Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF MAKING A PEROVSKITE LAYER AT HIGH SPEED
This invention was made with government support under Grant No. DE-
EE0008128 awarded by the Solar Energy Technologies Office, Department of
Energy. The government has certain rights in the invention.
FIELD
The present disclosure relates to depositing a perovskite solution at high
speed on a flexible substrate, drying the solution and, more particularly, a
novel
method of making perovskite layers and perovskite photovoltaic devices. The
present
disclosure further relates to methods of making a photovoltaic device on a
substrate at
high speed with a Perovskite solution. The present disclosure further relates
to the
composition of a Perovskite solution for use in making Perovskite layer and
Perovskite
photovoltaic devices at high speed.
BACKGROUND
Since their first report in 2009, rapid improvements have enabled halide
perovskite solar cells (PSCs) to become a promising technology for converting
light to
electricity as part of optoelectronic devices. To date, the power conversion
efficiencies
(PCEs) of solution-processed PSCs have been certified above 23 percent, which
is
higher than the current dominant photovoltaic technology that is based on
multicrystalline silicon (see National Renewable Energy Laboratories
Efficiency
Chart, https://www.nrel.gov/pv/assets/pdfs/pv-efficiency-chart.20181217.pdf
accessed
December 17, 2018). Whereas crystalline silicon is rigid, brittle, and
requires costly,
energy-intensive fabrication procedures, perovskites are flexible, easily
processed at
low temperatures, and up to a thousand times thinner. Furthermore, perovskites
are
solution-processable, which enables their manufacture with scalable, low-cost
methods. These attributes open new opportunities to integrate solar power
creatively
and inexpensively into previously inaccessible markets, such as electric
vehicles and
buildings. PSCs also have the important advantage of having minimal impact on
PCE
as temperature increases, unlike silicon based solar cells, which exhibit
significant
power loss in typical operating environments. PSCs advantages and high PCE put
them on the path to be the next generation technology for utility, commercial,
and
residential photovoltaic applications.
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Most top performing PSCs have been fabricated by a spin-coating method,
which is unsuitable for high throughput and scalable module production.
However,
several scalable film deposition techniques have been developed for PSC
fabrication,
such as doctor-blading, spray deposition, slot-die coating, gravure coating,
ink jet
printing, dip coating, chemical bath deposition, flexographic, and
electrodeposition.
See Stranks, S. D. and Snaith, H. J., Metal-halide perovskites for
photovoltaic and
light-emitting devices. Nat. Nanotechnol. 10, 391-402 (2015); Deng, Y. et al.,
Scalable
fabrication of efficient organolead trihalide perovskite solar cells with
doctor-bladed
active layers, Energy Environ. Sci. 8, 1544-1550 (2015); Yang, M. et al.,
perovskite
ink with wide processing window for scalable high-efficiency solar cells, Nat.
Energy
2, 17038 (2017); Barrows, A. T. et al., Efficient planar heterojunction mixed-
halide
perovskite solar cells deposited via spray-deposition, Energy Environ. Sci. 7,
2944-
2950 (2014); Hwang, K. et al., Toward large scale roll-to-roll production of
fully
printed perovskite solar cells, Adv. Mater. 27, 1241-1247 (2015); He, M. et
al.
Meniscus-assisted solution printing of large-grained perovskite films for high-
efficiency solar cells, Nat. Commun. 8, 16045 (2017); Chen, H., et al. A
scalable
electrodeposition route to the low-cost, versatile and controllable
fabrication of
perovskite solar cells, Nano Energy 15, 216-226 (2015); Kim, Y. Y. et al.,
Gravure-
Printed Flexible perovskite Solar Cells: Toward Roll-to-Roll Manufacturing,
Adv. Sci.
2019; and Deng, Y., et al., Vividly colorful hybrid perovskite solar cells by
doctor-
blade coating with perovskite photonic nanostructures, Mater. Horiz. 2, 578-
583
(2015), each of which is incorporated by reference in its entirety. A next
step towards
the scalable fabrication of PSCs is to develop methods to make the perovskite
layer
using high speed equipment suitable for high volume manufacturing. In order
for PSCs
to gain market share in existing solar markets the speed of production must be
fast
enough so that the capital equipment costs do not overly burden the ability to
scale up
for production and also so that the final cost of PSCs is competitive with the
already
mature manufacturing state of silicon-based solar cells. While the methods
cited
above are scalable in principle, they have not yet demonstrated deposition
speeds
necessary to produce low-cost PSCs that can compete with the current silicon
technologies. Forming uniform and defect free perovskite layers on flexible
multilayer substrates to make PSCs in a cost-effective manner remains a great
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challenge due to the complexity of depositing and drying a perovskite solution
with
high speed production equipment.
SUMMARY
In accordance with an embodiment of the present disclosure, a method of
making a perovskite layer is described comprising: providing a flexible
substrate;
providing a perovskite solution comprising an initial amount of solvent and
perovskite
precursor materials and having a provided solution temperature and a total
solids
concentration between 30 percent and 70 percent by weight of its saturation
concentration at the provided solution temperature; depositing the perovskite
solution
on the flexible substrate at a first location; removing a first portion of the
initial
amount of solvent from the deposited perovskite solution with a first drying
step
having a first drying step dwell time at a second location wherein the first
drying step
heats the deposited perovskite solution to a coated layer temperature and
increases the
total solids concentration of the perovskite solution to at least 75 percent
of its
saturation concentration at the coated layer temperature; and removing a
second
portion of the initial amount of solvent from the deposited perovskite
solution with a
second drying step having a higher rate of solvent evaporation than the first
drying
step during a second drying step dwell time at a third location that causes
saturation
and a conversion reaction in the deposited perovskite solution resulting in
perovskite
crystal formation or formation of a perovskite intermediate phase, wherein the
first
drying step dwell time is at least 5 times longer than the second drying step
dwell time.
In accordance with another embodiment of the present disclosure, a
method of making a photovoltaic device is described comprising: providing a
substrate; depositing a first carrier transport solution layer with a first
carrier transport
deposition device to form a first carrier transport layer on the substrate;
depositing a
Perovskite solution comprising solvent and perovskite precursor materials with
a
perovskite solution deposition device on the first carrier transport layer;
drying the
deposited Perovskite solution to form a Perovskite absorber layer; and
depositing a
second carrier transport solution with a second carrier transport deposition
device to
form a second carrier transport layer on the Perovskite absorber layer,
wherein the
deposited Perovskite solution is dried at least partially with a fast drying
device which
causes a conversion reaction and the Perovskite solution to change in optical
density
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by at least a factor of 2 in less than 0.5 seconds after the fast drying
device first acts
on the Perovskite solution.
In accordance with another embodiment of the present disclosure, a
Perovskite solution for making a Perovskite layer is described. The Perovskite
solution comprises a solvent, an organic Perovskite precursor material, and an
inorganic Perovskite precursor material, wherein the amount of solvent is
greater than
30 percent by weight and the Perovskite solution has a total solids
concentration that
is between 30 percent and 70 percent by weight of the Perovskite solution's
saturation
concentration at a solution temperature of from 20 to 25 degrees Celsius.
Various embodiments in accordance with the disclosure have the
advantage that a uniform perovskite layer can be manufactured at high speed on
a
flexible substrate, and in particular embodiments a flexible multilayer
substrate,
thereby enabling, e.g., low cost production of high efficiency solar cells
with low
equipment costs. Various further embodiments in accordance with the disclosure
have the advantage that a Perovskite photovoltaic device can be manufactured
at high
speed, thereby enabling, e.g., low cost production of a new class of photonic
devices
such as high efficiency solar cells. Various further embodiments in accordance
with
the disclosure have the advantage of providing Perovskite solutions that are
stable at
convenient handling and storage temperatures and which can be used to
manufacture
a uniform Perovskite layer at high speed thereby enabling low cost production
of high
efficiency solar cells with low equipment costs.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a cross section of a portion of a perovskite device
wherein multiple functional layers are shown on a flexible support;
FIGS. 2a, 2b, 2c, and 2d illustrate in cross sections the formation of the
perovskite layer on a portion of a multilayer flexible substrate after
important steps in
various embodiments of the disclosure. FIG. 2a shows the perovskite solution
on a
flexible multilayer substrate after the deposition of the perovskite solution.
FIG. 2b
shows a partially dry perovskite layer solution after the first drying step.
FIG. 2c
shows an immobile layer of perovskite crystals or intermediate phase on a
flexible
multilayer substrate after a second drying step. FIG. 2d shows the completed
perovskite layer on the flexible multilayer substrate after an annealing step;
and
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FIG. 3 is a schematic side view of an exemplary printing system for roll-
to-roll printing on a flexible multilayer substrate.
FIG. 4 is a schematic side view of an exemplary multi-station deposition
and drying device for roll-to-roll printing a photovoltaic device on a
flexible
multilayer substrate.
It is to be understood that the attached drawings are for purposes of
illustrating the concepts of the disclosure and may not be to scale. Identical
reference
numerals have been used, where possible, to designate identical features that
are
common to the figures.
DETAILED DESCRIPTION
The present disclosure is inclusive of combinations of the embodiments
described herein. References to "a particular embodiment" and the like refer
to
features that are present in at least one embodiment of the disclosure.
Separate
references to "an embodiment" or "particular embodiments" or the like do not
necessarily refer to the same embodiment or embodiments; however, such
embodiments are not mutually exclusive, unless so indicated or as are readily
apparent
to one skilled in the art. It should be noted that, unless otherwise
explicitly noted or
required by context, the word "or" is used in this disclosure in a non-
exclusive sense.
The example embodiments of the present disclosure are illustrated
schematically and not necessarily to scale for the sake of clarity. One of
ordinary skill
in the art will be able to readily determine the specific size and
interconnections of
the elements of the example embodiments of the present disclosure. It is to be
understood that elements not specifically shown, labeled, or described can
take
various forms well known to those skilled in the art. It is to be understood
that
elements and components can be referred to in singular or plural form, as
appropriate,
without limiting the scope of the disclosure.
Shown in FIG. 1 is a cross section of a portion of a perovskite device, 67.
The structure of the perovskite device 67 comprises a relatively thick (e.g.,
5 to 200
microns) flexible support 61 with several, much thinner, functional layers. On
top of
the flexible support 61 is first conducting layer 62, a first carrier
transport layer 63, a
completed perovskite layer 64d, a second carrier transport layer 65, and a
second
conducting layer 66. Support 61, along with layers 62 and 63 form a multilayer
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substrate 60 for perovskite layer 64, as further shown in Figs. 2a-2d. For
some
applications the first conducting layer 62 and the first carrier transport
layer 63 are
optically transparent in the frequency range that the perovskite layer 64d
converts
photons into electron-hole pairs, typically the visible frequency range. For
other
applications the second conducting layer 66 and the second carrier transport
layer 65
are optically transparent in the frequency range that the perovskite layer 64d
converts
photons into electron-hole pairs. For PIN photovoltaic devices the optically
transparent carrier transport layer transports holes and blocks electrons. For
NIP
photovoltaic devices the optically transparent carrier transport layer
transports
electrons and blocks holes. The methods for uniformly depositing a completed
perovskite layer 64d described in the disclosure apply to both NIP and PIN
structures.
The term "perovskite solution" is defined as a solution or colloidal
suspension that can be used to generate a continuous layer of organic-
inorganic
hybrid perovskite material (referred here as perovskite layer) with an ABX3
crystal
lattice where 'A' and 'B' are two cations of very different sizes, and X is an
anion that
coordinates to both cations. A perovskite solution comprises perovskite
precursor
material and solvent, and may also contain additives to aid in crystal growth
or to
modify crystal properties. Perovskite precursor material is defined as an
ionic species
where at least one of its constituents becomes incorporated into the final
perovskite
layer ABX3 crystal lattice. Organic perovskite precursor material are
materials whose
cation contains carbon atoms while inorganic perovskite precursor material are
materials whose cation contains metal but does not contain carbon.
For small quantities of perovskite solution, a high concentration of
precursor materials can be used when making high performance lab-scale
coatings.
However, when depositing perovskite solution at high speed on pilot scale or
full-
scale manufacturing equipment these high concentration solutions have been
found to
be unstable for the required duration to enable a uniform coating. Unstable
solutions
form non-colloidal solids in the solution prior to coating that inhibit the
deposition
and drying process and produce nonfunctional photovoltaic devices. Hence,
lower
concentrations of precursors must be specified for high speed coatings. Lower
concentration solutions require thicker wet coatings to achieve the
appropriate dry
thickness for the perovskite layer. For thicker wet coatings it has been found
that
simple drying methods do not produce a uniform coating suitable for functional
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photovoltaic devices. One reason for the non-uniformity is due to convective
flow in
the wet coating that leads to a highly non-uniform dry layer due to the
movement of
the liquid in the coated layer. Convective flow results from the evaporative
cooling at
the surface of the wet laydown that leads to strong thermal gradients in the
wet
coating. Convective flow increases as the thickness of the wet coating
increases and
also as the viscosity of the wet coating decreases. The very low viscosity of
the
perovskite solution coupled with the aforementioned need for a thick wet
coating to
enable high speed manufacturing makes it very challenging to make a uniform
dry
perovskite layer at high speed.
A second reason for the variability in the dry perovskite layer is variability
in the vapor concentration of the evaporating solvent above the wet coating.
Even small
differences in air flow above the wet coating cause significant changes in the
vapor
concentration above the wet coating resulting in non-uniformities in the dry
layer due to
spatial variations in the evaporation rate across and along the substrate. One
method
known by those skilled in the art of high speed drying of a coated film is to
blow a gas
across the surface of the wet film so that evaporating solvent is continuously
removed
thus reducing the variability in the vapor concentration above the wet
coating.
However, perovskite solutions typically have very low viscosity, e.g., less
than 10
centipoise (viscosity changes with applied shear), due to the nature of the
dissolved
solids and the limited selection of useful solvents and additives. The low
viscosity of
perovskite solutions causes blow marks in the dry layer when a gas is blown
across the
surface of the wet solution. Non-uniformity in the dry layer caused by blow
marks
makes the layer non-functional because discontinuities become electrical
shorts in
photovoltaic devices. Thinner wet laydowns reduce the non-uniformities caused
by
blowing air across the film but, as previously discussed, a relatively thick
wet laydown
is required when making a high speed deposition of perovskite solution.
A third reason for the variability in the dry perovskite layer is due to de-
wetting of the perovskite solution from the flexible multilayer substrate 60,
which
causes holes to form in the perovskite layer that severely degrade the
performance of
the completed perovskite device. Carrier transport layers used in perovskite
devices
may be hydrophobic to improve device performance and most perovskite solutions
tend to poorly wet the hydrophobic carrier transport layers. Perovskite
solution
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dewetting is exacerbated by depositing thinner layers of perovskite solution
and by
increasing the drying time.
To enable high speed production of a uniform perovskite layer, a novel
perovskite solution has been formulated using a large proportion (e.g., at
least 50
weight percent of total solvent, preferably at least 75 weight percent of
total solvent,
more preferably at least 90 weight percent of total solvent) of a low boiling
point
(e.g., less than 150 degrees Celsius, preferably less than 135 degrees
Celsius) solvent.
Using the novel drying method of the disclosure, a low boiling point solvent
can be
made to evaporate quickly from the perovskite solution after deposition on a
substrate
thus minimizing movement of the crystals that form as the perovskite solution
dries.
Solvents that do not strongly coordinate with the perovskite precursors
further enable
short annealing times. Short annealing times are desirable because they enable
higher
production speeds. Alcohol based solvents have been identified that do not
strongly
coordinate with the perovskite precursors, provide the proper solubility of
the
inorganic precursors, and have been shown to produce a perovskite solution
that is
stable for use in high volume manufacturing of perovskite layers and
photovoltaic
devices. Examples of alcohol-based solvents suitable for use at high
proportions in
the perovskite solution include 2-methoxyethanol, 2-ethoxyethanol, 2-
butoxyethanol,
2-isopropoxyethanol, methanol, propanol, butanol, and ethanol. Mixtures of
solvents
are envisioned for use in the perovskite solution to tune the evaporation
profile to
further optimize the drying process. Suitable solvent additives useful for
modifying
evaporation rate of the solvent, e.g., include dimethylformamide,
acetonitrile,
dimethyl sulfoxide, N-methyl-2-pyrrolidone, dimethylacetamide, gamma-
butyrolactone, phenoxyethanol, acetic acid, and urea.
The preferred perovskite solution is formulated with greater than 30 percent
by
weight of solvent (e.g., 30-82 percent by weight) and at least 18 percent by
weight of
solids (e.g., 18-70 percent by weight, preferably 25-60 percent by weight or
30-45
percent by weight), where the total solids concentration of the perovskite
solution is
between 30 percent and 70 percent by weight of its saturation concentration at
the
provided solution temperature. The preferred provided solution temperature is
between
20 and 50 degrees Celsius. The preferred solvent is an alcohol and has a
boiling point
less than 135 degrees Celsius. The preferred solvent is 2-methoxyethanol,
which has a
boiling point of 125 degrees Celsius. The disclosed perovskite solution
formulations
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have the advantage of providing perovskite solutions that are stable at
convenient
handling and storage temperatures of, e.g., from 20 to 50 degrees Celsius and
in
particular typical room temperatures of from 20 to 25 degrees Celsius, and
which can
be used to manufacture a uniform Perovskite layer at high speed thereby
enabling low
cost production of high efficiency solar cells with low equipment costs.
Uniform perovskite layers have been made at high production speeds with
the novel drying method and perovskite solution. However, it has been found
that the
time required for the perovskite solution to form homogeneous nuclei and grow
may
be longer than the time required to evaporate the low boiling point solvent in
such a
way as to produce a uniform perovskite layer. A uniform perovskite layer with
optimum sized crystals is needed to make perovskite devices with high
photovoltaic
energy output. A crystal growth modifier added to a perovskite solution with a
low
boiling point solvent have been found that optimize the performance of
perovskite
photovoltaic devices. A crystal growth modifier is defined as an additive that
either
alters the amount of time for homogeneous crystal growth or alters the rate of
homogeneous crystal growth when drying a perovskite solution. Examples of
crystal
growth modifiers that are especially useful in perovskite solutions for making
high
performance perovskite layers include dimethyl sulfoxide, N-methyl-2-
pyrrolidone,
gamma-butyrolactone, 1,8-diiodooctane, N-cyclohexy1-2-pyrrolidone, water,
dimethylacetamide, acetic acid, cyclohexanone, alkyl diamines, and hydrogen
iodide.
A preferred concentration of crystal growth modifier is less than about 10
percent by
weight of the coating solution (e.g., 0.01 to 10 percent by weight). A more
preferred
concentration of crystal growth modifier is less than about 2 percent by
weight of the
coating solution (e.g., 0.01 to 2 percent by weight).
Another additive for a perovskite solution that alters the perovskite layer
to improve the performance of perovskite devices is a crystal grain boundary
modifier. A crystal grain boundary modifier is defined as an additive that
improves
the quality of the grain boundary, for example be altering the electrical
properties of
the perovskite crystal grain boundary or reducing trap states at perovskite
crystal
grain boundary interfaces. Examples of crystal grain boundary modifiers that
are
especially useful in perovskite solutions for making high performance
perovskite
layers include choline chloride, phenethylamine, hexylamine, 1-a-
phosphatidylcholine, polyethylene glycol sorbitan monostearate, sodium dodecyl
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sulfate, Poly(methyl methacrylate), Polyethylene glycol, pyridine, thiophene,
ethylene
carbonate, propylene carbonate, fullerenes, poly(propylene carbonate), and
didodecyldimethylammonium bromide. A preferred concentration of crystal grain
boundary modifier is less than about 2 percent by weight of the coating
solution (e.g.,
0.01 to 2 percent by weight). A more preferred concentration of crystal grain
boundary modifier is less than about 0.2 percent by weight of the coating
solution
(e.g., 0.01 to 0.2 percent by weight).
Examples of inorganic perovskite precursors for making perovskite
solutions include lead (II) iodide, lead (II) acetate, lead (II) acetate
trihydrate, lead (II)
chloride, lead (II) bromide, lead nitrate, lead thiocyanate, tin (II) iodide,
rubidium
halide, potassium halide, and cesium halide. Examples of organic perovskite
precursors for making perovskite solutions include methylammonium iodide,
methylammonium bromide, methylammonium chloride, methylammonium acetate,
formamidinium bromide, and formamidinium iodide. To produce a high performance
perovskite device it is preferred that the organic perovskite precursor
material has a
purity greater than 99 percent by weight and the inorganic perovskite
precursor has a
purity greater than 99.9 percent by weight. The inorganic perovskite precursor
contains a metal cation and preferred metal cation is lead. In the preferred
embodiment the molar ratio of organic perovskite precursor material to
inorganic
perovskite precursor material is between one and three.
In one embodiment of the disclosure the perovskite solution comprises an
organic perovskite precursor material, an inorganic perovskite precursor
material, and
a solvent wherein the amount of solvent is greater than 30 percent by weight
and the
perovskite solution has a total solids concentration by weight that is between
30
percent and 70 percent of the perovskite solution's saturation concentration
at the
provided solution temperature (i.e., temperature the solution is maintained at
prior to
deposition of the solution onto the flexible substrate. In preferred
embodiments, the
solvent may comprise one or more alcohols and the preferred provided solution
temperature is between 20 and 50 degrees Celsius. In further preferred
embodiments,
it is preferred to have an amount of alcohol that is less than 50 percent by
weight and
a total solids concentration greater than 35 percent by weight. In another
preferred
embodiment the perovskite solution has an amount alcohol that is greater than
50
percent by weight and a total solids concentration less than 40 percent by
weight. In
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another preferred embodiment, the perovskite solution has a total solids
concentration
between 30 and 45 percent by weight and an amount of 2-methoxyethanol that is
greater than 55 percent by weight.
When the perovskite solution dries, perovskite crystals or the intermediate
precursor phase for hybrid perovskite crystals (intermediate phase) form. The
intermediate phase is a crystal, adduct, or mesophase that is not the desired
final crystal
lattice, which is ABX3. The intermediate phase, if present, is converted to
the desired
final crystal lattice by annealing. This formation process has been found to
be highly
sensitive to variations in the solvent vapor concentration above the wet layer
and to
convective flow in the wet layer of perovskite solution. A novel multistep
method has
been developed to form a uniform and functional perovskite layer at high
speed. FIGS.
2a, 2b, 2c, and 2d illustrate in cross sections the formation of the
perovskite layer on a
portion of a multilayer flexible substrate 60 after important steps in
embodiments of the
disclosure. FIG. 2a shows the layer of perovskite solution 64a on a flexible
multilayer
substrate 60 immediately after the deposition of the perovskite solution.
The flexible multilayer substrate 60 comprises a flexible support 61, a first
conducting layer 62, and a first carrier transport layer 63. However, in some
embodiments the flexible support is the first conducting layer. For example,
when a
metal foil is used for flexible support 61 it can provide the functionality of
the first
conducting layer 62. FIG. 2b shows a layer of the partially dry perovskite
solution 64b
on the flexible multilayer substrate 60 after a first drying step, hence the
thickness of
the layer of partially dry perovskite solution 64b is less than the thickness
of the layer
of perovskite solution 64a shown in FIG. 2a. FIG. 2c shows an immobile layer
of
perovskite crystals or intermediate phases 64c on a flexible multilayer
substrate 60
after a second drying step hence the thickness of the immobile layer
perovskite
crystals or intermediate phases 64c is less than the thickness of the layer of
the
partially dry perovskite solution 64b shown in FIG. 2b. FIG. 2d shows the
completed
perovskite layer 64d on the flexible multilayer substrate 60 after an
annealing step.
Examples of materials comprising the flexible support 61 include
polyethylene terephthalate (PET), thin flexible glass such as Corning Willow
Glass, polyethylene naphthalate (PEN), polycarbonate (PC), polysulfone, metal
foil
(e.g. copper, nickel, titanium, steel, aluminum, and tin), and polyimide. With
the
exception of using metal foil, the preferred thickness of the flexible support
61 is in
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range from 25 to 200 microns. When metal foil is used the preferred thickness
of the
metal foil is between 5 and 50 microns.
Examples of materials comprising the first conducting layer 62 when used
as the window for the photovoltaic device include transparent and
semitransparent
electrodes based on metal-nanowires and metal thin-films (see J. Mater. Chem.
A,
2016, 4, 14481-14508, which is incorporated by reference in its entirety);
metal mesh
and metal grid electrodes made with metal nanoparticles, particulate metal
paste, and/or
electroplating; Poly(3,4-ethylenedioxythiophene) (PEDOT) complex such as
poly(3,4-
ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS); doped and undoped
metal oxides such as tin oxide (doped with indium or fluorine), molybdenum
oxide, and
zinc oxide (doped with aluminum). A metal foil is preferred when the first
conducting
layer 62 is not on the window side. The metal foil can be made from a wide
range of
metals but is preferred to be selected from the group consisting of copper,
nickel, or
stainless steel. The metal foil may have more than one layer of metal such as
copper
coated with nickel or tin. The metal foil may also be part of a laminate
structure and
include plastic layers such as PET or polyimide and an adhesive interlayer.
Examples of materials comprising the first carrier transport layer 63 and
the second carrier transport layer 65 include poly(triaryl amine) (also known
as
Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)aminel), poly-(N-vinyl carbazole),
PEDOT
complex, Poly(3-hexylthiophene), Spiro-MeOTAD (also known as
N2,N2,N2',N2',N7,N7,N7',N7'-octakis(4-methoxypheny1)-9,9'-spirobi[9H-fluorene]-
2,2',7,7'-tetramine), fullerenes (e.g. fullerene-C60 and phenyl-C61-butyric
acid
methyl ester), graphene, reduced graphene oxide, copper(I) thiocyanate,
cuprous
iodide, and metal oxide (e.g. tin oxide, nickel oxide, cerium oxide,
molybdenum
oxide, and zinc oxide) and their derivatives. Carrier transport layers can be
hole
transport layers or electron transport layers depending on the desired
structure of the
solar cell, e.g. NIP or PIN. Many other carrier transport materials are known
by those
skilled in the art and are envisioned as possible materials for this
disclosure.
Many types of deposition and drying devices are known to those skilled in
the art and a variety of devices are envisioned to be configured to use the
methods
described in the embodiments of the disclosure. A high speed, roll-to-roll
(R2R)
deposition and drying device that conveys a flexible substrate from a roll
through the
device will enable production of a perovskite layer at low cost. FIG. 3 shows
a
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schematic of an exemplary R2R deposition and drying device 100 that will be
used to
describe preferred embodiments of the disclosure. Additional configurations
can be
adapted to enable the multistep process of the disclosure by those skilled in
the art. A
flexible multilayer substrate 60 is unwound from a unwind roll 10 and threaded
through a deposition (and first drying step) section 20, a fast drying (second
drying
step) section 30, a long duration heating section 40, and a short duration
heating
section 50, then wound onto a rewind roll 12. Other components in R2R
deposition
and drying devices known in the industry are considered useful for this
disclosure but
are not shown in FIG. 3. For example, a cooling section (not shown) may be
useful
prior to the rewind roll 12. The direction of movement of the flexible
multilayer
substrate 60 through the R2R deposition and drying device 100 is identified by
the
arrows in the unwind roll 10 and the rewind roll 12. A surface treatment
device 14
conditions the surface of the flexible multilayer substrate 60 prior to
deposition of the
perovskite solution. Surface treatment devices include corona discharge, ozone
(created, for example, with ultraviolet radiation), and plasma. Surface
treatment
devices can operate in ambient air, conditioned air (where temperature and
relative
humidity are controlled), oxygen, or inert gas such as nitrogen or argon.
The deposition (and first drying step) section 20 of the R2R deposition and
drying device 100 includes one or more conveyance rollers 24 to direct the
path of the
flexible multilayer substrate 60 so that it is correctly presented to the
deposition device
21 as well as correctly conveyed through the deposition section 20. Conveyance
rollers,
tensioning rollers, and web guidance rollers are typically used throughout
deposition
and drying devices to aid in conveying flexible substrates, controlling
tension and
position. A conveyance roller 13 is shown prior to the rewind roll 12 and
conveyance
rollers 41a-e are shown in the long duration heating section 40. To simplify
FIG. 3
additional rollers are not shown. Conveyance rollers may include air bearings
to
minimize or eliminate contact with the flexible multilayer substrate 60. Air
flotation
methods (not shown) known by those skilled in the art may also be used to
minimize or
eliminate contact between conveyance rollers and the flexible multilayer
substrate 60.
The deposition device 21 that deposits a layer of perovskite solution
comprising a solvent and perovskite precursor material to the flexible
multilayer
substrate 60 can be any number of known deposition devices but is preferred to
be
based on a slot die or gravure system (direct, reverse, or offset) deposition
device.
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Other deposition devices envisioned for use in the disclosure include spray,
dip coat,
inkjet, flexographic, rod, and blade. The perovskite solution is supplied to
the
deposition device 21 by methods and devices known by those skilled in the art
(not
shown). The deposited perovskite solution layer is partially dried in section
20 in a
first drying step by removing a first portion of solvent from the deposited
solution
while heating the deposited solution to a coated layer temperature. To
optimize the
drying conditions and to improve the wettability of the layer of perovskite
solution
64a deposited on to the flexible multilayer substrate 60 the temperature of
the
perovskite solution and the coating device is preferably controlled by a
temperature
controller (not shown). The setpoint for the temperature of the perovskite
solution 64a
deposited on the multilayer substrate 60 depends on the formulation of the
perovskite
solution. The preferred temperature range for the heated deposited perovskite
solution
in the first drying step is between 30 and 100 degrees Celsius and a more
preferred
temperature range is between 35 and 60 degrees Celsius. The thickness of the
.. perovskite solution 64a initially deposited on the flexible multilayer
substrate 60 is
preferably less than 10 microns to minimize nonuniformities created by
convective
flow in the coated layer and greater than 2 microns to enable sufficient
wetting of the
perovskite solution 64a with the flexible multilayer substrate 60. A backing
roller 22
or set of rollers is used to set the engagement, gap or load to the deposition
device 21.
To optimize drying conditions in the first drying step, the amount of air
flow around the wet coating on the multilayer substrate 60 can optionally be
controlled by constraining the movement of air above the wet coating with an
air flow
control device 27 such as screens, baffles or plenums. The temperature and
humidity
of deposition section 20 may be controlled by an environmental controller 25a
to
optimize the coating and drying conditions in deposition section 20. Optional
control
of the temperature of backing roller 22 is envisioned as well as control of
the
temperature of the flexible multilayer substrate prior to and subsequent to
the
deposition device 21 as depicted by plenums 23a and 23b, however, heated
rollers, or
heated fixed curved surfaces are also envisioned to control the temperature of
the
flexible multilayer substrate with conductive heating. Backing roller 22 can
act as a
substrate heating device that heats the flexible multilayer substrate. The
backing roller
22 can have fluid flowing through it to maintain a preset temperature. This
type of
roller is sometimes called a jacketed roller. The preferred range that a
substrate
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heating device heats the flexible multilayer substrate to prior to depositing
the layer
of perovskite solution is between 30 and 100 degrees Celsius.
The flexible multilayer substrate 60 enters a fast-drying section 30 with
the wet coating of the perovskite solution on the flexible multilayer
substrate 60 that
was applied by deposition device 21. The first drying step is defined by the
removal
of a first portion of perovskite solution in the region between the deposition
device 21
and the fast-drying section 30. The amount of solvent removed in the first
drying step
is an important factor in making a uniform coating. This first drying step is
affected
by: the length of the first drying region, which is the distance between the
deposition
.. location 26 and the entrance of the fast-drying section 30; the temperature
of
deposition section 20, the temperature, speed, surface energy, and surface
area of the
flexible multilayer substrate 60; the amount of air flow around the wet
coating of the
perovskite solution on the flexible multilayer substrate 60 in the first
drying region;
and the formulation of the perovskite solution. The preferred temperature of
the area
around the flexible multilayer substrate 60 and the perovskite solution is
between 30
and 100 degrees Celsius during the first drying step.
The fast-drying section 30 defines a second drying step where a second
portion of the solvent from the perovskite solution is removed, where the
second drying
step has a higher rate of solvent evaporation than the first drying step. Any
suitable
device that causes rapid solvent removal from the wet coating can be used and
may
include a non-contact drying device 31 or a contact drying device 32 where
contact is
defined by physically contacting the flexible multilayer substrate. Non-
contact drying
devices include air knives, infrared heaters, microwave heaters, convection
ovens,
Rapid Thermal Processors, and high energy photonic devices such as Xenon
lamps.
Contact drying devices include conduction heaters such as heated rollers or
station
curved plates that contact the side of the web opposite the wet coating. A non-
contact
drying device 31 used in the preferred embodiment of the disclosure is an air
knife that
blows gas, such as air or nitrogen, across the surface of the coating to lower
the solvent
vapor pressure and quickly remove the evaporating solvent. The temperature of
the gas
is optionally controlled (not shown). Some non-contact drying devices may
benefit by
the use of a nearby backing roller or rollers to control the spacing to the
non-contact
device 31 or to aid in drying the perovskite solution. The temperature and
humidity of
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the fast-drying section 30 may also be controlled by an environmental
controller 25b to
optimize the conditions of the second drying step.
The second drying step causes a conversion reaction in the perovskite
solution that is induced by the rapid evaporation of the solvent from the
solution
causing saturation of the solids and crystal formation or formation of an
intermediate
phase. The conversion reaction is typically readily visually apparent as it
changes the
color or optical density of the perovskite solution. The degree of color
change and
change in optical density of the perovskite solution depends on the type and
quantity of
perovskite precursors that are present in the deposited perovskite solution.
In order to
create a uniform perovskite layer the conversion reaction must be fast in the
second
drying step so that the movement of the crystals is minimized as they are
formed. The
conversion reaction that occurs in the second drying step causes the
perovskite solution
to have a large reduction in the transmission of visible light. Preferably,
the percent
transmission of visible light through the perovskite solution due to the
conversion
reaction in the second drying step is reduced by at least a factor of 2. The
percent
transmission of visible light is defined by the amount of visible light
leaving the sample
divided by the amount of visible light entering the sample and can be measured
by
known methods such as directing white light on the deposited perovskite
solution both
prior to entering and after exiting the second drying location. The percent
transmission
of visible light is determined by measuring the visible light intensity both
entering and
exiting the flexible multilayer substrate at the two locations. If the
flexible multilayer
substrate is opaque then a reflection measurement can be used to determine
percent
transmission of visible light through the perovskite solution.
Using an air knife as a drying device in the second drying step and both
lead (II) iodide and methylammonium iodine as perovskite precursors, it has
been
observed that the color of the coated layer changed from yellow to dark brown
in the
second drying step, indicating successful perovskite conversion. To achieve a
uniform
coating at high speed it has been determined that the conversion reaction, as
evidenced by the color change and change in percent transmission of visible
light,
must occur quickly, preferably in a second drying step dwell time of less than
0.5
seconds after the second drying device first acts on the perovskite solution.
When an
air knife is used as the drying device, the air knife first acts on the
perovskite solution
at the focal point of the air flow directed to the perovskite solution
residing on the
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multilayer substrate, which is defined by the intersection of a line drawn
from the
source of the air flow to the flexible multilayer substrate where the angle of
the line is
such that the line follows the air flowing from the air knife. For optical
drying
devices, for example, an infrared heater, the location where the drying device
acts on
the perovskite is defined by the location where a significant portion of the
optical
radiation first strikes the perovskite solution, i.e., more than 5 percent of
the optical
energy has impinged on the perovskite solution out of the total amount that
impinges
on the perovskite solution from the optical device. The temperature of the
layer of
perovskite solution 64b can be increased to speed the evaporation rate in the
second
drying step. The preferred temperature in the area around the flexible
multilayer
substrate and the perovskite solution is greater than 30 degrees Celsius
during the
second drying step.
The dwell time of the first drying step is also important to obtaining a
uniform coating at high speed. If the first drying step is too fast then
convective flow
in the layer of perovskite solution 64a creates artifacts, such as mottle, in
the
completed perovskite layer 64d. In addition, enough of the solvent must be
removed in
the first drying step so that the layer of perovskite solution can be dried
quickly in the
second drying step. If the first drying step does not remove enough solvent
prior to the
second drying step then nonuniformities in the coating, such as blow marks,
are
formed in the perovskite layer during the second drying step. Furthermore, if
too much
solvent is removed in the first drying step then solids form in the layer of
perovskite
solution that create artifacts and nonuniformities in the completed perovskite
layer. In
a preferred embodiment the first drying step has a dwell time that is at least
5 times
longer than the second drying step dwell time, preferably at least 10 times
longer.
To form a uniform perovskite layer on the flexible multilayer substrate it
has been found that between 40 percent and 75 percent of the initial amount of
solvent should preferably be removed from the perovskite solution in the first
drying
step to create a layer of partially dry perovskite solution 64b. This range is
bounded
by the need for an ink that is both stable for use in a production environment
and also
can be dried uniformly. For example, when using 2-methoxyethanol as a solvent
and
methylammonium lead iodide precursors with a total solids concentration of 33
weight percent, then 43 to 70 percent of the initial amount of the solvent
must be
removed in the first drying step to concentrate the perovskite solution to
between 46
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and 62 weight percent of solids. The amount of solvent and the total solids
concentration at the end of the first drying step can be measured by
monitoring the
wet thickness with a low coherence interferometer mounted at the end of the
first
drying step and calculating the perovskite solution total solids concentration
and
amount of solvent using the known the initial thickness and total solids
concentration
of the perovskite solution. In addition, the amount of solvent remaining after
the
second drying step should be less than 10 percent of the initial amount of
solvent, and
preferably less than 5 percent of the initial amount of solvent. Furthermore,
it is
preferred that the first drying step increases the total solids concentration
of the
perovskite solution to at least 75 percent of its saturation concentration
(measured in
weight percent solids), and more preferably to at least 90 percent of its
saturation
concentration, so that the subsequent conversion of the solution to a thin
film of
immobile crystals can occur rapidly in the second drying step.
After the second drying step, the perovskite solution has changed from a
.. solution or colloidal suspension to a layer comprised of immobile
perovskite crystals
or intermediates. However, to make a high performance photovoltaic device an
additional annealing step is typically required. The function of the annealing
step can
include the removal of residual solvents, the removal of excess volatile
perovskite
solution components, the growth of perovskite crystals, a
dissolution¨recrystallisation
process (Ostwald ripening effect) of the perovskite crystals, conversion of
intermediates to perovskite crystals, and changes in perovskite crystal
boundaries. In
the long duration heating section 40 of FIG. 3 the flexible multilayer
substrate is
conveyed over a series of conveyance rollers 41a-41e. The entire structure of
the long
duration heating section 40 is enclosed to maintain a consistent temperature
and air
flow that is maintained by the environmental controller 25c. In some
embodiments of
the disclosure there is more than one compartment (not shown) in the long
duration
heating section 40, each with a separately controlled temperature and air
flow.
The annealing time of the layer of immobile perovskite crystals or
intermediates 64c (FIG. 2c) is important for producing high performance
photovoltaic
.. devices. In various embodiments, e.g., the annealing step may include
heating the
Perovskite layer to a temperature greater than 90 degrees Celsius for at least
30
seconds. For a flexible support 61 that can withstand high temperatures
without
distorting, such as thin flexible glass, metal foil, polysulfone, and
polyimide,
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increasing the temperature of the long duration heating section 40 of FIG. 3
can reduce
the required time to make a high performance perovskite layer. For flexible
supports
that can withstand high temperatures the preferred temperature of the area
around the
flexible multilayer substrate and the perovskite layer is between 120 and 300
degrees
Celsius during the annealing step. For a flexible support 61 that cannot
withstand high
temperatures, such as PET, PC, and PEN, the area around the flexible
multilayer
substrate and the perovskite layer is preferred to be between 90 and 125
degrees
Celsius during the annealing step to minimize distortion of the flexible
support 61.
A rapid annealing device can be employed to reduce the length of the
heating section or to increase the production speed when using some perovskite
formulations. One method to reduce the long duration heating time is to
rapidly heat
one or more of the thin film coatings 62, 63, and 64c of the flexible
multilayer
substrate 60 to high temperature for a short duration (FIG. 2c). If the thin
film
coatings are heated directly without significantly heating the flexible
support 61 then
it is even possible to make high performance devices on low temperature
flexible
support 61 without the need for a very long oven. Short duration, high
temperature
heating of any of the thin film coatings 62, 63, and 64c does not distort a
low
temperature flexible support 61 because the dissipation of heat from the thin
film
coatings into the low temperature flexible support 61 is low due to the large
difference in thickness between them: the low temperature flexible support 61
is
typically more than 150 times thicker than the thin film coatings 62, 63, and
64c.
FIG. 3 shows that the flexible multilayer substrate is conveyed from the
long duration heating section 40 to the short duration heating section 50.
Short
duration heating section 50 contains a short duration heater 51, such as a
Rapid
Thermal Processing unit or a high energy photonic device, e.g. a Xenon lamp. A
backing roller 52 or set of rollers can be optionally used to set the gap to
the short
duration heater 51. The temperature and humidity of the short duration heating
section 50 may also be controlled by an environmental controller 25d to
optimize the
conditions of the short duration heating section 50.
For some embodiments of the disclosure the long duration heating section
is eliminated and only the short duration heating section 50 is used. For some
embodiments of the disclosure both the long duration heating section 40 and
the short
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duration heating section 50 are used. For some embodiments of the disclosure
only
the long duration heating section 40 is used.
The flexible multilayer substrate 60 moves at nearly a constant speed
through the R2R deposition and drying device 100 (FIG. 3). To clarify some
important locations in the R2R deposition and drying device 100, a first
location is
defined by the region where the perovskite solution is deposited on the
flexible
multilayer substrate 60 by the deposition device 21. A second location is
defined by
the region between the deposition device 21 and the fast-drying section 30. A
third
location is defined as the fast-drying section 30. A fourth location is
defined as the
region where the perovskite layer is heated in the annealing step by the
annealing
device. The fourth location in Fig. 3 is the long duration heating section 40
and may
include the region in the optional short duration heating section 50. The
flexible
multilayer substrate 60 in the R2R deposition and drying device 100 is
preferred to
move at a speed greater than 5 meters per minute and more preferred to be
greater
than 10 meters per minute as it moves from a first location to a second
location, and
from the second location to a third location. In a preferred embodiment of the
disclosure the perovskite layer is heated by an annealing device in an
annealing step
at the fourth location, wherein the flexible multilayer substrate is preferred
to move a
speed greater than 5 meters per minute and more preferred to move at a speed
greater
than 10 meters per minute from the third location to the fourth location.
Examples of
annealing devices for use in the annealing step include a convection oven, a
Rapid
Thermal Processor, a photonic device (e.g. an infrared radiation source or a
xenon
lamp), a heated roller, and a stationary heated curved surface.
In a preferred embodiment of the disclosure the flexible multilayer
.. substrate is moving at a constant speed from the first location to the
second location,
and moving at the same constant speed from the second location to the third
location,
and the second drying step causes a conversion reaction in the perovskite
solution that
changes the color of the perovskite solution.
Methods and devices (not shown in FIG. 3) are envisioned to contain and
control particulate contaminates for the entire R2R deposition and drying
device 100
or for one or more of the sections 20, 30, 40, and 50. Devices and methods to
clean
particulates from the flexible multilayer substrate include forced air, sticky
rollers,
and electrical discharge devices. Devices and methods to clean the air and to
maintain
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specified clean room conditions include forced air through HEPA filters and
positive
pressure in enclosures. Methods and devices to remove and condition solvent
vapors
are envisioned but not shown in FIG. 3 nor are devices to remove unwanted
gases or
byproducts such as ozone and nitric oxides. Static control devices are
commonly used
in devices that convey flexible webs but are not shown in FIG. 3.
FIG. 4 shows a schematic of an exemplary multi-station R2R deposition
and drying device 200 for roll-to-roll printing a photovoltaic device on a
flexible
substrate that will be used to describe preferred embodiments of the
disclosure. A
station of the multi-station R2R deposition and drying device 200 is defined
as
comprising a deposition section but other sections and devices may be part of
the
station. Additional configurations can be adapted to enable the multistep
process of
the disclosure by those skilled in the art to make some or all layers of
perovskite
devices, especially perovskite solar cells. While FIG. 4 shows five stations,
more or
less than five stations are envisioned for variations on preferred embodiments
of the
disclosure. For example, a multi-station R2R deposition and drying device with
three
stations (not shown) could be used to apply a first carrier transport layer, a
perovskite
absorber layer, and a second carrier transport layer in succession on top of a
flexible
substrate having a first electrode layer and a support layer. Another example
is a multi-
station R2R deposition and drying device with four stations (not shown) where
the
first electrode layer is formed on the flexible substrate in the first station
of the multi-
station R2R deposition and drying device prior to the deposition of the first
carrier
transport layer. In this example, the device is supplied with a flexible
substrate having
only a support layer. Alternatively, when the multi-station R2R deposition and
drying
device is provided with a flexible substrate having a support and a first
electrode layer,
the fourth station could be used to apply a second electrode layer on to the
second
carrier layer. A multi-station R2R deposition and drying device with more than
five
stations is envisioned to make photovoltaic devices that require additional
layers that
improve the performance or functionality of the photovoltaic devices.
In FIG. 4 a flexible support 61 is unwound from a unwind roll 10 and
threaded through five deposition sections 20a-e and five long duration heating
sections 40a-e, in a continuous inline process to make a perovskite device 67.
The
direction of movement of the flexible substrate 61 through the multi-station
R2R
deposition and drying device 200 is identified by the arrows adjacent to the
unwind
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roll 10 and the rewind roll 12. Additional devices after each deposition
section or
long duration heating section are envisioned and some are shown in FIG.4 and
described below. Each deposition section 20a-e deposits a functional solution
on to
the flexible support 61 at the associated deposition location 26a-e with a
deposition
device 21a-e. Each long duration heating section 40a-e heats the functional
solution
deposited by the associated deposition device to dry, cure, anneal, and/or
sinter the
functional solution. Typically, process setpoints for each long duration
heating
section 40a-e are different as they are optimized for the solution that is
deposited by
the associated deposition device. Likewise, the process configurations and
setpoints
for each deposition section 20a-e may also be different from each other.
A preferred embodiment of a multi-station R2R deposition and drying
device 200 the disclosure is described here in more detail. Deposition section
20a
deposits a first electrode solution on the flexible support 61 with a first
electrode
deposition device 21a. Long duration heating section 40a dries and sinters the
first
electrode solution to form a first electrode layer. The flexible substrate
with the first
electrode layer then travels to the deposition section 20b where a first
carrier transport
solution is deposited on the first electrode layer with a first carrier
transport deposition
device 21b. Long duration heating section 40b dries and sinters the first
carrier
transport solution to form a first carrier transport layer. The flexible
substrate with the
first electrode layer and the first carrier transport layer then travels to
the deposition
section 20c where a perovskite solution is deposited on the first carrier
transport layer
with a perovskite solution deposition device 21c. A first portion of the
initial amount
of solvent in the deposited perovskite solution is removed in section 20c in a
first
drying step, similarly as described for section 20 in FIG. 3. After deposition
section
20c, the flexible substrate travels through a second drying step fast drying
section 30,
where a second portion of the initial amount of solvent in the deposited
perovskite
solution is removed. Note that the description of the fast drying section
appears above
in the description of FIG. 3, wherein the second drying step causes a
conversion
reaction in the perovskite solution that is induced by the rapid evaporation
of the
solvent from the solution causing saturation of the solids and crystal
formation or
formation of an intermediate phase. After fast drying section 30, Long
duration
heating section 40c further dries and anneals the coated perovskite solution
to form a
perovskite layer. The flexible substrate with the first electrode layer, the
first carrier
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transport layer, and the perovskite layer then travels to the deposition
section 20d
where a second carrier transport solution is deposited on the perovskite layer
with a
second carrier transport deposition device 21d. Long duration heating section
40d
dries the second carrier transport solution to form a second carrier transport
layer. The
flexible substrate with the first electrode layer, the first carrier transport
layer, the
perovskite layer, and the second carrier transport layer then travels to the
deposition
section 20e where a second electrode solution is deposited on the second
carrier
transport layer with a second electrode deposition device 21e. Long duration
heating
section 40e dries the second electrode solution to form a second electrode
layer. The
.. flexible substrate with the five functional layers is then wound onto a
rewind roll 12.
Laser etching of thin films is known in the art and used here to create a
monolithic photovoltaic device as part of the inline continuous manufacturing
process. Between the long duration heating section 40a and deposition section
20b,
the flexible substrate travels through a laser etch unit 70a. Between the long
duration
heating section 40d and deposition section 20e, the flexible substrate travels
through a
laser etch unit 70d. Between the long duration heating section 40e and rewind
roll 12,
the flexible substrate travels through a laser etch unit 70e. Each laser etch
unit
contains a laser device 71a,d,e, and a laser etch backing roller 72a,d,e. The
laser etch
backing rollers 72a,d,e are used to ensure that the flexible support 61 is in
a known
location. A vision system (not shown) can be incorporated in one or more of
the laser
etch units 70a,d,e to increase the accuracy of the location that the laser
etches. A
control system (not shown) can be incorporated in one or more of the laser
etch units
70a,d,e to position the laser spots based on data collected. Feed forward and
feedback
may be used in the control system. Laser etch unit 70a removes a portion of
the first
electrode layer. Laser etch unit 70d removes a portion of the second carrier
transport
layer, a portion of the perovskite layer, and a portion of the first carrier
transport
layer. Laser etch unit 70e removes a portion of the second electrode layer, a
portion
of the second carrier transport layer, a portion of the perovskite layer, and
a portion of
the first carrier transport layer.
All of the further sections and elements shown in FIG. 3 and described
above are envisioned to be included in the preferred multi-station R2R
deposition and
drying device to make the perovskite layer, but are not shown in FIG. 4 for
clarity.
Some of the sections and elements shown in FIG. 3 are also envisioned for use
in
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making the other layers in the multi-station R2R deposition and drying device
but are
not shown in FIG. 4 for clarity. For example, a surface treatment device 14
may be
used to condition the of the flexible support 61 or one or more of the layers
made on
the flexible support 61 prior to entering each deposition section 20a-e, and
environmental controllers may be used for some or all of the deposition
sections
20a-e and long duration heating sections 40a-e.
The use of conveyance rollers and backing rollers for R2R machines have
been described above and only a small number of conveyance rollers 13a-e and
backing rollers 22a-e are identified in FIG. 4. Other conventional components
in R2R
deposition and drying devices are known in the industry are envisioned for use
in the
method of this disclosure but are not shown in FIG. 4.
EXAMPLE 1
A flexible multilayer substrate having a width of 25.4 cm was conveyed
through a R2R deposition and drying device made by Polytype Converting for 3
trials
at constant speeds of 30, 32, and 35 meters per minute. The Polytype
Converting
machine was modified as described below to enable the multistep drying method
of
the disclosure. The R2R deposition and drying device had an inline arrangement
for
conveying a continuous flexible substrate from an unwind roll through the
following
sections: a surface treatment device, a deposition and first drying step
section, a
second drying step fast drying section, and a long duration heating section.
The
flexible multilayer substrate with the perovskite layer was wound on a rewind
roll.
The flexible multilayer substrate had a polyester film as the flexible
support, a thin
layer of indium tin oxide as the first conducting layer, and poly(triaryl
amine) as the
first carrier transport layer. The surface treatment device was a corona
discharge
device that treated the coating surface of the flexible multilayer substrate
with ozone
prior to the deposition section. In the deposition section a 4.5 micron thick
wet
laydown of perovskite solution was deposited on to the flexible multilayer
substrate
using a gravure cylinder in direct mode as the deposition device. The gravure
cylinder
was heated to a temperature of 40 degrees Celsius and maintained at that
temperature
while the perovskite solution was deposited. The perovskite solution had 33
weight
percent solids with an equal molar mixture of lead (II) iodide and
methylammonium
iodide and a liquid comprising 99.25 percent by volume of 2-methoxyethanol and
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0.75 percent by volume of dimethyl sulfoxide. The saturation concentration of
the
perovskite solution is 62 weight percent solids at 20 degrees Celsius. The
distance
from the deposition location to the fast drying section was 1.4 meters and
defines the
region of the first drying step. The first drying step included heating the
substrate
with a fixed curved surface 0.4 meters in length that contacted the backside
of the
moving flexible multilayer substrate across its entire width. The fixed curved
surface
was maintained at 73.6 degrees Celsius. The second drying step, occurring in
the fast
drying section, included an air knife that blew nitrogen out of a 75 micron
wide slot
on to the perovskite solution to increase the rate of solvent evaporation from
the
deposited perovskite solution relative to the first drying step. The slot was
positioned
1.5 cm from the moving substrate and ran across the width of the moving
substrate.
The focal point of the air knife was positioned at the downstream end of the
fixed
curved surface at an angle of 20 degrees relative to the web, pointing away
from the
deposition location. Nitrogen gas was supplied to the air knife at a flow rate
of 40
standard cubic feet per minute. The long duration heating section consisted of
a
convection oven 18 meters in length set to a temperature of 120 degrees
Celsius.
In the first drying step for the three trials, up to 70 percent of the initial
solvent was removed, concentrating the perovskite solution to as high as 62
weight
percent solids. In the fast drying step for the trials, a conversion reaction
of the
perovskite solution was observed to occur between 0 and 8 centimeters
downstream
from the focal point of the air knife. The conversion reaction caused the
transparent
yellow perovskite solution to turn dark brown and become opaque, evidencing
that the
percent transmission of visible light through the perovskite solution was
reduced by a
factor of greater than 2. A uniform perovskite layer approximately 0.5 microns
thick
was formed on the flexible multilayer substrate. In further trials, removing
too little
solvent in the first drying step (e.g., less than 40 percent of the initial
amount of
solvent) led to discontinuous perovskite layers with significant mottle caused
by
heterogeneous nucleation of perovskite crystals during the fast drying step.
These
perovskite layers were also observed to have obvious defects caused by crystal
movement during the fast drying step. Removing too much solvent in the first
drying
step (e.g., greater than 75 percent of the initial amount of solvent) led to
discontinuous
perovskite layers with significant mottle caused by heterogeneous nucleation
of
perovskite crystals during the first drying step. The trial that produced the
most
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uniform perovskite layer was achieved with the trial run at 32 meters per
minute. For
this trial a second transport layer and a second conducting layer were
subsequently
deposited onto the perovskite layer to make functioning photovoltaic devices.
EXAMPLE 2
A flexible multilayer substrate having a width of 15.2 cm was conveyed
through a R2R deposition and drying device made by Eastman Kodak Company for 6
trials at the constant speeds of 11.9, 12.2, 12.5, 12.8, 13.1, and 13.4 meters
per
minute. The R2R machine was modified as described below to enable the
multistep
drying method of the disclosure. The R2R deposition and drying device had an
inline
arrangement for conveying a continuous flexible substrate from an unwind roll
through the following sections: a deposition and first drying step section, a
second
drying step occurring in a fast drying section, and a long duration heating
section. The
flexible multilayer substrate with the perovskite layer was wound on a rewind
roll.
The flexible multilayer substrate had a polyester film as the flexible
support, a thin
.. layer of indium tin oxide as the first conducting layer, and poly(triaryl
amine) as the
first carrier transport layer. In the deposition section a 4.5 micron thick
wet laydown
of perovskite solution was deposited on to the flexible multilayer substrate
using a
slot die as the deposition device. The slot die was heated to a temperature of
50
degrees Celsius and maintained at that temperature while the perovskite
solution was
deposited. The back side of the flexible support was also heated to a
temperature of
50 degrees Celsius in the deposition section using a temperature controlled
roller, and
maintained at that temperature while the perovskite solution was deposited.
The
perovskite solution had 33 weight percent solids with an equal molar mixture
of lead
(II) iodide and methylammonium iodide and a liquid comprising 99.25 percent by
volume of 2-methoxyethanol and 0.75 percent by volume of N-methyl-2-
pyrrolidone
with 0.4 milligrams per milliliter ofl-a-phosphatidylcholine as an additive.
The
saturation concentration of the perovskite solution is 62 weight percent
solids at 20
degrees Celsius. The distance from the deposition location to the fast drying
section
was 1 meter and defines the region of the first drying step. The first drying
step
included heating the substrate and deposited perovskite solution in a 0.7
meter section
of an oven, over which a screen was positioned 3 cm above the moving web to
limit
air turbulence in the first drying step. The oven was controlled to
approximately 35
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degrees Celsius. The second drying step, occurring I the fast drying section,
included
an air knife that blew nitrogen out of a 75 micron wide slot on to the
perovskite
solution to increase the rate of solvent evaporation from the deposited
perovskite
solution relative to the first drying step. The slot was positioned 1.5 cm
from the
moving substrate and ran across the width of the moving substrate. The air
knife was
immediately downstream of the first drying step, fixed at an angle of 25
degrees
relative to the web, pointing away from the deposition location. Nitrogen gas
was
supplied to the air knife at a flow rate of 40 standard cubic feet per minute.
The long
duration heating section consisted of a convection oven 11.88 meters in length
set to a
temperature of 120 degrees Celsius.
In the first drying step for the 6 trials, up to 70 percent of the initial
solvent was removed, concentrating the perovskite solution to as high as 62
weight
percent solids. In the fast drying step for the trials, a conversion reaction
of the
perovskite solution was observed to occur between 0 and 5 centimeters
downstream
from the focal point of the air knife. The conversion reaction caused the
transparent
yellow perovskite solution to turn dark brown and become opaque, evidencing
that
the percent transmission of visible light through the perovskite solution was
reduced
by a factor of greater than 2. A uniform perovskite layer approximately 0.5
microns
thick was formed on the flexible multilayer substrate. In further trials,
removing too
little solvent in the first drying step (e.g., less than 40 percent of the
initial amount of
solvent) led to discontinuous perovskite layers with significant mottle caused
by
heterogeneous nucleation of perovskite crystals during the fast drying step.
These
perovskite layers were also observed to have obvious defects caused by crystal
movement during the fast drying step. Removing too much solvent in the first
drying
step (e.g., greater than 75 percent of the initial amount of solvent) led to
discontinuous perovskite layers with significant mottle caused by
heterogeneous
nucleation of perovskite crystals during the first drying step. The trial that
produced
the most uniform perovskite layer was achieved with the trial run at 12.8
meters per
minute. For this trial a second transport layer and a second conducting layer
were
subsequently deposited onto the perovskite layer to make functioning
photovoltaic
devices with power conversion efficiency exceeding 10 percent.
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The multistep drying method described here has been found to produce
very uniform perovskite layers and enables reliable, high speed production of
low
cost, high efficiency perovskite devices. While the methods described here use
roll-
to-roll conveyance, a sheet fed system is envisioned for some of the
embodiments
where the substrate is provided to sections and devices in the form of a
sheet.
Perovskite devices include electromagnetic radiation sensors, photovoltaic
devices,
and light emitting devices. The invention has been described in detail with
particular
reference to certain preferred embodiments thereof, but it will be understood
that
variations and modifications can be effected within the spirit and scope of
the
invention.
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PARTS LIST
unwind roll
12 rewind roll
13 conveyance roller
13a-e conveyance rollers
14 surface treatment device
deposition/first drying step section
20a-e deposition sections for associated stations of the multi-station R2R
deposition and drying device
21 deposition device
21a first electrode deposition device
21b first carrier transport deposition device
21c perovskite solution deposition device
21d second carrier transport deposition device
21e second electrode deposition device
22a-e backing rollers for associated deposition sections
26a-e deposition locations for associated deposition sections
22 backing roller
23a-b air plenum
24 conveyance roller
25a-d environmental controller
26 deposition location
27 air flow control device
fast drying/second drying step section
31 non-contact drying device
32 contact drying device
long duration heating section
40a-e long duration heating sections for associated stations of the multi-
station
R2R deposition and drying device
41a-e conveyance roller
short duration heating section
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51 short duration heater
52 backing roller
60 flexible multilayer substrate
61 flexible support
62 first conducting layer
63 first carrier transport layer
64a layer of perovskite solution
64b layer of partially dry perovskite solution
64c immobile layer of perovskite crystals or intermediates
64d completed perovskite layer
65 second carrier transport layer
66 second conducting layer
67 perovskite device
70a,d,e laser etch units for associated stations of the multi-station R2R
deposition
and drying device
71a,d,e laser etch devices for associated laser etch unit
72a,d,e laser etch backing rollers for associated laser etch unit
100 roll-to-roll (R2R) deposition and drying device
200 multi-station R2R deposition and drying device
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