Group 13 selenide nanoparticles

Abstract

A method of preparing Group XIII selenide nanoparticles comprises reacting a Group XIII ion source with a selenol compound. The nanoparticles have an M.sub.xSe.sub.y Semiconductor core (where M is In or Ga) and an organic capping ligand attached to the core via a carbon-selenium bond. The selenol provides a source of selenium for incorporation into the semiconductor core and also provides the organic capping ligand. The nanoparticles are particularly suitable for solution-based methods of preparing semiconductor films.

Claims

1. A method of forming a semiconductor film, the method comprising: co-depositing CuSe nanoparticles and Group 13 selenide nanoparticles on a substrate; and, heating the substrate to a temperature sufficient to melt the CuSe nanoparticles and the Group 13 selenide nanoparticles; wherein the Group 13 selenide nanoparticles comprise a Group 13 selenide semiconductor having the formula M.sub.xSe.sub.y, wherein M is Ga or In, 0<x, and 0<y and an organic capping ligand bound to the nanoparticle by a carbon-selenium covalent bond and, wherein the temperature is sufficient to remove the organic capping ligand.

2. The method of claim 1 wherein the organic capping ligand is an alkyl, alkenyl, alkynyl, or aryl group.

3. The method of claim 1 wherein the semiconductor film is free of sulfur.

4. The method of claim 1 wherein the Group 13 selenide nanoparticles are prepared by a method comprising: reacting a Group 13 ion precursor with a selenol compound.

5. The method of claim 4 wherein the Group 13 ion precursor is a chloride, acetate, or acetylacetonate of a Group 13 element.

6. The method of claim 4 wherein the Group 13 ion precursor is selected from the group consisting of InCl.sub.3, In(OAc).sub.3, In(acac).sub.3, GaCl.sub.3, Ga(OAc).sub.3 and Ga(acac).sub.3.

7. The method of claim 4 wherein the selenol compound is an alkyl, alkenyl, alkynyl, or aryl selenol.

8. The method of claim 4 wherein the selenol compound contains 4 to 14 carbon atoms.

9. The method of claim 4 wherein the selenol compound is octane selenol.

10. The method of claim 4 further comprising adding a second selenium compound to the Group 13 ion precursor.

11. The method of claim 10 wherein the second selenium compound is a trioctylphosphine selenide.

12. The method of claim 4 wherein the selenide nanoparticles have diameters less than about 200 nm.

13. The method of claim 4 wherein the selenide nanoparticles have diameters of about 2 to about 100 nm.

14. The method of claim 4 wherein the selenide nanoparticles have diameters less than about 10 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the XRD pattern of indium selenide nanoscrolls synthesized in 1octadecene, showing broad peaks suggestive of nanoparticle materials. The peaks are compared to the cubic phase of InSe (K. H. Park et al., J. Am. Chem. Soc., 2006, 128, 14780).

(2) FIG. 2 shows TEM images of indium selenide nanoparticles synthesized in 1octadecene.

(3) FIG. 3 shows a thermogravimetric analysis (TGA) graph of indium selenide nanoparticles synthesized in 1-octadecene.

(4) FIG. 4 shows the XRD pattern of an indium selenide nanoparticle film annealed at 500 C. after deposition under nitrogen (black trace) and in the presence of CuSe nanoparticles at 500 C. under a selenium atmosphere (red trace). Annealing in the presence of CuSe nanoparticles enables the conversion to tetragonal phase CuInSe.sub.2.

(5) FIG. 5 shows a scanning electron microscopy (SEM) image showing large grains of copper indium diselenide, formed by annealing a blend of indium selenide and copper selenide in a selenium-rich atmosphere.

(6) FIG. 6 shows the XRD pattern of a copper indium diselenide film. The film was produced by rapid thermal annealing of a film of blended indium selenide and copper selenide nanoparticles, on a glass substrate, on a preheated hotplate under dry N.sub.2. The reflections match the peak positions of chalcopyrite CuInSe.sub.2 from the JCPDS database (00040-1487).

(7) FIG. 7 shows the XRD pattern of indium selenide nanoparticles synthesized in oleic acid/Therminol 66.

(8) FIG. 8 shows a TEM image of indium selenide nanoparticles synthesized in oleic acid/Therminol 66. The spherical particles are between 3-5 nm in diameter.

(9) FIG. 9 shows TGA of indium selenide nanoparticles synthesized in oleic acid/Therminol 66.

(10) FIG. 10 shows the XRD pattern of indium selenide nanoparticles synthesized in oleylamine.

(11) FIG. 11 shows a TEM image of indium selenide nanoparticles synthesized in oleylamine. The spherical nanoparticles are between 2-3 nm in diameter.

(12) FIG. 12 shows TGA of indium selenide nanoparticles synthesized in oleylamine.

(13) FIG. 13 shows the XRD pattern of gallium selenide nanoparticles synthesized in oleic acid/Therminol 66. The plot shows the as-synthesized material along with a film that was annealed under N.sub.2 for 10 minutes at 260 C. The reflections match the peak positions of monoclinic Ga.sub.2Se.sub.3 from the JCPDS database (00-044-0931).

(14) FIG. 14 shows TGA of gallium selenide nanoparticles.

(15) FIG. 15 shows the X-ray diffraction (XRD) pattern of gallium selenide nanoparticles synthesized in 1-octadecene/oleylamine. The higher angle reflections match the peak positions of monoclinic (-phase) Ga.sub.2Se.sub.3 from the JCPDS database (00-044-0931). Apparent splitting of the (200) peak suggests the as-synthesised material is crystallized in a lower order symmetry, pseudo-phase.

(16) FIG. 16 shows the thermogravimetric analysis (TGA) of gallium selenide nanoparticles.

(17) FIG. 17 shows the UV-visible absorption spectrum of gallium selenide, with a peak at around 495 nm (2.5 eV).

(18) FIG. 18 is a transmission electron microscopy (TEM) image of gallium selenide nanoparticles, on the order of 5 nm in diameter and with a spherical morphology

DETAILED DESCRIPTION

(19) As stated above, the present disclosure provides methods for producing Group XIII selenide nanoparticles by reacting a source of Group XIII ions with a selenol compound. The nanoparticles have a semiconductor core of a material represented by the formula M.sub.xSe.sub.y, wherein M is Ga or In, 0<x, and 0<y. Nanoparticles can be prepared with a high degree of monodispersity, allowing for populations of nanoparticles having uniform properties. For example, the disclosed methods can be used to prepare populations of nanoparticles having a narrow melting point range. The nanoparticles can be as small as 2 nm in diameter.

(20) The selenol compound provides a source of selenium ions that are incorporated into the nanoparticle core. The selenol compound also provides an organic capping ligand. As a result, the nanoparticles have an organic functional group bound to the semiconductor core by a SeC covalent bond. The organic functional group confers solubility and dispersibility of the nanoparticles in organic media. This renders the nanoparticles particularly suitable for use in inks and other formulations that can be used to deposit films of the nanoparticles onto substrates. Such inks can be used to fabricate printable solar cells on substrates, including flexible substrates. Once the nanoparticles are deposited, the organic group can be removed from the film at relatively a low temperature, enabling low temperature device sintering. The removal of the capping ligand by mild heating is virtually total and leaves no significant carbon residue on the particles after sintering, which enhances device performance.

(21) The disclosed nanoparticles are particularly suitable as precursors for CIGS-type materials for use in photovoltaic devices. For example, the Group XIII selenide nanoparticles can be blended with CuSe nanoparticles and processed to form Cu(In,Ga)Se.sub.2 thin films. The nanoparticles may be formulated into inks or pastes, which may then be deposited onto a substrate by a variety of techniques including printing (e.g., screen printing, ink jet printing, contact printing, gravure printing), or other methods, (e.g., simple doctor blade coating, spin coating, assisted aerosol spray coating, spray coating, but not restricted to such methods).

(22) The selenol ligand of the disclosed nanoparticles can provide a selenium-rich atmosphere during sintering of films made using the disclosed nanoparticles. Thus, in some embodiments, it is not necessary to include an additional Se source during sintering. Moreover, an advantage of using a selenol compound as a capping ligand, instead of another class of compounds, such as thiols, is that the resulting nanoparticles and semiconductor films are not contaminated with non-selenium chalocogens, such as sulfur.

(23) The Group XIII ion source, according to the disclosed methods, may be a Group XIII ion-coordination compound or a Group XIII salt. For example, the Group XIII ion source may be a metal chloride salt of a Group XIII element. Examples of ion-coordination compounds that may function as the Group XIII ion source include but are not limited to acetate (OAc) compounds and acetylacetonate (acac) compounds of Group XIII metals. Particular examples of Group XIII ion sources include InCl.sub.3, In(OAc).sub.3, In(acac).sub.3, GaCl.sub.3, Ga(OAc).sub.3, and Ga(acac).sub.3.

(24) Generally, any selenol compound may be employed. Preferably, the selenol compound is a volatile organoselenol compound. Reference to an organic compound as being volatile is well understood in the relevant technical field by the skilled person and generally refers to a compound that will vaporize at temperatures that are relatively low compared to other species with which the volatile compound is associated. In this way, using a volatile organoselenol compound provides the advantage of allowing the selenol to be easily and cheaply removed from the nanoparticles, for example by heating.

(25) The organoselenol compound may be represented by the formula RSeH, where R is a substituted or unsubstituted organic group. By substituted, it is meant that one or more hydrogen atoms bonded to a carbon atom may be replaced with a non-hydrogen atom. The organic group may be saturated or unsaturated. The organic group can be a linear, branched or cyclic organic group, which may be a carbocylic group or a heterocyclic group.

(26) The organic group is preferably an alkyl, alkenyl, alkynyl, and/or aryl. The organic group may be an alkyl, alkenyl or alkynyl group containing 2 to 20 carbon atoms, more preferably 4 to 14 carbon atoms and most preferably 6 to 12 carbon atoms. An exemplary selenol compound is 1-octane selenol. A further exemplary selenol compound is 1dodecane selenol or 1-dodecylselenol. Alternatively, the organic group may be an aryl group containing 4 to 14 carbon atoms. More preferably the organic group is an aryl group containing 6 to 10 carbon atoms, still more preferably 6 to 8 carbon atoms.

(27) At least a portion of the selenium ions in the nanoparticles may be provided by the selenol compound. Alternatively, or additionally, at least a portion of the selenium ions may be provided by an additional or secondary selenium source, such as, but not limited to trioctylphosphine (TOP) selenides.

(28) To form nanoparticles, the Group XIII ion source is dispersed in a solvent. The choice of solvent is not limited to any particular solvent. It is generally preferred that the solvent have a higher boiling point (e.g., around 200 C. or higher) than the reaction temperature so as to provide a medium in which the reactants can decompose and react. The solvent is typically a non-coordinating organic solvent, for example a saturated or unsaturated long-chain hydrocarbon solvent. Exemplary solvents include long chain, e.g., C.sub.8-C.sub.24, alkanes or alkenes, such as octadecene (C.sub.18H.sub.36), which has a boiling point in excess of 250 C. Other suitable solvents include high-boiling heat transfer fluids, such as Therminol 66, a modified terphenyl available from Solutia Inc., (St. Louis Mo.). According to some embodiments, a coordinating compound, such as a derivative of a fatty acid or a fatty amine, is used as a solvent, additive, or as a co-solvent to help solubilize the nanoparticles especially in organic solvents. Examples of such fatty acid and fatty amine derivatives are oleic acid and oleylamine, respectively. An example of a solvent mixture is a mixture of oleic acid and Therminol 66.

(29) According to certain embodiments, the Group XIII ion source is dispersed in the solvent and the solvent is heated to a first temperature. For example, the first temperature may be around 50 to 150 C. According to some embodiments, the first temperature is around 100 C. The dispersion of Group XIII precursor may be degassed using an inert gas at the first temperature.

(30) The selenol compound may be added to the dispersion at a second temperature or may be added at the first temperature. According to some embodiments, the temperature is dropped from the first temperature to a lower second temperature and the selenol compound is added to the dispersion. Following addition of the selenol compound, the temperature may be raised to a third temperature or the temperature may be maintained at the first or second temperatures. According to some embodiments, the third temperature is about 100 to about 160 C. Following addition of the selenol compound, the mixture may be maintained at the first, second, or third temperature for some period of time, ranging from several minutes to a few hours. For example, the mixture may be maintained for about 30 minutes to about 2 hours following addition of the selenol compound.

(31) According to some embodiments, a secondary selenium source, such as TOP/Se is added to the reaction. Upon addition of the selenium compound, the temperature may be changed to a fourth temperature. The temperature may be ramped up or down, pausing at intermediate temperatures before reaching the fourth temperature. Generally, the mixture will be maintained at one or more temperatures between about 100 C. and about 220 C. for between about 30 minutes and about five hours following the addition of the selenium compound.

(32) Nanoparticles can be isolated by precipitation and/or washing with a non-solvent, for example acetone or a mixture of toluene and acetone, followed by centrifugation or filtration. The preferred non-solvent depends upon the particular of reaction medium. The nanoparticles may be subjected to multiple washing steps. The disclosed methods can be further understood in view of the following examples.

Example 1: Synthesis of Indium Selenide Nanoparticles in 1-Octadecene

(33) A flask was charged with indium acetate In(OAc).sub.3 (2 g, 6.85 mmol) and 1octadecene (10 mL), degassed for 60 minutes at 100 C., and then backfilled with N.sub.2. The mixture was cooled to 75 C. and 1-octane selenol (4 mL, 22.4 mmol) was added quickly. The mixture was heated to 140 C., giving a cloudy dark red/orange solution. TOP/Se (6.25 mL of 1.71 M, 10.7 mmol) was injected into the flask at 12.5 mL hr.sup.1. Once the addition was complete, the temperature was raised to 160 C. for 1 hour. The mixture was cooled to 140 C. for 30 minutes and then cooled to room temperature. The solid was isolated by washing with methanol, then acetone, followed by centrifugation. The supernatant was discarded, and the brown solid was retained as the product.

(34) Elemental analysis for the product indicates: C 9.87%; H 1.53%; In 42.6%; Se 39.9%. The elemental ratio determined by chemical analysis corresponds to a material with formula InSe.sub.1.36. The 1-octane selenol ligand present on the particle surface contributes to the total amount of selenium found in the material. Referring to FIG. 1, the XRD pattern of the material (A) is characterized by broad diffraction peaks typical of nanoparticles. The peaks are matched to the cubic phase of InSe (B) (K. H. Park et al., J. Am. Chem. Soc., 2006, 128, 14780). TEM analysis, shown in FIG. 2, shows that the material is made of nanoscrolls, i.e. nanosheets that have scrolled into nanotubes, with an average length of 200 nm (best seen in FIG. 2A) and an average thickness of 40 nm (best seen in FIG. 2B). This phenomenon has been reported in other layered materials, such as sodium titanate (Formation, Structure, and Stability of Titanate Nanotubes and Their Proton Conductivity, A. Thorne et al., J. Phys. Chem. B, 2005, 109, 5439). TGA performed under nitrogen indicates that the material contains approximately 15% w/w of volatile organic ligands that are completely removed when the temperature reaches 500 C. (FIG. 3). The organic ligands help to disperse the nanoparticles in a range of solvents and can be removed by conventional, low temperature baking processes. FIG. 4A shows XRD of the InSe nanoparticles co-deposited with CuSe nanoparticles after annealing at 500 C. under N2.

(35) Baking at 500 C. in the presence of CuSe nanoparticulate material in a seleniumrich atmosphere converts the co-deposited nanoparticles to the tetragonal phase of CuInSe.sub.2 (FIG. 4B). Annealing in a selenium-rich atmosphere produces large grains, as observed using scanning electron microscopy (FIG. 5). Alternatively, a rapid thermal annealing process in an N.sub.2 atmosphere yields a tetragonal phase film of CuInSe.sub.2. FIG. 6A is an XRD pattern of a copper indium diselenide film produced by rapid thermal annealing of a film of blended indium selenide and copper selenide nanoparticles on a glass substrate on a preheated hotplate under dry N.sub.2. The reflections match the peak positions of chalcopyrite CuInSe.sub.2 from the JCPDS database (00-040-1487) (FIG. 6B).

Example 2: Synthesis of Indium Selenide Nanoparticles in Oleic Acid/Therminol 66

(36) In(OAc).sub.3 (2.921 g, 10.00 mmol), 18 mL of oleic acid and 18 mL of Therminol 66 were degassed for 90 minutes at 100 C. then backfilled with N.sub.2 and cooled to 45 C. 1-Octane selenol (9 mL, 50.5 mmol) was added quickly, upon which a white mass formed and the stirring became inhibited. The mixture was heated to 100 C. and the white solid melted. The temperature was reduced to 75 C., forming a cloudy, pale yellow solution.

(37) Separately, Se powder (1.244 g, 15.75 mmol) was dissolved in TOP (9.25 mL) under N.sub.2. The TOP/Se was added to the reaction solution and stirred for 30 minutes at 75 C. The solution was heated to 100 C. and held for 75 minutes, forming an orange precipitate in a clear solution. The mixture was allowed to cool to room temperature. The solid was isolated by washing with acetone, then toluene/acetone, followed by centrifugation. The supernatant was discarded, and the orange powder was retained as the product. Elemental analysis for the product indicates: C 24.45%; H 4.17%; In 30.74%; Se 38.69%. The elemental ratio determined by chemical analysis corresponds to a material with formula InSe.sub.1.83. The 1-octane selenol ligand present on the particle surface contributes to the total amount of selenium found in the material.

(38) The XRD pattern of the material is characterized by fairly broad diffraction peaks that are typical of nanoparticles (FIG. 7). TEM analysis shows that the nanoparticles are spherical and are between 3-5 nm in diameter (FIG. 8). TGA performed under nitrogen indicates that the material contains approximately 39% w/w of volatile organic ligands that are completely removed when the temperature reaches 350 C. (FIG. 9).

Example 3: Synthesis of Indium Selenide Nanoparticles in Oleylamine

(39) In(OAc).sub.3 (2.000 g, 6.85 mmol) and oleylamine (10 mL) were degassed for 1 hour at 100 C.; then the flask was backfilled with nitrogen. 1-Octane selenol (4 mL, 22.4 mmol) was added quickly at 75 C., then the mixture was heated to 140 C. TOP/Se solution (6.25 mL, 1.71 M, 10.7 mmol) was added at a rate of 12.5 mL h.sup.1. Once the addition was complete the temperature was raised to 160 C. and held for 2 hours. The solution was cooled to 120 C. and held for 4 hours and then allowed to cool to room temperature.

(40) The solid was isolated by washing with methanol, then toluene/methanol, followed by centrifugation. The supernatant was discarded, and the dark brown solid/paste was retained as the product. Elemental analysis for the product indicates: C 28.08%; H 4.95%; N 1.25%; In 30.18%; Se 31.38%. The elemental ratio determined by chemical analysis corresponds to a material with formula InSet.sub.1.51. The 1-octane selenol ligand present on the particle surface contributes to the total amount of selenium found in the material.

(41) The XRD pattern of the material is characterized by diffraction peaks that are typical of aggregated nanoparticles (FIG. 10). TEM analysis shows that the material is made of spherical nanoparticles between 2-3 nm in diameter (FIG. 11). TGA performed under nitrogen indicates that the material contains approximately 39% w/w of volatile organic ligands that are completely removed when the temperature reaches 500 C. (FIG. 12).

Example 4: Synthesis of Gallium Selenide Nanoparticles in Oleic Acid/Therminol 66

(42) Ga(acac).sub.3 (2.00 g, 5.45 mmol), Therminol 66 (10 mL) and oleic acid (10 mL) were degassed for 1 hour at 100 C. and then the flask was backfilled with N.sub.2 and cooled to room temperature. 1-Octane selenol (5 mL, 28.1 mmol) was added quickly and the clear yellow solution was left to stir at room temperature for 30 minutes. TOP/Se (5.1 mL, 1.71 M, 8.7 mmol) was injected quickly and the solution was stirred at room temperature for 30 minutes. The solution was heated to 100 C. and held for 30 minutes, then stirred at 125 C. for 30 minutes, then heated to 140 C. and stirred for 60 minutes. The solution was cooled to 100 C. and left to anneal for 4 hours, before cooling to room temperature.

(43) The solid was isolated by washing with methanol, then isopropanol, then dichloromethane/methanol, followed by centrifugation. The supernatant was discarded, and the yellow/cream colored sticky solid was retained as the product. Elemental analysis for the product indicates: C 30.93%; H 5.30%; Ga 19.70%; Se 42.55%. The elemental ratio determined by chemical analysis corresponds to a material with formula GaSe.sub.1.91. The 1octane selenol ligand present on the particle surface contributes to the total amount of selenium found in the material.

(44) The XRD pattern of the material is characterized by broad diffraction peaks that are typical of very small nanoparticles FIG. 13 (A). The XRD pattern of an annealed film of the nanoparticles, heated at 260 C. for 10 minutes under N.sub.2 shows better defined peaks (B) with peak positions matching those of monoclinic Ga.sub.2Se.sub.3 from the JCPDS database (00044-0931) (C). TGA analysis performed under nitrogen indicates that the material contains approximately 46% w/w of volatile organic ligands that are completely removed at around 500 C. (FIG. 14).

Example 5: Synthesis of Gallium Selenide Nanoparticles in 1Octadecene/Oleylamine

(45) Ga(acac).sub.3 (2.30 g, 6.27 mmol), 1-octadecene (10 mL) and oleylamine (5 mL) were degassed for 1 hour at 100 C. and then the flask was backfilled with N.sub.2 and cooled to 70 C. 1-Dodecane selenol (5.6 mL) was added quickly and the solution was heated to 140 C. TOP/Se (5.8 mL, 1.71 M, 9.9 mmol) was injected at a rate of 1 mL min.sup.1. The solution was heated to 220 C.; once the temperature reached 180-200 C., a colourless liquid was distilled off. The solution was held at 220 C. for 30 minutes, then cooled to room temperature.

(46) The solid was isolated by washing with isopropanol, then dichloromethane/isopropanol, followed by centrifugation. The supernatant was discarded, and the dark yellow solid (1.0 g) was retained as the product. The product dissolved readily in toluene. Elemental analysis for the product indicates: C 30.15%; H 4.89%; N 1.45%; Ga 23.06%; Se 39.85%. The elemental ratio determined by chemical analysis corresponds to a material with formula GaSe.sub.1.53. The 1-dodecane selenol ligand present on the particle surface contributes to the total amount of selenium found in the material.

(47) The X-ray diffraction (XRD) pattern of the material is characterized by broad diffraction peaks that are typical of small nanoparticles (FIG. 15). The higher angle reflections match the peak positions of monoclinic (-phase) Ga.sub.2Se.sub.3 from the JCPDS database (00-044-0931). Apparent splitting of the (200) peak suggests the material is crystallized in a lower order symmetry, pseudo- phase. TGA analysis performed under nitrogen indicates that the material contains approximately 62% w/w of volatile organic ligands that are completely removed at around 500 C. (FIG. 16). The UV-visible absorption spectrum (FIG. 17) shows an absorption peak around 495 nm (2.5 eV). The relative blue-shift in the absorption peak wavelength with respect to the bulk band gap of Ga.sub.2Se.sub.3 (2.3 eV539 nm for the phase) is suggestive of quantum confinement effects in the nanoparticle material. Transmission electron microscopy (TEM, FIG. 18) reveals that the nanoparticles are relatively monodisperse, with an approximately spherical morphology and an average nanoparticle diameter on the order of 5 nm.

Example 6: Larger Scale Synthesis of Gallium Selenide Nanoparticles in 1Octadecene/Oleylamine

(48) Ga(acac).sub.3 (11.50 g, 31.33 mmol), 1-octadecene (50 mL) and oleylamine (25 mL) were degassed for 1 hour at 100 C. and then the flask was backfilled with N.sub.2 and cooled to 70 C. 1-Dodecane selenol (28.0 mL) was added quickly and the solution was heated to 140 C. TOP/Se (29.0 mL, 1.71 M, 49.6 mmol) was injected at a rate of 1 mL min.sup.1. The solution was heated to 220 C. and held for 30 minutes. The solution was cooled to room temperature.

(49) The solid was isolated by washing with isopropanol, then dichloromethane/isopropanol, followed by centrifugation. The supernatant was discarded, and the yellow solid (6.9 g) was retained as the product. Characterization date were comparable to those of the smaller scale reaction in Example 5.

(50) The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the Applicants. It will be appreciated with the benefit of the present disclosure that features described above in accordance with any embodiment or aspect of the disclosed subject matter can be utilized, either alone or in combination, with any other described feature, in any other embodiment or aspect of the disclosed subject matter.