U.S. patent application number 13/818476 was filed with the patent office on 2013-08-29 for photovoltaic device and module with improved passivation and a method of manufacturing.
This patent application is currently assigned to ENERGY RESEARCH CENTRE OF THE NETHERLANDS. Invention is credited to Franciscus Theodorus Agricola, Lars Janssen, Jonas Koopmann, Menno Nicolaas Van Den Donker.
Application Number | 20130220396 13/818476 |
Document ID | / |
Family ID | 43778447 |
Filed Date | 2013-08-29 |
United States Patent
Application |
20130220396 |
Kind Code |
A1 |
Janssen; Lars ; et
al. |
August 29, 2013 |
Photovoltaic Device and Module with Improved Passivation and a
Method of Manufacturing
Abstract
A photovoltaic device, having an improved passivation of
surfaces, such as a circumferential outer wall and/or an aperture
wall of a back contact metal wrap-through photovoltaic device, for
example, into which the pn-junction of first and second
semiconductor layers extends. The passivation comprises a
passivating layer of a first type, covering at least part of such
wall substantially comprised by the depletion region across the
pn-junction; a passivating layer of a second type, covering at
least part of such wall comprised by the first semiconductor layer,
and a passivating layer of a third type covering at least part of
the outer wall comprised by the second semiconductor layer.
Inventors: |
Janssen; Lars; (Heerlen,
NL) ; Koopmann; Jonas; (Heerlen, NL) ; Van Den
Donker; Menno Nicolaas; (Heerlen, NL) ; Agricola;
Franciscus Theodorus; (Heerlen, NL) |
Assignee: |
ENERGY RESEARCH CENTRE OF THE
NETHERLANDS
Petten
NL
|
Family ID: |
43778447 |
Appl. No.: |
13/818476 |
Filed: |
August 24, 2011 |
PCT Filed: |
August 24, 2011 |
PCT NO: |
PCT/NL2011/000060 |
371 Date: |
May 7, 2013 |
Current U.S.
Class: |
136/244 ;
136/256; 438/98 |
Current CPC
Class: |
H01L 31/02161 20130101;
H01L 31/18 20130101; Y02P 70/50 20151101; H01L 31/0516 20130101;
H01L 31/02245 20130101; H01L 31/022425 20130101; H01L 31/1868
20130101; Y02E 10/50 20130101 |
Class at
Publication: |
136/244 ;
136/256; 438/98 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224; H01L 31/05 20060101 H01L031/05 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 24, 2010 |
NL |
2005261 |
Aug 24, 2011 |
NL |
PCT/NL2011/000060 |
Claims
1-30. (canceled)
31. A photovoltaic device, comprising: a layered structure having a
front surface for receiving optical radiation and a rear surface
opposite to said front surface, the layered structure having a
circumferential outer wall surrounding the layered structure; at
least one electrical contact arrangement positioned at the rear
surface, and optionally at the front surface, or at both the front
surface and the rear surface; a first semiconductor layer of a
first conductivity type, the first semiconductor layer extending
adjacent to the front surface and to the outer wall; at least one
electrical front contact positioned in communication with the front
surface and the first semiconductor layer, the at least one
electrical front contact being connected to the at least one
contact arrangement on the front surface, when the at least one
contact arrangement is present on the front surface; a second
semiconductor layer of a second conductivity type, said second
layer extending contiguous to the first layer and to the outer
wall; at least one electrical rear contact in communication with
the second semiconductor layer and the at least one electrical
contact arrangement at the rear surface; the layered structure
optionally comprising at least one aperture defined therein
extending from the front surface to the rear surface and being
bounded by an aperture wall, the aperture defining an electrical
contact path connecting the at least one electrical front contact
to the at least one electrical contact arrangement at the rear
surface; the conductivity types of the first semiconductor layer
and the second semiconductor layer being of opposite polarity
wherein the contact between the first semiconductor layer and the
second semiconductor layer is defined by a pn-junction and a
depletion region formed across the pn-junction, the pn-junction and
depletion region extending to the outer wall, and to the aperture
wall, when the at least one defined aperture is present; a
passivating layer of a first type covering the part of the outer
wall that is in contact with the pn-junction and depletion region;
a passivating layer of a second type covering the part of the outer
wall that is in contact with the first semiconductor layer, the
passivating layer of the second type being contiguous to the
passivating layer of the first type; a passivating layer of a third
type covering the part of the outer wall that is in contact with
the second semiconductor layer, the passivating layer of the third
type being contiguous to the passivating layer of the first type;
and when at least one aperture is defined, the passivating layer of
the first type extends into the at least one defined aperture and
covers the part of the aperture wall that is in contact with the
pn-junction and depletion region; the passivating layer of the
second type extends into the at least one defined aperture
contiguous to the passivating layer of the first type and covers
the part of the aperture wall contacting the first semiconductor
layer; and the passivating layer of a third type extends into the
at least one defined aperture contiguous to the passivating layer
of the first type and covers the part of the aperture wall
contacting the second semiconductor layer.
32. The photovoltaic device according to claim 31, wherein the
passivating layer of the second type extends over the front
surface.
33. The photovoltaic device according to claim 32, wherein the
passivating layer of the second type is identical to the
passivating layer of the first type.
34. The photovoltaic device according to claim 33, wherein the
passivating layers of the first and second type are integral.
35. The photovoltaic device according to claim 31, wherein the
passivating layer of the third type extends over the rear
surface.
36. The photovoltaic device according to claim 35, wherein the
passivating layer of the third type is identical to the passivating
layer of the first type.
37. The photovoltaic device according to claim 31, wherein the
passivating layers of the first and third type are integral.
38. The photovoltaic device according to claim 31, wherein the
passivating layer of the first type has a substantially neutral
effective surface charge density.
39. The photovoltaic device according to claim 38, wherein the
passivating layer of the first type comprises at least one of
amorphous silicon and annealed silicon dioxide.
40. The photovoltaic device according to claim 31, wherein the
passivating layer of the second type has an effective surface
charge density of a type opposite to and higher than the effective
surface charge density of the first semiconductor layer.
41. The photovoltaic device according to claim 31, wherein the
passivating layer of the third type has an effective surface charge
density of a type opposite to and higher than the effective surface
charge density of the second semiconductor layer.
42. The photovoltaic device according to claim 31, wherein the
first semiconductor layer is of an n-type conductivity and the
passivating layer of the second type comprises at least one of
Silicon Nitride, Silicon Dioxide, and wherein the second
semiconductor layer is of a p-type conductivity and the passivating
layer of the third type comprises at least one of Aluminium Oxide,
Titanium Dioxide, Hafnium Oxide.
43. The photovoltaic device according to claim 31, wherein the
first semiconductor layer is of a p-type conductivity and the
passivating layer of the second type comprises at least one of
Aluminium Oxide, Titanium Dioxide, Hafnium Oxide, and wherein the
second semiconductor layer is of an n-type conductivity and the
passivating layer of the third type comprises at least one of
Silicon Nitride, Silicon Dioxide, Silicon Carbide.
44. The photovoltaic device according to claim 31 having at least
one defined aperture, wherein the passivating layers form an
electrical insulating jacket covering the aperture wall of the at
least one defined aperture providing an electrical insulation of
the layered structure from an electrically conducting electrode
arranged in the at least one defined aperture.
45. The photovoltaic device according to claim 31, wherein the
layered structure comprises a planar semiconductor body, and
wherein each of the first semiconductor layer and the second
semiconductor layer are formed by doting of adjacent regions of the
semiconductor body.
46. The photovoltaic device according to claim 31, wherein the
layered structure comprises a thin-film body, and wherein each of
the first semiconductor layer and the second semiconductor layer
are applied on the thin-film body.
47. The photovoltaic device according to claim 31, further
comprising at least one further covering layer covering at least
one of the passivating layers of the first, second and third
type.
48. A method of manufacturing a photovoltaic device, the method
comprising the steps of: providing a layered structure having a
circumferential outer wall, a front surface for receiving optical
radiation and a rear surface opposite to the front surface having a
circumferential outer wall surrounding the layered structure;
positioning at least one electrical contact arrangement at the rear
surface, and optionally at the front surface, or at both the front
surface and the rear surface; extending a first semiconductor layer
of a first conductivity type adjacent to the front surface and to
the outer wall; positioning at least one electrical front contact
in communication with the front surface and the first semiconductor
layer, the at least one electrical front contact being connected to
the at least one contact arrangement on the front surface, when the
at least one contact arrangement is present on the front surface;
extending a second semiconductor layer of a second conductivity
type contiguous to the first semiconductor layer and to the outer
wall, the conductivity types of the first semiconductor layer and
the second semiconductor layer being of opposite polarity wherein
the contact between the first semiconductor layer and the second
semiconductor layer defines a pn-junction and a depletion region
formed across the pn-junction, the pn-junction and depletion region
extending to the outer wall; placing at least one electrical rear
contact in communication with the second semiconductor layer and
the at least one electrical contact arrangement at the rear
surface; applying a passivating layer of a first type to cover the
part of the outer wall that is in contact with the pn-junction and
depletion region; applying a passivating layer of a second type to
cover the part of the outer wall that is in contact with the first
semiconductor layer and to be contiguous to the passivating layer
of the first type; and applying a passivating layer of a third type
to cover the part of the outer wall that is in contact with the
second semiconductor layer and to be contiguous to the passivating
layer of the first type.
49. The method of claim 48, including the further steps of defining
at least one aperture in the layered structure extending from the
front surface to the rear surface and being bounded by an aperture
wall, the aperture defining an electrical contact path connecting
the at least one electrical front contact to the at least one
electrical contact arrangement at the rear surface, the contact
between the first semiconductor layer and the second semiconductor
layer defining the pn-junction and depletion region formed across
the pn-junction extending to include the aperture wall of the at
least one defined aperture; and applying a passivating layer of the
first type into the at least one defined aperture and covering the
part of the aperture wall that is in contact with the pn-junction
and depletion region; applying a passivating layer of the second
type into the at least one defined aperture contiguous to the
passivating layer of the first type and covering the part of the
aperture wall contacting the first semiconductor layer; and
applying a passivating layer of a third type into the at least one
defined aperture contiguous to the passivating layer of the first
type and covering the part of the aperture wall contacting the
second semiconductor layer.
50. The method according to claim 48, wherein the passivating layer
of the first type is applied from either one of the front surface
and the rear surface, followed by removal of the passivating layer
of the first type from either one of the front and rear surface and
from part of the layered structure substantially comprised by the
first semiconductor layer and the second semiconductor layer,
further applying the passivating layer of the second type from the
front surface to be contiguous to the passivating layer of the
first type and applying the passivating layer of the third type
from the rear surface to be contiguous to the passivating layer of
the first type.
51. The method according to claim 48, wherein the passivating layer
of the first type is applied from either one of the front surface
and the rear surface, followed by removal of the passivating layer
of the first type from either one of the front and rear surface and
from part of the layered structure substantially comprised by the
first semiconductor layer and the second semiconductor layer,
further applying the passivating layer of the second type from the
front surface, such to be contiguous to the passivating layer of
the first type and applying the passivating layer of the third type
from the rear surface, such to be contiguous to the passivating
layer of the first type, and either one of the passivating layer of
the second and/or the third type is removed from either one of the
front and rear surface.
52. The method according to claim 48, wherein the photovoltaic
device is of a front emitter design, the passivating layer of the
first type is applied from either one of the front surface and the
rear surface, followed by removal of the passivating layer of the
first type from either one of the front and rear surface and from
part of the layered structure substantially comprised by the first
semiconductor layer and the second semiconductor layer, further
applying the passivating layer of the second type from the front
surface to be contiguous to the passivating layer of the first type
and applying the passivating layer of the third type from the rear
surface to be contiguous to the passivating layer of the first
type, and the passivating layer of the second type is applied
before the passivating layer of the third type.
53. The method according to claim 48, wherein the photovoltaic
device is of a rear emitter design, the passivating layer of the
first type is applied from either one of the front surface and the
rear surface, followed by removal of the passivating layer of the
first type from either one of the front and rear surface and from
part of the layered structure substantially comprised by the first
semiconductor layer and the second semiconductor layer, further
applying the passivating layer of the second type from the front
surface to be contiguous to the passivating layer of the first type
and applying the passivating layer of the third type from the rear
surface to be contiguous to the passivating layer of the first
type, and the passivating layer of the third type is applied before
the passivating layer of the second type.
54. The method according to claim 48, wherein the passivating
layers of the first and second type are integral, and the integral
passivating layer is applied from the front surface after which the
passivating layer of the third type is applied from the rear
surface.
55. The method according to claim 48, wherein the passivating
layers of the first and third type are integral, and the integral
passivating layer is applied from the rear surface after which the
passivating layer of the second type is applied from the front
surface.
56. A plurality of photovoltaic devices according to claim 31
arranged in stacked operative relationship with one another to form
a multi junction photovoltaic device.
57. A plurality of photovoltaic devices according to claim 31
arranged in operative electrical connection to one another to form
a photovoltaic module.
Description
TECHNICAL FIELD
[0001] The present invention relates to photovoltaic devices and,
in particular, to back or rear contact device concepts using the
so-called metal wrap-through technology.
BACKGROUND
[0002] Photovoltaic devices or solar cells convert optical energy,
such as solar radiation, into electrical energy. These devices are
essentially composed of a layered structure, having a front surface
for receiving optical radiation and a back or rear surface opposite
to the front surface. The layered structure typically includes a
first semiconductor layer of a first conductivity type and a second
semiconductor layer of a second conductivity type extending
contiguous to the first layer. The first and second conductivity
types being of opposite polarity such that the first and second
layers are separated by a pn-junction or depletion layer, also
called active layer.
[0003] The first layer, extending adjacent to the front surface,
connects to at least one electrical contact at the front surface of
the layered structure, also called front contact or front
electrode. The second layer connects to at least one electrical
contact at the back or rear surface of the layered structure, also
called rear or back contact or rear or back electrode.
[0004] Photons contained in solar radiation, for example, impinging
the front surface of the photovoltaic device create electron-hole
pairs in the device. These charge carriers get separated as a
result of a voltage difference across the pn-junction, which
voltage difference is available at the front and rear contacts. By
connecting an electric load between the front and rear contacts, an
electric current will flow through this load.
[0005] A single solar cell produces a relative small amount of
electric energy with a relative low voltage difference between the
front and rear contacts. For practical use, several solar cells
have to be series connected to obtain a solar module having a
higher output voltage and/or several series connected cells have to
be parallel connected to obtain a solar module providing a higher
output current. As cells become larger and more efficient, the
output current increases and in order to limit resistive losses
within this wiring, the dimensions thereof across the front surface
increase. In practical photovoltaic devices and modules, among
others, the electrical wiring at the front surface, in the form of
bus bars or the like, contributes proportionally to shadowing
losses.
[0006] With the Metal Wrap-Through, MWT, technology small apertures
or vias are provided in the layered structure and extending from
the front surface to the rear surface. At the back or rear surface
of the layered structure a back or rear contact arrangement or back
or rear contact grids are provided. By metallizing the apertures,
i.e. by filling the apertures with an electrically conductive plug,
an electrical contact path is provided connecting a front contact,
or a plurality of front contacts of the solar cells, to the back
contact arrangement
[0007] It will be appreciated that the back contact arrangement can
be designed with appropriate dimensions to reduce electrical losses
in the contact wiring of the solar module without impeding the
effective amount of radiation at the front surface contributing to
the energy conversion.
[0008] To increase the effective amount of radiation at the front
surface for conversion into electrical energy, light trapping
schemes such as surface texturing are introduced. That is, the
front surface for collecting solar radiation is effectively
increased by a suitable surface treatment and/or provided with a
suitable anti-reflective coating. Making the back surface
reflective or diffuse reflective many of the photons reaching this
surface will be reflected or scattered towards the front again,
which provides additional opportunities to produce electron-hole
pairs in the photovoltaic device, i.e. increased generation of
electrical energy.
[0009] Among others, a significant loss effect known in
semiconductor devices is recombination of charge carriers at the
surface of a semiconductor layer. In order to prevent the surface
recombination of electrons and holes, passivating layers or films
are applied at the outer surfaces of a photovoltaic device. In the
area of photovoltaics, a passivating surface layer reduces surface
recombination by generating an electric surface field of opposite
polarity at the surface of a particular semiconductor layer.
[0010] The first and second semiconductor layers as well as the
pn-junction or depletion region formed there between may extend
into the circumferential outer wall of the layered structure.
Further, in a metal wrap-through cell, the first and second
semiconductor layers as well as the pn-junction or depletion region
formed there between extend into the aperture walls of the several
apertures produced in the layered structure. Accordingly,
passivation should not be restricted to the front and back surface
of the layered structure but should be applied also at the outer
circumferential wall and, in the case of a back contact metal
wrap-through photovoltaic cell, within the apertures.
SUMMARY
[0011] It is an object to provide a photovoltaic device, in
particular a back contact metal wrap-through photovoltaic and a
module comprising a plurality of photovoltaic devices with improved
passivation, and a method of manufacturing thereof.
[0012] In a first aspect, there is provided a photovoltaic device
comprising a layered structure having a circumferential outer wall,
a front surface for receiving optical radiation, a rear surface
opposite to the front surface, an electrical contact arrangement, a
first semiconductor layer of a first conductivity type, which first
layer extending adjacent to the front surface and having at least
one electrical front contact connected to the contact arrangement,
and a second semiconductor layer of a second conductivity type, the
second layer extending contiguous to the first layer and having at
least one electrical rear contact connected to the contact
arrangement. The first and second conductivity types being of
opposite polarity such that the first and second layers are
separated by a pn-junction, and the first and second layer
extending into the outer wall. A passivating layer of a first type
covering at least part of the outer wall substantially comprised by
the depletion region across the pn-junction. A passivating layer of
a second type covering at least part of the outer wall comprised by
the first semiconductor layer, the passivating layer of the second
type being contiguous to the passivating layer of the first, and a
passivating layer of a third type covering at least part of the
outer wall comprised by the second semiconductor layer, the
passivating layer of the third type being contiguous to the
passivating layer of the first type.
[0013] In a second aspect, there is provided a photovoltaic device,
comprising a layered structure having a front surface for receiving
optical radiation, a rear surface opposite to the front surface, an
electrical contact arrangement at the rear surface, a first
semiconductor layer of a first conductivity type, which first layer
extending adjacent to the front surface and having at least one
electrical front contact, and a second semiconductor layer of a
second conductivity type, which second layer extending contiguous
to the first layer and having at least one electrical rear contact
connected to the contact arrangement. The first and second
conductivity types being of opposite polarity such that the first
and second layers are separated by a pn-junction. The layered
structure comprising at least one aperture extending from the front
surface to the rear surface, the aperture being bounded by an
aperture wall and having an electrical contact path connecting the
at least one electrical front contact to the electrical contact
arrangement at the rear surface. A passivating layer of a first
type extending in the at least one aperture and covering at least
part of the aperture wall substantially comprised by a depletion
region across the pn-junction. A passivating layer of a second type
extending in the at least one aperture contiguous to the
passivating layer of the first type and covering part of the
aperture wall comprised by the first semiconductor layer, and a
passivating layer of a third type extending in the at least one
aperture contiguous to the passivating layer of the first type and
covering part of the aperture wall comprised by the second
semiconductor layer.
[0014] Both aspects may be combined resulting in a back contact
metal wrap-through photovoltaic cell device comprising passivation
of the circumferential outer wall according to the first aspect and
aperture passivation according to the second aspect.
[0015] Each of the passivating layers can be individually selected
and matched to a particular semiconductor layer exposed at the
circumferential outer wall of the layered structure and/or in an
aperture, for optimally reducing unwanted surface recombination at
the outer wall and/or in the apertures. In particular,
short-circuiting or shunting resistance loss due to leak currents
between the semiconductor layers and/or reverse breakdown effects
across the pn-junction or depletion area at the outer wall and/or
in the apertures, which both form an exposed wall or area of the
photovoltaic device, can be effectively prevented or reduced by
selecting a suitable passivation material, preferably having a
substantially electrically neutral effective surface charge
density. Thereby improving the efficiency of the photovoltaic
device.
[0016] For the purpose of the invention, passivation materials for
forming a passivating layer of the first type having a suitable
electrically neutral or substantially electrically neutral
effective surface charge density are amorphous silicon, annealed
silicon dioxide and others.
[0017] Suitable passivation materials for forming a passivating
layer of the second and third type at the first or second
semiconductor layer, respectively, should have an effective surface
charge density of a type opposite and higher than the corresponding
semiconductor layer.
[0018] For the purpose of the invention, it has been found that an
effective surface charge density Q.sub.f larger than 10.sup.-10
cm.sup.-2 provides an effective passivation, i.e. effectively
reduces the unwanted surface recombination of semiconductor layers.
Passivation materials of this type are, for example, Silicon
Nitride, Silicon Dioxide, Silicon Carbide, Titanium Dioxide,
Aluminium Oxide, Hafnium Oxide and others.
[0019] Silicon Nitride, Silicon Dioxide, and Silicon Carbide, for
example, are used to provide passivation of n-type semiconductor
material, whereas Aluminium Oxide, Titanium Oxide, and Hafnium
Oxide, for example, are used provide passivation of semiconductor
material of the p-type, for example.
[0020] It is noted that in a back contact metal wrap-through
photovoltaic device comprising passivation of the circumferential
outer wall and aperture passivation as disclosed above, at least
partly, different materials may be selected for the passivating
layers of the first, second and third type of the circumferential
outer wall and the apertures, respectively.
[0021] The passivating layer of the first type may cover part of
the circumferential outer wall and/or the aperture wall beyond the
depletion region across the pn-junction and both the passivating
layers of the second and third type may cover or overlap at the
passivating layer of the first type. For the purpose of the present
invention and the attached claims, the term contiguous used in
relation to the passivating layers has to be understood to include
at least partially overlapping layers, provided that the
passivating layer of the first type is overlapped by any or both of
the passivating layers of the second and third type. That is, any
or both of the passivating layers of the second and third type are
on top of the passivating layer of the first type, as seen from the
circumferential outer wall and/or the aperture wall.
[0022] Among others to simplify manufacturing of the photovoltaic
device, it is preferred to have the passivating layer of the second
type extending over the edge of the outer circumferential wall
adjacent to the front surface and/or the edge of the at least one
aperture adjacent to the front surface and covering the front
surface of the photovoltaic device. This passivating layer or film
at the same time may exhibit a transparent anti-reflective coating
for improving the collection of solar radiation, as already
discussed in the introduction.
[0023] In the case of a so-called front emitter design, in which
the pn-junction is provided essentially closer to the front surface
than to the back or rear surface of the photovoltaic device, it has
been found that shunting resistance loss and/or reverse breakdown
effects at the pn-junction at the outer circumferential wall and/or
in an aperture of the photovoltaic device, are already effectively
reduced by having the passivating layer of the first type identical
to the passivating layer of the second type. That is, the
passivating layer or film of the second type covers at least both
the area comprised by the first semiconductor layer or emitter and
the pn-junction area or depletion region.
[0024] Preferably, for ease of manufacturing of the photovoltaic
device, the passivating layers of the first and second type are
integral, i.e. it is a single layer or film which may be applied in
a single process step from the front surface. The passivating layer
of the third type may, at least partly, cover such an integral
passivating layer.
[0025] In the case of a so-called back or rear emitter design, in
which the pn-junction is provided essentially closer to the back or
rear surface than to the front surface of the photovoltaic device,
it has been found that shunting resistance loss and/or reverse
breakdown effects at the pn-junction in an aperture and/or at the
outer circumferential wall of the photovoltaic device, are already
effectively reduced by having the passivating layer of the first
type identical to the passivating layer of the third type. That is,
the passivating layer or film of the third type covers both the
area comprised by the second semiconductor layer or emitter and the
pn-junction area or depletion region.
[0026] Preferably, for ease of manufacturing of the photovoltaic
device, the passivating layers of the first and third type are
integral, i.e. it is a single layer or film which may be applied in
a single process step from the back surface. The passivating layer
of the second type may cover such an integral passivating
layer.
[0027] The electrical wiring or electric contact tracks at the
front surface and the electrical contact arrangement at the rear
surface may initially be applied before the application of the
respective passivating layers. From a manufacturing process point
of view, however, it is preferred to apply the passivating layer on
either surface first.
[0028] In the back contact metal wrap-through photovoltaic device,
in an embodiment thereof, the passivating layers in the at least
one aperture cover the aperture wall such to form an electrical
insulating jacket that provides an electrical insulation between
the layered structure, i.e. the aperture wall, and an electrically
conducting electrode or an electrically conducting plug arranged in
the at least one aperture.
[0029] Besides the passivating layers disclosed above, a further
covering layer or a stack of layers may be provided on top of a
passivating layer, to improve the efficiency of the photovoltaic
device.
[0030] An example of such a further covering layer in a back
contact metal wrap-through photovoltaic device is an insulating
layer extending in an aperture between the passivating layers and
the electrically conducting electrode from the front to the rear
surface of the device.
[0031] Other examples of further covering layers are Titanium
Nitride, Silicon Nitride, and the like.
[0032] The layered structure forming the photovoltaic device
according to the invention can be of any type known in practice
and, in particular, of the silicon bulk type or of the thin-film
type.
[0033] Silicon bulk type photovoltaic devices generally comprise a
crystalline silicon wafer or bulk semiconductor material, such as
monocrystalline silicon or poly- or multicrystalline silicon of an
n-conductivity type or a p-conductivity type, wherein the first and
second semiconductor layers are formed by suitably doting of
adjacent regions or volumes of the bulk. Photovoltaic devices of
this type are generally plate shaped.
[0034] A thin-film solar cell or photovoltaic device is produced by
depositing the semiconductor layers as thin layers (thin film) on a
substrate. The thickness range of such a layer is wide and varies
from a few nanometers to tens of micrometers. Cadmium Telluride,
CdTe, Copper Indium Gallium Selenide, CIGS, and Amorphous Silicon,
A-Si, are three thin-film technologies often used for outdoor
photovoltaic solar power production. Thin-film devices may have
varying shapes.
[0035] Photovoltaic devices of the silicon bulk type and the
thin-film silicon type are well known to the skilled person. For
the purpose of the present invention no further elucidation is
required.
[0036] In a third aspect, there is provided a method of
manufacturing a photovoltaic device comprising a layered structure
having a circumferential outer wall, a front surface for receiving
optical radiation and a rear surface opposite to the front surface.
The method comprising providing a first semiconductor layer of a
first conductivity type, the first layer extending adjacent to the
front surface and into the outer wall, and comprising at least one
electrical front contact. Providing a second semiconductor layer of
a second conductivity type, the second layer extending contiguous
to the first layer and into the outer wall, and comprising at least
one electrical rear contact, wherein the first and second
conductivity types being of opposite polarity such that the first
and second layers are separated by a pn-junction. Providing an
electrical contact arrangement. Electrically connecting the at
least one front and rear contact to the electrical contact
arrangement. The method further comprises the steps of covering at
least part of the circumferential outer wall substantially
comprised by a depletion region across the pn-junction by a
passivating layer of a first type; covering at least part of the
circumferential outer wall comprised by the first semiconductor
layer by a passivating layer of a second type contiguous to the
passivating layer of the first type, and covering at least part of
the circumferential outer wall comprised by the second
semiconductor layer by a passivating layer of a third type
contiguous to the passivating layer of the first type.
[0037] In a fourth aspect, there is provided a method of
manufacturing a photovoltaic device comprising a layered structure
having a front surface for receiving optical radiation and a rear
surface opposite to the front surface. The method comprising
providing a first semiconductor layer of a first conductivity type,
the first layer extending adjacent to the front surface and
comprising at least one electrical front contact. Providing a
second semiconductor layer of a second conductivity type, the
second layer extending contiguous to the first layer and comprising
at least one electrical rear contact, wherein the first and second
conductivity types being of opposite polarity such that the first
and second layers are separated by a pn-junction. Providing at
least one aperture extending from the front surface to the rear
surface, the aperture being bounded by an aperture wall. Providing
an electrical contact arrangement at the rear surface. Electrically
connecting the at least one rear contact to the electrical contact
arrangement, and providing an electrical contact path electrically
connecting the at least one electrical front contact through the at
least one aperture to the electrical contact arrangement. The
method further comprises the steps of covering at least part of the
aperture wall substantially comprised by a depletion region across
the pn-junction by a passivating layer of a first type; covering at
least part of the aperture wall comprised by the first
semiconductor layer by a passivating layer of a second type
contiguous to the passivating layer of the first type, and covering
at least part of the aperture wall comprised by the second
semiconductor layer by a passivating layer of a third type
contiguous to the passivating layer of the first type.
[0038] Both the method according to the third and fourth aspect may
be combined resulting in a method for manufacturing a back contact
metal wrap-through photovoltaic cell device comprising passivation
of the circumferential outer wall according to the third aspect and
aperture passivation according to the fourth aspect.
[0039] Surface passivation is generally applied using a Plasma
Enhanced Chemical Vapour Deposition, PECVD, process, preferably
followed by a post-deposition annealing process. However, for the
purpose of the present invention other known deposition processes
may be used, for example Atomic Layer Deposition, ALD, or a
so-called wet process, wherein the passivating material to be
deposited is contained in a liquid which is applied at a surface to
be passified. Passivating layers or films may also be applied using
a spraying method, for example.
[0040] The order in which the passivating layers of the first,
second and third type are applied may vary. In an embodiment, the
passivating layer of the first type, substantially covering the
depletion region across the pn-junction, is applied from either one
of the front surface and the rear surface, followed by a selective
etching step, for example, to remove the passivating layer of the
first type from the front or rear surface and from part of the
circumferential outer wall and/or the aperture wall substantially
comprised by the first semiconductor layer and/or the second
semiconductor layer. Next the passivating layer of the second type
may be applied from the front surface, such to be contiguous to the
passivating layer of the first type or the passivating layer of the
third type may be applied from the rear surface, such to be
contiguous to the passivating layer of the first type. It is noted
that the passivating layer of the second type and the third type
may, at least partially, overlap the passivating layer of the first
type. If required, the passivating layer of the second and/or the
third type may be removed from the front and/or rear surface, for
example by selective etching, if required.
[0041] The latter removal will not be applied, of course, if the
passivating layer of the second type should extend over the front
surface and the passivating layer of the third type should extend
over the rear surface, as discussed above. In the case of a
photovoltaic device of the front emitter design, it is preferred to
apply the passivating layer of the second type before the
passivating layer of the third type. In the case of a photovoltaic
device of the back or rear emitter design, it is preferred to apply
the passivating layer of the third type before the passivating
layer of the second type.
[0042] In an embodiment wherein the passivating layers of the first
and second type are integral, this passivating layer is applied
before the passivating layer of the third type is applied. In an
embodiment wherein the passivating layers of the first and third
type are integral, this passivating layer is applied before the
passivating layer of the second type is applied. The passivating
layers may be formed contiguous with a partial overlap.
[0043] In another embodiment, the passivating layers of the second
and third type may be formed first, for example from the front and
rear surface, respectively, after which at least part of the
passivating layers of the second and third type substantially
covering part of the circumferential outer wall and/or of the
aperture wall comprised by the depletion region across the
pn-junction is removed. This removal can be performed, for example,
by using a selective wet etching material applied at a brush or
sponge or the like which is selectively swept along the
circumferential outer wall and/or the aperture wall. After this
removal, the passivating layer of the first type is applied on the
exposed area, to be contiguous to the passivating layers of the
second and third type.
[0044] It will be appreciated that in the method according to the
invention passivating materials or compounds of the first, second
and third type are used as disclosed above. Suitable etching
materials for removing such passivating layers of the first, second
and third type are known to the skilled person and need no further
elaboration.
[0045] Based on the above, those skilled in the art will appreciate
that the application of the passivating layers is not limited to
the specific embodiments disclosed. After having applied the
passivating layers as disclosed, at least one further covering
layer covering at least one of the passivating layers of the first,
second and third type may be applied.
[0046] In a fifth aspect there is provided a photovoltaic module
comprising a plurality of electrically connected photovoltaic
devices of the types disclosed above.
[0047] Those skilled in the art will appreciate that the teachings
of the invention can also be applied with multi-junction or
heterojunction photovoltaic devices, comprising several stacked
cells of first and second semiconductor layers and their
corresponding p-n junctions. Such as a top cell, a middle cell and
a bottom cell, for example. Each junction or cell is tuned to a
different wavelength of light, thereby increasing efficiency of the
photovoltaic device as a whole.
[0048] The invention will now be described in more detail by means
of specific embodiments, with reference to the enclosed drawings,
wherein equal or like parts and/or components are designated by the
same reference numerals. The invention is in no manner whatsoever
limited to the embodiments disclosed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] FIG. 1 shows schematically, not to scale, partly in
cross-section, a portion of a substrate or wafer of a front emitter
silicon bulk type prior art photovoltaic device.
[0050] FIG. 2 shows schematically, not to scale, partly in
cross-section, a portion of a substrate or wafer of a front emitter
silicon bulk type prior art back contact metal wrap-through
photovoltaic device.
[0051] FIG. 3 shows schematically, not to scale, partly in
cross-section, the photovoltaic device of FIG. 1 comprising outer
wall passivation according to the invention.
[0052] FIG. 4 shows schematically, not to scale, partly in
cross-section, the photovoltaic device of FIG. 2 comprising
aperture wall passivation according to the invention.
[0053] FIG. 5 shows schematically, not to scale, partly in
cross-section, a portion of a substrate or wafer of a rear emitter
silicon bulk type photovoltaic device, comprising circumferential
outer wall passivation according to the invention.
[0054] FIG. 6 shows schematically, not to scale, partly in
cross-section, a portion of a substrate or wafer of a front emitter
silicon bulk type back contact metal wrap-through photovoltaic
device, comprising aperture wall passivation according to the
invention.
[0055] FIG. 7 shows schematically, not to scale, partly in
cross-section, a portion of a substrate or wafer of a front emitter
silicon bulk type back contact metal wrap-through photovoltaic
device comprising circumferential outer wall passivation and
aperture wall passivation according to an embodiment of the
invention.
[0056] FIG. 8 shows schematically, not to scale, partly in
cross-section, a portion of a substrate or wafer of a front emitter
silicon bulk type back contact metal wrap-through photovoltaic
device, comprising circumferential outer wall passivation and
aperture wall passivation according to an embodiment of the
invention.
[0057] FIG. 9 shows schematically, not to scale, partly in
cross-section, a portion of a substrate or wafer of a rear emitter
silicon bulk type back contact metal wrap-through photovoltaic
device, comprising circumferential outer wall passivation and
aperture wall passivation according to an embodiment of the
invention.
[0058] FIG. 10 shows schematically, not to scale, partly in
cross-section, the photovoltaic device of FIG. 8, comprising an
electrically conducting plug and insulating jacket.
[0059] FIG. 11 shows schematically, not to scale, partly in
exploded view, an example of a photovoltaic module comprising a
plurality of electrically connected back contact metal wrap-through
photovoltaic devices according to the invention.
DETAILED DESCRIPTION
[0060] FIG. 1 relates to a prior art photovoltaic device 10 of the
so-called front emitter design type, comprising a plate-shaped or
planar silicon substrate or wafer 11, having a front surface or
receiving surface 12 for receiving optical radiation, i.e. solar
radiation, a back or rear surface 13 opposite to the front surface
12 and a circumferential outer wall 14.
[0061] The substrate 11 comprises a semiconductor bulk. A first
volume or region of the bulk below the front surface 12 having a
doping of a first polarity or first conductivity type, such as an
n-type conductivity wherein electrons form the main charge
carriers, forms a first semiconductor layer 15 or emitter extending
adjacent to the front surface 12. A second volume or region of the
bulk between the first layer 15 and the rear surface 13 comprising
a doping of a second polarity or second conductivity type, opposite
to the first polarity or conductivity type, such as a p-type
conductivity wherein holes form the main charge carriers, forms a
second semiconductor layer 16 contiguous to the first semiconductor
layer 15.
[0062] The area or region separating the first and second
semiconductor layers 15, 16 is a pn-junction 17 across which a
depletion region 18 instantaneously forms, indicated by broken
lines. The first and second semiconductor layers 15, 16 and the
pn-junction 17 or depletion region 18 extend into the
circumferential outer wall 14 of the substrate or wafer 11. Or in
other words, the second semiconductor layers 15, 16 and the
pn-junction 17 or depletion region 18 are exposed at the
circumferential outer wall of the substrate or wafer 11.
[0063] At least one electrical contact 19 electrically contacts the
first layer or emitter 15 and is available from the front surface
12, also called a front surface contact. At least one electrical
contact 20 electrically contacts the second layer 16 and is
available from the back or rear surface 13, also called a rear
surface contact. Electrical energy provided by the photovoltaic
device 10 is available at the front and rear surface contacts 19,
20. To this end, at both surfaces 12, 13 of the substrate 11 a
contact arrangement 21 respectively 22 is provided, in the form of
an electric wiring and/or electrically conducting bus bars for
electrically connecting a plurality of front and rear surface
contacts 19, 20, respectively.
[0064] For producing a back contact metal wrap-through photovoltaic
device 25, the layered structure of first and second semiconductor
layers 15, 16 and the pn-junction 17 or depletion region 18
comprises at least one via or aperture 23 extending from the front
surface 12 to the rear surface 13, as shown in FIG. 2.
[0065] The at least one aperture 23 is bounded by an aperture wall
24. Although not shown for the purpose of clarity, the aperture 23
is filled with a plug of electrically conducting material providing
an electrical contact path from the at least one electrical front
contact 19 to an electrical contact arrangement at the rear surface
13. In practice, a plurality of apertures 23 are formed for
contacting a plurality of electrical front contacts 19 to the
electrical contact arrangement.
[0066] As shown in FIG. 2, the first and second semiconductor
layers 15, 16 and the pn-junction 17 or depletion region 18 extend
into the aperture wall 24 of the at least one aperture 23. Or in
other words, the second semiconductor layers 15, 16 and the
pn-junction 17 or depletion region 18 are exposed at the aperture
wall 24.
[0067] In practice, the first layer 15 or emitter has a thickness
in the micrometer range, such as 0.5 .mu.m, whereas the aperture 25
has a diameter of a few hundred micrometers, such as 200 .mu.m, for
example. Further, the front surface 12 may have a surface textured
structure (not shown) to increase the effective amount of radiation
at the front surface 12 for conversion into electrical energy.
Dependent on the manufacturing process of the photovoltaic device
10, the at least one aperture 23 may be drilled, such as laser
drilled, in the bulk before the first and second layers 15, 16 are
produced, or the at least one aperture 23 may be drilled in the
already manufactured layered structure 11.
[0068] Due to the drilling of the at least one aperture 23, and/or
etching of the front and rear surfaces 12, 13 and/or texturing of
the front surface 12, the edge 27 of the aperture 23 at the front
surface 12 and the edge 28 of the aperture 23 at the rear surface
13 may exhibit a rounded or bevelled or otherwise knurled edge
structure, resulting in a widened opening of the aperture at the
front and/or rear surface 12, 13, different from the sharp edges
27, 28 shown in FIG. 2. Likewise, the outer edges 29, 30 of the
circumferential outer wall 14 may be rounded or bevelled or knurled
in practice.
[0069] For the purpose of the invention, part of the aperture edges
27, 28 flush to the front surface 12 or the rear surface 13,
respectively, form part of the aperture wall 24 and part of the
outer edges 29, 30 flush to the front surface 12 or the rear
surface 13, respectively, form part of the circumferential outer
wall 14.
[0070] In accordance with the first aspect of the invention, as
schematically shown in FIG. 3, a photovoltaic device is provided,
for example a photovoltaic device 35 of the front emitter type,
having the layered structure as shown in FIG. 1 and having
passivating layers or films 31, 32 and 33 applied at the
circumferential outer wall 14 of the layered structure or substrate
11.
[0071] As shown in FIG. 3, a passivating layer 31 of a first type
covers at least part of the area of the circumferential outer wall
14 substantially comprised by the depletion region 18 across the
pn-junction 17. A passivating layer 32 of a second type covers at
least part of the area of the circumferential outer wall 14
comprised by the first semiconductor layer 15. A passivating layer
33 of a third type covers at least part of the area of the
circumferential outer wall 14 comprised by the second semiconductor
layer 16.
[0072] The passivating layer 32 of the second type is formed
contiguously to the passivating layer 31 of the first type and
extends partly over the outer edge 29 of the front surface 12. The
passivating layer 33 of the third type is formed contiguous to the
passivating layer 31 of the first type and extends partly over the
outer edge 30 of the rear surface 13. For the purpose of the
invention, the term contiguous used in relation to the passivating
layers 31, 32, 33 has to be understood to include adjoining
passivating layers as well as at least partially overlapping
passivating layers. That is one or both the passivating layers 32
and 33 may overlap the passivating layer 31 of the first type as
seen from the circumferential outer wall 14. That is, any or both
of the passivating layers 32 and 33 are on top of the passivating
layer 31 of the first type, as seen from the circumferential outer
wall 14.
[0073] FIG. 4 schematically shows a front emitter photovoltaic
device 40 of the back contact metal wrap-through type as shown in
FIG. 3, and comprising passivating layers or films 36, 37 and 38
applied at the aperture wall 24 of the aperture 23, in accordance
with the second aspect of the invention.
[0074] As shown in FIG. 4, a passivating layer 36 of a first type
covers at least part of the area of the aperture wall 24
substantially comprised by the depletion region 18 across the
pn-junction 17. A passivating layer 37 of a second type covers at
least part of the area of the aperture wall 24 comprised by the
first semiconductor layer 15. A passivating layer 38 of a third
type covers at least part of the area of the aperture wall 24
comprised by the second semiconductor layer 16.
[0075] In a like manner as disclosed with reference to the
passivation of the circumferential outer wall 14, the passivating
layer 37 of the second type is formed contiguously to the
passivating layer 36 of the first type and extends partly over the
aperture edge 27 of the front surface 12. The passivating layer 38
of the third type is formed contiguous to the passivating layer 36
of the first type and extends partly over the aperture edge 28 of
the rear surface 13. For the purpose of the invention, the term
contiguous used in relation to the passivating layers 36, 37, 38
has to be understood to include adjoining passivating layers as
well as at least partially overlapping passivating layers. That is
one or both the passivating layers 37 and 38 may overlap the
passivating layer 36 of the first type as seen from the aperture
wall 24. That is, any or both of the passivating layers 37 and 38
are on top of the passivating layer 36 of the first type, as seen
from the aperture wall 24.
[0076] Those skilled in the art will appreciate that passivation of
the circumferential outer wall 14 may also be applied in the back
contact metal wrap-through photovoltaic device 40 comprising
aperture wall passivation. In such an embodiment, in practice, the
passivating layers 31 and 36 are of the same first type, the
passivating layers 32 and 37 are of the same second type and the
passivating layers 33, 38 are of the same third type. However, its
is noted that the types of passivating layers 31, 32, 33 applied at
the circumferential outer wall 14 may differ from the types of
passivating layers 36, 37, 38 applied at the aperture wall 24,
because the passivating layers 36, 37, 38 may have to form an
electrical insulation between the layered structure, i.e. the
aperture wall 24, and an electrically conducting electrode or an
electrically conducting plug arranged in the at least one aperture
23.
[0077] FIG. 5 shows part of a photovoltaic device 41 of the rear
emitter type, in which the pn-junction 17 is provided essentially
closer to the rear surface 13 than to the front surface 12. The
circumferential outer wall 14 of the photovoltaic device 41 is
passified in accordance with the first aspect of the invention, in
a manner comparable to the front emitter device 35, as discussed
above and shown in FIG. 3.
[0078] FIG. 6 shows part of a back contact metal wrap-through
photovoltaic device 42 of the rear emitter type, comprising
aperture wall passivation according to the second aspect of the
invention, comparable to the front emitter device 40 discussed
above and shown in FIG. 4. In the embodiment of FIG. 6, the
circumferential outer wall of the photovoltaic device 42 may be
passified as shown for the photovoltaic device 41 in FIG. 5.
[0079] As will be appreciated by those skilled in the art, in the
photovoltaic devices illustrated in the FIGS. 3-6, the front
surface 12 and the rear surface 13 may be covered with further
passivating layers of a suitable type (not shown).
[0080] Among others to simplify manufacturing of the photovoltaic
device, it is preferred to combine passivation of the
circumferential outer wall and the aperture wall with surface
passivation of the front surface and/or the rear surface of a
photovoltaic device.
[0081] FIG. 7 shows a back contact metal wrap-through photovoltaic
device 45 wherein the passivating layers 32 respectively 37 of the
second type extending over the outer edge 29 of the outer
circumferential wall 14 or the aperture edge 27, as shown in FIGS.
3-6, cover the front surface 12 of the photovoltaic device,
indicated by reference numeral 46.
[0082] Likewise, the passivating layers 33 respectively 38 of the
second type extending over the outer edge 30 of the outer
circumferential wall 14 or the aperture edge 28, as shown in FIGS.
3-6, may cover the rear surface 13 of the photovoltaic device,
indicated by reference numeral 47.
[0083] As discussed above, the passivating layers 32 and 37 are,
preferably, of the same second type, such that a single passivating
layer 46 of the second type is applied. Besides its passivation
properties, the passivating layer or film 46 may form a transparent
anti-reflective coating for improving the collection of solar
radiation by the photovoltaic device. The passivating layers 32 and
37 are, preferably, of the same second type, such that a single
passivating layer 47 of the second type is applied.
[0084] In the case of a so-called front emitter design, such as
shown in FIGS. 1-4 and 7, it has been found that shunting
resistance loss and/or reverse breakdown effects at the pn-junction
17 at the outer circumferential wall 14 and/or in an aperture 23 of
the photovoltaic device of the back contact metal wrap-through
photovoltaic device, are already effectively reduced by having the
passivating layer of the first type 31, 36 identical to the
passivating layer of the second type 32, 37.
[0085] That is, the passivating layer or film of the second type
32, 37 covers both at least part of the area of the circumferential
outer wall 14 comprised by the first semiconductor layer 15 and at
least part of the area of the circumferential outer wall 14
substantially comprised by the depletion region 18 across the
pn-junction 17.
[0086] Preferably, for ease of manufacturing of the photovoltaic
device, the passivating layers of the first and second type are
integral, i.e. it is a single layer or film which may be applied in
a single process step from the front surface 12, for example. Such
as illustrated in FIG. 8 for a back contact metal wrap-through
photovoltaic device 50, wherein a single passivating layer 48 of
the second type extends over the front surface 12 of the
photovoltaic device 50 and the passivating layer 47 of the third
type extends over the rear surface 13 of the photovoltaic device
50. The passivating layer 47 of the third type is contiguous to the
passivating layer of the second type 48, that is the passivating
layers 47, 48 adjoin each other or the passivating layer 47 partly
or completely overlaps the passivating layer 48 at the
circumferential outer wall 14 and the aperture wall 24, as
disclosed above.
[0087] In the case of a so-called back or rear emitter design as
shown in FIGS. 5 and 6, for example, it has been found that
shunting resistance loss and/or reverse breakdown effects at the
pn-junction 17 in an aperture 23 and/or at the outer
circumferential wall 14 of the photovoltaic device, are already
effectively reduced by having the passivating layer of the first
type identical to the passivating layer of the third type. That is,
a passivating layer or film of the third type covers both the rear
surface 13 and the pn-junction 17, i.e. the depletion region 18 at
the circumferential outer wall 14 and the aperture wall 24.
Preferably, for ease of manufacturing of the photovoltaic device,
the passivating layers of the first and second type are integral,
i.e. it is a single layer or film 49 which may be applied in a
single process step from the rear surface 13, for example, such as
illustrated in FIG. 9 for a back contact metal wrap-through
photovoltaic device 55. The passivating layer 48 of the second type
is contiguous to the passivating layer 49 of the third type. That
is, the passivating layers 48, 49 adjoin each other or the
passivating layer 48 partly or completely overlaps or covers the
passivating layer 49 at the circumferential outer wall 14 and the
aperture wall 24.
[0088] The passivating layers of the first, second and third type
may have different thicknesses ranging from a few nm, like 5 nm for
ALD aluminium oxide, up to a few micrometers for silicon oxide. In
the case of a front emitter design, the passivating layers of the
first and second type may be thinner or less thick compared to the
passivating layer of the third type. In the case of a rear emitter
design, the passivating layers of the first and third type may be
thinner or less thick compared to the passivating layer of the
second type.
[0089] In the case of a back contact metal wrap-through
photovoltaic device, the passivating layers in the aperture 23 may
form an electrical insulating jacket covering the aperture wall 23
to provide electrical insulation of the layered structure 11 from
an electrically conducting electrode arranged in the at least one
aperture 23. However, inn order to optimally select the types of
passivating layers in the aperture, in an embodiment of the
invention, a separate insulating jacket may be provided in the
aperture 23 on top of the passivating layers, as seen from the
aperture wall 24, schematically as illustrated in FIG. 10.
[0090] FIG. 10 shows the back contact metal wrap-through
photovoltaic device 50 of FIG. 8, wherein an electrically
conducting plug 56 connects the front contact 19 to a contact
arrangement 26 at the rear surface 13. In the aperture 23, an
electrically insulating jacket 57 surrounds the plug 56.
[0091] Those skilled in the art will appreciate that further
layers, including further passivating layers, may be applied at a
passivating layer of the first, second or third type, to improve
the efficiency of a photovoltaic device. For the purpose of the
present invention, such further layers are not further
elucidated.
[0092] Instead of a silicon bulk wafer, the teachings of the
invention may be likewise applied to a photovoltaic device wherein
the layered structure comprises a thin-film body, and wherein the
first and second semiconductor layers are applied on the thin-film
body.
[0093] As discussed in the summary part, the several passivating
layers 31, 32, 33; 36, 37, 38; 46, 47, 48, 49 illustrated in the
drawings may be applied by any known surface passivation technique,
such as a Plasma Enhanced Chemical Vapour Deposition, PECVD,
process, preferably followed by a post-deposition annealing
process, etching techniques, by Atomic Layer Deposition, ALD, or by
a so-called wet process, wherein the passivating material to be
deposited is contained in a liquid which is applied at a surface to
be passified. Passivating layers or films may also be applied using
a spraying method, for example.
[0094] The order in which the passivating layers of the first,
second and third type are applied may vary. With reference to FIGS.
3, 4 and 5, 6, for example, in an embodiment, one or both the
passivating layers 31; 36 of the first type, substantially covering
the depletion region 18 across the pn-junction 17, are first
applied from either one of the front surface 12 or the rear surface
13, followed by a selective etching step, for example, to remove
the amount of passivating layer 31; 36 of the first type from the
front 12 or rear surface 13 and from part of the circumferential
outer wall 14 and/or the aperture wall 24 substantially comprised
by the first semiconductor layer 15 and/or the second semiconductor
layer 16.
[0095] It will be appreciated that in the case of a front emitter
design as shown in FIGS. 3 and 4, the passivating layer 31; 36 of
the first type is preferably applied from the front surface 12. In
the case of a rear emitter design, such as shown in FIGS. 5 and 6,
the passivating layer of the first type is preferably applied from
the rear surface 13.
[0096] Next, the passivating layer 32; 37 of the second type may be
applied from the front surface 12, such to be contiguous to the
passivating layer 31; 36 of the first type. The passivating layer
33; 38 of the third type may be applied from the rear surface 13,
such to be contiguous to the passivating layer 31; 36 of the first
type. It is noted that the passivating layer 32; 37 of the second
type and the passivating layer 33; 38 of the third type may, at
least partially, overlap the passivating layer 31; 36 of the first
type. If required, the passivating layer of the second 32; 37
and/or the third type 33; 38 may be removed from the front 12
and/or rear surface 13, for example by selective etching, if
required. For example to apply surface passivation at the front
surface 12 and/or the rear surface 13 using different types of
passivating layers compared to the passivating layers 31, 32, 33
and 36, 37, 38.
[0097] The latter removal will not be applied, of course, if the
passivating layer 32; 37 of the second type should extend over the
front surface 12 and the passivating layer 33; 38 of the third type
should extend over the rear surface 13, as shown in the embodiment
of FIG. 7, passivating layers 46 and 47, respectively.
[0098] In the case of a photovoltaic device of the front emitter
design, it is preferred to apply the passivating layer 32; 37 of
the second type before the passivating layer 33; 38 of the third
type, such to ensure that the passivating layer 33; 38 of the third
type is always on top of the passivating layer 32; 37 of the second
type, if same overlap. In the case of a photovoltaic device of the
back or rear emitter design, it is preferred to apply the
passivating layer 33; 38 of the third type before the passivating
layer 32; 37 of the second type, such to ensure that the
passivating layer 32; 37 of the second type is always on top of the
passivating layer 33; 38 of the third type, if same overlap.
[0099] In an embodiment of a photovoltaic device of the front
emitter design, wherein the passivating layers of the first and
second type are integral, such as the passivating layer 48 shown in
FIG. 8, this passivating layer 48 is applied from the front surface
12 before the passivating layer 49 of the third type is applied
from the rear surface 13.
[0100] In an embodiment of a photovoltaic device of the rear
emitter design, wherein the passivating layers of the first and
third type are integral, such as the passivating layer 49 shown in
FIG. 9, this passivating layer 49 is applied from the rear surface
13 before the passivating layer 48 of the second type is applied
from the front surface 12.
[0101] The passivating layers 48, 49 may be formed contiguous with
a partial overlap. Etching may be required between the successive
application steps, to remove passivating material from unwanted
areas of the circumferential outer wall 14 and/or the aperture wall
24, whenever required.
[0102] In another embodiment, the passivating layers 32; 37 of the
second and third 33; 38 type may be formed first, for example from
the front 12 and rear surface 13, respectively, after which at
least part of the passivating layers 32; 37 of the second and third
33; 38 type substantially covering part of the circumferential
outer wall 14 and/or of the aperture wall 24 comprised by the
depletion region 18 across the pn-junction 17 is removed. This
removal can be performed, for example, by using a selective wet
etching material applied at a brush or sponge or the like which is
selectively swept along the passivating layer(s) covering the
circumferential outer wall 14 and/or the aperture wall 24 at the
position at which the passivating layer 31; 36 of the first type is
to be applied. After this removal, the passivating layer 31; 36 of
the first type is applied on the thus exposed area, to be
contiguous to the passivating layers 32; 37 of the second and third
33; 38 type.
[0103] Suitable etching materials for removing passivating layers
of the first, second and third type are known to the skilled person
and need no further elaboration. Those skilled in the art will
appreciate that further steps may be required, such as cleaning
steps, for example.
[0104] Each of the passivating layers can be individually selected
and matched to a particular semiconductor layer exposed at the
circumferential outer wall of the layered structure and/or in an
aperture, for optimally reducing unwanted surface recombination at
the outer wall and/or in the apertures. In particular,
short-circuiting or shunting resistance loss due to leak currents
between the semiconductor layers and/or reverse breakdown effects
across the pn-junction or depletion area at the outer wall 14
and/or in the aperture wall 24, which both generally form an
exposed wall or area of the photovoltaic device, can be effectively
prevented or reduced by selecting a suitable passivation material,
preferably having a substantially electrically neutral effective
surface charge density.
[0105] For the purpose of the invention, passivation materials for
forming a passivating layer of the first type having a suitable
electrically neutral or substantially electrically neutral
effective surface charge density are amorphous silicon, annealed
silicon dioxide and others.
[0106] Suitable passivation materials for forming a passivating
layer of the second and third type at the first or second
semiconductor layer, respectively, should have an effective surface
charge density of a type opposite and higher than the corresponding
semiconductor layer.
[0107] For the purpose of the invention, it has been found that an
effective surface charge density Q.sub.f larger than 10.sup.-10
cm.sup.-2 provides an effective passivation, i.e. effectively
reduces the unwanted surface recombination of semiconductor layers.
Passivation materials of this type are, for example, Silicon
Nitride, Silicon Dioxide, Silicon Carbide, Titanium Dioxide,
Aluminium Oxide, Hafnium Oxide and others.
[0108] Silicon Nitride, Silicon Dioxide, and Silicon Carbide, for
example, are used to provide passivation of n-type semiconductor
material, whereas Aluminium Oxide, Titanium Oxide, and Hafnium
Oxide, for example, are used provide passivation of semiconductor
material of the p-type, for example.
[0109] It is noted that in a back contact metal wrap-through
photovoltaic device comprising passivation of the circumferential
outer wall and aperture passivation as disclosed above, at least
partly, different materials may be selected for the passivating
layers of the first, second and third type of the circumferential
outer wall and the apertures, respectively.
[0110] After having the passivation layers for passivation of the
circumferential outer wall 14 and the aperture wall 24 have been
applied, as discussed above, electrical wiring and the like may be
applied at the front and rear surfaces 12, 13 and further layers
may be applied on top these passivating layers, for example, using
any known deposition process. Such steps are well-known to the
skilled person.
[0111] The invention is likewise applicable with a photovoltaic
device comprising a plurality of stacked first 15 and second
semiconductor layers 16 and pn-junctions 17, also called
multi-junction or heterojunction photovoltaic devices, such that
for each pn-junction 17 and semiconductor layer 15, 16 optimal
passivating layer 31, 32, 33 or 36, 37, 38 or combinations of
passivating layers is applied, following the teachings above.
[0112] FIG. 11 shows an embodiment of a photovoltaic module 60
comprising a plurality of electrically connected back contact metal
wrap-through photovoltaic devices or solar cells 61 according to
the invention. A contact arrangement 62 on a support 64 is defined
to match the electrical pattern of the back contacts of the
photovoltaic devices 61. Solder paste 63 is applied to each of the
contact positions, indicated by small circles, for electrically
connecting the back contacts of each photovoltaic device 61 and the
contact arrangement 62. The photovoltaic devices 61 are positioned
onto the contact arrangement 62 such that the contact positions
match. By heating the solder paste 63, the back contacts are
electrically connected to the contact arrangement 62. A transparent
plate 65, such as a glass plate, covers and protects the
photovoltaic devices 61.
[0113] The invention may be practiced otherwise than as
specifically described herein, and the abovementioned embodiments
and examples are merely intended as an illustration to the skilled
reader. The scope of the invention is only limited by the appended
claims.
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