Patent Application
20140216555
Solar cells manufactured using one to three micron thin films of light absorbing semiconductors cost less than the solar cells manufactured using thicker (100-500 micron) silicon wafers because thin films use less material and, in general, can be manufactured at lower temperatures. The leading thin-film solar cell technologies are distinguished based on the light absorbing material and include (1) amorphous silicon thin film solar cells, (2) cadmium telluride (CdTe) thin film solar cells, and (3) copper indium gallium diselenide (CIGS) thin film solar cells. However, properties and characteristics of these light absorbing materials have hampered the development of solar cells based on these technologies, as further discussed below.
For example, amorphous silicon suffers from instability and low efficiency when exposed to sunlight, and the resulting stabilized efficiency for such solar cell modules rarely exceeds 10%.
CIGS has been demonstrated to have high laboratory efficiencies (20%). However CIGS thin films are difficult to deposit uniformly on large scale; they suffer from instability when exposed to moisture; and they contain indium, which is a scarce material. Furthermore, indium prices have increased by as much as a factor of eight in the last decade due to demand in electronics industry for this scarce metal.
CdTe is currently an important thin film solar cell technology. The cost of making CdTe solar cells has been lowered significantly in recent years. Although CdTe solar cells are simple and inexpensive to make, use of Cd necessitates cradle-to-grave recycling. Moreover, tellurium is a rare element.
Finally, limited tellurium and indium supplies may limit the annual production levels of both CdTe and CIGS solar cells, preventing adequate production levels to reach terawatt levels of solar cell power production.
Thus, there remains a need in the art for convenient methods using existing and/or new materials for making efficient solar cells.
Thin films of copper zinc tin sulphide (Cu2ZnSnS4; often abbreviated as CZTS) and copper zinc tin selenide (Cu2ZnSnSe4; often abbreviated as CZTSe) are emerging as potential alternatives to CdTe and CIGS as a solar cell material that contains only abundant and nontoxic elements. Overall power conversion efficiencies of 6.7% and nearly 9.6% were reached with solar cells based on thin films of CZTS and CZTSe, respectively. CZTS has been made by depositing copper, zinc and tin metals on a substrate using various physical deposition methods (sputtering, evaporation, etc.), and sulfurizing the resulting film at temperatures ranging from 400-700° C.
CZTS solar cells have been fashioned after the structure of the CIGS solar cells in which the CIGS absorber layer has been replaced with CZTS film. Similar to CIGS solar cells, CZTS has been deposited on molybdenum-coated soda-lime glass using one of the methods described above. Following, CZTS films have been coated with a thin cadmium sulfide (CdS) buffer layer, typically through chemical bath deposition (CBD). Next, Al doped ZnO or other transparent conducting oxide films have been deposited by sputtering.
However, there remains a need for new methods for making CZTS, CZTSe and CZTSSe, and new constructions for solar cells containing these materials. Disclosed herein are methods for making CZTS, CZTSe and CZTSSe in the form of, for example, colloidal dispersions (“inks”), solutions, and/or thin films.
In one aspect, the present disclosure provides a method of preparing a metal chalcogenide (e.g., a copper zinc tin chalcogenide). The method includes heating components including: at least one copper, zinc, and/or tin compound selected from the group consisting of chalcogenocarbamates, dichalcogenocarbamates, mercaptides, thiolates, dithiolates, thiocarbonates, dithiocarbonates, trithiocarbonates, and combinations thereof. Heating includes conditions effective to form a compound of the formula Cu2+x+zZn1-xSn1-zA4, wherein A represents one or more chalcogens (e.g. sulfur, selenium, or a combination thereof); −1≦x≦1; −1≦z≦1; and with the proviso that when x=z they are not equal to 1. Preferably the copper, zinc, and/or tin compounds are heated in the substantial absence of oxygen.
Exemplary copper dichalcogenocarbamates include those of the formula Cu2+(−A-(A)C—NR1R2)2, wherein each R1 and R2 independently represents H or an organic group in which R1 and R2 can optionally be joined to form one or more rings; and each A independently represents a chalcogen.
Exemplary zinc dichalcogenocarbamates include those of the formula Zn2+(−A-(A)C—NR1R2)2, wherein each R1 and R2 independently represents H or an organic group in which R1 and R2 can optionally be joined to form one or more rings; and each A independently represents a chalcogen.
Exemplary tin dichalcogenocarbamates include those of the formula Sn4+(−A-(A)C—NR1R2)4, wherein each R1 and R2 independently represents H or an organic group in which R1 and R2 can optionally be joined to form one or more rings; and each A independently represents a chalcogen.
Exemplary metal chalcogenides that can be prepared by such methods include, but are not limited to, those of the formulas Cu2+zZnSn1-zSySe4-y; Cu2ZnSnSySe4-y; Cu3ZnSySe4-y; CuZnSn2SySe4-y; Cu1+zZn2Sn1-zSySe4-y; CuZn2SnSySe4-y; Cu2Zn2SySe4-y; Zn2Sn2SySe4-y; Cu3+zSn1-zSySe4-y; Cu3SnSySe4-y; Cu2Sn2SySe4-y; Cu2+xZn1-xSnSySe4-y; Cu3+xZn1-xSySe4-y; and Cu1+xZn1-xSn2SySe4-y, wherein x and z are as defined above, and 0≦y≦4.
In one embodiment, the components can be heated in a solvent at a temperature of 125° C. to 300° C., optionally in the presence of an amine (e.g., oleylamine), to form the metal chalcogenide in the form of nanocrystals. Optionally, the nanocrystals can be coated (e.g., as a colloidal dispersion or solution) on a substrate and heated to form a film of the metal chalcogenide. Colloidal dispersions or solutions of such nanocrystals are also disclosed herein.
In another embodiment, the components can be applied to a substrate and heated at a temperature of 150° C. to 900° C., optionally in the presence of an amine (e.g., oleylamine), to form a film of the metal chalcogenide.
In another aspect, the present disclosure provides solar cells, and methods of making solar cells, that include a substrate and one or more copper zinc tin chalcogenide layers as disclosed herein. In certain embodiments the copper zinc tin chalcogenide is copper-deficient and is of the formula Cu2+xZn1-xSn SySe4-y, wherein: 0≦y≦4; and −1<x<0. In other certain embodiments, the copper zinc tin chalcogenide is copper-rich and is of the formula Cu2+xZn1-xSn SySe4-y, wherein: 0≦y≦4; and 0<x<1. In some embodiments the solar cells include one or more copper-deficient copper zinc tin chalcogenide layers and one or more copper-rich copper zinc tin chalcogenide layers. Optionally, the solar cells can further include a zinc sulfide, a tin oxide, and/or a zinc oxide buffer layer over at least one metal chalcogenide layer or layers.
In preferred embodiments, at least some of the methods of making and using metal chalcogenides disclosed herein can overcome at least some of the problems encountered by methods known in the art. For example, in certain embodiments, the methods disclosed herein for making metal chalcogenides employ commonly available materials and can preferably reduce undesirable wastes. For another example, in certain embodiments, the solar cells disclosed herein can reduce or eliminate at least some undesirable materials used in the construction of the cells.
Unless otherwise specified, “a,” “an,” “the,” and “at least one” are used interchangeably and mean one or more than one.
As used herein, the term “comprising,” which is synonymous with “including” or “containing,” is inclusive, open-ended, and does not exclude additional unrecited elements or method steps.
The above brief description of various embodiments of the present invention is not intended to describe each embodiment or every implementation of the present invention. Rather, a more complete understanding of the invention will become apparent and appreciated by reference to the following description and claims in view of the accompanying drawings. Further, it is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.
Metal chalcogenides, and methods of making and using metal chalcogenides, are disclosed herein. As used herein, the phrase “metal chalcogenides” is intended to include compounds that include at least one metal cation and at least one chalcogenide anion (e.g., sulfur, selenium, or a combination thereof). As such, the phrase metal chalcogenides can refer to a variety of compounds including, but not limited to, copper chalcogenides, copper zinc chalcogenides, copper tin chalcogenides, zinc tin chalcogenides, and copper zinc tin chalcogenides. Exemplary metal chalcogenides can be represented by the formula Cu2+x+zZn1-xSn1-zA4, wherein A represents one or more chalcogens; −1≦x≦1; −1≦z≦1; and with the proviso that when x=z they are not equal to 1.
Advantageously, the present disclosure provides methods in which metal chalcogenides can be prepared by heating suitable copper, zinc, and/or tin compounds selected from the group consisting of chalcogenocarbamates, dichalcogenocarbamates, mercaptides, thiolates, dithiolates, thiocarbonates, dithiocarbonates, trithiocarbonates, and combinations thereof (e.g., copper, zinc, and/or tin dichalcogenocarbamates) under conditions effective to form metal chalcogenides. For example, a wide variety of dichalcocarbamates can be used to prepare metal chalcogenides. Such dichalcocarbamates can often be prepared and purified by recrystallization according to known methods.
Useful dichalcogenocarbamates can include carbamate groups of the formula −A-(A)C—NR1R2, wherein each R1 and R2 independently represents H or an organic group in which R1 and R2 can optionally be joined to form one or more rings; and each A independently represents a chalcogen (e.g., sulfur, selenium, or a combination thereof). When R1 and/or R2 represent an organic group, preferably the organic group is a carbon-bound (i.e., the bond to the group is to a carbon atom of the organic group) organic group. In certain embodiments, the organic group is an aliphatic group such as a C1-C30 aliphatic group, in some embodiments a C1-C20 aliphatic group. In other certain embodiments, the organic group is a C1-C30 hydrocarbon moiety, and in some embodiments a C1-C20 hydrocarbon moiety.
As used herein, the term “organic group” is used for the purpose of this disclosure to mean a hydrocarbon group that is classified as an aliphatic group, cyclic group, or combination of aliphatic and cyclic groups (e.g., alkaryl and aralkyl groups). In the context of the present disclosure, suitable organic groups for metal chalcogenides or precursors thereof, as described herein, are those that do not interfere with the formation of such chalcogenides. In the context of the present disclosure, the term “aliphatic group” means a saturated or unsaturated linear or branched hydrocarbon group. This term is used to encompass alkyl, alkenyl, and alkynyl groups, for example. The term “alkyl group” means a saturated linear or branched monovalent hydrocarbon group including, for example, methyl, ethyl, n-propyl, isopropyl, tert-butyl, amyl, heptyl, and the like. The term “alkenyl group” means an unsaturated, linear or branched monovalent hydrocarbon group with one or more olefinically unsaturated groups (i.e., carbon-carbon double bonds), such as a vinyl group. The term “alkynyl group” means an unsaturated, linear or branched monovalent hydrocarbon group with one or more carbon-carbon triple bonds. The term “cyclic group” means a closed ring hydrocarbon group that is classified as an alicyclic group, aromatic group, or heterocyclic group. The term “alicyclic group” means a cyclic hydrocarbon group having properties resembling those of aliphatic groups. The term “aromatic group” or “aryl group” means a mono- or polynuclear aromatic hydrocarbon group. The term “heterocyclic group” means a closed ring hydrocarbon in which one or more of the atoms in the ring is an element other than carbon (e.g., nitrogen, oxygen, sulfur, etc.).
As a means of simplifying the discussion and the recitation of certain terminology used throughout this application, the terms “group” and “moiety” are used to differentiate between chemical species that allow for substitution or that may be substituted and those that do not so allow for substitution or may not be so substituted. Thus, when the term “group” is used to describe a chemical substituent, the described chemical material includes the unsubstituted group and that group with nonperoxidic O, N, S, Si, or F atoms, for example, in the chain as well as carbonyl groups or other conventional substituents. Where the term “moiety” is used to describe a chemical compound or substituent, only an unsubstituted chemical material is intended to be included. For example, the phrase “alkyl group” is intended to include not only pure open chain saturated hydrocarbon alkyl substituents, such as methyl, ethyl, propyl, tert-butyl, and the like, but also alkyl substituents bearing further substituents known in the art, such as hydroxy, alkoxy, alkylsulfonyl, halogen atoms, cyano, nitro, amino, carboxyl, etc. Thus, “alkyl group” includes ether groups, haloalkyls, nitroalkyls, carboxyalkyls, hydroxyalkyls, sulfoalkyls, etc. On the other hand, the phrase “alkyl moiety” is limited to the inclusion of only pure open chain saturated hydrocarbon alkyl substituents, such as methyl, ethyl, propyl, tert-butyl, and the like.
For example, useful copper dichalcogenocarbamates can include those of the formula Cu2+(−A-(A)C—NR1R2)2, wherein each R1 and R2 independently represents H or an organic group in which R1 and R2 can optionally be joined to form one or more rings; and each A independently represents a chalcogen (e.g., sulfur, selenium, or a combination thereof). When R1 and/or R2 represent an organic group, preferably the organic group is a carbon-bound (i.e., the bond to the group is to a carbon atom of the organic group) organic group. In certain embodiments, the organic group is an aliphatic group such as a C1-C30 aliphatic group, in some embodiments a C1-C20 aliphatic group. In other certain embodiments, the organic group is a C1-C30 hydrocarbon moiety, and in some embodiments a C1-C20 hydrocarbon moiety. In preferred embodiments, copper dichalcogenocarbamates include those of the formula Cu2+(−A-(A)C—N(C2H5)2)2, and each A independently represents a chalcogen (e.g., sulfur, selenium, or a combination thereof). Such preferred copper dichalcogenocarbamates include, but are not limited to, copper complexes of N,N-dimethyldithiocarbamate, N,N-dimethyldiselenocarbamate, N,N-dimethylthioselenocarbamate, N,N-diethyldithiocarbamate, N,N-diethyldiselenocarbamate, N,N-diethylthioselenocarbamate, N-undecyldithiocarbamate, N-undecyldiselenocarbamate, and N-undecylthioselenocarbamate.
For another example, useful zinc dichalcogenocarbamates can include those of the formula Zn2+(−A-(A)C—NR1R2)2, wherein each R1 and R2 independently represents H or an organic group in which R1 and R2 can optionally be joined to form one or more rings; and each A independently represents a chalcogen (e.g., sulfur, selenium, or a combination thereof). When R1 and/or R2 represent an organic group, preferably the organic group is a carbon-bound (i.e., the bond to the group is to a carbon atom of the organic group) organic group. In certain embodiments, the organic group is an aliphatic group such as a C1-C30 aliphatic group, in some embodiments a C1-C20 aliphatic group. In other certain embodiments, the organic group is a C1-C30 hydrocarbon moiety, and in some embodiments a C1-C20 hydrocarbon moiety. Such preferred zinc dichalcogenocarbamates include, but are not limited to, zinc complexes of N,N-dimethyldithiocarbamate, N,N-dimethyldiselenocarbamate, N,N-dimethylthioselenocarbamate, N,N-diethyldithiocarbamate, N,N-diethyldiselenocarbamate, N,N-diethylthioselenocarbamate, N-undecyldithiocarbamate, N-undecyldiselenocarbamate, and N-undecylthioselenocarbamate.
For a further example, useful tin carbamates can include those of the formula Sn4+(−A-(A)C—NR1R2)4, wherein each R1 and R2 independently represents H or an organic group in which R1 and R2 can optionally be joined to form one or more rings; and each A independently represents a chalcogen (e.g., sulfur, selenium, or a combination thereof). When R1 and/or R2 represent an organic group, preferably the organic group is a carbon-bound (i.e., the bond to the group is to a carbon atom of the organic group) organic group. In certain embodiments, the organic group is an aliphatic group such as a C1-C30 aliphatic group, in some embodiments a C1-C20 aliphatic group. In other certain embodiments, the organic group is a C1-C30 hydrocarbon moiety, and in some embodiments a C1-C20 hydrocarbon moiety. Such preferred tin dichalcogenocarbamates include, but are not limited to, tin complexes of N,N-dimethyldithiocarbamate, N,N-dimethyldiselenocarbamate, N,N-dimethylthioselenocarbamate, N,N-diethyldithiocarbamate, N,N-diethyldiselenocarbamate, N,N-diethylthioselenocarbamate, N-undecyldithiocarbamate, N-undecyldiselenocarbamate, and N-undecylthioselenocarbamate.
Conditions effective to form metal chalcogenides include heating suitable copper, zinc, and/or tin compounds selected from the group consisting of chalcogenocarbamates, dichalcogenocarbamates, mercaptides, thiolates, dithiolates, thiocarbonates, dithiocarbonates, trithiocarbonates, and combinations thereof (e.g., copper, zinc, and/or tin dichalcogenocarbamates), preferably in the substantial absence of oxygen (e.g., in vaccuo or under an inert atmosphere).
In some embodiments, the copper, zinc, and/or tin compounds selected from the group consisting of chalcogenocarbamates, dichalcogenocarbamates, mercaptides, thiolates, dithiolates, thiocarbonates, dithiocarbonates, trithiocarbonates, and combinations thereof (e.g., copper, zinc, and/or tin dichalcogenocarbamates) can be heated in a solvent (e.g., at a temperature of 125° C. to 300° C.) to form the metal chalcogenide. For certain embodiments, it is believed that the dichalcocarbamates decompose thermally to produce their corresponding sulfides. Properties of the dichalcocarbamates (e.g., melting and decomposition temperatures) depend on the metal, R1, and R2, and can be varied by changing these groups. In certain embodiments, the metal chalcogenide can be precipitated from the solvent in the form of nanocrystals.
For certain embodiments, when stoichiometric mixtures of copper, zinc, and tin dithiocarbamates are heated together they can decompose together to give CZTS. For example,
The amount of oleylamine injected to decompose these complexes can influence the availability of nucleation sites for growth of the nanocrystals. By varying the amount of oleylamine injected and the growth temperature, the average diameter of the nanocrystals can be tuned.
In a typical preparation, the contents of the flask can be heated to the desired temperature and a specific volume of oleylamine can be injected into this mixture. The nanocrystal size can be tuned by changing the temperature at which olelyamine is injected and the amount of oleylamine. For example, to prepare 2 nanometer diameter nanocrystals, 3 ml oleylamine can be injected into the flask at 150° C. to initiate nucleation and the nanocrystals can be allowed to grow for 4 minutes before quenching the contents of the flask by immersing the flask in water. For the preparation of 2.5 nanometer nanocrystals, a mixture of 1.5 ml oleylamine and 1.5 ml octadecene can be injected into the flask at 150° C. It should be understood that the quantities of reactants can be adjusted to produce the desired amount of nanocrystals.
To avoid premature decomposition of Sn(dedc)4 for preparations carried out above 150° C., Sn(dedc)4 can be dissolved in oleylamine and octadecene and injected into the flask with oleylamine, rather than dissolving and heating it to the reaction temperature with the other complexes. For example, for the preparation of 5 nanometer nanocrystals, Sn(dedc)4 can be dissolved in a mixture of 1.5 ml oleylamine and 1.5 ml octadecene and injected into the flask at 175° C. For the preparation of 7 nanometer nanocrystals, Sn(dedc)4 can be dissolved in a mixture of 0.75 ml oleylamine and 2.25 ml octadecene and injected into the flask at 175° C. All the other steps of the preparation and purification can, if desired, remain the same.
Nanocrystals can be precipitated from the dispersion using, for example, ethanol, and centrifuging for 5 minutes at 4000 revolutions per minute (rpm). The supernatant can be discarded and the nanocrystals can be redispersed in toluene. The precipitation and dispersion steps can optionally be repeated multiple times to wash out excess reactants. Finally, if desired, the nanocrystals can be dispersed in toluene and kept for further use.
In certain embodiments, the nanocrystals have an average particle size of 1 nanometer to 100 nanometers. In preferred embodiments, the nanocrystals have an average particle size of less than 2 nanometers to tens of nanometers.
The nanocrystals disclosed herein can be dispersed in various solvents (e.g., organic solvents or water) to form nanocrystal inks. Typically, the nanocrystals prepared as described herein can be readily dispersed in an organic solvent such as toluene to form a nanocrystal organic ink.
Alternatively, water based CZTS inks can be also prepared. Water based CZTS inks can be advantageous, for example, by avoiding the use of organic solvents. To make aqueous dispersions of CZTS nanocrystals, the organic ligands that stabilize CZTS nanocrystals in organic solvents can be stripped and exchanged by S2− ions. These ions surround the CZTS nanocrystals and can electrostatically stabilize the nanocrystals in aqueous solutions. In a typical ligand exchange procedure, the CZTS nanocrystals capped with oleylamine and dispersed in toluene in concentrations of 2 mg/mL can be contacted with a K2S solution in water and formamide. For 3 ml of 2 mg/mL CZTS nanocrystal dispersion 100 μL of 9-10 M K2S solution in water can be mixed with 1 mL of formamide and added to the CZTS nanocrystal dispersion in toluene. The organic toluene dispersion and aqueous K2S solution phase into separate layers. The two-phase mixture can be stirred, for example, at 1200 rpm for 90 minutes, resulting in the transfer of the CZTS nanocrystals capped with S2− ions from toluene to the aqueous phase. The toluene supernatant can be removed, for example, after centrifugation for 3 to 5 minutes at 4000 rpm. The CZTS nanocrystals can then be precipitated by addition of 1 mL of ethanol, centrifuged, washed, and redispersed in deionized water, for example, by sonication.
Nanocrystal inks can conveniently be coated on a substrate and heated to form a film of the metal chalcogenide. For example, thin films of CZTS, CZTSe and CZTSSe can be formed by coating surfaces of suitable substrates with nanocrystals and annealing the resulting nanocrystal film to form polycrystalline films. The surfaces can be coated from colloidal dispersions of nanocrystals (i.e., inks) using a variety of methods including, but not limited to, drop casting, spin coating, and/or dip coating.
CZTS nanocrystals (e.g., crystals with a diameter of 1 to 20 nm) can melt at temperatures much lower than bulk CZTS. Consequently, rapid grain growth at low temperatures is possible. Thus, CZTS nanocrystals can be coated onto a surface of a substrate and annealed using rapid thermal annealing at temperatures of 300 to 700° C. to provide a CZTS film. In rapid thermal annealing the temperature of the substrate and the film can be raised rapidly to the desired temperature (e.g., 300 to 700° C.) at rates of, for example, 1 to 5 degrees per second, then held at that temperature for a desired period of time for grain growth and recrystallization. This period of time may range from 0 to 1 hour, and typically are 5 to 15 minutes. The substrate and film are then cooled to room temperature, preferably at a rate slow enough to avoid film peeling or cracking due to thermal contraction, particularly if the substrate and the film have a high thermal expansion coefficient mismatch. Typical cooling rates may be 10° C. per minute. Annealing can be conducted in vacuum, under inert atmosphere such as nitrogen or argon, or even in a sulfidizing environment with H2S and sulfur vapor to replenish any sulfur that may escape the film during annealing. Brief rapid thermal annealing for 5 to 15 minutes as described herein typically does not reduce sulfur in the film. However, if films are annealed for times exceeding 1 hour, sulfur content in the films may decrease, and films may even become metallic. Conditions effective to achieve a particular balance between rapid crystallization and excessive sulfur loss can depend on a variety of factors, including, for example, the equipment being used. However, heating rates and annealing times can generally be adjusted to obtain CZTS films without significant sulfur loss. Thus, rapid thermal annealing can provide a method for obtaining large grained CZTS films, which can be advantageous, for example, in high throughput production.
Alternatively, thin films of metal chalcogenides can be formed on surfaces of suitable substrates directly from the metal complexes without forming the nanocrystal inks. For example, in some embodiments, the copper, zinc, and/or tin compounds selected from the group consisting of chalcogenocarbamates, dichalcogenocarbamates, mercaptides, thiolates, dithiolates, thiocarbonates, dithiocarbonates, trithiocarbonates, and combinations thereof (e.g., copper, zinc, and/or tin dichalcogenocarbamates) can be heated to a melt in the absence of a solvent (e.g., at a temperature of 150° C. to 900° C.), thus forming the metal chalcogenide. For one example, a mixture of suitable copper, zinc, and/or tin compounds (e.g., copper, zinc, and/or tin dichalcogenocarbamates) can be dissolved, dispersed, or suspended in a suitable solvent, the solvent mixture can be coated on a substrate, the solvent removed followed by heating to form a film of the metal chalcogenide. For certain embodiments, the dichalcocarbamates can optionally be heated in the presence of an amine (e.g., oleylamine).
Alternatively, the nanocrystal film can be annealed after ligand exchange to form a thin CZTS film (
For certain embodiments, the molten mixtures of the copper, zinc, and tin dithiocarbamates can be spread over desired substrates and subsequently decomposed to form CZTS films. Moreover, changing the groups attached to the carbamates can alter the melting temperature of the various complexes. However, there are some groups for which the complexes do not melt before decomposing, instead decomposing in their solid form to give the corresponding sulfide. CZTS films can be formed by dissolving the metal dithiocarbamate complexes in a solvent such as chloroform, placing a known amount of this solution on the substrate surface and heating the substrate. As the substrate is heated to temperatures above the boiling point of the solvent, the solvent evaporates, leaving behind a mixed powder of the metal complexes behind. As heating is continued, this mixture can subsequently melt to form a liquid mixture of the complexes. Further heating to higher temperatures can form solid CZTS. The presence of an amine additive (for example oleylamine, dodecylamine, etc.) in the solvent can decrease the decomposition temperature of these metal complexes and in some cases cause the complex to decompose even before the mixture reaches the melting temperature. A preferred method for making films from these complexes is to first dissolve stoichiometric or any desired amounts of the metal dithiocarbamate complexes in an organic solvent (e.g., chloroform, acetone, etc.) to form individual solutions (see
One approach is to spray a solution of these complexes on a heated substrate with temperatures ranging from 200° C. to 800° C. The metal complexes decompose into the corresponding sulfide as soon as they come in contact with the substrate. Another approach is to aerosolize the precursor solution containing the metal dithiocarbamates and place the heated substrate to be coated in the path of the aeresol particles which, upon impinging on the heated substrate, can decompose to form the film. Another approach is to aerosolize the precursor solution containing the metal dithiocarbamates, and heat the aeresol particles in flight to form CZTS particles. Placing a heated substrate in the path of these particles can form the film by impaction. Diselenocarbamate complexes can also be used in a similar manner as the dithiocarbamates to give metal selenides instead of sulfides. A mixture of these two types of complexes gives a final absorber layer containing both sulfur and selenium, for example, in the ratio of the initial precursor mix. This approach of making absorber layer directly from the metal complexes can avoid the intermediate step of forming the nanocrystal colloidal dispersion.
Finally, the solar cell architectures that can be realized using the CZTS film deposition methods described herein need not be limited to that in
One of skill in the art, particularly in view of the teachings of the present disclosure, can select desired ratios of copper, zinc, and/or tin dichalcocarbamates to provide metal chalcogenides of the formula Cu2+x+zZn1-xSn1-zA4, wherein A represents one or more chalcogens; −1≦x≦1; −1≦z≦1; and with the proviso that when x=z they are not equal to 1.
Changing relative amounts of the metal complexes in the reacting solution can change the composition of the nanocrystals. For example, copper tin sulfide (Cu3SnS4) can be prepared by replacing all Zn(dedc)2 with Cu(dedc)2. Using the methods described herein and only changing the proportions of the metal complexes in the reacting solution, nanocrystals of the formula Cu2+x+zZn1-xSn1-zSySe4-y can be prepared, wherein −1≦x≦1, 0≦y≦4, and −1≦z≦1. For example, it may be desired to make copper deficient or copper rich CZTS (e.g., Cu2+wZn1-wSnSySe4-y, wherein w is a small positive or negative number) to alter the carrier type or electronic doping in CZTS nanocrystals. Hereinafter we refer to all these films (CZTS, CZTSe, and CZTSSe) with differing Cu, Zn, Sn, S, and Se stoichiometry as CZTS for brevity. It is understood that the composition can be adjusted by using appropriate desired combinations and amounts of the corresponding metal thiocarbamates or the corresponding selenocarbamates.
Useful metal chalcogenides that can be prepared by the methods disclosed herein include, but are not limited to, those of the formulas Cu2+zZnSn1-zSySe4-y; Cu2ZnSnSySe4-y; Cu3ZnSySe4-y; CuZnSn2SySe4-y; Cu1+zZn2Sn1-zSySe4-y; CuZn2SnSySe4-y; Cu2Zn2SySe4-y; Zn2Sn2SySe4-y; Cu3+zSn1-zSySe4-y; Cu3SnSySe4-y; Cu2Sn2SySe4-y; Cu2+xZn1-xSnSySe4-y; Cu3+xZn1-xSySe4-y; and Cu1+xZn1-xSn2SySe4-y, wherein 0≦y≦4. Particularly useful metal chalcogenides include those of the formula Cu2ZnSnSySe4-y, wherein 0≦y≦4.
Other particularly useful metal chalcogenides include copper-rich copper zinc tin chalcogenides of the formula Cu2+xZn1-xSn SySe4-y, wherein: 0≦y≦4; and 0<x<1. Such copper-rich copper zinc tin chalcogenides can be useful for preparing p-doped metal chalcogenide layers.
Other particularly useful metal chalcogenides include copper-deficient copper zinc tin chalcogenides of the formula Cu2+xZn1-xSn SySe4-y, wherein: 0≦y≦4; and −1<x<0. Such copper-deficient copper zinc tin chalcogenides can be useful for preparing n-doped metal chalcogenide layers.
Metal chalocogenides as disclosed herein can be used, for example, to prepare solar cells. Disclosed herein are solar cells, and methods of making solar cells, that include a substrate and one or more copper zinc tin chalcogenide layers.
In certain embodiments the copper zinc tin chalcogenide is copper-deficient and is of the formula Cu2+xZn1-xSn SySe4-y, wherein: 0≦y≦4; and −1<x<0. In other certain embodiments, the copper zinc tin chalcogenide is copper-rich and is of the formula Cu2+xZn1-xSn SySe4-y, wherein: 0≦y≦4; and 0<x<1. In some embodiments the solar cells include one or more copper-deficient copper zinc tin chalcogenide layers and one or more copper-rich copper zinc tin chalcogenide layers. Optionally, the solar cells can further include a zinc sulfide, a tin oxide, and/or a zinc oxide buffer layer over at least one metal chalcogenide layer or layers.
The band gap of the CZTS nanocrystals determines the wavelengths of light that the nanocrystals absorb. CZTS has a bandgap of approximately 1.5 eV, ideal for making solar cells. Moreover, this bandgap makes the theoretical maximum efficiency of CZTS based solar cells nearly the same as that for CIGS and CdTe based solar cells.
Raman scattering from CZTS is unambiguous and can be used in addition to the above characterization methods to determine the phase purity of the nanocrystals.
The present invention is illustrated by the following examples. It is to be understood that the particular examples, materials, amounts, and procedures are to be interpreted broadly in accordance with the scope and spirit of the invention as set forth herein.
To prepare Cu(dedc)2, sodium diethyldithiocarbamate (9.0 g) in ethanol was added dropwise to 85 mg/ml copper chloride (4.23 g) solution in ethanol while constantly stirring. The black precipitate that formed upon reaction was filtered and washed multiple times with water before drying in a desiccator. Cu(dedc)2 crystals were purified by recrystallization from chloroform and dried overnight in vacuum before use. These crystals melted at 200° C.
To prepare Zn(dedc)2, sodium diethyldithiocarbamate (9.0 g) in ethanol was added dropwise to 68 mg/ml zinc chloride (3.38 g) solution in ethanol while constantly stirring. The white precipitate that formed upon reaction was filtered and washed multiple times with water before drying in a desiccator. Zn(dedc)2 crystals were purified by recrystallization from chloroform and dried overnight in vacuum before use. These crystals melted at 181° C.
To prepare Sn(dedc)4, sodium diethyldithiocarbamate (12.85 g) in ethanol (200 ml) was added dropwise to 50 mg/ml tin tetrachloride (2.5 g) solution in ethanol (50 ml) while constantly stirring. The orange precipitate that formed upon reaction was filtered and washed multiple times with water before drying in a desiccator. Sn(dedc)4 crystals were purified by recrystallization from acetone and dried overnight in vacuum before use. These crystals melted at 169° C.
CZTS nanocrystals were prepared in a nitrogen atmosphere using a Schlenk line apparatus. In a typical preparation, 18 ml octadecene and 2 ml oleic acid were mixed in a 100 ml three neck flask (
The amount of oleylamine injected to decompose these complexes influences the available nucleation sites for growth of the nanocrystals. By varying the amount of oleylamine injected and the growth temperature, the average diameter of the nanocrystals can be tuned.
In a typical preparation, the contents of the flask were heated to the desired temperature and a specific volume of oleylamine was injected into this mixture. The nanocrystal size was tuned by changing the temperature at which olelyamine was injected and the amount of oleylamine. For example, to prepare 2 nanometer diameter nanocrystals, 3 ml oleylamine was injected into the flask at 150° C. to initiate nucleation and the nanocrystals were allowed to grow for 4 minutes before quenching the contents of the flask by immersing the flask in water. For the preparation of 2.5 nanometer nanocrystals, a mixture of 1.5 ml oleylamine and 1.5 ml octadecene were injected into the flask at 150° C.
To avoid premature decomposition of Sn(dedc)4 for preparations carried out above 150° C., Sn(dedc)4 was dissolved in oleylamine and octadecene and injected into the flask with oleylamine rather than dissolving and heating it to the reaction temperature with the other complexes. For example, for the preparation of 5 nanometer nanocrystals, Sn(dedc)4 was dissolved in a mixture of 1.5 ml oleylamine and 1.5 ml octadecene and injected into the flask at 175° C. For the preparation of 7 nanometer nanocrystals, Sn(dedc)4 was dissolved in a mixture of 0.75 ml oleylamine and 2.25 ml octadecene and injected into the flask at 175° C. All the other steps of the preparation and purification remained the same.
The nanocrystals were precipitated from the dispersion using ethanol and were centrifuged for 5 minutes at 4000 revolutions per minute (rpm). The supernatant was discarded and the nanocrystals were redispersed in toluene. The precipitation and dispersion steps were repeated multiple times to wash out the excess reactants. Finally the nanocrystals were dispersed in toluene to prepare a nanocrystal organic ink that was kept for further use. Alternatively, water based inks could also be prepared as described herein above.
As the nanocrystals get larger, the peaks become better defined, sharper, and more intense. As an example,
Moreover, the compositions of various batches of nanocrystals were determined using inductively coupled plasma-mass spectroscopy (ICP-MS) as well as electron probe microanalysis (EPMA) and were consistent with the stoichiometry of Cu2ZnSnS4. Table 1 shows a typical result.
The XRD and Raman scattering from these films show that the films are CZTS. In fact, upon annealing, the XRD and Raman peaks of CZTS become sharper and more intense than before annealing, indicating that grain growth takes place. For example,
An alternative method for making CZTS films from the metal dithiocarbamate complexes avoids forming nanocrystals and offers the means to form the film by applying the complexes directly onto the surface of the substrate. Metal dithiocarbamate complexes have the useful property that they melt before they decompose. For example,
The formation of thin films of metal sulfides from the dithiocarbamate complexes was studied using an X-ray diffractometer with an in situ heating stage. A mixture of the Cu(dedc)2, Zn(dedc)2 Sn(dedc)4 (2:1:1 molar ratio) was heated on the heating stage while collecting data in regular intervals.
Dried powder of CZTS nanocrystals was heated inside a quartz capillary while it was under examination with X-rays of wavelength 0.3196 Å to monitor the structural changes.
In summary, the present disclosure illustrates the following embodiments:
A method of preparing a metal chalcogenide comprising heating components comprising: at least one copper, zinc, and/or tin compound selected from the group consisting of chalcogenocarbamates, dichalcogenocarbamates, mercaptides, thiolates, dithiolates, thiocarbonates, dithiocarbonates, trithiocarbonates, and combinations thereof; wherein heating comprises conditions effective to form a compound of the formula Cu2+x+zZn1-xSn1-zA4, wherein A represents one or more chalcogens; −1≦x≦1; −1≦z≦1; and with the proviso that when x=z they are not equal to 1.
The method of embodiment 1 wherein the at least one copper dichalcogenocarbamate is of the formula Cu2+(−A-(A)C—NR1R2)2, wherein each R1 and R2 independently represents H or an organic group in which R1 and R2 can optionally be joined to form one or more rings; and each A independently represents a chalcogen.
The method of embodiment 1 or 2 wherein the at least one zinc dichalcogenocarbamate is of the formula Zn2+(−A-(A)C—NR1R2)2, wherein each R1 and R2 independently represents H or an organic group in which R1 and R2 can optionally be joined to form one or more rings; and each A independently represents a chalcogen.
The method of any one of the preceding embodiments wherein the at least one tin dichalcogenocarbamate is of the formula Sn4+(−A-(A)C—NR1R2)4, wherein each R1 and R2 independently represents H or an organic group in which R1 and R2 can optionally be joined to form one or more rings; and each A independently represents a chalcogen.
The method of any one of the preceding embodiments wherein the chalcogen is selected from the group consisting of sulfur, selenium, and combinations thereof.
The method of any one of the preceding embodiments wherein each R1 and R2 independently represents hydrogen or a C1 to C30 aliphatic group.
The method of any one of the preceding embodiments wherein each R1 and R2 independently represents hydrogen or a C1 to C30 aliphatic moiety.
The method of any one of the preceding embodiments wherein x=0, and the compound is of the formula Cu2+zZnSn1-zSySe4-y, wherein 0≦y≦4.
The method of embodiment 8 wherein z=0, and the compound is of the formula Cu2ZnSnSySe4-y, wherein 0≦y≦4.
The method of embodiment 8 wherein z=1, and the compound is of the formula Cu3ZnSySe4-y, wherein 0≦y≦4.
The method of embodiment 8 wherein z=−1, and the compound is of the formula CuZnSn2SySe4-y, wherein 0≦y≦4.
The method of any one of embodiments 1 to 7 wherein x=−1, and the compound is of the formula Cu1+zZn2Sn1-zSySe4-y, wherein 0≦y≦4.
The method of embodiment 12 wherein z=0, and the compound is of the formula CuZn2SnSySe4-y, wherein 0≦y≦4.
The method of embodiment 12 wherein z=1, and the compound is of the formula Cu2Zn2SySe4-y, wherein 0≦y≦4.
The method of embodiment 12 wherein z=−1, and the compound is of the formula Zn2Sn2SySe4-y, wherein 0≦y≦4.
The method of any one of embodiments 1 to 7 wherein x=1, and the compound is of the formula Cu3+zSn1-zSySe4-y, wherein 0≦y≦4.
The method of embodiment 16 wherein z=0, and the compound is of the formula Cu3SnSySe4-y, wherein 0≦y≦4.
The method of embodiment 16 wherein z=−1, and the compound is of the formula Cu2Sn2SySe4-y, wherein 0≦y≦4.
The method of any one of embodiments 1 to 7 wherein z=0, and the compound is of the formula Cu2+xZn1-xSnSySe4-y, wherein 0≦y≦4.
The method of any one of embodiments 1 to 7 wherein
z=1, and the compound is of the formula Cu3+xZn1-xSySe4-y, wherein 0≦y≦4.
The method of any one of embodiments 1 to 7 wherein z=−1, and the compound is of the formula Cu1+xZn1-xSn2SySe4-y, wherein 0≦y≦4.
The method of any one of the preceding embodiments wherein conditions effective to form the compound comprise heating the components in the substantial absence of oxygen.
The method of any one of the preceding embodiments wherein conditions effective to form the compound comprise heating the components in a solvent at a temperature of 125° C. to 300° C., and wherein the formed compound is in the form of nanocrystals.
The method of embodiment 23 wherein the nanocrystals have an average particle size of 1 nanometer to 100 nanometers.
The method of embodiment 24 wherein the nanocrystals have an average particle size of 1 nanometer to 20 nanometers.
The method of any one of embodiment 23 to 25 further comprising coating the nanocrystals on a substrate and heating the nanocrystals under conditions effective to form a film of the compound.
The method of embodiment 26 wherein conditions effective to form the film comprise conditions for rapid thermal annealing.
The method of embodiment 26 wherein conditions effective to form the film comprise heating at a temperature below the melting point of the bulk compound.
The method of embodiment 26 wherein heating comprises heating at a temperature of 300° C. to 700° C.
The method of embodiment 29 wherein heating comprises heating at a temperature of 350° C. to 550° C.
The method of embodiment 26 wherein conditions effective to form the film comprise heating for a time of less than or equal to one hour.
The method of embodiment 31 wherein conditions effective to form the film comprise heating for a time of 5 minutes to 15 minutes.
The method of any one of embodiments 1 to 22 wherein the components are applied to a substrate, and wherein conditions effective to form the compound comprise heating the combined components at a temperature of 150° C. to 900° C. to form a film of the compound.
The method of any one of the preceding embodiments wherein conditions effective to form the compound comprise heating in the presence of an amine.
The method of embodiment 34 wherein the amine is selected from the group consisting of oleylamine, dodecylamine, and combinations thereof.
A colloidal dispersion of nanocrystals prepared by a method of any one of embodiments 23 to 25.
The colloidal dispersion of embodiment 36 wherein the dispersion is in the form of a nanocrystal organic ink.
The colloidal dispersion of embodiment 36 wherein the dispersion is in the form of a nanocrystal aqueous ink.
A solar cell comprising: a substrate; and a layer comprising a copper-deficient copper zinc tin chalcogenide over the substrate.
A solar cell comprising: a substrate; and a layer comprising a copper-rich copper zinc tin chalcogenide over the substrate.
A solar cell comprising: a substrate; a layer comprising a copper-deficient copper zinc tin chalcogenide over the substrate; and a layer comprising a copper-rich copper zinc tin chalcogenide over the copper-deficient copper zinc tin chalcogenide layer.
A solar cell comprising: a substrate; a layer comprising a copper-rich copper zinc tin chalcogenide over the substrate; and a layer comprising a copper-deficient copper zinc tin chalcogenide over the copper-rich copper zinc tin chalcogenide layer.
The solar cell of any one of embodiments 40 to 42 wherein the copper-rich copper zinc tin chalcogenide is of the formula Cu2+xZn1-xSn SySe4-y, wherein: 0≦y≦4; and 0<x<1.
The solar cell of any one of embodiments 39, 41, and 42 wherein the copper-deficient copper zinc tin chalcogenide is of the formula Cu2+xZn1-xSn SySe4-y, wherein: 0≦y≦4; and −1<x<0.
The solar cell of any one of embodiments 39 to 44 further comprising a zinc sulfide buffer layer over at least one metal chalcogenide layer or layers.
A method of making a solar cell, the method comprising: preparing a metal chalcogenide by a method according to any one of embodiments 1 to 35; and forming a layer comprising the metal chalcogenide over a substrate.
The method of embodiment 46 further comprising forming a zinc sulfide buffer layer over the metal chalcogenide layer.
The method of embodiment 46 further comprising forming a tin oxide buffer layer over the metal chalcogenide layer.
The method of embodiment 46 further comprising forming a zinc oxide buffer layer over the metal chalcogenide layer.
The complete disclosure of all patents, patent applications, and publications, and electronically available material cited herein are incorporated by reference. The foregoing detailed description and examples have been given for clarity of understanding only. No unnecessary limitations are to be understood therefrom. The invention is not limited to the exact details shown and described, for variations obvious to one skilled in the art will be included within the invention defined by the claims.
This application claims the benefit of U.S. Provisional Application No. 61/434,854, filed Jan. 21, 2011, which is hereby incorporated by reference in its entirety.
The present invention was made with government support under Agency Grant Nos. DMR-0819885 and CBET-0931145 from the National Science Foundation MRSEC. The Government has certain rights in this invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US12/21991 | 1/20/2012 | WO | 00 | 4/14/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61434854 | Jan 2011 | US |
