U.S. patent application number 13/773849 was filed with the patent office on 2014-08-28 for energy conversion and storage device and mobile electronic device containing same.
The applicant listed for this patent is Donald S. Gardner, Cary L. Pint. Invention is credited to Donald S. Gardner, Cary L. Pint.
Application Number | 20140239432 13/773849 |
Document ID | / |
Family ID | 51358529 |
Filed Date | 2014-08-28 |
United States Patent
Application |
20140239432 |
Kind Code |
A1 |
Gardner; Donald S. ; et
al. |
August 28, 2014 |
ENERGY CONVERSION AND STORAGE DEVICE AND MOBILE ELECTRONIC DEVICE
CONTAINING SAME
Abstract
An energy conversion and storage device includes an energy
storage component (530, 601) including a first electrode (611)
having a first plurality of channels (612) formed in a first region
(615) of a first material (617), a second electrode (621) adjacent
to but electrically isolated from the first electrode and having a
second plurality of channels (622) formed in a first region (625)
of a second material (627), and an electrolyte (650) within the
first and second pluralities of channels. The first electrode forms
a first interface (619) with the electrolyte and the second
electrode forms a second interface (629) with the electrolyte. The
energy conversion and storage device further includes a
photovoltaic component (520, 602) formed in a second region of the
first material.
Inventors: |
Gardner; Donald S.; (Los
Altos, CA) ; Pint; Cary L.; (Nashville, TN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Gardner; Donald S.
Pint; Cary L. |
Los Altos
Nashville |
CA
TN |
US
US |
|
|
Family ID: |
51358529 |
Appl. No.: |
13/773849 |
Filed: |
February 22, 2013 |
Current U.S.
Class: |
257/437 |
Current CPC
Class: |
H01L 31/053 20141201;
Y02E 60/13 20130101; H02S 40/38 20141201; H01G 11/26 20130101; H01G
9/20 20130101; Y02E 10/50 20130101 |
Class at
Publication: |
257/437 |
International
Class: |
H01L 27/142 20060101
H01L027/142 |
Claims
1. An energy conversion and storage device comprising: an energy
storage component comprising: a first electrode comprising a first
plurality of channels formed in a first region of a first material;
a second electrode adjacent to but electrically isolated from the
first electrode, the second electrode comprising a second plurality
of channels formed in a first region of a second material; and an
electrolyte within the first and second pluralities of channels
such that the first electrode forms a first interface with the
electrolyte and the second electrode forms a second interface with
the electrolyte; and a photovoltaic component formed in a second
region of the first material.
2. The energy conversion and storage device of claim 1 wherein: the
photovoltaic component comprises one or both of an anti-reflective
coating and a transparent electrically conductive layer.
3. The energy conversion and storage device of claim 1 wherein: the
first material comprises at least one of monocrystalline silicon,
polycrystalline silicon, amorphous silicon, germanium, gallium
arsenide, cadmium telluride, copper indium selenide, copper indium
gallium selenide, gallium indium phosphide, and copper indium
sulfide.
4. The energy conversion and storage device of claim 1 wherein: the
second material comprises at least one of monocrystalline silicon,
polycrystalline silicon, amorphous silicon, carbon, carbon-based
materials, and nickel foam.
5. The energy conversion and storage device of claim 1 wherein: the
photovoltaic component comprises a p-n junction; and the p-n
junction is the first interface with the electrolyte.
6. The energy conversion and storage device of claim 1 wherein: the
photovoltaic component comprises a p-n junction; and the p-n
junction constitutes a second interface separate from the first
interface with the electrolyte.
7. The energy conversion and storage device of claim 6 further
comprising: an electrically insulating layer between the energy
storage component and the photovoltaic component.
8. The energy conversion and storage device of claim 7 wherein: the
electrically insulating layer comprises silicon dioxide.
9. The energy conversion and storage device of claim 7 wherein: the
energy storage component and the photovoltaic component are
electrically connected using a through silicon via.
10. The energy conversion and storage device of claim 1 wherein:
the first material has a thickness of at least 100 microns; and a
photon-absorbing portion of the photovoltaic component constitutes
an upper layer of the first material beginning at a surface of the
first material and extending to a depth of no greater than 10
microns into the first material.
11. An energy conversion and storage device comprising: a
semiconductive substrate having a first side, a second side
opposite the first side, and a central bulk portion between the
first side and the second side; a photovoltaic device fabricated at
the first side of the semiconductive substrate; and an energy
storage component at the second side of the semiconductive
substrate, the energy storage component comprising a porous
semiconductive structure forming an interface with an
electrolyte.
12. The energy conversion and storage device of claim 11 further
comprising: a transparent electrically conductive layer over the
first side of the semiconductive substrate.
13. The energy conversion and storage device of claim 11 wherein:
the photovoltaic device comprises a p-n junction.
14. The energy conversion and storage device of claim 11 wherein:
the energy storage component is a pseudocapacitor comprising a
pseudocapacitive material; and the electrolyte comprises redox
couples that are accessible at energies below a silicon band gap
energy.
15. The energy conversion and storage device of claim 14 wherein:
the pseudocapacitive material is a solid-state material.
16. The energy conversion and storage device of claim 11 wherein:
the anti-reflective coating comprises a layer of dielectric
material.
17. The energy conversion and storage device of claim 11 wherein:
the anti-reflective coating comprises a textured region on a
surface of the first side of the semiconductive substrate.
18. A mobile electronic device comprising: a housing; a battery
contained within the housing; and an energy conversion and storage
device electrically connected to the battery, wherein the energy
conversion and storage device comprises: a semiconductive substrate
having a first side, a second side opposite the first side, and a
central bulk portion between the first side and the second side; a
photovoltaic device fabricated at the first side of the
semiconductive substrate, the photovoltaic device comprising an
anti-reflective coating; and an energy storage component at the
second side of the semiconductive substrate, the energy storage
component comprising a porous silicon structure forming an
interface with an electrolyte.
19. The mobile electronic device of claim 18 wherein: the
photovoltaic device further comprises a p-n junction.
Description
FIELD OF THE INVENTION
[0001] The disclosed embodiments of the invention relate generally
to energy storage, and relate more particularly to electrochemical
devices that collect and store energy.
BACKGROUND OF THE INVENTION
[0002] Modern societies depend on the ready availability of energy.
As the demand for energy increases, devices capable of efficiently
storing energy become increasingly important. As a result, energy
storage devices, including batteries, capacitors, electrochemical
capacitors (ECs), (including pseudocapacitors and electric
double-layer capacitors (EDLCs) (sometimes called ultracapacitors
or supercapacitors, among other names)), hybrid ECs, and the like
are being extensively used in the electronics realm and beyond. In
particular, capacitors are widely used for applications ranging
from electrical circuitry and power delivery to voltage regulation
and battery replacement. Electrochemical capacitors are
characterized by high energy storage capacity, rapid
charge/discharge ability, and large cycle lifetimes, as well as
other desirable characteristics including high power density, small
size, and low weight, and have thus become promising candidates for
use in several energy storage applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0003] The disclosed embodiments will be better understood from a
reading of the following detailed description, taken in conjunction
with the accompanying figures in the drawings in which:
[0004] FIGS. 1 and 2 are cross-sectional views of an energy storage
structure according to embodiments of the invention;
[0005] FIG. 3 is a depiction of an electric double layer formed
within a channel of a porous structure according to an embodiment
of the invention;
[0006] FIGS. 4a and 4b are images of, respectively, a surface and a
cross-sectional slice of a porous silicon structure;
[0007] FIG. 5 is a schematic representation of an energy conversion
and storage device according to an embodiment of the invention;
[0008] FIG. 6 is a cross-sectional view of an energy conversion and
storage device according to an embodiment of the invention; and
[0009] FIG. 7 is a schematic representation of a mobile electronic
device incorporating an energy conversion and storage device
according to an embodiment of the invention.
[0010] For simplicity and clarity of illustration, the drawing
figures illustrate the general manner of construction, and
descriptions and details of well-known features and techniques may
be omitted to avoid unnecessarily obscuring the discussion of the
described embodiments of the invention. Additionally, elements in
the drawing figures are not necessarily drawn to scale. For
example, the dimensions of some of the elements in the figures may
be exaggerated relative to other elements to help improve
understanding of embodiments of the present invention. Certain
figures may be shown in an idealized fashion in order to aid
understanding, such as when structures are shown having straight
lines, sharp angles, and/or parallel planes or the like that under
real-world conditions would likely be significantly less symmetric
and orderly. The same reference numerals in different figures
denote the same elements, while similar reference numerals may, but
do not necessarily, denote similar elements.
[0011] The terms "first," "second," "third," "fourth," and the like
in the description and in the claims, if any, are used for
distinguishing between similar elements and not necessarily for
describing a particular sequential or chronological order. It is to
be understood that the terms so used are interchangeable under
appropriate circumstances such that the embodiments of the
invention described herein are, for example, capable of operation
in sequences other than those illustrated or otherwise described
herein. Similarly, if a method is described herein as comprising a
series of steps, the order of such steps as presented herein is not
necessarily the only order in which such steps may be performed,
and certain of the stated steps may possibly be omitted and/or
certain other steps not described herein may possibly be added to
the method. Furthermore, the terms "comprise," "include," "have,"
and any variations thereof, are intended to cover a non-exclusive
inclusion, such that a process, method, article, or apparatus that
comprises a list of elements is not necessarily limited to those
elements, but may include other elements not expressly listed or
inherent to such process, method, article, or apparatus.
[0012] The terms "left," "right," "front," "back," "top," "bottom,"
"over," "under," and the like in the description and in the claims,
if any, are used for descriptive purposes and not necessarily for
describing permanent relative positions unless otherwise indicated
either specifically or by context. It is to be understood that the
terms so used are interchangeable under appropriate circumstances
such that the embodiments of the invention described herein are,
for example, capable of operation in other orientations than those
illustrated or otherwise described herein. The term "coupled," as
used herein, is defined as directly or indirectly connected in an
electrical or non-electrical manner. Objects described herein as
being "adjacent to" each other may be in physical contact with each
other, in close proximity to each other, or in the same general
region or area as each other, as appropriate for the context in
which the phrase is used. Occurrences of the phrase "in one
embodiment" herein do not necessarily all refer to the same
embodiment.
DETAILED DESCRIPTION OF THE DRAWINGS
[0013] In one embodiment of the invention, an energy conversion and
storage device comprises an energy storage component comprising a
first electrode having a first plurality of channels formed in a
first region of a first material, a second electrode adjacent to
but electrically isolated from the first electrode and having a
second plurality of channels formed in a first region of a second
material, and an electrolyte within the first and second
pluralities of channels. The first electrode forms a first
interface with the electrolyte and the second electrode forms a
second interface with the electrolyte. The energy conversion and
storage device further comprises a photovoltaic component formed in
a second region of the first material.
[0014] Solar cells (also known as photovoltaic (PV) devices)
directly convert light into electricity, and usually use physics
and technology similar to that used by the microelectronics
industry. A typical solar cell is composed of a thick
light-absorbing layer and a p-n junction that enables the
harvesting of photo-generated electrons. These materials are
typically coated with transparent conducting oxides and have an
efficiency that is determined by the amount of light absorbed
versus the number of resulting electron-hole pairs that are
converted to usable charge.
[0015] Silicon is a widely-used material in conventional solar
cells because it has optical properties that are nearly optimal in
terms of enabling maximum efficiency in a Generation I-type solar
device (which is typically more cost-effective than second- or
third-generation devices). Embodiments of the invention take
advantage of this property by integrating a solar cell with various
electrochemical devices that use silicon as the material for at
least one of their electrodes. At the same time, many materials
besides silicon may also be used for both solar cells and
electrochemical devices--some of which may be even better suited
for certain applications than silicon would be--and any of these
materials (examples of which will be given below) may be used to
advantage in various embodiments of the invention.
[0016] In a typical solar cell, the absorption of light into the
material (determined by the Beer-Lambert relation) follows an
exponential trend with thickness. Therefore, the active material
for absorption of solar radiation is only a few microns of material
on the surface of the solar device. Accordingly, since conventional
silicon wafer processing typically involves wafer thicknesses in
the range of 100-200 micrometers (hereinafter "microns," or
".mu.m") it is highly attractive to utilize the inactive material
in the solar cell (which will likely always be present due to the
high cost of ultra-thin materials) as a medium for storing energy
and improving the functionality of the materials in the solar
cell.
[0017] Existing solar cells are used to convert photons into
electrical energy and then are used to supply energy directly to an
electrical device or to feed it back into the electrical grid.
Unfortunately, this approach only supplies energy intermittently,
that is, when sunlight is available. In addition, the electrical
grid can only act as a storage unit for a limited amount of energy
before the cost exceeds the benefit and additional storage has to
be added. Furthermore, existing solar devices and energy storage
devices are produced in separate processing routes, making their
integration only possible by combining two isolated device
structures. Embodiments of the invention overcome the foregoing
issues by integrating the structures.
[0018] Although much of the discussion herein that is directed
toward energy storage devices (or toward energy storage components
of more comprehensive or multi-functional devices) will focus on
electrochemical capacitors, the "energy storage device" and "energy
storage component" designations explicitly include--in addition to
ECs--hybrid electrochemical capacitors (which, like electrochemical
capacitors, are discussed in more detail below) as well as
batteries, fuel cells, and similar devices that store energy.
Energy storage devices/components according to embodiments of the
invention can be used for a wide variety of applications, including
in personal computers (PCs), including desktop and laptop
(notebook) computers, tablet computers, cell phones, smart phones,
music players, servers, other electronic devices, automobiles,
buses, trains, airplanes, other transportation vehicles, home
energy storage, storage for energy generated by solar or wind
energy generators--especially energy harvesting devices--and many
others.
[0019] Electrochemical capacitors operate according to principles
similar to those that govern conventional parallel plate
capacitors, but certain important differences do apply. One
significant difference concerns the charge separation mechanism.
For one important class of ECs this typically takes the form of a
so-called electric double layer, or EDL, rather than the dielectric
of a conventional capacitor. The EDL is created at an interface
between an electrolyte and a high-surface area electrode by the
electrochemical behavior of electrons (or electronic holes) on one
side of the interface and ionic charge carriers on the other side,
and results in an effective separation of charge in spite of the
fact that the two layers within the double layer are so close
together. (Physical separation distances are on the order of a
single nanometer.) Thus, a typical EDL capacitor may be thought of
as storing charge in its EDL. Each layer of the EDL, which is
formed when a voltage is applied across the interface, is
electrically conductive--conduction is performed by ions in the
electrolyte and by electrons/holes in the electrode--but the
properties of the double layer prevent current from flowing across
the boundary between them. (The EDL is further discussed below in
connection with FIG. 3.)
[0020] As is true in conventional capacitors, capacitance in an EDL
capacitor is proportional to the surface area of the electrodes and
inversely proportional to the charge separation distance. The very
high capacitances achievable in an EDL capacitor are due in part to
the very high surface area attributable to the multi-channel porous
structure and to the nanometer-scale charge separation distance
attributable to the EDL, which arises due to the presence of an
electrolyte, as explained above. One type of electrolyte that may
be used in accordance with embodiments of the invention is an ionic
liquid. Another is an electrolyte comprising an ion-containing
solvent. Organic electrolytes, aqueous electrolytes, and
solid-state electrolytes are also possible.
[0021] Another class of electrochemical capacitor is the
pseudocapacitor, where, in addition to EDL capacitance, an
additional storage mechanism--one that is Faradaic and not
electrostatic in origin--can arise at the surface of certain types
of electrodes. The additional storage mechanism is typically
referred to as "pseudocapacitance," and is characterized by a
charge storage process that is similar to the operation of many
solid-electrode batteries. The two storage mechanisms complement
each other, leading to even greater energy storage potential than
is possible with EDL capacitance alone. Typically, one of the
electrodes of a pseudocapacitor is coated with a transition metal
oxide, a suitable conducting polymer, or a similar material that
makes up the active material where charge is stored. These
materials can be used with an electrolyte such as a potassium
hydroxide (KOH) solution; when the device is charged, the
electrolyte will react with the material and drive a charge
transfer reaction where energy is stored. More specifically, these
materials store most of their energy through highly-reversible
surface and near-surface electron transfer (e.g., redox (Faradaic))
reactions, which enable higher power than bulk storage in
conventional batteries due to the fast charge and discharge
kinetics.
[0022] It will be understood that pseudocapacitors may be
constructed using electrolytes other than the one mentioned above.
For example, ion-containing solvents such as Li.sub.2SO.sub.4 or
LiPF.sub.6 may be used as the electrolyte; these result in an
intercalation reaction that involves the insertion of a species
into the surface of the host structure without breaking any bonds.
This reaction, like the other pseudocapacitive reactions mentioned
earlier, results in a transfer of charge so it too is Faradaic and
considered a redox reaction (albeit a special type).
[0023] Hybrid electrochemical capacitors are energy storage devices
that combine the attributes of ECs and batteries. In one example,
an electrode coated with a lithium ion material is combined with an
electrochemical capacitor in order to create a device that has an
EC's rapid charge and discharge characteristics and a battery's
high energy density. On the other hand, hybrid ECs, like batteries,
have shorter expected lifespans than do electrochemical
capacitors.
[0024] Referring now to the drawings, FIGS. 1 and 2 are
cross-sectional views of an energy storage structure 100 that will
be used to guide an initial discussion introducing concepts and
structures that will aid in the understanding of embodiments of the
present invention. As illustrated in FIG. 1, energy storage
structure 100 comprises an energy storage device 101 and an
electrically conductive support structure 102. (In some embodiments
support structure 102 can be omitted.) Alternatively, as
illustrated in FIG. 2, energy storage structure 100 comprises
energy storage device 101 and a non-conductive support structure
102.
[0025] Energy storage device 101 comprises an electrically
conductive structure 110 and an electrically conductive structure
120 separated from each other by a separator 130 that is an
electron insulator and an ionic conductor. Separator 130 prevents
electrically conductive structures 110 and 120 from physically
contacting each other, thereby preventing an electrical short
circuit. (In other embodiments, for reasons discussed below, a
separator is not necessary and can be omitted.)
[0026] In some embodiments, at least one of electrically conductive
structures 110 and 120 comprises a porous structure containing
multiple channels, each one of which has an opening to a surface of
the porous structure. This feature is a result of an exemplary
process, described below, used to form the porous structure in
certain embodiments. As an example, the porous structure may be
formed within an electrically conductive or a semiconductive
material. Alternatively, the porous structure may be formed within
an insulating material (e.g., alumina) that has been coated with an
electrically conductive film (e.g., an atomic layer deposition
(ALD) conductive film such as titanium nitride (TiN), tungsten, or
ruthenium). In this regard, materials having greater electrical
conductivity are advantageous because they lower the energy storage
device's effective series resistance (ESR). In the illustrated
embodiments, both electrically conductive structure 110 and
electrically conductive structure 120 comprise such a porous
structure. Accordingly, electrically conductive structure 110
comprises channels 111 with openings 112 to a surface 115 of the
corresponding porous structure and electrically conductive
structure 120 comprises channels 121 with openings 122 to a surface
125 of the corresponding porous structure.
[0027] Various configurations of energy storage device 101 are
possible. In the embodiment of FIG. 1, for example, energy storage
device 101 comprises two distinct porous structures (that is,
electrically conductive structure 110 and electrically conductive
structure 120) that have been bonded together face-to-face with
separator 130 in between. As another example, in the embodiment of
FIG. 2 energy storage device 101 comprises a single planar porous
structure in which a first section (electrically conductive
structure 110) is separated from a second section (electrically
conductive structure 120) by a trench 231 containing separator 130.
One of the electrically conductive structures will be the positive
side of the device and the other electrically conductive structure
will be the negative side. Trench 231 may separate electrically
conductive structure 110 and 120 along a straight line, but
alternatively may separate them using a more complex shape such as
the meandering space between the fingers of two interdigitated
electrodes.
[0028] As an example, separator 130 could be a permeable membrane
or other porous polymer separator. In general, the separator
prevents the physical contact of anode and cathode (which could
cause an electrical malfunction in the device) while permitting the
transfer of ionic charge carriers. In addition to polymer
separators, several other separator types are possible. These
include non-woven fiber sheets or other non-woven separators,
liquid membranes, polymer electrolytes, solid ion conductors, glass
fiber, paper, ceramic, and the like. In some embodiments, non-woven
separators are concentrations of fibers that are either randomly
oriented or are arranged in a directional pattern.
[0029] It should be noted that the separator, although shown in
FIG. 2, may not be necessary in the configuration illustrated there
because, for example, support structure 102 could be used to
maintain a physical separation between structures 110 and 120. As
another example, electrically conductive structures 110 and 120
could each be attached to a ceramic package (not shown) that would
keep the two electrically conductive structures physically separate
from each other.
[0030] As an example, the porous structure of electrically
conductive structures 110 and 120 can be created by a wet etch
process in which a liquid etchant applied to a surface of the
electrically conductive structures etches away portions of the
electrically conductive structure in a way that is at least
somewhat similar to the way water is able to carve channels in
rock. This is why each one of the channels formed in this way has
an opening to the surface of the electrically conductive structure:
the wet etch method is incapable of creating fully-enclosed
cavities, i.e., cavities within the porous structure that have no
opening to the surface (like an air bubble trapped inside a rock).
This is not to say that those openings cannot be covered with other
materials or otherwise closed up because of the presence of or
addition of other materials--that is in fact likely to occur in
several embodiments--but, whether covered or not, the described
openings to the surface are a feature of each channel in each
porous structure according to at least one embodiment of the
invention. (One embodiment in which the openings may be covered up
is one in which a layer of epitaxial silicon as a location for
circuitry or other wiring is grown on top of the channels).
[0031] With the right etchant, it should be possible to make porous
structures having the described characteristics from a wide variety
of materials. Silicon in various forms--including metallurgical
grade silicon, monocrystalline silicon, polycrystalline silicon,
and silicon on insulator--is one material that works well. As an
example, a porous silicon structure may be created by etching a
silicon substrate with a mixture of hydrofluoric acid (HF) and
alcohol (ethanol, methanol, isopropyl, etc.). More generally,
porous silicon and other porous structures may be formed by such
processes as anodization and stain etching. Etching techniques
according to embodiments of the invention will be discussed in more
detail below. Some other materials (besides silicon) that may be
especially well-suited for energy storage devices according to
embodiments of the invention are porous germanium and porous
tin.
[0032] Possible advantages of using porous silicon include its
compatibility with existing silicon technology and its abundance in
the earth's crust. Porous germanium enjoys a similar advantage as a
result of existing technology for that material and, as compared to
silicon, enjoys the further possible advantage that its native
oxide (germanium oxide) is water-soluble and so is easily removed.
(The native oxide that forms on the surface of silicon may trap
charge, which is an undesirable result.) Porous germanium is also
highly compatible with silicon technology. Possible advantages of
using porous tin, which is a zero-band-gap material, include its
enhanced conductivity with respect to certain other conductive and
semiconductive materials.
[0033] Other materials may also be used for the porous structure,
including semiconducting materials such as gallium arsenide (GaAs),
indium phosphide (InP), boron nitride (BN), silicon carbide (SiC),
and alloys such as an alloy of silicon and germanium. Organic
semiconductors may also be used. In some embodiments the
semiconducting materials--or even insulating materials--may be
treated to make them electrically conductive (or more highly
conductive). An example is silicon that is degenerately doped with
boron. In addition to porous semiconducting substrates, porous
conducting substrates may also be used for ECs, including, in
certain embodiments, substrates composed of carbon or of metals
such as copper, aluminum, nickel, calcium, tungsten, molybdenum,
and manganese.
[0034] The etching used to make the porous structures may be
accomplished using an electrochemical etch that makes use of a
dilute mixture of HF and alcohol to form nanometer pores that can
extend through a significant portion of the substrate. As an
example, a porous structure such as porous semiconducting structure
110 or 120 may be prepared by applying an electrochemical etch
technique to a solid silicon wafer having an initial resistivity of
0.7 milli-ohm centimeters (m.OMEGA.-cm) using as the etchant one of
the HF mixtures referred to above. A current density in a range of
approximately 25 milliamps per square centimeter (mA/cm.sup.2) to
500 mA/cm.sup.2 may be used. (The area component in these values
refers to an area of the substrate surface before formation of the
pores.)
[0035] The foregoing discussion has made reference to porous
structures according to embodiments of the invention. These porous
structures, as mentioned, can be formed within a variety of
materials, including silicon (in various forms, including
metallurgical grade silicon, monocrystalline silicon,
polycrystalline silicon, and silicon on insulator), germanium,
GaAs, InP, BN, CdTe, tin, copper, aluminum, nickel, calcium,
tungsten, molybdenum, manganese, silicon carbide, organic
semiconductors, and silicon-germanium alloys. The material from
which the porous structure is made can, in at least some
embodiments, be doped with elements that increase its conductivity;
this may be done using standard techniques that are known in the
art. In one embodiment, the material in which the porous structure
is formed is silicon and the dopant species is boron, which may be
introduced into the silicon in a concentration of, for example,
10.sup.19 atoms/cm.sup.3. Other possible dopants include phosphorus
and arsenic (though these and other n-type dopants require an
illumination process during etching that p-type dopants do
not).
[0036] Embodiments of the invention that rely on electrochemical
etching as the channel creation technique have another reason for
introducing dopants into the material from which the porous
structure is to be made. Where silicon and an HF etchant are
involved, it is thought that a high electric field attracts holes
at defects and at the tip of the pores that aid the reaction
between the silicon and the fluorine from the etchant. It is
thought that the process involves the formation of SiF.sub.4
molecules in liquid form. The SiF.sub.4 gets pulled away and
eventually gets washed out of the channels, leaving hydrogen atoms
that bond to the sidewalls and also form H.sub.2 that then bubbles
away as a gas. Some hydrogen atoms remain; these bond with
remaining silicon atoms. This process etches the channel
(anisotropically) downward as opposed to expanding laterally in an
isotropic manner (which would simply polish the surface without
forming channels). Additional details, as best understood, are set
forth below (though it must be said that precise details of the
mechanism of porous silicon formation remain at least somewhat
unclear).
[0037] In general terms, during channel formation, direct
dissolution of the semiconductor almost always competes with
oxidation plus subsequent dissolution of the oxide. The etchant
(e.g., HF), therefore, has to be able to dissolve the oxide. A
second prerequisite for the dissolution reaction and thereby
channel formation in a semiconductor is the availability of
electronic holes. The silicon surface, in contact with aqueous HF
solutions, becomes saturated by hydrogen, depleted of electronic
holes, and tends to be chemically inactive with respect to the
electrolyte (this protects the channel sidewalls during the etching
process). If a voltage is applied to the electrodes, the holes
present in a silicon wafer start migrating towards the
silicon-electrolyte interface. At the interface, a hole removes one
silicon bond and thereby makes one silicon atom more susceptible
for interactions with the electrolyte. Eventually, the silicon atom
is transferred into the solution. The electrode decomposes into
areas with optimal current density and channels are formed in areas
with almost no current density. According to different models,
initiation of the channel growth could begin at micro-cavities,
structural defects, mechanically strained areas, or local
perturbations of the surface potential field.
[0038] Referring again to FIGS. 1 and 2, energy storage structure
100 further comprises (in the embodiment illustrated in FIG. 1) an
electrically conductive coating 140 on at least a portion of the
porous structure and in at least some of channels 111 and/or
channels 121. Such an electrically conductive coating may be
necessary in order to maintain or enhance the conductivity of the
porous structure, which can also lower the ESR, thereby improving
performance. For example, a device having lower ESR is able to
deliver higher power (which may be manifested in terms of greater
acceleration, more horse power, etc.). In contrast, higher ESR (a
condition that often prevails inside a typical battery) limits the
amount of available energy, at least partially due to the fact that
much of the energy is wasted as heat, which is a key consideration
for both long-term performance and safety.
[0039] Illustrated in FIGS. 1 and 2 is an electrolyte 150, which
gives rise to the EDL, as explained above. Electrolyte 150 (as well
as the other electrolytes described herein) is represented in the
drawings using a random arrangement of circles. This representation
is intended to convey the idea that the electrolyte is a substance
(liquid or solid, including gel-like materials) containing free
ionic charge carriers. The circles were chosen for convenience and
are not intended to imply any limitation as to the electrolyte
components or qualities, including any limitation with respect to
the size, shape, or number of the ionic charge carriers.
[0040] After the introduction of electrolyte 150, an electric
double layer is formed within the channels of the porous structure,
as depicted schematically in FIG. 3. In that figure, an electric
double layer 330 has been formed within one of channels 111. EDL
330 is made up of two components: the electrical charge of the
sidewalls of channel 111 (depicted as being positive in FIG. 3 but
which in other embodiments could be negative); and the free ionic
charge carriers in the electrolyte. EDL 330 thus provides a
separation of charge that is necessary in order for the capacitor
to function. As explained earlier, the large capacitance, and,
hence, energy storage potential, of EDL capacitors arises in part
due to the small (approximately 1 nanometer (nm)) separation
distance between electrolyte ionic charge carriers and the
electrode surface charge.
[0041] It should be noted that the FIG. 1 and FIG. 2 depictions of
the porous structures are highly idealized in that, to mention just
one example, all of channels 111 and 121 are shown as only
extending vertically. In reality the channels may branch off in
multiple directions to create a tangled, disordered pattern that
may look something like the porous structure shown in FIGS. 4a and
4b.
[0042] FIGS. 4a and 4b are scanning electron microscope (SEM)
images of, respectively, a surface and a cross-sectional slice of a
porous structure 400 (in this case porous silicon). As illustrated,
porous structure 400 contains multiple channels 411. It should be
understood that channels 411 are likely to twist and turn along
their lengths such that a single channel may have both vertical and
horizontal portions as well as portions that are neither completely
vertical nor completely horizontal but fall somewhere in between.
Note that in FIG. 4b, the channels extend near to but do not quite
reach a bottom of the etched structure, thus leaving a layer 402 of
un-etched silicon underneath the channels. In one embodiment,
un-etched layer 402 acts as a support structure for porous
structure 400 (and for the corresponding energy storage device, not
shown), and is thus the equivalent of support structure 102. In
some embodiments, as mentioned above, the support structure may be
omitted.
[0043] The foregoing discussion introduced solar cells and energy
storage devices. Part of that discussion highlighted various solar
cell shortcomings arising out of inefficiencies surrounding the
storage of the energy they produce. Those shortcomings are
addressed by embodiments of the invention that constitute an
integration into a single device of the two types of structures
that have been described above. An energy storage device--referred
to now as an energy storage component because of its presence in
the integrated structure--stores energy in the form of electrical
charge produced from photons harvested and converted by a solar
device, thereby improving the local storage capability of
illuminated regions of the solar cell. For example, energy produced
by the solar cell during sunny periods can be stored in the energy
storage component and then released after dark (or when the solar
cell is shaded by clouds or by trees) in order to maintain a more
constant output voltage.
[0044] Energy storage devices (e.g., electrochemical capacitors)
comprising porous electrodes fabricated in any suitable material
(for example as set forth herein: silicon, germanium, carbon, etc.)
can be monolithically fabricated as part of a solar cell, thereby
enabling the device both to convert light into energy and to store
the charge during brief or prolonged periods when the device is not
exposed to light. As an example, the energy storage device can be
fabricated onto the back side of a photovoltaic device. With the
two devices fabricated on opposite sides of a substrate, an
electrical connection can be achieved through the central bulk part
of the substrate with charge carriers (electrons or holes)
generated by the photovoltaic device.
[0045] FIG. 5 is a schematic representation of an energy conversion
and storage device 500 according to an embodiment of the invention.
As illustrated in FIG. 5, energy conversion and storage device 500
comprises a semiconductive substrate 510 having a side 511, a side
512 opposite side 511, and a central bulk portion 515 between sides
511 and 512. A photovoltaic device 520 is fabricated at side 511 of
the semiconductive substrate and an energy storage component 530 is
fabricated at side 512 of the substrate. In an embodiment, the
energy storage component comprises a porous semiconductive
structure forming an interface with an electrolyte.
[0046] In an embodiment, the semiconductive structure comprises
silicon. In an embodiment, the silicon acts as an absorber layer
(either p-type or n-type), the energy storage component is a
pseudocapacitor comprising a pseudocapacitive material (e.g., a
solid state material such as RuO.sub.2 or MnO.sub.2), and the
electrolyte comprises redox couples that are accessible at energies
below a silicon band gap energy (e.g., metal redox couples such as
V.sup.2+/3+ and Fe.sup.2+/3+). In another embodiment, the
photovoltaic device comprises a p-n junction.
[0047] In various embodiments, the electrolyte is a liquid or a gel
or a solid state electrolyte. In an embodiment, the photovoltaic
device comprises an anti-reflective coating (e.g., a layer of
dielectric material or a textured region on a surface of side 511
of semiconductive substrate 510).
[0048] FIG. 6 is a cross-sectional view of an energy conversion and
storage device 600 according to an embodiment of the invention. As
an example, energy conversion and storage device 600 can be similar
to energy conversion and storage device 500 that has just been
described, and many components of energy conversion and storage
device 600 that will be introduced below have corresponding
components in energy conversion and storage device 500. As an
example, FIG. 6 may be thought of as a more detailed illustration,
according to an embodiment of the invention, of the generalized
schematic of FIG. 5.
[0049] As illustrated in FIG. 6, energy conversion and storage
device 600 comprises an energy storage component 601. In various
embodiments, energy storage component 601 may take the form of a
battery, a hybrid EC capacitor, a pseudocapacitor, or another
energy storage device such as have been described above.
[0050] In various embodiments, energy storage component 601 may
take the form of a battery, a hybrid EC capacitor, a
pseudocapacitor, or another energy storage device such as have been
described above. As an example, consider an embodiment where the
energy storage component is a pseudocapacitor and material 617 is
silicon. An external potential difference applied to the
pseudocapacitor acts to reduce or oxidize either a solid-state
material (e.g., RuO.sub.2, MnO.sub.2) or an electrolyte into a
different valence state that can store the energy. Instead of
utilizing a p-n junction to drive the separation of electron-hole
pairs (which are then shuttled to an electrical circuit in a
standard PV device), one could simply utilize the silicon as the
absorbing layer (either p-type or n-type), and use an electrolyte
that results in a built-in potential across the
electrode-electrolyte interface. This potential can be utilized to
drive redox reactions through the injection of minority carriers
into the electrolyte or solid material located at the interface
with the semiconductor absorber. The electrolytes in this system
will have redox couples that are accessible at energies below the
silicon band gap energy. This includes many of the metal redox
couples such as V.sup.2+/3+ and Fe.sup.2+/3+, among others. Because
Si is used as the porous material in this electrochemical storage
system, one could integrate this design directly on the backside of
a Si layer that acts as the absorber layer for a Si solar device.
In this device, the system acts as a fully integrated conversion
and storage system with the electrolyte/electrode interface driving
the harvesting and storage reactions. This takes advantage of the
fact that both devices are composed of the same material (which is
silicon for the illustrated case but which could also be a variety
of other materials as discussed herein).
[0051] In the illustrated embodiment, energy storage component 601
is an electrochemical capacitor that can be similar to energy
storage device 101 that was first shown in FIG. 1. Accordingly,
energy storage component 601 comprises an electrode 611 having a
plurality of channels 612 formed in a region 615 of a material 617,
and an electrode 621 having a plurality of channels 622 formed in a
region 625 of a material 627. Electrode 621 is adjacent to but
electrically isolated from electrode 611. An electrolyte 650 is
located within channels 612 and 622 such that electrode 611 forms
an interface 619 with the electrolyte and electrode 621 forms an
interface 629 with the electrolyte.
[0052] Energy conversion and storage device 600 further comprises a
photovoltaic component/solar cell 602 formed in a region 616 of
material 617. Thus, in at least the illustrated embodiment, at
least one electrode of the energy storage component is formed in
the same material in which the photovoltaic component is formed.
This architecture, therefore, represents a fully-integrated
conversion and storage device that does not require the external
wiring between energy conversion and energy storage components that
a non-integrated system would require.
[0053] In the illustrated embodiment, PV component 602 comprises an
anti-reflective coating 671 and a transparent electrically
conductive layer 672. As an example, the transparent electrically
conductive layer may enable electrical contact to the photoactive
material in order to enable the charge to move through the external
circuit and apply a voltage across the storage device while
avoiding substantial losses. Other embodiments may include only one
or the other of these components, and still other embodiments may
omit both. For a bottom-illuminated device architecture, layer 672
(along with the rest of PV component 602) would be located on the
bottom of device 600 and energy storage component 601 would move to
the top. Anti-reflective coating 671 can be formed by creating a
textured surface or by using a thin layer of dielectric material.
The thickness of the dielectric material can be chosen such that
interference effects cause the wave reflected from the
anti-reflection coating to be out of phase with the wave reflected
from the body of the PV component (i.e., from material 617).
[0054] Material 617 can have a thickness of, for example, at least
100 microns. In an embodiment, a photon-absorbing portion of PV
component 602 constitutes an upper layer of material 617 beginning
at a surface of the material and extending to a depth of no greater
than 10 microns into the material.
[0055] Referring still to FIG. 6, in the general architecture
shown, the solar cell can include either: (i) a p-n junction, where
minority carriers are generated and then stored in the energy
storage component; or (ii) a thin absorbing layer where
electron-hole pairs diffuse to the electrode/electrolyte interface
(e.g., interface 619) and minority carriers are injected into the
electrolyte to drive electrochemical redox chemistry that stores
the charge. Redox couples may be important in this regard, e.g.,
Fe.sup.2+/Fe.sup.3+, V.sup.2+/V.sup.3+, etc. Thus, in an
embodiment, PV component 602 further comprises a p-n junction 675.
In the illustrated embodiment, p-n junction 675 forms an interface
within a depletion region 677 that is adjacent to anti-reflective
coating 671.
[0056] In a non-illustrated embodiment, the p-n junction exists at
the interface with the electrolyte in the porous electrode. In
other words, the effective p-n junction of the solar device is the
same interface that drives the energy storage, which is created by
the junction between the porous material and the electrolyte. As
electron-hole pairs are created by absorption in the material they
diffuse to the material's surface. The built-in field at the
liquid-semiconductor interface splits the electron and hole, and
the minority carrier (which is either an electron or a hole,
depending on the type of dopant in the material) drives an
electrochemical reaction. One such configuration comprises a porous
silicon piece, an electrolyte reservoir containing redox species,
and a metal counter-electrode. In this configuration, the porous Si
absorbs photons, and photo-excited electron-hole pairs diffuse to
the interface between the porous silicon and the electrolyte,
creating a depletion region. At the interface, the electron-hole
pairs are split to drive a redox reaction. An integrated "redox
battery" is thus created, where the charge is stored in a
liquid.
[0057] A more complex device could be designed such that the liquid
flows through the device and replenishes the electrolyte solution.
This will inhibit the build-up of an excessive concentration of
redox couples that have been "activated," and will allow the flow
of activated redox species to a location in the device where they
could be discharged. This would be a solar "flow battery."
[0058] In various embodiments, material 617 is a semiconductive
material that can be made porous (e.g., using an etching technique
as disclosed herein) and can also be made into a PV device. Such
materials include monocrystalline silicon, polycrystalline silicon,
amorphous silicon, germanium, gallium arsenide, cadmium telluride,
copper indium selenide, copper indium gallium selenide, gallium
indium phosphide, and copper indium sulfide, and in various
embodiments, material 617 comprises at least one of these
materials. Thus, for example, the material selected for the solar
cell may be electrochemically etched in order to form channels for
a porous electrode of the energy storage component.
[0059] Material 627 can be, but need not be, the same as material
617. In various embodiments, material 627 comprises at least one of
monocrystalline silicon, polycrystalline silicon, amorphous
silicon, carbon, other carbon-based materials, and nickel foam.
[0060] In an embodiment, energy conversion and storage device 600
comprises an electrically insulating layer 680 between energy
storage component 601 and photovoltaic component 602. As an
example, this can be a silicon-on-insulator (SOI)-like structure in
which a layer of silicon dioxide (or another electrical insulator)
is located at a desired depth below anti-reflective coating 671 (if
present; otherwise, at a desired depth below a surface of PV
component 602). The depth should be such that the insulating layer
is located many times deeper than the minority carrier diffusion
length (L.sub.D) for the solar device. In other words, the
thickness of the absorbing layer should be much greater than
L.sub.D. Circuitry can be designed on the device to connect the
energy storage component as needed. In an embodiment, the
electrical connection can be accomplished using through silicon
vias (TSV) 681. It should be noted that the "TSV" designation used
here is intended to cover any such connecting pathway, without
regard for whether it is formed in or passes through silicon or
some other material.
[0061] An energy conversion and storage device according to
embodiments of the invention can be used as an energy source for a
wide variety of devices that require energy, including mobile
electronic devices and the like. Referring still to FIG. 6, such a
device is represented by a load 690, which is connected to energy
conversion and storage device 600 through circuitry 691. In
operation, photons 692 (received from the sun or from some other
suitable light source) impinge on PV component 602 and some of them
are subsequently absorbed by material 617, thus generating an
electron-hole pair 693 according to mechanisms known in the art
that have previously been described herein. Contacts 694 and 695
are made to both the n-type and p-type sides of PV component 602,
with load 690 situated between them. Charge carriers that are
created on one side of the solar cell, or that have been swept
there by the internal electric field, may travel through circuitry
691, pass into energy storage component 601 (where, depending on
operating conditions, some or all of them may become "stored," that
is, may contribute to the capacitance or other energy storage
mechanism of the storage device) and/or may pass through the energy
storage component in order to provide power to load 690, and then
continue along the circuitry to contact 695, as indicated by arrows
698. Here they may recombine with a hole that was created on the
other side of the solar cell or was swept across the junction from
the first side after being created there. In an embodiment, contact
695 and/or contact 694 may comprise a multi-layer stack containing
titanium, palladium, and silver. Contacts of other compositions are
also possible, as known in the art. It should be understood that
load 690 may be powered by energy that had previously been stored
in energy storage component 601, by electricity flowing directly
from PV component 602, or by some combination of energy from both
sources.
[0062] A switch 697 operates to permit or to prevent the flow of
charge carriers 693 through circuitry 691. In an embodiment, switch
697 closes when the voltage is high and opens when it is low. In
other words, at times during which PV component 602 is receiving an
amount of light 692 sufficient to generate electricity, switch 697
will close in order to allow charge carriers 693 to flow through
circuitry 691 and be stored in energy storage component 601 and/or
provide power to load 690. Conversely, during times when the number
of impinging photons 692 are not sufficient to enable energy
generation, switch 697 will open so as to prevent charge carriers
from being drawn out of energy storage component 601 and back into
PC component 602, where they would otherwise contribute to a
possible undesired electroluminescence. Of course, if such
electroluminescence is desired, the operation of switch 697 may be
adjusted in order to enable it.
[0063] In one embodiment, an energy conversion and storage device
like those described above can be used as part of a mobile
electronic device such as a cell phone, a smart phone, a music
player (or another hand-held computing system), a laptop, a nettop,
a tablet (or another mobile computing system), a wristwatch, a
calculator, or the like. FIG. 7 is a schematic representation of a
mobile electronic device 700 according to an embodiment of the
invention. As illustrated in FIG. 7, mobile electronic device 700
comprises a housing 701, a battery 710 contained within the
housing, and an energy conversion and storage device 720 within
housing 701 and connected to battery 710 so as to be capable of
providing energy to the battery. In some embodiments, device 720
and/or battery 710 are further associated with an IC die (not
shown) located inside housing 701, where being "associated with"
the IC die can mean device 720 and/or battery 710 are integrated
into the IC die or its packaging in some fashion (e.g., by being
implemented on the die itself; by forming part of a
Package-on-Package (PoP) architecture or a system-on-chip (SoC)
architecture; etc.) As an example, energy conversion and storage
device 720 can be similar to energy conversion and storage devices
500 and/or 600, described above and shown in FIGS. 5 and 6. It
should be understood, however, that the depictions in the figures
of energy storage devices 500 and 720 are potentially incomplete in
that they omit certain details that would likely, or at least
possibly, be present in a finished device. These potentially
include one or more collectors attached to particular electrodes as
well as various packaging components.
[0064] As an example, the connection between energy conversion and
storage device 720 and battery 710 can include circuitry (not
shown) including components such as a voltage converter, as would
be apparent to one or ordinary skill in the art. Certain
embodiments of mobile electronic device 700 do not require a
battery because all of the energy required by the device is
supplied by energy conversion and storage device 720. In those
embodiments, battery 710 can be eliminated from the mobile
electronic device.
[0065] The IC die may comprise any type of integrated circuit
device. In one embodiment, the IC die includes a processing system
(either single core or multi-core). For example, the IC die may
comprise a microprocessor, a graphics processor, a signal
processor, a network processor, a chipset, etc. In one embodiment,
the IC die comprises a system-on-chip (SoC) having multiple
functional units (e.g., one or more processing units, one or more
graphics units, one or more communications units, one or more
signal processing units, one or more security units, etc.).
However, it should be understood that the disclosed embodiments are
not limited to any particular type or class of IC devices.
[0066] Although the invention has been described with reference to
specific embodiments, it will be understood by those skilled in the
art that various changes may be made without departing from the
spirit or scope of the invention. Accordingly, the disclosure of
embodiments of the invention is intended to be illustrative of the
scope of the invention and is not intended to be limiting. It is
intended that the scope of the invention shall be limited only to
the extent required by the appended claims. For example, to one of
ordinary skill in the art, it will be readily apparent that the
energy conversion and storage device and the related structures and
methods discussed herein may be implemented in a variety of
embodiments, and that the foregoing discussion of certain of these
embodiments does not necessarily represent a complete description
of all possible embodiments.
[0067] Additionally, benefits, other advantages, and solutions to
problems have been described with regard to specific embodiments.
The benefits, advantages, solutions to problems, and any element or
elements that may cause any benefit, advantage, or solution to
occur or become more pronounced, however, are not to be construed
as critical, required, or essential features or elements of any or
all of the claims.
[0068] Moreover, embodiments and limitations disclosed herein are
not dedicated to the public under the doctrine of dedication if the
embodiments and/or limitations: (1) are not expressly claimed in
the claims; and (2) are or are potentially equivalents of express
elements and/or limitations in the claims under the doctrine of
equivalents.
* * * * *