WO2008104087A1 - Chalcopyrite nanoparticles, processes for synthesis thereof and uses thereof - Google Patents

Chalcopyrite nanoparticles, processes for synthesis thereof and uses thereof Download PDF

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WO2008104087A1
WO2008104087A1 PCT/CA2008/000416 CA2008000416W WO2008104087A1 WO 2008104087 A1 WO2008104087 A1 WO 2008104087A1 CA 2008000416 W CA2008000416 W CA 2008000416W WO 2008104087 A1 WO2008104087 A1 WO 2008104087A1
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nanoparticles
process according
chalcopyrite
temperature
average particle
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PCT/CA2008/000416
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French (fr)
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Farid Bensebaa
Abdelillah Aouadou
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National Research Council Of Canada
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/032—Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312
    • H01L31/0322—Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312 comprising only AIBIIICVI chalcopyrite compounds, e.g. Cu In Se2, Cu Ga Se2, Cu In Ga Se2
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B19/00—Selenium; Tellurium; Compounds thereof
    • C01B19/002—Compounds containing, besides selenium or tellurium, more than one other element, with -O- and -OH not being considered as anions
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B19/00—Selenium; Tellurium; Compounds thereof
    • C01B19/007—Tellurides or selenides of metals
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G15/00—Compounds of gallium, indium or thallium
    • C01G15/006—Compounds containing, besides gallium, indium, or thallium, two or more other elements, with the exception of oxygen or hydrogen
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/70—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data
    • C01P2002/72—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data by d-values or two theta-values, e.g. as X-ray diagram
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
    • C01P2002/82—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70 by IR- or Raman-data
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
    • C01P2002/84—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70 by UV- or VIS- data
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
    • C01P2002/85—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70 by XPS, EDX or EDAX data
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/01—Particle morphology depicted by an image
    • C01P2004/04—Particle morphology depicted by an image obtained by TEM, STEM, STM or AFM
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/30—Particle morphology extending in three dimensions
    • C01P2004/32—Spheres
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/51—Particles with a specific particle size distribution
    • C01P2004/53—Particles with a specific particle size distribution bimodal size distribution
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/60—Particles characterised by their size
    • C01P2004/64—Nanometer sized, i.e. from 1-100 nanometer
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/541—CuInSe2 material PV cells

Definitions

  • the present invention relates to chalcopyrite nanoparticles, to processes for synthesizing such nanoparticles and to uses of such nanoparticles, preferably as semiconductor materials in photovoltaic films.
  • Chalcopyrite-based semiconductors have received much interest in recent years for their potential use as photovoltaic (PV) material in thin film based solar cells [1 ,2].
  • These chalcopyrite materials are usually designated by the general formulae CIGS or Cu(In x Ga 1- X)SySe 2 - V (where x varies from 0 to 1 and y varies from 0 to 2). They have the highest optical absorption in the solar spectrum region among inorganic photovoltaic materials.
  • CIGS based solar cells have been shown to provide the highest power efficiency among thin film technologies, approaching those of crystalline silicon based solar cells.
  • a band edge of 420 nm (2.95 eV) was also measured, which is outside the optimum solar absorption range.
  • Using a single source precursor Castro et al. [17] prepared colloidal CuInS 2 nanoparticles between 2.7 nm and 4 nm, although the fabrication process involves numerous steps, and is difficult and costly to scale-up. The visible absorption peak maximum was also shown to change from 530 to 563 nm through control of preparation parameters.
  • Yu et al. [34] produces CIGS nanoparticles from non-oxide precursors of Cu halides, In halides and sodium selenide, which may have an average particle size less than 5 nm and a particle size distribution of ⁇ 1-2 nm.
  • the structure and fabrication process of these two ternary chalcopyrites is relatively simple, providing the best possibility for cost effective and high efficiency thin film solar cells. This is particularly true when compared to their quaternary and pentanary homologues [1-3].
  • the best efficiency is obtained with a chalcopyrite film having an energy bandgap of about 1.13 eV [3].
  • Wide energy bandgaps (1.4 ⁇ Eg ⁇ 1.8 eV) obtained with Cu(In 1 Ga)S 2 structures are expected to give very high power efficiency since they match the solar spectrum quite well (see Table 1). Furthermore, wide bandgap gives rise to high open voltage solar cells, a well sought characteristic for high stability solar cells. Without using Ga and with only 3 elements (Cu, In and S or Se), it may be possible to prepare potentially efficient photovoltaic solar cells using quantum confinement of CuInS 2 or CuInSe 2 nanoparticles. Quantum confinement allows control the energy bandgap above the bulk value when particle size is reduced below the Bohr radius. This tunability in the energy bandgap combined with the variety of chemical compositions may be important parameters in providing high power efficiency. Furthermore, the porosity structure of the photovoltaic film will be beneficial for effective interfacing with the buffer layer and/or subsequent reaction with other elements (such as Na, Ga, Se and or S), if necessary.
  • other elements such as Na, Ga, Se and or S
  • Bohr radius of 2.4 nm [33] and 4.0 nm [14] have been reported for CulnSe 2 and CuInS 2 semiconductor materials, respectively.
  • Photovoltaic films based on particle size below Bohr radius are expected to have a larger bandgap and optimized solar spectrum absorption.
  • colloidal synthesis methodologies reported so far have a variety of shortcomings that prevent realization of this goal.
  • the synthesis and process are preferably scalable, cost effective and based on green chemistry principles.
  • film deposition steps are preferably based on non-vacuum and low temperature processing.
  • nanoparticles comprising a chalcopyrite of formula
  • the overall ratio of copper to indium of synthesized CIGS nanoparticles of the present invention may be varied to obtain so-called copper-poor, stoichiometric and/or copper-rich compositions.
  • w In Cu-poor CIGS nanoparticles, w ⁇ 1.
  • stoichiometric CIGS nanoparticles In stoichiometric CIGS nanoparticles, w > 1.
  • the value of w is preferably in a range of from about 0.6 to about 1.0, more preferably from about 0.8 to about 1.0.
  • the value of x is preferably 1.
  • the value of y is preferably 0 or 2.
  • nanoparticles of the present invention are smaller and have a narrower particle size distribution (more uniform particle size) than CIGS nanoparticles of the prior art.
  • Nanoparticles of the present invention are preferably colloidal.
  • Average particle diameter may be about 8 nm or less, about 5 nm or less, about 4 nm or less, or about 3 nm or less.
  • the average particle diameter is in a range of from about 2 nm to about 4 nm and/or the average particle diameter is less than the Bohr radius of the chalcopyrite.
  • Particle size distribution is preferably such that 100% of the nanoparticles have diameters less than double the Bohr radius of the chalcopyrite. Size distribution may be quantified using the dispersion ⁇ value.
  • the particle diameter of all particles may be within less than about 0.9 nm of the average particle diameter.
  • the particle size distribution may be so narrow that all the particles have particle diameters within about 0.7 nm of the average particle diameter. Particle size distribution may even be so narrow that all the particles have particle diameters within about 0.5 nm of the average particle diameter.
  • a process for preparing nanoparticles of a chalcopyrite comprising reacting CuX a 2 , MX b 3 and L 2 Y in water to form nanoparticles of a chalcopyrite of formula CU w (In x Ga 1-X )S y Se 2 . ⁇ wherein X a and X b are the same or different and are halogens, M is Ga or In, L is an alkali metal, Y is S or Se, and w, x and y are as defined above.
  • the halogen is preferably Cl.
  • X a and X b are preferably the same halogen.
  • CuX a 2 is preferably in the form of a hydrate.
  • M is preferably In.
  • the alkali metal is preferably Na.
  • the process is carried out in water.
  • the water is purified, for example by distillation, deionization or any other suitable method or combination of methods.
  • Water is preferably used in an amount to provide dilute solutions of reactants. Highly dilute solutions are particularly preferred, however, on a commercial scale a cost- benefit analysis may be undertaken to optimize water utilization. Dilute solutions of reactants contribute to a product having a smaller particle size.
  • the use of water as a solvent in the present process provides a number of advantages over organic solvents.
  • Water has a high dielectric constant and is very suitable for microwave heating. Water is cheaper, more easily recycled, safer and more environmentally friendly. Further, the use of water surprisingly leads to higher yields and/or better quality nanoparticles in terms of average size and especially size distribution. Furthermore, the use of water permits better control over product composition permitting direct synthesis of copper- poor, stoichiometric and copper-rich CIGS nanoparticles.
  • the process is preferably carried out at a temperature of about 100 0 C or less, for example, the temperature may be in a range of from about 80 0 C to about 100 0 C. Temperatures of about 9O 0 C or less are of particular note. Such temperatures are lower than temperatures used in many prior art processes. Lower temperature advantageously reduces energy costs and contributes to the formation of smaller nanoparticles having a narrower particle size distribution. Heating may be accomplished by any suitable means. Microwave healing advantageously permits shorter reaction times, preferably about 120 minutes or less, more preferably about 60 minutes or less, for example about 30 minutes. Shorter reaction time advantageously reduces energy costs and contributes to the formation of smaller nanoparticles having a narrower particle size distribution.
  • a surfactant may be used.
  • the surfactant is added to an aqueous mixture of CuX a 2 and MX b 3 before adding L 2 Y.
  • the amount of surfactant employed is greater, on a mole or atomic basis, than the amount of indium. More preferably, the surfactant to indium mole ratio at least 50: 1.
  • the surfactant is preferably water-soluble and preferably bonds weakly to the nanoparticles.
  • the surfactant can be easily removed from the nanoparticles before or during solar cell fabrication steps.
  • Water-soluble surfactants obviate the need for polymers to prevent agglomeration.
  • the water-soluble surfactant comprises a water-soluble mercaptan, for example mercapto-acetic acid (MAA). Reducing the possibility of agglomeration contributes to narrower particle size distribution.
  • MAA mercapto-acetic acid
  • An advantage of the process of the present invention is that particle size and particle size distribution are controllable. Dilution, temperature, heating time and/or the amount of surfactant may be adjusted to further control composition, particle size and/or particle size distribution. Optimum particle size and particle size distribution of the nanoparticles depends, at least in part, on the nature of the chalcopyrite and its Bohr radius. However, smaller average particle sizes and tighter particle size distributions are generally preferred because they provide denser, more uniform thin films. The process of the present invention permits formation of nanoparticles having smaller average particle sizes and tighter particle size distributions than processes of the prior art.
  • composition of the nanoparticles may be controlled by controlling CuX a 2 to MX b 3 ratio, overall concentration of CuX a 2 , MX b 3 and L 2 Y, temperature, rate of temperature increase, reaction time, MX b 3 to surfactant ratio, or a combination thereof.
  • cupric chloride dihydrate, indium chloride and sodium sulfide or selenide are reacted in water in the presence of a water soluble surfactant (e.g. MAA) at a temperature in a range of from about 8O 0 C to about 100 0 C for about 30 minutes or less to produce chalcopyrite nanoparticles having an average particle diameter of about 5 nm or less with substantially all of the nanoparticles having particle diameters within about 0.9 nm of the average particle diameter. Heating is accomplished wilh microwaves.
  • Particle-based CIGS films may be readily applied on various substrates
  • deposition e.g. glass, plastics
  • non-vacuum methods e.g. glass, plastics
  • Various methods of deposition may be used including eep coating, spin coating, solution drop, screen-printing, doctor- blading, ink-jet printing, and spray pyrolysis. These deposition techniques are suitable for large scale deposition of colloidal-based films. Lower melting point and small monodisperse nanoparticles-based CIGS films give rise to films similar in quality to those obtained by vacuum-based deposition techniques without the high cost and complexity.
  • Spray deposition techniques involve spraying suspensions, e.g. aqueous suspensions, of the nanoparticles on to a substrate.
  • concentration of the nanoparticles in the suspension may be used, for instance a concentration in a range of from about 0.05 mg/ml to about 150 mg/ml, or from about 5 mg/ml to about 100 mg/ml.
  • Any suitable deposition temperature may be used, for instance a temperature in a range of from about 3O 0 C to about 100 0 C, or from about 5O 0 C to about 8O 0 C, for example about 7O 0 C.
  • Spray pyrolysis has numerous advantages for the large scale deposition of these nanoparticles and their subsequent integration into photovoltaic devices.
  • spray deposition of nanoparticle solutions offers the advantage of lower processing temperatures.
  • annealing temperature may be lowered by 200 0 C or more, leading to lower thermal budget cost.
  • lower processing temperature permits use of low- cost substrates such as soda-lime glass (SLG) while alleviating sodium diffusion (and other contaminants) from the substrate towards the photovoltaic layer and also relieving thermal stress.
  • SSG soda-lime glass
  • Lower processing temperatures also permits use of polymeric substrates, without the limitation of weak power efficiency.
  • post-annealing may be performed on spray deposited nanoparticle films.
  • Post-annealing may be performed at a temperature up to about 580 0 C, particularly a temperature in a range of from about 200 0 C to about 55O 0 C, for example about 400°C.
  • it is possible to heat the substrate during the spray process preferably to a temperature up to about 400 0 C, thereby obviating the need for post-annealing at much higher temperatures.
  • nanoparticles produced in the present process are well dispersed, in some cases it may be desirable to prepare films of submicron particles.
  • Submicron particles may be prepared using a high-energy excitation method, such as ultrasonication or high-pressure homogenization and microfluidization.
  • a solution containing nanoparticles may be nebulized using a high frequency (few MHz range) ultrasonicator and then deposited onto a substrate [32].
  • nanoparticles of the present invention may be used to prepare single-graded or double-graded thin films.
  • Single-graded thin films may be prepared, for example, by first depositing a layer of Cu-poor CIGS nanoparticles of the present invention on a substrate (e.g. metal-coated glass).
  • This first layer preferably has a thickness in a range of from about 10 nm to about 1500 nm, more preferably about 500 nm.
  • a layer of stoichiometric CIGS nanoparticles of the present invention may be deposited on top of the previous layer.
  • This layer preferably has a thickness in a range of about 10 nm to about 2000 nm, more preferably about 1000 nm.
  • Double-graded thin films are also obtainable using CIGS nanoparticles of the present invention.
  • Double-graded thin films may be prepared, for example, by first depositing a layer of Cu-poor or Cu-rich CIGS nanoparticles of the present invention.
  • This first layer preferably has a thickness in a range of from about 10 nm to about 1500 nm, more preferably about 500 nm.
  • a layer of stoichiometric CIGS nanoparticles of the present invention may be deposited on top of the previous layer.
  • This layer preferably has a thickness in a range of about 100 nm to about 1000 nm, more preferably about 1000 nm.
  • a third layer of Cu-rich or Cu-poor CIGS nanoparticles of the present invention may be deposited on top of the layer of stoichiometric CIGS nanoparticles.
  • This third layer preferably has a thickness in a range of from about 10 nm to about 500 nm, more preferably about 100 nm.
  • nanoparticles of the present invention may be used to prepare single-graded or double-graded photovoltaic thin films.
  • Single-graded thin films may be prepared, for example, by first depositing a thin layer of CIGS nanoparticles having a diameter larger that the Bohr radius on a substrate.
  • the substrate may be, for example, coated glass.
  • Glass may be coated with, for example, one or more of transparent conductive oxide (TCO), metal, titanium dioxide (TiO 2 ) and cadmium sulfide (CdS).
  • TCO transparent conductive oxide
  • metal titanium dioxide
  • CdS cadmium sulfide
  • This first layer preferably has a thickness in a range of from about 10 nm to about 1000 nm, more preferably about 500 nm.
  • Nanoparticles of the present invention may be used in any application requiring such nanoparticles. For example, alone or incorporated with conductive polymers, they may be used to produce semiconductor films for photovoltaic devices, e.g. solar cells, light emitting diodes and photodetectors.
  • Fig. 2 depicts an EDXA spectrum of the as-synthesized CuInS 2 nanoparticles depicted in Fig. 1 ;
  • Fig. 3 depicts an XRD spectrum of as-synthesized CuInS 2 nanoparticles with a Cu-rich and Cu-poor composition
  • Fig. 4 depicts a simulation of an XRD spectra of CuInS 2 nanoparticles in which the Cu:ln ratio is 0.8:1 (copper poor);
  • Fig. 5 depicts an XRD spectrum of as-synthesized Cu(Ga 025 In 075 )Se 2 nanoparticles
  • Fig. 6 depicts a Raman spectrum of as-synthesized CuInS 2 nanoparticles
  • Fig. 7 depicts a UV-visible absorption spectrum of as-synthesized CuInS 2 nanoparticles
  • Fig. 8 depicts a UV-visible absorption spectrum of as-synthesized CuInSe 2 nanoparticles
  • Fig. 9 depicts an AFM (atomic force microscopy) image of a thin film of
  • Cupric chloride dihydrate (CuCI 2 -2H 2 O) (Merck) and indium chloride (InCI 3 )
  • the solution was then placed inside a microwave oven and heated to a temperature of about 9O 0 C for 30 minutes using controlled ramping of 2 minutes. The solution was allowed to cool. Volatiles were removed by rotary evaporation. Alternatively or additionally, centrifugation may be used to isolate the solid product. Resultant colloidal nanoparticles of CuInS 2 were then characterized to provide information about size, size distribution, composition and structure.
  • CuInSe 2 nanoparticles were also prepared using the method of Example 1 , except that sodium selenide (Na 2 Se) was used instead of Na 2 S, and the mass of sodium selenidewas adjusted based on its molecular weight.
  • Na 2 Se sodium selenide
  • Example 1 with the following changes. Gallium chloride (GaCI 3 , 17 mg) and indium chloride (InCI 3 , 94 mg) were added simultaneously. 120 mg of cupric chloride dihydrate (CuCI 2 ⁇ H 2 O) and 109 mg of sodium sulfide (Na 2 S) were used.
  • EDXA energy-dispersive X-ray analysis
  • Fig. 1 shows a TEM image of as-synthesized CuInS 2 nanoparticles, representative of the overall field of view.
  • the scale bar is 20 nm.
  • Ultra-fine, spherical and well dispersed particles with uniform size distribution are shown. Based on data from about 100 nanoparticles from the whole TEM field of view, an average particle diameter of about 3 nm was determined with all particles being within 0.4 nm of the average diameter.
  • EDXA Energy-dispersive X-ray analysis
  • XRD X-ray diffraction
  • Fig. 3 shows that typical diffraction features are observed, similar to those reported in the literature.
  • Three relaively broad diffraction peaks are detected around 28, 47 and 55 degrees. These three peaks correspond to the (112), (220) and (116) planes of tetragonal structure as reported in the literature [14].
  • the diffraction peak observed at around 16 degrees is relatively high compared to the other peaks.
  • This peak assigned to (110) plane is also observed in bulk CuInS 2 , although its intensity is relatively weaker.
  • ODC ordered defect compound
  • ⁇ (in radians) is the line width at an angle 2 ⁇ (in radians) and ⁇ is the X-ray wavelength (1.5406 A). Based on the line width of the (112) diffraction peak, the average diameter of the crystallite is estimated to be about 2.2 nm.
  • FIG. 6 shows the Raman spectrum of a non-annealed thin film of the CuInS 2 nanoparticles on a glass substrate using solution drop approach.
  • the presence of the two peaks atabout 295 cm “1 and about 332 cm “1 is reminiscent of Raman features observed for bulk CuInS 2 films.
  • the first peak at about 295 cm “1 is indicative of copper rich phase.
  • the fact that a distinct peak at about 305 cm "1 is not observed indicates that CuInS 2 is already of good quality even before annealing.
  • the peak at 305 cm “1 is two times more intense than the 295 cm "1 peak.
  • FIG. 7 and 8 show UV-visible absorption spectra of CuInS 2 and CuInSe 2 nanoparticles, respectively.
  • the edges of the absorption peak are measured at about 532 nm for CuInS 2 and at about 580 nm for CuInSe 2 .
  • Synthesized crystallite diameters of these two chalcopyrites are below the Bohr radius. This confirms that quantum dot material was obtained using the method of the present invention.
  • Example 8 Thin Films of Nanoparticles
  • Nanoparticle deposition on a cleaned glass slide was accomplished by spin coating CuInS 2 nanoparticles at 2500 rpm on to the slide.
  • Fig. 9 shows the AFM (atomic force microscopy) image of the spin-coated CuInS 2 film on the glass slide.
  • This CIGS film is uniform and smooth. Particle diameters between 50 nm and 100 nm could be seen. It's likely that these particles result from agglomeration of smaller nanoparticles during the spin coating process. Film thickness was about 100 nm. Although this film is relatively porous, the top layer is quite smooth, which may be a consequence of a lower melting point of the nanoparticles in comparison to their bulk counterparts.
  • TCO-coated glass substrate by doctor blading.
  • a 30 mg/ml aqueous suspension of CuInS 2 nanoparticles was spray deposited on to the TiO 2 film for about 10 minutes at a substrate temperature of about 7O 0 C to form a CulnS 2 -coated structure.
  • the CulnS 2 -coated structure was annealed at a temperature of about 400 0 C.
  • Fig. 10 shows the TEM of the CulnS 2 -coated structure.
  • the CuInS 2 layer is about 1 ⁇ m thick, the TiO 2 layer is about 5 ⁇ m thick and the TCO layer is less than 1 ⁇ m thick.
  • the TEM shows that the CuInS 2 layer is a high density, uniform film.

Abstract

Small-sized CIGS nanopartcles having a narrow particle size distributon and a good bandgap are made in large yield by a simple, scalable process. The process involves reacting CuXa2, MXb3 and L2Y in water to form nanoparticles of a chalcopyrite of formula CUw(InxGa1-X)SySe2-Y, wherein Xa and Xb are the same or different and are halogens, M is Ga or In, L is an alkali metal, Y is S or Se, w is a number from 0.8 to 1.2, x is a number from 0 to 1, and y is a number from 0 to 2. The nanoparticles are useful for producing semiconductor films for photovoltaic devices, e.g. solar cells, light emitting diodes and photodetectors.

Description

CHALCOPYRITE NANOPARTICLES, PROCESSES FOR SYNTHESIS THEREOFAND
USES THEREOF
Cross-reference to Related Applications
[0001] This application claims the benefit of United States Provisional application
USSN 60/903,837 filed February 28, 2007, the entire contents of which is herein incorporated by reference.
Field of the Invention
[0002] The present invention relates to chalcopyrite nanoparticles, to processes for synthesizing such nanoparticles and to uses of such nanoparticles, preferably as semiconductor materials in photovoltaic films.
Background of the Invention
[0003] Chalcopyrite-based semiconductors have received much interest in recent years for their potential use as photovoltaic (PV) material in thin film based solar cells [1 ,2]. These chalcopyrite materials are usually designated by the general formulae CIGS or Cu(InxGa1-X)SySe2-V (where x varies from 0 to 1 and y varies from 0 to 2). They have the highest optical absorption in the solar spectrum region among inorganic photovoltaic materials. CIGS based solar cells have been shown to provide the highest power efficiency among thin film technologies, approaching those of crystalline silicon based solar cells. A world best certified efficiency of 19.5% has been recently measured for a Cu(In1Ga)Se2 composition with an energy gap around 1.13 eV. [3]. However, processes used to obtain these highly efficient CIGS solar cells are costly and laborious requiring precise multi-elements co-deposition under high vacuum and high temperature conditions. Often post deposition treatments are also required involving toxic gases such as H2Se.
[0004] Several low temperature and non-vacuum approaches have been recently reported using chalcopyrite nanoparticles and microparticles [4-11]. A few of these approaches have been integrated into photovoltaic pilot plant scale with some relative success [4-8]. However, the physico-chemical properties of these particles are either too ill-defined to be effective and reproducible, or relatively large to allow smooth and uniform film preparation. For example, large particles may lead to highly porous films that may lead to short circuits. Control of particle features in the nanoscale is important to achieve high power efficiency in the range of those obtained using vacuum deposition. [0005] Various approaches of have been recently developed specifically to synthesize nanostructured CIGS materials [13-21 and 34]. There are very few cases where well monodispersed chalcopyrite nanopaiϋcles with diameters below 5 nm within a tight size distribution are obtained. Gurin [13] showed that using polyvinyl alcohol, CuInS2 nanoparticles can be obtained. Based on diffraction peak width, a crystallite size of 7 nm could be inferred, larger than measured particle size. Utilization of toxic gases (H2S and H2Se) and the presence of difficult to remove polymer coating are major weakness for PV applications. Using the TOPO approach [22] O'Brien's group showed that nanoparticles of CuInSe2 are obtained with diameter size in the range of 4.5 nm [15]. A band edge of 420 nm (2.95 eV) was also measured, which is outside the optimum solar absorption range. Using a single source precursor Castro et al. [17] prepared colloidal CuInS2 nanoparticles between 2.7 nm and 4 nm, although the fabrication process involves numerous steps, and is difficult and costly to scale-up. The visible absorption peak maximum was also shown to change from 530 to 563 nm through control of preparation parameters. Using a pyridine capping agent in an organic solvent, Yu et al. [34] produces CIGS nanoparticles from non-oxide precursors of Cu halides, In halides and sodium selenide, which may have an average particle size less than 5 nm and a particle size distribution of ±1-2 nm.
[0006] Photovoltaic cells based on bulk CuInS2 and CuInSe2 having energy bandgap values of 1.5 and 1.0 eV, respectively, have been shown to provide relatively high power efficiency [23, 24]. The structure and fabrication process of these two ternary chalcopyrites is relatively simple, providing the best possibility for cost effective and high efficiency thin film solar cells. This is particularly true when compared to their quaternary and pentanary homologues [1-3]. Calculation and experimental data obtained on these semiconductors with different bandgaps indicated that the highest efficiency solar cells are obtained with photovoltaic material having a bandgap between 1.2 and 1.8 eV, more specifically around 1.5 eV (see Table 1). However, in practice so far the best efficiency is obtained with a chalcopyrite film having an energy bandgap of about 1.13 eV [3].
[0007] Wide energy bandgaps (1.4 < Eg < 1.8 eV) obtained with Cu(In1Ga)S2 structures are expected to give very high power efficiency since they match the solar spectrum quite well (see Table 1). Furthermore, wide bandgap gives rise to high open voltage solar cells, a well sought characteristic for high stability solar cells. Without using Ga and with only 3 elements (Cu, In and S or Se), it may be possible to prepare potentially efficient photovoltaic solar cells using quantum confinement of CuInS2 or CuInSe2 nanoparticles. Quantum confinement allows control the energy bandgap above the bulk value when particle size is reduced below the Bohr radius. This tunability in the energy bandgap combined with the variety of chemical compositions may be important parameters in providing high power efficiency. Furthermore, the porosity structure of the photovoltaic film will be beneficial for effective interfacing with the buffer layer and/or subsequent reaction with other elements (such as Na, Ga, Se and or S), if necessary.
Table 1
Theoretical and laboratory efficiencies for the most common chalcopyrite solar cells
(based on Ref. [3, 23-26])
Figure imgf000004_0001
[0008] Estimated values of Bohr radius of 2.4 nm [33] and 4.0 nm [14] have been reported for CulnSe2 and CuInS2 semiconductor materials, respectively. A synthetic methodology to prepare nanoparticles with size distribution below double the Bohr radius, preferably below the Bohr radius, is thus an important factor. Photovoltaic films based on particle size below Bohr radius are expected to have a larger bandgap and optimized solar spectrum absorption. As mentioned above, colloidal synthesis methodologies reported so far have a variety of shortcomings that prevent realization of this goal.
[0009] Band grading as described by Contreras et al. [28] is generally recognized as the best approach to obtain high CIGS-based solar cells. Nanopartble-based solar cells allowing band control through variation of size andfor composition are suitable for constructing high efficiency solar cells using low-cost deposition techniques.
[0010] There are also other factors to consider for successful commercial application of any new material synthesis and process. For example, the synthesis and process are preferably scalable, cost effective and based on green chemistry principles. Furthermore, film deposition steps are preferably based on non-vacuum and low temperature processing.
[0011] There is a need in the art for new materials, synthetic methodologies and processes that fulfill one or more of the above factors. Summary of the Invention
[0012] It has now been found that small-sized CIGS nanoparticles having a very narrow particle size distribution and a good bandgap can be made in large yield by a simple, scalable process. Smooth thin films are obtainable from these nanoparticles even at room temperature.
[0013] Thus, there is provided nanoparticles comprising a chalcopyrite of formula
Cuw(lnxGai-x)SySe2-y, wherein w is a number from 0.5 to 1.2, x is a number from 0 to 1 and y is a number from 0 to 2, the chalcopyrite having a Bohr radius, the nanoparticles having an average particle diameter less than double the Bohr radius of the chalcopyrite, and substantially all of the nanoparticles having particle diameters within about 0.9 nm of the average particle diameter.
[0014] Advantageously, the overall ratio of copper to indium of synthesized CIGS nanoparticles of the present invention may be varied to obtain so-called copper-poor, stoichiometric and/or copper-rich compositions. In Cu-poor CIGS nanoparticles, w < 1. In stoichiometric CIGS nanoparticles, w = 1. In Cu-rich CIGS nanoparticles, w > 1. The value of w is preferably in a range of from about 0.6 to about 1.0, more preferably from about 0.8 to about 1.0. The value of x is preferably 1. The value of y is preferably 0 or 2.
[0015] Advantageously, nanoparticles of the present invention are smaller and have a narrower particle size distribution (more uniform particle size) than CIGS nanoparticles of the prior art. Nanoparticles of the present invention are preferably colloidal. Average particle diameter may be about 8 nm or less, about 5 nm or less, about 4 nm or less, or about 3 nm or less. In particularly advantageous embodiments, the average particle diameter is in a range of from about 2 nm to about 4 nm and/or the average particle diameter is less than the Bohr radius of the chalcopyrite.
[0016] Particle size distribution is preferably such that 100% of the nanoparticles have diameters less than double the Bohr radius of the chalcopyrite. Size distribution may be quantified using the dispersion σ value. Advantageously, the particle diameter of all particles may be within less than about 0.9 nm of the average particle diameter. The particle size distribution may be so narrow that all the particles have particle diameters within about 0.7 nm of the average particle diameter. Particle size distribution may even be so narrow that all the particles have particle diameters within about 0.5 nm of the average particle diameter. [0017] There is further provided a process for preparing nanoparticles of a chalcopyrite comprising reacting CuXa 2, MXb 3 and L2Y in water to form nanoparticles of a chalcopyrite of formula CUw(InxGa1-X)SySe2.^ wherein Xa and Xb are the same or different and are halogens, M is Ga or In, L is an alkali metal, Y is S or Se, and w, x and y are as defined above.
[0018] The halogen is preferably Cl. Xa and Xb are preferably the same halogen.
CuXa 2 is preferably in the form of a hydrate. M is preferably In. The alkali metal is preferably Na.
[0019] The process is carried out in water. Preferably, the water is purified, for example by distillation, deionization or any other suitable method or combination of methods. Water is preferably used in an amount to provide dilute solutions of reactants. Highly dilute solutions are particularly preferred, however, on a commercial scale a cost- benefit analysis may be undertaken to optimize water utilization. Dilute solutions of reactants contribute to a product having a smaller particle size.
[0020] Compared to processes such as described in Yu et al. [34], the use of water as a solvent in the present process provides a number of advantages over organic solvents. Water has a high dielectric constant and is very suitable for microwave heating. Water is cheaper, more easily recycled, safer and more environmentally friendly. Further, the use of water surprisingly leads to higher yields and/or better quality nanoparticles in terms of average size and especially size distribution. Furthermore, the use of water permits better control over product composition permitting direct synthesis of copper- poor, stoichiometric and copper-rich CIGS nanoparticles.
[0021] The process is preferably carried out at a temperature of about 1000C or less, for example, the temperature may be in a range of from about 800C to about 1000C. Temperatures of about 9O0C or less are of particular note. Such temperatures are lower than temperatures used in many prior art processes. Lower temperature advantageously reduces energy costs and contributes to the formation of smaller nanoparticles having a narrower particle size distribution. Heating may be accomplished by any suitable means. Microwave healing advantageously permits shorter reaction times, preferably about 120 minutes or less, more preferably about 60 minutes or less, for example about 30 minutes. Shorter reaction time advantageously reduces energy costs and contributes to the formation of smaller nanoparticles having a narrower particle size distribution. [0022] To reduce the possibility of nanoparticle agglomeration, a surfactant may be used. Preferably, the surfactant is added to an aqueous mixture of CuXa 2 and MXb 3 before adding L2Y. Preferably, the amount of surfactant employed is greater, on a mole or atomic basis, than the amount of indium. More preferably, the surfactant to indium mole ratio at least 50: 1. The surfactant is preferably water-soluble and preferably bonds weakly to the nanoparticles. Preferably, the surfactant can be easily removed from the nanoparticles before or during solar cell fabrication steps. Water-soluble surfactants obviate the need for polymers to prevent agglomeration. Preferably, the water-soluble surfactant comprises a water-soluble mercaptan, for example mercapto-acetic acid (MAA). Reducing the possibility of agglomeration contributes to narrower particle size distribution.
[0023] An advantage of the process of the present invention is that particle size and particle size distribution are controllable. Dilution, temperature, heating time and/or the amount of surfactant may be adjusted to further control composition, particle size and/or particle size distribution. Optimum particle size and particle size distribution of the nanoparticles depends, at least in part, on the nature of the chalcopyrite and its Bohr radius. However, smaller average particle sizes and tighter particle size distributions are generally preferred because they provide denser, more uniform thin films. The process of the present invention permits formation of nanoparticles having smaller average particle sizes and tighter particle size distributions than processes of the prior art. Further, the process of the present invention advantageous^ provides consistently high product yield (>90%) and is easily scalable in order to provide quantities of the product on a commercial scale. Furthermore, composition of the nanoparticles (i.e. Cu-poor, stoichiometric or Cu-rich) may be controlled by controlling CuXa 2 to MXb 3 ratio, overall concentration of CuXa 2, MXb 3 and L2Y, temperature, rate of temperature increase, reaction time, MXb 3 to surfactant ratio, or a combination thereof.
[0024] In particularly advantageous embodiments of the present process providing better quality nanoparticles, cupric chloride dihydrate, indium chloride and sodium sulfide or selenide are reacted in water in the presence of a water soluble surfactant (e.g. MAA) at a temperature in a range of from about 8O0C to about 1000C for about 30 minutes or less to produce chalcopyrite nanoparticles having an average particle diameter of about 5 nm or less with substantially all of the nanoparticles having particle diameters within about 0.9 nm of the average particle diameter. Heating is accomplished wilh microwaves. [0025] Particle-based CIGS films may be readily applied on various substrates
(e.g. glass, plastics) using non-vacuum methods [30, 31]. Various methods of deposition may be used including eep coating, spin coating, solution drop, screen-printing, doctor- blading, ink-jet printing, and spray pyrolysis. These deposition techniques are suitable for large scale deposition of colloidal-based films. Lower melting point and small monodisperse nanoparticles-based CIGS films give rise to films similar in quality to those obtained by vacuum-based deposition techniques without the high cost and complexity.
[0026] Spray deposition techniques involve spraying suspensions, e.g. aqueous suspensions, of the nanoparticles on to a substrate. Any suitable concentration of the nanoparticles in the suspension may be used, for instance a concentration in a range of from about 0.05 mg/ml to about 150 mg/ml, or from about 5 mg/ml to about 100 mg/ml. Any suitable deposition temperature may be used, for instance a temperature in a range of from about 3O0C to about 1000C, or from about 5O0C to about 8O0C, for example about 7O0C.
[0027] Spray pyrolysis has numerous advantages for the large scale deposition of these nanoparticles and their subsequent integration into photovoltaic devices. As a consequence of higher surface diffusivity and reduced melting temperatures, spray deposition of nanoparticle solutions offers the advantage of lower processing temperatures. Thus annealing temperature may be lowered by 2000C or more, leading to lower thermal budget cost. Further, lower processing temperature permits use of low- cost substrates such as soda-lime glass (SLG) while alleviating sodium diffusion (and other contaminants) from the substrate towards the photovoltaic layer and also relieving thermal stress. Lower processing temperatures also permits use of polymeric substrates, without the limitation of weak power efficiency.
[0028] If desired or required, post-annealing may be performed on spray deposited nanoparticle films. Post-annealing may be performed at a temperature up to about 5800C, particularly a temperature in a range of from about 2000C to about 55O0C, for example about 400°C. However, it is possible to heat the substrate during the spray process, preferably to a temperature up to about 4000C, thereby obviating the need for post-annealing at much higher temperatures.
[0029] Further, although nanoparticles produced in the present process are well dispersed, in some cases it may be desirable to prepare films of submicron particles. Submicron particles may be prepared using a high-energy excitation method, such as ultrasonication or high-pressure homogenization and microfluidization. For example, a solution containing nanoparticles may be nebulized using a high frequency (few MHz range) ultrasonicator and then deposited onto a substrate [32].
[0030] Furthermore, nanoparticles of the present invention may be used to prepare single-graded or double-graded thin films. Single-graded thin films may be prepared, for example, by first depositing a layer of Cu-poor CIGS nanoparticles of the present invention on a substrate (e.g. metal-coated glass). This first layer preferably has a thickness in a range of from about 10 nm to about 1500 nm, more preferably about 500 nm. Then, a layer of stoichiometric CIGS nanoparticles of the present invention may be deposited on top of the previous layer. This layer preferably has a thickness in a range of about 10 nm to about 2000 nm, more preferably about 1000 nm.
[0031] Double-graded thin films are also obtainable using CIGS nanoparticles of the present invention. Double-graded thin films may be prepared, for example, by first depositing a layer of Cu-poor or Cu-rich CIGS nanoparticles of the present invention. This first layer preferably has a thickness in a range of from about 10 nm to about 1500 nm, more preferably about 500 nm. Then, a layer of stoichiometric CIGS nanoparticles of the present invention may be deposited on top of the previous layer. This layer preferably has a thickness in a range of about 100 nm to about 1000 nm, more preferably about 1000 nm. A third layer of Cu-rich or Cu-poor CIGS nanoparticles of the present invention may be deposited on top of the layer of stoichiometric CIGS nanoparticles. This third layer preferably has a thickness in a range of from about 10 nm to about 500 nm, more preferably about 100 nm.
[0032] In another application, nanoparticles of the present invention may be used to prepare single-graded or double-graded photovoltaic thin films. Single-graded thin films may be prepared, for example, by first depositing a thin layer of CIGS nanoparticles having a diameter larger that the Bohr radius on a substrate. The substrate may be, for example, coated glass. Glass may be coated with, for example, one or more of transparent conductive oxide (TCO), metal, titanium dioxide (TiO2) and cadmium sulfide (CdS). This first layer preferably has a thickness in a range of from about 10 nm to about 1000 nm, more preferably about 500 nm. Then, a second layer of CIGS nanoparticles of the present invention may be deposited on the previous layer. Thickness of this second layer is preferably in a range of about 10 nm to about 2000 nm, more preferably about 1000 nm. A third layer of Cu-poor CIGS nanoparticles of the present invention may then be deposited. Thickness of this third layer is preferably in a range of about 10 nm to about 500 nm, more preferably about 100 nm. [0033] Nanoparticles of the present invention may be used in any application requiring such nanoparticles. For example, alone or incorporated with conductive polymers, they may be used to produce semiconductor films for photovoltaic devices, e.g. solar cells, light emitting diodes and photodetectors.
[0034] Further features of the invention will be described or will become apparent in the course of the following detailed description.
Brief Description of the Drawings
[0035] In order that the invention may be more clearly understood, embodiments thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which:
[0036] Fig. 1 depicts a TEM image spectrum of as-synthesized CuInS2 nanoparticles (scale = 20 nm);
[0037] Fig. 2 depicts an EDXA spectrum of the as-synthesized CuInS2 nanoparticles depicted in Fig. 1 ;
[0038] Fig. 3 depicts an XRD spectrum of as-synthesized CuInS2 nanoparticles with a Cu-rich and Cu-poor composition;
[0039] Fig. 4 depicts a simulation of an XRD spectra of CuInS2 nanoparticles in which the Cu:ln ratio is 0.8:1 (copper poor);
[0040] Fig. 5 depicts an XRD spectrum of as-synthesized Cu(Ga025In075)Se2 nanoparticles;
[0041] Fig. 6 depicts a Raman spectrum of as-synthesized CuInS2 nanoparticles;
[0042] Fig. 7 depicts a UV-visible absorption spectrum of as-synthesized CuInS2 nanoparticles;
[0043] Fig. 8 depicts a UV-visible absorption spectrum of as-synthesized CuInSe2 nanoparticles;
[0044] Fig. 9 depicts an AFM (atomic force microscopy) image of a thin film of
CuInS2 obtained by spin coating at 2500 rpm (image size = 2 μm x 2 μm); and, [0045] Fig. 10 depicts a TEM (transmission electron micrograph) cross-section of a CulnS2/Ti02/glass multilayered film (image size = about 9 μm x 9 μm).
Description of Preferred Embodiments
[0046] Example 1: Synthesis Of CuInS2 Nanoparticles
[0047] Cupric chloride dihydrate (CuCI2-2H2O) (Merck) and indium chloride (InCI3)
(Aldrich) were mixed thoroughly with a surfactant (mercapto-acetic acid (MAA)) (Sigma- Aldrich) in 200 ml deionized water to provide a dilute solution. Sodium sulfide (Na2S) (Aldrich) was then added to the solution. Cupric chloride dihydrate (100 mg), indium chloride (110 mg) and sodium sulfide (78 mg) were employed in amounts to provide a Cu:ln:S atomic ratio of about 1 :1 :2, with 20% excess copper. Further, the indium chloride and MAA were employed in amounts to provide an ln:MAA atomic/molecular ratio of about 1 :50.
[0048] The solution was then placed inside a microwave oven and heated to a temperature of about 9O0C for 30 minutes using controlled ramping of 2 minutes. The solution was allowed to cool. Volatiles were removed by rotary evaporation. Alternatively or additionally, centrifugation may be used to isolate the solid product. Resultant colloidal nanoparticles of CuInS2 were then characterized to provide information about size, size distribution, composition and structure.
[0049] Example 2: Synthesis Of CuInSe2 Nanoparticles
[0050] CuInSe2 nanoparticles were also prepared using the method of Example 1 , except that sodium selenide (Na2Se) was used instead of Na2S, and the mass of sodium selenidewas adjusted based on its molecular weight.
[0051 ] Example 3: Synthesis of Cu(Ga025ln0 7s)Se2 Nanoparticles
[0052] Cu(Ga0 25In075)Se2 nanoparticles were prepared using the method of
Example 1 with the following changes. Gallium chloride (GaCI3, 17 mg) and indium chloride (InCI3, 94 mg) were added simultaneously. 120 mg of cupric chloride dihydrate (CuCI2 ^H2O) and 109 mg of sodium sulfide (Na2S) were used.
[0053] Example 4: TEM and EDXA Characterization of Nanoparticles
[0054] To prepare samples for TEM (transmission electron microscopy) and
EDXA (energy-dispersive X-ray analysis) characterization, a dilute aqueous suspension of nanoparticles was prepared by adding a few drops of the nanoparticles to deionized water in a beaker and shaking vigorously. A drop of the resulting suspension was then deposited on a nickel grid and allowed to dry. TEM and EDXA analyses were performed using a Philips CM20200 kV electron microscope equipped with an Oxford Instruments energy-dispersive X-ray diffraction detector.
[0055] Fig. 1 shows a TEM image of as-synthesized CuInS2 nanoparticles, representative of the overall field of view. The scale bar is 20 nm. Ultra-fine, spherical and well dispersed particles with uniform size distribution are shown. Based on data from about 100 nanoparticles from the whole TEM field of view, an average particle diameter of about 3 nm was determined with all particles being within 0.4 nm of the average diameter.
[0056] Energy-dispersive X-ray analysis (EDXA) data shown in Fig. 2 confirmed the chemical composition of the nanoparticle. Based on peak intensity, an approximate Cu:ln:S atomic ratio of 1 : 1 :2 is inferred. For a more precise determination of stoichiometry, a reference sample could be used. Observed Ni peaks are assigned to the sample holder.
[0057] Example 5: XRD Characterization of Nanoparticles
[0058] XRD (X-ray diffraction) characterization was performed at ambient temperature with Cu-Ka radiation using a Bruker diffractometer D8. Powder samples were uniformly spread over a low background silicon holder.
[0059] Fig. 3 shows that typical diffraction features are observed, similar to those reported in the literature. Three relaively broad diffraction peaks are detected around 28, 47 and 55 degrees. These three peaks correspond to the (112), (220) and (116) planes of tetragonal structure as reported in the literature [14]. The diffraction peak observed at around 16 degrees is relatively high compared to the other peaks. This peak assigned to (110) plane is also observed in bulk CuInS2, although its intensity is relatively weaker.
[0060] XRD simulation on small (2.0, 2.5 and 3.0 nm) CuInS2 nanoparticles showed that the relative intensity of the peak at around 16 degrees is related to copper deficiency. As shown in Fig. 4, decreasing Cu/ln from 1 to 0.8 (copper-poor conditions) shows that the relative intensity of the peak at around 16 degrees increases. The peak at around 16 degrees disappears when a simulation is carried out under copper-rich conditions (Cu/ln > 1). [0061] State-of-the art CIGS photovoltaic cells have a complex multilayer structure. It is often assumed that a thin layer of an ordered defect compound (ODC), supposedly Cu(In1Ga)3Se5, is formed at the surface of the CIGS layer, next to the CdS buffer layer in the cell. The origin and relevance of copper deficiency will be discussed further in conjunction with Raman characterization data.
[0062] Line broadening of the XRD diffraction peaks has been used in the past to estimate the average crystallite size of nanoparticles. The size of the crystallites d; has been determined using the Debye-Scherrer equation:
dc = 0.9 λ/βcosθ
where β (in radians) is the line width at an angle 2Θ (in radians) and λ is the X-ray wavelength (1.5406 A). Based on the line width of the (112) diffraction peak, the average diameter of the crystallite is estimated to be about 2.2 nm.
[0063] Based on TEM and XRD data, it can be concluded that CuInS2 nanoparticles made by the method of the present invention consist of single crystallites. The difference in diameter between XRD and TEM data could be a result of a noncrystalline surface layer.
[0064] Example 6: Raman Characterization of Nanoparticles
[0065] A Dilor Raman system was used to collect Raman spectra. Fig. 6 shows the Raman spectrum of a non-annealed thin film of the CuInS2 nanoparticles on a glass substrate using solution drop approach. The presence of the two peaks atabout 295 cm"1 and about 332 cm"1 is reminiscent of Raman features observed for bulk CuInS2 films. The first peak at about 295 cm"1 is indicative of copper rich phase. The fact that a distinct peak at about 305 cm"1 is not observed indicates that CuInS2 is already of good quality even before annealing. In reported Raman characterization of thin films Of CuInS2 obtained using vacuum deposition and post annealing, the peak at 305 cm"1 is two times more intense than the 295 cm"1 peak.
[0066] Example 7: UV-Visible Characterization of Films of Nanopartcles
[0067] An HP 8453 UV-visible absorption spectrometer was used to collect UV- visible absorption spectra. Figs. 7 and 8 show UV-visible absorption spectra of CuInS2 and CuInSe2 nanoparticles, respectively. The edges of the absorption peak are measured at about 532 nm for CuInS2 and at about 580 nm for CuInSe2. This corresponds to an energy bandgap of about2.3 eV for CuInS2 and about2.1 eV for CuInSe2. Synthesized crystallite diameters of these two chalcopyrites are below the Bohr radius. This confirms that quantum dot material was obtained using the method of the present invention.
[0068] Example 8: Thin Films of Nanoparticles
[0069] Spin Coaling: Nanoparticle deposition on a cleaned glass slide was accomplished by spin coating CuInS2 nanoparticles at 2500 rpm on to the slide. Fig. 9 shows the AFM (atomic force microscopy) image of the spin-coated CuInS2 film on the glass slide. This CIGS film is uniform and smooth. Particle diameters between 50 nm and 100 nm could be seen. It's likely that these particles result from agglomeration of smaller nanoparticles during the spin coating process. Film thickness was about 100 nm. Although this film is relatively porous, the top layer is quite smooth, which may be a consequence of a lower melting point of the nanoparticles in comparison to their bulk counterparts.
[0070] Spray Deposition: A 5 μm thick porous TiO2 film was prepared on a
TCO-coated glass substrate by doctor blading. A 30 mg/ml aqueous suspension of CuInS2 nanoparticles was spray deposited on to the TiO2 film for about 10 minutes at a substrate temperature of about 7O0C to form a CulnS2-coated structure. After deposition, the CulnS2-coated structure was annealed at a temperature of about 4000C. Fig. 10 shows the TEM of the CulnS2-coated structure. The CuInS2 layer is about 1 μm thick, the TiO2 layer is about 5 μm thick and the TCO layer is less than 1 μm thick. The TEM shows that the CuInS2 layer is a high density, uniform film.
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[0072] Other advantages that are inherent to the structure are obvious to one skilled in the art. The embodiments are described herein illustratively and are not meant to limit the scope of the invention as claimed. Variations of the foregoing embodiments will be evident to a person of ordinary skill and are intended by the inventor to be encompassed by the following claims.

Claims

Claims:
1. Nanoparticles comprising a chalcopyrite of formula CUw(InxGaLx)SySe2-V, wherein w is a number from 0.5 to 1.2, x is a number from 0 to 1 and y is a number from 0 to 2, the chalcopyrite having a Bohr radius, the nanoparticles having an average particle diameter less than double the Bohr radius of the chalcopyrite, and substantially all of the nanoparticles having particle diameters within about 0.9 nm of the average particle diameter.
2. Nanoparticles according to claim 1 , wherein the average particle diameter is less than the Bohr radius of the chalcopyrite.
3. Nanoparticles according to claim 1 or 2, wherein substantially all of the nanoparticles have particle diameters within about 0.7 nm of the average particle diameter.
4. Nanoparticles according to any one of claims 1 to 3, wherein the average particle diameter is about 5 nm or less.
5. Nanoparticles according to any one of claims 1 to 3, wherein the average particle diameter is about 4 nm or less.
6. Nanoparticles according to any one of claims 1 to 5, wherein w is a number from 0.8 to 1.0.
7. Nanoparticles according to any one of claims 1 to 6, wherein x is 1 and y is 0 or 2.
8. A film comprising nanoparticles as defined in any one of claims 1 to 7.
9. Process for preparing nanoparticles of a chalcopyrite comprising reacting CuXa 2, MXb 3 and L2Y in water to form nanoparticles of a chalcopyrite of formula Cuw(lnxGai- x)SySe2-y, wherein Xa and Xb are the same or different and are halogens, M is Ga or In, L is an alkali metal, Y is S or Se, w is a number from 0.5 to 1.2, x is a number from 0 to 1 and y is a number from 0 to 2.
10. The process according to claim 9, wherein X8 and Xb are Cl and L is Na.
11. The process according to claim 9, wherein CuXa 2 is cupric chloride dihydrate, MXb 3 is indium chloride, L is Na and y is 0 or 2.
12. The process according to any one of claims 9 to 11 , further comprising presence of a water-soluble surfactant.
13. The process according to claim 12, wherein the surfactant comprises a water- soluble mercaptan.
14. The process according to claim 13, wherein the surfactant comprises mercapto- acetic acid.
15. The process according to any one of claims 12 to 14, wherein w is controlled by controlling MXb 3 to surfactant ratio.
16. The process according to any one of claims 9 to 14, wherein w is controlled by controlling CuXa 2 to MXb 3 ratio, overall concentration of CuXa 2, MXb 3 and L2Y, temperature, rate of temperature increase, reaction time, or a combination thereof.
17. The process according to any one of claims 9 to 16, performed at a temperature in a range of from 8O0C to 1000C.
18. The process according to claim 17, wherein the temperature is attained by microwave heating.
19. The process according to any one of claims 9 to 18, performed for a time of 30 minutes or less.
20. The process according to claim 9, wherein CuXa 2 is cupric chloride dihydrate, MXb 3 is indium chloride, L2Y is sodium sulfide or selenide, and wherein the process is performed at a temperature in a range of from 800C to 1000C for 30 minutes or less in presence of a water-soluble surfactant.
21. The process according to claim 20, wherein the temperature is attained by microwave healing.
22. The process according to claim 20 or 21 , wherein the surfactant is mercapto- acetic acid.
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