# Patent

## Title
Quantum dot solar cell

## Abstract

A solar cell may include a quantum dot and an electron conductor. A bifunctional ligand may be disposed between the quantum dot and the electron conductor. The ligand molecule may include an electron conductor anchor that bonds to the electron conductor and a first quantum dot anchor that bonds to the quantum dot. A hole conductor such as a conductive polymer may include a second quantum dot anchor.

## Background
PRIORITY
This application claims priority under 35 U.S.C. §119 to U.S. Provisional Application Ser. No. 61/031,631 entitled “QUANTUM DOT SOLAR CELL” filed Feb. 26, 2008, the entirety of which is incorporated herein by reference.


TECHNICAL FIELD
The disclosure relates generally to solar cells and more particularly to quantum dot solar cells.
SUMMARY
The disclosure relates generally to solar cells. In an illustrative but non-limiting example, the disclosure relates to a solar cell that has an electron conductor layer, a quantum dot layer, a bifunctional ligand layer and a hole conductor layer. The quantum dot layer may include a plurality of quantum dots. The bifunctional ligand layer may be coupled to the electron conductor layer and to the quantum dot layer, and may include a plurality of bifunctional ligands, at least some of which may have a structure selected from H2PO3—R—PO3H2, H2PO3—Ar—PO3H2 or H2N—Ar—PO3H2, where R is alkyl or hydroxyalkyl and Ar is aromatic. The hole conductor layer may be coupled to the quantum dot layer. In some cases, the electron conductor layer and/or the hole conductor layer may include a conductive polymer, but this is not required.
In another illustrative but non-limiting example, the disclosure relates to a solar cell that includes an electron conductor, a quantum dot, a bifunctional ligand and a hole conductor. The bifunctional ligand may include an electron conductor bonding moiety and a first quantum dot anchor. The hole conductor may include a second quantum dot anchor. The first quantum dot anchor and the second quantum dot anchor may independently be or otherwise include a phosphonic acid moiety.
In another illustrative but non-limiting example, the disclosure relates to a solar cell that includes an electron conductor, a quantum dot, a bifunctional ligand and a hole conductor. The bifunctional ligand may include an electron conductor bonding moiety as well as a quantum dot anchor. The hole conductor may include a pyridine moiety. The quantum dot anchor may be or include an amine moiety, while the electron conductor bonding moiety may be a phosphonic acid moiety.
The above summary is not intended to describe each disclosed embodiment or every implementation of the disclosure. The Description that follows more particularly exemplify various illustrative embodiments.



BRIEF DESCRIPTION OF THE FIGURES
The following description should be read with reference to the drawings. The drawings, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the disclosure. The disclosure may be more completely understood in consideration of the following description of various embodiments in connection with the accompanying drawings, in which:
FIG. 1 is a schematic cross-sectional side view of an illustrative but non-limiting example of a solar cell; and
FIG. 2 is a schematic cross-sectional side view of another illustrative but non-limiting example of a solar cell.



While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.
DESCRIPTION
The following description should be read with reference to the drawings, in which like elements in different drawings are numbered in like fashion. The drawings, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the invention. Although examples of construction, dimensions, and materials are illustrated for the various elements, those skilled in the art will recognize that many of the examples provided have suitable alternatives that may be utilized.
FIG. 1 is a schematic cross-sectional side view of an illustrative solar cell 10. In the illustrative example shown in FIG. 1, there may be a three-dimensional intermingling or interpenetration of the layers forming solar cell 10, but this is not required. The illustrative solar cell 10 includes a quantum dot layer 12. Quantum dot layer 12 may schematically represent a single quantum dot. In some cases, quantum dot layer 12 may be considered as representing a large number of individual quantum dots.
In the illustrative embodiment of FIG. 1, a bifunctional ligand layer 14 is provided, and may schematically represent a single bifunctional ligand, such as those discussed below. In some cases, bifunctional ligand layer 14 may represent a large number of individual bifunctional ligands, with at least some of the bifunctional ligands within bifunctional ligand layer 14 bonded to corresponding quantum dots within quantum dot layer 12. The illustrative solar cell 10 of FIG. 1 also includes an electron conductor layer 16. In some cases, electron conductor layer 16 may be an n-type conductor as discussed below. The illustrative solar cell 10 may further include a hole conductor layer 18. As discussed below, hole conductor layer 18 may be a p-type conducting electrode layer.
Bifunctional ligand layer 14 may, in some instances, be considered as being coupled to quantum dot layer 12 as well as being coupled to electron conductor layer 16. Similarly, hole conductor layer 18 may also be considered as being coupled to quantum dot layer 12. In some cases, two layers may be considered as being coupled if one or more molecules or other moieties within one layer are bonded or otherwise secured to one or more molecules within another layer. In some instances, coupling infers the potential passage of electrons from one layer to the next.
Quantum dot layer 12 may include one quantum dot or a plurality of quantum dots. Quantum dots are typically very small semiconductors, having dimensions in the nanometer range. Because of their small size, quantum dots may exhibit quantum behavior that is distinct from what would otherwise be expected from a larger sample of the material. In some cases, quantum dots may be considered as being crystals composed of materials from Groups II-VI, III-V, or IV-VI materials. The quantum dots employed herein may be formed using any appropriate technique. Examples of specific pairs of materials for forming quantum dots include, but are not limited to, MgO, MgS, MgSe, MgTe, CaO, CaS, CaSe, CaTe, SrO, SrS, SrSe, SrTe, BaO, BaS, BaSe, BaTe, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, HgO, HgS, HgSe, HgTe, Al2O3, Al2S3, Al2Se3, Al2Te3, Ga2O3, Ga2S3, Ga2Se3, Ga2Te3, In2O3, In2S3, In2Se3, In2Te3, SiO2, GeO2, SnO2, SnS, SnSe, SnTe, PbO, PbO2, PbS, PbSe, PbTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs and InSb.
FIG. 2 is a schematic cross-sectional side view of an illustrative solar cell 20 that is similar to solar cell 10 (FIG. 1). In some cases, a reflective and/or protecting layer 22 may be disposed over the hole conductor layer 18, as shown. The reflective and/or protecting layer 22 may be a conductive layer. In some instances, the reflective and/or protecting layer 22 may include a Pt/Au/C film as both catalyst and conductor, but this is not required. Alternatively, or in addition, a flexible and transparent substrate 24, shown at the lower side (in the illustrated orientation) of FIG. 2, may be an electron conductor such as an n-type electron conductor. The n-type electron conductor may be transparent or at least substantially transparent to at least some wavelengths of light within the visible portion of the electromagnetic spectrum.
As described with respect to FIG. 1, solar cell 10 may include a bifunctional ligand layer 14. In some cases, bifunctional ligand layer 14 may include a single bifunctional ligand or a large number of bifunctional ligands. A bifunctional ligand may, in some cases, be considered as improving electron transfer by reducing the energy barriers for electron transfer. A bifunctional ligand may provide a conduit so that electrons that are ejected by the quantum dot can travel to and through the electron conductor. A bifunctional ligand may, for example, secure the quantum dot relative to the electron conductor and/or any other related structure.
In some instances, a bifunctional ligand may include an electron conductor anchor that bonds to the electron conductor as well as a quantum dot anchor that bonds to the quantum dot. The bifunctional ligand may, for example, have a phosphonic acid moiety as the electron conductor anchor and may also have a phosphonic acid moiety as the quantum dot anchor. In some cases, the bifunctional ligand may have an amine moiety as the quantum dot anchor. In some instances, the quantum dot anchor may be selected in accordance with a particular type of quantum dot. The bifunctional ligand may, in some instances, be considered as having a structure that is one or more of H2PO3—R—PO3H2, H2PO3—Ar—PO3H2 or H2N—Ar—PO3H2, in which R is alkyl or hydroxyalkyl and Ar is aromatic.
The term “alkyl” refers to a straight or branched chain monovalent hydrocarbon radical having a specified number of carbon atoms. Alkyl groups may be unsubstituted or substituted with substituents that do not interfere with the specified function of the composition. The term “hydroxyalkyl” refers to an alkyl group bearing one or more hydroxyl groups. The term “aromatic” refers to unsaturated cyclic hydrocarbons containing one or more ring structures. Benzene is an example of an aromatic, and has a six carbon ring having three double bonds.
In some instances, the first quantum dot anchor and/or the second quantum dot anchor may be selected to bond well to a particular quantum dot. In some instances, Lewis Hard Soft Acid Base (HSAB) theory may be used to select a particular quantum dot anchor. For example, a soft Lewis acid prefers to bond with a soft Lewis base, while a hard Lewis acid prefers to bond with a hard Lewis base. In some instances, a borderline Lewis acid may prefer to bond with a borderline Lewis base.
As noted above, CdS, CdTe and CdSe are examples of quantum dots that may be employed in the solar cells discussed herein. Cadmium cation (Cd2+) is a soft Lewis acid, which means that quantum dot anchors that are or that include moieties that are soft Lewis bases may work well. Examples of suitable soft Lewis bases include mercapto, thio, thioether and phosphonic acid moieties. Following are examples of bifunctional ligands that may be considered as bonding well to quantum dots that include cadmium cations (Cd2+).
An illustrative but non-limiting example of a suitable bifunctional ligand is 1-hydroxyethane-1,1-diphosphonic acid (HEDP), which has the structure:




Another example of a suitable bifunctional ligand is propane-1,1-diphosphonic acid, which has the structure:




Another example of a suitable bifunctional ligand is benzene-1,4-diphosphonic acid, which has the structure:




As noted above, quantum dot anchors may be selected to bond well to a particular quantum dot. To illustrate, CuO, ZnO, Sb2S3, Bi2S3, PbS and PbSe are further examples of quantum dots that may be employed in the light sensitive assemblies discussed herein. Copper, zinc, antimony, bismuth and lead cations are borderline Lewis acids, which means that quantum dot anchors that are or that include moieties that are borderline Lewis bases may work well. Examples of suitable borderline Lewis bases include pyridine moieties and arylamine moieties.
An example of a bifunctional ligand that may be suitable for bonding to a borderline Lewis acid quantum dot is 3-amino phenyl phosphonic acid, which has the structure:




Referring back to FIG. 1, the illustrative solar cell 10 may include an electron conductor layer 16, which may be formed of any suitable material. In some cases, the electron conductor layer 16 may be an n-type electron conductor. The electron conductor layer 16 may be metallic, such as TiO2 or ZnO. In some cases, electron conductor layer 16 may be an electrically conducting polymer, such as a polymer that has been doped to be electrically conducting or to improve its electrical conductivity.
As discussed with respect to FIG. 1, the illustrative solar cell 10 may include a hole conductor layer 18 that is configured to reduce a quantum dot once the quantum dot has absorbed a photon and ejected an electron through the bifunctional ligand to the electron conductor. In some instances, the hole conductor may be a conductive polymer. In some cases, the conductive polymer may include a monomer that has an alkyl chain that terminates in a second quantum dot anchor. The conductive polymer may, for example, be or otherwise include a polythiophene that is functionalized with a moiety that bonds to quantum dots.
In some instances, the conductive polymer may include a monomer that has an alkyl group bearing a phosphonic acid moiety disposed at a terminal end of the alkyl group. The phosphonic acid moiety may bond to a quantum dot, particularly if the quantum dot is a soft acid quantum dot such as a cadmium-based quantum dot. Following are examples of suitable conductive polymers.
An illustrative but non-limiting conductive polymer may be formed from, or otherwise include, a monomer having the structure:




where m is an integer ranging from about 6 to about 12.
Another suitable example of a conductive polymer maybe formed from, or otherwise include, a monomer having the structure:




Another suitable example of a conductive polymer maybe formed from, or otherwise include, a monomer having the structure:




Another suitable example of a conductive polymer maybe formed from, or otherwise include, a monomer having the structure:




In some instances, the conductive polymer may include a monomer that has an alkyl group bearing a pyridine moiety disposed at a terminal end of the alkyl group. The pyridine acid moiety may bond to a quantum dot, particularly if the quantum dot includes a borderline acid element such as those discussed above. The following are examples of suitable conductive polymers.
Another suitable example of a conductive polymer maybe formed from, or otherwise include, a monomer having the structure:




where p is an integer ranging from about 6 to about 12.
Another suitable example of a conductive polymer maybe formed from, or otherwise include, a monomer having the structure:




Another suitable example of a conductive polymer maybe formed from, or otherwise include, a monomer having the structure:




Another suitable example of a conductive polymer maybe formed from, or otherwise include, a monomer having the structure:




An illustrative but non-limiting example of assembling a solar cell is described herein. A solar cell such as solar cell 10 (FIG. 1) may be formed by depositing and/or growing a suitable titanium dioxide layer on a glass substrate. The polymer, already functionalized with the appropriate quantum dot anchor, may be attached to the quantum dots. Then, the quantum dots/polymer structure may be attached to the titanium dioxide layer, for example, in a one-pot synthesis using the bifunctional ligands.
The disclosure should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the invention can be applicable will be readily apparent to those of skill in the art upon review of the instant specification.

## Claims

1. A solar cell comprising:
an electron conductor layer;
a quantum dot layer comprising a plurality of quantum dots;
a bifunctional ligand layer coupled to the electron conductor layer and to the quantum dot layer, the bifunctional ligand layer comprising a plurality of bifunctional ligands, at least some of the bifunctional ligands having an electron conductor anchor including a first phosphonic acid moiety and a quantum dot anchor including a second phosphonic acid moiety, the first phosphonic acid moiety and the second phosphonic acid moiety each having a structure selected from:

H2PO3—R—PO3H2, H2PO3—Ar—PO3H2 or H2N—Ar—PO3H2,
where R is alkyl or hydroxyalkyl and Ar is aromatic; and
a hole conductor layer coupled to the quantum dot layer.

2. A solar cell comprising:
an electron conductor layer:
a quantum dot layer comprising a plurality of quantum dots;
a bifunctional ligand layer coupled to the electron conductor layer and to the quantum dot layer, the bifunctional ligand layer comprising a plurality of bifunctional ligands, at least some of the bifunctional ligands having a structure selected from:

H2PO3—R—PO3H2, H2PO3—Ar—PO3H2 or H2N—Ar—PO3H2,
where R is alkyl or hydroxyalkyl and Ar is aromatic;
a hole conductor layer coupled to the quantum dot layer; and
at least some of the bifunctional ligands comprise

3. A solar cell comprising:
an electron conductor layer;
a quantum dot layer comprising a plurality of quantum dots;
a bifunctional ligand layer coupled to the electron conductor layer and to the quantum dot layer, the bifunctional ligand layer comprising a plurality of bifunctional ligands, at least some of the bifunctional ligands having a structure selected from:

H2PO3—R—PO3H2, H2PO3—Ar—PO3H2 or H2N—Ar—PO3H2,
where R is alkyl or hydroxyalkyl and Ar is aromatic;
a hole conductor layer coupled to the quantum dot layer; and
at least some of the bifunctional ligands comprise

4. A solar cell comprising:
an electron conductor layer;
a quantum dot layer comprising a plurality of Quantum dots;
a bifunctional ligand layer coupled to the electron conductor layer and to the quantum dot layer, the bifunctional ligand layer comprising a plurality of bifunctional ligands, at least some of the bifunctional ligands having a structure selected from:

H2PO3—R—PO3H2, H2PO3—Ar—PO3H2 or H2N—Ar—PO3H2,
where R is alkyl or hydroxyalkyl and Ar is aromatic;
a hole conductor layer coupled to the quantum dot layer; and
at least some of the bifunctional ligands comprise

5. A solar cell comprising:
an electron conductor layer;
a quantum dot layer comprising a plurality of quantum dots;
a bifunctional ligand layer coupled to the electron conductor layer and to the quantum dot layer, the bifunctional ligand layer comprising a plurality of bifunctional ligands, at least some of the bifunctional ligands having a structure selected from:

H2PO3—R—PO3H2, H2PO3—Ar—PO3H2 or H2N—Ar—PO3H2,
where R is alkyl or hydroxyalkyl and Ar is aromatic;
a hole conductor layer coupled to the quantum dot layer; and
at least some of the bifunctional ligands comprise

6. A solar cell comprising:
an electron conductor layer;
a quantum dot layer comprising a plurality of quantum dots;
a bifunctional ligand layer coupled to the electron conductor layer and to the quantum dot layer, the bifunctional ligand layer comprising a plurality of bifunctional ligands, at least some of the bifunctional ligands having a structure selected from:

H2PO3—R—PO3H2, H2PO3—Ar—PO3H2 or H2N—Ar—PO3H2,
where R is alkyl or hydroxyalkyl and Ar is aromatic;
a hole conductor layer coupled to the quantum dot layer; and
the hole conductor layer comprises a conductive polymer having as a repeating unit the following structure:




where m is an integer ranging from about 6 to about 12.

7. A solar cell comprising:
an electron conductor layer;
a quantum dot layer comprising a plurality of quantum dots;
a bifunctional ligand layer coupled to the electron conductor layer and to the quantum dot layer, the bifunctional ligand layer comprising a plurality of bifunctional ligands, at least some of the bifunctional ligands having a structure selected from:

H2PO3—R—PO3H2, H2PO3—Ar—PO3H2 or H2N—Ar—PO3H2,
where R is alkyl or hydroxyalkyl and Ar is aromatic;
a hole conductor layer coupled to the quantum dot layer; and
wherein the hole conductor layer comprises a conductive polymer formed from monomers having the structure:




where p is an integer ranging from about 6 to about 12.