U.S. patent application number 11/243492 was filed with the patent office on 2006-09-21 for formation of compound film for photovoltaic device.
This patent application is currently assigned to Nanosolar, Inc.. Invention is credited to Chris Eberspacher, Matthew R. Robinson.
Application Number | 20060207644 11/243492 |
Document ID | / |
Family ID | 37009046 |
Filed Date | 2006-09-21 |
United States Patent
Application |
20060207644 |
Kind Code |
A1 |
Robinson; Matthew R. ; et
al. |
September 21, 2006 |
Formation of compound film for photovoltaic device
Abstract
A compound film for an active layer of a photovoltaic device may
be formed in two or more sub-layers. A first sub-layer having a
first component of the active layer may be formed on a substrate
with a first process. A second sub-layer including a second
component of the active layer may then be formed using a second
process such that the first sub-layer is disposed between the
second sub-layer and the substrate. The second component has a
different chemical composition than the first component. The first
and/or second sub-layer may comprise one or more components in the
form of particles and/or globules. This procedure may be repeated
any number of times for any number of sub-layers so that active
layer can be built up sequentially. The different chemical
compositions of the components in the sub-layers can provide the
active layer with a graded bandgap. The components of the
sub-layers may include elements of group IB, and/or group IIIA. One
or more elements of group VIA can be incorporated into the
components of the sub-layers before, during or after formation of
the layers.
Inventors: |
Robinson; Matthew R.; (East
Palo Alto, CA) ; Eberspacher; Chris; (Palo Alto,
CA) |
Correspondence
Address: |
JOSHUA D. ISENBERG;JDI PATENT
809 CORPORATE WAY
FREMONT
CA
94539
US
|
Assignee: |
Nanosolar, Inc.
Palo Alto
CA
|
Family ID: |
37009046 |
Appl. No.: |
11/243492 |
Filed: |
October 3, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11081163 |
Mar 16, 2005 |
|
|
|
11243492 |
Oct 3, 2005 |
|
|
|
Current U.S.
Class: |
136/243 ;
257/E31.007; 257/E31.027 |
Current CPC
Class: |
Y02P 70/50 20151101;
H01L 31/0322 20130101; Y10S 977/786 20130101; C23C 18/1216
20130101; C23C 18/02 20130101; Y02P 70/521 20151101; H01L 31/03928
20130101; Y02E 10/541 20130101; Y10S 977/81 20130101; C23C 18/1258
20130101; H01L 31/0749 20130101 |
Class at
Publication: |
136/243 |
International
Class: |
H02N 6/00 20060101
H02N006/00 |
Claims
1. A method of forming a compound film for an active layer of a
photovoltaic device comprising the steps of: forming a first
sub-layer including a first component of the active layer on a
substrate with a first process; forming a second sub-layer
including a second component of the active layer using a second
process, wherein the second component has a different chemical
composition than the first component and wherein the first
sub-layer is disposed between the second sub-layer and the
substrate wherein the first and/or second component includes
particles.
2. The method of claim 1, further comprising, forming one or more
additional sub-layers including one or more additional components
of the active layer.
3. The method of claim 1 wherein the particles include
nanoparticles and/or nanoglobules.
4. The method of claim 1 wherein the first and/or second process is
a printing process.
5. The method of claim 1, further comprising depositing a one or
more addition sub-layers by a vapor deposition technique.
6. The method of claim 5 wherein the vapor deposition technique is
selected from the group of ALD, evaporation, sputtering, CVD, PVD,
electroplating and the like.
7. The method of claim 1 wherein the different chemical
compositions of the first and second components in the first and
second sub-layers provide the active layer with a graded
bandgap.
8. The method of claim 1 wherein the first and/or second components
include elements of group IB, and/or group IIIA.
9. The method of claim 8, further comprising the step of
incorporating an element of group VIA into the first and/or second
component.
10. The method of claim 1 wherein forming the first and/or second
sub-layer includes making one or more phase-stabilized precursors
in the form of fine particles, particles comprising at least one
metal oxide and reducing the metal oxides in a reducing
atmosphere.
11. The method of claim 1 wherein forming the first and/or second
sub-layer includes making single-phase mixed-metal oxide particles
with an average diameter of less than about 1 micron by preparing a
solution comprising Cu and In and/or Ga as metal-containing
compounds; forming droplets of the solution; and heating the
droplets in an oxidizing atmosphere to pyrolyze the contents of the
droplets to form single-phase copper indium oxide, copper gallium
oxide or copper indium gallium oxide particles.
12. The method of claim 1 wherein forming the first and/or second
sub-layer includes preparing a source material, depositing the
source material on the substrate to form a precursor, and heating
the precursor to form a film, wherein the source material includes
Group IB-IIIA containing particles having at least one Group
IB-IIIA phase, with Group IB-IIIA constituents present at greater
than about 50 molar percent of the Group IB elements and greater
than about 50 molar percent of the Group IIIA elements in the
source material.
13. The method of claim 1 wherein forming the first and/or second
sub-layer includes formulating a nano-powder material with a
controlled overall composition and having particles of one solid
solution, depositing the nano-powder material to form the first or
second sub-layer, and reacting in at least one suitable atmosphere
to form the first or second component.
14. The method of claim 1 wherein forming the first and/or second
sub-layer includes forming a liquid ink containing elements from
groups IB, IIIA and optionally VIA, spreading a film of the liquid
onto the substrate, and annealing the film to form the first or
second sub-layer.
15. The method of claim 14 wherein forming the liquid ink includes
forming non-oxide quantum nanoparticles containing elements from
group IB; and forming non-oxide quantum nanoparticles containing
elements from group IIIA; and optionally forming non-oxide quantum
nanoparticles containing elements from group VIA; intermixing the
non-oxide quantum nanoparticles from groups IB and IIIA and
optionally VIA wherein the non-oxide quantum nanoparticles are in a
desired particle size range of between about 0.1 nm and about 10 nm
in diameter, wherein, for each element, a majority of the mass of
the non-oxide quantum nanoparticles range in size from no more than
about 40% above or below an average particle size, or, if the
average particle size is less than about 5 nanometers, from no more
than about 2 nanometers above or below the average particle size,
and mixing the non-oxide nanoparticles to form a liquid that serves
as the ink
16. The method of claim 14 wherein forming the liquid ink includes:
forming nanoparticles from group IB; and intermixing the
nanoparticles from group IB with elements from group IIIA, wherein
the elements from group IIIA are in molten form, wherein the
nanoparticles from group IB comprise particles in a desired
particle size range of between about 0.1 nm and about 500 nm in
diameter, wherein a majority of the mass of the nanoparticles range
in size from no more than about 40% above or below an average
particle size, or, if the average particle size is less than about
5 nanometers, from no more than about 2 nanometers above or below
the average particle size; and mixing the nanoparticles with the
molten elements to form a liquid that serves as the ink.
17. The method of claim 14 wherein forming the liquid ink includes:
forming nanoparticles containing elements from groups IB, and IIIA
and optionally VIA, wherein the particles are in a desired particle
size range of between about 0.1 nm and about 500 nm in diameter,
wherein a majority of the mass of the nanoparticles range in size
from no more than about 40% above or below an average particle
size, or, if the average particle size is less than about 5
nanometers, from no more than about 2 nanometers above or below the
average particle size; and mixing the nanoparticles to form a
liquid that serves as the ink.
18. The method of claim 1 wherein forming the first or second
sub-layers includes: forming a molten mixture of one or more metals
of group IIIA and metallic nanoparticles containing elements of
group IB; and coating a substrate with a film formed from the
molten mixture.
19. The method of claim 1 wherein forming the first or second
sub-layers includes: obtaining core nanoparticles containing one or
more elements from group IB and/or IIIA; and coating the core
nanoparticles with one or more layers of metal from group IB, IIIA
or an element from group VIA in a controlled fashion such that the
resulting film of coated nanoparticles have a desired
stoichiometric ratio of elements forming an ink, paste, or paint
containing the nanoparticles; forming a thin film of the ink, paste
or paint on the substrate; and annealing the thin film.
20. The method of claim 1 wherein the substrate is an aluminum foil
substrate.
21. The method of claim 1, further comprising rapidly heating the
first and/second sub-layer and/or substrate from an ambient
temperature to a plateau temperature range of between about
200.degree. C. and about 600.degree. C.; maintaining the first
and/or second sub-layer and/or substrate in the plateau temperature
range for between about a fraction of a second and about 60
minutes; and reducing the temperature of the first and/or second
sub-layer and/or substrate.
22. The method of claim 1, further comprising: performing atomic
layer deposition of a group IB element and/or a group IIIA element
and/or a group VIA element onto the substrate, the first sub-layer
or the second sub-layer.
23. The method of claim 1, wherein forming the first and/or second
sub-layer includes formulating a dispersion of IB and IIIA
elemental nanoparticles, wherein a group IIIA element is
incorporated into the dispersion in the form of a suspension of
nanoglobules; depositing said dispersion onto a substrate to form a
film on the substrate; and reacting the film in a suitable
atmosphere.
24. The method of claim 23, further comprising iteratively
repeating said formulating, depositing and reacting steps two or
more times to form said compound film.
25. The method of claim 24 wherein, for at least one iteration, the
formulating step includes changing a relative concentration of the
IB, IIIA or VIA elemental nanoparticles such that a concentration
of the IB, IIIA or VIA elements within the compound film varies as
a function of depth.
26. The method of claim 24 wherein iteratively repeating said
formulating, depositing and reacting steps produces a concentration
of group IIIA elements that is relatively higher at and near a
front region and a back region of the compound film, and relatively
lower in a central region of the compound film, resulting in a
"saddle" profile for the concentration of the one or more group
IIIA elements as a function of depth within the compound film.
27. The method of claim 26 wherein the group IIIA elemental
nanoparticles include Gallium and/or Indium.
28. The method of claim 27 wherein iteratively repeating said
formulating, depositing and reacting steps produces a Gallium
concentration that is relatively high at or near the back
region.
29. The method of claim 26 wherein a Gallium concentration is
relatively lesser in the central region.
30. The method of claim 26 wherein the group IB element is copper
(Cu).
31. The method of claim 23, wherein the IIIA elemental
nanoparticles include Gallium
32. The method of claim 23 wherein the IIIA elemental nanoparticles
include Indium.
33. The method of claim 23 wherein the IIIA elemental nanoparticles
include Aluminum.
34. The method of claim 23 wherein the group IB elemental
nanoparticles include Copper.
35. The method of claim 23 further comprising, incorporating a
group VIA elemental material into the compound film.
36. The method of claim 35 wherein incorporating a group VIA
elemental material into the compound film includes exposing the
compound film to vapor and/or gas containing the group VIA
elemental material in conjunction with annealing.
37. The method of claim 35 wherein incorporating the group VIA
elemental material into the compound film includes the use of
nanoparticles containing a VIA group element in step of formulating
the dispersion.
38. The method of claim 35 wherein the group VIA elemental material
includes Selenium.
39. The method of claim 35 wherein the group VIA elemental material
includes Sulfur
40. The method of claim 35 wherein the group VIA elemental material
includes Tellurium.
41. The method of claim 1 wherein the compound film includes an
alloy of Copper and Gallium, wherein a ratio of Gallium to Copper
varies as a function of depth within the compound film.
42. The method of claim 1 wherein the compound film includes an
alloy of Copper and Indium wherein a ratio of Indium to Copper
varies as a function of depth within the compound film.
43. The method of claim 1 wherein the substrate is a flexible
substrate and printing the first and/or second sub-layers includes
the use of roll-to-roll manufacturing on the flexible
substrate.
44. A photovoltaic device having a light-absorbing layer, wherein
the light-absorbing layer is a compound film formed by the method
of claim 1.
45. The device of claim 44 wherein the light-absorbing layer
contains elements of groups IB, IIIA and VIA.
46. The device of claim 44 wherein a concentration of the IB, IIIA
or VIA elements within the light-absorbing layer varies as a
function of depth
47. The device of claim 44 wherein the light-absorbing layer is
disposed between a base electrode and a transparent electrode.
48. The device of claim 47, further comprising a junction partner
layer between the active layer and the transparent electrode.
49. The device of claim 48 wherein the junction partner layer
includes cadmium sulfide (CdS), zinc sulfide (ZnS), or zinc
selenide (ZnS) or some combination of two or more of these.
50. The device of claim 48 wherein the junction partner layer
includes an organic n-type polymer or small molecule.
51. The device of claim 48 wherein the base electrode includes a
layer of molybdenum proximate the junction partner layer.
52. The device of claim 47 wherein the transparent electrode
includes a layer of a transparent conductive oxide.
53. The device of claim 52 wherein the transparent conductive oxide
is indium tin oxide, fluorinated indium tin oxide, zinc oxide or
aluminum doped zinc oxide or a related material.
54. A method of forming a compound film comprising the steps of:
formulating a dispersion of IB and IIIA elemental nanoparticles,
wherein a group IIIA element is incorporated into the dispersion in
the form of a suspension of nanoglobules; depositing said
dispersion onto a substrate to form a layer on the substrate; and
reacting the layer in a suitable atmosphere; and iteratively
repeating said formulating, depositing and reacting steps two or
more times to form said compound film, wherein, for at least one
iteration, the formulating step includes changing a relative
concentration of the IB, IIIA or VIA elemental nanoparticles such
that a concentration of the IB, IIIA or VIA elements within the
compound film varies as a function of depth.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of
commonly-assigned, co-pending application Ser. No. 11/081,163,
entitled "METALLIC DISPERSION", which was filed on Mar. 16, 2005,
the entire disclosures of which are incorporated herein by
reference. This application is also related to commonly-assigned,
co-pending application Ser. No. 10/782,017, entitled
"SOLUTION-BASED FABRICATION OF PHOTOVOLTAIC CELL" which was filed
Feb. 19, 2004 and published as US Patent Application Publication
20050183767, and to commonly-assigned, co-pending application Ser.
No. 10/943,658 entitled "FORMATION OF CIGS ABSORBER LAYER MATERIALS
USING ATOMIC LAYER DEPOSITION AND HIGH THROUGHPUT SURFACE
TREATMENT," which was filed Sep. 18, 2004 and published as US
Patent Application Publication 20050186342, the entire disclosures
of both of which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention is related to formation of nanoparticles and
more particularly to formation of photovoltaic cells using
nanoparticle inks for the fabrication of IB-IIIA-VIA absorber
layers and where the absorber layers have a graded bandgap.
BACKGROUND OF THE INVENTION
[0003] Solar cells convert sunlight into electricity. These
electronic devices have been traditionally fabricated using silicon
(Si) as a light-absorbing, semiconducting material in a relatively
expensive production process. To make solar cells more economically
viable, solar cell device architectures have been developed that
can inexpensively make use of thin-film, light-absorbing
semiconductor materials such as
copper-indium-gallium-sulfo-selenide, Cu(In,Ga)(S,Se).sub.2, also
termed CI(G)S(S). Solar cells of this class typically have an
absorber layer sandwiched between an electrode layer and a junction
partner layer. The electrode layer is often Mo, while the junction
partner is often CdS or ZnS. A transparent conductive oxide (TCO)
such as zinc oxide (ZnO) is formed on the junction partner layer is
typically used as a transparent electrode. CIGS-based solar cells
have been demonstrated to have power conversion efficiencies
exceeding 19%.
[0004] A central challenge in constructing a CIGS-based solar cell
is that the components of the CIGS layer must be within a narrow
stoichiometric ratio in order for the resulting cell to be highly
efficient. Achieving precise stoichiometric composition over
relatively larger substrate areas is however difficult using
traditional vacuum-based deposition processes. For example, it is
difficult to deposit compounds and/or alloys containing more than
one element by sputtering or evaporation. Both techniques rely on
deposition approaches that are limited to line-of-sight and
limited-area sources, tending to result in poor surface coverage.
Line-of-sight trajectories and limited-area sources can result in
the non-uniform three-dimensional distribution of elements in all
three dimensions and/or poor film-thickness uniformity over large
areas. These non-uniformities can occur over the nano-meso, and/or
macroscopic scales. Such non-uniformity also alters the local
stoichiometric ratios of the absorber layer, decreasing the
potential power conversion efficiency of the complete device.
[0005] Alternative approaches to vacuum-based deposition techniques
such as sputtering and evaporation have been developed. In
particular, production of solar cells on flexible substrates using
semiconductor printing technologies provides a highly
cost-efficient alternative to conventional vacuum-deposited solar
cells. For example, T. Arita and coworkers [20th IEEE PV
Specialists Conference, 1988, page 1650] described a screen
printing technique that involved mixing and milling pure Cu, In and
Se powders in the compositional ratio of 1:1:2 and forming a screen
printable paste, screen printing the paste on a substrate, and
sintering this film to form the compound layer. They reported that
although they had started with elemental Cu, In and Se powders,
after the milling step the paste contained the CuInSe.sub.2 phase.
However, solar cells fabricated using the sintered layers had very
low efficiencies because the structural and electronic quality of
these absorbers were poor.
[0006] Screen-printed CuInSe.sub.2 deposited in a thin-film was
also reported by A. Vervaet et al. [9th European Communities PV
Solar Energy Conference, 1989, page 480], where a CuInSe.sub.2
powder was used along with Se powder to prepare a screen printable
paste. Layers formed by screen printing were sintered at high
temperature. A difficulty in this approach was finding an
appropriate fluxing agent for dense CulnSe.sub.2 film formation.
Solar cells made in this manner also had poor conversion
efficiencies.
[0007] U.S. Pat. No. 5,985,691 issued to B. M. Basol et al
describes another particle-based method to form a Group IB-IIIA-VIA
compound film. The described method includes the steps of preparing
a source material, depositing the source material on a base to form
a precursor, and heating the precursor to form a film. In that
method the source material includes Group IB-IIIA containing
particles having at least one Group IB-IIIA phase, with Group
IB-IIIA constituents present at greater than about 50 molar percent
of the Group IB elements and greater than about 50 molar percent of
the Group IIIA elements in the source material. The powder is
milled to reduce its particle size and then used in the preparation
of an ink which is deposited on the substrate in the form of a
precursor layer. The precursor layer is then exposed to an
atmosphere containing Group VIA vapors at elevated temperatures to
convert the film into the compound. The precursor films deposited
using this technique were porous and they yielded porous
CuInSe.sub.2 layers with small-grain regions as reported by G.
Norsworthy et al. [Solar Energy Materials and Solar Cells, 2000,
vol. 60, page 127]. Porous solar cell absorbers yield unstable
devices because of the large internal surface area within the
device, and small grains limit the conversion efficiency of solar
cells. Another key limitation of this method was the inability to
effectively incorporate gallium into the material. The
properly-distributed presence of gallium in a CIS film serves to
potentially broaden the bandgap of the semiconductor material,
thereby increasing the open circuit voltage of the solar cell, and
to promote the adhesion of the CIGS layer to a (Mo) electrode,
providing a back surface electric field which can improve the
collection of carriers. The absence of gallium decreases the
potential power conversion efficiency of the solar cell. In
practice, while gallium oxide particles can easily be produced, it
is very difficult to reduce gallium oxide, even at relatively high
temperatures, and in the absence of reduction, gallium oxide cannot
be effectively used as a precursor material for gallium in the
final film. Accordingly, in addition to poor stability, solar cells
made using the approach of Basol et al. had sub-optimal power
conversion efficiency.
[0008] Eberspacher and Pauls in U.S. Pat. No. 6,821,559 describe a
process for making phase-stabilized precursors in the form of fine
particles, such as sub-micron multinary metal particles, and
multi-phase mixed-metal particles comprising at least one metal
oxide. The preparation of particulate materials was described using
a range of methods including laser pyrolysis, atmospheric arc
evaporation, solution precipitation, chemical vapor reactions,
aerosol pyrolysis, vapor condensation, and laser ablation. In
particular, aerosol pyrolysis was used to synthesize mixed-metal
particulates comprising metal oxides formed as substantially solid
and spherical particulates. These particulate precursor materials
were then deposited onto large-area substrates in thin layers using
any of a variety of techniques including slurry spraying methods
such as pneumatic spraying with a pressurized gas nozzle, hydraulic
spraying with a pressurized slurry expelled through an orifice, and
ultrasonic spraying with a rapidly vibrating atomization surface. A
disadvantage of solar cell devices comprised of thin-film absorber
layers formed in this manner was the poor reproducibility of the
resulting device performance, and the porous form of the absorber
layer, which tends to result in poor device stability.
[0009] Bulent Basol in U.S. Published Patent application number
20040219730 describes a process of forming a compound film
including formulating a powder material with a controlled overall
composition and having particles of one solid solution. The powder
material is deposited on a substrate to form a layer on the
substrate, and this layer is reacted in at least one suitable
atmosphere to form the compound. According to one preferred
embodiment of that process, the compound film has a Cu/(In+Ga)
compositional range of 0.7-1.0 and a Ga/(In+Ga) compositional range
of 0.05-0.3. Due to the improved process window made available by
the phase space of a solid solution, the use of nanoparticles
comprised of a solid solution may improve the repeatability and the
overall yield of the thin-film deposition and solar cell production
process.
[0010] Using the solid-solution approach, gallium can be
incorporated into the metallic dispersion in non-oxide form--but
only with up to approximately 18 relative atomic percent
(Subramanian, P. R. and Laughlin, D. E., in Binary Alloy Phase
Diagrams 2.sup.nd Edition, edited by Massalski, T. B. 1990. ASM
international, Materials Park, Ohio, pp 1410-1412; Hansen, M.,
Constitution of Binary Alloys. 1958. 2.sup.nd Edition, McGraw Hill,
pp. 582-584.). The lack of a means to incorporate additional Ga
beyond that possible through a solid-solution (containing either
Cu+ Ga or In+Ga) restricts the potential performance of a device
constructed by this method. In particular, since the presence of
additional gallium in the light absorbing film can serve both to
widen the bandgap of the semiconductor material and to increase the
open circuit voltage of the solar cell, a lack of additional
gallium in the light-absorbing thin film tends to decrease the
potential power conversion efficiency of solar cells created in
this manner. Efficient CIGS solar cells benefit from achieving a
gallium ratio of up to 40 relative atomic percent. Furthermore, it
would be simpler to directly work with elemental metallic
nanoparticles rather than solid-solution metallic nanoparticles in
that the elements can be optimized individually and they are more
readily available in elemental form. However, no technique was
known in the prior art to create gallium nanoparticle powders
sufficient and adequate for semiconductor applications, in part
because gallium is molten near room temperature and therefore does
not lend itself to common techniques for creating nanoparticles in
the form of powders that are then dispersed in solution (as is
commonly done with the other elements). As a result, it was not
possible to directly incorporate gallium (or incorporate gallium in
a high percentage) into a metallic dispersion used to print the CIG
precursor of a CIGS solar cell.
[0011] Robinson and Roscheisen, in commonly-assigned, co-pending,
prior U.S. patent application Ser. No. 11/081,163, recently
developed a technique to incorporate any desired amount of gallium
into a nanoparticulate mixture used to form a compound film in a
photovoltaic device. In this approach, a mixture of elemental
nanoparticles composed of the IB, the IIIA, and, optionally, the
VIA group of elements is combined with a suspension of nanoglobules
of gallium to form a dispersion. The dispersion may be deposited
onto a substrate to form a layer on the substrate. The layer may
then be reacted in a suitable atmosphere to form the compound film
that can be used as a light-absorbing layer in a photovoltaic
device. However, this approach results in a compound film without
an intentionally graded bandgap.
[0012] It would be highly desirable to grade the bandgap of a CIGS
absorber layer by varying its composition as a function of depth,
since there are numerous advantages to varying the relative
concentrations of the components of the CIGS absorber layer. These
advantages include (1) improved open circuit voltage; (2) improved
short circuit current density; and (3) improved optoelectronic
quality in the absorber layer. A detailed discussion of these and
other advantages may be found in Olle Lundberg in "Band Gap
Profiling and High Speed Deposition of Cu(In, Ga)Se.sub.2 for Thin
Film Solar Cells", Comprehensive Summaries of Uppsala Dissertations
From the Faculty of Science and Technology 903, Acta Universitatis
Upsaliensis, Uppsala, Sweden 2003, which is incorporated herein by
reference.
[0013] In particular, the presence of higher concentrations of Ga
at the back of the absorber layer can also act as a carrier
reflector, directing carriers forward to the junction at the front
of the absorber layer. In addition, higher amounts of Ga deposited
at or near the back contact (e.g. near the Mo interface) of the
CIGS cell tend to improve device function by forming smaller grains
in the presence of Ga near the back contact region, where these
smaller grains are less-mechanically stressed, thus improving the
mechanical stability of the cell.
[0014] Further, a relatively high level of Ga in the middle of the
CIGS absorber layer tends to negatively impact device function, as
small CuGaSe.sub.2 grains form. These small grains tend to have a
high defect density and may act as sites for charge recombination
in the absorber layer.
[0015] Finally, high amounts of Ga deposited at or near the front
contact (e.g. near the TCO layer) of the CIGS cell promote improved
device function in two ways: (1) a higher bandgap (e.g. about 1.35
eV) near the front contact sets the voltage of the cell at a
relatively higher value than would otherwise exist, allowing the
absorption of more photons than would otherwise be possible, thus
further increasing the efficiency of light harvesting, and (2) such
a higher voltage couples with a lower current, resulting in fewer
I.sup.2R losses.
[0016] In the prior art, graded bandgap devices with graded
concentration profiles have been prepared using co-evaporation in a
vacuum from elemental and/or alloy sources, and have produced the
best performing CIGS solar cells recorded to date. For example,
Ramanathan and coworkers at the National Renewable Energy
Laboratory showed a cell having a 19.2% conversion efficiency with
a fill factor of 78.12%, Jsc=35.71 mA/cm.sup.2, and an open-circuit
voltage of 0.69 V using this approach (see K. Ramanathan et al.,
"Properties of 19.2% Efficiency ZnO/CdS/CuInGaSe.sub.2 Thin-Film
Solar Cells", Progress in Photovoltaics: Research and Applications.
Vol. 11 2003, pp. 225-230). However, this device was formed on a
relatively small substrate, and the compound film was formed over a
relatively long time. This fabrication method does not provide an
efficient approach for high-volume solar cell production.
[0017] Further, as described above, there are several challenges
and disadvantages associated with evaporation or other vacuum-based
deposition techniques for the CIGS absorber layer, including but
not limited to (a) relatively high production cost, (b) relatively
poor spatial and chemical uniformity of deposited compound films,
and (c) relatively low throughput, limiting the potential for
high-volume production. Moreover, the creation of a bandgap graded
absorber layer using evaporative sources requires a relatively
expensive real-time monitoring system to assess the relative
composition of the absorber layer as it is being constructed.
[0018] Thus, there is a need in the art for a method of forming a
material comprised of gallium-containing nanoparticulate CIGS
precursor materials, where the precursor materials can be
reproducibly, uniformly, and densely printed over large substrate
areas to form the absorber layer of a thin-film CIGS solar cell,
and where the absorber layer has a graded bandgap.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The teachings of the present invention can be readily
understood by considering the following detailed description in
conjunction with the accompanying drawings, in which:
[0020] FIGS. 1A-1E are a sequence of schematic diagrams depicting
the formation of a composition of matter according to an embodiment
of the present invention.
[0021] FIGS. 2A-2B are a sequence of cross-sectional schematic
diagrams illustrating the formation of a compound film using the
composition of matter according to an embodiment of the present
invention.
[0022] FIGS. 2C-2D are a sequence of cross-sectional schematic
diagrams illustrating the fabrication of a photovoltaic device
using the compound film according to an embodiment of the present
invention.
[0023] FIGS. 2E-2J are a sequence of cross-sectional schematic
diagrams illustrating formation of an absorber layer with an
iterative sequence according an alternative embodiment of the
present invention.
[0024] FIG. 2K is a schematic diagram illustrating roll-to-roll
manufacturing of photovoltaic devices according to an embodiment of
the present invention.
[0025] FIG. 3 is a schematic diagram of a photovoltaic cell
according to an embodiment of the present invention.
[0026] FIG. 4 is a graph illustrating concentration profiles for a
CIGS absorber layer that may be obtained according to embodiments
of the present invention.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0027] Although the following detailed description contains many
specific details for the purposes of illustration, anyone of
ordinary skill in the art will appreciate that many variations and
alterations to the following details are within the scope of the
invention. Accordingly, the examples of embodiments of the
invention described below are set forth without any loss of
generality to, and without imposing limitations upon, the claimed
invention.
[0028] According to embodiments of the present invention, a
compound film for an active layer of a photovoltaic device may be
formed in two or more sub-layers. A first sub-layer having a first
component of the active layer may be formed on a substrate with a
first process.
[0029] A second sub-layer including a second component of the
active layer may then be formed using a second process such that
the first sub-layer is disposed between the second sub-layer and
the substrate. The second component has a different chemical and/or
physical composition than the first component. This procedure may
be repeated any number of times for any number of sub-layers so
that active layer can be built up sequentially. The different
chemical compositions of the components in the sub-layers can
provide the active layer with a graded bandgap. The components of
the sub-layers may include elements of group IB, and/or group IIIA.
One or more elements of group VIA can be incorporated into the
components of the sub-layers before, during or after formation of
the layers. Although, for the sake of example, the active layer is
described as having first and second sub-layers, the active layer
may be made with any number of additional sub-layers and
corresponding components.
[0030] There are a number of different processes for forming the
any or all of the sub-layers. Many of these processes involve
"printing" a precursor film. Such precursor films may be made,
e.g., using particle-based inks. As used herein, the term particle
generally refers to a relatively small discrete portion of some
substance having a defined chemical composition. The term particle
is intended to encompass both discrete quantities of solid matter
and discrete quantities of liquid matter. The term "globule" is
often used herein to refer to particles of matter in the liquid
state. In addition, the expressions "nanoparticles" and
"nanoparticulates" as used herein refer to particles (in either the
liquid or solid state) characterized by nanometer-scale dimensions,
e.g. sizes ranging from about 1 nm to about 1000 nm. Nanoparticles
are discrete entities sized less than about 1000 nm, more
preferably less than about 500 nm, and still more preferably less
than about 250 nm. When the nanoparticles are sized less than about
10 nm, their chemical, physical, electronic, and optical properties
often change relative to that of bulk material, and at about the 10
nm or less size scale, nanoparticles are also called "quantum
dots". As used herein, the expression "nanoglobule" similarly
refers to globules (as that term is defined above) characterized by
nanometer-scale dimensions, e.g., sizes ranging from about 1 nm to
about 1000 nm.
[0031] The use of particles in forming such inks is useful in that
the particle sizes and compositions can be controlled to adjust the
properties of the ink and the resulting film. For example,
decreased particle size can significantly lower both the melting
point and the sintering temperature required (see e.g., C R M
Wronski, "The Size Dependence of the Melting point of Small
Particles of Tin" in the British Journal of Applied Physics vol.
18. No. 12, (December 1967) pp 1731-1737, IOP Publishing, Bristol,
UK; L. H. Allen, "Nanocalorimetry Studies of Materials: Melting
Point Depression and Magic Nanostructures" NNUN Abstracts
2002/Materials, Physics, Processes & Characterization, pp 40;
Zhang et al., 2000. "Size-dependent melting point depression of
nanostructures: Nanocalorimetric measurements." Phys. Rev. B 62
(15): 548-557; Lisecki et al. 2000. "Annealing Process of
Anisotropic Copper Nanocrystals." 2. Rods. Langmuir 16: 8807-8808).
Generally, reduction in the melting point is inversely proportional
to the particle radius, i.e., the smaller the nanoparticles, the
lower the melting point.
[0032] Smaller particles also tend to pack closer together and make
better contact with each other.
[0033] Reduction in size from bulk material to particles in about
the 500 nm regime can already show significant differences in
melting point and other altered physical and chemical properties.
With much smaller particle sizes, e.g. in the nanometer size range,
the surface area of particles will increase and nanoparticles will
be in intimate contact with each other. In addition, in the
nanometer size range, the reactivity of the particles and
interaction between nanoparticles will be enhanced. This can help
particles fuse together more easily, thereby enhancing the cohesion
of the resulting CIGS layer (W. H. Qi, et al. in "China--EU Forum
on Nanosized Technology" Beijing, P. R. China. December 2002. pp
86-92). This promotes coalescence between neighboring particles
during sintering.
[0034] By way of example, nanopaticulate-based inks containing
elements and/or solid solutions from groups IB, IIIA and optionally
VIA may be formed as described in commonly-assigned US Patent
Application publication 20050183767, which has been incorporated
herein by reference. A film of the liquid ink can be spread onto
the substrate and annealed to form the first or second layer. By
way of example the liquid ink can be made by forming and non-oxide
nanoparticles containing elements from group IB, group IIIA and,
optionally, group VIA and intermixing these nanoparticles and
adding them to a liquid to from an ink.
[0035] In one embodiment any or all of the sub-layers may be formed
by depositing a source material on the substrate to form a
precursor, and heating the precursor to form a film. The source
material may include Group IB-IIIA containing particles having at
least one Group IB-IIIA phase, with Group IB-IIIA constituents
present at greater than about 50 molar percent of the Group IB
elements and greater than about 50 molar percent of the Group IIIA
elements in the source material. Additional details of this
technique are described in U.S. Pat. No. 5,985,691 to Basol, which
is incorporated herein by reference.
[0036] In another embodiment any or all of the sub-layers may be
made from a precursor film containing one or more phase-stabilized
precursors in the form of fine particles comprising at least one
metal oxide. The oxides may be reduced in a reducing atmosphere. In
particular single-phase mixed-metal oxide particles with an average
diameter of less than about 1 micron may be used for the precursor.
Such particles can be fabricated by preparing a solution comprising
Cu and In and/or Ga as metal-containing compounds; forming droplets
of the solution; and heating the droplets in an oxidizing
atmosphere. The heating pyrolyzes the contents of the droplets
thereby forming single-phase copper indium oxide, copper gallium
oxide or copper indium gallium oxide particles. These particles can
then be mixed with solvents or other additives to form a precursor
material which can be deposited on the substrate, e.g., by screen
printing, slurry spraying or the like, and then annealed to form
the sub-layer. Additional details of this technique are described
in U.S. Pat. No. 6,821,559 to Eberspacher, which is incorporated
herein by reference.
[0037] In another embodiment, the sub-layers may be deposited using
a precursor in the form of a nano-powder material formulated with a
controlled overall composition and having particles of one solid
solution. The nano-powder material precursor may be deposited to
form the first, second layer or subsequent sub-layers, and reacted
in at least one suitable atmosphere to form the corresponding
component of the active layer. The precursor may be formulated from
a nano-powder, i.e. a powdered material with nano-meter size
particles. Compositions of the particles constituting the
nano-powder used in precursor formulation are important for the
repeatability of the process and the quality of the resulting
compound films. The particles making up the nano-powder are
preferably near-spherical in shape and their diameters are less
than about 200 nm, and preferably less than about 100 nm.
Alternatively, the nano-powder may contain particles in the form of
small platelets. The nano-powder preferably contains copper-gallium
solid solution particles, and at least one of indium particles,
indium-gallium solid-solution particles, copper-indium solid
solution particles, and copper particles.
[0038] Alternatively, the nano-powder may contain copper particles
and indium-gallium solid-solution particles. The nano-powder may
additionally contain Group VIA particles, e.g., particles of Se, S
and Te or their alloys or solid solutions.
[0039] The nano-powder may be mixed with well known solvents,
carriers, dispersants etc. to prepare an ink or a paste that is
suitable for deposition onto a substrate. Alternatively,
nano-powder particles may be prepared for deposition on a substrate
through dry processes such as dry powder spraying, electrostatic
spraying or processes which are used in copying machines and which
involve rendering charge onto particles which are then deposited
onto substrates. After precursor formulation, the precursor, and
thus the nano-powder constituents are deposited onto a substrate in
the form of a micro-layer, e.g., using dry or wet processes. Dry
processes include electrostatic powder deposition approaches where
the prepared powder particles may be coated with poorly conducting
or insulating materials that can hold charge. Examples of wet
processes include screen printing, ink jet printing, ink deposition
by doctor-blading, reverse roll coating etc. In these approaches
the nano-powder may be mixed with a carrier which may typically be
a water-based or organic solvent, e.g., water, alcohols, ethylene
glycol, etc. The carrier and other agents in the precursor
formulation may be totally or substantially evaporated away to form
the micro-layer on the substrate. The micro-layer can subsequently
be reacted to form the sub-layer. The reaction may involve an
annealing process, such as furnace-annealing, RTP or
laser-annealing, microwave annealing, among others. Annealing
temperatures may be in 350-600.degree. C. and preferably between
400-550.degree. C. If the micro-layer contains Group VIA particles,
the annealing atmosphere may be inert. Alternatively, reaction step
may employ an atmosphere with the vapors of at least one of Group
VIA elements (Se,S,Te) to provide a desired level of Group VIA
elements in the sub-layer. Further details of this technique are
described in US Patent Application Publication 20040219730 to
Bulent Basol, which is incorporated herein by reference.
[0040] In some embodiments, the group IIIA element may be provided
in molten form. For example, an ink may be made starting with a
molten mixture of Gallium and/or Indium (commercially available,
e.g., from Alfa Aesar of Ward Hill, Mass.). Copper nanoparticles
may then be added to the mixture, which may then be used as the
ink/paste. Copper nanoparticles are available commercially, e.g.,
from Materials Modification Inc., of Fairfax, Va.
[0041] Alternatively, the temperature of the Cu--Ga--In mixture may
be adjusted (e.g. cooled) until a solid forms. The solid may be
ground at that temperature until small nanoparticles (e.g., less
than about 100 nm) are present. Selenium may be added to the ink
and/or a film formed from the ink by exposure to selenium vapor,
e.g., before, during, or after annealing.
[0042] The nanoparticles may be in a desired particle size range of
between about 0.1 nm and about 500 nm in diameter, preferably
between about 0.1 nm and about 10 nm in diameter. For each element,
a majority of the mass of the non-oxide quantum nanoparticles may
range in size from no more than about 40% above or below an average
particle size, or, if the average particle size is less than about
5 nanometers, from no more than about 2 nanometers above or below
the average particle size. Methods to make nanoparticles of the
desired materials having the desired narrow particle size
distribution include controlling the reaction conditions under
which the nanoparticles are made or using size-selective
precipitation and other techniques (such as ultrafiltration and
membrane emulsification) to separate out particles in the desired
size range.
[0043] In other embodiments of the invention, the first or second
layer may be fabricated by forming a molten mixture of one or more
metals of group IIIA and metallic nanoparticles containing elements
of group IB and coating the substrate with a film formed from the
molten mixture. The molten mixture may include a molten group IIIA
element containing nanoparticles of a group IB element and
(optionally) another group IIIA element. By way of example
nanoparticles containing copper and gallium may be mixed with
molten indium to form the molten mixture. The molten mixture may
also be made starting with a molten mixture of Indium and/or
Gallium (commercially available, e.g., from Alfa Aesar of Ward
Hill, Mass.). Copper nanoparticles may then be added to the molten
mixture. Copper nanoparticles are available commercially, e.g.,
from Materials Modification Inc., of Fairfax, Va. Alternatively,
such nanoparticles can be produced using any of a variety of
well-developed techniques, including but not limited to (i)
electro-explosion of copper wire, (ii) mechanical grinding of
copper particles for a sufficient time so as to produce
nanoparticles, or (iii) solution-based synthesis of copper
nanoparticles from organometallic precursors. Alternatively, the
temperature of a molten Cu--Ga--In mixture may be adjusted (e.g.
cooled) until a solid forms. The solid may be ground at that
temperature until small nanoparticles (e.g., less than 5 nm) are
present. Selenium and/or sulfur may optionally be added to the
molten mixture, and/or a coating formed from the molten mixture
before, during or after coating the substrate. Additional details
of this technique are described in commonly assigned US Patent
Application publication 2005183768, which is incorporated herein by
reference.
[0044] Alternatively the first or second layer may be fabricated
using coated nanoparticles as described in commonly-assigned U.S.
patent application Ser. No. 10/943,657, which is incorporated
herein by reference. Various coatings could be deposited, either
singly, in multiple layers, or in alternating layers, all of
various thicknesses. Specifically, core nanoparticles containing
one or more elements from group IB and/or IIIA and/or VIA may be
coated with one or more layers containing elements of group IB,
IIIA or VIA to form coated nanoparticles. Preferably at least one
of the layers contains an element that is different from one or
more of the group IB, IIIA or VIA elements in the core
nanoparticle. The group IB, IIIA and VIA elements in the core
nanoparticle and layers may be in the form of pure elemental metals
or alloys of two or more metals. By way of example, and without
limitation, the core nanoparticles may include elemental copper, or
alloys of copper with gallium, indium, or aluminum and the layers
may be gallium, indium or aluminum. Using nanoparticles with a
defined surface area, a layer thickness could be tuned to give the
proper stoichiometric ratio within the aggregate volume of the
nanoparticle. By appropriate coating of the core nanoparticles, the
resulting coated nanoparticles can have the desired elements
intermixed within the size scale of the nanoparticle, while the
stoichiometry (and thus the phase) of the coated nanoparticle may
be tuned by controlling the thickness of the coating(s).
[0045] In certain embodiments of the invention, the sub-layers may
be annealed, either sequentially or simultaneously. According to
embodiments of the present invention, the sub-layer(s) may be
annealed by rapid heating from an ambient temperature to a plateau
temperature range of between about 200.degree. C. and about
600.degree. C. The temperature is maintained in the plateau range
for a period of time ranging between about a fraction of a second
to about 60 minutes, and subsequently reduced. Alternatively, the
annealing temperature could be modulated to oscillate within a
temperature range without being maintained at a particular plateau
temperature. This technique (referred to herein as rapid thermal
annealing or RTA) is particularly suitable for forming photovoltaic
active layers (sometimes called "absorber" layers) on metal foil
substrates, such as aluminum foil. Additional details of this
technique are described in U.S. patent application Ser. No.
10/943,685, which is incorporated herein by reference.
[0046] Other alternative embodiments of the invention utilize
techniques other than printing processes to form the absorber
layer. For example, a group IB, group IIIA and/or group VIA
elements can be deposited onto the top surface of a substrate
and/or onto the top surface of one or more of the sublayers of the
active layer by atomic layer deposition (ALD). For example a thin
layer of Ga may be deposited by ALD at the top of a stack of
sub-layers formed by printing techniques. By use of ALD, copper,
indium, gallium, and selenium can be deposited in a precise
stoichiometric ratio that is intermixed at or near the atomic
level.
[0047] Furthermore, by changing sequence of exposure pulses for
each precursor material, the relative composition of C, I, G and S
within each atomic layer can be systematically varied as a function
of deposition cycle and thus depth within the absorber layer. Such
techniques are described in US Patent Application Publication
20050186342, which is incorporated herein by reference.
Alternatively, the top surface of a substrate could be coated by
using any of a variety of vacuum-based deposition techniques,
including but not limited to sputtering, evaporation, chemical
vapor deposition, physical vapor deposition, electron-beam
evaporation, and the like.
[0048] In another embodiment, the various sub-layers of a
IB-IIIA-VIA compound film for use as a light-absorbing layer in a
photovoltaic device, may be formed using a composition of matter in
the form of a dispersion containing a mixture of elemental
nanoparticles of the IB, the IIIA, and, optionally, the VIA group
of elements, dispersed with a suspension of nanoglobules of
Gallium. Based on the relative ratios of input elements, the
gallium nanoglobule-containing dispersion can then have a
Cu/(In+Ga) compositional ratio ranging from 0.01 to 1.0 and a
Ga/(In+Ga) compositional ratio ranging from 0.01 to 1.0. This
technique is described in commonly-assigned U.S. patent application
Ser. No. 11/081,163, which has been incorporated herein by
reference.
[0049] As illustrated in FIG. 1A a mixture 106 of elemental
nanoparticles 102 of the IB, the IIIA, and, optionally, the VIA
group of elements is formed by combining nanoparticles 102
comprised of the element of the IB group of elements (e.g. Copper)
with nanoparticles comprised of the element of the IIIA group of
elements (e.g. indium) and optionally nanoparticles comprised of
the VIA group of elements (e.g. Selenium, Sulfur and/or
Tellurium).
[0050] The mixture of elemental nanoparticles 102 may optionally
include Aluminum, Tellurium, or Sulfur. For example, any or all of
these classes of nanoparticles may be optionally doped with one or
more inorganic materials. Examples of such inorganic materials
include but are not limited to Aluminum (Al), Sulfur (S), Sodium
(Na), Potassium (K), or Lithium (Li). The nanoparticles 102
typically have average diameters ranging from 10 nm to 1000 nm,
more preferably from 10 nm to 500 nm, and most preferably, from 10
nm to 100 nm. Preferably, the nanoparticles are less than about 500
nm in diameter, more preferably less than about 100 nm and even
more preferably less than about 50 nm. Various techniques may be
used for making such nanoparticles, including but not limited to
dry-synthesis techniques such as evaporation-condensation
(Granqvist and Buhrman, 1976, "Ultrafine metal particles", J.
Applied Physics 47 (5): 220-2219, which is incorporated herein by
reference) and the electro-explosion of wire (Tepper, "Nanosized
Alumina Fibers", Advanced Materials, American Ceramic Society
Bulletin, Vol. 80, No. 6, June 2001, which is incorporated herein
by reference), and wet chemical techniques, including but not
limited to thermal decomposition of organometallic precursors
(Hambrock et al, 2002, Chem. Commun. 68-69, which is incorporated
by reference) and metal salt reduction (Cai et al., 2004. Applied
Surface Science 226: 422-426; Capek, 2004. Adv. In Colloid and
Interface Science 110: 49-74, both of which are incorporated by
reference). In addition, nanoparticles of certain elements, such as
copper are commercially available from a variety of vendors in the
form of nano-powders.
[0051] The nanoparticles 102 are mixed together and with one or
more chemicals 104 including but not limited to surfactants
(surface active agents such as dispersants, anti-flocculants,
wetting agents and the like), polymers, binders, cross-linking
agents, emulsifiers, anti-foaming agents, dryers, solvents,
fillers, extenders, thickening agents, film conditioners,
anti-oxidants, flow agents, leveling agents, and corrosion
inhibitors.
[0052] Dispersants are used to prevent ultrafine flocculating
particles from coalescing, thus facilitating the suspension of
solid materials in a liquid medium and stabilizing the dispersion
thereby produced. If particle surfaces attract one another, then
flocculation occurs, resulting in aggregation and decreasing
stability and/or homogeneity. If particle surfaces repel one
another, then stabilization occurs, where fine particles do not
aggregate and tend not to settle out of solution as fast.
[0053] An efficient dispersing agent can typically perform pigment
wetting, dispersing, and stabilizing. Dispersing agents are
different depending on the nature of the ink/paint. Polyphosphates,
styrene-maleinates and polyacrylates are often used for aqueous
formulations whereas fatty acid derivatives and low molecular
weight modified alkyd and polyester resins are often used for
organic formulations.
[0054] Wetting agents lower the surface tension of the solvent in
which they dissolve, and keep the surface tension of an (aqueous)
medium low so that an ink interacts with a substrate surface.
Certain types of wetting agents are also used as dispersing agents.
Wetting agents typically contain both a hydrophobic carbon chain
and a hydrophilic polar group. The polar group can be non-ionic. If
the polar group is ionic, the charge can be either positive or
negative, resulting in cationic or anionic surfactants.
Zwitterionic surfactants contain both positive and negative charges
within the same molecule; one example is N-n-Dodecyl-N,N-diemthyl
betaine. Certain surfactants are often used as dispersant agents
for aqueous solutions. Representative classes include acetylene
diols, fatty acid derivatives, phosphate esters, sodium
polyacrylate salts, polyacrylic acids, soya lecithin,
trioctylphosphine (TOP), and trioctylphosphine oxide (TOPO).
[0055] Binders and resins are often used to hold together proximate
particles in a nascent or formed dispersion. Examples of typical
binders include acrylic monomers (both as monofunctional diluents
and multifunctional reactive agents), acrylic resins (e.g. acrylic
polyol, amine synergists, epoxy acrylics, polyester acrylics,
polyether acrylics, styrene/acrylics, urethane acrylics, or vinyl
acrylics), alkyd resins (e.g. long-oil, medium-oil, short-oil, or
tall oil), adhesion promoters such as polyvinyl pyrrolidone (PVP),
amide resins, amino resins (such as melamine-based or urea-based
compounds), asphalt/bitumen, butadiene acrylonitriles, cellulosic
resins (such as cellulose acetate butyrate (CAB)), cellulose
acetate proprionate (CAP), ethyl cellulose (EC), nitrocellulose
(NC), or organic cellulose ester), chlorinated rubber, dimer fatty
acids, epoxy resin (e.g. acrylates, bisphenol A-based resins, epoxy
UV curing resins, esters, phenol and cresol (Novolacs), or
phenoxy-based compounds), ethylene co-terpolymers such as ethylene
acrylic/methacrylic Acid, E/AA, E/M/AA or ethylene vinyl acetate
(EVA), fluoropolymers, gelatin (e.g. Pluronic F-68 from BASF
Corporation of Florham Park, N.J.), glycol monomers, hydrocarbon
resins (e.g. aliphatic, aromatic, or coumarone-based such as
indene), maelic resins, modified urea, natural rubber, natural
resins and gums, rosins, modified phenolic resins, resols,
polyamide, polybutadienes (liquid hydroxyl-terminated), polyesters
(both saturated and unsaturated), polyolefins, polyurethane (PU)
isocyanates (e.g. hexamethylene diisocynate (HDI), isophorone
diisocyanate (IPDI), cycloaliphatics, diphenylmethane disiocyanate
(MDI), toluene diisocynate (TDI), or trimethylhexamethylene
diisocynate (TMDI)), polyurethane (PU) polyols (e.g. caprolactone,
dimer-based polyesters, polyester, or polyether), polyurethane (PU)
dispersions (PUDs) such those based on polyesters or polyethers,
polyurethane prepolymers (e.g. caprolactone, dimer-based
polyesters, polyesters, polyethers, and compounds based on urethane
acrylate), Polyurethane thermoplastics (TPU) such as polyester or
polyether, silicates (e.g. alkyl-silicates or water-glass based
compounds), silicones (amine functional, epoxy functional, ethoxy
functional, hydroxyl functional, methoxy functional, silanol
functional, or cinyl functional), styrenes (e.g. styrene-butadiene
emulsions, and styrene/vinyl toluene polymers and copolymers), or
vinyl compounds (e.g. polyolefins and polyolefin derivatives,
polystyrene and styrene copolymers, or polyvinyl acetate
(PVAC)).
[0056] Emulsifiers are dispersing agents that blend liquids with
other liquids by promoting the breakup of coalescing materials into
small droplets and therefore stabilize the suspension in solution.
For example, sorbitan esters are used as an emulsifier for the
preparation of water-in-oil (w/o) emulsions, for the preparation of
oil absorption bases (w/o), for the formation of w/o type pomades,
as a reabsorption agent, and as a non toxic anti-foaming agent.
Examples of emulsifiers are sorbitan esters such as sorbitan
sesquioleate (Arlacel 60), sorbitan sesquioleate (Arlacel 83),
sorbitan monolaurate (Span 20), sorbitan monopalmitate (Span 40),
sorbitan monostearate (Span 60), sorbitan tristearate (Span 65),
sorbitan mono-oleate (Span 80), and sorbitan trioleate (Span 85)
all of which are available, e.g., from Uniqema of New Castle, Del.
Other polymeric emulsifiers include polyoxyethylene monostearate
(Myrj 45), polyoxyethylene monostearate (Myrj 49), polyoxyl 40
stearate (Myrj 52), polyoxyethylene monolaurate (PEG 400),
polyoxyethylene monooleate (PEG 400 monoleate) and polyoxyethylene
monostearate (PEG 400 monostearate), and the Tween series of
surfactants including but not limited to polyoxyethylene sorbitan
monolaurate (Tween 20), polyoxyethylene sorbitan monolaurate (Tween
21), polyoxyethylene sorbitan monopalmitate (Tween 40),
polyoxyethylene sorbitan monostearate (Tween 60), polyoxyethylene
sorbitan tristearate (Tween 61), polyoxyethylene sorbitan
mono-oleate (Tween 80), polyoxyethylene sorbitan monooleate (Tween
81), and polyoxyethylene sorbitan tri-oleate (Tween 85) all of
which are available, e.g., from Uniqema of New Castle, Del.
Arlacel, Myrj, and Tween are registered trademarks of ICI Americas
Inc. of Wilmington, Del.
[0057] Foam may form from the release of various gases during the
coating/printing process, especially if the printing process takes
place at high speeds. Surfactants may adsorb on the liquid-air
interface and stabilize it, accelerating foam formation.
Anti-foaming agents prevent foaming from being initiated, while
defoaming agents minimize or eliminate previously-formed foam.
Anti-foaming agents include hydrophobic solids, fatty oils, and
certain surfactants, all of which penetrate the liquid-air
interface to slow foam formation. Anti-foaming agents also include
silicate, silicone and silicone-free materials. Silicone-free
materials include microcrystalline wax, mineral oil, polymeric
materials, and silica- and surfactant-based materials.
[0058] Solvents can be aqueous (water-based) or non-aqueous
(organic). While environmentally friendly, water-based solutions
carry the disadvantage of a relatively higher surface tension than
organic solvents, making it more difficult to wet substrates,
especially plastic substrates. To improve substrate wetting with
polymer substrates, surfactants are added to lower the ink surface
tension (while minimizing surfactant-stabilized foaming), while the
substrate surfaces are modified to enhance their surface energy
(e.g. by corona treatment). Typical organic solvents include
acetate, acrylates, alcohols (butyl, ethyl, isopropyl, or methyl),
aldehydes, benzene, dibromomethane, chloroform, dichloromethane,
dichloroethane, trichloroethane, cyclic compounds (e.g.
cyclopentanone or cyclohexanone), esters (e.g. butyl acetate or
ethyl acetate), ethers, glycols (such as ethylene glycol or
propylene glycol), hexane, heptane, aliphatic hydrocarbons,
aromatic hydrocarbons, ketones (e.g. acetone, methyl ethyl ketone,
or methyl isobutyl ketone), natural oils, terpenes, terpinol,
toluene.
[0059] Additional components may include fillers/extenders,
thickening agents, rheology modifiers, surface conditioners,
including adhesion promoters/bonding, anti-gelling agents,
anti-blocking agents, antistatic agents, chelating/complexing
agents, corrosion inhibitors, flame/rust inhibitors, flame and fire
retardants, humectants, heat stabilizers, light-stabilizers/UV
absorbers, lubricants, pH stabilizers, and materials for slip
control, anti-oxidants, and flow and leveling agents.
[0060] To provide gallium for the composition, gallium nanoglobules
114 are formed by initially creating an emulsion 112 of liquid
gallium 108 in a solution 110 as shown in FIGS. 1B-1C. In
particular, gallium metal or gallium metal in a solvent with or
without emulsifier is heated to liquefy the metal, which is then
sonicated and/or otherwise mechanically agitated in the presence of
a solvent and optionally an emulsifier. Agitation can also be
carried out either mechanically or electromagnetically in the
presence of a solvent with or without a surfactant, dispersant,
and/or emulsifier. As gallium emulsifies, nanoglobules 114 are
formed in this environment. The emulsion 112 containing liquid
gallium nanoglobules 114 may be added to the mixture 106. In some
embodiments, the gallium nanoglobules 114 may first be manipulated
to form a solid-particulate, e.g., by quenching in an environment
either at or below room temperature. When quenched, liquid gallium
nanoglobules are converted to solid gallium nanoparticles. In
addition, the dispersion of liquid gallium in solution may be
modified. Techniques to modify the dispersion of liquid gallium in
a solution include stirring, quenching, sonication, and/or the
addition of melting point modifiers which can be used separately or
in concert to reduce the size and/or narrow the size distribution
of the Ga globules into a particular range within any of several
spatial scales including but not limited to the nanoscale,
sub-micron scale, and/or micron scale. Roughly spheroidal
nanoglobules can be formed with average diameters ranging from
about 10 nm to about 1000 nm. The gallium nanoglobules are then
mixed with the nanoparticle mixture 106 as shown in FIG. 1D,
forming a dispersion 116 as shown in FIG. 1E. Based on the relative
ratios of the input elements, the gallium nanoglobule-containing
dispersion 116 can then have a Cu/(In+Ga) compositional ratio
ranging from 0.01 to 1.0 and a Ga/(In+Ga) compositional ratio
ranging from 0.01 to 1.0.
[0061] To form one of the sub-layers of a light-absorbing layer,
the dispersion 116 may then be deposited onto a substrate 118 with
a contact layer 119 to form a layer 120 as shown in FIG. 2A, using
any of a variety of coating methods including but not limited to
dip coating, spin coating, web coating, doctor blade coating, cup
coating, spray coating, printing, and the like. These coating
methods may also be used for deposition of nano-particle based inks
formulated using any of the other techniques described above. The
uniformity of film thickness of the layer 120 can be improved by
use of any of a variety of printing techniques including but not
limited to microgravure, reverse microgravure, comma, slot/die
coating, lip coating, and the like. The substrate 118 may be
flexible, in the form of a metal foil (e.g. Al, Mo, Ti, or
stainless steel), a plastic foil, or a metallized plastic foil, or
the substrate may be rigid, e.g. a glass plate or other rigid
surface. Where the substrate 118 is made of a metal such as
aluminum, a contact layer 119 comprised of a different metal must
be also present, e.g., molybdenum, which may be disposed between
the substrate 118 and the film 120.
[0062] The layer 120 is then reacted in a suitable atmosphere to
form a compound film 122. For example, upon printing, the layer 120
is then heated to remove any solvent and other added organic or
volatile materials from the dispersion 116 and to anneal it into a
compound film 122 as shown in FIG. 2B. During this step, in the
case of a metal foil, the contact layer 119 inhibits the diffusion
of metal or other contaminants from the substrate 118 into the
compound film 122. Either during or after this heating step, the
layer 120 (if during the heating step) or compound film 122 (if
after the heating step) may be reacted in at least one suitable
atmosphere 124. By way of example, the suitable atmosphere 124 may
contain at least one of selenium, sulfur, or tellurium, and/or
hydride gases comprising Se and/or S and/or Te, or mixtures of
these gases.
[0063] One of the advantages of the use of nanoparticle-based inks
is that it is possible to vary the concentration of the elements
within the absorber layer by varying the relative elemental
concentrations of the nanoparticles that make up the ink. Thus, for
example, the concentration of gallium within the absorber layer may
be varied as a function of depth within the absorber layer. For
example, as depicted in FIGS. 2E-2J, the steps of FIGS. 2A-2B may
be repeated using inks with different relative concentrations
gallium and/or indium and/or copper. For example, as shown in FIGS.
2E-2F first compound film 122A may be formed from an initial
sub-layer 120A as described above with respect to FIGS. 2A-2B. The
first compound film may have a Cu:In:Ga ratio of 92:0:100 (i.e., 92
parts copper to 0 parts indium to 100 parts gallium). A second
sub-layer 120B may be then be formed from a nanoparticle ink having
a Cu:In:Ga ratio of 92:70:30 as shown in FIG. 2G. The second
sub-layer 120B may be annealed as shown in FIG. 2H to become part
of a modified compound film. A third sub-layer 120C may be formed
from a nanoparticle ink having a Cu:In:Ga ratio of 52:20:80 as
shown in FIG. 21. Third layer 120C may be annealed as shown in FIG.
2J to become part of a final compound film 122C. The annealing
process may be RTA, as discussed above. Furthermore, the annealing
process may also involve exposure of the compound film to a gas
such as H.sub.2, N.sub.2, H.sub.2Se or Se vapor.
[0064] Not all of the sub-layers need to be deposited using
nanoparticulate-based inks. In some embodiments, the third layer
120C or one or more of the other sub-layers may be deposited using
any of a variety of alternative deposition techniques including but
not limited to vapor deposition techniques such as ALD,
evaporation, sputtering, CVD, PVD, electroplating and the like.
Such techniques may also be used to deposit additional sub-layers
between the printed layers, e.g., between the first sub-layer 120A
and the second sub-layer 120B or between the second sub-layer 120B
and the third sub-layer 120C.
[0065] Embodiments of the invention are compatible with
roll-to-roll manufacturing, as depicted in FIG. 2K. Specifically,
in a roll-to-roll manufacturing system 200 a flexible substrate
201, e.g., aluminum foil travels from a supply roll 202 to a
take-up roll 204. In between the supply and take-up rolls, the
substrate 201 passes a number of applicators 206A, 206B, 206C, e.g.
microgravure rollers and heater units 208A, 208B, 208C. Each
applicator deposits a precursor for a sub-layer of a photovoltaic
device active layer, e.g., as described above. The heater units are
used to anneal the different sub-layers. In the example depicted in
FIG. 2K, three different sub-layers may be applied and each
sub-layer may be annealed before the next sub-layer is deposited.
Alternatively, all three sub-layers may be annealed at the same
time.
[0066] The result of these steps is an absorber layer 122C having a
gallium concentration that increases with distance from the
substrate 118. In the embodiments depicted with respect to FIGS.
2E-2J selenium may be incorporated within the absorber layer in
between thin film depositions (e.g., by exposure of the film to
Selenium vapor and/or H.sub.2Se gas in conjunction with the
annealing) or after the last thin film deposition, or both.
[0067] The total number of printing steps can be modified to
construct absorber layers with bandgaps of differential gradation.
For example, additional films (fourth, fifth, sixth, and so forth)
can be printed (and optionally annealed between printing steps) to
create an even more finely-graded bandgap within the absorber
layer. Alternatively, fewer films (e.g. double printing) can also
be printed to create a less finely-graded bandgap.
[0068] Referring again to FIGS. 2A-2D, the compound film 122 may be
used as the light-absorbing layer in a photovoltaic device. For
example, as shown in FIG. 2C a window layer 126 may be deposited
onto the compound film 122 as a junction partner between the
compound film 122 and a transparent conducting layer 128 as shown
in FIG. 2D. By way of example, the window layer 126 (sometimes
referred to as a junction partner layer) may include inorganic
materials such as cadmium sulfide (CdS), zinc sulfide (ZnS), zinc
hydroxide, zinc selenide (ZnSe), n-type organic materials, or some
combination of two or more of these or similar materials, or
organic materials such as n-type polymers and/or small molecules.
Layers of these materials may be deposited, e.g., by chemical bath
deposition (CBD) or chemical surface deposition, to a thickness
ranging from about 2 nm to about 1000 nm, more preferably from
about 5 nm to about 500 nm, and most preferably from about 10 nm to
about 300 nm.
[0069] The transparent conductive layer 128 may be inorganic, e.g.,
a transparent conductive oxide (TCO) such as indium tin oxide
(ITO), fluorinated indium tin oxide, zinc oxide (ZnO) or aluminum
doped zinc oxide, or a related material, which can be deposited
using any of a variety of means including but not limited to
sputtering, evaporation, CBD, electroplating, sol-gel based
coating, spray coating, chemical vapor deposition (CVD), physical
vapor deposition (PVD), atomic layer deposition (ALD), and the
like. Alternatively, the transparent conductive layer may include a
transparent conductive polymeric layer, e.g. a transparent layer of
doped PEDOT (Poly-3,4-Ethylenedioxythiophene), carbon nanotubes or
related structures, or other transparent organic materials, either
singly or in combination, which can be deposited using spin, dip,
or spray coating, and the like. Combinations of inorganic and
organic materials can also be used to form a hybrid transparent
conductive layer. Examples of such a transparent conductive layer
are described e.g., in commonly-assigned US Patent Application
Publication Number 20040187917, which is incorporated herein by
reference.
[0070] FIG. 3 depicts an example of an optoelectronic device 300,
e.g., a photovoltaic cell, incorporating a compound film fabricated
as described above. The device 300 includes a base substrate 302,
an optional adhesion layer 303, a base electrode 304, an absorber
layer 306 incorporating a compound film of the type described
above, a window layer 308 and a transparent electrode 310. By way
of example, the base substrate 302 may be made of a metal foil, a
polymer such as such as polyimides (PI), polyamides,
polyetheretherketone (PEEK), Polyethersulfone (PES), polyetherimide
(PEI), polyethylene naphtalate (PEN), Polyester (PET), related
polymers, or a metallized plastic. The base electrode 304 is made
of an electrically conducive material. By way of example, the base
electrode 304 may be of a metal layer whose thickness may be
selected from the range of about 0.1 micron to about 25 microns. An
optional intermediate layer 303 may be incorporated between the
electrode 304 and the substrate 302. The transparent electrode 310
may include a transparent conductive layer 309 and a layer of metal
(e.g., Al, Ag or Ni) fingers 311 to reduce sheet resistance.
[0071] As described with respect to FIGS. 2E-2J, the concentrations
of group IB, group IIIA and group VIA elements in the IB-IIIA-VIA
absorber layer 306 vary with respect to depth to achieve a graded
bandgap. For the purposes of the following discussion, the base
electrode 304 and/or adhesion layer 303 is sometimes referred to as
the "back" contact of the device 300. Similarly, the transparent
electrode 310 is sometimes referred to as the "front" contact of
the device 300. It is also noted that the base electrode 304 and
adhesion layer 303 are sometimes referred to as being at or near a
"back end" of the device 300. Consequently, regions of the absorber
layer 306 that are close to the base electrode 304 and adhesion
layer 303 are sometimes referred to as a "back region" of the
absorber layer 306. Similarly, the window layer 308 and transparent
electrode 310 are sometimes referred to as being at the "front end"
of the device 300 and regions of the absorber layer 306 proximate
the window layer 308 are sometimes referred to as a "front region."
A portion of the absorber layer 306 intermediate the front and back
regions is referred to herein as a "central region".
[0072] As described above, the sequential printing of discrete
nanoparticulate inks or paints where each ink has different
elemental and/or alloy concentrations can be used to form an
aggregate stack of compound films where, for example, the Ga
content is higher at and near both the front and back contacts of
the absorber layer 306, but minimized in the central region of the
absorber layer 306, resulting in a "saddle" profile for Ga
concentration as a function of depth within the absorber layer 306,
e.g., as shown in the graph depicted in FIG. 4. In the example
depicted in FIG. 4, an absorber layer may include a first Ga-rich
region 402 in the vicinity of the back end, a second Ga-rich region
404 in the vicinity of the front end, a relatively Ga-deficient
central region 406 between the first and second Ga-rich regions
402, 404 in the vicinity of the front end. By way of example, the
first Ga-rich region 402 may be characterized by a thickness of up
to about 500 nm and a Ga to In+Ga ratio of between about 10% and
about 90%. The second Ga-rich region 404 may between about 5 nm and
about 50 nm thick and have a ratio of Ga to In+Ga of between about
20% and about 80%. The central region 406 has a ratio of Ga to
In+Ga of less than about 5%. The central region 406 may be up to
about 1000 nm thick.
[0073] Furthermore, as described above, the sequential printing of
discrete nanoparticulate inks or paints where each ink has
different elemental and/or alloy concentrations can be used to form
an aggregate stack of compound films where the indium and/or copper
concentration varies as a function of depth within the aggregate
absorber stack. For example, the indium concentration in the
absorber layer may be characterized as a ratio of indium to copper.
In the first and second Ga-rich regions 402, 404, the ratio of In
to Cu may be roughly three parts indium to one part copper. The
overall indium level in the first gallium-rich region 402 and the
selenium-rich region 408 is typically less than in other regions of
the absorber layer. In the central region 406 the In:Cu ratio is
roughly 0.88-0.92 parts Cu to one part In. The concentration of Cu
may be measured as an atomic ratio of Cu to the group IIIA elements
(e.g., In and Ga). This ratio may be as large as 90%. The ratio of
Se to Cu may be roughly constant over most of the absorber layer
but may be slightly higher in a Se-rich region 408 near the front
of the device that overlaps partly with the second Ga-rich region
404 as a result of a slightly lower concentration of Cu.
[0074] The thicknesses and concentration ratios set forth above are
presented for the purpose of example and in no way limit the
invention. Those of skill in the art will recognize that the
particular concentrations and thicknesses may be adjusted to
optimize power conversion efficiency, open circuit voltage,
short-circuit current density, fill factor, grain size, charge
mobility and other functional and/or structural parameters of the
device 300.
[0075] Although in part of the preceding section the absorber layer
306 is described as including copper, indium and gallium, those of
skill in the art will recognize that the advantages of varying the
stoichiometric concentration as a function of depth may be extended
more generally to absorber layers of the IB-IIIA-VIA type. As such,
embodiments of the invention should not be limited to absorber
layers containing copper, indium and gallium. For example, by
varying their concentrations in discretely formed and sequentially
applied inks or paints, materials such as aluminum, tellurium,
sulfur, and/or other elements or alloys or compounds can also be
incorporated into the absorber layer where any or all of these
elements or alloys or compounds is tuned to have a varying
concentration as a function of depth in the absorber layer.
[0076] While the above is a complete description of the preferred
embodiment of the present invention, it is possible to use various
alternatives, modifications and equivalents. Any feature, whether
preferred or not, can be combined with any other feature, whether
preferred or not. Therefore, the scope of the present invention
should be determined not with reference to the above description
but should, instead, be determined with reference to the appended
claims, along with their full scope of equivalents. In the claims
that follow, the indefinite article "A", or "An" refers to a
quantity of one or more of the item following the article, except
where expressly stated otherwise. The appended claims are not to be
interpreted as including means-plus-function limitations, unless
such a limitation is explicitly recited in a given claim using the
phrase "means for."
* * * * *