Patent Application
20130000705
This invention relates to a photovoltaic device and a method of its manufacture.
Photovoltaic devices are formed by an array of photovoltaic cells fabricated in semiconductor wafers with appropriate electrical connection between them forming a readout circuit that collects photocurrent generated by multiple cells of the same cell unit (array). The readout circuit includes metal conductors from which the electric current is further transferred via so-called bus lines. A major factor in the performance of a photovoltaic device is the shading of the active area of the cell unit (i.e. its light collection surface) by the metal conductors and the bus lines, which all extend on top of the light collecting surface. The larger the surface area covered by the metal lines (metal conductors), the smaller the active, light collection area of the device. On the one hand, increasing the spacing between the metal lines (metal conductors), in order to reduce the shading, results in an increase in the cell series resistance with the direct outcome of increased resistive power loss. On the other hand, decrease of the spacing between the metal lines results in lower resistive power loss at the expense of larger shading loss.
A solution to the above problem is to use device configurations in which all the metal lines are located at the back side of the multi-cell device. However, this solution has certain drawbacks. The short wavelength photo generated electrons close to the top surface of the cell have to diffuse to the bottom side of the p-substrate through the few hundred microns of the wafer thickness with the penalty of certain loss due to recombination, resulting in certain decrease of cell efficiency. This also involves a more complex manufacturing process accompanied by higher cell price, and difficulty in effective cooling of the cell from the back side, since direct connection of the cell back side to the metal block is problematic because both polarities' cell contacts are on the same back side. This problem is aggravated in the case of concentrating systems. As a result, most currently manufactured solar cells have a metal contact to the n+ layer at the front side with resulting resistive and shading losses of about 12%.
There is a need in the art for a novel photovoltaic device which is configured for increasing the efficiency of photocurrent production. The configuration of the device of the present invention provides for desirably increased effective area for light collection (area of the outer, light collecting surface of the device, exposed to input light and capable of collecting the light), thus increasing the amount of collected light, as well as desirably reducing the total resistance within the device, thus facilitating the conversion of the collected light into the photocurrent. The present invention also provides for a cost effective technique for the fabrication of such a photovoltaic device, utilizing the general principles of lithography.
The main idea of the present invention is based on the understanding of the following. According to the conventional approach in the field of photovoltaic devices, a p-type wafer is formed with an upper n+ layer. The upper n+ layer is to be heavily doped to reduce the series resistance. This, however, reduces the lifetime of the photo generated carriers, thus less contributing to the generation of photocurrent. On the other hand, a lower level of doping results in higher series resistance, thus requiring smaller spaces between thick metals lines, typically about 1000 μm between the about 50 μm wide metal lines. As a result, about 5% of the surface of the device does not participate in the light collection being shaded by metal lines of the cells' readout circuit. As indicated above, these metal lines are further connected to one or more bus lines for transmitting electrical energy. The bus lines are wide wires, even wider than said metal lines. According to the conventional approach, the bus lines are also located within the surface area exposed to light, thus further shading a fraction of the effective area of the cell unit by about an additional 5%. The cumulative shading losses are accordingly about 10%.
According to the invention, the surface of a photovoltaic device (semiconductor wafer) is patterned to form an array of spaced-apart grooves, each having at least a bottom portion with side surfaces substantially perpendicular to the surface of the photovoltaic device. A photovoltaic cell structure (module) is created by providing a heavily doped semiconductor (typically n+) layer in the form of spaced-apart regions only inside the bottom portions of the grooves, while the rest of the photovoltaic cell's surface is doped with significantly lower concentration of n-type impurities, or it may be doped with p-type impurities. This configuration enables increased doping level only inside the bottom portions of the grooves, to thereby significantly reduce the resistance of the cell structure (e.g. by a factor of 2-4), which in turn allows for increasing a distance between the metal lines (extending perpendicular to, or generally intersecting with, the grooves' axis) to electrically connect the grooves. Indeed, provision of n+ regions only in such “vertical” grooves, whose regions are practically not light absorbing by themselves (due to the geometry of the bottom portion of the grooves which results in that practically light rays incident onto the bottom portion are only those of almost normal incidence, and thus a very small amount of light interacts with these regions), allows for increasing the level of doping. Preferably, these heavily doped n+ regions are further covered by a thin metal layer, which further reduces the series resistance, allowing for further increasing a distance between the perpendicular metal lines. Light reflected from the metal regions is further absorbed by side walls of the grooves, thus further increasing the device efficiency. The lightly doped n-type or alternatively p-type surface does not contribute to the resistive losses, as the distance between neighboring heavily doped n-type lines is about two orders of magnitude lower than the distance of about 1 mm between the metal lines in standard photovoltaic cells.
Increased distance between the perpendicular metal lines allows for reducing the number of these metal lines, for the lines of a given width and a given surface area of the photovoltaic cell, thus increasing the effective light collection area of the cell. On the other hand, the metal lines may be of varying width, such that each metal line is narrower at the central region of the cell and becomes wider towards the sides of the cell. These wider portions of the metal lines are thus located at the connection points of the lines to the bus lines. The bus lines can be “moved” outside the light collection surface of the photovoltaic cell further reducing the effect of shading, thus increasing the effective area of light collection.
Thus, the metal lines are allowed to be wider due to a need to provide a much smaller number of such lines per cell. Such metal lines can be configured with a varying width, e.g. can be of a trapezoid-like shape, with the width increasing from the center of the photovoltaic cell towards the periphery (sides) thereof, where the bus lines are placed. This configuration of the metal lines allows for further reduced shading of the light collection surface (increasing the effective area of the cell), while keeping the electrical resistance desirably low. The metal lines are configured to be relatively narrow at the central region of the cell area where the electric current is lower, and become wider at the periphery region of the cell close to the sides thereof where the current is higher (after collecting current from larger parts of the photovoltaic cell). Thus, the trapezoid or the like width-varying shape of the metal lines is appropriately selected to maintain substantially constant current density along the metal line while reducing the shading of the light collection surface. The fewer but wider trapezoidal-like shaped metal lines can be made significantly thicker than the much narrower lines in standard cells thus further reducing series resistance.
Thus, according to one aspect of the invention, there is provided a photovoltaic device comprising a semiconductor substrate having a patterned light collecting surface defining an array of spaced-apart substantially parallel first grooves, each having a bottom portion comprising a bottom surface and side walls extending from the bottom portion and being substantially perpendicular to the surface of the device, a doped semiconductor layer in the form of spaced-apart regions located on the bottom surfaces of the first grooves respectively.
The semiconductor substrate is typically a p-type wafer, and the doped semiconductor layer is a heavily doped n+-type layer. It should be stressed however that the same type of cells can be made of n-type silicon with p-type doping at the top surface.
Preferably, a metal layer in the form of spaced-apart regions is provided on top of the doped semiconductor regions respectively.
A bottom surface of the groove portion may and may not be planar. For example, it may have a tip-like shape.
Preferably, the first groove has a top portion extending from the sidewalls of the bottom portion and having a funnel-like shape (i.e. side walls of the top portion are tilted such that the cross sectional area of the top portion increases from the bottom portion towards the outer surface of the cell). As for the bottom portion of the first groove, it is shaped to define an elongated narrow cavity with the side walls thereof being substantially perpendicular to the bottom plane, i.e. perpendicular or forming a funnel like structure with a relatively small tilt of the side walls such that the cross sectional area of the bottom portion decreases from bottom surface towards the top portion.
The first grooves may be arranged with a space of about 10-20 micrometers between them.
A second array of metal lines which electrically connect the first grooves extend in a spaced-apart parallel relationship along a second axis substantially perpendicular to the first axis along which the first grooves are arranged. The metal lines (which are typically of a width of a few hundreds of micrometers) can be spaced a distance of at least a few centimeters from one another. The metal lines may be located in second wider grooves defined perpendicularly to the densely packed narrow grooves, extending along the bottom surfaces of the second grooves.
Preferably the metal lines are configured with a varying width, e.g. have a trapezoid-like geometry being narrower around the central part of the photovoltaic cell and wider at the sides (peripheral part) of the cell. In some embodiments, the width of the metal lines around the center of the cell is about one hundred micrometers, and the width at the sides of the cell may be close to one millimeter. It should be understood that the geometry/shape of the metal lines and the number and arrangement of these lines are selected in accordance with a desired resistance to be obtained along the lines for a desirably small number and low-density arrangement of such lines, to meet the requirements of the current density in the cell of given dimensions.
Considering a typical photovoltaic cell of size of 10 by 10 centimeters configured according to the present invention as described above, the metal lines may be spaced apart a distance of at least a few millimeters from one another. In some embodiments the metal lines may be spaced apart a distance of at least a few millimeters from one another. In some other embodiments, the metal lines may be spaced apart a distance of at least a few centimeters from one another. For example, the distance between the metal lines may be about 2.5 centimeters.
A bus line arrangement, composed of one or more bus lines, electrically connecting the metal lines, may be located in at least one (third) groove extending substantially parallel to the first axis.
According to some embodiments of the present invention, the bus line arrangement may be composed of two buses located outside the light collection surface at the opposite sides of the photovoltaic cell and substantially parallel to the first axis. Such bus lines are electrically connected to the metal lines extending along the second axis, intersecting with (e.g. being substantially perpendicular to) the first axis.
The effects of reduced series resistance and shading can reduce cumulative losses from about 12% to about 6%.
According to some other embodiments of the present invention, the photovoltaic cell is configured such as to eliminate a need for two bus lines, and uses a single bus line, extending substantially perpendicular to (generally intersecting with) the first axis along which the cell grooves of the photovoltaic device extend and carrying out the complete current collection from the photovoltaic device (from the array of cells). In the description below this single metal line is sometimes termed “bus line”, but it should be understood that it actually performs the function of perpendicular metal and bus lines of the alternative configurations. In this case, such a bus line is directly electrically connected to the metal regions in the cell grooves. This may result in lower manufacturing cost of the cell.
The above-described configurations of the photovoltaic device of the invention provides for obtaining combined series resistance and shading losses substantially not exceeding a few percentages, e.g. 6% or less. The efficiency loss may be as low as 5%.
According to another aspect of the invention, there is provided a photovoltaic device comprising a semiconductor substrate having a patterned surface defining an array of spaced-apart first grooves extending along a first axis, each groove having a bottom portion comprising a bottom surface and side walls extending from said bottom portion and being substantially perpendicular to said surface of the device, a doped semiconductor layer in the form of spaced-apart regions located on the bottom surfaces of said first grooves respectively; and an array of metal lines extending in a spaced-apart parallel relationship along a second axis substantially perpendicular to said first axis and being electrically coupled with said regions in the first grooves, said metal lines being spaced a distance of at least a few thousands of micrometers from one another.
According to yet another aspect of the invention, there is provided a photovoltaic device comprising: a semiconductor substrate having a patterned surface defining an array of spaced-apart first grooves, each having a bottom portion comprising a bottom surface and side walls extending from said bottom portion and being substantially perpendicular to said surface of the device; a doped semiconductor layer in the form of spaced-apart regions located on the bottom surfaces of said first grooves respectively; and a metal layer in the form of spaced-apart regions on top of said doped semiconductor regions respectively.
The invention also provides a method for manufacturing a photovoltaic device, the method comprising: patterning a light collecting surface of a semiconductor substrate, said patterning comprising creating a first array of first grooves arranged in a spaced-apart parallel relationship and extending along a first axis with a first distance between them, and a second array of second grooves arranged in a spaced-apart parallel relationship and extending along a second perpendicular axis with a second larger distance between them, each of the first grooves having a bottom surface and side walls extending from said bottom surface and being substantially perpendicular to said light collecting surface; forming a doped semiconductor layer in the form of spaced-apart regions located on the bottom surfaces of said first grooves respectively.
The formation of the doped semiconductor regions may be carried out in a single lithography step.
The method preferably further includes creation of a metal layer in the form of spaced-apart regions on top of the doped semiconductor regions in the grooves.
According to yet further aspect of the invention, there is provided a method for manufacturing a photovoltaic device characterized by combined series resistance and shading losses substantially not exceeding a few percentages, the method comprising patterning a light collecting surface of a semiconductor substrate, said patterning comprising creating a first array of first grooves arranged in a spaced-apart parallel relationship and extending along a first axis with a first distance between them, and a second array of second grooves arranged in a spaced-apart parallel relationship and extending along a second perpendicular axis with a second larger distance between them, each of the first grooves having a bottom surface and side walls extending from said bottom surface and being substantially perpendicular to said light collecting surface, and forming a heavily doped semiconductor layer in the form of spaced-apart regions located on the bottom surfaces of said first grooves respectively, and providing spaced-apart metal regions on top of said spaced-apart regions of the heavily doped semiconductor respectively.
As for the entire front surface of the device, except for the bottom of the trench, it may be formed with a lower concentration of n-type layer, for the collection of the photo generated free electrons in the p-substrate. Alternatively, the front surface may be p-type with slightly higher doping concentration than the p-type substrate, to form a potential barrier for the reduction of surface recombination.
In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:
Referring to
Such arrangement of the side walls 20 substantially perpendicular to the surface 14, or the so-called “substantially vertical” bottom portion 18 can be obtained by making the side walls 20 parallel to each other or arranged with a small angle between them. In the latter case, the configuration is such that a distance between the walls either increases or decreases along a direction from top to the bottom of the groove 16 (in other words, the bottom portion is either wider or narrower at the bottom thereof than at the top thereof). The bottom surface 19 may have a planar geometry, or a curved one, for example being shaped like a tip.
Preferably, the groove 16 has a top portion 26 extending from the side walls of the bottom portion and having a funnel-like shape (i.e. having tilted side surfaces extending along two intersecting planes) such that the top portion is wider at the distal part thereof (at the external surface 14 of the cell) than at the proximal part thereof (where it interfaces with the bottom portion). It should be understood that the provision of the funnel-like top portion is optional, and alternatively, light can be absorbed by planar surface regions 14 of the substrate in between the grooves 16.
When the surface 14 of the cell structure 10 is exposed to electromagnetic radiation, the side walls of the grooves 16 (e.g. including also tilted surfaces of the top groove portion 26 if any) operate as the active, light collecting area of the device, while the bottom surface of the bottom portion 18 of the groove practically does not collect incident radiation but acts as a reflective/scattering surface, where most of the radiation reflected from it hits the side walls and is absorbed by the semiconductor. The cross sectional dimension (width) a of the bottom portion 18 of the groove may be of a few micrometers (e.g. 1-5 μm), and the depth (height) b of this groove portion 18 may be 2-3 times larger, e.g. about 4-15 μm. As for the depth (height) of the top groove portion 26 it may be about 8-20 μm.
The device 10 further includes a heavily doped n+ layer in the form of spaced-apart regions 22 located only at the bottom of the groove portion 18. Preferably, the device 10 further includes a metal layer in the form of spaced-apart regions 24 on top of n+ regions 22. It should be understood, and is also indicated above that the present example refers to a semiconductor substrate which is typically a p-type wafer, and the doped semiconductor layer is a heavily doped n+-type layer; however the principles of the invention can be implemented or made of n-type silicon with p-type doping at the top surface.
Reference is made to
Reference is now made to
Reference is now made to
The semiconductor substrate of the photovoltaic device 10 of
More specifically, the metal lines L2 are configured with a trapezoid shape with width w1, e.g. of about a millimeter, at the sides of the cell and width w2 of about a few hundreds of micrometers at the center of the cell. The current collected by the metal lines L2 is accumulated from the center of the cell to the sides. The width of the metal lines L2 can therefore be narrow at the center of the cell w2 thus reducing shadowing of the light collection surface and contributing to the effective area of the cell. At the sides of the photovoltaic cell, the metal lines are wider in order to reduce the resistivity at higher current accumulated along the second axis of the cell and maintain a constant current density along the line L2.
It should be made clear that the trapezoidal-like shape of the metal lines L2 cannot be implemented in standard solar cell technology. This is because the metal lines, which are close to each other and thus exist in a large number, are kept at minimum width to reduce their shading.
Bus lines L3 are positioned at the sides of the photovoltaic cell and therefore practically do not screen a part of the light collection surface of the cell from incoming light. This is possible since the bus lines L3 are connected only to a few metal lines L2. These few metal lines are much wider than corresponding lines in the standard cell unit configuration, consequently they can be made much thicker with the result of significantly smaller series resistance. The soldering positions (nodes) may be larger without increasing the shadowing of the device.
The choice of distance d3 between the metal lines L2 and similarly the number of the metal lines is dictated mainly by the width of the grooves 16 and the metal lines L1 within the grooves. The wider the grooves, the lower the resistance of the cell structure, and the use of a fewer metal lines in the cell is sufficient. Still, wider grooves should preferably be deeper in order to reduce reflection of light from the n+ or metal layer on the bottom of the groove.
By providing the photovoltaic cell with appropriately wide and deep grooves 16 (for example grooves of 4 micrometers or more in width and 12 micrometers or more in depth in a 10×10 centimeters light collection surface), and providing metal lines L1 on the bottom of the grooves, the electrical conductivity within the cell might be sufficient to allow a single bus line for directly collecting the electrical energy from the grooves with no intermediate collection and transfer of energy by the perpendicular metal lines L2. Such an embodiment is illustrated in
The advantages/disadvantages of either one of the above described embodiments relative to others relate basically to the efficiency obtainable by the cell unit on one hand and the manufacturing costs of the cell unit on the other hand.
Table 1 shows the resistive and shading related efficiency losses of exemplary photovoltaic cells of the present invention constructed generally according to the embodiment illustrated in
(1) 10×10 cm2 for a photovoltaic device which does not utilize an additional light concentrator (1st, non-concentrating configuration), in this configuration d1 and d2 are both equal to 10 centimeters, and
(2) 10×3.3 cm2 for the device with ×10 concentrator (2nd, concentrating configuration), wherein d1 equals 10 centimeters and d2 equals 3.3 centimeters.
The table compares the calculated efficiency losses in percents resulting from the resistance of different elements and shading of the light collection surface of the cell unit for both the photovoltaic cells with no concentration and cells configured to work with 10 times concentration of light.
The efficiency loss is associated with two main effects, one is the resistance (R) of the materials in which electric current flows, and the other is shading (S) of the light collection surface. Table 1 shows the calculated values of efficiency loss, EL, associated with the following parameters: the electrical resistance of the semiconductor substrate, (R-substrate); the conduction/resistance of the grooves (in case of the configuration of the present invention) or the diffusion of charge carriers into the semiconductor in case of the conventional configuration, (R-groove); and the resistance of the metal lines L2 collecting the current, (R-metal). It should be noted that this comparative analysis does not include the resistance of the bus lines, because it is relatively low as these lines are much thicker than the other metal lines in the cell unit. The table also shows calculated losses resulting from shading of the light collection surface, S; and losses resulting from reflection of light from reflective parts of the cell unit, S-groove. The elements screening the light collection surface from incoming light and thus causing shading are formed by the metal layer at the bottom of the grooves, lines L1 which reflect a part of the incident light preventing its absorption (S-groove), the metal lines L2 collecting current (S-metal) and the buses L3 (S-bus). In the embodiment illustrated in
The resistance of the semiconductor substrate (R-substrate) is not affected by the configuration of the present invention, as compared to conventional configuration, and therefore this parameter is the same in the invented and conventional photovoltaic cell units. The present invention provides for improved efficiency of the photovoltaic cell unit by reducing the resistance and diffusion of charge carriers into the substrate by introducing the groove geometry, high n+ doping only within the grooves, and can also further improve to the efficiency owing to the provision of a metal layer on top of n+ regions at the bottom surface of the grooves (R-grooves). As indicated above, shading associated losses may be a result of light reflection from the metal layer on the bottom of the grooves (S-grooves), and from shading of the active area by the metal lines (S-metal) and the buses (S-bus). In a photovoltaic cell unit according to the conventional configuration the light collection surface actually does not contain any reflecting features and thus the shadowing losses caused by reflection are almost zero (S-grooves). This is while about 1% of the optical energy might be reflected from the metal layer at the bottom of the grooves of a photovoltaic cell according to the present invention. This estimate value of about 1% is much smaller than the number obtained from the relative width of this metal line with respect to the trench pitch. Two reasons are responsible for this deviation: A) most of the light reflected from the metal at the bottom of the trench is absorbed by the vertical trench walls; B) In the case of fixed solar panels, most of the time there is no direct line of sight from the sun to the metal line. Furthermore, in concentrated systems, most light reaches the cell at slanted angles. On the other hand, the present invention allows for significantly reducing the number and density of the metal lines L2, even eliminating a need for these lines at all (see
Reference is now made to
The above technique eliminates a silk printing step thus reducing manufacturing costs. There is direct correlation between the manufacturing process and the cells' layout. The groove dimensions under the thick metal buses are based on the preferred metallization process (metal on top of n+ regions in the grooves) on the one hand, and on the other hand are dictated by the goal of minimizing the junction area under the metal buses, as it does not contribute to the photocurrent and consumes forward bias diode current.
Thus, the present invention provides a novel configuration of a photovoltaic device and a method of its manufacture. This device has deep narrow and long grooves (groove portions 18), and possibly also funnel-shaped groove top-portions, in the front surface of the device with the heavily doped emitter (n+ layer) located only in the bottom of the grooves, and preferably also metal layer 24 deposited selectively on the bottom of the groove (on top of n+ regions) with the result that most of light reflected from these metal lines hits the side walls of the groove instead of escaping the device. The outer surface of the deposited metal layer 24 is preferably of a certain roughness (not smooth) in order to effect light scattering therefrom, and is also not necessarily flat but in the form of a tip. This structure (having deep narrow parallel groove portions 18 with the metal coated carrier emitter 22, 24, and possibly also having funnel shape impacts on the cell efficiency) reduces the shading in two aspects: (1) about 90% of the reflected light from the buried metal line hits the side walls of the grooves and is absorbed in the silicon, hence contributing to the photocurrent; and (2) the resistance of these buried metal lines is about three orders of magnitude lower than the heavily doped n+ line of the same dimensions. As a result, the spacing between the perpendicular metal lines (L2 in
Turning back to
Those skilled in the art will readily appreciate that various modifications and changes can be applied to embodiments of the invention as hereinbefore described without departing from its scope defined in and by the appended claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IL2010/000824 | 10/11/2010 | WO | 00 | 9/21/2012 |
| Number | Date | Country | |
|---|---|---|---|
| 61287036 | Dec 2009 | US |
