U.S. patent application number 11/931591 was filed with the patent office on 2008-09-25 for thin film photodetector, method and system.
Invention is credited to Philip D. Freedman.
Application Number | 20080230110 11/931591 |
Document ID | / |
Family ID | 38860708 |
Filed Date | 2008-09-25 |
United States Patent
Application |
20080230110 |
Kind Code |
A1 |
Freedman; Philip D. |
September 25, 2008 |
THIN FILM PHOTODETECTOR, METHOD AND SYSTEM
Abstract
A photodetector, comprises a first section comprising at least
one p-n junction that converts photon energy into a separate charge
carrier and hole carrier; and another section of semiconductors of
opposing conductivity type connected electrically in series and
thermally in parallel in a heat dissipating and electric generating
relationship to the cell to augment generation of electric energy
of the first section.
Inventors: |
Freedman; Philip D.;
(Alexandria, VA) |
Correspondence
Address: |
Philip D. Freedman PC
1449 Drake Lane
Lancaster
PA
17601
US
|
Family ID: |
38860708 |
Appl. No.: |
11/931591 |
Filed: |
June 9, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11781853 |
Jul 23, 2007 |
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11931591 |
Jun 9, 2008 |
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11381583 |
May 4, 2006 |
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11781853 |
Jul 23, 2007 |
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11627961 |
Jan 27, 2007 |
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11781853 |
Jul 23, 2007 |
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10127585 |
Apr 23, 2002 |
7208191 |
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11381583 |
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10127585 |
Apr 23, 2002 |
7208191 |
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11627961 |
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Current U.S.
Class: |
136/246 ;
136/256; 257/E23.11; 438/57 |
Current CPC
Class: |
H01L 35/32 20130101;
H01L 2924/0002 20130101; Y02E 10/50 20130101; B82Y 30/00 20130101;
H01L 31/18 20130101; H01L 31/0352 20130101; H02S 10/10 20141201;
H01L 2924/00 20130101; H01L 2924/0002 20130101; H01L 31/0725
20130101; H01L 23/373 20130101; H01L 31/0693 20130101; H01L
2924/3011 20130101 |
Class at
Publication: |
136/246 ;
136/256; 438/057 |
International
Class: |
H01L 31/0216 20060101
H01L031/0216; H01L 31/0248 20060101 H01L031/0248; H01L 21/02
20060101 H01L021/02 |
Claims
1-59. (canceled)
60. A solar cell comprising: at least one p-n junction that
converts photon energy into a separate charge carrier and hole
carrier; and at least one thin film electric interconnect on an
electric insulating and thermal transmissive substrate disposed in
a heat dissipating and electric generating relationship to the at
least one p-n junction.
61. The solar cell of claim 60, wherein the at least one thin film
electric interconnect on an electric insulating and thermal
transmissive substrate comprises a substrate with an aligned
thermal conductive upgraded SWNT coating or film.
62. The solar cell of claim 60, wherein the at least one thin film
electric interconnect on an electric insulating and thermal
transmissive substrate comprises a substrate with a thermal
conductive monolayer of upgraded SWNT film.
63. The solar cell of claim 60, wherein the at least one thin film
electric interconnect on an electric insulating and thermal
transmissive substrate comprises a substrate with a thermal
conductive monolayer of at least 80% SWNT.
64. The solar cell of claim 60, wherein the at least one thin film
electric interconnect on an electric insulating and thermal
transmissive substrate comprises a substrate with upgraded SWNT
comprising at least 90% SWNT.
65. The solar cell of claim 60, wherein the at least one thin film
electric interconnect on an electric insulating and thermal
transmissive substrate comprises a substrate with upgraded SWNT
comprising at least 95% SWNT.
66. The solar cell of claim 60, wherein the at least one thin film
electric interconnect on an electric insulating and thermal
transmissive substrate comprises a substrate with upgraded SWNT
comprising at least 99% SWNT.
67. The solar cell of claim 60, wherein the at least one thin film
electric interconnect on an electric insulating and thermal
transmissive substrate comprises a substrate with upgraded SWNT
comprising substantially aligned SWNT.
68.-78. (canceled)
79. A method of producing a photovoltaic cell, comprising forming a
thermal conductive film on an electric insulating and thermal
transmissive substrate and disposing the substrate with
semiconductors of opposing conductivity type connected electrically
in series and thermally in parallel in a heat dissipation and
electric generating relationship to at least one p-n junction that
converts photon energy into a separate charge carrier and hole
carrier.
80. The method of claim 79, comprising forming a thermal conductive
substantially monolayer film on the electric insulating and thermal
transmissive substrate.
81. The method of claim 79, comprising controlling substrate
exposure time or sublimation conditions to form a substantially
monolayer film on the electric insulating and thermal transmissive
substrate.
82. The method of claim 79, comprising applying a fullerene coating
to the substrate; and applying a selective bond disrupting energy
to the fullerene coating to cleave fullerene to fullerene molecular
bonds without cleaving fullerene to substrate bonds to form a
thermal conductive substantially monolayer fullerene layer on the
substrate.
83. The method of claim 79, comprising forming a substantially
monolayer film on the substrate by a method selected from the group
consisting of (i) are discharge process in the presence of a Group
VIIIb transition metal anode, (ii) a laser ablation process, (iii)
a high frequency plasma process, (iv) a chemical vapor deposition
(CVD) process and (v) a catalytic chemical vapor deposition (CCVD))
process to form the fullerene coating on the substrate; and
applying a selective bond disrupting energy to cleave fullerene to
fullerene molecular bonds without cleaving fullerene to substrate
bonds to form a thermal conductive, substantially monolayer
fullerene film on the substrate; and disposing the substrate in the
heat dissipation and electric generating relationship to or as part
of the at least one p-n junction that converts photon energy into a
separate charge carrier and hole carrier.
84. The method of claim 79, wherein a substantially monolayer film
is formed on the substrate by subliming a fullerene by heating to a
temperature from about 450.degree. C. to about 550.degree. C. at a
pressure less than about 1.times.10.sup.-8 torr, to produce a
fullerene coating on the substrate; applying a selected bond
disrupting energy to cleave fullerene to fullerene molecular bonds
without cleaving fullerene to substrate bonds to form a
substantially monolayer fullerene film on the substrate; and
disposing the substrate in the heat dissipation and electric
generating relationship to or as part of the at least one p-n
junction that converts photon energy into a separate charge carrier
and hole carrier.
85. The method of claim 79, wherein the substantially monolayer
film is formed on the substrate by dissolving in toluene, loading
the resulting solution into a resistively heated oven; placing the
oven into a vacuum chamber, evacuating to approximately 20.sup.-6
Torr. and heating the oven to about at least 450.degree. C. to
sublime a fullerene from the solvent onto the substrate surface to
produce a fullerene coating on the substrate; applying a selected
bond disrupting energy to cleave fullerene to fullerene molecular
bonds without cleaving fullerene to substrate bonds to form a
substantially monomolecular fullerene film on the substrate; and
disposing the substrate in the heat dissipation and electric
generating relationship to or as part of the at least one p-n
junction that converts photon energy into a separate charge carrier
and hole carrier.
86. The method of claim 79, wherein substantially monolayer film is
formed on the substrate by determining a target thickness for a
fullerene film; depositing a coating of fullerene molecules onto
the substrate; and removing layers of the coating to produce a
residual film of the target thickness.
87. The method of claim 79, wherein a fullerene substantially
monolayer film is formed on the substrate by determining a target
thickness for a fullerene film; depositing a SWNT coating onto the
substrate; and removing layers of the coating by selectively
breaking SWNT intermolecular bonds without breaking
SWINT-to-substrate bonding to produce a SWNT film of the target
thickness.
88-119. (canceled)
120. A photovoltaic cell comprising a photon to electric generating
structure that comprises a substrate having a support face having a
first electrode thereon and a second electrode spaced from the
first electrode by a plurality of layers including at least one
layer of a semiconducting material with an active junction
interface with a second layer of a second semiconducting type and a
cooling structure comprising semiconductors of opposing conductive
type coupled electrically in series and thermally in parallel by at
least one associated thin film, the cooling structure disposed in a
heat dissipating a electric generating relationship to the photon
to electric generating structure.
121. A system for generating electrical power from solar radiation,
comprising: a receiver comprising at least one photovoltaic cell
that can receive incidental solar energy or converting incident
solar energy into electrical energy and incidental solar energy in
the form of heat; and a thermoelectric element comprising an at
least one thermoelectric material layer disposed between an n-type
semiconductor and a p-type semiconductor in heat dissipating and
electric generating relationship to the receiver.
122-228. (canceled)
129. A photovoltaic system, comprising at least one photodetector
cell comprising a substrate having a support face having disposed
thereon a first electrode and a second electrode separated from the
first electrode by a plurality of layers comprising at least a
first layer of a first semiconducting type and at least a second
layer of a second semiconducting type with an active junction at an
interface of the first layer and second layer; and semiconductors
of opposing conductivity type connected electrically in series and
thermally in parallel in a heat dissipating and electric generation
relationship.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part application of
Freedman, Ser. No. 11/381,583 filed May 4, 2006 and Freedman Ser.
No. 11/687,961 filed Jan. 27, 2007, both of which are
continuation-in-part applications of Freedman Ser. No. 10/127,585,
filed 23 APR. 2002, now Pat. No. 7,208,191.
BACKGROUND OF THE INVENTION
[0002] The invention relates to a thin film photodetector, method
and system, particularly to a thin film thermoelectric configured
photodetector and more particularly to a photovoltaic cell with
integral structure.
[0003] A thin film photodetector, for example a photovoltaic cell
converts energy into electricity. Thin film photodectors find
application in photovoltaic devices including solar cells, infrared
sensors and photonic devices that absorb laser light that are
applied in high speed optical transmission systems, for example as
electro-absorption modulators, waveguide photodetectors, and
semiconductor Mach-Zender modulators.
[0004] A thin film photovoltaic cell photogenerates charge carriers
(electrons and holes) in a light-absorbing material, and separates
the charge carriers to a conductive contact that transmits
electricity. For example a photovoltaic cell detects photons
radiatively emitted by a light source. The cell converts the
incident photons to charge carriers (electrons and holes) in a
light absorbing material and the charge carriers are separated to a
conductive contact that transmits electricity.
[0005] The wavelength (.lamda.) of an incident photon is inversely
proportional to its photon energy and can be calculated from
.lamda.=hc/E where h is Planck's constant and c is the speed of
light. Photons with energy greater than the semiconductor bandgap
(E.sub.g) (typically ranging from 9.50 to 0.74 eV for photovoltaic
devices) excite electrons from the valence band to the conduction
band of the semiconductor material (interband transition). The
resulting electron-hole pairs are then collected and used to power
electrical loads. Photons with energy less than the semiconductor
bandgap cannot be converted to electrical energy and, therefore,
are parasitically absorbed as heat. In some systems, improved
photovoltaic conversion efficiency is attained by reducing the
amount of below bandgap energy that is parasitically absorbed. But
these mechanisms only depreciate the conversion efficiency of total
incident photon energy to electric power by diverting some energy
(albeit in the form of heat) away from the cell.
[0006] There is a need for an improved photodetector that converts
a higher proportion of available photon energy into useable
electric energy and method and system.
BRIEF DESCRIPTION OF THE INVENTION
[0007] The invention relates to an augmented photodetector that
converts a higher proportion of available photon energy into
useable electric energy and to a method and system.
[0008] In an embodiment the invention is a photodetector,
comprising: a first section comprising at least one p-n junction
that converts photon energy into a separate charge carrier and hole
carrier; and another section of semiconductors of opposing
conductivity type connected electrically in series and thermally in
parallel in a heat dissipating and electric generating relationship
to the first section.
[0009] In an embodiment, the invention is a method of making a
photodetector, comprising: forming at least one thin film electric
interconnect on an electric insulating and thermal transmissive
substrate; and disposing the substrate in a heat dissipating and
electric generating relationship to al least one p-n junction that
converts photon energy into a separate charge carrier and hole
carrier.
[0010] In an embodiment, the invention is a solar cell, comprising:
at least one p-n junction that converts photon energy into a
separate charge carrier and hole carrier; and at least one thin
film electric interconnect on an electric insulating and thermal
transmissive substrate disposed in a heat dissipating and electric
generating relationship to the at least one p-n junction.
[0011] In an embodiment, the invention is a method of making a
photodetector, comprising: providing a first section comprising at
least one p-n junction that converts photon energy into a separate
charge carrier and hole carrier; and positioning semiconductors of
opposing coductivity type connected electrically in series and
thermally in parallel in a heat dissipating and electric generating
relationship to the first section.
[0012] In another embodiment, the invention is a method of
producing a photovoltaic cell, comprising forming a thermal
conductive film on an electric insulating and thermal transmissive
substrate and disposing the substrate with semiconductors of
opposing conductivity type connected electrically in series and
thermally in parallel in a heat dissipation and electric generating
relationship to at least one p-n junction that converts photo
energy into a separate charge carrier and hole carrier.
[0013] In another embodiment, the invention is a method of making a
photodetector, comprising: providing a first section comprising at
least one p-n junction that converts photon energy into a separate
charge carrier and hole carrier; and positioning semiconductors of
opposing conductivity type connected electrically in series and
thermally in parallel in a heat dissipating and electric generating
relationship to the cell.
[0014] In another embodiment, the invention is an infrared sensor,
comprising; a first section comprising at least one p-n junction
that converts photon energy into a separate charge carrier and hole
carrier; and another section of semiconductors of opposing
conductivity type connected electrically in series and thermally in
parallel in a heat dissipating and electric generating relationship
to the cell.
[0015] In still another embodiment, the invention is a method of
making a structure, comprising providing a first section comprising
at least one p-n junction that converts photon energy into a
separate charge carrier and hole carrier; applying a patterned
discontinuous fullerene thin film to a substrate surface to form at
least one thin film interconnect; and positioning semiconductors of
opposing conductivity type connected electrically in series and
thermally in parallel by the at least one interconnect in a heat
dissipating and electric generating relationship to the first
section.
[0016] In still another embodiment, the invention is a photovoltaic
cell comprising a photon to electric generating structure that
comprises a substrate having a support face having a first
electrode thereon and a second electrode spaced from the first
electrode by a plurality of layers including at least one layer of
a semiconducting material with an active junction (J) interface
with a second layer of a second semiconducting type and a cooling
structure comprising semiconductors of opposing conductivity type
coupled electrically in series and thermally in parallel by at
least one associated thin film, the cooling structure disposed in a
heat dissipating a electric generating relationship to the photon
to electric generating structure.
[0017] In still another embodiment the invention is a system for
generating electrical power from solar radiation comprising a
receiver comprising at least one photovoltaic cell that can receive
incidental solar energy or converting incident solar energy into
electrical energy and incidental solar energy in the form of heat;
and a thermoelectric element comprising an at least one
thermoelectric material layer disposed between an n-type
semiconductor and a p-type semiconductor in heat dissipating and
electric generating relationship to the receiver.
[0018] In another embodiment, the invention is a thin film
photodetector comprising a photovoltaic cell with a thermoelectric
element, the thermoelectric element comprising p-type and n-type
semiconductors formed between opposing electric insulators and
opposing election conductors.
[0019] In another embodiment the invention is a thin film
photodetector comprising an at least one thermoelectric material
layer disposed between an n-type semiconductor and a p-type
semiconductor wherein the at least one thermoelectric material
layer comprises a fullerene thin film deposited on a surface of a
substrate.
[0020] In another embodiment, the invention is a thin film
photodetector comprising semiconductors of opposing conductivity
type coupled electrically in series and thermally in parallel by at
least one associated surface discontinuous patterned fullerene thin
film.
[0021] In another embodiment, the invention is a photovoltaic
system, comprising at least one photodetector cell comprising a
substrate having a support face having disposed thereon a first
electrode and a second electrode separated from the first electrode
by a plurality of layers comprising at least, a first layer of a
first semiconducting type and at least a second layer of a second
semiconducting type with an active junction at an interface of the
first layer and second layer; and semiconductors of opposing
conductivity type connected electrically in series and thermally in
parallel in a heat dissipating and electric generation
relationship.
BRIEF DESCRIPTION OF THE DRAWING
[0022] FIG. 1 is a front elevation view of a solar cell;
[0023] FIG. 2 is a perspective exploded view of a portion of a FIG.
1 solar cell;
[0024] FIG. 3 is a view from the underside of an electric
insulating and thermal transmissive substrate of the solar cell;
and
[0025] FIG. 4 is a top view an electric insulating and thermal
transmissive substrate that complements the FIG. 3 substrate.
DETAILED DESCRIPTION OF THE INVENTION
[0026] In an embodiment, the invention relates to a photodetector
for converting solar radiation to electrical energy. A photodector
converts incident photon energy into electricity. A photodetector
such as a photovoltaic cell can comprise a single crystalline
silicon material in which a PN junction is formed by the selective
introduction of elemental dopants into a semiconductor body. Doping
techniques such as diffusion and ion implantation can be used for
this purpose. Metallic electrodes can be placed on the surface of
the semiconductor body to form a current collection grid. In
operation, incident radiation onto the cell is absorbed within the
semiconductor body to create electron-hole pairs or carriers that
are separated by the PN junction and made available to energize an
external circuit.
[0027] Photvoltaic cells include solar cells that can produce
direct current electricity from the sun's rays. The current
electricity can be used variously for example, to power equipment
or to charge a battery. Radiation of photons having a threshold
energy level of approximately 1.12 electron volts or higher can
create electron-hole pair in a solar cell semiconductor material.
Photons of greater than threshold wavelength having lesser energy
may be absorbed by the cell as heat. Since only a percentage of
solar radiation is available for energy conversion and since the
maximum power of silicon photovoltaic cell is delivered at about
one-half volt rather than 1.12 volts, maximum energy conversion
without concentration of radiation is about 22%. However, in
practice, other losses reduce conversion to about 10% in typical
solar cells.
[0028] The efficiency of a solar cell is relatable to its thermal
content, descreasing with increasing temperature. Cooling can be
provided to a solar cell system by both active and passive systems.
Active cooling systems include Rankine cycle systems and absorption
systems, both of which require additional hardware and costs.
Passive cooling systems make use of three natural processes:
convection cooling; radiative cooling; and evaporative cooling from
outer surfaces exposed to the atmosphere.
[0029] The invention relates to an augmented photodetector, method
and system, particularly to a thin film configured photodetector.
While this invention does not depend on the following explanation,
it is believed that the configuration imparts a supplemental
electric generation to the photodetector by conversion of a
temperature gradient into electricity. A thermoelectric EMF is
created as a result of temperature difference between the materials
making up the photovoltaic cell. If the materials form a complete
loop, the EMF provides a continuous current flow. A voltage created
can be of an order of several microvolts per degree difference and
can be supplemental to the current generated by the cell p-n
junction.
[0030] In an embodiment, the invention relates to a multifunction
photovoltaic cell. A multifunction cell can attain higher total
conversion efficiency by capturing a larger portion of an incident
light spectrum. In one multijunction cell, individual cells with
different bandgaps are stacked on top of one another. In individual
cells are stacked so that light, for example solar energy falls
first on a material that has a largest bandgap. Photons not
absorbed in the first cell are transmitted to a second cell, which
then selectively absorbs a higher-energy portion of the light
radiation while remaining transparent to lower-energy photons.
These selective absorption steps continue through to a final cell,
which has the smallest bandgap.
[0031] A thin film photovoltaic cell such as a solar cell includes
a two component photoactive material; an electron acceptor and an
electron donor. The electron donor can be a p-type polymeric
conductor material, such as poly(phenylene vinylene) or
poly(3-hexylthiophene). The electron acceptor can be a
nanoparticulate material, such as, a derivative of fullerene (e.g.,
1-(3-methoxy carbonyl)-propyl-1-1-phenyl-(6,6) C61, known as PCBM).
Typically, silicone or gallium arsenide is used to fabricate a
solar cell can be used. Currently, much interest is directed to
cells based on other materials such as carbon fullerenes because of
their availability as thin films approaching a nanometer thickness.
In an embodiment, the present invention relates to a photovoltaic
cell that includes a fullerene material. Fullerene is a class of
carbon molecule having an even number of carbon atoms arranged in
the form of a closed hollow cage wherein the carbon-carbon bonds
define a polyhedral structure reminiscent of a soccer ball. The
most well studied fullerene is C.sub.60, Buckminster fullerene.
Other known fullerenes include C.sub.70 and C.sub.84. Also included
is the fullerene nanotube, particularly a single wall nanotube
(SWNT). A SWNT, is a hollow, tubular molecule consisting
essentially of sp.sup.2-hybridized carbon atoms typically arranged
in hexagons and pentagons. The SWNT can have a diameter in a range
of about 0.5 nanometer (nm) to about 3.5 nm and a length that can
be greater than about 50 nm.
[0032] A SWNT-containing fullerene product can be synthesized by an
arc discharge process in the presence of a Group VIIIb transition
metal anode, a laser ablation process, a high frequency plasma
process, a thermal decomposition process (a chemical vapor
deposition (CVD) process and a catalytic chemical vapor deposition
(CCVD) process) wherein fullerene is sublimed at a controlled
pressure and brought into contact with a heated catalyst, for
example as disclosed by Maruyama PN 20060093545, incorporated
herein by reference. These syntheses produce a distribution of
fullerene reaction products including a SWNT distribution of
diameters and conformations and amorphous and other carbon products
including multi-wall carbon nanotubes and metallic catalyst
residues. The distribution of reaction products varies depending on
the process and process operating conditions.
[0033] The fullerene can be deposited on a substrate surface using
a sputtering approach, by a sublimation technique, by spin coating
or by any other suitable technique. In one method, a
SWNT-containing fullerene coating can be applied onto a substrate
by a solution evaporation technique using solutions of fullerene
dissolved in non-polar organic solvents such as benzene, toluene,
etc. These processes form a physisorbed coating. In other
embodiments, the fullerene coating is applied to a substrate as the
fullerene is formed, for example by sublimation as hereinafter
described.
[0034] Features of the invention will become apparent from the
drawings and following detailed discussion, which by way of example
without limitation describe preferred embodiments of the
invention.
[0035] FIG. 1 is a schematic front elevation view of a
photodetector and FIG. 2 is a schematic exploded view of a FIG. 1
photodetector. The figures illustrate a preferred embodiment of the
invention. In FIG. 1 and FIG. 2, the photodetector is represented
by high efficiency solar cell 10.
[0036] With reference to FIGS. 1 and 2, solar cell 10 comprises an
upper section comprising an incident surface that comprises an
antireflective film 12 and an n-type material layer 14, p-type
material layer 16, upper electrode 18 and lower electrode 20. The
n-type layer 14 and p-type layer 16 can be any epitaxial structure
that forms a p-n junction of different semiconductor compositions.
While FIG. 1 and FIG. 2 show a single p-n junction the FIG. 1 p-n
junction can represent a plurality of p-n junctions. For example
the FIG. 1 structure can represent a top cell of gallium indium
phosphide, then a "tunnel junction" allot the flow of electrons
between the cells and a bottom cell of galluim arsenide. Another
cell can include a top cell of
n-AlInP.sub.2/n-GaInP.sub.2/p-GaInP.sub.2, a tunnel layer of
p-GaInAs/n-GaInAs and a bottom cell of n-GaInAs/p-GaInAs and
p+-GaAs substrate.
[0037] The p-n layers 14 arid 16 and electrodes 18 and 20 are
postured on a lower section that includes electric insulating and
thermal transmissive substrate or film 22, which is opposed by
corresponding electric insulating and thermal transmissive
substrate or film 24. Thin film electrical interconnects 26 and 28
are patterned to respective substrate 22 and 24 surfaces 30, 32 as
further shown in FIGS. 3 and 4. The interconnects 22 and 24 are
connected in a head to tail fashion by respective p-type
semiconductor layers 34 and n-type semiconductor layers 36 to form
a continuous electric transmissive pathway as hereinafter described
in detail with additional respect to FIGS. 3 and 4.
[0038] The electric insulating and thermal transmissive substrates
or films 22 and 24 can be the same or different material and each
cant be any suitable material that provides a path of relatively
low thermal impedance from surface 30 of substrate 22 through
surface 32 of substrate 24. In an embodiment, "electrically
insulating and thermally conductive" material is a material having
substantially no free electric charge to permit the flow of
electric current so that when a voltage is placed across the
material, no charge or current flows. Additionally, the material
has a thermal conductivity greater than its connective material;
for example such as the substrate 22 in the figures having a
thermal conductivity greater than patterned thin film electric
interconnect 26 and/or such as the substrate 24 having a thermal
conductivity greater than patterned thin film electric interconnect
28. Suitable materials include those with thermal conductivities
between 1 to 100 W/m.degree.K or 50 to 100 W/m.degree.K. In
embodiments, the electrically insulating and thermally conductive
material can comprise a material having a thermal conductivity
greater than 3 W/m.degree.K, desirably greater than 4 W/m.degree.K
and preferably greater than 20 W/m.degree.K. A high level of
thermal conductivity means the substrate 22 and 24 material allows
heat to pass through with ease and dissipates the heat evenly,
preventing the build up of problematic hot spots.
[0039] For example, the substrate 22 or 24 can be a material with a
thermal conductivity of greater than 3 W/m.degree.K. Some
appropriate materials include a ceramic material such as alumina
(Al.sub.2O.sub.3), aluminum nitride (AlN) beryllium oxide (BeO) or
beryllium nitride (Be.sub.3N.sub.2). Other suitable materials
include polymer film, epoxy cement film, polymer matrix such as a
thermoplastic or thermosetting polymer with or without a ceramic
filler, alumina, calcium oxide, titanium oxide, silicon oxide, zinc
oxide, silicon nitride, aluminum nitride, boron nitride materials
and mixtures thereof, silicone sponge, film, gel or grease, a
polycrystalline carbon including an appropriately doped fullerene
film, metallic oxide layer such as Al.sub.2O.sub.3 and a
thermoplastic molding material such as a polyester. The
electrically insulating and thermally conductive material can
include epoxy materials and epoxy glass laminates. Another suitable
"electrically insulating and thermally conductive" material is a
thin film high dielectric material impregnated with a fullerene
material.
[0040] An Al.sub.2O.sub.3 ceramic material is one preferred
electrically insulating and thermally conductive material. A
thermally conductive plastic substrate is another preferred
material; for example a thermoplastic or thermosetting polymer
matrix having dispersed thermally-conductive
electrically-insulating material and optionally a reinforcing
material. Polyphenylene sulfide is one suitable polymer. Thermally
conductive polymers selected from the group consisting of
polystyrene, polyurethane, polyvinyl chloride, polycarbonate,
polymethacrylate, polyethylene and polypropylene can be suitable.
The dispersed thermally-conductive, electrically-insulating
material can be selected from the group consisting of calcium oxide
titanium oxide, silicon oxide, zinc oxide, silicon nitride,
aluminum nitride, boron nitride and mixtures thereof. The
reinforcing material can be glass, inorganic minerals, or other
suitable material which strengthens the polymer matrix. A suitable
"electrically insulating and thermally conductive" can comprise a
base material having an electrically insulating property, for
example a silicone base and a thermally conductive filler.
[0041] In one aspect, the invention includes a relatively low
melting point material. For example, suitable substrate 22 or 24
materials can include polycarbonates and polymethacrylates. Even
polyethylene and polypropylene films may be selected as suitable.
These materials can import substantial lightweight and/or
flexibility properties.
[0042] In FIGS. 3 and 4, identical parts to the parts of FIGS. 1
and 2 are identified by the same numbers. FIG. 3 is a bottom view
of the upper electric insulating and thermal transmissive substrate
22 of solar cell 10 and FIG. 4 is a top view of lower electric
insulating and thermal transmissive substrate 24 of cell 10. With
reference to FIG. 1, FIG. 2, FIG. 3 and FIG. 4, electric insulating
and thermal transmissive substrate 22 is shown with a discontinuous
or patterned thin film 26 applied to the substrate 22 surface 30.
Electric insulating and thermal transmissive substrate 24 is shown
with a discontinuous or patterned thin film 28 applied to the
substrate 24 surface 32.
[0043] The structures 22 and 24 of FIG. 3 and FIG. 4 bear a
relationship to one another as illustrated in FIG. 2. The FIG. 2
view is an exploded view of the FIG. 1 solar cell 10. The substrate
22 in FIG. 3 is the FIG. 2 electrical insulator 22 oriented
180.degree. to disclose its underside to show the configuration of
thin film 26 on the substrate surface 30. The patterned surface of
FIG. 4 substrate 24 comprises a plurality of discontinuous thin
film applications 28 that form a system that complements and
interacts through layers 34 and 36 (FIGS. 1 and 2) with the second
and corresponding patterned thin films 26 on flat substrate surface
30. The FIGS. 1, 2, 3 and 4 taken together illustrate the
complementary alignment of thin film patterns 26 and 28. In FIG. 1
and FIG. 2, the FIG. 3 and FIG. 4 electrically insulating and
thermally conductive substrates 22, 24 are folded over together
with respective thin film patterned interconnects 26 and 28 facing
one another, to form opposing plates of a thermoelectric element.
The plates (interconnects 26 and 28) are alternately connected head
to tail to form a continuous pathway between electrodes 38 and 40.
The electrodes 38 and 40 are connected to load 42 as electrodes 18
and 20 are connected to load 44 (FIG. 1). While these figures show
separate load 42 and load 44, the FIG. 1 represents any correct
connection of circuits to loads, for example, circuits of
electrodes 38 and 40 can be connected to a single load in series or
parallel with a circuit of electrodes 18 and 20.
[0044] The upper portion of solar cell 10, including antireflective
film 12, n-type semiconductors 14, p-type semiconductors 16,
electrode 18 and electrode 20 and connecting circuit 44, comprises
a photovoltaic functioning module. When incident light excites the
photoactive material, electrons are released. The released
electrons are captured in the form of electrical energy within the
electric circuit 44 created between the electrodes 18 and 20. The
efficiency of the photoactive materials in generating electric
energy is relatable to its thermal content, decreasing with
increasing temperature.
[0045] A second module of the cell 10 comprises electric insulating
and thermal transmissive substrates 22 and 24, patterned thin film
electric interconnects 26, patterned thin film electric
interconnects 28, p-type semiconductor layers 34, n-type
semiconductor layers 36, electrodes 38 and 40 and circuit 44. The
electric insulating and thermal transmissive substrate 22 is shown
in a heat conductive relationship with the first module via
connection to electrode 22 and or a surface of p-type layer 16. In
a typical operation, much of the photon energy of incident light on
n-layer 14 is not converted to electric current energy but rather
is transferred as thermal energy to p-type layer 16. Electric
insulating and thermal transmissive substrate 22 initially
dissipates some of the thermal energy from the adjacent p-type
layer 16 to the cooler corresponding electric insulating and
thermal transmissive substrate 24. The substrate 24 acts as a heat
sink to set up a thermal gradient from the first module p-type
layer and second module electric insulating and thermal
transmissive substrate 22, across interconnects 26, 28 and layers
34, 36 to the cooling substrate 24.
[0046] The lower module generates electric current from the thermal
energy profile between electric insulating and thermal transmissive
substrate 22 and corresponding substrate 24. The patterned thin
film electric interconnects 26, patterned thin film electric
interconnects 28, p-type semiconductors 34 and n-type
semiconductors 36 are arranged to provide an end to end electron
conducting pathway. The thermoelectric effect of the temperature
gradient results in n-type semiconductors 36 of the second module
having excessive electrons and the p-type semiconductors 34 having
a deficiency, which results in a current flow with load 42. The
patterned structures 26 and 28 provide a complementary pathway
configuration that converts the thermal energy from the gradient
from substrate 22 to substrate 24 into electrical energy (Siebeck
effect). The current can be connected with the current through 44
either parallel or serially to supplement the current that is
directly generated from the cell 10 photovoltaic effect.
[0047] In an embodiment, the cell 10 represents a multijunction
cell. A multijunction cell can be constructed from a plurality of
independently made cells, at least one with a high bandgap and at
least one with a lower bandgap. Then the cells can be stacked, one
on top of the other. In another construction, one complete first
solar cell can be made and then layers for successive cells car be
grown or deposited on the first.
[0048] In an embodiment, a photovoltaic cell such as cell solar
cell 10, includes semiconductors of opposing conductive type
coupled electrically in series and thermally in parallel by at
least one associated patterned thin film electric interconnect, (26
or 28 in the figures), which can be a substantially monolayer film.
Preferably, the thin film is applied to a film substrate such as
the substrate 22 or 24 of the figures. In an embodiment, the
interconnect (26 or 28) thin film can be a fullerence monolayer. As
used herein, the term "monolayer" as applied to a film of fullerene
means a coating having approximately one layer of fullerene
molecules although the properties of the coating may not be
significantly affected if the film is slightly more than a molecule
thick. Moreover, while a monolayer of fullerene molecules generally
packs into a two-dimensional crystalline structure on the
substrate, a fullerene coating with minor lattice defects in the
monolayer may not alter the desirable properties of the fullerene
layer and would be considered a monolayer. Hence in this
application, "monolayer" or "monomolecular layer" means a
substantially monomolecular thick layer that can include some
molecular overlay and variation in diameter so that the thickness
can vary from about 0.5 nm to about 6 nm. Preferably, the monolayer
is less than 1 nm thick. In a desired embodiment, the monolayer is
less than 1 nm thick to about 3 nm. The monolayer exhibits
desirable and even in some instances, enhanced heat dissipating
properties without adding significant structure or profile to a
thermal energy generating component.
[0049] The patterned fullerene electric interconnect 26 or 28 is
formed by any suitable method, including a masked vapor deposition
process. A suitable vapor-deposition device comprises a reaction
chamber capable of maintaining vacuum or lower pressure and a
heater such as a resistance heater for vaporizing the fullerene
molecules. In one process, the fullerene is sublimed from a powder
by heating to a temperature greater than about 450.degree. C. under
low pressures, preferably less than about 1.times.10.sup.-6 torr.
Preferred sublimation temperatures are included in a range from
about 450.degree. C. to about 550.degree. C. In one process, the
fullerene powder is heated to a first lower temperature, preferably
from about 200.degree. C. to about 350.degree. C. to remove any
solvent or other impurities. In this process, the sublimation step
can be conducted at less of a reduced pressure but at a higher
temperature. However, it is preferred that the sublimation step is
conducted at lower pressure, preferably less than about
1.times.10.sup.-8 torr.
[0050] The heated fullerene molecules form a vapor-deposited film
26 or 28 on the substrate 22 or 24 surface 30 or 32. In these
methods, the film can be selectively applied to the substrate
surface 30 or 32 using a mask or lattice structure. Or the film can
be deposited, a mask or lattice structure applied and the film
selectively etched or otherwise removed to provide a fullerene thin
film 26 or 28 pattern of the invention. The mask can be a
sacrificial material such as a polycrystalline-silicon. In the
depositing step, the fullerene powder cain be placed in a porous
container or tube and the substrate 22 or 24 placed at the tube or
container opposite end. The substrate surface 30 or 32 is protected
while the powder is brought to sublimation temperature and
pressure. When the sublimation pressure and temperature are
reached, the substrate surface 30 or 32 is exposed while maintained
at a lower temperature. The fullerene vapor condenses onto the
substrate surface 30 or 32 and forms to the substrate surface
material.
[0051] In an embodiment, the substrate 22 or 24 is swept past a
fullerene powder source at a rate to provide desired condensation
and deposit. Exposure time and sublimation conditions can be
monitored by an appropriate device such as real-space STM atomic
imaging device to control deposition to a desired fullerene deposit
thickness on the substrate surface 30 or 32. One such method
comprises positioning a tunneling tip device at a desired detecting
position with respect to the substrate 22 or 24 arid controlling
application of the fullerene thin film to the substrate surface 30
or 32 according to the positioned tunneling tip device. In this
embodiment, control can be according to detection of a current
between the tip of the device and the fullerene thin film 26 or 28
depositing on the substrate surface 30 or 32.
[0052] In another embodiment, the fullerene thin film 26 or 28 is
deposited by sublimation from a solution. For example, the carbon
thin film can be applied by a Langmuir-Blodgett (LB) technique or
by solution evaporation using a solution of fullerene dissolved in
a non-polar organic solvent such as benzene or toluene. The
resulting solution is loaded into a resistively heated stainless
steel tube oven. The oven is placed into a vacuum chamber, which is
evacuated to approximately 10.sup.-6 Torr. The oven is then heated
to about 150.degree. C. for five minutes. A substrate is rotated
above the tube oven opening. The tube is then further heated to at
least 450.degree. C., preferably to approximately 550.degree. C. to
sublime the fullerene from the solvent onto the substrate surface
30 or 32.
[0053] After formation, the fullerene thin film 26 or 28 can be
polymerized by methods including photopolymerization, electron beam
polymerization, X-ray polymerization, electromagnetic
polymerization plasma polymerization, micro-wave polymerization
method and electronic polymerization. In electron beam
polymerization, an electron beam is irradiated from an electron
gun. The fullerene molecules are excited by the electron beam and
polymerized at an excited state, In X-ray polymerization, X-rays
are irradiated from an X-ray tube in place of an electron beam. The
fullerene molecules are excited by the X-rays and polymerized at
the excited state. These methods produce a fullerene polymer thin
film 26 or 28 consisting essentially of fullerene molecules bonded
together by covalent bonds.
[0054] Suitable plasma polymerization methods include a
high-frequency plasma method, a DC plasma method and an ECR plasma
method. A typical high-frequency plasma polymerization apparatus
can include a vacuum vessel with opposing electrodes. The
electrodes are connected to an outer high frequency power source. A
molybdenum boat accommodates fullerene starting material within the
vessel. The vessel is connected to an external resistance heating
power source. In operation, a low-pressure inert gas, such as
argon, is introduced into the vacuum vessel. After the vacuum
vessel 13 is charged with inert gas, current is supplied to
vaporize the fullerene to generate a plasma. The fullerene plasma
is illuminated by illuminating electromagnetic waves such as RF
plasma, to polymerize the fullerene molecules to deposit as a
fullerene polymer film. The amount of deposited thin film can be
controlled by control of the temperature of the substrate surface
30 or 32. Increasing the temperature, decreases the amount of
deposited film. Typically, the substrate surface 30 or 32 is
maintained at a temperature of 300.degree. C. or less. If plasma
power is of the order of 100 W, the temperature need not exceed
70.degree. C. Thickness of the deposited film can be measured to
control the film thickness.
[0055] As pointed out above, in one method the thin film 26 or 28
patterned structure can be fabricated by masking a substrate
surface 30 or 32 during a deposition procedure or by masking an
applied thin film during a subsequent etching step. In the first
instance, the mask can define deposition areas to create the
patterned areas of the structure of the invention. Typically, the
mask is a metal or a ceramic maternal. However, the mask can be
formed of any suitable material. The mask can be made of a material
that can be relatively easily removed, such as by physical removal,
dissolving in water or in a solvent, by chemically or
electrochemically etching, or by vaporizing through heating. The
deposition mask can be a metal oxide, such as silicon oxide or
aluminum oxide or water-soluble or solvent-soluble salts such as
sodium chloride, silver chloride, potassium nitrate, copper
sulfate, and indium chloride, or soluble organic materials such as
sugar and glucose. The mask material can also be a chemically
etchable metal or alloy such as Cu, Ni, Fe, Co, Mo, V, Al, Zn, In,
Ag, Cu--Ni alloy, Ni--Fe alloy and others, or base-dissolvable
metals such as Al can also be used. The mask can be made of a
soluble polymer such as polyvinyl alcohol, polyvinyl acetate,
polyacrylamide or acrylonitrile-butadiene-styrene. The removable
mask, alternatively, can be a volatile (evaporable) material, such
as PMMA polymer. These materials can be dissolved in an acid such
as hydrochloric acid, aqua regia, or nitric acid, or can be
dissolved away in a base solution such as sodium hydroxide or
ammonia. The removable layer or mask may also be a vaporizable
material such as Zn which can be decomposed or burned away by heat.
The mask can be added by physically placing it on the substrate
surface 30 or 32 (or on the deposited thin film 26 or 28), by
chemical deposition such as electroplating or electroless plating,
by physical vapor deposition such as sputtering, evaporation, laser
ablation, ion beam deposition, or by chemical vapor
decomposition.
[0056] In another aspect, the mask can be a metal oxide, such as
quartz or sapphire. The metal oxide can be stenciled or patterned
into the structures desired, such as holes, circles, and trenches.
In another aspect, the deposition targets can be formed by placing
an impurity, local defect, or stress on the substrate or the mask.
The impurity, local defect, or stress can be placed by x-ray
lithography, deep UV lithography, scanning probe lithography,
electron bean lithography, ion beam lithography, optical
lithography, electrochemical deposition, chemical deposition,
electro-oxidation, electroplating, sputtering, thermal diffusion
and evaporation, physical vapor deposition, sol-gel deposition, or
chemical vapor deposition. In yet another aspect, the location and
number of carbon thin films can be controlled by etching at desired
location and not etching at all or etching at different rates the
areas surrounding the desired area.
[0057] Additionally, methods of fabrication of the thin film
include lithographic techniques such as optical and scanning probe
lithography that fabricate a discontinuance or a structure at a
specific location on the substrate. Existing optical and scanning
probe lithographic technologies can be used to fabricate holes with
controllable diameter at precise locations on the substrate with
controllable depth. These methods include x-ray lithography deep UV
lithography, scanning probe lithography, electron beam lithography,
ion beam lithography, and optical lithography. Scanning Probe
Lithography can be used to fabricate structures, including the
holes, with precise control over the of the location and the
dimension of the hole. Optical lithography is a technology capable
of mass production of structures. Control of the location and
dimension of structures, such as the boles, can be performed with
precise control.
[0058] The thin film patterned substrate 22, 24 can be fabricated
by first depositing a thin film according to an above described
deposition process or by any other suitable process followed by
polymerization of the deposited thin film fullerene. And, the
fullerene patterned substrate can be formed and simultaneously
polymerized in the same disposition vessel by an exemplary
microwave polymerization, electrolytic polymerization or the like.
Various polymerization devices and processes are described in Ata
et al., U.S. Pat. No. 6,815,067 and Ramm et al., U.S. application
Ser. No. 10/439,359 (Publication 20030198021), each of which is
incorporated herein by reference in their respective entireties.
According to these references, a typical microwave polymerization
apparatus includes a molybdenum boat that accommodates fullerene
molecules as a starting material. Microwaves generate a depositing
fullerene polymer by excitation of vaporized fullerene molecules.
An electrolytic polymerization apparatus comprises an electrolytic
cell that includes a positive electrode and a negative electrode
connected to a potentiostat. A reference electrode is connected to
the same potentiostat so that a pre-set electric potential can be
applied across the positive/negative electrodes. Fullerene
molecules and a supporting electrolyte are charged into the cell.
The potentiostat applies a pre-set electrical energy the
positive/negative electrodes to form fullerene anionic radicals,
which precipitate as a thin fullerene film on the negative
electrode and fullerene polymer precipitates and is recovered by
filtration or drying and kneading into a resin to form a thin
fullerene polymer film.
[0059] In some applications of the invention, a thin monolayer
fullerene film or fullerene polymer film may be desirable to
provide the smallest and lightest possible structure that is an
effective conductive stricture without changing the electrical
insulator substrate properties. In these applications, a thin, even
mono-molecular layer can be applied according to one or more
procedures. One procedure takes advantage of strong fullerene to
substrate bond. The fullerene bond to a metal/semiconductor
substrate surface is stronger than inter molecular bonding among
fullerene molecules. Desorption temperature is related to bond
strength among fullerene molecules or between fullerenes and
substrate. Hence, strength of fullerene bonding can be estimated by
the temperature at which a fullerene desorbs. For multilayer
fullerene molecules on a substrate surface 30 or 32, fullerene
desorption temperature is between 225.degree. C. and 300.degree. C.
Hence, an applied temperature of higher than 225.degree. C.,
desirably at least350.degree. C. and in some applications up to
about 450.degree. C. will effect fullerene desorption without
disrupting the fullerene to substrate surface 30 or 32 bond. In one
process, desorption of excess fullerenes beyond a monolayer can be
achieved by heating at a temperature from about 225.degree. C. to
about 300.degree. C. In one procedure a fullerene monolayer film is
formed by depositing a thin film of fullerene molecules onto the
substrate surface 30 or 32 according to any of the above described
deposition procedures. Layers of the deposited thin film 26 or 28
are removed to produce a residual film of desired thickness. The
layers are removed by selectively breaking fullerene-to-fullerene
intermolecular bonds without breaking the fullerene-to-substrate
association or bonding and without subjecting the film or substrate
to injurious temperatures, by this mechanism, excess fullerene can
be removed beyond a desired thickness such as a monolayer, for
example by heating to a temperature sufficient to break the
fullerene--fullerene bonds without disrupting the fullerene
monolayer 26 or 28 that is applied to the substrate, surface 30 or
32.
[0060] Other methods of selectively breaking the fullerene
intermolecular bond include laser beam, ion beam or electron beam
selective irradiation. For example, an energetic photon laser beam,
electron bean or inert ion beam can be irradiated onto the
deposited substrate with a controlled energy that is sufficient to
break fullerene-to-fullerene intermolecular bonds without breaking
fullerene-to-substrate associations or bonds. The parameters of the
beam irradiation depend upon the energy, flux and duration of the
beam and also depend on the angle of the beam to the fullerene thin
film 26 or 28 deposit. In general, the energy of irradiation is
controlled to avoid fullerene molecule decomposition or reaction
and to avoid excessive local heating. For example, it is preferred
to operate a laser at an energy outside of the ultraviolet range
preferably in the visible or infrared range, to avoid reacting
fullerene molecules. On the other hand, the laser can be
effectively operated in the ultraviolet range to cleave fullerene
layers so long as operating conditions such as temperature,
pressure and pulsation are controlled. In a preferred embodiment,
the laser or other light source is operated in the visible or
infrared portion of the spectrum. Light intensity and beam size can
be adjusted to produce the desired desorption rate of fullerenes
beyond a desired layer thickness such as a monolayer thickness.
[0061] If a sublimation step is used to form the initial fullerene
thin film, the fullerene layers can be cleaved to a desired
thickness in the same vacuum chamber where the substrate surface is
cleaned and the fullerene thin film is deposited. Maintaining the
substrate under vacuum keeps it clean and reduces beam scattering
during irradiation. Additionally the vacuum can prevent fullerene
recondensation by removing desorbed fullerene from the irradiation
area.
[0062] An ion beam is generated by bombarding a molecular flow with
high energy electrons that produce an ionization. The ion beam can
be directed with electrodes. If an ion beam is used, beam energy
and flux should be low enough to avoid decomposing the fullerene or
forming higher-ordered fullerene molecules. For example,
acceleration, voltage can be as high as 3.0 kilovolts for some
applications. Desirably, the voltage is between 50 and 1000, and
preferably between about 100 and 300 volts. The beam current
density can be in the range of about 0.05 to 5.0 mA/cm.sup.2
(milliAmperes per square centimeter).
[0063] If a gas cluster ion beam is employed, ion clusters are used
that have an atomic mass approximating that of the fullerene
molecules. A C.sub.60 fullerene molecule has an atomic mass unit
(AMU) of 720. Beams of clustered ions approximating the mass of the
fullerene molecules can be used to inject energy into the
multilayer fullerene thin film to break the fullerene-to-fullerene
intermolecular bond without depreciating the fullerene molecules.
Clusters can be formed by expanding an inert gas such as argon,
through a supersonic nozzle followed by applying an electron beam
or electric are to form clusters.
[0064] The angle of incidence of a directed beam to the fullerene
thin film can be varied to control dissociation. In one embodiment,
a beam angle relative to irradiated target can be selected between
about 25.degree. and about 75.degree., preferably between
40.degree. and 65.degree.. When ion bean irradiation is used,
incident angle is determined by balancing factors such as removal
efficiency and precision.
[0065] In one aspect of the invention, it has been found that
fullerene thin films can be applied to certain substrates that
would otherwise be damaged by the conditions of thin film
application. For example, fullerenes cannot be applied to certain
lower melting substrates that would otherwise be damage because of
the high temperature requirements for fullerene sublimation.
According to this embodiment of the invention, a method of applying
a fullerene thin film to a substrate that melts at a temperature
lower than the application temperature of the fullerene thin film
(lower melting substrate) comprises first applying a fullerene thin
film to a first higher melting temperature substrate (melting at a
temperature higher than the application temperature of the thin
film) to produce a first fullerene thin filmed substrate. The first
fullerene thin film substrate is placed in contact with a lower
melting temperature substrate with a first surface in contact with
an exposed fullerene surface of the fullerene thin film substrate
to form a two substrate structure with intermediate fullerene thin
film between the substrates. A second fullerene deposit is then
applied to an exposed surface of the second substrate and the
intermediate fullerene deposit between the two substrates is
cleaved to produce two fullerene deposit substrates, one of which
is the lower melting temperature substrate. The intermediate
fullerene deposit functions to dissipate heat away from the lower
melting structure while the second deposit is applied at a
temperature that otherwise could damage the lower melting
substrate.
[0066] In an embodiment, the patterned structure 10 is a substrate
22 or 24 comprising deposited fullerene thin film 26 or 28 with or
without a property enhancing dopant. The fullerene pattern of the
of the invention can act as a hole transport thin film. The
performance characteristics of the hole transport thin film can be
determined by the ability of the fullerene to transport the charge
carrier. Ohmic loss in the fullerene thin film is related to
conductivity, which has a direct effect on operating voltage and
also can determines the thermal load transportable by the thin
film. By doping at least one of the fullerene hole transport thin
film patters 26 or 28 with a suitable acceptor material (p-doping),
the charge carrier density and hence the conductivity is
increased.
[0067] For example for some applications, the thin film fullerene
26 or 28 can be doped with a donor type (n-type) or acceptor type
(p-type) dopant. The dopant can be added to improve electric
conductivity and heat stability. In an embodiment, the dopant is a
polyanion. An alkali metal such as lithium, sodium, rubidium or
cesium is another preferred dopant. Other examples of preferred
dopants include alkali-earth metals such as calcium, magnesium, and
the like; quaternary amine compounds such as tetramethylammonium,
tetraethylammonium, tetrapropylammonium, tetrabutylammonium,
methyltriethylammonium and dimethyldiethylammonium. Preferably, the
fullerene is doped to have an increased charge carrier density and
effective charge carrier mobility for use as an element of a
thermoelectric element.
[0068] In one aspect, a hydrogenated form of an organic compound is
mixed as a dopant directly into the fullerene. The hydrogenated
form of the organic compound is a neutral, nonionic molecule that
can undergo complete sublimation. In the process, hydrogen, carbon
monoxide, nitrogen or hydroxy radicals are split off and at least
one electron is transferred to the fullerene or from the fullerene.
Also, the method can use a salt of the organic dopant. Suitable
organic dopants include cyclopentadiene, cycloheptatriene, a
six-member heterocyclic condensed ring, a carbinol base or
xanthene, acridine, diphenylamine, triphenylamine, azine, oxazine,
thiazine or thioxanthene derivative. After mixing of the dopant,
the mixture can be stimulated with radiation to transfer a charge
from the organic dopant to the fullerene.
[0069] The fullerene products of the above described syntheses
include only a proportion of SWNT product. An upgraded SWNT product
having enhanced thermal properties is desirable in some
thermoelectric applications. Processes to obtain a fullerene
product comprising an upgraded proportion of SWNT from a product of
the above syntheses include contacting a fullerene product in the
presence of a transition metal element or alloy under a reduced
pressure in an inert gas atmosphere. Some direct processes for
obtaining an upgraded SWNT product include catalytic laser
irradiation, heat treatment and CCVD processes. For example, one
SWNT product with less than about 10 wt % other carbon-containing
species can be produced by an all-gas phase method using a gaseous
transition metal catalyst and a high pressure CO as a carbon
feedstock. However, catalyst residue can be left as an impurity in
the product material.
[0070] Proportion of SWNT in a fullerene synthesis product can be
enriched in accordance with certain other procedures to provide an
improved and advantageous upgraded SWNT thermal coating and film.
In one procedure, a SWNT-containing reaction product is heated
under oxidizing conditions as described in Colbert et al. Pat. No.
7,115,864, incorporated herein by reference to provide a product
that is enriched in at least 80%, preferably at least 90%, more
preferably at least 95% and most preferably over 99% SWNT. In the
present application, a upgraded SWNT is a reaction product
comprising at least 80% pure SWNT. The upgraded SWNT has been found
to be particularly useful as a heat dissipating coating or film in
combination with a thermal energy generating component.
[0071] In the Colbert et al. upgrade, a SWNT-containing product
composition is heated in an aqueous solution of an inorganic
oxidant, such as nitric acid, a mixture of hydrogen peroxide and
sulfuric acid or potassium permanganate to remove amorphous carbon
and other contaminants. The SWNT-containing synthesis product can
be refluxed in an aqueous solution of the oxidizing acid at a
concentration high enough to etch away the amorphous carbon
deposits within a practical time frame, but not so high a
concentration that the SWNT material will be etched to a
significant degree. Nitric acid at concentrations from 2.0 to 2.6 M
is suitable. At atmospheric pressure, the reflux temperature of the
aqueous acid solution can be about 102.degree. C.
[0072] In a preferred upgrade process, a SWNT-containing product
can be refluxed in a nitric acid solution at a concentration of 2.6
M for 24 hours. The upgraded product can be separated from the
oxidizing acid by filtration. Preferably, a second 24 hour period
of refluxing in a flesh nitric solution of the same concentration
can be employed followed by filtration. Refluxing under acidic
oxidizing conditions may result in the esterification of some of
the nanotubes, or nanotube contaminants. The contaminating ester
material may be removed by saponification, for example, by using a
sodium hydroxide solution in ethanol at room temperature for 12
hours. Other conditions suitable for saponification of ester linked
polymers can be used. For example saponification can be
accomplished with a sodium hydroxide solution in ethanol at room
temperature for 12 hours. The SWNT-containing product can be
neutralized after the saponification step. Refluxing the
SWNT-containing product in 6M aqueous hydrochloric acid for 12
hours is one suitable neutralization.
[0073] After oxidation, saponification and neutralization, the
SWNT-containing product can be collected by settling or filtration
to a thin mat form of purified bundles of SWNT. In a typical
example, the upgraded SWNT-containing product is filtered and
neutralized to provide a black mat of upgraded SWNT about 100
microns thick. The SWNT in the mat may be of varying lengths and
may comprise individual SWNTs and of up to 10.sup.3 SWNT bundles
and mixtures of individual SWNTs of various thicknesses. A product
that comprises nanotubes that are homogeneous in length, diameter
and/or molecular structure can be recovered from the mat by
fractionation. The upgraded SWNT can then be dried, for example by
baking at 850.degree. C. in a hydrogen gas atmosphere to produce a
dry upgraded SWNT product.
[0074] According to one Colbert et al. procedure, an initial
cleaning in HNO.sub.3 can convert amorphous carbon in a SWNT
product to various sizes of linked polycyclic compounds. The base
solution ionizes most of the polycyclic compounds, making them more
soluble in aqueous solution. Then, the SWNT mat product can be
refluxed in HNO.sub.3. The SWNT product can be filtered and washed
with NaOH solution. Next, the filtered SWNT product is polished by
stirring in a Sulfuric acid/Nitric acid solution. This step removes
essentially all remaining material from the SWNT product that was
produced during the nitric acid treatment. Then, the SWNT product
is diluted and the product again filtered. The SWNT product is
again washed with a NaOH solution.
[0075] Smalley et al. Pat. No. 6,183,713 incorporated herein by
reference, discloses a method to make a SWNT reaction product by
laser vaporizing a mixture of carbon and one or more Group VIII
transition metals. Single-wall carbon nanotubes preferentially form
in the vapor. The SWNT product is fixed in a high temperature zone
where the Group VIII transition metal catalyzes further SWNT
growth. In one Smalley et at. embodiment, two separate laser pulses
are utilized with the second pulse timed to be absorbed by the
vapor created by the first pulse. Colbert et al. subjected a
Smalley et al. two laser method-produced SWNT product to refluxing
in nitric acid, one solvent exchange, and sonification in saturated
NaOH in ethanol. The product was neutralized and baked in a
hydrogen gas atmosphere at 850.degree. C. The procedure produced a
>99% pure upgraded SWNT that can be applied to a substrate to
form the upgraded SWNT film of the inventive thermal dissipating
surface.
[0076] An aligned nanotube, particularly aligned SWNT coating or
film is another preferred embodiment of the invention. Aligned
carbon nanotube arrays can be synthesized in a hot filament plasma
enhanced chemical vapor deposition (HF-PECVD) system. A variety of
substrates (metal, glass, silicon, etc) are first coated with
nickel nanoparticles and then introduced into the CVD chamber. The
method of nickel nanoparticle deposition defines the nanotube site
density. Standard aligned carbon nanotube arrays are produced on a
nickel sputtering-coated substrate, whereas low site-density carbon
nanotube arrays are produced on a nickel electric-chemical-coated
substrate.
[0077] The fullerene can include a thermal transfer enhancing
additive or dopant, for example encapsulation of one or more metal
atoms encapsulated inside a fullerene "cage" or NT. Examples
include Sc@C-82, Y@C-82, La@C-82, Gd@C-82, La-2@jC-80, Sc-2@C-84
and alkali metal, Fe, Cr and Ni and silicon-doped fullerene film
and NT.
[0078] Photovoltaic cells can be electrically connected in series
and/or in parallel to create a photovoltaic module. Typically, two
photovoltaic cells are connected in parallel by electrically
connecting the cathode of one cell with the cathode of the other
cell, and the anode of one cell with the anode of the other cell.
In general, two photovoltaic cells are connected in series by
electrically connecting the anode of one cell with the cathode of
the other cell. In an embodiment, one or more photovoltaic
functions are connected in series with one or more thermoelectric
functions by connecting a cathode of a cell of the photovoltaic
functions with a cathode of a thermoelectric function and an anode
of the photovoltaic functioning cell is connected with the anode of
the photovoltaic function.
[0079] When more power is required than a single cell can deliver,
cells can be grouped together to form modules or panels that can be
arranged in arrays. Such solar arrays have been used to power
orbiting satellites and other spacecraft and in remote areas as a
source of power for applications such as roadside emergency
telephones, remote sensing, and cathodic protection of pipelines.
Decline of cost of these panels or arrays is expanding the range of
cost-effective uses, for example to road signs, home power
generation pocket calculators and communication devices and even
for grid-connected electricity generation.
[0080] Solar cells have many applications. In one application, the
cells are used where electrical power from a grid is unavailable,
such as in remote area power systems, Earth-orbiting satellites and
space probes, consumer systems, e.g. handheld calculators or wrist
watches, remote radiotelephones and water primping applications.
More recently they are starting to be used in assemblies of solar
modules (photovoltaic arrays) connected to the electricity grid
through an inverter, often in combination with a net metering
arrangement.
[0081] While preferred embodiments of the invention have been
described, the present invention is capable of variation and
modification and therefore should not be limited to the precise
details of the Examples. Thus while the invention has been
described relative to a photovoltaic cell preferred embodiment,
other preferred embodiments may include infrared sensors, chemical
detectors, Photoresistors or light dependent resistors (LDR)
photodiodes, photocathodes, pyroelectric detectors, other types of
photovoltaic cells, which can operate in photovoltaic mode or
photoconductive mode, photomultiplier tubes containing a
photocathode, which emits electrons when illuminated, phototubes
containing a photocathode, which emits electrons when illuminated
and in general behalves as a photoresistor, phototransistor,
optical detectors that are effectively thermometers, responding
purely to the heating effect of the incoming radiation, such as
pyroelectric detectors, Golay cells, thermocouples and thermistors
and cryogenic detectors are sufficiently sensitive to measure the
energy of single x-ray, visible and near infra-red photons (Enss
2005). The invention includes changes and alterations that fall
within the preview of the following claims.
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