U.S. patent application number 15/308822 was filed with the patent office on 2017-08-10 for system and method for fabricating perovskite film for solar cell applications.
This patent application is currently assigned to Okinawa Institute of Science and Technology School Corporation. The applicant listed for this patent is Okinawa Institute of Science and Technology School Corporation. Invention is credited to Luis Katsuya ONO, Yabing QI, Shenghao WANG.
Application Number | 20170229647 15/308822 |
Document ID | / |
Family ID | 54392305 |
Filed Date | 2017-08-10 |
United States Patent
Application |
20170229647 |
Kind Code |
A1 |
QI; Yabing ; et al. |
August 10, 2017 |
SYSTEM AND METHOD FOR FABRICATING PEROVSKITE FILM FOR SOLAR CELL
APPLICATIONS
Abstract
A system and method for fabricating perovskite films for solar
cell applications are provided, the system including a housing for
use as a vacuum chamber, a substrate stage coupled to the top
section of the housing; a first evaporator unit coupled to the
bottom section of the housing and configured to generate BX.sub.2
(metal halide material) vapor; a second evaporator unit coupled to
the housing and configured to generate AX (organic material) vapor;
and a flow control unit coupled to the housing for controlling
circulation of the AX vapor. The dimensions of the horizontal
cross-sectional shape of the first evaporator unit, the dimensions
of the horizontal cross-sectional shape of the substrate stage, and
the relative position in the horizontal direction between the two
horizontal cross-sectional shapes are configured to maximize the
overlap between the two horizontal cross-sectional shapes.
Inventors: |
QI; Yabing; (Okinawa,
JP) ; ONO; Luis Katsuya; (Okinawa, JP) ; WANG;
Shenghao; (Okinawa, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Okinawa Institute of Science and Technology School
Corporation |
Okinawa |
|
JP |
|
|
Assignee: |
Okinawa Institute of Science and
Technology School Corporation
Okinawa
JP
|
Family ID: |
54392305 |
Appl. No.: |
15/308822 |
Filed: |
April 10, 2015 |
PCT Filed: |
April 10, 2015 |
PCT NO: |
PCT/JP2015/002041 |
371 Date: |
November 3, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61988547 |
May 5, 2014 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 51/0012 20130101;
H01L 51/44 20130101; C23C 14/548 20130101; C23C 14/545 20130101;
Y02E 10/549 20130101; H01L 51/42 20130101; C23C 14/24 20130101;
H01L 51/001 20130101; C23C 14/06 20130101 |
International
Class: |
H01L 51/00 20060101
H01L051/00; C07F 7/28 20060101 C07F007/28; H01L 51/44 20060101
H01L051/44; C23C 14/24 20060101 C23C014/24; C23C 14/54 20060101
C23C014/54 |
Claims
1: A system for fabricating a perovskite film for solar cell
applications, by using source materials AX and BX.sub.2, wherein
the AX is an organic halide material and the BX.sub.2 is a metal
halide material, wherein the halogen X in the AX and the halogen X
in the BX.sub.2 are the same element or different elements, the
system comprising: a housing for use as a vacuum chamber, the
housing having a side section along a vertical direction and top
and bottom sections along a horizontal direction; a substrate stage
coupled to the top section of the housing and configured to have a
stage surface facing vertically downward for a substrate to be
placed on; a first evaporator unit coupled to the bottom section of
the housing and configured to generate BX.sub.2 vapor; a second
evaporator unit coupled to the housing and configured to generate
AX vapor; and a flow control unit coupled to the housing for
controlling circulation of the AX vapor in the housing, wherein
dimensions of a horizontal cross-sectional shape of the first
evaporator unit, dimensions of a horizontal cross-sectional shape
of the substrate stage, and a relative position in the horizontal
direction between the two horizontal cross-sectional shapes are
configured to maximize an overlap between the two horizontal
cross-sectional shapes.
2: The system of claim 1, wherein the stage surface of the
substrate stage is configured to have an area for accommodating a
substrate having a size of 5 cm.times.5 cm or larger, wherein the
substrate is a one-piece substrate or a collection of a plurality
of substrates.
3: The system of claim 1, wherein the substrate stage, the first
evaporator unit, the second evaporator unit and the flow control
unit are configured to enable deposition of the BX.sub.2 vapor to
be substantially directional, following line-of-sight transfer from
the first evaporator unit to the substrate, while enabling
deposition of the AX to be substantially less directional based on
the AX vapor circulating in the housing.
4: The system of claim 1, wherein the flow control unit is
configured to control the circulation of the AX vapor to generate a
substantially uniform flow of the AX vapor over the substrate.
5: The system of claim 1, wherein the flow control unit includes a
fan system, a pump system or a combination thereof.
6: The system of claim 1, further comprising: a first shutter
provided below the substrate stage and configured to be moved to
expose or cover the substrate stage to control deposition of the
BX.sub.2 vapor onto the substrate; and a second shutter provided
above the first evaporator unit and configured to be moved to
expose or cover the first evaporator unit to control a flow of the
BX.sub.2 vapor.
7: The system of claim 1, wherein temperature of the substrate
stage is controlled to provide uniform cooling or heating to the
substrate in a range between -190.degree. C. to 200.degree. C.
8: The system of claim 7, wherein the temperature of the substrate
stage is controlled to have the substrate at a room temperature in
a range between 15.degree. C. to 25.degree. C.
9: The system of claim 1, wherein a first evaporation temperature
associated with the first evaporator unit is controlled to adjust a
first evaporation rate for generating the BX.sub.2 vapor.
10: The system of claim 9, wherein the first evaporator unit
includes a container for containing the BX.sub.2 in powder form and
a heating element configured to heat the container uniformly,
wherein the heating element is controlled to provide the first
evaporation temperature to adjust the first evaporation rate for
generating the BX.sub.2 vapor.
11: The system of claim 1, wherein a second evaporation temperature
associated with the second evaporator unit is controlled to adjust
a second evaporation rate for generating the AX vapor.
12: The system of claim 11, wherein the second evaporator unit
includes a container for containing the AX in powder form and a
heating element configured to heat the container uniformly, wherein
the heating element is controlled to provide the second evaporation
temperature to adjust the second evaporation rate for generating
the AX vapor.
13: The system of claim 1, wherein the second evaporator unit is
coupled to the side section of the housing.
14: The system of claim 1, wherein the second evaporator unit is
coupled to the bottom section of the housing.
15: The system of claim 14, further comprising: a shield between
the first evaporator unit and the second evaporator unit to reduce
thermal interference therebetween.
16: The system of claim 1, wherein the second evaporator unit
includes a valve or an evaporator shutter for controlling a flux of
the AX vapor exiting from the second evaporator unit.
17: The system of claim 1, further comprising: a gate valve coupled
between the housing and a pump unit for controlling pressure inside
the housing to a value optimal for a chemical reaction between the
source materials and for efficient use of the source materials.
18: The system of claim 17, wherein the gate valve is configured to
assume at least first and second positions, wherein the first
position is for use for controlling AX vapor pressure to stabilize
the circulation of the AX vapor and the second position is for use
for pumping out remaining vapor from the housing after deposition
is completed.
19: The system of claim 1, further comprising: a first monitor for
monitoring the BX.sub.2 vapor and thickness of the perovskite film
growing on the substrate.
20: The system of claim 1, further comprising: a second monitor for
monitoring the AX vapor.
21: The system of claim 1, wherein temperature of the housing is
kept at about 70.degree. C.
22: The system of claim 1, further comprising: a second housing for
use as a load-lock chamber; a second gate valve coupled between a
second pump unit and the second housing, the second gate valve and
the second pump being configured for controlling pressure inside
the second housing; a third gate valve coupled between the housing
and the second housing for controlling communication therebetween;
and a sample transfer system coupled to the housing for
transferring the substrate between the housing and the second
housing.
23: The system of claim 22, wherein the sample transfer system
includes: a mechanical device for holding and releasing an object;
and a rod coupled to the mechanical device for controlling movement
of the mechanical device.
24: The system of claim 23, wherein the second housing is
configured to store the substrate, and the second pump unit and the
second gate valve are controlled to have a predetermined pressure
level in the second housing while the third gate valve is closed;
after evaporation temperatures for the source materials are
controlled and the flow control unit is controlled to circulate the
AX vapor in the housing, the third gate valve is opened, and the
mechanical device is moved to reach and hold the substrate in the
second housing and moved back to the housing to release and place
the substrate on the substrate stage; and thereafter the third gate
valve is closed.
25: The system of claim 24, wherein after a predetermined thickness
of the perovskite film is attained, the third gate valve is opened,
and the mechanical device is moved to reach and hold the substrate
on which the perovskite film is grown in the housing, moved to the
second housing to release and place the substrate on which the
perovskite film is grown in the second housing, and moved back to
the housing; and thereafter the third gate valve is closed.
26: A method for fabricating a perovskite film for solar cell
applications, by using source materials AX and BX.sub.2, wherein
the AX is an organic halide material and the BX.sub.2 is a metal
halide material, wherein the halogen X in the AX and the halogen X
in the BX.sub.2 are the same element or different elements, and by
using a system comprising: a housing for use as a vacuum chamber,
the housing having a side section along a vertical direction and
top and bottom sections along a horizontal direction; a substrate
stage coupled to the top section of the housing and configured to
have a stage surface facing vertically downward for a substrate to
be placed on; a first evaporator unit coupled to the bottom section
of the housing and configured to generate BX.sub.2 vapor; a second
evaporator unit coupled to the housing and configured to generate
AX vapor; a flow control unit coupled to the housing for
controlling circulation of the AX vapor in the housing; a gate
valve coupled between the housing and a pump unit for controlling
pressure inside the housing; a first shutter provided below the
substrate stage and configured to be moved to expose or cover the
substrate stage; and a second shutter provided above the first
evaporator unit and configured to be moved to expose or cover the
first evaporator unit, the method comprising: controlling
temperature of the substrate stage for providing uniform cooling or
heating to the substrate; moving the first shutter to cover the
substrate; moving the second shutter to expose the first evaporator
unit; opening the gate valve to a first position; controlling a
first evaporation temperature associated with the first evaporator
unit to adjust a first evaporation rate for generating the BX.sub.2
vapor; controlling a second evaporation temperature associated with
the second evaporator unit to adjust a second evaporation rate for
generating the AX vapor; controlling the flow control unit to
control the circulation of the AX vapor; moving the first shutter
to expose the substrate; monitoring thickness of the perovskite
film growing on the substrate; moving the first shutter to cover
the substrate when the thickness of the perovskite film reaches a
predetermined thickness; terminating heating of the first and
second evaporator units; and opening the gate valve to a second
position to pump out remaining vapor inside the housing, wherein
dimensions of a horizontal cross-sectional shape of the first
evaporator unit, dimensions of a horizontal cross-sectional shape
of the substrate stage, and a relative position in the horizontal
direction between the two horizontal cross-sectional shapes are
configured to maximize an overlap between the two horizontal
cross-sectional shapes.
27: The method of claim 26, wherein the substrate stage, the first
evaporator unit, the second evaporator unit and the flow control
unit are configured to enable deposition of the BX.sub.2 vapor to
be substantially directional, following line-of-sight transfer from
the first evaporator unit to the substrate, while enabling
deposition of the AX to be substantially less directional based on
the AX vapor circulating in the housing.
28: The method of claim 26, wherein the stage surface of the
substrate stage is configured to have an area for accommodating a
substrate having a size of 5 cm.times.5 cm or larger, wherein the
substrate is a one-piece substrate or a collection of a plurality
of substrates.
29: The method of claim 26, wherein the controlling the temperature
of the substrate stage comprises controlling the temperature of the
substrate stage to have the substrate at a room temperature in a
range between 15.degree. C. to 25.degree. C.
30: The method of claim 26, further comprising placing the
substrate on the substrate stage prior to the controlling the
temperature of the substrate stage.
31: The method of claim 26, wherein the system further comprises: a
second housing for use as a load-lock chamber; a second gate valve
coupled between a second pump unit and the second housing, the
second gate valve and the second pump being configured for
controlling pressure inside the second housing; a third gate valve
coupled between the housing and the second housing for controlling
communication therebetween; and a sample transfer system coupled to
the housing for transferring the substrate between the housing and
the second housing, the method further comprising: storing the
substrate in the second housing; controlling the second pump unit
and the second gate valve to have a predetermined pressure level in
the second housing while the third gate valve is closed; opening
the third gate valve; controlling the sample transfer system to
reach and hold the substrate in the second housing and transfer the
substrate from the second housing to the housing, and to release
and place the substrate on the substrate stage; and closing the
third gate valve, wherein the opening through the closing the third
gate valve are carried out after the controlling the flow control
unit to control the circulation of the AX vapor and prior to the
moving the first shutter to expose the substrate.
32: The method of claim 31, further comprising: opening the third
gate valve; controlling the sample transfer system to reach and
hold the substrate on which the perovskite film is grown in the
housing and transfer the substrate on which the perovskite film is
grown from the housing to the second housing, and to release and
place the substrate on which the perovskite film is grown in the
second housing; and closing the third gate valve, wherein the
opening through the closing the third gate valve are carried out
after the opening the gate valve to a second position to pump out
remaining vapor inside the housing.
33: A perovskite film that has a perovskite structure having
ABX.sub.3 structure as a unit cell, where A is MA, FA or 5-AVA, B
is Pb or Sn, and X is Cl, I or Br, wherein an X-ray diffraction
spectrum of the perovskite film has a (110) plane peak, a (220)
plane peak and a (330) plane peak within a surface area of 5
cm.times.5 cm or larger.
34: The perovskite film of claim 33, wherein the perovskite film is
a CH.sub.3NH.sub.3PbI.sub.3-XCl.sub.X film, and the X-ray
diffraction spectrum of the perovskite film does not have a peak at
15.7.degree. within the surface area of 5 cm.times.5 cm or larger.
Description
TECHNICAL FIELD
[0001] The present invention relates to a system and a method for
fabricating perovskite film for solar cell applications.
BACKGROUND ART
[0002] A solar cell (also called a photovoltaic cell) is an
electrical device that converts solar energy directly into
electricity by using semiconductors that exhibit the photovoltaic
effect. Solar photovoltaics is now, after hydro and wind power, the
third most important renewable energy source in terms of globally
installed capacity. Constructions of these solar cells are based
around the concept of a p-n junction, wherein photons from the
solar radiation are converted into electron-hole pairs. Examples of
semiconductors used for commercial solar cells include
monocrystalline silicon, polycrystalline silicon, amorphous
silicon, cadmium telluride, and copper indium gallium diselenide.
Solar cell energy conversion efficiencies for commercially
available cells are currently reported to be around 14-22%.
[0003] High conversion efficiency, long-term stability and low-cost
fabrication are essential for commercialization of solar cells. For
this reason, a wide variety of materials have been researched for
the purpose of replacing conventional solar cell semiconductors.
For example, the solar cell technology using organic semiconductors
is relatively new, wherein these cells can be processed from liquid
solution, potentially leading to inexpensive, large scale
production. Besides organic materials, organometal halide
perovskites, CH.sub.3NH.sub.3PbX.sub.3, where X=Cl, Br, I or a
combination thereof, have recently emerged as a promising material
for the next generation of high efficiency, low cost solar
technology. In addition, they exhibit flexible properties that
enable innovative device structures, such as Tandem cells (e.g.
combination of PbX.sub.2, CH.sub.3NH.sub.3PbX.sub.3, and Pb-free
perovskites), gradient concentration cells, and other high
throughput structures. It has been reported that these synthetic
perovskites exhibit high charge carrier mobility and lifetime that
allow light-generated electrons and holes to move far enough to be
extracted as current, instead of losing their energy as heat within
the cell. These synthetic perovskites can be fabricated by using
the same thin-film manufacturing techniques as those used for
organic solar cells, such as solution processing and vacuum
evaporation techniques.
[0004] However, to date, it has been difficult to obtain large-area
highly uniform perovskite films based on the existing fabrication
techniques, and practical perovskite-based solar devices are
essentially non-existent. In view of ever increasing needs for
highly efficient and stable solar cells at low cost, a new
fabrication system and method are desired for producing large
scale, highly uniform perovskite films suited for solar cell
applications.
CITATION LIST
Non Patent Literature
[0005] NPL 1: Julian Burschka et al., Sequential deposition as a
route to high-performance perovskite-sensitized solar cells.
Nature, vol. 499, 316-320 (2013).
[0006] NPL 2: Mingzhen Liu et al., Efficient planar heterojunction
perovskite solar cells by vapour deposition. Nature, vol. 000, 1-8
(2013).
[0007] NPL 3: Dianyi Liu et al., Perovskite solar cells with a
planar heterojunction structure prepared using room-temperature
solution processing techniques. Nature Photonics, vol. 8 133-138
(2014).
[0008] NPL 4: Olga Malinkiewicz et al., Perovskite solar cells
employing organic charge-transport layers. Nature Photonics, vol. 8
128-132 (2014).
[0009] NPL 5: Nam-Gyu Park, Organometal Perovskite Light Absorbers
Toward a 20% Efficiency Low-Cost Solid-State Mesoscopic Solar Cell.
J. Phys. Chem. Lett. 2423-2429 (2013).
SUMMARY
[0010] According to an aspect of the invention, there is provided a
system for fabricating a perovskite film for solar cell
applications, by using source materials AX and BX.sub.2, wherein
the AX is an organic material having an organic element A selected
from a group consisting of methylammonium (MA), formamidinium (FA)
and 5-aminovaleric acid (5-AVA) and a halogen element X selected
from a group consisting of Cl, I and Br, or a combination of two or
more of the organic materials; and the BX.sub.2 is a metal halide
material having a metal element B selected from a group consisting
of Pb and Sn and a halogen element X selected from a group
consisting of Cl, I and Br, or a combination of two or more of the
metal halide materials, the system comprising: a housing for use as
a vacuum chamber, the housing having a side section along a
vertical direction and top and bottom sections along a horizontal
direction; a substrate stage coupled to the top section of the
housing and configured to have a stage surface facing vertically
downward for a substrate to be placed on; a first evaporator unit
coupled to the bottom section of the housing and configured to
generate BX.sub.2 vapor; a second evaporator unit coupled to the
housing and configured to generate AX vapor; and a flow control
unit coupled to the housing for controlling circulation of the AX
vapor in the housing, wherein dimensions of a horizontal
cross-sectional shape of the first evaporator unit, dimensions of a
horizontal cross-sectional shape of the substrate stage, and a
relative position in the horizontal direction between the two
horizontal cross-sectional shapes are configured to maximize an
overlap between the two horizontal cross-sectional shapes.
BRIEF DESCRIPTION OF DRAWINGS
[0011] FIG. 1 illustrates an example of the system configuration
for fabricating a perovskite film according to an embodiment.
[0012] FIG. 2 illustrates a vertical view from below with respect
to the cross-sectional plane indicated by X-X' in FIG. 1.
[0013] FIG. 3 illustrates a vertical view from above with respect
to the cross-sectional plane indicated by Y-Y' in FIG. 1.
[0014] FIGS. 4A and 4B illustrate examples of the second evaporator
unit 124 of the system in FIG. 1.
[0015] FIGS. 5A-5C illustrate an example of the first evaporator
unit 120 of the system in FIG. 1.
[0016] FIGS. 6A and 6B illustrate examples of the flow control unit
128 of the system in FIG. 1.
[0017] FIG. 7 illustrates another example of the system
configuration for fabricating a perovskite film according to an
embodiment.
[0018] FIG. 8 illustrates a side view of a third example of the
second evaporator unit of the system in FIG. 7.
[0019] FIG. 9 is a flowchart illustrating the fabrication method of
a perovskite film using the present system illustrated in FIG. 1 or
FIG. 7.
[0020] FIG. 10 schematically illustrates the deposition mechanism
according to the present system and method.
[0021] FIGS. 11A-11D illustrate a sequence of system configurations
including a load-lock chamber.
[0022] FIG. 12 is a flowchart illustrating the fabrication method
of a perovskite film using the present system including a load-lock
chamber.
[0023] FIG. 13 is a plot of the J-V curve representing the
photovoltaic device characterization of a solar cell including the
chloride iodide perovskite film,
CH.sub.3NH.sub.3PbI.sub.3-XCl.sub.X, grown by the present
fabrication system and method.
[0024] FIG. 14 is a plot showing the X-ray diffraction (XRD)
spectrum of the CH.sub.3NH.sub.3 PbI.sub.3-XCl.sub.X film with a
thickness of .about.50 nm.
[0025] FIG. 15 is a plot showing the X-ray diffraction (XRD)
spectra measured at 12 different locations of the
CH.sub.3NH.sub.3PbI.sub.3-XCl.sub.X film of .about.135 nm thickness
grown on a tin-doped indium oxide (ITO)/glass substrate with a 5
cm5 cm surface area.
[0026] FIG. 16 is a photo showing the atomic force microscopy (AFM)
image of the CH.sub.3NH.sub.3PbI.sub.3-XCl.sub.X film of .about.50
nm thickness grown on the ITO/glass substrate.
[0027] FIG. 17 is a plot showing the optical absorption of the
CH.sub.3NH.sub.3PbI.sub.3-XCl.sub.X film of .about.135 nm
thickness.
[0028] FIG. 18 shows photos of actual devices including the
CH.sub.3NH.sub.3PbI.sub.3-XCl.sub.X films of .about.50 nm thickness
and .about.135 nm thickness, respectively.
DESCRIPTION OF EMBODIMENTS
[0029] In view of ever increasing needs for highly efficient and
stable solar cells at low cost, this document describes a new
fabrication system and method for producing highly crystalline,
large scale, substantially uniform perovskite films suited for
solar cell applications. The present fabrication method may be
characterized as a hybrid of chemical vapor deposition and physical
vapor deposition techniques, wherein vapor sources and associated
parts in a vacuum chamber are configured to optimize the deposition
quality by utilizing material characteristics of each source
material. Here, the first category of source materials include
metal halide materials such as PbCl.sub.2, PbBr.sub.2, PbI.sub.2,
SnCl.sub.2, SnBr.sub.2, SnI.sub.2 and the like, and the second
category of source materials include methylammonium
(MA=CH.sub.3NH.sub.3.sup.+) compounds such as MACl, MABr, MAI and
the like, formamidinium (FA=HC(NH.sub.2).sub.2.sup.+) compounds
such as FACl, FABr, FAI and the like, and 5-aminovaleric acid
(5-AVA). (See, for example, Science 345, 295-298 (2014); Nature
517, 476-480 (2015).) An organometal halide perovskite structure is
the orthorhombic structure having the general ABX.sub.3 structure
as the unit cell, in which an organic element, MA, FA or 5-AVA,
occupies the site A; a metal element, Pb.sup.2+ or Sn.sup.2+,
occupies the site B; and a halogen element, Cl.sup.-, I.sup.- or
Br.sup.-, occupies the site X. In this document, AX represents an
organic material having an organic element A selected from a group
consisting of MA, FA and 5-AVA and a halogen element X selected
from a group consisting of Cl, I and Br, or a combination of two or
more of the organic materials. Here, a combination refers to a
mixture of two or more of the above organic materials, MAI and MACl
in mixed powder form, for example, which can be used for the
deposition if the respective evaporation temperatures fall within a
predetermined temperature range. A combination also refers to a
mixed compound of two or more of the above organic materials, such
as MAI.sub.(1-X)Cl.sub.X. Furthermore, in this document, BX.sub.2
represents a metal halide material having a metal element B
selected from a group consisting of Pb and Sn and a halogen element
X selected from a group consisting of Cl, I and Br, or a
combination of two or more of the metal halide materials. Here,
again, a combination refers to a mixture of two or more of the
above metal halide materials in mixed powder form, which can be
used for the deposition if the respective evaporation temperatures
fall within a predetermined temperature range. A combination also
refers to a mixed compound of two or more of the above metal halide
materials, such as Pb.sub.(1-X)Sn.sub.XI.sub.2. Examples of
implementations according to the present system and method are
described below with reference to accompanying drawings.
[0030] FIG. 1 illustrates an example of the system configuration
for fabricating a perovskite film according to an embodiment. The
system includes a housing 100 coupled to necessary parts. The
housing 100 can have a shape of substantially a hollow cylinder,
having a side section along the vertical direction and top and
bottom sections along the horizontal direction. The shape of the
housing 100 can be of substantially a hollow box having four
rectangular faces as the side section along the vertical direction,
one rectangular face as the top section along the horizontal
direction, and another rectangular face as the bottom section along
the horizontal direction. The shape of the housing 100 can be
adapted to have any shape as long as necessary parts can be
properly coupled to the housing 100. Each section has an internal
surface and an external surface. In FIG. 1, only the internal
surfaces of the housing 100 are illustrated. The housing 100 is
coupled to a pump unit 104 for generating near vacuum in the
housing 100, which is used as a vacuum chamber for the deposition
process. Examples of the pump unit 104 include a turbo molecular
pump. A gate valve 108 is coupled in this example between the pump
unit 104 and the housing 100 to control the pressure inside the
housing 100, wherein the open/close of the gate valve 108 can be
controlled manually, by use of a computer or any other suitable
means. The gate valve 108 can be positioned to adjust the pressure
inside the housing 100 to a value optimal for the chemical reaction
between the source materials and for efficient use of the source
materials. The pump unit 104 and the gate valve 108 can be coupled
to the bottom section, as illustrated in FIG. 1, to the side
section or any other suitable section of the housing 100. The
pressure inside the chamber may be monitored by a pressure gauge
over a full range, i.e., 110.sup.5.about.110.sup.-7 Pa. A substrate
stage 112 is coupled to the top section of the housing 100 and
configured to have a large stage surface facing downward for a
substrate 116 to be staged facing downward. The term "substrate" in
this document is referred to as a one-piece substrate or a
collection of multiple substrates in this document. The area of the
stage surface in the present system can be configured to
accommodate a large substrate, for example, 5 cm5 cm or larger, or
a multiple substrates with a total area of 5 cm5 cm or larger. The
shape of the stage surface 112 can be a circle, a square, a
rectangle, or any other shape, as long as such a large substrate or
multiple substrates can be accommodated. The temperature of the
substrate stage 112 is controlled to provide uniform cooling or
heating to the substrate 116. In a specific example, the
temperature of the substrate stage 112 is controlled to range from
-190.degree. C. up to 200.degree. C. A first evaporator unit 120 is
coupled to the bottom section of the housing 100, and is configured
for generating vapor of the metal halide material BX.sub.2. A first
evaporation temperature is associated with the first evaporator
unit 120, and is controlled to adjust a first evaporation rate for
generating the BX.sub.2 vapor. The dimensions of the horizontal
cross-sectional shape of the first evaporator unit 120, the
dimensions of the horizontal cross-sectional shape of the substrate
stage 112, and the relative position in the horizontal direction
between the above two horizontal cross-sectional shapes are
configured to maximize the overlap between the two horizontal
cross-sectional shapes. For example, the substrate stage 112 has a
horizontal cross-sectional shape of a 6 cm6 cm square; the first
evaporator unit 120 has a horizontal cross-sectional shape of a
circle with a 6 cm diameter; and the center of the square and the
center of the circle are vertically aligned. In another example,
each of the substrate stage 112 and the first evaporator unit 120
has a horizontal cross-sectional shape of a circle with a 10 cm
diameter; and the centers of these two circles are vertically
aligned.
[0031] A second evaporator unit 124 is coupled to the side section
of the housing 100 in this example, and is configured for
generating vapor of the organic material AX. As described in a
later example, the second evaporator unit 124 can be coupled to the
bottom section of the housing 100, separated from the first
evaporator unit 120 by a predetermined distance. A second
evaporation temperature is associated with the second evaporator
unit 124, and is controlled to adjust a second evaporation rate for
generating the AX vapor. The chamber body, i.e., the body of the
housing 100, may be kept at .about.70.degree. C. which helps to
reduce the adsorption of the AX vapor onto the chamber wall.
[0032] A flow control unit 128 is provided at the side section of
the housing 100 in this example in order to control the AX vapor
flow to circulate it effectively in the housing 100. The flow
control unit 128 is provided at the side section, substantially
opposite to the second evaporator unit 124 in this example.
However, the flow control unit 128 may be coupled to the housing
100 at any position with respect to the position of the second
evaporator unit 124 as long as it facilitates a substantially
uniform flow of the AX vapor over the substrate 116. The flow
control unit 128 may comprise one or more mechanical systems to
promote the circulation of the AX vapor in the housing 100.
Examples of the flow control unit 128 include a fan system, a pump
system, and a combination thereof. Examples of the pump system
include a foreline pump. In the combination example, the fan system
may be provided on the same side as and above the second evaporator
unit 124, and the pump system may be provided opposite to the
second evaporator unit 124. In another example, the flow control
unit 128 may be configured to have only the pump system, which is
coupled to the side section substantially opposite to the second
evaporator unit 124 and substantially leveled with the stage
surface of the substrate stage 112 to promote the uniformity of the
AX vapor over the substrate 116.
[0033] A first monitor 132 may be coupled to the top section of the
housing 100, in the proximity of the stage surface of the substrate
stage 112, in order to monitor the perovskite film thickness in
situ. The first monitor 132 can be used to monitor the vapor flow
of the metal halide material BX.sub.2 from the first evaporator
unit 120 as well as the film thickness. Monitoring the BX.sub.2
vapor flow helps assessing the deposition rate of the metal halide
material BX.sub.2. The sensor section of the first monitor 132 may
be configured to face downward, for example, as indicated in FIG.
1. A second monitor 134 may be coupled to the top section of the
housing 100 as indicated in FIG. 1 or to the side section of the
housing 100, in order to monitor the vapor flow of the organic
source AX from the second evaporator unit 124. Monitoring the AX
vapor flow helps assessing the deposition rate and flow speed of
the AX vapor. Depending on conditions, the sensor section of the
second monitor 134 may be configured to face upward, as indicated
in the example illustrated in FIG. 1, or sideways. Examples of the
first and second monitors 132 and 134 include a quartz crystal
thickness monitor, the temperature of which can be controlled to be
held at substantially the same temperature as the substrate stage
112 so as not to thermally disturb the deposition process. The
deposition rates and the film thickness in situ can be estimated
based on the monitored evaporation rates using the tooling factor
calculation. For example, in this calculation, the ratio between
the measured film thickness and the indicated film thickness (as
indicated by the monitored evaporation rates) is obtained during a
trial run; thereafter, the ratio can be used to obtain the in situ
film thickness by factoring in the evaporation rates during
deposition as observed by the monitors.
[0034] A first shutter 136 is provided just below the substrate
stage 112 and is configured to be moved to expose or cover the
substrate stage 112 to control the deposition of the BX.sub.2
molecules onto the substrate 116. A second shutter 140 is provided
just above the first evaporator unit 120 and is configured to be
moved to expose or cover the first evaporator unit 120 to control
the flow of the BX.sub.2 vapor.
[0035] FIG. 2 illustrates a vertical view from below with respect
to the cross-sectional plane indicated by X-X' in FIG. 1. The
internal side surface of the housing 100, the cross-sectional view
of the flow control unit 128, the bottom surface of the first
monitor 132 provided in the proximity to the substrate stage 112,
the bottom surface of the second monitor 134, and the bottom
surface of the first shutter 136 are illustrated in FIG. 2 as
projected onto the cross-sectional plane X-X'. The first shutter
136 is attached with a first rod 138, which is used to move the
first shutter 136 to expose or cover the substrate stage 112. A
push-pull linear motion device, for example, may be coupled to the
first rod 138 to provide longitudinal motion of the first shutter
136 along the axis of the first rod 138. Alternatively, the first
shutter 136, the first rod 138 and their peripheral parts may be
configured to control the exposing and covering of the substrate
stage 112 by the first shutter 136 based on rotation thereof along
the horizontal direction around a vertical axis, or any other
suitable motion. The bottom surface of the substrate stage 112 is
not visible when it is completely covered by the first shutter 136;
however, it is visible, as indicated by thick solid line in FIG. 2,
when the first shutter 136 is moved away to expose the bottom
surface of the substrate stage 112.
[0036] FIG. 3 illustrates a vertical view from above with respect
to the cross-sectional plane indicated by Y-Y' in FIG. 1. The
internal side surface of the housing 100, the cross-sectional view
of the second evaporator unit 124, and the top surface of the
second shutter 140 are illustrates in FIG. 3 as projected onto the
cross-sectional plane Y-Y'. The second shutter 140 is attached with
a second rod 142, which is used to move the second shutter 140 to
expose or cover the first evaporator unit 120. A push-pull linear
motion device, for example, may be coupled to the second rod 142 to
provide longitudinal motion of the second shutter 140 along the
axis of the second rod 142. Alternatively, the second shutter 140,
the second rod 142 and their peripheral parts may be configured to
control the exposing and covering of the first evaporator unit 120
by the second shutter 140 based on rotation thereof along the
horizontal direction around a vertical axis, or any other suitable
motion. The top surface of the first evaporator unit 120 is not
visible when it is completely covered by the second shutter 140;
however, it is visible, as indicated by thick solid line in FIG. 3,
when the second shutter 140 is moved away to expose the top surface
of the first evaporator unit 120.
[0037] FIGS. 4A and 4B illustrate examples of the second evaporator
unit 124 of the system in FIG. 1. Each example is illustrated as a
configuration with respect to the internal side surface of the
housing 100. FIG. 4A illustrates a cell evaporator, such as a
conventional Knudsen cell evaporator, having a cell 404
accommodating a crucible 408, which is a container where the AX
powder 412 can be contained. A heating element 416 is provided to
heat the crucible 408, hence the AX powder 412, to generate its
vapor. The second evaporation temperature is associated with the
second evaporator unit 124, and is controlled to adjust the second
evaporation rate for generating the AX vapor. Specifically, the
temperature of the heating element 416 is controlled to adjust the
AX evaporation rate, in this example. The cell evaporator also
includes an evaporator shutter 420 to control the AX vapor flux.
The cell evaporator of FIG. 4A is provided with an angle with
respect to the internal side surface of the housing 100, wherein
the angle can be adjusted to output the AX vapor efficiently. The
evaporator shutter 420 is provided to control the AX flux exiting
from the second evaporator unit 124 into the housing 100 to avoid
the high flux of the AX vapor hitting directly the substrate 116.
FIG. 4B illustrates another example of the second evaporation unit
124, which includes an ampule 424 that is a container to contain
the AX powder 428, and a heating element 432 provided to heat the
ampule 424, hence the AX powder 428, to generate its vapor. The
second evaporation temperature is associated with the second
evaporator unit 124, and is controlled to adjust the second
evaporation rate for generating the AX vapor. Specifically, the
temperature of the heating element 432 is controlled to adjust the
AX evaporation rate, in this example. The second evaporator unit
124 illustrated in the example of FIG. 4B further includes a duct
436 to guide the AX vapor flux into the housing 100. The duct 436
is configured to have a vertically extending section 438 in this
example to output the AX vapor vertically so that the flow control
unit 128 provided above the second evaporator unit 128 can
effectively circulate its flow in the housing 100. The duct 436 may
be coupled to an evaporator valve 440 for controlling the AX vapor
flow in a simple but timely manner.
[0038] FIGS. 5A-5C illustrate an example of the first evaporator
unit 120 of the system in FIG. 1. FIG. 5A illustrates a
cross-sectional side view, wherein the first evaporator unit 120
includes a crucible 504 that is a container to contain the BX.sub.2
powder 508 and a heating element 512 to heat the crucible 504,
hence the BX.sub.2 powder 508, to generate its vapor. Two end
terminals of the heating element 512 are held by two electric
feedthroughs 516, respectively, to communicate with the outside of
the housing 100, whereby the heating element 512 can be controlled
externally. The first evaporation temperature is associated with
the first evaporator unit 120, and is controlled to adjust the
first evaporation rate for generating the BX.sub.2 vapor.
Specifically, the temperature of the heating element 512 is
controlled to adjust the BX.sub.2 evaporation rate, in this
example. FIG. 5B illustrates a perspective view of the crucible
504, which has a dish shape in this example, with the diameter
close to the dimension of the substrate stage 112. The horizontal
cross-sectional shape of the crucible 504 can be a square, a
rectangle, an oval, a hexagon or any other shape, as long as the
area is configured to be close to the area of the substrate stage
112. As mentioned earlier, the dimensions of the horizontal
cross-sectional shape of the first evaporator unit 120, the
dimensions of the horizontal cross-sectional shape of the substrate
stage 112, and the relative position in the horizontal direction
between the above two horizontal cross-sectional shapes are
configured to maximize the overlap between the two horizontal
cross-sectional shapes. FIG. 5C illustrates a perspective view of
the heating element 512, which is a spiral-shaped tungsten filament
and tightly surrounds the dish-shaped crucible 504, in this
example. The heating element 512 can be formed in a mesh shape, a
meander shape, a zig-zag shape or any other shape, as long as it is
configured to heat the crucible 504 uniformly to control the
evaporation rate of the BX.sub.2 source. In the present example,
the diameter of the spiral is configured to be approximately the
same as the diameter D of the dish-shaped crucible 504 of FIG.
5B.
[0039] FIGS. 6A and 6B illustrate examples of the flow control unit
128, which may be coupled to the side section of the housing 100.
The example illustrated in FIG. 6A is a pump system, e.g., a
foreline pump, including a funnel 604, a duct 608, a valve 612 and
a pumping station 616, and being coupled to the side section of the
chamber 100 substantially opposite to the second evaporator unit
124. However, it can be coupled to the housing 100 at any position
with respect to the position of the second evaporator unit 124 as
long as the flow speed is controlled effectively to generate a
substantially uniform flow of the AX over the substrate 116. The
valve 612 may be coupled to the duct 608 to control the flow.
Another example illustrated in FIG. 6B is a fan system including a
fan 620, which is coupled to a rotary drive 624 coupled to a motor
628. In this example, the fan 620 is provided on the same side as
the second evaporator unit 124, just above it on the side section
of the chamber 100, so as to uniformly circulate the AX vapor over
the substrate 116. As mentioned earlier, the flow control unit 128
may comprise one or more mechanical systems to promote the
uniformity of the AX vapor flow over the substrate 116. Examples of
the flow control unit 128 include a fan system, a pump system and a
combination thereof. In the combination example, the fan system may
be provided on the same side as and above the second evaporator
unit 124, and the pump system may be provided substantially
opposite to the second evaporator unit 124. In another example, the
flow control unit 128 may be configured to have only the pump
system, which is coupled to the side section substantially opposite
to the second evaporator unit 124 and substantially leveled with
the stage surface of the substrate stage 112 to promote the
uniformity of the AX vapor over the substrate 116.
[0040] FIG. 7 illustrates another example of the system
configuration for fabricating a perovskite film according to an
embodiment. The system includes a housing 700 coupled to necessary
parts. The housing 700 serves as a vacuum chamber for the
deposition. A pump unit 704, a gate valve 708, a substrate stage
712 for a substrate 716 to be placed on, a first evaporator unit
720 for generating vapor of the metal halide material BX.sub.2, a
flow control unit 728, a first monitor 732, a first shutter 736,
and a second shutter 740 are similar to those parts explained
earlier with reference to FIG. 1 or adapted in accordance with the
present configuration while keeping the key functionalities by
those skilled in the art. For example, the pump unit 704 and the
gate valve 708 may be shifted in position and/or resized to
accommodate additional parts at the bottom section of the housing
700, as modified or designed by those skilled in the art based on
the system illustrated in FIG. 1. In the present system
configuration in FIG. 7, a second evaporator unit 724 is coupled to
the bottom section of the housing 700, separated from the first
evaporator unit 720 by a predetermined distance, and configured to
generate vapor of the organic material AX. A shield 725 is provided
between the first evaporator unit 720 and the second evaporator
unit 724 to reduce the thermal interference between them. A second
monitor 734 is provided just above the second evaporator unit 724
to monitor the AX vapor flow to assess its deposition rate.
[0041] Similar to the second evaporator unit 124 of FIG. 1,
examples of the second evaporator unit 724 include a cell
evaporator such as a Knudsen cell evaporator explained with
reference to FIG. 4A and an ampule-type evaporator explained with
reference to FIG. 4B. FIG. 8 illustrates a side view of a third
example of the second evaporator unit 724, which can be coupled to
the bottom section of the housing 700. Similar to the crucible-type
evaporator as used for the first evaporator unit 120, explained
with reference to FIGS. 5A-5C, the third example of the second
evaporator unit 724 includes a crucible 804 to contain the AX
powder 808 and a heating element 812 to heat the crucible 804,
hence the AX powder 808, to generate its vapor. Two end terminals
of the heating element 812 are held by two electric feedthroughs
816, respectively, to communicate with the outside of the housing
700, whereby the heating element 812 can be controlled externally.
The second evaporation temperature is associated with the second
evaporator unit 724, and is controlled to adjust the second
evaporation rate for generating the AX vapor. Specifically, the
temperature of the heating element 812 is controlled to adjust the
AX evaporation rate, in this example. The horizontal
cross-sectional shape of the crucible 804 can be a circle, a
square, a rectangle, an oval, a hexagon or any other shape. The
heating element 812 may be a spiral-shaped tungsten filament
tightly surrounding the crucible 804. The heating element 812 may
be formed in a mesh shape, a meander shape, a zig-zag shape or any
other shape, as long as it is configured to heat the crucible 804
uniformly to control the evaporation rate of the AX source
material. An evaporator shutter 820 is provided above the crucible
804 in this example. The horizontal dimensions of the evaporator
shutter 820 are configured to be larger than those of the crucible
804. The evaporator shutter 820 can be adjusted to cover the
opening of the crucible 804 to avoid the high flux of the AX vapor
exiting from the second evaporator unit 724 hitting directly the
substrate 716.
[0042] FIG. 9 is a flowchart illustrating the fabrication method of
a perovskite film using the present system illustrated in FIG. 1 or
FIG. 7. The reference numerals in FIG. 1 are used below for
explaining the process illustrated in FIG. 9; it should be noted
that the same process can be carried out using the system
illustrated in FIG. 7 as well. The substrate 116 can be initially
provided on the substrate stage 112 facing downward. Alternatively,
the substrate 116 can be provided on the substrate stage 112 later
in the process, as explained with reference to FIGS. 11A-11D later.
As mentioned earlier, the surface area of the substrate stage 112
in the present system can be configured to accommodate a large
substrate, for example, 5 cm5 cm or larger, or a multiple
substrates with a total area of 5 cm5 cm or larger. The inside of
the housing 100 is pumped to a predetermined vacuum level by using
the pump unit 104, and the housing 100 serves as a vacuum chamber.
The pressure inside the chamber can be monitored by a pressure
gauge over a full-range, i.e., 110.sup.5.about.110.sup.-7 Pa. In
the second evaporator unit 124 or 724, the evaporator shutter 420
of a cell evaporator illustrated in FIG. 4A, the evaporator valve
440 of an ampule-type evaporator illustrated in FIG. 4B, or the
evaporator shutter 820 of a crucible-type evaporator illustrated in
FIG. 8 is positioned to substantially cover the opening of the
second evaporator unit 124 or 724 to avoid the high flux of the AX
vapor exiting form the second evaporator unit 124 or 724 hitting
directly the substrate surface.
[0043] In step 904 of the process illustrated in FIG. 9, the
temperature of the substrate stage 112 is controlled to provide a
predetermined substrate temperature. The temperature of the
substrate stage 112 can be controlled to provide uniform cooling or
heating to the substrate 116, ranging from -190.degree. C. up to
200.degree. C., in the present system. As explained later,
experiments using substrates at various temperatures have suggested
that a solar device with the perovskite film grown with the
substrate at room temperature exhibits the best performance. Here,
the room temperature refers to a temperature in the range of
15.degree. C.-25.degree. C. In step 908, the first shutter 136,
which is provided just below the substrate stage 112, is moved to
cover the substrate 116, while the second shutter 140, which is
provided just above the first evaporator unit 120, is moved to
expose the first evaporator unit 120. The gate valve 108 coupled
between the housing 100 and the pump unit 104 is positioned to
adjust the pressure inside the housing 100 to a value optimal for
the chemical reaction between the source materials and for
efficient use of the source materials. In particular, the AX vapor
pressure inside the chamber is primarily determined by the gate
valve positioning. That is, as in step 912, setting the gate valve
108 to a first position, which may be predetermined, can help
stabilize the AX vapor circulation in the chamber. For example, a
relatively high pressure of .about.0.3 Pa may be applied and kept
substantially constant via the gate valve positioning during the
perovskite formation. In step 916, the first temperature associated
with the first evaporator unit 120, which is configured to source
the BX.sub.2 vapor, is controlled to adjust the first evaporation
rate for generating the BX.sub.2 vapor. For example, in the
crucible-type evaporator illustrated in FIGS. 5A-5C, the crucible
504 can be heated by the heating element 512 to a temperature that
generates the BX.sub.2 vapor at a predetermined first evaporation
rate. The BX.sub.2 rate can be monitored by the first monitor 132.
When the first evaporation rate of the BX.sub.2 material reaches a
certain rate, in step 920, the second temperature associated with
the second evaporator unit 124, which is configured to source the
AX vapor, is controlled to adjust the second evaporation rate for
generating the AX vapor. For example, in the Knudsen-type cell
evaporator illustrated in FIG. 4A, the crucible 408 can be heated
by the heating element 416 to a temperature that generates the AX
vapor at a predetermined second evaporation rate. The AX rate is
monitored by the second monitor 134. The second monitor 134 is used
to monitor the AX vapor flow to assess the evaporation rate and to
check if the flow is kept substantially constant inside the housing
100.
[0044] The present deposition process involves evaporation of two
materials with distinctively different evaporation temperatures.
For example, PbI.sub.2 typically evaporates at .about.250.degree.
C., while MAI evaporates at .about.70.degree. C. The organic
materials AX are typically highly volatile. In step 924, the flow
control unit 128 is controlled to adjust the flow speed of the AX
vapor to circulate it in the housing 100 and to promote the
uniformity of the AX vapor flow over the substrate surface 116. The
AX vapor pressure inside the chamber is primarily determined by the
gate valve positioning. That is, setting the gate valve 108 to the
first position can help stabilize the AX vapor circulation in the
chamber. Thus, the circulation of the AX vapor in the chamber is
optimized in the present system, based comprehensively on: (i) the
second evaporation temperature associated with the second
evaporator unit 124 for controlling the evaporation rate of the AX
material; (ii) the pressure inside the chamber adjusted by
positioning the gate valve 108 for controlling the AX vapor
pressure; and (iii) the flow control of the AX vapor by the flow
control unit 128.
[0045] In step 928, the first shutter 136, which is provided just
below the substrate stage 112, is moved to expose the substrate 116
to start the deposition of the BX.sub.2 molecules onto the
substrate 116. In step 932, the thickness of the perovskite film
growing on the substrate 116 is monitored in situ by the first
monitor 132, which is provided in the proximity of the stage
surface of the substrate stage 112. The temperature of the first
and second monitors 132 and 134 can be controlled to be held at
substantially the same temperature as the substrate stage 112 so as
not to thermally disturb the deposition process. In step 936, when
the film thickness reaches a predetermined thickness, the first
shutter 136 is moved to cover the substrate 116 to interrupt the
deposition of the BX.sub.2 molecules onto the substrate 116. In
step 940, the heating of the first evaporator unit 120 and the
second evaporator unit 124 is stopped. In step 944 the gate valve
108 is opened to a second position, which can be a maximum open
position, to pump out the remaining vapor from the chamber.
Experiments suggested that post annealing of the resultant
perovskite film is not necessary.
[0046] Physical vapor deposition is an example of fabrication
technique used in semiconductors, microelectronics and optical
industries. The source material is typically heated and vaporized
until its vapor pressure is high enough to produce a flux. The
deposition onto the substrate involves purely physical process such
as high-temperature vacuum evaporation with subsequent condensation
or plasma sputter bombardment. Thus, line-of-sight transfer is
typical for most of physical vapor deposition techniques, in which
the direction of the vapor flux of the source material is directed
toward the substrate. Since particles tend to follow a straight
path, films deposited by physical vapor deposition are generally
directional, rather than conformal. In contrast, in chemical vapor
deposition, chemical reaction takes place on the substrate surface
to produce the conformal uniform morphology.
[0047] In view of the conventional chemical and physical vapor
deposition techniques, the fabrication technique based on the
present system and method may be regarded as a hybrid of the two
techniques. FIG. 10 schematically illustrates the deposition
mechanism according to the present system and method. The reference
numerals for some of the system parts in FIG. 1 are used below for
explaining the deposition mechanism schematically illustrated in
FIG. 10; however, it should be noted that the explanations herein
are applicable as well to the system illustrated in FIG. 7. In FIG.
10, the stabilized evaporations are depicted after the process step
928 in FIG. 9, where the first shutter 136 (shown in dashed line)
was moved to expose the substrate 116. It is illustrated here that
the AX vapor 1054 circulates substantially uniformly to fill the
chamber. This is enabled by adjustments of the first evaporation
rate for generating the AX vapor via the temperature control of the
second evaporator unit 124 (omitted in the figure for simplicity),
the pressure inside the chamber via the positioning of the gate
valve 108 for controlling the AX vapor pressure, and the flow speed
of the AX vapor via the flow-speed control of the flow control unit
128. The BX.sub.2 material is heated uniformly in the first
evaporator unit 120 that has a large horizontal cross-sectional
area, and the resultant BX.sub.2 vapor 1058 travels substantially
vertically directing to the substrate 116, which has a surface area
close to the horizontal cross-sectional area of the first
evaporator unit 120. As mentioned earlier, the dimensions of the
horizontal cross-sectional shape of the first evaporator unit 120,
the dimensions of the horizontal cross-sectional shape of the
substrate stage 112, and the relative position in the horizontal
direction between the above two horizontal cross-sectional shapes
are configured to maximize the overlap between the two horizontal
cross-sectional shapes. For example, the substrate stage 112 has a
horizontal cross-sectional shape of a 6 cm6 cm square; the first
evaporator unit 120 has a horizontal cross-sectional shape of a
circle with a 6 cm diameter; and the center of the square and the
center of the circle are vertically aligned. In another example,
each of the substrate stage 112 and the first evaporator unit 120
has a horizontal cross-sectional shape of a circle with a 10 cm
diameter; and the centers of these two circles are vertically
aligned. Therefore, the deposition of the BX.sub.2 vapor is
substantially directional, following the line-of-sight transfer and
yet covering a large horizontal cross-sectional area. On the other
hand, the deposition of the AX vapor is substantially less
directional since the AX vapor is controlled to stay circulating
and filing the chamber. The BX.sub.2 vapor hits the substrate
surface 116 and is deposited thereon effectively based partially on
the good sticking coefficient and wettability of the BX.sub.2
material. Thereafter, the chemical reaction takes place between the
deposited BX.sub.2 and the AX vapor existent in the proximity of
the substrate surface 116. That is, according to the present system
and method, the perovskite film is formed by the chemical reaction
between the BX.sub.2 molecules deposited on the substrate 116 and
the AX molecules in the gas phase. Thus, the present system and
method allow for uniform chemical reaction on a large area of the
substrate surface 116, resulting in a large-scale and substantially
uniform perovskite film with high crystallinity even without
annealing. Here, the large-scale fabrication refers to the
formation of perovskite films with centimeter-scale uniformity or
even larger. Scaling-up of the fabrication is possible by keeping
the light-of-sight transfer of the BX.sub.2 deposition and the AX
vapor circulation in the chamber, and by simultaneous enlargement
of both the horizontal cross-sectional areas of the first
evaporator unit 120 and the substrate stage 112. Efficient chemical
reaction on the substrate surface 116 can be promoted, and thus the
speed of the film growth can be made significantly fast by
optimizing the evaporation rates of both source materials via the
respective temperature controls, the circulation of the AX vapor
flow, hence the AX incorporation ratio to the deposited BX.sub.2,
via the flow control, and the internal pressure via the gate valve
positioning, among various parameters. Thus, the present system and
method are configured to utilize the good sticking coefficient and
wettability of the BX.sub.2 material and the volatility of the AX
material. The resultant film thickness is primarily controlled by
the movement of the first shutter 136 to cover or expose the
substrate 116.
[0048] Therefore, the present fabrication process is inherently
different from a typical physical vapor co-deposition process. In a
typical physical vapor co-deposition process, two evaporators need
to be situated side-by-side with an angle so that both vapor flows
are directed at the substrate surface to have line-of-sight
transfer of both source materials. Accordingly, each of the vapor
flows reaches the substrate surface at an angle, limiting the
overlap region of the two vapor flows. That is, in a conventional
physical vapor co-deposition process, the stoichiometry of the
resultant perovskite film in the central region is different from
that in the edge region of the film. Therefore, the substrate size
is limited, and the crystallinity of the resultant perovskite film
tends to be of low quality even after annealing because of
non-uniform composition of the two source materials (i.e., BX.sub.2
and AX). Furthermore, the present system for the hybrid deposition
process includes the first evaporator unit 120 that has a large
horizontal cross-sectional area for evaporating the BX.sub.2
source, whereas in a conventional physical vapor co-deposition
system it is not possible to configure one of the evaporators to
have a wider opening than the other because the evaporators will
mechanically interfere with each other due to the side-by-side
positioning of the two evaporators with an angle. For example, the
monitor for the AX vapor will be influenced by the BX.sub.2
evaporation, which violates the operation principle of the typical
physical vapor co-deposition process. Yet furthermore, the present
system includes the flow control unit 128 to generate substantially
uniform flow of the AX vapor over the substrate surface 116,
thereby optimizing the AX incorporation ratio to the deposited
BX.sub.2, whereas a flow control unit does not lead to benefits for
physical vapor co-deposition because it is irrelevant to the
operation principles based on a purely physical process in the
molecular regime. Additionally, in a commercially available
physical vapor deposition system, the temperature range of the
substrate stage is limited from -10.degree. C. to 80.degree. C.,
whereas the present system can be configured to have a wider
temperature range from -190.degree. C. up to 200.degree. C.
[0049] Additional steps may be included in the fabrication process
to further improve the stoichiometry of perovskite films grown
based on the present system and method. For example, it may be
beneficial to include steps for suppressing the generation of
AX-rich region in the film. Specifically, during the warmup of the
second evaporator unit 124/724 for generating the AX vapor until a
predetermined evaporation rate is attained, deposition of the AX
molecules on the substrate 116/716 can occur. Although the first
shutter 136/736 is initially closed to cover the substrate 116/716
until the nominal deposition is started as illustrated in the
flowchart of FIG. 9, the AX vapor may swiftly move around the first
shutter 136/736 to reach the substrate surface 116/716 due to the
volatility of the AX material. Furthermore, the generation of the
AX vapor cannot be ceased immediately when the heating of the
second evaporator unit 124/724 is stopped at step 940 in the
process in FIG. 9. This is because it generally takes a substantial
period of time for the container containing the AX powder in the
second evaporator unit 124/724 to cool down. As a result, the
unknown concentration of the AX vapor in the chamber may generate a
perovskite film with topmost layers having the AX-rich
stoichiometry causing non-uniformity of the film.
[0050] One way to circumvent the problem associated with the
volatility of the AX material, especially during the ramp-up and
ramp-down of the evaporation temperature, is to use a second
housing, commonly known as a load-lock chamber. FIGS. 11A-11D
illustrate a sequence of system configurations including a
load-lock chamber for reducing the effect arising from the
volatility of the AX material that may cause non-uniformity of a
perovskite film. FIG. 11A illustrates a system configuration
including a first housing 1100 that serves as a main vacuum
chamber, coupled with a second housing 1160 that serves as a
load-lock chamber on one side and with a sample transfer system
1180 on the other side, wherein the second housing 1160 and the
sample transfer system 1180 are provided facing opposite to each
other. The first housing 1100 serves as a main vacuum chamber for
the deposition, similar to the housing 100 in FIG. 1 or the housing
700 in FIG. 7. The parts coupled to the first housing 1100 are
similar to those parts explained earlier with reference to FIG. 1
or 7, or adapted in accordance with the present purpose while
keeping the key functionalities as modified or designed by those
skilled in the art. That is, a first pump unit 1104, a first gate
valve 1108, a substrate stage 1112, a first evaporator unit 1120, a
first shutter 1136 and the other parts (omitted in the figure for
simplicity) in FIG. 11A are configured similarly to or adapted
correspondingly to the pump unit 104/704, the gate valve 108/708,
the substrate stage 112/712, the first evaporator unit 120/720, the
first shutter 136/736 and the other parts in FIG. 1/7,
respectively. The second housing 1160, which serves as a load-lock
chamber, is coupled to a second pump unit 1164 through a second
gate valve 1168, which are configured to control the pressure
inside the second housing 1160. The second housing 1160 is further
coupled to the first housing 1100 through a third gate valve 1172,
which is configured to control the communication between the second
housing 1160 and the first housing 1100. The sample transfer system
1180 includes a mechanical device 1184, such as a grab having
hinged jaws, for holding and releasing an object. Another example
of the mechanical device 1184 is a magnetically controlled unit for
holding and releasing a metal object. The mechanical device 1184 is
attached at one end portion of a rod 1185 in the sample transfer
system 1180, and is provided inside the first housing 1100. The
movement of the mechanical device 1184 is controlled by the
movement of the rod 1185, which is controlled manually, by a
computer or other suitable means. In FIGS. 11B-11D, the reference
numerals are omitted; however, the housings, the coupled parts and
respective functionalities are the same as those described above
with reference to FIG. 11A.
[0051] Initially, as illustrated in FIG. 11A, a substrate 1116 is
stored in the second housing 1160, and the second pump unit 1164
and the second gate valve 1168 are controlled to attain a
predetermined pressure level in the second housing 1160 while the
third gate valve 1172 is closed. Thereafter, the deposition can be
started in the first housing 1100, following the steps 904-924 in
the process illustrated in FIG. 9, for example. After controlling
the evaporation temperatures to reach desired evaporation rates of
the source materials in step 920 and controlling the flow control
unit to circulate the AX vapor in the first housing 1100 in step
924, the third gate valve 1172 is opened, and the mechanical device
1184 in the sample transfer system 1180 is moved to reach out and
hold the substrate 1116 in the second housing 1160, as illustrated
in FIG. 11B. Thereafter, the mechanical device 1184 holding the
substrate 1116 is controlled to move back to the first housing 1100
and release and place the substrate 1116 on the substrate stage
1112 facing downward, as illustrated in FIG. 11C. Thereafter, the
third gate valve 1172 is closed, and the mechanical device 1185 is
moved back to the original position, as illustrated in FIG. 11D.
Thereafter, the first shutter 1136 is moved to expose the substrate
1116 on the substrate stage 1112, as in step 928 of the process in
FIG. 9, and the deposition is started.
[0052] The reversed sequence of the system configurations, i.e.,
FIGS. 11D, 11C, 11B and 11A in order, can be carried out after the
completion of the deposition to reduce the excess deposition of the
AX molecules on top of the grown film. For example, the reversed
sequence to transfer the substrate 1116 with the grown film from
the first housing 1100 to the second housing 1160 can be carried
out after the heating of the first evaporator unit 1120 and the
second evaporator unit (omitted in the figure for simplicity) is
stopped in step 940, and the first gate valve 1108 is opened to a
second position, which can be a completely open position, in step
944 to pump out the remaining vapor from the first housing 1100.
That is, the third gate valve 1172 is opened, and the mechanical
device 1184 is moved to reach out and hold the substrate 1116 on
which the perovskite film is grown in the housing 1100, moved to
the second housing 1160 to release and place the substrate 1116 on
which the perovskite film is grown in the second housing 1160, and
moved back to the housing 1100; and thereafter the third gate valve
1172 is closed. The above sequence can carried out after the first
gate valve 1108 is opened to pump out the remaining vapor in step
944 so as to avoid contaminating the load-lock chamber with the
volatile AX material.
[0053] FIG. 12 is a flowchart illustrating the fabrication method
of a perovskite film using the present system including a load-lock
chamber. Sub-processes based on the use of the load-lock chamber
are added to the fabrication process illustrated in FIG. 9 to
improve the uniformity of the film by reducing the AX-rich regions
in the film, which may be caused by the excess deposition of the AX
molecules during, for example, the ramp-up and ramp-down of the
evaporation temperature. The first sub-process 1200 for
transferring the substrate 1116 from the second housing 1160, i.e.,
the load-lock chamber, to the first housing 1100, i.e., the main
chamber, is described above with reference to the sequence of the
system configurations illustrated in FIGS. 11A, 11B, 11C and 11D in
order. This first sub-process may be carried out after step 924 and
before step 928 in the process illustrated in FIG. 9. The second
sub-process 1204 for transferring the substrate 1116 with the grown
film from the first housing 1100, i.e., the main chamber, to the
second housing 1160, i.e., the load-lock chamber, is described
above with reference to the reverse sequence of the system
configurations illustrated in FIGS. 11D, 11C, 11B and 11A in order.
This second sub-process may be carried out after step 944 in the
process illustrated in FIG. 9.
[0054] The following describes some of the experimental results
obtained by using the present system and method for growing
perovskite films. Examples of using MAI for the AX source material
and PbCl.sub.2 for the BX.sub.2 source material are given
hereinafter, for growing chloride iodide perovskite films
CH.sub.3NH.sub.3PbI.sub.3-XCl.sub.X based on the system illustrated
in FIG. 1, wherein a dish-shaped crucible with a heating element,
as illustrated in FIG. 5A-5C, is used for the first evaporator unit
120 and a cell evaporator coupled to the side section of the
housing 100, as illustrated in FIG. 4A, is used for the second
evaporator unit 124. The first and second sub-processes by using a
load-lock chamber, as steps 1200 and 1204 in FIG. 12, are included
in the fabrication process for the present examples. The substrate
stage 112 is configured to accommodate a large substrate 116 having
dimensions of 5 cm5 cm. The following results pertain to the
perovskite films grown to thicknesses of .about.50 nm and
.about.135 nm.
[0055] FIG. 13 is a plot of the J-V curve representing the
photovoltaic device characterization of a solar cell including the
chloride iodide perovskite film,
CH.sub.3NH.sub.3PbI.sub.3-XCl.sub.X, grown by the present
fabrication system and method. The results for the film with a
thickness of .about.50 nm and the film with a thickness of
.about.135 nm are plotted with lines including squares and circles,
respectively. The measurements to obtain the J-V curves were
carried out under a simulated AM1.5G solar irradiation of 100
mW/cm.sup.2. The J-V curve for the .about.50 nm film shows that the
short circuit current density (Jsc) is 10.5 mA/cm.sup.2, the open
circuit voltage (Voc) is 1.06 V, and the fill factor (FF) is 0.566.
This sample has the power conversion efficiency (PCE) of about
6.3%. The J-V curve for the .about.135 nm film shows that the short
circuit current density (Jsc) is 17 mA/cm.sup.2, the open circuit
voltage (Voc) is 1.09 V, and the fill factor (FF) is 0.535. This
sample has the power conversion efficiency (PCE) of about 9.9%. All
six solar cells from the same batch of each film exhibited the
similar J-V performance, thereby indicating the device yield of
100%.
[0056] FIG. 14 is a plot showing the X-ray diffraction (XRD)
spectrum of the CH.sub.3NH.sub.3PbI.sub.3-X Cl.sub.X film with a
thickness of .about.50 nm. This XRD spectrum shows the organometal
halide perovskite characteristics having peaks at 14.0.degree.,
28.4.degree. and 43.1.degree. corresponding to the (110), (220) and
(330) planes of the orthorhombic structure. It should be noted that
the peak (110) is stronger than the (220) peak even without
annealing in the present fabrication process. In general, the
CH.sub.3NH.sub.3PbI.sub.3 phase formation is indicated by a peak at
15.7.degree. in the XRD spectrum; however, this peak is absent in
the present spectrum in FIG. 14. The absence of the peak at
15.7.degree. and the detection of the (330) peak together indicate
the high phase-purity and crystallinity of the
CH.sub.3NH.sub.3PbI.sub.3-XCl.sub.X film grown by the present
system and method. Further studies based on XRD measurements
indicated that the phase purity is optimal in the perovskite films
fabricated with the substrate temperature being at room temperature
(in the range of 15.degree. C.-25.degree. C.). The changes in
crystal morphology as a function of substrate temperature may be
ascribed to the temperature dependence of the sticking coefficient
of MAI on the substrate. The sticking coefficient is generally
defined as the fraction of the incident molecules from the source
that actually adhere to the substrate. MAI has chemical properties
that make its sticking coefficient high at low temperatures and low
at high temperatures. Thus, at low temperatures, e.g., lower than
-20.degree. C., the MAI sticking coefficient is high but partial
coverage of the perovskite on the substrate is likely to occur. At
high temperatures, e.g., higher than 80.degree. C., it is difficult
to form a perovskite film with suitable stoichiometry because of
the small sticking coefficient of MAI and the excess amount of
PbCl.sub.2.
[0057] FIG. 15 is a plot showing the X-ray diffraction (XRD)
spectra of the CH.sub.3NH.sub.3PbI.sub.3-XCl.sub.X film of
.about.135 nm thickness grown on a tin-doped indium oxide
(ITO)/glass substrate with a 5 cm5 cm surface area. The inset
illustrates 12 different locations over the sample selected for the
XRD measurements. This plot shows that the XRD spectra at the 12
different locations have similar intensities of the diffraction
peaks, indicating the organometal halide perovskite
characteristics, at 14.0.degree., 28.4.degree. and 43.1.degree.
corresponding to the (110), (220) and (330) planes of the
orthorhombic structure. The results confirm that uniformity and
high crystallinity of the perovskite films are attained over the
large substrate by using the present system and method.
[0058] FIG. 16 is a photo showing the atomic force microscopy (AFM)
image of the CH.sub.3NH.sub.3PbI.sub.3-XCl.sub.X film of .about.50
nm thickness grown on the ITO/glass substrate. The AFM image shows
that the typical root-mean square (RMS) roughness of the film of
.about.50 nm thickness is about 4.6 nm. Similarly, the typical RMS
roughness of the film of .about.135 nm was measured to be about 9
nm.
[0059] FIG. 17 is a plot showing the optical absorption of the
CH.sub.3NH.sub.3PbI.sub.3-XCl.sub.X film of .about.135 nm
thickness. A sharp rise at .about.780 nm corresponds to a bandgap
of 1.59 eV.
[0060] FIG. 18 shows photos of actual devices including the
CH.sub.3NH.sub.3PbI.sub.3-XCl.sub.X films of .about.50 nm thickness
and .about.135 nm thickness, respectively. The color is
semi-transparent light-orange in both cases.
[0061] While this document contains many specifics, these should
not be construed as limitations on the scope of an invention or of
what may be claimed, but rather as descriptions of features
specific to particular embodiments of the invention. Certain
features that are described in this document in the context of
separate embodiments can also be implemented in combination in a
single embodiment. Conversely, various features that are described
in the context of a single embodiment can also be implemented in
multiple embodiments separately or in any suitable subcombination.
Moreover, although features may be described above as acting in
certain combinations and even initially claimed as such, one or
more features from a claimed combination can in some cases be
exercised from the combination, and the claimed combination may be
directed to a subcombination or a variation of a
subcombination.
* * * * *