WO2011162865A2 - A nucleation promotion layer formed on a substrate to enhance deposition of a transparent conductive layer - Google Patents

A nucleation promotion layer formed on a substrate to enhance deposition of a transparent conductive layer Download PDF

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Publication number
WO2011162865A2
WO2011162865A2 PCT/US2011/033449 US2011033449W WO2011162865A2 WO 2011162865 A2 WO2011162865 A2 WO 2011162865A2 US 2011033449 W US2011033449 W US 2011033449W WO 2011162865 A2 WO2011162865 A2 WO 2011162865A2
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WO
WIPO (PCT)
Prior art keywords
layer
transparent conductive
containing material
nucleation promotion
conductive layer
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PCT/US2011/033449
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French (fr)
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WO2011162865A3 (en
Inventor
Klaus Schuegraf
Yashraj K. Bhatnagar
Brendan Mccomb
Jianshe Tang
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Applied Materials, Inc.
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Publication of WO2011162865A2 publication Critical patent/WO2011162865A2/en
Publication of WO2011162865A3 publication Critical patent/WO2011162865A3/en

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Classifications

    • 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
    • H01L31/02—Details
    • H01L31/0224—Electrodes
    • H01L31/022466—Electrodes made of transparent conductive layers, e.g. TCO, ITO layers
    • H01L31/022483—Electrodes made of transparent conductive layers, e.g. TCO, ITO layers composed of zinc oxide [ZnO]
    • 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
    • H01L31/036—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 crystalline structure or particular orientation of the crystalline planes
    • H01L31/0392—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 crystalline structure or particular orientation of the crystalline planes including thin films deposited on metallic or insulating substrates ; characterised by specific substrate materials or substrate features or by the presence of intermediate layers, e.g. barrier layers, on the substrate
    • 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
    • H01L31/054—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means
    • H01L31/056—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means the light-reflecting means being of the back surface reflector [BSR] type
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/52—PV systems with concentrators

Definitions

  • the present invention relates to methods and apparatus for forming a nucleation promotion layer prior to forming a transparent conductive layer on a substrate, more specifically, for forming a nucleation promotion layer prior to forming a transparent conductive layer on a substrate suitable for use in solar cell applications.
  • PV devices or solar cells are devices which convert sunlight into direct current (DC) electrical power.
  • PV or solar cells typically have one or more p-n junctions. Each junction comprises two different regions within a semiconductor material where one side is denoted as the p-type region and the other as the n-type region.
  • the p-n junction of the PV cell When the p-n junction of the PV cell is exposed to sunlight (consisting of energy from photons), the sunlight is directly converted to electricity through the PV effect.
  • PV solar cells generate a specific amount of electric power and cells are tiled into modules sized to deliver a desired amount of system power. PV modules are created by connecting a number of PV solar cells and are then joined into panels with specific frames and connectors.
  • ⁇ - Si microcrystalline silicon film
  • a-Si amorphous silicon film
  • poly-Si polycrystalline silicon film
  • a transparent conductive layer or a transparent conductive oxide (TCO) layer is often used as a top surface electrode, often referred as back reflector, disposed on the top of the PV solar cells.
  • the transparent conductive layer is also used between the substrate and a photoelectric conversion unit.
  • the transparent conductive layer must have high optical transmittance in the visible or higher wavelength region to facilitate transmitting sunlight into the solar cells without adversely absorbing or reflecting light energy.
  • transparent conductive layer As the materials selected for fabricating the transparent conductive layer and the substrates are different, adhesion at the interface always needs to be improved to enhance the interface bonding between the transparent conductive layer and the substrate. Furthermore, a certain degree of texture or surface roughness of the transparent conductive layer is also desired to assist sunlight trapping in the layers by promoting light scattering. Transparent conductive layers are desired to have high light scattering abilities so as to scatter light to the adjacent photoabsorbing layers to increase electric conversion efficiency.
  • the method includes forming a nucleation promotion layer on a substrate, wherein the nucleation promotion layer comprises a layer of metallic particles, and forming a transparent conductive layer on the nucleation promotion layer.
  • a method of forming a transparent conductive layer includes forming a nucleation promotion layer on a substrate, wherein the nucleation promotion layer comprises a layer of metallic particles having a thickness less than 500 A, wherein the metallic particles have a diameter between about 20 nm and about 40 nm, and forming a transparent conductive layer on the nucleation promotion layer, wherein the transparent conductive layer is an aluminum doped zinc containing layer.
  • a film stack for a PV solar cell includes a layer of metallic particles having a thickness less than about 500 A formed on a substrate as a nucleation promotion layer, wherein the metallic particles are selected from aluminum containing material, indium containing material, or tin containing material, and a transparent conductive layer disposed on the nucleation promotion layer.
  • Figure 1 depicts a schematic cross-sectional view of one embodiment of an apparatus in accordance with the invention
  • Figure 2 depicts a process flow diagram for depositing a nucleation promotion layer and a transparent conductive layer formed on the nucleation promotion layer in accordance with one embodiment of the present invention
  • Figures 3A-3C depict cross sectional views of a silicon-based thin film PV solar cell at different manufacture stages in accordance with one embodiment of the present invention
  • Figure 4 depicts an exemplary cross sectional views of a single type PV solar cell in accordance with one embodiment of the present invention.
  • Figure 5 depicts exemplary cross sectional views of a tandem type PV solar cell in accordance with one embodiment of the present invention.
  • the present invention provides methods for forming a nucleation promotion layer (NPL) to enhance deposition of a transparent conductive layer subsequently formed thereon.
  • the nucleation promotion layer (NPL) provides a good nucleation surface that provides desired nucleation sites that assist the transparent conductive layer to adhere thereon with a good adhesion and/or bonding therebetween.
  • the combination of the nucleation promotion layer (NPL) and the transparent conductive layer provides high film transparency, high haze, high light scattering and trapping as well as a desired surface morphology to provide high electric conversion efficiency for PV solar cells.
  • FIG. 1 is a sectional view of one embodiment of an apparatus 100 that may be utilized to perform a particle spray process according to the present invention.
  • the particle spray process forms a nucleation promotion layer on a substrate that may be utilized to provide desired nucleation sites for layers subsequently formed thereon with good interface bonding/adhesion.
  • the apparatus 100 may be a solvent ejection system or an air dispense system.
  • the apparatus 100 includes a processing chamber 122 having a top 130, a bottom 126, and sidewalls 142 that define a process region 124 in the interior volume of the processing chamber 122.
  • a stage 104 is disposed on the bottom 126 of the processing chamber 122 to a substrate holder 102 that holds a substrate 160 during processing.
  • the stage 104 is configured to move the holder 102 along the X and Y axis. Thus, the stage 104 moves the substrate 160 positioned thereon along the X and Y axis.
  • a mechanical booster pump 116 and a rotary pump 118 are coupled to the processing region 124 through a port formed in the sidewall 142 of the processing chamber 122 to maintain vacuum pressure during processing.
  • a particle generator 110 is coupled to the processing chamber 122 through the top 130 of the processing chamber 122 by a carrier pipe 128.
  • the carrier pipe 128 extends through the processing chamber top 130 to the processing region 124.
  • An injection nozzle 106 is disposed at an end 134 of the carrier pipe 128 to facilitate injection of particles toward a surface 132 of the substrate 160.
  • Multiple nozzles may be utilized to assist dispensing particles toward the surface 132 as needed.
  • the nozzles may be coupled to the generator by a common or individual carrier pipes. It is contemplated that the nozzles may direct the particles to the substrate 160 at different angles.
  • the nozzles may be arranged such that the holder 102 may only be need to be moved along a single axis.
  • a filter 112 may be optionally disposed between the processing chamber 122 and the particle generator 110 to assist in controlling the particle size generated by the particle generator 110. Particles are generated from the raw material 108 and are delivered through the nozzle 106 into the processing region 124. The particles are utilized to form a nucleation promotion layer on the substrate 160.
  • the raw material 108 may be fine ceramic particles, fine metallic particles, fine dielectric material particles or fine composite particles.
  • the raw material 108 may be metallic particles having an average diameter less than 50 nm, such as between about 20 nm and about 40 nm, for example about 30 nm.
  • the raw material 108 may be fine metallic particles, such as aluminum containing material, indium containing material, tin containing material, zinc containing material, tantalum containing material, titanium containing material, oxides thereof, alloys thereof, combinations thereof, or the like, having an average diameter in nanometer level, for example less than 50 nm, such as 30 nm.
  • the raw material 108 includes nanometer size metallic particles, such as aluminum containing material, indium containing material, tin containing material, having an average diameter of less than 50 nm, such as less than 30 nm.
  • One or more gas cylinders 1 14 and mass flow controllers (MFC) 120 are sequentially coupled to the particle generator 110 through a delivery line 136.
  • One cylinder 114 and MFC 120 are shown in Figure 1 for clarity of description.
  • the gas cylinder 1 4 provides carrier gases having a pressure sufficiently high enough to eject a desired amount of raw material 108 to the processing chamber 122.
  • the high carrier gas pressure from the gas cylinder 114 forms a raw material jet stream 138 having entrained raw material 108 that is dispensed toward the substrate surface 132.
  • the raw material 108 in the jet stream 138 forms a nucleation promotion layer on the substrate surface 132.
  • the flow and/or velocity of the stream 138 may be controlled by the mass flow controller (MFC) 120, carrier gas pressure or by the shape and/or opening diameter of the nozzle 106 or other flow control device.
  • the carrier gas provided by the gas cylinder 114 may be at least one of nitrogen gas (N 2 ), hydrogen gas (H 2 ), oxygen gas (0 2 ), fluorine gas (F 2 ), and inert gas, such as Argon (Ar), helium (He), neon (Ne), among others.
  • raw material 108 for example, metallic particles having an average diameter less than 50 nm, are disposed in the particle generator 110 as the source of the nucleation promotion layer deposition.
  • the carrier gas from the gas cylinder 114 is supplied into the particle generator 110.
  • the pressure and ejection rate of the carrier gas from the gas cylinder 114 are controlled to provide sufficient kinetic energy and momentum to accelerate the particles of raw material 108 into the processing chamber 122. Additionally, the pressure and ejection rate of the raw material 108 provides sufficient kinetic energy and momentum to promote bonding adhesion of the particles of raw material 108 to the substrate surface 132 without adversely damaging the underlying bulk substrate material.
  • the pressure of the carrier gas may be maintained at between about 20 PSI and about 40 PSI and the ejection rate of the carrier gas may be controlled at between about 250 meters per second (m/s) and about 1750 meters per second (m/s).
  • the gas cylinder 114 installed in the apparatus 100 may be replaced with a liquid tank to serve as a solvent ejection system.
  • the liquid tank may have liquid disposed therein.
  • the liquid disposed in the liquid tank may serve as solvent to mix with the raw material 108 in the particle generator 110.
  • the particle solvent mixture is then ejected toward the substrate surface 132 in liquid form to spray raw material 108 along with the solvent onto the substrate surface 132.
  • a layer of particles or powders is then formed on the substrate surface 132.
  • Suitable solvent that may be used to carry raw material 108 are alcohols and the like
  • the ejected raw material 108 impacts the substrate surface 132 with sufficient energy to remove contaminants or impurities on the substrate surface 132, if present.
  • the collision between the raw material 108 and the substrate surface 132 actives the substrate surface 132 as a result of the mutual collision.
  • the particles of raw material 108 impinge the substrate surface 132 and bond strongly thereto, thereby coating and/or depositing the nucleation promotion layer (NPL) on the substrate surface 132.
  • NPL nucleation promotion layer
  • the substrate 160 positioned on the substrate holder 102 is maintained at a low temperature substantially similar to the adjacent environment to eliminate temperature variation during processing.
  • the substrate 160 is maintained at a room temperature substantially similar to the adjacent environment.
  • the substrate 160 is maintained at a temperature at between about 0 degrees Celsius and about 50 degrees Celsius, such as between about 10 degrees Celsius and about 40 degrees Celsius, for example, about 25 degrees Celsius.
  • the low and steady processing temperature of the substrate 160 prevents the substrate 160 from undergoing excessive temperature fluctuation, thermal shock and/or expansion during deposition, thereby minimizing the stress induced between the coating and the underlying surface.
  • the consistent low processing temperature prevents the microstructure and surface roughness of the substrate surface 132 from thermal damage, thereby providing a uniform and consistent substrate surface condition.
  • the delivery of the raw material 108 to the substrate 160 is maintained until a desired thickness of the nucleation protection layer is reached.
  • the nucleation protection layer has a thickness between about 200 A and between about 500 A.
  • apparatus 100 described in Figure 1 is only an exemplary apparatus.
  • Other types of tool that can spray, eject, coat, or dispense particles onto the substrate surface such as a coating tool or an air dispense system, may also be utilized to spray particles onto the substrate.
  • the substrate 160 having the nucleation promotion layer (NPL) deposited thereon may be transferred to a PVD chamber for deposition of a TCO layer.
  • NPL nucleation promotion layer
  • Figure 2 is a flow diagram of one embodiment of a particle spray process 200 that may be practiced in the apparatus 100 or other suitable processing chamber to form a nucleation promotion layer (NPL) and a transparent conductive layer on the nucleation promotion layer (NPL).
  • Figures 3A-3C are schematic cross- sectional views of a portion of the substrate 160 utilized to form thin film PV solar cell corresponding to various stages of the deposition process 200.
  • the deposition process 200 may be illustrated for forming a nucleation promotion layer and a transparent conductive layer in Figures 3A-3C for forming solar cell devices, the deposition process 200 may be beneficially utilized to form other structures.
  • the process 200 begins at step 202 by transferring (i.e., providing) the substrate 160, as shown in Figure 3A, to an apparatus, such as the apparatus 100 in Figure 1.
  • the substrate 160 may be thin sheet of metal, plastic, organic material, silicon, glass, quartz, or polymer, or other suitable material.
  • the substrate 160 may have a surface area greater than about 1 square meters, such as greater than about 2 square meters.
  • the substrate 160 may be configured to form thin film PV solar cell, or other types of solar cells, such as crystalline, microcrystalline or other type of silicon-based thin films as needed.
  • a particle spray process is performed to form a nucleation promotion layer 302 on the substrate 160, as shown in Figure 3B.
  • the nucleation promotion layer (NPL) 302 is a thin layer of particles formed on the substrate 160 prior to the deposition of a transparent conductive layer.
  • the nucleation promotion layer (NPL) 302 assists promoting the nucleation of the transparent conductive layer subsequently formed thereon to have specific crystal grain orientations what will yield a particular surface morphology with desired textures and roughness that increase light scattering capability.
  • the nucleation sites formed in the nucleation promotion layer 302 may also assist grains of the transparent conductive layer subsequently formed thereon to have good positions to adhere thereto, thereby enhancing and promoting interface adhesion and bonding.
  • the nucleation promotion layer (NPL) 302 is a thin layer fabricated by particles.
  • the particles may be metallic powders that have an average diameter at nanometer level. Suitable examples of the particles are aluminum containing material, indium containing material, tin containing material, zinc containing material, tantalum containing material, titanium containing material, oxides thereof, alloys thereof, combinations thereof, or the like.
  • the particles utilized to fabricate the nucleation promotion layer (NPL) 302 has an average diameter between about 20 nm and about 40 nm, such as about 30 nm.
  • the particles used to form the nucleation promotion layer 302 are aluminum containing material, indium containing material or tin containing material, having an average diameter of between about 20 nm and about 40 nm.
  • particles provided from the raw material 108 disposed in the apparatus 100 may be ejected to the substrate surface.
  • the particles may be carried by gas or by solvent or any other suitable carrying media as needed.
  • the raw material 108 is continuously ejected to the substrate surface 132 until a desired thickness of the nucleation promotion layer 302 is formed on the substrate 160.
  • the particle spray process may be performed between about 30 seconds and 60 seconds and the thickness of the nucleation promotion layer 302 is controlled at between about 200 A and about 500 A.
  • a heating process may be performed.
  • the heating process may be performed to enhance adhesion of the particles of the nucleation promotion layer 302 within the nucleation promotion layer 302 itself, and to promote adhesion of the nucleation promotion layer 302 to the substrate surface 132 with good adhesion. It is believed that the thermal energy provided in the heating process may assist slightly melting the outer surface of the particles so as to assist the particles to adhere on the substrate surface 132 with better adhesion. Furthermore, the heating process may also drive out the solvent used to carry the particles to the substrate surface.
  • the heating process may be a radiant heat process, a thermal process, a curing process, a plasma treatment process, or any suitable heating process.
  • the heating process may heat the substrate 160 from room temperature to a high temperature but lower than glass softening temperature, such as about 300 degrees Celsius. In one embodiment, the heating process may heat the substrate between about 200 degrees Celsius and about 400 degrees Celsius.
  • a reactive sputter process is performed to form a transparent conductive layer 304 on the nucleation promotion layer 302 on the substrate 160, as shown in Figure 3C.
  • the transparent conductive layer 304 formed thereon may nucleate on the nucleation sites and adhere thereon to gradually grow the transparent conductive layer 304 on the nucleation promotion layer 302.
  • the transparent conductive layer may be fabricated by tin containing material, zinc containing material, tantalum containing material, titanium containing material, or the like.
  • the transparent conductive layer 304 is an aluminum doped zinc oxide layer having an aluminum dopant concentration between about 0.1 percent by weight and about 10 percent by weight, such as about 0.5 percent by weight and about 5 percent by weight. In one embodiment, the transparent conductive layer 304 may have a thickness between about 1000 A and about 10000 A.
  • the transparent conductive layer 304 may be formed in a manner that has high film transparency so as to assist passing light into the subsequent to-be-formed solar cell junctions, which will be further described below with referenced to Figures 4-5. Furthermore, high film transparency formed in the transparent conductive layer 304 helps reduce light loss traveling from the transparent conductive layer 304 to the subsequent to-be-formed solar cell junctions so as to maintain high current conversion efficiency to the photoelectric conversion unit.
  • the process gas mixture may be varied to supply different gases at step 206 for different process requirements and needs.
  • the gas mixture supplied at step 206 may include reactive gas, non-reactive gas, inert gas, and the like, as described above.
  • non-reactive gas include, but not limited to, inert gas, such as Ar, He, Xe, and Kr, or other suitable gases.
  • reactive gas include, but not limited to, 0 2 , N 2 , N 2 0, N0 2 , H 2 , NH 3 , H 2 0, among others.
  • a nucleation promotion layer 302 as a nucleation promotion layer on the substrate may provide a good nucleation surface that provides good nucleation sites that allow the transparent conductive layer 304 to form thereon with desired film structure.
  • the film stack of the nucleation promotion layer 302 and the transparent conductive layer 304 improves the haze (i.e., light scattering of near infrared (NIR) wavelength light and interface bonding and adhesion.
  • the film stack of the nucleation promotion layer 302 and the transparent conductive layer 304 may be applicable to any types of glass substrates including commercially available low iron float glass and soda lime glass, thus enabling the use of cheaper and more readily obtainable types of glass substrates for photovoltaic device manufacturing.
  • the transparent conductive layer 304 formed on the nucleation promotion layer 302 may also be fabricated by other suitable process, such as CVD process, metal plating process, coating process, or any suitable techniques available in the field.
  • Figure 4 depicts an exemplary cross sectional view of a single junction PV solar cell 400 having the nucleation promotion layer 302 and the transparent conductive layer 304 formed thereon in accordance with one embodiment of the present invention.
  • the substrate 150 may have the nucleation promotion layer 302 and the transparent conductive layer 304 consecutively formed thereon, as shown in Figure 4.
  • a first photoelectric conversion junction cell 408 is formed on the transparent conductive layer 304 disposed on the substrate 150.
  • the first photoelectric junction cell 420 includes a p-type semiconductor layer 402, a n-type semiconductor layer 406, and an intrinsic type (i- type) semiconductor layer 404 sandwiched therebetween as a photoelectric conversion layer.
  • An optional dielectric layer (not shown) may be disposed between intrinsic type (i-type) semiconductor layer 404 and the n-type semiconductor layer 406 as needed.
  • the optional dielectric layer may be a silicon layer including amorphous or poly silicon layer, SiON, SiN, SiC, SiOC, silicon oxide (Si0 2 ) layer, doped silicon layer, or any suitable silicon containing layer.
  • the p-type and n-type semiconductor layers 402, 406 may be silicon based materials doped by an element selected either from Group III or V.
  • a Group III element doped silicon film is referred to as a p-type silicon film, while a Group V element doped silicon film is referred to as a n-type silicon film.
  • the n-type semiconductor layer 406 may be a phosphorus doped silicon film and the p-type semiconductor layer 402 may be a boron doped silicon film.
  • the doped silicon films 402, 406 include an amorphous silicon film (a-Si), a polycrystalline film (poly-Si), and a microcrystalline film (pc-Si) with a thickness between around 5 nm and about 50 nm.
  • the doped element in semiconductor layers 402, 406 may be selected to meet device requirements of the PV solar cells 400.
  • the n- type and p-type semiconductor layers 406, 402 may be deposited by a CVD process or other suitable deposition process.
  • the i-type semiconductor layer 404 is a non-doped type silicon based film.
  • the i-type semiconductor layer 404 may be deposited under process conditions controlled to provide film properties having improved photoelectric conversion efficiency.
  • the i-type semiconductor layer 404 may be fabricated from i-type polycrystalline silicon (poly-Si), i-type microcrystalline silicon film (pc-Si), amorphous silicon (a-Si), or hydrogenated amorphous silicon (a- Si).
  • the back reflector 424 is disposed on the first photoelectric conversion junction cell 408.
  • the back reflector 414 may be formed by a stacked film that includes a back transparent conductive layer 410 and a conductive layer 412.
  • the conductive layer 412 may be at least one of Ti, Cr, Al, Ag, Au, Cu, Pt, or their alloys.
  • the back transparent conductive layer 410 may be fabricated from a material similar to the transparent conductive layer 304 formed on the substrate 150.
  • the back transparent conductive layer 410 may be fabricated from a selected group consisting of tin oxide (Sn0 2 ), indium tin oxide (ITO), zinc oxide (ZnO), or combinations thereof.
  • the back transparent conductive layers 410 may be fabricated from a ZnO layer having a desired Al 2 0 3 dopant concentration formed in the ZnO layer.
  • the incident light 401 provided by the environment is supplied to the PV solar cell 400.
  • the light passes through the transparent conductive layer 304 to the photoelectric conversion junction cell 408 in the PV solar cell 400 to absorb the light energy and convert the light energy into electrical energy by operation of the p-i-n junctions formed in the photoelectric conversion junction cell 408, thereby generating electricity or energy.
  • FIG. 5 depicts an exemplary cross sectional view of a tandum junction PV solar cells 500 having the nucleation promotion layer 302 and the transparent conductive layer 304 formed thereon in accordance with one embodiment of the present invention.
  • a wavelength selector reflector (WSR) 502 and a second photoelectric conversion junction cell 422 may be formed on the first photoelectric conversion junction cell 408.
  • the WSR layer 502 disposed between the first p-i-n junction 408 and the second p-i-n junction 422 is generally configured to have certain desired film properties.
  • the WSR layer 502 actively serves as an intermediate reflector having a desired refractive index, or ranges of refractive indexes, to reflect light received from the light incident side of the solar cell 500.
  • the WSR layer 502 also serves as a junction layer that boosts the absorption of the short to mid wavelengths of light (e.g., 280nm to 800nm) in the first p-i-n junction 408 and improves short-circuit current, resulting in improved quantum and conversion efficiency.
  • the WSR layer 502 further has high film transmittance for mid to long wavelengths of light (e.g. , 500nm to 1 100nm) to facilitate the transmission of light to the layers formed in the junction 422.
  • the WSR layer 502 may be a microcrystalline silicon layer having n-type or p-type dopants disposed within the WSR layer 502.
  • the WSR layer 502 is an n-type crystalline silicon alloy having n-type dopants disposed within the WSR layer 502. Different dopants disposed within the WSR layer 502 may also influence optical and electrical properties, such as bandgap, crystalline fraction, conductivity, transparency, film refractive index, extinction coefficient, and the like.
  • one or more dopants may be doped into various regions of the WSR layer 502 to efficiently control and adjust the film bandgap, work function(s), conductivity, transparency and so on.
  • the WSR layer 502 is controlled to have a refractive index between about 1.4 and about 3, a bandgap of at least about 2 eV, and a conductivity greater than about 10 " 3 S/cm.
  • the second p-i-n junction 422 may comprise a p-type microcrystalline silicon layer 416, an intrinsic type microcrystalline silicon layer 418 formed over the p-type microcrystalline silicon layer 416, and an n-type amorphous silicon layer 420 formed over the intrinsic type microcrystalline silicon layer 418.
  • the structure of the second conversion junction cell 422 is similar to the first photoelectric conversion junction cell 420 to assist absorbing light with different spectrum and retain light in the junction cells for a longer time to improve conversion efficiency.
  • An optional dielectric layer may be disposed on top of the n-type semiconductor layer 420 as needed.
  • the optional dielectric layer may be a heavily doped n-type semiconductor layer.
  • the doped silicon films 416, 420 include an amorphous silicon film (a-Si), a polycrystalline film (poly-Si), and a microcrystalline film (pc-Si) with a thickness between around 5 nm and about 50 nm.
  • the doped element in semiconductor layers 416, 420 may be selected to meet device requirements of the PV solar cells 500.
  • the p-type and the n-type 416, 420 may be deposited by a CVD process or other suitable deposition process.
  • the i-type semiconductor layer 418 is a non-doped type silicon based film.
  • the i-type semiconductor layer 418 may be deposited under process conditions controlled to provide film properties having improved photoelectric conversion efficiency.
  • the i-type semiconductor layer 418 may be fabricated from i-type polycrystalline silicon (poly- Si), i-type microcrystalline silicon film (pc-Si), amorphous silicon (a-Si), or hydrogenated amorphous silicon (a-Si).
  • nucleation promotion layer to provide nucleated sites for a transparent conductive layer subsequently formed thereon with good interface bonding and adhesion.
  • the nucleation promotion layer formed by a thin layer of particles advantageously produces a nucleation surface that can assist a transparent conductive layer subsequently formed thereon to adhere on the nucleation surface.
  • the nucleation promotion layer along with the transparent conductive layer may have an interface with good interface bonding and adhesion so that the photoelectric conversion efficiency and device performance of the PV solar cell can be efficiently improved.

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Abstract

Methods for forming a nucleation promotion layer prior to formation of a transparent conductive layer suitable for use in PV cells are provided. In one embodiment, the method includes forming a seed layer on a substrate by materials sputtered from a first target disposed in a reactive sputter processing chamber, and forming a transparent conductive layer on the seed layer by materials sputtered from a second target disposed in the reactive sputter processing chamber, wherein the first and the second target are fabricated by a containing material having dopants formed therein and dopant concentration formed in the first target is higher than the dopant concentration formed in the second target.

Description

A NUCLEATION PROMOTION LAYER FORMED ON A
SUBSTRATE TO ENHANCE DEPOSITION OF A TRANSPARENT CONDUCTIVE LAYER
BACKGROUND OF THE DISCLOSURE
Field of the Invention
[0001] The present invention relates to methods and apparatus for forming a nucleation promotion layer prior to forming a transparent conductive layer on a substrate, more specifically, for forming a nucleation promotion layer prior to forming a transparent conductive layer on a substrate suitable for use in solar cell applications.
Description of the Background Art
[0002] Photovoltaic (PV) devices or solar cells are devices which convert sunlight into direct current (DC) electrical power. PV or solar cells typically have one or more p-n junctions. Each junction comprises two different regions within a semiconductor material where one side is denoted as the p-type region and the other as the n-type region. When the p-n junction of the PV cell is exposed to sunlight (consisting of energy from photons), the sunlight is directly converted to electricity through the PV effect. PV solar cells generate a specific amount of electric power and cells are tiled into modules sized to deliver a desired amount of system power. PV modules are created by connecting a number of PV solar cells and are then joined into panels with specific frames and connectors.
[0003] Several types of silicon films, including microcrystalline silicon film (μο- Si), amorphous silicon film (a-Si), polycrystalline silicon film (poly-Si) and the like, may be utilized to form PV devices. A transparent conductive layer or a transparent conductive oxide (TCO) layer is often used as a top surface electrode, often referred as back reflector, disposed on the top of the PV solar cells. Alternatively, the transparent conductive layer is also used between the substrate and a photoelectric conversion unit. The transparent conductive layer must have high optical transmittance in the visible or higher wavelength region to facilitate transmitting sunlight into the solar cells without adversely absorbing or reflecting light energy. However, as the materials selected for fabricating the transparent conductive layer and the substrates are different, adhesion at the interface always needs to be improved to enhance the interface bonding between the transparent conductive layer and the substrate. Furthermore, a certain degree of texture or surface roughness of the transparent conductive layer is also desired to assist sunlight trapping in the layers by promoting light scattering. Transparent conductive layers are desired to have high light scattering abilities so as to scatter light to the adjacent photoabsorbing layers to increase electric conversion efficiency.
[0004] Therefore, there is a need for an improved process of forming a solar cell that has strong layer adhesion/bonding between the substrate and the transparent conductive layer and high overall cell performance.
SUMMARY OF THE INVENTION
[0005] Methods for forming a nucleation promotion layer prior to formation of a transparent conductive layer suitable for use in PV cells are provided in the present invention. In one embodiment, the method includes forming a nucleation promotion layer on a substrate, wherein the nucleation promotion layer comprises a layer of metallic particles, and forming a transparent conductive layer on the nucleation promotion layer.
[0006] In another embodiment, a method of forming a transparent conductive layer includes forming a nucleation promotion layer on a substrate, wherein the nucleation promotion layer comprises a layer of metallic particles having a thickness less than 500 A, wherein the metallic particles have a diameter between about 20 nm and about 40 nm, and forming a transparent conductive layer on the nucleation promotion layer, wherein the transparent conductive layer is an aluminum doped zinc containing layer.
[0007] In yet another embodiment, a film stack for a PV solar cell includes a layer of metallic particles having a thickness less than about 500 A formed on a substrate as a nucleation promotion layer, wherein the metallic particles are selected from aluminum containing material, indium containing material, or tin containing material, and a transparent conductive layer disposed on the nucleation promotion layer. BRIEF DESCRIPTION OF THE DRAWINGS
[0008] So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings.
[0009] Figure 1 depicts a schematic cross-sectional view of one embodiment of an apparatus in accordance with the invention;
[0010] Figure 2 depicts a process flow diagram for depositing a nucleation promotion layer and a transparent conductive layer formed on the nucleation promotion layer in accordance with one embodiment of the present invention;
[0011] Figures 3A-3C depict cross sectional views of a silicon-based thin film PV solar cell at different manufacture stages in accordance with one embodiment of the present invention;
[0012] Figure 4 depicts an exemplary cross sectional views of a single type PV solar cell in accordance with one embodiment of the present invention; and
[0013] Figure 5 depicts exemplary cross sectional views of a tandem type PV solar cell in accordance with one embodiment of the present invention.
[0014] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.
[0015] It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
DETAILED DESCRIPTION
[0016] The present invention provides methods for forming a nucleation promotion layer (NPL) to enhance deposition of a transparent conductive layer subsequently formed thereon. The nucleation promotion layer (NPL) provides a good nucleation surface that provides desired nucleation sites that assist the transparent conductive layer to adhere thereon with a good adhesion and/or bonding therebetween. The combination of the nucleation promotion layer (NPL) and the transparent conductive layer provides high film transparency, high haze, high light scattering and trapping as well as a desired surface morphology to provide high electric conversion efficiency for PV solar cells.
[0017] Figure 1 is a sectional view of one embodiment of an apparatus 100 that may be utilized to perform a particle spray process according to the present invention. The particle spray process forms a nucleation promotion layer on a substrate that may be utilized to provide desired nucleation sites for layers subsequently formed thereon with good interface bonding/adhesion. In one embodiment, the apparatus 100 may be a solvent ejection system or an air dispense system. The apparatus 100 includes a processing chamber 122 having a top 130, a bottom 126, and sidewalls 142 that define a process region 124 in the interior volume of the processing chamber 122. A stage 104 is disposed on the bottom 126 of the processing chamber 122 to a substrate holder 102 that holds a substrate 160 during processing. The stage 104 is configured to move the holder 102 along the X and Y axis. Thus, the stage 104 moves the substrate 160 positioned thereon along the X and Y axis. A mechanical booster pump 116 and a rotary pump 118 are coupled to the processing region 124 through a port formed in the sidewall 142 of the processing chamber 122 to maintain vacuum pressure during processing.
[0018] A particle generator 110 is coupled to the processing chamber 122 through the top 130 of the processing chamber 122 by a carrier pipe 128. The carrier pipe 128 extends through the processing chamber top 130 to the processing region 124. An injection nozzle 106 is disposed at an end 134 of the carrier pipe 128 to facilitate injection of particles toward a surface 132 of the substrate 160. Multiple nozzles may be utilized to assist dispensing particles toward the surface 132 as needed. In embodiments where multiple nozzles are utilized, the nozzles may be coupled to the generator by a common or individual carrier pipes. It is contemplated that the nozzles may direct the particles to the substrate 160 at different angles. In certain embodiments, the nozzles may be arranged such that the holder 102 may only be need to be moved along a single axis. [0019] A filter 112 may be optionally disposed between the processing chamber 122 and the particle generator 110 to assist in controlling the particle size generated by the particle generator 110. Particles are generated from the raw material 108 and are delivered through the nozzle 106 into the processing region 124. The particles are utilized to form a nucleation promotion layer on the substrate 160. In one embodiment, the raw material 108 may be fine ceramic particles, fine metallic particles, fine dielectric material particles or fine composite particles. In another embodiment, the raw material 108 may be metallic particles having an average diameter less than 50 nm, such as between about 20 nm and about 40 nm, for example about 30 nm. In an exemplary embodiment, the raw material 108 may be fine metallic particles, such as aluminum containing material, indium containing material, tin containing material, zinc containing material, tantalum containing material, titanium containing material, oxides thereof, alloys thereof, combinations thereof, or the like, having an average diameter in nanometer level, for example less than 50 nm, such as 30 nm. In one embodiment depicted in the present invention, the raw material 108 includes nanometer size metallic particles, such as aluminum containing material, indium containing material, tin containing material, having an average diameter of less than 50 nm, such as less than 30 nm.
[0020] One or more gas cylinders 1 14 and mass flow controllers (MFC) 120 are sequentially coupled to the particle generator 110 through a delivery line 136. One cylinder 114 and MFC 120 are shown in Figure 1 for clarity of description. The gas cylinder 1 4 provides carrier gases having a pressure sufficiently high enough to eject a desired amount of raw material 108 to the processing chamber 122. The high carrier gas pressure from the gas cylinder 114 forms a raw material jet stream 138 having entrained raw material 108 that is dispensed toward the substrate surface 132. The raw material 108 in the jet stream 138 forms a nucleation promotion layer on the substrate surface 132. The flow and/or velocity of the stream 138 may be controlled by the mass flow controller (MFC) 120, carrier gas pressure or by the shape and/or opening diameter of the nozzle 106 or other flow control device. In one embodiment, the carrier gas provided by the gas cylinder 114 may be at least one of nitrogen gas (N2), hydrogen gas (H2), oxygen gas (02), fluorine gas (F2), and inert gas, such as Argon (Ar), helium (He), neon (Ne), among others. [0021] In operation, raw material 108, for example, metallic particles having an average diameter less than 50 nm, are disposed in the particle generator 110 as the source of the nucleation promotion layer deposition. The carrier gas from the gas cylinder 114 is supplied into the particle generator 110. The pressure and ejection rate of the carrier gas from the gas cylinder 114 are controlled to provide sufficient kinetic energy and momentum to accelerate the particles of raw material 108 into the processing chamber 122. Additionally, the pressure and ejection rate of the raw material 108 provides sufficient kinetic energy and momentum to promote bonding adhesion of the particles of raw material 108 to the substrate surface 132 without adversely damaging the underlying bulk substrate material. In one embodiment, the pressure of the carrier gas may be maintained at between about 20 PSI and about 40 PSI and the ejection rate of the carrier gas may be controlled at between about 250 meters per second (m/s) and about 1750 meters per second (m/s).
[0022] Alternatively, the gas cylinder 114 installed in the apparatus 100 may be replaced with a liquid tank to serve as a solvent ejection system. The liquid tank may have liquid disposed therein. The liquid disposed in the liquid tank may serve as solvent to mix with the raw material 108 in the particle generator 110. The particle solvent mixture is then ejected toward the substrate surface 132 in liquid form to spray raw material 108 along with the solvent onto the substrate surface 132. After the solvent is evaporated from the substrate surface 132, a layer of particles or powders is then formed on the substrate surface 132. Suitable solvent that may be used to carry raw material 108 are alcohols and the like
[0023] The ejected raw material 108 impacts the substrate surface 132 with sufficient energy to remove contaminants or impurities on the substrate surface 132, if present. The collision between the raw material 108 and the substrate surface 132 actives the substrate surface 132 as a result of the mutual collision. Subsequently, the particles of raw material 108 impinge the substrate surface 132 and bond strongly thereto, thereby coating and/or depositing the nucleation promotion layer (NPL) on the substrate surface 132.
[0024] The substrate 160 positioned on the substrate holder 102 is maintained at a low temperature substantially similar to the adjacent environment to eliminate temperature variation during processing. In one embodiment, the substrate 160 is maintained at a room temperature substantially similar to the adjacent environment. In another embodiment, the substrate 160 is maintained at a temperature at between about 0 degrees Celsius and about 50 degrees Celsius, such as between about 10 degrees Celsius and about 40 degrees Celsius, for example, about 25 degrees Celsius. The low and steady processing temperature of the substrate 160 prevents the substrate 160 from undergoing excessive temperature fluctuation, thermal shock and/or expansion during deposition, thereby minimizing the stress induced between the coating and the underlying surface. The consistent low processing temperature prevents the microstructure and surface roughness of the substrate surface 132 from thermal damage, thereby providing a uniform and consistent substrate surface condition.
[0025] The delivery of the raw material 108 to the substrate 160 is maintained until a desired thickness of the nucleation protection layer is reached. In one embodiment, the nucleation protection layer has a thickness between about 200 A and between about 500 A.
[0026] It is noted that the apparatus 100 described in Figure 1 is only an exemplary apparatus. Other types of tool that can spray, eject, coat, or dispense particles onto the substrate surface, such as a coating tool or an air dispense system, may also be utilized to spray particles onto the substrate.
[0027] Alternatively, the substrate 160 having the nucleation promotion layer (NPL) deposited thereon may be transferred to a PVD chamber for deposition of a TCO layer. One PVD chamber in which a TCO layer may be deposited is described in United Patent Application Serial No. 12/628,569, filed December 1 , 2009, which is incorporated herein by reference.
[0028] Figure 2 is a flow diagram of one embodiment of a particle spray process 200 that may be practiced in the apparatus 100 or other suitable processing chamber to form a nucleation promotion layer (NPL) and a transparent conductive layer on the nucleation promotion layer (NPL). Figures 3A-3C are schematic cross- sectional views of a portion of the substrate 160 utilized to form thin film PV solar cell corresponding to various stages of the deposition process 200. Although the deposition process 200 may be illustrated for forming a nucleation promotion layer and a transparent conductive layer in Figures 3A-3C for forming solar cell devices, the deposition process 200 may be beneficially utilized to form other structures.
[0029] The process 200 begins at step 202 by transferring (i.e., providing) the substrate 160, as shown in Figure 3A, to an apparatus, such as the apparatus 100 in Figure 1. In the embodiment depicted in Figure 3A, the substrate 160 may be thin sheet of metal, plastic, organic material, silicon, glass, quartz, or polymer, or other suitable material. The substrate 160 may have a surface area greater than about 1 square meters, such as greater than about 2 square meters. Alternatively, the substrate 160 may be configured to form thin film PV solar cell, or other types of solar cells, such as crystalline, microcrystalline or other type of silicon-based thin films as needed.
[0030] At step 204, a particle spray process is performed to form a nucleation promotion layer 302 on the substrate 160, as shown in Figure 3B. The nucleation promotion layer (NPL) 302 is a thin layer of particles formed on the substrate 160 prior to the deposition of a transparent conductive layer. The nucleation promotion layer (NPL) 302 assists promoting the nucleation of the transparent conductive layer subsequently formed thereon to have specific crystal grain orientations what will yield a particular surface morphology with desired textures and roughness that increase light scattering capability. Furthermore, the nucleation sites formed in the nucleation promotion layer 302 may also assist grains of the transparent conductive layer subsequently formed thereon to have good positions to adhere thereto, thereby enhancing and promoting interface adhesion and bonding.
[0031] In one embodiment, the nucleation promotion layer (NPL) 302 is a thin layer fabricated by particles. The particles may be metallic powders that have an average diameter at nanometer level. Suitable examples of the particles are aluminum containing material, indium containing material, tin containing material, zinc containing material, tantalum containing material, titanium containing material, oxides thereof, alloys thereof, combinations thereof, or the like. The particles utilized to fabricate the nucleation promotion layer (NPL) 302 has an average diameter between about 20 nm and about 40 nm, such as about 30 nm. In one particular embodiment, the particles used to form the nucleation promotion layer 302 are aluminum containing material, indium containing material or tin containing material, having an average diameter of between about 20 nm and about 40 nm.
[0032] During deposition, particles provided from the raw material 108 disposed in the apparatus 100 may be ejected to the substrate surface. The particles may be carried by gas or by solvent or any other suitable carrying media as needed. The raw material 108 is continuously ejected to the substrate surface 132 until a desired thickness of the nucleation promotion layer 302 is formed on the substrate 160. In one embodiment, the particle spray process may be performed between about 30 seconds and 60 seconds and the thickness of the nucleation promotion layer 302 is controlled at between about 200 A and about 500 A.
[0033] Optionally, after the particles are spayed on the substrate surface 132 and a thin layer of the nucleation promotion layer 302 is formed on the substrate 160, a heating process may be performed. The heating process may be performed to enhance adhesion of the particles of the nucleation promotion layer 302 within the nucleation promotion layer 302 itself, and to promote adhesion of the nucleation promotion layer 302 to the substrate surface 132 with good adhesion. It is believed that the thermal energy provided in the heating process may assist slightly melting the outer surface of the particles so as to assist the particles to adhere on the substrate surface 132 with better adhesion. Furthermore, the heating process may also drive out the solvent used to carry the particles to the substrate surface. In one embodiment, the heating process may be a radiant heat process, a thermal process, a curing process, a plasma treatment process, or any suitable heating process. The heating process may heat the substrate 160 from room temperature to a high temperature but lower than glass softening temperature, such as about 300 degrees Celsius. In one embodiment, the heating process may heat the substrate between about 200 degrees Celsius and about 400 degrees Celsius.
[0034] At step 206, a reactive sputter process is performed to form a transparent conductive layer 304 on the nucleation promotion layer 302 on the substrate 160, as shown in Figure 3C. As the nucleation promotion layer 302 provides a thin layer of particles that may serve as nucleation sites, the transparent conductive layer 304 formed thereon may nucleate on the nucleation sites and adhere thereon to gradually grow the transparent conductive layer 304 on the nucleation promotion layer 302. In one embodiment, the transparent conductive layer may be fabricated by tin containing material, zinc containing material, tantalum containing material, titanium containing material, or the like. In one embodiment, the transparent conductive layer 304 is an aluminum doped zinc oxide layer having an aluminum dopant concentration between about 0.1 percent by weight and about 10 percent by weight, such as about 0.5 percent by weight and about 5 percent by weight. In one embodiment, the transparent conductive layer 304 may have a thickness between about 1000 A and about 10000 A.
[0035] In one embodiment, the transparent conductive layer 304 may be formed in a manner that has high film transparency so as to assist passing light into the subsequent to-be-formed solar cell junctions, which will be further described below with referenced to Figures 4-5. Furthermore, high film transparency formed in the transparent conductive layer 304 helps reduce light loss traveling from the transparent conductive layer 304 to the subsequent to-be-formed solar cell junctions so as to maintain high current conversion efficiency to the photoelectric conversion unit.
[0036] During sputtering, the process gas mixture may be varied to supply different gases at step 206 for different process requirements and needs. The gas mixture supplied at step 206 may include reactive gas, non-reactive gas, inert gas, and the like, as described above. Examples of non-reactive gas include, but not limited to, inert gas, such as Ar, He, Xe, and Kr, or other suitable gases. Examples of reactive gas include, but not limited to, 02, N2, N20, N02, H2, NH3, H20, among others. Accordingly, forming a nucleation promotion layer 302 as a nucleation promotion layer on the substrate may provide a good nucleation surface that provides good nucleation sites that allow the transparent conductive layer 304 to form thereon with desired film structure. The film stack of the nucleation promotion layer 302 and the transparent conductive layer 304 improves the haze (i.e., light scattering of near infrared (NIR) wavelength light and interface bonding and adhesion. Additionally, the film stack of the nucleation promotion layer 302 and the transparent conductive layer 304 may be applicable to any types of glass substrates including commercially available low iron float glass and soda lime glass, thus enabling the use of cheaper and more readily obtainable types of glass substrates for photovoltaic device manufacturing.
[0037] It is noted that the transparent conductive layer 304 formed on the nucleation promotion layer 302 may also be fabricated by other suitable process, such as CVD process, metal plating process, coating process, or any suitable techniques available in the field.
[0038] Figure 4 depicts an exemplary cross sectional view of a single junction PV solar cell 400 having the nucleation promotion layer 302 and the transparent conductive layer 304 formed thereon in accordance with one embodiment of the present invention. Similar to the structures depicted in Figures 3C, the substrate 150 may have the nucleation promotion layer 302 and the transparent conductive layer 304 consecutively formed thereon, as shown in Figure 4. After the transparent conductive layer 304 is formed on the substrate 150, a first photoelectric conversion junction cell 408 is formed on the transparent conductive layer 304 disposed on the substrate 150. The first photoelectric junction cell 420 includes a p-type semiconductor layer 402, a n-type semiconductor layer 406, and an intrinsic type (i- type) semiconductor layer 404 sandwiched therebetween as a photoelectric conversion layer. An optional dielectric layer (not shown) may be disposed between intrinsic type (i-type) semiconductor layer 404 and the n-type semiconductor layer 406 as needed. In one embodiment, the optional dielectric layer may be a silicon layer including amorphous or poly silicon layer, SiON, SiN, SiC, SiOC, silicon oxide (Si02) layer, doped silicon layer, or any suitable silicon containing layer.
[0039] The p-type and n-type semiconductor layers 402, 406 may be silicon based materials doped by an element selected either from Group III or V. A Group III element doped silicon film is referred to as a p-type silicon film, while a Group V element doped silicon film is referred to as a n-type silicon film. In one embodiment, the n-type semiconductor layer 406 may be a phosphorus doped silicon film and the p-type semiconductor layer 402 may be a boron doped silicon film. The doped silicon films 402, 406 include an amorphous silicon film (a-Si), a polycrystalline film (poly-Si), and a microcrystalline film (pc-Si) with a thickness between around 5 nm and about 50 nm. Alternatively, the doped element in semiconductor layers 402, 406 may be selected to meet device requirements of the PV solar cells 400. The n- type and p-type semiconductor layers 406, 402 may be deposited by a CVD process or other suitable deposition process.
[0040] The i-type semiconductor layer 404 is a non-doped type silicon based film. The i-type semiconductor layer 404 may be deposited under process conditions controlled to provide film properties having improved photoelectric conversion efficiency. In one embodiment, the i-type semiconductor layer 404 may be fabricated from i-type polycrystalline silicon (poly-Si), i-type microcrystalline silicon film (pc-Si), amorphous silicon (a-Si), or hydrogenated amorphous silicon (a- Si).
[0041] After the first photoelectric conversion junction cell 420 is formed on the transparent conductive layer 304, a back reflector 424 is disposed on the first photoelectric conversion junction cell 408. In one embodiment, the back reflector 414 may be formed by a stacked film that includes a back transparent conductive layer 410 and a conductive layer 412. The conductive layer 412 may be at least one of Ti, Cr, Al, Ag, Au, Cu, Pt, or their alloys. The back transparent conductive layer 410 may be fabricated from a material similar to the transparent conductive layer 304 formed on the substrate 150. Alternatively, the back transparent conductive layer 410 may be fabricated from a selected group consisting of tin oxide (Sn02), indium tin oxide (ITO), zinc oxide (ZnO), or combinations thereof. In one exemplary embodiment, the back transparent conductive layers 410 may be fabricated from a ZnO layer having a desired Al203 dopant concentration formed in the ZnO layer.
[0042] In operation, the incident light 401 provided by the environment is supplied to the PV solar cell 400. The light passes through the transparent conductive layer 304 to the photoelectric conversion junction cell 408 in the PV solar cell 400 to absorb the light energy and convert the light energy into electrical energy by operation of the p-i-n junctions formed in the photoelectric conversion junction cell 408, thereby generating electricity or energy.
[0043] Figure 5 depicts an exemplary cross sectional view of a tandum junction PV solar cells 500 having the nucleation promotion layer 302 and the transparent conductive layer 304 formed thereon in accordance with one embodiment of the present invention. In addition to the structures depicted in Figures 4, a wavelength selector reflector (WSR) 502 and a second photoelectric conversion junction cell 422 may be formed on the first photoelectric conversion junction cell 408.
[0044] The WSR layer 502 disposed between the first p-i-n junction 408 and the second p-i-n junction 422 is generally configured to have certain desired film properties. In one configuration, the WSR layer 502 actively serves as an intermediate reflector having a desired refractive index, or ranges of refractive indexes, to reflect light received from the light incident side of the solar cell 500. The WSR layer 502 also serves as a junction layer that boosts the absorption of the short to mid wavelengths of light (e.g., 280nm to 800nm) in the first p-i-n junction 408 and improves short-circuit current, resulting in improved quantum and conversion efficiency. The WSR layer 502 further has high film transmittance for mid to long wavelengths of light (e.g. , 500nm to 1 100nm) to facilitate the transmission of light to the layers formed in the junction 422. In one embodiment, the WSR layer 502 may be a microcrystalline silicon layer having n-type or p-type dopants disposed within the WSR layer 502. In an exemplary embodiment, the WSR layer 502 is an n-type crystalline silicon alloy having n-type dopants disposed within the WSR layer 502. Different dopants disposed within the WSR layer 502 may also influence optical and electrical properties, such as bandgap, crystalline fraction, conductivity, transparency, film refractive index, extinction coefficient, and the like. In some instances, one or more dopants may be doped into various regions of the WSR layer 502 to efficiently control and adjust the film bandgap, work function(s), conductivity, transparency and so on. In one embodiment, the WSR layer 502 is controlled to have a refractive index between about 1.4 and about 3, a bandgap of at least about 2 eV, and a conductivity greater than about 10" 3 S/cm.
[0045] The second p-i-n junction 422 may comprise a p-type microcrystalline silicon layer 416, an intrinsic type microcrystalline silicon layer 418 formed over the p-type microcrystalline silicon layer 416, and an n-type amorphous silicon layer 420 formed over the intrinsic type microcrystalline silicon layer 418. The structure of the second conversion junction cell 422 is similar to the first photoelectric conversion junction cell 420 to assist absorbing light with different spectrum and retain light in the junction cells for a longer time to improve conversion efficiency. In one embodiment, a p-type semiconductor layer 416, a n-type semiconductor layer 420, and an intrinsic type (i-type) semiconductor layer 418 sandwiched therebetween as a photoelectric conversion layer. An optional dielectric layer (not shown) may be disposed on top of the n-type semiconductor layer 420 as needed. In one embodiment, the optional dielectric layer may be a heavily doped n-type semiconductor layer. The doped silicon films 416, 420 include an amorphous silicon film (a-Si), a polycrystalline film (poly-Si), and a microcrystalline film (pc-Si) with a thickness between around 5 nm and about 50 nm. Alternatively, the doped element in semiconductor layers 416, 420 may be selected to meet device requirements of the PV solar cells 500. The p-type and the n-type 416, 420 may be deposited by a CVD process or other suitable deposition process. The i-type semiconductor layer 418 is a non-doped type silicon based film. The i-type semiconductor layer 418 may be deposited under process conditions controlled to provide film properties having improved photoelectric conversion efficiency. In one embodiment, the i-type semiconductor layer 418 may be fabricated from i-type polycrystalline silicon (poly- Si), i-type microcrystalline silicon film (pc-Si), amorphous silicon (a-Si), or hydrogenated amorphous silicon (a-Si).
[0046] Thus, methods for forming a nucleation promotion layer to provide nucleated sites for a transparent conductive layer subsequently formed thereon with good interface bonding and adhesion. The nucleation promotion layer formed by a thin layer of particles advantageously produces a nucleation surface that can assist a transparent conductive layer subsequently formed thereon to adhere on the nucleation surface. In this manner, the nucleation promotion layer along with the transparent conductive layer may have an interface with good interface bonding and adhesion so that the photoelectric conversion efficiency and device performance of the PV solar cell can be efficiently improved.
[0047] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

What is claimed is:
1. A method of sputter depositing a conductive contact layer, comprising:
forming a nucleation promotion layer on a substrate, wherein the nucleation promotion layer comprises a layer of metallic particles; and
forming a transparent conductive layer on the nucleation promotion layer.
2. The method of claim 1 , wherein the metallic particles are selected from a group consisting of aluminum containing material, indium containing material, tin containing material, zinc containing material, tantalum containing material, titanium containing material, oxides thereof, alloys thereof, and combinations thereof.
3. The method of claim 1 , wherein the transparent conductive layer is aluminum doped zinc containing material.
4. The method of claim 1 , wherein the nucleation promotion layer has a thickness between about 200 A and about 500 A.
5. The method of claim 1 , wherein the metallic particles have a diameter between about 20 nm and about 40 nm.
6. The method of claim 1 , wherein forming the nucleation promotion layer further comprises:
performing a heating process on the nucleation promotion layer prior to forming the transparent conductive layer.
7. The method of claim 6, wherein the heating process includes a heat radiation process, a thermal process, a curing process and plasma treatment process.
8. The method of claim 6, wherein the heating process heats the substrate to a temperature between about 200 degrees Celsius and about 400 degrees Celsius.
9. The method of claim 1 , wherein the layer of metallic particles is formed on the substrate by an air dispense system or a solvent ejection system.
10. A method of forming a transparent conductive layer, comprising:
forming a nucleation promotion layer on a substrate, wherein the nucleation promotion layer comprises a layer of metallic particles having a thickness less than 500 A, wherein the metallic particles have a diameter between about 20 nm and about 40 nm; and
forming a transparent conductive layer on the nucleation promotion layer, wherein the transparent conductive layer is an aluminum doped zinc containing layer.
11. The method of claim 10, wherein the metallic particles are selected from aluminum containing material, indium containing material, or tin containing material.
12. The method of claim 10, wherein forming the nucleation promotion layer further comprises:
performing a heating process on the nucleation promotion layer prior to forming the transparent conductive layer.
13. A film stack for a PV solar cell, comprising:
a layer of metallic particles having a thickness less than about 500 A formed on a substrate as a nucleation promotion layer, wherein the metallic particles are selected from aluminum containing material, indium containing material, or tin containing material; and
a transparent conductive layer disposed on the nucleation promotion layer.
14. The film stack of claim 13 further comprising:
a first photoelectric junction cell disposed on the transparent conductive layer, wherein the photoelectric junction cell further comprises:
a p-type semiconductor layer;
an intrinsic type semiconductor layer; and
a n-type semiconductor layer.
15. The film stack of claim 13, wherein the metallic particles have a diameter between about 20 nm and about 40 nm.
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Citations (4)

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KR20060047060A (en) * 2004-11-15 2006-05-18 한국과학기술원 Apparatus for photo-assisted low pressure metalorganic chemical vapor deposition and method for forming zinc oxide using the apparatus and doping zinc oxide using the apparatus
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Publication number Priority date Publication date Assignee Title
US6127202A (en) * 1998-07-02 2000-10-03 International Solar Electronic Technology, Inc. Oxide-based method of making compound semiconductor films and making related electronic devices
JP2000322932A (en) * 1999-05-14 2000-11-24 Fuji Photo Film Co Ltd Gel electrolyte, photoelectric transfer element, and photoelectric chemical battery
KR20060047060A (en) * 2004-11-15 2006-05-18 한국과학기술원 Apparatus for photo-assisted low pressure metalorganic chemical vapor deposition and method for forming zinc oxide using the apparatus and doping zinc oxide using the apparatus
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