g., phosphorus), such
that the
coating inhibits entry of n-dopant into the p-doped regions on the second
surface.
Optionally, the coating reduces entry of the n-dopant into regions coated by
the coating
by at least 99 %, optionally by at least 99.9 %, and optionally by at least
99.99 %.
Optionally, the coating is fully impermeable to the n-dopant.
The antireflective coating may be formed by any suitable method known to the
skilled artisan (e.g., chemical vapor deposition or plasma-enhanced chemical
vapor
deposition).
As exemplified hereinbelow in the Examples section, antireflective coatings
comprising silicon nitride and/or silicon oxynitride are particularly suitable
for
enhancing efficiency of photovoltaic cells when applied according to
procedures
described herein. However, application according to the procedures described
herein of
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antireflective coatings comprising other suitable substances (e.g., Ti02,
Zr02, Ta205) is
also contemplated.
The antireflective coating may comprise one or more layers. When more than
one layer is present, the different layers may differ, for example, in
refractive index (e.g.,
5 an upper layer having a lower refractive index than a lower layer) and/or
components
(e.g., one layer comprising silicon oxynitride and another layer comprising
silicon
nitride).
According to some embodiments, the method further comprises subjecting the
antireflective coating to a thermal treatment (e.g., heating), for example, a
thermal
10 treatment which increases a refractive index of the antireflective coating.
Thus, for
example, the refractive index of silicon nitride, an exemplary component of
antireflective coatings, is increased by thermal treatment [Winderbaum et al.,
"INDUSTRIAL PECVD SILICON NITRIDE: SURFACE AND BULK
PASSIVATION OF SILICON WAFERS", 19th European PVSEC, Paris, France, 2004,
15 576-579]. Optionally, the thermal treatment increases a refractive index of
at least a
portion (e.g., the lowermost portion, which is closest to the silicon
substrate) of the
antireflective coating by at least 0.05 (e.g., increasing from below 2.2 to at
least 2.25),
optionally by at least 0.1 (e.g., increasing from below 2.2 to at least 2.3),
and optionally
by at least 0.15 (e.g., increasing from below 2.2 to at least 2.35).
Thermal treatments may be used to dope a substrate, for example, by heating
the
substrate in the presence of a substance (e.g., gas, paste) which comprises a
dopant.
Hence, in some embodiments, the thermal treatment (e.g., a temperature in a
range of from 800 to 900 C, for 10 to 30 minutes) simultaneously dopes the
first surface
of the substrate with an n-dopant so as to form the second n+ layer described
hereinabove. Such embodiments advantageously allow for thermal treatment of
the
antireflective coating without increasing the number of procedures involved in
producing a photovoltaic cell. The simultaneous heat treatment of an
antireflective
coating and doping with an n-dopant is exemplified in the Examples section
hereinbelow. It is within the capabilities of a skilled artisan to determine
conditions
(e.g., temperature, time of treatment) which are suitable for both doping with
an n-
dopant and for optimizing the antireflective coating, based on the teachings
described
herein.
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In some embodiments, the antireflective coating applied to the second surface
is
characterized by a refractive index in a range of from 2.1 to 2.2. It is to be
understood
that such a refractive index refers to the coating when applied, e.g.,
following
application and before thermal treatment. Optionally, the refractive index is
increased
by thermal treatment, such that the refractive index in the produced
photovoltaic cell is
higher (e.g., in a range of from 2.15 to 2.4).
In some embodiments, the antireflective coating applied to the second surface
is
characterized by a graded refractive index which decreases from the direction
of an
interface with the substrate (i.e., the refractive index is highest at an
interface with the
substrate and lowest in regions of the coating which are farthest from the
substrate).
Optionally, the graded refractive index is in a range of from 1.7 to 2.25
(e.g., following
application and before thermal treatment). Optionally, the refractive index is
increased
by thermal treatment, such that a graded refractive index in the finished
photovoltaic cell
is higher (e.g., in a range of from 1.7 to 2.45).
An antireflective coating may optionally be applied to the first surface, for
example, subsequent to formation of the second n+ layer. Any suitable coating
(e.g.,
Ta205, Ti02, silicon nitride, silicon oxynitride) may be used. The
antireflective coating
applied subsequent to formation of the second n+ layer and the antireflective
agent
applied prior to the formation of the second n+ layer can be the same or
different.
Exemplary coatings comprise silicon nitride and silicon oxynitride, as for the
antireflective coating applied to the second surface.
The phrase "silicon nitride", as used herein, describes a family of substances
composed substantially of silicon and nitrogen, with various stochiometries of
Si and N
(e.g., Si3N4), although some amounts of additional atoms (e.g., hydrogen) may
be
present as impurities.
The phrase "silicon oxynitride" refers to SiNXOy,, wherein each of x and y is
a
positive number of up to 2 (e.g., between 0.1 and 2), and x and y are in
accordance with
the valence requirements of Si, N and 0. Some amounts of additional atoms
(e.g.,
hydrogen) may be present as impurities.
According to exemplary embodiments, the substrate is relatively thin and flat,
such that the substrate has two surfaces on opposing sides which serve as the
first and
second surfaces described herein.
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Silicon (e.g., silicon wafers) is an exemplary semiconductive substrate.
As is widely recognized in the art, "doping" is a process of impurity
introduction
in the semiconductor in which the number of free charge carriers in the doped
semiconductor material can be increased, and as a result, elevation of the
charge carrier
density in the doped semiconductor material is effected. "p-Doping" refers to
doping of
a semiconductor with a substance ("dopant") which is capable of accepting
weakly-
bound outer electrons from the semiconductor material. Thus p-doping, wherein
"p"
denotes positive, is a process of doping a semiconductor with an acceptor
material, or p-
type dopant, which forms "holes", or positive charges, in the semiconductor. n-
doping,
wherein "n" denotes negative, is a process of doping a semiconductor with an
electron
donating material, or n-type dopant, which forms negative charges in the
semiconductor.
As used herein, the term "dopant" refers to any element or compound, which
when present in the semiconductive substrate, results in p-type or n-type
conductivity.
A dopant which results in p-type conductivity is referred to herein as a "p-
dopant", and
is typically an acceptor of electrons, whereas a dopant which results in n-
type
conductivity is referred to herein as a "n-dopant", and is typically a donor
of electrons.
Boron is an exemplary p-dopant and phosphorus is an exemplary n-dopant.
Optionally, arsenic is used as an n-dopant. Other p-dopants and n-dopants that
are
suitable for use in PV cells are also contemplated.
According to optional embodiments, the semiconductive substrate is an n-type
semiconductor prior to the doping described hereinabove, which forms n+ and p+
layers.
In such embodiments, the photovoltaic cell has an n+-n-p+ structure, with an n
layer
between the n+ and p+ layers. "n+" denotes a layer with relatively strong
doping with an
n-dopant and "p+" denotes a layer with relatively strong doping with a p-
dopant, whereas
"n" denotes a layer with weaker doping with an n-dopant.
According to alternative embodiments, the semiconductive substrate is a p-type
semiconductor prior to the doping described hereinabove, which forms n+ and p+
layers.
In such embodiments, the photovoltaic cell has an n+-p-p+ structure, with a p
layer
between the n+ and p+ layers. "n+" denotes a layer with relatively strong
doping with an
n-dopant and "p+" denotes a layer with relatively strong doping with a p-
dopant, whereas
"p" denotes a layer with weaker doping with a p-dopant.
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As used herein, the phrase "variable throughout the first surface" describes a
surface in which the concentration of dopant in various regions on the surface
differs
from the concentration of dopant in other (e.g., adjacent) regions on the
surface. The
concentration of n-dopant at any location on the first surface may be
determined by
methods known in the art, for example, by sampling a thin slice of material
from the
surface of the substrate and determining its elemental composition.
Optionally,
secondary ion mass spectroscopy (SIMS) is used to determine the n-dopant
concentration. SIMS, a standard method of the art, is particularly suitable
for
determining local concentrations on a surface.
A further discussion of the variable concentrations of the dopant is provided
hereinunder.
The electric contacts may be formed according to methods well known in the
art.
In order to allow light to reach the substrate of the photovoltaic cell, the
contacts on at
least one surface (e.g., the first surface) are configured so as to reach as
much of the
surface as possible while shading the surface as little as possible. For
example, the
contacts may optionally be configured in a grid pattern.
Optionally, the photovoltaic cell is monofacial, wherein the contacts on one
surface are configured so as to allow light to pass through to the substrate,
as described
hereinabove, whereas the contacts on the other surface are not configured as
such. For
example, the surface may be completely covered by the electric contacts, as
such a
configuration provides ease of manufacture and high efficiency.
Alternatively, the photovoltaic cell is bifacial, wherein the contacts on both
surfaces are configured so as to allow light to pass through to the substrate,
thereby
allowing the photovoltaic cell to produce electricity from illumination on
either side of
the cell. As discussed hereinabove, and exemplified hereinbelow in the
Examples
section that follows, application of an antireflective coating on the second
surface as
described herein is particularly useful in increasing the efficiency of
bifacial
photovoltaic cells, by reducing reflection when the second (rear) surface is
illuminated.
According to some embodiments, the first n+ layer has a depth in a range of
0.4-2
m. Optionally, the depth is in a range of 0.6-1.2 m.
According to some embodiments, the first n+ layer is characterized by a sheet
resistance of less than 30 ohm. Optionally, the sheet resistance is less than
25 ohms,
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optionally less than 20 ohm, and optionally less than 15 ohm. According to
exemplary
embodiments, the sheet resistance is in a range of from about 13 ohm to about
25 ohm.
It is to be noted that the sheet resistance of an n+ layer is inversely
correlated to
the concentration of n-dopant. The relatively low sheet resistance of the
first n+ layer
described herein thus corresponds to a relatively high concentration of n-
dopant, which
can decrease the short circuit current and efficiency of a photovoltaic cell.
Thus, in exemplary embodiments, the second n+ layer, which replaces the first
n+
layer, is characterized by a higher sheet resistance than the relatively low
sheet
resistances described hereinabove for the first n+ layer.
According to some embodiments, the second n+ layer is characterized by a sheet
resistance in a range of 30-100 ohm. Optionally, the sheet resistance is in a
range of 40-
65 ohm. According to an exemplary embodiment, the sheet resistance is about 55
ohm.
According to some embodiments, the second n+ layer has a depth in a range of
0.2-0.7 m, and optionally in a range of 0.3-0.4 m.
According to exemplary embodiments, removing of the portion of the first n+
layer from the first surface comprises texturing the first surface.
As used herein, the term "texturing" means to make a surface more rough (e.g.,
resulting in peaks and troughs on the surface).
As used herein, the term "peak" refers to a region of the surface which is
higher
than adjacent regions, whereas the term "trough" refers to a region of the
surface which
is lower than adjacent regions.
According to some embodiments, the texturing generates peaks and troughs in
the first surface, wherein a concentration of the n-dopant remaining in the
first surface
following texturing is greater in the peaks than in the troughs. Accordingly,
the variable
concentration of the dopant throughout the surface is manifested in these
embodiments
by the different concentration of the dopant in the peaks and troughs. Thus,
the
concentration of n-dopant in the peaks will represent local maxima of the
concentration
on the surface of the substrate, whereas the concentration of n-dopant in the
troughs will
represent local minima. These maxima and minima of the concentration create a
variable concentration.
According to some embodiments, the concentration of the n-dopant in the second
n+ layer is greater in the peaks than in the troughs. Optionally, the
concentration of the
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n-dopant in the peaks is at least twice a concentration of the n-dopant in the
troughs.
Optionally, the concentration of the n-dopant in the peaks is at least 3
times, optionally
at least 5 times, and optionally at least 10 times a concentration of the n-
dopant in the
troughs.
5 According to some embodiments, a concentration of the n-dopant in the peaks
in
the second n+ layer is at least 5x1020 atoms/cm3. Optionally, the
concentration is at least
1021 atoms/cm3, optionally at least 2x1021 atoms/cm3, optionally at least
3x1021
atoms/cm3, and optionally at least 5x1021 atoms/cm3.
According to some embodiments, a concentration of the n-dopant in the troughs
10 in the second n+ layer is less than 1021 atoms/cm3. Optionally, the
concentration is less
than 0.5x1021 atoms/cm3, optionally less than 0.3x1021 atoms/cm3, optionally
less than
0.2x1021 atoms/cm3, and optionally less than 1020 atoms/cm3.
It is to be appreciated that a "high" concentration of n-dopant in the peaks
of
some embodiments with a greater concentration of n-dopant in the peaks than in
the
15 troughs thereof may be somewhat lower than a "low" concentration in the
troughs of
another embodiment with a greater concentration of n-dopant in the peaks than
in the
troughs thereof. According to some embodiments, removing the portion of the
first n+
layer from the first surface comprises etching the first surface to an average
depth in a
range of from 4 m to 12 m. Optionally, the depth is in a range of 6 m to 10
m.
20 According to some embodiments, the etching is effected by an alkaline
solution
(e.g., a solution that comprises sodium hydroxide).
In each of the methods described herein, the first n+ layer and the p+ layer
are
formed via any of the methods known in the art.
In some embodiments, whenever an n+ layer is deposited without forming
variable concentrations of the dopant throughout the surface, applying a film
comprising
an n-dopant to the first surface can alternatively be effected by any method
known in the
art. According to some embodiments, the first n+ layer and the p+ layer are
formed
simultaneously (e.g., by heating).
According to exemplary embodiments, the doping with the n-dopant so as to
form the first n+ layer and the doping with the p-dopant so as to form the p+
layer is
effected by applying a film comprising the p-dopant to the second surface,
applying a
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film comprising the n-dopant to the first surface, and heating the substrate,
thereby
simultaneously forming the first n+ layer and the p+ layer.
According to some embodiments, the film comprising the p-dopant and the film
comprising the n-dopant each comprise silicon dioxide. Silicon dioxide-based
films
may be selectively removed following the doping procedure by hydrofluoric
acid.
According to some embodiments, the film comprising the p-dopant comprises
boron oxide.
According to some embodiments, the film comprising the n-dopant comprises
phosphorus pentoxide (P205). Optionally, the film comprises at least 20 weight
percents
P205. As exemplified hereinbelow in the Examples section, the concentration of
phosphorus in the first n+ layer and the sheet resistance of the first n+
layer may be
readily controlled by selecting a suitable concentration of P205 in the doping
film.
Without being bound by any particular theory, it is believed that formation of
a
first n+ layer simultaneously with the formation of the p+ layer
advantageously prevents
formation of p+ regions within the n+ layer, which would detrimentally
increase
shunting. However, a concentration and depth of n-dopant which is particularly
suitable
for preventing formation of deleterious p+ regions may be higher than a
concentration
and depth of n-dopant which is particularly suitable for optimal performance
of the final
product. Hence, it is believed that by removing at least a portion of the
first n+ layer, the
n-dopant concentration in the n+ layer is reduced to a more suitable level for
a
photovoltaic cell.
According to another aspect of embodiments of the present invention, there is
provided a photovoltaic cell produced according to any of the methods
described herein.
Thus, according to some embodiments, there is provided a photovoltaic cell
comprising a semiconductive substrate, the substrate comprising an n+ layer on
a first
surface thereof and a p+ layer on a second surface thereof, the second surface
being
coated by an antireflective coating as described herein, and electrical
contacts attached
to each of the first surface and the second surface, wherein the first surface
is textured so
as to comprise peaks and troughs, and wherein a concentration of the n-dopant
in the n+
layer is greater in the peaks of the first surface than in the troughs of the
first surface.
It is to be appreciated that the "n+ layer" of the photovoltaic cells
described
herein corresponds to the "second n+ layer" which is discussed herein in the
context of
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the methods described herein. Thus, the n+ layer of the photovoltaic cells may
optionally be characterized by any of the features (e.g., depth, sheet
resistance, local n-
dopant concentration) described herein with respect to the second n+ layer.
Optionally, the photovoltaic cell is a bifacial photovoltaic cell.
The substrate optionally comprises silicon, the p-dopant optionally comprises
boron, and the n-dopant is optionally selected from the group consisting of
phosphorus
and arsenic, wherein phosphorus is an exemplary n-dopant.
According to some embodiments, the fill factor of the photovoltaic cell is at
least
75.5 %, optionally at least 76 %, optionally at least 76.5 %, and optionally
at least 77 %.
According to some embodiments, the efficiency of the photovoltaic cell is at
least 16.7 %, optionally at least 16.8 %, optionally, at least 16.9 % and
optionally at least
17 %.
According to some embodiments, the short circuit current density of the
photovoltaic cell is at least 0.033 amperes/cm2, optionally at least 0.0335
amperes/cm2,
and optionally at least 0.034 amperes/cm2.
The abovementioned physical parameters are determined by measurements at
standard test conditions used in the art to evaluate photovoltaic cells.
Standard test
conditions include solar irradiance of 1,000 W/m2, solar reference spectrum at
AM
(airmass) of 1.5 and a cell temperature 25 C.
Short circuit current density may be determined, for example, by measuring the
current (Iso) produced by the photovoltaic cell at short circuit (i.e.,
voltage = 0) using
standard techniques of the art. Open circuit voltage (Voc) may be determined
by
measuring the voltage across the photovoltaic cell at open circuit (i.e.,
current = 0) using
standard techniques.
Fill factor and efficiency may be determined by measuring the maximal power
output of the photovoltaic cell.
Thus, the fill factor is defined as the ratio between the maximal power and
the
product of short circuit current and open circuit voltage (Isc x Voo). The
maximal
power, Iso and Voc are determined as described hereinabove.
The efficiency may be determined by determining the maximal power as
described hereinabove, and dividing by the input light irradiance of the
standard test
conditions.
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It is to be appreciated that embodiments of the present invention do not
necessarily result in increased short circuit current density. Rather, as
exemplified
hereinbelow in the Examples section, it is the combination of a moderately
high short
circuit current density with an increased fill factor which results in the
high efficiencies
of photovoltaic cells according to embodiments of the present invention.
According to another aspect of embodiments of the invention, there is provided
a
photovoltaic array comprising a plurality of any of the photovoltaic cells
described
herein, the photovoltaic cells being interconnected to one another.
As used herein, the phrase "photovoltaic array" describes an array of
photovoltaic cells which are interconnected in series and/or in parallel.
Connection of
the cells in series creates an additive voltage. Connection of the cells in
parallel results
in a higher current. Thus, a skilled artisan can connect the cells in a manner
which will
provide a desired voltage and current.
The array may optionally further combine additional elements such as a sheet
of
glass to protect the photovoltaic cell from the environment without blocking
light from
reaching the photovoltaic cell and/or a base which orients the array in the
direction of a
source of light (e.g., for tracking the daily movement of the sun).
Optionally, an
inverter is present in order to convert the current to alternating current. A
battery is
optionally present in order to store energy generated by the photovoltaic
cell.
According to another aspect of embodiments of the present invention, there is
provided a power plant comprising the photovoltaic array described herein. The
power
plant optionally comprises a plurality of photovoltaic arrays positioned so as
to
maximize their exposure to sunlight.
It is to be appreciated that an optimal position and orientation of a
photovoltaic
array may depend on whether the photovoltaic cells therein are bifacial or
monofacial.
According to another aspect of embodiments of the present invention, there is
provided an electric device comprising the photovoltaic cell of claim 34. In
some
embodiments, the photovoltaic cells are a power source for the electric
device.
Exemplary applications of the photovoltaic cells and/or the solar arrays
described
herein include, but are not limited to, a home power source, a hot water
heater, a pocket
computer, a notebook computer, a portable charging dock, a cellular phone, a
pager, a
PDA, a digital camera, a smoke detector, a GPS device, a toy, a computer
peripheral
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24
device, a satellite, a space craft, a portable electric appliance (e.g., a
portable TV, a
portable lighting device), and a cordless electric appliance (e.g., a cordless
vacuum
cleaner, a cordless drill and a cordless saw).
According to another aspect of embodiments of the present invention, there is
provided a detector of electromagnetic radiation, the detector comprising any
photovoltaic cell described herein, wherein the electromagnetic radiation is
selected
from the group consisting of ultraviolet, visible and infrared radiation. The
detector may
be used, for example, in order to detect the radiation (e.g., as an infrared
detector) and/or
to measure the amount of radiation (e.g., in spectrophotometry).
It is expected that during the life of a patent maturing from this application
many
relevant doping techniques will be developed and the scope of the term
"doping" is
intended to include all such new technologies a priori.
As used herein the term "about" refers to 10 %
The terms "comprises", "comprising", "includes", "including", "having" and
their conjugates mean "including but not limited to".
The term "consisting of means "including and limited to".
The term "consisting essentially of' means that the composition, method or
structure may include additional ingredients, steps and/or parts, but only if
the
additional ingredients, steps and/or parts do not materially alter the basic
and novel
characteristics of the claimed composition, method or structure.
The word "exemplary" is used herein to mean "serving as an example, instance
or
illustration". Any embodiment described as "exemplary" is not necessarily to
be
construed as preferred or advantageous over other embodiments and/or to
exclude the
incorporation of features from other embodiments.
The word "optionally" is used herein to mean "is provided in some embodiments
and not provided in other embodiments". Any particular embodiment of the
invention
may include a plurality of "optional" features unless such features conflict.
As used herein, the singular form "a", "an" and "the" include plural
references
unless the context clearly dictates otherwise. For example, the term "a
compound" or
"at least one compound" may include a plurality of compounds, including
mixtures
thereof.
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Throughout this application, various embodiments of this invention may be
presented in a range format. It should be understood that the description in
range format
is merely for convenience and brevity and should not be construed as an
inflexible
limitation on the scope of the invention. Accordingly, the description of a
range should
5 be considered to have specifically disclosed all the possible subranges as
well as
individual numerical values within that range. For example, description of a
range such
as from 1 to 6 should be considered to have specifically disclosed subranges
such as
from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6
etc., as well
as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6.
This applies
10 regardless of the breadth of the range.
Whenever a numerical range is indicated herein, it is meant to include any
cited
numeral (fractional or integral) within the indicated range. The phrases
"ranging/ranges
between" a first indicate number and a second indicate number and
"ranging/ranges
from" a first indicate number "to" a second indicate number are used herein
15 interchangeably and are meant to include the first and second indicated
numbers and all
the fractional and integral numerals therebetween.
As used herein the term "method" refers to manners, means, techniques and
procedures for accomplishing a given task including, but not limited to, those
manners,
means, techniques and procedures either known to, or readily developed from
known
20 manners, means, techniques and procedures by practitioners of the chemical
and
physical arts.
It is appreciated that certain features of the invention, which are, for
clarity,
described in the context of separate embodiments, may also be provided in
combination
in a single embodiment. Conversely, various features of the invention, which
are, for
25 brevity, described in the context of a single embodiment, may also be
provided
separately or in any suitable subcombination or as suitable in any other
described
embodiment of the invention. Certain features described in the context of
various
embodiments are not to be considered essential features of those embodiments,
unless
the embodiment is inoperative without those elements.
Various embodiments and aspects of the present invention as delineated
hereinabove and as claimed in the claims section below find experimental
support in the
following examples.
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EXAMPLES
Reference is now made to the following examples, which together with the above
descriptions illustrate some embodiments of the invention in a non limiting
fashion.
EXAMPLE I
Exemplary preparation of photovoltaic cells
p-Type monocrystalline silicon pseudosquare substrates (125x125 mm) with a
resistivity of 1.6 ohm were used. The crystal orientation of the substrate
surface was
[100]. Saw damage was removed by means of etching in a solution of 25 % sodium
hydroxide. The substrates were then washed in peroxide-ammoniac solution.
A film of silicon dioxide containing 50 % (by weight) of boron oxide was
applied to the back side of the substrates employing a spin-on method using a
spin rate
of 3,000 rpm.
The substrates were divided into 3 experimental groups of 60 substrates. Films
of silicon dioxide containing 20 %, 25 % or 30 % (by weight) P205 were applied
to the
front surface of the substrates employing the spin-on method.
Diffusion of dopants into the substrate was performed by heating for 20
minutes
at a temperature of 1010 C under a nitrogen atmosphere. The resulting p+
layer on the
back side had sheet resistance of 25 ohm or less and a depth of approximately
1 m.
The resulting n+ layer on the front side exhibited sheet resistances of 25, 17
and 13 ohm
when phosphosilicate films of 20 %, 25 % and 30 %, respectively, of P205 were
used.
Sheet resistances were determined using a four probe method. The depths of the
n+ layers were determined by measuring sheet resistance and subsequently
removing
thin layers of the substrate by etching.
The oxide layers were then removed by a 10 % solution of hydrofluoric acid.
Simultaneous texturing of the front side of the substrate and removal of the
n+ layer was
performed by etching with an aqueous solution of 2 % sodium hydroxide and 4 %
isopropyl alcohol at 80 C. Etching was performed for 5, 10, 15, 25, 30 or 35
minutes.
The substrates were weighed before and after etching. The average depth of
etching was
determined according to a difference in weight before and after texturing.
An antireflective layer of titanium dioxide was then applied on the boron-
doped
surface using an atmospheric pressure chemical vapor deposition (CVD) method.
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A second diffusion of phosphorus into the substrate was performed by applying
a
film of phosphosilicate glass containing 50 % P205, and heating at a
temperature of 850
C for 20 minutes. The resulting n+ layer exhibited a sheet resistance of 55
ohm, and
had a depth of approximately 0.35 m. Phosphorus surface concentration was
determined as described above.
The film of phosphosilicate glass was removed by a 10 % solution of
hydrofluoric acid. The titanium dioxide film was resistant to the hydrofluoric
acid
solution. An antireflective layer of silicon nitride was then applied to the
front surface.
A contact pattern was applied to the both sides of the substrate employing a
screen printed process. PV-156 paste (DuPont) was used for the front contact;
a paste
developed by Monokristal (Stavropol, Russia) was used for the back contact.
Firing was
performed in a Centrotherm furnace.
Laser p-n junction separation was then performed at a distance of 0.2 mm from
the edge of the substrate. The parameters of solar cell performance were then
measured.
The results of the measurements are presented in Tables 1-3 hereinbelow. The
dependence of various parameters on average etching depth during texturing is
depicted
graphically in FIGs. 3-5.
Table 1: Mean values for solar cells prepared using 30 % P205 film
Sheet resistance of initial n+ layer = 13 ohm
Etching Short circuit Open circuit Shunt
Fill factor (FF) Efficiency
depth current (Isc) voltage (Voc) (%) (%) resistance (RSH)
m (amperes) mV (Ohm)
1.6 4.318 609 77.80 13.78 29
3.5 4.969 617 77.57 16.01 38
8 5.260 618 77.50 16.96 40
11 5.297 617 76.01 16.73 36
14 5.291 619 74.66 16.47 39
17 5.303 620 74.54 16.51 35
Table 2: Mean values for solar cells prepared using 25 % P205 film
Sheet resistance of initial n+ layer = 17 ohm
Etching Short circuit Open circuit Shunt
Fill factor (FF) Efficiency
depth current (Isc) voltage (Voc) % resistance % (RSH)
m (amperes mV ( ) ( ) (Ohm)
1.6 4.747 611 77.18 15.08 33
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3.5 5.158 620 77.14 16.60 40
8 5.201 620 76.79 16.68 47
11 5.344 624 75.55 16.95 30
14 5.225 621 75.04 16.40 42
17 5.305 622 74.09 16.45 30
Table 3: Mean values for solar cells prepared using 20 % P205 film
Sheet resistance of initial n+ layer = 25 ohm
Etching Short circuit Open circuit Shunt
Fill factor (FF) Efficiency
depth current (Isc) voltage (Voc) % resistance % (RSH)
m (amperes mV ( ) ( ) (ohm)
1.6 4.977 610 75.51 15.44 28
3.5 5.305 619 75.89 16.77 36
8 5.340 625 75.81 17.03 39
11 5.349 624 75.33 16.93 39
14 5.218 622 75.12 16.42 54
17 5.316 623 74.43 16.60 37
For several of the samples prepared, determination of phosphorus surface
concentration after both the first and second diffusion of phosphorus (i.e.,
in both the
first n+ layer and the second n+ layer) was performed using SIMS (secondary
ion mass
spectrometry). Based on these measurements, the concentration of phosphorus in
both
the peaks and in the troughs of the photovoltaic cells was estimated. The
expected
concentration in the troughs was the concentration measured after the second
diffusion
of phosphorus, whereas the expected concentration in the peaks was the sum of
phosphorus concentrations measured after the first and second diffusions. The
results
are summarized in Table 4.
Table 4: Mean values for phosphorous surface concentrations and expected
concentrations for peaks and troughs.
Initial sheet resistance (Ohm) 13 17 25
Surface concentration of phosphorus 20 20 20
after first diffusion atoms/cm3 __ X 10 -5x 10 -3x 10
Surface concentration of phosphorus 20
after second diffusion atoms/cm3 ~3 X 10
Expected concentration of phosphorus 21 21 21
for peaks, (atoms/cm) 1.1 X 10 -0.8 X 10 0.6 X 10
Expected concentration of phosphorus 20
3 X 10
-
for troughs, (atoms/cm)
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As a control, 25 solar cells were prepared as described in Russian Patent No.
2139601. In this procedure, an initial n+ layer was formed by applying a
silicon dioxide
film containing 15 % (by weight) P205 to the front surface. The resulting
initial n+ layer
had a sheet resistance of 35 ohm and a depth of 1.2 m. The mean values of the
parameters of the control solar cells were as follows: Voc = 616 mV, Jsc =
35.9
mA/cm2, efficiency = 16.2 %
As shown in FIG. 3, the short circuit current density (Jsc) of the solar cells
depended on the depth of etching during texturing, and was maximal at average
etching
depths of more than approximately 4 m.
As shown in FIG. 4, the fill factor (FF) of the solar cells depended on the
depth
of etching during texturing, and was maximal when the average etching depth
was less
than approximately 8 m.
As shown in FIG. 5, the efficiency of the solar cells depended on the etching
depth, and was maximal when the average etching depth was in a range of
approximately 4-12 m.
As shown in Tables 1-3 and in FIG. 5, the efficiency of the solar cells was
higher
than that of the efficiency of the control cells (16.2 %), and efficiencies of
over 17 %
were obtained. The relative gain in efficiency over control values was
approximately 3-
5 %.
These results show that the formation of an initial n+ layer and its removal
by
etching, as described hereinabove, results in high solar cell efficiency when
the etching
depth is within an optimal range for which relatively high values of both
short circuit
current and fill factor are obtained.
EXAMPLE 2
Effect of antireflective coatings on photovoltaic cell performance
Photovoltaic cells were prepared as described in Example 1 with an initial n+
layer having a sheet resistance of 25 ohm and an etching depth of 8 m. Laser
p-n
junction separation was performed at a distance of 0.2 mm from the edge of the
substrate.
As described in Example 1, an antireflective coating was applied to the boron-
doped surface before formation of the final n+ layer by phosphorus-doping, and
an
antireflective coating was applied to the final n+ layer following phosphorus-
doping.
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In one group, application of the antireflective layer on each side of the
photovoltaic cell comprised forming a 75 nm layer of titanium oxide
(refractive index =
2.2) using an atmospheric pressure chemical vapor deposition (CVD) method, as
described in Example 1.
5 In a second group, application of the antireflective layer on each side of
the
photovoltaic cell comprised forming a 60 nm layer of silicon nitride
(refractive index =
2.2) followed by forming an -80 nm layer of silicon oxynitride (refractive
index = 1.7)
using a plasma-enhanced chemical vapor deposition (PECVD) method.
The photovoltaic cells were than laminated with a poly(ethyl-vinyl acetate)
10 (EVA) film (refractive index = 1.45).
As a control, photovoltaic cells were also prepared as described in Russian
Patent
No. 2139601.
The performance of the photovoltaic cells was measured under both front
illumination (illumination of the n-doped surface) and back illumination
(illumination of
15 the p-doped surfaced). The effect of the antireflective layers on various
parameters of
photovoltaic cell performance is shown in Table 5.
Table 5: Mean values of solar cells with different antireflective coatings.
Short circuit Open circuit Fill factor, FF Efficiency
current, Iso voltage, Voc (%) (%)
(Amperes) mV
Front illumination
Control 5.334 615 73.25 16.17
Ti02 coating 5.346 625 75.66 17.01
Silicon nitride/silicon
ox nitride coating 5.358 626 75.68 17.08
Back illumination
Control 3.389 612 75.10 10.48
Ti02 coating 3.612 617 75.98 11.39
Silicon nitride/silicon
oxynitride coating 3.784 618 76.11 11.97
As shown in Table 5, the efficiency of the solar cells was higher than the
efficiency of the control cells for both front and back illumination. As
further shown in
Table 5, the silicon nitride/oxynitride antireflective coating provided
improved
efficiency relative to the Ti02 coating, particularly for back illumination.
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EXAMPLE 3
Measurements of effective minority carrier lifetime
In order to determine the effect of silicon nitride deposition on surface
recombination, the effective minority carrier lifetime was determined in p+-p-
p+
structures. p+-p-p+ structures were used instead of the n+-p-p+ structure of a
photovoltaic
cell in order to simplify interpretation of the experimental results.
4 samples were prepared from 1 ohm.cm silicon wafers, which were doped on
both sides with boron by applying a film of silicon dioxide containing 50 %
(by weight)
of boron oxide to the back side of the substrates, and then heating for 20
minutes at a
temperature of 1010 C under a nitrogen atmosphere. A 60 nm layer of silicon
nitride
(refractive index = 2.2) was then deposited on both sides of the wafer using a
plasma-
enhanced chemical vapor deposition (PECVD) method, and the wafer was then
subjected to thermal treatment at a temperature of 850 C for 20 minutes.
The lifetime values were determined from decay of injected carrier
concentration
at the various stages.
As shown in FIG. 6, the effective carrier lifetime values decreased after
silicon
nitride deposition and were then fully restored after thermal treatment.
These results indicate that thermal treatment of the antireflective coating
increases carrier lifetime by reducing surface recombination in the p+ layer,
thereby
improving photovoltaic cell performance.
EXAMPLE 4
Effect of coating refractive index on photovoltaic cell current
The effect of the refractive index of an antireflective coating on short
circuit
current density (Jsc) of a photovoltaic cell was calculated for 1-layer and 2-
layer
coatings.
For the purposes of the calculation, the photovoltaic cell was assumed to be a
silicon-based photovoltaic cell with theoretically maximal internal quantum
efficiency
and a smooth surface, within an optical medium with a refractive index of 1.45
(the
refractive index of poly(ethylene-vinyl acetate)).
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For a 1-layer coating, Jsc was calculated for each given refractive index of
the
coating as a function of coating thickness, and the Jsc at the optimal coating
thickness
(i.e., Jsc for a coating thickness at which Jsc is maximal) was determined.
For a 2-layer coating, Jsc was calculated for each given refractive index and
for
different thicknesses of the lower layer (the layer adjacent to the silicon
surface) of the
coating as a function of refractive index and thickness of the upper layer,
and the Jsc at
the optimal upper layer refractive index (i.e., Jsc for a coating thickness
and upper layer
refractive index at which Jsc is maximal) was determined.
As shown in FIG. 7, short circuit current density is highest when the
refractive
index of the antireflective coating (or of the lower layer of the
antireflective coating
when there is more than one layer in the coating) is at least approximately
2.3.
Although the invention has been described in conjunction with specific
embodiments thereof, it is evident that many alternatives, modifications and
variations
will be apparent to those skilled in the art. Accordingly, it is intended to
embrace all
such alternatives, modifications and variations that fall within the spirit
and broad scope
of the appended claims.
All publications, patents and patent applications mentioned in this
specification
are herein incorporated in their entirety by reference into the specification,
to the same
extent as if each individual publication, patent or patent application was
specifically and
individually indicated to be incorporated herein by reference. In addition,
citation or
identification of any reference in this application shall not be construed as
an admission
that such reference is available as prior art to the present invention. To the
extent that
section headings are used, they should not be construed as necessarily
limiting.
