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  • H01L31/0201—Arrangements for conducting electric current to or from the device in operations for device characterised by at least one potential jump barrier or surface barrier for solar cells or solar cell modules comprising specially adapted module bus-bar structures
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Definitions

• This disclosure relates in general to the field of photo voltaics and solar cells, and more particularly to back contact back junction thin solar cells and methods for
• solutions directed to process very thin solar cell structures may utilize a cell process during which the cell is fully supported by a host carrier throughout, or a cell process which utilizes a novel self-supporting, standalone, substrate with an accompanying structural innovation.
• Back junction/back contacted cell architecture is capable of very high efficiency - primarily because there is no metal shading on the front side and no emitter on the front which helps result in a high blue response, and also because of the potentially low metal resistance on the backside. It is known to those versed in the field that back contacted cell demands a very high minority carrier diffusion length to substrate thickness ratio (while a good criteria to have for any solar cell architecture including front contact cells, this is especially important for back contact cells). The ratio should typically be greater than five.
• the back contact back contact back junction three dimensional solar cell comprises a three-dimensional substrate.
• the substrate comprises a light capturing frontside surface with a passivation layer, a doped base region, and a doped backside emitter region with a polarity opposite the doped base region.
• a backside passivation layer is positioned on the doped backside emitter region.
• Backside emitter contacts and backside base contacts connected to metal interconnects and selectively formed on three-dimensional features of the backside of three-dimensional solar cell.
• FIGURES 1A and IB are side and top view of a reusable template
• FIGURES 2A and 2B are photographs of a silicon substrate
• FIGURE 3 is a process flow showing a fabrication process of making a back contact back junction thin solar cell
• FIGURES 4A - 4C are cross sectional diagrams of the making a reusable template
• FIGURES 5A through 5D are cross sectional diagrams of the making a substrate
• FIGURES 6A through 6H are cross sectional diagrams of a solar cell after key cell fabrication process steps of the flow of FIGURE 3;
• FIGURE 7 is a process flow for making an example solar cell
• FIGURES 8A through 8H are cross-sectional views of a solar cell after key processing steps of the fabrication process flow of FIGURE 7;
• FIGURE 9 is a process flow for making another example solar cell
• FIGURE 10 is a cross-sectional view of the final structure of the cell of FIGURE 9;
• FIGURE 11 is a process flow for making another example solar cell
• FIGURES 12A-12H are cross-sectional diagrams illustrating the cell after key fabrication steps of the process flow of FIGURE 11;
• FIGURE 13 illustrates the process flow of making another example solar cell
• FIGURE 14 is a cross-sectional view of the final structure of the cell of FIGURE
• FIGURE 15 illustrates the process flow of making another example solar cell
• FIGURE 16 is a cross-sectional view of the final structure of the cell of FIGURE 15;
• FIGURE 17 illustrates the process flow of making another example solar cell
• FIGURES 18A-18F are cross-sectional views of the cell after key processing steps of the flow of FIGURE 17;
• FIGURE 19 illustrates a process flow for making another example solar cell
• FIGURE 20 is a cross-sectional view of the final structure of the cell of FIGURE 19;
• FIGURE 21 illustrates a process flow for making another example solar cell
• FIGURE 22 is a cross-sectional view of the final structure of the cell of FIGURE 21;
• FIGURE 23 illustrates a process flow for making another example solar cell
• FIGURE 24 is a cross-sectional view of the final structure of the cell of FIGURE 23.
• the present disclosure provides structural solutions and fabrication process solutions for back contact back junction thin semiconductor solar cells.
• silicon such thin film silicon substrates (TFSS)
• other semiconductor materials such as germanium or gallium arsenide may also be used without departing from the scope of the disclosed structures and methods.
• Heterojunctions and multijunction solar cells using silicon or other semiconductor materials are also within the scope of the disclosed subject matter.
• the three-dimensional back contact back junction solar cells and fabrication methods are applicable to any three-dimensional shapes/structures or microstructures on a substrate but are shown and described as inverted pyramidal structures for explanatory purposes.
• the term "PyCell" herein refers to an inverted pyramidal structure contact back junction solar cell.
• the TFSS is released from the template using a sacrificial layer which not only is able to transfer the crystallinity from template to the TFSS, but is also easily removed selectively compared to the TFSS and the reusable template.
• a sacrificial layer is porous silicon, whose porosity can be modulated or graded to achieve both the aforementioned critical functions.
• FIGURES 1A and IB are cross-sectional and top views (respectively) of a PyCell reusable silicon template.
• FIGURE 1A illustrates the partial cross-section of the re-usable inverted pyramid template.
• the pyramid template cross-section as shown consists of large pyramid cavities and small pyramid cavities. Since the pyramid-shape cavities are chemically etched by anisotropic silicon etching, the angle between the sidewall and the top lateral plane is about 54.7°, which is angle between the (1 11) and (100) silicon crystallographic planes.
• FIGURE IB illustrates the top view of a fabricated template of the present invention.
• the structured silicon template consists of anisotropically etched large pyramid cavity and small pyramid cavity.
• the top opening size of the large cavities is in the range of ⁇ to 1mm and the top opening size of the small cavities is partial the size of the large cavities.
• the large cavity opening is about 200 ⁇ and small cavity opening is about ⁇ .
• the depth of the large cavities is about 140 ⁇ and the depth of the small cavities is about 70 ⁇ .
• FIGURES 2A and 2B are photographs of the backside and frontside views of a PyCell silicon substrate.
• FIGURE 2A illustrates the substrate backside, on where all the base and emitter contacts, junctions, metal fingers and busbars are made. Since the backside surface is not planar, the contacts and metal fingers are preferred to be placed on top surfaces, such as the top of the ridges.
• FIGURE 2B illustrates the substrate front side, which is also ATTORNEY DOCKET NO: SOLT036WO2
• the regular pyramidal structures provide effective light trapping effects.
• FIGURE 3 is a process flow showing a fabrication process of making a back contact back junction thin solar cell, PyCell-1.
• the process flow consists of three columns: Template, Substrate and Cell - which correspond to the template, substrate, and cell processing steps.
• FIGURES 4A through 4C are cross sectional diagrams of the making a reusable template after key fabrication process steps as it is manufactured according to the Template fabrication process of FIGURE 3.
• the template making process starts with using a mono- crystalline (100) silicon wafer.
• the starting wafer could be in circular, square, or pseudo- square shapes.
• This step involves forming a thin hard masking layer on the exposed wafer surfaces.
• the hard masking layer is used to masking the silicon surface areas that are not to be etched in the later steps.
• a proper hard masking layer includes, but is not limited to, thermally grown silicon oxide and low-pressure vapor phase deposited (LPCVD) silicon nitride.
• the next step involves a photolithography step, which consists of photoresist coating, baking, UV light exposure over a photomask, post baking, photoresist developing, wafer cleaning and drying.
• the pattern on the photomask is transferred to the photoresist layer.
• the patterned photoresist layer is used as a soft masking layer for the hard masking layer etching in the subsequent step.
• the next step involves further transferring the photoresist pattern to the hard masking layer underneath by chemical etching, such as etching a thin silicon oxide layer with buffered HF solution. Other wet etching methods and dry etching methods as known in semiconductor and MEMS wafer processing could also be used.
• the remaining soft masking layer i.e. the photoresist layer is removed and the wafer is cleaned.
• photoresist removal process examples include wet methods, such as using acetone or piranha solution (a mixture of sulfuric acid and hydrogen peroxide), or dry methods, such as oxygen plasma ashing.
• the wafers are batch loaded in an anisotropic silicon wet etchant, such as KOH solution.
• the typical etch temperature is in the range of 50°C to 80°C and etch rate is about 0.2 ⁇ / ⁇ to 1 ⁇ /min.
• the etched depth of the large pyramidal cavity is in the range of 50 ⁇ to 200 ⁇ .
• TMAH tetramethylammonium hydroxide
• TMAH tetramethylammonium hydroxide
• the KOH or TMAH silicon etch rate depends upon the orientations to crystalline silicon planes.
• the (11 1) family of crystallographic planes are etched at a very slow rate and they are normally the "stop" planes for anisotropic etching of a (100) silicon wafer with patterned hard mask.
• FIGURES 5A through 5D are cross sectional diagrams of the making a substrate after key fabrication process steps according to the substrate fabrication process of FIGURE 3.
• the structural features depicted in the cross sectional diagrams of FIGURES 5A through 5D are consistent unless otherwise noted.
• the first step starts with the pre-structured template on which a porous silicon layer is formed by electrochemical anodic HF etching of silicon on both the front and back sides of the template surfaces.
• the electrical current polarity is periodically switched between positive and negative currents so that each template side/surface is consecutively etched in order to form the bilayer or multi-layer porous silicon structure on both template sides.
• the current intensity is also changed in a controlled manner to form a porous silicon layer that preferably consists of two thin layers with different (low and high) porosities.
• the first thin porous silicon layer is on the top and is first formed from the bulk silicon wafer.
• the first thin layer preferably has a lower porosity of 15% to 30%.
• the second thin porous silicon layer is directly etched into the bulk silicon and is underneath the first thin layer of porous silicon.
• the 2 nd thin porous silicon layer preferably has a higher porosity in the range of 40% ⁇ 80%.
• the top lower porosity layer is used as a crystalline seed layer for high quality epitaxial silicon growth and the underneath higher porosity porous silicon layer is used for facilitating TFSS releasing due to its lower density of physical connections (between the epitaxial and bulk silicon interfaces) and its weak mechanical strength.
• a single porosity release layer with a progressively increased porosity from top to bottom can also be used.
• the top portion of the porous silicon layer has a low porosity of 15% to 30% and the lower portion of the porous silicon layer has a high porosity of 40% to 80%.
• the wafer is baked in a high temperature (at 950°C to 1200°C, and preferably in the range of 1050°C to 1150°C) hydrogen environment within the epitaxial silicon deposition reactor in order to form coalesced structures (with relatively large voids) within the higher-porosity buried porous silicon layer while forming a continuous surface seed layer of crystalline silicon on the top of the lower-porosity porous silicon layer.
• a mono-crystalline epitaxial silicon layer is deposited on the template, preferably in a high-throughput large-batch epitaxial furnace.
• the epitaxial layer may be in-situ doped. In this process flow the bulk base of the epitaxial layer ATTORNEY DOCKET NO: SOLT036WO2
• the inside layer may be n- (phosphorous) type doped with a doping level higher than the n-type base, such as to form the Front-Surface-Field (FSF), and the outer layer may be p (boron) doped to form the emitter region of the solar cell.
• the epitaxial layer is thin, preferably ⁇ . More specifically, the base doping may be achieved using phosphine (phosphorous imparting) gas in addition to silicon imparting (trichlorosilane - TCS) and other necessary gases, and when emitter growth is done toward the end of the epitaxial process the reactor may be programmed to switch to diborane (boron imparting) instead of phosphine.
• the thicknesses of the base and the emitter region should be optimized to give the best solar cell performance.
• a base thickness less than 100 ⁇ with doping between 5el4 and lei 7 cm-3 and an emitter thickness of less than 3 ⁇ with doping between lei 8 and 3e20 cm-3 are preferred.
• the emitter may also be multi-step with each step resulting in different concentration. This epitaxial structure can facilitate a high open circuit voltage (Voc) of the solar cell and thus a higher efficiency.
• a thin oxide layer is thermally grown on the epitaxial surface. The thin oxide layer is used for solar cell surface passivation and a masking layer for subsequent selective surface openings.
• FIGURE 5A illustrates the cross-sectional view after the thin oxidation step.
• a reinforcement layer is applied to fill the cavities and cover the entire top surfaces.
• the material of the reinforcement layer is required to withstand high temperatures, such as 200°C to 300°C for the subsequent front surface PECVD silicon nitride passivation process.
• Polyimide can be used for the said reinforcement purpose.
• the reinforcement layer may be applied by spray coating followed by curing or by a thin film vacuum lamination process.
• the reinforcement can be achieved by clamping carrier plate. The clamping can be done using vacuum forces or assisted or accomplished by means of an electrostatic chuck (ESC) or a mobile electrostatic carrier (MESC).
• FIGURE 5B shows the cross-sectional view after the backside reinforcement.
• the template edge top surface and bevel surface are mechanically polished so that the epitaxial silicon layer is physically disconnected from the template at its edges.
• the border of the PyCell TFSS may be further defined on template by shallow laser cutting partially through the epitaxial layer without damaging the underneath template surfaces.
• the reinforced epitaxial layer of silicon is released / separated from the template.
• the TFSS is released in an ultrasonic Dl-water bath.
• the TFSS is released by direct pulling with wafer backside and top epitaxial vacuum chucked.
• the released 3-D TFSS has a thickness in the range of in the range of 50 ⁇ to 200 ⁇ (depending on the inverted ATTORNEY DOCKET NO: SOLT036WO2
• a reinforced 3-D TFSS has a thickness in the range of 0.2 mm to 1 mm.
• the released TFSS backside surface is cleaned by short silicon etching using KOH or TMAH solutions to remove the silicon debris and fully or partially remove the quasi- mono-crystalline silicon (QMS) layer.
• the template is cleaned by using diluted HF and diluted wet silicon etch solution, such as TMAH and/or KOH to remove the remaining porous silicon layers and silicon particles.
• the template is further cleaned by conventional silicon wafer cleaning methods, such as SCI and SC2 wet cleaning (or the so-called RCA cleaning process) to removal possible organic and metallic contaminations.
• conventional silicon wafer cleaning methods such as SCI and SC2 wet cleaning (or the so- called RCA cleaning process) to removal possible organic and metallic contaminations.
• the template is ready for another re-use cycle.
• the reinforced PyCell substrate referred to as 3-D TFSS, is made and ready for the subsequent cell fabrication processes.
• FIGURES 6A through 6H are cross sectional diagrams of the solar cell PyCell-1 after key cell fabrication process steps as it is manufactured according to the fabrication process of FIGURE 3.
• the structural features depicted in the cross sectional diagrams of FIGURES 6A through 4H are consistent unless otherwise noted.
• the cross-sectional diagrams of the solar cell show the cell with the frontside (sunnyside) facing downwards and backside (non-sunny/contact side) facing upwards to better illustrate processing steps.
• a PECVD silicon nitride layer of preferably 60 nm to 100 nm thickness is deposited on the front side, which is the pyramid side (sunny side) of the PyCell substrate.
• the PECVD silicon nitride layer is used as a surface passivation layer and an anti-reflection coating (ARC) layer. Reduction of the carrier recombination at the surface defects is an important requirement in achieving high-efficiencies for crystalline silicon solar cells and it is even more critical for thin and large silicon substrates because of larger surface to volume ratio.
• the backside reinforcement layer may be fully removed, such as by de-clamping the MESC or MOVAC carrier plate, or removal of the deposited layer, such as the shown Polyimide layer.
• the backside reinforcement layer (such as the Polyimide layer) may be partially removed to partially expose the substrate backside surfaces so that junction, contacts, and metallization can be made.
• aligned laser ablation is used to open the emitter contact openings from the ridges on the backside.
• the laser ablation wavelength, pulse width and energy dose are required to selectively remove the dielectric layer (thin oxide layer in this case) without or with minimum damage to the silicon surface underneath.
• the cross-sectional view after the partial removal of the backside reinforcement layer and after the emitter contact opening is shown in FIGURE 6C.
• phosphorous liquid dopant is deposited on the surface by either blanket spraying or aligned printing. After curing the phosphorous liquid dopant, an aligned laser beam is directed to the base contact regions. The controlled laser beam opens the base contact by ablating the dielectric layers on top of the base contact regions as well as driving- in the phosphorous dopant to form n ++ base junctions.
• the cross-sectional view during the laser processing step is shown in FIGURE 6D.
• the remaining phosphorous dopant is removed in a wet etching process, such as using a diluted HF etchant.
• FIGURE 6E illustrates the formed emitter contact opening, base contact opening, and the n++ base junction.
• the emitter and base contact openings in the PyCell-1 flow are discrete, which means they are not connected to form continuous interdigitated line openings.
• inkjet printing of metal such as nickel (Ni) nanoparticle ink is conducted on the ridge surfaces to connect the contact openings into interdigitated emitter and base finger patterns.
• the printed Ni thin pattern also serves as the seed layer for subsequent thicker metal electroplating process.
• the Ni printed cross-sectional view is shown in FIGURE 6F and the electroplated thick Ni + Cu + Sn cross-sectional view is shown in FIGURE 6G.
• solar cell (PyCell-1) is completed and it may proceed to subsequent testing and module assembly steps.
• the backside reinforcement layer may be fully removed and a thin aluminum oxide passivation layer and a thin PVD aluminum mirror layer selectively deposited ATTORNEY DOCKET NO: SOLT036WO2
• FIGURE 6H shows the cross-sectional view of the PyCell-1 solar cell with improved backside surface passivation and added PVD Al back mirror.
• FIGURE 7 is a process flow of making another example of the PyCell solar cells (PyCell-2) according to the present invention.
• the reusable template and PyCell substrate making processes of PyCell-2 may be the same as PyCell-1.
• the cell process steps may also be same before the emitter contact opening step.
• the PyCell-2 design comprises continuous interdigitated line opening for both emitter and base contacts.
• the step of inkjet printing of metal nanoparticle ink is eliminated because the purpose of this Ni inkjet printing in PyCell-1 is to create continuous interdigitated line base and emitter patterns from the discrete contact openings.
• FIGURES 8A through 8H are cross-sectional views of a solar cell after key fabrication steps of the PyCell-2 cell process flow.
• FIGURES 8A through 8C are same as FIGURE 6A through 6C with the same processing steps and structure designs.
• the aligned laser beam shown in FIGURE 8D scans continuous interdigitated lines to make continuous interdigitated line base and emitter contact openings (shown in FIGURE 8E).
• the base junctions as shown also have a continuous interdigitated line shape.
• electroless plating of Ni followed by electroplating of Cu and Sn is conducted on the exposed silicon surfaces for form the emitter and base metal fingers and busbars, as shown in FIGURE 8G.
• FIGURE 8H illustrates the alternative embodiment of PyCell-2 that involves PECVD AI2O 3 passivation layer and PVD Al back mirror layer.
• the PECVD AI2O 3 passivation layer and PVD Al back mirror layer do not cover the emitter and base busbar contact areas, which are not shown in Figure 8(h).
• FIGURE 9 is a process flow for making another example of PyCell solar cells, PyCell-3A. Compared to PyCell-1, one difference of PyCell-3A is that the backside ATTORNEY DOCKET NO: SOLT036WO2
• FIGURE 10 is a cross-sectional view of the final structure of the PyCell-3A cell process flow of FIGURE 9 - with backside reinforcement and diffuse mirror layers shown.
• FIGURE 11 is a process flow for making another example of PyCell solar cells, PyCell-3B.
• the reusable template and PyCell substrate making processes of PyCell-3B may be the same as PyCell-1.
• Key embodiments of the PyCell-3B are that it has selective emitters and the backside reinforcement layer also serves as the back diffuse mirror layer.
• the base and emitter contact openings are discrete; therefore the step of inkjet printing of Ni nanoparticle ink is needed to create continuous interdigitated line base and emitter patterns from the discrete contact openings.
• FIGURES 12A-12H are cross-sectional diagrams illustrating the cell after key fabrication steps of the PyCell-3B cell process flow of FIGURE 11.
• the structural features depicted in the cross sectional diagrams of FIGURES 12A -12H are consistent unless otherwise noted.
• the cell process of PyCell-3B starts from the backside reinforced substrate and backside reinforcement layer also is used as a back mirror.
• a PECVD silicon nitride layer of 60 nm to 100 nm thick is deposited on the cell frontside, which is the pyramid side of the PyCell substrate.
• the PECVD silicon nitride layer is used as a surface passivation layer and an anti-reflection coating (ARC) layer.
• ARC anti-reflection coating
• the front side of the PyCell substrate is reinforced by depositing of a layer of supporting material, such as PV-grade EVA, Z68 or silicone.
• the depositing methods includes but not limited to thermal spraying and vacuum lamination.
• the backside reinforcement layer such as the PTFE layer
• the backside reinforcement layer may be partially or locally removed to expose partially the substrate backside surfaces that junction, contacts and metallization can be made.
• the partial removal of the backside reinforcement layer may be achieved by selective laser ablation.
• boron liquid dopant is deposited on the surface by either blanket spraying or aligned printing. After curing the boron liquid dopant, an aligned laser beam is directed to the emitter contact regions. The controlled laser beam opens the emitter contact by ablating the dielectric layer (thin silicon oxide) on top of the emitter regions as well as driving-in the boron dopant for forming the p ++ selective emitter junctions in the same time.
• the selective emitter p ++ junctions have higher doping concentration than the p + epitaxial emitter layer and the depth of the selective emitter junction may be deeper than the epitaxial emitter layer.
• FIGURE 12E a similar process is conducted to form the selective base junction.
• This process starts with phosphorous liquid dopant deposited on the surface by either blanket spraying or aligned printing. After curing the phosphorous liquid dopant, an aligned laser beam is directed to the base contact regions. The controlled laser beam opens the base contact by ablating the dielectric layer (thin silicon oxide) on top of the base emitter regions as well as driving-in the phosphorous dopant for forming n ++ selective emitter junctions at the same time.
• dielectric layer thin silicon oxide
• the remaining boron dopant is removed in a wet etching process, such as using a diluted HF etchant, and the selective base junction is fabricated - shown in FIGURE 12F.
• the selective base n ++ junction is formed by counter doping the epitaxial p + emitter layer and the base junction depth is deeper than the epitaxial emitter layer.
• FIGURE 12F illustrates the formed emitter contact opening, base contact opening, the p ++ selective emitter junction, and the n ++ base junction. It is to be noted that the emitter and base contact openings in the PyCell-3B flow are also discrete, which means they are not connected to form continuous inter digitated line openings.
• inkjet printing of nickel (Ni) nanoparticle ink is conducted on the ridge surfaces to connect the contact openings into interdigitated emitter and base finger patterns.
• the printed Ni thin pattern also serves as the seed layer for subsequent thicker metal electroplating process.
• the Ni printed cross-sectional view is shown in FIGURE 12G and the electroplated thick Ni + Cu + Sn cross-sectional view is shown in FIGURE 12H.
• the PyCell-3B solar cell is complete (shown in FIGURE 12H) and it may proceed to subsequent testing and module assembly steps.
• FIGURE 13 illustrates the process flow of making another example of PyCell solar cell, PyCell-4A.
• the reusable template and PyCell substrate making processes of PyCell-4A may be substantially similar as PyCell-1.
• the backside reinforcement layer also serves as a backside diffuse mirror purpose in PyCell-4A.
• the backside diffuse mirror reflects the pass-through light back into the silicon material in various directions.
• thermal sprayed PTFE backside reinforcement layer may also be used as a diffuse mirror layer simultaneously.
• the cell process steps may also be the same before the emitter contact opening step.
• the PyCell-4A design makes continuous interdigitated line opening for both emitter and base ATTORNEY DOCKET NO: SOLT036WO2
• the step of inkjet printing of Ni nanoparticle ink is eliminated because the purpose of this Ni inkjet printing in PyCell-1 is to create continuous interdigitated line base and emitter patterns from the discrete contact openings.
• the cell process flow of PyCell-4A may be the same as described for FIGURE 7 and 8, except the contact openings are continuous interdigitated line patterns and the inkjet printing of Ni nanoparticle step is eliminated.
• the back mirror effects provided by the backside reinforcement layer the need of applying PECVD AI2O 3 passivation layer and PCD Al layer on the backside is eliminated in PyCell-4A design.
• FIGURE 14 is a cross-sectional view of the final structure of the PyCell-4A cell process flow according to the present invention.
• the backside reinforcement and diffuse mirror layer are shown and the emitter and base contact openings and the base junction are continuous interdigitated line patterns.
• FIGURE 15 illustrates the process flow of making another example of PyCell solar cells, PyCell-4B.
• the reusable template and PyCell substrate making processes of PyCell-4B are mostly same as PyCell-1.
• the backside reinforcement layer also serves as a backside diffuse mirror purpose.
• the backside diffuse mirror reflects the pass- through light back into the silicon material in various directions.
• thermal sprayed PTFE backside reinforcement layer can also be used as a diffuse mirror layer simultaneously.
• the cell process flow of PyCell-4B is mostly same as the PyCell-3B cell process flow except the emitter and base contact openings and junctions are in continuous interdigitated line patterns.
• PyCell-4B also has selective emitter (p ++ ) junctions made in the same method as the PyCell-3B cell process, except the selective emitter design of the PyCell-4B is in continuous interdigitated line pattern.
• FIGURE 16 is a cross-sectional view of the final structure of the PyCell-4B cell process flow of FIGURE 15.
• the PyCell-4B cell has formed selective emitters.
• the backside reinforcement layer is also served as the diffuse mirror layer.
• the emitter and base contact openings and the base junction are continuous interdigitated line patterns.
• FIGURE 17 illustrates the process flow of making another example of PyCell solar cells, PyCell-4C.
• the reusable template and PyCell substrate making processes of PyCell-4C are mostly same as PyCell-1.
• the backside reinforcement layer also serves as a backside diffuse mirror purpose.
• the backside diffuse mirror reflects the pass- through light back into the silicon material in various directions.
• thermal sprayed PTFE backside reinforcement layer can also be used as a diffuse mirror layer simultaneously.
• the cell process flow of PyCell-4C is mostly same as the PyCell-4B cell ATTORNEY DOCKET NO: SOLT036WO2
• PyCell-4C also has selective emitter (p ++ ) junctions made in the same method as the PyCell-4B cell process.
• FIGURES 18A-18F are cross-sectional views after key processing steps of the PyCell-4C cell process flow according to the present invention.
• the cell process of PyCell- 4C starts from the backside reinforced substrate and backside reinforcement layer also is used as a back mirror.
• a PECVD silicon nitride layer of 60 nm to 100 nm thick is deposited on the front side, which is the pyramid side of the PyCell substrate.
• the PECVD silicon nitride layer is used as a surface passivation layer and an anti-reflection coating (ARC) layer.
• ARC anti-reflection coating
• the front side of the PyCell substrate is reinforced by depositing of a layer of supporting material, such as PV-grade EVA, Z68 or silicone.
• Depositing methods include, but are not limited to, thermal spraying and vacuum lamination.
• the backside reinforcement layer such as the PTFE layer, may be partially or locally removed to expose partially the substrate backside surfaces where junction, contacts, and metallization are formed. The partial removal of the backside reinforcement layer may be achieved by selective laser ablation.
• boron liquid dopant and phosphorous liquid dopant are deposited on the cell surface by an inkjet process.
• the printed of phosphorous and boron liquids may be conducted within a single print load, in which the said two types of liquid are dispensed from two separated printing nozzles. Alternatively, they may be printed in two subsequent printing loads with optional drying step in between. Next, both the printed phosphorous and boron dopant are fully dried and cured, such as in a convection oven.
• an aligned laser beam is directed to the emitter contact regions.
• the controlled laser beam opens the contacts by ablating the dielectric layer (thin silicon oxide) on top of the contact regions as well as driving-in the dopants for forming the p ++ selective emitter junctions and n ++ base junctions in the same time.
• the remaining boron and phosphorous dopants are removed in a wet etching process, such as using a diluted HF etchant, and the selective emitter junction and base junction are formed - shown in FIGURE 18E.
• the junctions and contact openings are in continuous interdigitated line pattern.
• the selective emitter p ++ junction has higher doping concentration than the p + epitaxial emitter layer and the depth of the selective emitter junction may be deeper than the epitaxial emitter layer.
• electroplating of thick Ni + Cu + Sb metal layers is conducted and plated metal fingers form interdigitated metal lines and base and emitter busbars.
• FIGURE 19 illustrates a process flow for making another example of PyCell solar cell, PyCell-5A.
• the reusable template and PyCell substrate making processes of PyCell-5A are mostly same as PyCell-1.
• the backside reinforcement layer also serves as a backside diffuse mirror purpose.
• the backside diffuse mirror reflects the pass- through light back into the silicon material in various directions.
• thermal sprayed PTFE backside reinforcement layer may also simultaneously be used as a diffuse mirror layer.
• the cell process flow of PyCell-5A is mostly same as the PyCell-4A cell process flow. However to further improve the back diffuse mirror effects; an additional diffuse mirror stack is applied on the cell backside.
• a low density PTFE layer is spray coated and covers the backside surface except the emitter and base busbar contact areas, which are covered by a shadow mask.
• a thin PVD Al layer is deposited on the backside and on top of the added PTFE layer, and the PVD Al serves as the back reflection mirror.
• the emitter and base busbar contact areas are also covered by a shadow mask during the PVD Al deposition process so that the emitter and base metals are not shunted by the PVD Al.
• FIGURE 20 is a cross-sectional view of the final structure of the PyCell-5A cell process flow described in FIGURE 19.
• the PyCell-5A cell has continuous interdigitated emitter and base contact line openings, enhanced back diffuse mirror with additional Low- density PTFE layer, and PVD Al layer on the backside.
• the PTFE and PVD Al layers do not cover the emitter and base busbar contact areas, which are not shown in FIGURE 20.
• PyCell-5A cell does not have selective emitters.
• FIGURE 21 is the process flow for making another example of PyCell solar cells, PyCell-5B.
• the reusable template and PyCell substrate making processes of PyCell-5B are mostly same as PyCell-1.
• the backside reinforcement layer also serves as a backside diffuse mirror.
• the backside diffuse mirror reflects the pass-through light back into the silicon material in various directions.
• a thermal sprayed PTFE backside reinforcement layer may also be used as a diffuse mirror layer simultaneously.
• the cell process flow of PyCell-5B is mostly same as the PyCell-4B cell process flow.
• an additional diffuse mirror stack is applied on the cell backside.
• a low density PTFE layer is spray coated and covers the backside surface except the emitter and base busbar contact areas, which are covered by a shadow mask.
• a thin PVD Al layer is deposited on the ATTORNEY DOCKET NO: SOLT036WO2
• the PVD Al serves as the back reflection mirror.
• the emitter and base busbar contact areas are also covered by a shadow mask during the PVD Al deposition process so that the emitter and base metals are not shunted by the PVD Al.
• FIGURE 22 is a cross-sectional view of the final structure of the PyCell-5B cell process flow of FIGURE 21. Shown, PyCell-5B cell has continuous interdigitated line emitter and base junction and contact openings, enhanced back diffuse mirror with additional Low-density PTFE layer, and PVD Al layer on the backside. The PTFE and PVD Al layers do not cover the emitter and base busbar contact areas, which are not shown in FIGURE 22. PyCell-5B cell has selective emitters.
• FIGURE 23 is a process flow for making another example of PyCell solar cell, PyCell-5C.
• the reusable template and PyCell substrate making processes of PyCell-5C are mostly same as PyCell-1.
• the backside reinforcement layer also serves as a backside diffuse mirror.
• the backside diffuse mirror reflects the pass-through light back into the silicon material in various directions.
• thermal sprayed PTFE backside reinforcement layer may also simultaneously be used as a diffuse mirror layer.
• the cell process flow of PyCell-5C is mostly same as the PyCell-4C cell process flow. However to further improve the back diffuse mirror effects; an additional diffuse mirror stack is applied on the cell backside.
• a low density PTFE layer is spray coated and covers the backside surface except the emitter and base busbar contact areas, which are covered by a shadow mask.
• a thin PVD Al layer is deposited on the backside and on top the added PTFE layer, and the PVD Al serves as the back reflection mirror.
• the emitter and base busbar contact areas are also covered by a shadow mask during the PVD Al deposition process, so that the emitter and base metals are not shunted by the PVD Al.
• FIGURE 24 illustrates the cross-sectional view of the final structure of the PyCell- 5C cell process flow in FIGURE 23. Shown, PyCell-5C cell has continuous interdigitated line emitter and base junction and contact openings, enhanced back diffuse mirror with additional Low-density PTFE layer, and PVD Al layer on the backside. The PTFE and PVD Al layers do not cover the emitter and base busbar contact areas, which are not shown in FIGURE 24. PyCell-5C cell has selective emitters.
• this disclosure describes the structure of back junction back contact crystalline three-dimensional solar cells deposited on a three dimensional structured re-usable template which acts as a high temperature capable carrier to enable on-template processes ATTORNEY DOCKET NO: SOLT036WO2
• reinforcement structures for the three dimensional cells and methods to apply them are described, which enable the use of an ultra-thin semiconductor layer for the three dimensional cell, thus reducing the overall cost and reducing the bulk lifetime requirement of the layer.
• the use of short laser pulses with sub-nanosecond pulse duration is described to accomplish the contact opening for at least one, preferably both polarities of the contacts to the above described three dimensional back junction back contact solar cell.
• the use of laser pulses is described to accomplish the doping of the contact areas for at least one, preferably both polarities of the contacts regions to the above described three dimensional back junction back contact solar cell.
• fabricating methods of making various types PyCell solar cells using an epitaxial silicon layer are provided.
• the term "ultra-thin” is referred to as a grown material thickness in the range of ⁇ ⁇ to 50 ⁇ .
• Two advantages of PyCell compared to a substantially planar solar cell that is made of same ultra-thin silicon layer (1) given a thickness of a specific range, a PyCell substrate is mechanically more robust than a substantially planar silicon substrate of the same thickness; (2) PyCell microstructures provide natural light trapping effects, so that the need of conducting a front surface texturing process is eliminated.
• an ultra-thin PyCell substrate saves epitaxial silicon material compared to a substantially planar ultra-thin epitaxial silicon solar cell.
• the pyramid-shaped side of the PyCell is used as the sunny side, which is referred to as the front side; while the ridge- shaped side of the PyCell is used as the contact, junction, and metallization side, which is referred to as the backside. Furthermore, all the base and emitter contacts, junctions, metal fingers, metal busbars are made on the backside of the PyCell solar cell.
• Additional aspects of the present disclosure include in-situ doping steps during epitaxial silicon growth are conducted to form in-situ doped Front Surface Field (FSF) and Emitter doping layer.
• FSF Front Surface Field
• the in-situ doing concentration profiles may be in steps or in gradient.
• Short pulsed (such as sub-nanosecond) laser ablation used to create discrete or continuous dielectric contact openings.
• Laser surface treatment used to perform the doping of the contact areas for at least one, preferably both polarities of the contacts regions to the above described three dimensional back junction back contact solar cell.
• the backside and the reinforcement layer serving as a diffused back mirror.
• selective emitters higher emitter doping inside emitter contact areas
• inkjet liquid dopant printing and direct-write laser doping processes are used to make selective emitters and base contacts.
• novel manufacturing methods using pulsed laser approach are provided.

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• 2010
  • 2010-12-09 WO PCT/US2010/059759 patent/WO2011072161A2/en active Application Filing
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