U.S. patent application number 11/312018 was filed with the patent office on 2006-05-18 for apparatus and method for photovoltaic energy production based on internal charge emission in a solid-state heterostructure.
Invention is credited to Eric W. McFarland.
Application Number | 20060102227 11/312018 |
Document ID | / |
Family ID | 22009251 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060102227 |
Kind Code |
A1 |
McFarland; Eric W. |
May 18, 2006 |
Apparatus and method for photovoltaic energy production based on
internal charge emission in a solid-state heterostructure
Abstract
An apparatus and method for solar energy production comprises a
multi-layer solid-state structure including a photosensitive layer,
a thin conductor, a charge separation layer, and a back ohmic
conductor, wherein light absorption occurs in a photosensitive
layer and the charge carriers produced thereby are transported
through the thin conductor through the adjacent potential energy
barrier. The open circuit voltage of the solar cell can be
manipulated by choosing from among a wide selection of materials
making up the thin conductor, the charge separation layer, and the
back ohmic layer.
Inventors: |
McFarland; Eric W.; (Santa
Barbara, CA) |
Correspondence
Address: |
Michael A. O'Neil;Michael A. O'Neil, P.C.
Suite 1030
5949 Sherry Lane
Dallas
TX
75225
US
|
Family ID: |
22009251 |
Appl. No.: |
11/312018 |
Filed: |
December 20, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10750015 |
Dec 31, 2003 |
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11312018 |
Dec 20, 2005 |
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10057223 |
Jan 25, 2002 |
6774300 |
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10750015 |
Dec 31, 2003 |
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60287205 |
Apr 27, 2001 |
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Current U.S.
Class: |
136/255 |
Current CPC
Class: |
H01L 51/4286 20130101;
H01L 31/0352 20130101; H01L 51/0073 20130101; Y02E 10/542 20130101;
Y02P 70/521 20151101; H01L 51/4226 20130101; H01L 31/07 20130101;
H01L 51/0038 20130101; H01G 9/2031 20130101; H01L 31/035281
20130101; Y02P 70/50 20151101; Y02E 10/549 20130101 |
Class at
Publication: |
136/255 |
International
Class: |
H01L 31/00 20060101
H01L031/00 |
Claims
1. A multi-layer solid-state device for producing electrical power
from light comprising: an electrically conductive layer having
first and second sides; a light energy conversion layer secured to
the first side of the electrically conductive layer; a charge
separation layer secured to the second side of the electrically
conductive layer; the electrically conductive layer for
ballistically transporting charge characters from the light energy
conversion layer to the charged separation layer thereby
eliminating the need for an electrolyte for producing electrical
power from light impinging upon the light energy conversion layer;
and the electrically conductive layer and the charged separation
layer defining a metal-insulator-semiconductor junction.
2. A multi-layer solid-state device for producing electrical power
from light comprising: an ultra-thin, two-sided, electrically
conductive front contact layer having first and second sides; a
light energy conversion layer comprising photosensitive means
secured to the first side of the electrically conductive layer; a
semiconductive charge separation layer having first and second
sides; the first side of the semiconductor charge separation layer
being secured to the second side of the electrically conductive
layer; the electrically conductive layer ballistically transporting
electrical energy from the light energy conversion layer to the
charged separation layer thereby eliminating the need for an
electrolyte when producing electrical power from light impinging on
the light energy conversion layer; and electrically conductive
metal back contact secured to the second side of the charged
separation layer; and the electrically conductive layer in the
semiconductor charged separation layer defining a
metal-insulator-semiconductor junction which maximizes power
output.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of
application Ser. No. 10/750,015 filed Dec. 31, 2003, currently
pending, the entire contents of which are incorporated herein by
reference; which is a continuation application of application Ser.
No. 10/057,223 filed Jan. 25, 2002, now U.S. Pat. No. 6,774,300,
the entire contents of which are incorporated herein by reference;
which claims priority of provisional application Ser. No.
60/287,205 filed Apr. 27, 2001, the entire contents of which are
incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention is directed to low cost, high
efficiency solar cell technology. More specifically, the present
invention is related to a method and apparatus for producing
photovoltaic energy using solid-state devices.
BACKGROUND OF THE INVENTION
[0003] Conventional photovoltaic cells convert sunlight directly
into electricity by the interaction of photons and electrons within
the semiconductor material. Most solid-state photovoltaic devices
rely on light energy conversion to excite charge carriers
(electrons and holes) within a semiconductor material and charge
separation by a semiconductor junction producing a potential energy
barrier. To create a typical photovoltaic cell, a material such as
silicon is doped with atoms from an element with one more or less
electrons than occurs in its matching substrate (e.g., silicon). A
thin layer of each material is joined to form a junction. Photons,
striking the cell, transfer their energy to an excited electron
hole pair that obtains potential energy. The junction promotes
separation of the electrons from the holes thereby preventing
recombinations thereof. Through a grid of physical connections, the
electrons are collected and caused to flow as a current. Various
currents and voltages can be supplied through series and parallel
arrays of cells. The DC current produced depends on the electronic
properties of the materials involved and the intensity of the solar
radiation incident on the cell.
[0004] Conventional solar cell technologies are based largely on
single crystal, polycrystalline, or amorphous silicon. The source
for single crystal silicon is highly purified and sliced into
wafers from single-crystal ingots or is grown as thin crystalline
sheets or ribbons. Polycrystalline cells are another alternative
which is inherently less efficient than single crystal solar cells,
but also cheaper to produce. Gallium arsenide cells are among the
most efficient solar cells available today, with many other
advantages, however they are also expensive to manufacture.
[0005] In all cases of conventional solid-state photovoltaic cells,
photon (light) absorption occurs in the semiconductor with both
majority and minority charge carriers transported within the
semiconductor; thus, both electron and hole transport must be
allowed and the band gap must be sufficiently narrow to capture a
large part of the visible spectrum yet wide enough to provide a
practical cell voltage. For the solar spectrum the ideal band gap
has been calculated to be approximately 1.5 eV. Conventionally,
expensive material and device structures are required to achieve
cells that provide both high efficiency and low recombination
probability and leakage.
[0006] A conventional solid-state solar cell, such as the one shown
in FIG. 1, may include structures such as a semiconductor junction,
heterojunction, interface, and thin-film PV's, which may be made
from organic or inorganic materials. In all of these devices the
necessary elements of these types of devices are a) photon
absorption in the semiconductor bulk, b) majority and minority
charge carrier transport in the semiconductor bulk, c) a
semiconductor band-gap chosen for optimal absorption of the light
spectrum and large photovoltages, and d) ideal efficiency limited
by open circuit voltages less than the semiconductor band-gap. The
photon absorption occurs within the bulk semiconductor and both
majority and minority carriers are generated and transported in the
bulk. For adequate absorbency, relatively thick, high quality
semiconductors are needed. However, defect free, thick, narrow
band-gap, materials are limited in numbers and expensive to
fabricate. In heterostructures a limited number of acceptable
compatible materials are available. Schottky barrier based devices
have been proposed in this class that rely, again, on absorption of
photons in the semiconductor bulk and use the Schottky barrier for
charge separation.
[0007] Another class of conventional solar cells are the
dye-sensitized photoelectrochemical solar cells as shown in FIG. 2.
These devices were derived from work on photoelectrochemical
electron transfer and are cathode/electrolyte/anode systems in
which a photoactive molecule is light activated and oxidized (or
reduced) by electron (or hole) transfer to the adjacent
semiconductor electrode. The charge transfer agents which replace
the transferred charge in the photoactive molecule are typically
molecules or atoms dissolved in a liquid electrolyte such that the
molecules or atoms receive charges from an electrode. Reduction is
performed by an electron donor in the liquid electrolyte. This
device is limited in its power output by the relative free energies
of electrons in the electrolyte and the semiconductor which limit
the photovoltage. The maximum photovoltage is limited by the
difference between the bottom of the conduction band edge and the
liquid electrolyte chemical potential. Additional inefficiencies
result from the required molecular diffusion of the donors to the
electrode as well as overpotential losses at the
electrode/electrolyte interface.
[0008] Another solid-state solar cell is the dye-sensitized
Schottky barrier solar cell as described in U.S. Pat. Nos.
4,105,470 and 4,190,950 by Dr. Skotheim. The Skotheim device is
similar to the above-mentioned photoelectrochemical cell except the
liquid electrolyte is replaced by a "reducing agent" layer, the
property of which is not precisely identified in either the '470
nor the '950 patent. Purportedly, as a means of removing the
band-gap restrictions of conventional PV's, an invention was
reported by Skotheim who proposed a solid-state Schottky barrier
device whereby a) photon absorption occurs in a photosensitive dye
deposited on the surface of a semiconductor, b) majority charge
carriers are injected directly into the conducting bands of the
adjacent semiconductor, c) the ionized photosensitizer is
neutralized by charges delivered by a reducing agent, d) a
conductor provides charge to the reducing agent, and e) the
Schottky barrier height will determine the device's ideal
efficiency and its height is determined by the interaction of the
dye and the semiconductor. However, as previously mentioned,
neither patent suggests the physical properties of the reducing
agent, and it is unclear whether the proposed devices disclosed in
the '470 and '950 patents can indeed yield the purported results.
In the proposed cell three separate molecular oxidation/reduction
electron transfer steps are required (one from the excited dye to
the adjacent semiconductor, one from the reducing agent to the dye,
and one from the conductor to the reducing agent). Thus an electron
must move from/to a conduction band to/from a molecular orbit twice
and from one molecular orbit to another one. An implementation of
the device was published using an organic hole transport material,
however, the performance and longevity were poor [ref: U. Bach, et
al., Nature, Vol. 395, October 1998, pg. 583-585].
[0009] Experimental work by the present inventor has demonstrated
that low energy molecular energy transfer at conducting surfaces
can lead to excited charge carriers that can be efficiently
transported through a conductor without energy loss (via ballistic
transport) and captured by an electrical barrier device wherein the
barrier height is determined in part by the electronic interactions
between the surface conductor and the barrier material.
[0010] Accordingly, a fundamentally different type of photovoltaic
device is provided by the present invention which can be easily
manufactured from a wide variety of inexpensive material, and which
may be, in practice, more efficient, the various embodiments of
which will be described in more detail below.
SUMMARY OF THE INVENTION
[0011] The preferred embodiment of the present invention described
herein is a multilayer solid-state structure wherein light
absorption occurs in photosensitive layer (molecules or
nanostructures) and the energetic charge carriers produced by the
absorption are transported ballistically, without significant
energy loss, through an ultra-thin conductor, to and over an
adjacent potential energy barrier that separates and stores the
charge for use as electrical power. The potential energy barrier
largely determines the device efficiency and can be optimized by
choice of the device materials.
[0012] In accordance with the preferred embodiment, a
photoexcitable molecular species or absorbing nanostructure is
deposited on an ultra-thin conductor, and following photoexcitation
excited charges are ballistically transported through the conductor
to the potential energy barrier (Schottky barrier) created at the
interface between the conductor and the charge collection layer (a
semiconductor). The ultra-thin conductor has, inter alia, three
specific functions: I) allows efficient ballistic transport of
charge carriers from the photosensitizer to the potential barrier
at the interface, II) directly provides replacement charges of the
opposite sign to the ionized photosensitizer, and III) influenced,
in part, by its interaction with the charge separation layer, the
magnitude of the potential energy barrier which determines, in
part, the maximum device power.
[0013] The essential components (e.g., layers) of the preferred
embodiment of the present invention include: 1) a photosensitive
layer where light energy is converted to electron and/or hole
excitation, 2) an adjacent ultra-thin conducting layer that
provides a pathway for ballistic transport of charges using high
efficiency conduction bands, and as a source of replacement charges
to the photosensitive layer; and 3) a charge separation and
collection layer such as an inorganic or organic semiconductor
affixed with a back side ohmic contact opposite the ultra-thin
conducting layer. The ohmic contact collects the charges
transported across the barrier. The addition of an anti-reflection
coating on top of the device is a highly practical embodiment of
the invention.
[0014] The present invention is advantageous over the
aforementioned dye-sensitized Schottky barrier solar cell structure
in that it has the advantage of potentially greater photovoltages
due to the ability to influence the barrier height by the choice of
a high (for n-type semiconductors) or low (for p-type
semiconductors) work function conductors at the surface, by the
choice of the semiconductor (type and doping level), and by the
surface treatment of the semiconductor prior to disposition of the
conductor to maximize the barrier height by affecting the
interface. Additional advantages of the present invention include
eliminating the need for a specific reducing agent or a minority
charge carrier transport material, and providing the ability to
choose from among a broad choice of charge separation layer
material to include both wide band-gap n and p type semiconductors.
In contrast to the prior art U.S. Pat. Nos. 4,105,470 and 4,190,950
by Skotheim, only two transfers of electrons to/from conduction
bands are required and no inter-molecular charge transfer is
necessarily required.
[0015] It is an object of the present invention to: 1) eliminate
the need for electrolytes and/or molecular reducing agents and/or
minority carrier conductors, 2) allow for a wider choice of the
conductor and charge separation layer, and 3) maximize by design of
the open circuit photovoltage.
[0016] It is another object of the present invention to increase
the efficiency of photovoltaic energy generation. More
specifically, light absorption can be optimized since a single
band-gap is not required for light absorption and a large number
and variety of materials with selectable spectral properties of
photoabsorbing molecules or structures can be utilized without the
need for compatibility with an electrolyte. Without the
overpotential losses of the electrochemical redox reactions (both
at the electrodes in the photoelectrochemical cell and by the
reducing agent charge transfer), higher efficiencies are also
possible. The ultra-thin conductor is used as an efficient
ballistic transport channel and to maximize the photovoltage as
determined by its effect on the barrier. The interaction between
the conductor and charge separation layer to influence the barrier
height, can be optimized by the choice of the conductor, charge
separation material, and interface preparation.
[0017] It is yet another object of the present invention to lower
the cost of generating photovoltaic energy. More specifically,
present solid-state P.V. systems are expensive due to the need for
high purity low defect silicon or other semiconductors with the
required band-gap, which have high manufacturing costs. The liquid
containing photoelectrochemical cells have reliability and
efficiency limits as well as restrictions on the dye stability and
reducing agent in solution, thus increasing their in-use costs.
Frequently, reactive species such as iodine must be used.
[0018] In is yet another object of the present invention to
increase the longevity of the solar cell devices by using stable
components. In the case of photoelectrochemical cells, most types
of feasible electrolytes are reactive and can erode or dissolve the
adjacent semiconductor or react with the dye, causing the device to
be unstable. By eliminating the need to use reactive components,
the present invention promotes the longevity of solar cells.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] A more complete understanding of the present invention may
be had by reference to the following Detailed Description when
taken in connection with the accompanying Drawings, wherein:
[0020] FIG. 1 is a graphic illustration of a conventional
solid-state solar cell;
[0021] FIG. 2 is an illustration of a conventional dye-sensitized
photoelectrochemical cell;
[0022] FIG. 3 is an illustration of the present invention in
accordance with the preferred embodiment;
[0023] FIG. 4 is an illustration of the present invention in
accordance with an alternative embodiment;
[0024] FIG. 5 is an illustration of the present invention in
accordance with another alternative embodiment;
[0025] FIG. 6 is an illustration of the present invention in
accordance with yet another alternative embodiment;
[0026] FIG. 7 is an illustration of the present invention in
accordance with yet another alternative embodiment;
[0027] FIG. 8 is an illustration of the present invention in
accordance with yet another alternative embodiment;
[0028] FIG. 9 is an illustration of the present invention in
accordance with yet another alternative embodiment;
[0029] FIG. 10 is an illustration of the present invention in
accordance with yet another alternative embodiment;
[0030] FIG. 11 is an illustration of the present invention in
accordance with yet another alternative embodiment;
[0031] FIG. 12 is an illustration of the present invention in
accordance with yet another alternative embodiment;
[0032] FIG. 13 is an illustration of the present invention in
accordance with yet another alternative embodiment;
[0033] FIG. 14 is an illustration of the present invention in
accordance with yet another alternative embodiment;
[0034] FIG. 15 is an illustration of the present invention in
accordance with yet another alternative embodiment;
[0035] FIG. 16 is an illustration of the present invention in
accordance with yet another alternative embodiment;
[0036] FIG. 17 is an illustration of the present invention in
accordance with yet another alternative embodiment; and
[0037] FIG. 18 is a current versus voltage plot of a device made in
accordance with the preferred embodiment of the present
invention.
DETAILED DESCRIPTION
[0038] Various embodiments of the present invention will be
described with reference to FIGS. 3-11. Although only a limited
number of embodiments of the invention are described hereinafter,
it shall be understood that the detailed discussion of the
embodiments is not intended to limit the present invention to those
particular embodiments.
[0039] FIG. 3 illustrates a photosensitized solid-state device in
accordance with the preferred embodiment of the present invention.
More specifically, the photosensitized solid-state device includes
a photosensitive layer 10, a front conducting layer 31, a charge
separation layer 39, a back conducting layer 30, and a load 32. The
front conducting layer 31 is preferably an ultra-thin metal film
(preferably in the nanometer range), while the back conducting
layer 30 is preferably an ohmic conducting layer. The charge
separation layer 39 has a determinable conduction band energy level
38 and a determinable valence band energy level 37. In an
alternative embodiment as shown in FIG. 14, the metal film may be
chemically treated to: 1) allow improved bonding of the photoactive
materials, and 2) provide partial isolation of the photoreceptor
from the conductor to optimize ballistic charge transfer compared
to other pathways to de-excitation of the excited photoreceptor. In
another alternative embodiment as shown in FIG. 16, the surface of
the device consisting of the photosensitive layer/metal film/charge
separation layer is highly contoured, porous, or otherwise shaped
to maximize the surface area and maximize the absorbance of
photons.
[0040] The front conducting layer 31 and the back conducting layer
30 preferably have determinable work function levels 34 and 33,
respectively. It is preferable that the material chosen to make up
the front conducting layer 31 has a higher work function (more
negative) than the Fermi level of the charge separation layer 39 if
the charge separation layer 39 is of an n-type semiconductor, or if
the charge separation layer 39 is of a p-type semiconductor, a
lower work function to facilitate formation of a Schottky barrier
25.
[0041] It is preferable that the front conducting layer 31 is of
the type of material that forms a Schottky barrier with the charge
separation layer 39 so as to maximize the power output of the
solid-state device. Such material (for an n-type barrier) may
include metals such as gold or platinum, or a non-metal material
such as organic conductor polythiophene or a metal oxide. For a
p-type barrier, such as one shown in FIG. 8, materials include low
work function conductors including aluminum and titanium. In
accordance with the preferred embodiment, the front conducting
layer 31 acts as a donor to the photo-oxidized surface species and
thereby eliminates the need for a redox active electrolyte, which
causes losses in the production of photovoltaic energy and
typically has mass transport limitations for current flow.
[0042] The charge separation layer 39 is preferably made of a
semiconductor material, or multiple semiconductors. Either
inorganic semiconductor materials (e.g., titanium dioxide, zinc
oxide, other metal and mixed metal oxides, moly sulfide, zinc
sulfide, other metal and mixed metal sulfides, silicon carbide,
etc.) or organic semiconductor materials, either hole conducting
(e.g., triphenyldiamine (TPD), poly (p-phenylene vinylene) (PPV),
poly (vinyl carbazole) (PVC), and their derivatives, etc.), or
electron conducting (e.g., conjugated oligothiophenes, oxadiazole
derivatives, etc.) may be used. In an alternative embodiment as
shown in FIG. 17, the charge separation layer 39 is made of an
insulator or insulator-semiconductor composite structure with the
key feature being alignment of the majority carrier bands with the
absorber donor level (in FIG. 3, 36 for n-type or in FIG. 8, 84 for
p-type). The photosensitizer layer 10 can be a dye or any energy
absorbing material or structure, and may include light absorbing
atomic or molecular species on a surface (e.g.,
cis-di(thiocyanato)-N,N-bis-(2,2-bipyridyl-4,4-dicarboxylic
acid)-Ru(II), phthalocyanines, carbocyanines, merbromin,
9-phenylxanthene, iron cyanate, etc.), or quantum structures (e.g.,
nanoparticles of CdS, CdSe, or other semiconductors, or metals, or
nanolayers of absorbing material). Additionally, multiple types
and/or layers of different photoactive species can be used on the
photosensitizer layer 10 to maximize the spectrum capture of
incident light. In an alternative embodiment, the photoactive
species may be imbedded in the front conductive layer to make a
single composite layer.
[0043] In fabricating the above-described structure, the
photosensitizer layer, the front and back conducting layers, and
the charge separation layer can be deposited by vapor deposition,
electrochemical deposition, deposition from solution or colloidal
suspension, or be produced by evaporative, extrusion, or other
conventional polymer manufacturing techniques. With specific regard
to the charge separation layer 39, it may be created with high
surface area using organic template molecules, or it can be nano-,
meso-, or macro-porous to increase the surface area. The conductor
and photoactive layers would then follow the contoured surface (see
FIG. 16).
[0044] In a specific fabrication example comprising the preferred
embodiment of the invention, a charge separation layer 39 of
titanium dioxide is deposited onto titanium foil (the ohmic back
contact 30). The charge separation layer 39 has a thickness ranging
between 100 nm and 500 nm and is deposited by electron beam
evaporation and/or by electroanodization of the titanium metal.
Gold is then deposited to the composite layer to a thickness of 10
nm to form the ultra-thin conductor.
[0045] The operation of the preferred embodiment will now be
discussed with reference to FIG. 3. The preferred embodiment of
FIG. 3 produces electrical power from a photon energy source based
on light energy conversion to charge excitation in a layer
containing photosensitive molecules or structures. More
specifically, a photon energy source 35 with energy h.nu., such as
sunlight, is incident upon the photosensitive layer 10. The energy
source excites electrons 36 located in the photosensitive layer 10
causing the electrons 36 to rise to a higher energy state. In
accordance with the preferred embodiment, electrons having an
energy level above the barrier height 25 (or slightly below if
tunneling occurs) pass through the front conducting layer 31 via
ballistic transport (ballistic transport refers to the transfer of
electrons through a medium in which there is a low or zero
scattering cross-section between the electrons and the medium
through which they are transferred). The process of charge
(electron) emission from the photoexcited absorber into and
ballistically across the conduction bands of the conductor and
charge separation layer is termed "Internal Charge Emission".
[0046] Once the electrons travel through the front conducting layer
31, they travel through the charge separation layer 39 towards the
back ohmic conducting layer 30 where they are stored with photon
derived excess potential energy for later use (dissipation) in
passing through the load 32. After losing their energy in the load
32 the electrons are returned to the front conducting layer 31. The
maximum photovoltage of the device, or open circuit voltage, is
determined by the potential barrier height between the front
conducting layer 31 and the charge separation layer 39. In
conventional Schottky solar cells (where the photons are absorbed
in the semiconductor band-gap) the same maximum voltage is possible
as determined by the barrier height, however, in the present
invention the choice of semiconductors is not limited to those with
solar spectrum absorbance. The voltage can be optimized or
influenced by selecting appropriate materials for the front
conducting layer 31 and the charge separation layer 39, and by
specific treatments of the interface. For example, on clean silicon
the Schottky barrier varies from approximately 0.4 eV to 0.8 eV as
the conductor work function increases from approximately -2.5 eV
(Ca) to -5.0 eV (Au) and on GaAs from 0.6 eV (for Mg) to 1 eV (for
Pt). Preparation of the interface and metal can also be used to
increase the barrier for Pd on titanium dioxide where treatment of
the metallic conductor Pd with oxygen causes an increase in the
barrier of nearly 0.5 eV. The design approach is to maximize the
barrier and still allow efficient carrier transport across the
barrier and efficient replacement of photosensitizer (PS) charge by
the conductor.
[0047] In accordance with an alternative embodiment, the charge
separation layer 39 may be a thin insulating layer (PS-MIM
configuration) wherein the conduction band edge and thickness of
the insulator are chosen to allow charge carriers from the
photoexcited state of the photosensitizer 10 to move to the back
contact and prevent current flow in the opposite direction.
[0048] In accordance with another alternative embodiment of the
present invention an additional layer of semiconductor is included
between the charge separation layer 39 and the back metal contact
(PS-MIS configuration). The conduction band edge and thickness of
the charge separation layer and the semiconductor type are chosen
to allow charge carriers from the photoexcited state of the
photosensitizer to move to the back contact and prevent current
flow in the opposite direction.
[0049] In accordance with another alternative embodiment as shown
in FIG. 4, the photosensitizer layer 10 is replaced with a layer of
photoactive material 40 comprising of clusters of atoms or
molecules, including doped or quantum structures (quantum wells,
nanoparticles, quantum dots, etc.), with dimensions engineered to
maximize light absorbency and ballistic electron transfer. One
advantage of this alternative embodiment is that the charged
electrons transferred need not move into or out of an atomic or
molecular system, which is the case when using a photosensitive
dye. Rather, the electrons travel in and out of degenerate levels
with less hindrance due to quantum state restrictions. A specific
example would be the deposition of CdSe or CdS nanoparticles
(.about.5 nm in dimension) on the conductor surface. These
semiconductor particles have been shown to have efficient capture
and efficient transfer to semiconductors. Interposing the conductor
ballistic transport will still allow charge transfer; however, the
particle can now be supplied with compensation charge directly from
the conductor.
[0050] In accordance with another alternative embodiment of the
present invention as shown in FIG. 5, the electrons 36 of the
photosensitizer layer 10 do not ballistically transport through the
front conducting layer 31. Rather, as the excited electrons 36
relax back to lower energy states, energy released from electrons
36 excites electrons 50 that reside in the front conducting layer
31. The excited electrons 50 may thereafter rise above the
conduction energy band 38 and flow towards the back conducting
layer 30.
[0051] In yet another alternative embodiment as shown in FIG. 6,
the front conducting layer 31 is selected from among either
conductors that have transparency characteristics, such as indium
tin oxide, or semi-transparent conductors (e.g., ultra-thin metal).
In this embodiment the photosensitizer layer can be deposited
between the front conducting layer 31 and the charge separation
layer 39, thereby eliminating the need for ballistic transport of
the electrons 36, while still maintaining the tenability of the
barrier height.
[0052] In accordance with another alternative embodiment of the
present invention as shown in FIG. 7, a doped semiconducting layer
70 having a doping type opposite that of the charge separation
layer 39 is placed between the front conducting layer 31 and the
charge separation layer 39. This particular embodiment effectively
increases the Schottky barrier level and thus the open circuit
voltage of the photovoltaic device as has been demonstrated in
conventional Schottky Barrier Solar Cells.
[0053] FIG. 8 shows yet another alternative embodiment of the
present invention wherein the charge carriers are ballistic holes
rather than electrons. The above-described operating principles of
the preferred embodiment (shown in FIG. 3) are symmetrically
applied in this instance.
[0054] FIG. 9 shows yet another alternative embodiment of the
present invention wherein the charge separation layer 39 is made of
a material having a narrow band-gap energy level (i.e., the
conduction band energy level is close to the valence band energy
level). The narrow band-gap property of the charge separation layer
allows for excitation of additional electrons 90 from the
underlying semiconductor material (as in a conventional Schottky
diode solar cell). The internal emission supplements the
photoexcitation of the photosensitizer layer 10 and thereby
produces additional energy.
[0055] FIG. 10 shows yet another alternative embodiment of the
present invention wherein an anti-reflection coating (ARC) layer
100 is added to the photosensitizer layer so as to increase the
absorbency of the photosensitizer layer and reduce the reflection
of incident light by keeping the photons within the structures. The
detailed design of these coatings is well-established
technology.
[0056] FIG. 11 shows a multilayer structure wherein multiple
structures of the preferred embodiment as shown in FIG. 3 is
deposited in a parallel fashion, separated by transparent spacer
112, to produce a superstructure that provides increased absorbency
and efficiency in producing photovoltaic energy. Although FIG. 11
shows a parallel combination of the preferred embodiment, it should
be noted that a serial combination is also possible and
feasible.
[0057] FIG. 12 shows an alternative embodiment where the absorption
of photo energy and injection of electrons may be performed with
different molecules or structures. More specifically, the photons
are absorbed in one or more photoactive molecules or structures 120
and relay their charge carriers 122 to a second layer or structure
121 with more efficient injection properties. This mimics natural
photosynthetic processes whereby multiple pigments are used to more
efficiently capture sunlight and relay the excited charges to
common collectors for further transport.
[0058] FIG. 13 shows an alternative embodiment where absorption
occurs in a quantum well 131 deposited on the surface. The
dimensions of the quantum well and the properties of the material
are chosen to optimally inject the charges.
[0059] FIG. 14 shows an embodiment where absorption occurs in
structure or molecule partially isolated from the conductor to
reduce coupling for optimal charge transfer. Examples include metal
oxides, silicon dioxide, titanium dioxide, aluminum dioxide,
organic chains and self-assembled monolayers deposited on the
surface prior to the photoabsorber. For example, a thin layer of
titanium dioxide (.about.1-5 nm) is deposited on the conductor
(Au). The photoactive merbromin is applied and forms a covalent
linkage through its active carboxylate moiety to the titanium
(C--O--Ti).
[0060] As previously discussed, in fabricating a device in
accordance with the preferred embodiment, a charge separation layer
39 of titanium dioxide is deposited onto titanium foil (the ohmic
back contact 30). The charge separation layer 39 has a thickness
ranging between 100 nm and 500 nm and is deposited by electron beam
evaporation and/or by electroanodization of the titanium metal.
Gold is then deposited to the composite layer to a thickness of 10
nm to form the ultra-thin conductor. The resulting current voltage
curves of the Schottky contact are shown in FIG. 18. Also shown in
FIG. 18 for comparison are devices using nickel instead of gold as
the ultra-thin conducting layer 31. An approximately 0.8 eV barrier
results.
[0061] In accordance with the alternative embodiment of FIG. 14, 2
nm of titanium dioxide is deposited onto the above-mentioned metal
conductor 31 as a partial isolation layer. Photoactive merbromin is
then applied and bonded covalently through its active carboxylate
moiety to the titanium (C--O--Ti) to complete the active
device.
[0062] FIG. 15 shows an alternative embodiment comprising a polymer
based device wherein a ballistic hole is injected into an
ultra-thin hole carrier. Polymer A in FIG. 15, (e.g.,
poly(p-phenylene vinylene), PPV) with its highest occupied
molecular orbital (HOMO) level lower in energy than the HOMO of a
second polymer (B in FIG. 15) hole conductor layered behind it. The
PPV provides a barrier to reverse hole transport serving the same
role as the Schottky barrier. More traditional Schottky barrier
devices have also been fabricated from polymer semiconductors and
would be configured as in the above embodiments.
[0063] Although preferred embodiments of the invention are
illustrated in the Drawings and described in the Detailed
Description, it will be understood that the invention is not
limited to the embodiments disclosed, but is capable of numerous
modifications and rearrangements of parts and elements without
departing from the spirit of the invention.
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