U.S. patent application number 15/000538 was filed with the patent office on 2016-05-12 for solar cell with dielectric back reflective coating.
The applicant listed for this patent is RCT SOLUTIONS GMBH. Invention is credited to STEFFEN KELLER, ADOLF MUENZER, REINHOLD SCHLOSSER, JAN SCHOENE, ANDREAS TEPPE.
Application Number | 20160133774 15/000538 |
Document ID | / |
Family ID | 44503057 |
Filed Date | 2016-05-12 |
United States Patent
Application |
20160133774 |
Kind Code |
A1 |
MUENZER; ADOLF ; et
al. |
May 12, 2016 |
SOLAR CELL WITH DIELECTRIC BACK REFLECTIVE COATING
Abstract
In a method for producing a solar cell, a layer stack of
dielectric layers is applied to a back of a solar cell substrate
and the layer stack is heated and is held at temperatures of at
least 700.degree. C. during a time period of at least 5 minutes.
The novel solar cell has a layer stack of dielectric layers on its
back. At least one of the dielectric layers of the layer stack is
densified so that its resistivity to firing-through of pastes with
glass components is enhanced.
Inventors: |
MUENZER; ADOLF;
(UNTERSCHLEISSHEIM, DE) ; TEPPE; ANDREAS;
(KONSTANZ, DE) ; SCHOENE; JAN; (REICHENAU, DE)
; SCHLOSSER; REINHOLD; (MUENCHEN, DE) ; KELLER;
STEFFEN; (KONSTANZ, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
RCT SOLUTIONS GMBH |
KONSTANZ |
|
DE |
|
|
Family ID: |
44503057 |
Appl. No.: |
15/000538 |
Filed: |
January 19, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13582502 |
Oct 1, 2012 |
9276155 |
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15000538 |
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PCT/DE2011/075036 |
Mar 3, 2011 |
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13582502 |
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Current U.S.
Class: |
136/256 |
Current CPC
Class: |
H01L 31/022425 20130101;
H01L 31/0547 20141201; H01L 31/02167 20130101; Y02E 10/547
20130101; Y02P 70/50 20151101; H01L 31/056 20141201; H01L 31/068
20130101; H01L 31/1804 20130101; Y02P 70/521 20151101; H01L 31/1864
20130101; Y02E 10/52 20130101 |
International
Class: |
H01L 31/054 20060101
H01L031/054 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 3, 2010 |
DE |
102010010221.0 |
Mar 5, 2010 |
DE |
102010010561.9 |
Jul 2, 2010 |
DE |
102010025983.7 |
Claims
1. A solar cell, comprising: a layer stack of a plurality of
dielectric layers disposed on a back of the solar cell; at least
one of said dielectric layers of said layer stack being a densified
layer having an enhanced resistivity to firing through of pastes
with glass components as compared with a resistivity of the
respective layer at a time immediately after a deposition
thereof.
2. The solar cell according to claim 1, wherein said layer stack
includes a silicon oxide layer with a thickness of less than 100
nm.
3. The solar cell according to claim 2, wherein the thickness of
said silicon oxide layer is between 5 nm and 100 nm.
4. The solar cell according to claim 3, wherein the thickness of
said silicon oxide layer is between 10 nm and 100 nm.
5. The solar cell according to claim 1, wherein said layer stack
includes a silicon nitride layer having a thickness of less than
200 nm.
6. The solar cell according to claim 5, wherein the thickness of
said silicon nitride layer is between 50 nm and 200 nm.
7. The solar cell according to claim 5, wherein the thickness of
said silicon nitride layer is between 70 nm and 150 nm.
8. The solar cell according to claim 1, wherein said layer stack
includes a silicon oxide layer and a silicon nitride layer, with
said silicon nitride layer being arranged on top of said silicon
oxide layer.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of patent application Ser.
No. 13/582,502, filed Oct. 1, 2012, which was a .sctn.371 national
stage of international patent application PCT/DE2011/075036, filed
Mar. 3, 2011, which designated the United States; this application
also claims the priority, under 35 U.S.C. .sctn.119, of German
patent applications Nos. DE 10 2010 010 221.0, filed Mar. 3, 2010,
DE 10 2010 010 561.9, filed Mar. 5, 2010, and DE 10 2010 025 983.7,
filed Jul. 2, 2010; the prior applications are herewith
incorporated by reference in their entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The invention concerns a solar cell with a dielectric back
reflective coating formed of a layer stack of dielectric layers
arranged on the back of the solar cell and a method for the
production of a solar cell, wherein a layer stack of dielectric
layers is applied to the back of a solar cell substrate, local
openings are formed in the layer stack, a metallic medium is
applied extensively to the layer stack, with the metallic medium
being partially injected into the local openings, the solar cell
substrate is fired, in order to form ohmic contacts in the local
openings. During firing any firing-through of parts of the metallic
medium located on the layer stack through the layer stack is
prevented.
[0003] In the domain of photovoltaics, the aim is to reduce the
expense of generating electricity. One way this can be done is by
increasing the efficiency of manufactured solar cells, and another
is by reducing the expense required to manufacture solar cells. An
improvement in efficiency requires that a greater proportion of
irradiated light quanta generates electron hole pairs and/or a
greater proportion of the generated electron hole pairs is
conducted away before they can recombine. The result is an
improvement in what is known as the quantum yield or quantum
efficiency.
[0004] There is particular potential for improvement in the red
spectral region, due to the comparatively greater absorption
lengths of the long-wave, red light components. Since ever-thinner
solar cell substrates, for example silicon discs, are being used in
industrial solar cell production, the red spectral region is also
becoming increasingly important. To improve quantum efficiency,
therefore, a metal layer is applied as optical reflector to the
back of the solar cell substrate, i.e. on a side of the solar cell
substrate facing away from incident light. As a result, long-wave
light incident on the front of the solar cell substrate is
reflected to the back of the solar cell substrate. This increases
the probability of absorption in the volume of the solar cell
substrate and hence the probability of the generation of an
electron hole pair. Without optical reflectors on the back of the
solar cell substrate, however, a greater proportion of light would
pass through the solar cell substrate without being absorbed. It
has, however, been shown that this type of metallic optical
reflector is associated with a high charge carrier recombination
rate at the interface of the metal with the solar cell substrate.
This can be circumvented by providing a dielectric reflective
coating on the back of the solar cell substrate instead of metallic
back reflectors. To this end, one or more dielectric layers are
applied on the back of the solar cell substrate. These are designed
in such a way that light quanta striking the dielectric layers are
reflected by the total-reflection effect. This effect replaces the
reflection of the light quanta to the optically denser medium which
occurs with metallic back reflectors.
[0005] With dielectric back reflective coatings of this type the
recombination rate of the charge carrier on the back can be
significantly reduced. Recombination rates of less than 500 cm/s
can be achieved. The full-area aluminum back contact, standard
until now, with a back field (often referred to as back surface
field), however, achieves only recombination rates in the order of
magnitude of 1000 cm/s. An ohmic metallic back contact used as back
reflector without back field even has recombination rates over
10.sup.6 cm/s.
[0006] To conduct away the electricity generated, an electrical
contacting of the back of the solar cell substrate is necessary.
However, this cannot be realized using dielectric layers.
Therefore, in addition to the dielectric back reflector, metallic
contacts must be provided. This can be done, for example, by
locally piercing the dielectric layers and forming metal contacts
in the openings formed. For example, the dielectric layers can be
pierced locally by means of laser beam evaporation and metal
contacts can be vacuum metalized. This way of forming back
contacts, however, is expensive by comparison with the printing
processes usually used in industrial solar cell production, such as
for example screen printing or spray printing processes. But the
printing processes used in industrial production cannot be used
unaltered in connection with dielectric layers for back contacting
of solar cell substrates. This is due to the fact that the pastes
used in these printing processes contain glass components, known as
glass frit. The effect of these is that the pastes are fired
through the dielectric layers in the firing process necessary to
form the contacts, thereby destroying them. The use of pastes which
contain no glass components has proven similarly problematic, since
contacts produced with such pastes have inadequate adhesion to the
solar cell substrate.
[0007] To prevent pastes containing glass frit from firing through
the dielectric layers, it is possible in principle to make the
dielectric layers so thick that this is prevented. However, this
brings with it a substantial additional production cost.
[0008] The improvement in efficiency achieved by the use of a
dielectric back reflective coating is thus now overcompensated by
the additional production cost associated with the dielectric back
reflective coating.
BRIEF SUMMARY OF THE INVENTION
[0009] Against the background described, the present invention is
based on the problem of providing a method which enables economical
dielectric reflective coating and contacting of a solar cell
back.
[0010] This problem is solved by a method as claimed.
[0011] The present invention is also based on the problem of
providing a solar cell with a dielectric reflective back which can
be economically produced.
[0012] This problem is solved by a solar cell as claimed.
[0013] Advantageous refinements are the subject matter of the
respective independent claims.
[0014] The method according to the invention provides that a layer
stack of dielectric layers is applied to the back of a solar cell
substrate. This layer stack is heated and held at temperatures of
at least 700.degree. C. for a period of at least 5 minutes. The
first-mentioned problem is already solved by a method which has
these features.
[0015] Surprisingly, it has emerged that as a result of the heating
and holding of the layer stacks as described at temperatures of at
least 700.degree. C., the resistivity of one or more dielectric
layers of the layer stack to a firing-through of pastes containing
glass components can be enhanced. Such an enhancement of
resistivity is referred to here for short as densification. So far,
it has not been clarified which processes take place in one or more
dielectric layers during the heating and holding at temperatures of
at least 700.degree. C. and lead to a densification of one or more
dielectric layers.
[0016] Preferably, the layer stack is held for a periods of at
least 10 minutes at temperatures of at least 700.degree. C.
[0017] The period during which the layer stack is held at
temperatures of at least 700.degree. C. can in principle be
interrupted by phases in which the layer stack is at temperatures
of less than 700.degree. C. So several time segments may be
provided in which the layer stack is held at temperatures of at
least 700.degree. C. Cumulatively, these time segments extend over
at least 5 minutes, preferably over at least 10 minutes.
[0018] It is preferable to use a silicon solar cell substrate as
solar cell substrate.
[0019] Advantageously a layer stack is applied which has a silicon
oxide layer with a thickness of less than 100 nm. Especially when
silicon solar cell substrates are used, this enables good
passivation of superficial defect states. The thickness of the
silicon oxide layer is preferably between 5 nm and 100 nm,
especially preferably between 10 nm and 100 nm. Said silicon oxide
layer can in principle be applied in any way known in the art. For
example, the silicon oxide layer can be applied using chemical
deposition from a vapor phase. If a silicon solar cell substrate is
used, the silicon oxide layer can be formed by thermal oxidation of
the silicon solar cell substrate.
[0020] In practice, it has proven effective to use a layer stack
which has a silicon nitride layer with a thickness of less than 200
nm. The silicon nitride layer can, for example, be applied by means
of chemical deposition from the vapor phase. In this case, in
particular, plasma-enhanced chemical vapor deposition (PECVD) or
low-pressure chemical vapor deposition (LPCVD) processes can be
used. Silicon nitride layers with a thickness of less than 200 nm
can be applied economically. Preferably, the thickness of the
silicon nitride layer is between 50 nm and 200 nm, especially
preferably between 70 nm and 150 nm.
[0021] It has been shown that silicon nitride layers can be
densified by being heated and held at temperatures of at least
700.degree. C. for a period of at least 5 minutes. As well as
silicon nitride layers, layers of silicon oxide, silicon carbide,
aluminum oxide, titanium oxide or tantalum nitride can also be
densified in this way.
[0022] Advantageously a layer stack is applied which has a silicon
oxide layer and a silicon nitride layer. In this case, it is
preferable firstly to apply the silicon oxide layer onto the back
of the solar cell substrate and then the silicon nitride layer on
the silicon oxide layer. It is especially preferable if the silicon
oxide layer is applied directly onto the solar cell substrate and
the silicon nitride layer is applied directly onto the silicon
oxide layer. This enables a wide-ranging dielectric passivation of
the back of the solar cell substrate, so that very low charge
carrier recombination rates can be realized on the back of the
solar cell substrate. At the same time, the layer stack, because of
the densified silicon nitride layer, has enhanced resistivity to
firing-through of pastes containing glass components.
[0023] Advantageously, after applying the layer stack on the back
of the solar cell substrate, dopant is diffused into the solar cell
substrate in a diffusion step and during this diffusion step the
layer stack is held for periods of at least 5 minutes at
temperatures of at least 700.degree. C. In this way, the
densification of at least one dielectric layer can be integrated
economically into the solar cell production process, since the at
least one dielectric layer can be densified during the diffusion
step, which is required in any case.
[0024] Preferably, the diffusion step is an emitter diffusion step.
This can in principle be arranged in any way known in the art. For
example, it may be an emitter diffusion from the gas phase, for
example a POCl.sub.3 diffusion, or a diffusion of dopant from
precursor layers (known as precursor diffusions). Depending on the
solar cell substrate used, the diffusion step can be in the form of
an n- or p-diffusion step.
[0025] During the diffusion step, the layer stack can be used as
diffusion mask for the back of the solar cell substrate. In this
way one sided emitter diffusion can be realized economically. This
works to advantageous effect in particular in the frequently-used
gas phase diffusions, for example in said POCl.sub.3 diffusion.
This is because, as a result of the one-sided emitter diffusion,
the edge insulation required for full-contact emitter diffusions is
no longer needed, thus reducing the cost of manufacture.
[0026] Advantageously, local openings are formed in the layer
stack. This can be done, for example, by means of laser beam
evaporation. Alternatively, a suitable etching paste can be applied
locally onto the layer stack, which can be pierced locally by
etching.
[0027] If the local openings are formed by means of laser beam
evaporation, it has proven effective to form the local openings as
local linear openings. Compared with a plurality of local,
quasi-punctiform openings, this is advantageous. This is due to the
fact that in laser beam evaporation the surface of the solar cell
substrate is damaged. The damage is more serious in the edge region
of the laser beam. As a result, with a plurality of
quasi-punctiform openings, a less favorable ratio of problematic
edge regions to good middle regions occurs than with linear
openings. Also, a metallization introduced into the linear openings
contributes to an increase in the transverse conductivity of the
back, which has an advantageous effect on the fill factors of the
manufactured solar cells. Optionally, to reduce the damage cause by
the laser beam evaporation, the openings can be over-etched, for
example with an alkaline etching solution or an etching solution
containing hydrofluoric acid.
[0028] If the local openings are formed by means of locally applied
etching paste, it is, however, advantageous to form the openings as
quasi-punctiform openings.
[0029] Alternatively, the local openings can be formed by a
metallic paste with a very high glass component being applied
locally onto the layer stack and being fired through it. Since the
layer stack has at least one densified dielectric layer, the high
glass component and an adaptation of the firing-through process is
necessary. In this variant embodiment the local openings are
already filled with metallic paste. In order to form electrically
conductive connection of the contacts arranged in the local
openings the layer stack can be printed flat with an ordinary
metallic paste. Since this has a smaller glass component, any
firing-through of the paste, which has been applied flat, through
the layer stack of the at least one densified dielectric layer is
prevented.
[0030] Preferably a metallic medium is applied extensively onto the
layer stack, with some of the metallic medium thereby being
injected into the local openings. This can, for example, be done by
means of a printing process of prior art, for example a screen
printing process. The application is extensive if the back-side
area of the solar cell substrate is covered at least 80% by the
metallic medium. In order to form ohmic contacts in the local
openings, the solar cell substrate is fired. During firing, any
firing-through of parts of the metallic medium located on the layer
stack through the layer stack is prevented. The firing parameters
such as temperature and time are to be selected accordingly. For
example, as metallic medium, metallic pastes or printing pastes or
a metallic fluid can be used. It is preferable to use
aluminum-based pastes or fluids, since in this way a local back
field can be formed in the regions of the local openings. This is
often referred to as a local back surface field and reduces the
charge carrier recombination in the regions of the local openings
and/or contacts. In the case of extensive back contacts, warping of
the solar cell often occurs. This is avoided, or the warping is at
least reduced, in the variant embodiment described, since contacts
are formed only in the local openings and thus it is only there
that the metallic medium comes into direct contact with the solar
cell substrate.
[0031] Advantageously, the back of the solar cell substrate is
etched before applying the layer stack, using a smoothing etching
solution or a polishing etching solution. In this way a smooth
surface can be prepared on the back of the solar cell substrate,
which has an advantageous effect on the reflection behavior of the
back of the solar cell substrate. A smoothing etching solution in
this instance means an etching solution by means of which the
surface of the solar cell substrate can be etched in such a way
that incident light with a wavelength of between 400 nm and 1000 nm
is reflected by at least 15% and less than 25%. A polishing etching
solution means an etching solution by means of which the surface of
the solar cell substrate can be etched in such a way that incident
light with a wavelength of between 400 nm and 1000 nm is least 25%
reflected.
[0032] Preferably the front side of the solar cell substrate is
textured. This can be done by means of an etching medium.
Especially preferably, this takes place by means of a texture
etching solution. As a result of the texturing, incident light is
injected increasingly obliquely into the solar cell substrate, so
that an increased proportion of light can be totally reflected to
the back of the solar cell substrate. This can improve the
efficiency of the manufactured solar cell.
[0033] Advantageously, the front of the solar cell substrate is
textured after applying the layer stack. During texturing, the
layer stack is used as etching mask for the back of the solar cell
substrate. In this way, one-sided texturing of the solar cell
substrate can be realized economically.
[0034] After the layer stack has been held for a period of at least
5 minutes at temperatures of at least 700.degree. C., preferably a
hydrogenous silicon nitride layer is deposited onto the front of
the solar cell substrate. The hydrogenous silicon nitride layer is
thus deposited on the front of the solar cell substrate after the
densification of at least one dielectric layer. This can for
example be done using a chemical deposition process known in the
art, from the vapor phase. Using the hydrogenous silicon nitride
layer, defect passivation can be carried out in the volume of the
solar cell substrate, as a result of which the efficiency of the
manufactured solar cells can be improved. Instead of passivation by
means of a hydrogenous silicon nitride layer, in principle any
other type of hydrogen passivation can be selected, for example
defect passivation by means of a hydrogen plasma.
[0035] The solar cell according to the invention has a layer stack
of dielectric layers arranged on the back of the solar cell. At
least one dielectric layer of this layer stack is densified.
[0036] A densified dielectric layer in the sense of the present
invention means a dielectric layer whose resistivity to a
firing-through of pastes with glass components is enhanced compared
with its resistivity at a time immediately following its
deposition.
[0037] A densified layer is obtained by heating the layer stacks
and holding the layer stacks at temperatures of at least
700.degree. C. for a period of at least 5 minutes.
[0038] It has proven effective to use a layer stack which has a
silicon oxide layer with a thickness of less than 100 nm.
Preferably it has a thickness of between 5 nm and 100 nm,
especially preferably between 10 nm and 100 nm.
[0039] Advantageously the layer stack has a silicon nitride layer
with a thickness of less than 200 nm. Preferably the thickness is
between 50 nm and 200 nm, especially preferably between 70 nm and
150 nm.
[0040] Silicon oxide layers and silicon nitride layers in the said
thicknesses can be deposited economically by means of a method
known in the art, for example chemical deposition processes from
the vapor phase. In the case of a silicon solar cell substrate, the
silicon oxide layer can be formed by thermal oxidation of the solar
cell substrate.
[0041] Advantageously, the layer stack has a silicon oxide layer
and a silicon nitride layer. The silicon nitride layer is
preferably arranged on the silicon oxide layer. Especially
preferably the silicon oxide layer is arranged directly on the
solar cell substrate and the silicon nitride layer is arranged
directly on the silicon oxide layer.
[0042] Advantageously, a flat back contact is arranged on the layer
stack, extending locally through the layer stack and contacting the
back of a solar cell substrate. To this end, several local openings
can be provided in the layer stack, through which the flat back
contact extends through the layer stack.
[0043] Preferably the back contact is formed from a metallic paste,
advantageously from an aluminum paste. This can, for example, be a
screen printing contact, which is preferably executed in a single
piece, thus has been applied in a single screen printing
process.
[0044] Advantageously the back contact has glass components. These
can, for example, be the glass frit usually found in screen
printing pastes. These glass components enable reliable adhesion of
the back contact on the layer stack.
[0045] The invention will next be explained in more detail on the
basis of some figures. Wherever expedient, elements with the same
effect have been given the same reference numbers.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0046] FIG. 1 shows a simplified diagram of a first embodiment of
the method according to the invention
[0047] FIG. 2 shows a simplified diagram of a second embodiment of
the method according to the invention
[0048] FIG. 3 shows a simplified diagram of a third embodiment of
the method according to the invention
[0049] FIG. 4 is a schematic diagram of a solar cell according to
the invention
[0050] FIG. 5 is a back view of the solar cell from FIG. 4 in a
schematic view.
DESCRIPTION OF THE INVENTION
[0051] FIG. 1 illustrates a first embodiment of the method
according to the invention. According to this, the solar cell
substrate is firstly textured 10 by means of a texture etching
solution. Next, the back of the solar cell substrate is etched in a
polishing etching solution 12 and cleaned 12 in a way known in the
art. Next, a silicon oxide layer is applied to the back of the
solar cell substrate 14. This can for example be done by a chemical
deposition from the vapor phase. Preferably, however, a silicon
solar cell substrate is used and the silicon oxide layer is grown
thermally or deposited in the plasma phase.
[0052] Next, a silicon nitride layer is applied 16 onto the silicon
oxide layer. The silicon oxide layer, together with the silicon
nitride layer, forms a layer stack, which effects a dielectric
reflective coating of the back of the solar cell substrate.
Together with the texturing present on the front side of the solar
cell substrate, this layer stack, as described above, brings about
an effective reflection of incident light to the back of the solar
cell substrate.
[0053] In the further course of the method the solar cell
substrates are cleaned 18 in a way known in the art, for example in
cleaning solutions containing hydrochloric acid and/or hydrofluoric
acid. This is followed by an emitter diffusion 20, in which the
layer stack consisting of the silicon oxide layer and silicon
nitride layer is heated and held for a period of at least 5 minutes
at temperatures of at least 700.degree. C., so that the silicon
nitride layer is densified 20. The emitter diffusion 20 can be
executed as phosphorus diffusion, provided the solar cell substrate
used has a p-volume doping. In this, as in all other embodiments,
however, n-doped solar cell substrates can also be used. The
emitter diffusion would then be executed as p-emitter diffusion,
for example as boron diffusion.
[0054] Next, a phosphorus emitter diffusion in a p-doped silicon
solar cell substrate will be assumed. This phosphorus diffusion can
for example be in the form of a POCl.sub.3 diffusion. The
embodiment in FIG. 1 is, however, also suitable for precursor
diffusions and is compatible with both continuous diffusion
processes and with diffusions conducted in batch mode.
[0055] During the emitter diffusion 20 the layer stack of silicon
oxide layer and silicon nitride layer serves as diffusion mask for
the back of the solar cell substrate. During the emitter diffusion
20, therefore, no dopant is diffused into the back of the solar
cell substrate. This obviates the need for edge insulation.
[0056] Following the emitter diffusion 20, a laser diffusion 32 can
optionally take place on the front side of the solar cell
substrate. This involves a laser beam being guided over the contact
structure of the front side. The contact structure is formed from
those regions in which the front contacts will be arranged at a
later time. Because the laser beam is guided over this contact
structure, an enhanced diffusion of dopant occurs into these
regions from a silicate glass formed on the surface of the silicon
solar cell substrate used during the emitter diffusion 20. If the
emitter diffusion 20 has been executed as phosphorus diffusion,
this is for example a phosphor silicate glass, from which
additional dopant is diffused locally into the front of the silicon
solar cell substrate. The laser diffusion 32 on the front thus
enables the formation of a selective emitter structure.
[0057] Next, local openings are formed in the layer stack 22. As
already explained above, this can be done for example by means of
laser beam evaporation or using a locally applied etching
paste.
[0058] Next, the silicate glass formed during the emitter diffusion
20 is etched 24 and thereby removed. In the case of a phosphorus
emitter diffusion, this will be a phosphor silicate glass.
[0059] Next, a hydrogenous silicon nitride layer is deposited 26
onto the front of the solar cell substrate. This enables, as
already explained above, a passivation of defect states in the
volume of the solar cell substrate.
[0060] Next, the front and back of the solar cell substrate are
metalized 28. Preferably this is done by means of screen printing
processes. In principle however, another method, in particular
another printing method, can be used. When the back is metalized,
it is preferable for a metallic paste to be applied extensively on
the back of the solar cell substrate and some of the metallic paste
to be injected into the local openings.
[0061] The metallic pastes applied during metallization 28 contain
glass components. In a subsequent co-firing 30 the metallic paste
arranged on the front is fired through the silicon nitride layer on
the front and sintered into the solar cell substrate, so that an
ohmic front contact is formed. The metallic paste containing glass
components and applied to the back is not fired through the back
silicon nitride layer during the co-firing 30, since this has been
densified 20 and is thus more resistant to firing-through. Because
of the glass components, the fired paste instead adheres reliably
to the layer stack. It is only in regions of the local openings, in
which the paste has been inserted, that sintering in of the
metallic paste in the back of the solar cell substrate and a
formation of ohmic contacts take place. Preferably, an aluminum
paste is used as metallic paste for the back, so that during the
co-firing 30 a local back field is formed in regions of the local
openings.
[0062] The embodiment in FIG. 1 thus represents an economical
method for the manufacture of solar cells with a dielectric back
passivation and local back field. The embodiment in FIG. 1 has
proven effective, especially in the manufacture of solar cells from
multi- or monocrystalline silicon discs.
[0063] In the embodiment in FIG. 2, a silicon solar cell substrate
is again used. To start with, this is etched 40 in a smoothing
etching solution and this also removes any saw damage on the solar
cell substrates. This results in a solar cell substrate which is
smoothly etched on both front and back.
[0064] Next, as in the embodiment in FIG. 1, a silicon oxide layer
is applied 14 directly onto the back of the solar cell substrate
and a silicon nitride layer is applied 16 onto the silicon oxide
layer.
[0065] This is followed by texturing 42 by means of a texture
etching solution. The layer stack formed on the back of the solar
cell substrate and consisting of the silicon oxide layer and the
silicon nitride layer thereby serves as etching mask, so that only
the front of the solar cell substrate is textured 42.
[0066] The rest of the method steps correspond to those for the
embodiment from FIG. 1.
[0067] In the embodiment from FIG. 2 the layer stack consisting of
silicon nitride layer and silicon oxide layer is thus not only used
as diffusion masking during the emitter diffusion 20, but also as
etching mask during the texturing 42. The expenditure for the
smooth and/or polishing etching of the back of the solar cell
substrate can be advantageously reduced as a result. The embodiment
in FIG. 2 has proven especially effective when monocrystalline
silicon discs are used as solar cell substrates for solar cell
manufacture.
[0068] In the embodiment from FIG. 3 a silicon solar cell substrate
is firstly textured 10 by means of a texture etching solution. This
is followed by the etching of the back, already known from FIG. 1,
in a polishing etching solution and cleaning 12.
[0069] Next, the solar cell substrate, which in this case is
executed as a silicon solar cell substrate, is thermally oxidized
52. The total surface of the solar cell substrate is thus covered
by a silicon oxide layer. Next, a silicon nitride layer is applied
54 on the back and therefore on the silicon oxide layer there.
[0070] Next, in the way already described in connection with FIG.
1, local openings are formed in the layer stack consisting of
silicon nitride layer and silicon oxide layer.
[0071] Furthermore, local contact openings are formed in the
silicon oxide layer on the front of the solar cell substrate 56.
The metallic front contacts are then formed in the regions of these
contact openings. The local contact openings can, for example, be
formed by means of laser beam evaporation. Alternatively, there is
the option of applying etching paste or another etching medium
locally.
[0072] If the local contact openings in the front and/or the local
openings in the layer stack are formed by means of laser beam
evaporation, it can be advantageous to remove the resultant laser
damage by etching, as provided by the optional method step 59. In
this case an alkaline etching solution, for example a KOH solution,
may be used.
[0073] During a subsequent cleaning 58 of the solar cell substrate,
the silicon oxide layer is retained. Therefore hydrofluoric acid is
not used during the cleaning 58. The cleaning 58 takes place using
hydrochloric acid instead.
[0074] Next, a dopant diffusion 60 takes place, in which the solar
cell substrates are heated and held for a period of at least 5
minutes at temperatures of at least 700.degree. C., so that the
silicon nitride layer is densified. In this dopant diffusion 60, as
in the case of the emitter diffusion in FIGS. 1 and 2, this can be
a p- or n-diffusion. It can also be carried out as continuous
diffusion or as diffusion in batch mode. During the diffusion 60
the layer stack consisting of silicon nitride layer and silicon
oxide layer on the back of the solar cell substrate again serves as
diffusion mask. On the front of the solar cell substrate the dopant
can penetrate through the local contact openings unhindered into
the solar cell substrate where it can effect a local doping of the
front. If solar cell substrates are used which already have a flat
emitter diffusion on the front side, the dopant diffusion 60 can
easily realize a selective emitter with heavily doped regions in
the regions of the local contact openings.
[0075] Alternatively, there is the option of making the silicon
oxide layer very thin and using it as diffusion-inhibiting layer,
so that during the dopant diffusion 60, dopant gets through the
front silicon oxide layer in reduced quantity into the solar cell
substrate where it can form a weak emitter doping. In the regions
of the local contact openings the dopant can, however, penetrate
unhindered into the solar cell substrate where it forms heavily
doped regions. The final result is a selective emitter structure
which can be realized with a single dopant diffusion 60.
[0076] The rest of the method steps correspond to those in FIG.
1.
[0077] FIG. 4 illustrates schematically in a sectional view an
embodiment of a solar cell according to the invention 70, which has
a solar cell substrate 72 which is provided on the front with a
texturing 73. On the back of the solar cell 70 a silicon oxide
layer 74 is provided, which is arranged directly on the solar cell
substrate 72. A densified silicon nitride layer 76 is arranged
directly on the silicon oxide layer 74. The silicon oxide layer 74
and the silicon nitride layer together form a layer stack with
local openings 78 through which an extensive back contact 80
extends and contacts the back of the solar cell substrate 72. On
the front side of the solar cell substrate 72 a further silicon
nitride layer 82 is provided as anti-reflection coating. Front
contacts 84 extend through this silicon nitride layer 82.
[0078] FIG. 5 shows a schematic back view of the solar cell from
FIG. 4. This shows the extensive back contact 80. This partially
overlaps a bus line 88, which is usually silvered and serves as
solder contact for the solar cell 70.
[0079] As can be seen in FIG. 5, the local openings 78 are executed
as linear openings, so that metallization lines 86 which extend
perpendicular to the bus line 88 are found in the openings. The
extensive back contact 80 is formed from a metallic paste and
contains glass frit. Because of this glass frit component, the flat
back contact 80 adheres reliably to the silicon nitride layer 76.
In a special variant embodiment the bus line can be interrupted in
places, so that individual collector sections are created, which
serve as solder contact.
[0080] The following is a summary list of reference numerals and
the corresponding structure used in the above description of the
invention:
[0081] 10 Texturing by means of texture etching solution
[0082] 12 Etching back in polishing etching solution and
cleaning
[0083] 14 Applying silicon oxide layer to the back
[0084] 16 Applying silicon nitride layer onto silicon oxide
layer
[0085] 18 Cleaning
[0086] 20 Emitter diffusion and densification of silicon nitride
layer
[0087] 22 Forming local openings in layer stack
[0088] 24 Etching silicate glass
[0089] 26 Deposition of hydrogenous silicon nitride layer on front
side
[0090] 28 Metallization of front and back
[0091] 30 Co-firing
[0092] 32 Laser diffusion contact structure on front
[0093] 40 Etching saw damage in smoothing etching solution
[0094] 42 Texturing front by means of texture etching solution
[0095] 52 Thermal oxidation of the solar cell substrate
[0096] 54 Applying silicon nitride layer on back
[0097] 56 Forming local contact openings in silicon oxide layer on
the front
[0098] 58 Cleaning while retaining silicon oxide layer
[0099] 59 Etching laser damage
[0100] 60 Dopant diffusion and densification of silicon nitride
layer
[0101] 70 Solar cell
[0102] 72 Solar cell substrate
[0103] 73 Texturing
[0104] 74 Silicon oxide layer
[0105] 76 Densified silicon nitride layer
[0106] 78 Opening
[0107] 80 Back contact
[0108] 82 Silicon nitride layer
[0109] 84 Front contact
[0110] 86 Metallization line
[0111] 88 Bus line
* * * * *