METHOD OF MANUFACTURING PHOTOVOLTAIC CELLS, PHOTOVOLTAIC CELLS PRODUCED THEREBY AND USES THEREOF

PROCEDE DE FABRICATION DE CELLULES PHOTOVOLTAIQUES, CELLULES PHOTOVOLTAIQUES PRODUITES SELON CE PROCEDE, ET UTILISATIONS DE CELLES-CI

English Abstract

Novel methods of producing photovoltaic cells are provided herein, as well as photovoltaic cells produced thereby, and uses thereof. In some embodiments, a method as described herein comprises doping a substrate so as to form a p+ layer on one side and an n+ layer on an another side, removing at least a portion of the n+ layer, and then forming a second n+ layer, such that a concentration of the n-dopant in the second n+ layer is variable throughout a surface of the substrate.

French Abstract

La présente invention concerne de nouveaux procédés de fabrication de cellules photovoltaïques, ainsi que des cellules photovoltaïques produites selon ces procédés et les utilisations de celles-ci. Dans certains modes de réalisation, un tel procédé consiste à doper un substrat de façon à former une couche dopée p+ sur une face et une couche dopée n+ sur l'autre face, à enlever au moins une partie de la couche dopée n+, puis à former une seconde couche dopée n+ de façon que la concentration de dopant n dans la seconde couche dopée n+ soit variable dans l'ensemble de la surface du substrat.

Claims

34
WHAT IS CLAIMED IS:
1. 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 said substrate;
b) doping a second surface of said substrate with a p-dopant so as to form a
p+ layer in said substrate;
c) removing a portion of said first n+ layer from said first surface of said
substrate, such that a concentration of said n-dopant remaining in said first
surface of
said substrate is variable throughout said first surface;
d) doping said first surface of said substrate with an n-dopant so as to form
a
second n+ layer, such that a concentration of said n-dopant in said second n+
layer is
variable throughout said first surface; and
e) forming electrical contacts on each of said first surface and said second
surface,
thereby producing said photovoltaic cell.
2. The method of claim 1, wherein said first n+ layer and said p+ layer are
formed simultaneously.
3. The method of claim 2, wherein said doping with said n-dopant so as to
form said first n+ layer and said doping with said p-dopant so as to form said
p+ layer is
effected by:
(i) applying a film comprising said p-dopant to said second surface;
(ii) applying a film comprising said n-dopant to said first surface; and
(iii) heating said substrate,
thereby simultaneously forming said first n+ layer and said p+ layer.
4. A method of producing a photovoltaic cell, the method comprising:
a) applying a film comprising a p-dopant to a second surface of a
semiconductive substrate;

35
b) removing said film comprising said p-dopant from a first surface of said
substrate and from an edge of said substrate;
c) applying a film comprising an n-dopant to said first surface;
d) heating said substrate, so as to simultaneously form a first n+ layer on
said
first surface and a p+ layer on said second surface of said substrate;
e) removing a portion of said first n+ layer from said first surface of said
substrate, such that a concentration of said n-dopant remaining in said first
surface of
said substrate is variable throughout said first surface;
f) doping said first surface of said substrate with an n-dopant so as to form
a
second n+ layer, such that a concentration of said n-dopant in said second n+
layer is
variable throughout said first surface; and
g) forming electrical contacts on each of said first surface and said second
surface,
thereby producing said photovoltaic cell.
5. The method of claim 4, wherein said film comprising said p-dopant
comprises boron oxide.
6. The method of any of claims 4 to 5, wherein said film comprising said n-
dopant comprises phosphorus pentoxide.
7. The method of any of claims 4 to 6, wherein said removing of said film
comprises washing said first surface of said substrate.
8. The method of any of claims 4 to 7, wherein said removing said film
comprises removing said film from an area bordering an edge of said substrate,
said area
having a width in a range of 0.1-1 mm.
9. The method of any of claims 1 to 8, wherein said first n+ layer is
characterized by a sheet resistance of less than 30 ohms.

36
10. The method of any of claims 1 to 9, wherein said first n+ layer has a
depth
in a range of 0.4-2 µm.
11. The method of any of claims 1 to 10, wherein said second n+ layer is
characterized by a sheet resistance in a range of 30-100 ohms.
12. The method of any of claims 1 to 11, wherein said second n+ layer has a
depth in a range of 0.2-0.7 µm.
13. The method of any of claims 1 to 12, wherein said removing said portion
of said first n+ layer from said first surface comprises texturing said first
surface.
14. The method of claim 13, wherein said texturing generates peaks and
troughs in said first surface, wherein a concentration of said n-dopant
remaining in said
first surface following texturing is greater in said peaks than in said
troughs.
15. The method of claim 14, wherein a concentration of said n-dopant in said
second n+ layer is greater in said peaks than in said troughs.
16. The method of claim 15, wherein a concentration of said n-dopant in said
peaks in said second n+ layer is at least twice a concentration of said n-
dopant in said
troughs in said second n+ layer.
17. The method of any of claims 15 to 16, wherein a concentration of said n-
dopant in said peaks in said second n+ layer is at least 5x10 20 atoms/cm3.
18. The method of any of claims 15 to 17, wherein a concentration of said n-
dopant in said troughs in said second n+ layer is less than 10 21 atoms/cm3.
19. The method of any of claims 1 to 18, wherein removing said portion of
said n+ layer from said first surface comprises etching said first surface to
an average
depth in a range of from 4 µm to 12 µm.

37
20. The method of any of claims 1 to 19, wherein said substrate is an n-type
semiconductor prior to forming said first n+ layer and said p+ layer.
21. The method of any of claims 1 to 19, wherein said substrate is a p-type
semiconductor prior to forming said first n+ layer and said p+ layer.
22. The method of any of claims 1 to 21, further comprising applying an
antireflective coating.
23. The method of claim 22, wherein said antireflective coating is applied to
said second surface subsequent to forming said p+ layer and prior to forming
said second
n+ layer.
24. The method of claim 22, wherein said antireflective coating is applied to
said first surface subsequent to forming said second n+ layer.
25. A photovoltaic cell produced according to the method of any of claims 1
to 24.
26. A photovoltaic cell comprising a semiconductive substrate, said substrate
comprising an n+ layer on a first surface thereof and a p+ layer on a second
surface
thereof, said n+ layer comprising an n-dopant and said p+ layer comprising a p-
dopant,
and electrical contacts attached to each of said first surface and said second
surface,
wherein said first surface is textured so as to comprise peaks and troughs,
and
wherein a concentration of said n-dopant in said n+ layer is greater in said
peaks
of said first surface than in said troughs of said first surface.
27. A photovoltaic cell produced according to the method of any of claims 4
to 8.
28. A photovoltaic cell comprising a semiconductive substrate, said substrate
comprising an n+ layer on a first surface thereof and a p+ layer on a second
surface

38
thereof, said n+ layer comprising an n-dopant and said p+ layer comprising a p-
dopant,
and electrical contacts attached to each of said first surface and said second
surface,
wherein said first surface is textured so as to comprise peaks and troughs,
wherein a concentration of said n-dopant in said n+ layer is greater in said
peaks
of said first surface than in said troughs of said first surface,
wherein said p+ layer and said n+ layer do not contact one another, and
wherein said p+ layer does not reach an edge of said second surface.
29. The photovoltaic cell of any of claims 27 to 28, characterized by a
specific shunt resistance of at least 4,750 ohm*cm2, wherein said specific
shunt
resistance is determined for a photovoltaic cell having an area in a range of
150-160 cm2
and a circumference in a range of 45-55 cm.
30. The photovoltaic cell of claim 28, wherein said p+ layer does not cover an
area bordering an edge of said substrate, said area having a width in a range
of 0.1-1
mm.
31. The photovoltaic cell of any of claims 26, 28 and 30, wherein said n+
layer is characterized by a sheet resistance in a range of 30-100 ohm.
32. The photovoltaic cell of any of claims 26, 28 and 30 to 31, wherein said
n+ layer has a depth in a range of 0.2-0.7 µm.
33. The photovoltaic cell of any of claims 26, 28 and 30 to 32, wherein a
concentration of said n-dopant in said peaks is at least twice a concentration
of said n-
dopant in said troughs.
34. The photovoltaic cell of any of claims 26, 28 and 30 to 33, comprising an
n+-n-p+ structure.
35. The photovoltaic cell of any of claims 26, 28 and 30 to 33, comprising an
n+-p-p+ structure.

39
36. The photovoltaic cell of any of claims 26, 28 and 30 to 35, further
comprising an antireflective coating.
37. The photovoltaic cell of any of claims 25 to 36, characterized by a short
circuit current density of at least 0.033 amperes/cm2.
38. The photovoltaic cell of any of claims 25 to 37, characterized by a fill
factor of at least 75.5 %.
39. The photovoltaic cell of any of claims 25 to 38, characterized by an
efficiency of at least 16.7 %.
40. The photovoltaic cell of any of claims 25 to 39, being a bifacial
photovoltaic cell.
41. A photovoltaic array comprising a plurality of the photovoltaic cell of
any
of claims 25 to 40, said plurality of photovoltaic cells being interconnected
to one
another.
42. A power plant comprising the photovoltaic array of claim 41.
43. An electric device comprising the photovoltaic cell of any of claims 25 to
40.

Description

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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.

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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) 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;
d) 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
e) forming electrical contacts on each of the first surface and the second
surface,
thereby producing the photovoltaic cell.
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) applying a film comprising a p-dopant to a second surface of a
semiconductive substrate;
b) removing the film comprising the p-dopant from a first surface of the
substrate and from an edge of the substrate;
c) applying a film comprising an n-dopant to the first surface;
d) heating the substrate, so as to simultaneously form a first n+ layer on the
first surface and a p+ layer on the second surface of the substrate;
e) 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;
f) 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

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g) forming electrical contacts on each of the first surface and the second
surface,
thereby producing the photovoltaic cell.
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) applying a film comprising a p-dopant to a second surface of a
semiconductive substrate;
b) removing the film comprising the p-dopant from a first surface of the
substrate and from an edge of the substrate;
c) applying a film comprising an n-dopant to the first surface;
d) heating the substrate, so as to simultaneously form a first n+ layer on the
first surface and a p+ layer on the second surface of the substrate;
e) removing at least a portion of the first n+ layer; and
f) forming electrical contacts on each of the first surface and the second
surface,
thereby producing the photovoltaic cell.
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, 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.
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, 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,

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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,
wherein the p+ layer and the n+ layer do not contact one another, and
wherein the p+ layer does not reach an edge of the second surface.
5 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, and
electrical contacts attached to each of the first surface and the second
surface,
wherein the p+ layer and the n+ layer do not contact one another, and wherein
the
p+ layer does not reach an edge of the second 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 method further comprises
forming a second n+ layer on the first surface subsequent to removing the
first n+ layer
and prior to forming the electrical contacts on the first surface.
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.

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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.
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-
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.

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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.
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 of the invention, the film comprising the p-
dopant and the film comprising the n-dopant are applied so as not to contact
one another.
According to some embodiments of the invention, the removing of the film
comprises washing the first surface of the substrate.
According to some embodiments of the invention, the washing comprises
applying a solution on the substrate using a spin-on method.
According to some embodiments of the invention, the spin-on method uses a spin
rate in a range of 400-4,500 rotations per minute.
According to some embodiments of the invention, the solution for the washing
comprises water and isopropyl alcohol.
According to some embodiments of the invention, removing the film comprises
removing the film from an area bordering an edge of the substrate, the area
having a
width in a range of 0.1-1 mm.

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According to some embodiments of the invention, the p+ layer of the
photovoltaic cell does not cover an area bordering an edge of the substrate,
the area
having a width in a range of 0.1-1 mm.
According to some embodiments of the invention, the method further comprises
applying an antireflective coating.
According to some embodiments of the invention, the photovoltaic cell further
comprises an antireflective coating.
According to some embodiments of the invention, the antireflective coating is
applied to the second surface subsequent to forming the p+ layer and prior to
forming the
second n+ layer.
According to some embodiments of the invention, the antireflective coating is
applied to the first surface subsequent to forming the second n+ layer.
According to some embodiments of the invention, the semiconductive substrate
is an n-type semiconductor prior to forming the first n+ layer and the p+
layer.
According to some embodiments of the invention, the semiconductive substrate
is a p-type semiconductor prior to forming the first n+ layer and the p+
layer.
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
characterized by a specific shunt resistance of at least 4,750 ohm*cm2,
wherein the
specific shunt resistance is determined for a photovoltaic cell having an area
in a range
2
of 150-160 cm and a circumference in a range of 45-55 cm.

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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.
1o BRIEF 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 (Isc) 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 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

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invention, wherein the sheet resistance of the first n+ layer of the cells was
13, 17 or 25
ohm; and
FIG. 5 is a is a graph showing the dependence of efficiency on etching depth
(in
micrometers) in photovoltaic cells produced according to an embodiment of the
5 invention, wherein the sheet resistance of the first n+ layer of the cells
was 13, 17 or 25
ohm.
DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION
The present invention, in some embodiments thereof, relates to energy
10 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 applying a film comprising a p-
dopant
and a film comprising an n-dopant to opposite sides of a substrate, efficiency
of the
photovoltaic cell can be enhanced by introducing a simple, inexpensive
procedure for
removing the film comprising a dopant from portions of the substrate, in
between the
applications of the two films. Thus, a film comprising a p-dopant or an n-
dopant is
applied to one side of the substrate, the reverse side and the edge of the
substrate are
cleaned (e.g., by washing the reverse side with a solution), thereby removing
the film
from that side (if any is present) and from the edge of the substrate, and the
other film is
then applied on the cleaned side. This procedure significantly reduces contact
between
the two films. Consequently, overlap between the doped layers that are formed
by these
two films (the n+ layer and p+ layer) is reduced, and shunts which reduce the
efficiency
of the photovoltaic cell are thereby avoided. This procedure is superior to
prior methods
of reducing overlap, such as laser separation, which tends to be costly and to
reduce the
working area of the photovoltaic cell.
The present inventors have therefore devised and successfully practiced a
novel
methodology for producing a photovoltaic cell, which involves a reduced number
of

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11
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-
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
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
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. p-dopant-containing film 2 is removed from the edge of substrate 1.
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.,

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12
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. Electrical contacts 9 are then formed on both sides of the substrate,
to form a
photovoltaic cell.
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. p-dopant-containing film 2 is removed from the edge of substrate 1.
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. 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 does not reach the edge of the
substrate as a
result of the washing step, and further because the p+ layer is protected by
the rear
antireflection coating when the second n+ layer is formed.

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13
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.
The above-described exemplary methods also form a non-symmetrical structure
in which one side is textured and the other side is smooth. 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 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.
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
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).

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14
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).
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;
b) doping a second surface of the substrate with a p-dopant so as to form a
p+ layer in the substrate;
c) 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;
d) 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
e) forming electrical contacts on each of the first surface and the second
surface.
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.
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"

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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
5 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
10 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
15 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 semi-conductive 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.
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

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16
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.
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,
optionally less than 20 ohm, and optionally less than 15 ohm. According to
exemplary
embodiments, the sheet resistance is in a range of between about 13 ohms to
about 25
ohms.
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.

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According to some embodiments, the second n+ layer is characterized by a sheet
resistance in a range of 30-100 ohms. Optionally, the sheet resistance is in a
range of
40-65 ohms. According to an exemplary embodiment, the sheet resistance is
about 55
ohms.
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
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.
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.

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According to some embodiments, a concentration of the n-dopant in the troughs
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
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.
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
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

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19
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.
In some embodiments, the film comprising the p-dopant and the film comprising
the n-dopant are applied so as not to contact one another, thus resulting in
reduced
shunting.
This is optionally effected by removing the film comprising the p-dopant from
a
first surface of the substrate and from an edge of the substrate.
It is to be appreciated that even in embodiments wherein the film comprising
the
p-dopant is not applied to the first surface and/or an edge, minute quantities
of the film
may inadvertently be present thereon. Such minute quantities may be
considerably
detrimental to the performance of a photovoltaic cell, by creating shunting.
Thus, it is to be understood that "removing the film" refers to any procedure
capable of removing any film which may be present, and does not require
demonstrating
that film is indeed present.
According to some embodiments, the film is removed from an area of the second
surface bordering an edge of the substrate. Optionally, such an area has an
average
width in a range of 0.1-1 mm. Thus, a film-free band (e.g., a 0.1-1 mm wide
band) is
formed around the perimeter of the second surface of the substrate. Such a
film-free
band further reduces the likelihood of overlap between p-dopant and n-dopant,
thereby
reducing shunting.
Optionally, removing the film is effected by washing the first surface of the
substrate with a solution. The washing may be performed such that film is
simultaneously removed from both the first surface and from an edge of the
substrate,
and optionally also from an area of the second surface which borders an edge.
According to some embodiments, the washing comprises applying a washing
solution on the substrate using a spin-on method. Optionally, the spin-on
method uses a
spin rate in a range of 400-4,500 rotations per minute (rpm), and optionally
in a range of
600-3,000 rpm. As exemplified hereinbelow in the Examples section, the spin-on
method simultaneously removes film from both the edge of the substrate and
from the
perimeter of the second surface, in addition to the first surface.
An exemplary solution for washing comprises water and isopropyl alcohol.

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In some embodiments, removing the film is effected via other methods known in
art.
As exemplified hereinbelow, the present inventors have surprisingly found that
the above-described procedure for removing a p-dopant-containing film, while
being
5 simple, convenient and inexpensive, provides photovoltaic cell performance
which is at
least as efficient as the performance obtained using more costly and
technically complex
standard methods for separating p+ and n+ layers (e.g., laser separation,
plasma etching).
Thus, the procedure described herein for removing a p-dopant-containing film
is
applicable for improving performance of a wide variety of photovoltaic cell
types.
10 Hence, according to another aspect of embodiments of the present invention,
there is provided a method of producing a photovoltaic cell, the method
comprising:
a) applying a film comprising a p-dopant (e.g., boron) to a second surface of
a semiconductive substrate (e.g., silicon);
b) removing the film comprising the p-dopant from a first surface of the
15 substrate and from an edge of the substrate (e.g., by washing the first
surface with a
liquid);
c) applying a film comprising an n-dopant (e.g., phosphorus, arsenic) to the
first surface;
d) heating the substrate, so as to simultaneously form a first n+ layer on the
20 first surface and a p+ layer on the second surface of the substrate;
e) removing at least a portion of the first n+ layer; and
f) forming electrical contacts on each of the first surface and the second
surface, thereby producing the photovoltaic cell.
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.

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21
In some embodiments, the method further comprises forming a second n+ layer
on the first surface subsequent to removing the first n+ layer and prior to
forming the
electrical contacts on the first surface. Removing all or nearly all of the
first n+ layer and
subsequently forming a second n+ layer with the desired properties may be less
difficult
and more reliable than removing only a portion of the first n+ layer. However,
in some
embodiments, no formation of a second n+ layer is performed. In such
embodiments, the
removal of the first n+ layer is optionally performed to a depth that is
correlated to the n+
layer thickness and doping profile so as to generate both regions (e.g.,
peaks) with high
n-dopant concentrations and regions (e.g., troughs) with low n-dopant
concentrations, as
described herein.
The dopant-containing films, applications thereof, removal of the film
containing
the p-dopant (e.g., washing procedure), and properties (e.g., depth, sheet
resistance) of
the final n+ layer (e.g., the second n+ layer) are optionally as described
hereinabove.
According to exemplary embodiments of the present invention, the above-
described novel procedure of removing an applied p-dopant-containing film from
the
first surface and from an edge is combined with the above-described novel
formation of
a variable concentration of n-dopant on the first surface.
Thus, according to an aspect of embodiments of the present invention, there is
provided a method of producing a photovoltaic cell, the method comprising:
a) applying a film comprising a p-dopant to a second surface of a
semiconductive substrate;
b) removing the film comprising the p-dopant from a first surface of the
substrate and from an edge of the substrate (e.g., by washing the first
surface with a
liquid);
c) applying a film comprising an n-dopant to the first surface;
d) heating the substrate, so as to simultaneously form a first n+ layer on the
first surface and a p+ layer on the second surface of the substrate;
e) 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;

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f) 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
g) forming electrical contacts on each of the first surface and the second
surface, thereby producing the 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, 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.
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, and electrical
contacts
attached to each of the first surface and the second surface, wherein the p+
layer and the
n+ layer do not contact one another, and wherein the p+ layer does not reach
an edge of
the second surface.
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, 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, 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 surface,
wherein the p+ layer and the n+ layer do not contact one another, and wherein
the p+
layer does not reach an edge of the second 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
the methods described herein. Thus, the n+ layer of the photovoltaic cells may

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23
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.
According to some embodiments, the specific shunt resistance of the
photovoltaic cell is at least 4,750 ohm*cm2 (i.e., ohm multiplied by cm2),
optionally at
least 5,500 ohm*cm2, and optionally at least 6,250 ohm*cm2. As specific shunt
resistance may depend on the area and shape of a photovoltaic cell, the
aforementioned
specific shunt resistance may be determined for a photovoltaic cell having an
area in a
range of 150-160 cm2 and being substantially square (i.e., having a
circumference in a
range of 45-55 cm).
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 AM
(airmass) of 1.5 and a cell temperature 25 C.
Short circuit current density may be determined, for example, by measuring the
current (Isc) 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.

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Thus, the fill factor is defined as the ratio between the maximal power and
the
product of short circuit current and open circuit voltage (Iso x Voo). The
maximal
power, Isc and Voc are determined as described hereinabove.
Specific shunt resistance may be determined, for example, by measuring the
current produced by the photovoltaic cell at various operating voltages, so as
to obtain
data describing the current as a function of operating voltage, and obtaining
the shunt
resistance from such data via non-linear regression, using techniques known in
the art.
The shunt resistance can then be multiplied by the area of the photovoltaic
cell to obtain
the specific shunt resistance. In order to assay photovoltaic cells with an
area in a range
of 150-160 cm2 and being substantially square, larger photovoltaic cells can
be cut to the
appropriate area and shape. 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.
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.
Without being bound by any particular theory, it is believed that embodiments
of
the methods and photovoltaic cells described herein in which a p-dopant-
containing film
is removed from the first surface and from an edge of the substrate are
particularly
suitable for obtaining an increased specific shunt resistance, and
consequently an
increased fill factor and efficiency.
According to some embodiments, the p+ layer of the photovoltaic cell does not
cover an area bordering an edge of the substrate thereof, the area having a
width in a
range of 0.1-1 mm.
According to some embodiments of each of the aspects described herein, an
antireflective coating is applied onto the substrate of the photovoltaic cell.
Various
antireflective coatings will be familiar to one of ordinary skill in the art.
The antireflective coating may be applied in more than one step. For example,
according to exemplary embodiments, an antireflective coating is applied to
the second
surface in one step, and applied to the first surface in another step.
Optionally, an

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antireflective coating applied to one surface is of a different composition
than an
antireflective coating applied to another surface.
According to some embodiments, the antireflective coating is applied to the
second surface subsequent to forming the p+ layer and prior to forming the
second n+
5 layer. As discussed hereinabove, such an application of an antireflective
coating may be
useful in preventing overlap between the p+ layer and the second n+ layer,
provided that
the antireflective coating is at least somewhat resistant to diffusion of the
n-dopant.
According to some embodiments, the antireflective coating is applied to the
first
surface subsequent to forming the second n+ layer.
10 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
15 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
20 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
25 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.

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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
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.

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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.
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
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
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
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
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
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

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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.
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.
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. A solution of 50 % water and 50 % isopropyl alcohol was applied
to the
front side of the substrates employing a spin-on method using a spin rate of
1,200 rpm
for 4 seconds. As a result of the application of water/isopropyl alcohol,
borosilicate
glass was removed from a narrow strip (approximately 0.6 mm wide) along the
edge of
the back side. The spin rate was then increased to 3,000 rpm, resulting in
complete
drying of the substrates.
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. A clean break
between
the films of borosilicate and phosphosilicate glass was observed along the
edge of the
back side (i.e, the two films did not come into contact).
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.

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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. In
all groups,
a textured ring along the edge of the back (boron-doped) side was clearly
visible after
texturing. As the boron-doped areas resisted texturing, this ring confirmed
that the
boron-doped area did not extend to the edge of the substrate.
An antireflective layer of titanium dioxide was then applied on the boron-
doped
surface using an atmospheric pressure chemical vapor deposition (CVD) method.
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.
After the contacts were formed, the parameters of solar cell performance were
measured. Laser p-n junction separation was then performed at a distance of
0.2 mm
from the edge of the substrate. The parameters were then measured again. The
results
of the measurements are presented in Tables 1-3 hereinbelow.

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The dependence of various parameters (before laser separation) 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)
Before Laser Separation
1.6 4.326 612 77.83 13.87 28
3.5 5.030 620 77.9 16.35 43
8 5.292 621 77.66 17.17 38
11 5.332 621 76.45 17.03 42
14 5.346 622 75.16 16.82 54
17 5.346 622 74.6 16.69 38
After Laser Separation
1.6 4.293 611 77.81 13.74 30
3.5 4.992 618 77.82 16.16 39
8 5.252 619 77.63 16.98 42
11 5.292 620 76.28 16.84 37
14 5.306 620 74.92 16.59 39
17 5.306 621 74.56 16.53 36
5 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 ( R sx)
%
m (amperes) mV ( ) ( ) (Ohm)
Before Laser Separation
1.6 4.771 615 77.31 15.26 40
3.5 5.188 621 77.13 16.72 37
8 5.230 624 77.14 16.94 52
11 5.362 624 75.92 17.09 32
14 5.260 625 75.26 16.65 41
17 5.346 622 74.49 16.67 37
After Laser Separation
1.6 4.735 614 77.32 15.13 34
3.5 5.149 620 77.15 16.57 40
8 5.190 622 77.12 16.75 48
11 5.322 623 75.94 16.94 31
14 5.220 623 75.21 16.46 43
17 5.306 621 74.42 16.50 31

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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 ( R sx)
%
m (amperes) mV ( ) ( ) (ohm)
Before Laser Separation
1.6 4.978 615 75.9 15.64 30
3.5 5.303 622 75.9 16.85 37
8 5.386 626 75.68 17.17 44
11 5.356 624 75.52 16.99 39
14 5.303 623 75.16 16.71 50
17 5.326 626 74.89 16.80 42
After Laser Separation
1.6 4.941 614 75.89 15.49 33
3.5 5.263 621 75.9 16.69 39
8 5.346 625 75.66 17.01 42
11 5.316 624 75.51 16.86 38
14 5.263 623 75.06 16.56 50
17 5.286 625 74.87 16.65 39
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 __ X 10 -3 X 10
Surface concentration of phosphorus 20
after second diffusion atoms/cm3 __ 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
for troughs, atoms/cm3 ~3 X 10

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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.
As further shown in Tables 1-3, laser p-n junction separation did not improve
the
shunt resistance to any significant extent, and the solar cell efficiency was
even reduced
by 0.6-0.8 % by laser separation, possibly due to a decrease of the area of
the working
surface.
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.

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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.

Representative Drawing