WO2012112821A1 - Solar absorbing films with enhanced electron mobility and methods of their preparation - Google Patents

Solar absorbing films with enhanced electron mobility and methods of their preparation Download PDF

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WO2012112821A1
WO2012112821A1 PCT/US2012/025529 US2012025529W WO2012112821A1 WO 2012112821 A1 WO2012112821 A1 WO 2012112821A1 US 2012025529 W US2012025529 W US 2012025529W WO 2012112821 A1 WO2012112821 A1 WO 2012112821A1
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nano
film
ink
sintering
nps
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Henry L. Lomasney
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Sandia Solar Technologies Llc
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    • H01L21/02524Group 14 semiconducting materials
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    • H01L21/02568Chalcogenide semiconducting materials not being oxides, e.g. ternary compounds
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    • H01L21/02612Formation types
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    • H01L21/02623Liquid deposition
    • H01L21/02628Liquid deposition using solutions
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    • H01L31/00Semiconductor 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
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    • H01L31/0256Semiconductor 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
    • H01L31/0264Inorganic materials
    • H01L31/032Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312
    • H01L31/0322Inorganic 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
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    • H01L31/0248Semiconductor 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
    • H01L31/0256Semiconductor 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
    • H01L31/0264Inorganic materials
    • H01L31/032Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312
    • H01L31/0326Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312 comprising AIBIICIVDVI kesterite compounds, e.g. Cu2ZnSnSe4, Cu2ZnSnS4
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    • H01L31/0248Semiconductor 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
    • H01L31/0352Semiconductor 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 their shape or by the shapes, relative sizes or disposition of the semiconductor regions
    • H01L31/035209Semiconductor 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 their shape or by the shapes, relative sizes or disposition of the semiconductor regions comprising a quantum structures
    • H01L31/035218Semiconductor 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 their shape or by the shapes, relative sizes or disposition of the semiconductor regions comprising a quantum structures the quantum structure being quantum dots
    • YGENERAL 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
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00Energy generation through renewable energy sources
    • Y02E10/50Photovoltaic [PV] energy
    • Y02E10/541CuInSe2 material PV cells

Definitions

  • CIS absorbers Although a variety of approaches have been taken to the deposition of CIS absorbers, they may be divided into two categories: physical vapor deposition and reactive annealing.
  • physical vapor deposition processes Cu - In complexes are vapor deposited from ternary (e.g., CuInSe2) or quaternary (e.g., Culni_ x Ga x Se 2 ) sources onto a heated substrate.
  • Deposition is generally followed by reactive annealing with sulfur or selenium.
  • reactive annealing techniques involve the deposition Cu and In layers, typically by sputtering, at the appropriate thickness on to a substrate that is generally at room temperature (e.g., a 20-25° C substrate).
  • Deposition is typically followed by a second step, where the multi-layer structure is annealed in the presence of selenium (i.e., reactive annealing). Energy conversion efficiency is not as high as with the physical vapor deposition processes, but the process is easier to manage.
  • RTP Rapid Thermal Processing
  • a production process that uses bulk CIGS is especially challenging since CIGS has a melting point around 1000° C. Most substrates cannot withstand such temperatures, and even glass substrates encounter serious warping problems. Furthermore, traditional production methods/alternatives involve high energy demands, require relatively long processing times, and they involve large capital outlays for production facilities.
  • U.S. Patent No.: 6,092, 669 the thin film precursor is provided in the form of metal layers that are subsequently exposed to reactive annealing with selenium.
  • This process has the drawbacks of high cost, required use of reactive annealing with selenium, and non-uniformity of product.
  • U.S. Patent No. 4,581,108 discloses an electrodeposition process to deliver the necessary metal precursor layers followed by the selenization of those layers. That process, however, has been shown to yield a CuInSe 2 absorber film that has poor adhesion to the back contact (ohmic) layer.
  • U.S. Patent No.: 7,582,506 to Basel discloses a process for
  • U.S. Patent Application No. 20090053878 discloses a method for producing a group IV semiconductor solar cell utilizing a colloidal silicon nanoparticle dispersion to lay down a silicon absorber precursor layer, and a flash lamp to sinter the precursor layer. While this process discloses the sintering of the silicon grain structure of the absorber layer, using a high intensity photonic energy source with the objective of a desired grain uniformity and densification of the absorber layer, it does not anticipate the
  • CIGS tetragonal chalcopyrite
  • the silicon sintering process disclosed therein provides gas phase dopants that are not compatible with CIGS formation processes employing colloidal nanoparticle dispersions of metal alloys having differing melting points and differing diffusion and reaction kinetics.
  • the design of the sintering regimen requires a focused engineering approach to the photonic energy regime, in order to achieve the uniform and complete formation of the tetragonal chalcopyrite, without any unreacted domains, and without damaging the structure of the absorber layer.
  • U.S. Patent Application 20070218657 discloses an approach to silicon semiconductor formation using colloidal nanoparticle technology, wherein irradiation by a laser is employed to fuse the nanoparticles.
  • the laser functions to provide a grain-size gradient through the cross section of the crystalline semiconductor layer.
  • the laser irradiation involved in that process also results in the evaporation of certain volatile species.
  • This disclosure provides a means for enabling a positive effect on the functional properties of semi-conducting crystalline structures that are provided by optimization of nano- ink compositions.
  • the use of nano-inks composed of colloidal nano-crystals prepared using the methodology described herein results in a highly conductive and functionally optimized solar absorber medium.
  • These colloidal nanocrystals are designed such that when combined by solvent release and heat treatment and/or sintering, a larger assembled matrices, whose properties are positively affected by their electron pathway interactions, result.
  • Organic ligands that are routinely used in synthesis of nanoparticles result in unacceptable inter-particle coupling, and/or a residue of electron trapping organic carbon residues and as a result the electron mobility is compromised.
  • the first is the use of a graphene matrix into which the nanoparticles are embedded in a manner that assures electron mobility
  • the second is the use of inorganic ligands of that is comprised of a molecular metal chalcogenide, which also favorably affects electron mobility.
  • these two mechanisms can optionally be used in a synergistic manner.
  • this disclosure addresses methods to positively affect the quantum efficiency of solar cells by optimizing the incident photon-to-electron conversion efficiency. Furthermore, the solar energy conversion efficiency of solar absorbing films is positively addressed by means of an engineered spray pyrolysis process for the application of the nano-ink.
  • This disclosure also sets forth a mechanism to improve the solar energy conversion efficiency and the economics of solar absorber production by means of an engineered photonic flash sintering process.
  • the engineered spray pyrolysis and engineered photonic flash sintering processes can be optionally used together or separately to improve the energy conversion efficiency of thin film solar absorber systems. Production processes that optionally incorporate any or all of these disclosed processes are disclosed. The process steps are arranged in sequences that generally are applicable to the production process. The use of each of these methods as a step in the production of solar absorbers is optional. Any one, or any multiple (including the use of all) of the techniques described herein may be employed for the enhancement of solar film production.
  • Figure 1 is Figure 1 from U.S. Patent No. 7,718,707 B2, which "shows a simplified diagram comparing surface area/volume to diameter for a set of Si nanoparticles.”
  • Light or Solar Absorber Layer All solar cells require a light absorbing material contained within the cell structure to absorb photons and generate electrons via the photovoltaic effect. Light penetrates such a composite material reaching the lower charged layer. There the energy causes atoms to release electrons, which drift to the upper layer, giving this region a net negative charge and the lower layer a net positive charge.
  • Nano-crystalline solar cells as used herein refer to thin-film light absorbing materials that are laid onto a supporting matrix of metal foil, polymer, ceramic or glass. These materials have a high surface area which can serve to increase internal reflections (thereby increasing the probability of light absorption).
  • the use of nanocrystals in such applications permits the design of architectures on the nanometer length scale, which is also the typical excitation diffusion length.
  • single-nanocrystal ('channel') devices can provide an array of single p-n junctions between the electrodes which are separated by a period of about a diffusion length. This enables solar cells having potentially high efficiency.
  • Chalcopyrite is a copper iron sulfide mineral that crystallizes in the tetragonal crystal structure.
  • the associated solar composition provides P- type absorber.
  • Rapid Thermal Annealing As used herein, refers to the heating of a semiconductor wafer over a relatively abbreviated time interval, in order to affect the electrical properties. This heating can activate, move or drive dopants from one region to another. It can change the film-to-film or film-to-surface interface. It can be used to densify a deposited film. In achieving these relatively abbreviated short term annealing objectives, a tradeoff is made in process uniformity, temperature distribution management and stress in the wafer. Unlike furnace annealing, RTP normally lasts only minutes in duration (e.g., several minutes, or about 2-4, 3-7, 4-10, 10-20, or 15-30 minutes). When compared to the engineered photonic energy pulse process, the RTP is many orders of magnitude longer in its exposure duration.
  • Photonic Energy the quantum of electromagnetic interaction that is contained in a photon, which is the basic unit of light. The energy and momentum of a photon depend only on its frequency (wavelength). The photon has no rest mass which allows for interaction at long distances.
  • CIGS Copper- indium-gallium-diselenide is a I-III-VI 2 compound tetrahedrally bonded semiconductor material. It is a solid solution of copper indium selenide (CIS) and gallium selenide.
  • CZTS - Copper-zinc-tin-selenide is a I-II-VI quaternary compound whose
  • thermodynamic stability region is small. It has an intrinsic p-type conductivity that can be attributed to the presence of CuZn antisites.
  • grain boundaries represent the region or juncture between adjacent grains.
  • the presence of foreign residues in grain boundary regions of the semiconducting media used in solar absorber applications are of importance relative to device performance.
  • grain size refers to the average grain size. In the context of chalcopyrite thin films, grain size is one of the parameters that will directly influence the solar cell efficiency. The traditional convention has maintained that a large average grain size provides higher overall solar efficiency. This is explained by the optimization of solar absorption combined with minimization of the grain boundaries. Grain size in CIGS absorbers generally has ranged from 0.2 microns to 1.7 microns (with the median grain size being in the range of 0.8 microns.
  • Pre-Sintering - As used herein, connotes a partial sintering phenomenon wherein the thermal regime that is imposed is not sufficient to grow the NP into the grain size that is ultimately desired, but where the growth of the nanoparticle is adequate to move the particle out of the catastrophic contamination domain.
  • catastrophic contamination domain refers to a core-shell nanoparticle architecture. It characterizes the region where the nanoparticle is a size range where the volume amount of organic capping ligand(s) (i.e., the shell) needed to provide the requisite colloidal stability becomes excessive in comparison to the volume of the nanoparticle core. The phenomenon is further explained in U.S. Patent No. 7,718,707 issued to Kellman, for example in Figure 1.
  • Pyrolysis - As used herein, is the decomposition or transformation of a compound that is brought about by the action of heat.
  • Ultrasonic Spray Pyrolysis - indicates a pyrolysis process wherein the use of a spray type deposition of a compound is involved.
  • Ultrasonic Spray Pyrolysis - indicates a spray pyrolysis process wherein ultrasonic energy enhances the atomization of the sprayed medium in a pyrolysis process.
  • Pre-Sintering - As used herein, connotes a partial sintering phenomenon wherein the thermal regime imposed is not sufficient to grow the NP into the grain size that is ultimately desired, but where the growth of the nanoparticle is adequate to move the particle out of the catastrophic contamination domain.
  • Microwave Enhanced Ligand Exchange is a nanoparticle ligand exchange that utilizes microwave energy to drive an exchange of a ligand associated with a nanoparticle with another ligand that the nanoparticle is in contact with.
  • Metallic Chalcopyrite Molecule - As used herein is the electron transmitting linking mechanism used to connect the semiconducting nanoparticles into a functional matrix.
  • chalcogenide complexes permit the preparation of conductive and even highly conductive arrays of nanocrystals as well as the mechanism to couple a functional nanoparticle array onto a graphene platelet.
  • MCCL Metallic Chalcogenide Capping Ligand
  • the metallic chalcogenide capping ligand can be affixed to the nanoparticles as they are in the nano-ink form.
  • Such a composition can be subsequently deposited onto a suitable substrate and exposed to subsequent thermal treatment, wherein the nanoparticles become bound into an adherent and robust functional film.
  • the pendant MCCL provides the attachment or linking mechanism.
  • NPs Inorganic Colloidal Nanocrystalline Nanoparticles
  • the core is the metallic alloy (e.g., CIS, CIGS, CZTS, and CdTe) that provides a semiconductor functionality, which in this case is the conversion of photons from the solar spectrum into electrons.
  • the shell is the medium which imparts the solubility or stability of dispersions in a liquid medium, and can be comprised of an organic ligand associated with the core, or a molecular metal chalcogenide complex associated with or liganded to the core (also denoted as a MCCL).
  • phase transfer solvents act as a chelate ligand of NPs and stabilize colloidal dispersions of the particles, the phase transfer solvent can also function as part of the shell.
  • Nanocrystal(s) (NC or NCs in the plural) - As used herein, refers to a crystalline particle with at least one dimension measuring less than 1000 nanometers. This can be suspended in an aqueous or organic solvent to yield an ink solution that can be applied to a surface, making it possible to literally paint a solar panel onto a substrate.
  • Graphene Nanoplatelets includes single atom quasi-planar graphene sheets with sp2 bound carbon atoms arranged in a lattice array.
  • the nano-platelet graphene is of nano dimension.
  • the graphene nano-platelet can consist of stacks of these single atom sheets. Functionality can be provided into these platelets by the introduction of oxygen into a fraction of the sp2 grapheme carbons to produce oxygenated carbons having an sp3 configuration and can be employed as covalent bonding site.
  • the sp2 grapheme carbons can be employed as covalent bonding site.
  • Graphene platelets are highly conductive and highly transparent to the solar light spectrum.
  • Nanoplatelets comprised of single layer carbon lattices that are non-conductive, or have significantly reduced conductivity relative to graphene nanoplatelets. These platelets are often the precursor to a graphene.
  • Graphene Host Conductor Medium indicates a graphene- filled or graphene-containing absorber layer as it is deposited and thermally processed such that its electron transport capability is enabled.
  • This medium which is often referred to as a "matrix” or “continuum,” provides a mechanism for optimizing the minority carrier diffusion length of a solar absorbing film.
  • IPCE Incident Photon-To-Electron Conversion Efficiency
  • nano-inks composed of colloidal nano- particle compositions results in a highly conductive and functionally enhanced/optimized solar absorber medium that when combined by solvent release, heat treatment, and/or sintering, results in larger assembled matrices whose properties are positively affected by their electron pathway interactions.
  • Two mechanisms are disclosed herein for improving the production and properties of thin film solar cells for electrical energy production, the use of a graphene matrix into which the nanoparticles are embedded and the use of nanoparticles with a molecular metal chalcogenide ligand, both of which favorably affect electron mobility and can be used separately or together in a synergistic manner.
  • a spray pyrolysis process for the effective application of nano-inks to form solar absorbers, including the nano-inks comprised of colloidal suspension of nanocrystals, is described herein. Sintering of the applied solar absorber to improve the solar energy conversion efficiency and the economics of solar absorber production by means of an engineered photonic flash process is also described.
  • the engineered spray pyrolysis and engineered photonic flash sintering processes can be used optionally together or separately, to improve the energy conversion efficiency of a thin film solar absorber system.
  • a step-by-step production process that optionally incorporates those processes is disclosed. These steps are arranged in the sequence that is generally applicable to the production process. The use of each of these methods as a step in the production of solar absorbers is optional. Any one, or any multiple (including the use of all) of the techniques described herein may be employed for the enhancement of solar film production.
  • the following describes two alternative means for achieving the electron mobility enhancement of a semiconducting medium for such applications as a solar cell.
  • One addresses the chalcopyrite nanoparticle array which is applied as a precursor medium and subsequently thermally processed to deliver the optimized medium.
  • the second incorporates graphene nano-platelets and the chalcopyrite nanoparticles.
  • h Plank's constant
  • m+ is the carrier effective mass
  • ft Plank's constant over 2 ⁇ (2 Pi)
  • ⁇ and ⁇ are the height of the tunneling barrier and the shortest edge-to-edge distance between the NCs, respectively.
  • formation conductive matrices with embedded nanoparticles/nanocrystals can facilitate the formation of solar absorbers/solar panels by permitting electrical communication between the nanoparticles/ nanocrystals in the matrix.
  • Such conductive matrices also facilitate the collection and delivery of electrical current generated by this solar absorber in a manner that improves the efficiency and capacity of such a device for converting light into electrical energy.
  • Inorganic colloidal preparations of nanocrystalline particles with precisely controlled compositions and morphologies may be employed for the preparation of thin film solar cells.
  • Such NPs provide useful physical and chemical properties and have found applications not only in solar cells but also in other devices.
  • An advanced NP synthesis that draws upon solution-processed colloidal building blocks is disclosed that addresses the challenge of devising a fabrication technique which results in a viable solid state solar absorber medium. This synthesis process produces a core/shell NP particle, where each individual NP has size- dependent properties of the respective absorber metal alloy or semiconductor entity.
  • a processing mechanism comprising the deposition of colloidal nano-inks that comprise nanoparticles associated with an organic capping ligand followed by solvent evolution, and thermal processing.
  • the objective of that process being the preparation of a solar absorber having a solar energy conversion efficiency within the absorber media's internal quantum efficiency limitation.
  • a second process similar to the first, with the exception that the capping ligand is a metal chalcogenide complex that serves to bond an array of nanoparticles into close proximity and permits functional interaction.
  • a third mechanism is provided by processes that incorporates NPs that have been associated with a graphene nanoplatelet, where the graphene nanoplatelet serves to contribute to an optimal transport pathway for the photon generated electrons
  • Colloidal nanomaterials that are applicable to the first above-mentioned application may be prepared by syntheses that involve the use surface ligands to stabilize the particles.
  • NPs having ligands with long (e.g., about C8 to about C18) hydrocarbon or fluorinated hydrocarbon chains or bulky organometallic molecules may be employed. These large molecules create highly insulating barriers around each NC core. After the deposition of such colloidal nanomaterials, the complete removal of undesirable stabilizing surface ligands (e.g., from the shell) is necessary, and this has proven to be problematic and expensive.
  • Those alternatives provide a colloidal nanostructure by the use of surface ligands that (i) adhere, covalently or non- covalently on the NP surface forming a shell and consequently provide colloidal stabilization, (ii) permit a functional electron communication matrix within a NP network, and (iii) supplement the functional performance of individual NPs either by permitting and/or promoting inter-particle behavior, or by their displacement, permitting NP interaction with a conductive host matrix such as graphene.
  • NP shell components e.g., hydrocarbon or organometallic ligands
  • ligand sites on a nanoplatelet e.g., a graphene nanoplatelet
  • NPs including those with a CIGS, CIS, CdTe, or CZTS core
  • a shell comprising a molecular metal chalcogenide complex e.g., Sn 2 S6 4"
  • both of these processes can be coupled.
  • the resulting complexes provide the requisite ligand stabilization of various nanostructures while enabling strong electronic coupling within the NP solids or within the graphene containing medium as disclosed below.
  • graphene matrices with embedded NP and/or NCs are prepared by the exchange of the organic and/or organometallic ligands on the surface of NPs with ligand sites on a graphene nanoplatelet or a graphene oxide nanoplatelet. This can be accomplished by contacting the graphene with NPs under conditions that permit ligand sites on a graphene and/or graphene oxide matrix to displace ligands present on the surface of the NP.
  • matrices with embedded nanoparticles/nanocrystals are prepared by interacting NPs having hydrocarbon ligands with one or more molecular metal chalcogenide complexes (e.g., Sn 2 S 6 4" ) followed by subsequent processing.
  • molecular metal chalcogenide complexes e.g., Sn 2 S 6 4"
  • nanoparticles, NPs with a chalcogenide ligand shell are formed (e.g., CIGS, CIS, CZTS, particles having Sn 2 S6 4" surface ligands for a shell ).
  • the particles are subsequently applied to a surface (e.g., the surface of a graphene containing composition) for the formation of a matrix.
  • a molecular metal chalcogenide complex is present, subsequent treatment, such as by mild heating, can convert the molecular metal chalcogenide complexes to semiconducting phases with embedded nanoparticles/nanocrystals.
  • the above described embodiments for preparing matrices with embedded nanoparticles employing graphenes and NPs comprising a molecular metal chalcogenide complex can be coupled, such as, for example, by separately preparing both materials and combining the products (e.g., in a nano-ink prior to its application to the surface of a nascent thin-film, or by separately delivering both materials to the surface upon which a thin-film solar absorber is being formed).
  • the graphenes comprising NPs and NPs comprising molecular metal chalcogenide complexes may be contacted simultaneously or sequentially with a surface and then processed (e.g., by heating) to obtain thin-films with embedded nanoparticles/nanocrystals as described below.
  • these matrices provide stabilization of various nanostructures while enabling strong electronic coupling within the NP/NC containing solids (e.g., the graphene nanoplatelet continuing medium).
  • a ligand (e.g., a molecular metal chalcogenide complex) exchange procedure is used to provide capped NPs (e.g., MCCL- capped NPs) that form stable colloidal suspensions or dispersion in water or other polar solvents
  • the exchange procedure involves a phase transfer of the newly formed NPs from a nonpolar (less polar) organic medium into a more polar solvent.
  • MCCL- capped NPs are formed by exchanging a molecular metal chalcogenide ligand (MCCL) for a hydrocarbon containing ligand associated with a NP (e.g. its core), phase transfer from non- polar or low-polarity solvents to a more polar solvent may occur.
  • solubilization of the NPs in a nonpolar solvent is provided by means of a nonpolar group (e.g., organic or organometallic groups) attached to the NP by a thiol ligand.
  • NPs having such non- polar groups in their shell may typically be dissolved in a nonpolar solvent at about 1 to about 20 mg/ml.
  • a molecular metal chalcogenide ligand complex which can displace such non-polar groups resulting in MCCL-capping, is facilitated by the nucleophilic nature of the NPs, the electrophilicity of the un-coordinated metal atoms at the NP surface, and the introduction of a host solvent medium that can interact with the hydrocarbon ligands.
  • the capped NPs Due to the colloid stabilizing effect of the molecular metal chalcogenide ligand (e.g., MCCL ligands), the capped NPs can be provided in water, DMSO, formamide, ethanolamine or a variety of other solvents (e.g., various glymes and other polar solvents).
  • solvents e.g., various glymes and other polar solvents
  • a property of MCCL ligands is their ability to transform into amorphous or crystalline metal chalcogenides (e.g., semiconductor or semi-conducting materials) upon mild thermal treatment.
  • amorphous or crystalline metal chalcogenides e.g., semiconductor or semi-conducting materials
  • Sn 2 S6 ⁇ neutralized by hydrazinium counter ions
  • N 2 H5 4 Sn 2 S6 ⁇ SnS 2 + 4N 2 H 4 + 2H 2 S.
  • the resulting SnS 2 phase crystallizes into pure electronic-grade semiconductor.
  • this ligand conversion process involves a total weight loss of only about 3.8%, the possibility of strain related crack formation is reduced.
  • the weight percentage of MCCLs that are needed to stabilize a colloidal NP is relatively small.
  • thermogravimetric data supports the conclusion that 2 to 10 weight % of MCCLs will stabilize colloidal NCs. Subsequent heating to 180°C, resulting in greater than 70%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5%, 99.7%, 99.8%, 99.9% or the complete disappearance of entities containing C-H, S-H, or N-H bonds.
  • NPs prepared using dodecanethiol and ODPA-HDA-TOPO octadecylphosphonic acid, hexadecylamine, and trioctylphosphine oxide
  • dodecanethiol and ODPA-HDA-TOPO octadecylphosphonic acid, hexadecylamine, and trioctylphosphine oxide
  • MCCL-capping may also impart substantial stability to NPs.
  • Thin films prepared with NPs having metal chalcogenide ligand complexes shells, have conductivities increased by several by orders of magnitude.
  • nanoparticle ink deposited films that are prepared from the alkanethiol (e.g., dodecanethiol) capped nanoparticles resulted in films that were highly insulating.
  • Conductivities ( ⁇ values) for those films are on the order of ⁇ 10 ⁇ 9 S cm -1 for 5-nm NPs (Au) and less than 10 ⁇ 12 S cm -1 for 5.5-nm NPs (CdSe).
  • Replacement of the dodecanethiol ligands with Sn 2 S6 4_ results in an increase in the conductivity by -11 orders of magnitude, approaching ⁇ values of -200 S cm -1'
  • the highest conductivity reported for Au NP solids with short-chain organic capping e.g., n-ethyl or n-butanethiol was less than 10 _1 S cm -1 .
  • the disappearance of the plasmonic absorption peak strongly supports the metallic nature of Sn 2 S 6 4 -capped Au NP solids. Reportedly there is a strong decrease in the mean interparticle distance from -1.6 nm for dodecanethiol-capped Au NCs to less than 0.5 nm for Sn 2 S 6 4 ⁇ -capped Au NCs.
  • the conductivity of Sn 2 S 6 4 ⁇ -capped Au NC solids is higher than the conductivities of conducting polymers and graphene-based composites.
  • MCCLs Metallic Chalcogenide Capping Ligands
  • NPs including NPs associated with a graphene (e.g., a graphene platelet).
  • MCCL-capped NPs can be employed to form a highly conductive solar absorbing continuum that can provide an effective (e.g., low resistance) pathway to the solar cell conductor media for electrons freed from their atoms/molecules by light interacting with the NPs. This is especially the case when the continuum is comprised of a properly deposited graphene film and metal molecular metal chalcogenide ligand capped NPs.
  • the removal of the undesirable organic ligand moieties from the NP is preferably achieved by prior reaction with a molecular metal chalcogenide ligand to form a MCCL-capped NP.
  • a molecular metal chalcogenide ligand to form a MCCL-capped NP.
  • NPs with a shell comprising organic ligands can interact effectively with graphenes, and particularly where they interact with more than one graphene (e.g., the NP becomes embedded or intercalated between layers of graphenes), it may not be necessary to displace all of the organic ligands as a sufficient path for electrons freed by light interacting with the NP to reach the conductor media of a solar cell may already be provided by the graphenes.
  • Such ligand-capped NP containing materials can be deposited as is (e.g., as part of a nano-ink) to yield a viable solar absorber layer that provides enhanced electron mobility between the solar absorber nanoparticles and the conductor media of the solar cell.
  • the graphene containing materials may be subject to thermal decomposition of the molecular metal chalcogenides complexes to further enhance the electrical conductive properties of the solar absorber layers and provide a further enhancement of the electron mobility between the solar absorber nanoparticles and the conductor media of the solar cell.
  • the electron-conducting graphene (CHRM) disclosed herein can be configured with a hole-conducting host (e.g., CuIni_ ⁇ Ga ⁇ Se 2 ) in order to form materials with distributed networks of p-n junctions. This provides a mechanism for solution based production process that makes this approach appealing for large-area, roll-to-roll production of thin-film solar cells.
  • CHRM electron-conducting graphene
  • nano-inks that can be formulated as nano-inks and subsequently processed into solar absorber thin films.
  • the nano-inks are also compatible with a variety of application processes.
  • nano-ink is deposited by spraying. This spraying can utilize an ultrasonic atomization process, which enhances the application process by providing improved atomization and aerosolization of the nano-ink droplets.
  • the solvents of the nano-inks described herein may also be compatible with a variety of subsequent processing steps. Depending on the solvent chosen, processing at different temperatures and under a variety of different conditions are possible.
  • Nano-ink containing compositions incorporating NPs having a core shell structure where the core is the chalcopyrite metal alloy and the shell is an organic capping ligand (sometimes denoted as an "OCL”) can be subject to a pyrolysis regime to eliminate substantially all of the organic shell.
  • the volume of this organic shell is estimated at 50% of that of the "as-applied" film (assuming that this film is aerosol sprayed onto a room temperature surface). In such case, this large percentage of organic shell material must egress from the absorber layer during the elevated temperature regime of the sintering process. The result is a substantial shrinkage, which can result in cracking of the absorber layer.
  • ultrasonic spray pyrolysis application of the compositions on to substrates at an elevated temperature advantageously avoids cracking of the solar absorber layer due to subsequent shrinkage.
  • ultrasonic spray pyrolysis application of the nano-ink containing compositions on to substrates at elevated temperatures contributes to the egress of the organic shell component and solvent during the application of the nano-ink containing composition. Because at lest a portion of the organic shell component and exits the absorber layer prior to the layer becoming set, the shrinkage it experiences is less than the same composition applied as an aerosol using compressed gas on a room temperature surface.
  • the absorber layer is pseudo- plastic at the time, at least a portion of the shell is volatilized, the absorber layer is more shrinkage tolerant, and again, less subject to cracking.
  • a pre-sintering regimen that mitigates the deleterious film quality consequences that are subsequently imparted upon the solar absorber film by the use of NP with an organic capping ligand (OCL ) shell.
  • OCL organic capping ligand
  • the OCL is an entity that is essential to the synthesis of the nano-ink colloid, and contributes to the formation of a stable dispersion of the NPs in a solvent
  • the OCL represents a significant volume of the nanoparticle (in the range of ten percent to 60 percent and higher). The demand for such a substantial volume of OCL is dictated by the complex stability constraints of nano-ink.
  • nano-ink deposition process results in growth of the nanoparticle to a size of 10 nanometers or more which is outside this catastrophic
  • Pre-sintering regimens are not intended to fully eliminate the entirety of the organic carbon trapping entities that arise from the OCL, but rather to reduce them to a manageable level. When OCL is reduced to a manageable level, subsequent thermal processing can achieve a viable solar absorber film.
  • the OCL is reduced by as much as an order of magnitude, and the nanoparticle is capped with an organic capping ligand that is in the range of a few nanometers thick. Removal of this remaining organic capping ligand (which still forms a nanoscale junction, or nanogap between the metal alloy nanoparticles) must be subsequently provided by the subsequent processing. This is done by a second phase of ablative sintering, which addresses this nanogap junction medium by illuminating the composite (precursor film that has been subject to pre- sintering) with one or more intense optical pulses that results in the destructive collapse of the nanogap.
  • This collapse is irreversible, occurring with the simultaneous ablation of the dielectric from the metal alloy nanoparticle junctions and ultimately the sintering of the metal alloy.
  • OCL Organic Capping Ligand
  • the nano-ink is deposited by spraying.
  • This spraying can utilize an ultrasonic atomization process, which enhances the application process by providing improved atomization and aerosolization of the nano-ink droplets.
  • the pre-sintering deposition process can be repetitious such that a multitude of thin layers (e.g., 3, 5, 7, 10, 25, 20 etc.) is deposited, with each layer being subjected to the pre-sintering phenomenon.
  • a multitude of thin layers e.g., 3, 5, 7, 10, 25, 20 etc.
  • This will yield a series of thin film layers, which in some embodiments will comprise a series of layers, each about 100-300nm thick or 100- 200nm thick.
  • the nanoparticles used herein which are of the form I-III -VI2 as well as the I- II-IV- VI are produced by the decomposition of single-source-precursors ("SSPs").
  • SSPs single-source-precursors
  • a preferred method for converting these SSP's into the nanoparticles used herein is microwave irradiation.
  • the nanocrystalline ink composition is prepared using the single molecular source precursor (SSP) rather than multiple compounds to contribute elements of the product chalcopyrite.
  • SSP single molecular source precursor
  • a single source precursor is used in the preferred methods to obtain all of the elements in the resulting chalcopyrite nanoparticle.
  • a single-source precursor is used to obtain all the elements (Cu, In, and Se) in the CuInSe 2 .
  • Such a nanoparticle production protocol makes possible the access to control of the nanoparticle sizes that are appropriate for the intended service that is described herein.
  • the diameters of the nanoparticles that are appropriate for use in the embodiments described herein can be controlled. This control is by the concentration of the SSPs. Increasing the concentration increases the diameter of the resulting nanoparticle. The diameters decrease with increasing concentration of thiol. Increasing the reaction temperature results in increased nanoparticle diameter.
  • Production of the nanoparticles used herein involves control of the growth to a range that is optimum for this application.
  • the range is betweenl-5 nm.
  • the SSP's used herein can optionally incorporate selenium and sulfur as an integral part of the molecular structure, with the result that a quantity of sulfur or selenium will be incorporated in the structure of the nanoparticle.
  • a Se nanoparticle is commercially available, for example from QuantumSphere, Inc. (Santa Anna, CA, USA).
  • the nano-inks described above can be applied to a substrate that is appropriate for thin- film production.
  • the preparation of suitable substrates is well known to persons who are knowledgeable in this field.
  • the nano-inks can be applied by spraying, printing, spin casting, roll-to-roll printing, flexo-printing, gravure printing, etc.
  • Graphene Oxide platelets have a wide range of chemical groups on their surface and edges, including reactive oxygen species such as hydroxyl and epoxy groups on their basal planes. Since there is such a wide range of chemical compositions present on the GO, there are many prospects for functionalizing, which can involve the formation of covalent bonds, ionic- pi-pi bonds, van der Waals interactions, etc.
  • One approach to the functionalization (in this case, nanoparticle modification) of these Graphene Oxide platelets utilizes orthogonal reaction of differing groups on the graphene oxide, whereby it is even possible to associate more than one type of NP with platelets through reactions selective for one or more different groups on the graphene oxide.
  • the synthesis process disclosed herein is directed toward a very fast and reliable synthesis protocol for achieving the NP/NC functionalization of a graphene platelet.
  • the metal nanoparticle is provided in the form of a core-shell (alkanethiol colloid stabilized) solution.
  • the solvent that is used in the process is typically a high boiling point member of the "glyme" family.
  • the solvent is a glyme. In additional embodiments the solvent is a monoglyme, diglyme, butyl-diglyme, tetraglyme. In another embodiment the solvent is selected from polyethylene glycol di-butyl ether, monoethylene glycol dimethyl ether, or diethylene glycol dibutyl ether. In another embodiment the solvent is a polyethylene glycol dibutyl ether (PEGDBE), whose boiling point is above 320 degrees C. The use of high boiling point solvents enables the ligand exchange onto the graphene.
  • PEGDBE polyethylene glycol dibutyl ether
  • PEGDBE and similar glymes
  • this solvent possesses a three coordinate complexant feature, which provides a means for translating the functional core to the residence site on the surface of the graphene.
  • Prior experience with an ultrafast microwave assisted deposition of naked nanoparticles on Graphene has been reported in the literature.
  • One such naked nanoparticle synthesis regime has been disclosed by Jasuja et al of the Kansas State University wherein the "naked" sites on a graphene oxide nanoplatelet provides a high density of bondable oxy-functional groups.
  • the microwave enable deposition of NPs (e.g., NPs with a CIGS core) onto graphene platlets comprises:
  • Oxidation of Graphite - Graphite is oxidized using the Hummers Method (i.e. a
  • Graphite Oxide The chemical reduction of the GO is the most common method but it is not the only method for the preparation graphenes. It is possible to produce thermodynamically stable carbon oxide by directly heating the GO in a furnace. Reduction can also be achieved using either of the following protocols: the Graphite Oxide (GO) can be subjected to a microwave environment (no electrolyte) wherein the thermal regime will reduce the GO (this requires an inert gas atmosphere); or, alternatively, the NP implantation regimen can be extended such that there is a subsequent reduction of the GO bearing NPs.
  • a microwave environment no electrolyte
  • the NP implantation regimen can be extended such that there is a subsequent reduction of the GO bearing NPs.
  • NPs e.g., CIGS NPs
  • CIGS NPs CIGS NPs
  • the transparent synthesis vessel allows the inert gas to sweep through the stirred reaction vessel, this vessel can be placed inside the microwave cavity and a fiber optic temperature probe is installed, at which point an argon purge is initiated and controlled from outside the oven.
  • the purge duration should be on order of 10 minutes.
  • the digestion vessel can be purged for approximately the same time prior to the seal of the cap. In this case it is desirable to provide a secondary barrier to protect the vessel contents from any subsequent contact with atmosphere during the remaining setup.
  • the possibility of maintaining an inert gas purge within the microwave cavity during the synthesis process should be considered.
  • Microwave Regimen A the time temperature and pressure regime for a typical
  • microwave synthesis is presented in Figure 1 of U.S. Patent No. 7,718,707. Note the rate of rise for the full power on regime, and the corresponding rate of temperature decay. As illustrated in that figure, the desired time interval at peak temperature can be achieved by providing a series of run cycles. The MW enabled "naked ion
  • the microwave protocol does not have to be continuous and microwave energy can be applied as appropriate for the process constraints including temperature and pressure.
  • the time needed for the association of the NPs with the graphene is about one to about ten minutes of total microwave exposure using SST-C as a solvent.
  • Solvent System-A is the a 1:1 blend of toluene and THF is employed. This is a relatively volatile solvent mixture.
  • THF a relatively volatile solvent mixture.
  • Solvent System-B has a considerably higher boiling point solvent, terpineol, with a boiling point of 220 degrees C.
  • the SST-C is a blend of a non-VOC solvent (tertbutylacetate) ,whose boiling point is the same as toluene, and PEGDBE, which acts as a "tail solvent” that has a higher boiling point (about 300 degrees C) and permits reactions at a higher temperature after the tertbutylacetate has been removed.
  • a non-VOC solvent tertbutylacetate
  • PEGDBE acts as a "tail solvent” that has a higher boiling point (about 300 degrees C) and permits reactions at a higher temperature after the tertbutylacetate has been removed.
  • Nanoparticles are typically suspended in a ratio of 95 parts (by weight) of solvent to 5 parts (by weight) of dry nano-ink powder (e.g., NPs). The mixture is sonicated for 5 minutes and the resulting nano-ink preparation is filtered.
  • dry nano-ink powder e.g., NPs
  • the thickness of an inorganic solar absorber layer plays a role in the optimization of the internal quantum efficiency of the resulting devices.
  • devices with thicker nanocrystal layers are reported to have lower power conversion efficiency, despite the increased photon absorption.
  • the internal quantum efficiency of CIS based devices having a size ranging from 150 nm to 540 nm decreased significantly as the film thickness increased.
  • the thin, most efficient devices exhibited internal quantum efficiencies as high as 40 percent, and this is observed with films that were deposited on the low end of the thickness range - namely 150 nm.
  • nanoplatelets provide a scaffold onto which thin NPs absorbers (e.g., a CIS type of absorber) can be applied, they provide a mechanism to utilize the observed correlation between layer thickness and efficiency. Because graphene platelets are essentially transparent, a multitude of layers of thin nanoparticle coated graphene platelets can be prepared as a single absorber film that can provide the desired level of solar absorbance without compromising the quantum efficiency of the absorber film.
  • thin NPs absorbers e.g., a CIS type of absorber
  • an engineered photonic flash for sintering is an optional embodiment that relates to thermal treatment to achieve ablation of organic carbon residues and sintering of the deposited nanoparticles by means of an engineered photonic pulse regimen.
  • Engineered photonic pulse regimens provide a means to manage the diffusion, reaction, nucleation, passivation and grain growth phenomena during the transition of the precursor to a solar absorber layer having a high optoelectronic quality.
  • the use of an engineered photonic flash regimen contributes to the cost effective production process for such thin film poly crystalline photovoltaic absorber layers.
  • Polycrystalline thin film absorber layers can consist of chalcopyrites, sphalerites, kesterites. This disclosure provides an approach to the production of these absorbers utilizing nanocrystalline precursor dispersions that are deposited onto a suitably prepared substrate.
  • the nano-crystal dispersion provides a uniform composition of metal precursors, which upon exposure to a precisely engineered thermal processing provided by an engineered photonic flash regimen delivers an optoelectronic layer.
  • the design of the photonic flash post-thermal processing regimen is comprised of two high energy and extremely rapid thermal pulse regimens.
  • the first is designed to provide the pyrolysis or thermal ablation of the organic products which must necessarily be removed from the nano-crystalline composite precursor media.
  • the second regimen is designed to provide the sintering of the nanoparticles into grains and permit the growth of same in such a manner to advance the formation of the solar absorber functionality objectives, which are provided by the simultaneous sintering, passivation, nucleation and grain growth.
  • the nano-crystalline film composite can incorporate additives such as dopants and passivation media, optionally in nano form that provide the means to enhance the opto-electronic performance.
  • the film can be converted to an absorber layer using an engineered photonic flash regimen.
  • the nanoparticle film layer is exposed to a properly engineered photonic pulse, which is of an intensity and duration sufficient to bring about the chemical reactions that are appropriate to achieve the objectives of its application.
  • a properly engineered photonic pulse which is of an intensity and duration sufficient to bring about the chemical reactions that are appropriate to achieve the objectives of its application.
  • said photonic pulse is delivered within a very short duration at a suitable intensity, and when such engineered photonic pulse consists of appropriate frequency and repeats, it can cause the nanoparticulate matrix to undergo the desired (and optimized) chemical reaction with suitable kinetics.
  • Such regimes simultaneously introduce conditions which are contra posed to those that would contribute to undesirable results in the resulting absorber layer (e.g., undesirable effects that would otherwise be brought about as a result of the variations in melting point or boiling point of the chemical reaction participants).
  • One embodiment described herein provides for fabrication of a thin film ternary chalcopyrite absorber layers comprised of copper indium gallium di-selenide, more often referred to as CIGS.
  • the chalcopyrite devices described herein are also referred to as I-III-VI2 devices, according to their constituent elemental groups.
  • Other embodiments provide for fabrication of thin film kesterite absorber layers comprised of copper, zinc, tin selenide more often referred to as CZTS.
  • the kesterite devices described herein are also referred to as I- II- IV- VI devices, according to their constituent elemental groups.
  • nanoparticles that are classified as chalcopyrites or kesterites may be produced using a composition of single molecular source ("single source precursors" (SSP's). These are combined into a suitable solution phase whereupon they are subsequently subjected to a specific thermolysis regime.
  • SSP's single molecular source precursors
  • One illustrative example of the approach to such processing involves the use of alkylthiol ligands in solvent, along with the appropriate SSP components, chalcopyrite nanoparticles that exhibit properties expected for chalcopyrite nanoparticles that are appropriate for use in this application.
  • organics that are associated with capping agents e.g., shell of an NP
  • surfactant materials that are used to produce and stabilize the nano-inks are removed by pyrolysis.
  • the pyrolysis of organics is achieved by the introduction of an energy pulse that delivers sufficient energy to a chalcopyrite containing precursor film to effect the removal of the organics from the composite, but at the same time the resulting thermal regime is not high enough to volatilize an unacceptably high amount of the chalcopyrite species.
  • the melting points of chalcopyrites (about 1,000° C) differ significantly from the other components, where Sulfur melts at about 115° C, Selenium at about 221° C, and Tellurium at about 450° C.
  • the removal of the organics must occur under suitably engineered and controlled conditions. It is possible to achieve those objectives by the appropriate selection of the capping agents associated with the nanoparticles, appropriate selection of surfactant materials in the nano-inks, and by the use of intense photonic light pulses which are of properly engineered duration and which are provided at an energy density in the range of 1-50 joules per cm ⁇ 2 .
  • the control of the volatilization of the selenium is necessary in order to mitigate undesired diffusion and the corollary volatilization of the Se that is provided in the nano- particulate medium of the nano-ink.
  • This regime provides the mechanism of CIGS formation and crystallization as the result of an intense photonic pulse from a xenon source.
  • the mechanism of CIGS formation, sintering, and crystallization using photonic pulses may be explained based on the melting point of CIG (550° C) and Selenium (217° C). Under these conditions the melting of the CIG and Se nanoparticles, the nucleation, passivation and grain growth occurs in a surprisingly short reaction time interval. Those processes take place within the sintering time interval, which occurs within a timeframe that is engineered, such that the desired absorber layer structure is achieved. In one embodiment this engineered time frame is within the range of microseconds to a few milliseconds, and accordingly this reaction interval is sufficiently short that precludes the oxidation of the constituent elements that are provided by the nanoparticulate precursor.
  • selenization is conducted without changing the microstructure of the materials as selenium diffuses into the CIG lattice.
  • substantially all of the selenium e.g., nano-selenium or nano particle selenium
  • the photonic pulse can form a CIGS film from a CIG precursor and Se nanoparticles in a very short time interval.
  • CGIS film formation can occur with pulses of as short as microsecond in duration.
  • Elemental analysis of the absorber layer that results from short pulse photonic sintering processes indicates that the Ga, Cu, In, Ca, and Se concentrations remain virtually constant through the entire cross-section of the absorber film. That confirms the short pulse photonic processes eliminate any grading of gallium to indium ratio in the CIGS structure, with the result that the film is a homogeneous, single phase material.
  • CIGS films are prepared using 20 Joules per cm 2 light intensity for a duration of 2 milliseconds. That brief and intense process results in a diffusion of selenium to form the tetragonal chalcopyrite structure within a uniform and relatively dense absorber layer, while at the same time altering the surface morphology to provide compact grains in the range of 0.3 to 1.0 micron, or 0.1 to 1 micron.
  • equipment produced by Novacentrix Corporation or Xenon Corporation which is capable of delivering the engineered photonic energy pulses at the energy level, pulse duration, wavelength, and pulse frequency is employed in the methods and processes described herein.
  • a method for the preparation of a nano-ink comprising: contacting a nanoparticle (NP) bearing hydrocarbon ligands with a phase transfer solvent and subject the resulting mixture to heat sufficient to sinter the NPs and displace at least some of the hydrocarbon ligands, wherein the NPs are increased in size by sintering.
  • NP nanoparticle
  • metallic chalcopyrite comprises copper indium selenide (CIS), Copper- indium-gallium-diselenide (CIGS) or copper-zinc-tin-selenide (CZTS).
  • NPs that have been increased in size by sintering are made to contact a graphene or graphene oxide platelet (GO-platelet) and subjected to heat sufficient for the nanoparticle to associate with surface of the graphene or GO-platelet or intercalate into said graphene or GO-platelet.
  • GO-platelet graphene oxide platelet
  • phase transfer solvent acts as a chelate ligand of said NP as a result of its coordination sites and its molecular
  • the solvent is a glyme (monoglyme), diglyme, butyl-diglyme, tetraglyme, polyethylene glycol di-butyl ether, monoethylene glycol dimethyl ether, or diethylene glycol dibutyl ether.
  • a nano-ink comprising one or more MCCL-capped NPs, where said NP have an average size in a range selected from about: 3nm-200nm, 3nm - lOOnm, 3nm-50 nm, 3nm-25 nm, 3nm-10 nm, 5nm-200nm, 5nm -lOOnm, 5nm-50 nm, 5nm-25 nm, 5nm-10 nm, lOnm- 200nm, lOnm-lOOnm, 10nm-50nm, 10nm-25nm, 20nm-200nm, 20nm -lOOnm, 20nm- 50nm, 15nm-25 nm, and 15nm-20nm.
  • a nano-ink comprising one or more MCCL-capped NPs.
  • a nano-ink comprising one or more NPs associated with or intercalated into a graphene.
  • the metallic chalcopyrite comprises copper- indium-gallium-diselenide (CIGS) and/or copper -zinc-tin-selenide (CZTS).
  • CGS copper- indium-gallium-diselenide
  • CZTS copper -zinc-tin-selenide
  • nano-inks of any of embodiments 21-26 further comprising a solvent.
  • the solvent comprises on or more of: a solvent that is more polar than hexane or toluene, an ether, a glyme (monoglyme), diglyme, butyl- diglyme, tetraglyme, or a solvent that has a boiling point greater than about 200, 225, 250, 275, or 300 centigrade.
  • a method for producing a thin-film solar absorber wherein the electron mobility is enhanced and electron traps are minimized comprising: a) applying a nano-ink according to any of embodiments 20-28 to a thin-film substrate to form a nano-ink film; and
  • nano-ink comprises an organic solvent
  • step of applying said nano-ink to said substrate further comprises removing the bulk of said organic solvent to form a dried nondensified precursor film that is to be subject to said pre-sintering pyrolysis.
  • nano-ink further comprises an additional amount of a metallic chalcogenide capping ligand.
  • any of embodiments 34-36 wherein said method further comprises thermal processing of the precursor film that: releases at least a portion of residual organic species that will compromise the energy conversion efficiency of the solar absorber formed from said nano-ink, and/or sinters the dried nondensified precursor film subject to said pre- sintering pyrolysis to achieve a final desired average grain size.
  • said method further comprises subjecting the precursor film to an engineered photonic pulse regimen that: releases at least a portion of residual organic species that will compromise the energy conversion efficiency of the solar absorber formed from said nano-ink, and/or sinters the dried
  • nondensified precursor film subject to said pre-sintering pyrolysis to achieve a final desired average grain size.
  • the thin-film solar absorber is comprised of monodisperse, passivated, non-aggregated semiconductor nanocrystals.
  • the nanoparticles present in said nano- ink comprise selenium and/or sulfur.
  • nanoparticles present in said nano- ink comprise sodium.
  • the duration of photonic energy pulse is between 100 picoseconds and 50 milliseconds.
  • the engineered photonic pulse regimen comprises one or more pulses, two or more pulses, three or more pulses, five or more pulses, ten or more pulses, or a series of pulses, and the number of pulses ranges from 1 to 100.
  • a method for sintering chalcopyrite nano-particles to produce a densified chalcogenide solar absorber layer having high optoelectronic quality comprising:
  • a precursor colloidal nanoparticle dispersion to a substrate, wherein the said precursor colloidal dispersion includes a plurality of chalcopyrite nanoparticles that are combined with an organic solvent;
  • the method of embodiment 60 where the engineering of the photonic energy regime delivers a photonic energy input that lasts for a sufficiently short time interval to preclude substantial diffusion of components in the chalcopyrite, while delivering an energy density sufficient to provide the required CIGS sintering and annealing.
  • the nano-ink comprises particles of a CZTS (a kesterite)or particles comprising CdTe.
  • a method of producing a solar absorber film wherein an exposure of a substrate to an engineered photonic regime removes substantially all organic contaminants prior to the deposition of the colloidal nano-particle precursor dispersion, wherein said contaminants have the ability to adversely affect the efficiency of the solar absorber film if not removed.
  • nanoplatelet species is a graphene platelet or a graphene oxide platelet.
  • bonding mechanism of MCCL-capped NPs on a graphene platelet is a pi:pi bond or a covalent bond.
  • absorber layer on graphene platelet are stacked so as to increase the solar absorbance provide by the multiple layers.

Abstract

The present disclosure describes economical and energy efficient solar absorbing thin- film devices having enhanced electron mobility produced by processes including spray pyrolysis, pre-sintering, and/or photonic flash sintering.

Description

SOLAR ABSORBING FILMS WITH ENHANCED ELECTRON MOBILITY AND METHODS OF THEIR PREPARATION
This application claims the benefit of U.S. Provisional Application No. 61/443,641, filed February 16, 2011, and entitled "Production of Solar Absorber Using Nano-Particle Precursors and Engineered Photonic Energy," the entirety of which is herein incorporated by reference.
BACKGROUND
In the early 1980's Boeing was able to demonstrate a solar photovoltaic device having ten percent energy conversion efficiency, using a CIS type of absorber layer. The production of this absorber involved two thermal regimes. The first was a Cu rich regime where elemental fluxes of Copper and Indium were deposited onto a 350° C substrate by thermal evaporation. In the second regime, the substrate was raised to 450° C. Among other things, the Boeing study provided insight into the relatively large tolerances in the Cu to In flux ratio that is possible in preparing CIS films, as long as the overall Cu-to-In ratio is less than 1. Subsequent research developed variations of that basic scheme directed at improved efficiency.
Although a variety of approaches have been taken to the deposition of CIS absorbers, they may be divided into two categories: physical vapor deposition and reactive annealing. In physical vapor deposition processes Cu - In complexes are vapor deposited from ternary (e.g., CuInSe2) or quaternary (e.g., Culni_x Gax Se2) sources onto a heated substrate. Deposition is generally followed by reactive annealing with sulfur or selenium. By contrast, reactive annealing techniques involve the deposition Cu and In layers, typically by sputtering, at the appropriate thickness on to a substrate that is generally at room temperature (e.g., a 20-25° C substrate). Deposition is typically followed by a second step, where the multi-layer structure is annealed in the presence of selenium (i.e., reactive annealing). Energy conversion efficiency is not as high as with the physical vapor deposition processes, but the process is easier to manage.
Other approaches for the deposition of solar absorber layer constituents, including the use of electrodeposition and plasma deposition have been employed. All of these methods, however, also require some form of post deposition annealing process that has been proven to be complex, expensive, and difficult to control.
Another process for the application and processing of solar absorber layer constituents is the use of Rapid Thermal Processing (RTP). This process provides Ga at the substrate interface. Because this does not provide a Ga matrix through the absorber layer, it does not contribute to an increased band gap. Instead, RTP processes looks to the incorporation of sulfur to increase the band gap. The Ga containing interfacial layer does improve uniformity and adhesion.
Certain phenomena are observed when utilizing those methods to produce solar absorber layers. For instance, in the above described processes, Ga is distributed non- uniformly through the film thickness and migrates to the substrate interface. When the opposite interface is Ga depleted, the resulting absorber layer functions as a CIS absorber. Cu and In also tend to diffuse to the free surface at a higher concentration (because they are more reactive with the Se), and as a consequence, the CuGa Se2 concentrates at the substrate surface.
Subsequent inert atmosphere annealing (at temperatures above 550° C) is thought to be necessary to create a defect structure that can facilitate In and Ga diffusion. In addition, most, if not all, of the above mentioned fabrication methods involving the use of toxic selenium, and processing under vacuum conditions, which is undesirable, is unavoidable.
A production process that uses bulk CIGS is especially challenging since CIGS has a melting point around 1000° C. Most substrates cannot withstand such temperatures, and even glass substrates encounter serious warping problems. Furthermore, traditional production methods/alternatives involve high energy demands, require relatively long processing times, and they involve large capital outlays for production facilities.
Traditional production approaches also are burdened with the problems of processes that do not lend themselves to consistency in their manufacturing methodology, and as a result the manufactured product is prone to high rejection rates and inconsistent performance.
Efforts to address some of these problems appear in patent literature. For example, in U.S. Patent No.: 6,092, 669 the thin film precursor is provided in the form of metal layers that are subsequently exposed to reactive annealing with selenium. This process, however, has the drawbacks of high cost, required use of reactive annealing with selenium, and non-uniformity of product. U.S. Patent No. 4,581,108 discloses an electrodeposition process to deliver the necessary metal precursor layers followed by the selenization of those layers. That process, however, has been shown to yield a CuInSe2 absorber film that has poor adhesion to the back contact (ohmic) layer. U.S. Patent No.: 7,582,506 to Basel discloses a process for
electrodepositing a precursor layer series that reportedly overcomes the adhesion problems between the absorber layer and ohmic contact layer of the process described in U.S. Patent No. 4,581,108. That disclosure does not provide a solution to the toxicity of the selenium or to the high cost of the production process.
Several published U.S. Patent applications also address issues arising in the production of solar voltaic products. U.S. Patent Application No. 20090053878 discloses a method for producing a group IV semiconductor solar cell utilizing a colloidal silicon nanoparticle dispersion to lay down a silicon absorber precursor layer, and a flash lamp to sinter the precursor layer. While this process discloses the sintering of the silicon grain structure of the absorber layer, using a high intensity photonic energy source with the objective of a desired grain uniformity and densification of the absorber layer, it does not anticipate the
complications that arise in the production of polycrystalline absorber layers such as those observed in tetragonal chalcopyrite (CIGS) structure. The silicon sintering process disclosed therein provides gas phase dopants that are not compatible with CIGS formation processes employing colloidal nanoparticle dispersions of metal alloys having differing melting points and differing diffusion and reaction kinetics. In the case of the chalcopyrite sintering and annealing, the design of the sintering regimen requires a focused engineering approach to the photonic energy regime, in order to achieve the uniform and complete formation of the tetragonal chalcopyrite, without any unreacted domains, and without damaging the structure of the absorber layer.
U.S. Patent Application 20070218657 discloses an approach to silicon semiconductor formation using colloidal nanoparticle technology, wherein irradiation by a laser is employed to fuse the nanoparticles. The laser functions to provide a grain-size gradient through the cross section of the crystalline semiconductor layer. The laser irradiation involved in that process also results in the evaporation of certain volatile species. Moreover, in such processes it is not clear that there is any substantial need for wavelength matching and a continuous wave laser can be employed.
In an article in the Journal of Electronic Materials issued in 2010, researchers at the University of California disclosed a process for producing chalcopyrite type crystalline solar absorber layers using nanoparticle dispersions and then sintering them with Intense Pulsed Light. In that case, the work delivered absorber layers having poor optoelectronic properties. This result illustrates the importance of the behavior of the organic capping ligand during the formation of the chalcopyrite semiconductor continuum as well as the proper engineering design of the photonic pulse, apparently related to the presence of unreacted CIG precursor.
SUMMARY
This disclosure provides a means for enabling a positive effect on the functional properties of semi-conducting crystalline structures that are provided by optimization of nano- ink compositions. The use of nano-inks composed of colloidal nano-crystals prepared using the methodology described herein results in a highly conductive and functionally optimized solar absorber medium. These colloidal nanocrystals are designed such that when combined by solvent release and heat treatment and/or sintering, a larger assembled matrices, whose properties are positively affected by their electron pathway interactions, result. Organic ligands that are routinely used in synthesis of nanoparticles result in unacceptable inter-particle coupling, and/or a residue of electron trapping organic carbon residues and as a result the electron mobility is compromised.
Two mechanisms are disclosed herein for overcoming the above-mentioned limitations and improving the production and properties of thin film solar cells for electrical energy production. The first is the use of a graphene matrix into which the nanoparticles are embedded in a manner that assures electron mobility, and the second is the use of inorganic ligands of that is comprised of a molecular metal chalcogenide, which also favorably affects electron mobility. Remarkably, these two mechanisms can optionally be used in a synergistic manner.
In addition to the foregoing, this disclosure addresses methods to positively affect the quantum efficiency of solar cells by optimizing the incident photon-to-electron conversion efficiency. Furthermore, the solar energy conversion efficiency of solar absorbing films is positively addressed by means of an engineered spray pyrolysis process for the application of the nano-ink. This disclosure also sets forth a mechanism to improve the solar energy conversion efficiency and the economics of solar absorber production by means of an engineered photonic flash sintering process. The engineered spray pyrolysis and engineered photonic flash sintering processes can be optionally used together or separately to improve the energy conversion efficiency of thin film solar absorber systems. Production processes that optionally incorporate any or all of these disclosed processes are disclosed. The process steps are arranged in sequences that generally are applicable to the production process. The use of each of these methods as a step in the production of solar absorbers is optional. Any one, or any multiple (including the use of all) of the techniques described herein may be employed for the enhancement of solar film production.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is Figure 1 from U.S. Patent No. 7,718,707 B2, which "shows a simplified diagram comparing surface area/volume to diameter for a set of Si nanoparticles." DETAILED DESCRIPTION
DEFINITIONS
The following definitions are provided
Light or Solar Absorber Layer - All solar cells require a light absorbing material contained within the cell structure to absorb photons and generate electrons via the photovoltaic effect. Light penetrates such a composite material reaching the lower charged layer. There the energy causes atoms to release electrons, which drift to the upper layer, giving this region a net negative charge and the lower layer a net positive charge.
Nano-crystalline solar cells as used herein refer to thin-film light absorbing materials that are laid onto a supporting matrix of metal foil, polymer, ceramic or glass. These materials have a high surface area which can serve to increase internal reflections (thereby increasing the probability of light absorption). The use of nanocrystals in such applications permits the design of architectures on the nanometer length scale, which is also the typical excitation diffusion length. In particular, single-nanocrystal ('channel') devices can provide an array of single p-n junctions between the electrodes which are separated by a period of about a diffusion length. This enables solar cells having potentially high efficiency.
Tetragonal Chalcopyrite Structure: Chalcopyrite is a copper iron sulfide mineral that crystallizes in the tetragonal crystal structure. The associated solar composition provides P- type absorber.
Rapid Thermal Annealing ("RTP"): As used herein, refers to the heating of a semiconductor wafer over a relatively abbreviated time interval, in order to affect the electrical properties. This heating can activate, move or drive dopants from one region to another. It can change the film-to-film or film-to-surface interface. It can be used to densify a deposited film. In achieving these relatively abbreviated short term annealing objectives, a tradeoff is made in process uniformity, temperature distribution management and stress in the wafer. Unlike furnace annealing, RTP normally lasts only minutes in duration (e.g., several minutes, or about 2-4, 3-7, 4-10, 10-20, or 15-30 minutes). When compared to the engineered photonic energy pulse process, the RTP is many orders of magnitude longer in its exposure duration.
Photonic Energy - the quantum of electromagnetic interaction that is contained in a photon, which is the basic unit of light. The energy and momentum of a photon depend only on its frequency (wavelength). The photon has no rest mass which allows for interaction at long distances. CIGS - Copper- indium-gallium-diselenide is a I-III-VI2 compound tetrahedrally bonded semiconductor material. It is a solid solution of copper indium selenide (CIS) and gallium selenide.
CZTS - Copper-zinc-tin-selenide is a I-II-VI quaternary compound whose
thermodynamic stability region is small. It has an intrinsic p-type conductivity that can be attributed to the presence of CuZn antisites.
Grain boundaries - As used herein, and in the context of the chalcopyrite thin- film applications, grain boundaries represent the region or juncture between adjacent grains. The presence of foreign residues in grain boundary regions of the semiconducting media used in solar absorber applications are of importance relative to device performance.
Grain Size - As used herein, grain size refers to the average grain size. In the context of chalcopyrite thin films, grain size is one of the parameters that will directly influence the solar cell efficiency. The traditional convention has maintained that a large average grain size provides higher overall solar efficiency. This is explained by the optimization of solar absorption combined with minimization of the grain boundaries. Grain size in CIGS absorbers generally has ranged from 0.2 microns to 1.7 microns (with the median grain size being in the range of 0.8 microns.
Pre-Sintering - As used herein, connotes a partial sintering phenomenon wherein the thermal regime that is imposed is not sufficient to grow the NP into the grain size that is ultimately desired, but where the growth of the nanoparticle is adequate to move the particle out of the catastrophic contamination domain.
Catastrophic Contamination Domain - As used herein, catastrophic contamination domain refers to a core-shell nanoparticle architecture. It characterizes the region where the nanoparticle is a size range where the volume amount of organic capping ligand(s) (i.e., the shell) needed to provide the requisite colloidal stability becomes excessive in comparison to the volume of the nanoparticle core. The phenomenon is further explained in U.S. Patent No. 7,718,707 issued to Kellman, for example in Figure 1.
Pyrolysis - As used herein, is the decomposition or transformation of a compound that is brought about by the action of heat.
Spray Pyrolysis - As used herein, indicates a pyrolysis process wherein the use of a spray type deposition of a compound is involved. Ultrasonic Spray Pyrolysis - As used herein, indicates a spray pyrolysis process wherein ultrasonic energy enhances the atomization of the sprayed medium in a pyrolysis process.
Pre-Sintering - As used herein, connotes a partial sintering phenomenon wherein the thermal regime imposed is not sufficient to grow the NP into the grain size that is ultimately desired, but where the growth of the nanoparticle is adequate to move the particle out of the catastrophic contamination domain.
Microwave Enhanced Ligand Exchange - As used in this disclosure, is a nanoparticle ligand exchange that utilizes microwave energy to drive an exchange of a ligand associated with a nanoparticle with another ligand that the nanoparticle is in contact with.
Metallic Chalcopyrite Molecule - As used herein is the electron transmitting linking mechanism used to connect the semiconducting nanoparticles into a functional matrix.
Molecular Metal Chalcogenide Complexes - As used herein, refers to the
semiconducting ligands that are used to "glue" the nanoparticles to form a continuum which can be converted to a semiconducting matrix upon gentle heat treatment. This continuum or matrix can be described as an inorganic nanocrystal solid. These molecular metal
chalcogenide complexes permit the preparation of conductive and even highly conductive arrays of nanocrystals as well as the mechanism to couple a functional nanoparticle array onto a graphene platelet.
Metallic Chalcogenide Capping Ligand (MCCL) - As used herein, a MCCL is specific form of molecular metal chalcogenide complex that is used herein as a "glue" to affix the inorganic semiconducting nanoparticles. As used herein, this moiety can attach the NPs to each other in a manner that affords efficient electron transmission. Alternatively, this moiety can be used to attach the NPs to a graphene platelet.
MCCL Nanoparticle Ink - As used herein, the metallic chalcogenide capping ligand can be affixed to the nanoparticles as they are in the nano-ink form. Such a composition can be subsequently deposited onto a suitable substrate and exposed to subsequent thermal treatment, wherein the nanoparticles become bound into an adherent and robust functional film. In such case, the pendant MCCL provides the attachment or linking mechanism.
Inorganic Colloidal Nanocrystalline Nanoparticles (denoted as "NPs" or singularly as an "NP") are nanoparticles having a core and shell construction. The core is the metallic alloy (e.g., CIS, CIGS, CZTS, and CdTe) that provides a semiconductor functionality, which in this case is the conversion of photons from the solar spectrum into electrons. The shell is the medium which imparts the solubility or stability of dispersions in a liquid medium, and can be comprised of an organic ligand associated with the core, or a molecular metal chalcogenide complex associated with or liganded to the core (also denoted as a MCCL). In some instances, where phase transfer solvents act as a chelate ligand of NPs and stabilize colloidal dispersions of the particles, the phase transfer solvent can also function as part of the shell.
Nanocrystal(s) (NC or NCs in the plural) - As used herein, refers to a crystalline particle with at least one dimension measuring less than 1000 nanometers. This can be suspended in an aqueous or organic solvent to yield an ink solution that can be applied to a surface, making it possible to literally paint a solar panel onto a substrate.
Graphene Nanoplatelets - As used herein, includes single atom quasi-planar graphene sheets with sp2 bound carbon atoms arranged in a lattice array. The nano-platelet graphene is of nano dimension. The graphene nano-platelet can consist of stacks of these single atom sheets. Functionality can be provided into these platelets by the introduction of oxygen into a fraction of the sp2 grapheme carbons to produce oxygenated carbons having an sp3 configuration and can be employed as covalent bonding site. Alternatively, the
functionalization can be provided by pi:pi bonding or van der Waals interactions. Graphene platelets are highly conductive and highly transparent to the solar light spectrum.
Graphene Oxide Nanoplatelets - As used herein are nano-platelets comprised of single layer carbon lattices that are non-conductive, or have significantly reduced conductivity relative to graphene nanoplatelets. These platelets are often the precursor to a graphene.
Graphene Host Conductor Medium (GHCM) - As used herein, indicates a graphene- filled or graphene-containing absorber layer as it is deposited and thermally processed such that its electron transport capability is enabled. This medium, which is often referred to as a "matrix" or "continuum," provides a mechanism for optimizing the minority carrier diffusion length of a solar absorbing film.
Incident Photon-To-Electron Conversion Efficiency (IPCE) - As used herein, is a measurement of a device's performance in which the short circuit current is measured as a function of the wavelength of the incident illumination. The IPCE data is essentially an external quantum efficiency (at zero bias) measurement that does not account for how much light is absorbed by the device. It is a measure of charge carriers extracted based on the number of photons that are illuminating the device. The significance of this measurement relative to this disclosure relates to its use in evaluating the Quantum Efficiency of thin solar absorber layers.
DESCRIPTION
Positive effects on the functional properties and the preparation/processing of semiconducting crystalline structures are provided as a result of optimizing colloidal nano-ink compositions used for solar cell production. The use of nano-inks composed of colloidal nano- particle compositions results in a highly conductive and functionally enhanced/optimized solar absorber medium that when combined by solvent release, heat treatment, and/or sintering, results in larger assembled matrices whose properties are positively affected by their electron pathway interactions.
Two mechanisms are disclosed herein for improving the production and properties of thin film solar cells for electrical energy production, the use of a graphene matrix into which the nanoparticles are embedded and the use of nanoparticles with a molecular metal chalcogenide ligand, both of which favorably affect electron mobility and can be used separately or together in a synergistic manner.
A spray pyrolysis process for the effective application of nano-inks to form solar absorbers, including the nano-inks comprised of colloidal suspension of nanocrystals, is described herein. Sintering of the applied solar absorber to improve the solar energy conversion efficiency and the economics of solar absorber production by means of an engineered photonic flash process is also described. The engineered spray pyrolysis and engineered photonic flash sintering processes can be used optionally together or separately, to improve the energy conversion efficiency of a thin film solar absorber system. A step-by-step production process that optionally incorporates those processes is disclosed. These steps are arranged in the sequence that is generally applicable to the production process. The use of each of these methods as a step in the production of solar absorbers is optional. Any one, or any multiple (including the use of all) of the techniques described herein may be employed for the enhancement of solar film production.
I. Matrix Designs Consisting of either NP Arrays or NP- Graphene Continuums.
The following describes two alternative means for achieving the electron mobility enhancement of a semiconducting medium for such applications as a solar cell. One addresses the chalcopyrite nanoparticle array which is applied as a precursor medium and subsequently thermally processed to deliver the optimized medium. The second incorporates graphene nano-platelets and the chalcopyrite nanoparticles.
While not wishing to be bound by any theory, it is believed that when two or more metallic or semiconducting nanoparticles are in close proximity to each other, their wave functions can couple forming states delocalized over several NCs or NPs or propagating throughout the entire solid. The quantum mechanical coupling energy can be approximated as: β « Y « exp[-(2m*AE/n2)1/2A ]
where h is Plank's constant , is the tunneling rate between two NP neighbors, m+ is the carrier effective mass, ft (i.e., Λ-bar) is Plank's constant over 2π (2 Pi), and ΔΕ and ΔΧ are the height of the tunneling barrier and the shortest edge-to-edge distance between the NCs, respectively. When bulky insulating organic and/or organometallic materials (e.g., hydrocarbon chains) are replaced with a medium that provides a conductive continuum, it can be seen that there is a substantial reduction of both ΔΕ and Δχ, thereby facilitating electronic communication between the NCs. From this it can be understood that formation conductive matrices with embedded nanoparticles/nanocrystals can facilitate the formation of solar absorbers/solar panels by permitting electrical communication between the nanoparticles/ nanocrystals in the matrix. Such conductive matrices also facilitate the collection and delivery of electrical current generated by this solar absorber in a manner that improves the efficiency and capacity of such a device for converting light into electrical energy.
Inorganic colloidal preparations of nanocrystalline particles with precisely controlled compositions and morphologies may be employed for the preparation of thin film solar cells. Such NPs provide useful physical and chemical properties and have found applications not only in solar cells but also in other devices. An advanced NP synthesis that draws upon solution-processed colloidal building blocks is disclosed that addresses the challenge of devising a fabrication technique which results in a viable solid state solar absorber medium. This synthesis process produces a core/shell NP particle, where each individual NP has size- dependent properties of the respective absorber metal alloy or semiconductor entity.
The process of solar absorption and subsequent electron transport in inorganic absorber media which are prepared using nano-ink solutions as described herein is facilitated in several alternative ways. First, a processing mechanism is provided comprising the deposition of colloidal nano-inks that comprise nanoparticles associated with an organic capping ligand followed by solvent evolution, and thermal processing. The objective of that process being the preparation of a solar absorber having a solar energy conversion efficiency within the absorber media's internal quantum efficiency limitation. A second process, similar to the first, with the exception that the capping ligand is a metal chalcogenide complex that serves to bond an array of nanoparticles into close proximity and permits functional interaction. A third mechanism is provided by processes that incorporates NPs that have been associated with a graphene nanoplatelet, where the graphene nanoplatelet serves to contribute to an optimal transport pathway for the photon generated electrons
Colloidal nanomaterials that are applicable to the first above-mentioned application may be prepared by syntheses that involve the use surface ligands to stabilize the particles. In some embodiments NPs having ligands with long (e.g., about C8 to about C18) hydrocarbon or fluorinated hydrocarbon chains or bulky organometallic molecules may be employed. These large molecules create highly insulating barriers around each NC core. After the deposition of such colloidal nanomaterials, the complete removal of undesirable stabilizing surface ligands (e.g., from the shell) is necessary, and this has proven to be problematic and expensive.
Failure to effect a complete removal can result in the creation of dangling bonds of unwanted species at the NP surface, which introduce undesirable charge-trapping centers in the final product.
Variations exist to the above mentioned processes, such as in the processing of PbSe nanoparticle films. Those films can be treated with dilute hydrazine solutions, or alternatively the linking of CdSe NCs with 1,4-phenylenediamine. Such processes are reported to provide conductive nanoparticle solids with carrier mobility (electron transmission) comparable to those of solution-processed organic semiconductors.
Herein disclosed are alternatives to achieve the above-mentioned second and third alternatives, which provide substantially improved performance. Those alternatives provide a colloidal nanostructure by the use of surface ligands that (i) adhere, covalently or non- covalently on the NP surface forming a shell and consequently provide colloidal stabilization, (ii) permit a functional electron communication matrix within a NP network, and (iii) supplement the functional performance of individual NPs either by permitting and/or promoting inter-particle behavior, or by their displacement, permitting NP interaction with a conductive host matrix such as graphene.
The synthetic approach for the preparation of matrices with embedded (e.g., attached or intercalated) nanoparticles/nanocrystals is based on the exchange of organic NP shell components (e.g., hydrocarbon or organometallic ligands) with ligand sites on a nanoplatelet (e.g., a graphene nanoplatelet). Alternatively, NPs (including those with a CIGS, CIS, CdTe, or CZTS core) with a shell comprising a molecular metal chalcogenide complex (e.g., Sn2S6 4" ) shell can be subject to an exchange reaction with sites on a nanoplatelet. Optionally, both of these processes can be coupled. The resulting complexes provide the requisite ligand stabilization of various nanostructures while enabling strong electronic coupling within the NP solids or within the graphene containing medium as disclosed below.
In one embodiment, graphene matrices with embedded NP and/or NCs are prepared by the exchange of the organic and/or organometallic ligands on the surface of NPs with ligand sites on a graphene nanoplatelet or a graphene oxide nanoplatelet. This can be accomplished by contacting the graphene with NPs under conditions that permit ligand sites on a graphene and/or graphene oxide matrix to displace ligands present on the surface of the NP.
In another embodiment, matrices with embedded nanoparticles/nanocrystals are prepared by interacting NPs having hydrocarbon ligands with one or more molecular metal chalcogenide complexes (e.g., Sn2S6 4" ) followed by subsequent processing. By contacting molecular metal chalcogenide complexes with the hydrocarbon ligand-containing NP under conditions where the complex displaces hydrocarbon ligands on the surface of the
nanoparticles, NPs with a chalcogenide ligand shell are formed (e.g., CIGS, CIS, CZTS, particles having Sn2S6 4" surface ligands for a shell ). The particles are subsequently applied to a surface (e.g., the surface of a graphene containing composition) for the formation of a matrix. Where a molecular metal chalcogenide complex is present, subsequent treatment, such as by mild heating, can convert the molecular metal chalcogenide complexes to semiconducting phases with embedded nanoparticles/nanocrystals.
In addition to their separate use, the above described embodiments for preparing matrices with embedded nanoparticles employing graphenes and NPs comprising a molecular metal chalcogenide complex can be coupled, such as, for example, by separately preparing both materials and combining the products (e.g., in a nano-ink prior to its application to the surface of a nascent thin-film, or by separately delivering both materials to the surface upon which a thin-film solar absorber is being formed). Alternatively, the graphenes comprising NPs and NPs comprising molecular metal chalcogenide complexes (e.g., MCCL-capped NPs) may be contacted simultaneously or sequentially with a surface and then processed (e.g., by heating) to obtain thin-films with embedded nanoparticles/nanocrystals as described below.
Once formed, these matrices provide stabilization of various nanostructures while enabling strong electronic coupling within the NP/NC containing solids (e.g., the graphene nanoplatelet continuing medium). In some embodiments, where a ligand (e.g., a molecular metal chalcogenide complex) exchange procedure is used to provide capped NPs (e.g., MCCL- capped NPs) that form stable colloidal suspensions or dispersion in water or other polar solvents, the exchange procedure involves a phase transfer of the newly formed NPs from a nonpolar (less polar) organic medium into a more polar solvent. For example, where MCCL- capped NPs are formed by exchanging a molecular metal chalcogenide ligand (MCCL) for a hydrocarbon containing ligand associated with a NP (e.g. its core), phase transfer from non- polar or low-polarity solvents to a more polar solvent may occur. In one embodiment, solubilization of the NPs in a nonpolar solvent is provided by means of a nonpolar group (e.g., organic or organometallic groups) attached to the NP by a thiol ligand. NPs having such non- polar groups in their shell may typically be dissolved in a nonpolar solvent at about 1 to about 20 mg/ml. The introduction of a molecular metal chalcogenide ligand complex, which can displace such non-polar groups resulting in MCCL-capping, is facilitated by the nucleophilic nature of the NPs, the electrophilicity of the un-coordinated metal atoms at the NP surface, and the introduction of a host solvent medium that can interact with the hydrocarbon ligands. Due to the colloid stabilizing effect of the molecular metal chalcogenide ligand (e.g., MCCL ligands), the capped NPs can be provided in water, DMSO, formamide, ethanolamine or a variety of other solvents (e.g., various glymes and other polar solvents). The fact that these NPs are stable when dispersed in such different solvents, suggests that negative charging of the NP surface is a general phenomenon responsible for colloidal stabilization of NP dispersions, such as MCCL-capped NPs in polar solvents.
Among the molecular metal ligand complexes that can be used to form MCCL-capped NPs, it has been reported that both CdSe and Au NCs capped with Sn2S6 4~ will maintain their size-dependent optical absorption features in the polar organic solvents such as DMSO, formamide, and ethanolamine. It has also been reported that Sn^^-capped CdSe NCs provide excitonic photoluminescence with a quantum efficiency of several percent. Those results are comparable to organic-capped NPs, which indicates good passivation of surface states by the MCCL ligands.
A property of MCCL ligands is their ability to transform into amorphous or crystalline metal chalcogenides (e.g., semiconductor or semi-conducting materials) upon mild thermal treatment. For example, at 180° C Sn2S6^ (neutralized by hydrazinium counter ions) will decompose (i.e., (N2H5)4Sn2S6→ SnS2 + 4N2H4 + 2H2S). As the process does not involve any nonvolatile or carbon-containing impurities, the resulting SnS2 phase crystallizes into pure electronic-grade semiconductor. Moreover, because this ligand conversion process involves a total weight loss of only about 3.8%, the possibility of strain related crack formation is reduced. Furthermore, the weight percentage of MCCLs that are needed to stabilize a colloidal NP is relatively small. For example, thermogravimetric data supports the conclusion that 2 to 10 weight % of MCCLs will stabilize colloidal NCs. Subsequent heating to 180°C, resulting in greater than 70%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5%, 99.7%, 99.8%, 99.9% or the complete disappearance of entities containing C-H, S-H, or N-H bonds. By way of comparison, NPs prepared using dodecanethiol and ODPA-HDA-TOPO (octadecylphosphonic acid, hexadecylamine, and trioctylphosphine oxide), (when studied using an IR spectra) remain nearly unaltered after 180° C annealing.
MCCL-capping may also impart substantial stability to NPs. Analytical
characterization has shown that the MCCL-capped semiconducting NPs do not easily sinter into a bulk phase.
Thin films prepared with NPs having metal chalcogenide ligand complexes shells, (e.g., MCCL-capped NPs) have conductivities increased by several by orders of magnitude. In one example, nanoparticle ink deposited films that are prepared from the alkanethiol (e.g., dodecanethiol) capped nanoparticles resulted in films that were highly insulating.
Conductivities (σ values) for those films are on the order of ~10~9 S cm-1 for 5-nm NPs (Au) and less than 10~12 S cm-1 for 5.5-nm NPs (CdSe). Replacement of the dodecanethiol ligands with Sn2S64_ results in an increase in the conductivity by -11 orders of magnitude, approaching σ values of -200 S cm-1' The highest conductivity reported for Au NP solids with short-chain organic capping (e.g., n-ethyl or n-butanethiol) was less than 10_1 S cm-1. The disappearance of the plasmonic absorption peak strongly supports the metallic nature of Sn2S6 4 -capped Au NP solids. Reportedly there is a strong decrease in the mean interparticle distance from -1.6 nm for dodecanethiol-capped Au NCs to less than 0.5 nm for Sn2S6 4~-capped Au NCs. The conductivity of Sn2S6 4~-capped Au NC solids is higher than the conductivities of conducting polymers and graphene-based composites.
Enhanced Electron Mobility Between Solar Absorbers And Conductor Media
Metallic Chalcogenide Capping Ligands (MCCLs) behaved as electron transporting "glue" for the NPs, including NPs associated with a graphene (e.g., a graphene platelet). Such MCCL-capped NPs can be employed to form a highly conductive solar absorbing continuum that can provide an effective (e.g., low resistance) pathway to the solar cell conductor media for electrons freed from their atoms/molecules by light interacting with the NPs. This is especially the case when the continuum is comprised of a properly deposited graphene film and metal molecular metal chalcogenide ligand capped NPs.
Where a graphene is employed in the solar absorber layer as the Conductive Host Receptor Medium (CHRM) into which the NPs are to be arrayed (embedded), the removal of the undesirable organic ligand moieties from the NP is preferably achieved by prior reaction with a molecular metal chalcogenide ligand to form a MCCL-capped NP. As discussed above, interaction of NPs with graphenes will also displace ligands, including undesirable organic ligands. Where NPs with a shell comprising organic ligands can interact effectively with graphenes, and particularly where they interact with more than one graphene (e.g., the NP becomes embedded or intercalated between layers of graphenes), it may not be necessary to displace all of the organic ligands as a sufficient path for electrons freed by light interacting with the NP to reach the conductor media of a solar cell may already be provided by the graphenes. Such ligand-capped NP containing materials, and particularly graphene containing MCCL-capped NPs, can be deposited as is (e.g., as part of a nano-ink) to yield a viable solar absorber layer that provides enhanced electron mobility between the solar absorber nanoparticles and the conductor media of the solar cell. Alternatively, the graphene containing materials may be subject to thermal decomposition of the molecular metal chalcogenides complexes to further enhance the electrical conductive properties of the solar absorber layers and provide a further enhancement of the electron mobility between the solar absorber nanoparticles and the conductor media of the solar cell.
Thermal decomposition of the MCCLs, or their hydrazinium derivatives (or easier to handle analogs) generate various chalcogenide phases with n- and p-type conductivity, phase- change properties etc. The electron-conducting graphene (CHRM) disclosed herein can be configured with a hole-conducting host (e.g., CuIni_^Ga^Se2) in order to form materials with distributed networks of p-n junctions. This provides a mechanism for solution based production process that makes this approach appealing for large-area, roll-to-roll production of thin-film solar cells.
Nano-ink Media
Many factors that contribute to determination of the ratio of shell to core of a colloid stabilized ink nanoparticle and thus the resulting nano-ink. Some factors include: the size of the core of an inorganic semiconductor nanoparticle, the physical characteristics of the ligand that comprises the shell, and the stability demands that the subsequent processing protocol imposes on the nanoparticle. In addition, as nanoparticles move into the region below 10 nanometers, the agglomeration energy increases and is a contributor to the catastrophic contamination domain described above.
This disclosure provides NPs that can be formulated as nano-inks and subsequently processed into solar absorber thin films. The nano-inks are also compatible with a variety of application processes. In one embodiment nano-ink is deposited by spraying. This spraying can utilize an ultrasonic atomization process, which enhances the application process by providing improved atomization and aerosolization of the nano-ink droplets.
The solvents of the nano-inks described herein may also be compatible with a variety of subsequent processing steps. Depending on the solvent chosen, processing at different temperatures and under a variety of different conditions are possible.
Simultaneous Deposition and Pyrolysis of Nano-ink Coatings To Provide Pre-Sintering (Grain Growth) of Nanoparticles, Solvent Volatilization, and Colloidal Ligand Shell Evolution From Nanoparticle Films.
Nano-ink containing compositions incorporating NPs having a core shell structure, where the core is the chalcopyrite metal alloy and the shell is an organic capping ligand (sometimes denoted as an "OCL") can be subject to a pyrolysis regime to eliminate substantially all of the organic shell. The volume of this organic shell is estimated at 50% of that of the "as-applied" film (assuming that this film is aerosol sprayed onto a room temperature surface). In such case, this large percentage of organic shell material must egress from the absorber layer during the elevated temperature regime of the sintering process. The result is a substantial shrinkage, which can result in cracking of the absorber layer.
In contrast to aerosol spray application of nano-ink containing compositions, ultrasonic spray pyrolysis application of the compositions on to substrates at an elevated temperature (above room temperature) advantageously avoids cracking of the solar absorber layer due to subsequent shrinkage. Unlike simple aerosol spray applications using a compressed carrier, ultrasonic spray pyrolysis application of the nano-ink containing compositions on to substrates at elevated temperatures contributes to the egress of the organic shell component and solvent during the application of the nano-ink containing composition. Because at lest a portion of the organic shell component and exits the absorber layer prior to the layer becoming set, the shrinkage it experiences is less than the same composition applied as an aerosol using compressed gas on a room temperature surface. Moreover, as the absorber layer is pseudo- plastic at the time, at least a portion of the shell is volatilized, the absorber layer is more shrinkage tolerant, and again, less subject to cracking.
Following the application of a nano-ink to a surface it may be treated by a pre-sintering regime. One example of a pre-sintering regimen that mitigates the deleterious film quality consequences that are subsequently imparted upon the solar absorber film by the use of NP with an organic capping ligand (OCL ) shell. While the OCL is an entity that is essential to the synthesis of the nano-ink colloid, and contributes to the formation of a stable dispersion of the NPs in a solvent, the OCL represents a significant volume of the nanoparticle (in the range of ten percent to 60 percent and higher). The demand for such a substantial volume of OCL is dictated by the complex stability constraints of nano-ink. The production of a solar absorber film using NPs with OCLs having a substantial volume introduces a number of problematic consequences. One is cracking of the film, and another is a significant residual organic carbon burden in the solar absorber film that is produced from such a nano-ink. An appreciation of a phenomenon reported in the literature called catastrophic contamination is desirable for an understanding of the relationship of the demand for OCLs as a function of nanoparticle size. Catastrophic contamination phenomenon explains the exponential demand for OCL, and the so called "catastrophic domain" that exists in the nanoparticle core range of 2-10 nanometers. This catastrophic contamination region is defined as the range where the volume of the OCL reaches a volume percent in the range of 60 percent and higher.
The consequences of employing nano-particle in the catastrophic contamination domain may be avoided where the nano-ink deposition process results in growth of the nanoparticle to a size of 10 nanometers or more which is outside this catastrophic
contamination region. During the deposition using pre-sintering as described herein, there is a coincident thermally induced growth of the core nanoparticles, which brings about the release of a substantial amount of the OCL during the deposition process. As a result there is a significant reduction in the propensity of the subsequently thermally processed solar absorber film to cracking during the subsequent thermal treatment of the film to optimize the solar absorbers physical properties. Pre-sintering regimens are not intended to fully eliminate the entirety of the organic carbon trapping entities that arise from the OCL, but rather to reduce them to a manageable level. When OCL is reduced to a manageable level, subsequent thermal processing can achieve a viable solar absorber film. As a result of this first stage sintering or pre-sintering, the OCL is reduced by as much as an order of magnitude, and the nanoparticle is capped with an organic capping ligand that is in the range of a few nanometers thick. Removal of this remaining organic capping ligand (which still forms a nanoscale junction, or nanogap between the metal alloy nanoparticles) must be subsequently provided by the subsequent processing. This is done by a second phase of ablative sintering, which addresses this nanogap junction medium by illuminating the composite (precursor film that has been subject to pre- sintering) with one or more intense optical pulses that results in the destructive collapse of the nanogap. This collapse is irreversible, occurring with the simultaneous ablation of the dielectric from the metal alloy nanoparticle junctions and ultimately the sintering of the metal alloy. By providing an effective method of removing the Organic Capping Ligand (OCL) this disclosure addresses a factor in the development of a viable production process for producing an efficient solar absorber film that is derived from a nano-ink solution.
In one embodiment of this process, the nano-ink is deposited by spraying. This spraying can utilize an ultrasonic atomization process, which enhances the application process by providing improved atomization and aerosolization of the nano-ink droplets.
In yet another embodiment, the pre-sintering deposition process can be repetitious such that a multitude of thin layers (e.g., 3, 5, 7, 10, 25, 20 etc.) is deposited, with each layer being subjected to the pre-sintering phenomenon. This will yield a series of thin film layers, which in some embodiments will comprise a series of layers, each about 100-300nm thick or 100- 200nm thick.
The nanoparticles used herein, which are of the form I-III -VI2 as well as the I- II-IV- VI are produced by the decomposition of single-source-precursors ("SSPs"). The reaction of which is facilitated by thermolysis of these precursors, although there are reports of these nanoparticles being formed by photolysis. A preferred method for converting these SSP's into the nanoparticles used herein is microwave irradiation. In one embodiment, the nanocrystalline ink composition is prepared using the single molecular source precursor (SSP) rather than multiple compounds to contribute elements of the product chalcopyrite. Thus, a single source precursor is used in the preferred methods to obtain all of the elements in the resulting chalcopyrite nanoparticle. For example, a single-source precursor is used to obtain all the elements (Cu, In, and Se) in the CuInSe2.
Such a nanoparticle production protocol makes possible the access to control of the nanoparticle sizes that are appropriate for the intended service that is described herein. In general the diameters of the nanoparticles that are appropriate for use in the embodiments described herein can be controlled. This control is by the concentration of the SSPs. Increasing the concentration increases the diameter of the resulting nanoparticle. The diameters decrease with increasing concentration of thiol. Increasing the reaction temperature results in increased nanoparticle diameter.
Production of the nanoparticles used herein involves control of the growth to a range that is optimum for this application. In one embodiment, the range is betweenl-5 nm.
The SSP's used herein can optionally incorporate selenium and sulfur as an integral part of the molecular structure, with the result that a quantity of sulfur or selenium will be incorporated in the structure of the nanoparticle. Optionally or in concert, it is possible to introduce an appropriate quantity of selenium or sulfur into the nanocrystalline ink by means of a Se nanoparticle. Such nanoparticle, having an average particle size of 80 nm, are commercially available, for example from QuantumSphere, Inc. (Santa Anna, CA, USA).
The nano-inks described above can be applied to a substrate that is appropriate for thin- film production. The preparation of suitable substrates is well known to persons who are knowledgeable in this field. The nano-inks can be applied by spraying, printing, spin casting, roll-to-roll printing, flexo-printing, gravure printing, etc.
Enhancing The Quantum Efficiency In Solar Absorbers Application By Embedding NPs Onto Graphene Platelets
A) The Use Of Microwave Energy To Provide Local Dielectric Heating During
Synthesis
Graphene Oxide platelets have a wide range of chemical groups on their surface and edges, including reactive oxygen species such as hydroxyl and epoxy groups on their basal planes. Since there is such a wide range of chemical compositions present on the GO, there are many prospects for functionalizing, which can involve the formation of covalent bonds, ionic- pi-pi bonds, van der Waals interactions, etc. One approach to the functionalization (in this case, nanoparticle modification) of these Graphene Oxide platelets utilizes orthogonal reaction of differing groups on the graphene oxide, whereby it is even possible to associate more than one type of NP with platelets through reactions selective for one or more different groups on the graphene oxide.
The synthesis process disclosed herein is directed toward a very fast and reliable synthesis protocol for achieving the NP/NC functionalization of a graphene platelet. The metal nanoparticle is provided in the form of a core-shell (alkanethiol colloid stabilized) solution. The solvent that is used in the process is typically a high boiling point member of the "glyme" family.
In one embodiment the solvent is a glyme. In additional embodiments the solvent is a monoglyme, diglyme, butyl-diglyme, tetraglyme. In another embodiment the solvent is selected from polyethylene glycol di-butyl ether, monoethylene glycol dimethyl ether, or diethylene glycol dibutyl ether. In another embodiment the solvent is a polyethylene glycol dibutyl ether (PEGDBE), whose boiling point is above 320 degrees C. The use of high boiling point solvents enables the ligand exchange onto the graphene. The use of PEGDBE (and similar glymes) may be particularly suited for this reaction, due to the fact that this solvent possesses a three coordinate complexant feature, which provides a means for translating the functional core to the residence site on the surface of the graphene. Prior experience with an ultrafast microwave assisted deposition of naked nanoparticles on Graphene has been reported in the literature. One such naked nanoparticle synthesis regime has been disclosed by Jasuja et al of the Kansas State University wherein the "naked" sites on a graphene oxide nanoplatelet provides a high density of bondable oxy-functional groups.
However Jasuja' s process is not feasible when the nanoparticles are capped with organic ligand shells that remain adherent to the NPs at the processing temperature that is dictated by conventional (relatively low boiling point) solvents. Such limitations would preclude the synthesis described herein. In the ligand exchange from core/shell nanoparticle to graphene oxide nanoparticles that is disclosed herein, it is observed that there can optionally be a co- reduction of the graphene oxide in the presence of the metal ions containing electrolyte. Since these graphene sheets are present during microwave exposure, they are expected to undergo a partial reduction as prolonged microwave exposure (in range of 10 minutes) has been shown to result in a mild thermal reduction of GO sheets.
Without wishing to be bound by theory, it is presently believed that the interaction of the electromagnetic waves with high dielectric solvent molecules results in a space confined uniform heating where: E is proportional to e' (tan delta)P2 ; and where P = microwave power; /= microwave frequency; e' is the dielectric constant of the solvent; and tan delta is the loss factor. The following are examples of some tan delta (permisivity) properties:
Ethylene glycol = 37, Water = 80, Sulfuric Acid = 100, Peroxide = 60, N-hexane =1.9
B) Microwave Enabled Deposition Of NPs Onto Exfoliated Graphene Platelets
A variety of synthetic paths for the immobilization of NPs onto graphene platelets may be envisioned, but the process will typically employ the steps recited in the embodiment given below. Thus, in one embodiment, the microwave enable deposition of NPs (e.g., NPs with a CIGS core) onto graphene platlets comprises:
1. Oxidation of Graphite - Graphite is oxidized using the Hummers Method (i.e. a
combination of sulfuric acid and potassium permanganate) to form graphite oxide (GO).
2. Reduction of Graphite Oxide. The chemical reduction of the GO is the most common method but it is not the only method for the preparation graphenes. It is possible to produce thermodynamically stable carbon oxide by directly heating the GO in a furnace. Reduction can also be achieved using either of the following protocols: the Graphite Oxide (GO) can be subjected to a microwave environment (no electrolyte) wherein the thermal regime will reduce the GO (this requires an inert gas atmosphere); or, alternatively, the NP implantation regimen can be extended such that there is a subsequent reduction of the GO bearing NPs.
3. Contacting Graphite Oxide with an Electrolyte Solution. The graphene oxide (either reduced or not) is brought into contact with the electrolyte solution (e.g., about 100 ml quantity), which provides a concentration in the range of about 1100 mg per liter. This mixture is next sonicated using a probe sonicator with a tip diameter of approximately ¼ inch and a frequency of about 20 KHz. The duration of sonication is in the range of three to five minutes.
4. Introduction of the Nanoparticles. NPs (e.g., CIGS NPs) are added to the electrolyte in a weight ratio of about 1:1 weight ratio versus the GO (i.e. 100 milligrams of nanoparticles). The sonication process described above is repeated.
5. In the case where an inert gas regimen is elected, the assembly of the microwave
transparent synthesis vessel allows the inert gas to sweep through the stirred reaction vessel, this vessel can be placed inside the microwave cavity and a fiber optic temperature probe is installed, at which point an argon purge is initiated and controlled from outside the oven. The purge duration should be on order of 10 minutes. As an alternative, the digestion vessel can be purged for approximately the same time prior to the seal of the cap. In this case it is desirable to provide a secondary barrier to protect the vessel contents from any subsequent contact with atmosphere during the remaining setup. The possibility of maintaining an inert gas purge within the microwave cavity during the synthesis process should be considered.
6. Microwave Regimen: A the time temperature and pressure regime for a typical
microwave synthesis is presented in Figure 1 of U.S. Patent No. 7,718,707. Note the rate of rise for the full power on regime, and the corresponding rate of temperature decay. As illustrated in that figure, the desired time interval at peak temperature can be achieved by providing a series of run cycles. The MW enabled "naked ion
implantation" process occurs in a relatively short time. The microwave protocol does not have to be continuous and microwave energy can be applied as appropriate for the process constraints including temperature and pressure. In one embodiment, the time needed for the association of the NPs with the graphene is about one to about ten minutes of total microwave exposure using SST-C as a solvent.
Solvents Used For The Nano-ink
A variety of solvents for solvent compositions can be employed in the preparation of nano-ink compositions. In one embodiment, denoted Solvent System-A (SST-A) is the a 1:1 blend of toluene and THF is employed. This is a relatively volatile solvent mixture. (The boiling point of THF is 66 degrees C. The boiling point of toluene is 100 degrees C). A second embodiment, Solvent System-B (SST-B), has a considerably higher boiling point solvent, terpineol, with a boiling point of 220 degrees C. The SST-C is a blend of a non-VOC solvent (tertbutylacetate) ,whose boiling point is the same as toluene, and PEGDBE, which acts as a "tail solvent" that has a higher boiling point (about 300 degrees C) and permits reactions at a higher temperature after the tertbutylacetate has been removed.
Nanoparticles are typically suspended in a ratio of 95 parts (by weight) of solvent to 5 parts (by weight) of dry nano-ink powder (e.g., NPs). The mixture is sonicated for 5 minutes and the resulting nano-ink preparation is filtered.
Quantum Energy Enhancement Thru Thickness Limited Performance Of Inorganic Nanocrystalline Photovoltaic Devices
The thickness of an inorganic solar absorber layer plays a role in the optimization of the internal quantum efficiency of the resulting devices. Remarkably, devices with thicker nanocrystal layers are reported to have lower power conversion efficiency, despite the increased photon absorption. For example, the internal quantum efficiency of CIS based devices having a size ranging from 150 nm to 540 nm decreased significantly as the film thickness increased. The thin, most efficient devices exhibited internal quantum efficiencies as high as 40 percent, and this is observed with films that were deposited on the low end of the thickness range - namely 150 nm. As nanoplatelets provide a scaffold onto which thin NPs absorbers (e.g., a CIS type of absorber) can be applied, they provide a mechanism to utilize the observed correlation between layer thickness and efficiency. Because graphene platelets are essentially transparent, a multitude of layers of thin nanoparticle coated graphene platelets can be prepared as a single absorber film that can provide the desired level of solar absorbance without compromising the quantum efficiency of the absorber film.
ENGINEERED PHOTONIC FLASH SINTERING
The use of an engineered photonic flash for sintering is an optional embodiment that relates to thermal treatment to achieve ablation of organic carbon residues and sintering of the deposited nanoparticles by means of an engineered photonic pulse regimen. Engineered photonic pulse regimens provide a means to manage the diffusion, reaction, nucleation, passivation and grain growth phenomena during the transition of the precursor to a solar absorber layer having a high optoelectronic quality. The use of an engineered photonic flash regimen contributes to the cost effective production process for such thin film poly crystalline photovoltaic absorber layers.
Polycrystalline thin film absorber layers can consist of chalcopyrites, sphalerites, kesterites. This disclosure provides an approach to the production of these absorbers utilizing nanocrystalline precursor dispersions that are deposited onto a suitably prepared substrate. The nano-crystal dispersion provides a uniform composition of metal precursors, which upon exposure to a precisely engineered thermal processing provided by an engineered photonic flash regimen delivers an optoelectronic layer.
In one embodiment the design of the photonic flash post-thermal processing regimen is comprised of two high energy and extremely rapid thermal pulse regimens. The first is designed to provide the pyrolysis or thermal ablation of the organic products which must necessarily be removed from the nano-crystalline composite precursor media. The second regimen is designed to provide the sintering of the nanoparticles into grains and permit the growth of same in such a manner to advance the formation of the solar absorber functionality objectives, which are provided by the simultaneous sintering, passivation, nucleation and grain growth. In this process the nano-crystalline film composite can incorporate additives such as dopants and passivation media, optionally in nano form that provide the means to enhance the opto-electronic performance.
DESCRIPTION OF EMBODIMENTS
Although the preceding detailed description contains many specific details for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations are within the scope of this disclosure. The following embodiments described herein are presented without imposing limitations on the claimed subject matter.
In embodiments where a composition of nanoparticles presents all of the elements that are required to form an efficient absorber layer, the film can be converted to an absorber layer using an engineered photonic flash regimen. In such an embodiment, the nanoparticle film layer is exposed to a properly engineered photonic pulse, which is of an intensity and duration sufficient to bring about the chemical reactions that are appropriate to achieve the objectives of its application. When said photonic pulse is delivered within a very short duration at a suitable intensity, and when such engineered photonic pulse consists of appropriate frequency and repeats, it can cause the nanoparticulate matrix to undergo the desired (and optimized) chemical reaction with suitable kinetics. Such regimes simultaneously introduce conditions which are contra posed to those that would contribute to undesirable results in the resulting absorber layer (e.g., undesirable effects that would otherwise be brought about as a result of the variations in melting point or boiling point of the chemical reaction participants).
One embodiment described herein provides for fabrication of a thin film ternary chalcopyrite absorber layers comprised of copper indium gallium di-selenide, more often referred to as CIGS. The chalcopyrite devices described herein are also referred to as I-III-VI2 devices, according to their constituent elemental groups. Other embodiments provide for fabrication of thin film kesterite absorber layers comprised of copper, zinc, tin selenide more often referred to as CZTS. The kesterite devices described herein are also referred to as I- II- IV- VI devices, according to their constituent elemental groups.
The production process described herein provides representative insight into a nanocrystalline ink composition that incorporates the metallic alloy Cu(Inx Gax) and Se nanoparticles. More specifically, nanoparticles that are classified as chalcopyrites or kesterites may be produced using a composition of single molecular source ("single source precursors" (SSP's). These are combined into a suitable solution phase whereupon they are subsequently subjected to a specific thermolysis regime. The result of these processing methods resulting in nanoparticles in the range of less than lOnm particle size, for example in the range of 2nm to lOnm. One illustrative example of the approach to such processing involves the use of alkylthiol ligands in solvent, along with the appropriate SSP components, chalcopyrite nanoparticles that exhibit properties expected for chalcopyrite nanoparticles that are appropriate for use in this application.
In the production of solar absorber layers from the NPs described herein, organics that are associated with capping agents (e.g., shell of an NP) and/or surfactant materials that are used to produce and stabilize the nano-inks are removed by pyrolysis. In one embodiment, the pyrolysis of organics is achieved by the introduction of an energy pulse that delivers sufficient energy to a chalcopyrite containing precursor film to effect the removal of the organics from the composite, but at the same time the resulting thermal regime is not high enough to volatilize an unacceptably high amount of the chalcopyrite species. It is noted that the melting points of chalcopyrites (about 1,000° C) differ significantly from the other components, where Sulfur melts at about 115° C, Selenium at about 221° C, and Tellurium at about 450° C.
Consequently, the removal of the organics must occur under suitably engineered and controlled conditions. It is possible to achieve those objectives by the appropriate selection of the capping agents associated with the nanoparticles, appropriate selection of surfactant materials in the nano-inks, and by the use of intense photonic light pulses which are of properly engineered duration and which are provided at an energy density in the range of 1-50 joules per cm ~2. The control of the volatilization of the selenium is necessary in order to mitigate undesired diffusion and the corollary volatilization of the Se that is provided in the nano- particulate medium of the nano-ink.
This regime provides the mechanism of CIGS formation and crystallization as the result of an intense photonic pulse from a xenon source.
The mechanism of CIGS formation, sintering, and crystallization using photonic pulses may be explained based on the melting point of CIG (550° C) and Selenium (217° C). Under these conditions the melting of the CIG and Se nanoparticles, the nucleation, passivation and grain growth occurs in a surprisingly short reaction time interval. Those processes take place within the sintering time interval, which occurs within a timeframe that is engineered, such that the desired absorber layer structure is achieved. In one embodiment this engineered time frame is within the range of microseconds to a few milliseconds, and accordingly this reaction interval is sufficiently short that precludes the oxidation of the constituent elements that are provided by the nanoparticulate precursor.
In another embodiment, selenization is conducted without changing the microstructure of the materials as selenium diffuses into the CIG lattice. In one embodiment substantially all of the selenium (e.g., nano-selenium or nano particle selenium) will be depleted from this matrix, under the subsequent processing. In such an embodiment it is importance that a sufficient amount of selenium source media be present to assure that all of the CIG precursor is selenated, since the presence of a minute amount of unreacted CIG precursor will adversely effect the absorber layer's photoactivity. That adverse effect, may be related to the properties of the light absorption by CIG and Se nanoparticles, which are black and thus they are very light absorbing. Those particles also have a very high surface area to mass ratio, and accordingly require a relatively small amount of energy to heat them. Considering those properties, the photonic pulse can form a CIGS film from a CIG precursor and Se nanoparticles in a very short time interval. In one embodiment, CGIS film formation can occur with pulses of as short as microsecond in duration.
Elemental analysis of the absorber layer that results from short pulse photonic sintering processes indicates that the Ga, Cu, In, Ca, and Se concentrations remain virtually constant through the entire cross-section of the absorber film. That confirms the short pulse photonic processes eliminate any grading of gallium to indium ratio in the CIGS structure, with the result that the film is a homogeneous, single phase material.
In one embodiment, CIGS films are prepared using 20 Joules per cm2 light intensity for a duration of 2 milliseconds. That brief and intense process results in a diffusion of selenium to form the tetragonal chalcopyrite structure within a uniform and relatively dense absorber layer, while at the same time altering the surface morphology to provide compact grains in the range of 0.3 to 1.0 micron, or 0.1 to 1 micron.
In one embodiment, equipment produced by Novacentrix Corporation or Xenon Corporation, which is capable of delivering the engineered photonic energy pulses at the energy level, pulse duration, wavelength, and pulse frequency is employed in the methods and processes described herein.
Certain Embodiments
1. A method for the preparation of a nano-ink comprising: contacting a nanoparticle (NP) bearing hydrocarbon ligands with a phase transfer solvent and subject the resulting mixture to heat sufficient to sinter the NPs and displace at least some of the hydrocarbon ligands, wherein the NPs are increased in size by sintering.
2. The method of embodiment 1, wherein the NP comprises a metallic chalcopyrite, a
kesterite, or CdTe.
3. The method of embodiment 3, wherein the metallic chalcopyrite comprises copper indium selenide (CIS), Copper- indium-gallium-diselenide (CIGS) or copper-zinc-tin-selenide (CZTS).
4. The method of any of embodiments 1-3, wherein the heat sufficient to sinter the NPs and displace at least some of the hydrocarbon ligands is applied by focused dielectric heating.
5. The method of embodiments 1-3, wherein the heat sufficient to sinter the NPs and displace at least some of the hydrocarbon ligands is applied by subjecting the sample to microwave energy.
6. The method of any of embodiments 1-5, wherein said NPs that have been increased in size by sintering are contacted with a metallic chalcogenide capping ligand and subjected to heat sufficient for the metallic chalcogenide capping ligand to displace at least a portion of the hydrocarbon ligands present on said NPs that have been increased in size by sintering, to produce a MCCL-capped NP.
7. The method of embodiment 6, wherein the heat sufficient for the metallic chalcogenide capping ligand to displace at least a portion of the hydrocarbon ligands is applied by focused dielectric heating. The method of embodiments 6, wherein the heat sufficient for the metallic chalcogenide capping ligand to displace at least a portion of the hydrocarbon ligands is applied by subjecting the sample to microwave energy.
The method of any of embodiments 1-5, wherein said NPs that have been increased in size by sintering are made to contact a graphene or graphene oxide platelet (GO-platelet) and subjected to heat sufficient for the nanoparticle to associate with surface of the graphene or GO-platelet or intercalate into said graphene or GO-platelet.
The method of embodiment 9, where at least a portion of the hydrocarbon ligands present on said NPs are displaced by the graphene or GO-platelet.
The method of embodiment 9 or 10, wherein the heat sufficient for the nanoparticle to associate with or intercalate into the graphene or GO-platelet is applied by focused dielectric heating.
The method of embodiment 9 or 10, wherein the heat sufficient for the nanoparticle to associate with or intercalate into the graphene or GO-platelet is applied by the introduction of microwave energy.
The method of any of embodiments 1-12, wherein said phase transfer solvent acts as a chelate ligand of said NP as a result of its coordination sites and its molecular
architecture, and which interacts with a microwave energy by a suitable coupling property that provides focused dielectric heating.
The method of embodiment 13, wherein said phase transfer solvent has three coordination sites.
The method of embodiment 13 or 14 wherein the chelate ligand properties of the phase transfer solvent results in an increase in the stability of a colloidal dispersion of the NPs. The method of any of embodiments 13- 15, wherein the presence of the phase transfer solvent results in a reduction in the amount of organic capping ligand that is required to maintain a dispersion of NPs of a given size
The method of embodiment 16, wherein the solvent is a glyme (monoglyme), diglyme, butyl-diglyme, tetraglyme, polyethylene glycol di-butyl ether, monoethylene glycol dimethyl ether, or diethylene glycol dibutyl ether.
The method of any of embodiments 13-17 wherein the solvent has a boiling point greater than about 200, 225, 250, 275, or 300 centigrade. The method of any of embodiments 1-18, wherein the nanoparticles present in the nano-ink have an average size in a range selected from about: 3nm-200nm, 3nm -lOOnm, 3nm-50 nm, 3nm-25 nm, 3nm-10 nm, 5nm-200nm, 5nm -lOOnm, 5nm-50 nm, 5nm-25 nm, 5nm-10 nm, 10nm-200nm, lOnm-lOOnm, 10nm-50nm, 10nm-25 nm, 20nm-200nm, 20nm -lOOnm, 20nm-50nm, 15nm-25 nm, and 15nm-20nm.
A nano-ink prepared by the method of embodiments 1-19.
A nano-ink comprising one or more MCCL-capped NPs, where said NP have an average size in a range selected from about: 3nm-200nm, 3nm - lOOnm, 3nm-50 nm, 3nm-25 nm, 3nm-10 nm, 5nm-200nm, 5nm -lOOnm, 5nm-50 nm, 5nm-25 nm, 5nm-10 nm, lOnm- 200nm, lOnm-lOOnm, 10nm-50nm, 10nm-25nm, 20nm-200nm, 20nm -lOOnm, 20nm- 50nm, 15nm-25 nm, and 15nm-20nm.
A nano-ink comprising one or more MCCL-capped NPs.
A nano-ink comprising one or more NPs associated with or intercalated into a graphene. The nano-ink of any of embodiment 22-23 where said NP have an average size in a range selected from about: 3nm-200nm, 3nm - lOOnm, 3nm-50 nm, 3nm-25 nm, 3nm-10 nm, 5nm-200nm, 5nm - lOOnm, 5nm-50 nm, 5nm-25 nm, 5nm-10 nm, 10nm-200nm, lOnm- lOOnm 10nm-50 nm, 10nm-25 nm, 20nm-200nm, 20nm -lOOnm, 20nm-50nm, 15nm-25 nm, and 15nm-20nm.
The nano-inks of any of embodiments 22-24, wherein the NP are comprised of a metallic chalcopyrite.
The nano-inks of any of embodiments 21-25, wherein the metallic chalcopyrite comprises copper- indium-gallium-diselenide (CIGS) and/or copper -zinc-tin-selenide (CZTS).
The nano-inks of any of embodiments 21-26 further comprising a solvent.
The nano-ink of embodiment 27, where the solvent comprises on or more of: a solvent that is more polar than hexane or toluene, an ether, a glyme (monoglyme), diglyme, butyl- diglyme, tetraglyme, or a solvent that has a boiling point greater than about 200, 225, 250, 275, or 300 centigrade.
A method for producing a thin-film solar absorber wherein the electron mobility is enhanced and electron traps are minimized comprising: a) applying a nano-ink according to any of embodiments 20-28 to a thin-film substrate to form a nano-ink film; and
b) subjecting the deposited nano-ink film to a pre-sintering pyrolysis.
The method of embodiment 29, wherein said pre-sintering pyrolysis causes the egress of organic components from the nano-ink film, other than graphenes that are optionally present, said egress of said organic components reducing the organic carbon associated with the nano-ink film and increasing the conductivity of the film by removal of those components.
The method of any of embodiments 29-30, where the pre-sintering of the nanoparticle results in the growth of grains of solar absorbing material, the reduction in grain boundaries, and the corresponding enhancement of electron mobility and/or the conductivity of the material resulting from applying said nano-ink on said thin-film substrate.
The method of embodiment 31, wherein the pre-sintering enhances electron mobility and minimizes the number of electron trapping species at the grain boundaries.
The method of embodiment 31 or 32. wherein the pre-sintering reduces the amount of organic residue at the grain boundaries.
The method of any of embodiments 29-33, wherein said nano-ink comprises an organic solvent, and the step of applying said nano-ink to said substrate further comprises removing the bulk of said organic solvent to form a dried nondensified precursor film that is to be subject to said pre-sintering pyrolysis.
The method of embodiment 34, wherein the nano-ink is a nano-ink according to embodiment 22 or embodiment 23.
The method of any of embodiments 28-34, wherein said nano-ink further comprises an additional amount of a metallic chalcogenide capping ligand.
The method of any of embodiments 34-36, wherein said method further comprises thermal processing of the precursor film that: releases at least a portion of residual organic species that will compromise the energy conversion efficiency of the solar absorber formed from said nano-ink, and/or sinters the dried nondensified precursor film subject to said pre- sintering pyrolysis to achieve a final desired average grain size. The method of any of embodiments 34-36, wherein said method further comprises subjecting the precursor film to an engineered photonic pulse regimen that: releases at least a portion of residual organic species that will compromise the energy conversion efficiency of the solar absorber formed from said nano-ink, and/or sinters the dried
nondensified precursor film subject to said pre-sintering pyrolysis to achieve a final desired average grain size.
The method of embodiment 37 or 38, wherein he final desired average grain size is in the range of about 0.12 microns to about 1 micron.
The method of any of embodiments 29-39, where one or more steps are preformed in a controlled environment having a controlled atmosphere and temperature; wherein said controlled atmosphere is selected independently for each step from: air, a vacuum, an inert gas atmosphere comprising one or more noble gases, and nitrogen alone or in combination, and wherein said controlled temperature is selected independently for each step.
The method of embodiments 38-40, wherein the energy density, pulse duration, pulse frequency and pulse repetitions of said engineered photonic pulse regimen are designed to introduce an intense ablative effect that releases residual organic species that will compromise the energy conversion efficiency of the solar absorber formed from said nano- ink, and/or contributes to the sintering of the dried nondensified precursor film subject to said pre-sintering pyrolysis to achieve a final desired average grain size.
The method of embodiment 41, wherein said regimen provides a means to eliminate a substantial percentage of the organic carbon, other than graphenes that are optionally present, from said precursor film.
The method of embodiment 42, wherein said substantial percentage of the organic carbon, other than graphenes that are optionally present, is greater than about 80%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5%, 99.7%, 99.8%, 99.9% or 99.95% if the organic carbon in the nondensified precursor film.
The method of any of embodiments 29-43, wherein the thin-film solar absorber is comprised of monodisperse, passivated, non-aggregated semiconductor nanocrystals. The method of any of embodiments 29-44, wherein the thin-film solar absorber is a nanocrytalline thin-film solar absorber and the nano-particles present in said nano-ink comprise of Group I-III- VI2 elements. The method of any of embodiments 29-45, wherein said nano-ink comprises a single type of nanoparticle. The method of any of embodiments 29-46, wherein the nanoparticles present in said nano- ink comprise selenium and/or sulfur.
The method of any of embodiments 29-46, wherein the nanoparticles present in said nano- ink comprise sodium.
The method of any of embodiments 38-48, where the engineered photonic pulse regime results in formation of a thin-film polycrystalline solar absorber having enhanced electron mobility architecture, wherein the pulse regimen delivers energy in a manner that provides for diffusion control, control of nucleation, passivation control, doping control, and control of grain growth.
The method of embodiment 49, where the energy delivered by the pulse regime does not damage the substrate on which the polycrystalline solar absorber is formed and/or the polycrystalline solar absorber.
The method of embodiment 50, wherein the damage to the substrate and/or the polycrystalline solar absorber is selected from the group consisting of cracking, pealing delamination, or chemical degradation.
The method of any of embodiments 38-51, wherein the photonic energy delivered in any portion of the pulse regimen is within the range from 1 to 100 joules per cm2 over a pulse duration from about 1 picoseconds to about 100 milliseconds.
The method of embodiment 52, wherein the energy delivered is from about 2 to 50 joules per cm2
The method of any of embodiments 38-53, wherein the photonic energy regime is such that the precursor reactions are carried out to completion such that there are substantially no unreacted precursors remaining in the thus sintered thin-film polycrystalline solar absorber layer.
The method of any of embodiments 38-54, wherein the photonic areal energy density is in the range of 0.05 joules per cm2 and 50 joules per cm2.
The method of any of embodiments 38-55, wherein the duration of photonic energy pulse is between 100 picoseconds and 50 milliseconds. The method of any of embodiments 38 - 56, wherein the engineered photonic pulse regimen comprises one or more pulses, two or more pulses, three or more pulses, five or more pulses, ten or more pulses, or a series of pulses, and the number of pulses ranges from 1 to 100.
The method of any of embodiments 38-57, where the engineering of the photonic energy regimen is employed to convert the nondensified thin-film precursor into a thin- film polycrystalline solar absorber layer having optoelectronic behavior.
The method of any of embodiments 38-58, wherein the engineered photonic pulse regimen employs a flash lamp that emits light within the wavelength range from about 300 nm to about 750 nm.
A method for sintering chalcopyrite nano-particles to produce a densified chalcogenide solar absorber layer having high optoelectronic quality comprising:
applying a precursor colloidal nanoparticle dispersion to a substrate, wherein the said precursor colloidal dispersion includes a plurality of chalcopyrite nanoparticles that are combined with an organic solvent;
removing the bulk of said organic solvent to form a non-densified chalcopyrite precursor film;
heating the non-densified chalcopyrite precursor film to cause a reduction in the amount of organic elements associated with the nondensified nanoparticulate chalcopyrite thin- film; and
subjecting the nondensified nano-particle precursor film to an engineered photonic pulse regimen which results in the formation of a chalcopyrite film having high
optoelectronic quality.
The method of embodiment 60 where the engineering of the photonic energy regime delivers a photonic energy input that lasts for a sufficiently short time interval to preclude substantial diffusion of components in the chalcopyrite, while delivering an energy density sufficient to provide the required CIGS sintering and annealing.
The method of any of embodiments 29-59, wherein the nano-ink comprises particles of a CZTS (a kesterite)or particles comprising CdTe. 63. A method of producing a solar absorber film wherein an exposure of a substrate to an engineered photonic regime removes substantially all organic contaminants prior to the deposition of the colloidal nano-particle precursor dispersion, wherein said contaminants have the ability to adversely affect the efficiency of the solar absorber film if not removed.
64. The method of any of embodiments 38-63, wherein the engineered photonic pulse is delivered by a laser.
65. The method of embodiment 66, wherein the laser is as eximer laser.
66. The method of any of embodiments 29-59, wherein the electron mobility is enhanced due to an enhanced proximity of the semiconducting nanoparticles through the presence of conductive metal chalcogenide ligands on the nanoparticles of said nano-ink.
67. The method of any of embodiments 29-59, wherein the electron mobility is enhanced due to the replacement of at least a portion of the hydrocarbon ligands on the nanoparticles of said nano-ink with metal chalcogenide ligands, which are subsequently converted into semiconductors by a photonic flash heating regimen.
68. The method of embodiment 67, wherein the nanoparticles are subsequently sintered into an inorganic matrix.
69. The method according to embodiment 68, wherein a solar absorber formed from said inorganic matrix having a Sn2Se4 chalcogenide ligand exhibits a conductivity that is increased by approximately eleven orders of magnitude relative to a solar absorber formed from a nanoparticle matrix where the ligand is dodecanethiol, but otherwise identical to said solar absorber formed from said inorganic matrix.
70. The method of any of embodiments 29-59, wherein the electron mobility is enhanced by the introduction of strong electronic coupling between the nanoparticles.
71. The method of any of embodiments 29-59, wherein the electron mobility is enhanced by mitigating the adverse effects of the highly insulating and electron trapping residues that are concentrated at the boundaries of the nanoparticles.
72. The method of any of embodiments 29-59, wherein the electron mobility is further
enhanced by the introduction of a conductive nanoplatelet species.
73. The method of embodiment 72, wherein the nanoplatelet species is a graphene platelet or a graphene oxide platelet. 74. The method of embodiment 73, wherein the bonding mechanism of MCCL-capped NPs on a graphene platelet is a pi:pi bond or a covalent bond.
75. The method of embodiment 73, wherein said MCCL particles are associated with or
intercalated into a graphene oxide platelet by a covalent bond or an ionic bond, or associated with localized oxygenated sites on the surface of the graphene.
76. The method of any of embodiments 73-75, wherein the nano-ink derived solar absorbing medium is further enhanced by an improvement in the quantum efficiency of the said absorber by the minimization of the thickness of a chalcopyrite absorber on the graphene platelet to produce multiple layers of minimized thickness.
77. The method of any of embodiment 76 where the multiple layers of said minimized
thickness of absorber layer on graphene platelet are stacked so as to increase the solar absorbance provide by the multiple layers.
78. The method of any of embodiments 76-77, wherein the thickness of each of said multiple layers on said graphene is platelet in range of 120nm to 150nm.
79. A thin-film solar absorber prepared employing the method of any of embodiments 29-78.

Claims

Claims:
1. A method for the preparation of a nano-ink comprising: contacting a nanoparticle (NP) bearing hydrocarbon ligands with a phase transfer solvent and subject the resulting mixture to heat sufficient to sinter the NPs and displace at least some of the hydrocarbon ligands, wherein the NPs are increased in size by sintering.
2. The method of claim 1 , wherein the NP comprises a metallic chalcopyrite, a kesterite, or
CdTe.
3. The method of claim 2, wherein the metallic chalcopyrite comprises copper indium selenide
(CIS), Copper-indium-gallium-diselenide (CIGS) or copper-zinc-tin-selenide (CZTS).
4. The method of any of claims 1-3, wherein said NPs that have been increased in size by sintering are contacted with a metallic chalcogenide capping ligand and subjected to heat sufficient for the metallic chalcogenide capping ligand to displace at least a portion of the hydrocarbon ligands present on said NPs that have been increased in size by sintering, to produce a MCCL-capped NP.
5. The method of any of claims 1-3, wherein said NPs that have been increased in size by sintering are made to contact a graphene or graphene oxide platelet (GO-platelet) and subjected to heat sufficient for the nanoparticle to associate with surface of the graphene or GO-platelet or intercalate into said graphene or GO-platelet.
6. The method of any of claims 1-3, wherein said phase transfer solvent acts as a chelate ligand of said NP as a result of its coordination sites and its molecular architecture, and which interacts with a microwave energy by a suitable coupling property that provides focused dielectric heating.
7. A nano-ink prepared by the method of claims 1.
8. A nano-ink comprising one or more MCCL-capped NPs.
9. A nano-ink comprising one or more NPs associated with or intercalated into a graphene.
10. A method for producing a thin- film solar absorber wherein the electron mobility is
enhanced and electron traps are minimized comprising:
a) applying a nano-ink prepared according to the method of claim 7 to a thin-film
substrate to form a nano-ink film; and
b) subjecting the deposited nano-ink film to a pre-sintering pyrolysis.
11. The method of claim 10, wherein said pre-sintering pyrolysis causes the egress of organic components from the nano-ink film, other than graphenes that are optionally present, said egress of said organic components reducing the organic carbon associated with the nano- ink film and increasing the conductivity of the film by removal of those components.
12. The method of claim 10, wherein said nano-ink comprises an organic solvent, and the step of applying said nano-ink to said substrate further comprises removing the bulk of said organic solvent to form a dried non-densified precursor film that is to be subject to said pre-sintering pyrolysis.
13. The method of claim 12, wherein said method further comprises subjecting the precursor film to an engineered photonic pulse regimen that: releases at least a portion of residual organic species that will compromise the energy conversion efficiency of the solar absorber formed from said nano-ink, and/or sinters the dried non-densified precursor film subject to said pre-sintering pyrolysis to achieve a final desired average grain size.
14. The method of claim 13, wherein the energy density, pulse duration, pulse frequency and pulse repetitions of said photonic pulse regimen are designed to introduce an intense ablative effect that releases residual organic species that will compromise the energy conversion efficiency of the solar absorber formed from said nano-ink, and/or contributes to the sintering of the dried non-densified precursor film subject to said pre-sintering pyrolysis to achieve a final desired average grain size.
15. The method of claim 13, wherein the photonic energy delivered in any portion of the pulse regimen is within the range from 1 to 100 joules per cm2 over a pulse duration from about 1 picoseconds to about 100 milliseconds.
16. The method of claim 15, wherein the energy delivered is from about 2 to 50 joules per cm2
17. The method of claim 13, wherein the engineered photonic pulse regimen comprises a series of pulses and the number of pulses ranges from 1 to 100.
18. A method for sintering chalcopyrite nano-particles to produce a densified chalcogenide solar absorber layer having high optoelectronic quality comprising:
applying a precursor colloidal nanoparticle dispersion to a substrate, wherein the said precursor colloidal dispersion includes a plurality of chalcopyrite nanoparticles that are combined with an organic solvent; removing the bulk of said organic solvent to form a non-densified chalcopyrite precursor film;
heating the non-densified chalcopyrite precursor film to cause a reduction in the amount of organic elements associated with the nondensified nanoparticulate chalcopyrite thin- film; and
subjecting the nondensified nano-particle precursor film to an engineered photonic pulse regimen which results in the formation of a chalcopyrite film having high
optoelectronic quality.
19. A method of producing a solar absorber film wherein an exposure of a substrate to an engineered photonic regime removes substantially all organic contaminants prior to the deposition of the colloidal nano-particle precursor dispersion, said contaminants having the ability to adversely affect the efficiency of the solar absorber film if not removed.
20. A thin- film solar absorber prepared employing the method of any of claims 10-19.
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