WO2013051895A4 - Metal oxide semiconductor-nanocarbon consolidated core-shell quantum dots and ultraviolet photovoltaic cell using it and fabrication process thereof - Google Patents

Metal oxide semiconductor-nanocarbon consolidated core-shell quantum dots and ultraviolet photovoltaic cell using it and fabrication process thereof Download PDF

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WO2013051895A4
WO2013051895A4 PCT/KR2012/008095 KR2012008095W WO2013051895A4 WO 2013051895 A4 WO2013051895 A4 WO 2013051895A4 KR 2012008095 W KR2012008095 W KR 2012008095W WO 2013051895 A4 WO2013051895 A4 WO 2013051895A4
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metal oxide
oxide semiconductor
nanocarbon
core
quantum dots
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WO2013051895A3 (en
WO2013051895A2 (en
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Won-Kook Choi
Dong Hee Park
Dong Ick Son
Byoung Wook Kwon
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Korea Institute Of Science And Technology
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0352—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions
    • H01L31/035209—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions comprising a quantum structures
    • H01L31/035218—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions comprising a quantum structures the quantum structure being quantum dots
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    • B82—NANOTECHNOLOGY
    • B82B—NANOSTRUCTURES FORMED BY MANIPULATION OF INDIVIDUAL ATOMS, MOLECULES, OR LIMITED COLLECTIONS OF ATOMS OR MOLECULES AS DISCRETE UNITS; MANUFACTURE OR TREATMENT THEREOF
    • B82B1/00—Nanostructures formed by manipulation of individual atoms or molecules, or limited collections of atoms or molecules as discrete units
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J13/00—Colloid chemistry, e.g. the production of colloidal materials or their solutions, not otherwise provided for; Making microcapsules or microballoons
    • B01J13/02—Making microcapsules or microballoons
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82B—NANOSTRUCTURES FORMED BY MANIPULATION OF INDIVIDUAL ATOMS, MOLECULES, OR LIMITED COLLECTIONS OF ATOMS OR MOLECULES AS DISCRETE UNITS; MANUFACTURE OR TREATMENT THEREOF
    • B82B3/00—Manufacture or treatment of nanostructures by manipulation of individual atoms or molecules, or limited collections of atoms or molecules as discrete units
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y40/00—Manufacture or treatment of nanostructures
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
    • H01G9/20—Light-sensitive devices
    • H01G9/2027—Light-sensitive devices comprising an oxide semiconductor electrode
    • H01G9/204—Light-sensitive devices comprising an oxide semiconductor electrode comprising zinc oxides, e.g. ZnO
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy

Definitions

  • the present invention relates to a method of preparing metal oxide semiconductor-nanocarbon core-shell consolidated quantum dots by chemically linking nanocarbon superior electrical properties to a metal oxide semiconductor and a method of fabricating a UV photovoltaic cell device by using the same.
  • a new type of quantum dots to a photovoltaic cell as a light absorbing layer
  • the rate of electron flow is accelerated and that of hole flow is inhibited.
  • the methods of the present invention can prepare the core-shell consolidated quantum dots having excellent power conversion efficiency and the photovoltaic cell using the same.
  • Photovoltaic cells convert sunlight into electricity, and since their energy source is in infinite supply and eco-friendly, their importance and attention thereon have been increased. Silicon photovoltaic cells have been conventionally used, but high production costs are required and there is a limitation to improve conversion efficiency of solar energy into electricity. Thus, photovoltaic cells using economically favorable organic materials, in particular dye sensitized photovoltaic cells come into the spotlight.
  • Dye sensitized photovoltaic cells use a semiconductor electrode being composed of metal oxide nanoparticles to which dye particles are absorbed, and thereby, are called photoelectrochemical photovoltaic cells. At this time, the semiconductor electrode is composed of a transparent substrate, a metal oxide and a light absorbing layer comprising dyes.
  • the semiconductor electrode When excited by absorbing sunlight, the semiconductor electrode transfers electrons to a conduction band of the metal oxide, leading to the delivery of electricity. However, all the excited electrons are not transferred to the conduction band of the metal oxide, and a back electron transfer phenomenon where the excited electrons convert into the ground state is occurred. This phenomenon leads to the decrease in power conversion efficiency of photovoltaic cells.
  • Japanese Patent Application Publication No. 2004-171969 disclose a method of using a porous TiO 2 thin film electrode to which alkyl carbonic acid and photosensitizing dyes as a semiconductor electrode are absorbed.
  • the method fails to prevent the occurrence of back electron transfer at the semiconductor electrode, and thus, it cannot expect the significant improvement of power conversion efficiency.
  • Korean Patent Application Publication No. 2007-70797 describes a semiconductor electrode which is made of a metal oxide layer to which dye particles are absorbed, and a photovoltaic cell comprising the same.
  • the semiconductor electrode is characterized in that a carbon nanotube having a hydrophilic anchoring functional group is attached to the surface of the metal oxide layer. It is possible to expect the improvement of power conversion efficiency to a certain extent due to the increase of photocurrent density, but its electron mobility is poor, which results in restricting the improvement of power conversion efficiency.
  • Korean Patent No. 10-1047476 discloses a method of fabricating quantum dot sensitized photovoltaic cells using supercritical fluids or subcritical fluids.
  • this method is to absorb quantum dots to a conductive thin film substrate coated with a metal oxide film having mesopores or a nanostructure by using supercritical fluids or subcritical fluids, not to improve physical properties of quantum dots themselves.
  • UV light only makes up about 5% of the total sunlight, but in case of photovoltaic cells under commercializing or studying, they do not employ this type of UV in sunlight. However, if UV photovoltaic cells using UV light is manufactured and combined with UV photovoltaic cells using visible and near-infrared lights, their efficiency can be easily improved.
  • UV photovoltaic cells using metal oxide semiconductor quantum dots in the UV range (3.0 eV or higher). Recently, it was reported UV photovoltaic cells having a ZnO-MW CNT hybrid structure in which MW CNT (multiwall-carbon nanotube) having high electrical conductivity is chemically linked to ZnO (3.4 eV) quantum dots (Son, D. I.; You, C. H.; Kim, W. T.; Jung, J. H.; Kim, T. W. Appl. Phys. Lett. 2009, 94, 132103).
  • the photovoltaic cells having a ZnO-MW CNT structure show only 1% of power conversion efficiency (PCE) due to the a level of electron transition because of the following reasons: they use the MW CNT that has been known to have relatively low electrical conductivity among nanocarbons; and they have a small number of Zn-O-C chemical binding relating to electron transport from the ZnO quantum dots. Therefore, there is much room for improvement.
  • PCE power conversion efficiency
  • the present inventors have endeavored to study and found the fact that if a metal oxide semiconductor-nanocarbon consolidated core-shell quantum dots to have a structure in which the surface of metal oxide semiconductor quantum dots is covered with nanocarbons through chemical binding between the metal oxide semiconductor quantum dots and the nanocarbons having superior electrical conductivity, it is possible to increase the number of metal-oxygen-metal (nanocarbon) chemical binding within the metal oxide semiconductor.
  • the present invention provides metal oxide semiconductor-nanocarbon consolidated core-shell quantum dots having a structure in which a metal oxide semiconductor forming a light absorbing layer is a core, and said core is covered with nanocarbons in a shell shape.
  • the present invention provides a UV photovoltaic cell comprising a metal oxide semiconductor-nanocarbon consolidated core-shell quantum dot as a single active layer, wherein the quantum dot has a structure in which a metal oxide semiconductor forming a light absorbing layer is a core, and said core is covered with a nanocarbons in a shell shape.
  • the present invention provides a method of fabricating a UV photovoltaic cell, comprising:
  • a first conductive polymer layer by coating a hydrophilic polymer on a transparent electrode substrate;
  • metal oxide semiconductors are prepared by using various component-based metal oxides and nanocarbon is chemically linked thereto, it is easy to regulate their corresponding band gap, which makes possible to fabricate a variety of photovoltaic cell diode.
  • Fig. 1 is a schematic diagram of synthesizing the zinc oxide-graphene quantum dots prepared in Example 1.
  • Fig. 2a is a TEM (transmission electron microscope) photograph of nano-sized powder that is prepared by removing a zinc oxide core from the zinc oxide-graphene quantum dots prepared in Example 1 and extracting pure graphene therefrom.
  • Fig. 2b is X-ray diffraction patterns of the zinc oxide-graphene quantum dots and graphene prepared in Example 1, showing that zinc oxide quantum dot cores grown in the directions of (100), (002) and (101) are formed and graphene is formed in the directions of (002) and (100).
  • Fig. 3 is a photoluminescence spectrum of the zinc oxide semiconductor core-shell quantum dots that are chemically linked to graphene in the quantum dots prepared in Example 1.
  • Fig. 4 shows energy levels between layer forming materials of a polymer hybrid UV photovoltaic cell comprising the zinc oxide-graphene quantum dots prepared in Example 1.
  • Fig. 5 is a schematic diagram of a polymer hybrid UV photovoltaic cell comprising the zinc oxide-graphene quantum dots prepared in Example 1.
  • Fig. 6 is the result comparing a current density-voltage (J-V) characterisitic curve observed for the zinc oxide-graphene quantum dots prepared in Example 1 with that of a conventional zinc oxide quantum dots.
  • Fig. 7 is a schematic diagram of synthesizing the zinc oxide-fullerene quantum dots prepared in Example 2.
  • Fig. 8a is a TEM photograph of nano-sized powder that is prepared by removing a zinc oxide core from the zinc oxide-fullerene quantum dots prepared in Example 2 and extracting pure fullerene therefrom.
  • Fig. 8b is X-ray diffraction patterns of the zinc oxide-fullerene quantum dots and graphene prepared in Example 2, showing that zinc oxide quantum dot cores grown in the directions of (100), (002) and (101) are formed and fullerene is formed in the directions of (111), (220) and (311).
  • Fig. 9 is a schematic diagram of a polymer hybrid UV photovoltaic cell comprising the zinc oxide-fullerene quantum dots prepared in Example 2.
  • Fig. 10 is photoluminescence spectra of the zinc oxide quantum dots and the zinc oxide-fullerene core-shell quantum dots.
  • Fig. 11a is a current density-voltage curve in case of using a conventional zinc oxide quantum dots as an absorbing layer.
  • Fig. 11b is current density-voltage curve in case of using the zinc oxide-fullerene core-shell quantum dots prepared in Example 2 as an absorbing layer.
  • Fig. 11c shows energy levels between layer forming materials of a polymer hybrid UV photovoltaic cell comprising the zinc oxide-fullerene quantum dots prepared in Example 2.
  • the present invention is characterized by metal oxide semiconductor-nanocarbon consolidated core-shell quantum dots having a structure in which a metal oxide semiconductor forming a light absorbing layer is a core and said core is covered with nanocarbons in a shell shape.
  • graphene, graphene sheet, fullerene, SW CNT, MW CNT and the like can be used, and it is preferable to use graphene or fullerene.
  • graphene in a curved form with a curvature so as to be chemically linked along to the core surface of a metal oxide semiconductor.
  • C60, C70, C76 or C84 fullerene it is preferable to use.
  • the metal oxide semiconductor nanoparticle forming a core and nanocarbons forming a shell have a structure where they are linked through the chemical binding with oxygen atoms.
  • the present invention provides metal oxide semiconductor-nanocarbon consolidated core-shell quantum dots having a structure in which the surface of the metal oxide semiconductor is covered with nanocarbons.
  • the metal oxide semiconductor and nanocarbon are integrated through the chemical binding.
  • nanocarbons showing high electron mobility. More preferably, two-dimensional graphene having a electron mobility of 2 ⁇ 10 6 cm 2 /Vs which is higher than that of noble metals such as copper and silver, and fullerene (diameter: about 0.7 nm) having a zero-dimensional spherical shape among nanocarbons can be used.
  • the metal oxide semiconductor of the present invention has a structure in which the surface of the quantum dot core is covered with nanocarbons, and thereby, the core and shell are integrated through the chemical binding.
  • the core and shell are integrated through the chemical binding.
  • the quantum dots as prepared above have a diameter of 8-15 nm, and preferably 10 nm.
  • the present invention provides a UV photovoltaic cell comprising the thus prepared metal oxide semiconductor-nanocarbon consolidated core-shell quantum dots as a single active layer.
  • the method of fabricating a UV photovoltaic cell using the new type of quantum dots according to the present invention typically comprises:
  • a first conductive polymer layer by coating a hydrophilic polymer on a transparent electrode substrate;
  • the method of fabricating a UV photovoltaic cell can be exemplified as follows.
  • the step of preparing a quantum dot alcohol solution can be carried out, for example, by dispersing oxidized graphite in a solvent, and mixing with a precursor of a metal oxide semiconductor, to thereby prepare metal oxide semiconductor-nanocarbon consolidated quantum dot powder, followed by dissolving the same in alcohol such as ethanol.
  • the step of forming a first conductive polymer layer can be performed by depositing a coating of a hydrophilic polymer on a transparent electrode substrate such as a glass substrate and drying the same.
  • the hydrophilic polymer suitable for this step can be selected from the group consisting of polyacetylene (PAC), poly(p-phenylene vinylene) (PPV), polypyrrole (PPY), polyaniline (PANI), polythiophene (PT), and poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS).
  • PAC polyacetylene
  • PV poly(p-phenylene vinylene)
  • PY polypyrrole
  • PANI polyaniline
  • PT polythiophene
  • PEDOT:PSS poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate)
  • the step of forming a second conductive polymer layer can be conducted by spray-coating a hydrophobic polymer on the first conductive polymer layer and hardening the same.
  • the hydrophobic polymer suitable for this step can be selected from the group consisting of CBP (4,4’-Bis(N-carbazolyl)-1,1’-bipheny) 1,4-bis(diphenylamino) benzene, TPB (Tetra-N-phenylbenzidine), NPD (N,N’-di-[(1-naphthyl)-N,N’-diphenyl]-1,1’-biphenyl)-4,4’-diamine), and TPD (N,N’-bis(3-methylphenyl)-N,N’-diphenylbenzidine).
  • the HOMO (highest occupied molecular orbital) energy level of such a second conductive polymer layer is applied onto the first conductive polymer layer.
  • the substrate on which the second conductive polymer layer is formed is coated with the metal oxide semiconductor-nanocarbon consolidated quantum dot solution prepared above, to thereby form a single active layer.
  • a supplementary layer for reducing a work function is formed on the single active layer.
  • alkali compounds such as LiF and Cs 2 CO 3 can be used, and it is preferable to use cesium carbonate.
  • a conventional metal electrode layer is then formed on the supplementary layer.
  • Ag, Al and the like can be used as a metal electrode, and it is preferable to use a low-priced Al electrode.
  • the metal electrode layer is formed, the fabrication of a UV photovoltaic cell is completed.
  • the metal oxide semiconductor-nanocarbon consolidated core-shell quantum dot when used as a light absorbing layer of the UV photovoltaic cell, the rate of electron transfer between the conductive layer of the metal oxide semiconductor and nanocarbons is increased during UV absorption, and thus a power conversion efficiency (PCE) of the UV photovoltaic cell (which was merely 1%) can be increased at least up to 2.3-4%. Therefore, the method of the present invention can be used in fabricating a highly efficient UV photovoltaic cell.
  • PCE power conversion efficiency
  • the thus fabricated UV photovoltaic cell shows 2- to 4-fold higher power conversion efficiency as compared with that using conventional quantum dots being composed of only a metal oxide semiconductor.
  • Such improved properties of the UV photovoltaic cell result from the use of metal oxide semiconductor-nanocarbon consolidated quantum dots according to the present invention as a single active layer.
  • the pseudo-metal oxide semiconductor-nanocarbon core-shell shaped particles used as a light absorbing layer according to the present invention are covered with nanocarbon having very high electron mobility, they show a very high rate of electron transfer, and thereby, it can be efficiently used in fabricating a photovoltaic cell with superior light properties. Further, in case of using the metal oxide semiconductor in which a band gap corresponds to the UV region or applying the same to a photovoltaic cell which is operated in the visible light region, it is possible to fabricate a highly efficient photovoltaic cell capable of absorbing the sunlight both in the UV and visible regions.
  • Fig. 1 is a schematic diagram of synthesizing the zinc oxide-graphene core-shell shaped quantum dots obtained above.
  • the zinc oxide-graphene quantum dots obtained above were synthesized as core-shell shaped nanoparticles.
  • the quantum dot nanoparticles and an X-ray diffraction pattern thereof were analyzed by using a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • the zinc oxide-graphene core-shell shaped quantum dots had an average diameter of about 10 nm.
  • Fig. 2b of the X-ray diffraction pattern in the case of the formed ZnO core, crystal faces of (100), (002) and (101) were observed, suggesting it is a polycrystalline ZnO nanoparticle.
  • peaks of (002) and (100) with significantly higher full width at half maximum (HWHM) were observed, which demonstrates that the ZnO nanoparticle was coved with the single layer of graphene.
  • Fig. 3 is a photoluminescence spectrum of the zinc oxide-graphene core-shell shaped quantum dots prepared above.
  • a Ti:Sapphire laser (wavelength: 365 nm) was used as an excitation light source, and peaks were observed at 379 nm (3.29eV), 406 nm (3.05 eV), and 432 nm (2.86 eV), respectively.
  • the peak at 379 nm was light emitting representing transition between a conduction band (CB) and a valence band (VB) of ZnO.
  • CB conduction band
  • VB valence band
  • the graphene in a semimetal state without any band gap due to the loading of 0.8% strain was changed into a semiconductor with the band gap of 250 meV which corresponded to the energy range of midinfrared.
  • the Fermi energy was 4.4 eV, and based on the Fermi energy, it divides into LUMO (lowest unoccupied molecular orbital) as a ⁇ *-antibinding orbital and HOMO (highly occupied molecular orbital) as a ⁇ -binding orbital.
  • an ITO (Indium Tin Oxide) thin film was deposited on the glass substrate, followed by forming an ITO electron pattern through an etching process.
  • the glass substrate was coated with poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PED OT:PSS) by using a spincoater at a rate of 4000 rpm for 40 seconds, to thereby obtain a first conductive polymer layer.
  • PED OT:PSS poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate)
  • the conductive polymer was hydrophilic, it was coated with a 0.5 ⁇ m hydrophilic filter so as to be uniformly deposited.
  • the glass substrate was dried at 110 o C for 10 minutes.
  • the glass substrate was coated with poly-(retra-N-phenylbenzidine) (Poly-TPD) by using a spincoater at a rate of 4000 rpm for 40 seconds, to thereby form a second conductive polymer layer.
  • Poly-TPD poly-(retra-N-phenylbenzidine)
  • the glass substrate was dried at 110 o C for about 30 minutes.
  • the thus prepared zinc oxide-graphene quantum dot powders (10 ml) were dissolved in ethanol at a proper ratio and washed by using an ultrasonic cleaner for 10 minutes.
  • the thus prepared zinc oxide-graphene quantum dot solution was deposited on the hardened second conductive polymer (poly-TPD) layer through spin coating by using a spincoater at a rate of 2000-6000 rpm for about 20-40 seconds.
  • the substrate was subjected to soft baking at 90 o C for about 10-30 minutes.
  • cesium carbonate (CsCO 3 ) powders 50 mg were dispersed in 10 ml of 2-ethoxyethanol, to thereby prepared a cesium carbonate solution.
  • the cesium carbonate solution was then deposited on the zinc oxide-graphene quantum dot layer through spin coating at a rate of 5000 rpm for about 30 seconds, followed by soft baking at 90 o C for about 10-30 minutes.
  • an Al electrode was deposited on each of the first conductive polymer (PEDOT:PSS) layer, second conductive polymer (poly-TPD) layer, zinc oxide-graphene quantum dot layer, and supplementary layer (cesium carbonate layer) by using a thermal evaporator in a thickness of 150 nm, to thereby fabricate a UV photovoltaic cell.
  • the thus fabricated UV photovoltaic cell has a band structure having properties as illustrated in Fig. 4
  • Fig. 5(a) shows current-voltage graphs of photovoltaic cells using conventional zinc oxide nanoparticels that are not linked to graphene.
  • UV at a wavelength of 365 nm electric power: 6 W
  • a light source 6 W
  • it showed an open circuit voltage (Voc) of 0.91 V
  • a saturation current density (Jsc) of 178.3 ⁇ A/cm 2
  • FF fill factor
  • PCE, ⁇ power conversion efficiency
  • Fig. 6 is a schematic diagram of synthesizing the zinc oxide-fullerene core-shell shaped quantum dots obtained above.
  • the zinc oxide-fullerene quantum dots obtained above were synthesized as core-shell shaped nanoparticles.
  • the quantum dot nanoparticles and an X-ray diffraction pattern thereof were analyzed by using a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • the zinc oxide-fullerene core-shell shaped quantum dot has a structure in which ZnO was grown as a shape of stable hexagon, and each length of the longest and shortest axes was 17 nm and 21 nm. Further, as shown in Fig.
  • the photovoltaic cell was fabricated according to the same method as described in Example 1 except that the zinc oxide-C60(fullerene) quantum dot power (10ml) was used as a quantum dot.
  • the thus fabricated UV photovoltaic cell has a band structure having properties as illustrated in Fig. 7.
  • Fig. 9 shows photoluminescence spectra of the zinc oxide quantum dots and zinc oxide-fullerene core-shell quantum dots.
  • the conventional zinc oxide quantum dots when exposed to UV at a wavelength of 380 nm around, a band-to-band emission curve (red) generated from the transition of excited electrons from a conduction band to a valence band.
  • the ZnO-fullerene(C60) core-shell quantum dots prepared in Example 2 it showed very low luminescence (black) intensity (it looks as if luminescence intensity was completely disappeared). In terms of an area ratio, there was about 99.8% of a decrease.
  • Fig. 10a is J-V curves of photovoltaic cells using conventional quantum dots fabricated at a rate of 2000, 3000 and 4000 rpm, respectively.
  • UV at a wavelength of 365 nm electric power: 6 W
  • a device fabricated at a rate of 4000 rpm it showed an open circuit voltage (Voc) of 0.91 V, a saturation current density (Jsc) of 178.3 ⁇ A/cm 2 , a fill factor (FF) of 0.23, and a power conversion efficiency (PCE, ⁇ ) of 1.86% at maximum.
  • Voc open circuit voltage
  • Jsc saturation current density
  • FF fill factor
  • PCE, ⁇ power conversion efficiency
  • Fig. 10b is J-V curves of photovoltaic cells using the zinc oxide-fullerene(C60) consolidated quantum dots as an absorbing layer.
  • Voc 0.64V
  • Jsc 295.2 ⁇ A/cm 2
  • FF 0.32
  • PCE ⁇ 3.02% at maximum.

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Abstract

Disclosed is a method of preparing metal oxide semiconductor-nanocarbon core-shell consolidated quantum dots by chemically linking nanocarbon having superior electrical properties to a metal oxide semiconductor and a method of fabricating a UV photovoltaic cell device by using the same. In case of applying such a new type of quantum dots to a photovoltaic cell as a light absorbing layer, the rate of electron flow is accelerated and that of hole flow is inhibited. Thus, the methods of the present invention can prepare the core-shell consolidated quantum dots having excellent power conversion efficiency and the photovoltaic cell using the same.

Description

METAL OXIDE SEMICONDUCTOR-NANOCARBON CONSOLIDATED CORE-SHELL QUANTUM DOTS AND ULTRAVIOLET PHOTOVOLTAIC CELL USING IT AND FABRICATION PROCESS THEREOF
The present invention relates to a method of preparing metal oxide semiconductor-nanocarbon core-shell consolidated quantum dots by chemically linking nanocarbon superior electrical properties to a metal oxide semiconductor and a method of fabricating a UV photovoltaic cell device by using the same. In case of applying such a new type of quantum dots to a photovoltaic cell as a light absorbing layer, the rate of electron flow is accelerated and that of hole flow is inhibited. Thus, the methods of the present invention can prepare the core-shell consolidated quantum dots having excellent power conversion efficiency and the photovoltaic cell using the same.
Photovoltaic cells convert sunlight into electricity, and since their energy source is in infinite supply and eco-friendly, their importance and attention thereon have been increased. Silicon photovoltaic cells have been conventionally used, but high production costs are required and there is a limitation to improve conversion efficiency of solar energy into electricity. Thus, photovoltaic cells using economically favorable organic materials, in particular dye sensitized photovoltaic cells come into the spotlight. Dye sensitized photovoltaic cells use a semiconductor electrode being composed of metal oxide nanoparticles to which dye particles are absorbed, and thereby, are called photoelectrochemical photovoltaic cells. At this time, the semiconductor electrode is composed of a transparent substrate, a metal oxide and a light absorbing layer comprising dyes. When excited by absorbing sunlight, the semiconductor electrode transfers electrons to a conduction band of the metal oxide, leading to the delivery of electricity. However, all the excited electrons are not transferred to the conduction band of the metal oxide, and a back electron transfer phenomenon where the excited electrons convert into the ground state is occurred. This phenomenon leads to the decrease in power conversion efficiency of photovoltaic cells.
Therefore, in order to improve the power conversion efficiency of photovoltaic cells by enhancing electrical conductivity of the semiconductor electrode, Japanese Patent Application Publication No. 2004-171969 disclose a method of using a porous TiO2 thin film electrode to which alkyl carbonic acid and photosensitizing dyes as a semiconductor electrode are absorbed. However, the method fails to prevent the occurrence of back electron transfer at the semiconductor electrode, and thus, it cannot expect the significant improvement of power conversion efficiency.
Further, Korean Patent Application Publication No. 2007-70797 describes a semiconductor electrode which is made of a metal oxide layer to which dye particles are absorbed, and a photovoltaic cell comprising the same. The semiconductor electrode is characterized in that a carbon nanotube having a hydrophilic anchoring functional group is attached to the surface of the metal oxide layer. It is possible to expect the improvement of power conversion efficiency to a certain extent due to the increase of photocurrent density, but its electron mobility is poor, which results in restricting the improvement of power conversion efficiency.
In order to overcome these problems, it has been developed sensitized photovoltaic cells using inorganic quantum dots that employ light having a wide wavelength range and are stable to sunlight (Mora-Sero, I.; Gimenez, S.; Fabregat-Santiago, F.; Gomez, R.; Shen, Q.; Toyoda, T.; Bisquert, J. Acc. Chem. Res. 2009, 42, 1848-1857).
In addition, Korean Patent No. 10-1047476 discloses a method of fabricating quantum dot sensitized photovoltaic cells using supercritical fluids or subcritical fluids. However, this method is to absorb quantum dots to a conductive thin film substrate coated with a metal oxide film having mesopores or a nanostructure by using supercritical fluids or subcritical fluids, not to improve physical properties of quantum dots themselves.
Therefore, there is a need to develop a method of improving power conversion efficiency of quantum dots by increasing electron mobility.
Generally, it has been well-known in the art that nanocarbons such as graphene, carbon nanotubes (CNT) and fullerene have excellent electron mobility. Among them, multi-wall carbon nanotubes (MW CNT) having semi-conductivity show lower electron mobility than metallic single wall carbon nanotubes (SW CNT). Ultraviolet (UV) light only makes up about 5% of the total sunlight, but in case of photovoltaic cells under commercializing or studying, they do not employ this type of UV in sunlight. However, if UV photovoltaic cells using UV light is manufactured and combined with UV photovoltaic cells using visible and near-infrared lights, their efficiency can be easily improved.
However, there are a few studies on UV photovoltaic cells using metal oxide semiconductor quantum dots in the UV range (3.0 eV or higher). Recently, it was reported UV photovoltaic cells having a ZnO-MW CNT hybrid structure in which MW CNT (multiwall-carbon nanotube) having high electrical conductivity is chemically linked to ZnO (3.4 eV) quantum dots (Son, D. I.; You, C. H.; Kim, W. T.; Jung, J. H.; Kim, T. W. Appl. Phys. Lett. 2009, 94, 132103). In this case, when the ZnO quantum dots absorb UV, electron-hole pairs are generated, excited electrons are rapidly transferred from a ZnO conduction band range to a carbon nanotube layer being chemically linked thereto, and then, the transferred electrons are captured at a cathode. On the other hand, in case of holes at a valence band, since the carbon nanotube layer acts as an energy barrier, their transfer to the cathode is hindered, and thereby, the holes transfer to an anode, leading to the increase in photovoltaic effect. In this case, such a photovoltaic effect varies depending on the number of nanocarbons being chemically linked to ZnO quantum dots and conductivity thereof.
Therefore, the photovoltaic cells having a ZnO-MW CNT structure show only 1% of power conversion efficiency (PCE) due to the a level of electron transition because of the following reasons: they use the MW CNT that has been known to have relatively low electrical conductivity among nanocarbons; and they have a small number of Zn-O-C chemical binding relating to electron transport from the ZnO quantum dots. Therefore, there is much room for improvement.

In order to overcome these problems, the present inventors have endeavored to study and found the fact that if a metal oxide semiconductor-nanocarbon consolidated core-shell quantum dots to have a structure in which the surface of metal oxide semiconductor quantum dots is covered with nanocarbons through chemical binding between the metal oxide semiconductor quantum dots and the nanocarbons having superior electrical conductivity, it is possible to increase the number of metal-oxygen-metal (nanocarbon) chemical binding within the metal oxide semiconductor.
It is an object of the present invention to provide metal oxide semiconductor-nanocarbon consolidated core-shell quantum dots.
It is another object of the present invention to provide a photovoltaic cell using the metal oxide semiconductor-nanocarbon consolidated core-shell quantum dots.
It is still another object of the present invention to provide a quantum dot sensitized photovoltaic cell with enhanced power conversion efficiency through the improvement of physical properties of quantum dots.
In accordance with the aspect thereof, the present invention provides metal oxide semiconductor-nanocarbon consolidated core-shell quantum dots having a structure in which a metal oxide semiconductor forming a light absorbing layer is a core, and said core is covered with nanocarbons in a shell shape.
Further, the present invention provides a UV photovoltaic cell comprising a metal oxide semiconductor-nanocarbon consolidated core-shell quantum dot as a single active layer, wherein the quantum dot has a structure in which a metal oxide semiconductor forming a light absorbing layer is a core, and said core is covered with a nanocarbons in a shell shape.
In addition, the present invention provides a method of fabricating a UV photovoltaic cell, comprising:
preparing a solution by adding the metal oxide semiconductor-nanocarbon consolidated quantum dots to alcohol;
forming a first conductive polymer layer by coating a hydrophilic polymer on a transparent electrode substrate;
forming a second conductive polymer layer by coating a hydrophobic polymer on the first conductive polymer layer;
forming a single active layer by coating the alcohol solution of the metal oxide semiconductor-nanocarbon consolidated quantum dots on the second conductive polymer layer;
coating the single active layer with cesium carbonate to form a supplementary layer; and
forming a metal electrode layer.
In case of a photovoltaic cell using the metal oxide semiconductor-nanocarbon core-shell quantum dots according to the present invention, it shows improved electron mobility due to the use of nanocarbons, leading to the significant increase in power conversion efficiency of the photovoltaic cell.
Further, if metal oxide semiconductors are prepared by using various component-based metal oxides and nanocarbon is chemically linked thereto, it is easy to regulate their corresponding band gap, which makes possible to fabricate a variety of photovoltaic cell diode.
Fig. 1 is a schematic diagram of synthesizing the zinc oxide-graphene quantum dots prepared in Example 1.
Fig. 2a is a TEM (transmission electron microscope) photograph of nano-sized powder that is prepared by removing a zinc oxide core from the zinc oxide-graphene quantum dots prepared in Example 1 and extracting pure graphene therefrom.
Fig. 2b is X-ray diffraction patterns of the zinc oxide-graphene quantum dots and graphene prepared in Example 1, showing that zinc oxide quantum dot cores grown in the directions of (100), (002) and (101) are formed and graphene is formed in the directions of (002) and (100).
Fig. 3 is a photoluminescence spectrum of the zinc oxide semiconductor core-shell quantum dots that are chemically linked to graphene in the quantum dots prepared in Example 1.
Fig. 4 shows energy levels between layer forming materials of a polymer hybrid UV photovoltaic cell comprising the zinc oxide-graphene quantum dots prepared in Example 1.
Fig. 5 is a schematic diagram of a polymer hybrid UV photovoltaic cell comprising the zinc oxide-graphene quantum dots prepared in Example 1.
Fig. 6 is the result comparing a current density-voltage (J-V) characterisitic curve observed for the zinc oxide-graphene quantum dots prepared in Example 1 with that of a conventional zinc oxide quantum dots.
Fig. 7 is a schematic diagram of synthesizing the zinc oxide-fullerene quantum dots prepared in Example 2.
Fig. 8a is a TEM photograph of nano-sized powder that is prepared by removing a zinc oxide core from the zinc oxide-fullerene quantum dots prepared in Example 2 and extracting pure fullerene therefrom.
Fig. 8b is X-ray diffraction patterns of the zinc oxide-fullerene quantum dots and graphene prepared in Example 2, showing that zinc oxide quantum dot cores grown in the directions of (100), (002) and (101) are formed and fullerene is formed in the directions of (111), (220) and (311).
Fig. 9 is a schematic diagram of a polymer hybrid UV photovoltaic cell comprising the zinc oxide-fullerene quantum dots prepared in Example 2.
Fig. 10 is photoluminescence spectra of the zinc oxide quantum dots and the zinc oxide-fullerene core-shell quantum dots.
Fig. 11a is a current density-voltage curve in case of using a conventional zinc oxide quantum dots as an absorbing layer.
Fig. 11b is current density-voltage curve in case of using the zinc oxide-fullerene core-shell quantum dots prepared in Example 2 as an absorbing layer.
Fig. 11c shows energy levels between layer forming materials of a polymer hybrid UV photovoltaic cell comprising the zinc oxide-fullerene quantum dots prepared in Example 2.
Hereinafter, the present invention will be described in more detail.
In accordance with an aspect, the present invention is characterized by metal oxide semiconductor-nanocarbon consolidated core-shell quantum dots having a structure in which a metal oxide semiconductor forming a light absorbing layer is a core and said core is covered with nanocarbons in a shell shape.
In case of the metal oxide semiconductor forming a core in the present invention, metal oxides whose light band gap capable of absorbing UV is 3.0 eV or higher can be used, and examples thereof may include TiO2, Nb-TiO2, Sb-TiO2, SnO2, ZnO, In2O3, CuO, MgZnO, MgO, In1-x(SnO2)x (0<x<0.15, ITO), Ga2O3 and BeO, F-SnO2, and preferably zinc oxide (ZnO).
As the nanocarbon forming a shell in the form of covering the metal oxide semiconductor, graphene, graphene sheet, fullerene, SW CNT, MW CNT and the like can be used, and it is preferable to use graphene or fullerene. At this time, it is preferable to use graphene in a curved form with a curvature so as to be chemically linked along to the core surface of a metal oxide semiconductor. Further, it is preferable to use C60, C70, C76 or C84 fullerene.
According to the present invention, the metal oxide semiconductor nanoparticle forming a core and nanocarbons forming a shell have a structure where they are linked through the chemical binding with oxygen atoms.
In order to form a structure for maximizing electron mobility by using a conventionally used metal oxide semiconductor as a core of a quantum dot and efficiently linking nanocarbons (for example, graphene or fullerene) with good electroconductivity thereto, the present invention provides metal oxide semiconductor-nanocarbon consolidated core-shell quantum dots having a structure in which the surface of the metal oxide semiconductor is covered with nanocarbons. Here, the metal oxide semiconductor and nanocarbon are integrated through the chemical binding. In this case, it is preferable to have a structure suitable to enhance the number of chemical binding between metal-oxygen-carbon (nanocarbon) within the metal oxide semiconductor.
In a preferred embodiment of the present invention, it is preferable to use ZnO as a metal oxide semiconductor, and to use carbon nanotubes having a two-dimensional, monolayer shape rather than carbon nanotubes having a linear structure as a nanocarbon. In order to achieve the purpose of the present invention, it is preferable to use nanocarbons showing high electron mobility. More preferably, two-dimensional graphene having a electron mobility of 2×106 cm2/Vs which is higher than that of noble metals such as copper and silver, and fullerene (diameter: about 0.7 nm) having a zero-dimensional spherical shape among nanocarbons can be used. As such, the metal oxide semiconductor of the present invention has a structure in which the surface of the quantum dot core is covered with nanocarbons, and thereby, the core and shell are integrated through the chemical binding. Here, as a preferred example, it is possible to prepare a ZnO-C60(fullerene) core-shell structure capable of maximizing the number of Zn-O-C chemical binding.
The quantum dots as prepared above have a diameter of 8-15 nm, and preferably 10 nm.
Meanwhile, the present invention provides a UV photovoltaic cell comprising the thus prepared metal oxide semiconductor-nanocarbon consolidated core-shell quantum dots as a single active layer.
The method of fabricating a UV photovoltaic cell using the new type of quantum dots according to the present invention typically comprises:
preparing a solution by adding the metal oxide semiconductor-nanocarbon consolidated quantum dots to alcohol;
forming a first conductive polymer layer by coating a hydrophilic polymer on a transparent electrode substrate;
forming a second conductive polymer layer by coating a hydrophobic polymer on the first conductive polymer layer;
forming a single active layer by coating the alcohol solution of the metal oxide semiconductor-nanocarbon consolidated quantum dots on the second conductive polymer layer;
coating the single active layer with cesium carbonate to form a supplementary layer; and
forming a metal electrode layer.
In a preferred embodiment of the present invention, the method of fabricating a UV photovoltaic cell can be exemplified as follows.
The step of preparing a quantum dot alcohol solution can be carried out, for example, by dispersing oxidized graphite in a solvent, and mixing with a precursor of a metal oxide semiconductor, to thereby prepare metal oxide semiconductor-nanocarbon consolidated quantum dot powder, followed by dissolving the same in alcohol such as ethanol.
The step of forming a first conductive polymer layer can be performed by depositing a coating of a hydrophilic polymer on a transparent electrode substrate such as a glass substrate and drying the same. The hydrophilic polymer suitable for this step can be selected from the group consisting of polyacetylene (PAC), poly(p-phenylene vinylene) (PPV), polypyrrole (PPY), polyaniline (PANI), polythiophene (PT), and poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS). Such a first conductive polymer layer is applied to the substrate so as to lower an energy barrier between the transparent electrode and the hydrophilic polymer and to increase mobility of holes in which a UV absorbing layer is generated.
Next, the step of forming a second conductive polymer layer can be conducted by spray-coating a hydrophobic polymer on the first conductive polymer layer and hardening the same. The hydrophobic polymer suitable for this step can be selected from the group consisting of CBP (4,4’-Bis(N-carbazolyl)-1,1’-bipheny) 1,4-bis(diphenylamino) benzene, TPB (Tetra-N-phenylbenzidine), NPD (N,N’-di-[(1-naphthyl)-N,N’-diphenyl]-1,1’-biphenyl)-4,4’-diamine), and TPD (N,N’-bis(3-methylphenyl)-N,N’-diphenylbenzidine). In order to increase hole mobility generated due to the presence of holes between energy levels of the light absorbing layer and the first conductive polymer layer, the HOMO (highest occupied molecular orbital) energy level of such a second conductive polymer layer is applied onto the first conductive polymer layer.
The substrate on which the second conductive polymer layer is formed is coated with the metal oxide semiconductor-nanocarbon consolidated quantum dot solution prepared above, to thereby form a single active layer.
In order to facilitate rapid transition of electrons generated at the light absorbing layer, a supplementary layer for reducing a work function is formed on the single active layer. Here, as a material suitable for the supplementary layer, alkali compounds such as LiF and Cs2CO3 can be used, and it is preferable to use cesium carbonate.
A conventional metal electrode layer is then formed on the supplementary layer. At this time, Ag, Al and the like can be used as a metal electrode, and it is preferable to use a low-priced Al electrode. When the metal electrode layer is formed, the fabrication of a UV photovoltaic cell is completed.
As such, when the metal oxide semiconductor-nanocarbon consolidated core-shell quantum dot is used as a light absorbing layer of the UV photovoltaic cell, the rate of electron transfer between the conductive layer of the metal oxide semiconductor and nanocarbons is increased during UV absorption, and thus a power conversion efficiency (PCE) of the UV photovoltaic cell (which was merely 1%) can be increased at least up to 2.3-4%. Therefore, the method of the present invention can be used in fabricating a highly efficient UV photovoltaic cell.
The thus fabricated UV photovoltaic cell shows 2- to 4-fold higher power conversion efficiency as compared with that using conventional quantum dots being composed of only a metal oxide semiconductor. Such improved properties of the UV photovoltaic cell result from the use of metal oxide semiconductor-nanocarbon consolidated quantum dots according to the present invention as a single active layer.
In case of the photovoltaic cell using the metal oxide semiconductor-nanocarbon core-shell consolidated quantum dot according to the present invention, since the pseudo-metal oxide semiconductor-nanocarbon core-shell shaped particles used as a light absorbing layer according to the present invention are covered with nanocarbon having very high electron mobility, they show a very high rate of electron transfer, and thereby, it can be efficiently used in fabricating a photovoltaic cell with superior light properties. Further, in case of using the metal oxide semiconductor in which a band gap corresponds to the UV region or applying the same to a photovoltaic cell which is operated in the visible light region, it is possible to fabricate a highly efficient photovoltaic cell capable of absorbing the sunlight both in the UV and visible regions.
Further, it is possible to select a variety of multi-component metal oxide semiconductors such as mono-, di-, tri-, tetra-, penta- and hexa-component, and when nanocarbon is chemically linked thereto, it is easy to regulate their corresponding band gap, which makes possible to fabricate a luminescence device having a different wavelength.

The present invention is further illustrated by the following examples. However, it shall be understood that these examples are only used to specifically set forth the present invention, rather than being understood that they are used to limit the present invention in any form.

Example 1
(1) Fabrication of zinc oxide-graphene quantum dots
To 40 ml of N,N-dimethylforamide, 40 mg of oxidized graphite was added and dispersed for 10 minutes by means of a homogenizer. On the other hand, 0.93 g of zinc acetate dehydrate [Zn(COO)22H2O] was added to 200 ml of N,N-dimethylforamide and subjected to stirring. After 10 minutes, the solution in which the oxidized graphite was dispersed was mixed with the solution of zinc acetate dehydrate [Zn(CH3COOH)22H2O], and then the resulting solution was stirred at 95℃, 150 rpm for 5 hours. The initial color of the solution was black, but it turned transparent after 30 minutes. After 1 hour, the solution was changed into hazy, followed by gradually turning to white. After 5 hours, gray powders were generated within the transparent solution. These powders were washed with ethanol and then with distilled water, followed by moderate drying in a 55℃ oven. As a result, zinc oxide-graphene core-shell shaped quantum dot were synthesized.
Fig. 1 is a schematic diagram of synthesizing the zinc oxide-graphene core-shell shaped quantum dots obtained above.

(2) Core-shell structure and property analysis of zinc oxide-graphene quantum dots
The zinc oxide-graphene quantum dots obtained above were synthesized as core-shell shaped nanoparticles. In order to examine the structure of the zinc oxide-graphene quantum dots, the quantum dot nanoparticles and an X-ray diffraction pattern thereof were analyzed by using a transmission electron microscope (TEM). As shown in Fig. 2a, the zinc oxide-graphene core-shell shaped quantum dots had an average diameter of about 10 nm. Further, as shown in Fig. 2b of the X-ray diffraction pattern, in the case of the formed ZnO core, crystal faces of (100), (002) and (101) were observed, suggesting it is a polycrystalline ZnO nanoparticle. In the case of graphene, peaks of (002) and (100) with significantly higher full width at half maximum (HWHM) were observed, which demonstrates that the ZnO nanoparticle was coved with the single layer of graphene.
Fig. 3 is a photoluminescence spectrum of the zinc oxide-graphene core-shell shaped quantum dots prepared above. A Ti:Sapphire laser (wavelength: 365 nm) was used as an excitation light source, and peaks were observed at 379 nm (3.29eV), 406 nm (3.05 eV), and 432 nm (2.86 eV), respectively. The peak at 379 nm was light emitting representing transition between a conduction band (CB) and a valence band (VB) of ZnO. On the other hand, in the case of graphene covering the ZnO quantum dot core, the graphene in a semimetal state without any band gap due to the loading of 0.8% strain was changed into a semiconductor with the band gap of 250 meV which corresponded to the energy range of midinfrared. In the case of graphene, the Fermi energy was 4.4 eV, and based on the Fermi energy, it divides into LUMO (lowest unoccupied molecular orbital) as a π*-antibinding orbital and HOMO (highly occupied molecular orbital) as a π-binding orbital. However, in case of ZnO-graphene being composed of Zn-O-C binding, when a molecular orbital energy level is calculated from the structure in which one oxygen atom is coupled to seven benzene rings (on the assume that a hydrogen atom is linked to the edge of the benzene ring), new LUMO, LUMO+1, and LUMO+2 are formed near the LUMO of pure graphene. The contribution levels of s orbital and p orbital within the LUMO : LUMO+1 : LUMO+2 molecular orbitals are calculated as 7.5% : 83.0%, 0.00% : 95.1%, 9.0% : 85.7%. Therefore, when considering the transition of electrons to a valance band as an O2p orbital of ZnO, a selection rate, that is, the change in movement amount (l) should be maintained as follows: Δl=±1. Thus, in case of graphene being coupled to the oxygen atom, it can be explained as transition from the s orbital (l=0) within the newly formed LUMO, LUMO+2 molecular orbitals to ZnO O2p (l=1). That is, the transition to ZnO O2p in LUMO or that to ZnO O2p in LUMO+2 corresponds to 432 nm (2.86 eV) and 406 nm (3.05 eV), respectively.

(3) Fabrication of a photovoltaic cell using zinc oxide-graphene quantum dots
In order to form an electrode on a glass substrate, an ITO (Indium Tin Oxide) thin film was deposited on the glass substrate, followed by forming an ITO electron pattern through an etching process. After that, the glass substrate was coated with poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PED OT:PSS) by using a spincoater at a rate of 4000 rpm for 40 seconds, to thereby obtain a first conductive polymer layer. At this time, because the conductive polymer was hydrophilic, it was coated with a 0.5㎛ hydrophilic filter so as to be uniformly deposited. After the coating, the glass substrate was dried at 110oC for 10 minutes.
After the first conductive polymer (PEDOT) layer was formed, the glass substrate was coated with poly-(retra-N-phenylbenzidine) (Poly-TPD) by using a spincoater at a rate of 4000 rpm for 40 seconds, to thereby form a second conductive polymer layer. Here, because Poly-TPD was hydrophobic, it was uniformly sprayed on to the substrate by using a 0.2㎛ hydrophobic filter. After that, the glass substrate was dried at 110oC for about 30 minutes.
Next, the thus prepared zinc oxide-graphene quantum dot powders (10 ml) were dissolved in ethanol at a proper ratio and washed by using an ultrasonic cleaner for 10 minutes. The thus prepared zinc oxide-graphene quantum dot solution was deposited on the hardened second conductive polymer (poly-TPD) layer through spin coating by using a spincoater at a rate of 2000-6000 rpm for about 20-40 seconds. The substrate was subjected to soft baking at 90oC for about 10-30 minutes. After the coating of the zinc oxide-graphene quantum dots, cesium carbonate (CsCO3) powders (50 mg) were dispersed in 10 ml of 2-ethoxyethanol, to thereby prepared a cesium carbonate solution. The cesium carbonate solution was then deposited on the zinc oxide-graphene quantum dot layer through spin coating at a rate of 5000 rpm for about 30 seconds, followed by soft baking at 90oC for about 10-30 minutes. Next, an Al electrode was deposited on each of the first conductive polymer (PEDOT:PSS) layer, second conductive polymer (poly-TPD) layer, zinc oxide-graphene quantum dot layer, and supplementary layer (cesium carbonate layer) by using a thermal evaporator in a thickness of 150 nm, to thereby fabricate a UV photovoltaic cell.
The thus fabricated UV photovoltaic cell has a band structure having properties as illustrated in Fig. 4

Test Example 1
Fig. 5(a) shows current-voltage graphs of photovoltaic cells using conventional zinc oxide nanoparticels that are not linked to graphene. Here, UV at a wavelength of 365 nm (electric power: 6 W) was employed as a light source. In case of a device fabricated at a rate of 4000 rpm, it showed an open circuit voltage (Voc) of 0.91 V, a saturation current density (Jsc) of 178.3 μA/cm2, a fill factor (FF) of 0.23, and a power conversion efficiency (PCE, η) of 1.86% at maximum. In case of a device fabricated at a relatively low rate of 2000-3000 rpm or at a rate of 5000 rpm or higher, its power conversion efficiency (PCE, η) was decreased to a range of 1.21-1.23 %.
On the other hand, as shown in Fig. 5(b), in case of a photovoltaic cell fabricated by using the core-shell consolidated quantum dots of zinc oxide-graphene prepared in Example 1 as a light absorbing layer at a rate of 4000 rpm, it showed similar properties as follows: Voc=0.84V, Jsc=178.9 μA/cm2, FF=0.24 and PCE η=1.8%. However, in case of that fabricated at a rate of 5000 rpm, it showed properties of Voc=0.99V, Jsc=196.4 μA/cm2, FF=0.24 and PCE η=2.33%. It was found that in case of a power conversion efficiency, the device of the present invention showed improved properties by about 125% due to the enhancement of current density as compared with the conventional device fabricated by using only zinc oxide as an absorbing layer at a rate of 4000 rpm.

Example 2
(1) Fabrication of zinc oxide-fullerene quantum dots
To 40 ml of N,N-dimethylforamide, 40 mg of oxidized fullerene (C60) was added and dispersed for 10 minutes by means of a homogenizer. On the other hand, 0.93 g of zinc acetate dehydrate [Zn(CH3COOH)22H2O] was added to 200 ml of N,N-dimethylforamide and subjected to stirring. After 10 minutes, the solution in which the oxidized graphite was dispersed was mixed with the solution of zinc acetate dehydrate [Zn(COO)22H2O], and then the resulting solution was stirred at 95℃, 150 rpm for 5 hours. The initial color of the solution was black, but it turned transparent after 30 minutes. After 1 hour, the solution was changed into hazy, followed by gradually turning to white. After 5 hours, gray powders were generated within the transparent solution. These powders were washed with ethanol and then with distilled water, followed by moderate drying in a 55℃ oven. As a result, zinc oxide-fullerene core-shell shaped quantum dot were synthesized.
Fig. 6 is a schematic diagram of synthesizing the zinc oxide-fullerene core-shell shaped quantum dots obtained above.

(2) Core-shell structure and property analysis of zinc oxide-fullerene quantum dots
The zinc oxide-fullerene quantum dots obtained above were synthesized as core-shell shaped nanoparticles. In order to examine the structure of the zinc oxide-fullerene quantum dots, the quantum dot nanoparticles and an X-ray diffraction pattern thereof were analyzed by using a transmission electron microscope (TEM). As shown in Fig. 7a, the zinc oxide-fullerene core-shell shaped quantum dot has a structure in which ZnO was grown as a shape of stable hexagon, and each length of the longest and shortest axes was 17 nm and 21 nm. Further, as shown in Fig. 7b of the X-ray diffraction pattern, in case of the formed ZnO core, crystal faces of (100), (002), (101), (102) and (110) were observed, suggesting it is a polycrystalline ZnO nanoparticle. In case of fullerene, peaks of (111), (220) and (311) with significantly higher full width at half maximum (HWHM) and low peak strength were observed at 2θ=10.9o, 2θ=17.74o and 2θ=20.76o, which corresponds to JCPDS No. 44-0058.

(3) Fabrication of a photovoltaic cell using zinc oxide-C60(fullerene) quantum dots
The photovoltaic cell was fabricated according to the same method as described in Example 1 except that the zinc oxide-C60(fullerene) quantum dot power (10ml) was used as a quantum dot.
The thus fabricated UV photovoltaic cell has a band structure having properties as illustrated in Fig. 7.

Test Example 2
Fig. 9 shows photoluminescence spectra of the zinc oxide quantum dots and zinc oxide-fullerene core-shell quantum dots. In case of the conventional zinc oxide quantum dots, when exposed to UV at a wavelength of 380 nm around, a band-to-band emission curve (red) generated from the transition of excited electrons from a conduction band to a valence band. However, in case of the ZnO-fullerene(C60) core-shell quantum dots prepared in Example 2, it showed very low luminescence (black) intensity (it looks as if luminescence intensity was completely disappeared). In terms of an area ratio, there was about 99.8% of a decrease. As shown in a figure inserted in Fig. 10, besides luminescence due to the band-to-band transition around at 380 nm, various types of luminescence was observed at 400 nm or higher, which suggests that this luminescence was generated from the transition of some electrons, that were transferred from the conduction band of ZnO to the LUMO orbital of fullerene, to the valence band of ZnO, and most electrons did not participated in the luminescence process and were captured at the cathode.
Fig. 10a is J-V curves of photovoltaic cells using conventional quantum dots fabricated at a rate of 2000, 3000 and 4000 rpm, respectively. Here, UV at a wavelength of 365 nm (electric power: 6 W) was employed as a light source. In case of a device fabricated at a rate of 4000 rpm, it showed an open circuit voltage (Voc) of 0.91 V, a saturation current density (Jsc) of 178.3 μA/cm2, a fill factor (FF) of 0.23, and a power conversion efficiency (PCE, η) of 1.86% at maximum. In case of a device fabricated at a relatively low rate of 2000-3000 rpm or at a rate of 5000 rpm or higher, its power conversion efficiency (PCE, η) was decreased to a range of 1.21-1.23 %.
Meanwhile, Fig. 10b is J-V curves of photovoltaic cells using the zinc oxide-fullerene(C60) consolidated quantum dots as an absorbing layer. In case of a device fabricated at a rate of 3000 rpm, it showed Voc=0.64V, Jsc=295.2 μA/cm2, FF=0.32 and PCE η=3.02% at maximum.

From the results of Examples and Test Examples described above, it has been found that in case of the photovoltaic cell using the metal oxide semiconductor-nanocarbon core-shell consolidated quantum dots according to the present invention, since the pseudo-metal oxide semiconductor-nanocarbon core-shell shaped particles used as a light absorbing layer according to the present invention are covered with nanocarbon having very high electron mobility, they show a very high rate of electron transfer, and thereby, it can be efficiently used in fabricating a photovoltaic cell with superior light properties. Further, in case of using the metal oxide semiconductor in which a band gap corresponds to the UV region or applying the same to a photovoltaic cell which is operated in the visible region, it is possible to fabricate a highly efficient photovoltaic cell capable of absorbing sunlight both in the UV and visible regions.

Specific terms used in the present description are given only to describe specific embodiments and are not intended to limit the present invention. Singular forms used in the present description include plural forms unless they apparently represent opposite meanings. The meaning of “including” or “having” used in the present description is intended to embody specific properties, regions, integers, steps, operations, elements and/or components, but is not intended to exclude presence or addition of other properties, regions, integers, steps, operations, elements, components and/or groups.

Claims (9)

  1. A metal oxide semiconductor-nanocarbon consolidated core-shell quantum dot having a structure in which a metal oxide semiconductor forming a light absorbing layer is a core, and said core is covered with a nanocarbon in a shell shape.

  2. The metal oxide semiconductor-nanocarbon consolidated core-shell quantum dot according to Claim 1, wherein the metal oxide semiconductor is zinc oxide.

  3. The metal oxide semiconductor-nanocarbon consolidated core-shell quantum dot according to Claim 1, wherein the nanocarbon is graphene or fullerene.

  4. The metal oxide semiconductor-nanocarbon consolidated core-shell quantum dot according to Claim 3, wherein the graphene is in a curved shape having a curvature.

  5. The metal oxide semiconductor-nanocarbon consolidated core-shell quantum dot according to Claim 3, wherein the fullerene is C60, C70, C76 or C84 fullerene.

  6. The metal oxide semiconductor-nanocarbon consolidated core-shell quantum dot according to Claim 1, wherein the metal oxide semiconductor nanoparticle forming a core is chemically linked to the nanocarbon forming a shell through the chemical binding to oxygen atoms.

  7. The metal oxide semiconductor-nanocarbon consolidated core-shell quantum dot according to Claim 1, wherein the quantum dot has a size of 8-15 nm.

  8. An ultraviolet (UV) photovoltaic cell comprising the metal oxide semiconductor-nanocarbon consolidated core-shell quantum dot according to any one of Claims 1 to 7 as a single active layer.

  9. A method of fabricating a UV photovoltaic cell, comprising:
    preparing a solution by adding the metal oxide semiconductor-nanocarbon consolidated quantum dot according to any one of Claims 1 to 7 to alcohol;
    forming a first conductive polymer layer by coating a hydrophilic polymer on a transparent electrode substrate;
    forming a second conductive polymer layer by coating a hydrophobic polymer on the first conductive polymer layer;
    forming a single active layer by coating the alcohol solution of the metal oxide semiconductor-nanocarbon consolidated quantum dot on the second conductive polymer layer;
    coating the single active layer with cesium carbonate to form a supplementary layer; and
    forming a metal electrode layer.

PCT/KR2012/008095 2011-10-07 2012-10-05 Metal oxide semiconductor-nanocarbon consolidated core-shell quantum dots and ultraviolet photovoltaic cell using it and fabrication process thereof WO2013051895A2 (en)

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KR101637952B1 (en) * 2014-11-20 2016-07-08 한국과학기술연구원 Core-shell type quantum dots structured metal Cobalt oxide-nanocabon composite, cathode active material for lithium secondary battery using it and fabrication process thereof
KR101643895B1 (en) 2014-11-20 2016-08-01 한국과학기술연구원 A photocatalyst using quantum of semiconductor-carbon nanomaterials as core-shell composite structure and its manufacturing method
KR101663046B1 (en) * 2015-02-23 2016-10-14 이화여자대학교 산학협력단 Plasmonic core-shell nanostructure, preparing method of the same, dye-sensitized solar cell including the same
KR101633451B1 (en) 2015-06-22 2016-07-08 한국과학기술연구원 Tunable light emitting diode using core-shell structured metal oxide-fullerene quantum dots and its preparing method
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