U.S. patent application number 12/108976 was filed with the patent office on 2008-10-30 for hybrid photovoltaic cells and related methods.
This patent application is currently assigned to NANOCO TECHNOLOGIES LIMITED. Invention is credited to James Harris, Nigel Pickett.
Application Number | 20080264479 12/108976 |
Document ID | / |
Family ID | 39672081 |
Filed Date | 2008-10-30 |
United States Patent
Application |
20080264479 |
Kind Code |
A1 |
Harris; James ; et
al. |
October 30, 2008 |
Hybrid Photovoltaic Cells and Related Methods
Abstract
Embodiments of the present invention involve photovoltaic (PV)
cells comprising a semiconducting nanorod-nanocrystal-polymer
hybrid layer, as well as methods for fabricating the same. In PV
cells according to this invention, the nanocrystals may serve both
as the light-absorbing material and as the heterojunctions at which
excited electron-hole pairs split.
Inventors: |
Harris; James; (Manchester,
GB) ; Pickett; Nigel; (East Croyden, GB) |
Correspondence
Address: |
GOODWIN PROCTER LLP;PATENT ADMINISTRATOR
EXCHANGE PLACE
BOSTON
MA
02109-2881
US
|
Assignee: |
NANOCO TECHNOLOGIES LIMITED
|
Family ID: |
39672081 |
Appl. No.: |
12/108976 |
Filed: |
April 24, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60926103 |
Apr 25, 2007 |
|
|
|
Current U.S.
Class: |
136/255 ;
438/94 |
Current CPC
Class: |
Y02E 10/549 20130101;
Y02P 70/50 20151101; H01L 51/0006 20130101; H01L 51/422 20130101;
H01L 51/0036 20130101; H01L 51/426 20130101; Y02P 70/521 20151101;
H01L 31/035227 20130101 |
Class at
Publication: |
136/255 ;
438/94 |
International
Class: |
H01L 31/00 20060101
H01L031/00; H01L 21/00 20060101 H01L021/00 |
Claims
1. A photovoltaic cell comprising: a. first and second electrodes;
b. a plurality of aligned semiconducting nanorods disposed between
the electrodes, each nanorod being electrically connected to the
first electrode and electrically insulated from the second; c. a
plurality of photoresponsive nanocrystals surrounding and bound to
the nanorods; and d. a semiconductor polymer surrounding the
nanorods and bound to the nanocrystals and to at least the second
electrode, whereby the nanocrystals act as heterojunctions
channeling a first charge carrier into the nanorods and a second
charge carrier into the polymer.
2. The cell of claim 1 wherein (i) the polymer is a hole-transfer
polymer, (ii) the first charge carrier is electrons, and (iii) the
second charge carrier is holes.
3. The cell of claim 2 wherein the polymer is
poly(3-hexylthiophene), polyphenylenevinylene or a derivative
thereof, or polyfluorene or a derivative thereof.
4. The cell of claim 1 wherein the nanorods are wide-bandgap
semiconductors.
5. The cell of claim 4 wherein the nanorods comprise at least one
of ZnO, SnO, and/or TiO.sub.2.
6. The cell of claim 1 wherein the nanorods are single-crystal
nanorods.
7. The cell of claim 1 wherein the nanorods have an aspect ratio of
at least 3.
8. The cell of claim 1 wherein the nanorods are bound to the
nanocrystal by a bifunctional capping agent.
9. The cell of claim 8 wherein the capping agent is mercaptoacetic
acid.
10. The cell of claim 1 wherein absorption of light by a
nanocrystal results in production of an exciton, the nanocrystal
having a largest spatial dimension no greater than an average
diffusion distance of the exciton.
11. The cell of claim 1 wherein the nanocrystals comprise at least
one of CuInSe.sub.2, CuInS.sub.2, CuIn.sub.1-xGa.sub.xSe.sub.2,
GaAs, InAs, InP, PbS, PbSe, PbTe, GaSb, InSb, CdTe and CdSe,
wherein 0.ltoreq.x.ltoreq.1.
12. The cell of claim 1 wherein the nanocrystals have extinction
coefficients of at least 100,000 M.sup.-1cm.sup.-1.
13. The cell of claim 1 wherein the semiconductor polymer is bound
to the nanocrystals but not to the nanorods.
14. A method of fabricating a semiconductor structure comprising
heterojunctions and being suitable for use in a photovoltaic cell,
the method comprising the steps of: a. providing a plurality of
nanorods and a plurality of photoresponsive nanocrystals capped
with a first capping agent; b. exposing the nanorods or the
nanocrystals to a second, bifunctional capping agent; c. thereafter
combining the nanocrystals with the nanorods, whereby the
nanocrystals bind to the nanorods via the bifunctional capping
agent; d. combining the bound nanorods and nanocrystals with a
functionalized monomer having a binding group, the binding group
(i) exhibiting a stronger affinity for the nanocrystals than the
first capping agent and (ii) exhibiting a weaker affinity for the
nanorods than the bifunctional capping agent, whereby the monomer
preferentially displaces the first capping agent so as to bind to
the nanocrystals but not to the nanorods; and e. polymerizing the
monomer.
15. The method of claim 14 further comprising the step of disposing
the nanorods between the first and second electrodes, the nanorods
each having one end in electrical contact with the first electrode,
and being electrically insulated from the second electrode through
a thin polymer layer at the other end.
16. The method of claim 14 further comprising, before step (a), the
step of growing a plurality of nanorods on a substrate providing
the first electrode.
17. The method of claim 16 further comprising, after step (e), the
step of depositing the second electrode, the second electrode being
electrically insulated from the nanorods through a thin polymer
layer.
18. The method of claim 14 wherein step (a) involves providing a
nanocrystal capped with a first capping agent containing at least
one binding functionality selected from the group consisting of
thiol, selenol, amine, phosphine, phosphine oxide, or an aromatic
heterocycle.
19. The method of claim 18 wherein the first capping agent is
octanethiol.
20. The method of claim 14 wherein step (b) comprises capping the
nanorods with the second capping agent, the first capping agent
having a weaker affinity for the nanocrystals than the second
capping agent, whereby the second capping agent preferentially
displaces the first capping agent so as to bind to the
nanocrystal.
21. The method of claim 14 wherein step (b) comprises exposing the
nanocrystals to the second capping agent, the second capping agent
replacing some but not all of the first capping agent on the
nanocrystals, whereby in step (c) the free ends of the second
capping agent bind to the nanorods.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefits of U.S.
Provisional Application Ser. No. 60/926,103, filed on Apr. 25,
2007, the entire disclosure of which is hereby incorporated by
reference.
FIELD OF THE INVENTION
[0002] The present invention relates to solar cells and their
fabrication, and in particular to nanorod-nanocrystal-polymer
hybrid solar cells
BACKGROUND
[0003] To create useful electrical current from electromagnetic
radiation, photovoltaic (PV) cells must absorb incident radiation
such that an electron is promoted from the valence band to the
conduction band (leaving a hole in the valence band), and must be
able to separate the electron and hole and deliver these charge
carriers to their respective electrodes before they recombine.
[0004] Many different strategies based on diverse materials have
been employed, with varying degrees of success, to realize these
basic behaviors with commercially satisfactory efficiency.
Representative devices include crystalline inorganic solar cells
(e.g., silicon, germanium, GaAs), nanocrystalline dye-sensitized
solar cells, semiconductor-polymer solar cells, nanoparticle solar
cells, and more recently, composite solar cells that incorporate
and combine the aforementioned components from other
strategies.
1. Inorganic Photovoltaics
[0005] Silicon is by far the most commonly used material for
fabricating inorganic photovoltaics. These cells rely on the
ability of silicon to absorb light and, consequently, to generate
an excited electron-hole pair that is then separated at a p-n
junction. The electric field set up by the p-n junction facilitates
this separation because of the way electrons and holes move through
materials: electrons move to lower energy levels while holes move
to higher energy levels.
[0006] Creation of p-n junctions generally involves
high-temperature processing in inert atmospheres to form very pure,
crystalline silicon wafers, which are inflexible and expensive.
Because silicon is an indirect semiconductor, a relatively thick
layer is typically needed to achieve a good level of absorption,
which increases material costs further. Efficiencies for the most
pure (and expensive) silicon photovoltaics are on the order of 20%;
efficiencies for the cheaper amorphous silicon cells are
approximately 5-10%.
[0007] Today's commercial PV systems can convert from 5% to 15% of
sunlight energy into electricity. These systems are highly reliable
and generally last 20 years or longer. The possibility of
fabricating solar cells by less expensive, lower-temperature
techniques is very attractive. Accordingly, nanocrystalline
dye-sensitized solar cells (DSSCs), semiconductor-polymer solar
cells and nanoparticle solar cells have enjoyed widespread
interest.
2. Polymer Photovoltaics
[0008] Semiconducting polymers can be used to make organic
photovoltaics. The properties of these polymers can be tuned by
functionalization of the constituent monomers. As such, a wide
range of polymers with suitable bandgaps, absorption
characteristics and physical properties is available. In order to
achieve separation of the electron-hole pair, organic photovoltaics
rely on donor-acceptor heterojunctions. In polymers, the
excited-state electron and hole are bound together, and travel
together, as a quasi-particle called an exciton. They remain
together until they encounter a heterojunction, which separates
them. Unfortunately, excitons are very short-lived and can only
travel about 10 nm before recombining. Hence, any photon absorbed
more than this diffusion length away from a heterojunction will be
wasted. Charge mobilities for polymers are typically low (0.5-0.1
cm.sup.2V.sup.-1s.sup.-1) compared to silicon, which is much higher
(1500 cm.sup.2V.sup.-1S.sup.-1). Current state-of-the-art polymer
photovoltaic cells have efficiencies of 1-2%. Although such
efficiencies are low, these materials hold promise for low-cost,
flexible solar cells.
3. Nanoparticle Photovoltaics
[0009] Inorganic nanoparticles (or nanocrystals) have been used to
prepare colloidal, thin-film PV cells that show some of the
advantages of polymer photovoltaics while maintaining many of the
advantages of inorganic photovoltaics. For example, such cells can
contain a bi-layer structure comprising a layer of donor and a
layer of acceptor nanoparticles, wherein the two layers exhibit
little intermixing, and both contribute to the measured
photocurrent. The strong photoconductive effect exhibited by these
devices suggests that these materials have a large number of
trapped carriers and are better described by a donor-acceptor
molecular model than by a p-n band model. Increased bandgap energy
compared to that of the bulk semiconductors minimizes the number of
carriers available, and spatial separation of the donor and
acceptor particles in different phases traps the excitons so that
they must split at the donor-acceptor heterojunction. There is no
band-bending, so splitting of the exciton is more difficult.
[0010] It should be stressed that simply blending the donor and
acceptor nanoparticles together will not create a film that
produces a photovoltage. The lack of selectivity at the electrode
towards one particle or another means that the electrodes can make
contact with both the donor and acceptor species. These species may
take the form of nanorods rather than nanospheres because nanorods
with high aspect ratios help to disperse the carriers. Quick
transfer of the exciton along the length of the nanorods improves
the chance of splitting the exciton at the donor-acceptor
heterojunction.
[0011] Solution processing of, for example, CdSe rods can achieve a
size distribution of 5% in diameter and 10% in length with an
aspect ratio of 20 and a length of 100 nm. The substantial control
available through solution processing allows for optimization of
the cell by variation of nanorod length and bandgap energy.
[0012] 4. Polymer-Nanocrystal Composite Photovoltaics
[0013] The combination of nanomaterials and polymer films has been
shown to give good power conversion efficiencies while affording
low-temperature solution processes for fabrication. In one
approach, nanomaterials are used to conduct charges while the
polymer is used as the absorbing material, or alternatively, the
nanomaterial serves as a chromophore, i.e., the light absorber, and
the semiconductor polymer is employed as a hole conductor. In the
former case, a wide-bandgap semiconductor (e.g., TiO.sub.2)
receives the excited electron from the conduction band of the
chromophoric polymer semiconductor; and in the latter case,
light-absorbing semiconductor nanocrystals absorb photons and
transfer the resulting negative charge to the transparent primary
electrode, while the semiconducting polymer transfers the holes to
the counter electrode. In both types of cell, a heterojunction
between the nanocrystal and the polymer separates the exciton
created in the nanocrystal or polymer. The electron is transferred
to the conduction band of the nanocrystal and the hole stays in the
valence band of the polymer, or the electron stays in the
conduction band of the nanocrystal, and the hole is transferred to
the valence band of the polymer.
[0014] 4.1 Wide-Bandgap Nanocrystal/Light-Absorbing Polymer
[0015] The active layer in a polymer-nanocrystal cell has two
components: a light absorber and a nanoparticulate electron
carrier. Typically, the light absorber is a p-type polymeric
conductor, e.g., poly(phenylene vinylene) or
poly(3-hexylthiophene), and the nanoparticulate electron carrier is
a wide-bandgap semiconductor such as ZnO or TiO.sub.2. In this
configuration, the polymer serves to absorb light, to transfer
electrons to the electron acceptor/carrier, and to carry holes to
the primary electrode. The electron acceptor accepts electrons and
transfers the electrons to the metal back contact.
[0016] The morphology of the phase separation is crucial. For
example, a bi-layer structure in which each layer has only one
component results in a cell with poor performance. The reason is
that the lifetime of the excited state of the light-absorbing
polymer is generally shorter than the transfer rate of the exciton
to the interface, and, consequently, the majority of the excitons
formed in the bulk of the polymer never reach the interface
separating electrons and holes, resulting in loss of photocurrent.
Morphologies in which a bulk heterojunction is formed tend to show
greater efficiencies. If the absorber and electron acceptor are in
intimate contact throughout the entire active layer, the shorter
exciton path length will result in increased electron transfer and
higher efficiencies. The best efficiencies obtained from cells of
this configuration are around 2%.
[0017] This technology shows promise, but there are obstacles to
overcome. One problem is incomplete absorption of the incident
radiation. The polymer -which absorbs light very strongly and is
referred to as a polymeric dye -has a large extinction coefficient
(>100,000 M.sup.-1cm.sup.-1), but due to low exciton migration
rates, the films must generally be thinner than 100 nm, which
contributes significantly to incomplete absorption. This effect can
be combated by means of an interdigitated array structure of donor
and acceptor species.
[0018] 4.2 Wide-Bandgap Nanocrystal/Light-Absorbing
Nanocrystals/Hole Transfer Polymer
[0019] A problem associated with the light-absorbing polymer
strategy is underutilization of available solar energy due to the
narrow absorption bandwidth of typical polymers. Approximately 40%
of the light (from about 600 nm out into the near IR) can be
wasted. An alternative configuration is to utilize nanocrystals as
light absorbers and electron carriers, and employ the polymer as a
light absorber and a hole carrier. CdSe nanorod and
tetrapod/polymer systems have demonstrated power-conversion
efficiencies of up to 1.7%. These systems have the advantage that
the absorption of the nanocrystal can be tuned via the size of the
nanocrystal, and systems that absorb essentially all of the
incoming radiation can therefore be fabricated.
[0020] Unfortunately, it is difficult to disperse inorganic
nanocrystals into a solution of monomers. The two phases tend to
agglomerate and minimize the electrical contact essential to form
the heterojunction which enables charge separation. Dispersion of
nanocrystals in polymer phases is an area of great interest.
[0021] Typically, the strategy employed for dispersing the
nanocrystals is to functionalize the nanocrystal with a capping
agent that has an organic tail, which enhances solubility in the
solvent in which the polymerization is carried out. Capping agents
for this purpose typically have a head-group with a strong affinity
for the nanocrystal; amine, carboxylate, phosphine, thiol,
phosphine oxide and phosphonic acid, for example, all bind
strongly. The organic tail of the capping agent should be
compatible with solvents in which the polymer is soluble. Long
hydrocarbon chains typically provide high solubility but are
non-conducting; accordingly, it is necessary to balance optimum
solubility against conductivity.
[0022] The most popular polymers used for composite studies are
PDFC, P3Ht and MEH-PPV (where PDFC refers to
-{poly[9,9-dihexylfluorenyl-2,7-diyl)-alt-co-(9-ethyl-3,6-carbazole)]}-,
P3Ht refers to poly(3-hexylthiophene), and MEH-PPV refers to
poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene)). Each
of these polymers has sites for functionalization, allowing the
manipulation of the valence/conduction band energies to achieve
optimal conditions for charge transfer to and from the
nanocrystals. It has been suggested that the capping agent may also
serve as the organic acceptor phase; for example, P3HT
functionalized with phosphonic acid groups has been shown to
isolate CdSe nanocrystals.
5. Dye-Sensitized Solar Cells
[0023] DSSCs incorporate a substrate which has been coated with a
transparent conducting oxide (which serves as the primary
electrode). The counter electrode may also be coated with a
transparent conducting oxide, but may also be a non-corrosive
metal, such as titanium coated with a very thin layer of platinum.
A porous layer of a wide-bandgap semiconductor (such as TiO.sub.2)
is deposited on the conductive surface of the primary electrode.
This porous layer is then coated with a dye having a strong
absorption in the visible region of the spectrum. To be optimally
effective, the dye concentration should be limited to a monolayer
of dye molecules. Because of this, a huge surface area is necessary
to accommodate enough dye to absorb all of the incoming light.
Therefore, nanocrystals (e.g., TiO.sub.2) are used to make the
highly porous films. Electrolyte containing a redox couple
(typically I.sup.-/I.sub.3.sup.-) is absorbed into the titania
layer. To complete the cell, the substrate bearing the primary
electrode and the sensitized titania layer is brought into
face-to-face contact with the counter electrode.
[0024] Typical dyes are inorganic-ruthenium-based, although organic
dyes are receiving increased interest. The dye absorbs visible
light, and the excited state injects an electron into the TiO.sub.2
conduction band. Before back electron transfer can occur, the
oxidized dye is reduced by a redox active species in solution
(typically I.sup.-/I.sub.3.sup.-), regenerating the dye. The
oxidized redox active species diffuses to the counter electrode,
where it is reduced, finishing the cycle and completing the
circuit. Work can be done by passing the injected electron through
an external load before allowing it to reduce the oxidized redox
active species at the counter electrode.
[0025] Inexpensive DSSC devices, which exhibit up to 10% energy
conversion efficiency, can be fabricated. There are many issues to
be addressed with this technology to improve performance and
stability, including replacing the best performing liquid
electrolytes with solid-state or higher-boiling electrolytes;
improving spectral overlap; using a redox mediator with a lower
redox potential; and lowering recombination losses due to poor
electron conduction through the nanoparticle TiO.sub.2 layer.
6. Hybrid Cells
[0026] Hybrid cells combine dye-sensitized titania, coated and
sintered onto a transparent semiconducting oxide, with a p-type
polymer that carries electrons to the oxidized dye. Since just one
polymer replaces the multi-component electrolyte, these cells can
be fabricated conveniently and reproducibly. Ruthenium
dye-sensitized, nanorod-based DSSCs tend to exhibit low efficiency,
however, because the lower surface area does not accommodate enough
dye to absorb all of the incident light. The most efficient dyes
found so far only have extinction coefficients on the order of
.about.20,000 M.sup.-1cm.sup.-1, and therefore a large surface area
is needed to bind enough dye to get maximal absorbance.
SUMMARY OF THE INVENTION
[0027] Aspects of the present invention provide a photovoltaic (PV)
cell comprising a semiconducting nanorod-nanocrystal-polymer hybrid
layer, as well as methods for fabricating the same. In PV cells
according to this invention, the nanocrystals serve both as the
light-absorbing material and as the heterojunctions at which the
excited electron-hole pairs (i.e., excitons) split. The nanorods
function as electron carriers and are electrically connected to the
anode of the cell, and the polymer acts as the hole carrier and is
electrically connected to the cathode of the cell.
[0028] One of the advantages of the invention lies in the use of
small particles, the nanocrystals, as both light absorber and
heterojunction. The resulting spatio-temporal proximity of exciton
generation and splitting entails a significant reduction in
recombination losses, compared, for example, with those of
conventional polymer PV cells, and consequently in higher
conversion efficiencies of photons into electricity. Embodiments of
the invention offer the additional advantages of mechanical
flexibility and low cost manufacturing processes.
[0029] Accordingly, in a first aspect, the invention provides a
photovoltaic cell containing two electrodes and, in between these
electrodes, a plurality of aligned semiconducting nanorods
surrounded by and bound to a plurality of photoresponsive
nanocrystals, and a semiconductor polymer surrounding the nanorods
and bound to the nanocrystals. The nanocrystals act as
heterojunctions channeling electrons into the nanorods and holes
into the polymer, or vice versa. The nanorods are electrically
connected to the first electrode, and electrically insulated from
the second electrode by a thin layer of polymer bound to the second
electrode. In various embodiments, the polymer is a hole-transfer
polymer, and consequently, the nanocrystals channel holes into the
polymer and electrons into the nanorods. In various embodiments,
the nanocrystals are bound to the nanorods by a bifunctional
capping agent, which can, for example, be mercaptoacetic acid. For
example, the nanorods may be grown on the first electrode, and the
other electrode can later be deposited on the
nanorod-nanocrystal-polymer layer in a manner ensuring insulation
of the nanorods from the second electrode.
[0030] Advantageous nanorods have aspect ratios (i.e., ratios of
the longest dimension to the shortest dimension of the particle) of
at least 3, and their shortest dimension is not greater than 100
nm. Preferred nanorods are single-crystalline. Suitable nanorod
materials according to the invention include, but are not limited
to, wide bandgap semiconductors such as, for example, ZnO, SnO, and
TiO.sub.2, whereby ZnO is the preferred material.
[0031] Suitable nanocrystals according to the invention include
semiconducting, monocrystalline or polycrystalline nanoparticles of
diameter not greater than 20 nm, which may (but need not) be
generally spherical in shape. Suitable nanocrystal materials
include, but are not limited to CuInSe.sub.2, CuInS.sub.2,
CuIn.sub.1-xGa.sub.xSe.sub.2 (where 0.ltoreq.x.ltoreq.1), GaAs,
InAs, InP, PbS, PbSe, PbTe, GaSb, InSb, CdTe and CdSe. Nanocrystals
with extinction coefficients of at least 100,000 M.sup.-1 cm.sup.-1
are preferred. In various embodiments, the largest spatial
dimension of the nanocrystals is no greater than the average
diffusion distance of the excitons created in the nanocrystal upon
absorption of light.
[0032] Suitable polymer materials include, but are not limited to,
poly(3-hexylthiophene), polyphenylenevinylene (PPV) and its
derivatives, and polyfluorene (PFO) and its derivatives. In various
embodiments, the polymer is bound to the nanocrystals but not to
the nanorods.
[0033] In a second aspect, the invention provides a method of
fabricating a semiconductor structure with heterojunctions; the
structure can be used in a photovoltaic cell. Embodiments of the
method involve providing a plurality of nanorods and a plurality of
photoresponsive nanocrystals capped with a first capping agent;
exposing the nanorods or the nanocrystals to a second, bifunctional
capping agent; then combining the nanocrystals with the nanorods so
that the nanocrystals bind to the nanorods via the bifunctional
capping agent; combining the bound nanorods and nanocrystals with a
functionalized monomer which has a binding group with (i) stronger
affinity for the nanocrystals than the first capping agent and (ii)
weaker affinity for the nanorods than the bifunctional capping
agent, so that the monomer preferentially displaces the first
capping agent and binds to the nanocrystals; and polymerizing the
monomer. The bifunctional capping agent can first bind to the
nanorods, and then bind to the nanocrystals, replacing some of the
first capping agent. Alternatively, the bifunctional capping agent
can first bind to the nanocrystals (replacing some of the first
capping agent), and then bind with its free ends to the nanorods.
In various embodiments, the first capping agent contains a thiol,
selenol, amine, phosphine, phosphine oxide, and/or aromatic
heterocycle functionality. A non-limiting example of a suitable
capping agent is octanethiol.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] The foregoing discussion will be understood more readily
from the following detailed description of the invention when taken
in conjunction with the accompanying drawings.
[0035] FIG. 1A schematically depicts an embodiment of a
nanorod-nanocrystal-polymer hybrid solar cell according to the
invention.
[0036] FIG. 1B is an enlarged schematic view of the three major
components of the hybrid semiconductor layer of FIG. 1A, and of
their interconnections.
[0037] FIG. 2A is a flow diagram detailing a method of fabricating
the structure depicted in FIG. 1A according to one embodiment.
[0038] FIG. 2B is a flow diagram detailing a method of fabricating
the structure depicted in FIG. 1A according to an alternative
embodiment.
[0039] FIG. 3 illustrates at a microscopic level some of the steps
of the method shown in FIG. 2 and the resulting products.
DETAILED DESCRIPTION OF THE INVENTION
1. Nanorod-nanocrystal-polymer Hybrid Structure
[0040] In polymer-based photovoltaics, excitons travel on average
of the order of 10 nm before recombining; accordingly, there is a
need to separate the excitons, i.e., to have them encounter a
heterojunction as soon as possible. This need is met in embodiments
of the present invention, in which nanocrystals (quantum dots)
serve as a bridge between a hole-transfer polymer and a
wide-bandgap semiconductor electron acceptor, thus constituting the
heterojunction, and serve simultaneously as the light absorber,
i.e., the place where the excitons are created. The diameter of a
nanocrystal according to the invention is approximately equal to,
or smaller than, the diffusion distance of an exciton. As a result,
an exciton generated in the nanocrystal will generally encounter
the interface of the nanocrystal with the electron acceptor or the
hole-transfer polymer within its average diffusion distance,
regardless of the direction in which it migrates. Consequently, the
exciton splits very efficiently, and recombination within the
nanocrystal occurs infrequently. The electron enters into the
wide-bandgap semiconductor, and the hole enters into the
polymer.
[0041] The structure of a PV cell 100 according to the invention is
illustrated in FIG. 1A. In between two electrodes, an anode 101 and
a cathode 103, a plurality of aligned wide-bandgap semiconductor
nanorods 106, which constitute the electron acceptor, is arranged.
As shown in the detail of FIG. 1A, the nanorods 106 are each
surrounded by photosensitive nanocrystals 109. The sensitized
nanorods, in turn, are surrounded by the hole-transfer polymer 112,
which fills the remaining space between the electrodes 101, 103.
The polymer 112 also forms a thin layer underneath the cathode 103,
which electrically isolates the cathode 103 from the sensitized
nanorods 106.
[0042] FIG. 1B shows how these three components are interconnected
in preferred embodiments of the invention. The nanocrystals 109 are
bound to the nanorods 106 by means of a bifunctional binding
molecule 115. In various embodiments, the bifunctional capping
agent 115 has thiol and carboxylate moieties. The thiol groups bind
preferentially to the nanocrystals 109, and the carboxylate groups
bind preferentially to the (metal oxide) nanorods 106. The
intervening chain should be short enough so that charge transfer
from nanocrystal 109 to nanorod 106 is not impeded. A
representative bifunctional capping agent 115 is mercaptoacetic
acid. The hole-transfer polymer 112 is directly bound to the
nanocrystals 109, but preferably not to the nanorods 106.
[0043] A representative, non-limiting example of a system of
nanorods, bifunctional molecules, nanocrystals, and polymers
comprises ZnO nanorods capped with mercaptoacetic acid,
CuInSe.sub.2 quantum dots, and poly(3-hexylthiophene).
[0044] 1.1 Nanocrystals 109
[0045] The semiconductor material used for nanocrystals in a
particular application depends on the suitability of valence and
conduction band energy levels. The conduction band should be of
sufficient energy to be able to inject electrons efficiently into
the nanorods, while the valence band should be of sufficiently low
energy to inject holes into the polymer valence band. The latter
constraint is generally straightforward to satisfy, as suitable
polymers having a higher-energy valence band than the nanocrystal
can readily be identified. Subject to the above constraints, the
bandgap of the nanocrystal should be small enough to allow for a
large portion of the solar spectrum to be absorbed. Suitable
nanocrystal materials include materials based on
copper-indium-diselenide and variants thereof, for example,
CuInS.sub.2, CuInSe.sub.2, or CuIn.sub.1-xGa.sub.xSe.sub.2 (wherein
0.ltoreq.x.ltoreq.1), as well as CdSe, GaAs, InAs, and InP.
[0046] Nanocrystals can be synthesized using techniques described,
for example, in U.S. Pat. No. 6,379,635 and co-pending U.S. patent
application Ser. Nos. 11/579,050 and 11/588,880, the entire
contents of which are hereby incorporated by reference.
[0047] A method for producing CIGS nanocrystals of any desirable
stoichiometry employing a selenol compound is disclosed in U.S.
Provisional Application Ser. No. 60/991,510, the entire content of
which is hereby incorporated by reference. Embodiments of the
method involve dispersing at least a first portion of a nanocrystal
precursor composition (comprising sources of at least one of Al,
Ga, and/or In, and at least one of Cu, Ag, Zn, and/or Cd) in a
solvent (e.g., a long-chain hydrocarbon solvent); heating the
solvent to a first temperature for an appropriate length of time;
adding a selenol compound to the solvent and heating the solvent;
adding a second portion of the nanocrystal precursor composition to
the reaction mixture; heating the mixture to a second temperature
higher than the first temperature over an appropriate length of
time; and maintaining the temperature for up to 10 hours. Once the
particles have been formed, the surface atoms of the particles will
typically be coordinated to a capping agent, which can comprise the
selenol compound employed in the method. If a volatile selenol
compound is used, this capping agent can be driven off with heating
to yield `naked` nanocrystals amenable to capping with other
coordinating ligands and further processing. Examples 1 and 2
provide further details regarding the implementation of this
method:
[0048] Example 1: Cu(I) acetate (1 mmol) and In(III) acetate (1
mmol) are added to a clean and dry RB-flask. Octadecene ODE (5 mL)
is added the reaction mixture heated at 100.degree. C. under vacuum
for 30 mins. The flask is back-filled with nitrogen and the
temperature raised to 140.degree. C. 1-octane selenol is injected
and the temperature falls to 120.degree. C. The resulting orange
suspension is heated with stirring and a transparent orange/red
solution is obtained when the temperature has reached 140.degree.
C. This temperature is maintained for 30 minutes, then IM
tri-octyl-phoshine selenide TOPSe (2 mL, 2 mmol) is added dropwise
and the solution heated at 160.degree. C. The PL is monitored until
it reaches the desired wavelength, after which it is cooled and the
resulting oil washed with methanol/acetone (2:1) 4-5 times and
finally isolated by precipitation with acetone.
[0049] Example 2 (Large Scale Production): A stock solution of
TOPSe was prepared by dissolving Se powder (10.9, 138 mmol) in TOP
(60 mL) under nitrogen. To dry, degassed ODE was added Cu(I)
acetate (7.89 g, 64.4 mmol) and In(III) acetate (20.0 g, 68.5
mmol). The reaction vessel was evacuated and heated at 140.degree.
C. for 10 min, backfilled with N.sub.2 and cooled to room temp.
1-Octane selenol (200 mL) was added to produce a bright orange
suspension. The temperature of the flask was raised to 140.degree.
C. and acetic acid distilled from the reaction at 120.degree. C. On
reaching 140.degree. C. the TOPSe solution was added dropwise over
the course of 1 hour. After 3 hours the temperature was raised to
160.degree. C. The progress of the reaction was monitored by taking
aliquots from the reaction periodically and measuring the
UV/Visible and photoluminescence spectra. After 7 hours the
reaction was cooled to room temperature and the resulting black oil
washed with methanol. Methanol washing was continued until it was
possible to precipitate a fine black material from the oil by
addition of acetone. The black precipitate was isolated by
centrifugation, washed with acetone and dried under vacuum. Yield:
31.97 g.
[0050] For the purpose of optimizing the composition, size, and
charge of the nanocrystals, they can be characterized by
conventional techniques, including, but not limited to, XRD,
UV/Vis/Near-IR spectrometry, SEM, TEM, EDAX, photoluminescence
spectrometry, and elemental analysis.
[0051] Some embodiments of the invention utilize nanocrystals with
extinction coefficients of at least 100,000 M.sup.-1cm.sup.-1. At
such high absorptivities, fewer nanocrystals are needed to achieve
the same overall absorption. Consequently, embodiments of this
invention based on these nanocrystals can benefit from increased
absorption without incurring losses in efficiency due to enhanced
recombination.
[0052] 1.2 Nanorods 106
[0053] Nanorods can be produced by direct chemical synthesis,
utilizing a suitable combination of ligands such as
trioctylphosphine oxide (TOPO) and various phosphonic acids, e.g.,
octadecylphosphonic acid, for shape control. Moreover, different
types of metal oxides can be grown in ordered nanorod arrays, using
techniques such as, for example, electrochemical etching of metal
foil, or substrate seeding followed by nanorod growth, in a
chemical bath, in a direction perpendicular to the substrate. See,
e.g., D. C. Olson et al., J. Phys. Chem. C, 2007, 111, 16640-16645;
and J. Yang et al., Crystal Growth & Design, 2007, 12/2562, the
disclosures of which are hereby incorporated by reference in their
entireties.
[0054] In preferred embodiments of the invention, the nanorods have
high aspect ratios exceeding 3, and are up to 200 nm long. A
preferred nanorod material is ZnO. Other materials that might be
suitable include SnO, TiO.sub.2, and other metal oxides.
[0055] As mentioned previously, the small size of the nanocrystals
greatly reduces recombination within the particle. In order to
further reduce recombination losses, preferred embodiments of the
invention utilize single-crystal nanorods. While in nanoporous
particle-based films, such as those employed in DSSC cells,
electrons percolate slowly through the film, enabling recombination
with the electrolyte to take place, electron transfer through
single-crystal nanorods is very fast, which limits the
recombination of electrons from the nanorods with holes in the
nanocrystals or the polymer.
[0056] In preferred embodiments and as discussed in greater detail
below, the nanorods are coated with a layer of a bifunctional
capping agent, which binds the quantum dots closely to the
nanorods, thereby preventing the semiconductor polymer from coming
into the proximity of the nanorod, which diminishes nanorod-polymer
recombination losses even further.
[0057] 1.3 Polymer 112
[0058] Polymer 112 should have a valence band energy that allows
holes to efficiently transfer from the nanocrystal valence band to
the polymer valence band. Suitable polymers include
poly(3-hexylthiophene), polyphenylenevinylene (PPV) and its
derivatives, and polyfluorene (PFO) and its derivatives. These
polymers are efficient hole-transfer polymers due to the high hole
mobility in organic materials.
2. Method for Fabricating a Nanorod-nanoparticle-polymer Hybrid
Structure
[0059] Hybrid semiconductor structures according to the invention
can be fabricated using low-cost deposition technologies, such as
printing, dip coating, or chemical bath deposition. An important
consideration regarding fabrication is control over where the
various pieces bind together. For example, binding of the polymer
to the nanorod would most likely result in substantial losses in
efficiency due to recombination. In preferred embodiments, the
nanocrystals are bound to both the nanorods and to the
semiconducting polymer to promote optimal performance as a
heterojunction, and the polymer is not directly bound to the
nanorods. This structure can be achieved with suitable capping
agents in appropriate processing steps.
[0060] FIGS. 2A and 2B illustrate representative process sequences
200A and 200B implementing embodiments of the present invention.
Some steps of these sequences, and the structures they result in,
are further illustrated in FIG. 3 at a microscopic level. In a
first step 202, nanorods are grown on an anodic substrate, e.g., by
printing seeds on the substrate and then growing the nanorods
perpendicularly to the substrate via a chemical bath. In this
structure, the nanorods are inherently in electrical contact with
the substrate. In subsequent steps, the nanocrystals and monomers
are introduced to the resulting film of aligned nanorods.
[0061] In step 204, nanocrystals capped with a (first) capping
agent which contains functionalities that bind weakly to the
nanocrystals are provided. Suitable functionalities include thiol,
selenol, amine, phosphine, phosphine oxide, and aromatic
heterocycles. Typically, the nanocrystals are dissolved in a
non-polar organic solvent. The capping agent serves to control
binding of the nanocrystals to the nanorods and the polymer; the
bond is reversible and the capping agent can later be exchanged for
other ligands. Examples of capping agents suitable for use with
CuInSe.sub.2 nanocrystals are octanethiol or pyridine.
[0062] In steps 206, 208, the nanorods are coated by the
nanocrystals, whereby the bond between nanorods and nanocrystals is
established via the bifunctional capping agent 115 (e.g.,
mercaptoacetic acid), which has strong binding groups for both the
nanorods and the nanocrystals. This can be accomplished in
different ways. In some embodiments, as illustrated in FIG. 2A and
FIG. 3, the nanorods are capped with the bifunctional capping agent
(step 206A), for example, by dipping the substrate with the
nanorods into a solution of the bifunctional capping agent. For
example, the capping agent may be bound to the nanorods via a
carboxylate functionality. The capped nanocrystals 302 are then
introduced to the film of capped nanorods 300 (step 208A), for
example, by dipping the rinced substrate with nanorods 300 into the
nanocrystal solution(s). At this stage, a fraction of the weak
capping agent of the nanocrystals is replaced by the stronger
binding groups of the bifunctional capping agent, e.g., the thiol
functionality of mercaptoacetic acid, which results in
nanocrystal-sensitized nanorods 304.
[0063] In alternative embodiments, as illustrated in FIG. 2B, a
solution of the capped nanocrystals in a non-polar organic solvent
is added to a solution of the bifunctional capping agent in a polar
organic solvent which is not miscible with the non-polar solvent,
and the solution is shaken to ensure good mixing (step 206B). The
nanocrystals undergo ligand exchange and transfer from a non-polar
organic phase to a polar organic phase. Subsequently, the substrate
with the aligned nanorods on the surface is dipped into the
nanocrystal solution or otherwise exposed to the nanocrystals (step
208B), whereby the nanorods bind the nanocrystals via a carboxylic
acid functionality of the capping agent. These embodiment likewise
result in nanocrystal-sensitized nanorods 304.
[0064] The monomers are functionalized (step 210) with a binding
group that has a stronger affinity for the nanocrystals than the
(first) nanocrystal capping agent, but a weaker affinity for the
nanorods than the bifunctional capping agent. Moreover, the
affinity of the binding group at the monomer for the nanocrystal is
preferably weaker than the affinity of the bifunctional capping
agent for the nanocrystal. The monomer functionality should not
interfere with the polymerization reaction. Binding groups with
suitable differential binding affinities are straightforwardly
identified by those of skill in the art without undue
experimentation based on the identities of the capping agents and
their substituents (e.g., whether they are unidentate or
multidentate, or on the presence of electron withdrawing groups,
etc.) and the size of the nanocrystal. The functionalized monomers
are then combined with the nanocrystal-sensitized nanorods (step
212), where they bind to the nanocrystals (but not the nanorods),
preferentially replacing the weak capping agent on the nanocrystal,
but leaving the nanorod-nanocrystal bond intact, resulting in
structure 306. A subsequent polymerization step 214 results in the
nanorod-nanocrystal-polymer semiconductor structure 308.
[0065] Finally, a metal cathode (e.g., Al) can be deposited on the
structure (step 216), for example, by sputtering or metal
evaporation, so that the nanorods form an array of aligned rods
deposited between two opposing electrodes. The polymer layer below
the cathode should be sufficiently thick to electrically isolate
the cathode from the nanorods.
[0066] Although the present invention has been described with
reference to specific details, it is not intended that such details
should be regarded as limitations upon the scope of the invention,
except as and to the extent that they are included in the
accompanying claims.
* * * * *