Patent Application
20080290368
1. Field of Invention
The present invention generally relates to semiconductor devices and more particularly to highly efficient photovoltaic (PV) cells with shallow emitters.
2. Description of Related Art
Crystalline silicon photovoltaic (PV) cells are generally produced from a silicon substrate doped with impurities to produce a p/n heterojunction. The p/n heterojunction may be produced by diffusion of either phosphorus or boron typically into the front side of a p-type or n-type semiconductor substrate. Fixed charges at the heterojunction, due to the boron and phosphorous atoms create a permanent dipole charge layer with a high electric field. A portion of the PV cell, between the front side and the p/n heterojunction is referred to as an emitter. When the PV cell is illuminated by light, photons of light energy produce electron-hole pairs and the high electric field of the p/n heterojunction provides charge separation. This displacement of free charges results in a voltage difference between the p and n regions of the substrate. When the p and n regions are connected to an electric circuit, an electric current flows. This electric current is collected from the PV cell by front and back side metal contacts.
Front and back side metal contacts are typically provided on the substrate through the use of screen printing technology in which a partially electrically conductive paste, which typically contains silver and/or aluminum is screen printed through a mask onto front and back surfaces of the substrate.
For the front side of the substrate, the mask typically has openings through which the conductive paste contacts the substrate surface. The front side mask is typically configured to produce a plurality of thin parallel line contacts connected to two or more thicker lines that are connected to, and extend generally perpendicular to the parallel line contacts. After spreading paste on the mask, the mask is removed and the wafer bearing the partially conductive paste is heated to dry the paste. The wafer is then “fired” in an oven and the paste enters a metallic phase, where at least part of it diffuses through the front surface of the substrate and into the substrate structure while a portion is left solidified on the front surface. The multiple thin parallel lines form thin parallel linear current collecting areas referred to as “fingers”, intersected by thicker perpendicular lines referred to as “bus bars”. The fingers collect the electrical current from the front side of the substrate and transfer it to the bus-bars. The bus bars can be connected to an electrical circuit.
Typically, the width and the height of each finger are approximately 120 microns and 10 microns respectively. While the fingers are sufficient to collect small electric currents from the substrate, the bus-bars are required to collect a much greater current from the plurality of fingers and therefore have correspondingly larger cross section and width.
On the back surface of the substrate, a partially conductive paste containing a composition of silver and aluminum is screen printed and dried in areas that are to act as electrical contacts. A partially conductive paste containing aluminum is then spread over the entire back surface of the substrate and partially overlaps edges of the above-mentioned contacts. This paste is then dried by heating. Then the substrate is subjected to “firing” in an oven, and part of the aluminum diffuses into the back surface of the substrate. This produces a highly doped p+ layer, or back surface field (BSF) at the back surface of the substrate. The aluminum also alloys with the silver/aluminum contacts in areas in which it overlaps the contacts. The silver/aluminum contacts thus appear as silver/aluminum pads among the back surface field of aluminum that is diffused into the back surface of the substrate. The silver/aluminum contacts collect electrical current from the rear side of substrate and act as electrical terminals for the back side of the substrate.
The area that is occupied by the fingers and bus bars on the front side of the substrate is known as the shading area because the non-transparent paste that forms the fingers and bus bars prevents solar radiation from reaching the surface of the substrate in this area. This shading area reduces the current producing capacity of the device. Modern solar cell substrate shading occupies 6-10% of the available active surface area. The presence of metal contacts on the front side and the silver/aluminum pads on the back side also results in a decrease of voltage generated by the substrate in proportion to the metallized area because diffusion of the contact paste into the front surface of the substrate has a detrimental effect on charge recombination. Conventional metallization techniques may also cause bowing of the substrate due to the difference in thermal expansion coefficients between silicon and silver/aluminum pastes. This can be a problem in thin solar cells, which may employ substrates less than 180 microns thick, making such cells fragile thus reducing production yield.
In addition conventional metallization techniques result in substantial losses in the emitter region. Therefore in order to increase conversion efficiency of solar cells that employ conventional screen printed metallization, emitter design parameters are often optimized in such a way that in light-illuminated areas doping concentration levels are as low as possible and the emitter is very thin. This provides for improved photon collection, especially in the blue spectral region. Doping concentrations and emitter thickness in areas under current collecting fingers and bus bars are usually substantially higher to provide for low resistance electric contact between the substrate and the fingers and bus bars without shunting the p/n heterojunction. In other words it has been desirable to make solar cells with a selective emitter that contains areas with different dopant concentration and different emitter thicknesses. Although the use of a selective emitter has proved to be effective in improving PV solar cell efficiency, implementation of a selective emitter in practice, is quite complicated.
U.S. Pat. No. 5,871,591 entitled Silicon Solar Cells made by a Self-Aligned, Selective-Emitter, Plasma-Etchback Process, to Ruby et al describes a process for forming and passivating a selective emitter. The process uses a plasma etch of a heavily doped emitter to improve its performance. The screen printed metallic patterns, so called grids of the solar cell, are used to mask the plasma etch so that only the emitter in the region between the grids is etched, while the region beneath the grids remains heavily doped to provide low contact resistance between the substrate and the screen printed metallic grids. This process is potentially low-cost because it does not require precision alignment between heavily doped regions and screen printed patterns. After the emitter is etched, silicon nitride is deposited by plasma-enhanced chemical vapor deposition, thereby creating an antireflection coating. The solar cell is then annealed in a forming gas. While this method allows fabricating a selective emitter and increased solar cell efficiency, it has a disadvantage in that selective emitter formation happens only after screen printed metallic patterns have been formed on the solar cell and thus is dependent on conventional screen printing metallization techniques.
U.S. Pat. No. 6,091,021 entitled “Silicon Solar Cells made by a Self-Aligned, Selective-Emitter, Plasma-Etchback Process” to Ruby et al describes photovoltaic cells and a method for making them wherein metalized grids of the cells are used to mask portions of cell emitter regions to allow selective etching of phosphorous-doped emitter regions. This self-aligned selective etching allows for enhanced blue response as compared to cells with uniform heavy doping of the emitter, while preserving heavier doping in the region beneath the gridlines needed for low contact resistance. This may replace expensive and difficult alignment methodologies used to obtain selectively etched emitters, and may be easily integrated with existing plasma processing methods and techniques. However, the proposed method requires that selective emitter formation be done only after screen printed metallization has been applied on the solar cell, making the process dependent on conventional screen printed metallization techniques.
U.S. Pat. Nos. 6,552,414 and 6,825,104 both entitled “Semiconductor Device with Selectively Diffused Regions” to Horzel et al. describe a PV cell having two selectively diffused regions with different doping levels. A first screen printing process is used to deposit a paste containing dopant on diffusion regions of a substrate to produce highly doped emitter regions. A second screen printing deposition of a metallization pattern is precisely aligned to ensure that a connection is made to the highly doped emitter regions. Again screen printing metallization techniques are required.
In accordance with one aspect of the invention, there is provided a photovoltaic semiconductor apparatus for use in forming a solar cell. The apparatus includes first and second adjacent oppositely doped volumes of semiconductor material forming a semiconductor heterojunction. The first doped volume acts as an emitter and has a front side for receiving light. The apparatus also includes a first passivation layer of material on the front side, the first passivation layer having a first outer surface and a plurality of openings therethrough defining corresponding unpassivated areas of the front side that are unpassivated by the first passivation layer. The apparatus further includes a first conductive anti-reflective coating on the first outer surface of the passivation layer and on the corresponding unpassivated areas of the front side.
The semiconductor heterojunction may be at least one of an ion-implanted heterojunction and a thermally diffused heterojunction.
The first doped volume may have a sheet resistivity of about 60 ohms per square to about 150 ohms per square and desirably has a sheet resistivity of about 80 ohms per square to about 150 ohms per square.
The first passivation layer may be comprised of at least one of SiO2, SiN4 and SiC.
The first passivation layer may have a thickness of about 10 nm to about 500 nm and preferably has a thickness of about 10 nm to about 50 nm.
The openings may have a width of about 50 micrometers to about 200 micrometers.
The openings in the first passivation layer may be arranged in parallel lines across the first outer surface.
The distance between parallel lines of openings in the second passivation layer may be about 500 micrometers to about 5000 micrometers.
The parallel lines may be connected by cross parallel lines to form a grid arrangement.
The grid arrangement may have meshes approximately about 500 micrometers to about 5000 micrometers square.
The first conductive anti-reflective coating may be continuous.
The first conductive anti-reflective coating may have a thickness of about 70 to about 280 nm.
The first conductive anti-reflective coating may be comprised of at least one of: InOx, SnOx, InSnOx, TiOx and ZnOx.
The first conductive anti-reflective coating may have a sheet resistivity of about 1 Ohm/sq. to about 30 Ohm/sq.
The apparatus may include a second passivation layer on the back side surface, the second passivation layer having a second outer surface having a second plurality of openings therethrough defining corresponding unpassivated areas of the second outer surface that are unpassivated by the second passivation layer.
The apparatus may also include a second conductive anti-reflective coating on the second outer surface of the second passivation layer and on the corresponding unpassivated areas of the second outer surface.
The second passivation layer may be comprised of at least one of SiO2, SiN4 and SiC.
The second passivation layer may have a thickness of about 10 nm to about 500 nm and preferably has a thickness of about 10 nm to about 50 nm.
The openings in the second passivation layer may have a width of about 50 micrometers to about 200 micrometers.
The openings in the second passivation layer may be arranged in parallel lines across the second outer surface.
The parallel lines may be spaced apart by about 500 micrometers to about 5000 micrometers.
The parallel lines may be connected by cross parallel lines to form a grid arrangement.
The grid arrangement may have meshes about 500 micrometers to about 5000 micrometers square.
The second conductive anti-reflective coating may be continuous.
The second conductive anti-reflective coating may have a thickness that is about at least as thick as the first conductive anti-reflective coating.
The second conductive anti-reflective coating may have a thickness of about 70 to about 500 nm.
The second conductive anti-reflective coating may be comprised of at least one of: InOx, SnOx, InSnOx, TiOx and ZnOx.
The second conductive anti-reflective coating may have a sheet resistivity of about 1 Ohm/sq. to about 30 Ohm/sq.
To employ the photovoltaic semiconductor apparatus in a solar cell, first and second electrodes are connected to the front and back sides of the apparatus. The first electrode includes a first optically transparent electrically insulating film having first and second opposite sides. The first side has a first adhesive for adhering the first film to the first conductive anti-reflective coating. The first electrode further includes a first plurality of conductors embedded in the first adhesive coating such that portions of the first plurality of conductors protrude from the first adhesive. The portions are soldered to the first conductive anti-reflective coating by an alloy coating on the portions to form ohmic connections between the first conductive anti-reflective coating and the portions of the first plurality of conductors such that electrons can pass between the unpassivated areas of the front side and the first plurality of conductors to permit an electric current generated by the photovoltaic semiconductor apparatus to be conducted by the first plurality of conductors.
The second electrode includes a second electrically insulating film having first and second opposite sides. The first side of the second film has a second adhesive for adhering the second film to the second conductive anti-reflective coating. The second electrode further includes a second plurality of conductors embedded in the second adhesive coating such that portions of the second plurality of conductors protrude from the second adhesive. The portions of the second plurality of conductors are soldered to the second conductive anti-reflective coating by an alloy coating on the portions to form ohmic connections between the portions of the second plurality of conductors and the second conductive anti-reflective coating such that electrons can pass between the unpassivated areas of the second outer surface and the second plurality of conductors to permit the electric current generated by the photovoltaic semiconductor apparatus to be conducted by the second plurality of conductors.
Instead of the second passivation layer and the second conductive anti-reflective coating, the apparatus may include a third doped volume adjacent the second doped volume on a side of the second doped volume opposite the semiconductor hetrojunction. The third doped volume has the same doping polarity as the second doped volume thereby forming an isotype junction with the second doped volume. The third doped volume also has a doping concentration greater than a doping concentration of the second doped volume and the third doped volume has a back side surface.
The apparatus may further include a second conductive anti-reflective coating on the back side surface of the third doped volume.
The second conductive anti-reflective coating may be continuous and uniform and may have a thickness that is about the same as, or greater than, a thickness of the first conductive anti-reflective coating.
The second conductive anti-reflective coating may have a thickness of about 70 to about 500 nm.
The second conductive anti-reflective coating may be comprised of at least one of: InOx, SnOx, InSnOx, TiOx and ZnOx.
The second conductive anti-reflective coating may have a sheet resistivity of about 1 Ohm/sq. to about 30 Ohm/sq.
A solar cell employing the apparatus with the third doped volume is formed by connecting first and second electrodes to the front and back surfaces of the apparatus.
The first electrode includes a first optically transparent electrically insulating film having first and second opposite sides. The first side has a first adhesive for adhering the first film to the first conductive anti-reflective coating. The first electrode further includes a first plurality of conductors embedded in the first adhesive such that portions of the first plurality of conductors protrude from the first adhesive coating. The portions are soldered to the first conductive anti-reflective coating by an alloy coating on the portions to form ohmic connections between the first conductive anti-reflective coating and the portions of the first plurality of conductors such that electrons can pass between the unpassivated areas of the front side and the first plurality of conductors to permit an electric current generated by the photovoltaic semiconductor apparatus to be conducted by the first plurality of conductors.
The second electrode includes a second electrically insulating film having first and second opposite sides. The first side of the second film has a second adhesive for adhering the second film to the second conductive anti-reflective coating. The second electrode further includes a second plurality of conductors embedded in the second adhesive such that portions of the second plurality of conductors protrude from the second adhesive coating.
The portions of the second plurality of conductors are soldered to the second conductive anti-reflective coating by an alloy coating on the portions to form ohmic connections between the portions of the second plurality of conductors and the second conductive anti-reflective coating such that electrons can pass between the back side surface of the third volume and the second plurality of conductors to permit the electric current generated by the photovoltaic semiconductor apparatus to be conducted by the second plurality of conductors.
In another embodiment, the second doped volume may have a back side surface and may include a second passivation layer on the back side surface and may further include a layer of aluminum on the second passivation layer. The layer of aluminum has a plurality of laser-fired current collecting contacts extending through the second passivation layer to the second doped volume.
A solar cell employing the apparatus with the layer of aluminum includes first and second electrodes connected to the front and back surfaces of the apparatus respectively. The first electrode includes a first optically transparent electrically insulating film having first and second opposite sides. The first side has a first adhesive for adhering the first film to the first conductive anti-reflective coating. The first electrode further includes a first plurality of conductors embedded in the first adhesive such that portions of the first plurality of conductors protrude from the first adhesive coating. The portions are soldered to the first conductive anti-reflective coating by an alloy coating on the portions to form ohmic connections between the conductive anti-reflective coating and the portions of the first plurality of conductors such that electrons can pass between the unpassivated areas of the front side and the first plurality of conductors to permit an electric current generated by the photovoltaic semiconductor apparatus to be conducted by the first plurality of conductors.
The second electrode includes a second electrically insulating film having first and second opposite sides. The first side of the second film has a second adhesive for adhering the second film to layer of aluminum. The second electrode further includes a second plurality of conductors embedded in the second adhesive such that portions of the second plurality of conductors protrude from the second adhesive coating. The portions of the second plurality of conductors are soldered to the aluminum layer by an alloy coating on the portions to form ohmic connections between the portions of the second plurality of conductors and the aluminum layer to permit the electric current generated by the photovoltaic semiconductor apparatus to be conducted by the second plurality of conductors through the aluminum layer and the laser-fired contacts to the second doped volume.
In accordance with another aspect of the invention, there is provided a method of making a photovoltaic semiconductor apparatus for use in forming a solar cell. The method involves forming a first plurality of openings in a first passivation layer on a front side of a first doped volume of semiconductor material of a semiconductor wafer having first and second adjacent oppositely doped volumes of semiconductor material forming a heterojunction, the first plurality of openings defining corresponding unpassivated areas of the first front side that are unpassivated by the first passivation layer. The method also involves forming a first conductive anti-reflective coating on a first outer surface of the first passivation layer and on the corresponding unpassivated areas of the front side.
Forming the first plurality of openings may involve causing each opening of the first plurality of openings to have a width of about 50 micrometers to about 200 micrometers.
Forming the first plurality of openings in the first passivation layer may involve arranging the first plurality of openings in parallel lines across the first outer surface.
The distance between parallel lines of openings in the first passivation layer may be about 500 micrometers to about 5000 micrometers.
Forming the first plurality of openings in the first passivation layer may involve arranging the first plurality of openings in parallel lines connected by cross parallel lines to form a grid arrangement.
The grid arrangement may have meshes of about 500 micrometers to about 5000 micrometers square.
Forming the first conductive anti-reflective coating may involve forming a first continuous conductive anti-reflective coating on the first outer surface and on the unpassivated areas of the front side surface.
Forming the first conductive anti-reflective conductive coating may involve causing the first conductive anti-reflective coating to have a thickness of about 70 nm to about 280 nm.
Forming the first conductive anti-reflective coating on the first outer surface and on the unpassivated areas of the front side surface may involve applying a material including at least one of InOx; SnOx, InSnOx; TiOx; and ZnOx.
Forming the first conductive anti-reflective coating may involve causing the first conductive anti-reflective coating to have a sheet resistivity of about 1 Ohm/Sq to about 30 Ohm/Sq.
The method may involve forming the heterojunction by at least one of ion-implanting and thermal diffusion.
The method may involve forming the first doped volume to have a sheet resistivity of about 60 ohms per square to about 150 ohms per square and desirably has a sheet resistivity of about 80 ohms per square to about 150 ohms per square
The method may involve forming the first passivation layer.
Forming the first passivation layer may involve forming a layer of at least one of SiO2, SiN4 and SiC on the front side.
Forming the first passivation layer may involve causing the first passivation layer to have a thickness of about 10 nm to about 500 nm and desirably about 10 nm to about 50 nm.
The method may involve forming a second plurality of openings in a second passivation layer on a back side surface of the second doped volume of the semiconductor material, the second plurality of openings defining corresponding unpassivated areas on the back side surface. The method may also involve forming a second conductive anti-reflective coating on an outer surface of the second passivation layer and on the unpassivated areas of the second back side surface.
Forming the second plurality of openings may involve causing each of the second plurality of openings to have a width of about 50 micrometers to about 200 micrometers.
Forming the second plurality of openings in the second passivation layer may involve arranging the second plurality of openings in parallel lines across the back side surface.
The distance between said parallel lines in the second passivation layer may be about 500 micrometers to about 5000 micrometers.
Forming the second plurality of openings in the second passivation layer may involve arranging the second plurality of openings in parallel lines connected by cross parallel lines to form a grid arrangement.
The grid arrangement may have meshes of about 500 micrometers to about 5000 micrometers square.
Forming the second conductive anti-reflective coating may involve forming a second continuous conductive anti-reflective coating on the outer surface of the second passivation layer and on the unpassivated areas of the back side surface.
Forming the second conductive anti-reflective conductive coating may involve causing the coating to have a thickness of about 70 nm to about 500 nm.
Forming the second conductive anti-reflective coating may involve coating the outer surface of the second passivation layer and the unpassivated areas of the back side surface with a material including at least one of InOx; SnOx, InSnOx; TiOx; and ZnOx.
Forming the second conductive anti-reflective coating may involve causing the second conductive anti-reflective coating to have a sheet resistivity of about 1 Ohm/Sq to about 30 Ohm/Sq.
The method may involve forming the second passivation layer.
Forming the second passivation layer may involve forming a layer of at least one of SiO2, SiN4 and SiC on the outer surface.
Forming the second passivation layer may involve causing the second passivation layer to have a thickness of about 10 nm to about 500 nm and desirably about 10 nm to about 50 nm.
The method may involve adhering an adhesive on an optically transparent electrically insulating film to the first conductive anti-reflective coating such that portions of an alloy coating on corresponding exposed portions of a first plurality of conductors embedded in the adhesive are disposed on the first conductive anti-reflective coating. The method may also involve heating the alloy coating while pressing the exposed portions against the first conductive anti-reflective coating to cause the alloy coating to solder the exposed portions of the first plurality of conductors to the first conductive anti-reflective coating to create ohmic connections between the first plurality of conductors and the first conductive anti-reflective coating.
The method may involve adhering a second adhesive on a second electrically insulating film to the second conductive anti-reflective coating such that portions of a second alloy coating on corresponding exposed portions of a second plurality of conductors embedded in the second adhesive are disposed on the second anti-reflective conductive coating. The method may further involve heating the second alloy coating while pressing the exposed portions of the second plurality of conductors against the second conductive anti-reflective coating to cause the second alloy coating to solder the exposed portions of the second plurality of conductors to the second conductive anti-reflective coating to create ohmic connections between the second plurality of conductors and the second conductive anti-reflective coating.
The method may involve forming a second conductive anti-reflective coating on a back side surface of a third doped volume on a side of the second doped volume opposite the semiconductor junction, the third doped volume having the same doping polarity as the second volume thereby forming an isotype junction and wherein the third doped volume has a doping concentration greater than a doping concentration of the second volume.
Forming the second conductive anti-reflective coating may involve forming a second continuous conductive anti-reflective coating on the back side surface of the third doped volume.
Forming the second conductive anti-reflective coating may involve causing the second conductive anti-reflective coating to have a thickness of about 70 nm to about 500 nm.
Forming the second conductive anti-reflective coating may involve coating the back side surface of the third doped volume with a material including at least one of InOx; SnOx, InSnOx; TiOx; and ZnOx.
Forming the second conductive anti-reflective coating may involve causing the second conductive anti-reflective coating to have a sheet resistivity of about 1 Ohm/Sq to about 30 Ohm/Sq.
The method may involve adhering an adhesive on an optically transparent electrically insulating film to the first conductive anti-reflective coating such that portions of an alloy coating on corresponding exposed portions of a first plurality of conductors embedded in the adhesive are disposed on the first conductive anti-reflective coating. The method may further involve heating the alloy coating while pressing the exposed portions against the first conductive anti-reflective coating to cause the alloy coating to solder the exposed portions of the first plurality of conductors to the conductive anti-reflective coating to create ohmic connections between the first plurality of conductors and the first conductive anti-reflective coating.
The method may involve adhering a second adhesive on a second electrically insulating film to the second conductive anti-reflective coating such that portions of a second alloy coating on corresponding exposed portions of a second plurality of conductors embedded in the second adhesive are disposed on the second conductive anti-reflective coating. The method may further involve heating the second alloy coating while pressing the exposed portions of the second plurality of conductors against the second conductive anti-reflective coating to cause the second alloy coating to solder the exposed portions of the second plurality of conductors to the second conductive anti-reflective coating to create ohmic connections between the second plurality of conductors and the second conductive anti-reflective coating.
The method may involve forming a second passivation layer on a back side surface of the second volume.
Forming the second passivation layer may involve forming a layer of at least one of SiO2, SiN4 and SiC on the outer surface.
Forming the second passivation layer may involve causing the second passivation layer to have a thickness of about 10 nm to about 500 nm and desirably about 10 nm to about 50 nm.
The method may involve forming a layer of aluminum on the second passivation layer.
Forming the layer of aluminum may involve forming the layer of aluminum by at least one of vapor deposition and sputtering.
Forming the layer of aluminum may involve forming the layer of aluminum such that the layer of aluminum has a thickness of about 1 micrometer to about 20 micrometers and desirably about 2 micrometers to about 10 micrometers.
The method may involve forming a plurality of laser-fired contacts in the layer of aluminum.
The method may involve adhering an adhesive on an optically transparent electrically insulating film to the first conductive anti-reflective coating such that portions of an alloy coating on corresponding exposed portions of a first plurality of conductors embedded in the adhesive are disposed on the front side. The method may further involve heating the alloy coating while pressing the exposed portions against the first conductive anti-reflective coating on the unpassivated areas to cause the alloy coating to solder the exposed portions of the first plurality of conductors to the conductive anti-reflective coating to create ohmic connections between the first plurality of conductors and the first conductive anti-reflective coating.
The method may involve adhering a second adhesive on a second electrically insulating film to the layer of aluminum such that a second alloy coating on corresponding exposed portions of a second plurality of conductors embedded in the second adhesive are disposed on the layer of aluminum. The method may further involve heating the second alloy coating while pressing the exposed portions of the second plurality of conductors against the aluminum layer to cause the second alloy coating to solder the exposed portions of the second plurality of conductors to the layer of aluminum to create ohmic connections between the second plurality of conductors and the layer of aluminum to permit current to flow between the second plurality of conductors and the second doped volume through the laser-fired contacts and the layer of aluminum.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
In drawings which illustrate embodiments of the invention,
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If the crystalline silicon wafer 15 is pre-doped to become p-type semi-conductor material, then the first doped volume 12 is usually formed by doping a front side of the wafer with phosphorous and if the silicon wafer is initially n-type, then its front side is usually doped with boron. Doping may be achieved by ion implantation which facilitates penetration of boron or phosphorous ions into the pre-doped semiconductor material providing for shallow emitter formation with sharp cut-off p/n junction barrier. These properties facilitate better p/n junction performance in charge separation and sensitivity in the blue spectral region. Subsequent annealing activates doped elements within the first doped volume on one side of the heterojunction 16 which defines the first and second doped volumes 12 and 14 within the silicon wafer 15. Alternatively, the first and second doped volumes 12 and 14 may be produced by conventional thermal diffusion of gas containing phosphorous or boron dopant atoms and subsequent annealing as described above. Desirably, the first doped volume 12 has a sheet resistivity of about 80 to about 150 ohms per square. Alternatively, the initially doped semiconductor material may be further doped by applying solid phosphorous or boron doping sources on the semiconductor material followed by subsequent firing diffusion and annealing. However, ion implantation techniques consume substantially less energy than conventional thermal diffusion processes and are therefore favoured over thermal diffusion techniques.
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After completing the third step shown in
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After having formed a plurality of openings in the second passivation layer 106, the second conductive anti-reflective coating 130 comprised of at least one of InOx, SnOx, InSnOx, TiOx or ZnOx is applied by chemical vapour deposition, sputter or other methods. Desirably, the second conductive anti-reflective coating 130 is formed across the surface defined by the second outer surface 108 of the passivation layer and the unpassivated areas 120, 122, 124, 126, and 128 to provide a continuous coating all across the back side of the wafer. Continuous means that there are no breaks in the second conductive anti-reflective coating 130 across the entire surface, even though the second conductive anti-reflective coating has a somewhat serpentine shape in cross section. Desirably, the second conductive anti-reflective coating 130 has a thickness that is about the same as or greater than the thickness of the first conductive anti-reflective coating 44. In this regard, the second conductive anti-reflective coating 130 may have a thickness of about 70 nanometers to about 500 nanometers. Desirably, the second anti-reflective coating has a sheet resistively of about 1 ohm per square to about 30 ohms per square.
With both the front side and back side of the apparatus prepared as described above, the apparatus is ready to receive first and second electrodes respectively. Referring to
A plurality of conductors, one of which is shown at 90, are embedded in the first adhesive coating 88 such that portions 92 protrude from the first adhesive coating 88. The portions 92 of the conductors 90 are soldered to the first conductive anti-reflective coating 44 by heating and pressing an alloy which may be provided as a coating pre-formed on the exposed portions of the conductors 90. The alloy may include a composition including at least two of Ag, Bi, Cd, Ga, In, Pb, Sb, Sn, and Zn. For example the alloy may include a composition including In, Sn, Ag in a proportion of about 47% In, about 51% Sn, and about 2% Ag. Alternatively, the alloy may include In and Sn in a proportion of about 48% In and about 52% Sn. The alloy may have a thickness of about 1 micron to about 5 microns and may have a melting temperature about 30° Celsius to about 200° Celsius. More particularly, the alloy may have a melting temperature of between about 60° Celsius and about 150° Celsius.
Soldering the portions 92 to the first conductive anti-reflective coating 44 forms ohmic connections between the portions 92 of the conductors, and the first conductive anti-reflective coating 44, such that electrons can pass between the unpassivated areas 34, 36, 38, 40, and 42 and the first conductive anti-reflective coating 44 and the portions 92 of the conductors embedded in the adhesive on the first electrode 80 to permit an electric current generated by the photovoltaic semiconductor apparatus 10 to be conducted by the conductors 90. The conductors 90 are connected to a bus bar 94 which acts as a first terminal that collects current from the conductors and enables the photovoltaic cell to be connected to an electrical circuit.
Further details of general and alternate constructions of the first electrode 80 may be obtained from applicant's International Patent Application published under International Publication Number WO 2004/021455A1, which is incorporated herein by reference.
The second electrode is shown generally at 140 and is applied to the second conductive anti-reflective coating 130. The second electrode 140 is similar to the first electrode 80, in that it includes a second electrically insulating film 142 having first and second opposite sides 144 and 146. The second insulating film need not be optically transparent. The first side 144 of the second film 142 has a second adhesive coating 148 for adhering the second film to the second conductive anti-reflective coating 130. A second plurality of conductors 150 are embedded in the second adhesive coating 148 such that portions 152 protrude from the second adhesive coating and are soldered to the second conductive anti-reflective coating 130 by heating and pressing an alloy coating thereon, as described above, to form ohmic connections between the portions of the conductors 150 and the second conductive anti-reflective coating 130. Electrons can therefore pass between the conductors 150, and the second conductive anti-reflective coating and the unpassivated areas (not shown in
Referring to
A second conductive anti-reflective coating 166 is provided on the back side surface 164 of the third doped volume 160. Desirably, the second conductive anti-reflective coating 166 is continuous and has a thickness that is about the same as or greater than the thickness of the first conductive anti-reflective coating 44. In this regard, the second conductive anti-reflective coating 166 may have a thickness of between about 70 nanometers to about 500 nanometers. The second conductive anti-reflective coating 166 may be comprised of at least one of InOx, SnOx, InSnOx, TiOx and ZnOx. Desirably, the second conductive anti-reflective coating has a sheet resistivity of between about 1 ohms per square to about 30 ohms per square.
The first and second electrodes 80 and 140 are secured to the front side of the apparatus and to the second conductive anti-reflective coating 166 of the third doped volume 160, respectively, in the same manner as described above in connection with
Referring to
The layer of aluminum 170 is then formed on the surface of the second passivation layer 174, using vacuum evaporation or sputtering techniques. The layer of aluminum 170 may be formed to have a thickness of about 1 micrometer to about 20 micrometers and more desirably to have a thickness of about 2 micrometers to about 10 micrometers.
The laser-fired contacts 172 are laser-fired into the layer of aluminum using conventional techniques that cause portions of the layer of aluminum 170 to burn through the second passivation layer 174 and form an alloy with the second doped volume 14, thereby creating a back surface field and current collecting contacts.
To form a solar cell using the semiconductor apparatus shown in
The present invention provides a photovoltaic cell that has a shallow emitter that is generally uniform in thickness and thus there is no need to selectively form emitter areas of different thicknesses. In addition, since the emitter is shallow, the apparatus is more responsive to blue light than devices with emitters of non-uniform thickness, making the overall device more efficient in converting light energy into electrical energy.
In addition, the methods and apparatus described herein do not require screen printing technology, which eliminates several time and energy consuming manufacturing steps and reduces susceptibility to bowing that can be caused by use of conductive pastes on the front and back surfaces of the cell.
In addition, the lack of any need for screen printing technology allows solar cells to be manufactured more quickly and at less cost.
In addition the avoidance of the use of screen printing technology allows the formation of substantially thinner emitters without risk of emitter shunting.
In addition the combination provided by the passivation layer with openings and the conductive anti-reflective coating facilitates efficient current collection while simultaneously providing semiconductor surface passivation
In addition, the methods and apparatus described herein allow the use of ion implantation as an alternative to thermal diffusion for hetero- and isotype junction formation thus decreasing manufacturing energy consumption and manufacturing costs.
Finally, the use of the conductive coatings on at least the front surface of the solar cell and the use of first and second electrodes soldered to the first and second conductive anti-reflective coating and the back side surface respectively obviates the need to precisely align the conductors on the electrodes with pre-printed contacts. Precise alignment of the electrodes so that the conductors on the electrodes align with pre-formed contacts on the front and back surfaces is not necessary, enabling a relaxation of manufacturing tolerances in solar cell manufacturing, which further decreases production costs.
While specific embodiments of the invention have been described and illustrated, such embodiments should be considered illustrative of the invention only and not as limiting the invention as construed in accordance with the accompanying claims.
