# Patent

## Title
Quantum dot solar cell

## Abstract

A solar cell including a quantum dot and an electron conductor, and a bifunctional ligand disposed between the quantum dot and the electron conductor. The bifunctional 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. 61/052,748 entitled “QUANTUM DOT SOLAR CELL” filed May 13, 2008, the entirety of which is incorporated herein by reference.


TECHNICAL FIELD
The disclosure pertains generally to solar cells and more particularly to quantum dot solar cells.
SUMMARY
The disclosure pertains generally to solar cells. In some instances, a solar cell may include quantum dots as light sensitizers. In an illustrative but non-limiting example, the disclosure pertains to a solar cell that includes an electron conductor layer, a quantum dot layer and a bifunctional ligand layer that couples the electron conductor layer and the quantum dot layer. A hole conductor layer is also coupled to the quantum dot layer. In some instances, the bifunctional ligand layer may include one or more of




where R1 is hydroxyl, alkoxy, halogen or NR2, with R2 being alkyl.
In another illustrative but non-limiting example, the disclosure pertains to a solar cell that includes an electron conductor layer, a quantum dot layer and a bifunctional ligand layer that couples the electron conductor layer and the quantum dot layer. A hole conductor layer is also coupled to the quantum dot layer. In some instances, the bifunctional ligand layer may include one or more of




where R1 is as defined above.
In another illustrative but non-limiting example, the disclosure pertains to a solar cell that includes an electron conductor layer, a quantum dot layer and a bifunctional ligand layer that couples the electron conductor layer and the quantum dot layer. A hole conductor layer is also coupled to the quantum dot layer. In some instances, the bifunctional ligand layer may include one or more of




where R1 is as defined above.
In another illustrative but non-limiting example, the disclosure pertains to a solar cell that includes an electron conductor layer, a quantum dot layer and a bifunctional ligand layer that couples the electron conductor layer and the quantum dot layer. A hole conductor layer is also coupled to the quantum dot layer. In some instances, the bifunctional ligand layer may include one or more of




The above summary is not intended to describe each disclosed embodiment or every implementation of the disclosure. The Detailed Description which follow more particularly exemplify these 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 detailed 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.
DETAILED 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.
The term “alkyl” refers to a straight or branched chain monovalent hydrocarbon radical having a specified number of carbon atoms. Examples of “alkyl” include, but are not limited to, methyl, ethyl, n-propyl, isopropyl, n-butyl, s-butyl, t-butyl, n-pentyl, n-hexyl, 3-methylpentyl, and the like.
The term “hydroxyl” as used herein refers to an —OH group. The term “halogen” as used herein refers to atoms found in Group VIIA of the periodic table, such as fluorine, chlorine, bromine and iodine.
The term “alkoxy” refers to a moiety that combines an oxygen atom with an alkyl group. In some instances, the oxygen molecule may bond to a molecule while the alkyl group is pendant. An example of an alkoxy is ethoxy, which is of the formula —OCH2CH3.
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, the electron conductor anchor may include a carboxylic acid moiety, an amide moiety, an ether or an acid halide. In some instances, a quantum dot anchor may be selected to bond well to a particular quantum dot. To illustrate, Ag2S, CdSe, CdTe and CdS are examples of quantum dots that may be employed in the light sensitive assemblies discussed herein.
An illustrative but non-limiting example of a suitable bifunctional ligand has the following structure:




where R1 is hydroxyl, alkoxy, halogen or NR2, with R2 being alkyl. Particular examples of bifunctional ligands of this structure include the following:




Another illustrative but non-limiting example of a suitable bifunctional ligand has the following structure:




where R1 is hydroxyl, alkoxy, halogen or NR2, with R2 being alkyl. Particular examples of bifunctional ligands of this structure include the following:




Another illustrative but non-limiting example of a suitable bifunctional ligand has the following structure:




where R1 is hydroxyl, alkoxy, halogen or NR2, with R2 being alkyl. Particular examples of bifunctional ligands of this structure include the following:




Another illustrative but non-limiting example of a suitable bifunctional ligand has the following structure:




where R1 is hydroxyl, alkoxy, halogen or NR2, with R2 being alkyl. Particular examples of bifunctional ligands of this structure include the following:




Another illustrative but non-limiting example of a suitable bifunctional ligand has the following structure:




or a stereoisomer thereof, and where R1 is hydroxyl, alkoxy, halogen or NR2, with R2 being alkyl. Particular examples of bifunctional ligands of this structure include the following:




It will be recognized that at least some of the bifunctional ligands described herein include carbon-carbon double bonds and thus some of the molecules may be considered as being either Z (same side) isomers or E (opposite side) isomers. In some cases, a solar cell including a number of bifunctional ligands may include only stereospecific bifunctional ligands, i.e., all Z isomers or all E isomers, for example. In some instances, a solar cell including a number of bifunctional ligands may include one or more Z isomers and one or more E isomers.
Another illustrative but non-limiting example of a suitable bifunctional ligand has the following structure:




where R1 is hydroxyl, alkoxy, halogen or NR2, with R2 being alkyl. Particular examples of bifunctional ligands of this structure include the following:




Another illustrative but non-limiting example of a suitable bifunctional ligand has the following structure:




where R1 is hydroxyl, alkoxy, halogen or NR2, with R2 being alkyl. Particular examples of bifunctional ligands of this structure include the following:




Another illustrative but non-limiting example of a suitable bifunctional ligand has the following structure:




where R1 is hydroxyl, alkoxy, halogen or NR2, with R2 being alkyl. Particular examples of bifunctional ligands of this structure include the following:




Another illustrative but non-limiting example of a suitable bifunctional ligand has the following structure:




where R1 is hydroxyl, alkoxy, halogen or NR2, with R2 being alkyl. Particular examples of bifunctional ligands of this structure include the following:




Another illustrative but non-limiting example of a suitable bifunctional ligand has the following structure:




or a stereoisomer thereof.

Another illustrative but non-limiting example of a suitable bifunctional ligand has the following structure:




or a stereoisomer thereof.

Another illustrative but non-limiting example of a suitable bifunctional ligand has the following structure:




or a stereoisomer thereof.

In some cases, a light sensitive assembly or a solar cell may include one or a number of bifunctional ligands. In some instances, a solar cell may include a number of bifunctional ligands that are isomers having the same molecular formula. These isomers may, for example, be dextrorotatory molecules, levorotatory molecules or a racemic mixture thereof. In some cases, a light sensitive assembly or solar cell may include a large number of bifunctional ligands representing a plurality of chemically different bifunctional ligands. Each group or subset of bifunctional ligands, either separately or in combination, may be dextrorotatory, levorotatory or a racemic mixture thereof.
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 cases, the polythiophene may be functionalized with a thio or thioether moiety.
An illustrative but non-limiting example of a suitable conductive polymer has




as a repeating unit, where R is absent or alkyl and m is an integer ranging from about 6 to about 12.
Another illustrative but non-limiting example of a suitable conductive polymer has




as a repeating unit, where R is absent or alkyl.
Another illustrative but non-limiting example of a suitable conductive polymer has




as a repeating unit, where R is absent or alkyl.
Another illustrative but non-limiting example of a suitable conductive polymer has




as a repeating unit, where R is absent or alkyl.
A solar cell may, for example, be assembled by growing nanoparticles of n-type semiconducting titanium dioxide on a glass substrate, optionally followed by a sintering process. Next, the quantum dots, the bifunctional ligands and the conducting polymer are synthesized. Finally, the solar cell may be assembled by combining the individual components in a one-pot synthesis.
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;
a bifunctional ligand layer coupling the electron conductor layer and the quantum dot layer, the bifunctional ligand layer including one or more of:




where R1 is hydroxyl, alkoxy, halogen or NR2, with R2 being alkyl; and
a hole conductor layer coupled to the quantum dot layer.

2. A solar cell comprising:
an electron conductor layer;
a quantum dot layer;
a bifunctional ligand layer coupling the electron conductor layer and the quantum dot layer;
a hole conductor layer coupled to the quantum dot layer; and
wherein the bifunctional ligand layer comprises one or more of:




where R2 is an alkyl moiety.

3. A solar cell comprising:
an electron conductor layer;
a quantum dot layer;
a bifunctional ligand layer coupling the electron conductor layer and the quantum dot layer;
a hole conductor layer coupled to the quantum dot layer; and
wherein the bifunctional ligand layer comprises one or more of:




where R2 is an alkyl moiety.

4. The solar cell of claim 1, wherein the bifunctional ligand layer comprises one or more of:



where R2 is an alkyl moiety.

5. A solar cell comprising:
an electron conductor layer;
a quantum dot layer;
a bifunctional ligand layer coupling the electron conductor layer and the quantum dot layer, the bifunctional ligand layer including one or more of:




where R1 is hydroxyl, alkoxy, halogen or NR2, with R2, being alkyl;
a hole conductor layer coupled to the quantum dot layer; and
wherein the hole conductor layer comprises




as a repeating unit, where R is absent or alkyl and m is an integer ranging from about 6 to about 12.