U.S. patent application number 13/693065 was filed with the patent office on 2013-04-18 for atypical kesterite compositions.
This patent application is currently assigned to E I DU PONT DE NEMOURS AND COMPANY. The applicant listed for this patent is E I Du Pont De Nemours and Company. Invention is credited to LYNDA KAYE JOHNSON, DANIELA RODICA RADU, H. DAVID ROSENFELD.
Application Number | 20130095602 13/693065 |
Document ID | / |
Family ID | 44121344 |
Filed Date | 2013-04-18 |
United States Patent
Application |
20130095602 |
Kind Code |
A1 |
JOHNSON; LYNDA KAYE ; et
al. |
April 18, 2013 |
ATYPICAL KESTERITE COMPOSITIONS
Abstract
This invention relates to processes for making kesterite
compositions with atypical Cu:Zn:Sn:S ratios and/or kesterite
compositions with unusually small coherent domain sizes. This
invention also relates to these kesterite compositions and their
use in preparing CZTS films.
Inventors: |
JOHNSON; LYNDA KAYE;
(Wilmington, DE) ; ROSENFELD; H. DAVID; (Drumore,
PA) ; RADU; DANIELA RODICA; (West Grove, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
E I Du Pont De Nemours and Company; |
Wilmington |
DE |
US |
|
|
Assignee: |
E I DU PONT DE NEMOURS AND
COMPANY
Wilmington
DE
|
Family ID: |
44121344 |
Appl. No.: |
13/693065 |
Filed: |
December 4, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12784799 |
May 21, 2010 |
8366975 |
|
|
13693065 |
|
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Current U.S.
Class: |
438/95 ;
252/501.1; 423/508; 423/511 |
Current CPC
Class: |
C01B 19/002 20130101;
C01G 19/006 20130101; C23C 18/1283 20130101; C23C 18/1204 20130101;
H01L 31/18 20130101; H01L 31/0326 20130101; Y02E 10/50 20130101;
C09D 11/52 20130101 |
Class at
Publication: |
438/95 ; 423/508;
423/511; 252/501.1 |
International
Class: |
H01L 31/032 20060101
H01L031/032; H01L 31/18 20060101 H01L031/18 |
Claims
1-4. (canceled)
5. A composition comprising a compound having a kesterite
structure, wherein the coherent domain size of the kesterite is
2-30 nm.
6. The composition of claim 5, further comprising one or more
copper-, tin-, or zinc-containing compounds selected from the group
consisting of binary copper chalcogenides, binary copper oxides,
binary tin chalcogenides, binary tin oxides, binary zinc
chalcogenides, ZnO, and CTS--Se.
7. The composition of claim 6, wherein the molar ratio of copper to
zinc to tin is about 2:1:1.
8. The composition of claim 6, wherein the molar ratio of copper to
(zinc plus tin) is less than one, and the molar ratio of zinc to
tin is greater than one.
9. The composition of claim 5, wherein the Cu/Zn ratio of the
kesterite is greater than about 3.1.
10. A process comprising: a) preparing an ink comprising: i) a
copper source selected from the group consisting of copper
complexes of nitrogen-, oxygen-, sulfur-, and selenium-based
organic ligands, and mixtures thereof; ii) a tin source selected
from the group consisting of tin complexes of nitrogen-, oxygen-,
carbon-, sulfur-, and selenium-based organic ligands, tin hydrides,
tin sulfides, tin selenides, and mixtures thereof; iii) a zinc
source selected from the group consisting of zinc complexes of
nitrogen-, oxygen-, sulfur-, and selenium-based organic ligands,
and mixtures thereof; iv) an elemental chalcogen selected from the
group consisting of: elemental S, elemental selenium, and mixtures
thereof; v) a solvent; iv) optionally, a chalcogen compound
selected from the group consisting of: R.sup.1S--Z, R.sup.1Se--Z
and mixtures thereof, with each Z independently selected from the
group consisting of: H and SiR.sup.5.sub.3; wherein each R.sup.1
and R.sup.5 is independently selected from the group consisting of:
hydrocarbyl and O-, N-, S-, halogen- and
tri(hydrocarbyl)silyl-substituted hydrocarbyl; b) heating the ink
to a temperature of 150-300.degree. C.; b) disposing the ink onto a
substrate; and c) heating the substrate and ink disposed thereon to
a temperature of 80-350.degree. C. to form a coated substrate
comprising CZTS--Se kesterite.
11. The process of claim 10, wherein the ratio of the total number
of moles of the chalcogen compound, the sulfur- and selenium-based
organic ligands, and the copper-, tin- and zinc-sulfides and
selenides to the total number of moles of the copper, tin and zinc
complexes is at least about 1.
12. The process of claim 11, wherein the ink comprises a chalcogen
compound.
13. The process of claim 10, wherein the elemental chalcogen
comprises sulfur and the molar ratio of elemental chalcogen is
about 0.2 to about 5 relative to the tin source.
14. The process of claim 10, wherein the nitrogen-, oxygen-,
carbon-, sulfur- and selenium-based organic ligands are selected
from the group consisting of: amidos; alkoxides; acetylacetonates;
carboxylates; hydrocarbyls; O-, N-, S-, halogen-, and
tri(hydrocarbyl)silyl-substituted hydrocarbyls; thio- and
selenolates; thio-, seleno-, and dithiocarboxylates; dithio-,
diseleno-, and thioselenocarbamates; and dithioxanthogenates.
15. The process of claim 10, wherein the process is conducted at
atmospheric pressure and the boiling point of the solvent is
greater than about 150.degree. C. at atmospheric pressure.
16. The process of claim 10, wherein the solvent is selected from
the group consisting of heteroaromatics, amines, and mixtures
thereof.
Description
FIELD OF THE INVENTION
[0001] This invention relates to processes for making kesterite
compositions with atypical Cu:Zn:Sn:S ratios and/or kesterite
compositions with unusually small coherent domain sizes. This
invention also relates to these kesterite compositions and their
use in preparing CZTS films.
BACKGROUND
[0002] Crystalline multinary-metal chalcogenide compositions
containing only non-toxic and abundant elements are of particular
interest in developing environmentally sustainable processes and
devices. Copper tin sulfide (Cu.sub.2SnS.sub.3 or "CTS") and copper
zinc tin sulfide (Cu.sub.2ZnSnS.sub.4 or "CZTS") are particularly
useful examples of this class of materials, and are of interest due
to their potential applications as small band-gap semiconductors,
as nonlinear materials, and as suitable candidates for photovoltaic
cell materials.
[0003] Thin-film photovoltaic cells typically use semiconductors
such as CdTe or copper indium gallium sulfide/selenide (CIGS) as an
energy absorber material. Due to toxicity of cadmium and the
limited availability of indium, alternatives are sought. CZTS
possesses a band gap energy of about 1.5 eV and a large absorption
coefficient (approx. 10.sup.4 cm.sup.-1), making it a promising
CIGS replacement.
[0004] Challenges in making CZTS thin-films are illustrative of the
general challenges that must be surmounted in making films of
crystalline multinary-metal chalcogenide compositions. Current
techniques to make CZTS thin films (e.g., thermal evaporation,
sputtering, hybrid sputtering, pulsed laser deposition and electron
beam evaporation) require complicated equipment and therefore tend
to be expensive. Electrochemical deposition is an inexpensive
process, but compositional non-uniformity and/or the presence of
secondary phases prevents this method from generating high-quality
CZTS thin-films. CZTS thin-films can also be made by the spray
pyrolysis of a solution containing metal salts, typically CuCl,
ZnCl.sub.2, and SnCl.sub.4, using thiourea as the sulfur source.
This method tends to yield films of poor morphology, density and
grain size. Photochemical deposition has also been shown to
generate p-type CZTS thin films. However, the composition of the
product is not well-controlled, and it is difficult to avoid the
formation of impurities such as hydroxides. The synthesis of CZTS
nanoparticles, which incorporate high-boiling amines as capping
agents, has also been disclosed. The presence of capping agents in
the nanoparticle layer may contaminate and lower the density of the
annealed CZTS film.
[0005] A hybrid solution-particle approach to CZTS involving the
preparation of a hydrazine-based slurry comprising dissolved Cu--Sn
chalcogenides (S or S--Se), Zn-chalcogenide particles, and excess
chalcogen has been reported. However, hydrazine is a highly
reactive and potentially explosive solvent that is described in the
Merck Index as a "violent poison."
[0006] Hence, there still exists a need for simple, low-cost,
scalable materials and processes with a low number of operations
that provide high-quality, crystalline CTS and CZTS films with
tunable composition and morphology. There also exists a need for
low-temperature routes to these materials using solvents and
reagents with relatively low toxicity.
SUMMARY
[0007] One aspect of this invention is a composition comprising a
compound having a kesterite structure, wherein the Cu/Zn ratio of
the kesterite is greater than about 3.1.
[0008] One aspect of this invention is a composition comprising a
compound having a kesterite structure, wherein the Cu/Zn ratio of
the kesterite is less than about 1.6.
[0009] One aspect of this invention is a composition comprising a
compound having a kesterite structure, wherein the coherent domain
size of the kesterite is 2-30 nm.
[0010] Another aspect of this invention is a process
comprising:
a) preparing an ink comprising:
[0011] i) a copper source selected from the group consisting of
copper complexes of nitrogen-, oxygen-, sulfur-, and selenium-based
organic ligands, and mixtures thereof;
[0012] ii) a tin source selected from the group consisting of tin
complexes of nitrogen-, oxygen-, carbon-, sulfur-, and
selenium-based organic ligands, tin hydrides, tin sulfides, tin
selenides, and mixtures thereof;
[0013] iii) a zinc source selected from the group consisting of
zinc complexes of nitrogen-, oxygen-, sulfur-, and selenium-based
organic ligands, and mixtures thereof;
[0014] iv) an elemental chalcogen selected from the group
consisting of: elemental S, elemental selenium, and mixtures
thereof;
[0015] v) a solvent;
[0016] iv) optionally, a chalcogen compound selected from the group
consisting of: R.sup.1S--Z, R.sup.1Se--Z and mixtures thereof, with
each Z independently selected from the group consisting of: H and
SiR.sup.5.sub.3; wherein each R.sup.1 and R.sup.5 is independently
selected from the group consisting of: hydrocarbyl and O-, N-, S-,
halogen-, and tri(hydrocarbyl)silyl-substituted hydrocarbyl;
b) heating the ink to a temperature of 150-300.degree. C.; b)
disposing the ink onto a substrate; and c) heating the substrate
and ink disposed thereon to a temperature of 80-350.degree. C. to
form a coated substrate comprising CZTS--Se kesterite.
DETAILED DESCRIPTION
[0017] Herein, the terms "solar cell" and "photovoltaic cell" are
synonymous unless specifically defined otherwise. These terms refer
to devices that use semiconductors to convert visible and
near-visible light energy into usable electrical energy. The terms
"band gap energy", "optical band gap", and "band gap" are
synonymous unless specifically defined otherwise. These terms refer
to the energy required to generate electron-hole pairs in a
semiconductor material, which in general is the minimum energy
needed to excite an electron from the valence band to the
conduction band.
[0018] Herein, element groups are represented utilizing CAS
notation. As used herein, the term "chalcogen" refers to Group VIA
elements, and the terms "metal chalcogenides" or "chalcogenides"
refer to materials that comprise metals and Group VIA elements.
Suitable Group VIA elements include sulfur, selenium and tellurium.
Metal chalcogenides are important candidate materials for
photovoltaic applications, since many of these compounds have
optical band gap values well within the terrestrial solar
spectra.
[0019] Herein, the term "binary-metal chalcogenide" refers to a
chalcogenide composition comprising one metal. The term
"ternary-metal chalcogenide" refers to a chalcogenide composition
comprising two metals. The term "quaternary-metal chalcogenide"
refers to a chalcogenide composition comprising three metals. The
term "multinary-metal chalcogenide" refers to a chalcogenide
composition comprising two or more metals and encompasses ternary
and quaternary metal chalcogenide compositions.
[0020] Herein, the terms "copper tin sulfide" and "CTS" refer to
Cu.sub.2SnS.sub.3; "copper tin selenide" and "CTSe" refer to
Cu.sub.2SnSe.sub.3; and "copper tin sulfide/selenide" and "CTS--Se"
encompass all possible combinations of Cu.sub.2Sn(S,Se).sub.3,
including Cu.sub.2SnS.sub.3, Cu.sub.2SnSe.sub.3, and
Cu.sub.2SnS.sub.xSe.sub.3-x, where 0.ltoreq.x.ltoreq.3. The terms
"copper tin sulfide", "copper tin selenide", "copper tin
sulfide/selenide", "CTS", "CTSe" and "CTS--Se" further encompass
fractional stoichiometries, e.g., Cu.sub.1.80Sn.sub.1.05S.sub.3.
That is, the stoichiometry of the elements can vary from a strictly
2:1:3 molar ratio.
[0021] Herein, the terms copper zinc tin sulfide and "CZTS" refer
to Cu.sub.2ZnSnS.sub.4; copper zinc tin selenide and "CZTSe" refer
to Cu.sub.2ZnSnSe.sub.4; and copper zinc tin sulfide/selenide and
"CZTS--Se" encompass all possible combinations of
Cu.sub.2ZnSn(S,Se).sub.4, including Cu.sub.2ZnSnS.sub.4,
Cu.sub.2ZnSnSe.sub.4, and Cu.sub.2ZnSnS.sub.xSe.sub.4-x, where
0.ltoreq.x.ltoreq.4. The terms "CZTS," "CZTSe," and "CZTS--Se"
further encompass copper zinc tin sulfide/selenide semiconductors
with fractional stoichiometries, e.g.,
Cu.sub.1.94Zn.sub.0.63Sn.sub.1.3S.sub.4. That is, the stoichiometry
of the elements can vary from a strictly 2:1:1:4 molar ratio.
Materials designated as CTS--Se and CZTS--Se may also contain small
amounts of other elements such as sodium. To date, the highest
efficiencies have been measured for copper-poor CZTS--Se solar
cells, where by "copper-poor" it is understood that the ratio
Cu/(Zn+Sn) is less than 1.0. For high efficiency devices, a molar
ratio of zinc to tin is greater than one is also desirable.
[0022] The term "kesterite" is commonly used to refer to materials
belonging to the kesterite family of minerals and is also the
common name of the mineral CZTS. As used herein, the term
"kesterite" refers to crystalline compounds in either the I4- or
I4-2m space groups having the nominal formula
Cu.sub.2ZnSn(S--Se).sub.4, and also "atypical kesterites" wherein
zinc has replaced a fraction of the copper or copper has replaced a
fraction of the zinc to give Cu.sub.cZn.sub.zSn(S--Se).sub.4
wherein c is greater than two and z is less than one; or c is less
than two and z is greater than one. The term "kesterite structure"
refers to the structure of these compounds. X-ray absorption
spectroscopy (XAS) reveals spectral features unique to the
kesterite form and allows for determination of the ratio of Cu to
Zn in the kesterite phase. This allows for the identification of
atypical kesterite compositions, which are clearly distinguished
from a mixture of separate sulfide phases producing the same
elemental ratios in aggregate. Control of stoichiometry in this
phase allows for control of electronic properties for improved
performance in a photovoltaic device.
[0023] As used herein, "coherent domain size" refers to the size of
crystalline domains over which a defect-free, coherent structure
may exist. The coherency comes from the fact that the
three-dimensional ordering is not broken inside of these
domains.
[0024] Herein, the term "metal salts" refers to compositions
wherein metal cations and inorganic anions are joined by ionic
bonding. Relevant classes of inorganic anions comprise oxides,
sulfides, carbonates, sulfates and halides. Herein, the term "metal
complexes" refers to compositions wherein a metal is bonded to a
surrounding array of molecules or anions, typically called
"ligands" or "complexing agents". The atom within a ligand that is
directly bonded to the metal atom or ion is called the "donor atom"
and, herein, often comprises nitrogen, oxygen, selenium, or
sulfur.
[0025] Herein, ligands are classified according to the "Covalent
Bond Classification (CBC) Method" (Green, M. L. H. J. Organomet.
Chem. 1995, 500, 127-148). An "X-function ligand" is one which
interacts with a metal center via a normal 2-electron covalent
bond, composed of 1 electron from the metal and 1 electron from the
X ligand. Simple examples of X-type ligands include alkyls and
thiolates. Herein, the term "nitrogen-, oxygen-, carbon-, sulfur-,
and selenium-based organic ligands" refers specifically to
carbon-containing X-function ligands, wherein the donor atom
comprises nitrogen, oxygen, carbon, sulfur, or selenium. Herein,
the term "complexes of nitrogen-, oxygen-, carbon-, sulfur-, and
selenium-based organic ligands" refers to the metal complexes
comprising these ligands. Examples include metal complexes of
amidos, alkoxides, acetylacetonates, acetates, carboxylates,
hydrocarbyls, O-, N-, S-, Se--, and halogen-substituted
hydrocarbyls, thiolates, selenolates, thiocarboxylates,
selenocarboxylates, dithiocarbamates, and diselenocarbamates.
[0026] As defined herein, a "hydrocarbyl group" is a univalent
group containing only carbon and hydrogen. Examples of hydrocarbyl
groups include unsubstituted alkyls, cycloalkyls, and aryl groups,
including alkyl-substituted aryl groups. Suitable hydrocarbyl
groups and alkyl groups contain 1 to about 30 carbons, or 1 to 25,
1 to 20, 1 to 15, 1 to 10, 1 to 5, 1 to 4, or 1 to 2 carbons. By
"heteroatom-substituted hydrocarbyl" is meant a hydrocarbyl group
that contains one or more heteroatoms wherein the free valence is
located on carbon, not on the heteroatom. Examples include
hydroxyethyl and carbomethoxyethyl. Suitable heteroatom
substituents include O-, N-, S-, halogen, and
tri(hydrocarbyl)silyl. In a substituted hydrocarbyl, all of the
hydrogens may be substituted, as in trifluoromethyl. Herein, the
term "tri(hydrocarbyl)silyl" encompasses silyl substituents,
wherein the substituents on silicon are hydrocarbyls. Herein, by
"O-, N-, S-, and Se-based functional groups" is meant univalent
groups other than hydrocarbyl and substituted hydrocarbyl that
comprise O-, N-, S-, or Se-heteroatoms, wherein the free valence is
located on this heteroatom. Examples of O-, N-, S-, and Se-based
functional groups include alkoxides, amidos, thiolates, and
selenolates.
Process for Forming Atypical Kesterites
[0027] One aspect of this invention is a process to form kesterite
with atypical Cu/Zn ratios and/or small coherent domains.
[0028] The process comprises:
a) preparing an ink comprising:
[0029] i) a copper source selected from the group consisting of
copper complexes of nitrogen-, oxygen-, sulfur-, and selenium-based
organic ligands, and mixtures thereof;
[0030] ii) a tin source selected from the group consisting of tin
complexes of nitrogen-, oxygen-, carbon-, sulfur-, and
selenium-based organic ligands, tin hydrides, tin sulfides, tin
selenides, and mixtures thereof;
[0031] iii) a zinc source selected from the group consisting of
zinc complexes of nitrogen-, oxygen-, sulfur-, and selenium-based
organic ligands, and mixtures thereof;
[0032] iv) an elemental chalcogen selected from the group
consisting of: elemental S, elemental selenium, and mixtures
thereof;
[0033] v) a solvent;
[0034] iv) optionally, a chalcogen compound selected from the group
consisting of: R.sup.1S--Z, R.sup.1Se--Z and mixtures thereof, with
each Z independently selected from the group consisting of: H and
SiR.sup.5.sub.3; wherein each R.sup.1 and R.sup.5 is independently
selected from the group consisting of: hydrocarbyl and O-, N-, S-,
halogen-, and tri(hydrocarbyl)silyl-substituted hydrocarbyl;
b) heating the ink to a temperature of 150-300.degree. C.; b)
disposing the ink onto a substrate; and c) heating the substrate
and ink disposed thereon to a temperature of 80-350.degree. C. to
form a coated substrate comprising CZTS--Se kesterite.
[0035] In some embodiments, the ratio of the total number of moles
of elemental chalcogen, sulfur- and selenium-based organic ligands,
and tin-sulfides and selenides to the total number of moles of the
copper, tin and zinc complexes is at least about 1. In some
embodiments, a chalcogen compound is present. In some embodiments,
the elemental chalcogen comprises sulfur and the molar ratio of
sulfur to tin source is about 0.2 to about 5, or about 0.5 to about
2.5.
[0036] In some embodiments, the nitrogen-, oxygen-, carbon-,
sulfur-, and selenium-based organic ligands are selected from the
group consisting of: amidos; alkoxides; acetylacetonates;
carboxylates; hydrocarbyls; O--, N--, S--, halogen- and
tri(hydrocarbyl)silyl-substituted hydrocarbyls; thio- and
selenolates; thio-, seleno-, and dithiocarboxylates; dithio-,
diseleno-, and thioselenocarbamates; and dithioxanthogenates.
[0037] Suitable amidos include: bis(trimethylsilyl)amino,
dimethylamino, diethylamino, diisopropylamino,
N-methyl-t-butylamino, 2-(dimethylamino)-N-methylethylamino,
N-methylcyclohexylamino, dicyclohexylamino,
N-ethyl-2-methylallylamino, bis(2-methoxyethyl)amino,
2-methylaminomethyl-1,3-dioxolane, pyrrolidino,
t-butyl-1-piperazinocarboxylate, N-methylanilino,
N-phenylbenzylamino, N-ethyl-o-toluidino,
bis(2,2,2-trifluoromethyl)amino, N-t-butyltrimethylsilylamino, and
mixtures thereof. Some ligands may chelate the metal center, and,
in some cases, comprise more than one type of donor atom, e.g., the
dianion of N-benzyl-2-aminoethanol is a suitable ligand comprising
both amino and alkoxide groups.
[0038] Suitable alkoxides include: methoxide, ethoxide,
n-propoxide, i-propoxide, n-butoxide, t-butoxide, neopentoxide,
ethylene glycol dialkoxide, 1-methylcyclopentoxide,
2-fluoroethoxide, 2,2,2,-trifluoroethoxide, 2-ethoxyethoxide,
2-methoxyethoxide, 3-methoxy-1-butoxide, methoxyethoxyethoxide,
3,3-diethoxy-1-propoxide, 2-dimethylaminoethoxide,
2-diethylaminoethoxide, 3-dimethylamino-1-propoxide,
3-diethylamino-1-propoxide, 1-dimethylamino-2-propoxide,
1-diethylamino-2-propoxide, 2-(1-pyrrolidinyl)ethoxide,
1-ethyl-3-pyrrolidinoxide, 3-acetyl-1-propoxide,
4-methoxyphenoxide, 4-chlorophenoxide, 4-t-butylphenoxide,
4-cyclopentylphenoxide, 4-ethylphenoxide,
3,5-bis(trifluoromethyl)phenoxide, 3-chloro-5-methoxyphenoxide,
3,5-dimethoxyphenoxide, 2,4,6-trimethylphenoxide,
3,4,5-trimethylphenoxide, 3,4,5-trimethoxyphenoxide,
4-t-butyl-catecholate(2-), 4-propanoylphenoxide,
4-(ethoxycarbonyl)phenoxide, 3-(methylthio)-1-propoxide,
2-(ethylthio)-1-ethoxide, 2-(methylthio)ethoxide,
4-(methylthio)-1-butoxide, 3-(methylthio)-1-hexoxide,
2-methoxybenzylalkoxide, 2-(trimethylsilyl)ethoxide,
(trimethylsilyl)methoxide, 1-(trimethylsilyl)ethoxide,
3-(trimethylsilyl)propoxide, 3-methylthio-1-propoxide, and mixtures
thereof.
[0039] Herein, the term acetylacetonate refers to the anion of
1,3-dicarbonyl compounds, A.sup.1C(O)CH(A.sup.2)C(O)A.sup.1,
wherein each A.sup.1 is independently selected from hydrocarbyl,
substituted hydrocarbyl, and O-, S-, and N-based functional groups
and each A.sup.2 is independently selected from hydrocarbyl,
substituted hydrocarbyl, halogen, and O-, S-, and N-based
functional groups. Suitable acetylacetonates include:
2,4-pentanedionate, 3-methyl-2-4-pentanedionate,
3-ethyl-2,4-pentanedionate, 3-chloro-2,4-pentanedionate,
1,1,1-trifluoro-2,4-pentanedionate,
1,1,1,5,5,5-hexafluoro-2,4-pentanedionate,
1,1,1,5,5,6,6,6,-octafluoro-2,4-hexanedionate, ethyl
4,4,4-trifluoroacetoacetate, 2-methoxyethylacetoacetate,
methylacetoacetate, ethylacetoacetate, t-butylacetoacetate,
1-phenyl-1,3-butanedionate, 2,2,6,6-tetramethyl-3,5-heptanedionate,
allyloxyethoxytrifluoroacetoacetate,
4,4,4-trifluoro-1-phenyl-1,3-butanedionate,
1,3-diphenyl-1,3-propanedionate,
6,6,7,7,8,8,8,-heptafluoro-2-2-dimethyl-3,5-octanedionate, and
mixtures thereof.
[0040] Suitable carboxylates include: acetate, trifluoroacetate,
propionate, butyrates, hexanoate, octanoate, decanoate, stearate,
isobutyrate, t-butyl acetate, heptafluorobutyrate, methoxyacetate,
ethoxyacetate, methoxypropionate, 2-ethyl hexanoate,
2-(2-methoxyethoxy)acetate, 2-[2-(2-methoxyethoxy)ethoxy]acetate,
(methylthio)acetate, tetrahydro-2-furoate, 4-acetyl butyrate,
phenylacetate, 3-methoxyphenylacetate, (trimethylsilyl)acetate,
3-(trimethylsilyl)propionate, maleate, benzoate,
acetylenedicarboxylate, and mixtures thereof.
[0041] Suitable hydrocarbyls include: methyl, ethyl, n-propyl,
i-propyl, n-butyl, i-butyl, sec-butyl, t-butyl, n-pentyl, n-hexyl,
n-heptyl, n-octyl, neopentyl, 3-methylbutyl, phenyl, benzyl,
4-t-butylbenzyl, 4-t-butylphenyl, p-tolyl, 2-methyl-2-phenylpropyl,
2-mesityl, 2-phenylethyl, 2-ethylhexyl, 2-methyl-2-phenylpropyl,
3,7-dimethyloctyl, allyl, vinyl, cyclopentyl, cyclohexyl, and
mixtures thereof.
[0042] Suitable O-, N-, S-, halogen- and
tri(hydrocarbyl)silyl-substituted hydrocarbyls include:
2-methoxyethyl, 2-ethoxyethyl, 4-methoxyphenyl, 2-methoxybenzyl,
3-methoxy-1-butyl, 1,3-dioxan-2-ylethyl, 3-trifluoromethoxyphenyl,
3,4-(methylenedioxy)phenyl, 2,4-dimethoxyphenyl,
2,5-dimethoxyphenyl, 3,4-dimethoxyphenyl, 2-methoxybenzyl,
3-methoxybenzyl, 4-methoxybenzyl, 3,5-dimethoxyphenyl,
3,5-dimethyl-4-methoxyphenyl, 3,4,5-trimethoxyphenyl,
4-methoxyphenethyl, 3,5-dimethoxybenzyl,
4-(2-tetrahydro-2H-pyranoxy)phenyl, 4-phenoxyphenyl,
2-benzyloxyphenyl, 3-benzyloxyphenyl, 4-benzyloxyphenyl,
3-fluoro-4-methoxyphenyl, 5-fluoro-2-methoxyphenyl,
2-ethoxyethenyl, 1-ethoxyvinyl, 3-methyl-2-butenyl, 2-furyl,
carbomethoxyethyl, 3-dimethylamino-1-propyl,
3-diethylamino-1-propyl, 3-[bis(trimethylsilyl)amino]phenyl,
4-(N,N-dimethyl)aniline, [2-(1-pyrrolidinylmethyl)phenyl],
[3-(1-pyrrolidinylmethyl)phenyl], [4-(1-pyrrolidinylmethyl)phenyl],
[2-(4-morpholinylmethyl)phenyl], [3-(4-morpholinylmethyl)phenyl],
[4-(4-morpholinylmethyl)phenyl], (4-(1-piperidinylmethyl)phenyl),
(2-(1-piperidinylmethyl)phenyl), (3-(1-piperidinylmethyl)phenyl),
3-(1,4-dioxa-8-azaspiro[4,5]dec-8-ylmethyl)phenyl,
1-methyl-2-pyrrolyl, 2-fluoro-3-pyridyl, 6-methoxy-2-pyrimidyl,
3-pyridyl, 5-bromo-2-pyridyl, 1-methyl-5-imidazolyl,
2-chloro-5-pyrimidyl, 2,6-dichloro-3-pyrazinyl, 2-oxazolyl,
5-pyrimidyl, 2-pyridyl, 2-(ethylthio)ethyl, 2-(methylthio)ethyl,
4-(methylthio)butyl, 3-(methylthio)-1-hexyl, 4-thioanisole,
4-bromo-2-thiazolyl, 2-thiophenyl, chloromethyl, 4-fluorophenyl,
3-fluorophenyl, 4-chlorophenyl, 3-chlorophenyl,
4-fluoro-3-methylphenyl, 4-fluoro-2-methylphenyl,
4-fluoro-3-methylphenyl, 5-fluoro-2-methylphenyl,
3-fluoro-2-methylphenyl, 4-chloro-2-methylphenyl,
3-fluoro-4-methylphenyl, 3,5-bis(trifluoromethyl)-phenyl,
3,4,5-trifluorophenyl, 3-chloro-4-fluorophenyl,
3-chloro-5-fluorophenyl, 4-chloro-3-fluorophenyl,
3,4-dichlorophenyl, 3,5-dichlorophenyl, 3,4-difluorophenyl,
3,5-difluorophenyl, 2-bromobenzyl, 3-bromobenzyl, 4-fluorobenzyl,
perfluoroethyl, 2-(trimethylsilyl)ethyl, (trimethylsilyl)methyl,
3-(trimethylsilyl)propyl, and mixtures thereof.
[0043] Suitable thio- and selenolates include: 1-thioglycerol,
phenylthio, ethylthio, methylthio, n-propylthio, i-propylthio,
n-butylthio, i-butylthio, t-butylthio, n-pentylthio, n-hexylthio,
n-heptylthio, n-octylthio, n-nonylthio, n-decylthio, n-dodecylthio,
2-methoxyethylthio, 2-ethoxyethylthio, 1,2-ethanedithiolate,
2-pyridinethiolate, 3,5-bis(trifluoromethyl)benzenethiolate,
toluene-3,4-dithiolate, 1,2-benzenedithiolate,
2-dimethylaminoethanethiolate, 2-diethylaminoethanethiolate,
2-propene-1-thiolate, 2-hydroxythiolate, 3-hydroxythiolate,
methyl-3-mercaptopropionate anion, cyclopentanethiolate,
2-(2-methoxyethoxy)ethanethiolate,
2-(trimethylsilyl)ethanethiolate, pentafluorophenylthiolate,
3,5-dichlorobenzenethiolate, phenylthiolate, cyclohexanethiolate,
4-chlorobenzenemethanethiolate, 4-fluorobenzenemethanethiolate,
2-methoxybenzenethiolate, 4-methoxybenzenethiolate, benzylthiolate,
3-methylbenzylthiolate, 3-ethoxybenzenethiolate,
2,5-dimethoxybenzenethiolate, 2-phenylethanethiolate,
4-t-butylbenzenethiolate, 4-t-butylbenzylthiolate,
phenylselenolate, methylselenolate, ethylselenolate,
n-propylselenolate, i-propylselenolate, n-butylselenolate,
i-butylselenolate, t-butylselenolate, pentylselenolate,
hexylselenolate, octylselenolate, benzylselenolate, and mixtures
thereof.
[0044] Suitable thio-, seleno-, and dithiocarboxylates include:
thioacetate, thiobenzoate, selenobenzoate, dithiobenzoate, and
mixtures thereof.
[0045] Suitable dithio-, diseleno-, and thioselenocarbamates
include: dimethyldithiocarbamate, diethyldithiocarbamate,
dipropyldithiocarbamate, dibutyldithiocarbamate,
bis(hydroxyethyl)dithiocarbamate, dibenzyldithiocarbamate,
dimethyldiselenocarbamate, diethyldiselenocarbamate,
dipropyldiselenocarbamate, dibutyldiselenocarbamate,
dibenzyldiselenocarbamate, and mixtures thereof.
[0046] Suitable dithioxanthogenates include: methylxanthogenate,
ethylxanthogenate, i-propylxanthogenate, and mixtures thereof.
[0047] For the chalcogen compounds, suitable R.sup.1S-- and
R.sup.1Se-- of R.sup.1S--Z and R.sup.1Se--Z are selected from the
above list of suitable thio- and selenolates.
[0048] Suitable SiR.sup.5.sub.3 include: SiMe.sub.3, SiEt.sub.3,
SiPr.sub.3, SiBu.sub.3, Si(i-Pr).sub.3, SiEtMe.sub.2,
SiMe.sub.2(i-Pr), Si(t-Bu)Me.sub.2, and Si(cyclohexyl)Me.sub.2.
[0049] Many of these ligands and chalcogen compounds are
commercially available or readily synthesized by the addition of an
amine, alcohol, or alkyl nucleophile to CS.sub.2 or CSe.sub.2 or
CSSe.
[0050] In some embodiments, a solvent is present and the boiling
point of the solvent is greater than about 100.degree. C. or
110.degree. C. or 120.degree. C. or 130.degree. C. or 140.degree.
C. or 150.degree. C. or 160.degree. C. or 170.degree. C. or
180.degree. C. or 190.degree. C. at atmospheric pressure. In some
embodiments, the process is conducted at atmospheric pressure.
Suitable solvents include: heteroaromatics, amines, and mixtures
thereof. Suitable heteroaromatics include pyridine and substituted
pyridines. Suitable amines include compounds of the form
R.sup.6NH.sub.2, wherein each R.sup.6 is independently selected
from the group consisting of: O-, N-, S-, and Se-substituted
hydrocarbyl. In some embodiments, the solvent comprises an
amino-substituted pyridine. In some embodiments, the solvent
comprises about 95 to about 5 wt %, or 90 to 10 wt %, or 80 to 20
wt %, or 70 to 30 wt %, or 60 to 40 wt % of the ink, based on the
total weight of the ink.
[0051] Suitable heteroaromatic solvents include: pyridine,
2-picoline, 3-picoline, 3,5-lutidine, 4-t-butylpyridine,
2-aminopyridine, 3-aminopyridine, diethylnicotinamide,
3-cyanopyridine, 3-fluoropyridine, 3-chloropyridine,
2,3-dichloropyridine, 2,5-dichloropyridine,
5,6,7,8-tetrahydroisoquinoline, 6-chloro-2-picoline,
2-methoxypyridine, 3-(aminomethyl)pyridine, 2-amino-3-picoline,
2-amino-6-picoline, 2-amino-2-chloropyridine, 2,3-diaminopyridine,
3,4-diaminopyridine, 2-(methylamino)pyridine,
2-dimethylaminopyridine, 2-(aminomethyl)pyridine,
2-(2-aminoethyl)pyridine, 2-methoxypyridine, 2-butoxypyridine, and
mixtures thereof.
[0052] Suitable amine solvents include: butylamine, hexylamine,
octylamine, 3-methoxypropylamine, 2-methylbutylamine, isoamylamine,
1,2-dimethylpropylamine, hydrazine, ethylenediamine,
1,3-diaminopropane, 1,2-diaminopropane,
1,2-diamino-2-methylpropane, 1,3-diaminopentane,
1,1-dimethylhydrazine, N-ethylmethylamine, diethylamine,
N-methylpropylamine, diisopropylamine, dibutylamine, triethylamine,
N-methylethylenediamine, N-ethylethylenediamine,
N-propylethylenediamine, N-isopropylethylenediamine,
N,N'-dimethylethylenediamine, N,N-dimethylethylenediamine,
N,N'-diethylethylenediamine, N,N-diethylethylenediamine,
N,N-diisopropylethylenediamine, N,N-dibutylethylenediamine,
N,N,N'-trimethylethylenediamine, 3-dimethylaminopropylamine,
3-diethylaminopropylamine, diethylenetriamine, cyclohexylamine,
bis(2-methoxyethyl)amine, aminoacetaldehyde diethyl acetal,
methylaminoacetaldehyde dimethyl acetal, N,N-dimethylacetamide
dimethyl acetal, dimethylaminoacetaldehyde diethyl acetal,
diethylaminoacetaldehyde diethyl acetal, 4-aminobutyraldehyde
diethyl acetal, 2-methylaminomethyl-1,3-dioxolane, ethanolamine,
3-amino-1-propanol, 2-hydroxyethylhydrazine,
N,N-diethylhydroxylamine, 4-amino-1-butanol,
2-(2-aminoethoxy)ethanol, 2-(methylamino)ethanol,
2-(ethylamino)ethanol, 2-(propylamino)ethanol, diethanolamine,
diisopropanolamine, N,N-dimethylethanolamine,
N,N-diethylethanolamine, 2-(dibutylamino)ethanol,
3-dimethylamino-1-propanol, 3-diethylamino-1-propanol,
1-dimethylamino-2-propanol, 1-diethylamino-2-propanol,
N-methyldiethanolamine, N-ethyldiethanolamine,
3-amino-1,2-propanediol, and mixtures thereof.
[0053] Preparing the ink typically comprises mixing the components
(i)-(v) by any conventional method. In some embodiments, the ink is
a solution; in other embodiments, the ink is a suspension or
dispersion. Typically, the preparation is conducted under an inert
atmosphere, taking precautions to protect the reaction mixtures
from air and light.
[0054] In some embodiments, the ink is prepared at about
20-100.degree. C., e.g., when the reagents are solids or have high
boiling points and/or when one or more of the solvents is a solid
at room temperature, e.g., 2-aminopyridine or 3-aminopyridine. In
some embodiments, all of the ink components are added together at
room temperature. In some embodiments, elemental chalcogen is added
last, following the mixing of all the other components for about
half an hour at room temperature. In some embodiments, the
components are added consecutively. For example, all of the
reagents except copper can be mixed and heated at about 100.degree.
C. prior to addition of the copper source, or all of the reagents
except tin can be mixed and heated at about 100.degree. C. prior to
the addition of the tin source.
[0055] The ink is heated to a temperature of greater than about
15.degree. C..degree. or 160.degree. C. or 170.degree. C. or
180.degree. C. or 190.degree. C. before coating onto a substrate.
Suitable heating methods include conventional heating and micowave
heating. In some embodiments, it has been found that this
heat-processing step aids the formation of CZTS--Se, with XAS
analysis of films formed from heat-processed inks indicating the
presence of kesterite upon heating the films at heating
temperatures as low as 120.degree. C. This heat-processing step is
typically carried out under an inert atmosphere. The ink produced
at this stage can be stored for extended periods (e.g., a couple of
months) without any noticeable decrease in efficacy. In some
embodiments, the molar ratio of copper to zinc to tin is about
2:1:1 in the ink.
[0056] The substrate can be rigid or flexible. In one embodiment,
the substrate comprises in layered sequence: (i) a base film; and
(ii) optionally, an electrically conductive coating. The base film
is selected from the group consisting of: glass, metals, and
polymeric films. Suitable electrically conductive coatings include:
metal conductors, transparent conducting oxides, and organic
conductors. Of particular interest are substrates of
molybdenum-coated soda-lime glass, molybdenum-coated polyimide
films, or molybdenum-coated polyimide films with a thin layer of a
sodium compound (e.g., NaF, Na.sub.2S, or Na.sub.2Se). In one
embodiment, the substrate comprises material selected from the
group consisting of metal foils, plastics, polymers, metalized
plastics, glass, solar glass, low-iron glass, green glass,
soda-lime glass, steel, stainless steel, aluminum, ceramics, metal
plates, metalized ceramic plates, and metalized polymer plates.
[0057] The ink is disposed on a substrate to provide a coated
substrate by any of a variety of solution-based coating and
printing techniques, including spin-coating, spray-coating,
dip-coating, rod-coating, drop-cast coating, roller coating,
slot-die coating, draw-down coating, ink-jet printing, contact
printing, gravure printing, flexographic printing, and screen
printing. The coating can be dried by evaporation, by applying
vacuum, by heating, or by combinations thereof. In some
embodiments, the substrate and disposed ink are heated at a
temperature from 80-350.degree. C., or 100-300.degree. C., or
120-250.degree. C., or 150-190.degree. C. to remove at least a
portion of the solvent, if present, and by-products derived from
the ligands and chalcogen source.
Atypical Kesterite Compositions
[0058] Additional aspects of this invention are atypical kesterite
compositions produced by the processes of this invention.
[0059] One aspect of this invention is a composition comprising a
compound having a kesterite structure, wherein the Cu/Zn ratio of
the kesterite is greater than about 3.1.
[0060] In some embodiments, the composition further comprises one
or more copper-, tin-, or zinc-containing compounds selected from
the group consisting of binary copper chalcogenides, binary copper
oxides, binary tin chalcogenides, binary tin oxides, binary zinc
chalcogenides, ZnO, and CTS--Se. In some embodiments, a molar ratio
of copper to zinc to tin is about 2:1:1 of the overall composition.
In some embodiments, a molar ratio of copper to (zinc plus tin) is
less than one, and a molar ratio of zinc to tin is greater than
one. In some embodiments, the heated layers comprise CZTS--Se
wherein the coherent domain size of the kesterite is 2-30 nm.
[0061] One aspect of this invention is a composition comprising a
compound having a kesterite structure, wherein the Cu/Zn ratio of
the kesterite is less than about 1.6.
[0062] In some embodiments, the composition further comprises one
or more copper-, tin-, or zinc-containing compounds selected from
the group consisting of binary copper chalcogenides, binary copper
oxides, binary tin chalcogenides, binary tin oxides, binary zinc
chalcogenides, ZnO, and CTS--Se. In some embodiments, a molar ratio
of copper to zinc to tin is about 2:1:1 of the overall composition.
In some embodiments, a molar ratio of copper to (zinc plus tin) is
less than one, and a molar ratio of zinc to tin is greater than
one. In some embodiments, the heated layers comprise CZTS--Se
wherein the coherent domain size of the kesterite is 2-30 nm.
[0063] One aspect of this invention is a composition comprising a
compound having a kesterite structure, wherein the coherent domain
size of the kesterite is 2-30 nm.
[0064] In some embodiments, the composition further comprises one
or more copper-, tin-, or zinc-containing compounds selected from
the group consisting of binary copper chalcogenides, binary copper
oxides, binary tin chalcogenides, binary tin oxides, binary zinc
chalcogenides, ZnO, and CTS--Se. In some embodiments, the
composition further comprises CuS, Cu.sub.2S, ZnS, and SnS.sub.2.
In some embodiments, a molar ratio of copper to zinc to tin is
about 2:1:1 of the overall composition. In some embodiments, a
molar ratio of copper to (zinc plus tin) is less than one, and a
molar ratio of zinc to tin is greater than one. In other
embodiments, the heated layers comprise CZTS--Se wherein the Cu/Zn
ratio of the kesterite is greater than about 3.1. In yet other
embodiments, the heated layers comprise CZTS--Se wherein the Cu/Zn
ratio of the kesterite is less than about 1.6.
[0065] The inks described above can be used to form CTS--Se or
CZTS--Se layers on a substrate. Typically, the ink is coated onto
the substrate by conventional means and then dried by applying a
vacuum, by evaporation or by heating at a temperature from
80-350.degree. C. Suitable substrates include metal foils,
plastics, polymers, metalized plastics, glass, solar glass,
low-iron glass, green glass, soda-lime glass, steel, stainless
steel, aluminum, ceramics, metal plates, metalized ceramic plates,
and metalized polymer plates.
[0066] For some inks, it has been found that CZTS--Se is formed
during this heating step, with XAS analysis indicating the presence
of kesterite.
[0067] More typically, the dried, coated substrate is annealed by
heating the substrate at about 350.degree. C. to about 800.degree.
C. The annealing step can be carried out under an inert atmosphere,
provided that the ratio of the total number of moles of the
chalcogen, chalcogen compound, sulfur-, and selenium-based organic
ligands, and tin-sulfides and selenides to the total number of
moles of the copper, tin and zinc complexes is at least about 1.
Alternatively, the annealing step can be carried out in an
atmosphere comprising an inert gas and reactive component selected
from the group consisting of: selenium vapor, sulfur vapor,
hydrogen, hydrogen sulfide, hydrogen selenide, and mixtures
thereof. Typically, at least a portion of the chalcogen can be
exchanged (e.g., S can be replaced by Se) by conducting the
annealing step in the presence of selenium vapor or hydrogen
selenide.
[0068] It has been found that CTS--Se and/or CZTS--Se can be formed
in high yield during the annealing step, as determined by XRD or
XAS. In some embodiments, annealed films consist essentially of
CTS--Se or CZTS--Se, according to XRD analysis.
[0069] CZTS--Se can serve as the absorber layer in a photovoltaic
cell. A CZTS--Se-based photovoltaic cell can be made by forming the
CZTS--Se layer on a photovoltaic substrate (e.g., Mo-coated SLG)
and then depositing a buffer layer, a transparent top contact
layer, electrode pads, and an antireflective layer in sequence onto
the annealed CZTS layer. Except for the absorber layer, the same
materials and processes used to assemble a CIGS-based photovoltaic
cell can be used to assemble a CZTS--Se-based photovoltaic
cell.
EXAMPLES
General
[0070] Materials
[0071] All reagents were purchased from Aldrich (Milwaukee, Wis.),
Alfa Aesar (Ward Hill, Mass.), TCI (Portland, Oreg.), or Gelest
(Morrisville, Pa.). Solid reagents were used without further
purification. Liquid reagents that were not packaged under an inert
atmosphere were degassed by bubbling argon through the liquid for 1
hr. Anhydrous solvents were used for the preparation of all
formulations and for all cleaning procedures carried out within the
drybox. Solvents were either purchased as anhydrous from Aldrich or
Alfa Aesar, or purified by standard methods (e.g., Pangborn, A. G.,
et. al. Organometallics, 1996, 15, 1518-1520) and then stored in
the drybox over activated molecular sieves.
[0072] Formulation and Coating Preparations
[0073] Substrates (SLG slides) were cleaned sequentially with aqua
regia, Miilipore.RTM. water and isopropanol, dried at 110.degree.
C., and coated on the non-float surface of the SLG substrate. All
formulations and coatings were prepared in a nitrogen-purged
drybox. Vials containing formulations were heated and stirred on a
magnetic hotplate/stirrer. Coatings were dried in the drybox.
Details of X-ray Analysis
XAS Analysis
[0074] XANES spectroscopy at the Cu, Zn and Sn K-edges were carried
out at the Advanced Photon Source at the Argonne National
Laboratory. Data were collected in fluorescence geometry at
beamline 5BMD, DND-CAT. Thin film samples were presented to the
incident x-ray beam as made. An Oxford spectroscopy-grade ion
chamber was used to determine the X-ray incident intensity
(I.sub.0). The I.sub.0 detector was filled with 570 Torr of N.sub.2
and 20 Torr of Ar. The fluorescence detector was a Lytle Cell
filled with Xe installed perpendicular to the beam propagation
direction. Data were collected from 8879 eV to 9954 eV for the Cu
edge. The high final energy was used in order to capture a portion
of the Zn edge in the same data set, to allow edge step ratio
determination as an estimate of Cu:Zn ratio in the film. The Zn
edge data were collected over the range 9557 eV to 10,404 eV. Sn
edge data covered the range of 29,000 eV to 29,750 eV. The data
energy scales were calibrated based on data from metal reference
foils collected prior to sample data collection. A second order
background was subtracted and the spectra were normalized. Data
from several Cu, Zn and Sn sulfide and oxide standards
(Cu.sub.2ZnSnS.sub.4, Cu.sub.2SnS.sub.3, CuS, Cu.sub.2S, CuO,
Cu.sub.2O, ZnS, ZnO, SnS, SnO and SnO.sub.2) were obtained under
the same conditions. Non-linear least squares fitting of a linear
combination of the appropriate standards to the spectra obtained
from the samples yielded the phase distribution for each
element.
XRD Analysis
[0075] Powder X-ray diffraction was used for the identification of
crystalline phases. Data were obtained with a Philips X'PERT
automated powder diffractometer, Model 3040. The diffractometer was
equipped with automatic variable anti-scatter and divergence slits,
X'Celerator RTMS detector, and Ni filter. The radiation was
CuK(alpha) (45 kV, 40 mA). Data were collected at room temperature
from 4 to 120.degree.. 2-theta; using a continuous scan with an
equivalent step size of 0.02.degree.; and a count time of from 80
sec to 240 sec per step in theta-theta geometry. Thin film samples
were presented to the X-ray beam as made. MDI/Jade software version
9.1 was used with the International Committee for Diffraction Data
database PDF4+ 2008 for phase identification and data analysis.
Identification of CZTS with Small Coherent Domain Size
[0076] Comparison of XRD and XAS data can identify films containing
nanoscale domains of Cu.sub.2ZnSnS.sub.4, kesterite, in which the
coherent domain size (d) is 2-30 nm. Detection of kesterite
crystalline material at this length scale by x-ray diffraction is
difficult due to diffraction peak broadening effects, particularly
on the weak lines required to differentiate kesterite from the
common impurity phase sphalerite (ZnS). However, x-ray absorption
spectroscopy (XAS) is not limited by the size of crystalline
domains over which a defect-free, coherent structure may exist.
Allowing the interpretation of x-ray diffraction peaks to be guided
by the XAS, peak broadening in the diffraction patterns suggests a
30 nm upper limit on domain size. At least two unit cells of
kesterite would need to be present in a domain in order for the
local environment of at least some of the atoms to resemble that of
bulk crystalline kesterite, and so give rise to the appropriate
spectrum. This puts a lower limit of 2 nm on the domain size.
Identification of CZTS--Se Kesterite with Atypical
Stoichiometries
[0077] In the case of XAS, one obtains the fraction of Cu atoms and
Zn atoms in the kesterite phase. This, along with a knowledge of
overall Cu:Zn ratio, which is determined as described above, allows
for determination of the ratio of Cu to Zn in the kesterite phase.
This is clearly distinguished from a mixture of separate sulfide
phases producing the same elemental ratios in aggregate.
Example 1
##STR00001##
[0079] Zinc diethyldithiocarbamate (0.5919 g, 1.635 mmol),
copper(II) dimethyldithiocarbamate (0.9930 g, 3.267 mmol), and
di-n-butyltinsulfide (0.4506 g, 1.701 mmol) were placed in a 40 mL
amber vial equipped with a stir bar. 4-t-Butylpyridine (4.4144 g)
was added, and the resulting mixture was stirred well. Next, 0.0520
g (1.621 mmol) of elemental sulfur was added. The reaction mixture
was stirred for .about.12 hr at room temperature and then .about.40
hr at a first heating temperature of 105.degree. C. Next, the
reaction mixture was stirred for .about.8 hr at a second heating
temperature of 190.degree. C. SLG coated substrate 1A was produced
via drop-coating according to the following procedure: While being
maintained at 105.degree. C. with stirring, a small portion of the
formulation was drawn into a pipette and then spread uniformly onto
the substrate, which was also heated to 105.degree. C. The coating
on the SLG slide was then dried in the drybox by raising the
temperature of the hotplate from 105.degree. C. to 170.degree. C.
for 0.5 h.
[0080] This procedure was repeated with the exception that the
coated substrate was dried at 210.degree. C. for 2 hr to produce
coated substrate 1B. This procedure was also repeated with the
exceptions that 2-aminopyridine was used as the solvent and a
treated SLG slide was used as the substrate to produce coated
substrate 1C.
[0081] Analysis of coated substrates 1A, 1B and 1C by XAS confirmed
the presence of CZTS with high Cu/Zn for 1A (Cu/Zn in
kesterite=6.45 for 1A) and low Cu/Zn for 1B and 1C (Cu/Zn in
kesterite=1.06 for 1B and 1.27 for 1C).
Example 2
##STR00002##
[0083] In the drybox, zinc diethyldithiocarbamate (0.842 mmol),
copper(II) acetate (1.58 mmol), and tin(II) acetylacetonate (0.846
mmol) were placed in a 20 mL vial equipped with a stir bar.
2-Aminopyridine (1.0058 g) was added, followed by 5.05 mmol of
(ethylthio)trimethylsilane and 0.814 mmol of sulfur. The reaction
mixture was stirred for .about.40 hr at a first heating temperature
of 105.degree. C. Next, the reaction mixture was stirred for
.about.8 hr at a second heating temperature of 190.degree. C. A
treated glass slide was coated via drop-coating to produce coated
substrate 2A in a manner similar to described above in Example 1,
with drying at 170.degree. C. for .about.0.5 h.
[0084] This procedure was repeated with the exception that the
3-aminopyridine was used as the solvent to produce coated substrate
2B. Analysis of coated substrates 2A and 2B by XAS confirmed the
presence of CZTS with low Cu/Zn for 2A and 2B (Cu/Zn in
kesterite=1.06 for 2A and 1.10 for 2B).
Example 3
##STR00003##
[0086] Zinc methoxyethoxide (1.638 mmol), copper(I) acetate (3.263
mmol), di-n-butyltinsulfide (1.631 mmol), and 2-mercaptoethanol
(6.885 mmol) were placed together in a 40 mL amber vial equipped
with a stir bar. 4-t-Butylpyridine (1.857 g) was added, and the
resulting mixture was stirred well. Next, 1.634 mmol of elemental
sulfur was added. The reaction mixture was stirred for .about.12 hr
at room temperature and then .about.40 hr at 105.degree. C. Next,
the reaction mixture was stirred .about.16 hr at 190.degree. C. A
SLG coated substrate 3A was produced via drop-coating according to
the procedure of Example 1, with drying at 250.degree. C. for 0.5
hr.
[0087] This procedure was repeated using 2-aminopyridine as the
solvent with the reaction mixture stirring .about.8 hr at a second
heating temperature of 170.degree. C., to produce coated substrate
3B. This procedure was repeated using 4-t-butylpyridine as the
solvent with the reaction mixture stirring .about.8 hr at a second
heating temperature of 160.degree. C., to produce coated substrate
3C. Coatings 3B and 3C were both dried at 170.degree. C. for 0.5 hr
and then annealed at 550.degree. C. for 0.5 hr.
[0088] Analysis of coated substrates 3A, 3B and 3C by XAS confirmed
the presence of CZTS with low Cu/Zn for 3A (Cu/Zn in kesterite=1.43
for 3A), high Cu/Zn for 3B (Cu/Zn in kesterite=2.70 for 3B) and low
Cu/Zn for 3C (Cu/Zn in kesterite=1.45 for 3C).
Example 4
##STR00004##
[0090] Following the procedure of Example 1, a formulation was
prepared using 0.1768 g (0.820 mmol) of zinc methoxyethoxide,
0.2047 g (1.630 mmol) of copper(II) methoxide, 0.4004 g (0.911
mmol) bis[bis(trimethysilyl)amino]tin(II), 0.8990 g (6.692 mmol) of
ethylthiotrimethylsilane, 0.0259 g (0.808 mmol) of elemental
sulfur, and 0.4700 g of 3,5-lutidine with a second heating
temperature of 165.degree. C. The coating was dried in the drybox
at 170.degree. C. for 0.5 h. Analysis of the coated substrate by
XAS confirmed the presence of CZTS with low Cu/Zn (Cu/Zn in
kesterite=1.18).
Example 5
##STR00005##
[0092] Zinc methoxyethoxide (1.638 mmol), copper(I) acetate (3.263
mmol), di-n-butyltinsulfide (1.631 mmol), and 2-mercaptoethanol
(6.885 mmol) were placed together in a 40 mL amber vial equipped
with a stir bar. 4-t-Butylpyridine (1.857 g) was added, and the
resulting mixture was stirred well. Next, 1.634 mmol of elemental
sulfur was added. The reaction mixture was stirred for .about.12 hr
at room temperature and then .about.40 hr at 105.degree. C. Next,
the reaction mixture was stirred .about.16 hr at 190.degree. C. A
SLG coated substrate 5A was produced via drop-coating according to
the following procedure: While being maintained at 105.degree. C.
with stirring, a small portion of the formulation was drawn into a
pipette and then spread uniformly onto the substrate, which was
also heated to 105.degree. C. The coating on the SLG slide was then
dried in the drybox by raising the temperature of the hotplate from
105.degree. C. to 250.degree. C. for 0.5 h.
[0093] This procedure was repeated using 3.714 g of
4-t-butylpyridine with .about.9 hr of heating during the second
heat at 190.degree. C. to produce coated substrate 5B, and using
3.714 g of 4-t-butylpyridine with .about.11 hr of heating during
the second heat at 190.degree. C. to produce coated substrate
5C.
[0094] Analysis of coated substrates 5A, 5B and 5C by XRD indicated
that CZTS may be present in these samples in at most trace amounts.
However, both Cu and Zn XAS spectra (Table 5-1) indicated the
presence of significant quantities of CZTS in all three coatings.
CuS, Cu.sub.2S and ZnS comprised the other copper- and
zinc-containing species, with only traces of ZnO present. Sn XAS
analysis indicated Sn(IV) sulfides (e.g., CZTS and SnS.sub.2) as
the predominate tin-containing compounds, comprising greater than
89% of all tin-containing compounds in all three coatings.
Comparison of the XRD and XAS data indicates that this route
produces films containing nanoscale domains of CZTS kesterite, in
which the coherent domain size (d) is 2-30 nm. The Cu/Zn ratio in
kesterite of coating 5A was 1.43.
TABLE-US-00001 TABLE 5-1 Cu and Zn XAS analysis. Cu Zn Ex CZTS CuS
Cu.sub.2S CZTS ZnS ZnO 5A 0.27 0.52 0.20 0.47 0.49 0.04 5B 0.34
0.45 0.21 0.28 0.69 0.03 5C 0.38 0.40 0.22 0.27 0.69 0.04
* * * * *