Abstract:
A solar cell is disclosed that may include a quantum dot, an electron conductor, and a bifunctional ligand disposed between the quantum dot and the electron conductor. The bifunctional ligand may include a first anchor group that bonds to the quantum dot and a second anchor group that bonds to the electron conductor. The solar cell may include a hole conductor that is configured to reduce the quantum dot once the quantum dot absorbs a photon and ejects an electron through the bifunctional ligand and into the electron conductor. The hole conductor may be a p-type polymer.

Description:
[0001]    This application claims priority under 35 U.S.C. §119 to U.S. Provisional Application Ser. No. 61/149,899 entitled “QUANTUM DOT SOLAR CELL” filed Feb. 4, 2009, the entirety of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The disclosure relates generally to solar cells and more particularly to quantum dot solar cells. 
       SUMMARY 
       [0003]    The disclosure relates generally to solar cells. In an illustrative but non-limiting example, the disclosure relates to a solar cell that includes a quantum dot layer, an electron conductor layer, an optional bifunctional ligand layer that is disposed between the quantum dot layer and the electron conductor layer, and a hole conductor layer that is disposed in contact with the quantum dot layer. The bifunctional ligand layer may include moieties that are configured to bond, either covalently or ionically, to the electron conductor as well as to quantum dots within the quantum dot layer. The hole conductor layer may be polymeric and may have pendant groups that are chemically similar to the moieties within the bifunctional ligand layer. 
         [0004]    In another illustrative but non-limiting example, the disclosure relates to a solar cell that includes a quantum dot, an electron conductor and a bifunctional ligand that is disposed between the quantum dot and the electron conductor. The bifunctional ligand may be a sulfur-based amino acid. A hole conductor having a pendant group that is chemically similar to the sulfur-based amino acid may be in contact with the quantum dot. 
         [0005]    In another illustrative but non-limiting example, the disclosure relates to a solar cell that includes a quantum dot, an electron conductor and a hole conductor having a pendant group including a sulfur-based amino acid. A bifunctional ligand that is chemically similar to the sulfur-based amino acid may be disposed between the quantum dot and the electron conductor. 
         [0006]    The above summary is not intended to describe each disclosed embodiment or every implementation of the disclosure. The Figures and Detailed Description which follow more particularly exemplify these embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0007]    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 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: 
           [0008]      FIG. 1  is a schematic cross-sectional side view of an illustrative but non-limiting example of a solar cell; and 
           [0009]      FIG. 2  is a schematic cross-sectional side view of another illustrative but non-limiting example of a solar cell. 
       
    
    
       [0010]    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 
       [0011]    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. 
         [0012]      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  are 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 further discussed below. The illustrative solar cell  10  may further include a hole conductor layer  18 . As discussed below, hole conductor layer  18  may, in some instances, be a p-type conducting electrode layer. 
         [0013]    In some cases, the electron conductor layer  16  may include or be formed so as to take the form of a structured pattern or array, such as a structured nano-materials or other structured pattern or array, as desired. The structured nanomaterials may include clusters or arrays of nanospheres, nanotubes, nanorods, nanowires, nano-inverse opals, or any other suitable nanomaterials or shapes as desired. The quantum dots is shown electrically coupled to or otherwise disposed on the electron conductor. In at least some embodiments, the quantum dots may be disposed over and “fill in” the structured pattern or array of the electron conductor, as shown in  FIG. 2 . 
         [0014]    It is contemplated that the electron conductor layer  16  may be formed of any suitable material. In some cases, the electron conductor layer  16  may be an n-type electron conductor. In some instances, the electron conductor layer  16  may be metallic, such as TiO 2  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. 
         [0015]    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, Al 2 O 3 , Al 2 S 3 , Al 2 Se 3 , Al 2 Te 3 , Ga 2 O 3 , Ga 2 S 3 , Ga 2 Se 3 , Ga 2 Te 3 , In 2 O 3 , In 2 S 3 , In 2 Se 3 , In 2 Te 3 , SiO 2 , GeO 2 , SnO 2 , SnS, SnSe, SnTe, PbO, PbO 2 , PbS, PbSe, PbTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs and InSb. 
         [0016]      FIG. 2  is a schematic cross-sectional side view of an illustrative solar cell that is similar to solar cell  10  ( FIG. 1 ). In some cases, a reflective and/or protecting layer may be disposed over the hole conductor layer, as shown. The reflective and/or protecting layer may be a conductive layer. In some instances, the reflective and/or protecting layer 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, 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. 
         [0017]    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. 
         [0018]    A variety of bifunctional ligands may be used. In some instances, a bifunctional ligand may be a molecule that is configured to bond to a quantum dot as well as to an electron conductor. A bifunctional ligand may, for example, include a first bonding site that bonds to a quantum dot and a second bonding site that bonds to an electron conductor. Examples of suitable bifunctional ligands include sulfur-based or sulfur-containing amino acids such as cysteine, homocysteine and isocysteine, the structures of which are shown below, respectively: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0019]    It will be appreciated that these are amphoteric molecules, and as such have both acidic and basic properties, depending on the pH of their environment. The structures below generically show an amino acid, at low pH, neutral pH and at high pH, respectively, in which R represents the amino acid&#39;s side chain: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0020]    As can be seen, at low pH (acidic conditions), amino acids such as cysteine, homocysteine and isocysteine are mostly in cationic form, and thus deprotonation of the carboxylic group is prevented. At high pH (basic conditions), amino acids are mostly in anionic form, and thus the carboxylic group is deprotonated. It will be appreciated, therefore, that the interactions between bifunctional ligand and electron conductor may be adjusted and/or regulated by controlling solution pH when forming the solar cell. 
         [0021]    In the cationic form (low pH), a covalent bond may form via esterification between the carboxylic group of the amino acid and an oxygen atom within a TiO 2  electron conductor. Conversely, in the anionic form (high pH), an anionic bond may form between the deprotonated (and thus negatively charged) carboxylic group of the amino acid and Ti 4+  (positively charged) ion present within the electron conductor. It will be appreciated that the positively charged titanium ion may be considered as being a surface defect. 
         [0022]    As discussed with respect to  FIG. 1 , the illustrative solar cell  10  may include a hole conductor layer  18 . A variety of hole conductor materials are contemplated. For example, hole conductor layer  18  may be formed of a p-type electrically conductive polymer. In some instances, hole conductor layer  18  may be formed of or otherwise include a monomer such as a polythiophene. An illustrative but non-limiting example of a suitable polythiophene that may be modified via esterification to include a pendant group is poly[3-6(hydroxyhexyl thiophene)], which has the following structure as a repeating unit: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0023]    In some cases, the polythiophene may be functionalized with a sulfur-based amino acid such as cysteine, homocysteine or isocysteine. In some instances, bifunctional ligand layer  14  may, for example, have a bonding moiety that is the same as, but distinct from, a pendant group within hole conductor layer  18 . It will be appreciated that in some instances, the amino acid used as the bifunctional ligand may, depending on pH, be in its cationic, neutral or anionic configuration. It will also be appreciated that in some cases, the particular amino acid used as the bifunctional ligand may not, as a result of its environmental pH, be in the same form as the same amino acid present as a pendant group in the hole conductor. In some instances, the amino acid used as the bifunctional ligand and the same (but distinct) amino acid used as the hole conductor pendant group may be referred to as being chemically similar. 
         [0024]    Some specific examples, in which one or more of the amino acids may be in cationic or anionic form, even though drawn in their neutral (pH 7) forms, are provided below. For example, in some cases bifunctional ligand layer  14  and hole conductor layer  18  may include, respectively: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0025]    In some instances, bifunctional ligand layer  14  and hole conductor layer  18  may include, respectively: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0026]    In some cases, bifunctional ligand layer  14  and hole conductor layer  18  may include, respectively: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    The above examples can be used in conjunction with any suitable quantum dots including, for example, CdSe, CdS and/or CdTe quantum dots. 
         [0027]    A structure such as solar cell  10  ( FIG. 1 ) may be formed in any suitable manner. It is contemplated that solar cell  10  can be formed by growing titanium dioxide nanoparticles on a glass substrate, followed by a sintering process to form electron conductor layer  16 . The hole conductor layer  18  may be formed by attaching, via an esterification process, the desired sulfur-based amino acid (e.g. cysteine) to a polythiophene such as poly[3-6(hydroxyhexyl thiophene)]. Bifunctional ligand layer  14  may be formed by attaching (either covalently or ionically) the desired sulfur-based amino acids to electron conductor layer  16 . Quantum dot layer  12  may then be formed by attaching quantum dots to bifunctional layer  14 . Finally, hole conductor layer  18 , which may be formed via the esterification process noted above, may be attached. 
         [0028]    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.