Method for improving ohmic contact behaviour between a contact grid and an emitter layer of a silicon solar cell

Abstract

The invention relates to a method for improving ohmic contact behaviour between a contact grid and an emitter layer of a silicon solar cell. The object of the invention is to propose a method for improving contact behaviour between the contact grid and the emitter layer of silicon solar cells, which method is used after the contacting of these solar cells and thus reduces the scrap quota of solar cells with faulty contacting. In order to achieve this object, a method is proposed which has the following method steps. First a silicon solar cell (1) is provided with the emitter layer, the contact grid (5) and a back contact (3). Then the contact grid (5) is electrically contacted by a contact pin matrix (8) or contact plate connected to one terminal of a current source and the back contact (3) is electrically connected by a contact device connected to the other terminal of the current source. Using the current source, at least one current pulse is induced along the forward direction of the silicon solar cell (1), the current pulse having a pulse duration of 1 ms to 100 ms and a current strength which is equivalent to 10 to 30 times the short-circuit current strength of the silicon solar cell (1). Two alternative methods are also proposed.

Claims

1. A method for improving the ohmic-contact behaviour between a contact grid and an emitter layer of a silicon solar cell, the method comprising: preparing a silicon solar cell having an emitter layer, a contact grid made of a metal paste and a rear contact; electrically contacting the contact grid of the silicon solar cell with a first contacting device connected to one pole of a current source; electrically contacting the rear contact of the silicon solar cell with a second contacting device connected to the other pole of the current source; and inducing at least one current pulse with a pulse length of 1 ms to 100 ms and a current magnitude corresponding to 10-to-30 times the short-circuit magnitude of the silicon solar cell measured under standard test conditions along the forward direction of the silicon solar cell.

2. The method of claim 1, wherein the first contacting device is a contact-pin matrix or a contact plate.

3. A method for improving the ohmic-contact behaviour between a contact grid and an emitter layer of a silicon solar cell, the method comprising: preparing a silicon solar cell having an emitter layer, a contact grid made of a metal paste and a rear contact; electrically contacting a subsection of the contact grid of the silicon solar cell by means of a contact brush or contact roller connected to one pole of a current source; electrically connecting the rear contact of the silicon solar cell by means of a contacting device connected to the other pole of the current source; and guiding the contact brush or the contact roller over the contact grid and inducing a current flow along the forward direction of the silicon solar cell by means of the current source and exposing the subsection to the current flow for 1 ms to 100 ms, the current having a magnitude corresponding to 10-to-30 times the short-circuit magnitude of the silicon solar cell reduced by the ratio of the area of the subsection to the area of the silicon solar cell measured under standard test conditions.

4. A method for improving the ohmic-contact behaviour between a contact grid and an emitter layer of a silicon solar cell, the method comprising: preparing a silicon solar cell having an emitter layer, a contact grid made of a metal paste and a rear contact; electrically contacting the contact grid of the silicon solar cell to one pole of a voltage source; connecting a contacting device, the contacting device being electrically connected to the other pole of the voltage source, to the rear contact of the silicon solar cell; applying, with the voltage source, a voltage directed opposite to the forward direction of the silicon solar cell, the applied voltage being lower than the breakdown voltage; guiding a point light source over the sun-facing side of the silicon solar cell while the voltage is applied; and point illuminating a subsection of the sun-facing side, inducing a current to flow in the partial area, where the current acts on the subsection for 1 ms to 100 ms, the current being of a magnitude corresponding to 10-to-30 times the short-circuit current magnitude of the silicon solar cell reduced by the ratio of the area of the subsection to the area of the silicon solar cell measured under standard test conditions.

5. The method of claim 4, wherein the point light source is a laser.

Description

(1) Embodiments of the invention are explained by means of drawings below. These show:

(2) FIG. 1 A schematic diagram of a silicon solar cell

(3) FIG. 2 A silicon solar cell with a contact-pin matrix placed on the contact grid

(4) FIG. 3 A silicon solar cell with a contact brush placed on the contact grid

(5) FIG. 4 A silicon solar cell with a contact roller placed on the contact grid

(6) FIG. 5 A silicon solar cell with the contact grid and a sun-facing side illuminated by a point light source.

(7) FIG. 1 shows a schematic simplified structure of a silicon solar cell, 1. This has an absorber layer, 2, consisting of a p-doped silicon layer and an n-doped silicon layer, whereby a pn junction that typically exhibits a diode character is formed at the transition between the two layers. On a side of the silicon solar cell facing away from the sun, 1, a usually planar rear contact, 3, is applied to the p-doped silicon layer. On an opposite, sun-facing side of the silicon solar cell, 1, a dielectric silicon-nitride layer, 4, is usually arranged on the n-doped silicon layer. A contact grid, 5, consisting of metal paste is built up on this as described above. The contact grid, 5, of common silicon solar cells, 1, consists of busbars, 6, and contact fingers, 7, running transversely to them. As a result of the diode character of the pn junction, the silicon solar cell, 1, has forward direction and a breakdown voltage. In addition, the silicon solar cell, 1, is characterized by a short-circuit strength under standard test conditions (illuminance: 1,000 W/m.sup.2, temperature of the silicon solar cell: 25° C.). A person having ordinary skill in the art will be familiar both with these parameters and with the standard test conditions, so they are not explained in detail here. Such a silicon solar cell, 1, is the starting point for the method according to the invention.

(8) FIG. 2 shows the silicon solar cell, 1, with the contact grid, 5, schematically, whereby only the busbars, 6, and no contact fingers, 7, are illustrated in the figure. The side of the silicon solar cell, 1, facing away from the sun is not visible in the figure. After the silicon solar cell, 1, has been prepared, a first contacting device made up of a contact-pin matrix, 8, is placed on the contact grid, 5, whereby all contact pins, 9, of this matrix are electrically connected together. The contact pins, 9, rest on the contact fingers, 7, of the contact grid, 5. In principal, however, only the busbars, 6, or both the busbars, 6, and the contact fingers, 7, can also be contacted. Here, the contact pins, 9, can be sprung pins so that reliable contacting of the contact grid, 5, is ensured. Ideally, the contact pins, 9, are distributed homogeneously over the contact grid, 5. Alternatively, instead of the contact-pin matrix, 8, a contact plate can also be placed on the contact grid, 5, so that this is contacted over its whole area by the contact plate.

(9) The next step of the method envisages placing a second contacting device on the rear contact, 3. This is not shown in the figure. The second contacting device can be, for example, a matrix of sprung pins or a metal plate for full-area contacting of the rear contact, 3. In terms of procedure, the second contacting device can also be put in place before the first contacting device.

(10) The contact-pin matrix, 8, is connected to one pole of a current source. The second contacting device is connected to the second pole of that current source. The current source is not shown in the figure.

(11) After putting the contact-pin matrix, 8, and the second contacting device in place, the current source induces a current pulse, whereby the current flows along the forward direction of the silicon solar cell, 1. The current pulse has a pulse length of 1 ms to 100 ms and a current magnitude in the range of 10-to-30 times the short-circuit current magnitude of the silicon solar cell, 1. In addition to the application of a current pulse, the application of a pulse sequence is also possible in principal. In this case, the individual current pulses can be of identical magnitudes. The pulses can also be of different magnitudes so that, for example, a current pulse with a pulse length of 50 ms and a current magnitude of 10 times the short-circuit current strength is first applied, followed by a current pulse with a pulse length of 5 ms and a current magnitude of 30 times the short-circuit current density. A silicon solar cell, 1, with a dimension of 15 cm×15 cm usually has a short-circuit current magnitude of 5-10 A. In this case, current magnitudes of 50-300 A are achieved for a current pulse in the method according to the invention. Contacting the contact grid, 5, with a contact-pin matrix, 8, or a contact plate ensures that current pulses with such high current magnitudes are distributed homogeneously over the whole silicon solar cell, 1, and the serial resistances associated with the lateral dimensions along the contact grid, 5, can be neglected.

(12) At a low contact resistance between the contact grid, 5, and the emitter layer, a current pulse with currents and pulse lengths of the aforementioned orders of magnitude cause only slight heating at the transition between the contact grid, 5, and the emitter layer. The heating is essentially attributable to the power loss over the contact resistance. Slight heating has no influence on the ohmic-contact behaviour of the transition. However, if there is a high contact resistance between the contact grid, 5, and the emitter layer, the current pulse will result in a much higher power loss and therefore in a much greater heating of the transition. It is assumed that temperatures of the same order of magnitude as the temperatures used in the tempering process occur locally in the areas with small contact area between the contact grid, 5, and the emitter layer. This can cause local melting of the contact grid, 5, or the metal paste, which causes the metal paste to diffuse further through the silicon-nitride layer, 4, resulting in an increase in the contact area between the contact grid, 5, and the emitter layer and thus reducing the contact resistance. Limiting the time of the current pulses ensures that the heating-up time is limited so that the metal paste does not diffuse further through the emitter layer and destroy the structure of the absorber layer, 2, or even diffuse through to the rear contact, 3, of the silicon solar cell, 1, thereby short-circuiting the same.

(13) The method according to the invention can be modified such that the contact grid, 5, is not in contact with a contact-pin matrix, 8, or a contact plate, but that a contact brush, 10 (FIG. 3), or a contact roller, 11 (FIG. 4), is placed on a subsection of the contact grid and is moved along the contact grid, 5.

(14) FIG. 3 shows the silicon solar cell, 1, whereby a subsection of the contact grid, 5, is electrically contacted by a contact brush, 10, connected to one pole of a current source. Only the busbars, 6, and not the contact fingers, 7, of the contact grid, 5, are illustrated. The rear contact, 3, is electrically connected by a contacting device connected to the other pole of the current source, whereby the contacting device and the rear contact, 3, are not illustrated. After placing the contact brush, 10, on the contact grid, 5, the former is moved along the contact grid, 5. Here, the current source induces current pulses along the forward direction of the silicon solar cell, 1. The current pulses have a pulse length of 1 ms to 100 ms and a current of a magnitude corresponding to 10-to-30 times the short-circuit magnitude for the silicon solar cell, 1, reduced by the ratio of the area of the subsection to the area of the silicon solar cell, 1. For example, the subsection contacted by the contact brush, 10, can cover a quarter of the contact grid, 5. The magnitude of the current pulse applied in this case is around 2.5-to-7.5 times the magnitude of the short-circuit current. Alternatively to the pulsed current, a continuous current can also be used, whereby the time for which it is applied is regulated by the rate of forward motion of the contact brush.

(15) The contact brush, 10, is guided continuously along the contact grid, 5. However, it is also possible to set down the contact brush, 10, on a subsection of the contact grid, 5, and to apply at least one current pulse first. The contact brush, 10, is then guided to another subsection of the contact grid, 5, and then at least one current pulse is applied. These steps are repeated according to the dimensions of the contact grid, 5. The bristles of the contact brush, 10, are designed such that no damage (such as scratches) is caused to the surface of the silicon solar cell, 1, while the contact brush, 10, is in motion.

(16) FIG. 4 shows the silicon solar cell, 1, from FIG. 3 with the subsection of the contact grid, 5, electrically contacted by the contact roller, 11, instead of by the contact brush, 10. Only the busbars, 6, of the contact grid are illustrated. After placing the contact roller, 11, on the contact grid, 5, the former is moved along the contact grid, 5. Here, the current source induces current pulses along the forward direction of the silicon solar cell, 1. The current pulses have a pulse length of 1 ms to 100 ms and a current of a magnitude corresponding to 10-to-30 times the short-circuit magnitude for the silicon solar cell, 1, reduced by the ratio of the area of the subsection to the area of the silicon solar cell, 1. If, for example, the contact roller, 11, is designed such that the subsection encompasses around 10% of the contact grid, 5, a current magnitude of the order of one-to-three times the short-circuit current magnitude must be used for the current pulse. Alternatively to the pulsed current, a continuous current can also be used, whereby the time for which it is applied is regulated by the rate of forward motion of the contact roller.

(17) The contact roller, 11, is also designed such that no damage (such as scratches) is caused to the surface of the silicon solar cell, 1, while the contact roller, 11, is in motion.

(18) FIG. 5 shows an arrangement for an alternative method according to the invention. The alternative method comprises the following process steps: First, a silicon solar cell, 1, is prepared with the emitter layer, the contact grid, 5, and the rear contact, 3. The contact grid, 5, is then electrically connected to one pole of a voltage source and the rear contact, 3, is electrically connected to the other pole of the voltage source, whereby a point contacting of the contact grid, 5, is sufficient. Thus, placing a contact strip, 12, on the ends of the busbars, 6, suffices, for example. A voltage is then applied opposite to the forward direction of the silicon solar cell, 1, by the voltage source. The voltage should be less than the breakdown voltage of the silicon solar cell, 1. While this voltage is applied, a point light source, 13, is guided over the sun-facing side of the silicon solar cell, 1. The point illumination of subsections of the sun-facing side of the silicon solar cell, 1, generates a light-induced current that is amplified by the applied voltage source, which causes the aforementioned heating of the transition between the contact grid, 5, and the emitter layer. Here, it is an advantage that the contact grid, 5, need only be point contacted. Furthermore, selected areas of the silicon solar cell, 1, can be processed in a targeted manner. Thus, for example, areas with increased contact resistance can be detected by means of imaging processes, such as electroluminescence or CoreScan, and subjected to targeted treatment.

(19) The point light source 13 is executed as a laser, although the method is not restricted to this. In this case, the laser can generate pulsed or continuous laser radiation. Diode lasers or CO.sub.2 lasers can be used as low-cost variants, for example.

(20) For a common silicon solar cell, an improvement of the ohmic-contact behaviour between the contact grid and the emitter layer is achieved with a reverse voltage of 12 V, a laser output of 0.5 W, a laser wavelength of 1,062 nm and a laser spot size of 60 μm, for example, whereby the current generated is of the order of 100 mA. If the silicon solar cell is scanned with a laser motion rate of 10 m/s and a line spacing of 1 mm, the process time is approximately 3 s.

LEGEND

(21) 1 Silicon solar cell 2 Absorber layer 3 Rear contact 4 Silicon-nitride layer 5 Contact grid 6 Busbar 7 Contact finger 8 Contact-pin matrix 9 Contact pin 10 Contact brush 11 Contact roller 12 Contact strip 13 Point light source