RU2407108C2 - Concentrating solar cell - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: concentrating solar cell (8) is in form of a rectangle with ratio of length of sides between 1 and 1.5. Said element has a substrate (3), a multilayer structure (4) formed on the substrate (3), with a central photosensitive area (12), a contact layer (9), a solid lower electrode (2) and an upper electrode in form of a contact grid, having at least one current-collecting bus (10), lying on the perimeter of the photosensitive area (12), and current-collecting strips (11). The current-collecting strips (11) equidistantly come from at least one current-collecting bus (10) at and angle 35°-55° to the lateral face (13) of the solar cell (8). The current-collecting strips (11) are parallel each other within each of four segments (a, b, c, d), lying between mutually perpendicular planes, passing through the middle of opposite sides of the rectangular solar cell (8).

EFFECT: invention enables design of a concentrating solar cell, having high efficiency owing to reduced resistance of the top contact and reduced ohmic loss.

11 cl, 7 dwg

Description

The invention relates to semiconductor devices, in particular to devices for converting light energy into electrical energy, and can be used in concentrator photovoltaic module installations.

When developing solar cells operating at high solar radiation concentrations, one of the key points is the design of the upper metal electrode. On the one hand, in order to reduce electrical losses at the contact, it must provide a minimum ohmic resistance, on the other hand, the contact grid must provide minimal shading of the photosensitive region of the solar cell. Given that in solar concentrator cells the distribution of solar radiation on the surface of the photosensitive region occurs unevenly, the task of optimizing the topology of the contact grid becomes even more difficult.

от центра к краю.A known concentrator solar cell (see US patent No. 4320250, IPC H01L 31/04, published March 16, 1982), comprising a substrate of a semiconductor wafer, a first collector electrode formed on the back surface of the substrate, a doped semiconductor layer formed on the upper surface of the substrate, a contact grid formed on a doped semiconductor layer, and a second metal electrode formed by electroplating on the contact grid. The second metal electrode has a width of the order of less than 25 μm, a thickness of at least more than the width and, therefore, sharp vertical contours. Mentioned contact grid has the shape of a circle and is made in the form of radially diverging rays from the center of the circle to the edges. In this case, each beam is divided into two at a distance

from center to edge.

The known element allows for uniform collection of the generated charge carriers over the entire area of the solar cell. The disadvantages of the known solutions include the strong shadowing created by such a contact system.

Known solar concentrator cell for use in concentrators (see US patent No. 4227940, IPC H01L 31/06, published 10/14/1980), which includes, as a rule, a round plate made of silicon, having a conductivity of from 0.5 to 1.5 Ohm · cm and having a back and top surface. The plate has a diameter of about 2 inches, an aluminum layer formed on the back surface and embedded in a silicon wafer to form a p + layer, an n + layer formed on the upper surface of the plate, a multilayer metal contact structure made on the back surface and providing contact to the p + layer , a multilayer metal contact structure formed on the upper surface and providing contact to the n + layer. The multilayer metal contact structure formed on the upper surface is made in the form of two axisymmetric tires having a circle shape and about 300 radially diverging wedge-shaped strips. Approximately 150 wedge-shaped strips extend from the center region to the edges of the contact grid, intersecting both contact buses, approximately 150 more rays exit the first contact bus and connect to the second contact bus located on the periphery of the contact grid. Another circle, having a very small diameter, is located in the center of the photosensitive region, while not in contact with the rays.

The disadvantages of this design are the large area of shading, as well as the inefficient collection of generated charge carriers in the center of the photosensitive region, where the strips of the contact grid, made in the form of radial rays, do not touch the central circle.

Known three-junction concentrator solar cell (see K. Nishioka, T. Takamoto, T. Agui, M. Kaneiwa, Y.Uraoka, T. Fuyuki. - Solar Energy Materials and Solar Cells. - 90, 2006, 1308-1321), including a lower electrode, a germanium substrate with a transition formed in it, a transition based on the GalnAs compound, a transition based on the GalnP compound, a wide-gap window, a GaAs contact layer, and an upper silver electrode 5 μm thick. The upper electrode and the GaAs contact layer located beneath it are formed in the form of a rectangular grid, on both sides of which are collector bars, 0.85 mm wide and 7 mm long. The distance between the two mentioned tires is 7 mm. The two mentioned tires are connected by 55 current-collecting tracks with a width of 7 μm and a distance between tracks of 120 μm.

The disadvantage of this design is that it is optimized for solar radiation distributed uniformly over the area of the solar cell, while in a concentrator solar power plant, solar radiation incident on the solar cell is distributed unevenly, with a maximum concentration in the center of the solar cell.

Closest to the claimed solution in terms of technical nature and the combination of essential features is the design of a concentrator solar cell adopted as a prototype (see B. Galiniana, C. Algora, I. Rey-Stolle. - Comparison of 1D and 2D analysis of the front contact influence on GaAs concentrator solar cell performance. - Solar Energy Materials and Solar Cells. - 90, 2006, 2589-2604), including a substrate with a heterostructure formed on it, a contact layer, a lower solid electrode and an upper electrode made in the form of a contact grid. The contact grid mentioned above forms a photosensitive region of the solar cell in the form of a square with sides 1 mm in size. The contact grid includes current collector bars 100 μm wide, located along the perimeter of the photosensitive region, and corner current collector strips 5 μm wide, emerging from the current collector bars perpendicular to the sides of the square and pairwise connected at their ends. Said current collector strips extending from the middle of each edge of the photosensitive region are connected in the form of a cross. Current collection tracks and tires have a specific surface resistance of 0.35 Ohm / □.

The known design of the concentrator solar cell prototype allows you to achieve lower losses associated with the shading of the photosensitive region of the solar cell, compared with previous versions of the contact grid. The disadvantages of this technical solution include the insufficiently high coefficient of performance (COP) of a concentrator solar cell operating at high degrees of solar energy concentration (500-1000 suns). The reason for this drawback is the non-optimal design of the contact grid. At a concentration of solar radiation, the main part of the incident light falls on the central region of the solar cell, and current collection is carried out from its corners. For this geometry of the contact system, charge carriers collected in the center of the photosensitive area need to make the maximum path from the center to the edges and further along the collector bars to the corners of the solar cell, which, given the electrical resistance of the contact paths, leads to an additional voltage drop and, therefore, decrease Efficiency of a solar cell.

The objective of the proposed technical solution is to create a concentrator solar cell having increased efficiency by reducing the resistance of the upper contact and, therefore, reducing ohmic losses.

The problem is solved in that the concentrator solar cell is made in the form of a rectangle with a ratio of side lengths ranging from 1 to 1.5, and includes a substrate, a multilayer structure formed on a substrate with a central photosensitive region, a contact layer, a solid bottom electrode and upper electrode in the form of a contact grid. The contact grid contains at least one current collector bus located along the perimeter of the photosensitive region, and current collector strips. Current collector strips equidistantly exit from at least one current collector bus at an angle of 35-55 ° to the sides of the said rectangle and parallel to each other within each of the four segments lying between mutually perpendicular planes drawn through the middle of the opposite sides of the said rectangle. At least in one corner of the rectangle, a collector is attached to the collector bus, for example, by ultrasonic (thermosonic) welding or soldering.

In a concentrator solar cell, the collector strips of one segment can be offset relative to the collector strips of an adjacent segment along the dividing line of these segments by a distance d = 1 / 2D, where D is the distance between the proximal ends of the neighboring strips.

Current collector strips of two adjacent segments can be located symmetrically with respect to a plane passing along the dividing line of these segments, while each current collector strip of one segment can be connected to the current collector strip of the adjacent segment symmetrical to it. In particular, the connection of symmetrical strips of adjacent segments can be performed by pairing.

The proximal end of each collector strip of one segment can be separated by a gap from the proximal end of the collector strip symmetrical to it of the adjacent segment.

The photosensitive region in the concentrator solar cell can be made in the shape of a circle; in the form of a circle flattened on four sides of the said rectangle; in the shape of a square with cut corners.

The contact grid may contain four identical axisymmetric slip rings, separated by a gap near the symmetry axes of the rectangle.

Most preferred is the case where the current collector strips equidistantly exit from at least one current collector bus at an angle of 45 ° to the sides of said rectangle.

In the claimed design of the concentrator solar cell, the minimum length of the contact strips from the center of the solar cell is provided, i.e. areas with a maximum concentration of solar radiation, to the collector bus, as a result of which losses on the resistance of the contact grid are minimized. This advantage allows to increase the efficiency of the inventive concentrator solar cell in comparison with the known concentrator solar cell prototype.

The rectangular shape of the concentrator solar cell is due to the need to cut the semiconductor wafer into individual cells. The optimal shape of a concentrator solar cell is a square (the aspect ratio is 1), since the projection of the incident concentrated solar radiation on the solar cell has the shape of a circle or a circle flattened on four sides, then with an increase in the aspect ratio of the rectangle to 1.5, the fraction of the solar cell area increases not participating in the generation of charge carriers, which leads to inefficient use of the material. The angle at which the collector strips are located to the sides of the said rectangle is determined from the conditions for ensuring the minimum electrical resistance for the part of the current generated in the central region of the solar cell, where the concentration of solar radiation reaches its maximum value. The optimal value for this angle is a value of 45 °. When using angles smaller or larger than 35-55 °, losses on the electrical resistance of the contact grid increase significantly, and the competitive advantage is lost compared with the prototype. The same distance between the collector strips and their parallelism are determined by the need to ensure uniform collector.

The claimed invention is illustrated by drawings, where:

figure 1 shows a side view of a concentrator solar cell prototype in cross section along aa;

figure 2 shows a top view of a concentrator solar cell prototype;

figure 3 shows a side view of the inventive concentrator solar cell in cross section along BB;

figure 4 shows a top view of the inventive concentrator solar cell with offset current collector strips and four axisymmetric current collector buses;

figure 5 shows a top view of the inventive concentrator solar cell with symmetrically located open collector strips and four axisymmetric collector buses;

Fig.6 shows a top view of the inventive concentrator solar cell with symmetrically located collector strips connected by proximal ends and one collector bus;

Fig.7 shows a top view of the inventive concentrator solar cell with symmetrically located collector strips connected by pairing with the proximal ends, and one collector bus.

The concentrator solar cell prototype 1 includes a lower electrode 2, a substrate 3, a heterostructure 4 formed on it, a contact layer 5, an upper electrode consisting of a current collector bus 6 and current collector strips 7. The design of the upper electrode of the concentrator solar cell prototype is visible in figure 2 .

The concentrator solar cell 8 (see FIG. 3) includes a lower electrode 2, a substrate 3, a heterostructure 4 formed on it, a contact layer 9, an upper electrode (see FIG. 4), consisting of four collector buses 10 and collector strips 11. The photosensitive region 12 of the solar cell 8 may be in the shape of a circle (see FIGS. 4, 6), the shape of a circle flattened on four sides of said rectangle (see FIG. 7); the shape of a square with cut corners (see figure 5), which allows to increase the photoactive region 12 of the solar cell 8. In each of the four segments a, b, c and d of the photosensitive region 12, the current collection strips 11 are parallel to each other. Contact slip strips 11 are located in segments a, b, c and d at an angle of 35-55 ° to the side face 13 of the solar cell 8. For a more uniform current collection, slip strips 11 (see Fig. 4) of one segment (for example, a) can be offset relative to the current-collecting strips 11 of the adjacent segment (for example, b) along the dividing line of these segments by a distance d = 1 / 2D, where D is the distance between the proximal ends 14 of the adjacent strips 11. Current-collecting strips 11 (see Fig. 5) of two neighboring segments (for example, a and b) can be located symmetrically relative a plane passing through the section of these segments. In the event that the implementation of the contact grid with contact strips 11 is technologically too complicated, each current collector strip 11 of one segment (for example, a) can be connected to a symmetrical current collector strip 11 of an adjacent segment (for example, b). In particular, the connection of symmetrical strips 11 of adjacent segments (for example, b and c) can be performed by pairing 15 (see Fig.7). As shown in FIG. 5, the proximal end 14 of each current collection strip 11 of one segment (e.g., b) can be separated by a gap 16 from the proximal end 14 of the symmetrical current collection strip 11 of an adjacent segment (e.g., c), which allows to increase the photoactive area. In each corner of the square, a current collector 17, for example, made in the form of a gold wire, is attached to the collector bus 10 by ultrasonic or thermosonic microwelding.

The increase in efficiency of the inventive concentrator solar cell is confirmed by a numerical evaluation. The design of the contact grid of the concentrator solar cell prototype 1 is presented in figure 2, and the design used in the numerical evaluation of the contact grid of the inventive solar cell 8 is presented in figure 4. The size of the photosensitive region 12 in both cases will be 1 mm (the length of the side of the square in figure 2 and the diameter of the circle in figure 4). The width of the contact strips 7 and 11 is 5 μm, the specific surface resistance is 0.35 Ohm / □. The width of the slip rings 10 located along the perimeter of the photosensitive region 12 (figure 4) is 100 μm. Given the symmetry of the contact grid pattern of the concentrator solar cell prototype 1 and the concentrator solar cell 8, the task of determining the voltage drop will be solved only for the region of 1/8 of the total area of the solar cell and highlighted by a thick dashed line in figure 2 and 4. Considering the symmetry the distribution of the intensity of solar radiation, as well as the similarity of the geometry of the contact strips 7 and 11, the differences in the voltage drop across the contact strips 7 and 11 shown respectively in FIG. .2 and FIG. 4 can be neglected. Thus, the task boils down to determining the voltage drop across the collector buses 6 and 10 for the two above-mentioned contact grid designs. The magnitude of the voltage drop across the collector bus 6 and 10 between the two contact strips 11 (n-1 and n) is

where J _k is the magnitude of the current flowing along the k-th current collection strip 11; m = 5 - the number of slip strips 11 in the considered area; r _{n is the} resistance of the portion of the collector bus 6, 10 between the contact strips 11 (n-1 and n), which can be expressed as

where l _n is the length of this section of the collector bus 6, 10; W _n is its width in the case of the design of the contact grid shown in figure 2, or the effective width for the case of the design of the contact grid shown in figure 4; ρ _s is the specific surface resistance. Given the formula (1), the total voltage drop across the collector bus 6, 10 can be expressed as follows:

Assuming that the concentrated solar radiation incident on solar cells 1 and 8 is distributed according to the Gauss law, and also that the short circuit current density at one sun is 14 mA / cm ² , calculation by formula (3) at a concentration of 1000 suns gives a voltage drop of 60 mV and 20 mV, respectively, for solar cell 1 and solar cell 8. Thus, the claimed design of the concentrator solar cell 8 allows three times to reduce losses on the resistance of the contact grid at high levels of solar concentration radiation, which can give an increase in the absolute value of the efficiency by 0.5-1%. An additional positive effect may be a decrease in the cost of the concentrator solar cell 8 by reducing its size without reducing the area of the photosensitive region 12.

Claims (13)

1. A concentrator solar cell made in the form of a rectangle with a ratio of side lengths ranging from 1 to 1.5, and comprising a substrate, a multilayer structure formed on the substrate, with a central photosensitive region, a contact layer, a solid lower electrode and an upper electrode in the form of a contact grid containing at least one current collector bus located along the perimeter of the photosensitive region, and current collector strips equidistantly emerging from at least one current collector bus at an angle scrap 35 ° -55 ° to the sides of the said rectangle and parallel to each other within each of the four segments lying between mutually perpendicular planes drawn through the midpoints of the opposite sides of the said rectangle, while a collector is attached to the collector busbar in at least one corner of the said square . 2. The solar cell according to claim 1, characterized in that said current collector strips of one segment are offset relative to the current collector strips of an adjacent segment along the dividing line of these segments by a distance d = 1 / 2D, where D is the distance between the proximal ends of adjacent strips. 3. The solar cell according to claim 1, characterized in that said current collector strips of two adjacent segments are located symmetrically with respect to a plane passing along the dividing line of these segments. 4. The solar cell according to claim 3, characterized in that each current collector strip of one segment is connected to a current collector strip symmetrical to it of the adjacent segment. 5. The solar cell according to claim 4, characterized in that the connection of the aforementioned symmetrical strips of adjacent segments is performed by pairing. 6. The solar cell according to claim 1, characterized in that the proximal end of each collector strip of one segment is separated by a gap from the proximal end of the symmetrical collector strip of the adjacent segment. 7. The solar cell according to claim 1, characterized in that the photosensitive area is made in the form of a circle. 8. The solar cell according to claim 1, characterized in that the photosensitive area is made in the form of a circle, flattened on four sides of the said square. 9. The solar cell according to claim 1, characterized in that the photosensitive area is made in the form of a square with cut corners. 10. The solar cell according to claim 1, characterized in that the contact grid contains four identical collector busbars, separated from each other by a gap near the axis of symmetry of the said square. 11. The solar cell according to claim 1, characterized in that the collector strips equidistantly exit at least one collector bus at an angle of 45 ° to the sides of the said square. 12. The solar cell according to claim 1, characterized in that the collector is made of gold. 13. The solar cell according to claim 1, characterized in that the solar cell is made in the form of a square.

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