Method and device for stabilizing a photovoltaic silicon solar cell

Abstract

The invention relates to a method for stabilizing a photovoltaic silicon solar cell, including a regeneration step in which a semiconductor substrate of the solar cell which are heated to at least 50 C. is injected with charge carriers. The invention is characterized in that a degradation step is carried out before the regeneration step, wherein the solar cell is subjected to radiation, in particular laser radiation, having an illumination intensity of at least 5.000 W/m2 and the solar cell is simultaneously cooled. The invention also relates to a device for stabilizing a photovoltaic silicon solar cell.

Claims

1. A method for stabilizing a photovoltaic silicon solar cell, comprising: in a regeneration step: injecting charge carriers in a semiconductor substrate of the solar cell by subjecting the solar cell to radiation, which is at least 50 C., and in a degradation step carried out before the regeneration step: converting material regions of the solar cell in an annealed state into a degraded state by subjecting the solar cell to radiation having a radiation intensity of at least 5,000 W/m.sup.2 and at a same time as actively cooling the solar cell; and the regeneration step is carried out at a higher temperature of the solar cell than the degradation step.

2. The method as claimed in claim 1, wherein in the degradation step, subjecting the solar cell to laser radiation having an irradiation intensity of at least 10,000 W/m.sup.2 and at the same time the solar cell is actively cooled.

3. The method as claimed in claim 1, wherein in the degradation step, cooling the solar cell to a temperature of less than 100 C.

4. The method as claimed in claim 1, further comprising carrying out the degradation step for a time duration in the range of 0.1 s to 10 s.

5. The method as claimed in claim 1, further comprising in the regeneration step, actively cooling the solar cell at a same time as injecting charge carriers in the solar cell.

6. The method as claimed in claim 5, further comprising at least one of in the regeneration step, cooling the solar cell to a temperature in the range of 90 C. to 500 C., or carrying out the regeneration step for a regeneration duration T of less than 3 s.

7. The method as claimed in claim 1, further comprising measuring a temperature of the solar cell, and selecting at least one of an irradiation intensity or an active cooling such that the temperature of the solar cell does not exceed a predefined maximum temperature.

8. The method as claimed in claim 1, further comprising in the degradation step, irradiating the solar cell in a degradation region that covers only a partial region of a surface of the solar cell.

9. The method as claimed in claim 8, further moving the degradation region relative to the solar cell, or irradiating the solar cell progressively in a plurality of spatially different degradation regions.

10. The method as claimed in claim 8, wherein in the degradation step the irradiation is carried out in a degradation region extending at least over a width of the solar cell, and at edges of the solar cell the irradiation is carried out with a lower edge intensity, relative to a central-region intensity with which regions at a distance from the edges are irradiated.

11. The method as claimed in claim 1, wherein during the regeneration step the solar cell is actively cooled to a temperature of less than 250 C. and the solar cell is subjected to a radiation intensity of at least 10,000 W/m.sup.2, and a total process duration of degradation step and regeneration step is less than 50 s.

12. The method as claimed in claim 1, wherein during the regeneration step the solar cell is actively cooled to a temperature in the range of 250 C. to 400 C. and the solar cell is subjected to a radiation intensity of at least 10,000 W/m.sup.2, and a total process duration of degradation step and regeneration step is less than 50 s.

13. The method as claimed in claim 9, wherein in an initial region, in which the degradation region covers a first edge of the solar cell, the irradiation is carried out with an initial intensity, and in an end region, in which the degradation region, after the irradiation of the initial region, covers a second edge opposite to the first edge of the solar cell, the irradiation is carried out with an end intensity lower than the initial intensity, and in a central region, in which the degradation region, after the irradiation of the initial region and before the irradiation of the end region, covers a central region of the solar cell, the irradiation is carried out with a medium intensity, which is less than the initial intensity and greater than the end intensity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further preferred features and embodiments are explained below with reference to exemplary embodiments and the figures, in which:

(2) FIG. 1 shows the three-state model for the states degenerated (D), regenerated (R) and annealed (A), as already explained above;

(3) FIG. 2 shows a first exemplary embodiment of a device according to the invention comprising a degradation radiation source and a separate regeneration radiation source;

(4) FIG. 3 shows a second exemplary embodiment of a device according to the invention comprising an actively cooled mount for a solar cell;

(5) FIG. 4 shows an illustration of a solar cell being subjected to radiation, wherein the intensity decreases toward the edges of the solar cell;

(6) FIG. 5 shows an illustration with the solar cell being subjected to radiation, wherein the radiation is moved in a strip-shaped region across the surface of the solar cell; and

(7) FIGS. 6A-6C show a development of the exemplary embodiment in accordance with FIG. 5 with different intensities at the beginning and at the end of the processing step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) All of the figures show schematic illustrations that are not true to scale. In the figures, identical reference signs designate identical or identically acting elements.

(9) FIG. 2 illustrates a first exemplary embodiment of a device for stabilizing a photovoltaic silicon solar cell. The device comprises a regeneration radiation source 1 for subjecting a silicon solar cell 2 that is to be stabilized to regeneration radiation.

(10) Furthermore, the device comprises a degradation radiation source 3 for subjecting the solar cell 2 to degradation radiation, and also an active cooling unit 4, which in the present case is configured in a manner comprising two fans 4a and 4b.

(11) The degradation radiation source 3 and the fans 4a and 4b are arranged and configured cooperatively in such a way that the solar cell 2 in the degradation position illustrated in FIG. 2 is able to be subjected to the degradation radiation and is able to be cooled simultaneously.

(12) The device in accordance with FIG. 2 furthermore comprises a conveyor belt 5, in order to convey the solar cell 2 into the degradation position illustrated and a regeneration position illustrated by dashed lines.

(13) Furthermore, additional fans 5a and 5b are provided such that it is possible for the solar cell to be actively cooled by blowing during the regeneration process as well.

(14) The device in accordance with FIG. 2 enables, in particular, a reliable and at the same time shorter stabilization method compared with previously known methods, since, in the regeneration position of the silicon solar cell 2 illustrated in FIG. 2, by use of the degradation radiation source 3 the solar cell is able to be subjected to high light intensity and it is ensured at the same time by the fans 4a and 4b due to the active cooling that a predefined maximum temperature of the silicon solar cell 2 is not exceeded.

(15) In the present case, the degradation radiation source is configured as a matrix of diode lasers comprising 30 lasers per rows, with 30 rows, which generate light in the wavelength range of 850 nm. Light in the wavelength range of 808 nm or 980 nm can likewise be used. Matrices of diode lasers having a different number of lasers per row and number of columns can likewise be used. It is likewise possible to use just one laser, the output beam of which is correspondingly spatially expanded and homogenized with regard to the intensity by optical means. At the surface of the silicon solar cell, in the degradation step, light having an intensity of 50,000 W/m.sup.2 is generated by the degradation radiation source 3. At the same time, ambient air is blown at the solar cell by the fans 4a and 4b, such that the solar cell does not exceed a temperature of 60 C.

(16) A rapid and reliable degradation is ensured as a result. In particular, a regeneration is precluded or at least considerably reduced due to the comparatively low process temperature of the solar cell. Given these process parameters, a reliable degradation can be carried out with a process time of less than 10 s.

(17) Afterward, the solar cell is conveyed into the regeneration position illustrated by dashed lines by the conveyor belt 5. In said regeneration position, the solar cell is subjected to radiation from the regeneration radiation source 1, wherein here, too, by the fans 5a and 5b the solar cell is cooled by ambient air being blown at it, but in such a way that the solar cell has a temperature in the range of 120 C. to 250 C.

(18) The regeneration radiation source comprises a plurality of halogen lamps for generating radiation.

(19) Alternatively, the regeneration process can also be carried out with other set-ups, in particular as known from the prior art cited in the introduction. What is essential is that, for a reliable and at the same time rapid degradation in the degradation step, the silicon solar cell 2 is simultaneously subjected to radiation having a high intensity and actively cooled.

(20) In an alternative set-up, instead of the fans 4a and 4b, spray nozzles for spraying a cooling liquid mist onto the silicon solar cell 2 are arranged at the degradation position. Via a cooled cooling liquid tank, cooling liquid is fed to the two spray nozzles by a pump, such that the active cooling is effected by spraying on the solar cell, such that the solar cell does not exceed a predefined temperature during the degradation step.

(21) In a further alternative, a cooled gas, for example cooled ambient air or other types of gas, such as cooled argon gas, for example, is fed to the fans 4a and 4b, such that more efficient cooling is possible. Furthermore, it is advantageous to feed a particularly pure gas and/or purified, in particular filtered, ambient air, in particular argon gases, as described above, by the fans for cooling purposes, in order to avoid contamination of the solar cell and the process space.

(22) Furthermore, it is advantageous for the components illustrated in FIG. 2 to be arranged in a housing.

(23) FIG. 3 illustrates a second exemplary embodiment of a device according to the invention. In order to avoid repetition, only the essential differences in comparison with the device in accordance with FIG. 2 are discussed:

(24) The cooling unit 4 of the device in accordance with FIG. 3 is configured as an actively cooled mount, in the present case as a metallic block, also referred to as chuck. This mount preferably has openings for sucking the solar cell onto the mount in a manner known per se. Furthermore, the mount has cooling lines, through which cooling liquid is pumped by a pump in order to cool the mount to a predefined temperature. In particular, the mount 4 can be embodied in accordance with or at least substantially like corresponding blocks which are configured for measuring solar cells, in particular for bright characteristic curve measurement (so-called measurement blocks).

(25) This exemplary embodiment has the advantage that the active cooling unit 4 has a significantly larger thermal mass compared with the silicon solar cell 2, such that the temperature of the silicon solar cell 2 corresponds to the temperature of the active cooling unit 4 in a very reliable manner the silicon solar cell 2.

(26) In one advantageous configuration, the active cooling unit is displaceable, in particular displaceable by a motor, toward the right and left, as indicated by the arrows. As a result, in a kind of reciprocating operation, on one side a solar cell can be received, then the solar cell can be brought to the illustrated position for carrying out the process step and then be brought to an output position for example at the opposite side of the solar cell.

(27) In one advantageous development, the device in accordance with FIG. 3 comprises an optical temperature measuring and control unit 7, illustrated by dashed lines. The temperature measuring and control unit is connected to the degradation radiation source 3 and controls the power thereof in such a way that the solar cell in the degradation step that the solar cell has a temperature in the range of 50 C. to 60 C.

(28) In the case of the device in accordance with FIG. 3, by changing the cooling power with which the mount is cooled and/or by changing the radiation power with which the degradation radiation source 3 acts on the silicon solar cell 2, it is possible to carry out both the degradation step and, after the conclusion of the degradation step, subsequently the regeneration step. What is essential in this case, in particular, is that the degradation step is carried out at a lower temperature of the silicon solar cell compared with the temperature of the silicon solar cell 2 during the regeneration step.

(29) It is likewise possible to use the device in accordance with FIG. 3 exclusively for the degradation step and then to carry out the regeneration in a region (not illustrated) of the device which comprises a regeneration radiation source. This has the advantage that the respective regions of the device can be optimized for the degradation step, on the one hand, and the regeneration step, on the other hand.

(30) It lies within the scope of the invention, for carrying out the degradation, to subject the silicon solar cell to degradation radiation over the whole area or to subject only a partial region of the solar cell to degradation radiation, wherein the partial region is advantageously displaced over the solar cell, such that as a result the entire solar cell was subjected to degradation radiation and/or a plurality of partial regions are progressively subjected to degradation radiation, such that as a result the entire solar cell was subjected to degradation radiation. Advantageous configurations thereof are explained below with reference to FIGS. 4 to 6A-6C:

(31) FIG. 4 shows a plan view from above of a silicon solar cell 2 which is subjected to degradation radiation over the whole area. Since central regions of the solar cell are connected to correspondingly temperature-regulated regions of the solar cell toward all sides, a higher energy input is typically possible here, compared with edge regions, at which only a lower heat dissipation takes place via the edge. In order to avoid an excessively increased temperature in the edge regions of the silicon solar cell, it is therefore advantageous to subject the edge regions to a lower intensity compared with a central region. FIG. 4 illustrates by dashed lines the edge regions R at the four edges of the solar cell, which are at a distance X in the range of 0.5 cm to 5 cm, in particular a distance of approximately 1 cm from the respective edge. As described above, the degradation radiation source is advantageously configured in such a way that the silicon solar cell is subjected to a higher intensity in the central region M compared with the intensity in the edge regions R. In particular, the intensity in the edge regions is lower than in the central region M preferably by approximately 5%, more particularly preferably by approximately 10%, wherein here typically due to the heat conduction there is a fluid temperature transition between the central region M and the edge regions R, that is to say that the temperature change does not proceed in a step-like manner.

(32) An irradiation intensity adapted in this way in accordance with FIG. 4 is particularly advantageous if the silicon solar cell is not connected to a considerably larger thermal mass, for example if the solar cell, as in the device in accordance with FIG. 1, is cooled by blowing and, in particular, if the conveyor belt, in a manner known per se, as in the case of continuous furnaces, for example, consists merely of two comparatively thin belts or cords, which thus have only a small thermal mass and exhibit little heat dissipation.

(33) In the case of a configuration of the cooling unit in accordance with FIG. 2, i.e. with the use of a cooling block with integrated active cooling or with active cooling by feeding a cooling liquid, a thermal mass of the mount configured as a cooling block is typically higher by a multiple compared with that of the silicon solar cell, such that in this case, also owing to a very good thermal contact due to the rear side of the solar cell bearing on the mount, in particular bearing thereon substantially over the whole area, an inhomogeneous temperature distribution of the solar cell during the degradation process is not present or is present at least to a substantially smaller extent. Consequently, a homogeneous irradiation over the whole area can be used here. A structurally simple configuration is achieved as a result. In order to increase the temperature homogeneity, it is possible, of course, in this case, too, to use a spatially inhomogeneous irradiation intensity, in particular in accordance with FIG. 4.

(34) In an alternative exemplary embodiment to irradiation over the whole area, in particular irradiation over the whole area in accordance with FIG. 4 during the degradation process, in a manner illustrated in FIG. 5, only a strip-like degradation region 6 of the silicon solar cell is subjected to degradation radiation, wherein the degradation region 6 is moved relative to the silicon solar cell 2 in accordance with the direction V, such that the entire surface of the silicon solar cell 2 was subjected to degradation radiation as a result. A continuous method can thereby be realized in an advantageous manner, in particular by virtue of the fact that, given a spatially fixed degradation region 6, the solar cell is moved oppositely to the direction V in FIG. 5.

(35) In order to optimize the respective process conditions, firstly the degradation step is carried out. The regeneration is carried out in a subsequent, separate method step. This regeneration can comprise subjecting the whole area to regeneration radiation or likewise just subjecting a partial region of the solar cell to regeneration radiation, wherein the partial region is moved relative to the solar cell, such that the entire solar cell was subjected to regeneration radiation as a result.

(36) This has the advantage that respectively optimum process conditions for degradation, on the one hand, and regeneration, on the other hand, can be predefined in a particularly reliable manner.

(37) A structurally advantageously simple configuration results from the fact that, in an alternative configuration, the irradiation of the partial region illustrated in FIG. 5 is used for degradation in a first partial region, situated on the left in FIGS. 6A-6C, and for regeneration in a second partial region, situated on the right:

(38) This makes use of the circumstance that the solar cell, proceeding from a starting temperature, heats up as a result of being subjected to radiation during the relative displacement of the region in accordance with the direction V with respect to the silicon solar cell. Process parameters can thus be chosen such that a temperature profile as illustrated schematically below FIG. 5 results within the region 6 in FIG. 5:

(39) In this schematic, qualitative illustration, the location x is plotted against the temperature T of the solar cell. Since the region 6 is displaced from right to left in the illustration in accordance with FIG. 5 and, in this exemplary embodiment, the solar cell initially has a low temperature (for example approximately room temperature, in particular approximately 20 C.), the solar cell is heated due to being subjected to radiation. The parameters are chosen, then, in such a way that a limit temperature T.sub.G is exceeded in a boundary region X.sub.G, such that substantially a degeneration takes place below said limit temperature and substantially or increasingly a regeneration takes place above said limit temperature. As a result, degradation and regeneration can likewise be carried out in two successive, separate process steps, wherein the boundary Xg between degeneration and regeneration is likewise displaced with the displacement of the partial region relative to the silicon solar cell, such that a complete regenerated solar cell results after the complete displacement of the partial region over the entire solar cell. This variant enables a structurally simple configuration since, in particular, it is necessary to use just one radiation source and a spatially compact set-up is furthermore ensured. In return, however, it is necessary to accurately adjust and comply with the process parameters. In one advantageous development, the solar cell is cooled in the region lying to the left of the boundary X.sub.G in accordance with FIG. 5, in particular is subjected to a cooling gas blown at it, in order to bring about the temperature difference in the degeneration region 6.

(40) In method variants in which, separately, firstly the solar cell is completely degraded and then the solar cell is completely regenerated, a greater robustness and insusceptibility to faults vis--vis fluctuations in the process parameters are typically ensured.

(41) FIGS. 6A-6C illustrate one exemplary embodiment of the degradation step, in which, as also described with regard to FIG. 5, a degradation region 6, in which the solar cell is subjected to degradation radiation, is moved in accordance with a direction V relative to the silicon solar cell, such that the entire surface of the silicon solar cell is subjected to degradation radiation as a result. Here, too, in order to take account of the deviating thermal behavior of the edge regions of the solar cell, firstly in the state illustrated in FIG. 6A, in which the degradation region covers a first edge, situated on the right, of the solar cell 2, the solar cell is subjected to degradation radiation with a fine initial intensity increased in this initial region. This compensates for the fact that, in said initial region, the rest of the solar cell typically still has a lower temperature and can therefore be subjected to a higher intensity in order to attain a desired temperature range.

(42) In a central region, since the degradation region is at a distance X.sub.1, in particular a distance of approximately 1 cm, from the edge situated on the right and thus the initial region, the solar cell is subjected to degradation radiation with a medium intensity, lower than the initial intensity. This is illustrated in FIG. 6B.

(43) At the end of the degradation step, as illustrated in FIG. 6C, when the degradation region 6 covers the opposite edge, situated on the left in FIG. 6C, of the solar cell, the solar cell is subjected to degradation radiation with an end intensity, lower than the initial intensity and lower than the medium intensity.

(44) A particularly high consistency of the temperature of the solar cell can be achieved as a result. Such a method can likewise be employed in the regeneration step. Since the temperature sensitivity is typically higher in the regeneration step than in the degradation step, it is particularly advantageous to employ the configuration as described above in the regeneration step.