U.S. patent application number 11/271203 was filed with the patent office on 2006-05-11 for pallet based system for forming thin-film solar cells.
This patent application is currently assigned to DayStar Technologies, Inc.. Invention is credited to John R. Tuttle.
Application Number | 20060096635 11/271203 |
Document ID | / |
Family ID | 36337215 |
Filed Date | 2006-05-11 |
United States Patent
Application |
20060096635 |
Kind Code |
A1 |
Tuttle; John R. |
May 11, 2006 |
Pallet based system for forming thin-film solar cells
Abstract
The present invention provides a photovoltaic thin-film solar
cell produced by a providing a pallet based substrate to a series
of reaction chambers layers can be sequentially formed on the
pallet based substrate.
Inventors: |
Tuttle; John R.;
(Mechanicville, NY) |
Correspondence
Address: |
HISCOCK & BARCLAY, LLP
2000 HSBC PLAZA
ROCHESTER
NY
14604-2404
US
|
Assignee: |
DayStar Technologies, Inc.
|
Family ID: |
36337215 |
Appl. No.: |
11/271203 |
Filed: |
November 10, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60626843 |
Nov 10, 2004 |
|
|
|
Current U.S.
Class: |
136/262 ;
136/264; 136/265; 438/95 |
Current CPC
Class: |
Y02P 70/50 20151101;
H01L 31/072 20130101; H01L 31/1876 20130101; C23C 14/568 20130101;
Y02E 10/541 20130101; Y02P 70/521 20151101; H01L 31/0749 20130101;
H01L 31/0322 20130101 |
Class at
Publication: |
136/262 ;
136/264; 136/265; 438/095 |
International
Class: |
H01L 31/00 20060101
H01L031/00 |
Claims
1. An apparatus for manufacturing a photovoltaic device comprising
a means for providing a plurality of pallets holding multiple
substrate pieces in sequence to a plurality of reaction zones
including at least: a zone capable of providing an environment for
deposition of a semiconductor layer; and a zone capable of
providing an environment for depositing a p-type absorber
layer.
2. The apparatus for manufacturing a photovoltaic device of claim 1
further comprising a means for providing in sequence a substrate to
a plurality of reactor zones for preparing said substrate.
3. The apparatus for manufacturing a photovoltaic device of claim 1
further comprising a first processing zone capable of providing an
environment for transition of the substrate from an ambient
environment to the processing environment.
4. The apparatus of claim 3 wherein the substrate transitions, in
part or whole, from atmospheric pressure to reduced pressure
consistent with the subsequent processing environment.
5. The apparatus for manufacturing a photovoltaic device of claim 1
further comprising a processing zone capable of providing an
environment for deposition of a barrier layer.
6. The apparatus of claim 5 wherein the barrier layer comprises a
thin conductor or very thin insulating material.
7. The apparatus for manufacturing a photovoltaic device of claim 1
further comprising a processing zone capable of providing an
environment for deposition of a conductive back contact layer.
8. The apparatus of claim 7 wherein the deposition of a conductive
back contact layer comprises a metallic layer.
9. The apparatus of claim 8 wherein the metallic layer is comprise
conductive metals chosen from the group consisting of molybdenum,
titanium, tantalum, or other acceptable metals or alloys.
10. The apparatus of claim 9 wherein the metallic layer is
molybdenum.
11. The apparatus for manufacturing a photovoltaic device of claim
1 further comprising a processing zone capable of providing an
environment for deposition of alkali materials.
12. The apparatus of claim 11 wherein the alkali materials are
Na-VII or Na.sub.2-VII.
13. The apparatus for manufacturing a photovoltaic device of claim
1 further comprising a processing zone capable of providing an
environment for deposition of a semiconductor layer.
14. The apparatus of claim 13 wherein the semiconductor layer
comprises Group I, III, VI elements.
15. The apparatus of claim 14 wherein the semiconductor layer
comprises CuGaSe.sub.2, CuAlSe.sub.2, or CuInSe.sub.2 alloyed with
one or more of the I, III, VI elements.
16. The apparatus of claim 15 wherein the semiconductor layer
comprises CuGaSe.sub.2.
17. The apparatus for manufacturing a photovoltaic device of claim
1 further comprising a processing zone capable of providing an
environment for deposition of a semiconductor layer wherein the
layer comprises precursor materials.
18. The apparatus of claim 17 wherein the precursor materials
comprise Group I, III, VI elements.
19. The apparatus of claim 18 wherein the precursor materials
comprise a I-(IIIa,IIIb)-VI.sub.2 layer.
20. The apparatus of claim 19 wherein the precursor materials
comprise one or more of the elements of a I-(IIIa,IIIb)-VI.sub.2
layer where the 0.0<IIIb/(IIIa+IIIb)<0.4.
21. The apparatus of claim 19 wherein the precursor materials
comprise one or more of the alloys of a I-(IIIa,IIIb)-VI.sub.2
layer where the 0.0<IIIb/(IIIa+IIIb)<0.4.
22. The apparatus of claim 19 wherein the semiconductor layer
comprises a CIGS absorber layer comprising
In.sub.1-x:Ga.sub.x:Se.sub.2 where x ranges between 0.2 to 0.3
wherein the thickness ranges from about 1 .mu.m to about 3
.mu.m.
23. The apparatus of claim 22 where the CIGS absorber layer is
formed by the delivery of type I, III and VI precursor metals where
Cu, In.sub.1-x, Ga.sub.x, and Se.sub.2 layers are sequentially
deposited on the substrate.
24. The apparatus of claim 22 where the CIGS absorber layer is
formed by the delivery of the type I, III and VI precursor metals
where Cu, In.sub.1-x, Ga.sub.x, and Se.sub.2 layers are
sequentially deposited on the substrate and then synthesized into
an alloy mixture with a thermal treatment.
25. The apparatus of claim 22 where the CIGS absorber layer is
formed by the delivery of type I, III and VI precursor metals where
an Cu:Ga.sub.x layer is separately synthesized, and then
co-deposited with an In.sub.x-1 layer and Se.sub.2 layer on a
substrate.
26. The apparatus of claim 22 where the CIGS absorber layer is
formed by the delivery of type I, III and VI precursor metals where
a Cu:Ga.sub.x layer is separately synthesized, and then
co-deposited with an In.sub.1-x layer and Se.sub.2 layer on a
substrate; and then synthesized into an alloy mixture with a
thermal treatment.
27. The apparatus of claim 22 where the CIGS absorber layer is
formed by the delivery of type I, III and VI precursor metals where
an Cu:Ga.sub.x:In.sub.x-1 layer is separately synthesized, and then
co-deposited with an Se.sub.2 layer on a substrate.
28. The apparatus of claim 22 where the CIGS absorber layer is
formed by the delivery of type I, III and VI precursor metals where
a Cu:Ga.sub.x:In.sub.x-1 layer is separately synthesized, and then
co-deposited with an Se.sub.2 layer on a substrate; and then
synthesized into an alloy mixture with a thermal treatment.
29. The apparatus for manufacturing a photovoltaic device of claim
1 further comprising a processing zone capable of providing an
environment for thermal treatment of one or more layers.
30. The apparatus of claim 29 wherein the treatment occurs in the
pressure range of 10.sup.-6 torr up to atmospheric pressure and
temperature range of 300.degree. C. to 700.degree. C.
31. The apparatus for manufacturing a photovoltaic device of claim
1 further comprising a processing zone capable of providing an
environment for deposition of an n-type semiconductor layer.
32. The apparatus of claim 31 wherein the n-type semiconductor
layer is discrete.
33. The apparatus of claim 32 wherein the discrete layer comprises
one or more of Group II-VI, III-VI elements.
34. The apparatus of claim 32 wherein the discrete layer materials
comprise one or more of the following groups (In,Ga).sub.y(Se,S,O)
and (Zn,Cd) (Se,S,O).
35. The apparatus of claim 32 wherein the discrete layer materials
comprise one or more of the following materials chosen from the
group consisting of (In,Ga).sub.2Se.sub.3, (In,Ga).sub.2S.sub.3,
ZnSe, ZnS, and ZnO.
36. The apparatus of claim 32 wherein the n-type semiconductor
layer is formed by diffusion of a dopant species into the p-type
absorber layer.
37. The apparatus of claim 36 wherein the dopant species is chosen
from the group consisting of one or more Group II or III
elements.
38. The apparatus of claim 37 wherein the dopant species comprises
either Zn or Cd.
39. The apparatus for manufacturing a photovoltaic device of claim
1 further comprising a processing zone capable of providing an
environment for deposition of an insulating transparent oxide
layer.
40. The apparatus of claim 39 wherein the insulating transparent
oxide layer comprises one or more materials from Group II-VI or
II-IV-VI.
41. The apparatus of claim 39 wherein the insulating transparent
oxide layer comprises one or more materials ZnO or ITO.
42. The apparatus for manufacturing a photovoltaic device of claim
1 further comprising a processing zone capable of providing an
environment for deposition of a conducting transparent layer.
43. The apparatus of claim 42 wherein the conducting transparent
layer comprises one or more materials from Group II-VI or
II-IV-VI.
44. The apparatus of claim 42 wherein the conducting transparent
layer comprises one or more materials ZnO, Cd.sub.2SnO.sub.4 or
ITO.
45. The apparatus for manufacturing a photovoltaic device of claim
1 further comprising a first processing zone capable of providing
an environment for transition of the substrate from the processing
environment back to the ambient environment.
46. The apparatus of claim 45 wherein the substrate transitions, in
part or whole, from atmospheric pressure to the reduced pressure
consistent with the subsequent processing environment.
47. A method for manufacturing a photovoltaic device comprising
providing a pallet, capable of holding a substrate, in sequence to
a plurality of reactor zones wherein said plurality of zones
includes at least one zone capable of providing an environment for
depositing a p-type absorber layer.
48. A method for manufacturing a photovoltaic cell comprising: a.
providing a plurality of substrate pieces affixed to a pallet
carrier means; b. depositing a conductive film on the surface of
said plurality of substrate pieces; c. wherein the conductive film
includes a plurality of discrete layers of conductive materials; d.
depositing at least one p-type semiconductor layer on the
conductive film, wherein the p-type semiconductor layer includes a
copper indium diselenide based alloy material; e. depositing an
n-type semiconductor layer on the p-type absorber layer forming a
p-n junction.
49. A pallet system for production of photovoltaic devices
comprising: a. a pallet base with a first side and a second side
where disposed on said first side of said pallet base is a
plurality of regularly disposed target areas; wherein each of said
plurality of disposed target areas has means for fixing a work
substrate in a removable fashion; b. indexing means disposed on
said pallet base allowing control of the positioning of said pallet
base; c. fixing means is magnetic means; d. magnetic means have
thermal reservoir capacity disposed evenly over said designated
target areas; e. work substrate is a magnetic material such as
stainless steel; f. fixable means is mechanical.
50. A pallet system for production of photovoltaic devices
comprising: a. a pallet base with a first side and a second side
where said first side has a plurality of regularly disposed target
areas disposed on said first side of said pallet base; b. a
plurality of regularly disposed target areas disposed on said
second side of said pallet base; wherein each of said plurality of
disposed target areas has means for fixing a work substrate in a
removable fashion; c. indexing means disposed on said pallet base
allowing control of the positioning of said pallet base.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Patent Application Ser. No. 60/626,843, filed Nov. 10, 2004.
FIELD OF THE INVENTION
[0002] The invention disclosed herein relates generally to the
field of photovoltaics and more specifically to the product and
method of manufacturing thin-film solar cells using a pallet based
system to prevent the formation of defects during deposition.
BACKGROUND OF THE INVENTION
[0003] The benefits of renewable energy are not fully reflected in
the market price. While alternative energy sources such as
photovoltaic (PV) cells offer clean, reliable, and renewable
energy, high product costs and lack of production reliability have
kept these devices from being a viable commercial product. With the
demand for energy going up, the world demand for alternatives to
present energy sources is increasing.
[0004] Although relatively efficient thin-film PV cells can be
manufactured in the laboratory, it has proven difficult to
commercially scale manufacturing processes with the consistent
repeatability and efficiency critical for commercial viability.
Moreover, the cost associated with manufacturing is an important
factor preventing the broader commercialization of thin-film solar
cells. The lack of an efficient thin-film manufacturing process has
contributed to the failure of PV cells to effectively replace
alternate energy sources in the market.
[0005] Thin-film PV cells can be manufactured according to varied
designs. In a thin-film PV cell, a thin semiconductor layer of PV
materials is deposited on a supporting layer such as glass, metal,
or plastic foil. Since thin-film materials have higher light
absorptivity than crystalline materials, thin-film PV materials are
deposited in extremely thin consecutive layers of atoms, molecules,
or ions. The typical active area of thin-film PV cells is only a
few micrometers thick. The basic photovoltaic stack design
exemplifies the typical structure of a PV cell. In that design, the
thin-film solar cell comprises a substrate, a barrier layer, a back
contact layer, a mixed type semiconductor source layer, an absorber
layer, an n-type junction buffer layer, an intrinsic transparent
oxide layer, and a conductive transparent oxide layer. Compounds of
copper indium gallium diselenide (CIGS) have the most promise for
use in absorber layers in thin-film cells and fit within the
classification of copper-indium selenium class, called CIS
materials. CIGS films are typically deposited by vacuum-based
techniques.
[0006] Thin-film manufacturing processes suffer from low yield due
to defects in the product that occur during the course of
deposition. Specifically, these defects are caused by contamination
occurring during processing and materials handling, and the
breakage of glass, metal, or plastic substrates. Thus, a process
for manufacturing thin-film solar cells that both limits potential
contamination during processing and concurrently minimizes
substrate breakage is desired in the art.
[0007] Currently, cells are manufactured using a multi-step batch
process wherein each product piece is transferred between reaction
steps. This transfer is bulky and requires the reaction in chambers
to be cycled. A typical process consists of a series of individual
batch processing chambers, each specifically designed for the
formation of various layers in the cell. Problematically, the
substrate is transferred from vacuum to air--and back
again--several times. Such vacuum breaks may result in
contamination of the product. Thus, a process that minimizes vacuum
breaks is desired in the art.
[0008] While an alternate system uses a series of individual batch
processing chambers coupled with a roll-to-roll continuous process
for each chamber, the discontinuity of the system and the need to
break vacuum continues to be a major drawback. Additionally, the
roll-to-roll process may impose flexing stress on substrates,
resulting in fracturing and breakage. Such defects compromise layer
cohesiveness and may result in a zero yield.
[0009] Also contributing to the low yield in PV cell manufacturing
is the requirement of high-temperature deposition processes. High
temperatures are generally incompatible with all presently known
flexible polyimide or other polymer substrate materials.
[0010] For example, U.S. Patent Application 2004/0063320, published
by Hollars on Apr. 1, 2004, discloses a general methodology for
continuously producing photovoltaic stacks using a roll-to-roll
system. As discussed above, this process requires the application
of flexing stress to the substrate. This stress potentially results
in fractures and breakage. Fractures or breakage reduce high
quality stack structures and lower manufacturing yield. Thus, to be
a commercially viable process, the disclosed system requires a
flexible substrate for the production of the stack. However, no
currently known flexible polymer materials can withstand the
high-temperature deposition process. Therefore, a process that does
not impose flexing stress on the substrates, where the substrates
can withstand the high-temperature deposition process, is desired
in the art. So a process for manufacturing PV work pieces
effectively, and capable of large scale production are needed.
SUMMARY OF THE INVENTION
[0011] The present invention provides a photovoltaic device
produced by providing a pallet-based substrate to a series of
reaction chambers where sequentially a barrier layer, a back
contact layer, a semiconductor layer or layers, alkali materials,
an n-type junction buffer layer, an intrinsic transparent oxide
layer, a transparent conducting oxide layer and a top metal grid
can be formed on the pallet.
[0012] A method is further disclosed for forming a photovoltaic
device in a continuous fashion by employing a train of the pallet
based holders loaded with work pieces. In this embodiment, a series
of pallets are passed at a defined rate through a reactor having a
plurality of processing zones, wherein each zone is dedicated to
one production step stage of device manufacture.
[0013] These production steps may include: a load or isolation zone
for substrate preparation; environments for depositing a barrier
layer, a back contact layer, semiconductor layer or layer and
alkali materials; an environment for the thermal treatment of one
or more of the previous layers; and an environment for the
deposition of: an n-type compound semiconductor wherein this layer
serves as a junction buffer layer, an intrinsic transparent oxide
layer, and a conducting transparent oxide layer. In a further
embodiment, the process may be adjusted to comprise greater or
fewer zones in order to fabricate a thin film solar cell having
more or fewer layers.
[0014] A pallet type system may be employed where a plurality of
work pieces are held as a pallet and a plurality of pallets are
processed though a continuous reactor step apparatus. This pallet
based system allows continuous processing of smaller work pieces
and alternative materials handling steps, such as pallet stacking
in intermediate or final steps.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 shows an embodiment of a thin-film solar cell
produced by the production technology of the present invention.
[0016] FIG. 2 schematically represents a reactor for forming solar
cells.
[0017] FIG. 3 shows a plurality of work piece substrates on a
device capable of affixing the substrates onto a carrier that also
has means that allow the pieces to be advanced in a precise fashion
through the production apparatus.
[0018] FIG. 4 illustrates one embodiment of a substrate being fed
from left to right through a process in accordance with the present
invention.
[0019] FIG. 5A shows an embodiment of the processing method wherein
two substrates are fed and processed simultaneously by a sequential
sputter-evaporate in accordance with the present invention.
[0020] FIG. 5B shows a top view of an embodiment of the processing
method wherein two substrates are fed and processed simultaneously
by a sequential sputter-evaporate/sputter-evaporate process.
[0021] FIG. 6 illustrates another embodiment of a process in
accordance with the invention wherein zones further comprise one or
more sub-zones.
[0022] FIG. 7 shows a schematic of the pallet used in the present
invention populated with a plurality of substrate work pieces.
[0023] FIG. 8 shows a schematic of a cartridge used to stack a
plurality of substrates in a controlled environment.
[0024] FIG. 9 shows a schematic production technique employing the
cartridge system to allow discontinuations in a photovoltaic
manufacturing process.
DETAILED DESCRIPTION OF THE INVENTION
General Photovoltaic Stack Designs
[0025] The present invention employs a new production apparatus to
produce photovoltaic devices. Of course, the particular apparatus
will depend upon the specific photovoltaic device design, which can
be varied. However, the base premise is that each photovoltaic
device has a photovoltaic device or thin-film solar cell 100
comprises a substrate 105, a barrier layer 110, a back contact
layer 120, a semiconductor layer 130, an alkali materials 140,
another semiconductor layer 150, an n-type junction buffer layer
160, an intrinsic transparent oxide layer 170, and a transparent
conducting oxide layer 180. This stack of layers, according to the
present invention, will be made on a plurality of substrates
arrayed on a pallet 700, shown in FIG. 7. The individual substrate
pieces 710 will be arrayed on the pallet, fixed by a fixture
means.
General Apparatus Configurations
[0026] A first embodiment of the invention is an apparatus for
manufacturing a photovoltaic device comprising a means for
providing a plurality of pallets holding multiple substrate pieces,
in sequence, to a plurality of reaction zones. These reaction zones
include at least a zone capable of providing an environment for
deposition of a semiconductor layer, and a zone capable of
providing an environment for depositing a precursor p-type absorber
layer.
[0027] FIG. 7 shows a schematic top view of a pallet. The pallet
provides a holding basis 700 for a plurality of small PV work piece
substrates 710, or working substrates fixedly attached to the
pallet in a pre-determined manner so that the individual work
pieces are presented in each treatment chamber in a precise and
controllable fashion. The pallet itself is engineered so that the
position of the pallet can be precisely determined. The pallet also
has a means for allowing attachment to a drive means to advance the
pallet through the treatment chamber. Materials of the body of the
pallet are chosen so that they are thermally stable and do not
interact with the treatment or deposition materials used in the
reaction or deposition chamber.
Fixturing Means
[0028] Furthermore, the means for securing the work pieces to the
pallet are releasable. In some instances the means for affixing the
work piece is magnetic, either because the substrate of the work
piece is itself ferro-magnetic, or with an overlay that hold the
individual pieces to the body of the pallet. A mask may be employed
to hold each piece where there is a frame and a plurality of panes
that allow deposition through said pane on the work piece
substrates.
[0029] FIG. 4 shows as the plurality of pallets are processed
through the series of treatment chambers 420, 430 used to deposit
the multiple required layers for a photovoltaic device, the train
of reaction/deposition chambers may be fed by loaded cartridges
containing a plurality of pallets 440 (see also FIG. 8), and after
the desired layers are deposited and processed the finished work
pieces are collected in a cartridge 450 and stored for further
processing or manufacturing steps (see also FIG. 9).
[0030] In an alternative method, the starting piece cartridge 440
feeds a system that might provide surface treatment of the
substrate, deposition of the junction layer, deposition of the
alkali-containing semiconductor source layer, deposition of a
p-type absorber layer, and then formation of the junction and
n-type layer, and a buffer so that the worked-on pieces can be
taken from the controlled reaction train and inventories for
further processing.
Method of Thin Film Manufacturing
[0031] One form of the invention provides a method for
manufacturing a photovoltaic device comprising the step of
providing a pallet, capable of holding a substrate, in sequence to
a plurality of reactor zones wherein the plurality of zones
includes at least one zone depositing a precursor p-type absorber
layer.
[0032] In another form, the invention provides a method for
manufacturing a photovoltaic cell comprising the steps of providing
a plurality of substrate pieces affixed to a pallet carrier means,
depositing a conductive film on the surface of said plurality of
substrate pieces, wherein the conductive film includes a plurality
of discrete layers of conductive materials, depositing at least one
p-type absorber layer on the conductive film, wherein the p-type
semiconductor absorber layer includes a metal based alloy material,
e.g., alloys of Cu, In, Ga, and possibly Se or S, and depositing an
n-type semiconductor layer on the p-type semiconductor layer
forming a p-n junction.
[0033] A series of treatment chambers are provided where each
chamber provides a specific treatment environment, as well as the
means for depositing specified materials onto the working surface,
or interface of the work pieces being processed in order to produce
a specific layer deposition or layer treatment. Each of these
treatment chambers allow a means to transport the work pieces
(marked on the pallet being made into the photovoltaic device) to
be transported from the first designed chamber, through the
sequential plurality of chambers, until the work piece has been
made into the designed photovoltaic stack.
[0034] This plurality of reaction or treatment chambers provided
with a transport mechanism may also include one or more isolation
chambers that ensure effective reactants are maintained in
specifically desired chambers and do not contaminate downstream
processes. This isolation system is particularly important in the
formation of the semiconductor layers of the photovoltaic device,
where relatively small amounts of material determine whether a
layer is a p-type or n-type semiconductor. This carrier may be
configured with referencing means to ensure the work pieces are
positioned within the production apparatus at defined
positions.
[0035] FIG. 3 shows a plurality of work piece substrates 310 on a
device capable of affixing the substrates onto a carrier 320, that
also has means that allow the pieces to be advanced in a precise
fashion through the production apparatus. These pallets are
generally flat and have means for holding a plurality of work
pieces on a surface of the pallet so as to present each of the work
piece surfaces to a deposition source or treatment source.
General Photovoltaic Stack Designs
[0036] The present invention employs a new production apparatus to
produce photovoltaic devices. Of course, the particular apparatus
will depend upon the specific photovoltaic device design, which can
be varied.
[0037] Viewing FIG. 1, all layers are deposited on a substrate 105
which may comprise one of a plurality of functional materials, for
example, glass, metal, ceramic, or plastic. Deposited directly on
the substrate 105 is a barrier layer 110. The barrier layer 110
comprises a thin conductor or very thin insulating material and
serves to block the out diffusion of undesirable elements or
compounds from the substrate to the rest of the cell. This barrier
layer 110 may comprise chromium, titanium, silicon oxide, titanium
nitride and related materials that have the requisite conductivity
and durability. The next deposited layer is the back contact layer
120 comprising non-reactive metals such as molybdenum. The next
layer is deposited upon the back contact layer 120 and is a p-type
semiconductor layer 130 to improve adhesion between an absorber
layer and the back contact layer. The p-type semiconductor layer
130 may be a I-III.sub.a,b-VI isotype semiconductor, but the
preferred composition is Cu:Ga:Se; Cu:Al:Se or Cu:In:Se alloyed
with either of the previous compounds.
[0038] In this embodiment, the formation of a p-type absorber layer
involves the interdiffusion of a number of discrete layers.
Ultimately, as seen in FIG. 1, the p-type semiconductor layers 130
and 150 combine into a single composite layer 155 which serves as
the prime absorber of solar energy. In this embodiment, however,
alkali materials 140 are added for the purpose of seeding the
growth of subsequent layers as well as increasing the carrier
concentration and grain size of the absorber layer 155, thereby
increasing the conversion efficiency of the solar cell. The layers
are then thermally treated at a temperature of about 400.degree.
C.-600.degree. C.
[0039] After the thermal treatment, the photovoltaic production
process is continued by the deposition of an n-type junction buffer
layer 160. This layer 160 will ultimately interact with the
absorber layer 155 to form the necessary p-n junction 165. A
transparent intrinsic oxide layer 170 is deposited next to serve as
a hetero-junction with the CIGS absorber. Finally, a conducting
transparent oxide layer 180 is deposited to function as the top of
the electrode of the cell. This final layer is conductive and may
carry current to a grid carrier that allows the current generated
to be carried away.
Alternative Pallet Based Manufacturing Schemes
[0040] FIG. 2 schematically represents a reactor 200 for forming
solar cells. A substrate 205 is fed left to right through the
reactor. The reactor 200 includes one or more processing zones,
referred to in FIG. 2 as 220, 230, 240 and 250, wherein each
processing zone comprises an environment for depositing materials
on a substrate 205. The zones are mechanically or operatively
linked together within the reactor 200. As used herein, the term
environment refers to a profile of conditions for depositing or
reacting a material layer or mixture of materials on the substrate
205 while the substrate 205 is in a particular zone.
[0041] Each zone is configured according to which layer of the
solar cell is being processed. For example, a zone may be
configured to perform a sputtering operation, including heat
sources and one or more source targets.
[0042] Preferably, an elongated substrate 205 is passed through the
various processing zones at a controllable rate. It is further
contemplated that the substrate 205 may have a translational speed
of 0.5 m/min to about 2 m/min. Accordingly, the process internal to
each of the zones is preferably tuned to form the desired
cross-section given the residence time the material is proximate to
a particular source material, given the desired transport speed.
Thus, the characteristics of each process, such as material and
process choice, temperature, pressure, or sputtering delivery rate,
etc., may be chosen to insure that constituent materials are
properly delivered given the stack's residence time as determined
by the transport or translation speed.
[0043] According to the invention, the substrate 205 may be
transported continually through the process in a palletized fashion
in a "picture frame" type mount for indexing and transportation
through the process, the latter of which is illustrated in FIG. 3.
Referring to FIG. 3 one substrate or group of substrates 310 are
mounted on a pallet 320 that translates through one or more zones
330 and 340 on track 350. In alternate embodiments the process may
further comprise a second substrate or set of substrates placed in
a back to back configuration with substrate 310.
[0044] It is contemplated that the background pressure within the
various zones will range from 10.sup.-6 torr to 10.sup.-3 torr.
Pressures above base-vacuum (10.sup.-6 torr) may be achieved by the
addition of a pure gas such as Argon, Nitrogen or Oxygen.
Preferably, the rate R is constant resulting in the substrate 205
passing through the reactor 200 from entrance to exit without
stopping. It will be appreciated by those of ordinary skill in the
art that a solar cell stack may thus be formed in a continuous
fashion on the substrate 205, without the need for the substrate
205 to ever stop within the reactor 200.
[0045] The reactor in FIG. 2 may further comprise vacuum isolation
sub-zones or slit valves configured to isolate adjacent process
zones. The vacuum isolation sub-zones or slit valves are provided
to facilitate the continuous transport of the substrate between
different pressure environments.
[0046] The reactor shown in FIG. 2 is a plurality of N-processing
zones 220, 230, 240 and 250. However, it should be recognized by
one skilled in the art that the reactor may comprise zones 220,
230, 240, 250 . . . N zones. The load/unload zones 210/211 comprise
zones that can be isolated from the rest of the reactor and can be
open to atmosphere.
[0047] In a preferred embodiment, the process may further comprise
a substrate 206 that runs back-to-back with substrate 205. In this
embodiment substrates 206 and 205 are oriented in a back-to-back
configuration and run through zones 220, 230, 240, and 250
performing identical process operations 222/221, 232/231, 242/241
and 252/251.
[0048] FIG. 5A shows a top illustration of a portion of a reactor
500 processing substrates 501 and 502 in a back-to-back fashion and
also illustrates a sequential sputter-evaporate process isolated by
zone 511. To achieve back-to-back processing, heat sources 503 for
substrate 501 are mirrored as heat sources 507 for substrate 502.
Likewise, sputtering source 504, heat sources 505, and evaporative
sources 506 for substrate 501 are mirrored for substrate 502 as
sputtering source 508, heat sources 509, and evaporative sources
510.
[0049] FIG. 5B shows a top illustration of a portion of a reactor
512 processing substrates 521 and 522 in a back-to-back fashion
with a sequential sputter-evaporate/sputter-evaporate process. As
in FIG, 5A, sputter sources 534 for substrate 521 are mirrored as
sputter sources 528 for substrate 522. Likewise, heat sources 523
and 526, evaporative sources 524 and 527, and sputtering source 525
for substrate 521 are mirrored for substrate 522 as heat sources
529 and 532, evaporative sources 530 and 533, and sputtering source
531. Hence, with the simple duplication of heat and material
sources, solar cell production may be effectively doubled within
the same machine.
Specific Processing Steps
[0050] Of course, the method steps for producing a particular PV
article depends upon the specific design of that article. CIGS
based PVs will have a different production method than Si based
systems. The present invention is not so limited to one PV type and
in general any PV could be made with the technology of the
invention.
[0051] In cases of CIGS, the specific steps might include: loading
a pallet based substrate through an isolated loading zone or like
unit 210. In various embodiments, the isolation zone 210 is
contained within the reactor 200. Alternatively, the isolation zone
210 may be attached to the outer portion of the reactor 200. The
first processing zone 210 may further comprise a substrate
preparation environment to remove any residual imperfections at the
atomic level of the surface. The substrate preparation may include:
ion beam, deposition, heating, or sputter-etching. These methods
are known in the art and will not be discussed further.
[0052] A second processing zone may be an environment for
depositing a barrier layer for substrate impurity isolation,
wherein the barrier layer provides an electrically conductive path
between the substrate and subsequent layers. In a preferred
embodiment, the barrier layer comprises an element such as chromium
or titanium delivered by a sputtering process. Preferably, the
environment comprises a pressure in the range of about 10.sup.-6
torr to about 10.sup.-2 torr at ambient temperature.
[0053] A third processing zone downstream from the previous zones
comprises an environment for the deposition of a metallic layer to
serve as a back contact layer. The back contact layer comprises a
thickness that provides a conductive path for electrical current.
In addition, the back contact layer serves as the first conducting
layer of the solar cell stack. The layer may further serve to
prevent the diffusion of chemical compounds such as impurities from
the substrate to the remainder of the solar cell structure or as a
thermal expansion buffer between the substrate layer and the
remainder of the solar cell structure. Preferably, the back contact
layer comprises molybdenum, however, the back contact layer may
comprise other conductive metals such as aluminum, copper or
silver.
[0054] A fourth zone provides an environment for deposition of a
p-type semiconductor layer. As used herein, this layer may serve as
an epitaxial template for absorber growth. Preferably, the p-type
semiconductor layer is an isotype I-IIIVI.sub.2 material, wherein
the optical band gap of this material is higher than the average
optical band gap of the p-type absorber layer. For example, a
semiconductor layer may comprise Cu:Ga:Se; Cu:AI:Se or alloys of
Cu:In:Se with either of the previous compounds. Preferably, the
materials are delivered by a sputtering process at a background
pressure of 10.sup.-6 to 10.sup.-2 torr and at temperatures ranging
from ambient up to about 300.degree. C. More preferably,
temperatures range from ambient to about 200.degree. C.
[0055] A fifth zone, downstream from the previous zones, provides
an environment for the deposition of alkali materials to enhance
the growth and the electrical performance of a p-type absorber.
Preferably, the alkali materials are sputtered, at ambient
temperature and a pressure range of about 10.sup.-6 torr to
10.sup.-2 torr. Preferably, the material comprises NaF, Na.sub.2Se,
Na.sub.2S or KCl or like compounds wherein the thickness ranges
from about 150 nm to about 500 nm.
[0056] A sixth zone, also downstream from the previous zones, may
comprise an environment for the deposition of another semiconductor
layer comprising p-type absorber precursor materials. In a
preferred embodiment, the sixth zone may further comprise one or
more sub-zones for the deposition of the precursor materials. In
one embodiment, the semiconductor layer is formed by first
delivering precursor materials in one or more contiguous sub-zones,
then reacting the precursor materials into the final p-type
absorber in a downstream thermal treatment zone. Thus, especially
for CIGS Systems, there may be two material deposition steps and a
third thermal treatment step in the format of the layer.
[0057] In the precursor delivery zones, the layer of precursor
materials is deposited in a wide variety of ways, including
evaporation, sputtering, and chemical vapor deposition or
combinations thereof preferably, the precursor material may be
delivered at temperatures ranging from about 200.degree.
C.-300.degree. C. It is desired that the precursor materials react
to form the p-type absorber as rapidly as possible. As previously
discussed, to this end, the precursor layer or layers may be formed
as a mixture or a series of thin layers.
[0058] A manufacturing device may also have seventh processing zone
downstream from previous processing zones for the thermal treatment
of one or more of the previous layers. The term multinaries
includes binaries, ternaries, and the like. Preferably, thermal
treatment reacts previously unreacted elements or multinaries. For
example, in one embodiment it is preferred to have Cu, In, Se, and
Ga in various combinations and ratios of multinary compounds of
elements as the source for deposition on the work piece. The
reactive environment includes selenium and sulfur in varying
proportions and ranges in temperature from about 400.degree. C. to
about 600.degree. C. with or without a background inert gas
environment. In various embodiments, processing time may be
minimized to one minute or less by optimizing mixing of the
precursors. Optimal pressures within the environment depend on
whether the environment is reactive or inert. According to the
invention, within the thermal treatment zone, the pressures range
from about 10.sup.-5 to about 10.sup.-2 torr. However, it should be
noted that these ranges depend very much on the reactor design for
the stage, the designer of the photovoltaic device and the
operational variables of the apparatus as a whole.
[0059] The reactor may have an eighth processing zone for the
formation of an n-type semiconductor layer or junction partner. The
junction layer is selected from the family II-VI, or IIIx VI. For
example, the junction layer may comprise ZnO, ZnSe, ZnS, In, Se or
In.sub.NS deposited by evaporation, sublimation or chemical vapor
deposition methodologies. The temperatures range from about
200.degree. C. to about 400.degree. C.
[0060] Additionally, the process may also have a ninth zone having
an environment for deposition of an intrinsic layer of a
transparent oxide, for example ZnO. According to the invention, the
intrinsic transparent oxide layer may be deposited by a variety of
methods including for example, RF sputtering, CVD or MOCVD.
[0061] In various embodiments, the process further has a tenth zone
with an environment for the deposition of a transparent conductive
oxide layer to serve as the top electrode for the solar cell. In
one embodiment for example, aluminum doped ZnO is sputter
deposited. Preferably, the environment comprises a temperature of
about 200.degree. C. and a pressure of about 5 millitorr.
Alternatively, ITO (Indium Tin Oxide) or similar may be used.
[0062] In one embodiment, as described above, the reactor may
comprise discrete zones wherein each zone corresponds to one layer
of photovoltaic device formation. In a preferred embodiment
however, zones comprising similar constituents and or environment
conditions may be combined thereby reducing the total number of
zones in the reactor.
[0063] For example, in FIG. 6, zone 610 comprises sub-zones 611 and
612, zone 615 comprises sub-zones 616 and 617, and zone 620
comprises one zone, wherein each zone and sub-zone comprises a
predetermined environment. In this example, a material A may be
deposited in sub-zone 611 and a different material B may be
deposited in sub-zone 612, wherein the environment of sub-zone 612
downstream from material A differs from the environment in sub-zone
611. Thus, the substrate 605 may be subjected to a different
temperature or other process profiles while in different regions of
the same zone 610. According to this embodiment, the zone may be
defined as having a predetermined pressure, and a zone may include
one or more regions, sub-zones, or phases therein, with each
sub-zone configured to deposit or react a desired material or
materials within the same pressure environment.
[0064] The substrate 605 may then be passed to chamber 615, where
material C is deposited within sub-zone 616, and material D is
deposited in sub-zone 617. Finally, the substrate 605 reaches a
zone 620, where a single material E is deposited.
[0065] As will be appreciated by those of ordinary skill in the
art, the reactor 600 may be described as having a series of zones
disposed between the entrance and exit of the reactor along a path
defined by the translation of the substrate. Within each zone, one
or more constituent environments or sub-zones may be provided to
deposit or react a selected target material or materials, resulting
in a continuous process for forming a solar cell stack. Once the
substrate enters the reactor, the various layers of a solar stack
are deposited and formed in a sequential fashion, with each
downstream process in succession contributing to the formation of
the solar cell stack until a finished thin film solar cell is
presented at the exit of the reactor.
[0066] While the present technique has been couched in terms of
CIGS based photovoltaic stack designs, it must be understood that
the technique may also be employed for the production of other
photovoltaic designs including production of silicon based systems
such as those discussed in state of the art. For instance, it would
be possible to use or include carbon or germanium atoms in
hydrogenated amorphous silicon alloys in order to adjust their
optical bandgap. For example, carbon has a larger bandgap than
silicon and thus inclusion of carbon in a hydrogenated amorphous
silicon alloy increases the alloy's bandgap. Conversely, germanium
has a smaller bandgap than silicon and thus inclusion of germanium
in a hydrogenated amorphous silicon alloy decreases the alloy's
bandgap.
[0067] Similarly one could incorporate boron or phosphorus atoms in
hydrogenated amorphous silicon alloys in order to adjust their
conductive properties. Including boron in a hydrogenated amorphous
silicon alloy creates a positively doped conductive region.
Conversely, including phosphorus in a hydrogenated amorphous
silicon alloy creates a negatively doped conductive region.
[0068] Hydrogenated amorphous silicon alloy films are prepared by
deposition in a deposition chamber. Heretofore, in preparing
hydrogenated amorphous silicon alloys by deposition in a deposition
chamber, carbon, germanium, boron or phosphorus have been
incorporated into the alloys by including in the deposition gas
mixture carbon, germanium, boron or phosphorus containing gases
such as methane (CH.sub.4), germane (GeH.sub.4), germanium
tetrafluoride (GeF.sub.4), higher order germanes such as digermane
(Ge.sub.2 H.sub.6), diborane (B.sub.2 H.sub.6) or phosphine
(PH.sub.3). See for example, U.S. Pat. Nos. 4,491,626, 4,142,195,
4,363,828, 4,504,518, 4,344,984, 4,435,445, and 4,394,400. A
drawback of this practice, however, is that the way in which the
carbon, germanium, boron or phosphorus atoms are incorporated into
the hydrogenated amorphous silicon alloy is not controlled. That
is, these elements are incorporated into the resulting alloy in a
highly random manner thereby increasing the likelihood of
undesirable chemical bonds.
[0069] Thus, in cases where PV devices are manufactured, and
specific and controlled reaction and or deposition conditions are
required to produce the films of the PV, the present invention
technology will be useful.
* * * * *