U.S. patent application number 12/971232 was filed with the patent office on 2011-07-07 for device for fabricating a photovoltaic element with stabilised efficiency.
This patent application is currently assigned to Universitat Konstanz. Invention is credited to Giso Hahn, Axel Herguth, Martin Kas, Ihor Melnyk, Gunnar Schubert.
Application Number | 20110162716 12/971232 |
Document ID | / |
Family ID | 38169429 |
Filed Date | 2011-07-07 |
United States Patent
Application |
20110162716 |
Kind Code |
A1 |
Herguth; Axel ; et
al. |
July 7, 2011 |
DEVICE FOR FABRICATING A PHOTOVOLTAIC ELEMENT WITH STABILISED
EFFICIENCY
Abstract
A method and device for fabricating a photovoltaic element with
stabilized efficiency is proposed. The method comprises the
following steps: preparing a boron-doped, oxygen-containing silicon
substrate; forming an emitter layer on a surface of the silicon
substrate; and a stabilization treatment step. The stabilization
treatment step comprises keeping the temperature of the substrate
during a treatment time within a selectable temperature range
having a lower temperature limit of 50.degree. C., preferably
90.degree. C., more preferably 130.degree. C. and even more
preferably 160.degree. C. and an upper temperature limit of
230.degree. C., preferably 210.degree. C., more preferably
190.degree. C. and even more preferably 180.degree. C., and
generating excess minority carriers in the silicon substrate during
the treatment time, for example, by illuminating the substrate or
by applying an external voltage. This method can be used to
fabricate a photovoltaic element, e.g. a solar cell or a solar
module having an efficiency which is stable at a value higher than
that of photovoltaic elements fabricated without the stabilization
treatment step.
Inventors: |
Herguth; Axel; (Konstanz,
DE) ; Schubert; Gunnar; (Konstanz, DE) ; Hahn;
Giso; (Konstanz, DE) ; Melnyk; Ihor;
(Konstanz, DE) ; Kas; Martin; (Konstanz,
DE) |
Assignee: |
Universitat Konstanz
Konstanz
DE
|
Family ID: |
38169429 |
Appl. No.: |
12/971232 |
Filed: |
December 17, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12225337 |
Sep 18, 2008 |
|
|
|
PCT/EP2007/002502 |
Mar 21, 2007 |
|
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12971232 |
|
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Current U.S.
Class: |
136/261 ;
219/443.1; 257/E31.001; 438/57 |
Current CPC
Class: |
H01L 31/1804 20130101;
Y02P 70/50 20151101; H01L 31/1864 20130101; Y02E 10/547 20130101;
Y02P 70/521 20151101 |
Class at
Publication: |
136/261 ; 438/57;
219/443.1; 257/E31.001 |
International
Class: |
H01L 31/0264 20060101
H01L031/0264; H01L 31/18 20060101 H01L031/18; H05B 3/68 20060101
H05B003/68 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 21, 2006 |
DE |
10 2006 012 920.2 |
Claims
1. A device for fabricating a photovoltaic element with stabilized
efficiency or for stabilizing the efficiency of a photovoltaic
element, wherein the device is adapted to perform the following
steps: receiving a boron-doped, oxygen-containing silicon substrate
with an emitter layer formed on a surface of the silicon substrate;
characterized in that the device is further adapted to perform a
stabilization treatment step, comprising: keeping the temperature
of the substrate during a treatment time within a selectable
temperature range having a lower temperature limit of 50.degree. C.
and an upper temperature limit of 230.degree. C. and generating
excess minority charge carriers in the silicon substrate during the
treatment time.
2. The device according to claim 1, wherein the silicon substrate
comprises electrical contacts formed thereon, wherein the device is
further adapted to, in the step of generating excess minority
carriers, apply an external electrical voltage to the contacts.
3. The device according to claim 2, wherein the device is adapted
to apply the voltage in the conducting direction of the pn junction
formed with the silicon substrate and the emitter layer.
4. The device according to claim 2, wherein the applied voltage is
higher than 0.4 V, preferably higher than 0.6 V and more preferably
higher than 0.7 V.
5. The device according to claim 2, wherein the device is adapted
such that the silicon substrate is substantially not illuminated
during the application of the external voltage.
6. The device according to claim 2, wherein the device is adapted
such that the treatment time t in minutes during which the
substrate is held within the selectable temperature range is given
by: t .gtoreq. a ( y + b ) c * exp ( x ( T + 273 ) ) ##EQU00002##
where T is the average temperature of the selectable temperature
range in .degree. C. during the treatment time, y is the current
density through the photovoltaic element brought about by the
applied voltage in A/cm.sup.2 and a=4.247*10.sup.-14, b=0.00286,
c=0.887 and x=12550, preferably a=3.272*10.sup.-14, b=0.00352,
c=0.934 and x=12800.
7. The device according to claim 1, wherein the device is adapted
to simultaneously receive a plurality of silicon substrates which
have been previously stacked one above the other in a space-saving
manner.
8. The device according to claim 1, wherein the device is adapted
to, in the step of generating the excess minority carriers,
illuminate the silicon substrate.
9. The device according to claim 8, wherein the illumination takes
place using light having a wavelength shorter than 1180 nm.
10. The device according to claim 8, wherein the illumination takes
place using light having a radiation intensity higher than 10
W/m.sup.2, preferably higher than 100 W/m.sup.2 and more preferably
higher than 1000 W/m.sup.2.
11. The device according to claim 8, wherein the treatment time t
in minutes during which the substrate is held within the selectable
temperature range is given by: t .gtoreq. a ( y + b ) c * exp ( x (
T + 273 ) ) ##EQU00003## where T is the average temperature of the
selectable temperature range in .degree. C. during the treatment
time, y is the radiation intensity in kW/m.sup.2 and
a=2.298*10.sup.-11, b=0.399, c=1.722 and x=11100, and preferably
a=5.355*10.sup.-11, b=0.355, c=1.349 and x=11000.
12. The device according to claim 1, wherein the device is adapted
to simultaneously receive a plurality of silicon substrates which
have been previously encapsulated in a module.
13. The device according to claim 1, wherein the device comprises a
voltage source for applying a voltage corresponding to a desired
voltage to be applied to a single solar cell multiplied by the
number of solar cells connected in series and received within the
device.
14. The device according to claim 1, wherein the device comprises a
hot plate and/or a suitably heatable room for storing the silicon
substrate and keeping it on an elevated temperature during the
stabilization treatment step.
15. A photovoltaic element comprising a boron-doped,
oxygen-containing silicon substrate having an efficiency-stabilized
state, wherein the photovoltaic element has a high efficiency such
as can be achieved by annealing, characterized in that the
efficiency of the solar cell drops by less than 5% relatively,
preferably less than 2% relatively, under illumination.
16. A photovoltaic element with stabilized efficiency, fabricated
by a process comprising the steps of: providing a boron-doped,
oxygen-containing silicon substrate; forming an emitter layer on a
surface of the silicon substrate; performing stabilization
treatment, comprising: keeping the temperature of the substrate
during a treatment time within a selectable temperature range
having a lower temperature limit of 50.degree. C. and an upper
temperature limit of 230.degree. C.; and generating excess minority
charge carriers in the silicon substrate during the treatment
time.
17. The photovoltaic element according to claim 16, fabricated by a
process further comprising the step of forming electrical contacts
on the silicon substrate, wherein the step of generating excess
minority carriers involves applying an external electrical voltage
to the contacts, and wherein the treatment time t in minutes during
which the substrate is held within the selectable temperature range
is given by t .gtoreq. a ( y + b ) c * exp ( x ( T + 273 ) )
##EQU00004## where T is the average temperature of the selectable
temperature range in .degree. C. during the treatment time, y is
the current density through the photovoltaic element brought about
by the applied voltage in A/cm.sup.2 and a=4.247*10.sup.-14,
b=0.00286, c=0.887 and x=12550, preferably a=3.272*10.sup.-14,
b=0.00352, c=0.934 and x=12800.
18. The photovoltaic element according to claim 16, wherein the
step of generating the excess minority carriers comprises
illuminating the silicon substrate, and wherein the treatment time
t in minutes during which the substrate is held within the
selectable temperature range is given by: t .gtoreq. a ( y + b ) c
* exp ( x ( T + 273 ) ) ##EQU00005## where T is the average
temperature of the selectable temperature range in .degree. C.
during the treatment time, y is the radiation intensity in
kW/m.sup.2 and a=2.298*10.sup.-11, b=0.399, c=1.722 and x=11100,
and preferably a=5.355*10.sup.-11, b=0.355, c=1.349 and x=11000.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 12/225,337, filed on Sep. 18, 2008, which is a
national phase entry under 35 U.S.C. .sctn.371 of International
Application No. PCT/EP2007002502, filed on Mar. 21, 2007, published
in English, which claims priority from DE 10 2006 012 920.2 filed
Mar. 21, 2006, all of which are incorporated herein by
reference.
[0002] The present invention relates to a method and a device for
fabricating a photovoltaic element with stabilized efficiency. In
particular, the present invention relates to a method and a device
for fabricating a solar cell based on a boron-doped,
oxygen-containing silicon substrate or a photovoltaic module in
which such solar cells are encapsulated.
[0003] Solar cells are used to convert light directly into
electrical energy. For this purpose, a region having the
correspondingly opposite doping is formed on a p- or n-type
semiconductor. A pn junction is produced. On exposure to light,
charge carrier pairs are produced, these being spatially separated
by the potential gradient formed by the pn junction. The charge
carriers separated in this way can then diffuse to a surface of the
solar cell and be supplied to an external circuit by metal contacts
formed there.
[0004] Crystalline silicon is a semiconductor frequently used to
fabricate solar cells. A distinction is made between
multicrystalline (or polycrystalline) and monocrystalline silicon.
Since monocrystalline silicon has no grain boundaries which act as
impurities and thus serve as recombination centers, it has a higher
material quality compared to multicrystalline silicon, resulting in
a higher potential efficiency for solar cells fabricated therefrom.
However, as a result of the energy-intensive additional process
steps required for drawing the silicon single crystal, the
fabrication costs for monocrystalline silicon are higher compared
to multicrystalline silicon.
[0005] Nowadays, principally two crystal drawing methods are used
to fabricate monocrystalline silicon. Silicon wafers fabricated by
the float-zone (FZ) method have the highest quality but are the
most expensive and are primarily used in the electronics field. The
Czochralski (Cz) method is less expensive and is therefore suitable
for use for the mass production of solar cells. In the Czochralski
method, silicon is melted in a crucible. A single-crystal seed
crystal is brought in contact with the melt and is then drawn
upwards by slowly turning away from the melt. In this case, silicon
from the melt crystallizes on the surface of the (seed) crystal and
a cylindrical single crystal is formed, which can typically have a
diameter of up to 30 cm and a length of several meters. This single
crystal is then cut into wafers about 200-300 .mu.m thick.
[0006] Nowadays, Cz silicon of the p-semiconductor type to be used
for fabricating solar cells is usually doped with boron due to
process technology advantages. For this purpose, boron is dissolved
in the silicon melt during the Cz method. As a result of its good
solubility behavior, boron becomes incorporated in the silicon
crystal drawn from the melt. A largely homogeneous boron-doped
silicon crystal is obtained.
[0007] In the conventional Cz method, oxygen is frequently
dissolved in the silicon melt, having been released from the
crucible which frequently consists of quartz (silicon oxide). The
oxygen is likewise incorporated into the silicon crystal.
[0008] When a solar cell fabricated from Cz silicon is illuminated
and/or an external voltage is applied thereto, excess minority
carriers are generated therein and an electric current flows
therein. In this case, the oxygen atoms incorporated into the
silicon together with the boron atoms used for the doping seem to
form defects which may act as electrically active impurities and
may negatively influence the electrical properties of the solar
cell. As a result of the formation of such defects, the material
quality of the Cz silicon substrate deteriorates during the first
operating hours of the solar cell and the efficiency of the solar
cell drops until it reaches saturation at a certain end value. This
phenomenon is designated as "carrier induced degradation" of Cz
silicon; it was discovered in 1972 and has been the subject of
intensive research until the present day.
[0009] At an advantageous boron concentration of about 1*10.sup.16
cm.sup.-3 and a typical oxygen concentration in Cz silicon of 5 to
10*10.sup.17 cm.sup.-3, the efficiency of a solar cell under
operating conditions typically degrades within a few hours by up to
3% absolute. A loss of 1% absolute frequently observed in Cz
silicon solar cells already represents a loss of more than 6%
relative at an efficiency of 16.5% which is typical of industrially
fabricated solar cells based on Cz silicon prior to degradation.
The degradation and therefore the loss of efficiency during the
initial operation of the solar cells, is greater the higher the
boron and/or the oxygen concentration.
[0010] Hitherto in the prior art, substantially two general
approaches are known as to how the degradation can be reduced or
avoided. The first approach is based on minimizing the oxygen
contamination of the silicon melts during the Cz method. To this
end, the so-called MCz (Magnetic Czochralski) method was developed
whereby contact between the silicon melt and quartz crucible is
avoided during the production of the crystal by complex magnetic
field technology, resulting in a lowering of the oxygen content and
therefore a reduction in the degradation. However, as a result of
the more complex fabrication, MCz silicon is considerably more
expensive than normal Cz silicon.
[0011] The other approach is based on reducing the boron
concentration in the silicon crystal. Calculations have shown that
an optimal efficiency for the solar cell can be achieved by using a
silicon substrate having a boron concentration of around
1*10.sup.16 cm.sup.-3. In order to reduce the considerable
degradation which occurs at these boron concentration, Cz silicon
substrates having a lower boron concentration are frequently used
at the present time in the industrial production of solar cells. As
a result, the degradation is reduced and although the efficiency of
the solar cell directly after its fabrication is lower than that
with a doping of around 1*10.sup.16 cm.sup.-3, a higher stable
efficiency can be achieved after degradation.
[0012] In order to completely avoid the problem of degradation,
attempts are currently also being made to replace boron by other
dopants such as, for example, gallium. However, as a result of its
solubility behavior in silicon, gallium has the decisive
disadvantage that a homogeneous distribution in the entire crystal
is extremely difficult to achieve. Thus, large quantities of
rejects would have to be expected on the industrial scale so that
this variant cannot be considered to be feasible industrially so
far.
[0013] Another procedure involves using phosphorus as dopant and
thus using n-type silicon as substrate material. However, such
substrate material is unusual in the existing photovoltaic industry
and would require a modification of the entire production
process.
[0014] It has been established that the degradation of Cz silicon
is reversible by a so-called annealing step. In such an annealing
step the Cz silicon wafer or the ready-processed solar cell is
heated for a few minutes to temperatures in the range of around
230.degree. C. It was observed that the original efficiency of the
solar cell or the original minority carrier diffusion length in the
silicon substrate before degradation can be achieved again through
such an annealing step. The boron-oxygen defects formed during the
degradation apparently dissolve as a result of the annealing step.
We thus talk of metastable defects. However, it was observed that
an annealed solar cell degrades again under illumination or when
current flows through the cell, i.e. under conditions such as occur
as standard, for example, during operation of the solar cell. The
annealed state thus appears not to be stable in a solar cell under
operating conditions.
[0015] For an overview of the effects observed so far in connection
with degradation and the attempts at solutions to avoid this made
so far, reference is made to Progress in Photovoltaics: Research
and Applications, 2000; 8; 537-547.
[0016] There may be a need to avoid the problems described above.
Particularly, there may be a need to provide a method and a device
for fabricating a photovoltaic element in which the efficiency of
the photovoltaic element can be stabilized at a high level.
[0017] This need may be achieved by a subject-matter as defined in
claim 1. Advantageous embodiments are specified in the dependent
claims.
[0018] A method according to the invention for fabricating a
photovoltaic element such as, for example, a single solar cell or
an entire solar cell module is proposed, comprising the following
steps: preparing a boron-doped, oxygen-containing silicon substrate
and forming an emitter layer on a surface of the silicon substrate.
The method according to the invention is thus characterized in that
it comprises an additional stabilization treatment step, comprising
keeping the temperature of the substrate during a treatment time
within a selectable temperature range and generating excess
minority charge carriers in the silicon substrate during the
treatment time. The temperature range in this case has a lower
temperature limit of about 50.degree. C. and an upper temperature
limit of about 230.degree. C.
[0019] The silicon substrate used in the method according to the
invention can be a Cz silicon wafer which, for example, is
boron-doped with a concentration of about 1*10.sup.16 cm.sup.-3 to
3*10.sup.16 cm.sup.-3. Conditional of manufacturing, such a wafer
has a specific oxygen concentration of, for example 5*10.sup.16
cm.sup.-3 to 3*10.sup.18 cm.sup.-3. However, other boron-doped
oxygen-containing silicon substrates can also be used such as, for
example, multicrystalline silicon wafers or silicon layers
deposited from the gaseous or liquid phase on a supporting
substrate.
[0020] In order to produce the potential gradient required for the
charge carrier separation in a solar cell, according to the
invention an emitter is formed on the surface of the silicon
substrate. This is a region which usually comprises a semiconductor
type opposite to the semiconductor type of the substrate.
Boron-doped silicon is usually a p-type semiconductor. The
formation of an emitter layer on a substrate surface can be
achieved, for example, by superficial in-diffusion of an n-type
dopant such as, for example, phosphorus into the substrate. Such
diffusion is typically carried out at temperatures above
800.degree. C. However, it is also possible to use other methods
for producing an n-doped layer. For example, an additional n-doped
layer can be deposited on the substrate surface from a gaseous or
liquid phase. Furthermore, the layer must not completely cover the
substrate surface. It can be sufficient if only partial region of
the substrate surface at the front and/or back of the silicon
substrate are covered with the n-doped layer.
[0021] Alternatively, the boron-doped silicon substrate can also be
an n-type semiconductor, e.g. if it is overcompensated with
phosphorus. In this case, the emitter is a p-doped region and can
be produced, for example, by in-diffusion or in-alloying of boron
or aluminum. The case where the emitter and the substrate are of
the same type of semiconductor can also be achieved, for example,
if the two regions have highly different band structures so that
band bending is established at their interface, which effects the
desired potential gradient.
[0022] Excess minority carriers can be generated in the silicon
substrate during the stabilization treatment step in various ways,
as described in detail further below. For example, in addition to
the equilibrium carrier concentration which depends principally on
the dopant concentration, excess minority carriers can be generated
in the p-type silicon substrate in which electrons are available as
minority carriers, by exposure to light or by producing a current
through the photovoltaic element by applying an external
voltage.
[0023] At the same time, i.e., within the treatment time, the
temperature of the substrate must be kept within the aforesaid
temperature range. With increasing treatment time, the
efficiency-stabilizing effect successively increases. The longer
the treatment time, the higher the efficiency at which the
photovoltaic element remains stable in a subsequent operation. In
this context, it appears not to be significant if the treatment
time is temporarily interrupted. For example, the illumination of
the substrate or the application of the external voltage used to
generate the excess minority carriers can be temporarily
interrupted and resumed at a later time. The temperature can also
be temporarily reduced below 50.degree. C. Only the entire duration
of the treatment during which the substrate is held in the
temperature range and excess minority carriers are generated in the
substrate is important for the efficiency-stabilizing effect
achieved.
[0024] It has been established that the desired effect of
stabilizing the efficiency of the photovoltaic element to be
achieved by the method according to the invention can be achieved
with increasing substrate temperatures within a shorter treatment
time. The lower limit of the temperature is thus preferably
selected as 90.degree. C., more preferably 130.degree. C., even
more preferably 160.degree. C. It has been further established that
as the temperature approaches an upper limit of 230.degree. C., the
efficiency stabilizing effect is reduced. Thus, the upper
temperature limit of the temperature range is preferably selected
as 210.degree. C., more preferably 190.degree. C. and even more
preferably 180.degree. C.
[0025] According to one embodiment, the method according to the
invention further comprises a step of forming electrical contacts
on the silicon substrate, wherein the step of generating excess
minority carriers involves applying an external electrical voltage
to the contacts. In other words, electrical contacts are formed on
the solar cell to be fabricated and these are then used in turn to
apply a voltage to the solar cell. The voltage is preferably
applied in the conducting direction of the pn junction formed with
the silicon substrate and the n-doped layer. As a result of the
applied voltage, minority carriers are thus injected via the space
charge region of the pn junction into the region of the
respectively opposite semiconductor type. The
efficiency-stabilizing effect of the method according to the
invention increases with increasing voltage in the conducting
direction.
[0026] The voltage can be selected to be higher than 0.4 Volts,
preferably higher than the voltage at which the solar cell has its
maximum power point in normal operation, for example, higher than
0.6 Volts, and more preferably higher than the open circuit voltage
of the solar cell in normal operation, for example, higher than 0.7
Volts. Normal operation of the solar cell should be understood in
this connection as operation under standard conditions (25.degree.
C., illumination with an AM 1.5 spectrum).
[0027] Preferably, the silicon substrate is substantially not
illuminated during the application of the external voltage, i.e. in
the previously described embodiment in which the excess minority
carriers are injected by a current produced by applying an external
voltage, the stabilization treatment step can be carried out in the
dark. It is sufficient to heat the solar cells already provided
with contacts in the desired temperature range and apply an
external voltage to the contacts. Thus, no expensive equipment
requiring a large amount of space is required for this embodiment.
The solar cells can be stacked one above the other in a
space-saving manner in a suitably temperature-controlled room and
merely need to be connected to an inexpensive voltage source.
[0028] The method is particularly advantageous if it is applied to
photovoltaic modules which have already been connected. In this
case, a plurality of solar cells is generally interconnected within
the module, partly in series and partly in parallel. The module
itself can have electrical contacts which are easily contacted from
outside. Consequently, it is sufficient to connect the entire
module to an external voltage source whose voltage corresponds to
the desired voltage to be applied to a single cell multiplied by
the number of solar cells connected in series within the module. An
embodiment of the present method according to the invention of
particular economic interest is thereby possible whereby finished,
ready-to-operate solar modules are subjected to the stabilizing
treatment step before ultimately being delivered to the end
consumers by storing them for example in a suitably
temperature-controlled room whilst applying the corresponding
voltage for a predetermined treatment time. The costs incurred in
addition to a normal fabrication method for solar modules by the
stabilizing treatment step according to the invention are thus
minimal. The method according to the invention is therefore
economically extremely interesting since a stable efficiency can be
achieved for the fabricated solar modules with minimal increased
costs, which is about 0.5-2% absolute higher in the long term
compared with conventional solar modules which degrade appreciably
within the first few operating hours.
[0029] According to a further embodiment of the method according to
the invention, the step of generating excess minority carriers
comprises illuminating the silicon substrate. For example, the
silicon substrate can be illuminated from the side which serves as
the side facing the sun in the finished solar cell. If the photon
energy of the light used for the illumination is higher than the
energy band gap of silicon, excess minority carriers are generated
in the silicon by the illumination. The illumination is preferably
carried out using light at a wavelength of less than 1180 nm and
having a radiation intensity higher than 1000 W/m.sup.2.
Conventional lamps such as halogen lamps can thus be used for the
illumination.
[0030] In this embodiment, it is not necessary for the method
according to the invention for the silicon substrate to be
contacted electrically. The stabilizing treatment step can be
carried out, for example, before the electrical contacts are
applied to the solar cell. In this case, however, care must be
taken to ensure that during the subsequent metallization no
temperatures substantially higher than 200.degree. C. are used
which could cancel out the stabilizing effect. For example, the
metallization could be carried out by vapor deposition of metal
contacts.
[0031] According to a further embodiment of the method according to
the invention, the treatment time t, measured in minutes, during
which the substrate is held within the selectable temperature range
is selected according to the following conditions:
t .gtoreq. a ( y + b ) c * exp ( x ( T + 273 ) ) ##EQU00001##
where T is the average temperature of the selectable temperature
range in degrees centigrade during the treatment time, a, b, c and
x are constants.
[0032] For the case where the stabilizing treatment step is carried
out whilst applying an external voltage without illuminating the
photovoltaic element, y is the current density of the current
produced in the cell in A/cm.sup.2. The treatment time t is
preferably selected to that it satisfies the above equation for
a=4.247*10.sup.-14, b=0.00286, c=0.887 and x=12550. Experiments
conducted by the applicant have revealed that sufficient
stabilization of the efficiency of the photovoltaic element is
achieved with this minimum treatment time. Compared with a
conventional, degraded, non-stabilized solar cell, such a
stabilized solar cell has an open circuit voltage which is
increased by around 50% of the maximum increase in the open circuit
voltage attainable by annealing. Even better stabilization of the
efficiency is achieved if the treatment time is selected to be
longer so that the above inequality is satisfied for
a=3.272*10.sup.-14, b=0.00352, c=0.934 and x=12800. An
approximately 75% increase in the open circuit voltage can thus be
achieved.
[0033] For the other case where the stabilizing treatment step is
carried out by illuminating the photovoltaic element and without
applying an external voltage, y is the illumination intensity in
kW/m.sup.2. The treatment time t is preferably selected so that it
satisfies the above equation for a=2.298*10.sup.-11, b=0.399,
c=1.722 and x=11100. Experiments conducted by the applicant have
revealed that sufficient stabilization of the efficiency of the
photovoltaic element is achieved with this minimum treatment time.
Compared with a conventional, degraded, non-stabilized solar cell,
such a stabilized solar cell has an open circuit voltage which is
increased by around 50% of the maximum increase in the open circuit
voltage attainable by annealing. Even better stabilization of the
efficiency is achieved if the treatment time is selected to be
longer so that the above inequality is satisfied for
a=5.355*10.sup.-11, b=0.355, c=1.349 and x=11000. An approximately
75% increase in the open circuit voltage can thus be achieved.
[0034] As has already been mentioned, the efficiency-stabilizing
effect of the method increases with increasing treatment time. The
specified limits for the treatment time t tend to be economically
determined limits rather than technically determined limits and
were selected so that the best possible efficiency-stabilizing
effect can be achieved within a treatment time which is acceptable
from economic aspects. Studies carried out by the applicant have
revealed that satisfactory stabilization of the efficiency can be
achieved at treatment temperatures selected to be correspondingly
high, e.g. above 140.degree. C. with treatment times of 30 minutes
or, at even higher temperatures, 20 minutes.
[0035] From economic aspects, it is desirable for the treatment
time to be as short as possible. A lengthening of the stabilizing
step still produces a slight improvement in the stabilized
efficiency but this is no longer in proportion to the increased
costs thereby incurred. It is therefore preferable to select the
treatment time to be shorter than a day, preferably shorter than
five hours and even more preferably shorter than one hour.
[0036] According to a further embodiment, the method according to
the invention comprises a last high-temperature process step with
process temperatures above 230.degree. C., wherein the
stabilization treatment step follows the high-temperature process
step. The high-temperature process step can be any process step
such as is usually used in the fabrication of photovoltaic
elements, in particular solar cells. For example, it can comprise
the aforesaid diffusion step to form the n-doped layer. Mostly
however, this diffusion step is not the last high-temperature
process step. Other processes can follow where, for example, a
metallization process step, in which metal contacts are applied to
the silicon substrate and then fired in, can be the last
high-temperature process step. In the industrial production of
solar cells, metal contacts are frequently printed onto the silicon
substrate using a metal-containing paste in the screen printing
method and fired into the substrate at temperatures above
600.degree. C. According to present knowledge, for the functioning
of the method according to the invention, i.e. for the
stabilization of the efficiency it is necessary or at least
favorable that the step of keeping the substrate in the elevated
temperature range and simultaneously generating excess minority
carriers is carried out after a last high-temperature process step
since another high-temperature process step, i.e. heating the
substrate to more than 230.degree. C. could cancel out the
efficiency-stabilizing effect achieved by the treatment. However,
it is not important that the treatment time directly follows the
last high-temperature step. After the last high-temperature step,
initially any other temperature steps can be carried out below
200.degree. C., for example, before the temperature of the
substrate is set to the aforesaid temperature range and the excess
minority carriers are generated.
[0037] According to another embodiment, the method according to the
invention comprises a step of encapsulating the silicon substrate
in a module, wherein the stabilization treatment step follows the
encapsulation step. In other words, the method is used to fabricate
a solar cell module where the ready-processed, ready-to-operate
solar cells are encapsulated in a module before they are subjected
to the treatment at elevated temperature and the excess minority
carrier generation.
[0038] This embodiment has the advantage that the stabilizing
treatment step can be carried out simultaneously for a plurality of
solar cells. However, care must be taken to ensure that the
temperature range is suitably selected within the stabilizing
treatment step, i.e. that the module is not damaged. For example,
above a temperature of 180.degree. C., the cabling used to connect
the individual solar cells can become detached since the solder
used liquefies. Temperatures above about 140.degree. C. can damage
the module since the EVA laminating film frequently used becomes
damaged above such a temperature.
[0039] According to a further aspect of the invention, a method for
stabilizing the efficiency of a photovoltaic element comprising a
boron-doped, oxygen-containing silicon substrate is proposed. In
this case, a finished conventional photovoltaic element, e.g. a
solar cell or an entire solar module is subjected to the
stabilizing treatment step described above and thereby stabilized
at an efficiency which approximately corresponds to the efficiency
to be achieved by annealing. The features of the previously
described embodiments of the method of fabrication can also be
applied appropriately to this stabilizing method.
[0040] According to yet another aspect of the invention, a
photovoltaic element comprising a boron-doped, oxygen-containing
silicon substrate is proposed, having an efficiency-stabilized
state, wherein the photovoltaic element has a high efficiency such
as can be achieved by annealing, where the efficiency of the solar
cell drops by less than 5% relatively, preferably less than 2%
relatively, under illumination. The photovoltaic element according
to the invention can be obtained, for example, by the method of
fabrication described above.
[0041] Further details, features and advantages of the invention
are deduced by the person skilled in the art from the following
description of preferred embodiments and the appended figures.
[0042] FIG. 1 shows a three-state model with which the operating
mode of the method according to the invention may be explained.
[0043] FIG. 2 shows a graph giving the measurement results relating
to the change in the absolute efficiency of a solar cell fabricated
according to the invention as a function of the treatment time of
the stabilizing treatment step at various treatment temperatures
between 45.degree. C. and 85.degree. C. when a current is induced
in the solar cell in the dark.
[0044] FIG. 3 shows a graph giving the measurement results relating
to the change in the efficiency of a solar cell fabricated
according to the invention as a function of the treatment time of
the stabilizing treatment step at a treatment temperature of
120.degree. C. and under illumination of about 1000 W/m.sup.2
without applying an external voltage.
[0045] A model which can be used to substantiate the
efficiency-stabilizing effect achieved by the method according to
the invention will be put forward hereinafter with reference to
FIG. 1. However, it is noted that exact causal relationships
resulting in the efficiency stabilization achieved by the method
according to the invention were not yet understood in detail at the
time of writing the present patent application. The proposed model
and the measurement results given to support this should thus not
in any way restrict the scope of protection as defined by the
appended claims.
[0046] Hitherto, two different states were known for boron-doped
oxygen-containing Cz silicon in a simplified model. The first state
given as (A) in FIG. 1 is hereinafter designated as "annealed
state". In this state, there appear to be no or very few impurities
caused by boron and oxygen, which promote recombination and thus
cause a deterioration in the efficiency of a solar cell, or these
impurities are electrically inactive. In state (A) oxygen contained
in the silicon crystal acts only weakly as a recombination-active
centre. Since state (A) is usually measured directly after
annealing, i.e. a temperature treatment in the dark which undoes
the degradation, this is designated as "annealed".
[0047] The second state given as (B) in FIG. 1 is designated as
"degraded state". The degradation is presumably triggered by the
formation of a defect in which complexes are formed from
interstitial oxygen and substitutional boron. The formation of
boron-oxygen complexes takes place under illumination or under
current flow in particular at temperatures below 50.degree. C.,
such as are typical during normal operation of a solar cell. In
contrast to state (A), the degraded state (B) exhibits strongly
recombination-active impurities which decisively reduce the
effective diffusion length of the minority carriers and are thus
responsible for the deterioration in the electrical properties of
the solar cell.
[0048] Studies conducted by the applicant of the present patent
application suggest that in addition to these two known states,
another state (C) also exists in Cz silicon. This state will be
designated hereinafter as "regenerated state". In this state, as in
state (A) there appear to be no or very few recombination-active
centers or these are electrically inactive. In contrast to the
annealed state (A), the regenerated state (C) according to findings
so far is stable in time during subsequent operation of the solar
cell under illumination or under current flow.
[0049] The method according to the invention shows a way of
transferring the photovoltaic element from the degraded state (B)
to the regenerated state (C) where the electrical properties of the
solar cell recover again as far as a level which substantially
corresponds to the initial or annealed state (A).
[0050] The assumption of an additional third state, i.e. the
transition from the hitherto known two-state model to the
three-state model put forward here appears to be necessary since
studies made by the applicant reveals that the solar cell appears
to "know" whether it is in the degradable annealed state (A) or in
the stable regenerated state (C). This seems to require that
microscopically different states (A) and (C) exist even if no
detailed microscopic findings relating to its composition are
available for the newly introduced regenerated state (C).
[0051] A direct differentiation of states (A) and (C) has not yet
been made on account of the similar electrical properties. In order
to determine whether states (A) or (C) are present in a Cz silicon
crystal or how the two states are divided, all defects in state (A)
must be transferred to state (B), i.e. the solar cell must be
degraded. If the degraded final state is now compared with that of
complete degradation from the annealed state (A) (i.e. after an
annealing step at above 200.degree. C.), the deviation is a measure
for the population of states (A) and (C).
[0052] The individual states (A), (B) and (C) seem to be able to go
over partly into one another depending on illumination, current and
temperature conditions. In this context, a transition from (A) to
(B) is designated as degradation. The inverse transition from (B)
to (A) is designated as annealing. A transition from (B) to (C)
such as can be achieved by the method according to the invention is
designated as regeneration. The inverse transition from (C)
directly to (B) has so far not been observed experimentally.
According to studies made by the applicant under operating
conditions, solar cells fabricated by the method according to the
invention were stable over at least 137 h with regard to their
efficiency, the efficiency remaining substantially at the level of
the annealed state. It is thus assumed that the regenerated state
(C) is largely temporally stable. A transition from the regenerated
state (C) to the annealed state (A) can be achieved by an annealing
step at about 230.degree. C. which is usually carried out for 10 to
30 minutes and is also designated as annealing. The inverse
transition from (A) directly to (C) has not yet been observed
experimentally.
[0053] The reaction path from (A) to (B) (degradation) can be
excited by illumination and/or induced current flow but also has a
strong temperature dependence. It is therefore described as
thermally assisted. According to findings so far, the annealing
reactions from (B) to (A) and (C) to (A) are purely thermally
activated, i.e. the reaction is intensified with increasing
temperature. If no new degradation is forced in this case, the
system can be transferred completely to state (A) with a certain
temperature-time combination. The regeneration reaction from (B) to
(C) appears to be thermally assisted, i.e. it appears to be
activated by illumination and/or induced current flow but proceeds
considerably more rapidly at higher temperatures.
[0054] The transitions between the individual states compete with
one another. According to findings so far, which transition
dominates depends principally on the temperature conditions and
possibly on the concentration of excess minority carriers. Under
the conditions such as those selected for the method according to
the invention, the transition from the degraded state (B) to the
regenerated state (C) dominates. The more the silicon crystal is
transferred to state (C), the more extensively the efficiency of a
corresponding solar cell is stabilized.
[0055] Studies such as those shown hereinafter among others with
reference to the graphs plotted in FIGS. 2 and 3 have yielded
relationships as to how the efficiency-stabilizing effect of the
method according to the invention depends on process
parameters.
[0056] FIG. 2 shows the loss of efficiency in absolute percent as a
function of the treatment time for various temperatures when the
solar cell under study is regenerated under a current flow of 20
mA/cm.sup.2 in the dark. It can be seen that the degradation
effected approximately at zero time and the associated loss of
efficiency are approximately recovered again in the course of the
regeneration treatment time and the regeneration proceeds faster,
the higher the selected regeneration temperature. Further studies
have shown that this effect also appears to continue at higher
temperatures up to higher than 160.degree. C. No noticeable
regeneration was observed at temperatures below 50.degree. C. At
65.degree. C., regeneration proceeds very slowly and even after 160
hours, the loss of efficiency due to the degradation has only
approximately halved. Whereas at 85.degree. C., extensive
regeneration still requires more than 80 hours, at 110.degree. C.
it occurs after only about 2 hours, at 140.degree. C. this time is
reduced to about 40 minutes and at 160.degree. C. about 25 minutes
was measured (measurements for 110.degree. C., 140.degree. C. and
160.degree. C. are not plotted in FIG. 2).
[0057] A similar effect was observed when the solar cell was
illuminated with light instead of having current passed through it
in the dark as in the case described above. In this case, it was
established that the treatment time required to achieve a specific
regeneration is substantially proportional to the illumination
intensity. A light source which is ten times stronger can
approximately accelerate the stabilizing treatment process by a
factor of 8. The proportionality appears to hold up to an
illumination intensity of at least 1000 W/m.sup.2, under even
stronger illumination the acceleration decreases and appears to
reach saturation.
[0058] FIG. 3 shows results of measurements which show the
efficiency of a solar cell fabricated according to the invention as
a function of the treatment time. The solar cell was illuminated at
120.degree. C. with about 1000 W/m.sup.2 using a halogen lamp. No
external voltage was applied. It can be seen that almost complete
regeneration is achieved after about 35 min.
[0059] With regard to the temperature dependence, it was
established that the regeneration effect of the method according to
the invention appears to decrease from a temperature of about
190.degree. C. This is interpreted at the present time in that from
this temperature, the competition due to the transition from the
degraded state (B) or from the regenerated state (C) to the
degradable annealed state (A) increases and consequently, the
system no longer moves principally into the regenerated state (C)
but also partly into the non-stable annealed state (A).
[0060] Direct regeneration from the annealed state (A) into the
regenerated state (C) without the detour via the degraded state (B)
has not been observed so far. If a minority carrier excess is
brought about in addition to maintaining at elevated temperature
(as in the studies shown in FIG. 2 and FIG. 3), it is observed
that, starting from an annealed initial state (A), the solar cell
initially degrades, but then successively recovers in the further
course of the stabilization treatment step and goes over into the
regenerated state (C).
[0061] However, the aforesaid operating principle can be used to
deduce how it can be determined whether a finished solar cell was
fabricated using the method according to the invention: the solar
cell fabricated according to the invention is in the
efficiency-stabilized regenerated state (C) as described above.
However, it can be transferred to the non-stable annealed state (A)
by an annealing step at temperatures of about 230.degree. C. If,
after such an annealing step, a solar cell under study degrades
during subsequent normal operation to an efficiency below the
original stable efficiency for the same solar cell, it can thus be
ascertained that the solar cell was originally fabricated using the
method according to the invention and was stabilized with regard to
its efficiency.
[0062] Finally, exemplary embodiments of the method according to
the invention are described in which the stabilizing step was
integrated in a conventional method for fabricating a solar
cell.
[0063] A commercially available Cz silicon wafer is initially
subjected to etching and cleaning to remove surface cutting damage
produced in the wafer by cutting the Cz silicon crystal. An
n-conducting emitter layer is then diffused into the surface of the
wafer by POCl.sub.3 diffusion. Grid front contacts were then
printed onto the emitter layer by screen printing using a
silver-containing thick-film paste and a screen of corresponding
geometry. After the thick-film paste had dried in a continuous
furnace, an aluminum-containing thick-film paste was printed onto
the back side of the wafer to form the back contact. Both the front
and the back contact are then fired into the wafer surface at a
temperature between 700 and 900.degree. C. in a continuous furnace.
After this last high-temperature step, the solar cell is in
principle ready to operate, i.e. it has the pn junction required to
separate charge carrier pairs generated by light and front and back
contacts to transport the charge carrier pairs thus separated.
[0064] The ready-to-operate solar cell is now subjected to the
stabilizing treatment step according to the invention. In this
case, the solar cell is placed on a hot plate and heated, for
example, to a temperature of 160.degree. C. At the same time, the
solar cell is illuminated by a halogen lamp on its front side, thus
generating excess minority carriers in the solar cell. The solar
cell is held in this state for about 30 minutes. It can then cool
down and be supplied to further processing, e.g. wiring and
encapsulation in modules.
[0065] In an alternative embodiment, the ready-to-operate solar
cell is connected in series with other solar cells via contact
strips soldered onto its front and back contacts and encapsulated
into a module using an EVA film (ethylene vinyl acetate). The
contact strips are electrically connected to connections for the
module. The connections of the module are then connected to an
external voltage source whose voltage is selected such that a
voltage of about 0.7 Volts is applied to each of the solar cells
connected in series. The modules thus set under voltage are then
stored for about one hour in a suitably heated room at, for
example, 140.degree. C.
* * * * *