U.S. patent application number 13/890592 was filed with the patent office on 2014-06-05 for use of dopants with different diffusivities for solar cell manufacture.
This patent application is currently assigned to VARIAN SEMICONDUCTOR EQUIPMENT ASSOCIATES, INC.. The applicant listed for this patent is VARIAN SEMICONDUCTOR EQUIPMENT ASSOCIATES, INC.. Invention is credited to Nicholas P.T. Bateman, Atul Gupta, Christopher Hatem, Deepak Ramappa.
Application Number | 20140154834 13/890592 |
Document ID | / |
Family ID | 41054059 |
Filed Date | 2014-06-05 |
United States Patent
Application |
20140154834 |
Kind Code |
A1 |
Bateman; Nicholas P.T. ; et
al. |
June 5, 2014 |
USE OF DOPANTS WITH DIFFERENT DIFFUSIVITIES FOR SOLAR CELL
MANUFACTURE
Abstract
A method of tailoring the dopant profile of a substrate by
utilizing two different dopants, each having a different
diffusivity is disclosed. The substrate may be, for example, a
solar cell. By introducing two different dopants, such as by ion
implantation, furnace diffusion, or paste, it is possible to create
the desired dopant profile. In addition, the dopants may be
introduced simultaneously, partially simultaneously, or
sequentially. Dopant pairs preferably consist of one lighter
species and one heavier species, where the lighter species has a
greater diffusivity. For example, dopant pairs such as boron and
gallium, boron and indium, phosphorus and arsenic, and phosphorus
and antimony, can be utilized.
Inventors: |
Bateman; Nicholas P.T.;
(Reading, MA) ; Gupta; Atul; (Beverly, MA)
; Hatem; Christopher; (Hampton, NH) ; Ramappa;
Deepak; (Somerville, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
VARIAN SEMICONDUCTOR EQUIPMENT ASSOCIATES, INC. |
Gloucester |
MA |
US |
|
|
Assignee: |
VARIAN SEMICONDUCTOR EQUIPMENT
ASSOCIATES, INC.
Gloucester
MA
|
Family ID: |
41054059 |
Appl. No.: |
13/890592 |
Filed: |
May 9, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12397542 |
Mar 4, 2009 |
8461032 |
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13890592 |
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61033873 |
Mar 5, 2008 |
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61095010 |
Sep 8, 2008 |
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Current U.S.
Class: |
438/87 |
Current CPC
Class: |
H01L 31/0236 20130101;
Y02E 10/547 20130101; H01L 33/025 20130101; Y02P 70/50 20151101;
H01L 31/0682 20130101; H01L 21/2658 20130101; Y02P 70/521 20151101;
H01L 31/1864 20130101; H01L 31/022441 20130101; H01L 21/26513
20130101; H01L 31/03529 20130101 |
Class at
Publication: |
438/87 |
International
Class: |
H01L 33/02 20060101
H01L033/02 |
Claims
1. A method of creating a desired dopant profile in a solar cell,
comprising: selectively implanting a first dopant and a second
dopant at least partially simultaneously into a region of a surface
of said solar cell using patterned ion implantation, wherein said
region is less than an entirety of said surface and wherein said
second dopant has a similar conductivity and higher diffusivity
than said first dopant; and performing a thermal cycle to diffuse
said first dopant and said second dopant into said solar cell,
wherein said second dopant diffuses deeper into said solar cell
than said first dopant; and applying contacts to said solar cell,
wherein said contacts are applied to said region with high
concentration of said first dopant.
2. The method of claim 1, wherein said first dopant and said second
dopant are selected from the group consisting of nitrogen,
phosphorus, arsenic, antimony, bismuth, boron, aluminum, gallium,
and indium.
3. The method of claim 1, wherein a flood implant is performed.
4. The method of claim 1, wherein said first dopant is arsenic and
said second dopant is phosphorus.
5. The method of claim 1, wherein said first dopant is gallium and
said second dopant is boron.
6. The method of claim 1, wherein said patterned ion implantation
comprises implanting through a stencil mask.
7. A method of creating a desired dopant profile in a solar cell,
comprising: selectively implanting a first dopant and a second
dopant at least partially simultaneously into a region of a surface
of said solar cell through a stencil mask, wherein said region is
less than an entirety of said surface and wherein said second
dopant has a similar conductivity and higher diffusivity than said
first dopant; and performing a thermal cycle to diffuse said first
dopant and said second dopant into said solar cell, wherein said
second dopant diffuses deeper into said solar cell than said first
dopant; and applying contacts to said solar cell, wherein said
contacts are applied to said region with high concentration of said
first dopant.
8. The method of claim 7, wherein said first dopant and said second
dopant are selected from the group consisting of nitrogen,
phosphorus, arsenic, antimony, bismuth, boron, aluminum, gallium,
and indium.
9. The method of claim 7, wherein a flood implant is performed.
10. The method of claim 7, wherein said first dopant is arsenic and
said second dopant is phosphorus.
11. The method of claim 7, wherein said first dopant is gallium and
said second dopant is boron.
12. A method of creating a desired dopant profile in a solar cell,
comprising: selectively implanting a first dopant and a second
dopant at least partially simultaneously into a region of a surface
of said solar cell using patterned ion implantation, wherein said
region is less than an entirety of said surface and wherein said
second dopant has a similar conductivity and higher diffusivity
than said first dopant, wherein said region with high concentration
of said first dopant is configured for use with contacts on said
solar cell.
13. The method of claim 12, wherein said first dopant and said
second dopant are selected from the group consisting of nitrogen,
phosphorus, arsenic, antimony, bismuth, boron, aluminum, gallium,
and indium.
14. The method of claim 12, wherein a flood implant is
performed.
15. The method of claim 12, wherein said first dopant is arsenic
and said second dopant is phosphorus.
16. The method of claim 12, wherein said first dopant is gallium
and said second dopant is boron.
17. The method of claim 12, wherein said patterned ion implantation
comprises implanting through a stencil mask.
18. The method of claim 12, further comprising: performing a
thermal cycle to diffuse said first dopant and said second dopant
into said solar cell, wherein said second dopant diffuses deeper
into said solar cell than said first dopant; and applying contacts
to said solar cell, wherein said contacts are applied to said
region with high concentration of said first dopant.
Description
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/397,542, filed Mar. 4, 2009, which claims
priority of U.S. Provisional Patent Application Ser. No.
61/033,873, filed Mar. 5, 2008, and U.S. Provisional Patent
Application Ser. No. 61/095,010, filed Sep. 8, 2008, the
disclosures of which are hereby incorporated by reference.
FIELD
[0002] This invention relates to dopant profiles, and, more
particularly, to dopant profiles in solar cells.
BACKGROUND
[0003] Ion implantation is a standard technique for introducing
conductivity-altering impurities into semiconductor substrates. A
desired impurity material is ionized in an ion source, the ions are
accelerated to form an ion beam of prescribed energy, and the ion
beam is directed at the surface of the substrate. The energetic
ions in the beam penetrate into the bulk of the semiconductor
material and are embedded into the crystalline lattice of the
semiconductor material to form a region of desired
conductivity.
[0004] Solar cells are typically manufactured using the same
processes used for other semiconductor devices, often using silicon
as the substrate material. A semiconductor solar cell is a simple
device having an in-built electric field that separates the charge
carriers generated through the absorption of photons in the
semiconductor material. This electric field is typically created
through the formation of a p-n junction (diode), which is created
by differential doping of the semiconductor material. Doping a part
of the semiconductor substrate (e.g. surface region) with
impurities of opposite polarity forms a p-n junction that may be
used as a photovoltaic device converting light into
electricity.
[0005] FIG. 3 shows a first embodiment of a solar cell, and is a
cross section of a representative substrate 300. Photons 301 enter
the solar cell 300 through the top surface 305, as signified by the
arrows. These photons pass through an anti-reflective coating 310,
designed to maximize the number of photons that penetrate the
substrate 300 and minimize those that are reflected away from the
substrate.
[0006] Internally, the substrate 300 is formed so as to have a p-n
junction 320. This junction is shown as being substantially
parallel to the top surface 305 of the substrate 300 although there
are other implementations where the junction may not be parallel to
the surface. The solar cell is fabricated such that the photons
enter the substrate through a heavily doped region, also known as
the emitter 330. In some embodiments, the emitter 330 may be an
n-type doped region, while in other embodiments, the emitter may be
a p-type doped region. The photons with sufficient energy (above
the bandgap of the semiconductor) are able to promote an electron
within the semiconductor material's valence band to the conduction
band. Associated with this free electron is a corresponding
positively charged hole in the valence band. In order to generate a
photocurrent that can drive an external load, these electron hole
(e-h) pairs need to be separated. This is done through the built-in
electric field at the p-n junction. Thus any e-h pairs that are
generated in the depletion region of the p-n junction get
separated, as are any other minority carriers that diffuse to the
depletion region of the device. Since a majority of the incident
photons are absorbed in near surface regions of the device, the
minority carriers generated in the emitter need to diffuse across
the depth of the emitter to reach the depletion region and get
swept across to the other side. Thus to maximize the collection of
photo-generated current and minimize the chances of carrier
recombination in the emitter, it is preferable to have the emitter
region 130 be very shallow.
[0007] Some photons pass through the emitter region 330 and enter
the base 340. In the scenario where the emitter 330 is an n-type
region, the base 340 is a p-type doped region. These photons can
then excite electrons within the base 340, which are free to move
into the emitter region 330, while the associated holes remain in
the base 340. Alternatively, in the case where the emitter 330 is a
p-type doped region, the base is an n-type doped region. In this
case, these photons can then excite electrons within the base 340,
which remain in the base region 340, while the associated holes
move into the emitter 330. As a result of the charge separation
caused by the presence of this p-n junction, the extra carriers
(electrons and holes) generated by the photons can then be used to
drive an external load to complete the circuit.
[0008] By externally connecting the emitter region 330 to the base
340 through an external load, it is possible to conduct current and
therefore provide power. To achieve this, contacts 350, typically
metallic, are placed on the outer surface of the emitter region and
the base. Since the base does not receive the photons directly,
typically its contact 350b is placed along the entire outer
surface. In contrast, the outer surface of the emitter region 330
receives photons and therefore cannot be completely covered with
contacts. However, if the electrons have to travel great distances
to the contact, the series resistance of the cell increases, which
lowers the power output. In an attempt to balance these two
considerations (the distance that the free electrons must travel to
the contact, and the amount of exposed emitter surface 360) most
applications use contacts 350a that are in the form of fingers.
FIG. 4 shows a top view of the solar cell of FIG. 3. The contacts
are typically formed so as to be relatively thin, while extending
the width of the solar cell. In this way, free electrons need not
travel great distances, but much of the outer surface of the
emitter is exposed to the photons. Typical contact fingers 350a on
the front side of the substrate are 0.1 mm with an accuracy of
+/-0.1 mm. These fingers 350a are typically spaced between 1-5 mm
apart from one another. While these dimensions are typical, other
dimensions are possible and contemplated herein.
[0009] A further enhancement to solar cells is the addition of
heavily doped substrate contact regions. FIG. 5 shows a cross
section of this enhanced solar cell. The cell is as described above
in connection with FIG. 3, but includes heavily doped contact
regions 370. These heavily doped contact regions 370 correspond to
the areas where the metallic fingers 350a will be affixed to the
substrate 300. The introduction of these heavily doped contact
regions 370 allows much better contact between the substrate 300
and the metallic fingers 350a and significantly lowers the series
resistance of the cell. This pattern of including heavily doped
regions on the surface of the substrate is commonly referred to as
selective emitter design.
[0010] A selective emitter design for a solar cell also has the
advantage of higher efficiency cells due to reduced minority
carrier losses through recombination due to lower dopant/impurity
dose in the exposed regions of the emitter layer. The higher doping
under the contact regions provides a field that repels the minority
carriers generated in the emitter and pushes them towards the p-n
junction.
[0011] The embodiment shown in FIG. 3 requires contacts on both
sides of the substrate, thereby reducing the available area of the
front surface through which photons may pass. A cross section of a
second embodiment of a solar cell 400 is shown in FIG. 6.
Fundamentally, the physics of this embodiment is similar, in which
a p-n junction is used to create an electric field which separates
the generated electron hole pairs. However, rather than create the
p-n junction across the entire substrate, as done in the previous
embodiment, the junctions are only created in portions of the
substrate 400. In this embodiment, a negatively doped silicon
substrate 410 is used. In certain embodiments, a more negatively
biased front surface field (FSF) 420 is created by implanting
addition n-type dopants in the front surface. This front surface is
then coated with an anti-reflective material 430. This front
surface is often etched to create a sawtooth or other non-planar
surface, so as to increase surface area. The metallic contacts or
fingers 470 are all located on the bottom surface of the substrate.
Certain portions of the bottom surface are implanted with p-type
dopants to create emitters 440. Other portions are implanted with
n-type dopants to create more negatively biased back surface field
450. The back surface is coated with a dielectric layer 460 to
enhance the reflectivity of the back surface. Metal fingers 470a
are attached to the emitter 440 and fingers 470b attaches to the
BSF 450. FIG. 7 shows one commonly used configuration of the metal
fingers on the back surface. This type of cell is known as an
interdigitated back contact (IBC) solar cell.
[0012] With current energy costs and environmental concerns, solar
cells are becoming increasingly important. Any reduced cost to the
manufacturing or production of high-performance solar cells or any
efficiency improvement to high-performance solar cells would have a
positive impact on the implementation of solar cells worldwide.
This will enable the wider availability of this clean energy
technology.
[0013] Current solar cell design is limited by the dopant profiles
that can be achieved by diffusing dopants into the silicon of the
solar cell. Thermal diffusion has limited process parameters to
control a dopant profile, such as time, temperature, and ramp
speed. These thermal diffusion process parameters may not allow the
desired tailoring of a dopant profile in a solar cell to achieve
solar cell performance requirements. Furthermore, various dopants
diffuse differently under a thermal diffusion process. The dopant
that is selected may limit possible tailoring of the dopant profile
in a solar cell. Accordingly, there is a need in the art for
improved dopant profiles solar cells and, more particularly, a
method that uses dopants with different diffusivities to tailor
dopant profiles in a solar cell.
SUMMARY
[0014] The issues in the prior art are alleviated by the disclosed
method. By utilizing two different dopants, each having a different
diffusivity, the dopant profile of the substrate can be tailored as
required. For example, in the case of solar cells, it may be
advantageous to have a greater dopant concentration near the
surface of the substrate, while still establishing a deep
moderately doped region. By introducing two different dopants, such
as by implantation, furnace diffusion, or paste, it is possible to
create the desired dopant profile. In addition, the dopants may be
introduced simultaneously, partially simultaneously or
sequentially. Dopant pairs may consist of two species of differing
diffusivity. In some embodiments, the dopant pairs may consist of
one lighter species and one heavier species, where the lighter
species has a greater diffusivity. For example, dopant pairs such
as boron and gallium, boron and indium, phosphorus and arsenic, and
phosphorus and antimony, can be utilized.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] For a better understanding of the present disclosure,
reference is made to the accompanying drawings, which are
incorporated herein by reference and in which:
[0016] FIG. 1A-1C is a set of graphs showing dopant distribution
profiles for phosphorus and arsenic before and after anneal;
[0017] FIG. 2 is a graph comparing dopant concentration versus
dopant depth for arsenic and phosphorus;
[0018] FIG. 3 shows a cross section of a solar cell of the prior
art;
[0019] FIG. 4 shows a top view of the solar cell of FIG. 3;
[0020] FIG. 5 shows a cross section of a solar cell of the prior
art using selective emitter design;
[0021] FIG. 6 shows a cross section of a second type of solar cell
of the prior art; and
[0022] FIG. 7 shows a bottom view of the solar cell of FIG. 6.
DETAILED DESCRIPTION
[0023] The embodiments of the process described herein may be
performed by, for example, a beam-line ion implanter or a plasma
doping ion implanter. Such a plasma doping ion implanter may use RF
or other plasma generation sources. Other plasma processing
equipment or equipment that generates ions also may be used.
Thermal or furnace diffusion, pastes on the surface of the solar
cell substrate that are heated, epitaxial growth, or laser doping
also may be used to perform embodiments of the process described
herein. Furthermore, while a silicon solar cell is specifically
disclosed, other solar cell substrate materials also may benefit
from embodiments of the process described herein.
[0024] The manufacture of high efficiency solar cells has many,
often conflicting, requirements. For example, screen printed
contacts typically require specific dopant profiles for proper
solar cell performance. These contacts require high interstitial
dopant concentrations at the surface of the substrate. Current
solar cell manufacturing methods use fired paste contacts with a
single dopant, such as phosphorus, to achieve this high
concentration. However, achieving a high surface concentration may
require a high concentration of the dopant to be introduced
throughout the emitter of the solar cell. This high concentration
of dopant throughout the emitter increases carrier recombination
and, consequently, may lower cell efficiency. Thus, contacts
between the emitter and the contacts are improved by introducing
high dopant concentrations near the contact regions. However, the
introduction of this dopant throughout the emitter degrades solar
cell performance.
[0025] Another issue is that recombination of e-h pairs at the
surface of a solar cell typically limits solar cell efficiency.
This recombination can be reduced by repelling minority carriers
from the surface of the solar cell. One way to repel minority
carriers is to put a shallow, high concentration layer of dopant at
the surface of the solar cell. This layer needs to remain in place
throughout any subsequent thermal processing. However, previous
methods would diffuse this dopant layer throughout the emitter,
reducing its effectiveness.
[0026] While the above criteria require a shallow, high
concentration at the surface of the substrate, other criteria may
require a deeper dopant concentration. One example of such a solar
cell criteria is a p-n junction. Efficiency is enhanced if the p-n
junction is located deep within the substrate, away from the
surface of the substrate. The presence of a deep dopant profile
also may lower series resistance of the solar cell. This deeper
dopant concentration has been previously performed using a
high-diffusivity dopant. Such a high-diffusivity dopant may not
allow high concentration of the dopant at the surface of the cell
without introducing an excessive number of dopant atoms into the
silicon, thereby increasing recombination.
[0027] By implanting dopants with similar conductivities and
differing diffusivities in a solar cell, dopant profiles may be
tailored or the thermal steps required to manufacture a solar cell
may be minimized. The number of overall thermal steps required also
may be reduced. Furthermore, using dopants with different
diffusivities may allow both a high surface concentration of a
dopant and a high concentration of a dopant deeper in the solar
cell, while having the same conductivity throughout the implanted
region.
[0028] Crystalline silicon solar cell manufacturing typically
requires thermal process steps. During thermal process steps, such
as an anneal, dopants in the silicon will diffuse and any dopant
distribution will change. FIG. 1 is a set of graphs showing
representative dopant distribution profiles for phosphorus and
arsenic. In FIG. 1A, phosphorus is implanted, while in FIG. 1B,
arsenic is implanted. As illustrated in FIG. 1C, the phosphorus
dopant distribution 100 and the arsenic dopant distribution 101
change after the anneal, with each diffusing to greater depths.
[0029] At least two different dopants are introduced into the
silicon matrix of the solar cell. Solar manufacturing has not
previously used the different diffusion properties of two or more
different dopants to tailor dopant profiles or to optimize solar
cell performance. Current solar cell manufacturing methods use only
a single dopant.
[0030] These at least two dopant species each behave differently
during thermal processing. A first dopant species may have a high
diffusivity and will diffuse relatively extensively during thermal
processing. The second dopant species may have a similar
conductivity and may have a lower diffusivity and will diffuse
substantially less than the first dopant species during thermal
processing. Examples of pairs of n-type dopants are phosphorus and
arsenic, phosphorus and antimony, phosphorus and bismuth, and
nitrogen, in conjunction with any of the above mentioned dopants.
Example of a pairs of p-type dopants are boron and gallium, boron
and indium, and boron and aluminum. Other dopant species may be
used and this process is not solely limited to the dopants
disclosed herein. Similarly, other combinations of the above
mentioned dopants, which are not listed herein, may also be used.
In one instance, the first dopant in the silicon matrix limits the
diffusivity of the second dopant by competing for interstitial or
substitutional sites in the crystal lattice. This may result in the
second dopant diffusing less than the first dopant. In another
embodiment, more than two dopants are used to tailor the dopant
profile in the solar cell.
[0031] In one instance, during subsequent thermal process steps
after the dopants are introduced, such as diffusion or annealing,
the first dopant will diffuse more than the second dopant. This
enables flexibility in tailoring a dopant profile in a solar cell.
For example, using a first and second dopant with different
diffusivities may allow charge collection, series resistance, or
other solar cell parameters to be optimized. The first dopant may
form a deep pn-junction and the second dopant may provide a high
dopant concentration at the surface of the solar cell. This may
enable various contact designs and may reduce recombination at the
surface of the solar cell. Such a configuration also increases
charge collection (by minimizing surface recombination) and lowers
series resistance.
[0032] In one embodiment, the first and second dopants are
introduced using ion implantation. A patterned or non-patterned
implant may be performed. A patterned implant may use a hard mask
or a stencil mask, for example. These two dopants may be introduced
at least partially simultaneously using a cluster or molecular
implant or by using one or more gases with a tool with no mass
separation, such as a plasma doping tool, or a tool that performs a
flood implant. For example, a single gas containing both species
can be ionized. For example, molecules such as, but not limited to,
AsP (arsenic monophosphide), As.sub.2P.sub.2 (diarsenic
diphosphide), As.sub.3P (Triarsenic phosphide), and AsP.sub.3
(arsenic triphosphide) may be used for a molecular implant.
Implantation of the four enumerated molecules will result in atomic
arsenic ions, and atomic phosphorus ions, where each has a
different diffusion rate. Alternatively, two different gaseous
species can be ionized simultaneously, such as boron and gallium,
or phosphorus and arsenic, yielding ions from each species, which
are then at least partially simultaneously implanted.
[0033] If the two dopants are introduced at least partially
simultaneously, the two dopant species will either have similar
depth profiles due to the implant parameters, or they will have
dopant profiles where the heavier dopant species has a shallower
profile than the lighter dopant species due to the larger size of
the heavier dopant species. If the heavier dopant species has a
lower diffusivity, such as with arsenic (as compared with
phosphorus), the lighter dopant species will have a deeper
as-implanted profile and the heavier dopant species will have a
shallower as-implanted profile.
[0034] In one particular embodiment, a flood implant is used where
the ions are not mass analyzed. Both atomic dopant species and
molecular dopant species are accelerated. For example, ionized AsP
molecules and atomic phosphorus can both be implanted. In this
case, the atomic phosphorus atoms would diffuse deepest. Phosphorus
atoms from the ionized molecule would be somewhat shallower.
Finally, the arsenic atoms from the ionized molecule would be much
shallower. Therefore, the atomic dopant species may have a deeper
as-implanted dopant profile than the molecular dopant species.
[0035] In other embodiments, the two dopants also may be introduced
using a chained implant where one dopant is implanted prior to the
other dopant. In one particular embodiment, the lighter dopant,
such as phosphorus or boron, is implanted first, as it will diffuse
deeper into the substrate. The heavier dopant, such as arsenic or
gallium, is then implanted. The heavier dopant is likely to be
shallower in its diffusion. Furthermore, the earlier implanted ions
may occupy some of the available interstitial locations, further
limiting the ability of the second implanted species to diffuse
deep into the substrate.
[0036] In another particular embodiment the heavier dopant can be
implanted first (such as As) so as enable amorphization of the
substrate over the implanted depth. The lighter dopant such as
phosphorous is then implanted. Due to the amorphized substrate, the
lighter dopant diffusion is reduced. Following an anneal a more
abrupt profile of the lighter dopant can be obtained, which can be
beneficial in certain aspects of the cell.
[0037] As seen in FIG. 1C, the substrate is annealed after the
dopants are introduced into the substrate to activate the dopants
and remove any damage to the lattice of the substrate. In the
embodiment of FIG. 1, the diffusion profile of the arsenic 101 will
be much less than the diffusion profile of the phosphorus 100.
Thus, the surface peak dopant profile may be maintained using the
arsenic 101 while the deeper dopant profile may be accomplished
using the phosphorus 100.
[0038] Two dopants also may be introduced using furnace diffusion.
In some embodiments, a single gas that contains both dopants, such
as those enumerated above, may be introduced during at least one
process step. In other embodiments, two or more gases that each
contains one dopant may be introduced in at least one process step.
During the diffusion, the high-diffusivity dopant, such as
phosphorus or boron, will diffuse deeply into the silicon of the
solar cell to, for example, form a p-n junction. The
low-diffusivity dopant, such as arsenic or gallium, will remain
near the surface of the solar cell. To further tailor the dopant
profile in the solar cell, one of the two dopant gases may be
introduced after one dopant gas has already at least partly
diffused into the silicon of the solar cell or after the diffusion
of one dopant gas is complete.
[0039] In another embodiment, at least one paste is used to obtain
the desired dopant profile in the solar cell. In some embodiments,
the paste is applied before the antireflective coating has been
applied. In other embodiments, paste is applied after the
antireflective coating has been applied. Therefore, when paste is
applied to the surface of the solar cell, in some embodiments, the
paste is applied directly to the substrate, while in other
embodiments, the paste is applied to the antireflective coating. In
some embodiments, the paste is applied on top of the coating and
fired. The heat allows the paste to eat through the coating and
reach the substrate, which it diffuses into. The solar cell
substrate is coated with the at least one paste that contains one
or both dopants. In some embodiments, a paste containing two or
more dopants is used. When the substrate is placed in a furnace,
the dopants on the surface will diffuse into the substrate. The
high-diffusivity dopant will diffuse more than the low-diffusivity
dopant. In other embodiments, each paste contains only a single
dopant and at least two pastes are used. In this embodiment, the
heating process may be repeated to apply the second paste
containing a second dopant.
[0040] In yet another embodiment, one dopant is introduced by ion
implantation and the second dopant is introduced by diffusion while
the damage caused by implanting the first dopant is annealed. This
may form a retrograde or a flat dopant profile for the dopant
introduced by ion implantation and a surface-peaked dopant profile
for the diffused dopant. The higher-diffusivity dopant would form a
deeper dopant profile than the lower-diffusivity dopant in this
embodiment. The selection of which dopant is introduced by ion
implantation and which dopant is introduced by diffusion is
application specific. Similarly, the order in which the dopants are
introduced can also be varied. For example, for a surface peaked
monotonically decreasing profile, the high diffusivity species
should be implanted. Alternatively, for a profile that has a
retrograde feature at some depth, it may be advantageous to implant
the low diffusivity species. The disclosure is not limited to a
particular configuration or order of operations.
[0041] FIG. 2 is a graph comparing dopant concentration versus
dopant depth for arsenic and phosphorus. This graph may correspond
to a secondary ion mass spectrometry (SIMS) profile. As seen in
FIG. 2, the arsenic 200 retains a shallow profile while the
phosphorus 201 diffuses deeply into the silicon.
[0042] In one particular embodiment, the parameters of the anneal
are configured to further tailor the dopant profile using at least
two dopant species.
[0043] The terms and expressions which have been employed herein
are used as terms of description and not of limitation, and there
is no intention in the use of such terms and expressions of
excluding any equivalents of the features shown and described (or
portions thereof). It is also recognized that various modifications
are possible within the scope of the claims. Other modifications,
variations, and alternatives are also possible. Accordingly, the
foregoing description is by way of example only and is not intended
as limiting.
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