U.S. patent application number 12/978801 was filed with the patent office on 2011-08-04 for formation of cigs absorber layer materials using atomic layer deposition and high throughput surface treatment on coiled flexible substrates.
Invention is credited to Craig Leidholm, Martin R. Roscheisen, Brian M. Sager.
Application Number | 20110189815 12/978801 |
Document ID | / |
Family ID | 34861045 |
Filed Date | 2011-08-04 |
United States Patent
Application |
20110189815 |
Kind Code |
A1 |
Sager; Brian M. ; et
al. |
August 4, 2011 |
FORMATION OF CIGS ABSORBER LAYER MATERIALS USING ATOMIC LAYER
DEPOSITION AND HIGH THROUGHPUT SURFACE TREATMENT ON COILED FLEXIBLE
SUBSTRATES
Abstract
An absorber layer may be formed on a substrate using atomic
layer deposition reactions. An absorber layer containing elements
of groups IB, IIIA and VIB may be formed by placing a substrate in
a treatment chamber and performing atomic layer deposition of a
group IB element and/or one or more group IIIA elements from
separate sources onto a substrate to form a film. A group VIA
element is then incorporated into the film and annealed to form the
absorber layer. The absorber layer may be greater than about 25 nm
thick. The substrate may be coiled into one or more coils in such a
way that adjacent turns of the coils do not touch one another. The
coiled substrate may be placed in a treatment chamber where
substantially an entire surface of the one or more coiled
substrates may be treated by an atomic layer deposition process.
One or more group IB elements and/or one or more group IIIA
elements may be deposited onto the substrate in a
stoichiometrically controlled ratio by atomic layer deposition
using one or more self limiting reactions.
Inventors: |
Sager; Brian M.; (Menlo
Park, CA) ; Roscheisen; Martin R.; (San Francisco,
CA) ; Leidholm; Craig; (Sunnyvale, CA) |
Family ID: |
34861045 |
Appl. No.: |
12/978801 |
Filed: |
December 27, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10943658 |
Sep 18, 2004 |
7858151 |
|
|
12978801 |
|
|
|
|
10782545 |
Feb 19, 2004 |
7115304 |
|
|
10943658 |
|
|
|
|
Current U.S.
Class: |
438/95 ;
257/E31.008 |
Current CPC
Class: |
C25D 7/0614 20130101;
Y02E 10/541 20130101; H01L 31/1876 20130101; C23C 16/4581 20130101;
H01L 31/1864 20130101; Y02P 70/521 20151101; C23C 16/405 20130101;
H01L 31/0392 20130101; H01L 31/0322 20130101; Y02P 70/50 20151101;
C23C 16/458 20130101; C23C 16/545 20130101; C25D 17/02 20130101;
H01L 31/022425 20130101; C23C 16/45544 20130101; H01L 31/03926
20130101 |
Class at
Publication: |
438/95 ;
257/E31.008 |
International
Class: |
H01L 31/0272 20060101
H01L031/0272 |
Claims
1. A method for forming an absorber layer containing elements of
groups IB, IIIA and VIA, comprising the steps of: atomic monolayer
resolution tuning of a bandgap grading of a precursor layer on a
substrate, wherein the tuning occurs with spatial uniformity and
resolution through deposition of partial atomic monolayers with
aggregate growth rate directly proportional to a number of reaction
cycles rather than the pressure or concentration of precursor gases
in the chamber, with control over film thickness, film uniformity,
and conformality; wherein after exposure to a precursor agent and
then a reducing agent, atoms from each of the partial atomic
monolayers occupy only a portion of all deposition sites on a
target surface; repeating atomic monolayer deposition until a
desired atomically-graded deposition profile is formed in the
precursor layer.
2. The method of claim 1 wherein the absorber layer is between
about 25 nm and about 5000 nm thick.
3. The method of claim 1 wherein the absorber layer is between
about 25 nm and about 3000 nm thick.
4. The method of claim 1 wherein the absorber layer is between
about 100 nm and about 2000 nm thick.
5. The method of claim 1 wherein the absorber layer is between
about 500 nm and about 2000 nm thick.
6. The method of claim 1 wherein the absorber layer is between
about 1000 nm and about 2000 nm thick.
7. The method of claim 1 wherein the substrate is coiled in the
treatment chamber.
8. The method of claim 7 wherein the precursor layer is formed by
atomic layer deposition carried out in a stoichiometrically
controlled ratio using one or more self-limiting reactions
involving precursor gases of the group IB and group IIIA elements
in a mix ratio that translates into a deposition ratio of the group
IB and IIIA elements on the substrate, and/or by an atomic layer
deposition sequence involving two or more self-limiting single
species deposition reactions with precursor gases of the group IB
and group IIIA elements; and incorporating an element of group VIA
into the absorber layer.
9. The method of claim 7 wherein the element of group VIA is
selenium or sulfur.
10. The method of claim 7 wherein incorporating the element of
group VIA into the absorber layer includes exposing the film to
selenium vapor, sulfur vapor, H.sub.2Se, H.sub.2S, one or more
other selenium- or sulfur-containing compounds, or combinations or
mixtures of two or more of these.
11. The method of claim 7 wherein incorporating the element of
group VIA into the absorber layer involves using one or more
precursor gases containing one or more elements of group VIA.
12. The method of claim 11 wherein the element of group VIA is
incorporated into the absorber film through a sequence of atomic
layer deposition steps.
13. The method of claim 11 wherein the sequence of atomic layer
deposition steps includes the use of one or more metal organic
precursors containing selenium, sulfur, H.sub.2Se, H.sub.2S, one or
more other selenium- or sulfur-containing compounds, or
combinations or mixtures of two or more of these.
14. The method of claim 13 wherein the one or more metal organic
precursors containing selenium or sulfur include dimethyl selenide,
dimethyl diselenide, or diethyl diselenide.
15. The method of claim 14 wherein incorporating the element of
group VIA takes place either on a monolayer by monolayer basis, or
periodically, with an exposure period substantially longer than a
monolayer deposition cycle, or at the end of an absorber layer
deposition sequence.
16. The method of claim 1 wherein performing atomic layer
deposition includes exposing the substrate to one or more
precursors of copper, indium, and/or gallium, and/or aluminum,
and/or selenium, and/or sulfur.
17. The method of claim 16 wherein the precursors include one or
more Cu(I) compounds, one or more Cu(II) compounds, CuCl, copper
iodide, or other copper halides, one or more copper diketonates,
Cu(II)-2,2,6,6,-tetramethyl-3,5,-heptanedionate (Cu(thd).sub.2)),
Cu (II) 2,4-pentanedionate, Cu(II) hexafluoroacetylacetonate
(Cu(hfac).sub.2), Cu(II) acetylacetonate (Cu(acac).sub.2), Cu(II)
dimethylaminoethoxide, one or more copper ketoesters, one or more
organocopper precursors containing Si or Ge, one or more other
organocopper precursors and combinations or mixtures of the above,
indium chloride, indium iodide, one or more other indium halides,
dimethylindium chloride, trimethylindium, indium 2,4-pentanedionate
(indium acetylacetonate), indium hexafluoropentanedionate, indium
methoxyethoxide, indium methly(trimethylacetyl)acetate, indium
trifluoropentanedionate, one or more organoindium precursors
containing Si or Ge, one or more other organoindium precursors, and
combinations or mixtures of the above, diethylgallium chloride,
gallium triiodide, one or more other gallium halides, Ga (III)
2,4-pentanedionate, Ga (III) ethoxide, Ga(III)
2,2,6,6,-tetramethylheptanedionate, tris(dimethylaminogallium),
gallium (I) salts, gallium chloride, gallium fluoride, gallium
iodide, gallium acetate, other gallium (I)-based organometallic
precursors, one or more organogallium precursors containing Si or
Ge, one or more other organogallium precursors and combinations or
mixtures of the above, aluminum chloride, aluminum iodide, or other
halides, dimethylaluminum chloride, one or more aluminum butoxides,
aluminum di-s-butoxide ethylacetoacetate, aluminum diisopropoxide
ethylacetoacetate, aluminum ethoxide, aluminum isopropoxide,
aluminum hexafluoropentanedionate, Al(III) 2,4,-pentanedionate,
AI(III) 2,2,6,6-tetramethyl-3,5-heptanedionat-e, aluminum
trifluoroacetate, trisisobutylaluminum, aluminum silicate, one or
more organoaluminum or organometallic precursors containing Si or
Ge, one or more other organoaluminum or other organometallic
precursors and combinations or mixtures of the above.
18. The method of claim 16 further comprising, after exposing the
substrate to one or more organometallic precursors of copper,
indium, and/or gallium, and/or aluminum, and/or selenium, and/or
sulfur, exposing the substrate to one or more reducing agents or
proton-donor compounds.
19. The method of claim 18 wherein the one or more reducing agents
or proton donor compounds include water (H.sub.2O), hydrogen
peroxide (H.sub.2O.sub.2), methanol, ethanol, isopropyl alcohol,
one or more butyl alcohols, one or more other alcohols, carbon
monoxide (CO), Oxygen gas (O.sub.2), formalin, or combinations or
mixtures of two or more of these.
20. The method of claim 1 wherein the precursor layer is formed by
performing atomic layer deposition of one or more group IB elements
and one or more group IIIA elements and includes an atomic layer
deposition sequence involving two or more self-limiting single
species deposition reactions with precursor gases of the group IB
and group IIIA elements and varying the sequence of exposure pulses
of the precursor gases.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of commonly-assigned
co-pending U.S. patent application Ser. No. 10/943,658, which was
filed Feb. 19, 2004, the entire disclosure of which are
incorporated herein by reference. This application is a
continuation-in-part of commonly-assigned co-pending U.S. patent
application Ser. No. 10/782,545, which was filed Feb. 19, 2004, the
entire disclosures of which are incorporated herein by reference.
This application is also related to commonly-assigned, co-pending
application Ser. no. 10/782,233, titled "ROLL-TO-ROLL ATOMIC LAYER
DEPOSITION METHOD AND SYSTEM", which was filed Feb. 19, 2004, 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, titled "SOLUTION-BASED FABRICATION
OF PHOTOVOLTAIC CELL", which was filed Feb. 19, 2004, the entire
disclosures of which are incorporated herein by reference. This
application is also related to commonly-assigned co-pending
application Ser. No. 10/943,685, titled "FORMATION OF CIGS ABSORBER
LAYERS ON FOIL SUBSTRATES" (Attorney Docket NSL-038), which is
filed Sep. 18, 2004, the entire disclosures of which are
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention is directed to the deposition and/or
surface treatment of thin films on large area substrates and more
specifically to atomic layer deposition in a high-throughput
production system.
BACKGROUND OF THE INVENTION
[0003] Low-cost production of solar cells on flexible substrates
using printing or web coating technologies is promising highly
cost-efficient alternative to traditional silicon-based solar
cells.
[0004] A typical Copper-Indium-Gallium-diSelenide (CIGS) solar cell
structure includes a back electrode followed by a layer of
molybdenum (Mo). A CIGS absorber layer is sandwiched between the Mo
layer and a CdS junction partner layer. A transparent conductive
oxide (TCO) such as zinc oxide (ZnO.sub.x) or tin oxide (SnO.sub.2)
formed on the CdS junction partner layer is typically used as a
transparent electrode.
[0005] A central challenge in constructing a highly efficiency
CIGS-based solar cell is that the elemental components of the CIGS
layer must be present in a fairly narrow stoichiometric ratio
relative to one another in order for the resulting cell to be
highly efficient. It is difficult to control the stoichiometric
ratios and achieve high volume production with prior art
techniques. Furthermore, it would be highly desirable to vary the
composition of a CIGS absorber layer as a function of depth. 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.
[0006] More specifically, higher amounts of Ga deposited at or near
the back contact (e.g. near the Mo interface) of the CIGS cell
promotes improved device function in two ways: (1) smaller grains
form in the presence of Ga in this back region, and these smaller
grains are less-mechanically stressed at the back contact, thus
improving the mechanical stability of the cell and (2) the presence
of higher levels of CuGa at the back of the absorber layer also
acts as a carrier reflector, directing carriers forward to the
junction at the front of the absorber layer.
[0007] Second, a relatively high level of Ga in the middle of the
CIGS absorber layer negatively impacts device function, as small
CuGaSe grains form. These small grains tend to have a high defect
density and act as sites for charge recombination in the absorber
layer.
[0008] Third, high amounts of Ga deposited at or near the front
contact (e.g. near the TCO layer) of the CIGS cell can promote
improved device function in two ways: (1) a higher bandgap (e.g.
1.35 eV) near the front contact sets the voltage of the cell at a
relatively higher value than would otherwise exist, and (2) such a
higher voltage couples with a lower current results in the same
power conversion efficiency but with fewer I.sup.2R losses.
[0009] In the prior art, graded bandgap devices with graded
concentration profiles have been prepared using co-evaporation 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).
[0010] Unfortunately, there are several challenges and
disadvantages associated with evaporation as a deposition technique
for the CIGS absorber layer. For example, it is difficult to
deposit many compounds and alloy compositions by evaporation.
Evaporation deposition is limited to line-of-sight and limited-area
sources, which tends to result in poor surface coverage and spatial
non-uniformity. Line-of-sight trajectories and limited-area sources
can also result in poor film-thickness uniformity over large areas.
In addition, evaporation deposition techniques typically have
relatively few processing parameters that can be varied to tune the
properties of the resulting film. Furthermore, the intermixing of
the elements from evaporative sources occurs not only atomically
but also as the interaction of discrete atomic clusters, resulting
in locally non-uniform deposition in all three dimensions. Such
non-uniformity also alters the local stoichiometric ratios of the
absorber layer, decreasing the potential power conversion
efficiency of the complete device. Further, the use of evaporation
can result in uneven surfaces that can further degrade the device
performance.
[0011] Moreover, the use of evaporation as a deposition mechanism
to grade the bandgap provides at best a rough tuning. It is
extremely difficult, if not impossible to reproducibly control the
gradation of the individual components of the absorber layer at the
atomic or sub-monolayer level. Such reproducible control would be
desirable for the formation of precisely structured bandgap
gradients in the absorber layer. Reproducible, atomic-level
gradation of the relative amounts of deposited elements and/or
alloys would allow a higher-yield optimization of the completed
device. Finally, 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.
[0012] Chemical vapor deposition (CVD) of copper indium di-sulfide
(CIS) films was first reported in 1992, when a single-source
precursor was used in metal organic CVD to produce a
photo-responsive light absorbing film, though not in a solar cell
(see Nomura, R., Seki, Y. and H. Matsuda. 1992. Preparation of
CuInS.sub.2 Thin Films by Single-source MOCVD Process using
Bu.sub.2In(SPr)Cu(S.sub.2CNPr.sub.2). J. Mater. Chem. 2 (7):
765-766. Since the elements in the film derived from a single
source, the stoichiometric ratio of these elements in the resulting
films could not be altered relative to one another, leading to the
inability to tune and optimize the optoelectronic and electronic
properties of the deposited film. Further, no bandgap grading was
possible. Finally, sulfur rather than selenium was used in the
films, which limits the potential for large grain growth. In
particular, large grains only form in sulfur-containing CIS films
in Cu-rich regions. In such copper-rich regions, copper sulfide
forms, and this compound needs to be removed e.g. with a potassium
cyanide etch for proper device function.
[0013] More recently, Atomic Layer Deposition (ALD) has been used
in the fabrication of a film comprised of aggregates of CIS-coated
titania spheroids as the active layer of inorganic solar cells (see
e.g., Nanu, M., Schoonman, J. and A. Goossens. 2004. Inorganic
Nanocomposites of n- and p-type Semiconductors: A New Type of
Three-Dimensional Solar Cell. Adv. Mater. (5): 453-455 and Nanu,
M., Reijnen, L., Meeester, B., Schoonman, J. and A. Goossens. 2004.
Chemical Vapor Deposition 10 (1): 45-49) which are incorporated
herein by reference. In this approach, the aggregated titania
spheroids were coated with 25 nm of CIS film using ALD, a
high-quality thin-film deposition technique based on sequential,
self-limiting surface reactions. Atomic layer deposition works by
exposing a substrate sequentially to two or more reactant vapors or
solutions and maintaining the substrate temperature within, for
example, a temperature range that depends on the chemistry of the
particular ALD reaction. A typical ALD process involves a sequence
of two different and alternating surface reactions involving two
different gaseous reactants referred to herein as A and B. The ALD
system is typically purged of reactant gas in between reactions
with a non-reactive purge gas C, and/or is pumped clean of the
reactant gases. Sequencing the reactions provides precision in the
rate of deposition and allows the use of highly reactive reactants.
With each reactant exposure, a self-limiting reaction occurs on the
surface of the substrate if the substrate temperature is, for
example, within the right temperature range, or if alternative
energy sources are provided, such as energetic ions or molecules or
atoms, ozone, plasma, UV light, etc.
[0014] ALD can control the thickness of deposited films at the
level of an atomic or sub-atomic layer. Thus films deposited by ALD
tend to be uniform over large areas. In addition ALD allows
deposition of conformal films on structures having very high aspect
ratios (e.g., >>10). A wide variety of materials may be
deposited by ALD, including semiconductors, metals, oxides,
nitrides, and other materials. ALD techniques can thus deposit thin
films one atomic layer at a time, in a "digital" fashion. Such
"digital" build-up of material greatly simplifies thickness
control, thus reducing both the complexity and cost of thin film
deposition.
[0015] Many industries, such as the optoelectronics industry, can
benefit from the high uniformity, high aspect ratio conformal
coating abilities and low cost of ALD. Unfortunately, prior art ALD
systems have mostly been made for semiconductor wafer processing,
which is oriented to batch processed wafer handling systems.
Although existing ALD systems are suitable for the semiconductor
industry, they are unsuitable for high volume manufacturing of
large area devices such as photovoltaic cells. Current systems are
typically designed to coat small area wafers. Scaling up systems
that coat a small area at a time might not be practical for coating
large area sheets, panels or rolls of material. ALD may be too slow
and expensive overall, if only small area batch processing can be
performed. Further, surface treatments such as annealing, drying,
and exposure to reactive gases cannot be carried out at high-volume
for large-area substrates when the surface treatments and/or
reactions take place in a relatively smaller treatment chamber.
[0016] In particular, current ALD deposition approaches for the
construction of thin-film absorber materials in CIS solar cells are
limited to relatively thin films, e.g. less than 25 nm, since
thicker deposits of absorber material are impractical to form due
to the slow rate of deposition. Thin films of CIS material are not
optimal as an absorber component for solar cells, since the
majority of the light reaching a 25 nm thick absorber layer is not
absorbed by that material, limiting the potential power conversion
efficiency of such a solar cell. Further, CIS absorbers have a more
narrow bandgap and tighter processing conditions than CIGS
absorbers, where Ga is incorporated into the crystal structure of
the absorbing semiconductor layer. Finally, to achieve an efficient
power conversion efficiency, the relative ratios of the individual
elements comprising a CIS or CIGS film within a solar cell require
a graded distribution through the absorber layer depth. This is
very difficult or impossible to achieve with such a thin (e.g. 25
nm) absorber layer, for which it is inherently challenging to
establish initial elemental gradients with a proper distribution,
and where atomic intermixing at that length scale will tend to
minimize any elemental gradients.
[0017] Thus, there is a need in the art, for a high throughput
method and system for fabricating thick films of elementally-graded
and optimized CIGS solar cells produced by ALD.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The teachings of the present invention can be readily
understood by considering the following detailed description in
conjunction with the accompanying drawings, in which:
[0019] FIG. 1 is a 3-dimensional exploded view schematic diagram of
a high throughput atomic layer deposition system according to an
embodiment of the present invention.
[0020] FIGS. 2A-2D are a sequence of schematic diagrams
illustrating formation of a CIGS absorber layer according to an
embodiment of the present invention.
[0021] FIG. 3 is a schematic diagram of a photovoltaic cell
according to an embodiment of the present invention.
[0022] FIG. 4 is a graph illustrating concentration profiles for a
CIGS absorber layer.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0023] 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 exemplary embodiments of the invention
described below are set forth without any loss of generality to,
and without imposing limitations upon, the claimed invention.
[0024] According to embodiments of the present invention, an
absorber layer may be formed on a substrate using atomic layer
deposition reactions. An absorber layer containing elements of
groups IB, IIIA and VIB may be formed by placing a substrate in a
treatment chamber and performing atomic layer deposition of a group
IB element and/or one or more group IIIA elements from separate
sources onto a substrate to form a film. A group VIA element is
then incorporated into the film and annealed to form the absorber
layer. The absorber layer may be greater than about 25 nm thick.
According to embodiments of the present invention, the absorber
layer thickness between about 25 nm and about 5000 nm, more
preferably 25 nm to 3000 nm, even more preferably 100 nm to 2000
nm, still more preferably, 500 nm to 1000 nm and most preferably
1000 nm to 2000 nm. In some embodiments, a group IB element and two
different group IIIA elements (e.g., indium and gallium) may be
deposited using different sources. In such embodiments the absorber
layer may be between about 1 nm and about 5000 nm thick.
[0025] In atomic layer deposition systems, wherever the reactant
gases A, B come in contact with substrate, good coating may be
achieved. Consequently, ALD may be readily scaled to coating large
substrate areas in each reaction step. Thus, the throughput of an
ALD system may be increased by massively scaling the substrate
surface area processed during each step (as opposed to scaling up
the step speed for each deposition cycle and/or processing many
substrates in parallel via, e.g. the number of ALD reaction
chambers) This can be achieved by coiling a flexible substrate
(e.g., metal or alloy foil, e.g. Al, metalized plastic foil,
otherwise coated foils, foils with pre-deposited/processed surface
structure and/or patterning, laminates, etc.) in an ALD chamber in
such a way that adjacent `turns or windings` of the foil on the
carrier roll, cassette, or carousel do not touch one another. Gaps
between adjacent layers of the coiled substrate allow the reactant
gases to flow or diffuse into the gaps between adjacent turns and
thus reach and be adsorbed or react on the substrate surface(s) to
be coated. Each step of a sequence of one or more ALD reactions may
then be applied to a massive surface area of the substrate. The
basic concept behind the embodiments of present invention is an
implementation in which each step may be applied to the entire
surface area of the substrate, e.g., to an entire roll. The same
approach, with appropriate modifications, can be used to scale up
other surface treatment techniques such as substrate annealing,
drying, anodization, electro-deposition, electro-polymerization,
electro-polishing, cleaning, exposing to chemicals to treat the
surface (e.g. selenization of a substrate using H.sub.2Se gas or Se
vapor), solution treatments, treatments that require electric
fields/current/voltage, etc. Consequently, an entire CIGS absorber
layer as well as a window layer and front and back contacts may be
formed in one chamber without having to remove the substrate
between steps.
[0026] According to embodiments of the present invention, an
absorber layer for an optoelectronic device (e.g., a photovoltaic
cell) may be fabricated by atomic layer deposition of an absorber
layer material containing elements from groups IB, IIIA and VIA on
a coiled substrate. The coiling of the substrate increases the
surface area that can be treated at one time, thereby increasing
throughput and yield. In general one or more group IB elements
and/or one or more group IIIA elements are deposited by atomic
layer deposition in a stoichiometrically controlled ratio using one
or more self limiting ALD reactions. Two or more precursor gases of
the group IB and group IIIA elements may be intermixed in a mix
ratio that translates into a deposition ratio of the group IB and
IIIA elements on the substrate. Alternatively, the group IB and
group IIIA elements may be deposited by an atomic layer deposition
sequence involving two or more self-limiting single species
deposition reactions with precursor gases of the group IB and group
IIIA elements. In either case, the group VIA element may be
deposited by performing a reaction that incorporates an element of
group VIA into the absorber layer. In addition, embodiments of the
present invention include combination deposition sequences
involving both single species deposition reactions and mixed
species deposition reactions.
[0027] One way to achieve high-volume production in ALD systems of
the type described herein is to perform an ALD reaction on an
entire roll of substrate material at one time. To do this, it would
be useful to fit the entire length of a roll into the ALD chamber
so that the A and B half-reactions can be performed without having
to remove the substrate from the chamber. In another alternative
embodiment, an entire roll of substrate material may be treated at
one time by using a system of the type depicted in FIG. 1. The
system 100 includes a surface treatment chamber 102 and a carousel
104 for coiling a flexible substrate 106 in a way that allows gaps
between adjacent turns of the coil. Gas sources 108, 110 and 112
provide ALD reactants A, B and purge gas C to the chamber 102. One
or more robots 114 place stackable spacers 116 on the carousel 104.
An exhaust system 111 removes gas or liquid from within the chamber
102. The gas pressure within the chamber 102 may be adjusted by
appropriate control of the gas sources 108, 110 and 112 and the
exhaust system 111.
[0028] By way of example, the chamber 102 may include an inner wall
103 and an outer wall 105. The substrate 106 may be wound outside
the chamber 102 on the carousel 104, which fits between the inner
wall 103 and the outer wall 105. The carousel 104 may be loaded
into the chamber 102 through the top (or side). A lid 107 seals the
top of the chamber 102. By way of example, the chamber 102 may also
include equipment for pre-treatment of the substrate 106 by plasma,
UV-ozone, heat (e.g., infrared), corona or combinations thereof. In
addition, the chamber 102 may include equipment for performing one
or more treatment and/or coating steps that are performed prior or
subsequent to atomic layer deposition that is performed in the
chamber 102, including but not limited to substrate cleaning,
annealing, drying, and/or exposure to reactive gas such as Se vapor
or H.sub.2Se. Such post-ALD steps may include passivation, or
coating the ALD treated substrate 106 with, e.g., an organic or
inorganic material. In addition, the chamber 102 may be equipped
with additional gas low inlets, heaters (e.g., infrared heaters,
light sources, or ultraviolet radiation sources, sources for
energetic particles such as plasma ions, ozone, etc.) or cooling
mechanisms, such as fluid filled tubes or peltier effect
(thermoelectric) elements. Furthermore, the chamber 102 may be part
of a much larger coating line that may include other equipment for
performing pre-ALD and post-ALD treatment of the substrate 106. The
wound substrate can also be surface-treated in the absence of ALD
processes, e.g. by carrying out substrate cleaning, and/or
annealing, and/or drying, and/or exposure to reactive gas (such as
Se vapor or H.sub.2Se) in the absence of ALD. Further, the wound
substrate can also be surface-treated in the absence of ALD
processes, e.g. by carrying out anodization, electrodeposition,
electroplating, electropolishing, and/or other reactions in the
absence of ALD. The carousel 104 and/or surrounding chamber may
include an optional substrate temperature control element (e.g.,
heating and/or cooling element) to maintain the temperature of the
substrate 104 within a specified range.
[0029] Gas sources 108, 110, supply reactant gases A and B for
sequential atomic layer deposition processes that occur in the
chamber 102. Gas source 112 may supply an optional non-reactive
purge gas C, e.g., an inert gas such as argon or argon.
Alternatively, or in addition, reactant gases A and B (or better
reactant gases A and B each mixed into a carrier gas such as
nitrogen (N.sub.2) or argon) could be removed via pumping. The gas
sources 110, 112, 116 may selectively supply either reactant gas A
or reactant gas B and/or purge gas C though one or more gas lines
and one or more valves 113. Although three gas sources are depicted
in FIG. 1 for the sake of example, any number of gas sources may be
coupled to the chamber 102 as required to perform the desired
reaction or reactions. As described above, a temperature control
element may be disposed in the chamber 102 or on the carousel 104
to control the temperature of the substrate 106 and/or chamber 102.
Alternatively or in addition, other energy sources could be used,
such as energetic particles (from plasma, Ozone, etc.), UV light,
etc. At the right range of temperature and/or presence of other
energetic species and pressure each reactant gas A, B may
participate in a half-reaction at the surface of the substrate 106.
When the two half-reactions are performed sequentially a very thin
layer of material, e.g., as little as one atomic layer or (more
common) part of one monolayer, may be deposited on the substrate
106 as a result of the two half-reactions.
[0030] A typical ALD process involves a sequence of two different
and alternating surface reactions involving two different gaseous
reactants. The first reaction exposes the substrate to a pulse of a
precursor gas containing molecules or atoms of interest that are to
be deposited. Upon application of a pulse of precursor gas, the
entire surface within the ALD chamber becomes saturated with
chemisorbed molecules of the precursor gas. The atoms of interest
attach the precursor gas molecules to deposition sites on the
substrate surface. The second reaction exposes the substrate and
attached precursor gas molecules to a pulse of second gas,
typically a reducing agent, such as hydrogen, which reacts with the
attached precursor gas molecules and removes undesired components
of the precursor gas leaving the atoms of interest attached to the
surface at the deposition sites.
[0031] The ALD system is typically purged of reactant gases in
between these reactions with a non-reactive purge gas, such as
argon or nitrogen, which serve to remove excess chemical species
from the reaction chamber. The separate and pulsed application of
the second precursor gas followed by the purge with non-reactive
gas ensures that no gas-phase reactions take place in the
gas-phase. Rather, chemical reactions occur on exposed surfaces
within the ALD reaction chamber. The preceding sequence may be
repeated with the original precursor gas or with a different
precursor gas. Such a technique may readily be applied to formation
of CIGS absorber layers.
[0032] ALD thus permits a IB-IIIA-VIA absorber layer (e.g., a CIGS
layer) to be built up layer-by-layer, using stepwise deposition of
partial atomic monolayers during each application cycle, with the
aggregate growth rate directly proportional to the number of
reaction cycles rather than the pressure or concentration of
precursor gases in the chamber. ALD techniques can thus deposit
thin films one atomic layer at a time, in a "digital" fashion. Such
"digital" build-up of material greatly simplifies thickness
control, thus reducing both the complexity and cost of thin film
deposition. As a result, ALD provides a means for the uniform
deposition of large surface areas within the ALD chamber with
substantial control over film thickness, film uniformity, and high
conformality, even for substrates with features exhibiting very
high aspect ratios (e.g. 100:1). Importantly, while the ALD process
is typically carried out at a deposition rate of less than 1 nm per
deposition cycle, and is thus a relatively slow process compared to
other CVD deposition techniques, the ALD process can nevertheless
be scaled to ultra-high surface areas within a reaction chamber by
using suitably wound and coiled substrates, using techniques such
as those described herein. As ALD provides for a uniform and
conformal coating even over high surface areas, the use of ALD
eliminates the inconstant evaporation rates commonly experienced
during co-evaporation, sputtering, or CVD processing of metals and
metal organic precursors.
[0033] Some high-volume batch processes, e.g., chemical bath
deposition (CBD) and atomic layer deposition (ALD), could
potentially coat or otherwise treat both sides of the coiled
substrate 106 at one time. However, it may be desirable to coat
only one side of the substrate 106. Coating or otherwise treating
both sides can result in waste of valuable reactants or may lead to
extra processing steps such as removing unwanted coatings. To avoid
such waste or undesired processing, two substrates may be attached
together "back-to-back" to form a dual substrate having, in effect,
two front sides with the back sides protected against undesired
treatment. Preferably, the substrates are attached in a manner that
allows them to be separated from each other after processing. By
way of example the substrates may be attached with a low-strength
adhesive or electrostatic film applied to the back side of one or
both substrates. Alternatively, an edge where the two substrates
join may be sealed, e.g., with a tape, so that reactants cannot
reach the back sides during processing. The dual substrate may then
be wound into a coil and coated such that both front surfaces are
treated while the back surfaces are not. Processing the substrate
in this fashion may reduce the waste of reactants and may increase
the area of the substrate that can be processed at one time.
[0034] As set forth above, coiled-substrate ALD techniques, such as
those described above, may be used to deposit an absorber layer
material for an optoelectronic device such as a solar cell. By way
of example, absorber layer material may include Cu with In or Ga
and Se or S in a stoichiometric ratio of approximately
CuIn.sub.1-xGa.sub.x(S, Se).sub.2, where x is between 0 and 1. It
should also be understood that group IB, IIIA, and VIA elements
other than Cu, In, Ga, Se, and S may be included in the description
of the IB-IIIA-VIA alloys described herein, and that the use of a
hyphen ("-" e.g., in Cu-Se or Cu-In-Se) does not indicate a
compound, but rather indicates a coexisting mixture of the elements
joined by the hyphen. Where several elements can be combined with
or substituted for each other, such as In and Ga, or Se, and S, in
embodiments of the present invention, it is not uncommon in this
art to include in a set of parentheses those elements that can be
combined or interchanged, such as (In, Ga) or (Se, S). The
descriptions in this specification sometimes use this convenience.
Finally, also for convenience, the elements are discussed with
their commonly accepted chemical symbols. Group IB elements
suitable for use in the method of this invention include copper
(Cu), silver (Ag), and gold (Au). Preferably the group IB element
is copper (Cu). Group IIIA elements suitable for use in the method
of this invention include gallium (Ga), indium (In), aluminum (Al),
and thallium (Tl). Preferably the group IIIA element is gallium
(Ga) and/or aluminum (Al) and/or indium (In). Group VIA elements of
interest include selenium (Se), sulfur (S), and tellurium (Te), and
preferably the group VIA element is either Se or S.
[0035] The proper choice of precursor materials is important for
the ALD process to proceed effectively. Appropriate materials
typically exhibit the following features: (I) sufficient volatility
at the reaction temperatures, thermal stability with minimal or no
self-decomposition, significant reactivity with the second
precursor (reducing agent), and substantial insolubility of both
precursors in both the product film and the underlying substrate.
Limited solubility can however be tolerated if the out-diffusion of
a precursor material is rapid enough to go to completion during a
short purging period. Limited thermal stability can also be
tolerated if the temperature ranges for the deposition processes
are well controlled.
[0036] For the ALD-based synthesis of Mo layers, any of a variety
of precursor materials can be used, including but not limited to
molybdenum chloride, molybdenum iodide, or other halides,
molybdenum ethoxide, molybdenum VI oxide bis(2,4-pentandedionate),
molybdenum hexacarbonyl, molybdenum disilicide, and other
organomolybdenum or organometallic precursors (for example
containing Si or Ge), and combinations or mixtures of the
above.
[0037] For the ALD-based synthesis of tungsten layers, any of a
variety of precursor materials can be used, including but not
limited to tungsten chloride or other halides, tungsten ethoxide,
tungsten silicide, and other organotungsten or organometallic
precursors (for example containing Si or Ge), and combinations or
mixtures of the above.
[0038] For the ALD-based synthesis of vanadium layers, any of a
variety of precursor materials can be used, including but not
limited to vanadium chloride, vanadium iodide or other halides,
vanadium tri-n-propoxide oxide, vanadium triisopropoxide oxide,
vanadium trisisobutoxide, vanadium III 2,4-pentanedionate, vanadium
IV oxide bis(2,4-pentanedionate), vanadium IV oxide
bis(heacafluoropentanedionate), vanadium IV oxide
bis(benzoylacetonate), and other organovanadium or organometallic
precursors (for example containing Si or Ge), and combinations or
mixtures of the above.
[0039] For the ALD-based synthesis of silicon dioxide layers, a
wide variety of organometallic precursors are available, including
but not limited to hexafluorosilicates, metasilicates,
orthosilicates, and other organosilicon or organometallic
precursors (for example containing Ge), and combinations or
mixtures of the above.
[0040] For the ALD-based synthesis of chromium layers, any of a
variety of precursor materials can be used, including but not
limited to chromium chloride, chromium iodide, or other halides,
chromium III benzoylacetonate, chromium (III)
heaxafluoropentanedionate, chromium III isopropoxide, chromium III
2,4-pentanedionate, chromium III 2,2,6,6-tetramethylheptanedionate,
chromium III trifluoropentanedionate, chromium II acetate, chromium
III acetate, chromium III 2-ethylheaxonate, and other
organochromium or organometallic precursors (for example containing
Si or Ge), and combinations or mixtures of the above.
[0041] For the ALD-based synthesis of CIGS absorber layers, there
are several possible precursors for each of the elements in the
absorber layer. For copper, suitable precursors include but are not
limited to Cu(I) and Cu(II) compounds such as CuCl, copper iodide,
or other copper halides, copper diketonates (e.g.
Cu(II)-2,2,6,6,-tetramethyl-3,5,-heptanedionate (Cu(thd).sub.2)),
Cu (II) 2,4-pentanedionate, Cu(II) hexafluoroacetylacetonate
(Cu(hfac).sub.2), Cu(II) acetylacetonate (Cu(acac).sub.2), Cu(II)
dimethylaminoethoxide, copper ketoesters, other organocopper or
organometallic precursors (for example containing Si or Ge)), and
combinations or mixtures of the above. For indium, suitable
precursors include but are not limited to indium chloride, indium
iodide, or other indium halides, dimethylindium chloride,
trimethylindium, indium 2,4-pentanedionate (indium
acetylacetonate), indium hexafluoropentanedionate, indium
methoxyethoxide, indium methyl(trimethylacetyl)acetate, indium
trifluoropentanedionate, and other organoindium or organomettalic
precursors (for example containing Si or Ge), and combinations or
mixtures of the above. For gallium, suitable precursors include but
are not limited to diethylgallium chloride, gallium triiodide, or
other gallium halides, Ga (III) 2,4-pentanedionate, Ga (III)
ethoxide, Ga(III) 2,2,6,6,-tetramethylheptanedionate,
tris(dimethylaminogallium), and other organogallium or
organometallic precursors (for example containing Si or Ge), and
combinations or mixtures of the above. For aluminum, suitable
precursors include but are not limited to aluminum chloride,
aluminum iodide, or other halides, dimethylaluminum chloride,
aluminum butoxides, aluminum di-s-butoxide ethylacetoacetate,
aluminum diisopropoxide ethylacetoacetate, aluminum ethoxide,
aluminum isopropoxide, aluminum hexafluoropentanedionate, Al(III)
2,4,-pentanedionate, Al(III) 2,2,6,6-tetramethyl3,5-heptanedionate,
aluminum trifluoroacetate, trisisobutylaluminum, aluminum silicate,
and other organoindium or organometallic precursors (for example
containing Si or Ge), and combinations or mixtures of the
above.
[0042] ALD-based synthesis of CIGS absorber layers may also
(optionally) use a metal organic precursor containing selenium such
as dimethyl selenide, dimethyl diselenide, or diethyl diselenide or
a sulfur-containing metal organic precursor, or H.sub.2Se or
H.sub.2S, or other selenium- or sulfur-containing compounds, and
combinations or mixtures of the above.
[0043] Furthermore, it may be advantageous to increase the
concentration of Ga in the CIGS film, for example at the front of a
CIGS absorber layer, by depositing a layer of Ga onto a
just-deposited CI, CIG, or CIGS film, where the CI, CIG, or CIGS
film was deposited using any of a variety of techniques, including
but not limited to solution-based printing, sputtering,
evaporation, and the like. To do so, ALD can be carried out using
any of a range of suitable Ga precursors, including but are not
limited to diethylgallium chloride, gallium triiodide, or other
gallium halides, Ga (III) 2,4-pentanedionate, Ga (III) ethoxide,
Ga(III) 2,2,6,6,-tetramethylheptanedionate,
tris(dimethylaminogallium), and other organogallium or
organometallic precursors (for example containing Si or Ge), and
combinations or mixtures of the above.
[0044] For the ALD-based synthesis of cadmium sulfide, any of a
variety of precursor materials can be used, including but not
limited to cadmium chloride, cadmium iodide, or other halides,
cadmium 2,4-pentanedionate, cadmium acetate, cadmium formate,
dimethylcadmium, and other organocadmium or organometallic
precursors (for example containing Si or Ge), and combinations or
mixtures of the above.
[0045] Other examples of specific A and B precursors and substrate
temperature ranges for both in ALD of cadmium sulfide (CdS) include
the following shown in Table I:
TABLE-US-00001 TABLE I Substrate Temperature Precursor A Precursor
B Range (.degree. C.) Elemental Cd Elemental S 350-450 (source temp
320.degree. C.) (source temp 90-120.degree. C.) CdCl.sub.2 H.sub.2S
(5 sccm) 480 (source temp 470.degree. C.)
[0046] Such CdS ALD is described e.g., by A. Kytokivi, A. et al.,
in MRS Symp. Proc. 222: 269-273, 1991, which is incorporated herein
by reference.
[0047] For the ALD-based synthesis of Zinc oxide, any of a variety
of precursor materials can be used, including but not limited to
zinc chloride, zinc iodide, or other halides, zinc
N,N-dimethylaminoethoxide, zinc methoxyethoxide, zinc
2,4-pentanedionate, zinc 2,2,6,6-tetramethyl-3,5-heptanedionate,
zinc acetate, zinc bis(hexamethyldisilazide), and other organozinc
or organometallic precursors (for example containing Si or Ge), and
combinations or mixtures of the above.
[0048] Other examples of specific A and B precursors and substrate
temperature ranges for both in ALD of Zinc Oxide (ZnO) include the
following shown in Table II below:
TABLE-US-00002 TABLE II Substrate Temperature Precursor A Precursor
B Range (.degree. C.) Dimethyl Zinc Trimethyl Ammonium 120-350
Diethyl Zinc Trimethyl Ammonium 120-350
[0049] Such ZnO ALD is described e.g., by V. Lujala, in "Atomic
layer epitaxy growth of doped zinc oxide films from organometals"
Applied Surface Science 82/83: pp 34-40, 1994, which is
incorporated herein by reference. To react any of the above
precursor materials on the substrate surface, ALD reactions require
an additional reactant, often a reducing agent or proton-donor
compound. This compound can be introduced concurrently with the
first (precursor) reactant (especially if the compounds do not
cross-react prior to interacting with one another at the substrate
surface), or the introduction of the second reactant can be made
subsequent to the introduction of the initial (precursor material).
When an organometallic precursor is hydrated, a proton-donor
compound may not be necessary. Reducing/proton-donating compounds
include but are not limited to water (H.sub.2O), methanol, ethanol,
isopropyl alcohol, butyl alcohols, and other alcohols, and
combinations or mixtures of these materials, as well as carbon
monoxide (CO).
[0050] Oxygen gas (O.sub.2) is also typically used as a second
reactant, as is a mixture of H.sub.2O and H.sub.2O.sub.2. For
certain precursors, especially hexafluoro-pentanedionate (HFPD)
precursors such as copper (II) hexafluoro-pentanedionate, indium
hexafluoro-pentanedionate, and gallium hexafluoro-pentanedionate,
formalin (37% formaldehyde, and 15% methanol in distilled deionized
water) is often used as the reducing agent while nitrogen gas
(N.sub.2) is used as the purge gas.
[0051] For these surface reactions, an inert gas (such as nitrogen,
argon, helium) is typically used as the purge gas, while hydrogen
gas is often used as a reducing agent.
[0052] In some situations, a seed layer, e.g., of platinum or
palladium may be deposited on the substrate before ALD with these
precursors.
[0053] During the deposition process, a typical ALD cycle consists
of 1-2 seconds of a first metal organic precursor pulse, followed
by a 1-2 second purge, 1-2 seconds of a second metal organic
precursor pulse, followed by a 1-2 second purge, (optionally) a 1-2
second pulse of a third metal organic precursor, followed by a 1-2
second purge, and (optionally) a 1-2 second pulse of a fourth metal
organic precursor, followed by a 1-2 second purge, then
(optionally) a 1-2 second pulse of a fifth metal organic precursor,
and finally (optionally) a 1-2 second pulse of a sixth metal
organic precursor. More generally, the duration of the pulse and/or
purge cycles range from 0.001 seconds to 60 seconds, more
preferably from 0.01 to 20 seconds, and most preferably from 0.1 to
10 seconds.
[0054] The temperature used during ALD typically ranges from
150.degree. C. to 600.degree. C. depending upon the chemistry and
physical properties of each precursor material.
[0055] Selenium and/or sulfur may be incorporated into the absorber
layer in any of several ways. In one approach, ALD may be carried
out within each monolayer of a nascent absorber layer deposit using
precursor gases that may include a metal organic precursor
containing selenium such as dimethyl selenide, dimethyl diselenide,
or diethyl diselenide and/or a sulfur-containing metal organic
precursor, or H.sub.2Se or H.sub.2S. In this case, selenium is
incorporated on a monolayer-by-monolayer basis as the absorber film
is built up through sequential deposition steps. In another
approach, ALD of selenium-containing compounds can be carried out
on a periodic basis where the selenium precursor deposition
frequency is less than that required for initial incorporation into
each monolayer. In yet another alternative approach, a nascent
absorber layer can be exposed to selenium using either H.sub.2Se,
H.sub.2S or selenium vapor. In this case, selenization can be
carried out either (a) on a monolayer by monolayer basis, or (b)
periodically, where the exposure period is longer than a monolayer
deposition cycles, or (c) at the end of the absorber layer
deposition sequence. Combinations and/or variations of these steps
can be carried out. To carry out selenization and/or sulfurization,
the film, if deposited on a flexible substrate, can be wound into a
coil and the coil can be coated so that the entire roll is exposed
at the same time, substantially increasing the scalability of the
Se vapor exposure process through such a high-volume batch process,
e.g., as described above.
[0056] An additional absorber layer reduction reaction may also
optionally be performed after each metal organic precursor pulse or
after a sequence of two or more pulses of precursor gas. Each of
these approaches has its advantages and drawbacks. For example,
when each precursor pulse is followed by a reduction reaction, the
reduction process is likely to be more effective since reactions
take place on a relatively high surface area to volume ratio.
However, the addition of repeated reduction steps can slow the
overall process speed. On the other hand, if reduction is carried
out only after a series of precursor pulses, the reduction process
will likely be less effective since reactions take place on a
relatively lower surface area to volume ratio, but eliminating the
repeated reduction steps may increase the overall process speed.
Thus some combination of these two approaches might work better for
a given situation. Alternatively, reduction can be carried out upon
completion of the deposition of the other elemental components of
the absorber layer. A further complication arises from the need to
ensure that the materials that form during the deposition process
are not locked into a stochiometry and/or phase that cannot be
later transformed into the intended target phase and stochiometry.
Thus the deposition process should be monitored and/or guided by
the phase transformations that may occur as different combinations
of elements are deposited within the nascent absorber film.
[0057] Embodiments of the invention are especially advantageous for
the construction of solar cells based on CIGS absorber layers. For
example, 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. 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 with the product deposit. Both of these features can provide
benefits (such as improved power conversion efficiency) as the
product deposited serves within the absorber layer of a solar cell.
These benefits are much more difficult to achieve with conventional
co-evaporation, sputtering, or solution-based deposition of
inorganic materials used for the absorber layers of solar cells, as
both the spatial uniformity and the potential resolution of
atomically-graded deposition profiles of these processes is
considerably less than for ALD-based deposition. Further, the
reproducibility of non-ALD-based processes is less than that of
ALD-based deposition.
[0058] Atomic layer deposition of a CIGS absorber layer material
may proceed by the sequence illustrated in FIGS. 2A-2D.
Specifically, as shown in FIG. 2A, a substrate 202 may have
deposition sites 204 where atoms may attach. After exposure to a
copper precursor and reducing agent, copper atoms C occupy a
portion of the sites 204 as shown in FIG. 2B. Other sites remain
unoccupied. After exposure to an indium precursor and reducing
agent, indium atoms I occupy a portion of the unoccupied sites 204
leaving other sites unoccupied as shown in FIG. 2C. After exposure
to a gallium precursor and reducing agent, gallium atoms G occupy a
portion of the unoccupied sites 204 as shown in FIG. 2D. In these
depositions, the amount of material deposited may be controlled,
e.g., by varying the number and sequence of relative exposure
pulses of one precursor gas to another precursor gas, e.g. for the
copper, indium and/or gallium precursor. With ALD it is possible to
precisely control the stoichiometric ratio in the CIGS absorber
layer in several different ways. Specifically, the sequence of
atomic layer depositions of the CIGS components can be varied in a
way that controls the relative stoichiometric ratios of the
elemental components of the deposited film. For example, a
copper:indium stoichiometric ratio of 4:3 may be achieved by a
deposition sequence of four copper depositions and three indium
depositions, such as C-I-C-I-C-I-C or C-C-C-C-I-I-I or
C-C-I-I-I-C-C and the like, where "C" represents copper deposition
and "I" represents Indium deposition. The sequence may be repeated
as often as necessary to get the desired thickness. Here it is
assumed that the copper and indium depositions deposit
approximately equal numbers of atoms. Those of skill in the art
will be able to devise other sequences that take into account
variations in the deposition ratios. Furthermore, deposition
sequences involving more than two different CIGS components may be
sequentially used, e.g. serial deposition of cycles of C-I-G or
C-I-G-S, or C-I-A-S, or C-I-G-Se-Su, and so forth. In addition,
deposition sequences involving more than two different CIGS
components may be performed in parallel, e.g. two or more
metal-organic precursors can be introduced simultaneously in the
same deposition pulse. I a preferred mode, parallel deposition of
more than one metal-organic precursor occurs without reaction of
those precursors prior to the surface reaction/deposition.
[0059] In other embodiments of the invention, ALD may be used to
control the stoichiometric ratio of a CIGS absorber layer as a
function of depth. For example, if a 4:3 copper:indium ratio is
desired over a first desired thickness at the bottom of the CIGS
absorber layer and a 3:4 copper:indium ratio is desired over a
second desired thickness higher up in the CIGS absorber layer, then
the C-I-C-I-C-I-C sequence may be repeated until the first desired
thickness is achieved and a sequence I-C-I-C-I-C-I may be repeated
until the second desired thickness is achieved. Those of skill in
the art will be able to devise sequences that take into account
variations in the deposition ratios or that use more than two
different CIGS components. Stoichiometric ratios may also be varied
as a function of depth by using a deposition sequence in which the
particular pulse sequence for different precursor gases vary.
[0060] Once the various CIGS components (or more generally
IB-IIIA-VIA components) have been deposited by ALD they are usually
annealed to form an absorber layer for a device. The annealing may
be implemented by flash heating, also called rapid thermal
processing. In particular, the substrate and absorber layer
components may be flash heated at a rate of between about 5
C..degree./sec and about 5.degree. C..degree./sec to a plateau
temperature of between about 200.degree. C. and about 550.degree.
C. The plateau time for the flash heating process may last between
about 2 minutes and about 10 minutes. Such processing allows
annealing of the absorber layer without damaging substrates that
would otherwise be damaged by high temperature thermal processing.
The combination of ALD and rapid thermal processing also allows for
sharp transitions in the relative concentrations of the
constituents of the absorber layer. The rapid thermal processing is
typically performed only once at the end of the absorber layer
deposition process. Selenization or sulfurization may be performed
before or during annealing or both.
[0061] Although the preceding section describes deposition of
copper, indium and gallium by ALD, some of the components of the
absorber layer may be deposited by techniques other than ALD. It is
noted here that suitable control of the desired properties of the
resulting absorber layer may be achieved even if only one component
of the absorber layer is deposited by ALD. The other components may
be deposited by conventional means.
[0062] A IB-IIIA-VIA alloy absorber layer formed as described above
can be used in an optoelectronic device 300, e.g., as shown in FIG.
3. The device 300, which may be a solar cell, generally includes a
substrate or base layer 302, a base electrode 304, an absorber
layer 306, a window layer 308, and a transparent electrode 310. The
base layer 302 may be made from a thin flexible material suitable
for roll-to-roll processing By way of example, the base layer may
be made of a metal foil, such as titanium, a polymer such as such
as polyimides (PI), polyamides, polyetheretherketone (PEEK),
Polyethersulfone (PES), polyetherimide (PEI), polyethylene
naphtalate (PEN), Polyester (PET), 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 a layer of
stainless steel, aluminum, or molybdenum, e.g., about 0.5 micron to
about 25 microns thick. An optional adhesion layer 303 may
facilitate bonding of the electrode 304 to the substrate 302. By
way of example, the adhesion layer 303 may be vanadium, chromium,
tungsten or silicon dioxide.
[0063] The absorber layer 306 may include material containing
elements of groups IB, IIIA, and VIA. Preferably, the absorber
layer 306 includes copper (Cu) as the group IB, Gallium (Ga) and/or
Indium (In) and/or Aluminum as group IIIA elements and Selenium
(Se) and/or Sulfur (S) as group VIA elements. The absorber layer
306 may be fabricated using a sequence of atomic layer depositions
on the base electrode 304. The absorber layer 306 may be about 1000
nm thick. By using atomic layer deposition as described above, the
absorber layer 306 may be deposited at a temperature compatible
with the underlying substrate 302 and electrode 304. Furthermore,
the elemental ratios of the IB, IIIA and VIA elements in the
absorber layer 306 may be precisely controlled.
[0064] The window layer 308 is typically used as a junction partner
for the absorber layer 306. By way of example, the junction partner
layer may include cadmium sulfide (CdS), zinc sulfide (ZnS), or
zinc selenide (ZnSe) or some combination of two or more of these.
Layers of these materials may be deposited, e.g., by chemical bath
deposition or chemical surface deposition, to a thickness of about
50 nm to about 100 nm. The combination of the absorber layer 306
and the window layer 308 is sometimes referred to as an absorber
layer.
[0065] The transparent electrode 310 may include a transparent
conductive layer 309, e.g., a transparent conductive oxide (TCO)
such as zinc oxide (ZnO) or aluminum doped zinc oxide (ZnO:Al),
which can be deposited using any of a variety of means including
but not limited to sputtering, evaporation, CBD, electroplating,
CVD, PVD, ALD, and the like. If the substrate is flexible and the
deposition technique is ALD or CBD or the like, a coiled/wound
flexible substrate can be exposed so that the entire roll is
processed at one time, e.g., as described above.
[0066] Alternatively, the transparent conductive layer 309 may
include a transparent conductive polymeric layer, e.g. a
transparent layer of doped PEDOT (Poly-3,4-Ethylenedioxythiophene),
which can be deposited using spin, sip, or spray coating, and the
like. PSS:PEDOT is a doped, conducting polymer based on a
heterocyclic thiophene ring bridged by an diether. A water
dispersion of PEDOT doped with poly(styrenesulfonate) (PSS) is
available from H. C. Starck of Newton, Mass. under the trade name
of Baytron.RTM. P. Baytron.RTM. is a registered trademark of Bayer
Aktiengesellschaft (hereinafter Bayer) of Leverkusen, Germany. In
addition to its conductive properties, PSS:PEDOT can be used as a
planarizing layer, which can improve device performance. A
potential disadvantage in the use of PEDOT is the acidic character
of typical coatings, which may serve as a source through which the
PEDOT may chemically attack, react with, or otherwise degrade the
other materials in the solar cell. Removal of acidic components in
PEDOT can be carried out by anion exchange procedures. Non-acidic
PEDOT can be purchased commercially. Alternatively, similar
materials can be purchased from TDA materials of Wheat Ridge,
Colo., e.g. Oligotron.TM. and Aedotron.TM..
[0067] In addition to the transparent conductive layer 310, the
transparent electrode 310 may further include a layer of metal
(e.g., Ni, Al or Ag) fingers 311 to reduce the overall sheet
resistance.
[0068] For the optoelectronic devices of the type shown in FIG. 3,
an optional encapsulant layer (not shown) may provide environmental
resistance, e.g., protection against exposure to water or air. The
encapsulant may also absorb UV-light to protect the underlying
layers. Examples of suitable encapsulant materials include one or
more layers of polymers such as THZ (e.g. Dyneon's THV220
fluorinated terpolymer, a fluorothermoplastic polymer of
tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride),
Tefzel.RTM. (DuPont), Tefdel, ethylene vinyl acetate (EVA),
thermoplastics, polyimides, polyamides, nanolaminate composites of
plastics and glasses (e.g. barrier films such as those described in
commonly-assigned, co-pending U.S. patent application Ser. No.
10/698,988, to Brian Sager and Martin Roscheisen, filed Oct. 31,
2003, and entitled "INORGANIC/ORGANIC HYBRID NANOLAMINATE BARRIER
FILM"), and combinations of the above.
[0069] Embodiments of the present invention encompass situations
where the concentrations of group IB, group IIIA and group VIA
elements in the IB-IIIA-VIA absorber layer 306 vary with respect to
depth. 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 506 intermediate the front and back
regions is referred to herein as a "central region".
[0070] In one embodiment, the absorber layer 306 may include Cu,
Ga, and/or In and Se and/or S. As described above, the Ga content
may be tuned to be 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 and a Se-rich region 408 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.
[0071] 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 In to one part Cu. The overall In level in the
first Ga-rich region 402 and the Se-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 higher in the Se-rich region
408. For example over the first Ga-rich region 402, central region
406 and second Ga-rich region 404, the absorber layer may have a
composition of Cu.sub.0.9Ga.sub.1-xIn.sub.xSe.sub.2. At the Se-rich
layer 408, the composition may be CuIn.sub.3yGa.sub.3-3ySe.sub.5.
In the specific case where y=0, the Se-rich layer 408 may have a
composition given by CuIn.sub.3Se.sub.5. The Se-rich layer may be
about 20 nm thick.
[0072] 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.
[0073] 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.
[0074] The advantages of the coiled substrate approach of the
embodiments of the present invention may be illustrated by a
numerical example. Consider a coiled substrate that can fit into
16'.times.16'.times.12' space. Assume that the coil has an inner
diameter of 1 meter (e.g., for a hexagonal carousel, the distance
from the center to an edge of a hexagon). Assume that the coil has
an outer diameter of 3 meters and that the width of the coil is 2
meters and the carousel is a little wider, e.g., 2.4 meters wide.
Each turn of the coiled substrate is 1 meter long between the edges
of the hexagon initially and 3 meters long when fully wound. The
average length of each turn of the coiled substrate is thus 6
sides.times.2 m/side. If the substrate has a thickness of 0.025 mm
and adjacent turns of the coiled substrate are 1 mm apart, then the
coiled substrate would have about 2 m/1.025 mm=.about.2,000 turns
about the carousel. The total area of the coiled substrate would be
2,000.times.2 m.times.6.times.2 m=48,000 m.sup.2. In this numerical
example, suppose that a deposition requires 25 repetitions of the
four step sequence ACBC, i.e., filling the chamber with reactant A
(e.g., a precursor), purge with inert gas C, fill with reactant B
(e.g., a reducing agent), purge with inert gas C. If each purge,
pump or fill step takes 10 minutes, the throughput may be estimated
as the total area divided by the total number of steps and the time
per step, e.g., 48,000 m.sup.2/10 min/step/100 steps=48
m.sup.2/min>500 square feet per minute. If the time for each
step can be reduced to only 1 minute, the throughput may be
increased to >5,000 square feet per minute. Because of this
relatively improved deposition rate, a thick (e.g. 20 nm to as much
as 2000 nm or more) CIGS absorber layers can be cost- and
time-effectively deposited at high production volume. This
contrasts with the prior art, where, due to the limited deposition
rate of flat substrates, ALD was used only for very thin films on
extremely thin absorber (ETA) cells.
[0075] Furthermore, it is possible to form most or all of the
layers of an optoelectronic device by ALD in one chamber without
having to remove the substrate from the chamber between steps.
Specifically, with respect to a device of the type depicted in FIG.
3, the electrode layer 304 may be made of molybdenum. By way of
example, molybdenum may be deposited by ALD, e.g., using MoCl.sub.5
and Zinc (see e.g., M. Juppo, M. Vehkamaki, M. Ritala, and M.
Leskela, Deposition of molybdenum thin films by an alternate supply
of MoCl.sub.5 and Zn, Journal of Vacuum Science & Technology A
16 (5), (1998) 2845, which is incorporated herein by reference).
The absorber layer 306 may be deposited by ALD as described above,
e.g., with respect to FIGS. 2A-2D, and then annealed by rapid
thermal processing.
[0076] The window layer 308 may be Cadmium Sulfide (CdS) deposited
by ALD. Cadmium Sulfide may be more reliably deposited by Chemical
Surface Deposition, a technique described, e.g., in McCandless, B.
E. and W. N. Shafarman. "Chemical Surface Deposition Of Ultra-Thin
Cadmium Sulfide Films for High Performance and High Cadmium
Utilization", 3rd World Conference on Photovoltaic Energy
Conversion, Osaka, Japan, 2003, which is incorporated herein by
reference. CdS can also be deposited by a liquid-based atomic layer
epitaxy, as described e.g. in T. E. Lister and J. L. Stickney,
"Formation of the first monolayer of CdSe on Au(111) by
Electrochemical ALE", Appl. Surface Science, 107 (1996), 153; and
T. E. Lister, and J. L. Stickney, "CdSe Deposition on Au(111) by
Electrochemical ALE," Appl. Surface Sci., 103 (1996) 153.), the
disclosures of both of which are incorporated herein by reference.
Even with such liquid based deposition techniques, the window layer
308 may be deposited in the same chamber as the other layers if the
deposition chamber is suitably configured to allow for filling and
draining of liquid phase reactants. Alternatively, the window layer
308 may be made of materials other than CdS that may be deposited
by ALD. Examples of such materials include ZnO, Zn(O,S), ZnSe,
In.sub.2S.sub.3, TiO.sub.2, Ta.sub.2O.sub.5, and Al.sub.2O.sub.3,
as described e.g., in the Sterner dissertation and other references
cited herein.
[0077] Finally, the transparent conductive layer 309 may be ZnO
deposited by ALD using diethyl zinc as a reactant and water vapor
(H.sub.2O) as a reducing agent. ZnO deposition by ALD is described
e.g., by J. W. Elam et al, "Properties of ZnO/Al.sub.2O.sub.3 Alloy
Films Grown Using Atomic Layer Deposition Techniques" Journal of
The Electrochemical Society, vol. 150 no. 6, pp G339-G347 (2003),
which is incorporated herein by reference. Alternatively, the
transparent conductive layer 309 may be a conductive polymeric
layer, e.g., of the types described above, deposited from a
solution, e.g., by any of a variety of coating methods including
but not limited to contact printing, top feed reverse printing,
bottom feed reverse printing, nozzle feed reverse printing, gravure
printing, microgravure printing, reverse microgravure printing,
comma direct printing, roller coating, slot die printing, meiyerbar
coating, lip direct coating, dual lip direct coating, capillary
coating, ink-jet printing, jet deposition, spray deposition, and
the like. The transparent electrode 310 may further include a layer
of metal (e.g., Ni, Al or Ag) fingers 311 to reduce the overall
sheet resistance.
[0078] In embodiments of the present invention, the scaling of the
ALD process is geared towards surface area maximization, not
necessarily process step speed, thus leaving enough time for each
step. Consequently, pumping, purging and filling can be ensured to
be high quality, thus minimizing loss of coating quality by
intermixing gases, etc. Although the whole process may take a
considerable period of time to complete, a vast surface area of
substrate may be coated at one time.
[0079] 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. 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. 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."
* * * * *