U.S. patent application number 12/643419 was filed with the patent office on 2010-10-07 for copper delafossite transparent p-type semiconductor materials for dye sensitized solar cells.
This patent application is currently assigned to APPLIED MATERIALS, INC.. Invention is credited to Nety M. Krishna, Omkaram Nalamasu, Kaushal K. Singh, Michael Snure, Ashutosh Tiwari.
Application Number | 20100252108 12/643419 |
Document ID | / |
Family ID | 42269297 |
Filed Date | 2010-10-07 |
United States Patent
Application |
20100252108 |
Kind Code |
A1 |
Singh; Kaushal K. ; et
al. |
October 7, 2010 |
COPPER DELAFOSSITE TRANSPARENT P-TYPE SEMICONDUCTOR MATERIALS FOR
DYE SENSITIZED SOLAR CELLS
Abstract
Methods for fabrication of copper delafossite materials include
a low temperature sol-gel process for synthesizing CuBO.sub.2
powders, and a pulsed laser deposition (PLD) process for forming
thin films of CuBO.sub.2, using targets made of the CuBO.sub.2
powders. The CuBO.sub.2 thin films are optically transparent p-type
semiconductor oxide thin films. Devices with CuBO.sub.2 thin films
include p-type transparent thin film transistors (TTFT) comprising
thin film CuBO.sub.2, as a channel layer and thin film solar cells
with CuBO.sub.2 p-layers. Solid state dye sensitized solar cells
(SS-DSSC) comprising CuBO.sub.2 in various forms, including
"core-shell" and "nano-couple" particles, and methods of
manufacture, are also described.
Inventors: |
Singh; Kaushal K.; (Santa
Clara, CA) ; Nalamasu; Omkaram; (San Jose, CA)
; Krishna; Nety M.; (Sunnyvale, CA) ; Snure;
Michael; (Salt lake City, UT) ; Tiwari; Ashutosh;
(Sandy, UT) |
Correspondence
Address: |
APPLIED MATERIALS;C/O PILLSBURY WINTHROP SHAW PITTMAN LLP
P .O . BOX 10500
MCLEAN
VA
22120
US
|
Assignee: |
APPLIED MATERIALS, INC.
Santa Clara
CA
|
Family ID: |
42269297 |
Appl. No.: |
12/643419 |
Filed: |
December 21, 2009 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61203336 |
Dec 19, 2008 |
|
|
|
Current U.S.
Class: |
136/261 ;
257/E31.003; 438/85 |
Current CPC
Class: |
H01G 9/2031 20130101;
Y02E 10/542 20130101; C25B 1/55 20210101; H01L 51/4226 20130101;
Y02P 70/50 20151101; Y02E 10/549 20130101 |
Class at
Publication: |
136/261 ; 438/85;
257/E31.003 |
International
Class: |
H01L 31/0256 20060101
H01L031/0256; H01L 31/18 20060101 H01L031/18 |
Claims
1. A solid state dye sensitized solar cell comprising: p-type
semiconductor material with the general composition and
stoichiometry of CuBO.sub.2; an n-type semiconductor material; a
dye; and a pair of electrodes; wherein at least one of said pair of
electrodes is optically transparent, and wherein said n-type
semiconductor material, said p-type semiconductor material and said
dye are between said pair of electrodes.
2. A solar cell as in claim 1, wherein said n-type semiconductor
material forms a network of pores and said p-type semiconductor
material is within said pores.
3. A solar cell as in claim 2, wherein said p-type semiconductor
material comprises nanoparticles.
4. A solar cell as in claim 1, wherein said p-type semiconductor
material forms a network of pores and said n-type semiconductor
material is within said pores.
5. A solar cell as in claim 1, wherein said n-type semiconductor
material and said p-type semiconductor material are in the form of
core-shell particles.
6. A solar cell as in claim 5, wherein said core-shell particles
comprise a p-type semiconductor core and a n-type semiconductor
shell.
7. A solar cell as in claim 5, wherein said core-shell particles
comprise a n-type semiconductor core and a p-type semiconductor
shell.
8. A solar cell as in claim 5, wherein said dye is on the surface
of said core-shell particles.
9. A solar cell as in claim 1, wherein said n-type semiconductor
material and said p-type semiconductor material are configured as
nano-couple particles, each of said nano-couple particles
comprising a p-type semiconductor particle, a n-type semiconductor
particle, and a polymer binding together said p-type semiconductor
particle and said n-type semiconductor particle.
10. A solar cell as in claim 9, wherein said dye is at the
interface between said p-type semiconductor particle and said
n-type semiconductor particle.
11. A solar cell as in claim 1, wherein said n-type semiconductor
is TiO.sub.2.
12. A solar cell as in claim 1, wherein said n-type semiconductor
material is a material selected from the group consisting of
TiO.sub.2, ZnO and Zr0.sub.2.
13. A solar cell as in claim 1, wherein said dye is a
ruthenium-based dye.
14. A solar cell as in claim 1, wherein said dye is selected from
the group consisting of ruthenium-based dyes, copper-based dyes and
iron-based dyes.
15. A solar cell as in claim 1, wherein said at least one of said
pair of electrodes is glass coated with a film of transparent
conductive oxide.
16. A method of fabricating a solid state dye sensitized solar
cell, said method comprising: providing a first transparent
electrode; forming a layer on said first transparent electrode,
said layer comprising p-type semiconductor material with the
general composition and stoichiometry of CuBO.sub.2 and n-type
semiconductor material; providing a second transparent electrode;
and applying said second transparent substrate to said layer,
wherein said first transparent substrate and said second
transparent substrate are parallel.
17. A method as in claim 16, further comprising, before said
forming, depositing a semiconductor thin film on said first
transparent electrode.
18. A method as in claim 16, further comprising, before said
applying, depositing a semiconductor thin film on said second
transparent electrode.
19. A method as in claim 16, wherein said forming said layer
includes: forming a mesoporous layer of n-type semiconductor on
said first transparent electrode; and applying dye to the surfaces
of said mesoporous layer.
20. A method as in claim 16, wherein said forming said layer
includes: forming a mesoporous layer of n-type semiconductor on
said first transparent electrode; and filling the pores of said
mesoporous layer with nanoparticles of p-type semiconductor.
21. A method as in claim 16, wherein said forming said layer
includes: forming a mesoporous layer of n-type semiconductor on
said first transparent electrode; and filling the pores of said
mesoporous layer with p-type semiconductor powder using a sol-gel
process and heating.
22. A method as in claim 21, wherein said forming said layer
includes, after the filing the pores, dying the filled mesoporous
layer.
23. A method as in claim 16, wherein said forming said layer
includes: forming a mesoporous layer of p-type semiconductor on
said first transparent electrode; and filling the pores of said
mesoporous layer with nanoparticles of n-type semiconductor.
24. A method as in claim 16, wherein said layer comprises
core-shell particles coated in dye.
25. A method as in claim 16, wherein said layer comprises
nano-couples with dye at the interface between the particles in
said nano-couple.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Ser. No. 61/203,336 filed Dec. 19, 2008, incorporated
by reference in its entirety herein.
FIELD OF THE INVENTION
[0002] The present invention relates to transparent p-type
semiconductor materials, more specifically methods of manufacture
of copper delafossite transparent p-type semiconductor material and
devices comprising said copper delafossite material, including
solar cells and transparent thin film transistors.
BACKGROUND OF THE INVENTION
[0003] Transparent conductive oxides (TCOs), such as doped zinc
oxide, indium tin oxide (ITO) and indium molybdenum oxide are
widely used as conductive optically transparent electrodes. These
oxides exhibit both high electrical conductivity and optical
transparency, in the visible spectrum. However, all of these oxides
are characterized as n-type materials and their use is accordingly
limited. In order to expand the use of TCOs to applications such as
solar cells, transparent transistors, transparent light emitting
diodes (LEDs), ultraviolet (UV) detectors, etc. there is a need for
optically transparent conductive p-type materials which are
compatible with the existing n-type TCOs. There is also a need for
transparent p-type semiconductor materials that can be incorporated
in devices with low cost substrates that may limit process
temperatures. Furthermore, there is a need for methods and
apparatuses for forming these materials.
[0004] In recent years, dye-sensitized solar cells (DSSCs) have
received considerable attention as a cost-effective alternative to
conventional solar cells. DSSCs operate on a process that is
similar in many respects to photosynthesis, the process by which
green plants generate chemical energy from sunlight. Central to
these cells is a thick semiconductor nanoparticle film (electrode)
that provides a large surface area for the adsorption of light
harvesting organic dye molecules. Dye molecules absorb light in the
visible region of the electromagnetic spectrum and then "inject"
electrons into a nanostructured semiconductor electrode. This
process is accompanied by a charge transfer to the dye from an
electron donor mediator supplied by an electrolyte, resetting the
cycle. DSSCs based on liquid electrolytes have reached efficiencies
as high as 11% under AM 1.5 (1000 W m.sup.-2) solar illumination.
However, a major problem with these DSSCs is the evaporation and
possible leakage of the liquid electrolyte from the cell. This
limits the stability of these cells and also poses a serious
problem in the scaling up of DSSC technology for practical
applications.
[0005] Presently, tremendous efforts are being focused on
fabricating solid state DSSCs (SS-DSSCs) by replacing liquid
electrolytes with solid electrolytes such as molten salts, organic
hole transport materials, and polymer electrolytes. However, most
of the SS-DSSCs suffer from the problems of short-circuit and mass
transport limitations of the ions, and so have low conversion
efficiencies compared with the liquid version. There is a need for:
solid electrolyte materials for making stable, high efficiency
SS-DSSCs; process tools for making said solid electrolyte
materials; new designs of SS-DSSCs comprising said solid
electrolyte materials; and manufacturable methods of making said
materials and said SS-DSSCs.
SUMMARY OF THE INVENTION
[0006] Embodiments of this invention include methods for
fabrication of Cu delafossite materials, equipment for said
fabrication, devices including said materials and methods of making
said devices.
[0007] Certain embodiments of the present invention are processes
for making Cu delafossite materials including: a low temperature
sol-gel process for synthesizing CuBO.sub.2 materials; a process
which controls the band gap of the CuBO.sub.2 material by
controlling the particle size; a process for making ultrafine
powders of CuBO.sub.2 capable of penetrating a dyed porous
TiO.sub.2 network; a process for forming TiO.sub.2--CuBO,
"core-shell" nanoparticles; a process for forming
TiO.sub.2--CuBO.sub.2 "nano-couples"; and a pulsed laser deposition
(PLD) process for forming thin films of CuBO).
[0008] Certain embodiments of the present invention are equipment
for fabricating CuBO.sub.2 materials including a nanopowder
production system.
[0009] Certain embodiments of the present invention are devices
comprising Cu delafossite materials including: a transparent thin
film transistor comprising thin film CuBO.sub.2 as a channel layer;
p-i-n and p-n solar cells comprising thin film CuBO.sub.2 as a
p-layer; and solid state-dye sensitized solar cells (SS-DSSCs)
comprising CuBO.sub.2 in various forms, including "core-shell" and
"nano-couple" particles. In some embodiments of the present
invention a SS-DSSC comprises: p-type semiconductor material with
the general composition and stoichiometry of CuBO.sub.2; an n-type
semiconductor material; a dye; and a pair of electrodes; wherein at
least one of the pair of electrodes is optically transparent, and
wherein the n-type semiconductor material, the p-type semiconductor
material and the dye are between the pair of electrodes.
[0010] Certain embodiments of the present invention are methods of
making Cu delafossite-containing devices including: fabricating a
SS-DSSC by impregnating a dyed porous TiO.sub.2 network with
ultrafine CuBO, powders; fabricating a SS-DSSC using a sol-gel
technique to deposit CuBO.sub.2 particles into the pores of a
TiO.sub.2 network; fabricating a SS-DSSC by preparing a porous
network of CuBO.sub.2 in which TiO.sub.2 particles are embedded;
fabricating a SS-DSSC by using TiO.sub.2--CuBO.sub.2 "core-shell"
nanoparticles; and fabricating a SS-DSSC by using
TiO.sub.2--CuBO.sub.2 "nano-couples".
[0011] Certain embodiments of the present invention are methods of
fabricating devices including thin films of copper delafossite
materials.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] These and other aspects and features of the present
invention will become apparent to those ordinarily skilled in the
art upon review of the following description of specific
embodiments of the invention in conjunction with the accompanying
figures, wherein:
[0013] FIG. 1 is a schematic flow diagram illustrating the
CuBO.sub.2 sol-gel process, according to embodiments of the
invention;
[0014] FIG. 2 is a graph showing the variation of the indirect
band-gap of CuBO.sub.2 with particle size;
[0015] FIG. 3 is a schematic of equipment for nanopowder
production, according to embodiments of the invention;
[0016] FIG. 4 is a schematic cross-section of a single-junction
p-i-n solar cell with a copper delafossite p-layer, according to
embodiments of the present invention;
[0017] FIG. 5 is a schematic cross-section of a multiple junction
p-i-n solar cell with copper delafossite p-layers, according to
embodiments of the present invention;
[0018] FIG. 6 is a schematic cross-section of a p-n solar cell with
a copper delafossite layer within the p-layer, according to
embodiments of the invention;
[0019] FIG. 7 is a schematic cross-section of a CuBO.sub.2
transparent thin film transistor (TTFT), according to embodiments
of the present invention; and
[0020] FIG. 8 is a scanning electron micrograph of a CuBO.sub.2
TTFT, according to embodiments of the invention.
[0021] FIG. 9 is a graph of the instantaneous photocurrent J.sub.ph
and dark current J.sub.d of CuBO.sub.2 vs. potential for a
CuBO.sub.2 pellet fabricated according to embodiments of the
invention;
[0022] FIG. 10 is a schematic diagram showing the energy levels of
TiO.sub.2, the ground and excited states of Ruthenium (Ru-535) dye,
and a CuBO.sub.2 thin film fabricated according to embodiments of
the invention;
[0023] FIG. 11 is a schematic diagram of a solid-state DSSC,
according to embodiments of the invention;
[0024] FIG. 12 is a graph of the photocurrent-voltage
characteristics of a CuBO2-based DSSC, with an inset showing the
conversion efficiency (.eta.) of the cell as a function of time,
measured from a DSSC made according to embodiments of the
invention;
[0025] FIGS. 13A & 13B are schematic diagrams showing a
mesoporous TiO.sub.2 film with (A) larger size CuBO.sub.2 particle,
(B) nanosize CuBO.sub.2 powder, prepared by methods according to
embodiments of the invention;
[0026] FIG. 14 is a graph showing differential scanning calorimetry
(DSC) for citrate gel used for preparing CuBO.sub.2, according to
embodiments of the invention;
[0027] FIGS. 15A, 15B & 15C are schematic diagrams showing: (A)
citrate sol filling in the pores of a mesoporous TiO.sub.2 film,
(B) gel inside a mesoporous TiO.sub.2 film, and (C)
interpenetrating network of TiO.sub.2 and CuBO.sub.2 nanosize
particles, according to embodiments of the invention;
[0028] FIG. 16 is a schematic diagram of a DSSC with a network of
nanosize CuBO.sub.2 particles, according to embodiments of the
invention;
[0029] FIGS. 17A & 17B are schematic diagrams of
TiO.sub.2--CuBO.sub.2 "core-shell" nano particles--(A) shows
particles with TiO.sub.2 as the core and CuBO.sub.2 as the shell
layer and (B) shows particles with CuBO.sub.2 as the core and
TiO.sub.2 as the shell layer, according to embodiments of the
invention;
[0030] FIG. 18 is a schematic diagram of a DSSC made with
TiO.sub.2--CuBO.sub.2 "core-shell" particles, according to
embodiments of the invention;
[0031] FIG. 19 is a schematic diagram of a TiO.sub.2 and CuBO.sub.2
"nano-couple", according to embodiments of the invention; and
[0032] FIG. 20 is a schematic diagram illustrating the steps
involved in the synthesis of TiO.sub.2--CuBO.sub.2 "nano-couples",
according to embodiments of the invention.
DETAILED DESCRIPTION
[0033] The present invention will now be described in detail with
reference to the drawings, which are provided as illustrative
examples of the invention so as to enable those skilled in the art
to practice the invention. Notably, the figures and examples below
are not meant to limit the scope of the present invention to a
single embodiment, but other embodiments are possible by way of
interchange of some or all of the described or illustrated
elements. Moreover, where certain elements of the present invention
can be partially or fully implemented using known components, only
those portions of such known components that are necessary for an
understanding of the present invention will be described, and
detailed descriptions of other portions of such known components
will be omitted so as not to obscure the invention. In the present
specification, an embodiment showing a singular component should
not be considered limiting; rather, the invention is intended to
encompass other embodiments including a plurality of the same
component, and vice-versa, unless explicitly stated otherwise
herein. Moreover, applicants do not intend for any term in the
specification or claims to be ascribed an uncommon or special
meaning unless explicitly set forth as such. Further, the present
invention encompasses present and future known equivalents to the
known components referred to herein by way of illustration.
[0034] The examples provided herein are directed primarily to
CuBO.sub.2 materials; however, many of the concepts are applicable
to other Cu delafossite materials, for example CuAlO.sub.2,
CuGaO.sub.2 and CuInO.sub.2. Furthermore, the examples of devices
provided herein are directed to solar cell devices and transparent
thin film transistors; however, for similar reasons, other devices
can also benefit from incorporating transparent p-type
semiconducting materials, including transparent thin film
photovoltaics, transparent p-n diodes, visible and ultraviolet
photodetectors, devices for photoelectrolysis for hydrogen
production, and other devices for displays and low-E glazing
applications. AgBO.sub.3, TIBO.sub.3 and alloys of
Cu.sub.1-xAg.sub.xBO.sub.2 p-type transparent semiconductors can be
used, in addition to CuBO.sub.2. The sol-gel process used for
synthesizing the CuBO.sub.2 powders can be modified for synthesis
of thin-films by solution deposition techniques such as: dip
coating, spray coating, ink jet printing or spin coating. This
modified sol-gel process can be used as a low temperature technique
for depositing thin films on a wide variety of substrates including
ceramic, single crystal and temperature sensitive substrates such
as glass, metal foil, and plastics.
[0035] Herein, unless indicated otherwise, the terms copper boron
oxide and CuBO.sub.2 are used interchangeably to refer to optically
transparent p-type semiconductor materials which have the general
composition and stoichiometry of CuBO.sub.2 and the delafossite
crystal structure.
[0036] Processes for Synthesizing CuBO.sub.2 Powders &
Films
[0037] A new technique has been developed for synthesizing
CuBO.sub.2 powders via a low temperature wet process. FIG. 1 shows
a schematic flow diagram of the processing technique. In this
technique CuO and B.sub.2O.sub.3 are dissolved in nitric acid and
water, respectively, and the two solutions are combined to form a
homogeneous solution (110). In order to achieve a stoichiometric
powder, the molar ratio of Cu to B is 1:1. Citric acid is then
added to the solution in a 2:1 citric acid to Cu molar ratio (120).
Citric acid is a chelating agent which bonds to the metal ions--one
citric acid molecule chelates one copper and one boron atom. The
solution was then diluted with de-ionized water increasing the
volume by 10 times (130). This solution is then refluxed for
approximately 18 hrs. at 100.degree. C. (140). After refluxing, the
solution is evaporated creating a gel network (150). Further
heating of the gel to a temperature in the range of 160 to
200.degree. C. results in an exothermic reaction (combustion)
producing the CuBO.sub.2 powders (160). Reacted powders are
calcined at a temperature in the range of 300 to 500.degree. C. for
approximately 2 hrs. to remove any residual carbon.
[0038] Dilution of the citric acid solution was done in order to
prevent metal precipitation during refluxing. The reason for the
excess citric acid is because the citric acid is not fully
dissociated into ions in solution, and better quality films are
produced when all of the Cu and B are chelated; although, this must
be balanced with minimizing excess carbon formed from the citric
acid during combustion of the gel. In summary, the ratio of citric
acid to Cu should be in excess of 1:1, and a ratio of approximately
2:1 is found to provide satisfactory results.
[0039] Furthermore, the band-gap of CuBO.sub.2 particles may be
adjusted by controlling the particle size. Copper delafossite
powders are synthesized from gels, as described above. The particle
size of the resulting CuBO.sub.2 powders is controlled by varying
the temperature of the gel to solid reaction. As described above,
this reaction occurs in the range of 160 to 200.degree. C. under
ambient conditions. In order to reduce the reaction temperature,
the system pressure is reduced--the gel to solid process is carried
out by heating under vacuum. For example, the gel to solid process
was carried out at 70 and 100.degree. C. under vacuum at 50 Torr.
The color of the as synthesized powders is an indicator of the
process temperature, i.e. lower process temperature gives smaller
particles and blue shifted color. For example, the powder formed at
70.degree. C. under vacuum is blue in color, whereas the powder
formed at 160.degree. C. in air is red-brown in color. Furthermore,
post annealing or calcining of powders causes particle size to
increase and results in red shifting. All powders calcined at
500.degree. C. are a red-brown indicating similar particle size.
Powder samples of CuBO.sub.2 produced at lower temperature showed
slightly higher band gaps. FIG. 2 shows a plot of the indirect
band-gap of CuBO.sub.2 as a function of the average particle size.
As the average particle size decreases from 200 nm to about 100 nm,
the indirect band gap increases from 2.4 eV to 2.6 eV. Note that
the 200 nm particles were formed using the above process at
70.degree. C. under vacuum and the 200 nm particles were formed at
200.degree. C. under ambient conditions. The indirect band gap was
measured using an ultraviolet-visible spectrophotometer. The
particle size was measured by x-ray diffraction using Scherrer's
formula which relates diffraction peak broadening to crystallite
size. Note that as the particle size is reduced further, the band
gap will continue to increase.
[0040] One inch targets were prepared using the nanopowders
described above by pressing the powder under 5 MPa, followed by
pressing in an isostatic press for 20 minutes under 20 MPa. These
press targets were then placed in a vacuum chamber of a pulsed
laser deposition system.
[0041] Substrates, such as transparent conducting oxide coated
glass, were ultrasonically cleaned in three organic solvents in the
order: acetone, isopropanol, and then methanol. The substrates were
then rinsed in clean methanol and dried with dry air. The
substrates were then mounted onto the substrate heater of the
pulsed laser deposition system. The vacuum chamber containing the
deposition target and substrate was sealed and evacuated to a
vacuum of 1.times.10.sup.-6 Torr. Once a pressure of
1.times.10.sup.-6 Torr was reached, the target was cleaned in situ
by ablating the surface. A laser energy of 2 J/cm.sup.2, laser
pulse frequency of 10 Hz and target rotation speed of 18.degree./s
were used for this cleaning process. To ensure all surface
contamination was removed, the target was ablated with two full
rotations. After cleaning the substrate, the chamber was again
evacuated to 1.times.10.sup.-6 Torr and the substrate heated (to
500.degree. C.) at a rate of 10.degree. C./min. Prior to
deposition, the substrate was held at a constant temperature for 10
minutes. Due to the low temperature requirements of the TCO coated
glass substrate, low deposition temperatures were used. After 10
minutes, high purity O.sub.2 gas was introduced into the chamber
with a partial pressure roughly within the range of 1 mTorr to 0.1
Torr. Once the O.sub.2 pressure reached equilibrium the deposition
process was started. Targets were ablated using a KrF excimer laser
with a photon wavelength of 248 nm and pulse duration of 25 ns. A
laser energy of 2 J/cm.sup.2, laser pulse frequency of 10 Hz and
target rotation speed of 18.degree./s were used for deposition.
These conditions resulted in a growth rate of 0.5 .ANG./pulse. Film
thicknesses were varied from 80 to 500 .ANG.. After the deposition
was complete, the substrate was cooled in an O.sub.2 atmosphere in
order to prevent reduction of the deposited film. Reduction of the
copper delafossite films is undesirable since it results in excess
oxygen vacancies and decomposition of the films. When room
temperature was reached, films were removed from the vacuum chamber
and stored under vacuum to reduce contamination.
[0042] When substrates are used which are tolerant of higher
processing temperatures, the deposition temperature may be varied
over a wider range, including higher temperatures. In these
circumstances, the film properties may be optimized by varying
deposition temperature and oxygen pressure over wider ranges. Some
examples are provided below. In alternative embodiments of the
present invention, sapphire, silicon, or other substrates tolerant
of high processing temperatures may be used. When depositing
CuBO.sub.2 on one of these high temperature substrates, the
deposition process follows the same general steps as described
above with the exception of the deposition temperatures and oxygen
pressures. The deposition temperature and oxygen partial pressure
may be varied between 350 and 700.degree. C. and 10.sup.-6 and
10.sup.-1 Torr, respectively, to determine optimum growth
conditions. For example, for a CuBO.sub.2 channel transistor on a
silicon substrate, deposition temperature and oxygen pressure of
550.degree. C. and 10.sup.-1 Torr, respectively, were found to be
ideal for device performance.
[0043] Typical CuBO.sub.2 thin films deposited using the above
techniques are nanocrystalline, with a grain size of approximately
20 nanometers. Optical transmission is in excess of 50% over the
measured wavelength range of 200 to 900 nanometers. Values of
direct and indirect bandgaps were estimated to be roughly 4.5 eV
and 2.4 eV, respectively. Electrical conductivity was measured to
be roughly 1.5 Scm.sup.-1. The material is p-type, with estimated
carrier Hall mobility of approximately 100
cm.sup.2V.sup.-1s.sup.-1. The material has the general composition
and stoichiometry of CuBO.sub.2 and the delafossite crystal
structure.
[0044] The transparent semiconducting copper delafossite thin films
may be used in a variety of devices, for example: transparent light
emitting diodes (LEDs), ultraviolet (UV) detectors, solar cells,
transparent transistors, etc. Some specific examples of devices are
provided below.
[0045] However, there are some applications in which very small
CuBO.sub.2 particles, smaller than the typically 200 nanometer size
particles produced by the sol-gel process described above, are
desired. A laser assisted fabrication system was designed and
fabricated for preparing nano-sized CuBO.sub.2 powders. Using this
system, it is possible to continuously produce nano-scaled powders
under well-defined and stable conditions.
[0046] FIG. 3 shows the schematic design of the laser assisted
fabrication system. The main part of the evaporation chamber 210 is
a continuously rotating disc 215 with a ring-shaped channel along
its rim containing the raw powder. The laser beam 220 is focused
through an inlet tube onto the revolving powder surface and
evaporates the raw material. This technique is very similar to the
PLD technique described above for making nanocrystalline films
(grain size .about.20 nm) of CuBO.sub.2. However, in this case,
instead of directly depositing the ablated material (in the form of
plasma) on a substrate held at high temperature, the ablated
material is blown out of the area of the interaction between the
laser radiation and the powder target by a constantly flowing inert
gas. Due to the steep temperature gradient between the hot
evaporation zone and the surrounding atmosphere, nucleation,
condensation, and coagulation proceeds very quickly. This results
in the formation of ultrafine particles. The constant flow of the
inert gas is maintained to rapidly dilute the emerging droplets
making the formation of hard agglomerates by edge melting of
droplets improbable.
[0047] During one revolution of the disc, the evaporated material
is automatically refilled by a refilling unit 230 and the surface
of the fill is flattened by a scraper. Thereby, a continuously
regenerated powder surface is fed to the laser beam, ensuring
stable and reproducible process conditions. The evaporation chamber
210 is connected in a gas tight manner to the filtering chamber 240
through a system of glass tubes 245. A gas extraction fan 250,
attached to the filtering chamber 240 by a flange, provides for the
constant flow of the process gas, which is ingested below the
evaporation zone. By this gas flow, the nanoparticles will be
dragged into the filtering chamber 240, where they will be
separated from the aerosol on a cylindrical paper 260 or metal bag
filter. Any particles that fall from the filter 260 are collected
in nanopowder container 270. Nanoparticles of approximately 20.+-.5
nanometers diameter were made using this system. This system may be
used to make nanoparticles with a distribution centered about a
diameter ranging from 5 to 500 nanometers. As discussed above, with
reference to FIG. 2, the band gap of the CuBO.sub.2 particles is
larger for smaller particles.
[0048] Solar Cells with CuBO.sub.2 P-Layer
[0049] Using the low temperature processes described above, p-type
copper boron oxide may be incorporated into a wide variety of solar
cells where a transparent p-layer is desired. For example, FIG. 4
shows a single junction amorphous silicon solar cell on a
transparent substrate with a transparent p-type copper boron oxide
p-layer 330. In more detail, the solar cell in FIG. 4 comprises a
glass/flexible substrate 310, a transparent conductive layer 320
such as a thin film of n-type TCO, a thin film of p-type copper
boron oxide 330, an amorphous silicon absorber layer 340, an n-type
amorphous silicon thin film 350 and a back contact 360. The back
contact 360 may be formed of aluminum, aluminum with 1% silicon or
nickel, for example. The copper boron oxide film 330 is typically
between 8 and 100 nanometers thick.
[0050] FIG. 5 shows an example of a multiple junction solar cell.
The multiple junction solar cell is the same as the single junction
solar cell of FIG. 3, except for a second p-i-n stack. The top
stack may have a different absorber to the lower stack, for example
the i-layer for the upper stack 470 is microcrystalline silicon and
the i-layer for the lower stack 440 is amorphous silicon. In more
detail, the solar cell in FIG. 5 comprises: a glass/flexible
substrate 410; a transparent conductive layer 420 such as a thin
film of n-type TCO; a first stack including a thin film of p-type
copper boron oxide 430, an i-layer 440 and an n-layer 450; a second
stack including a thin film of p-type copper boron oxide 460, an
i-layer 470 and an n-layer 480 and a back contact 490. The copper
boron oxide films 430 and 460 are between 8 and 100 nanometers
thick.
[0051] FIG. 6 shows a solar cell with a p-layer 530 comprising an
absorber material and a thin film of p-type copper delafossite. The
absorber material may be comprised of materials such as copper
indium selenide (CIS), copper indium gallium selenide (CICS),
Cu(In,Ga)(S,Se).sub.2 (CISSe), CdTe, Cu.sub.2ZnSnS.sub.4, or other
II-VI binary and ternary compounds. The copper delafossite film may
be either between the absorber and the conductive layer 520 or
between the absorber and the cadmium sulfide n-type layer 540. In
more detail, the solar cell in FIG. 6 comprises a glass/stainless
steel/polymer substrate 510, a conductive layer 520 such as a thin
film of molybdenum metal, a p-layer 530, an n-layer 540 such as a
film of cadmium sulphide, a TCO/buffer layer 550 such as films of
ITO/zinc oxide, an anti-reflective layer 560 such as a film of
magnesium fluoride and metal contacts 570. Note that the substrate
510 may typically be 1.5 millimeters thick. The molybdenum layer
520 is typically 0.5-1.5 microns thick and may be sputter deposited
on the substrate. The p-layer 530 is typically 1.5-2.0 microns
thick and the absorber material may be deposited by a wet chemical
process; within the p-layer, the copper delafossite film is
typically 0.02 microns thick and may be deposited by the laser
ablation process described above. The cadmium sulphide n-layer 540
is typically 0.03-0.05 microns thick and may be deposited by a
chemical bath deposition (CBD) process. The ITO/zinc oxide layer
550 is typically 0.5-1.5 microns thick and may be deposited using
wet chemical or radio frequency sputtering processes. The magnesium
fluoride anti-reflective layer 560 is typically 0.1 microns thick
and may be electron beam evaporated. The metal contacts 570 may be
made of nickel/aluminum, may have a thickness somewhere in the
range of 0.05-3.00 microns depending on the solar cell geometry,
and may be electron beam evaporated.
[0052] Furthermore, as described earlier, the p-type copper boron
oxide in the examples of solar cells given above may be replaced by
other p-type copper delafossite materials, such as CuAlO.sub.2,
CuGaO.sub.2 and CuInO.sub.2. Yet further, for purposes of improving
the quantum efficiency of the solar cell, the p-type copper boron
oxide thin film may be replaced by two thin films: a film of a
copper delafossite material, and a film of a second material such
as p-type amorphous silicon, p-type microcrystalline silicon or
p-type microcrystalline silicon carbide.
[0053] Transparent Thin Film Transistors (TTFTs) with CuBO.sub.2
Channel Layers
[0054] Transparent thin film transistors (TTFT's) have recently
become of great interest for invisible microelectronics and drivers
for organic and flat panel displays. All transistor components--the
gate, gate dielectric, drain, source and a transparent oxide
semiconductor channel layer--can all be made from stable and
transparent oxide materials. However, only n-type oxide
semiconductor TTFT's have been widely demonstrated with great
success. For most applications complementary p-type TTFT's are
required. CuBO.sub.2 is a viable p-type wide hand gap semiconductor
for making p-type TTFTs.
[0055] FIG. 7 shows a schematic cross-section of a YTFT with a
p-type CuBO.sub.2 channel 640. The TTFT comprises a substrate 610,
a gate 620, a gate insulator 630, a p-type CuBO.sub.2 channel 640,
a source 650 and a drain 660. The substrate 610 may be glass or
some other rigid material such as a polymer. The gate 620 may be a
TCO such as ITO. The gate insulator 630 may be a dielectric such as
Al.sub.2O.sub.3, HfO.sub.2, ZrO.sub.2 and rare earth oxides. The
channel 640 is typically 100-300 nanometers thick. In fabricated
TTFTs the channel layer length was varied between 100 and 500 .mu.m
and the width between 0.5 and 5 mm. The CuBO.sub.2 channel 640 may
be deposited by pulsed laser deposition (PLD) onto the insulator
layer 630. The deposition parameters are the same as those
described above. The source contact 650 and drain contact 660 may
be a 10 nm thick metal layer, such as platinum, or a 100 nm thick
transparent conducting oxide (TCO) layer, such as ITO,
aluminum-doped zinc oxide and fluorine-doped tin oxide. Drain and
source contacts 650, 660 were sputtered on top of the channel layer
640.
[0056] FIG. 8 shows a scanning electron micrograph of a single TTFT
such as shown in FIG. 7. FIG. 8 is a top view of a device in which
the gate 620, CuBO.sub.2 channel 640, drain 650 and source 660 are
imaged.
[0057] The deposition temperature for the CuBO.sub.2 channel 640
plays a very critical role on device performance. In an ideal field
effect transistor (FET) the dielectric layer (gate insulator) has
very high resistance since a finite resistance allows current
leakage through the dielectric layer banning device performance.
For the CuBO, channel TTFTs, the resistance between the gate and
source depends sensitively on the CuBO.sub.2 thin film deposition
temperature. The higher the deposition temperature the lower the
gate source resistance. The temperature dependence of gate to
source resistance is due to diffusion of Cu and B elements through
the dielectric layer reducing the resistance. This is not the only
consideration for selecting an ideal deposition temperature since
there is a minimum thermodynamic temperature required to form
CuBO.sub.2. A deposition temperature of 550.degree. C. may be the
ideal balance between these temperature requirements. However, the
substrate material may limit the deposition temperature to
approximately 500.degree. C.
[0058] In order to prevent the diffusion of parasitic elements like
Cu and B into the dielectric layer a diffusion barrier (not shown
in FIG. 7) may be added between the channel 640 and dielectric
layer (gate insulator) 630. The diffusion barrier is a thin film
deposited on the dielectric layer 630, the CuBO, film (channel
layer) 640 being formed on top of the diffusion barrier layer. The
diffusion barrier should be thin, non-reactive with the dielectric
material and CuBO.sub.2 and impede diffusion of Cu or B into the
dielectric layer. Diffusion barriers based on transition metal and
rare earth oxides, like Ta.sub.2O.sub.5, or transition metal and
rare earth nitrides, like TaN, may be suitable. The use of such
barriers may reduce current leakage through the gate
dielectric.
[0059] Furthermore, as described earlier, the p-type copper boron
oxide in the examples of TTFTs given above may be replaced by other
p-type copper delafossite materials, such as CuAlO.sub.2,
CuGaO.sub.2 and CuInO.sub.2.
[0060] Solid State Dye-Sensitized Solar Cells Using CuBO.sub.2
[0061] In order to be useful in DSSCs, a p-type semiconductor and a
dye are required to have the following properties: (i) the p-type
material must be transparent throughout the visible spectrum, where
the dye absorbs light (in other words the semiconductor must have a
large band-gap), (ii) a method must be available for depositing the
p-type material without dissolving or degrading the monolayer of
dye on the TiO.sub.2 nanocrystallites (n-type semiconductor), (iii)
the dye must be such that its excited level is located above the
bottom of the conduction band of TiO.sub.2 and the ground level
below the upper edge of the valence band of the p-type material.
This condition is essential for ensuring the separation of
photo-generated electron-hole pairs.
[0062] CuBO.sub.2 may be used as a hole collector in TiO.sub.2
based DSSCs. To extract holes from the dye, the valance band edge
of the material should be above the ground level of the dye. To
determine whether this condition is satisfied by CuBO.sub.2, its
flat band potential and valence band edge were determined by
performing photoelectrochemical characterization. Electrochemical
measurements were carried out in 1M solution of KOH (pH 12) using a
standard three electrode device. The three electrodes were, a
CuBO.sub.2 pellet, a large platinum counter electrode and a
saturated calomel reference electrode (SCE) to which all potentials
were quoted. Note that the CuBO.sub.2 pellet is a small disc
prepared by: pressing calcined CuBO.sub.2, prepared as described
above using the sol-gel process, with a uniaxial hydraulic press in
a circular dye; and further densifying the pellet by isostatic
pressing at 30,000 Psi for 20 minutes. The electrolyte was
continuously flushed with pure nitrogen gas. FIG. 9 shows the
current-voltage curves, both in the dark as well as under
illumination. As can be seen in FIG. 9, the appearance of the
photocurrent (J.sub.ph) started at a potential V.sub.ON of +0.21 V
and increased in the cathodic direction, which is typical of p-type
behavior. The potential V.sub.ON can be reasonably considered as
the potential (V.sub.lb) that corresponds to the position of the
valence band of the material. The valence band position of
CuBO.sub.2 was estimated using the known equation:
E.sub.VB=4.75+eV.sub.fb+0.059(pH-pH.sub.pzzp)
pH.sub.pzzp is the pH at the point of zero zeta potential (pzzp)
and was found to be 8.2. Thus the results showed that the valence
band is located at .about.5.2 eV below vacuum (0.46 eV vs. SCE.
[0063] In FIG. 10 the energy level diagrams of TiO.sub.2 {R.
Memming, "Solar energy conversion by photoelectrochemical
processes", Electrochemical Acta. 25, 77-88 (1980)}, ruthenium dye
[cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate)-ruthenium(-
II), hereafter referred as Ru-535 dye {Nazeeruddin et al.,
"Engineering of Efficient Panchromatic Sensitizers for
Nanocrystalline TiO.sub.2-based Solar Cells", Journal of the
American Chemical Society 123, 1613-1624 (2001)}], and CuBO.sub.2
pellet, made as described above, are shown. The excited energy
level of the dye lies at 0.66 eV above the conduction band of
TiO.sub.2 while the ground state of the dye lies 0.39 eV below the
valence band of CuBO.sub.2. As is evident, these energy level
positions satisfy the condition for charge separation of
photo-generated electron-hole pairs very well. So, if an
electron-hole pair is generated in the dye, the electron will
readily be injected into the conduction band of TiO.sub.2 and the
hole to the valence band of CuBO.sub.2. FIG. 10 shows the energy
levels of Ru-535 dyes only; however, there are several other dyes
which can favorably satisfy the energy level requirement. This is
discussed in more detail below.
[0064] A prototype DSSC was fabricated and its performance and
conversion efficiency were evaluated. A schematic diagram of the
cell is shown in FIG. 11. The cell was fabricated as follows. First
of all a thin solid film of TiO.sub.2 720 (30-100 nm thick) was
deposited on an electrically conductive indium tin oxide (ITO)
coated glass plate 710 by spray pyrolysis. This was followed by the
pressing of a mesoporous TiO.sub.2 layer 730 onto the sprayed
TiO.sub.2 layer 720. For this, 500 mg of TiO.sub.2 nano-powder
(diameter .about.5 nm) was suspended in 10 mL of pure ethanol by
stirring for several hours followed by 10 minutes of sonication
using a titanium horn immersed in the suspension. The slurry was
spread onto the surface of the ITO/TiO.sub.2 substrate by tape
casting using a spacer layer of scotch tape (10 .mu.m thick). The
resulting layer of ethanol/TiO.sub.2 was allowed to dry in the
ambient atmosphere. The very loose film of particles that resulted
was then pressed between two steel plates at 100 kg/cm.sup.2 for 2
minutes. Under such pressures, films compress significantly,
decreasing from an initial porosity of over 90% to about 70% (see
FIG. 6). The TiO.sub.2 layers were then heated for 2 hours at
500.degree. C. in air. The films were cooled to room temperature
and were dyed by immersing them in a 5.0.times.10.sup.-4 M solution
of cis-bis(thiocyanate)bis(2,2-bipyridyl4,4-dicarboxylate)
ruthenium(II) in ethanol for 6 hours. For the hole collector
coating 740, a few drops of CuBO.sub.2 suspension in ethanol were
placed on the dyed TiO.sub.2 film and spin coated at 800 rpm for 2
min. On the dyed TiO.sub.2/CuBO.sub.2 layer, a thin layer of
graphite was applied for better electrical contact between the
electrode and a back contact. A conducting indium tin oxide (ITO)
coated glass plate 760 with a thin layer (.about.50 nm) of dense
CuBO.sub.2 750 was used as the back contact. The dense back contact
750 was applied to the ITO coated glass plate 760 by PLD.
Alternatively, deposition of back contact 750 may be by sputter
deposition, molecular beam epitaxy (MBE) pulsed electron beam
deposition, electron beam evaporation, other physical vapor
deposition techniques, and sol-gel/chemical deposition
techniques.
[0065] The energy conversion efficiency was measured under
simulated sunlight (AM 1.5, 100 mWcm.sup.-2 illumination). FIG. 12
shows a typical photocurrent density vs. voltage curve for the
CuBO.sub.2 based DSSC. The values of the open-circuit voltage
(V.sub.oc), short circuit photocurrent density (I.sub.sc) and fill
factor (FF) are 550 mV, 1.6 mA cm.sup.-2 and 0.61, respectively.
The solar cell's overall energy conversion efficiency
(.eta.=FF.times.V.sub.oc.times.I.sub.sc/P.sub.in) was calculated to
be 0.53%. The stability of the solid-state DSSC was determined by
the computer controlled measurements of the photocurrent vs.
voltage characteristics under continuous illumination for 15 days
(360 hrs). Measurements were performed at regular intervals of six
hours. Over a period of 15 days only a decrease of 2% in the
conversion efficiency of the cell was observed (see inset of FIG.
12).
[0066] Fabrication of Solid-State DSSC by Preparing Ultrafine
Powders of CuBO.sub.2 which Can Penetrate Through the Dyed Porous
TiO.sub.2 Network
[0067] FIG. 13A shows the situation where CuBO.sub.2 particles 820
are too large to efficiently penetrate the mesoporous TiO.sub.2
network 810 (pore size .about.50 nm-100 nm), this is the case for
CuBO.sub.2 powders (particle size .about.200 nm) produced by the
sol-gel process described above. In contrast, FIG. 13B shows the
situation where the CuBO.sub.2 particles 830 are sufficiently small
(.about.20 nm) to penetrate the dyed TiO.sub.2 network 810. DSSCs
were fabricated using nano-sized particles of CuBO.sub.2, as
produced by the nanopowder tool described above and the method
described above, resulting in devices in which the CuBO.sub.2
particles have penetrated the dyed TiO.sub.2 network. Using these
nanosized powders, a higher fraction of p-type oxide enters inside
the pores of the mesoporous TiO.sub.2, which results in higher
conversion efficiency. Detailed measurements of the energy
conversion efficiency were performed as described above, and were
in the range of 0.6-1.0%. (Note that a variation of this method is
to dye the structure after CuBO.sub.2 particles have been added to
a mesoporous TiO.sub.2 structure, rather than dying the mesoporous
TiO.sub.2 structure before adding the CuBO.sub.2 particles.)
[0068] Fabrication of Solid-State DSSC by Depositing p-Type Oxide
into the Pores of a TiO.sub.2 Network by a Sol-Gel Technique
[0069] CuBO.sub.2 material was deposited inside the pores of a
TiO.sub.2 network by a sol-gel technique, as shown in FIGS. 15A,
15B & 15C. This process has two variations. In the first
variation, CuBO.sub.2 material is deposited inside the pores of a
dye-coated TiO.sub.2 network. FIG. 14 shows differential scanning
calorimetry (DSC) data of the citrate gel used for preparing
CuBO.sub.2. FIG. 14 shows that the decomposition of the gel to
CuBO.sub.2 occurs at .about.160.degree. C. The decomposition
temperature of Ruthenium dye (Ru-535) is 250.degree. C., showing
that CuBO.sub.2 can be deposited in the pores of a dye-coated
TiO.sub.2 network by this sol-gel technique without damaging the
Ru-535. To deposit the CuBO.sub.2 within the pores, the first step
is to introduce dilute citrate sol 920 containing chelated
Cu.sup.2+ and B.sup.3+ ions into the pores of dye-coated TiO.sub.2
network 910 (see FIG. 15A). This is followed by evacuation of the
sol-soaked TiO.sub.2 film at about 80.degree. C. to convert the sol
to gel. The evacuation is done by applying a vacuum in order to
keep the temperature low enough to avoid damage of the dye. The
above two steps are repeated several times to fill the pores with
the desired amount of gel 930 (see FIG. 15B). After this, the
mesoporous TiO.sub.2 containing gel is heated to 160.degree. C. At
this temperature the gel decomposes to form CuBO.sub.2 powder 940
which is likely to be uniformly dispersed inside the TiO.sub.2
network 910 (see FIG. 15C). However, CuBO.sub.2 formed at this
temperature is quite amorphous, which may negatively impact the
performance of the cell. Detailed measurements of the energy
conversion efficiency were performed as described above, and were
in the range of 0.5-0.9%.
[0070] In the second variation of the sol-gel based process,
CuBO.sub.2 material was deposited inside the pores of an uncoated
(without dye) TiO.sub.2 network structure. The process described
above is followed, with the following differences. In this
variation there is the freedom to increase the temperature of the
system to 500.degree. C. (because the dye has not yet been added to
the cell yet) for the purpose of increasing the crystallinity of
the CuBO.sub.2. The uniformly dispersed TiO.sub.2--CuBO.sub.2
system was annealed in flowing oxygen to compensate for any oxygen
non-stoichiometry of the TiO.sub.2--CuBO.sub.2 material system
because of the carbonaceous byproducts of the gel-decomposition.
The TiO.sub.2--CuBO.sub.2 system was dyed by immersion in the dye
solution for 6-12 hours. Because of the tendency of these materials
to become porous, there is some spacing between CuBO.sub.2
particles in the pores in the network and the walls of the
TiO.sub.2 network. Dye molecules reach these spaces due to
capillary action. The resulting structure is similar to that of
FIG. 15C, except the CuBO.sub.2 particle size will likely be
larger. There are several parameters that will have to be optimized
such as the amount of CuBO.sub.2, the amount of dye, the pore size
of the TiO.sub.2 network, etc. Detailed measurements of the energy
conversion efficiency were performed as described above, and were
in the range of 0.5-0.9%.
[0071] Fabrication of Solid-State DSSC by Preparing a Porous
Network of CuBO.sub.2 in which TiO.sub.2 Particles can be
Embedded
[0072] A solid-state DSSC was fabricated by first preparing a
porous network of CuBO.sub.2 and then creating an interpenetrating
network of TiO.sub.2 nano-particles. Most of the work on DSSC has
been done by making porous networks of TiO.sub.2 and then inserting
electrolyte inside the pores. However, when using solid p-type hole
collectors, the order of the fabrication steps may be reversed.
This approach will be specially significant for CuBO.sub.2 based
hole collectors, where the particles or grains have an inherent
tendency to grow larger in size. The grain/particle growth occurs
over time at temperature by solid state diffusion and Oswald
ripening. FIG. 16 shows a schematic diagram of this DSSC cell
structure.
[0073] In FIG. 16, the DSSC consists of a glass substrate 1010 with
a coating of transparent conducting oxide (ITO) 1015. A thin (50
nm) dense coating of CuBO.sub.2 1020 is deposited over the
substrate 1010, 1015 by a pulsed laser deposition (PLD) technique
using the same protocol as described above. After this, a
mesoporous CuBO.sub.2 layer 1030 is deposited over the dense
CuBO.sub.2 layer 1020. For this, a small amount of CuBO.sub.2
powder (average diameter .about.100 nm) is suspended in pure
ethanol by stirring for several hours and then spread onto the
surface of the substrate. The resulting layer of ethanol/CuBO.sub.2
powder is dried in ambient atmosphere followed by pressing between
two steel plates at 100 kg/cm.sup.2 for 2-5 minutes. Under such
pressures, CuBO.sub.2 layers 1030 compress significantly. The
CuBO.sub.2 layers 1030 are then heated for 2-5 hrs at 500.degree.
C. in air. For inserting TiO.sub.2 nanoparticles 1040 inside the
pores of the CuBO.sub.2 layer 1030, a few drops of TiO.sub.2
suspension in ethanol is placed on the CuBO.sub.2 layer 1030 and
spin coated at about 1000 rpm for 2 min. This step is repeated
several times to insert the desired amount of TiO.sub.2
nano-particles 1040 in the pores of the CuBO.sub.2 layer 1030. This
is followed by annealing at 500.degree. C. in air. The
interpenetrating network of TiO.sub.2 and CuBO.sub.2 thus obtained
is coated by dye by dipping it in an ethanolic solution of dye. In
the final step, the DSSC is assembled by placing a transparent
conducting electrode 1060 with a dense layer of TiO.sub.2 1050 over
it. Slight pressure applied to compress the DSSC is sufficient to
ensure good electrical contact. Detailed measurements of the energy
conversion efficiency were performed as described above, and were
in the range of 0.6-1.0%.
[0074] Fabrication of Solid-State DSSC by Using
TiO.sub.2--CuBO.sub.2 "Core-Shell" Nano-Particles
[0075] FIGS. 17A & 17B show TiO.sub.2--CuBO.sub.2 "core-shell"
nano-particles that were designed specifically to enable a new
method of forming a DSSC. In conventional DSSCs, the device is
fabricated in such a manner that the dye layer lies in between the
n-type semiconductor and the hole collector layer. However, in some
embodiments of the present invention, dye is not coated on the
interface of TiO.sub.2 and CuBO.sub.2, rather it is coated on the
surface of TiO.sub.2--CuBO.sub.2 "core-shell" particles.
[0076] TiO.sub.2--CuBO.sub.2 "core-shell" particles were prepared.
These "core-shell" particles were used for fabricating DSSCs by
sensitizing the outer surface of the "core-shell" particles with a
dye 1150. Two different kinds of "core-shell" particles were
synthesized: (i) TiO.sub.2 1110 as core and CuBO.sub.2 1120 as
shell layer (FIG. 17A), and (ii) CuBO.sub.2 1130 as core and
TiO.sub.2 1140 as shell layer (FIG. 17B). For preparing
"core-shell" nano-particles with a TiO.sub.2 core, TiO.sub.2
nano-particles were washed with a dilute nitric acid solution
(pH.about.4) and a small amount of tetraethyl orthotitanate
(Ti(OC.sub.2H.sub.5).sub.4) was added so as to barely coat the
nano-particles. After stirring for an hour, the pH of the
suspension was adjusted to 5-6, and acidic nitric acid solutions of
CuO.sub.2 and B.sub.2O.sub.3 were added drop wise to the suspension
while stirring. To encourage "core-shell" nanoparticle formation
and to avoid the formation of solid CuBO.sub.2, the rate of
addition of drops was kept low, and if needed the pH of the
TiO.sub.2 nanoparticle suspension was adjusted to change the rate
of CuBO.sub.2 formation. The amount of CuO.sub.2 and B.sub.2O.sub.3
needed to obtain shells of desired average thickness was determined
experimentally, and the shell thickness measured directly using
transmission electron microscopy. For preparing "core-shell"
nano-particles with a CuBO.sub.2 core, CuBO.sub.2 nano-particles
were washed with dilute nitric acid (pH.about.4) and sufficient
tetraethyl orthotitanate was added so as to barely coat the
nano-particles. (The CuBO.sub.2 cores are manufactured using one of
the methods described above, depending on the desired particle
size.) After stirring for an hour, additional aliquots of
orthotitanate were added until the desired shell thickness was
obtained. Final products were washed with deionized water and
characterized with TEM and SEM to evaluate "core-shell" morphology
and overall size and shape distributions.
[0077] To assemble the DSSC shown in FIG. 18, a transparent
conducting electrode 1210 with a thin coating of TiO.sub.2 1220 was
dipped in dilute nitric acid to chemically activate the electrode
surface and make a good seal between it and the active device
layers. A concentrated suspension of "core-shell" particles 1230
was mixed with electron transfer dye solution in ethanol and spray
dried onto the surface. A small amount of dilute nitric acid
solution of CuO.sub.2 and B.sub.2O.sub.3 was spray dried over this
and then the CuBO.sub.2 coated 1240 transparent conducting counter
electrode 1250 was placed on top. The entire structure was heated
for several minutes at around 80.degree. C. in a vacuum environment
to remove solvent and anneal the layers. Detailed measurements of
the energy conversion efficiency were performed as described above,
and were in the range of 1.0-1.2%.
[0078] Fabrication of Solid-State DSSC by Using
TiO.sub.2--CuBO.sub.2 "Nano-Couples"
[0079] In this case TiO.sub.2 and CuBO.sub.2 are held together in
couples using a polymer connector. These "nano-couples" are capable
of simultaneously harvesting light and separating charge. A
schematic diagram of the TiO.sub.2--CuBO.sub.2 "nano-couple" is
shown in FIG. 19. The "nano-couple" includes a pair of TiO.sub.2
1310 and CuBO.sub.2 1320 particles held together by a polymer
spacer 1330. The polymer spacer 1330 also provides an interface
between the particles where ionizable light absorbing dyes 1340 are
accumulated. The electrons and holes generated in the dye 1340 are
able to quickly transfer to TiO.sub.2 1310 and CuBO.sub.2 1320
particles, respectively.
[0080] "Nano-couples" are synthesized starting with TiO.sub.2 and
CuBO.sub.2 nanoparticles as raw materials and using the steps
described in FIG. 20. (The CuBO.sub.2 nanoparticles are
manufactured using one of the methods described above, depending on
the desired particle size.) To start with, TiO.sub.2 nano-particles
are bound to a column of hydrophilic chromatography media (such as
provided by BioRad, Hercules, Calif.). The column is washed with
ethanol to remove loose nano-particles. After this, the column is
again washed with 1 M carboxyethylphosphoric acid (CEPA) in ethanol
to functionalize the exposed TiO.sub.2 nanoparticle surfaces with
carboxylic acid groups. The column is unpacked for use as slurry.
1-Ethyl-3[3-dimethylaminopropyl]carbo-diimide hydro-chloride (EDAC)
and N-hydroxysulfo-succinimide (SNHS) are added to the slurry (1
equivalent for each carboxylic group on the surface of the
nano-particles) and stirred for 30 minutes at room temperature.
Then PEG coupler, NH.sub.2--PEO.sub.n--NH.sub.2, is added to the
slurry and stirred overnight at room temperature. The slurry is
washed with ethanol to remove unreacted material but retain
TiO.sub.2 nano-particles. In a separate column CuBO.sub.2
nano-particles bound to their support are reacted with CEPA to
functionalize them partially with carboxylic acid groups. They are
eluted with an aqueous ethanol gradient, and dialyzed to remove
unreacted materials. This purified material is applied to the
slurry containing TiO.sub.2 nano-particles and EDAC, and SNHS is
added to link the particles together. The recovered material is
dialyzed to remove any residual CEPA, EDAC, and SNHS. Ruthenium
complexes are attached to the PEG linker in the "nano-couples" by
reacting with sodium hydride in dimethylformamide (DMF), via the
procedure outlined in Zhang et al., "Oxidation chemistry of
poly(ethylene glycol)-supported carbonylruthenium(II) and
dioxoruthenium(VI) mesotetrakis (pentafluorophenyl) porphyrin",
Chemistry 12, 3020-3031 (2006). By systematic manipulation of the
length and structure of the spacer polymer, the separation between
the particles and the amount of dye inserted between them is
varied. TiO.sub.2--CuBO.sub.2 "nano-couples" are used to fabricate
DSSCs by using the same protocol as described above for the
"core-shell" particles. Detailed measurements of the energy
conversion efficiency were performed as described above, and were
in the range of 1.0-1.4%.
[0081] Ru-535 has been used as an example of a suitable
ruthenium-based sensitizing dye for use in the DSSCs described
above; however, other dyes may be used, including low cost dyes.
Some examples of alternative dyes are copper and iron based dyes,
such as Cu(3).sub.2[PF.sub.6] or FeL.sub.2(CN).sub.2. Furthermore,
a nonaqueous solvent with high dielectric constant, such as
hydrazine, can be used to enhance the adhesion of the dye to
semiconducting electrodes, such as the CuBO2 and TiO2. (Excess
adhesion promoter can be removed by applying a vacuum or
evaporation at elevated temperature.)
[0082] TiO.sub.2 has been used as an example of a suitable n-type
semiconductor material for use in the SS-DSSCs described above.
However, other materials may be used as an alternative to TiO.sub.2
including ZnO and ZrO.sub.2. For example: ZnO and ZrO.sub.2 may be
used in place of TiO.sub.2 in forming "core-shell" nanoparticles
with copper boron oxide; and ZnO and ZrO.sub.2 nanoparticles may be
used in place of TiO.sub.2 nanoparticles in forming "nano-couples"
with copper boron oxide nanoparticles.
[0083] Furthermore, as described earlier, the p-type copper boron
oxide in the examples of SS-DSSCs given above may be replaced by
other p-type copper delafossite materials, such as CuAlO.sub.2,
CuGaO.sub.2 and CuInO.sub.2.
[0084] In alternative embodiments of the present invention the
deposition of the copper boron oxide thin films may include
sputtering techniques, molecular beam epitaxy (MBE), pulsed
electron beam deposition, electron beam evaporation, other physical
vapor deposition techniques and sol-gel deposition techniques. The
citrate sol-gel process used to form copper boron oxide powders may
be adapted to synthesize copper boron oxide thin films. Sol-gel
solutions are prepared as described above. After refluxing, the
solvent is partially evaporated creating a viscous liquid. The
viscous liquid is then used to deposit copper boron oxide thin
films by solution deposition techniques such as, dip coating, spray
coating, ink jet printing or spin coating. Deposited films are
dried at approximately 50.degree. C. and additional coats may be
deposited, if needed to achieve a desired thickness thin film. The
dried sol-gel coating is sintered between 70 and 200.degree. C.
under vacuum between 5 and 50 Torr to form the copper boron oxide
thin film. After formation of the copper boron oxide film the
sintering temperature may be increased to in the region of
300-600.degree. C. (depending on temperature restrictions due to
substrate type, etc.) to densify the film.
[0085] Although the copper boron oxide material of the present
invention has been described as having a delafossite crystal
structure, alternative crystal structures may also exist for this
material, including hexagonal close packed (HCP).
[0086] Although the present invention has been particularly
described with reference to embodiments thereof, it should be
readily apparent to those of ordinary skill in the art that changes
and modifications in the form and details may be made without
departing from the spirit and scope of the invention.
* * * * *